A novel type of metal-on-polymer micromachined thermopile

We report the fabrication and characterization of a novel type of metal-on-polymer micromachined thermopile. The thermopile is based on a design that consists of many three-dimensional mesas that are coupled in series. These mesas are produced by lithographically structured SU-8 deposited on silicon wafers. Identical copies were replicated into thermoplastic materials by injection moulding. By evaporative coating of Alumel and Chromel from two equal but opposite angles the two alloys will, due to shadowing effects, overlap each other only at the top of the mesas and between them. These two areas will therefore form the hot and cold solder junctions. Typical height and width of a mesa were 200 /spl mu/m and 300 /spl mu/m, respectively. The deposited metal was about 100 nm thick at the top, less metal would not form a conductive coating over the whole mesa structure. Scanning electron microscopy was used to investigate the surfaces of the micromachined mesas in SU-8 and thermoplastics, which in both cases showed very smooth surfaces. We measured the generated thermovoltage over many thermocouple elements coupled in series, and evaluated the voltage dependency of mesa height and number of mesas. These pilot tries demonstrate that our concept for a low-cost polymer-based thermopile element generates a measurable thermovoltage. More detailed experiments will be carried out in order to further characterize the polymer thermopile, especially with respect to sensitivity, noise equivalent power and response time, all three important figures of merit for a thermopile. Such a thermosensing element can be used in various applications where miniaturization is essential, for instance, infrared detection of handheld gas sensors, liquid flow sensors for microfluidics, or calorimetry for monitoring chemical or biological reactions.