The Tencor FLX-2350FP is designed to measure the shape change caused by the film deposition and calculate the corresponding film stress. The stress is calculated from the curvature change caused by the addition of the film. The key point is that the FLX tool measures the curvature of the wafer caused by the film deposition. The film stress is calculated using elasticity equations. Since stress is force normalized by area, it is a field variable, and therefore it cannot be measured directly; only the effect of stress is measurable.
The tool determines stress by measuring the curvature change of pre- and post-deposition of the film. This difference in curvature is used to calculate stress by way of Stoney’s equation, which relates the biaxial modulus of the substrate, thickness of the film and substrate, and the radius of curvatures of pre- and post-process. Curvature is measured by directing a laser at a surface with a known spatial angle. The reflected beam strikes a position sensitive photodiode. The geometry of the film is recorded by scanning the surface.
- Fim stress measurement
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