![H. Pilo, IEDM 200617
Read Stability – Static Noise Margin (SNM)
VDD
1
PR
VL
VR
Read SNM[1]
AXR
VR (V)
NR
0.5
90nm simulation
0
0
0.5
1
VL (V)
• Read SNM is typically the most stringent constraint
[1] E. Seevinck, JSSC 1987
18
9](https://image.slidesharecdn.com/lecture10-sram2-131116054124-phpapp02/85/Lecture10-sram2-9-320.jpg)
![SRAM Design – Read Stability
Load
WL
VDD
PL
AXL
PR
AXR
‘1’
V>0
Access NL
NR
NPD
BL
Retention fluctuations
BL
Read margin and
retention margin
[Bhavnagarwala, IEDM’05]
19
Read Stability – N-Curve
•
A, B, and C correspond to the two
stable points A and C and the metap
stable point B of the SNM curve
•
When points A and B coincide, the
cell is at the edge of stability and a
destructive read can easily occur
20
10](https://image.slidesharecdn.com/lecture10-sram2-131116054124-phpapp02/85/Lecture10-sram2-10-320.jpg)
![H. Pilo, IEDM 200621
Write Stability – Write Noise Margin (WNM)
VDD
1
90nm simulation
PR
VL
VR
AXR
VR (V)
NR
0.5
WNM[1]
0
0
0.5
1
VL (V)
• Write stability is becoming more stringent with scaling
• Optimizing read and write stability at the same time is difficult
[1] A. Bhavnagarwala, IEDM 2005
22
11](https://image.slidesharecdn.com/lecture10-sram2-131116054124-phpapp02/85/Lecture10-sram2-11-320.jpg)
![Write Stability – BL/WL Write Margins
1.2
1.2
1
0.8
0.6
Voltage (V)
(
V o ltag e (V )
1
BL
0.4
0.2
WM
0
0.00E+00
-0.2
2.00E-08
0.8
0.6
0.4
WM
WL
0.2
0
4.00E-08
6.00E-08
Time (s)
8.00E-08
1.00E-07
0.00E+00
-0.2
2.00E-08
4.00E-08
6.00E-08
8.00E-08
1.00E-07
Time (s)
Highest BL voltage under which write is possible when BLC is kept
precharged (left)
Difference between VDD and lowest WL voltage under which write is
possible when both bit-lines are kept precharged (right)
Can be directly measured in large memory arrays via BL currents
23
Write Stability – Write Current (N-Curve)
[1] C. Wann et al, IEEE VLSI-TSA 2005
24
12](https://image.slidesharecdn.com/lecture10-sram2-131116054124-phpapp02/85/Lecture10-sram2-12-320.jpg)
![6-T SRAM Static/Dynamic Stability
Read Margin
SNM: pessimistic
Write Margin
WNM: optimistic
[1] E. Seevinck, JSSC 1987; [2] A. Bhavnagarwala, IEDM 2005
27
H. Pilo, IEDM 200628
14](https://image.slidesharecdn.com/lecture10-sram2-131116054124-phpapp02/85/Lecture10-sram2-14-320.jpg)
Lecture10 sram2
- 1. EE241 - Spring 2008 Advanced Digital Integrated Circuits Lecture 10: SRAM Announcements Homework 1 Due February 26 Homework 2 Next week 2 1
- 2. Agenda SRAM stability metrics Retention Read Write Dynamic Options for scaling Column assist techniques Migration away from 6T Technology options Departure from SRAM (eDRAM) 3 SRAM Read/Write/Retention Margins 2
- 3. SRAM Scaling Trends ITRS Effective Cell Reported Effective Cell 2 SRAM Cell Size (um ) 2 SRAM Cell Size (um ) 100 ITRS Single Cell Reported Individual Cell Reported Cell in Array 10 1 0.1 10 1 0.5x effective cell area scaling difficult 0.1 700 600 500 400 300 200 100 80 70 60 10090 50 40 30 300 Technology N d ( ) T h l Node (nm) 200 100 90 80 70 60 50 40 30 Technology N d ( ) T h l Node (nm) Individual SRAM cell area able to track ITRS guideline Array area deviates from ITRS guideline at 90nm Memory design no longer sits on the 0.5x area scaling trend! 5 On-Die L3 Cache siz (MB) ze Memory Scaling Server processors 10 Itanium® It i ® Processors Xeon® Processors 1 180 160 140 120 100 80 60 Technology Node ( ) gy (nm) • • • • Memory latency demands larger last level cache (LLC) Memory is more energy-efficient than logic LLC approaches 50% chip area for desktop and mobile processors LLC approaches 80% chip area for server processors Vivek De, Intel 2006 6 3
- 4. 6-T SRAM Cell • Improve CD control by unidirectional poly • Relax critical layer patterning requirements • Optimizing design rules is key 7 SRAM cell design trends BL BLB IEDM 02 0.46x1.24μm V DD ’ GND WL Cell in 90nm (1μm2) • • • • • Cell in 65nm (0.57μm2) Improve CD control by unidirectional poly Relax critical layer patterning requirements Optimizing design rules is key Shorter bitline enables better cycle time and/or array efficiency Full metal wordline with wider pitch achieves better RC 8 4
