The ps-laser micro-machining technology for substrate-removed HgCdTe IRFPAs is researched in this paper. By analysis the microstructure morphologies under different pulse energy, the ablation threshold is fitted. Based on the analysis of ablation threshold, we optimize the laser parameters and etch grooves on the HgCdTe chip which is substrate removed and thinned to 5μm. The FPAs experiment test results show that thermal effected area was about 10μm away from the grooves, and the laser has no effect on the ROIC under the HgCdTe chip. The results are important to verify the optimal strategy of ps-pulsed laser for robust operation of substrate-removed HgCdTe IRFPAs and for laser micromachining industrial applications.
As a narrow bandgap semiconductor, the preparation of surface passivation layers on HgCdTe film epilayers is essential in the process of device fabrication. Most new infrared detectors use the mesa structure. A stable and reproducible passivation technology which meets the surface uniform cover of the high aspect ratio mesa is particularly important. Atomic layer deposition (ALD) is a new type of accurate surface thin film preparation technique, which has several characteristics such as depositing large-area uniform films, making the film thickness control at nanometer level feasible, and lower deposition temperature. ALD-ZnS film is prepared on the HgCdTe IRFPAs chip at 65°. I-V and R-V curves are similar to that of IRFPAs with CdTe thermal passivation. This shows that ALD ZnS film has a good potential application in the passivation of high aspect ratio mesa-array HgCdTe devices.
This paper reports the development of 2000×256 format SWIR HgCdTe/Si FPA with multiple-choice gain (i.e. multiple-choice charge handling capacity) for hyperspectral detection. The spectral resolution is about 8nm. To meet the demands of variable low flux detection within each spectral band in the short wave infrared range, low dark current, low noise, variable conversion gains and high SNR (Signal to Noise Ratio) of FPA are needed. In this paper, we fabricate 512×512 pixel 30μm pitch SWIR HgCdTe diode array on Si by using a novel stress-release construction of HgCdTe chip on Si. Moreover, we design low noise, variable conversion gain and large dynamic range read-out integrated circuit (ROIC) and hybridized the ROIC on the HgCdTe diode array on Si substrate. There are 8-choice gains which can be selected locally according to the incident flux to meet high SNR detection demand. By high-accuracy splicing 4 512×512 HgCdTe/Si FPA we get mosaic 2000×512 FPA, and characterizations have been carried out and reveal that the array dark current densities on an order of 10-10A/cm2, quantum efficiency exceeding 70%, and the operability of 99.5% at operating temperature of around 110K. The SNR of this FPA achieved 120 when illuminated under 5×104photons/pixel.
HgCdTe is one of the dominating materials for infrared detection. To pattern this material, our group has proven the feasibility of SiO2 as a hard mask in dry etching process. In recent years, the SiO2 mask patterned by plasma with an auto-stopping layer of ZnS sandwiched between HgCdTe and SiO2 has been developed by our group. In this article, we will report the optimization of SiO2 etching on HgCdTe. The etching of SiO2 is very mature nowadays. Multiple etching recipes with deferent gas mixtures can be used. We utilized a recipe containing Ar and CHF3. With strictly controlled photolithography, the high aspect-ratio profile of SiO2 was firstly achieved on GaAs substrate. However, the same recipe could not work well on MCT because of the low thermal conductivity of HgCdTe and CdTe, resulting in overheated and deteriorated photoresist. By decreasing the temperature, the photoresist maintained its good profile. A starting table temperature around -5°C worked well enough. And a steep profile was achieved as checked by the SEM. Further decreasing of temperature introduced profile with beveled corner. The process window of the temperature is around 10°C. Reproducibility and uniformity were also confirmed for this recipe.
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.
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.
An innovative heterojunction photodiode structure in HgCdTe-on-Si long-wavelength (LW) infrared focal plane array (IRFPA) detector is investigated in this paper. The quantum efficiency and the photoresponse of devices have been numerically simulated, using Crosslight Technology Computer Aided Design (TCAD) software. Simulation results indicate that in contrast to the p+-on-n homojunction photodiode, the heterojunction photodiode effectively suppresses the
crosstalk between adjacent pixels and interface recombination between HgCdTe active region and
buffer layer on Si substrate. And in the range of the LW-band, the quantum efficiency of the heterojunction photodiode increases by 35.5%. Furthermore, the heterojunction photodiode acquires the narrow-band response spectrum desired in the application of the LW IRFPA detectors
as the p+-on-n homojunction photodiode with the optical filter. Finally, the smaller bulk resistance of its heavily doped N-type layer ensures the uniformity of the pixel series resistance in the large format IRFPAs.
