Angstrom--Unit of length.  One angstrom = 1 Å = 10^-10 meters = 0.0000000001 meters = 0.1 nanometers.  Angstroms are the units generally used when discussing wavelengths of light.

Back-Illuminated CCD--(see also Front-Illuminated CCD)  Back-illuminated just means that the light is coming in the back side, rather than the front side.  To make the light hit the backside, the CCD is flipped over and made very, very thin (about 10-15 microns or about 1/10th the thickness of a typical human hair!).  This is essentially what we can do here at the lab--take a front-illuminated CCD and make it back-illuminated by flipping it over and making it thin!  (See the schematic below.)

Schematic of a typical back-illuminated CCD 

processed at the Imaging Technology Lab

Ball Application--Small gold balls, about the diameter of a human hair (0.004 inches) and a little taller than the thickness of a sheet of tissue paper (approximately 35 microns), are put down on the pads of a CCD to prepare the device for bump bonding.  During bump bonding, these gold balls are forced into the indium bumps on a substrate (see bump or flip chip bonding for more).  The gold balls help make the electrical connection from the CCD bond pads to the substrate.

Bump or Flip Chip Bonding--In our lab, bump or flip chip bonding is used to make a connection from the CCD to its silicon support substrate (see picture at left, substrate traces being aligned to the CCD bond pads).  When the CCD is flipped upside-down onto the substrate, the gold balls applied to the CCD bond pads get smooshed (thanks to applied heat and pressure) into the matching indium bump pads on the silicon substrate.  This makes a solid connection between the CCD and the substrate so that electricity can get to the CCD even though it is upside down.

CCD--CCD stands for charge-coupled device.  A CCD is a detector made on a silicon wafer.  Due to the physical nature of silicon, photons of light that hit it generate electrons in the silicon.  The job of the CCD is to collect these electrons in its "light buckets" (called pixels) during the length of the exposure to light. The more light falling on a particular "light bucket" or pixel, the more electrons that pixel will contain.  The buckets then transfer their electrons (think 'water bucket brigade') out to the CCD controller (which contains the electronics to control the CCD) and on to the computer.  The computer regenerates the image and voila!

CCD Bond Pads--The pads, or 'bond pads', are how the electrical connections are made on a CCD.  The bond pads are connected to traces (think of traces as wires) on the CCD.  The trace/wire carries the electricity to the pixels and other CCD components (amplifiers, etc.) so that the device can function.  CCDs need electricity too!

Charge Transfer Efficiency--When electrons (charge) are transferred from pixel to pixel, not every electron makes it into the next pixel.  A few electrons are left behind in each transfer.  Just like in the CCD Movie above, not all of the rain water in each bucket makes it into the successive bucket when transferred.  A few drops are always left over in the initial bucket.  How well this transfer is done, i.e. the ratio of how many electrons are transferred to how many were actually detected, is called charge transfer efficiency (CTE).  Typically, this number is around 99.9990% for good quality CCDs, where perfect charge transfer is 100%.  When a lot of charge is left behind with each transfer, a device is said to have bad CTE.  If the CTE is bad enough, you can see streaks, as in the image at left.  These streaks are caused by charge/electrons being left behind after transfer.

Coating--The inside of some potato chip bags have a metal coating.  This coating is actually a thin sheet of metal that is deposited on the plastic.  We do the same thing to thinned CCDs!  Coatings are deposited in a vacuum chamber, in which nearly all atmospheric air is evacuated (similar to the environment of space).  In the accompanying picture, the lower chip has a coating of magnesium fluoride, while the upper chip is thinned but is without a coating.  

Dicing--(See also die, below.) The action of slicing a silicon wafer into its respective parts called die.  The picture shows a wafer on the left which, after dicing, is cut into four die (on right).  Due to the precision needed for such a feat, a special saw, similar to a circular saw only with a blade approximately 2 inches in diameter, is used for dicing silicon.  

Dewar--CCDs operate at their optimum when cold.  Dewars are made for just that purpose.  Astronomers want only optimized equipment to collect the most light possible from distant objects.  For the CCD to run at its best, it needs to be cold--as cold as liquid nitrogen (77 Kelvin/-321°F/-196°C).  CCDs generate electrons just from being warm (by warm, we mean room temperature), without exposure to light.  Also, the chamber where the CCD resides needs to be under vacuum.  The CCD will be below the freezing point of water, once cooled.  Water vapor in the atmosphere would condense on the surface of the device (frost!), if the atmospheric air was not removed, and affect the images.   

Die--(also see Dicing above) A single detector (or substrate, whatever the case may be) that is no longer part of its silicon wafer.  A silicon wafer has many die on it, many separate detectors or substrates.  Once the single detectors/substrates are sliced from the wafer, they are referred to as die.  The picture at left shows one CCD die, a Loral 512FT CCD, which used to be part of a 4-inch silicon wafer; and, one Loral 512FT substrate die, which was also part of a 4-inch silicon wafer.

Front-Illuminated CCD--(see Back-Illuminated CCD also)  Light enters the front side of the device on a front-illuminated CCD.  All CCDs are front-illuminated until they are processed and become back-illuminated.  A front-illuminated CCD is thus a CCD die that has been put into a package.  Front-illuminated die are not coated (at least by us), not thinned, not oxidized (they already have an oxide), and not bump-bonded.  (At left, CCD on right is packaged as front-illuminated; on left, a back-illuminated CCD.)

