диафрагмированные волноводные фильтры / 762bc564-9bb3-4210-acae-3243da88eb35

Daniel López Cuenca, Jan Hesselbarth

Institut für Hochfrequenztechnik

University of Stuttgart

70569 Stuttgart, Germany

Abstract — A new method for coupling from an on-chip microstrip line to a rectangular waveguide oriented perpendicular to the chip is presented. The coupling is realized by a 3-pole bandpass structure, containing an on-chip microstrip resonator, a dielectric spherical resonator and a waveguide cavity resonator. Measurements show a transition loss of 2.6 dB at 64 GHz and less than 3 dB loss over a 4.5% bandwidth. Simulations indicate that less than 0.2 dB of this loss are due to radiation leakage. The structure occupies small on-chip area und provides features for electrical test before assembly and for simple yet precise assembly.

Keywords — millimeter-wave; on-chip; chip to waveguide transition; dielectric resonator; spherical dielectric resonator

I. INTRODUCTION

A growing number of applications in communications and sensing use frequencies in the millimeter-wave bands of the spectrum. This trend drives innovative concepts for the connection of integrated circuits with waveguides. Beyond established technologies which connect chips to planar boards using wire-bonds or flip-chip bonds, chips can connect directly to dielectric or metal waveguides. Small chip-area consumption and a cost-efficient assembly process are of importance.

Connections between integrated circuits and perpendicularly oriented dielectric rod or tube waveguides have been proposed [1],[2]. Here, an on-chip antenna couples to the waveguide which is positioned in the near-field of the antenna. A similar concept was proposed for a circular metallic waveguide filled with low-permittivity dielectric [3]. Similarly, the on-chip antenna was proposed to couple directly into the open end of a rectangular waveguide oriented perpendicularly to the chip surface [4]. In all these concepts, the on-chip antenna is often narrowband and of low efficiency. The coupling from chip to waveguide is prone to unwanted radiation leakage. Furthermore, alignment of all parts is rather involved.

Connections with laterally aligned chip and waveguide have been proposed. With a dipole radiator close to the edge of the chip, a low-loss connection to a suspended image guide was demonstrated [5]. The chip can also extend into the E- plane of a rectangular metal waveguide, where the fundamental waveguide mode couples to the chip by means of an on-chip E- probe [6] or an on-chip dipole [7].

This work was supported in part by the German Research Foundation (DFG) under grant HE6429/5.

Golzar Alavi

Institut für Nanound Mikroelektronische Systeme University of Stuttgart

70569 Stuttgart, Germany

This paper elaborates the idea to couple from on-chip microstrip to rectangular waveguide through an on-chip dielectric spherical resonator. On the chip side, the coupling to the alumina ceramic (Al2O3) dielectric resonator sphere is enhanced by an on-chip microstrip quarter-wave resonator, whereas on the waveguide side, an iris-coupled halfwavelength waveguide resonator increases the coupling strength. By careful tuning of these three resonators, a rather wide bandpass transmission is obtained. The alignment of the spherical dielectric resonator on the chip is facilitated by a shallow square crate precisely etched into the back-end-of-line dielectrics at the chip surface. The sphere uses a mode which minimizes radiation leakage, leading to low-loss transmission. The sphere together with the microstrip excitation structure occupies only a small area on the chip surface. Before mounting the sphere on the chip, the excitation structure behaves like an open-circuit resonance, therefore, the on-wafer circuitry can be tested using conventional 50 Ω ground-signal- ground (GSG) wafer probes [8].

II. DESIGN OF THE TRANSITION

Fig. 1 shows the on-chip microstrip to waveguide transition, designed for operation around 64 GHz. The structure consists of three resonators (microstrip resonator, spherical dielectric resonator, waveguide cavity resonator) and connects a 50 Ω microstrip line to a WR15 rectangular waveguide.

A. Microstrip Resonator

The strong electric field at the open end of the quarter-wave microstrip resonator (Fig. 2) enhances the coupling to the spherical dielectric resonator. The dielectric and ohmic loss of this microstrip resonator represents a large part of the overall loss of the chip-to-waveguide transition. Without the dielectric sphere, the resonator around resonance frequency shows a large impedance to ground. Therefore, chip circuits connected to the microstrip line can be tested using a GSG probe. Capacitive ground probe pads are provided in the design.

B. Dielectric Spherical Resonator

The position of the dielectric sphere close to the microstrip resonator is precisely fixed by means of a shallow crate etched into the microstrip dielectric substrate. The microstrip ground and the waveguide end form together a parallel metal plate structure which prevents the specifically chosen resonance mode of the sphere from unwanted radiation [9] (Fig. 3).

z

Fig. 1. Overall view of the microstrip-to-waveguide transition.

Fig. 2. Quarter-wavelength microstrip resonator. The dielectric of the microstrip BEOL structure is a low-permittivity spin coat polymer

(Benzocyclobutene) of 24 μm thickness (εr = 2.55; tan δ = 0.008 @ 60 GHz).

Conductor is made from AlSiCu alloy (σ = 3·107 S/m). D_1, D_2, W_1 and L are the design parameters. The GSG probe pads at the input allow for circuit testing before the sphere is placed.

