Abstract
Although the ion-sensitive field-effect transistor (ISFET) concept has existed for over 30 years, practical applications are still emerging very slowly. In this thesis, we will point out some practical problems that were the limiting factors in the breakthrough of commercialization of ISFET and try to study and provide the appropriate solutions. Three critical issues of ISFET are : (1) The lack of a suitable ion-sensitive membrane as the ISFET gate material with sufficient sensitivity and stability; (2) the encapsulation of the electronics functions from exposure to the sample liquids; and (3) the need of a bulky conventional reference electrode is not compatible to miniaturized chip and not convenient for in-vivo biomedical applications. Although each of the above-mentioned problems has been investigated respectively, very rare researchers have offered a total solution to solve all of those issues at the same time. In this dissertation, a monolithic ISFET hydrogen ion (pH) sensor with an integrated miniaturized solid-state reference electrode and electrical backside contacts structure will be presented and its electrochemical high performance also will be verified.Firstly, we will compare the sensing characteristics of two inorganic insulators, silicon nitride and tantalum oxide, as the ion-sensitive membranes of pH-ISFET. According to the results of their respectively fabricated pH sensors characterized in standard pH solution, the tantalum oxide (Ta2O5) insulator based ISFET has higher linear pH sensitivity (56-57mV/pH) and lower drift level than silicon nitride (Si3N4) insulator based ISFETs. These results agree very well with the theoretical value of the site-binding theory.Secondly, backside electrical contacts were constructed to facilitate its protection from chemical attack by the test solutions. These backside electrical contacts were fabricated using silicon bulk micromachining with P+ etching stop and double-side alignment lithography techniques.Finally, to eliminate the need of a separate reference electrode and facilitate the use of ion-sensitive field-effect transistor (ISFET), an all-solid-state reference electrode integrated with ISFET in one chip has been developed. A novel agarose-stabilized KCl-gel membrane was introduced to serve both as a polymer-supported solid reference electrolyte and an ionic bridge for Ti/Pd/Ag/AgCl electrode. This new planar integrated reference electrode has not only eliminated the fabrication problems associated with the filling of the reference liquid electrolyte into a miniature micromachined cavity, but also has many excellent performance characteristics with respect to the reference potential stability and its insensitivity to the changes of pH values and Cl- ion concentrations in the sample solutions under test. Calibrated against the commercial macro Ag/AgCl reference electrode the new miniaturized reference electrode shows its cell potential variation was ±0.9~1.4mV (equivalent to about ±0.015~0.023pH) in one hour, less than 2mV variation over pH4 to pH10, and almost insensitive to changes in Cl- ion concentration (about 0.02~0.25mV/pKCl). The planar solid-state reference electrode also shows a very small offset voltage of 0.45mV and reproducible to within 0.5mV among the batch fabricated electrodes.ISFET pH-sensor of SiO2/Ta2O5 gate insulator and through chip backside electrical connections with integrated all-solid-state reference electrode, characterized in standard pH solutions show a linear sensitivity of 56mV/pH, these results agree very well with the theoretical value of 59mV/pH. Dynamic response and hysteresis characteristics of the pH-sensor are also studied and discussed.