**7. Future IR Technology Directions**

vice is defined by the lateral diffusion length of minority carriers (holes), as illustrated in Fig‐ ure 24 (b). A 100 K increase in the BLIP temperature has been demonstrated [68]. SLS-based detectors with nBn design and special processing schemes showed dark current reduction of two orders of magnitude (at 77 K) in comparison to conventional photodiode processing tech‐ niques. Quantum efficiency and shot-noise-limited specific detectivity are comparable to cur‐ rent SLS-based p-i-n diodes. While nBn detectors have been demonstrated, focal plane arrays

A microbolometer is a resistive element fabricated from a material that has a very small thermal capacity and a large temperature coefficient of resistance (TCR). The absorbed IR ra‐ diation is converted into heat which changes the resistance of the microbolometer such that measurable electrical signals can be detected.The spectral response of the bolometer is flat across the IR spectrum, since the sensing mechanism isindependent of the photoexcited car‐ riers jumping across an energy band-gap. A schematic representation of a bolometer pixel on top of a ROIC is shown in Figure 25 details of which can be found in reference 70. The pixel design shows a resonant cavity formed by an absorbing layer suspended above a re‐ flecting metal layer. The cavity is used to amplify the absorptance of the incident IR radia‐ tion. The microbridge is supported by two beams and is thermally isolated from the ROIC to

based on InAs/GaSb SLS detectors with nBn designs are being developed [68-69].

**6.5. Microbolometers**

180 Optoelectronics - Advanced Materials and Devices

increase the sensitivity of the microbolometer.

amorphous silicon technologies.

**Figure 25.** Schematic cross sectional and top views of Microbolometer IR Detector [70].

The microbolometer based on Si-MEMS device structure has been under development for over 20 years with support from DARPA and the Army. Most microbolometer structures utilize VOx and amorphous silicon thin film technologies. Companies such as Raytheon, BAE Systems and DRS Technologies are developing and producing 17 micron pixels in 640x 480 and larger arrays using VOx [71-72]. L3 Communications and CEA-LETI are developing and producing 640x480 arrays with 17 micron unit cells using amorphous-Silicon technolo‐ gy [73-74]. Figure 26 presents examples of the microbolometer structures using VOx and

Further advancement in imaging systems requires solutions for many fundamental and technological issues related to wide field of view (FOV), resolution, pixel pitch, optics, mul‐ ticolor, form-factor, low SWaP, and low cost. This section highlights two projects currently fielded through the Microsystems Technology Office (MTO) of the Defense Advanced Project Agency (DARPA) to prepare for these advancements. The first is called AWARE (Advanced Wide Field of View Architectures for Image Reconstruction and Exploitation) and addresses several fundamental issues that will enable technologies for wide field-ofview, pixel scaling, broadband and multiband imaging [75-77]. The second project is LCTI-M (Low Cost Thermal Imager-Manufacturing) which is addressing a cost effective solution for manufacturing microbolometers.

Traditional detector arrays are typically designed for a narrow band of wavelengths due to in‐ adequate absorption and charge collection from photons with varying wavelengths. Broad‐ band absorption is usually inadequate due to quantum efficiency (QE) roll-off. To design a detector with high QE, low NETD and high operating temperature across a very broad band of wavelengths, say visible to 5 um range, traditional detector design would be less than opti‐ mum. A detector design that can accomplish these goals is based on a photonic pillar-type ar‐ chitecture. Photonic crystals are relatively well understood and have been demonstrated for applications like VCSELs, which are similar to photovoltaic detectors. Sub-wavelengthsize semiconductor pillar arrays within a single detector can be designed and structured as an en‐ semble of photon trapping units to significantly increase absorption and QE for a wide band of wavelengths. Each sub-element in each pixel can be a 3D photonic structure fabricated using either a top-down or bottom-up process scheme. The sub-element architecture canbe of differ‐ ent shapes such as pyramidal, sinusoidal or rectangular [75].

Using unique pyramidal and pillar topologies etched into the photon absorbing layer, re‐ searchers have demonstrated 3D photon trapping, achieved significant reduction in dark current and established uniform QE (see Figure 27). This is the first demonstration of broad‐ band performance in a single infrared detector using a pillared microstructure in a semicon‐ ducting material. The broadband technology has been demonstrated independently in II-VI and III-V based epitaxial materials. This achievement paves the way to replace multiple cameras with one [75]. It also gives the ability for hyper spectral sensing that will enable bet‐ ter target discrimination compared to a single narrow band camera. The high performance at 200 K compared to traditional 80 K operation allows for a smaller SWaP design, since high power and large cryogenic coolers can be replaced by low power miniature coolers. Such cameras would have significant impact on smaller platforms.

