Imposition of defined states of stress on thin films by a wafer-curvature method; validation and application to aging Sn films

• A wafer-curvature method has been developed to subject thin films to defined loads.
• Two X-ray diffraction techniques were employed for the analysis of stresses.
• The wafer-curvature method was validated by application to a W film.
• Application to a potentially whiskering Sn film did not lead to whisker growth.
• The study confirms that stress gradients are essential for inducing whisker formation.

A wafer-curvature method has been developed to subject thin films, deposited on (Si) substrates, to well defined and controllable loads in a contact-free manner. To this end, a custom-made glass pan (i.e. a roof-less cylinder with a connection piece for vacuum tubes) connected to a needle valve, a vacuum pump and a pressure gauge has been used as an experimental setup. By fixing the coated Si wafer on top of the glass cylinder and evacuating the glass cylinder to a defined low-pressure, a state of stress is imposed in the thin film due to bending of the wafer. It has been shown that the (initial) stress state of a film and its change, due to its bending with the help of the wafer-curvature method, can be analyzed accurately close to the wafer center by application of one of two independent X-ray diffraction techniques: i) conventional X-ray diffraction stress analysis (i.e. application of the well known sin2ψ-method) to reflections originating from the film and ii) determination of the radii of curvature by rocking curve measurements utilizing reflections originating from the substrate. The validation of this stress-imposition method has been carried out with a tungsten film of 500 nm thickness, since tungsten is known to be (practically) intrinsically elastically isotropic. Further, the method has been applied to an electro-deposited, potentially whiskering, aging Sn film of 3 μm thickness where a combination of both stress-measurement techniques is essential for the determination of initial and (by wafer bending) imposed stresses. The results of the aging experiment of the Sn film under load have been discussed with respect to the current whisker-growth model.