In this video we will consider the important process of ion implantation. a process that is an indispensable part of the fabrication of modern MOS transistors. And specifically, we will concentrate on one application of ion implantation, namely the adjustment of the threshold voltage value. The main uses of ion implantation, are to make source and drain extensions. Which we have seen, and will see in a moment again. to make Halo regions. Which we have also seen. And to adjust the, the threshold voltage value. Here is a device with source and drain extensions, shown here and here. I remind you, these are used to avoid having the bulky source and drain regions, which have to be bulky to give you low resistance. From being too close to the channel, because then that would result in charge sharing and dibble effects. In addition we have the so called these halo regions which are regions doped heavier then the substrate and this helps contain the depth of the depletion region for the same reason. To, to reduce the short channel effects. And finally there is an implanted region here, which modifies the substrate doping in order to adjust the value of the threshold. And we will say more about this in a minute. Now we will discuss implantation which results in a change of the doping horizontally in this picture, in parallel with the surface. And this will be called the lateral direction and also implementation that changes the doping vertically in this picture or in the transverse direction. Now if we make a device with the same extensions and the same halo adjustment with a shorter channel length. Then these two regions will apploach, approach each other. And you see that they kind of merge, and now the entire substrate below the channel is highly doped. This effect will be important to incorporate of course in the threshold voltage because the threshold voltage depends on substrate doping. We will see how this is done shortly. Here is a three dimensional picture of a ironing planted device. the vertical access is dopping concentration. And the x, dimension is the horizontal dimension along the channel. And this one here is the depth of the, is the depth below the surface. So the, the surface corresponds to y equals 0, unless you go down below the surface, it's like this. Now, the doping of the source and terrain regions is high, near the surface, and it goes down, for reasons that we will see. Because these regions are implanted and the implant results in a variation of the doping concentration with depth. When the N type doping becomes sequel to the P type doping concentration of the substrate. The two cancel each other out and the net doping goes to 0, which on the logarithmic axis such that the one we have here, means that this actually goes down to minus infinity. So the this dip here defines the juntion boundary. Now the substrate doping is shown here, you can see it is somewhat higher than 10 to the 17th centimeter to the minus 3. This is a threshold adjust implant next to the surface these are the extension regions. And these are the halo implants, and as I have already mentioned if you now make the channel length shorter now the halo implants approach each other and the substrate doping becomes large throughout the region under the channel. So let us now talk about the so called High-Low Profile. To motivate, let us start with an un-implanted device always, and most it has a flat pan voltage given by Fie ms minus Q0 over Cox, as we know. for an N plus poly gate, pi ms is about minus one volt. with the clean processes today, Q zero prime is practically zero, and you can neglect it.so the total value of Vfb turned aught to be about minus 1 volt. Now the threshold voltage turned aught to be V plus V0 plus gamma squared 2 V0. Now we want this to be positive, let's say we need it to be about 0.4 volts because we want to make sure that the device is off, for digital applications when VGS is zero. want VGS minus Vt to be as large as possible to give you a large current when the device is on, you don't want to make Vt zero very large. So 0.4 is a compromise between too low a value and too high a value. Now, if this is minus 1 vote, be at least minus 1 vote and you want the results to be plus 0.4 if you make a quick calculation, you'll find that gamma must be very large. And to make gamma very large, you would have to use a large substrate doping, and then undesirable things happen. First of all, you have a large buddy effect because of a large gamma. And you also have large junction capacitances is because the depletion regions are narrow and the junction capacitances are large. So ideally you do not want to dope this up straight heavily throughout it's extent. A better way to do it is just change to 0 if you can. So in other words you make this one to 0 prime negative and with another minus sign here becomes positive. And it makes the total Vfb less negative and can give you the right value of the threshold. Unfortunately, there's no way to just change to 0, what you do is you attempt the p implant, which should be as shallow as possible to add negatively charged ion. Which is equivalent, if they are very close to the surface, to make Q0 prime or negative. So the result then, is a device with an implanted region. And in this simplified picture, I show the implant as existing over. The finite depth, and that is a clearly defined boundary between the implant and the substrate. The implant characteristics are the effective dose, which tells you the number of implanted ions per unit area. Which is 10^12 per square centimeter of gate surface area. And the average kinetic energy of the ions, the way the ions are implanted is they are accelerated via an electric field, then they pinch on the surface they enter with high kinetic energy. which they gradually lose as they hit the crystal lattice. So, the kinetic energy, as they leave the implanter, is measured in tens or hundreds of kiloelectron volts. So these are implant characteristics that will help you determine the final implant you'll end up with. And if you plot the doping concentration versus depth from the surface, you get a constant concentration for the, substrate that you start with. And then you end up with a distribution of implanted ions that looks almost like a Goshun like this. And sub i, which is a function of y. Now both of these implants are of the same type, so you add N A B The initial substrate doping plus an 1 and at the end of the total doping after the implant. Now because this is too complicated to handle in simple modules the whole thing is approximated be a step like this so the final value of the step is NAB, the un implant that the substrate doping. And this one here is Nab plus Ni, where Ni is a constant value that tends to approximate this varying profile of Ni of y over here. So this is called high to low as you go from the surface to the bulk, you go, start from the high concentration go to a low concentration. Now, let's assume strong inversion operation. as you remember, the as you increase VSB, because of the body effect, the threshold increases. Now, initially, if you assume you have a channel next to the surface, initially if you don't have much of VSP, the depletion region depth would be small. And the entire depletion region would be inside the implant, which corresponds to high substrate doping, and then you can expect large body affect. But as you increase VSB eventually the depletion region extends outside the implant. And now as you vary VSB, the bottom of the depletion region moves inside the lower doped substrate, in other words, where the doping is only NAB. And then it turns out this is equivalent to a smaller body effect. Now if we denote by V sub I, the value of VSB at which the bottom of the depletion region reaches the bottom of