Highly efficient polishing of polycrystalline CVD diamond via atmosphere inductively coupled plasma
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摘要: 作为一种典型的难加工材料,多晶金刚石的抛光存在着材料去除率低、损伤引入较多、难以获得亚纳米级粗糙度等诸多问题。采用一种基于大气电感耦合等离子体的非接触式加工方法,在纯氩等离子体中引入氧气作为反应气,激发产生高活性氧自由基,并在多晶金刚石表面不同位点处发生差异化刻蚀,最终实现多晶金刚石的高效抛光。研究结果表明:随着含氧等离子体辐照的进行,多晶金刚石表面晶粒尖端位点被快速去除,晶粒间的高度差大幅下降,且金刚石表面粗糙度Sa在30 min内从10.10 μm降低至93.70 nm,材料去除率可达34.4 μm/min,远高于传统的机械或化学机械抛光方法。拉曼光谱与X射线衍射谱分析表明,该抛光方法未引入非晶碳或新应力损伤,不改变多晶金刚石表面晶粒取向。该方法可作为一种多晶金刚石粗抛技术,与化学机械抛光、紫外激发抛光等精抛工艺相结合,显著提升多晶金刚石的综合抛光效率。
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关键词:
- 多晶金刚石 /
- 高效抛光 /
- 大气电感耦合等离子体 /
- 高活性氧自由基
Abstract: Objectives: As a typical difficult-to-machine material, polycrystalline CVD diamond often requires sophisticated and time-consuming polishing methods to achieve a smooth surface. In this study, a non-contact polishing method based on atmospheric inductively coupled plasma is investigated for the polishing of polycrystalline CVD diamond. Several indicators, including surface roughness, surface chemical composition, inner crystal structure, and material removal rate (MRR), are measured during the polishing process to evaluate the surface quality and polishing efficiency. Methods: The atmospheric inductively coupled plasma device, consisting of a plasma working zone, radio frequency (RF) power, electric sparker, RF match, mass flow controller, and water cooler, is used for polishing experiments. Oxygen is used as a reaction gas and added to the pure argon plasma, generating highly active oxygen radicals. The polycrystalline CVD diamond sample is fully exposed to oxygen-containing plasma irradiation to gain sufficient activation energy and react with the oxygen radicals. Once the processing is completed, the RF power is turned off, the oxygen supply is stopped, and the diamond is raised and cooled with pure argon shielding to protect the sample from being etched by ambient oxygen at high temperatures. Laser scanning confocal microscopy and scanning electron microscopy (SEM) are used to observe the