Hidekazu Uchida

Application of a cubic-like mesoporous silica film to a surface photovoltage gas sensing system

Abstract A self-ordered cubic-like mesoporous silica film has been successfully fabricated in a metal–insulator–semiconductor (=Au/SiO 2 (cubic-like meso)/Si 3 N 4 /SiO 2 /Si) device based on the surface photovoltage (SPV) system and applied to an NO gas sensor. The self-ordered cubic-like mesoporous silica film is synthesized by using as a template in spin coating a nonionic poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO 100 –PPO 65 –PEO 100 ) type triblock copolymer surfactant. The sensing characteristics of the self-ordered cubic-like mesoporous SPV system have been investigated by repeated exposure to 100 ppm NO gas and standard air, as well as observation of the alternating (photo) current, which resulted from the physical adsorption and chemical interactions between detected NO gas and the self-ordered cubic-like mesoporous film. In sensing NO gas, this cubic-like mesoporous SPV system exhibits a response nearly five times larger than that of a simple SPV sensor without mesoporous silica film. Even at room temperature, this mesoporous SPV system exhibits a recoverable response. These results can be explained by the characteristics of the cubic-like mesoporous silica film including large surface area and a bi-continuous mesopore structure. This kind of mesoporous film has a great potential for application to highly sensitive and responsive gas sensors.

Integrated taste sensor using surface photovoltage technique

Abstract A surface photovoltage (SPV) technique has been applied to construct a taste sensor by combining modified LB (Langmuir-Blodgett) methods to immobilize taste-sensitive membranes. The contactless approach of the SPV method provides a simple sensing system with considerable patterning flexibility. Several kinds of artificial lipid membranes are monolithically integrated on a semiconductor surface as taste-sensitive materials and the surface potential change caused by the reactions with taste substances is detected by scanning a light beam along the semiconductor surface. First it is shown that the uniformly oriented lipid membranes exhibit different responses to five taste substances, with high sensitivity and fast response rate. Then, a preliminary experimental result to identify commercial drinks is demonstrated.

Integrated biosensor employing a surface photovoltage technique

Abstract A monolithically integrated biosensor is constructed using a surface photovoltage (SPV) technique combined with a new patterning method for multiple enzyme integration. The SPV method provides a contactless sensing system leading to patterning flexibility. Photolithographical patterning methods using a water-soluble photocrosslinkable polymer (copolymer of dimethylacrylamide and cinnamoyloxyethylmethacrylate) are applied to immobilize the enzyme on a semiconductor surface. For bonding the enzyme membrane to the semiconductor surface, photoreactive poly-(meta azide styrene) is used, which bonds covalently with both the enzyme membrane and substrate. A pen-printing method has also been proposed for the patterning of enzyme films, which provides a simple method suitable for mass production.

Highly sensitive taste sensor with a new differential LAPS method

A new differential measurement method for a LAPS (light-addressable potentiometric sensor) has been developed and applied to fabricate an integrated taste sensor with artificial lipid membranes as the ion-sensitive material. The differential measurement procedure is based on a time-sharing technique, which makes it possible to achieve a very sensitive and highly stabilized response due to the noise-compensation effect. Sensitivity enhancement is further achieved by cancelling the base component of the differential response current. These techniques improve the sensitivity by at least two orders of magnitude compared to a conventional LAP system. The sensor shows highly sensitive responses to various taste substances, which makes it possible to identify a sweet taste through pattern-recognition routines. Miniaturization of the LAPS is also attained by using a small metal pseudo-reference electrode instead of a glass electrode.

A novel semiconductor NO gas sensor operating at room temperature

Abstract A novel NO gas sensor workable at room temperature was proposed and discussed the response mechanism. It is based on a Si diode structure consisting of Pd–Pt/WO 3 /p-Si/Al. WO 3 film was deposited by reactive sputtering and crystallized by thermal annealing. The gas detection was carried out by measuring vertical direction current between Pt–Pd electrode and Al ohmic contact. The sensor showed sensitivity to NO gas at the concentration lower than 50 ppm at room temperature. The 90% response time was less than 3 min and good reversibility was observed without the shift of the base level current. This sensor may lead to the development of integrated sensors on a single Si chip.

