Nanocrystalline Piezoresistive Polysilicon Film Obtained By Aluminum Induced Crystallization For Pressure Sensing Applications
Patil, Suraj Kumar
MetadataShow full item record
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 withstand AIC annealing temperatures in excess of 500 °C for couple of hours. This facilitated the use of multilayer PI-2611 as our substrate and sacrificial material. We have obtained nanocrystalline polysilicon films with average grain sizes of 45-55 nm at temperatures ranging from 400 °C to 500 °C with annealing time of 60 min utes, and an average grain size of 50 nm at 500 °C for a shorter annealing time of 30 minutes. An additional advantage of this process is that the polysilicon films are simultaneously doped p-type, thereby eliminating any additional doping step. By varying the aluminum (Al) and a-Si layer thicknesses, annealing temperature and duration, the growth of polysilicon grains ranging from few tens of nanometers to tens of microns in diameter can be effectively obtained. Additionally, exploring the piezoresistive properties of the above mentioned low temperature nanocrystalline polysilicon thin films deposited on polyimide substrate for pressure sensing applications was another vital aspect of this research. In order to achieve this firstly, arrays of MEMS based pressure sensors were successfully fabricated on polyimide substrate. Secondly, an atomic force microscope (AFM) in contact mode with a modified probe-tip was used to apply differential pressures. Low pressures (lesser than atmospheric pressure) were successfully applied onto the sensors using AFM. Thirdly, higher pressures (greater than 4 times the atmospheric pressure) were applied onto the sensors by using a load-cell coupled with a nano-positioner. The design of the pressure sensor characterization set-ups and subsequent experimental procedures are described in this work. Finally, experimental characterization of fabricated MEMS pressure sensors on polyimide substrate employing polysilicon resistors obtained by AIC were performed to measure their pressure sensitivity responses.