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Library of Congress Cataloging‑in‑Publication Data
Names: Kurinec, Santosh K., editor. | Walia, Sumeet, editor.
Title: Energy efficient computing & electronics : devices to systems / edited
by Santosh K. Kurinec and Sumeet Walia.
Other titles: Energy efficient computing and electronics
Description: Boca Raton : CRC/Taylor & Francis, [2019] | Series: Devices,
circuits, & systems | Includes bibliographical references and index.
Identifiers: LCCN 2018042978| ISBN 9781138710368 (hardback : alk. paper) |
ISBN 9781315200705 (ebook)
Subjects: LCSH: Electronic apparatus and appliances--Power supply. | Computer
systems--Energy conservation. | Low voltage systems. | Wireless
communication systems--Energy conservation.
Classification: LCC TK7868.P6 E54 2019 | DDC 621.381028/6--dc23
LC record available at https://lccn.loc.gov/2018042978
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Energy Efficient Computing Electronics Devices to Systems Devices Circuits and Systems 1st Edition Santosh K. Kurinec
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- 6. Devices, Circuits, and Systems Series Editor Krzysztof Iniewski Emerging Technologies CMOS Inc. Vancouver, British Columbia, Canada PUBLISHED TITLES: 3D Integration in VLSI Circuits: Implementation Technologies and Applications Katsuyuki Sakuma Advances in Imaging and Sensing Shuo Tang and Daryoosh Saeedkia Analog Electronics for Radiation Detection Renato Turchetta Atomic Nanoscale Technology in the Nuclear Industry Taeho Woo Biological and Medical Sensor Technologies Krzysztof Iniewski Biomaterials and Immune Response: Complications, Mechanisms, and Immunomodulation Nihal Engin Vrana Building Sensor Networks: From Design to Applications Ioanis Nikolaidis and Krzysztof Iniewski Cell and Material Interface: Advances in Tissue Engineering, Biosensor, Implant, and Imaging Technologies Nihal Engin Vrana Circuits and Systems for Security and Privacy Farhana Sheikh and Leonel Sousa Circuits at the Nanoscale: Communications, Imaging, and Sensing Krzysztof Iniewski CMOS: Front-End Electronics for Radiation Sensors Angelo Rivetti CMOS Time-Mode Circuits and Systems: Fundamentals and Applications Fei Yuan Design of 3D Integrated Circuits and Systems Rohit Sharma
- 7. Diagnostic Devices with Microfluidics Francesco Piraino and Šeila Selimović Electrical Solitons: Theory, Design, and Applications David Ricketts and Donhee Ham Electronics for Radiation Detection Krzysztof Iniewski Electrostatic Discharge Protection: Advances and Applications Juin J. Liou Embedded and Networking Systems: Design, Software, and Implementation Gul N. Khan and Krzysztof Iniewski Energy Harvesting with Functional Materials and Microsystems Madhu Bhaskaran, Sharath Sriram, and Krzysztof Iniewski Gallium Nitride (GaN): Physics, Devices, and Technology Farid Medjdoub Graphene, Carbon Nanotubes, and Nanostuctures: Techniques and Applications James E. Morris and Krzysztof Iniewski High-Speed and Lower Power Technologies: Electronics and Photonics Jung Han Choi and Krzysztof Iniewski High-Speed Devices and Circuits with THz Applications Jung Han Choi High-Speed Photonics Interconnects Lukas Chrostowski and Krzysztof Iniewski High Frequency Communication and Sensing: Traveling-Wave Techniques Ahmet Tekin and Ahmed Emira High Performance CMOS Range Imaging: Device Technology and Systems Considerations Andreas Süss Integrated Microsystems: Electronics, Photonics, and Biotechnology Krzysztof Iniewski Integrated Power Devices and TCAD Simulation Yue Fu, Zhanming Li, Wai Tung Ng, and Johnny K.O. Sin Internet Networks: Wired, Wireless, and Optical Technologies Krzysztof Iniewski Introduction to Smart eHealth and eCare Technologies Sari Merilampi, Krzysztof Iniewski, and Andrew Sirkka PUBLISHED TITLES:
- 8. Ionizing Radiation Effects in Electronics: From Memories to Imagers Marta Bagatin and Simone Gerardin IoT and Low-Power Wireless: Circuits, Architectures, and Techniques Christopher Siu Labs on Chip: Principles, Design, and Technology Eugenio Iannone Laser-Based Optical Detection of Explosives Paul M. Pellegrino, Ellen L. Holthoff, and Mikella E. Farrell Low Power Circuits for Emerging Applications in Communications, Computing, and Sensing Fei Yuan Low Power Emerging Wireless Technologies Reza Mahmoudi and Krzysztof Iniewski Low Power Semiconductor Devices and Processes for Emerging Applications in Communications, Computing, and Sensing Sumeet Walia Magnetic Sensors and Devices: Technologies and Applications Kirill Poletkin and Laurent A. Francis Medical Imaging: Technology and Applications Troy Farncombe and Krzysztof Iniewski Metallic Spintronic Devices Xiaobin Wang MEMS: Fundamental Technology and Applications Vikas Choudhary and Krzysztof Iniewski Micro- and Nanoelectronics: Emerging Device Challenges and Solutions Tomasz Brozek Microfluidics and Nanotechnology: Biosensing to the Single Molecule Limit Eric Lagally MIMO Power Line Communications: Narrow and Broadband Standards, EMC, and Advanced Processing Lars Torsten Berger, Andreas Schwager, Pascal Pagani, and Daniel Schneider Mixed-Signal Circuits Thomas Noulis Mobile Point-of-Care Monitors and Diagnostic Device Design Walter Karlen Multisensor Attitude Estimation: Fundamental Concepts and Applications Hassen Fourati and Djamel Eddine Chouaib Belkhiat PUBLISHED TITLES:
- 9. Multisensor Data Fusion: From Algorithm and Architecture Design to Applications Hassen Fourati MRI: Physics, Image Reconstruction, and Analysis Angshul Majumdar and Rabab Ward Nano-Semiconductors: Devices and Technology Krzysztof Iniewski Nanoelectronic Device Applications Handbook James E. Morris and Krzysztof Iniewski Nanomaterials: A Guide to Fabrication and Applications Sivashankar Krishnamoorthy Nanopatterning and Nanoscale Devices for Biological Applications Šeila Selimovic´ Nanoplasmonics: Advanced Device Applications James W. M. Chon and Krzysztof Iniewski Nanoscale Semiconductor Memories: Technology and Applications Santosh K. Kurinec and Krzysztof Iniewski Noise Coupling in System-on-Chip Thomas Noulis Novel Advances in Microsystems Technologies and Their Applications Laurent A. Francis and Krzysztof Iniewski Optical, Acoustic, Magnetic, and Mechanical Sensor Technologies Krzysztof Iniewski Optical Fiber Sensors: Advanced Techniques and Applications Ginu Rajan Optical Imaging Devices: New Technologies and Applications Ajit Khosla and Dongsoo Kim Organic Solar Cells: Materials, Devices, Interfaces, and Modeling Qiquan Qiao Physical Design for 3D Integrated Circuits Aida Todri-Sanial and Chuan Seng Tan Power Management Integrated Circuits and Technologies Mona M. Hella and Patrick Mercier Radiation Detectors for Medical Imaging Jan S. Iwanczyk Radiation Effects in Semiconductors Krzysztof Iniewski PUBLISHED TITLES:
- 10. Reconfigurable Logic: Architecture, Tools, and Applications Pierre-Emmanuel Gaillardon Semiconductor Devices in Harsh Conditions Kirsten Weide-Zaage and Malgorzata Chrzanowska-Jeske Semiconductor Radiation Detection Systems Krzysztof Iniewski Semiconductor Radiation Detectors, Technology, and Applications Salim Reza Semiconductors: Integrated Circuit Design for Manufacturability Artur Balasinski Sensors for Diagnostics and Monitoring Kevin Yallup and Laura Basiricò Smart Grids: Clouds, Communications, Open Source, and Automation David Bakken Smart Sensors for Industrial Applications Krzysztof Iniewski Soft Errors: From Particles to Circuits Jean-Luc Autran and Daniela Munteanu Solid-State Radiation Detectors: Technology and Applications Salah Awadalla Structural Health Monitoring of Composite Structures Using Fiber Optic Methods Ginu Rajan and Gangadhara Prusty Technologies for Smart Sensors and Sensor Fusion Kevin Yallup and Krzysztof Iniewski Telecommunication Networks Eugenio Iannone Testing for Small-Delay Defects in Nanoscale CMOS Integrated Circuits Sandeep K. Goel and Krishnendu Chakrabarty Tunable RF Components and Circuits: Applications in Mobile Handsets Jeffrey L. Hilbert VLSI: Circuits for Emerging Applications Tomasz Wojcicki Wireless Medical Systems and Algorithms: Design and Applications Pietro Salvo and Miguel Hernandez-Silveira Wireless Technologies: Circuits, Systems, and Devices Krzysztof Iniewski PUBLISHED TITLES:
- 11. Wireless Transceiver Circuits: System Perspectives and Design Aspects Woogeun Rhee X-Ray Diffraction Imaging: Technology and Applications Joel Greenberg FORTHCOMING TITLES: Compressed Sensing for Engineers Angshul Majumdar Energy Efficient Computing: Devices, Circuits, and Systems Santosh K. Kurinec and Sumeet Walia Radio Frequency Integrated Circuit Design Sebastian Magierowski Spectral Computed Tomography: Technology and Applications Katsuyuki Taguchi, Ira Blevis, and Krzysztof Iniewski PUBLISHED TITLES:
- 12. Energy Efficient Computing & Electronics Devices to Systems Krzysztof Iniewski Managing Editor Edited by Santosh K. Kurinec Sumeet Walia
- 13. CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2019 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed on acid-free paper International Standard Book Number-13: 978-1-138-71036-8 (Hardback) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www. copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging‑in‑Publication Data Names: Kurinec, Santosh K., editor. | Walia, Sumeet, editor. Title: Energy efficient computing & electronics : devices to systems / edited by Santosh K. Kurinec and Sumeet Walia. Other titles: Energy efficient computing and electronics Description: Boca Raton : CRC/Taylor & Francis, [2019] | Series: Devices, circuits, & systems | Includes bibliographical references and index. Identifiers: LCCN 2018042978| ISBN 9781138710368 (hardback : alk. paper) | ISBN 9781315200705 (ebook) Subjects: LCSH: Electronic apparatus and appliances--Power supply. | Computer systems--Energy conservation. | Low voltage systems. | Wireless communication systems--Energy conservation. Classification: LCC TK7868.P6 E54 2019 | DDC 621.381028/6--dc23 LC record available at https://lccn.loc.gov/2018042978 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com
