wideband frequency patch antenna seminar report.doc

Design of MEMS-based wideband frequency reconfigurable microstrip patch antenna

VISVESVARAYA TECHNOLOGICAL UNIVERSITYJnana Sangama, BELGAUM - 590018

TECHNICAL SEMINAR ON --------------------------------------------------------------------------------------------------

Design of MEMS-Based Wideband Frequency Reconfigurable Microstrip Patch Antenna--------------------------------------------------------------------------------------------------

Submitted in partial fulfillment of the Technical Seminar8th Semester, ECE

Under the Guidance ofDR. C.R. BYRA REDDY

(Asst. Professor, Dept of E & C)

Submitted by

PAWAN KUMAR 1BI09EC063

Department of Electronics and communication Engineering

BANGALORE INSTITUTE OF TECHNOLOGYBANGALORE

K.R. Road, Bangalore – 560004

2012-2013

1 ELECTRONICS AND COMMUNICATION ENGINEERIMG


BANGALORE INSTITUTE OF TECHNOLOGY(Affiliated to Visvesvaraya Technological University)

K. R. Road, V.V. Puram, Bangalore-560004

Department of Electronics and Communication Engineering

CERTIFICATE

This is to certify that the seminar report entitled “A New Design of MEMS-Based Wideband Frequency Reconfigurable Microstrip Patch Antenna” presented by PAWAN KUMAR bearing USN 1BI09EC063, student of final year is in partial fulfillment for the course of Bachelor of Engineering in Electronics and Communication Engineering of the Visvesvaraya Technological University during the academic year 2012-13.

Evaluated By

DR. C.R. BYRAREDDY

Signature of the H.O.D Asst. Professor

Name of the student: PAWAN KUMAR

University Seat Number: 1BI09EC063

Date:


50


ACKNOWLEDGEMENT

The satisfaction and euphoria that accompanies the successful

completion of any task would be incomplete without the mention of the people

who made it possible. I wish to express gratitude to the guide DR. C.R.BYRA

REDDY Asst. professor, electronics and communication department, for his

valuable suggestions and guidance through the preparation of seminar. I am

grateful for his belief on me that I would present the seminar as per

expectation and for giving me a free hand in setting schedules and deadlines.

My profound gratitude to H.O.D Dr. PRASAD K. V and Principal Dr. SURESH

K.R for providing the necessary infrastructure. I am also thankful to all the

teaching and non-teaching staff who has directly or indirectly contributed

towards the seminar.

A large amount of material has been obtained from the internet and

numerous other incredible sources. I thank all those unseen faces whose

contribution to my venture has been invaluable.

I thank my beloved parents for their blessings. It is their prayers that

have helped me in preparation for the seminar. Finally I thank god for his

blessings on me.

PAWAN KUMAR



CONTENTS

1. ABSTRACT 5

2. INTRODUCTION 6

3. ANTENNA CONFIGURATION 7

4. OPERATING PRINCIPAL 8

5. MEMS ACTUATOR CONFIGURATION AND DESIGN 12

6. FABRICATION PROCESS 17

7. SIMULATION RESULTS 21

8. CONCLUSION 25

9. REFRENCES 26



Design of MEMS-Based Wideband Frequency Reconfigurable Microstrip Patch Antenna

Abstract

A new micromachined wideband frequency reconfigurable microstrip patch antenna is demonstrated in this paper. The tunable antenna system is fabricated by a combination of surface and bulk micromachining processes.Thermal actuation is employed to precisely adjust the operating frequency with voltage ranges compatible with today’s CMOS circuitries. A bulk micromachined silicon membrane containing thermal actuators serves as a platform for antenna patch. The patch is excited by a 50Ω feed line through proximity coupling.

The out-of-plane motion of the thermal actuators normal to the antenna surface deflects the patch downward respect to the fixed ground plane. This deflection reduces the air gap height between the antenna patch and feed line and consequently, resonant frequency of the antenna. The effect of initial air gap height on achievable tuning range and bandwidth of the antenna is also investigated and the optimum range is presented. The up-state operating frequency of the antenna is 22.61GHz which continuously can be lowered to 22.17GHz. A tuning range of 440MHz was achieved by a CMOS compatible actuation voltage of 1.5V. The bandwidth of the antenna for up and down-state positions are 1.34 and 1.27GHz respectively. The RF performance of the antenna and its mechanical behavior is investigated by FEM analysis and satisfactory results are obtained within the tuning range.



Introduction

One of the most desirable features of smart antennas in high-performance wireless communication systems is the ability of precisely adjusting the operating frequency, while retaining the same radiation pattern within the tuning range. Such antennas have improved the flexibility and performance of cordless communication systems by eliminating the need for various antenna systems operating at different frequencies in applications like as reconfigurable sensor networks and smart tracking systems.

