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，包括在半导体工业中用于等离子体沉积和等离子体蚀刻。等离子体可以看作至少三个过程的组合：(1)在一个电极上施加电压，同时另一个电极接地，在电极之间产生电场；(2)电场将电子加速到非常高的能量（1 ~ 20ev）；(3)在这些高能量下，电子与带中性电荷的分子反应，形成更多的电子、离子、自由基和激发态分子。(4)自由基之间、与其他气相分子、或与反应器固体表面上的分子发生反应；(5)离子通过等离子体鞘层加速，对固体表面产生高能轰击（高达几百eV），从而影响在那里发生的表面反应。目前，大多数蚀刻和沉积工艺的发展和控制主要是由经验方法控制的，这些方法检查了特定反应器的工艺条件（功率，压力，气体成分，频率）对最终效果的影响（如蚀刻速率，晶圆上的均匀性，蚀刻方向等）。对特定过程的更基本的理解将加快开发过程（特别是新的或实质上不同的等离子体过程），并有助于理解这些过程的控制。为了获得这样一个基本的理解并最终对整个系统建模，需要对发生的每个单独过程进行定量的理解和/或测量。本研究项目的目的是建立和使用一种新的方法来测定低频（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蚀刻和沉积速率，而低频放电产生更高的固体表面离子轰击能，从而改善表面效应，如更好的蚀刻方向性。PI的方法是用高频信号扰动低频放电，然后测量和分析低频周期内高频阻抗作为时间的函数。这些阻抗将被解释为等离子体作为一个电路，给出放电中的电子浓度作为时间的函数。该技术将允许实验人员定量地确定在特定蚀刻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器中发生的电子-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.