Precise timing for perfect thin films
Empa researchers have developed a process that enables piezoelectric thin films to be produced in high quality and at low temperatures on insulating substrates for the first time. This new process, called SFP-HiPIMS, opens up decisive prospects for the semiconductor industry, quantum and photonics technologies. The timing of the processes is the key to a breakthrough that overcomes previous technical limitations.
Piezoelectric thin films are indispensable components in a wide range of electronic applications. They are used in frequency filters, sensors, actuators and tiny energy converters. Their ability to convert electrical voltage into mechanical movement and vice versa makes them a central component of modern communication technology. However, the production of these highly sensitive layers is a complex matter. Their quality determines the performance and durability of the end products.
HiPIMS process with new possibilities
High Power Impulse Magnetron Sputtering (HiPIMS) is a well-known process for coating substrates with high-density layers. High-energy pulses are generated in a vacuum chamber environment, which knock atoms out of the target material. These atoms are deposited as a thin film on the substrate. For piezoelectric applications, such as aluminium nitride coatings, the process has not yet offered an optimal solution. This is because along with the desired target ions, argon ions from the process gas also reach the substrate, which leads to undesirable inclusions.
Avoiding argon inclusions
Argon, a common process gas in magnetron sputtering, is chemically inactive, but it can remain in the coatings in the form of inclusions. This is problematic for piezoelectric coatings as they are operated under high electrical voltages. Even small amounts of argon lead to an electrical breakdown and jeopardise the functionality of the components. In classic HiPIMS process control, however, it is difficult to eliminate the argon ions as they hit the substrate at the same time as the target ions.
Timing as a decisive factor
Under the leadership of Sebastian Siol, the team led by PhD student Jyotish Patidar has developed precise timing in order to accelerate only the target ions and avoid argon inclusions. As the argon ions are faster and reach the substrate first, the accelerating voltage is applied to the substrate with a delay. At this point, the argon ions have already flown past and can no longer penetrate the growing layer. This results in piezoelectric layers of outstanding quality that were previously not possible with HiPIMS.
A new standard for sensitive substrates
The researchers call this innovative process “Synchronised Floating Potential HiPIMS”. Particularly noteworthy is the possibility of creating layers on non-conductive substrates such as glass or sapphire. Normally, no electric fields for ion control can be applied to such substrates. By utilising the so-called “electron shower”, which is generated with the magnetron pulse, ions can nevertheless be accelerated at the right moment. The substrate is briefly negatively charged so that the desired ions are introduced in a targeted manner.
Practical relevance for semiconductor and quantum technologies
Lower process temperatures protect the sensitive components in semiconductor production and enable the coating of temperature-sensitive components. At the same time, a high layer density and associated resistance is achieved, which is crucial for a long service life of the components. The possibility of depositing layers on insulating substrates also opens up completely new applications in photonics and quantum technologies that previous methods could not achieve.
Cooperations and next steps
The Empa team has not reached the end of its research with these successes. The group is already working on optimising the process with machine learning and high-throughput experiments. At the same time, collaborations are being established with other research institutions and industrial partners in order to bring the technology into application. Research into ferroelectric thin films, which pose similar challenges in terms of precision and material purity, is already underway.