 reactive evaporation deposition. We already know of the simplest and oldest process and what we have seen further to this that using a filament made of tungsten or tantalum as a heating source as a resistance heater it will be easy to evaporate some low melting point metal or alloy and that is just for metallization purpose. But if we like to have wider use of this evaporation process say for example, we are interested immediately in interest may be directed along this use of this what we call hard metals or hard coating and we know that it is needless to say that this titanium, zirconium, hafnium from group 4B, 5B we have here vanadium, niobium, tantalum and also here we have 6B chromium, molybdenum and tungsten in addition to that we have aluminium, we have silicon for example. So, what will be our goal from the engineering point of view to have one of the compound or what we know as hard coating that if we can produce out of this metal and that is no more this element but these are the carbides, carbides of all those bearing aluminium or we can have nitride, we can have boride or even oxide. So, the question comes here if we are interested in titanium carbide or zirconium carbide or chromium nitride or titanium nitride or tungsten carbide how we can deposit this thing or say for example, aluminium oxide something like that. So, these are the material of immediate interest because they have those required properties means hardness, high temperature hardness, wear resistance, low friction many of those can offer low friction and high wear resistance and most importantly chemical stability, oxidization resistance and so many properties which are definitely attractive from the standpoint of mechanical function of this materials. So, the question here arises whether we can use this evaporation process that means, this vacuum evaporation followed by condensation and deposition whether we can apply directly those processes for this kind of compound which are formed by this group of elements and that is the point to be noted and considered. So, here what we find that reactive evaporation deposition, but first of all let us look into direct evaporation. So, direct evaporation means here that we have to have a heat source heating source and in that case directly this will be evaporated or titanium nitride will be evaporated or aluminium oxide will be evaporated, but here the question is that this material must melt and then starts evaporating that means, it can be straight solid to vapour or solid liquid to vapour this path also it can follow, but the main issue here is the energy density or the power density of the heat source. Now in this case what we find that normal filament simple filament which are very simple in their construction it can be in the form of a boat or a foil or it can be a wire forming a basket within which we can place a crucible, but this will not give the required power density for melting or vaporization of this thing and as a result the process is limited by its power density. Now here comes the electron beam gun. So, this electron beam gun in rip as a replacement of this filament which can be a foil or it can be a wire that electron beam gun that means, it is actually an electron emitter. So, this electron emitter will have emission of electron and which will be finely focused on the surface of the substrate or the surface of the material to be evaporated and in that case we expect a high power density and that can be used for melting and evaporation of the material. So, basically this electron beam gun beam gun it is actually it consists of just cathode and anode these are the essential elements of in the construction of this electron gun. So, we can put it like this say this is one wire in the form of wire and here we have what we call this substrate or the workpiece this is the workpiece and here what we have this is called workpiece accelerated workpiece accelerated gun and between these two we have low tension supply. So, between these two we have L T supply, but between these two we have high tension supply. So, that is the high tension supply. So, this is just for heating this coil or this wire and because of this there will be thermo ionic emission thermo ionic emission and with this emission we can have a finely focused. So, this is just like a focusing coil it can be an electrostatic lens electrostatic lens which will do the necessary focusing and through this we can have this beam which is finely focused and that will be useful for melting or evaporation of this. So, this is actually workpiece accelerated gun that means, the high tension is applied between this cathode. So, this is cathode and this is going to be the anode in this case. You have another option that is called self-accelerated gun self-accelerated gun EB gun. So, in this case we do have this wire which is functioning as the cathode and here of course, we have the lenses focusing lenses electrostatic lens and then we have one aperture and through this again we have this focus beam and that can be incident. So, it is an incidental beam and that is going to fall on this workpiece and here we can have one magnetic lens also that is for deflecting this beam. So, this is magnetic lens and this is just one plate with one hole one slit. So, this is anode and this is one electrostatic lens. So, this is self-accelerated electron gun. So, the advantage of this will be that it can be it is more flexible in determining this deposition rate and the evaporation rate. So, those are better handled by this and also this magnetic lens can be used for deflecting this beam. So, over this entire source we can have uniform melting and evaporation of the material. So, we have this electron beam gun and accelerated beam gun and then what we have what we call bent beam gun. So, bent beam gun means suppose we have one source in the form of a rod this is the material in the form of a rod and here we have this electron beam gun this is the eb gun and here it is just not the focused beam will fall on the surface, but it will be deflection of this electron stream that means, this will be a