Microelectromechanical systems (MEMS) technology is an emerging technology that has revolutionized the implementation of RF components with reduced feature size and enhanced performance. Recently demonstrated frequency reconfigurable microstrip patch antennas employing MEMS technology are low-power, low cost and low size antennas that have shown excellent functionality within the tuning range.

This technology has also enabled the integration of antenna system with RF analog functions into a compact module.However, one of the challenging issues associated with these miniaturized antennas is the dependency of resonant frequency on fabrication process tolerances which leads to a deviation in operating frequency. This undesirable effect can be suppressed by the tuning capability of these smart antennas. In this way, the operating frequency of the antenna can be precisely adjusted on the desired value by means of some MEMS-based actuation mechanisms. Some methods have been proposed in literature for frequency reconfigurable microstrip patch antenna design. In a CPW stub which MEMS bridge type capacitors are placed periodically is attached to one of the radiating slots of the microstrip patch antenna. By application of a bias voltage between MEMS capacitors and CPW stub the capacitance loading and hence the resonant frequency of the antenna can be tuned. A tuning range of 300 MHz by an actuation voltage range of 0-11.9V was achieved. Authors proposed a circular microstrip patch antenna which operating frequency was tuned through adjusting the air gap height under the suspended patch by applying an electrostatic force between the patch and fixed ground plane. By this way with an actuation voltage of 165 V a tuning range of 270 MHz was achieved.



This paper presents a novel wideband MEMS-based frequency reconfigurable microstrip patch antenna. Thermal actuators are choose as driving mechanism in order to operate at voltage levels compatible with CMOS circuitries. The upstate operating frequency of the antenna is 22.61GHz. A tuning range of 440MHz was achieved by an actuation voltage of 1.5V. The bandwidth of the antenna in up and down-state (3μm downward deflection of patch) positions are 1.34 and 1.27GHz, respectively.

Antenna Configuration A schematic view of the proposed micromachined antenna is shown in figure 1(a). It consists of two segments; a micromachined silicon chip (upper segment) and a 500-μm thick RF-module (lower segment). For fabrication of upper segment which includes thermal actuators, meandered springs and 4-μm-thick suspended silicon membrane a process combination of surface and bulk micromachining is used. The hot-arms of the thermal actuators are formed by surface micromachining over the silicon chip, while the suspended silicon membrane, meandered springs and cold-arms are fabricated by bulk micromachining. The dimension of the suspended silicon membrane (εr=11.9) is 4×4mm2 and serves as a platform for a 3×3mm2 thin golden patch. The lower segment incorporates the proximity feed system. Mounting the silicon chip over the RF-module introduces an air gap of 114μm between antenna patch and feed line.

Figure 1(a) Schematic view of the proposed micromachined antenna.



Figure 1(b) Conceptional operating principal of the antenna.

Operating Principle The dielectric material type and its thickness have a significant effect on resonant frequency of the microstrip patch antennas. In literature, many methods have been proposed for reconfigurable microstrip patch antenna design.

Among these, the effective method is the idea of introducing an adjustable air gap between the antenna’s patch and ground plane. In this way, by controlling the height of the air gap, the effective dielectric constant and consequently, operating frequency of the antenna can be tuned. Another beneficial outcome of this method is that the air gap enhances the bandwidth of the antenna.

For the proposed microstrip patch antenna system, the dielectric layer is composed of epitaxial silicon (silicon membrane), air gap cavity and RF-module. The air gap cavity gives the antenna frequency tuning capability and increases the bandwidth.

A conceptional operating principal of the antenna is shown in figure 1(b). As the picture suggests, downward deflection of the suspended silicon membrane by thermal actuators reduces the air gap height and hence changes the effective dielectric constant of the antenna. Any variation in the effective dielectric constant influences the operating frequency and bandwidth of the antenna.



Figure 2 shows the resonant frequency shift of the antenna versus the air gap height variations. As the figure indicates, with decreasing the air gap, resonant frequency of the antenna decreases.

To investigate the effect of initial air gap height and its variations on the achievable tuning range a finite element analysis is done.First, we designed an antenna with initial air gap of 500μm operating at 29.96GHz. Decreasing the air gap height reduces the resonant frequency and degrades the return loss of the antenna.

Here the maximum reduction of the air gap height for a return loss better than 10dB is 350μm which yields a frequency shift of 5.66GHz. Two other antennas were designed with initial air gaps of 280 and 120μm operating at frequencies of 26.66 and 23.11GHz, respectively. Simulation results are summarized in table I. This table reveals that the 23.11 GHz antenna with initial air gap of 120μm provides larger frequency shift as a function of downward deflection of the patch.

TABLE I SIMULATION RESULTS OF THE ANTENNAS WITH DIFFERENT INITIAL AIR GAPS.



