The range of products and processes that need a vacuum system is too extensive to describe. This section, therefore, will briefly describe the most important vacuum process (technically and commercially)-thin film deposition.
Film deposition by thermal evaporation was first reported in 1887. In the past 50 years, the number of vacuum deposition techniques has multiplied and the thin film uses have grown exponentially. Many modern products for consumer, commerce, military, medical, or research applications depend on thin films (see side bar.)
- Thin-Film Deposition Methods
- Film Structure/Properties
- Vapor Pressures/Melting Points
Thin-Film Deposition Methods
The methods used to deposit thin films are split into: Physical Vapor Deposition (PVD) Chemical Vapor Deposition (CVD) depending on the underlying principles causing film deposition. A PVD method evaporates or sputters a material, producing a gaseous plume or beam that deposits a film on the substrate. (Our Material Deposition Tables suggest which PVD techniques applies to what material). A CVD method uses reactive, volatile compounds that decompose on a heated substrate. The starting materials are often organo- or hydrido-compounds that pyrolyse at relatively low temperatures into a non-volatile (film) component and a pumpable vapor/gas. Both methods sub-divide into a variety of techniques with auxiliary mechanisms to achieve some goal.
Products of Thin Films
A few examples of products that would not exist, or be as effective, without vacuum thin film deposition processes.
- Consumer: CDs and DVDs; aluminized plastic food packaging; camera lenses; mirrors; optical coatings for windows, glasses, and sunglasses
- Commerce: Computer hard-drives and GMR read heads; headlamp reflectors; architectural glass; tool hard-coatings; diamond-like films; semiconductor chip production; organic electronics and displays; transparent conducting layers; magnetic memory; advanced solar panels; MEMS accelerometers
- Military: Aircraft canopies; night vision goggles; FLIR vision devices
- Medical: Eximer lasers for eye surgery; passivated prosthetic joints; coated stints; super-insulation for MRI magnets
- Research: Neutron beam guides; laser mirrors; optical filters; super-lattices; rugates; high/low Tc superconductors; SQUID magnetic detectors; quantum dots
Physical Vapor Deposition
The major PVD techniques are Sputtering, E-Beam Evaporation, and Thermal Evaporation.
The principles of sputtering are described here. In all the variations of sputtering, the bulk material (from which thin films are made) is called the target or cathode---the latter indicating its electrical potential during sputtering.
Early diode sputter sources used contoured targets to shape the electrical field to optimize the plasma. But the diode's inefficiency and complex target has meant it has been mostly superseded by sources with magnetic fields near the target. Three significant effects were noted: the target shapes are now disks, tubes, or rectangles; electron loss from the plasma is reduced; and the electron's path length in increased. The resulting magnetron sputter source's denser plasma greatly increased deposition rates and target utilization.
RF/DC/pulse DC Sputter Power Supplies
Electrically conducting metal, alloy, and compound targets cause no ion charging issues and are sputter with DC power. Sputtering a material with poor electrical conductivity using DC power leads to charge build-up. Energetic ions are buried in the target's surface but its resistivity prevents neutralization by electrons from adjacent electrodes. The target charge acts as a electrical barrier preventing further ion bombardment. Poor electrical conductors are sputtered using RF or pulsed DC power. The RF's reversal of electrode polarity or the negative surface bias present during the pulsed DC's 'off' period, causes the highly mobile electrons from the plasma to quickly flood and neutralize the surface.
The designation regular/reactive differentiates (regular) sputtering with inactive Ar gas and reactive sputtering with a gas mixture such as Ar/O2 or Ar/N2. Reactive sputtering has two important applications: (1) when target and film must have different chemical composition (e.g., sputtering a Ti target in Ar/N2 to give TiN films); and (2) when a compound target decomposes during sputtering and its stoichiometry (see Stoichiometry) must be restored.
If the central and outer magnets in a magnetron sputter source have similar strengths, then the flux lines from one pole will all terminate at the other pole. This arrangement is called a balanced magnetron and is the most commonly used. By contrast, if the outer magnets are stronger than the central magnet, some fraction of the flux lines do not terminate at the other pole and an unbalanced magnetron is created. The additional flux, roughly perpendicular to the target, passes through the substrate.
