cantoni.txt

 We start from where we stopped yesterday. Yesterday, if you remember, we started to introduce the deposition technique by evaporation. First, we produced an ethical heating. Ethical heating means you just heat by a wire of a current your material to be evaporated. You heat up to the evaporation temperature and so you can control the evaporation. The key parameter is evaporation temperature because what you can control and measure by thermocouple is the evaporation temperature. We have seen advantages and disadvantages of this kind of evaporation and then we move to molecular mean epitaxy. Molecular mean epitaxy is thermal operation with the objective of making the fields reduction. We demonstrated that we need what essentially two constraints. First, very high vacuum. Why? High vacuum in order to reduce the contamination inside the fields. Second, very slow grow. Very slow grow in order to give a monolayer the time to form before want a second one here okay so because the process is very slow this means that the rate is very small and by consequence also the rate of contaminants must be very small so the pressure must be very small typical molecular epithets is made in ultra vacuum that means 10 to minus 10 and then torr or glass. Now we move to what is a basic evaporation source for MB. Typically a basic evaporation source for MB is made by a wire where current will flow. So you have a current generator, typically up to six ampere. And this is the wire, the external piece. Then you have an internal pin that is connected to a material to be evaporated. That can be contained in a crucible or not. We'll see the difference. This pin connected to your, in this case, crucible, can be connected or not to a voltage source, to a positive voltage source, from 0 to 3 kilowatts. We'll see why. This is just a picture with the three beams. It's difficult to see, but the two here are connected by a wire. The other one contains a crucible. What is the idea? The crucible contains the evaporating source. You have a current flowing along the filament, and this will hit this crucible. The easiest possibility is just the wire is circulating around the crucible, touching the crucible. Remember that in vacuum there is no convection, only conduction, so direct contact, or irradiation. If it is direct contact, it can hit the crucible. Clearly, the crucible must be insulating, because otherwise it will shortcut all the filament. The idea is, if a filament is made in a spiral way, inside there you have your crucible. Inside the crucible, there is the material. Here is the first option. In this case, the power that must convert into heating is just electrical power of the current. That is just, just this one. But maybe it's not enough, because maybe in order to arrive to the Td temperature, to the, sorry, to the evaporation temperature, you have not enough power released by this, or the resistance is given by the filament. So it's fixed. Maybe we'll not increase, we'll decrease sometimes, because due to the use, the filament will evaporate, so we decrease the diameter. No, it's increased, okay. Anyway, this is just a fixed value. You can increase the current clearly, but up to a given value, you have no more power sources or maybe much expensive. You cannot think about using a 10 ampere current. So you can use another technique. That is, imagine that your wire is not in contact with the crucible. Your wire is around the crucible. That is, the crucible is not touched by the wire. Imagine the crucible is conducting and you put your crucible at high voltage positive. What do you have? The filament will be... there is a current inside the filament. By the thermionic effect, some electron will be emitted from the filament. It is a typical effect. This electron will be emitted by the filament with which voltage, with which kinetic energy? very small because the filament will stay between ground and few volts. But now this electron will sense the electric field due to this voltage source. In other words, if you have your filament at, you know, one volt and here you have a crucible, really, I just designed this for the true situation, but just to be sure that is plus one kilovolt the kinetic energy of the electron arriving here one kilo and so it's a way in order to increase the sitting effect this voltage can arrive to three kilos so in this case the power released is not due to the current refinement but to the emission gradient from from the fire meter to you. Okay. It's a way in order to increase the power drastically increase the power without using a larger. So power current source. Okay. It depends on material. Maybe it's enough. This solution maybe is not enough. Okay. Okay, I can use crucible or not. Maybe in many cell you have your filament and inside the filament a rod of the material won't deposit. A rod is just a cylinder with a tungsten wire that is sustaining it. You put this rod at 30 volts, that's enough. Why you can use rod or why you can use crucible? Rod is best solution. Why? Because you have just the material to be evaporated. So the only point to be heated is the material to be evaporated. But it works only for solid sources, because if the source will melt before the operation like gold, you cannot use it. So, for example, you can use a rod of platinum or iron, because typically when you deposit iron on platinum, the position temperature, the evaporation temperature is in the solid state. So you have just a rod. But maybe you have to deposit gold or silver. In this case, it will melt. So you need a crucible in order to contain this melting. Or maybe your material is not in form of a solid rod, a solid cylinder, but in form of powder, grime, grain, dust. So you need something to contain it. The third option is when you are depositing an insulating material. Because if you have an insulating material, and you are bombarding an insulating material, you will charge the material. By charging the material, you will create a potential barrier that will stop other electrons to arrive. If you send electrons on the insulating rod, the retrum will stay on the external surface and so will be negatively biased. So in this case, use a conducting crucible containing new material to be deposited. You hit the crucible by a retrum bombardment. By conduction, the crucible will hit the insulating substance. It's another opportunity. So in these three cases, you need to use crucible. must be conductive and made by material that is UHV compatible. Means material, we discussed yesterday about this, that when you hit they do not evaporate. So the pressure vapor of this material is much larger than the one of the material to be deposited. Okay. The quantity in the crucible will decrease with the time and because of that the consistency of evaporation will increase no with the time because there is less things to yes yes so it decreases with time the time for the operation uh yeah okay the point is yes it's truly with the reason for this reason typically what you use you have a thickness monitor i will explain on next week