diamond morphology, while the Sa roughness at different scales is characterized by scanning white interferometry and atomic force microscopy (AFM). The material removal rate is measured using an ultra-precision balance, and surface temperature is detected by an infrared imager. Raman spectroscopy and X-ray diffraction (XRD) are separately used to characterize the surface chemical composition and crystal orientation of the diamond before and after polishing. Results: The results of diamond treatment under different plasma irradiation conditions show that pure argon plasma tends to deteriorate the surface quality, while oxygen-containing plasma achieves significant polishing performance, removing grain tips and protruding structures. The morphology evolution of diamonds during the polishing process reveals that the initial surface is extremely rough, with severe surface irregularities and angular grain structures. After exposure to oxygen-containing plasma, the elevated grain structures are gradually removed, and the sharp edges are rounded. As the plasma irradiation time increases, some protruding sites are further eliminated, revealing the remaining cavities formed during the crystal growth process. Ultimately, the various crystal grains on the diamond surface maintain a consistent height, forming a relatively smooth surface. The fluctuating grain boundaries, which are widespread among the crystal grains, inhibit the formation of a smoother surface. The polishing effect can be explained by the principle of plasma-based atom-selective etching (PASE). During the polishing process, carbon atoms with different bonding states are randomly distributed on the diamond surface. Among them, carbon atoms at the grain tips, which only bond with atoms below and possess less stable bonds compared to those in the substrate, have lower activation energy. Consequently, tip carbon atoms preferentially react with active oxygen radicals due to their lower activation energy, accounting for the removal of grain tips during the initial polishing stage. As the