Electrons spiraling along these field lines are lost from the plasma; there are, however, beneficial effects with an insulating substrate. The electrons make ions in the process gas near the substrate. In addition, those electrons hitting the substrate give it negative bias. The result is that the substrate's surface is bombarded by ions that help density the film as it deposits. This is a form of ion-assisted deposition (IAD) (see IAD). The unbalanced magnetron disadvantage is that its less-dense plasma lowers target utilization and sputter rates.
In e-beam evaporation, free electrons generated by thermionic emission from a filament hit the evaporant's surface. Dissipation of the high-energy, high-current beam causes a temperature rise and, at a suitable vapor pressure, a plume of evaporant.
Under regular e-beam evaporation, the chamber pressure is as low as possible to prevent chemical reaction with the film or bulk evaporant. Under carefully controlled partial pressures of reactive gases, reactive e-beam evaporation gives films of different chemical composition to the bulk material.
Thermal evaporation is a major thin film deposition technique, particularly in R&D applications where the low installation costs and inexpensive, disposable evaporant "containers" are clear advantages. The shapes and sizes of the boats, boxes, crucibles, baskets, filament, etc., can be seen in Evaporation Sources. The disadvantages are precise temperature control may not be simple, and refractory metals sometime alloy, unexpectedly, with evaporants (e.g., evaporating Al from a W boat).
Evaporation vs. Sublimation
Almost all information about thin film deposition characterizes material transfer from bulk-to-film as evaporation. The correct usage of evaporation covers the "change of state" from a liquid to gas. A "change of state" from a solid to gas should be called sublimation. In general thin film work, however, the physical state of the bulk material is of little consequence and is probably unknown. Throughout these Tech Notes, the word evaporation covers both phenomena.
A common misconception is that an evaporant's vapor pressure somehow changes markedly during a transition from sublimation to evaporation. That is, a solid evaporant at its melting point has a different vapor pressure when compared to the liquid form at its melting point. This is simply not true-for any material, the vapor pressure versus temperature curve is smooth at all temperatures. To give an example from everyday experience: in a glass containing ice cubes and water at 0°C, both phases have exactly the same vapor pressure.
Effusion Cell Evaporation
Effusion cells come in many different designs classed as near-ideal, open-tube, conical, nozzle-jet, point-source, etc. with different names (including Knudsen cells, K-cells, and proprietary names). The main differences between them are beam intensity (which affects film deposition rate) and angular distribution (which determines film thickness uniformity). Good explanations of effusion cell characteristics are found in Chapter V of J.E. Mahan's book Physical Vapor Deposition of Thin Film (2000) together with Chapter 1 of Maissel and Glang's book "Handbook of Thin Film Technology" (1970).
High Temperature Effusion Cells
Many elements and some inorganic compounds attain VPs suitable for fast deposition between 600ºC and 1500ºC. Effusion cells, which are essentially ceramic or carbon crucibles heated by external resistance or induction heaters, are a popular choice is this temperature range. Temperatures can be well controlled giving stable deposition rates and the crucible hold moderately large volume of evaporant.
Low Temperature Effusion Cells
In the last decade a huge interest has developed in electrically conducting organic films for displays and circuits. The evaporation temperature range for these materials is 200ºC to 500ºC. Neither resistive thermal sources nor 'high temperature' effusion cells provided adequate thermal stability and a new class of effusion cells, specifically designed for this low temperature range, has been developed.
Pulse Laser Deposition (PLD)
This is a flash evaporation technique. A short, high energy pulse from an eximer laser strikes the solid's surface. The fraction of energy absorbed creates a thermal pulse that rapidly spreads into the bulk evaporant, perpendicular to the surface. A directed plume completely vaporizes a thin 'layer'. The technique is particularly successful at producing stoichiometric films of complex compounds, for example: high Tc superconductor YBCO; bio-compatible calcium hydroxy-apatite; and alloys with components that have very different VPs. A good introduction to many aspects of pulse laser deposition is given in Mahan's book noted above.
Ion Beam Sputtering
Essentially, ion beam sputtering is a version of diode sputtering. In place of the normal plasma, a variable energy, wide ion beam source provides ions that are accelerated toward the target. The target and substrate are typically parallel with the ions injected at 45º. Ejected target atoms deposit on the substrate as a film. Using an active background gas, reactive ion beam sputtering is also possible. The technique has been very successful in applications requiring thin films of magnetic materials. (The magnetic fields of normal magnetron sputter sources are partially or completely shunted by magnetic targets. Only magnetrons equipped with very high strength magnets will operate.)
