Polycrystalline lead selenide: the resurgence of an old infrared detector

OPTO-ELECTRONICS REVIEW 15(2), 110 117 DOI: 10.2478/s11772-007-0007-7 Polycrystalline lead selenide: the resurgence of an old infrared detector G. VERGARA *, M.T. MONTOJO, M.C. TORQUEMADA, M.T. RODRIGO, F.J. SÁNCHEZ, L.J. GÓMEZ, R.M. ALMAZÁN, M. VERDÚ, P. RODRÍGUEZ, V. VILLAMAYOR, M. ÁLVAREZ, J. DIEZHANDINO, J. PLAZA, and I. CATALÁN Centro de Investigación y Desarrollo de la Armada (CIDA), Arturo Soria 289, 28033 Madrid, Spain The existing technology for uncooled MWIR photon detectors based on polycrystalline lead salts is stigmatized for being a 50-year-old technology. It has been traditionally relegated to single-element detectors and relatively small linear arrays due to the limitations imposed by its standard manufacture process based on a chemical bath deposition technique (CBD) developed more than 40 years ago. Recently, an innovative method for processing detectors, based on a vapour phase deposition (VPD) technique, has allowed manufacturing the first 2D array of polycrystalline PbSe with good electro optical characteristics. The new method of processing PbSe is an all silicon technology and it is compatible with standard CMOS circuitry. In addition to its affordability, VPD PbSe constitutes a perfect candidate to fill the existing gap in the photonic and uncooled IR imaging detectors sensitive to the MWIR photons. The perspectives opened are numerous and very important, converting the old PbSe detector in a serious alternative to others uncooled technologies in the low cost IR detection market. The number of potential applications is huge, some of them with high commercial impact such as personal IR imagers, enhanced vision systems for automotive applications and other not less important in the security/defence domain such as sensors for active protection systems (APS) or low cost seekers. Despite the fact, unanimously accepted, that uncooled will dominate the majority of the future IR detection applications, today, thermal detectors are the unique plausible alternative. There is plenty of room for photonic uncooled and complementary alternatives are needed. This work allocates polycrystalline PbSe in the current panorama of the uncooled IR detectors, underlining its potentiality in two areas of interest, i.e., very low cost imaging IR detectors and MWIR fast uncooled detectors for security and defence applications. The new method of processing again converts PbSe into an emerging technology. Keywords: uncooled, low cost, infrared sensors, lead selenide. 1. Introduction * e-mail: gvergarao@oc.mde.es Polycrystalline PbSe was one of the first IR detectors used successfully during the WW II. After that, important efforts were made along the 50 s and 60 s trying to develop better and more complex devices and to understand their physics [1 10]. At this time, exhaustive works were done for optimizing preparation methods of sensitive PbSe thin layers. The result is the current standard PbSe technology based on a CBD method [11,12]. PbSe polycrystalline films exhibited a number of drawbacks which limited their applications in the IR imaging systems: significant 1/f noise, poor long term stability, bad photoresponse uniformity, high dielectric constant. Poor understanding of the physics involved in the photoconduction process, the appearance of new and more promising materials sensitive to IR (InSb, SiPt, CMT, thermal or lately, quantum devices (QWIPs)) and technological limitations associated with the CBD method deviated the efforts of scientists and technicians towards other directions, relegating lead salts to a few elementary, and very specific applications. Along the 70 s, the research activity on lead salts decreased considerably and only a very limited number of applications have kept alive the old PbSe technology during the last 30 years, mainly at the US. Today, its physics is still bad understood and the standard processes used for manufacturing detectors have evolved so little that, at present, the biggest format commercially available is a linear array of 256 elements. Recently, an innovative technology for processing polycrystalline PbSe has been developed at CIDA. The most remarkable advantages associated with it [13 15]: good reproducibility, good uniformities, long term stability, fully compatible with plain (no textured) Si substrates, viability studios show that it is compatible with existing Si CMOS technology, simple and affordable technology, 110 Opto-Electron. Rev. 15, no. 2, 2007

