Next-generation packaging for fiber optics & MEMS
01/01/2002
New opportunities for thick film and LTCC
BY S.J. HOROWITZ, D.I. AMEY, B.E. TAYLOR and M. DOYLE
Next-generation packaging for fiber optic and MEMS modules will require improved levels of mechanical, thermal and environmental stability combined with increased capability to integrate electrical and optical functions. Additionally, all of this must be delivered to the market faster and at lower system cost. Thick film and low temperature co-fired ceramic (LTCC) materials, effective in harsh-environment military and high-volume automotive and wireless applications, possess the required attributes. This article reviews the properties of thick film and LTCC packaging and interconnect materials compared to the needs for packaging fiber optic and MEMS modules.
Thick Film and LTCC Technology
Thick film and LTCC materials have a long history of meeting the most demanding electronic packaging and interconnect requirements.1,2 The technology is well-established for high-performance low-volume applications, such as those in military and space applications, and high-volume, cost-sensitive applications, such as those in the automotive and portable wireless industries. Advances in IC technology drive packaging and interconnect technology by increasing the number of input/output connections, thermal dissipation and the physical size of die. Thus, cost-effective packaging solutions must have fine-feature patterning capability, high thermal dissipation and the flexibility to reliably interconnect bare and packaged ICs using a variety of technologies. Innovative integrated packaging will require mixing analog, digital, RF, MEMS and optical technologies and embedding passive components and functions within multilayer substrates. Advanced thick film and LTCC materials meet many of these requirements today, and can become an enabler for fiber optic and MEMS packaging.
 Table 1. Needs for fiber optic and MEMS packaging. |
Thick film materials are mixtures of fine metal, glass and/or ceramic powders dis-persed in an organic medium that is applied to a non-conducting substrate by a process such as screen printing.1 The choice of the inorganic phase determines the functionality of the thick film composition. Metal or metal alloy inorganic phases form conduc-tors, and metal alloy or ruthenium com-pounds form thick film resistors. Glass or glass-ceramic inorganic phases are used to form multilayer dielectrics, encapsulants or high dielectric constant capacitor layers. By combining successive prints of conductor and dielectric, complex multilayer intercon-nects can be constructed. Adding resistors, capacitors or coplanar or multilayer induc-tors enables integral components or func- tions, such as filters used in RF applications. Substrates are typically 96 percent or 99 percent alumina, firing at 850 to 900°C in a controlled profile in a conveyor belt furnace. This "low" firing temperature allows use of high conductivity metallurgies. AlN and BeO substrates are also used in some specialized applications.
 Figure 1. The market for optical components was about $2B in 1999, growing to $14B in 2004, based on the mid-year 2001 forecast. |
The LTCC material we have worked with is composed of inorganic materials with chemistry similar to a thick film dielectric, but it is cast as a film rather than formulated into a screen printable composition.2 In the LTCC process, an unfired tape replaces the screen printable dielectric composi-tion. The tape is cast from a slurry of the same inorganic components used to formulate thick film dielectric composi-tions. Each layer of tape is blanked to size and registration holes are punched. Vias are formed in the dielectric by laser or mechanical punching or drilling, and the conductor traces and via fills are screen printed or photo-defined. Including resistors, capacitors or inductors enable integral components or functions that, in turn, reduce the number of discrete passives that need to be placed. When all layers have been punched, printed and inspected, the tape layers are registered, laminated and co-fired. The co-firing process (firing the dielectric and conductor at the same time) involves fewer firing steps than the conventional sequential thick film process. Larger sized panels, Ag-based conductor metallurgy, and constrained sintering2 continue to lower costs.
