Ion Assisted Deposition of Thin Films

Deposition of thin films by thermal and electron beam (e-beam) evaporation processes was discussed in the previous two Blogs. 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 water pick up and poor mechanical properties. Typically, the substrate is heated to several hundred degrees Celsius during coating to mitigate this effect, but it is by no means eliminated. Ion assisted deposition (IAD) and ion plating mitigate many of these problems by providing enhanced energy to the evaporated atoms and ion cleaning the substrate [1].

For optical coatings, problem with porous films is that they can subsequently absorb moisture, which changes the refractive index of the layer(s), and can cause shifts in the center wavelength with changes in ambient temperature and humidity.  Low density also limits mechanical durability to some extent, although these films can typically meet most of the MIL-SPEC durability and environmental requirements.  Furthermore, the requirement to heat the components during processing can limit substrate material choice, and also introduce stress in the substrate due to thermal cycling.

Figure 1 below shows the placement of an ion source in a typical deposition chamber. Bombardment prior to deposition is used to sputter clean the substrate surface. Bombardment during the initial deposition phase can modify the nucleation behavior of the depositing material such as nucleation density. During deposition the bombardment is used to modify and control the morphology and properties of the depositing film such as film stress and density. It is important, for best results, that the bombardment be continuous between the cleaning and the deposition portions of the process in order to maintain an atomically clean interface [1]. IAD adds a high energy ion beam that is directed at the part to be coated.  These ions impart energy to the deposited material, acting almost like an atomic sized hammer, resulting in a higher film density than achieved with purely evaporative methods.

The higher density of IAD coatings generally gives them more mechanical durability, greater environmental stability and lower scatter than conventional evaporated films.  Furthermore, the amount of energetic bombardment can be varied from zero to a maximum level on a layer by layer basis, giving the process tremendous flexibility.  For example, while IAD is not compatible with some of the commonly used materials in the infrared, it can be used solely on the outermost layer to yield an overall coating with superior durability.  The ion energy can also be used to modify the intrinsic stress of a film during deposition.  In some cases, this can change the film stress from tensile to compressive, which can help to maintain substrate surface figure, especially when depositing thick infrared coatings.

Several types of ion source are available, including Kaufman type, end Hall type, cold and hollow cathode types. Each source differs in how ions are created and accelerated to the substrate and ion energy distribution. All these sources ionize inert gases such as argon and krypton to bombard the substrate. Ions are extracted by several methods, grid extraction (Kaufman type) where ions are relatively monoenergetic or from a broad beam ion source (end Hall and hollow cathode) having a spectrum of ion energies. Figures 2 and 3 show cross sections of Kaufman and end Hall ion sources [1,2,3]. The Kaufman source is gridded to control energy of exiting ions. Ion current available from the ion source are determined by source parameters, such as gas pressure, cathode power, anode potential, geometry, etc. The accelerator grid serves two purposes: 1) to extract the ions from the discharge chamber, and 2) to determine the ions trajectories, i.e. focusing. Table 1 shows operating parameters for this ion source.

Table 1. Maximum argon ion beam current

Discharge voltages for end Hall ion sources typically range from 40 – 300V. Discharge currents for hollow cathode designs range from 30 μA – 5A with discharge voltages up to 16 kV [4]. This unit is also used for ion etching and ion sputtering.

A wide range of properties of evaporated (and other PVD processes as will) coatings improve with increased density and packing density resulting from ion bombardment. Effects of IAD are no more apparent than in moisture stability and stress in optical coatings [2]. Moisture can readily penetrate low density films deposited using low energy processes, causing a distinct shift in optical properties (refractive index, absorption), as shown in Figure 4. We see that moisture shift in the refractive index is significant for as deposited HfO2 films while negligible for IAD films.

Stress in evaporated thin films deposited without IAD is generally tensile. Compressive stress is desirable in thin films to enhance mechanical properties and reduce cracking, although all stress should be kept to a minimum. By removing loosely bound atoms IAD increases film density and can change stress from tensile to compressive. The capability to tune stress is particularly valuable in multilayer films. Thus it is possible to vary stress of alternating layers from tensile to compressive, thus achieving very low stress in the resulting structure.

However, with IAD there can be too much of good thing and defects can form during deposition. As a result of this process ion energy can be given up to the growing layer either at the surface of in the underlying regions [5]. Atom displacements responsible for lattice damage (voids, lattice modification) are produced by energy deposition in bulk regions of the film. Contrast this with surface smoothing described previously. Additionally, inert gases can also be driven into the film. An extension of this effect is ion implantation used extensively in semiconductor technology. Fortunately, the threshold energy needed for bulk defect formation is higher than the threshold for surface driven processes. Care must thus be taken not to use excessive ion energy or bulk defects will be created.