polishing process continues, carbon atoms with fewer covalent bonds fade away, and the bonding states become equivalent in local areas, contributing to a progressively smoother surface. However, carbon atoms at the crystal boundary zones have complex arrangements and bonding characteristics, which restrict the differential removal of carbon atoms and hinder the overall polishing effect. The roughness evolution results show that the final Sa roughness of polycrystalline CVD diamond is reduced to 93.70 nm over a 400 μm × 400 μm area and 21.40 nm over a 20 μm × 20 μm area after polishing for 30 minutes. In some smooth areas, the roughness is as low as 2.53 nm, while in the crystal boundary zones, it reaches 31.30 nm. The presence of the crystal boundary zone hinders the global polishing performance. After 30 minutes of polishing, the surface roughness stabilizes, with the MRR also stabilizing at 34.4 μm/min. Surface composition analysis indicates a tensile stress of 1.5309 GPa on the diamond surface, and the polishing process dose not introduce new stress or amorphous carbon contaminants. XRD spectra show that there is no change in the crystal grain orientation of the diamond before and after polishing, demonstrating that PASE acts on all crystal planes of polycrystalline diamond (PCD) without preference for any specific crystal plane. Conclusions: Atmospheric inductively coupled plasma (ICP) can be used to efficiently smooth polycrystalline CVD diamond based on the principle of PASE. During the polishing process, oxygen is introduced as a reaction gas, generating oxygen radicals that selectively etch the diamond by preferentially removing carbon atoms with low activation energy, thus rapidly smoothing the surface. The Sa roughness of diamond is reduced from 10.10 μm (400 μm × 400 μm) and 338.00 nm (20 μm × 20 μm) to 93.70 nm and 21.40 nm, respectively, after 30 minutes of polishing. As polishing progresses, the MRR sharply decreases and eventually stabilizes at around 34.4 μm/min. The surface chemical composition and crystal grain orientation of polycrystalline CVD diamond remain consistent before and after polishing. Overall, atmospheric ICP can be an efficient pre-polishing method to significantly improve the overall polishing efficiency of polycrystalline CVD diamond. -
