Resolving Abnormal Target Erosion in High Frequency Magnetron Discharge

射频磁控溅射是制造各种高质量薄膜的重要技术。在射频溅射过程中消耗的陶瓷靶材是薄膜涂层材料的来源，因此有效的靶材消耗是降低制造成本的迫切需要。对射频磁控溅射过程中陶瓷靶蚀刻机制的不完全了解导致靶利用率低（~30%）。该奖项支持旨在理解射频磁控管放电及其如何诱导目标消耗的基础研究。在复杂的射频放电过程中获得的新知识将有助于等离子体科学和技术，并最终为有效的射频磁控管设计提供指导。研究结果预计将使目标材料的使用率增加一倍。这将对许多工业领域产生积极影响，包括大面积光学涂层、能量存储、显示器、太阳能和半导体器件。这些领域的年市场规模已超过5000亿美元，并在持续增长。该项目将大大降低薄膜的制造成本，从而加强美国的校企合作和经济竞争力。它还将通过吸引和培训科学和工程专业的研究生和本科生，为劳动力发展做出贡献。射频磁控溅射本质上是由随时间和空间变化的非均匀电磁场决定的。导致靶材溅射的带电粒子与非均匀场发生复杂的相互作用。理解磁场约束下的高频等离子体放电是一个挑战，具有很强的科学吸引力。尽管科学界对高频等离子体放电有浓厚的兴趣，但射频磁控管溅射的知识非常有限，这导致射频磁控管不令人满意。事实上，电流磁控管是为实现直流（DC）溅射的最佳性能而设计的，直流（DC）溅射用于像金属这样导电的目标材料。当同样的磁控管用于绝缘体目标的射频溅射时，它是非常无效的，并且产生了与直流溅射完全不同的异常侵蚀剖面。具体来说，直流溅射下蚀刻密集的区域在射频溅射下蚀刻最少。为了了解异常射频靶腐蚀的机制并填补知识空白，研究小组假设了一种“局部电荷效应”，该效应假设射频磁控管放电中的电子优先积聚并在靠近磁极的绝缘体靶表面保持不动，以吸引离子溅射这些区域。这一假设是意想不到的，从现有的知识磁控溅射，因此是潜在的变革。本研究包括两个主要任务：1)射频磁控管放电建模；2)通过建模和实验测试，通过设计高效的射频磁控管来验证“局部电荷效应”。

Radio frequency (RF) magnetron sputtering is an essential technology for manufacturing a broad variety of high-quality thin films. The ceramic targets, the source of the thin film coating material, consumed during the RF sputtering process are expensive, thus efficient target consumption is highly desirable in lowering manufacturing costs. Incomplete understanding of the ceramic target etching mechanisms occurring during RF magnetron sputtering results in poor target use rates (~30%). This award supports fundamental research aimed at understanding RF magnetron discharges and how they induce target consumption. New knowledge obtained on the complicated RF discharge processes will contribute to plasma science and technology, and will ultimately provide guidance on efficient RF magnetron design. Results from the research are expected to double the use rates of target materials. This will have a positive impact on many industrial fields including large-area optical coatings, energy storage, displays, solar energy, and semiconductor devices. The annual market of these fields has reached over $500 billion and is growing continuously. This project will strengthen university-industry collaborations and economic competitiveness of the U.S. by greatly reducing manufacturing costs of thin films. It will also contribute to workforce development by attracting and training graduate and undergraduate students in science and engineering.RF magnetron sputtering is essentially determined by the non-uniform electromagnetic fields that vary with time and in space. Charged particles that lead to the sputtering of the target undergo complicated interactions with the non-uniform fields. Understanding high-frequency plasma discharges under the confinement of magnetic fields is a challenge and holds strong scientific appeal. Despite the scientific community's strong interest in high-frequency plasma discharges, knowledge in RF magnetron sputtering is very limited, which results in unsatisfactory RF magnetrons. In fact, current magnetrons are designed for achieving optimum performance in direct current (DC) sputtering, which is used for target materials that are electrically conducting like metals. When this same magnetron is used for RF sputtering of insulator targets, it is extremely ineffective and produces an abnormal erosion profile completely different from that in DC sputtering. Specifically, the intensively etched region under DC sputtering is the least eroded under RF sputtering. To understand the mechanisms of the abnormal RF target erosion and fill the knowledge gap, the research team hypothesizes a 'localized charge effect', which assumes that electrons in RF magnetron discharges preferentially accumulate and stay immobile on the insulator target surface near the magnet poles to attract ions to sputter these regions. This hypothesis is unexpected from the existing knowledge on magnetron sputtering and is thus potentially transformative. The research includes two major tasks: 1) modeling of RF magnetron discharges, and 2) verifying the 'localized charge effect' by designing a highly efficient RF magnetron through modeling and experimentally testing.