Qi Hua Fan

Diamond growth by chemical vapour deposition

This paper reviews the growth of diamond by chemical vapour deposition (CVD). It includes the following seven parts: (1) Properties of diamond: this part briefly introduces the unique properties of diamond and their origin and lists some of the most common diamond applications. (2) Growth of diamond by CVD: this part reviews the history and the methods of growing CVD diamond. (3) Mechanisms of CVD diamond growth: this part discusses the current understanding on the growth of metastable diamond from the vapour phase. (4) Characterization of CVD diamond: we discuss the two most common techniques, Raman and XRD, which have been intensively employed for characterizing CVD diamond. (5) CVD diamond growth characteristics: this part demonstrates the characteristics of diamond nucleation and growth on various types of substrate materials. (6) Nanocrystalline diamond: in this section, we present an introduction to the growth mechanisms of nanocrystalline diamond and discuss their Raman features.This paper provides necessary information for those who are starting to work in the field of CVD diamond, as well as for those who need a relatively complete picture of the growth of CVD diamond.

Diamond coating on steel with a titanium interlayer

Abstract Diamond coating on steel by microwave plasma chemical vapour deposition (MPCVD) is investigated. Direct growth of diamond film on steel substrate is possible, but its adhesion is poor because of the formation of a rather thick graphite layer at the diamond/steel interface. Calculation is performed in order to select a suitable interlayer. The results show that titanium is one of the promising candidates. Under optimized process conditions, adherent diamond film is coated on the steel with a titanium interlayer approx. 2 μm thick. Pull-off tests and micro-indentation tests are performed to evaluate the coating adhesion.

Microwave plasma chemical vapour deposition diamond nucleation on ferrous substrates with Ti and Cr interlayers

Abstract Diamond-coated steel is considered an important issue in synthetic diamond technology due to the great economical importance of enhancing the wear resistance and surface hardness of commercial Fe-based alloys. However, direct diamond coating by chemical vapour deposition (CVD) is rather problematic—adhesion and growth are seriously affected. The use of interlayers is a common approach to minimise these problems. This work reports an investigation on the establishment of good nucleation and growth conditions of diamond films by microwave plasma CVD (MPCVD) on ferrous substrates coated with Ti and Cr interlayers. Commercial grade ferrous substrates were pre-coated with commercial interlayers by sputtering (Ti, Cr) and electroplating (Cr) techniques. Steel substrates led to better results than iron cast substrates. The best films were obtained on Ti pre-coated steel substrate. The results on Cr interlayers pointed to the advantage of electroplating over the physical vapour deposition (PVD) sputtering. From the two selected parameter sets for diamond deposition, the one using lower power level conducted to the best results. Initial roughness and growth parameters were found to counteract on the uniformity of the diamond films. The morphology was studied by scanning electron microscopy (SEM), the roughness was estimated by profilometry, while diamond quality and stress state were evaluated by μ-Raman spectroscopy.

Chromium interlayers as a tool for enhancing diamond adhesion on copper

Abstract Diamond films on copper suffer from poor adhesion, mainly due to the lack of affinity between carbon and copper. In this study a chromium (Cr) interlayer has been employed to improve the adhesion between the diamond films and the copper substrates. The Cr interlayer, a carbide-forming material, plays the role of an adhesive between the diamond film and the copper substrate. A hot filament chemical vapour deposition (HFCVD) system, modified to enable the substrate to be negatively biased has been used to deposit diamond. It was observed that diamond nucleation together with the deposition rate increased with bias time. A bias time of 5 min was sufficient to increase diamond nucleation and enhance the growth rate. The diamond films deposited exhibited predominantly 〈111〉 orientation, as evident from the SEM micrographs. Diamond films grown on the Cr-coated Cu substrates gave reasonable adhesion values. Pull-off tests showed that the adhesion was better than the strength of the adhesive employed, ∼14 MPa. Scratch tests revealed a critical load of ∼7 N with unbiased samples, while the critical load increased significantly to ∼38 N for samples pre-treated at –250 V for 15 min. Furthermore, it was found from Raman analysis that films with better adhesion exhibited greater Raman shift of the 1332 cm −1 diamond peak. The overall results suggest that Cr is an effective interlayer for producing adherent diamond coatings onto Cu substrates.

Residual stresses in chemical vapour deposited diamond films

Abstract In this paper we report the determination of residual stresses in diamond films grown on Si(100) using a plate bending theory and a bi-metal theory combined with micro-Raman spectroscopy. Raman spectra show that with an increase in the film thickness, the characteristic diamond line shifts from higher wave numbers (>1332 cm −1 ) to lower ( −1 ), indicating a change of compressive to tensile bi-axial stress with increase in the film thickness. A plate bending theory and a bi-metal theory are used to determine the distribution of the stress induced by the thermal mismatch. The modelled results show that the bi-axial stress decreases linearly along the film growth direction and the stress at the film/substrate interface decreases when the film becomes thicker. The difference from the Raman results is attributed to intrinsic stress.

