# Ultra-wideband coplanar microstrip transitions for connectorizing microwave photonic components based on thin polymer and driven by microstrip electrode

Mohammed El Gibari, Yann Mahe, Dominique Averty, Cyril Lupi, Hong Wu Li

Li

► To cite this version:

Mohammed El Gibari, Yann Mahe, Dominique Averty, Cyril Lupi, Hong Wu Li. Ultra-wideband coplanar microstrip transitions for connectorizing microwave photonic components based on thin polymer and driven by microstrip electrode. German Microwave Conference IEEE (GeMiC 2011), Mar 2011, Darmstadt, Germany. pp.159-162. [hal-00795203](https://hal.science/hal-00795203v1)

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# Ultra-Wideband Coplanar Microstrip Transitions for Connectorizing Microwave Photonic Components Based on Thin Polymer and Driven by Microstrip Electrode

Mohammed El-Gibari, Yann Mahé, Dominique Averty, Cyril Lupi and Hongwu Li

Institut de Recherche en Electrotechnique et Electronique de Nantes Atlantique (IREENA, EA 1770 and FR CNRS 2819)

Faculté des Sciences et Techniques, 2, rue de la Houssinière, BP 92208
44322 Nantes Cedex 3, France
mohammed.el-gibari@univ-nantes.fr

**Abstract**— We present ultra-wideband coplanar-microstrip transitions studied both experimentally and by numerical simulations. They are very useful for characterizing and connectorizing microwave photonic devices realized from materials in thin film and driving by microstrip line, such as very promising electro-optic modulators based on polymer. Built on BCB polymer, coplanar-microstrip back-to-back transitions present a bandwidth over 70 GHz while the bandwidth of coplanar connector-microstrip back-to-back transitions can covers the range from DC to 30 GHz.

**Index Terms:** back-to-back GCPW-MS-GCPW transitions, Ultra-wide bandwidth, coplanar connector, polymers, thin film.

## I. INTRODUCTION

The evolution of telecommunications is accompanied in particular for RF systems by a rise in frequency of operation, and for the optoelectronic systems, by an increase in the flows of information [1]. This evolution generates new requirements on components and functions for signal processing, especially constraints of integration. In order to ensure a good transmission quality, the integrity of the signal must be preserved throughout the transmission. The electro-optic modulator is one of the key elements in the optical transmission chains for very high bit-rate. Its microwave interconnections and driving circuits are elements to take into account in order to achieve the maximum bandwidth allowed by its intrinsic physical properties, especially the electro-optic coefficient and phase matching between microwave and lightwave in the used materials.

We propose, in this paper, the study of coplanar-microstrip and coplanar connector-microstrip transitions optimized for

characterization and connectorization of electro-optic modulators based on polymer. These transitions may be also employed in microwave circuits and components where coplanar lines and microstrip lines coexist. The study and the validation of these transitions were carried out in two stages: initially, we optimize back-to-back GCPW-MS-GCPW (Grounded Coplanar Waveguide-Microstrip lines) transitions without making via-hole in the substrate and without patterning the bottom ground plane. These transitions use the electromagnetic coupling between the bottom and top ground planes, which simplifies the manufacturing and facilitates the characterization, by means of coplanar probes, of microwave photonic devices realized from materials in thin film and whose driving electrode is a microstrip line. Optimized transitions have a bandwidth exceeding 70 GHz and a lower cutoff frequency of 700 MHz. The second part concerns the study of the transitions for component connectorization using coplanar connectors. At this stage, patterning of the bottom ground plane was necessary to maintain the characteristic impedance to 50 Ω by optimizing the dimensions of the transitions for easy installation of the coplanar connectors. The results of electromagnetic simulations with HFSS show that we can have a very broadband bandwidth from DC to over 30 GHz. These transitions based on BCB polymer are optimized to be used directly with the coplanar connectors.

## II. GCPW-MS-GCPW TRANSITIONS

To show the feasibility of the microwave circuits and ultra-high frequencies optoelectronic components during the step of development, we employ, in the majority of cases, a probe station system for its facility of use. This essential step allows settle certain problems involved in the structure itself

<footnote>The authors would like to thank the ANR (French National Research Agency) which support the work through the project ModPol</footnote>
before adding connectors. In electro-optic modulators based on polymer, the chromophores responsible for the electro-optic effect are oriented perpendicularly to the substrate as they are generally poled by Corona effect or with contact electrodes. To obtain maximum electro-optic effect, the driving electric field must be oriented as the chromophores. Consequently, a microstrip line is suitable to apply the driving signal. So, a transition between coplanar and microstrip lines (CPW-MS) is indispensable to characterize components. This transition may be also employed in all circuits to link coplanar and microstrip lines.

