Molecular beam mass spectrometry of microwave activated plasmas used in diamond chemical vapour deposition

现在可以使用称为“化学气相沉积”（CVD）的气相化学反应将金刚石薄涂层沉积到选定的基底上。在CVD中，含碳分子（如甲烷气体）与其他气体（如氢气和氩气）的混合物一起进入真空室，在那里它们由微波发生器提供能量。这种能量加热气体，使它们分解并扩散到表面，形成钻石涂层。反应持续的时间越长，金刚石涂层就越厚。这种金刚石涂层开始在切削工具、耐磨涂层、医疗植入物、光学和散热器中得到应用。但是缺少的是对CVD生长过程背后的化学原理的更深入的理解。没有这一点，薄膜的性能就不能优化，工艺开发就变成了一个反复试验的问题。布里斯托尔金刚石CVD小组在了解用于沉积金刚石薄膜的复杂等离子体的化学性质方面具有丰富的经验。1994年，我们设计并建造了第一代仪器，能够测量金刚石沉积过程中等离子体中重要的稳定和自由基物质的浓度——这是一台分子束质谱仪，它可以通过采样探针从等离子体中吸收气体，并将其传递到识别每种气体成分的探测器中。我们已经非常成功地使用这个仪器来理解在各种金刚石生长气体混合物中发生的过程，并根据它的发现发表了10篇论文。然而，这种独特的设备有许多局限性，当我们最近开始尝试探测商业上更重要的微波等离子体环境时，这一点变得最明显。我们现在希望在这些经验教训的基础上，开发一种新的、更通用的、更灵敏的分子束质谱仪（MBMS）实验。改进的第二代反应堆- mbms系统将带来哪些新科学？我们将首次在兆瓦级反应器中对生长中的金刚石膜表面的工艺气体进行采样，该反应器的设计接近工业标准。由于仔细考虑了孔板尺寸，在各种压力区域的传播距离，大大提高了泵送速度，并结合了适当设计的斩波器，我们预计与以前的设备相比，灵敏度增益将达到100倍。仅在基本的CH4/H2（和CH4/Ar/H2）气体混合物的背景下，这应该为我们提供了第一次仔细研究反应物质（如C， CHx, C2, C2Hx, C3, C3Hx /）的相对（和绝对）丰度的机会，并作为工艺条件的函数。到目前为止，我们开发的等离子体化学模型和热化学反应机制，以及我们最近与实验进行的所有比较，仍然有许多公认的局限性/可以通过这种实验进行测试，验证或改进，必要时。我们还可以使用新系统探测迄今为止由于压力过高或对低浓度物种缺乏灵敏度而无法诊断的气体混合物。例如，我们希望研究B和N在气相中的作用，用于掺杂金刚石制造电子器件。我们还想研究Ar和CH4在沉积所谓的超晶金刚石薄膜中的作用，这些薄膜目前被建议作为生化传感器和其他探测器的候选材料。

It is now possible to deposit thin coatings of diamond onto a substrate of choice using a gas phase chemical reaction called 'Chemical Vapour Deposition' (CVD). In CVD, a carbon containing molecule, such as methane gas, is passed into a vacuum chamber along with a mixture of other gases, such as hydrogen and argon, where they are supplied with energy from a microwave generator. This energy heats the gases and causes them to break apart and diffuse to a surface to form a diamond coating. The longer the reaction continues, the thicker the diamond coating becomes. Such diamond coatings are beginning to find applications in cutting tools, wear-resistant coatings, medical implants, optics and heat spreaders. But what is missing is a deeper understanding of the chemistry behind the CVD growth process. Without this, the film properties cannot be optimised and process development becomes a matter of trial-and-error. The Bristol diamond CVD group has a great deal of experience in understanding the chemistry of the complex plasmas used to deposit diamond films. In 1994 we designed and built the first generation of an apparatus capable of measuring the concentrations of the important stable and radical species within these plasmas during diamond deposition - this is a molecular beam mass spectrometer, which can suck gas from the plasma through a sampling probe and pass it into a detector which identifies each gaseous component. We have used this apparatus very successfully to understand the processes occurring in various diamond-growing gas mixtures, and have published 10 papers based on its findings. However, this unique apparatus has a number of limitations, which became most obvious when we recently began trying to probe commercially more important microwave plasma environments. We now wish to build on these lessons and develop a new, more versatile, and much more sensitive, molecular beam mass spectrometer (MBMS) experiment. What new science will an improved 2nd generation reactor-MBMS system enable? For the first time, we will be sampling process gas at the surface of the growing diamond film in a MW reactor / which has been designed to approach industry standards. We anticipate a >1000x sensitivity gain compared to our previous apparatus, as a result of careful consideration of orifice sizes, propagation distances in the various pressure regions, much improved pumping speeds, and the incorporation of a properly-designed chopper. Just in the context of the basic CH4/H2 (and CH4/Ar/H2) gas mixtures, this should afford us the opportunity to make the first careful studies of the relative (and absolute) abundances of reactive species like C, CHx, C2, C2Hx, C3, C3Hx / in parallel, and as a function of process conditions. The plasma chemical model and thermochemical reaction mechanism that we have developed thus far, and which has been used in all of our most recent comparisons with experiment, still has a number of recognised limitations / which can be tested, validated or improved, as necessary, by experiments of this kind. We can also use the new system to probe gas mixtures that have so far eluded diagnosis, due to the pressure being too high or lack of sensitivity for low concentration species. For example we wish to study the role of B and N in the gas phase, for use in doping diamond to make electronic devices. We also want to study the role of Ar and CH4 in depositing so-called ultrananocrystalline diamond films, which are currently being suggested as candidates for biochemical sensors and other detectors.