Geiger-mode APD Single-Photon Cameras for 3D Laser Radar Imaging

Transcription

1 Geiger-mode APD Single-Photon Cameras for 3D Laser Radar Imaging Mark A. Itzler, Mark Entwistle, Xudong Jiang, Mark Owens, Krystyna Slomkowski, Sabbir Rangwala Princeton Lightwave Inc. US Route 13 South Cranbury, NJ Abstract We describe the design and performance of shortwave infrared 3D imaging cameras with focal plane arrays (FPAs) based on Geiger-mode avalanche photodiodes (GmAPDs) with single photon sensitivity for laser radar imaging applications. The FPA pixels incorporate InP/InGaAs(P) GmAPDs for the detection of single photons with high efficiency and low dark count rates. Based on the design of the GmAPD detectors, FPAs have been optimized for source wavelengths near either 1. μm or 1.5 μm. We present results and attributes of fully integrated camera sub-systems with 32 x 32 and 128 x 32 formats, which have 1 μm pitch and 5 μm pitch, respectively. We also address the sensitivity of the fundamental GmAPD detectors to radiation exposure, including recent results that correlate detector active region volume to sustainable radiation tolerance levels. For GmAPD detector designs that are compatible with our existing FPA platform, these discrete device results indicate that sensors with our current level of performance can be designed to tolerate radiation exposure of at least 8 krad with acceptable levels of dark count rate elevation. TABLE OF CONTENTS 1. INTRODUCTION GMAPD FPA AND CAMERA DESIGN X 32 CAMERA PERFORMANCE X 32 CAMERA PERFORMANCE RADIATION TOLERANCE OF REDUCED VOLUME GMAPD DETECTORS SUMMARY OF FUTURE DIRECTIONS ACKNOWLEDGMENTS REFERENCES BIOGRAPHY INTRODUCTION High-performance three-dimensional (3D) imaging technologies promise to deliver disruptive capabilities by providing a much greater wealth of information than that obtained from conventional two-dimensional (2D) imaging techniques. In particular, explicit high-resolution range (i.e., depth) information can eliminate ambiguities that are often present in 2D imagery, and 3D imaging enables advanced functionality such as the imaging of targets behind foreground obscurants that is generally not possible using traditional 2D intensity imaging approaches. Laser radar techniques, which employ optical signals with micron-scale wavelengths, allow for vastly higher resolution than traditional radar measurements. With sub-nanosecond scale laser pulse widths, range resolution on the scale of centimeters can be readily achieved. Furthermore, by using arrays of detectors with single-photon sensitivity, laser radar systems can operate at very large stand-off distances with very high area coverage rates. Focal plane arrays based on single-photon-sensitive Geiger-mode avalanche photodiodes (GmAPDs) were first implemented in laser radar systems about a decade ago [1,2], and more recent system demonstrations have shown the ability to map hundreds [1] to thousands [2] of square kilometers per hour, which represents as much as an order of magnitude advantage in mapping rate over other competing laser radar systems. Beyond their use in laser radar systems, GmAPD arrays are also attractive for deployment in very long range free space optical communication systems. This application presents a quintessential example of a photon-starved environment in which the single-photon sensitivity of GmAPDs is highly desirable, and the requirements for signal acquisition and tracking in free space communications are similar in many respects to those of single-photon imaging. Initial pioneering work on GmAPD-based FPAs was carried out in the early 2s by researchers at MIT Lincoln Laboratory [3,4,5]. More recently, the maturation and industrialization of this technology by the present authors has led to the development of high performance sensors [6,7] with commercial availability of these sensors in turnkey camera systems [8]. Core camera functionality is provided by arrays of GmAPDs hybridized to CMOS readout integrated circuits (ROICs) that provide independent time-of-flight measurements for each pixel and operate at frame rates as high as ~185 khz for 32 x 32 sensors and ~115 khz for 128 x 32 sensors. For both formats, average single-photon detection efficiencies beyond 4% (including all optical transmission and microlens array losses) are achieved for average dark count rates of less than 1 khz. Near-neighbor pixel optical crosstalk is <1%, and timing jitter is < 5 ps rms. Robust hermetic packaging of the FPA sensor provides high reliability operation in harsh environments, and the camera sub-systems based on these FPAs incorporate FPGA-based sensor electronics, flexible GUI software, and industry-standard data interfaces /14/$ IEEE /14/$ IEEE 1

2 A key attribute on our roadmap for future generations of GmAPD 3D imagers is the push to larger array formats. Larger formats will necessarily be accompanied by reduced pixel pitch, which will in turn drive the implementation of smaller volume detectors in each pixel. Aside from enabling smaller pitch arrays, the reduction of the active volume of these detectors offers improvements in many pixel performance parameters that scale with active region size. Dark count rate is expected to reduce proportionally with the detector volume, and optical crosstalk between pixels which results from single photon emission during avalanche events also scales with the cross-sectional area of the active regions. Additionally, the radiation tolerance of these detectors is expected to scale with device active volume, and this issue is particularly critical in considering the deployment of GmAPD array-based sensors on space platforms supporting both imaging and free space communications applications. The primary challenge posed by implementing smaller active regions is greater difficulty in maintaining high optical coupling efficiency. In this paper, we describe the state-of-the-art in the performance of GmAPD-based 3D imaging cameras with 32 x 32 format as well as our more recent development of cameras with a 128 x 32 format. We present camera-level performance maps for dark count rate (DCR) and photon detection efficiency (PDE), as well as results for optical crosstalk and camera timing jitter. We then summarize the results of recent work done to fabricate GmAPDs with reduced active volumes and study the scaling of their performance for dark count rate and radiation tolerance. In particular, we find a close correlation of radiation tolerance levels with the inverse area of the detector active region. For GmAPD detector designs that are optically compatible with our existing FPA platforms, these discrete device results indicate that GmAPD sensors can be designed to tolerate radiation exposure of at least 8 krad with acceptable levels of dark count rate elevation. 