Physical Vapour Deposition of Thin Films (all content)

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Contents

Aims

By the end of this TLP, you should be able to:

• Explain how Evaporation can be used as a deposition technique, and know what external factors have an effect.
• Know how temperature affects the necessary vapour pressure of the material for evaporation to occur.
• Describe the basic Sputtering technique, and the difficulties which it presents.
• Discuss the complications which arise when the film is to be made of an alloy or compound.
• Explain the process of laser ablation and how it can improve on other Physical Vapour Deposition (PVD) techniques.
• Know how energy contributes to the structure and properties of the final film.
• Describe the 3 basic growth modes and what conditions favour each.

Before you start

There are no specific requirements.

The practicalities of coating mechanics are covered here.

It may also be useful to look at this simulation of epitaxial growth.

Introduction

In this TLP, thin films will be defined as solid films formed from a vapour source:

   Built up as a thin layer on a solid support (substrate) by controlled condensation of individual atomic, molecular, or ionic species.

This can lead to structures which are far from equilibrium so understanding how different deposition techniques can lead to different growth mechanisms and structures is essential in being able to control the properties of the film.

The applications of these films can vary depending on the thickness, among other properties:

• Low thickness
  • Optical interference effects
  • Electron tunnelling
  • High resistivity
• High surface to volume ratio
  • Gas absorption
  • Diffusion
  • Catalytic activity
• Microstructural control
  • High hardness
  • Optical absorption
  • Corrosion protection

Because of their many controllable aspects, thin films play a major role in microelectronics, communications, protective coatings, optics and the medical industry.

There is always continued pressure for advances in size reduction, uniformity, purity, reproducibility and manufacturing speed.

One of the main methods for forming these films is through Physical Vapour Deposition (PVD). In PVD a vapour is generated from a source and travels to a substrate, where there is nucleation and growth of the solid film materials. The two main ways of forming this vapour flux are through Evaporation and Sputtering. A common, general set up for this process is shown schematically below:

Simple deposition will lead to amorphous film material unless the depositing atoms have enough energy to rearrange themselves into a more thermodynamically stable crystalline structure. If the film growth is epitaxial then it is forming on a lattice-matched crystalline substrate and so will be crystalline from the start.

This is a multidisciplinary subject which includes vacuum engineering, fluid dynamics, plasma physics and molecular simulations.

Evaporation theory

One method of generating the vapour flux is through evaporation (heating a solid or liquid source). For evaporation to occur the heating must lead to sufficient vapour pressure (typically between 0.1-1 Pa). This often requires melting of the source, but not necessarily.

The Clausius-Clapeyron equation for solid-vapour and liquid-vapour equilibrium is an approximation (based on the vapour being a perfect gas) that is often used as a starting point to describe the connection between temperature and pressure.

The plot below is based on this, and gives the vapour pressure of the material at a given temperature. Click on the line that you want to analyse and use the scroll bar to see how far you can reduce the temperature whilst keeping an effective vapour pressure.

As you can see, a small variation in temperature leads to a large change in vapour pressure. Vapour flux (atom arrival rate per unit area per unit time) is linked to the vapour pressure, so we need very precise temperature monitoring in order to control the vapour flux and hence the film growth rate.

This process is carried out in a vacuum, and the evaporated atoms have relatively low energies (~ 0.1-0.3 eV)

Evaporation Techniques

Simple Resistance Evaporation

The source (known as the charge) is held in an electrically conductive boat or crucible, supported in a coil, or wrapped around a rod. This support is then heated by passing a current through it.

This method is reliable and relatively cheap due to the lack of complex components. However, the heating of the support can lead to desorption / evolution of impurities which will be incorporated into the growing film. There is also limited control over the temperature of the charge, and hence the deposition rate, so this technique is most widely used for non-critical applications.

Electron Beam Evaporation

In this scenario, the charge is heated directly using an electron beam.

This can lead to higher purity films since the crucible / support is not heated and may be water-cooled. There may also be some ionisation or activation of the depositing vapour flux as it passes through the electron beam.

Sputtering

Sputtering is an etching process. The source (known as the target) is bombarded with a high energy species, leading to the ejection of a vapour flux. Sputter deposition therefore uses this flux as the vapour source for film growth. It principally consists of atoms, with a range of energies, travelling away from the target at random angles. Sputtering is a purely physical process and is most simply modelled by assuming elastic binary collisions. The Sputter Yield (S) is defined as the average number of sputtered atoms per collision and it is affected by many variables. Forming an exact relationship between these parameters and the sputter yield is very difficult. Some of the contributing factors are:

• The momentum and energy transfer coefficients
• The temperature of the target
• The bond strength of the target atoms
• The incident particle energy
• The incident angle at the target surface

Typically, the target is bombarded with noble gas ions such as Argon (with energies in the range of 100-500eV). In this case, it is found that S~1 for most metals.

DC Glow Discharge Sputtering

The most common method of sputter deposition uses a self-sustaining discharge in a low pressure inert gas. The sputtering target is the cathode and ejects secondary electrons. These collide with the inert gas atoms, which become positively charged and accelerate towards the target. This causes sputtering on impact.

In order to maintain the discharge, the gas pressure needs to be high enough that the secondary electrons collide with and ionise gas atoms before they are lost to the surroundings. The sputtered flux then has to travel through this gas in order to reach the substrate. This scatters it, meaning that the setup is quite inefficient.

The following animation shows the different stages of the sputtering process:

If the required target is not electrically conductive, then a Radio Frequency voltage can be used to develop a negative potential on an insulating target surface.

