Technical Insight
Near-IR focal-plane arrays improve camera performance
The recent improvement in performance of InGaAs-based detectors and cameras has opened up a wide range of applications, according to Austin Richards of Indigo Systems.
Advances in focal-plane array (FPA) design, along with the availability of higher quality III-V material, have enabled the fabrication of high-performance, commercial 2D arrays using InGaAs photovoltaic detectors. These advanced InGaAs FPAs have become instrumental in the creation of highly sensitive near-infrared (NIR) cameras that deliver excellent image quality, rivaling that associated with silicon CCD detectors, but with a spectral sensitivity that is shifted into the NIR band, from 900 to 1700 nm.
The ability to produce high-quality images of objects in the 900-1700 nm waveband with high sensitivity opens up a range of applications, including laser-beam characterization, agricultural and petrochemical inspection, forensics, NIR imaging spectroscopy and astronomy. InGaAs detectors offer high quantum efficiencies (around 85%) in the 900-1680 nm band, unlike the competing technologies of lead-oxysulfide vidicons and coated CCD cameras. At wavelengths longer than 1100 nm silicon becomes transparent, making silicon-based cameras ineffective unless coated with a wavelength-shifting material. This coating results in a quantum efficiency of about 1-2% in the 1100-1700 nm band, which is useful in laser-beam profiling applications where intensities are high, but not in many other applications.
High sensitivity is important, since NIR scenes often span a wide dynamic range and light signals can be quite low in intensity, especially in NIR imaging spectroscopy where only a small portion of the InGaAs sensors passband is admitted. InSb FPAs have high quantum efficiency in the NIR band, but require cooling to cryogenic temperatures using mechanical cryocoolers that cost about $10,000. Their spectral response is from 1 to 5.5 µm, so the sensor must be combined with a bandpass filter to make a camera that operates solely in the NIR band. Because the bandgap energy of InGaAs is much higher than InSb, InGaAs FPAs can operate at temperatures around ambient (25 °C) with noise performance comparable to InSb sensors at liquid-nitrogen temperatures. The thermoelectric coolers cost about $20, making the overall camera cost much lower.
The FPA sandwich
The FPAs are made from InGaAs photodiode arrays fabricated on 3 inch, MOCVD-grown InGaAs-on-InP epiwafers. The 30 x 30 µm2 detectors are made by diffusing zinc (a p-type dopant) through a diffusion mask and into an n-type (S- or Si-doped) InGaAs epilayer. After forming ohmic contacts, a total of 82,000 truncated-cone-shaped bumps of indium are deposited onto the contact metal pads to make a 320 x 256 pixel array. The same number of indium bumps are deposited onto a silicon mixed-signal IC die, which Indigo Systems designs and offers as a standard readout IC (ROIC) product.
After a proprietary cleaning process, the indium bumps on the InGaAs detector dies are fused to the bumps on the ROIC wafer under pressure in a cold-forming process. The technicians probe the ROIC bond pads and check for operational hybrid interconnection, before wire-bonding the FPA to a motherboard inside a dewar package.
The combination of an anti-reflection (AR) coating on the BK-7 glass or quartz window of the package, and an AR coating deposited directly on the detectors, results in extremely high transmissivity in the 1300-1550 nm range. The vacuum package also incorporates thermoelectric cooling to stabilize the FPA at an appropriate operating temperature, typically between 0 and 20 °C.
The FPAs that are incorporated into cameras have 99.5% operability and few cluster defects. In addition, algorithms have been developed that substitute neighboring good pixels for the few isolated bad pixels. These semiconductor and software advancements result in an image quality that competes favorably with silicon CCD detectors.
