Mechanical properties of diamond thin films

Abstract

The mechanical properties of diamond films deposited via hot filament chemical vapour deposition have been determined using a range of techniques, and related to the composition and morphology of the diamond films as determined by laser Raman spectroscopy. As the quality of the film increases, its hardness (as determined by the volume law of mixtures hardness model) also increases until it is larger than values often reported for polycrystalline bulk material, a consequence of the very small grain size in the films. Coating adhesion, as determined from indentation adhesion tests, also appears to improve with coating quality. Variations in the behaviour of the friction coefficient between diamond films and diamond and steel counterfaces are less well defined, but it appears that the surface morphology of the film is important in dictating the behaviour rather than the quality of the diamond. These results are discussed in the context of the potential use of diamond coatings in tribological applications.

MST/1695     

Polarization properties of thin films of diamond

Thin films transparent to optical radiation offer polarization properties that are enhanced when the thickness of the film is an odd multiple of the quarter-wavelength. The transmission and reflection properties of a 1.16-μm-thick film of diamond realized by plasma-assisted chemical vapor deposition have been studied at 10.6 μm. A compact polarizer built with four films at a Brewster angle revealed an extinction ratio of better than 1:1000 of the S polarization. The interest in optics in which parasitic-reflected or transmitted beams do not exist is pointed out. The high damage threshold of diamonds opens the possibility of controlling the polarization characteristics of high-power lasers used, for example, for soldering and cutting applications.     

Hydrogen induced microstructural variation in diamond and diamond like carbon thin films

Hydrogen plays a crucial role in the growth of micro-crystalline diamond (MCD) and diamond like carbon (DLC) thin films grown by plasma assisted chemical vapour deposition (PACVD) processes. It selectively etches graphite phase and helps in stabilizing the diamond phase. The presence of various hydrocarbon species in the plasma and their reaction with atomic, excited or molecular hydrogen on the substrate surface decide the mechanism of diamond nucleation and growth. Several mechanisms have been proposed but the process is still not well understood. Control of hydrogen and other deposition parameters in the PACVD process leads to deposition of yet another class of materials called diamond like carbon. By varying the concentration of hydrogen it is possible to produce purely amorphous carbon films on the one hand and amorphous hydrogenated carbon films (with as high as 60% hydrogen) on the other. Very hard, optically transparent and electrically insulating films characterize the diamond like behaviour. The proportion of hydrogen and its bonding with carbon or hydrogen in the film can be varied to obtain very hard to very soft films which could be optically transparent or opaque. The microstructure of these films have been investigated by a large number of techniques. The results show interesting situations. This paper reviews the work on the role of hydrogen on the growth, structure and properties of MCD and DLC thin films.     

Bias enhanced nucleation of diamond thin films in a modified HFCVD reactor

In this study we investigated the nucleation of synthetic diamond thin films on Si substrates by double bias enhanced Hot Filament Chemical Vapour Deposition (HFCVD) method. First, we investigated the influence of the bias voltage and secondly the influence of the nucleation time under different bias voltages. The bias voltage was varied from −120 V up to −220 V as well as the nucleation time was changed from 30 up to 120 min in order to obtain the optimized nucleation conditions for following growth of continuous diamond layer. Samples were analyzed by Scanning Electron Microscopy (SEM) and Raman Spectroscopy. SEM was used for determination of cluster sizes and their distribution on the Si surface, while Raman Spectroscopy for determination and analysis of carbon phases.     

Macrotexture and growth chemistry in ultrananocrystalline diamond thin films

We have determined the average preferred crystalline orientation of thin ultrananocrystalline diamond (UNCD) films using X-ray diffraction. The grain size and lattice parameters of the films were also calculated. We show how these characteristics change markedly with the gas chemistry used during growth, adding either 0-20% nitrogen or 0-15% hydrogen to the argon-rich, argon and methane microwave plasma used. We discuss how these changes give evidence that there is a competing growth mechanism between C₂ dimer mediated growth and the more widely used methyl radical growth process. Finally, we identify an additional X-ray diffraction peak, dependent on both the substrate used and growth conditions, as silicon carbide. We discuss these results in the context of the growth mechanisms of ultrananocrystalline diamond.     

Correlation between diamond grain size and hydrogen incorporation in nanocrystalline diamond thin films

Hydrogen-incorporated nanocrystalline diamond thin films have been deposited in microwave plasma enhanced chemical vapour deposition (CVD) system with various hydrogen concentrations in the Ar/CH₄ gas mixture. The bonding environment of carbon atoms was detected by Raman spectroscopy and the hydrogen concentration was determined by elastic recoil detection analysis. Incorporation of H₂ species into Ar-rich plasma was observed to markedly alter the microstructure of diamond films. Raman spectroscopy results showed that part of the hydrogen is bonded to carbon atoms. Raman spectra also indicated the increase of non-diamond phase with the decrease in crystallite size. The study addresses the effects of hydrogen trapping in the samples when hydrogen concentration in the plasma increased during diamond growth and its relation with defective grain boundary region.     

