15 Physical Vapor Deposition Process

Dr. Ayushi Paliwal and Dr. Monika Tomar

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1. Introduction

 

A thin film is defined as a low-dimensional material created by condensing, one-by-one, atomic/molecular/ionic species of matter on a substrate. An innumerous variety of thin film materials, their deposition, fabrication techniques, spectroscopic and optical characterization increases the utility in the production of devices. The possible techniques used for thin film deposition are classified broadly into two categories (Fig. 1):

  • Physical Deposition Process
  • Chemical Deposition Process

 

Physical method covers the deposition techniques which depend on the evaporation or ejection of the material from a source, i.e. evaporation or sputtering, whereas chemical methods depend on physical properties. Structure-property relationships are the key features of such devices and basis of thin film technologies. Underlying the performance and economics of thin film components are the manufacturing techniques on a specific chemical reaction. Thus chemical reactions may depend on the chemical formation of the film from the medium, and typical methods involved are electroplating, chemical reduction plating and spin coating/ dip coating of chemical solutions, chemical vapour deposition etc.

 

Physical Vapor Deposition (PVD) is basically a collective set of vaporization processes that are used to deposit thin layers (atomic level) of material, typically ranging from few nanometers to several micrometers. These processes are eco-friendly vacuum deposition techniques and consist of three fundamental steps:

 

Material vaporization from a solid source assisted by high temperature vacuum or gas phase plasma.

 

Transfer of the vapour to the surface of the substrate in vacuum or partial vacuum. Condensation of the vapour onto the substrate to form thin films.

 

Common Examples of PVD process are: Pulsed Laser Deposition, Sputtering, Thermal Evaporation, e-beam evaporation etc.

 

All the methods, though differ from each other in the growth and deposition methods, yet follow the same above mentioned three fundamental steps.

 

Detailed study of each technique is given in subsequent sections:

 

1.1  THERMAL EVAPORATION:

 

One of the simplest Physical Vapor Deposition processes is the Thermal Evaporation method.

 

In Thermal Evaporation, an electric resistance heater is used to melt the material in a vacuum chamber till the atoms at the surface of the material gain sufficient energy and leave the surface as shown in figure 1. At this point, they traverse throughout the chamber, at thermal energy ( < 1 eV), and will get deposited on a substrate which is situated above the evaporating target material (at an average distance of 200 mm to 1 m).

High vacuum is required for thermal evaporation for two main reasons:

 

1) to allow the vapor to reach the substrate without reacting with or getting scattered against other gaseous phase atoms in the chamber,

 

2) to reduce the consolidation of residual gas impurities in the vacuum chamber.

 

In this process, the chamber pressure must be kept below the level (5 × 10-5 mbar) such that the mean free path (i.e. the average travelling distance for an atom or a molecule in the vacuum chamber before it collides with any other particle, disturbing its direction to some extent) is longer than the distance between substrate and the evaporation source. Tungsten or Molybdenum filament is utilised to heat the source material for evaporation as shown in figure 2.

OPTIMIZATION OF THE FILM:

  • The film purity depends on the purity of the source material and the quality of the vacuum.
  • Thicknesses of the film differ due to the geometry of the chamber.
  • Collision with the residual gases leads to non- uniform thickness.

ADVANTAGES OF THERMAL EVAPORATION:

  • It is simple and cheap.
  • Less substrate surface damage.
  • Excellent purity of films

DISADVANTAGES OF THERMAL EVAPORATION:

 

Considerable wastage of source material during the process. Confined to low melting point metals.

 

Evaporation of Dielectric materials is not possible.

 

The amount of material deposited is also constrained by the filament. Poor density of the film.

 

Poor adhesion of the film.

 

Step coverage is harder to improve.

 

CRYSTAL MONITOR:

 

In order to monitor the thickness of the deposited film and also to control the rate of evaporation, a Quartz crystal is used.

 

Crystal needs to be cleaned or changed periodically.

