APPLICATION OF SPUTTERING TO THE FABRICATION OF NEUTRON DETECTOR ANODES. Page: 17 of 27
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BNWL-553
thus, the probability of an ionizing collision with a gas atom.
Since the local ion density will depend on the magnetic field
strength, C4) a contoured magnetic field can favorably influence
deficiencies in plasma uniformity. This process may also work
adversely, For example, the magnetic field surrounding the
filament may distort an otherwise satisfactory plasma. The dis-
tortion may be minimized by increasing the filament-cathode
spacing, reducing the filament current, and/or using noninduc-
tively wound filaments.
Composition
Two considerations are important in regard to deposit
composition-impurity pickup in the sputtering process, and
preservation of alloy composition from cathode to deposit.
Impurity pickup is minimized by utilizing a high vacuum sput-
tering chamber, baking to remove water vapor and other gases
adsorbed on the walls, and backfilling with research grade inert
gas. The necessity of these precautions can be appreciated by
noting that each atomic layer of the deposited metal is in turn
exposed to the chamber atmosphere; in contrast to conventional
processes such as casting, in which only a small fraction of
the metal is exposed.
When reactive metals are being sputtered, they quickly
purify the atmosphere by gettering. Thus, impurity pickup in
the deposit can be reduced by a preliminary sputtering oper-
ation, using a shutter to protect the substrate.
In addition to impurity elements in the atmosphere, the
inert gas itself may be incorporated into the deposit. An
inert gas atom adsorbed on the substrate may be buried by the
sputtered metal before it has a chance to escape. The amount
of pickup by this mechanism depends upon the sticking coeffi-
cient of the gas, and thus can be greatly reduced by increas-
ing the substrate temperature. The low pressure operating
capability of the triode apparatus is beneficial in this regard.10
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McClanahan, E.D.; Moss, R.W. & Busch, R. APPLICATION OF SPUTTERING TO THE FABRICATION OF NEUTRON DETECTOR ANODES., report, January 1, 1967; Richland, Washington. (https://digital.library.unt.edu/ark:/67531/metadc1029001/m1/17/: accessed May 1, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.