High aspect ratio diamond microelectrode array for neuronal activity measurements

Diamond exhibits several attractive properties for bio-sensing applications. In particular its high bio inertness, high electrochemical stability, and optical transparency provide diamond with high interests for neural activity study. The purpose of this study is the realisation of microelectrodes arrays (MEA) in diamond for neurons slices study. Due to a cellular lysis on the edge of the tissue slices studied, electrodes have to be at least 60 μm in height even though the electrode surface has to be minimised in order to achieve good signal noise rate. Silicon MEA with metal contacts were realised using (Deep Reactive Ion Etching) DRIE and coated with Nanocrystaline Diamond (NCD) using (Bias Enhanced Nucleation) BEN technique. We focus the study on the understanding of the BEN nucleation process on such high aspect ratio electrodes. Several parameters such as slope of the substrate, conductivity and chemical nature of the substrate were studied in order to enable selective nucleation necessary to fabricate diamond MEAs. The study leads to the optimised development of a processing route enabling the selective coating of the active tips of high aspect ratio MEAs without altering the electrical insulation between probes.     

Transparent diamond microelectrodes for biochemical application

In this investigation a technology has been developed to use diamond electrodes in Micro Electrode Arrays (MEAs) on a transparent sapphire substrate, thus combining the outstanding electrochemical properties of boron-doped diamond electrodes (BDD) with the transparency needed for simultaneous fluorescence analysis. Nanodiamond films were grown on double side polished sapphire substrates by hot filament CVD (HFCVD) and Bias Enhanced Nucleation (BEN). A simple four microelectrode array (quadrupole) has been fabricated, fully characterized in optical and electrochemical properties, and tested with adrenal chromaffin cells, identifying amperometric spikes by the four recording diamond electrodes, corresponding to the oxidation current of catecholamine molecules.     

Silicon substrate preparation for epitaxial diamond crystals

Using the microwave plasma-assisted chemical-vapour deposition technique (MPCVD), we study the role of the (100) silicon substrate preparation prior to the ultra short bias enhanced nucleation (USBEN) step via various pre-treatments. We study the effect of the silicon HF cleaning coupled with the hydrogen plasma exposure in order to determine the efficiency of these pre-treatments to eliminate the native oxide layer on the silicon surface prior to the bias step. We show that the residual oxygen content in the gas phase during the hydrogen plasma exposure can strongly affect the nucleation density because of the formation of an oxide layer which is detrimental for the synthesis of highly oriented diamond (HOD) films. Moreover, we study the effect of the carburation step, often used for the synthesis of HOD films and show that it raises the percentage of epitaxial crystals and the crystal density. Thus, we show that the achievement of high epitaxial crystal density is not only due to the BEN parameters but also to the silicon substrate preparation. As a conclusion, it appears that the silicon substrate preparation prior to BEN is fundamental for controlling the quality of epitaxial diamond films.     

Surface study of iridium buffer layers during the diamond bias enhanced nucleation in a HFCVD reactor

The BEN nucleation of diamond on iridium substrates has been studied in a hot filament reactor. Without a prior BEN stage, no diamond nucleation could be detected. Nucleation is promoted only if a BEN step is applied before the CVD growth with nucleation densities up to 5 10⁹ cm⁻². The present study focuses on the early stages of BEN to better understand its specific role. In this way, samples have been in situ characterized using electron spectroscopies (XPS, AES, ELS) and further investigated by HR-SEM, AFM, Nano-Auger and Raman spectroscopy. A very different behaviour in the interface formation has been observed, as compared to silicon. First, a substrate roughening takes place during the cleaning step. Second, the formation of a graphite layer was systematically observed, with or without the BEN stage, in the early stages of CVD synthesis. Its crystallinity has been studied from the Raman experiments. The study of the XPS Ir 4f peaks supports a weak chemical bonding between graphite and iridium. Finally, after the BEN stage, spatially resolved Nano-Auger and Raman measurements revealed the presence of diamond nanocrystals.     

