Observations & Opportunities: Nanoscale Challenges As Chipmakers Produce ICs with 10nm & 7nm Features

Remember the sub-micron-featured ICs in PCs just a few decades ago? That was doable. Some chipmakers did complain a bit about technical challenges as they retooled for smaller-featured optical lithography requirements (180nm, 90 nm, 65 nm, 45 nm, 32 nm, etc.) and more demanding processing but they all knew they could make it work, profitably. With 10nm or smaller, however, profitability is and manufacturing are more challenging technology leaps. This Blog explores some emerging IC and MEMS manufacturing advances and challenges.

The Bleeding Edge
The world’s most demanding and sophisticated high-volume thin-film deposition occurs during the wafer fabrication of nano-scaled ICs and some MEMS devices. MEMS (Micro-Electro-Mechanical Systems) typically are a few generations behind ICs smaller critical dimensions but the MEMS devices are increasingly used in mission-critical applications which demands precision and repeatability. MEMS must provide that requisite physical interface to the world on micro and nano scales. Since MEMS incorporate electrical and/or optical I/Os and/or other electronic circuitry, this obviously impacts design and manufacturing efforts.

Remember the “Hype Cycle for Emerging Technologies, 2017” from Gartner Inc.[1]. Here it is again below. Consider that virtually none of these anticipated high-growth market opportunities will be possible without more computing power in ever smaller packages and that increasingly includes AI (Artificial Intelligence) to enable previously unattainable results. To get this performance, chipmakers and computer makers need faster processors and more memory, the latter preferably being fast and rugged. One given: vacuum-enabled thin-film processing for deposition, plasma etching and cleaning demands will be tougher than ever.

The hype cycle for emerging technologies. Source: Gartner Inc., Stamford, CT

 

We’ve all heard about the options being evaluated when silicon ICs can no longer do the job. Nanosheets and carbon nanotube transistors are possibilities but this is an industry that usually resists change in its wafer fabrication processes. That does not mean they won’t do it if necessary. There is also the requisite support infrastructure needed to maintain and tweak process machines for ≤10nm production.

Parallel Production
It will take years to transition from mainstream IC manufacturing to EUV lithography and even longer if non-silicon devices are used. But ≤10 nm devices will not lessen the need for currently maxed out existing optical lithography wafer fabs that will likely keep producing “standard” ICs and MEMS for decades since those devices will be needed for mainstream applications. Recently, more 3D deices with many layers help cram more into chips.

The emerging ≤ 10nm generation is beyond most chipmaker’s comfort zones. We’ll explore some of the challenges these semiconductor manufacturers face as early production of chips with feature sizes at 10nm and 7nm begins, perhaps using EUV lithography, if it is ready, or pushing forward with older but proven familiar lithography technology awaiting EUV reality with high wafer-per-hour throughputs.

Now a nomenclature caution: chipmakers are being creative with marketing terminology when it comes to exact “nodes” that they are actually using now, what is planned and with 10nm or 7nm claims are not being truly comparable. “Node” being a somewhat vague way of describing “industry standard” references to the geometries being fabricated. The bottom line: feature sizes are shrinking one way or another so producing more transistors per mm2 is a given.

Mark Bohr is an Intel Senior Fellow and director of process architecture and integration at Intel Corporation. During the recent Technology and Manufacturing Day presentations, pointed out Intel’s innovation enabled technology pipeline carries through to 7nm, 5nm and 3nm feature sizes.

Intel’s innovation enabled technology pipeline.

 

Clearing Up the Node Naming Mess
Bohr posits, “Let’s clear up the node naming mess. Moore’s Law, as stated by our co-founder over half a century ago, refers to a doubling of transistors on a chip with each process generation. Historically, the industry has been following this law, and has named each successive process node approximately 0.7 times smaller than the previous one – a linear scaling that implies a doubling of density. Thus, there was 90 nm, 65 nm, 45 nm, 32 nm – each enabling the packing of twice the number of transistors in a given area than was possible with the previous node.

“But recently – perhaps because of the increasing difficulty of further scaling – some companies have abandoned this rule, yet continue to advance node names, even in cases where there was minimal or no density increase. The result is that node names have become a poor indicator of where a process stands on the Moore’s Law curve.”