- 14. xi Contents Preface............................................................................................................................................ xiii Editors...........................................................................................................................................xvii Contributors.................................................................................................................................. xix Section I Emerging Low Power Devices 1. A FinFET-Based Framework for VLSI Design at the 7 nm Node..................................3 Vinay Vashishtha and Lawrence T. Clark 2. Molecular Phenomena in MOSFET Gate Dielectrics and Interfaces.........................51 S. Arash Sheikholeslam, Hegoi Manzano, Cristian Grecu, and Andre Ivanov 3. Tunneling Field Effect Transistors....................................................................................67 Amir N. Hanna and Muhammad Mustafa Hussain 4. The Exploitation of the Spin-Transfer Torque Effect for CMOS Compatible Beyond Von Neumann Computing...................................................................................93 Thomas Windbacher, Alexander Makarov, Siegfried Selberherr, Hiwa Mahmoudi, B. Gunnar Malm, Mattias Ekström, and Mikael Östling 5. Ferroelectric Tunnel Junctions as Ultra-Low-Power Computing Devices............... 157 Spencer Allen Pringle and Santosh K. Kurinec Section II Sensors, Interconnects, and Rectifiers 6. X-ray Sensors Based on Chromium Compensated Gallium Arsenide (HR GaAs:Cr)....................................................................................................................... 167 Anton Tyazhev and Oleg Tolbanov 7. Vertical-Cavity Surface-Emitting Lasers for Interconnects....................................... 195 Werner H. E. Hofmann 8. Low-Power Optoelectronic Interconnects on Two-Dimensional Semiconductors.................................................................................................................215 D. Keith Roper 9. GaN-Based Schottky Barriers for Low Turn-On Voltage Rectifiers.........................239 Nishant Darvekar and Santosh K. Kurinec
- 15. xii Contents 10. Compound Semiconductor Oscillation Device Fabricated by Stoichiometry Controlled-Epitaxial Growth and Its Application to Terahertz and Infrared Imaging and Spectroscopy................................................................................................ 267 Takeo Ohno, Arata Yasuda, Tadao Tanabe, and Yutaka Oyama Section III Systems Design and Applications 11. Low Power Biosensor Design Techniques Based on Information Theoretic Principles............................................................................................................287 Nicole McFarlane 12. Low-Power Processor Design Methodology: High-Level Estimation and Optimization via Processor Description Language.............................................301 Zheng Wang and Anupam Chattopadhyay 13. Spatio-Temporal Multi-Application Request Scheduling in Energy-Efficient Data Centers.........................................................................................................................343 Haitao Yuan, Jing Bi, and MengChu Zhou 14. Ultra-Low-Voltage Implementation of Neural Networks...........................................379 Farooq Ahmad Khanday, Nasir Ali Kant, and Mohammad Rafiq Dar 15. Multi-Pattern Matching Based Dynamic Malware Detection in Smart Phones......421 V. S. Devi, S. Roopak, Tony Thomas, and Md. Meraj Uddin Index..............................................................................................................................................443
- 16. xiii Preface Performance of electronic systems are limited by energy inefficiencies that result in over- heating and thermal management problems. Energy efficiency is vital to improving per- formance at all levels. This includes transistors to devices and to large Internet Technology and electronic systems, as well from small sensors for the Internet-of-Things (IoT) to large data centers in cloud and supercomputing systems. The electronic circuits in computer chips still operate far from any fundamental limits to energy efficiency. A report issued by the Semiconductor Industry Association and Semiconductor Research Corporation bases its conclusions on system-level energy per bit operation, which are a combination of many components such as logic circuits, memory arrays, interfaces, and I/Os. Each of these con- tributes to the total energy budget. For the benchmark energy per bit, as shown in Figure 1, computing will not be sustainable by 2040. This is when the energy required for comput- ing is estimated to exceed the world’s estimated energy production. The “benchmark” curve shows the growing energy demand for the system level energy per bit values of mainstream systems. The target system curve uses the practical lower limit system level energy per bit value, set by factors such as materials. The Landauer limit curve uses the minimal device energy per bit value provided by the Landauer’s Principle that relates to the Second Law of Thermodynamics to computation. As such, significant improvement in the energy efficiency of computing is needed. There is a consensus across the many technologies touched by our ubiquitous computing infrastructure that future performance improvements across the board are now severely limited by the amount of energy it takes to manipulate, store, and critically transport data. Revolutionary device concepts, sensors, and associated circuits and architectures that will greatly extend the practical engineering limits of energy-efficient computation are being investigated. Disruptive new device architectures, semiconductor processes, and emerging FIGURE 1 Estimated total energy expenditure for computing, directly related to the number of raw bit transitions. Source: SIA/SRC (Rebooting the IT Revolution: A Call to Action, Semiconductor Industry Association, September 2015).
- 17. xiv Preface new materials aimed at achieving the highest level of computational energy efficiency for general purpose computing systems need to be developed. This book will provide chap- ters dedicated to some of such efforts from devices to systems. The book is divided into three sections each consisting of five chapters. Section I is dedicated to device level research in developing energy efficient device structures. Non-planar finFETs dominate highly scaled processes, such as sub-20 nm, CMOS pro- cesses, due to their ability to provide lower leakage and enable continued power supply scaling (VDD). Chapter 1 gives a comprehensive overview of the finFET-based predic- tive process design kit (PDK) that supports investigation into both the circuit as well as physical design, encompassing all aspects of digital design. Prevention of various deg- radation mechanisms in transistors is the key to increase the reliability and efficiency of electronic systems. Chapter 2 discusses the understanding of molecular phenomena at the MOSFET channel/dielectric interface, focussing on ZrO2 system aimed at minimizing the degrading mechanisms in transistors. Chapter 3 provides an important insight into the nanotube Tunneling field effect transistors (TFETs) device, which promise to exhibit steep slope faster than the Boltzmann limit of 60 mV/dec. TFETs address two major challenges faced by aggressively scaled conventional CMOS technology; scaling the supply voltage (VDD) and minimizing the leakage currents. Chapter 4 gives an in-depth introduction and potential of spin based devices. It gives an overview of spintronic devices, circuits, and architecture levels that include thermally assisted (TA)-MRAM, STT-MRAM, domain wall (DW)-MRAM, spin-orbit torque (SOT)-MRAM, spin-transfer torque and spin Hall oscillators, logic-in-memory, all-spin logic, buffered magnetic logic gate grid, ternary con- tent addressable memory (TCAM), and random number generators. A large bottleneck for energy efficiency has long been the information storage units, which typically rely on charge-storage for programming and erasing. Chapter 5 describes the promise of fer- roelectric tunnel junctions (FTJs) as memristors. Memristor-based logic systems for XOR, XNOR, full-adder, DAC, and ADC outperform CMOS with as low as 50% the delay and 0.1% the power consumption. Section II deals with sensors, interconnects, and rectifiers aimed at consuming lower power. Chapter 6 provides insight into X-ray sensors based on chromium compensated GaAs for the development of modern X-ray imaging systems. Chapters 7 and 8 deal with opti- cal interconnects, which are actively being pursued to reduce power requirements and increase speed. Chapter 7 provides an overview of the challenges and developments of various types of Vertical-Cavity Surface-Emitting Laser (VCSEL)-based interconnects. Optoelectronic interactions at interconnective hetero-interfaces between nanoparticles and two-dimensional semiconductors, motivated by their enhancement of electronic and photonic properties are described in Chapter 8. Chapter 9 investigates AlGaN/GaN het- erostructures based Schottky diodes with low turn-on voltage of Vf < 0.4V, and high break- down voltage of Vbr > 400V for applications in energy efficient 230V AC-DC rectifiers. In Chapter 10, the authors have discussed the stoichiometry-controlled crystal growth tech- nique and its application to compound semiconductor oscillation devices for extending to THz region The THz wave generators can be used in applications of non-destructive evaluation, safe for human tissues. Section III has five chapters aimed at low power system designs. The imminent concern of inherent physical noise with low power biosensing mixed sig- nal CMOS technology is addressed in Chapter 11. The information rate and bit energy have been incorporated into a design methodology for detecting weak signals in the presence
- 18. xv Preface of fixed system noise. Chapter 12 provides an overview of the high-level processor archi- tecture design methodologies using Architecture Description Languages (ADLs). Chapter 13 focuses on challenging problems related to energy-efficient Cloud Data Centers (CDC). It presents how to minimize the total cost of a CDC provider in a market, and how to migrate to green cloud data centers (GCDCs). The authors propose a Temporal Request Scheduling algorithm (TRS) that can achieve higher throughput and lower grid energy cost for a GCDC. Chapter 14 presents an innovative brain inspired approach of implement- ing neural network in hardware implementation for ultra-low-voltage implementation of the perceptron and the inertial neuron. The final chapter (Chapter 15) addresses a large system-level problem of malware attacks to mobile devices. It presents a multi-pattern matching based dynamic malware detection mechanism in smart phones as an alternative to machine learning based methods. The proposed mechanism is more efficient and uses fewer resources. Thus, this book brings together a wealth of information that will serve as a valuable resource for researchers, scientists, and engineers engaged in energy efficient designs of electronic devices, circuits, and systems. The editors express their sincere appreciation to the authors who have contributed their knowledge and expertise to this book. Special thanks to Nora Konopka and Erin Harris of Taylor & Francis Group/CRC Press for their publishing efforts and coordinating with the authors. The authors also express their sin- cere appreciation for Joanne Hakim, project manager of Lumina Datamatics, for coordi- nating the production of this book.
