A versatile plasma source at the core of tomorrow's innovations

Without plasma technologies, there would be no smartphones, no chips, no SIM cards. Without them, it would be impossible to manufacture with extreme precision the miniaturized components that equip our modern electronic devices. Discreet yet revolutionary, these technologies are shaping today’s microelectronics industry and are making their mark across numerous industrial sectors. It is within this dynamic context and rapidly expanding market that the resonant network antennas developed at the Swiss Plasma Center (SPC), come into play, an innovation that emerged unexpectedly.

Eureka!
The concept of resonant network antennas was sparked by a technology from a completely different scientific field: Magnetic Resonance Imaging (MRI). These medical devices use radio-frequency waves to excite the nuclear spins of atoms in the body, producing detailed scans. But in 2003, Philippe Guittienne, researcher at the SPC, saw something more. Inspired by his wife’s PhD thesis, he had a bold idea: what if the same principle could be used to generate plasma?

That spark of inspiration led to the development of resonant network antennas, a plasma source that generates high-energy electrons which cause exotic gas chemistry without heating the gas or surfaces. This breakthrough preserves the quality of the materials, making it a perfect fit for ultra-precise manufacturing.

The influence of this innovation is far-reaching. While plasma is often associated with fusion energy, its applications extend well beyond. Some examples include space satellite design and plasma thrusters, coatings for jet turbine blades, thin film deposition for solar cells, roll-to-roll packaging, plasma welding, and waste treatment, to name some. In addition, in the medical field, bioplasmas are used for sterilisation, skin wounds, and even cancer therapy. However, perhaps the most transformative impact is inmicroelectronics, the foundation of our digital world.

A multi-trillion-dollar market
Chip manufacturers use plasma to imprint electronic circuits at the nanoscale level. These circuits consist of arrays of millions of transistors fabricated on a silicon wafer. To isolate individual transistors from each other, the silicon is etched away following a pattern previously deposited as a photoresistive layer.

In the past, this process was done using acid-based "wet etching", which dissolved the exposed silicon pattern in all directions, widening the pattern as the acid penetrates into the silicon. This effect prevents the imprint of fine details on the wafer. Therefore, manufacturing circuits on a nanometric scale requires the use of a different technique.

Resonant network antennas could have a big impact on large area plasma processing for microelectronics

Enter plasma etching: instead of acids, it uses ions accelerated across the plasma sheath, which forms spontaneously in contact with the silicon surface. Then, the plasma ions bombard the silicon only in the perpendicular direction. This “dry” etching produces well-defined features, thus providing a high-fidelity reproduction of the circuit pattern.

But there’s a catch: traditional monolithic plasma sources struggle to maintain uniformity over large areas. As chipmakers push for ever-larger silicon wafers, conventional technology simply can’t keep up. This is where resonant network antennas could change the game. Unlike previous plasma sources, these resonant antennas distribute energy evenly across vast surfaces, enabling precision plasma etching at an industrial scale.

This technology might be an opportunity for Europe to enter a multi-trillion-dollar global industry currently led by the USA and Asia. “Resonant network antennas could have a big impact on large area plasma processing for microelectronics”, declares Alan Howling, former researcher at the SPC and co-author of the book.

Authors of the book

• Philippe Guittienne is currently a physicist at the Swiss Plasma Center in the Low Temperature Plasma Physics and Applications group of the SPC. He is the founder of Helyssen Sàrl. for the development and commercialisation of resonant network antennas as plasma sources.
• Alan Howling is a Senior Scientific collaborator and former researcher and lecturer at the Swiss Plasma Center.
• Ivo Furno is currently Adjunct Professor at the EPFL and the leader of the Low Temperature Plasma Physics and Applications group of the SPC.

Tech Corner
Resonant Network Antennas

The generation of plasma, the fourth state of matter, requires various components. First, it requires a gas such as argon, nitrogen, or hydrogen. In regular conditions these gases are not in a plasma state. A strong external electromagnetic field is applied in the range of radio-frequency (RF) waves to ionize the atoms and molecules of the gas: Electrons follow the electric field, while ions remain almost unperturbed due to their high inertia. These electrons produce a cascade of collisions with atoms of the gas and ionize them, leading to the generation of the plasma. Finally, an external magnetic field can be used to shape and confine the plasma.

Traditionally, there are two main techniques for generating plasma: capacitively- and inductively-coupled plasma sources. Although these two technologies are commonly used in industry, their asymptotic reactor impedance and plasma non-uniformity impose limitations for treatment of large substrates. Resonant network antennas can overcome these limitations thanks to their distributed resonant structure.

Resonant network antennas consist of a mesh of parallel inductive and capacitive elements, whose overall circuit exhibits a set of resonant modes. The capacitance is chosen so that the electric current resonates at radio frequencies of a few MHz. The associated oscillating magnetic fields induce electric fields in the surrounding low-pressure gas, which electrically breaks down to form a plasma of ions and electrons. The real impedance at the resonance frequency simplifies the power matching and minimises the high input currents inherent in the upscaling of conventional capacitively- or inductively-coupled devices.

Network antennas can be built in two basic configurations: planar (also called “ladder”) and cylindrical (also called “birdcage”) configurations. Planar configurations are used to create flat plasma that can cover large surfaces. This configuration is particularly suited for plasma processing of silicon wafers, solar cell arrays, and flat panel displays, as well as for the treatment of glass in architecture, for example. On the other hand, cylindrical or “birdcage” design is best suited for volume treatment of multiple items, such as biocompatible barriers on medical implants, as well as for plastic activation for the adhesion of paint on automobile components. All these applications represent an enormous and rapidly growing market.