Nanomanufacturing: High-Throughput, Cost-Effective Deposition of

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Nanomanufacturing: high-throughput, cost-effective deposition of atomic scale thin films via atmospheric pressure spatial atomic layer deposition Kevin P Musselman, Chukwuka F. Uzoma, and Michael S. Miller Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03077 • Publication Date (Web): 11 Nov 2016 Downloaded from http://pubs.acs.org on November 14, 2016

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Chemistry of Materials

Nanomanufacturing: high-throughput, cost-effective deposition of atomic scale thin films via atmospheric pressure spatial atomic layer deposition Kevin P. Musselman*a, Chukwuka F. Uzomaa, Michael S. Millerb a

Waterloo Institute for Nanotechnology, Dept. of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Ave West, Waterloo ON, N2L 3G1, Canada b Angstrom Engineering Inc., 91 Trillium Dr., Kitchener ON, N2E 1W8, Canada KEYWORDS: Atmospheric pressure spatial atomic layer deposition, nanomanufacturing, thin films, high throughput, coating, printing ABSTRACT: The demand for materials and devices with dimensions on the nanometer scale continues to increase. To meet this demand, high-throughput, cost-effective methods for depositing nanoscale thin films are needed. In the last few years, atmospheric pressure spatial atomic layer deposition (AP-SALD) has emerged as a potential nanomanufacturing method that is scalable, open air, and operates at modest temperatures that are compatible with flexible substrates. In this perspective, we compare AP-SALD to other high-throughput techniques for depositing nanometer scale thin films, including gravure printing, screen printing, knife-overedge coating, slot-die coating, inkjet printing, spray deposition, as well as high-throughput sputtering and evaporation. While APSALD does not provide the same patterning capabilities as some of these printing techniques, it offers multiple advantages: it produces continuous, conformal coatings with few defects; it requires minimal thermal treatment of the deposited materials; it provides atomic scale thickness control; it facilitates tuning of material properties; and no vacuum chamber is required, which simplifies maintenance requirements and minimizes the operating cost. Areas for further development are identified, which will allow these advantages to be leveraged: new precursors need to be developed to enable deposition of a wider variety of materials; precursor recycling should be examined; and AP-SALD systems that are high-throughput (roll-to-roll coating speeds of tens or hundreds of meters per minute) and low-maintenance need to be further developed and tested.

INTRODUCTION Continued miniaturization has lead to the use of functional nanomaterials in many devices and applications, and it is expected that nanoscale materials will ultimately be integrated into all parts of our daily lives to make us happier, healthier, and safer. Nanoscale energy harvesting systems, such as photovoltaic solar cells and piezoelectric generators, will collect energy from our bodies and environment; nanoscale wireless technologies will transmit this power to integrated circuits and allow them to communicate; and connected sensors and flexible displays that we interact with will be fabricated using nanomaterials. While continued advancements in vacuum science and lithographic techniques have enabled the development of nanoscale devices, the cost associated with these techniques is prohibitive. As discussed by Liddle et al. recently, the economic viability of a commercial nanotechnology device depends on the product of its value (price) and production capacity (throughput).1 Traditional vacuum and batch processes for producing nanomaterials are characterized by limited throughput, which hinders the production capacity. Notably, the desire to integrate smart devices into our clothing, packaging, built environment, and even ourselves, as part of the burgeoning “internet of things”, has placed greater focus on highthroughput production of nanomaterials, even for more com-

plex devices that previously leveraged a high value/price to ensure profitability. If this future of flexible electronics, renewable energy, and networks of smart sensors is to become a reality, high-throughput nanomanufacturing methods that are cost-effective need to be developed. In addition to high throughput, open air and low temperatures are other characteristics associated with “cost-effective” techniques. For flexible electronics, for example, traditional rigid substrates are replaced with low-cost plastics, and the maximum temperature that a polymer substrate can typically withstand ranges from 115 to 320 °C, with most below 270 °C.2 With these criteria in mind, it is clear that many laboratoryscale techniques (vacuum-based deposition, spin-coating, etc.) are not ideal for cost-effective, high-throughput nanomanufacturing. Recently, spatial atomic layer deposition (SALD) has emerged as a scalable alternative for producing nanoscale thin films.3 In conventional atomic layer deposition (ALD), a substrate is exposed sequentially to precursor gases, which contain the different elements to be deposited. Where conventional ALD separates precursors in time via alternating exposure and purging cycles, often in a vacuum chamber, SALD separates the precursors in space and moves the substrate between the different precursors, which are being supplied constantly, to reproduce the sequential exposure steps. This has been

