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Module 1: Photolithography

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Introduction to Photolithography

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So, if you see the screen lithography, lithography actually means to carve from a single stone. You have a single stone and if you make something out of single stone, it is called lithography. ,‘litho’ and ‘graphic’. So, litho means single stone, graphic means to carve, it is a Greek word called lithography. Now, if we use photons, it becomes photolithography, photo litho and graphy. So, it is a single word, photolithography. So, let us understand what is the importance or how we can design or pattern different materials on a substrate using a photolithography technique. So, a very important part in photolithography is your photoresist, photoresists. There are two types of photoresists. One is called positive photoresist, second is called negative photoresist and positive photoresist again depending on the viscosity, there are is a different kind, thick photoresist, thin photoresist. Same The same thing holds true for negative photoresist as well, thick and thin. Now let us see what is positive photoresist?. If I expose only some of the area of photoresist let us say, I am exposing only the region which is not shaded by red colour, which is this particular box, so this box and this box, it is transparent and the one in the centre is a opaque. So, if I expose this particular photoresist which is my positive photoresist with UV light, UV light, what is it? UV light. A and I developed this photoresist, develop, what will happen? I will have a silicon or oxidized silicon wafer with photoresist only in the centre region. So, what will happen here? What happened is, the region which is not exposed to the photoresist, which was not exposed became stronger. You see here. W, while the photoresist that got exposed through this transparent layer got weaker. So, you can see here there is no photoresist. So, what is the a characteristics of a positive photoresist, that the area which is not exposed to UV light will become stronger..
So, on an oxidized silicon wafer, if we spin coat negative photoresist and we expose this oxidized silicon wafer with negative photoresist using UV light. After exposing if I take the wafer and develop it, that means we will develop the photo resist, what will happen? The area which was not exposed got weaker and the area which was exposed got stronger, you can see here. Tthat means when we use positive photoresist, then we had a pattern like this which is shown in this figure A and when we use, this is when we use positive and when we use negative photoresist, then we got pattern which is shown in B. So, depending on the photoresist, positive or negative, we will have different pattern either A or B., A, we will get if the photoresist is positive and what we said? The area which is not exposed got stronger. If the photoresist is negative, then the area which is not exposed gets weaker. Easy? So, a very important point when you understand photolithography is your photoresist. So now, let us understand one more thing and that is called the mask. So, let me give you a very simple example, you will understand. Assume this is completely shaded, is red, complete red like this. This box is dark. A and this pattern is on a glass substrate, glass. It can be Fe2O3, glass and what we see here is that the centre portion is opaque to light while the portion on the sides are transparent. What we see here is that the side portions are transparent and the one in the middle is opaque. Now, why I am drawing this, there is a reason. You see the maximum area in this mask, maximum area in this mask, is it dark or transparent? This is transparent, this one is transparent, this one is dark or opaque. That means light cannot pass through this, light cannot pass through the opaque region. Since the maximum of the field of this mask is bright, we call this as a bright field mask, bright field mask.
However, if I have a mask where except this area everything else is dark, everything else is opaque. Assume that everything else is opaque, except the one in the central region. So, if everything else is (okay) opaque that means that maximum region of this mask is opaque, this is called dark dark-field makemask, it is dark, opaque . Opaque means light cannot pass through it, is dark. Only the region in the centre is, the region in the centre is transparent. Which region? This region. Region The region which is in the centre is transparent remaining everything is opaque. Assume that everything is opaque. This is your dark field mask, dark field mask. So, you have seen two types of mask, one is a bright field, another one is the dark field. Alright sSo, photoresist two type, positive, negative. Mask two type, bright field and dark filed. Now let us understand how can we use a photoresist and what are the steps for lithography. So, let us assume that we have a silicon substrate. I will draw it on the screen you can see silicon wafer. This is our first step. If you can see the screen please. Now, form where silicon dioxide came, because this wafer is exposed to air, it may be possible that there is a very thin layer of silicon dioxide present on the silicon wafer. What will happen? This that water will, before you put on the hot bed after rinsing with DI, you have to dry the wafer, dry silicon wafer with nitrogen gas. First step is to take silicon wafer, dip in HF, then you rinse in DI water, dry it with silicon, a dry silicon wafer with nitrogen and then place the silicon wafer on the hot plate. Why hot plate? To remove any moisture present on the silicon wafer. Ones you perform this step, then let us grow silicon dioxide using wet etching and for growing silicon dioxide and wet etching you will use a horizontal tube tube furnace.
