1. DC Sputtering Disadvantage #1 Low secondary electron yield
from Vossen (1991), Table I, p. 23

2. 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

3. DC Sputtering Disadvantage #2
A dc plasma is only effective for sputtering conductive samples
cathode
anode
electron
ion + -
Vdc

4. 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.

5. RF Sputtering
Sputtering DC RF
Magnetron
Sputtering
Microwave
(ECR)

6. Replace dc bias with RF bias No net current flows
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

7. RF Sputtering
Amplitude ~ kV
Frequency ~ MHz
In practice, MHz is used due to government communications regulations (International Telecommunications Union)

8. 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

9. RF Sputtering
The electron current charging the capacitor is much greater than the ion current discharging it
Current (mA)
from Ohring, Fig. 3-19, p. 122

10. A dc bias develops that is about ½ of the peak-to-peak rf voltage
RF Sputtering
A dc bias develops that is about ½ of the peak-to-peak rf voltage
from Vossen (1991), Fig. 9, p. 26

11. The dc bias establishes zero net current over one complete rf cycle
RF Sputtering
The dc bias establishes zero net current over one complete rf cycle
Current (mA)
from Ohring

12. The cathode fall is equal to the dc bias
RF Sputtering
The cathode fall is equal to the dc bias
from Dobkin, Fig. 6-2, p. 152

13. 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)
Electron bombardment and damage of sample at anode
Sputtering chamber walls

14. 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

15. 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

16. 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

17. 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

18. Confine electrons closer to the cathode
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

19. Magnetron Sputtering
Electrons will experience a drift called the ExB drift analogous to the Hall effect
from Vossen (1991), Fig. 24, p. 40

20. Magnetron Sputtering
from Powell, Fig. 3.12(a), p. 71
Electrons will accumulate at one side of the electrode causing nonuniform sputtering

21. Solution 1: rotate the magnetic fields
Magnetron Sputtering
Solution 1: rotate the magnetic fields
from Vossen (1991), Fig. 27,
p. 45
from Powell, Fig. 3.12(b), p. 71

22. Magnetron Sputtering
from Vossen (1991), Fig. 28, p. 46

23. Solution 2: use a magnetron
Magnetron Sputtering
Solution 2: use a magnetron
from Vossen (1991), Fig. 30, p. 47
from Mahan, Fig. VI.4, p. 157

24. Magnetron Sputtering
from Powell, Fig. 3.13, p. 72

25. Magnetron Sputtering
from Mahan, colorplate I.5

26. 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

27. 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)

28. 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

29. 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

30. Sputtering Targets
from Ohring, Table 3-6, p. 119

31. Sputtering Targets
from Ohring, Table 3-6, p. 120

32. Evaporation versus Sputtering
from Ohring, Table 3-7, p. 132

33. 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