Laser-assisted stress reduction in molybdenum microstructures for CMOS compatible MEMS integration

Abstract

The fusion of micro-cantilevers over complementary-metal-oxide-semiconductor (CMOS) wafers (with driving electronics) is essential integration requirement for the development of portable sensing devices. In general, the stress gradient in metal cantilevers (after release) lead to structure warping and limits the performance and stability of the final fabricated devices. This work is focused to solve the requirement of (i) having a stress-free metal based micro-electro-mechanical structures (MEMS) without using high-temperature annealing technique and (ii) process compatibility with vapor HF release. We demonstrate a laser-assisted stress controlling scheme to achieve free-hanging metal cantilever without affecting the underneath (essential requirement for CMOS-MEMS integration) deposited layers. The molybdenum (Mo) metal is deposited by physical vapor deposition (PVD) technique at the temperature of 200 °C and then patterned to obtain the microcantilever beams of different length. High-pulse-energy excimer laser (wavelength =248 nm with pulse duration =24 ns) is used to anneal these metal cantilevers and their residual stress is measured by analyzing the final displacement of the released microbeams. Different laser energy density ranging from 200 × 10−3 J/cm2 to 450 × 10−3 J/cm2 is used to study the optimized parameters in the annealing process. Proposed pulsed laser-annealing (PuLA) technique reduces not only the stress gradient but also aid stress-tuning ability that has numerous advantages especially in electrostatic and piezoelectric MEMS devices. Furthermore, we also present the impact of laser action on the grain microstructures and electrical conductivity. Results show that molybdenum microstructures with localized laser heat treatment achieve a reduced surface roughness (from 7.5 nm to 3.8 nm) and improved electrical conductivity. Results demonstrate that the proposed PuLA scheme is a promising technique for (i) eliminating stress in metal interconnects in CMOS-MEMS integrations, (ii) building a stress-free and smooth surface micromechanical platform for N/MEM (resonators, sensors and actuators) devices and (iii) establishing molybdenum as an anti-diffusion barrier in Cu interconnect technology owing to its excellent bulk property.