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We then remove the remaining resist, and perform an activation/anneal/diffusion step, also sometimes called the"drive-in". The purpose of this step is two fold. We want to make the n-tank deep enough so that we can use it for ourp-channel MOS, and we want to build up an implant barrier so that we can implant into the p-substrate region only. Weintroduce oxygen into the reactor during the activation, so that we grow a thicker oxide over the region where we implanted thephosphorus. The nitride layer over the p-substrate on the LHS protects that area from any oxide growth. We then end up withthe structure shown in . Now we strip the remaining nitride. Since the only way we can convert from p to n is to add a donor concentration which is greater than the background acceptor concentration, we had to keep the doping in the substrate fairlylight in order to be able to make the n-tank. The lightly doped p-substrate would have too low a threshold voltage for goodn-MOS transistor operation, so we will do a adjust implant through the thin oxide on the LHS, with the thick oxide on the RHS blocking the boron from getting intothe n-tank. This is shown in , where boron is implanted into the p-type substrate on the LHS, but isblocked by the thick oxide in the region over the n-well. Next, we strip off all the oxide, grow a new thin layer of oxide, and then a layer of nitride . The oxide layer is grown only because it is bad to grow directly on top of silicon, as the different coefficients of thermal expansion between the two materials causes damage to thesilicon crystal structure. Also, it turns out to be nearly impossible to remove nitride if it is deposited directly on tosilicon. The nitride is patterned (covered with photoresist, exposed, developed, etched, and removal of photoresist) to make two areaswhich are called "active" . (This is where we will build our transistors.) The wafer is then subjected to ahigh-pressure oxidation step which grows a thick oxide wherever the nitride was removed. The nitride is a good barrier foroxygen, so essentially no oxide grows underneath it. The thick oxide is used to isolate individual transistors, and also tomake for an insulating layer over which conducting patterns can be run. The thick oxide is called field oxide (or FOX for short) . Then, the nitride, and some of the oxide are etched off. The oxide is etched enough so that all of the oxide under thenitride regions is removed, which will take a little off the field oxide as well. This is because we now want to grow thegate oxide, which must be very clean and pure . The oxide under the nitride is sometimes called sacrificial oxide, because it is sacrificed in the name of ultra performance. Then the gate oxide is grown, and immediately thereafter, the whole wafer is covered with polysilicon . The polysilicon is then patterned to form the two regions which will be our gates. The wafer is covered once again withphotoresist. The resist is removed over the region that will be the n-channel device, but is left covering the p-channeldevice. A little area near the edge of the n-tank is also uncovered . This will allow us to add some additional phosphorus into the n-well, so that we can make acontact there, so that the n-well can be connected to . Back into the implanter we go, this time exposing the wafer to phosphorus. The poly gate, the FOX and the photoresist all blockphosphorus from getting into the wafer, so we make two n-type regions in the p-type substrate, and we have made our n-channelMOS source/drain regions. We also add phosphorous to the contact region in the n-well so as the make sure we get good contact performance there . Note that the formation of the source and drain were performed with a self-aligning technology . This means that we used the gate structure itself to define where the two insideedges of the source and drain would be for the MOSFET. If we hadmade the source/drain regions before we defined the gate, and then tried to line the gate up right overthe space between them, we might have gotten something that looks like what is shown in . What's going to be the problem with this transistor? Obviously, if thegate does not extend all the way to both the source and the drain, then the channel will not either, and the transistor will never turn on! We could trymaking the gate wider, to ensure that it will overlap both active areas, even if it is slightly misaligned, but then youget a lot of extraneous fringing capacitance which will significantly slow down the speed of operation of the transistor . This is bad! The development of the self-aligned gate technique was one of the big breakthroughswhich has propelled us into the VLSI and ULSI era. We pull the wafer out of the implanter, and strip off the photoresist. This is sometimes difficult, because the act of ionimplantation can "bake" the photoresist into a very tough film. Sometimes an rf discharge in an atmosphere is used to "ash" the photoresist, and literally burn it off the wafer! We now apply some more PR, and this timepattern to have the moat area, and a substrate contact exposed, for a boron p+ implant. This is shown in . We are almost done. The next thing we do is remove all the photoresist, and grow one more layer of oxide, which coverseverything, as shown in . We put photoresist over the whole wafer again, and pattern for contactholes to go through the oxide. We will put contacts for the two drains, and for each of the sources, make sure that the holesare big enough to also allow us to connect the source contact to either the p-substrate or the n-moat as is appropriate .
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