Microwave Characterization and Modeling of Multilayered Cofired Ceramic Waveguides Microwave Characterization and Modeling of Multilayered Cofired Ceramic Waveguides Daniel Stevens and John Gipprich Northrop Grumman Corporation Electronic Sensors and Systems Division P.O. Box 1521, MS 3K11 Baltimore, Maryland 21203 Phone: 410-765-2832 Fax: 410-765-2116 e-mail: Stevens.Dan@postal.essd.northgrum.com Abstract This paper examines, with the aid of an Electromagnetic (EM) Field Solver (High Frequency Structure Simulator, HFSS 1 ) the performance of via sidewall rectangular waveguide structures in a cofired ceramic substrate, and compares the modeled results to the modeled performance of a conventional solid conductor waveguide. The comparisons are made on the basis of insertion loss, reflection loss, and waveguide cutoff frequency. In addition, HFSS simulations were performed to determine the crosstalk between two adjacent waveguides that share a common metal via fence sidewall, as well as two adjacent waveguides with separate, closely spaced, via sidewalls. In order to facilitate testing, a transition from stripline to cofired ceramic waveguide was developed. Finally, the authors present measured results of a via sidewall rectangular waveguide structure fabricated as a Low Temperature Cofired Ceramic (LTCC) substrate, which demonstrate very good agreement with the modeled performance. Key words: Waveguide, W/G resonators, X-band, LTCC, and EM Simulation. 1. Introduction Cofired ceramics have found increasing acceptance in the packaging of various microwave integrated circuits. One reason for this increased usage is that the electrical properties of cofired ceramics have reached the point that microwave transmission lines and other planar microwave structures (such as couplers and filters) can be fabricated with reasonably low Radio Frequency (RF) losses. Due to the way cofired ceramics are processed and fabricated, these microwave transmission line circuits and structures have been limited primarily to planar configurations. These structures are typically realized as strip transmission lines, such as microstrip lines, coplanar waveguides, and buried striplines. However, the RF losses of these strip transmission lines, while reaching tolerable levels, are still much higher than that of most traditional microwave substrates (such as Duroid 2 and ceramics). In this paper, the researchers investigate the feasibility of using waveguide structures in cofired ceramics since such structures, due to their wider conductors, offer lower RF losses than strip transmission lines, particularly at higher microwave and millimeter wave frequencies. This difference in loss becomes even more significant for applications requiring a small ground plane spacing. Figure 1 shows a comparison of rectangular waveguide and 50 ohm stripline loss for various ground plane spacings. 43
Intl. Journal of Microcircuits and Electronic Packaging 1.2 Freq=10 GHz, ger=6, r=6, Tand=0.002, Rho=3 1.0 0.8 0.6 0.4 Stripline 0.2 Waveguide 0.0 10 20 30 40 50 60 70 80 90 100 Figure 1. Loss (db/in) vs ground plane spacing (mils). Figure 2B. Simulation of solid conductor W/G. 2. Conventional Rectangular Waveguide Conventional rectangular waveguide 3 consists of four solid conductor walls, a top and a bottom conductor, and two vertical sidewall conductors. A typical rectangular waveguide, of horizontal dimension A, vertical dimension B, and length L, is shown in Figure 2A. A ceramic filled waveguide of g r = 6.1, A = 0.25, B = 0.10, and L = 0.70 has a cutoff frequency for the dominant TE 10 mode of 9.6 GHz, allowing propagation of Ku Band frequencies (12-18 GHz) with minimal RF losses. The next higher order mode, the TE 20, is cutoff for frequencies below 19.3 GHz. Figure 2B shows an EM simulation of the frequency response of a solid conductor waveguide with these dimensions. B L 3. Cofired Ceramic Rectangular Waveguide A rectangular waveguide may be constructed in cofired ceramic with two parallel planar conductors serving as the top and the bottom waveguide conductors, connected together with two metal filled via fences that serve as the sidewalls of the waveguide. Figure 3A shows this construction. If the spacing of the vias within the via fence is less than the 1/10 of the guide wavelength, then a negligible amount of the RF signal escapes the guide structure, resulting in low RF transmission losses. If the via spacings are too large, then a significant potential difference can develop across adjacent vias resulting in radiation outside the guide structure. Figure 3B shows an EM simulation of the frequency response of the via sidewall waveguide with g r = 6.1, A = 0.25, B = 0.10, and L = 0.70. The via diameter is 0.006 and the via spacing within the via fence is 0.03 (center to center), or approximately 1/10 the guide wavelength. The frequency response of the via sidewall waveguide agrees closely with that of the solid sidewall waveguide of Figure 2B. It was found that a spacing of 0.25" between the inside edges of the sidewall via fences produced the same cutoff frequency as the solid conductor waveguide. A Figure 2A. Solid conductor waveguide model. Figure 3A. Via sidewall waveguide model. 44
