Using the laser diffraction method described on another page, we were actually able to look for striation development in real-time during spinning. We configured the input laser on one side and used a video camera to capture the diffractogram in real-time (1/30 second time increments). The spinning wafer was located at the point labeled "Striated Wafer" and the laser was directed at a point away from the center.

At the same time a different laser was directed at the center of the wafer for measuring the fluid thinning and evaporation behavior. This interferometric fluid measurement technique is described elsewhere.

A typical thickness vs. time plot determined from interferometric monitoring of the thinning fluid behavior is shown below.  This particular curve shows the thinning of an SOG spin coated at 3000 rpm.  The vertical line represents the time at which striations began to develop as determined from frame by frame analysis of the video monitoring of the diffraction pattern as described above:
 

This onset time correlates very closely with the point when fluid flow is starting to become less important in controlling thinning behavior. After about this point, solvent evaporation contributes more thinning than the fluid flow -- thus striation defects form chiefly during the "coating drying" stage of coating formation.

This work was presented at the American Ceramics Society Annual Meeting, in April 2000. Further details of this work can be found in our conference proceedings paper:

 
D. E. Haas and D. P. Birnie, III, “Real-Time Monitoring of Striation Development During Spin-On-Glass Deposition”, to appear: Proc. Am. Ceram. Soc. Symposium on Sol-Gel Processing, (2000).