Hazards and controls at the Sandia National Laboratories microelectronics development laboratory Page: 5 of 6
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Phosphorous, using phosphine, is implanted during polysilicon gate fabrication. Arsenic,
using arsine, is implanted during N+ source/drain fabrication. Personal monitoring for inorganic
arsenic is in conformance with the arsenic standard.
CHEMICAL VAPOR DEPOSITION (CVD)
A CVD process is used to form the polysilicon gates. Amorphous polysilicon is deposited
using low pressure chemical vapor deposition (LPCVD) in a vertical thermal reactor. The
precursors are silane and hydrogen. Silane is heated to 8000 C. yielding silicon and hydrogen. It
dry etches the polysilicon not covered by resist. Phosphorus is implanted using a 15% phosphine
85% hydrogen gas.
The silicon nitride spacer is deposited onto the wafer in a vertical thermal reactor using a
low pressure chemical vapor deposition (LPCVD) process. Silicon nitride grows onto the oxide
layer. Chemical vapor deposition requires the decomposition of a gas to generate the chemical
species of interest which is deposited on the wafer in solid form. The precursors for silicon nitride
are dichlorosilane, nitrogen and ammonia. The byproducts of the decomposition, hydrochloric
acid and ammonium chloride are pumped to a burnbox. The burnbox heats the chemicals to
produce more benign chemicals. The result of the combustion is then pumped to a scrubber.
There is continuous monitoring for dichlorosilane and ammonia.
The CVD process is also used in forming the first interlevel dielectric. A plasma
enhanced tetraethyl orthosilicate (PETEOS) multiple deposition/etch deposits an oxide layer
which conforms to the surface topography of the wafer. The precursor is tetraethyl orthosilicate
(TEOS). The reaction byproducts are water vapor and ethylene (C2H4).
CVD is also used to form tungsten via fills. After titanium and a titanium nitride liner are
sputtered onto the wafer for tungsten via fill fabrication, tungsten is deposited using a chemical
vapor deposition process. The precursors are typically tungsten hexafluoride, silane and
hydrogen. The byproducts are hydrofluoric acid in gas form, silicon tetrafluoride and hydrogen.
These are pumped under negative pressure to a burn box and scrubber. The wafers are then
planarized with chemical mechanical polishing and the tungsten studs are recessed using a sulfur
hexafluoride with an argon plasma.
CVD is used for the final passivation. A complex circuit design may require 15 or more
layer patterns to create the final integrated circuit (IC). The final passivation step contains a
chemical vapor deposition (CVD) of p-glass. Phosphosilicate glass (PSG) contains phosphorus.
The precursors for this reaction are silane, oxygen, nitrogen, and phosphine. The reaction
byproducts are water vapor and undecomposed phosphine and silane. This is exhausted into a
burnbox for combustion, decomposition and oxidation and then scrubbed. The toxic gas
monitoring system monitors continuously for silane and phosphine.
Metal deposition is used to form titanium silicide (TiS2). First, titanium is sputtered onto
the wafers. RF is used to sputter the titanium onto the wafer in a stainless steel chamber. The
titanium target is bombarded with argon ions to release titanium ions. These atoms form a film on
Titanium silicide is formed in a rapid thermal annealer with a nitrogen atmosphere.
The unreacted titanium is wet etched in an ammonium hydroxide/peroxide solution. The final
titanium silicide is formed using a rapid thermal annealer with argon. Titanium alloys with silicon
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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. (https://digital.library.unt.edu/ark:/67531/metadc675631/m1/5/: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.