B. Völkel

Determination of the specific resistance of individual freestanding ZnO nanowires with the low energy electron point source microscope

A low energy electron point source microscope is used to determine the electrical conductivity of freestanding ZnO nanowires. The nanowires were contacted with a manipulation tip and I-V curves were taken at different wire lengths. From those, the specific resistance was calculated and separated from the contact resistance. By comparing the specific resistances of ZnO nanowires with diameters between 1100 and 48nm, a large surface contribution for the thin nanowires was found. A geometric model for separation between surface and bulk contributions is given.

Influence of secondary electrons in proximal probe lithography

This article describes the limitations of proximal probe lithography due to electrons that are mirrored by the electric field between the tip and the surface. The incident beam generates two kinds of electrons at the sample surface: primary electrons which are elastically backscattered and secondary electrons which are produced in the resist/substrate system. The electric field confines the electrons emanating from the surface. The electron trajectories are bent in such a way that the electrons impinge on the sample surface in the vicinity of their origin. These reflected electrons contribute to the exposure of the resist and therefore, limit the resolution. For hexadecanethiol monolayers on gold substrates, we have measured the energy distribution of the mirrored electrons and the secondary electron yield as a function of the primary energy. With near edge x-ray absorption fine structure spectroscopy, we have investigated the relevance of low energy electrons in the exposure of hexadecanethiol films. Sim...

Emission properties of electron point sources

Abstract Emission properties and stability of ultrasharp 〈111〉-oriented tungsten tips were studied in a combined field ion/electron microscope and with an electron energy analyzer. In particular the divergence angle ϕ of the emitted electrons is important for novel developments in electron microscopy. It is found that this divergence angle depends only weakly on the geometry of the ultimate tip (monomer, trimer) but shows a strong variation with overall tip radius, with dull tips providing smaller divergence angles. For emission currents up to 100 nA we found only a slight dependence of the divergence angle on current. At higher currents, however, we observed a distinct increase of this angle, which is attributed to Coulomb interactions between the electrons. The energy distribution of the field-emitted electrons showed only weak deviations from standard field-emission theory.

Electrode modification by electron-induced patterning of self-assembled monolayers

Gold electrodes were coated with self-assembled monolayers of 1-1′-biphenyl-4-thiol(C6H5–C6H4–SH, BPT) and 1-octadecanethiol (CH3–(CH2)17–SH, ODT) and patterned by proximity printing with stencil masks using electron energies of 300 eV and area doses of 40 000 μC/cm2 (BPT) as well as 10 000 μC/cm2 (ODT). The subsequent copper deposition in an electrochemical cell revealed that e-beam patterned alkanethiol behaves opposite to that of e-beam patterned biphenyl. ODT acts as a “positive template” leading to copper deposition only on the irradiated parts. BPT on the other hand acts as a “negative template,” where the irradiated and therefore cross-linked biphenyl layer exhibits a blocking behavior, and hence copper is only deposited on the nonirradiated parts. The influence of the deposition potential and the copper ion concentration of the electrolyte on the process has been investigated to find the optimal parameters for producing nanostructures with high selectivity.

Holographic imaging of macromolecules

The low energy electron point source microscope (kendroscope) is used to acquire in-line holograms of rodlike macromolecules of phthalocyaninato polysiloxane (PcPS). The molecules were placed on a new class of substrates, thin microstructured silicon membranes that have slits about 100 nm wide which can be “bridged” by the molecular chains. The resulting in-line holograms of PcPS were recorded digitally and reconstructed numerically via a Kirchoff–Helmholtz transform. The mechanisms of image formation and uncertainties relating to the interpretation of the holograms are discussed.

Low energy electron proximity printing using a self-assembled monolayer resist

Abstract A simple method for high resolution ( nm ) lithography is reported. We use electrons with energies ranging from 100–300 eV emitted by tungsten field emission tips for proximity printing of stencil masks. A comparison with other parallel fine line techniques, like proximity printing and projection lithography with x-rays, high energy electrons or ions, reveals the specific advantages and restrictions of our method. The masks are made of ≈ 100 nm thick silicon membranes structured by e-beam lithography and reactive ion etching (RIE). Free standing gratings with periods down to 100 nm serve as test patterns for proximity printing with gaps of ≈15μ m . due to the short penetration depth of the low energy electrons, ultrathin resist systems are needed. We have chosen self-assembled monolayers (SAMs) of hexadecane thiol on gold. The monolayer resist is degraded by the exposure, the structures can be transferred by wet chemical etching. Periodic gold structures below 100 nm lines and spaces have been generated this way. It was found, that the SAM resist of hexadecane thiol can be used as a positive or negative tone resist depending on the exposure dose.

Nanostructuring of alkanethiols with ultrasharp field emitters

We have investigated the electron beam exposure of self‐assembled monolayers (SAM) of alkanethiols on gold. SAM resist systems are of growing interest for proximal probe lithography due to their high chemical selectivity though being as thin as a monolayer. For this purpose a custom built proximal probe lithography device was used. The electrons emanate from an ultrasharp tungsten tip, providing extraordinary field emission properties—small emission angle and low emission energy. In contrast to common scanning tunneling microscope (STM) lithography our system can be operated not only in tunneling mode, but also in true field emission mode, i.e., at much larger tip‐to‐sample distances (typically 10–100 nm). Therefore the latter mode allows much higher writing speeds, as the tip does not have to follow the sample topography as accurately as in the STM mode. The first results obtained by exposure in the field emission mode are presented.

Nanometer Resolution Holography with the Low Energy Electron Point Source Microscope

Holography with the low energy electron point source (LEEPS) microscope has been explored for imaging of unstained biomolecules. DNA ropes are prepared on microfabricated sample supports and imaged with coherent 40 eV electrons emitted from a W(111) point source. We reconstruct the electron wave front and show that the correct source-to-object distance can be determined by analyzing the focus of the reconstruction. Upon variation of the source position, the reconstruction shows reproducible object features with an object focus located at the correct tip-to-object distance with a depth of sharpness of ∼40 nm. It is argued that reconstruction artifacts can be identified by their focus behavior and that only features in the first focus – coming from the screen – truly relate to the objects structure.

Optimization of the low energy electron point source microscope: imaging of macromolecules

Optimal conditions for low energy electron point source microscopy are investigated by the simulation and numerical reconstruction of holograms of phthalocyaninato polysiloxane, PcPS, a rod-like macromolecule. The effects of the electron energy, width of the electron beam and the detector size on the spatial resolution in the reconstructed images are modeled. We find that for electron energies around 200eV, with the specimen 0.1 microm from the source, a screen recording the image in a cone of at least 15 degrees half angle (7 cm lateral dimension at 10 cm from the source) with at least 512 x 512 pixel and 8-bit resolution will result in near atomic resolution.