Lithography Patterns and Inversions

Lithography Patterns and Inversions

Today I’d like to lead you through an exercise in spatial comprehension. Or maybe it’s 3-D visualization. I’m not sure what the correct term is – but what I’m talking about is inversions of patterns. The goal of this post is to describe how to make a mask when you know what your end product should look like. What is a mask, you say? In general terms, a mask is a clear sheet that is patterned with opaque areas. You use it in the cleanroom for lithographic processes – that is, processes that transfer a pattern to the product you are working on. Is that gibberish? Let me show you were the spatial skills come into play…

Say I want to make a glass slide with tiny gold wires on it. This is going to be an integral part of Iron Man’s flying suit, so it’s important to get these tiny wires correct. This is what we want:

Figure: Glass Slide with Gold Wires

Now how do we get those gold wires on that glass slide? There are several ways, but let’s say we are using a peel-off method (also known as adhesive lithography, if we are going to be technical). We start with a bare slide.

Figure: Clean Slide

Then we are going to coat the whole slide with gold (Iron Man is rich, obviously, so this won’t be a problem).

Figure: Slide Coated with Gold Layer

Then we are going to take a sticky stamp, place the sticky stamp on the slide, and peel off the areas we don’t want. So that we are left with:

Figure: Slide with Gold Wires. Again. In Case You Forgot. Iron Man's Counting On You, Remember....

So here’s our first problem. What is our stamp going to look like? Our sticky stamp is going to peel off the areas we don’t want – so the sticky stamp needs to be the inverse of the wire pattern. So our sticky stamp, with the peeled-off gold stuck to it, looks like this:

Figure: Stamp with Gold Stuck to Raised Areas

The areas with the gold stuck to it are the raised areas. Now our second problem: we know what the stamp needs to look like, how do we make it? Well, we are going to make it out of sticky rubber, (most stamps are rubber – even simple arts-and-crafts type stamps) so we need a mold. We will make a mold, and then pour our sticky rubber in the mold, let it cure, and then peel it out of the mold.

Now you know when you peel something out of the mold, the sunken areas of the mold become the raised areas of the part. In other words, the mold needs to be the inverse of the stamp. But the stamp was already the inverse of the pattern we wanted, so the inverse of the inverse is…

Figure: Mold (dark green) with raised resist (light green)

Back to where we started. Light green areas are rasied. Dark green is sunken. This pattern for the mold is the same pattern we want for the final product – with an inverse stamping step in between.

Now for the next problem. We know what we want this mold to look like for pouring in the rubber – but how do we make it? Well, say the gold wires we want are very tiny. One good way to make a very tiny mold, is to make it out of a plastic (much easier than a very tiny metal mold). So a common method of mold-making is to pattern tiny plastic features on a silicon wafer. How do we get tiny plastic features on a silicon wafer? We use a material called SU-8 resist. SU-8 is a liquid originally, and then when exposed to certain type of light, it polymerizes into a solid plastic. The reason the material is called a “resist,” is because once it is exposed to light and hardens, it resists being washed away. (For those who know their cleanroom lingo – a resist is also a material that protects the layer beneath it by resisting chemical etchants – but in this case, we don’t have layers, and the next step is just a chemical washing step.)

We start with a bare silicon wafer:

Figure: Clean Silicon Wafer

And then we coat it with SU-8 resist.

Figure: Silicon Coated with Layer of SU-8

What are we up to, fifth or sixth problem now? Man, this is a long process. We have a layer of liquid plastic on the wafer now, but how do we make it into hardened plastic features in the pattern we want for our mold? The important thing to know about SU-8 is that it is a negative photoresist. That means that the areas light HITS is where it hardens and STAYS RASIED. The areas in SHADOW from the light remain liquid, and can be washed away. (There are resists which are positive resists, which means that the areas the light hits are washed away, and the shadowed areas remain as the raised features). So what we need is a mask, that will allow the light to go where we want it, and make the SU-8 harden in the pattern we desire.

A mask is (as I said in the beginning) a clear sheet with patterned opaque areas. We need to design a mask that lets the light shine through where we want rasied features, and shadows the areas we don’t want in our mold.

So what will the mask look like? If you refer back to what we want our mold to look like, the mask will look like this:

Figure: Mask - Dark Areas Create Shadow, Clear Areas Let in the Light

And if you’ll notice, because SU-8 is a negative resist, that mask patten is the inverse of the mold pattern.

Whew! I think we’ve got it now. We’ve gone through these steps:

Gold Wire Pattern on Slide = Inverse Pattern on Stamp = Original Wire Pattern on Mold = Inverse Pattern on Mask

And just to show you that I mean what I say, here is the mask I just had made for me this week:

Figure: Actual Shadow Mask

There’s a lot of additional features on that mask because my research requires various things, but right up there at the top you see the pattern of wires that this post used. Now, if only I really could make Iron Man’s suit with that pattern… 🙂

So in the end, how do you design a mask, given the output pattern you want on the final product? It’s a long series of inversions, and depends on the manufacturing processes that you use. Just be sure to keep track of the steps. And if you have an asymmetrical pattern, remember that some of those steps require a mirror image as well as an inverse image. So keep those spatial skills sharp!

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