Question

QUESTION 1

1. What material can we use to mask an implantation (control where implantation will occur)

   	1.	Photoresist is the preferred mask since it is easiest to create and to remove, and as long as it is thick enough it is effective at blocking the implant when it is not needed.
   	2.	A patterned metal film is the best mask since not only can it block an implant it can also help drain off the implanted charge. An oxide, nitride, or photoresist film can charge up since they are electrical insulators.
   	3.	This is a trick question since the best mask is to pattern the implant itself by turning the beam on and off depending on whether it is over a region needing an implant or not. This is referredtoas maskless implantation.
   	4.	A patterned dielectric film like oxide or nitride can stand up to the energetic ions used during an implantation.

10 points   

QUESTION 2

1. What is and what causes channeling?

   	1.	Channeling is when designers earmark convenient space for a signal that many gates need access to. The clock signal is a classic example of a signal that needs to be channeled.
   	2.	Channeling is what happens when an implant stops inside a wafer.
   	3.	Channeling is the belief that a person's body has being taken over by a spirit for the purpose of imparting wisdom. Credit: Sergey Nivens Shutterstock. Jul 31, 201(http://www.livescience.com/38561-channeling.html)
   	4.	Channeling is when implanted ions go much deeper through the substrate by finding open paths (channels) between the atoms. In silicon the [110] direction has hexagonal paths that allow much deeper penetration by ions.

10 points   

QUESTION 3

1. When would a process designer at Intel decide to do implantation over a diffusion?

   	1.	Implantation allows much more precise control of dose, depth, and distribution profile of the implanted impurities than diffusion.
   	2.	An implantation would be chosen if the diffusion furnace wasn't available.
   	3.	An implantation would be chosen if a designer had not learned about the diffusion process.
   	4.	An implantation would be chosen when a diffusion process was not acceptable.

10 points   

QUESTION 4

1. When would a process designer at Intel decide to do diffusion over an implantation?

   	1.	Diffusion would be chosen if a designer had not learned about the implantation process.
   	2.	A designer might choose a diffusion instead of an implantation in order to avoid purchasing more expensive implantation equipment.
   	3.	If a designer wanted to implant an impurity that could not be ionized and therefore not be able to be accelerated to high energies in an implanter.
   	4.	A diffusion can create very deep impurity distributions. Implanters can not reach energies over several MeV, and heavy implants can damage a wafer.

Answer 1

Ans 1. Option (1) as photoresist is easiest to create and remove and it can block implantation where it is not needed.

Ans 2. Option (4) Channeling definition as given in options.

Ans 3. Option (1) due to following advantages:-

a. Done at around room temperature.

b. Precise control of ion dose (no. of ions per cm^{​​​​2}​​​​) by beam current.

c. Doping layers can be implanted without disturbing previous diffused regions due to low temperature associated with ion implantation.

Ans 4. Option (3) is correct as per reason given in question.