Figure 2 Resonant freq shift of the antenna versus the air gap height variations

Figure 3. Resonant frequency shift of the 23.11 GHz antenna by decreasing the air gap height.

Figure 3 depicts the resonant frequency shift of the 23.11 GHz antenna by decreasing the air gap height.



Figure 4. Effect of the initial air gap height on bandwidth of the antenna

Another simulation is carried out to investigate the effect of initial air gap on bandwidth of the antenna. Figure 4 presents the simulation result. As the figure suggests for air gap heights around 120μm, the antenna bandwidth is about 5.74%. By further decreasing the air gap height, the bandwidth of the antenna decreases almost monotonically. Based on afore mentioned simulation results, the initial air gap of the proposed antenna system is decided to be around 120μm. the value of achievable tuning range is dependent on the downward deflection produced by thermal actuators which are discussed in the next section. As the downward deflection of thermal actuators increases, the tuning range of the antenna increases too.



MEMS ACTUATOR CONFIGURATION AND DESIGN

A. Electro-thermal actuator configuration An air gap height, a large actuation voltage is required hence electro-thermal actuation is used instead of electrostatic actuation as a driving mechanism. Electro-thermal microactuators have the advantage of achieving higher deflections with considerable force by actuation voltage ranges compatible with today’s CMOS circuitries and can be fabricated by IC-compatible fabrication techniques.

In our study a horizontal U-shape electro-thermal actuator that can achieve vertical motion is presented. The schematic view of the proposed thermal actuator is shown in figure 5. It consists of two layers separated by an air gap between them; the upper layer, which actuation voltage is applied, is called hot arm and is fabricated by surface micromachining of a polysilicon layer over the epitaxial silicon. The lower layer is called cold arm and is formed by bulk micromachining of epitaxial layer of silicon. These two layers are connected to each other on the tip of the microactuator by a shuttle through a very thin layer of silicon nitride which serves as an electrical isolator. By applying an actuation voltage, two hot-arms elongate and deflect the tip of the actuator downward. The dimensions and material properties of the thermal actuator are given in tables II and III, respectively.

TABLE II: DIMENSIONS OF THERMAL ACTUATOR

TABLE III: MATERIAL PROPERTIES OF THE THERMAL ACTUATOR



Power dissipation and thermal distribution are of most challenging issues associated with electro-thermal microactuator design. From the proposed antenna standpoint, as the downward deflection produced by thermal actuator increases, the tuning range of the antenna increases too.

Figure 5. The schematic view of the proposed thermal actuator

Figure 6 shows the frequency shift of the antenna respect to air gap height variations. As shown on the figure a tuning range of 440MHz can be achieved by a downward deflection of 3μm from 114μm up-state position.



Figure 6. Frequency shift of the proposed antenna respect to air gap height variations

Figure 7. Vertical downward deflection curve of the thermal actuator respect to applied voltages



Figures 7 and 8 show the vertical downward deflection and power dissipation curves of the thermal actuator respect to applied voltages, respectively. As indicated on the figures, the respective actuation voltage and power dissipation of the thermal actuator for a 3μm deflection are 1.5V and 2mW.

Figure 8. Power dissipation curve of the thermal actuator respect to applied voltages

Thermal distribution over the actuator for a 1.5V applied actuation voltage is illustrated in figure 9.



Figure 9. Thermal distribution over the actuator for a 1.5V applied actuation voltage

B. Mechanical stability considerationsAny suspended structure, is subject to unwanted mechanical noises such as vibrations and shocks. To have a mechanically stable system, the designer needs to shift the mechanical natural frequencies of the suspended structure to a higher range respect to frequency range of mechanical noises.In this design the suspended silicon membrane is mechanically fixed through a number of suspension systems which are a combination of thermal actuators and meandered springs. The mechanical functionality and power dissipation of the antenna structure is mainly dependent on the number of suspension systems connected to the silicon membrane.

To investigate the mechanical resonant modes of the structure, a finite element analysis is done for 4, 6 and 8 numbers of suspension systems connected to the silicon membrane. Simulation results are summarized in table IV.



TABLE IV: natural frequencies of antenna structure for Different numbers of suspension systems connected to the silicon Membrane

Table IV implies that increasing the number of connected suspension systems, enhances the mechanical robustness of the structure. However, the power dissipation increases. Hence for a high mechanically stable antenna structure, the number of suspension systems is choose by considering mechanical vibration and noise frequency range and power dissipation constraints of the mobile systems employing the antenna.

FABRICATION PROCESSFabrication process of the proposed antenna structure is CMOS compatible in all of the fabrication steps. Figure 10 presents the fabrication process flow.

The process begins with a chemical-physical polishing of a silicon substrate to a height of 120μm.

A 0.75-μm-thick silicon-oxide layer is grown by thermal oxidation and formed as sacrificial layer (a).



