Download Lecture # 22

Lecture # 22
o Film purity of evaporated films
o Methods employed in Heated/Thermal Evaporation sources
‐ The 4 types of Thermal Evaporation sources
o Estimating the Temperature of Resistance Heaters
o Electron‐beam evaporation: Process & its basic principle
Ref. :The Materials Science of Thin Films, by Milton Ohring
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Film purity of evaporated films
o Chemical purity of evaporated films is dependent on the nature and level of impurities that
(1) are initially present in the source,
(2) are present in the heater, crucible, or support materials, and
(3) originate from the residual gases present in the vacuum system
o During deposition, molecules of both the evaporant and residual gases impinge on the substrate in parallel, independent events
o Ratio of Residual gas molecule impingement rate Ji to Ji
Ci =
Evaporant molecule impingement rate (Je) is a Je
measure of Gas impurity concentration (Ci) :
o Gas molecule impingement rate Ji : J ⎛⎜ mc ⎞⎟ = 3.51 × 10 22 P
i
2
cm
.s ⎠
M gT
⎝
(P in Torr, Mg in gram, and T in K)
o Evaporant molecule impingement rate Je can be obtained from deposition rate (cm/s) :
d&
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ρN A &
Je =
d
Me
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Film purity of evaporated films
J i 5.82 × 10 −2 PM e
oThus, the Gas impurity concentration (Ci) : Ci =
=
Je
M g T ρd&
¾Very high oxygen incorporation occurs at residual gas pressures (P) of 10‐3 torr
¾This is utilised
in reactive evaporation processes where intentionally
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introduced oxygen serves to promote metal‐oxide films
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Heated evaporation sources
‰ 3 methods: Resistance, induction, and electron‐beam heating
‰ Resistance based sources: Joule heating of metal filaments
‰ Such heaters must reach the temperature of the evaporant while having a negligible vapor pressure in comparison
‰ Ideally, they should not contaminate, react with, or alloy with the evaporant, or release gases such as O2, N2, H2 at Tevap
1. Tungsten Wire Sources: individual or multiply stranded wires twisted into helical or conical shapes (Good till 2200K):
¾ Helical coils are used for metals that wet tungsten readily
(the metal evaporant wire is wrapped around or hung from the tungsten strands and the molten beads of metal are retained by surface tension forces)
¾ Conical baskets are better to contain poorly wetting materials 16-03-2016
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2. Refractory Metal Sheet Sources: Tungsten, Tantalum, and Molybdenum sheet metal heaters
‰ Used to evaporate powder mixtures of metals
‰ Fabricated into a variety of shapes including the dimpled strip, boat, and deep‐folded configurations
‰
3.
‰
‰
They require low‐voltage high‐current power supplies (10V, 200 A)
Sublimation Furnaces:
Used to evaporate sulfides, selenides, and some oxides efficiently
The evaporant powder materials pressed and sintered into pellets and heated by surrounding radiant heating sources
‰ To avoid the spitting and ejection of particles caused by evolution of gases trapped within the source, the baffled/encapsulated heating assemblies are used; These avoid direct line‐of‐sight access to substrates. (Tantalum sheet is readily cut, bent, and spot welded to form heaters, radiation shields, supports, and current bus strips)
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1. Tungsten Wire Sources
2. Refractory Metal Sheet Sources
Helical coils
4. Crucible Sources
Conical baskets
3. Sublimation Furnaces
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(http://www.testbourne.com)
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4. Crucible Sources: Cylindrical cups composed of oxides, pyrolytic BN, graphite, and refractory metals heated by external tungsten‐wire resistance heating elements wound to fit snugly around them
¾ Alternatively, high‐frequency induction heating is also used
Induction heating resembles a transformer: the powered primary is a coil of water‐cooled copper tubing that surrounds the crucible. High‐frequency currents are induced in either a conducting crucible or evaporant material serving as the secondary, resulting in heating
Example of induction heating: Al is commercially evaporated from BN or BN/TiB2 composite crucibles
Tungsten wire resistance heater in the form of a conical basket that is encased in Al2O3 or refractory oxide to form an integral crucible‐heater assembly.
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Estimating the Temperature of Resistance Heaters
‰ Electrical power (P) supplied & the filament temperature reached are related as P is simply i2R or alternately by V2/R where i, V and R
are the current, voltage, and resistance, respectively ‰ For a wire filament of length L and 2
2
P
=
i
R
=
i
ρ (T ) L / AC
cross‐sectional area Ac, P is
‰ Using Power law dependence of resistivity ρ(T) on temperature T,
⎡ T ⎤
ρ (T ) = ρ (0) ⎢
⎥
(
0
)
T
⎣
⎦
n
n
L ⎛ T ⎞ ρ(0) is the value resistivity at reference ⎜⎜
⎟⎟
P = i ρ ( 0)
AC ⎝ T (0) ⎠ temperature T(0), and n is a constant~1
‰ In tungsten, at T(0) = 293 K, ρ(0) = 5.5 × 10‐8 Ω‐m, n = 1.20
⇒
2
‰ T can be calculated from the resistor dimensions & power delivered
‰ Alternatively, T can also be estimated from Steafan’s‐Boltzman law:
(
Pr = εσAS T 4 − T (0) 4
)
(Assumption: Psupplied=Pradiated)
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(ε=emissivity; As=filament Surface Area)
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Electron Beam Evaporation
‰ Disadv. of resistively heated sources: contamination by crucibles, heaters, and support materials & limitation of relatively low input power levels. This makes it difficult to deposit pure films or evaporate high‐melting‐point materials at appreciable rates. ‰ Electron‐beam (e‐beam) heating eliminates these disadvantages
In principle, this type of source enables evaporation of virtually all materials over a wide range of practical rates
‰ The evaporant is placed in either a water‐cooled crucible or in the depression of a water‐cooled copper hearth ‰ Purity of the evaporant is assured because only a small amount of charge melts or sublimes so that the effective crucible is the unmelted skull material next to the cooled hearth. For this reason there is no contamination of the evaporant by Cu hearth
‰ Multiple source units are available for either sequential or parallel
deposition of more than one material (e.g., alloys)
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Process of Electron Beam Evaporation
‰ Electrons are thermionically
emitted from heated filaments that are shielded from direct line of sight of both the evaporant
charge and substrate
Film contamination from the heated cathode filament is eliminated this way ‰ The cathode potential is biased negatively at 4 to 20 kV w.r.t. a nearby grounded anode, and this accelerates the electrons
‰ In addition, a transverse magnetic field is applied that deflects the electron beam in a 270° circular arc and focus it on the hearth and evaporant charge at ground potential
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(Top view)
‰ Disadv.
(www.temescal.net)
(Side view)
Multi-hearth electronbeam evaporation unit
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Basic principle of e‐beam evaporation
‰ The thermionically emitted electron beam is accelerated to a high kinetic energy and directed towards the evaporation material
‰ As the e‐beam strikes the evaporation material, the electrons lose their energy quickly
‰ The kinetic energy of the electrons is instantly converted into thermal energy through interactions with the evaporation material ‰ The thermal energy so‐produced heats up the evaporant kept in the crucible causing it to melt or sublimate
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‰ The vapor‐cloud near the evaporant is viscous in nature
‰ The region beyond this dense cloud is at much lower pressure (molecular flow). ‰ Thus, instead of evaporant particles being beamed from various points on the flat source surface, they appear to originate from the perimeter of the viscous cloud
‰ Thus, the effective or virtual source plane has sort of moved away from the melt surface toward the substrate
‰ The effective source‐substrate distance is taken as hv (not h)
(The ratio hv/h=0.7, depends on the evaporation rate)
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