WEBVTT Kind: captions Language: en 00:00:31.360 --> 00:00:36.920 Hello everyone, welcome to lecture 34 of the online course on Nanophotonics, 00:00:36.920 --> 00:00:43.160 Plasmonics and Metamaterials. Today's lecture will be on Nanofabrication. So, 00:00:43.160 --> 00:00:51.600 in this lecture we will see the most mostly used nanofabrication methods and we will discuss about 00:00:51.600 --> 00:00:58.040 thin films and different ways of making thin films like physical methods, chemical methods and 00:00:58.040 --> 00:01:04.280 epitaxy and then we will conclude this lecture. So, this particular module will give you a brief 00:01:04.280 --> 00:01:11.680 overview of how different fabrication methods can help you realize the structures that we have 00:01:11.680 --> 00:01:17.600 studied in this particular course. So, it will not be a very extensive overview or in detail 00:01:17.600 --> 00:01:26.360 study of the different fabrication methods. I will try to catch and touch upon different topics so 00:01:26.360 --> 00:01:33.040 that you have basic idea of how to realize the structures that we have discussed. 00:01:33.040 --> 00:01:38.880 So, when we talk about nanofabrication, nanofabrication lacks a clear definition 00:01:38.880 --> 00:01:44.560 and it is not very distinctly separated from the term microfabrication. So, 00:01:44.560 --> 00:01:50.080 new methods are constantly developed to make electronic components smaller starting 00:01:50.080 --> 00:01:57.240 from micrometers to nanometers and the same techniques can also be used for nanophotonics, 00:01:57.240 --> 00:02:07.120 plasmonics and metamaterials. So, microfabrication that those are typically done for larger parts 00:02:07.120 --> 00:02:14.560 will become nanofabrication when the parts shrink and the dimensions become almost comparable to 00:02:14.560 --> 00:02:23.600 nanometers ok. So, the basics remain similar. Now, integrated circuit production drives this 00:02:23.600 --> 00:02:30.560 particular technology and they provide extra advantages in the areas like photonics. Tools 00:02:30.560 --> 00:02:38.760 and techniques keep evolving with time there is a constant research and updation going on to 00:02:38.760 --> 00:02:46.240 make the process more robust so that there are less fabrication error and the yield increases 00:02:46.240 --> 00:02:53.720 and there are also challenges coming from continuous shrinking of the component sizes. So, 00:02:53.720 --> 00:03:00.000 the process has resulted in smaller and smaller electronic parts with widespread benefits that 00:03:00.000 --> 00:03:07.760 is why you see all these devices they are becoming very lightweight and small compact these days. So, 00:03:07.760 --> 00:03:14.720 that is because of the advancement in nanotechnology that allows the scientists 00:03:14.720 --> 00:03:20.760 and engineers to fabricate structures which are really really small. Now, 00:03:20.760 --> 00:03:28.080 if you look into nanofabrication there are mainly three parts thin films lithography and etching. 00:03:28.080 --> 00:03:33.080 So, thin films as you can see there will be a substrate and you can develop a thin film on top 00:03:33.080 --> 00:03:40.600 of that this film thickness can be typically few to several nanometers ok or even micrometers ok 00:03:40.600 --> 00:03:48.680 those are thin films. Then you have lithography lithography is basically a process of making 00:03:48.680 --> 00:03:57.400 patterns ok. So, you can see it starts with a silicon substrate which is coated with a photo 00:03:57.400 --> 00:04:03.080 or e-beam resist. We will come into the details of this in the next lecture, but let us quickly 00:04:03.080 --> 00:04:11.080 go through what it looks like. Then you will basically have a hard or soft mask which actually 00:04:11.080 --> 00:04:19.600 got the pattern that you want to develop and then when you expose UV or electron beam ok your your 00:04:19.600 --> 00:04:27.320 photoresist ok if they are positive photoresist they get developed ok and then they can be etched 00:04:27.320 --> 00:04:35.440 away by some kind of solvent and this is the pattern of the resist that remains ok. 00:04:35.440 --> 00:04:42.400 So, after that you can evaporate material and that material fills in this particular gap and then 00:04:42.400 --> 00:04:48.800 when you remove this photoresist completely these are the patterns which are basically 00:04:48.800 --> 00:04:57.480 developed on the silicon. So, you can this is done by putting this in a lift of solvent. So, 00:04:57.480 --> 00:05:05.080 all these are the standard processes in nanofabrication technology. So, we will look 00:05:05.080 --> 00:05:12.440 into this in the next lecture. Today we will be mainly covering thin films and in the last lecture 00:05:12.440 --> 00:05:19.040 of this module we will be covering etching ok. So, here you can see that you have got a circuit 00:05:19.040 --> 00:05:27.240 material that needs to be etched and you develop a photoresist first on that ok and then you can 00:05:27.240 --> 00:05:33.560 expose these areas of the circuit material to the etch they can etch away those circuit 00:05:33.560 --> 00:05:38.840 material and this is what you get after etching. So, these are the 3 methods that 00:05:38.840 --> 00:05:46.600 actually help you develop those structures that you have seen in this particular course ok. So, 00:05:46.600 --> 00:05:51.880 these as you can see these methods are very much standardized and optimized for semiconductor 00:05:51.880 --> 00:05:59.520 industry and that is where people are actually aiming towards making photonics material CMOS 00:06:00.160 --> 