| AVS 60th International Symposium and Exhibition | |
| MEMS and NEMS | Monday Sessions |
| Session MN+AS+SS-MoM |
| Session: | Fabrication and Multi-scale Interactions of Materials, Surfaces, and Interfaces at the Micro- and Nano-scale |
| Presenter: | S. Park, University of California San Diego |
| Authors: | A. Erten, University of California San Diego S. Park, University of California San Diego E. Briggs, University of California San Diego D. Martin, University of California San Diego Y. Takeshita, University of California San Diego T. Martz, University of California San Diego A.C. Kummel, University of California San Diego |
| Correspondent: | Click to Email |
Ion-Sensitive Field Effective Transistors (ISFETs) are used for measuring the activity and concentration of ions in solutions. ISFETs are modified Metal-Oxide-Semiconductor FET (MOSFET), which utilize changes in the floating potential on the gate insulator to modulate the current between source and drain. When employed as a pH sensor, ISFETs are operated at constant source-drain current by modulating the potential of the solution via a reference electrode of ISFET. The overall potential change at gate is a direct measurement of the solution pH. Adding an additional gold electrode near the gate region of the ISFET allows the total alkalinity to be determined, which has critical applications in oceanography to study ocean acidification as well as temporary variability in the marine CO2 system. The gold electrode is used to generate protons (H+) which react with the OH-(aq) thereby neutralizing the pH, which is being monitored by the ISFET gate. This has been successfully demonstrated by using a front side contact to the gold electrode. In a simple 0.5 M NaCl solution, the differences in total alkalinity with 1 milimolar precision were measured within 15 second as predicted by simple device models. However, for practical applications, a backside contact to the front side gold electrode is needed so that all the circuitry and wire bonds can be protected from the solution. In this study, a method for fabrication of through-silicon via contacts for front side electrodes in ISFET dies is discussed. To form backside contacts for front side gold electrodes, it requires patterning and deep etching of a chip with an extremely corrugated topology. The ISFET dies already have two backside quasi-through chip vias for backside contact to the source and drain regions, and this non-planar surface obviates the ability of conventional photoresist coating methods to form a uniform film. A new patterning technique was developed for through-chip etching on highly non-planar surfaces using a roll-on photoresist film to overcome the challenges presented by non-planar surfaces. An oxygen plasma was employed to clean the surface and enhance the adhesion between substrate and photoresist film. In comparison with spray coating and spin coating techniques, roll-on photoresist film method showed significantly improved uniformity and adhesion. This method was also employed to protect the substrate from etch plasma. Reactive ion etching was used to etch away oxide layer before gold deposition and Bosch process. Using through-silicon Bosch etching, a via could be made and gold coating could be employed to contact the front side electrode of ISFET.