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--- microscope:sem:coating [2013/07/11 05:18] – mcmaster
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-Ways to deposit material:
-  * Physical vapor deposition (PVD)
-    * Sputtering: electric field rips ion off of sacrificial electrode ("target") and deposits them on sample in plasma cloud
-    * Evaporation: material to deposit is heated to give off metal in gas form that condenses on the sample surface
-  * Chemical vapor deposition (CVD): generally not used for SEM sample prep?
-This article focuses on PVD.
-====== Sputtering ======
-"The most common arrangement for a D.C. (Direct Current) sputter coater is to make the negative cathode the target material to be sputtered typically gold, platinium or with high vacuum sputter coaters, metals such as chromium and iridium)" [Principles]
-Sputtering should be done in pure argon, not air.  "Always use high purity argon gas of the grade known as 'White spot' this will ensure fast sputter rate and good pump down time" [Principles]
-"Average coating times will be of the order of 2 -3 minutes using V = 2.5KV and I = 20 mA" [Principles]
-===== Sample machine spec =====
-"For a typical modern magnetron sputter coater" [Principles]:
-  * Vacuum: 8 x l0-2 to 2 x l0-2 mbar (60 mTorr to 15 mTorr)
-  * Sputtering Voltage: 100V to 3Kv
-  * Current: 0 to 50mA
-  * Deposition: 0 to 25 nm/min
-  * Grain Size: < 5 nm
-  * Temperature Rise: < 10°C
-====== Evaporation ======
-Material is typically evaporated in either a "boat" or a filament.
-===== Boat =====
-A boat is basically a strip of conductive material with an indentation in the center to hold the material to be evaporated.  Current is applied to both ends of the boat to heat it.  Boats are usually made of tungsten, a high melting point metal, because it needs to remain solid despite evaporating material inside of it.
-Sample is typically mounted upside-down above the boat so that it deposits evenly.
-===== Filament =====
-Filaments are similar except that the material is placed inside a spiral.  This makes placing the sample a little easier as the evaporation can drift downward.
-====== Coating material ======
-Factors in choosing a coating material:
-  * Do you have equipment to sufficiently heat it?
-  * Do you have equipment to achieve the necessary vacuum?
-  * Does it need to be removed?
-  * Will depositing it alter the sample?
-===== Platinum =====
-Good coating material but difficult to remove.  Usually sputtered?
-===== Carbon =====
-Reasonable coating material that is easy to remove.  Typically evaporated.
-===== Titanium =====
-===== Copper =====
-Could possibly work?  People sputter Cu without too much trouble in medium vacuum (10E-6 torr)
-===== Tungsten =====
-Might work but generally not used because its difficult to find something to evaporate it from
-===== Aluminum =====
-Probably not usable due to rapid surface oxidation
-====== References ======
-Sputter Coater Principles ("Principles"): http://www.emsdiasum.com/microscopy/technical/datasheet/sputter_coating.aspx