Microwave Characterization and Modeling of Multilayered Cofired Ceramic Waveguides The simulation for Figure 3A used radiation boundaries placed at the outer sides of the ceramic substrate in order to absorb any signal energy escaping the waveguide structure. Figure 3C shows a plot of the magnitude of the electric field as the wave propagates along the guide. It can be seen that the fields are well contained by the via fence sidewalls. copper. Figure 4A shows the model of the solid sidewall waveguide resonator, and Figure 4B shows the resonant response. Figure 4A. Solid wall W/G resonator model. Figure 3B. Simulation of via sidewall W/G. Figure 4B. Solid wall W/G resonator simulation. Figures 5A and 5B show the model and response of the via sidewall waveguide resonator using 0.006 diameter vias spaced 0.02 apart. An additional set of simulations were also performed to the above parameters except with a reduced waveguide height, B, of 0.02". Figure 3C. E of via sidewall waveguide. 4. Via Sidewall Rectangular Waveguide RF Losses In order to quantify the effect of a via sidewalls on the RF losses of a cofired ceramic waveguide, a half-wavelength resonator4 was characterized for a solid conductor waveguide and the via sidewall waveguide. The resonators were designed for a resonant frequency of 10 GHz using A = 0.36", B = 0.18", gr = 6.1, and an iris spacing of 0.32". The iris opening selected was 0.04". The simulation used a dielectric loss of 0.002 and a metal resistivity three times that of Figure 5A. Via sidewall W/G resonator model. 45
Intl. Journal of Microcircuits and Electronic Packaging 5. Crosstalk Between Two Adjacent Waveguides Figure 5B. Via sidewall W/G resonator simulation. Table 1 summarizes the resonator simulation results, including the resonant frequency, the 3dB bandwidth, the loss, the unloaded Q, and the attenuation in db/inch. Equations (1) and (2) were used for the calculation of unloaded Q and attenuation. From Table 1, it can be seen that the via sidewalls add little, if any, to the RF losses for vias spaced less than one-tenth the guide wavelength. In this Table, Tand = 0.002, and Rho = 3 copper. Crosstalk between adjacent transmission lines is an important issue for RF assemblies packaged in cofired ceramics. The ability to obtain at least 40 db of isolation is frequently necessary for many applications. For strip transmission lines, it is common practice to enclose RF signal conductors within via fences, or to place a via fence between two adjacent conductors, in order to provide the needed isolation. For even higher isolation, a double via fence can be used, as well as separating adjacent conductors electrically far apart to reduce coupling. The HFSS model shown in Figure 6A was examined to determine the crosstalk between adjacent waveguide structures using a common via fence sidewall. This waveguide has dimensions of A = 0.25", B = 0.10", a length of 0.50" and an g r = 6.1. The via diameter is 0.006" and via spacings of 0.02, 0.04 and 0.06 were simulated. Table 1. Summary of waveguide resonator simulation results. Wall type B dim. F RES, GHz B 3dB, MHz Loss @ Fc Q U Atten (db/in) Solid 0.18 9.95 23.5 32.7 433 0.197 Via 0.18 9.92 23.9 28.4 432 0.197 Solid 0.02 9.93 43.4 29.9 237 0.360 Via 0.02 9.91 43.7 30.0 233 0.364 Q U =Q L /(1- S 21 ) (1) Figure 6A. Common via sidewall crosstalk model. where Q L =F RES /B 3dB and Attenuation = 8.686p / Q U L, (in db per unit length) (2) Figure 5C shows the loss of the waveguide for various spacing of the vias used to construct the waveguide sidewalls for an X-band structure. It can be readily seen that a via spacing up to 30 mils (center to center) provides similar loss to a solid sidewall waveguide (that is a spacing = 0 mils). Figure 6B. Common via fence crosstalk simulation. Figure 7A shows a similar model except it uses two separate via fences, separated 0.02" apart, between the adjacent waveguides. Simulation results are shown in Figures 6B and 7B, for the common via sidewall and the separate via sidewall models, respectively. For a 0.02 via spacing, the common via fence sidewall provides approximately Figure 5C. W/G loss vs sidewall via spacing. The International Journal of Microcircuits and Electronic Packaging, Volume 22, 50 Number db isolation 1, First between Quarter adjacent 1999 (ISSN waveguides 1063-1674) while 46
