Two-Dimensional Self-Assembly Of Block Copolymer Films And Its Application In AFM Nanolithography

The ability of block copolymers to self-assemble into a variety of high regular domains on the 10-100 nm scale depending on the polymer species involved, the molecular weight, the average composition, and the thermotropic/lyotropic processing characteristics makes them especially suited for nanotechnical applications. Unlike any other candidate materials, their nanotechnical applicability stems from not only the scale of the microdomains but also the tunability of the microdomains in shape, orderliness, orientation, size, and period afforded by changing the influential parameters mentioned above. In the past decade, block copolymer thin films have attracted much more attention than block copolymer bulk because many new nanotechnical applications, for example fabrication of high density arrays for data storage, electronics, and molecular separation.In the thesis, we first focus on the pathway and mechanism of the self-assembly of poly(styrene-ethylene/butylene-styrene) block copolymer (SEBS) thin film upon"annealing"in the vapor of cyclohexane, a selective solvent for majority poly(ethylene/butylene) block. For the contemporary microscopy, it is usually difficult or even impossible to perform an in-situ measurement to follow a dynamic self-assembly process under a certain field. To meet the challenge, we develop an AFM tip relocation technique with an ability of relocating the same 1×1μm² area after an ex-situ treatment so that one can image a sample, treat the sample ex-situ so that its morphology evolves, quench the sample, taking another image of the same area, and so on and so forth to obtain the pathway clue. In such a way, ex-situ imaging can take the place of in-situ imaging. Given that the thickness of SEBS may have influence on the self-assemble process, we also present a method which enables polymer film thickness, ranging from nano- to micromenter scale, to be facilely determined by measuring the height of an artificially created film step on smooth substrates with atomic force microscopy (AFM). It is known that AFM has the highest vertical resolution - less than 0.2 nm for most commercial systems, and thus the precision of film thickness measurements by AFM should be, in principle, extremely high.Second, based on the AFM ex-situ study of SEBS thin films with different thicknesses upon annealing in the vapor of cyclohexane, it is found that: 1) for the films with thickness of less than 11 nm, the microphase-separated morphology is not well-defined and the as-cast morphology does not change upon annealing in cyclohexane vapor; 2) for the films with thickness ranging from 13 to 17 nm, the self-assembly of SEBS films depends on the evaporation rate of the solvent. Generally speaking, the pathway information on the self-assembly from poorly ordered short cylinders, to semi-disorder phase, to spheres and then to well-ordered hexagonal spheres was obtained; 3) for the films with thickness ranging from 20 to 44 nm, the whole ordering process consists of (1) the cyclic transitions between poorly ordered cylinders and semidisordered phase via poorly ordered spheres, during which the orderliness of cylinders gradually improves, and (2) the pinching-off from enough-ordered cylinders into hex-spheres. In addition, the potential application of this solvent annealing technique to block copolymers at different molecular weight for tailoring diameters of the wellordered monolayer spherical microdomains from 18 to 38 nm and period from 30 to 61 nm has been demonstrated. That believe the use of very simple and cost efficient solvent annealing technique to produce and tune well ordered monolayer spherical structures on the tens of nanometers scale can make this kind of material a promising candidate for using as template for the creation of nanostructures.Third, an AFM tip hammering nanolithography was developed by using a vibrating AFM tip in Tapping-mode as a nano-hammer to forge SEBS block copolymer monolayer thin films after being annealed in the vapor of cyclohexane, a selective solvent for majority poly(ethylene/butylene) block, to transform their microstructure from as-cast poorly-ordered cylinders into well-ordered hexagonal spheres. Experimental results demonstrated that such structure-tailored thin films enabled macroscopic AFM tip writing to be performed in their surface: imprint and embossed patterns could be generated with a sub-20 nm linewidth resolution at low cost and high speed. In addition, as SEBS is known as a material with shape memory capability, we demonstrate that the generated lithographic patterns"HIT"can be erased within 5 minutes by thermal annealing at 70°C. Such a repteable writing and erasing capability has a promising application for high-density data storage.Finally, the dynamic behaviors of indentation recovery of the written nanopatterns were investigated at different temperatures. It was found a linier relationship exists between the recovery percentage and logarithm of time during the recovery process. Based on extrapolation method, an Arrhenius equation in the form of lg (t/t₀) = (ΔE-βε) / 2.303RT was proposed. The results suggest that a lower temperature can be used to preserve these written patterns as long as possible and a higher temperature can be used to ease the patterns generated as quickly as possible.