h2rg focal plane array and camera performance update

H2RG Focal Plane Array and Camera Performance Update

Richard Blank*1, Selmer Anglin1, James W. Beletic1, Sid Bhargava1, Ryan Bradley2, Craig A. Cabelli*1, Jing Chen1, Don Cooper1, Rick Demers1, Michael Eads1, Mark Farris1, William Lavelle1, Gerard Luppino2, Eric Moore2,

Eric Piquette1, Raphael Ricardo1, and Min Xu1, and Majid Zandian1

1Teledyne Imaging Sensors, 5212 Verdugo Way, Camarillo, CA 93012, USA 2GL Scientific, 3367 Waialae Avenue, Honolulu, HI 96816, USA

ABSTRACT Teledyne’s H2RG focal plane arrays have been widely used in scientific infrared and visible instruments for ground-based and space-based telescopes. The majority of applications use the H2RG with 2.5 micron cutoff HgCdTe detector pixel at an operating temperature of ~77 K (LN2). The exceptionally low dark current of the 2.5 micron H2RG allows for operation at higher temperatures which facilitates simplified instrument designs and therefore lower instrument cost. Performance data of 2.5 micron H2RG arrays at 77K, 100 K, and 120 K are presented and are discussed as a function of detector bias and pixel readout rate. This paper also presents performance data of 1.75 micron and 5.3 micron H2RG focal plane arrays and discusses some of the inherent performance differences compared to 2.5 micron cutoff arrays. A complete infrared camera system that uses the H2RG focal plane array and SIDECAR ASIC focal plane electronics is introduced. Keywords: HgCdTe detector array, H2RG ROIC, hybrid CMOS, infrared, camera

1. INTRODUCTION

More than 80 science grade and/or flight grade H2RG focal plane arrays (FPAs) have been delivered to ground-based and space-based observatories since its introduction in 2002. Its high performance and reliability have been extensively tested and verified in the laboratory and on sky by many organizations around the world. The H2RG signature performance trademarks include high quantum efficiency, low noise, ultra-low dark current, low crosstalk, and low latency1 - 6. The high level of functionality, including window and guide modes, non-destructive readout, low-speed and high-speed operation, as well as several enhanced clocking modes, including sub-frame integration as well as different reset schemes including pixel-by-pixel, line-by-line, and global reset, make the H2RG a highly flexible and versatile FPA and deployable for a variety of applications. In 2011, the H2RG FPA achieved NASA technical readiness level nine (TRL-9) operating in space for a U.S. defense application.

This paper is organized as follows. In section 2 we provide a summary of the key H2RG features and its mechanical and electrical interfaces. Section 3 presents performance and reliability improvements since the SPIE ATI meeting in San Diego in 2010. Section 4 discusses FPA performance as a function of operating temperature, detector reverse bias, and pixel read out rate. In section 5 we present the performance of Teledyne’s three standard product H2RG FPAs with cutoff wavelengths of 1.75 micron (near-infrared, NIR), 2.5 micron (short-wave infrared, SWIR), and 5.3 microns (mid-wave infrared, MWIR) and discuss FPA performance differences between the different cutoff wavelengths. Section 6 introduces a complete camera system developed by GL Scientific in collaboration with Teledyne.

2. H2RG INTERFACES

The H2RG hybrid FPA represents a high performance, large format detector for a variety of scientific applications in the X-ray, near-UV, visible, and IR. Further details of the H2RG key features and functionality are described in detail elsewhere3. * Correspondence: [email protected] and [email protected]

2.1. H2RG Configurations

The H2RG FPA is available in the following configurations: (a) Ground-based astronomy (GBA) H2RG, (b) flight metal H2RG, and (c) flight silicon carbide (SiC) H2RG. In each configuration the H2RG hybrid, i.e. the detector and readout integrated circuit (ROIC) structure is the same. The differences between the three configurations are in the packaging and the available capabilities and features of the H2RG ROIC. Figure 1 portrays the three H2RG configurations, and Table 1 provides a comparison of the properties and capabilities offered by each.

(a) Ground based astronomy (GBA) H2RG: Teledyne’s H2RG GBA package employs an Invar (for HyViSI™ visible detectors) or molybdenum (for HgCdTe infrared detectors) pedestal, a wirebond ceramic with 90-pin grid array (PGA), and a flex cable with a PGA socket and 92-pin Hirose connector. This package allows 1-, 4-, or 32-output H2RG operation.

(b) Flight metal package: This package employs an Invar or molybdenum pedestal, a wirebond ceramic with 36-pin PGA, flight-qualified capacitors, and a flex cable with PGA socket and 37-pin MDM connector (1- and 4-output operation only).

(c) Flight silicon carbide (SiC) package: This package employs a SiC pedestal. A wrap-around printed circuit board (PCB) contains flight-qualified capacitors, heater resistors, a temperature sensor, and an Airborn Nano 85-pin connector. The PCB terminates into a flexible cable that wraps around the side of the pedestal and routes the traces to the bondpads that are wirebonded to the H2RG ROIC. This package allows 1-, 4-, and 32-output operation.

Figure 1. (a) GBA, (b) flight metal, and (c) flight SiC packages offered by Teledyne for the H2RG hybrid FPA

Teledyne has benefitted from a long partnership with GL Scientific in the design and manufacturing of packaging for infrared and visible FPAs and SIDECAR ASIC focal plane electronics (FPE), for both ground-based and space-based applications. The package design evolution leveraged the knowledge and experience gained from previous package designs. The three package designs described in this paper have been demonstrated in dozens (SiC) to scores (flight metal) to hundreds (GBA) of H2RG FPAs.