2. GMAPD FPA AND CAMERA DESIGN GmAPD Design and Integration into Photodiode Arrays The single-photon sensitivity of the sensor arrays described in this paper is provided by high-performance GmAPDs. The schematic cross-section at the left side of Figure 1 illustrates the basic design of the active device in each pixel. The wavelength range of sensitivity is determined by the material composition of the absorber region. We have developed cameras optimized for use with laser wavelengths between ~.9 and 1.1 μm in which an appropriate quaternary InGaAsP absorber is employed, as well as cameras with sensitivity between.9 and 1.6 μm compatible with more eye-safe sources near 1.5 μm in which a ternary InGaAs layer provides absorption over this wavelength range. A second critical region of the device structure is the multiplication region in which avalanche multiplication occurs. Operation of an APD in Geiger-mode entails biasing the device above its avalanche breakdown voltage into a metastable state in which a single photo-excited carrier can trigger a macroscopic pulse of avalanche charge that is easily sensed using threshold detection electronics GmAPD schematic device design Indium bump GmAPD pixel 32 x 32 GmAPD array anode contact SiN x passivation p + -InP diffused region i-inp cap multiplication region n-inp charge n-ingaasp grading i-ingaasp absorption n + -InP buffer n + -InP substrate anti-reflection coating cathode contact optical input Electric field Figure 1 Schematic illustration of GmAPD device design (left) incorporated into a single array pixel (center) which is ultimately integrated into a larger format photodetector array (right). 2

3 connected to the APD. Following the threshold detection, the avalanche must be quenched. In this mode of operation, strictly speaking there is no well-defined gain since the avalanche is self-sustaining and the total number of carriers generated during an avalanche depends more on the quenching circuit than on the properties of the device itself. However, we can estimate an effective gain of ~1 5 to 1 6 carriers generated during each avalanche prior to quenching. In order to establish the internal electric field profile required for Geiger-mode operation (e.g., as illustrated to the right of the device schematic in Figure 1), a suitable distribution of dopant charge in the APD structure is critical. This basic APD device structure is incorporated into an array geometry in which isolation trenches are etched between neighboring pixels (see center of Figure 1) to reduce optical crosstalk, which is discussed further below. Indium bumps are deposited on the anode contact of each active region to facilitate hybridization of the photodiode array (PDA) to an appropriate readout integrated circuit (ROIC) by flip chip assembly. Cathode connections are provided around the perimeter of the pixel array. A photograph of a completely processed 32 x 32 PDA is shown on the right side of Figure 1. Design of GmAPD Focal Plane Array The principal elements of the assembled FPA module are illustrated schematically at the left side of Figure 2. The imaging functionality of the module is dictated by three semiconductor chips: (i) the InGaAsP GmAPD PDA just described; (ii) a custom silicon CMOS ROIC; and (iii) a GaP microlens array (MLA). Following the hybridization of the GmAPD PDA to the CMOS ROIC, we align and attach the MLA to the back surface of the PDA to increase the overall FPA optical fill factor. For the 32 x 32 FPA format, we have developed a passive alignment technique employing matched alignment fiducials on the PDA and MLA that provides for micron-scale alignment of the MLA to the PDA active areas. For the smaller pitch 128 x 32 FPAs, the MLA is aligned actively by monitoring the photocurrent generated when the sensor array is illuminated through the MLA. A high-performance optical epoxy is used to attach the MLA to the PDA with relatively rapid curing, and the process has been confirmed to be robust with respect to elimination of possible shifting of the MLA position after alignment. Based on the characterization of FPA pixel-level photon detection efficiency before and after MLA attachment, the effective fill factor for broad illumination of the FPA is found to be ~75%. Electrical connections to the ROIC bond pads are provided by automated wire bonding to corresponding signal traces on the ceramic interposer board (center of Figure 2). The interposer traces are then wire-bonded to a bond pad shelf in the ceramic housing, with connections leading to the pin grid array in the base of the package. A two-stage thermoelectric cooler maintains a temperature differential of up to 55 C; with a typical camera chassis ambient temperature near room temperature, the detector array in the FPA is generally operated at a temperature in the range of -25 C to -2 C. The fully assembled FPA is shown on the right side of Figure 2 just prior to attachment of the hermetic lid with a window allowing optical access to the sensor array. Camera Construction and Functionality The FPA serves as the sensor engine of the camera head, which consists of a modular design based on three circuit boards: (i) the FPA board contains the FPA sensor and circuitry to control its power and temperature regulation; (ii) the FPGA board contains an Altera FPGA and a microcontroller that provide extensive on-board functionality through firmware programming; and (iii) the interface board provides an industry-standard CameraLink interface to the system computer, power regulation, and external clock and trigger inputs. The camera head requires only a single DC source between 12 and 36 V, from which all required biases and power levels are generated internally. The camera head is controlled by a high-performance personal computer with flexible GUI software, which includes algorithms and data structures to support the realtime storage of raw sensor data that reaches rates in excess of 4 Gb/s when the sensors are running at their maximum frame rates. Sensor firmware incorporates comprehensive monitoring functions and real-time signal diagnostics, Lid MLA GmAPD PDA ROIC Ceramic interposer TEC Housing Figure 2 Schematic illustration of element stack-up in FPA assembly (left), with photograph of assembled chip stack on interposer (center) that is incorporated into full FPA sensor package (right). 3

