Beyond Moore’s Law Print E-mail
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Written by W.R. Bottoms, Ph.D.   
Monday, 31 March 2008 19:00

SiP-based system-level integration resolves CMOS scaling limits.

The electronics industry is nearing the limits of traditional CMOS scaling. Although predictions that Moore’s Law has reached its limits have been heard for years, they have proved premature. We are now, however, nearing the basic physical limits to CMOS scaling, and the price elastic growth of the industry can no longer continue based solely on Moore’s Law scaling. New materials and device architectures in development will eventually provide a path to increased density, increased performance and lower cost beyond the capability of CMOS-based circuits. There will, however, be a time lapse between the slowing of traditional CMOS scaling and the rollout of architectures and materials that can support Moore’s Law scaling. In the meantime, as scaling becomes more difficult, packaging innovations are taking up the slack (Figure 1).

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The International Technology Roadmap for Semiconductors (ITRS) defines functional diversification, incorporating system-level functions into a single package, as “more than Moore.” This approach enables continued rapid progress in functional density during a period where traditional CMOS scaling cannot keep pace and new architectures are not yet ready. A second key contribution of packaging to maintaining the pace of functional density scaling is 3-D integration. Both these innovations are accomplished through integration of multiple circuit types into a single device using system-on-chip (SoC) and system-in-package (SiP) technologies (Figure 2). As electronics becomes more consumer-dominated, the most important of these will be SiP. ITRS defines SiP as “a combination of multiple active electronic components of different functionality, assembled in a single unit that provides multiple functions associated with a system or sub-system. SiPs may optionally contain passives, MEMS, optical components and other packages and devices.”

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SiP technology enables the efficient use of three dimensions through innovation in packaging and interconnect. The result supports continued increased functional density and decreased cost per function. Although there will be some applications where SoC represents the better alternative, SiP provides advantages over SoC in most market segments. The importance of these advantages varies with different applications. They include:

  • Small and custom form factors.
  • Decreased weight.
  • Reduced power consumption.
  • High functional density.
  • High frequency operation.
  • Large memory capacity.    
  • High reliability.
  • Low package cost.
  • Lower development cost, greater integration flexibility, lower NRE cost and lower product cost compared to SoC.
  • Rapid time to market.
  • Wireless connectivity (GPS, Bluetooth, cellular, etc.).

SiP is not a replacement for the high-level, single-chip, silicon integration of SoC. It is complementary, and some complex SiP products will contain SoC components. SiP technology is evolving from a specialty used in a narrow set of applications to a high-volume technology with wide-ranging impact on electronics. The broadest adoption of SiP to date has been for stacked memory/logic devices and small modules used to integrate mixed-signal devices and passives. Both these applications are driving high volume in very cost-competitive markets. SiP has rapidly penetrated many market segments, including consumer electronics, mobile devices, automotive controls and sensors, computing, networking, communications and medical electronics. SiP’s benefits vary by market segment but share some elements. Time-to-market, size, power requirements and cost have resulted in SiP’s strongest initial penetration: mobile communications. Unit shipments have been rising at approximately 25% per year; this growth is forecast to continue.

Challenges for SiP

Traditional single-chip packaging and system-level interconnect have limitations in interconnect density, thermal management, bandwidth and signal integrity that can be met only with new approaches. SiP technology is the most important new technology to address these limitations. Nonetheless, there are still a number of challenges, the most critical of which are:

  • Interconnection capable of maintaining power integrity for actives.
  • Performance and reliability of electronic systems are limited by the ability of on-chip and off-chip system-level interconnections to maintain power integrity during operation.
  • Interconnect inductance, high current requirements, increasing frequency and decreasing operating voltage all increase the difficulty.

SiP technology enables improvement in each of these parameters, but challenges must be addressed if SiP is to meet its potential.

Thermal dissipation. Inadequate thermal dissipation imposes the most serious bottleneck to SiP performance. Not only does the thermal dissipation technology dictate the chip junction temperature and subsequently its performance, the thermal technology’s size and cost will limit packaging density, size, cost and performance of SiP-based products. Thermal dissipation is also the key limiter to 3-D stacking of microprocessors and other high-power/density ICs.

