Diamond tips and cantilevers for the characterization of semiconductor devices

Highly boron doped diamond tips on diamond cantilevers have been prepared by means of the moulding technique and been tested for applications in scanning probe microscopy measurements for the characterization of semiconductor devices. Tips doped with ca 1% boron show a resistivity of some 10⁻³ Ωcm; their radius of curvature is ca 20 nm. With respect to wear resistance, they are superior to either nitride probes also fabricated by the moulding technique or to tapping mode silicon tips. Finally, first measurements revealed the suitability of such probes for applications in scanning spreading resistance microscopy, scanning capacitance microscopy and nanopotentiometry measurements.     

Diamond nucleation on iridium: Local variations of structure and density within the BEN layer

The diamond nuclei generated by the bias enhanced nucleation (BEN) on iridium are gathered in well defined areas (“domains”). In atomic force microscopy (AFM) measurements they become manifest in a 1 nm downward step. The fine structure of the carbon layer inside and outside these domains has been studied by small spot Auger electron spectroscopy (AES), high resolution transmission electron microscopy (HRTEM), AFM and lateral force microscopy (LFM). The Auger spectra of the carbon KLL peak taken in an ultra high vacuum setup revealed diamond features inside and more graphitic features outside the domains. The comparison with the intensity of the Auger signal originating from the underlying Ir film indicates a carbon coverage inside the domains which is only by about 20% lower than outside. Cross section HRTEM images after BEN and after a short growth step, as well as AFM measurements after softly etching off the carbon layer, give no indication that the Ir had experienced a major modification by the domain formation process. Combining the information deduced from these experiments we conclude a significantly higher density of the carbon matrix within the domains. The lower friction forces measured in these regions by LFM confirm the interpretation of a material with higher elastic modulus. The present results allow to refine our understanding of structure and formation of the BEN layer on Ir.     

Microscopic study of electrical properties of CrSi<Subscript>2</Subscript> nanocrystals in silicon

Semiconducting CrSi₂ nanocrystallites (NCs) were grown by reactive deposition epitaxy of Cr onto n -type silicon and covered with a 50-nm epitaxial silicon cap. Two types of samples were investigated: in one of them, the NCs were localized near the deposition depth, and in the other they migrated near the surface. The electrical characteristics were investigated in Schottky junctions by current-voltage and capacitance-voltage measurements. Atomic force microscopy (AFM), conductive AFM and scanning probe capacitance microscopy (SCM) were applied to reveal morphology and local electrical properties. The scanning probe methods yielded specific information, and tapping-mode AFM has shown up to 13-nm-high large-area protrusions not seen in the contact-mode AFM. The electrical interaction of the vibrating scanning tip results in virtual deformation of the surface. SCM has revealed NCs deep below the surface not seen by AFM. The electrically active probe yielded significantly better spatial resolution than AFM. The conductive AFM measurements have shown that the Cr-related point defects near the surface are responsible for the leakage of the macroscopic Schottky junctions, and also that NCs near the surface are sensitive to the mechanical and electrical stress induced by the scanning probe.     

Evaluation of the nanotube intrinsic resistance across the tip-carbon nanotube-metal substrate junction by Atomic Force Microscopy

Using an atomic force microscope (AFM) at a controlled contact force, we report the electrical signal response of multi-walled carbon nanotubes (MWCNTs) disposed on a golden thin film. In this investigation, we highlight first the theoretical calculation of the contact resistance between two types of conductive tips (metal-coated and doped diamond-coated), individual MWCNTs and golden substrate. We also propose a circuit analysis model to schematize the «tip-CNT-substrate» junction by means of a series-parallel resistance network. We estimate the contact resistance R of each contribution of the junction such as R_tip-CNT, R_{CNT-substrate} and R_{tip-substrate} by using the Sharvin resistance model. Our final objective is thus to deduce the CNT intrinsic radial resistance taking into account the calculated electrical resistance values with the global resistance measured experimentally. An unwished electrochemical phenomenon at the tip apex has also been evidenced by performing measurements at different bias voltages with diamond tips. For negative tip-substrate bias, a systematic degradation in color and contrast of the electrical cartography occurs, consisting of an important and non-reversible increase of the measured resistance. This effect is attributed to the oxidation of some amorphous carbon areas scattered over the diamond layer covering the tip. For a direct polarization, the CNT and substrate surface can in turn be modified by an oxidation mechanism.     

