在通電時錫晶粒方向與錫晶界對Cu6Sn5生成之影響

Abstract

隨著電子元件微小化與多功能化的趨勢，晶片的元件密度與I/O數需求也越來越高，使得半導體製程技術將面臨種種挑戰。而以往的半導體製程技術已漸漸達到了物理極限，將難以繼續微縮下去。因此，為了滿足未來的種種需求，3D封裝技術將是未來半導體發展的關鍵技術。 銲錫微凸塊(microbump)是3D IC技術的可行結構之一，其與傳統覆晶銲錫凸塊的最大差異點在於凸塊尺寸縮小至20微米，銲錫所占體積百分比大幅降低。所以銲錫內所含的錫晶粒數量將會受限，甚至在一個微凸塊中只會存在著一兩個錫晶粒。此外，在文獻中指出，鎳原子在錫晶粒內的擴散是高度非等向性的，c軸與電子流的角度差異會造成銲錫凸塊形成截然不同的破壞模式。因此，錫晶粒方向的研究對於3D IC微凸塊的可靠度會是相當重要的議題。然而，在通電的情況下，銅原子在不同錫晶粒方向下擴散的影響還沒有被報導。 本研究利用銲錫高度為30微米的覆晶銲錫Cu/SnAg/Cu結構來模擬微凸塊的情形，並利用背向散射電子繞射儀(EBSD)來分析錫晶粒的方向。在140 PoPC，1.17 x 10P4P A/cmP2P通電測試後，發現銅原子在錫晶粒內的擴散非等向性差異雖然沒有鎳來的高，但不同方向的錫晶粒仍可造成相當顯著的差異。若錫晶粒的c軸平行電子流方向，會使陰極端的介金屬化合物及金屬墊層快速溶解，並在陽極端產生CuR6RSnR5R累積，且此現象在扇貝狀Cu6Sn5的channel關閉後仍會發生；但若c軸垂直電子流方向的話，CuR6RSnR5R會沿著錫晶界成長，且成長的速率與晶界的角度有關，而此現象在關閉CuR6RSnR5的channel後將會變得不顯著。因此除了錫晶粒的方向外，錫晶界對於介金屬化合物的生成也扮演著相當重要的腳色。

As electronic devices become more functional and miniaturization, the microelectronic industry is facing a lot of challenges. The fabrication process is encountering many physical limitations. In order to keep up with Moore’s law, the 3D-IC packaging technology is a promising solution to the limitatons. Microbumps are one of the possible solutions for 3D-IC packaging. The bump diameter shrinks to 20μm and the bump height decreases to about 10-15μm . As a result, the number of solder grains in a microbump is limited, and these few grains may affect the reliability of microbumps. It is well known that the diffusion is the key factor for intermetallic compounds (IMCs) formation, void formation and electromigration (EM). Previous researches reported that the diffusion of Ni in solder is highly anisotropic. The different Sn grain orientations can cause different failure modes for solder joints with Ni under bump metallization (UBM) during current stressing. Therefore, the Sn grain orientation is a critical problem for the reliability of 3D IC microbumps. However, no researches reported the anisotropic diffusion of Cu in solder under EM. In this study, we used flip-chip samples with 30μm bump height to study the Sn grain orientation effect on Cu diffusion by Electron Backscatter Diffraction (EBSD). The stressing condition was 1.17 x 10P4P A/cmP2 Pat 140 PoPC. Even though the anisotropy of Cu in Sn is not as large as Ni in Sn, the effect of Sn grain orientation on Cu-Sn IMC formation was still obvious. If the c-axis of Sn grain was aligned with along electron flow, the IMCs and the Cu UBM dissolved quickly at cathode side and the IMCs accumulated at anode side. Even when the channels of scallop CuR6RSnR5R closed, this phenomenon was unchanged. But if the c-axis of Sn grain was normal to electron flow, the CuR6RSnR5R formed along Sn grain boundary and the growth rate depended on the angle of grain boundary. Therefore, the Sn grain boundary also played an important role in the IMCs formation besides the Sn grain orientation.