And we have a silicon dioxide which is about 1 micron grown on the silicon wafer, this is my silicon dioxide, this is a silicon wafer, this is my step number 2. Silicon, silicon dioxide. Next step is I will use a photoresist. Now, let us take the example of positive photoresist. So, how can we have a layer of photoresist on oxidized silicon wafer? We can have a layer of photoresist on oxidized silicon wafer by spin coating the photoresist. So, this is your spin coated photoresist. What kind of photoresist we are using? Positive photoresist. Easy? Now, in certain cases you will see that the positive or negative photoresist does not adhere to silicon dioxide surface. That means, it will not stick to silicon dioxide surface and that is why in some sometimes what we do is after silicon, after sf oxidized silicon wafer is there by growing silicon dioxide in using thermal oxidation technique, we scot a thin layer of HMDS, HMDS. This HMDS will create a site of adhering or improving the addition of positive photoresist or negative photoresist with silicon dioxide or purge in an other words, it creates a, it improves the addition of the photoresist on any surface or most of the surface. Do you got it? So, silicon wafer, we clean the silicon wafer, then we perform the HF, then we rinse it and do the drying, then hot plate, then we can grow silicon dioxide. Of course, there are RCA1 and RCA2 cleaning if you want to perform RCA1 as it to cleaning it, you have a new silicon wafer, you may not like to go for RCA1, RCA2. So, you have first step, then you grow silicon dioxide, after that you coat a thin layer of HMDS. HMDS thin layer which will create a site for improving the addition of photoresist. After that you have your photoresist. So, this layer is very thin in particular maybe I have drawn a too thick layer here. So, if in reality, this layer is very- very thin layer like this. This is HMDS. So, and then you have a positive photoresist, so after positive photoresist, just for this positive photoresist is spin coated, spin coated.
So, spin coating of positive photoresist we perform, so that we have the photoresist on silicon dioxide wafer. Now next step would be you load the mask on this positive photoresist. Load a mask, but before we load a mask, there is one more step. So, let us understand again. You have your silicon wafer, then you perform your cleaning steps, so this is step 1, then in step 2 you grow silicon dioxide and step 3 you spin coat HMDS, then step 4 is you have your positive photoresist. After HMDS you can again heat the wafer at 95 95-degree centigrade for 1 minute and then you take the wafer, spin coat positive photoresist. After that, the next step which is step number 5 would be soft bake. The soft bake is done at 95 95-degree centigrade for 1 minute on hot plate. So, after soft bake the next step would be you load the mask, load the mask on an oxidized silicon wafer, where you have positive photoresist. A and let us say my mask looks like this. Now what you can say, the maximum field is bright, field is bright and that is why this would be a bright field mask, bright field mask. This will be my sixth step. Let us quickly recall. First The first step is silicon, second step is to grow silicon dioxide, now I am skipping the washing step which is already known that you dip the wafer in HF and remove silicon dioxide and then rinse it with DI, then dry with nitrogen, then you put on the hot plate, then take the wafer, grow silicon dioxide, then after silicon dioxide you have HMDS layer, after HMDS you haver positive photoresist, after positive photoresist you have soft bake at 95 degree centigrade for 1 minute on hot plate, then you load the mask. A and in this case it is a bright field mask. After loading a bright field mask, we can expose this wafer with UV light, expose this wafer with UV light. When you expose this wafer with UV light, the next step would be to develop the wafer. So, what I will do if I say this wafer which is exposed by UV light is called A.
