Materials Chemistry and Physics 63 (2000) 153–156 Study on SnO 2 /Al/SiO 2 /Si ISFET with a metal light shield Chung-Lin Wu a , Jung-Chuan Chou b,∗ , Wen-Yaw Chung a , Tai-Ping Sun c , Shen-Kan Hsiung a a Department of Electronic Engineering, Chung Yuan Christian University, Chung-Li, 320, Taiwan b Institute of Electronic and Information Engineering, National Yunlin University of Science and Technology, Touliu, Yunlin 640, Taiwan c Department of Electrical Engineering, National Chi Nan University, Chi Nan 545, Taiwan Received 11 June 1999; received in revised form 13 September 1999; accepted 28 September 1999 Abstract For conventional open-gate FET-based sensors, such as the ISFETs, the influence of light exposure is very sensitive. This drawback leading to change in the electrical characteristics. To reduce light-induced instability, the optimized ISFET structure with a metal light shield is investigated in this study. We used aluminum as a light shield and tin oxide as a pH sensitive layer to develop the ISFET devices with SnO 2 /Al/SiO 2 /Si and compared to SnO 2 /SiO 2 /Si ISFET sensors. The data show that ISFETs with an aluminum as a light shield can maintain a linear pH response of about 56–58mV per unit pH in the pH range between 2 and 10, and have effectively decreased light sensitivity tested under 15 mW (room light about 0.3 mW) irradiation at wavelength of λ = 550 nm compared to the ISFETs without an aluminum light shield. ©2000 Elsevier Science S.A. All rights reserved. Keywords: ISFET; Tin oxide; Light shield; pH response 1. Introduction Since Bergveld [1] reported on ion sensitive field effect transistors (ISFET) for measuring ion concentrations in so- lutions, various kinds of chemical sensors have been devel- oped which are based on semiconductor technology. Inor- ganic materials such as SiO 2 , Si 3 N 4 , Al 2 O 3 , and Ta 2 O 5 are known to be suitable pH sensitive gate membranes for IS- FETs. These pH sensitive layers are high band gap materi- als. Thus, light will penetrate into these materials and result in the generation of carriers in the silicon substrate [2]. This effect is a serious drawback with respect to the appli- cation of illuminated FET-based sensors. This leads to the shifts of the threshold voltage, depending on the intensity and the spectral distribution of the light penetrating into the Si substrate [3–6]. In order to prevent this serious problem, Voorthuyzen and Bergveld [7] presented a method to elimi- nate the traps which are generated due to light exposure, and reduce the threshold voltage instability. Gimmel [8] used a Pt layer as a light shield to reduce the photosensitivity, but the pH response of their structure was nonlinear. Tin oxide, prepared using thermal evaporation or sputter- ing, as a pH-sensitive material for ISFET applications was first studied in our laboratory [9–11]. Due to the wide band ∗ Corresponding author. Tel.: +886-05-5342601/ext. 2500; fax: +886-05-5312029. E-mail address: choujc@pine.yuntech.edu.tw (J.-C. Chou). gap of tin oxide 3.5eV, the light sensitivity of a tin oxide gate ISFET cannot be eliminated. In this study, we used aluminum as a light shield to develop the ISFET devices: SnO 2 /Al/SiO 2 /Si. This structure not only can maintain a lin- ear pH response of about 56–58 mV per unit pH in the pH range between 2 and 10 but also can effectively reduce light sensitivity. 2. Experimental procedures Four photomasks were designed for the fabrication of the pH-ISFET, and 81 pH-ISFET sensors were fabricated on a four-inch silicon wafer. The channel length and width are 50 and 1000 m, respectively. The fabrication processes are summarized as follows: 1. Starting material: 4 inch diameter, 8–12  cm, (100), p-type silicon wafer. 2. thermal oxide: 5000 Å. 3. 1st photomask and oxide etching. thermal oxide: 5000 Å. 4. Phosphorus ion implantation: dose 10 15 cm −2 . 5. 2nd photomask and oxide etching. 6. Silicon dioxide: 1000 Å and silicon nitride: 1000 Å. 7. 3rd photomask and etching. 8. Al sputtering: 5000 Å. 9. 4th photomask and Al etching. 0254-0584/00/$ – see front matter ©2000 Elsevier Science S.A. All rights reserved. PII:S0254-0584(99)00217-5