SnO₂-Pd as a Gate Material for the Capacitor Type Gas Sensor ^†

Abstract

The article describes the result of the use SnO₂-Pd thin films as a gate for structure measured ppb range of NO₂ gas by the capacitive method. The technological aspects of fabrication SnO₂-Pd gate and one comparison by metrological parameters with the classical Pd gate field effect sensor are discussed. The use of SnO₂-Pd material allows improvement in sensitivity of NO₂ by an order of magnitude compare the classical Pd based gate field effect sensors.

1. Introduction

Using of SnO₂ material for fabrication field effect gas sensors based on Schottky diode effect firstly describing in work [1], somewhat later, a study of the sensitivity of field-effect sensors to NO₂ began [2]. But still remains an important issue measurement of sub-ppb concentrations of NO₂ in such areas as medicine, environmental control and explosives detection. Materials based on SnO₂ are widely used to measure NO₂ by resistive type MOX sensors, but MOX sensors margin of stable sensitivity is limited by sub-ppm range. The further increase in sensitivity to NO₂ is possible in the technological combination of well proven material using nowadays in MOX sensors and high sensitivity of field-effect sensors.

2. Experimental

In [3] was shown that the characteristics of field-effect sensors strongly depend on the composition metal-dielectric transition layer, which is determined by the materials of metal gate and insulator dielectric and methods of their deposition. For a new capacity type sensor manufacturing n-type silicon substrate with a 0.1 μm SiO₂ layer thickness was used. Layer of SnO₂ is additionally deposited through the shadow mask on the SiO₂ film by the method of magnetron sputtering. This method of deposition allows you to form films with a high effective surface area. The Pd film with a thickness of 100 nm was coated SnO₂ film through the shadow mask by pulse laser deposition method. The Pd-SnO₂-SiO₂-Si structure was basis of capacitor type gas sensor which photo present on Figure 1 and cross-section scheme of structure on Figure 2, respectively. In parallel with the SnO₂-Pd based sensor, a similar series of capacity sensors without SnO₂ layer, only with a 100 nm Pd gate, was made for comparing changes of gas-sensitive characteristics.

Figure 3 presents the capacitance-voltage characteristic for Pd gate sensor at different heating temperatures. The capacitance-voltage characteristics for SnO₂-Pd and Pd base sensors at temperatures of 170 °C, 140 °C and 100 °C was study and present in Figure 4. Also sensitivity to NO₂ at different temperatures present on Figure 5 and Figure 6. On Figure 4 and Figure 5 can be seen that the slope of the characteristic decreases with increasing temperature, therefore, the sensitivity of the sensor should decrease; this effect is explained by the shift of C-V characteristic during exposed to gas. The Figure 5 shows the time responding capacity sensor with SnO₂-Pd gate to 108 ppb NO₂ at different temperature and possible to see decreasing response time and sensitivity during increasing working temperature of sensor. This behavior is standard for the field effect sensor.

The using of the new type SnO₂-Pd gate in capacity type sensor gives possible to raise sensitivity by almost ten times but disadvantage for such approach is increasing response and relaxation times of the sensor. Possible approach for improving response and relaxation times is using pulse temperature mode increasing diffusion rate through the gate present in work [4]. Extrapolation of present measurement results suggests stable detection NO₂ concentrations by the Pd-SnO₂-SiO₂-Si structure in the region of 1 ppb and less.