 it is not coming here ok. In this session, we discuss about CVD reactions. What is meant by chemical vapour deposition? In this particular process, some of the gaseous reactants are admitted into a reactor and they are thermally activated and near the substrate surface or on the substrate surface a chemical reaction takes place leading to deposition of a solid material and a reaction product which is gaseous in nature. It means we have two gaseous reactants and under certain condition of temperature and pressure in the CVD reactor, we are going to have one C plus D and this C is deposited as a solid and this is actually the coating and the gaseous phase which is actually the reaction product, this is leaving the reactor. So, this is basically a chemical vapour deposition process. Now, here we see what we see that this is a CVD reactor schematically shown, this is the location of the substrate and the gaseous reactants which are admitted into this reactor in a very controlled manner, they are coming in the vicinity of the substrate and the necessary reaction takes place over the surface and as a result of that a solid phase is deposited and as an outcome of this reaction, we have this by product which should leave this reactor that is what we have shown here. Now, this application of CVD that means this chemical vapour deposition, we have various applications that means it can be directly used just like a coating on a substrate the way we see it, it is the substrate on which a coating can be deposited which has some functional task. Then it can be also used for producing coated particle that means there are certain particle and which can be also coated by this CVD process, this coating material can be also covered, it can also cover this coated particle. It can be also used for making free standing part that means on a substrate we can have a coating a metal thick layer which can be deposited by CVD and later this substrate can be disposed just keeping this as the coating material and that is actually the material for engineering use. CVD can be also used for making the powder, here we see that this coating it is a solid material solid layer, but this material can be also converted in the form of dust or powder and it can have various application. Also instead of a cover coverage or having some dust, one single crystal can be also grown and it can be also used in various application. A filament say for example, a tungsten filament can be also made by this process, it can be as an extension of this CVD process we can also use it for making fiber or whisker. Now important things are the process parameters to have proper control on the process, final composition of this product, this coating its structure and all the functional properties there must be close control or some process parameters. And these parameters are namely composition of the reactant gas mixture in what ratio they are admitted in the reactor. Then comes the flow rate of this reactant gas, it can be so many liters per minute or SCCM standard cubic centimeter per minute. Then the process pressure inside the CVD chamber we have to also control the process it can be atmospheric, it can be also atmospheric ranging from very low to very medium or high. More importantly we see here temperature of the substrate, because it is the goal of this process to have this reaction on the surface. That means, this A plus B which is actually converted into C plus D, D goes out leaves the reactor as the reaction gaseous product, but D has to be deposited and this D means here the temperature of the substrate is extremely important otherwise this synthesis may not take place on the substrate. Then one thing also we should consider that means this is the physical and chemical characteristics of the substrate surface, because we have to know that this is going to be this substrate surface this is going to be the receptor surface. So, naturally here physical compatibility and chemical compatibility to this two are the requirement to have proper deposition that means, sticking of this reacting species over this surface. So, that necessary reaction can be initiated here we try to understand the various reaction zones in a CVD process. What we see this is actually the substrate over that coating is gradually growing and this is the main gas flow which is admitted, but from this fluid mechanics point of view there had to be a stagnant boundary layer which does not move which does rather stick to this. Now, at the very initial phase of substrate this stagnation occurs at this substrate surface and once this coating grows then this boundary layer moves up and it now keeps the contact with the coating. So, what happens here we find that reaction zone that means this is the reaction zone one that means in the boundary layer within the boundary layer there we can have some reaction. Then this is the reaction zone two that is the phase boundary between the stagnation layer and the solid phase coating. Then we have the interface the interface between the coating and the substrate and then this is the zone five that is the substrate as a whole. Now, zone one and the main gas stream here we can see if we have certain favorable condition that means in this zone and in the main gas flow what is going to happen we may have homogeneous reaction and nucleation in vapour phase and that may lead to flaky and non-adherent coating which is most undesirable in any CVD process when we like to have a well adherent functional coating. Then we arrive at this zone two much expected heterogeneous reaction that means here either it is between this vapour phase and the solid surface of the substrate or once the coating grows it is between the coating and this stagnant boundary layer that means the vapour phase and the coating that that contact layer. So, here we want the much expected heterogeneous