DC Sputtering Disadvantage #1 Low secondary electron yield

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

2. DC Sputtering Disadvantage #1Low 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 1016Wcm for quartz gives 109 Volts. This cannot be achieved in practice.

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

6. 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 ~ 0.5-1 kV • Frequency ~ MHz • In practice, 13.56 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. 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. RF Sputtering • The dc bias establishes zero net current over one complete rf cycle Current (mA) from Ohring

12. 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) • Low ionization levels • 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. 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. 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. 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