2.1.1. GBA Package Teledyne’s package for GBA H2RG FPAs is a well-established package design that has been proven on hundreds of H2RG devices. The package is of a modular design, which enables easy assembly of focal plane mosaics and straightforward replacement of individual SCAs within a mosaic. The package features gold-plated invar or molybdenum pedestal for Teledyne’s CMOS hybrid visible silicon imager (HyViSI) or infrared hybrid detector, respectively. A visible HyViSI silicon detector is mounted directly onto the Invar pedestal. In the case of an infrared hybrid detector, because the coefficient of thermal expansion (CTE) of molybdenum is different from that of the silicon ROIC of the hybrid detector, thermal stresses would be induced in the detector if directly mounted onto the molybdenum pedestal. A Balanced Composite Structure (BCS) is sandwiched between the hybrid and molybdenum in order to counteract the induced stresses and enable the hybrid detector to be thermo-mechanically reliable over many hundreds of thermal cycles. The GBA package has three copper-tungsten (CuW) feet which provide a 3-point pseudo-kinematic mount of the SCA and also serve as the primary thermal conduction pathway to an FPA plate. A 4-leg design preceded the current 3-leg design, but has been obsoleted. The detector is wirebonded to a multilayer ceramic pin grid array (PGA) attached to the underside of the pedestal and held in place with 4 spring-loaded retainer clips. A narrow-profile bond ledge along the bondpad edge of the H2RG ROIC enables near-buttability of SCAs on the bondpad edge. The other three edges are close-buttable (less than 2.5 mm between active pixels from adjacent arrays). The underside of the ceramic is populated with a set of commercial filtering capacitors. Flexcircuits of various lengths mate to the 90-pin PGA and terminate on the other end in a 92-pin Hirose connector enabling 1-, 4-, or 32-output H2RG operation.

2.1.2. Flight Metal Package Teledyne offers a three-side close buttable, fourth-side near-buttable H2RG FPA package that is flight-qualified and qualified for space applications. It is a mechanically robust, proven FPA package that enables 1- and 4-output H2RG operation tailored for ultra-low power consumption and is used in the NASA James Webb Space Telescope (JWST) Fine Guidance Sensor instrument. As with the GBA package, the pedestal can be Invar or gold-plated molybdenum, and is mounted to an FPA plate through three CuW feet which provide a 3-point pseudo-kinematic mount and are also the primary thermal conduction pathway. The mounting feet allow small adjustments in focal surface positions to allow planarization of a mosaic FPA to high precision. Electrical interconnection is provided by a flex circuit with a flight rated multilayer wirebond ceramic PGA which has 36 pins, four spring-loaded retainer clips, and a lateral retention feature in the center of the ceramic that holds the ceramic to the pedestal and prevents relative lateral motion between the pedestal and ceramic during spacecraft launch. The other end of the flex cable has a 37-pin Micro-D (MIL-DTL-83513 ) connector. MIL-DTL-83513 series connectors are available in flight-qualified versions and have been used extensively in space-flight hardware. There are two types of filtering capacitors, tantalum and ceramic, both of which meet the requirements of EEE-INST-002 and MIL-STD-883, NASA’s standard specifications for electronic component selection, screening, and qualification testing.

2.1.3. Flight SiC Package The SiC flight package is Teledyne’s latest FPA package for the H2RG and, as with the GBA version, facilitates all features of the H2RG. While molybdenum is flight proven and commonly used, its high density makes mosaics of molybdenum-based FPAs relatively heavy. SiC is much lower density alternative to molybdenum, is also flight proven, and has excellent thermal conductivity. In addition, SiC has mechanical and thermal properties similar to those of ceramic composite materials used for instrument benches and support structures; thus, the use of SiC based packages greatly simplifies interfaces to higher assembly levels. SiC has a CTE that closely matches the CTE of silicon from room temperature to deep-cryogenic temperatures as low as 30 K. Thus, the good CTE match between the silicon carbide pedestal and silicon H2RG ROIC induces almost no stress in the hybrid/pedestal interface. A 3-point pseudo-kinematic mount is provided by mounting feet, which in the flight SiC package are machined out of titanium and mated with CuW (or molybdenum) spacers which provide the thermal conduction pathway to the FPA plate. Electrical interconnection is provided by a wirebond rigid-flex circuit which “wraps around” a bond-ledge stiffener on one edge of the SiC pedestal to the pedestal underside, terminating in a high-density, 85-pin connector (of the Airborn Nano series) that is fastened to the pedestal. The filtering capacitors (mounted onto the pedestal underside) are similar to those on the flight molybdenum package, and are selected, screened and qualified with virtually identical methodology. A calibrated and flight-qualified Cernox temperature sensor is mounted onto the package, enabling FPA temperature to be measured on the FPA package as well as on the H2RG ROIC.

2.2. Comparison of Teledyne H2RG FPA Packages Table 1 provides a comparison of the key features and capabilities of each of the FPA packages described in this section.

Table1: H2RG FPA Package Comparison; note that the flight SiC package can be manufactured with a molybdenum pedestal if required for a specific space mission

GBA H2RG SCA FLIGHT MOLYBDENUM H2RG SCA FLIGHT SiC H2RG SCA

COMPARISON OF TELEDYNE'S H2RG

SENSOR CHIP ASSEMBLIES

Dimensions 38.89 mm x 40.46 mm x 7.62 mm 38.99 mm x 41.26 mm x 7.62 mm 42.39 mm x 39.24 mm x 12.19 mm

Mass (incl. hybrid detector) 80 grams 80 grams 55.9 grams

Package Flatness <5 microns <5 microns <5 micronsParallelism of detector mounting surface and pedestal mounting interface to FPA <5 microns <5 microns 5 microns

Emissivity Gold-plated pedestal Gold-plated pedestal SiC (can be gold-plated)