作为最宝贵的矿物材料之一,金刚石内部的碳原子以sp3形式杂化形成强的碳碳共价键,键能高达347.5 kJ/mol,具有很强的化学惰性,常温常压下耐酸耐碱,难以被氧化,只有在高温环境下才能够被强氧化剂等的熔融体轻微氧化或溶解[1]。此外,金刚石独特的成键特点和晶体结构也使其具有极为优越的物理特性,例如:高达
2200 W/(m∙K)的热导率,5.47 eV的带隙宽度[2],几乎在全波段均具有极高的光学透过率以及高达100 GPa的维氏硬度[3-4]。这些特性使得金刚石成为优异的散热衬底、半导体、光学窗口和耐磨损涂层材料,并广泛应用于电子器件、超精密加工等诸多领域[5-8]。目前,高质量单晶金刚石的生产成本高,且生长技术还不够成熟,难以获得大尺寸的晶圆,无法满足光电器件的应用需求。因而,成本更低、尺寸更大、性能相近的多晶金刚石成为市场应用主流[9]。然而,多晶金刚石生长过程中会发生晶粒的竞争性生长,这往往会引入较多的位错、孪晶等内部缺陷,且表面上各晶粒杂乱分布,起伏严重,无法满足各应用领域对金刚石高表面质量的要求[10-11]。另一方面,多晶金刚石极强的化学惰性和极高的硬度也使得其成为典型的难加工材料。若采用传统的机械抛光加工金刚石,则过程中往往伴随着大载荷和高转速,不可避免地引入划痕、坑点、非晶层等表面与亚表面损伤[12]。化学机械抛光可以借助氧化还原试剂或碱性胶体溶液,在金刚石抛光过程中引入化学反应,能够在较小载荷和转速的条件下使金刚石表面粗糙度降低至3 nm以下,实现多晶金刚石的精抛光。但这种方法往往也需要数小时的加工时长,加工效率较低,限制了多晶金刚石的工业化应用[13-14]。因此,研究开发出高效低损伤的多晶金刚石抛光方法对于促进其应用发展具有重要意义。
除了传统的机械抛光和化学机械抛光方法,一些新的方法被相继提出并用于多晶金刚石的抛光。WATANABE等[15]结合紫外诱导光化学反应,氧化金刚石上层碳原子,并使碳原子在高温和高压作用下以CO/CO2气体形式去除,能够在60 min内将粒径<1 μm的多晶金刚石表面粗糙度Ra从30 nm降低至5.2 nm,材料去除率可达2.0 μm/h,可实现多晶金刚石的精抛光。 LIANG等[16]提出采用三维高速动态摩擦的抛光方法实现多晶金刚石的抛光,该抛光方法借助金刚石的高速滑动和多维运动避免了抛光过程中严重的单向划痕并提升了抛光效率,最终将多晶金刚石的表面粗糙度Ra降低至5.7 nm(60 μm × 60 μm),局部非晶界区粗糙度甚至小于0.5 nm。然而,该抛光方法存在复杂冗长的前段研磨工序,大大降低了多晶金刚石的抛光效率。LU等[17]采用高速旋转的陶瓷结合剂金刚石砂轮诱导多晶金刚石表面石墨化,并通过机械和热氧化作用去除石墨化黑层,当金刚石砂轮转速达到
1350 r/min时,多晶金刚石表面粗糙度Sa降低至0.548 nm(30.0 μm × 30.0 μm),但这种高速旋转的研磨抛光方法会在金刚石基底中引入含有层错和微裂纹的损伤层,使得金刚石的表面质量恶化。为进一步提高多晶金刚石的抛光效率,陆续出现了一些高效的粗抛光工艺。OGAWA等[18]对比纳秒脉冲激光和飞秒脉冲激光加工多晶金刚石,发现利用飞秒脉冲激光可以使多晶金刚石表面粗糙度Ra降低至0.022 μm,材料去除率为0.004 mm3/s,且表面几乎无石墨层存在。RALCHENKO等[19]利用金刚石微颗粒在强超声波的冲击作用下对金刚石表面造成损伤去除,在5 min内使多晶金刚石表面粗糙度Ra从5 μm左右降低至0.5 μm左右,显著提升了多晶金刚石的粗加工效率。上述多晶金刚石抛光方法大多局限于利用机械应力或石墨化转变去除的方式实现多晶金刚石材料的抛光,去除过程中往往会引入机械作用诱导的亚表面损伤缺陷或非金刚石碳,影响多晶金刚石原有的材料特性。相比之下,无应力的抛光方法不涉及材料的塑性变形去除,且不会引入形变层和残余应力,具有获得高品质表面的优势,有望实现多晶金刚石的无损伤抛光。在本研究中,采用一种基于大气电感耦合等离子体的抛光方法来实现多晶金刚石的高效抛光。该抛光方法基于等离子体原子选择性刻蚀的机理,即材料粗糙表面杂乱分布有成键状态不同的原子,原子活化能因其不同成键状态存在差异,成键更少的原子活化能更低,等离子体中的反应性活性粒子优先与这些活化能低的原子发生反应去除,直到表面原子成键状态均一,实现抛光效果[20-21]。实验中,采用高温含氧等离子体,借助高活性氧自由基对金刚石表面不同成键状态碳原子的差异化刻蚀去除,能将多晶金刚石表面粗糙度Sa从10.1 μm迅速降低至93.7 nm,材料去除速率可达34.4 μm/min。多晶金刚石抛光后无其他非晶碳和新应力损伤引入,表面晶粒取向不发生变化。因此,该方法可作为一种多晶金刚石粗抛光新技术,结合化学机械抛光、紫外激发抛光等精抛光技术,大幅提升多晶金刚石的综合抛光效率。
