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Structured metallization on glass: A comparative study of resonance and propagation characteristics at 24GHz.
Keywords: glass substrate, 24 GHz, integrated passive devices
The SCHOTT glass AF32_eco is an alkaline free glass with a dielectric constant of eps' = 5.1 and a comparatively low dielectric loss of tan(delta) = 0.0049 at 5 GHz. It is produced in numerous thicknesses down to 0.025 mm. In this work, several transmission lines of different length as well as ring resonators were designed for a characteristic impedance of 50 Ohm and an AF32_eco substrate height of 0.1 mm. A microstrip line topology was chosen, since its propagation characteristics are very sensitive to the dielectric substrate used. Therefore, it is very useful for the qualitative evaluation of different substrate materials. Further, a comparison of microstrip lines of different length allows determining the attenuation per unit length for a given dielectric and metallization. Also ring resonators were designed with a resonance close to 24 GHz. These ring resonators are weakly coupled to microstrip lines on both sides, separated by small gaps. While the radius of the ring influences the resonance frequency, the width of the gaps between the ring and the microstrip feeding lines are influencing the coupling into the resonator. The smaller the gap, the stronger is the coupling, but at the same time, the loaded quality factor of the resonator is decreasing. Besides that, the quality factor of the resonator contains information on the dielectric loss of the substrate and of the quality of the metallization. A qualitative comparison allows to evaluate and to distinguish between different metallization methods on the same glass substrate. Contact pads in coplanar waveguide topology were included to the design for RF characterization by means of on-wafer probes. Radial stubs ensure a capacitive coupling of the coplanar waveguide’s ground planes to the one of the microstrip line. All designs were conducted using CST Studio Suite. The glasses were metallized and structured at different commercial and academic partners. The metallization consists of copper and adhesive agents with a thickness of at least 6 µm. The differences of these theoretically identical structures are investigated in terms of the metallization’s quality, propagation and resonating properties. As a reference, all structures were also realized on well-established Rogers RO3003 substrates with a substrate thickness of 0.125 mm. As a result, the performance of the best metallized glasses of the SCHOTT glass AF32_eco comes very close to the one of the Rogers RO3003 substrate both for the attenuation of the microstrip line (78 dB/m for RO3003 and 83 dB/m for AF32_eco) as well as for the quality factor of the ring resonator (Q=73 for RO3003 and Q=68 for AF32_eco). This is a surprising result, since the dielectric loss of the PTFE-based substrate in RO3003 (eps' = 3.0 and tan(delta) = 0.0013 at 10 GHz) is almost five times lower than for the glass AF32_eco. This proves that this glass is well suited as substrate material for GHz applications. The reason for the excellent performance is probably due to the extremely smooth glass surface with rms values smaller than 1 nm as well as the well defined glass-to-metal interface on such a smooth surface.
Martin Letz, senior principal scientist
SCHOTT AG
Mainz, Rheinland-Pfalz
Germany


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