Characterization of sige-detector arrays
for visible-NIR imaging sensor applications
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Citation
Ashok K. Sood ; Robert A. Richwine ; Adam W. Sood ; Yash R. Puri ;
Nicole DiLello ; Judy L. Hoyt ; Tayo I. Akinwande ; Nibir Dhar ;
Raymond S. Balcerak ; Thomas G. Bramhall; Characterization of
SiGe-detector arrays for visible-NIR imaging sensor applications.
Proc. SPIE 8012, Infrared Technology and Applications XXXVII,
801240 (May 20, 2011). Copyright © 2011 SPIE Digital Library
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http://dx.doi.org/10.1117/12.889205
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SPIE
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Final published version
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http://hdl.handle.net/1721.1/73942
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policy and may be subject to US copyright law. Please refer to the
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Characterization of SiGe-Detector Arrays for Visible-NIR Imaging Sensor
Applications
Ashok K. Sood, Robert A. Richwine, Adam W. Sood and Yash R. Puri
Magnolia Optical Technologies Inc., 52-B Cummings Park, Woburn, MA 01801
Nicole DiLello, Judy L. Hoyt and Tayo I. Akinwande
Microsystems Technology Laboratories, MIT, Cambridge, MA 02139
Nibir Dhar
DARPA/MTO, 3701, North Fairfax Drive, Arlington, VA 22203
Raymond S. Balcerak
Ray S. Balcerak, LLC, VA 22203
Thomas G. Bramhall
DARPA Programs Office, US Army, AMSRD, Redstone Arsenal, AL 35898
ABSTRACT
SiGe based focal plane arrays offer a low cost alternative for developing visible- near-infrared focal plane arrays that will cover the spectral band from 0.4 to 1.6 microns. The attractive features of SiGe based foal plane arrays take advantage of silicon based technology that promises small feature size, low dark current and compatibility with the low power silicon CMOS circuits for signal processing. This paper discusses performance characteristics for the SiGe based VIS-NIR Sensors for a variety of defense and commercial applications using small unit cell size and compare performance with InGaAs, InSb, and HgCdTe IRFPA’s. We present results on the approach and device design for reducing the dark current in SiGe detector arrays. The electrical and optical properties of SiGe arrays at room temperature are discussed. We also discuss future integration path for SiGe devices with Si-MEMS
Bolometers.
Performance Modeling for SiGe Vis-NIR Sensor
Incendiary events provide large amounts of energy in the SWIR spectral region. Spectrally, muzzle flashes approximate a blackbody spectrum of 800K to 1200K sources, with additional high radiance peaks that can occur at the emission lines for water and for various incendiary chemicals that comprise the propellant. Consequently, smaller pixel sizes can be employed without significantly compromising performance (detection of flash events). Flash durations are in the 1-3 millisecond range, so the full frame time based integration time of 33 msec used for night imaging cannot be used. Since flash events provide such high SNR’s, the spectral bands can be segmented to provide additional information used to identify the threat. Two and three-color outputs can be used to approximate the flash temperature and discriminate live fire from non-threats such as solar or lunar glints. Spectral tags (chemical additives) used for ID of friendly fire can be identified with this two to three-color sensor.
Muzzle flashes consist of an intermediate flash and a brighter secondary flash (unless suppressed) as shown in figure 1.The secondary flash is shown in red and can be approximated as a 1200-1400K blackbody in the SWIR band[1-5]. The primary flash in blue is approximated as an 800-1000K source. For the flash simulations (SNR plots), a conservative 1000K blackbody source is used.
