However, it is the biochemical and biosensor applications that are attracting piezoresistive cantilevers most. They have been used as environmental sensor [16], biosensor [17], biochemical sensor [18], in DNA sequencing [19], biomolecular force sensor [20] and immunosensor [21]. Nevertheless, the sensitivity and resolution of piezoresistive detection is generally an order of magnitude less than optical method due to low piezoresistive coefficients and the large noise. Piezoresistor cantilevers are vulnerable to thermal effects such as thermal deflection because of temperature increase by Joule heating. Thus, characterisation of Joule heating in piezoresistive microcantilevers is necessary to improve their accuracy. Recently, Chui et al.
[22] proposed a highly effective method of reducing thermal sensitivity in piezoresistive sensors by taking advantage of the dependence of the piezoresistive coefficient of silicon on crystallographic orientation.Piezoresistive microcantilevers were traditionally fabricated from single crystalline silicon substrate with the piezoresistor element created by selectively doping the substrate with a suitable dopant. However, later studies found that for MEMS piezoresistors, polysilicon offers a number of advantages over single-crystalline silicon, including the ability to be deposited on a wide range of substrates [10]. The polycrystalline silicon also exhibits piezoresistivity, but the gauge factor is much smaller than that of single crystalline.
Thus, to improve the sensitivity and resolution of piezoresistive microcantilevers, efforts have been made to use soft material cantilever or use single crystalline Carfilzomib silicon as piezoresistor to achieve high piezoresistive coefficients [23]. To this use, application of silicon dioxide as substrate and single crystalline silicon as piezoresistor was proposed. However, silicon dioxide microcantilevers fabricated from surface micromachining technology can integrate only polysilicon piezoresistors, which suffer from low piezoresistive coefficients and high noise [21]. In recent days, SOI wafers have been used to fabricate silicon dioxide microcantilevers with etched single crystalline silicon piezoresistors to improve the sensitivity and the resolution [24].
The low Young��s modulus of silicon dioxide combined with the high piezoresistive coefficients of single crystalline silicon piezoresistor presents an ideal Cilengitide solution to improve the sensitivity of piezoresistivity microcantilevers. However, silicon dioxide cantilevers have a major drawback in form of Joule heating produced by the piezoresistor encapsulated inside.