1. 实验材料与方法
1.1 实验原料
实验采用由直流电弧等离子体喷射化学气相沉积工艺制备的多晶金刚石样品,厚度为1 mm,尺寸为3 mm × 3 mm。样品表面为晶粒竞争性生长原始形态,未经过其他研磨与抛光处理。金刚石样品在实验前经无水乙醇、纯水各超声清洗10 min。实验过程中采用高纯Ar、O2作为等离子体气体源(纯度≥99.999%,深圳市华特鹏特种气体有限公司)。
1.2 实验设备及具体操作
实验采用的大气电感耦合等离子体设备如图1所示。
设备主要由6个部分组成,即等离子体工作区、射频电源、电火花点火器、匹配器、质量流量控制器和水冷机。等离子体工作区主要有样品台、同轴玻璃炬管以及射频线圈。加工时,在玻璃炬管外管通入Ar作为冷却气,内管通入Ar和O2分别作为载流气和反应气,打开电火花点火器提供种电子,载流气在射频磁场的作用下发生电离,产生稳定的等离子体,并进一步实现反应气的解离与电离。金刚石样品放置于等离子体正下方,被等离子体射流完全覆盖,其表面碳原子能够与等离子体中活性粒子充分接触。停止加工时,关闭射频电源和O2,并将样品迅速上移,用纯氩屏蔽冷却,以防止大气中的O2在高温下与金刚石表面发生刻蚀作用。
1.3 实验参数
在金刚石抛光过程中,所采用的具体实验参数如表1所示。其中,气体流速单位以标准状态下每分钟流出气体体积(L)计算,用slm表示;或以标准状态下每分钟流出气体体积(cm3)计算,用sccm表示。
表 1 实验参数Table 1. Experimental parameters实验参数 具体条件 玻璃炬管直径 d /mm 内炬管(内直径/外直径):14/16
外炬管(内直径/外直径):18/20射频电源功率 Q /W 1200 射频电源频率 f /MHz 27.12 载流气流速 v1 /slm 1.5 冷却气流速 v2 /slm 18 反应气流速 v3 /sccm 60 工作距离 l /mm 15 在等离子体作用下,金刚石表面馈入大量热量,表面温度在短时间内快速上升,具体温度变化曲线如图2所示。激发等离子体后,金刚石样品表面平均温度迅速上升,仅100 s温度便达到
1400 ℃,之后温度缓慢上升,直到金刚石表面达到热平衡状态,温度稳定在1500.2 ℃左右。抛光结束后,关闭等离子体,金刚石表面温度也迅速下降。1.4 表征手段
为分析抛光过程中材料的去除率,采用精度为0.01 mg的超精密天平(XSR105, Mettler Toledo)来测量多晶金刚石抛光前后的质量,其材料去除率计算公式为:
$$ R_{\mathrm{MRR}}=\frac{\mathrm{ }\Delta m}{\rho\times S\times t} $$ (1) 式中:$ \Delta m $为抛光前后金刚石样品质量差,ρ 为金刚石密度(ρ = 3.5 g/cm3),S为金刚石抛光面积,t为抛光时间。
抛光过程中多晶金刚石表面温度采用红外热成像仪(Flir T660)测定,测定前采用热电偶对多晶金刚石表面热辐射率进行校准,校准值为0.70。等离子体中各粒子强度分别采用光学发射光谱(Ocean Optics USB2000 + UV-VIS-ES)来表征。
多晶金刚石表面形貌变化采用激光扫描共聚焦显微镜(Keyence VK-X1000)和扫描电子显微镜(ZEISS Merlin)来观察,表面不同范围内粗糙度Sa分别用扫描白光干涉仪(Taylor Hobson CCI)和原子力显微镜(Bruker Dimension Edge)来表征分析。
采用532 nm激光源的拉曼光谱仪(Horiba LabRAM HR Evolution)检测抛光前后金刚石表面化学成分的变化。此外,采用X射线衍射仪(Rigaku Smartlab)来分析抛光前后金刚石表面晶粒取向。
2. 实验结果与讨论
2.1 抛光结果
图3所示为等离子体光学发射光谱的分析结果。由图3可知:纯氩等离子体激发后会产生Ar自由基、N2 + 、O2 + 和O + ,其中Ar自由基是由Ar电离产生的,而N2 + 、O2 + 和O + 等离子来自空气中O2与N2的电离。相较于纯氩等离子体,含氧等离子体在777.3 nm处还存在一个特征峰,该特征峰对应于氧自由基的存在,即O2的引入会促进氧自由基的产生,而高活性氧自由基能够与多晶金刚石表面上的碳原子在高温下发生化学反应,生成CO或CO2,从而实现多晶金刚石的材料去除。
