Xilinx 2.5D FPGAs

Liam Madden, corporate VP of Xilinx, gave the keynote presentation to kick off the 2011 IMAPS 44th Int. Symp. on Microelectronics a few weeks ago in Long Beach CA.
Last fall Xilinx announced a single layer, multi chip silicon interposer for its 28nm 7 series FPGAs. Looking at module assembly, first the four 28nm chiplets are mounted on a 25 �? 31 mm , 100μm thick, silicon interposer with 45μm pitch microbumps and 10μm TSV. The interposer is then assembled on a 35 �? 35mm BGA with 180μm pitch C4 bumps. The FPGA slices are connected by ~10,000 connections created on the silicon interposer. Compared to connections on a PWB, the interposer interconnect technology provides over 100�? the die-to-die connectivity bandwidth per watt, at one-fifth the latency.

Madden indicated that the use of an interposer (known as 2.5D) was the right choice for FPGAs since the "10,000 routing connections" if they would have been TSV in the FPGA slices, would have used up valuable chip area making the chips larger and more costly than they are now.

TSMC is fabricating the chip and the interposer and bumping the interposer, while Amkor is bumping the chip and doing the module assembly. Madden gladly showed one of the modules to the packed audience:

[Madden showing Virtex 7 module with James Lu ( RPI) and GS Kim (CEO of EPworks) looking on]

Madden indicated that the Virtex 7 HT will consist of 3 FPGA slices and two 28gbps SerDes chips on an interposer capable of operating at 2.8Tb/sec. In their paper "Advanced Thermal study of Very High Power TSV Interposer and Interconnects for 28nm Technology FPGA," Xilinx details the thermal study of TSV interposer technology for high performance 28nm logic die mounted on a silicon interposer with Cu-filled TSV. Based on DOE experimental results optimized TIM material, underfill, bump pitch and passive heat sinks were selected resulting in the following optimized thermal results at 55°C or 75°C ambient. Simulation results confirm that for the selected passive heat sink the high power FPGA package is thermally reliable and meets thermal specs.

Xilinx also reported on quality and reliability in their paper "Quality and Reliability of 3D Interposer and Fine Pitch Solder Micro-bumps for 28nm Technology." Microbump (FPGA to interposer) resistance was measured from Kelvin structure measurements.

Interposer stress and delamination risk were carefully studied through simulation and thermal cycling. Simulation results indicated that the overall stresses in the silicon, SiO₂ insulator and copper via are below the fracture strength of the given materials.

During package level reliability testing, the 3 main factors evaluated were type of underfill, top die thickness and interposer cleaning . Test samples were exposed to level 5 preconditioning, HTOL (high temp operating life) and TC (temp cycling). Reliability results showed that higher Tg underfills passed all tests . Failures were observed with lower Tg underfills. In addition proper interposer cleaning and die thickness reduction were necessary to prevent delamination.

Wafer applied underfills for 3D

IMEC addressed the "Use of Wafer Applied Underfill for 3D Stacking." In the case of die-to-package UF one is looking to mitigate the CYE differences between the laminate package substrate, the ~100μm solder bumps, the silicon die, and the package overmolding — whereas in the case of die to die assembly such as 3D structures the underfill has to mitigate the CTE differences between the 2 silicon die and the ~ 10μm microbumps. Thus there are different requirements for the two.

In 3D packaging the main challenges for underfill are the narrow gaps between the chips and/or the chip and the substrate (~10μm) and the fine pitch between the bumps (i.e. 20μm). The use of capillary underfill (CUF) is time-consuming as requires excess space around the die for the dispense action. In the case of no flow underfill (NUF) the materials is dispensed on the substrate before the die stacking. Materials need to be transparent so you can see the alignment marks during the flip chip operation, dispense timing since this is still done for each individual die and underfill/filler entrapment between the bump and the pad.
For 3D, CUF is not an option due to the narrow gap and the fine bump pitch. NUF is a better choice but suffers from the transparency requirement and dispense volume control (i.e. excess underfill can be thicker than the bumps and thus hinder chip to chip bonding and/or squeezing out excess underfill can "backside overflow" (see pic below) which can contaminate backside pads.

Wafer applied underfill is considered a strong candidate for 3D because theoretically it can significantly increase throughput. It can be done by either spin coating or dry film lamination.

