Hazards and controls at the Sandia National Laboratories microelectronics development laboratory Page: 3 of 6
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The wafers are cleaned in exhausted wet benches with solutions that may contain sulfuric
acid, hydrofluoric acid, ammonium hydroxide or other corrosive chemicals. A cassette containing
several wafers is placed into the bath for cleaning. The cassette is then removed, placed into a
rinse tank, and rinsed with deionized water. They are rinsed until all traces of chemicals are
removed. The cassette is then placed into the spin rinse dryer where the wafers are dried.
Automatic chemical processing systems are also used for cleaning. The potential exposure to
chemicals is limited to filling the canisters and changing the filters. Personal samples for inorganic
acid exposure are collected using low flow samplers and washed silica solid sorbent tubes in
accordance with NIOSH method 7903.
Before field oxidation, the wafers are cleaned with a sulfuric acid/hydrogen peroxide
solution followed by a hydrofluoric acid solution. Next, they are cleaned in Standard Clean 1
(SC-1), ammonium hydroxide/peroxide and hydrochloric acid/peroxide followed by Standard
Clean 2 (SC-2), hydrochloric acid/hydrogen peroxide.
After field oxidation, the wafers are cleaned in a hydrofluoric acid solution and rinsed. Then, the
nitride is stripped in hot phosphoric acid.
Similar cleans are used before gate oxidation, before nitride sidewall nitride deposition,
before titanium is sputtered onto the wafers for titanium silicide, and before the first interlevel
dielectric multiple deposition/etch.
The wafers are cleaned with N-methyl 2-pyrrolidone (NMP) before tungsten via fills and
before and after metal stack deposition. Passive dosimeters are used for personal monitoring of
Photolithography is used to produce each of the 15 mask levels of the wafer. To form the N+
source/drain regions, the wafers are coated with photoresist, exposed, developed and UV
Before photoresist coating, a wafer is first baked and primed in a vapor prime oven.
Vapors of an adhesion promoter, usually hexamethyldisilazane (HMDS) are drawn into the
chamber, coating the wafer's surface. After priming, the chamber is exhausted into the solvent
exhaust system. The wafer is transported from the priming chamber to the photoresist coating
chamber where photoresist (resin, UV sensitive initiator and solvent) is spun onto the wafer.
After the photoresist has been spread over the wafer, the edge of the spinning wafer is sprayed
with an "edge-bead remover" (propylene glycol monomethyl ether acetate). Passive dosimeters
are used for personal monitoring. Glycol ethers with known reproductive toxicity are not used in
the MDL. The waste photoresist and edge-bead remover that is spun off the wafer is collected in
a can under the coating chamber for disposal as hazardous waste. The entire process has local
exhaust ventilation. The wafer is then aligned to a mask in the exposure system. A UV light is
shown through the mask onto the wafer, exposing the photoresist under the transparent portions
of the mask. The wafer is then transported into a spin chamber for spray developing. While the
wafer spins, an alkaline developer (dilute tetramethyl ammonium hydroxide) is dispensed onto the
surface. The developer solution dissolves the photoresist in the areas that were exposed to the
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Benton, M.A. Hazards and controls at the Sandia National Laboratories microelectronics development laboratory, article, March 1, 1997; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc675631/m1/3/: accessed October 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.