- 20. xvii Editors Santosh K. Kurinec is a professor of electrical and microelectronic engineering at Rochester Institute of Technology (RIT), Rochester, New York. She received a PhD in physics from University of Delhi, India and worked as a scientist at the National Physical Laboratory, New Delhi, India. She worked as a postdoctoral research associate in the Department of Materials Science and Engineering at the University of Florida, Gainesville, Florida where she researched thin metal film composites. Prior to joining RIT, she was assistant profes- sor of electrical engineering at Florida State University/Florida A&M University College of Engineering in Tallahassee, Florida. She is a Fellow of Institute of Electrical and Electronics Engineers (IEEE), Member APS, NY State Academy of Sciences, and an IEEE EDS Distinguished Lecturer. She received the 2012 IEEE Technical Field Award. In 2016, she received the Medal of Honor from the International Association of Advanced Materials (IAAM). She was inducted into the International Women in Technology (WiTi) Hall of Fame in 2018. She has worked on a range of materials covering magnetic, ferroelectrics, semiconductors, photonic luminescent thin films, and metal com- posites for device applications. Her current research activities include nonvolatile memory, advanced integrated circuit materials and processes, and photovoltaics. She has over 100 publications in research journals and conference proceedings. She can be reached at Santosh.kurinec@rit.edu. Sumeet Walia is a senior lecturer and a Vice Chancellor’s Fellow at RMIT University, Melbourne, Australia. He is an expert in mate- rials engineering for nanoelectronics, sensing, and wearable devices. He has been recognized as one of the Top 10 Innovators under 35 in Asia-Pacific by the MIT Technology Review in 2017, was awarded the Victorian Young Achiever Award for Research Impact in 2017, and named among the most innovative Engineers in Australia in 2018 by Engineers Australia. He can be reached at waliasumeet@gmail.com or sumeet.walia@rmit.edu.au.
- 22. xix Contributors Jing Bi Faculty of Information Technology Beijing University of Technology Beijing, China Anupam Chattopadhyay School of Computer Science and Engineering Nanyang Technological University Singapore Lawrence T. Clark School of Electrical, Computer, and Energy Engineering Arizona State University Tempe, Arizona, USA Mohammad Rafiq Dar Department of Electronics and Instrumentation Technology University of Kashmir Srinagar, India Nishant Darvekar Electrical and Microelectronic Engineering Rochester Institute of Technology Rochester, New York, USA V. S. Devi Department of Computer Science Indian Institute of Information Technology and Management – Kerala (IIITM-K) Thiruvananthapuram, India Mattias Ekström Department of Electronics KTH Royal Institute of Technology Kista, Sweden Cristian Grecu Department of Electrical and Computer Engineering University of British Columbia Vancouver, British Columbia, Canada Amir N. Hanna Department of Electrical Engineering King Abdullah University of Science and Technology (KAUST) Thuwal, Saudi Arabia Werner H. E. Hofmann Institute of Solid State Physics and Center of Nanophotonics Technical University of Berlin Berlin, Germany Muhammad Mustafa Hussain Department of Electrical Engineering King Abdullah University of Science and Technology (KAUST) Thuwal, Saudi Arabia Andre Ivanov Department of Electrical and Computer Engineering University of British Columbia Vancouver, British Columbia, Canada Nasir Ali Kant Department of Electronics and Instrumentation Technology University of Kashmir Srinagar, India Farooq Ahmad Khanday Department of Electronics and Instrumentation Technology University of Kashmir Srinagar, India Santosh K. Kurinec Electrical and Microelectronic Engineering Rochester Institute of Technology Rochester, New York, USA
- 23. xx Contributors Hiwa Mahmoudi Institute of Electrodynamics, Microwave and Circuit Engineering TU Wien Vienna, Austria Alexander Makarov Institute for Microelectronics TU Wien Vienna, Austria B. Gunnar Malm Department of Electronics KTH Royal Institute of Technology Kista, Sweden Hegoi Manzano Department of Condensed Matter Physics University of the Basque Country UPV/ EHU Barrio Sarriena s/n, 48330 Leioa, Bizkaia, Spain Nicole McFarlane Min H. Kao Department of Electrical Engineering & Computer Science Tickle College of Engineering The University of Tennessee Knoxville, Tennessee, USA Takeo Ohno Department of Innovative Engineering Oita University Oita, Japan Mikael Östling Department of Electronics KTH Royal Institute of Technology Kista, Sweden Yutaka Oyama Department of Materials Science and Engineering Tohoku University Sendai, Japan Spencer Allen Pringle Electrical & Microelectronic Engineering Rochester Institute of Technology Rochester, New York, USA S. Roopak Department of Computer Science Indian Institute of Information Technology and Management – Kerala (IIITM-K) Thiruvananthapuram, India D. Keith Roper Ralph E. Martin Department of Chemical Engineering and Microelectronics-Photonics Graduate Program and Institute for Nanoscience and Engineering University of Arkansas Fayetteville, Arkansas, USA Siegfried Selberherr Institute for Microelectronics TU Wien Vienna, Austria S. Arash Sheikholeslam Department of Electrical and Computer Engineering University of British Columbia Vancouver, British Columbia, Canada Tadao Tanabe Department of Materials Science and Engineering Tohoku University Sendai, Japan
- 24. xxi Contributors Tony Thomas Department of Computer Science Indian Institute of Information Technology and Management – Kerala (IIITM-K) Thiruvananthapuram, India Oleg Tolbanov Functional Electronics Laboratory Tomsk State University Tomsk, Russia Anton Tyazhev Functional Electronics Laboratory Tomsk State University Tomsk, Russia Md. Meraj Uddin Department of Computer Science Indian Institute of Information Technology and Management – Kerala (IIITM-K) Thiruvananthapuram, India Vinay Vashishtha School of Electrical, Computer, and Energy Engineering Arizona State University Tempe, Arizona, USA Zheng Wang Shenzhen Institute of Advanced Technology Chinese Academy of Sciences Beijing, China Thomas Windbacher Institute for Microelectronics TU Wien Vienna, Austria Arata Yasuda Department of Creative Engineering National Institute of Technology Tsuruoka College Tsuruoka, Japan Haitao Yuan School of Software Engineering Beijing Jiaotong University Beijing, China MengChu Zhou Helen and John C. Hartmann Department of Electrical and Computer Engineering New Jersey Institute of Technology Newark, New Jersey, USA
- 26. Section I Emerging Low Power Devices
- 28. 3 1 A FinFET-Based Framework for VLSI Design at the 7 nm Node Vinay Vashishtha and Lawrence T. Clark CONTENTS 1.1 Introduction.............................................................................................................................4 1.1.1 Chapter Outline..........................................................................................................5 1.2 ASAP7 Electrical Performance..............................................................................................5 1.3 Lithography Considerations..................................................................................................7 1.3.1 Lithography Metrics and Other Considerations for Design Rule Determination.............................................................................................................7 1.3.1.1 Critical Dimension Uniformity (CDU).....................................................7 1.3.1.2 Overlay..........................................................................................................8 1.3.1.3 Mask Error Enhancement Factor (MEEF) and Edge Placement Error (EPE)....................................................................................................8 1.3.1.4 Time-Dependent Dielectric Breakdown (TDDB).....................................9 1.3.2 Single Exposure Optical Immersion Lithography.................................................9 1.3.3 Multi-Patterning Approaches................................................................................. 10 1.3.3.1 Litho-Etchx (LEx)......................................................................................... 10 1.3.3.2 Self-Aligned Multiple Patterning (SAMP).............................................. 11 1.3.3.3 Multiple Patterning Approach Comparison..........................................12 1.3.4 Extreme Ultra Violet Lithography (EUVL)............................................................15 1.3.4.1 EUVL Necessity.........................................................................................15 1.3.4.2 EUVL Description and Challenges......................................................... 16 1.3.4.3 EUVL Advantages.....................................................................................17 1.3.5 Patterning Cliffs........................................................................................................17 1.3.6 Design Technology Co-Optimization (DTCO)..................................................... 18 1.4 Front End of Line (EOL) and Middle of Line (MOL) Layers..........................................18 1.5 Back End of Line (BEOL) Layers.........................................................................................20 1.5.1 Self-Aligned Via (SAV) and Barrier Layer.............................................................21 1.5.2 EUV Lithography Assumptions and Design Rules.............................................22 1.5.3 Multiple Patterning (MP) Optical Lithography Assumptions and Design Rules..............................................................................................................23 1.5.3.1 Patterning Choice.......................................................................................23 1.5.3.2 SADP Design Rules and Derivations......................................................24
- 29. 4 Energy Efficient Computing & Electronics 1.1 Introduction Recent years have seen finFETs dominate highly scaled (e.g., sub-20 nm) complementary metal-oxide-semiconductor (CMOS) processes (Wu et al. 2013; Lin et al. 2014) due to their ability to alleviate short channel effects, provide lower leakage, and enable some continued VDD scaling. However, availability of a realistic finFET-based predictive process design kit (PDK) for academic use that supports investigation into both circuit, as well as physical design, encompassing all aspects of digital design, has been lacking. While the finFET-based FreePDK15 was supplemented with a standard cell library, it lacked full physical verification, layout vs. schematic check (LVS) and parasitic extraction (Bhanushali et al. 2015; Martins et al. 2015) at the time of development of the PDK described in this chapter. Consequently, the only available sub-45 nm educational PDKs are the planar CMOS-based Synopsys 32/28 nm and FreePDK45 (45 nm PDK) (Stine et al. 2007; Goldman et al. 2013). The cell libraries available for those processes are not very realistic since they use very large cell heights, in contrast to recent industry trends. Additionally, the static random access memory (SRAM) rules and cells provided by these PDKs are not realistic. Because finFETs have a 3-D structure and result in significant density impact, using planar libraries scaled to sub-22 nm dimensions for research is likely to give poor accuracy. Commercial libraries and PDKs, especially for advanced nodes, are often difficult to obtain for academic use and access to the actual physical layouts is even more restricted. Furthermore, the necessary non-disclosure agreements (NDAs) are unmanageable for 1.6 Cell Library Architecture....................................................................................................25 1.6.1 Gear Ratio and Cell