Vacuum Observations & Perspectives

In this weblog series, we’ll examine some emerging and evolving opportunities for vacuum-centric equipment, materials, processes and R&D. Also, we’ll look at some interesting applications and technology trends. Thinking outside the box, if you will, with a broad perspective.

Vacuum use is only likely to increase going forward, especially when producing very small and complex components like ICs and MEMS for sophisticated products. Doing so in a vacuum is the only practical way to prevent gaseous and particulate contamination. Vacuum is absolutely essential for semiconductor production.

Semiconductor Challenges—Manufacturing Is Difficult

The familiar semiconductor industry is changing, significantly, now. These changes provide many opportunities for those companies involved that can meet the challenging technical and investment demands. The microchip industry itself is always continuing its quest for ever smaller geometries (circuit features) to get more devices produced per wafer area. That’s how Moore’s Law has keep innovation and performance increases alive.

By constantly pushing new frontiers of equipment, materials, and processes, chipmakers produce the ubiquitous integrated circuit (IC) chips at reasonable prices. Historically, jumps in wafer size were tied to photolithography advances that enabled smaller feature sizes. The result was more advanced semiconductor chips with greater performance. Transistors are much smaller now and circuits more complex but greater performance results. But they are extremely difficult to manufacture.

The greatest demand for computer chips now is in mobile devices, wireless, IoT and automotive areas. Laptops, tablets and smartphones all benefit. Since desktop PC performance is already greater than what most customers actually need, mobile devices are often a primary focus. For products where space and extreme performance are not necessary, conventional IC manufacturing fabs will be cranking out those chips for years. You certainly don’t need state-of-the-art microprocessors in your home’s thermostat.

Monolithic ICs

Semiconductor manufacturing, often considered a mature industry, needs near-perfect thin films and truly advanced lithography patterning along with sophisticated deposition and etching process technologies to produce the active and passive elements in ICs—the core of most high tech products worldwide. Planar 2D production is mainstream now in semiconductor manufacturing but some high-profit, in-demand chips are now made with 3D circuitry. Conventional optical photolithography now appears to be at the end of its affordable smaller dimensions potential but there is a promising new exposure system: EUVL.

2D, 3D & EUV Lithography

To produce more chips in the same wafer surface space, you need smaller chips that provide the functionality but in smaller dimensions. Greater functionality and faster performance are the norms. If the latest EUVL (Extreme UltraViolet Lithography) tools prove ready for prime time with high-volume production next year, the quality and flatness of the many deposited thin film layers will become even more critical than they are now. EUV needs a stringent vacuum environment. See “THE USE OF EUV LITHOGRAPHY IN CONSUMER MICROCHIP MANUFACTURING” at for some insights.

In July 2017, ASML Holding N.V. (ASML) President and Chief Executive Officer Peter Wennink said, “In EUV lithography, we have integrated an upgraded EUV source into a TWINSCAN NXE:3400B lithography system in our Veldhoven [The Netherlands] facility and achieved the throughput specification of 125 wafers per hour on this system. Now, with all key performance specifications demonstrated, we focus on achieving the availability that is required for high-volume manufacturing as well as further improving productivity.”

That sounds promising but the actual EUV masks and photoresists seem somewhat problematic from published comments and reports. In July 2017, BACUS (formerly the Bay Area Chrome Users Society) noted, “Recently, readiness of the EUVL infrastructure for the high volume manufacturing (HVM) has been accelerated [1]. EUV source availability, the first showstopper against EUVL HVM, has been dramatically increased and close to the targets for HVM insertion. Mask defectivity, another focus area for the HVM, has also been concerned. Due to the difference in mask and optics appropriate for the wavelength between EUV and ArF lithography, specialized metrology tools are required in EUVL. However, current DUV and e-beam inspection tools are easy to miss the printable phase defects in EUV mask since the lights of corresponding wavelengths cannot penetrate multilayers (MLs)[2,3]. Therefore, the actinic review system is essential to provide defect free EUV masks.” See more details at

The Samsung R&D experts who authored this are betting on EUV and 7nm lithography to take some IC foundry business away from the leading foundry producer TSMC (Taiwan Semiconductor Manufacturing Co.) but TSMC is also planning on introducing 7nm EUV devices next year. We’ll see. EUV keeps getting promised as “soon” but the dates keep slipping. Reuters noted, “But the firm lags well behind Taiwan’s TSMC in contract manufacturing: TSMC held a market share of 50.6 percent last year compared with Samsung’s 7.9 percent, according to research firm IHS. It also trailed U.S.-based Global Foundries, which had a 9.6 percent share, and Taiwan-based UMC’s 8.1 percent.”