00:06:06.720 compatible. So, that the same foundry with some minor modification can help us fabricate those 00:06:06.720 --> 00:06:16.480 nanophotonic structures or plasmonic structures. If we are going for totally different materials 00:06:16.480 --> 00:06:23.000 that might be difficult for mass production at this current stage when the foundry is not 00:06:23.000 --> 00:06:30.400 supporting those kind of material fabrication. So, if you look into the thin film industry, 00:06:30.400 --> 00:06:38.280 thin film size science is basically a broad field which is used in various industries ok 00:06:38.280 --> 00:06:46.680 and thin films are ultra thin layers that can be found in products such as eyeglasses ok screens 00:06:46.680 --> 00:06:52.920 and vehicles and these are different kind of purposes something like reducing reflection 00:06:52.920 --> 00:07:00.280 preventing damage and altering properties. In tiny computer parts these thin films can control 00:07:00.280 --> 00:07:08.440 electricity and help you switch ok. So, another important application you can think of is the 00:07:08.440 --> 00:07:15.200 gate direct link in transistors which is often less than 10 nanometer thick. So, those are also 00:07:15.200 --> 00:07:23.440 thin films. So, here you can see applications of thin film in different different disciplines. 00:07:23.440 --> 00:07:28.520 So, if you think of optics it can give you anti reflection coating, highly reflected 00:07:28.520 --> 00:07:33.920 coating something like laser mirrors you can have interference filters beam splitter then 00:07:33.920 --> 00:07:39.760 thin film polarizers integrated optics and so on. If you think of optoelectronics you can 00:07:39.760 --> 00:07:46.840 use them for photo detectors image transmission optical memories LCD TFTs and all this. If you 00:07:46.840 --> 00:07:53.160 think of electronics they can be like passive thin film elements like resistors condensers 00:07:53.160 --> 00:08:00.600 and interconnects you can think of active thin film elements like transistors and diodes ok and 00:08:01.320 --> 00:08:09.880 integrated circuits like VLSI circuits they also use a lot of thin films. Then CCD charge coupled 00:08:09.880 --> 00:08:18.840 device you can think of chemistry where catalysis electro catalysis biocatalysis photo catalysis 00:08:18.840 --> 00:08:27.560 all those things are the application you can think of sensors you can think of magnetic applications 00:08:27.560 --> 00:08:32.960 engineering and processing and also new materials I will not read out all of them, but you can 00:08:32.960 --> 00:08:39.120 always see that there are lot of applications of thin films. They are also very useful in 00:08:39.120 --> 00:08:45.480 biomedicine where you can have neurological sensors you can have claddings for a depot 00:08:45.480 --> 00:08:51.200 pharmacy and biocompatible implant coatings. So, all these things are possible. So, 00:08:51.200 --> 00:08:58.400 there are vast application of thin film technology not only in the area of semiconductor or photonics 00:08:58.400 --> 00:09:06.000 they can actually have applications in lot more other disciplines ok. They can be used for 00:09:06.000 --> 00:09:13.800 alternative energy or green energy something like in solar collectors or solar cells and so on ok. 00:09:13.800 --> 00:09:21.280 So, as I mentioned earlier this thin films can be really really thin and they can be from 10 00:09:21.280 --> 00:09:28.240 to 1000 nanometers. So, typically 10 nanometer to 1 micron that is what we call them as thin 00:09:28.240 --> 00:09:37.480 films ok and these are for visible and infrared light and for extreme UV light the films will 00:09:37.480 --> 00:09:43.960 become even more thinner because extreme UV light has got a very short wavelength and in 00:09:43.960 --> 00:09:50.360 that case you can actually use thin films of 1 to 2 nanometer thickness and these are typically 00:09:50.360 --> 00:09:57.200 used in photo masks will come into this. So, right now just keep this new terms in mind 00:09:57.200 --> 00:10:04.360 we will discuss about this new terms soon in this lecture or in the subsequent lectures. Now, thin 00:10:04.360 --> 00:10:12.960 film properties change based on how they are made. So, the key properties include how well they coat 00:10:12.960 --> 00:10:20.920 the surfaces, their density and their electrical behavior and density affects how light bands in 00:10:20.920 --> 00:10:29.160 the film and stress can wrap or crack it. So, this is the effect of density when the thin films are 00:10:29.160 --> 00:10:36.280 made from compounds the films make up can vary depending on how it is created and for single 00:10:36.280 --> 00:10:42.960 element thin films it is not usually of a worry and when you develop thin films the impurities 00:10:42.960 --> 00:10:49.720 grain size and how tightly the elements of the particles are packed they also affect how well 00:10:49.720 --> 00:10:57.800 the thin film will conduct electricity. So, their conductivity depends on all these parameters. 00:10:57.800 --> 00:11:04.640 Now, how do you make thin films? So, the process of making thin films can be broadly classified 00:11:04.640 --> 00:11:12.920 into 2 categories physical methods and chemical methods. Physical methods involve transferring the 00:11:12.920 --> 00:11:20.400 material from the source to the substrate without changing its chemical state. So, it is a physical 00:11:20.400 --> 00:11:27.240 method. On the other hand when you think of chemical method it the film is basically created 00:11:27.240 --> 00:11:34.840 as a byproduct of some chemical reaction. So, let us look into the physical methods first. 