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Novel Low-Cost On-Chip CPW Slow-Wave Structure

for Compact RF Components and mm-Wave Applications

Guoan Wang, Wayne Woods, Hanyi Ding, and Essam Mina

IBM Semiconductor Research and Development Center

1000 River Street, Essex Junction VT 05452

[email protected]

Abstract

In this paper, an ideal slow wave coplanar waveguide

(CPW) structures with low losses, moderate impedance and

CMOS fabrication technology have been developed. The slow

wave CPW transmission line structures were achieved

through IBM 0.13 m technology with multi-layer metals.

The CPW were implemented with narrower signal line or

wider separation between signal and ground plane to increase

the inductance per unit length, while metal strips on another

metal layers cross under/above the CPW lines, which are

orthogonal to signal propagation direction. Losses reduction

using via bars to increase the thickness of the signal metal

layer, structures with metal strip options (above, under andboth CPW) to increase the capacitance per unit length of the

CPW and provide more flexibility for the proposed structure,

effect of the metal strips pitch are also discussed. The slow

wave structure discussed in this paper can shrink the side

dimension of the mmwave passive components by up to 35%.

Introduction

Future hand-held and ground communications systems as

well as communications satellites will require very low

weight, volume and power consumption in addition to higher

data rates and increased functionality. Improvements in the

size and component count have been achieved by increasing

the level of integration. Despite many years of research, there

is a technological barrier for the IC industry to furtherintegration. Components, such as high-Q inductors,

capacitors, varactors and ceramic filters, play a limiting role

in further reducing the size. Passive components are

indispensable in RF systems and are used for matching

networks, LC tank circuits (in VCOs), attenuators, power

dividers, and filtering, switching, decoupling purposes and as

reference resonators. Right now, large percentage of the board

area is taken up by the off-chip passives. For instance, 90-

95% of components in a cell phone are passive components,

taking up 80% of the total transceiver board area, and

accounting for 70% of the cost. To reduce the space taken up

by the passives, very small discrete passive components and

the integration of the passive components are needed. Slowwave transmission lines are promising candidates for size

reduction of microstrip circuit components, and it is very

attractive to develop a complementary CPW slow wave

structure for the miniaturization of MICs and MMICs.

Metal-insulator-semiconductor (MIS) CPW lines can

achieve very high slow wave factors, but suffer form low

impedance values and high insertion loss, making MIS CPW

impractical at higher frequencies. Research have been done to

improve the loss by introducing cross-tie periodic structures

or by inhomogeneously doping the semiconductor, but these

methods need additional fabrication processes [1][2]. Slow

wave CPW periodic structure with arms was also developed

for filter applications [3], in the scheme, each unit cell

consists of a narrow signal line that enhances the inductance

per unit length while two branched arms located in the slots of

the CPW enlarge the capacitance to ground. Slow wave

structure with discontinuous microstrip lines were also

developed [4], the discontinuous line is made by placing a

wide and short line and a narrow and short line in turn since

the step discontinuity provides the line with additional

inductance and capacitance. Other method developing slow

wave structures includes using inductive loaded line and

capacitive loaded line by changing either the shape of theground plane or the signal plane [5]. However, none of them

was fabricated using standard CMOS fabrication process.

John Long etc. [6] developed on-chip slow wave CPW

structure with specific design.

This paper spent efforts on developing an ideal slow wave

CPW structures with low losses, moderate impedance and

CMOS fabrication technology, and provided thoroughly study

on the factors affecting slow wave effect. From the

transmission line theory, the wavelength, phase velocity and

characteristics impedance are given respectively as Equation

[1-3]:

f

v= (1)

LCv

1= (2)

C

LZ =0 (3)

From the above equations, the wavelength can be made

smaller while the characteristic impedance is kept unchanged

by increasing L and C with the same ratio. Both inductance

and capacitance of microstrip are related to line width:

inductance increases with decreasing line width, whereas

capacitance increases with increasing line width. In this

paper, the slow wave CPW transmission line structures were

achieved through IBM 0.13 m technology with multi-layermetals. As shown in Fig. 1. the CPW were implemented with

narrower signal line or wider separation between signal and

ground plane to increase the inductance per unit length, while

metal strips on another metal layers cross under/above the

CPW lines, which are orthogonal to signal propagation

direction so there will be no induced current in the metal

strips along signal propagation direction and avoid

electromagnetic interference, meanwhile, the capacitance per

unit length of the CPW is increased due to capacitive

coupling between the CPW and cross above/under metal

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strips. The proposed method offers several advantages. First,

the ground planes of the CPW lines remain unperturbed so

there will have little effect on CPW wave propagation. And

second, the proposed structure can be matched with different

impedance over a wide range of frequencies up to 120 GHz

by using different metal layer options. Third, the proposed

structure is a low-cost on-chip slow wave solution with

standard CMOS fabrication process. And finally, current

transmission line model has included metal fill effects, so itcan be used to design such kind of slow wave structure with

provided Pcell which has already passed the DRC and LVS

check. Different structures with different dimensions and

metal layers have been developed, simulation results have

shown that the capacitance and inductance per unit length of

the proposed lines can be increased up to 2 times of the

conventional structures. The developed structures can be used

to develop mm-wave passive components with twice of the

size reduction.

