- Email: [email protected]
Development of a 1K × 1K, 8–12 μm QWIP array
Infrared Physics & Technology 50 (2007) 234–239 www.elsevier.com/locate/infrared
Development of a 1K · 1K, 8–12 lm QWIP array M. Jhabvala b
a,*
, K.K. Choi b, C. Monroy b, A. La
a
a NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA US Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD 20783, USA
Available online 17 November 2006
Abstract In the on-going evolution of GaAs quantum well infrared photodetectors (QWIPs) we have developed a 1024 · 1024 (1K · 1K), 8–12 lm infrared focal plane array (FPA). This 1 megapixel detector array is a hybrid using an L3/Cincinnati Electronics silicon readout integrated circuit (ROIC) bump bonded to a GaAs QWIP array fabricated jointly by engineers at the Goddard Space Flight Center (GSFC) and the Army Research Laboratory (ARL). We have integrated the 1K · 1K array into an SE-IR based imaging camera system and performed tests over the 50–80 K temperature range achieving BLIP performance at an operating temperature of 57 K. The GaAs array is relatively easy to fabricate once the superlattice structure of the quantum wells has been defined and grown. The overall arrays costs are currently dominated by the costs associated with the silicon readout since the GaAs array fabrication is based on high yield, wellestablished GaAs processing capabilities. One of the advantages of GaAs QWIP technology is the ability to fabricate arrays in a fashion similar to and compatible with silicon IC technology. The designer’s ability to easily select the spectral response of the material from 3 lm to beyond 15 lm is the result of the success of band-gap engineering and the Army Research Lab is a leader in this area. In this paper we will present the first results of our 1K · 1K QWIP array development including fabrication methodology, test data and imaging capabilities. 2006 Elsevier B.V. All rights reserved.
1. Introduction Recently, remarkable advances in the development of GaAs QWIP focal plane arrays have been made. It was only in the late 1980s that single element, angle lapped QWIPs were developed and used [1]. In the 1990s rapid development from single element QWIPs to 512 · 640 arrays [2–4] occurred across a broad spectrum of the near to far infrared. With the concurrent development of large format silicon readout ICs it has been a relatively simple task for QWIP technology to keep pace. We recently had successfully developed a narrow band 8.4–9 lm, 1K · 1K QWIP array which was hybridized to a Rockwell Scientific TCM 8050 ROIC [5]. The array was designed jointly by engineers at NASA’s Goddard Space Flight Center (GSFC) and the Army Research Laboratory (ARL) and *
Corresponding author. Tel.: +1 301 286 5232; fax: +1 301 286 1672. E-mail addresses: [email protected], murzy.d.jhabvala@ nasa.gov (M. Jhabvala). 1350-4495/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2006.10.029
was also jointly fabricated in Goddard’s Detector Development Laboratory. It is most important for NASA scientific missions to obtain spectral information in our investigations of the Earth and universe. It is this spectral information that provides the clues to many unanswered questions. Unfortunately narrow band detectors, which QWIPs tend to be, pretty much preclude multi-wavelength spectroscopy. To this end, we have developed a broad band, large format QWIP array as part of NASA’s Earth Science Technology program for Advanced Component Technology development [6]. The applications for QWIP arrays are numerous. At GSFC some of these potential applications include global warming studies which includes environmental monitoring of troposphere and stratosphere temperatures and identifying trace chemicals; CO2 absorption; tree canopy energy balance measurements; measuring cloud layer emissivities, droplet/particle size, composition and height; SO2 and aerosol emissions from volcanic eruptions; tracking dust
M. Jhabvala et al. / Infrared Physics & Technology 50 (2007) 234–239
particles (from the Sahara Desert, e.g.); coastal erosion; ocean/river thermal gradients and pollution; analyzing radiometers and other scientific equipment used in obtaining ground truthing and atmospheric data acquisition; ground based astronomy, and; temperature sounding. Defense and military applications include surveillance, target identification, FLIR, fire control and mine detection [7]. The potential commercial applications are quite diverse. The utility of QWIP arrays in medical instrumentation is well documented [8] and may become one of the most significant QWIP technology drivers. The success of OmniCorder Technologies use of 256 · 256 narrow band QWIP arrays for aiding in the detection of malignant tumors is quite remarkable. Other potential commercial applications for QWIP arrays include location of forest fires and residual warm spots; location of unwanted vegetation encroachment; monitoring crop health; monitoring food processing contamination, ripeness and spoilage [9]; locating power line transformer failures in remote areas; monitoring effluents from industrial operations such as paper mills, mining sites and power plants; IR microscopy; searching for a wide variety of thermal leaks and not least of all; locating new sources of spring water. Recently, Freund and his colleagues [10] have been conducting experiments to simulate the effects of the extreme pressures acting on granite cubes to simulate precursor earthquake events. A Goddard built QWIP camera using a Lockheed Martin 256 · 256, 8–8.5 lm, EQWIP [11] was used to detect IR photonic emission during the compressive loading of the granite. 2. QWIP array design and fabrication The design and fabrication of the 1K · 1K QWIP array is straightforward and relatively simple. The architecture of the ROIC determines the pixel pitch which is 25 lm for the L3/Cincinnati Electronics ROIC. The ARL has pioneered the technique [12] of corrugating the QWIP array with V-
