Cooled and uncooled infrared detectors for missile seekers

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1 Cooled and uncooled infrared detectors for missile seekers Rami Fraenkel, Jacob Haski, Udi Mizrahi, Lior Shkedy and Itay Shtrichman SemiConductor Devices P.O. Box 2250, Haifa 31021, Israel Ephraim Pinski RAFAEL P.O. Box 2250, Haifa 31021, Israel ABSTRACT Electro-optical missile seekers pose exceptional requirements for infrared (IR) detectors. These requirements include: very short mission readiness (time-to-image), one-time and relatively short mission duration, extreme ambient conditions, high sensitivity, fast frame rate, and in some cases small size and cost. SCD is engaged in the development and production of IR detectors for missile seeker applications for many years. 0D, 1D and 2D InSb focal plane arrays (FPAs) are packaged in specially designed fast cool-down Dewars and integrated with Joule-Thomson (JT) coolers. These cooled MWIR detectors were integrated in numerous seekers of various missile types, for short and long range applications, and are combat proven. New technologies for the MWIR, such as epi-insb and XBn-InAsSb, enable faster cool-down time and higher sensitivity for the next generation seekers. The uncooled micro-bolometer technology for IR detectors has advanced significantly over the last decade, and high resolution - high sensitivity FPAs are now available for different applications. Their much smaller size and cost with regard to the cooled detectors makes these uncooled LWIR detectors natural candidates for short and mid-range missile seekers. In this work we will present SCD's cooled and uncooled solutions for advanced electro-optical missile seekers. Keywords: Infrared detector, InSb, XBn, Micro-bolometers, Missile seekers 1. INTRODUCTION Electro-optical missile seekers pose exceptional requirements on infrared (IR) detectors. SCD has been engaged for more than 3 decades in development and production of arrays for missile seeker applications. For cooled detectors over 15 different solutions were fielded starting with single element, 1D vector arrays and in the last decade 2D arrays integrated with Joule-Thompson (JT) coolers. Emerging technologies such as High Operating Temperature (HOT) XBn or Epi- InSb MWIR devices can provide new directions for reducing cool down time or improving sensitivity. The uncooled micro-bolometer technology at SCD has also reached the threshold for seeker requirements in terms of sensitivity and spatial uniformity. The lower cost and size and the almost immediate "mission readiness" make these detectors natural candidates for short to mid-range seekers with considerably wider proliferation. The uncooled 17µm pitch family has recently been expanded with the High Sensitive (HS) grade and with the addition of two new formats: A compact 384x288 (QVGA) version with minimal footprint that will address Size Weight and Power (SWaP) sensitive applications, and a large 1024x768 (XGA) FPA for high resolution and wide Field of View (FOV). The paper is organized as follows: In section 2 we elaborate on the system requirements for seeker detector arrays. Many of the demands are common to both cooled and uncooled detectors, but some are unique to each technology. In section 3 we show how SCD's long and rich legacy of InSb cooled detectors fulfills these harsh requirements with over 15 different solutions. In the following section we present the spectrum of the possible uncooled detectors emphasizing the high sensitivity, high frame rate and low residual non uniformity (RNU). In the last part we present future trends and solutions that are based on HOT MWIR technologies (epi-insb or XBn) enabling faster cool-down time or high resolution, and on higher sensitivity uncooled detectors for the next generation seekers.

