Nanopatterning via Solvent Vapor Annealing of Block Copolymer Thin

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Nanopatterning via Solvent Vapor Annealing of Block Copolymer Thin Films Cong Jin, Brian C. Olsen, Erik J. Luber, and Jillian M. Buriak Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02967 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 9, 2016

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Chemistry of Materials

Submission for special issue on methods and protocols for materials chemistry

Nanopatterning via Solvent Vapor Annealing of Block Copolymer Thin Films Cong Jin,1,2 Brian C. Olsen, 1,2 Erik J. Luber, 1,2 Jillian M. Buriak1,2*

1

Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, AB

T6G 2G2, Canada 2

National Institute for Nanotechnology, National Research Council Canada, 11421

Saskatchewan Drive, Edmonton, AB T6G 2M9, Canada

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Abstract

The self-assembly of block copolymers to generate nanopatterns is of great interest as an inexpensive approach to sub-20 nm lithography. Compared to thermal annealing, solvent vapor annealing has several intriguing advantages with respect to the annealing of thin films of block copolymers, particularly for polymers with high interaction parameters, χ, and high molecular weights. In this methods paper, we describe a controlled solvent vapor flow annealing system with integrated in-situ microscopy and laser reflectometry, as well as a feedback loop that automatically controls the solvent vapor flow rate, based upon real-time calculations of the difference between thickness setpoint and the observed film thickness. The feedback loop enables precise control of swelling and deswelling of the polymer thin film, the degree of swelling at the dwell period, and preprogrammed complex multi-step annealing profiles. The insitu microscope provides critical insight into the morphological evolution of the block copolymer thin films over a broad area of the sample, revealing critical information about terraced phases, on the scale of tens and hundreds of microns, during the annealing process. This device could be a powerful tool for understanding and optimizing solvent annealing by providing multiple sources of in-situ information, at both the micro- and nanoscale.

Introduction Nanolithography is the key technology for patterning of semiconductor devices, and has, up to this point, relied upon remarkable advances in photolithography.1,2 Sub-20 nm photolithography is, however, facing its most extreme physical limits in terms of a number of metrics, including cost.3 The self-assembly of block copolymers (BCP) is of great interest for producing 2 ACS Paragon Plus Environment

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nanopatterns on semiconductors, as it is compatible with existing semiconductor manufacturing, and could enable low-cost, high throughput manufacturing.4–13 Along with extreme UV lithography, electron beam lithography, and nanoimprint lithography, block copolymer selfassembly, referred to as directed self-assembly (DSA) in the International Technology Roadmap for Semiconductors (ITRS), is being considered as a viable alternative patterning strategy for next generation lithography.14 Owing to the nature of the bottom-up processing, block copolymer self-assembly is able to generate large scale periodic features with sizes as small as 3 nm, at low cost.15–20 Block copolymers themselves are relatively inexpensive materials, and comprise a well-studied family of polymers with two or more homopolymer segments (blocks), connected via covalent bonds.21,22 The feature size, spacing, and shapes of the resulting self-assembled nanostructures can be tuned by choosing block copolymers with varying chain lengths, molecular weights, and chemical composition.6,21 After two decades of development, block copolymer self-assembly in thin films has been shown to be capable of generating patterns of interest to the semiconductor industry, such as ordered hexagonal dot arrays, square arrays, bends, jogs, circles, and Tjunctions.7,23–30 A few examples of the many nanopatterns accessible via block copolymer selfassembly are shown in Figure 1. These thin film block copolymer nanopatterns can then be transferred to a substrate surface via etching, or deposition.31–36 Fabrication of field effect transistors,37–40 contact hole applications,41–45 phase change memory,46,47 and bit patterned media48–51 using block copolymer self-assembly technology has been demonstrated. In addition to their obvious applicability to semiconductor device applications, self-assembled block copolymer nanopatterned films are also of great interest for applications in filtration,52,53 tissue and cellular interfacing,54–56 plasmonics,57–59, photonics,60 and sensing.61 3 ACS Paragon Plus Environment


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Figure 1. Annealed BCP thin films on patterned substrates. (a) Pt line patterns formed from PSb-P2VP (23.5k-10.4k)-thin films. (b) PS-b-PMMA/PS/PMMA blend forms a 90º bend on a chemically patterned surface. Adapted with permission from ref 23. Copyright 2005 from the American Association for the Advancement of Science. (c) Pt concentric ring patterns formed from PS-b-P2VP (50k-16.5k). Adapted with permission from ref 33. Copyright 2008 from the American Chemical Society. (d) Ordered silica dots formed from PS-b-PDMS (51.5 kg/mol, fPDMS = 16.5%), on a patterned substrate formed from e-beam lithography of HSQ. Adapted with permission from ref 62. Copyright 2005 from the American Association for the Advancement of Science.