something like this. So, where it is actually 102 70 degree deflection it is 270 degree deflection that will be done to have proper location and focusing of this point over this surface. So, this is one thing very useful for all this vacuum evaporation process. So, it is called bent beam gun it is also electron beam then we have what we call piercing gun. So, here what we have just we have the cathode in the form of a disc and behind this we have the heater and that is also low tension that is low tension and here we have a shield. So, this is the heater this is the cathode cathode and then what we have here we have shielding and then what we have here this is actually the anode. So, we can show this anode just like this it is also have having an aperture in the central position something like this. So, this is anode. So, what we expect here that this electron beam that will emerge emit it starts emitting from this surface and here we have the focusing coil and this is going to be the piece it may be the source material which is placed and that here it can be pin pointed and that is the source material which needs to be evaporated. So, this is piercing gun which can be used for this evaporation purpose. So, cathode heater and between these two anode and this this heater we can also have this high tension. So, this will be just an emitter of electron and from that this will be finally, it will arrive on this surface and do the necessary melting or evaporation. So, this is about the use of gun the main idea here is to obtain a high power density and that is done by this electron gun. However, though we use this electron gun, but we are not free of one problem and that is the problem of variation of coating thickness. Now, this can be illustrated one can immediately understand. Suppose if we have just one rod it is a cylindrical rod and which is having this orientation it is a vertical orientation and over that this is the distance over which so this is the position and let us assume that this is a 0 degree and if we just try to show that this is just we can have segmentation here. So, 10, 20, 30, 40 and 50 degree so that is the angle divergent similarly, we can also write we can also show in this direction. So, this is also 10, 20, 30, 40 and 50. Now, what we can show further to this is something interesting in that. So, what we can see here that if we show with this diagram these are just subdivision of this distance this way we can see. So, we have segmentation we have this segmentation. Now, if we consider this as 100 percent here so this is just 10, 20, 30, 40, 50, 60, 70, 80, 90 and that is 100 percent. Now, what is the significance of this diagram? Significance of this diagram would be evident if we just superimpose another distribution so it will be something like this. So, this is so what we have shown by this red mark that is very important that means, when it is just above this rod that is the evaporant then just vertically above we have 100 percent coating thickness. Now, when we have divergence of angle by 10 degree it becomes 90 percent when it is 20 degree it becomes this is 80, this is 90, 70 and this is 60 percent. Similarly, when it becomes 50 degree divergent angle then it becomes just 20 percent. So, this way we can see the variation in thickness and this variation of thickness that becomes stiffer when we use an electron beam gun it becomes stiffer. Now, another way it can be also expressed if we have a point source and with that point source if we have a substrate here which is the receptor surface then we can also have this variation in coating thickness which can be also expressed by one relation and say this is the variation and this is actually the distance which is given by h. So, that is the distance h and here the thickness is thickness is t. So, thickness is t 0. So, and say at a distance x from this point at a distance x this thickness this is actually t. So, another relation we can also write that is also given by 1 plus x by h square to the power 3 by 2 and from this also we get another relationship that means, this value of t that will be less as the value of x is increasing. So, this is actually a basic problem with this source, but to improve this situation little bit what can be done we can have some gas scattering by introducing argon gas and thereby it will be no more a line of sight and at the same time we can have some kind of mechanical movement. It can be a reciprocatory, it can be a rotary, it can be a sun planetary movement, it can be 3 stage or 4 stage rotation that means, sun, planet and satellite movement. So, within the system it is a mechanical device mechanism which can be incorporated to improve this so called stiff variation in the coating thickness and that can be properly handled. So, this is one dimension of the problem. However, what we find limitation of direct evaporation what we mean by this that means, if we are interested say for example, aluminium oxide coating direct evaporation that means, this material will be melted evaporated and then that will again be deposited it will arrive on the surface of the substrate there it get condensed and finally, it is available obtained in the form of a film. So, solid to solid so it is a solid in the bulk form and now it is the solid in the film form in the coating form. So, this only this formation changes, but it remains solid. Now, there are some of the material so what is the requirement? Requirement is very clear on one side we have evaporation and on the other side we may have unfortunately dissociation. Now, the temperature in question where this evaporation takes place it depends upon also the vapour pressure and the material basic material property. So, once we know this thing at any cost this evaporation point that should not be above this dissociation point that means, evaporation temperature should be always below this dissociation temperature. Otherwise, what is going to happen this aluminium that will split this aluminium and oxygen that will that is called fragmentation and then they will arrive here as aluminium and oxygen and they can combine and