235
grooves, which reflect normal incidence light 90 into the QWIP. The pixel geometry design is shown in Fig. 1. Corrugated QWIPs (C-QWIPs) rely on inclined sidewalls to reflect normal incident light into parallel propagation. Assuming the angle of the corrugation sidewalls were 45 instead of 50, the unpolarized external quantum efficiency g, calculated from the decay of light along the optical path, can be expressed as [13] 4n 1 eap 2at gða; p; tÞ ¼ tþ 1e ð1Þ þ K0 ; 2 2a ð1 þ nÞ p where n = 3.34 (the refractive index of GaAs), p is the corrugation period, t is the height of the corrugations, a is the absorption coefficient the vertically polarized, parallel propagation light, and K0 is the internal unpolarized quantum efficiency created by the sidewalls at the ends of the corrugations. For infinitely long corrugations, K0 will be zero, but its value increases with decreasing pixel size. The geometry of a C-QWIP is fixed by the ratio t/p, which is an important structural parameter. Within the 45 sidewall approximation, when t/p = 0.5, the corrugation cross-section is a triangle. Detectors with a fixed t/p will have a fixed projection area fill factor S (=2t/p) available for light reflection i.e. the ratio of the normal incident radiation that is reflected to the total surface incident radiation. For the triangular corrugations (S = 1), the maximum g is 0.5 · 0.71 · S = 0.36, which is half of the unpolarized unit incident power times the transmission coefficient of the substrate. For the trapezoidal corrugation with S = 0.5, the maximum g is 0.18. For a given p, S can be increased by using a thicker active material. The noise equivalent temperature difference, NEDT is determined by the ratio of Ip to the dark current Id [14,15]. Hence, the detection sensitivity depends on g/Id rather than g alone. Furthermore, background limited performance BLIP is achieved only when Ip > Id. Due to the reduced active volume in C-QWIPs, Id is reduced by a factor (p t)/p = (1 S/2). Therefore, when comparing with other couplings that have
Fig. 1. Cross-section of a C-QWIP pixel.
236
M. Jhabvala et al. / Infrared Physics & Technology 50 (2007) 234–239
no dark current reduction, as with a grating, a new quantity g 0 is defined, where g0 g
I d ð45Þ p g ¼ : ¼g Id p t 1 S=2
ð2Þ
The value of g 0 is the quantum efficiency normalized to the full geometrical dark current. The mesas, shown in Fig. 1, are created by wet etching through a square mesa mask. With this approach, there is undercut in one of the directions, which significantly shortens the length of the corrugations and reduces the material fill factor. It is this undercutting which makes it infeasible to produce a viable 11 lm thick triangular corrugation (which would have a larger QE) with a single chemical etching. When one etches 11 lm into the material, the undercut in the other direction will eliminate the majority of the active material. Our numerical analysis modeling predicts the total quantum efficiency combined from both sets of sidewalls to be 35% Table 1 Characteristics of the L3/Cincinnati Electronics Array format Pixel size Number of video outputs Full well capacity Pixel rate Conversion gain Integration time Lens/system Power dissipation Cold operating temperature
1.024 · 1.024 25 lm · 25 lm 8 13 million electrons (0.2 lV/e) 7.5 MHz/channel 750 electrons/DN selectable from 16 lsec to 16 msec f/2/f/1.32 220 mW below 70 K
for the unthinned devices. This level of performance is comparable to that with 11 lm tall corrugations even though the actual superlattice thickness is 8 lm. Our QWIP array fabrication process requires three masks for (1) the detector mesa formation; (2) the ohmic metal pixel contacts and; (3) the insulator/reflector layer definition. The process begins with a wafer cleaning procedure followed by photoresist deposition and the exposure of the mesa definition mask. A phosphoric peroxide acid etches the GaAs mesas. The second masking step defines the lift-off areas of deposited Ge/Au/Ag/Au metal followed by a rapid thermal anneal. The third mask step defines the areas for lifting off the insulator and sidewall metal reflector. This sidewall reflector is mainly to prevent possible infrared absorption in the epoxy through the fringing optical field. This third step can be eliminated depending on the epoxy backfill requirement of the subsequent hybridization process. The entire QWIP wafer fabrication process can be completed in less than 4 days. Five complete 1K · 1K die were fabricated on a single 4-in. wafer but 6-in. diameter wafers are just as available. After fabrication we performed some diagnostic tests and then sent the wafer to L3/Cincinnati Electronics for the hybridization process. Indium bumps are applied, the wafer is diced and candidates are bump bonded to the silicon readout. In order to test the detector characteristics a single detector element was also included on the wafer. The characteristics of this particular ROIC are identified in Table 1. The L3/Cincinnati Electronics ROIC was reconfigured for our QWIP array (it is generally used for their InSb products) and as such was the first time they had attempted
Fig. 2. Photograph of QWIP 4 in. GaAs wafer (top left), QWIP sub-array (top right) and of QWIP hybrid.
