Resolving Abnormal Target Erosion in High Frequency Magnetron Discharge

射频磁控溅射是制备各种高质量薄膜的关键技术。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对于降低制造成本是非常必要的。由于对射频磁控溅射过程中陶瓷靶材刻蚀机理的认识不够深入，导致靶材利用率较低(~30%)。该奖项支持旨在了解射频磁控放电及其如何引起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，并最终将为高效的射频磁控管设计提供指导。这项研究的结果有望使目标材料的使用率翻一番。这将对包括大面积光学涂层、储能、显示器、太阳能和半导体器件在内的许多工业领域产生积极影响。这些油田的年市场规模已超过5000亿美元，而且还在持续增长。该项目将通过大幅降低薄膜制造成本，加强美国的产学研合作和经济竞争力。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来促进劳动力发展。射频磁控溅射基本上是由随时间和空间变化的非均匀电磁场决定的。导致靶溅射的带电粒子与非均匀电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项挑战，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，但对射频磁控溅射的了解非常有限，导致射频磁控管的情况并不令人满意。事实上，电流磁控管是为了在直流溅射中实现最佳性能而设计的，直流溅射用于金属等导电的靶材。当同样的磁控管用于绝缘体靶材的射频溅射时，它是非常无效的，并且产生了与直流溅射完全不同的反常侵蚀轮廓。具体来说，直流溅射下的密集刻蚀区域在射频溅射下受到的侵蚀最小。为了了解异常射频目标侵蚀的机制并填补这一知识空白，研究小组假设了一种“局域电荷效应”，即假设射频磁控放电中的电子优先积累并保持不动，在靠近磁极的绝缘体目标表面上吸引离子溅射这些区域。这一假设与现有的关于磁控溅射的知识相比是出乎意料的，因此具有潜在的变革性。本研究主要包括两个方面的工作：1)对射频磁控管放电进行建模；2)通过设计一种高效的射频磁控管，通过建模和实验测试来验证“局域电荷效应”。

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.