 So we start with the other kind of evaporation which is essentially related to still evaporation but not using heating, resistive heating. Here is the second possibility of evaporating a material and the process is called electron beam evaporation. The basic idea of electron beam evaporation is the following. I have a some kind of a source of electrons which can be from filament. Then you have an extractor voltage. Similar like implanter you have seen the start source as well as the extractor. Then there is some kind of a beam forming aperture which also has some voltages applied on that and bendings are possible. Then this electron beam passes through a magnetic system. Sometimes directly through electric system it is possible. If it is only 90 degree shifts but since I want 270, I need both electromagnets as well as electric fields to bend the beam. The beam bending is essentially because of the Lorentz force we cross beam. So accelerated beam not really very large energy, 10 keV energy is all that is needed. Typically energy used in electron beam is 5 keV but source normally can go up to 10 keV. So this beam, electron beam, some people have electron beam coming from the vertical region itself. The kind of system given in plumber has right side so I also copied it. That is it. No more. The way I have done it myself was a slightly different version of this. So this electron beam which is accelerated by potential of 10 keV and therefore it has a 10 keV energy is so focused at 270 degree beam angle that in a crucible which is which has internally which has a graphite hearth as it is called you are the material which is you want to evaporate. It can be pieces, it can be powder, it can be wires. Anything can be put there. It is a crucible there and of course there is a possibility of this beam forming aperture. Some voltages are given so the beam can be scanned as well. X, Y. Some raster can be given on that. But normally not needed because this aperture is very small. Once it starts heating this, the energy from the electrons is delivered to the metal or water's material and if the temperature rise because of that is sufficient enough it melts and if it is still higher it evaporates. Normally you need some cooling system in this because it is a very large temperature 1800 degree centigrade or above. Therefore you need a huge cooling system and say the outer vessel is of copper many things. Now so this can be replacing the source of either the coil or the basket which I did resistive heater. Same this only thing now it is a crucible through which material can evaporate. Is that clear? Material can evaporate. The target is again above. The vacuum is as much as possible because so that the mean free path is larger than the distance and at an angle it will also have the lesser thickness compared to this. To avoid this as I said you have a planetary system the big dome which has number of wafers attached there which themselves can rotate and whole dome rotates. So that uniformity or film thickness is roughly possible. Perpendicular we do 90 degree maybe shift but there is an advantage of 270 beam degree read plumber's book. Okay you know normally this system this may be just corner of the system. So I do not want to bring anything which may dig as and actually impinge on this. So I want to keep my beam as pure as possible. So if I am pushing from outside the rest of the system does not affect the hearth system. So that is the idea. This e-beam is of course is a little costlier system but one biggest advantage of e-beam is since it is 10 K e-beam energy can be provided it can evaporate many things not necessarily metals. Of course metals it can not metals of which are 3000 typically molybdenum, platinum, rhodium, pallodium, tungsten they are not the material which even electron beam can probably melt. You can by increasing the energy but then the system will become bicker and whole effort will be waste of money. So here is something you want there is nothing that figure great. So what is the advantage disadvantage of this system e-beam is a complex and costlier than simpler operation system. Simple operating system is very cheap comparatively. A Indian company can give you as low as 3.5 lakhs or so a way of operating whereas if you buy from abroad any Edward or Varian or anyone it may cost you 12 to 15 lakhs even otherwise and e-beam may be even costlier. Typical as I say extractor voltage is 5 to 10 KVs the temperatures if you increase 10 KV some of the temperatures people have reached up to 3000 but typically it should be around 2200 to 2400 is all that temperature normal otherwise the cooling system requires huge amount there because if you have 3000 degree hearth you need huge cooling actually and that may cost you so people do not make is possible but normally it is not used for 3000 degree centigrade heating. Now the advantage being this since it can go higher temperatures it can operate all resistive heated system materials and that apart it can also evaporate nickel, platinum, titanium, vanadium, zirconium, tungsten if it goes to 3000, tungsten, pantalum, rhodium, gallium, ishenium everything. It can also evaporate of course depends on the if you have 10 normally most beams are 5 KV then this of course higher materials will not be able to evaporate. You can also operate alumina, SiO, SiO2, tin oxide, titanium oxide and many of such oxides. The major problem with electron beam is since it is 5 KV even if you are now saying that electrons are only heating the crucible the one believes that electrons do not disperse but in real life they may scatter from variety of parts of the system itself from the crucible age from the age of the where the magnet