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Packaging Conference Addresses Challenges, Opportunities in New Technologies

Friday, December 18th, 2015

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By Jeff Dorsch, Contributing Editor

On the second day of the 12th annual 3D ASIP conference, the heavy hitters came out to talk. Attendees heard presentations from executives of Amkor Technology, the Defense Advanced Research Projects Agency (DARPA), Northrop Grumman, Taiwan Semiconductor Manufacturing, Teledyne Scientific & Imaging, and Xilinx, among other companies.

The day began with Pioneer Awards presented to Mitsumasa Koyanagi of Tohoku University and Peter Ramm of Fraunhofer EMFT. Those two men then gave talks on their involvement in 3D packaging technology over the decades.

“It started with DRAM in 1974,” Koyanagi recalled.

Ramm reviewed various European initiatives in the field, including the development of InterChip Vias (ICVs), a precursor to through-silicon via (TSV) technology, and the concept of known good die.

Suresh Ramalingam of Xilinx discussed the attributes of Silicon-less Interconnect Technology (SLIT), which the chip company developed in cooperation with Siliconware Precision Industries (SPIL), the IC assembly, bumping, and testing contractor.

“It’s still a silicon platform,” he pointed out. SLIT promises to connect multiple die in a package without resorting to TSVs. “Wafer warpage is a big issue,” Ramalingam noted.

Amkor’s Mike Kelly followed Ramalingam. “There’s a kind of upturn or resurgence in 2.5D, driven by high-bandwidth memory,” he said.

Amkor is offering the Silicon-less Interposer Module (SLIM) as its TSV alternative technology, according to Kelly, while also providing Silicon Wafer Integrated Fan-out Technology (SWIFT) as another packaging alternative to TSV-based interconnections.

KC Yee of TSMC, filling in for an absent presenter, spoke at length about the foundry’s Integrated Fan-Out (InFO) wafer-level packaging technology. “InFO eliminates silicon, TSVs, interposers,” he said. At the same time, InFO “reduces cost,” he asserted.

DARPA’s Daniel Green spoke about the agency’s Diverse Accessible Heterogeneous Integration (DAHI) program, which succeeded its Compound Semiconductor Materials on Silicon (COSMOS) program.

He was followed by Augusto Gutierrez-Aitken of Northrop Grumman Aerospace Systems. “DAHI is not in competition with CMOS,” he said. NGAS is developing a foundry for heterogeneous integration projects, inviting in companies and universities to participate in the research and development.

Teledyne Scientific’s Miguel Urteaga spoke about his company’s CS-STACK 3D stacking chip program. “We’re looking to get the highest III-V performance we can,” he said.

Managing Dis-Aggregated Data for SiP Yield Ramp

Monday, August 24th, 2015

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By Ed Korczynski, Sr. Technical Editor

In general, there is an accelerating trend toward System-in-Package (SiP) chip designs including Package-On-Package (POP) and 3D/2.5D-stacks where complex mechanical forces—primarily driven by the many Coefficient of Thermal Expansion (CTE) mismatches within and between chips and packages—influence the electrical properties of ICs. In this era, the industry needs to be able to model and control the mechanical and thermal properties of the combined chip-package, and so we need ways to feed data back and forth between designers, chip fabs, and Out-Sourced Assembly and Test (OSAT) companies. With accelerated yield ramps needed for High Volume Manufacturing (HVM) of consumer mobile products, to minimize risk of expensive Work In Progress (WIP) moving through the supply chain a lot of data needs to feed-forward and feedback.

Calvin Cheung, ASE Group Vice President of Business Development & Engineering, discussed these trends in the “Scaling the Walls of Sub-14nm Manufacturing” keynote panel discussion during the recent SEMICON West 2015. “In the old days it used to take 12-18 months to ramp yield, but the product lifetime for mobile chips today can be only 9 months,” reminded Cheung. “In the old days we used to talk about ramping a few thousand chips, while today working with Qualcomm they want to ramp millions of chips quickly. From an OSAT point of view, we pride ourselves on being a virtual arm of the manufacturers and designers,” said Cheung, “but as technology gets more complex and ‘knowledge-base-centric” we see less release of information from foundries. We used to have larger teams in foundries.” Dick James of ChipWorks details the complexity of the SiP used in the Apple Watch in his recent blog post at SemiMD, and documents the details behind the assumption that ASE is the OSAT.

With single-chip System-on-Chip (SoC) designs the ‘final test’ can be at the wafer-level, but with SiP based on chips from multiple vendors the ‘final test’ now must happen at the package-level, and this changes the Design For Test (DFT) work flows. DRAM in a 3D stack (Figure 1) will have an interconnect test and memory Built-In Self-Test (BIST) applied from BIST resident on the logic die connected to the memory stack using Through-Silicon Vias (TSV).

Fig.1: Schematic cross-sections of different 3D System-in-Package (SiP) design types. (Source: Mentor Graphics)

“The test of dice in a package can mostly be just re-used die-level tests based on hierarchical pattern re-targeting which is used in many very large designs today,” said Ron Press, technical marketing director of Silicon Test Solutions, Mentor Graphics, in discussion with SemiMD. “Additional interconnect tests between die would be added using boundary scans at die inputs and outputs, or an equivalent method. We put together 2.5D and 3D methodologies that are in some of the foundry reference flows. It still isn’t certain if specialized tests will be required to monitor for TSV partial failures.”

“Many fabless semiconductor companies today use solutions like scan test diagnosis to identify product-specific yield problems, and these solutions require a combination of test fail data and design data,” explained Geir Edie, Mentor Graphics’ product marketing manager of Silicon Test Solutions. “Getting data from one part of the fabless organization to another can often be more challenging than what one should expect. So, what’s often needed is a set of ‘best practices’ that covers the entire yield learning flow across organizations.”

“We do need a standard for structuring and transmitting test and operations meta-data in a timely fashion between companies in this relatively new dis-aggregated semiconductor world across Fabless, Foundry, OSAT, and OEM,” asserted John Carulli, GLOBALFOUNDRIES’ deputy director of Test Development & Diagnosis, in an exclusive discussion with SemiMD. “Presently the databases are still proprietary – either internal to the company or as part of third-party vendors’ applications.” Most of the test-related vendors and users are supporting development of the new Rich Interactive Test Database (RITdb) data format to replace the Standard Test Data Format (STDF) originally developed by Teradyne.

