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 ion + - Vdc

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 RF Magnetron Sputtering Microwave (ECR)

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

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

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

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

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

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

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

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

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

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

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

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

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