industry metrology
When SiC substrates are formed by slicing discs from an ingot and polishing them with a machine, defects are created, such as scratches. 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
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.
Tool capabilities
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.
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.
October 2012
www.compoundsemiconductor.net 33
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
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
Figure
4.Scratches on substrates can spawn yield-killing defects, such as carrots,in the epitaxial film.
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