The Market for Optical Components
Deregulation of telecommunication operations in most developed nations and technological innovation have facilitated widespread use of the Internet, e-mail, data transmission, cable networks and wireless telephony. These developments have driven an exponential growth in user traffic and fueled a strong demand for greater network capacity or bandwidth. To meet this demand, telecommunications service providers have been installing optical networks because they provide greater capacity more cost effectively than traditional copper networks. The extensive deployment of optical networks has increased demand for optical and optoelectronic components, as well as packaging technology to meet the unique needs of these new applications. According to market research firm RHK, the market for optical components was about $2B in 1999 growing to $14B in 2004 (Figure 1).3 This forecast represents the view at mid-2001 in a year of great volatility. These forecasts were revised downward again during the fourth quarter of 2001, but show renewed growth beginning in the second half of 2002.
 Figure 2. The dielectric constant of LTCC material tape, 99 percent and 96 percent alumina are invariant over a broad frequency range (GHz). |
The key needs of this rapidly growing market, from the perspective of packaging, are shown in Table 1. Fiber optic packaging requires broadband performance from DC to greater than 100 GHz, as well as high-density interconnect and excellent thermal performance. Precision alignment of optical components sets a new standard for dimensional and property stability. As optical and optoelectronic modules may require combining digital, analog and optical components using a variety of assembly technologies, and in some cases hermetic packaging, interconnect technologies that facilitate integration will become enablers.
Materials for Fiber Optic Packaging
A low-loss LTCC tape system was developed to meet the needs of broadband wireless applications, which are similar to the needs of fiber optic and optoelectronic packaging. Figures 2 and 3 illustrate some electrical properties of the material.4 Many PWB materials have high loss tangents at frequencies above a few GHz. Lower loss tangents impart higher performance at high frequencies. Conventional thick film hybrid circuit technologies and LTCC materials have the broadband performance required for fiber optic and optoelectronic packaging.
 Table 2. Characteristics of thick film and LTCC materials. |
Fine-line precision patterning is impor-tant because of its impact on the capability of a system to reliably and efficiently interconnect ICs with large numbers of I/Os. Fine-line precision patterning also impacts reproducibil-ity and property consistency, particularly at high frequencies.
 Figure 3. LTCC material tape has low loss performance to millimeter wave frequencies. |
Photo patterning screen printable conductors and dielectrics have been available for conventional hybrid circuit applica-tions on pre-fired 96 percent alumina substrates for many years. Recently that technology has been extended to pattern-ing on unfired LTCC tape to facilitate 50-µm lines and spaces on any layer.5
Figure 4 shows a co-fired Ag conductor patterned on the surface of an LTCC tape laminate. Photo-imageable LTCC tape is also available, offering parallel processing of vias and formation of channels and other features.6
 Figure 4. Fine line pattern made on LTCC material tape. The line pitch is 100 µm and can be patterned on any layer. |
Figure 5 compares the temperature coefficient of expansion (TCE) of materials used to manufacture integrated circuits with those of ceramic and organic packaging materials.7 LTCC, Al2O3 and other ceramic packaging materials have TCEs that are close to those of Si, GaAs and InP, while organic PWB materials have much higher TCEs. The close TCE match min-imizes mechanical stress and allows the use of larger die than possible with organic laminates. Minimizing thermal mismatch imparts increased mechanical integrity, decreased variation of properties with temperature, and increased ability to integrate analog, digital and optical technologies.
 Figure 5. Temperature coefficient of expansion (TCE) of materials used to manufacture ICs, packaging and interconnect. |
Laser sources generate heat during operation, yet they must operate within a very close temperature range to maintain fre-quency control in dense wave division multiplexing (DWDM) systems. Figure 6 compares the through-plane thermal con-ductivity of ceramic and organic printed circuit materials.8 Alumina substrates have 100X the thermal conductivity of organic PWB materials. LTCC has 20X the thermal conduc-tivity of organic laminates. (Alumina demonstrates 27 W/mK [90 to 123X organic], FR-4 is 0.22 to 0.3, and LTCC is 3.0 to 5.0.) Higher thermal conductivity simplifies thermal design and significantly improves circuit life and reliability. Thermal vias further enhance thermal performance.9,10
 Figure 6. Thru-plane thermal conductivity of ceramic and organic PWB materials. |
Many optical components today require hermetic packag-ing. While conventional technologies for hermetic packaging are expensive, low-temperature brazing combined with LTCC materials offer a lower cost alternative for achieving reliable hermetic packages.11 Cavities in LTCC structures allow parti-tioning of components requiring hermeticity, lower brazing temperatures allow inclusion of embedded passives, and elimination of plating reduces cost and the need to address environmental issues associated with plating chemicals.
