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 character- istics. 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 complemen- tary 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 mal or lately, quantum devices (QWIPs)) and technological limitations associated with the CBD method deviated the Polycrystalline PbSe was one of the first IR detectors used efforts of scientists and technicians towards other direc- successfully during the WW II. After that, important efforts tions, relegating lead salts to a few elementary, and very were made along the 50’s and 60’s trying to develop better specific applications. Along the 70’s, the research activity and more complex devices and to understand their physics on lead salts decreased considerably and only a very lim- [1–10]. At this time, exhaustive works were done for opti- ited number of applications have kept alive the old PbSe mizing preparation methods of sensitive PbSe thin layers. technology during the last 30 years, mainly at the US. To- The result is the current standard PbSe technology based on day, its physics is still bad understood and the standard pro- a CBD method [11,12]. cesses used for manufacturing detectors have evolved so PbSe polycrystalline films exhibited a number of draw- little that, at present, the biggest format commercially backs which limited their applications in the IR imaging available is a linear array of 256 elements. systems: Recently, an innovative technology for processing • significant 1/f noise, polycrystalline PbSe has been developed at CIDA. The • poor long term stability, most remarkable advantages associated with it [13–15]: • bad photoresponse uniformity, • good reproducibility, • high dielectric constant. • good uniformities, Poor understanding of the physics involved in the photoconduction process, the appearance of new and more • long term stability, promising materials sensitive to IR (InSb, SiPt, CMT, ther- • fully compatible with plain (no textured) Si substrates, • viability studios show that it is compatible with existing Si CMOS technology, *e-mail: [email protected] • simple and affordable technology,

110 Opto-Electron. Rev. 15, no. 2, 2007 • compatible with complex multilayer structures such as reaching an adequate performance/affordability ratio. The interference filters (spectral discrimination feature advances along the last five years have been impressive, monolithically integrated). but still will be necessary to overcome important obstacles. Big efforts are being made at CIDA for processing the The road towards affordability started more than twenty first imaging device of PbSe monolithically integrated with years ago. Along eighties and nineties, the important its read out electronics circuitry. The imaging detector un- hi-tech companies around the world began a rush for devel- der processing will incorporate the last advances in elec- oping and commercializing less expensive and better unco- tronics, including active pixel sensor (APS) concept with oled IR sensors. All of them focused their efforts on ther- analogue/digital signal mixing. Without any doubt, such at- mal imaging sensors. As a consequence, today, uncooled is tainment will turn PbSe into a major actor among the synonymous of thermal. At present, numerous companies uncooled IR detectors. Meanwhile, this new technology still spend big amount of resources on developing enable has made possible to process devices such as the first 2D technologies to increase sensor performances and to reduce FPAs of polycrystalline PbSe on plain (no textured) silicon manufacture costs. [16,17]. Thermal FPAs are monolithic or hybrid devices and This technology overcomes some of the most important their evolution have been tightly linked to the microelec- inconveniences presented by the standard CBD method. On tronics golden rule, Moore’s law [22,23]. the other hand, the effects associated to the material nature During this time, the performance increment and the (high 1/f noise frequency knee or high dielectric constant) cost reduction strategies have been based on the following can, today, be minimized using specific circuitry and the points: high processing capabilities supplied by modern electron- • Processing on larger areas. At present, the most ad- a ics. All these facts open new and important perspectives, vanced uncooled technologies (VOx and -Si micro- ³ reviving PbSe sensors and converting it again, more than bolometers) use large area Si wafers ( 8”) as standard 50 years later, into a very promising IR sensitive material. substrates. It is an important driven factor for reducing manufacture costs. Section 2 reviews the panorama of uncooled and low Reduction of pixel and array sizes. In addition to per- cost imaging IR detectors and their issues in terms of cost • formance improvements there are further benefits reduction and increased performances. Section 3 is dedi- linked to the pitch size shrink, smaller detector sizes, cated to give a brief description of the CIDA’s PbSe tech- meaning more detectors per wafer, higher yields and nology (sec. 3.1), to present its potential advantages in smaller and more affordable optics. terms of low cost and affordability (sec. 3.2) and finally, to • Increase process yields. Improvements in wafer han- propose PbSe as a candidate for filling the existing gap in dling, process reproducibility, etc. the field of uncooled photonic MWIR detectors. • Testing reduction. The goal was to reduce testing for 2. Uncooled thermal detectors. A journey minimizing touch labour improving reliability of each process and diminishing scrap. towards affordability After the efforts done along all these