 

STEPS TO USE CRYSTAL MONITOR:

  • Switch ON the crystal monitor.
  • Depending on the material to be deposited, select the film number.
  • Enter the density of the material, Acoustic impedance, Tooling factor.
  • When evaporation begins, press the Start button(set the thickness display to zero and open the shutter).
  • Once the desired thickness is achieved close the shutter mechanically.
  • Press the Stop button.

 

1.2   E-BEAM EVAPORATION:

 

E-Beam evaporation is a physical vapor deposition process in which an intense beam of electron is generated from a filament and is guided via electric and magnetic fields to strike source material and vaporize it within a vacuum environment as evident from figure 3.

It is a potent deposition process as the user can evaporate materials that are very hard or impossible to process using standard thermal evaporation. Some of the examples of such materials are high-temperature materials like gold and titanium, and also ceramics such as silicon dioxide and alumina.

 

Generally extreme end range of pressure is required for running e-beam evaporation process so as to allow employment of a wide ion beam source for the film densification and other modifications f the property at the same time.

 

For generating electron beam, an electric current is applied to a filament in presence of high electric field. Electrons in the filament escape and accelerate away due to this electric field. Then, these electrons are guided by magnets to build a beam and then get directed towards the source material placed on a crucible. The electron beam’s energy is transferred to the material and thus the material starts evaporating. Many metals, like aluminium, melt first and then begin evaporating, while ceramic materials get sublimated. The vapours of the evaporated material, then, travel out of the crucible and thus the substrate is coated.

 

To increase the crucible lifetime, it is put up in a water cooled copper hearth during evaporation.

 

ADVANTAGES OF E-BEAM EVAPORATION :

 

e-beam evaporation source allows deposition of material in larger amounts. Also, higher deposition rate is achieved.

 

The e-beam source can be equipped with a carousel of multiple pockets for depositing multiple targets sequentially without breaking the vacuum. High purity of the deposited films.

 

DISADVANTAGES OF E-BEAM EVAPORATION:

 

Difficult to be controlled.

 

Incapable of performing cleaning of the surface. Difficult to improve step coverage.

 

1.3 PULSED LASER DEPOSITION:

 

One of the PVD methods, i.e. Pulsed Laser Deposition method has become increasingly popular in the past few years as it is easy to use and successful in deposition of materials with complex stoichiometry. PLD is one of the best techniques to grow high-quality functional oxides thin films.

Here, a high power ultra-violet wavelength pulsed laser beam is directed in a vacuum chamber (roughly 10-6 torr) for target ablation. The ejected material forms a plasma plume which then extends away from the surface of the target and interacts with the chamber’s atmosphere until it arrives at the substrate, where it gets deposited as a thin film.

 

A laser beam of sufficient energy is focused on a target, the target material goes into vapour state directly without melting. The vaporized species in front of the target are seen as glowing plasma, called the plume. The plume interacts with the processing gas (generally reactive gas) available in the deposition chamber and the ablated matter is condensed on the surface of the substrate as a thin film. If the laser beam is intense enough, it can ablate the hardest and most heat resistant material also.

 

In PLD, there is a provision for heating the substrate to assist nucleation and allow growth of crystal.

 

Also, to aid controlling the composition of film, a background gas such as oxygen can be used.

 

PROCESSING STEPS:

 

The PLD process can be divided into following five stages:

 

Absorption of laser on the surface of target. Laser ablation of the target and plasma debut. Dynamics of the plasma.

 

Deposition of the ablated target material on the substrate surface. Nucleation and growth of the film on the surface of substrate.

 

All these steps are crucial for the formation of crystalline, uniform and stoichiometric film.

 

THIN FILM DEPOSITION PARAMETERS:

  • Laser parameter: Consists of laser fluence (J/cm2), laser energy, and the ionisation degree of the material to be ablated.
  • Surface Temperature: Nucleation density generally gets decreased with increase in the temperature.
  • Surface of the substrate: Roughness of the substrate surface , any miscut can affect the growth of the film.
  • Background Pressure: This is needed for ensuring stoichiometric transfer from the target material to the film.

