High Performance MEMS Inertial Instruments Fabricated on LTCC Substrates

Draper Laboratory's current generation of high performance, MEMS inertial instruments are assembled from a discrete sensor in a ceramic chip carrier, an ASIC control chip in a plastic ball grid array package, one or more Op Amps, and a dozen or more passive components.

The MEMS device in each of these instruments exhibits a minute capacitance shift, in response to an acceleration or Coriolis force, which changes the gap between its proof mass and sense electrode. Consequently, the physical implementation and mechanical stability of interconnect between components, strongly influences instrument performance.

Changes in temperature or humidity may cause the instrument circuit board to expand or contract more than the ceramic sensor package, to a degree that the resulting stresses deform the sensor die. This deformation can change the nominal gap distance between sensor proof mass and sense electrode.

If the thermal conductivity of the substrate is low and the ambient temperature changes rapidly, the resulting temperature gradients induce thermal mismatch stresses, which also deform the sensor die. Both gyroscope and accelerometer sensors are susceptible to vibration induced errors, especially at frequencies near their structural resonances.

If the instrument circuit board is not sufficiently rigid, then it could induce vibration errors even if the instrument is mounted on a vibration isolation structure.

The sensor and control ASIC are mounted so as to minimize their interface path length and associated parasitic and coupling capacitances. Any change in these capacitance values appears as an inertial input, hence temperature or humidity induced changes in the thickness or permittivity of the dielectric layers must be minimized.

The I/O impedance of MEMS sensors is extremely high, so variations in leakage resistance on the instrument circuit board can adversely affect scale factor and isolation of drive and sense signals.

This paper will utilize simple electrical and mechanical models to examine the effect of candidate substrate materials on instrument performance. We have elected to build high performance instruments on low temperature cofired ceramic circuit (LTCC) boards. These boards are fabricated in eight layers, with Dupont 951 green tape and silver based metallization. They are approximately one inch in diameter by 0.050 inches thick, and have 23 braze attached I/O pins, which connect to a copper flex tape interconnect. The instruments are mounted on a system stable member by three 1 mm screws that fit through machined, braze attached eyelets.