Microwave Characterization and Modeling of Multilayered Cofired Ceramic Waveguides the separate via fence sidewall provides approximately 80 db isolation 5. Figure 8A. Stripline to W/G transition model. Figure 7A. Separate via sidewall crosstalk model. The height of the top quarterwave waveguide was subsequently increased to B = 0.05" in order to raise its impedance and thereby minimize its effect on the frequency response of the matched transition. Figure 8B shows the response of the matched transition. Figure 7B. Separate via fence crosstalk simulation. Figure 8B. Stripline to W/G transition model. 6. Cofired Ceramic Waveguide Prototype A stripline to waveguide transition was designed to facilitate testing of a via sidewall waveguide structure. A waveguide height of B = 0.011" was chosen since it would provide a sufficiently high loss that could be accurately measured. An E-plane probe transition was attempted, however this proved very unefficient in coupling to the reduced height waveguide. Instead, an end-feed transition from stripline to waveguide was developed. In this transition, the stripline conductor is connected directly to the common center conductor of two stacked waveguides. Figure 8A shows the stripline to waveguide transition. If the stripline ground plane spacing is made equal to twice the waveguide height, then the ground plane step discontinuity at the stripline to waveguide junction is eliminated. In order to couple the signal into the lower waveguide, a short was placed in the top waveguide a quarter of the guide wavelength from the stripline to waveguide junction. A prototype via sidewall rectangular waveguide was fabricated in Low Temperature Cofired Ceramic (LTCC) in order to validate the performance of the proposed waveguide structure. A rectangular waveguide with dimensions A = 0.36" and B = 0.011" was built using 0.006" diameter vias spaced 0.02" apart for the sidewalls. Figure 9A shows the layout of the LTCC test substrate used for the prototype waveguide. The test circuit is a length of reduced height waveguide 2.0 inches long with two transitions to stripline at each end. Figure 9A. Prototype LTCC W/G w/transitions. 47
Intl. Journal of Microcircuits and Electronic Packaging Figure 9B shows the simulated performance and Figure 9C shows the measured response. The measured insertion loss is less than - 1.5 db over an 8.0 to 12.0 GHz frequency band and measured -1.2 db at 10 GHz. This value agrees closely to the simulated value of - 1.1 db based on the material properties of the LTCC material. References 1. High Frequency Structure Simulator (HFSS), Hewlett Packard Company, Westlake, California. 2. Duroid, Rogers Corp., Chandler, Arizona. 3. Samuel Laio, Microwave Devices and Circuits, pp. 102-119, Prentice Hall, 1990. 4. G. L. Mathaei, L. Young, and E.M.T Jones, Microwave Filters, Impedance Matching Networks, and Coupling Structures, pg. 243, Artech House, Dedham, Massachusetts, 1985. 5. H. Uchimura, T. Takenoshita, and M. Fuji, Development of the Laminated Waveguide, IEEE MTT-S Digest, pp. 1811-1814, 1998. About the authors Figure 9B. Prototype LTCC W/G simulation. S21 S11, S22 Daniel Stevens received his B.S. Degree from the Georgia Institute of Technology in 1983. He joined the Westinghouse Electric Corporation in Baltimore, Maryland in 1983, where he has worked in the area of high power solid state microwave transmitter design for airborne radar applications. In 1996, he joined Northrop Grumman s Electronic Sensors and Systems Division in Baltimore as a Senior engineer and is currently involved in the active aperture T/R module development Group. Figure 9C. Measured data for prototype W/G. 7. Summary and Conclusion Based on the results of the modeled and measured performance, the researchers have concluded that the waveguide structure with via fences serving as sidewalls is an acceptable alternate transmission line structure to strip transmission lines for cofired ceramic substrates. These waveguide structures can be embedded into multilayer cofired ceramic assemblies without significant crosstalk, and efficiently transitioned to other transmission line structures. In addition, these waveguide structures, due to relatively wider conductor widths, result in lower RF losses when compared to strip transmission lines with similar ground plane spacing. Similar results have been recently reported 5 for higher frequency waveguide structures. John Gipprich joined Westinghouse Electric Corporation in Baltimore, Maryland in 1959, as a participant in the Westinghouse/Johns Hopkins Work Study Program. He received his B. S. and M. S. Degrees in Electrical Engineering from Johns Hopkins University, in 1965 and 1971, respectively. Since 1963, Mr. Gipprich has worked in the antenna and microwave areas and has been involved in microwave circuit and subsystems design. In 1996, he joined the Northrop Grumman Electronic Sensors and Systems Division in Baltimore. Currently, he is an Advisory engineer in the active aperture module engineering Department and is responsible for T/R module development and microwave multilayer cicuit designs. Mr. Gipprich is a member of IMAPS, IMAPS National Technical Committee, and IEEE/MTT-S. In 1987, he served as Chairman of the Baltimore IEEE AP-MT Chapter. 48