Wafer Sizes Matter

First, there are wafer size considerations. Today, 200mm wafer fabs are typically running at capacity with some new 200mm fabs being built but some essential 200mm fabrication production tools are in short supply and expensive. Some of these fabs once made high-volume state-of-the-art ICs but have transitioned to more profitable proprietary and/or lower volume chips. Supporting 200mm will be necessary for the foreseeable future.

Per Christian G. Dieseldorff, Industry Research & Statistics Group, SEMI, at SEMICON West 2017, “Driven by mobile and wireless applications, IoT (Internet of Things), and automotive, the 200mm market is thriving.  Many of the products used in these applications are produced on 200mm wafers, so companies are expanding capacity in their facilities to the limit, and there are nine new 200mm facilities in the pipeline. Looking only at IC volume fabs, the report shows 188 fabs in production in 2016 and expanding to 197 fabs by 2021. China will add most of the 200mm capacity through 2021, with 34 percent growth rate from 2017 to 2021, followed by South East Asia with 29 percent and the Americas with 12 percent.”

The 300mm fabs, once expected to displace 200mm fabs, are now competing with 200mm in some markets where smaller volumes can make 300mm efforts more expensive. But 300mm is running at capacity too with the most advanced chips. There also has been the realization by the major semiconductor manufacturers that many improvements to optimize 300mm manufacturing are possible which delays the costly transition to building factories that handle 450mm wafers.

Finally, 450mm seems to be a wafer size that can wait, perhaps until 2020 per SEMI []. Also, the New York-based 450mm global consortium, G450C, is defunct. Samsung, and others, are now stacking many layers of transistors on the same memory die with smaller transistors, so the need for 450mm wafers is not as urgent now although companies are still exploring 450mm options for the future.

Then, of course, there is the high-end leading edge efforts to switch to EUV lithography in the quest to produce even more ICs per unit area on a wafer. EUV emerged when x-ray proved problematic years ago. X-ray lithography was first proposed by H. Smith and Spears at MIT. []. There are recollections of the frantic x-ray lithography efforts several years ago that never reached mainstream production status. X-ray lithography promised > 1nm feature sizes, far smaller than the EUV efforts of today will produce.

Mainstream x-ray lithography simply had too many issues at the time: dangerous x-ray sources as well as expensive masks and resists that were problematic. Proposed x-ray synchrotron radiation sources required very long times to reach acceptable low vacuum levels which is problematic for volume production lines that cannot stop operating for maintenance. For some perspective on x-ray lithography’s origins, check out “X-ray lithography: Some history, current status and future prospects” by Juan R. Maldonado and Martin Peckerarat

Note: Upcoming weblogs will address IC lithography issues and why MEMS devices are essential in mobile devices and many other products.

Deposition Technology: Electron Beam Evaporation

Electron beam (e-beam) evaporation involves heating the material by an electron beam as opposed to thermal evaporation which uses resistive heating (discussed in previous Blog). A typical e-beam evaporation system is shown schematically in Figure 1. E-beam heated sources have two major benefits:

  • Very high power density and as a result, a wide range of control over evaporation rates from very low to very high
  • Evaporant is contained in a water cooled Cu hearth, thus eliminating the problem of crucible contamination

While it will not be discussed in any detail, the two major types of e-beam sources (guns) are thermionic and plasma types (details of these guns can be found in reference 1). Deposition rate in this process can be as low as 1 nm/min to as high as a few μm/min. Material utilization efficiency is high relative compared to other PVD methods and the process offers structural and morphological control of films (discussed shortly). Due to the very high deposition rate, this process has been used in industrial applications for wear resistant and thermal barrier coatings in aerospace industries, hard coatings for cutting and tool industries, and electronic and optical films for semiconductor industries and thin film solar applications.

Note that one of the major problems with these processes is the low energy of the evaporated atoms, which can cause problems in the thin films such as poor adhesion to the substrate, reduced density, porous and columnar microstructure, increased moisture pick up and poor mechanical properties. This will be addressed in the next Blog.