Protein-modified nanocrystalline diamond thin films for biosensor applications

Diamond exhibits several special properties, for example good biocompatibility and a large electrochemical potential window, that make it particularly suitable for biofunctionalization and biosensing. Here we show that proteins can be attached covalently to nanocrystalline diamond thin films. Moreover, we show that, although the biomolecules are immobilized at the surface, they are still fully functional and active. Hydrogen-terminated nanocrystalline diamond films were modified by using a photochemical process to generate a surface layer of amino groups, to which proteins were covalently attached. We used green fluorescent protein to reveal the successful coupling directly. After functionalization of nanocrystalline diamond electrodes with the enzyme catalase, a direct electron transfer between the enzyme's redox centre and the diamond electrode was detected. Moreover, the modified electrode was found to be sensitive to hydrogen peroxide. Because of its dual role as a substrate for biofunctionalization and as an electrode, nanocrystalline diamond is a very promising candidate for future biosensor applications.     

Dielectric and Raman spectroscopy of MWCVD diamond thin films

The dielectric properties of diamond thin films obtained on silicon substrates by microwave plasma-assisted chemical vapour deposition (MWCVD) have been measured in the frequency range from 0.1 to 10³ kHz at different temperatures up to 150 C. The experimental results have been discussed in terms of the many body theory for dielectric relaxation in solids. Dielectric parameters as well as the d.c. conductivity of the samples have been correlated with the morphology and diamond content in the films, respectively detected from scanning electron microscopy (SEM) and Raman spectroscopy. The calculated activation energies for the dielectric relaxation mechanism agree with those obtained from other measurement techniques used in the electrical characterization of diamond films.     

The growth of epitaxial thin nickel films on diamond

Thin Ni films were deposited in vacuum onto (001) diamond surfaces at various temperatures between 22 and 600 °C. The films were usually in the “island” or “channel” stages of growth and were studied by transmission electron microscopy. At 200 °C epitaxy was observed. At temperatures between 500 and 600 °C the Ni islands were elongated along the 〈110〉 directions and were of rectanguaar shape.     

Growth of diamond thin films by chemical vapour deposition

Deposition of diamond thin films on non-diamond substrates at low pressures (<760 torr) and low temperatures (<2000°C) by chemical vapour deposition (CVD) has been the subject of intense research in the last few years. The structural and the electrical properties of CVD diamond films grown on p-type 〈111〉 and high-resistivity (>100 kΩ-cm) 〈100〉 oriented silicon substrates by hot filament chemical vapour deposition technique are described in this review paper.     

Enhanced nucleation and growth of diamond thin films using a nanodiamond monolayer

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

热丝CVD方法中气体状态参数的二维模拟计算

使用在连续介质中建立的二维流场数学模型，模拟计算了在用HFCVD方法生长金刚石薄膜过程中影响气体温度分布的多个沉积工艺参数，研究了气体的速度和体密度的空间分布．结果表明，在优化工艺参数条件下，高温热丝的热阻塞作用导致气体状态参数的不均匀空间分布；在热丝附近气体的速度大而靠近反应腔体侧壁处小；热丝附近处气体体密度减小而靠近冷反应腔体侧壁处增加，采用绝热或高等温边界的反应腔管道壁可以消除气体的热绕流现象，有利于大面积金刚石薄膜的快速均匀生长．     

Electron emission characterization of diamond thin films grown from a solid carbon source

Diamond films of various morphologies and compositions have been deposited on silicon substrates by a plasma-enhanced chemical transport (PECT) process from a solid carbon source. Electron emission efficiency of these diamond films is related to their morphology and composition. The electric field required to excite emission in a boron-doped polycrystalline diamond film ranged between 20 to 50 MV m⁻¹. In an undoped conducting nanocrystalline diamond composite film, the field was as low as 5–11 MV m⁻¹. The cold field electron emission of these films is confirmed from the Fowler-Nordhelm plots of the data. Enhancement of electron emission by band-bending and by the nanocrystalline microstructure are discussed. New diamond emitters made of nanocrystalline boron-doped diamond composite are proposed.     