2 2. SYSTEM REQUIREMENTS FOR SEEKERS' DETECTOR ARRAYS Harsh environmental conditions, battlefield and atmosphere effects as well as the limitations related to missile seeker design, pose significant challenges on the IR detectors embedded inside the systems. Conflicts between requirements may also occur frequently. The most prominent and well known is the one between sensitivity and resolution at a given size constraint. This should be solved by carefully selecting a working-point with desired tradeoffs provided that the IR thermal detectors are well suited for this task. Harsh environmental conditions in the immediate vicinity of the detector include: Launch shock, high acceleration, vibrations and noise Poor heat exchange and poor heat removal capability out of the seeker's unit High dome and/or optics/optics-chassis temperature Intense temperature transients within the time after launch EMI/RFI interferences Wide range of temperatures and high humidity values during long storage periods. Battlefield environment is also an important factor to consider. It has implications on the detector properties such as poor visibility due to smoke and dust as well as dazzling, blooming, high intensity countermeasures, explosions, etc. The limitations related to the seekers design have implications on the detectors' requirements. These are mainly: cost, size, weight, power consumption and the amount of available cooling gas reservoir vessel per mission length (this last issue is obviously related only to cooled detectors). Mission profile, combat situation, design constraints and the above combat environment define the requirements and performances of IR thermal detectors to be installed in the seekers. Many of them are common to both the cooled and the uncooled detectors: High Sensitivity - low as possible NETD (Noise Equivalence Temperature Difference) i.e. enhancing the signal to maximum and reducing temporal noise as much as possible. Very low residual spatial noise (RNU), with provisions in the detector to support this result, is also related to this key requirement. High resolution- small pixel size with large number of pixels in the array. Yet for many seekers applications, small arrays (e.g.384x288 elements) are preferable since they are still suitable for the mission with the advantage of low cost and low Size, Weight and Power (SWaP). Small size in many of the seekers applications, reducing size is important due to the overall seeker's volume constraints. The combination of small size pixels and relatively small number of elements (e.g. 384x288) is generally preferred. Image stability manifested in signal response as well as spatial and temporal noise, before launch and in the course of the flight. Low power consumption an important requirement both to reduce the seeker heating and for saving energy to enable longer operation time. High frame rate and controllable exposure time are beneficial both for dynamic applications and temporal noise reduction. Spectral range optimization in accordance with the atmospheric windows. Since the imaging applies to relatively long range atmospheric path, the detector spectral filter should accommodate the atmospheric window for optimal performance Minimum bad pixels where their map of clusters depends on the seeker algorithm requirements. Supporting large field of view - this is required in some applications where either 'stiff-neck' configuration is used or the angle between the missile axis and the line of sight (LOS) to target at any point of the trajectory is large. Relatively high resolution is still required for optimal seeker performance. Long storage time with little degradation including vacuum integrity. Real time continues measurements of detector parameters to be used with signal processing algorithms, for instance FPA or package case temperature.

3 On top of the requirements stated above, we should add special requirements that apply to either cooled or uncooled detectors. A highly desired requirement for a cooled detector is: Very short cooling time to image (of the order of a few seconds) for most applications the readiness of the missile seeker to be operational- ready to launch, must be almost immediate. In this sense, increasing the detector's working temperature well above the standard liquid nitrogen temperature without giving up performances may improve readiness time. Uncooled detectors also have their unique specific requirements on top of the common ones stated above: Short time constant in which the signal is attenuated it is important to reduce blurring due to high dynamics such as high relative motion of the whole scene or moving objects as imaged on the detector plane. NETD and spatial non-uniformity stability over time and over the immediate-environment temperature changes. This applies to the FPA itself, the package case and the opto-mechanical elements at the detector vicinity, that emit non-imaged radiation on the FPA. The immediate-environment temperature usually increases rapidly upon turning on the seeker's power supply (particularly after launch). Fast TEC Stability This may effect the readiness of the missile seeker to be operational and ready to launch. Supporting TEC-less operation TEC-less configuration reduces the seeker power consumption (the TEC is not a negligible energy consumer ( and it simplifies the seeker design by eliminating the need for temperature control. Supporting Shutter-less operation- Helps simplifying the seeker design and enhances reliability by eliminating mechanical parts. Obviously, conflict between the various requirements may occur. Consequently, a creative, accurate and careful design is needed to optimize the performance and to balance the detectors' parameters so that they will meet the seekers demanding needs. When selecting a detector for a planned seeker, all the above requirements must be considered. There are always tradeoffs to be performed between the different parameters where the detector selected must provide optimal compliance for all system scenarios and requirements. The variety of detectors and technologies available allow a wide range of seekers for use in different applications. 3. COOLED DETECTORS For the cooled segment SCD has accumulated over 30 years of experience with combat proven detectors for missile seekers. The main challenges were to achieve the following goals: High performance: fast cool down time, high resolution, good and stable image uniformity ("one time" Non Uniformity Correction) and exceptional reliability. Flexibility in adaptation of the mechanical configuration to the customer's system constraints Very long lifetime with minimal or no maintenance Minimal space (low SWaP) Short cool down time and fast "mission readiness" is critical for missile seeker applications. Over the years SCD has developed unique design tools for fast cool down Dewar. They are based on combination of sophisticated simulations and meticulous mechanical design. There is also a tradeoff with other important issues such as harsh environmental conditions and "design to cost". In Figure 1 we show a typical cool down profile of the FPA and the cold shield. For obvious reasons both components' cooling rate is of vast importance. Another critical feature is image uniformity and its stability with time. This is due to the fact that we can't perform NUC during flight duration and should rely on the initial correction parameters ("one time NUC"). It is well known that InSb FPAs exhibit exceptional short range and long range image stability even after a series of on/off operations. Figure 2

