Processes for fabricating thin-film type-S thermocouples on metal and silicon nitride substrates (see figure) have been developed. (A type-S thermocouple comprises one leg made of platinum and the other leg made of an alloy of platinum with a rhodium content of 10 percent.) Thin-film type-S thermocouples can be used to measure temperatures on the surfaces of metal and ceramic components and are intended, especially, for use as monitoring and control sensors in the development and operation of advanced engines.
The fabrication processes involve numerous steps. Combining and summarizing some steps for clarity, the processes can be described as follows:
The process for fabrication on a metal substrate begins with polishing and cleaning the substrate. The clean substrate is precoated (by vapor deposition) with NiCoCrAlY to a thickness between 50 and 120 µm. The precoated substrate is cleaned and oxidized to form an electric surface layer of a α-Al2O3.
A photoresist is applied and patterned with an opening for the Pt leg of the thermocouple, and the Pt leg is deposited by sputtering to a thickness between 2 and 3 µm. After removal of photoresist and further cleaning, a new photoresist mask for the Pt/Rh leg of the thermocouple is formed, then the Pt/Rh leg is deposited to a thickness of 2 to 3 µm. The second photoresist mask is removed, the workpiece is cleaned, and a protective overcoat of Al2O3is deposited on vulnerable portions of the thermocouple legs to a thickness between 1.5 and 2.0 µm.
Transition Pt and Pt/Rh wires with a diameter of 3 mils (≈76 µm) are welded to the outer contact ends of the Pt and Pt/Rh thermocouple films, respectively. Then external 0.020- or 0.040-in. (0.5- or 1.0-mm)-diameter sheather lead wires of Pt and Pt/Rh, respectively, are welded to the outer ends of the Pt and Pt/Rh transition wires. The transition wires and their welded junctions are covered with a ceramic cement.
The process for fabrication on a silicon nitride substrate also begins with polishing. The polished substrate is thermally oxidized to form silicon dioxide and silicon oxynitride, then etched in hydrofluoric acid to remove the silicon dioxide and leave a layer of silicon oxynitride 60 to 100 Å thick. The remaining steps of forming the Pt and Pt/Rh legs, attaching wires, and protective overcoating are similar to those described above for fabrication on a metal substrate, except an anneal is performed before attaching the transition wires.
This work was done by Raymond H. Niska, Nicholas F. Schmidt, and Joseph R. Hoge of Allied-Signal Aerospace Co. for Lewis Research Center.For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Manufacturing/Fabrication category, or circle no. 145 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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