4 including time-averaged grayscale passive imagery, time-offlight data histograms, and sensor performance monitors for temperature and bias control. The raw data from the camera consists of 13-bit timestamps indicating photon time-of-arrival at each pixel to a resolution as high as.25 ns. Once a pixel fires during a given frame, it is disabled until all the pixels are re-armed for the next frame. Pixels that do not detect an event during a given frame will output the terminal count of the pixellevel counters. The maximum number of time bins during each frame is 8, and the highest resolution operation (i.e.,.25 ns time bins) corresponds to 2 μs range gates. A range extension function allows the camera to function with longer time bins so that the 32 x 32 camera can operate with range gates as long as 1 μs, and the 128 x 32 camera can have range gates up to 4 μs in duration. These range gate settings can be changed on a frame-by-frame basis X 32 CAMERA PERFORMANCE In this section, we summarize the principal performance attributes of the 32 x 32 GmAPD camera. The following subsections contain data for sensor-level performance maps of the dark count rate (DCR) and photon detection efficiency (PDE), a description of optical crosstalk behavior, and results for the sensor timing jitter. Details of the test set used to obtain these performance results are summarized in Ref. [8]. 32 x 32 Camera DCR and PDE The most fundamental trade-off in GmAPD operation is that between the DCR and PDE. Higher PDE can be obtained by applying a larger excess bias voltage (i.e., the amount of voltage by which the bias exceeds the APD breakdown voltage), but this results in a higher DCR as well. The DCR performance of all pixels in a 32 x 32 camera operated at Number of Pixels Dark Count Rate (khz) Avg DCR = 2.2 khz σ(dcr) =.4 khz Avg PDE = 3.5% Number of Pixels Avg PDE = 3.5% σ(pde) = 4.% Avg DCR =2.2 khz 1% 14% 18% 22% 26% 3% 34% 38% 42% 46% 5% Photon Detection Efficiency 54% Figure 3 Dark count rate performance for 32x32 GmAPD camera operating at 1.6 μm. 4 Figure 4 Photon detection efficiency performance for 32x32 GmAPD camera operating at 1.6 μm.

5 -25 C is illustrated in Figure 3 by an array-level DCR map (top of figure) along with the accompanying histogram (bottom of figure). The average DCR of this array is 2.2 khz with an rms variation of.4 khz for an operating point corresponding to an average PDE of 3.5%. This average DCR corresponds to <.5% probability that any given pixel of the array registers a dark count during a 2 μs range gate. As the histogram shows, every pixel in the array has a DCR of less than 4 khz, and the tight distribution is roughly Gaussian. Residual performance non-uniformity is related to both epitaxial material variation, which is generally quite gradual and occurs over distances of many pixels, as well as process-related variation, which is responsible for the slightly lower DCR typically observed around the perimeter of the array. A corresponding performance map for the PDE of this camera is illustrated in Figure 4. PDE data were obtained using a pulsed diode laser source collimated across the entire FPA and calibrated to.1 photon per pixel area per pulse with a calibration uncertainty of ~5%. The rms variation about the measured average PDE of 3.5% is 4.%, and this variation is qualitatively similar to that seen in the DCR performance. These PDE values are asmeasured and account for all optical losses, including the less-than-unity fill factor of the microlens array. 32 x 32 Camera Crosstalk During avalanche events in GmAPDs, charge acceleration in the multiplication region can give rise to hot carrier luminescence that results in the emission of photons. Although the probability of photon emission is low on the order of one photon per 1 4 to 1 5 charges the fact that all neighboring pixels are sensitive to single photons creates the possibility of crosstalk in which emitted photons trigger temporally and spatially correlated detections at neighboring pixels due to a primary pixel avalanche initiated by a signal photon. The coupling of crosstalk photons can occur by line-of-sight travel to nearest-neighbor pixels or by reflections from the back side of the PDA substrate to more distant neighbors. The spectrum of these emitted photons has been found to have a large peak at a wavelength corresponding to an energy just below the bandgap of the InP multiplication region material, as well as a longer wavelength blackbody tail [9]. We can characterize the crosstalk properties of the GmAPD FPAs by illuminating a single pixel with a short (sub-ns) pulse at a known time and then looking for correlated avalanches in the rest of the array. However, it is also possible to obtain the same information by analyzing dark count data: given that dark counts are infrequent, we can look for correlations in the dark count data timestamps and positions to determine the crosstalk properties of the arrays. We have verified that we obtain equivalent results from these two approaches; however, the dark count analysis is more convenient because it does not require optical coupling to a single pixel, and it allows us to use events from every single pixel in the array. Cross talk probability per pixel.4%.3%.2%.1% 3%PDE 25%PDE 2%PDE T = 248 K Cumulative crosstalk 7.4% 5.9% 4.7%.