Quantitative study of epitaxial CVD diamond deposits: Correlations between nucleation parameters and experimental conditions

In the case of diamond deposit obtained by microwave plasma assisted chemical vapour deposition technique (MPCVD) where the bias enhanced nucleation (BEN) was used to initiate diamond islands on silicon substrate, we simultaneously studied nucleation parameters such as crystal density and epitaxial ratio according to main synthesis conditions. These ones were relative to in situ pretreatment steps occurring before diamond growth, i.e. plasma etching and bias sequences. The nucleation parameters were studied by the high resolution SEM associated to image analysis techniques on homogeneous 1 cm² samples.We observed that hydrogen etching duration clearly modified the epitaxial ratio without any change on the crystal density. So an optimal epitaxial ratio was reached for a moderate hydrogen etching while crystal density remained quite constant. The bias step was analysed in terms of duration and electrical behaviour (voltage and intensity) in relation to the plasma density that we were able to modify by physically confining the discharge. Bias duration clearly modified crystal density and epitaxial ratio. In later case, we observed a short optimal duration (between 30–90 s) for a 120 V bias voltage depending on the etching. We showed too that for a given bias duration the epitaxial ratio was all the more high as the voltage is low. The study of crystal density in relation to electrical characteristics of bias step showed that the more important parameter for nucleation is the electrical charge density (including intensity and time) and not the voltage, since nucleation density of 10⁸ cm^− 2 can be maintained for voltage close to 35 and 50 V respectively if the plasma power density during the bias is higher or if the BEN duration is longer.     

Influence of silicon carbide interlayer evolution on diamond heteroepitaxy during bias enhanced nucleation on silicon substrates

In order to improve the crystalline quality of diamond films produced by microwave plasma assisted chemical vapour deposition (MPCVD), the structural evolution of the silicon carbide interlayer during the bias nucleation step has been investigated by reflection high energy electron diffraction (RHEED). Here we highlight the fact that the carbonisation pre-treatment induces a strong extension of the silicon carbide lattice in the direction perpendicular to the surface. This extension gives a lattice constant close to that of silicon. Then, during bias enhanced nucleation, the carbide lattice relaxes. At the same time, this modification is accompanied by an increase of the surface roughness and by a progressive polar misorientation of the silicon carbide. All these transformations could be responsible for the observed drop of the diamond epitaxial ratio when the duration of the bias step is extended. Finally, we found that a lower methane concentration in the plasma slows down this carbide transformation, allowing us to obtain a promising 37% epitaxial ratio.     

Silicon carbide films from polycarbosilane and their usage as buffer layers for diamond deposition

Thin films of polycarbosilane (PCS) were coated on a Si (100) wafer and converted to silicon carbide (SiC) by pyrolyzing them between 800 and 1150 °C. Granular SiC films were derived between 900 and 1100 °C whereas smooth SiC films were developed at 800 and 1150 °C. Enhancement of diamond nucleation was exhibited on the Si (100) wafer with the smooth SiC layer generated at 1150 °C, and a nucleation density of 2 × 10¹¹ cm^− 2 was obtained. Nucleation density reduced to 3 × 10¹⁰ cm^− 2 when a bias voltage of − 100 V was applied on the SiC-coated Si substrate. A uniform diamond film with random orientations was deposited to the PCS-derived SiC layer. Selective growth of diamond film on top of the SiC buffer layer was demonstrated.     

Nucleation and growth of diamond films on aluminum nitride by hot filament chemical vapor deposition

Nucleation and growth of diamond films on aluminum nitride (ALN) coatings were investigated by scanning electron microscopy, Raman spectroscopy and scratch test. ALN films were grown in a magnetron sputtering deposition. The substrates were Si(111) and tungsten carbide (WC). Chemical vapor deposition (CVD) diamond films were deposited on ALN films by hot filament CVD. The nucleation density of diamond on ALN films was found to be approximately 10⁵ cm⁻², whereas over 10¹⁰ cm⁻² after negative bias pre-treatment for 35 min was −320 V, and 250 mA. The experimental studies have shown that the stresses were greatly minimized between diamond overlay and ALN films as compared with WC substrate. The results obtained have also confirmed that the ALN, as buffer layers, can notably enhance the adhesion force of diamond films on the WC.     

