Louis Hellemans

Characterization of a point‐contact on silicon using force microscopy‐supported resistance measurements

A conductive atomic force microscope (AFM) tip based on B‐implanted diamond has been developed for the determination of the spatial distribution of charge carriers in semiconducting structures. The characteristics of this tip have been determined by studying the current–voltage behavior as a function of substrate resistivity and tip load. From this work a model of the electrical properties of the microcontact is emerging. It includes an Ohmic contribution to the overall resistance, which is related to the plastically deformed area, and contributions from a barrier. The tip imprints have been imaged with AFM and their physical dimensions are seen to match the requirements of the model. From resistance measurements on uniformly doped silicon a calibration curve has been established which can be used as a standard to convert measured resistance into resistivity.

Cross-sectional nano-spreading resistance profiling

The nano-spreading resistance profiling (nano-SRP) method has been developed and improved such that it can now be used as an accurate tool for quantitative two-dimensional carrier profiling. Instrumental improvements include the use of batch-fabricated, conducting diamond-coated silicon probes, and a low-noise logarithmic current amplifier. The spatial resolution (10 nm), the dynamic range (1014–1020 atoms/cm3), and the sensitivity (1014 atoms/cm3) of the nano-SRP technique are illustrated by profiling a wide range of state-of-the-art device structures. Two-dimensional measurements of the carrier distribution inside fully processed metal–oxide–semiconductor transistors with gate lengths varying from 2 μm down to 0.25 μm illustrate the strength of the technique to map present and future devices. The nano-SRP method currently has sufficient resolution to demonstrate the small asymmetry in the source/drain profiles from transistors in which the sample was not rotated during the 7° implant. The electrical tra...

Evaluating probes for “electrical” atomic force microscopy

The availability of very sharp, wear-proof, electrically conductive probes is one crucial issue for conductive atomic force microscopy (AFM) techniques such as scanning capacitance microscopy, scanning spreading resistance microscopy, and nanopotentiometry. The purpose of this systematic study is to give an overview of the existing probes and to evaluate their performance for the electrical techniques with emphasis on applications on Si at high contact forces. The suitability of the characterized probes has been demonstrated by applying conductive AFM techniques to test structures and state-of-the-art semiconductor devices. Two classes of probes were examined geometrically and electrically: Si sensors with a conductive coating and integrated pyramidal tips made of metal or diamond. Structural information about the conductive materials was obtained by electron microscopy and other analytical tools. Swift and nondestructive procedures to characterize the geometrical and electrical properties of the probes p...

The fabrication of photonic band gap materials with a two-dimensional defect

Colloidal crystals with three-dimensional periodicities in the refractive index have a photonic band gap (PBG) in which electromagnetic waves are forbidden. We present a method to fabricate stacked colloidal crystals containing a two-dimensional defect as a middle layer by combining vertical deposition method with the Langmuir–Blodgett (LB) technique. The defect layer introduces an impurity mode within the optical stop band, which is observed as a defect peak (pass band) in the optical density spectrum. The result shows that the combination of vertical deposition with LB technique provides a way for introducing defect modes in PBG materials.

Dynamical and mechanical study of immobilized microtubules with atomic force microscopy

The dynamics of the assembly/disassembly reaction play an important role in the study of microtubule (MT) polymers. The polymerization rate constant (2±1×10−3 s−1) of this process was determined on the basis of atomic force microscopy images of air‐dried samples, taken at discrete time intervals. The decay of tubulin oligomers (rings) is ten times faster, and consequently is not a rate‐limiting step. The time dependence of turbidity measurements is compared with atomic force microscopy results expressed as the average MT length and mass per unit surface. Further, the elastic behavior of microtubules under liquid is analyzed as a function of a cross‐linking agent’s concentration (glutaraldehyde). Recording force–distance curves allows the indentation to be determined, from which Young’s modulus E is derived. Extrapolating these data provides the intrinsic stiffness of the MTs (E=3.1±0.9 MPa). MTs prepared in the presence of the drugs taxol and taxotere have values of, respectively, 1.3±0.6 and 2.7±1.2 MPa ...

