E.S. ten Have

Moulded photoplastic probes for near-field optical applications

The inexpensive fabrication of high‐quality probes for near‐field optical applications is still unsolved although several methods for integrated fabrication have been proposed in the past. A further drawback is the intensity loss of the transmitted light in the ‘cut‐off’ region near the aperture in tapered optical fibres typically used as near‐field probes. As a remedy for these limitations we suggest here a new wafer‐scale semibatch microfabrication process for transparent photoplastic probes. The process starts with the fabrication of a pyramidal mould in silicon by using the anisotropic etchant potassium hydroxide. This results in an inverted pyramid limited by < 111 > silicon crystal planes having an angle of ∼ 54°. The surface including the mould is covered by a ∼ 1.5 nm thick organic monolayer of dodecyltrichlorosilane (DTS) and a 100‐nm thick evaporated aluminium film. Two layers of photoplastic material are then spin‐coated (thereby conformal filling the mould) and structured by lithography to form a cup for the optical fibre microassembly. The photoplastic probes are finally lifted off mechanically from the mould with the aluminium coating. Focused ion beam milling has been used to subsequently form apertures with diameters in the order of 80 nm. The advantage of our method is that the light to the aperture area can be directly coupled into the probe by using existing fibre‐based NSOM set‐ups, without the need for far‐field alignment, which is typically necessary for cantilevered probes. We have evidence that the aluminium layer is considerably smoother compared to the ‘grainy’ layers typically evaporated on free‐standing probes. The optical throughput efficiency was measured to be about 10−4. This new NSOM probe was directly bonded to a tuning fork sensor for the shear force control and the topography of a polymer sample was successfully obtained.

Photoplastic near-field optical probe with sub-100 nm aperture made by replication from a nanomould.

Polymers have the ability to conform to surface contours down to a few nanometres. We studied the filling of transparent epoxy‐type EPON SU‐8 into nanoscale apertures made in a thin metal film as a new method for polymer/metal near‐field optical structures. Mould replica processes combining silicon micromachining with the photo‐curable SU‐8 offer great potential for low‐cost nanostructure fabrication. In addition to offering a route for mass production, the transparent pyramidal probes are expected to improve light transmission thanks to a wider geometry near the aperture. By combining silicon MEMS, mould geometry tuning by oxidation, anti‐adhesion coating by self‐assembled monolayer and mechanical release steps, we propose an advanced method for near‐field optical probe fabrication. The major improvement is the possibility to fabricate nanoscale apertures directly on wafer scale during the microfabrication process and not on free‐standing tips. Optical measurements were performed with the fabricated probes. The full width half maximum after a Gaussian fit of the intensity profile indicates a lateral optical resolution of ≈ 60 nm.