Application of Thin-Film Amorphous Silicon to Chemical Imaging
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0910-A20-01
Application of Thin-Film Amorphous Silicon to Chemical Imaging Tatsuo Yoshinobu1, Werner Moritz2, Friedhelm Finger3, and Michael J. Schoening3,4 1
Department of Electronic Engineering, Tohoku University, 6-6-05 Aza-Aoba, Aramaki, Aoba-ku, Sendai, 980-8579, Japan 2
Humboldt University Berlin, Brook-Taylor-Str. 2, Berlin, 12489, Germany
3
Research Centre Juelich, Juelich, 52425, Germany
4
University of Applied Sciences Aachen, Division Juelich, Ginsterweg 1, Juelich, 52428, Germany ABSTRACT A thin-film amorphous silicon (a-Si) deposited on a glass substrate was employed as a semiconductor material for the chemical imaging sensor, which can visualize the distribution of ion concentration in a solution. The sensing properties and the spatial resolution of the a-Si sensors were investigated. Nearly-Nernstian pH sensitivities and submicron resolution were demonstrated, which suggests the superior performance of the chemical imaging sensor based on thin-film a-Si. INTRODUCTION The chemical imaging sensor [1] is a semiconductor-based chemical sensor that can visualize the spatial distribution of ion concentration in the solution. It is based on the principle of the light-addressable potentiometric sensor (LAPS) [2,3], which is a field-effect sensor with an electrolyte-insulator-semiconductor (EIS) structure as shown in figure 1. A dc bias voltage is applied to the EIS system so that a depletion layer is induced at the insulator-semiconductor interface. The width of the depletion layer is dependent not only on the applied bias voltage, but also on the ion concentration of the solution in contact with the sensing surface. The measuring principle of the LAPS is similar to that of the EIS capacitance sensor [4,5], in which the capacitance of the EIS system is measured to determine the ion concentration. In the case of the LAPS, the semiconductor layer is illuminated with a modulated light to induce an ac photocurrent as a sensor signal. In contrast to the EIS capacitance sensor, which gives an average value over the whole sensing surface, the measured area of the LAPS is defined by illumination and therefore, spatially resolved measurements are possible [1, 6]. In the chemical imaging sensor, a focused laser beam scans the sensing area to obtain a map of photocurrent, which
Figure 1. Schematic of the light-addressable potentiometric sensor (LAPS). represents the spatial distribution of the ion concentration. The LAPS is also a basis of an integrated multisensor [7-10] in which more than one measuring point on the sensing surface is individually accessed by the light beam. The spatial resolution is an important factor in both chemical imaging applications and multisensor applications. The spatial resolution determines the smallest size of structures that can be visualized by the chemical imaging sensor and it limits the density of measuring points on the multisensor surface. It has been demonstrated both experimentally and theoretically [11-13] that the spatial resolution is determined by the beam size and t
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