Fabrication of submicron scale vertically aligned diamond rods by mask-free oxygen plasma etching

A mask-free plasma etching process is described to fabricate 6 μm long submicron diamond rods (SDRs) in long conical shape. Polished polycrystalline diamond is etched in oxygen plasma ignited at a pressure of 10 mTorr by radio-frequency power of 100 W at 13.56 MHz. Each SDR is a bi-crystal, consisting of two diamond crystallites of micron size. The SDR is coated with a Fe₂O₃ layer, as characterized by Auger electron spectroscopy, X-ray photoemission microscopy, and transmission electron microscopy. We propose that a “self-forming” mask of Fe₂O₃ is generated during the etching process in which iron atoms sputtered from the substrate holder are deposited and oxidized on the diamond surface forming “micromask” that protects the underlying diamond and promotes the formation of SDRs.     

A significant improvement of both yield and purity during SWCNT synthesis via the electric arc process

This paper deals with the optimisation of the single walled carbon nanotube (SWCNT) synthesis by the electric arc technique using so-called heterogeneous anodes filled with Ni and Y catalysts along with either graphite (large-grain or small-grain) or diamond powders. The various carbon nanophases produced were analyzed using high-resolution transmission electron microscopy. Plasma physical properties were determined by emission spectroscopy and were correlated to the variation in the carbon products formed. Using large-grain (100 μm) graphite powder corresponded to standard conditions since able to generate impurity-rich SWCNT material resembling that usually described in literature. However, replacing the large-grain graphite powder by small-grain graphite powder (∼1 μm) resulted in a dramatic increase in both the yield and purity of the SWCNTs obtained. On the other hand, a similar result was obtained by using diamond powder (grain size ∼1 μm) instead of the small-grain graphite powder. The results are explained via the erosion modes of the anodes with respect to the apparent density of the powder mixtures filling their cavities. Maintaining a steady plasma composition and a CI/NiI concentration ratio higher than 10⁸ are identified as two conditions required for optimising SWCNT synthesis.     

Study of an on-line precision microgroove generating process on silicon wafer using a developed ultra-thin diamond wheel-tool

This study presents a novel and economical method for precisely developing an ultra-thin diamond grinding wheel-tool and using the finished wheel-tool to on-line fabricate crisscross microgrooves on silicon wafer. The wheel-tool blank is made of diamond grain of 0-2 μm grade via a designed micro co-deposition. A non-continuous cathode design, in which current crowding effect can be suppressed, is used to obtain a diamond wheel-tool with good surface characteristics. With abrasive content of 8 g/l, a suitable interval chip-pocket of 2-3 μm can be generated. The grinding wheel blank is thinned and dressed simultaneously down to a thickness of 15 μm using micro wire Electro Discharge Dressing (w-EDD). The finished wheel-tool is directly utilized to grind the crisscross microgrooves on the silicon wafer using ‘high-speed and fast-shallow grinding’ technique. A grinding depth of 0.5 μm per stroke is exactly controlled to ensure that the removal mechanism transfers to a ductile grinding mode. The width, depth and surface roughness R_a of the microgrooves are 15 μm, 9 μm and 0.087 μm, respectively.     

Synthesis of large single crystal diamond using combustion-flame method

A combustion flame method is used to synthesize large single crystal diamond in ambient atmosphere. The basic of this technique was originally described by Hirose and Kondo in 1988 [Hirose H, Komaki K. Eur Pat Appl 1988:EP324538]. The advantage of this method is the high growth rate of diamond films, which is about 60 μm/h [Alers P, Hanni W, Hintermann HE. A comparative study of laminar and turbulent oxygen-acetylene flames for diamond deposition. Diam Relat Mat 1992;2:393-6]. The diamond can grow on itself to achieve large single-crystal. Negative substrate-bias effects on diamond growth have been investigated. Diamonds films were characterized by scanning electron microscopy, Raman spectroscopy, and atomic force microscopy in tapping mode. For given conditions, diamond coatings with highly oriented {1 0 0} crystal facets were produced. Large singles crystals diamonds were obtained. The sizes of these crystals vary between 80 and 90 μm. These results are discussed with respect to the competing events occurring during the heteroepitaxial growth of diamond.     