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Table 1. Commonly used block copolymers used for self-assembly in thin films for nanopatterning applications. Abbreviation

PS-b-P2VP

Full Name polystyrene-block-poly(2-vinylpyridine)

PS-b-P4VP

polystyrene-block-poly(4-vinylpyridine)

Chemical Structure n

m

N

n

m

N

PS-b-PMMA

polystyrene-block-poly(methyl methacrylate) n

m O

PS-b-PDMS

polystyrene-block-polydimethylsiloxane

Si m

PS-b-PEO

polystyrene-block-poly(ethylene oxide)

O

O n

O m

PS-b-PLA

n

polystyrene-block-poly(d,l-lactide)

O n

m O

The repertoire of patterns, shapes, and order that can be accessed through block copolymer self-assembly is described in several reviews on the subject.5,6,21,25,25,27,31,48,63–70 The starting point for the prediction of an equilibrium product of self-assembly of a given block copolymer would be the phase diagram, if known.5,63–66,71–73 Well-studied diblock copolymers that represent the workhorse materials for much of the nanopatterning work in the literature are shown in Table 1. The phase behavior of a given block copolymer depends upon three variables related to the polymer composition: the degree of polymerization (N), the interaction parameter of the two 5 ACS Paragon Plus Environment


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blocks (Flory-Huggins interaction parameter, χ), and the volume fraction of the minority block, f.21,71,73 Moreover, when compared to bulk, self-assembly of block copolymer thin films can realize a greater variety of patterns and motifs, resulting in part from the influence of surface energetics at the substrate-polymer and polymer-air interfaces. Furthermore, film thickness and defect formation and annihilation are also linked, which add another level of complexity to pattern formation.21,26,66,68,72,74–76 Initial film thickness plays a critical role with respect to the equilibrium nanopatterns that can form in the film since the height of the film has a profound effect on the formation of formation of block copolymer domains (vide infra).77–80 On the nanoscale, through the use of chemical and topological patterns, however, one can induce longrange alignment of block copolymer self-assembly and reduce the defect density of nanopattern formation through what is called graphoepitaxy (examples shown in Figures 1a-c).23,26,62,72,81,82 These patterns act to guide the self-assembly during the annealing process, nudging or directing the block copolymers into a more ordered configuration. Because of the number of variables at play, newcomers to the field of thin film block copolymer self-assembly are suggested to start with a published recipe, one that describes surface preparation, film thickness, the annealing process (vide infra), and characterization.

Background: Annealing of Thin Films of Block Copolymers The key to driving the self-assembly process is the annealing step of the thin film, during which the block copolymers undergo nanoscale phase segregation.5,21 Thin films of the block copolymer of interest are typically spin-coated onto a substrate in order to produce smooth films of controlled thicknesses. The substrate surface should be cleaned beforehand in an appropriate manner, with perhaps additional functionalization with a brush layer, or other chemical means.83– 6 ACS Paragon Plus Environment

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88

As-spun films are kinetically trapped in a disordered state due to fast evaporation of solvent,

and represent the starting point for self-assembly. By far, the two most widely used techniques for annealing of a block copolymer thin film are thermal and solvent vapor annealing, although other recently described and less-developed approaches deserve mention, including light- or laser-promoted photothermal annealing,89–91 the use of shear forces,92–94 electric fields,95–98 solvent immersion,99,100 and magnetic fields101–103 to induce long-range alignment. This methods and protocols paper will start with a brief description of the most basic approach towards annealing, thermal annealing, as it requires relatively simple apparatus to perform. The paper will then shift focus and concentrate primarily on solvent vapor annealing of thin films of block copolymers to generate nanopatterns on surfaces. Solvent vapor annealing offers flexibility in terms of the choice of solvent and conditions (temperature, vapor pressure and time), is fast and easy to monitor in-situ via spectroscopic means, does not contribute to the thermal budget of the semiconductor processing stream, and may be the sole route for annealing of block copolymers with high χ values or of high molecular weights (a useful class of polymers that are normally troublesome to anneal thermally).66,104–106

Thermal Annealing Thermal annealing involves the heating of a thin film of a block copolymer on a substrate to a temperature above the glass transition temperature, Tg, and below the order-disorder transition temperature (ODT), to enhance polymer diffusivity.23,81,82,107,108 The thermal energy increases the mobility of the block copolymer chains, and with sufficient time, allows the polymers to reach a lower energy configuration. For each polymer, the optimal annealing conditions will differ as they depend upon the chain lengths, volume fraction of each block, the χ parameter, film 7 ACS Paragon Plus Environment


