Imaging scatterometry for flexible measurements of patterned areas.

Characterization of micro/nano-textured surfaces is time consuming using scanning probe and electron microscopy techniques. Scatterometry, where the intensity of scattered light is used as a 'fingerprint' to reconstruct a surface, is a fast and robust method for characterization of gratings. However, most scatterometry techniques are measuring the averaged signal over an area equal to the spot size of the light source. In this paper we present the imaging scatterometry technique, which is capable of locally measuring topographic parameters of gratings spanning an area down to a few µm(2) with nm accuracy. The imaging scatterometer can easily find areas of interest on the cm scale and measure multiple segments simultaneously. We demonstrate two imaging scatterometers, one built into an optical microscope and one in a split configuration. The two scatterometers are targeted characterization of mm(2) and cm(2) areas, respectively, and both setups are validated using nano-textured samples.

Alignment-free characterization of 2D gratings.

Fast characterization of 2D gratings is demonstrated using a Fourier lens optical system and a differential optimization algorithm. It is shown that the grating-specific parameters such as the basis vectors and the angle between them, along with the alignment of the sample, such as the rotation of the sample around the x, y, and z axis, can be deduced from a single measurement. More specifically, the lattice vectors and the angle between them have been measured, while the corrections of the alignment parameters are used to improve the quality of the measurement and, hence, reduce the measurement uncertainty. Alignment-free characterization is demonstrated on a 2D hexagonal grating with a period of 700 nm and a checkerboard grating with a pitch of 3000 nm. The method also can be used for automatic alignment and in-line characterization of gratings.

Cell membrane conformation at vertical nanowire array interface revealed by fluorescence imaging.

The perspectives offered by vertical arrays of nanowires for biosensing applications in living cells depend on the access of individual nanowires to the cell interior. Recent results on electrical access and molecular delivery suggest that direct access is not always obtained. Here, we present a generic approach to directly visualize the membrane conformation of living cells interfaced with nanowire arrays, with single nanowire resolution. The method combines confocal z-stack imaging with an optimized cell membrane labelling strategy which was applied to HEK293 cells interfaced with 2-11 µm long and 3-7 µm spaced nanowires with various surface coatings (bare, aminosilane-coated or polyethyleneimine-coated indium arsenide). We demonstrate that, for all commonly used nanowire lengths, spacings and surface coatings, nanowires generally remain enclosed in a membrane compartment, and are thereby not in direct contact with the cell interior.

Structural phase control in self-catalyzed growth of GaAs nanowires on silicon (111).

Au free GaAs nanowires with zinc blende structure, free of twin planes and with remarkable aspect ratios, have been grown on (111) Si substrates by molecular beam epitaxy. Nanowires with diameters down to 20 nm are obtained using a thin native oxide layer on the Si substrates. We discuss how the structural phase distribution along the wire length is controlled by the effective V/III ratio and temperature at the growth interface and explain how to obtain a pure twin plane free zinc blende structure.

Tuning InAs nanowire density for HEK293 cell viability, adhesion, and morphology: perspectives for nanowire-based biosensors.

Arrays of nanowires (NWs) are currently being established as vehicles for molecule delivery and electrical- and fluorescence-based platforms in the development of biosensors. It is conceivable that NW-based biosensors can be optimized through increased understanding of how the nanotopography influences the interfaced biological material. Using state-of-the-art homogenous NW arrays allow for a systematic investigation of how the broad range of NW densities used by the community influences cells. Here it is demonstrated that indium arsenide NW arrays provide a cell-promoting surface, which affects both cell division and focal adhesion up-regulation. Furthermore, a systematic variation in NW spacing affects both the detailed cell morphology and adhesion properties, where the latter can be predicted based on changes in free-energy states using the proposed theoretical model. As the NW density influences cellular parameters, such as cell size and adhesion tightness, it will be important to take NW density into consideration in the continued development of NW-based platforms for cellular applications, such as molecule delivery and electrical measurements.

Influence of the oxide layer for growth of self-assisted InAs nanowires on Si(111).

The growth of self-assisted InAs nanowires (NWs) by molecular beam epitaxy (MBE) on Si(111) is studied for different growth parameters and substrate preparations. The thickness of the oxide layer present on the Si(111) surface is observed to play a dominant role. Systematic use of different pre-treatment methods provides information on the influence of the oxide on the NW morphology and growth rates, which can be used for optimizing the growth conditions. We show that it is possible to obtain 100% growth of vertical NWs and no parasitic bulk structures between the NWs by optimizing the oxide thickness. For a growth temperature of 460°C and a V/III ratio of 320 an optimum oxide thickness of 9 ± 3 Å is found.