So, what I will do if I say this wafer which is exposed by UV light is called A Tthen I had to dip this A in a beaker which has a photoresist developer. A, so I am nd dipping the wafer which is exposed with UV light, of course, I will remove the mask and then I will take the wafer and dip the wafer in PR developer. When I do this step, what I will have is, I will have my oxidized silicon wafer and photoresist pattern as shown in this figure. You see what happened? From here since this is a positive photoresist, it is a positive photoresist, what happens? The area which were not exposed got stronger. See the opaque area, the area of S of the photoresist that was not exposed by UV light got stronger, you see here. Next step is that I want to now dip this wafer in silicon dioxide. After you develop it, the next step would be, after you develop this wafer, the next step would be hard bake, hard bake. Hard bake is done at 120 120-degree centigrade for 1 minute on a hot plate, hard bake is done at 120 degree centigrade for 1 minute on a hot plate. So, once I perform hard bake, the next step would be I will dip this wafer in BHF. If I dip this wafer in BHF, BHF is the etchant for silicon dioxide. So, what will happen? We will have a wafer which looks like the figure that is shown here and if you have carefully observed what happened is, that wherever the photoresist was there, the silicon dioxide below the photoresist is protected. Why the silicon dioxide which was not protected by photoresist got etched.
So, we have patterned silicon dioxide on the silicon wafer. But do we require positive photoresist? We do not require positive photoresist. So, next step would be you dip this wafer in acetone. Now acetone is called photoresist stripper. To strip photoresist, you have to dip the wafer in acetone. See if I dip the wafer in acetone, what would I have? I would have my photoresist stripped off be and I would be felt with wafer as shown in the schematic. So, this is why silicon dioxide pattern on the silicon wafer. Easy? This is how the photolithography would work. So, this is how the photolithography would work. So, let us again quickly see. First step would be silicon wafer, second step would be oxidized silicon or silicon dioxide grown on silicon wafer with the help of thermal oxidation, next step would be to spin coat HMDS on this followed by placing the wafer on a hot plate at 100 degree centigrade for 1 minute, then you have spin coat of photoresist followed by soft bake, 95 degree centigrade 1 minute hot plate. Next step would be to load the mask, in this case it is bright field mask. Next step is exposed with UV light, so UV light exposure. Next step would be unload the mask and develop this wafer. So, when you develop the wafer in photoresist developer, what you will have? You will have this particular pattern where the unexposed area, see the unexposed area, will become stronger, because it is a positive photoresist. Next step would be you dip this hard bake, hard bake is at 120 degree centigrade, 1 minute hot plate and then you can dip the wafer in BHF. If you dip the wafer in BHF, then the area which is protected by the photoresist will be stronger, the area which is not protected by photoresist will be weaker and finally you dip the wafer in acetone, which is a stripper for photoresist, you will have a pattern shown here. So, right from wafer which looks like this to the wafer which looks like this, we can pattern different kind of materials using a photolithography technique.
So, if you see the photolithography process, a photolithography process is to print features on a wafer directly or by using photoresist. Generally, features on top surface of any sample are patterned using photoresist by exposing to UV light development, etching of the targeted layer. So, what is the reason of running photolithography, so that we can know that if there is an oxidized silicon wafer and you want to have a gold patch on this particular wafer, everything I want is a gold. Maybe I do not want gold everywhere except in this area. So, how can I get this kind of pattern? I can obtain this kind of pattern from this oxidized silicon wafer where you have coated gold or like this and you perform lithography, so to reach step number 2. Because only this area does not have gold, in this area everywhere there is a gold, so I want to protect this particular area and I want to remove the gold from the area which is not required to be protected with gold. If I want to do this step that can be used for measuring EEG, it can be an EEG electrode after some modification, then I need to use the lithography system. So, as we have discussed if you see the steps, the first step is always wafer cleaning followed by pre-bake and primer coating, see wafer cleaning and then I said that we place the wafer on a hot plate to remove the water vapour that is your prebake and primer coating, primer coating I said HMDS. So, this is a primer that will help to improve the addition of the photoresist onto the wafer. Then we have photoresist spin coating, either positive or negative, followed by soft bake, soft bake is at 95 degree centigrade 1 minute hot plate. Then alignment and exposure that means loading the mask and then next step would be UV exposure. So, alignment is loading the mask, exposure is UV exposure followed by developer. So, developer or development of what? Development of photoresist. After develop of photoresist you have to go for hard bake, hard bake is done at 120 degree centigrade for 1 minute on hot plate, followed by pattern inspection. That means how the pattern has come after you have performed the photolithography.