“Every chip maker, when referring to a process node, should disclose its logic transistor density in units of MTr/mm2 (millions of transistors per square millimeter) as measured by this simple formula. Reverse engineering firms can readily verify the data.

Intel Transistor Density Formula

 

“There is one important measure missing: SRAM cell size. Given the wide variety of SRAM-to-logic ratios in different chips, it is best to report SRAM cell size separately, next to the NAND+SFF density metric.

“By adopting these metrics, the industry can clear up the node naming confusion and focus on driving Moore’s Law forward,” added Bohr.

Intel’s 10nm
Before the Consumer Electronics Show (CES 2017) began, Intel Corp. kicked off the event with announcements in automated driving, 5G technologies and virtual reality — or the more advanced merged reality.

“The pace of technology improvement is accelerating faster than ever,” said Intel CEO Brian Krzanich. “Moore’s Law is at the center of this acceleration. Technology is extending far beyond consumer electronics, defining almost every aspect of our lives, and transforming industries.”

Krzanich described the company’s vision for VR and merged reality technology and how these technologies will reinvent the experiences of travel, work safety and productivity, and sports and gaming. He explained that Intel’s first 10nm-powered 2 in 1 PC with Intel’s next generation processor, codenamed Cannon Lake, made it possible. Intel’s Cannon Lake 10nm technology is said to have the world’s tightest transistor and metal pitches, created with hyper scaling, for the highest density in the industry.

Intel’s Cannon Lake 10nm technology is said to have the world’s tightest transistor and metal pitches

 

During Intel’s Technology and Manufacturing Day last year, Kaizad Mistry, Corporate Vice President, Technology and Manufacturing and Co-Director, Logic Development, said that Intel’s 10nm technology is a full generation ahead of other “10nm” processes. Mistry added, “Hyper scaling extracts the full value of multi-patterning schemes and and allows Intel to continue the benefits of Moore’s Law economics.”

2018—The Year of 10nm Critical Dimensions and EUV Lithography
From several published reports over the past year, 2018 is the year of ≤10nm with EUV (Extreme UltraViolet) but we’ve heard that before.

There are just a handful of chipmakers now who can afford the new 10nm, 7nm and smaller lithography equipment. Taiwan Semiconductor Manufacturing Company (TSMC) is challenging Samsung Electronics in the race for ≤7nm process volume production in 2018. TSMC has many orders for fabricating chips with 7nm processing for mobile communications, high-performance computing and AI (artificial intelligence) applications. Apple and Qualcomm are among TSMC’s major customers, with the foundry house contracted to fabricate all the advanced Apple A12 processor chips for the 2018 next-generation iPhones. TSMC will use extreme ultraviolet (EUV) technology for its 7nm+ process and step up EUV deployments in the 5nm and 3nm processes. TSMC will outpace Samsung in 7nm volume production in 2018.

TSMC is building a 5nm production fab (Fab 18) in the Southern Taiwan Science Park this year. It will also invest US$20 billion on a 3nm fab in the park, with construction starting in 2020. Their 5nm process will be an extension of its 7nm process, targeting mobile communication, high-performance computing, AI, and machine learning (ML) applications.

Samsung is building a 7nm production line in Hwaseong, Korea. The company is negotiating with U.S. and Chinese customers over new projects. Samsung spun off its wafer foundry as an independent business unit in 2017. It will launch 4nm processing in 2020 against TSMC’s 5nm node, before starting volume production of 7nm process in 2018 and developing 6nm and 5nm processes in 2019, according to sources.

Samsung says it will start manufacturing chips with circuitry widths of 7nm by using EUV tech in the second half of 2018. TSMC also said hat its chip manufacturing process using EUV technology will be the “most advanced technology in foundry industry” in 2018 in terms of density, performance and power.

Meanwhile, GLOBALFOUNDRIES’ new 7LP 7nm FinFET process technology targets high-performance, power-efficient System-on-Chips (SoCs) for demanding, high-volume applications. This provides world-class performance, power, area and cost advantages through 7nm scaling. Based on a 3D FinFET transistor architecture imaged with optical lithography but it is EUV compatible where needed. 7LP delivers more than twice the logic and SRAM density, and either >40% performance boost or >60% total power reduction, compared to 14nm foundry FinFET offerings.