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thickness, and treatment of the substrate surface.16,23,62,109,110 Thermal annealing is usually performed in vacuum or in an inert gas atmosphere to avoid oxidation and decomposition of the organic polymer. Apparatus for thermal annealing includes use of a vacuum oven, a hot plate in an inert atmosphere glove box, or a tube furnace with either vacuum or inert gas flow. Temperatures are generally in the range of 120-250 °C, and the time required is on the order of hours-to-days.23,33,74,81,109,111–113 It has been recently reported, however, that thin films of block copolymers can be successfully annealed in ambient atmosphere (air) via fast heating approaches (seconds to minutes) such as microwave heating,114–118 photothermal laser heating,89–91 and rapid thermal annealing.119,120 Thermal annealing has the advantage of being compatible with current integrated circuit manufacturing practices.120 Block copolymers with high χ values or high molecular weights are difficult to anneal thermally in any reasonable annealing time, as higher annealing temperatures and longer times are required to enable disentanglement of the polymer strands of this class of block copolymers, which can result in thermal decomposition of the polymer.104,105,121–124

Solvent Vapor Annealing The basic principle of solvent vapor annealing is simply the exposure of a block copolymer thin film to a solvent vapor; if the solvent has a similar value of Hildebrand solubility parameter with one or more of the blocks (often referred to as a ‘good’ solvent), the polymer layer absorbs the solvent and swells, resulting in a thicker film. Within the swollen film, the glass transition temperature of BCP thin film drops to well below room temperature, and the polymer chains increase in mobility, and plasticize. The Flory-Huggins interaction parameter of the block copolymer, χ, drops due to shielding of one or more of the blocks by solvent. Unlike thermal 8 ACS Paragon Plus Environment

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annealing, solvent vapor annealing may be used to access non-equilibrium, metastable phases through control of evaporation rate and other parameters.8,66,105,125–129 Annealing of thin films of polystyrene−block-polybutadiene−block-polystyrene (PS-b-PB-b-PS) triblock copolymers with solvent vapor was initially reported by Thomas and co-workers in 1998 as an alternative to thermal annealing.130 The resulting films showed improved order of the self-assembled block copolymer cylindrical and lamellar nanopatterns as compared to thermal annealing. The same year, Libera and co-workers studied the influence of solvent evaporation on the order and orientation of cylinder-forming PS-b-PB-b-PS triblock copolymer nanopatterns over large areas.131 This annealing method became increasing popular after Fukunaga and co-workers and Russell and co-workers demonstrated that lamellar and cylindrical structures with long-range order over single grains (single domains) could be achieved via solvent vapor annealing.132–134 Since these early reports of solvent vapor annealing, this approach has been widely used for many block copolymer systems. Solvent vapor annealing is incredibly versatile as a wide range of solvents, with different solubility parameters, vapor pressures and other characteristics, can be used. Because it can be carried out well below the glass transition temperature of a given block copolymer, this annealing method is useful for heat-sensitive polymers, and has been partnered with thermal annealing (called solvothermal annealing), which can produce results that differ from a unique solvent or thermal anneal.99,114,115,136–138

Apparatus for Solvent Vapor Annealing Although solvent vapor annealing has been used for almost two decades, there is no standard annealing apparatus or set-up, meaning that reproducibility and comparison of results may be difficult. There are two main categories that define the methods for carrying out solvent vapor 9 ACS Paragon Plus Environment


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annealing, shown in Figure 2. All start with a block copolymer thin film on a substrate (very often atomically smooth, native oxide-capped silicon). As mentioned earlier, the block copolymer is almost always spin-cast from a dilute solution, often using toluene or THF as the solvent. Spin-casting is practical because of its control over the film thickness, which is critical for reproducibility of the self-assembly process. The simplest set-up to carry out solvent annealing is one that uses a sealed chamber containing a solvent reservoir and the sample of a polymer thin film-on-substrate, held at room temperature with no additional controls over temperature. This method, termed static or ‘jar’ annealing, is effective, and with practice, can be used to produce self-assembled thin films of block copolymers, albeit in a mostly empirical manner. Simple but serviceable examples include using a beaker, crystallization dish with a flat glass cover, or dessicator, enclosing a vial of solvent and the sample.105,128,129,131,132,135 More sophisticated versions of the static solvent vapor annealing apparatus use a custom-designed chamber with a transparent window that allows for in-situ ellipsometry or reflectometry to monitor the film thickness during the annealing process, shown in Figure 2a. The annealing chamber volume, surface area of solvent reservoir, and possible leakage of solvent vapor from the annealing chamber all influence the swelling speed and final degree of swelling, and thus in-situ analysis is critical for reproducibility and to study intermediate degrees of swelling. This type of annealing apparatus has no direct control over the swelling rate during the annealing, and since solvent vapor annealing is sensitive to humidity and room temperature fluctuations, precise reproducibility becomes an issue - an example using insitu ellipsometry of a block copolymer thin film-on-silicon in a static annealing system as a crude temperature monitor is shown in the Supporting Information, Figure S1b. The annealing results may, therefore be influenced by gross parameters such as the weather, room temperature 10 ACS Paragon Plus Environment

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fluctuations, and seasonal building temperature changes. Indeed, we have found noticeable effects due to heating of the building during the spring and fall, when outside temperatures fluctuate on either side of what is considered ‘room temperature’.

