Josh Kvavle

Introduction

The purpose of my research was to fabricate air/dielectric interface diffraction grating couplers. This report will explain the obstacles that I encountered, and the resulting direction that my research took. In summary, soon after I began my research, a very expensive laser that I depended on for this research was sent away for repairs, so I began doing characterization of a polymer.

Initial Work

I began work on this research during the fall semester of 2004. My process had been characterized and I achieved promising results from the start. My process went as follows. First I coated a silicon wafer with photoresist and using holographic interference of an ultraviolet laser, patterned a diffraction grating into the photoresist. See my initial proposal for the motivations for fabrication of such devices.

Once the pattern was made on the surface of the wafer, I used a reactive ion etching machine to reduce the height and width of the diffraction grating, so that the silicon beneath the resist would be exposed, exposing it for proper adhesion of the dielectric film. This left me with a semi-sinusoidal grating resting on the silicon.

The next step was to deposit the dielectric, in this case Silicon Nitride was a good option, because of its relatively high index of refraction. The high-low index interface, we hoped, would create a higher grating coupling efficiency. The film was deposited with a Plasma Enhanced Chemical Vapor Deposition machine to the thickness of about 50 nm.

To remove the photoresist from beneath this film, we used the RIE to etch channels into the film all the way down to bare silicon. The exposed the grating beneath the PECVD film. Using a strong acid mixture called Piranha, the photoresist was etched out from beneath the film, leaving thousands of small tunnels in the Silicon Nitride. This completed the Fabrication of this air/dielectric/air diffraction grating.

To verify the results we used a scanning electron microscope, to get a cross section of the air gratings. The results came out positive. I had indeed fabricated these gratings. The cross section also showed that the etch time to reduce the height of the photoresist had been too long, and the air tunnels were only 5nm high, we were hoping for larger tunnels. My next step would be to characterize the process further and make diffraction gratings that could be used as couplers.

Equipment Failures

With those goals in mind, I began to again try and produce some diffraction gratings with the holographic laser interference process. The Laser that I used for this had some major problems, and I was unable to continue further with characterizing these air gratings for better results.

In the view of these “technical difficulties” which lasted the whole winter semester, and still hadn’t been resolved when I graduated, my mentor provided me with another task to work on.
Characterization of Polycarbonate

I was given some polycarbonate pellets and masters thesis from a previous student, and the charge to find how to mix these chemicals, and produce a usable polycarbonate film for a waveguide material. It sounds easy, but it turned out to be more difficult that I thought. I finally did end up find the right chemicals, temperatures, spin speed, and glassware needed for this characterization, and was able to mix up some polycarbonate, and began trying to use it in experiments.

One of the tasks that I focused on was to try and find a wet etch technique for this polycarbonate. It turned out that the chemical seemed to soak up the etchant and then peel off upon removal from the etchant. I could find no acid or solvent mixture that would reduce the thickness of this polymer without removing the film entirely or not affecting it at all.
Characterization of PMMA

Another task that I was given was to repeat the process that was developed a few years earlier on how to deposit a polymer called PMMA into the etched out core of a D shaped fiber. I was again given some chemicals, and some instructions. I had some initial problems with air bubbles forming when spinning this polymer, but finally found the right consistency, and was able to repeat the mixing of this polymer.

Conclusions

Research can be very interesting, and challenging. Often time when a tool isn’t available for use, research is either delayed or doesn’t happen at all. I learned the valuable lesson that it is important to be ready to change directions quickly when a project had obstacles that necessitate a change in course. The air core diffraction gratings may have been delayed, but I learned more about characterizing the process of chemical mixtures, and how to budget time for important research while working on other important responsibilities. My experience with Dr. Schultz as my mentor was great. He was encouraging, and gave me a lot of insight and guidance in the direction I should go. He was also patient with my failures, and helped to create new tasks when I needed them. Doing this research was a wonderful experience.