Height.....................................................................................25 1.6.2 Fin Cut Implications.................................................................................................26 1.6.3 Standard Cell Middle of Line (MOL) Usage.........................................................27 1.6.4 Standard Cell Pin and Signal Routing...................................................................29 1.6.5 Library Collaterals....................................................................................................31 1.6.6 DTCO Driven DR Changes Based on APR Results.............................................32 1.7 Automated Place and Route with ASAP7.........................................................................33 1.7.1 Power Routing and Self-Aligned Via (SAV)..........................................................33 1.7.2 Scaled Layout Exchange Format (LEF) and QRC TechFile.................................33 1.7.3 Design Experiments and Results............................................................................35 1.8 SRAM Design........................................................................................................................37 1.8.1 FinFET Implications and Fin Patterning...............................................................38 1.8.2 Statistical Analysis....................................................................................................39 1.8.3 SRAM Cell Design and DTCO Considerations....................................................40 1.8.3.1 MOL Patterning..........................................................................................40 1.8.3.2 1-D Cell Metallization...............................................................................41 1.8.3.3 Stability and Yield Analysis.....................................................................41 1.8.4 Array Organization and Column Design.............................................................44 1.8.5 Write Assist................................................................................................................45 1.9 Chapter Summary................................................................................................................45 References........................................................................................................................................46
- 30. 5 A FinFET-Based Framework for VLSI Design at the 7 nm Node large university classes, and the plethora of design rules can distract from the key points. NDAs also make it difficult for the publication of physical design as these may disclose proprietary design rules and structures. This chapter focuses on the development of a realistic PDK for academic use that overcomes these limitations. The PDK, developed for the N7 node even before 7 nm processes were available in the industry, is thus predictive. The predictions have been based on publications of the continually improving lithography, as well as our estimates of what would be available at N7. The original assumptions are described in Clark et al. (2016). For the most part, these assumptions have been accurate, except for the expectation that extreme ultraviolet (EUV) lithography would be widely available, which has turned out to be optimistic. The background and impact on design technology co-optimization (DTCO) for standard cells and SRAM comprises this chapter. The treatment here includes learning from using the cells originally derived in Clark et al. (2016) in realistic designs of SRAM arrays and large digital designs using automated place and route tools. 1.1.1 Chapter Outline The chapter first outlines the important lithography considerations in Section 1.3. Metrics for overlay, mask errors and other effects that limit are described first. Then, modern liquid immersion optical lithography and its use in multiple patterning (MP) techniques that extend it beyond the standard 80 nm feature limit are discussed. This sets the stage for a discussion of EUV lithography, which can expose features down to about 16 nm in a single exposure, but at a high capital and throughput cost. This section ends with a brief overview of DTCO. DTCO has been required on recent processes to ensure that the very limited possible structures that can be practically fabricated are usable to build real designs. Thus, a key part of a process development is not just to determine transistor and interconnect structures that are lithographically possible, but also ensuring that successful designs can be built with those structures. This discussion is carried out by separating the front end of line (FEOL), middle of line (MOL), and back end of line (BEOL) portions of the process, which fabricate the transistors, contacts and local interconnect, and global interconnect metallization, respectively. The cell library architecture and automated placement and routing (APR) aspects comprise the next section, which with the SRAM results, comprise most of the discussion. The penultimate section describes the SRAM DTCO and array development and performance in the ASAP7 predictive PDK. The final chapter section summarizes. 1.2 ASAP7 Electrical Performance The PDK uses BSIM-CMG SPICE models and the value used are derived from publicly available sources with appropriate assumptions (Paydavosi et al. 2013). Drive current increase from 14 to 7 nm node is assumed to be 15%, which corresponds to the diminished Idsat improvement over time. In accordance with modern devices, saturation current was assumed to be 4.5× larger than that in the linear region (Clark et al. 2016). A relaxed 54 nm contacted poly pitch (CPP) allows a longer channel length and helps
- 31. 6 Energy Efficient Computing & Electronics with the assumption of a near ideal subthreshold slope (SS) of 60 mV/decade at room temperature, along with a drain-induced barrier lowering (DIBL) of approximately 30 mV/V. P-type metal-oxide-semiconductor (PMOS) strain seems to be easier to obtain according to the 16 and 14 nm foundry data and larger Idsat values for PMOS than those for n-type metal-oxide-semiconductor (NMOS) have been reported (Wu et al. 2013; Lin et al. 2014). Following this trend, we assume a PMOS to NMOS drive ratio of 0.9:1. This value provides good slew rates at a fan-out of six (FO6), instead of the traditional four. Despite the same drawn gate length, the PDK and library timing abstract views sup- port four threshold voltage flavors, viz. super low voltage threshold (SLVT), low voltage threshold (LVT), regular threshold voltage (RVT), and SRAM, to allow investigation into both high performance and low-power designs. The threshold voltage is assumed to be changed through work function engineering. For SRAM devices the very low leakage using both a work function change and lightly doped drain (LDD) implant removal. The latter results in an effective channel length (Leff) increase, GIDL reduc- tion and, the overlap capacitance reduction. The drive strength reduces from SLVT to SRAM. The SRAM Vth transistors are convenient option for use in retention latches and designs that prioritize low-standby power. In addition to typical-typical (TT) models, fast-fast (FF) and slow-slow (SS) models are also provided for multi-corner APR opti- mization. Tables 1.1 and 1.2 show the electrical parameters for single fin NMOS and PMOS, respectively, for the TT corner at 25°C (Clark et al. 2016). The nominal operating voltage is VDD = 700 mV. TABLE 1.1 NMOS Typical Corner Parameters (per fin) at 25°C Parameter SRAM RVT LVT SLVT Idsat (μA) 28.57 37.85 45.19 50.79 Ieff (μA) 13.07 18.13 23.56 28.67 Ioff (nA) 0.001 0.019 0.242 2.444 Vtsat (V) 0.25 0.17 0.10 0.04 Vtlin (V) 0.27 0.19 0.12 0.06 SS (mV/decade) 62.44 63.03 62.90 63.33 DIBL (mV/V) 19.23 21.31 22.32 22.55 TABLE 1.2 PMOS Typical Corner Parameters (per fin) at 25°C Parameter SRAM RVT LVT SLVT |Idsat| (μA) 26.90 32.88 39.88 45.60 |Ieff| (μA) 11.37 14.08 18.18 22.64 |Ioff| (nA) 0.004 0.023 0.230 2.410 Vtsat (V) −0.20 −0.16 −0.10 −0.04 Vtlin (V) −0.22 −0.19 −0.13 −0.07 SS (mV/decade) 64.34 64.48 64.44 64.94 DIBL (mV/V) 24.10 30.36 31.06 31.76
- 32. 7 A FinFET-Based Framework for VLSI Design at the 7 nm Node 1.3 Lithography Considerations Photolithography, hereinafter referred to simply as lithography, in a semiconductor industry context, refers to a process whereby a desired pattern is transferred to a target layer on the wafer through use of light. Interconnect metal, via, source-drain regions, and gate layers in a CMOS process stack are a few examples of the patterns defined, or “printed,” using lithography. A simplified pattern transfer flow is as follows. From among the pattern information that is stored in an electronic database file (GDSII) corresponding to all the layers of a given integrated circuit (IC) design, the enlarged pattern, or its photographic negative, corresponding to a single layer is inscribed onto a photomask or reticle. The shapes on the photomask, hereinafter referred to as mask, define the regions that are either opaque or transparent to light. Light from a suitable source is shone on the mask through an illuminator, which modifies the effective manner of illumination, and passes through the transparent mask regions. Thereafter, light passes through a projection lens, which shrinks the enlarged pattern geometries on the mask to their intended size, and exposes the photoresist that has been coated on the wafer atop the layer to be patterned. The photoresist is developed to either discard or retain its exposed regions corresponding to the pattern. This is followed by an etch that removes portions of the target layer not covered by the photoresist, which is then removed, leaving behind the intended pattern on the layer. Both lines and spaces can be patterned through this approach with some variations in the process steps. Lithography plays a leading role in the scaling process, which is the industry’s primary growth driver, as it determines the extent to which feature geometries can be shrunk in suc- cessive technology nodes. Lithography is one of the most expensive and complex procedures in semiconductor manufacturing, with mask manufacturing being the most expensive pro- cessing steps within lithography (Ma et al. 2010). Both complexity and the number of masks used for manufacturing at a node affect the cost, and an increase in either of these can increase the cost to the point of becoming the limiting factor in the overall cost of the product. As in any other manufacturing process, the various lithography steps also suffer from variability. The lithographic resolution determines the minimum feature dimension, called critical dimension (CD), for a given layer and is based on the lithography technique employed at a particular technology node. Design rules (DRs) constitute design guidelines to minimize the effects from mask manufacturability issues, the impact of variability and layer misalignment, and ensure printed pattern fidelity to guarantee circuit operation at good yield. Thus, ascertaining these DRs requires consideration of the following lithography-related metrics that can cause final printed pattern on a layer to deviate from the intent and/or result in reliability issues. 1.3.1 Lithography Metrics and Other Considerations for Design Rule Determination 1.3.1.1 Critical Dimension Uniformity (CDU) Critical dimension uniformity (CDU) relates to the consistency in the dimensions of a feature printed in resist. CD variations arise due to a number of factors—wafer temperature and photoresist thickness, to name a few. It is defined by CDU CDU CDU CDU E F M = + + 2 2 2 2 , (1.1)
