Evaluation of the SBRC 256x256 InSb focal plane array and preliminaryspecifications for the 1024x1024 InSb focal plane array

A. M. Fowler and I. Heynssens

National Optical Astronomy ObservaftriesttTucson, Arizona 85726

ABSTRACT

The evolution of Santa Barbara Research Center (SBRC) InSb focal plane arrays (FPA) for astronomy has beensignificant since the introduction of the SBRC 58x62 array in the mid 1980's. A 256x256 array, whose performance isevaluated herein, is currently available from SBRC, and a 1024x1024 InSb FPA is under development by tbe U.S. NavalObservatory (USNO), National Optical Astronomy Observatories (NOAO), and SBRC. This represents an increase by afactor of ..3OO in the number of pixels in less than 10 years, with attendant increase in performance as well. The availabil-ity of high performance infrared arrays has opened up the field of infrared astronomy. As this jper shows, the 256x256InSb EPA is a great improvement over its predecessor in all areas of performance. Its most striking improvement is thecomplete absence of a latent or residual image after exposure to very high fluxes. Because of its low node capacitance,which results in very low read noise, the full well capacity is somewhat compromised.

1. INTRODUCTION

InSb photovoltaic detectors have been available for many years but it was the introduction of 2-d InSb FPAs in themid 1980's that revolutionized infiired astronomy. As Dr. Ian Gatley said at the 1988 AAS meeting in Austin, these newfocal planes give the modem infrared astronomer a performance increase over previous detectors of 5 orders of magnitude.The first indications of this were presented by many different authors at the Infrared Astronomy with Arrays conference1 inHawaii in 1987. Herein, we present the performance of the current 256x256 InSb FPA and discuss the upcoming1024x1024 FPA. The most striking improvement in the 256x256 FPA is the complete absence of a latent or residual imageafter exposure to very high fluxes. In the past this has been a serious limitation to flat fielding and accurate photometry.Residual image effects also seriously compromise spectroscopy and frequency switched imaging where strong emissionlines in one observation can show up as weak features in the next observation. The performance demonstrated by thepresent 256x256 device has resulted in a project between NOAO, USNO, and SBRC to develop the next genezation1024x1024 InSb FPA which is discussed in more detail later in this paper.

At NOAO, we have installed these arrays in the new Cryogenic Optical Bench and upgraded the Cryogenic Spec-trometer from 58x62 too. With these instruments and the new high speed readout electronics, the observatory will providethe most up to date state of the art capabilities in imaging and spectroscopy in the 1-5 micron region to our users. Wehave tested two of the 256x256 focal planes with consistent results Our results are also similar to those of the RochesterGroup (1946-03) presented herein.

2. DEVICE DESCRWrION

Like all high performance focal planes, this device is a hybrid consisting of a detector die indium bump bonded to asilicon readout. In this case the readout is the CRC463, a PMOS design produced by the Hughes Technology Center.PMOS was chosen for the readout design because it is considered to have lower noise and suits the detector biasingrequirements. The readout has four outputs organized as interleaved columns rather than in quadrants, which is the case inanother popular design. The unit cell is designed as a source follower, the only design which has demonstrated the lownoise and stability required for scientific applications. A schematic representation of this readout is shown in Figure 1. Shiftregisters are used for row and column enables, so selective readout is not available in this device as it was in the 58x62,but non-destructive readout is provided.

ttOperawl by the Association of Universities for Research in Astronomy, Inc. under cooperative agreement with the NationalScience Foundation.

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Table 1 lists the characterislics of the 256x256 device. We list both the SBRC specified performance and what wehave achieved at NOAO. Our results are for two devices and we believe they represent what can be generally expectedfrom this part.