bOptimizing atomic force microscopy for characterization of diamond-protein interfaces

Atomic force microscopy (AFM) in contact mode and tapping mode is employed for high resolution studies of soft organic molecules (fetal bovine serum proteins) on hard inorganic diamond substrates in solution and air. Various effects in morphology and phase measurements related to the cantilever spring constant, amplitude of tip oscillations, surface approach, tip shape and condition are demonstrated and discussed based on the proposed schematic models. We show that both diamond and proteins can be mechanically modified by Si AFM cantilever. We propose how to choose suitable cantilever type, optimize scanning parameters, recognize and minimize various artifacts, and obtain reliable AFM data both in solution and in air to reveal microscopic characteristics of protein-diamond interfaces. We also suggest that monocrystalline diamond is well defined substrate that can be applicable for fundamental studies of molecules on surfaces in general.     

Optimizing atomic force microscopy for characterization of diamond-protein interfaces

Atomic force microscopy (AFM) in contact mode and tapping mode is employed for high resolution studies of soft organic molecules (fetal bovine serum proteins) on hard inorganic diamond substrates in solution and air. Various effects in morphology and phase measurements related to the cantilever spring constant, amplitude of tip oscillations, surface approach, tip shape and condition are demonstrated and discussed based on the proposed schematic models. We show that both diamond and proteins can be mechanically modified by Si AFM cantilever. We propose how to choose suitable cantilever type, optimize scanning parameters, recognize and minimize various artifacts, and obtain reliable AFM data both in solution and in air to reveal microscopic characteristics of protein-diamond interfaces. We also suggest that monocrystalline diamond is well defined substrate that can be applicable for fundamental studies of molecules on surfaces in general.     

Inhomogeneous DNA bonding to polycrystalline CVD diamond

Double stranded deoxyribonucleic acid (ds-DNA) layers, bonded to hydrogen terminated polycrystalline diamond, are characterized by scanning electron (SEM), fluorescence (FM), and atomic force microscopy (AFM). DNA grafting has been achieved using photochemical bonding of ω-unsaturated 10-amino-dec-1-ene molecules. SEM detects local variations of electron affinities on polycrystalline diamond, revealing distinct grain structures. FM applied on fluorescence labeled ds-DNA show laterally varying intensities of typically 20%, which resembles also grain structure as detected by SEM. Contact and tapping-mode AFM characterization reveal a tilted DNA bonding to diamond and a dense layer formation which gives rise to smoothening of surface properties. The lateral density variation of DNA is attributed to local variations of the photo-electron emission efficiency which affects the photochemical attachment chemistry of amine-linker molecules to diamond.     

Friction force microscopy study of diamond films modified by a glow discharge treatment

A glow discharge treatment technique has been developed which enables control of the surface roughness and morphology of diamond films for applications in optical and electrical components. A conventional hot filament chemical vapour deposition (CVD) system was used to deposit the diamond films onto silicon substrates via a three-step sequential process: (i) deposition under normal conditions; (ii) exposure to either a pure hydrogen plasma or 3% methane in an excess of hydrogen using DC-bias; and (iii) diamond deposition for a further 2 h under standard conditions. The frictional characteristics and roughness of the film surfaces were investigated by atomic force microscopy (AFM) and the morphology and the growth rates determined from scanning electron microscope images. Lateral force microscopy (LFM) has revealed significant differences in frictional behaviour between the high quality diamond films and those modified by a glow discharge treatment. Friction forces on the diamond films were very low, with coefficients ∼0.01 against silicon nitride probe tips in air. However, friction forces and coefficients were significantly greater on the DC-biased films indicating the presence of a mechanically weaker material such as an amorphous carbon layer. A combination of growth rate and frictional data indicated that the exposure to the H₂ plasma etched the diamond surface whereas exposure to CH₄/H₂ plasma resulted in film growth. Re-Nucleation of diamond was possible (stage iii) after exposure to either plasma treatment. The resultant friction forces on these films were as low as on the standard diamond film.     