Figure 2. Two different types of solvent vapor annealing apparatus. (a) Static solvent vapor annealing setup. (b) Solvent vapor flow annealing system.

The second general family of solvent vapor annealing apparatus is more sophisticated, and is equipped with flow control, a purge line, in-situ sample thickness monitoring (Figure 2b and Table S1). Solvent flow enables greater control over the degree of swelling, D, (defined as the 11 ACS Paragon Plus Environment


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ratio of swollen film thickness to initial film thickness), and much improved reproducibility. The first use of solvent vapor flow annealing system, to the best of our knowledge, was carried out by Knoll and co-corkers in 2004, who studied the dependence of morphology on the thickness of the film, of a PS-b-PB-b-PS triblock copolymer.139 In 2007, Russell and co-workers followed with a study of cylinder-forming PI-b-PLA diblock copolymer in a solvent vapor flow annealing system.140 Their results revealed that solvent vapor annealing was an effective way to achieve nanopatterns of perpendicular cylinders. They also studied the influence of annealing parameters on the order of cylinder-forming PS-b-P4VP thin film using the same setup.141 The results were promising, and showed that longer annealing times with the vapor of a selective solvent (THF, a ‘good’ solvent for the PS block) can not only improve local order, but can also improve the longrange order (as registered by an increase the size of the domains, or grains) of the resulting nanopatterns. In 2012, Epps and co-workers used a solvent vapor flow annealing system and showed that the rate of solvent removal from swollen block copolymer thin films influenced the final morphology.142 The same year, Ross and co-workers reported a study of the resulting morphology of a PS-b-PDMS block copolymer using a combination of solvents for annealing in a controlled solvent vapor flow annealing system with reflectometry to measure film thickness in-situ.104 In this system, three mass flow controllers were used to adjust flow rates of toluene vapor, n-heptane vapor and pure nitrogen gas, independently. With precise control over solvent vapor pressure and the ratio between the two solvents, this annealing apparatus was shown to reproducibly tune the morphology of a given block copolymer thin film. When combined with a sufficiently fast quench to terminate the anneal, long-range order was achieved in 30 s.136 Leighton and co-workers examined solvent vapor annealing of cylinder-forming PS-b-PLA in a custom designed all-metal solvent flow annealing system with an incorporated in-situ thickness 12 ACS Paragon Plus Environment

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monitor (reflectometry).143 The results obtained using this system, thanks to the added capability of the in-situ thickness monitor, showed that there was a narrow window (with respect to the degree of swelling) for the annealing of this cylinder-forming PS-b-PLA, which was close to the order-disorder transition condition.

Figure 3. The profile and plan view of terraced phases of a thin film of polystyrene-blockpolybutadiene-block-polystyrene (14k-73k-15k), showing different morphologies in areas of varying thicknesses. (a-c) AFM phase images of BCP thin films. The drawn white lines are contour lines, and all images are 2 × 2 µm2. Adapted with permission from ref 139. Copyright 2004 from the American Institute of Physics.

Another variant of solvent vapor flow annealing apparatus uses grazing-incidence small-angle X-ray scattering (GISAXS) to provide in-situ analysis of the resulting crystallinity of a thin film.144–147 Most recently, Ross and co-workers reported a GISAXS study of polystyrene-blockpolydimethylsiloxane (PS-b-PDMS) thin films in a newly designed controlled solvent vapor flow



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annealing system.148 The results shed light on the complex kinetic and thermodynamic factors at play during swelling and deswelling in solvent vapor annealing. Ellipsometry and reflectometry, provides extremely useful in-situ information regarding the thin film thickness and therefore the degree of swelling in the film resulting from solvent uptake. These techniques are most often implemented over large areas (hundreds of microns) without lateral resolution, which forces the assumption of a uniform film thickness over that area. However, during the annealing process, the film can become terraced, forming regions of different microstructures with different thicknesses (Figure 3).78,124,139,149 Since these terraced phases are related to the local thickness of the film, data obtained from ellipsometry, reflectometry or other non-mapping thickness techniques will therefore only be able to return an average or effective thickness over a large area. In order to provide additional information, exsitu optical microscopy of thin films can be carried out to complement the in-situ data, but the thin film samples need to be quenched before optical microscopy analysis. Herein, we describe a solvent vapor flow annealing apparatus with precise control over the degree of film swelling, enabled by feedback control linking in-situ monitoring of film thickness with solvent vapor flow. An integrated optical microscope provides additional critical information about macroscale properties of the film (such as micron-scale terraced phase formation). Feedback control enables precise, dial-in control over the degree of swelling of the film to enable reproducible solvent annealing of block copolymer thin films. This combination of precise control over thin film annealing, combined with information obtained from in-situ optical microscopy and laser reflectometry enables detailed understanding of solvent vapor annealing of block copolymer thin films in both nanometer thickness regimes (z-scale), and broad, millimeterscale areas in the x- and y-directions on the substrate. These insights are useful for rational 14 ACS Paragon Plus Environment