M. Jhabvala et al. / Infrared Physics & Technology 50 (2007) 234–239
to develop this configuration. This array was one of two QWIP arrays successfully hybridized as well as being their technology pathfinders. The second array performed markedly better than the first but the fact that both arrays performed as imagers is a credit the L3 team. Shown below in Fig. 2 are images of the QWIP wafer, QWIP array region and the hybrid.
237
0.4
GAIN
0.3
0.2
0.1
3. Experimental results A block diagram of the camera system is shown below in Fig. 3. Data acquisition is based around an SE-IR, Corp. (Goleta, CA) custom system. The quantum well structure is shown in Fig. 4 below along with the measured spectral response (the cladding and contact layers have been omitted for simplicity). The mean spectral response of the detectors in the FPA was measured by imaging the output slit of a grating monochromator onto the FPA and collecting images of the monochromatic light from 7.5 to 12 lm at 0.1 lm intervals. The spectral response was measured at biases of 1.0, 2.0, 3.0 and 4.0 V and an operating temperature of 57 K. Shown below in Fig. 5 is the measured noise gain of the single detector used to determine the photoconductive gain,
0 0
1
2
3
4
Fig. 5. Measurement of noise current as a function detector bias.
g. By measuring the unity bandwidth noise current versus the dark current shot noise, g is easily computed from g ¼ i2n =ð4qI d Þ: In our current configuration the photoconductive gain was 0.20 at a detector bias of 3.0 V. Clearly, as we are able to increase the detector bias we would see improved performance
ELECTRONICS CONTROL DISPLAY
IMAGE DISPLAY
CRYOCOOLER CLOCKS
TEST PATTERN GENERATOR AND LVDS OUTPUTS
DC BIASES 8 CHANNEL OUTPUTS
DATA ACQUISITION SYSTEM
TEMPERATURE CONTROLLER AND MONITOR DISPLAY
Fig. 3. Block diagram of the camera system.
Relative Spectral Response 1.1 1.0 0.9
10 Å n = 0.75x1018 cm-3
GaAs
40 Å n = 0.75x1018 cm-3
In0.1Ga0.9As
18
-3
10 Å n = 0.75x10 cm 700 Å
undoped
GaAs Al0.08Ga0.92As
0.8 0.7
x 106
6
QWIP BIAS (V)
OPTICS
QWIP ARRAY
5
Vd = 1.0
0.6 0.5
Vd = 2.0
0.4
Vd = 3.0
0.3 0.2
Vd = 4.0
0.1 0.0 6
8
10 Wavelength
12
14
Fig. 4. Quantum well structure and spectral response of QWIP sub-array measured on the FPA.
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M. Jhabvala et al. / Infrared Physics & Technology 50 (2007) 234–239
Fig. 6. QWIP images showing the word ‘‘QWIP’’ written with a damp cotton swab (left) and thermal gradients in the dish of water after soldering iron encounter (right).