reaction and this actually governs the growth rate of coating and properties of the coating. We have zone three four five where various solid phase reaction may take place and this can be phase transformation precipitation recrystallization and grain growth that means if we get back to this figure we have this three four five this is the solid phase and here we can expect this reaction in the solid phase. Now we should put our concentration in zone four this is actually the diffusion zone this is the diffusion zone between three and five and this is actually the interface between the coating and the substrate. This diffusion is extremely important and it must be it must take place in a very controlled manner so that we can have transportation of this material across the coating and the substrate, but at the same time not to promote a very severe very severe diffusion leading to a very very thick diffusion layer which may have different characteristics with respect to coating as well as with respect to the substrate. So, that thick layer formation that required to be avoided. Now we go to this section classification of the CVD reaction. Now there are various types of CVD reaction, but ultimately which one should be utilized that depends upon the ultimate end product what we like to have and also availability of the reactant in what form it is available most in I mean intelligent way to would be to find out the reactant which are available as commercial product and then the temperature of the reaction which we can allow because it depends upon the selection of the substrate or say physics and chemistry of the substrate. So, this temperature of the CVD reaction that is also restricted considering the substrate material. We go to this first one thermal decomposition reaction. This reaction can be illustrated by this general form where M is the metal part of this compound and X that is non-metal it can be one halogen or it can be one hydride that means it is hydrogen and when we simply raise the temperature of this one in the vapor phase and under a certain condition of pressure it is expected that there will be a separation of this metal from this non-metal part and this will be deposited on the coating as a solid phase and this is going to leave the reactor. So, here as we have said this is the gaseous compound which we can have as a commercial product and which can be evaporated to this near this substrate where this necessary reaction that will take place and this is going to leave the reactor. Now, this use of this thermal decomposition reaction results in relatively pure coating. So, this is one I mean one point of attraction where we have to use this process for getting this metal in the form of a deposit just by splitting that compound. We go to this various examples it is actually diborin which will be splitted into boron as solid and hydrogen this will leave the reactor. We may consider another example that means silicon deposition from silane it is also a hydrogen hydride. So, we can also split silicon as a deposit I mean releasing hydrogen. There is another compound called nickel carbonyl it is a low temperature decomposition is possible just at 200 it will split into nickel as a film and this CO in the form of gas. We can also see that methane can be also used for this civility if it is thermally excited activated we can have splitting of this hydrogen and carbon this carbon can be deposited and this can be used in many of the well established heat treatment process just like carburizing that means, enriching the surface of any material most likely it is still by carburizing process. This is reduction reaction now considering the chemical characteristics and requirement of pressure and temperature it may not be always possible to have this deposition of a material simply by thermal decomposition. And here we have to take help of hydrogen and hydrogen will do the necessary reduction of metal just splitting this halogen and come making a hydride of that halogen that means, in case of chlorine it could be a hydrogen chloride in case of iodine it can be hydrogen iodide and like this. So, here we feel see that this reduction that means, getting this metal in the form of a coating is possible by reduction of hydrogen and this is to leave the reactor. We can see those examples one of the very interesting CVD is most commonly used as getting a film or coating of tungsten and here this tungsten hexafluoride is commercially available at around 500 to 550 the reaction will be product favored and by this hydrogen it is possible to move this reaction in the forward direction releasing this tungsten and also for with the formation of hydrogen fluoride. Similarly, we have molybdenum pentachloride and this molybdenum pentachloride also can undergo this reduction process by this hydrogen and here it is hydrogen chloride which comes like the reaction product. Deposition of silicon from silicon tetrachloride that can be also utilized by this reduction of hydrogen. We can have another one boron chloride. So, we have numerous examples and all this whether it is tungsten, molybdenum, silicon or boron each has its own use just like a metal or it can interact with the substrate making one composite having the desired property to do the necessary function. So, these are the few examples of CVD reaction by hydrogen reduction. Now, we go to this exchange reaction. Exchange reaction is also known as displacement reaction in this case say this is one metal halide it can be a metal halide and we have one gas in the form it is E that is one element it is the gas gaseous phase and then there will be exchange of this position of x by E that means, E is displacing x and releasing x in the form of gas. So, this is called exchange reaction and we can use it in actual chemical vapour deposition just if we follow this following examples it is