INTEGRATED MICRO-COAXIAL PASSIVE COMPONENTS FOR MILLIMETER-WAVE ANTENNA FRONT ENDS

Kenneth Vanhille, Milan Lukic, Sebastien Rondineau, Dejan Filipovic, Zoya Popovic Department of Electrical and Computer Engineering

University of Colorado, Boulder, CO 80309-0425, U.S.A. vanhille@colorado.edu, zoya@colorado.edu

ABSTRACT Miniature 250 to 700-μm tall copper air-filled rectangular coaxial line with inner conductor supported by periodic dielectric straps is demonstrated. The measured loss is as low as 0.08dB/cm at 37GHz, while the isolation between two parallel lines with center-to-center separation of 350μm is better than 60dB. A number of quasi-planar components are designed to be fabricated on the same 6-inch wafer using a sequential micro-fabrication process with 5 to 10 layers of copper. This paper presents an overview of several components that have been demonstrated in the air-micro-coax technology: low-loss densely-packed lines with a range of characteristic impedances; patch-type antenna elements; branch-line hybrids; cavity resonators; a Buttler matrix beamforming network. In addition to these components, the paper also discusses dividers/combiners and broadband hybrids, with an overview of the limitations and advantages of the technology. KEY WORDS Coaxial cables, millimetre-waves, directional coupler, low loss, microfabrication, wafer-scale integration. 1. Introduction Coaxial lines offer low loss, good shielding and isolation, and small form factor for low and medium-power applications, but their performance degrades with frequencies increasing into the millimetre-wave range. At W-band, the loss of standard 1-mm coaxial cable is a fraction of a dB/cm and the cable is not electrically thin. Assuming the dielectric loss is much smaller than conductor loss, the approximate well-known expression for the attenuation coefficient in terms of distributed transmission line parameters is

rLCR εα ∝≈

''

2'

,

even when the dielectric inside the cable is perfect. Thus, for a given size and metal, an air-filled coaxial line will have the lowest loss [1]. Such a line can be fabricated using standard machining techniques with limited

bandwidth and frequency limitations [2], depositing a large number of nickel layers to form micro-rectangular coaxial cables [3-5], or using sequential deposition of high-aspect ratio copper layers [6]. The latter method, referred to as Polystrata, was demonstrated to give good performance for several Ka-band components [7-10]. This paper focuses on an overview of a library of Polystrata-based components designed for the frequency range from 20 to 100GHz. 2. Basic properties of Polystrata coaxial lines The micro-rectangular coaxial structures are fabricated by deposition of a small number (5-10) uniform copper layers which are 10 to 50μm thick. The middle layer contains the center conductor supported on thin periodic dielectric straps. The sacrificial photo-resist is drained through release holes in the top and side walls (Fig. 1).

Figure 1. SEM photographs of a portion of a Polystrata wafer with a multitude of micro-coaxial lines and components. The high-aspect ratio 50-um thick copper layers are seen clearly.

200 μm200 μm200 μm

Figure 2. Schematic of Polystrata microcoaxial line, showing the copper layers which comprise the outer and inner conductors, the dielectric support periodically spaced at around 300um, and periodic resist-drain holes in top and side walls.

Figure 3. Measured and calculated transmission coefficient amplitude for a 50-ohm line from DC to 40GHz. The measured data is based on a coaxial resonator measurement, and a coaxial line resonator is shown in the inset. A schematic of a micro-coaxial line used for designing all the components presented here is shown in Fig.2. The periodic straps do not affect the loss appreciably and the release holes do not radiate. The size and spacing of the straps, the size and position of the release holes and the coaxial line dimensions are determined using full-wave electromagnetic simulations. The simulations were performed under fabrication-constrained parameter variations to obtain optimal geometrical parameters [1]. The measured loss on a 50-ohm line is shown in Figure 3. One of the advantages of the micro-coaxial lines shown here is that lines with many different characteristic impedances can be fabricated on the same wafer, enabling new types of coaxial components. Fig.4 shows calculated attenuation for lines with geometries as shown in Fig.2 and for different characteristic impedances. The characteristic impedance is varied by changing the cross-sectional rectangular shape aspect ration, and the parameter a in the figure is the longer side of the outer

conductor rectangle. The impedance range that is possible with a 1.5:1 fabrication aspect ratio is 25 ohms to 75 ohms.

