Driven by concerns over hydrocarbon fuel resources, costs, and environmental impact, energy has moved into the spotlight. New energy sources, and conserving energy, impacts nearly every aspect of our power-dependent lives.
Improving the efficiency of energy generation is critical to every society. Many of those improvements come directly or indirectly from electronic devices, and those devices are made possible by advances in semiconductor processing.
More specifically, they are made possible by plasma processes for etching and deposition. The following covers how plasma etch and deposition processes are helping solve one of the most pressing energy issues: green technology.
What is Green Energy Technology?
Green energy technology is sustainable, clean, and environmentally-conscious technology. We can think of it as a subset of what is more popularly just called Green Technology.
Where Green Technology includes aspects such as recycling, building design, and organic food production, Green energy is more focused on renewable energy sources such as solar and energy storage (e.g. batteries and hydroelectric).
One particular set of materials has a very significant impact on green energy – wide bandgap semiconductors.
Why Are Wide Bandgap Power Semiconductors Important to Green Technology?
Power semiconductors are used in all aspects of energy generation, transmission, and usage. These devices are nearly ubiquitous as they appear in nearly every application that is manipulating power (e.g. AC/DC converters or inverters). Nearly every modern piece of electronics has some power device content.
Traditionally, power semiconductors are made using silicon (Si). More recently, the industry has moved to a newer technology that bypasses some of the inherent restrictions of Si power semiconductors.
These new materials are characterized by their wide bandgap properties – the energy required to cause a material to change from being an insulator to a conductor. Although it may seem counter-intuitive that a material that requires more energy to change from insulator to conductor is actually a more energy-efficient device, the wide bandgap (WBG) feature allows them to handle more power than materials with smaller band gaps such as Si or GaAs.
The most well-known of the wideband materials is silicon carbide (SiC), and there is a well-established market for SiC power devices (e.g. MOSFETs, Schottky diodes). The newer material in this area is gallium nitride (GaN), the same material that gives white light LEDs significant energy savings over incandescent lighting.
In this case, it is the GaN HEMT (high electron mobility transistor) devices that are changing the information and communication industry with power savings. These savings go into servers and base stations, as well as mobile phone chargers.
Wide bandgap (WBG) power semiconductors are faster, smaller, and more reliable power components that handle higher power and larger breakdown voltages. An important ancillary benefit is that they can operate at higher temperatures, reducing the requirements for larger and heavier cooling systems. For electric vehicles, this benefit is critical as it has a cascading effect; lighter weight means either small batteries (lower cost) or longer drive distances, both encouraging electric vehicle adoption. The benefits of WBG power semiconductors include:
In other words, WBG power semiconductors enable smaller systems and improve energy efficiency for green energy technologies.
GaN and SiC are not the only materials pushing the power saving limits. There are other materials with even wider band gaps that are being seriously explored for their energy benefits. Gallium oxide (Ga2O3) is one material aspiring to change the market with its 47% increase in bandgap over GaN. Similarly, diamond is receiving serious research attention for its unmatched thermal conductivity and a bandgap that is yet another 10% larger than Ga2O3.
Plasma Processing and Wide Bandgap Power Semiconductors
Wide bandgap power semiconductor devices are fabricated using plasma etch and deposition systems. Fabrication details vary based on device structure and the specific materials used in the device. A wide range of process conditions is necessary to meet the range of device structures.
For example, GaN HEMT devices grown epitaxially on SiC for radio frequency (rf) power applications; these need both a very slow, shallow, controllable low-damage etch for the device frontside, and a fast etch for the backside via through the SiC substrate. Other devices such as SiC MOSFETs require a well-defined trench profile.
Deposition processes are also varied and range from low damage, controllable SiNx plasma enhanced chemical vapor deposition (PECVD) to high rate encapsulation with stress control. Some groups explore the low-temperature deposition regime of high-density plasma chemical vapor deposition (HDPCVD).
Whatever the process, the performance is critical to not only making working devices but making them with a high yield. Despite the advantages these materials provide for green energy, there are still cost considerations. This means not only must all the process requirements be met, but they must also be met while reducing costs. Etch and deposition performance such as within-wafer, wafer-to-wafer uniformity, and yields must be market competitive.
The Future of Green Technology
WBG power semiconductors are at the cutting edge of energy. They’re delivering on promises from improving energy generation efficiency (e.g. matching inverters to specific solar panels) to conserving energy with reduced energy loss during power conversion (e.g. ac/dc or dc/ac).
Their larger operating voltages, faster switching speeds, and higher temperature capability versus traditional silicon power devices make them very attractive. These materials continue to gain traction in the market with demonstrated reliability and savings. They are poised to make a critical contribution to reducing the planet’s energy profile.
Plasma processing technology is one of the enabling components for this change.