% Distance from primary avalanche (in 1 µm) Figure 5 Crosstalk performance for 32x32 GmAPD camera operating at 1.6 μm. In Figure 5 we present the results of crosstalk analysis for 1, frames of dark count data from a 32 x 32 camera. The analysis consists of identifying any dark counts that occur within 5 ns of each other. The graphic at the top of the figure is a compilation of all data in which we have assumed that the primary avalanche occurs in the center of this map (i.e., at the blue pixel labelled with ). The values and corresponding color coding indicate the relative incidence of crosstalk events at any given distance and direction from the primary avalanche. There are geometric patterns to the data that are found to be very consistent and are related to specific geometric features in the arrays themselves, e.g., attributes of the etched isolation trenches and the nature of reflections from different positions on the back surface of the PDA. The plot at the bottom of the figure shows the per-pixel probability of a crosstalk event as a function of the distance of the crosstalk pixel from the primary avalanche pixel. The crosstalk data indicates a roughly monotonic decay in crosstalk probability with increased distance, although there are specific pixel positions that result in locally higher crosstalk probabilities 5

6 due to the nature of reflections within the PDA. In addition to individual pixel crosstalk probabilities, the cumulative probability for a crosstalk event to occur at any location surrounding the primary avalanche is another relevant metric to consider. As indicated in the legend in the Figure 5 plot, the cumulative crosstalk is well under 1% for an average array PDE of 3%. The optical intensity is calibrated to provide a mean photon number of.1 photons per pixel area per pulse. We perform characterization of two types of camera timing jitter: (i) intraframe jitter, for which we assess the variation in time stamps across all array pixels within a single frame; and (ii) interframe jitter, which is the variation in time stamp values for a given pixel across many consecutive frames. 32 x 32 Camera Timing Jitter The last of the critical camera performance parameters covered in this section is the timing jitter, which is a measure of the uncertainty in time stamp values reported for photon arrival times. Beyond the quantization error introduced by the temporal widths of the counter time bins (e.g.,.25 ns for the highest resolution camera operation), there can be additional timing uncertainty caused by temporal fluctuations in the ROIC timing circuitry and camera electronics. Timing jitter is assessed using a gain-switched laser diode to generate short ~.1.2 ns pulses, and the pulsed output is collimated to uniformly illuminate the sensor array area. Number of Counts Number of Counts E+5 1.E+5 8.E+4 6.E+4 4.E+4 2.E+4.E+.1 photons/pulse Single frame.1 photons/pulse 1, frames Time Bin σ ~ 175 ps Measured Data Gaussian Fit Time Bin Measured Data Gaussian Fit σ ~ 38 ps Figure 6 Intraframe (top) and interframe (bottom) timing jitter performance for 32x32 GmAPD camera. 6 At the top of Figure 6, we show the measured intraframe jitter for a 1.6 μm 32 x 32 camera operating at ~3% PDE. Essentially all detection events occur in the two neighboring time bins 44 and 45. There are a few isolated counts that occur at somewhat later time bins e.g., one count each in bins 412 and 413 which are caused by crosstalk events. We can calculate the rms variation represented by the measured count distribution using either a Gaussian fit or a strictly numerical calculation of the standard deviation. In either case, we find the timing jitter to have an rms deviation of σ ~ 175 ps. The bottom of Figure 6 illustrates data obtained for interframe jitter in which we have included counts for all pixels in the array for 1, consecutive frames. In this case, counts are accumulated across several neighboring time bins, and a numerical calculation of the interframe timing jitter yields σ ~ 38 ps. However, it is important to note that these data include all system-level contributions to the timing jitter since there has been no correction for other sources of timing uncertainty. We have seen that for measurements of 1.55 μm 32 x 32 cameras with more stable pulsed lasers, the interframe jitter yielded in those measurements was of the same magnitude as the intraframe jitter i.e., 15 to 2 ps. Therefore, we suspect that interframe jitter reported in Figure 6 is a worst-case estimate and that these data may be dominated by the timing jitter of the 1.6 μm source used in these measurements. Moreover, this frame-to-frame variability is not well-described by a Gaussian fit, which suggests that some systematic (i.e., nonstochastic) variations are inherent in this measurement due to the test electronics and the readout circuitry. Nevertheless, as for the data at the top of Figure 6, a strictly numerical calculation of the standard deviation for the interframe timing distribution provides a jitter value comparable to the estimate obtained from the (somewhat inadequate) Gaussian fit. Ultimately, the timing jitter of the camera contributes to the overall range resolution of the laser radar system in which the sensor is deployed. Given that light travels about 25 cm in 1 ns, overall timing jitter of σ ~ 4 ps represents a round-trip rms uncertainty of about 5 cm. Given the sub-ns timing of the GmAPD cameras, system jitter will often be dominated by jitter inherent in the laser pulse generation as well as the laser pulse width, which is often 1 ns or longer X 32 CAMERA PERFORMANCE This section summarizes performance attributes of the larger format 128 x 32 camera and includes results for DCR, PDE,

