1.What is the fabrication process? 1) Well implantation; 2) Field implantation; 3) Field oxide growth; 4) Polysilicon deposition and patterning; 5) Junction implantation; 6) Metal deposition and patterning. 2. What are the major problems associated with a single threshold-voltage-adjust implant?N-well and p-substrate generally have different doping levels.The major problem with a single threshold-adjust implant is that the doping level of the n well is higher than optimum. This higher doping level increases the junction capacitances and the body effect of the transistors in the well. 3. What is the reason for using a field implant and why is it often not needed in the well regions? Field implant (boron) is implanted under the field -oxide everywhere except in the well regions. This implant guarantees that the silicon under the field-oxide will never invert when a conductor over the field-oxide has a large voltage. If this implant were not performed, leakage currents could occur between the junctions of separate n-channel transistors in the substrate region. Therefore,field implant provides a better device isolation. Field implant is not necessary for n-well because the heavier doping of the n-well (compared to that of the substrate) normally guarantees that the silicon will never invert under the field-oxide in these regions. 4. Why is polysilicon rather than metal used to realize gates of MOS transistors? 1. Polysilicon has replaced metal gates mainly due to its higher melting temperature which allows for better annealing before metalization steps start. 2. Polysilicon is semiconductor. Its work function can be modulated by adjusting the type and level of doping and thus easy to achieve low threshold voltage. Work function is not easy to modulate for metal. It requires different metals for PMOS and NMOS in order to obtain low threshold voltage, which increases fabrication complexity. 3. Polysilicon has relatively few defects comparing to metal. 4. It is preferable to deposit the gate material before certain high-temperature steps in order to make better-performing transistors. Such high temperature steps would melt some metals, limiting the types of metal that can be used in a metal-gate-based process.