Much of our work is aimed at developing new technologies which benefit astronomy and imaging science in general. On this page we describe some of the more important of these technologies. Also see our References section for technical papers.
The process of optimizing a detector for backside illumination in order to improve its quantum efficiency and range of spectral response is referred to as backside processing. This is the largest area of expertise at ITL and accounts for a significant amount of our effort. Backside processing refers to the sequence of steps required to convert a front illuminated CCD (as fabricated at a semiconductor manufacturer's foundry) into a highly optimized detector suitable for low light level scientific and industrial imaging.
While most of our processing is performed at the die level, we have also developed a process for hybridizing and backside processing 150 mm wafers. CCD wafers are hybridized to aluminum nitride ceramic substrates with indium bumps and laser drilled vias and then thinned as a complete wafer. Backside coating and dicing follow, yielding fully buttable backside devices which are no larger than the original CCD die. This represents a true 4-side buttable technology.
The individual backside process steps are described below.
Our back-illuminated CCDs are flip chip bonded or hybridized to mount their front side against a stable support substrate. The flip chip process also allows electrical interconnection from the CCD front side bonding pads to matching pads on the substrate. Gold bumps are placed on the CCD bonding pads prior to flip chip bonding.
We also have a fully automated stud bumping machine which applies bumps at the wafer level. We use flip chip bonding machines to align the device with the substrate and apply the necessary heat and pressure. We have four bonders capable of bonding device larger than 6" including detectors, wafers, and substrates.
A very important goal of our CCD optimization has been to produce a flat and stable imaging surface . Our target flatness is to maintain the imaging surface to within <10 microns of a plane, peak-to-valley. The flip chip process allows this flatness to be maintained because the CCD is forced against a custom silicon support substrate. Profilometry of thinned CCDs show we can meet this flatness specification as well as exceed it when required.
After flip chip bonding, the CCD is attached to the silicon substrate only by the adhesion of the bumps. To ensure mechanical stability, we underfill epoxy between the CCD and substrate.
Larger devices are more difficult to underfill due to the large area involved. We have developed ways to underfill large devices, such as 4kx4k CCDs and 150 mm wafers, without producing bubbles of air (voids) under the chip. Air bubbles, although not visible at the underflow stage due to device thickness, become problematic after a device is thinned.
The heart of CCD optimization is backside etching or thinning.
We have designed and constructed a suite of linear agitation thinning machines which move the CCD in an acid bath. Uniformity to better than one micron is obtained with this method. Etching is done in an acid mixture selective to p+ silicon. Thinning is accomplished on a die basis. We use a wax border to protect the front-side device circuitry and substrate traces from being attacked by the acid. We have successfully thinned devices as large as a 10kx10k 9-micron pixel CCD, which is the largest devices which fits on a 150 mm silicon wafer.
We use a second acid etch to remove any remaining p+ silicon and to remove stains which sometimes form on the surface. This etch is non-selective and can therefore be used to thin into the epitaxial layer to tailor device thickness, if necessary. Resolution can also be improved by ultra-thinning the device to eliminate any field-free region in the CCD. We make use of this fact to ultra-thin devices to be used in applications requiring the highest possible spatial resolution.
One of the most significant quantum efficiency losses of back illuminated CCDs is reflection off their back surface. The thinning process creates a mirror-like finish with an extremely high specular reflectivity. This reflection loss approaches 60% in the UV. The application of a thin film antireflection (AR) coating directly onto the CCD back surface can therefore significantly increase QE.
The Imaging Technology Laboratory has pioneered the development of high efficiency AR coatings for back illuminated CCDs. We have found several suitable materials for silicon AR coatings, including hafnium oxide (HfO2) and magnesium fluoride (MgF2). We routinely produce single layer HfO2 and double-layer HfO2-MgF2 AR coating for our devices. These coatings provide nearly 100% QE at selected wavelengths with very high QE throughout the entire near-UV to near-IR spectral region. The picture below shows our primary vacuum chamber.
For permanent backside charging, we apply a Chemisorption Coating (developed at ITL) to produce a net negative charge on the detector back surface. There is no backside damage using this process which would cause QE-temperature instabilities and/or reduce the maximum QE obtained. We routinely achieve flat field uniformity of better than 5% (measured at 400 nm) with Chemisorption Charged devices.
A wafer dicing saw is needed to cut (or dice) the individual detectors from the round wafers on which they are fabricated. We also dice the flip chip bonding substrates produced on silicon wafers.
We have two saws for dicing wafers up to 6-inches in diameter. The saw has automation capabilities and can be used to dice a number of materials, including silicon, glass, and ceramics. It has been fitted with a custom high-conductance water-feed system which reduces static charge build-up during dicing which can lead to Electrostatic Discharge (ESD) failure of the sensitive detectors.
Packaging of our back illuminated CCDs is accomplished by mounting the thinned CCD with its substrate to a metal package. We have designed each silicon substrate to fit into a particular Kovar or Invar package. Each electrical trace on the substrate leads from an indium bump to a wire bonding pad. Commercial Kovar packages are not flat enough for the large area CCDs require in spectrographs and for wide field mosaics. By replacing all these packages with custom Invar carriers, we will routinely achieve flat and stable imaging surfaces as well as provide a much simpler I/O connection in the dewar.
Standard thermo compression gold ball bonding is used to wire bond from the pad to an I/O pin. To check the reliability of our wire bonds, we pull test each to the required MIL-STD specification.
We characterize individual die as well as entire lot runs at the wafer level before proceeding with backside processing. Our tests include DC shorts and opens, AC functionality, cosmetics, CTE, and absolute gain. We perform these tests as low as -60C.
The UA Imaging Technology Laboratory, with original funding from the National Science Foundation, has developed a cold CCD wafer probing system to quantitatively characterize scientific CCDs at the wafer and die level. The goals of this system are to evaluate devices for front and back illuminated packaging, to provide rapid feedback to CCD manufacturers concerning device performance, particularly for new devices and device technologies, and to allow cryogenic and imaging characterization of other devices and device structures. We have adapted our system to handle many die and wafer sizes.
We have developed software and hardware systems for detector characterization, usually aimed at fully testing CCDs for scientific applications.
We also provide detector testing as a service to the community for a nominal fee. Our goal is to provide rapid feedback of device performance using well proven and calibrated techniques. All the testing is automated and script driven, requiring no operator intervention once the device is set up. We have developed an automated system using Python and LabVIEW programming for measuring quantum efficiency (QE), read noise (down to 1.0 electrons), gain, full well capacity, linearity, photoresponse non-uniformity, dark current and dark-current non-uniformity, and charge transfer efficiency (CTE).