Reactive Sputtering Research

Many sputter-PVD coatings of interest are compounds. They could be deposited by co-sputtering from multiple sources or by using compound targets. Often the compound target material and the deposited thin film are electrically insulating making DC sputtering and substrate biasing unsuitable, so RF sputtering must be used instead.

When sputtering compounds, it can be difficult to obtain the correct chemical stoichiometry in the deposited thin film even when using a stoichiometric target. The sputter plasma can break up the compound into its constituent elements and volatile species (e.g. nitrogen, oxygen, hydrogen) can be pumped away or otherwise lost to the process. This leads to the deposited thin film being deficient in these elements, i.e. sub-stoichiometric.

To combat this problem a gas containing the deficient elements can be flowed into the chamber along with the inert sputter gas to top-up the lost elements. The gas must be sufficiently chemically reactive that it either reacts with the sputtered flux or deposited film directly, or once it has been activated or ionized by the plasma generated by the sputter source or a bias excitation. This is Reactive Sputtering. The amount, or more precisely the partial pressure, of the reactive gas can be controlled to tune the stoichiometry of the thin film coating to the application. Typical reactive gases include nitrogen, oxygen, hydrogen and hydrocarbons, such as methane. This is a flexible and commonly used technique, although it suffers from some disadvantages. RF sputtering typically produces less than half the deposition rate than DC sputtering. Plus compounds typically have lower sputter yields than metallic targets which further reduces efficiency. RF energy is also more complicated to implement than DC and is more difficult to scale up to higher powers.

As an alternative, the reactive gas (or gases) can be used to supply the entire proportion of certain elements of the thin film, while the other elements are DC sputtered from metallic/conductive targets. This offers increased deposition rates as the targets can be sputtered efficiently at high rate, while the reactive gas contributes to the rate by supplying a significant proportion of the growing thin film. Another advantage is that the stoichiometry can be personalized over a much wider range, or even graded from pure metal. However, as the metallic target surface also reacts with the gas, process control is usually more demanding with this technique.

Reactive sputtering is widely utilized in the production of hard coatings, hydrogenated-DLC films, thin-film resistors and dielectrics, semiconductors, glass and optical coatings, solar cells, fuel cells, thin-film batteries, and decorative coatings, to name but a few. The technique has huge commercial and scientific importance.

Kurt J. Lesker has been manufacturing PVD tools for reactive sputtering since the 1980s, having supplied hundreds of tools to the world's leading research institutions. Because of this, our team of dedicated applications, design and vacuum experts have developed unrivalled know-how about the creation and application of reactively sputtered thin films.

Successful reactive sputtering relies on a stable process environment and usually the ability to manage a resistive or dielectric film coating the system internals.

The total chamber pressure and particularly the partial pressure of the reactive gas must be tightly controlled to maintain the required stoichiometry in the depositing thin film. This is even more important when DC sputtering from metallic targets as great care must be taken to avoid poisoning the target.

As the partial pressure of the reactive gas increases, more of the metallic target surface reacts with it to form the compound, which eventually starts to lower the sputter rate and the reactive gas consumption of the process is reduced - if the reactive gas flow is not quickly reduced the target surface can quickly transition to the compound state, i.e. become poisoned.

Reactive sputtering also suffers from hysteresis making it hard to recover from poisoning, and often the process must be repeated with new substrates. If correct film composition can only be obtained by operating within the transition region, a reactive gas controller is required to maintain the reactive gas partial pressure at the correct value. Such a system might employ optical plasma monitoring and/or monitor the target voltages.

The anode surfaces must not be allowed to become insulated by a compound coating otherwise the sputter discharge will extinguish (the 'disappearing anode' effect). A dielectric coating will build up charge on its surface from the plasma, which can produce electrical arcs causing damage to the target and/or the substrate. Pulsed-DC bias on the target and/or substrate can be used to mitigate this problem. HIPIMS can also be used, often to good effect as the and providing further options to personalize the process to the application.

Reactive sputter process and its applications are very varied. We know this and fully understand the need for a configurable thin film deposition tool to allow for materials engineering and personalization of the films based on the particular and often unique application.

We understand that tool personalization that addresses a researcher's unique requirements is critically important and we have therefore developed and support an extensive capabilities portfolio including:

• DC, Pulsed-DC, HiPIMS and RF power supplies for reactive magnetron sputtering and substrate bias
• HiPIMS for deposition of very smooth (low surface roughness) films and features positive KICK for control of ion energies
• Ion sources for substrate cleaning prior to deposition
• RF bias for optimum gas reactivity
• Patented Mag Keeper™ magnetron sputter sources with wide deposition rate capability, able to operate at low sputtering pressures. High strength magnet options for sputtering of magnetic materials
• Optimized process chamber geometry to enable excellent film uniformity
• Load lock options for quick sample entry and maintaining process vacuum integrity
• Upstream and downstream multi-channel pressure control allowing repeatable, stable sputtering conditions
• Integrated reactive gas controller options for partial pressure feedback control
• Multi-gas supply manifold with personalized reactive gas injection to suit application
• Heating and cooling substrate platens
• In-situ film measurement to accurately monitor real-time deposition parameters
• Fully integrated film recipe and system control using our Lesker eKLipse™ process control platform for precise, repeatable deposition conditions

We want to hear from you. Our thin film experts and service support team are eager to help enable your important research.