Free-standing diamond film preparation using copper substrate

Abstract Free-standing diamond films are prepared using copper substrates by microwave plasma chemical vapour deposition (MPCVD). A two-step growth method for stress relief is proposed to obtain relatively thin (∼10 μm) films, while thicker films (>20 μm) can be prepared by direct deposition. The film cracking mechanism is discussed. The free-standing diamond films show similar Raman spectra at surface side and back side. A carbide interface between the diamond film and the substrate is avoided as expected.

Evaluation of residual stresses in chemical-vapor-deposited diamond films

In this article we report the determination of residual stresses in diamond films grown on Si (100) by using a plate bending theory and a bimetal theory combined with micro-Raman spectroscopy. Raman spectra show that with increase in the film thickness the characteristic diamond line shifts from higher wave numbers (>1332 cm−1) to lower (<1332 cm−1), indicating a change in the nature of residual stress with the film growth. A plate bending theory and a bimetal theory are used to determine the distribution of residual stress induced by thermal mismatch. The modeled results show that the thermal stress decreases linearly along the film growth direction and the stress at the film/substrate interface decreases once the film becomes thicker. The difference from the Raman results is attributed to intrinsic stress, originating from lattice mismatch between the diamond and Si at the growth temperature and from a variation in microstructure of the film with its growth evolution.

Role of surface pre-treatment in the CVD of diamond films on copper

Abstract Diamond films have been deposited on copper substrates using hot-filament chemical vapour deposition (HFCVD). In order to improve the nucleation density, several methods of surface pre-treatment and substrate biasing have been investigated. These included polishing the substrates using a number of diamond powders and diamond pastes followed by ultrasonic cleaning. We show that the nucleation density on copper is highly dependent on the particle size in the polishing materials and on the polishing duration. Negative d.c.-biasing enhances more effectively the diamond nucleation on copper than the abrasion process. This method is also much more controllable, reliable and reproducible. High quality diamond films on copper have been produced via HFCVD using a precursor gas mixture of 1% methane in hydrogen. The as-deposited diamond films were characterised for film morphology, crystallinity, film quality and phase purity by scanning electron microscopy (SEM) and Raman spectroscopy. Raman spectroscopy analysis revealed an intense diamond peak at around 1332 cm −1 and nearly no graphite band. Diamond crystals of predominantly 〈111〉 orientation were evident from SEM analysis. Both the diamond phase purity and the nucleation density were enhanced in films deposited by the bias-enhanced nucleation (BEN) method as compared to the diamond deposited on abraded copper substrates.

Novel Time-modulated Chemical Vapor Deposition Process for Growing Diamond Films

Smooth polycrystalline diamond films were deposited onto silicon substrates using a newly developed time-modulated chemical vapor deposition (TMCVD) process. The distinctive feature of the TMCVD process involves pulsing the hydrocarbon gas, methane, at different flow rates for varying durations into the vacuum reactor during the chemical vapor deposition (CVD) process. Generally, CVD diamond films display nonuniformity in the crystal sizes and surface roughness along the film growth profile. The TMCVD method was specifically developed to (i) deposit smooth films, (ii) control film microstructure and morphology, and (iii) improve film reliability. We show that the TMCVD process produces diamond films with improved surface smoothness as compared to films of similar thickness produced by conventional CVD method under similar conditions. Surprisingly perhaps, the TMCVD method gave growth rates much higher than the conventional CVD method without reducing the film quality as revealed by the SEM micrographs and micro-Raman spectra.

A cross-corner effect in a rectangular sputtering magnetron

Electron trajectories, ionization distribution, and magnetic field in a conventional rectangular sputtering magnetron cathode are simulated in order to understand the mechanism of a cross-corner effect, which is a common phenomenon associated with rectangular magnetron cathodes and which limits the target utilization. It is found that once the magnetic field in end region of the cathode is different from that in straightway, the cross-corner effect exists. Using a fourth-order Runge–Kutta method, the electron trajectories are simulated, showing that the electrons may drift much faster in the end region than in the straightway and pass quickly to cross-corner region. A Monte-Carlo method is employed to simulate ionization distribution and to quantitatively predict target erosion. The results show denser ionization in the cross-corner region, causing more intensive erosion in that area. We demonstrate that by properly modifying the magnet field in the end region, the electron drift velocity and ionization distribution can be controlled and the cross-corner effect may be significantly reduced.