Deposition of single elements by evaporation is straight forward. Deposition of alloys with two or more components, however, can be a challenge due to different vapor pressures and different evaporation rates. The solution to this problem is to use multiple sources, one for each constituent of the alloy. Additionally, the material evaporated from each source can be a metal, alloy or compound. Challenges of this process are calibrating deposition rates for each source, ensuring that the adatom beam from each source uniformly coincides with the beams from the other sources and that deposited atoms are sufficiently blended and obtaining uniform density of each material over the substrate surface.

Difficulties of direct evaporation to form oxides, nitrides, fluorides, carbides, etc. are due to fragmentation of vaporized compounds. Reactive evaporation can overcome many of these problems. Here metals are evaporated in the presence of reactive gas. A schematic of a typical reactive evaporation system is also shown in Figure 1. The problem here is that most oxides are substoichiometric due to the low energy of the evaporated adatoms and that deposition rates can suffer. In most cases stoichiometric films are deposited only as low deposition rates. Reaction kinetics (i.e., low energy of evaporated atoms/molecules) prohibit full oxidation or full reaction with the reactive species, which limits the potential applications of these films as optical coatings in particular. To this end, activated reactive evaporation (ARE) was developed [1,2]. This process generally involves evaporation of a metal or alloy in the presence of a plasma of a reactive gas, hence the term “activated”. For example, TiC and TiN coatings are deposited by evaporation Ti in a C2H2 or N2 plasma respectively. The plasma has the following roles:

  • Enhance reactions that are necessary for deposition of compound films
  • Modify growth kinetics and, as a result, structure/morphology of deposits

Applications for evaporated thin film coatings include:

  • Multilayer optical coatings
  • Metallization
  • Large area metal mirror coatings for telescopes
  • Metallization for roll coating
  • Decorative coatings
  • Hard coatings
  • Tribological coatings
  • Thin film solar cells
  • Diffusion barriers
  • Thermal barrier coatings
  • Organic thin films

Evaporation was one of the first processes used extensively for the deposition of thin films. This process enabled the use of thin films in a wide variety of applications, which lowered manufacturing costs and expanded functionality of bulk materials. Large area coverage was now possible. Evaporation and related processes are still used extensively to synthesize a wide variety of thin film coating materials.

A full range of evaporated optical coatings is being produced, including AR on glass and plastic, mirror coatings, beam splitters, filters, transparent conductive coatings, dichroic coatings, attenuation coatings and fiber optic coatings [3]. Coating of large areas, as well as high speed web coating, are possible with e-beam systems. Figure 2 shows the evaporated 8m Al Gemini telescope mirror coating. These have come a long way from the evaporated quartz/MgF2/cryolite/Al coatings in 1936 [4].


  1. S Ismat et al., Chapter 4, Handbook of Deposition Technologies for Films and Coatings, Science, Applications and Technology, 3rd Ed., Peter M Martin Ed., Elsevier (2010).
  2. R F Buhshah & C Deshpandey, in Physics of Thin Films, J L Vossen and M H Francombe (Eds.), Academic Press (1987).
  3. See
    J Strong, J Opt Soc Am 26 (1936) 73.

Deposition Technology: Thermal Evaporation

This article sponsored by Maynards


Deposition technology is the backbone of thin film technology. It is based essentially on the history of improved adatom energetics, higher deposition rates, thickness uniformity, large area coverage, and of course, cost. This technology can be tracked from thermal evaporation and electrochemical deposition in the 1930’s (starting as early as the 1870’s [1]) to advanced processes such as atomic layer deposition (ALD) and high power impulse magnetron sputtering (HIPMS). Due to the low energy of evaporated atoms, quality, environmental stability and durability of thin films was initially a major problem. Additionally, since these are nonequilibrium processes, virtually and material composition is possible. Each process has made significant contributions to possible compositions, mechanical properties, electrical properties, optical properties, durability, environmental stability, manufacturability, and advanced applications. And evaporation technology is no exception. Additionally, a wide range of complex multilayer structures, substrate sizes and shapes, complex compositions, functionality, decorative coatings, microstructures and nanostructures is now possible.

