Wafer Level Temporary Bonding/Debonding for Thin Wafer Handling Applications

This paper presents the latest results at IMEC in the area of temporary wafer level bonding and debonding for thin Silicon wafer applications. This work fits in the framework wafer level 3D integration, which typically requires wafer thinning down to thicknesses below 200 um, even down to 15um. At these thicknesses, a wafer becomes very difficult to handle and process. The wafers are very fragile and tend to bend either under their own weight or due stress relaxation after thinning. The proposed solution is to temporarily bond the wafer to a supporting carrier preferably before or otherwise after thinning. This carrier can support the thin wafer during further process steps until the thin wafer is permanently bonded to its final destination. Depending on the application, Silicon or glass wafers can be used as a carrier. Glass is advantageous when using backside alignment while the use of Silicon avoids bow of the wafer stack due to the CTE mismatch between Silicon and glass. The temporary bonding material should be compatible with the temperature budget of subsequent processing, and should debond easily.

We will report on several materials, which are being debonded either by acetone dissolution or heating. Successful wafer level bondings and debondings were performed with wax, and several sacrificial polymers. Acetone dissolution is limited by the peripheral access to the adhesive between the bonded wafers, thus requiring die level debonding or the use of a perforated carrier. Thermally assisted release occurs either by mechanical sliding above the flow point or thermal decomposition. Remaining organic residues after thermal decomposition have been analyzed by contact angle measurement and Auger electron spectroscopy analyses. Different cleaning methods to remove these residues are evaluated.