ICT2014 Abstracts

This paper describes the design and fabrication of free-standing high aspect ratio thermocouples (3D TCs) for an uncooled infrared sensor, using MEMS process technologies. In contrast to the conventional horizontal (2D) thermocouple design, our 3D TCs consist of vertically oriented coaxial thermoelectric legs covered by an infrared absorber plate. Using photoassisted electrochemical etching we fabricated an array of high aspect ratio macropores in a silicon substrate, serving as a mold for the 3D TCs. The resulting 3D TCs have diameters and lengths of 5 µm and 515 µm, respectively, and are arranged in a square array with a lattice pitch of 75 µm. Since heavily doped semiconductors provide a good thermoelectric figure of merit, boron (p) - and phosphorous (n) -doped polysilicon are used as thermoelectric materials. A sputtered metal layer interconnects the outer and inner p- and n-polysilicon legs and forms the hot junction. Subsequently, the bulk silicon substrate between the 3D TCs is removed by front-side wet chemical etching and a dielectric multilayer IR absorber is deposited. The thermoelectric properties of the materials were investigated on lateral test structures whereby Seebeck coefficients of 200 µV/K and 150 µV/K were recorded for p- and n-polysilicon, respectively. Based on the above-mentioned geometrical parameters, the electrical responsivity of a 3D TC is estimated to be 520 V/W in vacuum. The high aspect ratio of the 3D TCs of up to 100:1 leads to an increased thermal resistance and thus a higher electrical responsivity compared to conventional 2D design. Further, the coaxially mounted thermoelectric legs lead to small lateral geometries of the 3D TCs, which allow a close-packed arrangement and thus a setup of a high-resolution focal plane array. These features represent the 3D TCs innovative uniqueness and open new ways for uncooled low-cost and high-resolution thermal imagers.