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Wafer Stress Release Etching Process
Wafer backside grinding is conducted in the latter stage of IC manufacturing to ensure a desirable thinness of the wafer, and facilitate subsequent wafer dicing and high-density packaging. For example, wafers must be ground and polished from 600 ~ 700 μm to less than 180 μm in SmartCard designs. Wafer back grinding leads to stress and warpage, so therefore, if there is excessive stress on the wafer, the components on the wafer front would be significantly impacted. This is why a mixture of HF and HNO3 is necessary for silicon wet etching to eliminate the stress and damage layer wafer (Stress & Damaged Layer Removal).
This particular process is characterized by the following:
This particular process is characterized by the following:
Figure 1、Etching uniformity values of a continuous process on the production line (Source: GPTC)1. Etching Uniformity
The 8-inch Wet Bench automation equipment designed and manufactured by GPTC is perfect for silicon wafer etching. Results indicate that a specially designed tank with improved flow field can successfully overcome loading restrictions, an accompanying issue with the cassette. Also, etch uniformity of 8-inch silicon wafers can reach within 5% (Figure 13). Because wafer etching is a type of batch process, a wet bench unit can produce 40,000 pieces of wafer a month to significantly improve productivity and reduce costs.
Figure 2、Before-and-after wafer etching (Source: GPTC).2. Eliminating Stress and Warpage
Defects resulted from silicon wafer back grinding include: grinding damage layer, crystal defects (Crystal Defect) and micro-cracks (Micro-crack). Silicon wet etch process can significantly eliminate stress, and damage on the layer, thus bolstering the strength of the wafer and chip. Figure 14 is a photograph of an 8-inch wafer before and after etching: on the right is a wafer before etching. Wafer back grinding has led to warpage, making it unable to lie levelly on the workbench; on the left is a wafer after etching. Stress and warpage have been eliminated, allowing the wafer to lie levelly on the bench.
Figure 3、Wafer surface after a stress relief etching process by SEM inspection (Source: GPTC)3. Micro-roughness surface etching increases metal deposition adhesion
In an instance where backside metal deposition is necessary after back grinding to increase adhesion, special agents can be added into the etching solution, allowing micro-roughness on wafer surface. This process increases metal deposition adhesion. See Figure 15 for a photo of the wafer examined by a SEM inspection equipment after it is etched.