Ruijun Ding

128 × 128 long-wavelength/mid-wavelength two-color HgCdTe infrared focal plane array detector with ultralow spectral cross talk

High temporal and spatial coherent simultaneous long-wavelength/mid-wavelength (LW/MW) two-color focal plane array (FPA) infrared detection is the cutting-edge technique for third-generation infrared remote sensing. In this Letter, HgCdTe LW/MW two-color infrared detectors were designed and fabricated. The top long-wavelength and bottom mid-wavelength infrared planar photodiodes were processed by selective B(+)-implantation after etching the long-wavelength epilayer into a curvature and exposing the mid-wavelength layers for the implantation of the n region of the MW photodiode by a micro-mesa array technique. A 128×128  MW/LW HgCdTe infrared FPA detector is fabricated photo-lithographically by simultaneous nonplanar B(+)-implantation of the LW and MW photodiodes, passivation and metallization of the sidewalls, mesa isolation, and flip-chip hybridization with a read-out integrated circuit. The inner mechanisms for suppressing the cross talk and improving photoresponse have been carried out by combining experimental work with numerical simulations.

A study of MBE growth and thermal annealing of p-type long wavelength HgCdTe

The results of MBE growth and annealing of p-type HgCdTe are described in the paper. It is found that the surface morphology is sensitive to the growth temperature. HgCdTe epilayers showed excellent lateral uniformity in x-values as well as in thickness, relative deviations for x and thickness over a 2 in. wafer were found to be 0.18% and 2.19%, respectively. The hole concentrations in a range of (1-2) x 10 16 cm -3 with hole mobilities higher than 600 cm 2 V -1 s -1 were obtained by p-annealing. MBE grown p-HgCdTe epilayers were successfully incorporated into 32 x 32 focal plane arrays detectors.

Numerical simulation of refractive-microlensed HgCdTe infrared focal plane arrays operating in optical systems

The optoelectronic performance of the mid-wavelength HgCdTe infrared focal plane array (IRFPA) with refractive microlenses integrated on its CdZnTe substrate has been numerically simulated. A reduced light-distribution model based on scalar Kirchhoff diffraction theory was adopted to reveal the true behavior of IRFPAs operating in an optical system under imaging conditions. The pixel crosstalk obtained and the energy-gathering characteristics demonstrated that the microlenses can delay the rise in crosstalk when the image point shifts toward pixel boundaries, and can restrict the major optical absorption process in any case within a narrow region around the pixel center. The dependence of the microlenses’ effects on the systems properties was also analyzed; this showed that intermediate relative aperture and small microlens radius are required for optimized device performance. Simulation results also indicated that for detectors farther from the center of the field of view, the efficacy of microlenses in crosstalk suppression and energy gathering is still maintained, except for a negligible difference in the lateral magnification from an ordinary array without microlenses.

A high-performance readout circuit (ROIC) for VLWIR FPAs with novel current mode background suppression

This study designs a 32×32 readout integrated circuit (ROIC) for N-on-P very long wavelength (VLWIR) detectors. The new ROIC has the ability to operate with current mode background suppression in two steps, and increases the integration time and the signal-to-noise ratio (SNR) of image data. To improve the linearity and provide tight control of the detector bias, a buffered direct injection (BDI) unit cell is used as input circuit. By means of a sample and hold circuit, the infrared focal plane array (IRFPA) can be operated in snapshot mode and rolling mode. The simulation results confirm these advantages. With the 5.0V power supply, the VLWIR ROIC provides the output dynamic range over 2.0V and the well capacity more than 1×108e−. The skimming error is less than 0.5%. The final chip will be fabricated with HHNEC 0.35um 1P4M process technology, and the pixel occupies a 60um×60um area. It can be operated at the temperatures of 50K.

Investigations on a Multiple Mask Technique to Depress Processing-Induced Damage of ICP-Etched HgCdTe Trenches

A multiple mask technique, integrating patterned silicon dioxide (SiO2) film over patterned thick photoresist (PR) film, has been investigated as a method to perform mesa etching for device delineation and electrical isolation of mercury cadmium telluride (HgCdTe) third-generation infrared focal-plane arrays. The multiple mask technique was achieved by standard thick PR photolithography, SiO2 film deposition to cover the thick PR patterned film, and etching the SiO2 film at the bottom region after another photolithography process. The dynamic resistance in the zero-bias and low-reverse-bias regions of HgCdTe photodiode arrays isolated by inductively coupled plasma (ICP) etching with the multiple mask of patterned SiO2 and patterned thick PR film underneath was improved one- to twofold compared with a simple mask of patterned SiO2. It is suggested that the multiple mask technique is capable of maintaining high etching selectivity while reducing the side-wall processing-induced damage of ICP-etched HgCdTe trenches. The results show that the multiple mask technique is readily available and shows great promise for etching HgCdTe mesa arrays.

