Penrose Design for e-beam lithography

This week I was working with a magnetic force microscopy (MFM). It’s like an Atomic Force Microscopy (AFM), but can also see the magnetic domains of a sample… I suppose I also need to explain what an AFM is… an AFM is a type of Scanning Probe Microscopy (SPM). (I know, in this job abreviations are everywhere).

An SPM It’s a microscopy that takes pictures of samples aproaching a very small needle to the surface of the sample. Because the end of the needle has only one atom, It can sense the atomic force of the sample surface.

The next video is something just amazing. It’s a SPM working inside a Scanning Electron Microscopy. So you can use the electron microscopy to see how the SPM works!

Well, I was working with one of these when I found something in the sample I have been studying. A mark in one of the corners! This is the mark. In the left side is the surface and in the right side is the magnetic domains inside the mark.

One of the most beautiful things I have ever saw. And, probably there is not too many images of that around there (and probable any with the magnetic domains). Do you realize the size of the human hair in comparison? And want to know the most amazing thing? We made it, it’s something that was there intentionally.

I wasn’t aware of it because is a part of the design used by the people in charge of fabrication of the samples. It’s called Penrose-like design for alignement or electronic beam litography. This is another example of how they will look alike.

Ok ok, very funny. Cool and whatever you want. What is a Penrose design and why they need it?

To keep it simple, a Penrose Tiling is an aperiodic tiling of the space. Just that. It’s more complex than that, but it’s just what we need now.

And why it’s usefull here?

Well, according to wikipedia it’s main properties are:

• It is non-periodic, which means that it lacks any translational symmetry.
• It is self-similar, so the same patterns occur at larger and larger scales. Thus, the tiling can be obtained through “inflation” (or “deflation”) and any finite patch from the tiling occurs infinitely many times.
• It is a quasicrystal: implemented as a physical structure a Penrose tiling will produce Bragg diffraction and its diffractogram reveals both the fivefold symmetry and the underlying long range order.

Hmm that means two things. The first one is that if we use x-rays to watch this tilings, we will see a very precise pattern that will reveal us if the tiling is really a Penrose pattern or not. And the second thing has to be with the simetry. Being aperiodic means that there is no way of move or transform the pattern an actually make it look like himself.

And why these properties are good for making chips?

It has to be with the way we do chips. The way of doing chips implies preparing a pattern with the devices we want to impress into a silicon wafer. To do that, we use a electron beam to create a picture of the devices into a substrate by removing parts of it (e-beam  lithography).

This process is very complicated and requieres extreme precision. And that is where the Penrose designs are being used. The Patterns are added to the design of the chips to allow the calibration and allignement of the e-beam system. In the video they only show one process of lithography, but what happens when you have to repeat it? You need a way of knowing where exactly where you are. And the Penrose design is there for it. If you want to read more about it, here is a Thesis from the university of Glasgow 2009 about it.

Amazing.

Almost forgot. If you want to play with Penrose tilings, here there are two nice programs to use:

http://stephencollins.net/penrose/

http://condellpark.com/kd/quasig.htm