4 RNU [% of DR] RNU [% of DR] shows this stability for duration of 7 hours (short range) and 8 days (long range) after the initial correction. The RNU hardly changes with negligible deterioration. Figure 1: Cool down profile of the FPA and Cold Shield of InSb detector cooled to 87K At 8:00 At 15: First Operation (16:10) After 24 hours (16:00) After 40 hours (8:00) After 8 days (8:00) Well Fill [%] Well Fill [%] Figure 2: RNU stability of InSb detector for short range (left) and long range (right) We can shorten the "mission readiness" time frame even further by applying a dynamic real time dark current compensation method that was introduced elsewhere [1]. This method uses prior knowledge of the offset dependence on the FPA temperature. It can numerically update the offset coefficients to compensate for the FPA temperature drift. Since this offset correction is purely computational, it can be performed without losing sight of the observed scenery which is mandatory for missile seeker applications. The idea of compensating for the dark current variation is probably general but the specific temporal behavior is typical for each detector.

5 Figure 3: RNU vs. FPA temperature of InSb detector before (left) and after (right) applying dark current compensation The results of the algorithm are demonstrated in Figure 3: each pixel in corrected according to the pre measured behavior with temperature as elaborated in [1]. For large FPA temperature variations shown here the compensation is not as good as the offset method but nevertheless the improvement of the uniformity is dramatic especially for a large drift (e.g. almost one order of magnitude for 91K). We now discuss the technological evolvement of the detectors along the years. We start with the 1 st generation based on a 128 elements vector. The Dewar is cooled by a JT cooler and the unique design ensures extremely fast cool down time. It is also optimized for low cost production and low maintenance. The next generation is based on the Blue Fairy (BF) ROIC with a 320x256 format and 30µm pitch [2]. This array already supports most of the features that were laid out in section 2: High sensitivity: temporal NETD < F/3 and extremely low RNU: below 0.04% std/dr for a wide range of 5% - 90% of the dynamic range. Small size: unique design with minimal thermal load. Image stability: real time implementation of column Correlated Double Sampling (CDS). High frame rate and controllable exposure time: implementation of the Integrate while Read (IWR) mode and the special feature of "combined mode" for extending the effective dynamic range [2]. The maximum frame rate is 350Hz. Long storage time: designed for more than 15 years without getter activation. The next step was the introduction of the digital detectors [3, 4]. Detectors based on a digital FPA are considered to be very attractive due to their many advantages over detectors with an analogue FPA, and this is expressed especially in missile seeker applications: Lower level of readout noise due to immunity of the analogue signal to external noise Higher linearity Less sensitivity to external ambient conditions Higher long term stability of the Residual Non Uniformity (RNU) Removal of the requirement to develop low noise electronics in the system The distance between the detector and the system electronics can be increased up to several meters without affecting performance. The integration of the detector into the system is much simpler and faster All these new features combined can support a high sensitivity compact detector. This is demonstrated in Figure 4 for the 480x384 format implemented in Sabastian480 detector. Other JT cooled detectors for missile seekers include 640x512 and 1280x1024 formats with 15µm pitch.

