Surface stress sensor using MEMS-based Fabry-Perot interferometer for label-free biosensing

We have recently developed a surface stress sensor based on a microelectromechanical system Fabry-Perot interferometer integrated with a photodiode, which utilizes the nonlinear optical transmittance change in the Fabry-Perot interference to enhance the sensitivity of the surface stress. The novel signal transducing technique from the surface stress caused by molecular adsorption to the electrical readout is carried out in three steps, namely, mechanical deflection, transmittance change, and photocurrent change. Parylene-C was used as a deformable membrane in the sensor because of its high optical transmittance, low Young's modulus, and negligible residual stress, which provides a flat freestanding membrane. The theoretical minimum detectable surface stress of the proposed sensor was predicted to be −1 μN/m, which is two orders of magnitude greater than that of the piezoresistor-embedded surface stress sensor. The Fabry-Perot surface stress sensor was fabricated using a semiconductor fabrication process and sacrificial release process. An encapsulating structure for the Fabry-Perot cavity that uses a dry film resist by a roll-to-roll technique enabled us to use wet condition for detection of biomolecules. Amino-methyl-functionalized parylene was coated on the parylene-C membrane for immobilization of the biomolecules via electrostatic coupling between biomolecules and the amino group. The photocurrent was demonstrated to shift by 23.7 nA at a 3-V reverse bias voltage with immobilized antibodies. This Fabry-Perot sensor allows the use of a universal biochemical sensing platform in a label-free manner.