FPA Attachment Interface 4-40 nuts, 3 places, thread onto CuW stud/pin

4-40 nuts, 3 places, thread onto CuW stud/pin

M3 fasteners, 3 places, thread into titanium pin or

M5 fasteners, 3 places, thread directly into insert in SiC pedestal

Spacer shim material W/Cu (80:20) W/Cu (80:20) TZM Molybdenum or W/Cu (80:20)Spacer shims serialized (lasermarked) NO YES YESBondledge Stiffener Material N/A N/A TitaniumSiC Pedestal Insert Material N/A N/A InvarHandling Tool Features YES YES YESFPA Installation Tool Features YES YES YESVented Fasteners YES YES YES

H2RG capabilities available All H2RG ROIC signals available 2- and 4-output operation only All H2RG ROIC signals available

Connector and Pinout 90-pin PGA on package and 92-pin Hirose connector on flexcircuit

36-pin PGA socket and 37-pin MDM connector on flexcircuit 85-pin AirBorn Nano MIL-DTL-32139

Shield Line Pins Supplied on Connector NO NO YESConnector meets NASA EEE-INST-002 NO YES YESOn-Package Capacitors meet NASA EEE-INST-002 NO YES YES

On-Package Temperature Sensor NO NO YES; Lakeshore CERNOX CX1080-SD-20L

On-Package Heater Resistors NO NO YES

Thermal design flexibility YES YES YES

Vibration NO YES YESThermal YES YES YESOutgassing NO YES YES

Connectors, shorting plugs, wear savers Commercially available Commercially available Commercially availableFlight qualification of EEE parts NO YES YES

COST

GLOBAL PROPERTIES

MECHANICAL DETAILS

ELECTRICAL DETAILS

THERMAL

TESTING

3. PERFORMANCE AND RELIABILITY Since the introduction of the H2RG in 2002, Teledyne has continued to improve the performance and reliability of the H2RG FPA. As for most semiconductor devices, performance, quality, and reliability typically improve when producing larger quantities. However, the most significant improvements are typically triggered by either technology breakthroughs or result from failure investigations following problems that have been encountered. Figure 2 shows the evolution of the quality, performance, and reliability of the H2RG FPA over the past 10 years in a qualitative manner.

Figure 2. H2RG performance and reliability improvements between 2002 and 2012 BCS reliability improvement and substrate removal in 2005: As part of the initial product development, Teledyne had to address an issue pertaining to the thermal cycling reliability of the BCS in the FPA stack-up which could ultimately lead to delamination of the FPA. After the root cause was understood, Teledyne subsequently modified the relevant process steps, and carried out extensive thermal cycle tests from room-temperature to 30 K and thermal shocks from room-temperature to 77 K. After thousands of temperature cycles and thermal shocks on several parts and after shipments of several hundred H2RG FPAs not a single array displayed any indication of the original problem. During 2004 / 2005, Teledyne also developed and qualified a process to completely remove the CdZnTe substrate. Substrate removed HgCdTe H2RGs are proven to provide higher QE overall compared to substrate-on HgCdTe and flat spectral response into the visible. It also dramatically reduces the effect of cosmic ray detection and virtually eliminates fringing between the substrate and the detector surface1, 3. Improvement of dark current operability in 2007: An improved molecular beam epitaxy (MBE) growth technique of HgCdTe on CdZnTe substrates led to a significant reduction of hot pixels and higher overall yields. Although the median dark current of H2RG FPAs had already been very low in the early days of the H2RG product life cycle (e.g. less than 0.01 electronics per second per pixel at 77 K for a 2.5 micron cutoff detector), the hot pixels caused a reduction in dark current operability and therefore a loss in overall operability of the FPA. The improved MBE process yielded dark current operability of greater than 99.9% with a pixel considered operable if dark is current less than 0.1 electron per second per pixel at 77 K for a 2.5 micron cutoff FPA. The resulting yield improvement enabled Teledyne to reduce its price for standard product H2RG FPAs by 35%. Reduction of readout noise in 2008: Detailed modeling of the H2RG FPA noise performance as a function of detector processing parameters enabled Teledyne to undertake and validate process changes that resulted in a significant reduction in readout noise. Typical correlated double-sampling (CDS) noise prior to 2008 was 18 to 20 electrons for

Substrate Removal• Higher quantum efficiency in IR• Visible response (with high QE)• Less susceptible to cosmic rays• Eliminates fringing in the substrate

Reliability Improvement• Improved Balanced Composite

Structure (BCS)

Process Improvements• Lower dark current• Eliminate dark current tail• Higher operability• Higher yields

Dark CurrentLowered meanEliminated tail

H2R

G V

isib

le -

IR A

rray

Per

form

ance

and

Rel

iabi

lity

Initial Technology Development2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Product Phase 3Product Phase 1 Product Phase 2

Process Improvement• Lower interpixel capacitance

Process Improvement• Lower readout noise

SiC Package• Low mass

Reliability Improvement• Bake stability• Long term storage reliability



Structure (BCS)



Structure (BCS)






H2R

G V

isib

le -

IR A

rray

Per

form

ance

and

Rel

iabi

lity

Initial Technology Development2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Product Phase 3Product Phase 1 Product Phase 2Initial Technology Development2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Product Phase 3Product Phase 1 Product Phase 2