Preparation and characterization of boron doped diamond electrodes on diamond anvil for in situ electrical measurements under high pressure

A layer of boron doped diamond (BDD) film was deposited selectively on a diamond anvil and employed as electrodes for measuring the electrical resistivity of matter under high pressure. Both heavily doped and lightly doped electrodes were characterized by Raman spectroscopy and scanning electron microscopy. Though the BDD film electrodes contain sp² carbon, it is still suitable for in situ high pressure electrical measurements. The dependability of diamond film electrodes was tested at high pressure up to 36 GPa, by measuring the electric resistance of C60 fullerene powder, and no damage of the electrodes was observed.     

Optical characterization of surface roughness of diamond by spectroscopic ellipsometry

Vacuum ultraviolet spectroscopic ellipsometry (SE) was performed on high-temperature and high-pressure (HPHT) synthesized Ib diamond samples with different polishing grades of different surface roughness, R_a [nm], which was measured by atomic force scanning microscopy (AFM). Observations showed that SE spectra systematically changed as a function of surface roughness. From this experimental result, we estimated the ideal dielectric function (DF) of diamond with a flat surface. Using this estimated DF, the optical thickness of a roughness layer was evaluated only from the SE data. It was shown that SE is an effective means of evaluating the thickness of a roughness layer and also the degree of surface modification without damaging the diamond surface, unlike the tip scanning of AFM measurements.     

Analysis of piezoresistive properties of CVD-diamond films on silicon

The piezoresistive properties of CVD-diamond are still very much in discussion since not only the materials energy band structure properties have to be considered but also the grain boundaries and internal stress distribution. Here, the experimental piezoresistive properties of CVD-diamond-on-silicon layers for free standing structures have been investigated comprehensively. The longitudinal gauge factor k_l has been extracted using freestanding diamond cantilevers on silicon. The piezoresistors have been grown selectively onto the surface of diamond cantilevers near the mechanical suspension and doped with boron (acceptor). The electrical contacts are based on the tunneling mechanism with a silicon-based multilayer metalization leading to a linear IV-characteristic. Gauge factor values, k_l, have been extracted on various structures with different doping concentrations and diamond film quality (highly oriented and textured, textured, randomly oriented), depending on temperature (room temperature, −350°C) and intrinsic stress. Highly oriented and textured films with grain sizes between 3 and 10 μm have been used to realize ‘single grain’ resistor structures enabling the investigation of grain boundaries in the electrical current path of the piezoresistor. Raman measurements have been performed to measure the intrinsic stress in the diamond grains. Gauge factors, k_l of between 4 and 28 have been extracted. Largest k_l values were observed on piezoresistors on highly oriented and textured diamond (HOD) films. Results of this work have been used in piezoresistive sensor applications.     