hits out. So all beams are not fully focused though we try some is scattering some is secondary electrons creation. Now these some of these electrons can go and hit target. Now any high energy beam or high energy electrons if they hit at least an MOS device in particular it may actually charge the MOS that means the oxide charge will increase or minus charge will increase which essentially means threshold will shift. So it is called radiation damage. So 5 KV is a small energy the radiation damage in space is much more higher there is a one MEV energy charge particles are there in space in Van Allen's belt but even 5 KV but the distance here is very low and therefore there is a damage and the first our effort to study this electron beam damage was in 1987-88 and one of my BTEC student did India's first radiation damage studies so just to tell you because he is now with Cadence nothing to do with evaporation. Cadence US he is a manager in some group. So this radiation damage was our great project we did from 87 to 93 we developed a new technology for radiation hardness but all said and done our first effort was on electron beam damages we just want to see how much damage it gives and to a great surprise because our oxides are not that good compared in those days in 80s our damage was sufficiently high we used to see huge CV shifts so we first realize that yes charging is happening charging is happening. The next possible process in which we can do evaporation but many other thing is to use of a plasma in IC manufacturing you look at the list again there may be many more I did not add everything one of the features of plasma processing in IC that you have what is called as plasma implantation sputtering essentially is based on plasma process you can also deposit any material using CVD which is plasma and ion CVD you can always etch any material using plasma and there is no liquid there and therefore it is called dry etching of course in dry also there can be reactive and non-reactive etching but essentially plasma etching and of course one can do analyzation of silicon itself using plasmas so you can go with in oxide using plasma analyzation the problem is plasma is similar if it hits too much to a high energy then it may actually create a damage but most plasma damages can be annealed in the plasma itself that is the fun part think of it by acid plasma itself can anneal the much of the damages so what is plasma plasma is the first state of matter which is very I mean not so much start from first standard we keep saying three states of matter solid liquid gases but in fact 99 percent of the universe is in this state that is called plasma state and we hardly talk about it is not that is a interesting thing what is plasma well it consists of its please take it always plasma is charged neutral this has to be understood plasma is charged neutral so there will be electrons there will be ions and there will be neutrals but net charge is 0 charge neutrality holds even if there is negative charge positive charge and neutrals this has to be understood there are all this this is something which people do not realize but this has to be understood that plasma is still neutral however it should be noted as I said plasma is neutral if we want a material to change its state if you are we I forgot that or may be if one wants a material to change its state from solid to liquid or liquid to gas actually energy required is very small 10 milli electron volt 10 milli milli or 10 to power minus 2 electron volt per particle okay. So very small energy is required to liquefy solid to liquid and liquid to gas but if you want to convert gas into its plasma then you need roughly 1 to 30 electron volt per particle energy to actually create a plasma so plasma creation is much higher energy requirement compared to conversion from solid to liquid or liquid to gas is that point clear that why plasmas are not so very common because you will require higher energy per particle to create plasmas why are we so keen about plasma I already listed that all processes which we now going to do in next few days all are plasma based okay. So today probably I wish to actually tell you what is a plasma why it is so crucial okay and what is the funda issue on that okay once you understand plasma then you can think all designs of plasma systems are understood another issue which plasma provides you is called cold processes the ambient temperature can be even as low as room temperature or at best less than 300 to 50 to 200 degree centigrade. So it is a cold process but we have said the temperature it gives is how much it can give to 3000 plus okay which means some way energy must be provided okay even if the ambient temperature is not large the energy is higher which means KT is higher means T which is called electron temperature is 10 to power 5 10 to power 4 degree centigrade so it is the electron temperature which we are rising and not the ambient temperature this is the difference between normal processes which is like diffusion what we do 800 degree furnace 1200 degree furnace there is nothing called 1200 is everything is below 300 but internally there is a huge energy provided which rises the temperature electron temperature to 10 to power 4 or 10 to power 5 and it can create therefore thousands of degrees of centigrade of temperatures so that is how the plasmas have become very popular. In all our diffusion process in plant process we say anneal or any temperature time cycle what does it do what did it do is driven the impurities from its original implant position or diffusion that means profile changes all your circuit analysis was based on a given profile okay now we almost plan all of that