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achieved using systems based on circulating flexible substrates, rotating substrate cylinders, and linear “zoneseparated” gaseous precursor regions,3,4 as illustrated schematically in Figure 1. The substrate passes through a region with “Precursor A”, adsorbing a surface layer of the precursor, then passes through an inert gas before passing through a region with “Precursor B”, which reacts with the layer of “Precursor A” to form a monolayer of the desired material. Different SALD approaches have been detailed in recent review articles.3,5

Figure 1 Schematic illustrations of spatial atomic layer deposition techniques based on (a) circulating flexible substrates, (b) rotating substrate cylinders, and (c) linear “zone-separated” precursor regions. All approaches spatially isolate precursor gases A and B from one another.

Atmospheric pressure spatial atomic layer deposition (APSALD) is an open-air variant of the SALD approach where a

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substrate is passed in close-proximity to a manifold (typical separation less than 100 µm) with alternating parallel flows of precursor gases, as shown in Figure 2a.5,6 Typically an inert carrier gas (e.g., N2) is bubbled through liquid precursors and vapors of the precursors and the inert gas are separately fed to the gas manifold. In the SALD approaches illustrated in Figure 1, large physical distances, as well as differential pressures and pumping, ensure separation of the precursors. With APSALD, the close-proximity of the substrate to the gas streams ensures separation of the precursors, allowing them to be placed more closely together. The precursor flows are separated by flows of inert gas/purge streams (“I” in Figure 2b), which act as effective diffusion barriers in the close-proximity arrangement when the inlet and exhaust flows are balanced appropriately. This keeps the precursors isolated, even for precursor zone spacings of a few millimeters. The substrate can be passed closely underneath the flow of precursor “A”, adsorbing a surface layer of the precursor (Figure 2b). It then passes underneath the flow of inert gas “I”, flushing away excess precursor “A” before passing under the flow of precursor “B”, which reacts with the layer of “A” to form a monolayer of the desired material (Figure 2c). Any by-products of the reaction are removed by a subsequent flow of inert gas. The substrate can be moved back and forth under these alternating flows to deposit pinhole-free films with atomic scale thickness control. Alternatively, the manifold can be moved above a stationary substrate or the precursor and inert flows can act as gas bearings, allowing substrates to be floated over the surface at a suitable distance. Importantly, as no purging of a vacuum chamber is required (as in conventional ALD), APSALD deposition can be two orders of magnitude faster than conventional ALD.5 The close spacing of the substrate and precursor manifold also effectively seals off the reaction zones, making the process independent of the surrounding environment and allowing operation in atmospheric conditions. Due to the open-air nature of the technique, it has sometimes been referred to as “atmospheric ALD”. Its ability to quickly deposit thin films over large areas in air at modest temperatures (typically 100 to 350 °C) makes AP-SALD very promising for roll-to-roll production of next-generation, costeffective electronic devices and flexible electronics. Different AP-SALD systems have been developed, including those based on linear manifolds,3,7-9 such as the one illustrated schematically in Figure 2, as well as systems based on rotating substrates.3,10 The first applications of nanomaterials produced by SALD included zinc oxide and aluminum oxide films for transistors and ring oscillators,7,11,12 as well as passivation layers for solar cells.10 Over the past several years, the Device Materials Group at the University of Cambridge, of which Dr. Musselman was a member, has introduced AP-SALD films into nextgeneration optoelectronics devices. Linear close-proximity AP-SALD systems (illustrated schematically in Figure 2), which were based on systems developed by the Eastman Kodak Company, were used. The work focused on the deposition of a variety of binary oxide films, including ZnO, TiO2, Cu2O, Al2O3, and Nb2O5 using established water-based thermal processes. TiO2 and ZnO films were employed as charge-selective layers in organic solar cells.8,13 The compact, pinhole-free APSALD films resulted in performances equivalent to devices with films produced using higher-temperature methods. APSALD Cu2O films were employed in Cu2O/ZnO solar cells as a back-surface field layer.14


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Chemistry of Materials AP-SALD has also demonstrated the ability to carefully tune the properties of the deposited materials. Novel AP-SALD films will undoubtedly continue to be developed at laboratory scale, both to validate the potential use of these films in cost-effective next-generation devices, and as a materials chemistry tool to study the underlying device operating mechanisms. However, the route to widespread commercial use of AP-SALD remains unclear. It is promoted as a scalable, open-air nanoscale coating technique, but how does it compare to other techniques with similar strengths, and what scientific and technological advancements are needed?