compatible with complex multilayer structures such as interference filters (spectral discrimination feature monolithically integrated). Big efforts are being made at CIDA for processing the first imaging device of PbSe monolithically integrated with its read out electronics circuitry. The imaging detector under processing will incorporate the last advances in electronics, including active pixel sensor (APS) concept with analogue/digital signal mixing. Without any doubt, such attainment will turn PbSe into a major actor among the uncooled IR detectors. Meanwhile, this new technology has made possible to process devices such as the first 2D FPAs of polycrystalline PbSe on plain (no textured) silicon [16,17]. This technology overcomes some of the most important inconveniences presented by the standard CBD method. On the other hand, the effects associated to the material nature (high 1/f noise frequency knee or high dielectric constant) can, today, be minimized using specific circuitry and the high processing capabilities supplied by modern electronics. All these facts open new and important perspectives, reviving PbSe sensors and converting it again, more than 50 years later, into a very promising IR sensitive material. Section 2 reviews the panorama of uncooled and low cost imaging IR detectors and their issues in terms of cost reduction and increased performances. Section 3 is dedicated to give a brief description of the CIDA s PbSe technology (sec. 3.1), to present its potential advantages in terms of low cost and affordability (sec. 3.2) and finally, to propose PbSe as a candidate for filling the existing gap in the field of uncooled photonic MWIR detectors. 2. Uncooled thermal detectors. A journey towards affordability Along the last 30 years, the technology of IR detectors has grown very fast, achieving an outstanding degree of development, inconceivable few years ago. Without any doubt, the most outstanding event has been the revolution led by thermal uncooled IR detectors. Thanks to the last technological advances in this field, the dream of creating devices able to work at room temperature, close to the BLIP condition, is now a reality [18,19]. The global IR detector industry has been shaken by thermal uncooled devices. This market is growing annually by ~25% [20] and huge efforts are being made for improving sensor performances and reducing manufacture costs. The objective is to bring low/medium performance IR imagers to a volume market in a short period of time. The applications are multiple: personal imagers, medical thermography, automotive enhanced vision systems, atmospheric pollutant control, homeland security, smart environmental comfort systems for domestic use, etc. The predictions foresee an enormous widespreading of very low cost imaging IR sensors in a near future [21]. The conversion of IR sensors into a mass market product depends on reaching an adequate performance/affordability ratio. The advances along the last five years have been impressive, but still will be necessary to overcome important obstacles. The road towards affordability started more than twenty years ago. Along eighties and nineties, the important hi-tech companies around the world began a rush for developing and commercializing less expensive and better uncooled IR sensors. All of them focused their efforts on thermal imaging sensors. As a consequence, today, uncooled is synonymous of thermal. At present, numerous companies still spend big amount of resources on developing enable technologies to increase sensor performances and to reduce manufacture costs. Thermal FPAs are monolithic or hybrid devices and their evolution have been tightly linked to the microelectronics golden rule, Moore s law [22,23]. During this time, the performance increment and the cost reduction strategies have been based on the following points: Processing on larger areas. At present, the most advanced uncooled technologies (VO x and a-si microbolometers) use large area Si wafers (³ 8 ) as standard substrates. It is an important driven factor for reducing manufacture costs. Reduction of pixel and array sizes. In addition to performance improvements there are further benefits linked to the pitch size shrink, smaller detector sizes, meaning more detectors per wafer, higher yields and smaller and more affordable optics. Increase process yields. Improvements in wafer handling, process reproducibility, etc. Testing reduction. The goal was to reduce testing for minimizing touch labour improving reliability of each process and diminishing scrap. After the efforts done along all these years, the results are excellent in terms of both performance and cost reduction. So, today the largest format commercially available is a microbolometer of 640 512 elements, but all important actors commercialize systems or devices with a standard format of 320 240 elements. For volume applications, all manufacturers have unanimously chosen a standard format of 160 120 elements, in some cases with a TEC-less feature, integrated in the read out integrated circuit. Regarding cost, during the last years it was notably reduced but it is still out of the affordability threshold for volume consumption and the market continues dominated by defence and security applications. The manufacture cost reduction must go on, but the activity has reached a point where the cost reduction rate is decreasing. Up to now, the main driven forces (wafer size increment, pixel size reduction, testing reduction and yield increase) are close to their limits. Shrinkage of pixel size also reduces the number of photons collected by each pixel, it increases the statistical noise level and reduces the available signal dynamic range. At present, in the microbolometer case, a point has been reached at which it no longer makes sense to attempt to reduce FPA pixel dimensions. Pixel size Opto-Electron. Rev. 15, no. 2, 2007 G. Vergara 111