Effect of bias enhanced nucleation on the nucleation density of diamond in microwave plasma CVD

The variation of diamond nucleation density as a function of the conditions of bias enhanced nucleation (BEN) were studied. The nucleation density increased with microwave power, but decreased with the substrate temperature. The nucleation density also increased with bias voltage above 60 V, and had a maximum around 100 V. The crystal growth of diamond took place when either the bias voltage was high or the deposition time was long. The shift of C_1s energy measured by X-ray photoelectron spectroscopy indicated that the ratio of carbon sp³ bonds in the amorphous carbon and/or SiC phases formed before the nucleation of diamond, increased around the bias voltage of 100 V, which seemed to be the reason for enhancement of diamond nucleation by bias voltage. A simple computer simulation was performed in order to understand the effect of BEN conditions on the nucleation of diamond. The simulation reproduced the experimentally observed changes of nucleation density and particle size.     

Effect of titanium metal in the prenucleation of ultrananocrystalline diamond film growth at low substrate temperature

Ultrananocrystalline diamond (UNCD) film is usually grown in methane–argon plasma unlike methane–hydrogen plasma conventionally used to deposit microcrystalline diamond film. The prenucleation and growth mechanism of these two types of diamond films are different as well. The present study introduces titanium metal powder during ultrasonication of silicon substrate to enhance the nucleation density of UNCD. A titanium thin film was also used at the interface to find the effect of metal on the growth of diamond film. The nucleation density of as-grown film was estimated from the FE-SEM images. After 20 min of growth, nucleation density reaches to 10¹¹/cm² on a surface pretreated by titanium mixed nanodiamond powder. Raman study was carried out for qualitative analysis of different carbon phase present in the UNCD films. X-ray photoelectron spectroscopy (XPS) was used to understand the growth mechanism by detecting the formation of carbon phase and metal carbide formation at the surface after stopping the growth at different time intervals.     

Synthesis of highly oriented CVD diamond films by ultra short bias enhanced nucleation step

Highly oriented diamond films have been deposited on silicon substrate by the MPCVD technique (microwave plasma assisted chemical vapour deposition) using an ultra short bias enhanced nucleation process (so called USBEN). We focus our attention on two points: the homogeneity of the deposit in order to perform a precise characterisation whatever surface location (on 1×1 cm² of single silicon substrate); and the simplification of the successive steps usually performed in the BEN process. This is carried out by optimising the microwave cavity and the d.c. discharge extension and by keeping the pretreatments just necessary to obtain high nucleation density with an acceptable epitaxial ratio and a good homogeneity. This leads to a drastic reduction of the bias time of only 30 s for low bias voltage. As we obtain a highly oriented diamond film with a short bias pretreatment without preliminary carburation step, we discuss the substrate transformation under a weak bombardment duration of ions having a quite low energy. We think that the bias step probably consists to a slight modification of the substrate surface.     

