The sounding rocket experiment FIRE (Far-ultraviolet Imaging Rocket Experi ment) will improve the science community’s ability to image a spectral region hitherto unexplored astronomically. The imaging band of FIRE (≈900 to 1,100 Å) will help fill the current wavelength imaging observation hole existing from ≈620 Å to the GALEX band near 1,350 Å. FIRE is a single-optic prime focus telescope with a 1.75-m focal length. The bandpass of 900 to 1100 Å is set by a combination of the mirror coating, the indium filter in front of the detector, and the salt coating on the front of the detector’s microchannel plates. Critical to this is the indium filter that must reduce the flux from Lyman-alpha at 1,216 Å by a minimum factor of 10–4. The cost of this Lyman-alpha removal is that the filter is not fully transparent at the desired wavelengths of 900 to 1,100 Å.

A cutaway view shows the Detector Assembly and Filter. The indium filter sits just in front of the detector plates in the light beam (yellow cone) at the orange ring.
Recently, in a project to improve the performance of optical and solar blind detectors, JPL developed a plasma process capable of removing carbon contamination from indium metal. In this work, a low-power, low-temperature hydrogen plasma reacts with the carbon contaminants in the indium to form methane, but leaves the indium metal surface undisturbed. This process was recently tested in a proof-of-concept experiment with a filter provided by the University of Colorado. This initial test on a test filter showed improvement in transmission from 7 to 9 percent near 900 Å with no process optimization applied. Further improvements in this performance were readily achieved to bring the total transmission to 12% with optimization to JPL’s existing process.

A low-power, hydrogen plasma treatment is generated in a PlasmaTherm RIE etcher using a mixture of argon and hydrogen gas. The gas ratio is optimized in order to control the following variables: bias voltage, atomic hydrogen content, and substrate temperature. Low bias voltage is required to avoid mechanically degrading the filters by sputtering the indium foil. High atomic hydrogen content is required to enhance the carbon removal rate. Low substrate temperature is required to avoid deformation of the indium foil due to sagging. Those variables are optimized around MFC (mass flow controller) setpoints of 25 sccm argon and 7 sccm hydrogen.

This work was done by Harold F. Greer and Shouleh Nikzad of Caltech, and Matthew Beasley and Brennan Gantner of the University of Colorado 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
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Plasma Treatment To Remove Carbon From Indium UV Filters

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This article first appeared in the September, 2012 issue of NASA Tech Briefs Magazine.

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