Study of the Kinetics and Mechanism of Rapid Self-Assembly in Block Copolymer Thin Films during Solvo-Microwave Annealing

Abstract

Microwave annealing is an emerging technique for achieving ordered patterns of block copolymer films on substrates. Little is understood about the mechanisms of microphase separation during the microwave annealing process and how it promotes the microphase separation of the blocks. Here, we use controlled power microwave irradiation in the presence of tetrahydrofuran (THF) solvent, to achieve lateral microphase separation in high-χ lamellar-forming poly­(styrene-b-lactic acid) PS-b-PLA. A highly ordered line pattern was formed within seconds on silicon, germanium and silicon on insulator (SOI) substrates. In-situ temperature measurement of the silicon substrate coupled to condition changes during “solvo-microwave” annealing allowed understanding of the processes to be attained. Our results suggest that the substrate has little effect on the ordering process and is essentially microwave transparent but rather, it is direct heating of the polar THF molecules that causes microphase separation. It is postulated that the rapid interaction of THF with microwaves and the resultant temperature increase to 55 °C within seconds causes an increase of the vapor pressure of the solvent from 19.8 to 70 kPa. This enriched vapor environment increases the plasticity of both PS and PLA chains and leads to the fast self-assembly kinetics. Comparing the patterns formed on silicon, germanium and silicon on insulator (SOI) and also an in situ temperature measurement of silicon in the oven confirms the significance of the solvent over the role of substrate heating during “solvo-microwave” annealing. Besides the short annealing time which has technological importance, the coherence length is on a micron scale and dewetting is not observed after annealing. The etched pattern (PLA was removed by an Ar/O2 reactive ion etch) was transferred to the underlying silicon substrate fabricating sub-20 nm silicon nanowires over large areas demonstrating that the morphology is consistent both across and through the film.

Simulation methods for solvent vapor annealing of block copolymer thin films

“…Solvent vapor annealing (SVA) has become a leading processing method in producing long range ordered structures in thin film block copolymer (BCP) systems. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] SVA is particularly important for BCPs with high Flory-Huggins interaction parameter w, [1][2][3][6][7][8][9][10][19][20][21][22][23][24][25][26][27][28] which are of interest for making microdomain patterns with small period but which have high order-disorder transition (ODT) temperatures requiring high annealing temperatures. SVA also enables access to non-bulk morphologies such as spheres or lamellae formed from a bulk-cylindrical BCP.…”

Evolution in surface morphology during rapid microwave annealing of PS‐b‐PMMA thin films

“…The orientation of BCPs during these rapid annealing methods has been generally controlled in concert with a neutral substrate surface to generate perpendicular cylinders in polystyrene‐ block ‐poly(methyl methacrylate) (PS‐ b ‐PMMA) thin films . Morris and coworkers have investigated the kinetics of the solvent‐assisted microwave annealing, but knowledge regarding the structural evolution of the surface morphology during microwave annealing without solvents remains sparse. Herein, we report on how microwave annealing conditions impact the surface morphology developed in cylinder‐forming PS‐ b ‐PMMA films on unmodified silicon wafers.…”

Nanomanufacturing: A Perspective

“…350,351 These limitations represent design constraints, but are unlikely to impede the adoption of this technology in manufacturing. 352 The time taken for diblock systems to order can be relatively long 353 and increases with increasing molecular weight, 354,355 but recent work using high-temperature, 356,357,358,359 solvent annealing, 360,361 or a combination, 362,363 indicates that this is unlikely to be a serious issue. Reducing chain entanglement with brush block copolymers is also an effective strategy.…”