hydrogen sulphide this is in the form of E and zinc this is in this form. So, here also we can find that this metal is converted into zinc sulphide releasing hydrogen free. So, this is exchange of this position and this Z n taking up the position of hydrogen and hydrogen is set free it is in the form of gas. Similarly, replacement of chlorine by oxygen this is also possible that means, this chlorine is displaced by oxygen giving this oxide of tin and releasing chlorine. Instead of element E what we have considered so far one element E it participating in this exchange or displacement, but instead of the element one gaseous compound can also be very active and participate in this exchange reaction which is shown here by this general form A E is the gaseous reactant and M x which can donate this metal and it is going to be a combination of M E instead of just metal and this is going to be a compound having some interest in many engineering application and A x come in the form of a gas and which is the byproduct. So, here we can find out this example that tungsten hexafluoride which was reduced in the previous example by hydrogen reduction, but now it is both have been brought together this tungsten hexafluoride and hydrogen sulphide resulting in formation of tungsten disulfide and H F and this tungsten disulfide has numerous use in mechanical application in tribological application. We can take another consider another example titanium tetrachloride and in this case it is methane and they are also exchange of the position between chlorine and carbon. So, carbon is taking up this position and chlorine is actually occupying the position of carbon and we have here titanium carbide and the result is also the formation of hydrogen chloride. This is called disproportionation reaction. In this reaction what happens the compound which is thermally activated, but in this case what we see that we can have the metallic form, but at the same time this compound actually gain another state that means, its oxidation number can increase and decrease. Let us see this example M X where we can have this results in metallic M and now it is M X 2 formally it was just M X that means, 1 atom of oxygen. Now it is 2 atom of X 2 which can be a chloride which can be anything. Similarly, we can 3 see here it is M X so that means, this original M X can lead to this reaction or it can be this one or it can be even this one it depends upon the chemical properties of this material and also the bond strength of the reaction product. So, it can be just metallic M and M X 3 or it can be metallic M and M X 4. So, here we see it is T I C L 2 and T I C L 2 can be converted into metallic titanium plus titanium tetrachloride. So, it is a chloride higher chloride and this is a lower chloride it is T I C L 2 and this is T I C L 4. Similarly, we can also have deposition of silicon from silicon iodide. So, as it is excited it results in splitting silicon, but with the resulting silicon iodide which is having 4 atoms of iodine in combination with silicon. We have here coupled reaction coupled reaction means actually 2 reactions are combined it can be one exchange reaction it can be one reduction reaction or thermal decomposition 2 are combined to have one overall CVD reaction. Let us consider this example deposition chemical vapour deposition of aluminum oxide using titanium trachloride as the source material with help of carbon dioxide and hydrogen. So, here we see that overall reaction is as follows it is a L C L 3 C O 2 and hydrogen they put together. So, this is the reactant side and we can see what are the thing are going to form on the product side. We see that synthesis of aluminum oxide as a solid it will deposit in the form of the coating and with the formation of carbon monoxide reaction product H C L that is also a reaction product, but if we split it see in detail that reaction one is actually the reduction reaction that means, C O 2 plus H 2 O that become C O plus H 2 O. So, that is the reaction reduction reaction which takes place in the first step and now comes this vapour of this H 2 O that comes in contact with a L C L 3 and it is going to be a hydrolysis of a L C L 3 which results in formation of a L 2 O 3 and 6 H C L. So, in this case we finally, if we add them we get back to this original overall CVD reaction and here what we see a reduction reaction plus one exchange reaction that leads to overall reaction for CVD of aluminum oxide. Here we see that coating CVD reactions are used to deposit a coating of choice a coating with desired property a coating which is expected to function either mechanically or otherwise and here we find at least two types of coating which can be deposited by this CVD process. One we call as overlay coating exactly what does it mean in the overlay coating we find that all the resources material they actually supplied from the external agent that means, in this case of deposition of a metal carbide coating we have the metal halide and methane they are supplied. So, metal donor and carbon donor both are coming from some outside agent and then you have the necessary reaction with the formation of this MCC. So, best example would be that titanium tetrachloride gas and that means, this vapor plus methane gas which will lead to titanium carbide solid and H C L and this we call overlay coating that means, both of them are coming from some external source. Now, we come to this diffusion coating in diffusion coating the one of the component of the coating is supplied by the external agent. However, the substrates participate actively without this participation no fruitful coating can develop or grow. So, here we can give this example. So, this is MCX that means, this is the metal halide MC stands for the metal and it can be reduced by hydrogen leaving this MC and this will be released and this should escape that reaction