Abstract:
A new and improved approach using vertical combs for the differential capacitive sensing of
acceleration using silicon-on-insulator (SOI) wafer technology is presented. The design, which
supports a ± 30 g operational range demonstrates the enhanced linear range that is achievable
using an SOI approach, which overcomes the limitations of the dissolved wafer process (DWP)
technology based on the diffusion layer depth in silicon, used for the fabrication of MEMS
accelerometers. Two technological approaches are compared, one having the same thicknesses
of fixed and movable interdigitated fingers (DWP) and the other having different thicknesses,
using an SOI wafer. A remarkable sensitivity improvement, by a factor of up to two, is achieved
for the capacitance change per unit g (deltaC g−1) using the new differential SOI design
presented here. The bandwidth of the device is also improved significantly in the SOI design,
compared to the DWP design. Furthermore, the effect of temperature (−40 ◦C to 125 ◦C) has an
almost negligible influence on (deltaC) due to the built-in differential concept of comb-type
fingers compared to DWP technology. Cross-axis sensitivity is also very low due to the stable,
robust design of the accelerometer offering less stiction sideways and better pull-in stability.
Simple analytical relations for dynamically changing overlap capacitance are derived and
presented. These analytical results are compared with simulations using Coventorware software
and are in agreement to within 1.8% in a linear range of operation up to a tip deflection of
8.5 μm. The effects of fabrication tolerances, including all the process steps, on the sensitivity
(deltaC g−1) and bandwidth (BW), are also studied and the results presented. An overall figure
of merit (FOM) is included which is better in this study compared to the available literature.
Furthermore, only two wafers are needed to fabricate the proposed differential capacitive
sensors, compared to conventional push–pull three plate systems using three wafers. A separate
deep reactive ion etching (DRIE) step on two different wafers offers better debris removal as
compared to the interdigitated fingers cut by a single DRIE step.
