IR detection with uncooled focal plane arrays. State-of-the art and trends

Transcription

1 Contributed paper OPTO-ELECTRONICS REVIEW 12(1), (2004) IR detection with uncooled focal plane arrays. State-of-the art and trends J.L. TISSOT* ULIS, BP2, Veurey Voroize, France The emergence of uncooled detectors has opened new opportunities for IR detection for both military and commercial applications. Development of such devices involves a lot of trade-offs between the different parameters that define the technological stack. These trade-offs explain the number of different architectures that are under worldwide development. The key factor is to find a high sensitivity and low noise thermometer materia³ compatible with silicon technology in order to achieve high thermal isolation in the smallest area as possible. Ferroelectric thermometer based on hybrid technology and electrical resistive thermometer based (microbolometer) technology are under development. However, ferroelectric materia³ suffers from the difficulty to achieve a high figure of merit from thin film that is needed for monolithic structure development. Besides, the microbolometer technology, well adapted for thin film process, leads to higher performance at the expense of more complex readout integrated circuit design. LETI and ULIS have been chosen from the very beginning to develop first, a monolithic microbolometer technology fully compatible with commercially available CMOS technology and secondly, amorphous silicon based thermometer. This silicon approach has the greatest potential for reducing infrared detector manufacturing cost. After the development of the technology, the transfer to industrial facilities has been performed in a short period of time and the production is now ramping up with ULIS team in new facilities. LETI and ULIS are now working to facilitate the IRFPA integration into equipment in order to address a very large market. Achievement of this goal needs the development of smart sensors with on-chip advanced functions and the decrease in manufacturing cost of IRFPA by decreasing the pixel pitch and simplifying the vacuum package. We present in this paper the new designs for readout circuit and packages that will be used for and arrays with a pitch of 35 µm and advanced results on 35 µm pixel pitch arrays. Thermographic application needs high stable infrared detector with a precise determination of the amount of absorbed infrared flux. Hence, infrared detector with internal temperature stabilized shield has been developed and characterised. The results will be presented. Keywords: amorphous silicon, microbolometr, focal plane arrays, NETD. 1. Introduction Uncooled microbolometer is going to take a large part of infrared imaging application business. Many developments are under progress and lightweight low power camera is already available making the uncooled microbolometer systems a viable alternative to many cooled IR systems. Development efforts in uncooled focal plane array are now basically going in two directions: arrays for military and high-end commercial applications with the highest possible performance, arrays for commercial applications with the lowest possible cost. The goal of these efforts could be summarized in being able to decrease the pixel pitch keeping the same, or higher, level of performance. We present hereafter a brief worldwide overview of uncooled infrared focal plane array (IRFPA) developments. * jl.tissot@ulis-ir.com 2. Uncooled infrared focal plane worldwide development Many worldwide developments are under progress mostly in USA and Europe and mainly supported by the Departments of Defense concerned by the possibility to have low cost, low power infrared imaging camera for goggles, portable weapon sights or driver s vision enhancers. However, uncooled IR detectors are typically dual-use since the same infrared modules could be used for both commercial and military applications The United States of America The United States of America has been active in uncooled infrared focal plane arrays since the early 1980s with the work of Honeywell on vanadium oxide based microbolometers and Texas Instrument on ferroelectrics ceramics and amorphous silicon based detectors. Figure l shows im- Opto-Electron. Rev., 12, no. 1, 2004 J.L. Tissot 105