zone. Now, this MC which will be reduced to MC and so, this is the thing which is actually arriving here and then we have MC from this source outside source and MS participates from this side and we have MC-MS combination and this is called a diffusion coating. So, then diffusion coating substrate participates the externally applied metal donor that participates and we have this a formation of MC-MS combination. Example we can have say boron chloride reduction releasing free boron and now this boron can go inside and then you have cross diffusion of iron making FeB plus Fe2B and this boride of iron has many mechanical properties which can be judicially used and this surface have some augmentation or improvement of property. We can consider another example where chlorine is chromium is reduced by hydrogen releasing chromium which will be deposited on the substrate and this is a still substrate and this is still substrate have iron and carbon. So, in this case carbon and iron they are sharing some of their lattice position and then we have a combined carbide of chromium iron in the form of Cr1 minus X and FeX7 C3. So, here chromium from the outside and iron from the substrate they combine together to form a complex carbide in this form. Now it comes to pseudo diffusion coating. So, in this case what we do first we start the process just like this one. So, the titanium can form titanium carbide by picking up carbon from graphite and later on this diffusion process can be switched over to overly coating process. So, at the beginning CH4 is not supplied, but in the later half CH4 is supplied and this becomes a overly process while the initiation is done by diffusion process. So, we have diffusion coating and overly coating. Now these are the source material mostly they are form in the they are available in the form of halide and this halide their thermodynamic property will dictate what should be the temperature, what should be the pressure at what condition we can have the metal removal metal coating or some compound of the metal which can be also used as a coating. So, selection of source material selection of source material means the decomposition should not take place before the this is evaporated. That means, in simple language decomposition temperature should be higher than that evaporation temperature because in most of the cases these are available in the form of liquid which needs evaporation. So, this evaporation point and decomposition point we should pay attention here. If it is so that decomposition is can occur at a lower temperature then naturally process pressure should be reduced to reduce the evaporation point and to make the decomposition temperature greater than evaporation. Normally this decomposition of reaction that is takes place well within 1200 and this is one of the temperature of technical interest while conducting the CVD. Now this comes the reaction rate in any CVD process or operation one would be interested in that what would be the reaction rate that means at what rate this if we have A plus B is equal to C plus D which comes in the form of coating and this is the gas escaping and this is also in the form of gas that means one would have immediate interest at what rate this conversion is taking place that rate and at the same time to what extent we can convert this A and B to C and D what proportion of A and B will be converted into C and D and at the same time what is the spontaneity of the reaction that means whether this reaction is product favored or not whether we can carry forward the reaction with some driving force. So, here to have this reaction in the forward direction what is immediately needful that means this is stated by the collision theory that the reacting molecules must have collision and they must have collision with sufficient energy and also this collision should not only take place with sufficient energy, but with correct orientation. So, we find there are some necessary condition and when we put all the necessary condition we get the sufficient condition for this reaction to occur and to reaction to move in the product direction. So, here we find this is collision frequency which is very important parameter and this collision frequency depends on the concentration of the reactants. Here we find that the temperature of where it is activated or the activation energy that has a big role to play if we see that this is a distribution of the energy of different molecules in the different energy level and this is more or less the fraction of the molecules having different level of energy. Now, these we have one shape of this distribution at a particular temperature T 1 and when we move to a higher temperature T 2 we can see that with the threshold point that means threshold energy necessary for this reaction to occur we can see that at this point we get a more large fraction of the molecules which are having that threshold energy that means we can say by activation through this rise of temperature we can also have more number of collisions. Now, this is actually the energy barrier that means for this reaction to happen A plus B plus C plus D which becomes A C plus B D. So, A B plus C D should become A C plus B D and to make it happen the whole thing has to be the whole thing it must attend this transition state and where this exchange will take place and finally it becomes the product. So, this is actually the activation energy when the reaction moves in the product direction. So, here we find that if we have low activation energy we have large number of molecules participating in that collision and we have a first reaction and if it is high requirement is high we have low which leads to slow reaction. This is molecular orientation unless we have this thing in the correct orientation then the requirement is not totally fulfilled. So, it is not only the question of threshold