W_I

0.2L_R

Fig. 4. TE011 waveguide resonator on a WR15 waveguide size. L_I, W_I, W_S and L_S are the design parameters. A_1 depends on the rest of the geometry for the sphere to touch both the slot and the step at the same position and self-align with the structure. All the waveguide structure is considered to be milled with a 0.35 mm radius tool, except the slot which is milled with a

0.25mm radius tool.

TABLE I. DIMENSIONS (IN MM) OF THE DESIGNED TRANSITION

Ground plane bottom

Fig. 3. Resonance mode of the spherical resonator. A 1.588 mm diameter alumina sphere (εr = 9.8; tan δ = 0.0006 @ 60 GHz) is used for this design.

Fig. 5. Simulated S-parameter magnitudes of the transition (port 1: microstrip, port 2: rectangular waveguide). Blue lines show results without material loss (i.e., radiation leakage only). Black and grey line results include conductor and dielectric loss and radiation. Microstrip feed-lines are deembedded.

C. Waveguide Resonator

The dielectric spherical resonator couples with the rectangular waveguide through a slot iris. In order to further enhance this coupling effect, a TE011 half-wavelength cavity resonator is formed in the waveguide end by an inductive iris. The waveguide end provides some steps to help mechanical alignment between waveguide and sphere. Fig. 4 shows the waveguide resonator in detail. Dimensions of the transition are given in Table 1.

III. SIMULATIONS AND MEASUREMENTS

Simulations were carried out with the FEM frequency domain solver of CST Microwave Studio. S-parameter magnitudes of the transition are shown in Fig. 5. Simulations with and without material loss are shown, indicating that radiation leakage is indeed very small, less than 0.2 dB in passband. The simulated insertion loss is less than 2.5 dB (best value 1.7 dB at 65 GHz) over a bandwidth of 3.8 GHz (5.9%).

Fig. 6. Test structure for end-to-end measurements including two chip-to- waveguide transitions. Top: Concept (all dimensions in mm). Center: Photograph of the complete setup. The waveguide is positioned with the help of an xyz-positioner, as are the GSG probes. Bottom: top view (waveguide removed) on one sphere with the microstrip feedline to the GSG wafer probe.

Fig. 7. Measured end-to-end transition and comparison with the simulated values in the same conditions.

Measurements of the transition are performed in an end-to- end configuration. As shown in Fig. 6, a WR15 waveguide structure milled from brass metal is attached to two transitions, where the respective on-wafer microstrip feeds are connected to GSG wafer probes. The effect of the microstrip feeds is known from the measurement of a long through line and is subtracted from the end-to-end measurements shown in Fig. 7. This figure also shows the results of a simulation of the measured end-to-end structure.

The measured transmission (Fig. 7) shows an insertion loss of less than 6 dB (that is, each transition with less than 3 dB since the loss contribution of the WR15 waveguide is very small) over a bandwidth of 2.9 GHz (4.5%), with a best insertion loss value of 5.1 dB at 64 GHz (less than 2.6 dB for each transition).

This measured transmission of better than -3 dB from onchip microstrip line to rectangular waveguide around 65 GHz compares well with reported transmission of -11 dB from chip to dielectric rod waveguide at 120 GHz [2] and, in another case, transmission of -6 dB from on-chip microstrip to rectangular waveguide at 160 GHz [1].

The discrepancies between measurement and simulation (Fig. 7) can be in part attributed to the permittivity value of the alumina sphere, which was assumed to be 9.8 in the design and simulation of the transition, but in fact it is 10.05 as it has been determined later from a single resonator measurement of the sphere in a test setup on the same wafer. Another likely reason for the differences may be due to bad contact between the milled waveguide parts at the inductive irises. For manufacturing reasons, the waveguide was not split in E-plane into two identical parts, but is built from one milled part and a flat plate.

A particular feature of the proposed structure is that the circuit connected to the microstrip feed (e.g., an amplifier) can be tested on-wafer before assembly (i.e., before mounting the sphere) in a standard 50 Ω environment with a standard GSG probe. This is made possible by the microstrip quarter-wave resonator which represents an open circuit at the frequency of operation.

A measurement of a test structure with a microstrip resonator but without sphere is shown in Fig. 8. At a frequency of about 61 GHz, both ports are matched to -13 dB whereas the transmission is -2.3 dB. A simple line of this length would have an insertion loss of 0.6 dB, therefore some loss can be attributed to the resonator. One-port measurements of the resonator indicate a resonance frequency of 63 GHz for the resonator alone without the sphere, which is 3 GHz lower than the expected resonance from simulations with the same conditions. This discrepancy is likely due to through-contact failure of some of the vias at the quarter-wave resonator that affected some parts of the wafer, including this test structure.

IV. CONCLUSION

A low-loss transition from an on-chip microstrip line to a rectangular waveguide oriented perpendicular to the chip is presented. The coupling is realized by a 3-pole bandpass structure offering about 5% bandwidth. The measured insertion loss of the transition at 64 GHz is 2.6 dB. The transition occupies small chip area and provides features simplifying assembly and electrical test. These advantageous properties make the proposed transition suitable for low-loss interconnects in millimeter-wave systems, e.g., between chip and antenna.

Resonator behind the probe

Fig. 8. Test configuration for the microstrip quarter-wave resonator without sphere. Top: Photograph. The left probe 2 connects to an open ended microstrip line, whereas the probe 1 connects to a microstrip line feeding a quarter-wave microstrip resonator (as depicted in Fig. 1). Bottom: measured S-parameter magnitudes.

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