these are the system that measure the rate during the position or maybe just before and after because you have to correct these yes it's a problem so we will manage i h plus to keep it constant or not and not typically difficult to see you just measure the rate you know that when the rate becomes too small you know you have to feel okay maybe you can also have a expression by eye but it's really difficult because they are inserting the shrouds in order so This evaporation source is only for voluco-renewal epitaxy or in general for thermal evaporation? It can be used for any evaporation. Yes, typically for the evaporation we told yesterday, typically you don't need so pure evaporation. So typically the wire is enough. Also because the evaporation explained yesterday is something cheap. In this case, it's something more important because you need a better uniformity, so it's better to use this kind of sources. We will see today other two kinds of evaporation, in this case it will be completely different. There is another problem in this case that if you eat the crucible, you will not eat only the crucible, but you eat everything around the crucible. For example, a tungsten rod. But by conduction also, all the parts outside, if you are switching on a MV cell and you touch by hand, it's hot. It's a problem not because you touch, but because it will degas. So in principle, if you heat this system, everything will be heated, everything will degas. So, large number of contaminants, worse pressure. How you can try to solve this problem by cooling water? You have a cooling water flowing around all the parts except the crucible that must be not heated in order to cool it down. So typically if you look at a cell, a cell have three connectors, as in this case, one for a voltage and two for the current. Or maybe just two connectors because maybe you don't use the voltage. typically a rotary movement in order for the shutter and two tubes for in and out of the water. Cooling water is needed in order to avoid completely hot system. It happened to me one time the water stopped and so the system was very hot and the pressure was very large. Okay, so this is made for molecule of the epitaxy, but as you follow your comment, this is not so the molecule of the epitaxy is not so different from thermal evaporation we explained yesterday. Simply it's different the objective. In this case, you must stay very slow, very controlled, but it's exactly as thermal evaporation. They use the same, use a wire, so the power given by the current, in order to heat the material. Now I speak a second technique of evaporation that is different. In this case, the power released to the material to evaporate it is due to an electron beam. Electron beam is essentially accelerated electrons. It seems similar to what is explained there, but it's not. the source is the same you have a filament with a flow of current flowing this will emit electrons by thermionic effect this filament will be referred not to ground but to a negative voltage so that here you have a grid with zero voltage and you have a small electric field here this is needed in order to extract these electrons Then what you have, you have here some accelerating electrons, accelerating means if you have an electrode at high voltage, you will accelerate electrons. And then a magnetic field perpendicular to here in order to rotate this beam. So in this case, it was not a beam, it was simply electron emitted and accelerated, but like a mass in this case what you can obtain is an electron beam it is a beam of electrodes we forgive the kinetic energy and because electrodes are a charged particle they suffer the lawless force and so they can be rotated by an electromagnetic field is more complicated than there. This an NB cell is something this dimension. This is something this dimension. So in this case this will have a given energy with impact of evaporation source. So which is the, so essentially what explained, which is the first advantage. Before I told you that when you hit the first when you have a filament like in this case is the filament is emitting thermionic electron or simply is it the filament with the gas the gas means will evaporate it's true that i choose total that is hard to evaporate but something will evaporate if you switch on a filament chamber you will see the pressure increase where close to the material to be deposited. So you will contaminate the evaporant flux. In this case it's not because in this case the filament is in another place where the material to be evaporated. That is the red zone. So in this case you remove the problem of contamination from the filament because you move the filament in another region of the system so you can pump. Moreover, this beam is a true beam, means something with a diameter of a few millimeters, or maybe less than millimeters. So this beam will arrive directly on the material. So we leave only a local portion of the material to evaporate. in this case where you eat everything the crucible the material the rod everything in this case you do not hit the crucible because the beam does not arrive on the crucible it arrived just on the material so in this case even the contamination from the crucible due to the fact the crucible is hotter is reduced is avoided so in this sense is a technique that is guarantees a larger purity than before clearly it's more complicated because you need an electron optics and you need a magnetic field okay electron optics daniela will explain to later optics when i think you speak about electron beam lithography essentially if you think to classical optics in classical optics you have lenses as mirrors okay that are able to focus the focus light ray in electrostatic you have essentially the same but instead of lenses you have a capacitor capacitor is able to change the dimension of an electron beam this is called electron optics the views are similar and maybe Daniela I think will explain something about this. Okay, so the advantage of the nickel is the purity. Because only Europe, for example, in this case, you have some crucible. This is the material is put inside this and this is a picture of the electron beam. That arrive directly here, the material evaporates. Okay, I think in polypharma you will see one evaporator. There is one evaporator. By point of view of physics, the energy released by the beam is due not only for making the material sublimation or evaporation, but you can also use, even in this case, the material can be in a solid liquid state. When you insert the material, it's in a solid state anyway because at room temperature everything is solid. But then it can melt. The energy released by the electron beam is due to material sublimation. The fact that the atoms evaporated are not zero kinetic energy. They have a kinetic energy. You can also radiation heat loss for the material. Why? Because if you have a heated element, you will irradiate heat. So yeah, heat radiation in vacuum, not convection, but heat radiation. But not only, not only if you heat your material, it's true that