Figure 2 (a) AP-SALD manifold with alternating flows of precursor gases. (b) The substrate first moves underneath precursor A, absorbing a monolayer of the precursor, and then through the purging, inert (I) gas flow. (c) The substrate then moves underneath precursor B, which reacts with A to form a monolayer of the desired material.

Alloys can be produced by AP-SALD, simply by introducing multiple precursors. Illiberi et al., for example, produced AlxZn1-xZnO and InxGayZnzO alloys.15,16 As a member of the Optoelectronics Group and Device Materials Group at the University of Cambridge, Dr. Musselman and his colleagues leveraged this capability to control the properties of AP-SALD films in a variety of next-generation solar cells and lightemitting diodes (LEDs) and probe the underlying operating mechanisms. A water-ammonia mixture was used as the oxidizing precursor to dope ZnO with nitrogen and vary its carrier concentration by several orders of magnitude. This nitrogendoped ZnO was used to identify prominent interfacial recombination channels in colloidal quantum dot solar cells17 and to clarify the factors influencing exciton dissociation at polymermetal oxide interfaces in hybrid solar cells.18 Magnesium was introduced into ZnO to control its bandgap, and this alloy was used to demonstrate and mitigate losses in colloidal quantum dot solar cells,19 Cu2O/Zn1-xMgxO solar cells,20-21 organometal halide perovskite LEDs,22 and hybrid blue polymer LEDs.23 Low turn-on voltages, higher luminance, and sharp, color-pure electroluminescence were obtained in new organometal halide perovskite LEDs, for example, by replacing the poly(9,9dioctylfluorene) electron injector with Zn1-xMgxO.22 The ability to carefully control film thickness via AP-SALD proved particularly useful. It was shown that optimization of the Zn1xMgxO thickness could negate the detrimental impact of a Schottky barrier at Zn1-xMgxO/ITO interfaces.21 More recently, TiO2-polymer hybrid solar cells were produced and thin (approximately 10 nm) AP-SALD Nb2O5 interlayers were introduced between the TiO2 and polymer to produce an energy cascade structure.24 Thus, in addition to being an open-air, low-temperature, and potentially high-throughput technique,

DISCUSSION 1. Comparison to conventional open-air deposition methods We start by discussing AP-SALD in the context of established, open-air, nanoscale deposition techniques. To emphasize techniques relevant to nanoscale devices, we focus our discussion on methods that have demonstrated the ability to coat or print films with thicknesses less than 100 nm. We note that inherent to “printing” is the ability to transfer a complex pattern, whereas coating does not infer this. Both classes of technique are relevant to nanomanufacturing and are discussed here. Established techniques include gravure printing, screen printing, knife-over-edge coating, slot-die coating, inkjet printing, and spray deposition, which are illustrated schematically in Figure 3. While this set of techniques is not exhaustive, it represents some of the most common nanomanufacturing methods that have been developed to-date for the fabrication of large-area flexible electronics, and next-generation polymer solar cells, transistors, sensors, and LEDs. Gravure printing makes use of a gravure cylinder with engraved micro cells, which are filled with a functional ink, either from a reservoir beneath the rotating cylinder or from a top nozzle dispenser, as shown in Figure 3a. A doctor blade is used to remove extra ink and the ink is directly transferred to the substrate (e.g., a roll-to-roll plastic, which is often referred to as a “web”). Alternatively, the engraved micro cells can be located on a planar plate, with a cylindrical substrate configuration.25 A pattern can be transferred directly from the engraved micro cells, or the substrate surface and ink can be designed so that the droplets flow together to form a continuous film before the solvent evaporates. In screen printing, ink is pressed through a porous screen (typically made of fabric or stainless steel) onto a substrate using a squeegee. In rotary screen printing (Figure 3b), which is compatible with roll-toroll production, the ink is held inside the rotating cylindrical screen with a fixed internal squeegee. Importantly, the screen can be designed to replicate a desired pattern. In knife-overedge coating (Figure 3c), an ink bath is placed in front of a sharp blade (knife) at a fixed distance above the surface of the web. As the substrate moves underneath the knife, ink is removed, leaving a thin wet film with thickness equal to roughly half the knife-substrate gap width.26 Slot-die coating is shown schematically in Figure 3d. A slot-die system contains an ink distribution chamber, which delivers the ink to a feed slot and subsequently to the web. The wet film thickness is controlled by the flow rate, coating width, and web speed. Inkjet printing makes use of thermal, piezoelectric, or electrohydrodynamic actuation to eject materials in the form of droplets of colloidal or chemical solutions from a micrometer-sized inkjet nozzle aperture.2 Figure 3e shows a schematic of a piezoelectric inkjet nozzle. Finally, spray deposition involves forcing a col-