InGaAs camera performance
Because Indigo Systems designs and fabricates its own ROICs and detectors, and then integrates them into its line of cameras, the company controls the supply chain, assuring that the FPAs have an ROIC optimized for use with InGaAs detectors. One of the ROIC chips used with InGaAs detectors - the ISC9809 - is designed for ultra-low-noise operation in very-low-background applications, i.e. environments in which there is a very weak signal from the target object. The ISC9809 has the additional feature that the integration capacitor used in each unit cell on the focal plane is selectable, enabling 20 times gain changes without appreciable noise increases. Indigo Systems manufactures three varieties of InGaAs cameras. The highest performance camera is the Phoenix, which has a 14-bit dynamic range in its A/D converter. This enables the user to select exposure times as short as several microseconds and as long as 660 ms -a change in sensitivity of approximately five orders of magnitude. The on-chip gain can also be switched by a factor of 20. The tremendous operational range of camera sensitivity means that the camera can be used both for the extremely low light levels typical of applications in astronomy, and for the high-brightness conditions encountered in laser-beam measurement.
The ISC9809 FPA has four separate out-puts, and can stream image data at rates up to 40 megapixels per second into a computer. This corresponds to frame rates of 346 Hz at the full 320 x 256 array size, enabling high-time-resolution recording of very dynamic events such as missile launches and explosions. In one embodiment of the Phoenix camera system, two serial data links transmit image data from the camera to a PC equipped with a special frame grabber board and 1 gigabyte of RAM, allowing large data bursts to be acquired. A new version of the Phoenix camera contains a 640 x 512 pixel InGaAs array for applications needing higher sensor resolution.
Unmasking art forgeries
Art dealers, museums and collectors now have a new tool to protect themselves from multi-million dollar liabilities caused by elaborate forgeries. Works of art can be authenticated by imaging through surface pigments to detect the presence of any "underdrawings" beneath the top layer. The art conservator can thus verify that the style of the underdrawing matches that of the artist s other works, and can also detect the possible existence of another painting beneath. Silicon CCD detectors have historically been used to image through art materials such as dirty varnish that can obscure the painting, but standard silicon detectors begin to lose their effectiveness at about 1000 nm, and cut off completely at 1100 nm. In contrast, InGaAs sensors can be used to view through traditional paint pigments, which exhibit a high degree of transparency at wavelengths longer than 1100 nm.
Figure 1 shows an oil pigment test panel imaged with visible and NIR sensors. The horizontal black lines are made by a variety of media, including pencil and charcoal. All the pigments show some degree of transparency in the 900-1680 nm band, with the exception of the black pigment, which contains carbon particles that are highly absorbing across the NIR band. Imaging through black pigment requires X-ray techniques.
The ability to produce high-quality images of objects in the 900-1700 nm waveband with high sensitivity opens up a range of applications, including laser-beam characterization, agricultural and petrochemical inspection, forensics, NIR imaging spectroscopy and astronomy. InGaAs detectors offer high quantum efficiencies (around 85%) in the 900-1680 nm band, unlike the competing technologies of lead-oxysulfide vidicons and coated CCD cameras. At wavelengths longer than 1100 nm silicon becomes transparent, making silicon-based cameras ineffective unless coated with a wavelength-shifting material. This coating results in a quantum efficiency of about 1-2% in the 1100-1700 nm band, which is useful in laser-beam profiling applications where intensities are high, but not in many other applications.
High sensitivity is important, since NIR scenes often span a wide dynamic range and light signals can be quite low in intensity, especially in NIR imaging spectroscopy where only a small portion of the InGaAs sensors passband is admitted. InSb FPAs have high quantum efficiency in the NIR band, but require cooling to cryogenic temperatures using mechanical cryocoolers that cost about $10,000. Their spectral response is from 1 to 5.5 µm, so the sensor must be combined with a bandpass filter to make a camera that operates solely in the NIR band. Because the bandgap energy of InGaAs is much higher than InSb, InGaAs FPAs can operate at temperatures around ambient (25 °C) with noise performance comparable to InSb sensors at liquid-nitrogen temperatures. The thermoelectric coolers cost about $20, making the overall camera cost much lower.