7 7 crosstalk, and timing jitter, analogous to the previous section covering the 32 x 32 format camera. Aside from the difference in number of pixels, the 128 x 32 sensors have a 5 μm pixel pitch, which is half of the 1 μm pitch used in the 32 x 32 sensors. Beyond the differences in format and pixel size, many of the camera functions are very similar for both camera types. 128 x 32 Camera DCR and PDE Characterization of the DCR and PDE performance for the 128 x 32 format cameras is analogous to that described in the previous section for the 32 x 32 cameras. In Figure 7, we show the array-level maps for both DCR and PDE, along with corresponding histograms of their performance distribution, for 1.6 μm arrays operated at -2 C. Given that the 5 μm pixel pitch for these arrays is half that of the 32 x 32 arrays, the direction in the array with four times as many pixels has a physical dimension twice as large (i.e., 6.4 mm) as the dimensions of the smaller arrays (i.e., 3.2 mm). Along the length of this longer dimension, the array is more susceptible to pixel performance variation, as is evident in the performance maps in the figure. However, performance distributions are still reasonably narrow, with the observed variation in the long direction of the array arising from a breakdown voltage gradient of ~.6 V. There are about three dozen pixels in the lower right corner of the array that are not operable due to lack of hybridization between the PDA and the ROIC. Essentially all other pixels are operable with DCR of less than 5 khz, 95% of the pixels have DCR < 1 khz, and the average DCR is ~5 khz at an average PDE of 32.5%. The 18 μm diameter active regions of the 128 x 32 FPAs have an inherently lower DCR than the 34 μm diameter active regions of the 32 x 32 FPAs. However, much tighter optical tolerances and consequently higher microlens array coupling losses for the smaller detectors in the 128 x 32 Figure 7 Dark count rate (top) and photon detection efficiency (middle) performance maps, along with corresponding performance histograms (bottom) for 128 x 32 GmAPD camera operating at 1.6 μm and -2 C Number of Pixels Dark Count Rate (khz) Avg DCR = 5.1 khz σ(dcr) = 3.9 khz Avg PDE = 32.5% % 15% 2% 25% 3% 35% 4% 45% 5% 55% Number of Pixels Photon Detection Efficiency Avg PDE = 32.5% σ(pde) = 6.4% Avg DCR = 5. khz

8 Crosstalk Probability per pixel %.8%.6%.4%.2% PDE = 3% PDE = 25% PDE = 2% T = 253 K Cumulative crosstalk 9.8% 7.9% 6.%.% Distance from primary avalanche (in 1 µm) Figure 8 Crosstalk performance for 128x32 GmAPD camera operating at 1.6 μm. FPAs require operation at a higher excess bias voltage to achieve the same PDE of ~3%. This is the dominant factor explaining the higher average DCR value found in Figure 7 for the 128 x 32 FPA relative to the DCR seen in Figure 3 for the 32 x 32 FPA. 128 x 32 Camera Crosstalk The crosstalk performance of the 128 x 32 FPA is qualitatively similar to that demonstrated for the 32 x 32 FPA in the previous section, as is seen by comparing the data presented in Figure 8 with that in Figure 5. However, if device designs were equivalent, the crosstalk of the 128 x 32 array would be higher because of the closer spacing of its active regions (as dictated by halving the pixel pitch). Although the fact that the active regions of the 128 x 32 FPA have a smaller diameter 18 μm, as opposed to the 34 μm diameter of the 32 x 32 FPA active regions would tend to decrease the crosstalk approximately linearly with the diameter for line-of-sight coupling, the solid angle that the 8 active region presents to emitted crosstalk photons scales as 1/R 2 where R is the distance between pixels. This explains the factor of two increase in the nearest neighbor line-ofsight crosstalk probability per pixel (at a distance of 1 on the x-axis) seen in the plot at the bottom of Figure 8 relative to comparable data shown in Figure 5 for the 32 x 32 FPA. However, the crosstalk at further neighbors has been effectively reduced by incorporating a suitable epitaxial filter layer [11] between the substrate and buffer of the GmAPD structure (refer to the schematic at the left side of Figure 1) so that a significant fraction of the crosstalk photons are absorbed before they can be reflected, without attenuating the input signal photons. As in the case of the 32 x 32 camera, the cumulative crosstalk is under 1% for an average array PDE as high as 3%. We note that the addition of a similar filter layer to the 32 x 32 FPA will provide reduced crosstalk relative to that demonstrated in Figure 5, and this design improvement is in progress for these smaller format sensors. 128 x 32 Camera Timing Jitter The timing jitter for the 128 x 32 camera is qualitatively similar to that found for the 32 x 32 camera, although the specific jitter values are higher, primarily because there is more timing variation in the larger format ROIC associated with more challenging clock distribution to four times as many pixel on a physically larger CMOS chip. The intraframe jitter is ~35 ps, and the interframe jitter is in the range of ~5 6 ps. 5. RADIATION TOLERANCE OF REDUCED VOLUME GMAPD DETECTORS A key element of any general roadmap for the development of future generations of GmAPD 3D imaging sensors will include larger format arrays. Scaling to larger formats will require further reduction in pixel pitch and a corresponding decrease in the size of the detector active region in each pixel. Even for arrays of the current generation formats (e.g., 32 x 32 and 128 x 32), there is additional motivation for the implementation of small active regions because many performance parameters improve with the reduction of device size. In particular, dark count rate can be expected to scale with the volume of the active region. Optical crosstalk between pixels should also reduce because the cross section presented by neighboring pixels will shrink. A similar argument applies with regard to radiation tolerance since a smaller device volume will have a lower probability for experiencing radiation damage given a fixed particle fluence. The primary tradeoff encountered with smaller active regions is the greater difficulty in achieving high efficiency optical coupling. In the context of GmAPD arrays, this poses a challenge in the design, alignment, and attachment of microlens arrays consistent with high effective fill factor in focal plane arrays with smaller active regions in each pixel.