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- 34. "W CHAPTER XIX. ARTHUR'S PLAN. HY, the madam is going to drive, too, and I've come to harness; there'll be a row somewhere," John said. "Can't help it," Charles replied. "Mr. Arthur wants the phaeton, and will have it for all of madam." "Yes, I s'p'o' so. Wall, I'll go and tell her," was John's rejoinder, as he started for the house, where Mrs. Tracy was just drawing on her long driving gloves and admiring her new hat and feather before the glass. Dolly looked almost as young, and far prettier, than when she came to the park, years before. A life of luxury suited her. She had learned to take things easily, and the old woman with the basket might now come every day to her kitchen door without her knowing it. She aped Mrs. Atherton, of Brier Hill, in everything, and had the satisfaction of knowing that she was on all occasions quite as stylish- looking and well-dressed as that aristocratic lady whom she called her intimate friend. She had also grown very proud and very exclusive in her ideas, and when poor Mrs. Peterkin, who was growing, too, with her million, ventured to call at the park, the call was returned with a card which Dolly's coachman left at the door. Since the night of her party, and the election which followed, when Frank was defeated, she had ignored the Peterkins, and laughed at what she called their vulgar imitation of people above them, and when she heard that Mary Jane had hired a governess for her two children, Bill and Ann Eliza, she scoffed at the airs assumed by come-up people, and wondered if Mrs. Peterkin had forgotten that she was one of Grace Atherton's hired girls. Dolly had certainly forgotten the Langley life, and was to all intents and purposes the
- 35. great lady of the park, who held herself aloof from the common herd, and taught her children to do the same. She had seen Jerry enter the house that morning with a feeling of disapprobation, which had not diminished as the day wore on and still the child staid, and what was worse, Maude was not sent for to join her. "Not that I would have allowed it, if she had been," she said to herself, for she did not wish her daughter intimate with one of whose antecedents nothing was known, but Arthur might at least have invited her. He had never noticed her children much, and this she deeply resented. Maude, who knew of Jerry's presence in the house, had cried to go and play with her, but Mrs. Tracy had refused, and promised as an equivalent a drive in the phaeton around the town. And it was for this drive Dolly was preparing herself, when John came with the message that she could not have the phaeton, as Mr. Arthur was going to take Jerry home in it. Usually Arthur's slightest wish was a law in the household, for that was Frank's order; but on this occasion Dolly felt herself justified in rebelling. "Not have the phaeton! That's smart I must say," she exclaimed. "Can't that child walk home. I'd like to know? Tell Mr. Tracy Maude has had the promise of a drive all day, and I am ready, with my things on. Ask him to take the Victoria; he never drives." All this in substance was repeated to Arthur, who answered, quietly: "Let Mrs. Tracy take the Victoria. I prefer the phaeton myself." That settled it, and in a few moments Jerry was seated at Arthur's side, and skimming along through the park, and out upon the highway which skirted the river for miles. "This is not going home, and grandma will scold," Jerry said. "Never mind grandma—I will make it right with her. I am going to show you the country," Arthur replied, as he chirruped to the fleet
- 36. pony who seemed to fly along the smooth road. No one who saw the tall, elegant-looking man, who sat so erect, and handled the reins so skillfully, would have suspected him of insanity, and more than one stopped to look after him and the little girl whose face looked out from the white sun bonnet with so joyous an expression. On the homeward route they met the Victoria, with John upon the box, and Mrs. Tracy and Maude inside. "There's Maude! Hallo, Maude—see me! I'm riding!" Jerry called out, cheerily, while Maude answered back: "Hallo, Jerry!" But Mrs. Tracy gave no sign of recognition, and only rebuked her daughter for her vulgarity in saying "Hallo," which was second class and low. "Then Nina St. Claire is second class and low, for she says 'Hallo,'" was Maude's reply, to which her mother had no answer. Meanwhile the phaeton was going swiftly on toward the cottage, which it reached a few minutes after the furnace whistle blew for six, and Harold, who had been working there, came up the lane. There were soiled spots on his hands, and on his face, and his clothes showed marks of toil, all of which Arthur noticed, while he was explaining to Mrs. Crawford that he had taken Jerry for a drive, and kept her beyond the prescribed hour. Then, turning to Harold, he said: "And so you work in the furnace?" "Yes, sir, during vacation, when I can get a job there," Harold answered, and Mr. Tracy continued: "How much do you get a day?" "Fifty cents in dull times," was the reply, and Arthur went on: "Fifty cents from seven in the morning to six at night, and board yourself. A magnificent sum, truly. Pray, how do you manage to spend so much? You must be getting rich."
- 37. The words were sarcastic, but the tone belied the words, and Harold was about to speak, when his grandmother interrupted him, and said: "What he does not spend for us he puts aside. He is trying to save enough to go to the High School, but it's slow work. I can do but little myself, and it all falls upon Harold." "But I like it, grandma. I like to work for you and Jerry, and I have almost twenty dollars saved," Harold said, "and in a year or two I can go away to school, and work somewhere for my board. Lots of boys do that." Arthur was hitching his pony to the fence, while a new idea was dawning in his mind. "Fifty cents a day," he said to himself, "and he has twenty dollars saved, and thinks himself rich. Why, I've spent more than that on one bottle of wine, and here is this boy, Amy's son, wanting an education, and working to support his grandmother like a common laborer. I believe I am crazy." He was in the cottage by this time—in the clean, cool kitchen where the supper table was laid with its plain fare, wholly unlike the costly viands which daily loaded his board. "Don't wait for me, Harold must be hungry," he said, adding quickly: "Or stay if you will permit me, I will take a cup of tea with you. The drive has given me an appetite, and your tea smells very inviting." It was a great honor to have Arthur Tracy at her table, and Mrs. Crawford felt it as such, and was very sorry, too, that she had nothing better to offer him than bread and butter and radishes, with milk, and a dish of cold beans, and chopped beets, and a piece of apple pie saved for Harold from dinner. But she made him welcome, and Jerry, delighted to return the hospitality she had received, brought him a clean plate and cup and saucer, and asked if she might get the best sugar-bowl and the white sugar. Then, remembering the beautiful flowers which had adorned the table at
- 38. Tracy Park, she ran out, and gathering a bunch of June pinks, put them in a little glass by his plate. When all was ready and they had taken their seats at the table, Mrs. Crawford closed her eyes reverently and asked the accustomed blessing which in that house preceded every meal. Jerry's amen was a good deal louder and more emphatic than usual, while she nodded her head to Arthur, with an expression which he understood to mean, "You know now what you ought to say, instead of that long prayer," and he nodded back that he did so understand it. Arthur enjoyed the supper immensely, or pretended that he did. He ate three slices of bread and butter; he drank three cups of tea; he even tried the beans and the beets, but declined the radishes, which, he said, would give him nightmare. When supper was over and the table cleared away, he still showed no signs of going, but asking Mrs. Crawford to take a seat near him, he plunged at once into the business which had brought him there, and which, since he had seen Harold in his working-dress and heard what he was trying to do, had grown to be of a two-fold nature. He was very lonely, he said, and the little taste he had had of Jerry's society had made him wish for more, and he must have her with him a part at least of every day. "In short," he said, "I should like to undertake her education myself until she is older, when I will see that she has the proper finishing. She tells me she hates the district school, with Bill Peterkin and his warts—" "Trying to kiss me," Jerry interrupted, as, open-eyed and open- mouthed, she stood, with her hand on his shoulder, listening to him. "Yes, trying to kiss you, though I do not blame him much for that," Arthur said, with a smile, and then continued: "She is ambitious enough to want a governess like Ann Eliza Peterkin and my brother's daughter, but I am better than a dozen governesses. I can teach her all the rudiments of an English education, with French and German, and Latin, too, if she likes; and my plan is, that she
- 39. shall come to me every day, except Saturdays and Sundays, at ten in the morning, get her lessons and her lunch with me, and return home at four in the afternoon. Would you like it, Cherry?" "Oh-h-oh!" was all the answer Jerry could make for a moment, but her cheeks were scarlet, and tears of joy stood in her eyes, until she glanced at Harold; then all the brightness faded from her face, for how could she accept this great good and leave him to drudge and toil alone? "What is it, Cherry?" Mr. Tracy asked; and, with a half sob, she replied: "I can't go without Harold. If I get learning, he must get learning, too," and leaving Arthur, she crossed over to the boy, and putting her arm around him, looked up at him with a look which in after years he would have given half his life to win. "I shall not forget Harold," Arthur hastened to say, "and I have something better in store for him than reciting his lessons to me. When the High School opens in September, he is going there, and if he does well he shall go to Andover in time, and perhaps to Harvard. It will all depend upon himself, and how he improves his opportunities. What! crying? Don't you like it?" Arthur asked, as he saw the tears gathering in Harold's eyes and rolling down his cheeks. "Yes, oh, yes; but it don't seem real, and—and—I guess it makes me kind of sick," Harold gasped, as, freeing himself from Jerry's encircling arm, he hurried from the room, to think over this great and unexpected joy which had come so suddenly to him. With his naturally refined tastes and instincts the dirty furnace work was not pleasant to him, neither were the many menial duties he was obliged to perform for the sake of those he loved. How to get an education was the problem he was earnestly trying to solve, and lo! it was solved for him. For a moment the suddenness of the thing overcame him, and he sat down upon a block of wood in the
- 40. yard, faint and bewildered, while Arthur made his plan clear to Mrs. Crawford, saying that what he meant to do was partly for Jerry's sake and partly for the sake of the young girl who had been his early love. "I always intended to take care of you," he said; "but things go from my mind, and I forget the past as completely as if it had never been. But this will stay by me, for I shall have Cherry as a reminder, and if I am in danger of forgetting she will jog my memory." For a moment Mrs. Crawford could not speak, so great was her surprise and joy that the good she had thought unattainable was to be Harold's at last. And yet something in her proud, sensitive nature rebelled against receiving so much from a stranger, even if that stranger were Arthur Tracy. It seemed like charity, she said. But Arthur overruled her with that persuasive way he had of converting people to his views; and when at last he left the cottage it was with the understanding that Jerry should commence her lessons with him the first week in September, and that Harold should enter the High School in Shannondale when it opened in the autumn.
