Abstract
The need of low-loss substrate materials with stable dielectric performances over a wide frequency range is a strong
requirement when working at millimeter frequencies, where standard dielectric substrates exhibit prohibitive losses. In
this contribution, the authors focus their attention on a polymer material, the benzocyclobutene (BCB), having a low
dielectric constant equal to 2.65, and a low loss tangent, equal to 0.0008 at 1 GHz, with a stable dielectric behavior
over a broad frequency range, up to THz. The use of BCB is proposed here as innovative dielectric substrate for
millimeter-wave printed circuits. At this purpose, an accurate dielectric characterization of BCB is firstly performed up
to 65 GHz to demonstrate the low-loss and stable behavior of the material. Measurements of dielectric parameters are
executed on test circuits, which are realized on BCB substrate by a in-house fabrication technology able to produce
BCB substrate of arbitrary thicknesses. The developed BCB fabrication technology is applied to the design and realize
a 60 GHz microstrip reflectarray.
1.Introduction
Research efforts are devoted to the investigation of alternative materials and fabrication technologies for the realization of efficient low-loss radiating systems well working at millimeter frequencies but requiring minimal increase in cost. Potential materials with excellent performances extending throughout millimeter-wave frequencies can be found in the polymer category. Table I summarizes the relevant properties of different polymers for high-frequency applications. To guarantee good performances as dielectric substrate at millimeter waves, the material should have low losses and a low dielectric constant, stable within the operating frequency range, so the values of permittivity and dissipation factor should be considered, but also the thermal stability of these parameters is relevant for a proper material selection. Among the dielectric polymers reported in Table I, Parylene-N shows the lowest values of dielectric constant and loss tangent, but a higher coefficient of thermal expansion, so the BCB has been selected in this paper as giving better electrical features with respect to the remaining candidates, guaranteeing also a strong dielectric stability versus temperature. Furthermore, low production costs have been obtained by developing an in house fabrication technique which requires small quantity of BCB polymer. BCB is a promising organic material showing stable permittivity values and low losses over a broad millimeter-wave range [6]. The producer [6] claims a dielectric constant r = 2.65, with a few percent variations between 10 GHz and 1.5 THz, and a loss tangent of values between 0.0008 and 0.002 within the frequency interval from 1 MHz to 10 GHz. BCB has already been successfully applied in literature as a covering film for packaging and interconnections on a silicon substrate [9-10]. In this paper, the use of BCB as substrate material on planar microstrip structures is investigated in order to overcome difficulties related to large losses at high microwave range. As a first task, the dielectric characterization of BCB is performed in terms of both dielectric constant and loss tangent, over a broad millimeter frequencies band. BCB is used, for the first time, as dielectric substrate on a CBCPW and the extraction of dielectric parameters is obtained from on-wafer scattering measurements performed on a CBCPW test structure. The technological process used to realize the BCB-based microstrip structure is further validated by realizing and experimentally testing a 60 GHz reflectarray antenna on BCB substrate


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