- 5. SRAM Cell Trends 0.242μm2 cell in 45nm from TSMC (IEDM’07) 0.346μm2 cell in 45nm from Intel (IEDM’07) 9 More SRAM Trends 0.15μm2 cell in 32nm from TSMC (IEDM’07) 10 5
- 6. Ion/Ioff: Cell Read and Leakage H. Pilo, IEDM 200611 SRAM Cell/Array Hold stability WL VDD Read stability M2 Write stability M5 Read current (access time) Access Transistor Pull down M1 BL Pull up Q M4 Q M6 M3 BL 12 6
- 7. SRAM Design – Hold (Retention) Stability Load WL VDD PL AXL PR ‘1’ ‘0’ AXR Access NL NR NPD BL BL Data Retention Leakage Scaling trend: Increased gate leakage + degraded ION/IOFF ratio Lower VDD during standby PMOS load devices must compensate for leakage 13 SRAM Cell Mismatch ΔVTh ∝ K. Zhang, Intel 1 Cox WL Due to RDF 14 7
- 8. The Data-Retention Voltage (DRV) of SRAM VDD 0 M5 V1 Leakage current M2 M4 DRV Condition: 0 M3 V2 M6 VDD Leakage current ∂V1 ∂V2 = Left inverter ∂V1 ∂V2 When Vdd scales down to DRV, the voltage transfer curves (VTC) of the internal inverters degrade to such a level that retention static noise margin (SNM) of the SRAM cell reduces to zero. , when VDD = DRV Right inverter VTC of SRAM cell inverters 0.4 VDD=0.4V 2 0.3 0.2 V =0.18V DD 0.1 VTC1 VTC2 0 Qin, ISQED’04 0 0.1 0.2 V1 (V) 0.3 0.4 15 Monte-Carlo Simulation of DRV Distribution 300 250 Histogram of cell # VDD M1 200 150 100 50 0 0 50 100 150 200 250 Simulated DRV of 1500 SRAM cells (mV) 300 16 8
- 9. H. Pilo, IEDM 200617 Read Stability – Static Noise Margin (SNM) VDD 1 PR VL VR Read SNM[1] AXR VR (V) NR 0.5 90nm simulation 0 0 0.5 1 VL (V) • Read SNM is typically the most stringent constraint [1] E. Seevinck, JSSC 1987 18 9
- 10. SRAM Design – Read Stability Load WL VDD PL AXL PR AXR ‘1’ V>0 Access NL NR NPD BL Retention fluctuations BL Read margin and retention margin [Bhavnagarwala, IEDM’05] 19 Read Stability – N-Curve • A, B, and C correspond to the two stable points A and C and the metap stable point B of the SNM curve • When points A and B coincide, the cell is at the edge of stability and a destructive read can easily occur 20 10
- 11. H. Pilo, IEDM 200621 Write Stability – Write Noise Margin (WNM) VDD 1 90nm simulation PR VL VR AXR VR (V) NR 0.5 WNM[1] 0 0 0.5 1 VL (V) • Write stability is becoming more stringent with scaling • Optimizing read and write stability at the same time is difficult [1] A. Bhavnagarwala, IEDM 2005 22 11
- 12. Write Stability – BL/WL Write Margins 1.2 1.2 1 0.8 0.6 Voltage (V) ( V o ltag e (V ) 1 BL 0.4 0.2 WM 0 0.00E+00 -0.2 2.00E-08 0.8 0.6 0.4 WM WL 0.2 0 4.00E-08 6.00E-08 Time (s) 8.00E-08 1.00E-07 0.00E+00 -0.2 2.00E-08 4.00E-08 6.00E-08 8.00E-08 1.00E-07 Time (s) Highest BL voltage under which write is possible when BLC is kept precharged (left) Difference between VDD and lowest WL voltage under which write is possible when both bit-lines are kept precharged (right) Can be directly measured in large memory arrays via BL currents 23 Write Stability – Write Current (N-Curve) [1] C. Wann et al, IEEE VLSI-TSA 2005 24 12
- 13. H. Pilo, IEDM 200625 SRAM Design – Read/Write Stability Load WL Cell Stability VDD PL AXL Cell Trip Voltage PR AXR ‘1’ V>0 Access NL Read U R d Upset t Occurs Cell Read Voltage NR NPD BL BL Technology Scaling H. Pilo, ISSCC’2005 Read margin is typically the most stringent constraint Cell read voltage must stay below cell trip voltage Harder to achieve with process induced variations Noise margin degraded with technology scaling 26 13
- 14. 6-T SRAM Static/Dynamic Stability Read Margin SNM: pessimistic Write Margin WNM: optimistic [1] E. Seevinck, JSSC 1987; [2] A. Bhavnagarwala, IEDM 2005 27 H. Pilo, IEDM 200628 14
- 15. Next Lecture SRAM design techniques Alternatives to planar 6T SRAM 29 15