a priori okay now that means any process which I do later should be such that it should not actually affect the process which earlier I had done which means the temperature should be as low implant I did anneal 800 but then I will never go beyond 800 okay and now I will always be less than so DT product at 300 degree is so small 10 to power minus 16 minus 17 into T is the DT so you can say practically nothing changes at room these temperatures so therefore these are called cold processes they do not change any profiles or there is no annealing going on there is only localized heating going on and you are doing whatever you otherwise would have wanted to do okay so that is why plasmas are very very important that they give you cold systems but very high temperature increase possibilities is that okay all of you KT KT is the energy T is increasing no it will not be plasmas are take from me once for all plasmas are neutral they start charging the whole you will get a shock of your life you know so please never think no system is chargeable it must provide some ground sometimes or otherwise also it will create a neutrality like semiconductors you put a voltage here your mass capacitor will not charge you you know okay then charge neutrality always holds it is locally we may say space charge but still N and P are same N plus N D P plus N is N plus NA as in semiconductor charge neutrality holds this is important all analysis is based only on this for a transient case there may be a possibility what you are thinking but these cases are steady state so we are not looking in transient many things can happen okay they are reversible processes they are non-reversible processes so we will not discuss that you want specially we have another hours for you hours for you transient process KT K is 1.23 10 to power minus 23 that is key we go is a divided by minus 20 to keep that temperature just like energy to KV may there are no 10 KV cut extractor like I am an energy may be it not temperature divide Corona K say it not electron temperature the body excessive there are 3 zones of discharge in a normal plasma system and this is called the IV characteristics of plasma you draw it and then I will discuss this is very important this is the fundamental of plasma where do you use plasma very often you blight of course this is the figure given probably in plumber as well I may be any anything you on Google this figure is most common IV characteristics of a plasma is the most common figure available in with associated plasma I is in amps V is in volts the scale is 0 to 2000 and it is 10 to power minus 8 or minus 7 to plus 10 to power 9 or 10 to power 12 amps huge scale these are log scales so shown on a log scale can be made on that all that means all energy is not imparted as I say that net energy I push it is not received by this this is a energy conservation so not all energy will be delivered so not all temperature will rise to 10 to power 4 is that clear energy is proportional to KT is that correct so T will rise definitely the energy but all energy is not transferred to the material this has to be understood there is exchange mass both energy and momentum are to be conserved so not all energy is transferred so there are 3 zones of discharge of a gas okay I should have forgot I did not write this is a plasma is only created from a gas gas to plasma state okay so we are now looking for dark discharge glow discharge and out discharge there are 3 zones in a plasma so if you look at the first part okay around 10 to power 1 micron per so current I do not see any plasma is called dark dark means no plasma so all dark zones are called plasma free zones so there may be ions but they may not be electrons okay is that point clear they may be ions there but they are not enough electrons only when electrons and ions come together there is some light is seen that is called glow okay so dark zones are those where there are no free electrons to interact okay so this initial region is called dark region okay so let me come back I will just show you from say this onward to something like 1000 amps there is a region which is essentially called glow region where plasma exists most important for us glow okay and in glow also there are some peculiar things we have seen in some part is called normal glow the other is called abnormal glow okay why I showed you this specific abnormal glow because only this region is what is used in IC fabrication so called this abnormal zone which actually it should have followed something like this normally okay it did not some way it peaked up again and this is the zone where I is high and V is high is essentially what is of in a glow is of interest to us that is the whole plasma processing rest on only this zone called abnormal glow ahead of this we find the world so much charges inside the tube or inside the area the conductive falls drastically so the voltage becomes very very low but the currents are extremely high and there you actually see arcs if you have air and you just apply 30 volts bring it close it will spark spark is an odd discharge okay is that clear but that is essentially people believe it is air actually air molecules do not break so easily it is the moisture which actually picks up the arc okay so you must remember it is the moisture which actually acts up okay so anyway so these are again we are I repeat my only interest is in this zone for all IC processing in the glow region also I am more interested in something not good or worldwide abnormal I do not want normal glues I want abnormal glues and only this region I may prefer to work on so all my processing should be maintained somewhere to create abnormal glues now let us this I