Determination of Striation Defect Formation Onset
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: