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Solvothermal Annealing of Block Copolymer Thin Films Kevin W. Gotrik, and Caroline A Ross Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl4021683 • Publication Date (Web): 01 Oct 2013 Downloaded from http://pubs.acs.org on October 7, 2013
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Nano Letters
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Solvothermal Annealing of Block Copolymer Thin Films
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Kevin W. Gotrik1, C. A. Ross1* 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 6 0
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Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 *Address correspondence to
[email protected].
Abstract A two-stage annealing process for block copolymer films was introduced consisting of a solvent vapor exposure followed by a thermal cycle. By heating the film but not the chamber, changes in the ambient vapor pressure of the solvent were avoided. Films of block copolymers and homopolymers showed transient non-monotonic swelling behavior immediately after solvent exposure that was dependent on how the thin film was cast before the anneal. Thermal cycling of the solvent-swelled block copolymer films during the solvent anneal caused the films to deswell in 1 – 10 s and produced well-ordered microdomains in templated 45.5 kg/mol and 51.5 kg/mol polystyrene-block-polydimethylsiloxane films annealed in toluene and n-heptane vapors for total process times of 30 s – 5 min.
Keyword: PS-PDMS, self-assembly, Solvent anneal, Solvothermal, block copolymer, directed self assembly
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Block copolymer (BCP) self-assembly produces periodic microdomain features with dimensions of a few nm and above, making it a candidate for next generation lithography1–4. By appropriate selection of the chemistries for each of the blocks in the BCP, a wide variety of applications are enabled, such as filtration membranes5,6, high Li-ion conductivity electrolytes7,8, nanowire9 or nanoparticle10 growth templates, and hard masks for bit patterned media11 or transistor contact hole interconnects12,13. Most lithography applications of BCPs require precise control over the location and size of the microdomains; and to accomplish this, the selfassembly is directed by chemical or topographical substrate features made by top-down lithography methods such as optical lithography or scanning electron beam lithography (SEBL). Directed self-assembly (DSA) of BCPs is therefore under intense study for high resolution nanolithography. 2
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The period of the microdomains in the BCP is given by 𝐿0 = 𝑎𝑁 3 𝜒 6 in the strong segregation limit, where 𝑎 is the Kuhn monomer length, 𝑁 is the degree of polymerization and 𝜒 the FloryHuggins interaction parameter14,15. The driving force for microphase separation increases with 𝜒𝑁 so to obtain microphase separation for small period BCPs, a high 𝜒 is required. Commonly studied polystyrene-block-polymethylmethacrylate (PS-PMMA) has 𝜒 ~ 0.06 at room temperature limiting achievable periodicities to >~ 24 nm16, but high-𝜒 BCPs, such as the one used in this work, polystyrene-block-polydimethylsiloxane (PS-PDMS, 𝜒 ~ 0.26 at 300 K), can exhibit periodicities ~10 nm or lower making them candidates for reaching the very small feature sizes required for next generation lithography17. The annealing process used to promote microphase separation is a key component of any application of BCPs in nanoscale pattern generation. Thermal annealing may be used in which the BCP films are heated above the glass transition temperature (Tg) but below the orderdisorder transition temperature (ODT) to promote microphase separation. This can pose a problem for high-𝜒 BCPs as they can have ODTs far above the temperatures at which
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degradation or chain scission can occur18. Also, the diffusion coefficient decreases rapidly as 𝜒 increases, leading to long annealing times to achieve microphase separation. Solvent vapor annealing (SVA)19 can address these problems by allowing solvent molecules to interact with
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and swell a BCP film, reducing the diffusive energy barrier20 and enabling microphase separation at low temperatures. In addition, SVA can produce non-bulk morphologies by selectively swelling one block, effectively changing the volume fraction and driving morphological changes21,22, for example spheres or lamellae may be produced from a bulk cylinder-forming BCP. Moreover, domain size and spacing can vary with the solvent vapor composition and pressure23,24, and the solvent removal rate can affect orientation and correlation length of microdomains allowing additional levels of control not available in thermal annealing25–28. SVA and thermal annealing are often applied for several hours, producing highly-ordered periodic structures over large areas with low defectivity29. Shorter annealing times are desirable for manufacturing, and this has recently been explored through annealing processes that combine SVA and thermal annealing. Kim et al. described the effects of a solvent anneal followed by a thermal anneal for large molecular weight BCPs30. Both Zhang et al.31 and Borah et al.32 using microwave heating of a sample in a solvent rich environment, and Park et al.,33 by applying heat to a metal SVA chamber, achieved templated self-assembly of BCPs within minutes. However, identifying the mechanism of the combined annealing process is complicated, because temperature affects not only the diffusion coefficient directly but also the solvent vapor pressure and the solubility of solvent in the film. In this article, the effects of a combined thermal and solvent anneal are quantified by heating the swollen BCP film without changing the temperature of the surrounding solvent vapor environment, and by tracking the swelling ratio of thin films of PS-PDMS in situ as they are exposed to solvent vapor and subsequently to a thermal cycle. Total annealing times as low as 30 s were sufficient to produce