The FPA sandwich
The FPAs are made from InGaAs photodiode arrays fabricated on 3 inch, MOCVD-grown InGaAs-on-InP epiwafers. The 30 x 30 µm2 detectors are made by diffusing zinc (a p-type dopant) through a diffusion mask and into an n-type (S- or Si-doped) InGaAs epilayer. After forming ohmic contacts, a total of 82,000 truncated-cone-shaped bumps of indium are deposited onto the contact metal pads to make a 320 x 256 pixel array. The same number of indium bumps are deposited onto a silicon mixed-signal IC die, which Indigo Systems designs and offers as a standard readout IC (ROIC) product.
After a proprietary cleaning process, the indium bumps on the InGaAs detector dies are fused to the bumps on the ROIC wafer under pressure in a cold-forming process. The technicians probe the ROIC bond pads and check for operational hybrid interconnection, before wire-bonding the FPA to a motherboard inside a dewar package.
The combination of an anti-reflection (AR) coating on the BK-7 glass or quartz window of the package, and an AR coating deposited directly on the detectors, results in extremely high transmissivity in the 1300-1550 nm range. The vacuum package also incorporates thermoelectric cooling to stabilize the FPA at an appropriate operating temperature, typically between 0 and 20 °C.
The FPAs that are incorporated into cameras have 99.5% operability and few cluster defects. In addition, algorithms have been developed that substitute neighboring good pixels for the few isolated bad pixels. These semiconductor and software advancements result in an image quality that competes favorably with silicon CCD detectors.
InGaAs camera performance
Because Indigo Systems designs and fabricates its own ROICs and detectors, and then integrates them into its line of cameras, the company controls the supply chain, assuring that the FPAs have an ROIC optimized for use with InGaAs detectors. One of the ROIC chips used with InGaAs detectors - the ISC9809 - is designed for ultra-low-noise operation in very-low-background applications, i.e. environments in which there is a very weak signal from the target object. The ISC9809 has the additional feature that the integration capacitor used in each unit cell on the focal plane is selectable, enabling 20 times gain changes without appreciable noise increases. Indigo Systems manufactures three varieties of InGaAs cameras. The highest performance camera is the Phoenix, which has a 14-bit dynamic range in its A/D converter. This enables the user to select exposure times as short as several microseconds and as long as 660 ms -a change in sensitivity of approximately five orders of magnitude. The on-chip gain can also be switched by a factor of 20. The tremendous operational range of camera sensitivity means that the camera can be used both for the extremely low light levels typical of applications in astronomy, and for the high-brightness conditions encountered in laser-beam measurement.
The ISC9809 FPA has four separate out-puts, and can stream image data at rates up to 40 megapixels per second into a computer. This corresponds to frame rates of 346 Hz at the full 320 x 256 array size, enabling high-time-resolution recording of very dynamic events such as missile launches and explosions. In one embodiment of the Phoenix camera system, two serial data links transmit image data from the camera to a PC equipped with a special frame grabber board and 1 gigabyte of RAM, allowing large data bursts to be acquired. A new version of the Phoenix camera contains a 640 x 512 pixel InGaAs array for applications needing higher sensor resolution.
Unmasking art forgeries
Art dealers, museums and collectors now have a new tool to protect themselves from multi-million dollar liabilities caused by elaborate forgeries. Works of art can be authenticated by imaging through surface pigments to detect the presence of any "underdrawings" beneath the top layer. The art conservator can thus verify that the style of the underdrawing matches that of the artist s other works, and can also detect the possible existence of another painting beneath. Silicon CCD detectors have historically been used to image through art materials such as dirty varnish that can obscure the painting, but standard silicon detectors begin to lose their effectiveness at about 1000 nm, and cut off completely at 1100 nm. In contrast, InGaAs sensors can be used to view through traditional paint pigments, which exhibit a high degree of transparency at wavelengths longer than 1100 nm.
Figure 1 shows an oil pigment test panel imaged with visible and NIR sensors. The horizontal black lines are made by a variety of media, including pencil and charcoal. All the pigments show some degree of transparency in the 900-1680 nm band, with the exception of the black pigment, which contains carbon particles that are highly absorbing across the NIR band. Imaging through black pigment requires X-ray techniques.