Fig. 1 Schematics of slow wave coplanar waveguide

Structures and Principle

As shown in Figure 1, the slow wave CPW structure

including floating metal strips cross over and cross under the

CPW, each section of the slow wave CPW can be described

by an L-C lumped element model, where L and C are the line

inductance and capacitance of the section, respectively.

Ideally, the high impedance section is designed to have line

inductance nL and line capacitance C/n, so that its

characteristic impedance is higher by a factor of n2 than the

low impedance section with line inductance L/n, and line

capacitance nC. The higher inductance requires aproportionally smaller line capacitance because the velocity is

fixed by the dielectric constant. In the physical layout of the

transmission line, higher inductance requires more distance

between the signal and ground paths which reduce line

capacitance. When the two sections are cascaded together, the

series inductance is dominated by the high impedance section,

while the capacitance is dominated by the low impedance

section. This causes a simultaneous increase of the both L and

C, resulting in an increased delay of the signal [6]. In the

section without the floating metal strips crossing over or

crossing under the CPW, the inductance and capacitance per

unit length of the CPW is mainly decided by the gap between

the ground and signal plane of the CPW; in the section with

crossover and cross under metal strips, the capacitance per

unit length is increased tremendously due to the small

distance between the metal strips and CPW, while the

inductance will also reduced simultaneously if the metal strips

is connected with the ground plane of the CPW.

Fig. 2 shows the equivalent circuit of the slow wavestructure. As shown in the figure, when there is metal strips

crossing over or crossing under CPW, the capacitance will

increase, and this will cause velocity of signal propagation

reduce, so will the wavelength. With slow wave structure, the

compact mmwave passive components can be designed. For

example, it can be used to design compact branch line coupler

as shown in Fig. 3, branch line coupler have four arms each

has a electrical length of quarter wavelength, the dimension of

the coupler is highly dependent on the wavelength.

Fig. 2 Equivalent circuit of slow wave CPW

Fig. 3 Schematics of a branch line coupler

Fig. 4 shows the cross section of the back end of line in a

0.13 m IBM process technology. In this paper, CPW is

implemented with MQ layer, while the cross under layer is

M2 and crossover layer is LY to get high coupling

capacitance between the metal strips and CPW due to the

relative smaller distance between MQ and M2 layer and MQ

layer has reasonable thickness to reduce the metal loss. The

distance between M2 and MQ is less than 1 m while thedistance between MQ and LY is about 4 m. EM simulations

for different slow wave CPW structures have been done using

Ansoft HFSS full wave 3D simulator, and the results are

discussed in the next section. Five different cases are

discussed, first, slow wave CPW and traditional CPW are

simulated and compared; second, the effect of the floating

metal strips pitch is studied; third, as shown in Fig. 5, metal

strips are connected with ground plane of the CPW, the effect

is studied; Fourth, metal strips cross options are discussed, the

effect of cross under metal only and both crossover and

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crossover metals are simulated and compared; and the final

case is that using combined MQ and top via metal as CPW to

improve the insertion loss of the CPW.

Fig. 4 Cross section of metal layers in a BiCMOS

technology

Fig. 5 Revised slow wave CPW structure

Results and Discussions

In this section, simulation results for slow wave CPW

structures are shown. All the CPW structures are implemented

in the back end of line using 0.6 m thick copper.

1. Slow wave CPW vs. traditional CPW

To demonstrate the slow wave effect of the structure as

shown in Fig. 1, simulations are firstly done for single CPW

with and without the cross under layer. Fig. 6 shows the

results comparison for the both cases, the cross line shows the

calculated capacitance per unit length for traditional CPW

with signal layer width of 6 m and the spacing between thesignal and ground plane of 4 m, the circle line shows the

slow wave CPW with floating metal strips crossing under the

CPW, the width of the metal strips is 2 m and the spacing is

2 m. As shown in the Fig. 6, at 100 GHz, the capacitance per

unit length increased 121% from 0.1619nF/m to 0.3591nF/m.

2. Pitch effect

To check the effect of the metal strips pitch (metal strip

width plus spacing between strips), simulation have been

done for slow wave CPW with cross under layer with 4 m

and 10 m pitch. The CPW has signal width of 6 m and the

distance between the signal plane and ground plane is 4 m.

The result for capacitance per unit length comparison is

shown in Fig. 7. For the structure with smaller pitch, the slow

wave effect is better. At 100 GHz, the capacitance per unit

length increased 11% from 0.3235nF/m to 0.3591nF/m. To

further increase the slow wave effect, the pitch can be shrunk

to the minimum width and space for the MQ layer in the

practical applications.

Fig. 6 Capacitance per unit length comparison between

slow wave CPW and traditional CPW

Fig. 7 Capacitance per unit length comparison between slow

wave CPWs with different pitch

3. Floating strips or connect with CPWIn order to increase the capacitance per unit length of the

slow wave CPW structure, the floating metal strips are

connected with ground plane of the CPW as shown in Fig. 5.