Thermal evaporation is a physical vapor deposition (PVD) process that goes back as far as the 1850’s and was the first technology to facilitate numerous thin film applications [1,2]. The first evaporated thin films were probably prepared by Faraday in 1857 when he exploded metal wires in a vacuum [3]. Deposition of thin metal films in vacuum by thermal heating was initiated in 1887 by Nahrwold and employed by Kundt in 1888 to measure refractive indices of these films [4,5]. As early as the 1920’s it was initially used to metallize auto parts, metallize mirrors (including large telescope mirrors), deposit antireflection coatings, and is still used today for a number of advanced applications [3]. Production of fully dense coatings and self-supported shapes by high rate PVD processes started around 1961.

PVD processes, including thermal and electron beam evaporation involve atom-by-atom transfer of material from a source to a substrate. The three general steps in the formation of a deposit can be summarized as:

  • Synthesis of the material to be deposited:
    • Transition from a condensed phased (solid or liquid) to a vapor phase
    • For compounds, a reaction between the components of the compound, some of which may be introduced into the chamber as a gas or vapor
  • Transport of the vapors between source and substrate
  • Condensation of vapors (and gases) followed by film nucleation and growth

In evaporation and other PVD processes, steps 1 and 3 can be independently controlled, giving the process a high degree of flexibility in controlling structure, properties and deposition rate.

Thermal evaporation (TE) involves resistive heating a solid material inside a high vacuum chamber, taking it to a temperature which vaporizes the material, while electron beam (e beam) evaporation uses an electron beam to heat the source (next Blog). In TE, metal material (in the form of wire, pellets, shot) is fed onto heated semimetal (ceramic) evaporation boats. A pool of melted metal forms in the boat cavity and evaporates into a cloud above the source. Alternatively the source material is placed in a crucible which is radiatively (radio frequency-RF) heated by a resistive filament or the source material may be hung from the filament itself (filament evaporation). MBE is an advanced form of thermal evaporation. Figure 1 shows a schematic of a thermal evaporation system and Figure 2 shows a thermal evaporation source.

This process requires a good vacuum (generally < 10-4 torr), however, even a relatively low vapor pressure is sufficient to raise a vapor cloud inside the chamber. This evaporated material now constitutes a vapor stream, which traverses the chamber and condenses on the substrate, forming a coating or film.

In most processes, source material is heated to its melting point and is liquid, and is usually located in the bottom of the chamber, often in some sort of upright crucible. The metal vapor then rises above this bottom source, and substrates are held inverted in appropriate fixtures at the top of the chamber. Multiple sources are used to deposit alloys, and care must be taken since each material evaporates at a different rate. Although many elements evaporate, a number of materials such as Cr, Cd, Mg, As and C sublime and elements such as Sb, Se and Ti are borderline between evaporation and sublimation [2]. Sn, Al, Ga and Pb have low vapor pressures, just above their melting point while materials such as Cr have high vapor pressures. Although most elements vaporize as atoms, Sb, Sn, C and Se can vaporize in clusters of atoms [2].

Deposition rates for evaporated materials are generally very high, which minimizes incorporation of impurities into the film and can result in a very fine grain microstructure. Microstructure, durability and environmental stability of thin films to a large extent depends on the energy of the atoms arriving at the substrate. While deposition rates can be very high, thermally evaporated atoms arrive at the substrate with an energy of ~ tenths of eV. Adatom energetics are thus not impressive, which can result in a porous microstructure and poor film adhesion. As a result, high substrate temperatures or ion assist are often required to achieve fully dense and adherent films. Ion assist will be addressed in a future Blog.


  1. Historical Timeline of Vacuum Coating and Vacuum/Plasma Technology, Society of Vacuum Coaters: of vacuum coating/History-of-Vacuum-Coating.cfm*.
  2. Donald Mattox, Handbook of Physical Vapor Deposition, Noyes (1998).
  3. S Ismat et al., Chapter 4, Handbook of Deposition Technologies for Films and Coatings, Science, Applications and Technology, 3rd Ed., Peter M Martin Ed., Elsevier (2010).
  4. F Soddy, Proc R Soc Lond 78 (1967) 429.
  5. I Langmeir, J Am Chem Soc 35 (1913) 931.

