Issue



SoP for Multifunctional System Packages


07/01/2005







THROUGH THIN FILM COMPONENT INTEGRATION IN CONTRAST TO SIP WITH STACKED IC MODULES

BY RAO TUMMALA AND P. MARKONDEYA RAJ

The role of packaging is changing from traditional interconnections of discrete components to thin film component integration. It started at Georgia Tech in 1993, and is now everywhere - on CMOS ICs as overlay, in SiP modules, and in SOP boards. The system-on-package (SoP) is a system concept with this package integration, in contrast to SiP, a modular concept. The SoP, a system-centric technology, is based on embedded thin film components in organic boards or packages, and together with SiP modules, SoC devices, and battery and user interface, leads to multi-functional systems in the short term and mega-function systems in the long run. It seeks to integrate disparate technologies to achieve multiple system functions into a single package, while providing ultra-small form factor. This is in contrast to SiP, which is IC-centric and starts with CMOS chips and their 3-D stacking in either bare-chip form or packaged-chips form, leading to modules. Thus, SiP is a subset of SoP. SoP accomplishes this by system-package-IC co-design with embedded thin film RF, optical, digital, and sensor components1,2, leading to component densities of more than 2,500/cm2.


Figure 1. PRC’s law for the systems of the next $1T industry.
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Alternate approaches, such as SoC, MCM, and SiP, depend on CMOS for system functions and packaging for wiring. The packaging role in these is the same as in the past - interconnect ICs or components. CMOS is suitable for transistors and bits, and for certain other components such as VCO and LNA, but is not an optimal platform for several other functional components like capacitors, inductors, filters, antennas, and optical waveguides. The SoP paradigm goes one step further in overcoming this fundamental limitation of SoC and SiP, leading to a new law for system integration (Figure 1). This concept leads not only to miniaturization, but also lower cost and higher performance and system reliability. It is also consistent with the emerging convergent computing, biomedical, communication, and consumer systems trend. SoP can be described as “the package is the system, not the bulky board.” Several barriers and challenges need to be addressed to realize this highly integrated functional package. Figure 2 shows the research focus at Georgia Tech’s Packaging Research Center (PRC), spanning from mixed-signal design, multiple-function fabrication and integration of RF, optical, digital and sensor components, mixed-signal test, and mixed reliability to address these barriers.


Figure 2. Research focus at PRC to realize SoP.
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SoP Integration for Computing Systems

New and advanced system integration concepts in signal and power integrity, EMI and ultra-fine-pitch packages with ultra-low loss materials enable multi-gigabit data transmission in the package, leading to new system architectures. To enable this, digital system integration at PRC is focusing on advanced dielectric, conductor, and embedded capacitor materials, including thin, low-loss dielectrics like BCB, A-PPE; 50- to 100-µm-pitch escape routing using 10- to 15-µm stacked microvia structures, and 5- to 10-µm ultra-fine lines, and novel, low CTE, high modulus and large-area composite substrates replacing conventional organic substrates.

The smaller noise tolerance at lower operating voltages of future high-speed processors necessitates low-impedance power supply (<0.5 mΩ) over broad frequency band. At high frequencies, it becomes important to place decoupling capacitors as close as possible to the fast switching load device. Surface mount ceramic components, current workhorses of the electronics industry, are bulky and ineffective for high-frequency decoupling above a few hundred MHz because of their excessive inductance. Thin oxide decoupling on the chip has the lowest inductance, but occupies valuable silicon real estate. PRC pioneered embedded decoupling in the package with high-k and thin nanocomposite dielectrics in the package in the mid-1990s. For higher capacitance densities, there is an increasing trend to integrate inorganic thin films into organic packages.3,4 High-k, high capacitance density ceramics films are generally deposited by high-temperature processing or costly vacuum technologies that are expensive and incompatible with organic packages, so low-cost chemical solution methods are more attractive. Different embedded capacitor technologies are compared in Figure 3. PRC has demonstrated embedding high-k thin film capacitors (>1 µF/cm2) over a large area with high yield in organic packages using low-cost hydrothermal and sol-gel barium/strontium titanate thin films.


Figure 3. Embedded decoupling with different technologies (left). Organic compatible BaTio3 thin film (200-nm) integration with hydrothermal synthesis (right).
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SoP for Wireless Communication Systems

The demand for increasingly higher rates of data, voice, and video drives RF technology to higher frequencies. Emerging high-performance applications as personal communication networks, wireless local area networks (WLAN), and RF-optical networks impose stringent specifications never addressed before in noise, linearity, power consumption, size, weight, and cost. The wireless integration limits of Si are better handled by SoP. RF components such as capacitors, filters, antennas, switches, and high-frequency and high-Q inductors are best fabricated on the package rather than on silicon. Most organic materials have increased losses at high frequencies, making it extremely difficult to build high-performance, low-cost mm-wave front ends. For example, FR-4 is well-known for its high loss above 10 GHz. Current RF component integration is confined to thick film LTCC and LCP packages, owing to the design rules and integration capability.