Figure 4. Calculated attenuation coefficient for rectangular coaxial cables of different characteristic impedances possible with the Polystrata technology. The parameter a is the longer side of the rectangular cross-section of the outer conductor. An important design and fabrication parameter is the connection of the miniature coaxial lines to other circuits and to test equipment. Figure 5 shows a portion of a Polystrata wafer under test (courtesy Mr. Dan Fontaine, BAE Systems). Figure 5 (bottom) shows the detail of a connection to a standard-pitch CPW probe, where the otherwise horizontal coaxial line is bent at a right angle so that the open end is exposed to the probe. Most of the measurements shown in this paper are performed with this kind of a probe connection and a standard vector network analyzer.

Figure 5. Photograph of standard CPW probes positioned to connect to the micro-coaxial line components on a 6-inch silicon wafer (top). Detail of probe connection to micro-coaxial line shows the complexity required for a low return loss connection.

δs=0, tanδ=0δs=0, tanδ=.045δs≠0, tanδ=.045

δs=0, tanδ=0δs=0, tanδ=.045δs≠0, tanδ=.045

δs=0, tanδ=0δs=0, tanδ=.045δs≠0, tanδ=.045

δs=0, tanδ=0δs=0, tanδ=.045δs≠0, tanδ=.045

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250

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350400500

600700

Zc [Ω]

αc

[dB

/cm

] a in μm is the parameter

200

250

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350400500

600700

Zc [Ω]

αc

[dB

/cm

] a in μm is the parameter

Although the interconnect in Figure 5 can be optimized for excellent performance, it does not allow easy connections to, e.g. active circuits. Figure 6a shows a connection of micro-coax to a CPW line on the host substrate, while Figure 7a shows a possible method of flip-chip integration. Both connections were simulated using Ansoft HFSS full-wave software, and Figure 6b shows the return loss and the insertion loss of such a connection as a function of length of CPW transition.

(a)

(b) Figure 6. (a) A micro-coax-to-CPW interconnect on host substrate. (b) Simulated return loss and insertion loss as a function of length of CPW transition.

(a)

(b) Figure 7b demonstrates that a CPW connection can be designed with an insertion and return loss acceptable well into W-band. It should be noted that because of the small size of this coaxial cable, the TEM mode is dominant up to frequencies in the 400-500GHz range, depending on the characteristic impedance and exact dimensions. 3. Micro-coaxial component library The low-loss, well-shielded, high-frequency lines shows in the previous section can be densely packed to form single-layer or multi-layer components. The purpose of this section of the paper is to systematically present a number of components that have been designed, along with measurement data for most of the components. Figure 8 shows a SEM photograph of a single-layer set of resonators, hybrids and calibration standards. The components were designed using HFSS, with details given in [7-10]. The couplers are designed with 35-ohm and 50-ohm lines and several tee-compensation techniques. The measured and simulated results for the branch-line coupler are shown in Figure 9. The measurements were obtained with four 50-ohm probes, where two of the probes were used as terminations. An off-wafer set of SOLT calibration standards provided raw data, which was then de-embedded using an in-house TRL algorithm [11]. Figure 9b shows the obtained amplitude and phase balance for a coaxial branch line coupler.

S11 S21 S22

Figure 8. SEM photograph of quasi-planar TE resonators (left), branch-line hybrids (middle) and transmission-line resonators and calibration standards (right) on a single wafer.

Figure 9. A quarter of a HFSS computational model for a branch-line coupler using two layers of Polystrata (top). The measured and simulated performance for this coupler (bottom). Another component present in many millimetre-wave front ends is a divider/combiner network. Figure 10 shows a 1:3 divider in Polystrata designed to operate in W-band with equal splitting. The divider is optimized using HFSS and in order to obtain good matching, several different characteristic impedances were used in the design. The transmission coefficient is -5dB with a return loss of -20dB or better from 75 to 100GHz. This device is currently being fabricated.

Figure 10. Layout of a W-band 3-way divider-combiner in coaxial technology. The characteristic impedance of the lines is varied in this devices in order to obtain a simulated return loss better than -20dB from 75 to 100GHz. Another component useful in oscillators and filters is a high-quality factor (Q) resonator. In Polystrata, the resonator is implemented as a quasi-planar TE resonator which is a half-wavelength square, Figure 11a. Such a resonator has an unloaded Q-factor which depends on the metal, dielectric and radiation loss, as well as the height of the resonator in the non-resonant direction. For a 250-um tall resonator, the measured unloaded Q-factor at Ka-band using 250-um tall Polystrata is 450 (Figure 11b).

