Self-assembled monolayers (SAMs) are well-known and have been intensively studied for many years, partly because of their interesting properties and partly because of interesting perspectives for potential applications as functional, ultrathin coatings [2
]. Due to their functionality SAMs play an important role for the construction of sensors [6
] or, e.g., the controlling of cell adhesion [8
]. Patterning of self-assembled monolayers on the nanometer scale is easily performed by sequential lithographic techniques that are well-established in the literature.
Electron beam lithography allows the desorption or destruction of molecules of a SAM layer, line by line [9
]. Advanced scanning force microscopy (SFM) techniques allow not only the imaging of the topography of surfaces but also the spatially resolved study of surface properties, such as the electrical, elastic, tribological and wear properties [11
]. At the same time, scanning-force-microscopy-based lithographic techniques allow the structuring and patterning of surfaces with a lateral resolution down to the nanometer scale [24
]. The advantage of techniques such as electron beam lithography or SFM-based lithography is their high lateral resolution and their reproducibility; their major disadvantage is the fact that they rely on sequential writing processes, which are very time consuming and require expensive equipment. For patterning larger areas on the nanometer scale, e.g., for the fabrication of nanopatterned, biofunctional templates, easy-to-use, cheap and fast techniques allowing the parallel fabrication of billions of nanostructures are required.
Phase separation of binary polymer blend solutions during a spin-coating process produces nano- and micropatterns on large areas in a fast and scalable fashion. This phase separation has been intensively studied over the past two decades and allows the formation of complex layered or lateral micro- or nanoscale structures [31
]. These structures can be used for many applications, such as antireflection coatings [39
], photovoltaic devices [40
], organic light-emitting diodes (OLED) [42
] and more. Polymer phase separation in thin films can be obtained by methods such as spin coating [31
] and Langmuir–Schaefer deposition [45
]. In the case of the spin-coating technique it is possible to guide the morphogenesis by employing a prepatterned solid template in order to form layout-defined structures [46
]. However, so far there is no direct way to use the resulting polymer blend film as a lithographic mask, because the formed structure contains both lateral and layered phase separations [49
]. Special techniques, such as UV curing have to be combined to make the film ready for lithographic applications [52
]. Zemla et al. [52
] describe a technique where after cross-linking one polymer, the other one is removed, and a protein is adsorbed at the free surface areas. The second polymer, however, cannot be dissolved due to the cross-linking and remains on the substrate. Kawamura et al. [53
] use the difference in resistance to photo-etching between the two polymers in the blend to remove the component with less stability under photo-irradiation. The remaining micropatterned polymer layer has a thickness of about 3 nm, albeit without a well-defined surface chemistry.
Here, we are aiming for a lateral polymer phase morphology that can be completely removed by a selective solvent to make the substrate available for well-defined chemical surface modification. This can be achieved by inserting a silane SAM, which then exposes a functional group. The preparation process of the SAM should not affect the remaining polymer mask, such that it can protect the substrate during the procedure and can be removed afterwards. For the spin-coating of polymer blend films, there are many parameters and conditions, such as the concentration of the polymer solution, the spin rate, and the surface property of the substrate, among others, that affect the final morphology of the polymer blend film. Some examples of both the influence of the substrate [54
] and the solution parameters [49
] can be found in the recent literature. We found that the formed polymer blend structures in our case are also strongly dependent upon the relative humidity during the demixing. The relative humidity influences the interaction of the two polymer phases and the affinity of the polymers to the substrate [59
]. This effect has to be distinguished from the formation of so-called breath figures, which are formed at high relative humidity (over 60%) due to water condensation on the evaporatively cooled polymer solution [60
]. The breath figure technique can be applied to generate nearly hexagonal arrays of holes [61
] or for the fabrication of 3-D structures [62
]. Water droplets are introduced into the polymer solution film and leave behind holes after the film has solidified. These breath figure structures can be found both in films of one-polymer systems, such as PMMA in THF, and of polymer-blend systems.
In this article we present a method to obtain a polymer-blend film with a purely lateral phase morphology, which means that the blend separates completely into two lateral phases. The introduction of a small amount of water during the spin coating process is crucial for obtaining this purely lateral morphology. Either of the two different polymers can be dissolved independently afterwards by using a selective solvent. The remaining morphology is later on applied directly as a lithographic mask to fabricate nanopatterned self-assembled-monolayer (SAM) templates. Performed at higher humidity, our technique combines polymer-blend phase separation with the breath-figure formation. A three-phase lithographic mask is formed in one process step, giving the opportunity to produce a SAM template with three different chemical functionalities.