in many respects especially an increase in quantum efficiency. The average quantum efficiency of the QWIP structure was calculated by varying a blackbody source from 20 C to 60 C and the measured spectral response: Z Q ¼ SðkÞW ðkÞ dðkÞ: Q is the spectral photon density at f/1.32, S(k) the measured relative spectral response from 7.5 to 12 lm, and W(k) the spectral radiant emittance from the blackbody source. The peak quantum efficiency at 10 lm was 16.4% with 3.0 V of detector bias at detector temperature of 65.5 K. In Fig. 6 are two images we selected. The first image shows the word ‘‘QWIP’’ which was drawn on the lab coat by with a slightly dampened cotton swab. The second image was taken as we dipped a hot soldering iron pinpoint tip into a dish of water. The picture illustrates the thermal dissipation of the localized heated water. With the SE-IR data acquisition we are able to acquire 30 Hz video, which is a marked improvement over our previous camera systems. We expect to deploy this system after some further refinement into airborne earth observing missions primarily for environmental monitoring. 4. Summary We have successfully developed and operated a broadband LWIR 1K · 1K GaAs QWIP array. The design, fabrication, hybridization and testing were accomplished through a highly collaborative effort between Goddard, the Army Research Lab and L3/Cincinnati Electronics with each group contributing their own particular technological expertise. Even though the effort is the culmination of many years of QWIP research and development, we as a group were able to develop this device with minimal funding and in a relatively short time frame. The ease of fabri-
cation, the relatively high device yield, the compatibility with existing silicon processing techniques and the compatibility with a variety of available readout ICs make QWIP technology very attractive for many applications. Our calculations, based on the collected data yielded an NEDT of .023 C under our operating conditions. We were experiencing excess noise which we are trying to eliminate. However, this noise may be an intrinsic component of the ROIC and hence the NEDT of this array may be limited due to this shortcoming. We are planning a number of aircraft missions utilizing this camera both domestically and in Asia. Currently, the operating temperature is below the solid nitrogen point so we are liquid helium based at the moment but we are pursuing retrofitting the camera with a mechanical cooler. Once we are able to decouple this instrument from the liquid helium source and rely solely on wall power for cooling the value of this imaging system will be more apparent and hopefully exploited by the science community. We are also very hopeful that the commercial/public sector will capitalize on this technology. Acknowledgements This work was supported by NASA’s Earth Science Technology Office. We would like to express our gratitude to Janice Buckner of ESTO; to John Devitt, Dave Forrai, Bob Fischer, Darrel Endres of L3/Cincinnati Electronics and to Mark Stegall of SE-IR Corp. We would like to thank Joe Adams and Fred Wang of the Detector Systems Branch at Goddard for their valuable support. References [1] B.F. Levine, C. Bethea, G. Hasnian, V. Shen, E. Pelve, R. Abbott, S. Hsieh, Appl. Phys. Lett. 56 (1990) 851–853. [2] M. Jhabvala, K. Forrest, R. Kaipa, Technology 2001, San Jose, CA, December 3–5, 1991.
M. Jhabvala et al. / Infrared Physics & Technology 50 (2007) 234–239 [3] H. Schneider, P. Koidl, M. Walther, J. Fleissner, R. Rehm, E. Diwo, K. Schwarz, G. Weimann, Infrared Phys. Technol. 42 (2001) 283–289. [4] M. Jhabvala, S. Gunapala, D. Reuter, K.K. Choi, S. Bandara, J. Liu, A. La, S. Banks, J. Cho, T. Hwang, S. Tsay, D. Rafol, H. Huet, N. Chauvet, T. Huss, Infrared Phys. Technol. 44 (2003) 445–447. [5] M. Jhabvala, K. Choi, A. Goldberg, A. La, S. Gunapala, SPIE, San Diego, CA, August 2003. [6] M. Jhabvala, S. Gunapala, D. Reuter, K.K. Choi, S. Tsay, NASA Earth Science Technology Office Advanced Component Technology, Proposal Award, 2003. [7] A. Goldberg, K. Choi, M. Jhabvala, A. La, P. Uppal, M. Winn, in: Proceedings of SPIE, Orlando, 2003. [8] M. Fauci, R. Breiter, W. Cabanski, W. Fick, R. Koch, J. Ziegler, S. Gunapala, Infrared Phys. Technol. 42 (June) (2000) 337–344.
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[9] R. Gaughan, Biophotonics International, March 2003, pp. 20–21. [10] F. Freund, Friedemann Freund, M. Jhabvala, A. La, P. Shu, S. Tsay, D. Ouzounov, Y. Fei, AGU 2002 Spring Meeting Section Tectonophysics. [11] P. Mitra, A. Brouns, F. Case, J. McCurdy, in: SPIE Conference 4820, July 2002. [12] A. Goldberg, K. Choi, N. Das, A. La, M. Jhabvala, R. Bailey, K. Vural, in: Proceedings of the 1999 IRIS Specialty Group Meeting on Detectors, ERIM, 1999. [13] K.K. Choi, J. Appl. Phys. 73 (1993) 5230. [14] K.K. Choi, C.J. Chen, D.C. Tsui, J. Appl. Phys. 88 (2000) 1612. [15] K.K. Choi, C.J. Chen, A.C. Goldberg, W.H. Chang, D.C. Tsui, SPIE 3379 (1998) 441.