For electrically isolating between hot and cold arms, a 200-nmthick layer of silicon-nitride is coated by PECVD process and structured (b).

For formation of hot-arms and shuttle a 2-μm thick polysilicon layer is deposited and structured by RIE process (c).



The hot-arms are released by etching the sacrificial layer in a gas phase HF (d).

To formation of the patch, a 150nm gold layer is deposited through lift-off process (e).



A bulk etching on the back side of the silicon substrate is performed by KOH solution to form a 4-μm-thick silicon membrane (f).

Finally, by the aid of a very thick photo resist layer, the RIE is performed to form the cold arms, meandered springs and floating silicon membrane (g)

Anchor

deposited patch



SIMULATION RESULTS

Return loss of the proposed antenna is plotted in figure 11 for various applied voltages.

Figure 11. Return loss of the antenna for various applied voltages

It is obvious from the figure that the resonant frequency shifts downward with increasing the bias voltage. At 0V, antenna is in up-state position and resonates at 22.61GHz. As the bias voltage increases, the antenna provides a downward frequency shift, at 1.5V the suspended patch antenna deflects downward about 3μm and resonates at 22.17GHz. A tuning range of about 440MHz is achieved by an actuation voltage range of 0-1.5V. The antenna demonstrates a wide bandwidth of 1.34GHz in up-state and 1.27GHz in down-state (3μm downward deflection of the patch) positions. The radiation pattern for total gain at E and H planes for different resonant frequencies is illustrated in figure 12.



E-plane H-plane

Figure 12. Radiation pattern for total gain at E and H-planes: (a) Up-state



E-plane H-plane

Figure 12. Radiation pattern for total gain at E and H-planes: (b) 2μm deflection



E-plane H-plane

Figure 12. Radiation pattern for total gain at E and H-planes: (c) 3μm deflection

As the figure suggests, the antenna provides stable radiation characteristic within the tuning range.



CONCLUSION

An electro-thermally driven wideband frequency reconfigurable microstrip patch antenna fabricated by micromachining techniques has been proposed in this paper which has a stable radiation pattern within the tuning range.Thermal actuators are choose as driving mechanism in order to operate at voltage levels compatible with CMOS circuitries.

The RF performance and mechanical functionality of the antenna is investigated by FEM simulations and satisfactory results are obtained. The up-state operating frequency of the antenna is 22.61GHz which continuously can be lowered to 22.17GHz. A tuning range of 440MHz is achieved by an actuation voltage range of 0-1.5V. The tuning range of the proposed antenna can also be increased by further downward deflection of the patch by thermal actuators.

Simulation results show that the bandwidth of the antenna in up-state position is about 1.34 GHz and 1.27GHz in down-state (3μm deflection from up-state) position. The MEMS-based wideband frequency reconfigurable antenna of this study can be a promising choice for realization of low-cost, light-weight and MMIC compatible antenna systems.



REFERENCES

[1] Emre Erdil, K. T., Mehmet Unlu, Ozlem Aydin Civi, and Tayfun Akin, "Frequency Tunable Microstrip Patch Antenna Using RF MEMS Technology." IEEE transactions on antennas and propagation 55(4):1193-1196, April 2007.[2] Ramadoss, R. J. J. a. R., "A MEMS-based electrostatically tunable circular microstrip patch antenna" J. Micromech. Microeng. 17: 1-8,2007.[3] Balanis, C. A., Antenna theory: analysis and design. New York: Wiley, 1997, p.722-775.[4] J.S. Dahele, Sen. Mem, and K.F. Lee, "Theory and experiment on microstrip antennas with air gaps" 1EE PROCEEDINGS, Vol. 132, Pt. H, No. 7, December 1985.[5] Caglar Elbuken, Nezih Topaloglu, Patricia M. Nieva,, Mustafa Yavuz, Jan P. Huissoon, "Modeling and analysis of a 2-DOF bidirectional electro-thermal microactuator," Microsyst Technol:713-722, 16 January 2009.[6] D. Yan, "Mechanical Design and Modeling of MEMS Thermal Actuators for RF Applications," Master of Applied Science, Waterloo, Waterloo, 2002.[7] Tai-Ran Hsu, MEMS and Microsystem: design, manufacture and nanoscale engineering second ed. vol. 1. New Jersey: John Wiley & Sons, Inc., 2008, pp. 245-284.[8] Jorge Varona, Margarita Tecpoyotl-Torres, Jesus Escobedo-Alatorre, Anas A. Hamoui, "Design and Fabrication of a MEMS Thermal Actuator for 3D Optical Switching Applications," IEEE/LEOS Summer TopicalMeetings: 31-32, July 2008.[9] Vijay K. Varadan, K. J. V., and K.A. Jose, RF MEMS and their applications, Wiley, 2002, pp. 1-105,344-364.


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wideband frequency patch antenna seminar report.doc