said it so I now show of course the upper figure right now you do not look at it that I repeat again if I apply voltage across the in a tube or in a system across two ends so apply electric field is that clear there is a gas apply voltage across okay if this applied voltage is larger than the breakdown voltage what is breakdown I define breakdown is the voltage at which electrons starts ionizing the gas ions okay so if electrons and ions start forming that is called breakdown so onset breakdown is not total onset of plasma is called breakdown voltage so if you are applied voltage is larger so what will be the dark region when your plasma has not been initiated is that correct plasma has not been so your voltages are looks higher but not sufficient carriers are made available to you and therefore there is no plasma onset okay that region is called dark region no onset of plasmas okay electrons emitted at cathode now you can see if I apply cathode one where is the negative potential and the other is positive so cathode is negative potential so electrons are emitted from cathode because opposite polarity carriers will go towards anode somewhere this figure can be seen but there are many other things shown there we will discuss them separately this is my cathode this is my anode electrons will travel towards anode is that correct this figure you do not draw now because we will come back to draw that I wrongfully draw here thinking that I may not give you this but then I decided to write so that figure has anyway made okay now if you electrons emitted at cathode travel to anode and then they fix ionic collisions okay now you can see here I have a gas some electrons stuck and ions are created okay which side ions will try to move towards cathode electrons will travel towards anode this has to be understood okay now is that okay so I last line I start now electrons emitted at cathode travel towards anode and had fixed ionic collisions then such created ions travel towards cathode because they are negatively positively charged so they travel towards cathode some of these ions when they travel what is the force on them to travel the electric field which I applied is that correct electric field which I applied so they pick up energy any charged carrier going through electric field will pick up energy how much is that Q into V okay so charge into voltage is the energy and what is the force Q into electric field is the force on them Q is not electron charge now ion charge so this energy is larger is that correct if these ions come and hit the cathode their energy you have a cathode so it will also ionize that region or something and it will create what we call as secondary electrons one electrons was anyway coming because cathode is releasing electrons going towards anode but this impinge also will create quite a loose energy and when it loses energy it will create electrons these are called secondary electrons these ions produce secondary electrons from cathode surface glow at this cathode is called cathode glue why now there is there is ions neutrals electrons everything possible so near cathode very thin region you actually have a glue so if you see actually to when it starts the cathode becomes slightly violet and then suddenly it lose that because then one sustained it does not need that okay so this is called cathode glue but now think what is happening and this is self-sustaining ions will come electrons will come out and some steady state will reach so you will have a constant cathode thin cathode glow you apply voltage some ions will lose some new ions some new electrons will go again I am so average balance and please remember again plasma is neutral this never go against that is that clear this man I am impinging and home him if the current in the external circuit is now large let us say because your ions that region has now lower conductivity okay sorry larger conductivity are you increase the voltage outside outside then you have a huge plasma electrons and ions and this region is called sorry this was wrongfully put here this one sorry this is mean mean couple of this was something which was the we said to that discharge glow discharge and this is the odd discharge odd discharge if you apply large currents or large voltages the air or whatever gas will break down instantaneously and that is called odd discharge okay forget about this I am sorry you come from here I do not know how this I wrote here but anyway in good system as shown we have now this is some now you draw this now you look back to this figure which is most important okay this has a low pressure gas in a chamber okay it can be even tube cathode ray tube can be anything or in a system which is back evaporate evacuated so I have a cathode I have a node I applied a voltage positive with reference to cathode so electrons start coming and this as I say some ions strike cathode and may create a glow there which is thin glow but it is called cathode glow however there is a space in which there is no collisions what is this is because of I have written down below but can be what can you think this region between the ionic region or the glow region there is a gap between cathode glow or cathode to this region and which is called crooks dark space crooks dark space that is why s is always written by me crooks his name is crooks this occurs because electrons now this is you think of it why I suddenly thought of this what was the word written above so what does improve mean free path is that clear to you so if the distance from cathode to the next glow electrons do not interact with ions okay because they are the mean free path okay once they reach that mean free path now they see gas and now