Thick Nanocrystalline Diamond Research Articles

Synthesis of High Quality Transparent Nanocrystalline Diamond Films on Glass Substrates Using a Distributed Antenna Array Microwave System

Diamond is a material of choice for the fabrication of optical windows and for protective and anti-reflecting coatings for optical materials. For these kinds of applications, the diamond coating must have a high purity and a low surface roughness to guarantee a high transparency. It should also be synthesized at low surface temperature to allow the deposition on low melting-point substrates such as glasses. In this work, the ability of a Distributed Antenna Array (DAA) microwave system operating at low temperature and low pressure in H2/CH4/CO2 gas mixture to synthesize nanocrystalline diamond (NCD) films on borosilicate and soda-lime glass substrates is investigated aiming at optical applications. The influence of the substrate temperature and deposition time on the film microstructure and optical properties is examined. The best film properties are obtained for a substrate temperature below 300 °C. In these conditions, the growth rate is around 50 nm·h−1 and the films are homogeneous and formed of spherical aggregates composed of nanocrystalline diamond grains of 12 nm in size. The resulting surface roughness is then very low, typically below 10 nm, and the diamond fraction is higher than 80%. This leads to a high transmittance of the NCD/glass systems, above 75%, and to a low absorption coefficient of the NCD film below 103 cm−1 in the visible range. The resulting optical band gap is estimated at 3.55 eV. The wettability of the surface evolves from a hydrophilic regime on the bare glass substrates to a more hydrophobic regime after NCD deposition, as assessed by the increase of the measured contact angle from less than 55° to 76° after the deposition of 100 nm thick NCD film. This study emphasizes that such transparent diamond films deposited at low surface temperature on glass substrate using the DAA microwave technology can find applications for optical devices.

Thermal conductivity of nano- and micro-crystalline diamond films studied by photothermal excitation of cantilever structures

Polycrystalline diamond films have unique structural and thermal properties that make them suitable for use in extreme environments. Recently, they have been utilized in accelerator beamlines for electron stripping due to their unique combination of mechanical and thermal properties. Thermal conductivities of nanocrystalline diamond (NCD) and microcrystalline diamond (μCD) films were characterized using photothermally actuated bimaterial cantilevers. Approximately one micrometer thick NCD and μCD cantilevers were fabricated from microwave plasma enhanced chemical vapor deposition grown polycrystalline diamond (PD) films. A layer of gold was sputtered on the diamond film surfaces to make bilayer cantilevers and the thermal response time was measured by photothermally exciting the bilayer cantilevers, causing them to deflect. Finite element thermomechanical modeling of the deflection dynamics in response to photothermal actuation was performed to determine the NCD and μCD thermal diffusivities and conductivities. By fitting the simulated and experimentally observed response times, thermal conductivities of 10 and 60 W/(m-K) were extracted for the NCD and μCD samples, respectively. Expected changes in thermal conductivity of PD in higher temperature regimes are also discussed. In addition to applications of PD films as electron stripping foils, these findings also have implication in fields such as micro/nano-electromechanical systems.

Inkjet-Printed High-Q Nanocrystalline Diamond Resonators.

Diamond is a highly desirable material for state-of-the-art micro-electromechanical (MEMS) devices, radio-frequency filters and mass sensors, due to its extreme properties and robustness. However, the fabrication/integration of diamond structures into Si-based components remain costly and complex. In this work, a lithography-free, low-cost method is introduced to fabricate diamond-based micro-resonators: a modified home/office desktop inkjet printer is used to locally deposit nanodiamond ink as ∅50-60 µm spots, which are grown into ≈1 µm thick nanocrystalline diamond film disks by chemical vapor deposition, and suspended by reactive ion etching. The frequency response of the fabricated structures is analyzed by laser interferometry, showing resonance frequencies in the range of ≈9-30 MHz, with Q-factors exceeding 104 , and (f0 × Q) figure of merit up to ≈2.5 × 1011 Hz in vacuum. Analysis in controlled atmospheres shows a clear dependence of the Q-factors on gas pressure up until 1 atm, with Q ∝ 1/P. When applied as mass sensors, the inkjet-printed diamond resonators yield mass responsivities up to 981 Hz fg-1 after Au deposition, and ultrahigh mass resolution up to 278 ± 48 zg, thus outperforming many similar devices produced by traditional top-down, lithography-based techniques. In summary, this work demonstrates the fabrication of functional high-performance diamond-based micro-sensors by direct inkjet printing.

