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Plasma etching is a powerful technique for transferring high-resolution lithographic patterns into HgCdTe material with low etch-induced damage, and it is important for fabricating small-pixel-size HgCdTe infrared focal plane array (IRFPA) detectors. P- to n-type conversion is known to occur during plasma etching of vacancy-doped HgCdTe; however, it is usually unwanted and its removal requires extra steps. Etching at cryogenic temperatures can reduce the etch-induced type conversion depth in HgCdTe via the electrical damage mechanism. Laser beam-induced current (LBIC) is a nondestructive photoelectric characterization technique which can provide information regarding the vertical and lateral electrical field distribution, such as defects and p–n junctions. In this work, inductively coupled plasma (ICP) etching of HgCdTe was implemented at cryogenic temperatures. For an Ar/CH4 (30:1 in SCCM) plasma with ICP input power of 1000 W and RF-coupled DC bias of ∼ 25 V, a HgCdTe sample was dry-etched at 123 K for 5 min using ICP. The sample was then processed to remove a thin layer of the plasma-etched region while maintaining a ladder-like damaged layer by continuously controlling the wet chemical etching time. Combining the ladder etching method and LBIC measurement, the ICP etching-induced electrical damage depth was measured and estimated to be about 20 nm. The results indicate that ICP etching at cryogenic temperatures can significantly suppress plasma etching-induced electrical damage, which is beneficial for defining HgCdTe mesa arrays.

The dark currents of two short wave (SW) HgCdTe infrared focal plane arrays (IRFPA) detectors hybridized with direct injection (DI) readout and capacitance transimpedance amplifier (CTIA) with long time integration were investigated. The cutoff wavelength of the two SW IRFPAs is about 2.6 μm at 84 K. The dark current densities of DI and CTIA samples are approximately 8.0 × 10−12 A/cm2 and 7.2 × 10−10 A/cm2 at 110 K, respectively. The large divergence of the dark current density might arise from the injection efficiency difference of the two readouts. The low injection efficiency of the DI readout, compared with the high injection efficiency of the CTIA readout at low temperature, makes the dark current density of the DI sample much lower than that of the CTIA sample. The experimental value of injection efficiency of the DI sample was evaluated as 1.1% which is consistent with its theoretical value.

A highly anisotropic etching process with low etch-induced damage is indispensable for advanced HgCdTe (MCT) infrared focal plane array (IRFPA) detectors. The inductively coupled plasma (ICP) enhanced reactive ion etching technique has been widely adopted in manufacturing HgCdTe IRFPA devices. An accurately patterned mask with sharp edges is decisive to accomplish pattern duplication. It has been reported by our group that the SiO2 mask functions well in etching HgCdTe with high selectivity. However, the wet process in defining the SiO2 mask is limited by ambiguous edges and nonuniform patterns. In this report, we patterned SiO2 with a mature ICP etching technique, prior to which a thin ZnS film was deposited by thermal evaporation. The SiO2 film etching can be terminated at the auto-stopping point of the ZnS layer thanks to the high selectivity of SiO2/ZnS in SF6 based etchant. Consequently, MCT etching was directly performed without any other treatment. This mask showed acceptable profile due to the maturity of the SiO2 etching process. The well-defined SiO2 pattern and the etched smooth surfaces were investigated with scanning electron microscopy and atomic force microscope. This new mask process could transfer the patterns exactly with very small etch-bias. A cavity with aspect-ratio (AR) of 1.2 and root mean square roughness of 1.77 nm was achieved first, slightly higher AR of 1.67 was also get with better mask profile. This masking process ensures good uniformity and surely benefits the delineation of shrinking pixels with its high resolution.

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.

We report results of a detailed study of electronic transport in n-on-p junctions formed by 150-keV boron-ion implantation in vacancy-doped p-type Hg0.769Cd0.231Te without postimplantation thermal annealing. A mobility spectrum analysis methodology in conjunction with a wet chemical etching-based surface removal approach has been employed to depth profile the transport characteristics of the samples. In the as-implanted samples, three distinct electron species were detected which are shown to be associated with (a) low-mobility electrons in the top 220-nm surface-damaged layer (E1: μ80K = 2940 cm2/Vs), (b) the B-ion implantation region in the top 500-nm region (E2: μ80K = 7490 cm2/Vs), and (c) high-mobility electrons in the n-to-p transition region at a depth of 600 nm to 700 nm (E3: μ80K = 25,640 cm2/Vs). Due to the maximum magnetic field employed (2 T), hole carriers from the underlying vacancy-doped p-type region were detected only after the removal of the top 220 nm of the profiled sample (μ80K = 126 cm2/Vs), revealing fully p-type character 800 nm below the original sample surface. A comparison of the extracted E2 electron concentration and calculated B-impurity profile suggests that the n-type region is due primarily to near-surface implantation-induced lattice damage.

A novel mask technique utilizing patterned silicon dioxide films 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 films were deposited at temperature of 80°C, and a procedure for patterning and etching of HgCdTe was developed by standard photolithography and wet chemical etching. Scanning electron microscopy (SEM) showed that the surfaces of inductively coupled plasma (ICP) etched samples were quite clean and smooth. Root-mean-square (RMS) roughness characterized by atomic force microscopy (AFM) was less than 1.5 nm. The etching selectivity between a silicon dioxide film and HgCdTe in the samples masked with patterned silicon dioxide films was greater than 30:1. These results show that the new masking technique is readily available and promising for HgCdTe mesa etching.