The format of infrared focal plane arrays (IRFPAs) is continuously increasing, and the spatial non-uniformity in the irradiance affects the result of large-format IRFPAs’ responsivity measurement. When placed in the test system, different pixels on the IRFPA receive different radiation flux due to the specific geometric setup of the dewar, resulting signal response uniformity. This is not the inherent characteristics of the IRFPA and thus should be corrected. The existing methods of correcting irradiance spatial non-uniformity simply consider the solid angle subtended by the aperture of the cold shield viewing from the center of a pixel. However, these methods are only applicable for the case of an infinite blackbody behind the cold shield. This paper presents a more comprehensive and appropriate correction, taking account of the specific geometric setup of the dewar, especially a dewar window of finite size. The relative irradiance received by different pixels on an IRFPA, as a correction factor, is simulated directly through matrix calculations. We apply the FOV correction profile to a 640 x 512 HgCdTe IRFPA with a pitch of 25μm, finding that responsivity ratio of the peripheral pixels to the central pixels on the IRFPA has changed from an original value of 88.0% to 96.4% after correction. This method has been proven to be useful and effective in obtaining more accurate description of the IRFPA performance for further analysis.
N2 atmosphere annealing process to recover ICP etching induced damage on p type mercury vacancy HgCdTe film has been exploited in this paper. ICP etching and N2 atmosphere annealing processes were carried out on a series of Hgvacancy-doping p-type HgCdTe samples. The carrier transport and lifetime properties of these samples were characterized by Hall measurement and microwave reflectance method respectively. P-to-n electrical damage of the surface HgCdTe film induced by the ICP etching process was deduced from the polarity inversion of Hall coefficient. The carrier transport and lifetime properties were similar to those of the non-etched samples, indicating that the surface HgCdTe electrical damage was recovered partially by annealing at 210°C for 2 hours. I-V and R-V characteristics curves of photodiodes fabricated on the etched and N2 atmosphere annealing processed MCT samples were also comparable to those of photodiodes on the non-etched MCT samples in the following experiments. These results show that N2 atmosphere annealing process is a readily available and promising recovering technique for HgCdTe ICP etching induced damage.
Generally the electrical interconnectivity between The Mercury Cadmium Telluride (MCT) infrared focal plane array
(IRFPA) device and circuit takes the flip chip technology using indium bump as a connection medium. In order to improve
the reliability of the interconnectivity indium melting is a common packaging technique at present. This technique is called
reflow soldering. The heating is transferred to the indium bump by heating the device and circuit. This heating process will
persist about 10 minutes resulting in the MCT material going through a 10 minutes high temperature baking course. This
baking process will strongly degenerate the characteristic of the MCT device. Under this circumstance this article gives a
new heating technique for indium bump which is call induction heating melting technique. This method realizes the
selective heating. While the indium bump is melted by the conduction heating the semiconductor material such as MCT
can’t be heated.
This paper analyzes the dark current characteristic of two different sizes at different temperature. The wavelength is 12 μm. We find that the main mechanism of the dark current is band-to-band tunnel current at 20 k-30 k. By fitting the ideal factor, we find that at temperature of 80 k-100 k, the dark current is a mixture of diffusion current, g-r current. We find the large size is better by comparing the R0A-V curves. At 45 k-60 k, the dark current is a mixture of trap-assisted tunnel current and band-to-band tunnel current. An interesting thing is that in this temperature area, the different R0-V curves have a same sharp cross which does not exist in the 20 k-30 k area or 80 k-100 k area. At this point the trap-assisted tunnel is equal to the band-to-band tunnel. We calculated the trap density of depletion region which is about 1.38×1011 cm-3 at 55 k.
The weakness of HgCdTe materials is a weak Hg–Te bond, which results in bulk, surface and interface instabilities.
Usual surface preparation methods for bulk HgCdTe wafers are chemical etch with bromine-in-methanol (Br-MeOH).
