Bandpass filters help extract desired signals from a portion of frequency spectrum while rejecting unwanted signals. In addition, they can symmetrically or asymmetrically modify the amplitude and/or phase of a signal. They are essential to a host of commercial and military systems and have evolved over the years, so much so that a review of available filter design and materials technologies can aid engineers faced with developing high-performance bandpass filters for X-band applications.

In complex EM environments, where signals must be strictly confined to allotted bandwidths to minimize interference, optimized RF/microwave component performance is critical, with the filter being among the most important of these components. Of the numerous configurations of filters, the planar waveguide, transmission-line structure in microstrip is well suited for high-precision thin-film, manufacturing techniques, passive microwave component manufacturing, and module assembly techniques.

In this configuration, dielectric material is sandwiched between metallized conductor(s) with a patterned metallization circuit conductor on top and blanket ground-plane on the back. Although the resultant, electromotive (E) and magnetic (H) fields provide for quasi-transverse electromagnetic (TEM) propagation due to the dielectric being asymmetric with respect to the top circuit conductor (touching the conductor only on one side), microstrip geometry provides for good power handling, moderate radiative loss (with reasonable crosstalk), and reasonable dispersion over a wide characteristic impedance range (about 15 to 150 Ohms).

Filter performance figures-of-merit (FOM) are designated by S-param-eters, bandwidth, center frequency, ripple, rejection, group delay, and power-handling capability. With a specification set using these FOMs, calculation-intensive EM modeling and optimization can be performed. An appropriate transfer function, which closely approximates a desired filter response (Chebyshev, Bessel, elliptic, etc.), can be selected and an iterative FOM optimization process performed. The resulting filter structures can be a cascade of end-coupled, edge-coupled, interdigitated, half-wavelength-long, and open-circuited resonators. The offset parallel positioning of the resonators has a symmetry that allows for coupling along the length equal to one-quarter wavelength at the center frequency of the filter. Adjusting the offset symmetry, spacing, width, length, and thickness of the conductor resonators and the dielectric constant (e_r) and thickness of the “sandwiched” dielectric provide for optimizing the FOMs. Examples of completed edge-coupled and interdigitated high-frequency filter structures are shown in Fig. 1 and Fig. 2, respectively.

For manufacturing a filter, once an optimized design is achieved, the appropriate conductor artwork is generated and conventional thin-film processing is used to deposit the metallization patterns required. Filled viaholes were used in these filter layouts to allow access with ground-signal-ground (G-S-G) probes on the microstrip conductor top side. These filter designs also featured polyimide-supported air-bridges for conductor interconnects. Measurements of material e_r and loss tangent (tan Δ) were made with a commercial HP8510 vector network analyzer (VNA) from Agilent Technologies from 18 to 25 GHz using an open resonator with the E-field parallel to each of the two principal in-plane axes of the substrates.

Parallel-plate techniques were used to derive e_r and tan Δ from capacitance and dissipation factors, respectively. The 99.6-percent, 0.015 x 4.5 x 3.75-in. Al₂O₃ ceramic substrate was cleaned, sputter-metalized with TiW/Au (1000-Angstroms/2500-Angstroms), and electroplated with 3.75 µm gold (Au). Substrates were diced to nominal dimension of 4.40 x 3.70 in. to provide isolated top and bottom electrodes. An inductance-capacitance-resistance (LCR) meter/ fixture was used to perform capacitance measurements with subsequent calculation of e_r using dielectric thickness, electrode area, and measured capacitance.

Surface roughness was measured with a contact profilometer in a similar 12-point array on each plate. On each of 50 Al₂O₃ substrates, thickness measurements were made in a 12-point array distributed over the 4.40 x 3.70 in. area, and two length and width measurements were made on the major and minor axes of the substrates. Table 1 shows e_r, tan Δ., and thickness distribution data for the 50 Al₂O₃ substrates with “within-substrate” data for the “best” and “worst” individual substrates. The surface roughness was superior to the material requirements data (MRD).

Table 1 shows the e_r and tan Δ results for a high-quality 99.6-percent Al₂O₃ ceramic dielectric. The MRD for e_r was 9.9 and tan Δ was 0.0001 at 1 kHz and 1 MHz (ASTM D150). The manufacturer-reported e_r (including tolerances) was slightly higher, but in reasonable agreement with the 20-kHz measurements for the (50) plate lot statistical data. However, the 20-kHz “best” to “worst” range of 9.78 to 10.2 (Δ = 0.42) for e_r exceeded the reported data of 9.8 to 10.0 (Δ = 0.2). The 17-to-25-GHz measurements trended similarly with respect to the MRD and had a “best” to “worst” range of dielectric constant with Δ = 0.25. The tan Δ for the (50) plate lot statistical data was approximately three to four times the MRD. The measured thickness range and mean thickness in the (50) plate lot was slightly higher than the nominal MRD of 0.015 in. (0.381 mm) and tolerance was 0.369 to 0.393 mm.

The Al₂O₃ dielectric substrate properties that are desirable for superior microwave electrical performance are low loss tangent (low dielectric loss), high resistivity, isotropic/uniform dielectric constant, uniform/smooth surface and a uniform thickness. The characteristics of the dielectric should be similar in the as-received and processed conditions and stable over frequency and temperature. Isotropy of these properties are especially important in microstrip circuits to minimize fringing capacitance issues in fine line/gap precision coupled structures such as filters and Lange couplers.

The major contributors to dielectric losses are conduction and polarization effects. Conduction losses typically are attributable to conduction band-to-valence band electron transitions and can be attributed to impurities, secondary phases, and point defects that disturb the Al and O sublattice symmetries. Polarization losses are an indication of the efficiency of the dielectric at propagating the EM field. The dissipation factor (tan Δ) is the assessment of the dielectric loss. The efficiency of the alignment of the electric dipole (ions-electrons) in the Al₂O₃ dielectric as the time-variant E field propagates through the dielectric will affect loss characteristics. The polycrystalline Al₂O₃ may have preferred crystallographic orientations and grain size distributions that can promote/inhibit this alignment thereby affecting the dielectric constant and loss tangent. Non-uniform dielectric properties can lead to increased radiative loss, increased crosstalk, localized variations in characteristic impedance (mismatch effects), degradation in signal time delay and rise time and the appearance of intersymbol interference (ISI) effects.¹ These conduction and polarization losses increase as temperature and frequency increase.

For the microstrip planar waveguide geometry, the dielectric media in which the conductor exists is non-homogeneous; for both the signal and ground conductors, one surface is in contact with the Al₂O₃ dielectric (e_r of approximately 9.9) and the remaining surfaces are in contact with air (e_r = 1.0). This geometry supports quasi-TEM wave propagation with a significant portion of the EM-signal (field) propagating in the Al₂O₃ dielectric substrate. The critical RF-current-carrying regions are the metal/ceramic interface regions of the signal traces and the ground planes. In microwave applications, microstrip dielectrics are typically high-quality, low-loss materials, and conductor loss dominates the total loss performance. However, the contributions of dielectric losses to the total losses will increase with increasing frequency. Therefore, manufacturing processes that can modify the dielectric/conductor interface (plasma and/or chemical cleaning, annealing, conductor deposition parameters) through surface chemical, crystallographic defects, and interdiffusion modifications can have an effect on performance. Although acceptably uniform, the Al₂O₃ thickness was higher than the MRD, which could lead to unanticipated microstrip circuit characteristic impedance increases if not identified. For high quality 99.6-percent Al₂O₃ (R_rms of about 0.05 µm) dielectric losses due to surface roughness effects are not influential (for Au or Cu conductors) until reaching 100 GHz and higher frequencies.

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