Spectroscopic Ellipsometry of Nanocrystalline Diamond Film Growth.

With the retention of many of the unrivaled properties of bulk diamond but in thin-film form, nanocrystalline diamond (NCD) has applications ranging from micro-/nano-electromechanical systems to tribological coatings. However, with Young’s modulus, transparency, and thermal conductivity of films all dependent on the grain size and nondiamond content, compositional and structural analysis of the initial stages of diamond growth is required to optimize growth. Spectroscopic ellipsometry (SE) has therefore been applied to the characterization of 25–75 nm thick NCD samples atop nanodiamond-seeded silicon with a clear distinction between the nucleation and bulk growth regimes discernable. The resulting presence of an interfacial carbide and peak in nondiamond carbon content upon coalescence is correlated with Raman spectroscopy, whereas the surface roughness and microstructure are in accordance with values provided by atomic force microscopy. As such, SE is demonstrated to be a powerful technique for the characterization of the initial stages of growth and hence the optimization of seeding and nucleation within films to yield high-quality NCD.

Preparation and optical properties of nanocrystalline diamond coatings for infrared planar waveguides

The high index of refraction planar optical waveguides operating in the near infrared spectrum were prepared from the intrinsic hydrogenated amorphous silicon (a-Si:H) layer coated by nanocrystalline diamond (NCD). The a-Si:H layers were grown by radio frequency plasma enhanced chemical vapor deposition on glass substrates from silane and hydrogen mixture at the substrate temperature about 250°C followed at 210°C by the NCD coating grown by the microwave plasma enhanced chemical vapor deposition from hydrogen rich plasma with adding methane and carbon dioxide at the substrate temperature. The optical absorptance spectra, Raman spectra and scanning electron microscopy reveal the diamond character of the NCD coatings as well as the stability of a-Si:H layer during NCD deposition. The critical issue of the NCD coated a-Si:H optical waveguide is the optical absorption in NCD coating. The Finite Element Method simulations show that the near infrared optical absorption coefficient of the NCD coating must be reduced below 10/cm to reduce the optical attenuation below 1dB/cm in the planar waveguide with the 50nm thick NCD coating.

CVD 다이아몬드가 코팅된 알루미늄 방열판의 방열 특성

Nanocrystalline diamond(NCD) coated aluminium plates were prepared and applied as heat sinks for LED modules. NCD films were deposited on 1 mm thick Al plates for times of 2 - 10 h in a microwave plasma chemical vapor deposition reactor. Deposition parameters were the microwave power of 1.2 kW, the working pressure of 90 Torr, the CH4/Ar gas ratio of 2/200 sccm. In order to enhance diamond nucleation, DC bias voltage of �90 V was applied to the substrate during deposition without external heating. NCD film was identified by X-ray diffraction and Raman spectroscopy. The Al plates with about 300 nm thick NCD film were attached to LED modules and thermal analysis was carried out using Thermal Transient Tester (T3ster) in a still air box. Thermal resistance of the module with NCD/Al plate was 3.88 K/W while that with Al plate was 5.55 K/W. The smaller the thermal resistance, the better the heat emission. From structure function analysis, the differences between junction and ambient temperatures were 12.1 o C for NCD/Al plate and 15.5 o C for Al plate. The hot spot size of infrared images was larger on NCD/Al than Al plate for a given period of LED operation. In conclusion, NCD coated Al plate exhibited better thermal spreading performance than conventional Al heat sink.