and NETD (f/1.0) are limited by fundamental reasons to ~20 µm and ~10 mk, respectively. Current VO x technology is close to the limit. Pixel sizes of 20 µm and NETD (f/1.0) < 25 mk have been reported. á-si technology is also close to the pixel size limit but there is few room for NETDs improving (today, ~40 mk). The image detected by an FPA will always be limited by the collection optics and the interface to the physical world. Once the pixel-to-pixel sampling distance satisfies the Nyquist criteria for the primary spatial frequencies allowed by the aperture and the residual detector noise, further oversampling, i.e., reducing the pixel-to-pixel separation, adds little benefit. Traditional strategies to reduce the cost of thermal detector manufacture are close to give up and the cost reduction process will be mainly lead by cheaper packaging and optics and manufacture yield improvements. Packaging It is probably the most relevant parameter in terms of cost of an IR sensor. An important part of the price of an IR FPA is the cost of its package. In the case of a cooled detector, the cost of the cooler plus its cryogenic assembly and packaging can be more than 60% of total cost, including electronics. In the case of uncooled detectors, their total price is much lower but the percentage of the package cost is today more or less the same than that of cooled detectors. This percentage tends to increase because die price is decreasing faster than package price. In the case of thermal devices, the packaging is one of the most important issues associated to the technology. In fact, the die has to be isolated from any thermal leakage, and vacuum is required to minimize any air thermal convection. By practical reasons, good vacuum need to be guaranteed during a long period of time (at least 10 years). As a consequence, high quality packages are required. It represents an important constriction for lowing costs. At present, important research activities are being focused on development of advanced low cost packaging of thermal IR sensors. The most innovative approaches to the problem are on wafer vacuum seal [24 28] techniques. Several companies are working on it and up to now the results obtained are very promising. More work, however, needs to be done and the impact of this technology on the final cost of the device should be evaluated. This technique demands more processing, consumes some space of total wafer area and the low volume enclosed limits the device lifetime. Optics Thermal detectors operate in the LWIR range. In terms of affordability it is an important drawback because the optics is traditionally expensive in this spectral band. Big efforts are being made to study new materials applied to affordable LWIR optics. Recently, it has been reported a new industrial molding process called GASIR to manufacture low cost LWIR optics for applications in commercial high volume thermal imaging [29,30]. It is soon to evaluate the cost reduction of these new technologies but today the activities devoted to reduce costs of LWIR optics are an important part of the strategy designed to reach the affordability demanded by mass market products. Processing The processing yields are improving continuously. Standard thermal detectors are MEMS consisting of microbridges where one of the critical parameters is the optical cavity formed by the absorbing material layer and a reflector placed in the substrate. The technologies associated with their manufacture, however, are sophisticated and demand very expensive equipment and specialized technicians. Automatic wafer handling, improvement in processing reproducibility, reduction of array sizes, optimization of test procedures, etc., are activities carried out with the objective to increase yields and reduce manufacture costs. Summarizing, today manufacture costs reduction is a key issue to bring thermal sensors to volume applications. During the last 15 years, impressive advances have been done in this direction. However, facts such as packaging in vacuum, optics cost and technological complexity represent important limiting factors for reaching the affordability threshold necessary to turn uncooled into a mass market product. 