The result comparison between the traditional slow wave

structure and the novel structure is compared in Fig. 8. At 100

GHz, the capacitance per unit length increased 28% from

0.3591nF/m to 0.4593nF/m. Hence, the new structure with

metal strips connected with CPW ground plane instead of

floating has better slow wave effect.


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wave CPWs

4. Crossunder, Crossover or Both

To check the further effect of metal strips with both

crossover and cross under options, simulations and analysisare done in this section. First, the slow wave CPW is built

with 6 m wide MQ layer and 4 m spacing between ground

and signal plane, while the metal strips crossing under the

CPW and connected with ground plane of the CPW; and then

another metal layer crossing over the CPW is added to the

structure and simulated. The pitches for both the crossover

and cross under metal strips are 4 m. The capacitance per

unit length comparison is shown in Fig. 9, At 100 GHz, the

capacitance per unit length increased 3% from 0.4593nF/m to

0.4702nF/m. Hence, the new structure with crossover and

cross under metal strips has better slow wave effect, the

improvement is slight in this simulation is mainly because that

the crossover layer is further away from CPW (~4 m)

compared with the distance between the cross under layer andCPW (~0.6 m). In the practical design, metal layer can be

selected purposely to get equal distance between the

crossover layer and CPW and CPW and cross under layer.


wave CPWs

5. Metal loss improvement

Since the MQ layer is less than 1 m thick, the metal loss

for the CPW built on MQ layer is relative high. In order to

reduce the metal loss for the CPW, slow wave CPW structure

built with MQ layer combined with M2 layer connected with

metal via bars as shown in Fig. 10. In this structure, the

thickness of the CPW metal layers is doubled, thus reduces

the metal loss. The width of the signal layer is still 6 m and

the spacing between the signal and ground plane is 4 m. Themetal strips crossing under the CPW are M1 layer with the

pitch of 4 m. The capacitance comparison between the new

structures and the slow wave structure built with MQ layer

only is shown in Fig. 11. And Fig. 12 shows the insertion loss

comparison for the two structures.

Fig. 10 Structure of slow wave CPW with improved insertion

loss

Fig. 11 Capacitance per unit length comparison between slowwave CPWs with different thickness


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S-parameters vs. Frequency

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0 20 40 60 80 100 120

Frequency (GHz)

S12(dB)

Slow wave CPW

Slow wave CPW with improved loss

Fig. 12 Insertion loss comparison between thin slow wave

CPW (MQ) and thick slow wave CPW (M2 and MQ

combined with metal via bars)

At 100 GHz, the capacitance per unit length is increased

20% from 0.36nF/m to 0.4303nF/m; this capacitance

changing is mainly due to the larger thickness of the CPW. As

shown in the Fig. 12, the metal loss is greatly improved

especially at higher frequency for slow wave CPW built with

thick metal option. At 100 GHz, the insertion loss reduced

from 1.22 dB to 1.03 dB.

Conclusions

A novel on-chip slow wave CPW structure which has

metal strips crossing over and crossing under is provided in

this paper. The factors affecting the slow wave effect are

discussed: pitch of the floating metal strips, metal strips

crossing options (over, under and both), metal strips

connected with ground plane of the CPW, and solution to

improve the insertions loss of the slow wave structure.

Results show that the smaller the metal strips, the better the

slow wave effect. Detail results are shown in the paper, and a

CPW structure with better slow wave effect is proposed, thestructure has both crossover and cross under metal strips

which are connected with the ground plane of the CPW. The

proposed slow wave structure can be used for compact

mmwave passive components design.

Acknowledgments

This work is supported by IBM semiconductor research

and development center.

References

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waveguide on semi-insulating GaAs substrate," Electronic

Letters, vol. 17, no. 25, pp. 940-941, Dec. 1981.

2. K. Wu and R. Vahldieck, "Hybrid-mode analysis of

homogeneously and inhomogeneously doped low-loss

slow-wave coplanar transmission line," IEEE Transactions

on Microwave Theory and Techniques, vol. 39, no. 8, pp.1348-1360.

3. James Sor, Yongxi Qian and Tatsuo Itoh, " A novel low-

loss slow-wave CPW periodic structure for filter-

applications," IEEE MTT-S conference, Phoenix AZ

2001.

4. Kae-Oh Sun, Sung-Jin Ho, Chih-Chuan Yen, and Daniel

van der Weide, " A compact branch-line coupler using

discontinuous microstrip lines," IEEE Microwave and

Wireless Components Letters, vol. 15, no. 8, pp. 519-520.

5. Kaixue Ma, Jianguo Ma, Manh Anh Do and Kiat Seng

Yeo, "Experimentally investigating slow-wave

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CPW periodic cells," 2005 IEEE MTT-S conference,Long Beach, CA 2005.

6. Tak Shun Dickson Cheung and John Long, "Shielded

passive devices for silicon-based monolithic microwave

and millimeter-wave integrated circuits," IEEE Journal of

Solid State Circuits, Vol. 41, No. 5, May 2006.


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