CDMS at Case Tungsten Thin Films

What and Why?

At Case, we study the superconducting properties of tungsten thin films. A superconductor is a material that has zero electrical resistance across it when its temperature is below its critical temperature (specific to the material). A thin film is a layer of material that is no thicker than a micrometer. CDMS uses tungsten thin films in the creation of the transition edge sensors for the phonon detection part of our WIMP detectors. When a particle collides with an atom in the detector, it deposits a small amount of heat. The small amount of heat brings the tungsten above its critical temperature, drastically changing the resistance across the film (from zero to its non-superconducting resistance). This drastic change in resistance is part of what we measure.

The goal of our tungsten thin film research at Case is to study how varying different parameters in the thin film growth (A.K.A thin film making) process affects the thin film's critical temperature. With a better understanding of these films, CDMS-II could fabricate its detectors more quickly and more economically by improving predictability and uniformity of the critical temperature.

Growing Thin Films

We grow our thin films in Room 12 of the Rockefeller Physics Building. This room is a cleanroom shared by multiple research groups at Case and is overseen by Professor Kathy Kash.

There is more than one way to grow thin films; at Case, we use a Plasma Sciences, Inc. sputtering machine (see picture below). Silicon wafers are placed on a rotating stage inside a vacuum chamber. The chamber is pumped on by a turbo pump to reach high vacuum pressures. The chamber is then backfilled with argon gas and a potential difference is created between the silicon wafer and a "target" - in our case, tungsten. The argon atoms ionize and are accelerated toward the target until they collide with the atoms of the target. Through the exchange of momentum, a thin, uniform layer of the target's atoms are showered onto the wafer. You can learn more about this process by reading about it at Wikipedia's sputtering entry.

Testing Thin Films

The critical temperature of each thin film that is grown is found by mounting it in our dilution refrigerator and taking four-wire resistance measurements while varying the temperature. This data clearly shows at what temperature the the thin film becomes superconducting (see example of data below).

We are also intersted in studying how the critical temperature might change with time. We have taken data over a timespan of a few months to see the effects.

We also use X-ray diffraction to determine what phase the tungsten is. There are two phases of interest for us, alpha and beta, each having different critical temperatures. X-ray diffraction is a tool that can be used to predict the critical temperature without ever having to actually manipulate the temperature of the film.

Conclusions

Our tungsten thin film study is in its early stages, but progress has already been made in learning how to vary the critical temperature of the films and how to grow thin films with more predictable critical temperatures. Much of this research was done for the "senior projects" of two physics majors at Case. You can learn more about our work by reading Brandon Bachler's senior project report.

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