energy and collision, but at the same time correct orientation has to be also attained. So, this is one example what we see here say A no plus O 3 ozone and nitric oxide and that will become A no 2 plus O 2 this is possible when N of A no approaches one oxygen atom at end of this O 3 molecule. That means, this has to approach one of those which is situated at the end and then we have this activated complex and then we have this O 2 that means this O will be now attached to this A no making it A no 2 and this is O 2 which will be a stable state. If it does not happen that means, if it is the case that O 3 approaches the O from atom O 3 approaches that means, if O 3 approaches O atom then this reaction is not possible also. If A no approaches the central O atom of O 3 getting back if this N approaches the central atom then also that reaction will not take place that means, it has to approach one N atom. So, this is the illustration how this orientation comes into picture and it plays a very decisive role. So, finally, we find the rate of reaction is governed by activation energy, frequency of collision, temperature and correct orientation of collision. So, this is called Aronius equation where K is the reaction rate, A is the frequency factor and E A is the activation energy universal gas constant and this temperature at that point of activation. So, here A is significant in that this frequency factor related to the number of collisions and fraction of collisions with correct orientation. So, this actually tells us gives us the impression what is the level of collision and how many collision can take place with this correct orientation. And this part second part of this equation E into E minus E to the power E A by RT that is actually the significant number fraction showing that fraction of molecules having at least that threshold energy required for the reaction to occur. So, from this we can get a plot. So, from this plot either it is possible to calculate the activation energy from the rate constant or we can also calculate the rate constant if the activation energy are known. So, both are important information to make the thing happen and to take the reaction in the forward direction. Now, what we can find here that if we consider those principle and apply in a severe reaction just like a reduction reaction, we can often write this K as the reaction rate as dM dt that means, how much conversion either change in mass of product or change in mass of the reactant. And this is given by R into E to the power minus E A by RT into since these are gaseous phase we put this partial pressure in this case P M x y that means, partial pressure of this halide say for example, to the power A and partial pressure of hydrogen to the power B. So, this concentration of this reactants that will give us some indication that means, at what rate this reaction will proceed. So, these are already explained this notations. Now come one important diagram Ellingham diagram of CVD reaction and that give us a guidance how to choose the particular temperature for this CVD reaction. So, here we see that change in free energy is related to change in enthalpy and change in entropy by this equation. And for a standard state of one atmosphere we can put this super fix O here. So, this is a equation now neglecting the the effect of heat capacity this equation can be rewritten as a first step of approximation. Now, from this looking at from this thermodynamic table it will be possible to find out this G o t G 0 t is equal to in terms of this enthalpy of reaction and this change in entropy of that means, the entropy of reaction. So, this is one way of finding the assessing the value which is a function of temperature where this reaction is favored. Now here what we find that for a non standard state we can write that delta G t is equal to delta G 0 t o plus R t into L n q. In fact, what we can write here if we consider such kind of CVD reaction this is the stoichiometric coefficient of this gaseous reactant this is also gaseous reactant this is the coating that means, that is the solid phase and this is the byproduct of this reaction this is also gaseous phase. Then in terms of partial pressure we can write that this is going to P d to the power d divided by P a to the power a into P b to the power b. Now from this we get a clear indication that this is called equilibrium constant. So, equilibrium constant give us a clear idea that what is the whether the reaction is product favored or reaction is reactant favored. So, from this we can find out a particular condition where the value of k can be adjusted in favor of product that means, we can also write this k substituting they in the form of this relation P b to the power b that means, here we call it equilibrium constant and here we call this is as the reaction quotient this as the reaction quotient this we called as the equilibrium constant and this we call the reaction quotient. Now when this so, this is actually moving in this direction. So, unless it attains the equilibrium we express this conversion of this reactant to product by this term what we call reaction quotient and when equilibrium is reached then this q becomes equal to k. Now this relation we also write here what we have seen that delta G 0 reaction that is equal to actually delta H 0 reaction minus T delta H 0 reaction that means, in a CVD reaction this is the change in free energy this is the change in enthalpy of the reaction and this is the change in entropy of the reaction. Now to have the reaction in the product direction that means, product favored it should be always negative. So, this equilibrium constant this value of this equilibrium constant which we write as this is the reactant side and this is the product side we call it like this P d to the power d that is the index divided by P a to the power a into P b to the