Homework on Speed Limits in Chips, 2001 version (solutions)

Feel free to work on the homework in groups. The work you hand in, however, should reflect your understanding of the material and be in your own words. Students who turn in identical (or close to identical) homework assignments will be asked to explain their answers orally to the TA or prof. A student who cannot explain how he or she arrived at a given answer will be charged with academic dishonesty.

You should justify all of your answers for full credit.

Increasing Interconnect Speed (from Dr. Lu's Lecture)

1.	Dr. Lu showed a slide of a memory chip, consisting of a capacitor and a transistor. The capacitor appears to extend much "deeper" into the chip than does the transistor. Explain why it is desirable for capacitors used in electronic memory to be so deep. Capacitance in a memory chip is a good thing, so the capacitors are built "deeper" in order to increase their area.
2.	Thanks to the wonderful education you received at Rensselaer, you land a job as director of development at BuyTheBest Computer Corporation. Your company does not fabricate your own chips, but seeks the best chips made by other companies for incorporation into your computers. Thanks to your experience in Science of IT, you recognize that further improvement of your computers will require some new technology and that traditional silicon technology is approaching its limits. Your minions have each researched startup firms that are developing chips based on new approaches. Explain why each of the following firms should or should not be investigated further as a source of chips for ultra-fast computers. Which firm do you think is the most promising as a supplier for your business? Which would you buy stock in? Golden Chips, which "takes the 'rare' out of rare metals," is currently testing chips using copper wiring rather than aluminum wiring. They hope to eventually use gold and silver as interconnect materials as well. Good idea; all of these materials have lower resistances, which would reduce rise times. Gold and silver will be tougher to work with due to migration issues. Up in the Air Microelectronics is currently developing a technique to etch out the oxide insulator between capacitor plates and between interconnects, leaving air as the insulator instead. Good idea; air is not as good a dielectric as the oxide and would lower capacitance, which would also give reduced rise times. Skyscraper Chip Fab, Inc., which is "taking chips to new heights", has a prototype of a 3D chip that stacks chips on top of each other. Useful, but there are serious heat issues due to reduced surface area proportions. Super Chip Solutions uses high-T_c superconductors as interconnects in their chips. Lowers resistance, but they still need to be very cold (liquid nitrogen) in order to operate effectively. X-ray Visionaries, Inc. develops chips with shorter "gate" regions in transistors by using x-ray diffraction to produce the masks when growing chips. This works, but it is currently a very expensive and time-consuming process.
3.	What was the most interesting thing you learned from Dr. Lu's lecture?