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.