power b. Now here we find that the role of this partial pressure of the reactants and product this is number 1, number 2 that means, the partial pressure then comes the temperature and the pressure that means, the system pressure all this 3 can affect this value of k and it is obvious higher the value of k means more is the favorable reaction for the product formation that means, reaction will be more and more product favored higher the value of k. So, this value of k can be little bit disturbed and disturbance can be done just by adjusting the value of this P d P a or P b. Say for example, if we put more a that means, increasing the partial pressure of a then the reaction will this reaction will move in the forward direction. Similarly, if we have some arrangement for quick a escapement of this gaseous product from this reactor similarly, there will be short fall of this d that means, this partial pressure will fall similarly, it will adjust itself and reaction will also move in this direction. Similarly, this temperature temperature means when we have an endothermic reaction, endothermic reaction means if we have so, heat will be absorbed. So, if we increase the CVD temperature then also reaction will move in the forward direction. Now, pressure means here say a and b these are the a moles of a and b moles of b and this is d. So, if we find out the number of molecules of a and b which is actually transformed into d. So, if d is greater than a plus b that means, here you have certain increase in volume and in that case if we reduce the pressure increase in volume will be favored by a reduce of pressure and then the reaction will move in this direction. However, if it is contrary to that if d is less than a plus b that means, number of molecules is less in that case this reaction will be favored that means, it will move in the forward direction if we increase the pressure. So, this is the way this value of k can be also adjusted it is to the advantage of CVD reaction that means, to make the reaction more and more product favored that means, favorability towards product can be achieved by just adjusting this partial pressure by regulating the temperature and by regulating the total pressure inside the reactor. Now, comes spontaneity spontaneity of the reaction and this is decided by value of delta G 0 reaction and here what we find this is represented by this reaction minus T delta S reaction. Now, we can have illustration like this, this side we have temperature and this is delta G 0 this is plus and minus. Now, we can have four situations suppose this is one this is one exothermic reaction and in exothermic reaction always delta H reaction it is negative. Now, if this change of entropy is positive that means, this is negative and this is also negative. So, curve takes this form. So, all along throughout we have a negative delta G. So, the reaction is spontaneous, but it can be also like this we have this delta H that means, enthalpy of reaction is negative, but however for some reason delta S O that is positive. So, in that case reaction can be product flavor provided this whole I mean this enthalpy and this entropy their total effect should be negative and to have that it is obvious that temperature should be on this side that means, at a lower temperature we can have this reaction to move in the forward direction. If it is higher temperature then this side will be very positive and which can neutralize and the total delta 0 will be negative positive and the reaction cannot be product favored it will be reactant fred favored. So, this is for exothermic reaction when we have endothermic reaction it can be like this. So, here it is always absorbing the heat. So, here this delta H O reaction minus delta S O reaction which is equal to delta G 0 reaction this is the equation. Now, in this case this is already positive. Now, if we have this value which is positive in that case at a very high temperature we can have. So, when we have such situation in this case what we have that this is the actually the formation here delta H reaction is positive, but if we go to high temperature with entropy in that case we find that the combined effect of this enthalpy and entropy that is found out to be negative and the reaction goes in the forward direction. That means, for a endothermic reaction we can also have the reaction as product favored provided the reaction is the whole operation is conducted at a high temperature. So, that this part is negative and that neutralizes the positivity of delta H O and the whole effect will be negative and in that case the free energy of the reaction happens to be negative. So, we find in summary that this effect of low temperature on exothermic reaction to carry forward in the product direction and the influence of high temperature on this effect of delta G 0 that means, free energy of formation also to carry forward this reaction in the product direction. So, this is the Ellingham diagram which shows that the stability of metal halides with respect to the by product of this CVD reaction and here we find how to find out the temperature where actually this reaction is possible or feasible. So, finally, we come to this summary this summary is like this principle of chemical vapour deposition is mentioned application of CVD and various process parameters controlling the CVD are outlined. Various types of CVD reaction may be utilized. However, the selection depends on type of reactants available and the desired end product the influence of activation energy concentration of reactants reaction temperature collision frequency and orientation are also highlighted. The role of temperature concentration of reactants and that of product and pressure on equilibrium constant of CVD reaction is also seen one can also understand the change in free energy of a CVD reaction as an index of product favourability.