Diamond Deposition Process Research Articles

Scaling behavior of microwave reactors and discharge size for diamond deposition

Several microwave reactors of various sizes were studied to quantify the size scaling behavior of microwave plasma-assisted CVD (PACVD) diamond deposition. All the systems studied were microwave resonant cavity based systems using predominantly hydrogen based discharges. The input variables considered include the microwave frequency (2.45 GHz and 915 MHz), the microwave power level, and the discharge chamber size (5–28 cm diameter). The substrate sizes for the various reactors studied ranged from 2.5 cm diameter substrates for the small reactor size to 15 cm diameter substrates for the large reactor size. The variables quantified in the diamond deposition process and reactor included the microwave discharge power density, discharge size, discharge gas temperature, as well as, measurements of diamond deposition area and rate. The discharge power density versus pressure and discharge size is quantified by experimental measurements and numerical modeling. Results show that as the discharge size increases the discharge power density decreases significantly at a given pressure.

Wireless optical diagnostic apparatus for analyzing diamond thin film CVD process under high gravity conditions

A wireless optical emission diagnostic apparatus has been developed for in-situ diagnosis of the high gravity (HG) plasma properties during diamond chemical vapor deposition using H 2–CH 4 gas mixture. Optical emission spectroscopy (OES) enabled us to identify the emitting chemical species and to qualitatively investigate the plasma composition changes as a function of process parameters, including gravity, up to 100 g. During the diamond deposition process, the light emitted from the individual chemical species existing in the plasma is analyzed as overall emission spectra. The developed diagnostic system is not intrusive and allows a real-time probing of the plasma inside the reactor. The purpose of this paper is to describe the developed plasma diagnostic apparatus, including the new wireless data transfer communication system, which has been designed to improve both stability and reliability of traced plasma characteristics, together with some preliminary results.

Nanocrystalline diamond from carbon nanotubes

Structural phase transformation from multiwalled carbon nanotubes to nanocrystalline diamond by hydrogen plasma post-treatment was carried out. Ultrahigh equivalent diamond nucleation density above 1011 nuclei/cm2 was easily obtained. The diamond formation and growth mechanism was proposed to be the consequence of the formation of sp3 bonded amorphous carbon clusters. The hydrogen chemisorption on curved graphite network and the energy deposited on the carbon nanotubes by continuous impingement of activated molecular or atomic hydrogen are responsible for the formation of amorphous carbon matrix. Diamond nucleates and grows in the way similar to that of diamond chemical vapor deposition processes on amorphous carbon films.

Diamond coatings on electrical-discharge machined hardmetals

The influence of electrical-discharge machining (EDM) of WC–Co cemented carbides on the structural features and adhesion strength of subsequently deposited chemical vapor deposition (CVD) diamond coatings is investigated. Two different surface conditions are analyzed for a low binder-content hardmetal substrate. One variant corresponds to multi-pass sequential EDM, whereas the second one, used here as a reference, is attained through grinding and polishing. In all the cases, identical substrate chemical etching and arc-plasma CVD standard diamond deposition processes are applied. It is found that fine-executed EDM of hardmetals allows the deposition of diamond coatings exhibiting similar structural and adhesion characteristics to those obtained on conventionally machined cemented carbides substrates. Although surface integrity of the substrate together with quality and residual stresses of the diamond film are slightly affected by EDM, adhesion strength corresponding to this surface condition is retained because EDM-induced isotropically rough surfaces enhance mechanical interlocking of the diamond coating to the hardmetal substrate.

Direct current bias effects in RF induction thermal plasma diamond CVD

Substrate biasing is an emerging technique extensively used in diamond and cBN chemical vapor deposition (CVD) processes. Direct current bias voltages between -400 V and +500 V are here applied to a diamond deposition surface in an RF induction thermal plasma CVD system. This is made possible by introducing a high-impedance filter network, eliminating the time-varying voltage drop across the plasma-substrate junction and suppressing the radio-frequency interference. A traditional limitation of thermal RF systems is thereby overcome. Negative. bias conditions enhance the initial diamond nucleation density. Positive bias improves the diamond quality and augments the film growth rate. A threefold increase in linear growth rate is attained at +500 V as compared to the unbiased case. In conjunction with optical emission spectroscopic diagnostics, the substrate is used as an electrical and thermal probe. Contrary to do arcjet CVD, there is no secondary discharge created in the RF system at positive bias voltage. Also, the role of ion bombardment at negative bias is shown to be of little importance. It is inferred that the predominant mechanism leading here to changes in the diamond deposit under bias conditions is the promotion and suppression of electron emission from the growing diamond.

