Slits for imaging spectrometers can now be fabricated to a precision much greater than previously attainable. What makes this possible is a micromachining process that involves the use of microlithographic techniques. This micromachining process supplants a prior machine-shop process.
In the specific application that gave rise to this development, there is a requirement to make imaging-spectrometer slits 27 μm wide and 1.7 cm long. In the prior machine-shop process, the slits were formed by electrical-discharge machining (EDM). The slit widths could not be maintained accurate to within less than about 12 μm, and there was some long-range drift over the 1.7-cm slit lengths. The present micromachining process affords about 100× the precision of the EDM process, with corresponding reductions in the tolerances for slit-width error and long-range drift.
An overview of the micromachining process for fabricating slits consists of the following steps:
- Grow low-stress silicon nitride via low-pressure chemical vapor deposition (LPCVD) on both sides of a silicon wafer.
- In a photolithographic subprocess, spin the front-side silicon nitride coated wafer with a photoresist, expose the photoresist through an optical mask to define the opening to be formed, and develop the photoresist to transfer the pattern into the resist.
- Transfer the photoresist pattern via a dry etch, such as a reactive ion etcher (RIE), through the exposed nitride.
- Repeat steps 2 and 3 on the backside of the wafer with a pattern to define the opening for the nitride window.
- Etch through the exposed silicon using an aqueous solution of potassium hydroxide or another suitable strong base.
- Deposit an opaque, low-stress layer of a suitable metal (e.g., titanium/gold) on both sides.
This work was done by Daniel Wilson, James Kenny, and Victor White of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-42378, volume and number of this NASA Tech Briefs issue, and the page number.