Doping

From Home Semiconductor Fabrication
Jump to: navigation, search

From Sam's "Semiconductor Fabrication Basics Background Theory" video: "OK, let's talk about doping next. So I'm gonna start with diffusion, which is a very simple method of doping, it has some downsides which I'll talk about in a second, it's commonly done inside a tube surface, and it's basically you're driving in atoms of phosphorous, arsenic or boron or whatever your dopant material is using a lot of heat. So the steps to do this, these are the steps to do it with a liquid source, theres liquid source, gas source and solid source. Solid source you have to use like BOCl3, gas source you gonna have diborane and phosphene gas, and they pump it in a chamber where the diffusion takes place. But with liquid source the steps we take to do that is: first we clean the wafer and if you etch it with HF after you clean it, you're going to want to put a very thin layer of SiO2 onto it so you have a hydrophilic surface and the next step will go nicely, if you have a hydrophobic surface caused by the HF then you won't be able to spin-coat the wafer very easily, it will bead up and it won't go well, so I recommend if you have to do an HF step right before it, do that and then do the first part of the RCA clean or something like that to leave a small passivation layer, then the next step you are going to put a couple of drops of either a phosphouric acid or boric acid solution depending on if you are doing you know N-type or P-type doping, and you're going to spin-coat that onto your wafer, and then right after you do that you don't want to leave it on the wafer too long before you do the pre-deposition step because you can get like hazing and things like that because of the excess oxygen thats going to come into play here. The next step known as predeposition places a high concentration of the heavier phosphorous or boron atoms and they come out of the acid and it places them right over the area where you want to do the diffusion and this can be done on a hot plate where you just place your wafer on top of the hotplate on like a piece of tin foil or something on a couple of hundred degrees C for 15 minutes or so or it can be done up to a thousand degrees C very quickly I'm sure theres also a rapid thermal process for doing this. After you do that you'll notice that on top of the wafer there's a thin, it will be thick actually, oxide layer that will form because your phosphorous spin on source is basically acting as a spin-on glass, and after the predeposition that will form glass on it, so you can put that in a weak HF bath maybe 1 or 2 percent, that will get rid of all the excess phosphorous film and also that unwanted oxidation on top of it. Then you'll do your drive-in step, and this takes place at 1200deg C optimally, it can start as low as 950 degrees C but you really want 1200 degrees C for about 45 minutes, and you want a very nitrogen rich atmosphere to do this drive-in diffusion step in. Once you take it out of the furnace, we talked earlier about depletion and pile-up, anytime you put this wafer in a furnace there's going to be some oxidation, whether you want it or you don't and that oxidation is going to change your distribution of dopants, so you have to be careful about that, so these graphs they indicate that at the surface here on an N-type diffused wafer will have a very high doping concentration as we progress that will drop of quite steeply because of the pile-up and then decrease after that. But then on the surface of a P-type wafer, it's depleted of those P-type charge carriers because of the depletion mechanism. One mostly unwanted side-effect of the diffusion is lateral diffusion, because ideally if we wanted to create a N- or P-type well right here, the atoms would diffuse straight down, and that would be awesome, but they don't, they diffuse to the sides, as much as they diffuse straight down, this is normally unwanted and this is one of the main reasons for the ion implantation technology being developed, which I will talk about in a second. There are some devices that actually use this to their advantage, there are LDMOS transistors, laterally diffused metal oxide semiconductor field effect transistors, and those use lateral diffusion to I believe they use it in like high power RF applications, microwave RF applications, and it allows them to put higher voltages through the device of this RF energy, they can put higher voltages before breakdown occurs, which is interesting.

That pre-deposition step, you better look at that, once we spin on the film on top of it right here, then that pre-deposition step will put a high concentration in this case of phosphorous atoms right here, and right here and then they are driven in to create these N-type wells.

This is the datasheet for emulsitone dopant this is for Borofilm 100 what they call it, and it says in here, well this you could basically assume, at higher temperatures you're going to have a lower sheet resistance at the end, and the same with longer times [???] but more importantly if we look at the atmosphere if you do this in ambient atmosphere so what's that you know like 70% nitrogen 20% oxygen so thats close to right here, thats the sheet resistance we're gonna have across the wafer, but if we do it in a purely nitrogen atmosphere, which it's done close to that commercially in a fab they'll pump nitrogen through it, we can decrease the overal sheet resistivity which is great, so it's very easy to create an atmosphere inside of a tube furnace, which is what these steps are normally done in, you basically just blow the nitrogen into the furnace, either perpendicularly or parallel to the wafers themselves, I think in here it says perpendicularly is best, but depending on what material you're blowing it, it differs. So the more nitrogen you have during the diffusion step, the better electrical characteristics you can get at the end.

Last thing I want to talk about under doping is ion implantation, which is a incredibly complicated and dangerous and expensive and massive technology which eliminates this lateral diffusion issue, it also allows for extremely precise control over the distribution of the dopants, not onl the depth but also their concentration at that given depth. To explain that I'm gonna go to a book that has some better drawings than I could do, this is "Microchip fabrication" by Peter Van Zant, so here is your typical ion implantation system. First it's got an ion source, so there'll be phosphene or diborane gas that will come in here, it will be ionized into a plasma state, and then that will be electrostatically focused into a beam, and there's a mass analyzing magnet, and these are typically 90 degrees, somewhere a little bit more than 90 degrees, and this is set-up so that you can select which ions you want to implant, the ions that are heavier than what you want will not be able to make this sharp turn along the magnet and will hit the wall, the ions that are heavier [lighter?] will go too far and smash into the other side of the wall, and the ions that are just the right weight of what you want will make the 90 degree bend and go straight through, and then they're accelerated electrostatically and there's some magnetic lenses and such that give you a nice focusable and deflectable beam, and these super high energy ion beams can be in the MeV range and these machines are massive, then this focused beam can be scanned across your wafer in a raster scan style so that it can implant these ions into the wafer very uniformly. This is the mass analyzing magnet I was talking about, so we can select what ions we want based on their weight. This is how electrostatic beam deflection works, we can choose between ions that we want and other gasses that are not charged by passing them through electrostatic plates like this, and we can deflect only the ions into a wafer that we're implanting and then the other random gases that are impurities will continue straight across and be unaffected, and those can be trapped somewhere else and extracted. And this is the beam scanning I was talking about, so that we can coat the entire wafer and implant these ions.

Ion implantation can also be used to very precisely control and modify threshold voltages of field effect devices like a MOSFET and can also be used to implant isolation slots, this is typically used in Gallium Arsenide high electron mobility field effect transistors so for microwave applications"


Easiest Dopant Sources For Home Chip Labs[edit]

The first 2 options are the easiest and will yield the best results but are the most expensive. The second 2 are harder but can yield good results with lots of experimentation. You may have problems with the acids spinning onto the wafers evenly so you will need to experiment with mixing them with alcohols as a solvent. IPA works alright for phosphoric acid but not nearly as good as anything from Filmtronics.

  • Emulsitone Spin on dopants, Borofilm 100, Phosphoro Film (high minimum order quantity)
  • Filmtronics Spin on glass (oxide) dopants, P509 (manageable minimum order quantity)
  • Phosphoric Acid, N type dopant
  • Boric acid (aq) 5%-10% by mass, P type dopant