Measurement of Electron Concentrations in Low Frequency Glow Discharges by High-Frequency (4-15MHz) Impedance Analysis: Research Initiation Award

射频等离子体用于各种工艺，包括用于半导体工业的等离子体沉积和等离子体刻蚀。等离子体可以被看作是至少三个过程的组合：(1)向一个电极施加电压，而另一个电极接地，在两个电极之间产生电场；(2)电场将电子加速到很高的能量(1-20 eV)；(3)在这些高能下，电子与中性带电分子反应，形成更多的电子、离子、自由基物种和受激发的分子。(4)自由基与其他气相分子相互作用，或与反应器固体表面上的分子反应；以及(5)离子通过等离子体鞘加速，对固体表面产生高能轰击(高达数百eV)，影响那里发生的表面反应。目前，大多数蚀刻和沉积过程的开发和控制主要由经验方法控制，这些方法检查最终产生的效果(如蚀刻速度、晶片上的均匀性、方向等)。作为特定反应堆的工艺条件(功率、压力、气体成分、频率)的函数。对特定工艺的更基本的了解将加快开发进程(特别是新的或本质上不同的等离子体工艺)，并有助于理解对这些工艺的控制。为了获得这样的基本理解并最终对整个系统进行建模，需要对发生的每个单独的过程进行定量的理解和/或测量。本研究项目的目标是建立和使用一种新的方法来确定低频(40-400赫兹)放电中电子浓度随时间的变化。高频放电产生更快的刻蚀和沉积速率，而低频放电产生更高的离子轰击固体表面的能量，从而改善表面效应，如更好的刻蚀方向性。PI的方法包括用高频信号扰动低频放电，然后在低频周期内测量和分析高频阻抗作为时间的函数。这些阻抗将被解释为等离子体是一个电路，以给出放电中的电子浓度作为时间的函数。这项技术将使实验人员能够定量地确定在特定的刻蚀或沉积放电中发生的电子浓度。电子浓度测量将有助于预测等离子体反应堆中发生的电子-分子反应的速率。这些测量可以与其他信息相结合，以确定对蚀刻或沉积速率和特性的最终影响。

Radio-frequency plasmas are used for a variety of processes including uses in the semiconductor industry for plasma deposition and plasma etching. The plasma can be viewed as the combination of at least three processes: (1) a voltage is applied to one electrode while another one is grounded creating an electric field between the electrodes; (2) the electric field accelerates electrons to very high energies (1 to 20 eV); (3) at those high energies, electrons react with neutrally charged molecules to form more electrons, ions, free radical species, and excited molecules. (4) the free radicals react with each other, other gas phase molecules, or with the molecules on solid surfaces of the reactor; and (5) ions are accelerated through the plasma sheathes to produce high energy bombardment (up to several hundred eV) of the solid surfaces which influence the surface reactions occurring there. At the present time the development and control of most etch and deposition processes is mainly governed by empirical approaches that examine the final effect created (such as etching rate, uniformity across a wafer, etch directionally, etc.) as a function of the process conditions (power, pressure, gas composition, frequency) for a particular reactor. A more fundamental understanding of a particular process would speed up the development process (especially of new orsubstantially different plasma processes) and aid in understanding the control of these processes. To obtain such a fundamental understanding and to ultimately model the whole system, a quantitative understanding and/or measurement of each individual process occurring is needed. The objectives of this research project are to establish and use a new method to determine electron concentrations as function of time in low frequency (40 - 400 Hz) discharges. High frequency discharges produce faster etch and deposition rates while low frequency discharges produce higher ion bombardment energies of solid surfaces which leads to improved surface effect such as better etching directionality. The PI's method involves perturbing the low frequency discharge with high frequency signals, and then measuring and analyzing the high-frequency impedance as a function of time within the low frequency cycle. These impedances will be interpreted in terms of the plasma as an electrical circuit to give the electron concentrations in the discharge as a function of time. The technique will allow experimenters to determine quantitatively the electron concentrations occurring in specific etching or deposition discharges. Electron concentration measurements will help in predicting rates of the electron-molecule reactions occurring in the plasma reactor. These measurements can be combined with other information to determine the ultimate effects on etching or deposition rates and characteristics.