Magnetron Sputtering

Physical vapour deposition (PVD) is a vacuum coating technique which works on the basis of vaporising a solid target with the intention of the vapour re-condensing onto a nearby substrate in the form of a thin film. The most common form of PVD is magnetron sputtering and has been used extensively to produce thin metallic films for industry due to its ease of scalability and high deposition rates.

Figure 3.1. Schematic view of a planar magnetron. Kr+ _{are positive krypton ions and T and T}+ _{are target atoms and ions}

respectively. Dashed lines denote the magnetic field lines which form an electron trap. N and S are the north and south poles of permanent magnets housed underneath the target.

A typical planar magnetron is shown in figure 3.1. A sputtering process begins by injecting a process gas, usually a noble gas, into the vacuum chamber at low pressure, typically between 10-3 _to

10-1 _{mbar. The target of the magnetron is biased with a large negative voltage so that electrons can}

be emitted into the vacuum via field emission. If an electron and gas atom collide with sufficient energy then the gas atom will become positively ionised and release a secondary electron. The probability for ionisation to occur is increased if there are large numbers of both electrons and gas atoms confined to a small volume. Therefore, magnets are housed under the target of the magnetron to create an electron trap utilising closed E × B drift currents close the target surface i.e. where they are most needed. Positively charged gas ions are then accelerated towards the negatively charged target with enough impact energy to eject (sputter) target atoms. More secondary electrons are emitted from the target as atoms are sputtered away. The secondary electrons released from the

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target surface are accelerated through the target potential and arrive at the plasma with high kinetic energies and can initiate further ionisation of the process gas. Thus, sputter rates are largest directly beneath the E × B drift in a region of the target known as the racetrack [19]. Ejected target atoms follow a parabolic trajectory towards the substrate material. If the target material is ejected with enough energy, or the plasma is dense enough, then the target material can become ionised.

Magnetrons can be in either a planar or cylindrical design however both operate using the same principles. Variations of the planar magnetron are typically used to deposit on a flat substrate whereas a cylindrical magnetron would be better suited to depositing on the inside of a curved surface, such as an RF cavity.

There are a number of different power supplies which can be used to control the magnetron sputtering process. Direct current (DC) is the simplest. A DC power supply is used to provide a constant voltage to negatively bias the target of the magnetron. DC Magnetron sputtering provides uniformity of coating and high deposition rates but does not produce sputtered material with high kinetic energies.

Pulsed DC magnetron sputtering is an adaption of DC sputtering which applies the DC sputter voltage as pulses rather than a constant voltage. Pulsed DC magnetron sputtering was first developed to enable reactive sputtering of dielectrics that were unsuitable for use with DC sputtering. A pulsed DC power supply operates by providing a pulsed negative sputtering voltage to the target, and then a positive voltage, of the order of 5 to 20 % of the sputter voltage, is applied between each negative pulse. Thus, the alternating polarity of the voltage allows the surface of a dielectric target to partially discharge and helps to prevent arcs [20]. Pulsed DC magnetron sputtering can also be effective to deposit conducting materials as current densities at the target surface can be up to double of those produced by DC magnetron sputtering [21]. Pulsed DC discharges are typically characterised by the duty cycle of the process. The duty cycle is given as a percentage equal to 100 times the length of a pulse multiplied by its frequency. Pulsed DC magnetron sputtering usually operates with a duty cycle of approximately 50 to 90 %.

RF magnetron sputtering is another technique developed to sputter dielectric material. RF power supplies operate at 13.56 MHz and supply an AC voltage to the target. The biasing of the cathode results in a plasma containing both sputter gas ions and electrons. The heavy sputter gas ions are not mobile enough to alternate from cathode to anode before the polarity of the RF field changes. The lighter electrons can switch between cathode and anode before the polarity of the RF field changes and results in the target being negatively biased with respect to the substrate and chamber due to the relatively small surface area of the target [22].

15 100 200 300 400 500 600 1 10 100 I ( A) Power (W) Pk. HiPIMS DC Av. HiPIMS 100 200 300 400 500 600 300 350 400 450 500 550 600 HiPIMS DC Vav (V) Power (W)

Figure 3.2. Peak power of HiPIMS pulses are two orders of magnitude higher than that of DC sputtering. Black lines are plasma current and red lines are the bias voltage supplied to the target cathode.

HiPIMS is a variant of pulsed DC magnetron sputtering which utilises long off times to achieve very large plasma currents. The duty cycle of a HiPIMS discharge is typically as small as 0.1 to 5 %, but the peak plasma current can be as much as two orders of magnitude larger than that of DC magnetron sputtering, as shown in figure 3.2. The evolution of a HiPIMS discharge is quite well understood. Normal operation of HIPIMS uses off times that are so long that there is no longer an E × B drift between pulses because of the lack of an E field. Thus, the plasma density decreases during the off time. The high voltage at the beginning of each pulse causes the low density plasma to become highly negative, containing large numbers of secondary electrons with energies of up to 100 eV [23]. This initial stage lasts for approximately 10 µs. The next phase of the discharge is a rapid increase in the plasma density as the high energy electrons become trapped in the renewed E × B drift; resulting in increased sputtering rate, more secondary electrons and ramping of the plasma current [24]. The plasma current stops ramping once the number of neutrals is diminished [25]. Self-sputtering can occur as large fractions of the target material become ionised [26]. After the peak, the plasma current starts to drop till the end of the pulse.

Benefits of the HiPIMS process are reported to be dense films with few voids and with good film adhesion [27]. The main downside of the process is a small deposition rate a sensitivity to the process gas pressure and the pulse settings i.e. length of pulse and repetition rate. A stable plasma cannot be maintained if the off time is set to too long. Pre-ionisers can be used to maximise the length of the off time by maintaining a low density plasma between pulses.

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