IMEC challenged 9 global underfill suppliers with the following criteria:

– Uniform material thickness ( < 30μm, target 10μm)
– Gap fill for <40μm bump pitch
– Transparent to allow alignment
– Tacky at ambient temp
– Low cure temp, usable up to 250°C

One spin coat and two dry film materials were submitted for testing. After initial testing IMEC was left with one spin on material and two dry films.

After fabrication of test vehicles IMEC daisy chain yields of 0% eliminated the "hybrid dry film" and resulted in 20%-50% yields for the remaining epoxy spin on and dry film. The latter two materials are being considered for further development.

3D activity at ITRI

John Lau and co-workers from ITRI gave several presentations on the various aspects of 3D IC that they are working on at ITRI, many of them tied to their 3DIC test vehicle. [ see IFTLE 52, "3D and Adv Pkging at ICEP 2011"]

In their paper "Oxide Liner, Barrier and Seed Layers and Cu Plating of Blind TSVs on 300 mm Wafers for 3D IC Integration" they focused on their process development for TSV filling. They use an AMAT PECVD to deposit TEOS SiO₂. At 180°C deposition temperature they find that step coverage is improved by higher temp, higher Rf power, lower pressure, and lower TEOS flow.

For barrier layer and seed, a AMAT self ionized plasma PVD system is used for Ta barrier and Cu seed. They achieved < 50 pA leakage current between 10μm �? 60μm TSV. In their paper "Thin Wafer Handling of 300mm Wafers for 3D IC Integration" IRTI points out that if your dicing tape adhesive strength is "too strong" it may strip immersion gold off of the chip pads. In their presentation "Wafer bumping and Characterization of Fine Pitch Lead Free Solder Micro bumps on 300mm wafers for 3DIC integration" they offer that the difference in volume between FC solder balls and micro bumps is > 20�? and the smaller and thus IMC and Kirkendall void formation issues are more pronounced for the smaller bumps. For this reason ITRI does not reflow the micro bumps before joining and the micro bump assembly is usually fluxless to reduce the chance of entrapping flux during solder reflow. Underfills are more critical for micro bumps. UBM thickness is > 10�? less and the budget for undercutting the micro bumps is much smaller meaning that the process windows are smaller.

For all the latest on 3D IC and advanced packaging stay linked to IFTLE……….

We have previously reported that the IEEE International 3DIC Conference was moved from its initial October 2011 date outside Tokyo to Jan 31st 2012 in Osaka due to the unfortunate earthquake/tsunami events of this past year. The US program committee, which was scheduled to hold the 2012 even in San Francisco in October 2012, has decided to postpone their event till 2013 in deference to the unusual events surrounding the 2011 Japan meeting. We strongly recommend support of the coming meeting in Osaka.

For the second year the IEEE 3D Test workshop was held in conjunction with ITC (IEEE International Test Conference), with Yervant Zorian of Synopsys as general chair and Erik Jan Marinissen of IMEC as program chair. More than 125 attendees attended 11 sessions which covered all of the mainline test issues: Executive views (Synopsys, ASE, Samsung, Teradyne,Cascade, Cadence ect.); 3D electronic Design Automation (EDA); wafer probing; standardization; and challenges and solutions for wide IO DRAM stacking.

Unlike a few years ago where Universities were the main groups involved with developing 3D test protocols, the 3D Test workshop list of corporate sponsors now says all we need to say about the desire for the world’s major design and test houses being involved.

Eric Strid of Cascade Microtech looked at the status of probing:

Brandon Noia and Krishnendu Chakrabarty of Duke University looked at "Methodology for Pre-bond Test of TSVs and Breakpoints in High Performance 3D-SICs." If we assume the following is the accepted 3D Manufacturing/Test flow:

The goal is to detect TSV defects prior to bonding (pre-bond) but we are faced with:
— Pre-bond TSVs are single ended
— Current probe technology: Minimum pitch 35μm but TSVs will be â??¤5μm with pitch of â??¤10μm and densities of â??¥10,000/mm²

TSV test can be done by BIST (built-in self test) and DfT (design for test).