by electron beam, we are not heating the crucible in the surrounding, but you can heat the crucible surrounding simply by conduction. Okay. So in this case, it's not completely true that this technique does not heat the crucible. But in this case, the heating of the crucible was direct. In this case, indirect by conduction through the material, so it is reduced. Even in this case, use cooling water in order to reduce this problem. Okay, typically, heat conduction through the material is the most relevant contribution. Here I'm indicating some powers that can be used for different materials. In particular, the electrics typically use powers smaller than meters, because the two large power the electric related like a ceramic can break it physically break clearly there is nothing this is a general rule you cannot find any technique perfect with all the advantage which are the disadvantage uh beam curling and uniform non-uniform beam density. What does it mean that you have an electron beam? Electron beam is something can be directed like a light ray. You use this magnetic field, but you must be sure that if the beam is generated this way, you will make a 270 rotation arrive this way, perpendicular to the crucible. This is not so easy because maybe there will be some disturbance and it will arrive not perpendicular on the crucible. Or it could change by time. So maybe we move this way in a random way. Okay? This means that the field of position will not be variable, can be variable with time and not uniform. Second problem. In this case, you are hitting too much. But maybe in this case, you have the opposite problem. You are just eating a small portion of your material. So the eating is not uniform on the material. So the use of the material is not uniform. You can find then your material with some holes where the beam arrives. This means changing, for example, the position rate. How you can do? You can use exactly what was used in the old TV. Now it's everything CD and so on. But before there was a beam, cathodic beam. You can use magnets because as you use one magnet to make this rotation, you can also use magnets in order to move the beam where you want. So for example, correct the first problem and change the position when the beam impacts in the second phase. So you have some controller, manual controller or electrical controller in order to move the beam where you want. Clearly it's more complicated in this case, just as which on and off. In this case you have to take care not only of the power released, but also the position of the beam. visually in this case the visual inspection is fundamental this is the second issue the third issue is third effort um maybe this electron beam after the focusing and so on will impact on the material with very very large kinetic energy up to 10 kilo electron volt this can create evaporation, but also ionization. So maybe the atoms evaporated are also ionized. So on the substrate will impact not only neutral atoms, but also ionized atoms. This is good or not, it depends. It can be good. We'll see later that the use of ions against a a substrate can help for example for cleaning the substrate or increasing the energy on this of the atom the substrate and so this makes easier to have a layer growth so in this case it can be positive but clearly is a problem you have considered case by case the third if the the electron other one the 10,000 and to vote and thank you to vote. You can also generate x-rays. So you're the materials are source of x-rays even in the case of x-rays. They're not as others because they are low x-rays low energy. But and so typically the walls of the chamber shield so use if you stay outside, have any problem but maybe these x-rays can damage some fields in particular oxide fields where there are some energies of bonding they are not so large okay so it's an extra problem okay advantage of this is a value of a beam of operation the true advantage is that larger purity. Second advantage is larger rate than thermal evaporation. We come to another technique used for field deposition. This technique is called Pulse Laser Deposition. Instead of using an electron beam, you used a laser beam, so a light beam. In this case, you have the laser beam hitting on your material. This will produce what is called a flash evaporation. That is very short and intense evaporation. Its attention is pulsed laser deposition, not continuous. Electron beam is continuous. This is continuous. This is pulsed. So you are able to concentrate a very large power in each pulse to make evaporate the material. You have a high power laser beam situated outside the vacuum chamber that is focused through a window to the target material by what is light, you can use a phosphorous beam. Which is the first advantage of this technique? Electron beam evaporation, you have an electron beam. Electron beam must stay in vacuum. Otherwise, if you have an electron beam in the air, it will scatter with the air particles, and so will stop immediately. So electron beam must be in vacuum. Means you have to generate the electron beam, move the electron beam, focus, and have it in vacuum. This means that the machine is very large. In the case of light, you can do what you want with laser light outside the chamber. You produce the laser outside the chamber, you just need a window, transparent window, to make the beam enter in the chamber. So maybe your laser, in my lab, the laser is just like this desk. But it's outside. is the true advantage of passive deposition that is a current with it is the passive position is the only technique between thermal operation technique that maintains stoichiometry means that if you are depositing a target that is a multi-component target like baryon titanate of last time you cannot use thermal evaporation because the different vapor pressure You cannot use e-beam evaporation for the same reason, but in this case you can use. You can replicate the same stoichiometry. Why? Because in this case the thermodynamics is different. In this case the energy released by the laser is so large that all elements will evaporate independently on the on the bubble pressure because the energy released is so large that everything is evaporating in a stomach structure method way so for example if you want to deposit volume titanate you start from volume titanate you have a target okay you have a source of volume titanate that is produced by chemistry you hit by laser and you replicate volume time okay this is the only thermal technique that can achieve this because the faster and intense heating of the system um okay this is just a list of possible lasers that you use keeping the absorption of material leading to the operation are in ultraviolet range so you need ultraviolet lasers. You can also use not ultraviolet lasers, but use what is called frequency multiplication. They are optical system in order to multiply the frequency. And so you can move from a laser in infrared to a laser in ultraviolet. This laser is very dangerous because the power released is very, very large. Laser