Vacuum Sealing Adds Life to Scientific CCDs

Dr. Hugh Cormican, Andor Technology

Perhaps a key question for any scientific imaging application is how long the CCD camera will last before the chip underperforms. One major “camera killer” is condensation on the chip. Most cameras leave the factory virtually blemish-free, but over time, they pick up moisture, hydrocarbons and other gas contaminants. CCD performance will then decline proportionally. Eventually, it becomes so poor that the camera is virtually unusable.

One way to preserve the integrity of the chip is to house the sensor in a vacuum environment. However, it is important to understand that all vacuums are not the same, and that the technology behind vacuums can seriously affect camera life.

For example, one strategy is to place the CCD sensor inside a simple O-ring-sealed metal housing. To protect it from moisture, some designers even go so far as to place silica gel bags inside the Dewar — but this does not protect a state-of-the-art sensor at subzero temperatures. The problem is outgassing. Plastic and O-rings trap large amounts of gas, including moisture and hydrocarbons, which are steadily released over months and even years. Resulting electrochemical reactions will ultimately destroy the CCD. These materials cannot be effectively purged of these gases, which is why O-rings are used only in actively pumped systems.

One solution is to use sensors with windows, usually added in a dry-gas environment. This works well for applications where modest cooling is required (say at 0 °C); however, moisture problems will still occur. As a result, the windows cannot be antireflection-coated because the coating would just degrade with time. With this type of imager, the “stated” peak quantum efficiency should be downgraded by at least 8 percent. Even then, moisture, etc., will continue to build up on the window until it obscures the image. The system will require vacuum-pumping, which means sending the camera back to the manufacturer regularly.

A permanent hermetic vacuum should be considered for high-end spectroscopy and imaging cameras. The sensor system should also use metal, glass and ceramic Dewars, such as the one depicted in this cross section.

Hermetic sealing

Instead, a permanent hermetic vacuum should be considered for high-end spectroscopy and imaging cameras. The sensor system should also use metal, glass and ceramic Dewars that have been treated to improve on their already low level of outgassing. Performance improves because the temperature of the chip can be reduced significantly. Better cooling (to –95 °C) translates into less dark current and fewer blemishes. The permanent hermetic vacuum ensures that peak quantum efficiency and cooling will not degrade, even after years of operation.

As a side benefit, elimination of condensation problems means that the system also can use a single window with antireflection coating, which further protects against any degradation in peak quantum efficiency.

Meet the author

Hugh Cormican is managing director of Andor Technology in Belfast, Northern Ireland; e-mail: h.cormican@andor-tech.com.