Indium--Atomic number 49 on the Periodic Table of Elements.  Indium is a silvery metal that is very soft and malleable.  It has a low melting temperature and very good cyrogenic (very low temperature, well below the freezing point of water) properties.  We use indium to make bumps on our substrates for flip chip bonding.  (See Bump or Flip Chip Bonding above.

IR--Infrared.  Wavelengths of light from 1 micron to 1 mm.  The infrared wavelengths of light are beyond what our eyes sense.  We do have another way to sense infrared light--our sense of touch!  We sense infrared light as heat--as the heat from a campfire, from a cup of hot coffee, and even from the Sun.

Lapping--Lapping is a process that sloughs off material--here, silicon.  Lapping is accomplished on a "lap" (no, not what Fido likes to sleep upon).  Think of a lap as a round table that rotates as you sand it down (see a picture of a lap at top left).  Except, it's not the table you are sanding but a wafer stuck to the table top.  And just like the table, you want the wafer to be very flat and uniform.  The picture shows the back of a lapped (left) and unlapped (right) wafer.)

Micron--A unit of length.  1 micron = 1 µm = 10^-6 meters = 0.000001 meters = one millionth of a meter.  A typical human hair is about 100 microns.  A sheet of tissue paper is about 25 microns thick.

Oxidation--Process by which an oxide (here, silicon dioxide, SiO₂, or glass) is grown.  Basically, oxygen creates a bond to atoms in a material.  Raw silicon, due to its physical properties, grows a native oxide instantly upon exposure to air.  This native oxide, as it is referred to, is approximately 5-50 Angstroms (5-50x10^-9 meters or 5-50 billionths of a meter) in thickness.  Our oxidation process, with use of an oven that basically steams (much like you steam a lobster or vegetables) that device, enables a high quality oxide to grow on the surface.  The right combination of water, oxygen, pressure, and temperature make this happen.  A number of other things oxidize, besides silicon.  For example, iron produces an oxide upon exposure to air--something we commonly refer to as rust.

Pixel--CCD's main elements.  Pixels are like buckets that collect the light 'raining' on the surface of a CCD (see CCD and the CCD MOVIE above).  Pixels of a typical CCD are each 15 microns square.  Many pixels make up the imaging area of a CCD--many little buckets collecting light.  CCDs are described by the number of pixels they have.  A "4kx4k CCD" has 4096 by 4096 pixels, for a grand total of almost 17 million individual light buckets!

Probing--Probing is a way to test the CCD, making the electrical connections to run it while it is on a wafer or in die form.  We receive CCDs on wafers, and sometimes as die.  But, not every CCD we receive works or works well!  Problems in manufacturing, shipping, or handling of these sensitive devices sometimes results in their demise.  We probe all incoming CCDs so that we do not waste our time working on a device that is 'dead' or sub-par.

Quantum Efficiency (QE)--The quantum efficiency (QE) of front-illuminated CCDs is around 40%.  This means that 40 out of 100 incident photons were detected.  The Imaging Technology Lab back-illuminated CCDs have a peak quantum efficiency upwards of 90%--90 out of 100 photons of a certain wavelength are detected!  On the other hand, photographic film and the human eye have a peak QE of about 10%. 

Silicon--Silicon, atomic number 14 on the periodic table, is a semiconducting material from which integrated circuits (computer chips of all types--processors, memory chips, etc.; CCDs; transistors; etc.) are created.  Silicon is not found in its pure raw form in nature, but mostly in combination with other elements, as in sand and quartz.  A type of sand called quartzite is purified to create silicon for use by the semiconductor industry.

Silicon Dioxide--SiO₂.  Most commonly referred to as glass.  See oxidation.

Substrate--A CCD is bump bonded to a silicon substrate.  The substrate supports the CCD during and after thinning.  Without the substrate, the CCD would be prone to breakage.  A thinned CCD is only about 0.0005" or 10-15 microns thick (1/10th of the thickness of a human hair!).  The substrate also brings out the electrical connections from the front side CCD bond pads to make packaging easier.

Thinning--The etching away of silicon from the backside of a CCD.  Thinning is done in a large bath of acid (see left).  The device is mounted on a support (see left) and agitated.  Wax is used to mount the device to its support, since the acid does not attack wax.  When the acid has eaten away a certain amount of silicon, the CCD that was once 0.010" thick becomes approximately 0.0005" thick!  (That acid must be hungry.) 

UV--Ultraviolet.  Wavelengths of light from 4-400nm (one nm = 1 nanometer = 10^-9 meters).  The Sun produces ultraviolet light.  Certain wavelengths of ultraviolet light give us a suntan (especially in Arizona!).

Wafer--All semiconductor devices are fabricated on wafers.  Round wafers are used in order to simplify automated handling during manufacturing.  CCDs are typically fabricated on 4", 5", or 6" wafers.  The semiconductor industry as a whole is currently moving toward 8" wafers, although one day wafers will be as large as an LP or record (12" in diameter)!  The larger the wafer, the larger the number of devices that can fit on one wafer--thus reducing manufacturing costs.

Wire Bonding/Bonds-- As part of the packaging process, an electrical connection must be made from the substrate to the package.  The best way to do this is by applying a thin (0.002" or 50 microns thick) gold wire to the substrate and looping it up and over to the package.  The bond made to the package could either be to a circuit card, as in the schematic at the top of the page, or to a pin of the package.