The uncooled sensor is a thermal detector that can operate at an elevated temperature without the need of temperature reduction to enhance its responsivity, detectivity, and other radiometric characteristics. The detector materials can be selected to operate at or above room temperature (25-35°C) without degradation. Since the devices measure a change in conductance or some other temperature-dependent parameter, they must be temperature-stabilized at or near the selected operating temperature (usually plus or minus one degree). Thus, if the system is out of doors and the ambient temperature is -0°C, the thermoelectric cooler must be put into the heat mode and the temperature of the detector must be raised to the set point of 25°C. Conversely, if the ambient is 85°C, the thermoelectric cooler must be put into cooling mode to reduce the detector temperature to 25°C. Thus the thermoelectric cooler is thermodynamically reversible.

  At present there are three detector materials under development. Two of them, vanadium oxide (VOX) and barium strontium titanate (BST) are in production. The VOX makes use of resistance change with temperature. Low-mass micro-bridges of low thermal conductance material are formed to support a thin layer of VOX, which has small dimensions for the pixels on 50-micron or 25-micron centers. The thermal radiation is absorbed by the VOX and it changes resistance based on the amount of radiation that hits it. A voltage is applied on the detector and the delta voltage is collected as a signal. Temperature variations can be detected to within 10 to 100 millikelvins. The sensitivity is characterized as the NETD (noise equivalent temperature difference), such as 25 millikelvins for a 50-micron pixel using an f/1.0 optical system or lens.

  The second detector material, BST, uses a change in the dielectric constant of the material with temperature changes and the resulting change in the capacitance is termed the ferroelectric effect. Raytheon/TI is the only manufacturer of this material. All other suppliers use VOX.

  Yet another material, silicon, looks promising. It has good sensitivity and a relatively flat response for sensitivity from 15 to 40°C. Thermoelectric cooling may not be required since this material would work well in an air-conditioned office, where the temperature is tightly controlled. The VOX and BST are better suited to an outside environment.

  The uncooled detectors sense radiation in the 8- to 20-micron spectral region. Therefore, they generate excellent thermal imagery around room temperature. (A 300-degree K black body peaks at 10 microns.)

  Marlow provides a single-stage thermoelectric cooler integrated with a ceramic package with multiple metallization layers. It takes the signals from the inside of the package through the walls of the package to the outside pins. This enables the cooler, which is also called a temperature stabilizer, to gain a heat sink as part of the package. After placement of the uncooled IRFPA matrix on the thermoelectric cooler, the customer ball-bonds the detector signal leads from the focal plane to the pads inside the package. The package is evacuated to 10-6 TORR vacuum levels and is then tipped off. The "uncooled detector" is then used in a camera box with associated electronics to create and present thermal imagery for an operator to see at night.

Advantages of Thermoelectrics for this Application

  The advantages are: small size, low weight, no moving parts, no vibration in the focal plane, no noxious gases, and good power efficiency. The devices are also as reliable as semiconductors, and rugged. They can operate in a vacuum, in any orientation, and are inexpensive to produce in large production quantities. They can be built inside the ceramic header/heat sink and they lend themselves to robotic automated production processes.

Specific Marlow Advantages

  Marlow has been the common denominator in all makers of this material for over a decade, since the invention of the technology. We have accumulated a vast store of knowledge in solving the uncooled detector manufacturer's problems with optimized and efficient thermal solutions.