JOURNAL OF ELECTRONIC SCIENCE AND TECHNOLOGY, VOL. 8, NO. 4, DECEMBER 2010 351

CPW Circulators with Barium Ferrite Thin Films Yuan Wang, Bin Peng, Wan-Li Zhang, and Ke Tan

AbstractA miniaturized circulator using barium

ferrite films with a coplanar waveguide (CPW) structure is designed and optimized by high frequency electromagnetic field simulations based on finite element methods. The best circulation performance of the film circulator based on 10 m thick barium ferrite thin films is obtained with an insertion loss of 0.13 dB and an isolation of 22.89 dB around 36.9 GHz. The microwave characteristics of film circulators with CPW and CPW with ground (CPWG) structures have been compared. The influences of the gap between the ground and the signal line, and the ferromagnetic resonance line width on the microwave properties are also studied.

Index TermsCirculators, coplanar waveguide, millimeter wave, passive microwave components.

doi: 10.3969/j.issn.1674-862X.2010.04.011

1. Introduction With the development of the modern communication

systems, microwave circulator, as one of the most important microwave components, is demanded to be miniaturized and integrated with microwave circuits. Hexagonal barium ferrite, especially M-type barium ferrite BaM (BaFe12O19), which has high uniaxial magnetocrystalline anisotropy, high coercivity, and high resistivity, is widely used in millimeter wave devices[1]-[3].

Wen[4] firstly brought forward the theory of the coplanar waveguide (CPW). The CPW structure has a signal line and a ground (GND) in an identical plane, which is well matched to the monolithic microwave integrated circuit (MMIC). The CPW structure has many benefits. For instance, they facilitate easy shunt as well as series surface mounting of active and passive devices, and they can reduce the radiation loss[5]. Unlike the microstrip, CPW has little parasitic loss between surface mounted components

Manuscript received December 20, 2009; revised April 27, 2010. This work was supported by the National Basic Research Program of China under Grant No. 61363Z06.1.

Y. Wang, B. Peng, W.-L. Zhang, and K. Tan are with the State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China (e-mail: abysswae@uestc.edu.cn, bpeng@uestc.edu.cn, and wlzhang@ uestc.edu.cn).

and an underlying ground plane. Therefore, the circulator with a CPW structure will be easily fabricated using a low-cost lithography process.

At the same time, it is known that a circulator with a CPW structure is well matched to the technology of MMIC. So, it is important to design a circulator with a CPW structure using BaM films. Ogasawara et al.[6] and Koshiji et al.[7] have shown how to modify the design method of stripline circulator to that of CPW circulator with the bulk ferrites. Oshiro et al.[8] have proposed a circulator with a YIG/CPW/YIG sandwiched structure. Later, circulators with CPW structure, which have a ground plane on alumina substrates, have been proposed by Dehlinger and Zahwe[9],[10]. In this work, film circulators with different CPW structures are compared and investigated.

2. Theoretical Study From the theoretical results obtained by Bosma[11],

design rules are achieved, and then the dimensions of a functional Y-junction stripline circulator can be calculated. All the elements of the scattering matrix are expressed in term of the Greens function. The parameters of design rules are the characteristics of magnetic material, polarization, and circulation frequency[12].

The electric field intensity in the disk is assumed to have a z-component only, so the specific gyrotropic permeability is given by

00

0 0 1

ii

=

where i is 1 , and are the Polder tensor elements of ferrite materials.

The effective permeability of ferrite (Polder tensor) eff can be introduced by

( )2 2eff = . Two circulation conditions can be analytically

expressed as

1,1

eff2R

f

= (1) ( )

eff

13 1.84

= (2)

JOURNAL OF ELECTRONIC SCIENCE AND TECHNOLOGY, VOL. 8, NO. 4, DECEMBER 2010 352

where R is the radius of the central disc conductor, f is the circulation frequency, is the permittivity, 1,1 is the Bessels solution of the first resonance mode of ferrite disks.

The internal field of the ferrite films, Hi, is defined by

0i aH H H N M= + z s

(a

(3)

where H0 is the external field, Ms is the saturation magnetization, Nz is the demagnetizing factor (Nz=1 for a thin film), and Ha is the anisotropy field. For thin film circulator, it is reasonable that H0=0 and Nz=1, then

i a sH H M= . (4)

3. Design of Circulator with CPW The thickness of the BaM films and the alumina

substrate are 10 m and 500 m, respectively. The simulation parameters of the BaM films are: the dielectric constant r=13 and the saturation magnetization Ms=240 kA/m. Based on the theory of stripline circulators, the stripline circulators are designed firstly and then optimized using high frequency structure simulation (HFSS) software.