 

In Pulsed laser deposition, there are three possible growth modes based on the above deposition parameters:

 

(i) Step-flow growth – Miscuts on the substrate lead to atomic steps on its surface. Thus, atoms land on the surface of the substrate and get diffused to a step edge before being nucleated. The growing surface is viewed as steps moving across the surface.

 

(ii) Layer-by-Layer growth – In this mode, islands nucleate on the substrate surface till a critical island density is attained. When more material is added, the islands keep growing till the islands coalesce into each other. Hence, the surface has a large density of cavities. When more material is added to the substrate surface, the atoms get diffused into such cavities to complete the layer. This process is repeated for depositing subsequent layers.

 

(iii) 3D growth – this growth is similar to layer-by-layer growth, apart from the fact that once an island is formed, nucleation of the additional island takes place on top of the first island. Thus, the surface gets roughened every time a material is imparted.

 

FACTORS INFLUENCING DEPOSITION RATE:

 

Target material.

 

Energy of the pulsed laser.

 

Distance between target and substrate.

 

Background gas type (Eg: oxygen, argon etc.). Chamber pressure.

 

ADVANTAGES OF PLD:

 

Capability for stoichiometric transport from target material to substrate surface. In other words, procreation of the exact chemical composition of complex materials in the deposited film.

 

Relatively high rate of deposition

 

Use of laser as an external source of energy leads to a highly clean process without filaments. Therefore, both inert and reactive gases can be used as background gases while depositing film.

 

Carousel for holding multiple targets enables deposition of multilayer films without the need of breaking the vacuum in while changing the material in-between.

 

DISADVANTAGES OF PLD:

 

Chances of Non uniform thickness and varied composition across the deposited film as the plasma plume formed during laser ablation process is extremely forward directed.

 

However, this problem can be tackled by rastering of the laser spot across the target material or by substrate rotation during deposition.

 

Area of deposited material is quite small ( ~ 1 cm2) compared to that required for several industrial applications.

 

Deposition of novel materials requires periodic empirical optimization of deposition parameters.

  1. SPUTTERING:

 

Sputtering is another PVD technique to deposit thin film of a material onto a substrate.

 

First, gaseous plasma (a plasma is an ionised gas consisting of positive ions and free electrons, and neutral. It is the fourth state of matter) is created in the vacuum chamber, and then the ions are accelerated from this plasma into some target source i.e. Bombardment of high speed particles with the source material.

 

This leads to momentum transfer from the particles to the atoms at the surface, and this imparts sufficient energy to the surface atoms to escape away.

 

These ejected atoms can then travel to a substrate and thus the film gets deposited.

 

CONSIDERATIONS:

 

Several factors need to be considered for sputtering deposition-

  • Creation, Control and Directing of high speed particle stream.
  • Interaction of such particles with the material source surface and emission yield.
  • Deposition of ejected target atoms on the substrate
  • Quality of the deposited film.

So, in sputtering process, the target material and the substrate are placed in a vacuum chamber. Voltage is applied in between such that target acts as cathode and substrate as anode.

 

Ionization of gas (a chemically inert, heavy gas such as argon in general) results in creation of plasma.

 

This sputtering gas then bombards the target and sputters off its surface.

 

GENERATION AND CONTROL OF THE PLASMA:

 

Generation of ions can take place by the collision of neutral atoms with high energy electrons. The velocity and energy of the ions determine the interaction of the target and the ions.

 

As ions are charged particles, they can be controlled by electric and magnetic fields.

 

In the beginning of the process, a stray electron near the cathode is accelerated towards the anode and collides with a neutral gas atom, changing it to a positively charged ion.

 

This results in two electrons which then collide with other gas atoms and ionize them producing a cascading action till the gas breaks down.

 

The breakdown voltage is dependent on the chamber pressure and the distance between cathode and the anode.

At very low pressure, there are not sufficient collisions between atoms and electrons to hold plasma.

