Physical mechanism characterization and verification of High-Uniformity Ultra-Thin wafer thinning

Wafer thinning technology is crucial for the current development of 3D chip stacking technology. The appropriate wafer thinning process determines the yield and reliability. In the current semiconductor industry production. Dry etching is commonly and extensively utilized in the wafer thinning process, which is critically important for achieving superior wafer surface quality and excellent thickness uniformity. However, few studies have investigated the control methods for wafer TTV (Total Thickness Variation) during ultra-thin wafer thinning processes and their corresponding physical mechanisms. This work designed a freely adjustable gas homogenization device (GHD) capable of precisely regulating the uniformity of dry etching on wafers. To study the physical mechanisms by which GHD affects etching uniformity, the finite element analysis is first used to simulate the impact of GHD for distribution of F radicals (F*) and SF⁺ ions generated by the plasma discharge during the etching process, Simulation results demonstrate that the TTV during wafer etching is predominantly governed by the distribution of SF⁺ ions across the wafer surface. The gap and height parameters of the GHD significantly influence SF⁺ ion flux modulation. When the gap progressively widens until complete GHD removal, the SF⁺ ion flux at the wafer edge increases from 5.21 × 10¹⁸ m⁻²·s⁻¹ to 7.27 × 10¹⁸ m⁻²·s⁻¹. Conversely, introducing the GHD and elevating its height induces a pronounced reduction in edge SF⁺ ion flux, decreasing from 2.41 × 10¹⁸ m⁻²·s⁻¹ to 7.16 × 10¹⁸ m⁻²·s⁻¹. Moreover, experiments validate the reliability of simulation results, the effects of GHD parameters on the wafer etching uniformity are also analyzed during the dry etching thinning experiments. Ultimately, a bonded wafer is used to test the result of optimized GHD and is thinned as small as 4 μm. While the overall wafer total thickness variation (TTV) is controlled around 1 μm, which is similar to bare wafer. A good wafer surface quality is also achieved.