An overview of wafer-level packaging technologies developed at the University of Michigan is presented. Two
sets of packaging technologies are discussed: (i) a low temperature wafer-level packaging processes for
vacuum/hermeticity sealing, and (ii) an environmentally resistant packaging (ERP) technology for thermal and
mechanical control as well as vacuum packaging.
The low temperature wafer-level encapsulation processes are implemented using solder bond rings which are
first patterned on a cap wafer and then mated with a device wafer in order to encircle and encapsulate the device at
temperatures ranging from 200 to 390 °C. Vacuum levels below 10 mTorr were achieved with yields in an optimized
process of better than 90%. Pressures were monitored for more than 4 years yielding important information on
reliability and process control.
The ERP adopts an environment isolation platform in the packaging substrate. The isolation platform is
designed to provide low power oven-control, vibration isolation and shock protection. It involves batch flip-chip
assembly of a MEMS device onto the isolation platform wafer. The MEMS device and isolation structure are
encapsulated at the wafer-level by another substrate with vertical feedthroughs for vacuum/hermetic sealing and
electrical signal connections. This technology was developed for high performance gyroscopes, but can be applied to
any type of MEMS device.
Acoustic phenomena during nanochannel machining by <i><b>fs</b></i> laser pulses are found to
have an unexpected strong influence on the machining efficiency. Analysis of acoustic
nodes that strongly limit machining efficiency allows strategies to be identified for
fabrication of high aspect ratio channels. Based on an analytic solution for node
formation, it is found that increasing the speed of acoustic transmission can produce a
two-fold increase in the length of the channels; this can be accomplished by maximizing
the mole fraction of hydrogen in the gas phase. The model is further reinforced by the
effects of varying pressure.