나노결정질 다이아몬드가 코팅된 SiC 마모시험기 볼

Nanocrystalline diamond(NCD) coated SiC balls were applied in a ball-on-disk tribometer. After seeding in an ultrasonic bath containing nanometer diamond powders, 2.2 µm thick NCD films were deposited on sintered 3 mm diameter SiC balls at 600 o C in a 2.45 GHz microwave plasma CVD system. Bare ZrO2 and SiC balls were prepared for comparison as test balls. Tribology tests were performed in air with pairs of three different balls and mirror polished steel(SKH51) disk. The wear tracks on balls and disks were examined by optical microscope and alpha step profiler. Under the load of 3 N, the friction coefficients of steel against ZrO2, SiC and NCD-coated balls were between 0.4 and 0.8. After a few thousands sliding laps, the friction coefficient of NCD-coated balls dropped from 0.45 to below 0.1 and maintained thereafter. Under a higher load of 10 N or 20 N with a long sliding distance of 2 km, ZrO2 and SiC balls exhibited the similar friction coefficients as above. The friction coefficient of NCD-coated balls was less than 0.1 from the beginning and increased to above 0.1 steadily or with some fluctuations as sliding distance increased. NCD coating layers were found worn out after long duration and/or high load sliding test, which resulted in the friction coefficient higher than 0.1.

Role of high microwave power on growth and microstructure of thick nanocrystalline diamond films: A comparison with large grain polycrystalline diamond films

In this work, we study the growth habit of nanocrystalline diamond (NCD) films by exploring the very high power regime, up to 4kW, in a 5kW microwave plasma chemical vapour deposition (MPCVD) reactor, through addition of a small amount of nitrogen and oxygen (0.24%) into 4% CH4 in H2 plasma. The coupled effect of high microwave power and substrate temperature on NCD growth behaviour is systematically investigated by varying only power, while fixing the remaining operating parameters. When the power increases from 2kW to 4kW, resulting also in rise of the Si substrate temperature higher than 150°C, the diamond films obtained maintain the NCD habit, while the growth rate increases significantly. The highest growth rate of 4.6μm/h is achieved for the film grown at 4kW, which represents a growth rate enhancement of about 15 times compared with that obtained when using 2kW power. Possible factors responsible for such remarkable growth rate enhancement of the NCD films are discussed. The evolution of NCD growth characteristics such as morphology, microstructure and texture is studied by growing thick films and comparing it with that of large grain polycrystalline (PCD) films. One important characteristic of the NCD films obtained, in contrast to PCD films, is that irrespective of deposition time (i.e. film thickness), their grain size and surface roughness remain in the nanometer range throughout the growth. Finally, based on our present and previous experimental results, a potential parameter window is established for fast growth of NCD films under high power conditions.

Effects of Various Seeding Methods on Multi-Layered Diamond Coatings Deposited via Hot Filament Chemical Vapor Deposition

Multi-layer alternating nanocrystalline diamond (NCD) layer and polycrystalline diamond (PCD) layer was successfully deposited on pretreated tungsten carbide (WC) substrates with various seeding sizes (<0.1μm synthetic, <0.5μm synthetic, <0.25μm natural, <0.5μm natural, and <1μm natural) diamond with and without hammering by silicon carbide. X-rays penetrate through the coating to the substrate from XRD method was able to show strong peaks of diamond relative to WC despite the diamond film being 4μm thick only. It is found that substrates with no hammering produce stronger signals. The coating was cross sectioned and analysed using field emission scanning electron microscopy showing the multi-layer with NCD grains that has coalesced and columnar structure for PCD. None of the diamond coating delaminated during cross sectioning showing good adhesion. Raman was able to capture data from the 1-1.6μm thick NCD layer only while AFM measured the extreme low roughness of the NCD surface.