Ion sources are also used for ion beam sputter deposition of thin films, but that will be addressed in a future Blog.

Figure 1. Ion source placement in deposition chamber.

Figure 2. Kaufman ion source geometry [3]
Figure 3. End Hall ion source geometry [1]
Shift in refractive index for evaporated HfO2 films with and without IAD [2]
Reference:

  1. Donald M. Mattox, in Handbook Deposition Technologies for Films and Coatings, 3rd Ed., P M Martin Ed., Elsevier (2009).
  2. D. E. Morton, V. Fridman, 41st Annual Technical Conference Proceedings of the SVC, 297 (1998).
  3. South Bay Technology, Applications Laboratory Report 123.
  4. J Alessi et al., Brookhaven Laboratory Report 102494 -2013 CP.
  5. A R Gonzalex-Elipe, F Yubero & J M Sanz. Low Energy Ion Assisted Film Growth, Imperial College Press (2003).

 

AI & Virtual Reality—Let The Games (and Work) Begin!

Artificial Intelligence (AI) and Virtual Reality (VR) have enormous potential for entrepreneurs, designers, software experts, video gaming developers and high-tech manufacturers. Some common AI and VR essentials include very powerful computing power in extremely small/tiny spaces and exceptionally high definition displays—real or projected. There will be a requirement for many sensors and actuators too. If today’s marketplace prices are any indication, consumers, businesses and militaries will pay handsomely for the very best. But that presents several challenges including terminology.

AI terminology seems to be described somewhat uniformly but what a given company describes as AI can vary enormously. VR has several common descriptive variants since enhanced reality is more affordable to manufacture and more affordable for consumers. In recent announcements, Microsoft has announced Windows Mixed Reality headsets as part of a new thrust, “We are on a mission to help empower every person and organization on the planet to achieve more, and one of the ways we are doing that is through the power of mixed reality,” said Alex Kipman, Technical Fellow at Microsoft. This is a follow to their earlier HoloLens headsets. The technology behind it will allow its flagship operating system to use the latest generation of Windows 10 hardware devices and software for augmented and virtual reality technologies experiences.

Caption Windows 10 Mixed Reality headsets from partners Lenovo, Acer, Dell & HP

 

Google’s Alphabet’s X division has moved on to refining their technology with the Google Glass 2.0 headsets the result. Google is targeting business and manufacturing applications that will help boost productivity. The original Google Glass headsets had some appeal but some problems that were addressed for the version 2.0 headsets.

Caption: Google Glass 2.0 headset

 

Augmented Reality

“AR is a live direct or indirect view of a physical, real-world environment whose elements are “augmented” by computer-generated or extracted real-world sensory input such as sound, video, graphics or GPS data. It is related to a more general concept called computer-mediated reality, in which a view of reality is modified (possibly even diminished rather than augmented) by a computer. Augmented reality enhances one’s current perception of reality, whereas in contrast, virtual reality replaces the real world with a simulated one.[1][2] Augmentation techniques are typically performed in real time and in semantic context with environmental elements, such as overlaying supplemental information like scores over a live video feed of a sporting event,” according to Wikipedia. [https://en.wikipedia.org/wiki/Augmented_reality]

Apple is excited about Augmented Reality and will soon introduce such capabilities soon on iPhones and iPads. There are rumors that they may intro glasses too. What is for real there is Apple’s ARKit, a set of software developer tools for creating augmented reality apps or iOS.

According to Apple developers, “Apps can use Apple’s augmented reality (AR) technology, ARKit, to deliver immersive, engaging experiences that seamlessly blend realistic virtual objects with the real world. In AR apps, the device’s camera is used to present a live, onscreen view of the physical world. Three-dimensional virtual objects are superimposed over this view, creating the illusion that they actually exist. The user can reorient their device to explore the objects from different angles and, if appropriate for the experience, interact with them using gestures and movement.” With their track record on iOS Apps for iPhones and iPads, that may give them an edge.

Not to be overlooked, however, is the now famous and impressive Oculus Rift virtual reality headset, now a Facebook company. It’s still expensive but very competitive. “Content for the Rift is developed using the Oculus PC SDK, a free proprietary SDK available for Microsoft Windows (OSX and Linux support is planned for the future).[62] This is a feature complete SDK which handles for the developer the various aspects of making virtual reality content, such as the optical distortion and advanced rendering techniques”, according to Wikipedia.