So, when you talk about the first step which is wafer cleaning and pre-bake, it consists of Bubble Jet, High Pressure Rinse followed by Sonication. So, bubble jet is using N2 plus H2O and sonication is done at 1120. 5 megahertz frequency. Followed by prebake which is a high temperature baking to remove moisture as we have already discussed after water cleaning process. After wafer cleaning what you do? You rinse the wafer in DI water and then you dry the wafer using nitrogen and then you prebake it at high temperature to remove any moisture content. Now primer, is as we already discussed is to improve the photoresist addition and in this case we are using HMDS which stands for Hexamethyldisilazane. So, after the primer, we can use 100 degree centigrade, we can also sometime use a high temperature like 200 degree centigrade, time would be 1 minute. In this case, you can see here resist coating, so this is the step resist coating, soft bake, then expose, after expose you have a developer. So, if it is a positive photoresist, you can see that the unexposed area which is this area got stronger and the exposed area got weaker when you develop the wafer.
So, this is the same thing that the photoresist is nothing but a polymer solid organic material that can be used for transferring the design to the wafer surface. It changes its photo solubility due to photochemical reaction to expose UV light. You can see here, if you have a positive photoresist coated on a substrate and you use a bright field mask, what will happen that, if it is a positive photoresist, then the unexposed region would be stronger. Where, if it is a negative photoresist, then the unexposed region will be weaker. So, this is after development we have seen that. So, what are the other parameters of a photoresist or characteristics of photoresist? It should have a high etch rate. Actually, photoresist should not be having high etch, it should have a high etch resistance and absolutely good adhesion. Etch resistance means if you dip the wafer in let us say BHF, this silicon, the photoresist should not get affected. What I mean by this is. Let us see this case. You have a wafer which after you perform the photolithography and hard bake you have photoresist in this area for example. This is your photoresist. Then you perform hard bake, hard bake. Now, I want to etch silicon dioxide from this area. What I will do? I will dip this wafer in BHF. If I dip the wafer in BHF, what will happen? If I dip this wafer in BHF, what I am expecting that I would have silicon dioxide etch from this area while the silicon dioxide which was protected by the photoresist would not get etch. Also, simultaneously, I am expecting that my photoresist should show a high etch resistance. What does it mean that this photoresist should not get etch in BHF when I am dipping the wafer in BHF, which is meant to a silicon dioxide and photoresist should not get affected. That is your high etch resistance.
So, when you discuss about the photoresist in high etch resistance that is what it means. You can see here high etch resistance, and good adhesion. Wafer held onto the vacuum and then there is a dispenser which will dispense about 3 to 5 millilitre photoresist. Initially we do slow spin, then we ramp up the system at a high rotations per minute which is rpm to have uniform coating. Higher the rpm, thinner the photoresist; closer the rpm, thicker the photoresist coating on the wafer. Photoresist is spread by (centri) centrifugal force and the quality measures are time, thickness, speed, uniformity, particle defects. There are several kind of photoresist right from SU-8, to AR-N 4200, 4, 4300, 4400, while positive photoresist are either Shipley or AZ-2212.. In this module what we have covered, we have covered silicon dioxide. But in actual world, you may want to have a conductive electrode pattern onto the oxidised silicon wafer which you can use as a patch electrode. We can put on the scalp and you can measure EEG signal because that is what we are interested in, in this particular course which is on the neuroscience introduction. A and particularly, when you talk about neuro instrumentation that depends on the how good your electrode is, so to capture the signal from your scalp.

