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200mm & 300mm Processes & Characterization for Face to Back Flow Chart for Wide I/O
Keywords: 3D integration, face to back, 200 & 300mm
3D integration so far has often been investigated through a face to face point of view: the top die FEOL is in front of bottom die FEOL. This allows a dense connectivity between both dies, but TSV are mandatory on the bottom die for each external exchange. Another flow chart, which main application is identified as the Wide I/O, is “face to back”: the FEOL of the top die faces to the back side of the bottom dies. To communicate between top & bottom, interconnections then TSVs are needed. Advantage is that the bottom die, for Wide I/O the logic dies, faces directly to the board allowing a rapid communication with external. Also, the Wide I/O interface delivers high bandwidth at relatively low power consumption level. The objective of this paper is to show latest integration at Leti, both on 200mm & 300mm wafers, using face to back integration. If we compare both flow charts, using via middle technology, the main challenge consists of the temporary bonding. The temporary adhesive needs to be at least 50µm thick, even more depending on staking technology on board, and in presence of the macro connection. The choice of the adhesive is crucial for the final stability of the stack during back side process and also during debonding, even more on 300mm wafers. Technical developments are introduced in the paper. A specific focus is done on temporary adhesive & the associated thermal stability of the stack. Impact of the temporary bonding on copper pillar is evaluated. Assembly of best processes for a full integration on daisy chain wafers, both 200 & 300 wafers is described. Finally, a comparison of electrical datas (resistance, capacitance, isolation) for both wafer diameters is given. Morphological characterization finalizes this first integration and leads to further potential improvements.
Severine Cheramy, Laboratory Manager

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