2 IR detection with uncooled focal plane arrays. State-of-the art and trends Fig. 1. Industrial configuration evolution of uncooled infrared detector manufacturer in the United States. portant industrial organization evolution along the past twenty years. Three technologies are used in USA: vanadium oxide technology previously developed by Honeywell and now used by Raytheon Vision Systems, DRS and BAe and more recently by Indigo. High performance is already demonstrated with this technology and the challenge is now to be able to reduce the pixel pitch down to 25 µm. Raytheon [1] and DRS [2] shown the first results with a pixel pitch of 25 µm and a double deck structure and BAe with a pixel pitch of 28 µm and a more conventional simple deck structure [3]. The second challenge is to suppress the focal plane temperature stabilisation in order to simplify the detector integration in a camera. Besides, DRS has developed a radiometric version of its pixels array and delivers its to infrared thermometry equipment manufacturer. ferroelectric BST technology previously developed by Texas Instrument and now used by Raytheon Commercial Infrared. The hybrid detector structure of this technology limits the achievable thermal insulation of the absorber and thermometer and then the performance. Effort is now focused on the development of a monolithic structure in order to achieve higher thermal insulation. The difficulty is to carry out high figure of merit [4] from thin layer of a ceramic material. amorphous silicon technology previously developed by Texas Instruments [5] and now used by Raytheon Commercial Infrared. This technology is much more oriented on low price commercial applications. Another company, ICC, is recently involved in another amorphous silicon microbolometer technology coming with an exclusive license agreement from DSTO (Australian Ministry of Defense) upon the support of which this technology was developed in Australia The rest of the world Some developments are under progress in Canada and Japan. INO in Canada is involved in a vanadium oxide technology development similar to those developed in USA [6]. In Japan, Mitsubishi develops with success an attractive silicon p-n junction based technology. Besides, NEC is working since four or five years on vanadium oxide technology with the Honeywell patent license. 3. ULIS technology The development of the microbolometer technology is done in France by a unique structure composed of CEA/LETI for advanced microbolometer process development and ULIS for packaging development, process industrialisation and IRFPA production. ULIS is a subsidiary of SOFRADIR and CEA that has been set up to develop and industrialize uncooled focal plane arrays. CEA/LETI has been involved in amorphous silicon uncooled microbolometer development since 1992 with French MoD funding support. LETI chose from the very beginning to develop amorphous silicon based monolithic microbolometer technology fully compatible with commercially available CMOS technology wafers. Monolithic architecture and silicon compatibility are two key characteristics for being able to produce low cost IR detectors. Besides its silicon technology compatibility, amorphous silicon presents many other advantages. First, it enables manufacturing of thin suspended membrane combined with short leg lengths resulting in high mechanical strength structures which could sustained high vibration rates and high mechanical shocks. This reduced mechanical susceptibility to vibration or shock solicitation is obviously important for many military or commercial applications. Secondly, the electrical property of amorphous silicon does not 106 Opto-Electron. Rev., 12, no. 1, COSiW SEP, Warsaw

3 Contributed paper show any phase transition inside the usual equipment operating temperature range. In consequence, amorphous silicon resistance is monotonously decreasing with temperature increase and thermoelectric cooler could be suppressed by using smart electronics readout in a simple way. The first generation of amorphous silicon based microbolometer technology developed by CEA/LETI is well mastered and very fast industrial transfer has been performed in Since this industrial transfer, CEA/LETI and ULIS are still working for improving the electro-optical performances and reducing the cost of the uncooled focal plane arrays. These developments are presented in the following roadmaps that cover the microbolometer and packaging technologies and the products. 