Transcript Vacuum Evaporation Lecture 8 G.J. Mankey

Vacuum Evaporation
Lecture 8
G.J. Mankey
[email protected]
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Monolayer Time
Sticking Coefficient
S = # adsorbed / # incident
Impingement rate for air:
Z = 3 x 1020 P(Torr) cm-2 s-1
Area of an adsorption site:
A  1 Å2 = 10-16 cm2
• The monolayer time is the time for
one atomic layer to adsorb on the
surface: t = 1 / (SZA).
• At 3 x 10-5 Torr, it takes about one
second for a monolayer to adsorb
on a surface assuming a sticking
coefficient, S = 1.
• At 10-9 Torr, it takes 1 hour to form
a monolayer for S = 1.
• For metals at room temperature
S = 1, so the vapor pressure should
be >10-6 Torr.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Vapor Pressure Curves
• The vapor pressures of most
materials follow an Arrhenius
equation behavior:
PVAP = P0 exp(-EA/kT).
• Most metals must be heated to
temperatures well above 1000 K
to achieve an appreciable vapor
pressure.
• For PVAP = 10-4 mbar, the
deposition rate is approximately
10 Å / sec.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Physical Evaporation
Substrate
Flux
Boat
Evaporant
High Current
Source
• A current, I, is passed through
the metal boat to heat it.
• The heating power is I2R, where
R is the electrical resistance of
the boat (typically a few ohms).
• For boats made of refractory
metals (W, Mo, or Ta)
temperatures exceeding 2000º C
can be achieved.
• Materials which alloy with the
boat material cannot be
evaporated using this method.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Limitation of Physical Evaporation
• Most transition metals, TM,
form eutectics with refractory
materials.
• The vapor pressure curves
show that they must be heated
to near their melting points.
• Once a eutectic is formed, the
boat melts and the heating
current is interrupted.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Electron Beam Evaporator
• The e-gun produces a beam of
electrons with 15 keV kinetic
Substrate
energy and at a variable current of
up to 100 mA.
• The electron beam is deflected
e-beam
Flux
270º by a magnetic field, B.
Evaporant
• The heating power delivered to a
Crucible small (~5mm) spot in the evaporant
B
is 15 kV x 100 mA = 1.5 kW.
cooling
• The power is sufficient to heat
most materials to over 1000 ºC.
• Heating power is adjusted by
e-gun
controlling the electron current.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Wire Evaporator
substrate
filament
cooling
shroud
0-12V
1-2 kV
• This is a "mini" version of the
electron beam evaporator.
• The entire assembly fits
through a 2 3/4 " OD Flange.
• Electrons from the heated
filament bombard a 2 mm
wire that is held at a large
positive bias.
• The power supply is operated
in a current limiting mode
and the heating power is
P = VbiasIemission.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Wire Basket
cooling
shroud
1-20 V
• Direct or alternating current is
passed through a pre-fabricated
helical wire container.
• Evaporant placed in the helix is
heated by contact and
irradiation.
• Heating power is of the order of
100 W or more with a refractory
helix with 0.1 - 0.5 mm diameter
wire.
• Works for Ag, Au, Cu, Cr, Mn,
etc.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Knudsen Cell
cooling
shroud
• The crucible is heated by a coil
or heater surrounding it.
• Crucibles are usually made of
boron nitride, alumina, or
graphite.
• Since there is a large amount of
heat, the device is constructed
of low outgassing materials and
a large amount of cooling is
necessary.
1-20 V
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Measuring and Calibrating Flux
?
• Many fundamental physical properties are
sensitive to film thickness.
• In situ probes which are implemented in
the vacuum system include a quartz crystal
microbalance, BA gauge, quadrupole mass
spectrometer, Auger / XPS, and RHEED.
• Ex situ probes which measure film
thickness outside the vacuum system
include the stylus profilometer,
spectroscopic ellipsometer, and x-ray
diffractometer.
• Measuring film thickness with subangstrom precision is possible.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Quartz Crystal Microbalance
Frequency
Measurement
Conversion to
Thickness
Display
Substrate
Quartz
Crystal
Flux
• The microbalance measures a shift
in resonant frequency of a
vibrating quartz crystal with a
precision of 1 part in 106.
• fr = 1/2p sqrt(k/m) f0(1-Dm/2m).
• For a 6 MHz crystal disk, 1 cm in
diameter this corresponds to a
change in mass of several
nanograms.
• d = m / (rA), so for a typical metal
d  10 ng / (10 g/cm3*1 cm2) =
0.1 Angstroms.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center