Vacuum Deposition of Thin Films

Vacuum deposition is a family of processes used to deposit layers of material atom-by-atom or molecule-by-molecule on a solid surface. A high vacuum (either as a starting point or during the actual deposition) is required for deposition of thin films by physical vapor deposition (PVD), pulsed laser deposition (PLD), molecular beam epitaxy (MBE), chemical vapor deposition (CVD), cathodic arc deposition, vacuum polymer deposition and atomic layer deposition (ALD).  PVD processes include sputtering (diode, triode, magnetron, thermal and electron beam evaporation, MBE and arc vapor deposition and ion plating.  CVD processes include conventional CVD, plasma enhanced CVD (PECVD), low pressure CVD (LPCVD) and ALD. To review, the vacuum environment serves one or more purposes:

  • Reducing particle density so that the mean free path for collision is long, thus preserving the energy of the particles
  • Reducing particle density of undesirable atoms and molecules (contaminants)
  • Providing a low pressure plasma environment
  • Providing a means for controlling gas and vapor composition
  • Providing a means for mass flow control into the processing chamber.

All of the above are critical to achieve high quality thin films, control deposition parameters and manage reproducibility of film properties and microstructure.  PVD processes move atoms from a source and condense them on a substrate, by means of plasma bombardment or by evaporation.   Thermal and electron evaporation are based on the vapor pressure of the elements.  A diagram of a simple evaporation chamber is shown in Figure 1. At equilibrium vapor pressure conditions (pressure, temperature, etc.), as many atoms return to the surface as leave the surface. Vapor pressure is extremely temperature dependent and vapor pressures of different elements at a given temperature can differ by orders of magnitude.  Elements vaporize (evaporate) as atoms or molecules at temperatures that create vapor pressures below the equilibrium vapor pressure, which depends on the phase diagram of the material.

As a result of this low energy, to achieve good adhesion to the substrate and a dense microstructure, they cannot experience energy robbing collisions with gas atoms in the chamber.   Additionally, to achieve a dense microstructure, energy of evaporated particles is combined with energetic bombardment of the substrate by an ion source.  The rate at which evaporated particles reach the substrate (deposition rate) is generally very high, particularly for metals, compared to other deposition processes.  This makes thermal and electron beam evaporation a competitive process for thin film production.  It should be noted that the composition of evaporated or bulk compounds may not be replicated at the substrate due to a difference in evaporation rates of the individual constituents.  However, metals can be coevaporated to achieve a specific composition.

Additionally, the substrate can be heated to improve film quality. A wide variety of evaporation systems and sources are available and can be reviewed in columns, articles and the Product Showcase published in VT&C.  Properties of evaporated thin films (microstructure, density, environmental stability) will be compared to those of other deposition processes in a future Blog.

Sputtering on the other hand employs an energetic plasma of inert gas ions (Ar, Kr, Xe, etc.) to bombard a source (or target) material and knock off atoms from the source via momentum transfer and deposited them onto a substrate.  Figure 2 shows a diagram of a simple sputtering chamber.  Ar is the most common sputtering gas.

Atoms sputtered from an alloy surface are deposited in the ratio of the target composition.  Metals, semiconductors, insulators and alloys can all be deposited by sputter deposition.  Due to the short mean free path of the sputtered particles, source-substrate separation is kept as small as possible, without affecting the thickness uniformity of the material on the substrate.  The sputtering process is a complex interaction between the plasma, gas atoms, sputtered particles, reflected particles and even the geometry of the deposition chamber.  Additionally, the substrate can be heated to improve film quality. A wide variety of oxides, nitrides, carbides, fluorides, borides can be deposited by reactive sputtering.  Deposition rates for sputtering are generally lower than rates for evaporated thin films.  Sputter deposition rate can be enhanced by magnetic focusing of the plasma (magnetron sputtering).  Sputtered thin films generally have improved adhesion to the substrate and control of microstructure compared to evaporated films (with no external energy enhancement).  A wide variety of articles and Columns describing all aspects of sputter deposition can be found in VT&C.

In pulsed laser deposition (PLD) high power laser pulses ablate a small amount of material from a solid target when a focused laser beam in absorbed within a small area of the target surface.  A schematic of a PLD system is shown in Figure 3.  Ablation involves breaking chemical bonds in the target and material is subsequently evaporated in a specific flux distribution.  The composition of the ablated flux is complex, composed of ions, molecules, neutral atoms and free radicals. The ablated species condenses on the substrate at an angle opposite to the target, forming the thin film.  Excimer lasers are frequently used in this process.  Stoichiometry of the target material is preserved in the PLD process.  PLD produces high quality films, such as semiconductors, high TC superconductors, superconductors, ceramics, ferroelectrics, multilayer films and polymers.  Alloy formation is also possible.  As with sputtering, the deposition process is complex, combining laser fluence, pulse duration, pulse repetition, preparation conditions and chamber configuration (target-substrate separation, substrate temperature, background gas and pressure).  This process has several disadvantages: complex transmitting and focusing systems are involved, sensitive to laser wavelength, low energy conversion efficiency, size of deposited films is limited (rastering the beam helps) and residual particulates.  However, the high quality of the resulting film can compensate for these shortcomings.

