While interviewing SSEC on its TSV clean and TSV reveal processes, and Dow Corning on its temporary bond/debond process, the name “Dynaloy” kept popping up in reference to chemistries used for cleaning residues in all these process steps.
I realized that cleaning steps for TSV fabrication and subsequent backside process steps for 2.5D and 3D IC are something I’ve never talked about on 3D InCites. An interview was definitely in order, and so I put in a call to Dynaloy’s technology manager, Kim Pollard, who was delighted to get on her soap box (pun intended) and educate me (and my readers) about the importance of cleans, and about Dynaloy’s role in this. Dynaloy, a subsidiary of Eastman Chemical, develops and creates chemistries specifically for the removal of photoresist and other polymers like those used for temporary bond adhesives.
“Reliability is all about clean, “says Pollard. For example, there are some who after etch think a TSV is “clean enough”, but any particles or polymer residue remaining on the TSV sidewall can result in defects in the fill later. With regard to temporary bond and debond, compatibility with the film frame post debond has to be considered. Residue remaining on the active wafer can also affect reliability. Additionally, residue remaining on carrier wafers needs to be cleaned if they are going to be re-used.
“It all comes down to risk,” said Pollard. “When I think of high volume manufacturing (HVM), I would want a rigorous set of processes that guarantees me that I don’t have to worry about device reliability down the line or have to integrate a cleans process later.” She calls cleaning steps “insurance.”
Another reason for the necessity of integrating clean steps, rather then leaving them to the end, is the changes in feature sizes over the past 10 years. As we move from larger feature sizes to Angstrom precision, the focus shifts from just simply cleaning to ‘surface engineering’. “It’s about cleaning and leaving a surface ready for the next step,” explained Pollard. “When we plan our formulations, we don’t just plan for cleaning and compatibility during cleaning steps. We plan for the end result step.”
Trying to cut costs by using incompatible or old chemistries developed for legacy processes can also cause reliability issues. Pollard shared an anecdote about a Taiwanese company that recently approached Dynaloy to solve a problem they were having with stripping solutions. They were trying to use older formulations that were created for larger features, and causing damage in smaller geometries. Clearly, the chemistries had met their maximum time in the industry and have to be replaced.
Cost of ownership is another issue. Pollard says customers always want to consider cost of a cleaning solution per liter, rather than per wafer cleaned. “If you have a solution that gives better yield and cleans twice the number of wafers by the time of its disposal, but costs 1.5x the competition, isn’t it a better cost of ownership?” she asked. In this case, performance data exceeds the competition and it makes more sense to consider cost per wafer rather than cost per liter.
According to Pollard, as we get closer to production, the necessity for clean processes has come to the forefront and is now being addressed “in a very sincere way.” She says she is very skeptical that anyone will be able to get away with a no-clean process, whether they’re talking about TSV fabrication or any subsequent backside process steps. ~ F.v.T.