A study of silicon Schottky diode structures for NOx gas detection

Abstract A silicon Schottky diode structure was applied for detecting nitride oxide gases at room temperature. The Pt–Pd/Si/Al structure was employed successfully to detect NO2 gas concentration for as low as 6 ppm at room temperature. This sensor also showed useful response to NO gas, but the sensitivity was lower than its sensitivity to NO2 gas. Fabrication of the diode on a porous silicon surface enhances NO2 gas sensitivity, but the response time becomes longer. This structure provides a convenient technique to manufacture miniaturized and integrated sensors.

An organic pollution sensor based on surface photovoltage

Abstract A surface photovoltage (SPV) device, which is sensitive to surface pH, was applied to the fabrication of an organic pollution sensor. Trichosporon cutaneum, designated for use as a biochemical oxygen demand (BOD) sensor by the Japanese Industrial Standard, was employed as the immobilized biocatalyst. A flow cell of the system consists of an SPV device and a microbial membrane immobilized with T. cutaneum between membrane filters. The pH measurement by the SPV device and fabrication of SPV-based sensor is discussed and characterized by comparison with the 5 day BOD test (BOD5) and sensors used in the measurement of BOD (BODs). Response time was 25 min, and a microbial membrane can be used more than 14 weeks. Though there is a different substrate specificity, the system was applicable to BOD measurement of some real waste water and shows good agreement with BODS and BOD5.

High speed chemical image sensor with digital LAPS system

A high speed two-dimensional surface photovoltage (SPV) sensing system based on a digital data processing was developed and applied to an in-situ monitoring of chemical images. The SPV signal generated by a scanning light spot was directly memorized in a computer and signal integration of all measurement points was carried out in parallel in allocated memories by numerical calculation, which made it possible in principle to reduce the measurement time as short as the scanning time of the light spot. For the formation of the image of 6400 data points, the proposed system needs about 30 min which is at least one order of magnitude faster than that of a conventional analog SPV system.

NO Gas Sensor Based on Surface Photovoltage System Fabricated by Self-Ordered Hexagonal Mesoporous Silicate Film

The first reported NO gas sensor based on a surface photovoltage (SPV) semiconductor device system is fabricated with a metal/SiO2 (self-ordered hexagonal mesoporous)/Si3N4/SiO2/Si structure (MIS). A size controlled silicate mesoporous film is successfully synthesized by spin coating on a Si3N4/SiO2/Si silicon wafer using poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (Pluronic P123=EO20PO70EO20) triblock copolymers as a template. The characteristics of the mesoporous films were investigated by X-ray diffraction (XRD) and transmission electron microscope (TEM). The sensing properties of the self-ordered hexagonal mesoporous SPV system have been investigated by repeated exposure to NO gas and air. The changes in the average value and phase of the AC photocurrent (Iph) have been observed after exposure of the films to 100 ppm NO gas. The response of the alternative photocurrent results from the physical adsorption and chemical interaction between detected NO gases and the self-ordered hexagonal mesoporous film.

Highly sensitive semiconductor NOx gas sensor operating at room temperature

Abstract The novel semiconductor NOx gas sensor based on a heterojunction structure workable at room temperature has been investigated. The sensor consists of Pt/Tin-dioxide(SnO2)/n-Si/p+-Si/Al in which vertical direction current between Pt and Al electrode was measured with applying reverse direction bias voltage on Pt electrode. All the films including SnO2 with the thickness of 50–200 nm were deposited on an epitaxial layer of n-Si over p+-Si substrate by RF sputtering method. As a result, the current in the sensor decreased when the gas flow was switched from dry air to mixed gas of air and NOx. Clear response was obtained at the NOx gas concentration as low as 1 ppm at room temperature, while the almost no response was observed for the n-Si, p-Si, and p-Si/n+-Si substrate. The generation of large change in current for the sensor was considered that the barrier height change or conductivity change of SnO2 gas sensitive layer may cause the modulation of the depletion layer at the n/p+-junction of Si substrate.