Characterization of Low Loss LTCC Materials at 40 GHz
DuPont Microcircuit Materials
Research Triangle Park, NC
The dramatic increase in the application of microwave technologies due to the growth in wireless communications has created many challenges for interconnect and packaging technologies. The majority of wireless equipment has used conventional organic printed circuit board technologies, with some extension in their capabilities to handle higher frequencies and lower cost. It is becoming increasingly clear, however, that such technologies do not address all the commercial and technical needs of the market. Specific issues relate to the RF performance of circuits due to limitations in environmental and dimensional stability of organic materials. To address the needs of higher performance applications a new low loss, low temperature cofired ceramic (LTCC) system has been developed based on an alumina-filled glass and designed for ease of processing and cofired silver compatibility.1 Data on the properties of this new system, 943-A5 Green Tape, at frequencies to 23 GHz, were described and benchmarked against 99 percent alumina and other low loss ceramic material. The emergence of higher frequency wireless applications, such as local multipoint distribution systems (LMDS) in the 27 to 37 GHz frequency range, and the introduction of new interconnection materials, resulted in the need to characterize the material at higher frequencies and to benchmark its properties. Testing was undertaken to characterize the materials to 40 GHz.
943-A5 GREEN TAPE
Green Tape 943-A5 is a Au, Ag and mixed metal compatible low loss, low thermal coefficient of expansion (TCE), lead-free glass/ceramic tape. Typical key properties are listed in Table 1.
LTCC materials have an excellent combination of properties, especially for high frequency applications where stable, uniform properties over a broad frequency and environmental range are required. This performance has been shown in testing of the 951 Green Tape system benchmarked with PWB materials exposed to 85°C/85 percent RH.2
HIGH FREQUENCY TESTING
Previous high frequency characterizations were performed with metallized microstrip T-pattern resonant circuits.3 Testing up to 20 GHz was performed using this method to compare the low loss system attenuation with prior data obtained for other materials. For the 40 GHz data the testing was performed using an open resonator measurement technique on unmetallized samples for ease of testing, and lower frequency data was obtained on the same samples using a resonant cavity method.
Figure 1 shows the attenuation data from the T pattern testing for 951 Green Tape with cofired silver conductor, 943 Green Tape with cofired silver conductor, unpolished 99 percent alumina with thin film gold over Ti/W conductor and glass reinforced polytetrafluoroethylene (PTFE) printed wiring board with one-ounce copper clad conductor. In this testing the conductor widths for the tape and thin film samples were comparable, while the PTFE board, due to its low dielectric constant, required a greater line width to maintain a 50 Ω impedance for the network analyzer measurement. Note that this difference favors the PWB material, for if comparable line widths were used, the PWB conductor attenuation, and hence the overall attenuation, would be greater than shown.3
Measurements in the 1 to 5 and 15 to 25 GHz range were made as benchmarks for the higher frequency dielectric characterization. Measurements below 5 GHz were made using a Damaskos Model 601 Thin Sheet Rectangular Cavity.4 The measurements from 15 GHz to 40 GHz were made using a Damaskos Model 600T Open Resonator. An HP 8510B network analyzer was used for measurements below 26.5 GHz and the 40 GHz measurements used a Scientific Atlanta 1783 receiver, Hughes millimeter-wave mixers and an HP 8762A synthesized signal generator driving a multiplier. All testing was performed under laboratory ambient conditions and all samples were 0.025" to 0.031" thick.
Figure 2 shows the loss tangent of the low loss 943 Green Tape vs. 96 percent and 99 percent alumina, and two of the newer PWB dielectrics being offered as cost-effective options for wireless applications rather than the more expensive conventional microwave laminates. It is apparent that 943 Green Tape dielectric loss is much lower than the polyester/glass dielectric loss and is comparable with the PTFE/ceramic dielectric, as well as the alumina materials. Note that the tape materials have higher thermal conductivity and much more stable environmental properties than the organic materials. Figure 3 shows an expanded scale plot of the loss tangent omitting the higher loss polyester/glass material.
WIRELESS APPLICATION OF 943 GREEN TAPE
National Semiconductor Corporation performed an evaluation of a variety of LTCC dielectric and conductor materials and construction techniques.5 A resonator structure was used and test results in terms of Q at a resonance in the 2 GHz range were examined to define the highest performance resonator structure in terms of both physical construction and the material system. The 943 Green Tape system was shown to provide the highest performance in this resonator application in the 2 to 3 GHz range with a Q of twice that of the baseline design using 951 Green Tape. Figure 4 shows these Q measurements indicating the improvement due to the low loss tape. As part of this evaluation, National Semiconductor used the 943 Green Tape to duplicate an 850 MHz frequency synthesizer circuit, which has been routinely fabricated using 951 Green Tape. No problems were experienced in production with the new tape. However, their performance evaluation did not show a significant difference between the two materials since the conductor loss, not dielectric loss, is dominant at this frequency.
Ceramic technology solutions have been described which provide benefits in enhanced performance and lower cost in the 1 GHz to 40 GHz ranges required for evolving wide bandwidth communication systems. Testing performed to characterize a new low loss 943 Green Tape LTCC material at 40 GHz has been described and its properties compared to 96 percent and 99 percent alumina, 951 LTCC Green Tape, and PTFE and polyester-based organic dielectric materials for microwave applications. Additional information may be obtained at www.dupont.com/mcm.
1. P.C. Donohue, et al., "A New Low Loss Lead Free LTCC System for Wireless and RF Applications," Proceedings of the 1998 International Conference on Multichip Modules and High Density Packaging, 1998, pp. 196199.
2. D.I. Amey and S.J. Horowitz, "High Frequency Performance of Ceramic and Printed Wiring Materials Under Varying Temperature and Humidity Conditions," Proceedings of the 11th European Microelectronics Conference, 1997, pp. 269276.
3. D.I. Amey and S.J. Horowitz, "Tests Characterize High Frequency Material Properties," Microwaves & RF, August 1997.
4. Damaskos Inc., P.O. Box 469, Concordville PA, 19331, 610-358-0200.
5. M.R. Ehlert and R. Draudt, "Performance of LTCC Materials Systems in 2 GHz Resonator Applications," Proceedings of the International Conference on High Density Packaging and MCMs, 1999, pp. 279283.
DuPont Microcircuit Materials, Research Triangle Park, NC (919) 248-5752.
Circle No. 304