GLOBALFOUNDRIES 7LP 7nm FinFET process technology platform produces high-performance, power-efficient SoCs in demanding, high-volume applications

 

With 100 million transistors per square millimeter density, or more, being normal now, the world’s most demanding and sophisticated high-volume thin-film deposition requirements during wafer fabrication of ICs and MEMS devices only get more demanding. Wafer fabrication toolmakers must continue to refine their machines, perhaps even including AI at some point to handle new data requirements and process variations needed by customers. With the high stakes for all involved, every process must be exceedingly accurate and repeatable.

Reference:
1. Terrence Thompson, Vacuum Technology & Coating, “Faster Changes and the Implications” blog, August 2017, posted on Aug. 23, 2017, Observations & Opportunities.
2. Steve Hansen, Vacuum Technology & Coating, Vol. 18, No. 2, Feb. 2017, “Miniature Analyzers and IonTraps From Compact to MEMS”, pp. 12-16.
3. Donald M. Mattox, Vacuum Technology & Coating, Vol. 5, No. 12, Dec. 2004, SVC Educational Guide, “Applications: Low-Pressure CVD and PECVD: Plasma-Enhanced Chemical Vapor Deposition (PECVD)”, pp. 28-30.

Note: The many vacuum-centric deposition, coating, etching and cleaning processes that Vacuum Technology & Coating has covered recently, as well as those going back almost 15 years, are worth exploring. These articles serve as invaluable basic primers while others describe leading-edge applications. These are free articles, and magazine issues, to peruse and educate. Visit http://www.vtcmag.com and explore the magazine’s online viewing or downloadable PDF issues. Use the Search button to find specific topics.

Next time: We’ll take a look at some new MEMS sensors and why they are essential.

Magnetron Sputtering: Basics

The last Blog introduced magnetron sputtering and the importance of low pressure operation for improved energy of ejected species. Magnets and their configuration with respect to the target determine many of the operating parameters and conditions. The strength of the magnetics affects the voltage at which the cathode operates. Operating voltage decreases with increased magnetic field strength. The operating voltage also decreases with erosion through the target because the surface of the target gets closer to the magnets (increasing magnetic field strength). For this reason there is a limit to target thickness that can practically be used. Thicker nonmagnetic targets can be used with stronger magnetics. Ferromagnetic materials and ferrites present a problem with magnetrons and tend to shunt the magnetic field and reduce magnetic field at the surface. Hence very high target voltages result if the target can be lit at all. Unless the magnetics are exceptionally strong, sputtering targets of magnetic materials must be much thinner than nonmagnetic materials (typically < 0.16 cm).

Both direct current (DC) and radio frequency (RF) power sources can be used with most magnetron cathodes. Pulsed DC and low and mid frequency power supplies are also used in magnetron sputtering. New concepts such as DC with RF overlay are also being developed for a number of coating applications (TCO’s for example).

Magnetron sputtering is particularly well suited for reactive deposition. Virtually any oxide, nitride and carbide thin film material can be deposited. Fluorides present a problem due to reflected neutrals off the target. The operating reactive gas partial pressure depends on deposition rate (target power), target material, chamber pressure and substrate target spacing. Two techniques are used to tune the reactive gas partial pressure, a hysteresis curve that plots target voltage against percentage of reactive gas and target voltage against reactive gas partial pressure [1,2]. Figure 1 shows a typical hysteresis plot for an oxide coating. Target voltage remains constant or increases slightly with increased oxygen, and then plummets at a transition concentration. The surface of the target is oxidized for percentages higher than the transition voltage and in the “metal” mode below the transition. Metallic and substoichiometric coatings result on the left side of the transition. If the percentage oxygen is too high, the target will become “poisoned” and an insulating crust will form on its surface. Deposition rate will decrease significantly and arcing may occur, introducing particulates into the growing film. The resulting coating may be less dense and or poor quality. The operating point is just on the cusp of the transition. The coating is fully stoichiometric and the deposition rate is optimum. Note that if the oxygen is decreased, there will be a hysteresis effect due to cleaning up of the surface oxide. The other technique actually determines the partial pressure of reactive gas. As shown in Figure 2. Reactive gas pressure is plotted against reactive gas flow. The operating point occurs just before the partial pressure increases with flow. The operating point is the flow at which the reactive gas is no longer “reacting” with the target material. There is also a hysteresis effect for this technique.

