IWLPC (Wafer-Level Packaging) Conference Proceedings


Temporary Bonding Material Total Thickness Variation (TTV)

Authors: A. Jacobs, M. Privett, G. Brand
Company: Brewer Science, Inc.
Date Published: 11/5/2013   Conference: IWLPC (Wafer-Level Packaging)


Abstract: The total thickness variation (TTV) of a coating is the difference between the maximum and minimum thickness values obtained from scan patterns or a series of point measurements of the coating. TTV is commonly expressed in micrometers or angstroms, but more often in micrometers for thick temporary bonding materials and applications. Obtaining uniform coating thickness in the temporary bond line has become increasingly important due to the mechanical grinding steps necessary to thin a device wafer. Poor TTV of the temporary bonding material results in high and low points in the coated film, which translates to a poor TTV in the temporary bond line. The downstream effects can be drastic, such as the copper nails in through-silicon via (TSV) processes being unevenly revealed, damaged, or destroyed, ultimately impacting yield and permanent bonding performance. In general, the thin wafer handling industry requires a post-coating TTV for temporary bonding materials of less than 5 µm. This value is calculated with no more than a 3-mm exclusion from the edge of the wafer. Typically, spin speed, acceleration, and spin time determine the final film thickness. However, using a modified spin process carried out at specific times can help level the edge crown commonly observed in thicker coatings. Multistage solvent removal baking helps to dry the film and prevent damage caused by rapid solvent evaporation, such as voids or pinholes. Dispense methods can also contribute to improved TTV. Static, dynamic, or radial dispense methods may be used, depending on wafer size, exhaust flow in the spin-coating tool, and the flash point of coating material’s solvent system. Using spin coating as a deposition technique will inevitably produce films exhibiting some degree of coating thickness variation. However, by carefully tuning the parameters available in the bonding process, the remaining TTV from the coating step can be leveled by a combination of heat, pressure, vacuum, and time. All of these parameters can influence the post-bonding TTV necessary to achieve a highly uniform thinned device wafer.

Key Words: 

total thickness variation, TTV, Brewer Science, WaferBOND, ZoneBOND



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