Infrared Technology and Applications XXXVII, edited by Bjørn F. Andresen, Gabor F. Fulop, Paul R. Norton, Proc. of SPIE Vol. 8012, 801240 · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.889205
Figure 1: Plot of Flash Spectra as a function of spectral band
The dark current numbers used in the analysis were taken from measurements of Ge-on-Si photodiodes fabricated for high-speed on-chip integrated photonic applications [6]. These diodes were not optimized for imaging applications. The test diodes have sizes in the range of 10x10 to 200 x 200 microns. As the size of the detector is reduced below 10 microns further improvement in the detector dark current is expected, and will improve the sensor performance. These preliminary results show that with SiGe IR Sensor with f/2 optical system, one can detect targets of interest from over 1 km distance. This can have significant applications for a variety of defense missions. The detector sizes investigated here are for SiGe camera with (3, 5 and 7 microns) pixels. These preliminary simulations were performed to show the capability of megapixel SiGe NIR cameras.
Figure 2: Signal to noise ratio (SNR) as a function of dark current for SiGe IRFPA for three unit cells
size ranging from 3, 5 and 7 micron for temperature region of interest
As part of the simulations, detector dark currents were varied from 20 to 0.01 nano-Amps (nA). The spectral band used was 0.4-1.6 microns and the average quantum efficiency in the band was 60%. An optical diameter of 2
inches, a focal length of 5 inches, and a 3 micron pixel provide a detector IFOV linear distance of 1.0” at 1 km and, assuming a 2000x2000 format array, an IFOV of 2.7 degrees or 155 feet at 1 km.
Figure 2 summarizes multiple model runs using these parameters. The source radiance simulated is an extended blackbody from 900 K to 1500K which approximates some of the targets of interest. The red plots are for the 3 micron pixel, the green for the 5 micron pixel and the blue for the 7 micron pixel. Muzzle flash (primary and secondary) can be approximated as extended blackbody sources in the 900-1500K.
Figure 3: NEDR as a function of Dark Current for pixel size of 20, 10 and 8.5 microns for VIS-NIR spectral band
(0.4 to 1.6)
Dark currents in SiGe detectors can be reduced by reducing the pixel size since dark currents track with the volume of the pixel. Reductions in size are advantageous for resolution; however, since the nightglow level is so low, a large pixel size or at least a large collection area is required. If a high density (high fill factor) is achieved in these SiGe arrays, electronic pixel binning of small pixels can be used to achieve a reasonable sensitivity (NEI). If not, a microlens can be used to focus 20-25 micron collection area onto a smaller pixel.
These two cases are investigated here using NEI (noise equivalent irradiance) and NEDR (noise equivalent differential reflectance) as performance metrics. NEI is in units of photons/sec-cm2. NEDR is defined as the difference in target reflectance that provides a signal-to-noise of one under “standard” nightglow irradiance. In figure 3, the three plots are for a 20, 10 and 8.5 micron pixel SiGe detector.
The preliminary analysis shows that a SiGe Sensor with room temperature operation has the ability to detect targets at 1Km and other plume detection at longer ranges. The results clearly demonstrate that SiGe sensor will have a variety of applications for Army missions. We are evaluating variety of ways to reduce the dark current, while increasing the signal collection from each of the pixel. One of the approaches that have been used very effectively in HgCdTe and InSb is the use of smaller junction and lateral collection[1-5]. We have carried out preliminary simulation for the SiGe detector array. Figure 4 presents the plot of SNR vs. dark current for f/2 optics for SiGe sensor with pixel sizes (collection areas) of 15, 30 and 40 um showing improvement in extended target SNR with collection area. Figure 5 presents the analysis of the signal to noise ratio (SNR) as a function of dark current for a variety of target temperatures. We are continuing to refine the model to include smaller detectors i.e. 5 micron or less unit cell sizes. This analysis shows the importance of extending the cutoff wavelength to capture more 900K energy. For hotter sources, the wavelength requirement can be relaxed.
Figure 4: Signal to noise ratio (SNR) as a function of dark current for SiGe IRFPA
Figure 5: SNR vs. dark current for various target temperatures (radiances) for SiGe detector array The analysis shows that a SiGe Sensor with room temperature operation has the ability to detect targets at 1Km and beyond and other plume detection at longer ranges. The results clearly demonstrate that SiGe sensor will have a variety of applications for a variety of defense applications.
