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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 , we show oxide thickness as a function of time for growth with steam. Dry does not behave too much differently, the rate is just somewhat slower. On top of the oxide, we are now going to deposit yet another material. This is silicon nitride, 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:
We have to come up with some way of selectively illuminating certain portions of the photoresist. Anyone who hasever seen a projector know how we can do this. But, since we want to make small things, not big ones, we will change around our projector so that it makes a smallerimage, instead of a bigger one. The instrument that projects the light onto the photoresist on the wafer is called a projection printer or a stepper . As shown in , the stepper consists of several parts. There is a light source (usually a mercury vaporlamp, although ultra-violet excimer lasers are also starting to come into use), a condenser lens to image the light source onthe mask or reticle . The mask contains an image of the pattern we are trying the place on the wafer. The projection lens then makes a reduced (usually 5x)image of the mask on the wafer. Because it would be far too costly, if not just plain impossible, to project onto the wholewafer all at once, only a small selected area is printed at one time. Then the wafer is scanned or stepped into a new position, and the image is printed again. If previous patterns have already been formed onthe wafer, TV cameras, with artificial intelligence algorithms are used to align the current image with the previously formed features. The stepper moves the whole surfaceof the wafer under the lens, until the wafer is completely covered with the desired pattern. A stepper is notcheap. Usually, TI or Intel will fork over several million dollars for each one! It is one of the most important pieces ofequipment in the whole IC fab however, since it determines what the minimum feature size on the circuit will be.
After exposure, the photoresist is placed in a suitable solvent, and "developed". Suppose for our example thestructure shown in is what results from the photolithographic step. The pattern that was used in the photolithographic (PL) step exposed half of our area to light, and so the photoresist (PR)in that region was removed upon development. The wafer is now immersed in a hydrofluoric acid (HF) solution. HF acid etchessilicon nitride quite rapidly, but does not etch silicon dioxide nearly as fast, so after the etch we have what we see in . We now take our wafer, put it in the ion implanter and subject it to a "blast" of phosphorus ions . The ions go right through the oxide layer on the RHS, but stick in the resist/nitride layer on the LHS of our structure.
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