My Research

Well, I’ve finally gotten my first research paper submitted and accepted. It will appear in this year’s Proceedings of SPIE. The title of the paper is “Drag-a-drop: a characterization tool for immersion lithography.”

So my friend Mitch asked me, “Have you reached the stage where you can describe your work in English? If so I would love to know what it is about. ” I’m not sure how to describe it consisely to someone that doesn’t have a basic knowledge of optical lithography, but I’ll give it a try.

There is a good article in Wikipedia that outlines photolithography in general, and another on immersion lithography, which is my current area of research. The first section of the photolithography article isn’t too complicated, so that should give the uninitiated a general idea of how it works. The main idea in immersion lithography is that the size of feature you can make on the silicon wafer is limited by several factors, including the wavelength of light you are using, and the refractive index of whatever is in between the lens and the wafer. The industry has just about reached the limit on getting a smaller wavelength of light (currently an Argon-Flourine laser of 193 nm) and so they are looking to increase the refractive index of the medium between the lens and the wafer. This medium is generally air, which has a refractive index of 1.0. So in place of the air, they have tried injecting water between the lens and the wafer. Water has a refractive index of about 1.3 at 193 nm, so this can give a substantial improvement on feature size.

So now we come to my research. Injecting a drop of water (or some other fluid) causes several new problems. Before you could move the lens over the wafer as fast as you wanted, since the air between doesn’t really care how fast you are moving. With water between the lens and the wafer though, things change. If you move too fast, the water will break up. The stability of the water depends on such properties as the height of the lens, the diameter of the lens, the coatings on the wafer and on the lens, the surface tension and viscosity of the water, etc. My research is to try and understand this system to better optimize the immersion lithography process.