“The collaboration across the semiconductor ecosystem placed features in RITdb that understand the end-to-end data needs including security/provenance,” explained Carulli. Figure 2 shows that since RITdb is a structured data construct, any data from anywhere in the supply chain could be easily communicated, supported, and scaled regardless of OSAT or Fabless customer test program infrastructure. “If RITdb is truly adopted and some certification system can be placed around it to keep it from diverging, then it provides a standard core to transmit data with known meaning across our dis-aggregated semiconductor world. Another key part is the Test Cell Communication Standard Working Group; when integrated with RITdb, the improved automation and control path would greatly reduce manually communicated understanding of operational practices/issues across companies that impact yield and quality.”

Fig.2: Structure of the Rich Interactive Test Database (RITdb) industry standard, showing how data can move through the supply chain. (Source: Texas Instruments)

Phil Nigh, GLOBALFOUNDRIES Senior Technical Staff, explained to SemiMD that for heterogeneous integration of different chip types the industry has on-chip temperature measurement circuits which can monitor temperature at a given time, but not necessarily identify issues cause by thermal/mechanical stresses. “During production testing, we should detect mechanical/thermal stress ‘failures’ using product testing methods such as IO leakage, chip leakage, and other chip performance measurements such as FMAX,” reminded Nigh.

Model but verify

Metrology tool supplier Nanometrics has unique perspective on the data needs of 3D packages since the company has delivered dozens of tools for TSV metrology to the world. The company’s UniFire 7900 Wafer-Scale Packaging (WSP) Metrology System uses white-light interferometry to measure critical dimensions (CD), overlay, and film thicknesses of TSV, micro-bumps, Re-Distribution Layer (RDL) structures, as well as the co-planarity of Cu bumps/pillars. Robert Fiordalice, Nanometrics’ Vice President of UniFire business group, mentioned to SemiMD in an exclusive interview that new TSV structures certainly bring about new yield loss mechanisms, even if electrical tests show standard results such as ‘partial open.’ Fiordalice said that, “we’ve had a lot of pull to take our TSV metrology tool, and develop a TSV inspection tool to check every via on every wafer.” TSV inspection tools are now in beta-tests at customers.

As reported at 3Dincites, Mentor Graphics showed results at DAC2015 of the use of Calibre 3DSTACK by an OSAT to create a rule file for their Fan-Out Wafer-Level Package (FOWLP) process. This rule file can be used by any designer targeting this package technology at this assembly house, and checks the manufacturing constraints of the package RDL and the connectivity through the package from die-to-die and die-to-BGA. Based on package information including die order, x/y position, rotation and orientation, Calibre 3DSTACK performs checks on the interface geometries between chips connected using bumps, pillars, and TSVs. An assembly design kit provides a standardized process both chip design companies and assembly houses can use to ensure the manufacturability and performance of 3D SiP.

—E.K.

Solid State Watch: July 31-August 6, 2015

Friday, August 7th, 2015
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3DIC Technology Drivers and Roadmaps

Monday, June 22nd, 2015

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By Ed Korczynski, Sr. Technical Editor

After 15 years of targeted R&D, through-silicon via (TSV) formation technology has been established for various applications. Figure 1 shows that there are now detailed roadmaps for different types of 3-dimensional (3D) ICs well established in industry—first-order segmentation based on the wiring-level/partitioning—with all of the unit-processes and integration needed for reliable functionality shown. Using block-to-block integration with 5 micron lines at leading international IC foundries such as GlobalFoundries, systems stacking logic and memory such as the Hybrid Memory Cube (HMC) are now in production.

Fig. 1: Today’s 3D technology landscape segmented by wiring-level, showing cross-sections of typical 2-tier circuit stacks, and indicating planned reductions in contact pitches. (Source: imec)

“There are interposers for high-end complex SOC design with good yield,” informed Eric Beyne, Scientific Director Advanced Packaging & Interconnect for imec in an exclusive interview with Solid State Technology. ““For a systems company, once you’ve made the decision to go 3D there’s no way back,” said Beyne. “If you need high-bandwidth memory, for example, then you’re committed to some sort of 3D. The process is happening today.” Beyne is scheduled to talk about 3D technology driven by 3D application requirements in the imec Technology Forum to be held July 13 in San Francisco.

Adaptation of TSV for stacking of components into a complete functional system is key to high-volume demand. Phil Garrou, packaging technologist and SemiMD blogger, reported from the recent ConFab that Hynix is readying a second generation of high-bandwidth memory (HBM 2) for use in high performance computing (HPC) such as graphics, with products already announced like Pascal from Nvidia and Greenland from AMD.

For a normalized 1 cm2 of silicon area, wide-IO memory needs 1600 signal pins (not counting additional power and ground pins) so several thousand TSV are needed for high-performance stacked DRAM today, while in more advanced memory architectures it could go up by another factor of 10. For wide-IO HVM-2 (or Wide-IO2) the silicon consumed by IO circuitry is maybe 6 cm2 today, such that a 3D stack with shorter vertical connections would eliminate many of the drivers on the chip and would allow scaling of the micro-bumps to perhaps save a total of 4 cm2 in silicon area. 3D stacks provide such trade-offs between design and performance, so the best results are predicted for 3DICs where the partitioning can be re-done at the gate or transistor level. For example, a modern 8-core microprocessor could have over 50% of the silicon area consumed by L3-cache-memory and IO circuitry, and moving from 2D to 3D would reduce total wire-lengths and interconnect power consumptions by >50%.

There are inherent thresholds based on the High:Width ratio (H:W) that determine costs and challenges in process integration of TSV:

-    10:1 ratio is the limit for the use of relatively inexpensive physical vapor deposition (PVD) for the Cu barrier/seed (B/S),

-    20:1 ratio is the limit for the use of atomic-layer deposition (ALD) for B/S and electroless deposition (ELD) for Cu fill with 1.5 x 30 micron vias on the roadmap for the far future,

-    30:1 ratio and greater is unproven as manufacturable, though novel deposition technologies continue to be explored.