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loidal or chemical solution through a nozzle to create a fine plume of droplets, as shown in Figure 3f. Different mechanisms, including a carrier gas, ultrasonic vibration, and electrostatic charging have been used to assist with forming the droplets and directing them to the surface to be coated.27-29 Solvent can evaporate in transit, resulting in shrinking of the droplets to a very small (i.e. macromolecular) scale. Some of the key capabilities of these deposition techniques, along with those of AP-SALD are summarized in Table 1. Some commonalities are observed. All of the techniques are roll-to-roll compatible, at least in principle. Previous work with AP-SALD has predominantly been performed using rigid glass substrates, although depositions on polymer substrates were also demonstrated,9,53 and it is feasible that this could be extended to kilometer-long webs. Most of the techniques do not require a controlled environment for operation. AP-SALD is typically performed in a fume-hood or ventilated cabinet to prevent exposure of the operator to the precursor gases employed, however with proper design of exhaust systems within the gas manifold, it could be possible to relax this need for a controlled environment. In some aspects, AP-SALD is disadvantageous compared to the other techniques listed in Table 1. Like knife-over-edge coating (and spray coating to a large extent), it lacks the ability to print a pattern. In contrast, patterns (e.g. lines) can be deposited by gravure printing and screen printing with resolutions approaching 20-30 µm. In slot-die coating, an internal mask (shim) defines the feed slot width, allowing stripes to be printed in addition to complete coatings, albeit with inferior resolution. The key advantage of inkjet printing relative to the other techniques summarized in Table 1 is its ability to directly pattern lines or single droplets on various substrates. This can eliminate the need for lithography and etching, making inkjet printing attractive for cost-effective manufacturing of devices. Inkjet printing is also characterized by a good print resolution. The most feasible way to produce a pattern using AP-SALD is by pre-patterning the substrate, and indeed this was demonstrated by Levy et al. who used poly(methyl methacrylate) as a growth inhibitor to selectively deposit ZnO by AP-SALD on certain areas.7 Thus while novel AP-SALD systems may be developed to allow direct patterning of deposited materials, it seems more likely in the short term that APSALD will be combined with established printing techniques to produce requisite patterns. The range of materials that can be deposited by AP-SALD is also limited. The more established techniques listed in Table 1 have been used to deposit a wide variety of materials, including metals, dielectrics, organic and inorganic semiconductors, graphene, carbon nanotube layers, and electrolytes for devices such as solar cells, transistors, LEDs, and sensors, as summarized in Table 1. Previous work on AP-SALD has focused on metal oxides, so the study of metal and polymer depositions by AP-SALD is required, such as recent work on plasma-enhanced AP-SALD of silver.54 It is important to point out that most of the techniques in Table 1 make use of solution-based precursors (e.g. conductive nanoparticles or polymers in a suitable solvent) such that the development of ink formulations for large-area substrates is somewhat complementary amongst these techniques. AP-SALD differs from these techniques in its use of gaseous precursors. Gaseous precursors are essential to allow single layers of the precursor molecules to react, as shown in Figure 2c. This atomic layer-

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Figure 3 Schematic illustrations of some established open-air nanoscale deposition techniques.


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Table 1. Comparison of open-air nanoscale deposition techniques Property

Gravure

Screen

Knife-overedge

Slot-die

Inkjet

Spray

AP-SALD

Roll-to-roll compatible

Yes2

Yes30

Yes26

Yes2

Yes26

Yes26

Yes

Controlled environment

Yes2

No2

No31

No2

No2

No26

Ventilation required

Print resolution

20 µm2,32

30 µm2

-

200 µm2

15 to µm2,33

Millimeters26

-

20

< 1 µma,34 Material wastage

Yes2

Yes2

No26

No26,35

No2

Some26

Yes

Maximum deposition speed

1000 m/min26,35

100 m/min2,26

100 m/min26,35

1000 m/min26,35

10 m/min26

100 m/min26

Unknown

Reported nanoscale thickness

15-20 nm2,36

40 nm37

20 nm31

28 nm38

70 nm39

10 nm28,29,40

< 5 nm

Usage examples

Transistors,25,32,41 solar cells,42 flexible electronics,36 ring oscillators, LEDs, and sensors2