Thin Film Deposition Technologies: Introduction to Thin Films and Processes

Thin films are deposited by a wide variety of processes. However, we begin this blog first with the advantages of using thin films and deposition processes applications. Thin film is the general term used for coatings that are used to modify and increase the functionality of a bulk surface or substrate. Typical thicknesses range from 50 nm to 10 μm. They are used to protect surfaces from wear, improve lubricity, improve corrosion and chemical resistance, modify optical and electrical properties and provide a barrier to gas penetration. In many cases thin films do not affect the bulk properties of the substrate material. They can, however, totally change the optical, electrical transport, and thermal properties of a surface or substrate, in addition to providing an enhanced degree of surface protection. Thin film deposition technology and the science have progressed rapidly in the direction of engineered thin film coatings and surface engineering [1]. Plasmas are used more extensively. Accordingly, advanced thin film deposition processes have been developed and new technologies have been adapted to conventional deposition processes. The market and applications for thin film coatings have also increased astronomically, particularly in the biomedical, display and energy fields.

Thin films have distinct advantages over bulk materials. Because most processes used to deposit thin films are nonequilibrium in nature, the composition of thin films is not constrained by metallurgical phase diagrams. Crystalline phase composition can also be varied to certain extent by deposition conditions and plasma enhancement. Virtually every property of the thin film depends on and can be modified by the deposition process and not all processes produce materials with the same properties. Microstructure, surface morphology, tribological, electrical, and optical properties of the thin film are all controlled by the deposition process. A single material can be used in several different applications and technologies, and the optimum properties for each application may depend on the deposition process used. Since not all deposit technologies yield the same properties or microstructures, the deposition process must be chosen to fit the required properties and application. For example, diamond-like carbon (DLC) films are used to reduce the coefficient of friction of a surface and improve wear resistance, but they are also used in infrared optical and electronic devices. Titanium dioxide (TiO2) is probably the most important and widely used thin film optical material and is also used in photocatalytic devices and self-cleaning windows, and may have important applications in hydrogen production. Zinc oxide (ZnO) has an excellent piezoelectric properties but is also used as a transparent conductive coating and spintronics applications. Silicon nitride (Si3N4) is a widely used hard optical material but also has excellent piezoelectric response. Aluminum oxide (Al2O3) is a widely used optical material and is also used in gas barriers and tribology applications. The list goes on…..

Thin films thus offer enormous potential due to the following:

  • Creation of entirely new and revolutionary products
  • Solution of previously unsolved engineering problems
  • Improved functionality of existing products; engineering, medical and decorative
  • Production of nano-structured coatings and nanocomposites
  • Conservation of scarce materials
  • Ecological considerations – reduction of effluent output and power consumption

Engineered materials are the future of thin film technology. Engineered structures such as superlattices, nanolaminates, nanotubes, nanocomposites, smart materials, photonic bandgap materials, molecularly doped polymeres and structured materials all have the capacity to expand and increase the functionality of thin films and coatings used in a variety of applications and provide new applications. New advanced deposition processes and hybrid processes are being used and developed to deposit advanced thin film materials and structures not possible with conventional techniques a decade ago. For example, until recently it was important to deposit fully dense films for all applications, but now films with engineered porosity are finding a wide range of new applications. Hybrid processes, combining unbalanced magnetron sputtering and filtered cathodic arc deposition for example, are achieving thin film materials with record hardness.

Organic materials are also playing a much more important role in many types of coating structures and applications, including organic electronics and organic light emitting devices (OLED). These materials have several advantages compared to inorganic materials, including low cost, high deposition rates, large area coverage, and unique physical and optical properties. It is also possible to molecularly dope and form nanocomposites with organic materials. Hybrid organic/inorganic deposition processes increase their versatility, and applications that combine organic and inorganic films are increasing.

In addition to traditional metalizing and glass coating, large area deposition, decorative coating and vacuum web coating have become important industrial processes. Vacuum web coating processes employ a number of deposition technologies and hybrid processes, most recently vacuum polymer deposition (VPD) and have new exciting applications in thin film photovoltaics, flexible displays, large area detectors, electrochromic windows, and energy efficiency.