Modeling the effect of oxygen on vapor phase diamond deposition inside micro-trenches

For the same feed gas composition, radical species concentrations vary with depth inside micro-trenches during diamond deposition. However, the feed gas compositions are typically used for predicting diamond deposition vs. non-diamond deposition. So, any type of modeling for predicting diamond deposition quality and uniformity inside trenches and onto three-dimensional objects is not possible. In this regard, a simple modeling approach utilizing a recently constructed C–H–O ternary diagram, based on radical species composition by Eaton and Sunkara [Eaton and Sunkara, Diam. Relat. Mater., 9 (2000) 1320], demonstrated the diamond deposition process inside trenches. Specifically, the effect of oxygen in the feed gases on the depths at which diamond can be deposited is modeled under both Knudsen and Fickian diffusion conditions and compared with experimental observations. The results suggested that certain feed gas phase compositions (C/O ratio) help increase the aspect ratio (>2.0) at which diamond deposition can be achieved.

Diamond films grown by a 60-kW microwave plasma chemical vapor deposition system

In order to use chemical vapor deposition (CVD) diamond films for electronic devices, it is necessary to establish technologies for producing diamond wafers with controlled quality. Most of existing diamond CVD systems are, however, designed primarily for laboratory use. To cross the technological gap between the commercial production and the laboratory experiments, the current CVD technologies of diamond must be scaled up and upgraded. Development of large-scale diamond deposition processes was undertaken by using a microwave plasma CVD system, equipped with a 915-MHz, 60-kW generator for generating a large-size plasma. Polycrystalline diamond films were deposited from a hydrogen/methane gas mixture with typical gas pressures and substrate temperatures of 80–120 torr and 800–1050°C, respectively. It was found that depending on the growth conditions, the deposited films have various surface morphologies. Some of the samples have well-defined {111} and {100} facets of up to tens of micrometers in size. The Raman spectra had an intense main peak due to diamond at 1333 cm −1 without a trace of non-diamond carbon. The film quality in terms of Raman spectra was relatively uniform across the samples of 100 mm in diameter. Both 〈111〉 and 〈001〉 textured diamond films were obtained by selected growth conditions.

Adherent diamond coatings on cemented carbide substrates with different cobalt contents

A new process has been employed for preparing diamond coatings on cemented carbide substrates. Before depositing the diamond coatings, a diffusion barrier interlayer was prepared by solid diffusing boron into the substrates and this interlayer has the function of suppressing catalytic effect of Co on non-diamond formations. It was confirmed that on cemented carbide substrates with Co contents ranging from 3 to 8%, high quality adherent diamond coatings could be deposited. The formation of the boronized interlayer changed the catalytic character of the substrate surface and the interlayer functioned as a diffusion barrier layer against outward Co diffusion. The latter beneficial effect was found to be accompanied by phase transformations in the interlayer, from one composed of mainly WC, CoW 2B 2 and CoB phases to that of CoWB, WC and Co 2B phases, during the high temperature diamond deposition processes.

Time-resolved plasma diagnostics for a better understanding of the improvement of pulsed MWPACVD of diamond

The deposition of diamond layers from CH4-H2 microwavedischarge operating in pulsed mode has been achieved. It has been shownthat the variation of the time parameters of the process (frequency andduty cycle) leads to noticeable modifications of the deposited layers.From plasma diagnostic measurements, the change in plasma composition hasbeen determined and correlated with the quality and growth rate of thediamond thin films. Particular attention has been paid to theconcentration of H-atoms, CH and C2 radicals and their evolution duringthe discharge regime and the afterglow. Indeed, these species are wellknown either as agents for graphite etching (H), or diamond precursors(CHx imaged by CH) or graphite precursors (C2Hx imaged by C2).Optimum values of the power pulse repetition rate (500 Hz) and duty cycle(50%) have been found which are correlated with the variation of therelative concentrations of H, CH and C2 with time, especially duringthe afterglow. It has been shown that these optimum conditions correspondto a minimization of C2 in the afterglow while H and CH concentrationsremain high enough to continue the diamond deposition process after thepower is switched off.

