Chuck Marks in Sol-Gel Coatings

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Chuck Marks in Sol-Gel Coatings
Dunbar P. Birnie, III

After noticing puzzling color variations (indicating thickness difference patterns) in sol-gel coatings that we had been depositing on glass (to make multilayer interference filters and other devices) --- see photo at right + click to enlarge --- we (myself, in collaboration with Prof. Zelinski and with the help of Dr. Melpolder of Kodak) designed experiments to reach a better understanding of how these thickness patterns formed during the spin-coating process.The figure at left just below shows some subtle patterning in coating appearance that formed on a glass wafer, where the pattern of back-side-contact with the vacuum chuck matches the pattern shown. All areas were transparent, but showed rather subtle reflectance appearances. Ellipsometry measurements determined that the darker areas were physically thicker than the lighter areas in this picture. The darker (thicker) areas corresponded with areas that had good physical contact with some material in the vacuum chuck. Areas that were above vacuum grooves or were prevented from making physical contact (e.g. by hanging out over chuck edge) were thinner.

        For comparison, the figure at the right (slightly different magnification) shows the appearance of the vacuum chuck that was used when making the coating shown above (the ruler is in inches). The vertical and horizontal lines are tiny grooves machined in the flat metal surface; the o-ring protrudes slightly above the level of the metal mesa, thus preventing good wafer/chuck contact in the outer region (other than where the o-ring touches the wafer), so the cross-shaped chuck mark feature is mostly evident near the center of the wafer.
        The most unusual part of these thickness variations is that there was never physical contact on the top side of the substrate, only contact on the back side using the vacuum chuck of the spin coater. Thus, there must be some form of "communication" between the metal in contact on the back side and the coating solution on the top side.
       Our experiments tested the effect of substrate type (glass, silicon, and thin polymer) on coating uniformity, especially in patterns related to the vacuum grooves in the chuck holding the wafers in place during spinning. In addition, we tested spin duration, solution temperature, and firing temperature.

This figure shows the chuck mark created when coating the very thin plastic substrate. This material was much thinner than the glass sample shown above, so it was able to flex down and form better contact with more of the metal vacuum chuck parts. One assumes that the areas above the vacuum grooves was also bowing down somewhat as well. Interestingly, these areas over the vacuum lines were still the thinnest parts of coating on this sample (i.e. fluid was not simply being collected in the low spots --- on the contrary in fact!). Because the much thinner substrate allowed better thermal conductivity "communication" between the coating solution and the vacuum chuck, then the chuck mark was printed with much higher detail -- presumably because local temperature differences were more pronounced.
In general, we found that coatings that were on substrate areas that had their back side in direct contact with the metal of the vacuum chuck were systematically thicker than regions over vacuum grooves. We also noted some differential edge effects where corners and edges of these metal regions had even thicker coatings. These two observations were suggested to be attributed to solvent evaporation effects. Our final explanation was that solvent evaporation was producing noticable cooling of the solution and wafer top-side and that this surface cooling was being counter-balanced by heat flow effects from the metal vacuum wafer holder. And, rather minute temperature differences from place to place across the substrate surface were impacting the thickness of coating formed from our sol-gel solutions. In other words:

        (1) areas without metal contact were more significantly affected by the evaporative cooling effect,
        (2) the lower temperature in these areas caused a lower evaporation rate there too, and
        (3) this lower evaporation rate translated into lower end-point coating thicknesses.

This explanation is completely compatible with Meyerhofer's seminal model that explained the connection between solvent volatility and final coating thickness.

The above findings are described in more detail in:

D. P. Birnie, III, B. J. J. Zelinski, S. P. Marvel, S. M. Melpolder, and R. Roncone, "Film/Substrate/Vacuum-Chuck Interactions During Spin Coating", Optical Engineering, 31, 2012-2020 (1992).