CHARACTERIZATION AND COMPARISON OF LITHIUM NIOBATE
AND QUARTZ SURFACE ACOUSTIC WAVE DEVICES
by
BABURAO GHONGADE
B.E., Solapur University, 2014
A thesis submitted to the Graduate Faculty of the
University of Colorado Colorado Springs
in partial fulfillment of
the requirements for the degree of
Master of Science
Department of Electrical and Computer Engineering
2017
ii
This thesis for the Master of Science degree by
Baburao Ghongade
has been approved for the
Department of Electrical and Computer Engineering
by
T.S. Kalkur, Chair
Carlos De Paz Araujo
Heather Song
Date: 12/14/2017
iii
Ghongade, Baburao (M.S., Electrical Engineering)
Characterization and Comparison of Lithium Niobate and Quartz Surface Acoustic
Wave Devices
Thesis directed by Professor T.S. Kalkur
ABSTRACT
Sensors became the very important part of our technological life now a day.
Its accuracy and reliability became the matter of success or failure, death or life.
That’s why we all are trying to make these sensors as accurate and reliable as possible. At the same time, we need them affordable as well. We can make them cheap but reliable and accurate by using latest and advanced technology we ever known.
Basically, sensors give us very important numerical data regarding the condition or the behavior of the devices. To get this data most of the sensors need wired connections to the computers and power supply. This wiring obviously need to be done very accurately and smartly so that it will not disturb or interfere with sensors performance. This indirectly make system more complex and less robust.
On the other hand, wireless systems are handy, robust and futuristic.
Talking about wireless temperature sensors, they are required by the almost every scientific field. A temperature sensor plays an important role in various applications. For an example, regulating the temperature is a very important part of the processes used in the chemical industries, pharmaceutical industries, bio medical and almost all the electrical and electronics industries. For such important fields, the responsiveness and accuracy of the sensors can be critical.
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In this thesis, we have compared and characterized the existing (in use)
Quartz SAW devices with the newly designed and manufactured Lithium Niobate
SAW devices which can be compatible with wireless interrogation unit for wireless temperature sensing.
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DEDICATION
To My Lovely and Encouraging Family
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ACKNOWLEDGEMENTS
I want to appreciate and be thankful to the people who were there for me and supported me during this beautiful journey.
I am really thankful to my adviser Dr. T.S. Kalkur, for his encouragement, whose expertise, excellent guidelines and enormous support made it possible for me to study and work on the topic that was of great interest to me. It was a great experience working along with him. I would also like to thank all my professors for their confidence and support.
I would like to express my deepest gratitude to Mr. Bogdan Crivin for all his help in the laboratories. My sincere thanks to Eva Wynhorst for her kindness and support. Thanks to
Gauri, the most amazing person and the best friend I have met, who has been there with me for all times.
Finally, I would like to thank my parents for their tired less effort and enormous encouragement throughout my masters.
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CONTENTS
CHAPTER
1 INTRODUCTION 1
1.1 History ...... 1
1.2 Types of temperature sensors ...... 3
1.2.1 Contact Temperature Sensors ...... 3
1.2.2 Non-contact Temperature Sensors ...... 5
1.3 Temperature sensors in use ...... 6
1.4 Acoustic Wave Devices ...... 8
1.5 Objective and Scope of Thesis ...... 9
1.6 Novelty of proposed work ...... 8
2 SURFACE ACOUSTIC WAVE DEVICES 10
2.1 Getting familiar with SAW ...... 10
2.2 Development phases of SAW devices ...... 11
2.2.1 Wedge and Comb model ...... 11
2.2.2 Mortley’s model ...... 12
2.2.3 Constant pitch model ...... 13
2.2.4 Dispersive IDT model ...... 13
2.3 Modern SAW device model ...... 14
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2.3.1 Aperture(a) ...... 15
2.3.2 Acoustic Wavelength (λ) ...... 15
2.3.3 Metallization Thickness (h) ...... 15
2.3.4 Number of fingers in IDT ...... 15
2.4 Working Principle of SAW ...... 16
2.5 Types of SAW devices ...... 16
2.5.1 Resonator SAW devices ...... 16
2.5.2 One Port Resonator ...... 16
2.5.3 Two Port Resonator ...... 19
2.5.4 Reflective Delay Line Device ...... 23
2.6 Materials for SAW devices ...... 25
2.6.1 Properties of commonly used piezoelectric SAW substrates ...... 26
2.6.2 Quartz (SiO2) ...... 26
2.6.3 Lithium Niobate(LiNbO3) ...... 28
2.6.4 Physical Characteristics ...... 30
3 WAFER PROCESSING AND MAPPING 31
3.1 What is wafer? ...... 31
3.1.1 Ingot- the foundation for wafers ...... 31
