BS IEC 63068-3:2020 pdf download – Semiconductor devices — Non-destructive recognition criteria of defects in silicon carbide homoepitaxial wafer for power devices Part 3: Test method for defects using photoluminescence
4 Photoluminescence method
4.1 General Defects with characteristic PL features shall be evaluated by PL method. The following descriptions concern such defects in n/n + -type 4H-SiC homoepitaxial wafers with an off-cut angle of 4° along the direction of [1 1 2 0], where their PL images are obtained by detecting emission wavelengths longer than 650 nm:
– individual linear defects exhibiting bright line images, e.g. BPDs;
– individual planar defects exhibiting dark contrast images, e.g. stacking faults, propagated stacking faults, stacking fault complexes, and polytype inclusions. When emission wavelengths from 400 nm to 500 nm are used for the defect detection, stacking faults exhibit bright contrast images. Defects without characteristic PL features or with weak PL contrasts against SiC area with no defects should be evaluated by other test methods such as optical inspection and X-ray topography. Those defects include micropipes, TSDs, TEDs, scratch traces, particle inclusions, bunched-step segments, and surface particles.
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 Measuring equipment
4.3.1 .1 PL imaging system Measuring equipment for PL imaging of defects in 4H-SiC homoepitaxial wafers is shown in Figure 1 . The measuring equipment consists of light source, focusing optics, optical filter, CCD, wafer stage, controller/processor, and dark box. Each component shall have the performance specified below. Different wafer specifications and defect types will require an optimum setup of light source, focusing optics and optical filter to acquire distinct PL features that are to be analysed. Therefore, a combination of light source, focusing optics and optical filter for a specific application needs to be prepared.
4.3.1 .2 Light source
A gas discharge lamp, such as a mercury-xenon lamp, and diode lasers with a specific emission wavelength are used as a typical source of photons for electronic excitation. When a white light from a gas discharge lamp is used for electronic excitation, suitable optical filters for the light source shall be used to obtain excitation light with a suitable wavelength band for PL imaging. The suitable wavelength of excitation light shall be selected to be equal to or greater than the bandgap energy of 4H-SiC. For example, an emission line of 31 3 nm or 365 nm from a mercury-xenon lamp is suitable for electronic excitation of 4H-SiC.BS IEC 63068-3 pdf download.