In a joint presentation between Cadence, IMEC and TSMC entitled Automation of DfT Insertion and Interconnect Test Generation for 3D Stacked ICs", Sergej Deutsch of Cadence concluded:

• 3D test challenges include pre-bond and post-bond testing
• 3D-DfT architecture
  — I/O wrap and test-only pads for pre-bond testing
  — Serial and parallel test access mechanisms
  — Test turns: to bypass upper dies in stack
  — Test elevator mode: for test paths to/from upper dies
  — DRAM top control interface
• 3D wrapper insertion flow
  — Inserts 1500-style wrappers and 1149.1 for bottom die
  — Includes controls for I/O wrap and DRAM testing
  — Generates input to run ATPG
• Industrial case study concludes: negligible area costs of 3D wrapper

Etienne Racine of Mentor Graphics gave a look at TSMC’s RF12 reference flow for die stacked on interposer.

For wide IO DRAM they offer the following:

Larry Smith from SEMATECH’s Standards group discussed the 3D Enablement Center, which was announced December 2010 by SEMATECH, SIA, and SRC. It is designed to meet SIA member needs in high performance, mobile, analog, mixed signal, MEMS, fabless, fablite, IDMs. Their mission: "Enable industry-wide ecosystem readiness for cost effective TSV-based 3D stacked IC solutions." Members include: ASE, Altera, ADI, LSI, NIST, ON Semi, Qualcomm, Hynix, CNSE, GlobalFoundries, Hewlett Packard, IBM, Intel, Samsung, TSMC, and UMC. Initial focus is on wide IO DRAM for mobile and high-performance applications:

Erik Jan Marinissen of IMEC updated the group on the status of IEEE P1838 the “3D-Test Standardization Study Group” chartered with defining the standards in 3D test and DfT. Their current project is P1838: “Standard for Test Access Architecture for Three-Dimensional Stacked Integrated Circuits”.

Focus:
— Generic test access to and between dies in a multi-die stack
— Prime focus on stacks with TSV-based interconnects

Test:
— Pre-bond, mid-bond (partial stack), post-bond (complete stack)
— Intra-die circuitry and inter-die interconnects
— Pre-packaging, post-packaging, board-level situations

Die-centric standard:
— Die-level features comprise a stack-level architecture
— Compliance to standard pertains to a die (not to the stack)
— Enables interoperability between die and stack maker(s)
— Standard does not address stack/product-level challenges/solutions (e.g. boundary scan for board-level interconnect testing)
— However, standard should not prohibit application thereof

Two standardized components:
— 3D test wrapper hardware per die
— Description + description language

Scan-based:
— Based on and works with digital scan-based test access

Leverage existing DfT wherever applicable/appropriate:
— Test access ports (such as IEEE 1149.x)
— On-die design-for-test (such as IEEE 1500)
— On-die design-for-debug (such as IEEE P1687)

Standard does not mandate:
— Specific defect or fault models
— Test generation methods
— Die-internal design-for-test

Further info can be obtained here: 3D-Test WG or here: Project P1838.

For all the latest on 3D IC integration and advanced packaging stay linked to IFTLE……….

Moving to 450mm has several advantages, the main being that the area is 2.25Ã?? larger. This obviously means that more chips can be cut from one wafer, and less material is lost at the edges. The last conversion process from 200mm to 300mm wafers began in 2000, creating price reductions of around 30%-40% per unit. The elite of the global semiconductor industry now plan to move to 450mm wafers where a cost gain of 25%-30% is hoped to be achieved.

Although the initial goals from the triumvirate of Intel/Samsung and TSMC called for a 450mm pilot line to be ready in 2012 [link], it does look like things are finally getting off the ground.

Intel, GlobalFoundries, IBM, TSMC, Samsung create 450mm initiative

New York State has entered into agreements for $4.4 billion in investments over the next 5 years from Intel, IBM, GlobalFoundries, TSMC, and Samsung to create a 450mm consortium and manufacturing center there. Reportedly this will create close to 7000 jobs, 2500 of which would be high-tech. (Hopefully these reported job numbers are more realistic than what have been reported recently for "green jobs" and jobs created from the "stimulus.")

The facilities will be located in CNSE (College for Nanoscale and Science Engineering) "Albany NanoTech," CNSE’s Smart System Technology & Commercialization Center in Canandaigua, SUNY-Utica, and IBM sites in East Fishkill and Yorktown Heights. New York State will invest $400 million in CNSE in Albany over a 5-year period.

The joint 450mm project will focus on transforming existing 300mm technology into the new 450mm technology. These technology developments "may facilitate the possibility of building a 450mm production line in New York state."