Development of a 1K · 1K, 8–12 lm QWIP array M. Jhabvala b
a,*
, K.K. Choi b, C. Monroy b, A. La
a
a NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA US Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD 20783, USA
Available online 17 November 2006
Abstract In the on-going evolution of GaAs quantum well infrared photodetectors (QWIPs) we have developed a 1024 · 1024 (1K · 1K), 8–12 lm infrared focal plane array (FPA). This 1 megapixel detector array is a hybrid using an L3/Cincinnati Electronics silicon readout integrated circuit (ROIC) bump bonded to a GaAs QWIP array fabricated jointly by engineers at the Goddard Space Flight Center (GSFC) and the Army Research Laboratory (ARL). We have integrated the 1K · 1K array into an SE-IR based imaging camera system and performed tests over the 50–80 K temperature range achieving BLIP performance at an operating temperature of 57 K. The GaAs array is relatively easy to fabricate once the superlattice structure of the quantum wells has been defined and grown. The overall arrays costs are currently dominated by the costs associated with the silicon readout since the GaAs array fabrication is based on high yield, wellestablished GaAs processing capabilities. One of the advantages of GaAs QWIP technology is the ability to fabricate arrays in a fashion similar to and compatible with silicon IC technology. The designer’s ability to easily select the spectral response of the material from 3 lm to beyond 15 lm is the result of the success of band-gap engineering and the Army Research Lab is a leader in this area. In this paper we will present the first results of our 1K · 1K QWIP array development including fabrication methodology, test data and imaging capabilities. 2006 Elsevier B.V. All rights reserved.
1. Introduction Recently, remarkable advances in the development of GaAs QWIP focal plane arrays have been made. It was only in the late 1980s that single element, angle lapped QWIPs were developed and used [1]. In the 1990s rapid development from single element QWIPs to 512 · 640 arrays [2–4] occurred across a broad spectrum of the near to far infrared. With the concurrent development of large format silicon readout ICs it has been a relatively simple task for QWIP technology to keep pace. We recently had successfully developed a narrow band 8.4–9 lm, 1K · 1K QWIP array which was hybridized to a Rockwell Scientific TCM 8050 ROIC [5]. The array was designed jointly by engineers at NASA’s Goddard Space Flight Center (GSFC) and the Army Research Laboratory (ARL) and *
Corresponding author. Tel.: +1 301 286 5232; fax: +1 301 286 1672. E-mail addresses: [email protected], murzy.d.jhabvala@ nasa.gov (M. Jhabvala). 1350-4495/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2006.10.029
was also jointly fabricated in Goddard’s Detector Development Laboratory. It is most important for NASA scientific missions to obtain spectral information in our investigations of the Earth and universe. It is this spectral information that provides the clues to many unanswered questions. Unfortunately narrow band detectors, which QWIPs tend to be, pretty much preclude multi-wavelength spectroscopy. To this end, we have developed a broad band, large format QWIP array as part of NASA’s Earth Science Technology program for Advanced Component Technology development [6]. The applications for QWIP arrays are numerous. At GSFC some of these potential applications include global warming studies which includes environmental monitoring of troposphere and stratosphere temperatures and identifying trace chemicals; CO2 absorption; tree canopy energy balance measurements; measuring cloud layer emissivities, droplet/particle size, composition and height; SO2 and aerosol emissions from volcanic eruptions; tracking dust
M. Jhabvala et al. / Infrared Physics & Technology 50 (2007) 234–239
particles (from the Sahara Desert, e.g.); coastal erosion; ocean/river thermal gradients and pollution; analyzing radiometers and other scientific equipment used in obtaining ground truthing and atmospheric data acquisition; ground based astronomy, and; temperature sounding. Defense and military applications include surveillance, target identification, FLIR, fire control and mine detection [7]. The potential commercial applications are quite diverse. The utility of QWIP arrays in medical instrumentation is well documented [8] and may become one of the most significant QWIP technology drivers. The success of OmniCorder Technologies use of 256 · 256 narrow band QWIP arrays for aiding in the detection of malignant tumors is quite remarkable. Other potential commercial applications for QWIP arrays include location of forest fires and residual warm spots; location of unwanted vegetation encroachment; monitoring crop health; monitoring food processing contamination, ripeness and spoilage [9]; locating power line transformer failures in remote areas; monitoring effluents from industrial operations such as paper mills, mining sites and power plants; IR microscopy; searching for a wide variety of thermal leaks and not least of all; locating new sources of spring water. Recently, Freund and his colleagues [10] have been conducting experiments to simulate the effects of the extreme pressures acting on granite cubes to simulate precursor earthquake events. A Goddard built QWIP camera using a Lockheed Martin 256 · 256, 8–8.5 lm, EQWIP [11] was used to detect IR photonic emission during the compressive loading of the granite. 2. QWIP array design and fabrication The design and fabrication of the 1K · 1K QWIP array is straightforward and relatively simple. The architecture of the ROIC determines the pixel pitch which is 25 lm for the L3/Cincinnati Electronics ROIC. The ARL has pioneered the technique [12] of corrugating the QWIP array with V-
