New ways to determine film thickness

Tuesday, July 10, 2007

Today we dissolved a white polystyrene foam "hot cup" in toluene, then spin coated a slide quite thickly with it. We increased the thickness of the film by decreasing the spin speed of the spin coating instrument. Thickness, of course, is relative. Normally, our 1.4% polystyrene solution is spin coated onto the glass slide at 120,000 rpm to a thickness of about 100 nm; with the dissolved polystyrene foam cup, we were trying to achieve a thickness of about 1 micrometer. We reduced the RPM to 600,000 for one film and 300,000 for the second film. We then used an instrument called a Profilometer to determine the film thickness. This instrument employs a different principle and mechanism than the X-Ray reflectometer, and it is only successful at measuring thicknesses approximating 0.5 micrometers.


After spin coating the film onto the glass slide, a razor blade is used to make a scratch in the surface of the film down to the glass slide's surface. The slide is then placed in the Profilometer, and the stylus of the instrument is placed in contact with the surface of the film. The path of the stylus across the film is determined with the help of a computer and the instrument software. The stylus stays in contact with the film surface until it encounters the scratch, when it descends into then up from the furrow created by the razor blade. The differences in depth are displayed on the computer screen, and the operator selects the points at the top of the film and at the bottom of the furrow (glass surface) to be compared. Once the point selection is complete, the depth of the film is calculated. The film I prepared was 0.936 micrometers thick..not bad for a first try!






Here is more information on how a profilometer works.

The rest of Tuesday was spent preparing films of the 1.4% polystyrene and measuring their thickness using the X-Ray Reflectometer. Each scan, after the zeroing process detailed yesterday, takes 90 minutes. This makes for long days!