TSV Processing Results

The researchers at imec have evaluated different ways of connecting TSV to underlying silicon, and have determined that direct connections to micro-bumps are inherently superior to use of any re-distribution layer (RDL) metal. Consequently, there is renewed effort on scaling of micro-bump pitches to be able to match up with TSV. The standard minimum micro-bump pitch today of 40 micron has been shrunk to 20, and imec is now working on 10 micron with plans to go to 5 micron. While it may not help with TSV connections, an RDL layer may still be needed in the final stack and the Cu metal over-burden from TSV filling has been shown by imec to be sufficiently reproducible to be used as the RDL metal. The silicon surface area covered by TSV today is a few percents not 10s of percents, since the wiring level is global or semi-global.

Regarding the trade-offs between die-to-wafer (D2W) and wafer-to-wafer (W2W) stacking, D2W seems advantageous for most near-term solutions because of easier design and superior yield. D2W design is easier because the top die can be arbitrarily smaller silicon, instead of the identically sized chips needed in W2W stacks. Assuming the same defectivity levels in stacking, D2W yield will almost always be superior to W2W because of the ability to use strictly known-good-die. Still, there are high-density integration concepts out on the horizon that call for W2W stacking. Monolithic 3D (M3D) integration using re-grown active silicon instead of TSV may still be used in the future, but design and yield issues will be at least comparable to those of W2W stacking.

Beyne mentioned that during the recent ECTC 2015, EV Group showed impressive 250nm overlay accuracy on 450mm wafers, proving that W2W alignment at the next wafer size will be sufficient for 3D stacking. Beyne is also excited by the fact the at this year’s ECTC there was, “strong interest in thermo-compression bonding, with 18 papers from leading companies. It’s something that we’ve been working on for many years for die-to-wafer stacking, while people had mistakenly thought that it might be too slow or too expensive.”

Thermal issues for high-performance circuitry remain a potential issue for 3D stacking, particularly when working with finFETs. In 2D transistors the excellent thermal conductivity of the underlying silicon crystal acts like a built-in heat-sink to diffuse heat away from active regions. However, when 3D finFETs protrude from the silicon surface the main path for thermal dissipation is through the metal lines of the local interconnect stack, and so finFETs in general and stacks of finFETs in particular tend to induce more electro-migration (EM) failures in copper interconnects compared to 2D devices built on bulk silicon.

3D Designs and Cost Modeling

At a recent North California Chapter of the American Vacuum Society (NCCAVS) PAG-CMPUG-TFUG Joint Users Group Meeting discussing 3D chip technology held at Semi Global Headquarters in San Jose, Jun-Ho Choy of Mentor Graphics Corp. presented on “Electromigration Simulation Flow For Chip-Scale Parametric Failure Analysis.” Figure 2 shows the results from use of a physics-based model for temperature- and residual-stress-aware void nucleation and growth. Mentor has identified new failure mechanisms in TSV that are based on coefficient of thermal expansion (CTE) mismatch stresses. Large stresses can develop in lines near TSV during subsequent thermal processing, and the stress levels are layout dependent. In the worst cases the combined total stress can exceed the critical level required for void nucleation before any electrical stressing is applied. During electrical stress, EM voids were observed to initially nucleate under the TSV centers at the landing-pad interfaces even though these are the locations of minimal current-crowding, which requires proper modeling of CTE-mismatch induced stresses to explain.

Fig. 2: Calibration of an Electronic Design Automation (EDA) tool allows for accurate prediction of transistor performance depending on distance from a TSV. (Source: Mentor Graphics)

Planned for July 16, 2015 at SEMICON West in San Francisco, a presentation on “3DIC Technology Past, Present and Future” will be part of one of the side Semiconductor Technology Sessions (STS). Ramakanth Alapati, Director of Packaging Strategy and Marketing, GLOBALFOUNDRIES, will discuss the underlying economic, supply chain and technology factors that will drive productization of 3DIC technology as we know it today. Key to understanding the dynamic of technology adaptation is using performance/$ as a metric.

Silicon Technology Extensions shown at MRS Spring 2015

Monday, June 1st, 2015

By Ed Korczynski, Sr. Technical Editor, Solid State Technology/SemiMD

In the spring meeting of the Materials Research Society held recently in San Francisco, Symposium A: Emerging Silicon Science and Technology included presentations on controlling the structure of crystalline spheres and thin-films. Such structures could be used in future complementary metal-oxide semiconductor (CMOS) devices and in photonic circuits built using silicon.

Alexander Gumennik, et al., from the Massachusetts Institute of Technology, presented on “Extraordinary Stress in Silicon Spheres via Anomalous In-Fiber Expansion” as a way to control the bandgap of silicon and thus enable the use of silicon for photodetection at higher wavelengths. A silica fiber with a crystalline silicon core is fed through a flame yielding spherical silicon droplets via capillary instabilities. Upon cooling the spheres solidify and expand against the stiff silica cladding generating high stress conditions. Band gap shifts of 0.05 eV to the red (in Si) are observed, corresponding to internal stress levels. These stress levels exceed the surface stress as measured through birefringence measurements by an order of magnitude, thus hinting at a pressure-focusing mechanism. The effects of the solidification kinetics on the stress levels reached inside the spheres were explored, and the experimental results were found to be in agreement with a pressure-focusing mechanism arising from radial solidification of the spheres from the outer shell to the center. The simplicity of this approach presents compelling opportunities for the achievement of unusual phases and chemical reactions that would occur under high-pressure high-temperature conditions, which therefore opens up a pathway towards the realization of new in-fiber optoelectronic devices.