00:11:34.840 --> 00:11:42.160 So, when you talk about physical methods there are 2 widely used methods for adding thin films 00:11:42.160 --> 00:11:50.120 one is evaporation another is sputtering. So, this methods work like a special kind of painting with 00:11:50.120 --> 00:11:57.200 the vaporized material adding tiny particles to a particular substrate that will be the 00:11:57.200 --> 00:12:03.240 surface that you will be coating with the thin material. Evaporation or sputtering are part 00:12:03.240 --> 00:12:10.800 of a group called physical vapor deposition or PVD. In PVD the materials are transformed 00:12:10.800 --> 00:12:18.560 into vapor and then layered onto a surface one atom at a time and these processes are often 00:12:18.560 --> 00:12:28.360 carried out in high vacuum chambers that prevent interference from gases in the surrounding. So, 00:12:28.360 --> 00:12:34.400 if we look into the evaporation method evaporation method involves heating 00:12:34.400 --> 00:12:41.480 the source material a lot until it turns into vapor and vapor pressure plays a very important 00:12:41.480 --> 00:12:48.120 role here because all materials evaporate at specific temperatures as you can see here. 00:12:48.120 --> 00:12:55.680 So, this is basically the pressure it is given in millibar this one is in torr and 00:12:55.680 --> 00:13:02.560 this is the temperature scale ok. So, that is in centigrade and that is in Kelvin ok. So, 00:13:02.560 --> 00:13:08.600 and here are the materials like tungsten, gold, magnesium there are many more all 00:13:08.600 --> 00:13:14.680 listed here ok. So, this is basically a vapor pressure versus temperature curve for different 00:13:14.680 --> 00:13:21.760 kind of metals. So, commonly used materials barely evaporate at room temperature right. 00:13:21.760 --> 00:13:31.040 So, you can see that gold with a very low vapor pressure below 10 to the power minus 15 torr at 00:13:31.040 --> 00:13:40.480 room temperature ok. Now, when you see how the vapor pressure for gold is changing ok 00:13:40.480 --> 00:13:47.720 with temperature. So, you can see that at 1500 degree centigrade it actually changes 00:13:47.720 --> 00:13:56.560 to 100 milli torr ok. And the vapor pressure also depends on the position in the vapor stream. So, 00:13:56.560 --> 00:14:02.720 the pressure decreases as the vapor moves away from the source, make sense. 00:14:02.720 --> 00:14:10.480 So, the closest it is to the source it will be the highest vapor pressure. So, vapor pressure on the 00:14:10.480 --> 00:14:17.920 surface where the film forms can be estimated from the vapor flow patterns. And to calculate 00:14:17.920 --> 00:14:24.080 the film growth rate pressure connects to the rate of atoms hitting the surface. So, 00:14:24.080 --> 00:14:31.000 that way you can calculate the rate at which your thin film will be growing. Often instead 00:14:31.000 --> 00:14:36.160 of directly measuring the temperature and vapor pressure deposition rate is controlled 00:14:36.160 --> 00:14:43.000 by adjusting the power to the source. So, that is also another method to adjust 00:14:43.000 --> 00:14:52.880 how much the temperature how much thickness will be deposited. So, among the two methods of 00:14:52.880 --> 00:15:00.880 evaporation you can see here one method is called resistively heating evaporation process. So, 00:15:00.880 --> 00:15:05.580 in this particular process there are two methods. So, this is the one the other one 00:15:05.580 --> 00:15:11.560 will show in the next slide. So, in this one the palette of the source material that is here 00:15:11.560 --> 00:15:17.200 it is actually put in a metal container like a boat ok and it is heated using a 00:15:17.200 --> 00:15:24.200 strong electric current around 100 amperes. So, this is particularly called resistively 00:15:24.200 --> 00:15:32.960 heated evaporation. So, it is very simple needing only a low voltage DC source, 00:15:32.960 --> 00:15:39.680 but it has got some drawbacks. Drawback something like it is inefficient and there are chances of 00:15:39.680 --> 00:15:45.880 potential contamination due to other parts also getting heated up and evaporating along 00:15:45.880 --> 00:15:51.840 with the source. So, this boat material or other parts also may get heated up and they 00:15:51.840 --> 00:15:56.480 they will also vaporize along with the source material. So, this is how it works the source 00:15:56.480 --> 00:16:04.160 material or evaporation material is here the filament or the heater element is here. 00:16:04.160 --> 00:16:11.360 So, it hits this one up the vaporized material particles they actually go and settle down on 00:16:11.360 --> 00:16:17.720 the substrate which is kept right on top of it and this is done in a vacuum chamber. The 00:16:17.720 --> 00:16:23.160 second method is called a electron beam heating ok where the heated electrons are 00:16:23.160 --> 00:16:30.680 basically accelerated by high voltage. So, here you see the amount of voltage requirement goes 00:16:30.680 --> 00:16:36.520 very very high it is around 10 kilovolt and then it is focused this particular electron 00:16:36.520 --> 00:16:43.160 beam is focused on to the source palette. So, this happens again inside a vacuum. So, 00:16:43.160 --> 00:16:51.560 the electrons can be controlled by magnetic field ok and without heating the gas molecules. 