- 41. A CHAPTER XX. THE WORKING OF ARTHUR'S PLAN. S Arthur was wholly uncommunicative with regard to his affairs, and as Mrs. Crawford kept her own counsel, and bade Harold and Jerry do the same, the Tracy's knew nothing whatever of the plan until the September morning when Jerry presented herself at the park house, and was met in the door-way by Mrs. Frank, who was just going out. Very few could have resisted the bright little face, so full of childish happiness, or the clear, assured voice, which said so cheerily: "Good-morning, Mrs. Tracy. I'm come to school." But, prejudiced as she was against the girl, Mrs. Tracy could resist any thing, and she answered, haughtily: "Come to school! What do you mean! This is not a school-house, and if you have any errand here, go round to the other door. Only company come in here." "But I'm company. I'm going to get learning; he told me to come," Jerry answered, flushed and eager, and altogether sure of her right to be there. Before Mrs. Frank could reply, a voice, distinct and authoritative, and to which she always yielded, called from the top of the stairway inside: "Mrs. Tracy, if that is Jerry to whom you are talking, send her up at once. I am waiting for her." Jerry did not mean the nod she gave the lady as she passed her to be disrespectful, but Mrs. Frank felt it as such, and went to her own room in a most perturbed state of mind, for which she could
- 42. find no vent until her husband came in, when she stated the case to him, and asked if he knew what it meant. But Frank was as ignorant as herself, and could not enlighten her until that night, after he had seen his brother, and heard from him what he was intending to do. "God bless you, Arthur. You don't know how happy you have made me," Frank said, feeling on the instant that a great burden was lifted from his mind. Jerry was to be educated and cared for, and would probably receive all that the world would naturally concede to her if the truth were known. He believed, or thought he did, that Gretchen had never been his brother's wife, though to believe so seemed an insult to the original of the sweet face which looked at him from the window every time he entered his brother's room. Jerry was a great trouble to him, and he would not have liked to confess to any one how constantly she was in his mind, or how many plans he had devised in order to atone for the wrong he knew he was doing her. And now his brother had taken her off his hands, and she was to be cared for and receive the education which would fit her to earn her own livelihood, and make her future life respectable. No particular harm was done her after all, and he might now enjoy himself, and cast his morbid fancies to the winds, he reflected, as he went whistling to his wife's apartment, and told her what he heard. For a moment Dolly was speechless with astonishment, and when at last she opened her lips, her husband silenced her with that voice and manner of which she was beginning to be afraid. It was none of their business, he said, what Arthur did in his own house, provided, they were not molested, and if he chose to turn schoolmaster, he had a right to do so. For his part, he was glad of it, as it saved him the expense of Jerry's education, for if Arthur had not taken it in hand, he should, and Dolly was to keep quiet and let the child come and go in peace.
- 43. After delivering himself of these sentiments, Frank went away, leaving his wife to wonder, as she had done more than once, if he, too, were not a little crazy, like his brother. But she said no more about Jerry's coming there, except to suggest that she might at least come in at the side door instead of the front, especially on muddy days when she was liable to soil the costly carpets. And Jerry, who cared but little how she entered the house, if she only got in, came through the kitchen after the second day, and wiped her feet upon the mat; and once, when her shoes were worse than usual, took them off, lest they should leave a track. It is not our intention to linger over the first few months of Jerry's school days at Tracy Park, but rather to hasten on to the summer four years after her introduction to Tracy Park as Arthur's pupil. During all that time he had never once seemed to be weary of the task he had imposed upon himself, but, on the contrary, his interest had deepened in the child who developed so rapidly under his training that he sometimes looked at her in astonishment, marveling more and more who she was, and from whom she had inherited her wonderful memory and power to grasp points which are usually far beyond the comprehension of a child of ten, or even twelve, and which Maude Tracy could no more have mastered than her brother, the stupid Jack, whose intellect had not grown with his body. There was a tutor now at Tracy Park for Jack, but Maude had been transferred to Arthur's care. This was wholly due to Jerry, who alone could have induced him to let Maude share her instruction. Arthur did not care for Maude. She was dull, he said, and would never have her lessons. But Jerry coaxed so hard that Arthur consented at last, and when Jerry had been with him about three years, Maude became his pupil, and that of Jerry as well, for nearly every day when the lessons were over, the two little girls might have been seen sitting together under the trees in the park, or in some corner of the house, Maude puzzled, and perplexed, and worried, and Jerry anxious, decided, and peremptory, as she went over and over again with what was so clear to her and so hazy to her friend.
- 44. "Oh, dear me, suz, what does ail you?" she said one day, with a stamp of her foot, after she had tried in vain to make Maude see through a simple sum in long division. "Can't you remember first to divide, second multiply, third subtract, and fourth bring down?" "No, I can't. I can't remember anything, and if I could, how do I know what to divide or what to bring down? I am stupid, and shall never know anything," was Maude's sobbing reply, as she covered her face with her slate. Maude's tears always moved Jerry, who tried to comfort her with the assurance that if she tried very hard, she might some time know enough to teach a district school. This was the height of Jerry's ambition, to teach a district school and board around; but Maude's aspirations were different. She was rich. She was to be a belle and wear diamonds and satins like her mother; and it did not matter so much whether she understood long division or not, though it did hurt her a little to be so far outstripped by Jerry, who was younger than herself. To Arthur, Jerry was a constant delight and surprise, and nothing astonished or pleased him more than the avidity with which she took up German. This language was like play to her, and by the time she was ten years old she spoke, and read, and wrote it almost as well as Arthur himself. "It takes me back somewhere, I can't tell where," she said to him; "and I seem to be somebody else than Jerry Crawford, and I hear music and see people, and a pale face is close to me, and my head gets all confused trying to remember things which come and go." Only once after her first day at the park had she enacted the pantomime of the sick woman and the nurse, and then she had done it at Arthur's request. But it was not quite as thrilling as at first; the him for whom the dying woman had prayed was omitted, and the whole was mixed with the Tramp House, and the carpet-bag, and Harold, who was now a youth of seventeen, and a student at
- 45. the high-school in Shannondale, where he was making as rapid progress in his studies as Jerry was at the park. But Harold's life was not as serene and happy as Jerry's, for it was not pleasant for him to hear, as he often did, that he was a charity student, supported by Arthur Tracy. Such remarks were very galling to the high-spirited boy, and he was constantly revolving all manner of schemes by which he could earn money and cease to be dependent. All through the long summer vacations, he worked at whatever he could find to do, sometimes in people's gardens, sometimes on their lawns, but oftener in the hay-fields, where he earned the most. Here Jerry was not unfrequently his companion. She liked to rake hay, she said; it came natural to her, and she had no doubt she inherited the taste from her mother, who had probably worked in the fields in Germany. One afternoon, when Jerry knew that Harold was busy in one of Mr. Tracy's meadows, she started to join him, for he had complained of a headache at noon, and had expressed a fear that he might not be able to finish the task he had imposed upon himself. The road to the field was by the Tramp House, which looked so cool and quiet, with its thick covering of woodbine and ivy over it, that Jerry turned aside for a moment to look into the room which had so great a fascination for her, and where she spent so much time. Indeed, she seldom passed near it without going in for a moment and standing by the old table which had once held her and her dead mother. Things came back to her there, she said, and she could almost give a name to the pale-faced woman who haunted her so often. As she entered the damp, dark place now, she started with an exclamation of surprise, which was echoed by another, as Frank Tracy sprang up and confronted her. It was not often that he visited the Tramp House, and he would not have confessed to any one his superstitious dread of it, or that, when he was in it, he always had a feeling that the dead woman found there years ago would start up to accuse him of his deceit and hypocrisy. Could he have had his way he would have pulled the building down; but it was not his, and
- 46. when he suggested it to Arthur, as he sometimes did, the latter opposed it, saying latterly, since Jerry had been so much to him; "No, Frank; let it stand. I like it, because, but for it, Jerry might have perished with her mother, and I should not have had her with me." So the Tramp House stood, and grew damper and mustier each year, as the moss and ivy gathered on the walls outside, and the dust and cobwebs gathered on the walls within. These, however, Jerry was careful to brush away, for she had a play-house in one corner, and a little work-bench and chair, and she often sat there alone and talked to herself, and the woman dead so long ago, and to others whose faces were dim and shadowy, but whom she felt sure she had known. Very frequently she went through the process of cleaning up, as she called it, and her object in stopping there now was, in part, to see if it did not need her care again. "Oh, Mr. Tracy! are you here? How you scared me! I thought it was a tramp!" she said, as he came toward her. "Do you come here often?" he asked, as he offered her his hand. "Yes, pretty often. I like it, because mother died here, and sometimes I feel as if she would make it known to me here who she was. I talk to her and ask her to tell me, but she never has. Oh, don't you wish she would?" Frank shuddered involuntarily, for to have Jerry told who she really was, was the last thing he could desire, but as a criminal is said always to talk about the crime he has committed and is hiding, so Frank, when with Jerry, felt impelled to talk with her of the past and what she could remember of it. Seating himself upon the bench with her at his side, he said: "And you really believe the woman found here was your mother?" "Why, yes. Don't you? Who was my mother, if she wasn't?" and Jerry's eyes opened wide as they looked at him.
- 47. "I don't know, I am sure. Does my brother talk of Gretchen now?" was the abrupt reply. "Yes, at times," Jerry answered; "and yesterday, after I sang him a little German song, which he taught me, he had them pretty bad— the bees in his head, I mean; that is what he calls it when things are mixed; and he says he is going to write to her, or her friends." "Write to her! I thought he had given that up. I thought he——Did he say, 'Write to her friends?'" Frank gasped, as he felt himself grow cold and sick with this threatened danger. Arthur had seemed so quiet and happy with Jerry, and had said so little of Gretchen, that Frank had grown quite easy in his mind, and the black shadow of fear did not trouble him as much as formerly. But now it was over him again, and grew in intensity as he questioned the child. "Have you ever tried to find out who Gretchen is?" he asked, at last. "No," she replied, "but I guess she is his wife." "Yes," Frank said, falteringly, "his wife; and where do you think she lived?" "Oh, I know that. In Wiesbaden. He told me so once, and it seems as if I had been there, too, when he talked about it, and I hear the music and see the flowers, and a white-faced woman is with me, not at all like mother, who, they say, was ugly and dark; black as a nigger, Tom told me once, when he was mad. Was she black?" Mr. Tracy made no reply to this, but said, suddenly: "Jerry, do you like me well enough to do me a great favor?" "Why, yes, I guess I do. I like you very much, though not as well as I do Harold and Mr. Arthur. What do you want?" was Jerry's answer. After hesitating a moment, Mr. Tracy began.