Absorption and dispersion of the field induced dielectric increment in caprolactam–cyclohexane solutions

The nonlinear dielectric effect in solutions of e‐caprolactam in cyclohexane is studied by a new differential technique over the frequency range of 2 to 100 MHz. The frequency dependent real and imaginary parts of the field induced permittivity increment are obtained. Cole–Cole plots show a concentration dependent dielectric relaxation, to which a single Debye term is fitted. The relation of relaxation times with concentration is not consistent with a simple dimerization mechanism. Static permittivity measurements indicate the presence of polar associates. An expression is derived including those contributions to the observed nonlinear effect, which are frequency independent within the range studied. Calibration measurements on diethyl ether–cyclohexane solutions are reported and the effect measured in pure cyclohexane is discussed.

Optical properties and orientation of arrays of polystyrene spheres deposited using convective self-assembly

We have prepared face centered close packed arrays of polystyrene spheres using convective self-assembly. The optical properties of the arrays of spheres were studied and revealed high quality crystals that could be prepared using this method. Optical transmission measurements over a wide spectral range reveal the presence of multiple diffraction peaks. Their use for checking the quality of the arrays is proposed. The orientation of the arrays formed was studied using optical diffraction and atomic force microscopy. Both indicate the formation of oriented crystals. This was until now only reported for epitaxial growth of colloidal crystals. The oriented crystals were used to measure their photonic band structure.

One‐ and two‐dimensional carrier profiling in semiconductors by nanospreading resistance profiling

Measurement of the carrier concentrations in silicon with lateral and in‐depth resolution on the sub‐100 nm scale has been demonstrated with an atomic force microscope (AFM) using conducting tips. The technique determines the local spreading resistance and hence inherits the attractive features of the conventional spreading resistance profiling (SRP) technique and will be referred to as nano‐SRP. For instance, the calibration curve established by measuring homogeneously doped substrates indicates a dynamic range of concentrations between 1014 and 1019 cm−3 and a monotonic relation between resistance and resistivity, similar to a conventional SRP calibration curve. In the present study, W‐coated diamond tips are used at a precisely controlled force of 70 μN, leading to a contact radius of 50 nm as determined from AFM analysis of the resulting imprints. The drastic reduction of the contact size implies that one can measure directly on a vertical cross section of the structure and overcome some limitations c...

Immobilizing and imaging microtubules by atomic force microscopy

Microtubules isolated from pig brains have been immobilized on an inorganic substrate for use in AFM studies. The method employs 4-aminobutyldimethylmethoxysilane and glutaraldehyde to activate a silicon wafer for binding the biopolymer. The covalent bond ensures the positional stability of the tubules on the substrate, and allows reproducible scanning probe experiments. Microtubules have been imaged both by atomic force and scanning tunneling microscopy, yielding results very similar to electron microscopy. The average apparent height of the tubules is smaller than observed with transmission electron microscopy (25 nm) and is smaller in buffer solution (10 nm) than in air (15 nm). The biopolymer surface is softer under buffer than in air. The highest resolution was obtained with the tapping mode where surface features as small as 10 nm in X and Y have been resolved. Gold-coated tubules bound on silicon have been successfully imaged by STM, while images of uncertain origin were generated for tubules deposited on graphite. It is shown that artefacts imaged on a blank graphite surface can easily be confounded with collapsed tubules.

Influence of oxygen on the formation of ripples on Si

An extensive study of the ripple formation on Si is presented. The ripples are characterized with atomic force microscopy (AFM) as a function of sputter depth and also the effect of the introduction of oxygen gas near the sample is investigated. Based on these results, a two‐step model is proposed for the formation of the ripples on Si. The first step relates to the formation of small topography (seeds) caused by the heterogeneity of the internal layer. The second step relates to the rapid development of these seeds into the regular ripple structure. The driving force for the latter is the surface oxidation of the different faces of the ripples. The validity of the separation in two steps is additionally checked by studying the effect of deliberately roughened samples. The chemically induced topography with a height of no more than 10 nm is shown to move forward the transient region quite severely suggesting that the origin of the roughness does not have any influence on the subsequent ripple formation. I...