Spontaneous formation of ordered micro-wrinkles on a thermosensitive resist

Self-organizing patterns with micrometer-scale features are promising for applications in photonics and bioengineering. Their spontaneous formation reduces the number of required processing steps. Here, we report an approach to spontaneously form stochastic patterns in thin skin layers on top of thermosensitive resist and an approach to align the micro-wrinkles. The skin layer is generated on top of poly(phthalaldehyde) (PPA) during a plasma etch step. When subsequently heated, the skin layer as well as PPA buckles into micro-wrinkles. Moreover, defects or purposely created edges in the skin layer/PPA induce the micro-wrinkles to orientate themselves perpendicular to the edge. For example, uniaxially aligned wrinkles are formed under the guidance of a straight step. Methods for spontaneously forming periodic micro-/nanostructures have received considerable attention for lithography-free patterning applications [1-2]. It is in particular very attractive as it can be simply scaled to large area, potentially enabling higher throughput and lower cost than conventional top-down lithography processes. These techniques have primarily exploited phase separation of block copolymers or strain-induced wrinkling of polymeric films, such as polydimethylsiloxane (PDMS) [3]. Block copolymers are effective for patterning sub-100-nm features [4]. Thin-film wrinkling can form structures with periodicities ranging from 0.4–10 μm [2-3], and thus it is promising for photonic and optoelectronic applications in the near-infrared and visible regions. Although wrinkling approaches have remarkable control and tunability over pattern formation, aligned patterns are more challenging to realize. Here, we first report an approach to spontaneously form surface micro-wrinkles in PPA, a polymer widely used for dry lithography. Figure 1 shows the fabrication process of the surface micro-wrinkles. First, the thin layer (20-200 nm) of PPA is exposed to a C4F8/He plasma. During this step a hydrofluorocarbon (HFC) composite is formed by the C4F8 plasma which modifies the surface [5]. Therefore, a skin layer composed of hydrogen, fluorine and carbon is generated on top of PPA. When subsequently heated to 135 °C, compressive stress in the skin layer induces buckling thereby deforming the underlying PPA. Figure 2 shows the result of the fabricated micro-wrinkles. On a uniformly flat surface, the micro-wrinkles on the underlying PPA substrate form an arbitrary pattern consisting of randomly oriented domains with periodicity of 1.6–3 μm for an underlying PPA thickness of 136 nm (Fig 2a). We can control the periodicity of the micro-wrinkles by adapting the PPA thickness. The periodicity decreases with decreasing the PPA thickness (Fig 2b). AFM images (Fig 3) of the micro-wrinkles show sinusoidal patterns with a peak-to-peak amplitude of about 150 nm. Additionally, by introducing a step (Fig 4a), the micro-wrinkles self-orient and form an ordered pattern (Fig 4). It is found that the micro-wrinkles preferentially orientate in one direction. For example, when a straight line is created in the skin layer, the micro-wrinkles are ordered in parallel (Fig 4b). When two steps are introduced, the micro-wrinkles can bend with the two ends perpendicular to the two steps, respectively (Fig 4c).