the implant, in other words, depth d sub I over here. Then, when you plot VT versus VSB, it turns out you get two branch curves. Which can be represented as one value of a the threshold versus VSB, when the implant, when the depletion region is inside the implant, and another one when it is outside. So you may have two different expressions, one for VT1 and one for VT2. And use one or the other, depending on the value of Vsp. But in reality, because the actual doping profile is not a step profile like this, but it is like that, you can get equally good results, or even better, by using a simple modification to the classical body. effect expression, which is this one. You simply add the term K2VSB, you made K2 negative, so what this does is when VSB is large, this term becomes significant and subtracts from what you would normally have which would be continue like this. Subtracts from that and gives you an approximation with this behavior over here. So this model is widely used. Now let's talk about the opposite case, the low-high profile. now instead of an acceptor implant, we use a donor implant. Which means that if we have a p-, p-type substrate here, we have, we implant Donor ions, and depending on the relative concentration between n and that the p, the final implanted region here can end up being p with lower doping concentration, or even n. For now we will assume it stays p so we have this is the initial substrate doping, this is the implanted donor concentration. So, you have to take this and subtract that, because they are of the opposite type, to get the overall concentration. And again, you can approximate this like that for a simple model. So as you go from the surface towards the balk? You start with the low and you go to high. Now this is why this is called a low high profile, the final, the approximate doping concentration you end up in the implant with the difference between the initial substrate concentration you started with minus an I who and I is an approximation. It's an operative value over here. The threshold is still given by the same expression as before, only now because you encounter the low doping and then the high doping has VSB increases and eventually the depletion rate can extend outside the implant. That is when you see the large concentration and gamma becomes more important. So this k2, it has now a positive value for the low/high profile in order to make this one an effective correction to the rest of the body effect equation. Now, real profiles of course are as I mentioned. The approximate Gaussian curves. They are too complicated to get manageable analytical results, so people use approximations. And these approximations work because of several reasons. First of all, the variation of the doping with depth is gradual. It's not true that as you increase Vsb in the depletional region. Becomes deeper and deeper. Suddenly, you reach the limit point between the implanted and the unimplanted region. You just see a gradually changing doping concentration with depth, nothing drastic happens. Also the details of the Qb variation are obscured by the integral in our drift equation. I'll remind you that the drain current has a drift term and a diffusion term. And the drift term looks like this. And what matters int his current is not QB itself, it's only the integral of it. So the details of QB prime are obscured inside this integral. The diffusion equation looks like this. Here, you do see QB by itself. But in both of these equations, the main terms, which is the first-order term here and the square-law terms over there, the first-order term here. All of these terms do not involve Q'B prime, and that is why If you don't model Qb prime very accurately, you can still get accurate expressions for the count. Now's some models use an effective doping, which is made a fraction of the surface potential in an effort to model implanted devices. Now If you do this, if you've got an effective doping that depends on the surface potential, effectively you're making the body effect coefficient a function of Vsb. This is just a mathematical convenience to help you model the device. In other models, what they do is they make an effective doping a function of Vgb. Again, this is just a mathematical convenience. Of course, it doesn't mean that the concentration itself changes as you change VGB, but in equations that we used to have that relates VGB to the surface potential. there is a term that depends on substrate doping. They make that term a function of VGB. And now for a given VGB, you can use the same equation to get the value of the surface potential, which we used to do for the old region model. Some problems that can occur with the iron implantation, if the device is not designed properly or sometimes even if it is. you can get a log I versus Vgs, here we emphasize the weak inversion region, and when you have a large Vsb then you get the curve on the right, here. And this happens because for large Vsb the bottom of the depletion region is outside the implant,it is in the lightly doped substrate. And a lively dope substrate means that you have a small gamma, and therefore a small factor n, which affects the slope of the weak inversion region, so it looks like that. Now, when VSB is 0, the depletion region is shallow and it is inside the implant. And then gamma, because of the large doping consideration there, gamma is large and n is large and you get a smaller slope. But in addition, as you vary VGS the depletion region depth varies, an it moves over various parts of the implant. In other words, it goes over, a region where the doping concentration of the implant gradually changes. From high to low. And, therefore, as you increase VGS, the slope even changes. So when you see things like that, there may be an indication that the implant has something to do with such effects. Now, pMOS devices are like nMOS devices but here they have a p type source and drain, an n type body. And when these regions are implanted, the pole is also implanted, so the pole typically p, p plus. And if you implant. This one with extra donors you can adjust the effective body doping near the surface and modify the threshold of the pMOS devices. I would very briefly like to mention buried-channel devices. These are devices that are not used today. They used to be used for, to make depletion-mode devices, and in device research they are being considered even today. Now, the implant type for such devices is the same as low to high, you remember to get the low to high profile. You start with a, a P-type substrate and you implant donors. But now, in this case, we implant with even more donors than we have acceptors, so in other words, the implant, here, the peak of NI is above NAB. This is the substrate concentration. This is the implant concentration versus depth. So then in order to get the com, combined concentration of the two, you have to subtract the two. And because this is larger in [INAUDIBLE] than this one, you end up with a net donor concentration near the surface. And and accept the consideration in the bulk. So, the device looks like it is p type in the bulk and N type near the surface. Now, depending on the value of the gate potential, you can even deplete the surface here. And then you end up with a channel that is formed by the n type region, except for the depleted part near the top. So you have a channel where the current flows below the surface. So this is called buried channel operation and in some cases, it has been reported to lead to lower flicker noise. In this video, we discussed the iron implantation and where it is used. And then we concentrated on one of it's uses, which is the adjustment of the threshold voltage. In the next video we will continue with another use of an implantation to make halo implants.