3.1.2 The lapping and polishing process ...... 32
3.2 Wafer processing ...... 33
3.3 Knowing the wafer ...... 36
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3.3.1 Wafer nomenclature ...... 37
3.3.2 Wafer characteristics ...... 38
3.4 Calibrating the Network Analyzer ...... 38
3.5 Wafer mapping ...... 40
4 CHARACTERIZATION AND COMPARISON 43
4.1 SAW device specification ...... 44
4.2 Frequency response of temperatures ...... 44
4.3 Quality factor ...... 46
4.4 Bandwidth ...... 48
4.5 S-parameter characterization ...... 50
4.6 Temperature Coefficient of Frequency ...... 52
5 CONCLUSION AND FUTURE WORK 53
5.1 Conclusion ...... 53
5.2 Future work ………………………………………………………………………………………… 53
REFERENCES 55
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TABLES
Table
1.1 Most common temperature sensors in the market [13] ...... 7
1.2 Types of sensors and their application [13] ...... 7
2.1 Properties of SAW piezoelectric materials [26] ...... 26
2.2 Characteristics of Lithium Niobate and Quartz [20] [29] ...... 30
3.1 Physical characteristics of the Lithium Niobate wafer ...... 38
4.1 Specification of Lithium Niobate devices ...... 44
4.2 Specification of Quartz devices [15] ...... 44
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FIGURES
Figure
1.1 Galileo Thermometer [5] ...……………………………………………………………………………… 2
1.2 Bimetallic Temperature Sensor [1] …………………………………………………………………...2
1.3 Weld pad surface temperature thermocouple probe [3] ……………………………………… 4
1.4 Infrared temperature sensors thermometer CT series [2] ………...... 6
2.1 Rayleigh wave generation methods with (a) Wedge and (b) Comb transducer [23] 12
2.2 Wedge delay line [23] …………………………………………………………………………………… 12
2.3 Planar SAW pulse compressor [23] ……………………………………………………………….. 13
2.4 Interdigital SAW transducer by White and Voltmer, 1965 [23] ………………………….13
2.5 Interdigital pulse compressor [23] ………………………………………………………………… 14
2.6 Schematic of Surface Acoustic Wave Device [27] …………………………………………….. 14
2.7 One Port SAW Resonator [12] ……………………………………………………………………… 17
2.8 One-port SAW equivalent at resonance [22] ……………………………………………………. 17
2.9 ADS schematic of one-port device …………………………………………………………………. 18
2.10 S-parameter response of one-port SAW schematic …………………………………………… 18
2.11 Phase response of one-port SAW schematic ……………………………………………………. 19
2.12 Two Port SAW Resonator [12] ……………………………………………………………………… 20
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2.13 Two-port SAW equivalent at resonance [22] …………………………………………………… 20
2.14 ADS schematic of two-port device …………………………………………………………………. 21
2.15 S-parameter response of two-port SAW schematic ………………………………………….. 22
2.16 Phase response of two-port SAW schematic …………………………………………………… 22
2.17 Reflective Delay Line Device [25] ………………………………………………………………… 23
2.18 Crystalline structure of (a) alpha-quartz, (b) beta-quartz [21] …………………………… 27
2.19 (a) Molecular structure of SiO2, (b) Atoms with compressed force, (c) Atoms with
expanding force [22] ……………………………………………………………………………………... 28
2.20 Crystalline structure of Lithium Niobate [9] …………………………………………………… 29
3.1 Ends cut off from elongated cylindrical structure or ingot [7] ………………...... 32
3.2 Polished wafers ready for process [7] ……………………………………………………………… 33
3.3 Inter Digital Transducer [18] …………………………………………………………………………. 34
3.4 Microscopic image of actual IDT ……………………………………………………………………. 34
3.5 Flowchart of a fabrication process [15] ……………………………………………………………. 35
3.6 Actual Lithium Niobate wafer after post processing …………………………………………. 36
3.7 Wafer with labelled sections [7] ……………………………………………………………………… 37
3.8 Lithium Niobate undertesting ……………………………………………………………………….. 40
3.9 Mapped wafer-1 …………………………………………………………………………………………… 41
3.10 Mapped wafer-2 …………………………………………………………………………………………… 41
4.1 Frequency response of Lithium Niobate sensors for varying temperature ………….. 45
4.2 Frequency response of Quartz sensor for varying temperatures ………………………… 45
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4.3 Quality factor vs Temperature for Lithium Niobate ………………………………………….. 47
4.4 Quality factor vs Temperature for Quartz ………………………………………………………… 48
4.5 Plot of Bandwidth vs Temperature for Lithium Niobate …………………………………… 49
4.6 Plot of Bandwidth vs Temperature for Quartz ………………………………………………….. 49
4.7 Plot of S11-parameter vs Temperature of Lithium Niobate ………………………………… 50
4.8 Plot of S11-parameter vs Temperature of Quartz ……………………………………………… 51
4.9 S-parameter responses of oscilloscope ……………………………………………………………. 51
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CHAPTER 1
INTRODUCTION
1.1 History
The sensation of hot and cold are fundamentals to the human experience, finding the ways to measure it has challenged many great minds. As we know heat is a measure of energy in a body or material, the more energy, the hotter it is. But unlike other physical quantities such as mass and length, it’s been difficult to measure.