Characterization of diamond thin films deposited by a CO2 laser-assisted combustion-flame method

Diamond thin films were deposited by a CO₂ laser-assisted O₂/C₂H₂/C₂H₄ combustion-flame process. The effect of the deposition parameters, in particular the laser wavelength and power, on the film surface morphology, microstructure and phases present was the primary focus of the work. The laser power was set at 100, 400 and 800 W while the wavelength was varied and set at 10.591 µm in the untuned condition and set at 10.532 µm to resonantly match the CH₂-wagging vibrational mode of the C₂H₄ molecule when in the tuned condition. When the laser was coupled to the combustion flame during deposition the diamond film growth was enhanced as the lateral grain size increased from 1 µm to greater than 5 µm. The greatest increase in grain size occurred when the wavelength was in the tuned condition. Scanning transmission electron microscopy images from focused-ion beam cross-sectioned samples revealed a sub-layer of smaller grains less than 1 µm in size near the substrate surface at the lower laser powers and untuned wavelength. X-ray diffraction results showed a more intense Diamond (111) peak as the laser power increased from 100 to 800 W for the films deposited with the tuned laser wavelength. Micro-Raman spectra showed a diamond peak nearly twice as intense from the films with the tuned laser wavelength.     

金刚石薄膜生长速度研究

在电子辅助热丝CVD中,研究刀具预处理对金刚石薄膜生长速度的影响。在保持生长条件不变的前提下,经酸腐蚀处理的刀具的侧、背面镀铜能使金刚石薄膜的生长速度从没有镀铜时的4μm／h增加到镀铜后的10．6μm／h。镀铜处理提高了刀具的电导率,使得热丝发射的电子在偏压电场的作用下,在刀具表面附近聚集,加速氢气和丙酮的裂解,从而提高金刚石薄膜生长速度。SEM和Raman测试结果表明,高速生长的金刚石薄膜仍然具有很高质量。     

Comparative study on dry etching of polycrystalline diamond thin films

The reactive ion etching (RIE) technique was used to etch polycrystalline diamond thin films. In this study we investigate the influence of process parameters (total pressure, rf power, gas composition) of standard capacitively coupled plasma RIE system on the etching rate of diamond films. The surface morphology of etched diamond films was characterized by Scanning Electron Microscopy and the chemical composition of the etched film part was investigated by Raman Spectroscopy.We found that the gas composition had a crucial effect on the diamond film morphology. The use of CF₄ gas resulted in flatter surfaces and lateral-like etching, while the use of pure O₂ gas resulted in needle-like structures. Addition of argon to the reactant precursors increased the ion bombardment, which in turn increased the formation of non-diamond phases. Next, increasing the rf power from 100 to 500 W increased the etching rate from 5.4 to 8.6 μm/h. In contrast to this observation, the rise of process pressure from 80 to 150 mTorr lowered the etching rate from 5.6 down to 3.6 μm/h.     

沉积参数对化学气相沉积纳米金刚石薄膜的影响

用强电流直流伸展电弧化学气相沉积金刚石薄膜装置,在CH4-Ar和CH4-H2-Ar气氛中沉积了纳米金刚石薄膜,研究了沉积气氛中H2加入量和沉积压力对金刚石薄膜显微组织和生长机制的影响.沉积气氛中H2含量对金刚石薄膜的表面形貌、晶粒尺寸和生长速度有显著影响,随着H2含量增加,金刚石晶粒尺寸增大,薄膜生长速度提高.在1%CH4-Ar气氛中沉积的纳米金刚石薄膜,晶粒尺寸细小,薄膜表面形貌光滑平整.在1%CH4-少量H2-Ar气氛中沉积的金刚石薄膜,晶粒尺寸小于100nm,薄膜表面形貌较平整.随着沉积压力提高,金刚石薄膜的生长速度增大.用激光Ram an对金刚石薄膜进行了表征.     

Field emission characteristics of high-energy ion-irradiated polycrystalline diamond thin films

Diamond thin films have been synthesized by hot-filament chemical vapor deposition process using a mixture of methane and hydrogen gases. The samples were subjected to very high-energy ion irradiation (100 MeV Au⁷⁺ ions). The field emission characteristics of ion-irradiated samples have been studied. High emission currents and low turn-on and threshold fields were obtained for ion-irradiated samples. The as-deposited and the ion-irradiated samples have been characterized by X-ray Diffraction, Scanning Electron Microscopy and Micro-Raman Spectroscopy techniques and the resulting changes are correlated with field emission results.     

Illumination-induced surface charges on thin diamond films at different wavelengths

Contact angle studies of pure water droplets were carried out on thin diamond films. The films were deposited on a fresh Si 111 wafer by plasma assisted chemical vapour deposition (PACVD) and confirmed to be diamond by Raman spectroscopy and scanning electron microscopy. Advancing and receding contact angles were measured as a function of droplet volume by using different incident wavelengths: red, green and violet. Large differences in the contact angles were observed, depending on the wavelength of the incident light. These differences are usually related to the existence of illumination-induced surface charges or electromagnetically active inhomogeneities in the diamond films. The results show that this technique is sufficiently sensitive to correlate changes in the contact angle with changes in the wavelength used.