University of Birmingham Ka-Band LTCC Stacked Substrate Integrated Waveguide Bandpass Filter

A Ka-band substrate integrated waveguide bandpass filter has been designed and fabricated using low temperature co-fired ceramic (LTCC) technology. The in-house developed SICCAS-K5F3 material with a permittivity of 6.2 and a loss tangent of 0.002 was used. The size and surface area of the proposed bandpass filter are reduced by exploiting vertical coupling in vertically laminated three-dimensional structures. The coupling between adjacent cavities is realized by a narrow slot. A vertical transition structure between the coplanar-waveguide feed line and the substrate integrated waveguide is adopted to facilitate the internal signal connection. The demonstrated third-order filter has a compact size of 6.79 mm × 4.13 mm × 1.34 mm (0.63 𝜆 0 × 0.38 𝜆 0 × 0.12 𝜆 0 ) and exhibits good performance with a low insertion loss of 1.74 dB at 27.73 GHz and a 3 dB fractional bandwidth of 10 %.


Introduction
Substrate integrated waveguide (SIW) has received enormous attention due to its low loss and excellent compatibility with planar circuits [1,2].It preserves the advantage of rectangular waveguide and also has merits such as low profile, ease of packaging, being lightweight, and low fabrication cost.Therefore, SIW is a promising candidate structure for high performance microwave and millimeter-wave circuits.SIW is normally composed of waveguide structures defined by two rows of metallic via arrays in a dielectric substrate and sandwiched between two parallel conductor layers [3].Apart from traditional print circuit board technology, SIW components could also be fabricated by low temperature cofired ceramic (LTCC) process.Owing to the excellent dielectric, mechanical, and thermal properties, LTCC has been widely used in high density integrated circuits package [4][5][6].Moreover, LTCC technology is very suitable for forming stacked structures to reduce the area of the circuit.
High performance and miniaturized bandpass filter (BPF) is an essential component in modern wireless communication system.With a BPF, the required channel spectrum is selected while the unwanted spectrum is rejected.Various BPFs are implemented by diverse transmission line technologies such as microstrip [7] and SIW [8].Direct-coupled or cross-coupled SIW cavities are common in millimeter-wave filter designs [9,10].Compared with in-plane arranged SIW cavities, multilayer stacked SIW cavities are more compact.
In the design of high frequency transmission circuits, the internal signal connection and transition are vitally important, especially in multilayer LTCC package.Generally, the transition takes either horizontal or vertical form.Coplanarwaveguide-(CPW)-to-stripline vertical transitions have been researched extensively in recent years.In previous report, a CPW-to-stripline vertical transition was utilized for a V-band LTCC system-on-package applications [11].A broadband CPW-to-stripline transition which is capable of operation up to 100 GHz was also presented for LTCC module [12].However, few works were reported on the CPWto-CPW vertical transition for LTCC based SIW BPF.
In this paper, a compact Ka-band LTCC SIW BPF with a via-based CPW-to-CPW vertical transition is presented.The SIW cavities are vertically stacked to allow miniaturization.As a key element of the circuit integration, the CPW transition is adopted and analyzed.To validate the proposed scheme, a third-order SIW BPF in Ka-band was designed and fabricated using LTCC technology.The simulation result and measurement result are in good agreement.Competitive performance has been shown as compared with other multilayer SIW filters.It should also be noted that an in-house developed LTCC material was used in this work.

BPF Design
2.1.Overview of Structure.The coupling topology of the proposed third-order SIW BPF is depicted in Figure 1. S represents the source excitation, L represents the Load, and R1, R2, and R3 represent three SIW resonant cavities.  is the external quality factor.k 12 and k 23 are the coupling coefficients.
The architecture of the proposed LTCC BPF is illustrated in Figure 2. The third-order BPF is built in four layers of LTCC substrates and four layers of metallization.The three resonators are vertically coupled to each other through slots which are made in the metallization layers and close to the wall.The coupling between adjacent SIW resonant cavities is magnetic in nature.A CPW structure is adopted as the excitation ports for measurements using the common ground-signal-ground (GSG) probe.In order to bring the two CPW ports to the same circuit level, a CPW-to-CPW vertical transition structure at port 2 is used to facilitate the measurement.
The dielectric substrates are based on the SICCAS-K5F3 LTCC material with a permittivity of 6.2 and a loss tangent of 0.002.This material has been in-house developed at Shanghai Institute of Ceramics, Chinese Academy of Sciences.The thickness of each layer in this design is 0.3 mm, formed by 10 sheets of green tapes.10 m silver layer is screen-printed on the LTCC substrates acting as the top and bottom enclosure planes for the cavities.

SIW Resonant Cavity.
The resonant cavity is composed of laminated dielectric substrate, parallel conductor layers, and metal via arrays.To design a SIW cavity operating at a given frequency, the following formula is adopted.
where c is the speed of light in vacuum and   is the dielectric constant of the LTCC substrate.L and W are the length and width of the SIW resonant cavity and d and p represent the diameter of the metal via and the periodic length of the metal via array, respectively.Based on these formulas, the initial dimensions of the SIW cavity are determined.