SEMATECH

Since 2006, the SEMATECH ISMI organization has been looking at the early stages of the 450mm transition, including developing standards for the wafers, automation, and getting agreement on the development of the 450mm processing tools. Last year, the entire ISMI organization moved to Albany, from Austin, and the state of New York invested an estimated $300 million in the 450mm program at ISMI. Intel headed up the ISMI 450mm program, and has been on point for many of the negotiations with the tool suppliers.

The announcement of the Global 450 Consortium consolidates the 450mm effort into one consortium, with access to the new CNSE Fab West building now under construction at the CNSE campus.

TSMC 450mm announcements

Earlier in the year, TSMC reported that the problems with 450mm were not technical but rather economic [link]. Recently TSMC reiterated that a pilot line at Fab 12 Phase VI starting with 20nm process technology, would be timed around 2013/2014, and a production line set for Fab 15 following around 2015/2016 [link]. "The timing for the Albany 450mm line and the TSMC line […] will coincide with each other, or be very close," the company claims.

Intel 450mm announcements

In late 2010 Intel announced that as par of a $6B-$8B investment, it was upgrading several US facilities with the ability to handle 450mm wafers. The Hillsboro, OR facility D1X is scheduled for 2013.

Intel says it will make the Albany site its "450mm East Coast headquarters," implying their D1X fab on the West Coast, which was "built with 450mm in mind," could be beyond an initial pilot-line.

IMEC announces 450mm

Not to be outdone, IMEC’s president/CEO Luc van den Hove laid out a timeline that begins in 2012 with 450mm wafer tool and metrology testing, 450mm process development between about 2013 and 2015, and advanced production starting in about 2016.

Van den Hove proposed the early work covering early metrology, process elements, wafer characterizations of stain, uniformity, and performance will be done in IMEC’s present 300mm wafer fab which is 450mm compatible. Phase Two which will require full process flow will require its own clean room which will probably require a significant extension of the existing pilot wafer fab at IMEC’s Leuven site. He said IMEC was looking at various options to accomplish that since construction would be required to begin prior to 2015.

Where does this leave Micron?

Mark Durcan, president/COO of Micron, is on record as saying that they are not a big proponent for 450mm saying that Micron would have to ”re-tool” the entire company to move to 450mm. He indicated that 450mm would have to prove a "2.5Ã?? cost advantage over 300mm" [link].

Following the NY consortium announcement, Micron quickly announced an expansion of its Boise Idaho R&D center with plans to make the facility "450mm-compatible." [link]

Where is End Game?

According to roadmaps, the 2015 450mm pilot lines coincide with what is expected to be the 10nm node, and 2017-2019 could be 7nm or less. As we have stated before, it is unclear to IFTLE how many players will have the financial or technical wherewithal to continue to proceed with scaling technology to these levels.

Having said that, it certainly looks like 450mm is moving forward for those with the financial capability. So those of us involved in the packaging segment of the industry should begin looking at what will be necessary to move packaging technology to 450mm.

For all the latest on 3D IC Integration and advanced packaging stay linked to IFTLE……..

The 2011 Semicon Taiwan Embedded Substrate Forum "Bridging the Last Mile of Heterogeneous Integration" was chaired by Dr. Kuo-Ning Chiang, Professor, Director-Advanced Packaging Research Center, NTHU.

One of the main messages from the forum is that embedding passives into package substrates is just beginning for applications ranging from consumer electronics to routers. Embedded active in substrate has seen few commercial implementations but it is thought will grow in importance with time.

JISSO has defined "embedded substrate" as containing embedded active or passive components or passive functions that are fabricated as part of the substrate fabrication process.

Jan Vardaman of TechSearch presented an overview of the embedded substrate market. Advantages of embedded components are:

– Small form factor (reduced Z-height), enables reduced board thickness
– Improved performance
– Shorter electrical path, EMI reduction, integrated passives
– Shielding advantages for RF components
– Embedded die technologies appropriate for:
– Lower value, high yielding die where high interconnect density is required on both sides of the substrate
– RF modules where embedding tested die allows high density SMT on top

There are still concerns about:

– Patent issues
– Handling thin die
– Solder joint reliability of buried joints
– Cost (embedded die cost vs. die in package mounted on board)
– Concerns about liability
– Test (how to test after embedding component?)
– Inspection (how to inspect an embedded component?)