during this combination that is the point where we may face problem. We have seen there are certain materials say calcium fluoride SiO then magnesium fluoride B 2 O 3 boron oxide these are some of the materials mean there can be many more, but this is just by way of illustration what we can what we like to say here that for this material we can handle this evaporation very easily and this dissociation will not occur and it will not create any problem. So, evaporation can be done very easily much before the dissociation takes place, but for other material this is not that easy and in that case what we can what we end up with say for example, aluminium oxide when it is a solid it undergoes evaporation and finally, it comes like a condensate on the substrate and there we have it is no more aluminium oxide, but it is aluminium Al 2 O 3 minus x. So, this is actually the final outcome if we go for this direct evaporation and that is becomes the basic problem and root of the problem is obvious that means, the sticking coefficient of oxygen evaporation rate of oxygen and then the ability to combine with the surface. So, these are the basic things which are actually responsible for this substoichiometric result on this aluminium oxide. So, this is actually a major issue now to get rid of this what is normally done normally what is done we have to have one chamber that is the evaporation chamber and within this evaporation chamber what is done we have to inject O 2 while this aluminium oxide is being evaporated. So, this deficiency of oxygen during this condensation of this aluminium oxide what we see from that. So, this is the substrate. So, this oxygen will arrive here and that will actually make up this deficiency of 3 minus x. So, we need to have supply of O 2 with a with a controlled partial pressure. So, that ultimately when it is a direct evaporation though with it is a direct evaporation finally, we end up with just not with this one, but AL 2 O 3 this is one way we can handle this problem another one is what we call. So, it is supply of O 2 in a controlled manner that means, with a controlled partial pressure. So, here P O 2 has to be properly determined and that should be admitted to have to make up this deficiency. Here we call post annealing so, post evaporation annealing. So, this is also not uncommon and sometimes it is done also. So, AL 2 O 3 minus x so, that will go so that means, this is a coating which is having this is the substrate and this is a coating with AL 2 O 3 minus x and here on this we have this is done after this deposition. Here also we can have supply of O 2 so, that this x value will fall and it will come to 0 and finally, we get AL 2 O 3. So, this substoichiometric formation of the coating that is one of the major problem another one also we cannot just ignore this that means, melting of this compound which is ceramic already known for its high melting point and other characteristics. So, for melting and evaporation what we need we need high power density. So, for those we need high power density and sometimes it may not be that convenient to handle this thing. So, for that what we need we need another process what we called reactive evaporation. So, this reactive evaporation what can be done in this case we can have a material in this form say this is the vacuum chamber and here what we have this is the feeder rod that means, the metal which is need to be evaporated and that is fed in this way and here this is actually a separator. So, this separator is useful in that so, here what we have we have one electron gun and with this electron gun we can deflect this beam and it will be allowed to pass through the slit and this is a separator and this separator is used and here what we have in addition to this is actually the substrate. Say this is the metal rod metal rod to be evaporated and that is the deflected electron beam. Now, here we can maintain two pressure because this is the electron gun and where we need a pressure of better than 10 to the power minus 3, but here to have now to have this evaporation it is just not evaporation it is actually reactive evaporation. So, what we need here this deposition of the material and this is just the coating and this coating may be this is the metal and from this side we have one reactive gas reactive gas. Say we can say we can have here for example, titanium, chromium or say zirconium or even aluminium say for example, and on this side we can have a reactive gas it can be N 2, it can be CO 2 or it can be C 2 H 2. So, with this we can have a reaction the vapor is actually arriving here and this gas which will be admitted that will also arrive here and as a result we expect a reaction to occur on the surface and definitely with that we can have a reaction product that means, it is a reactive layer of this compound and which need not to be directly evaporated. Obviously, what we have to have control of this partial pressure so, naturally this one it has to it must have this is the vacuum system. So, this is actually the evaporation chamber. So, what is to be maintained here all these gases which are admitted for that we need MFC that means, the mass flow controller which will allow the calculated quantity of material into this chamber and at the same time the pressure is maintained and the downstream side by proper throttling of the valve on the downstream side followed which just following which what we have all the entire vacuum system. So, here actually we have a pool of material in fact, since it is the electron so, what we have in case in this case we have a thin plasma sheath. So, on this top of that on this top of that we have a thin plasma sheath just in this top of the molten pool so, it is actually pool formation. So, here we have a thin plasma sheath. So, this way we can also have reactive evaporation. So, this will solve our problem and we can get very easily the coating of choice and which can be mostly comes from those compounds of the hard metals. However, the problem this reactive sputtering whatever we have just now discussed this is also not free of problem free of problem means