3. VPD PbSe: an affordable and MWIR uncooled detector There are good imaging uncooled detectors working in both, short wavelength (SW) and long wavelength (LW), IR spectral bands. The first one are photonic detectors (InGaAs, CMT, etc.) and the second one are thermal detectors (VO x and á-si microbolometers, PLZT, BST, etc.). However, and even though MWIR is the most interesting spectral window for multiple applications, there is not a mature technology of uncooled MWIR imaging detectors. The reasons are related to fundamental limitations imposed by the mechanisms involved in the detection process. A MWIR sensitive device must choose between one of the two main working principles, thermal or photonic. Thermal detectors optimized for MWIR radiation require a smaller space between the absorbing material and the substrate compared to that required for LWIR. That makes difficult to obtain high efficiency devices. Intensive work is currently underway to improve device performance in this spectral band. It is remarkable the good results obtained by other alternative technologies such as microcantilever based arrays [31,32]. On the other hand, MWIR photonic devices require the use of narrow band gap materials. Thermal generation and recombination in narrow gap semiconductors at near room temperature is determined by the Auger mechanism [33,34]. Intensive efforts have been made in the past and still are currently underway to improve the performance of MWIR uncooled photodetectors. So, some scientists have approached the problem using different strategies: reduction of thermal generation rate by proper selection of semiconductor material, doping, G-R 112 Opto-Electron. Rev. 15, no. 2, 2007 2007 SEP, Warsaw

suppression by non equilibrium operation mode or reduction of detector physical volume, preserving the field of view and the quantum efficiency [35]. At present, multiple groups are working on artificial narrow gap semiconductors, based on type II, type III superlattices, QDP, Hg 1 x Cd x Te and non equilibrium devices [36,37]. All these technologies present high cost and technological complexity and all of them, besides, are still immature, with uniformity and reproducibility problems. Paradoxically, nature gives a solution for overcoming most of the problems associated with photonic sensors and narrow band gap materials, polycrystals. The suppression of Auger mechanism and the reduced dark current found in some polycrystalline thin films such as lead salts are direct consequences of having multiple intergrain depletion regions and potential barriers. However, this natural solution has been traditionally relegated by facts such as lack of understanding of their physics and of the mechanisms associated to the detection of photons, lack of reproducibility, issues related to their morphology (roughness, thermal mismatch with substrates, etc.) which have prevented to process more advanced IR detectors due to technological difficulties during their hybridization, incompatibility with Si technologies, etc. 3.1. New PbSe VPD based technology description CIDA s technology is based on a thermal deposition in vacuum (VPD) method followed by specific sensitization process. A detailed description can be found in Ref. 38. The vacuum deposition of PbSe is an old and well known technique for processing IR detectors of polycrystalline PbSe [4,10,12]. It was widely accepted that CBD techniques yielded better uniformity of photoresponse and longer term stability in comparison with the evaporative method [39]. The innovations in material processing introduced by CIDA s group along more than 10 years of continuous research have improved the performances of detectors processed by VPD in such a way that their uniformity and long term stability is today comparable or better than those processed with the standard CBD method. But, the real advantages of the new VPD based method, compared with the traditional CBD method, reside in that it permits to use big area Si substrates with complex structures patterned on it, including specific CMOS read out electronics. The new PbSe processing method represents a substantial advance and a qualitative leap respecting to the existing PbSe technology. It is possible to find in the literature numerous works and patents describing or claiming PbSe detectors interfaced [33,40 42] or monolithically integrated [43,44] with CMOS circuitry. However, in most cases the technologies and methods described correspond to the manufacture of small format detectors (linear, multielement, etc.) coupled or hybridized with some type of specific multiplexed electronics. Even in case of monolithic integration, the methods of detector processing demand to use specific and complex features such as textured coatings in order to avoid structural damage in the layers during the CBD deposition and sensitization process. At this point, it is important to point out that the monolithic integration of lead salt detectors cannot be considered as an unique task. Although PbSe and