 

At very high pressures, there exist so many collisions that electrons do not have sufficient time to gather energy between collisions to become able to ionize the atoms.

 

GLOW DISCHARGE FORMATION:

 

At the beginning, there is small flow of charges (current). As the charges multiply, the current increases rapidly, but the supply voltage remains constant.

 

Eventually, enough ions and charges are available for self-sustaining plasma.

 

Some electron-atom collision produce light in place of electrons and ions, and hence the plasma also glows followed by a voltage drop (normal glow).

 

If we further increase the input power, the current density becomes uniform across the cathode and then the abnormal discharge regime exists. Here, the sputtering process functions.

 

The sputtering process can be broadly divided into DC sputtering and RF-sputtering. Besides, the same basic principle for DC sputtering and RF sputtering except different power sources (i.e. DC and RF power supplies).

 

a) DC SPUTTERING:

DC sputtering uses a DC gaseous discharge. The power supply is a high-voltage DC source. Ions bombard the target (cathode) source, while substrate and chamber walls may act as anode.

 

The main control is the energy range of the ions.

 

LIMITATIONS OF THE DC SPUTTERING:

  • High pressures required for attaining plasma can possibly degrade the quality of the film.
  • Only little fraction of gas is changed to ions.
  • Low deposition rates.
  • Limited to materials of resistivity less than 106 ohms (no insulators as they require impossibly large voltage for sustaining plasma).

AC PLASMAS:

 

Since the effective resistance of dielectrics varies with the frequency of applied current (impedance), thus a capacitor-coupled AC plasma sputtering system holds lower impedance and thus requires reasonable voltage for sustaining current through the target.

 

At low frequencies, as both the electrons and ions react to the change in voltage polarity, crucially a DC plasma (changing sides) dominates.

 

Above frequencies ~1 MHz, since the ions are heavier, only the lighter electrons are able to follow the change in voltage.

 

The high mobility of electrons renders them to be more energetic and thus increases density of the plasma.

 

SELF-BIASING OF THE TARGET:

 

The target automatically tends to bias itself more negatively than the anode as a consequence of the difference in mobility of ions and electrons.

 

The reason is that high mobility of electrons enables them for a larger electron flow through the anode while the less mobile ions provide a lower current to the cathode.

 

Due to this asymmetry, a negative biased current is created through the plasma.

 

b) RF SPUTTERING:

In RF sputtering, there is a cathode (target) and anode, in series with a blocking capacitor(C). The capacitor is included in an impedance-matching network utilized for optimizing power transfer from RF source (often constant at 13.56 MHz) to the plasma discharge. The blocking capacitor C is used in the circuitry for developing necessary DC self-bias.

 

ADVANTAGES OF RF SPUTTERING:

 

RF sputtering has advantage over DC: as building up of ions is prevented for electrically insulating targets by avoiding constant negative voltage on the cathode.

 

Operating pressure can be low for sustaining plasma.

 

DISADVANTAGES OF RF SPUTTERING:

 

Deposition rates are lower.

 

Power supplies are expensive.

 

c) MAGNETRON SPUTTERING:

This is magnetically aided discharge. As we know, in DC and RF sputtering, electric field is perpendicular to the target surface. However, in the Magnetron arrangement, a permanent magnet (or electromagnet) is added to the system for creating magnetic flux lines parallel to the target surface. This magnetic field concentrates and intensifies the plasma immediately above the target due to the trapping of electrons near the target surface. This leads to enhanced ion bombardment (without the need of increasing operating pressure as it degrades film quality after a certain point) and sputtering rate for both DC and RF.

 

MATCHING NETWORK:

 

This is the typical matching network for RF plasma coupling.

  1. Summary

 

In this module we have learnt about:

  1. The fundamental physics of the Physical Vapour Deposition process.
  2. Detailed study of Various types of PVD methods and the difference in their growth process :
  • Thermal Evaporation
  • E-beam Evaporation
  • Pulsed Laser Deposition
  • Sputtering

 

3. Sputtering

 

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