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This module is based upon the Connexions module entitled Photolithography by Bill Wilson.

Actually, implants (especially for moats) are usually done at a sufficiently high energy so that the dopant(phosphorus) is already pretty far into the substrate (often several microns or so), even before the diffusion starts. Theanneal/diffusion moves the impurities into the wafer a bit more, and as we shall see also makes the n-region grow larger.

"The n-region"! We have not said a thing about how we make our moat in only certain areas of the wafer. Fromthe description we have so far, is seems we have simply built an n-type layer over the whole surface of the wafer. This would bebad! We need to come up with some kind of "window" to only permit the implanting impurities to enter the silicon waferwhere we want them and not elsewhere. We will do this by constructing an implantation "barrier".

To do this, the first thing we do is grow a layer of silicon dioxide over the entire surface of the wafer. Wetalked about oxide growth when we were discussing MOSFETs but let's go into a little more detail. You can grow oxide in eithera dry oxygen atmosphere, or in a an atmosphere which contains water vapor, or steam. In [link] , we show oxide thickness as a function of time for growth with steam. Dry O 2 does not behave too much differently, the rate is just somewhat slower.

A plot of oxide thickness as a function of time.

On top of the oxide, we are now going to deposit yet another material. This is silicon nitride, Si 3 N 4 or just plain "nitride" as it is usually called. Silicon nitride is deposited through a method calledchemical vapor deposition or "CVD". The usual technique is to react dichlorosilane and ammonia in a hot walled low pressurechemical vapor deposition system (LPCVD). The reaction is:

Silicon nitride is a good barrier for impurities, oxygen and other things which do not want to get into the wafer. Take alook at [link] and see what we have so far. A word about scale and dimensions. The silicon wafer is about250 μm thick (about 0.01") since it has to be strong enough not to break as it is being handled. The two deposited layersare each about 1 μm thick, so they should actually be drawn as lines thinner than the other lines in the figure. This wouldobviously make the whole idea of a sketch ridiculous, so we will leave things distorted as they are, keeping in mind that thedeposited and diffused layers are actually much thinner than the rest of wafer, whichreally does not do anything except support the active circuits up on top.

Initial wafer configuration.

Now what we want to do is remove part of the nitride, so we can make our n-well, but not put inphosphorous where do not want it. We do this with a processes called photolithography and etching respectively. First thing we do is coat the wafer with yetanother layer of material. This is a liquid called photoresist and it is applied through a processcalled spin-coating. The wafer is put on a vacuum chuck, and a layer of liquid photoresist is sprayed uncap of thewafer. The chuck is then spun rapidly, getting to several thousand RPM in a small fraction of a second. Centrifugal forcecauses the resist to spread out uniformly across the wafer surface. The solvent for thephotoresist is quite volatile and so the layer of photoresist dries while the wafer is still spinning, resulting in a thin,uniform coating across the wafer [link] .

Questions & Answers

Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Chemistry of electronic materials. OpenStax CNX. Aug 09, 2011 Download for free at http://cnx.org/content/col10719/1.9
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