Study On The Synthesis Of High-grade Microcrystalline Diamond Powders By Chemical Vapor Deposition

Microcrystalline diamond powders with the grain size of0.1~54μm have been widelyused in the super precision lapping, and polishing processes. Almost all the industrialdiamond powders are manufactured by high pressure and high temperature methods.However, typically with this method, the equipment is complicated, and its manipulatingsynthetic parameter is critical, leading to the inability to grow isolated crystals with ultra-finegrain sizes. Hence, most microcrystalline diamond powders (<38μm) are fabricated bycrushing the large-sized diamonds, and the fabrication process is very complex and mayspend too much time and energy for purifying and selecting the diamonds form impurities.Moreover, such powders are usually of low quality with irregular shapes, even after thepowders are treated by the shaping technology. These imperfections have negative effects onthe surface quality during the polishing or lapping process, therefore, it is necessary to exploita new method for improving the morphology and quality of microcrystalline diamondpowders. On the other hand, chemical vapor deposition (CVD) has attracted considerableattention due to the synthetic diamond crystals with well-defined morphology and size, andsmooth surfaces，together with the simple fabrication process and low energy consumption.Hence, it is possible and reasonable to produce the well-faceted microcrystalline diamondpowders by CVD technology. The researches on CVD technology are mainly concentrated onthe fabrication and application of diamond films and large single crystal diamonds, while forthe technique of CVD microcrystalline diamonds, there are very few studies and urgent needsto explore their growth mechanism, quality control, synthesis technology, andcharacterization and so on. In the present work, the hot filament CVD (HFCVD) is adopted,with the purpose of developing the high-grade CVD diamond powders, synthesizing a great many of cube-octahedral microcrystalline diamonds simultaneously but independently on thelarge-area substrate, and inhibiting the film growth, as well as trying to reduce the formationof twined-or poly-crystals and lattice distortion. We carry out an in-depth study to exploresuch novel synthesis technology of microcrystalline diamonds and reveal their growthmechanism. The main innovative accomplishments in this dissertation can be concluded asfollows:1. Study on the growth physical fields in the synthesis of microcrystalline diamonds byHFCVD. The three-dimensional finite volume method and simulation software FLUENT,together with coupling three heat transfer mechanisms including radiation, conduction andconvection, are adopted to predict the profiles of the growth physical distributions inside theHFCVD reactor. We perform a systemic study into the influences of the arrangement offilaments and inlets on the substrate temperature, the gas-phase temperature, and the gasdensity fields, and optimize these arrangements for improving the uniformity of the physicalfields. The simulation results show that filament radius has a decisive role in the substratetemperature, and the number of filaments is correlative to the deposition area directly.Besides, multiple inlets arranged evenly could increase slightly the density of active atomsand improve greatly the homogeneity of the gas velocity field around the substrate. Upon thepreferred filaments and inlets arrangements obtained from simulation results, the growthenvironment of each crystal tends to the same, which leads to the synthesis of particles withthe homogenous morphology and size.2. Study on the growth mechanism in the synthesis of microcrystalline diamonds withthe seeding method by HFCVD. HFCVD technique is employed to regrow the conventionalmicrocrystalline diamond powders. We firstly reveal the mechanism on the regrowth for agreat many seeds simultaneously and uniformly, elimination of the morphologicalimperfections of seeds, development of the cube-octahedral single crystals, and inhibition ofthe film growth. Based on that, a novel seeding method, spraying the photoresist solutionwith the seeds toward the substrate using a spin coater machine, is proposed, which is provedto resolve effectively the agglomerations and unevenly distribution of W1~W15seeds.Subsequently, we mainly discuss the effects of active pressure, substrate temperature, acetoneconcentration, and growth duration on the basic growth characteristics of microcrystalline diamonds including their morphology, purity, homoepitaxial growth rate and the inhibitionthe film growth. The results present that the pressure of4500Pa can effectively