James E. Robinson

Bonding for 3-D Integration of Heterogeneous Technologies and Materials

Modern electronic applications demand more and more complex, multifunctional microsystems with performance characteristics which can only be achieved by using best-of-breed materials and device technologies. Three-dimensional (3-D) integration of separate, individually complete device layers provides a way to build complex heterogeneous microsystems without compromising the system performance and fabrication yield. In the 3-D integration approach, each device layer is fabricated separately using optimized materials and processes. The layers are bonded and interconnected through area array vertical interconnects with lengths on the order of microns. This paper will review bonding techniques for high density area array 3-D integration of integrated circuits, focusing on techniques suitable for die-to-die and die-to-wafer bonding configurations.

Gated IR Imaging with 128 × 128 HgCdTe Electron Avalanche Photodiode FPA

The next generation of IR sensor systems will include active imaging capabilities. One example of such a system is a gated-active/passive system. The gated-active/passive system promises long-range target detection and identification. A detector that is capable of both active and passive modes of operation opens up the possibility of a self-aligned system that uses a single focal plane. The detector would need to be sensitive in the 3-5 μm band for passive mode operation. In the active mode, the detector would need to be sensitive in eye-safe range, e.g. 1.55 μm, and have internal gain to achieve the required system sensitivity. The MWIR HgCdTe electron injection avalanche photodiode (e-APD) not only provides state-of-the-art 3-5 μm spectral sensitivity, but also high avalanche photodiode gain without minimal excess noise. Gains of greater than 1000 have been measured in MWIR e-APDs with a gain independent excess noise factor of 1.3. This paper reports the application of the mid-wave HgCdTe e-APD for near-IR gated-active/passive imaging. Specifically a 128x128 FPA composed of 40 μm pitch, 4.2 μm to 5 μm cutoff, APD detectors with a custom readout integrated circuit was designed, fabricated, and tested. Median gains as high as 946 at 11 V bias with noise equivalent inputs as low as 0.4 photon were measured at 80 K. A gated imaging demonstration system was designed and built using commercially available parts. High resolution gated imagery out to 9 km was obtained with this system that demonstrated predicted MTF, precision gating, and sub 10 photon sensitivity.

SWIR hyperspectral detection with integrated HgCdTe detector and tunable MEMS filter

Hyperspectral imaging in the infrared bands is traditionally performed using a broad spectral response focal plane array, integrated in a grating or a Fourier transform spectrometer. This paper describes an approach for miniaturizing a hyperspectral detection system on a chip by integrating a Micro-Electro-Mechanical-System (MEMS) based tunable Fabry Perot (FP) filter directly on a photodetector. A readout integrated circuit (ROIC) serves to both integrate the detector signal as well as to electrically tune the filter across the wavelength band. We report the first such demonstration of a tunable MEMS filter monolithically integrated on a HgCdTe detector. The filter structures, designed for operation in the 1.6-2.5 μm wavelength band, were fabricated directly on HgCdTe detectors, both in photoconducting and high density vertically integrated photodiode (HDVIP) detectors. The HDVIP detectors have an architecture that permits operation in the standard photodiode mode at low bias voltages (≤0.5V) or in the electron avalanche photodiode (EAPD) mode with gain at bias voltages of ~20V. In the APD mode gain values of 100 may be achieved at 20 V at 200 K. The FP filter consists of distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity and a silicon nitride spacer membrane for support. Mirror stacks fabricated on silicon, identical to the structures that will form the optical cavity, have been characterized to determine the optimum filter characteristics. The measured full width at half maximum (FWHM) was 34 nm at the center wavelength of 1780 nm with an extinction ratio of 36.6. Fully integrated filters on HgCdTe photoconductors with a center wavelength of approximately 1950 nm give a FWHM of approximately 100 nm, and a peak responsivity of approximately 8 × 104 V/W. Initial results for the filters on HDVIP detectors exhibit FWHM of 140 nm.