However, Galileo is reported to have built a device that showed changes in the temperature in 1592. This device (figure 1.1), appears to have sealed glass tube that is filled with water and some floating glass bubbles. These bubbles are filled with a colored liquid mixture which may contain alcohol, or water mixed with food color. The little hanging things with these bubbles are metal tags that has a number and degree engraved in it which indicates a temperature. In fact, they are precisely calibrated counterweights. [4]
In 1612 the Italian scientist Santorio Santorii invented the temperature measuring device known as thermoscope in which the liquid was sealed inside a glass tube which is pretty much the primitive form of today’s thermometer. Also, he was the first inventor to put a numerical scale on the instrument. Finally, in 1714, Gabriel Fahrenheit invented the first mercury thermometer which was more precise and modern than the previous devices.
Another important discovery was bimetallic temperature sensor (figure 1.2) invented in late 19th century. It uses the differential expansion of two metal strips bonded together. Change in tempera- true creates bending that can be used to activate a thermostat or a gauge like those used in gas grills
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Figure 1.1: Galileo Thermometer [5]
Figure 1.2: Bimetallic Temperature Sensor [1]
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At the beginning of 19th century, electricity was an exciting area of scientific investigation, and scientists soon discovered that metals varied in their resistance and conductivity. In 1821, the inventor called Thomas Johann Seebeck discovered that a voltage is created by joining the ends of two different metals which are held at different temperatures. Further Jean-Charles Athanase Peltier discovered that this thermocouple effect is reversible and can be used for cooling. In the same year Humphrey Davey gave the relation between electrical resistivity and temperature.
Five years later, Becquerel proposed platinum-platinum thermocouple for temperature measurement. Platinum based resistance temperature detector invented by
C.H. Meyers, measures the electrical resistance of a length of platinum wire and it is the most accurate type of temperature sensor. The 20th century witnessed the greatest invention of all i.e. silicon semiconductor. [6]
1.2 Types of temperature sensors
Temperature sensors measure the amount heat energy or even coldness generated by the body or object allowing us to sense any change to the temperature producing analog or digital output. There are various types of temperature sensor and all of them have varied characteristics depending on the application they are used in. A temperature sensor consists of two basic physical types:
1. Contact Temperature Sensor
2. Non-contact Temperature Sensor
1.2.1 Contact Temperature Sensors
They are needed to be in the physical contact with the object being sensed and use conduction to monitor changes in temperature. They can be used to sense solids, liquids
4 or gases over the wide range of temperatures. These kinds of sensors should be used whenever we can make good thermal contact with the object and if the expected temperature is ranging from -40◦C to 1700◦C (approximately). The term used here, ’good thermal contact means the sensor and the object to be measured should be at same temperature. We can maintain this physical contact by means of soldering, gluing, clamping or sometimes welding the sensor on the object. The high temperatures like
1700◦C, platinum alloy thermocouples start losing its calibration and the insulating material and wires may start to soften. For this kind of situation there are some specialized devices like type B platinum and tungsten-rhenium thermocouples can be used. Even at lower temperatures like -40◦C we may face some cryogenic problems.
The most popular widely used contact temperature sensors are mercury thermometers, thermistors, thermocouples and resistance temperature detectors (RTDs).
They are typically encapsulated in a protective ceramic or metal sheath, known as thermo- well, so that they can easily pass through a process barrier and also be easily pulled out for maintenance or calibration without letting anybody exposed to these adverse temperature conditions.
Figure 1.3: Weld pad surface temperature thermocouple probe [3]
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The device shown in the above picture is designed for the industrial application to measure surface temperature by means of weld, braze or simply clamp. In this, the flexible stainless-steel pad can be attached to the any flat or curved surface for measuring the temperature of the object.