Table 1: SBRC 256x256 InSb FPA CHARACTERISTICS

Specification Measured Performance

HFPA Structure

Number of Pixels 256 (H) x 256 (V)65,536 elements

PixelSize 3Ojimx3OimDetector Active Area 28 jim x 28 jimGeometric Fill Factor 87%Effective Fill Factor 100%Readout Type PMOS-SFD - 4 OutputsJR Detector Thinned InSbWavelength Range 0.9 - 5 microns

HFPA Performance

Operating Temperature 50K 35KDark Current < 400 c/sec -O.1 c/secNoise <lOOerms 26ermsQuantum Efficiency >80 % @ 1.7 pm 90% @ 1.0 jim

-.-92%@ 1.25 jim'90%@ 1.65 jim.85%@ 2.2 jim80%@331im**

Defective Pixels < 3% < 100 in a 250 x 250 AreaScience Grade Device Only

** University of Rochester Data

The detector material used in these devices is low doped InSb developed for SIRTF and the 58x62 astronomy pro-gram. It has high quantum efficiency at low temperatures and low capacitance, and therefore low noise. SBRC is nowusing diamond turning, instead of lapping, to thin the InSb, producing superior thickness control and correspondingresponse uniformity. The low detector capacitance results in low full well capacity which will be discussed later. The otherstriking improvement is in the passivation of the InSb, which has eliminated the need for a gate electrode. This hasimproved the yield, as the gate was a source of single point failure in the detector die, and has completely solved the resi-dual (latent) image and stability problems which plagued the 58x62 devices. In spite of the new passivation, these devicesmust still be stored in a vacuum to prevent contamination problems. This will be discussed further in the section on darkcurrent.

3. GAIN, LINEARITY, AND NOISE

The system electrical gain is used for calculating the detector node capacitance, evaluating readout elecirical unifor-mity, and establishing the linear operating range. To determine the gain, the drain voltage DDUr v&ied and the changein output voltage is noted. The data derivative is used to determine the system electrical gain and'uniformity shown in his-togram form in Figure 2. Uniformity, given by the ratio of the standard deviation to the mean, is —0.8%.

Photon Gain, Read Noise, and Linearity are determined by the Mean-Variance test.2 The relevant equation, shown insummary, is as follows:

= N + Sg

where V = Variance of measured output signalS = Mean of the measured output

NR = System read noise in digital units

g = System gain in ADUs/electron

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The readnoise and conversion gain determinedin this way are not subject to interpretationby assuming a node capacitance.Therefore this test is an accurate representation of device performance. Our readnoise results, shown in Figure 3, weretaken using the reset-read-read techni3 which is the standard readout procedure at NOAO. Linearity is evaluated bysubtracting the readnoise from the Mean-Variance data and plotting the result on log-log scale as shown in Figure 4. Thescattering at low flux is due to noise and the limited number of data points for Statistical smoothing.

4. CAPACITANCE AND FULL WELL CAPACITY

In the pursuit of lower read noise, considerable effort has been directed toward lowering the detector node capaci-tance. In this device that was accomplished by using low doped InSb material, small pixel size, and eliminating the gateelectrode. At 600 my of applied bias, we measured a node capacitance of 0.06 pf, as determined by using the thta from thegain and mean-variance tests. Unfortunately this low capacitance also results in a lower full well capacity.

We have defined full well capacity as the point where linearity starts to deviate from a straight line. It does not meanthe detector stops responding, only that it is not substantially linear. The test was performed by taking a series of dataframes at intervals of 100 ADU's (-'1500 photons/sec). Successive frames were subtracted from each other and histogramswere made of these difference frames. A non-full well result, shown in Figure 5a, is symmetric in shape while the onset ofsaturation, shown in 5b, starts to show distortion on the left side. This is an easy and very sensitive test to determine whenparts of the array are reaching saturation. Table 2 is a summary of our results.

Table 2: FULL WELL CAPACITY

Applied Full WellVbias Capacity

600 my •400,000 electrons700 my —150,000 electrons800 my '2OO,OOO electrons

The difference between the expected full well capacity, 360,000 electrons per volt of bias, and the measured results is dueto charge pumping at the detector node. We have measured -4W my shift when the reset transistor is turned off and theremainder, .2OO my, is due to charge pumping from the isolated node between the reset and the enable transistor, the unitcell source follower capacitance to the common column bus, and other clock crosstalk sources near the unit cell.

