Jeffrey D. Wilbur
	Graduate Student Researcher B.S. Chemical Engineering, 2002 Yale University Research Interest: Research | Resume | Publications
Jeffrey D. Wilbur 201G Gilman Hall UC Berkeley  Dept. of Chemical Engineering Berkeley, CA 94720 USA Lab Phone 510-643-5037 Email Me!

Research Description:

I am developing a new technique for characterizing the grain structure of block copolymer thin films, which we call guided wave depolarized light scattering (GWDLS).

There are several extant techniques for studying grain structure in this type of film in position space (cross sectional TEM, SEM, AFM), and some work has been done to study the evolution of grains over time using time lapse AFM [Harrison et al. Macromolecules 2000, 33, 857], but there exists no complimentary reciprocal space technique. GWDLS is intended to fill this void. It is based on the well established bulk depolarized light scattering technique, in which the tendency of block copolymer samples with anisotropic grains to rotate the polarization of light that passes through them can be used to find the average grain size and shape within. As a reciprocal space experiment, it provides spatially averaged data on grain structure over the (1 cm) interaction path length. In order to gather comparable statistically significant data, a very large number of images would need to be collected using the above position space techniques. This precludes the statistical study of grain structure over a large area in real time, since these techniques are unable to capture more than a few square microns without losing the resolution necessary to identify and distinguish between grains. We intend to use GWDLS while performing in situ anneals on block copolymer thin films to give us an instantaneous measure of the grain structure within, in order to better understand the kinetics of the ordering process and how it is affected by the large interfacial area of this confined geometry.

The key to GWDLS is achieving a significant interaction path length between our laser beam and the sample, which is under 1 micron in thickness. Instead of passing the beam straight through the film as would be done in the analagous bulk experiment, the light instead is sent through the film laterally, allowing us to arbitrarily choose a path length for scattering. Getting light to travel in this way through a sub-micron film is not a trivial task. Our technique uses the film as an optical waveguide, which is the same type of optical structure used in fiber optics cables. The polarized (trans-magnetic) light is coupled into one side of the film, and then travels within the film, confined between the top and bottom interfaces because its angle of incidence upon them is less than the critical angle for total internal reflection. As it passes through the film, any anisotropic grains present will scatter some of the light into the perpendicular (trans-electric) plane of polarization. The amount of light depolarized is related to the average size of these grains. The signal is then coupled out of the film after about 1 cm of scattering, and a crossed polarizer and detector are used to measure the fraction of light depolarized by interaction with the film, giving us a information about the grains through which the light passed.

We have proven the efficacy of this technique and performed a preliminary theoretical analysis. We are now working toward enabling real time measurements during anneals, and further developing our theory to determine grain size and shape from our data.

 
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