Because the bromine etch forms Te-enrich HgCdTe surface due to the depletion of Hg and Cd. This Te-enrich surface
layer is easily oxidized when exposed to air or acids, and this native oxide is one of the main reasons degrading the
passivation properties. Besides the surface of the material will have more damages during the ion implantation, which is
used to process a junction. In this way, appropriate surface passivation is required and is very important for HgCdTe
photovoltaic detectors. This paper presents the different surface treatment methods on the I-V characteristics of HgCdTe
infrared photovoltaic detectors. The results of the experiments show that the performance of the diodes with surface
treatment by the bromine-in-ethanol etching and lactic acid in glycol solution oxidation is better than other diodes, which
have no surface treatment or only bromine-in-ethanol etching. So it proves that the process of surface treatment in
appropriate methods can improve the quality of surface passivation.
A novel mask technique, combining high selectivity silicon dioxide patterns over high aspect-ratio
photoresist (PR) patterns has been exploited to perform mesa etching for device delineation and electrical
isolation of HgCdTe third-generation infrared focal plane arrays (IRFPAs). High-density silicon dioxide film
covering high aspect-ratio PR patterns was deposited at the temperature of 80°C and silicon dioxide film
patterns over high aspect-ratio PR patterns of HgCdTe etching samples was developed by standard
photolithography and wet chemical etch. Scanning electron microscopy (SEM) shows that the surfaces of
inductively coupled plasma (ICP) etched samples are quite clean and smooth. The etching selectivity between
the novel mask and HgCdTe of the samples is increased to above 32: 1 while the side-wall impact of etching
plasma is suppressed by the high aspect ratio patterns. These results show that the combined patterning of
silicon dioxide film and thick PR film is a readily available and promising masking technique for HgCdTe
mesa etching.
The polarity inversion of laser beam induced current (LBIC) signal at low temperature and high
laser power density in As-doped p-type HgCdTe is investigated in this paper. It is found that the
polarity of LBIC signal reverses at 87 K compared to that at 300 K and the high laser power density is
also an important factor in inducing the LBIC signal reverse. The results demonstrate that the shape of
the LBIC signal profile is strongly dependent on the temperature of the device and the laser irradiation.
To provide a reasonable analysis for this interesting fact, a photocarrier spreading mode is presented in
this paper.
This paper aims to analysis the characteristics of VLWIR HgCdTe detectors with n-on-p implanted planar junction. We
use the variable area test structures, which are used as an important tool to access the quality of the material, process and
surface passivation in HgCdTe device technology. Through analyzing the relation between the inverse of the zero-bias
resistance-area product of a diode and its perimeter-to-area ratio, we can distinguish the contributions of bulk and surface
effects, and calculate the minority carrier diffusion length, which can reflect the conditions of the HgCdTe epitaxy.
According to the results, we find the VLWIR HgCdTe detectors have abnormal current-voltage phenomenon at a low
temperature, which may be the results of a parasite p-n junction. Besides, through data analysis and curves fitting, we
find the surface current of the VLWIR HgCdTe diodes at 80K is nearly comparable with the bulk current.
Hydrogen-based dry plasmas, generated in inductively coupled plasma reactors have been demonstrated to be very
effective in fabricating high fill-factor mesa of Hg1-xCd xTe multi-layer hetero-structure material for infrared focal plane
array applications. To obtain reasonable dry etching process for Hg 1-xCd xTe, it is essential to investigate the physical,
chemical, and electrical characteristics of the surface. This paper explores the effect of varying the plasma process
parameters on the surface of Hg 1-xCd xTe. The surface chemical analysis was carried out using spot X-ray photoelectron
spectroscopy (XPS), the surface roughness was measured by atomic force microscopy (AFM), and p-to-n type
conversion depth was assessed by a reliable current-voltage test of a designed structure basing on material-chip
technology concept and a convenient technology of cross-section surface potential imaging (SPM). At last, Hg 1-xCd xTe
etched surfaces with roughness low and mechanical or electrical damage free were achieved.
The high-density inductively coupled plasma etching technique was applied to HgCdTe, while using the RF-powered wafer electrode to provide low plasma energy. By using a CH4/H2/N2/Ar chemistry the HgCdTe etch profiles were studied as a function of mask selectivity, chamber pressure, gas ratio and ICP power. The etch rate was found to decrease as etch depth increasing. The LBIC and I-V measurements were employed to investigate the electrical damage of HgCdTe material caused by plasma bombardment.