9 Dark Count Rate (khz) 1 1 Avg of three 25 μm dia. Avg of three 1 μm dia..1 5% 1% 15% 2% 25% Photon Detection Efficiency Figure 9 Dependence of DCR on PDE at 225 K for devices designed for detection of 1.55 μm photons with active region diameters of 25 μm and 1 μm. DCR vs. PDE Improvement with Reduced Volume We have developed a number of approaches for fabricating GmAPDs with reduced active volumes, and while there are details beyond the scope of this paper, one general approach is to employ the fundamental device design summarized in Figure 1 with appropriate reduction of physical dimensions. To assess improvements in DCR with size reduction, we compared the DCR vs. PDE dependence of devices with different active region diameters. As an example, we present in Figure 9 measurements for DCR vs. PDE for several devices designed for detection of 155 nm photons with active region diameters of 25 μm and 1 μm. These devices were packaged in transistor-style packages with fiber pigtails to facilitate measurement at a temperature of 225 K. Although there is some variation among the devices tested of each size, a comparison of the average performance for three devices demonstrates that the DCR scales by approximately the area ratio (i.e., ~6) at a given value of PDE. The fact that the measured data shows a DCR ratio somewhat less than the area ratio may be explained by the lower coupling efficiency achieved with the smaller 1 μm diameter devices. Radiation Tolerance Results In past radiation testing of discrete GmAPDs with a device design comparable to that discussed in this paper (see Figure 1), it has been found that devices with 25 μm diameter active regions could survive ~1 krad radiation exposure but degraded dramatically for exposures beyond this (e.g., 3 krad and higher). These earlier tests were carried out by the present authors [12] as well as researchers at JPL [13] using protons with energies of ~5 MeV at a fluence of ~ p/cm 2. A five-year Martian mission requires detector survivability for a total radiation dose of 1 krad, where this dose is based on assumptions of a displacement damage dose (DDD) of MeV/g for the InGaAsP 9 absorption layer and a fluence of p/cm 2, which includes a radiation design factor of 2 [14]. The marginal radiation tolerance of the 25 μm diameter detectors relative to the Martian mission requirements was one motivation for investigating the behavior of GmAPDs with reduced active volumes. We have recently performed expanded testing of devices with 4, 8, and 18 μm diameter active regions with exposure to 5 MeV protons at fluences corresponding to radiation levels of 1, 3, 6, 1, and 15 krad. Our results indicate that the tolerance of these devices increases inversely proportionally with the device volume. This result is consistent with the simple argument that radiation tolerance should improve with reduction in the device cross-section. We did not have the ability to perform repeated tests on the same device following subsequently larger doses (i.e., a step stress approach). Therefore, tests were performed by allocating a specific number of devices (typically three) of each active region size to be subjected to a particular exposure level. All devices had an initial high-temperature DCR (Hz) DCR (Hz) 1E+7 1E+6 1E+5 1E+4 1E+3 1E+2 1E+1 1E+ 1E+7 1E+6 1E+5 1E+4 1E+3 1E+2 1E+1 1E+ Post irradiation Pre irradiation 1.6 μm 18 μm dia. 1 Krad T = 233 K Excess Bias (V) Pre irradiation Post irradiation 1.6 μm 18 μm dia. 3 Krad T = 233 K Excess Bias (V) Figure 1 Dependence of dark count rate on excess bias for 18 μm diameter pre-irradiation (blue dashed curves) and post-irradiation (red solid curves) for doses of 1 krad (top) and 3 krad (bottom).

10 DCR (Hz) 1E+7 1E+6 1E+5 1E+4 1E+3 1E+2 1E μm 8 μm dia. 6 Krad T = 233 K Post irradiation Pre irradiation DCR (Hz) 1E+7 1E+6 1E+5 1E+4 1E+3 1E+2 1E μm 4 μm dia. 15 Krad T = 233 K Post irradiation Pre irradiation 1E Excess Bias (V) 1E Excess Bias (V) DCR (Hz) 1E+7 1E+6 1E+5 1E+4 1E+3 1E+2 1E+1 1E+ 164 nm 8 μm dia. 1 Krad T = 233 K Post irradiation Pre irradiation Excess Bias (V) Figure 11 Dependence of dark count rate on excess bias for 8 μm diameter pre-irradiation (blue dashed curves) and post-irradiation (red solid curves) for doses of 6 krad (top) and 1 krad (bottom). burn-in to screen out infant mortalities and were then tested for dark count rate as a function of excess bias voltage over a range from V to ~ V. (An excess bias of 3 V corresponds to a PDE of roughly 3%.) These preirradiation tests confirmed that all devices of a given size exhibited comparable behavior prior to irradiation. Devices were unbiased and at room temperature during irradiation. In Figure 1, we illustrate the behavior of the dark count rate as a function of excess bias for devices with an 18 μm diameter active region. Pre-irradiation measurements are indicated as blue dashed curves, and post-irradiation data are plotted as red solid curves. The top graph of the figure demonstrates that for a dose of 1 krad, two devices exhibit fairly modest increases in DCR of ~5X and 2X, while the third device experienced no increase. The lower graph shows that for two devices exposed to 3 krad, both experience dramatic DCR increases of two to three orders of magnitude. These results suggest that the maximum tolerable dose for detectors of this size is between 1 and 3 krad. Figure 12 Dependence of dark count rate on excess bias for 4 μm diameter