are four categories of danger. Typically, laser diodes of pointers are i think in the second category means that they are dangerous for the direct light or reflected light in this case it is also dangerous for the diffused light okay so for example if you use lasers don't use rings because they can reflect but in this case you need to use glasses. Glasses that filter this kind of light. A further danger is that it's ultraviolet, so you cannot see. If you see the light, at least you see where the light is. But if it's not visible, you don't see. So you must enter with the glasses. glasses. Okay, the laser is produced by a system entered through a window, optically transparent, that must be transparent to both visible, because you have to see where the laser impacts, and ultraviolet light, because it's ultraviolet light. How you can understand where the laser impact is ultraviolet? Because typically you have in parallel a laser pointer visible, like red that is in the initial phase the system is prepared in such a way that the pointer and the two laser point to the same point in same position so when you switch on the laser you also switch on the pointer and so you can see where the laser impacts question of names in pld and this pattern you call target the source. Why? Because it's the place where the laser hits. So in the PLD and sputtering nomenclature, when I speak about target is a synonymous for the source. Okay. The absorbing beam energy is converted into thermal, chemical, mechanical energy causing water, the excitation of the atoms that try to evaporate. This evaporation is not made in an electron beam, but forms a plasma. What is a plasma? We discuss electron plasma with Sputtering. Plasma is made by ions and electrons. These energy is so large that it can unite some atoms. So we have three populations at least. ions, the population, electros, neutrons. All the three mover according to particular law. Essentially, you can consider each one as a gas, a separate gas that interacts between them as gases is called the plasma. We see plasma later. But in a way, PID is a plasma. And you can look at this is just a picture. source, target, substance. The laser arrives here, we produce an operation that forms this plume, it's called plume because of the shape, it's formed by a plasma and we deposit the substance. Okay, the color of the plume tells you which reaction we have, so which is the composition. okay you can deposit for a single homogeneous target what you want that is you can replicate this stoichiometry maybe also a more complicated stoichiometry for example some superconductors are 4 by 4 or 5 components you can do by this technique this technique can be used for superconductors Maybe oxygen does not work so well because even with PLB oxygen tends to be lost. So maybe if you try to start from this target, we arrive to a film of value, or x with x smaller than 3. So what you can do in this case, you can insert oxygen in the atmosphere, in the chamber, a partial pressure of oxygen that will compensate the lost oxygen. So for example, by the time we go in my, in my system was done from this target with a partial pressure of oxygen. Okay, so you want to know because the bonding of oxygen with the other two element is easier to get. you're releasing a lot of energy if some of this energy comes to break this bonding you lose oxygen it's typically a typical problem of oxygen even in other materials okay this is just an example of a typical machine which you have here you have the substrate a red if you have the laser beam is hit the target in blue we produce the plume that is a big machine why because the laser is big okay this is just a few mathematics because we like mathematics how to calculate the the peaks and evaporation of a water in this case, the things that evaporation per unit time is not relevant because it's fast. It's better to understand which is the thickness of operation, pair, pulse. So in this case, the equation are very easy. You start an equation, you know, you obtain the thickness of operation rate in thickness per second. If you know the length, the time length of a pulse, you can find the thickness evaporator per pulse. Then depends on the frequency of the laser, you understand how many, which is the thickness deposited for second. Okay. Pulse are typically few nanosecond of length, because you need a very fast and daintiness pulse to provoke this evaporation. These just examples. So this number is not relevant. Centimeter per second is something exaggerated. But the true reason is that what is important is this. You can deposit up to 10 nanometer for pulse. If you have a frequency of five hertz, means five to 50 nanometers per second. Okay. Here, I do not consider geometrical factors. Clearly, if you have a geometrical factor, we'll consider the fact that it can lose some atoms. But if you don't consider it, this is the position of eight. Okay, just some work to anticipate one argument we'll take in the last lecture, but I think it's better to do it here because it's something too related to PLD. The question is, imagine I want to deposit a film that is crystalline. And imagine I want to know which is the deposition rate, that is, how many monolayers are deposited per second. Okay. Why? Because if you send a first shot of the laser, maybe it will not deposit a full monolayer. Maybe it will deposit more shots. We need more shots in order to deposit the monolayers. Okay. So what you can do, you use electron diffraction. Now electron diffraction is something that you don't know. But think, you know what is electron diffraction? Davison-Gamma experiment. You know that if you send an electron beam on a crystal structure, you will see a diffraction pattern. That's all what you need. You are sending an electron beam to your substrate. If it's crystalline, it will diffract as a diffraction pattern. That's all you need. Then in the solid-state course, you will understand which is the broskal lattice, what does it mean. But for the moment, it's what you need. if you have a crystal surface, not structure, sorry, a crystal structure of the surface, you will see a diffraction pattern. If the crystal structure of the surface is absent, there is no symmetry, you don't have any diffraction pattern. That's all. You have an electron gun. It will send this electron beam on the substrate, and we collect the diffraction pattern on a screen by a cable. This can be done during the position. Why can be done during the position? Because as you can see, the plume, this is the red one, and the green electron beam do not interact. So you can make this measurement during the position. You can calculate the position rate during the position itself. That is the best condition because you are measuring not before and maybe something can change but during maybe this one imagine to start with one monolayer and this is a structure it will present a diffraction imagine to look at one of the points with larger intensity on that screen that correspond to a diffraction to some diffraction. So you have here. Now