The structure of GCPW-MS-GCPW transitions studied in this paper is shown in Figure 1. The choice of this topology has been studied previously [2]. In order to reduce the driven voltage of modulator, we fixed thickness of the polymer at 10 µm, typical value for this type of components [3]. Transitions are realized on commercial BCB polymer (benzocyclobutene) known to have very low permittivity and loss tangent (ε<sub>r</sub>=2.65, tanδ=0.0025). From the thickness fixed and the dielectric constant of the BCB polymer, we used the software Linecalc Agilent Advanced Design System (ADS 2006 version) to determine the dimensions of the transition in order to have a characteristic impedance of 50 Ω for both the microstrip section and the GCPW section: the width of the central strip W = 20 µm, the coplanar gap G = 13 µm. The length of the microstrip line L is 1 cm.

<figure><img src="image_2.png" /><figcaption>Figure 1. The structure of the studied back-to-back GCPW-MS-GCPW transitions</figcaption></figure>

The influences of the other parameters of this transition, such as the thickness of the substrate and the dimensions of coplanar pads, have been studied in detail in [2]. We have shown that the thinness of the substrate facilitates the conversion of the coplanar mode into microstrip mode and pushes the excitation of resonance peaks in the high frequencies. We have also shown the importance of the low permittivity to broaden the bandwidth of the transitions [4].

A. Choice of the coplanar length : GCPW-MS-GCPW transitions on the thick NH9338 substrate

To show the importance of optimizing the coplanar length (L1) of the transition shown in Figure 1, we present here the measurement and simulation results of this transition built on the commercial NH9338 substrate of 254 µm thickness. Figure 2 illustrates the simulated and experimental results of S-parameters of the GCPW-MS-GCPW transition for several coplanar lengths (L1). The dimensions of these transitions are: W = 530 µm, G = 200 µm, S = 1 mm and L = 2 cm. For characterization, we measured the S-parameters with a network analyzer Agilent E8364B whose bandwidth covers

the frequency range from 10 MHz to 50 GHz, using the full two port SOLT (Short-Open-Load-Thru) calibration technique. The whole system is controlled by the software Wincal Cascade. For the measurement of transitions on the NH9338 substrate, SUSS Microtec probes 50A3N500GSG (bandwidth of 40 GHz, pitch of 500 µm and pad of 30 µm) and a calibration substrate SUSS Microtec CSR-4 were used, while for the structures on polymer substrates, we used Cascade probes I40AGSG250 (bandwidth of 40 GHz, pitch of 250 µm and pad of 12 µm) and a Cascade AE-101-190 calibration substrate.

To get simulation results in accordance to measurements, the exact experimental conditions should be taken into account, including even the probes used for the characterization. In HFSS software, we used the lumped port. The skin effect must also be taken into account.

<figure><img src="image_3.png" /><figcaption>Figure 2. Measurements and simulations of the back-to-back GCPW-MS-GCPW transitions with L1 = 1 mm (left) and L1 = 5 mm (right)</figcaption></figure>

The experimental results obtained with commercial NH9338 substrate show that a very broad bandwidth can be obtained when the length of the coplanar line is short. Indeed, when L1 = 1 mm, the 3-dB bandwidth of the back-to-back transition ranges from 8 GHz to 30 GHz with a value of return loss lower than -10 dB. This bandwidth covers only the range from 2 GHz to 15 GHz when L1 = 5 mm because of the excitation of a spurious parasitic resonance (cf. Figure 2). We can also note that the lower cut-off frequency is due to electromagnetic coupling between the backside ground of the microstrip and the coplanar ground via capacitive effect. With a given substrate, by enlarging the coplanar pad surface, their length L1 or/and their width S, one can reduce this lower cut-off frequency but decrease also the parasitic resonance frequencies. There exists a tradeoff between the lower cutoff frequency and the spurious high resonance frequency. So, in order to have a very high bandwidth with a polymer substrate, the coplanar pads are S=1 mm wide and L1=1 mm long.

