A micromachined surface stress sensor has been fabricated and integrated off chip with a low-noise, differential capacitance, electronic readout circuit. The differential capacitance signal is modulated with a high frequency carrier signal, and the output signal is synchronously demodulated and filtered resulting in a dc output voltage proportional to the change in differential surface stress. The differential surface stress change of the Au(111) coated silicon sensors due to chemisorbed alkanethiols is for 1-dodecanethiol (DT) and for 1-butanethiol (BT). The estimated measurement resolution ( bandwidth) is (DT: and BT: ) and as high as (DT: and BT: ) with system optimization.
REFERENCES
A commercially available sensor reports a displacement resolution of ( bandwidth) (Ref. 38).
Compared to cantilever beam dimensions from (Refs. 5–8 and 39) and plate dimensions presented here.
For all devices tested (a) nominal plate thickness , (b) , and (c) . Szilard (Ref. 40) recommends .
Defined as tension and compression .
The surface stress signal is defined as and the noise signal is . To determine the detection limit, set , and calculate , where . The effective spring constant is calculated from Hooke’s law , where is a point force applied to the plate center. For a rectangular plate, , where for (Ref. 40), resulting in . The estimated squeeze-film damping is (Refs. 41 and 42).
The polysilicon is doped using boron ion implantation (, , , ) followed by annealing at for in a 100% environment.
Peak occurs at corresponding to (111) direction for Au.