HgCdTe and silicon detectors and FPAs for remote sensing applications

Photon detectors and focal plane arrays (FPAs) are fabricated from HgCdTe and silicon in many varieties. With appropriate choices for bandgap in HgCdTe, detector architecture, dopants, and operating temperature, HgCdTe and silicon can cover the spectral range from ultraviolet to the very-long-wavelength infrared (VLWIR), exhibit high internal gain to allow photon counting over this broad spectral range, and can be made in large array formats for imaging. DRS makes HgCdTe and silicon detectors and FPAs with unique architectures for a variety of applications. Detector characteristics of High Density Vertically Integrated Photodiode (HDVIP) HdCdTe detectors as well as Focal Plane Arrays (FPAs) are presented in this paper. MWIR[λc(78 K) = 5 μm] HDVIP detectors RoA performance was measured to within a factor or two or three of theoretical. In addition, 256 x 256 detector arrays were fabricated. Initial measurements had seven out of ten FPAs having operabilities greater than 99.45% with the best 256 x 256 array having only two inoperable pixels. LWIR [λc(78K)~10 μm] 640 X 480 arrays and a variety of single color linear arrays have also been fabricated. In addition, two-color arrays have been fabricated. DRS has explored HgCdTe avalanche photo diodes (APDs) in the λc = 2.2 μm to 5 μm range. The λc = 5 μm APDs have greater than 200 DC gain values at 8 Volts bias. Large-format to 10242 Arsenic-doped (Si:As, λc ~ 28 μm), Blocked-Impurity-Band (BIB) detectors have been developed for a variety of pixel formats and have been optimized for low, moderate, and high infrared backgrounds. Antimony-doped silicon (Si:Sb) BIB arrays having response to wavelengths > 40 μm have also been demonstrated. Avalanche processes in Si:As at low temperatures (~ 8 K) have led to two unique solid-state photon-counting detectors adapted to infrared and visible wavelengths. The infrared device is the solid-state photomultiplier (SSPM). A related device optimized for the visible spectral region is the visible-light photon counter (VLPC). The VLPC is a nearly ideal device for detection of small bunches of photons with excellent time resolution. Finally, DRS makes imaging arrays of pin-diodes utilizing the intrinsic silicon photoresponse to provide high performance over the 0.4-1.0 μm spectral range operating near room temperature. pin-diode arrays are particularly attractive as an alternative to charge-coupled devices (CCDs) for space applications where radiation hardening is needed. In addition, wire grid micropolarizers have been demonstrated and two color doped silicon detectors using diffractive microlenses are being developed. Precision alignment of sensor chips with respect to a base mounting plate has been demonstrated to be within 2 μm. A similar technique is also utilized to align single large detectors for sounder applications in focal plane arrays (FPAs). FPAs for space applications with the associated cold and warm electronics and packaging/cables have been fabricated.

HDVIP for low-background-flux and high-operating-temperature applications

An overview of the DRS HDVIP architecture for realization of large-area infrared focal plane arrays (IRFPAs) is given. Improvements needed to meet more stringent application requirements are discussed and modeled. Both theoretical and experimental data are presented.

Au- and Cu-doped HgCdTe HDVIP detectors

Detector characteristics of Au- and Cu-doped High Density Vertically Integrated Photodiode (HDVIP) detectors are presented in this paper. Individual photodiodes in test bars were examined by measuring I-V curves under dark and illuminated conditions at high bias values. Noise as a function of frequency has been measured on Au- and Cu-doped MWIR [λc(78 K) = 5 μ] HDVIP HgCdTe diodes at several temperatures under dark and illuminated conditions. No excess currents are observed above the photocurrents for reverse bias values out to 500 mV. Both Au- and Cu-doped detectors measured at 85 K, exhibit gain values between 40 and 50 at 8 V reverse bias. Gain values fell in this same range even when the flux incident on each type of detector was varied. The excess noise factor for the Cu-doped detectors ranged from 1.35 to 1.69 depending on the incident flux. Variation is probably due to measurement error. The noise at 8 V reverse bias is white for the Cu-doped detectors. The Au-doped detectors exhibited 1/f noise at 8 V reverse bias. At higher frequencies where the noise spectrum was quasi-white, the excess noise factor for the Au-doped detector was in the 1.0 to 1.5 range.

