Mercury cadmium telluride staring arrays for mid-IR detection

P. Knowles I.M.Baker

Indexing terms: Mercury cadmium telluride (CMT/MCT), Infrared detectors

Abstract: Mercury cadmium telluride (CMT or MCT) has for many years been the preferred material for high performance long wave infrared detectors. For mid-IR applications, CMT is also used for many simple photoconductive detectors. The majority of these applications are in instrumentation, industrial process control, thermography or radiometry. CMT is also commonly used for thermal imaging through the 3-5 pm atmospheric window. Advanced detectors for this application are generally two-dimensional ‘staring’ arrays. GEC-Marconi Infra-Red (GMIRL) specialises in the production of CMT detectors. The paper describes the most advanced 3 - 5 ~ staring array produced, which is a 384 x 288 element array for the Euclid programme. Designed for use in conjunction with 2 x 2 optical microscan to produce a full 768 x 576 TV standard image at 50Hz field rates, it is capable of resolving scene temperature differences below 20mK. Special features include the small pixel pitch of 2 0 ~ and the relatively high operating temperature of 120K. The operating temperature is beyond the capability of competing technologies based on InSb or PtSi.

1 Introduction

High quality thermal imagery has been made possible through the development of several key technologies, including infrared detector arrays, miniature cryogenic coolers, optical materials and components and high density silicon circuits.

The most striking developments of recent years have taken place in infrared focal plane arrays. The trend has been towards larger numbers of detector elements on the focal plane, with a multiplexer chip performing functions such as time delay and integration, anti- blooming and time division multiplexing. A complex focal plane assembly can be serviced by a relatively small number of electrical connections. However, the design of integrated circuits capable of operating within 0 IEE, 1997 IEE Proceedings online no. 19971489 Paper first received 23rd Decmber 1996 and in revised form 23rd June 1997 The authors are with GEC-Marconi Infra-Red Limited, PO Box 217, Millbrook Industrial Estate, Southampton SO1 5 OEG, UK

the constraints of size and power dissipation imposed by the imaging system is problematic; also, it is fre- quently at the limits of silicon VLSI technology.

The mechanical interface between a silicon chip and an infrared detector array also requires novel process- ing methods for its implementation. For instance, flip- chip (indium bump) bonding is widely used. GEC- Marconi Infra-Red (GMIRL) uses the unique loophole process for the hybridisation of detector arrays with multiplexers.

The detector materials which have been developed into large 2D solid state arrays include CMT, InSb, PtSi, extrinsic silicon, pyroelectrics, silicon microbo- lometers and multiple quantum well structures (princi- pally in GaAs). The first three are the most highly developed for military and commercial applications in the 3 - 5 p waveband, and CMT in particular has unique advantages when the highest radiometric per- formance is required in combination with minimal cooling or the shortest time to operation from ambient temperature.

2 Background

The majority of current generation thermal imaging systems employ long wave CMT detectors with electro- mechanical scanning in at least one dimension to gener- ate the required image format from linear detector arrays [l]. This type of system is commonly described as a FLIR, or forward looking infrared imager, espe- cially in the US. FLIRs have evolved into imagers with full TV frame rates and spatial resolution, combined with temperature resolution down to 0.1”C.

In engineering terms, the elimination of mechanical scanning represents a substantial saving in weight, cost and electrical power. The potential advantages to be derived from the development of full TV format arrays are therefore great, combining improved performance with reduced complexity, and much effort has been applied to the development of large 2D arrays. For thermal imaging through the 3-5 pm atmospheric win- dow, CMT is commonly used but is in competition with other materials, notably InSb and platinum sili- cide. This type of detector is generally a two-dimen- sional ‘staring’ array employing a silicon read-out chip. An example of imagery from a 256 x 256 InSb array is given in Fig. 1. The choice of detector may be influ- enced by several factors such as sensitivity, uniformity, array size, cost and export controls. In some circum- stances, the time to operation from initiation of the cooling cycle may be critical, for which the highest operating temperature is clearly an important driver.