Process Improvement• Lower interpixel capacitance

Process Improvement• Lower readout noiseProcess Improvement• Lower readout noise



Reliability Improvement• Bake stability• Long term storage reliability

SWIR H2RGs at 77 K. The improved process yielded arrays with 10 to 12 electrons of CDS noise and Fowler-32 noise as low as 3 electrons1, 3. Reduction of inter-pixel capacitive coupling in 2009: It is well known that focal plane arrays that use a source follower detector in the unit cell such as the H2RG are prone to electrostatic crosstalk from interpixel capacitance (IPC)7. All three components of the pixel, i.e. the detector, the indium bump, and the ROIC contribute to IPC. With significantly improved shielding in the ROIC, Teledyne was able to virtually eliminate the ROIC IPC which reduced the total IPC of the H2RG pixel from 10 - 12% to 5 - 7%, which is equivalent to less than 1% of electrostatic crosstalk between nearest neighbors. Introduction of SiC as packaging material in 2010: Mostly targeting space flight applications, Teledyne in collaboration with GL Scientific introduced and qualified SiC as packaging material for the H2RG. The key advantages of SiC as packaging for space flight applications are summarized eleswhere3. Improved storage reliability in 2011: It was first noted by the JWST project that after an extended time of storage (on the order of a few years) at room-temperature, the dark current operability of the H2RG FPA would decrease. Although the effect was very small and only observed on 2 out of 14 flight arrays with a 2.5 micron cutoff, the effect was more pronounced on the 5.3 micron H2RG arrays and most flight arrays showed 1 to 2% of the pixels being affected. Although flight H2RG arrays for JWST still met performance requirements, there was concern that by the time JWST would launch the H2RG arrays would no longer meet flight requirements. A multi-month effort was undertaken in 2011 to determine the root cause by the JWST Detector Degradation Failure Review Board with personnel from TIS, NASA Goddard Space Flight Center and other organizations. The root cause was identified and found to be caused by breakdown of the diffusion barrier between the detector indium bump and the underlying contact metal and HgCdTe. Dark current increase was associated with either direct exposure of the HgCdTe junction region to high concentrations of indium, or damage introduced by stress from the indium reaction, or both. Based on these observations, Teledyne transferred its proven dielectric metal stack process from its high-background FPA detector process to its ultra-low background astronomy FPA manufacturing process. This so called “bake-stable” process has been implemented and verified on H1RG arrays under the JWST reliability improvement program and H2RG arrays for ground based astronomy fabricated in 2011 and 2012. Results show that the excellent and well established device performance of the H2RG FPA remains unchanged, however, with much improved long-term storage reliability This bake stable process is now baseline for all HgCdTe arrays manufactured by Teledyne. Persistence reduction in 2012 (ongoing): While readout noise, quantum efficiency, and crosstalk are at near theoretical performance levels, users have pointed out that in certain cases instrument performance can be limited by image persistence (also referred to as image lag or latency), i.e. pixels show some remnant charge after exposure to light even when the light has been turned off. This phenomenon has been observed by various groups in HgCdTe detectors and is discussed in detail by Roger Smith et. al.8. Teledyne has invested significant resources to reduce latency and has identified a process that has the potential to significantly reduce latency. Figure 3 shows a comparison of image lag of Teledyne’s current baseline process (data in green) and a new process in development (shown in red). Although

Product Spec

1 2 3 4 5 6 7 8 9 10

Frame Number

Measurement Limit

Figure 3: Image persistence for the current H2RG baseline process vs. improved process in development. The latent image is the CDS signal measured on the second frame (i.e., the difference of the first latent frame from the second latent frame) after performing two resets following the removal of the illumination source. Integration time for each post-exposure frame is equal to the integration time used to provide the ~80% full well signal frame. Cumulative and differential signal as percent of illumination signal vs. time is shown.

very small, i.e. less than 0.05% of the original exposure, the current H2RG detector process shows a detectable latent image still after several tens of seconds. In comparison, the new process in development shows immeasurable persistence after 10 seconds.

4. FPA PERFORMANCE DISCUSSION Performance data of the 2.5 micron (SWIR) H2RG FPA have been discussed and published in great detail by several groups1 - 7. Most of this data has been obtained for 2.5 micron cutoff H2RG typical operating conditions, i.e. 250 mV detector reverse bias at 100,000 pixel / second / output read out rate and an operating temperature of ~77 K. In this section we discuss H2RG performance data as a function of detector reverse bias (section 4.1), operating temperature (section 4.2), and pixel readout rate (section 4.3). All test data have been taken at Teledyne’s dedicated astronomy test laboratory using either discrete analog or SIDECAR ASIC FPE.

4.1. H2RG FPA performance as a function of detector reverse bias Historically, 2.5 micron H2RG FPAs have been operated under about 250 mV reverse bias. This reverse bias setting was a good compromise for maintaining high dark current operability and obtaining reasonable full well. With the reduction in pixel defects by the new MBE process introduced in 2007 (see section 3 in this paper), the H2RG can now be operated at higher reverse bias to boost performance without sacrificing dark current operability. An increase in the photodiode reverse bias causes an increase in the depletion width of the detector which decreases the detector capacitance, which causes a decrease in readout noise and an increase in full well under otherwise equal conditions. Table 2 shows summary performance data for a 2.5 micron SWIR H2RG FPA of the latest process generation at 250 mV, 500 mV, 750 mV, and 1,000 mV reverse bias. The transimpedance gain was measured for each reverse bias and was taken into account when calculating full well and readout noise in electrons. An increase in reverse detector bias from 250 mV to 1,000 mV can boost the full well by 70% and decrease readout noise by more than 10% at the same time. The impact in dark current operability is shown in table 2 and visualized in figure 4.