Chemical Vapor Deposition (CVD)
CVD (chemical vapor deposition) is a primary thin film deposition process in the semiconductor industry. Various CVD techniques are identified by groups (initial letters), for example: MOCVD (metal organic); PECVD (plasma enhanced); PACVD (plasma-assisted); APCVD (atmospheric pressure); LPCVD (low pressure); UHVCVD (ultrahigh vacuum); etc. Chapter 7 of D.L. Smith's book Thin Film Deposition-Principles and Practice gives an extensive description of general CVD processes and practices.
The initials in each case give an indication of a major operational characteristic. For example: MOCVD is basically the thermal degradation of a volatile metal-organic vapor. However, both "M" and "O" have liberal interpretations since MOCVD covers Si/Ge deposition and includes hydrides and carbonyls as the "organic" part. The chemistry, in particular MOCVD process, can be complex, involving pyrolysis, oxidation, hydrolysis, reduction, and displacement. In another example, PECVD, while similar to MOCVD, uses microwave or RF-generated plasmas in the vapor to facilitate a desired chemical change. Films form at lower substrate temperatures, making PECVD the preferred technique for films or substrates that are temperature sensitive. One important semiconductor processing application of PECVD is making silicon oxynitride films.
Alternative ways of categorizing thin film deposition describe the film's structure or the focus on techniques that improve the film's properties.
The atomic- and nanometer-level structure of a deposited film has a profound influence on its chemical, optical, mechanical, electrical, and magnetic properties. The word morphology is a general description for this low-level structure. A film's detailed morphology depends on many factors present during the film's growth, including: chemical/physical properties of the depositing material; substrate temperature, flatness, and contamination; deposition rate; process or residual gas pressure; surface diffusion; film growth mode; residual stress in the film; and match between the film's and substrate's lattice parameters. Chapter 5 of Smith's book, mentioned earlier, gives a particularly good description of film morphology considerations.
Stoichiometry issues arise when depositing films of chemical compounds. Known rules cover the proportions in which elements combine to form compounds. For example, zinc oxide has the formula ZnO. However, depositing a film from bulk ZnO using (regular) sputtering or thermal evaporation causes oxygen loss, giving the film a stoichiometry of ZnO1-x. That is, it is a mixture of ZnO and some portion of free metal. To correct this loss, a reactive deposition form with additional oxygen is used.
Zinc oxide's lack of stoichiometry during film deposition is well established. However, the range of oxides, sulfides, etc., that give non-stoichiometric films is not widely recognized and frequently is the source of variable or "weird" film properties.
A film's thickness is frequently critical to desired performance. For example, anti-reflection coatings, giant magneto-resistance devices, neutron beam guides, and optical filters will not function with the correct film thickness. Equally important is the uniformity of that thickness across the area of interest. With the exception of atomic layer deposition (see below), all deposition techniques can produce films with some level of non-uniformity. Approaches to reducing thickness variations in PVD methods include optimizing the bulk material's throw distance to the substrate, substrate rotation, and/or planetary motion. For CVD methods the approaches include laminar 'showerhead' gas introduction and substrate rotation.
Ion Assisted Deposition (IAD)
IAD uses a wide-diameter inert gas ion gun to provide relatively low-energy impact on a film being deposited by evaporative or sputter techniques. The technique is well established for high-quality optical coatings where it modifies intrinsic film stress, improves film adhesion, and reduces porosity (densifies). The last one reduces moisture adsorption, and hence vacuum- to-air shifts. But in all, IAD enables production of multilayer optical components with low absorption and stable refractive indexes.
Atomic Layer Deposition (ALD)
ALD is a self-limiting CVD process that is rapidly becoming the preferred technique for depositing high-k dielectric oxides. The growth of a metal oxide consists of two reaction steps. In the first the metal compound precursor is allowed to react with the surface and in the second it reacts with the some source of oxygen. Between steps a purge gas removes excess metal compound precursor and reaction by-products. An example is the ALD deposition of aluminum oxide where the overall reaction 2Al(CH3)3 + 3H2O = Al2O3 + 6CH4 is split into two steps: 1. AlOH* + Al(CH3)3 = AlOAl(CH3)2* + CH4 2. AlCH3* + H2O = AlOH* + CH4 where * indicates the surface species Alternating pulsing the metal precursor and the oxidant with gas purges between them allows precise multiple layer formation. Although MOCVD is a faster process, ALD's surface reaction nature (without a gas-phase component) gives superior step coverage and film uniformity.. ALD is particularly suitable for surfaces containing 'trenches' or 'vias'.
