Static fatigue – crack growth causing delayed failure under constant stress and involving stress corrosion cracking – was investigated in polysilicon MEMS using two different surface-micromachined devices. One exploited residual tensile stresses to create stress concentrations at micromachined notches, and the other involved a single-edge notched beam specimen integrated with an electrostatic comb-drive microactuator. Tests with both devices revealed that polysilicon is not susceptible to static fatigue in humid environments. However, when a relatively thick (45 to 140 nm) surface oxide was thermally grown on the microactuator devices, the specimens demonstrated delayed fracture in a humid ambient, presumably due to static fatigue of the surface SiO2. The stress intensities at the resulting cracks in the SiO2 were then sufficient to cause catastrophic crack propagation through the polysilicon specimens. The implications of our data on the issue of fatigue in polysilicon is discussed.

A commercially fabricated aluminum nitride chip carrier was evaluated for packaging various types of MEMS inertial sensors. They were successfully assembled and vacuum-sealed within AlN chip carriers and their pressures have remained stable for over one year. Aging tests were conducted under electrical bias at 85°C and 85 %RH. The leakage currents were not as stable as those measured in alumina chip carriers and post test inspection of the AlN parts revealed etching of the ceramic between conductors.

The present work describes the selective titanium oxide formation based on electron-beam (e-beam) induced carbon deposition used as a mask against chemical dissolution. Under ideal conditions the C-deposits act as a negative resist to hinder the titanium oxide dissolution at e-beam treated locations. Carbon patterns were written in a scanning electron microscope at different electron doses on titanium oxide electrochemically grown in 1 M NaOH by potentiostatic experiments. Subsequently, the untreated areas were chemically removed in 0.5 % HF leaving the C-protected TiO2 patterns at the Ti surface. The selectivity of the technique depends on several factors such as the electron dose during masking and the chemical parameters. Under ideal conditions, it is demonstrated that direct patterning of surfaces as well as the fabrication of microstructures can be achieved using such technique.