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highly ordered microdomain assemblies on a substrate that was prepatterned with a sparse post array. We first describe the solvothermal anneal process depicted in Figure 1. A nitrogen carrier
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gas was bubbled through a mixture of toluene and n-heptane (5:1 by volume) to create a saturated vapor stream in the annealing chamber at an ambient temperature of 23°C which consisted of vapor pressures of approximately 19.5 and 7.8 Torr of toluene and n-heptane respectively in nitrogen at atmospheric pressure34. This solvent ratio was found previously to be non-selective between the two blocks, i.e. it produced thin film microdomains characteristic of the bulk morphology of the PS-PDMS. Films of 45 kg/mol PS-PDMS (fPDMS= 0.32) annealed in this mixed-solvent vapor produced in-plane cylinders whereas 51.5 kg/mol PS-PDMS (fPDMS= 0.16) produced close-packed spheres35. PS or PDMS brush-treated Si substrates were coated with a block copolymer film (~45 nm thick) and placed in the solvent vapor-filled annealing chamber on top of a heater (a hot surface igniter) which was initially unheated. After the film had swelled in the solvent vapor for 30 – 300 s at room temperature, a voltage was applied to the hot surface igniter which heated the films at a rate that increased with the input voltage (5 – 20 V). The solvent vapor conditions were kept constant during the heating step. This process allowed the BCP film to simultaneously experience both solvent swelling and an increased temperature, i.e. the process enabled the BCP to undergo a short term ‘solvothermal’ anneal. The film swelling was measured using spectral reflectometry every 0.1 s throughout the anneal, starting within 1 s of introduction of the film into the vapor. Swelling ratios greater than about 1.3 were investigated in order to obtain self-assembly within a few minutes36. The advantage of this system was the sequential two step nature of the process it enabled, in which the thin film was first exposed to a saturated solvent vapor at ambient temperature (23°C) and then exposed locally to heat without raising the temperature of the entire chamber or altering the solvent annealing chamber vapor pressure. This helped decouple the effects of solvent and heating on the final BCP morphology.
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Figure 1: A schematic of the annealing system used for solvothermal annealing, which enabled a solvent vapor anneal and a thermal quenching step to remove the solvent in the film. A N2 carrier gas passed through a solvent bubbler to provide a saturated stream of solvent vapor to an annealing chamber where a Si substrate coated with a polymer film rested on a silicon nitride resistive heater controlled by a variable voltage regulator. In situ spectral reflectometry monitored film thickness changes throughout the entire anneal.
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Figure 2: Non-monotonic swelling behavior of polymer thin films (PS-PDMS, PS, PDMS of initial thickness D0 = 45, 82, 72 nm) exposed to saturated solvent vapors consisting of a mixture of toluene and n-heptane. The molecular weight and the spin speed used to make each film are given. (a) The film swelling vs. log(time) after introduction of the films into the solvent vapor in the chamber. D is the film thickness. (b) Swelling of PS-PDMS with D0 = 45 nm spin cast from two different solvents and spin speeds. The insets show the swelling ratio on a linear scale.
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First, the transient swelling behavior of the thin films was explored for 45 kg/mol PSPDMS and for two homopolymer films of PS and PDMS. High MW homopolymers were used here, especially in the case of PDMS, to prevent dewetting. Figure 2a shows non-monotonic
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swelling behavior after the polymer films were exposed to the solvent vapor. The initial linear regime in this log-log plot indicated a fast mass uptake typical of Case 1 diffusion, where there is no mechanical resistance of the polymer to the diffusing solvent37. The higher rates of solvent incorporation and swelling found in the PDMS compared to the other polymers was attributed to its low glass transition temperature Tg and correspondingly higher free volume and diffusivity at ambient temperature. The drop in swelling ratio after 10 s was attributed to a situation in which after a critical amount of solvent entered the film the polymers became sufficiently mobile to relax part of the free volume that was kinetically trapped during spin casting and reduce entanglements, analogous to Case 2 diffusion for glassy polymeric films. Other studies have shown initial drops in solvent mass after the initial mass uptake and swelling of thin films.38 Swelling ratio may also decrease as strain energy (remaining after spin casting) is relaxed, promoting the release of the absorbed solvent. After 30 – 50 s the swelling ratio began to increase as solvent continued to incorporate into the film, reaching values of up to ~3. These three regions of initial mass uptake, polymer relaxation, and steady-state swelling behavior have also been observed during in situ studies of toluene incorporation into polystyrene-blockpoly(ethlyene oxide) (PS-PEO)39. A dip in the swelling ratio was not observed if the thin films were inserted into the chamber prior to a gradual introduction of solvent vapor because the system took approximately 15 min to reach steady-state vapor conditions and the films swelled monotonically during this time36. Figure 2b demonstrates the importance of the spin casting conditions on the swelling transient for 45 nm thin films of 45 kg/mol PS-PDMS. Two samples are shown, one cast from propylene glycol methyl ether acetate (PGMEA, boiling point 146 ˚C) at higher spin speed and the other from cyclohexane (boiling point 81 ˚C) at lower spin speed, both spun for 30 s. During
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spin-casting the PS-PDMS film from the PGMEA dried within ~6 s (based on observations of the color change during casting) but the film from the cyclohexane dried in