say we are interested in deposition of TIC by reactive evaporation. So, obviously, most reasonably we like to have this titanium vapor plus say supply of acetylene and that can give us to TIC with liberation of hydrogen. Experience shows, but this way we can also have say T i plus N 2 we can have 2 T i N similarly, A L plus 3 O 2 to A L we can also have A L to O 3 what is very important here delta G of all these reactions this delta G this value. So, whether this value is highly negative or not so, highly negative that will give a clear message clear information with what is it is actually the ease or difficulty of the reaction we have to carry forward the reaction in this direction. So, whether it is the ease of the reaction or the difficulty of the reaction. So, here this delta G value we have to determine for each reaction considering this free energy change from this side to that. So, which one is stable so, these issues must be taken into consideration. Now, when that is decided we can see that whether it is easy to have this reaction or it is just really difficult to have such kind of reaction or formation of the coating. Experience say that from this all investigation that this getting T i C with a low deposition rate low deposition rate low deposition rate say for example, 1 to 5 angstrom per second this is a low deposition rate and that is possible just by this T i C 2 H 2 to C i and this is T i C plus H 2 that is possible in the range of 300 to 500 degree centigrade and with T i C that means, this value is 1. So, here C by T i that is equal to 1. So, that is in of our immediate interest that catering to N N ratio whether that is 1 or less than 1, but now if we like to push this deposition rate. So, here it is 1 to 1.5 suppose we like to push it in the order of say 100 angstrom per second for example, or it can be say even 500 then that is the beginning of the problem and there we to push this thing we have to also increase the rate of evaporation of titanium here and also the flow rate of C 2 H 2 that is also that has to be also done, but in this case our experience is that with that we never get a C by T i equal to 1, but this hardly we can achieve, but in most of the cases that is going to be less than 1. That means, in this case the free energy of formation or the activation energy that is not favouring the formation of this particular titanium carbide and for that we have to find out another path or another route to solve this problem and that is known as activated reactive evaporation. That means, in simple language what we have to do here that to activate those thing that means, ionization of this metal and gas in this case what we see it is just in the atomic state this reaction is going to take place. So, this is thermally activated now what is done in this case we can also mention that if we like to have this is in this maybe with such a high rate compared to this is a very low rate and this is reasonably high. So, in that case if we like to have this kind of thing we must increase the temperature, but then the basic purpose of this PVD say evaporation that is actually lost because here the whole idea is not to raise the temperature of the substrate which are temperature sensitive and in that case the we cannot handle this substrate and the substrate will be damaged thermally. So, that is just not possible. So, we have to we ought to activate the whole process and that is known as activated reactive evaporation and with this activated reactive evaporation we have to have certain modification in the evaporation chamber. So, this is the evaporation this is the vacuum chamber or say this is the deposition chamber. So, here we have a separator. So, here we have the rod that means, the feeding rod which is supplying the stream of metal and then we must have this separator here we have this slit. And then what we have here we have this electron beam gun e b and with this e b we can focus this stream and this is actually a bent beam gun. So, it is focused here and the location of the substrate, substrate must be also located here that is the location of the substrate. So, this is the location of the substrate. Now, what we have here we have one like it is like a burner which is supplying this gas. So, the gas can be admitted from this side and here we have a burner like thing. So, it is something like a burner from each side we have this burner. So, this is actually radially arranged. So, gas is supplied radially. So, this is actually the front view. So, this is actually the supply gas that means, this is reactive gas. So, this reactive gas is supplied radially from each direction. So, this is the supply of the gas, but what is important here that here we must have one electrode and this is this looks like a ring. It looks like a ring that means, this is almost like a ring this is one electrode which is actually positively biased. So, gas supply this is actually the substrate. So, only the thing what has been changed here modified that this is the actually the source material. So, that is the source material. So, this way what we can see here that this electrode which is positively biased that means, what we have here we have a plasma sheath just here we have a plasma sheath thin plasma sheath and there we have this secondary electrons which will be drawn in this reaction zone. So, this is actually the reaction zone. So, what we also want to like to have that means, in this zone in this zone that is actually the reaction zone. So, here what we like to have this stream of metal vapour and the stream of this reactive gas those will have some kind of collisional ionization that means, this electrons secondary electrons that will be drawn in this reaction zone by this positively biased probe and that is one electrode. So, this is the electrode. So, which is going to have which is going to attract this secondary secondary electrons low energy electrons here and this electrons while in being attracted they will have collision with the stream of metal vapour and this gas thereby it can be ionized. So, both this reactive gas it can be it can be acetylene it