The threshold voltage time dependence and the technological difficulties associated with the packaging of miniature reference electrodes limit the commercial viability of all the ISFET applications intended for the long term pH monitoring in chemical analyses. Above all, daily sensor calibrations cannot be proposed to realise fully automated systems for on-field measurements. In the last years, in order to overcome these drawbacks, different approaches have been pursued by imposing special requirements for sensor design, packaging, and signal compensations. Several solutions include on-chip fabrication of an Ag/AgCl electrode with microfabrication techniques, physical and chemical treatments of the sensing membrane. As an alternative, differential arrangements, consisting of an ISFET and a reference FET (REFET) with the same ISFET characteristics but insensitive to the pH variations, have been incorporated in the same device structure. Moreover, in these configurations the traditional liquid-junction glass reference electrode has been replaced from a metal electrode (platinum or gold). In this work, the design and the characterisation of a microsystem, employing differential sensing by means of a innovative read-out CMOS circuit, is reported. The sensing section consists of a standard ISFET/REFET structure with a Si3N4/SiO2 double layer as ISFET gate insulator and with a buffered hydrogel layer as REFET insensitive membrane. The core of the system is a signal conditioning circuit based on switched-capacitors technique which provides a differential read-out between the sensor and the REFET so as to reduce the drift effects. The circuit has been fabricated in Thesys/AMS 0.8ƒÝm CMOS technology and designed for signal resolution of 1/100 pH. The power consumption is only 5.3mW with a standard voltage supply of 5V making it suitable for battery-supplied applications. The complete microsystem has been mounted in a custom type package where the CMOS processing circuit has been coupled with the electrochemical probe
A CMOS ISFET Microsystem for pH Detection in Differential Mode
Soncini, Giovanni;Dalla Betta, Gian Franco;Lorenzelli, Leandro
2002-01-01
Abstract
The threshold voltage time dependence and the technological difficulties associated with the packaging of miniature reference electrodes limit the commercial viability of all the ISFET applications intended for the long term pH monitoring in chemical analyses. Above all, daily sensor calibrations cannot be proposed to realise fully automated systems for on-field measurements. In the last years, in order to overcome these drawbacks, different approaches have been pursued by imposing special requirements for sensor design, packaging, and signal compensations. Several solutions include on-chip fabrication of an Ag/AgCl electrode with microfabrication techniques, physical and chemical treatments of the sensing membrane. As an alternative, differential arrangements, consisting of an ISFET and a reference FET (REFET) with the same ISFET characteristics but insensitive to the pH variations, have been incorporated in the same device structure. Moreover, in these configurations the traditional liquid-junction glass reference electrode has been replaced from a metal electrode (platinum or gold). In this work, the design and the characterisation of a microsystem, employing differential sensing by means of a innovative read-out CMOS circuit, is reported. The sensing section consists of a standard ISFET/REFET structure with a Si3N4/SiO2 double layer as ISFET gate insulator and with a buffered hydrogel layer as REFET insensitive membrane. The core of the system is a signal conditioning circuit based on switched-capacitors technique which provides a differential read-out between the sensor and the REFET so as to reduce the drift effects. The circuit has been fabricated in Thesys/AMS 0.8ƒÝm CMOS technology and designed for signal resolution of 1/100 pH. The power consumption is only 5.3mW with a standard voltage supply of 5V making it suitable for battery-supplied applications. The complete microsystem has been mounted in a custom type package where the CMOS processing circuit has been coupled with the electrochemical probeI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