With Augmented Reality (AR), one can immerse themselves in games, education, work, movies or whatever. The big question, however, is will this be just a short-term fad or will it be enduring like the personal computer or smartphone? We just don’t know yet but the question of industry-wide standards versus proprietary platforms will likely be an issue. We do know that familiar tech giants are betting on VR and AR for current and future products. What is certain is that thin-film technologies are essential for manufacturing the LCD, OLED or other screens used in the requisite headsets. If you thought that building 4K or 8K HDTVs was challenging, VR and ER in small form factors will make it interesting. The VR headset options abound.

A Crowded Market

“By 2016 there were at least 230 companies developing VR-related products. Facebook has 400 employees focused on VR development; Google, Apple, Amazon, Microsoft, Sony and Samsung all had dedicated AR and VR groups. Dynamic binaural audio was common to most headsets released that year. However, haptic interfaces were not well developed, and most hardware packages incorporated button-operated handsets for touch-based interactivity. Visually, displays were still of a low-enough resolution and frame-rate that images were still identifiable as virtual. On April 5, 2016, HTC shipped its first units of the HTC VIVE SteamVR headset. This marked the first major commercial release of sensor-based tracking, allowing for free movement of users within a defined space. “ per Wikipedia [https://en.wikipedia.org/wiki/Virtual_reality_headset].

Is AI Ready?

“We are at an inflection point in the development and application of AI technologies,” according to the Partnership on AI [https://www.partnershiponai.org/introduction/]. “The upswing in AI competencies, fueled by data, computation, and advances in algorithms for machine learning, perception, planning, and natural language, promise great value to people and society.

“However, with successes come new concerns and challenges based on the effects of those technologies on people’s lives. These concerns include the safety and trustworthiness of AI technologies, the fairness and transparency of systems, and the intentional as well as inadvertent influences of AI on people and society.

“On another front, while AI promises new capabilities and efficiencies, the advent of these new technologies has raised understandable questions about potential disruptions to the nature and distribution of jobs. While there is broad agreement that AI advances are poised to generate great wealth, it remains uncertain how that wealth will be shared broadly. We do, however, also believe that there will be great opportunities to harness AI methods to solve important societal challenges.

“We designed the Partnership on AI, in part, so that we can invest more attention and effort on harnessing AI to contribute to solutions for some of humanity’s most challenging problems, including making advances in health and wellbeing, transportation, education, and the sciences.”

Some founding members of this Partnership on AI include Apple Inc., Amazon, DeepMind, Facebook, Google (Android, Chrome), IBM (Watson computer), Intel Corp., Microsoft, Sony (PlayStation) and the The Association for the Advancement of Artificial Intelligence (AAAI).

What Is Needed

In future blogs, we’ll revisit some specific AI and VR applications of interest and challenges.

Observations & Opportunities

Faster Changes and the Implications

We are indeed in the midst of rapidly changing times. Some of today’s global issues and challenges are the result of using various technologies without thinking the impact through. Regardless, science and technology will be essential for solving these problems.

Something to keep in mind while contemplating tackling anything new with science and technology. It is prudent to consider the impact of any new product or manufacturing process itself, all of the equipment and materials involved, and what happens throughout the world as advanced technologies permeate virtually everything in our business or personal lives.

What’s Really New

We all have some readily available topics in mind when “new” technologies are mentioned but we probably overlook several areas of great potential. Below is the thought-provoking Hype Cycle for Emerging Technologies, 2017 chart from Gartner Inc. Every topic on the chart is significant and there are overlapping interactions between these new technologies that are likely unpredictable. Someone invariably uses a product or technology for something totally overlooked by the original technology inventors and proponents—and unintended consequences result.

Hype Cycle for Emerging Technologies, 2017. Source: Gartner July 2017

Gartner, Inc., of Stamford, Connecticut, is a global research and advisory company. It helps business leaders in every industry and enterprise with some objective insights needed to make the right decisions with analyses and predictions that can prove insightful.

As Gartner states in its “Gartner Identifies Three Megatrends That Will Drive Digital Business Into the Next Decade” [http://www.gartner.com/newsroom/id/3784363], “The emerging technologies on the Gartner Inc. Hype Cycle for Emerging Technologies, 2017 reveal three distinct megatrends that will enable businesses to survive and thrive in the digital economy over the next five to 10 years. According to Mike J. Walker, research director at Gartner, “Artificial intelligence (AI) everywhere, transparently immersive experiences and digital platforms are the trends that will provide unrivaled intelligence, create profoundly new experiences and offer platforms that allow organizations to connect with new business ecosystems.”

The Hype Cycle for Emerging Technologies report is the longest-running annual Gartner Hype Cycle, providing a cross-industry perspective on the technologies and trends that business strategists, chief innovation officers, R&D leaders, entrepreneurs, global market developers and emerging-technology teams should consider in developing emerging-technology portfolios.