图4所示为采用纯氩等离子体处理后,金刚石粗糙表面未能得到改善,表面质量甚至会进一步恶化,没有抛光效果,如图4a所示;而经含氧等离子体处理后,多晶金刚石表面产生了明显的抛光效果,在激光扫描共聚焦显微镜下能够观察到相对平整的表面,金刚石原始表面上竞争性生长的晶粒尖端和凸起结构被去除,表面粗糙度Sa显著降低,具体如图4b所示。
图5所示为多晶金刚石在不同抛光时间段的表面扫描电子显微镜图像,左侧图像为大标尺下的表面形貌,右侧图像为局部区域放大图像。如图5a所示,多晶金刚石初始表面极其粗糙,表面形貌起伏严重,晶粒结构棱角分明。图5b则表明,在含氧等离子体辐照多晶金刚石表面30 s后,上层凸出的晶粒尖端结构被逐渐去除,晶粒棱角也逐渐变钝。继续延长辐照时间至2 min,金刚石表面晶粒高度差整体下降,形成相对平坦的表面,如图5c所示。随着辐照时间的进一步延长,金刚石表面局部高位点也被进一步去除,表面主要由不同晶粒及其晶界组成,局部区域还存在晶粒生长过程中残留的间隙,如图5d~图5f所示。抛光达到30 min时,金刚石表面各晶粒达到较光滑的水平,但晶粒之间还存在起伏的晶界区,金刚石表面难以达到全局光滑。
在含氧等离子体抛光多晶金刚石过程中,金刚石原始粗糙表面上杂乱分布有大量成键状态不同的碳原子。在金刚石晶粒尖端处,碳原子仅与下层碳原子相连,同基底碳原子相比,这些碳原子往往成键较少,反应活化能更低,当等离子体充分辐照表面时,高活性氧自由基会优先与这类碳原子发生反应,使得晶粒尖端在抛光初始阶段被优先去除,金刚石表面高低起伏下降。继续延长抛光时间,表面上成键较少的碳原子数量下降,在局部区域碳原子成键状态趋向均一,金刚石表面整体上也渐趋平坦。然而,多晶金刚石各晶粒之间存在晶界缺陷,该处碳原子排列及成键方式复杂,高活性氧自由基难以有效实现该区域内碳原子的差异化去除,限制了多晶金刚石全局抛光的效果。
2.2 粗糙度演变分析
图6展示了抛光过程中金刚石表面的粗糙度变化,测量区域为400 μm × 400 μm。如图6a所示,多晶金刚石原始样品存在高达150.00 μm的表面起伏,表面粗糙度Sa为10.10 μm。图6b显示,抛光30 s后多晶金刚石表面粗糙度略微下降至9.74 μm,但表面起伏大幅下降。图6c~图6e表明:随着抛光的进行,金刚石表面粗糙度开始显著下降,抛光至10 min时,金刚石表面粗糙度已降低至324.00 nm。图6f显示,继续延长抛光时间,可以明显观察到多晶金刚石表面各晶粒和彼此之间的界面,此时金刚石表面粗糙度Sa已下降至93.70 nm。通过30 min的等离子体辐照处理,多晶金刚石表面粗糙度显著降低,实现了多晶金刚石的高效抛光。
为深入了解多晶金刚石的抛光过程,采用原子力显微镜进一步分析金刚石表面更小尺度(20 μm × 20 μm)的形貌及表面粗糙度Sa变化情况,具体结果如图7所示。由图7a可知:多晶金刚石原始样品表面分布有随机取向的晶粒,晶粒棱角分明,彼此之间有较大的高度差,表面粗糙度Sa为386.00 nm。如图7b和图7c所示,经过30 s抛光处理后,金刚石表面粗糙度略微下降至362.00 nm;而抛光2 min后,金刚石内部晶粒之间的微孔隙有所暴露,导致局部区域产生坑状结构,此时金刚石表面不同晶粒之间的高度差依然较大,金刚石表面粗糙度为96.20 nm。图7d和图7e表明:抛光5和10 min后,金刚石表面晶粒间的高度差不再明显,表面粗糙度分别降低至30.20和29.10 nm。图7f显示,当抛光30 min时金刚石表面粗糙度最低,达到21.40 nm。此时表面无晶界区(3 μm × 3 μm)A与B的表面粗糙度Sa可分别达到2.53和3.60 nm,而晶界区C(3 μm × 3 μm)的表面粗糙度Sa为31.30 nm。虽然抛光30 min后多晶金刚石表面依然存在有广泛的晶界区和一定的晶粒高度差,但通过该抛光方法,多晶金刚石的表面粗糙度可以在短时间内发生显著改善,局部区域粗糙度也可以降低至纳米水平,能够实现多晶金刚石的高效粗抛光。
抛光过程中不同范围内表面粗糙度Sa变化如图8所示。由图8可知:随着抛光时间的延长,多晶金刚石表面粗糙度显著下降。抛光20 min时,金刚石表面小范围内平均粗糙度稳定在20.00 nm左右。此时,金刚石表面大范围内粗糙度仍在继续变化。当抛光30 min时,金刚石表面大范围内平均粗糙度降为130.00 nm左右。继续延长时间,大范围和小范围内粗糙度均不会发生显著改善,达到含氧等离子抛光多晶金刚石极限。
2.3 材料去除率分析