Takayoshi Katahira of Nokia addressed embedded technology from the mobile device perspective.

Embedding technology can either be face up:

– Cavity cut-out
– Component placement
– Lamination
– Laser drilling
– Plating

Or it can be face down, where the component is soldered in place and then buried. E-B2IT is seen as the leading technology of this kind.

Since 3D eWLB and RCP fan out technologies enable the same merits as substrate-based embedding, these can also be called "active embedding." Nokia sees high IO active embedding into packaging substrate coming soon.

Top-Bottom interconnection and top patterning enabling 3D assembly will be suitable for:

– Standard memories with high-pin count
– DDR2 Quad Channel or DDR3
– WLCSPs
– Passives

Many passives are mounted on mainboard for smartphones. Capacitors tends to be used in the greatest numbers. Soldering embedded caps has a clear advantage in process cost.

Bruce Su of ASE presented chip embedding as a technology evolution after bumping. ASE is developing "advanced Embedded Assembly Solution Integration" or aEASI as shown below:

EASI currently has the following design rules:

For all the latest on 3D IC integration and advanced packaging stay linked to IFTLE……….

The cellphone continues to pull in the functionality of digital cameras, PDAs, GPS navigators, mobile TV and numerous other applications. It is quickly becoming thedominant market driver for virtually all of these functions.

Earlier this summer market research firm Forward Concepts issued a report, "Cellular handset and chip markets ’11: An in-depth analysis of cellphones and their chips," which indicates that cellular handset shipments grew 12% in 2010 to 1.5 billion units. It includes some interesting points that those in high end packaging should study carefully.

As we know, smartphones are expected to grow at an accelerated rate (15.4%) to the 318M unit level this year. The report provides extensive forecasts for all handset types and virtually all cellphone chips through 2015. Though Samsung and Apple are growing faster, Nokia continues to be the leading handset vendor. Nokia’s average handset selling price is among the lowest because of their huge share of the low-end markets in China, India and Africa. Nokia is still the largest vendor of smartphones, but Apple is catching up, as illustrated in the graph below:

In terms of chip revenue coming from cellular handsets, Qualcomm remains the "big dog" with 23% of the market. If we include TI, Infineon and ST-Ericsson, we can account for more than 50% of the chip revenue in this market:

From the chart below we see that the display, the baseband chip and the image sensor account for more than 50% of the component value:

Predicting memory supplier consolidation

Anyone following the goings-on in the 3D IC market space would have to agree that Elpida has been at the forefront of the technology [see IFTLE 67] Several of these 3D practitioners such as Elpida, Samsung, and Micron (Aptina) also are in the memory business. A recent article attributed to Bloomberg Business Week [link] proposes that memory chip-makers ProMOS Technologies, Powerchip Technology and Elpida Memory, "burdened by debt, losses and falling prices, are under increasing pressure to seek mergers or exit the industry."

Reportedly, Elpida, which is $4.6B in debt, is "producing chips that sell for less than they cost to make." Micron, market leader Samsung, and Hynix Semiconductor are the only DRAM makers among the top eight generating a profit. DRAM makers as a group have lost 19% of their market value this year, according to Bloomberg, which quotes a financial analyst’s doubts: "I find it difficult to believe they [Elpida] are going to survive this downturn […] Consolidation is inevitable for survival in this industry."

The Japanese government’s interest in maintaining an on shore supply of memory chips might limit who can acquire Elpida. Toshiba is thought of as a logical choice if such a merger is required.

According to recent announcements, Elpida Memory is considering cutting back production at its Hiroshima facility and sending more work to Taiwan partner Rexchip, its JV with Powerchip [see "Elpida shifting output to Taiwan, blames yen and ASPs]. Reports indicate up to 40% of Elpida’s domestic capacity (50,000 wafer/month, 300mm wafers) could go to Rexchip would be producing the majority of Elpida’s output. The Taiwan partner would produce commodity DRAM, while the Elpida Hiroshima plant would focus on memory for smartphones, according to the Nikkei daily.

No matter the outcome, IFTLE hopes these business issues do not impact the outstanding work Elpida is doing in 3D IC.

Next week we will finish updates from SEMICON Taiwan.

For all the latest on 3D IC and advanced packaging stay linked to IFTLE……….