Combined AFM–SEM study of the diamond nucleation layer on Ir(001)

During bias enhanced nucleation (BEN) of diamond on iridium the nucleation centres are gathered in discrete islands — the so called “domains”. The topographic signature of these domains has been clarified in the present study by two different concepts. First scanning electron microscopy (SEM) and atomic force microscopy (AFM) were combined to take images with both techniques of a small identical area on a standard BEN sample. In spite of the 2–3 nm deep roughening of the iridium it turned clearly out that the surface shows a 1 nm deep depression within the domains compared with the surface of the surrounding layer. On a second sample which did not show the normal roughening the domains could be identified directly from AFM images. The topographic signature of the domains was the same. Conductive AFM measurements showed that inside and outside the domains the carbon nucleation layer behaves like a high resistivity dielectric sustaining fields up to 10⁷ V/cm. Finally, the temporal development of the domain patterns was studied by consecutive biasing steps on one sample. Depending on the local ion bombardment conditions we observed lateral growth or shrinkage on the same sample. This result suggests that domain formation is a continuous process during the whole BEN procedure starting from a local nucleation event and subsequent lateral expansion.     

Scanning Near-Field Ellipsometry Microscopy: imaging nanomaterials with resolution below the diffraction limit

We introduce a simple Scanning Near-Field Ellipsometer Microscopy (SNEM) setup to address the rapidly increasing need for simple, routine optical imaging techniques with resolution well below the diffraction limit. Our setup is based on the combination of commercially available atomic force microscope (AFM) and ellipsometry equipment with gold-coated AFM tips to obtain near-field optical images with a demonstrated resolution below λ/10. AFM topographical data, obtained in contact mode, and near-field optical data were acquired simultaneously using a combined AFM-ellipsometer. The highly enhanced field due to lightning-rod effects and localized surface plasmons excited at the end of the gold-coated tip allowed us to resolve and identify metallic nanoparticles embedded in poly(methyl methacrylate) as well as microphases in microphase-separated block copolymer films.     

Gas sensing properties of nanocrystalline diamond films

Toxic gas sensing device with metal electrodes built into nanocrystalline diamond (NCD) is investigated. The NCD morphology is controlled via seeding and/or deposition time. The surface properties and morphology of NCD are studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). AFM measurements reveal increase in NCD surface area by up to 13%. Gas sensing properties of H-terminated NCD device show high sensitivity towards oxidizing species where the surface conductivity is increased by an order of magnitude for humid air and by three orders of magnitude for COCl₂. The surface conductivity exhibits a small decrease to reducing spices (CO₂, NH₃).     

Microscopic study of field emission from diamond particles

The field emission properties and microscopic characteristics of diamond particles (DPs) are studied by scanning electron microscopy (SEM), secondary electron spectroscopy (SES), scanning tunneling microscopy/spectroscopy (STM/STS), scanning field emission microscopy (SFEM) and field emission electron microscopy (FEEM). DPs with an average size of 1 μm were spin-coated onto a tungsten substrate. After the spin-coat process, an undoped thin diamond layer was grown on the DP surfaces by the conventional microwave plasma-assisted CVD method. Consequently, the total emission current measured on the DP-seeded substrate (1×1 cm²) was approximately 1 mA under an electric field of 3.5 kV/mm. The DPs with the CVD diamond overcoat have facet edges of diamond, which are sometimes covered with small crystallites. The SES spectrum for these samples showed that the surface of DPs with a CVD diamond overcoat has essentially a NEA surface. The SFEM was able to image the distribution of field emission sites by scanning the STM tip with fixed tip height. The SFEM and FEEM images suggest that some particular DPs contribute to field emission and the emission occurs from the top site of the DP. The modification of surface property and electric field at the top site or edge of the DP affects the effective field emission.     

Spectroscopic investigations of diamond/hydrogen/metal and diamond/metal interfaces

The formation of metal contacts on diamond surfaces is of decisive importance for the material's application in semiconductor devices. We present an investigation of the barrier heights of metals on diamond (111) single crystal surfaces by a combination of spectroscopic and structural methods. Two metals with different work functions and chemical reactivities, aluminium and gold were chosen as model systems for Schottky and ohmic contacts, respectively. Particular attention was paid to the role hydrogen termination of diamond has for the metal film morphology and the barrier heights. To this end diamond/metal contacts on hydrogenated and dehydrogenated diamond were investigated. Core-level spectroscopy was used to monitor chemical reactions between diamond and metal, metal-induced band bending, and the barrier heights. The morphology of the evaporated metal films was investigated by low energy electron diffraction (LEED), atomic force microscopy (AFM), and secondary electron microscopy (SEM).     

