Long-Term Stability of Thermally-Carbonized Porous Silicon Humidity Sensor
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Long-Term Stability of Thermally-Carbonized Porous Silicon Humidity Sensor
J. Tuura, M. Björkqvist, J. Salonen and V-P. Lehto Department of Physics, University of Turku, FI-20014 ABSTRACT Thermally-carbonized porous silicon humidity sensor showed ageing affecting electrical characteristics. During the first month the variations in electrical characteristics were very distinctive. The decline in the sensitivity of the sensor after three months storage was found to be 37%, however, the sensitivity was still over 200%. When aged, the sensor stabilized and only minor variations on capacitance were noticeable. Sensitivity, on other hand, remains nearly constant. The hysteresis of the sensor did not change remarkably during storage. The variations on capacitance values at different relative humidities during storage were measured as a function of detection frequency. This showed an interesting feature of the ageing of the sensor, which is also discussed.
INTRODUCTION Porous silicon (PSi) is an attractive candidate for gas and humidity sensing purposes due to its large specific surface area and compatibility with existing silicon based integrated circuit production technologies [1,2]. The full utilization of PSi in sensing applications has been limited, because of its natural tendency to oxidize [3]. This oxidation or so called ageing, causes longterm changes on the properties of PSi. Anyhow, PSi has been applied on a wide range of scientific areas ranging from medical and pharmaceutical to electrical and optical applications [1,4-8]. Since the first reported prototype of thermally-carbonized porous silicon (TCPSi) humidity sensor [9], efforts have been made to optimize the properties of such sensor. Annealing has been applied in processing steps, as it decreases hysteresis effectively by enlarging pores of the PSi. A drawback of annealing is, that the sensitivity of the sensor decreases [10]. However, the sensitivity remains over 200%. To improve the reproducibility and the treatment efficiency, the carbonization procedure contains a two-step thermal treatment of the porous layer. In the first part of the treatment, low temperature (500 oC) and continuous flush of acetylene and nitrogen gas mixture have been applied to obtain a hydrocarbon termination of the surface. The second part consists of the high temperature (>800 oC) treatment with nitrogen flush only, which forms the silicon carbide surface. In this work, we report long-term stability studies of a TCPSi humidity sensor for the first time. Changes in the electrical characteristics of the sensor were monitored in this study. The observed quantity of the sensor is capacitance, because it is more sensitive to a wide range of different relative humidities than resistance at corresponding conditions. Also, the capacitance does not vary as much as the resistance during ageing. Therefore the variations of resistance are excluded from the discussion in this paper.
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As the capacitance was measured several times during a one year period, the sensor was st
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