Photolithography

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"Photolithography, also termed optical lithography or UV lithography, is a process used in microfabrication to pattern parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate. A series of chemical treatments then either engraves the exposure pattern into, or enables deposition of a new material in the desired pattern upon, the material underneath the photo resist. For example, in complex integrated circuits, a modern CMOS wafer will go through the photolithographic cycle up to 50 times. " - Wikipedia


From Sam's "Semiconductor Fabrication Basics Background Theory" video: "That photo-resist is a photosensitive paint, we then have a mask above it that blocks light in some portions and allows light to go through in other portions. Intense DUV deep ultraviolet light is passed through that masked, and the light is stopped in this section and the light continues trough in these sections. This is a negative acting photo-resist, meaning that the portions that are exposed to light become polymerized and then after they are developed they don't wash away and they become structures. So the light is blocked in this little section in the middle therefore washes away after being developed and we're left with a hole. Now photo-lithography is extremely intricate and there are techniques to expose feature sizes that are incredibly small in the tens of nanometers ranges and below and these sizes are even smaller than the wavelength of the light used to expose them its absolutely incredible. You'd expect that the masks used are very expensive and they are! The masks are made out of I believe chrome plates that are written onto directly with focused electron beam and a set of masks to make an IC is hundreds and hundreds of thousands of dollars its absolutely nuts. There's 2 different types of masks, theres light field and dark field (sometimes called clear field or light field) and light field masks, the majority of them is clear its transparent, and the features you want to transfer are dark, and thats what this masks up here is, and then theres dark field masks where the majority of the mask is dark and the feature you want to transfer is light and then theres 2 kinds of structure, hole and island, this is a hole and the inverse of it would be an island if this portion was polymerized and these 2 were un-polymerized. So this is just a quick thing you should be able to memorize. Theres a couple different types of photo-resist theres of course negative and positive, theres also light-sensitive and energy sensitive there are resists that are, the majority of them traditionally are UV sensitive, but theres also direct e-beam writing and writing and theres also X-ray methods for doing photo-lithography. Within the photo-resist theres 4 types of chemicals: polymer, solvent, sensitizer and sometimes additives. The polymer is the actual light-sensitive part, its the thing that changes structure, it becomes polymerized or becomes soluble based on the light that hits it, and the solvent is I don't know maybe like an alcohol or something like that, and that allows it to be coated in very thin layers onto the silicon wafer, that allows for spin coating basically, and then theres sensitizers and these are specific to the brand of photo-resist you buy I guess, and this allows the company to fine-tune the actual chemical reaction thats taking place during exposure, the method of polymerization basically, and the final thing are additives, so sometimes they'll add colored dyes into the photo-resist and other chemicals that the company desires that increase resolution capability, process yield, things like that, maybe its an additive that allows them to manufacture the photo-resist cheaper, possibly to make it have longer shelf life, possibly to focus the sensitized light region, so it only works on a specific part of the DUV spectrum, so this kind of thing would be additives inside the photo-resist. As far as characteristics of photo-resist, because you might think that negative and positive photo-resist are basically interchangeable but you just have to change your masks, thats not the case at all, negative and positive photo-resist are extremely different. Using positive photo-resist will give you higher resolution, higher aspect ratio, aspect ratio is the ratio between the smallest feature you can make and the thickness of the spin-on photo-resist. So positive wins in that category, but negative is easier to apply, has better adhesion, and negative also has a faster exposure speed, so you can run through an automated process faster and faster so negative is kind of better in those respects and is easier to work with. Positive same trend with the resolution, higher resolution, also higher quality, lower pinhole and impurity count, positive will flow into small steps into SiO2 and stuff much easier and it gives you better coverage in that respect, but positive is also more expensive, as far as removing the photo-resist after its been developed positive can be removed with simple solvents, some are water soluble, some alcohols and stuff. And the negative generally you need chlorinated substances, more nasty acids and stuff like that. The photo-resist strippers for negative are a little bit more nasty.