they see ions and they may and by them they might have already acquired sufficient energy and plasma can be created is that what layer so that is crooks dark space it is essentially the mean from mean free path distance from cathode where no collisions are possible this crooks dark space is a very important is also called there is a voltage drop across it why voltage drop it has no ions there okay only electrons and so larger conduct smaller conductivity so there is a voltage drop there is called cathode fall or potential across this is called cathode fall and that potential that region is also in our plasma system is called sheath sh e a t h I will come back to it sheath this crooks space is called sheath so there is a sheath potential is that clear to you every sheath and what is in sheath there are no ions there is that clear electrons are now traversing in the mean free path ions are yet to reach this so secondary electrons main electrons both are not able to interact and they travel that small region shown larger just to show for it but it is a very close mean free path may be a few centimeters and therefore you see a dark space there which is called took star space okay in this region there may be positive ions but there are no electrons which can interact okay so if there is no electrons and ions then there is no globe is that what clear to you if there are no electrons and ions together plasma what what did I define plasma equal number of electrons ions and possibly some neutrals is plasma if electrons cannot interact they move out so in that region there is ions but they are not interacting with electrons so this is the place where there is no globe how much it can be the length of the mean free path the electrons that region is called crooks dark space or later we define that space as sheath okay sheath okay so in a low discharge system as we are next to cathode glow a region called crooks dark space is in this region we have only okay in this region we have only positive ions and no electrons is that correct not only positive ions but no electrons so no plasma okay you just add this last you just be this is the beach coming up what I need to start building it please remember this in between part or discharge part should be part of I need this explanation but maybe in some mood I wrote that but I just want to clarify so we are not looking into this figure essentially this figure from here okay because most of the electrons are leading that place. So the why it is very small because most of the electrons from cathode leave, second electrons are leaving but some electrons are coming, some ions are reaching, okay. So some way is possible there, okay. So is that crook space is clear? So let us talk further and this is very important region for us. This is the most important region for us. This is called sheath, S H E A T H. This crooks large space region is called sheath, okay. Which is, which is, where is that? It is always close to cathode, okay. It is very close to cathode. And again I repeatedly saying it, once again I say its thickness is roughly the mean free distance of electrons traversing towards anode without collisions. Is that correct? That is the typical dark space will be. However as it crosses the mean free path, you are still electric field applied. So what will happen to these electrons? They will further accelerate. Energy lagivie, a field lagavane, they will get force and they will pick up the energy. As electrons gather energy from the field, they are capable now to interact with plasma. Please remember when ionization can take place, at least few E V per particle energy is given by that. That is the plasma requirement. Is that correct? So till this electrons are able to get that much energy, you cannot create plasma. Is that clear? So it interacts with ions and creates plasma, okay. And this is the glow region. Once it interacts ions and electrons available to you, this region is again, which region? Glow region. And among these 2, among the glow region I said there are 2 possible glow regions. One is normal, the other is abnormal. But that will wait, okay. Okay. After ionizing the gas, electrons do not sufficiently have enough energy. Once you, why they lose energy? Because they have spent their energy in ionization. But they are still traveling towards anode. So what will happen? The electrons which will come out of this glow will not be having enough energy. Is that their last energy? That is how they created ions. As they come out, since they have no energy, they cannot ionize. Is that clear? So just a minute, I will just show you a figure. As they come out of this plasma, beyond this the electrons do not have sufficient energy to ionize, okay. They will pick up now. But still they are, so they will travel for some distance before they acquire sufficient energy again and starts ionizing. Is that clear? This gap between glow, this glow and this glow is given the name Faraday's dark space. Is that correct? The person who is aware, Maibab, if Faraday would not have been there, probably electrical engineering would not have been there, you would have been saved. So this Faraday's dark space, actually Faraday did this experiment. The name was given later. Faraday was also looking for discharge. The potential which allows sustenance of plasma, after ionizing the gas electron, do not have sufficient energy to create further ionization. So some part is left dark, which is called Faraday's dark space. Beyond that electrons, now they are closer to anode. So they are very highly energetic to create plasma as they reach anode and this region is called positive glow. The word is positive and the earlier we said negative is because