GmAPDs pre-irradiation (blue dashed curves) and post-irradiation (red solid curves) for a dose of 15 Krad. Similar data are presented for 8 μm diameter devices in Figure 11. Prior to irradiation, these devices exhibit a DCR on the order of.5 to 1 khz for excess bias values in the range of 2 to 3 V. The top graph of the figure illustrates that following 6 krad irradiation, the devices experience a DCR increase of roughly 5 1X, but absolute values remain well below 1 khz and are consistent with operational requirements. In contrast, the data in the bottom graph signify catastrophic failure of the two devices subjected to a dose of 1 krad (the lack of response for excess bias values up to 3 V indicates device failure). We therefore conclude that the maximum tolerable dose for 8 μm diameter detectors is between 6 and 1 krad. Sustainable Radiation Level (krad) μm Scaling with inverse area 8 μm 5 MeV protons p/cm 2 = catastrophic damage = tolerable degradation 18 μm 25 μm Device diameter (μm) Figure 13 Summary of radiation exposure levels resulting in tolerable degradation and catastrophic damage as a function of GmAPD active region diameter. The dashed curve is a fit to the data assuming that sustainable radiation level scales with the inverse of the area of the device active region. 1

11 Finally, data for three devices with 4 μm diameter active regions are presented in Figure 12. For the maximum exposure of 15 krad used in this study, these devices show less than 1X increase in DCR and maintain a value of ~5 khz or less for an excess bias corresponding to ~3% PDE. underpinning of next generation cameras based on this technology. ACKNOWLEDGMENTS As a summary of the results from this study, we plot in Figure 13 the results for tolerable degradation (i.e., < 1X increase in DCR) and catastrophic damage (>>1X increase in DCR) for the three active area diameters used along with a datapoint for legacy results from 25 μm diameter devices. We then fit these data by assuming that the sustainable radiation level scales with the inverse of the area of the device active region. The data agree well with this assumption and suggest that the 4 μm diameter devices could survive exposures up to about 3 krad before exhibiting catastrophic failure. This extracted dependence provides useful guidance in specifying the maximum device diameter that can be expected to be consistent with mission requirements for a given sustainable radiation level. The practicality of smaller device areas will be determined by the feasibility of designing and assembling appropriate microlens arrays for devices with the desired dimensions. 6. SUMMARY OF FUTURE DIRECTIONS As motivation for the work on reduced volume GmAPD active regions, we alluded to our roadmap for future developments, which includes larger format arrays with smaller pitch and reduced active regions. The performance improvements afforded by reduced active regions can be leveraged in a number of different ways. The reduction of DCR demonstrated in Figure 9 may be intrinsically valuable for applications requiring sub-khz DCR performance. However, in other cases, higher DCR levels may be acceptable, and the fact that they could be achieved at higher temperature poses the prospect for higher temperature operation (e.g., room temperature) of InP-based GmAPD arrays with greatly reduced power dissipation given the relaxation of cooling requirements. Alternatively, other applications may require higher PDE, in which case reductions in DCR can be traded for higher PDE operation. In the push towards smaller pitch arrays, crosstalk can become a critical limitation, and the reduction of GmAPD active region will allow smaller pitch designs to become viable. Interestingly, the optical challenge of coupling to smaller active regions actually becomes less severe with smaller pitch formats if the ratio of pixel dimension to active region dimension decreases. Other challenges will also become more acute as pixel pitch reduces, such as the design of desired ROIC pixel functionality with smaller pixel areas. Notwithstanding these future challenges, the results described in this paper demonstrate a maturing of first generation GmAPD-based cameras with 32 x 32 and 128 x 32 formats for laser radar 3D imaging as well as the progress of development activities that will provide the 11 We wish to acknowledge valuable discussions with Bill Farr at NASA Jet Propulsion Laboratory (JPL) and partial support from JPL for development work on the reduced volume detectors under JPL subcontract number REFERENCES [1] M. A. Albota, B. F. Aull, D. G. Fouche, et al., Threedimensional imaging laser radars with Geiger-mode avalanche photodiode arrays, MIT Lincoln Laboratory Journal, vol. 13, no. 2, p (22). [2] B. F. Aull, A. H. Loomis, D. J. Young, et al., Threedimensional imaging with arrays of Geiger-mode avalanche photodiodes, Proc. of the SPIE 5353, p (24). [3] George Gray, High Altitude Lidar Operations Experiment (HALOE) Part 1, System Design and Operation, Proc. of Military Sensing Symposium, Active Electro-Optic Systems, paper AH3, San Diego, CA, Sep 211. [4] Robert Knowlton, Airborne Ladar Imaging Research Testbed, MIT Lincoln Laboratory Tech Notes, 211 ( TechNote_ALIRT.pdf) [5] K. A. McIntosh, J. P. Donnelly, D. C. Oakley, A. Napoleone, S. D. Calawa, L. J. Mahoney, K. M. Molvar, E. K. Duerr, S. H. Groves, and D. C. Shaver, InGaAsP/InP avalanche photodiodes for photon counting at 1.6 μm, Appl. Phys. Lett. 81, p (22). [6] J.P. Donnelly, E.K. Duerr, K.A. McIntosh, E.A. Dauler, D.C. Oakley, S.H. Groves, C.J. Vineis, L.J. Mahoney, K.M. Molvar, P.I. Hopman, K.E. Jensen, G.M. Smith, and S. Verghese, Design considerations for 1.6-μm InGaAsP-InP Geiger-mode avalanche photodiodes, IEEE J. Quantum Electron. 