We will continue with a previous module in which we have talked about photoresist and we will continue on lithography and then we will take one example to help you out how to fabricate an EEG electrode or a metal electrode on an oxidized silicon substrate, you can also use instead of oxidized silicon substrate you can use glass, you can use polymer and there different substrates are available on which you can design this particular electrode. Now, continuing to our last topic I told about the mask and I have shown you a bright field mask and dark field mask. So, the one that I am holding right now in my hand is a bright field mask as you cannot see so I will put on my a background as my shirt and you can now see the glass that I am holding which is a 5 inch glass and there is some pattern on the glass. So, this is you see most of the part is transparent and only the pattern which is here in the centre it is dark. So, the field is bright and pattern is dark this is a bright field mask bight field mask this is what we were talking about we have crome glass or crome mask on which there is a pattern and the field is bright field mask. But if you have something like this which I am again holding you can very clearly see that this is a dark field mask and only two places you will be able to that there are two holes here and here, here and here so you can see here there are two transparent areas while the majority of the area majority filed is dark because everywhere there is chrome except this two holes. So, this is your dark field mask, it is very clear to understand and very easy to recognize a bright filed versus dark field mask and importance of bight field versus dark field mask we will discuss when we talk about how to take out the context from the electrodes, I will teach you about that particular part.

So, coming back to the photoresist if you see the screen, we were talking about positive and negative photoresist and we also discussed that in positive photoresist the explorer or exposed area will become weaker and unexposed area will become stronger, in case of positive photoresist, the unexposed region will become stronger. So, this is an example that if you have a bright field mask which is shown here we the pattern that shows like plus and if you use positive photoresist the unexposed area would be stronger and you can see here that the unexposed area is stronger while the exposed area which is this area gets weaker in case of positive photoresist. When you use positive photoresist with the bright filed mask, this will what will happen you use positive photoresist bright field or dark field does not matter the concept that you need to remember is that the unexposed region would be stronger. Now, if you take a negative photoresist what will happen the unexposed region is weaker. See, the unexposed region of the mask in case of negative photoresist will be weaker and the exposed region which is this region will be stronger you can see here the photoresist is intact in this region, in this case, the photoresist is edged from the area which is exposed and there photoresist is intact in the area which is not exposed. The same thing is done in this sentence there exposer to UV light removes resist that is positive photoresist you see exposer of UV light, exposer of UV light will be in this area this area here wherever it is the bright field.

So, the exposer of UV light will remove the photo photoresist when we talk about positive and when we talk about negative photoresist then exposer to UV light will retain the photoresist. See it is retaining the photoresist same thing. So, different authors write differently. Some will write at the unexposed region of the in case a positive photoresist would become stronger in case of negative photoresist unexposed region would become weaker some people will lie that exposed region in case of positive photoresist will become weaker and exposed region in case of negative photoresist will become stronger, some people will like that exposer to UV light removes resist in case of positive exposer to UV light detains photoresist in case of negative at the point remains same. So, now if you see the illustration of positive and negative photoresist even though this overall the schematic is small still you will be able to understand see you start with a sub striate so this is my first step then off course, you clean the substrate that is but obvious you need to follow it. Second is that you are applying a photoresist, resist film, the third one is you use some mask this is second where the photoresist is applied, third one is using a mask. Now, if it is positive you can see the mask something like this, it comes out like this and then there is a one-line here and this area that we are showing is dark which is some pattern is there, like this. This is our mask pattern which is this one. Now, if I if the photoresist that you have spin coated which is this one, so what will happen first is your substrate, second is your spin coat photoresist then perform soft lithograph, soft baking which is at 90 degree centigrade 1 minute on hot plate, then you load their mask either mask pattern is as shown in schematic, then if you use positive photoresist you will retain the area which is not exposed and the exposed region will be edged and finally you have to you what you can do you have to hard bake it before you develop the wafer.