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optimization of the annealing procedures for directed self-assembly. All software and drawings needed to implement this feedback controlled solvent vapor annealing system are available as open

source

software

through

Github

in

the

bcolsen/BCPID

repository

(https://github.com/bcolsen/BCPID).

Results and discussion Experimental details for many of the processes described here are found in the Supporting Information (SI), including the preparing, annealing, and etching of PS-b-PDMS thin films, metallization and etching PS-b-P2VP and PS-b-P4VP thin films, as well as detailed descriptions and videos of metallization, plasma etching, thermal annealing, controlled solvent flow annealing, and static solvent vapor annealing (a selected summary is found in Table 2).

Table 2: List of supporting materials.

Experimental Procedure

Supporting materials

Photographs of experimental apparatus

Figure S2,

General experimental procedures for silicon wafer handling, block copolymer solution preparation, metallization and plasma treatments, and related procedures

Pages S6-S8

Vacuum oven thermal annealing

Movie S1, Figure S7

Static solvent vapor annealing

Movie S2, Figure S8

Controlled solvent vapor flow annealing

Movie S3, Figure S9

Metallization of PS-b-P2VP/P4VP

Movie S4, Figure S10 15

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Oxygen plasma etching of PS-b-PDMS diblock copolymers

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Movie S5, Figure S11

Description of apparatus A functional flow diagram, schematic, and photographs of our controlled solvent vapor flow annealing apparatus are shown in Figure 4. The chamber volume is less than 1.5 cm3, which is sufficient to fit a “standard” block copolymer thin film sample (1 cm × 1 cm). The sample chamber is mounted to a thermoelectric plate capable of both heating and cooling the chamber. The bubbler is immersed in a water bath to help maintain a constant temperature of the solvent. While solvent annealing at a constant flow, the swelling degree will still fluctuate with room temperature changes. Figure 5a shows the swelling profile of a 25-nm-thick PS-b-PDMS (31k14.5k)/30 wt % PS (10k) film on native-oxide-capped silicon annealed at 1.2 sccm of Ar flow mixed with 20 sccm flow of THF-saturated gas from the bubbler, at room temperature. By mixing the PS-b-PDMS (31k-14.5k) a with an appropriate amount of PS, thin films from this blend yield a hexagonal packed dot pattern upon annealing.78 The degree of swelling reached ~2.24 soon after exposure to the solvent vapor flow, but decreased gradually to 2.08 at the 2000 s mark. The entire swelling profile of this sample as well as two other anneals conducted under the same condition are shown in Figure S4. In order to control the degree of swelling of the polymer film during annealing, a feedback loop linking a precise and responsive thickness measurement technique with solvent vapor flow was developed (Figure 4). Laser reflectometry has been used in other systems to determine film thickness, and was adopted here.104,146 It is a simple and effective way to determine film thickness in real time to obtain the degree of swelling in the polymer film: To determine the thickness of the film, the laser reflectivity is correlated to a pre16 ACS Paragon Plus Environment

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calculated table (as shown in the Supporting Information) based an initial ex-situ ellipsometry thickness measurement.

Figure 4. (a) A functional flow diagram of the controlled solvent vapor flow annealing system. (b) A schematic of controlled solvent vapor flow annealing apparatus. (c) Cross section drawing of the annealing chamber in (b). (d,e) Two photographs of the apparatus.



