A very-low cross-axis sensitivity piezoresistive accelerometer with an electroplated gold layer atop a thickness reduced proof mass

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This paper presents fabrication and testing of a high performance quad beam silicon piezoresistive Z-axis accelerometer with a very-low cross-axis sensitivity. Cross-axis sensitivity in piezoresistive accelerometers, mainly caused by the asymmetric structural design is an important issue primarily for high performance applications. In the present study, symmetry of the structure is achieved by shifting the center of mass of the proof mass toward the beam plane by selectively electroplating a 20μm thick gold layer atop the proof mass and simultaneously reducing the silicon thickness from backside of the proof mass surface. Silicon shadow mask technique was used to deposit a Cr/Au seed layer on the selected regions of the proof mass followed by an electroplating process to achieve the 20μm thick gold layer. Theoretical analysis shows that for an in-plane acceleration, the rotation of the thickness reduced proof mass with the electroplated gold layer is reduced by 69% compared to the accelerometer device with normal all-silicon proof mass keeping the resonant frequency of both the structures nearly equal. The accelerometer device was realized by a bulk micromachining technique using a 5% dual doped tetra methyl ammonium hydroxide (TMAH) solution. A new figure of merit called the performance factor (PF) defined as the ratio of the product of the prime-axis sensitivity and the square of the resonant frequency to the maximum cross-axis sensitivity is used as a quantitative index to evaluate the fabricated sensors with already reported devices. Test results of a fabricated device with 30μm flexure thickness show a very-low cross-axis sensitivity of 0.316μV/Vg for an in-plane acceleration and a PF of around 2900MHz2.