Table 2. SWIR H2RG FPA performance as a function of detector reverse bias

Reverse Bias [mV]

Median [e-/sec]

Operability: < 1e-/sec

Operability: < 0.1e-/sec

Full Well [Ke-] CDS [e-] Fowler-32 [e-

]

250 0.002 99.90% 99.72% 109 10.9 3.2

500 0.011 99.90% 99.56% 154 10.2 3.1

750 0.014 99.90% 99.47% 179 9.6 3.1

1000 0.013 99.89% 99.39% 185 9.4 3.0

Dark Current Noise

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

-0.02 0.18 0.38 0.58 0.78 0.98

Idark (e-/pixel/sec)

Num

ber

of P

ixel

s

Median = 0.002 e-/sec

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

-0.02 0.18 0.38 0.58 0.78 0.98

Idark (e-/pixel/sec)

Num

ber

of P

ixel

s

Median = 0.013 e-/sec

Figure 4. Dark current histograms of a SWIR H2RG FPA at 250 mV reverse bias (left) and 1,000 mV reverse bias (right)

4.2. H2RG FPA performance as a function operating temperature Some instrument designs, in particular for space flight applications, require excellent FPA performance at higher temperatures, which are either driven by power budgets or financial budgets. For this purpose, Teledyne has tested FPA performance of a 2.5 micron H2RG FPA at 77 K, 100 K, and 120 K. The summary results are shown in table 3. The transimpendence gain was measured independently at each temperature and taken into account for all data reported in electrons. From 77 K to 100 K, the median dark current was still below Teledyne’s dark current measurement capability, which is around 0.002 electronics / second / pixel for H2RG FPAs. Dark current operability only decreased by 0.15% for a dark current operability requirement of 1 electron / second / pixel or 0.56% for an 0.1 electron / second / pixel requirement. Figure 5 shows an Arrhenius plot of the median dark current as a function of inverse temperature. At the highest temperatures on the Figure 5 plot, the dark current is limited by the generation-recombination (GR) process, transitioning to another mechanism near 105K. The low temperature dark current limiting process (~0.001 e-/s/pixel) is not well understood, but may be related to background light or testing limitations, weakly temperature dependent surface currents, or potential drifts.

Table 3. H2RG performance at 77 K and 100 K operating temperature, 250 mV detector reverse bias, and 105 pixel / second / output readout rate

Temperature [K]

Median [e-/sec]

Operability: < 1e-/sec

Operability: < 0.1e-/sec

Full Well [Ke-] CDS [e-] Fowler-32 [e-

]

77 0.002 99.9 99.72 109 10.9 3.2

100 0.002 99.75 99.16 98 10.7 3.6

Dark Current Noise

Figure 5. Median SWIR H2RG array dark current as a function of inverse operating temperature (1,000 / Temperature). The 2.5 micron FPA dark current is limited at high temperatures (100 - 200K) by a GR mechanism, but transition to a less-understood limiting process for temperatures below 100K, where dark currents are <0.01 electron / second / pixel. Recent SCA #16589 achieved dark current median of 0.002 e-/s at 100K.

1.E-4

1.E-3

1.E-2

1.E-1

1.E+0

1.E+1

1.E+2

1.E+3

1.E+4

1.E+5

1.E+6

1.E+7

4 6 8 10 12 14 161000 / Temperature [1/K]

Dar

k C

urre

nt (e

lect

rons

/sec

/pix

el)

TIS H2RG#11 (UofH)

2.5um PEC (TIS)

'dirt' current background

Surface GR (TIS)

TIS H2RG#16589

120 K 100 K 77 K

4.3. H2RG FPA performance as a function of pixel read out rate The H2RG can be programmed via its serial register to run either in slow mode or in fast mode. Pixel rates in slow mode can be set as high as 500 kilopixel / second / output (1, 4, or 32 outputs), and up to 10 megapixel / second / output (1, 4, or 32 outputs) in fast mode. Slow mode is intended to support low background applications, such as night-time astronomical observations. Fast mode allows for full 2K × 2K frame rates of up to 76 Hz as required for solar astronomy or Earth observing. Both output modes are supported by Teledyne’s standard SIDECAR ASIC focal plane electronics controller kits.

4.3.1. Slow mode operation CDS read noise of a SWIR H2RG FPA has been measured at 100, 200, 300, 400, and 480 kilopixel / second / output at 77 K at 250 mV reverse bias. Pixel settling was verified at each readout rate. Figure 6 shows that the median H2RG FPA readout noise is nearly constant as a function of pixel read out rate in slow mode. This observation enables the possibility of acquiring more reads within a given integration time and therefore reducing readout noise with multiple sampling while keeping all other parameters constant.

Figure 6. Median CDS noise of a SWIR H2RG FPA at a function of pixel read out rate (data was taken at 77 K, and 250 mV reverse detector bias)

4.3.2. Fast mode operation In order to run at MHz pixel readout rates, the CTIA column buffers of the H2RG are enabled and Class A/B output amplifiers are utilized for buffered mode operation. The internal timing of the H2RG also changes to transfer an entire line of pixels into the column buffers at once, instead of accessing a single column at a time as it is done in slow mode operation. In fast mode, the H2RG ROIC integrates down vs. integrate up in slow mode. When using SIDECAR ASIC FPE is the FPA controller in fast mode, the SIDECAR’s 12 bit analog to digital converters (ADCs) must be used in order to accommodate the higher pixel rates, vs. the 16 bit ADCs for slow mode operation. For sustained data output rates needed beyond simple functional verification, the output data is sent over CamLink™. Teledyne has delivered systems for space surveillance and solar astronomy with the H2RG and SIDECAR ASIC operating in fast mode at up to 10 MHz pixel readout rate. Readout noise was measured in reset-read as well as reset-read-read (CDS) operation using the H2RG and SIDECAR ASIC operating in fast mode at the 10 MHz pixel rate. A median readout noise of 72 and 65 electrons was obtained for reset-read and CDS operation, respectively. Figure 7 shows readout noise map and histogram for CDS operation. The fact that there is not a large difference between CDS and reset-read noise is due to the fast mode readout noise being mostly dominated by the kTC noise of the column buffers. In fast mode a CDS acquisition can only remove the kTC noise from the pixel but not from the column buffer.