We have derived the radius of central conductor R=0.95 mm from the material physic parameters. Numerical results of the stripline structure are transposed to a coplanar waveguide. To realize this step, a discussion of different electric and magnetic lines between stripline and coplanar waveguide should be held.

Fig. 1 shows the schematic distribution of electric line and magnetic line for stripline and CPW. The electric field and the magnetic field are shown by solid lines and broken lines, respectively. As shown in Fig. 1, the electric field is stronger in the area sandwiched by the signal line (LINE) and a GND plane in a substrate, whereas CPW has a signal line (LINE) and a GND plane in the same level with a small gap between LINE and GND plane. The electric field of stripline structure is strong in the vertical direction while that of the CPW structure is strong in the horizontal direction.

From the observation of the differences between the stripline and CPW, it can be clearly seen that the field in the centre of signal line must be modified. Zahwe et al.[9] has proposed a metallic plane which is not connected to the GND plane to modify the field line, thereby it can be realized that the electric and magnetic field of CPW structure is similar to a microstrip circulator, whose structure is schematically illustrated in Fig. 2 (c).

As shown in Fig. 2 (a) and (b), BaM films are directly deposited on the alumina substrate without the non-connected planes, and the GND plane is deposited on the opposite side of the substrate (CPW with ground structure).

GND

Fig. 1. Schematic distributions of electric field (solid lines) and magnetic field (broken lines): (a) stripline and (b) CPW.

)

Fig. 2. Schematic illustrations of the circulator with CPW structure: (a) top view, (b) cross-section of the CPWG structure, and (c) cross-section of the CPW structure.

4. Simulation and Discussion 4.1 Effects of the Two Structures with a Thin Layer of

Barium Ferrite Thin Film From the parameter studies, the dimensions of a CPW

thin film circulator are obtained as follows: the radius of the central conductor is R=0.95 mm, the width of the signal line is W=0.27 mm, the space between the signal line and GND plane is G=0.15 mm, the space between the GND plane and the signal line is S=0.16 mm, and the radius of the lower non-connected plane is RC=1.1 mm.

The simulation results of these two film circulators are shown in Fig. 3. It can be seen that the circular frequency of the circulator with the CPW structure is lower than that of the circulator with the CPWG structure. The transmission parameter S21 and the isolation parameter S31 of these two circulators maintain in the same level. However, the magnitude of the return loss S11 of the circulator with the CPW structure increases markedly. The different microwave behaviors of the circulator with CPWG and without the ground CPW structure can be interpreted by the transverse high-order mode resonance, fringing field, and the electromagnetic coupling.

(b)

(c)

Substrate

BaM thin film

Signal line

GNDSubstrate

GND

R

G

W

S

Port1

Port2 Port3

GNDGND

GND

A B

Signal line

BaM thin film

Substrate

GND

(b)

(c)

A B

(a)

GND

GND

LINE LINEGND GND

X

Z

(a) (b)

Z

X

WANG et al.: CPW Circulators with Barium Ferrite Thin Films 353

36.5 36.6 36.7 36.8 36.9 37.0

-25

-20

-15

-10

-5

0

S11 S31 S21

S-pa

ram

eter

s (dB

)

Frequency (GHz) (a)

36.0 37.5 39.0 40.5 42.0 43.5 45.0-28

-24

-20

-16

-12

-8

-4

0

0

2

4

6

8

10

Frequency(GHz)

S11 S21 S31S-

para

met

ers (

dB)

(b) Fig. 3. Simulation results of the film circulators of two different structures: (a) CPW structure and (b) CPWG structure .

(a) (b) (c) (d)

Fig. 4. Schematic illustrations of the modes on a CPWG: (a) CPW mode, (b) MSL mode, (c) first higher-order MSL mode, and (d) image-guide-like mode.

0 100 200 300 400 500-18

-16

-14

-12

-10

-8

-6

-4

-2

0

S-pa

ram

eter

s(dB

)

Gap(um)

S11(dB) S21(dB) S31(dB)

Fig. 5. Influences of the gap on the S-parameters.