4. Microbolometer technology development 4.1. First generation technology under production Development of this technology is now limited to some adjustments needed by the use of more recent equipments that will be installed in ULIS production line. These equipments will mainly enhanced the mastering of the technology in terms of wafer uniformity, reproducibility and throughput. Typical NETD performances achieved with this technology are about 85 mk for a pixel pitch of 45 µm and f/1 and 60 Hz frame rate operation Development of second-generation technology CEA/LETI and ULIS are working on performance increase of the device by pushing the design rules to enhance the achievable thermal insulation. The key characteristics of our uncooled technology is the very thin amorphous silicon based microbridge structure previously described [7] which leads to a smali thermal time constant close to sin 45 µm pixel pitch. To take advantage of this behaviour, CEA/LETI has defined a new technological stack, fully compatible with the industrialized technology in order to improve thermal insulation at a pixel level and to decrease 1/f noise level. This, so-called, second-generation amorphous silicon microbolometer technology exhibits dramatically enhanced sensitivity and enables the decrease in a pixel pitch to 35 µm, keeping a high level of performance. However, pixel structure is still using the standard surface-micromachined single-level membrane technology. As far as we have a great margin with thermal time constant, thermal capacitance evolution is still acceptable unless thermal time constant is kept compatible with video frame rate. Typical characteristics of this second generation technology for a 35 µm pixel-pitch, are summarized in Table 1. We can notice from these experimental data that: the thermal insulation R th has been increased by a factor value greater than 3. similarly, the NETD figure has been improved by a factor of 5 due to the previous R th enhancement and to an extra 60% l/f noise reduction resulting from the optimization of the detector design (architecture improvement in the pixel as well as technological design rules shrink). as far as the time constant ô th of the 1 st generation technology exhibits a tremendous margin regarding usual video frame rate, it has been possible to increase R th keeping a fully usable time constant close to 10 ms for a 35 µm pixel pitch. despite the pitch reduction from 45 to 35 µm and an associated dramatic drop of a pixel area, a fill factor larger than 80% and high optical efficiency in 8 14 µm wavelength range have been maintained. This second-generation microbolometer technology is now mastered in CEA/LETI [8] pilot line and has been transferred to ULIS in The production is now ramping up in the new ULIS facilities. Improvement based on pixel design and readout integrated circuit design is still forecasted for the future. 5. Product development The product roadmap takes into account array formats needed by the applications with the smallest pixel pitch compatible with the available microbolometer technology µm pixel pitch radiometric device A radiometric version has been derived from the standard product using the same device integrated with an internal shield limiting the field of view to f/1.4 (see Fig. 2). The shield aperture is 16.3 mm in diameter and mm above a focal plane. These new devices exhibit uniform noise and responsivity characteristics. A histogram of the NETD data for the first array is presented in Fig. 3. The mean NETD is 147 mk and the same microbolometer bias is used for 20 C to35 C or 20 C to100 C blackbody reference temperatures. The fixed pattern noise measured for 65 C background after a two-point correction at 20 C and 100 C (584 µv rms) Table 1. First- and second-generation (a-si) technology comparison IRFPA Pitch (µm) R th 10 6 (K/W) ô th (ms) NEDT (mk) Comments 1 st generation (a-si) bolometer ULIS industrial process 2 nd generation (a-si) bolometer Transferred to ULIS in 2003 Opto-Electron. Rev., 12, no. 1, 2004 J.L. Tissot 107

4 IR detection with uncooled focal plane arrays. State-of-the art and trends Fig. 2. Radiometric version of the /45 µm device. Fig. 5. NETD distribution of a 35-µm pitch IRFPA. Fig. 3. NETD distribution (f/1.4, 30 Hz, 300). is close to the temporal noise (509 µv rms), (see Fig. 4). An extensive characterization procedure is under progress to demonstrate its stability against environment temperature fluctuations Smart 35-m pixel pitch devices The development and the mastering of the second-generation microbolometer technology enable to decrease the pixel pitch to 35 µm keeping the same level of a performance. Taking profit from this possibility we have designed and D arrays with a number of innovative on-chip features to simplify the use of this focal plane keeping a very smal silicon ROIC area down to 0.7 cm 2 for the array, in order to reduce wafer-level processing costs per die. This new is designed to fulfil medium resolution and low cost applications. One of the most promising functions under development is the possibility to compensate the non-uniformity pixel by pixel. At power on, the detector acquires its pixel non-uniformity coefficients and stores them in on-chip memory for performing the current compensation during