We are evaluating variety of ways to reduce the dark current, while increasing the signal collection from each of the pixel. One of the approaches that have been used very effectively in HgCdTe and InSb is the use of smaller junction and lateral collection. We have carried out preliminary simulation for the SiGe detector array. We are continuing to refine the model to include smaller detectors such as 5 micron unit cell sizes.
Measured Characteristics of Ge-on-Si Photodiodes
To efficiently access the VIS-NIR band for various applications, high-Ge-content Si1-xGex (x > 0.5) layers are required. To reduce power consumption and improve the signal-to-noise ratio, the diodes must have a low leakage current in reverse bias. Previous studies have demonstrated Ge-on-Si photodiodes with epitaxial material grown by ultra-high vacuum chemical vapor deposition [7] and by low-pressure chemical vapor deposition [6]. For imaging applications, the most important parameter of these devices is a low dark current.
In previous studies, diodes have been made by selectively growing Ge in oxide windows on a Si substrate [8, 9, 10, 11]. The selective-area growth serves to reduce the threading dislocations arising from the lattice mismatch between the Si and Ge, yielding a higher quality Ge film and a lower leakage current. High-quality passivation of the Ge surface is another key aspect of fabricating detectors with low dark current. Passivation of Ge MOSFETs has been studied extensively and materials have been found to yield low Dit and good electrostatic performance [12, 13]. This work studies the effect of diode passivation on dark current. We investigate the leakage current of germanium photodiodes grown by low-pressure chemical vapor deposition (LPCVD) using an Applied Materials epitaxial reactor. This study specifically examines the effect of a post-metallization nitrogen anneal on the dark current.
Figure 6: A cross-sectional schematic of a Ge-on-Si photodiode. The Ge is 2 μm thick and passivated
with a low temperature oxide.
To simplify the processing in this study, a blanket epitaxial film of germanium was used. 2 μm of germanium was grown epitaxially on a p+ Si substrate. The first step of the growth was done at a low temperature (365°C) to ensure a smooth surface morphology. This seed layer was doped p-type to ~2 x 1018 cm-3. The next step of the growth was performed at a higher temperature (750°C) to achieve a higher growth rate while maintaining a smooth Ge film. The wafers then received an in-situ cyclic anneal between 800°C and 450°C to reduce the threading dislocation density. The resulting Ge film used in a similar process had a threading dislocation density of ~2 x 107 cm-3 [14]. The wafers were subsequently implanted with phosphorus and activated at 550°C to create a vertical pin junction. Next, the wafers were passivated with a low temperature oxide (LTO) and contacted with metal. Following metallization, the wafers received an optional anneal in nitrogen for 45 minutes at 400°C. A cross-sectional schematic of the diode can be seen in figure 6.
Figure 7 shows the current-voltage characteristics for a 10 x 10 μm square device. The dark current is reduced from 10 μA for diodes without a post-metallization anneal to 8 nA for a sample with a 400°C anneal. To further
investigate this effect, Ge-on-Si capacitors were made with LTO as the dielectric. They were annealed at the same temperature as the diodes. The anneal was observed to shift the flat band voltage to higher voltages, indicating that the anneal changes the fixed charge at the LTO/Ge interface. Without an anneal, the Ge surface is depleted, causing a high surface recombination velocity. After an anneal at 400°C, the surface is accumulated with holes, which reduces the recombination velocity and decreases the dark current. This effect is illustrated in Figure 8.
Figure 7: Current-voltage characteristics for 10 x 10 μm2 devices with and without a 400°C
post-metallization anneal in N2 for 45 minutes. At -1 V, the dark current is reduced by ~1000X with a 400°C
anneal.
Figure 8: Cross-sectional schematic diagrams of the diode before and after anneal. On the left, the area
under the LTO is depleted. On the right, the area under the LTO is accumulated with holes.
