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The University of Arizona Imaging Technology Laboratory (ITL) is able to provide the following services to projects within the University of Arizona as well as to external Sponsors. Our expertise has focused on providing these services for astronomy-related projects, we often work on other types of scientific and industrial imaging projects. Please contact Michael Lesser at mlesser@email.arizona.edu for further information.

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 efforts. Backside processing refers to the sequence of steps required to convert a front illuminated CCD or CMOS imaging sensor(as fabricated at a semiconductor manufacturer's foundry) into a highly optimized detector suitable for low light level imaging.

CCD Wafer Probing

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.

Our probe systems are show below.

Electroglas 4090u+ cold wafer prober.
Electroglas 2001x probe station with dark enclosure and cold chuck.
Signatone manual prober.

Wafer Dicing

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.

Our 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. A unique feature of our DISCO DAD3650 saw is that it has been custom fitted with an IR camera which allows dicing silicon wafers and die from the backside.

DISCO DAD3650 dicing saw with IR camera.
Wafer taping related dicing preparation equipment.
View from the dicing saw camera of a CCD through the back surface.

Hybridizing – Flip Chip Bonding

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 devices larger than 6“ including detectors, wafers, and substrates.

Stud bumping machine

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.

Epoxy underfill stations

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.

Flip chip bonder or hybridizer

Silicon Sensor Etching

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 may 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.

Laminar flow clean wet benches

Thin Films Deposition and Analysis

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.

Thin films vacuum coating chamber
Filmetrics Thin Films Analyzer

Sensor Packaging and Wire Bonding

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 required 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.

Wire bonder for sensor packaging.
Dage 4000 wire pull tester.

Metrology

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Sensor Cleaning

We provide sensor cleaning services due to the delicate nature of imaging devices.

Spin cleaning system for the critical cleaning of imaging sensors.
Optical microscope for inspection and cleaning of sensor surfaces.
Stainless steel oven for sensor cleaning.

Sensor Characterization

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).

Cryogenic test dewar at an optical test bench