Technology Today

2015 Issue 1

Vertically Integrated HgCdTe-based Sensor Manufacturing

Mercury cadmium telluride (HgCdTe) is the most versatile and widely-used material for infrared (IR) sensors. Yet, no commercial vendor satisfies the defense industry’s needs for this material. Raytheon Vision Systems (RVS) has established a vertically-integrated capability for HgCdTe-based technology that begins with the raw materials and extends to the completed infrared (IR) focal plane array (FPA) sensor module, providing full end-to-end control of the process. In particular, the vertically integrated approach allows RVS to tailor the HgCdTe material characteristics for any specialized application, and it provides short-loop feedback in support of design innovation and material optimization.

HgCdTe, Near Universal IR Sensor Material

Figure 1. Advantages of using HgCdTe

HgCdTe is a narrow-gap semiconductor whose wavelength sensitivity is tuned by adjusting the relative amounts of Hg and Cd in its chemical composition. Figure 1 shows a unit cell of the crystal structure and summarizes the characteristics that make this material advantageous and widely adaptable for IR detection. The material is grown epitaxially1 upon substrates of cadmium zinc telluride (CdZnTe). Also, in a multilayer structure, each HgCdTe layer is epitaxial with respect to the previous layer, taking advantage of the fortuitously small change in lattice parameter with composition. Figure 2 illustrates RVS’s dual-band HgCdTe FPA technology. The first layer is the n-type band-12 absorber the second layer is a p-type cap, and the third is the n-type band-2 absorber. The photocurrents from the two bands are read out sequentially by switching the bias voltage applied to the top of the mesa. The many details of this design have been optimized through repeated iterations of a cycle of numerical modeling, HgCdTe layer growth, and device testing. The development process would have been longer and more challenging without a vertically integrated manufacturing approach.

HgCdTe has proven to be broadly applicable at wavelengths ranging from less than 2 microns (µm) to more than 16 µm, in an impressive variety of device architectures. High-performance IR sensors are by far the dominant application for HgCdTe, and the principal users are government agencies. Serving many Department of Defense (DoD) and civil customers, RVS has built photoconductive, photovoltaic, dualband, avalanche, heterodyne and hyperspectral detectors in HgCdTe, and array pixel counts up to 4,000 × 4,000 have been produced. Most designs are intended for the atmospheric windows at 3–5 µm or 8–12 µm wavelengths.

Figure 2. Schematic cross-section of the

Vertically Integrated Approach

The detector branch of the HgCdTe vertical integration process, shown on the left side of Figure 3, begins with the growth of CdZnTe boules for substrates. By growing its own semiconductor materials, RVS takes vertical integration to its deepest level and sets itself apart from the majority of wafer processing enterprises. Although component manufacturers using other semiconductors commonly rely on vendor-supplied wafers, RVS recognized at the beginning that this would not be optimum for developing HgCdTe-based components. In the early years (from 1978 to 1998), in-house growth was motivated by the rapidly developing state of the technology, and by the extremely close connection between material characteristics and device performance. Innovations and progress were dependent on frequent adjustments to the material growth parameters. More recently, in-house growth has enabled RVS the ability to maintain the highest material quality, and to customize the layer structures for multiple applications. Because HgCdTe material is critical to many of our principal product lines, and comparable material is not available externally, RVS continues to supply its own wafers.

Boule growth starts with the raw materials, polycrystalline ultrapure CdTe and ZnTe, loaded into a carbon-coated quartz crucible. The crucible is mounted into an evacuated quartz ampoule, which is placed in a cylindrical furnace. Large-crystal boules are produced by mixing and melting the ingredients, followed by recrystallizing with the vertical gradient freeze method. Standard boule diameters are 92 and 125 millimeter (mm).

The boule substrate material is then sawn into slices, diced into squares, and polished to prepare the surface for epitaxial growth. Typical substrate sizes are 6 centimeters (cm) × 6 cm, although sizes up to 8 cm × 8 cm have been produced.

The HgCdTe layers are grown on top of the substrate by molecular beam epitaxy (MBE), which employs molecular beams to deposit material on the substrate in an ultrahigh vacuum chamber. The composition of a layer is determined by the fluxes of the beams, which in turn are controlled by the temperatures of the sources from which material is evaporating. Spectroscopic ellipsometry (SE) provides feedback to control the composition, while the temperature of the substrate is held at the optimum value with high precision. Dopant sources emit indium for n-type and arsenic for p-type doping during growth. Shutters in front of the sources are used to turn the beams on and off. The entire growth procedure is automated, with each step being programmed in advance.

An alternative technique is to use a silicon (Si) wafer as the substrate. The tradeoff in using Si is between creating a larger wafer with a lower cost substrate and accepting a higher defect density in the HgCdTe material due to the layer/substrate lattice mismatch. A special sequence of buffer layers between the Si and the HgCdTe has been developed to mitigate this problem. The selection of substrate depends on the specific application and both substrates continue to be used at RVS.

After growing the HgCdTe layers, the wafers are nondestructively evaluated against multiple quality specifications. They are then conveyed to the FPA processing line, where the sensing elements (pixels) are formed by photolithographic steps, including mesa etching, surface passivation, metal contact deposition, and indium bump formation. After wafer dicing, the FPAs are ready for mating to the readout integrated circuits (ROICs).

The ROIC branch of the process is shown in the lower right of Figure 3. The ROICs are designed and modeled at RVS using the latest software tools. For each pixel on the detector array, there is a corresponding unit cell on the ROIC to collect the photocurrent and process the signal. Each design is delivered to a silicon foundry for fabrication. RVS then receives and screens the ROIC wafers, after which they are diced and ready for mating with the FPA.

Figure 3. Process flow for vertically integrated HgCdTe focal plane array manufacturing

The FPA and ROIC process branches converge at the hybridization step, where the HgCdTe array and the ROIC die are mated together, as shown in the center of Figure 3. The industry’s most advanced flip-chip bonders, utilizing laser alignment and submicron-scale motion control, bring the two chips together. The indium bumps on all of the pixels form the mechanical bonds that join the pair of chips securely. FPAs with a pixel pitch as small as 10 µm are routinely aligned and hybridized with high yield. Next, each FPA with attached ROIC is tested according to a defined protocol, and if performance meets requirements, it is installed in a sensor module. Associated packaging and electronics are designed and assembled at RVS to complete the integrated manufacturing process.

Summary

The vertically integrated manufacturing process developed at RVS continues to be the foundation of our strong position in the IR sensor market. Because device performance depends critically on material characteristics, and since no external supply of HgCdTe wafers will meet our needs, the ability to supply our sensor fabrication line with material grown in-house has been a vital part of a complete, vertically integrated process.

1Epitaxial means that the crystal structure of the layer is aligned to that of the substrate.
2Band 1 and band 2 are the two ranges of IR wavelengths in which the device is sensitive, typically 3–5 µm and 8–12 µm, respectively. Each band requires its own IR-absorbing layer.

David R. Rhiger, Ph.D.

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