1. Homework: CMOS Fabrication Digital IC Design : Martin Page 62 2.1 Discuss briefly the relationships between an ion beam’s acceleration potential, the beam current, and the time of implantation on the resulting doping profile. Ans: Acceleration potential determines depth of implantation;beam current and exposure time determine dosage 2.2 Place the following processing steps in their correct order: metal deposition and patterning, field implantation, junction implantation, well implantation, polysilicon deposition and patterning, field-oxide growth. Ans: well implantation, field implantation, field-oxide growth, polysilicon deposition and patterning, junction implantation, metal deposition and patterning 2.3 What are major problems associated with a single threshold- voltage-adjust implant? Ans: NWELL and psubstrate generally have different doping levels, may be difficult to achieve required threshold voltages by a single adjustment step: for optimal well doping adjust thresholds separately

2. Homework: CMOS Fabrication Digital IC Design : Martin Page 62 2.4 What is the reason for using a field implant and why is it often not needed in the well regions? Ans: Implant guarantees that Si under FOX will never invert when large voltages are in vicinity; better device isolation 2.5 What are the major trade-offs in using a wet process or a dry process when growing thermal SiO 2 ? Ans: Wet process faster since water vapor diffuses in Si faster than oxygen gas but lower quality porous oxide results. Dry process gives higher density oxide 2.6 Why is polysilicon used to realize gates of MOS transistors rather than metal? Ans: Poly gates (as opposed to metal) permit a self-aligned process, eliminating need for another mask.Poly mask defines transistor channel area 2.7 Why can a microcircuit not be annealed after metal has been deposited? Ans:Annealing is a high temp process, metal distribution may be adversely affected

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