Consumers have witnessed a dramatic increase in picture resolution and decrease in price for digital cameras in recent years. This trend, however, has not been seen in long-wavelength IR cameras. Mercury cadmium telluride is the most widely used sensor material in this band. Unfortunately, the lack of large-area wafer substrates combined with material difficulties has limited its ability to supply high-resolution cameras. Because of the importance of this imaging capability in defense, science, and industry, there is a great need for an alternative material technology.
Among the most promising candidates is the quantum-well IR photodetector (QWIP). Based on the mature gallium arsenide technology, QWIP focal plane array (FPA) cameras are amenable to low-cost and high-volume production. Today, QWIP cameras with resolution as high as 0.33 megapixels are commonly available in the commercial market. However, QWIPs are still evolving and can improve. For example, QWIP material absorbs light only when the optical electric field is vertical to the material layers. To detect light under normal incidence, the conventional approach is to use diffraction gratings. Although this approach is an elegant one, high diffraction efficiency is nevertheless difficult to achieve over a limited pixel area. Furthermore, one study1 has shown that when the pixel size is less than 25μm across, the entire pixel volume acts as a resonant cavity in defining the overall electromagnetic field. A small change in the pixel geometry or substrate thickness can change the detector quantum efficiency (QE) drastically. Although large QEs using this grating technique have been reported in the literature, there is little evidence that high performance can reliably be achieved given the normal manufacturing tolerances. From our limited testing of commercial products and third-party test data, the QE is typically less than 5% in these FPAs.
To improve the optical coupling in QWIPs, we have investigated corrugated QWIPs (C-QWIPs).2 The C-QWIP uses inclined micromirrors on the detector sidewalls to reflect light into parallel propagation, thus creating the vertical field. Rigorous electromagnetic field simulations indicate that resonant cavity effects do not play a role in this detector structure.2 Figure 1 shows the pixel geometry.
Figure 1. The figure on the left shows a 4” C-QWIP wafer containing five 1024 ×1024 focal plane arrays (FPAs). The figure on the right shows individual pixels with inclined sidewall reflectors.
C-QWIP FPAs improve the technology both in performance and in manufacturability. Because reflection is more effective in redirecting the light, the QE is larger. Reflection is also independent of wavelength. This means that the detector will preserve the natural absorption spectrum of the material, which can be much wider than the grating coupling bandwidth. Without the need for matching the material wavelength to the grating cavity modes in the detector, it is possible to apply the same pixel geometry and production process to all QWIP designs. This allows the simultaneous production of FPAs having a wide range of wavelength bands without jeopardizing QE. In the absence of the fine grating features, it also enables the use of standard photolithographic techniques for faster, less-expensive, and very large format production. With all these benefits, QWIP technology will be more capable of closing the gap between the infrared and the visible in high-resolution imaging.
We have produced a number of 1-megapixel C-QWIP FPAs operating in different wavelength bands ranging from 8.3 to 11.6μm to demonstrate its promise.3 Figure 1 shows a typical processed wafer containing five FPA die. Due to various system considerations, different QEs were designed in these FPAs. Figure 2 compares the calculated and measured QEs for five designs. The experimental results generally agree with the designed values. This close relationship will ensure consistent performance during mass production. These QE values, as well as the measured bandwidths, are about three times larger than that of the tested commercial QWIP FPAs. Figure 3 compares the detection spectrum of a C-QWIP FPA with that of the building material, and Figure 4 shows a typical outdoor IR image.
Figure 2. The calculated and the observed quantum efficiency (QE) of five FPA designs. The dashed line serves as a visual guide, on which the two are equal.
Figure 3. The natural spectral response lineshape observed from an edge-coupled test detector and the measured detection spectrum from the corresponding FPA.
Figure 4. An image taken by a 1024×1024 C-QWIP FPA with 10.2μm cutoff.
To conclude, we have shown that the C-QWIP technology is advantageous and can be the technological driver of a new infrared industry in providing large quantities of high-sensitivity, high-resolution, and affordable long-wavelength IR cameras. With further research and development, it may even catch up with the trend experienced by the commercial CCD cameras. In the near future, we are planning to increase the FPA resolution to 4 megapixels and to detect two colors simultaneously.
This work was supported by the Missile Defense Agency.
US Army Research Laboratory (ARL)
Kwong-Kit Choi is the senior research scientist for physical science. His current research interests are IR physics and FPA fabrication and demonstration.
L-3 Communications Cincinnati Electronics
David Forrai is a senior electro-optics engineer working on systems, analysis, and test of emerging IR imaging technology.
2. K. K. Choi, C. Monroy, V. Swaminathan, T. Tamir, M. Leung, J. Devitt, D. Forrai, D. Endres, Optimization of corrugated-QWIPs for large format, high quantum efficiency, and multi-color FPAs, Proc. SPIE 6206, pp. 62060B, 2006. doi:10.1117/12.663960