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Film Stress Principle

Thin Film Deposition when a thin film is deposited onto a substrate like a wafer, it is usually done at elevated temperatures. After deposition, the substrate cools down to room temperature. The differences in thermal expansion coefficients between the film and the substrate causes mechanical stress.

This effect and the intrinsic film stress may cause the wafer to bow as well as induce cracks, voids, hillock formation and film lifting, yield loss and poor reliability issues.

Measurement Principle

FSM pioneered in the laser scanning technique for stress measurements. Film Stress measurement is based on Stoney's Equation, where the change of radius of curvature for the wafer is measured by a laser scanner. This is done in 2 steps. Typically, a bare wafer is prescanned ( First Scan ) followed by Film deposition, The wafer is remeasured in a postscan ( 2nd scan ). The radius of curvature change is then calculated

Types of Film Stress Tools

1) Ambient Systems : Monitors residual stress after a film deposition at room temperature. Typically this is used in R+D and Production Monitoring

2) Variable Temperatures: Here Film Stress is monitored during a heat cycle at a user selectable ramping and cooling rate. FSM provides tools for variable temperature Stress Hysteresis Studies from ambient nto up to 1000 degrees C. Such variable temperature tools are typically used for thin film characterization and new process development.

FSM also offer variable temperature tools in standard resistive heating chuck ( FSM 500TC) , or for more advanced research, measurements are done in sealed Rapid Thermal Annealing chambers, which are designed for operation under vacuum or inert gas environment( FSM 900TC-vac)

Applications

FSM provides tools to measure wafer topography with 2D and 3D maps, before and after thin film deposition to characterize film stresses. FSM provides tools to measure wafer topography from room temperature to 1100 C, under vacuum, to characterize stress hysteresis, and materials desorption at elevated temperatures.