can be nitrogen it can be hydrogen sulfide it can be even carbon dioxide whatever may be the ultimate compound we are interested in that gas will be also ionized and in this ionized form their ability to react that will be easy and then it simply facilitates this reaction to occur at a much lower temperature which otherwise could not have been possible had it been only thermally activated. So, by this what we are doing just by bringing a probe 5 minutes by just bringing this probe what we are going to do here just getting this secondary electrons which ionizes this reactive gas and this stream of metal vapour. So, in that ionized state these two reactive spaces has a better chance to react and with the formation of stoichiometric titanium carbide even with a high rate. So, that is the advantage of activated reactive evaporation. Now, what we have also biased evaporation in this biased evaporation what we have we can also have little modification of this chamber. So, you have to have the vacuum system here also we have the separation this is the partition we do have here this feeder rod that means, the filler material which can be evaporated by this electron beam and this is the electron beam which will be deflected through 270 degrees. So, this is E b that is the source material. So, it is separating two chamber and here at the top what we have that is the substrate placed and here we can have either reactive gas or it can be just gas also another gas for this plasma. So, this is reactive gas and also we have admittance of argon which has to be used for initiation of this plasma. Now, what is the specialty of this process here of course, the substrate has to be negatively biased substrate. So, this is the substrate. So, we have this vapour flux which can go on this side, but however what we can do just by negative polarization we can also have this argon ion which will be splitted by this secondary electron if this is the shell this chamber well if this acts like anode then this secondary electron that will be attracted and in the process this argon neutral which is present here that will be splitted and this argon will be splitted as so into electron and argon ion. Now, this argon ion will be attracted over this surface and if it will do the necessary impingement. So, it is going to strike this surface. Now, what are those advantages? The advantages are many fold number one it is very similar to what we call ion etching, ion etching what we use in sputtering, but this is not a sputtering. So, material stream that is generated by this vapour flux by this electron beam. However, the substrate can be cleaned before evaporation. So, in situ we can have a high quality surface and it is almost a virgin surface free of all sort of contaminant and also oxide layer and after that so here reasonably we must have one shutter between this two. So, this is going to be a shutter blocking this flow of flux. So, this is actually one shutter. So, what can be done? So, at the very beginning this argon ion that will cause sputtering of the top surface of the substrate thereby that surface will be cleaned and that material may be deposited on the shutter. So, once that etching period is over we can remove this one and then this vapour flux that will be falling on this surface and this becomes a receptor surface, but during this period also we can keep the substrate biasing, but during ion etching the bias voltage will be more. However, during the deposition process bias should be kept at a low rate so that the sputtering this deposited material which is evaporated and condensed that should not be totally re-sputtered and the material should not be totally eroded. So, during the evaporation process we can also have we can also have some kind of substrate biasing and here we can have this argon ion impingement. Here the whole idea is to have densification of the coating this is number 1. If there are some loose particle that can be also pulled out that can be removed from there and then also we can neutralize some of the stress for some reason if there be any tensile stress and also if it is necessary some residual compressive stress of course, in a very very controlled manner that can be also induced on this surface. So, the whole idea here is that modification of the surface it is the same material which can be deposited without biasing and now with biasing we can have the basic morphology of the substrate whether it is a columnar grain whether it is a open column or porous column or it is a dense column we can also try to transformed into equiaxed structure and energy available will be useful, but this can be also extended for reactive sputtering. So, reactive sputtering that will be also reactive evaporation that means that is going to be biased reactive evaporation. So, the material which will be deposited this flux of metal and then we have the reactive gases and then we have this ion energy of this argon which is impinging on this surface and that will facilitate the formation of this reaction product in the form of a film on this surface with good and adequate property requirement. So, in summary what we can summarize immediately that reactive sputtering is essential when the direct sputtering is difficult because of the lack of stoichiometry it is because of the simple reason that the metal and non-metal part they cannot combine in the stoichiometric form and as a result there is a substoichiometric formation in the product of this evaporation and this can be handled by reactive evaporation, but in some reactive evaporation what we have seen that because of this limitation of this reaction and the free energy restriction the activation energy restriction the reaction cannot be conducted with the result of stoichiometric formulation in that case by activating the materials in the ionic state this reaction can be conducted and this activation facilitated this reaction to occur. We have also seen that this biased reactive evaporation or biased evaporation that helps by just ion impingement on the coated surface the film property and the morphology can be improved remarkably.