00:16:51.560 --> 00:16:58.040 This particular method of electron beam evaporation is pretty efficient and here 00:16:58.040 --> 00:17:04.320 accurately heating of the source can be obtained. So, that it minimizes the contamination and 00:17:04.320 --> 00:17:11.560 therefore, it is a good process of developing in a high quality thin films, but only problem is that 00:17:11.560 --> 00:17:19.840 it requires a complex high voltage power source. And safety also becomes important here because you 00:17:19.840 --> 00:17:26.400 are dealing with very high voltage. Now, despite this electron beam evaporation is widely used in 00:17:26.400 --> 00:17:33.640 thin film research and development the reason you get a very high quality film. Now, the next method 00:17:33.640 --> 00:17:41.120 of developing thin film is called sputtering. So, sputtering also uses a vacuum system with 00:17:41.120 --> 00:17:48.960 an excited gas plasma ok. So, this is the typical setup for sputtering. So, 00:17:48.960 --> 00:17:58.800 here you can see that there is a gas inlet this is the outlet and the sputtering gas comes in 00:17:58.800 --> 00:18:08.080 and it hits the cathode sputtering target ok. So, that knocks out the metallic particles or 00:18:08.080 --> 00:18:15.280 atoms and they actually go and settle down on the substrate forming a thin film ok. 00:18:15.280 --> 00:18:22.800 So, here you can see how it works. So, plasma ions so these are the plasma ions which are so this 00:18:22.800 --> 00:18:29.800 is plasma and then the sputtering gas enters and the plasma ions are basically directed at 00:18:29.800 --> 00:18:37.040 the cathode and they could knock the neutral atoms of the surfaces. You can see here M is 00:18:37.040 --> 00:18:45.160 the metal that is been knocked off ok and these atoms they collect on all surfaces including 00:18:45.160 --> 00:18:53.440 the substrate. So, you can see a thin film being growing on this it is because of this ok. Now, 00:18:53.440 --> 00:19:01.320 in this particular method unlike evaporation it does not depend on heating ok. So, 00:19:01.320 --> 00:19:08.960 what is the method of atoms coming out? The the atoms are basically ejected by momentum. 00:19:08.960 --> 00:19:17.800 So, this leads into much denser films ok. However, there are some benefits with sputtering because it 00:19:17.800 --> 00:19:24.400 is more flexible and that is why it is popular in the industries compared to evaporation. Heating 00:19:24.400 --> 00:19:29.280 is not required high voltage requirement is not there. So, it is pretty good. Targets 00:19:29.280 --> 00:19:37.080 and plasma sources in sputtering can be made of different shapes for various coating 00:19:37.080 --> 00:19:43.720 setups something like circles rectangles or any other unusual shape you can coat ok. 00:19:43.720 --> 00:19:52.320 Sputtering can happen upward downward or sideways while evaporation happens only upward ok. So, 00:19:52.320 --> 00:19:57.880 these are the flexibilities with the sputtering technique. Moreover, the plasma power in the 00:19:58.400 --> 00:20:08.000 sputtering comes from a DC or RF source ok. So, for metal targets DC sputtering works whereas, 00:20:08.000 --> 00:20:16.600 for insulating targets RF sputtering is used where the target acts as the capacitor for the plasma. 00:20:16.600 --> 00:20:20.880 We will not go into too much of detail, but it is good to know what are the different techniques 00:20:20.880 --> 00:20:28.560 being used for developing what kind of films. So, RF excitation needs a matching network due 00:20:28.560 --> 00:20:37.680 to variable impedance and it is used for both conductive and non-conductive targets ok. In 00:20:37.680 --> 00:20:46.440 sputtering the atoms collide before reaching the substrate due to plasma gas making it less 00:20:46.440 --> 00:20:55.360 directional than evaporation. So, these are couple of pros and cons of sputtering as compared to 00:20:55.360 --> 00:21:01.440 evaporation. There is a variation to it which is called reactive sputtering and in reactive 00:21:01.440 --> 00:21:10.240 sputtering what happens a bit of reactive gas like oxygen or nitrogen mixes with the argon gas 00:21:10.240 --> 00:21:19.960 to create compounds from ejected target molecule ok. So, the setup looks like this you have got a 00:21:19.960 --> 00:21:24.920 power supply here this is the target which is kept this is the plasma formation you have got 00:21:24.920 --> 00:21:30.520 argon gas and also you get some reactive gas and this is the vacuum chamber ok. 00:21:30.520 --> 00:21:40.000 And from the target these are knocked off and then the materials or nanoparticles which are 00:21:40.000 --> 00:21:49.440 ejected from this target gets reacted with this reacting gas and then the compound gets deposited 00:21:49.440 --> 00:21:54.600 on the substrate. So, during this process as you can see the atoms are ejected from 00:21:54.600 --> 00:22:03.240 the target by energized ions to form a plasma that is directed to the substrate under a high 00:22:03.240 --> 00:22:12.280 vacuum. Argon is commonly used as the sputtering gas and the sputtering is carry out with DC 00:22:12.280 --> 00:22:19.280 power source and RF alternating current or ion assisted deposition there are 3 methods. So, 00:22:19.280 --> 00:22:26.280 mainly DC and RF sputtering are used and in this particular case of reactive sputtering 00:22:26.280 --> 00:22:33.360 a reactive gas such as oxygen are used for getting oxides. You can use nitrogen if you 00:22:33.360 --> 00:22:39.280 want to create thin films of nitrides ok. They are also passed to the reaction chamber 00:22:39.280 --> 00:22:45.880 along with the argon gas that you can see here and this reactive gases react with the target atoms 00:22:45.880 --> 00:22:53.000 in the plasma to form the desired composition and then they get deposited in the film as I explained 