Cool CCDs Make a Small Spectrometer

Brent D. Johnson, Senior News Editor

Component integration of CCDs has been a persistent problem with designing spectrometers.

Many devices need specialized interface cards in the spectrometer's computer to operate the CCD. Newer, compact computers have little or no room for expansion cards.

To simplify spectroscopy, Acton Research Corp. in Acton, Mass., has combined a spectrometer and CCD detector in one package and developed a USB interface for easy connectivity.

"The historic market for these devices is the laser jocks," said Dick Merk, vice president of Acton's Instrument Div. However, he said the new InSpectrum system should be able to reach a broadening market that will include industrial Raman spectrometry, fluorescence spectrometry, light source characterization and plasma diagnostics for semiconductors.

One feature that Merk believes is critical to the InSpectrum's performance is its Hamamatsu CCD detectors. He said the fundamental issue is that the Hamamatsu CCD packaging lends itself well to integration. In addition, while other CCDs require designers to integrate extensive additional cooling into their products, the Hamamatsu detectors have a built-in cooling capability of -22 °C below ambient.

John Gilmore, product manager for solid-state image sensors at Hamamatsu in Bridgewater, N.J., said the company builds a thermoelectric cooling unit into the hermetically sealed package. This means that only the chip gets cold, instead of the surrounding ceramic material. The confined cooling improves efficiency.

Acton's InSpectrum uses four types of Hamamatsu CCDs, including both front- and backlit devices, all with 1024 elements over 25 mm.

The InSpectrum spectroscopy system integrates a Hamamatsu CCD within the box, so the user does not have to deal with external components.

Gilmore said that in normal full-frame transfer CCDs, polysilicon electrodes that are necessary for charge transportation absorb a lot of light, reducing quantum efficiency, especially in the UV region where the light doesn't penetrate as deeply.

Backlit devices flip the chip over and illuminate it from the back side to bypass the electrodes, resulting in high quantum efficiency in both the UV and visible ranges.

Hamamatsu also tunes the thickness of the oxide layer to control the reflection instead of using antireflective coatings as other companies do.

Merk said the front-illuminated CCD has a Lumogen coating to enhance the UV response, but it is not as good as the performance of the backlit CCD.