A process that includes photolithography, liftoff, etching, and sputter deposition has been developed to enable the fabrication of thin, finely patterned layers of gold, platinum, and other difficultto- etch materials in advanced miniature sensors and associated electronic circuitry. Heretofore, photolithography has been used in conjunction with liftoff and etching to produce finely detailed structures in easy-to-etch materials. The present process is needed because conventional photolithography cannot be used to pattern difficult-to-etch materials and the alternative processes heretofore available for patterning difficult-to-etch materials are limited to spatial resolution of about 0.005 in. (≈0.13 mm) or coarser.
The process (see figure) includes some new steps plus some steps from prior processes that are used with modifications and in a different sequence. The sequence is the following:
- Copper is deposited over an entire face of a substrate of suitable material (e.g., alumina). If the substrate material is one to which copper does not adhere well, aluminum or nickel can be used instead of copper. The thickness of the copper (or aluminum or nickel, as the case may be) should be made roughly three times that of the final desired difficult-to-etch metal layer.
- The copper is covered with positive photoresist.
- The photoresist is soft-baked.
- The photoresist is exposed through a photomask in the desired pattern.
- The photoresist is developed.
- It is hard baked at 110 °C for 30 minutes.
- The entire workpiece is washed with a half-and-half mixture of nitric acid and water to remove the copper from the regions in which the difficult-toetch metal is to be deposited. This removal of copper is facilitated by a slight undercutting of the photoresist. The most significant aspect of this process is the use of copper as the liftoff material. Partly because of the rapidity with which copper is etched, it is possible to control the amount of undercutting of the photoresist and to ensure that the undercutting occurs uniformly and cleanly. Undercutting is very important to the success of this process: it expedites removal of unwanted material later in the process and increases the sharpness of the final pattern of difficultto- etch metal.
- The substrate holder is precooled to 10 °C before the piece is loaded into the sputtering system.
- The difficult-to-etch metal is sputterdeposited at a relatively low power density (0.14 W/cm2) over the entire substrate. In the regions from which the copper and photoresist were previously removed, difficult-toetch metal becomes deposited directly onto the substrate. During the deposition of the difficult-to-etch metal, the substrate is cooled to prevent the carbonization of the photoresist; this is necessary because carbonization would make the subsequent removal of the photoresist more difficult.
- The workpiece is washed with acetone to remove all the photoresist plus the portion of difficult-toetch metal deposited on top of the photoresist.
- The workpiece is washed with the nitric acid/water mixture to etch away all the remaining copper, leaving the desired difficult-to-etch metal pattern.
This work was done by Charles A. Blaha of Akima Corp. for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Manufacturing category.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17079.