of what? Glow is neutral. So why it was given name negative, positive? Closer to cathode, negative glow, closer to anode, positive glow, no other difference. We will say that that a neutral level. The glow closer to cathode was given a name negative glow, closer to anode is given positive glow. Now there is something which I wanted to tell you about the, if you have written down, this is something, this is what has happened now overall. Either cathode fall or cathode glow, crooks dark space which I call sheath, then there is a negative glow, then there is a Faraday dark space and then there is a positive column towards anode. This is essentially a cross sectional view of the plasma. So let us now do, is that okay, all of you, this is what we have shown, we drawn the figure. I am just trying to say how much, what is so far we talked about. I do not know where is this figure but may be Abhitha said like that. So as I said earlier, the regions, region of interest to VLSI people is not really oil dark spaces or non-dark spaces or glow, only glow region near cathode is of interest to me, in fact, only glow region closer to cathode is of interest to me, is that correct? As electrons create plasma near cathode after crook star space, initially the area of plasma because fewer electrons are coming and they interact with ions. So the area of the plasma is smaller than the available cathode area, I repeat. Of course a slightly different way, this area comparatively is smaller than the initially that area is smaller because electrons are ionized. So initial area is smaller as you reach, start growing, getting a glow. Now this is an issue, the potential which allows sustenance of plasma is a sheath potential and this glow is called normal glow. However, in case I increase the power, I increase the current. So if I increase the power now that is IV product, beyond this where it started, okay. What will happen if I increase the electron current, I mean current means externally, the number of electrons which I am emitting will increase. So ionization will be stronger. So finally the total glow area will be same as that of cathode. So initial glow area say, because initially what was it? Plasma. But as it starts ionizing again heavily, it starts falling down once again. So initial glow say Nichiaya, till the time the new ionization starts, the resistance starts increasing of the system. So voltage starts rising. But at certain potential ionization is very heavy, it falls down again. And that region where it initially starts rising and falls down again is called abnormal glow region, abnormal glow region. However as power increases which in turns increases the current density at cathode, this results in increased secondary electron emission. In this small power region, voltage is higher and current is also high. This results in a stronger plasma creation then and finally resistance goes down or the current goes down. And this region is stronger plasma region is called abnormal region. And please take it, this is the only region of interest coming. So what does it mean? I will have to operate at higher current density if I want abnormal glues. Of course we will further increase current. What will happen? The arc discharge will start. Currents are very high but the voltage is because why it is, the ions are so many. So the voltage drop becomes very low and almost huge amount of current flows. 10 to the power 9 amps the current. So arcs can be as high as 10 to the power 9 amps. So please do not, that is why it is a shock. So please remember this is the most crucial part to understand in plasma that after all that I only have a gas, I will apply voltage, I will analyze it and see to it I am in a normally region which is of my interest which is abnormal glow. Now with all the actual system may we will do little more mischief. Here is some more mischief for you. Of course as I say after this glow if you further increase current arc will start but that is not very important. Since the distance to anode, to reach anode , is that clear? So how far your anode has nothing to do with the cathode region? Is that clear? Is that clear? So let us do what further I say. Since distance to reach anode has no direct impact on glow near cathode we can reduce this length of the tube of distance from cathode to anode. Bringing anode closer to cathode. So you bring your cathode anode closer to anode. So first of all that positive glow will go away. If we reduce it then dark space, ferrari will also go away. Or part of the negative glow may also go away if you bring too close. But that is fine. Even part of negative glow reduces this will have a picture of plasma of this kind. This is what it will look like. If I reduce the distance between anode and cathode then I may have a plasma touch to anode. And this may be negative because negative glow, now it cannot be called negative also because it is also positive as well as negative. But negative glow become also part of this. And what is in between this plasma and the cathode? Sheath. And what is the importance of sheath? There is a potential drop there. Is that correct? It is called sheath potential. What kind of voltages so far I have applied? DC. Is that clear? I have applied DC voltage. Typically how much voltage should be applied? It should be 100 to 300 volts. At what pressure? Because that will be decided by pressure. Why it is decided by pressure? Not just mean 3 bar this is one additional feature. Something else it gives. If your vapor, if the pressure is low what does it mean? Number of gas molecules are less okay. So to create a current density she will require larger than this. Is that clear to you? So you have to understand