Cantilever Array for Chemical Sensing: Development of a Moisture-Independent Sensor with Possible Application in Explosive Detection

Chapter 1 Introduction As our civilization is becoming more technologically advanced, demand for information in every aspect of day-to-day life has grown tremendously. Sensors and sensing technology play a critical role in this process of information gathering. The need for cost effective and portable sensors for chemical and biological sensing has increased in the last decade, with new applications in the areas of healthcare (genetics, diagnostics and drug discovery), environment and industrial monitoring, quality control as well as security and threat evaluation. Advances in MEMS (Micro-Electro-Mechanical Systems) and nanotechnology have opened up many novel sensing technologies that have the potential to satisfy this in- creasing demand. The small size of these sensors offers many desirable properties like high surface-to-volume ratio, large scale manufacturing using IC fabrication techniques, and arraying capability for multiplexed measurements. Surface stress sensing is one such new sensing technology that has been investigated extensively in the recent years. Surface stress sensors using micromachined silicon cantilevers have been demonstrated in chemical and biological sensing [1, 2]. Recent advances in the design and fabrication of microcantilever structures, that are capable of detecting extremely small mechanical stresses, and mass addition, offer the promising prospects of chemical, physical, and biological sensing with unprecedented sen- sitivity and dynamic range. The spring constant of a microcantilever is generally on the order of 10−3−100 N/m, which allows the detection of extremely small forces (10−12−10−9 N) [3]. Noting that the force to break a single hydrogen bond is on the order of 10−12 N, one can appreciate the level of sensitivity. The key to the high sensitivity of microcantilevers is the enormous surface-to-volume ratio, which leads to amplified surface stress. There are three cantilever sensing modes: 1) heat sensing, 2) mass sensing, and 3) surface stress change sensing. Tiny amount of heat generation or absorption on a bi- morph cantilever surface can deflect the cantilever structure by the well-known bimetallic principle. Molecular adsorption on a cantilever surface can be detected by measuring its resonance frequency shift. Also, differential surface stress caused by preferential molec- ular interaction on one side of cantilever surface deflects the cantilever structure. By functionalizing cantilevers with target specific receptors such as metals, polymers, and 1