Figure 9. A photograph of a Ka-band quasi-planar 250-um tall resonator and its measured and simulated frequency response, which results in unloaded Q factors of 500 (simulated) and 450 (measured).

f=94GHzL=5.3mmW=2.9mm

f=94GHzL=5.3mmW=2.9mm

Figure 12. Measured and simulated performance of a slot-loaded patch antenna shown in the inset. Another important element for front ends is an antenna. Because of the low profile of Polystrata, lack of high-permittivity dielectric, and proximity of ground plane, the antenna design is a challenge. Figure 12 (inset) shows a demonstrated Polystrata antenna detailed in [10]. The radiator is a patch supported with two metallic posts that are located in the plane of minimum electric field. The feed is an L-shaped recta-coax with its inner conductor supported by the dielectric strap. The designed cavity backing reduces coupling between the neighboring elements, but with a drawback of a somewhat narrower bandwidth compared to that a conventional patch. The cavity height is about 700um. Two slits on the patch are used to tune the antenna resonant impedance to 50 ohms. Square and rectangular holes are used to allow the removal of the photoresist in the release process. 4. More complex components An example of a more complex component is a 4x4 Butler matrix for beam forming, the photograph of which is shown in Figure 13. Four branch-line hybrids, laid out as two stacked pairs, are joined with an interconnect section. To the best of our knowledge, this is the first demonstration of a Ka-band recta-coax Butler Matrix. The component contains numerous sections of different impedance lines, right-angled crossovers and delay lines for the appropriate phasing. A full-wave analysis of a complete 4x4 Butler matrix is computationally intensive and practically not possible with standard resources. Therefore, the complete matrix is divided into a number of two and four port constitutive components. As these are significantly smaller, a full-wave analysis therefore can be easily executed. By cascading the respective S-matrices, a combined circuit/full-wave simulation within the environment of a selected circuit simulator (Ansoft Designer in this work) is obtained. As shown in Figure 13b, measurements confirm the design methodology.

(a)

(b) Figure 13. (a) Photograph of fabricated 4x4 Buttler matrix and (b) Measured and simulated performance for one of the channels. One addition step in complexity is to increase the number of antennas feeds, and increase the level of integration by adding an antenna array. The topology of a 16x16 Butler matrix along with the 4x4 antenna array with integrated phase correcting network is shown in Figure 14. This array has 16 distinct beams, and the challenging testing is currently in progress. As can be seen, very complex and intensive integration of numerous multi-level components is undertaken in this case.

Figure 14. Photograph of an integrated fabricated 16x16 Buttler matrix and 16-element antenna array. 5. Discussion and conclusions In conclusion, this paper reviews numerous miniature wafer-scale integrated passive coaxial components for millimetre-wave front ends. The air-filled coaxial technology is attractive because of its low loss, high

packing density, high isolation and small form factor. In addition, scaling of devices in frequency is straightforward and small footprints can be obtained at lower microwave frequency using the same basic coaxial line. To illustrate this scaling, directional couplers were designed at 6GHz and 26GHz (a frequency ratio of over 1:4). The resulting hybrids are shown in Figure 15. and are not drawn to scale for clarity. The length of the 6-GHz hybrid is only 2 times larger than the 26-GHz design, and four 26-GHz hybrids fill the footprint of a single 6-GHz design.

(a)

(b) Figure 15. (a) 6-GHz branch-line hybrid with a footprint of 5.45 x 5.45mm, using a 250-um 50-ohm square coax as the baseline. The coaxial line can meander in a dense meander since the line has excellent isolation and the bend reactances can be easily compensated. (b) 26-Ghz branch-line hybrid using the same coaxial line has only a few meanders and a footprint of 2.76x2.76mm. Beyond 26GHz, the line thickness is such that meandering is not reasonable. Acknowledgements The authors acknowledge the support of Gil Potvin and Dan Fontaine at BAE Systems under a DARPA 3DMERFS subcontract. This paper would not have been possible without the Rohm and Haas Electronics Polystrata Technology and continuous technical discussions and support of David Sherrer, Dr. Chris Nichols and Dr. Jean-Marc Rollin.

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