A micromachined surface stress sensor based on a thin suspended crystalline silicon circular plate measures differential surface stress changes associated with vapor phase chemisorption of an alkanethiol self-assembled monolayer. The isolated face of the suspended silicon plate serves as the sensing surface treated with a receptor layer sensitive to a target molecule, in this case Au(111). Chemisorption of an alkanethiol on the gold coated silicon surfaces results in plate bending. Plate displacements, measured with a phase scanning interferometer, indicate a differential surface stress change for 1-dodecanethiol.
REFERENCES
For all devices tested, (a) nominal plate thickness , (b) , and (c) . Szilard (Ref. 23) recommends .
Defined as tension for and compression for .
Average residual stress of layers measured (FLX-2320-S, Toho Technology Corp.) in separate experiments on diameter silicon wafers (average of five measurements) with average compressive stress of .
A solution of with boundary conditions at . The variable represents a uniform lateral pressure (Ref. 26) and is calculated by assuming that the initial bending is identical to the bending due to . The center deflection due to , , (Ref. 26), is equated to the initial bending resulting in and . The term is then replaced with .
Deflection , where replaced with first three terms of series expansion. The quadratic equation is solved for .
Peak occurs at corresponding to the (111) direction for Au.
Comparison between cantilever beam dimensions from Refs. 6–10 and plate dimensions presented here.
