Surface micromachined polyimide scanning thermocouple probes

This paper describes micromachined scanning thermocouple probes that exploit the low thermal conductivity and the high mechanical flexibility of polyimide as a structural material. They are surface micromachined using a low-temperature six-mask process suitable for appending to a CMOS fabrication sequence. The probes are 200-1000-μm long, 40-120-μm wide, and of varying thickness. They are assembled by a flip-over approach that eliminates the need for dissolving the substrate wafer or removing the probe from it. Temperature sensing is provided by thin-film Ni/W or chromel/alumel thermopiles embedded in the polyimide, which provide Seebeck coefficients of 22.5 and 37.5 μV/K per junction, respectively. Modeling results indicate that the low thermal conductivity of polyimide causes the temperature drop along the probe length to be much higher than with other candidate materials such as Si or SiO₂, which contributes to improved thermal isolation of the sample and higher temperature sensitivity of the probe. However, the response time of the probe is compromised, and the measured -3 dB bandwidth of the probes is ≈500 Hz. A sample scan is presented