Focused Ion Beam fabrication of defined scalable roughness structures
Authors: Hemmleb Matthias , Berger Dirk , Dziomba Thorsten 
Published: December 2016

Focused ion beam (FIB) technology is a reliable tool for the defined local surface modification on the nanoscale and therefore a promising technique to “write” a predefined texture on a point‐by‐point basis. FIB applications allow flexible scaling of surface patterns both laterally as well as in their height and have therefore the potential to become a versatile tool for purpose‐tailored roughness standards.

For surface texture creation by FIB, both real measurement data of actual surfaces and artificially defined surface models can be used. Before FIB is applied to create the desired surface roughness, the input topography data needs to be converted into control commands of the FIB instrument. For this purpose, an automated procedure has been developed. This includes the conversion of the resolution of the given surface topography data into the resolution of the FIB patterning engine and the vertical segmentation of this surface topography into equidistant height layers that are later written by FIB milling (subtractive) or deposition (additive). In addition, this software allows the projection of the data on parametric surfaces (i.e. cylindrical surfaces or spheres). Finally, all data are integrated together with supporting tasks, like orientation marks, finder grid and identifier into a patterning script, which allows the automated “writing” of the structure by FIB. While FIB deposition is not material‐dependent, the milling rate strongly depends on the specimen material. The correct milling dose has therefore to be determined experimentally before the creation of the roughness fields. In addition, the pixel size of the FIB patterning layers has to be chosen with respect to the diameter of the used focused ion beam.

In the initial tests, the actual roughness data used as input data for replication by FIB were real AFM (Atomic Force Microscopy) measurements with a nearly symmetric distribution of height values; Sq (root‐mean‐square roughness) values around 100 nm and Sz (peak‐to‐peak) values around 1000 nm. The FIB reproductions were then investigated by AFM in the same way, thus allowing best possible high‐resolution comparison of input and result. Tests were successfully performed both with a higher‐frequency and a lower‐frequency roughness to check the FIB limits. While in the beginning, both deposition and milling were tested with success, milling was chosen in the following, as it can be applied faster and thereby reduces machine hours and thus costs.

The figures show a polished Si‐specimen with a FIB‐milled roughness area of about 180 µm x 180 µm. The similarity of the FIB‐created roughness and the model data is apparent, while a closer look reveals that the highest frequency components are not reproduced by FIB. As these frequencies of rather small amplitudes are only on top of the dominating surface texture, their absence does not alter the key roughness values significantly, that agree with those of the model data within 10 % in this case.

The AFM investigations showed that FIB structuring allows the reproduction of a given surface texture on different substrates, resulting in a homogeneous, isotropic roughness. Tests with a more precise milling depth calibration proved that amplitudes are reproduced with only 1 % to 3 % deviation from the chosen model data.

Furthermore, these works prove that FIB is a unique tool to rescale a given roughness in all three directions, opening a wide range of applications, both to the smaller and to the larger sizes, over a rather broad range of dimensions: Towards lower vertical scales, tests with amplitudes downscaled down to 1/30 (i.e. Sq ~ 3 nm) were performed successfully. In the opposite direction, lateral scales were enlarged by a factor of 5 so the roughness is composed of spatial frequencies that can well be measured by most optical surface measurement techniques. Larger roughness fields require the application of advanced stitching techniques; roughness structures with a size up to 290 µm x 290 µm were already created successfully. For many practical applications, the performance of topography measurement techniques on curved surface needs to be characterized. In order to address this issue, defined roughness fields again of 290 µm x 290 µm were transferred successfully onto cylinders and spheres (both with a diameter of about 1 mm) by FIB and applied for characterization of a broad scope of instruments, including AFM, WLI (White Light Interferometry) and optical 3D microscopes.

The authors like to thank André Felgner and Peter Krebs (both PTB) for extensive AFM and CLSM measurements. This work is partly supported by the EMRP JRP IND59 “Microparts” jointly funded by the EMRP participating countries within EURAMET and the European Union.