Uncovering Imperfections In SiC
The miniscule scratches and pits found on the surface of SiC substrates spawn yield-killing defects in the epiwafers. It is not possible to pick-up these minor imperfections and track their consequences with conventional light-scattering inspection tools, but this can be done with our detection systems employing confocal optics and differential interferometry, argues Yuji Asakawa from Lasertec Corporation.
To reduce carbon dioxide emissions and meet the rapidly growing energy demand in emerging economies, engineers are installing more eco-friendly energy sources and trying to increase the efficiency of many different products. These efforts can be aided by the introduction of a range of new technologies that increase the efficiency of electronic devices.
Today, the vast majority of electronic devices are made from silicon. This material is relatively cheap to process, but it cannot deliver device efficiencies as high as equivalents built from wide bandgap semiconductors. These alternatives promise to enhance electric power conversion efficiency in a wide range of applications, including electric vehicles, inverters for solar systems and air-conditioning.
A handful of companies have already brought diodes and transistors made from wide bandgap materials to market, but in general, this industry is still in the development stage. For sales to rise, the quality of these products must improve and their production costs fall, so that they can be seen as genuine competitors to the silicon incumbents.
Introducing inspection tools that set a new benchmark for scrutinizing substrates and epiwafers will help to make better products and drive up device yields. At Lasertec Corporation, an inspection tool manufacturer with headquarters in Yokohama, Japan, we have launched a range of tools that do just this: the SICA61 series, which is designed to inspect SiC material and is discussed in the remainder of this article; and TROIS, an instrument for scrutinizing other wide bandgap materials, which is described in the box “Inspecting and reviewing wide bandgap semiconductors", which can be found on below.
Lasertec’s SICA series is capable of identifying scratches with a depth of less than 1 nm in SiC substrates
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.
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
One of the great strengths of using confocal optics is that it is able to capture accurate information of a sample surface, because the detector does not receive light reflected from the back surface of the substrate. Very small pits and shallow scratches with a depth of just 1 nm or less can be uncovered, thanks to the incorporation of differential interferometry. In comparison, inspection tools based on light scattering are generally unable to resolve such small deficiencies in SiC material, which are known to impair device performance. We launched the SICA61 in 2009, and last year we addressed the request for a higher throughput tool with the addition of the SICA6X. This is designed to detect and review defects generated during each process of SiC production – from SiC crystal growth to planarization to epitaxial film growth – and to evaluate the entire production processes with high efficiency.
When SiC substrates are formed by slicing discs from an ingot and polishing them with a machine, defects are created, such as scratches (see Figure 2). Even if they are shallow, they impact the quality of the epitaxial layer by inducing other types of defects to form, such as step bunching and continuous carrots (See Figures 3 and 4). Those defects that are subsequently formed adversely impact device performance. What’s more, during device manufacturing, the activation annealing and thermal oxidation processes lead to growth of step bunching near scratches.
Figure 2. Defects present in the ingots and that result from the slicing and lapping processes can degrade device performance
Figure 3. It is possible to observe step-bunching growth during the annealing process. Images of a rapid change in surface morphology have been observed by Tadaaki Kaneko and his team from Kwansei Gakuin University, Japan. Images (a), (b) and (c) reveal the changes in the wafer when the temperature exceeded 1500 °C. The inset in (c) is an atomic force microscopy image that confirms that the defects in the orange circles are pits
Figure 4. Scratches on substrates can spawn yield-killing defects, such as carrots, in the epitaxial film.
Consequently, it is clearly desirable to eliminate as many scratches as possible during the chemical mechanical polishing (CMP) process. One way to do this is to employ our SICA range of tools in this process, because they can detect shallow surface scratches. With SICA, it is possible to review the results of polishing processes under different conditions, simplifying efforts to determine the optimum polishing process for SiC wafers. The improvements that result will be highly valued, because SiC is a very hard material – it takes a lot of time and cost to polish it and make it flat.
Another form of surface deficiency in SiC substrates, small pits, has been found to shorten the time-dependant dielectric breakdown of MOSFETs. However, this effect can be mitigated by planarization of step bunching over the surface of epitaxial wafers using CMP. This implies that one route to increasing the long-term reliability of SiC devices begins by capturing detailed information of the surface conditions of the epiwafers. Our SICA tools are ideal for this task, because they can generate a plot of the distribution of step bunching over the entire surface of a wafer. In addition, they have sufficient sensitivity to reveal very small pits without interference from step bunching.
An additional attractive feature of our tool is that it has an alignment trace function for accurately locating positions of defects in different processes. This allows the user to track the impact of an individual defect in a substrate on the quality of the epitaxial film grown close to it.
Specifically, the alignment trace function is capable of performing three insightful tasks: Identifying what types of defect have a large impact on electrical characteristics of a device, and how many of them are acceptable; determining what types of defects can be the root cause of yield-killing defects; and enabling the formation of a grading structure for detecting and describing defects in substrates, epiwafers and devices.
The unique capabilities of our tool are already creating something of a stir within the SiC industry. Device manufacturers are starting to consider using our tool for incoming inspections, and the SICA series is already playing a role in the SiC supply chain. Its deployment is sure to rise over the coming decade, enabling it to play a key role in improving the bang-per-buck of SiC power electronics.
The author thanks Hirokazu Seki, Takeshi Higuchi and Genichiro Kamiyama for their help in compiling this article.
Inspecting and reviewing wide bandgap semiconductors
In addition to its tools dedicated to the inspection of SiC substrates and epiwafers, Lasertec offers an inspection and review system for transparent wafers called “TROIS series". Designed to detect and analyse defects on wide bandgap semiconductors, the TROIS series features the same optics as the SICA tools, and allows the user to carry out inspections without interference from the hetero-epitaxial film. One of the strengths of the TROIS series is that the wavelength of its light source can be varied, ensuring optimal performance for various wafers. To meet requirements for production of large-size wafers, the latest model of the series, TROIS33, is capable of inspecting wafers up to 8 inches in diameter. Maximum throughput is six wafers per hour (for 6-inch wafers).
The SICA inspection tool can identify many forms of defect, including: (a) a pit in the centre of a big bump (b) a V-shaped pit (c) a bump (d) a hole (e) cracks around the wafer edge