Bringing Silicon Microsystems to Space : Manufacture, Performance, and Reliability

The incorporation of extremely compact multifunctional microsystems is a highly profitable long-term approach in spacecraft design. These systems bring substantial launch-cost reductions, and enable exciting space exploration and science missions. Silicon microsystems technology is an adequate choice for the multifunctional microsystem development. However, the development of basic microsystems technology cannot be financed within application-specific space missions. Rather, the microsystems technology should be matured through fundamental research. Silicon microsystems technology was used to develop a cold gas microthruster system suitable for minute movements of spacecraft (low Δv). In a hybrid integration, the system unit contains three silicon microsystem parts with four individual thrusters in total, together with external control electronics. The total mass is 0.35 kg. Further integration will result in a mass of 0.08 kg. Complete system integration means that all package and interconnection levels are integrated into the silicon microsystem units. Several vital issues must be addressed, e.g. the reliable bonding of silicon wafers, the microfabrication process compatibility, and the manufacture process sequence. A graphical tool is introduced for process sequence evaluation. Wafer bonding is used as fabrication process, assembly tool, and packaging technique. The quality and reliability of the bonded interfaces must be assessed in order to secure the operation of the microsystems in space. Therefore, statistical methods for burst test evaluation have been developed. Weibull fracture probability functions have been derived in order to interpret the bond quality. In addition, rank-sum tests on spot series and analysis of variance are performed for bond quality diagnostics. The dependence on annealing temperature and surface-activation are presented, together with diagnosed degradation of insufficiently annealed bonds due to different spaceflight environments (thermal cycling, vibration, γ-irradiation)