Reflow flip-chip bonding technology for infrared detectors

Based on the self-alignment principle, a new reflow flip-chip bonding technology for infrared detectors is proposed. By optimizing the dimensions between the under bump metallization (UBM) and the indium bump, 10 µm tall spherical indium balls are achieved firstly. Then the technical parameters of heating temperature and surface pre-treatment are discussed. Thereafter, a new reflow flip-chip bonding technology is applied to the infrared focal plane array (IRFPA) and it results in a 6.7% of the total bad pixel percentage which is dramatically decreased compared with the thermo-compression one of 41.9%. The deduced fatigue life of the IRFPA bonded by the new reflow flip-chip bonding technology is four times longer than that of the thermo-compression one.

CH4/Ar/H2/SF6 Plasma Etching for Surface Oxide Removal of Indium Bumps

Plasma etching for surface indium oxide removal by methane/argon/hydrogen/sulfur hexafluoride (CH4/Ar/H2/SF6) mixture has been implemented. The morphology of the indium bumps was not deteriorated after the plasma etching. High-resolution O 1s x-ray photoelectron spectroscopy (XPS) proved that the In–O component decreased from 44.5% for the nonetched sample to 10.8% for the sample after plasma etching. The surface modification of the indium bumps might be in the form of doped fluorine according to the XPS results. The zero-bias resistance derived from current–voltage (I–V) measurements for plasma-etched infrared detectors was comparable to that for nonetched ones, indicating that such plasma treatment is suitable for processing sensitive materials such as mercury cadmium telluride.

A preliminary study on MBE-grown HgCdTe two-color FPAs

This paper presents the recent progress on the study of device processings at multilayer HgCdTe film for integrated two-color (SWIR/MWIR) n-p-P-P-N detector arrays. The four-layer p-P-P-N heterostructures Hg1-xCdxTe film needed to achieve two color detector arrays was grown by molecular beam epitaxy (MBE) on (211)B oriented GaAs substrates. The secondary ion mass spectroscopy (SIMS) data for the HgCdTe film was obtained. The p-type layer on top of a thin P-type potential barrier layer and the SWIR P-on-N homojunction photodiode formed in-situ during MBE growth using indium impurity doping was processed into the MWIR planar photodiode by selective B+-implantation. The preliminary 256×1 linear arrays of SWIR/MWIR HgCdTe two-color FPAs detector were then achieved by mesa isolation, side-wall passivation and contact metallization. At 78K, the average R0A values of SWIR and MWIR are 3.852×105 Wcm2 and 3.015×102 Wcm2, and the average peak detectivities Dλp* are 1.57×1011cmHz1/2/W and 5.63×1010 cmHz1/2/W respectively. The SWIR photodiode cut-off wavelength is 3.04μm and the MWIR photodiode cut-off wavelength is 5.74μm, quite consistent with the initial device design. The SWIR response spectrum of the two-color detector with a distinct fall-off at shorter wavelength regime was discussed especially.

Inductively coupled plasma etching of HgCdTe IRFPAs detectors at cryogenic temperature

To fabricate various advanced structures with HgCdTe material, the Inductively Coupled Plasma enhanced Reactive Ion Etching system is indispensable. However, due to low damage threshold and complicated behaviors of mercury in HgCdTe, the lattice damage and induced electrical conversion is very common. According to the diffusion model during etching period, the mercury interstitials, however, may not diffuse deep into the material at cryogenic temperature. In this report, ICP etching of HgCdTe at cryogenic temperature was implemented. The etching system with cryogenic assembly is provided by Oxford Instrument. The sample table was cooled down to 123K with liquid nitrogen. The mask of SiO2 with a contact layer of ZnS functioned well at this temperature. The selectivity and etching velocity maintained the same as reported in the etching of room temperature. Smooth and clean surfaces and profiles were achieved with an optimized recipe.

Modeling of LWIR nBn HgCdTe photodetector

The nBn structure with an electron barrier sandwiched by n-type cap and absorber layers was predicted to suppress the Shockley-Read-Hall (SRH) generation-recombination processes and surface leakage. The MCT nBn structure has been studied by several groups to implement high operating temperature (HOT) device. In this report, the numerical analysis of the Hg1-xCdxTe nBn device in LWIR region (x=0.225) is performed utilizing Crosslight APSYS. The detector performance characterized by dark current, photo-current and detectivity is optimized by adjusting structural parameters such as Cd component and doping of each layer under various biases. Among the parameters, the trade-off between ΔEc and ΔEv is most intensively affected by Cd component of the barrier which was modified carefully and accomplished firstly. Furthermore, the effect of the trap density and trap energy level on the device performance is also investigated especially according to the processing techniques. At 110K, the optimized detectivity of the LWIR MCT nBn device reaches 7.5×1010 cmHz1/2/W in this report, comparable with that of the DLPH device (7.6×1010 cmHz1/2/W). The novel nBn HgCdTe structure is potentially valuable in LWIR region since the controllable p-doping issue is circumvented and passivation process is simplified.