Investigation of chemical kinetics and energy transfer in a pulsedmicrowave H2/CH4 plasma

We present a modelling study of pulsed H2/CH4 microwaveplasmas obtained under moderate pressure discharge conditions in a tubularquartz reactor. The transport in the reactor was described using a Nusseltmodel for a radially quasi-homogeneous plasma. The thermal behaviour of theplasma was modelled by distinguishing a single heavy species energy mode andthe electron translation mode. The chemistry was described using a 30species-130 reaction model. The time variations of the electron energydistribution function, the species concentrations and the gas temperature weredetermined by solving the coupled set of the electron Boltzmann equation,species kinetics equations and a total energy equation. Some of the resultsobtained from the present model were compared to measurements previouslycarried out on the plasmas considered. Good agreement was obtained for thetime variations of the gas temperature, the relative concentration of theH-atom and the intensities of the Hα and the argon 750 nm emissionlines. The effect of the duty cycle on the time-averaged composition andtemperatures of the discharge was also studied. Results showed that moderatepressure H2/CH4 pulsed discharges obtained at duty cycles of less than20% show different behaviour than those obtained at higher duty cycles. Inparticular, while the plasma reaches the permanent periodic regime in lessthan 2 pulse-periods, i.e. 60 ms, for duty cycle values of less than 20%,long-time-scale density variations of hydrocarbon species, ions and electronsare obtained when this parameter is greater than 20%. The model was also usedto determine if the use of a pulsed regime may bring some improvements inplasma-assisted diamond deposition processes. For this purpose we analysed thevariation with duty cycle of the time-averaged populations of the H-atom andCH3 that represent the key species for diamond deposition. Results showedthat pulsed discharges with small duty cycle, of typically less than 20%,lead to a substantial enhancement of the time-averaged dissociation yield. Onthe other hand, the CH3 concentration exhibits a strong decrease with theduty cycle. The methyl concentration in the investigated pulsed discharge isgenerally smaller than in continuous wave discharges obtained in the samereactor. These results indicate that short-pulse discharges would favour theformation of films with higher Raman quality, while long duty cycle pulseddischarges would enable deposition at higher growth rates.

A new field of phase diagrams of stationary nonequilibrium states

Diamond is a metastable phase, while graphite is the stable phase in low pressure equilibrium phase diagrams of carbon. There seemed to be no way to get diamond from graphite under low pressures. However, a new activated low pressure diamond deposition process has emerged. A new concept of phase diagrams of stationary nonequilibrium states on the basis of nonequilibrium thermodynamics can be used for explanation of the “thermodynamic paradox” that the diamond growth with graphite etching takes place under activated low pressure. The agreement between CALPHAD prediction and experiments is excellent.

CVD diamond grown by microwave plasma in mixtures of acetone/oxygen and acetone/carbon dioxide

Abstract Diamond has been deposited on silicon and molybdenum substrates by microwave plasma enhanced chemical vapor deposition technique in acetone/oxygen and acetone/carbon dioxide mixtures. A narrow C/O ratio around 1:1 was found necessary for diamond to be deposited under our deposition conditions. Diamond of good quality was deposited at rates exceeding 25 μm/h. By the use of high power density microwave plasmas, diamond deposition using these two mixtures has been achieved at substrate temperatures up to around 1300°C. A comparison between the diamond deposition process using these two mixtures without additional hydrogen gas and the traditional diamond deposition process using the mixture of methane and hydrogen will be made.