00:22:53.000 --> 00:23:00.360 before. So, while forming perovskite oxide films multiple targets with different elements are 00:23:00.360 --> 00:23:10.320 simultaneously sputtered which are reacted with oxygen and they deposit as the desired film. Since 00:23:10.320 --> 00:23:17.240 the target elements and oxygen exhibit a large electronegativity difference the formed ions can 00:23:17.240 --> 00:23:22.920 be negatively charged and they can be accelerated towards the substrate due to the difference in the 00:23:22.920 --> 00:23:31.120 potential of the negatively charged target and the grounded substrate ok. So, the substrate is 00:23:31.120 --> 00:23:40.440 grounded ok. This ionic fluxes possibly act as sputtering ions to re sputter the growing thin 00:23:40.440 --> 00:23:48.560 films onto the substrate or modify the composition of the films or etching the substrate ok. 00:23:48.560 --> 00:23:58.200 So, the next method to develop thin film is pulsed laser deposition. So, here the name itself tells 00:23:58.200 --> 00:24:06.520 you that you are going to use a laser pulse ok for depositing thin films. So, how it works? So, 00:24:06.520 --> 00:24:15.040 pulsed laser deposition or PLD employs brief powerful laser pulses which are as short as few 00:24:15.040 --> 00:24:20.360 nanoseconds to remove the target material. So, you can see in the diagram here you have got a 00:24:20.360 --> 00:24:26.760 laser pulse you focus it and you hit the target ok. And then there is a plasma plume that goes 00:24:26.760 --> 00:24:33.120 towards the substrate and it gets deposited. So, the laser energy here is focused on the target 00:24:33.120 --> 00:24:42.360 subs surface causing rapid material evaporation and atom injection. So, that gives you this 00:24:42.360 --> 00:24:51.800 plasma plume ok and the atoms then they gather on the surface to give you that thin film. So, 00:24:51.800 --> 00:24:58.960 what is the key advantage of this PLD? It can be performed in ultra high vacuum or with various 00:24:58.960 --> 00:25:08.960 pressures and gases due to the laser source being external to the vacuum chamber. So, that that is 00:25:08.960 --> 00:25:16.880 the flexibility it provides and the main advantage of PLD is its ability to remove target materials 00:25:16.880 --> 00:25:25.320 in stoichiometric manner that treating all atoms equally due to high laser fluency and rapid 00:25:25.320 --> 00:25:33.400 ablation. So, that way you are able to deposit uniform films using pulsed laser deposition. 00:25:33.400 --> 00:25:40.160 So, PLD is particularly suited for complex ceramic films like yttrium barium copper 00:25:40.160 --> 00:25:50.680 oxide YBCO lead zirconate titanate PZT and different other carbides oxides nitrides ok that 00:25:50.680 --> 00:25:57.240 are challenging to deposit using other methods. So, you can actually deposit this kind of films 00:25:57.240 --> 00:26:05.320 using PLD method. So, as I told you that the main advantage of PLD is that it is non discriminatory 00:26:05.320 --> 00:26:15.280 and it gives you stoichiometric removal of target atoms ok and it facilitates and it comes 00:26:15.280 --> 00:26:22.200 from rapid ablation and high laser fluency. So, quickly this evaporation takes place and it takes 00:26:22.200 --> 00:26:29.280 place from all from the entire target. So, that happens very very quickly. So, 00:26:29.280 --> 00:26:36.240 with that we more or less cover all the basic methods in this particular physical methods of 00:26:36.240 --> 00:26:45.080 depositing thin films. Now, let us look into the chemical methods of depositing thin films. As we 00:26:45.080 --> 00:26:51.120 mentioned before that chemical methods will deposit the thin films as a byproduct of some 00:26:51.120 --> 00:26:57.960 chemical reaction. So, chemical methods they offer uniform well covered and stoichiometric 00:26:57.960 --> 00:27:05.280 films, but different gases and chambers are often required for each type of film. 00:27:05.280 --> 00:27:12.600 So, that makes it very very specific. So, so and it is that makes it also costly if you are 00:27:12.600 --> 00:27:18.640 experimenting in different films because and using different gases. So, you need different 00:27:18.640 --> 00:27:25.280 different chambers for different film types. So, the most common chemical method is chemical 00:27:25.280 --> 00:27:32.200 vapor deposition where gas precursors enter a chamber and the high substrate temperature 00:27:32.200 --> 00:27:40.920 prompts a reaction to build the desired film. So, different types of CVDs are there one is called 00:27:40.920 --> 00:27:48.280 low pressure CVD, you have atmospheric pressure CVD, you have plasma enhanced CVD and atomic layer 00:27:48.280 --> 00:27:55.760 deposition. So, CVD requires low pressure and high substrate temperature that ensures that the 00:27:55.760 --> 00:28:03.840 reaction occurs only on the substrates surface and not in the gas phase which could lead to 00:28:03.840 --> 00:28:12.840 particle formation and surface deposition. So, here is the setup typically used for low 00:28:12.840 --> 00:28:20.560 pressure CVD. So, in this particular case of low pressure CVD the reactor consists of a quartz 00:28:20.560 --> 00:28:29.560 tube that you see here this quartz tube that is connected to a pump ok and the gas inlet is used 00:28:29.560 --> 00:28:36.760 to introduce the reactant gases as well as the gases needs to purge the system. You typically 00:28:36.760 --> 00:28:44.520 nitrogen is used for purging and the wafers are loaded through the door which is on the left. So, 00:28:44.520 --> 00:28:53.560 you can open this door and you can load this sample ok. In low pressure system the wafers can 00:28:53.560 --> 00:29:02.520 be placed closer as you can see here a furnace. So, this is a three zone furnace that is there 00:29:02.520 --> 00:29:12.800 from the three sides ok top back and bottom ok. And this particular one actually encompasses the 00:29:12.800 --> 00:29:21.000 quartz tube and this heats up the chamber which drives the reaction rate faster. Now, what is 00:29:21.000 --> 00:29:28.120 the advantages of low pressure CVD? The first thing is it is a relatively simple design. So, 00:29:28.120 --> 00:29:34.600 excellent economy it gives you high throughput and very good uniformity of the thin films. 