Interpret the new challenges brought by silicon carbide testing

Automotive electrification is driving demand for SiC (silicon carbide) power chips, but it also poses challenges in finding and identifying defects in these chips.

At the same time, there is a growing awareness of how immature SiC technology is, how much work still needs to be done, and how quickly it needs to be done. Automakers are vigorously promoting electric vehicles, and the transition from 400V to 800V battery systems is accelerating the transition of electric vehicle power modules from IGBTs to SiC devices. SiC demand is also growing exponentially, but only if these devices operate flawlessly.

Frank Heidemann, vice president and head of technology at National Instruments (NI), said: "The power semiconductor market is undergoing significant changes due to the rapid growth of electric vehicles (EVs) and renewable energy. This shift is driving the need for increased efficiency, especially in It’s in the automotive field, leading to the emergence of wide-bandgap technologies such as SiC and gallium nitride.”

SiC devices have several properties that make them a better choice than silicon-based IGBT devices.

"Power density, higher voltages and attractive thermal performance are the three things that really make SiC-powered devices attractive to people who are making very efficient motor drives or very dense motor drives or power conversion circuits." Jay Cameron, senior vice president of power ICs at Wolfspeed, said, “We see many applications that require a lot of power but in a smaller or lighter form factor. So if you are looking for a lightweight system that uses less copper for heat dissipation, with SiC, you There’s an opportunity to trade off voltage versus current while still maintaining high power levels.”

Power electronics also help reduce weight, thereby increasing the vehicle's driving range. SiC-based power modules require fewer ICs and do not require much cooling, which reduces required heat dissipation costs. These modules perform a series of basic voltage conversions between various battery systems, between charging stations and battery systems, and between electric motors and battery systems.

IGBT devices have been the primary IC supporting these functions in 400V battery systems. Engineers have already begun switching from IGBTs to SiC devices in an effort to reduce overall power module costs, but this shift is accelerating as battery vehicles move from 400V to 800V. SiC operates at voltages up to 1,200V.

Figure 1: System diagram of Mitsubishi iMiEV showing the location of modules using power ICs. Source Wikimedia Commons, Creative Commons License BY-SA 3.0

To meet the growing demand for SiC, the industry needs to increase production. This means solving the manufacturing challenges that have long hindered SiC production. These challenges include high equipment costs as well as defect and reliability issues. To address cost issues, SiC substrate manufacturers are moving from 150mm wafers to 200mm wafers. However, this expected exponential growth creates challenges for SiC device screening, which requires innovation from manufacturers as well as inspection and test suppliers.

“Testing these wide-bandgap devices at end-of-line (EoL, i.e., testing at the end of the manufacturing process of wafers, packages, modules, systems) presents unique challenges because they exhibit different failures compared to conventional devices mechanisms and model silicon devices,” said NI’s Heidemann. "Additionally, testing their reliability requires high voltage environments of up to 2,000 volts or more, posing significant challenges to EoL test systems for which previous designs were not designed."

The manufacturing process of SiC sometimes creates defects that affect basic functional and performance characteristics and therefore need to be screened through inspection and electrical testing. High voltage and high current testing requires a carefully designed test system that can deliver the necessary current and voltage while also providing protection when the inevitable short circuit occurs.

Until now, this screening has been done in small batches. Scaling up will require innovative screening approaches to ensure effectiveness and cost-effectiveness.

Inspection and measurement methods

The main difference between silicon and SiC power ICs relates to the growth of the substrate. As a uniform crystal structure, silicon has few subsurface defects. In contrast, SiC is grown by chemical vapor deposition, which can lead to various subsurface defects such as stacking faults and microtubules. Crystal defects continue to propagate during subsequent epitaxial growth. Additionally, because SiC is a brittle material, it is more prone to surface defects such as scratches and pits, which can affect overall wafer performance.

In addition, SiC wafers are easily cracked during processing, and more cracks will be generated during chip cutting, and the cracks will expand. Therefore, inspection throughout the wafer and packaging process is critical.