imagine that you are starting to grow. Some atoms will arrive on the surface, but will not form a monolayer. So it will be some disordered surface. In this case, you can have diffraction, but not everywhere. So the intensity of the diffracted spot will decrease. Then you have a situation more complicated. essentially, half of the surface covered, half not, there is no symmetry, and so the diffraction disappears. Then you continue, you continue populating this layer. The diffraction increases. Up to filling the complete layer, diffraction becomes maximum. They are called RID. RID stands for Reflection Ia-Neg-Gel-L-D. This encode has read oscillation to each maximum correspond the full layer. If you know the time from year to year of the number of passes you need in this case, for example, maybe you need one, two, three, four passes to obtain this layer or five passes. So what now? This is the first one, one, two, three, four. So you need you know that in this case the position rate is one layer each four passes you have measured in the position rate no way of using formulas can be approximated is it just a measurement. And what you've said in this case is this graph measured by looking at the intensity one spot on the camera and see if you can find the rate of the position by metric okay it's a way to detect the piece okay as before we came to the advantage of disadvantage as before advantages Mass evaporation rate is very large. You can achieve larger rates than thermal evaporation. 10 to 4 times faster. Can be very fast. The flash evaporator is very flash, very short time, few nanoseconds. Means that when you you evaporate this from the target, it will evaporate everything. This means that if you start from a target with this composition, you continue to have the same composition. Oxygen typically is lost after the evaporation, but during the flash evaporation, you will evaporate everything. One problem of other techniques, like sputtering, is that sometimes if you evaporate these two the composition of target can change. Imagine it is easier to evaporate barium and titanate. At the end, you will have a target that is pore of barium to reach of titanate. So we evaporate something different. In this case it's not, because once you evaporate, you evaporate everything with the chiometer. The same for the film I already explained. In the film, because you have the simultaneous evaporation of everything, you will have, apart from oxygen, the simultaneous deposition of oxygen. The stoichiometry is conserved. Sorry, but why oxygen? You said that the bonds are unicare, so it goes away. But why it is in the chamber? Why the stoichiometry changes? Because during evaporation, in the plume, oxygen tends to be lost. And so, stoichiometry decreases because instead of barium-tide T O3, you can have barium T O2, for example. But where does oxygen go? Whichever. It's a gas. It's a gas. Because at variants with barium-titanium, if they detach, they are solid states, oxygen in this case is a gas. So it's dispersed as a gas. Or maybe it tends to go in the gas phase because it's a gas. Okay so main advantage, very fast stoichiometry conservative. But from the picture before of the plume, you have seen that this is like a plume of a bird. It's very directional. So this means that if you remember yesterday that we brought the evaporation the job and determine as cause of the trend. Yes, and we told a true source of a source as any one to one in this case and is from I think eight to 12 is very directional means that you will regard just a small area of substrate but if you have a large substrate what you have to do because sometimes you need to deposit of few inches substrate pld is not a good technique if you want to deposit a large substrate use a technique like maybe molecular bf ethyl or electron b evaporation not pld you can use pld why because in principle the plume is very small you can use pld if you use some tricks one is you can rotating and translating the substrate because the plume is very small this is the service but you move on your rotator you can make the position quite everywhere you can rotate it translating the laser scanning a target to uniform target erosion what does it mean imagine this you have your sample the target and your laser This is the plume. If the laser continues to arrive at this point, and then you will have a hole here, like an electron beam, and so the target erosion will be no uniform. What you can do? You can move this or rotate in order to make, typically in the LED system, if you see if this is a target, the laser point scans in this way make a rotation make a movement rotation movement and so on up to the end then come back okay you can move this or you can also move the laser point you can have external optics mirrors moving on mirrors and you can move the laser point because remember that the plume is perpendicular to the point where the laser right If you want to move, for example, a plume on the era, you have to move the laser. So you have essentially three translation possible laser target sample. Moreover, this can be rotated. This can be rotated. If you mix all the movement, you can arrive to uniformity up to, I think, two inches. So uniformity is not a traditional technique, but can be achieved. So this is a, or if you want, for example, to increase uniformity, you can increase the distance. As we explained today, if you increase the distance, the rate decreases, but the LD have a large rate, so maybe it's not a problem. And you can increase uniformity, which is the problem, the dimension of the machine. So this is a problem, but not so large problem can be solved. There is the worst problem of PID. Do you target your target obtained by chemistry production? You will contain your material. But maybe it will not be so pure that is doing the production. Some air bubbles are embedded here, very small. So it's typical of what's produced by chemistry. Maybe the material is some porosity. Now you are heating by laser. By heating by laser, you are heating also these bubbles. These bubbles will increase the volume, will explode. By explosion, they risk to take with them particulates of the target. Particulates means instead of evaporating atom by atom or molecule by molecule, you are evaporating blocks of the substrate up to 10 microns. These will deposit on the substrate. So you risk you have your substrate contaminated by pieces of a target. So it's not going to be right, but this structure completely wrong because it's just pieces, pieces. If you look by microscope, you can see some of them. This is the worst problem. You can manage changing, for example, the power of the laser, maybe heating the sub-synod to try to make this to reduce dimension. But it's a problem. Typically it's a problem. When you deposit by PLD, you have to look at the sample and check that there are no particulates. Okay. Is the worst problem of PLD. Okay. So just summarizing this part, thermal operation, electrical heating, uniform, isotropic as you want. Uh, not so pure. Molecular beam epithets are