Youzheng Zhou

Design, fabrication and characterization of a two-step released silicon dioxide piezoresistive microcantilever immunosensor

This paper presents the design, fabrication and characterization of a silicon dioxide piezoresistive microcantilever immunosensor fabricated on silicon-on-insulator (SOI) wafers. The microcantilever consists of two strips of single crystalline silicon piezoresistors sandwiched in between two silicon dioxide layers. A theoretical model for the laminated microcantilever with a discontinuous layer is deduced using classic laminated beam theory. A two-step release method combining anisotropic and isotropic etching is developed to suspend the microcantilever, and the fabrication results show an excellent yield. The residual stress-induced free bending of the microcantilever and the stress caused by self-heating of the piezoresistors are discussed. The microcantilever sensor is characterized as an immunosensor using specific binding of antigen and antibody. These methods and some conclusions are also applicable to the development of other piezoresistive sensors that use laminated structures.

A Front-Side Released Single Crystalline Silicon Piezoresistive Microcantilever Sensor

This paper presents the design, fabrication, and characterization of a piezoresistive microcantilever sensor fabricated on silicon-on-insulator (SOI) wafers. The microcantilever consists of two silicon dioxide supporting layers and a single crystalline SOI layer in-between. The piezoresistors are implanted in the surface of the SOI layer to exploit its large piezoresistive coefficients. Laminated beam theory is employed to design the microcantilevers and the piezoresistors. A front-side releasing method is developed to suspend the microcantilevers by isotropically etching the substrate beneath the microcantilevers from the front-side of the wafers using SF6 plasma. The features of SOI wafers and the front-side releasing enable high uniformity and high yield for the fabrication of piezoresistive microcantilever sensors. The sensors are validated using specific binding reaction of antigen and antibody of immunoglobulin G on the sensor surface, and the experimental results show that they are promising for portable and integrated sensing applications.

Label-free detection of p53 antibody using a microcantilever biosensor with piezoresistive readout

We report the detection of p53 antibody as a biomarker for early-stage cancer diagnosis using a microcantilever biosensor with piezoresistive readout. The accumulation of p53 antibody in human sera is found strongly involved in a variety of human cancers, thus simple and fast detection of the level of p53 antibody is of importance in clinical application. In this work, p53 antigen is immobilized on the surface of the microcantilever as a recognition probe to detect p53 antibody by measuring the deflection of the microcantilever using integrated piezoresistors, which is caused by the changes of the surface stress as a result of the specific bioaffinity between the antigen and the antibody. Quantitative detection of p53 antibody ranging from 20ng/ml to 20µg/ml has been achieved. Compared with conventional ELISA or immunofluorescene assay, the microcantilever sensor has the advantages of label-free operation, fast detection, and low cost.

A silicon microcantilever biosensor fabricated on SOI wafers using a two-step releasing approach

This paper reports the fabrication of a piezoresistive microcantilever biosensor using a unique two-step front-side releasing method, which combines anisotropic etching and isotropic etching. The microcantilever sensors are fabricated on single crystalline SOI wafers to exploit the high piezoresistive coefficients. The anisotropic etching exposes the sidewalls beneath the microcantilevers, and the following isotropic etching undercuts the substrate and suspends the microcantilevers. Thin and uniform microcantilevers with a yield of 100% have been successfully fabricated. Significant improvement in releasing efficiency of 10 times higher than the conventional backside releasing has been achieved. Detection of Immunoglobulin G (IgG) specific binding down to 0.2 mug/ml with the microcantilever sensor demonstrates the potential applications as a biosensor for biomolecular specific binding detection.

Trimethylamine detection using microcantilever chemical sensors functionalized with a self-assembled monolayer

We report the detection of gas phase trimethylamine (TMA) using a microcantilever chemical sensor (MCS). The MCS is made from silicon-on-insulator (SOI) wafers and consists of two silicon dioxide layers and a single-crystallinesilicon piezoresistor sandwiched in-between. The MCS surface is chemically functionalized with a self-assembled monolayer (SAM) of 11-mercaptoundecanoic acid (11-MUA) via Au-SH covalent bonding on the microcantilever. The 11-MUA SAM adsorbs TMA through hydrogen bonding between the SAM and TMA molecules, resulting in the microcantilever deflection caused by the changes in the surface stress as a result of the hydrogen bonding based chemisorption. The limit of detection (LoD) 1.65 µg/L (0.6 ppm) is achieved.