Solar cells,37,43 OLEDs,44 transistors45

Solar cells31

Solar cells,38 transistors,46 OLEDs47

Transistors, LEDs, solar cells, memory devices, and sensors33,39,48,49

Solar cells,27,28 transistors,50 gas sensors,51,52

Solar cells,8,10,1314,,17-21,24

transistors,7,11,12,15 LEDs22,23

a

With electrohydrodynamic jetting

by-layer growth would be impractical to achieve via the deposition of monolayers of precursors from solutions. It should be possible to leverage the development of precursors for conventional ALD and chemical vapor deposition. However, the requirements for AP-SALD precursors do differ from those for conventional ALD. In particular the precursors should have suitably high vapor pressures to create a vapor in the absence of vacuum (e.g. via bubbling of the liquid precursor). Not surprisingly, the most commonly used AP-SALD precursors todate are conventional ALD and CVD precursors characterized by high vapor pressures (e.g. 12.2 mm Hg for diethyl zinc and 9 mm Hg for trimethylaluminium at room temperature). For lower vapor pressures, vaporization of the precursor can be assisted with heating or nebulization. For high-throughput nanomanufacturing, volatile precursors that rapidly coat substrates and react at low temperatures (compatible with polymer substrates) will be essential. As such, AP-SALD precursor development and testing are other key areas requiring attention. Present-day AP-SALD systems also typically waste precursor materials in the form of unreacted exhaust gases. They are not alone in this regard, as gravure printing, screen printing, and spray coating also result in material wastage, as noted in Table 1. Compared to conventional ALD systems, however, AP-SALD systems do have the advantage that material is not deposited on vacuum chamber walls. If the precursor gases

and their unreacted exhaust are reliably isolated, it could be possible to recycle the precursors to eliminate wastage. Arguably the greatest uncertainty related to AP-SALD in the context discussed here, is the maximum speed at which AP-SALD can be performed. Commercial SALD systems for depositing Al2O3 photovoltaic passivation layers have been developed by SoLayTec and Levitech, and throughputs on the order of 4000 wafers per hour (approximately 10 m/min) were reported.3 In the previous AP-SALD work in Cambridge cited earlier, substrate speeds of 3 m/min were commonly used and the actual throughput was lower, as the substrates needed to be oscillated back and forth through the limited number of precursor channels to produce films of suitable thickness. Ali et al. used a similar manifold design to deposit AP-SALD films on a roll-to-roll polymer.9 They used a web speed of 0.42 m/min, and multiple cycles were again required to deposit a film of sufficient thickness. In contrast, roll-to-roll gravure printing has the highest throughput compared to other mass printing techniques, with printing speeds as high as 1000 m/min or 60 m2s-1 reported.25 Screen printing, knife-over-edge coating, slot-die coating, and spray deposition have also been reported to operate at similarly high speeds, as summarized in Table 1. High-throughput AP-SALD manifolds that contain many precursor channels, as illustrated in Figure 4, can be developed to avoid the need to oscillate the substrate back and forth, enabling continuous web motion in one direction. For example, a linear roll-to-roll AP-SALD system that provided 5



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ALD cycles was developed by Yersak et al. and was used to deposit Al2O3 on flexible substrates at web speeds of 30 to 120 m/min.55 In another approach, researchers at TNO in the Netherlands developed a roll-to-roll close-proximity system based on a rotating drum that supplies the precursor gases. The web is pulled over the drum (providing exposure to the precursors) and the drum is rotated in the opposite direction of the web to increase the number of cycles encountered by the web.3 These web speeds demonstrated for AP-SALD thus far are slow compared to the maximum speeds indicated for most of the techniques in Table 1. However, it is noted that for some of the nanoscale films reported in Table 1, the deposition was performed at a much slower speed than the maximum value indicated in the table (e.g. the 28 nm slot-die coated ZnO films were deposited at 2 m/min not 1000 m/min). Nonetheless further understanding of the AP-SALD chemisorption processes for a wider variety of precursors and development of new APSALD manifolds that enable higher speeds will likely be needed if AP-SALD is to become a ubiquitous nanomanufacturing method. Fortunately, work done by companies such as Lotus Applied Technology, ASTRaL, and Cambridge Nanotech to scale SALD systems for roll-to-roll coating will provide useful guidance for similar scaling of AP-SALD systems.

Figure 4 A possible high-throughput AP-SALD manifold design that provides 20 ALD cycles.