very similar. Electron beam is more pure, difficult to control. PLD can preserve stoichiometry very fast, but it can never particulates, which we will choose the pencil. So if you're interested in the policy, for example, superconduct made by many elements. Does it matter for particulates? You get it if you want to deposit the know and alloy of just two elements. Maybe you can put a third and be by two different sources. You don't use the. Yeah, it is just a mention of two operations technique that the first is just historical one that is not so used. But it's mentioned. You have a source. In this case, you are depositing not on a subject, but on a film. So it's just a film. It is moved across these like the old magnetic films on the tapes. The tapes used when there was a child before the CD and so on. the whole magnetic. In this case, they used these in order to deposit. All this is called ion beam assisted evaporation. Essentially, a thermal evaporation, this is an evaporation, where you send an ion beam, which is a beam made of ions, on the substrate. Why? Because ions can help to clean the substrate. We will see in the next lecture. That's just to mention of two possible different ways in order to make thermal operation. But the most relevant are electrical heating, MB, eBIM, and PLD. In the laboratory, typically, you find these. Okay? Clear? Any questions? Okay, so we can move to start the next lecture. that is sputtering. Okay. We move to the next lecture that deals with sputtering. Fogbecker forgets about thermal aberration, but stays inside the physical position physical uh vapor deposition technique what does it mean that the way to make the position is to remove atoms from the target even in this case you call it target and move to the source so the target is in this case is a solid state every time target is solid state you remove from the target and put in a vapor phase, this vapor phase will condensate to the substance. The idea is the same of thermal operation. What is changed is the way in which we are removing the atom from this target moving to the vapor phase. In this case what you have is use an ion bombardment of the target in order by scuttling, like the acid scuttling, to make the atom be removed. Essentially, the target with some atoms, an ion's arrive, will physically remove the atom from the target. And as before, we'll describe, I think tomorrow, some different kinds of sputtering. So some different ways to use this mechanism, to perform this mechanism. Okay, we start with what is pattern basic configuration. You have a source, you call the target, and you have a substance. The substance typically is grounded. The source is not. Okay, the surface of the target, the T-R solid, is bombarded by energetic ions to forming a plasma because you have ions, these ions will bombard your substrate, they will produce emission of atoms, other ions or electrons, all them form a plasma. These atoms will move in this region between source and substrate and will deposit on the substrate. This is the process. physical mechanism because it's more physical than before before was thermodynamics in this case it's mechanics just cut it up this is a picture of the tube here the thermal operation this is yellow because of heating of the filament and heated filaments we have in the yellow color red in this case this color comes from the plasma in the case you have two evaporators switched on at the same time they are evaporating different materials so different colors of the plasma it doesn't mean that you look at them the main differences with respect to evaporation processes the medium in the case of evaporation you have vacuum maybe you had oxygen in the case of pld for example but apart from this exception yeah vacuum better vacuum you have lower level contaminants you have so maybe is ultra vacuum definitely in this case if you need sputtering you need a gas to use the to spatter atoms so inside the chamber you cannot have vacuum you you must have a gas. Otherwise, spartan cannot work. So in this case, inside the medium, you have a ionized gas that will become a plasma, instilled in the vacuum. What does it mean? Lower purity. Intrinsically, spartan is a lower purity technique. Because if you have a gas, the gas can be pure after a given level. If you have, for example, one millibar of gas, the purity level cannot be better than 10 to minus 5 millibar of contaminants. So this means that you have contaminants. In this sense, but it is not so suitable for ultra-pure fields. Electrodes. Before, what are the electrodes? Before, they were the source and the substrate. by they were just places one was heated the other not that's all in this case they are active what does it mean they are put electrical voltage and we see that the voltage of these electrodes is a key point in order to activate the sparkling process the temperature is very relevant the temperature is room temperature because in this case the evaporation process does not takes place because of heating but because of different mechanisms So the target can stay at room temperature. This is the reason for which you don't need to melt. Every material apart from mercury is solid at room temperature. So you have only solid sources. So you don't need crucibles, just solid sources. Instead, they have high temperature. First advantage is solid. you have no degassing because of high temperature from the surrounding. You don't need cooling water. This is the key point. You can tell which is the advantage. You are reducing contamination, but here you are increasing contamination. We have our balance between the two. Third, as in PLD, Stereo is concerned in the case of multi-element deposition. You can deposit volume tight end by sputtering, and we will see why. This is not so good as PLD. PLD is just a perfect replication. Stereo can be a replication with some tricks, but you can. With thermal evaporation, you cannot. For example, this spattery used for deposit this material. This material is an alloy of cobalt, iron, boron. Cobalt and iron are magnetic materials. With the addition of boron, this magnetic alloy is one of the magnetic alloys more used. It can be deposited by spattery. It's a ternary, but the replication is perfect. We will see why. Okay, coming back to the basic mechanism. You have two actors. The cathode, that is the target of the material to be deposited. It is negatively biased, of few kilovolts. While the anode, that is the substrate, that is grounded. It can be even biased, cooled, heated, but this depends on the specific process. process in general this basic configuration and this is an example here you have a pump people vacuum why because anyway the chamber will not stay in atmospheric pressure we stay in control the pressure of oxygen of sorry of gas we'll have a gas inlet that are gauge controlling the pressure and then the pumping that will balance the ingoing flowing of gas and the outgoing flowing not the controller anyway the position is made not in the vacuum, but in vacuum, not in the state of pressure. Okay. You