Disadvantages of PE-PLD

• Additional level of cost and complexity. • Limited area of deposits.

Chapter 3

Methodology

This chapter presents the methodology used throughout this thesis, with an in depth description of the apparatus used in PE-PLD, alongside all diagnostic techniques used, both for plasmas and films. Lastly, numerical codes used to simulate various aspects of the plasma processes important to PE-PLD are described in appropriate detail.

3.1

Experimental set-up

Figure 3.1 shows the internals of the deposition chamber, schematically and with pho- tographs. The entire process occurs within a 6 arm vacuum chamber, with a vacuum achieved by two pumps, with deposition occurring under vacuum created by a scroll pump (Edwards nxDS 15i) able to maintain a base pressure of 1.5 Pa; with a further turbo pump (Pfeiffer TPU 170) used to achieve a lower base pressure of down to 5 × 10−6 Pa. Pure oxygen gas (99.99% from BOC) is supplied to the chamber via mass flow controllers (MKS Instruments), operating typically at around 5 sccm to 10’s of sccm, but additional gases may be added for spectroscopy or other reasons. Gas pressure is controlled during deposition by changing the flow rate of the gases, with additional fine control of the pressure by controlling screw valves leading to the vacuum pumps.

During deposition an ICP is created by supplying power to a 5 turn copper coil on the top side of the chamber which is separated from the gas by a 2.54 cm quartz barrier. Power comes from a 13.56 MHz RF power supply (Ceasar 1330), able to provide up to 1 kW, supplied through a Pi-type matching network (Meidan). This chamber is based upon the GEC reference cell [92], a device used within the global community for ease of comparison

between experiments. The plasma may also be run as a CCP, by using the same power supply but applied to the bottom stainless steel electrode, via an Advanced Energy auto-matching network. 4 cm 4 cm 14 cm 10 cm Nd: YAG laser Steel electrode Dielectric To ground/power Substrate holder Target holder Plasma Quartz barrier 5-turn copper coil

Figure 3.1: Top: Schematic of PE-PLD apparatus. Bottom: Picture of apparatus in YPI labs, from within vacuum chamber.

Targets (Zn, Cu etc.), which are 2.54 cm square in size, are held within the chamber on a manually rotatable stage which can be controlled from outside of the chamber, with the target held within the two electrodes, just off from the centre point of the chamber. Samples used are metals of at least 99.99% purity (from Testbourne), and are held in place via small

screws around the outside of the target. Substrates are held in place on a stationary holder on the opposite flange to the target holder, with this substrate holder having the capability to be cooled from the external cooling system, which also manages to electrodes and vacuum pumps. Substrates are held in place via a series of screws around its circumference similar to the targets, and substrates used within this work are quartz (SiO2), silicon (Si), stainless

steel, and plastics. Target and substrate holders can hold targets of a 254 mm diameter. Before entering the chamber substrates are cleaned via treatment with acetone, then an ultrasonic bath of isopropanol at 40 to 50oC for at least 10 minutes to remove any blemishes and impurities on the surface.

The laser used is a Continuum Minilite laser, which is a frequency doubled Nd:YAG laser operating at 532 nm, with a pulse length of 5 ns, and total beam energy of 35 mJ per pulse, operating at a repetition rate of 10 Hz. The laser alignment is controlled by mirrors, and focused by a lens with a focal length of 50 cm, entering the chamber through a window with an anti-reflective coating for 532 nm, finally incident onto the target. The focal spot size of the laser on the target if it was normal to the laser is 1mm in diameter, however due to in practice being offset at an angle of 45o, the spot size is not perfectly circular.

All components within the set-up are controlled by a digital delay generator (Stanford Instruments), with the timings as follows; Firstly the ICP coil is powered and the plasma

Matching

network

Power

supply

Digital delay

generator

Nd:YAG

laser

_Vacuum

chamber

Grounded

electrode

Powered

coil

Mass flow

controllers

Screw

valve

Gasses in

To vacuum

pumps

Optics

is allowed to reach an equilibrium for 8 ms, with this point assumed as it is much longer than any process known to generate plasma species under such conditions. After this 8 ms the laser flash lamp is powered, generating gain within the lasing medium (Nd:YAG rods) which reaches its maximum at 150 µs, after which point the laser q-switch is triggered causing laser emission and ablation of target material into a plasma plume. This plume travels to the substrate, a distance of 4 cm, which from spectroscopy and simulations has been determined to take approximately 2 ms [2], and then after this total of 10 ms the power for the plasma is stopped. In order to be in line with the repetition rate of the laser, no power is applied to the system for a total of 90 ms which after said the point the cycle begins again; This gives the entire system a duty cycle of 10% [2]. A full representation of all apparatus used in PE-PLD can be seen in Figure 3.2.