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Microelectronic catalytic gas sensor for detecting hydrocyanic acid (hcn) S. Sazhin - Nizny Novgorod State Technical University 'Russia; L.Vadova - N.N. State Technical University 'Russia Contact

1. Introduction

Recently large attention is given to development microelectronics gas sensors, generating an electrical signal, which can be posed in the correspondence with concentration of gas, interesting for us. Among various types of these sensors the greatest research and practical interest represent catalytic sensors, using a chemical reactions at gas/solid interface. The success these devices is determined both theoretical, and applied researches in this area.

In most devices based on temperature or calorimetric measurements the temperature measurement sensor has such a size and thermal inertia that it may disturb the environment to be measured.

An example of such a calorimetric sensor is the platinum wire coil developed by CERCHAR in 1960. This coil is electrically heated to about 1000° C. In the presence of both methane and oxygen, a thermal catalytic reaction takes place on the surface of the wire. The temperature of the wire increases and so does its electrical resistance. For low concentrations the resistance variation is proportional to the gas concentration.

This sensor has several drawbacks: the electrical resistance is low; the cost is high because of hand manufacturing of the platinum coil; loss of heat by thermal conduction along the legs of coil is about a third of the total electric power consumption; the coil has a diameter of 80 mm and it is difficult to reduce this size because of the manufacturing process.

We designed and tested a new platinum microsensor with small energy consumption for detection of hydrocyanic acid. It is produced by microelectronics processes and eliminates the drawbacks presented above.

2. Device Description and Fabrication Process

At development the sensor the following engineering requirements were taken into account: low cost, small losses of heat, good stability to high temperatures and compatibility with a process engineering of a thin film. The sensor works by a principle of registration of heat evolved at course of catalytic oxidizing reaction of hydrocyanic acid on a surface of responsive layer. The touch sensitive structure represents a silicon wafer 400 mm in thickness on which with the help of modern technological receptions the countermeasure feeler is shaped on the basis of Pt films. It is known, that Pt has high catalytic activity in relation to a major series of chemical reactions, including a reaction of a catalytic oxidation. At the same time Pt rather inconvertible behaves in conditions of heightened temperatures and hostile environments. Platinum film serves as a heater to maintain the catalyst at the desired reaction temperature and as resistance thermometer to measure any temperature rise due to the catalytic gaseous reaction.

In order to optimize the active area of the sensor and the mechanical stability, the platinum film is meandered (Fig 1). Connections were made with gold ball bonding. The lamina of a dielectric SiO₂-Si₃N₄-SiO₂ realizes separation and heat insulation of a sensor from a silicon substrate that allows to avoid draw leaks of heat into a surrounding medium and to lower heating up power. Due to a membranous construction of the sensor and low thermal conductivity of dielectric layer it is only 10 mW enough for heating a sensor up to operating temperature.

Fig 1: Design and dimensions of platinum sensor structure

The process steps used to fabricate the device are of microelectronics type: photolithography and thin-film deposition by vacuum evaporation. The source material of the sensor is a silicon wafer, 400mm in thickness, with an additional 1,6mm thick SiO₂-Si₃N₄-SiO₂ dielectric on both sides. To build up the meander structure of catalytic metallic layer the method photolithography was used, for what on a surface of a stratum of a dielectric was ploted positive photoresist, and with the help of appropriate template after exhibiting on a surface there was a positive figure. At the following stage by a method of laser dispersion in vacuum, at first deposited a thick layer of titanium, for best adhesion to a surface of a dielectric, then stratum of platinum. After plotting a stratum of metal for final shaping of figure used a method explosive photolithography. After the feeler was obtained, from reverse of a slice on a surface of silicon, with the help of photoresist formed negative figure of windows, under etching channel. As a sheeting used a lamina of carbon, which was ploted by a method of laser dispersion of a graphite target in vacuum. After the sheeting was put, photoresist with a stratum of carbon was deleted, the windows under etching channels in silicon were thus opened.

The chemical etching channel was produced by a method of dynamic etching with HNO₃ : HF : CH₃COOH (10 :1 :1).

