Yet Another Way to Use BCB
John Baliga, Contributing Editor -- Semiconductor International, 7/1/2006
Benzocyclobutene (BCB) is a widely used dielectric in wafer-level packaging and redistribution applications. Researchers at the Automation and Robotics Research Institute at the University of Texas at Arlington have performed proof-of-concept work on a novel flip-chip on chip method in which photo-BCB is used as an adhesive that performs some of the functions of an underfill. They presented results of their work at this year's IMAPS Device Packaging Conference.
The key to this method is using a lead-free solder that can reflow at a suitable curing temperature for the BCB. If the BCB layer thickness is greater than the as-deposited height of the bump but less than the bump height after reflow, face-to-face electrical contacts can be made (Figure ). Curing the BCB with the wafers in face-to-face contact bonds the wafers and protects the electrical contacts. If reflow is done during this cure, the bumps should form a contact.
The solder chosen for this work is 95% tin and 5% gold, which has a reflow temperature of 230°C. It was deposited in four layers: two 600 nm layers of tin and two 20 nm layers of gold, using e-beam evaporation with liftoff. One of the gold layers was on top to inhibit oxide growth.
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| Solder stacks can reflow during a face-to-face cure, providing electrical contact. |
Another concern was the possibility of BCB flowing into the cavity, which would interrupt the formation of the contact. No such flow was observed, even though a bond force over 3000 N was used on 4 in. wafers.
Flip-chip on chip is an established interconnection method that typically does not require the complexity of this process. The potential advantages of this process are reduced bond heights and high interconnection densities. In the presented work, the BCB layer thickness was only 3 µm, and base pad diameters were as small as 40 µm.
Some of the other vertical chip interconnection technologies that have comparable bond heights and interconnection densities require very flat surfaces, which are typically achieved using processes like CMP. This BCB-based process has the potential of wider process windows with lower cost.
Reliable, high-density interconnection between two die is not likely to be easy or cheap. Processes like this one, however, provide hope that upcoming requirements for reliability, density and cost can be met simultaneously.
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