Mechanical properties of MWPECVD diamond coatings on Si substrate via nanoindentation

The mechanical properties of polycrystalline diamond coatings with thickness varying from 0.92 to 44.65 μm have been analysed. The tested samples have been grown on silicon substrates via microwave plasma enhanced chemical vapour deposition from highly diluted gas mixtures CH₄-H₂ (1% CH₄ in H₂). Reliable hardness and elastic modulus values have been assessed on lightly polished surface of polycrystalline diamond films.The effect of the coating thickness on mechanical, morphological and chemical-structural properties is presented and discussed. In particular, the hardness increases from a value of about 52 to 95 GPa and the elastic modulus from 438 to 768 GPa by varying the coating thickness from 0.92 to 4.85 μm, while the values closer to those of natural diamond (H = 103 GPa and E = 1200 GPa) are reached for thicker films (> 5 μm). Additionally, the different thickness of the diamond coatings permits to select the significance of results and to highlight when the soft silicon substrate may affect the measured mechanical data. Thus, the nanoindentation experiments were made within the range from 0.65% to 10% of the film thickness by varying the maximum load from 3 to 80 mN.     

Growth and field emission study of a monolithic carbon nanotube/diamond composite

Carbon nanotube (CNT)/diamond composite has been fabricated by hot filament chemical vapor deposition on a silicon substrate using iron as catalyst. The material characteristics of this monolithic structure were examined by electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and electron energy loss spectroscopy. The composite material shows the presence of carbon nanotubes of several microns in length together with conspicuous diamond microcrystals of sizes ranging from 0.5 to 2.0 μm. The CNTs protrude from the diamond microcrystals and become entangled around them as they grow. This monolithic CNT/diamond composite provides an intrinsic heat dissipation mechanism for CNTs during field emission and exhibits low turn on field, large field enhancement factor, and an excellent current stability over a period of 44 h.     

Conversion of carbon nanotubes to diamond by spark plasma sintering

The diamond phase has been converted directly from carbon nanotubes by spark plasma sintering (SPS), at 1500 °C under 80 MPa pressure, without any catalyst being involved. Well-crystallized diamond crystals, with particle sizes ranging from 300 nm to 10 μm were obtained. After sintering at 1200 °C, the tips of the carbon nanotubes were found to be open and the conversion from carbon nanotubes to diamond started. The mechanism for carbon nanotube to diamond conversion in SPS may be described as that from carbon nanotubes to an intermediate phase of carbon nano-onion, and then to diamond. It is believed that the plasmas generated by the low-voltage, vacuum spark, via a pulsed DC in the SPS process, played a critical role in the low pressure diamond formation. This SPS process provides an alternative approach to diamond synthesis.     

Growth of high-quality carbon nanotubes on free-standing diamond substrates

Carbon nanotubes (CNTs) were grown on diamond-coated Si substrates and free-standing diamond wafers to develop efficient thermal interface materials for thermal management applications. High-quality, translucent, free-standing diamond substrates were processed in a 5 kW microwave plasma chemical vapor deposition (CVD) system using CH₄ as precursor. Ni and Ni-9%W-1.5%Fe catalyst islands were deposited to nucleate CNTs directly onto the diamond substrates. Randomly-oriented multi-walled CNTs forming a mat of ∼5 μm thickness and consisting of ∼20 nm diameter tubes were observed to grow in a thermal CVD system using C₂H₂ as precursor. Transmission electron microscopy and Raman analyses confirmed the presence of high-quality CNTs on diamond showing a D/G peak ratio of 0.2-0.3 in Raman spectra.     

