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As part of NASA's Earth Observing Fleet Landsat has been monitoring the planet's health
for over 40 years. With stunning images in multiple wavelengths, Landsat provides valuable
data to farmers, scientist, city planners, as well as the public. And behind these images
lies QWIP, the Quantum Well Infrared Photodector, which is processed here in the Detector Development
Lab at NASA Goddard. Similar to the sensor in your digital camera or cell phone, the
QWIP detector is designed to be more sensitive and to detect specific wavelengths. Let me
show you how these high-end detectors are created. The spectrum of light contains many
wavelengths of which the colors of visible light are a small portion. We have built and
packaged detectors to cover wavelengths from Microwave to X-ray The basic goal of a detector
is to absorb the energy from a region of these wavelengths of light and turn it into an electrical
signal. The first step for us is to take the wavelength, resolution, and sensitivity requirements
of the mission and turn that into a detector design using CAD software. The layout is sliced
into individual layers or masks that will be combined during fabrication to create the
final detector device. Welcome to the Detector Development Lab. This is a unique, Class 100
clean room where we have the agility and technology to develop first-of-a-kind detectors as well
as the experience and the process control to turn those into flight qualified products.
All of the specialized equipment in this lab is used for 1 of 3 general purposes - Lithography,
Etching, and Deposition. Together these 3 processes make up our basic toolset. The first
basic process is called Lithography. Don't adjust your picture, the yellow lights provide
protection for photoresist, a photosensitive polymer that is used to define the detector
geometries to as small as 1 micron. The first step in lithography is to apply photoresist
to the substrate. This is done by spinning on a liquid suspension of the polymer in the
spin-coater. The coated wafer is taken to the photo-mask aligner, where the chrome-on-quartz
mask that was created by the design and layout is placed in extremely close proximity to
the substrate and aligned to existing patterns with in a micron of accuracy. A UV light source
exposes the photoresist through the mask, transferring the pattern into the photoresist.
Finally the substrate is developed, rinsing away the exposed areas of the photoresist.
The next step is to use this protective mask and etch to remove material from the exposed
areas of the substrate. We have a wide variety of methods for removing materials including
wet chemical etchants, reactive plasmas, and ion sputtering. In one of the most fascination
examples, the Deep-Reactive-Ion Etcher uses short etching and passivation cycles in a
high-power ICP chamber to etch silicon 100's of microns deep with 100:1 aspect ratios.
After etching, the photoresist mask is stripped off in a solvent and the substrate is ready
for deposition of the next layer of material. Deposition is the third basic process in our
toolset and it covers a wide range of specific processes and materials. During deposition,
a thin film of material is added back to the surface of the detector's substrate. Materials
run the gamut from insulating dielectrics to metal-nitrides, to pure metals and even
superconductors. Here in the Atomic-Layer-Deposition system, single atomic layers of materials
can be built up one-by-one for precise control of thickness and uniformity. Once the deposition
is complete the substrate goes back to lithography to pattern the new layer of material. In building
up the designed detector we start with a substrate such as silicon or gallium arsenide wafers.
Next, numerous iteration of these 3 basic process steps are applied to the substrate
in order to build up the materials and geometry needed to detect the energy of interest such
as IR, define the pixels and allow electrical read-out, while meeting all the scientific
requirements for the mission. Here is a gallium arsenide wafer with 16 QWIP arrays that has
completed fabrication. The substrate is taken to our packaging lab where it is diced into
individual dies. The die are screened, then meticulously cleaned and indium-bump bonded
directly to a specialized Read-Out Integrated Circuit, which converts the raw electrons
generated in each detector pixel to a signal that can be interfaced to the instrument's
computers. Finally, the detector and ROIC are bonded to the necessary fixtures and PC
boards and the detector subsystem is ready for integration with the rest of the scientific
instrument. The QWIP detector on Landsat 8 is just one example of many missions supported
by the DDL. We have built and packaged detectors to cover wavelengths from X-ray to microwave,
in support of missions such as ASTRO-H, SWIFT, HAWC, Suzaku, and JWST. Each detector was
specifically designed to meet the science and mission requirements, developed, fabricated,
packaged, tested, and delivered for integration. These unique capabilities at Goddard have
helped NASA to stay on the cutting edge of instrument development and scientific discovery.
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