- 48. "There are certain reasons why I ought to know if my brother writes to Gretchen, or her friends, or any one in Germany, especially Wiesbaden. A letter of that kind might do me a great deal of harm; if he should write to any one in Germany, you would, perhaps, be asked to post the letter, as he never goes to town?" He said this interrogatively, and Jerry answered him, promptly: "I think he would give it to me, as I post nearly all his letters." "Yes, well; Jerry, can you keep a secret, and never tell any one what I am saying to you?" was Frank's next remark, to which Jerry responded: "I think I should tell Harold, and, perhaps, Mr. Arthur." "No, no, Jerry, never!" and Frank laid his hand half menacingly upon the little girl's shoulder. "I have been kind to you, have paid for your board to Mrs. Crawford ever since you have been there"— He felt how mean it was to say this, and did not at all resent Jerry's quick reply: "Yes, but Mrs. Peterkin says you do not pay enough." "Perhaps not," he continued; "but if Mrs. Crawford is satisfied, it matters little what Mrs. Peterkin thinks. Jerry, you must do this for me," he went on rapidly, as his fears kept growing. "You must never tell any one of our conversation, and if my brother writes that letter soon, or at any time, you must bring it to me. Will you do it? Great harm would come if it were sent—harm to me, and harm to Maude, and"— "To Maude!" Jerry repeated. "I would do anything for Maude. Yes, I will bring the letter to you if he writes one. You are sure it would be right for me to do so?" Frank had touched the right chord when he mentioned his daughter's name, for during the years of close companionship the two little girls had learned to love each other devotedly, though naturally Jerry's was the stronger and less selfish attachment of the
- 49. two. To her Maude was a queen who had a right to tyrannize over and command her if she pleased; and as the tyranny was never very severe, and was usually followed by some generous act of contrition, she did not mind it at all, and was always ready to make up and be friends whenever it suited the capricious little lady. "Yes, I will do it for Maude," she said again; but there was a troubled look on her face, and a feeling in her heart as if, in some way, she was false to Arthur in thus consenting to his brother's wishes. But, she reflected, Arthur was crazy, so people said, and she herself knew better than any one else of his many fanciful vagaries, which, at times, took the form of actual insanity. For weeks he would seem perfectly rational, and then suddenly his mood would change, and he would talk strange things to himself and the child, who was now so necessary to him, and who alone had a soothing influence over him. Only the day before, he had been unusually excited, after listening to a simple air which he had taught her, and which, at his request, she sang to him after Maude had gone out and left them alone. "I could swear you were Gretchen, singing to me in the twilight, and across the meadow comes the tinkle of the bells where the cows and goats are feeding," he said to her, as he paced up and down the room. Then, stopping suddenly, he went up to her, and pushing her hair from her forehead, looked long and earnestly into her face. "Cherry," he said at last, using the pet name he often gave her, "you are some like Gretchen as she must have been when of your age. Oh, if you only were hers and mine! But there was no child; and yet—and yet—" He seemed to be thinking intently for a moment, and then, going to a drawer in his writing-desk which Jerry had never seen open before, he took out a worn, yellow letter, and ran his eye rapidly over it until he found a certain paragraph, which he bade Jerry read.
- 50. The paragraph was as follows: "I have something to tell you when you come, which I am sure will make you as glad as I am." Jerry read it aloud slowly, for the handwriting was cramped and irregular, and then looked up questioningly to Arthur, who said to her: "What do you think she meant by the something which would make me glad as she was?" "I don't know," Jerry answered him. "Who wrote it? Gretchen?" "Yes, Gretchen. It is her last letter to me, and I never went back to see what she meant, for the bees were bad in my head and I forgot everything, even Gretchen herself. Poor little Gretchen! What was the idea which came to me like a flash of lightning, in regard to this letter, when I heard you sing? It is gone, and I cannot recall it." There was a worried, anxious look on his face as he put the letter away, and went on talking to himself of Gretchen, saying he was going to write her again, or her friends, and find out what she meant. The next day Jerry met Frank in the Tramp House, as we have described, and gave him the promise to bring him any letter directed to Germany which Arthur might entrust to her. But the promise weighed heavily upon her as she walked slowly on toward the field where Harold was at work, and where she found him resting for a moment under the shadow of a wide-spreading butternut. He looked tired and pale, and there was an expression on his face which Jerry did not understand. Harold was not in a very happy frame of mind. Naturally cheerful and hopeful, it was not often that he gave way to fits of despondency, or repining at his humble lot, so different from that of the boys of his own age, with whom he came in daily contact, both at school and in the town.
- 51. Dick St. Claire, his most intimate friend, always treated him as if he were fully his equal, and often stood between him and the remarks which boys make thoughtlessly, and which, while they mean so little, wound to the quick such sensitive natures as Harold's. But not even Dick St. Claire could keep Tom Tracy in check. With each succeeding year he grew more and more supercillious and unbearable, pluming himself upon his position as a Tracy of Tracy Park, and the wealth he was to inherit from his Uncle Arthur. For the last year he had been at Andover, where he had formed a new set of acquaintances, one of whom was spending the vacation with him. This was young Fred Raymond, whose home was at Red Stone Hall, in Kentucky, and whose parents were in Europe. Between the two youths there was but little similarity of taste or disposition, for young Raymond represented all that was noble and true, and though proud of his State and proud of his name, he never assumed the slightest superiority over those whom the world considered his inferiors. He was Tom's room-mate, and hence the intimacy between them which had resulted in Fred's accepting the invitation to Tracy Park. If anything had been wanting to complete Tom's estimate of his own importance this visit of the Kentuckian would have done it. All his former friends were cut except Dick St. Claire, while Harold was as much ignored as if he had never existed. Tom did not even see him or recognize him with so much as a look, but passed him by as he would any common day laborer whom he might chance to meet. All through the summer days, while Harold was working until every bone in his body ached, Tom and his friend were enjoying themselves in hunting, fishing, driving, or rowing, or lounging under the trees in the shady lawns. That afternoon, when Jerry joined him in the hayfield, Tom and the Kentuckian had passed him in their fanciful hunting-suits, with their dogs and guns, but though Harold was within a few yards of them, Tom affected not to see him, and kept his head turned the other way, as if intent upon some object in the distance. Leaning upon his rake, Harold watched them out of sight, with a choking sensation in his throat, as he wondered if it would always be
- 52. thus with him, and if the day would never come when he, too, could know what leisure meant, with no thought for the morrow's bread. "I am Tom's superior in everything but money, and yet he treats me like a dog," he said, as he seated himself upon the grass, where he sat fanning himself with his straw hat. When Jerry appeared in view he brightened at once, for in all the world there was nothing half so sweet and lovely to him as the little blue-eyed girl who sat down beside him, and, nestling close to him, laid her curly head upon his arm. "I've come to help you rake the hay," she said, "for grandma told me you had a headache at noon, and could'nt eat your huckleberry pie. I am awfully sorry, Harold, but I ate it myself, it looked so good, instead of saving it for your supper. It was nasty and mean in me, and I hope it will make me sick." But Harold told her he did not care for the pie, and was glad that she ate it if she liked it. Then he questioned her of the park house and of Arthur, asking if the bees were often in his head now, or had she driven them out. "No, I guess I haven't. They were awful yesterday," Jerry replied. "He was talking of Gretchen all the time. I wonder who she was. Sometimes I look at her until it seems to me I have seen her or something like her, a paler face with sadder eyes. How he must have loved her, better than you or I could ever love anybody; don't you think so?" Harold hesitated a moment, and then replied: "I don't know, but it seems to me I love you as much as one could ever love another." "Phoo! Of course you do; but that's boy love; that isn't like when you are old enough to have a beau!" and Jerry laughed merrily, as she sprang up, and, taking Harold's rake, began to toss the hay about rapidly, bidding him sit still and see how fast she could work in his place.
- 53. Harold was very tired, and his head was aching badly, so for a time he sat still, watching the graceful movements of the beautiful child, who, it seemed to him, was slipping away from him. Constant intercourse with a polished man like Arthur Tracy had not been without its effect upon her, and there was about her an air which with strangers would have placed her at once above the ordinary level of simple country girls. This Harold had been the first to detect, and though he rejoiced at Jerry's good fortune, there was always with him a dread lest she should grow beyond him, and that he should lose the girl he loved so much. "What if she should think me a clown and a clodhopper, as Tom Tracy does?" he said to himself, as he watched her raking up the hay faster, and quite as well as he could have done himself. "I believe I should die." It was impossible that Jerry should have guessed the nature of Harold's thoughts, but once, as she passed near him, she dropped her rake, and going up to him, wiped his forehead with her apron, and, kissing him fondly, said to him: "Poor, tired boy, is your head awful? You look as if you wanted to vomit! Do you?" "No, Jerry," Harold answered, laughingly. "I am not as bad as that. I was only wishing that I were rich and could give you and grandma a home as handsome as Tracy Park. How would you like it?" "First-rate, if you were there," Jerry replied; "but if you were not I shouldn't like it at all. I never mean to live anywhere without you; because, you know I am your little girl, the one you found in the carpet-bag, and I love you more than all the world, and will love and stand by you forever and ever, amen!" She said the last so abruptly, and it sounded so oddly, that Harold burst into a laugh, and taking up the rake she had dropped, began his work again, declaring that the headache was gone, and that he was a great deal better.