Improvement of field emission performance on nitrogen ion implanted ultrananocrystalline diamond films through visualization of structure modifications

The relationship between the electron field emission properties and structure of ultra-nanocrystalline diamond (UNCD) films implanted by nitrogen ions or carbon ions was investigated. The electron field emission properties of nitrogen-implanted UNCD films and carbon-implanted UNCD films were pronouncedly improved with respect to those of as-grown UNCD films, that is, the turn-on field decreased from 23.2 V/μm to 12.5 V/μm and the electron field emission current density increased from 10E−5 mA/cm² to 1 × 10E−2 mA/cm². The formation of a graphitic phase in the nitrogen-implanted UNCD films was demonstrated by Raman microscopy and cross-sectional high-resolution transmission electron microscopy. The possible mechanism is presumed to be that the nitrogen ion irradiation induces the structure modification (converting sp³-bonded carbons into sp²-bonded ones) in UNCD films.     

Sharpening of CVD diamond coated tools by 0.5−10 keV Ar ion beam

CVD diamond coated tungsten carbide tools have been used for cutting and drilling of soft materials such as aluminum and copper alloys. However, it is very difficult to obtain a tool having a sharp tip of the order of sub-μm by mechanical abrasive polishing methods. Therefore, we applied ion beam processing for sharpening the cutting edge of diamond coated tungsten carbide tools. Result shows that it is possible to obtain a 20-80 nm order tip width of a CVD diamond coated knife processed by a 0.5-10 keV Ar⁺ ion beam, and the sharpening speed of a tip of the knife depends on the ion beam energy. Namely, a tip width of a knife processes by a 1.0 keV Ar⁺ ion beam at an ion dose of 2.7 × 10²⁰ ions/cm² becomes 20 nm, and a tip width of a knife processed by a 10 keV Ar⁺ ion beam at an ion dose of 5.4 × 10¹⁹ ions/cm² becomes 40 nm. However, a facet with an apex angle in the range of 60-100° was formed on the cutting edge of a knife with an initial apex angle of 55°, and we found that the facet angle can be controlled by choosing ion beam energy of 0.5-10 keV. Moreover, results show that the processed knife machined by a 0.5 keV Ar⁺ ion beam has very smooth rake and flank faces, and also has a small line edge roughness of the cutting edge compared to those of the sharpened knife by a 1.0-10 keV Ar⁺ ion beam.     

Anisotropic dry etching of boron doped single crystal CVD diamond

Semiconducting boron doped single-crystal CVD diamond has been patterned using aluminum masks and an inductively coupled plasma (ICP) etch system. For comparison insulating HPHT diamond samples were also patterned using the same process. Diamond etch rates above 200 nm/min were obtained with an O₂/Ar discharge for a gas pressure of 2.5 mTorr using 600 W RF power. We have accomplished the fabrication of structures with a minimum feature size of 1 μm with vertical sidewalls in both CVD and HPHT diamond. The ICP etching produced smooth surfaces with a typical root-mean-square surface roughness of 3 nm. The dependence of etch rate on bias voltage was somewhat different for the two types of diamond. However, for all samples both the etch rate and anisotropy were found to improve with increasing bias voltage.     

Comparative study of homoepitaxial single crystal diamond growth at continuous and pulsed mode of MPACVD reactor operation

Homoepitaxial growth of single crystal diamond by microwave plasma chemical vapor deposition in pulsed regime of a 2.45 GHz MPACVD reactor operation at pulse repetition rates of 150 and 250 Hz was investigated. The high quality CVD diamond layers were deposited in the H₂-CH₄ gas mixture containing 4% and 8% of methane, gas pressures of 250 and 260 Torr and substrate temperature of 900 °C without any nitrogen addition. The (100) HPHT single crystal diamond seeds 2.5 × 2.5 × 0.3 mm (type Ib) were used as substrates. At pulse repetition rate 150 Hz the high quality single crystal diamond was grown with growth rate of 22 μm/h. The comparison of the single crystal diamond growth rates in CW and pulsed wave regimes of MPACVD reactor operation at microwave power density 200 W/cm³ was made. It was found that at equal power density, the growth rate in pulsed wave regime was higher than in CW regime. Differences in single crystal diamond growth for two sets of experiments (with continuous and pulsed wave regimes) were explained.     

