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Title: Fabrication of ISFET microsensor by diffusion-based Al gate NMOS process and determination of its pH sensitivity from transfer characteristics
Authors: Khanna, V K
Keywords: Chemical Sensor;Ion Sensor;pH sensor;ChemFET;ISFET;Analytical Device
Issue Date: Mar-2012
Publisher: NISCAIR-CSIR, India
Abstract: The present paper describes the fabrication of ISFET (Ion-Sensitive Field-Effect Transistor) by a four-mask, thermal diffusion-based process. The sequence of physical and chemical processes for ISFET fabrication has been elaborated. In the first photomasking step, the regions for source and drain diffusion have been opened. The second photolithography defined the gate area. In the third photolithographic step, contact windows have been opened, and in the fourth photolithographic step, the metal pattern has been delineated. After completion of the fabrication process, the wafer has been diced into chips, which have been mounted on ceramic substrates to provide electrical connections for source, drain and substrate. Except for the gate region, the whole chip and wire bonds have been protected with insulating epoxy. For process characterization, current-voltage characteristics of MOSFET test devices simultaneously fabricated on the same wafer have been measured for gate-source voltages from –5 to +5 V. These were found to be N-channel, depletion-mode devices indicating similar behaviour for ISFETs. The pH-response of ISFET has been evaluated by drawing its IDS-VGS characteristics after immersion in standard buffer solutions and applying the gate-source voltage through Ag/AgCl reference electrode. From these transfer characteristics, pH-sensitivity of ISFET has been determined by finding the gate-source voltage necessary to ensure constant IDS, VDS condition. Technological shortcomings of this work have also been pointed out, and necessary remedial measures have been suggested.
Page(s): 199-207
ISSN: 0975-1041 (Online); 0019-5596 (Print)
Appears in Collections:IJPAP Vol.50(03) [March 2012]

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