Radio Frequency Magnetron Sputtering

In this study all magnetic films were grown by Radio Frequency (RF) magnetron sputtering using a Nordiko NM2000 system. The system is schematically shown in Figure 3.2. The sputtering system was configured to operate in the sputter-up mode, where the substrates were mounted 6cm directly above the target electrode. The sputter-up configuration had the advantage of not suffering from the problem of eroded target fragments falling on the substrate electrode and thus affecting the depositing film. Both the substrate and target electrodes were 15cm in diameter and were water-cooled. The Nordiko NM2000 consisted of three target electrodes (referred to as 1,2,3) which were mounted on a rotatable carousel. This provided the facility to sputter from three different target materials separately, and therefore allowed the deposition of multi-layered films. A grounded stainless steel shielding plate ensured that only the target below the substrate electrode was being sputtered from. In this study all films were grown using target electrode 1 unless otherwise stated. The importance of this is discussed in Section 5.8.1. The main chamber is attached to a mechanical hoist in order that the vacuum chamber could be lifted clear of the base of the machine to provide access for the mounting of targets and general maintenance. Two Viton seals were used to provide a vacuum seal for the main chamber as shown Figure 3.2. The rotatable shutters were positioned to provide shielding for either the target or substrate from the plasma. This allowed the target or substrate to be sputter-cleaned prior to the deposition of the film. A stainless steel clamping ring was used to secure the target to the copper electrode which ensured good electrical and thermal contact. To prevent sputtering from the clamping ring itself, an over-sized earthing shield was mounted directly above to prevent the plasma from coming into contact with the ring. The earthing shield also prevented the sputtered material from being deposited onto the insulator which isolated the target electrode from ground. Substrates were mounted

Grounded Rotatable Shutters _Matching Network Substrate Electrode Target Electrode Matching Network

High Vacuum Baffle Valve

Nitrogen Baffle Trap Penning Gauge B1

Diffusion Pump _{Rotary Pump}

Pirani Gauges A1 A2 Roughing Valve Foreline Valve Needle Valve Argon Inlet Radio Frequency Power Amplifier Relay Switch Viton Seal Earthing Clips Base

Figure 3.2: A schematic representation of the Nordiko NM2000 sputtering system used to sputter deposit the amorphous thin films.

on a removable copper substrate platter which interlocked firmly into the substrate electrode with a copper gasket which insured good thermal and electrical contact.

The power from the RF amplifier could be directed either to the target or the substrate electrode by means of a relay switch through a matching circuit. This allowed the impedance of the power supply and electrode to be matched, so the required power could be delivered efficiently. Directing the power to the substrate electrode allowed the substrates to be sputter etched if needed before the deposition of the film. The sputtering gas used was zero-grade high purity argon (99.99%); the gas flow was controlled by a needle valve and by partially opening the high vacuum baffle valve.

The high vacuum pumping system consisted of a diffusion pump (Balzers DIF 200) which was backed by a mechanical rotary pump (Balzers DUO 030A). A liquid nitrogen trap was situated between the high vacuum baffle valve and the diffusion pump so as to prevent oil or water vapour from entering the chamber. The chamber could be isolated from the pumping system by the high vacuum baffle value which allowed the chamber to be vented to atmospheric pressure without the requirement of shutting the pumping system down. The baffle valve also allowed the chamber to be gradually exposed to the high vacuum pumping system. This was important since the chamber was only evacuated to a pressure of 10-2 _{mTorr using the mechanical rotary pump, whereas the high vacuum pumping system was in the}

low 10-8 _{Torr region. Any sudden opening of the high vacuum valve would have resulted in oil vapour}

passing into the chamber, and disrupting the flow of the diffusion pump. Pneumatic valves were used to isolate or open the rotary pump to either the chamber or the diffusion pump. The chamber pressure was monitored by a Pirani gauge (A2) in the range 102_-10-4 _{mTorr and by a cold cathode Penning}

gauge (B1) in the range ~10-5_-10-8 _{Torr. The foreline (backing) pressure for the diffusion pump was}

monitored using a second Pirani gauge (A1).

All removable items (earthing shields, shutters, etc.) within the chamber were shot-blasted using a fine glass bead prior to the growth of any FeSiBC films. This was to remove any foreign material which had previously been grown in the chamber. The sputtering process deposited material on all surfaces within the chamber, and therefore it could also sputter material back off these surfaces thereby contaminating the films (secondary sputtering). All items were de-greased after shot-blasting and handled with clean-room gloves. The chamber was periodically baked out at 700_{C to reduce out-}

gassing, and hence improve the vacuum. The contaminants within the system were monitored using a residual gas analyser (Balzers QMG64 RGA).

Use of a magnetron source increased the growth rate by ensuring that a high density of electrons exists near the surface of the target. The field from the magnetron forms a race track from where the majority of the material is sputtered. The downside to this is that the target erodes at a much higher rate in these regions, making very little use of the majority of the target. The growth rate is dependent mainly upon the pressure and power at which the films are deposited. The growth rate is also affected by substrate and target separation, but this was always fixed at 6cm. Increasing the sputtering power increases the growth rate and therefore erosion of the target, but it also has the effect of raising the temperature of the target and substrate. Increasing the pressure will also increase the sputtering rate at low pressures, but this will level off and even decrease at higher pressures because the various particles are slowed down by inelastic collisions. The magnetic properties of the films can be highly sensitive to pressure, power and the temperature at which they are deposited.