Whatever method one uses to determine the operating reactive gas pressure, it should be monitored as the target ages. Generally less reactive gas will be needed with increased erosion of the target. It should be noted that magnetron power supplies are available that incorporate reactive gas flow control.

Figure 1. Hysteresis plot of target voltage versus percentage of oxygen.
Figure 2. Plot of reactive gas partial pressure against gas flow. The operating point is the flow at which the reactive gas is no longer “reacting” with the target material [2].

Magnetron sputtering is also particularly well suited for deposition of multilayer thin film structures, including optical designs, electrochromic coatings, nanolaminates, superlattices, tribological coatings and barrier coatings. High quality thin film optical materials have been deposited since the 1970’s. A wide range of optical thin film materials has been developed. The list includes most oxides, nitrides, carbides, transparent conductive materials, semiconductors and many polymers. Fluorides are best left to evaporation processes and hybrid processes. The main advantage is that these materials can be deposited using both nonreactive and reactive processes with excellent control of composition, layer thickness, thickness uniformity, and mechanical properties.

Additionally, deposition of multilayer structures, including combinations of the materials listed above, is straight forward using multiple cathodes and reactive gases. A small sampling of the optical materials deposited by planar magnetron sputtering is listed in Table 1. Deposition of alloys, oxinitrides, alloy oxinitrides, carbon nitrides is straightforward. Use of multiple cathodes and reactive gases also makes deposition of multilayer structures possible. It is possible to deposit virtually any type of optical design, from the simplest antireflection coating to complex beam splitters and multiple cavity filters. Also, since sputtering is a nonequilibrium process, alloys and compounds not found in binary and ternary phase diagrams can be deposited.

Optical and quartz crystal monitoring are used to deposit the exact thickness; however, the process is often so reproducible that many layers can be deposited by simply timing them.

Magnetron sputtering is the most widely used deposition process to achieve high quality thin films and coatings. Modern cathodes offer very good target utilization and high deposition rates. This type of sputtering is used extensively in large area deposition. Large area single and multilayer coatings are deposited by magnetron sputtering using large cathodes and arrays of large cathodes. Magnetron cathodes are also used in in-line and roll-to-roll processes (addressed in a future Blog). Figure 3 shows an array of magnetron cathodes used for deposition of large area optical coatings and Figure 4 shows an array of solar concentrator panels with magnetron sputtered high reflection coatings.

Figure 3. Array of large magnetron cathodes used for large area optical coatings.
Figure 4. Array of solar concentrator panels with magnetron sputtered high reflection coatings.

Advanced techniques such as HPPMS, HPIMS, and dual magnetron sputtering are breaking new ground in thin film deposition and will be addressed in a future Blog.

Reference:

  1. Handbook of Deposition Technologies for Films and Coatings, P M Martin Ed., Wiley/Scrivener (2009).
  2. J A Thornton, Ann. Rev. Mater. Sci, 7 (1977) 239.
  3. W D Sproul and B E Sylvia, SVC 45th Annual Technical Conference Proceedings (2002) 11
  4. D A Glocker et al., SVC 47th Annual Technical Conference Proceedings (2004) 183.
  5. W D Sproul, SVC 35th Annual Technical Conference Proceedings (1992) 236.

