James David Zook

Sealed-cavity resonant microbeam pressure sensor

A quasi-digital pressure sensor based on polysilicon resonant microbeams has been demonstrated. Pressure sensitivities of nearly 4000 counts per second per psi have been attained on a 10 psi device with a base frequency of 233 000 Hz. Short-term stability as low as 0.01 ppm of the base frequency is typical. The microbeams are fabricated with their own integral vacuum cavities, allowing high-Q operation in the differential pressure mode or in contact with liquids such as silicone oil. Design considerations include the effects of internal strain and lead to a push-pull layout configuration independent of microbeam strain or diaphragm thickness. Fabrication technology incorporates fine-grained polysilicon, surface micromachining, bulk micromachining, and reactive sealing. Packaging into precision avionics headers is being used for preliminary testing. Testing results indicate suitability for precision avionics, industrial, and commercial applications. Optical methods have been used to test resonant microbeam pressure sensors and verify the push-pull design methodology. Testing methods developed under this effort include electrostatic drive/piezoresistive sensing, optical drive/optical sensing, substrate piezoelectric drive/optical sensing, and electrostatic drive/laser vibrometer sensing. Wafer-level testing of 200 μm×46 μm×1.9 μm microbeams shows an average fundamental frequency of 553 150 and first overtone of 1 332 550 Hz. The standard deviations across the wafer are 0.15 and 0.10%, respectively. The internal strain and effective thickness can be determined with high resolution. Laser vibrometer measurements through the microbeam shell verify the fundamental frequency and reveal at least ten overtones up to 25 MHz.

Electromagnetic linear actuators with inductive position sensing

Abstract Magnetically driven linear micro-actuators with 1 mN output force and hundreds of microns of total travel have been realized using LIGA-like processing techniques. These devices are driven at resonance frequencies of around 350 Hz and exhibit quality factors of about 300 in air. Exploiting wire-wound electromagnets, 300 μm resonant displacements have been demonstrated with drive signal currents of about 1 mA and power dissipation of less than 200 μW. In addition, the self inductance of the coils in these actuators varies linearly with plunger displacement. The inductance changes of about 1 μH μm −1 may be used for inductive position sensing and as a measure of output force.

Sealed-cavity resonant microbeam accelerometer

Abstract Resonant microbeam strain-sensing elements have been combined with a highly symmetric multiwafer silicon microstructure to form the first micromachined accelerometer with a polysilicon resonant microbeam pickoff. The MARIMBA (micromachined resonant integrated microbeam accelerometer) uniquely combines bulk-micromachining with advanced surface-micromachining techniques and a multiple-wafer structure to produce an all-silicon acceleration sensor capsule with a direct acceleration-to-digital frequency output. The microbeams are fabricated from thin films of fine-grained polysilicon with integral polysilicon vacuum encapsulation, electrostatic drive and piezoresistive sense. The silicon microstructure features a dual open-web suspension system formed monolithically with the silicon proof mass and frame. Silicon caps provide squeeze-film gas damping, overrange protection, and environmental protection. Initial devices have been fabricated with scale factors greater than 700 Hz g −1 on ±20 g devices with base microbeam frequencies of 500–700 kHz. Root Allan variances below 0.1 Hz and seven-day stability measurements less than 2 ppm of the base frequency on test microbeams indicate that milli- g stability and micro- g sensitivities are attainable.