235
grooves, which reflect normal incidence light 90 into the QWIP. The pixel geometry design is shown in Fig. 1. Corrugated QWIPs (C-QWIPs) rely on inclined sidewalls to reflect normal incident light into parallel propagation. Assuming the angle of the corrugation sidewalls were 45 instead of 50, the unpolarized external quantum efficiency g, calculated from the decay of light along the optical path, can be expressed as [13] 4n 1 eap 2at gða; p; tÞ ¼ tþ 1e ð1Þ þ K0 ; 2 2a ð1 þ nÞ p where n = 3.34 (the refractive index of GaAs), p is the corrugation period, t is the height of the corrugations, a is the absorption coefficient the vertically polarized, parallel propagation light, and K0 is the internal unpolarized quantum efficiency created by the sidewalls at the ends of the corrugations. For infinitely long corrugations, K0 will be zero, but its value increases with decreasing pixel size. The geometry of a C-QWIP is fixed by the ratio t/p, which is an important structural parameter. Within the 45 sidewall approximation, when t/p = 0.5, the corrugation cross-section is a triangle. Detectors with a fixed t/p will have a fixed projection area fill factor S (=2t/p) available for light reflection i.e. the ratio of the normal incident radiation that is reflected to the total surface incident radiation. For the triangular corrugations (S = 1), the maximum g is 0.5 · 0.71 · S = 0.36, which is half of the unpolarized unit incident power times the transmission coefficient of the substrate. For the trapezoidal corrugation with S = 0.5, the maximum g is 0.18. For a given p, S can be increased by using a thicker active material. The noise equivalent temperature difference, NEDT is determined by the ratio of Ip to the dark current Id [14,15]. Hence, the detection sensitivity depends on g/Id rather than g alone. Furthermore, background limited performance BLIP is achieved only when Ip > Id. Due to the reduced active volume in C-QWIPs, Id is reduced by a factor (p t)/p = (1 S/2). Therefore, when comparing with other couplings that have
Fig. 1. Cross-section of a C-QWIP pixel.
236
M. Jhabvala et al. / Infrared Physics & Technology 50 (2007) 234–239
no dark current reduction, as with a grating, a new quantity g 0 is defined, where g0 g
I d ð45Þ p g ¼ : ¼g Id p t 1 S=2
ð2Þ
The value of g 0 is the quantum efficiency normalized to the full geometrical dark current. The mesas, shown in Fig. 1, are created by wet etching through a square mesa mask. With this approach, there is undercut in one of the directions, which significantly shortens the length of the corrugations and reduces the material fill factor. It is this undercutting which makes it infeasible to produce a viable 11 lm thick triangular corrugation (which would have a larger QE) with a single chemical etching. When one etches 11 lm into the material, the undercut in the other direction will eliminate the majority of the active material. Our numerical analysis modeling predicts the total quantum efficiency combined from both sets of sidewalls to be 35% Table 1 Characteristics of the L3/Cincinnati Electronics Array format Pixel size Number of video outputs Full well capacity Pixel rate Conversion gain Integration time Lens/system Power dissipation Cold operating temperature
1.024 · 1.024 25 lm · 25 lm 8 13 million electrons (0.2 lV/e) 7.5 MHz/channel 750 electrons/DN selectable from 16 lsec to 16 msec f/2/f/1.32 220 mW below 70 K
for the unthinned devices. This level of performance is comparable to that with 11 lm tall corrugations even though the actual superlattice thickness is 8 lm. Our QWIP array fabrication process requires three masks for (1) the detector mesa formation; (2) the ohmic metal pixel contacts and; (3) the insulator/reflector layer definition. The process begins with a wafer cleaning procedure followed by photoresist deposition and the exposure of the mesa definition mask. A phosphoric peroxide acid etches the GaAs mesas. The second masking step defines the lift-off areas of deposited Ge/Au/Ag/Au metal followed by a rapid thermal anneal. The third mask step defines the areas for lifting off the insulator and sidewall metal reflector. This sidewall reflector is mainly to prevent possible infrared absorption in the epoxy through the fringing optical field. This third step can be eliminated depending on the epoxy backfill requirement of the subsequent hybridization process. The entire QWIP wafer fabrication process can be completed in less than 4 days. Five complete 1K · 1K die were fabricated on a single 4-in. wafer but 6-in. diameter wafers are just as available. After fabrication we performed some diagnostic tests and then sent the wafer to L3/Cincinnati Electronics for the hybridization process. Indium bumps are applied, the wafer is diced and candidates are bump bonded to the silicon readout. In order to test the detector characteristics a single detector element was also included on the wafer. The characteristics of this particular ROIC are identified in Table 1. The L3/Cincinnati Electronics ROIC was reconfigured for our QWIP array (it is generally used for their InSb products) and as such was the first time they had attempted
Fig. 2. Photograph of QWIP 4 in. GaAs wafer (top left), QWIP sub-array (top right) and of QWIP hybrid.