The etching channel in silicon slices, with the planar metal their face planar metal or semiconducting structures, is the difficult technological task. The local depth etching of silicon requires a reliable chemical guard of that part of a surface of silicon, which should not be exposed to etching during long time. This time is defined by a velocity of etching of silicon (4 m m /min), and thickness of channel (250m m), and makes approximately 1hour. We have found, that the stratum of carbon by a thickness 500-600 A, is a reliable guard during etching on stretch of all time of fabrication of a membrane structure.

For calibration of the temperature characteristics of the sensor was used thermostat, which allowed to install and to support by a constant temperature in a range from room up to 150С° . Temperature was measured by the mercury thermometer. The resistance was defined as a ratio of a voltage drop to a flowing past current. For this purpose were used the ampermeter (À) and voltmeter (V). Such installation allows to measure a resistance at small currents.

Fig 3: Temperature assotiation of a resistance.

3. Experimental

The measurements of sensor performances were spent on installation consisting of three main bodies (Fig 4):
• Systems of preparation and feeding of a gas mixture of necessary structure;
• Complex of electrical measuring instruments, power supply and blocks of control;
• The measuring patron with a test structure.

Fig 4: The scheme of experimental installation 1-minicompressor, 2-air filter, 3-rotameter, 4-sensor, 5-power source, 6-ampermeter, 7-voltmeter, 8-XY-recorder

The system of preparation and feeding of a gas mixture consists of gas and air lines. The air line includes the minicompressor, air filter and rotameter, permitting to inspect a stream of an air in litres at one o'clock in the correspondence from it by a preliminary graduation. Gas line also consists from minicompressor, filter and rotameter, through which stream of an air acts in glass flask with HCN, mixs up in it with pairs of a hydrocianic acid and through other orifice of glass flask acts on an entrance of the three-running crane , which ensures in one position passing of a gas mixture in cooling, and in the friend - the gas is added in a common stream of an air continuously acting in the measuring patron.

For security is fixed of low temperature of a hydrocianic acid in view of its force volatility the glass flask with HCN puted in a glass with cold water (6° С).

The scheme of electrical measurements allowed to heat up a researched structure to working temperature (140° С ) by a passage through it of a direct current, governed on magnitude. Working temperature was installed on a value of a resistance by a preliminary graduation.

The modification of temperature of a stratum of platinum called by a response of oxidation HCN on it of a surface, reduced in magnification of a resistance of a stratum and registered by the voltmeter on a modification of a voltage drop on it. The XY recorder allowed to observe a kinetics of processes happening in a researched structure at graduated feeding of gas.

The researches have revealed an association of magnitude of a signal of the response of the sensor from concentration HCN. On Fig.5 the static characteristic of the sensor is shown, whence it is visible, that the magnitude of the response tends to saturation at concentration > 10 ppm. At concentration HCN, were in a range from 0,1 up to 1 ppm the linear concentration association of sensitivity of the sensor is observed. On Fig.6 the dynamic characteristic of the sensor is reduced at graduated concentration of a hydrocianic acid give on the sensor.

Fig 5: The static characteristic of the 1 - 0,2 ppm, 2 - 1 ppm

Fig 6: The dynamic characteristic of the sensor

As it is visible from Fig.6, the time of the response makes about 0,1 s and at magnification of concentration does not vary almost, and the time of a relaxation at magnification of concentration will increase. By work with concentration HCN up to 0,1 ppm for all period of trials (1 month) was not remarked any of a modification of parameters of the sensor. At the same time long periodic action HCN reduced in a drop of sensitivity of the sensor on 10-20%.

On the basis of this sensor a model of a portable gas analyzer on hydrocyanic acid with microprocessor control is developed now. The sensor is included in a shoulder of the measuring bridge, which voltage amplifies by the precision operational amplifier. The strengthened analog signal moves on ÀCP, which on interruption forms on a bus of data a digital signal. The digital information is treated by the processor in the correspondence with the control program.

In a gear the devices light and chime, and also seven-segment indicator working in a condition of dynamic scanning, for map of exceeding of the given level of concentration of a hydrocianic acid are stipulated. The knot of heat allows program to heat up, to operate and to govern temperature of a transmitter, supporting it at the given level.

The gas transmitter is installed through a cable in the remote head. A method of a selection of gas - continuous diffusion. On the left wall of a gear there is a plug for connection to the personal computer on a sequential data channel of a type RS-232C with a velocity of interchanging of 9600 baud.

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