- 55. M CHAPTER XXI. MRS. TRACY'S DIAMONDS. RS. TRACY was going to have a party—not a general one, like that which she gave when our readers first knew her, and Harold Hastings stood at the head of the stairs and bade "the ladies go this way and the gentlemen that." Since she had become a leader of fashion, she had ignored general parties and limited her invitations to a select few, which, on this occasion, numbered about sixty or seventy. But the entertainment was prepared as elaborately as if hundreds had been expected, and the hostess was radiant in satin, and lace, and diamonds, as she received her guests and did the honors of the occasion. The September night was soft and warm, and the grounds were lighted up, while quite a crowd collected near the house to hear the music and watch the proceedings. Mrs. Tracy would have liked to have Jerry in the upper hall, where Harold had once stood. "It would help to keep the child in her place," she thought. But her husband promptly vetoed the proposition, saying that when Jerry Crawford came to the park house to an entertainment it would be as a guest, and not as a waiter. So a colored boy stood in the upper hall, and a colored boy stood in the lower hall, and there were colored waiters everywhere, and Dolly had never been happier or prouder in her life; for Governor Markham and his wife, from Iowa, were there, and a judge's wife from Springfield—all guests of Grace Atherton, and, in consequence, hidden to the party. Another remarkable feature of the evening was the presence of Arthur in the parlors. He had known both Governor Markham and his wife, Ethelyn Grant, and had been present at their wedding, and it
- 56. was mostly on their account that he had consented to join in the festivities. Jerry, it is true, had done a great deal toward persuading him to go down, repeating, in her own peculiar way, what she had heard people say with regard to his seclusion from society. "You just make a hermit of yourself," she said, "cooped up here all the time. I don't wonder folks say you are crazy. It is enough to make anybody crazy, to stay in one or two rooms and see nobody but Charles and me. Just dress yourself in your best clothes and go down and be somebody, and don't talk of Gretchen all the time! I am tired of it, and so is everybody. Give her a rest for one evening, and show the people how nice you can be if you only have a mind to." Jerry delivered this speech with her hands on her hips, and with all the air of a woman of fifty; while Arthur laughed immoderately, and promised her to do his best not to disgrace her. Jerry's anxiety was something like that of a mother for a child whose ability she doubts; and, after her supper was over, she took her way to the park house to see that Arthur was dressed properly for the occasion. "It would be like him to go without his neck-tie and wear his every-day boots," she thought. But she found him as faultlessly gotten up as he well could be in his old-fashioned evening dress, which sat rather loosely upon him, for he had grown thinner with each succeeding year. Jerry thought him splendid, and watched him admiringly as he left the room and started for the parlors, with her last injunction ringing in his ears: "Not a word out of your head about Gretchen, but try and act as if you were not crazy." "I'll do it, Cherry. Don't you worry," he said to her, with a little reassuring nod, as he descended the stairs.
- 57. And he kept his promise well. There was no word out of his head about Gretchen, and no one ignorant of the fact would ever have suspected that his mind was unsettled as he moved among the guests, talking to one and another with that pleasant, courtly manner so natural to him. A very close observer, however, might have seen his eyes dilate and even flash with some sudden emotion when his brother's wife passed him and her brilliant diamonds sparkled in the bright gas-light. The setting was rather peculiar, but Mrs. Tracy liked it for the peculiarity, and had never had it changed. She was very proud of her diamonds, they were so large and clear, and she had the satisfaction of knowing that there were no finer, if as fine, in town. She seemed to know, too, just in what light to place herself in order to show them to the best advantage, and at times the gleams of fire from them were wonderful, and once Arthur put his hand before his eyes as she passed him, and muttering something to himself moved quickly to another part of the room. This was late in the evening, and soon after he excused himself to those around him, saying it was not often that he dissipated like this, and as he was growing tired he must say good-night. The next morning Charles found him looking very pale and worn, with a bad pain in his head. He had not slept at all, he said, and would have his coffee in bed, after which Charles was to leave him alone and not come back until he rang for him, as he might possibly fall asleep. It was very late that morning when the family breakfasted, and as they lingered around the table, discussing the events of the previous night, it was after eleven o'clock when at last Mrs. Tracy went up to her room. As she ascended the stairs, she caught a glimpse of Harold disappearing through a door at the lower end of the hall, evidently with the intention of going down the back stairway and making his exit from the house by the rear door, rather than the front. Mrs. Tracy knew that he was sometimes sent by his grandmother on some errand to Arthur, and giving no further thought to the matter
- 58. went on to her own room, which her maid had put in order. All the paraphernalia of last night's toilet was put away, diamonds and all. Contrary to her usual custom, for she was very careful of her diamonds, and very much afraid they would be stolen, she had left them in the box on her dressing bureau. But they were not there now. Sarah, who knew where she kept them, had put them away, of course, and she gave them no more thought until three days later, when she received an invitation to a lunch party at Brier Hill. "I shall wear my dark blue satin and diamonds," she said to her maid, who was dressing her hair, but the diamonds, when looked for, were not in their usual place. Sarah had not put them away, nor in fact had she seen them at all, for they were not upon the bureau when she went to arrange her mistress' room the morning after the party. The diamonds were gone, nor could any amount of searching bring them to light, and Mrs. Tracy grew cold, and sick, and faint, and finally broke down in a fit of crying, as she explained to her husband that her beautiful diamonds were stolen. She called it that, now, and the whole household was roused and questioned as to when and where each had last seen the missing jewels. But no one had seen them since they were in the lady's ears, and she knew she had left them upon her bureau when she went down to breakfast. She was positive of that. No one had been in the room, or that part of the house, except Tom, Fred Raymond, Charles, and Sarah. Of these the first two were not to be thought of for a moment, while the last two had been in the family for years, and were above suspicion. Clearly, then, it was some one from outside, who had watched his or her opportunity and come in. "Had any one been seen about the house at that hour?" Frank asked, and Charles remembered having met Harold Hastings coming out of the rear door; "but," he added, "I would sooner suspect myself than him." And this was the verdict of all except Mrs. Tracy, who now recalled the fact that she, too, had seen Harold "sneaking through the door
- 59. as if he did not wish to be seen." That was the way she expressed herself, and her manner had in it more meaning even than her words. "What was Harold doing in the house? What was his errand? Does any one know?" she asked, but no one volunteered any information until Charles suggested that he probably came on some errand to Mr. Arthur; he would inquire, he said, and he went at once to his master's room. Arthur was sitting by his writing desk, busy with a letter, and did not turn his head when Charles asked if he remembered whether Harold Hastings had been to his room the morning after the party. "No, I have not seen him for more than a week," was the reply. "But he must have been here that morning," Charles continued. "Try and think." "I tell you no one was here. I am not quite demented yet. Now go. Don't you see you are interrupting me?" was Arthur's rather savage response, and without having gained any satisfactory information, Charles returned to the group anxiously awaiting him. "Well?" was Mrs. Tracy's sharp interrogatory, to which Charles responded: "He does not remember what happened that morning; but that is not strange. He was very tired and unusually excited after the party, and when he is that way he does not remember anything. Harold might have been there a dozen times and he would forget it." "Bring the boy, then. He will know what he was doing here," was Mrs. Tracy's next peremptory remark, and her husband said to her, reproachfully: "Surely you do not intend to charge him with the theft?" "I charge no one with the theft until it is proven against him; but I must see the boy and know what he was doing here. I never liked this free running in and out by those people in the lane. I always
- 60. knew something would come of it," Mrs. Tracy said, and Charles was dispatched for Harold. He found him mowing the lawn for a gentleman whose premises joined Tracy Park, and without any explanation told him that he was wanted immediately at the park house. "But it is noon," Harold said, glancing up at the sun. "And there is Jerry coming to call me to dinner." "Better come at once. Jerry can go with you, if she likes," Charles said, feeling intuitively that in the little girl Harold would find a champion. Harold left his lawn mower, and explaining to Jerry that he had been summoned to the park house, whither she could accompany him if she chose, he started with her and Charles, whom he questioned as to what was wanted with him. "Were you in the park house the morning after the party? That would be Tuesday," Charles asked. "Yes, I went to see Mr. Arthur Tracy, but could get no answer to my knock," Harold promptly replied, while his face flushed scarlet, and he seemed annoyed at something. He could not explain to Charles his motive in going to see Arthur, as, now that the first burst of indignation was over, he felt half ashamed of it himself. On the afternoon of the day of the party he had been at Grassy Spring, helping Mrs. St. Claire with her flowers, and after his work was done he had gone with Dick into the billiard- room, where they found Tom Tracy and his friend, young Raymond. They had come over for a game, and the four boys were soon busily engaged in the contest. Harold, who had often played with Dick, and was something of an expert, proved himself the most skillful of them all, greatly to the chagrin of Tom, who had not recognized him even by a nod. Dick, on the contrary, had introduced him to Fred Raymond with as much ceremony as if he had been the Governor's son, instead of the boy who sometimes worked in his mother's
- 61. flower garden. And the Kentuckian had taken him by the hand and greeted him cordially, with a familiar: "How d'ye, Hastings? Glad to make your acquaintance." There was nothing snobbish about Fred Raymond, whose every instinct was gentlemanly and kind, and Harold felt at ease with him at once, and all through the game appeared at his best, and quite as well bred as either of his companions. When the play was over Dick excused himself a moment, as he wished to speak with his father, who was about driving to town. As he staid away longer than he had intended doing, Tom grew restless and angry, too, that Fred should treat Harold Hastings as an equal, for the two had at once entered into conversation, comparing notes with regard to their standing in school, and discussing the merits of Cicero and Virgil, the latter of which Harold had just commenced. "We can't wait here all day for Dick," Tom said. "Let us go out and look at the pictures." So they went down the stairs to a long hall, in which many pictures were hanging—some family portraits and others copies of the old masters which Mr. St. Claire had brought from abroad. Near one of the portraits Fred lingered a long time, commenting upon its beauty, and the resemblance he saw in it to little Nina St. Claire, the daughter of the house, and whose aunt the original had been. The portrait was not far from the stairway which led to the billiard-room, and Harold, who had remained behind, and was listlessly knocking the balls, could not help hearing all that was said: "By the way, who is that Hastings? I don't think I have seen him before; he is a right clever chap," Fred Raymond said, and Tom replied, in that sneering, contemptuous tone which Harold knew so well, and which always made his blood boil and his fingers tingle with a desire to knock the speaker down: "Oh, that's Hal Hastings, a poor boy, who does chores for us and the St. Claires. His grandmother used to work at the park house, and so Uncle Arthur pays for his schooling, and Hal allows it, which I
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