Photoresponse measurements were also taken on larger devices. Figure 9 shows the photoresponse of a 20 x 20 μm2 device at three different bias points. The device was annealed at 400°C. The measurement is done with a lensed
fiber with a 3 μm2 spot size. The diode is large enough such that all of the light is coupled into the device. At 1550 nm, the diode has a responsivity of ~0.3 A/W. The lack of voltage dependence indicates that the device is fully depleted at 0 V and has a high internal electric field due to the steep doping profiles.
Figure 9: The responsivity of a 20 x 20 μm2 photodiode. The responsivity at 1550 nm is ~0.3 A/W.
In summary, we have fabricated germanium diodes with a dark current of 8 nA at -1 V. These detectors use low temperature oxide as the surface passivation material. The inclusion of a post-metallization N2 anneal reduces the dark current by 1000X for small area devices.
Integration of SiGe VIS-NIR Arrays with Silicon Microbolometers
As the technology for low dark current SiGe detector arrays is developed, further effort will be needed for
fabricating high density large format SiGe VIS-NIR focal plane arrays. The front side illuminated process has been developed by DRS for other IR sensor application such as HgCdTe, Si-MEMS based Microbolometers. The process flow shown in figure 13 provides various steps for the fabrication Process.
The fabrication and the integration process shown in figure 10 is shown with Silicon on Insulator (SOI) wafers. The SOI wafers have a thin, CMOS quality silicon layer on top, and a buried oxide layer. Detector p+ base layer and the intrinsic (i) layer are deposited by Si-Ge epitaxy (step1). Vias into Si-Ge are etched by RIE and the buried oxide layer provides the etch stop. Next we deposit an oxide layer to provide dielectric isolation for the via structure. Doped polysilicon deposition completes the top layer (n+) of the photodiode structure (p-i-n) and provides a conductive path to the opposite site of the detector (step 3). A silicon handle is bonded to the detector wafer (step 4) to provide support during the etching and thinning (step 5). The buried oxide layer provides an excellent stop (selectivity > 1000:1) when using EDP for silicon etch. Next, vias are opened though the oxide to access the top detector layer (n+) and the p+ base layer. This step is followed by via metal fill.
The detector wafer will be ready for bonding to CMOS. Direct bond interconnect brings to bear state of the art 3-D interconnect technology: wafer scale, low temperature process, highest density of electrical connections and ultra- small pitch, allowing the pixel scale reductions. Following bonding, the handle wafer is removed and pixel isolation etch completes the detector array (step 9).
Similarly, we are also exploring integrating SiGe VIS-NIR detectors with SI=MEMS device. The potential
advantage of integrating SiGe devices with Silicon based bolometers is the use of the silicon base process and hence the integration can be accomplished on large silicon wafers. Magnolia is evaluating the integration process using both VOx based bolometers and amorphous silicon based bolometer, as shown in figure 11.
Figure 10: Process Flow for fabrication of VIS-NIR Front side illuminated SiGe FPA & CMOS ROIC
Figure 11: Si-MEMS bolometer architectures for small pixel designs. (SPIE Papers by DRS and L3)
SUMMARY
SiGe based Vis-IR Focal Plane Arrays offer a low cost alternative for developing near IR sensors that will not require any cooling and can operate in the visible and NIR bands. The attractive features of SiGe based IRFPA’s will take advantage of Silicon based technology, that promises small feature size and compatibility with the low power silicon CMOS circuits for signal processing. A feasibility study of an infrared sensor based on Ge-on-Si photodiodes indicates promise for IR focal plane arrays with a spectral cutoff wavelength of 0.4 to 1.6 microns. Selective epitaxial growth, reduction of the diode size, and optimization of the diode structure to reduce tunneling leakage are expected to further reduce the dark current in these devices. We have also discussed potential
application of integrating SiGe-VIS-NIR arrays with Si-MEMS devices. Magnolia effort continues towards reducing the dark current further by another order of magnitude.
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