Fabio  Carta and others from Columbia University working with researchers from IBM showed results on “Excimer Laser Crystallization of Silicon Thin Films on Low-K Dielectrics for Monolithic 3D Integration.” This research supports the “Monolithic 3D” (M3D) approach to 3D CMOS integration as popularized by CEA-LETI, as opposed to the used of Through Silicon Vias (TSV). M3D requires processing temperature below 400°C if copper interconnects and low-k dielectric will be used in the bottom layer. Excimer laser crystallization (ELC) takes advantage of a short laser pulse to fully melt the amorphous silicon layer without allowing excessive time for the heat to spread throughout the structure, achieving large grain polycrystalline layer on top of temperature sensitive substrates. The team crystallized 100nm thick amorphous silicon layers on top of SiO2 and SiCOH (low-k) dielectrics. SEM micrographs show that post-ELC polycrystalline silicon is characterized by micron-long grains with an average width of 543 nm for the SiO2 sample and 570 nm for the low-k samples. A 1D simulation of the crystallization process on a back end of line structure shows that interconnect lines experience a maximum temperature lower than 70°C for the 0.5 μm dielectric, which makes ELC on low-k a viable pathway for achieving monolithic integration.

Seiji  Morisaki, et al., from Hiroshima Univ, showed results for “Micro-Thermal-Plasma-Jet Crystallization of Amorphous Silicon Strips and High-Speed Operation of CMOS Circuit.” The researchers used micro-thermal-plasma-jet (µ-TPJ) for zone melting recrystallization (ZMR) of amorphous silicon (a-Si) films to form lateral grains larger than 60 µm. By applying ZMR on a-Si strip patterns with widths <3 µm, single liquid-solid interfaces move inside the strips and formation of random grain boundaries (GBs) are significantly suppressed. Applying such strip patterns to active channels of thin-film-transistors (TFTs) results in a demonstrated field effect mobility (µFE) higher than 300 cm2/V*s because they contain minimal grain-boundaries. These a-Si strip pattern were then used to characteristic variability of n- and p-channel TFTs and CMOS ring oscillators. The strip patterns showed improved uniformities and defect densities, in general. A 9-stage ring oscillator fabricated with conventional TFTs had a maximum frequency (Fmax) of operation of 58 MHz under supply voltage (Vdd) of 5V which corresponds to a 1-stage delay (τ) of 0.94 ns, while strip channel TFTs demonstrated 108 MHz Fmax and τ decreased to 0.52 ns.

Ebrahim  Najafi, et al., from the California Institute of Technology, showed how “Ultrafast Imaging of Carrier Dynamics at the p-n Junction Interface” based on scanning ultrafast electron microscopy (SUEM) combines the spatial resolution of an electron probe with the temporal resolution of an optical pulse to enable unprecedented studies of carrier dynamics in spatially complex geometries. Observing the behavior of carriers in both space and time provides direct imaging of carrier excitation, transport, and recombination in the silicon p-n junction and the ability to follow their spatiotemporal behavior. Carrier separation on the surface of the p-n junction extends tens of microns beyond the depletion layer, as explained by a model using a ballistic-type transport. With the invention of SUEM, it should now be possible to study density profiles and electric potentials at surfaces and interfaces at the ultrafast time scale with the spatial resolution of the electron probe.

As a reminder, the Call For Paper for the MRS Fall 2015 meeting closes on June 18.

—E.K.

Monolithic 3D processing using non-equilibrium RTP

Friday, April 17th, 2015

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By Ed Korczynski, Senior Technical Editor, Solid State Technology

Slightly more than one year after Qualcomm Technologies announced that it was assessing CEA-Leti’s monolithic 3D (M3D) transistor stacking technology, Qualcomm has now announced that M3D will be used instead of through-silicon vias (TSV) in the company’s next generation of cellphone handset chips. Since Qualcomm had also been a leading industrial proponent of TSV over the last few years while participating in the imec R&D consortium, this endorsement of M3D is particularly relevant.

Leti’s approach to 3D stacking of transistors starts with a conventionally built and locally-interconnected bottom layer of transistors, which are then covered with a top layer of transistors built using relatively low-temperature processes branded as “CoolCube.” Figure 1 shows a simplified cross-sectional schematic of a CoolCube stack of transistors and interconnects. CoolCube M3D does not transfer a layer of built devices as in the approach using TSV, but instead transfers just a nm-thin layer of homogenous semiconducting material for subsequent device processing.

Fig. 1: Simplified cross-sectional rendering of Monolithic 3D (M3D) transistor stacks, with critical process integration challenges indicated. (Source: CEA-Leti)

The reason that completed transistors are not transferred in the first place is because of intrinsic alignment issues, which are eliminated when transistors are instead fabricated on the same wafer. “We have lots of data to prove that alignment precision is as good as can be seen in 2D lithography, typically 3nm,” explained Maud Vinet, Leti’s advanced CMOS laboratory manager in an exclusive interview with SST.

As discussed in a blog post online at Semiconductor Manufacturing and Design (http://semimd.com/hars/2014/04/09/going-up-monolithic-3d-as-an-alternative-to-cmos-scaling/) last year by Leti researchers, the M3D approach consists of sequentially processing:

  • processing a bottom MOS transistor layer with local interconnects,
  • bonding a wafer substrate to the bottom transistor layer,
  • chemical-mechanical planarization (CMP) and SPE of the top layer,
  • processing the top device layer,
  • forming metal vias between the two device layers as interconnects, and
  • standard copper/low-k multi-level interconnect formation.

To transfer a layer of silicon for the top layer of transistors, a cleave-layer is needed within the bulk silicon or else time and money would be wasted in grinding away >95% of the silicon bulk from the backside. For CMOS:CMOS M3D thin silicon-on-insulator (SOI) is the transferred top layer, a logical extension of work done by Leti for decades. The heavy dose ion-implantation that creates the cleave-layer leaves defects in crystalline silicon which require excessively high temperatures to anneal away. Leti’s trick to overcome this thermal-budget issue is to use pre-amorphizing implants (PAI) to completely dis-order the silicon before transfer and then solid-phase epitaxy (SPE) post-transfer to grow device-grade single-crystal silicon at ~500°C.

Since neither aluminum nor copper interconnects can withstand this temperature range, the interconnects for the bottom layer of transistors need to be tungsten wires with the highest melting point of any metal but somewhat worse electrical resistance (R). Protection for the lower wires cannot use low-k dielectrics, but must use relatively higher capacitance (C) oxides. However, the increased RC delay in the lower interconnects is more than offset by the orders-of-magnitude reduction in interconnect lengths due to vertical stacking.

