Research Academics  
 
   
 

Basic Modules for Chemical Sensors
 
Contact Person : Dr.-Ing. H-D. Ngo

Introduction

Interdigital capacitors: In the field of chemical sensors fabricated in thin film technology the IDC (interdigital capacitor) is an often used structure for signal conversion (Fig. 1). The change of the resistance or the variation of the dielectric constant of a sensitive coating can be measured by it. Small structure widths are essential for measurements in the high resistance area or for high sensitivity in capacitive measurements. IDC-structures made of NiCr/Au with dimensions down to 2.5 µm electrode width have been fabricated.

 

Principel

Fig. 1: Schematic of a chemical sensor with IDC and sensitive coating

Directly heated sensor modules

The response of a chemical sensor, e. g. the selectivity, sensitivity and the response time can be optimised by choosing an appropriate operating temperature of the sensitive layer. A direct temperature control of the gas sensitive area can be achieved by using a resistance heater on the chip. Fig. 3 shows the concept of a sensor chip with an IDC structure on the front side and a heating resistor on the back side. Based on this concept a directly heated sensor module has been fabricated for operating temperatures up to 450°C (Fig. 2). Two different designs can be offered with optimised temperature uniformities for 200°C and 380°C with a maximum temperature deviation across the sensitive area of less than 2.8 %. Heating powers of 1.2 W and 3.5 W are needed, respectively . For the temperature range up to 100°C a similar device has been developed with smaller dimensions and both, the heater and IDC are located on the same side of the chip (Fig. 4). The directly heated sensor modules represent a flexible and inexpensive way of temperature control for chemical sensors. A capacitve CO2-sensor with an operating temperature of 70°C has been realized recently.

 

    

Fig 3: Fabricated sensor chip without upper glass plate. Left: SEM-Picture, Right: Photograph of the device

 

Device for calorimetric measurements

A common method for detecting gases is the measurement of the energy of a catalytic reaction between a sensitive layer (the catalyst) and the gas. For this application a micromachined sensor module consisting of a heater and a temperature sensor situated on a thin membrane has been developed. Fig. 5 shows a schematic of the device, known as a "pellistor". By using thin membranes a low power consuming module can be realized. A temperature of 250°C can be achieved with a heating power of 100 mW for a membrane size of 1.25x1.25 mm2. The pellistor is suitable for implementation in sensor arrays (Fig. 6).

 

Pellistor  Pellistor

Fig. 4: left: Schematic of the pellistor. Chip dimensions: 4x4 mm2.
right:  Sensor array mounted on a TO 8 header. One sensor is red-hot because of the high temperature ( 750 °C)