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The physical layout of the reflectometer and the details of the calculated table are described in the Supporting Information. The green and black solid lines in Figure 5a show a preprogrammed swelling profile, and the measured degree of swelling, respectively, of a 25-nm-thick PS-bPDMS (31k-14.5k)/30 wt % PS (10k) film. The pre-programmed swelling profile was set using a rate of 0.3 D/min (e.g. the degree of swelling increases 0.3 every minute) ramp to a set point of D = 2.20, and then held constant for a prescribed length of time (1500 s). The system was then rapidly purged at the 30-minute mark to arrest the annealing process and kinetically trap the morphology of the block copolymer film. More detailed data including the gas flow profile and feedback control profile of this annealing are plotted in Figure S2. Figure 5b and 5c shows four annealing profiles with different set points of D, from 2.05 to 2.20, and four annealing profiles with different swelling speeds from 0.04 D/min to 0.3 degree of swelling/min. The degree of swelling can be held precisely (D ± 0.03) for several hours or more (limited only by the volume of solvent in the bubbler). In order to deswell films as quickly as possible, the volume of the chamber was only 1.5 mm3, enabling a deswell time of less than 2 seconds at 20 sccm, permitting kinetic trapping of the thin film morphology at a precise time (Figures 5a-c). To contrast with the swelling profiles that can be obtained with the solvent flow-controlled system, Figure 5d shows the swelling curve for a 25-nm-thick film of PS-b-PDMS (31k14.5k)/30 wt % PS (10k) in a static solvent vapor annealing system. The structure of this simple, sealed annealing chamber was reported in our previous paper,150 and consists of a home-built sealed aluminum chamber with two optically transparent windows to enable ellipsometric determination of sample thickness (annealing procedure and device setup can be found in SI, Figure S2b, and Movie S2). The shape of the swelling curve was determined by several factors, such as the surface area of the solvent reservoir, the humidity of the ambient air, and temperature 18 ACS Paragon Plus Environment

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of the room. The degree of swelling of the polymer film was monitored via ellipsometry, and while a given morphology (or combination of morphologies) was reproducible for a given degree of swelling, the rate of swelling varied from experiment-to-experiment. For example, the dip in the degree of swelling at 3500 s in Figure 5d was determined to be the result of a usual room temperature fluctuation of about 2 °C (due to the building temperature control). With the feedback loop and temperature control in the flow-controlled system, reproducibility may be less of an issue.



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Figure 5. Swelling profile of a 25-nm-thick PS-b-PDMS (31k-14.5k)/30 % wt PS (10k) film annealed by THF vapor in two different annealing apparatus: (a-c) controlled solvent vapor flow annealing, and (d) static solvent vapor annealing. (a) Feedback controlled solvent vapor flow annealing and constant solvent flow annealing (Ar and bubbler flow are set to be 20 sccm, then the Ar flow is reduced to 1.2 sccm.) (b) Four swelling profiles with different degree of swelling: 2.05, 2.10, 2.15, and 2.20 at the same swelling rate (0.3 D/min). The dwell time was set to be 1500 s. (c) Four swelling profiles with the same degree of swelling and dwell time (500 s), but 20 ACS Paragon Plus Environment

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different swelling rate (from 0.300 to 0.04 D/min). (d) Swelling profile in a static solvent vapor annealing system.

Controlling and observing the degree of swelling As stated earlier, the thickness of the pre-annealed film is critical not only with respect to the resulting nanoscale morphology, but on the micron-scale as well. For the dot forming block copolymer used in this article, if the film is too thin, thin wetting layers or bare substrate regions (referred to hereafter as wetting layer in accordance with previous literature)139 will form. If the sample is too thick, double- and triple-layer regions will result. Inconsistencies in as-spun thickness as well as mass transport due to repeated swelling of the film can result in formation of both wetting layers and multilayers within the same sample (Figure S5). An example, in what was intended to be a single-layer thin film, is shown in Figure 6. In this example, a 25-nm-thick PS-b-PDMS (31k-14.5k)/30 wt % PS(10k) film was solvent annealed, resulting in mostly singlelayer films, but the low magnification SEM micrograph, Figure 6b, shows some small regions of wetting layers (darkest areas), and double layers (lightest areas). Figures 6c-e show high magnification SEM micrographs of the three different areas for conclusive identification.



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Figure 6. (a) Cross-sectional schematic showing formation of terraced phases of wetting, single and double layer morphologies, in an annealed PS-b-PDMS film, with an initial thickness of 25 nm. (b) Low magnification SEM micrograph of a solvent vapor-annealed thin film of PS-bPDMS following CF4/O2 plasma treatment, an established high-fidelity conversion of the PDMS block to silica, accompanied by simultaneous removal of the PS. (c) High magnification SEM micrograph of the majority single layer (resulting hexagonal silica dot pattern after CF4/O2 plasma treatment). (d) High magnification SEM image of double layer area (resulting silica honeycomb dot pattern after CF4/O2 plasma treatment). (e) High magnification SEM micrograph of an area that shows wetting layer formation (no silica dot pattern seen after CF4/O2 plasma treatment).

