Sunday, July 21, 2013

IFTLE 155 2013 IEEE ECTC Part 2 Temporary Bonding

Continuing our look at key presentations from the 2013 IEEE ECTC Conference.

IMEC and Brewer Science reported on the “Integration and Manufacturing Aspects of Moving from  Waferbond HT 10.10  to ZoneBOND in Temporary Wafer Bonding…”
 Temporary wafer bonding has become a key element in the emergence of 3D Stacked IC technologies. In the past approaches such as IMECS have relied on the “thermal slide process” to debond the thinned wafer from the carrier.  There was a desire to move away from this process for several reasons including:
(1) stresses that are generated can cause cracks in thinned wafers, especially those containing TSV.
(2) slide debond normally conducted > 200 C which accelerates solder diffusion .
(3) cannot slide debond if already on dicing frame since no dicing tapes can take temps > 200 C.

This led to the development of the ZoneBOND process which has been described previously [ See IFTLE 61 and IFTLE 90]

Integration Changes Required for the ZoneBOND process
In the standard process sequence edge trimming is done to the device wafer prior to temporary bonding. If this is done in the ZoneBOND process the find that adhesive is trapped in the trimmed region of the wafer and clogs the grinding wheel during backside thinning.

A new integration scheme is proposed where edge trimming occurs after the bonding step as shown below.

Debonding is performed at room temperature in a SUSS DB12T debonder. First, a wet edge preparation is required in order to eliminate the high adhesion area of the adhesive layer between the 2 substrates. Then, the thin device wafer still bonded to the carrier is laminated onto a dicing tape on frame. Next, the room temperature peel off debonding separates the thin wafer from the carrier while the thin wafer is still on tape and on frame. A final cleaning step on tape is performed to remove adhesive residues from the device wafer.
CEA Leti reported on their comparison between the  “WSS and ZoneBond Temporary Bonding Techniques…”
The ZoneBOND technique requires silicon carriers that are treated with an antistick layer with an edge exclusion to ensure the adhesion. Temporary glue can be deposited either on the device, either on the carrier. Bonding is achieved under elevated temperature and separation requires a specific soaking of the bonded pairs to preliminary remove the adhesive from the edges.

The WSS system requires transparent carriers as the temporary adhesive is cured after bonding by UV exposure. Separation is enabled by a laser exposure which modifies a sensitive layer (named LTHC for Light to Heat Conversion) that has been deposited before bonding on the carrier.

Pros and cons of the processes are given in the attached table.
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