M. Jhabvala et al. / Infrared Physics & Technology 50 (2007) 234–239
to develop this configuration. This array was one of two QWIP arrays successfully hybridized as well as being their technology pathfinders. The second array performed markedly better than the first but the fact that both arrays performed as imagers is a credit the L3 team. Shown below in Fig. 2 are images of the QWIP wafer, QWIP array region and the hybrid.
237
0.4
GAIN
0.3
0.2
0.1
3. Experimental results A block diagram of the camera system is shown below in Fig. 3. Data acquisition is based around an SE-IR, Corp. (Goleta, CA) custom system. The quantum well structure is shown in Fig. 4 below along with the measured spectral response (the cladding and contact layers have been omitted for simplicity). The mean spectral response of the detectors in the FPA was measured by imaging the output slit of a grating monochromator onto the FPA and collecting images of the monochromatic light from 7.5 to 12 lm at 0.1 lm intervals. The spectral response was measured at biases of 1.0, 2.0, 3.0 and 4.0 V and an operating temperature of 57 K. Shown below in Fig. 5 is the measured noise gain of the single detector used to determine the photoconductive gain,
0 0
1
2
3
4
Fig. 5. Measurement of noise current as a function detector bias.
g. By measuring the unity bandwidth noise current versus the dark current shot noise, g is easily computed from g ¼ i2n =ð4qI d Þ: In our current configuration the photoconductive gain was 0.20 at a detector bias of 3.0 V. Clearly, as we are able to increase the detector bias we would see improved performance
ELECTRONICS CONTROL DISPLAY
IMAGE DISPLAY
CRYOCOOLER CLOCKS
TEST PATTERN GENERATOR AND LVDS OUTPUTS
DC BIASES 8 CHANNEL OUTPUTS
DATA ACQUISITION SYSTEM
TEMPERATURE CONTROLLER AND MONITOR DISPLAY
Fig. 3. Block diagram of the camera system.
Relative Spectral Response 1.1 1.0 0.9
10 Å n = 0.75x1018 cm-3
GaAs
40 Å n = 0.75x1018 cm-3
In0.1Ga0.9As
18
-3
10 Å n = 0.75x10 cm 700 Å
undoped
GaAs Al0.08Ga0.92As
0.8 0.7
x 106
6
QWIP BIAS (V)
OPTICS
QWIP ARRAY
5
Vd = 1.0
0.6 0.5
Vd = 2.0
0.4
Vd = 3.0
0.3 0.2
Vd = 4.0
0.1 0.0 6
8
10 Wavelength
12
14
Fig. 4. Quantum well structure and spectral response of QWIP sub-array measured on the FPA.
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Fig. 6. QWIP images showing the word ‘‘QWIP’’ written with a damp cotton swab (left) and thermal gradients in the dish of water after soldering iron encounter (right).
