Micro/Nano-Physical and Chemical Architecture of Polymer Surfaces by Ultra-Violet Exposure and Scanning Probe Microscopy

Abstract

Intentional manipulation on the micro- and nano-scales of surface structures and chemistry on three model polymers, by UV exposure and scanning probe microscopy methods, has been the focus in this study. A general framework has been built, upon which the 'marriage' of the two techniques could be explored in order to refine and improve nano-scale manipulation as a future enabling technology. It has been shown how the early developments in microscopy and manipulation have paved the way for research in the broad area of surface and interface science and technology, and has provided the setting for the present work. Nano-mechanical effects must be taken into account in the case of manipulation of a soft surface such as a polymer. The outcomes of manipulation were found to depend on both in- and out-of-plane forces acting at the point of contact. Therefore quantification of the interactions that stimulate deformational modes have been described. The responses of the lever to changes in scan direction and (in contact) force loading, and for isotropic friction in the x-y plane, have been defined. A linear relationship between the normal force loading and the in-plane friction force has been adopted as a first approximation. A simple description of the functional dependence of the frictional force over a large dynamic range of force loadings is inadequate; thus the emphasis has been placed on describing and explaining phenomenological trends. In the multi-asperity regime for 'hard' surfaces a linear dependence is often observed at high loads. UV patterning of polymeric surfaces on the micro scale has been described in chapter 3. It has been shown that the irradiation process (either via the excimer laser or a UV lamp) is capable of physical and chemical alteration of the surface. Topographical, micro-mechanical and lateral/chemical differentiation has been observed for all three polymer surfaces. For instance, P(tBuMA) and polyimide surfaces exhibit shrinkage, while the PDMS sample exhibits swelling. By using tips with varying chemistries and working in air and under water, the adhesive, lateral force and snap-on features were analysed in greater detail. Surface adhesion, snap-on features and lateral forces all increased for P(tBuMA) as a result of irradiation for all tip combinations, when analyzed in air. Due to a native oxide layer being present on all as-received tip surfaces (except when Au-coated), a uniform hydrophilic surface is created, resulting in a similar tip-to-surface interaction for all tips. Polyimide was examined in air with similar results. However when analyzed in water, the lateral force contrast was reversed due to the capillary interaction being eliminated, thus revealing the underlying tip-surface interactions. Details of the snap-on features revealed long-range attraction prior to snap-on in air, but repulsion was observed in water. The repulsion is due to a combination of electrostatic and van der Waals interactions. The PDMS surface, on the other hand, exhibited increased adhesion and lateral force on the unirradiated surface in air and on the irradiated surface in liquid. Those results are a consequence of the irradiation process; PDMS surfaces undergoing UV irradiation in the presence of air generally become hydrophilic as a consequence of surface activation and oxidation, causing hydroxylation of the surface and a change in surface chemistry. Thus the meniscus force is likely to account for the reversal in trends. It has been demonstrated in chapter 4 that it is possible to create three-dimensional structures on a soft polymeric surface on the nanometer scale using the SPM probe.

In the cases of P(tBuMA) and PDMS samples, it was shown that manipulation required levers with spring constants up to 1 nNnm-1, whereas imaging could take place with no further alteration of the surfaces with levers with spring constants less than 0.5 nNnm-1. The P(tBuMA) sample was not only able to be altered physically by creating various wells and troughs, but also chemically as revealed by friction loop analysis showing a lower lateral force within a well. Elastic recovery of up to 90% and a 'planeing' effect of the tip over the surface were also evident. An increase in the height of material build-up and depth of troughs was observed during the acquisition of f-d curves at varying loading forces. It was also shown that a decrease in the angle of attack (relative to the horizontal), caused an increase in the lateral force and a decrease in lithographic efficiency. Lithographic results on the PDMS surface revealed stick-slip phenomena in both the fast and slow directions of travel. Friction loop analysis revealed that the lateral force increased with the strength of trapping of the tip. An increase in the yield of stick-slip features, including an increase in dynamic stick-slip amplitude, spacing of stick points/lines and subsequent breakdown just prior to slipping has also been documented. The surface exhibited significant in-plane relaxation, i.e., the stick point was displaced by the tip due to lateral forces being imposed. Relaxation ranged from ~ 1.5 µm for weak trapping and increased to ~ 5.2 µm for strong trapping toward the end of a stick-slip cycle, at a loading force of 1113 nN. Thus there was an increase in trapping force with cycle progression. An increasing loading force resulted in an increase in in-plane displacement and a greater spacing between the stick lines in the slow scan direction. A decrease in trough length in the fast scan direction is also observed as a result of an increase in static friction with normal force, resulting in a greater surface relaxation and shorter track length for sliding friction. Stick-slip behaviour was found to be dependent on loading force, attack angle, scan speed and image resolution. An increasing loading force resulted in an increase in lateral force, decrease in the trough number per unit stage travel, increase in excavated depth and increased spacing between troughs. The attack angle affected trough number and spacing as a result of the out-of-plane force component being a significant contributing factor when the fast scan direction was aligned with the y-direction. No visible effect on lateral force was observed, however, presumably due to the active feed-back loop being able to adjust the force loading back to its set point. As the scan speed increased, the lateral force and stick-slip amplitude increased, whereas the number of troughs per unit stage travel decreased. By increasing the number of scan lines along the y-direction (increase in image resolution), the trough depth increased with a decrease in trough number.

A comparison of the three polymers and a clean silicon surface revealed the dependence of in-plane deformation and lateral force on surface properties. PDMS showed the greatest degree of indentation, with polyimide and silicon surfaces being essentially incompressible. Friction loop data show a lateral force of 868 nN and 1421 nN at the start and end, respectively, of the cycle on the PDMS surface, 316 nN on the P(tBuMA), and 158 nN for the polyimide and silicon surfaces for a constant loading force of 972.5 nN. There was a linear relationship between loading force and lateral force for all surfaces. The coefficient of friction was found to be ca. 5.3, 6.2 and 6.8 for the beginning, middle and end, respectively, of the cycle on the PDMS surface, ca. 1, 0.46 and 0.26, on P(tBuMA), PI and silicon surfaces respectively. The project has shown that the lithographic efficiency is not only dependent on the methodology utilized in the manipulation process, but that there is also a contribution from the properties of a polymer surface itself. Irradiation and tip-induced manipulation alters the surface both physically and chemically, however the properties of the polymer also influences the outcome, as in the case of PDMS inducing a stick-slip mechanism. The overall outcome of the project has extended and enhanced the scientific basis for micro/nano-scale lithographic processing as a basis for hypothetical future enabling technologies. With SPM facilities now being available in most R&D institutions their importance can be realized and full capabilities be exploited. The many emerging fields of research and development, particularly those with a focus on nano-scale surface/interface engineering, can benefit from some form of controlled and purposeful manipulation of various organic and inorganic surfaces. A union of relevant techniques and expertise, will allow further future exploration and exploitation of novel technologies.