years, the results are excellent in terms of both performance and cost reduc- Along the last 30 years, the technology of IR detectors has tion. So, today the largest format commercially available is grown very fast, achieving an outstanding degree of devel- a microbolometer of 640×512 elements, but all important opment, inconceivable few years ago. Without any doubt, actors commercialize systems or devices with a standard the most outstanding event has been the revolution led by format of 320×240 elements. For volume applications, all thermal uncooled IR detectors. Thanks to the last techno- manufacturers have unanimously chosen a standard format logical advances in this field, the dream of creating devices of 160×120 elements, in some cases with a TEC-less fea- able to work at room temperature, close to the BLIP condi- ture, integrated in the read out integrated circuit. Regarding tion, is now a reality [18,19]. cost, during the last years it was notably reduced but it is The global IR detector industry has been shaken by still out of the affordability threshold for volume consump- thermal uncooled devices. This market is growing annually tion and the market continues dominated by defence and by ~25% [20] and huge efforts are being made for improv- security applications. ing sensor performances and reducing manufacture costs. The manufacture cost reduction must go on, but the ac- The objective is to bring low/medium performance IR tivity has reached a point where the cost reduction rate is de- imagers to a volume market in a short period of time. The creasing. Up to now, the main driven forces (wafer size in- applications are multiple: personal imagers, medical ther- crement, pixel size reduction, testing reduction and yield in- mography, automotive enhanced vision systems, atmo- crease) are close to their limits. Shrinkage of pixel size also spheric pollutant control, homeland security, smart envi- reduces the number of photons collected by each pixel, it in- ronmental comfort systems for domestic use, etc. The pre- creases the statistical noise level and reduces the available dictions foresee an enormous widespreading of very low signal dynamic range. At present, in the microbolometer cost imaging IR sensors in a near future [21]. The conver- case, a point has been reached at which it no longer makes sion of IR sensors into a mass market product depends on 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 tivities devoted to reduce costs of LWIR optics are an im- µm and ~10 mK, respectively. Current VOx technology is portant part of the strategy designed to reach the close to the limit. Pixel sizes of 20 µm and NETD (f/1.0) < affordability demanded by mass market products. 25 mK have been reported. á-Si technology is also close to the pixel size limit but there is few room for NETDs improv- Processing ing (today, ~40 mK). The image detected by an FPA will al- The processing yields are improving continuously. Stan- ways be limited by the collection optics and the interface to dard thermal detectors are MEMS consisting of micro- the physical world. Once the pixel-to-pixel sampling dis- bridges where one of the critical parameters is the optical tance satisfies the Nyquist criteria for the primary spatial fre- cavity formed by the absorbing material layer and a reflec- quencies allowed by the aperture and the residual detector tor placed in the substrate. The technologies associated noise, further oversampling, i.e., reducing the pixel-to-pixel with their manufacture, however, are sophisticated and de- separation, adds little benefit. mand very expensive equipment and specialized techni- Traditional strategies to reduce the cost of thermal de- cians. Automatic wafer handling, improvement in process- tector manufacture are close to give up and the cost reducing reproducibility, reduction of array sizes, optimization tion process will be mainly lead by cheaper packaging and of test procedures, etc., are activities carried out with the optics and manufacture yield improvements. objective to increase yields and reduce manufacture costs. Summarizing, today manufacture costs reduction is a Packaging key issue to bring thermal sensors to volume applications. It is probably the most relevant parameter in terms of cost During the last 15 years, impressive advances have been of an IR sensor. An important part of the price of an IR done in this direction. However, facts such as packaging in FPA is the cost of its package. In the case of a cooled de- vacuum, optics cost and technological complexity repre- tector, the cost of the cooler plus its cryogenic assembly sent important limiting factors for reaching the afforda- and packaging can be more than 60% of total cost, includ- bility threshold necessary to turn uncooled into a mass mar- ing electronics. In the case of uncooled detectors, their total ket product. price is much lower but the percentage of the package cost is today more or less the same than that of cooled detectors. 