1.2.2 Non-contact Temperature Sensors
These types of sensors use convection and radiation to sense the changes in temperature and can be used to detect liquids and gases that emit radiant energy. Even though the non-contact temperature sensors are available in different names and styles, basically they all are radiation thermometers that works on the Planck’s principle of thermal radiation. They can be found in different forms such as radiation pyrometers, optical pyrometers, IR pyrometers, IR thermometers. These devices can be battery- powered portables, firmly mounted or online process monitoring devices. This type of devices has their area of application a bit broader than contact temperature devices because they do not necessarily have to be at same temperature as that of the object to be measured. The non-contact temperature sensors are suitable if any of the following situation present,
• Mobile object
• Contact would damage either sensor or object due to extremely hot, abrasive
condition.
• The object is distant or unable to access like in outer space
• Contact with the object leads to undesirable temperature change
• A huge area measurement is required. [24]
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Figure 1.4: Infrared temperature sensors thermometer CT series [2]
The device in the above picture is infrared temperature sensor. It is designed for measuring temperatures from -50◦C to 2200◦C. The infrared radiation emitted by the object is used for measurement. As this is non-contact technology, the device perform wear-free and hence can be used reliably for longer duration. [2] These categories can also be sub divided in to Electro- mechanical, Resistive and Electronic. Sensors can further be divided in to two categories like wired and wireless. Wireless sensors can operate in a wide range of environments and provide benefits in cost, size, power, flexibility and distributed intelligence compared to wired ones. Wireless sensor technology allows Micro-Electro-
Mechanical Systems Sensors (MEMS) to be integrated with signal conditioning and radio units to form motes (they are as tiny as dust particles, and consist of a wireless transceiver which has a remote sensor). Today’s MEMS include pressure, temperature, humidity and various piezo and capacitive transducers for position, velocity, proximity, acceleration and vibration measurements. Another advantage for wireless devices is the feasibility of installation in places where wiring is impossible. Wired devices are very reliable and stable.
However, wireless technology provides lower installation costs as compared to wired devices.
1.3 Temperature sensors in use
In the past few years the sensor technology improved a lot. We have created many devices by using these amazing technologies.
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Table 1.1: Most common temperature sensors in the market [13]
Properties Thermocouple Thermistor RTD Integrated Silicon
Temperatu- -270 to 1800 ◦C -250 to 900 ◦C -100 to 450 ◦C -55 to 150 ◦C re range
Accuracy ± 0.5 ◦C ± 0.01 ◦C ± 0.1 ◦C ± 1 ◦C
Ruggednes Larger gauge wires RTDs are more The thermistor This can be as s make it more prone to damage element can be rugged as any rugged. The due to vibrations. covered in many other IC pack- insulation This is because ways but the ages materials that are they have typically popular among used helps in 26 to 30 AWG leads them is to increase the which are prone to enclose in glass. sturdiness of the breakage. Generally, they sensor. are difficult to handle but nor affected by shock or vibrations.
Sensitivity 10s of µ V/◦C 0.00385 Ω/Ω/◦C Several Ω/Ω/◦C Based on the technology that is -2mV/◦C
Form of Voltage Resistance Resistance Voltage, Current output or digital
Table 1.2: Types of sensors and their application [13]
Sensor Type Application
Thermocouple Extreme temperature sensing, gas chromatography, chemical reactions, biophysics, IC engine temperatures, metal cutting research
Thermistor Bridge temperature sensing, liquid levels, fluid velocity, thermal conductivity gas chromatography, thermal conductivity cells, vacuum manometers, anemometer
RTD Bridge temperature sensing, process control, calibration and cold junction compensation
Silicon Based Personal computers, general electronics, cellular phones, HVAC, battery management
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Not every sensor is useful in every application, each of them are designed by keeping some purpose or necessity in mind such as the thermocouples are built for wide range of temperatures, RTDs are known for their excellent linearity, thermistors are known to be accurate and silicon sensors are easy to implement and install in the circuit.
1.4 Acoustic Wave Devices
Acoustic wave devices are described by the mode of wave propagation through or on a piezo- electric substrate. Acoustic waves are distinguished primarily by their velocities and displacement directions. Various combinations are possible, depending on the material and boundary conditions. Transverse or shear waves shows particle displacement that are normal to the direction of the propagation of the wave and which can be polarized so that the particle displacement is either normal to or parallel to the sensing surface. Shear horizontal wave motion signifies transverse displacement polarized parallel to the sensing surface; shear vertical motion indicates transverse displacement normal to the surface.