onstrated [75]. The characterization shows excellent results with 99.8 % response operability

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**Figure 28.** a) Schematic illustrationof the 3D integrated AWARE Lambda-Scale LWIR FPA design; b) Micrograph of the

Another DARPA program, LCTI-M addresses the development of an advanced low cost room temperature IR cameras based upon cell phone CMOS camera technology, where the imaging sensor, optics and electronics are fabricated at the wafer level. IR imaging capabili‐ ty, such as thermal cameras, provides significant advantages in terms of visibility and target detection in all weather conditions making it a vital tool for day/night operations. However, the cost of thermal camerasis one of the key factors limiting the availability of high perform‐ ance IR imagers at consumer level. Further, current form-factors are unacceptable for new applications in smaller handheld devices (such as PDAs) and glasses similar to Google Glass. Availability of very low cost and small form-factor IR cameras will enable a variety of

Nanotechnology and science of emerging materials and material designs have stirred up a slew of research that has significant impact on sensors and many other electronic devices. Nanostructures offer very thin absorption layers due to many intricate designs such as plas‐ monics and metamaterials to concentrate photons and enhance electric field. The advantages of thinner absorber in a photodetector means shorter carrier transit time, thus high speed detectors and greater structural compatibility with ever-shrinking electronic devices. Many advances in nanomaterials for detectors have been made allowing for very low cost technol‐

DARPA has funded several new approaches for detector development using carbon nano‐ tube, graphene, nanoparticles and other nanomaterials. These researches demonstrate high potential for future detector technologies that could be very beneficial for both, mili‐

In this chapter, we have discussed growth, fabrication and characteristics of mainstream infra‐ red materials and devices on a variety of substrates. We have discussed SWIR band of interest

applications such as fire-fighting, security, medical and gaming industry.

ogy. A detailed review of nano-based detector research is given in reference [78].

within 50 % of the median.

MEMS capacitor array cross section [75].

tary and commercial sectors.

**8. Summary**

**Figure 27.** (Left) Illustration of pillared photonic detector architecture; (right) Micrograph of pyramidal PT structure fabricated in epitaxial InAsSb.

The pixel scaling effort is developing very high density LWIR and MWIR FPAs with pix‐ el dimensions approaching the Nyquist- limit. Unlike visible sensors where the pixel size has been reduced to 1.4 um, the scaling of infrared pixels is much more difficult. As the pixel size is reduced, "bump-bonding", ROIC, signal integrating capacitor and signal to noise ratio become difficult. Achieving very small pixels however, will enable larger FPAs with small optics and cold shield, better resolution and yielding a huge reduction in SWaP. In one of the approaches, three layers (detector array, ROIC and MEMS capacitor array) are being developed separately, followed by integrating individual cells via indium bumps and through silicon vias (TSV).

To achieve high sensitivity (say < 30 mK) LWIR FPAs with 5 μm pixels require large amounts of integrated charge to be accommodated in a very small unit cells. For a 5 μm pla‐ nar unit cell, the charge capacity in standard ROIC technology is less than 1 million elec‐ trons, whereas 8 to 12 million electrons are required for good sensitivity – a reason why small pitch IR detectors are not available today. As an enabler for this small pitch LWIR de‐ tector, the challenge of charge storage in small pixels is being addressed by fabricating MEMS capacitors suited to a 3D ROIC design. The MEMS capacitor array can be fabricated in a separate 8" wafer. This technology yielded 20 million electrons in a 5 micron unit cell. This breakthrough will pave the way for small pitch FPAs to operate with very high sensi‐ tivity. Figure 28(b) shows a Transmission Electron Micrograph (TEM) picture of a portion of the MEMS capacitor array. Using the High Density Vertically Integrated Photodetector (HDVIP) technology, a fully functional 1280X720, 5 μm unit cell LWIR FPA has been dem‐ onstrated [75]. The characterization shows excellent results with 99.8 % response operability within 50 % of the median.

**Figure 28.** a) Schematic illustrationof the 3D integrated AWARE Lambda-Scale LWIR FPA design; b) Micrograph of the MEMS capacitor array cross section [75].

Another DARPA program, LCTI-M addresses the development of an advanced low cost room temperature IR cameras based upon cell phone CMOS camera technology, where the imaging sensor, optics and electronics are fabricated at the wafer level. IR imaging capabili‐ ty, such as thermal cameras, provides significant advantages in terms of visibility and target detection in all weather conditions making it a vital tool for day/night operations. However, the cost of thermal camerasis one of the key factors limiting the availability of high perform‐ ance IR imagers at consumer level. Further, current form-factors are unacceptable for new applications in smaller handheld devices (such as PDAs) and glasses similar to Google Glass. Availability of very low cost and small form-factor IR cameras will enable a variety of applications such as fire-fighting, security, medical and gaming industry.

