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:

  1. Grow low-stress silicon nitride via low-pressure chemical vapor deposition (LPCVD) on both sides of a silicon wafer.
  2. 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.
  3. Transfer the photoresist pattern via a dry etch, such as a reactive ion etcher (RIE), through the exposed nitride.
  4. Repeat steps 2 and 3 on the backside of the wafer with a pattern to define the opening for the nitride window.
  5. Etch through the exposed silicon using an aqueous solution of potassium hydroxide or another suitable strong base.
  6. 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

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Refer to NPO-42378, volume and number of this NASA Tech Briefs issue, and the page number.

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This article first appeared in the June, 2008 issue of NASA Tech Briefs Magazine.

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