5. DARK CURRENT AND LATENT IMAGE RESULTS

The dark current is significantly less than in the previous generation of devices. Contamination is still a problemwhich must be recognized if the lowest results are to be achieved. Figure 6 is an image from an array with increased darkcurrent, also known as the picwre frame effect, due to contamination. Fortunately this can be eliminated by baking. Theprocedure used at NOAO is to vacuum bake the device at 80'C for approximately 2 weeks. Time is a very importantparameter as 1 week was not enough to completely erase the effect. The selection of 80'C was somewhat arbitrary andlower temperatures might work as well. The baking not only eliminates the picwre frame but generally lowers dark currentover the whole array. Figure 7 is a dark current histogram and an image taken at 550my of reverse bias. Dark current asa function of applied bias performance is given in Table 3.

Table 3: Dark Current versus Applied Bias

Applied 'Dark

Vbias (electrons/sec)

550 my 0.1600mv 0.14700mv 0.44750mv 1.10800mv 3.12

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The brighter regions in the upper left and lower right corners (Figure 7) are thought to be due to the shift register carry outdriver but that hasn't been confirmed. If so, it might be possible to eliminate it by connecting "Last C" and "Last R" to V3.

Latent images are a problem which has plagued IR as well as CCD arrays for many years. It is due to a non-flathandsurface condition caused by charge trapping in the oxidellnSb interfe. This is discussed in some detail in a paper bySolomon, et al. in this proceedings.4 SBRC has a new passivation technique which eliminates this effect and maintains thesurface potential at a level which produces very low dark currenL Figure 8a is an image of a very saturated spot (>1O xfull well) while Figure 8b is a 100 second dark image taken immediately after closing the dark slide in the test dewar. Fig-ure 8c is the difference of this first dark after subtracting the average of several standard darks in an attempt to enhance theimage. At the KPNO 1.3m telescope, we have imaged Vega (1O photon—sec1 in the K band) onto the array and thenrun a series of 100 second darks to further evaluate latent image effects. We have been unable to detect any after images, asdemonstrated by the images in Figures 8b and 8c, regardless of how we expose the device. This is the most significanttechnical breakthrough made by SBRC in pmducing this new array.

6. RESPONSE UNIFORMITY AND FILL FACTOR

Compensation for non uniform response is achieved by flat fielding using the sky or other suitable flat illuminationsource. In situations where the background flux is high and the array is operating near full well, it is important that all pix-els approach this level in a similar manner. Figure 9 is a normalized response histogram which shows a Stddev/Mean ratioor uniformity of 4%.

Array fill factor is yet another important performance parameter. If the array does not have 100% fill factor, flux islost and the point source point spread function will be distorted. Therefore the image of a point source must be spread overseveral pixels, if accurate intensity information is to be recovered by computational means. This reduces the signal/noiseand field of view, and increases observation time. As one can see in Figures lOa and lOb, the fill factor in these devices is4OO%. Figure lOa is the summed response of several pixels as a spot is scanned across them. If there were a loss of signalbetween detectors this plot would show dips in the signal level, which is not the case. Figure lOb is a response plot of asingle pixel, showing the extent of crosstalk between pixels using a spot which is diffraction limited at slightly less than thesize of the pixel. Again the profile is smooth and shows no apparent loss of flux.

7. ADVANCED LARGE ARRAY DEVELOPMENT IN InSb

Based on our test results for the current generation of InSb arrays, we have considered the potential for even largerarrays. SBRC indicated that devices of 1024x1024 pixels were the next reasonable step. SBRC/Hughes Technology Centerhad already produced arrays in PtSi and InSb in the 640x480 format for other customers. Gemini and other large tele-scopes, which are designed for diffraction limited imaging, will need very large arrays if a reasonable instrument field ofview is to be achieved. In spectroscopy, arrays of much greater length are needed to increase spectral coverage at high reso-lution. Recently the USNO in Flagstaff expressed an interest in large arrays for astrometry in the infrared. They have beenengaged in the development of such arrays in the visible for the same reason. Also the new generation of large telescopestelescopes, which are designed for diffraction limited imaging, will need very large arrays if a reasonable instrument fieldof view is to be achieved.

Out of this has come the ALADIN project which will be jointly funded by NOAO and USNO with SBRC to developa 1024x1024 focal plane array. Figure 11 illustrates the size of the new array in relation to the present 256x256 device. Thetentative specifications for this array are given in Table 4.

