Effect of heat treatment on high sensitivity of Al2O3 sensing membrane using the ALD for pH-ISFET

The ion-sensitive field effect transistor (ISFET) of chemical sensors has been used due to many reasons which have the advantages of small size, light weight and fast response by semiconductor processing. In recently, the sensing membranes applied to high-k materials such as Al2O3, ZrO2, HfO2, TiO2 and Ta2O5 for pH-ISFET were investigated because of the high sensitivity [1]. The Al2O3 film among the high-k materials have been deposited as the methods of sputter and atomic layer deposition (ALD). Generally, the Al2O3 film deposited method of sputter has the problem with thickness control and non-uniformity, while method of ALD has the advantage such as high density, low impurity, excellent step coverage, easily controlled thickness as the atomic scale, good uniformity at the large area and low deposition temperature [2]. The Al2O3 film used sensing membrane deposited as method of ALD was performed the heat-treatment of subsequent process step, it is important parameter that determines the sensitivity of the Al2O3 pH-ISFET [3]. In this paper, the Al2O3 film of sensing membrane has been fabricated by using the ALD to be applied in the pHISFET and its characteristics of heat-treatment have been investigated. Substrate was 4inch p-type (100) silicon wafer. 20nm Al2O3 film was deposited on 35nm silicon oxide was grown by dry oxidation processing. The MOSFET devices having a gate of 2500 um width and 50 um length were fabricated by using the semiconductor processes and the gate of FET consisted of thick silicon oxide and thick Al2O3 film. Annealing process of Al2O3 film was performed in O2 ambient for 40m at a temperature of 500, 600, 800 and 900 °C. The structure of fabricated Al2O3 pH-ISFET based on the MOSFET device is described in Fig.1. To confirm of pH sensitivity, the drain currents of Al2O3 pH-ISFET were measured in the pH solution (pH10 → pH7 → pH4). The commercialized reference electrode of the Ag/AgCl and liquid-junction filled with 3 M KCl was used to establish the solution potential in the pH solution. Fig. 2 shows the variation of drain current in pH solution heat-treated at 600°C. Drain current of Al2O3 pH-ISFET at pH10, pH7 and pH4 were 0.273 mA, 0.331 mA and 0.396 mA, respectively. The results measured drain current variation of Al2O3 pHISFET with annealing temperatures of 500, 600, 800 and 900°C were described in Fig. 3. From the measured results, we have found that the sensitivity variation of Al2O3 pH-ISFET was more increased at 900 °C than the other condition of annealing temperature, because it was increased the number of O-sitesin the Al2O3 film of sensing membrane. The Al and Ocompositon ratio of the Al2O3 film was a most importantparameter that determines the sensitivity of the Al2O3 pH-ISFET.References [1] Sang-Kwon Lee, Young-Soo Sohn, Sie-Young Choi,Sensor Letters, 9 (2011), 3-6[2] Stefan Jakschik, Uwe Schroeder, Thomas Hecht, MartinGutsche, Harald Seidl, Johann W. Bartha, Thin SolidFilms 425 (2003) 216-220.[3] Dae-Hyuk Kwon, Byung-Woog Cho, Chang-Soo Kim,Byung-Ki Sohn, Sensor and Actuators B, 34 (1996),441-445. Fig. 1. The scheme of Al2O3 pH-ISFET based on theMOSFET device. Fig. 2. The drain current of Al2O3 pH-ISFET in pH