this. That pressure also decides what are the voltages. So to create this kind of a, so I could have directly shown you sputtering from here. This is what they use. So normally the DC discharge normally occurs at pressures around 3 into 10 to power minus 2 tall. And typically allowed voltages are 100 to 300. If you reduce pressure 2 things may happen. As you said mean 3 path will increase. But larger the mean 4 path means what? Lesser collisions. So even less plasma. So do not think that reducing pressure is all good. It has advantage in the sense. It is purity. Purity is maintained. But I do not want to. So what I will do? I should do therefore what I, you said it. First actually vacuum as much as possible and then backfill it with gas. So if you reduce the pressure we may have 2 issues. Mean 3 path increases and unlikely event of collision and hence creation of plasma may be inhibited. So too small a pressure or too small a larger a vacuum is not advised. Though at times in other system where the operation we said reduce, reduce, do not reduce here. Because essentially it will not create sufficient secondary electrons. So no plasma. But that can be, you are short of electrons now. So if I provide you another source of electrons, all that you are looking for additional electrons to come. So then I said okay if that is your worry I will apply your filament heated electrode which I will call cathode and it will limit electrons. Your short of electrons can be probably taken care and then what can I do? I can further reduce the pressure is that. So if I want to do lower pressure as much then I need additional electron source which is from the filaments. Here is that figure. Everything what I wrote here is available in mostly in the books as well. It is written in my own language. So here is what a typical system will look like. I may have this anode cathode here, additional anode cathode here. I may apply minus potential here and I may have anode here and I may have a filament which may give me electrons. Sir heat kiya electrons mile plasma tayaar ho gaya. This is called plasma confinement. And now vacuum can be even militars okay 10 to the power minus 3, 2 into 10 to the power minus 4 it can even go better vacuums. For better this okay. But additionally you have to do few things. Now all along all this time I was talking to you that there is a DC voltage I am applying. Isn't it? DC voltage. So all the plasmas are what so far we discussed are called DC plasmas. We are done DC DC we apply and whatever charge discharge gas happened we say it is a DC plasma. But in all our required system we do not use DC plasmas. Actually there is a DC diode sputtering but that is rarely used but possible. But normally we will use RF sputtering or RFHEU. So how does plasma is available to you in RF? The reason why it is yes or no is this. If you apply DC it is fine but if you apply very low frequency AC bias instead of DC. So what does mean low frequency means? For a while cathode will become anode, anode will become cathode but very slowly because your frequency is very low okay. So the plasma during the time when it was one end will only go from here to here. Because it will create sheath here then it will create sheath here. But it will keep oscillating. But essentially equivalent is saying you have a DC discharge okay. Though this keeps moving but it has still a equivalent of DC discharge. However if you increase the frequency it cannot follow this movement. Whole bunch of plasma cannot go so fast okay. So there may not be plasma at all because if the voltage keeps changing there is no time for ions to interact with electrons they keep moving other side. So there may not be any ionization at all if I apply an RF voltage okay. But as I said we want to use RF discharges okay. Ions may not follow the fields, RF fields. But electrons can no? So the advantage we take it. Electrons follow these fields, RF fields and pick up energies and slowly start ionizing the gas okay. Because once they will go here and come back and hit again. Again create ionize they move up this side. We will move back by the time cycle ask you to come back. So electrons will keep moving and may finally because of acquiring the, because the distance is small there is sufficient energy they can acquire in every transition and it will keep ionizing the gas. This is called RF discharge. This is called RF discharge okay. Now it is very interesting field data should be known to you. RF discharge is something also do with pressures. Why pressure? Do you believe that higher the lower pressure higher frequencies are sustainable or vice versa? I am asking you a question if I reduce pressure should I have to go on lower frequency or higher frequency? Okay do not think too much. It is seen that RF discharge can occur at the pressure which is inversely proportional to sustaining frequencies okay 1 upon f. So if you have lower pressure you must work at higher frequencies okay. This mode of plasma creation is called RF plasma or RF discharge. This technique of RF discharge is the technique used in all deposition and etching processes in IC fabrications. But they do not have to because electrons will ionize them. I do not want ions to move anyway. I want plasma to be sustained but what would happen if they would have moved then I will have to change the polarity to come back and I do not want them to move anyway. This electrons will go and come back and they will keep giving energy because they will pick up energy from the field and will hit the ions after a while they will ionize. Then you have a plasma. So it is called RF plasma okay that is in DC that would happen. I want to move but in RF it does not move. So