Jesus Garcia, Advanced Energy Industries, Inc., Fort Collins, CO
In the vacuum coating industry, in particular magnetron sputtering, it has been understood and well documented through many experiments that Radio Frequency (RF) sputtering can provide several benefits to the deposition process. Firstly, there are certain materials that are difficult or even impossible to sputter with DC or low frequency. Certain materials are so insulative that the charge buildup of ions on the target surface repel the sputter ions and will eventually bring the sputter process to a halt. With RF sputtering, the electrons respond to the varying electric field, but the heavy ions do not. Therefore, there is a net DC bias that develops, and this is what causes the ions to accelerate and sputter the target material. This means that RF actually has a higher relative sputter rate for insulative materials than conductive materials which is, in general, the opposite of low frequency sputtering. This phenomenon opens up a class of insulative materials that is not typically possible with low frequency sputtering, such as Al₂O₃, Quartz, etc. Another benefit of RF sputtering is the properties of the deposited materials that can be achieved are unique and often superior to low frequency. This is related to the increased plasma density, ion energy, and plasma sheath physics.
Although there are several benefits to RF sputtering, there are known issues as well. The RF power must be matched to the process to deliver usable power. This matching is unique to each material and process. In the world of conductors and semiconductors, the RF sputter rate is significantly less than the low frequency rate. Historically, there has not been a high-power RF power supply available. Two of the most significant issues for large area RF sputtering has been the question of compatibility with rotatable magnetrons and the uniformity of the deposited material.
This presentation will review the physics and benefits of RF sputtering, the history of implementation and products developed, and the challenges to scale up to large area manufacturing.