Reprinted from Sensors April 1988
Pyroelectrics for Smart Munitions Alan P. Doctor, Microwatt Applications Inc.
There is a growing requirement for low cost, shock resistant, high reliability infrared detectors. A driving force behind this specification is the military, with its smart munitions (SM) programs. Smart munitions are devices that seek, locate, and guide themselves to their targets, which are usually tanks. The principle is one of using the IR energy such objects radiate, creating hot targets against a cooler background. The radiated energy is proportional to the object's temperature in Kelvin, raised to the fourth power.
σ = Stefan Boltzman Constant 5.67x10-12 W/m2K4 Smart munitions can be launched or dropped from an airborne platform or shot from a cannon or mortar. As the munitions descend over the target area, they are slowed by a parachute, which is attached off center in order to cause the SM to spin eccentrically. This spin allows the SM to scan an area, looking for its target. Once it locates a target it fires a charge, launching the actual projectile at the target. Pyroelectric detectors are thermal devices with an optical response determined by the absorption property of the material and/or by a coating applied to the receiving surface. An output current is produced whenever the active area perceives a temperature difference. The detector is inherently AC coupled to the background, responding only to changes in IR energy. Since the detectors require no bias voltage, current, or cooling, both the electronic and the mechanical designs are simplified. CONSTRUCTION MATERIALS Four construction materials were studied extensively: triglycine sulfate (TGS,) lithium tantalate (LiTaO3,) lead zirconate titanate (PZT,) and polyvinylidene fluoride (PVdF) which was found to offer the best performance. TGS, with the highest degree of detectivity, is hygroscopic and is not pyroelectric above its 47°C Curie point. It is a fairly inexpensive crystal to produce, but its limitations reduce its suitability for military applications. There have been doped TGS materials with Curie points near 100°C, but these show the same water sensitivity, as well as a decrease in IR detector performance. LiTa03 is an excellent material, with a 600°C Curie point. It is insensitive to moisture and temperature and has been used successfully for many years. At about ;$3.00/g, however, the base material is expensive. Furthermore, it must be prix- These are difficult to manufacture, hard processed into thin plates with large areas. to handle, and fragile; they have trouble meeting military standards for shock and vibration. The price of the ceramic PZT is about a tenth that of LiTa03. However, it has a very large piezoelectric output signal that limits its signal-to-noise ratio in vibration and shock environments. Its dielectric constant is quite large; detectors made of the material have large capacitances that result in longer time constants. PVdF, a new pyroelectric material, is renowned for its mechanical strength and chemical inertness. It is available in large area sheets in thickness’ from 6 to 150 µm. Its Pyroelectric properties are developed after a quasi crystallinity is formed in the film by a stretching process that permanently realigns and reorients the organic molecules. As is the case with all pyroelectric materials, PVdF must be poled to become active. This can be done at any time during the processing, but it is usually performed on the "raw material" at temperatures above 100°C. The other crystalline, or "hard," materials have bulk pyroelectric, thermal, and electronic properties that are larger or better than those of PVdF. However, because of the thinness of the film, a pyroelectric detector made of PVdF has a performance comparable to that of traditional detectors. It costs less (about $0.01/cm2) than other pyroelectric materials. It does not require extensive and costly material processing such as slicing, dicing, lapping, or polishing. It requires only the application of electrodes to define the
Reprinted from Sensors April 1988 active detector areas. Its low mass and mechanical resiliency permit it to meet military environment requirements, including high-g launch. Devices with rectangular, circular, and even angular geometries are possible. In practice, arrays with element geometries of 0.2 by 0.2 mm and spacings of 0.1 mm are the minimum practical size, although smaller devices have been fabricated. The upper limit on detector size is limited by handling and packaging constraints, although quad arrays with active areas of up to 10.0 by 10.0 mm have been available for some time. The minimum element size is governed by the capabilities of the photolithographic process and the input capacitance, which, when coupled to the amplifier capacitance, will cause a reduction in output signal. However, current mode amplification will negate this effect. The minimum spacing is determined by the thermal diffusion length of the material. The material properties of PVdF, and its use in very thin films, permits element spacings much smaller than those possible with other materials. Unique properties of PVdF permit the development of low-cost multispectral arrays. THE DETECTOR IN OPERATION In practice, the pyroelectric element is coupled to the outside world by means of an integrated low-noise buffer amplifier. Because the detectors behave like capacitors with a very large impedance and low signal levels, the design of these amplifiers is critical. Two techniques for amplification are commonly used; voltage mode and current mode. In the voltage mode scheme, the output signal or current from the pyroelectric element produces a voltage across the buffer amplifier's input resistor. This high impedance level signal is transformed with an FET source follower into a signal with a more usable low impedance. In this configuration, the frequency response of the detector is determined by the RC constant of the detector and the input load resistor, and the output signal level is simply the product of the pyroelectric current and the 'lumped" impedance of the detector and the load resistor. The current mode amplification scheme uses an operational amplifier connected as a current-to-voltage converter where the current gain or output voltage is determined by the impedance of the feedback loop. Each amplification scheme has limitations; the application should determine the choice. The piezoelectric output from these detectors is a potential source of noise. Several techniques have been developed to reduce or eliminate this problem. One method is to define piezoelectric "areas" in proximity to the active IR detector. These have an output that can be used to null the piezo output from the active areas. Other techniques include the differential connection of pairs of active devices to cancel the microphonic signals. A comparison of the material properties and electrical output of the various materials can be seen in Tables 1 and 2. The current responsivity is a measure of the efficiency of converting optical power to electric current. The voltage responsivity is given by an FET voltage mode amplifier. In determining the noise current due to the loss tangent of the material, the loss tangent is defined as the ratio between the resistive and capacitive components of the material's impedance vector. Summing the noise factors for the device and dividing by the responsivity gives the NEP. The D', a figure of merit used to compare detectors, is derived by dividing the square root of the area of the detector in cm by the NEP. The sixth entry in Table 2 is a comparison of the piezoelectric voltage output due to acceleration, shock, or vibration. Of the materials considered, PVdF has the highest voltage and current responsivity, D', and the lowest signal due to acceleration. The electronics are assembled using surface mounted devices (SMDs) on ceramic substrates with thin or thick film conductors. In the past, these circuits were fabricated using chip or wire technology. SMDs cost less and are more reliable in high-g applications, since there are no wire bonds to fail. The individual components are also 100 percent testable, and ease of assembly lends them to high-volume production. Pyroelectric detectors and associated electronics can be delivered in a variety of packages, including various TO styles and specialty flatpaks. Since the devices and the integral electronics do not require cooling, the packaging does not have to be evacuated or backfilled with a low thermal mass gas; a nitrogen backfill at atmospheric pressure will suffice. Packaging reliability is thus increased and cost reduced when compared to the cooled detectors. In summary, pyroelectric PVdF film can solve many of the problems associated with pyroelectric detectors used in smart munitions. The film technology can meet the cost, performance, and high-g reliability requirements for such programs.
Reprinted from Sensors April 1988
Figure 1. Smart munitions use the IR energy that objects radiate to locate their targets, usually tanks. Shown is such a device sweeping an area. The cube at upper right represents black box electronics. The squares at lower right represent areas seen by the detector at one particular point in time-instantaneous fields of view.
Figure 2. Shown it an enlarged version of the pyroelectric detector contained in the smart munitions of Figure I. The detector it a themes! device with an optical response determined by the absorption property of the material and/or by a coating applied to the receiving surface.
Reprinted from Sensors April 1988