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Table 4: ALADIN 1024x1024 InSb Focal Plane Characteristics

HFPA Structure

Numb& of Pixels 1024 (H) x 1024 (V)1,048,576 elements

Pixel Size 25-30 p.m squareEffective Fill Factor 100%Readout Type CMOS or PMOS using a SFDNumber of Outputs 32IR Detector Thinned InSbWavelength Range 0.8 - 5 microns

HFPA Performance

Operating Temperature 35KDark Current < 1 c/secNoise <25 e rmsQuantum Efficiency >80 % 0.8 to 5 micronsDefective Pixels <0.5%

No Bad Rows or Columns

The readout will be designed as four independent quadrants with eight outputs per quadrant. This is equivalent tohalf the number of outputs per pixel of the present 256x256 device. To compensate for this, the new device will bedesigned to operate at speeds of less than 1 isec per pixel per output. The detector will be designed with no gaps so thearray will appear to be a seamless 1024x1024 array. The separate quadrants will improve the yield of lesser devices whichwill reduce the overall cost in the future. Care wifi be taken in the readout design to reduce and shield the LED emissionfrom the output amplifiers and shift register. A CMOS approach is tentatively being considered but that will not bedecided until all analysis is complete. The use of larger pixels, possible use of higher doped detector material, and controlof charge pumping wifi all be considered in the design to improve the full well capacity.

The program is to proceed in three phases with the readout design to be first, followed by the detector developmentand hybridization. It is expected that the project show feasibility in twelve to eighteen months with the third phaseprototype production in the following year. At this point in time proposals have been received and are being evaluated.

8. CONCLUSIONS

The present 256x256 SBRC InSb FPA is a significant improvement over the original 58x62 EPA. It clearly has nomajor faults and only its full well capacity is a slight limitation. The very low dark current and lack of latent image effectsmake it the best array available today for spectroscopy and frequency switched imaging. The performance of this device isresponsible for the program to further increase the field of view by developing the 1024x1024 device.

9. ACKNOWLEDGEMENTS

We gratefully acknowledge the contributions of Jim Phillips and Alan Hoffman of SBRC in this work. In addition wewant to acknowledge Sidney Wolff at NOAO and our collaborators at the USNO in Flagstaff, F. Vrba and H. Ables,without whose support the 1024x1024 development would not have been possible.

10. REFERENCES

1. C.G. Wynn-Williams and E.E. Becklin, eds., "Infrared Astronomy with Arrays," Proceedings of the Workshop onGround-based Astronomical Observations with Infrared Array Detectors, University of Hawaii, Institute forAstronomy, Honolulu, Hawaii, (March 1987).

2. A.M. Fowler and R.R. Joyce, "Status of the NOAO Evaluation of the Hughes 20x64 Si:As Impurity BandConduction Array," SPIE instrwnentation in Astronomy Vii 1235 (March 1990).

3. A.M. Fowler and Ian Galley, "Noise reduction strategy for hybrid JR focal plane arrays," SPIE infrared Sensors:Detectors, Electronics, and Signal Processing 1541 (July 1991).

4. S.L. Solomon, J.D. Garnett, and H. Chen, "Investigation of charge trapping effects in InSb focal plane arrays,"SPIE- Infrared Detectors and Instrwnentazion 1946 (April 1993).

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30 ISPIE Vol. 1946 Infrared Detectors and Instrumentation (1993)

500 12.150E-5 2.200E-5 2.250E-5 2.300E-5 2.350E-5

Vo'ts in per Oigtai Unit Out

Figure 2

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Rgure 8 100-Seccnd Ds

Minus 100-Sccmd Daà Average

SPIE Vol. 1946 Infrared Detectors and Instrumentation (1993)131

.1 .2 .3Dark Current eectrons/pxe1/sec)

Figure 7agwc 7 Da± cuuait formance - Pt-Bike

Figure 8*. Saturated Spot Figure 8b. Fiist 100-Second Da Image

Normalized Response

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Spot Scan Results

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Spot Scan Results

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1111111111111 111111111111 111111 ff11 11111111 H 111111 11111

1024 x 1024

Figure 11. Illustration of 1024x 1024 FPA with Current 256x256FPA

32 / SPIE Vol. 1946 Infrared Detectors and Instrumentation (1993)

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