Monday, July 27, 2009

A Switch to Ellipsometry

Since I am running a bit short on time, we decided to move this setup today (Wednesday) from the spectroscopy setup to the ellipsometer setup to try and characterize the beam. Since the beam was probably too wide at the waist (in between the two parabolic mirrors), we placed the Si wafer in front of the receiver to look at the distribution at this position. A sketch of this is shown below (a very rough sketch... just to show placement of components).

The dotted black line again represents the excitation beam, the pink components represent the lens and lens arm, which are attached to the translational stage, shown as the grey square. The darker grey squares are the emitter and receiver for the THz signal, the blue ovals are the parabolic mirrors, and the black disc represents a mirror which the excitation beam reflects off. Finally, the grey disc again represents the Si wafer.

After taking an x- and y-lateral scan over the Si wafer, I found some very interesting looking profiles, especially in the x-direction. A plot showing both directional scans is shown below.

Clearly the x-direction is very strange, while the y-direction almost looks like an upside-down parabola. The features in the x-directional scan, I think, help to reinforce the idea that perhaps the incident angle does not have much effect on the profile of the beam and that possibly the alignment of the actual microscope is what should be adjusted in order to correct any sort of skewness. In terms of the y-direction, it would be nice to take a wider range of data; and by this, I simply mean having more data points on the edges of the scan. I hope to do this tomorrow after adjusting the lens and stage.

Overall, it is very interesting to see such an odd profile in the x-direction -- and I think the most interesting thing is that the scan is extremely smooth, as opposed to some more "rigid" scans.

I also should mention that the lock-in was much less noisy in this setup than in the spectroscopy setup. Perhaps this has to do with the THz setup in general, the excitation beam, or something else. Also note that the relative amplitude is much less than what we saw in the spectroscopy setup, where we would see amplitudes of 4 or 5 mV, here we see just over 2 mV.

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