Materials Science and Technology of Polymers at the University of Twente

Scanning Thermal Lithography for Nanopatterning of Polymers

The aim of this Thesis is the development of scanning thermal lithography (SThL) as an alternative nanotechnology tool for scanning probe lithography (SPL) of soft condensed matter at the sub 50 nanometer length scale. Heated atomic force microscopy (AFM) cantilevers are used as spatially controlled highly localized heat sources for the thermochemical modification of tert-butyl (meth)acrylate based polymer film platforms.  The surface exposed tert-butyl esters of these polymer platforms are known to be thermally cleavable to yield carboxylic acid moieties at the air exposed interface that are readily available for subsequent wet chemical grafting reactions with relevant (bio)molecules. The length scales of heat transport, as well as effects from the proximity of solid substrates supporting the polymer films on the heated probe tip interface temperature, are discussed in detail. In addition, a systematic study on the effect of copolymer film composition on the resolution of SThL is presented, starting from poly(styrene)-block-poly(tert-butyl acrylate) (PS‑b‑PtBA) films, followed by specifically tailored tert-butyl ester exposing reactive surfaces.

A short introduction to the topics discussed in this Thesis and the motivation for the research described herein are provided in Chapter 1.

In Chapter 2a literature overview on the general topics discussed in this Thesis is presented. Cantilever design, probe tip temperature calibration and an overview of scanning thermal microscopy applications are highlighted.

In Chapter 3transient heat flow effects for a heated cantilever-tip assembly in contact with a polymer film are described. The heated probe-induced surface crystallization of amorphous poly(ethylene terephthalate) (PET) films revealed a steep short range temperature gradient close to the tip-polymer contact interface. In addition, thermal expansion measurements in poly(dimethylsiloxane) (PDMS) films with varying thickness values provided insight in the long range thermal transport during typical non steady state nanoscale thermal analysis (NanoTA) conditions. The results obtained emphasize the importance of taking heat transport from the heated cantilevers in contact with a polymer film to the environment into account for optimizing the resolution of SThL.

In Chapter 4heated probe tip-sample interface temperature (Ti) deviations from the calibrated Tifor NanoTA measurements on thin polymer films are described. With decreasing film thickness an increase in measured softening temperature by NanoTA was observed for films on good thermally conductive solid supports (e.g. silicon or glass). This effect was more pronounced for silicon compared to glass, which is ascribed to the ~ 100 times higher thermal conductivity of silicon compared to glass. The close proximity of the highly thermally conductive solid support resulted in effective lowering of Ti. The deviation of Tifrom the calibrated Tiwas several tens of degrees Celsius for 50 nm thick poly(styrene) (PS), polycarbonate (PC) and poly(methyl methacrylate) (PMMA) films on silicon substrates. Significant silicon substrate proximity effects occurred for polymer film thicknesses below several hundreds of nanometers.

In Chapter 5the melting behavior of poly(isoprene)-block-poly(ferrocenyldimethylsilane) (PI-b-PFS) crystals deposited on hydrophilic (native oxide surface of silicon) and hydrophobic (highly ordered pyrolytic graphite) surfaces was described. Following an in situ isothermal temperature controlled AFM study, the heatable AFM cantilevers were introduced as a novel tool for the rapid nanoscale thermal analysis of the PI-b-PFS crystalline architectures. NanoTA measurements on the 15 nanometer thick PI-b-PFS crystals deposited on silicon were found to be below the physical limit of the technique. This was ascribed to i) the low sensitivity of the heated cantilevers with respect to the small sample penetration of thin samples and ii) the decreased probe tip interface temperature in close proximity of the silicon substrate. The latter effect is in full agreement with the results presented in Chapter 3and Chapter 4.

Reactive imprint lithography (RIL) was introduced in Chapter 6as a new one-step approach for the combined thermal chemical surface functionalization and topographical patterning of PS-b-PtBA block copolymer films. Imprinting above the tert-butyl ester deprotection temperature yielded topographically patterned polymer films with carboxylic acid functional groups exposed at the surface. Well established wet chemical grafting reactions were used to confirm the availability of the formed carboxylic acid groups for bioconjugation. Besides the introduction of RIL as an interesting complementary approach for the preparation of reactive (bio)interfaces in a one-step process, the feasibility of thermal chemical surface functionalization of tert‑butyl ester protected acrylates was also demonstrated. Hence Chapter 6formed one of the pillars for an in depth investigation of exploiting SThL on tert-butyl ester containing polymer film platforms.

Following the successful large area thermal chemical surface functionalization with RIL, Chapter 7continues with exploring SThL for the localized spatially controlled thermochemical surface functionalization of PS-b-PtBA films. First the thermolysis kinetics and thermal deprotection mechanism of PS-b-PtBA were established followed by the introduction of SThL. The smallest domain sizes obtained remained above the critical sub 100 nm length scale, which was ascribed to the poor thermomechanical properties of the block copolymer films at the relatively high temperatures used during SThL (~ 250 °C). Hence the formed domains are the result of thermomechanical surface deformation combined with the localized thermochemical deprotection of the PtBA block. 

Therefore in Chapter 8 the development of crosslinked tert-butyl ester protected carboxylic acid groups containing (meth)acrylate based copolymer substrates for SThL is described. Crosslinking the polymer films enabled the development of thermochemical SThL that does not result in thermomechanical surface deformation. The difference in thermal deprotection mechanism for methacrylate and acrylate based films (depolymerization versus ester deprotection, respectively) was discussed in detail. The higher apparent activation energy (Ea) observed for tert-butyl ester deprotection in the tert-butyl acrylate system compared to the Eafor poly(tert‑butyl methacrylate) depolymerization significantly enhances the resolution of thermochemical surface functionalization via SThL. The chemical functionality of the deprotected tert-butyl acrylate domains (i.e. acrylic acid domains) was confirmed by fluorescence microscopy following wet chemical derivatization with a fluorescent dye. The highest achieved resolution obtained with SThL on the crosslinked tert-butyl acrylate based films is approximately 20 nm, which is below the radius of curvature of the used probes.

The work described in this Thesis contributes significantly to the development of SThL. Deepened insights in the thermal transport routes as well as in probe tip-sample interface temperatures for heated cantilevers in contact with a polymer film were obtained. Differences in thermal deprotection mechanism for tert-butyl methacrylate and tert-butyl acrylate based polymer film platforms emphasize the importance of understanding the thermal deprotection mechanism in detail in order to achieve the highest possible resolutions for SThL. Future directions envisaged to enhance the further development SThL should focus on the incorporation of multiple functionalities within the polymer film platforms as well as on increasing the patterning throughput. In addition, sharper probe tips with tip radii of curvature smaller than 30 nanometer (e.g. carbon nanotube tips) compared to the commercially available cantilevers used throughout this Thesis, are expected to further enhance the resolution.