XP Cooling Technology

Details

Published on Thursday, 15 December 2011 19:53

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 Features & Benefits

• Guaranteed camera lifetime vacuum
• Single AR coated input window
• -85°C TE cooling without liquid assistance

Cameras from Princeton Instruments are the only deep-cooled, CCD-based systems in the world that provide guaranteed high-vacuum performance for the entire working lifetime of the detector. Exclusive XP technology is available in both the Princeton Instruments PIXIS® and ProEM product line. Dark current rates of 0.001 to 0.002 e-/p/s, along with system read noise as low as <1 e-rms for the ProEM and 2.5 to 2.6 e- rms for the PIXIS to give all users truly superb sensitivity.

The Need to Be Cool
The classic struggle to minimize dark current in high-performance CCDs is well known. Thermally generated dark current, which grows linearly as pixel area increases, is inversely proportional to temperature. As a rule of thumb, it is halved for every 5 to 7°C of additional cooling. Keeping a detector’s dark current — and associated dark noise —low is one of the elements critical to the success of scientific imaging and spectroscopy applications.

Cool Then, Cool
Now Historically, several methods have been available to achieve deep cooling, such as the use of thermoelectric devices, liquid nitrogen, or external cryogenic compressors. Of these techniques, thermoelectric cooling offers the most convenience — but only if designed and implemented properly. Presently, there are several other manufacturers offering cameras that claim -70 to -90°C cooling. However, these systems must employ forced circulation of water as cold as +10°C to achieve it. The additional requirement for external water circulation eliminates the biggest benefit of thermoelectric cooling — convenience. Moreover, a +10°C water supply is not particularly common in a typical lab setting. Even when it is available, cold water can lead to unwanted problems (e.g., condensation).

The Vacuum
To achieve deep cooling, a world-class vacuum chamber needed to be designed to house the CCD. The contents of the vacuum chamber had to be minimized to keep the thermal load and out-gassing as small as possible. An innovative “getter” material was used to absorb any residual out-gassing, albeit minute, that might occur over time. The XP design also called for customized UHV (ultrahigh vacuum) components made of stainless steel. Additionally, the metalglass interface between the chamber and the input optical window was brazed, hermetically sealing the entire detector. Next came the thermoelectric cooler itself. Princeton Instruments engineers worked to create a Peltier device that would keep the CCD at a very low temperature while also stabilizing a “control” point to minimize the variation in dark current. Princeton Instruments addressed these criteria by designing a “feedback” circuit that can provide ±0.05°C stability across a wide temperature range.

Manufacturing Expertise
While designing the world’s best CCD vacuum technology involved overcoming a number of formidable challenges, all the hard work would be for naught if the XP detectors could not be manufactured reliably. Fortunately, years of manufacturing experience at Princeton Instruments made this task a relatively easy endeavor, even though the process is actually quite complex. For starters, all vacuum parts are stored in special, “dry” containers until they are handled. The vacuum processing is done using oil-less "molecular" pumps. During vacuum processing, which is one of the longest steps in the detector assembly, the seals are continually checked for leaks. Once assembled, every detector undergoes a strictly defined, comprehensive inspection to ensure that rigorous specification limits are met. This process ensures the best performance and reliability available on the market today.

Single window for maximum QE and sharper images
Princeton Instruments cameras utilize only one anti-reflective coated window; this reduces the chance of multiple reflections, stray light and interference patterns in the optical path degrading the image. Princeton Instruments uses only windowless CCDs and handles them in a strict clean room environment. While this obviously involves greater time and expense, it is the only way to produce cameras which deliver the ultimate in sensitivity, spatial resolution and image MTF.