With an integrated resonance exciting heater and a self-sensing piezoresistor, resonant micro-cantilever bio/chemical sensors are optimally designed and fabricated by micromachining techniques. This study is emphasized on the optimization of the integrated heating resistor. Previous research has put the heater at either the cantilever clamp end, the midpoint or the free end. Aiming at sufficiently high and stable resonant amplitude, our research indicates that the optimized location of the thermal-electric exciting resistor is the clamp end instead of other positions. By both theoretical analysis and resonance experiments where three heating resistors are placed at the three locations of the fabricated cantilever, it is clarified that the clamp end heating provides the most efficient resonance excitation in terms of resonant amplitude, Q-factor and resonance stability. Besides, the optimized combination of dc bias and ac voltage is determined by both analysis and experimental verification. With the optimized heating excitation, the resonant cantilever is used for biotin–avidin-specific detection, resulting in a ±0.1 Hz ultra-low noise floor of the frequency signal and a 130 fg mass resolution. In addition to resonance excitation, the heater is used to heat up the cantilever for speed-up desorption after detection that helps rapid and repeated sensing to chemical vapor. The clamp end is determined (by simulation) as the optimal heating location for uniform temperature distribution on the cantilever. Using the resonant cantilever, a rapid and repeated sensing experiment on dimethyl methylphosphonate (DMMP) vapor shows that a short-period heating at the detection interval significantly quickens the signal recovery and enhances the sensing repeatability.

Featuring a huge surface-to-volume ratio, synthesized SBA-15 mesoporous silica is functionalized by inner-channel-wall modification of sensing groups for highly specific chemical-vapor detection at trace level. With the developed sensing material loaded on resonant microcantilevers, the specifically adsorbed chemical-vapor molecules act as an added mass to shift the cantilever resonant frequency for gravimetric sensing signal readout. Two kinds of sensing materials for trinitrotoluene (TNT) and ammonia/amine are respectively prepared by inner-wall layer-by-layer grafting functionalization. By using hexafluoro-2-propanol-functionalized mesoporous silica (HFMS), experimental results show highly specific and rapid detection of TNT vapor, with a ppt-level detection limit; functionalized with a carboxyl (COOH) group, the mesoporous silica is loaded onto the cantilever resonating sensor that experimentally exhibits an ultrafine detection limit of tens of ppb to ammonia/amine gases.

According to the demand for an ultrasensitive mass sensor for bio/chemical molecular detection, resonant cantilever sensors are developed for detection in an air environment. Both a piezoresistive bridge and a metal coil are integrated in the cantilever for signal sensing and Lorentz-force resonance excitation, respectively. Compared with conventional first flexure mode resonance, measurement results for the second mode resonance show an improved mass-sensing resolution from 0.17 pg to 0.06 pg due to the higher quality factor. For further improving the resolution, an optimized electromagnetic excitation method specifically for the second resonant mode is proposed and developed. The optimized method provides a two-point excitation that matches the second mode shape function of the cantilever deflection and excites the second mode more efficiently. Compared to a cantilever with a conventional excitation method, the optimally excited cantilever improves the quality factor from 307 to 857. Based on the experimental results for the optimally excited second mode resonant sensor, 29 × 10−15 g resolution for in-air mass sensing is achieved. The developed second mode resonant cantilever sensor with piezoresistive sensing element integrated on-chip is promising for use in high-performance portable biological/chemical sensing applications.