It is well known that CPWG has four main propagating modes, including CPW mode, MSL (microstrip-like) mode, first higher-order MSL mode, and image-guide-like mode, which are shown in Fig. 4 (a), (b), (c), and (d), respectively[13].

The dominant CPW mode has its electric field components as sketched in Fig. 4 (a). The MSL mode resembles parallel-plate transmission line mode, which is

shown in Fig. 4 (b). If the top GND planes are wide, it may excite the next high-order microstrip-like mode as shown in Fig. 4 (c). If the dielectric substrate is even wider, this structure can support an image-guide-like propagating mode, which is shown in Fig. 4 (d).

According to Fig. 3 (b), if the top GND plane is too narrow for the signal line, then the CPWG structure is easy to excite the microstrip-like mode. If the top GND plane is too wide for the signal line, the normal working modes can be supplanted by the first higher-order MSL mode and image-guide-like mode. When the resonance frequency is in the range of frequency band, this effect can increase the standing wave ratio so as to make the return losses even smaller than that of the CPW structure. Another reason may be the effect of electromagnetic coupling, which is produced by the eddy-current on the interface between the substrate and the barium ferrite thin film[14]. The BaM thin films have the same characteristics with semiconductor in some aspects. The resistivity of BaM is bigger than that of alumina, which could lead to the differences of carrier concentration in two dielectric materials. The density of free carrier concentration in alumina is more than that in BaM thin film. This effect induces large inductive current in the interface between alumina and BaM thin film by electromagnetic coupling, which may alternatively induce the medium molecular polarization and lattice collision when the microwave signal is transmitting. The polaron interactions decrease the return loss. The last reason may be that the effect of fringing field can not be neglected[15], which could decrease the phase velocity. At the same time, the first higher-order modes should be taken into consideration.

4.2 Effects of the Gap on the S-Parameters The influence of the gap (space between the GND plane

and the signal line) on the transmission characteristics with the CPWG structure is investigated. The gap varies from 50 m to 500 m. We use the HFSS to calculate the return loss S11, the insertion loss S21, and the isolation S31, which are shown in Fig. 5.

The device gets the best performance at the gap of 100 m with the insertion losses less than 1 dB. From 100 m to 200 m, the isolation and the return losses decline rapidly and the insertion losses increase minimally. Outside this range, the S-parameters fluctuate with the width of the gap. Therefore, the gap of our circulator is set to be 100 m.

4.3 Effects of the H on the Insertion Loss In the CPWG type, the thickness of the barium thin film

and the gold conductor are 10 m and 2 m, respectively. The ferromagnetic resonance linewidth (H) is set to be 40 kA/m. The insertion losses are 1.21 dB at about 40 GHz. The effect of the H on insertion losses can be seen by setting the perfect conductor and the dielectric loss tangent to zero. The results are presented in Fig. 6.

S11 S21 S31

0

510152025

S-pa

ram

eter

s (d

B)

36.5 36.6 36.7 36.8 36.9 37.0 Frequency (GHz)

S11 S21 S31

0

48

1216202428

S-pa

ram

eter

s (dB

)

36.0 37.5 39.0 40.5 42.0 43.5 45.0 Frequency (GHz)

S11 (dB) S21 (dB) S31 (dB)

02468

1012141618

S-pa

ram

eter

s (dB

)

0 100 200 300 400 500Gap (m)

JOURNAL OF ELECTRONIC SCIENCE AND TECHNOLOGY, VOL. 8, NO. 4, DECEMBER 2010 354

0 16 32 48 64 80-0.6

.5

-0.4

-0.3

-0.2

.1

0.0

-0

-0

H(kA/m)

Inse

rtion

loss

(dB

)

Fig. 6. Influences of the H on the insertion losses.

From the simulation results, the magnetic losses are not the overwhelming factor in circulator based on the 10 m thin film. The insertion losses increase with H. When the H is 80 kA/m, the insertion loss is about 0.55 dB. The CPWG circulator has the same characteristics with the CPW circulator. Benevent[16] indicated that the conductor losses are predominant with regard to the insertion losses by focusing on the attenuation constants based on the thin film stripline circulator. We find this rule also holds true for the CPW circulator through simulation.