PbS are both lead salts, the sensitization process of PbSe requires much higher temperatures (~400 C) than PbS (below 200 C) sensitization, making its monolithic integration with CMOS electronics a serious technological challenge. An evidence of the limitations imposed by the traditional CBD based technology is that, at present, the biggest format commercially available is a linear detector with 256 elements interfaced with a specific MUX. After studying the main limitations imposed by the CBD method and the identification of numerous potential advantages associated with a VPD based method (better controllability, more simplicity, affordability and uniformity in big areas and compatibility with multiple substrates), CIDA s group started to develop its own PbSe VPD based technology. With the main premise of using Si substrates as standard substrate, they were defined four main directions of research: optimization of a thermal evaporation in vacuum method, improving layer uniformities, developing specific methods for enhancing layers adherence to plain polished Si and making the deposition method compatible with conventional lithographic methods, development of a completely new sensitization method with the idea of obtaining high sensitivity and uniform detectors. At this point, the main task was to identify the chemistry involved during this step and to decrease as much as possible the maximum treatment temperature in order to avoid potential problems during the monolithic integration with CMOS circuitry, optimization of the detector passivation layers in order to increase the device long term stability, to provide to the detector some spectral capability developing technologies aimed to process the detectors on complex multilayer structures such as interference filters. The technological development covered a wide range of fields and after more than ten years of uninterrupted work the outstanding results were obtained. Following are described the most important achievements associated with the processing method developed: PbSe layer thickness uniformity (on 4 Si wafers) better than 2%, good adherence on different type of polished substrates (Si, sapphire, Al 2 O 3, etc.), compatible with lift-off and other photolithographic processes. Features as small as 5 5 µm were patterned, chemistry of CIDA s PbSe sensitization process was studied and a mechanism of PbSe sensitization was proposed [38]. It is widely accepted that oxygen incorporated to the PbSe crystal lattice is the main responsible for the photoconductive behaviour of PbSe [1,3 6]. The sensitization process developed is based on the fact that halogens act as transport agents during the recrystalliza- Opto-Electron. Rev. 15, no. 2, 2007 G. Vergara 113

tion of the PbSe promoting a quick formation of large PbSe microcrystals. During the recrystallization process, oxygen is incorporated in situ to the PbSe microcrystal lattice in a very efficient way, turning the insensitive as-evaporated layers in high sensitive material, figure 1 shows a typical detectivity distribution curve obtained for an 8 8 array. More than 85% of the detectors have detectivities ranging between 2 and 4 10 9 Hz 1/2 cmw 1, regarding long term stability, electrooptical figures are being periodically measured using a reduced set of detectors. They are stored in the standard ambient of laboratory without any specific prevention of inert or vacuum envelopes. Typically, after more than 70.000 hrs (8 years) the signal variation has been around 15%, they were processed detectors monolithically integrated with interference filters [14,15] modifying the natural spectral response of PbSe. Figure 2 shows a schematic diagram of the sensor processed on an interference filter, including the PbSe layer. The substrate used in this kind of devices is sapphire with both sides polished. Figure 3(a) shows the natural spectral response of our standard PbSe. Figure 3(b) corresponds to the spectral response of the device deposited on an interference filter. This curve fits quite accurately with the convolution (dashed) between the transmittance curve of the interference filter and the PbSe spectral response, they were processed detectors on Si wafers and complex structures patterned on it. So, the first PbSe x-y addressed devices (2D) reported have been manufactured [16,17]. Figure 4 shows a schematic layout of the cross section of an x-y addressed FPA structure, including the PbSe semiconductor and the passivation layer. Fig. 2. Structure of a spectrally selective detector. Fig. 3. Natural response of a standard PbSe detector at room temperature (a) and spectral response of a device with interference filter (b), curve 1 experimental results and curve 2 convolution of PbSe spectral response whit the filter transmission curve. Fig. 1. Histogram describing the standard detectivity distribution curve for an 8 8 array. Fig. 4. Cross-section diagram of an x-y addressed FPA. 114 Opto-Electron. Rev. 15, no. 2, 2007 2007 SEP, Warsaw

