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1 Sputtercoating:a brieftechnicalbackground
6.2,it is usefulto knowabout
Beforeyouunpack,set up andoperateyourHummer@
works. lt isn'tabsolutely
necessary
that
sputtercoatingandhowyournewinstrument
you readthischapter.However,
your
herewillhelpyou understand
the information
Hummerandoperateit better.
lntroduction
producesan image.
ln electronmicroscopy
floodingelectrons
overa specimen
Typically,specimens
are poorelectrical
examinedin an electronmicroscope
conductors
andwillaccumulate
a negativechargefromthe electronflood. Thischarge
distortsthe image.To minimize
and producethe bestimages,specimens
distortion
shouldhavea conductive
surface.Evensamplesthataresomewhatconductive
will
producea betterimageif theirconductive
produce
will
a betterimageif their
conductivity
is increased.
Coatingthe specimen
withmetal,usuallya precious
metalsuchas gold,palladium
or
an alloyof thetwo can increasesurfaceconductivity.
lf appliedcorrectly,
the coating
won'timpairresolution
or surfacedetail.Severalcoatingtechniques
havebeen
developedandare routinelyusedin preparing
samplesfor electronmicroscopy,
Two
commontechniques
are evaporative
coatingandsputtercoating.
Evaporativecoating
Evaporative
coatinginvolvessublimation
of a metalat hightemperature
and high
vacuum.The metalvapor"sprays"
the specimen,
adhering
to any exposedsurfaces.
Thistechniqueis directional,
andan irregular-shaped
specimenmayneedto be tilted
and rotatedto be totallycovered.Sincethe metalis hot,somespecimens
can be
damaged.Further,sincemetalparticlesizeand coatingthicknessare hardto control,
the resultsheavilydependon operator
skill,technique
andcare.
Sputtercoating
Sputtercoating,usedin AnatechHummers,
is a coldprocessin whichionsimpacting
a
metalsourcedislodgemetalatoms.Theatomsdispersethroughout
the process
chamberin a waythatadequately
coatsirregular-shaped
specimens
withouttiltingor
rotatingthem. Sincethe atomsare cool,thereis no thermaldamage.
ln contrastto the "spray''
of relatively
largeclumpsof metalin the evaporative
process,
sputtering
processthatcloudsof metalatoms.Thisallowsthe uniformity
is microscopic
andthicknessof sputtercoatingsto be easilycontrolled.In general,sputtering
produceshighqualityand repeatable
coatingscompared
process.
to the evaporative
Sputtercoatingprovidestwo additional
benefits.First,sputtering
is donein a "soft"
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