experimental investigations of microstrip distributed mems

International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME

International Conference on Communication Systems (ICCS-2013) October 18-20, 2013 B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India Page 222

Experimental Investigations of Microstrip Distributed MEMS Transmission Line Phase Shifter

V Janardhana1, S Pamidighantam2, N Chattoraj1, J S Roy3, K S Reddy4, R G Kulkarni5,

Kamaljit Rangra6

1Faculty/Electronics & Communication, Birla Institute of Technology, Mesra, Ranchi, India 2Birla Institute of Technology (BIT-P), Hyderabad, India

3School of Electronics Engineering, KIIT University, Bhubaneswar, Orissa, India 4Department of Informatics, University of Oslo, Oslo

5HMC-Lab, Bharat Electronics Limited, Bangalore, India 6Central Electronics Engineering Research Institute, Pilani, India

[email protected], [email protected], [email protected], [email protected],

[email protected], [email protected], [email protected]

ABSTRACT: In this work RF MEMS phase shifter on Microstrip DMTL is attempted for the first time. Microstrip conductor is lifted from the ground plane to form a parallel plate bridge capacitance between the Microstrip conductor & ground. The measured and simulated results of the structure are presented in a table below:

Frequency (GHz)

Model Length (µm) Phase Shift ( ) Loss (dB)

10 3606 Simulated Measured Simulated Measured 9° 5° 0.41 3.46

KEYWORDS: Microstrip, Phase shifter, DMTL

I. INTRODUCTION Voltage & current on the air filled Microstrip line (Fig. 1) are uniquely defined as the dominant mode is TEM. With the inductance L & capacitance per unit length C of the transmission line, the characteristic impedance and phase velocity can be written as [1, 4, 11]:

cCCLC

CL

Z o1

(4.1)

LCV

oo

11

(4.2)

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International Conference on Communication Systems (ICCS-2013) October 18-20, 2013 B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India 23

Fig. 1: Electromagnetic Field of Air-Filled Microstrip-TEM mode

Equation (4.1) indicates that the analysis of the characteristic impedance can be reduced to the determination of the capacitance per unit length C the capacitance per unit length of an ideal air-filled parallel plate transmission line of effective width weff & height h reads

hwC eff

o (4.3)

The effective width weff can be calculated from

92.0

h2weπ2ln

hh2ww eff (4.4)

Microstrip Inductance per unit length L can be written as [1,2,3,4]:

wh

μLeff

o (4.5)

II. THEORY & DESIGN

The structure of MDMTL phase shifter (Fig. 2) consists of CPW input and output feed lines with 50 Ω impedance. The DMTL line is lifted above the ground that forms a parallel plate bridge capacitance with ground plane. The equivalent circuit of MDMTL is shown in Fig. 4.

Fig. 2: 3D view of Microstrip Distributed MEMS Transmission Line Phase Shifter MDMTL



Fig. 3: CST - Electromagnetic MDMTL Simulation Model

DC bias line is under the MDMTL line (Fig. 3) to pull it down through electrostatic actuation, with bridge and the DC bias lines acting as two electrodes. The MDMTL Bridge in an initial position has a capacitance Cup, and subsequently Cdn, with the ground plane, when it is pulled down through DC electrostatic actuation. The change in the capacitance alters the phase velocity of the electromagnetic field leading to a phase shift.

Fig. 4: Equivalent Lumped Element Model of MDMTL

Substituting (4.3) & (4.4) in (4.1), the characteristic impedance of the air-filled microstrip transmission line is calculated from:

)2(92.0

22ln1

2

5.0)( hw

hwe

hw

airZo

oo

(4.6)

Hammerstad and Jensen have derived an accurate formula for characteristic impedance oZ of a Microstrip transmission line and are given below [5,6,9,11,12]:

2

121ln

2)(

wh

wh

hwFHamZ o

o (4.7)

7528.0

1 wh666.30exp6π26

hwF

Where o = 120 is the wave impedance in free space.



Table 1: Microstrip Impedance Calculation from Various Formulae

From (4.4), (4.1) & (4.6), (4.7), (4.5) & (4.3) - weff, Zo, Zo (air), Zo (Ham) L, & C, are calculated respectively for various heights go (h).