With this technology, focal plane arrays up to 32 32 elements have been processed [45]. Figure 5(a) shows a picture of the front view of a 32 32 x-y addressed array of PbSe. In Fig. 5(b), an image taken with a camera of a 16 16 PbSe array is shown. The electronics and optics designed and fabricated for testing the 16 16 array is now under development for a 32 32 array. The new technology is fully compatible with CMOS technology. CMOS circuitry withstands the temperatures and the corrosive atmosphere used during the sensitization process of the PbSe layer [13]. The method developed makes possible to process monolithic devices without any fundamental limitation. In Fig. 6(a), a schematic layout is shown. A picture of a CMOS test device with sensitized PbSe can be observed in Fig. 6(b). At present, the first monolithic devices manufacture is in progress. Summarizing, the VPD solves most of the inconveniences associated with the CBD processing method as demonstrated manufacturing the first 2D FPAs on untextured Si. The technology developed paves the way for achieving more complex detectors, including monolithic devices and multicolour sensors. 3.2. VPD PbSe: a very low cost solution It may be considered pretentious to compare the new PbSe technology, with a modest degree of development, with other much more advanced uncooled technologies and very well positioned in the domain of low cost IR imaging. Today, a format of 160 120 elements is considered, the smallest size recommendable for imaging in very low cost applications for mass market commercialization. Due to the high amount of elements in the array it is mandatory a reduced pitch and an integrated/hybridized electronics. In the Fig. 5. Front view picture of an x-y 32 32 PbSe addressed array (a) and image taken with an IR camera of 16 16 PbSe array (b). It can be seen an extended hand. polycrystalline PbSe case, the smallest pitch processed until date has been 100 100 µm2 [45] and the monolithic integration process is still in a prototype level. So, there is a long trip to walk but the effort can be very profitable if the following considerations are taken: Packaging. As mentioned before, the packaging cost is an important part of the total cost of an IR detector. In case of thermal uncooled it is mandatory to use long term, high quality vacuum packages because of the requirement of thermal convection minimization. It is not the case for photonic detectors. The mechanisms involved in the detection process are completely different, the thermal convection is no longer a big issue and, as a consequence, the package requirements are much more moderated. Generally speaking, uncooled photonic detectors take an important advantage in terms of packaging quality and device cost reduction compared to thermal uncooled. Fig. 6. Structure of a processed CMOS wafer. Opto-Electron. Rev. 15, no. 2, 2007 115 G. Vergara

Optics. VPD PbSe works in the MWIR spectral region. Optics in this spectral band is much more affordable than that used in LWIR. For instance, a Si lens used in SWIR or MWIR costs, typically, from 7 to 10 times less than a Ge or AMTIR lens. These differences in cost are being reduced thanks to new materials and new molded technologies but still are, and will be, important. Again, PbSe presents a potential advantage in terms of cost compared to uncooled thermal detectors. Processing. In sec. 3, the new PbSe technology is described. It complies with the imperative of being an all Si technology. In addition, other remarkable facts are its technological simplicity, reproducibility and suitability for volume productions (it is easily extended to big substrates, high yields, etc.). High sensitivity. Thermal uncooled detectors present detectivities typically one order of magnitude below VPD PbSe. It is a serious limitation for numerous low cost applications which demand spectral selectivity. PbSe would take advantage again for those applications where signals to be detected are weak or need to be spectrally separated. 4. Conclusions It is a remarkable fact that, after several decades of continuous investment on both scientific knowledge and technological development, there is still a lack of suitable uncooled, MWIR sensitive, photonic detectors. It is even more unexpected if it is taken into account that it is a very demanded product, with important commercial, security and defence applications such as personal imagers, environmental control, low cost seekers, sensors applied to active protection systems (APS), sniper detection, enhanced vision systems (EVS) for automotive, etc. In this work, we have shown good arguments to consider VPD-PbSe based devices as the only detectors that, at present, combine affordability with fast response (photonic), MWIR sensitivity and high detectivity. 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