Recovery of stiction-failed MEMS structures using laser-induced stress waves

Stiction, or adhesion between suspended structures and the underlying surface, is a hurdle in batch fabricating long, freestanding MEMS structures. A novel technique is presented in this paper to release stiction. In this technique, a nanosecond rise time stress wave is launched on the backside of the Si substrate by impinging a 2.5 ns-duration Nd:YAG laser pulse onto a 3-mm-dia area. The compressive stress wave propagates through the Si substrate and arrives at the site of several stiction-failed cantilevers on the front Si surface. The compressive stress wave propagates through the cantilevered structures and is reflected into a tensile wave from their free surfaces. The returning tensile wave pries off the interface, releasing the cantilevers. The procedure is demonstrated on a MEMS chip with stiction-failed cantilevers with varying lengths from 100 μm to 1000 μm. The threshold laser energy to release stiction increased linearly with cantilever lengths. Beam recovery began at a laser fluence of 11 kJ/m² laser energy. 70% of the tested beams had been recovered after impingement with a fluence of 26 kJ/m². After the highest applied laser fluence of 40 kJ/m², 90% of the tested beams had been recovered. No damage to the structures or surrounding features was observed below 40 kJ/m². Because of rather low laser fluence, no thermal damage to the back surface of Si was noted. Since it literally takes few seconds to release stiction, the proposed technique can be implemented in MEMS foundry, and for repair of in-use stiction failed MEMS devices.