Title :
Design of a single-chip pH sensor using a conventional 0.6-μm CMOS process
Author :
Hammond, Paul A. ; Ali, Danish ; Cumming, David R S
Author_Institution :
Dept. of Electron. & Electr. Eng., Univ. of Glasgow, UK
Abstract :
A pH sensor fabricated on a single chip by an unmodified, commercial 0.6-μm CMOS process is presented. The sensor comprises a circuit for making differential measurements between an ion-sensitive field-effect transistor (ISFET) and a reference FET (REFET). The ISFET has a floating-gate structure and uses the silicon nitride passivation layer as a pH-sensitive insulator. As fabricated, it has a large threshold voltage that is postulated to be caused by a trapped charge on the floating gate. Ultraviolet radiation and bulk-substrate biasing is used to permanently modify the threshold voltage so that the ISFET can be used in a battery-operated circuit. A novel post-processing method using a single layer of photoresist is used to define the sensing areas and to provide robust encapsulation for the chip. The complete circuit, operating from a single 3-V supply, provides an output voltage proportional to pH and can be powered down when not required.
Keywords :
CMOS analogue integrated circuits; chemical sensors; ion sensitive field effect transistors; pH measurement; passivation; system-on-chip; 0.6 micron; 3 V; CMOS analog circuit; CMOS process; battery-operated circuit; bulk-substrate biasing; differential measurements; encapsulation; floating-gate structure; ion-sensitive field-effect transistor; pH-sensitive insulator; passivation layer; photoresist; reference FET; silicon nitride; single-chip pH sensor; system-on-chip; threshold voltage; ultraviolet radiation; CMOS process; Circuits; FETs; Insulation; Passivation; Resists; Robustness; Semiconductor device measurement; Silicon; Threshold voltage; 65; CMOS analog circuit; ISFET; SoC; encapsulation; ion-sensitive field-effect transistor; pH; system-on-chip;
Journal_Title :
Sensors Journal, IEEE
DOI :
10.1109/JSEN.2004.836849