Diamond deposition in low-pressure acetylene flames: in situ temperature and species concentration measurements by laser diagnostics and molecular beam mass spectrometry

Diamond deposition in a flat, premixed acetylene–oxygen–argon flame at 50 mbar was investigated to characterize the reactive gas phase in the vicinity of the substrate. For this, flames with and without a substrate present were analyzed as a function of stoichiometry; also, the distance between the substrate and the burner was varied. Optimal conditions for the deposition of diamond films were found for oxygen–acetylene ratios of 1.3 and 1.4 and at distances between substrate and burner of 8, 9, and 10 mm. The flame structure in this region was investigated. In particular, gas temperature and OH radical concentrations were measured by laser-induced fluorescence (LIF). Furthermore, hydrogen atoms were monitored using three-photon excitation and subsequent fluorescence detection. Molecular beam mass spectrometry was employed to obtain an overview of stable species and hydrocarbon intermediates. The results provide a substantial experimental basis for comparison with theoretical models and are consistent with earlier observations, which stress the importance of H and CH 3 for the diamond deposition process. In addition, the observations indicate the participation of hydrocarbon species with more than 2 carbon atoms, e.g., C 3H 3, C 4H 3, and C x H 2 with x = 4, 6, or 8, in the gas-phase reactions controlling the deposition of diamond; an active role for these species in diamond chemical vapor deposition (CVD) has not been discussed before. As a first interpretation, diamond formation seems to be controlled by a counterbalance between OH and hydrocarbon intermediates at a position in the flame where sufficient H-atoms and CH 3 radicals are present to support diamond film growth.

Free-standing CVD diamond wafers for thermal management by d.c. arc jet technology

Because of the unfavorable mechanical properties of diamond, the source and type of stresses present must be identified and controlled at every stage of the CVD diamond deposition process in order to develop a high-yield, large-wafer-scale fabrication process. In this paper, we report on the types of defect and stress encountered in free-standing CVD diamond films deposited by d.c. arc jet deposition. Substrate materials and designs, temperature non-uniformity, and process conditions all contribute to the type and magnitude of stresses present in a film. Results of mechanical strength testing by biaxial flexure are presented. Strength sample sets consisted of 20 disks from each diamond wafer. Results from over 50 wafers are summarized. Weibull analysis of strength data is presented, with a description of the effects of thickness and sample orientation. It is shown that by implementing tight process control and through judicious selection of a substrate material, d.c. arc jet technology is capable of producing 175 mm diameter wafers at high yield with uniform properties.

Diamond microstructures for optical micro electromechanical systems

We have used hot filament chemical vapour deposition (HFCVD) to fabricate diamond microstructure components for optical micro electromechanical systems (MEMS). In order to demonstrate the wide application range for diamond technology we have made components for different applications such as diffractive optics, laser-to-fibre alignment, and active cooling of high power devices. The free-standing polycrystalline diamond devices were grown on moulds of silicon and fused silica, creating replicas of the moulds. The structured silicon-on-insulator (SOI) wafers as replicating moulds for diamond deposition makes it possible to create shapes that can be very useful for laser-to-fibre alignment. The two-step SOI diamond deposition process enables the creation of long and precise capillaries or capillary arrays with a flat top-side here intended for cooling of high power electronic devices. Furthermore, microstructured fused silica has been introduced as mould material, which gives new possibilities in micro-optical mould design since fused silica technology is much more common in optical industry.

The status and future of diamond thin film FED

Abstract Field emission displays have emerged as a leading contender of low-power flat panel applications. As one of the promising field emission cathodes (FEC), diamond thin film has some unique advantages over its conventional microtip counterpart because of the superlative physical and chemical properties of diamond and the simplicity of cathode manufacturing. Over the last year, FEPET, Inc., one of the leading organizations in developing FEC, has made tremendous progress in its cathode emission performance, film processing, and understanding of the emission mechanism. Currently, an emission site density (ESD) of 2×10 5 sites per cm 2 and current density of 100 mA/cm 2 have been achieved at an extraction field of about 10 V/μm. We also developed a novel patterning process that allows emission patterning without applying any post-deposition processing to the film to degrade its emission properties. The improvement in film emission properties is found closely related to a diminishing diamond peak and a strongly accentuated graphitic or amorphous carbon shift in the UV-Raman spectra. High-resolution atomic force microscopy (AFM) analysis also showed an association of the emission sites with conductive–insulating–conductive regions of the surface. With the achievable ESD today, diamond film FEC can already find many practical applications such as low- to medium-resolution displays and backlights for liquid crystal display (LCDs). For high-resolution color displays, significant improvement in emission site density is still necessary, and a glass substrate with high-quality surface becomes indispensable. The use of high-temperature glass applied for polysilicon active matrix LCDs (AMLCDs) or a low-temperature diamond thin film deposition process needs, therefore, to be developed.