Typically, there is a preferred deposition technique for each material and type of film required. A very involatile element or compound may make thermal evaporation unfavorable. Perhaps the element responds to DC sputtering and the compound to RF/pulse DC sputtering or reactive DC sputtering. There are, however, many elements and compounds for which thermal or e-beam evaporation is preferred. Practical deposition systems are frequently made more versatile by installing two or more different techniques in the same chamber. In such an arrangement, a process requiring both evaporation and sputtering for co-deposition, or sequential deposition, is done without substrate re-location or breaking vacuum.
Rather than different techniques, some applications need multiple examples of the same technique. This is particularly true of sputtering, where up to nine guns have been mounted in one chamber, or effusion cells for organic materials, where as many as ten cells have been installed in one chamber.
One critical aspect of multiple or combined techniques in one deposition system is "cross-talk". The "plume" of one material must not deposit on (and contaminate) the material in a separate deposition source. This is achieved by careful design of sources, shields, shutters.
With the exclusion of ALD, all evaporation sources are mounted with the vapor plume's axis vertically up. This avoids spillage if the material is molten at its evaporation temperature.
Sputter source targets never melt and the source's orientation is un-restricted. Two common arrangements for multiple sputter sources are "parallel" and "convergent". Parallel sources have their axes (the normal to the target plane's surface) parallel. This is a common arrangement in box coaters where a number of substrates is mounted on a rotating platen and the platen moves to locate a substrate's center over each source's center in turn. This arrangement suits sequential layer deposition. Convergent sources have axes that meet at a point in space that coincides with the substrate surface's center-point. This arrangement is particularly useful when co-depositing different materials.
One common chamber combination is a load lock (LL) and main chamber (MC). The small volume LL has its own pumping system and O-ring sealed door. It is connected to the MC by a large diameter gate valve. A substrate is loaded into the vented LL which is then pumped. When the LL is at high vacuum, the gate valve is opened and the substrate is mechanically transferred into the MC (which remains at high vacuum throughout the transfer stage). Some LLs are fitted with heater stages, plasma cleaning devices, or other substrate preparation devices so the "gassy" processes do not contaminate the main deposition chamber.
More complex combinations interconnect UHV chambers for, perhaps, MBE deposition and surface science analysis, or high vacuum chambers for plasma etching, metal deposition, organic deposition, mask storage, etc., in a cluster arrangement. Of recent interest is the combination of an ALD chamber (isolated by a gate valve and suitable substrate transporter unit) and a PVD chamber, allowing the overlaying of different film types.
The recent huge increase of interest in: reactive metal films; organic electronics films; and light-emitting organic displays (OLEDs), has led to strong growth in another chamber combination. The issue here is, these film cannot be exposed to the atmosphere without a protective coating or sealing. These coating/sealing operations are however, not vacuum operations but are done under an inert gas. This led to a demand for glove boxes interfaced to load locks. The films are transferred from the LL to glove box under positive argon atmosphere and are sealed/coated before venting the glove box to air.
Vapor Pressures & Melting Points
Two firmly held convictions exist when discussing evaporation as a thin film technique.
An element's equilibrium vapor pressure (VP) is related to its melting point.
Al and Mg melt only 10°C apart, yet have VP that differ by a factor of 109. Although Gd and Ga melt over 1280°C apart, at the same temperature their vapor pressures are roughly a factor of 20 different.
Yes, if one assumes an acceptable evaporation rate requires a VP of 1 x 10-2 Torr and plots the temperature needed to reach this VP versus melting point, the result for some elements is a quasi-straight line. However, a quick count shows ~23 elements of interest in deposition deviate by having VPs 100 times higher or lower at their melting points.
Yes, a few alloys (e.g,. Ni/Cr 80/20%) evaporate, giving films with almost the bulk alloy's stoichiometry. But the VPs of Ni and Cr are less than one decade apart at any temperature. In addition, chromium's slightly higher VP probably compensates for its lower mole fraction (actually its activity in the evaporating bulk). By contrast, any alloy of Al/Mg would always give a greatly enriched Mg film.
Two pairs of metals demonstrate how wrong these convictions are.
|Metal||Melting Point °C||VP at M.pt Torr|
|Al||660.32||3 x 10-9|
|Gd||1312||1 x 10-2|
|Ga||(at 1312)||2 x 10-1|