Nanodiamond-mediated crystallization in fibers of PANI nanocomposites produced by template-free polymerization: Conductive and thermal properties of the fibrillar networks

The detonation nanodiamond is a novel versatile nanomaterial with tunable properties and surface chemistry. In this work, we report on a template-free method to synthesize polyaniline based nanocomposite fibers during a chemical oxidative precipitation polymerization where the cooperative interactions between nanodiamond and polyaniline nucleates trigger the final morphology of the nanocomposite. FE–SEM and TEM observations evidence the prominent growth of fibril-like structures assembled in 2-D networks of tightly woven, partially oriented fibers. Optical and Raman spectroscopy and X-ray diffraction analyses reveal that the polymer chains are in a protonated emeraldine form and organize themselves in a highly ordered 3-D spatial arrangement. Conductivity measurements performed on isolated fibers by a conductive tip of an AFM apparatus highlight that the diamond filler does not affect the conductive properties of the polyaniline matrix while increases the thermal stability of the polymer as confirmed by TGA studies.     

Application of two- and four-point contact probes to various monocrystalline diamond surfaces

Point contact methods using metallic needles are widely applied for electrical measurements on the diamond semiconductor surfaces because of the difficulties of alloyed ohmic contact preparation. Contradictions and discussions in the literature concerning the resistance values and the shape of I–V characteristics made necessary a detailed study of the point contact measurements. In this work two- and four-point methods were applied to HPHT bulk crystals and MPCVD monocrystalline epitaxial layers in order to evaluate the applicability of these methods, to estimate the current conductivity character of such metal tip–diamond junctions, and to measure the electrical resistance of these crystals. Some samples underwent various treatments: annealing in vacuum; hydrogenation in molecular hydrogen; deuteration in RF and MW plasmas. Conductivity character of the contacts was determined and the applicability of point contacts was demonstrated. In contrast to some earlier results, only linear I–V characteristics were found. Conducting superficial layer connected with the presence of hydrogen was not observed. After the microwave deuterium plasma treatment, just the contrary, a highly resistive superficial layer posed obstacles before the measurements.     

Heteroepitaxial diamond on iridium: New insights on domain formation

Topography and chemical evolutions of the iridium surface in the successive steps of bias-enhanced nucleation and growth were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), and nano-Auger analyses. This sequential approach, which was performed in localized areas at the nanoscale, provides three new experimental and complementary concepts that can enhance the knowledge of nucleation pathways on iridium. First, SEM imaging at low-acceleration voltage enables the detection of diamond nuclei or stable precursors after the BEN step. Second, domains consist of iridium furrows that are covered by an amorphous carbon overlayer, with a thickness of 6.8 nm, according to AFM and X-ray photoemission spectroscopy data. Third, SEM observations also suggest a close relationship between furrows created under ion bombardment and domains in our study conditions. These results prompted us to propose a scheme that describes the topography and surface chemistry of domains.     

Characterization of single 1.8-nm Au nanoparticle attachments on AFM tips for single sub-4-nm object pickup

This paper presents a novel method for the attachment of a 1.8-nm Au nanoparticle (Au-NP) to the tip of an atomic force microscopy (AFM) probe through the application of a current-limited bias voltage. The resulting probe is capable of picking up individual objects at the sub-4-nm scale. We also discuss the mechanisms involved in the attachment of the Au-NP to the very apex of an AFM probe tip. The Au-NP-modified AFM tips were used to pick up individual 4-nm quantum dots (QDs) using a chemically functionalized method. Single QD blinking was reduced considerably on the Au-NP-modified AFM tip. The resulting AFM tips present an excellent platform for the manipulation of single protein molecules in the study of single protein-protein interactions.