Wenzhou Ruan

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.

Synthesis of Carbon Nanotubes on Suspending Microstructures by Rapid Local Laser Heating

A chemical vapor deposition (CVD) method using local infrared laser heating is developed for synthesis of carbon nanotubes (CNTs) on suspending microstructures. Adjusting the laser power and the focused beam spot enables local illumination and heating of suspending microstructures to high temperature for CNT growth. Rapid synthesis of multiwalled CNTs on suspending microstructures is successfully achieved. This method provides uniform temperature distribution for self-confined CNT growth, with structure size adaptability. It avoids the need of global heating in conventional CVD synthesis and protects temperature sensitive devices from being damaged. The CNTs are typical in a porous foam manner with the potential in sensor 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.

Localized Synthesis of Carbon Nanotube Films on Suspended Microstructures by Laser-Assisted Chemical Vapor Deposition

A laser-assisted chemical vapor deposition (LCVD) method has been developed for in situ synthesis of carbon nanotube (CNT) films on suspended microstructures. Focused laser beams are used to heat locally the suspended microstructures with low thermal mass and low thermal dissipation to high temperatures for localized CNT growth. Other substrate areas than the microstructures remain at low temperatures, preventing the devices on the substrate from being destroyed by high temperatures. The synthesizing parameters and the influences on CNT morphology and structures are systematically investigated and optimized, and solutions for uniform temperature distribution are proposed. Upon optimization, uniform, localized, and rapid growth of CNT synthesis has been achieved on suspended microstructures, and aligned CNTs with length and uniformity comparable to conventional hot-wall CVD have been successfully obtained. The experimental results show LCVD is a promising technology for in situ and localized synthesis of CNT films on suspended microstructures for CNT-CMOS (complementary metal oxide semiconductor) integration.

In-Situ Heat Capacity Measurement of Carbon Nanotubes Using Suspended Microstructure-Based Microcalorimetry

This paper reports a method for measuring the heat capacity of as-grown carbon nanotubes (CNTs) using a microcalorimeter. The microcalorimeter consists of a double-layer suspended silicon dioxide microstructure and two silicon resistors sandwiched in-between the silicon dioxide layers. CNTs for heat capacity measurement are locally synthesized on the surface of the microstructure using laser-assisted chemical vapor deposition. The CNTs and the microcalorimeter are heated to a high temperature with a silicon resistor, and the temperature of the microcalorimeter with CNTs is measured with using the other silicon resistor. The heat capacity of the CNTs is obtained by dividing the measured effective heating power by the rate of temperature increase. The suspended configuration and the low thermal conductivity of silicon dioxide achieve low thermal dissipation, which together with the minute thermal capacity enable significant temperature changes. Using this sensitive microcalorimeter, the heat capacity of as-grown CNTs, 6-14 nJ/K from 340 to 440 K, is measured in-situ without the need of CNT transfer and pretreatment, avoiding damage to the CNT samples. This microcalorimeter is also applicable to in-situ measurement of structure-related thermal properties of porous materials.

A Microcalorimeter Integrated With Carbon Nanotube Interface Layers for Fast Detection of Trace Energetic Chemicals

Detection of trace explosives is still a challenging task because of the extremely low vapor concentrations. This paper reports a new microcalorimeter for detection of trace explosives with low detection limit and fast detection rate. The microcalorimeter consists of a suspended micromembrane with integrated heaters and thermistors and a carbon nanotube (CNT) interface layer in-situ synthesized on the membrane surface. Due to the large surface areas, the CNT interface layer improves adsorption to target chemical molecules. By operating the microcalorimeter in differential scanning calorimetry mode and differential thermal analysis mode, trace chemical detection is achieved through heating the adsorbed chemicals to deflagration and measuring the induced thermal response features and the total heat. The microcalorimeter is verified by explosive detection, and an equivalent limit of detection of 2.6 pg has been achieved by extrapolating the measurement results. Theoretically, detection is distinguishable upon 10-s fast exposure to saturated explosive vapors. These preliminary results demonstrate the ability of the microcalorimeter in detection of trace energetic chemicals.

Detection of trace energetic substance vapors using carbon nanotube network-based thermal sensors

We have developed a microsensor using a suspended microbridge and a carbon nanotube networks (CNN) that is in-situ synthesized on the microbridge for detection of trace energetic substances. The energetic substance vapors are adsorbed onto the CNN surfaces and are heated to micro deflagration using an integrated heater on the microbridge. The involved temperature changes are measured with an integrated thermistor to obtain the thermal spectra for vapor discrimination. The huge surface areas of the CNNs significantly improve the adsorption efficiency and the limit of detection (LOD), and the tiny thermal mass allows distinct temperature changes to be generated. A LOD of 13 pg has been achieved for 2,4,6-trinitrotoluene (TNT) detection.