the following images acquisition and readout sequences. This automatic mode of operation could be changed to an external driving mode with non-uniformity coefficients stored in an external memory. The video output will be available on analogical or digital format with an on-chip 12 bits (2 6) ADC. Most of the biases are generated inside of the ROIC for friendly user operation. Using an internal bias reference, we have implemented circuit architecture to decrease the sensitivity of the device to the rms noise on the external analogical bias supply. Several microbolometer IRFPAs have been integrated under vacuum package and the usual electro-optical tests were performed under the standard conditions including an operating temperature of 300 K through a f/1-limiting aperture. Figure 5 shows a typical NETD histogram highlighting the high uniformity of the IRFPA characteristics. 6. Packaging development Metallic package is the first generation of package used to integrate microbolometer chip. Their mechanical strength is well adapted to harsh environment and their design could be easily modified to add internal shield or different com- Fig. 4. Temporal noise map and a fixed pattern noise map. 108 Opto-Electron. Rev., 12, no. 1, COSiW SEP, Warsaw

5 A development of radiometric product shows high level on uniformity achieved with a standard technology. New package technologies and advance 35 µm pixel pitch detector are under mastering in order to decrease a detector cost. Acknowledgments Contributed paper Fig. 6. Metallic packages (a) and (b), and ceramic package (c) for uncooled IRFPA. ponent. However, their cost remains a large part of the total detector cost and with the possibility to decrease detector chip size; this part will be greater in the future. In a consequence, a less expensive package technology is under development to integrate the new generation of detector chip. Ceramic packages are, using available technologies, developed and produced for a high volume chip. Only the process used to assemble chip carrier and window carrier is adapted to take into account the required over 10-year lifetime under vacuum. An automatic assembling machine will be used to decrease manufacturing cost by suppressing manual operations and increasing throughput. Beside, these advanced packaging techniques, CEA/ LETI are working on the development of wafer level packaging [9] in order to achieve the ultimate reduced manufacturing cost. This goal will be achieved by developing a silicon package (chip carrier and IR window [10]) instead of welding infrared window on the top of the microbolometer array wafers developed elsewhere. This last technique involves an increase in the microbolometer chip dimension that is not compatible with the manufacturing cost goal we have. 7. Conclusions After a brief review of the main development activity in the field of uncooled microbolometer technology, we have presented improvement in our technological stack. We obtained significant results in term of signal-to-noise ratio improvement. We took advantage of our low thermal time constant characteristics on a standard technology that paves the way to increase a thermal resistance keeping a thermal time constant compatible with a video frame rate. A new pixel design participates to reduce the noise level enabling the possibility to achieve high performance in spite of pixel pitch reduction. A NETD down to 30 mk has been demonstrated on a laboratory prototype with 35 µm pixel pitch. The authors would like to thank the DGA/DTCO and the Ministere de 1'Economie, des Finances et de 1'Industrie for supporrting these studies and the Uncooled Teams of LETI LIR and ULIS who took part in them. We also thank the Sofradir staff for their contribution. References 1. D. Murphy, M. Ray, R. Wyles, J. Asbrock, N. Lum, J. Wyles, C. Hewitt, A. Kennedy, D. Van Lue, J. Anderson, D. Bradley, R. Chin, and T. Kostrzewa, High sensitivity 25 µm microbolometer FPAs, Proc. SPIE 4721, (2002). 2. P.E. Howard, J.E. Clarke, A.C. Ionescu, C. Li, and J.C. Stevens, DRS U Vox uncooled IR focal plane, Proc. SPIE 4721, (2002). 3. B.S. Backer, N. Butler, M. Kohin, M. Gurnee, J.T. Whitwam, and T. Breen, Recent improvements and developments in uncooled systems at BAE SYSTEMS North America, Proc. SPIE 4721, (2002). 4. C.M. Hanson and H.R. Beratan, Thin-film ferroelectrics: breakthrough, Proc. SPIE 4721, (2002). 5. United State Patent N 5,367,167, Nov. 22, T.D. Pope, H. Jerominek, C. Alain, C. Cayer, B. Tremblay, C. Grenier, P.A. Topart, S. LeClair, F. Picard, C. Larouche, B. Boulager, A. Martel, and Y. Desroches, Commercial and custom , 251 l and pixel bolometric FPAs, Proc. SPIE 4721, (2002). 7. C. Yedel, J.L. Martin, J.L. Ouvrier-Buffet, J.L. Tissot, M. Yilain, and J.J. Yon, Amorphous silicon based uncooled microbolometer IRFPA, Proc. SPIE E. Mottin, A. Bain, J.L. Martin, J.L. Ouvrier-Buffet, J.J. Yon, J.P. Chatard, and J.L. Tissot, Uncooled amorphous silicon technology: high performance achievement and future trends Proc. SPIE (2002). 9. E. Mottin, J.L. Tissot, and L. Letellier, Proc. AMAA 2003 Conference. (to be published). 10. E. Mottin, J.L. Martin, J.L. Ouvrier-Buffet, M. Yilain, A. Bain, J.J. Yon, J.L. Tissot, and J.P. Chatard, Enhanced amorphous silicon technology for microbolometer arrays with a pitch of 35 µm, Proc. SPIE 4369, (2001). Opto-Electron. Rev., 12, no. 1, 2004 J.L. Tissot 109

