Megapixel QWIPs Deliver Multi-color Performance
Objects at room temperature have their most intense emission between 8 and 9 μm, which enables infrared cameras operating in this range to deliver detailed images in complete darkness. The best of these cameras can cover a wider spectral band, from 3 to 15 μm, and have focal plane arrays (FPAs) with individual photon detectors. These imagers typically use narrow-bandgap semiconductors. Interband absorption converts the incident light into charged carriers that provide the electric signals that ultimately form the image.
However, difficulties associated with the growth and processing of narrow-bandgap materials reduces the yield of these detectors and increases their cost. These problems provide the motivation to develop artificial, low-effective-bandgap structures containing multiple quantum well (MQW) heterostructures formed from GaAs-based material systems that are easier to grow and process.
Developments at the Jet Propulsion Laboratory (JPL), CA, have led to the fabrication of megapixel singleband detectors, such as GaAs/AlGaAs-based 1024 × 1024 pixel long-wavelength infrared (LWIR) QWIP detector arrays, and mid-wavelength infrared (MWIR) arrays that use GaAs/InGaAs/AlGaAs structures to detect shorter wavelengths. The MWIR arrays operate in the 4-5.5 μm spectral range, and their long-wavelength counterparts detect the 8-9 μm band. Both detectors have a 19.5 μm pixel pitch but an actual pixel size of 17.5 × 17.5 μm. The detectors have been combined with matching silicon readout integrated circuits (ROICs) using indium bumps to form cameras that operate at a frame rate of 30 Hz.
Although singleband QWIPs can deliver infrared images (figure 1), they are inferior to cameras that deliver images containing information from different spectral bands. A number of years ago JPL fabricated a four-band, spatially-separated QWIP FPA camera in which each band fulfilled a different need. One band detected forest fires, another pollution, a third recorded cloud characteristics, and a fourth monitored weather patterns. This detector, which had a 640 × 512 pixel focal plane, also had a unique feature: the four infrared bands were independently and simultaneously readable on a single imaging array. This gave the FPA an advantage over competing broadband detectors, such as HgCdTe, that have a relatively narrow spectral response.
Each of the bands that formed the four-band camera detected radiation only in that band, and this enabled the detector to have little or no spectral cross-talk. The design of this detector also enabled simultaneous read mode operation of the array, as opposed to the frame read mode used by other multiband FPAs. These advantages eliminated the need for moving parts, and this reduced the size of the instrument, its weight, mechanical and optical complexity, and power requirements.
The four detector bands were defined by a deep trench etch process, and the unwanted spectral bands were eliminated by electrically shorting the top detectors with gold-coated reflective gratings that provided light coupling to the QWIP stack (figure 2). The FPAs were then combined with 640 × 512 pixel silicon CMOS ROICs. The imager operated at a bias of -1.5 V, covered the 4-6 μm, 8.5-10 μm, 10-12 μm, and 13-15 μm spectral bands, and was mounted onto an 84-pin leadless chip carrier. One frame of a video, taken with the camera cooled to 45 K, is shown in figure 3.
This four-band camera has been used to investigate the environmental impact of vegetation burning in Africa, and was modified by NASA s Goddard Space Flight Center to form a hyperspectral infrared camera operating over 209 wavelength bands. Detection over many bands was achieved by covering the array with several narrowband spectral filters that each selected a different spectral range.
Although this form of camera delivers multipleband performance, each pixel detects only a single spectral band. Consequently, these cameras produce images with separate regions corresponding to the intensities at different spectral ranges. This can be seen in the image of the soldering iron in figure 3. To form images that have spectral information for every band at every point in the image requires that several separate pictures are taken with the camera pointing in slightly different directions, before the results are added together. This approach can be used for certain applications, such as imaging pollution from a satellite, but it is clearly advantageous for every pixel in the array to simultaneously detect multiple spectral ranges.
JPL is now developing dualband cameras that contain an array of pixels that simultaneously detect light in the MWIR and LWIR spectral regions. These cameras are suitable for many applications. For example, the military can use them to determine the absolute temperature of a target with unknown emissivity and to differentiate between missiles, warheads, and decoys. The imagers can also be used for earth and planetary remote sensing, astronomy, and medical applications. These monolithically integrated pixel-collocated FPAs also eliminate the need for the beam splitters, filters, moving filter wheels, and rigorous optical alignment that are required in dualband systems that use two separate singleband FPAs, or in broadband FPA systems with filters.
JPL s dualband infrared FPAs use GaAs-based QWIPs, because this technology offers a narrowband response, wavelength selection, and stability. The photosensitive MQW region of each QWIP is transparent to other wavelengths, giving the imager an important advantage over conventional interband detectors. This spectral transparency also enables dualband FPAs using QWIPs to deliver negligible spectral cross-talk.
The 320 × 256 dualband QWIPs developed by JPL are similar to the singleband devices already described, but the two detectors are separated by an intermediate GaAs layer (figure 4). The device structures and the contact layers were grown in situ during a single MBE growth run. Individual pixels for the dualband QWIP detector arrays are fabricated using a process similar to that used for their singleband counterparts, but via holes are added to access the
silicon ROIC s electrical connections.
The process begins by dry etching through the photosensitive GaAs/InGaAs/AlGaAs MQW layers into the 0.5 Âµm-thick heavily n-type doped GaAs intermediate contact layer. This forms the MWIR 320 × 256 pixel detector arrays and via holes to access the detector common contact. LWIR pixels are then fabricated along with additional via holes to access these pixels, before a thick insulating layer is deposited and contact windows are opened at the bottom of each via hole and on the top surface. Once this is completed, ohmic contacts are added before excess metal is removed with a lift-off process.
JPL has recently produced 20 dualband detectors, each containing FPAs that have a pitch of 40 Âµm and an actual pixel size of 38 × 38 μm on a 4 inch GaAs wafer. The 320 × 256 pixel detectors were formed by using indium bonding to combine the arrays with CMOS ROICs.
Although the dualband cameras have yet to be tested, JPL s work has shown one route to fabricating detectors that require a single exposure to determine the temperature of an object. This work could lead to imagers that can detect three separate bands on a single pixel. This would improve the accuracy of temperature measurements and consequently the reliability of weather forecasting - a task that will be increasingly important if extreme weather becomes more common.
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