Fig. 5: Simulated S- Parameter and Smith Chart for MDMTL Model With go (h)= 1.3 µm

At S11 peak the transmission line acts as a quarter wave transformer and hence Zl = Zo at this point.

Γ1Γ150Z

in

inl

(4.8)

From Fig. 5, S11 peak is 28 dB, = 0.043 and hence load impedance Zl = 53 Ω from (4.8), it can be observed from the Table 1 that for go = 1.3 µm the calculated value for Zo(Ham) is 42 Ω, hence it is underestimating the Zl, as the simulate value is 53 Ω. From Fig. 5, The value of impedance Zo(Ham) needs to be multiplied by 1.26.

go (µm) (h) weff (µm) Zo (Ω) Zo(air) (Ω) Zo(Ham) (Ω) L (nH) C (pF) 3.5 23 88 55 82 184 60 2.5 21 62 43 67 144 76 1.5 19 37 29 47 97 114 1.3 18 32 25 42 86 129 1.2 18 30 24 39 80 138 1 18 25 20 33 69 161

0.8 17 20 17 27 56 195



go (µm) Zo (Ham) (Ω)

3.5 103

2.5 84

1.5 59

1.3 53

1.2 49

1 41

0.8 34 Table 2: Corrected Microstrip Impedance

Zo (Ham) from Table 1 is multiplied by 1.26 times to arrive at Zo (Ham) values at Table 2. Phase Shift: The phase shift is found from [8]:

=

ldnlup

o

Z1

Z1

cZfπ2 (4.9)

A. MDMTL Single Bridge-3 Cascaded Units The microstrip bridge consists of center conductors with width w = 15µm. The length of the bridge is 3 x1202 µm. The total length of the structure L = 5257 µm.

Fig. 6: CST Model of Single Beam MDMTL three units cascaded

Fig. 7: Simulated & Measured S-Parameter of Single Beam MDMTL

Three units cascaded Model at go = 3.5 µm



S11 peak 12.8 dB & 10.0 dB of measured & modeled structure respectively, the load impedance from (4.8) is 71 Ω & 84 Ω respectively. The S11 Null of measured from Fig. 7, is closer compared to the simulated S11 Nulls the reason being the height of the transmission line go from the ground plane in the fabricated structure is less & non uniform compared to go in the modeled structure.

Fig. 8: Simulated Phase shift of Single Beam MDMTL

Three units cascaded at go = 3.5 µm & 2.3 µm

Fig. 9: Optical Microscope Picture of Fabricated Three Cascaded

Single Beam MDMTL phase shifter

Fabrication: Surface micromachining technology process is used, unlike bulk micromachining where a substrate is selectively etched to produce structures, surface micromachining is based on the deposition and etching of different structure layers. Corning glass substrate is used. The detailed technology used for fabricating the structure is discussed in [13] Measurement: Wafer level measurements are carried out using Cascade Microtech probe station along with Agilent PNA 8362 B Network analyzer.

Fig. 10: Line loss for length L: Calculated from (4.10), Simulated & Measured loss for

Single Beam MDMTL Three Cascaded Units at go = 2.3 µm.

-15

-10

-5

0

0 5 10 15 20

Microstrip 3 Cascaded Single Beam 2.3 SimulatedMicrostrip 3 Cascaded Single Beam 2.3 CalculatedMicrostrip 3 Cascaded Single Beam 34V Measured

Frequency - (GHz)

Loss

-(d

B)



B. Ohmic Losses Within the conductors, losses result from the finite conductivity of the metal. The following approximate expression is found sufficient for loss in most situations [2,11,12].

m/dBwZR686.8α

o

sc (4.10)

Where sR = σωμ2

is metal wall surface resistance, w is width of the conductor.

C. SWR Loss The line losses increase when SWR is greater than 1:1, S11 > 27 dB, > 0.04 the net effect of standing waves on transmission line is to increase the average value of current and voltage compared to the matched line. An increase in current rises I2R ohmic losses in the conductor and increase in voltage increases E2/R losses in dielectric - Line loss increase with frequency, since the conductor resistance is related to skin effect and because of dielectric loss rise with frequency. The measured loss includes the loss due to SWR loss, ohmic losses and also includes the additional loss due to the difference in conductivity of the conductors (Bridge). The material used for fabrication of conductors may have less conductivity than the aluminum conductivity of 3.56 X 107 Siemens / met. The additional loading effects on the conductor due to the change in current distribution on the transmission line in the fabricated structure also add to the loss. This is not accounted for in the formulation of loaded line loss. These are the reasons for the measured loss to be more compared to simulated loss.