含氧等离子体抛光多晶金刚石过程中可以实现超高的材料去除率,具体结果如图9所示。在抛光前30 s过程中,多晶金刚石的材料去除率可以达到44.3 μm/min。随着抛光的进行,金刚石的材料去除率逐步下降,直到抛光20 min时,材料去除率达到34.4 μm/min,此时材料去除率趋于稳定。在多晶金刚石抛光初始阶段,样品表面未经任何加工处理,表面存在随机性分布的晶粒,各晶粒之间存在有比较大的高度差,表面起伏波动严重。当等离子体辐照于金刚石表面时,样品与等离子体接触面积达到最大,且由于晶粒尖端部位具有更高的去除率,此时的材料去除率最大。延长抛光时间至2 min,材料去除率发生显著下降,这是由于初始抛光过程中金刚石表面粗糙度迅速下降,晶粒尖端被去除,金刚石表面的高度差逐渐缩小,使得等离子体与样品接触面积变小。同时,金刚石表面高效率去除位点也随之减少。而后,多晶金刚石表面渐趋平坦,表面整体上高速率去除位点趋于稳定,材料去除率下降趋势也逐渐平缓,直到达到含氧等离子体抛光多晶金刚石的抛光极限,此时,金刚石的材料去除率也达到稳定水平。
2.4 组成分析
多晶金刚石经含氧等离子体抛光后表面化学成分和内部应力的变化通过拉曼光谱仪来检测分析。如图10所示,拉曼光谱测量范围为
1200.0 ~1800.0 cm−1,抛光前,多晶金刚石在1329.8 cm−1处存在金刚石sp3的特征峰,相较于IIa型天然金刚石位于1332.5 cm−1的金刚石特征峰发生了一定偏移[19]。金刚石应力与特征峰偏移之间的关系为[22]:
$$ \mathit{\sigma}\mathrm{=-0.567}\mathit{\mathrm{\Delta}v} $$ (2) 式中:σ为内部应力,Δv = v −
1332.5 cm−1,v为金刚石特征峰的波数。基于该关系式可以计算出金刚石内部存在1.5309 GPa的拉应力。该拉应力主要源于多晶金刚石内部晶粒竞争性生长,而紧邻金刚石特征峰处存在一宽峰,该宽峰可能是由金刚石内部存在的sp2缺陷或者氮杂质引起的[23-24]。经含氧等离子体抛光后,多晶金刚石的特征峰位置没有发生偏移,表明抛光过程中不会引入新应力。此外,在测量范围内也没有检测到其他非金刚石峰的存在,表明采用含氧等离子体处理多晶金刚石不会在表面引入其他非金刚石碳[25],去除过程中不改变金刚石原有结构。借助X射线衍射仪进一步分析了多晶金刚石表面抛光前后的晶粒取向,结果如图11所示。抛光前后多晶金刚石表面各晶粒取向没有发生改变,主要有位于44.12°的(111)晶面、位于75.47°的(220)晶面、位于91.64°的(311)晶面和位于119.63°的(400)晶面[26]。其中,(220)晶面始终占主导地位,其次为(111)晶面,而(400)晶面对应的特征峰强度很低,表明多晶金刚石表面该晶面成分很少。此外,在含氧等离子体作用于多晶金刚石过程中,材料去除率始终维持在很高水平,而多晶金刚石表面晶粒取向却不会因材料的去除而产生显著变化,表明高活性氧自由基刻蚀多晶金刚石无晶面选择性。金刚石表面碳原子不因所处晶面而具有不同的反应速率,其反应速率快慢仅与碳原子在金刚石表面上的成键状态有关,当金刚石表面碳原子成键状态不一致时,高活性氧自由基会优先与成键数量较少的碳原子反应,逐步改变金刚石表面碳原子差异化成键状态,最终使得表面碳原子成键状态趋于一致,达到抛光的效果。
3. 结论
采用大气电感耦合等离子体成功实现了多晶金刚石的高效抛光,该抛光方法可以作为高效的前段工序用于多晶金刚石的粗抛光,可大大缩短多晶金刚石的抛光时间,并显著提升多晶金刚石的综合抛光效率。研究所得结论如下:
(1)高温含氧等离子体能够对金刚石表面产生抛光效果,主要是由于该等离子体中高活性氧自由基能够对多晶金刚石产生选择性刻蚀反应,优先刻蚀表面高位点碳原子,从而实现多晶金刚石表面快速平坦化。
(2)抛光过程中多晶金刚石表面粗糙度Sa可以在30 min内从大范围(400 μm × 400 μm)10.10 μm和小范围(20 μm × 20 μm)338.00 nm迅速降低至93.70 nm和21.40 nm,且抛光过程中无需任何前段预处理工艺。
(3)随抛光过程的进行,多晶金刚石材料去除率先迅速下降,而后缓慢降低并最终稳定在34.4 μm/min,实现了多晶金刚石的高效抛光。
(4)综合分析抛光前后多晶金刚石表面成分及组成可知,采用含氧等离子体抛光多晶金刚石不会在其表面引入其他的非金刚石碳,且过程中没有进一步增加金刚石内部的拉应力,不改变金刚石表面的晶粒取向。
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表 1 实验参数
Table 1. Experimental parameters
实验参数 具体条件 玻璃炬管直径 d /mm 内炬管(内直径/外直径):14/16
外炬管(内直径/外直径):18/20射频电源功率 Q /W 1200 射频电源频率 f /MHz 27.12 载流气流速 v1 /slm 1.5 冷却气流速 v2 /slm 18 反应气流速 v3 /sccm 60 工作距离 l /mm 15 -
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