Diamond nucleation and growth under very low-pressure conditions

The synthesis of thin diamond films using various chemical vapor deposition methods has received significant attention in recent years due to the unique characteristic of diamond, which make it an attractive candidate for a wide range of applications. In order to grow diamond epitaxially, the proper control of diamond nucleation on mirror-polished Si is essential. Adding the negative bias voltage to the substrate is the most popular method. This paper has proposed a new method to greatly enhance the nuclear density. Under very low pressure (1 torr), the high-density nucleation of diamond is achieved on mirror-polished silicon in a hot-filament chemical vapor deposition (HFCVD). Scanning electron microscopy has demonstrated that the nuclear density can be as high as 10¹⁰–10¹¹ cm⁻². Raman spectra of the sample have shown a dominant diamond characteristic peak at 1332 cm⁻¹. The pressure effect has been discussed in detail and it has been shown that the very low pressure is a very effective means to nucleate and grow diamond films on mirror-polished silicon. Extraordinary pure hydrogen (purity=99.9999%) was used as the source. Compared with the highly pure hydrogen (purity=99.99%), we found that the density of nucleation was greatly increased. The residual oxygen in the hydrogen displayed a very obvious negative effect on the nucleation of diamond, although it can accelerate the growth of diamond. Based on these results, it was suggested that the enhanced nucleation at very low pressure should be attributed to an increased mean free path, which induced a high density of atomic hydrogen and hydrocarbon radicals near the silicon surface. Atomic hydrogen can effectively etch the oxide layer on the surface of silicon and so greatly enhance the nucleation density.     

Chemical and structural transformations of silicon submitted to H2 or H2/CH4 microwave plasmas

Silicon substrates are often used to synthesize polycrystalline diamond films by microwave plasma assisted chemical vapour deposition technique (MPCVD). In the case of highly oriented diamond films, several steps are employed to carefully prepare the silicon surface (pre-treatment steps), to nucleate diamond crystals (nucleation step) and to thick the film (growth step). In this study, we characterize {100} silicon substrates and diamond released from its silicon substrate by electronic microscopies (TEM and SEM), by Atomic Force Microscopy (AFM) and by X-ray photoelectron spectroscopy (XPS), to follow the substrate transformations after each step, particularly the formation and the evolution of the silicon carbide and to characterise the diamond films grown on the carburised silicon. We show that according to the experimental conditions and the level of surface/gas contamination by carbon and silicon species, isolated islands or continuous β-SiC compound are formed over the silicon surface and can generate defects such as voids or strip structures that influence the subsequent diamond nucleation and growth.     

Domain formation in diamond nucleation on iridium

Domain formation in epitaxial diamond nucleation on Ir(001) surfaces using the bias-enhanced nucleation (BEN) procedure has been studied. Bright areas of up to several microns lateral size with negligible topographic contrast are observed by scanning electron microscopy (SEM) after ion bombardment. When a growth step is applied after BEN, these domains develop into islands of identical shape consisting of epitaxial diamond with a high local area density of oriented grains. Outside the domains the nucleation density is either orders of magnitude lower or the grains are completely non-oriented. The diamond nuclei or precursors which are formed during the BEN step proved to be very stable: They still yielded oriented diamond islands when the samples were stored in air for 1 year before the growth step. Electron backscatter diffraction (EBSD) patterns taken from inside and outside the domains immediately after BEN did not show any significant difference. This allows the conclusion that the modification of the iridium crystal lattice accompanied with diamond nucleation is either very faint or only restricted to a very thin layer at the surface. Kelvin probe force microscopy (KPFM) measurements indicate a reduced work function within the domains.     

Morphological study of thin films: Simulation and experimental insights using horizontal visibility graph

We studied the morphological nature of various thin films such as silicon carbide (SiC), diamond (C), germanium (Ge), and gallium nitride (GaN) on silicon substrate Si(100) using the pulsed laser deposition (PLD) method and Monte Carlo simulation. We, for the first time, systematically employed the visibility algorithm graph to meticulously study the morphological features of various PLD grown thin films. These thin-film morphologies are investigated using random distribution, Gaussian distribution, patterned heights, etc. The nature of the interfacial height of individual surfaces is examined by a horizontal visibility graph (HVG). It demonstrates that the continuous interfacial height of the silicon carbide, diamond, germanium, and gallium nitride films are attributed to random distribution and Gaussian distribution in thin films. However, discrete peaks are obtained in the brush and step-like morphology of germanium thin films. Further, we have experimentally verified the morphological nature of simulated silicon carbide, diamond, germanium, and gallium nitride thin films were grown on Si(100) substrate by pulsed laser deposition (PLD) at elevated temperature. Various characterization techniques have been used to study the morphological, and electrical properties which confirmed the different nature of the deposited films on the Silicon substrate. Decent hysteresis behavior has been confirmed by current-voltage (IV) measurement in all the four deposited films. The highest current has been measured for GaN at ～60 nA and the lowest current in SiC at ～30 nA level which is quite low comparing with the expected signal level (μA). The HVG technique is suitable to understand surface features of thin films which are substantially advantageous for the energy devices, detectors, optoelectronic devices operating at high temperatures.     