6 XVIII International Scientific and Engineering Conference on Photoelectronics and Night Vision Devices Conference is organized by: The State Scientific Center of Russian Federation of FSUE RD&P Center Orion The conference will be held in Moscow by SSC RF RD&P Center Orion in May 25 28, Conference topics: Semiconductor photodetectors and focal plane arrays (physics researches of photodetection and new materials; technology, including electron and ion-plasma ones; cooling systems and signal processing systems); Thermal photodetectors and focal plane arrays (physics researches, technology, cooling systems and signal processing methods); Night vision devices (based on image intensifiers and thermal vision ones); Microelectronics for photodetective assemblies and thermal imagers; New trends and recent achievements in IR photoelectronics and night vision devices. International Program Committee Chairman: A.M. Filachev, RD&P Center Orion Russia Vice-chairmen: V.P. Ponomarenko, RD&P Center Orion Russia A.I. Dirochka, RD&P Center Orion Russia Scientific secretary: S.G.Kin, RD&P Center Orion Russia Commitee members: Zh.I. Alferov PTI RAS A.L. Aseev IPS SD RAS S.N. Bagaev IPS SD RAS A.S. Bugaev MIPT, Russia I.S. Gibin SSIOS, Russia Y.V. Gulyaev IRE RAS A.V. Elyutin IRM, Russia V.P. Ivanov FSSC GIPO, Russia V.I. Ryzhi University of AIZU, Japan Lester J. Kozlowski Rockwell Science Center, USA O.N. Krochin PI RAS Kumar Vikram Solid-State Phys. Lab., India N.N. Kudryavtsev MIPT, Russia V.I. Pokrishkin CCB Peleng, Byelorussia G.N. Popov CCB Tochpribor, Russia A. Rogalski WAT, Poland L.D. Saginov RD&P Center Orion E.Y. Salaev Institute of Physics, Azerbaijan F.F. Sizov ISP, Ukraine R.M. Stepanov NRI Electron, Russia Official languages of Conference are Russian and English The conference program provides for oral and poster reports. The proceedings of the conference will be published in Proceeding of SPIE Submission of abstracts Abstracts are presented in English. Please, send one-page high-quality hardcopy version of the whole abstract with typing area mm 2, and type font 12 pt. The paper title must be typed at the beginning of the page, then lower, names and first names of authors, then the name of the organization. The abstracts should be duplicated by by files in WinWord-2000 or WinWord-97 format within the standard book document class. The report text in English is typed in accordance with the requirements of Proceeding of SPIE and diskette is presented at registration. Abstracts should be sent to: Alexander I. Dirochka, State Scientific Center of Russian Federation RD&P Center Orion, 46/2, Enthuziastov Sh., Moscow, , Russia. Phone: 007 (095) , 007 (095) Fax: 007 (095) Please, indicate correspondence data (mail address, phone, fax, ), Oral or Poster presentation and a brief biography of a principal author ( words). Abstracts Deadline: February 10, 2004 Registration and fee ATTENTION: Program committee members are released from registration fee payment. A registration fee including participation in a scientific and social program, publishing abstracts and processing of the conference, lunch etc. will be 135 EUR. Foreign participants can pay fees the first working day of the conference. The Conference will be held in the RD&P Center Orion at the address: 9, Kosinskaya St. Moscow (near Vykhino subway station) Organization committee: Chairman A.M. Filachev, RD&P Center Orion Vice chainrmen: M.D. Korneeva, RD&P Center Orion L.Ya. Grinchenko, RD&P Center Orion Executive secretary A.I. Dirochka, RD&P Center Orion Committee members: I.M. Avseenko Russian Agency of Conventional Armaments E.I. Akopov SPIE RUS V.D. Bugaev MIPT, Russia K.A.Volkov RD&P Center Orion, Russia N.M. Evtichiev Government of Moscow V.M. Proskurin RD&P Center Orion, Russia A.V. Putilov Ministry of Industry, Russia M.I. Romanishina RD&P Center Orion, Russia L.D. Saginov RD&P Center Orion, Russia A.M. Tokarev RD&P Center Orion, Russia 110 Opto-Electron. Rev., 12, no. 1, COSiW SEP, Warsaw