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.
We study the 1/f noise currents and dark currents in LWIR HgCdTe photodiodes with different passivation. The diodes are fabricated by ion implanting boron on MBE HgCdTe with x=0.2173. One kind of photodiodes was passivated by ZnS and the other kind was passivated by CdTe/ZnS. Both dark currents and 1/f noise currents were measured at several reverse bias voltages. The measured dark currents of the photodiodes are analyzed using current model fitting methods. The different dark current components, such as diffusion current, generation-recombination current, trap assisted tunneling current and band-to-band tunneling current, at various biases voltages can be separated from the measured dark currents. The measurement results demonstrate that the dominant mechanism that produces 1/f noise in HgCdTe photodiodes with either passivation is tunneling. When the reverse bias voltages are less than 200mv, the main mechanism that produces 1/f noise is trap assisted tunneling. In this case, the 1/f noise currents of the photodiodes passivated by ZnS are smaller than those passivated by CdTe/ZnS. When the reverse biases are larger than 200mv, the band-to-band tunneling currents of the photodiodes passivated by ZnS are much larger than the photodiodes passivated by CdTe/ZnS. And the 1/f noise currents of the ZnS passivated photodiodes are larger than the different passivated one. In order to investigate the effect of surface passivation on the stability of two kinds of diodes, R-V characteristics and 1/f noise of the diodes were measured after vacuum baking for 10 hour at 80°C, the photodiodes passivated by CdTe/ZnS show higher performance compared with the diodes passivated by ZnS after baking.
The HgCdTe photovoltaic detectors passivated by single ZnS layer and dual (CdTe+ZnS) layers were fabricated in same wafer. The fabricated devices were characterized by measurements of the diode dark I-V characteristics and low-frequency noise. The dual-layer passivated diodes showed the better performance compared to the single layer passivated diodes, and modeling of diode dark current mechanisms indicated that the performance of the diodes passivated by single ZnS were found to be strongly affected by tunneling current related to the surface defects, By the analysis of X-ray reciprocal space maps, It was found the Qy scan direction broadening of HgCdTe epitaxial layer passivated by ZnS was wider after passivation, which confirmed the existence of defects in the surface of HgCdTe epitaxial layer passivated by ZnS.
Reflection Fourier transform infrared (RFTIR) measurements were performed on backside-illuminated InSb infrared focal plane arrays for thinning process monitoring. InSb detector film thickness and its variation across the film can be measured from the RFTIR spectra taken by this contactless and non-destructive technique.
An obvious minimum was observed in the reflectance spectrum of ion implanted Hg1-xCdxTe. The anodic oxidation method and beveling technique were used to learn the spectrum changing with the depth. This minimum even can be observed when a layer with thickness thicker than the ion range was removed from the implanted surface. Considering the high electron concentration of the implanted layer, this phenomenon was explained by using the model of reflection of layered media in which the refractive index changes with the depth. By numerical fitting, the depth profile of the carrier density and refractive the index of the ion implanted layer were obtained.
The transmission spectra and response spectra were measured in the same small region of Hg1-xCdxTe material over a temperature range 85K < T < 300K. The relationship between the cut-on wavenumber of the transmission spectra and the cut-off wavenumber of response spectra in the same temperature is shown and the empirical expression is deduced as following: vcutoff equals 2.407 X 10-2 + 1.401vcuton - 1.199vcuton2.
The rates of growth on oxide films on polished Hg1-xCdxTe surfaces exposed to room air are obtained by measuring the ellipticity of polarized light reflected from the surfaces with the ellipsometer. Plots of thickness vs. the logarithm of the time in room air are linear after about 1500 minutes with slopes of 15 angstroms/decade. Immediately after polishing the native oxide film is proximately 10 angstroms thick and increase in thickness by about 36 angstroms after one week. Measurements utilizing polarized light are made of the increase in film thickness with time on Hg1-xCdxTe surfaces immersed in water. The regular of the anodization voltage versus time was given. With ellipsometer, the thickness of anodic oxides film grown on Hg1-xCdxTe is determined. Further details and discussion will be presented.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.