Ultrathin glass is well suited for use as interposers in semiconductor packaging applications.

BY JUILA GOLDSTEIN, Senior Associate Analyst, NanoMarkets, Glen Allen, VA

Flexible glass seemed like a natural fit for the display industry, combining the impermeability of glass with the flexibility of plastic. In 2012 it appeared as though flexible and ultrathin glass companies were going to benefit from the explosion of touchscreens in displays of all sizes, but the market made an abrupt turnaround. Now suppliers of ultrathin and flexible glass are looking for applications beyond displays to bring in revenue in the next few years, and one of the places they are looking is in semiconductor packaging.

For those who approach flexible glass from the point of view of a display, an application where the glass is hidden between layers of silicon and other materials may not seem to make a lot of sense. As far as NanoMarkets can tell, no one really thought about semiconductor packaging as a use for flexible glass until the display application began to fail. The flexible glass sector itself was firmly focused on displays until then and the semiconductor packaging sector had probably never considered flexible glass as an option.

Nonetheless, using ultrathin glass in semiconductor packaging may actually be a very good idea, even though its optical properties and flexibility may be irrelevant in this application.

The Role of glass in interposers

For many years the semiconductor packaging industry has been developing packages that are smaller, thinner, and lighter than what has come before. Ultrathin glass, 30 to 100 μm, may be able to further progress toward this goal.

The target application is 2.5D or 3D multi-chip or chip scale packages (CSP), where semiconductor chips are placed in close proximity or stacked on top of each other to provide a space-saving configuration. Such packages traditionally use a layer of thinned silicon as an interposer to connect chips to each other and to the underlying organic substrate. Silicon has the advantage of being a familiar material with a well-established infrastructure in the semiconductor packaging industry, but it does have some drawbacks, the major one being cost.

FIGURE 1. A 30 μm thick flexible glass interposer made by Schott Glass.

Glass may be preferable to silicon as an interposer because it is a less expensive material, it can be provided in thin sheets (silicon has to be ground and polished to the proper thickness) and it is thermally insulating. Silicon is a semicon- ductor, not an electrical insulator, which can cause problems with crosstalk between chips. FIGURE 1 shows a 30 μm thick flexible glass interposer made by Schott Glass.

Silicon conducts heat better than glass, making the semiconductor industry a bit suspicious of the ability of glass to conduct heat sufficiently to avoid hot spots in sensitive ICs. The answer is in the through-glass vias (TGV), channels drilled through the interposer that are filled with metal (usually copper) and form electrical connections between the chip and the organic substrate. Solid filled vias act like heat pipes to provide a path for heat conduction.

The potential cost advantages of glass can best be achieved using large sheets of glass, thus allowing facilities to process more units in parallel than is possible with silicon wafers. The largest possible cost savings of using flexible glass is realized if it can be integrated into a roll- to-roll production process. Several suppliers are producing flexible glass on rolls, but the semiconductor industry is not necessarily prepared to process it.

Re-evaluating the supply chain

While glass may be a compelling interposer material from the point of view of glass makers, lack of infra- structure in this application is a real problem. In order for glass to be useful as an interposer, someone needs to drill vias through the glass and metallize them, and it is not yet clear who that would be. Several industries could participate in the supply chain, but there are barriers in all cases:

• Semiconductor packaging houses: The industry is not used to working with glass and is not inclined to do so. It is very resistant to change and may be especially averse to implementing R2R processing. Convincing semiconductor packaging facilities to process glass will clearly be an uphill battle.
• Flat-panel display manufacturers: These companies have experience with glass but have not historically had anything to do with semiconductor packaging. It may be possible to build awareness in this sector, but the flat panel display industry prefers to sell large pieces of glass.
• Printed circuit board manufacturers: The PCB industry currently makes organic interposers, geared toward applica- tions where fine pitch is not required. Glass suppliers might be able to work with the PCB industry, which is used to large panels, if they want to supply sheets of glass. It still may be difficult, however, to implement very thin glass using this approach. It also will probably be difficult to integrate TGV production into a PCB-like process flow.

Organizations that are promoting ultrathin glass interposers are attempting to address the infrastructure challenge:

• Georgia Tech: The Packaging Resource Center at Georgia Tech has been working with industry partners on glass interposers since 2010 and has moved from initial trials with 180-μm thick glass down to the thinnest products that today’s glass suppliers are producing. The PRC is working with major glass suppliers such as Corning and Schott, who are interested in flexible glass interposers.

The PRC has been working on transferring the technology from prototype to low volume, and perhaps eventually high volume, commercial production. It has made some real progress in developing the technology and moving proto- typing from labs into industry, but admits that the greatest challenge in moving forward is lack of infrastructure to support the transition.

nMode solutions that is partially funded by Asahi Glass Company, is providing some missing segments in the supply chain. Triton has developed a production process to create through glass vias (TGVs) that is sufficient for today’s 2.5D applications and it is making interposers for MEMS, RF, and optics at its manufacturing facility in Carlsbad, CA. According to Triton, the major advantage it provides over silicon is the ability to produce solid filled, hermetic TGVs.

Existing commercial products use glass interposers from Triton, but this is much thicker glass, typically 0.3mm or greater. The glass is cut into wafers, matching the form factor of silicon but not requiring backgrinding. This provides the convenience of a process that fits easily into existing manufacturing lines but doesn’t take advantage of glass’ potential to provide thinner interposers at much lower cost than silicon. Triton can make large panels of 0.1-mm glass with TGVs, but customers do not know how to handle it and may not be inclined to learn.

NanoMarkets understands the potential advantage thin glass would have as an interposer, but is not especially optimistic about its future, especially in the near term. It seems very unlikely that flexible glass will be able to generate large revenues in this space, even if penetration rates get large. Each product uses a very small amount of glass compared to what would be needed for even a smart phone display.

The semiconductor packaging industry may be an even more difficult environment for introducing new processes than the display industry, and we know flexible glass has had challenges there. Still, we feel this sector is worth keeping an eye on to see if glass has an opportunity to succeed where silicon has not.