Magnetron Sputtering

The principle used here is to add a magnetic field at the target surface. This means that the secondary electrons which are ejected from the target are in a region of crossed electric and magnetic fields, leading to cycloidal motion. This traps the electrons near the target surface, prolonging their residence time and enhancing the probability of collisions such that a denser discharge can be maintained down to lower pressures. This greatly increases the deposition rates.

Due to the nature of the magnetic field, the electrons are trapped within a specific region on the target, and so this is where sputtering occurs most heavily. This leads to a distinctive ‘racetrack’ region where the target is worn down much faster.

There are many possible magnetron geometries, but the rectangular planar one shown above is very common. Cylindrical cathodes allow uniform erosion of the target surface, improving material utilisation.

Comparisons and Complications

Energy

As seen above, the evaporated atoms will have relatively low energies which correspond mainly to their thermal energies. This is typically between 0.1-0.3 eV.

However, sputtered atoms can have a much larger range of energies, from 5-50eV. This large range can have an effect on the uniformity of the film.

The effect of the energy of the incoming (source) species will be discussed in the next section.

Alloys and Compounds

If an alloy source is used, then the different components will have different vapour pressures and so the vapour fluxes will not be equal. One way of avoiding this is to use co-evaporation from multiple charges, but this rapidly becomes difficult to control and maintain accurately. Compounds may be evaporated directly, but the high temperatures encourage dissociation.

Sputter deposition is much more flexible as using multiple targets, alloy or compound targets is more feasible.

Another effective method for compound film growth is the use of Laser Ablation (also known as Pulsed Laser Deposition). In this scenario, the vapour flux is created by firing high energy, focused laser pulses onto the surface of the target. This produces a plume of ablated target material. The laser typically has a wavelength of 200-300nm, and the pulse lasts between 6-12ns. The most efficient plume production occurs when the laser strikes the target surface at 45 degrees.

The speed of the heating process means that all the components evaporate simultaneously – avoiding fractionation.

Laser Ablation simply requires a vacuum chamber, a support for the target, and a window for the laser. Unfortunately, the laser can be quite expensive and difficult to scale up to industrial applications.

Growth Modes

When using evaporation, the flux travels in a straight line and so the deposition is line-of-sight only. This can lead to macroscopically shadowed regions, which are reduced by rotating the substrate during the deposition process.

When sputtering, the deposition is not line of sight (due to scattering in the intervening gas) but shadowing still occurs on an atomic scale. Variables which affect the degree and orientation of the shadowing are, among others, the background pressure in the chamber and the average angle from which the sputtered atoms approach.

If the atoms have low energies then they will remain in their initial positions. If the atoms have higher energies, then their thermal mobility may be high enough for surface diffusion. This allows the atoms to rearrange themselves into a lower energy conformation. A typical energy threshold to mobilise atoms on the surface is around 5eV. Hence, sputtered atoms typically have enough energy for surface diffusion to take place, whilst evaporated ones don’t.

If the energy is high enough for surface diffusion to take place then once the stable nuclei form on the substrate surface, they may coalesce. This leads to 3 possible modes of film growth.

Summary

Thin films can be constructed by the deposition of vapourised atoms onto a substrate surface. There are 2 principle methods of generating the flux for Physical Vapour Deposition (PVD)  – Evaporation and Sputtering. Both of these involve the use of a vacuum chamber.

If the incoming vapour flux has low energy, then the resultant film will have shadowed regions. However, if there is enough energy for surface diffusion to occur then the film will rearrange into one of 3 possible structures – depending on the relative bonding energies between the film and substrate atoms.

Deposition of films made from alloys or compounds are more difficult, as the different elements will not necessarily react to the environment in the same way. One way around this issue is a process known as Pulsed Laser Deposition (PLD). This involves heating the source material instantaneously using a focussed laser in order to avoid fractioning.

Questions

Quick questions

You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

1. Why are substrates often rotated during the deposition process?

   	a	To increase the speed of deposition.
   	b	To allow compounds to be deposited.
   	c	To help give a uniform thickness.
   	d	To help cool the substrate surface.

2. What is an advantage of using electron beam evaporation over resistive heating evaporation?

   	a	Lower contamination levels from the heating vessel.
   	b	Lower cost.
   	c	Increased speed of deposition.
   	d	Compounds can be evaporated.

3. Why is argon often used as the gas in the sputtering set up?

   	a	It is reactive.
   	b	It is inert.
   	c	It is heavy.
   	d	It is easily ionised.

4. What effect does changing the angle of deposition have on the film?

   	a	Makes the coating less uniform.
   	b	Makes the coating more uniform.
   	c	‘Tilts’ the shadowed regions and columns to a different angle.
   	d	No effect.

5. What conditions are most likely to lead to Layer Growth?

   	a	Film atoms are more strongly bound to each other than to the substrate.
   	b	Film atoms are more strongly bound to the substrate than to each other.
   	c	Low atomic energy (<5eV).
   	d	High misfit strain between the substrate and the film.

Open-ended questions

The following questions are not provided with answers, but intended to provide food for thought and points for further discussion with other students and teachers.

6. Explain how Pulsed Laser Deposition allows coating of precise compositions of alloys and compounds.

Going further

Books

M. Ohring, The Materials Science of Thin Films, Academic Press, 1992, ISBN: 0-12-524990-X

Academic consultant: Zoe Barber (University of Cambridge)
Content development: Maddie Hyde
Photography and video:
Web development: Lianne Sallows and David Brook

This DoITPoMS TLP was funded by the UK Centre for Materials Education and the Department of Materials Science and Metallurgy, University of Cambridge.