Using polysilicon as a gate contact instead of metal in CMOS
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The document discusses the use of polysilicon as a gate contact in CMOS technology, detailing its advantages over metal, including lower threshold voltage, higher thermal resistance, and the ability to achieve a self-aligned gate process. It highlights the fabrication steps of CMOS technology and emphasizes the efficiency, low power consumption, and ease of integration in various digital and analog applications. Polysilicon's properties, such as modifiable work function through doping and lower fabrication complexity, make it a preferable choice in VLSI chip design.
Using polysilicon as a gate contact instead of metal in CMOS
- 1. Page 1 Using polysilicon as a gate contact instead of metal in CMOS Al- Mustansiriya university college of engineering Computer Department Eng.Ansam Hadi Rashed October 2018
- 2. Page 2 CMOS Technology Complementary metal oxide semiconductor (CMOS technology) is used to construct ICs and this technology is used in digital logic circuits, microprocessors, microcontrollers and static RAM.CMOS technology is also used in several analog circuits like data converters, image sensors and in highly integrated transceivers. The main features of CMOS technology are low static power consumption and high noise immunity.
- 3. Page 3 The transistor draws significant power only during switching between ON & OFF states. So, MOS devices do not generate as much waste heat as other forms of logic. Due to this reason, this technology most widely used and is implemented in VLSI chips
- 4. Page 4 Fabrication of CMOS large-scale integration (VLSI) technology is based on silicon, which has bulk electrical resistance between that of a conductor and an insulator, When n-type and p-type materials are put together, CMOS is formed. 1-Start with blank wafer (typically p-type where NMOS is created)
- 6. Page 6 5-Etch 6- Strip Photoresist 7-n-well
- 7. Page 7 8-Strip Oxide 9- Polysilicon (self-aligned gate technology) 10- Polysilicon Patterning
- 8. Page 8 11- Self-Aligned Process 12- N-diffusion/implantation
- 9. Page 9 13-Strip off oxide 14- P-Diffusion/implantation
- 11. Page 11 Why is polysilicon used as a gate contact instead of metal in CMOS?? 1- Self-aligned Gate Process used in fabrication process of polysilicon gate that prevent parasitic overlap input capacitance between gate-drain Cgd and capacitance between gate-source Cgs. Therefore the device with aligned gate will be faster because the gate is 3 times smaller, and full speed ,that will not happened in metal gate.
- 12. Page 12 2- The doping process of the drain and source require very high temperature more than (800°C) polysilicon would not melt under this temperature ,but If Al is used as a gate material, it would melt under such high temperature. This is because the melting point of Al is approximately 660°C. 3-Using metal as gate material resulted in high threshold voltage compared to polysilicon.
- 13. Page 13 4- Polysilicon is a semiconductor, its work function can be modulated by adjusting the level of doping. 5-Polysilicon cheapest also available. 6-Fabrication steps with polysilicon gate less than in metal.