Using Vacuum Technology to Cool CCDs
- Monday, 01 July 2013
Imagine a CCD camera operating on a long exposure and seeing only 1 electron per pixel every 16 minutes. That equates to dark current of less than 0.000001 electrons/pixel/sec. Imagine the same camera delivering less than 1.75 electrons readout noise with minimal hot spots and blemishes while delivering a peak QE of 77%. To hit these specs it would need to be cooled significantly.
Raptor Photonics recently launched a new camera called the Kingfisher V that delivers this type of performance using a new breed of Sony ICX sensors. At resolutions up to 9.1 megapixels, this camera will certainly command a lot of interest in the scientific market. So what is the secret? Raptor has launched this camera using a proprietary new vacuum technology, called PentaVac™, that will enable cooling down to -111°C delta giving the Kingfisher V one of the lowest dark currents for a CCD in the market today.
It’s a well-known fact that cooling a CCD will significantly reduce the associated noise with the chip. In fact, the cooler you can get the chip the better the performance. Most applications only require minimum cooling for enhanced performance, but high end applications like single molecule detection require ultimate sensitivity. Applications requiring long exposures will see an increase in the noise over time, so these applications demand cooling. Minimum thermal impedance leads to maximum efficiency, enabling these long exposures. There are various degrees of cooling a CCD, from passive cooling, to active cooling, but few camera companies have the ability to reliably cool to -80°C or beyond.
So why do you need to deep cool? There are several contributing factors to the noise experienced by a CCD, the main ones being dark current and readout noise. Thermal energy alone is enough to excite electrons into the image pixels and these cannot be distinguished from the actual image photoelectrons. This process generates noise and is called “dark current.” For every 6 or 7°C of cooling, there is about twice the reduction in the total dark current generation rate. This, of course, has its limits. Most CCDs don’t function well below -120°C as at this temperature electron mobility in silicon is greatly reduced. Cooling to -110°C is entirely feasible and the result is virtually a dark current free image for long periods of exposure time. Reduction in dark current by lowering temperature can be seen in Figure 1.
Often forgotten, or deliberately avoided, dark current distribution on CCDs can also be an issue. Most measurements for dark current indicate the average dark current for the complete 2D CCD array and in some cases the median dark current value is used. For any CCD there will be a distribution of dark current values across the array, i.e. each pixel will have a slightly different value and every device will have a few “Hot” pixels with relatively high dark current. In applications that require long exposures these “Hot” pixels can distort images or provide false measurements. Deep cooling also benefits the often ignored pixels by reducing their dark current beyond detectable limits, making more of the CCD array available for accurate scientific image detection.
So the case for cooling a CCD well below ambient temperature is easy to make. But as anyone who has sipped a cold drink on a warm day knows, a cold surface will cause moisture to condense out of the air. Moisture can damage the CCD, but when cooling below zero the moisture turns to frost. Frost can quickly destroy a CCD by damaging the surface structure or even breaking the bond wires. To prevent this, the CCD chip needs to be placed in a chamber with the moisture removed by some method, thermally isolating the sensor from its environment. The best solution is to completely remove the air by creating a vacuum. This also allows the CCD to get colder as there is no air to facilitate conduction or convection heat to and from the sensor. The problem with a vacuum is that it typically requires a much heavier chamber and window.
Cooling can be achieved in several ways with liquid nitrogen, thermoelectric coolers (TECs), or mechanical pumps (cryo-coolers). Liquid nitrogen has been a tried and tested way of deep cooling, but it is messy, cumbersome and potentially dangerous. It also doesn’t easily allow the camera to be orientated in any direction.
TECs have become more common in recent years. Thermoelectric cooling uses the Peltier effect to create a thermal difference between the junction of two different conducting materials. A Peltier cooler is a solid-state active heat pump which transfers heat from one side of the device to the other, with consumption of electrical energy, depending on the direction of the current. Coolers can have multiple stages (1 up to 7) or multiple coolers may be used. There are various means of removing the heat from the hot side of the peltier: passive air, forced air, or circulating coolant tap water/ chilled coolant circulation.