Fig. 11: - Phase Shift for length L- 5257 µm, Calculated from (4.9), Simulated & Measured

for Single Beam MDMTL Three Cascaded Units at go = 3.5 µm & 2.3 µm

D. Phase Shift Fig. 11, The simulated phase shift values between two bridge positions are taken from Fig. 8. The calculated values of phase shift are from (4.9). Phase shift has not increased due to effective length increase of cascaded structure. This is due to the fact that the equivalent inductance per unit length of transmission line also changes

-18-16-14-12-10

-8-6-4-20

0 4 8 12 16 20

Microstrip Three Cascased Single Beam SimulatedMicrostrip Three Cascaded Single Beam CalculatedMicrostrip Three Cascaded Single Beam Measured 34V

Frequency - (GHz)

Ph

ase

Shif

t -(

Deg

rees

)



along with equivalent capacitance per unit length of transmission line, when the bridge is moved down, even though at a lesser rate than the capacitance change . III. CONCLUSION The advantage of low loss in microstrip could not be fully taken in this design due to height go = 3.5 µm of the microstrip from ground & subsequent mismatch along the structure with input impedance –Table 2 & resulting in standing waves that in turn increases the loss. The loss can be minimized with the optimum height go = 1.2 µm so that there is impedance match–Table 2. Phase shift of 5 is achieved for height go = 3.5 µm & 2.3 µm, the loss can be minimized as discussed, however the phase shift will be in the same range as the line inductance also changes along with the line capacitance even though at a lesser rate. IV. ACKNOWLEDGMENTS

The authors would like to thank Bharat Electronics, Bangalore, India, for their cooperation, by providing their facility for fabrication & measurements. We would also thank Prof. K C Gupta, university of Colorado for his invaluable guidance. CST - Microwave Studio software is used for simulation. REFERENCES [1] Ramesh Garg, "A Microstrip design guide ", IIT Kanpur, 1978. [2] R. K. Hoffmann, “Handbook of Microwave Integrated Circuits ", Artech House, Inc., Norwood, MA, 1987. [3] Bharathi Bhat, “Stripline-Like Transmission Lines for Microwave Integrated Circuits ", Centre for Applied Research in Electronics Indian Institute of Technology, New Delhi, Wiley Eastern Limited, 1989. [4] D.M.Pozar, “Microwave Engineering ", Addison-Wesley Publishing Company, Reading, MA, 1990. [5] Dr. E.H.Fooks, Dr R.A. Zakarevicius, " Microwave Engineering using Microstrip Circuits", prentice Hall of Australia Pty Ltd., 1990. [6] Paul H. Young, “Microstrip Design Laboratory ", Senior Member, IEEE, 1991. [7] K.C. Gupta, Ramesh Garg, Inder Bahl, Prakash Bhartia," Microstrip Lines and Slotlines", Second Edition, Artech House, Inc., 1996. [8] Nicolas Scott Barker, “Distributed MEMS Transmission Lines ", A dissertation, University of Michigan, 1999. [9] T.C.Edwards & M.B.Steer, "Foundation of Inter connect & microstrip design", John Wiley & Sons, 2000. [10] Hector J. De Los Santos, “RF MEMS Circuit Design for Wireless Communication ", The Artech House Micro electromechanical Systems (MEMS) Series, 2002. [11] Gunter Kompa, "Practical Microstrip design and applications ", ARTECH HOUSE, INC., 2005. [12] Kai Chang, “Encyclopedia of RF and Microwave Engineering", John Wiley & Sons, Volume I, 2005. [13] Laha, R., B. Sriram, A. T. Kalghatki, K. Natarajan, and S. Pamidighantam, “Design, development and characterization of surface micromachined passive components for radar applications," Proceedings of International Radar Symposium (IRSI), Bangalore, India, Dec. 2005.

experimental investigations of microstrip distributed mems

Technology

microstrip line

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h w f1

mdmtl line

h w eff4

microstrip dmtl

india institute of technology

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