Is 2020 A Lost Year?
I recall, growing up in Hell’s Kitchen in NYC, the old-timers in the neighborhood used to explain that some years in one’s life were simply to be viewed as “lost years”. Remember, this was coming from the generation that lived through the Spanish Flu pandemic ~ 1918 that reportedly killed some 50 million globally.
Some hundred years later, we are living through something similar but certainly not (yet) as devastating. Hopefully few of us have had this virus and/or have had loved ones physically affected by it, but even for those who haven’t had the pandemic impact their or their families’ health, it has affected nearly all of us in other ways. Lost jobs and lost opportunities, lost high school and college graduations, student-athletes losing a year of training and competition, and all of us losing the opportunity to share time with each other at professional conferences this year.
While many of us missed the social interaction that we normally got from attending the IEEE ECTC, for one person in particular the situation around this year’s “virtual” conference was a real bummer, a real ill-timed “lost year”.
Many of you know Christopher Bower from professional interactions over the years. I first met Chris in the early 1990s while I was still working for Dow Chemical, with my offices at MCNC (Microelectronic Center of NC). There I met two young MCNC scientists, Chris Bower and Alan Huffman. We had similar interests and hit it off personally. When it was apparent that their direct management showed little interest or capability in helping them career-wise, I took them under my wing, as a mentor if you will. In the ensuing years, I got to watch both of them grow professionally. One of my first recommendations to both of them was to become involved with the ECTC conference and they both listened. The photo of the three of us ((Bower to my left, and Huffman to my right) was taken at ECTC 2010.
Flash forward to 2016 and Alan was ECTC General Chair. 2020 was to be Chris’ year.
Chris put in his time on the ECTC program committee which is a six or seven-year volunteer commitment. He moved his way up the ladder and in 2020 reached the pinnacle as General Chair. This is the year he was supposed to get the Presidential Suite in the hotel, throw the General Chairs’ party, and bask in the applause of the 2000+ attendees thanking him for his years of work that got him to this point. Then came the pandemic. Sorry, Chris, you have done a great job, but as luck would have it you finished up during a “lost year”. But things could be worse; it could be 1918!
Now… finishing up this year’s look at the virtual ECTC 2020…
A STAR/IME – Gang Bonding for Multi-die Stacking
An interesting presentation on Cu-Cu bonding was “Development of Multi-die Stacking with Cu-Cu interconnects using a Gang Bonding Approach” by A Star/IME.
The major challenges in Cu-Cu bonding include oxide formation, bump height uniformity, and surface roughness. The IME assembly approach is shown below:
The Cu bumps on the die are covered with a layer of non-conductive film (NCF) prior to the die stacking process. The non-conductive film offers
the protection of the Cu surface and it also helps to clean the copper surface prior to Cu-Cu joint formation via gang bonding. The thickness of the non-conductive film should be optimized to avoid excessive spreading, which can cause tool contamination and voids inside the package. The wafer is mounted on the dicing frame; the carrier is removed, and the wafer is cleaned and is ready for singulation as shown.
The substrate temperature during the stacking process should be optimized to minimize thermal exposure of the NCF and avoid its early crosslinking.
The dies are stack on top of each other in a conventional flip-chip bonder before the wafer populated with many die stacks is sent to the gang bonder for final Cu-Cu joint formation. The compliant layer present during the compression bonding is there to take up the bump height non-uniformity which they found to be from 4 to 14µm for the 4-die stack. Force, time, and temperature were optimized to achieve reproducible and reliable Cu-Cu interconnect.
Hopefully, we will all be able to meet together again next summer at ECTC 71 in San Diego.
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