Nanocrystalline Piezoresistive Polysilicon Film Obtained By Aluminum Induced Crystallization For Pressure Sensing Applications

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dc.contributor Patil , Suraj Kumar en_US 2011 -03 -03T21 :52 :29Z 2011 -08 -24T21 :44 :24Z 2011 -03 -03T21 :52 :29Z 2011 -08 -24T21 :44 :24Z 2011 -03 -03 January 2010 en_US
dc.identifier.uri http : / /hdl .handle .net /10106 /5472
dc.description.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 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 . en_US
dc.language.iso en en_US
dc.publisher Materials Science & Engineering en_US
dc.title Nanocrystalline Piezoresistive Polysilicon Film Obtained By Aluminum Induced Crystallization For Pressure Sensing Applications en_US
dc.type Ph .D . en_US


Nanocrystalline Piezoresistive Polysilicon Film Obtained By Aluminum Induced Crystallization For Pressure Sensing Applications. Available electronically from http : / /hdl .handle .net /10106 /5472 .

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