321 IEE Proc.-Optoelectron., Vol. 144, No. 5, October 1997

Fig. 1 (0 1996 GEC-Marconi Sensors Ltd)

Thermal image taken with 256 x 256 pixel midwave staring array

Fig.2 (0 1996 GEC-Marconi Infra-Red Ltd)

384 x 288 CMT hybrid detector array

Detailed comparisons of CMT, InSb, and PtSi are summarised in [2, 31. In essence, the ‘industry standard’ for CMT and InSb arrays in the US is 256 x 256 or 320 x 240, on a typical pitch of 30pm, while full TV (640 x 480) and even larger arrays (1024 x 1024) have been demonstrated in both materials. In Europe, CMT technology has been demonstrated by Sofradir of France [4] with arrays up to 320 x 240 on 30pm pitch and, in GMIRL, up to 384 x 288 ( 2 0 ~ pitch), reflect- ing the European TV format ([5] and Fig. 2).

The choice of CMT, InSb or PtSi for a specific appli- cation is determined by a number of factors. PtSi, due to its low quantum efficiency and spectral response characteristic, performs well in high background condi- tions. In normal conditions, its sensitivity is generally limited to around lOOmK whereas CMT and 1nSb detectors achieve noise equivalent temperatures of 10- 20mK. The advantages of PtSi are very large array size and excellent intrinsic uniformity (one-sigma value of 0.2%), requiring electronic uniformity correction to a depth of perhaps only 8 bits.

Selection among CMT and InSb is more difficult, since basic sensitivities are similar and there is a wide range of applications where both serve well. For main- stream applications at 80 K operating temperature,

328

InSb has superior intrinsic uniformity (one-sigma val- ues of 1% compared with a few percent in CMT). Even after electronic correction, this leads to superior resid- ual nonuniformity or spatial noise, which is a signifi- cant factor in overall system performance. However, InSb detectors are seldom operated above 100K, whereas background limited operation can be preserved in CMT up to 120-130K, by optimising the alloy com- position and tuning the spectral response for the higher operating temperature. This is important where mini- mal cooling power, or time to operation, are key sys- tem parameters, High temperature operation has been fully exploited in GMIRL’s 384 x 288 CMT array development.

3 384 x 288 CMT detector array

The 384 x 288 element array has the largest element count of any hybrid detector array produced in Europe to date. Development of this detector was undertaken by GMIRL in support of RTP 8.1 of the Euclid pro- gramme, as part of the development of an affordable lightweight imager with full TV resolution. The design aim was to produce the smallest possible 1/2 TV array, operating at the highest possible temperature.

The small size of the optical field allows the use of appropriately small optics, which contributes to low cost and weight; and the high operating temperature (120-130K) achieved through the use of an optimised CMT detector cut-off permits the use of an ultra-small Stirling cycle cryocooler which is under development for the programme. By this means it will be possible to maintain the operating temperature with an input of 1-2W compared with 3 4 W to maintain 80K, with proportional savings in battery life or weight.

The small overall size of 6mm by 8mm for the pho- todiode matrix is achieved with an array pitch of 20pm. CMT material for the array is grown on lattice matched CdZnTe substrates (4% ZnTe) by sliding-boat liquid phase epitaxy. The acceptor concentration of Hg-vacancies is controlled to mid-1 0l6 by annealing. Detector dies are mined from a precise stratum of the epilayer by removing the substrate and polishing both epitaxial surfaces to achieve a final thickness of 6.5 2 0.2pm, and these are glued to planarised multiplexers by a process which ensures a uniform and ultra-thin glue layer.

Loophole diode formation is achieved in a single photolithographic stage. Ion beam milling is used to replicate a matrix of via holes in the developed resist mask into the underlying CMT, until the metal contact pads of the subjacent multiplexer are exposed. An annular region of CMT surrounding the loophole is converted to n-type in the milling process, forming an array of cylindrical p-n junction diodes. Before lift-off of the resist, metal is deposited into the holes to form ohmic contacts between the n-CMT and the multi- plexer inputs (Fig. 3). The final stage of manufacture is the deposition of a quarter-wave blooming layer. A detailed account of the loophole process is given in

The multiplexer for the 384 x 288 array employs a lpm double-level metal CMOS process. The Si archi- tecture includes a simplified variant of the pixel circuit illustrated in Fig. 4. Fully staring arrays contain two storage capacitors in the pixel to permit simultaneous stare and read operations: signal charge is transferred

[I, 61.

