This chapter describes the physical vapour deposition equipment, target manufacturing techniques and the bulk and surface characterisation methods employed during analysis of the advanced coating systems and the investigation of steered arc motion on
segmented targets.
3.1. The physical vapour deposition equipment.
3.1.1. The electromagnetic steered arc evaporation PVD unit.
This sub-section outlines the design and construction of the experimental
electromagnetic steered arc facility at the MRI, Sheffield Hallam University and the nature of its associated control systems and power supplies. The unit was principally designed and built by Dr P.J.Walke during a Ph.D study on the fundamental motion of steered arcs (1) with contributions on the cathode assembly design by Dr R.New and its manufacture by Messrs R.Day and G.Robinson.
Figure 3.1. Schematic of the electromagnetic steered arc evaporation apparatus.
PHOTO D E T E C T O R AND INTERFACE ARC 1 0 0 P S U B A L L A S T R E S I S T O R NAIN CHAHBER INTERLOCK GAS FLOW RE G U L A T O R COOLING L I N E S l j z UJ <E h -or <n O UTER COIL P S U INNER COIL P S U C O N T R O L L E RGAS FLOW
JZ,A[JCI 1IU C IIU U
The apparatus (figure 3.1) consists of a central cylindrical aluminium PVD chamber mounted upon a modified base plate of an existing Genevac evaporation system. Vacuum pressure is attained by means of a 20cm throat diffusion pump backed by a rotary vane pump, together capable of a pumping capacity of approximately 13001s"! and a typical base pressure below lx lO ^ m b a r . Entrance to the chamber can be achieved
through a circular top plate by which the chamber is also earthed. Four orthogonal ports were positioned on the sides of the chamber each consisting of a square plate through which a number of feed-throughs were mounted (figure 3.2).
• Port 1 - Provided an access to the cathode assembly.
• Port 2 - Provided a circular viewing port allowing photographic or optical analysis. • Port 3 - Provided a water cooled electrical feed-through that supplies current and
supports a circular anode positioned in front of the cathode.
• Port 4 - Provided electrical and gas feed-throughs and a connection to a pressure transducer.
The electrical feed-through in port four was designed to accommodate a rotating motion and a connection to anode potential through a ballast resistor. This allowed a high current cathodic arc to be struck by making a momentary short between the supply voltage and the earth via the rotary movement of a Monel spike. The main arc current was generated using an "ARC-100" commercial supply manufactured by ELMA Technik GmbH, capable of delivering fixed current outputs between 0 and 100A at potentials of up to 100 Volt.
The magnetic steering of the arc upon a path of continuously variable radius (in circular geometry) was achieved by two sets of water cooled electro-magnetic coils mounted behind the cathode. The inner coil was driven by a constant current power supply (currents up to 20A) with feedback stabilisation, whilst the outer set was supplied by a similar unit capable of current up to approximately 10A. Independent control of the inner and outer coil current meant that substantial movement of the position of the normal field component zero and consequently the arc radius (usage of nearly the entire cathode area) could be achieved .
Figure 3.2. Primary feedthroughs on electromagnetic steered arc PVD chamber. WINDOW fo □ o] CAHERA IG NITION F E E D -T H R O U G H CATHODE ANODE \ F E E D -T H R O U G H OUTER COIL A BORON N ITR ID E RING POWER AND COOLING L I N E S
The cathodes consisted of 045mm or 0150mm diameter x 5mm thick discs of the material under investigation, secured into an assembly by means of an aluminium ring fixed by a number of counter-sunk bolts around its circumference (figure 3.3). The joint between the cathode material and the retaining ring was covered by a boron nitride disc which acted as a passive arc barrier and prevented arc burrowing at the cathode / ring interface. The cathode and retaining ring were mounted on a copper cooling plate (thermocouple monitored) which also served as a vacuum seal between the main body of the assembly and the cathode mounting. Power was applied to the cathode by direct connection to the cooling plate.
During evaporation trials, argon (99.999% purity) was generally introduced into the system to obtain a working pressure of the order of 3xl0"2mbar and aid arc initiation and an increase in arc stability. This was achieved by an MKS Instruments 250 gas pressure controller combined with a MKS Baratron 122A pressure transducer and a MKS 248 solenoid gas flow control valve. Prior to steered arc evaporation, a random arc conditioning of the cathode for a duration of 2-3 minutes at 100 A arc current was always initiated to remove surface oxides and obtain reproducible type II cathode spot (2) data.
Figure 3.3. Steered arc cathode assembly. B O R O N N I T R I D E C E R A H I C S T U D R I N G C A P C A T H O D E R E T A I N I N G R I N G C O P P E R P L A T E WA TE R S E A L ’O' R I N G C O N T E R - S U N K M WATER O U ' L E T
The typical process parameters and field profiles used during evaporation trials are shown in table 3.1. and figure 3.4.
Table 3.1. Segmented target steered arc evaporation trial parameters
Process parameters
0 45mm TiZr targets central & offset
interfaces
0150mm TiZr, TiMo & ZrMo targets central
interfaces
Base pressure < lxlO'^mbar < lxlO'^mbar
Working pressure 3xl0"2mbar 3xl0"2mbar
Working gas Argon Argon
Inner coil current 19.9 Amp 15.0 Amp
Outer coil current 10.2 Amp 6.0 Amp
Arc current 85 Amp 85 Amp
Arc radius 15mm 2 0mm
Normal field gradient 18.2 mTcm"l 12.1 mTcm"l Transverse field
strength
11.6 mT 11.2 mT
Start temperature 25°C 25°C
Cathode conditioning 5 mins random arc 5 mins random arc
Figure 3.4. Typical steered arc evaporation magnetic field profiles.