SEM Coating
Ways to deposit material:
- Physical vapor deposition (PVD)
- Sputtering: electric charges electrons which them bumps material 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.
Contents
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]
"DC sputtering is common for conductors. RF sputtering can be used for insulators too"
A magnet is often placed around the target: "Lorenz forces force electrons in the plasma into a helical path. The longer path results in more collisions with gas atoms (and thus more ions) [SJSU]
Usually DC but sometimes RF power. [SJSU] cites 13.5 MHz
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]
"Step coverage is a measure of how uniform thickness is over the topography of the surface" [SJSU]. Higher pressure gives a more even surface. Having a larger target also helps to create a more even film [Lab 7]
Putting a negative bias on substrate can produce better films [Lab 7]
Distance between target and substrate is typically a few cm [Techniques]
Optimal temperature varies on a few parameter but 100 mTorr is a good rule of thumb [Techniques, Lab 7]. [SJSU] says its closer to 110 mTorr for Ar
Reactive sputtering can deposit non-metallic compounds [SJSU] """
- Oxides from O2 gas: Al2O3, Ta2O5, SiO2
- Nitirdes from N2 or NH3: TaN, TiN, Si3N4
- Carbides from CH3 or CH4: TiC, WC, SiC
- Also combinations such as oxycarbides, oxynitrides
"""
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.
Carbon
Can be deposited by quickly vaporizing a few strands. This is done at about 200 mTorr. Going lower to 150 mTorr with a higher current burst can be more gentle on samples but may not give as good of a surface [EFFA]
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.
"Filaments work well with materials that melt, wet the filament, and then evaporate uniformly in all directions. With materials that sublime you get shadowing from the wire"
Coating material
Factors in choosing a coating material:
- What type of detector are you going to use? Secondary emission and other properties vary from material to 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
Chrome
Less BSE than other materials but must be stored under vacuum to avoid corrosion and requires high vacuum to sputter [Principles]
Iridium
Requires high vacuum to sputter [Principles]