M3D Roadmaps

Leti shows data that M3D transistor stacking can provide immediate benefit to industry by combining two 28nm-node CMOS layers instead of trying to design and manufacture a single 14nm-node CMOS layer:  area gain 55%, performance gain 23%, and power gain 12%. With cost/transistor now expected to increase with sequential nodes, M3D thus provides a way to reduce cost and risk when developing new ICs.

For the industry to use M3D, there are some unique new unit-processes that will need to ramp into high-volume manufacturing (HVM) to ensure profitable line yield. As presented by C. Fenouillet-Beranger et al. from Leti and ST (paper 27.5) at IEDM2014 in San Francisco, “New Insights on Bottom Layer Thermal Stability and Laser Annealing Promises for High Performance 3D Monolithic Integration,” due to stability improvement in bottom transistors found through the use of doping nickel-silicide with a noble metal such as platinum, the top MOSFET processing temperature could be relaxed up to 500°C. Laser RTP annealing then allows for the activation of top MOSFETs junctions, which have been characterized morphologically and electrically as promising for high performance ICs.

Figure 2 shows the new unit-processes at <=500°C that need to be developed for top transistor formation:

*   Gate-oxide formation,

*   Dopant activation,

*   Epitaxy, and

*   Spacer deposition.

Fig. 2: Thermal processing ranges for process modules need to be below ~500°C for the top devices in M3D stacks to prevent degradation of the bottom layer. (Source: CEA-Leti)

After the above unit-processes have been integrated into high-yielding process modules for CMOS:CMOS stacking, heterogeneous integration of different types of devices are on the roadmap for M3D. Leti has already shown proof-of-concept for processes that integrate new IC functionalities into future M3D stacks:

1)       CMOS:CMOS,

2)       PMOS:NMOS,

3)       III-V:Ge, and

4)       MEMS/NEMS:CMOS.

Thomas Ernst, senior scientist, Electron Nanodevice Architectures, Leti, commented to SST, “Any application that will need a ‘pixelated’ device architecture would likely use M3D. In addition, this approach will work well for integrating new channel materials such as III-V’s and germanium, and any materials that can be deposited at relatively low temperatures such as the active layers in gas-sensors or resistive-memory cells.”

Non-Equilibrium Thermal Processing

Though the use of an oxide barrier between the active device layers provides significant thermal protection to the bottom layer of devices during top-layer fabrication, the thermal processes of the latter  cannot be run at equilibrium. “One way of controlling the thermal budget is to use what we sometimes call the crème brûlée approach to only heat the very top surface while keeping the inside cool,” explained Vinet. “Everyone knows that you want a nice crispy top surface with cool custard beneath.” Using a laser with a short wavelength prevents penetration into lower layers such that essentially all of the energy is absorbed in the surface layer in a manner that can be considered as adiabatic.

Applied Materials has been a supplier-partner with Leti in developing M3D, and the company provided responses from executive technologists to queries from SST about the general industry trend to controlling short pulses of light for thermal processing. “Laser non-equilibrium heating is enabling technology for 3D devices,” affirmed Steve Moffatt, chief technology officer, Front End Products, Applied Materials. “The idea is to heat the top layer and not the layers below. To achieve very shallow adiabatic heating the toolset needs to ramp up in less than 100 nsec. In order to get strong absorption in the top surface, shorter wavelengths are useful, less than 800 nm. Laser non-equilibrium heating in this regime can be a critical process for building monolithic 3D structures for SOC and logic devices.”

Of course, with ultra-shallow junctions (USJ) and atomic-scale gate-stacks already in use for CMOS transistors at the 22nm-node, non-equilibrium thermal processing has already been used in leading fabs. “Gate dielectric, gate metal, and contact treatments are areas where we have seen non-equilibrium anneals slowly taking the place of conventional RTP,” clarified Abhilash Mayur, senior director, Front End Products, Applied Materials. “For approximate percentages, I would say about 25 percent of thermal processing for logic at the 22nm-node is non-equilibrium, and seen to be heading toward 50 percent at the 10nm-node or lower.”

Mayur further explained some of the trade-offs in working on the leading-edge of thermal processing for demanding HVM customers. Pulse-times are in the tens of nsec, with longer pulses tending to allow the heat to diffuse deeper and adversely alter the lower layers, and with shorter pulses tending to induce surface damage or ablation. “Our roadmap is to ensure flexibility in the pulse shape to tailor the heat flow to the specific application,” said Mayur.

Now that Qualcomm has endorsed CoolCube M3D as a preferred approach to CMOS:CMOS transistor stacking in the near-term, we may assume that R&D in novel unit-processes has mostly concluded. Presumably there are pilot lots of wafers now being run through commercial foundries to fine-tune M3D integration. With a roadmap for long-term heterogeneous integration that seems both low-cost and low-risk, M3D using non-equilibrium RTP will likely be an important way to integrate new functionalities into future ICs.

3D memory for future nanoelectronic systems

Wednesday, June 18th, 2014

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By Ed Korczynski, Sr. Technical Editor

The future of 3D memory will be in application-specific packages and systems. That is how innovation continues when simple 2D scaling reaches atomic-limits, and deep work on applications is now part of what global research and development (R&D) consortium Imec does. Imec is now 30 years old, and the annual Imec Technology Forum held in the first week of June in Brussels, Belgium included fun birthday celebrations and very serious discussions of the detailed R&D needed to push nanoelectronics systems into health-care, energy, and communications markets.

3D memory will generally cost more than 2D memory, so generally a system must demand high speed or small size to mandate 3D. Communications devices and cloud servers need high speed memory. Mobile and portable personalized health monitors need low power memory. In most cases, the optimum solution does not necessarily need more bits, but perhaps faster bits or more reliable bits. This is why the Hybrid Memory Cube (HMC) provides >160Gb/sec data transfer with Through-Silicon Vias (TSV) through 3D stacked DRAM layers.

