Compaction of PDMS due to proton beam irradiation

Complete text of publication follows. This work is about the detailed investigation of the changes of the surface topography, the degree of compaction/shrinkage and its relation to the irradiation fluence and the structure spacing in poly(dimethylsiloxane) (PDMS) patterned with 2 MeV proton microbeam. Sylgard 184 kit (Dow-Corning) was used to create the PDMS samples. The density of the PDMS samples was determined with pycnometer. The penetration depth for 2 MeV protons is ∼85 μm, the PDMS layer was ∼95 μm thick, so the incident protons stop in the PDMS, they do not reach the substrate. The irradiations have been performed at the nuclear microprobe facility at ATOMKI. The irradiated periodic structures consisted of parallel lines with different widths and spacing. To achieve different degrees of compaction, each structure was irradiated with five different fluences. The surface topography, the phase modification of the surface, and the connection between them were revealed using an atomic force microscope (AFM PSIA XE 100). The shrinkage data were obtained from the topography images. The structures with different line widths and spacing show different degrees of compaction as a function of irradiation fluence. By plotting them in the same graph (Fig. 1) it is clearly seen that the degree of compaction depends on both the irradiation fluence and the distance of the structures. The fluence dependence of the compaction can be explained with the chemical changes of PDMS. When an energetic ion penetrates through the material it scissions the polymer chain, whereupon among other things volatile products form. In the case of PDMS, these are mainly hydrogen, methane and ethane gases that can be released from PDMS. The irradiated volume shrinks due to significant structural change during which silicate derivatives (SiOx) are formed. The phase change and the corresponding surface topography was compared and studied at all applied irradiation fluences. It was concluded that at low fluences the surface topography does not follow the phase change that corresponds to the irradiation pattern. At higher fluences the surface topography follows the irradiation pattern, i.e. the phase change, quite well but not perfectly. The irradiated regions are compacted significantly but their bottom is not flat. Between the irradiated regions, the unirradiated areas show a regular curved surface. This can be explained with the developed stress caused by compaction due to irradiation. During irradiation the irradiated areas start to shrink while the unirradiated areas try to remain unchanged. The topographical transition between the two phases is not a step function, but it is continuous due to the rubbery nature of PDMS. The PBW technique, that utilizes a focused MeV ion beam, is capable of the production of devices in PDMS, which have curved-edge relatively shallow, a couple of microns deep, structures on the surface. The curvature of the surface and the degree of compaction can be adjusted with the applied ion fluence and with the distance of the structures. By choosing the proper irradiation parameters short or even long range symmetrical surface curvatures can be achieved.