From a manufacturing perspective, reproducibility, consistency, and avoidance of multiple self- assembled nanopatterns will be critical, particularly across the large area of a 300-mm wafer.151 Optical microscopy provides real-time measurement of terraced phases correlated directly with film thickness across large areas of the sample, as shown in Figure 7, and Movie S6. For future lithographical applications, minimization of large scale defects such as regions of 22 ACS Paragon Plus Environment

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undesired thicknesses is critically important. Much remains to be learned about the terraced phases formation in block copolymer annealing, in the context of manufacturing and scale-up. The in-situ microscopy capability allows for monitoring and, perhaps in the future, control of the terraced phase morphology during the entirety of the solvent vapor annealing process. Control software was developed to allow multiple annealing steps, including ramping up and down and dwelling at different degree of swelling (Figure 7), enabling cyclic annealing recipes152 that have recently been suggested to provide a fine degree of control over the selfassembly process. To demonstrate the utility of the integrated optical microscope, coupled with control over the degree of swelling (enabled by the feedback loop), Figure 7 demonstrates multistep annealing. The figure shows a simple three-step annealing process of a 25-nm-thick PS-bPDMS (31k-14.5k)/30 wt % PS(10k) thin film on a silicon substrate: The first step is a 250 s dwell at a degree of swelling of 2.4; the second step is a decrease in swelling degree to 2.0, which is then held constant for a 150 s; the third step is an increase of the degree of swelling to 2.3, holding this value for 150 s. The ramp rate for these three steps are all 0.3 D/minute, indicated by the solid red line. Far more complicated cycling profiles are, of course, possible. The optical micrographs are shown in images T0-T7 with corresponding times of capture indicated on the annealing curve. The contrast in the optical micrographs is due to thin film interference. At this thickness, thicker films appear darker, and thinner films are lighter. The sharp contrast between the phases is due to the discrete equilibrium thickness corresponding to the underlying terraced phase (Figure 6a and Figure 3). The area fraction of regions seen as white circular spots in the microscope (referred to as here as wetting layers) and dark circular areas (double layer regions) have been tabulated using thresholding and are plotted along the annealing profile. A video showing the evolution of thin film morphology throughout this 23 ACS Paragon Plus Environment


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annealing process can be found in the Supporting Information, Figure S12 and Movie S6. As seen in these optical micrographs, immediately after spin casting, the block copolymer thin film is relatively uniform, although weak contrast variations are observed across the field of view, which are ascribed to thickness fluctuations of the film. Upon commencing the solvent vapor anneal, the block copolymer film immediately begins to increase in thickness as it absorbs solvent, from 25 nm at 8.7 s to 29 nm at 13.8 s. Along with the increase in film thickness, nonuniformity of the film thickness begin to increase significantly, as evidenced by the increased contrast between light and dark regions (as seen in the optical micrographs, Movie S6). The wetting layers (white circular spots) appear at ~189 s, and the area fraction of wetting layers increases from 4.2 % of the total area at 189 s to 11.1 % at 242 s (degree of swelling: 2.1, Figure 7T1). The wetting layers then progressively shrink as the block copolymer thin film continues to swell, and completely disappears at a degree of swelling of 2.4 at 377 s (Figure 7T2). At these higher degrees of swelling, double layers (dark circular areas) start to appear, and increase in size during the 250 s dwell, from 0.2 % to 8.1 % (Figure 7T3) of the total film area. From T3 (619 s) to T4 (700 s), the degree of swelling of this block copolymer thin film was then decreased from 2.4 to 2.0, and the double layer area of the thin film decreased from 8.1 % to 2.6 % (Figure 7T4). Upon a second stage of dwelling, the area of double layers decreased (from 2.6 % to 0.5 %), and the wetting layers reappear, increasing from 0.5 % (Figure 7T4) to 9.3 % (Figure 7T5) of the total area. The wetting layer area reached its maximum at 888 s (12.7 %, 2.2 degree of swelling) but started to shrink afterward. At the end of the solvent annealing, the block copolymer thin film was quenched, with a resulting double layer area fraction of 1.8 % and wetting layer area fraction of 7.5 %.


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Careful investigation of Figure 7 and Movie S6 reveals that the area fractions of wetting, single, and double layers can change significantly despite maintaining a constant degree of swelling. As mentioned previously, when using an optical interference method without lateral resolution to measure the degree of swelling of a BCP thin film, it is generally necessary to assume that the film has a uniform thickness over the measurement area. When there are terraced phases present, only an effective thickness is measured resulting in an effective degree of swelling. Therefore, as the film equilibrates to a given degree of swelling setpoint, the relative fractions of wetting, single and double layers can change while the effective thickness stays constant. This is an important consideration for interpreting in-situ optical thickness measurements, and highlights the importance of coupling these measurements with optical microscopy.



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Figure 7. The evolution of the terraced phases within a 25-nm-thick PS-b-PDMS (31k-14.5k)/30 wt % PS(10k) film during a three-step annealing process. See Supporting Information for a full video capture of the entire annealing process. Note: the gray line mark at middle left and two whitish spots in every image are due to the contamination of the microscope lens.


