Relationship Between Wafer-Level Warpage and Cu Overburden Thickness Controlled by Isotropic Wet Etching for Through Si Vias

Abstract

In this paper, we developed an isotropic wet etching process in a capsule-type bevel etch chamber to reduce a Cu overburden of through Si via (TSV) for less wafer-level warpage with 300 mm wafers. We report the relationship between the wafer-level warpage and the Cu overburden thicknesses controlled by the isotropic wet etching with diluted solution of hydrogen peroxide and sulfuric acid, which is widely used for Cu wet etching. After Cu filling by electroplating, there are humps at the top of the TSVs; therefore, the isotropic wet etching can be considered as a solution to etch away the Cu overburden without any damages on the TSVs. We modified the capsule-type bevel etch chamber to avoid serious attack on TSVs at the center area of the wafer caused by the etchant delivery path. We also adjusted the process parameters to have a controllable Cu etch rate. The etch rate of ∼0.2 µm/s and the uniformity of ∼3% were achieved. The overburden was able to be etched up to 3 µm from the initial Cu overburden. While the Cu overburden decreased during the isotropic wet etching, the TSVs were protected from the etchant because of the humps at the top of the TSVs. After the Cu electroplating, there was a grain size difference between the Cu at TSV and the Cu at field area. Because the microstructural difference caused a galvanic corrosion during the wet etching, the etch rate of the adjacent Cu around TSV was faster than the Cu at any other area. That resulted in exposure of dielectric layer at the adjacent area around TSVs when the Cu overburden was etched heavily. It may be another protection mechanism of TSV during the isotropic wet etching. The wafer-level warpage of the wafer with the Cu overburden etched up to 3 µm after the annealing decreased by 50% from that of the wafer with the initial Cu overburden. The wafer-level warpage exhibited a linear relationship with the Cu overburden thickness controlled by the isotropic wet etching.Index Terms-Cu filling, Cu overburden, galvanic corrosion, isotropic wet etch, microstructure of Cu, through Si via (TSV), wafer warpage, wafer-level packaging. publication by Associate Editor B. Dang upon evaluation of reviewers' comments.The authors are with the Institute

Fabrication of silicon-embedded low resistance high-aspect ratio planar copper microcoils

“…Furthermore, the process described in Ref. 44 involves differential etch rate between Cu protrusion from TSVs and Cu over burden outside TSVs to achieve the selectivity while in our case the scenario is totally different where a blanket universal cover of Cu overburden is formed over the Cu microcoil and this has been removed by sodium persulfate etching the parameters of which has been determined precisely using the orthogonal DoE. The comparison table for existing Cu wet etchants given in Ref.…”

Design and fabrication of micro-coils for NMR micro probes

“…Next, only room temperature was used, which yielded good results with controlled etching and well defined traces for the micro-coils with 3. wet etch procedure is the first one that reports Cu wet etching using sodium persulphate solution alone, avoiding the usage of any toxic or acidic agents. Choi et al have reported controlled wet etching of copper overburden but their etching solution consisted of sulphuric acid and also required a special capsule chamber for the Cu etching process [102]. However, in our case we avoided the use of any acid and no special apparatus and/or equipment set up was required.…”

“…Wet etching of copper has been well-known for a long time and reported in detail earlier [100][101][102][103]. Choi et al have reported controlled wet etching of copper overburden obtained from electroplating of TSVs, their etching solution using sulphuric acid solution in a special capsule chamber [102].…”

“…Fabrication of High Aspect ratio (HAR) Planar Micro-coils embedded in silicon involves simple immersion of the wafer in the etchant solution and taking out the wafer after a specific time, followed by thorough rinsing. Furthermore, the process described by Choi et al [102] relies on the differential etch rate between the Cu protrusions from TSVs and the Cu overburden outside TSVs to achieve the desired selectivity. In our case the scenario is totally different: a blanket universal cover of Cu overburden is formed over the Cu micro-coil and this has been removed by sodium persulphate etching, the parameters of which have been determined precisely using the orthogonal DoE.…”