Nanocrystalline piezoresistive polysilicon film obtained by aluminum induced crystallization for pressure sensing applications

Abstract

The overall objective of our research was to integrate various sensors on to a single flexible substrate for multi-sensory information gathering. Additional capabilities could be incorporated towards the realization of 'smart skin' for simultaneous and real time sensing of various mechanical, biological and chemical stimuli. Recent research venues are dictated by the trend of shifting from conventional silicon (Si) substrates to lower weight, low profile, structurally robust and lower cost flexible substrates. These flexible substrates easily conform to non-planar objects, could be batch fabricated at lower cost and enable multilayer construction. This would eventually evolve into seamless assimilation of sensors for various stimuli onto a single flexible substrate for plethora of applications in consumer electronics, robotics, medical prosthetics, surgical instrumentation, structural health monitoring and industrial diagnostics to name a few. Pressure sensors currently find numerous applications in the field of automobiles (airbag deployment, tire pressure monitoring systems (TPMS), fuel systems etc.), smart cell phones (microphones, touch screens etc.) and various biomedical devices. The pressure sensor selection criterion is strictly based on the requirements of specific pressure range and resolution. It is also dependent on the environment (temperature, medium etc.) the sensor would be deployed in. Some commonly used pressure sensor designs include absolute, gauge and differential/tactile types. All of the above sensors could either employ piezoresistive, piezoelectric, capacitive or optical readout methodologies for sensing applied pressure. Piezoresistor-based, differential pressure sensor designs are most commonly used because of their (i) versatility, (ii) relatively simple construction, (iii) linear responsivity with applied pressure, (iv) long-term stability, and (v) maturity of the technology. There has been a growing interest in the development of various sensors that often require deployment of planar micro to nano-scale sized sensors on flexible substrates such as polyimide, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and stainless steel (SS). Current work describes the use of piezoresistive-based differential pressure sensors on a flexible polyimide substrate. Our design uses a suspended diaphragm with piezoresistive sensing based on a Wheatstone bridge circuitry. The measurement resolution can be effectively controlled by the diaphragm geometry and size, whereas the diaphragm thickness and the micromachined gap under the diaphragm determine the range. The surface micromachining used here would also facilitate stacking of different sensors (viz. infrared, pressure, chemical, biological) on a single flexible substrate, conforming to the underlying object. For our current application, the aim was to measure low pressure changes ranging from few tens of a pascal (Pa) to few tens of kPa. Fabrication processes on a wide variety of flexible substrates are dictated by their lower glass transition temperatures (Tg). This critical restriction more often requires low temperature film deposition and device fabrication techniques in order to use them as substrates. Polysilicon being CMOS compatible is used both as a mechanical and an electrical material in many sensor designs, as it makes the integration of the sensor with read-out circuitry readily feasible. Since polysilicon also exhibits a relatively high piezoresistive gauge factor, it is also preferred over its metal counterparts. However, conventional polysilicon deposition techniques typically require high temperatures, which are incompatible with polyimide substrates. The work presented here is a low temperature method for obtaining polysilicon piezoresistive thin films using aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) film. A very important step involving the curing of polyimide PI-2611 was successfully developed to with