B. GCPW-MS-GCPW transitions on thin film polymers

The simulations results of this transition based on BCB polymer, for several aluminum thicknesses are shown in Figure 3. A very wide bandwidth can be obtained when increasing the thickness of aluminum. With 4 µm of aluminum this bandwidth is only limited by the excitation of spurious resonance because of dimensions of the coplanar pads. So, a 71 GHz bandwidth is obtained with a metallization of 4 µm and 50 GHz even with 1 µm. Thanks to the substrate thinness, the capacitive effect between the bottom ground plane and the upper ground plane of the coplanar line is big, so, the low cutoff frequency is decreased to 700 MHz. We recall that this last depends on the surface of the coplanar pads, the thickness and the permittivity of the substrate [2, 4].
<figure><img src="image_4.png" /><figcaption>Figure 3. Calculated S-parameters of back-to-back GCPW-MS-GCPW transition on BCB polymer with different metallization thicknesses</figcaption></figure>

For realization on thin film polymers, we present in Figure 4 the measurement and simulation results obtained with 10-µm BCB polymer substrate, deposited by spin-coating on a 380 µm thick metallized silicon wafer. For the upper metallization, this transition has an aluminium layer of only 0.4 µm due to a problem of adhesion. The other dimensions of the transition measured with a profilometer are : W = 18 µm et G = 15 µm.

<figure><img src="image_5.png" /><figcaption>Figure 4. S-parameters measurement and simulation of a back-to-back GCPW-MS-GCPW transition on BCB polymer substrate</figcaption></figure>

We can notice that the insertion loss of the transition is as high as 3 dB due to the ohmic effect of the thin metal layer. We also note that S21-parameter curve has a very low slope thanks to the very low loss tangent of the BCB polymer and the use of a metallized silicon wafer. A very good agreement between the experimental and simulation results was obtained.

### III. COPLANAR-MICROSTRIP TRANSITION FOR CONNECTORIZATION WITH A COPLANAR CONNECTOR

Once the GCPW-MS-GCPW transition on thin film polymer was validated by measurements with probe station, we are interested in studying how to use this transition with a coplanar connector. We chose the end launch coplanar connector, Model #1492-04A-5 of SouthWest Microwave. The diameter of its center contact is 127 µm and its coplanar gap is 254 µm. So, the center conductor width and the coplanar gap of the transition to optimize must be sufficiently broad to install easily the connector.

The thickness of BCB polymer being fixed at 10 µm, a segmentation of the bottom ground plane is needed to

maintain the impedance matching when the width of the center conductor increases.

Different transitions of this type were studied in the literature [5-7]. Most of these transitions are based on a smooth transformation of the field by gradually changing the physical boundary conditions. The objective is to maintain the impedance matching at 50 Ω in all sections of the transition.

The proposed transition for connectorization with the coplanar connector is shown in Figure 5. In this transition, $W_1$, $W_2$ and $W_3$ indicate the different widths of the signal conductor in different sections, $G_1$, $G_2$ and $G_3$ indicate the different values of the coplanar gap, $L1$ the length of the CPW line and the $L2$ the length of the GCPW line. For each section, the width of the signal conductor on the top plane gradually decreases from the width needed for the coplanar connector to the final width of the MS line optimized to have a characteristic impedance of 50 Ω. At the beginning of the transition, the bottom ground is designed to have a tapered slot. This step is necessary to facilitate the excitation of the CPW fundamental mode and a gradual installation of the MS fundamental mode at the grounded coplanar line (GCPW). This is possible thanks to the thinness of the BCB polymer lower than the coplanar gap. Indeed, the small thickness of the dielectric substrate presents a double advantage for a good transition without via-holes. On one hand, it can easily create an electromagnetic coupling between the bottom ground plane of microstrip line and coplanar pads by capacitive effect. On the other hand, the low ratio of the thickness and the coplanar gap allows a fast conversion of the field between the CPW and MS modes.

