There is an increasing demand to build highly sensitive, low-G, microscale acceleration sensors with the ability to sense accelerations in the nano-G (10-8 m/s2) regime. To achieve such sensitivities, these sensors require compliant mechanical springs attached to large masses. The high sensitivities and the difficulty in integrating robust mechanical stops into these designs make these parts inherently weak, lacking the robustness to survive even the low level accelerations encountered in standard handling, from release processing, where supporting interlayers present during fabrication are etched away, through packaging. Thus, the process of transforming a MEMS-based acceleration sensor from an unreleased state to a protected functional state poses significant challenges. We summarize prior experiences with packaging such devices and report on recent work in packaging and protecting a highly sensitive acceleration sensor that optically senses displacement through the use of sub-wavelength nanogratings. We find that successful implementation of such sensors requires starting with a clean and robust MEMS design, performing careful and controlled release processing, and designing and executing a robust handling and packaging solution that keeps a fragile MEMS device protected at all times.
This work utilized advanced engineering in several fields to find solutions to the challenges presented by the integration of MEMS/NEMS with optoelectronics to realize a compact sensor system, comprised of a microfabricated sensor, VCSEL, and photodiode. By utilizing microfabrication techniques in the realization of the MEMS/NEMS component, the VCSEL and the photodiode, the system would be small in size and require less power than a macro-sized component. The work focused on two technologies, accelerometers and microphones, leveraged from other LDRD programs. The first technology was the nano-g accelerometer using a nanophotonic motion detection system (67023). This accelerometer had measured sensitivity of approximately 10 nano-g. The Integrated NEMS and optoelectronics LDRD supported the nano-g accelerometer LDRD by providing advanced designs for the accelerometers, packaging, and a detection scheme to encapsulate the accelerometer, furthering the testing capabilities beyond bench-top tests. A fully packaged and tested die was never realized, but significant packaging issues were addressed and many resolved. The second technology supported by this work was the ultrasensitive directional microphone arrays for military operations in urban terrain and future combat systems (93518). This application utilized a diffraction-based sensing technique with different optical component placement and a different detection scheme from the nano-g accelerometer. The Integrated NEMS LDRD supported the microphone array LDRD by providing custom designs, VCSELs, and measurement techniques to accelerometers that were fabricated from the same operational principles as the microphones, but contain proof masses for acceleration transduction. These devices were packaged at the end of the work.