are producing a current flowing between the electrodes, so the material for the cathode is deposited from the anode. What I told you before, a current of water, a current of atoms, okay, Okay, that can be a nice day if you ever between these two, you have a voltage drop because you're seeing that the subset is a grounded them. The cathode is a negative biased. So it's a capacitor. If you have a charger inside the capacitor, the charge will move. And so we produce a current. The gas used is typically argon. Argon is an inner gas. We see other gases, but the gas must be inner because it must not react. Why argon? Because argon is an inner gas, it's not so expensive. You can also use krypton, for example, but it's very expensive. The role of this gas is just to make the sputtering process. No more than this. There is no chemistry about this. And the pressure is from 1 to 10 milliters. It's not atmospheric, but it's not so large vacuum. And it seems it's a medium to sustain the electric discharge. Now we'll see what is the electric discharge. First, two words about the plasma. In order to understand the plasma, you just need one course. Because plasma is very complicated. What is a plasma? plasma is defined as a quasi-neutral gas made of electron, ions, neutral molecules, neutral atoms, a lot of materials, exhibiting a collective behavior. What does it mean? That you have not one electron speaking with an ion, but you have a lot of electrons speaking between themselves, a lot of ions speaking between themselves, So two clouds and the two clouds speaking one to each other. It's a fourth state of the matter that corresponds to the fact that electrons or ions make a collective motion. All the motions of them are related. It's a description of this kind of material, this kind of gases, in some conditions. Essentially what you have, you have that the collective motion of electrons and ion clouds and use electric forces between them. It happens when, not every time, in this room there is not plasma, when you have a very large fraction of the United Spaces. If you have a very large fraction of ions, this behavior can start. Okay? Okay, but they beat the plasma physics is something that we need one course when it was a student courses plasma physics and they were two courses on plasma. Okay, which is the main difference between a gas like in his room and plasma. plasma is made by ions and electrons so it's electrically conducting. and it's sensitive to many epifields because it's electric. You can tell, but if I have one ion and one electron, it's true they are conducting, but they form a neutral gas. Yes, but in this case, the area of electrons is separated by the area of ions. So each one is a single charge. Second, you have two more independent elements. that is electrons is one element ions one other element each one is characterized as a gas by density temperature velocity and interaction is a many body interaction so not two electrons speak each other but many electrons speak each other the description is completely different Just examples, Northern light in the north of Europe, south of Europe, or Southern light. Lightning, we have a different potential between clouds and earth. What is the lighting? The formation of a path, a conductive path that emits light, because plasma emits light. All this is a tokamak. It is one of the attempts to make nuclear fusion. Because in the case of nuclear fusion, you have temperatures so large that walls cannot sustain this temperature. So the idea is to use nuclear fusion in plasmas. Because plasmas are electrical charged, they can be contained, not by walls, by magnetic fields. So magnetic confinement of a plasma. Just examples of plasma. Plasma is a state present in the nature. This is something that I have not the chance to see, but you can see it going north. This everyone is able to see. So it's not a strange state, but it's not a so-known state because it's not studied in books. So the point is, in the thing we stop on this, how to produce a plasma. Why an insulating gas is converted into an electrical conductive medium by a DC voltage? Because if you insert argon inside the material, argon is neutral gas. So there is no reason for which it will become a plasma. But you need to have a plasma in order to make sputtering. So you have to find a condition to produce a plasma for a gas. This charge across this gas can be viewed as an electrical breakdown, like the electrical breakdown in an insulating solid. You know that if you have a capacitor, you apply a too large voltage, it will break the electric. And so you create a conductive path. the same. The lightning is the electrical down of the space which is clouds in the earth. You start with water. The yellow ones are electrons, the blue ones are ions. Remember that this is grounded, this is negative. You start with, imagine you have an electron near the cathode for some reason because of emission random emission just one this electron will be accelerated towards the anode by the applied electric field because you have a field of few kilovolts so we'll accelerate it will accelerate from where to where clearly from here to here because of the voltage but this is not iron it's just electric imagine you have these electrons this is what we do if there is vacuum nothing but if there is a gas they can hit the argon atoms that can be ionized because remember this electron is a few electron volt kinetic energy so they are enough to ionize the argon atoms so what you have is you have electrons these electrons are moving to this voltage this is back can be matched as a current because you have a flow of electrons this electron will ionize the argon atom they will cross because there is an atmosphere inside after ionization you start one electron pure neutral atom you obtain two electrons plus an ion you assume that the ion is just plus one not double unization now what you have more electrons that are continuing to be accelerated towards the um the anode. During the acceleration, they impact other ions. The ions are accelerated toward water, toward the cathode that is the target. So in this way, starting from one electron, can attain a cloud of electrons in the cloud of ions the ions will impact on the target and will make the ejection of water of atoms from the target they want to deposit but also they are very energetic so they can also also produce the impact of ions, that is, they not only they scatter atoms, but they scatter and ionize atoms. So they produce atoms, ions and secondary electrons. They are these electrons produced by ionization of the target. These secondary electrons will add to the other electrons, generating an avarage current. That creates an electric breakdown. up to a given point, this current is so large that it is a conducting current. The medium becomes conducting. The medium, not the medium, sorry. The argon, which is called the medium, becomes conducting and just creates a plasma. This is a way to create a plasma. You understand that in order to create a plasma, you need argon. Because in vacuum, you