초경합금에 나노결정질 다이아몬드 코팅 시 금속 중간층의 효과

For the coating of diamond films on WC-Co tools, a buffer interlayer is needed because Co catalyzes diamond into graphite. W and Ti were chosen as candidate interlayer materials to prevent the diffusion of Co during diamond deposition. W or Ti interlayer of 1 μm thickness was deposited on WC-Co substrate under Ar in a DC magnetron sputter. After seeding treatment of the interlayer-deposited specimens in an ultrasonic bath containing nanometer diamond powders, 2 μm thick nanocrystalline diamond (NCD) films were deposited at 600oC over the metal layers in a 2.45 GHz microwave plasma CVD system. The crosssectional morphology of films was observed by FESEM. X-ray diffraction and visual Raman spectroscopy were used to confirm the NCD crystal structure. Micro hardness was measured by nano-indenter. The coefficient of friction (COF) was measured by tribology test using ball on disk method. After tribology test, wear tracks were examined by optical microscope and alpha step profiler. Rockwell C indentation test was performed to characterize the adhesion between films and substrate. Ti and W were found good interlayer materials to act as Co diffusion barriers and diamond nucleation layers. The COFs on NCD films with W or Ti interlayer were measured as less than 0.1 whereas that on bare WC-Co was 0.6~1.0. However, W interlayer exhibited better results than Ti in terms of the adhesion to WC-Co substrate and to NCD film. This result is believed to be due to smaller difference in the coefficients of thermal expansion of the related films in the case of W interlayer than Ti one. By varying the thickness of W interlayer as 1, 2, and 4 μm with a fixed 2 μm thick NCD film, no difference in COF and wear behavior but a significant change in adhesion was observed. It was shown that the thicker the interlayer, the stronger the adhesion. It is suggested that thicker W interlayer is more effective in relieving the residual stress of NCD film during cooling after deposition and results in stronger adhesion.

Diamond-Modified AFM Probes: From Diamond Nanowires to Atomic Force Microscopy-Integrated Boron-Doped Diamond Electrodes

In atomic force microscopy (AFM), sharp and wear-resistant tips are a critical issue. Regarding scanning electrochemical microscopy (SECM), electrodes are required to be mechanically and chemically stable. Diamond is the perfect candidate for both AFM probes as well as for electrode materials if doped, due to diamond's unrivaled mechanical, chemical, and electrochemical properties. In this study, standard AFM tips were overgrown with typically 300 nm thick nanocrystalline diamond (NCD) layers and modified to obtain ultra sharp diamond nanowire-based AFM probes and probes that were used for combined AFM-SECM measurements based on integrated boron-doped conductive diamond electrodes. Analysis of the resonance properties of the diamond overgrown AFM cantilevers showed increasing resonance frequencies with increasing diamond coating thicknesses (i.e., from 160 to 260 kHz). The measured data were compared to performed simulations and show excellent correlation. A strong enhancement of the quality factor upon overgrowth was also observed (120 to 710). AFM tips with integrated diamond nanowires are shown to have apex radii as small as 5 nm and where fabricated by selectively etching diamond in a plasma etching process using self-organized metal nanomasks. These scanning tips showed superior imaging performance as compared to standard Si-tips or commercially available diamond-coated tips. The high imaging resolution and low tip wear are demonstrated using tapping and contact mode AFM measurements by imaging ultra hard substrates and DNA. Furthermore, AFM probes were coated with conductive boron-doped and insulating diamond layers to achieve bifunctional AFM-SECM probes. For this, focused ion beam (FIB) technology was used to expose the boron-doped diamond as a recessed electrode near the apex of the scanning tip. Such a modified probe was used to perform proof-of-concept AFM-SECM measurements. The results show that high-quality diamond probes can be fabricated, which are suitable for probing, manipulating, sculpting, and sensing at single digit nanoscale.

Impact of nitrogen doping on growth and hydrogen impurity incorporation of thick nanocrystalline diamond films

A much larger amount of bonded hydrogen was found in thick nanocrystalline diamond (NCD) films produced by only adding 0.24% N2 into 4% CH4/H2 plasma, as compared to the high quality transparent microcrystalline diamond (MCD) films, grown using the same growth parameters except for nitrogen. These experimental results clearly evidence that defect formation and impurity incorporation (for example, N and H) impeding diamond grain growth is the main formation mechanism of NCD upon nitrogen doping and strongly support the model proposed in the literature that nitrogen competes with CHx (x = 1, 2, 3) growth species for adsorption sites.

Diamond overgrown InAlN/GaN HEMT

In this work the technology and characterization of nanocrystalline diamond (NCD) films directly grown on InAlN/GaN HEMTs is presented. Optimization of GaN based HEMT process steps including metallization stacks is discussed. A fully processed InAlN/GaN HEMT structure with 7 nm barrier has been overgrown in a temperature range of 750 °C to 800 °C with a 500 nm thick nanocrystalline diamond film in a Hot Filament CVD system. First results of semi-enhancement mode of DC and RF HEMT operation are reported. The grown NCD films were characterized by SEM, TEM, and Raman spectroscopy. Although no direct thermal conductivity measurements are conducted yet; the performed experiments shows the compatibility of growing high quality NCD films, several microns thick, on InAlN/GaN HEMTs as a potential material for heat extraction purposes.