00:29:34.600 --> 00:29:43.480 But there are some disadvantages also. So, you can say what are the disadvantages? They 00:29:43.480 --> 00:29:50.960 are susceptible to particle contamination. So, it requires frequent cleaning of this 00:29:50.960 --> 00:29:59.000 setup and also you need to compensate for gas depletion effects. Next methods we will see is 00:29:59.000 --> 00:30:09.200 plasma enhanced CVD. Now single wafer process chambers for plasma CVD look somewhat similar 00:30:09.200 --> 00:30:20.040 to this low power CVD ok. Adjustment figure here it shows a schematic that tells you how exactly 00:30:20.040 --> 00:30:29.800 single wafer plasma chamber will look like. So, as with the single wafer LP CVD chambers 00:30:29.800 --> 00:30:36.440 here also the precursor gas is fed to the chamber using the shower head ok. So, 00:30:36.440 --> 00:30:44.920 you see here and that ensure the uniformity of the precursor concentration over the wafer phase. So, 00:30:44.920 --> 00:30:52.480 this is where the wafer is kept and this is basically on a heated plate ok. So, 00:30:52.480 --> 00:31:02.280 the wafer sits on a heated plate and the byproduct gases are exhausted through this outlet which is 00:31:02.280 --> 00:31:13.320 below the wafer level ok. Now with direct exposure RF PECVD systems typically employ shower head as 00:31:13.320 --> 00:31:20.360 an electrode for the introduction of the RF energy to create the plasma that is see here ok. 00:31:20.360 --> 00:31:27.680 The precursor entering the plasma undergoes electron molecule collisions producing high 00:31:27.680 --> 00:31:35.080 energy excited molecules and molecular fragments that absorb on the substrate surface and deposit 00:31:35.080 --> 00:31:46.040 the film. So, these are the gas inlets ok. So, the next method is atomic layer deposition. So, 00:31:46.040 --> 00:31:57.640 atomic layer deposition or ALD it starts by a pulse ok of metal organic precursor gas into a 00:31:57.640 --> 00:32:04.000 deposition chamber as you can see here this is the first stage. Under certain conditions the gas will 00:32:04.000 --> 00:32:13.280 react with the surface species of the substrate in a self limiting reaction that is terminated 00:32:13.280 --> 00:32:20.440 when the surface runs out of the reactants. So, that is why it is called self limiting 00:32:20.440 --> 00:32:28.640 reaction and the excess gas is purged in the next step with a neutral gas. So, 00:32:28.640 --> 00:32:35.480 as you can see here nitrogen is being used for doing the purging you can also use argon 00:32:35.480 --> 00:32:42.840 depending on the process requirements. So, the second reactant is then introduced into the 00:32:42.840 --> 00:32:50.480 chamber in the third stage. So, here the second one is introduced and again that starts reacting 00:32:50.480 --> 00:32:58.120 with the surface species like that and the excess reactant is again purged in the fourth step. 00:32:58.120 --> 00:33:05.000 So, that that gives you one particular cycle and then it is repeated. So, you can actually 00:33:05.000 --> 00:33:12.040 atomic layer deposition as the name tells you we can actually deposit layer by layer of atoms and 00:33:12.040 --> 00:33:21.280 grow your thin film. So, in an ideal ALD process one atomic layer of material is deposited in each 00:33:21.280 --> 00:33:29.200 cycle that is what it does. So, you can actually precisely control the thickness of the films using 00:33:29.200 --> 00:33:33.640 this particular method. The number of cycles will then determine the overall thickness of 00:33:33.640 --> 00:33:41.080 the film okay and ALD deposited films are highly conformal and they can be used for coating and 00:33:41.080 --> 00:33:49.960 encapsulating of complex geometries because you have complete control on the thickness of 00:33:49.960 --> 00:33:57.160 the films that are being deposited ok. So, with that we move on to the third method 00:33:57.160 --> 00:34:04.680 of depositing thin films that is epitaxy. So, when we talk about epitaxy it basically 00:34:04.680 --> 00:34:10.480 this word comes from a Greek word which means ordered upon. So, epitaxy means basically the 00:34:10.480 --> 00:34:18.200 growth of a single crystal film on top of a crystalline substrate. So, for most of 00:34:18.200 --> 00:34:23.920 the thin film application which includes soft or hard coating, protective coating, 00:34:23.920 --> 00:34:31.080 material coating it has little importance. However, when you go for semiconductor thin 00:34:31.080 --> 00:34:37.640 film technology this becomes very very crucial the crystalline growth becomes very very crucial. 