It is not possible to describe (even briefly) all thin film deposition processes. We finish with chemical vapor deposition (CVD).  CVD is parent to a family of processes whereby a solid material is deposited from a vapor by a chemical reaction occurring on or near a heated substrate.   Precursor gases (often diluted in carrier gases) are delivered into the reaction chamber at approximately ambient temperatures. As they pass over or come into contact with a heated substrate, they react or decompose forming a solid phase which and are deposited onto the substrate. The substrate temperature is critical and can influence what reactions will take place.

By varying process conditions, including substrate material, substrate temperature and composition of reactive gas mixture, total pressure gas flows, etc., materials can be grown with a wide range of physical, tribological and chemical properties.  Thin films deposited by CVD are uniform in thickness and properties and dense.  A diagram of a simple CVD system is shown in  Figure 4.

A CVD reactor consists of several basic components:

  • Gas delivery system – For the supply of precursors to the reactor chamber
  • Reactor chamber – Chamber within which deposition takes place
  • Substrate loading mechanism – A system for introducing and removing substrates, mandrels etc
  • Energy source – Provide the energy/heat that is required to get the precursors to react/decompose.
  • Vacuum system – A system for removal of all other gaseous species other than those required for the reaction/deposition.
  • Exhaust system – System for removal of volatile by-products from the reaction chamber.
  • Exhaust treatment systems – In some instances, exhaust gases may not be suitable for release into the atmosphere and may require treatment or conversion to safe/harmless compounds.
  • Process control equipment – Gauges, controls etc to monitor process parameters such as pressure, temperature and time. Alarms and safety devices would also be included in this category.

CVD coatings are typically fine grained, impervious, high purity and very hard.  CVD materials include semiconductors, metals and alloys, carbides, borides, elements, oxides and intermetallics. They are usually only a few microns thick and are generally deposited at fairly slow rates, usually of the order of a few hundred microns per hour.  The various CVD cousins are mentioned above.

 

 

 

What is Vacuum and How it is Created, Controlled and Used

The importance of a high vacuum environment for deposition of high quality thin films was introduced in part 1 of the series.  Virtually all thin film deposition processes require an initial high vacuum or ultrahigh vacuum in an air tight chamber, which require sophisticated vacuum pumping systems, gas control and pressure control systems reported in Vacuum Technology & Coating Magazine (VT&C).  Note that all aspects of thin film deposition, characterization, application design can be found in VT&C.

All physical and laser deposition processes move atoms or molecules from a source and deposit them onto a substrate.  These processes generally require that:

  • A plasma be used to energize gas atoms (usually an inert gas) and eject atoms or molecules from the source
  • Ejected particles are not significantly impeded (scattered) by gas atoms in the deposition chamber.  Particles lose energy with each collision with a gas atom.

This can only be accomplished in a high vacuum environment, that is, particles have a long mean free path (MFP) between collisions so they can make it to and be deposited onto the substrate with sufficient energy to adhere and coalesce onto the substrate.  Vacuum is essentially the absence of gas molecules.  The best vacuum can be found in outer space.


A vacuum pumping system, which may include a series of vacuum pumps and subchambers, must be able to evacuate each chamber volume in a reasonable length of time, and as a result, the throughput and pumping speed of the vacuum pumping system must be designed accordingly. The figure below shows a large deposition chamber that was located at Pacific Northwest National Laboratory. A high vacuum pumping system includes combinations of a roughing pump, high vacuum pump (diffusion pump, cryopump, turbomolecular pump, ion pump), cold trap to freeze out gases, chamber bake out, pressure measurement and control, reactive gas manifold and gas mixing control.  The deposition chamber must be leak tight to prevent contaminants and air degrading the vacuum environment and film quality.  The inner surface of the chamber must be kept pristine so that contaminants (gases and poorly adhering materials from previous depositions) are not liberated.  Leaks and outgassing introduce contaminants into the chamber which can react with materials from the deposition source, deposit loosely bound material onto the substrate and complicate control of gas pressure.  To achieve this the inner surfaces are heated during pump down, a process known as degassing.

 

The Figure below shows a schematic of a vacuum pumping system [1]. The roughing pump is used first to reduce chamber pressure to ~ 100 mTorr.  The high vacuum pump takes over at this pressure and completes evacuation of the chamber.  Note that the high vacuum pump is not capable of evacuating the chamber starting at atmospheric pressure.  If necessary, the walls of the chamber are then baked out.  Many chambers have a cold trap positioned before the high vacuum pump to freeze out contaminant gases and improve both vacuum quality and pump down speed.  The high vacuum pump is throttled before process and reactive gases are introduced to prevent overloading the pump and for control of chamber pressure during deposition.

The chamber is now ready for deposition of thin films (described in a future blog). The partial pressure of as many as four gases can be controlled by the gas control system.  Again, for more information on this equipment, refer to VT&C columns, articles and Product Showcase.  Baratron manometers (capacitance manometer) are the heart of pressure control systems.  Gases are introduced through flow controllers with flows determined by partial pressure of each gas (measured by Baratrons) and pumping speed.  A typical deposition pressure ranges from 1 – 5 mTorr.  The controller adds inert and reactive gases as required for the composition of the thin film.

Reference:

1. Donald M Mattox, Handbook of Physical Vapor Deposition (PVD) Processes, Noyes (1998).