industry metrology
The upward curve for SiC Today, the SiC power device market is significantly bigger than that for GaN, its biggest wide bandgap rival. According to the market analysis firm Yole Développement of Lyon, France, SiC device sales should be worth $71 million in 2012, and will grow throughout the decade to reach almost $1 billion by 2020. During the next few years, 6-inch substrates for SiC devices will appear on the market from more and more substrate suppliers, which are planning to ramp their production capacity. Announcements at the 2011 International Conference on SiC and Related Materials indicate that substrate production capacity, evaluated in terms of surface area, will increase twenty-fold from 2011 to 2015. During this timeframe, device manufacturers are projecting a four-fold increase in the use of SiC substrates.
These substrates are being used to make several different classes of SiC power device, which are in various stages of development and production. European chipmaker Infineon launched SiC diodes back in 2001, while SiC MOSFETs are still in their infancy. US firm Cree has recently launched this form of transistor onto the market, but prices are high, and it will take several years for the SiC MOSFET to become a mainstream device.
All makers of SiC devices can improve the quality of their products by reducing the density of various types of performance-degrading defects. These can cause devices to fail or malfunction – either initially or after many hours of use – and have led some to question the long-term reliability of SiC devices. Additional challenges that must be overcome in order for SiC devices to have long-term commercial success in the power electronics market are increasing production yield and driving down costs.
Classifying defects
Three major types of defects are found in SiC, each originating from a different production process: Slicing of ingots can lead to crystallographic damage in the substrates; planarization processes can generate fabrication defects; and defects can be found in the epilayers, resulting from imperfect film growth. All forms of defect can drag down device production yields, so it is of paramount importance to detect them with high accuracy and then take effective countermeasures.
SiC quality will then increase, spurring greater commercial success, thanks to higher yields. This will not only result in better devices, but cheaper ones too – substantial savings are possible, because a typical 4-inch epitaxial wafer currently costs thousands of dollars.
Efforts to improve the quality of SiC substrates and
epiwafers have been underway for many years. They have generally involved the use of inspection tools that expose relatively large defects – including epitaxial defects such as triangles, downfalls and carrots. These imperfections are often the cause of initial malfunctioning in SiC devices. However, these conventional inspection tools rarely reveal minor defects, such as pits and scratches underneath gate oxidation films. These undetected deficiencies may well have an impact on the long-term reliability and performance of SiC MOSFETs.
Recently, we addressed this shortcoming with the launch of the SICA61, the first inspection tool designed to detect minor defects with high accuracy. It is based on two of our core technologies – confocal optics and differential interferometry (see Figure 1 for details).
Figure
1.The Lasertec SICA series features confocal optics and differential
interference.SiC substrates are transparent,so light reflects from their back surface and can inhibit attempts to detect features on the wafer
surface.This stray light is rejected with a confocal system,because only light from the sample surface is focused by the objective lens and passes through the pinhole in front of the
detector.The SICA tools also have a much higher resolution and contrast than conventional instruments,thanks to less interference from scattered light
Figure
2.Defects present in the ingots and that result from the slicing and lapping processes can degrade device performance
October 2012
www.compoundsemiconductor.net 31
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