School of Chemistry, University of Bristol, Bristol BS8 1TS, UK.
J Phys Chem A. 2012 Sep 27;116(38):9447-58. doi: 10.1021/jp306191y. Epub 2012 Sep 18.
The spatial distributions and relative abundances of electronically excited H atoms, OH, CH, C(2) and C(3) radicals, and CO molecules in microwave (MW) activated CH(4)/CO(2)/H(2) and CO/H(2) gas mixtures operating under conditions appropriate for diamond growth by MW plasma enhanced chemical vapor deposition (CVD) have been investigated by optical emission spectroscopy (OES) as a function of process conditions (gas mixing ratio, incident MW power, and pressure) and rationalized by reference to extensive 2-dimensional plasma modeling. The OES measurements clearly reveal the switch in plasma chemistry and composition that occurs upon changing from oxygen-rich to carbon-rich source gas mixtures, complementing spatially resolved absorption measurements under identical plasma conditions (Kelly et al., companion article). Interpretation of OES data typically assumes that electron impact excitation (EIE) is the dominant route to forming the emitting species of interest. The present study identifies a number of factors that complicate the use of OES for monitoring C/H/O plasmas. The OH* emission from EIE of ground state OH(X) radicals can be enhanced by excitation energy transfer from metastable CO(a(3)Π) molecules. The CH* and C(2)* emissions can be boosted by chemiluminescent reactions between, for example, C(2)H radicals and O atoms, or C atoms and CH radicals. Additionally, the EIE efficiency of each of these radical species is sensitively dependent on any spatial mismatch between the regions of maximal radical and electron density, which itself is a sensitive function of elemental C/O ratio in the process gas mixture (particularly when close to 1:1, as required for diamond growth) and the H(2) mole fraction.
利用光学发射光谱（OES）研究了微波（MW）激活的 CH4/CO2/H2和 CO/H2气体混合物中电子激发的 H 原子、OH、CH、C2和 C3自由基以及 CO 分子的空间分布和相对丰度，这些条件适用于通过 MW 等离子体增强化学气相沉积（CVD）生长金刚石。研究过程中考察了工艺条件（气体混合比、入射 MW 功率和压力）对 OES 的影响，并通过与广泛的二维等离子体模型相结合进行了合理化。OES 测量清楚地揭示了等离子体化学和组成的转变，这种转变发生在从富氧源气体混合物向富碳源气体混合物的转变过程中，补充了在相同等离子体条件下的空间分辨吸收测量（Kelly 等人，相关文章）。OES 数据的解释通常假设电子碰撞激发（EIE）是形成感兴趣发射物种的主要途径。本研究确定了一些因素，这些因素使得 OES 用于监测 C/H/O 等离子体变得复杂。基态 OH（X）自由基的 EIE 可以增强 OH发射，因为来自亚稳态 CO（a3Π）分子的激发能量转移。CH和 C2*发射可以通过例如 C2H 自由基和 O 原子之间的化学发光反应，或 C 原子和 CH 自由基之间的化学发光反应而增强。此外，这些自由基物种的 EIE 效率对自由基和电子密度最大区域之间的任何空间失配非常敏感，而这种空间失配本身又是过程气体混合物中元素 C/O 比（特别是当接近 1:1 时，如金刚石生长所需）和 H2摩尔分数的敏感函数。