CDS Noise vs Pixel Clock Frequency

8

10

12

14

16

18

20

50 100 150 200 250 300 350 400 450 500

Pixel Clock Frequency (KHz)

CDS

Noi

se (e

-)

Figure 7: H2RG FPA standard product cutoff wavelengths are aligned with atmospheric transmission windows

5. H2RG STANDARD PRODUCTS AT 1.75 MICRON, 2.5 MICRON AND 5.3 MICRON CUTOFF In order to provide the highest quality and performance at the lowest possible price, Teledyne has developed H2RG FPA standard products at cutoff wavelengths of 1.75 microns, 2.5 microns, and 5.3 micron that match the atmospheric transmission windows and filter bands J and H for 1.75 micron cutoff (also referred to as near infrared, NIR), K for 2.5 micron cutoff (also referred to as short wave infrared, SWIR), and L and M for 5.3 micron cutoff (also referred to as midwave infrared, MWIR), respectively (see figure 8). The H2RG standard product FPA performance specifications are summarized in table 4.

Figure 8: H2RG FPA standard product cutoff wavelengths are aligned with atmospheric transmission windows

Table 4. H2RG Standard Product performance specifications9

(1) There are 2040 x 2040 pixels for light detection plus 4 rows and columns of reference pixels on each side of the array (2) At 100 kHz pixel read-out rate, unbuffered, 32 outputs. Does not include external current source; power has to be optimized with respect to the system in which the device is used (3) At 10 MHz pixel read-out rate, buffered, 32 outputs (4) Crosstalk includes both optical (charge diffusion) and electrical (interpixel capacitance) components

J1.1-1.4

H1.5-1.8

K2.0-2.4

L3.0-4.0

M4.5-5.1

Common Astronomical Filters

Wavelength [μm]

J1.1-1.4

H1.5-1.8

K2.0-2.4

L3.0-4.0

M4.5-5.1

Common Astronomical Filters

Wavelength [μm]

0

50000

100000

150000

200000

20 40 60 80 100 120

CDS Noise (e-)

Num

ber

of P

ixel

s

Median = 64.8 e-

(5) A pixel is considered operable if QE ≥ 35%, dark current ≤ 0.1 e-/sec, and single correlated double sample (CDS) noise is ≤35 e- (6) Maximum variation (peak-to-valley) to best fit plane

Value Parameter Unit1.7μm 2.5μm 5.3μm

Array Format (1) 2048 x 2048 pixel, 18 μm pitch

Number of Outputs # Programmable 1, 4, 32

Frame rate Hz 3 (slow mode, 480 Kpix/sec/output, 32 outputs) 76 (fast mode, 10 Mpix/sec/output, 32 outputs)

Power Dissipation mW ≤ 4 (2) / ≤ 300 (3)

Detector Material HgCdTe

Detector Substrate CdZnTe - Removed

Cutoff wavelength (50% of peak QE): 1.75μm: @ 120 K 2.5μm: @ 77 K 5.3μm: @ 37 K

μm 1.65 - 1.80 2.45 - 2.65 5.1 - 5.5

Mean Quantum Efficiency (QE) at 800 nm % ≥ 50 (goal is ≥ 70) ≥ 70 (goal is ≥ 80)

Mean Quantum Efficiency (QE) at 1,000 nm % ≥ 50 (goal is ≥ 70) ≥ 70 (goal is ≥ 80)



Mean Quantum Efficiency (QE) at 2,000 nm % 0 ≥ 70 (goal is ≥ 80)

Mean Quantum Efficiency (QE) at 3,500 nm % 0 0 ≥ 70 (goal is ≥ 80)

Mean Quantum Efficiency (QE) at 4,400 nm % 0 0 ≥ 70 (goal is ≥ 80)

Median Dark current: 1.7μm: @ 0.25 V bias and 120 K 2.5μm: @ 0.25 V bias and 77 K 5.3μm: @ 0.18 V bias and 37 K

e-/s ≤ 0.05 (goal is ≤ 0.01)

Median Readout Noise, correlated double sampling (CDS) at 100 KHz pixel readout rate

e- ≤ 30 (goal is ≤ 15) ≤ 18 (goal is ≤ 12) ≤ 15 (goal is ≤ 12)

Median Readout Noise, reset - read at 10 MHz pixel readout rate ≤ 100 (goal is ≤ 70)

Well Capacity at 0.25 V bias (0.175V bias for 5.3μm cutoff) e- ≥ 80,000 (goal is ≥ 100,000) ≥ 65,000 (goal is ≥ 85,000)

Crosstalk (4) % ≤ 2 (goal is ≤ 1) ≤ 4 (goal is ≤ 2)

Operability (5) % ≥ 95 (goal is ≥ 99)

Cluster: 50 or more contiguous inoperable pixels % ≤ 1 (goal is ≤ 0.5) of array

SCA Flatness (6) μm ≤ 20 (goal is ≤ 10)

The detector design employs Teledyne’s planar dual heterostructure with an HgCdTe absorber for all three standard product cutoff wavelengths as shown in figure 9. The HgCdTe detector layers are deposited on CdZnTe substrates by

MBE. Following a thorough characterization mapping and screening of the HgCdTe wafers, the arrays of photodiodes are subsequently processed using standard semiconductor microprocessing techniques.

n - HgCdTe:In

p - HgCdTe:As (implant)

HgCdTe Buffer

Cap

Surface Passivation

CdZnTe substrate (removed)

In bump interconnect

H2RG ROIC

n - HgCdTe:In

p - HgCdTe:As (implant)

HgCdTe Buffer

Cap

Surface Passivation


In bump Interconnect

H2RG ROIC

n-HgCdTe:In

p-HgCdTe:As(implant)

HgCdTe Buffer

Cap

Surface Passivation


MBE Growth Direction

Detector Band Diagram

n-HgCdTe:In

p-HgCdTe:As(implant)

HgCdTe Buffer

Cap

Surface Passivation


MBE Growth Direction

Detector Band Diagram

Figure 9: The TIS planar hybrid infrared detector cross-section (left) and band diagram (right). Section lines show the implementation of CdZnTe substrate removal.