is that clear? So what is the all these crux? Is that okay? You have to I mean that is what I kept saying I am not that these are not known to many but I only want to impinge on you these are relevant basics okay. Whether you like or you do not like but I believe you should know. Where is the people other than us are you looking for plasmas? I did not get you. Yes rocket propulsion is one or third another one. We are trying to generate electricity, m h d project. You need to start the plasma at 6000 degree centigrade. Now first sun temperature law energy and then the efficiency is 30 percent. So 6000 degree creation energy 30 percent up to return correct. m h d hydrodynamics you have a lot of game to play. You may become trillionaire if you succeed. Of course the imagery system is right now have here in government of all 5, 6 big governments have spent 10 trillion dollars and now succeeded. So they are firing now on the hydrogen bomb inside. So that the 6000 is reached. Now the safety of that if you fire the hydrogen bomb yeah sun temperature will reach but then everything will remain or that also will go. So there are tricks. So something I belong to DAE. I was in Tata Institute of Industrial Research. So I know much about atomic energies. Okay. Here is some RF system looks like. There is a in this case we do not call it in order of cathode. We say powered electrode where RF source is applied and the other is grounded electrode and it so happens that you have sheath on both sides and we define a potential VP in the plasma and anything any point in between we say floating surface potential VF. We will discuss this later. This is an RF system. We are not shown you here also. There will be a filament source to maintain electrons once awhile. Okay. So that I have not shown. I am right now only showing plasma system part. If I plot power versus distance whatever is the sheath potential must be same as plasma potential. The drop here total drop to 0 is RF. So jitna potential VP hai hutna sheath square hai. The difference here is please take it. The the way these two figures are these areas we called powered electrode area and also we have ground electrode area AP and AG. The first case we looked into is AP equal to AG. So uniform sheath sizes with reference to plasma. So we say the sheath potential is same as plasma potential VP. However in many a times I may do this. What is that I have done here? The plasma powered area or the powered anode or this a smaller area than the ground ones shown as an example here. Or to say AP is smaller than AG and then if I plot the power I will maybe next time I will show you more. There will be a negative VDC across at the close end of the power. There will be a voltage drop VDC then the plasma potential and finally it goes to 0 at the ground. Please remember this DC potential here average DC potential is minus. We will come we will use this and therefore I am not detailing it but just take it. So this is called asymmetric electrodes. So many of the RF systems are asymmetric electrodes. This is symmetric. This is asymmetric. We will come back to this part last if you draw a figure I will just show you the equivalent circuit for this and we will stop for the day. What is the plasma essentially equivalently you can see. You look at this figure or this figure. But that is 1 upon omega C depends on frequency capacity. There is a resistance associated in the sheath. There is a resistance associated in the plasma. There is a resistance associated a lower sheath. So if I look at it here is the figure. If the frequencies are smaller around 2 megahertz or lower the equivalent circuit of a plasma is RS1 the top sheath plasma resistance and the lower sheath resistance. And as someone wanted it yes if you are too large actually are in parallel C so whatever frequency is important we will decide which one to stay. If the frequencies are larger than 2.5 megahertz but smaller than 65 megahertz this appears more like a capacitance. Sheath acts like a capacitance in between plasma is why it is always resistance. Ions electrons conducting havoc. So at a much higher frequency it acts like a RC circuit and at lower frequency it acts like a resistive circuits. This is equivalent of a plasma. Before we quit I may just tell you all our frequency ranges for all RF systems. One can always say 65 megahertz or I can have kilohertz or whatever frequencies but there are limitations of using some frequencies. These are called firstly there is a wireless standards which will not allow use to wireless bands. Then there is a problem of the industrial band which will not allow certain frequencies to be used by other people. So there is some open band left. So typically all RF or any frequency systems only 2 frequencies we use 454.5 kilohertz. 454.5 kilohertz is one frequency given to us. The other is 13.5 megahertz. The other is 13.5 megahertz. These are the 2 frequencies allowed for any integrated circuit manufacturers. Is that clear? So all system will either work at 454 kilohertz in which case this is the equivalent. If it is 13.56 megahertz this is the equivalent. Up to 65 megahertz this circuit is valid. Beyond that surface properties will take care. So is that okay? So only 2 frequencies 454.5 and 13.5 megahertz. These are the 2 bands given 2 frequency given to us for any of the integrated circuit manufacturer has to use all over the world. It is not us. It is all over the world. So you buy only these 2 frequency sources will be available. So this gives you some idea of plasma. Now next time we will use this to create sputtering system, deposition system, etching system. Same thing. Just play games. It will do one or not the other. See you then.