6 Figure 4: Compact 480x384 digital detector for missile seeker application 4. UNCOOLED DETECTORS A new generation of high-performance uncooled detector arrays, with 17 and 25 µm pitch, improved sensitivity, and extended spectral response were developed by SCD. This development brings the uncooled infrared technology very close to the performance of traditional second generation cooled LWIR detectors, and enables a new range of applications [5-7]. Specifically these arrays can fulfill the requirements of low to mid end missile seeker applications with the obvious benefits of low SWaP and cost. 1. SWaP considerations: SCD's micro-bolometer product line consists of various formats and sensitivity grades. The basic ROIC architecture of the 17µm pitch family follows closely the successful framework of the previous 25 µm pitch designs, the new capabilities of the 0.18µm CMOS process support an internal "coarse NUC" (Compensation) mechanism and a more sophisticated interface management unit. This in turn considerably facilitates the user interface and was implemented in 3 formats. The basic format is the compact 384x288 (QVGA) version (either 17µm or 25µm pitch) with minimal footprint that can address Size Weight and Power (SWaP) sensitive seekers. This is followed by the 640x480 (VGA) 17µm pitch format, and finally the 1024x768 (XGA) format with 17µm pitch, for platforms requiring high resolution and wide Field of View (FOV). Table 1 summarizes the main specification of the 17µm detectors. All packages are designed to ensure vacuum level integrity of at least 14 years (in suitable ambient temperature) and the base of the package is used as mounting surface and features high accuracy design in order to ensure proper parallelism. The package was tested to withstand temperature cycles, random and sine vibration and shocks in alignment with the stringent demands of seeker applications. It was also tested under extreme mechanical stress with no apparent damage. Recently we have introduced a ceramic package that enables further reduction of the footprint and weight of the detector. The reduction was achieved by several means. Transformation to a ceramic package allows for lower pitch between the pins. We have also eliminated the need for a vacuum pipe and pumping is performed in an especially designed vacuum assembly machine. The package size and weight were further reduced by removing the TEC. The reduced weight and size compared with a metallic package are important for space limited seekers. The ceramic package is in its final stages of development for the compact 384x288 (QVGA) and for the 640x480 (VGA) TECLESS version.

7 Counts Table 1: Basic specification of the 17µm micro-bolometer detector family 2. Sensitivity and Time Constant: One of the main features described in Section 2 is the temporal sensitivity. In the previous year special effort was devoted to the improvement of the temporal NETD or SNR of the 17µm pitch pixel. This was achieved via pixel architecture and process modifications, and the outcome is the 17µm High Sensitive (HS) and Very High Sensitive (VHS) pixel. Currently only the HS is implemented in a product. In Figure 5 we demonstrate the temporal NETD distribution measured for F/1 optics at the frame rate of 60Hz with the VGA format. The peak of the distribution is around 15mk for VHS, 23mk for the HS version and 40mk for the standard detector version. The HS penalty is manifested in a longer time constant, but due to the relatively low thermal capacitance it is still below 14msec, the thermal time constant of the Standard pixel is <9msec, and for the VHS pixel is less then 16msec. Each of these pixel designs can be implemented in the various format of the product line (QVGA, VGA and XGA). There is an obvious tradeoff between NETD and time constant that should be considered by the system designer VHS HS Standard NETD(mK) Figure 5: Uncooled 17µm pitch VGA detector temporal NETD (F/1, 60Hz) of the VHS, HS, and Standard pixels

