LEDs Provide Stable Spectroscopic Light Sources

To estimate the concentration of various species in plasmas, scientists frequently employ broadband absorption spectroscopy. The xenon arc or deuterium lamps used as light sources in the devices, however, display high spatial and spectral variations. The resulting fluctuation of the absorption baseline prevents accurate measurement of substances with low densities or with overlapping absorption bands.

The experimental setup used for plasma analysis incorporates an LED light source. Courtesy of Gilles Cunge, CNRS-LTM.

Researchers at the Centre National de la Recherche Scientifique-Laboratoire des Technologies de la Microélectronique (CNRS-LTM) in Grenoble, France, have used LEDs successfully in place of lamps in a broadband absorption spectroscopy system. Their modification allows accurate analysis of low-pressure, high-density plasmas as well as of time-resolved density measurements.

Their setup incorporated an LED encapsulated in an air-cooled strip of copper for temperature stabilization and connected to a 20-mA current source. A lens with a 0.9-cm focal length collimated the light from the diode; the light then passed through a 40-cm-diameter inductively coupled plasma reactor. The reactor permitted assessment of the technique’s usefulness in etching applications. Focused by a lens with a 3-cm focal length, the light passed to a detector composed of a 50-cm monochromator and a 1024-element photodiode array. With the detector, the investigators determined the carbon difluoride radical density in fluorocarbon plasma under illumination from a 255-nm LED. They calculated that radical densities as low as 5 × 10¹¹ cm^–3 could be detected with a signal-to-noise ratio of 5:1.

For time-resolved measurement of chlorine gas density in chlorine plasma, the researchers used a single-photodiode detector and 350-nm LEDs. Modulating the LED current at 6 kHz eliminated plasma emission. At a gas pressure of 20 mtorr and a plasma strike of 500 W, the team determined chlorine density within 3 mt and with a time resolution of 10 ms, an order of magnitude better than results obtained using a xenon arc light source.

Other applications include environmental testing, where it could allow low-cost, accurate assessment of atmospheric pollutants. Passing transmitted light through a monochromator and adjusting integration times could increase the setup’s detection limit in other settings.

(Applied Physics Letters, 6 Dec. 2007, 231503)