Download Magnetron Sputtering

DC Sputtering Disadvantage #1
Low secondary electron yield
from Vossen (1991), Table I, p. 23
DC Sputtering Disadvantage #1
Low secondary electron yield
• For example:
• d = 0.1
•10 ions required to produce one
secondary electron
• Each electron must produce 10 ions
• I = 16 eV
• cathode fall = 160 V
10 ions
1 electron
DC Sputtering Disadvantage #2
• A dc plasma is only effective for
sputtering conductive samples
cathode
anode
electron
+ V dc
ion
DC Sputtering Disadvantage #2
• Typical ion currents striking the
cathode are on the order of 1 mAcm-2
• To draw a current density of J
through a film of thickness t and
resistivity r, the cathode needs a
voltage
V = rtJ
• Hence, a typical film thickness of 1
mm and resistivity of 1016 Wcm for
quartz gives 109 Volts. This cannot be
achieved in practice.
RF Sputtering
Sputtering
DC
Magnetron
Sputtering
RF
Microwave
(ECR)
RF Sputtering
• Replace dc bias with RF bias
• No net current flows
• Can use insulating source and target
materials
from Mahan, Fig. VI.3, p. 156
RF Sputtering
• Amplitude ~ 0.5-1 kV
• Frequency ~ MHz
• In practice, 13.56 MHz is used due
to government communications
regulations (International
Telecommunications Union)
RF Sputtering
• In RF discharges, a blocking
capacitor is placed on the cathode so
that a dc bias is built up with each RF
cycle
from Mahan, Fig. VI.3, p. 156
RF Sputtering
Current (mA)
• The electron current charging the
capacitor is much greater than the ion
current discharging it
from Ohring, Fig. 3-19, p. 122
RF Sputtering
• A dc bias develops that is about ½ of
the peak-to-peak rf voltage
from Vossen (1991), Fig. 9, p. 26
RF Sputtering
Current (mA)
• The dc bias establishes zero net
current over one complete rf cycle
from Ohring
RF Sputtering
• The cathode fall is equal to the dc
bias
from Dobkin, Fig. 6-2, p. 152
Disadvantages of DC or RF Sputtering
• Inefficient secondary electron
process
• Low plasma densities
• Low ionization levels
• Low discharge currents or ion
bombardments
• Low sputtering rate
• Slow etching or deposition
• Long mean free path of secondary
electrons (10’s cm)
• Low ionization levels
• Electron bombardment and
damage of sample at anode
• Sputtering chamber walls
Magnetron Sputtering
• Use a magnetic field (~ 200 – 500
G) to contain the secondary electrons,
and therefore the plasma, close to the
cathode
• An electron moving in a magnetic
field B experiences a force
F = e v x B sinq
Magnetron Sputtering
• The velocity component tangential
to the B field is unaffected, so
electrons actually move in a helical
path around the magnetic field lines
from Ohring, Fig. 3-20, p. 124
Magnetron Sputtering
• The frequency of rotation is called
the Larmor, cyclotron, or gyro
frequency and is given by:
w = eB/m
• Radius of rotation is:
r = mv/eB
• For electrons, r ~ few mm
• For ions, r >> system dimensions
• Ions are essentially unaffected by the
magnetic field
Magnetron Sputtering
• Electrons are trapped by the field
lines increasing their time spent
within the plasma and increasing the
probability of ionization
from Vossen (1991), Fig. 25, p. 44
Magnetron Sputtering
• An improved configuration places
the magnetic field parallel to the
sample surface
• Confine electrons closer to the
cathode
from Vossen (1991), Fig. 26, p. 44
Magnetron Sputtering
• Electrons will experience a drift
called the ExB drift analogous to the
Hall effect
from Vossen (1991), Fig. 24, p. 40
Magnetron Sputtering
from Powell, Fig. 3.12(a), p. 71
• Electrons will accumulate at one
side of the electrode causing
nonuniform sputtering
Magnetron Sputtering
• Solution 1: rotate the magnetic fields
from
Vossen
(1991),
Fig. 27,
p. 45
from Powell, Fig. 3.12(b), p. 71
Magnetron Sputtering
from Vossen (1991), Fig. 28, p. 46
Magnetron Sputtering
• Solution 2: use a magnetron
from Vossen (1991), Fig. 30, p. 47
from Mahan, Fig. VI.4, p. 157
Magnetron Sputtering
from Powell, Fig. 3.13, p. 72
Magnetron Sputtering
from Mahan, colorplate I.5
Magnetron Sputtering
• Can also have many different
magnetron geometries as long as the
ExB path forms a closed loop
• For example, the length of the
magnetron can be several meters to
allow coating of very large surfaces
from Powell, Fig. 3.15, p. 74
Magnetron Sputtering
• Electrons are trapped for several
trips around the ExB loops above the
cathode (magnetic tunnel)
• Increased ionization (ni/n ~ 10-4 to
10-2)
• Higher plasma density (ni ~ 1011
cm-3)
• Increased ion bombardment (4-60
mA/cm2)
• Higher deposition rates (~ 1
mm/min for Al)
• Lower Ar pressures (0.5 – 30 mT)
• Lower dc voltages (300 – 700 V)
or RF voltages (< 500 V amplitude)
Sputtering Advantages
• Can deposit refractory metals
• High deposition rate
• Sputtered particle energy ~ 3-5 eV
>> evaporated particles
• Higher surface mobility in
condensing particles
• Smooth and conformal film
morphologies
• Sputtering sources are typically of
relatively large area
• Can sputter alloys
Sputtering Advantages
Alloy Targets
• An alloy target may have different
sputtering yields for different elements
• The difference in sputtering yields among
elements is typically smaller than their
differences in vapor pressure
• An element with a low sputtering yield
will build up on the target compared to an
element with a high sputtering yield
• Surface composition of target achieves an
equilibrium condition where sputtering
composition is the same as the target
composition
• This is an advantage of sputtering
compared to thermal evaporation
Sputtering Targets
from Ohring, Table 3-6, p. 119
Sputtering Targets
from Ohring, Table 3-6, p. 120
Evaporation versus Sputtering
from Ohring, Table 3-7, p. 132
PVD Summary
• Solid or molten sources
• Source atoms enter the gas phase by
physical mechanisms (evaporation
or sputtering)
• Gaseous source particles are
transported through a reduced
pressure environment
• Generally, an absence of chemical
reactions in the gas phase and at the
substrate surface