8 3. Mission readiness: An extremely important aspect is the ability to operate the detector at any given FPA temperature enabling minimal mission readiness time of the missile seeker. The NETD of the detector should be kept low at any FPA temperature. The FPA can be then set to the instantaneous temperature of the system (or a bit higher), enabling fast stabilization of the TEC controller loop. This configuration also decreases dramatically the power consumption by the TEC driver, assuming a short mission time. In Figure 6 we show the variation of the NETD as a function of FPA temperature. The behavior is relatively smooth with a plateau below 45 0 C that continues even for lower temperatures. For higher FPA temperatures there is some deterioration which is due mainly to the increased contribution of the ROIC readout noise. Still, even at 65 0 C the NETD is relatively low Relative NETD (n.u.) FPA temperature [C] Figure 6: Relative NETD vs. FPA temperature In summary, the versatility of the formats and sensitivity grades of the uncooled product line enable the system house to optimize the seeker design. For seekers with 'stiff-neck' the standard pixel with the lowest available time constant should be selected. This will reduce blurring due to seeker vibration. When large FOV with high resolution is required, the XGA format is the best candidate for the mission (see next section). The combination of XGA and short time constant enables the seeker to maintain LOS with the target during the flight. For a small gimbaled seeker, QVGA format should be selected, the HS pixel will support the low NETD with a high F/# optics, keeping the volume of the seeker as small as possible. 5. FUTURE TRENDS A major challenge for missile seeker applications is to achieve faster mission readiness. SCD s new epi-insb detectors, grown by molecular beam epitaxy, have a BLIP temperature of ~110 K at F/3 [8]. This enhanced operating temperature reduces the required cooling power by ~20 % compared with the conventional 77 K operation. For a very high operating temperature, we have developed the new XBn-InAsSb detector with a 4.2 μm cut-off wavelength [9-11]. This detector exhibits a BLIP temperature of ~175K at F/3 and a reduction in cooling power of ~60 %. These HOT detectors enable an improved range of solutions, including faster cool-down time and mission readiness, longer mission times, and higher cooler reliability. We can also exploit their reduced dark current to obtain an enhanced signal to noise ratio at lower operating temperatures when required. In Figure 7, the V-curve for epi-insb at 95 K is similar to that of planar InSb at 80 K. The V-curve is a plot of the RNU (at a well fill of 45 %) as the FPA temperature is varied from its calibration temperature. It is a measure of the stability of the FPA image quality with respect to temperature drift, where a wider curve corresponds to a greater stability. Epi-InSb at 95 K has a width of 7 K at RNU=0.1% and is actually more stable than planar InSb at 80 K, whose width is 4K. At 80

9 RNU (Std/DR) (%) NETD (mk) Operability (%) K the V-curve for epi-insb is much wider than that for planar InSb, indicating the robustness of its RNU to temperature drift. This can be utilized in seekers for performing NUC at relatively high temperature during cool-down Epi-InSb Planar InSb Temperature (K) Figure 7: Comparative V-curve of bulk InSb and Epi InSb NETD Operability Temperature (K) Figure 8: NETD and Operability of XBn Figure 8 shows the temperature dependence of the NETD of the at F/3.2, and the pixel operability, for a 15 μm pitch XBn FPA. The operability was determined with similar criteria to those used for the InSb FPA. The NETD and operability only begin to change above 170 K, consistent with the BLIP temperature of 175 K. Obviously the mission readiness time will improve dramatically compared with conventional bulk InSb. For the uncooled detectors, the future trend is to increase the resolution and the field of view. Recently we have presented a large format ( ) detector array, with 17μm pitch [12]. This detector combines high resolution and large format, together with low NETD of better than 35mK (at F/1, 30Hz). We estimate a Recognition Range for a NATO target of better than 4 km at all relevant atmospheric conditions, which is better than standard 2nd generation scanning array cooled detector. A new type of detector package enables improved stability of the Non-Uniformity Correction (NUC) to environmental temperature drifts. 6. SUMMARY AND CONCLUSIONS In this paper we reviewed SCD's legacy of cooled and uncooled detectors optimized for missile seekers. The cooled detectors evolved for almost 3 decades spreading from a single element throughout state-of-the-art 2D arrays. The recently introduced HOT detectors offer a real benefit in terms of cool down time and mission readiness. The uncooled detectors have improved immensely in terms of sensitivity and resolution in recent years, and today they can offer a low SWaP and affordable solution for low to mid end seekers. For both detector types, the combination of material technology, sophisticated ROIC and package design offer the optimal solutions for missile seeker system designers. ACKNOWLEDGEMENTS The work presented here was supported over the years by the Israeli Ministry of Defense (IMoD) and the Israeli Ministry of Trade & Industry (MOITAL). We are in debt to a large group of engineers and technicians that conducted this work for almost 3 decades. Their dedicated work and contribution to the development and production of the detectors is highly appreciated.

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