AP-SALD also offers a number of advantages compared to the other open-air nanoscale deposition techniques listed in Table 1. Defect related challenges (e.g., pinholes) associated with contact printing techniques (e.g., gravure) are largely avoided with AP-SALD. The surface-self-limited nature of ALD not only results in compact, continuous films, but should also enable the conformal coating of rough or porous surfaces. Nanoporous anodic aluminum oxide films have been coated using chamber-based SALD, although non-uniform coatings were obtained for the highest aspect ratio features (small nanopore diameters or short exposure times).56 AP-SALD has been used to deposit conformal coatings on macro-scale porous polypropylene fabrics57 and recently on nanorod arrays.58 This is in sharp contrast to the other methods listed in Table 1, which are ‘top-down’ methods limited to the deposition of planar films. A notable exception is electrospray deposition, where it has been shown that the charged macromolecular droplets can adhere preferentially at uncovered areas of the grounded substrate, avoiding pinholes and enabling some degree of conformal coating of non-planar substrates.29 But the degree of conformal coating is expected to be smaller for electrospray deposition than AP-SALD and is limited to the coating of conductive substrates. The need to control the substratemanifold spacing in AP-SALD does however place a limitation on the types of surfaces that can be coated. Particularly, non-flat substrates present a challenge for close-proximity APSALD, although this is the case for many of the deposition techniques listed in Table 1.

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While many of the depositions in Table 1 take place at room temperature, the wet films produced typically require thermal treatment, e.g. for inkjet printing of conductive inks, a 30 minute sintering step and/or high temperature exposure (>250°C) is typically required to remove the solvent and make the printed material conductive.59 These are significant obstacles to high-throughput roll-to-roll fabrication. As Perelaer et al. noted, “a sintering time of 30 minutes and a web speed of 1 m/s mean that the production line is required to be at least 1.8 km in length”,59 and as noted earlier, these high sintering temperatures are incompatible with common polymer substrates. However, room temperature techniques have also been reported. For ink-jet printed conductive inks, for example, the solvent can be removed by gradual room temperature evaporation or by chemical treatments.33 The temperature requirements for AP-SALD depend heavily on the particular material being deposited and precursors used. Substrate temperatures of 300 °C, 300 °C, and 225 °C, for example, were required to deposit TiO2, Nb2O5, and Cu2O films respectively.24,53 This has implications for substrate compatibility, however the required thermal treatment is at least typically limited to during the film deposition. Much lower deposition temperatures have also been demonstrated with AP-SALD, such as the coating of ZnO and Zn1-xMgxO on organometal halide perovskites at only 60 °C for LEDs and the deposition of Al2O3 at 150 °C for hybrid solar cell studies.22,24 Finally, a key distinguishing feature of AP-SALD is the ability to deposit atomic scale thin films. While film thicknesses ranging from 10 nm to 70 nm are reported in Table 1 for the other techniques, these typically represent the achievable thickness limit. Screen printing, for example, was used to deposit a uniform 40 nm active layer for an organic solar cell,37 but it is more typically used to deposit layers with wet thicknesses of tens or hundreds of micrometers and thin and homogeneous layers are challenging to obtain.30,33 Knife-overedge coating can be used to produce very smooth films,26 however nanoscale coatings have often been relatively rough. For example, ZnO films produced for polymer solar cells by knife-over-edge coating of ZnO nanoparticle ink on indiumtin-oxide-coated plastic had uneven thicknesses of 20 +/- 10 nm when dry.31 The control of layer thickness is typically problematic with inkjet printing,25 although in some instances the printed films have been very uniform. Graddage et al., for example, made use of the “coffee ring effect” to produce 70 nm poly(vinylpyrrolidinone) films with a root-mean-square roughness less than 1 nm.39 Spray deposition has been employed to deposit very thin, ‘quasi-monomolecular’ polymer layers and carbon nanotube layers, however, challenges associated with controlling droplet size and behavior can make it difficult to prepare films with a smooth surface.26 The degree of thickness control that is achievable with AP-SALD is unequalled by existing open-air, high-throughput techniques. Growth rates of less than 0.5 nm per cycle are standard and can be accurately controlled. Notably, unlike the ‘top-down’ deposition approaches summarized in Table 1, the surfaceself-limited ‘bottom-up’ nature of AP-SALD can provide excellent thickness uniformity. In depositions of Zn1-xMgxO films by AP-SALD, for example, a thickness variation of only 3% was observed over an area of several square centimeters.60 This thickness uniformity is similar or better than that reported for industrial-scale, vacuum-based, roll-to-roll systems.61,62 Careful control of the flows, however, is required to avoid


