Vacuum and Hermetic Packaging of MEMS using Solder

Abstract

This work explores the use of solder as a material for wafer-level vacuum packaging of MicroElectroMechanicalSystems (MEMS). Two bonding techniques were developed and characterized: a standard solder bond and an advanced solder bond based on transient liquid phase (TLP) bonding. Solder was also used as a release layer as well as bond layer for forming transferred thin-film packages. Several different standard solder alloy / under bump metallization combinations were used for wafer bonding. Only the Au-Sn solder alloy, with its low tin content, proved to be compatible with the thermal limitations of commercial wafer bonders. The bond is formed at 300 °C in under an hour and has a shear strength of 28 MPa. It was used to create packages (2.3 mm X 2.3 mm X 0.5 mm) with integrated Pirani gauges. The pressures were as low as 200 mTorr and showed a worst-case leak rate of 1.5.10-15 atm.cc.s-1. TLP solder bonding was investigated because it is more compatible with the long thermal time-constant of commercial wafer bonders. Au-In and Ni-Sn TLP solder bonds were used to create vacuum packages (2.3 mm X 2.3 mm X 0.5 mm) with integrated Pirani gauges. The Ni-Sn and Au-In packages were formed at 300 °C and 200 °C, have measured shear strengths of 12.4 and 24.4 MPa, showed package pressures of 200 mTorr and 150 mTorr, and worst-case leak rates of 1.7.10-15 atm.cc.s-1 and 0.1.10-15 atm.cc.s-1. The design rules for creating bonds with these techniques are presented. Outgassing and getter activation were studied. Package pressures were reduced to 20 mTorr by outgassing for 24 hours before bonding. It was shown that titanium getters can be activated at 200 °C, enabling a MEMS vacuum packaging process with a maximum temperature of 200 °C. Solder was used to transfer thin-film electroplated nickel packages as small as 250 μm wide, 250 μm long, and 20 μm thick. A thin nickel film was electroplated over a lead-free solder transfer layer on a carrier wafer and then simultaneously bonded and transferred to a device wafer at 300 °C for 1 hour with a yield of greater than 99%.