00:34:37.640 --> 00:34:43.760 So, this is what we mean by crystalline growth. So, this is not epitaxial, but this is epitaxial 00:34:43.760 --> 00:34:50.760 ok. So, it is a crystalline growth ordered and crystalline growth. Now note that when 00:34:50.760 --> 00:34:58.320 we talk about the other thin films if they are not crystalline they are typically amorphous ok. So, 00:34:58.320 --> 00:35:05.320 amorphous films are preferred over thin films due to the simpler production ok used when 00:35:05.320 --> 00:35:11.000 their properties can meet the application requirement. So, if you are able to provide 00:35:11.000 --> 00:35:15.800 a amorphous film that can serve your purpose you stick to that you do not need to develop 00:35:15.800 --> 00:35:21.600 a crystalline film or a very ordered film because that makes it very expensive ok. 00:35:21.600 --> 00:35:28.320 Now when we talk about the optical coatings where the refractive index control is crucial 00:35:28.320 --> 00:35:34.640 you actually use amorphous films ok that works for you. Metal films are commonly used in amorphous 00:35:34.640 --> 00:35:41.760 form for consistent electrical conductivity and optical reflectivity. While amorphous materials 00:35:41.760 --> 00:35:49.560 they lack distinct electronic band structure due to their randomness crystals process defines band 00:35:49.560 --> 00:35:56.800 structure with their ordered arrangements and crystallization can occur through annealing ok, 00:35:57.360 --> 00:36:05.760 but epitaxy is a more formal process for creating high quality crystalline films. So, 00:36:05.760 --> 00:36:15.440 here is an example of method of epitaxy which is metal organic CVD. So, this is very very similar 00:36:15.440 --> 00:36:24.560 to LPCVD that you have seen it grows thin layers by sending special gases over the substrate. 00:36:24.560 --> 00:36:30.600 Now if you see the shower head here ok in this particular figure it is designed for equal 00:36:30.600 --> 00:36:36.720 distribution of gaseous precursors through the chamber onto the wafer. So, these are different 00:36:36.720 --> 00:36:46.760 gases which comes in ok. It allows a quick and uniform diffusion of gas molecules onto a rotating 00:36:46.760 --> 00:36:54.880 heated wafer. So, this the wafer which is being on a rotating platform and which is also heated. So, 00:36:54.880 --> 00:37:03.560 nitrogen gas is typically used for purging the system and argon gas can be used as the 00:37:03.560 --> 00:37:10.120 carrier of the metal organic precursors. These are the wafer loading window this is 00:37:10.120 --> 00:37:17.280 the exhaust ok and this is where the resistance heater is that heats up the platform ok. So, 00:37:17.280 --> 00:37:27.720 you can actually see that this this particular wafer at rotates on this platform at a speed up 00:37:27.720 --> 00:37:35.840 to 1000 rpm. The film thickness which is grown epitaxially that means, orderly ok is mainly 00:37:35.840 --> 00:37:42.680 controlled by deposition time and the growth rate. And growth rate is also strongly affected 00:37:42.680 --> 00:37:48.160 by the growth pressure in the chamber substrate temperature and the precursor flow rate. So, 00:37:48.160 --> 00:37:54.560 these are the factors they they actually decide what will be the growth rate of the film. 00:37:54.560 --> 00:37:59.960 The other method is called molecular beam epitaxy. This is an epitaxy method where 00:37:59.960 --> 00:38:07.600 thin film deposition of single crystals take place. So, in this particular method it is 00:38:07.600 --> 00:38:13.280 widely used in manufacturing semiconductor devices something like transistors and 00:38:13.280 --> 00:38:19.880 it is considered as one of the fundamental tools for development of nanotechnology. So, 00:38:19.880 --> 00:38:27.480 MBE molecular beam epitaxy is used to fabricate diodes and and MOSFETs MOS means metal oxide 00:38:27.480 --> 00:38:36.200 semiconductor FETs ok working at microwave frequencies ok and to manufacture the lasers 00:38:36.200 --> 00:38:45.800 used to read optical disks such as CD and DVDs. So, I will go into the process soon. So, remember 00:38:45.800 --> 00:38:51.600 that MBE is considered to be one of the cleanest, but also one of the most technically challenging 00:38:51.600 --> 00:38:59.240 and demanding process because this MBE growth takes place in ultra high vacuum environment. 00:38:59.240 --> 00:39:07.640 So, this is the chamber which is a ultra high vacuum UHV chamber. So, an experimental setup 00:39:07.640 --> 00:39:14.840 as you can see here it consists of two or more Knudsen efficient efficient cells or you can 00:39:14.840 --> 00:39:21.160 say them as K cells which are located at the bottom of this ultra high vacuum chamber and 00:39:21.160 --> 00:39:26.960 they are aligned towards the center of the chamber like this ok where the sample holder 00:39:26.960 --> 00:39:32.720 with a substrate. So, this is the sample holder where the substrate is actually located. Now, 00:39:32.720 --> 00:39:39.840 each individual K cells they contain different elements in ultra pure solid form. 00:39:39.840 --> 00:39:46.560 So, when you say ultra pure the purity is typically 99.999 percent ok. So, what kind 00:39:46.560 --> 00:39:54.600 of materials like selenium, bismuth ok they are used in the thin film synthesis. Now, how it 00:39:54.600 --> 00:40:01.680 works the process of MBE growth starts by heating this K cells to appropriate temperature until the 00:40:01.680 --> 00:40:12.040 elements in each cell reach a sublimation point ok. And then the shutters over here as you can 00:40:12.040 --> 00:40:17.240 see the shutters this is open shutter open shutter and this is a closed shutter. So, these shutters 00:40:17.240 --> 00:40:24.440 are then open and the physical vapor from each K cell diffuses through the chamber until it 00:40:24.440 --> 00:40:32.320 reaches the substrate and get deposited and this is how thin films are being formed. Now remember 00:40:32.320 --> 00:40:39.440 that for more uniform growth substrate can be continuously rotated at low rotation speed. 