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mixing of the precursors in the gas phase and produce a consistent film thickness. A closely related technique is atmospheric pressure chemical vapour deposition (AP-CVD). In contrast to AP-SALD where the precursors are spatially isolated, reacting only on the substrate surface, chemical vapour deposition mixes the precursors in the gas phase (often in a tube furnace), which typically results in higher deposition rates. In fact, AP-SALD systems are sometimes operated in a CVD mode, where the precursor and exhaust flows are adjusted to allow some mixing of precursors in the gas phase and produce higher deposition rates.60 AP-CVD has been employed to deposit a variety of materials including metal oxides,63-65 sulphides,66 and twodimensional materials such as graphene and MoS2,67-69 even in monolayer form. Most studies have utilized glass and metal substrates, with typical deposition temperatures greater than 400 °C,70 although depositions can take place at lower temperatures. Polymer coatings, for example, have been deposited on flat substrates and porous textiles in ambient conditions using plasma-enhanced AP-CVD.71 The atmospheric nature of APCVD facilitates roll-to-roll processing, although most work in this respect has focused on metal foils due to the higher temperatures used. However, similarities with AP-SALD mean that the established deposition techniques summarized in Table 1 can also acts as benchmarks for AP-CVD, and AP-CVD faces some of the same scalability challenges discussed here for AP-SALD. There are other techniques, particularly solution-based ones such as Langmuir-Blodgett film deposition and convective assembly, that are capable of depositing atomic scale films in open-air.72,73 These techniques rely on the assembly of building blocks (e.g. nanoparticles, molecules) into twodimensional films and efforts are being made to extend them to roll-to-roll fabrication.74 However, these techniques typically require laborious preparation and it is less clear whether uniform films with controllable thickness could ultimately be produced with suitably high throughput. As such, these methods are not discussed in detail here, but further research to identify maximum deposition speeds possible with these techniques is important. 2. Comparison to vacuum-based high-throughput deposition methods It is presumptuous to dismiss vacuum-based techniques when discussing high-throughput nanomanufacturing. Vacuum chambers typically limit roll-to-roll webs to widths smaller than that desired for large-scale production, however the vacuum industry has a long and well-established history and continues to make strides in increasing the throughput achievable with vacuum-based systems. Table 2 summarizes some roll-to-roll vacuum deposition systems from Applied Materials, the development of which has been driven by the demand for flexible technology (touch screen displays, flexible displays, etc.) and metallized thin film flexible packaging. All of the systems in Table 2, which make use of physical vapor deposition techniques, are designed for roll-to-roll production and can accommodate polymer webs wider than 1 m. Furthermore; high-throughputs comparable to those of the open-air systems in Table 1 are possible with some of these vacuum systems. The use of high vacuum in all of the systems in Table 2, however, adds complexity related to system maintenance and the replacement of substrate rolls and source materials. Even

though polymer webs tens of kilometers long can be accommodated in some industrial scale systems,75 if throughputs of 10-1000 m/min are ultimately used, frequent roll changes are required. The ‘mean time before cleaning’ is another important factor for roll-to-roll systems as they are scaled. While time-consuming changes of rolls/source materials and vacuum cleanliness are not issues for open-air techniques, these methods have their own maintenance requirements. For example, “relatively time consuming cleaning procedures” are associated with rotary screen printing30 and “clogging of nozzles is a serious issue of inkjet printing”.33 In continuously operated gravure printing systems, frequent replacement of gravure cylinders is required to maintain print quality.2 The corresponding maintenance requirements for future highthroughput AP-SALD systems are as of yet unknown. Mixing of the precursors in the gas phase can result in unwanted material deposition on the manifold that requires periodic cleaning. It is not clear to what extent the mean time before cleaning can be reduced as AP-SALD systems are scaled. But in general the system downtime is expected to be much smaller for open-air techniques, where maintaining vacuum quality is not a concern. Generally a greater initial cost is also associated with vacuum equipment, however the ultimate production cost depends both on the initial cost and the system’s reliability. APSALD systems have the potential advantage of a relatively low initial cost, so the remaining challenge is to demonstrate reliable high-throughput fabrication. CONCLUSIONS AND OUTLOOK In this perspective we discussed atmospheric pressure spatial atomic layer deposition (AP-SALD) as a potential highthroughput, cost-effective nanomanufacturing technique. It compares favorably to existing open-air nanoscale deposition techniques in terms of thickness control, film uniformity, and thermal processing requirements; and the lack of a vacuum chamber should, in theory, maintain simplicity and mitigate maintenance requirements; however, much uncertainty remains and significant development is required: (i) More precursors need to be developed and tested to enable coating of a wider variety of materials (e.g. metals and polymers) at lower temperatures compatible with common polymer substrates. Direction can be taken from previous work done to deposit different materials by conventional ALD, although additional requirements need to be met for high-throughput AP-SALD systems. Gas streams with high precursor concentrations will need to be delivered reliably, and the exposure times (of the substrate to the alternating precursor streams) will need to be very brief if throughputs on the order of tens to hundreds of metres per minute are to be achieved. Desired precursor properties for high-throughput AP-SALD therefore include: (1) high vapor pressure to easily created concentrated gas streams; (2) high sticking coefficient on relevant substrates (plastics, transparent conducting oxides, etc.) to ensure rapid formation of precursor monolayers; and (3) high reaction efficiency to ensure the formation of the desired material within a short exposure time. (ii) Exhaust management within AP-SALD systems is another important factor for optimization. It will allow ventilation requirements to be relaxed and may enable precursor recycling to limit material wastage.