inhibit anyunnecessary nucleation, and temperature of800~900°C and carbon concentration of1.3~1.4%most probably lead to the formation of well-faceted diamonds with smooth surfaces, as wellas1A bias current has a positive impact on both the inhibition the film growth andimprovement of the growth rate. Upon the preferred deposition parameters, themorphological imperfections of seeds have been covered by the new-synthesized diamondsgradually, and the improved final crystals exhibit high-grade well-faceted morphology.3. Study on the growth mechanism in the synthesis of microcrystalline diamonds withthe self-nucleation method by HFCVD. The self-nucleation method is proposed to control thenucleation density. We focus on the nucleation and growth mechanisms based on theinhibition of film growth, which is very different from the mechanisms for conventionaldiamond films. Subsequently, the scratching pretreatment for substrates is applied in thiswork, and the nuclei with a low density （106cm-2） can be distributed evenly by greatlyreducing the scratching time. The nucleation and growth experiments are developed from thetheory of orthogonal design for optimizing and analyzing the deposition parameters. Theexperimental results show that the active pressure is the most critical factor in the nucleationprocess. A high pressure （>3000Pa） leads to the nuclei with the euhedral diamond faces,based on which the most final crystals present a cube-octahedral, Wulff-polyhedron andicosahedron morphology, and they have a narrow grain size distribution at an optimal groupof growth parameters （3Kpa pressure,2.0%carbon concentration,950°C substratetemperature,4A bias current, deposition time≤120min）.4. Study on the effect of boron doping on the growth characteristics of microcrystallinediamonds by HFCVD. The trimethyl borate （C3H9BO3） is used as the source of boron andintroduced into the hydrogen-acetone gas mixture. We establish the relationships between theboron concentration and the growth characteristics of microcrystalline diamonds and theinhibition of film growth. With the seeding method for depositing microcrystalline diamonds,a no-doping condition is found to suppress the occurrence of spontaneous nucleation,however, leads to a relatively low growth rate, which obviously increase the repairingduration for the imperfections of seeds, particularly in the case of5μm seeds. Fortunately, with the boron-doping, the homoepitaxial growth rate is higher than that without the doping;moreover, the boron clusters could insert into the defect spaces of diamond lattices, whichfacilitates the elimination of morphological imperfection of seeds. However, the heavilyboron-doping generally results in an increase in the agglomeration of spontaneous nucleationand an obvious decrease in the purity of diamonds. On the contrary, the500ppm boron-doping provides an appropriate growth environment to eliminate the morphologicalimperfections of original seeds at a higher growth rate, and keep a relatively high purity ofdiamond. Besides, with self-nucleation method, the500ppm boron-doping is also a preferredcondition that could help to enhace the growth rate of diamonds.5. Fabrication and quality assessment of CVD microcrystalline diamond powders. Forthe seeding and self-nucleation method of depositing microcrystalline diamonds, theoptimized deposition parameters are can be summarized as follows. With the seeding method,the CVD diamond powders with the average grain size of2~13μm can be obtained from thedifferent-sizes seeds by the no-doping or lightly boron-doping technology. With the self-nucleation method, the CVD diamond powders with the average grain size of0.3~2μm canbe obtained by the lightly boron-doping technology. Subsequently, CVD freestandingdiamonds are obtained by chemical etching to remove silicon wafers and then collect crystalswith high-speed centrifugation method. On the other hand, the performances of CVDmicrocrystalline diamond powders by the seeding or self-nucleation method, including theparticle size composition, crystal morphology, and diamond purity, are evaluated by theindustry standard of conventional diamond powders in our country (JB/T7990-2012) andmorphology indexes of high-grade single crystal diamonds. The detection results show thatthe70~80%crystals among the two types of CVD powders exhibit a cubo-octahedral oricosahedron morphology with smooth surfaces and no obvious growth defects. Obviously,the morphology and surface quality of CVD microcrystalline diamond powders haveadvantages over that of the powders obtained from crushing the large-size HPHT diamonds,and such well-faceted powders will be more suitable for machining high precision products.