Thus we see that each thin film deposition process can be used for a range of applications and that some are more conducive than others with respect to certain applications and materials. The following processes with be reviewed:

  • Thermal and electron beam deposition
  • Magnetron sputtering and its cousins
  • Ion assisted deposition
  • Pulsed laser deposition
  • Chemical vapor deposition and its cousins
  • Atomic layer deposition
  • Ion plating
  • Cathodic arc deposition
  • Vacuum polymer deposition
  • Vacuum web coating

This series of Blogs will describe deposition processes with respect to the following criteria:

  • Adatom energetics varies significantly with deposition process and must be taken into account. Some processes require additional components to increase energy.
  • Substrate temperature is critical.
  • Thickness uniformity of the film on the substrate
  • Microstructure
  • Materials usage
  • Deposition rate.
  • Process scale up
  • Materials, Some processes are work better with certain substrate materials that others



  1. Peter M Martin, Introduction to Surface Engineering and Functionally Engineered Materials, Wiley/Scrivener (2011).

How vacuum affects quality and microstructure of thin films

Density and microstructure of thin films depends on a number of factors, but primarily on the energy of the species (adatoms) incident on the substrate. Microstructure of the thin film includes the crystalline structure, morphology, density, defects and inclusions, voids and grain structure. Microstructure can strongly influence physical properties such as strength, toughness, ductility, hardness, corrosion resistance, high/low temperature behavior or wear resistance.  Electrical properties also depend on microstructure. As we described in an earlier Blog, adatom energies range from a few tenths of an eV for thermal evaporation to tenths to hundreds of eV for sputtering. Adhesion of the thin film to the substrate also critically depends on energy of the species. Columns and articles on how vacuum affects thin film performance, as well as vacuum equipment, can be found in virtually every issue of VT&C.

Because they are nonequilibrium processes, each atomistic deposition process has the potential to deposit materials that vary significantly from the source material in composition, microstructure, mechanical properties, tribological properties, and physical properties (conductivity, optical properties, etc.), depending on deposition conditions. The resulting films may have high intrinsic stress, high concentration of point defects, extremely fine grain size, highly oriented microstructures, metastable phases, incorporated impurities, and micro to macro porosity. These properties have significant influence on the physical, corrosion resistance, and mechanical and tribological properties of the deposited film [1].

Microstructure can be varied over wide ranges depending on deposition process and conditions.  To fully understand thin film microstructure it will be instructive to first elucidate film growth processes: how the film evolves during growth and the major factors that affect nucleation, growth, and microstructure. Defects are also an integral aspect that must be understood since they can severely degrade film performance. Microstructure of a thin film evolves as it nucleates and grows on the substrate surface. How a thin film grows is not a trivial process, and depends on a number of factors, including deposition process, substrate surface quality, temperature, energy of incident particles, angle of incidence, etc. These factors are all interconnected, and as a result, it is hard to segregate the dependence on vacuum. Additionally, film growth mechanisms can be complicated by the fact that not only is the species from the source deposited, but additional processes such as resputtering, reflected neutrals, shadowing and ion implantation can occur.

The focus here will be on role of energy of the adatoms and impurities resulting from the vacuum during deposition (note however that vacuum is only one of many factors that determine film quality). As discussed earlier, atoms and molecules ejected from the source collide with and scatter off gas atoms (inert and reactive) in the region between source and substrate. How the film grows on the substrate depends critically on the energy is has when it reaches the substrate and how this energy is utilized on the surface of the substrate. We see for sputtering that inert gas (Ar, Kr, etc.) pressure influences film structure through indirect mechanisms such as thermalization of deposited atoms with increased pressure and increased component of atom flux due to gas scattering [2-7]. Reduction of gas pressure results in more energetic particle bombardment which densifies the film, while the opposite is true for increased pressure. The structure zone model (SZM) attempts to quantify this relationship, and differs for each deposition process. Basic SZMs used to predict film microstructure depend on chamber pressure, substrate temperature, deposition rate and substrate placement [3].

While beyond the scope of this Blog, SZMs have been revised and refined over the last two decades, and incorporate ion bombardment, grain structure, film thickness effects, substrate roughness, angle of incidence, and external substrate bias. Ion bombardment effects are important both in sputtering and evaporation. However, ion bombardment is an integral part of sputtering (DC and RF) processes but must be introduced externally into evaporation processes. Here the floating potential replaces chamber pressure. This makes sense since particle energy can be related to both pressure and substrate bias (expect more energetic bombardment at low pressures and higher bias:           VS ~ 1/P). Zone T is widened compared to zone 1, which can be interpreted as increased ion bombardment enhancing adatom mobility similar to higher substrate temperatures.

How does film microstructure affect mechanical properties of thin films?