Comparative electron diffraction study of the diamond nucleation layer on Ir(001)

The carbon layer formed during the bias enhanced nucleation (BEN) procedure on iridium has been studied by different electron diffraction techniques. In reflection high energy electron diffraction (RHEED) and low energy electron diffraction (LEED) the carbon nucleation layer does not give any indication of crystalline diamond even if the presence of domains proves successful nucleation. In contrast, X-ray photoelectron diffraction (XPD) shows a clear C 1s pattern when domains are present after BEN. The anisotropy in the Ir XPD patterns is reduced after BEN while the fine structure is essentially identical compared to a single crystal Ir film. The change in the Ir XPD patterns after BEN can be explained by the carbon layer on top of a crystallographically unmodified Ir film. The loss and change in the fine structure of the C 1s patterns as compared to a single crystal diamond film are discussed in terms of mosaicity and a defective structure of the ordered fraction within the carbon layer. The present results suggest that the real structure of the BEN layer is more complex than a pure composition of small but perfect diamond crystallites embedded in an amorphous matrix.     

Epitaxial Ir layers on SrTiO3 as substrates for diamond nucleation: deposition of the films and modification in the CVD environment

Iridium films on SrTiO₃(001) have recently proven to be a superior substrate material for the heteroepitaxy of diamond thin films by chemical vapour deposition in the effort towards the realization of single crystal diamond films. In this paper we report on the growth and structural properties of iridium (Ir) films deposited by electron-beam evaporation on SrTiO₃(001) surfaces varying the deposition temperature between 280 and 950°C. The films were studied by scanning electron microscopy, atomic force microscopy and X-ray diffraction. At the highest temperature film growth proceeds via three-dimensional nucleation, coalescence and subsequent layer-by-layer growth. The resulting films show a cube-on-cube orientation relationship with the substrate and a minimum mosaic spread of 0.15°. Towards lower deposition temperatures the orientation spread increases only slightly down to ∼500°C while the surface roughness, after passing through a maximum at ∼860°C, decreases significantly. For the lowest temperatures (below 500°C) the mosaic spread rises accompanied by the occurrence of twins until the epitaxial order is lost. Plasma treatment in the diamond deposition reactor at high temperature (920°C) yields low nucleation densities and modifies the Ir surface. At the same time {111} facets show a significantly higher structural stability as compared with {001} facets. Nucleation at 700°C results in highly aligned diamond grains with low mosaic spread and a vanishing fraction of randomly oriented grains, proving the superior properties of Ir films on SrTiO₃ for diamond nucleation as compared with pure silicon substrates.     

Heteroepitaxial diamond nucleation and growth on silicon by microwave plasma-enhanced chemical vapor deposition synthesis

Heteroepitaxial diamond films were successfully nucleated and deposited on 1-inch diameter Si(001) substrates by microwave plasma-enhanced chemical vapor deposition (MPECVD). The precursor gases for the synthesis were methane and hydrogen. Before the application of a negative d.c. bias to the substrate, an in-situ carburization pre-treatment on the silicon was found to be an indispensable step towards the heteroepitaxial diamond on the silicon. Morphologies of the films were characterized by scanning electron microscopy (SEM). Interface observations based on the cross-sectional HRTEM directly reveal the heteroepitaxial diamond nucleation phenomena in detail. No interlayers of silicon carbide and/or amorphous carbon phases were observed. Tilt and azimuthal misorientation angles between the heteroepitaxial diamond crystals and the substrate were determined by combining the Ewald sphere construction in the reciprocal lattice space and the selected area diffraction (SAD) patterns taken across the interface.     