Signal bandwidth. Even though bandwidth is often better than for single-chip packages, inadequate chip I/O bandwidth is the third serious challenge to the realization of ultimate performance. Losses resulting from package substrate properties, crosstalk and impedance mismatches are exacerbated as off-chip bandwidth per channel increases and signal noise budgets decrease. Perhaps the greatest issue is the inability of the small transistor to drive off chip impedance at high speed. SiP technologies address these limitations and offer major improvements, but much development work remains.

SiP Evolution

SiP technology builds from the state-of-the-art in single-chip packaging, with its advanced wirebond and flip-chip processes, by integrating new technologies to support system-level integration. Emerging technologies that will be combined with this base include wafer-level packaging, die stacking, package stacking, through-silicon vias (TSV), 3-D packaging, printable circuits, thinned wafers, and embedded actives and passives. The current technical solutions for 3-D packaging include wire bonding, face-to-face bonding and multilayer TSV structures (Figure 3).

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Combining these technologies into SiP devices provides a mechanism for cost-effective functional diversification. These technologies enable SiP to provide the necessary continuous increase in functional density and decrease in cost per function. Market demands will result in the integration of more components (e.g., passives, MEMS, optical and even bio components) into a single package. The long-term vision for SiP is the optimized heterogeneous integration of wireless, optical, fluidic, bio elements/interfaces, as well as integrated shielding and heat sinks. This goal requires new materials and control of their interactions on the micrometer and nanometer scale.

Numerous concepts for 3-D SiP packaging are emerging, driven largely by the demands of portable consumer products. One of the most important is wafer-level packaging. WLP is an emerging technology, used for both single-chip packaging and SiP, where all elements of a package are within the boundary of the die and all packaging processes performed prior to wafer singulation into individual circuits. WLP development was motivated by the recognition that WLP technology (i.e., parallel processing on the wafer) addresses the need to increase performance and functionality, while reducing system size, power and cost. WLP technology with and without a redistribution layer (RDL) is used for a variety of products where the small size, thickness and weight are important product differentiators. This technology will provide significant cost reductions as it matures and production volume increases.

The combination of WLP and wafer/die stacking approaches leads to a large number of variations in WLP technology used for SiP. The highest levels of integration are achieved through 3-D packaging. Die stacking has been used for consumer products, such as cellphones, for several years, with wire bonding used to connect the stacks to the package substrates. An important new technology is through-silicon vias, which allow more efficient die stacking and 3-D integration. These developments lead to more complex packages for both single and multi-die WLPs (Figure 4).

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The use of TSV as a base for SiP requires solutions for both the thermal density associated with a “cube” of transistors (rather than the planar array of traditional CMOS) and the incorporation of passive devices required for system-level integration. Prototypes have been developed addressing both requirements. Microfluidic components with a form factor suitable for lamination into a device stack with TSV interconnect have been fabricated. Passive networks have also been fabricated that are compatible with TSV interconnected die stacks (Figure 5).

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Innovations in SiP and WLP technologies depend on the integration of progress in materials and equipment made in all segments of the industry. The successful integration of all of these elements provides a rich portfolio of capabilities in the era of “more Moore” (continued CMOS scaling) and “more than Moore” (the addition of functional diversification). Some of the advanced packaging elements include:

  • New materials such as nanoparticles to lower processing temperatures and nanotubes for improved thermal and electrical conductivity.
  • High-density, low-cost packaging substrates.
  • Wafer thinning, singulation and handling.
  • Embedded and integrated passives and actives.
  • Co-design tools.
  • Equipment for advanced packaging.

Innovations in SiP have been accelerating, as this technology becomes a major enabler for a large class of products in the consumer-driven marketplace. Many issues remain that require continued research and development. Today we have not proven the reliability of package-level system integration for complex systems; we do not have a proven strategy for repair and rework of SiP-based products; and we have not resolved the test access and test contactor challenges associated with the high frequency of future devices that will exceed 15GT/s.

As 3-D SiP packaging architectures evolve, advanced co-design tools linked with modeling and simulation capability must be in place to facilitate an effective collaborative environment between system, device and packaging engineers. New materials must be developed to meet the requirements of these new SiP architectures and for meeting (changing) environmental regulatory requirements. As an integrator of components and technologies from different areas, SiP will become the primary architecture for high-value, system-level products for consumer products, before proliferating into products in all major market segments.

W.R. (Bill) Bottoms, Ph.D., is chairman and CEO of NanoNexus (nanonexus.com) and chairs the Packaging Technology Working Group (TWG) for the iNEMI Roadmap and the ITRS Roadmap for Assembly and Packaging; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

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