Sputter Deposition of Thin Films: Introduction

Sputtering, in its simplest form, is the ejection of atoms by the bombardment of a solid or liquid target by energetic particles, mostly ions. It results from collisions between the incident energetic particles, and/or resultant recoil atoms, with surface atoms. One of the major advantages of this process is that sputter-ejected atoms have kinetic energies significantly larger than evaporated materials. The growing film is subjected to a number of energetic species from the plasma. Figure 1 shows a comparison of the energetics of thermal evaporation and sputtering processes for Cu [1]. Figure 2 shows the general sputtering process. In this process, atoms or molecules of a solid material (a target or sputtering source) are ejected into a gas form or plasma due to bombardment of the material by energetic gas ions and deposited on a substrate above or to the side of the target. A vacuum is required to initiate a plasma whose ions bombard the target. The sputtering process is essentially a momentum exchange, shown in Figure 3 [2], between the gas ions; the more intense and concentrated the plasma in the region of the target, the higher the atom removal rate (or deposition rate). The number of atoms ejected per incident ion is called the sputtering yield, and is dependent on the energy of the incident ion. A measure of the removal rate of surface atoms is the sputter yield Y, defined as the ratio between the number of sputter ejected atoms and the number of incident projectiles. The mass of the ion is important compared to the mass of the atoms in the target. Momentum transfer is more efficient if the masses are similar. Inert gases are used to generate the plasma. The most common (and cheapest) sputtering gas is argon (Ar), followed by krypton (Kr), xenon (Xe), neon (Ne), and nitrogen (N2). There will be negligible sputtering with light atomic weight gases such as hydrogen and helium. Reactive gases typically used are oxygen (O2), nitrogen, fluorine (F), hydrogen (H2), and hydrocarbons (methane, butane, etc.).

Figure 1. Comparison of thermal energy distributions for Cu evaporated at 1300 K and energy distribution of sputtered Cu [1]

The major sputtering techniques are diode, planar magnetron, cylindrical magnetron, high power impulse magnetron, and ion beam sputtering. All these methods have a number of variations. Diode sputtering, shown in Figure 2, is the simplest configuration of this family. Both RF (poorly conductive targets) and DC (conductive targets) power is applied to the sputtering target. This type of sputtering is typically performed at higher chamber pressures than its cousins, 0.5 to 10 Pa (5 X 10-3 – 0.1 torr). Substrate heating is often required to obtain high quality adherent films. RF sputtering has the advantage of higher deposition rates and a wider range of materials that can be deposited. Both methods, however, can be used for reactive deposition. As with all sputtering processes, deposition rate depends on a number of factors, such as chamber pressure, power to the target and substrate target spacing.

Figure 2. Diagram of the sputtering process.

 

Figure 3. Diagram showing momentum exchange in the target during the sputtering process [2].

As the name implies, planar magnetron sputtering in its simplest form utilizes a flat sputtering target in a cathode enclosure. Magnetron targets can be as small as 1 inch or as large as several meters. Figure 4 shows the geometry of a basic planar magnetron cathode. Magnets (called magnetics) are placed under the target in various configurations to confine the plasma or spread the plasma above the region of the target. The magnetic lines of force focus the charged gas atoms (ions). The stronger the magnetic field, the more confined the plasma will be (consequences are discussed below). The magnetics focus the plasma at the surface of the target and an erosion pattern, commonly called a “racetrack”, is formed as sputtering proceeds. Magnet configuration is different for planar circular, planar rectangular, external and internal cylindrical magnetrons. Figure 5 shows an erosion pattern of a conventional planar magnetron cathode. This pattern is narrow and materials usage was poor, but second and third generation magnetrons use target material much more efficiently. Target utilization of cylindrical magnetrons can be as high as 80%. Other designs such as a rotating circular magnetrons and full face erosion magnetrons also have high target utilization.

Figure 4. Magnet geometry in a planar magnetron cathode.

 

Figure 5. Erosion racetrack in a magnetron sputtering target.

 

The advantages of magnetron sputtering are:

  • Wide range of materials can be deposited
  • DC and RF reactive processes possible
  • Lower chamber pressures
  • Higher deposition rates than diode sputtering processes
  • Dense, high quality thin films
  • Large areas can be covered
  • Good thickness uniformity
  • Substrate heating not required in many cases
  • Amenable to a wide range of substrates, including plastics
  • Deposition conditions easily controlled
  • Several cathode configurations possible

 

Disadvantages are:

  • Poor materials usage (this is improving with modern cathode designs)
  • Lower deposition rates than electron beam evaporation, ion beam sputtering, cathodic arc and CVD processes

 

The next Blog will address other magnetron designs and applications.

 

Reference:

  1. Handbook of Deposition Technologies for Films and Coatings, 3rd Ed., P M Martin Ed., Elsevier (2009).
  2. Handbook of Deposition Technologies for Films and Coatings, R. Bunshaw Ed., Noyes (1994).

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).

 

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).

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

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

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

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

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


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

 

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

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

Reference:

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

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)