Causes of Abnormal Black Spots on the Back Edge of Silicon Epitaxial Wafers
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The electrical parameters, defects, and uniformity of epitaxial wafers are directly related to the key parameters of the device, which in turn determine the yield of the device. In order to improve material utilization to a greater extent, customers have put forward higher requirements for the edges of external extension sheets, and reducing edge defects has become an important implicit requirement. The fog defects and slip lines that often occur at the edge of epitaxial wafers have received strong attention, while other abnormal phenomena at the edge, such as black spots on the back edge, have also attracted attention. During the inspection of the surface and back of the epitaxial wafer, the inspector found abnormalities in the back edge of the silicon epitaxial wafer. The abnormal area appears as black patches under fluorescent light, as shown in Fig.1-a, while under strong light, it reflects white light, as shown in Fig.1-b, resembling foreign object contamination.
Fig. 1 Edge detection of anomal silicon epitaxial wafers under fluorescent lamp (a) and strong light lamp (b)
The production personnel were unable to remove the black spots through water washing, acid washing, and mechanical scraping, proving that they were not caused by surface pollutants. Secondly, no defects were found in the silicon single crystal through standard corrosion testing, so the primary defects in the crystal rod were also excluded. In addition, the white threads on the edge of the substrate are commonly found on substrates from different manufacturers, but they have not caused black spots.
After repeated observation, it was found that the black spots appeared at the edge of the back sealing layer and stopped at the back sealing area, and it is preliminarily suspected to be related to the chamfering process of the substrate. After the restoration chamfering process, it was found that there may be excessive grinding of the chamfering wheel from the starting point to the end point during the substrate chamfering process, and the location of excessive grinding is consistent with the position where black spots appear. Further confirmed that this phenomenon is related to the substrate chamfering process. In addition, when observing the abnormal location under a microscope, it was found that the abnormal area was not smooth, but appeared corrosive. If it is caused by excessive grinding of chamfers, the abnormal area should not have this corrosive appearance.
By comparing various epitaxial furnace processes and considering the phenomenon of corroded edges, it was found that the epitaxial furnace process included HCL polishing, which resulted in abnormal wafers, while the epitaxial furnace process did not include HCL polishing, resulting in 0 abnormal wafers.
Therefore, the fundamental reason for the black spots is the interaction between excessive grinding in the substrate chamfering process and the epitaxial HCL process. Excessive local grinding during the chamfering process leads to abnormal edge structure of the substrate (consistent with the black spot position), forming surface damage areas; The intervention of HCL in the epitaxial process can lead to corrosion and different silicon absorption rates in the damaged area, resulting in abnormal color. Due to HCL not reacting with SiO2, black spots only appear at the edges (exposing the silicon material), while the back sealing layer is not affected. The observed corrosion edge morphology under the microscope further confirms this mechanism.
It should be noted that the black spot only exists at the edge removal area and will not affect the device process.
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