Transverse bias-enhanced nucleation of diamond in hot filament chemical vapor deposition

The nucleation density of diamond films was greatly enhanced by inserting a transverse bias into hot filament vapor deposition chamber, which differs from the conventional used longitudinal bias method. In-situ Optical Emission Spectroscopy (OES) technique was used to investigate the diamond deposition process. The relative optical emission intensities of the atomic hydrogen and CH radical to argon near the substrate were calculated. The results show that the improvement of the nucleation density is due to the product of much more atomic hydrogen and CH radicals.

Diamond deposition on titanium and iron substrates pretreated in N 2–C 2H 2 plasma

Diamond deposition on pure titanium and iron substrates pretreated in N 2–C 2H 2 plasma without any mechanical surface pretreatment was studied. High quality diamond film was deposited on such pretreated substrates by CVD using combustion flame. After the pretreatment, a C–C bond was detected on the modified substrate, and graphitic carbon formation on the substrate was suppressed during diamond deposition. The surface of the modified substrate also became slightly rough, so the anchor effect between the film and the substrate was expected. This suppression and the anchor effect would indicate a remarkable improvement of adhesion of the film to the substrate. In particular, the diamond deposition process on the plasma-pretreated iron substrate was compared in detail with that on the substrate scratched with diamond powder. It was found that diamond was directly grown from the carbonitrided surface layer of the plasma pretreated substrate.

Characterization and electron field emission from diamond coatings deposited by multiple laser process

Abstract A new diamond deposition process utilizing a plasma and a variety of interactions from a multiple laser system has been demonstrated, with WC/Co substrates. The process is conducted in open air and does not involve hydrogen. Structural characterization of the diamond coatings, which have exceptional adhesion to cutting tool inserts, indicates a cubic diamond structure. Tungsten and cobalt atoms are incorporated into the film and a layer depleted in cobalt exists at the diamond-WC/Co interface. Electron field emission current densities, useful for flat panel displays of 6 mA/cm2 at an applied voltage of 3000 V for a film-anode distance of 20 μm has been measured.

Application of heat treatment and dispersive strengthening concept in interlayer deposition to enhance diamond film adherence

Two different deposition processes were carried out to enhance adherence of diamond films on WC+3~5%Co substrate with Ti-Si as the interlayer. One process can be called two-step diamond deposition process. Another process can be called interlayer heat treatment process. Diamond films were deposited by a microwave plasma chemical vapor deposition system. Ti and Si interlayer are deposited by DC sputter and an E-gun, respectively. Film morphologies, interface structure and film quality were examined by SEM, XRD, Auger electron spectroscopy and Raman spectroscopy. The residual stresses and adhesion strengths of the films were determined by Raman spectroscopy and indentation adhesion testing, respectively. Comparing the regular one-step diamond deposition process with the present two different new processes, the average dP/ dX values, which are a measure of the adherence of the film, are 354 kgf/mm, 494 kgf/mm and 787 kgf/mm, respectively. In other words, the interlayer heat treatment process gives the best film adherence on average. For the two-step diamond deposition process, the interlayer thickness and the percent diamond surface coverage of the first diamond deposition step are the main parameters, and there exists an optimum Ti thickness and percent diamond coverage for the best film adherence. The main contribution to better film adherence is not a large difference in residual stress, but is due to the following reasons. The interlayer heat treatment can transform amorphous Si to polycrystalline Si, and may form strong TiC and SiC bonding. The polycrystalline Si and the diamond particles from the first diamond deposition step can be an effective seeds to enhance diamond nucleation.