“We’re not adding 70-80% more bits like we used to per generation, or even the 40% recently,” explained Mark Durcan, chief executive officer of Micron Technology. “DRAM bits will only grow at the low to mid-20%.” With those numbers come hopes of more stability and less volatility in the DRAM business. Likewise, despite the bit growth rates of the recent past, NAND is moving to 30-40%  bit-increase per new ‘generation.’

“Moore’s Law is not over, it’s just slowing,” declared Durcan. “With NAND, we’re moving from planar to 3D, and the innovation is that there are different ways of doing 3D.” Figure 1 shows the six different options that Micron defines for 3D NAND. Micron plans for future success in the memory business to be not just about bit-growth, but about application-specific memory solutions.

Fig. 1: Different options for Vertical NAND (VNAND) Flash memory design, showing cell layouts and key specifications. (Source: Micron Technology)

E. S. Jung, executive vice president Samsung Electronics, presented an overview of how “Samsung’s Breaking the Limits of Semiconductor Technology for the Future” at the Imec forum. Samsung Semiconductor announced it’s first DRAM product in 1984, and has been improving it’s capabilities in design and manufacturing ever since. Samsung also sees the future of memory chips as part of application-specific systems, and suggests that all of the innovation in end-products we envision for the future cannot occur without semiconductor memory.

Samsung’s world leading 3D vertical-NAND (VNAND) chips are based on simultaneous innovation in three different aspects of materials and design:

1)    Material changed from floating-gate,

2)    Rotated structure from horizontal to vertical (and use Gate All Around), and

3)    Stacked layers.

To accomplish these results, partners were needed from OEM and specialty-materials suppliers during the R&D of the special new hard-mask process needed to be able to form 2.5B vias with extremely high aspect-ratios.

Rick Gottscho, executive vice president of the global products group Lam Research Corp., in an exclusive interview with SST/SemiMD, explained that with proper control of hardmask deposition and etch processes the inherent line-edge-roughness (LER) of photoresist (PR) can be reduced. This sort of integrated process module can be developed independently by an OEM like Lam Research, but proving it in a device structure with other complex materials interactions requires collaboration with other leading researchers, and so Lam Research is now part of a new ‘Supplier Hub’ relationship at Imec.

Luc Van den hove, president and chief executive officer of Imec, commented, “we have been working with equipment and materials suppliers form the beginning, but we’re upgrading into this new ‘Supplier Hub.’ In the past most of the development occurred at the suppliers’ facilities and then results moved to Imec. Last year we announced a new joint ‘patterning center’ with ASML, and they’re transferring about one hundred people from Leuven. Today we announced a major collaboration with Lam Research. This is not a new relationship, since we’ve been working with Lam for over 20 years, but we’re stepping it up to a new level.”

Commitment, competence, and compromise are all vital to functional collaboration according to Aart J. de Geus, chairman and co-chief executive officer of Synopsys. Since he has long lead a major electronic design automation (EDA) company, de Geus has seen electronics industry trends over the 30 years that Imec has been running. Today’s advanced systems designs require coordination among many different players within the electronics industry ecosystem (Figure 2), with EDA and manufacturing R&D holding the center of innovation.

Fig. 2: Semiconductor manufacturing and design drive technology innovation throughout the global electronics industry. (Source: Synopsys)

“The complexity of what is being built is so high that the guarantee that what has been built will work is a challenge,” cautioned de Geus. Complexity in systems is a multiplicative function of the number of components, not a simple summation. Consequently, design verification is the greatest challenge for complex System-on-Chips (SoC). Faster simulation has always been the way to speed up verification, and future hardware and software need co-optimization. “How do you debug this, because that is 70% of the design time today when working with SoCs containing re-used IP? This will be one of the limiters in terms of product schedules,” advised de Geus.

Whether HMC stacks of DRAM, VNAND, or newer memory technologies such as spintronics or Resistive RAM (RRAM), nanoscale electronic systems will use 3D memories to reduce volume and signal delays. “Today we’re investigating all of the technologies needed to advance IC manufacturing below 10nm,” said Van den hove. The future of 3D memories will be complex, but industry R&D collaboration is preparing the foundation to be able to build such complex structures.

DISCLAIMER:  Ed Korczynski has or had a consulting relationship with Lam Research.

Blog review February 3, 2014

Monday, February 3rd, 2014

Ira Feldman provides an interesting perspective on last month’s SEMI Industry Strategy Symposium. He notes that numerous speakers including Jon Casey (IBM) and Mike Mayberry (Intel) stated that scaling will continue below the 10 nm process node perhaps to 5 or 7 nm. However, the question raised by both the speakers and the audience was at what cost will this scaling be achieved.

“Long live the FinFET,” says Zhihong Liu, Executive Chairman, ProPlus Design Solutions, Inc. In this blog post, he describes how designers will have to seek out new tools and methodologies to overcome FinFET design challenges. One example is the adoption of giga-scale parallel SPICE simulators to harness circuit simulation challenges in FinFET designs. Traditional SPICE simulators don’t have the capacity and lack sufficient performance to support FinFET designs, while FastSPICE simulators likely will not meet accuracy requirements, he writes.

Adele Hars of Advanced Substrate News reports that STMicroelectronics will soon be announcing a “major foundry player” that will be both a dual FD-SOI manufacturing source for ST, plus an open source for the industry. This important piece of news came out of the company’s Q4 and FY13 presentation in Paris on January 28th.

Phil Garrou finishes up his review of the IMAPS 2013 meeting, with an analysis of Xilinx/SPIL results from their 2.5D 28nm FPGA program, a review of the Copper TSV work presented by Nanyang/IME, Canon’s FPA-5510iV and FPA-5510iZ TSA steppers designed to support high density processes and the implementation of 2.5 & 3D technology, and a report on the embedded technology being developed by AT&S.

GLOBALFOUNDRIES, Open-Silicon and Amkor demo 2.5D test vehicle

Friday, November 22nd, 2013

GLOBALFOUNDRIES, Open-Silicon and Amkor Technology have jointly exhibited a functional system-on-chip (SoC) solution on a 2.5D silicon interposer featuring two 28nm logic chips, with embedded ARM processors. The jointly developed design is a test vehicle that showcases the benefits of 2.5D technology for mobile and low-power server applications. The companies recently demonstrated the functioning SoC at ARM TechCon in Santa Clara, CA.