3. VPD PbSe: an affordable and MWIR This percentage tends to increase because die price is de- uncooled detector creasing faster than package price. In the case of thermal devices, the packaging is one of There are good imaging uncooled detectors working in the most important issues associated to the technology. In both, short wavelength (SW) and long wavelength (LW), fact, the die has to be isolated from any thermal leakage, IR spectral bands. The first one are photonic detectors and vacuum is required to minimize any air thermal con- (InGaAs, CMT, etc.) and the second one are thermal detec- vection. By practical reasons, good vacuum need to be tors (VO and á-Si microbolometers, PLZT, BST, etc.). guaranteed during a long period of time (at least 10 years). x However, and even though MWIR is the most interesting As a consequence, high quality packages are required. It spectral window for multiple applications, there is not a represents an important constriction for lowing costs. mature technology of uncooled MWIR imaging detectors. At present, important research activities are being fo- The reasons are related to fundamental limitations imposed cused on development of advanced low cost packaging of by the mechanisms involved in the detection process. A thermal IR sensors. The most innovative approaches to the MWIR sensitive device must “choose” between one of the problem are on wafer vacuum seal [24–28] techniques. two main working principles, thermal or photonic. Several companies are working on it and up to now the re- Thermal detectors optimized for MWIR radiation results obtained are very promising. More work, however, quire a smaller space between the absorbing material and needs to be done and the impact of this technology on the the substrate compared to that required for LWIR. That final cost of the device should be evaluated. This technique makes difficult to obtain high efficiency devices. Intensive demands more processing, consumes some space of total work is currently underway to improve device performance wafer area and the low volume enclosed limits the device in this spectral band. It is remarkable the good results ob- lifetime. tained by other alternative technologies such as microcan- Optics tilever based arrays [31,32]. On the other hand, MWIR Thermal detectors operate in the LWIR range. In terms of photonic devices require the use of narrow band gap mate- affordability it is an important drawback because the optics rials. Thermal generation and recombination in narrow gap is traditionally expensive in this spectral band. Big efforts semiconductors at near room temperature is determined by are being made to study new materials applied to afford- the Auger mechanism [33,34]. Intensive efforts have been able LWIR optics. Recently, it has been reported a new in- made in the past and still are currently underway to im- dustrial molding process called GASIR® to manufacture prove the performance of MWIR uncooled photodetectors. low cost LWIR optics for applications in commercial high So, some scientists have approached the problem using dif- volume thermal imaging [29,30]. It is soon to evaluate the ferent strategies: reduction of thermal generation rate by cost reduction of these new technologies but today the ac- 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 reduc- avoid structural damage in the layers during the CBD depo- tion of detector physical volume, preserving the field of sition and sensitization process. At this point, it is important view and the quantum efficiency [35]. At present, multiple to point out that the monolithic integration of lead salt detec- groups are working on artificial narrow gap semiconductors cannot be considered as an unique task. Although PbSe tors, based on type II, type III superlattices, QDP, and PbS are both lead salts, the sensitization process of PbSe Hg1–xCdxTe and non equilibrium devices [36,37]. All these requires much higher temperatures (~400 C) than PbS (be- technologies present high cost and technological complex- low 200 C) sensitization, making its monolithic integration ity and all of them, besides, are still immature, with unifor- with CMOS electronics a serious technological challenge. mity and reproducibility problems. An evidence of the limitations imposed by the traditional Paradoxically, nature gives a solution for overcoming CBD based technology is that, at present, the biggest format most of the problems associated with photonic sensors and commercially available is a linear detector with 256 ele- narrow band gap materials, polycrystals. The suppression ments interfaced with a specific MUX. of Auger mechanism and the reduced dark current found in After studying the main limitations imposed by the some polycrystalline thin films such as lead salts are direct CBD method and the identification of numerous potential consequences of having multiple intergrain depletion re- advantages associated with a VPD based method (better gions and potential barriers. However, this natural solution controllability, more simplicity, affordability and unifor- has been traditionally relegated by facts such as lack of un- mity in big areas and compatibility with multiple sub- derstanding of their physics and of the mechanisms associ- strates), CIDA’s group started to develop its own PbSe ated to the detection of photons, lack of reproducibility, is- VPD based technology. With the main premise of using Si sues related to their morphology (roughness, thermal mis- substrates as standard substrate, they were defined four match with substrates, etc.) which have prevented to pro- main directions of research: cess more advanced IR detectors due to technological diffi- • optimization of a thermal evaporation in vacuum culties during their hybridization, incompatibility with Si method, improving layer uniformities, developing spe- technologies, etc. cific methods for enhancing layers adherence to plain polished Si and making the deposition method compati- 3.1. New PbSe VPD based technology description ble with conventional lithographic methods, • development of a completely new sensitization method CIDA’s technology is based on a thermal deposition in with the idea of obtaining high sensitivity and uniform vacuum (VPD) method followed by specific sensitization detectors. At this point, the main task was to