5. Conclusions We have compared the two structures and investigated

the influences of H and gap on transmission parameters. The circulator with the CPW structure has good performances on the return losses while the circulator with the CPWG structure has good performance on the isolation. If the width of the side ground is effectively controlled, the transverse high-order mode resonance can be effectively dominated out of the frequency band, thereby a better result on the insertion losses than Fig. 3 (b) can be achieved. The reasons why the CPWG circulator has smaller return loss is analyzed. Three factors caused this phenomenon are: the transverse higher-order resonance, electromagnetic coupling at the interface between the BaM films and the alumina substrate, and the effect of fringing field.

The circulator with the CPWG structure has some deficiencies. For instance, if the thickness of substrate is too larger than that of BaM thin film, the CPWG circulator is hard to realize circulation. The reason is that / decrease with the decrease of the ratio of the ferrite thickness to the overall thickness[17].

The simulation results illustrate that the conductor losses are still the main factor in the return losses of the CPWG structure. It also has the same characteristics with the stripline circulator with magnetic thin film.

It is possible to realize miniature and multifunctional microwave components with magnetic thin films through simulation results. The coplanar technologies yet have two obstacles[18]: the c-axis orientation of thin films and the thermal process. But with the development of integration

technology, the fabrication of circulators with 10-m barium ferrite thin films will come ture.

0

0.1

Inse

rtion

loss

es (d

B)

0.20.3

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Vittoria, MMIC circulator using hexaferrites, IEEE Trans. on Magnetics, vol. 37, no. 4, pp. 2389-2391, 2001.

0.40.50.6

[2] X. Zuo, H. How, S. Somu, and C. Vottoria, Self-biased circulator/isolator at millimeter wavelengths using magnetically oriented polycrystalline strontium M-type Hexaferrite, IEEE Trans. on Magnetics, vol. 39, no. 5, pp. 3160-3162, 2003.

H (kA/m)

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[6] N. Ogasawara and M. Kaji, Coplanar-guide and slot-guide junction circulators, Electronics Letters, vol. 7, no. 9, pp. 220-221, May 1971.

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[9] O. Zahwe, B. Sauviac, J. P. Chatelon, A. S. Dehlinger, M. Leberre, and S. Perrot, Towards a miniaturized circulator with magnetic thin film, in Proc. of the 37th European Microwave Conf., Munich, 2007, pp. 274-276.

[10] A. S. Dehlinger, M. Leberre, E. Benevent, H. Hassane, D. Givord, V. Larrey, and D. Vincent, Development of millimeter wave integrated circulator based on barium ferrite, Material Science and Engineering C, vol. 27, no. 13, pp. 755-758, Jul. 2008.

[11] H. Bosma, On stripline Y-junction at UHF, IEEE Trans. on Microwave Theory and Techniques, vol. MTT-12, pp. 61-72, Jan. 1964.

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[13] R. N. Simons, Coplanar Waveguide Circuits, Components, and Systems, New York: John Wiley& Sons Inc., 2001, ch. 4.

WANG et al.: CPW Circulators with Barium Ferrite Thin Films 355

[14] H. How, S. A. Oliver, S. W. McKnight, P. M. Zavracky, N. E McGruer, C. Vittoria, and R. Schmidt, Theory and experiment of thin-film junction circulator, IEEE Trans. on Microwave Theory and Techniques, vol. 46, no. 11, pp. 1645-1653, Nov. 1998.

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Yuan Wang was born in Anhui Province, China, in 1984. He received the B.S. degree from Anhui University, Hefei, in 2007 and he is currently pursuing the M.S degree with the State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China (UESTC). His research interests include microwave materials and components.

Bin Peng was born in Sichuan Province, China, in 1975. He received his B.S. degree in magnetic materials and devices in 1998 and the Ph.D. degree in microelectronics and solid-state electronics in 2007, all from UESTC, Chengdu. Currently, he is an associate professor with UESTC. His research interests include electronic information materials and devices.

Wan-Li Zhang was born in Chongqing, China, in 1966. He received his Ph.D. degree in electrical materials from UESTC in 2008. Currently, he is a professor with UESTC. His research interests include electronic information materials and devices.

Ke Tan was in Chongqing, China, in 1984. He received his B.S degree from UESTC in 2006. He is currently pursuing the M.S degree with the State Key Laboratory of Electronic Thin Films and Integrated Devices, UESTC. His research interests include microwave materials and components.

1. Introduction2. Theoretical Study3. Design of Circulator with CPW4. Simulation and Discussion5. ConclusionsReferences