Performance and modeling of the MWIR HgCdTe electron avalanche photodiode

The operation of the mid-wave infrared (MWIR) HgCdTe cylindrical electron injection avalanche photodiode (e-APD) is described. The measured gain and excess noise factor are related to the to the collection region fill factor. A 2D diffusion model calculates the time dependent response and steady state pixel point spread function for cylindrical diodes, and predicts bandwidths near 1 GHz for small geometries. A 2 μm diameter spot scan system was developed for point spread function and crosstalk measurements at 80 K. An electron diffusion length of 13.4 μm was extracted from spot scan data. Bandwidth data are shown that indicate bandwidths in excess of 300 MHz for small unit cells geometries. Dark current data, at high gain levels, indicate an effective gain normalized dark density count as low as 1000 counts per μs per cm2 at an APD gain of 444. A junction doping profile was determined from capacitance-voltage data. Spectral response data shows a gain independent characteristic.

MEMS based tunable infrared sensors

A low temperature MEMS process integrated with an infrared detector technology has been developed. The integrated microsystem is capable of electrically selecting narrow wavelength bands in the range from 1.6 to 2.5 μm within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The integrated fabrication process is compatible with two-dimensional infrared focal plane array technology. The demonstration prototypes consist of both HgCdTe SWIR photoconductive as well as high density vertically integrated photodiode (HDVIP®) detectors, two distributed Bragg mirrors formed of Ge-SiO-Ge, an air-gap optical cavity, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors indicates a wide tuning range and high percentage transmission. Tuning is achieved with a voltage of only 7.5 V, and the FWHM ranged from 95-105 nm over a tuning range of 2.2 μm to 1.85 μm. The same MEMS filters, though unreleased, and with the sacrificial layer within the optical cavity, have been fabricated on planarised SWIR HDVIP® photodiodes with FWHM of less than 60 nm centred at a wavelength of approximately 1.8 μm. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to optimize the filter geometry in terms of fill factor, mirror displacement versus applied voltage, and membrane bowing.

Visible response of λc=2.5µm HgCdTe HDVIP detectors

Cu-doped HDVIP detectors with different cut-off wavelengths are routinely manufactured. The DRS HDVIP detector technology is a front-side-illuminated detector technology. There is no substrate to absorb the visible photons as in backside-illuminated detectors and these detectors should be well suited to respond to visible light. However, HDVIP detectors are passivated using CdTe that absorbs the visible light photons. CdTe strongly absorbs photons of wavelength shorter than about 800 nm. Detectors with varying thickness of CdTe passivation layers were fabricated to investigate the visible response of the 2.5-μm-cutoff detectors. A model was developed to predict the quantum efficiency of the detectors in the near infrared and visible wavelength regions as a function of CdTe thickness. Individual photodiodes (λc = 2.5 μm) in test bars were examined. Measurements of the quantum efficiency as a function of wavelength region will be presented and compared to the model predictions.

Visible response of λ c =2.5´m HgCdTe HDVIP detectors

Cu-doped HDVIP detectors with different cut-off wavelengths are routinely manufactured. The DRS HDVIP detector technology is a front-side-illuminated detector technology. There is no substrate to absorb the visible photons as in backside-illuminated detectors and these detectors should be well suited to respond to visible light. However, HDVIP detectors are passivated using CdTe that absorbs the visible light photons. CdTe strongly absorbs photons of wavelength shorter than about 800 nm. Detectors with varying thickness of CdTe passivation layers were fabricated to investigate the visible response of the 2.5-μm-cutoff detectors. A model was developed to predict the quantum efficiency of the detectors in the near infrared and visible wavelength regions as a function of CdTe thickness. Individual photodiodes (λc = 2.5 μm) in test bars were examined. Measurements of the quantum efficiency as a function of wavelength region will be presented and compared to the model predictions.