At this weekâ€™s GaTech Global Interposer Technology Workshop (GIT) in Atlanta, the pervasive theme appeared to be whether a change in substrate is required to lower overall costs and help driveÂ HVM (high volume manufacturing) applications. Certainly conference chair Rao Tummala, industry visionary whose name is synonomous with microelectronic packaging, feels the time is right to take a serious look at glass interposers both for their superior electrical performance and their promise of lower costs. The PB substrate manufacturers are also taking a serious look at this market and proposing that they can drive their technology to the required dimensions and electrical performance, though many skeptics including IFTLE are taking a â€śshow meâ€ť attitude about these claims.
Status in Silicon
The Yole Developpement presentation pointed out that while 2.5D silicon interposer technology was fully underway at TSMC and GlobalFoundries, UMC and SPIL supposedly are near initiation, all of the rumored â€śdriver applications,â€ť like the Apple A7, the next gen Qualcomm phone, the Sony PS4, ST Microâ€™s â€śWiomingâ€ť application processor, wide IO memory and the next generation Altera FPGA (see discussion below) have been, at the very least, postponed. While no one would openly reveal what the current and proposed future costs are, it is believed that all of these postponements are due to cost which certainly is not yet meeting the mobile phone requirements of less than 1 cent per sq mm proposed by Qualcommâ€™s Matt Nowak (i.e this is roughly $550 for a 300mm wafer of interposers).
While Yole has identified at least 10 products moving towards commercialization, all of them currently require so called high density interposers (i.e. 1um L/S and as small as 10um TSV). Currently these dimensions Â can only be fabricated using front end dual damascene type processing available only at silicon foundries and more recently the OSAT, SPIL.
While Yole is still projecting a greater than $1B in revenue from 2.5D TSV activity by 2017 (activity revenues including TSV etching, filling, RDL, bumping, wafer test & wafer level assembly), these projections only hold if the current â€śpostponed applicationsâ€ť are quickly commercialized.
During the Amkor presentation Ron Huemoeller indicated that lowering cost could come from elimination of backside RDL on the interposers by arranging pin out on the top side high density interconnect.
Huemoeller sees high end applications being dominated by silicon, mid end applications like graphics possibly using glass and the low end applications (yet unidentified) being wide open. He sees GPU + HBM (high bandwidth memory) being adopted in 2015 and tablets and processors adopting interposer solutions the following year.
In terms of organic â€śinterposersâ€ť he indicates that Shinko and Semco are in limited sampling ofÂ 2/2 (L/S) and Kyocera 5/5. He labels Unimicron as in â€śearly development.â€ť
After making the standard IFTE argument that 2.5/3DIC was needed to combat the costs of continued scaling and that system level cost savings could pay for interposer costs,Â Dave McCann of GlobalFoundries indicated that GF was achieving near 100% yields with reticle sized interposers having 4 layers of high density interconnect.
McCann predicted we would see voltage regulator function on future interposers. He also described a program between Global (chip and silicon interposer), Open-Silicon (design), Cadence (EDA tools) and Amkor (assembly and test), which produced a functional processor vehicle featuring two 28nm ARM Cortex-A9 processors connected on a 2.5D silicon interposer built on a 65nm manufacturing flow. The program demonstrated first-time functionality of the processor, interposer, substrate and the die-to-substrate assembly process. The design tools, process design kit (PDK), design rules, and supply chain are now in place for other activities.
What will be the Glass Interposer Infrastructure? Â
Inherently most believe that all things being equal, glass should be a lower cost interposer solution since it can be processed in large format. However, one interesting question from the audience was â€śâ€¦why are silicon and glass wafer the same price then ?â€ť
Although the data from experts like Professor Kim from KAIST confirms that glass is a better electrical performance solution, especially for Rf applications, the major issue is that a complete infrastructure is not yet in place to manufacture such glass interposers.
The big 3 glass producers ( Corning, Asahi Glass and Schott) have been listening to Tummala for several years now and all 3 are involved with his GaTech interposer consortium which has been promoting the use of glass. They each have their own technologies for forming TSV, but the infrastructure appears to stop there. While Corningâ€™s Windsor Thomas said they are nearly ready to ship rolls of glass containing TSV, the question expressed by many in the audience was â€śship to who?â€ť Schott is in the same position.
Asahi Glass (AGC), however, appears to be a step ahead, having announced the formation of Triton, to produce and deliver circuitized glass interposers (link). This certainly will help AGC better understand the market and what the technology limitations are.
Neither the â€śpanel of experts,â€ť the speakers or the audience had a convincing argument as to whether traditional flat panel LCD manufacturers or PCB houses would be better at handling the metallization and singulation issues that still remain with glass panels (or rolls). LCD manufacturers use aluminum, not copper and we are told by Thomas of Corning, â€śHave absolutely no interest in this technology at all.â€ť The PCB industry appears interested (certainly by their presence at this meeting) but would have to change nearly every unit operation and material that they currently use in order to meet the advanced requirements of 2.5D interposers.
What can PCB based Interposers Deliver
PCBâ€™s are of course the first interposers, i.e most of the BGA substrates that exit today are PCB technology. So really the question that is being asked is â€ś â€¦can PCB technology ever produce thin film silicon dimensions?â€ť
Hu of Unimicron indicated that moving toward 2/2 (L/S) in polymeric PCB technology was doable but would require a move from wet processing to dry processing, the use of stepper lithography, embedded copper linesÂ and a change of core material to minimize warpage. Even if 2um lines and spaces were possible, this would have to be done in a class 100 clean room (more cost !) and does not address the TSV and catch pad dimension issue which really determines how many layers of interconnect are needed. If materials are changed and a move to thin film processes and equipment and facilities are needed, IFTLE questions whether costs will be considerably lower.
Koizumi of Shinko showed data on a 200um glass core PCB with a 5/0/5 build up process ( 30um polymer/18um thick Cu per layer) . When diced such structures resulted in what they called â€śSe-wa-reâ€ť (loosely translated back split) which was fracture through the glass core layer due to the stresses built up on both sides of the core.
Enough encouraging data was shown to reasonably conclude that both glass and PCB should continue to be examined to fully understand their ultimate capabilities and costs.
Altera 2.5D Postponed
For those of you who havenâ€™t noticed, the move of Altera to Intel to build FPGAs using its 14-nm FinFET process technology [link] basically terminated the intentions of Altera to commercialize FPGAs using the TSMC CoWoS process as previously disclosed [link].
This is certainly another setback for 2.5D commercialization.
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