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Sub-Surface Damage

Material Name: Silicon
Record No.: 103
Primary Chemical Element in Material: Si
Sample Type: Wafer
Uses: Etching
Etchant Name: None
Etching Method: Dry etching
Etchant (Electrolyte) Composition: No data
Procedure (Condition): No data
Note: All commercially available grinding systems use a two-step process including a coarse grinding that performs the bulk of the material removal and a subsequent fine grinding. The first grinding step is performed with a rough grinding wheel (small mesh) in order to remove the bulk of the Si at high speed (in the order of a few ìm per second). Yet, it causes deep SSD due to the brittle nature of the Si wafer in combination with the big grit size. This damage layer is typically confined to the first 30-20 µm below the ground surface (Fig 1). A fine grinding step is then performed to remove this damaged layer created by the coarse grinding step and reduce surface roughness by means of grids with smoothest roughness than that ones used for the first step. This step provides a mirror like surface that is the final finish of the wafer backside. Fine grinding step provide the highest wafer and die strength as the high grit wheel removes the most subsurface damage. As a rule, as the grit increases, the wafer strength and smoothness improves, while the wafer warpage and subsurface damage decreases. However, there is a remaining defect band near the surface. Although the fine grinding is used to remove the SSD from the rough grinding, it also introduces its own damage, though in a much smaller range, normally a few microns deep or even below 1ìm. The residual defects cause stress in the thinned wafer, leading to an additional bow and often broken wafers during handling or further processing. This means that additional thinning is necessary to remove the remaining defect layer and surface roughness after mechanical grinding. This can be done by either chemical mechanical polishing (CMP), dry etching (ADP) or wet chemical etching.
Reference: Isabella Para, Thermal dissipation improvement by new technology approach: study, development and characterization, PHD Thesis, POLITECNICO DI TORINO, 2017, pp. 4-5.


Figure 1: Sub-Surface Damage stack (top) and SEM image of cracks (fracture zone) introduced because of the rough grinding process (bottom).