William Ray Evans and Dr. David Allred, Department of Physics and Astronomy
The Extreme Ultraviolet (EUV), that portion of the spectrum with photon energies of ~30 to ~300 eV is a scientifically interesting part of the spectrum for many reasons.
First of all, there are many applications for EUV optics. The desire to continue “Moore’s law” development of faster, denser, and more powerful memory and logic chip is demanding lithography capable of making the smaller lines. EUV optics have the promise to help make this possible. Biologists and medical scientists are also finding ways to use EUV microscopes to better analyze individual living cells. Astronomers have always been interested in studying the sky with whatever methods they could use. For a long time EUV astronomy was impossible, because EUV light does not penetrate the atmosphere. However, now, with space-based telescopes, EUV astronomy is becoming a reality.
Secondly, the EUV is interesting for basic science reasons. Its interaction with matter is not well understood. In order to take advantage of EUV optics, we have to be able to build optical components (like lenses and mirrors) that are reliable in the EUV. Most materials are opaque in the EUV, so lenses really aren’t an option. Mirrors aren’t much easier, but they are possible if we understand the optical properties of different materials well. The trouble is there aren’t many materials that we understand well in the EUV.
The BYU EUV research group has been studying a number of different materials as potential EUV reflectors. The materials we have most recently been working with are thorium and thorium dioxide. Some of our earlier work has shown that these should be very good reflectors in certain parts of the EUV. We have been studying different ways to determine the optical constants for these materials.
My project was to determine the effectiveness of a different method of depositing our thin films, possibly changing the optical constants for our films, hopefully making them more reflective. This method is called Biased Sputtering.
Sputtering is the method we use to deposit most of our thin films. Basically, in our case, we mount a piece of thorium metal onto a “sputter gun.” This is a vacuum chamber in which we create strong electrical fields. We place the gun in a vacuum chamber with the samples we want to coat. We then pump all the air out, and allow argon gas in. The high voltage difference between the negative and positive parts of the chamber ionizes some of the argon gas, turning it into a plasma. The negative voltage impressed on the thorium “target” extracts argon ions and accelerates them towards the thorium. The energetic argon ions “sputter” atoms of thorium off of the metal, coating surfaces in the line of sight of the target, including the samples.
Normally, the sample holder and the chamber are electrically grounded. However, we can also set the sample holder to a negative “bias voltage” (hence “biased sputtering”). This negative bias voltage pulls positive argon ions out of the plasma and slams them in to the film. The analogy is cold-working a piece of metal, pounding it to make it smoother, denser, and harder. In the EUV and x-rays, higher densities yield an n, the index of refraction, which deviates more from vacuum. This is what I wanted to test, whether or not our bias voltage would be able to noticeably change our values of n.
There are some problems, however, with trying to study a large number of samples in the EUV. First of all, EUV light sources are not common. To study our samples, we usually go to the Advanced Light Source in Berkeley, CA. This requires a lot of time, preparation, and money. Also, when dealing with short wavelengths of light, any contaminants or roughness in the mirror can seriously alter our measurements. These are problems that would have greatly increased the complexity of the project.
We found a possible alternative. Instead of studying the films in the Extreme Ultraviolet, we could study them in the Visible and Near Ultraviolet. This is a very different part of the spectrum, and the optical constants would be very different, but we were investigating was a change in optical constants, which should be apparent even in the Visible. At BYU we have several Spectroscopic Ellipsometers, which are made to study optical properties of different materials in the Visible and Near UV. In using these longer wavelengths of light, we wouldn’t have to worry as much about roughness and dust. (We still have to worry about them, but not as much, and it’s not as complicated.) With the ellipsometers, finding the optical constants from the measurements is also straightforward.
We made and measured nine different samples. They were deposited at a variety of bias voltages between 0 V and 75 V, and at a variety of thicknesses. We found that, within the limits of our techniques, there was no relationship between bias voltage and n. It appeared that our bias voltage didn’t change the films at all. The samples with different thicknesses also had the same values of n. We also found that our values of n matched well with what had been published before. It is possible that higher voltages might cause a change, but above 75 V, the electric potential started to discharge through our plasma, damaging our samples. It is also possible that there was some error in our fitting technique, but we do not believe that to be significant due to the uniformity of our results.
This research has served as the basis of my senior honors thesis, and will be presented at the 2006 BYU Spring Research Conference as well as the 2006 International Conference on Metallurgical Coatings and Thin Films, and will be published in the peer-reviewed journal Thin Solid Films. In the end, we have reached a much better understanding of the optical properties of reactively sputtered ThO2 thin films. We also determined that biased sputtering does not appear to alter the optical constants of ThO2. We have also found what we believe to be a useful technique in measuring the optical constants of thin films in the visible by means of spectroscopic ellipsometry.