Micromachining shrinks sensors

With micromachining techniques coming of age, Dave Wilson looks at what’s what in the big world of small sensors, and reviews some applications for tiny sensing devices.

According to a recent report from SRI, the first uses of silicon solid state microsensors were in high cost aerospace and military applications.

As prices have dropped, and fabrication technologies have evolved, silicon pressure sensors have entered automotive, process control and medical applications. Silicon accelerometers are now poised for growth in automotive and industrial uses, as designers find a home for them in air-bag actuation systems, ride control and vibration monitors for machines.

Better yet, whole new types of sensors are under development.

Take the field of fluid mechanics, for example, Here, there is a growing need for small, accurate triple hot wire probes for the simultaneous measurement of three perpendicular velocity components in turbulent gas flows. But using conventional anemometer techniques, however, the triple wire approach is very expensive and their structure of the sensor is relatively large.

In one project at the KTH Instrumentation Laboratory in Sweden, however, developers have fabricated a new type of anemometer using micromachining techniques that aims to solve some of the cost problems.

Developed in a co-operative project with Volvo Aero Corporation, the surface micromachined pressure sensors are arranged in different planes for the measurement of the velocity components of turbulent gas flows (Figure 1).

THE SPEED OF SOUND

For its part, the Micro Mechanical Devices Group at the Mesa Research Institute at the University of Twente in the Netherlands, working in conjunction with Sennheiser, has also concentrated on the development of a flow sensor, this time for the measurement of the particle velocity of a sound wave.

Although an acoustic wave is mostly seen as a pressure wave, an acoustic wave actually consists of a pressure and a particle velocity wave. The acoustical pressure can be seen as a temporary increase and decrease in the number of particles in a certain small volume. When the number of particles increases in this volume, particles have to enter the volume.

DIRECT MEASUREMENT

Until a few years ago, it wasn’t possible to measure the particle velocity directly, but with the new sensor, dubbed the Microflown, particle velocity measurements can be made in a very convenient way. The Microflown itself consists of two or three beams of silicon nitride. On top of these cantilevers, a metal pattern is fabricated that acts like a resistor. By heating the metal patterns with an electrical current, a temperature profile is created. An applied flow causes a difference in temperature between the upstream and downstream sensors. The change in resistance of both resistors can be measured and leads to an electrical output proportional to the applied flow. In the case of an acoustical signal, the particle velocity creates an electrical signal representing the particle velocity.

Other investigators are focusing on biotechnology applications. Biosensors could potentially find commercial application in the areas of industrial process control, agriculture, veterinary medicine, defence, and environmental pollution monitoring equipment. But their greatest use is in health care – especially in patient monitoring.

At the Advanced Surface Spectroscopy section of the Navel Research Laboratory in the US, researchers are concentrating on the development of sensors that use microfabricated transducers to detect the minute forces with which antibodies bind to particular molecules. Researchers there hope that this will lead to the development of portable, highly sensitive chemical and biological sensors. One such device, called the Force Amplified Biological Sensor (FABS) uses a solid-phase immunoassay to measure the concentration of a biologically-active analyte.

It takes advantage of the fact that antibodies can bind very selectively to one molecule (Figure 2).

In the related field of Liquid Chromatography (LC), micromachined sensors will play a part too. Today, liquid Chromatography is one of the most widely used methods for separation and analysis of chemical compounds. But to monitor the conditions in an LC system and to detect sample components as they leave the separation column, several detector techniques are needed.

Research conducted at IMC in Sweden, has led to the use of micromachining to build a new type of all-in-one microanalysis system called the ‘CombiCell’. The design integrates sensors for the measurement of seven physical and chemical parameters: pressure, temperature, flow rate, conductivity, pH, UV-absorbtion and fluorescence in one single detector cell.

SANDWICH STRUCTURE

Based on a sandwich structure of three microchips, the lower chip in the sensor contains the structures for the measurement of the pressure, temperature, conductivity and flow rate of a liquid flow. The middle chip defines the fluid flow cell by means of a channel structure that has been etched in silicon, and includes mirrors for the optical elements. The pH sensor chip is mounted in a cavity on the topside of the channel chip (Figure 3).

Despite their advantages, one of the problems in developing such novel sensors is that the CAD techniques developed for helping designers build macroscopic structures may not work at the microscopic level. For that reason, some developers, like those at the Massachusetts Institute of Technology in Cambridge, Massachusetts, USA are working on developing CAD packages specifically for designers of micromachined parts. They are also creating macromodels of some micromechanical systems that can be used by developers of such systems. With the correct tools in place, developing such systems could become a lot easier in the future.

SRI Consulting. Tel: 0181 256 1406

Massachusetts Institute of Technology. Tel: +1 617 253 6869

IMC. Tel: +46 13 28 25 00

The Navel Research Laboratory. Tel: +1 202 404 3384

University of Twente. Tel: +31 53 48 99 111

KTH Instrumentation Laboratory. Tel: +46 8 10 08 58