Porous Silicon Organic Vapor and Humidity Sensor
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ABSTRACT This paper presents new organic vapor sensitive device using anodized porous silicon (PS). The sensor has aluminum (Al)/PS/p-Si/AI Schottky diode structure and sensitivity at room temperature in 2600 ppm acetone, methanol, 2-propanol and ethanol is about 4, 5, 10 and 40 times respectively. The sensitivity in 800-2600 ppm ethanol vapor is 2 to 40 times. The diode sensor can be converted into an Al/PS/Al resistor sensor by switching the electrical contacts, and the sensitivity is about 500 times for a humidity change of 43-75%. All sensors have response time of about 0.5 min. The sensitivity is stable with time and the PS sensor can be integrated into VLSI Si devices to form novel microelectronic systems. INTRODUCTION Porous silicon (Si) has many novel and unique properties such as high surface area (>200 m2/cm 3) and chemical activity, high sensitivity and efficiency in light absorption, and light emitting. It can have numerous applications including opto-electronic devices, micro-electromechanical systems (MEMS) and sensors [1-5]. The design and fabrication of porous Si (PS) devices are also compatible with the mature monolithic Si very-large-scale-integrated (VLSI) technologies. However, until now little research has been done on the porous Si devices. In this paper, we present our new results on the systematic study of a novel PS based organic gas and humidity sensor. EXPERIMENT Boron doped 15-25 a-cm p-Si wafers were chemically cleaned. Aluminum (Al) film of about 1 gm thick was deposited onto the back side of the wafers and annealed at 450°C/30min in dry nitrogen to obtain an uniform ohmic contact. This step is critical in obtaining uniform PS layer by anodization. The Al film was protected with photoresist, and PS layer was formed by anodizing the front Si surface in dilute HF-ethanol solution (HF:C 2H5 OH = 1:1), with platinum electrode and at a current density of 5-15 mA/cm 2 for 5-10 min in room light and temperature. For better passivation of the PS surface, some PS samples were further boiled for 5-10 min in dilute nitric acid solution (HNO 3 :DI=1:9). The photoresist was removed and Al circular contacts with diameter of 1-3mm and thickness of 15-270 nm were then deposited onto the surface of the PS layer to form Al/PS/p-Si/Al "Schottky-like" diodes. To measure the organic vapor sensing characteristics, the diodes were placed in a closed chamber filled with fixed concentration (0-2600 ppm) of vapor mixtures of ethanol, acetone, 2-propanol, and methanol, respectively. Humidity sensitivity was measured in water vapor with 43-75% relative humidity. Sensor responses were studied by measuring the current-voltage (I-V) characteristics of the 183 Mat. Res. Soc. Symp. Proc. Vol. 459 @1997 Materials Research Society
diodes, and sensitivity of sensor was defined as the ratio of diode current in organic gas and in air for fixed forward applied voltage. Poly-Si film with thickness of 670 nm was also used to form the PS layer. The poly-Si film was formed by thermal decomposition of silane in a LPCVD reactor
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