Microwave activation of electrochemical processes: High temperature phenol and triclosan electro-oxidation at carbon and diamond electrodes

The electrochemical oxidation of phenolic compounds in aqueous media is known to be affected by the formation of electro-polymerized organic layers which lead to partial or complete electrode blocking. In this study the effect of high intensity microwave radiation applied locally at the electrode surface is investigated for the oxidation of phenol and triclosan in alkaline solution at a 500 μm diameter glassy carbon or at a 500 μm × 500 μm boron-doped diamond electrode. The temperature at the electrode surface and mass transport enhancement are determined by calibration with the Fe(CN)₆^3−/4− redox system in aqueous 0.3 M NaOH and 0.2 NaCl (pH 12) solution. The calibration shows that strong thermal and mass transport effects occur at both glassy carbon and boron-doped diamond electrodes. The average electrode temperature reaches up to 390 K and mass transport enhancements of more than 20-fold are possible. For the phenol electro-oxidation at glassy carbon electrodes and at a concentration below 2 mM a multi-electron oxidation (ca. 4 electrons) occurs in the presence of microwave radiation. For the electro-oxidation of the more hydrophobic triclosan only the one-electron oxidation occurs. Although currents are enhanced in presence of microwave radiation, rapid blocking of the electrode surface in particular at high phenol concentrations still occurs.     

Fabrication of three-dimensional Cu/Ni multilayered microstructure by wet process

Three-dimensional (3D) microstructures were formed using a wet process consisting of the formation of a multilayered microstructure and selective etching of the other layers of the structure. Cylindric Cu/Ni multilayered microstructures of 50 μm in diameter and 30 μm in height were formed by electroplating and photolithography. Cu layers were then etched selectively in acidic thiourea solution. The redox reaction of the multilayered microstructure in the solution was investigated electrochemically. The shapes of three-dimensional microstructures were affected by the surface morphology of the deposited films. Various three-dimensional microstructures having controlled geometric unevenness were obtained using films having smooth surface morphology.     

Effect of deposition temperature and quality of free-standing diamond substrates on the properties of RF sputtering ZnO films

Highly c-axis oriented ZnO film is often deposited on diamond substrates by RF magnetron sputtering and widely used for high frequency surface acoustic wave (SAW) devices. Deposition temperature is a key factor affecting the quality of the ZnO film. Different quality polished free-standing diamond films prepared by DC Arc Plasma Jet were used as the substrates to deposit ZnO films at different temperatures. Effect of the deposition temperature and the quality of the diamond films on the properties of the ZnO films were investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that highly c-axis oriented ZnO films can be much easier deposited on the optical-grade diamond films with < 111> preferred orientation than the tool-grade diamond films with < 220> preferred orientation. The optimal deposition temperature is 200 °C for highly c-axis oriented and lower roughness ZnO films. Acoustic phase velocity of more than 10,000 m/s for the SAW devices based on the ZnO/optical-grade free-standing diamond films was obtained.     

Field emission property of arrayed nanocrystalline diamond

Arrays of nanocrystalline diamond (NCD) stripes were fabricated by plasma etching of a NCD film. Electron field emission (EFE) of NCD arrays with 100-μm-wide stripes separated by different spacings was analyzed. The NCD arrays had higher EFE efficacy than the non-patterned blanket NCD film. The turn-on electric field (E_on) decreased from 5.4 V/μm^-1 for the blanket NCD film to 4.2, 4.4 and 4.7 V/μm^− 1 for the NCD arrays with 100, 500 and 1000 μm of spacing, respectively. Both the effective emitting area and the field enhancement factor for the NCD emitters were increased by patterning. The enhanced EFE from arrayed NCD stripes was possibly attributed to the edge effect and reduction of electrostatic screening.     