The test vehicle features two ARM Cortex-A9 processors manufactured using GLOBALFOUNDRIES’ 28nm-SLP (Super Low Power) process technology. The processors are attached to a silicon interposer, which is built on a 65nm manufacturing flow with through-silicon-vias (TSVs) to enable high-bandwidth communication between the chips.

Open-Silicon provided the processor, interposer, substrate, and test design, as well as the test and characterization of the final product. GLOBALFOUNDRIES provided the PDKs (process design kits), interposer reference flow and manufactured both the 28nm ARM processors and the 65nm silicon interposer with embedded TSVs. Amkor provided the package-related design rules and manufacturing processes for back-side integration, copper pillar micro-bumping, and 2.5D product assembly. GLOBALFOUNDRIES and Amkor collaborated closely throughout the project to develop and validate the design rules, assembly processes, and required material sets.

The companies developed the custom SoC to help overcome some of the challenges associated with bringing 2.5D technology to market. The 2.5D system features the following characteristics:

· Logic die including dual-core ARM Cortex-A9 CPUs, as well as DDR3, USB and AXI bridge interfaces

· A special EDA reference flow designed to address the additional requirements of 2.5D design, including top-level interposer design creation and floor planning, as well as the increased complexity of using TSVs, front-side and back-side bumps, and redistribution layer (RDL) routing

· Support for additional verification steps brought on by 2.5D design rules

· Custom die-to-die IO for better area and power characteristics providing a maximum of 8GB/s full-duplex data-rate across the two die through the silicon interposer

· A development board with memory, boot-ROM, and basic peripherals to demonstrate the die-to-die interface functionality through software running on the CPUs embedded in the logic dies

· A test methodology consisting of Boundary Scan and Loopback modes

· Package-related design rules, back-side integration, copper pillar micro-bumping, and 2.5D product assembly by Amkor Technology, a leading supplier of outsourced semiconductor packaging and test services.

GLOBALFOUNDRIES said this demonstrates the value of its open and collaborative approach to delivering next-generation chip packaging technologies, which it calls “Foundry 2.0,” which is aimed at enabling an open supply chain through collaboration with ecosystem partners and customers. This approach allows GLOBALFOUNDRIES’ customers to choose their preferred supply chain partners, while leveraging the experience of ecosystem partners who have developed deep expertise in design, assembly and test methodologies. This open and collaborative model is expected to deliver lower overall cost and less risk in bringing 2.5D technologies to market.

“As the fabless-foundry business model evolves to address the realities of today’s dynamic market, foundries are taking on increasing responsibility for enabling the supply chain to deliver end-to-end solutions that meet the requirements of the broad range of leading-edge designs,” said David McCann, vice president of packaging R&D at GLOBALFOUNDRIES. “To help address these challenges, we are driving our ‘Foundry 2.0’ collaborative supply chain model by engaging early with ecosystem partners like Open-Silicon and Amkor to jointly develop solutions that will enable the next wave of innovation in the industry.”

The companies demonstrated first-time functionality of the processor, interposer, and substrate designs, and the die-to-substrate (D2S) process used by the supply chain resulted in high yields. The design tools, process design kit (PDK), design rules, and supply chain are now in place and proven for 2.5D interposer products from GLOBALFOUNDRIES, Amkor, and Open-Silicon.

“This project is a testament to the value of an open and collaborative approach to innovation, leveraging expertise from across the supply chain to demonstrate progress in bringing a critical enabling technology to market,” said Ron Huemoeller, senior vice president of advanced product development at Amkor Technology. “This collaborative model will offer chip designers a flexible approach to 2.5D SoC designs, while delivering cost savings, faster time-to-volume, and a reduction in the technical risk associated with developing new technologies.”

“We are pleased to be at the forefront of making 2.5D a reality with our foundry and OSAT partners,” said Dr. Shafy Eltoukhy, vice president of technology development at Open-Silicon. “This approach will allow designers to choose the right technology for each function of their SoC while simultaneously enabling finer grain and lower power connectivity than traditional packaging solutions along with reduced power budgets for next-generation electronic devices.”

Marrying diversification, innovation with high-volume manufacturing – the MEMS puzzle

Tuesday, October 8th, 2013

By Sara Verbruggen

Initiated by Apple’s launch of the iPhone, the subsequent explosive growth of the smartphone market has provided the MEMS industry with one of its biggest opportunities to supply high-volume demand. But if motion sensing in our portable electronics – enabled by accelerometer and gyroscope MEMS applications for example – is the tip of the iceberg for MEMS technology how can the semiconductor industry ensure that high volume markets like consumer electronics benefit from all that MEMS potentially has to offer.

As the MEMS industry evolves, in terms of further diversification of device applications in higher volumes, this creates manufacturing challenges.

‘Organizations like MIG are helping to set standards across classes of devices in terms of specifications, rating, test interfaces, and system interfaces, and this is a great advancement in helping the industry to grow. On the manufacturing side though it is unlikely that a “standard” MEMS flow will emerge even within individual foundries except for very specific and limited types of MEMS – Invensense NF Process is an example of an attempt at this,’ comments Silex Microsystems’ VP of marketing and strategic alliances Peter Himes.

The emergence of MEMS technology over the last decade into high volume markets – consumer electronics especially – has presented the semiconductor industry with the challenge of designing and fabricating devices with different functionalities (as opposed to focusing on scaling down while ramping performance). This has paved the way for electronics in industries as diverse as healthcare, energy, security and environment. The long-term growth of MEMS depends on functional diversification but also being able to manufacture devices for these various applications in significant volumes and bringing down cost.

More than Moore techniques and processes

Wafer-scaling fabrication and process technologies, to enable these ‘More than Moore architectures’ are beginning to become established in MEMS manufacturing, for high volume markets.