  • It is generally desirable for the film to have high density, low porosity, and tightly bound grain structure
  • Columnar structure is a strong function of deposition temperature
  • A highly columnar or porous film may not be as hard or corrosion resistant as a dense, small grained and defect free film
  • Stable optical properties are observed for dense, small grained, or amorphous films
  • Structural defects tend to increase with increased thickness
  • Columnar structure is required for a number of applications, such as piezoelectric and ferroelectric and magnetic thin films
  • Compressive stress generally increases microhardness and microfracture toughness
  • Interfacial phases and associated microporosity and contaminants all degrade coating adhesion

Unless there is a highly specialized application that requires porous films, special film microstructure or films with large columnar structure, highest quality films are dense, tightly packed and smooth. Thus we conclude that virtually all properties of PVD films depend on vacuum (pressure) at some level.



  1. Peter M Martin, Introduction to Surface Engineering and Functionally Engineered Materials, Wiley/Scrivener (2011).
  2. J Greene, in Multicomponent and Multilayered Films for Advanced Microtechnologies: Techniques, Fundamentals and Devices, O Auciello and J Engemann, eds., Kluwer (1993).
  3. John A. Thornton, Ann. Rev. Mater. Sci, 7 (1977) 239.
  4. R. Messier et al., J. Vac. Sci. Technol., A2(2) (1984) 500.
  5. Russell Messier, J. Vac. Sci. Technol., A4(3) (1986) 490.
  6. B. A. Movchan and A. V. Demchishin, Phys. Met. Metallogr., 28 (1969) 83.
  7. J. V. Sanders, in Chemisorption and Reactions on Metal Films, J R Anderson, ed., Academic Press (1971).

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.


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

Why do we need high vacuum for deposition of thin films and what does vacuum do?

Thin films are deposited by a number of processes, including sputtering (DC, AC, diode, magnetron), thermal and electron beam evaporation, molecular bean epitaxy (MBE), chemical vapor deposition (CVD), pulsed laser deposition (PLD) and atomic layer deposition (ALD).  Although the mechanism for deposition for these processes differ significantly, they all have one thing in common:  they require a high vacuum environment.  Vacuum deposition is a family of processes used to deposit layers of material atom-by-atom or molecule-by-molecule on a solid surface. These processes operate at pressures well below atmospheric pressure (i.e., vacuum). The deposited layers can range from a thickness of one atom up to millimeters, forming freestanding structures. Multiple layers of different materials can be used, for example to form optical coatings. The process can be qualified based on the vapor source; physical vapor deposition uses a liquid or solid source and chemical vapor deposition uses a chemical vapor.


Here V = volume of vessel or vacuum chamber, n = number of particles (atoms, molecules), R = universal gas constant and T = temperature.  Thus, we see that pressure decreases when the number of particles in the chamber decreases.  Why is this important?  All deposition processes essentially move an atom or molecule from a source to a substrate and condense atoms on a substrate.  These processes will not work if the pressure is above a certain value, i.e., there are too many gas atoms or molecules in the chamber which degrade the transport of atoms from the deposition source.   As will be addressed in the future, the quality and microstructure of the thin film depends critically on a low pressure.

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.

Sputtering processes employ a plasma to bombard and eject atoms from the source.  Atoms are ejected with a specific energy.  Collisions of atoms ejected by the deposition source with gas atoms in the chamber reduce the energy of the ejected atoms, which results in poor film adhesion to the substrate and with high porosity.  Atmospheric gas atoms (oxygen, nitrogen, hydrogen, etc.) present in the deposition chamber can contaminate the thin film and even change its composition if not eliminated to very low levels.  Deposition processes such as sputtering employ a plasma to eject particles from the source.  A plasma can only form at low pressures (~ mTorr).   In the reactive deposition process a reactive gas (oxygen, nitrogen, hydrogen, fluorine, hydrocarbon, etc.) is introduced into the deposition chamber to react with the ejected source particles to form compounds such as oxides, nitrides, fluorides, carbides, etc.  These reactive gases must be held at a pressure low enough to form a plasma and conform to the above criteria.  High vacuum in the deposition chamber also provides the means to introduce and control gases located externally at a higher pressure.

Can any atmospheric process (i.e., non-vacuum) achieve high quality thin films?  The answer is generally “no”.  The only possible exception is electroplating, where metal ions dissolved from a source are transported in an ionic solution from the source to the substrate.  Atmospheric plasmas are used for a variety of applications, except for deposition of thin films.  Atoms or molecules cannot be ejected from a source or transported from a source to a substrate by any other method.

(Note: The above section contains corrected equations as of 5/5/17)