IEE Proc -0ptoelectron , Vol 144, No 5, October 1997

between capacitors at the end of each stare period, to be read out during the following stare period. In this way virtually 100% duty cycle is available for the star- ing function.

via interconnection doped volume in-type) \ CdtigTe monoiith (p-type1

I

30

x - E

25- w z -

20

aluminiuk pod Fig.3 Loophole array process (0 1996 GEC-Marconi Infra-Red Ltd)

-

-

-

transer gate

dig reset? reset 2 Vdd

output Fig.4 (0 1996 GEC-Marconi Infra-Red Ltd)

Pixel circuit diagramforfully staring array

In the 384 x 288 chip, one of the capacitors is omit- ted and alternating stare and read periods are employed. This approach is compatible with the Euclid demonstrator, which employs optical microscan to dither the infrared image by 1/2 pixel in a 2 x 2 raster in order to double the spatial resolution of the imager. The inactive periods during the microscan movement can be synchronised with the read period of the multi- plexer.

temperature, mK Fig.5 384 x 288 N E T D histogram

The simplified pixel circuit allows the single storage capacitor, which limits the stare time of the chip and controls the NETD performance, to be maximised within the 20pm pitch at 0.3pF, and the corresponding background-limited NETD performance is around 14mK. The measured performance of these arrays at 80K is characterised by a median NETD around 17-20mK (Figs. 5 and 6). In this example, 99% of the elements exhibit NETD better than 27mK. Perfonn- ance remains strongly photon noise limited up to 120K operating temperature, and degrades by only 10mK up to 140K (Fig. 7).

O O O - * . I ~~$~~~~ g 8 percent less than ordinate

Fig.6 384 x 288 NETD distribution

35 r

151 I I 1

100 110 120 130 ILO 150 160 operating temperature, K

N E T D performance as a function of operating temperature Fig. 7 (0 1996 GEC-Marconi Infra-Red Ltd) For Euclid hybrid-P3 ( 5 . 0 ~ cutoff at 120K)

4 Future

The successful demonstration of the 384 x 288 array provides some valuable clues to the feasibility of larger arrays. Affordability remains a key factor for successful exploitation, and the overall array size is the most important cost driver. Close examination of the 20pm pitch array (Fig. 8), using a selective etch technique to reveal the positions of the p-n junctions, indicates that, with appropriate measures to control array thickness and process conditions, arrays of isolated photodiodes may be made on a pitch of 17 or even 1 5 ~ . At the same time, the use of more advanced Si processing with 0.6 or 0 . 8 ~ design rules, is likely to permit compres- sion of the pixel circuit to a similar pitch without loss of charge storage capacity or sensitivity.

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Fig. 8 (0 1996 GEC-Marconi Infra-Red Ltd) Junction reveal process for Euclid hybrid

Photograph of defect-etched 384 x 288 array

It is therefore reasonable to anticipate, within a few years, a full TV staring array of 768 x 576 pixels with an active matrix perhaps no larger than 12” by 9mm, and operating at 120K. A detector array of this type would enable the construction of a high perform- ance thermal imaging camera of similar size and com- plexity to a conventional video camcorder.

5 Acknowledgments

The authors would like to thank Alenia of Italy and the United Kingdom Defence Research Agency for their support and cooperation in detector development programmes, and colleagues in the development team at GEC-Marconi Infra-Red for help in the preparation of this article. The thermal picture in Fig. 2 was kindly supplied by GEC-Marconi Sensors Ltd, who devel- oped the imager.

References

BAKER, I.M., HASTINGS, M.P., HIPWOOD, L.G., JONES, C.L., and KNOWLES, P.: ‘Infrared detectors for the year 2000’, GEC Rev., 1995, 10, (3), pp. 148-160 NORTON, P.R.: ‘Infrared image sensors’, Opt. Eng., 1991, 30, (II), pp. 1649-1663 NORTON, P.R.: ‘Infrared image sensor status’, Proc. SPIE, - 1994, 2274, pp. 82-92 DESTEFANIS. G.L.: ‘HeCdTe infrared diode arravs’. Semicon- , , ductor Sei. Technol., 1991y6, pp. C88-C92 BAKER, I.M., CRIMES, G.J., and LOCKETT, R.J.: ‘CMOS/ HgCdTe 2D array technology for staring systems’. SPIE Aero- sense conference, Orlando, USA, April 1996 BAKER, I.M., CRIMES, G.J., PARSONS, J.E., and O’KEEFE, E.S.: ‘CdHgTe-CMOS hybrid focal plane arrays - A flexible solution for advanced infrared systems’, Proc. SPZE, 1994, 2269, pp. 636-645

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