One of the unique features of HgCdTe is that its lattice constant only slightly varies when the Hg to Cd content in the ternary HgCdTe alloy is varied in order to achieve the desired bandgap and cutoff wavelength. This unique feature is the reason why HgCdTe is the high performance absorber material of choice for infrared light between approximately 1.5 microns and 16 microns. When HgCdTe is grown by MBE on CdZnTe substrates, the lattice constant of CdZnTe is perfectly matched at a Cd-to-Hg ratio of 30% for the MWIR cutoff, i.e. at around 5.3 micron.

Figure 10: Bandgap and cutoff wavelength as a function of Cadmium mole-fraction of HgCdTe10

Performance metrics have many aspects in common for the three standard product FPAs. Dark current is generally very low at less than 0.1 to 0.01 electron / second / pixel while operability is commonly above 99%. Some difference in performance between bands can be seen in the expected readout noise performance and quantum efficiency (QE). These differences arise from common challenges when manufacturing compound semiconductors and have to do with the semiconductor bandgap and interface engineering. While generally narrow bandgap semiconductors present challenges

such as tunneling mechanisms for dark current and Auger recombination which can limit dark current and QE performance, the wide bandgap devices also have their own materials challenges, including managing interface charge and forming ohmic contacts (particularly at cryogenic temperatures). The interface charge engineering is particularly crucial in order to achieve high QE in the short wavelengths for the 1.75um SCAs. Likewise, rectifying contacts for wide-bandgap diode arrays can give rise to excess readout noise. Teledyne is continually making efforts to master the engineering challenges of these materials to extract the best possible performance. Typical performance data for science grade H2RG FPAs with 1.75 micron, 2.5 micron, and 5.3 micron cutoff wavelenghts are shown in tables 5, 6, and 7 respectively. All data shown in tables 5 - 7 were taken at 100 kHz pixel readout rate, unbuffered, 4-output mode. A pixel is considered operable if QE ≥ 35%, dark current ≤ 0.1 e-/sec, and CDS readout noise is ≤ 35 e-.

Table 5: Typical science grade (SG) NIR (1.75 micron cutoff) H2RG FPA performance

Cluster: 50 or more contiguous inoperable pixel % ≤ 1% of array ≤ 0.5% of array 0.01

58

Quantum Efficiency (QE) at 1000 nm % ≥ 50 ≥ 70 57

Quantum Efficiency (QE) at 800 nm % ≥ 50 ≥ 70

Crosstalk % ≤ 2 ≤ 1

Well Capacity at 0.25 V bias e- ≥ 80,000 ≥ 100,000

Median Read Noise (single CDS) e- ≤ 30 ≤ 15

Median Dark current @ 0.25 V bias and 120 K e-/s ≤ 0.05 ≤ 0.01

Quantum Efficiency (QE) at 1230 nm % ≥ 70

Cutoff wavelength @ 120 K (50% of peak QE) μm 1.65 - 1.80 1.70 - 1.75

Operability % ≥ 95 ≥ 99

Power Dissipation mW ≤ 0.5≤ 1.0

≥ 80

0.297

1.72

73

0.001

18.92

95,563

0.9


98.32

MeasuredParameter Unit Specification Goal

Table 6: Typical SG SWIR (2.5 micron cutoff) H2RG FPA performance




99.58

83

0.005

9.43

120300

0.9

0.426

2.53

≤ 0.5≤ 1.0

≥ 70

Power Dissipation mW






≥ 55 ≥ 70



83


Quantum Efficiency (QE) at 800 nm %

Table 7: Typical SG MWIR (5.3 micron cutoff) H2RG FPA performance


84


Quantum Efficiency (QE) at 800 nm % ≥ 55 ≥ 70








Power Dissipation mW ≤ 0.5≤ 1.0

≥ 70

0.255

5.33

93

0.01

11.49

81,785

2.8


99.28


6. HXRG INFRARED CAMERA In partnership with GL Scientific, Teledyne has developed and productized a compact, turn-key, high performance, highly configurable, and affordable infrared camera system. The HxRG IR Camera is designed to accommodate H1RG, H2RG, and H4RG-10 and -15 FPAs and cryogenic SIDECAR ASIC FPEs, either in a single-SIDECAR or dual-SIDECAR configuration to support up to 64 output operation of the H4RG FPAs and / or to support high-speed, fast mode operation of the H2RG of up to 76 Hz full 2K × 2K framerate. The HxRG IR Camera can be delivered as a basic system for laboratory testing purposes or can be deployed directly on a telescope for astronomical observations. The basic system consists of a Stirling cooler cryostat (SCC) holding the HxRG FPA and cryogenic SIDECAR ASIC FPE (see figure 10a). A Sunpower Stirling cooler is attached to a compact cryostat via a vibration isolation mount to minimize mechanical vibration. Mechanical stability of less than 1 micrometer has been demonstrated on sky9. The temperatures of the HxRG FPA and SIDECAR ASIC are controlled separately and can be set from 60 K to 200 K ± 0.01 K and 120 K to 200 K ± 0.01 K, respectively. The SCC includes an HxRG to SIDECAR ASIC flexcable with light-tight feedthrough and a vacuum feedthrough flexcircuit that connects the cryogenic SIDECAR ASIC to the external, on-cryostat SIDECAR Acquisition Module (SAM). An on-board micro-Pirani vacuum gauge monitors the cryostat pressure from atmosphere to 10-5 Torr. An internal, cooled, activated-charcoal getter maintains cryostat vacuum when disconnected from vacuum pump. Light-tight, tongue-in-groove seals are used for all internal mechanical interfaces to detector and radiation shields. The Stirling cooler requires only electrical power and chilled water or glycol to remove waste heat from cooler body. No compressors or pressurized gas lines are required. Depending on the needs in the laboratory or at the telescope, Teledyne and GL Scientific provide and integrate the following auxiliary equipment:

• Lakeshore model 336 4-channel temperature controller and cables to cryostat • Solid State Cooling Systems Thermorack 650 water chiller with hoses to connect to cryostat • Power supplies for SIDECAR ASIC focal plane electronics and SAM • Custom integrated lab stand with 19 inch rack mount base for convenient packaging of all system components • Varian mini-Task AG81 dry, turbo-molecular vacuum pumping station with vacuum isolation valve safety

interlock, vent port, and electrically isolated vacuum fittings and hose to connect to cryostat

Figure 11 Stirling Cooler Cryostat (SCC) based HxRG IR Camera. (a) Basic system, (b) H2RG – single SIDECAR ASCI FPE module, (c) complete IR Camera system with lab stand and vacuum pump station

The SCC is designed to conveniently interface with additional optics and laboratory equipment. Teledyne and GL Scientific offer to supply and fully integrate an F/8 optics module (OM) and a optics calibration module (OCM). The OM consists of cooled 1:1 Offner relay reimaging optics with cold Lyot stop, and motorized, six-position filter wheel with J, H, and K filters plus blank (figure 12). The OCM employs a cryogenic 4 inch integrating sphere which is either illuminated with a fiber from e.g. a monochromator or with LEDs, and offers target projection capability, and NIST calibrated reference detectors for absolute in-situ flux monitoring (figure 13).

Stirling Cooler Cryostat (SCC) containing the H2RG focal plane array and the SIDECAR ASIC focal plane electronics

Integrated Lab Stand/Cart

Stirling cooler and vacuum gauge power supplies

Power supply for Teledyne electronics: Agilent E3640A

Lakeshore temperature controller, model 336, 4-channel controller

Solid state cooling systems Thermorack 650 water chiller with hoses to connect to SCC

Turbomolecular Vacuum Pumping Station

Figure 12 Stirling Cooler Cryostat (SCC) integrated with a cooled 1:1 Offner relay optics module (OM) with cold Lyot stop and motorized 6-position filter wheel

(a) (b)

(c)

Figure 13 Stirling Cooler Cryostat (SCC) integrated with optics calibration module (OCM)

7. REFERENCES [1] G. Finger, R. Dorn, S. Eschbaumer, D. Ives, L. Mehrgan, M. Meyer, J. Stegmeier, “Recent Performance

Improvements, Calibration Techniques and Mitigation Strategies for Large-format HgCdTe Arrays”, Proceedings of Scientific Detector Workshop 2005, Garching (2009)

[2] Roger Smith, Gustavo Rahmer, David Hale, Elliott Koch, “HgCdTe Noise from µHz to kHz”, Proceedings of Scientific Detector Workshop 2005, Garching (2009)

[3] R. Blank, S. W. Anglin, J. W. Beletic, Y. Bai, S. Buck, S. Bhargava, J. Chen, D. Cooper, M. Eads, M. Farris, D. N. B. Hall, K. W. Hodapp, W. Lavelle, M. Loose, G. Luppino, E. Piquette, R. Ricardo, T. Sprafke, B. Starr, M. Xu, and M. Zandian, “The HxRG Family of High Performance Image Sensors for Astronomy”, Astronomical Society of the Pacific Conference Series, Vol 437, Solar Polarization 6, 383 (2011)

[4] D. N. B. Hall, K. W. Hodapp, D.L. Goldsmith, C. Cabelli, A. K. Haas, L. J. Kozlowski, and Kadri Vural, “Characterization of 5 micron HgCdTe arrays for low-background astronomy”, Optical and IR Telescope Instrumentation and Detectors, Proceedings of SPIE, vol. 4008 (2000).

[5] G. Finger, J.W. Beletic, R. Dorn, M. Meyer, L. Mehrgan, A.F.M. Moorwood, J. Stegmeier, “Conversion gain and interpixel capacitance of CMOS hybrid focal plane arrays”, Proceedings of Scientific Detectors for Astronomy 2005, p. 447, Springer (2006)

[6] G. Finger, R. Dorn, S. Eschenbaumer, D. N. B. Hall, L. Mehrgan, M. Meyer, and J. Stegmeyer, “Performance evaluation, readout modes, and calibration techniques of HgCdTe Hawaii-2RG mosaic arrays”, Proceedings of the SPIE, Vol. 7201 (2008).

[7] G. Finger, J.W. Beletic, R. Dorn, M. Meyer, L. Mehrgan, A.F.M. Moorwood, J. Stegmeier, “Conversion gain and interpixel capacitance of CMOS hybrid focal plane arrays”, Proceedings of Scientific Detectors for Astronomy 2005, p. 447, Springer (2006)

[8] R. Smith, M. Zavodny, G. Rahmer, M. Bonati “A theory for image persistence in HgCdTe photodiodes”, Proceedings of the SPIE, Vol. 7021 (2008)

[9] Cleared for public release by the DoD Office of Security Review, Case# 10-S-1868 (2012) [10] G. L. Hansen, J. L. Schmidt, T. N. Casselman, J. Appl. Phys. 53/10, p. 7099 (1982) [11] H. Lin, personal communication (2012)

LED Winston Cone

REF DET Mount

4-in diameter Integrating Sphere

OAP1

OAP2LED Winston Cone

REF DET Mount

4-in diameter Integrating Sphere

OAP1

OAP2

h2rg focal plane array and camera performance update

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