SEMI’s chief marketing officer Tom Morrow says: ‘To be competitive in high-volume MEMs markets, 8” production equipment and economies will be, if not already, needed. Deep reactive ion etch (DRIE) “tuned” for MEMs technologies are also required, coupled with advanced cleaning solutions such as plasma. Bonding is, with DRIE, the other key MEMS-specific technology, used for wafer level capping and wafer level packaging.’ Critical concerns include providing good hermetic solutions to maintain performance of sensitive moving parts like gyros, while taking up less area on the wafer with bond lines. ‘The bonding process tends to take time, so throughput is typically low. Room temperature bonding and temporary bonding are areas of major improvement,’ adds Morrow.

DRIE and wafer bonding are the technologies subject to significant process improvement as both technologies are increasingly used in the mainstream semiconductor industry for 3D-TSV. In addition packaging and bonding technologies today support increasing standardization.

‘While contact and proximity aligners remain prominent lithography tools for MEMs, there is some movement towards projection steppers for better CD uniformity and automated 8” volume production,’ according to Morrow. Tools also need to be able to handle thin wafers and manufacturers also demand better overlay precision.

TSV is a critical technology, agrees Silex Microsystems’ Peter Himes. The company has specialised in TSV integration into MEMS since 2005 when its Sil-Via technology went into first production. This process, developed for the mobile industry, consisted of an all-silicon interposer for 2.5D integration of a MEMS microphone and ASIC onto a silicon substrate which was then solder- bumped and mounted directly onto the PCB.

‘Since then, we have been developing more TSV options for our customers, including TSV for buried cavity MEMS, TSV for capping solutions of either MEMS or CMOS, and both metal TSV and TGV through glass substrates for RF and power applications,’ says Himes.

As MEMS companies increasingly move beyond competing on manufacturing technology to competing on functionality, more of TSV/WLP packaging solutions will become widely-used platforms, predicts Yole Développement. This would also make more use of the outsourced infrastructure to reduce costs and speed-up development time.

‘Today, a few MEMS companies such as VTI, STMicroelectronics, Robert Bosch or MEMSIC have successfully implemented 3D wafer-level packaging concepts by using TSV/TGV vertical feedthrough, redistribution layers, and bumping processes to directly connect the silicon part of the MEMS/sensor to the final motherboard but without using a ceramic, leadframe, or plastic package. We believe this trend will be accelerated even further with the shift to 200mm wafer manufacturing for MEMS: it just makes sense to use wafer-level packaging, because as soon as you can add more dies on a wafer, it is more cost-effective,’ says Eric Mounier from Yole.

AMAT’s Mike Rosa points out that wafer-scale integration techniques, to enable more device functionality on a per die area basis, in combination with system-on-chip technologies to enable greater intelligence on die is becoming a standard requirement for more advanced MEMS.  ‘The end-users (system integrators – like Apple or Samsung for example) now require the MEMS device to do a lot more of the signal processing than has traditionally been the case – hence MEMS designers have to include more signal processing (CMOS) capability on die,’ says Rosa.

Fabless model

The fabless approach in the MEMS industry is now well-established, where, in order to speed up MEMS development device cycles, foundry companies partner with designers to provide them with process modules around which designers can develop MEMS devices.

But for the fabless model to facilitate the development of more differentiated and disruptive MEMS and to ensure companies remain competitive manufacturers need to be able to embrace and adopt new manufacturing processes and material technologies – which accompany disruptive new MEMS devices. ‘In the foundry space, it’s the foundry partner who is strongest in technology development that will win market share – this there is already a clear ‘pecking order’ with the big three foundries today and that is for a very good reason,’ says Rosa.

Silex is an example of a successful business servicing the fabless segment, through its program with AMFitzgerald. ‘The fact is that new companies cannot afford the cost of building a MEMS manufacturing line, and need a foundry infrastructure to get their products to market,’ says Himes.

Several key factors point to a strengthening fabless market in the long term, he observes. These include an ongoing reduction in overall development times for MEMS over the past two decades, lowering the time to market for new MEMS devices ‘though Yole is correct in saying that it needs to come down further,’ he adds. Increasingly fabless start-ups are driving innovation in MEMS-based functionality. ‘The percentage of MEMS revenues which comes from components not on the market before 2006 has been steadily growing, pointing to increased diversity and expansion of the MEMS- enabled market,’ says Himes pointing to a recent iSuppli presentation.

‘In terms of what works, Silex’s systematic SmartBlock-based approach toward process integration coupled with our defined new product introduction (NPI) process has proven to be the best way for us to manage the risk and uncertainty which comes with any process development. While customers always want shorter time to full production, an early focus of our customer programs is to get the customer fully functional samples as early as possible so that the rest of the component or system can be developed,’ Himes explains.

According to Mounier a successful fabless model relies on a MEMS designer, or similar business, finding a reliable foundry working on the long term. ‘Depending on the application, the foundry will have to be competitive on cost (consumer, automotive) or performances (defense, industrial applications). However, as many new MEMS devices are emerging in for new applications, such as touchscreens and flat speakers, MEMS foundries must be able to think about adapting the customer design to their own process flow.’

The RocketMEMS program run by AMFitzgerald & Associates is a good example. The company has defined a product design platform for rapidly commercializing semi-custom MEMS devices (pressure sensors is the first area) based on a pre-qualified manufacturing flow at Silex. ‘We think that this is an efficient path toward design enablement that can avoid the “one product, one process” paradigm in the long term,’ says Himes. Customers would be prioritizing time to market and customized form-fit-function over fully customized and optimized MEMS process flow. ‘We can envision many more such programs being set up worldwide, and thereby expanding the capability of doing MEMS design from the PhD level down to a broader class of component design engineers,’ he adds.

There are various challenges in the MEMs industry, owing to both the required process craftsmanship seen in advanced devices and the sheer proliferation of device types. Morrow observes: ‘Foundries continue to address these challenges through process capability improvement, and are benefitting from a maturing design process ecosystem that understands the need for integration with manufacturing, particularly in high-volume segments such as inertial sensors, microphones, and optical MEMs. Lower volume products, highly specialized device types, unique packaging or ASIC integration requirements seem to support IDM-type manufacturing.’

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