Significance of coating stress on substrate bow in large area processing of MCM

The substrate bow in a large area processing (LAP) was simulated using a finite element analysis (FEA). The structures considered were aluminum (Al) and glass substrates of various thicknesses, and a coating from Photosensitive film thickness. It was found that the deflection of a large area substrate, e.g., 400 mm², could not always be obtained from the linear, small deflection theory even if the curvature might be small and the stress-strain behavior in the linear elastic regime. In this case, the nonlinear, large deflection theory had to be adopted. Also, the gravity effect from the substrate weight turned out to be very significant and had to be incorporated as well. The simulation incorporating these two factors agreed well with the experimental data, which was generated by spin coating and curing the BCB formulation on Al substrates, 400×400×1.27 mm. As a means of flattening out the curvature, subjecting a vacuum underneath the substrate was simulated. Significant reduction of the substrate deflection was observed by applying only a very small vacuum. This result suggested that the use of double-stick tape on the bottom of the substrate, for example, might also of feasible to completely eliminate the bow