Nanotechnology and science of emerging materials and material designs have stirred up a slew of research that has significant impact on sensors and many other electronic devices. Nanostructures offer very thin absorption layers due to many intricate designs such as plas‐ monics and metamaterials to concentrate photons and enhance electric field. The advantages of thinner absorber in a photodetector means shorter carrier transit time, thus high speed detectors and greater structural compatibility with ever-shrinking electronic devices. Many advances in nanomaterials for detectors have been made allowing for very low cost technol‐ ogy. A detailed review of nano-based detector research is given in reference [78].

DARPA has funded several new approaches for detector development using carbon nano‐ tube, graphene, nanoparticles and other nanomaterials. These researches demonstrate high potential for future detector technologies that could be very beneficial for both, mili‐ tary and commercial sectors.

### **8. Summary**

band performance in a single infrared detector using a pillared microstructure in a semicon‐ ducting material. The broadband technology has been demonstrated independently in II-VI and III-V based epitaxial materials. This achievement paves the way to replace multiple cameras with one [75]. It also gives the ability for hyper spectral sensing that will enable bet‐ ter target discrimination compared to a single narrow band camera. The high performance at 200 K compared to traditional 80 K operation allows for a smaller SWaP design, since high power and large cryogenic coolers can be replaced by low power miniature coolers.

**Figure 27.** (Left) Illustration of pillared photonic detector architecture; (right) Micrograph of pyramidal PT structure

The pixel scaling effort is developing very high density LWIR and MWIR FPAs with pix‐ el dimensions approaching the Nyquist- limit. Unlike visible sensors where the pixel size has been reduced to 1.4 um, the scaling of infrared pixels is much more difficult. As the pixel size is reduced, "bump-bonding", ROIC, signal integrating capacitor and signal to noise ratio become difficult. Achieving very small pixels however, will enable larger FPAs with small optics and cold shield, better resolution and yielding a huge reduction in SWaP. In one of the approaches, three layers (detector array, ROIC and MEMS capacitor array) are being developed separately, followed by integrating individual cells via indium

To achieve high sensitivity (say < 30 mK) LWIR FPAs with 5 μm pixels require large amounts of integrated charge to be accommodated in a very small unit cells. For a 5 μm pla‐ nar unit cell, the charge capacity in standard ROIC technology is less than 1 million elec‐ trons, whereas 8 to 12 million electrons are required for good sensitivity – a reason why small pitch IR detectors are not available today. As an enabler for this small pitch LWIR de‐ tector, the challenge of charge storage in small pixels is being addressed by fabricating MEMS capacitors suited to a 3D ROIC design. The MEMS capacitor array can be fabricated in a separate 8" wafer. This technology yielded 20 million electrons in a 5 micron unit cell. This breakthrough will pave the way for small pitch FPAs to operate with very high sensi‐ tivity. Figure 28(b) shows a Transmission Electron Micrograph (TEM) picture of a portion of the MEMS capacitor array. Using the High Density Vertically Integrated Photodetector (HDVIP) technology, a fully functional 1280X720, 5 μm unit cell LWIR FPA has been dem‐

Such cameras would have significant impact on smaller platforms.

fabricated in epitaxial InAsSb.

182 Optoelectronics - Advanced Materials and Devices

bumps and through silicon vias (TSV).

In this chapter, we have discussed growth, fabrication and characteristics of mainstream infra‐ red materials and devices on a variety of substrates. We have discussed SWIR band of interest that involves InGaAs, SiGe and HgCdTe based technologies and their applications. We also discussed the technologies and applications of MWIR, LWIR and multi-color devices for the 3-5 and 8-14 micron bands. Some of the key work discussed includes InSb, HgCdTe, and III-V based nBn and Strained Layer Super Lattice (SLS). Discussion of thermal bolometer devices provide introduction to future low cost LWIR technology. Each of these technologies has a place in the infrared band where a variety of detector configurations are being used. We also discussed the application of photonic type structures to IR detectors with broadband spectral response and high operating temperatures. It was shown that sub-wavelength size semicon‐ ductor pillar arrays can be designed and structured as an ensemble of photon trapping units to significantly increase absorption and QE over a wide band of wavelengths. It is anticipated that the current research and development presented in Section 7 will enable a host of new technologies for a variety of defense and commercial applications.

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Although numerous research activities are ongoing in the area of nanoscience and tech‐ nology, we briefly made comments on such technologies to make readers aware of vari‐ ous research activities.