Due to their high throughput, engineers rely primarily on optical inspection systems during the SiC manufacturing process. Many companies offer specialized SiC optical inspection tools that include inspection and classification capabilities.

Metrology is not so simple. Metrology feedback involves a variety of parameters that process engineers need to measure, including substrate flatness and thickness, lattice orientation, electrical resistance, and surface roughness. These, in turn, require a diverse set of systems.

“White light interferometer (WLI) profilometers are used in quality assurance/quality control at wafer manufacturer sites to measure wafer roughness (sub-nanometer) for Si, GaN and SiC,” said Sandra Bergmann, white light interferometer product manager at Bruker . “SiC substrates are more challenging to produce. Due to their higher hardness, polishing is more difficult. Therefore, WLI is crucial to optimize/track the polishing process.”

SiC devices can be implemented in planar or trench technology. WLI is particularly useful for trench depth gauging.

“For high-aspect-ratio trench depth measurements during high-voltage IC processes, WLI can resolve resolutions from 2μm openings to 40μm depth,” said Bergmann. “It enables parallel inspection of all trenches within the field of view, making it non-destructive. We typically use a 5X objective with a 0.5mm² interrogation field. We also provide complete variation in depth along the trench throughout the field of view. "

Wafer inspection needs to consider surface defects and sub-surface defects, the latter being particularly important for SiC.

“Optical inspection technology is used for defect detection, while X-ray and photoluminescence are used for metrology,” said Burhan Ali, Inspection Product Marketing Manager at Onto Innovation. “The challenge with optical inspection is that it can effectively find surface defects at high throughput, but it quickly runs out of steam when it comes to subsurface crystalline defects. In these cases, photoluminescence technology has been proven Subsurface crystal defects on SiC substrates and epitaxial layers can be effectively detected.”

Inspections occur throughout the assembly process. Optical methods have the advantages of high throughput and low equipment investment and are the preferred method. But optics are limited to surface defects. For detecting moderate to high densities of subsurface defects, X-ray is the solution of choice because it can run in 2D at high speeds. Acoustic inspection, meanwhile, can easily detect delamination but requires immersing the part in water.

“Manual, optical, X-ray inspection are all non-destructive methods,” said George Harris, vice president of global test services at Amkor Technology. “Basic X-ray inspection is very useful for checking package integrity. Most of the defect patterns can be easily identified by Cross-section and scanning electron microscopy."

Inspections are not limited to electrical issues. It can also be used to identify defects that may affect thermal management.

“In packaging, most electrical defects are related to wires crossing/contacting the molding process and causing short circuits,” said Brad Perkins, product line director at Nordson. “Thermal protection also needs to be considered, which is why engineers check chip connections because it’s part of thermal management. Too much void, too high a total void percentage, or too much delamination can cause hot spots in the mold, which can lead to premature failure "Since many power devices are used in high-reliability applications (automotives, trains, windmills, etc.), the cost of failure can be very high, so it is very cost-effective for manufacturers to check for defects that can lead to premature field failure."

Figure 2: X-ray used for void inspection. SourceNordson

Test Methods

Volume production of SiC is relatively new, as is its use in automotive applications. Therefore, the industry is designing rigorous testing processes to ensure quality and reliability. Testing was performed at a variety of temperatures, voltages and frequencies. This is critical because defects may appear benign at lower frequencies and voltages, but then become apparent at higher frequencies and/or voltages.

Due to their analog nature, power ICs require functional and performance testing. For power ICs, testing is divided into static testing and dynamic testing, that is, DC testing and AC testing. Static testing is performed at room temperature, while dynamic testing is performed at elevated temperatures.

"Static testing is no longer a challenge because the device under test (DUT) is tested in a steady state." said Fabio Marino, General Manager of Advantest Italy. "That means low power. Even if it's ultra-high voltage, it's going to be low current, and if it's ultra-high current, it's going to be low voltage. The real challenge for the engineering community is dynamic testing. Dynamic testing is extremely high power because it Testing the transition of the DUT from the ON state to the OFF state and vice versa means very high currents at very high voltages, but it's very high power."