Researchers Develop Forward-Looking Polymer With Immediate Applications

Sally Cole Johnson, Contributing Editor -- Semiconductor International, 2/4/2008 7:57:00 AM

Responding to the industry’s call for new advanced packaging and interconnect materials to support emerging flip-chip and optical interconnect technologies, Rensselaer Polytechnic Institute (RPI, Troy, N.Y.) researchers teamed up with Polyset Co. (Mechanicville, N.Y.) to develop a polymer that should help solve redistribution layer challenges and ease future transitions from conventional lithography processes to next-generation on-chip nanoimprinting.

The new polymer, polyset epoxy siloxane (PES), is appealing because it allows chipmakers to eliminate several steps from their production and packaging processes, according to Toh-Ming Lu, the R.P. Baker Distinguished Professor of Physics at Rensselaer, who led the research team. The research was reported in the January issue of the Journal of Vacuum Science and Technology B.

RPI researchers said the PES polymer improves the resolution of nanoimprint lithography. (Source: RPI)

Photolithography uses a mixture of light and chemicals to produce intricate micro- and nano-scale patterns on silicon. As part of the process, a thin polymer film is used to create a redistribution layer to ease the signal propagation delay and protect the chip from a variety of environmental and mechanical factors. PES can be used for the redistribution layer in ultraviolet (UV) on-chip nanoimprinting lithography technology, which is in the early phases of development, Lu explained. And the consistency of using PES in conventional technology, then continuing to use PES while academia and industry test and gradually migrate to next-gen devices, should help ease the transition.

“PES offers significant performance enhancements, particularly in the redistribution process, which is the first step of wafer-level packaging using a flip-chip approach, ” Lu said. “PES has a much lower cure temperature and less water uptake. And you can forget about irritating B-stage problems (an issue with some polymers that occurs in a secondary stage in the reaction of thermosetting that causes undesirable changes) — they’re eliminated with this polymer. You put this polymer down and it’s done. If you do it again and add more layers, it doesn’t change anything.”

Benzocyclobutene (BCB) and polyimide (PI) are polymers typically used for redistribution layers because of properties such as low water absorption, thermal stability, low curing temperature, low thermal expansion, a low dielectric constant and low leakage current.

The new polymer, PES, compares well with incumbent polymers. View a larger version of this table. (Source: RPI)

PES boasts several advantages over BCB and PI. First, PES relies on a cationic polymerization process, rather than the more traditional radical polymerization processes, so it’s both fast and stable. It cures at 165°C — about 35% cooler than either BCB or PI. This translates into lower overhead costs for manufacturers, Lu pointed out. Other advantages include its low water uptake rate of <0.2%, which is far less than BCB or PI. Its dielectric strength is also superior compared with BCB and PI. And PES adheres well to copper and can easily be made less brittle if desired. These properties make PES a very promising material for redistribution layer application and UV imprint lithography, Lu said.

“You can use either a thermal or UV cure for packaging applications. We’ve tested this polymer in every way you can imagine for packaging and on-chip applications,” Lu said. “If one day you want to use imprint techniques to do patterning for packaging and also large-area flat panel displays, this is a relatively inexpensive way to do it.”

Lu sees huge potential for PES in applications such as optical devices, biotechnology devices MEMS, in addition to lithography, imprinting and flat-panel displays.

This research was funded through the New York State Foundation for Science, Technology and Innovation.