42, p (26). [7] S. Verghese, J. P. Donnelly, E. K. Duerr, K. A. McIntosh, D. C. Chapman, C. J. Vineis, G. M. Smith, J. E. Funk, K. E. Jensen, P. I. Hopman, D. C. Shaver, B. F. Aull, J. C. Aversa, J. P. Frechette, J. B. Glettler, Z. L. Liau, J. M. Mahan, L. J. Mahoney, K. M. Molvar, F. J. O Donnell, D. C. Oakley, E. J. Ouellette, M. J. Renzi, and B. M. Tyrrell,, Arrays of InP-based avalanche photodiodes for photon counting, IEEE J. Sel. Topics in Quantum Electron. 13, p (27). [8] M.A. Itzler, M. Entwistle, M. Owens, K. Patel, X. Jiang, K. Slomkowski, S. Rangwala, P.F. Zalud, T. Senko, J. Tower, J. Ferraro, Geiger-mode avalanche photodiode focal plane arrays for three-dimensional imaging LADAR, Proc. of the SPIE 788, 788C (21). [9] M. A. Itzler, M. Entwistle, M. Owens, K. Patel, X. Jiang, K. Slomkowski, S. Rangwala, P. F. Zalud, T. Senko, J. Tower, J. Ferraro, Comparison of 32 x 128 and 32 x 32

12 Geiger-mode APD FPAs for single photon 3D LADAR imaging, Proc. of the SPIE 833, 833G (211). [1] see [11] Richard D. Younger, K. Alex McIntosh, Joseph W. Chludzinski, Douglas C. Oakley, Leonard J. Mahoney, Joseph E. Funk, Joseph P. Donnelly, and S. Verghese, Crosstalk Analysis of Integrated Geiger-mode Avalanche Photodiode Focal Plane Arrays, Proc. of the SPIE 732, 732Q (29). [12] Mark A. Itzler and Xudong Jiang, unpublished. [13] Richard D. Harris, William H. Farr, and Heidi N. Becker, Degradation of InP-based Geiger-mode avalanche photodiodes due to proton irradiation, J. Modern Optics 58, p (211). [14] Bill Farr, NASA Jet Propulsion Laboratory, private communication BIOGRAPHY Mark Itzler is CEO and CTO of Princeton Lightwave Inc. He received a Ph.D. in Physics from the University of Pennsylvania and pursued post-doctoral research at Harvard University from 1992 to His current technical focus is on semiconductor photodetectors, particularly in the areas of single photon counting and avalanche photodiodes. In 1996, he joined Epitaxx Inc., where he held successive management roles and led development programs for telecommunications photodetectors. Following Epitaxx s acquisition by JDSU, Dr. Itzler became CTO and VP of R&D of the Epitaxx Division. In 23, he joined Princeton Lightwave as CTO, and he became the company s CEO in 212. Dr. Itzler is an IEEE Fellow. Mark Entwistle is the Director of Product Development at Princeton Lightwave Inc. He holds a B.S. degree in electrical engineering and has over 27 years of experience in a wide range of engineering disciplines. From 1986 to 1996 he worked for a military contractor in design and test engineering for airborne and ground-based instrumentation systems. In 1996 he joined Epitaxx Optoelectronics Inc., where he was Manager of Test Engineering. Since joining Princeton Lightwave in 23, Mr. Entwistle has specialized in developing electronic sub-systems and integrating sensor-level devices into high-performance camera systems. Xudong Jiang is a Principal Scientist at Princeton Lightwave Inc. He received a Ph.D. in solid state physics in 1995 from Peking University. He is involved in the design, analysis and characterization of photodetectors, particularly InP-based single photon avalanche photodiodes. Prior to joining Princeton Lightwave, he was a Distinguished Member of Technical Staff and Manager of Laser Engineering and Operations at Multiplex, Inc., working on the development and manufacturing of 98 nm pump lasers and electroabsorption modulated lasers (EMLs). Dr. Jiang is a member of the IEEE. Mark Owens is Director of Packaging and Process Engineering at Princeton Lightwave Inc. He received his B.S. degree in Physics from SUNY at Stony Brook in He worked at Gage Lab Corp. as a Metrology Engineer before joining the Epitaxx Division of JDSU in 1999, where he served as a Quality Engineer. Since joining Princeton Lightwave in 25, he has functioned in a succession of development and programmatic roles, with current responsibilities spanning process development, package integration and reliability of laser and photodetector products. He is a US Air Force veteran and a member of IMAPS and ASQ. Krystyna Slomkowski is a Senior Staff Engineer at Princeton Lightwave Inc. She received a B.A. in Microbiology from Douglass College/Rutgers University in Since joining Princeton Lightwave in 24, Ms. Slowkowski has focused on outsourced foundry management for the development and production of III-V compound semiconductor devices. Prior to joining Princeton Lightwave, she was the Manager of Process Development at the Epitaxx Division of JDSU. Sabbir Rangwala is President and COO at Princeton Lightwave Inc. He received a Ph.D. in Mechanical Engineering from the University of California at Berkeley, after which he joined Bell Laboratories to develop high power laser diodes for undersea operation. Following management positions at AT&T and consulting work with Deloitte Consulting, he returned to the photonics industry when he joined the Epitaxx Division of JDSU to run product development in the Active Components group. In 23, he joined Princeton Lightwave as VP of Product Development and Operations and became the company s President and COO in