00:40:39.440 --> 00:40:48.120 So, when I say low rotation speed is typically 1 or 2 rpm ok and by utilizing a stepper motor which 00:40:48.120 --> 00:40:55.000 can be attached to this magnetic manipulator. So, this is how in this form it can be slowly rotated 00:40:55.000 --> 00:41:03.920 and the electrons which come from this reed gun ok. So, this is electron diffraction gun. So, 00:41:03.920 --> 00:41:10.960 you can see that it actually incident electron at a very low angle and you can 00:41:10.960 --> 00:41:17.760 get the defected electron and you can observe the pattern here. And this pattern can actually 00:41:17.760 --> 00:41:24.760 reveal the quality of the film that is being developed and also you can measure the film 00:41:24.760 --> 00:41:33.640 thickness starting from single monolayer ok. So, as I told this particular characterization 00:41:33.640 --> 00:41:42.720 is done by using reflection high energy electron deflection that is RHEED ok. So, 00:41:42.720 --> 00:41:48.040 in this particular method the final composition of the film will depend on the temperature and 00:41:48.040 --> 00:41:56.240 the surface atomic structure of the substrate as well as it will also depend on the flux ratio 00:41:56.880 --> 00:42:02.680 of individual components which are reaching the substrate. So, this is how you can mix things up 00:42:02.680 --> 00:42:10.920 and get a thin film of a particular composite material. Selective opening of the structures 00:42:11.640 --> 00:42:19.520 shutters of each K cell they will ensure growth using only specific elements. So, 00:42:19.520 --> 00:42:25.920 you can also do one by one kind of just by opening the shutters you can do one layer 00:42:25.920 --> 00:42:30.880 first and then the next layer and so on. And you can characterize the thin film that 00:42:30.880 --> 00:42:38.760 is getting generated by this RHEED method ok. So, this particular table sums up all the techniques 00:42:38.760 --> 00:42:45.560 that we have studied till now. So, you can see the methods are being mentioned here. So, 00:42:45.560 --> 00:42:52.440 evaporation sputtering, pulsed laser deposition, low power CVD plasma enhanced CVD, ALD, MOCVD, 00:42:52.440 --> 00:42:58.080 and MBE and you can see the parameters on which they are discussed. So, 00:42:58.080 --> 00:43:06.640 you have substrate temperature deposition energy pressure step coverage defect density uniformity 00:43:06.640 --> 00:43:11.600 deposition rate and the materials that are used and what are the different applications. 00:43:11.600 --> 00:43:17.160 So, this actually gives you a very good idea. And what is important to see here that all these 00:43:17.160 --> 00:43:23.520 methods more or less are very very uniform other than pulsed laser deposition that does not give 00:43:23.520 --> 00:43:30.560 a very good uniformity. And you can see that this is pulsed laser deposition is also slow 00:43:30.560 --> 00:43:36.200 and these are first evaporation and sputtering are first deposition method these are physical 00:43:36.200 --> 00:43:44.720 methods. In chemical methods LPCVD and PECVD are first methods ok. And you can also see the 00:43:44.720 --> 00:43:51.640 substrate temperature in this case you can have a wide range in case of physical method in LPCVD 00:43:51.640 --> 00:43:58.440 it should be high PECVD it should be moderate. And these are the materials that can be used. So, 00:43:58.440 --> 00:44:04.080 when you talk about MBE these are typically compound semiconductors like gallium arsenide, 00:44:04.080 --> 00:44:11.240 indium phosphide and aluminum gallium arsenide. MOCVD you can again use the similar 00:44:11.240 --> 00:44:18.320 kind of compound semiconductors. Atomic layer deposition is useful for alumina, hafnium oxide, 00:44:18.320 --> 00:44:25.560 silicon dioxide and certain metals okay. Then silicon dioxide can be also deposited by low 00:44:25.560 --> 00:44:33.360 pressure CVD or plasma enhanced CVD okay. Polysilicon can be done by PECVD ok. Pulsed 00:44:33.360 --> 00:44:39.080 laser deposition can do complex compounds as we mentioned though it is poor or slow, 00:44:39.080 --> 00:44:45.600 but this method only allows to develop this kind of complex compounds like PZT YBCO and 00:44:45.600 --> 00:44:53.240 other ferroelectric materials and so on. So, these are the different applications. So, 00:44:53.240 --> 00:45:00.280 optical thin films as you can see evaporation and sputtering they are the most commonly used 00:45:00.280 --> 00:45:09.200 for making thin films that can be used for photonics application ok. Then MOCVD they 00:45:09.200 --> 00:45:15.680 can manufacture optoelectronic devices and MBE they are also in the useful for 00:45:15.680 --> 00:45:20.400 research and development in epitaxy and other optoelectronics components. So, 00:45:20.400 --> 00:45:27.640 this particular chart should actually give you a complete overview of the different processes 00:45:27.640 --> 00:45:33.600 in thin film growth and what are their pros and cons and what are their applications. 00:45:33.600 --> 00:45:39.940 So, with that thank you for your attention and we will stop here and in the next lecture we 00:45:39.940 --> 00:45:45.760 will look into the methods of lithography and pattern transfer of how to make a particular 00:45:45.760 --> 00:45:52.560 pattern on your for your device. And if you have got any queries regarding this lecture you 00:45:52.560 --> 00:46:24.520 can always drop an email to the similar address mentioning MOOC on the subject line. Thank you.