identify process. A detailed description can be found in Ref. 38. the chemistry involved during this step and to decrease The vacuum deposition of PbSe is an old and well known as much as possible the maximum treatment tempera- technique for processing IR detectors of polycrystalline ture in order to avoid potential problems during the PbSe [4,10,12]. It was widely accepted that CBD tech- monolithic integration with CMOS circuitry, niques yielded better uniformity of photoresponse and lon- • optimization of the detector passivation layers in order ger term stability in comparison with the evaporative to increase the device long term stability, method [39]. The innovations in material processing intro- • to provide to the detector some spectral capability de- duced by CIDA’s group along more than 10 years of con- veloping technologies aimed to process the detectors on tinuous research have improved the performances of detec- complex multilayer structures such as interference fil- tors processed by VPD in such a way that their uniformity ters. and long term stability is today comparable or better than The technological development covered a wide range of those processed with the standard CBD method. But, the fields and after more than ten years of uninterrupted work real advantages of the new VPD based method, compared the outstanding results were obtained. Following are de- with the traditional CBD method, reside in that it permits to scribed the most important achievements associated with use big area Si substrates with complex structures patterned the processing method developed: on it, including specific CMOS read out electronics. • PbSe layer thickness uniformity (on 4” Si wafers) better The new PbSe processing method represents a substan- than 2%, tial advance and a qualitative leap respecting to the existing • good adherence on different type of polished substrates PbSe technology. It is possible to find in the literature nu- (Si, sapphire, Al2O3, etc.), compatible with lift-off and merous works and patents describing or claiming PbSe de- other photolithographic processes. Features as small as tectors interfaced [33,40–42] or monolithically integrated 5×5 µm were patterned, [43,44] with CMOS circuitry. However, in most cases the • chemistry of CIDA’s PbSe sensitization process was technologies and methods described correspond to the man- studied and a mechanism of PbSe sensitization was pro- ufacture of small format detectors (linear, multielement, etc.) posed [38]. It is widely accepted that oxygen incorpo- coupled or hybridized with some type of specific multi- rated to the PbSe crystal lattice is the main responsible plexed electronics. Even in case of monolithic integration, for the photoconductive behaviour of PbSe [1,3–6]. The the methods of detector processing demand to use specific sensitization process developed is based on the fact that and complex features such as “textured” coatings in order to 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 micro- crystal lattice in a very efficient way, turning the insen- sitive 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 4×109 Hz1/2cmW–1, • regarding long term stability, electrooptical figures are Fig. 2. Structure of a spectrally selective detector. being periodically measured using a reduced set of detectors. They are stored in the standard ambient of labo- ratory 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. 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 Fig. 5. Front view picture of an x-y 32×32 PbSe addressed array (a) fundamental limitation. In Fig. 6(a), a schematic layout is and image taken with an IR camera of 16×16 PbSe array (b). It can shown. A picture of a CMOS test device with sensitized be seen an extended hand. 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 inconve- polycrystalline PbSe case, the smallest pitch processed un- niences associated with the CBD processing method as til date has been 100×100 µm2 [45] and the monolithic inte- demonstrated manufacturing the first 2D FPAs on untex- gration process is still in a prototype level. So, there is a tured Si. The technology developed paves the way for long trip to walk but the effort can be very profitable if the achieving more complex detectors, including monolithic following considerations are taken: devices and multicolour sensors. • Packaging. As mentioned before, the packaging cost is an important part of the total cost of an IR detector. In 3.2. VPD PbSe: a very low cost solution case of thermal uncooled it is mandatory to use long term, high quality vacuum packages because of the re- It may be considered pretentious to compare the new PbSe quirement of thermal convection minimization. It is not technology, with a modest degree of development, with the case for photonic detectors. The mechanisms in- other much more advanced uncooled technologies and very volved in the detection process are completely differ- well positioned in the domain of low cost IR imaging. To- ent, the thermal convection is no longer a big issue and, day, a format of 160×120 elements is considered, the as a consequence, the package requirements are much smallest size recommendable for imaging in very low cost more moderated. Generally speaking, uncooled photo- applications for mass market commercialization. Due to the nic detectors take an important advantage in terms of high amount of elements in the array it is mandatory a re- packaging quality and device cost reduction compared duced pitch and an integrated/hybridized electronics. In the to thermal uncooled.

Fig. 6. Structure of a processed CMOS wafer.

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