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Table 2. High-throughput vacuum-based nanomanufacturing systems

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AMAT System

SmartWeb75

TopBeam61

TopMet62

Deposition method

Sputtering

Electron beam evaporation, sputtering

Thermal evaporation

Vacuum requirement

High vacuum

High vacuum

High vacuum

Materials deposited

Most metals and dielectrics

Most metals and dielectrics

Most metals, SiOx, ZnS

Substrate types

R2R polymers

R2R polymers

R2R polymers

Substrate width

Up to 1.5 m

> 2.8 m

> 4.4 m

Throughput

0.5 to 25 m/min

30 to 840 m/min

Up to 1200 m/min

(iii) Integration of AP-SALD systems with other printing techniques will enable substrate patterning and patterned film growth. (iv) True high-throughput deposition at coating speeds on the order of tens to hundreds of meters per minute needs to be reliably demonstrated with AP-SALD. A thorough understanding of the AP-SALD chemisorption process is required and high-throughput manifolds need to be developed accordingly to demonstrate the coating of kilometer-long webs at high speeds. Technical challenges are anticipated when scaling AP-SALD systems as suggested in Figure 4. Accurate micron-scale control of the substrate-manifold spacing, as well as uniform heating of the precursor manifold and substrate, are essential for the deposition of uniform films by closeproximity AP-SALD. It will be important to maintain the control and uniformity of these parameters, as systems are scaled for high-throughput. (v) As AP-SALD systems are scaled, it will also be essential to monitor maintenance and cleaning requirements and ensure that high-throughput systems are designed in a manner that maximizes reliability and minimizes the frequency of servicing. Even with these gains, high-throughput AP-SALD will undoubtedly possess strengths and weaknesses compared to other techniques. The structural, electrical, and optical properties of the films produced using the various techniques discussed here, for example, have not been considered and would surely influence a technique’s suitability for a particular application. But for applications that require particularly uniform or conformal nanoscale films with precise thickness control, APSALD is extremely attractive. With further development, it should be possible to manufacture excellent films with APSALD at a very low cost and large scale. Given the importance of nanomaterials in many critical applications such as energy generation, energy storage, and smart sensor networks, the development of high-throughput AP-SALD technology has the potential to have a significant impact on the field of nanomanufacturing and our well being.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources KM acknowledges a University of Waterloo Startup Grant.

ACKNOWLEDGMENT KM acknowledges a University of Waterloo Startup Grant.

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BIOGRAPHY Kevin Musselman is an assistant professor at the University of Waterloo. He received his Ph.D. in Materials Science and Metallurgy with Judith Driscoll at the University of Cambridge in 2010. As the Hertha Ayrton Junior Research Fellow in Materials Science, he worked with Sir Richard Friend in the Department of Physics at the University of Cambridge. He produced metal oxide thin films with tuneable properties by atmospheric pressure spatial atomic layer deposition and used these films to study the operating mechanisms of a variety of next-generation photovoltaics and light-emitting diodes. In 2015, he joined the Department of Mechanical and Mechatronics Engineering at the University of Waterloo. His research focuses on developing scalable processes for manufacturing nanomaterials and using these materials in a variety of devices.



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Authors are required to submit a graphic entry for the Table of Contents (TOC) that, in conjunction with the manuscript title, should give the reader a representative idea of one of the following: A key structure, reaction, equation, concept, or theorem, etc., that is discussed in the manuscript. Consult the journal’s Instructions for Authors for TOC graphic specifications.

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Nanomanufacturing: High-Throughput, Cost-Effective Deposition of

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