Effect of XeF laser treatment on structure of nanocrystalline diamond films

Nanocrystalline diamond (NCD) films were deposited on Si substrates by microwave plasma-enhanced chemical vapor deposition (MPECVD) using methane/hydrogen/oxygen (30/169/0.2 sccm) as process gases. Subsequently a thin (0.33 μm) and a thick (1.01 μm) NCD films were irradiated with XeF excimer laser ( λ = 351 nm) with 300 and 600 mJ cm − 2 of energy densities in air. The NCD films became rougher after laser irradiations. Fraction of graphitic clusters decreased but oxygen content increased in the thin NCD film after laser irradiation. Opposite phenomena were observed for the thick NCD films. Effect of laser irradiation to oxygenation and graphitization of NCD films was correlated with structural properties of free surface and grain boundaries of the thin and thick NCD films.

Role of oxygen additive on hydrogen impurity incorporation in nanocrystalline diamond films fabricated by microwave plasma chemical vapor deposition

AbstractIn this work, we study the influence of oxygen additive in the gas phase on hydrogen impurity incorporation into thick nanocrystalline diamond (NCD) films. Various diamond samples were grown on large silicon wafers of 5.08 cm diameter by adjusting the amount of oxygen and nitrogen additives into a conventional CH4/H2 plasma while keeping the other parameters constant using a 5‐kW microwave plasma‐assisted chemical vapor deposition (MPCVD) reactor. The morphology, crystalline quality, and hydrogen impurity content of the produced diamond samples were characterized using scanning electron microscopy, micro‐Raman spectroscopy, and Fourier transform infrared (FTIR) spectroscopy, respectively. The Raman and the FTIR spectra of the diamond samples indicate that on increasing the amount of oxygen additive in the gas phase, the crystalline quality of the NCD films is significantly enhanced, while their bonded hydrogen content decreases drastically. We conclude that a small amount of oxygen addition has an enhancement effect on the overall quality of diamond films ranging from polycrystalline to nanocrystalline in two ways: improving the diamond crystalline quality and suppressing hydrogen impurity incorporation.

ZnO/나노결정다이아몬드 적층 박막 SAW 필터

A surface acoustic wave (SAW) filter structure was fabricated employing thick nanocrystalline diamond (NCD) and thick ZnO films on Si wafer. The NCD film was deposited in an gas mixture by microwave plasma chemical vapor deposition method. The ZnO film was formed over the NCD film in an RF magnetron sputter using ZnO target and gas. On the top of the two layers, copper film was deposited by the RF sputter and inter digital transducer (IDT) electrode pattern (line/space : ) was defined by the photolithography including a lift-off etching process. The fabricated SAW filter exhibited the center frequency of 1.66 GHz and the phase velocity of 9,960 m/s, which demonstrated that a giga Hertz SAW filter can be realized by utilizing the nanocrystalline diamond thin film.

Fabrication of a film bulk acoustic wave resonator from nano-crystalline diamond

A proposed new type of diamond-based frequency device has been fabricated for the first time through the use of nano-crystalline diamond as the thin film component of a Film Bulk Acoustic Resonator (FBAR) device. The new fabrication process uses Si MEMS-based fabrication techniques on 200 nm thick nano-crystalline diamond deposited on a 4 inch silicon wafer. After completely removing the silicon substrate below the device by the BOSCH process, one port resonators with a diaphragm structure have been fabricated. All the devices produced in the first fabrication run operated successfully with a high frequency response of 3.5 GHz.

