Thin film stress measurement
|Thin film stress measurement
Thin film stress measurement is calculated based on the change in radius of curvature of a substrate that is caused by the stress in a deposited thin film.
The Flexus 2320-S uses laser light sources to scan the surface of the wafer in order to measure the radius of curvature of substrates/wafers.
The Dektak XT uses a stylus in contact with the surface of the wafer to trace the diameter in order to measure the radius of curvature. The Flexus is the primary tool to use for extracting film stress values. The Dektak XT can be used for samples that can't be measured using by using the Flexus (transparent substrates or very rough, non reflective surfaces).
Method of operation
Thin film stress is calculated by measuring the change in radius of curvature of a substrate caused by the deposition of a thin film on the substrate. The thin film must only be on one side of the sample (front), because if it is on both, it will balance the stress. This requires measurement of the original substrate radius of curvature followed by a second curvature measurement after the film of interest has been deposited on the front (single side) of the substrate. If the film is deposited on both sides of the substrate, it must be removed on the back.
- Thin film stress can be an important parameter when fabricating devices. If film stress is too high, the film may have delamination and adhesion issues.
- For devices where free standing beams or membranes are released, stress of the released film or film stack is critical. For beams (cantilevers), stress gradients through the thickness of the film will cause the beam to curve. For released membranes, a slightly tensile film stress is desired since compressive stress will cause membranes to buckle.
-  Toho Technology Stress Measurement Theory