Future Power Electronics Overcoming Challenges in SiC Production

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For decades, silicon (Si) has formed the backbone of semiconductor technology - from microprocessors to power electronics. However, with growing technical demands, the material's properties are increasingly reaching their physical limits.

In the search for solutions, materials with a wide bandgap (WBG), including silicon carbide (SiC) and gallium nitride (GaN), are increasingly being used. The bandgap describes the energy difference between the upper end of the valence band and the lower end of the conduction band. Silicon (Si) has a relatively narrow band gap of 1.1 electron volts (eV), while SiC and GaN have 3.3 and 3.4 eV respectively. These properties mean that WBG materials behave more like insulators and can operate at higher voltages, frequencies and temperatures. They are therefore suitable for power conversion applications such as electric vehicles and renewable energy.

Silicon Carbide (SiC)

SiC is not new and has been produced as an abrasive for more than a century. However, the material has other advantages due to its attractive properties: it is suitable for high-performance applications with high voltage. Physical properties such as high thermal conductivity, high drift velocity and high breakdown field strength give SiC designs very low losses, faster switching speeds and smaller geometries than Si MOSFETs or IGBTs. Many in the industry see SiC as a competitive advantage because it can increase efficiency while reducing size, weight and cost. Since SiC systems operate at higher frequencies, the passive components are smaller, and since losses are lower, less thermal protection is required. This leads to the desired higher power densities that many modern applications demand.

In addition to the choice of material, new chip/die mounting techniques in SiC power components also help to dissipate heat from the components. Techniques such as sintering create a strong bond between the chip and the substrate and ensure a reliable connection. This improves heat transfer efficiency and thermal behavior. SiC is generally used for high voltages (>650 V), although it comes into its own at 1,200 V and above and is the best solution for PV inverters and electric vehicle charging. The material is also an important enabler for semiconductor-based (solid-state) transformers, where semiconductors replace magnets.

Challenges in Production

The production of SiC is not easy as the granules must be very pure and SiC boules (semiconductor blanks) require a high degree of consistency. As SiC material can never be liquid, crystals cannot grow from a melt and therefore require carefully controlled pressure in a vapor phase process known as sublimation. This involves placing SiC powder in a furnace and heating it to over 2,200 °C, where it sublimates and crystallizes at a nucleus. However, growth rates are slow - up to 0.5 mm per hour. Due to the extreme hardness of SiC, it is difficult to cut even with a diamond saw, which makes the production of wafers more difficult than with silicon. Although other techniques can be used, these can introduce defects into the crystals.

As SiC is a highly defective material and doping is a challenge, it is not easy to produce larger wafers with few defects. Nevertheless, companies such as Onsemi are now routinely producing 8-inch substrates.

Supporting Research

Onsemi recognizes the importance of science in the development of semiconductor technology. Research work is being carried out on SiC:


 

• Resistance to cosmic radiation

• Modeling the intrinsic lifetime of the gate oxide

• Characterization of the SiC/SiO2 interface and lifetime modeling

• Extrinsic population (screening)

• Epitaxy and substrate defects

• Degradation of the body diode

• Reliability when blocking high voltages (HTRB)

• Specific performance indicators for edge termination, avalanche robustness and short circuit

• Design for high dv/dt robustness

• Surge currents

In addition, Onsemi invested USD 8 million in a cooperation with the Onsemi Silicon Carbide Crystal Center (SiC3) at Pennsylvania State University (PSU). The aim of this collaboration is to further advance the development of silicon carbide technologies. In addition, Onsemi is cooperating with at least six other educational institutions in Europe to expand research and development in this field.

The Manufacturing Advantage

Onsemi has a fully integrated SiC device supply chain that provides complete control over all aspects of the process and associated quality - from blank to customer. The process begins in New Hampshire with the growth of single crystal SiC material to which a thin epitaxial layer is then added. This is followed by several processing steps for the die and packaging. The end-to-end process in the onsemi-fab enables comprehensive testing and supports root cause analysis. The goal is highly reliable products without defects.

Transparency and control over each individual step makes it easy to adjust capacity to growing demand. In addition, the process can be optimized to maximize yield and control costs. McKinsey & Company recognized the benefit of a vertically integrated supply chain and confirmed that "vertical integration in SiC wafer and device manufacturing can improve yields by 5 to 10%".

Five Steps to Success

To overcome the SiC-specific challenges, Onsemi applies a five-step methodology. This solves issues such as substrate and epitaxial defects, body diode degradation, high voltage blocking reliability and application-specific performance. Gate oxide integrity (GOI) is critical in this process and is an area where the five-step approach is applied:

1. Control - Instruments such as a control plan, statistical process control and a failure mode and effects analysis (FMEA) are used, data is collected and used to improve the process.

2. Improvement - Defects in the substrate or in the epitaxial layer as well as metallic impurities and particles can impair the GOI. Continuous improvement reduces the occurrence of such defects.

3. Testing and screening - Visual and electrical inspections identify defective chips. Substrates are scanned and scanning continues during wafer processing to detect defects at each stage. Electrical tests are performed at wafer level, including burn-in and wafer sorting.

4. Characterize - QBD (Charge to Failure) tests are used to measure GOI quality as more details are visible. The tests have shown that SiC is 50 times better than silicon in intrinsic QBD performance. QBD tests are performed in production and wafers are rejected if they do not meet a predefined acceptance criterion.

5. Qualify and extract models - The intrinsic performance of the gate oxide is evaluated using time-dependent dielectric breakdown stress (TDDB). Gate bias and temperature are combined to stress the SiC MOSFETs and the times to failure are recorded. The Weibull statistical distributions are then used to extract the lifetime.

The SiC difference

Onsemi recognizes the critical role of SiC in the future of power electronics - particularly in power conversion in the electric vehicle and renewable energy sectors. This understanding is driving investment in manufacturing capacity and product innovation to ensure that SiC reaches its full potential as quickly as possible.

As a vertically integrated supplier, the entire process is under one roof and in our hands - something no one else can claim. This controls costs and ensures flawless products for vehicle manufacturers and industrial customers. (mr)

*Catherine De Keukeleire is Director Reliability & Quality Assurance, Wide Bandgap at onsemi