just have electrons accelerated, but that's all. The reaction between argon electrons are necessary to sustain the plasma. What you can have, the simplest one, is elastic or inelastic scattering with argon. Elastic means just scattering, inelastic with productionization. You can also have, as we see tomorrow, chemical reaction, because sometimes you can also use reactive gases like oxygen. In this case, you can have something more, because you can also have chemical effects. But these are for tomorrow. For today, just consider this kind of reaction. So what do you have? You have two electrodes. they are the target and the substrate facing at a given distance d between the two electrons and each one is an area a which are the geometric constraints the distance must be large enough in order to allow electrons to increment the kinetic energy for unised neutral gas because if they are too close one electron we start from one one electron we start from one electrode and I like the other without making any sketching or any unization. So you need a distance large enough in order to produce unization. For what concerns the area? Of the surface, the letters is must be large enough to contain the plasma is the same situation of a capacitor when you study capacitor in the first here, you assume the capacitor is infinite. If you assume capacitor is infinite, the electric field is perpendicular. But if the capacitor is not infinite, you have these border effects. In this case, this means that the plasma can be also outside this. The plasma outside this will not be good in order to produce sputtering. So it's better to confine the plasma as in this situation. Means the plates must be as large as possible. Okay. Okay. Imagine you have this system in which there is two vectors with a distance d. We assume that the second condition is satisfied. All the plasma is contained inside. And you assume you have the two vectors with a distance d. You can calculate this current. There is a calculation in the end of the... What is this current? This current is the discharge current flowing between the vectors emersed in low-pressure gas due to what? True. Alpha is the ionization coefficient, probability of ionization of an argon atom per unit length, that is, proportional gas pressure. Larger is the pressure, larger is the density of argon atoms, larger is the probability of ionization. Why gamma is the secondary electron emission coefficient? number of second letter I meet the cut the incident I told you that if you have an iron incident can eject atoms but also unite but you see second electrons we can demonstrate I don't make the demonstration because it's not so relevant for our scope sir but if you want to look is enough appendix these relate the current i test what is the current i is a function of these parameters okay which is the current the current of the discharge of the plasma what you want this current is present anyway because alpha is present because there is a gas gamma is present but maybe it can be very small What we want is what is called an avalanche. That is the breaking of the gas, the medium, the break of the electric, you can tell. That is, have this current turning to infinity. This is called the situation in which you have a breaking of your system. At this charge current, what you look at is a light, like a lighting. writing, but this continues because once this is produced, it will continue forever. Okay, so you start the plasma and then plasma stays there. It's not passed as before. It's like electron beam evaporation. It continues, but you have to need to understand how to switch on this. How to switch on? you have to take this expression. If the denominator is equal to zero, you have what is called the breakdown. Of course, when the discharge current I becomes infinite. So tomorrow we see what does that mean, this equation, and how we can achieve this condition operatively. This is how we have to act on the pressure on the distance in order to achieve this situation. That means plasma is produced. this mechanism is a self-sustained, it is enough secondary electrons to generate the ionization, essentially. You have enough secondary electrons that will ionize argon atoms, that will make sputtering of the atoms on target, that will produce the same number of secondary electrons, like a reaction chain. A self-sustaining reaction chain. One, you start, you will continue. And they will produce light. Because in this case, what you see is light and lightning. You see light. Light is better not to look too much, because it will contain also ultraviolet components. So if you look too much, it's like looking at the sun. So typically in this machine you have a window with a cover. You look at the window because the way in order to understand if this plasma is switched on, it's just looking. Then you close in order to avoid the radiation. Radiation is dangerous as the sun, if you continue to look at the sun, no more than this. But if you have to stay close this machine for one hour is better to avoid. Okay. Note the gas we use, argon, is not dangerous because it's not explosive. Oxygen is a problem because it's explosive. Argon is not inert gas. Almost no problem at all. There is only one problem. The problem is any gas, which is the danger of any gas, asphyxia. Because if you have a bottle of argon, this argon inside of coming in the wall chamber stays in the room, and the room is completely closed, at the end, this argon will substitute the oxygen. And so you have a problem of asphyxia. The problem for any gas, nitrogen, everything. As a matter of fact, in many laboratories, there is an oxygen meter. This is a small machine that measures the percentage of oxygen inside the room. If it's too large, there is a danger of explosion. If it is too small, there is the risk of asphyxia. But typically, it's not a problem, just to tell you. Because apart from what you want to use, in order to deposit such material there is also problem of space cost but also safety because maybe some technique can be less usable for other ones for safety for example laser apart for using glasses you need to stay in a room completely closed just one door all the windows must be closed because nobody must enter or look into so if you have this kind of room you can otherwise better use thermal operation that have no problem at all or sputtering the same as party once you close the window it's not the problem the wind of the room the chamber is not a problem laser is different because laser comes from outside the chamber so you have to close everything in the the room. When you go to polyfab, I don't know, Marco Asa will show you, you can ask Marco Asa to show you the, you know, the laser, I don't know if it's installed, but take him to show you sputtering machine, okay? I think a sputtering machine will show you. Okay. Any question? Nothing? Okay, Tomorrow we will continue sputtering and get finished sputtering. And then we will fry the chemical vapor deposition and don't use the characterization. Ok, that's all. See you tomorrow.