Nanocrystalline diamond resonator array for RF signal processing

Two-dimensional arrays of weakly coupled radio frequency (RF) nanomechanical resonators were designed and fabricated in 50–150 nm thick nanocrystalline diamond films using electron beam lithography and dry etch techniques. We present a novel resonator design that allows us to reduce fabrication tolerances and to confine the frequency spread of individual resonators within ∼ 0.6% over the entire modal frequency range up to 100 MHz. Due to an optimized plasma enhanced chemical vapor deposition (PECVD) nanocrystalline diamond deposition process, high quality factors ( Q > 8000) have been demonstrated for particular modes of vibrations in single nanomechanical resonators. 2D arrays of coupled resonators exhibit an acoustic band, formed by the splitting of the individual resonant modes. The propagation of a wide-band signal within the array has been demonstrated using a high magnification optical imaging setup with two independently controlled scanning lasers. In order to alleviate various sources of disorder in resonator arrays and to demonstrate their potential for RF signal processing, nanomechanical resonator fabrication techniques and resonator designs will be detailed and compared in this manuscript.

Infrared absorption study of hydrogen incorporation in thick nanocrystalline diamond films

We present an infrared (IR) optical absorbance study of hydrogen incorporation in nanocrystalline diamond films. The thick nanocrystalline diamond films were synthesized by microwave plasma-assisted chemical vapor deposition and a high growth rate about 3.0μm∕h was achieved. The morphology, phase quality, and hydrogen incorporation were assessed by means of scanning electron microscopy, Raman spectroscopy, and Fourier-transform infrared spectroscopy (FTIR). Large amount of hydrogen bonded to nanocrystalline diamond is clearly evidenced by the huge CH stretching band in the FTIR spectrum. The mechanism of hydrogen incorporation is discussed in light of the growth mechanism of nanocrystalline diamond. This suggests the potential of nanocrystalline diamond for IR electro-optical device applications.

Effects of processing conditions on the growth of nanocrystalline diamond thin films: real time spectroscopic ellipsometry studies

Abstract Real time spectroscopic ellipsometry has been applied to characterize the preparation of ∼2000-A thick nanocrystalline diamond films on Si substrates by microwave plasma-enhanced chemical vapor deposition. Diamond films prepared under different conditions, including variations in the substrate temperature and CH4+H2+O2 gas mixture, have been studied. In addition to providing accurate Si substrate surface temperatures for diamond film growth via an ellipsometry-based calibration, the real time approach yields information on the following processes: (i) generation of initial substrate damage by the seeding process used to enhance diamond nucleation, (ii) annealing of the seeding damage that occurs upon heating the substrate to the deposition temperature, and (iii) the structural evolution of the diamond film throughout the nucleation and bulk film growth regimes. In the nucleation regime, the following information on the diamond film can be extracted: (i) the diamond mass thickness evolution with time, (ii) the sp2 C and void volume fractions in the nucleating layer, and (iii) the thickness at which nuclei make contact, the latter providing an estimate of the nucleation density. In the bulk layer growth regime, the time evolution of the following information can be extracted: (i) the bulk layer thickness, (ii) the surface roughness layer thickness, (iii) the mass thickness of sp2 C in the bulk layer, and (iv) the void volume fraction in the bulk layer. In the nucleation regime, we find that high quality diamond nanocrystals form under a wide range of gas compositions for substrate temperatures above 700°C. It is proposed that nucleation is controlled by a disordered carbon phase embedded in the substrate surface during the seeding process, in which the Si wafer is abraded with diamond powder. Above 700°C, the disordered C at the substrate surface is believed to recrystallize in the diamond phase upon exposure to an initial H2 plasma prior to diamond film growth. A common feature of diamond growth under all deposition conditions is the incorporation of a large concentration of sp2 C when diamond nuclei coalesce (typically after a thickness of 200–400 A). The maximum concentration occurs under conditions for which poorer quality bulk films are obtained, including low substrate temperatures ( 0.02). The sp2 C is trapped within ∼300 A of the substrate interface, but under optimum conditions of growth (i.e., the diamond growth region of the CHO gas phase diagram), little additional sp2 C forms with continued growth after the first 300 A. The formation of sp2 C in the coalescence stage has been attributed to shadowing effects that lead to a reduction of atomic H relative to C-containing precursors arriving at shadowed surfaces. For the nanocrystalline diamond films studied here, it is found that films having the lowest volume fraction of sp2 C at the end of the 2000 A deposition also exhibit the lowest void volume fraction and the smoothest surfaces.