Photo-resist is normally spin-coated onto the wafer, so there'll be a vacuum chuck where the wafer will be placed onto the chuck and a vacuum will suck it down onto the platform so its nice and flat, and then theres either static or dynamic dispense, where an arm will come across and dispense the photo-resist either when it is static when the wafer is not spinning, or dynamic the wafer will start spinning and then dispense, and then spin up to a higher RPM normally in the 3000-5000 RPM range and that will make a nice even coating of the photo-resist right across the wafer, and then the excess will fly off and -its not recycled of course- but it will fly off and be collected. Before the photo-resist is spun on, theres a number of surface treatments that take place mainly just to get rid of all the water on this, theres dehydration baking, RCA cleans and stuff like that are not really used prior to this because they allow a hydrophilic surface, but I believe like an HF bath would make sense, because if theres no previous steps that would be screwed up by that, an HF bath would leave a nice hydrophobic surface for the photo-resist to be applied onto, then it will be spun, it will be dried, depending on if its negative or not there might be another soft-bake or hard-bake step and developed and dried.

Aright lets talk about alignment and exposure techniques: So the first exposure techniques were basically contact, and thats where the mask on top was lowered down so it was touching the surface of the wafer, UV light was exposed through, and that was your exposure, and theres a couple of issues associated with that one of which is since you actually have a physical connection between your mask and your wafer, transfer of contamination, the mask does not last as long, you can scratch up the surface of the wafer, things like that, that lent to the development of proximity exposure systems where the mask is lifted off the wafer by a very small gap in between them, so the mask and wafer are in very close proximity, that eliminates some of the problems with contact, but as we get to smaller and smaller feature sizes, that little gap between the mask and the surface of the wafer really degrades the actual resolution that we can get, you know there's some diffraction, and some of the light is not passed through that gap as it should be, some of the light is set there off at different angles, and the image can be skewed so your critical dimensions are not quite what you wanted them to be. Next we have projection and stepper, stepper is the most complicated optical setup basically, and these use reticles or masks and projection systems so that the light source is further away from the wafer, its basically similar to - it's a reverse enlarger if you ever worked in a dark room developing black and white photographs, it's the opposite idea of an enlarger. So you have a light source and then right below it you have your mask, and then you have focusing lenses that focus that down and then you have your chip that gets exposed, a piece of silicon and then a stepper can do very high quality images, the mask contains the image for one chip, and that gets focused down onto an entire wafer, and then the wafer is stepped so that that image for one of the chips is exposed onto multiple locations on the wafer, and then the whole wafer is filled up with that exposed image onto it. So these ones right here are optical exposure methods, and then theres energy or non-optical exposure methods right here, these are done in vacuum, or at least e-beam is. so depending on the photoresist, some are sensitive to x-rays and e-beams, cool thing you can do with electron beam is that's actually a direct writing method where no mask is used, and where you have a very focusable and finely deflectable electron beam you can use that to write directly on the photoresist surface, it's just like a CRT it's like a raster scan and it wil draw out the image with this e-beam, I think it has to go over the image a few times to get it actually burnt into that photo-resist."

Low Budget Maskless DLP Projector Lithography for Home Chip Labs[edit]

Maskless Photolithography with DLP Projector - 10um Feature Sizes VIDEO

The ambient light in labs set up for photolithography are heavily UV and DUV filtered (yellow lighting) so that photoresists are not exposed by mistake while processing.


Positive Photoresists[edit]

"With positive photoresists, UV light strategically hits the material in the areas that the semiconductor supplier intends to remove. When the photoresist is exposed to the UV light, the chemical structure changes and becomes more soluble in the photoresist developer. These exposed areas are then washed away with the photoresist developer solvent, leaving the underlying material. The areas of the photoresist that aren’t exposed to the UV light are left insoluble to the photoresist developer. When working with positive photoresists in the semiconductor manufacturing industry, you receive an identical copy of the pattern, which is exposed as a mask on the wafer." Source

Negative Photoresists[edit]

"With negative resists, exposure to UV light causes the chemical structure of the photoresist to polymerize, which is just the opposite of positive photoresists. Instead of becoming more soluble, negative photoresists become extremely difficult to dissolve. As a result, the UV exposed negative resist remains on the surface while the photoresist developer solution works to remove the areas that are unexposed. This leaves a mask that consists of an inverse pattern of the original, which is applied on the wafer." Source