High-density component integration on low-cost organic platforms is best handled with ultra-low-loss, thin dielectrics like LCP and BCB. Stringent thermal stability requirements (TCC and TCR <20 ppm/°C) and ultra-low loss for dielectrics confine the choice of passive component materials to relatively few systems. Resistors represent up to 50% of the passive components on the board. Candidates for embedded resistors in the low resistivity range include NiCr, CrSi, Ni-P, and TaN films, while polymer thick films still appear to be strong candidates for higher resistivity needs. Embedded RF functions with various thin film passive components like TaN transistors and Ta2O5 capacitors have been demonstrated.5,6 High-Q on-chip inductors are difficult to accomplish because of the high parasitic capacitance of thin film dielectrics, high conductor resistance, and lossy substrate. Although, several semiconductor and MEMS packaging technologies are emerging to improve Q of on-Si inductors, the highest Q factors reported are much lower than the 250 to 500 achieved in the package.7 Fabricating an antenna directly on the package has the advantages of reduced feeder loss and size of the entire module. With the high-Q multilayer passives, shown in Figure 4, wideband and low-loss interconnects, board-compatible embedded antennas, reconfigurable modules using MEMS and efficient partitioning of MMICs, and electronic band gap structures for RF-digital noise isolation, PRC is moving toward complete RF system integration.


Figure 4. Examples of RF component integration at PRC.
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Multifunction Systems

New applications are calling for higher data processing and computing capability, resulting in a merger of communication and computing capabilities. Optoelectronics, which today finds use primarily in the back plane and is used for high-speed board interconnects, is moving onto the package as chip-to-chip, high-speed interconnections replacing copper, thereby addressing both the resistance and cross-talk issues of electronic ICs. The focus of optoelectronics SoP is the heterogeneous integration of optically active devices such as lasers, detector arrays, and laser amplifiers, and optical passives such as waveguides, gratings, and beamsplitters onto electrically interconnected mixed-signal SoP (Figure 5).2,8


Figure 5. Examples of optical component integration at PRC.
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An immediate application of SoP with RF/digital/opto-integration is a miniaturized, single package that can support computing, wireless, and consumer functions. Since SoP can also include MEMS-based sensors, in addition to digital and RF functions in one microminiaturized package, one can visualize a highly integrated multifunctional system like an “electronic pill” made of SoP technology that can be taken daily to sense, monitor, and analyze bodily fluids and functions, while communicating this information wirelessly and transparently to the body by local and satellite RF to anywhere in the world.

References

  1. R. Tummala, “SOP: Microelectronic Systems Packaging Technology for the 21st Century,” Advancing Microelectronics, Vol. 26, No. 3, May-June 1999, pp. 29-37.
  2. Rao Tummala, et al., “SOP for Miniaturized Mixed-signal Computing, Communication and Consumer Systems of the Next Decade,” CPMT Transactions on Advanced Packaging, May 2004, pp. 250-267.
  3. R. Ulrich, L. Schaper, “Decoupling with Embedded Capacitors,” CircuiTree, July 2003, Vol. 16, No. 7, p. 26.
  4. Devarajan Balaraman, et al., “Exploring the Limits of Low Cost, Organics-compatible High-k Ceramic Thin-films for Embedded Decoupling Applications,” Accepted for 55th IEEE Electronic Components and Technology Conference, May, 2005, Orlando, Fl.
  5. Kai Zoschke, et al., “Thin Film Integration of Passives - Single Components, Filters, Integrated Passive Devices,” Proceedings, 54th Electronic Components and Technology Conference, 2004, pp. 294-301.
  6. Carchon, G., et al., “Multilayer Thin-film MCM-D for the Integration of High-performance RF and Microwave Circuits,” Components and Packaging Technologies, IEEE Components, Packaging and Manufacturing Technology Transactions, Part A: Packaging Technologies, IEEE Transactions on Volume 24, No. 3, Sept. 2001, pp. 510 - 519.
  7. S. Dalmia, et al., Liquid Crystalline Polymer Based Lumped-element Bandpass Filters for Multiple Wireless Applications, 2004 IEEE MTT-S International Microwave Symposium Digest, 2004, pt. 3, p 1991-4 Vol. 3.
  8. Z. Huang, et al., “Embedded Optical Interconnections Using Thin Film InGaAs MSM Photodetectors,” Electronics Letters, Vol. 38, 2002, pp. 1708.

RAO TUMMALA, Ph.D, professor and director, and P. MARKONDEYA RAJ, research engineer, may be contacted at Georgia Institute of Technology, Packaging Research Center, 813 Ferst Drive, NW, Atlanta, GA 30332-0560; e-mail: rtummala@ece.gatech.edu and raj@ece.gatech.edu.