in many respects especially an increase in quantum efficiency. The average quantum efficiency of the QWIP structure was calculated by varying a blackbody source from 20 C to 60 C and the measured spectral response: Z Q ¼ SðkÞW ðkÞ dðkÞ: Q is the spectral photon density at f/1.32, S(k) the measured relative spectral response from 7.5 to 12 lm, and W(k) the spectral radiant emittance from the blackbody source. The peak quantum efficiency at 10 lm was 16.4% with 3.0 V of detector bias at detector temperature of 65.5 K. In Fig. 6 are two images we selected. The first image shows the word ‘‘QWIP’’ which was drawn on the lab coat by with a slightly dampened cotton swab. The second image was taken as we dipped a hot soldering iron pinpoint tip into a dish of water. The picture illustrates the thermal dissipation of the localized heated water. With the SE-IR data acquisition we are able to acquire 30 Hz video, which is a marked improvement over our previous camera systems. We expect to deploy this system after some further refinement into airborne earth observing missions primarily for environmental monitoring. 4. Summary We have successfully developed and operated a broadband LWIR 1K · 1K GaAs QWIP array. The design, fabrication, hybridization and testing were accomplished through a highly collaborative effort between Goddard, the Army Research Lab and L3/Cincinnati Electronics with each group contributing their own particular technological expertise. Even though the effort is the culmination of many years of QWIP research and development, we as a group were able to develop this device with minimal funding and in a relatively short time frame. The ease of fabri-
cation, the relatively high device yield, the compatibility with existing silicon processing techniques and the compatibility with a variety of available readout ICs make QWIP technology very attractive for many applications. Our calculations, based on the collected data yielded an NEDT of .023 C under our operating conditions. We were experiencing excess noise which we are trying to eliminate. However, this noise may be an intrinsic component of the ROIC and hence the NEDT of this array may be limited due to this shortcoming. We are planning a number of aircraft missions utilizing this camera both domestically and in Asia. Currently, the operating temperature is below the solid nitrogen point so we are liquid helium based at the moment but we are pursuing retrofitting the camera with a mechanical cooler. Once we are able to decouple this instrument from the liquid helium source and rely solely on wall power for cooling the value of this imaging system will be more apparent and hopefully exploited by the science community. We are also very hopeful that the commercial/public sector will capitalize on this technology. Acknowledgements This work was supported by NASA’s Earth Science Technology Office. We would like to express our gratitude to Janice Buckner of ESTO; to John Devitt, Dave Forrai, Bob Fischer, Darrel Endres of L3/Cincinnati Electronics and to Mark Stegall of SE-IR Corp. We would like to thank Joe Adams and Fred Wang of the Detector Systems Branch at Goddard for their valuable support. References [1] B.F. Levine, C. Bethea, G. Hasnian, V. Shen, E. Pelve, R. Abbott, S. Hsieh, Appl. Phys. Lett. 56 (1990) 851–853. [2] M. Jhabvala, K. Forrest, R. Kaipa, Technology 2001, San Jose, CA, December 3–5, 1991.
M. Jhabvala et al. / Infrared Physics & Technology 50 (2007) 234–239 [3] H. Schneider, P. Koidl, M. Walther, J. Fleissner, R. Rehm, E. Diwo, K. Schwarz, G. Weimann, Infrared Phys. Technol. 42 (2001) 283–289. [4] M. Jhabvala, S. Gunapala, D. Reuter, K.K. Choi, S. Bandara, J. Liu, A. La, S. Banks, J. Cho, T. Hwang, S. Tsay, D. Rafol, H. Huet, N. Chauvet, T. Huss, Infrared Phys. Technol. 44 (2003) 445–447. [5] M. Jhabvala, K. Choi, A. Goldberg, A. La, S. Gunapala, SPIE, San Diego, CA, August 2003. [6] M. Jhabvala, S. Gunapala, D. Reuter, K.K. Choi, S. Tsay, NASA Earth Science Technology Office Advanced Component Technology, Proposal Award, 2003. [7] A. Goldberg, K. Choi, M. Jhabvala, A. La, P. Uppal, M. Winn, in: Proceedings of SPIE, Orlando, 2003. [8] M. Fauci, R. Breiter, W. Cabanski, W. Fick, R. Koch, J. Ziegler, S. Gunapala, Infrared Phys. Technol. 42 (June) (2000) 337–344.
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[9] R. Gaughan, Biophotonics International, March 2003, pp. 20–21. [10] F. Freund, Friedemann Freund, M. Jhabvala, A. La, P. Shu, S. Tsay, D. Ouzounov, Y. Fei, AGU 2002 Spring Meeting Section Tectonophysics. [11] P. Mitra, A. Brouns, F. Case, J. McCurdy, in: SPIE Conference 4820, July 2002. [12] A. Goldberg, K. Choi, N. Das, A. La, M. Jhabvala, R. Bailey, K. Vural, in: Proceedings of the 1999 IRIS Specialty Group Meeting on Detectors, ERIM, 1999. [13] K.K. Choi, J. Appl. Phys. 73 (1993) 5230. [14] K.K. Choi, C.J. Chen, D.C. Tsui, J. Appl. Phys. 88 (2000) 1612. [15] K.K. Choi, C.J. Chen, A.C. Goldberg, W.H. Chang, D.C. Tsui, SPIE 3379 (1998) 441.