Coupling Slots and Feeding
Structure.According to the general Chebyshev BPF theory, the normalized coupling coefficients and the external quality factors can be obtained by following formula.
To minimize the occupation area, the resonant cavities are vertically stacked rather than cascaded laterally.The coupling between adjacent SIW cavities is realized by a narrow slot close to the metal via array.Since the SIW cavities are coupled together via the magnetic slot, the central frequency peak of the SIW cavities splits into two discrete peaks with frequencies f 1 and f 2 , respectively.The lower resonant frequency point f 1 and the higher frequency point f 2 are obtained from the simulation by ANSYS HFSS.The coupling coefficient k 12 between the adjacent cavities is extracted using the following formula: With the combination of the formula and ANSYS HFSS simulation, the coupling coefficient related to the slot dimension is achieved.The relationship between the extracted coupling coefficient and coupling slot length SLOT L is depicted in Figure 3.When the coupling slot length is increased, the coupling coefficients become larger.Accordingly, the coupling coefficients can be tuned by the length, width, and position of the slot.The final values have been achieved by optimization using the full-wave simulator.
The external quality factor   depends on the input and output excitation structure.This structure is composed of a section of 50 ohm CPW line and a slot dipole for coupling to the SIW cavity as shown in the inset of Figure 4.The external quality factor   can be extracted using following formula: where  0 is the resonant frequency of the structure shown in Figure 4 and Δ ±90 ∘ is the frequency bandwidth corresponding to the phase shift of ±90 ∘ with respect to the absolute phase at  0 .Using ANSYS HFSS simulation, the external quality factor   can be derived.The external quality factor of the feeding structure is determined by the slot dipole length, width, and offset position.The relationship between the external quality factor and the slot dipole offset S OFFSET is  depicted in Figure 4.When the S OFFSET value is increased, the external quality factor becomes smaller.The final values have been achieved by optimization in ANSYS HFSS.

CPW-to-CPW Vertical Transitions.
Microwave and millimeter-wave component measurement are often accomplished using GSG on-wafer probes.The back-to-back CPW structure is commonly utilized.As illustrated in Figure 5, the proposed CPW-to-CPW vertical transition structure is composed of two sets of metal vias and three sections of CPW transmission lines using a two-layer LTCC substrate.The thickness of the top layer is 0.1 mm; the thickness of the bottom layer is 0.9 mm.The diameter of the via hole is 0.09mm and the metal used is silver.
To maintain a good transmission and reflection characteristics, the CPW line is designed to be 50 ohm in Ka band.The width of the signal line is 0.25 mm, and the gap is 0.066 mm.The structure is simulated with ANSYS HFSS software.As depicted in Figure 6, the CPW vertical transition structure has a return loss better than 15 dB from 20 to 36 GHz.The simulated insertion loss is less than 0.61 dB.
Based on the above analysis, the CPW-to-CPW vertical transition structure is adopted at port 2 and optimized to miniaturize the multilayer SIW filter.Figure 7 shows the simulated performance of the whole SIW filter.It is observed that the length of the CPW line has a noticeable impact on the reflection coefficients.This is mainly caused by the imperfect impedance matching due to the introduction of the vertical transition structure.When the CPW line length is 0.6 mm, the return loss is over 20 dB.After full-wave simulation and optimization, the final dimensional parameters of the proposed SIW BPF are presented in Figure 8 and Table 1.filters were obtained after debinding at 450 ∘ C for 2 h and sintered at 900 ∘ C for 25 min.Owing to the shrinkage in the process, the designed value and fabricated dimension had a slight discrepancy.The photograph of the fabricated filter is exhibited in Figure 9(a), while the microscopic image of the CPW port is presented in Figure 9(b).The fabricated filter is 6.79 mm×4.13 mm×1.34 mm (0.63 0 × 0.38 0 × 0.12 0 ) in size.

Measurement.
The LTCC SIW filter sample was measured using a Keysight N5245A Vector Network Analyzer (VNA) and a Cascade M150 probe station with two GSG probes after a SOLT calibration.The measured and simulated S-parameters of the SIW filter are plotted in Figure 10.
The measurement results are in good agreement with the simulation results.The measured insertion loss is 1.74 dB at the center frequency of 27.73 GHz.The 3 dB bandwidth of the filter is 10% from 26.30 GHz to 29.08 GHz.Some discrepancies were observed in the return loss and transmission at the upper band edge.These are mainly due to the fabrication tolerance.The performance of the SIW BPF in this work, as compared with a few other reported results, is presented in Table 2.It is demonstrated that the SIW filter has a compact size, low insertion loss, and good comparable bandwidth.

Conclusions
In this paper, a Ka-band multilayer SIW BPF with CPW-to-CPW vertical transition is designed and fabricated using an in-house developed LTCC material and process.The coupling is realized by inductive slots between different layers.The CPW-to-CPW vertical transition structure is analyzed and

Figure 1 :Figure 2 :
Figure 1: Coupling topology of the proposed filter.

12 Figure 3 :
Figure 3: Relationship between the coupling coefficients k 12 and the coupling slot length SLOT L.

Figure 4 :
Figure 4: Relationship between the external quality factor   and the offset value S OFFSET.

3. 1 .
LTCC Fabrication Process.The dielectric material used during the LTCC fabrication process was SICCAS-K5F3 developed by Shanghai Institute of Ceramics, Chinese

Figure 5 :Figure 6 :
Figure 5: The model of the proposed CPW vertical transition structure.

Figure 7 :Figure 8 :Figure 9 :Figure 10 :
Figure 7: Effect of the length of the CPW line on the S-parameters of the SIW filter.

Table 1 :
Dimensions of the SIW filter (mm).