Magnetron sputtering devices

Originally plasma devices were simply metal plates separated by a dielectric material, usually a gas, however the magnetron incorporates a ring of magnets behind the metal target causing the electrons to remain in a dense halo close to the target. This dense region of electrons generates a more dense plasma than was previously possible without magnets, and allows operation at much lower pressures. They were developed originally for the deposition of functional thin films in industry, with the films being deposited by the sputtered neutral target atoms [109]. For the purposes of growing fuzz they provide a good source of ions which can be drawn to a negatively biased sample. The experimental process in this thesis used a magnetron in a non-conventional way, by using a W target and a He gas the deposition is very low, with a sputter yield of ∼0.003 atoms/ion (with ion energies of 300 eV) (compare with typical values of >0.4 for argon ions on a typical metal target) [108].

LPDs provide a high flux of ions, and as a by-product of this high flux the bombarded samples attain sufficient temperatures for fuzz to occur without the need for additional heating. However, this limits the control over the temperature as it is coupled to the plasma parameters. In order to change the temperature the plasma conditions have to be changed. This makes it difficult to correctly analyse a temperature range whilst keeping either a constant He+ ion fluence or constant ion bombardment energy. Magnetron sput- tering plasma sources produce a much lower incident ion flux on the W sample compared

with typical LPDs, thereby preventing excessive heating by the ion bombardment, and hence decoupling the surface temperature from the plasma properties, making it possible to control the temperature of the sample by utilising a separate heating method. This enables the study of the temperature requirements of fuzzy tungsten whilst keeping other parameters constant.

The lower fluxes found in magnetron sputtering devices (MSDs) imply that for equiv- alent fluences an MSD would have to be run for much longer times, though this can still be used as a tool to study the fuzz formation in the early stages, an area fairly overlooked. MSDs offer other benefits over LPDs as comparatively, MSDs are much smaller in scale, and they are also cheaper and simpler to set-up, thus if fuzzy tungsten can be produced in such a device, possibilities are opened for much more research to be conducted on the phenomenon.

Experimental Setup

In this section the experimental apparatus used throughout this research will be explained. It will begin with an overview of the experimental rig, as it stood for the majority of the experiments, including an explanation of the vacuum chamber, the pumping system, the magnetron sputtering source, the sample heater, and the temperature sensors. The plasma diagnostic techniques used are also explained in this section. A brief description of the devices used at the University of California at San Diego (UC San Diego) will also be provided.

3.1

The apparatus

All of the experiments based at the University of Liverpool were carried out in a cylin- drical, stainless steel vessel supplied by Gencoa Ltd, 600 mm in length, and 388 mm internal diameter. A photo of the rig and a schematic for the most common set-up are shown in figures 3.1 and 3.2, respectively. This set-up was used for chapter 6, but only differs slightly from the other sections. Where it does differ, the changes to the set-up will be explained. The chamber was pumped using both a rotary pump (Edwards E2M40), and a turbomolecular pump (LEYBOLD Vacuum Turbovac 1000) in series. The base pressure that could be achieved was of the order of 10−4 Pa. Helium gas of 99.9995% purity (supplied by BOC) was fed into the chamber through a needle valve and the pres- sure monitored using three pressure gauges. A Pirani gauge (BOX Edwards APG100) monitored the pressure from atmosphere down to 10−2 Pa, an inverted magnetron gauge (BOC Edwards) monitored pressures in the range 10−2₋10−4 Pa, and lastly a capacitance monometer (MKS BaratronOR Type 627) monitored the working pressure with a typical range of 1−6 Pa. The pressure gauges are shown in figure 3.2 labelled as P1, P2, and P3, respectively.

A V-TechTM 150 magnetron sputtering source (Gencoa Ltd.) mounted on one side of the chamber was used to generate the plasma. The axial position could be varied by ∼50 mm. The sample holder and heater was situated facing the magnetron on the same

Figure 3.1. A photo of the experimental apparatus used for the majority of this thesis. Labelled parts are: a, the vacuum chamber, b, the IR pyrometer, c, the sapphire window, d, the feedthrough for the heating cables and the rod that held the sample holder, e, the magnetron, f, the feedthrough for the sample bias, g, the pirani gauge, h, the inverted magnetron gauge, i, the Baratron gauge, j, the He gas input line, k, the turbo controller, l, the rotary pump controller, m, the pressure gauge displays, n, the magnetron power supply, o, the sample heater power supply, and p, the sample bias power supply. The pumping system is behind the rig in this picture.

axis, mounted on a long cylindrical pipe allowing the wires to pass through the back of the chamber. The temperature of the samples were either monitored by an IR Pyrometer (CTLM-3H1CF4-C3, Micro-Epsilon UK Ltd.) through a sapphire window, or by type-k thermocouples. A HEAT-2PS power supply (PREVAC) connected to a filament behind the samples was used to heat the samples to the required temperature.