Servo Corporation of America - All rights reserved
Servo Corporation has been involved with the various aspects of thermal detection application
since 1946. Currently we are the leading supplier of thermal detectors to both the railroads for
implementation in wheel bearing fault detection systems and to the aerospace industry for use in
satellite Earth sensor assemblies. Servo's experience with thermal detectors includes thermistor
bolometers based on high work function (TCR) oxide material and pyroelectric effect in
ferroelectric material. Specific products include:
Model 1350 Thermistor IR Detector
Model 1375 Immersed Thermistor IR Detector
The thermistor detector is a thermal detector where the absorption of optical or infrared energy leads to a temperature rise. Since
the thermistor has a very large negative temperature coefficient of about 4%/oC, a large voltage responsivity results when the
detector is appropriately biased. The thermistor detector can also be used over the entire wavelength range of UV to far IR since
the temperature rise is dependent on the energy absorbed and not the photon wavelength directly.
Servo has been manufacturing thermistor bolometer IR detectors for nearly 50 years. Many thousands of devices been produced.
The broad band optical (from UV to far IR wavelengths) and electrical response (from dc), as well as inherent ruggedness, have
made this the detector of choice in many classical remote sensing applications. These include Earth horizon sensors for satellite
attitude determination, Earth Resources Monitoring, non-contact IR thermometry and various industrial applications. In
operation, the incident optical power causes an instantaneous change in the temperature of the element, thus changing its
Until recently, these detectors have only been fabricated using a labor intensive ceramic technology where thin flakes of the
thermistor material were bonded to substrates. The Model 1350 and Model 1375 Thermistor detectors are examples of detectors
using this technology. The device performance parameters in these detectors such as responsivity, noise and response time, are
very dependent on operator skill and experience, and processing conditions.
Servo has developed a manufacturing method whereby the detectors are produced with modern semiconductor processing
techniques. The core of the process is the ability to sputter deposit the active thermistor material onto suitable substrates while
retaining it's high TCR. These films are deposited from a single target consisting of hot pressed Nickel, Cobalt and Manganese
oxides. Processes to form the production of sputtering targets as well as delineation of the films have been developed and are being
utilized at Servo.
Model 1501CM Single Element Pyroelectric Detector, Current Mode
Model 1501VM Single Element Pyroelectric Detector, Voltage Mode
Model 1508 Pyroelectric Linear Array
Pyroelectric detectors, made of Lithium Tantalate (LTO) are much more sensitive devices than either the traditional bolometers as
noted above, or thermopiles which are used on some Earth sensors. Servo began building pyroelectric detectors using proprietary
processing techniques, as commercial products several years ago, but early on noted that they would be very effective if used as an
Earth sensor. The performance advantages of this material are in its large pyroelectric current output, and high temperature
stability. Made into an array using hybridized preamplifiers, the device can easily be interfaced to standard electronics circuits.
Because of the nature of the pyroelectric effect, a radiant contrast must be created between the detector and some known
temperature source. This can be accomplished by means of a mechanical chopper, scanning the detector across a surface or
sensing movement of the target.
For the past several years Servo has maintained an ongoing research development program which has resulted in the development
of a pyroelectric copolymer which can be spun cast in very thin layers. This material P(VDF-TrFE) in the ratio of 75:25, was
chosen for its high Curie temperature of 160 C, making it suitable for median temperature application, and for its ability to be
corona poled without mechanical stretching. The copolymer has a high pyroelectric output and is ideal for manufacture as it can be
uniformly spun onto a thermal isolation structure to thickness' of .3 microns or lower, chemically delineated before curing, and
delineated using Reactive Ion Etch (RIE) afterward. The film thickness is controlled by adjusting the solution viscosity and spin
speed. This process eliminates the bump-bonding requirement plus it is essentially insensitive to changes in ambient temperature
over a wide range
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