 Figure 7. This Bluetooth radio circuit contains embedded passives (Courtesy of Ericsson). |
Thick film and LTCC packages are associated with advanced technologies for embedded passive components. Those advantages have been incorporated in a number of miniature multifunction modules for portable wireless applications where buried inductors and capacitors form filters and other functions. Figure 7 is an Ericsson Bluetooth radio circuit that achieved reduced size through inclusion of embedded passives in the design, and was configured for BGA mounting for a small footprint. Another LTCC Bluetooth design, developed by National Semiconductor, and also incorporating embedded passives, was shown to be one half the size and 20 percent less expensive than an equivalent designed for laminate technology.12
Conclusion
Table 2 shows the needs for fiber optic packaging compared to the properties of thick film and LTCC tape materials. Many opportunities exist to increase performance and decrease size with designs that integrate electrical and optical functions using LTCC technology. AP
References
- W. Borland, "Thick Film Hybrids," Electronic Materials Handbook, Vol. 1, ASM International, New York, 1989, p. 332.
- C.R.S. Needes, D.I. Amey and S.J. Horowitz, "Cost Effective Solutions for High Density Interconnect and RF Modules using Low Temperature Cofired Ceramic Materials," Proceedings of 2000 IEMT, April 2000.
- RHK, Telecommunication Industry Analysis.
- D.I. Amey and S.J. Horowitz, "Characterization of Low Loss LTCC Materials at 40 GHz," Proceedings of IMAPS 1999 Intl. Symp. on Microelectronics, 1999, pp. 89-93.
- M.A. Skurski et. al., "Thick-Film Technology Offers High Packaging Density," Microwaves and RF, February 1999.
- B.E. Taylor, L. Bidwell and A. Lawrence, "New Photoimageable LTCC Technology for Making a Wide Range of Ceramic Architectures and Circuits," Proceedings of the International Conference on High Density Interconnect and Systems Packaging, 2001, pp. 87-92.
- Don E. Harrison and Christy J. Moratis, "Ceramics, Glass, and Micas," Handbook of Materials and Processes for Electronics, Charles A. Harper, ed., McGraw Hill, New York, 1970, 6-12 through 6-42.
- D.I. Amey et. al., "Low Loss Tape Materials System for 10 to 40 GHz Application," Proceedings of IMAPS 2000 Intl. Symp. on Microelectronics, 2000, pp. 654-658.
- J. C. Crumpton, V. E. Cofield and R. J. Bacher, "Through-hole Plugs and Thermal Vias," Proceedings of IMAPS 2000 Intl. Symp. on Microelectronics, 2000, pp. 325-329.
- T.R. Poulin and L.T. Nguyen, "Green Tape Via Design-FEM and Proposed Design Philosophy," Proceedings of the Electronic Component and Technology Conference, 1993, pp. 904-909.
- R. Keusseyan et. al., "Hermetic Packaging for Optoelectronic and High Frequency Applications," Proceedings of 2001 Nepcon Conference.
- Ray Brown, "RF Module Solutions for Wireless Applications," Workshop Notes 2000 IEEE MTT-S International Microwave Symposium, June 2000.
S.J. Horowitz, marketing manager, D.I. Amey, research fellow, B.E. Taylor, research fellow, and M. Doyle, marketing manager, can be contacted at DuPont Microcircuit Materials, Research Triangle Park, NC 27709; 919-248-5752; Fax: 919-248-5208; E-mail: [email protected].
null