Space-Based Advanced Imaging Technology

The Advanced Imaging Technology (AIT) program area has had extensive experience in designing and assuring reliable operation of devices in a space environment. In carrying out these programs, the AIT program draws on the extensive experience and capability of the Lincoln Laboratory Engineering Division to provide structural, thermal, electrical, optical, and vibration design and assurance of critical components.

Some examples of AIT space-based science programs are given below.

The Advanced Satellite for Cosmology and Astrophysics (ASCA) was a joint NASA and Japanese Institute for Space and Astronautical Science (ISAS) mission, aimed at space-based X-ray astronomy observations. This mission was launched in February 1993. The AIT program provided the Solid-State Imaging Spectrometer
(SIS) charge-coupled devices (CCDs). Each SIS array was composed of four thinned 420 × 420–pixel CCDs positioned accurately with respect to each other on a hybrid ceramic board. Each CCD was furnished with a narrow trough to guide small-sized charge packets away from many of the radiation-induced traps expected to be formed from space radiation over the life of the mission. This was the first deployment of this radiation-mitigation strategy in AIT-fabricated devices. The SIS devices also contained a unique n+ diode on the back surface of the devices to remove photoelectrons that were not controlled by the CCD gates and so would add unwanted charge to the devices if left untreated.

The Space-Based Visible (SBV) mission was a Department of Defense program to evaluate the efficacy of observation of other satellites from a space-based (versus ground-based) platform. This mission launched in 1996 with the SBV instrument riding on a larger Midcourse Space Experiment (MSX) satellite.

Chandra Great ObservatoryFigure 1. Chandra Great Observatory. (Photo: NASA.)

Chandra is one of the NASA Great Observatories, whose goal is high-resolution imaging of X-ray astronomical targets from space. The AIT program area developed and provided the Advanced CCD Imaging Spectrometer (ACIS) focal-plane array for Chandra. The ACIS instrument is an array of ten 1 k × 1 k deep-depletion CCDs arranged in two arrays (a 1 × 6 linear array and a 2 × 2 area array). Lincoln Laboratory developed and fabricated the CCDs, using radiation-hard technology, and the focal plane array bench, fabricated from beryllium to exacting tolerances.

Figure 1. Chandra ACIS focal plane assembly.Figure 2. Chandra ACIS focal plane assembly.

NASA’s Chandra Great Observatory is shown in Figure 1, and the Advanced CCD Imaging Spectrometer (ACIS) focal plane array—the observatory’s principal imaging device—is shown in Figure 2. Each of the 10 CCD imaging arrays (the squares visible in the photo) is about one inch square. Two of the 10 CCDs were back-illuminated (BI) devices, fabricated using a high-temperature oxidation and annealing for treatment of the back surface, which enabled high quantum efficiency for very low-energy (down to 100 eV) X-ray photons. The two BI devices are the dark ones in the 1 × 6 spectrometer array shown in Figure 2.

Accurate placement of all devices was required in all three directions. Lincoln Laboratory developed a method for placing devices accurately, while also allowing for a specific device to be removed without affecting neighbor devices if it were damaged in assembly; this technology provided assurance that the assembly task could be completed on time with the expected number of devices and with good yield.

Charge-coupled devices are sensitive to environmental radiation found in space, in particular, to damage from high-energy protons. These particles cause displacement damage in the silicon near the spot where they are absorbed. The high-resistivity silicon needed for deep-depletion X-ray imagers is especially sensitive to proton radiation damage, as explained in the section on deep-depletion imagers. Mitigation of this radiation damage involves implantation of a narrow (two-micron-wide) trough in the bottom of all pixels to keep small charge packets from spreading; this limits exposure to only those traps that are located in the trough. A second strategy is to operate the device at an appropriate temperature so that charge traps, once filled, remain filled and therefore are not able to absorb more photocharge. For the Chandra devices and operational conditions, this optimum temperature was close to –100°C. Both of these strategies were implemented in Chandra.

Very early in the mission, Chandra experienced a large unexpected dose of radiation that was caused by relatively low energy protons (~100 keV) focused on the focal plane by the X-ray mirror assembly. These protons caused damage to the front-illuminated devices, but caused insignificant damage to the back-illuminated devices, which were about 50 microns thick. The thickness of the substrate silicon of the latter devices effectively shielded the sensitive CCD charge-transfer channels (near the front surface of the device) from the damaging protons.

The Suzaku mission, launched in July 2005, was a cooperative venture between ISAS/JAXA of Japan and NASA of the United States. The spacecraft’s goal is imaging astrophysical targets in the X-ray wavelength band. One of the three instruments is CCD based, using devices designed and fabricated by the AIT program area. These CCD imagers have a novel charge-injection register at the top of the device to inject a measured amount of charge into the device that is used to fill radiation-induced traps. This procedure has reduced the rate of radiation-induced performance degradation on orbit.

The Extreme Ultraviolet Variability Experiment (EVE) was launched in February 2010. The goal of this mission is to study the EUV radiation from the Sun and to better understand and predict its behavior. The EVE CCDs fabricated at Lincoln Laboratory are back-illuminated devices using a novel back-surface passivation treatment deposited by a molecular-beam evaporation system. This back-surface treatment was found to be surprisingly robust and stable even for substantial doses of ultraviolet radiation.



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