BS IEC 63068-3:2020 pdf free download – Semiconductor devices – Non-destructive recognition criteria of defects in silicon carbide homoepitaxial wafer for power devices
4.2 Principle
PL images of defects are captured and transformed into a digital format. In the course of this process, an SiC homoepitaxial wafer is irradiated with excitation light whose energy is greater than the bandgap of 4H-SiC crystals, and the resulting PL is collected and recorded as a PL image of a specified area of the wafer including defects. PL is detected using an optical image sensor such as a CCD image sensor, and PL image is usually acquired using an optical filter which transmits a specific range of PL appropriate for the detection of each type of defect. Then, the obtained PL image (digital image) is processed by manipulating the grey levels of the image. Through a specified scheme of image analysis, the image information is reduced to a set of values which are specific to the detected defects. A greyscale image is produced from the original digital image of defects in the wafer. This image can be converted into a binary image (thresholding). The size and shape of defects are measured, and the distribution and number of defects within a specified area of wafer are calculated.
NOTE The size of planar and volume defects extending along the off-cut direction depends on the thickness of homoepitaxial layer. Details of such defects and the method of estimating the size of their PL images are described in Annex A and 4.6.2, respectively.
4.3.1 .5 Uniformity and constancy
A combination of light source and focusing optics should be optimized to achieve sufficient uniformity of the excitation light intensity on the wafer surface. The PL intensity at each point on the epitaxial layer is adjusted in an appropriate range so that defects are clearly detected. Uniformity of excitation light intensity can be achieved using hardware and/or software. The spectral and power distributions of the excitation light are maintained constant during the whole measurement period.
4.3.2 Wafer positioning and focusing
Wafers shall be positioned in the plane of Cartesian coordinate system (X–Y) or cylindrical coordinate system (R–θ). The third axis (Z) is the optical axis of image capturing system. The Z-axis is perpendicular to the plane and its point of intersection with the plane shall be the point of focus. The distance between the front-end portion of image-capturing optics and the wafer surface shall be constant, independent of the thickness of the wafers, so that focusing and magnification are not mutually adversely affected.
4.3.3 Image capturing
The PL imaging system is typically composed of a light source, focusing optics, CCD image sensor as an optical digital sensor, lighting-geometry adjustment system, wafer stage and light-tight enclosure. A dark box or a rack housing is often used to prevent the interference by external illumination. The spatial resolution of the PL imaging system shall be high enough to capture distinct features of small size defects. The image information is digitized directly within the optical image sensor unit. To ensure the repeatability and reproducibility of the image capturing procedure, parameter settings should be carried out at a regular interval. This can be performed using specified reference wafers, for example, silicon or silicon carbide wafers.