So, after you develop the wafer and then you can hard bake it so the point is you go for photoresist coating, then soft bake, expose, then develop and then hard bake. Hard bake is done at 120 degree centigrade for 1 minute on hot plate and then you can eth or strip the photoresist by dipping the wafer in acetone. Now this is case of when the photoresist is positive, but if the photoresist is negative what will happen? Again you have to coat the photoresist, soft bilk, load the mask, expose the wafer then develop the wafer. In this case, when you develop the wafer you will see that the unexposed region becomes stronger and the unexposed region will become weaker and the exposed region will become stronger. This is what you can see here and then what you have to do? Again you have to perform after developing hard bake after hard bake you have to dip the wafer in acetone so then you dip this wafer in acetone, then you will achieve this pattern. So, this is how positive, negative photoresist works. But the point is how you will coat or the photoresist onto the wafer? And you have seen the wafer in the previous modules of silicon wafers and oxidized silicon wafers. So, you have to first take the wafer to clean it or and rinse it which is washing step and then you load the wafer on to the vacuum chuck which is here and then there is a spindle which is connected this whole chuck is connected to vacuum pump and your wafer is loaded on to this vacuum chuck is loaded it makes a hard contact. So, now the wafers is holded, this is just for you to understand that there is a here there is a gap which is shown there is no gap, it is just to show you that there is a hole which will, through which the vacuum will be generated and are and also these lines are there through which vacuum will be generated, this will help the wafer to stick to the vacuum chuck and then there is a spindle which will rotate.

So, when you load the wafer, the photoresist is dispensed and the initially it is a slow RPM to have a uniform deposition, followed by the thickness that you want. As you can see here that if you increase the spin speed that is rotations per minute of this particular spindle, what will happen? Rotations per minute increase means thickness is decreasing, as you increase the rotation, the thickness will decrease as you decrease the rotation, the thickness would increase. Now here the chart is showing for different material which is SU-8 material SU-8 is another negative photoresist we will discuss about SU-8 at some point of time and then depending on the viscosity we have a different thickness of SU-8 pattern is possible. So, here let us see that what is the characteristic of photoresist, the properties that it should have a high etch resistance and good addition so you should stick well to the wafer. The second one is the wafer is held onto the vacuum, the photoresist is dispensed on the substrate, it can be silicon, it can be oxidized silicon, it can be glass, it can be polymer. Then there is a slow spin at 500 RPM to have a uniform deposition or uniform spreading, finally, we have to ramp the spin coater from 500 all the way to 5000. It depends on what kind of thickness you desire. Photoresist is spin coated or spreaded by centrifugal force and the quality that we need to measure are time, speed, thickness, uniformity and particles as well as defects.

So, the particle defects would be there if the photoresist is not clean, that is why we require a clean room, we require a particular protocol to follow so that to have a clean photoresist. And you can very clearly see that this spin speed or thickness is inversely proportional to the square root of spin speed. You can also further see that the higher viscous photoresist coated with lesser thickness, while higher spin speed will result in a higher thickness of the photoresist. Then we talk about, so if you recall what we said that in photolithography, the first step is you take a substrate and then perform wash, washing of wafer, then second step is spin coat of photoresist, third step is soft bake. So, and then fourth step is the mask loading, mask loading and UV exposure, fifth step would be developer, sixth step would be hard bake, let me write down and finally after you do this step, you have to edge if there is some metal and if you want to remove this or strip the photoresist, you can go for acetone. So, these are the steps. (Refer Slide Time: 12:10) So, why we require soft bake? So, this slide shows the reason of using soft bake. First is that it will help in partial evaporation of the photoresist. You see photoresist comes in a semisolid or in a liquid form and