Design and Characterization of CMOS Micromachined Piezoresistive Accelerometers

In this article, we present the design and characterization of monolithically integrated CMOS micromachined accelerometers utilizing polysilicon piezoresistors. Despite their lower piezoresistive effect compared to their singlecrystal counterpart, we achieve enhanced sensitivity by implementing thin support beams to amplify the piezoresistive change upon detection. To facilitate a comprehensive comparison, three designs with proof mass sizes of <inline-formula> <tex-math notation="LaTeX">$100 \times 100,150 \times 150$ </tex-math></inline-formula>, and <inline-formula> <tex-math notation="LaTeX">$200 \times 200 \mu \mathrm{m}^2$ </tex-math></inline-formula> and a proof-mass thickness of <inline-formula> <tex-math notation="LaTeX">$8.3 \mu \mathrm{m}$ </tex-math></inline-formula> were fabricated. These designs were realized using a <inline-formula> <tex-math notation="LaTeX">$0.35 \mu \mathrm{m}$ </tex-math></inline-formula> CMOS process followed by post-CMOS wet etching processes. The measured results revealed out-ofplane resonant frequencies of 8.125,4.015, and 2.260 kHz for the respective designs. The <inline-formula> <tex-math notation="LaTeX">$200-\mu \mathrm{m}$ </tex-math></inline-formula> design exhibited the highest sensitivity, measuring <inline-formula> <tex-math notation="LaTeX">$317.7 \mu \mathrm{V} / \mathrm{g} / \mathrm{V}$ </tex-math></inline-formula> for out-of-plane acceleration, while the <inline-formula> <tex-math notation="LaTeX">$100-\mu \mathrm{m}$ </tex-math></inline-formula> design demonstrated a sensitivity of <inline-formula> <tex-math notation="LaTeX">$38.6 \mu \mathrm{V} / \mathrm{g} / \mathrm{V}$ </tex-math></inline-formula>. In addition, the <inline-formula> <tex-math notation="LaTeX">$200-\mu \mathrm{m}$ </tex-math></inline-formula> design displayed a sensing resolution of 15 mg, accompanied by a temperature dependence of <inline-formula> <tex-math notation="LaTeX">$-2.60 \mu \mathrm{V} / \mathrm{g} / \mathrm{N} / ^{\circ} \mathrm{C}$ </tex-math></inline-formula>. The results demonstrate the promising potential of CMOS piezoresistive accelerometers for various sensing applications.