A new regime for high rate growth of nanocrystalline diamond films using high power and CH4/H2/N2/O2 plasma

In this work, we report high growth rate of nanocrystalline diamond (NCD) films on silicon wafers of 2 inches in diameter using a new growth regime, which employs high power and CH₄/H₂/N₂/O₂ plasma using a 5 kW MPCVD system. This is distinct from the commonly used hydrogen-poor Ar/CH₄ chemistries for NCD growth. Upon rising microwave power from 2000 W to 3200 W, the growth rate of the NCD films increases from 0.3 to 3.4 μm/h, namely one order of magnitude enhancement on the growth rate was achieved at high microwave power. The morphology, grain size, microstructure, orientation or texture, and crystalline quality of the NCD samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction, and micro-Raman spectroscopy. The combined effect of nitrogen addition, microwave power, and temperature on NCD growth is discussed from the point view of gas phase chemistry and surface reactions.     

Fabrication of flat silicon surfaces using ion-exchange particles in ultrapure water

A silicon etching process using an ultrafine particle dispersion is proposed. The ultrafine particles contain quaternary ammonium hydroxide groups as ion-exchange groups, giving an alkaline dispersion. In the etching process, the fine particles and impurity ions can be easily separated by filtration or dialysis. Dialysis led to a decrease in the concentration of impurities in the dispersion, which included heavy metal ions and alkali metal ions known to result in a rough etched surface and affect the electronic properties of the semiconductor. Our proposed method is applicable to the surface planarization of silicon single crystals, manufacture of semiconductor devices, and fabrication of MEMS (micro-electro and mechanical systems). In addition, the etching waste can be reused after removal of the impurity ions by dialysis. Thus, the method has low environmental burden. Using the proposed alkaline etching dispersion, cathodic electrochemical etching of a silicon single crystal was demonstrated. The etching characteristics, properties of the etched surface, and effects of particle size were evaluated. The roughness of a 2 μm × 2 μm etched p-Si(0 0 1) surface was measured to be 0.1228 nm Ra (center line average roughness) by AFM.     

Selective formation of porous layer on n-type InP by anodic etching combined with scratching

The selective formation of porous layer on n-type InP (0 0 1) surface was investigated by using scratching with a diamond scriber followed by anodic etching in deaerated 0.5 M HCl. Since the InP specimen was highly doped, the anodic etching proceeded in the dark. The potentiodynamic polarization showed the anodic current shoulder in the potential region between 0.8 and 1.3 V (SHE) for the scratched area in addition to the anodic current peak at 1.7 V (SHE) for the intact area. The selective formation of porous layer on the scratched are was brought by the anodic etching at a constant potential between 1.0 and 1.2 V (SHE) for a certain time. The nucleation and growth of etch pits on intact area, however, took place when the time passed the critical value.The cross section of porous layer on the scratched area perpendicular to the or [1 1 0] scratching direction had a V-shape, while the cross section of porous layer on the scratched area parallel to the or [1 1 0] scratching direction had a band structure with stripes oriented to the or direction. Moreover, nano-scratching at a constant normal force in the micro-Newton range followed by anodic etching showed the possibility for selective formation of porous wire with a nano-meter width.     

PEG-directed microwave-assisted hydrothermal synthesis of spherical α-Ni(OH)2 and NiO architectures

Spherical α-Ni(OH)₂ architectures were synthesized by the microwave-assisted hydrothermal technique using PEG-6000 as the surfactant. NiO architectures with similar morphology were obtained by a simple thermal decomposition process of the precursor α-Ni(OH)₂ at 400 °C for 2 h and were confirmed by the X-ray diffraction (XRD) analysis. Scanning electron microscopy (SEM) revealed that the synthesized spherical α-Ni(OH)₂ and NiO architectures were composed of stacked lamellar sheets and transmission electron microscopy (TEM) showed that the α-Ni(OH)₂ and NiO architectures were polycrystalline. The effect of the PEG-6000 concentration on particle size was investigated and it was found that the average particle size of α-Ni(OH)₂ architectures decreased from 4.689 μm at C_PEG=2 mmol L⁻¹ to 3.907 μm at C_PEG=4 mmol L⁻¹, and the corresponding average particle size of NiO decreased from 2.818 μm to 2.492 μm. The optical absorption band gap of NiO architectures was determined to be about 2.7–3.0 eV by UV–vis spectroscopy.