A kinetic model of diamond nucleation and silicon carbide interlayer formation during chemical vapor deposition

The presence of thin silicon carbide intermediate layers on silicon substrates during nucleation and the early stages of diamond deposition have been frequently reported. It is generally accepted that the intermediate layer is formed by the bulk diffusion of carbon atoms into the silicon carbide layer and the morphology and orientation of the diamond film subsequently grown on the intermediate layer are strongly affected by that layer. While there have been considerable attempts to explain the mechanism for intermediate layer formation, limited quantitative data are available for the layer formation under the operating conditions conducive to diamond nucleation.This study employs a kinetic model to predict the time evolution of a β-SiC intermediate layer under the operating conditions typical of diamond nucleation in hot filament chemical vapor deposition reactors. The evolution of the layer is calculated by accounting for gas-phase and surface reactions, surface and bulk diffusions, the mechanism for intermediate layer formation, and heterogeneous diamond nucleation kinetics and of its dependence on the operating conditions such as substrate temperature and inlet gas composition. A comparison between the time scales for intermediate layer growth and diamond nuclei growth is also performed. Discrepancies in published adsorption energies of gaseous hydrocarbon precursors on the intermediate layer—ranging from 1.43 to 4.61 eV—are examined to determine the most reasonable value of the adsorption energy consistent with observed saturated thicknesses, 1–10 nm, of the intermediate layer reported in the literature. The operating conditions that lead to intermediate layer growth followed by diamond deposition vs. those that yield heteroepitaxial diamond nucleation without intermediate layer formation are discerned quantitatively. The calculations show that higher adsorption energies, 3.45 and 4.61 eV, lead to larger surface number densities of carbon atoms, lower saturated nucleation densities, and larger intermediate layer thicknesses. The observed saturated thicknesses of the intermediate layer may be reproduced if the true adsorption energy is in the range of 3.7–4.5 eV. The intermediate layer thickness increases by increasing substrate temperature and inlet hydrocarbon concentration and the dependence of the thickness on substrate temperature is especially significant. Heteroepitaxial diamond nucleation without intermediate layer formation reported in experimental results can be readily explained by the significant decrease of the intermediate layer thickness at lower substrate temperatures and at higher diamond nucleation densities. Further, the present model results indicate that the intermediate layer thickness becomes saturated when growing diamond nuclei cover a very small surface area of that layer.     

Nanometer rough,sub-micrometer-thick and continuous diamond chemical vapor deposition film promoted by a synergetic ultrasonic effect

In this paper we report on a surface treatment to seed substrates for the promotion of diamond nucleation. This surface treatment consists of an ultrasonic abrasion process using poly-disperse slurry composed of a mixture of small diamond particles (<0.25 μm) and larger particles (>3 μm) which may consist of diamond, alumina, titanium, etc. Whereas ultrasonic abrasion with a mono-disperse diamond slurry results in a diamond nucleation density of ∼2–3×10⁸ particles/cm², treatment with poly-disperse slurries results in diamond nucleation density of values up to ∼5×10¹⁰ particles/cm². This effect was found to display a similar effectiveness on a variety of substrates such as silicon, sapphire, quartz, etc. The enhancement in diamond nucleation is interpreted by a ‘hammering’ effect whereby the larger particles insert very small diamond debris onto the treated surface, thus increasing the density of nuclei onto which diamond growth takes place during the chemical vapor deposition process. By increasing the nucleation density to values of ∼5×10¹⁰ particles/cm², continuous diamond films of thickness of less than ∼100 nm were grown after only 5 min of deposition. The roughness of continuous diamond films grown on substrates treated at optimum conditions obtains values of 15–20 nm. The effect of ultrasonic treatment on silicon substrates and the deposited films was investigated by atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.