“The Emerging Technologies Hype Cycle is unique among most Gartner Hype Cycles because it garners insights from more than 2,000 technologies into a succinct set of compelling emerging technologies and trends. This Hype Cycle specifically focuses on the set of technologies that is showing promise in delivering a high degree of competitive advantage over the next five to 10 years,” added Walker.

Their Hype Cycle graphic, above, touches upon many exciting technologies including IoT (Internet of Things), autonomous vehicles, nanotube electronics and many other key categories. The obvious thread that ties all of these things together is ubiquitous computing and communications access plus advanced software. Gartner’s take on when these technologies reach mainstream adoption seems reasonable today but unforeseen events, or just some major company’s very secret product launch plans, could significantly alter those predictions.

Just recall Apple Inc.’s launch of the iPhone a decade ago. We already had our cell phones but Apple’s elegant design plus marketing spin on what consumers wanted, or could be convinced that they needed and could use, proved remarkable. The iPhone changed mobile information access and use in profound ways as well as computing in general.

Today’s plethora of smart phones, tablets and laptops are all influenced by the iPhone to varying degrees. With numerous companies like Apple and Google pursuing Augmented Reality (AR) and/or Virtual Reality (VR) to make information access and usage better while improving work or entertainment experiences, these technologies can have both predicted and unintended consequences too.

The Internet is essential now for most businesses and consumers which has driven its enormous growth. However, Internet security concerns are something that requires ongoing vigilance to avoid viruses, malware and scams to protect its viability. It also requires products with built-in protection and the ability to upgrade protection as new threats emerge.

AI or VR features are already appearing in more consumer products as well as the Internet of Things (IoT) for consumers and businesses as well. VR and ER plus more powerful personal computers, smartphones and tablets are certainly changing how many work and play. Not everyone will need and/or use all of these things but the impact will be felt by everyone. If you want to check the weather, you probably get more useful information quicker on your smartphone or tablet than on your PC.

A pixel comparison chart of SD (standard definition), Full HD (1080p), 4K Ultra HD and 8K Ultra HD displays. Image by By Libron – Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=25976260

Whether it is your laptop, desktop, smartphone, tablet, 4K or soon 8K TV, automobile dashboard, thermostat or any other product with an electronic display, it is obvious that display technology continues to advance too. Many were content with HDTV (1080p) until the 4K TVs appeared and the better pictures combined with a marketing push made 4K Ultra HD TV a must-have product for many people. Soon the 8K push will begin as soon as enough cameras are available for content production, movies at first and TV later. Of course, Blu-ray and other disc player manufacturers will have to add upconversion for the 8K sets. Since high-speed Internet is still not available everywhere, discs will prove useful for a while.

AI technologies will be the most disruptive class of technologies over the next 10 years due to radically increased computational power, near-endless amounts of data, and unprecedented advances in deep neural networks; these will enable organizations and governments with AI technologies to harness data in order to adapt to new situations and solve problems that no one has ever encountered previously.

Walker believes that enterprises seeking leverage in this theme should consider the following technologies: Deep Learning, Deep Reinforcement Learning, Artificial General Intelligence, Autonomous Vehicles, Cognitive Computing, Commercial UAVs (Drones), Conversational User Interfaces, Enterprise Taxonomy and Ontology Management, Machine Learning, Smart Dust, Smart Robots and Smart Workspace. This will require some serious homework to grasp the implications and the best ways to use it. These new tools may raise concerns about George Orwell’s novel Nineteen Eighty-Four becoming real by some observers. Of course the novel was just fiction.

Note: Upcoming weblogs will address why MEMS devices are essential in mobile devices and many other products and why vacuum technology is essential for their manufacturing.

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 http://www.pitt.edu/~budny/papers/23.pdf 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 https://spie.org/Documents/Membership/BacusNewsletters/BACUS-Newsletter-July-2017.pdf

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 [http://www.semi.org/en/node/50856]. 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. [https://www.researchgate.net/publication/299496830_X-ray_Lithography_Some_History_Current_Status_and_Future_Prospects]. 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 https://www.researchgate.net/publication/299496830_X-ray_Lithography_Some_History_Current_Status_and_Future_Prospects.

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].

Reference:

  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 http://www.evaporatedcoatings.com/index.php?WTX=gaw.
    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.

Reference:

  1. Historical Timeline of Vacuum Coating and Vacuum/Plasma Technology, Society of Vacuum Coaters: svc.org/history 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

 

Reference:

  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.

 

Reference:

  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).

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.