<figure><img src="image_6.png" /><figcaption>Figure 5. Structure of the GCPW-MS-GCPW transition. (a) view of the transition studied. (b) Top view of the transition. (c) View of the bottom ground plane</figcaption></figure>

For mechanical fixing on the substrate, the end launch connector is equipped with screws in its corners. So, for our application we take advantage of these holes in order to obtain the DC contact. We used the RT/Duroid 6010 substrate which has a permittivity similar to that of silicon. So, the transition was designed on BCB polymer (10 µm of thickness, $ε_r=2.65$)
and tanδ= 0.0025) deposed on RT/Duroid 6010 substrate (635 μm of thickness, εr=10.2 and tanδ= 0.0035).

The signal line widths were determined first. The value of W3 was determined firstly to have a microstrip characteristic impedance of 50 Ω. This width depends essentially on the thickness of BCB polymer deposed (10 µm) and its permittivity (2.65). So, W3 is fixed at 20 µm. Secondly, to simplify the installation of the coplanar connector, we fixed the value of W1 at 220 µm. We design a smooth transition by gradually changing the physical boundary conditions, so insertion losses are minimized. Simulations were realized for several different cases, the parameters of the most efficient transition are as follows: W1 = 220 µm, G1 = 110 µm, W2 = 30 µm, G2 = 8 µm. The coplanar gap becomes critical to maintain the characteristic impedance at 50 Ω. Starting from W2 we consider the zone of GCPW line, W3 = 20 µm, G3 = 13 µm. In order to achieve a progressive transition, the coplanar lengths L1 and L2 are chosen to minimize the losses caused by the fast change of the widths of the coplanar line (from 220 µm to 20 µm). According to the various simulations realized, we took 1 mm and 300 µm respectively for L1 and L2. The total length (L) of the back-to-back transition is 15 mm in order to facilitate the placement of the coplanar connector.

<figure><img src="image_7.png" /><figcaption>Figure 6. Calculated S-parameters of back-to-back GCPW-MS-GCPW transition on BCB polymer deposed on RT/Duroid 6010 substrate</figcaption></figure>

Figure 6 shows that a very broad bandwidth from 700 MHz to 32 GHz is expected with the station system equipped with GSG probes. Currently, we are working to realize this transition in order to characterize it with the coplanar connector model #1492-04A-5 of SouthWest Microwave.

IV. CONCLUSION

We have presented in this article an ultra-wide bandwidth back-to-back transitions between coplanar and microstrip lines. These transitions are widely requested to be used in

packages, on-wafer measurements of microstrip based MMICs, and also in the interconnections in hybrid circuit including both microstrip and CPW.

In the first part, we have shown measurement and simulation results of GCPW-MS-GCPW transitions without making via-hole in the substrate and patterning the bottom ground plane. These transitions, using the electromagnetic coupling between the bottom and top ground planes, simplifies the manufacturing and facilitates the characterization with GSG probe station. We saw that it's possible to have a very large bandwidth up to 70 GHz with the BCB polymer.

In the second part, we continued to optimize this transition on the same polymer to be used with a coplanar connector model #1492-04A-5 of SouthWest Microwave. We have shown that the back-to-back transition presents an ultra-wide bandwidth over 30 GHz.

ACKNOWLEDGMENT

The authors wish to thank Prof Jean-Pierre Vilcot with the Institute of Electronics, Microelectronics and Nanotechnology of Lille for having provided us the BCB polymer, Mr. Marc Brunet with IREENA for his help in microwave characterization with probe station and Prof Serge Toutain with IREENA for helpful discussion.

REFERENCES

[1] A. Vilcot, B. Cabon and J. Chazelas, "Microwave Photonics from Components to Applications and Systems," Kluwer Academic Publishers, 2003.

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[4] M. El Gibari, D. Aerty, C. Lupi, M. Brunet, H. W. Li and S. Toutain, "Ultra-Broad bandwidth and low-loss GCPW-MS transitions on low-k substrates," Electronics Letters, vol. 46, pp. 931-933, June 2010.

[5] G. Straub, P. Ehret and W. Menzel, "On-Wafer Measurement of Microstrip-Based MMICs without Via Holes," IEEE. MTT-S Int. Microwave Symposium Digest, vol. 3, Jun. 1996, pp. 1399-1402.

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[7] L. Zhu and K. Melde, "On-Wafer Measurement of Microstrip-Based Circuits with a Broadband Vialess Transition," IEEE. Trans. on Advanced Packaging, vol. 29, pp. 654-659, Aug. 2006.