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Figure 8. Example of controlled solvent vapor flow annealing of a 22-nm-thick PS-b-PDMS (22.5k-4.5k) thin film. (a) High magnification SEM micrograph of the silica dot patterns formed from an annealed BCP thin film, following CF4/O2 plasma treatment. (b) Low magnification SEM micrograph of a single grain of hexagonal silica dots. The hexagonal pattern can be seen upon expanding (zooming in) on the image. Insert: 2DFFT. (c) Low magnification SEM micrograph of annealed BCP thin film surface with single (majority phase), and double layers (light circles). (d) Swelling profile of the annealing profile. 27 ACS Paragon Plus Environment


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An example of the utility of the controlled solvent vapor flow annealing system is shown in Figure 8, which demonstrates very fast annealing of large ordered areas of a thin film of PS-bPDMS (22.5k-4.5k). The annealing profile, Figure 8d, is short, and starts with a pre-programmed swelling profile at a rate of 0.3 D/min to ramp up to D = 1.75. The film was then held constant for 900 s, followed by a purge at time = 1050 s. The annealed thin film was shown to be majority single layer dot pattern, comprised of 11.3 nm-diameter dots with a pitch of 22 nm, within large ordered grains of over 50 µm2 in area (Figures 8a and b). The film has a small quantity of double layer areas, which was determined to be 3.5 % of the total area (Figure 8c).

Solvent Vapor Annealing and Defect Density For semiconductor device applications, control over and quantification of the resulting defect density of a nanopattern is critical.7,153–155 The International Technology Roadmap has specified a key metric for directed self-assembly: the defectivity of a nanopattern must be less than 0.01 cm-2, which is orders of magnitude below the state-of-the-art.7,20 Research to optimize the annealing of thin films of block copolymers is therefore critical. With respect to solvent vapor annealing, it has been reported that the rate of swelling, the degree of swelling, annealing time, and deswelling rate all influence the resulting defect density.66,124,138,142,146,148 In terms of quantifying the defect density of a nanopattern formed via the self-assembly of a block copolymer, a well-described and automated method that extracts statistically relevant data would be preferred. A number of published papers on the topic of analysis of quantification of defect density have been published.81,81,114–116,155,156 A recent publication by our group described the development of freeware that enables automated classification and quantification of defects in a block copolymer-derived nanopattern, as well as related characteristics like line-edge roughness 28 ACS Paragon Plus Environment

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and correlation lengths.149 An example of the output of this automated application is shown in Figure 9. The application, along with a manual and demonstration/test images, can be downloaded and free of charge, as directed in the original publication.

Figure 9. Example of automated defect analysis of SEM images of Pt nanopatterns formed from the cylindrical morphology of a self-assembled block copolymer films of PS-b-P2VP (A+D 50kb-16.5k, B+E 44k-b-18.5k, and C+F 32.5k-b-12k) patterns that can be carried out using the automated application described in reference 149. Reprinted with permission from reference 149. Reproduced with permission from PLoS.

Conclusions For nanopatterning via block copolymer self-assembly (directed self-assembly) to be a viable commercial method for nanolithographic applications for semiconductor devices, unprecedented control over the annealing process is required. Detailed, fundamental insights are required to 29 ACS Paragon Plus Environment


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enable the deep understanding required to permit rational optimization. In this paper, we have described apparatus to enable solvent vapor annealing with dial-in feedback control to precisely control the degree of swelling, tailor the swelling profile, and rate of swelling and deswelling of a thin film. The integration of an in-situ optical microscope provides critical information as to the evolution of the micromorphology of the film during the anneal, which complements the film thickness data, as formation of terraced phases can and does occur. Because terraced phase formation is reversible, exquisite control over the degree of swelling allows for optimization of the solvent vapor annealing, to reach the highest level of desired morphology. This tool can be used by the community to tailor and optimize annealing procedures in a manner that is reproducible, while providing instructive data as to intermediate states along the annealing pathway.

ASSOCIATED CONTENT Supporting Information Photographs and descriptions of solvent vapour apparatus; plots showing relationship between temperature and solvent flow, and thickness of a block copolymer thin film; SEM micrographs of different thin film samples, general experimental details, and screen grabs from the supporting videos showing experimental procedures (PDF).

The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION 30 ACS Paragon Plus Environment

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Corresponding Author *E-mail: [email protected] (J.M.B) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS This work was supported by grants from the Natural Sciences and Engineering Research Council (NSERC,

grant

numbers

RGPIN-283291-09,

RGPIN-2014-05195),

Alberta

Innovates

Technology Futures (fellowship to CJ, and grant number AITF iCORE IC50-T1 G2013000198), and the Canada Research Chairs program (CRC 207142). Electron microscopy was carried out NRC-NINT. Minjia Hu is thanked for taking videos and pictures.

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Nanopatterning via Solvent Vapor Annealing of Block Copolymer Thin

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