A wave propagating through the substrate is called a bulk wave. The most popular bulk acoustic wave (BAW) devices are the thickness shear mode (TSM) resonator and the shear- horizontal acoustic plate mode (SH-APM) sensor. If the wave propagates on the surface of the substrate, it is known as a surface wave. The most widely used surface wave devices are the surface acoustic wave sensor and the shear-horizontal surface acoustic wave
(SH-SAW) sensor, also known as surface transverse wave (STW) sensor.
All Acoustic wave devices basically are the sensors and sensitive to the perturbations of many different physical parameters. Any change in the characteristics of the path over which the acoustic wave propagates will result in a change in outcome. All the sensors will work fine in gaseous or vacuum environments but only some of them will work efficiently when they are in influence with liquids. The SH-APM, SH-SAW and TSM all
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generate waves that propagate primarily in the shear horizontal motion. These shear horizontal waves do not radiate good enough energy in to liquids, allowing liquid operation without excessive damping. On the other hand, the SAW sensor has a substantial surface normal displacement that radiates compression waves into the liquid, thus causes excessive damping. The devices that uses waves propagating below the velocity of sound in the liquid can be an exception. Regardless of the displacement components, such modes do not radiate coherently and are thus comparatively undamped by liquids.
Other acoustic waves that are promising for sensors are the flexural plate wave (FPW),
Love wave, surface-skimming bulk wave (SSBW), and Lamb wave. In this thesis, we have focused on various aspects of the surface acoustic wave devices. [11]
1.5 Objective and Scope of Thesis
The primary and important goal of the thesis is to perform the study of sensors, primarily temperature sensors. To gain the deep knowledge of surface acoustic waves and surface acoustic wave devices along with their working principle and different types is going to be our secondary but equally important goal. Once the previous goals were achieved, we can aim to our principle goal and motivation of this thesis that is, to characterize the newly designed and fabricated lithium niobate SAW wafer and also compare its characteristics with existing quartz SAW wafer.
1.6 Novelty of proposed work
During the research of this thesis, we have come across various papers and literatures of quartz and lithium niobate describing various aspect of their application. The main and important aspect of these devices is being used as a temperature sensors. By considering the importance of this topic, we have decided to come up with the comparative study of the quartz SAW devices and lithium niobate SAW deices. This novel idea of the thesis is going to be beneficial for the further studies of the SAW devices as a temperature sensor.
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CHAPTER 2
SURFACE ACOUSTIC WAVE DEVICES
2.1 Getting familiar with SAW
In 1965, the first Surface Acoustic Wave (SAW) devices were built, introducing unbelievable versatility and the reason behind this, the propagation path was available to access to the components for generating and receiving the waves. In the next three decades, these devices have been evolved with tremendous rate. A huge number of device types were introduced in to the market and they are now pervasive in applications ranging from RADAR, communication systems to consumer electronics.
The existence of the basic type of surface acoustic wave, in an isotropic material was first introduced by Lord Rayleigh in 1885 and therefore the wave often known as Rayleigh wave. This wave propagates along the plane surface of a half-space, with the particle in the sagittal planar motion (the plane containing the surface normal to the direction of propagation) and amplitude decreasing with depth. Lord Rayleigh was curious about the seismic signals observed following a ground shock. He proved that a late component, following the expected signal because of bulk longitudinal and transverse waves could be explained by the presence of the slower surface wave.
Later, Augustus Loves substantial work includes a study of shear surface waves, with motion perpendicular to the sagittal plane. Today, this wave known as Love wave. Love also proved that a Rayleigh- type wave, with sagittal particle motion, could exist in a layered system. Further research while working with Earthquake Research Institute, Tokyo, in
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1920s showed that a series of higher modes could exist. The first higher mode is called as the Sezawa wave that has been used in the SAW devices. The interest in surface acoustic waves for electronics applications is quite recent, originally originated from RADAR discoveries.
During World War II, RADAR became very important technology and much classified research. In the post war world, research on pulse compression was revealed. It was proven that the range capability of a RADAR can be substantially improved if the radiated pulse is elongated with un- changed power levels and preferably with same bandwidth, determines its resolution. The resolution is nothing but the smallest change of the quantity measured that the sensor can detect. It was contemplating that this would be achieved by transmitting a chirp pulse (whose frequency is time dependent). The receiver would have a matched filter to optimize the signal to noise ratio along with a dispersive delay line that delays various receiving frequencies by different amount so that they can arrive at the output simultaneously. [23]
2.2 Development phases of SAW devices
2.2.1 Wedge and Comb model
The most common methods at that time for generating surface waves were the
Wedge and the Comb. In these methods, a bulk wave is generated by a piezoelectric plate transducer and eventually converted into the Rayleigh waves. In the case of Combs method, the generated waves would travel in both directions.
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Figure 2.1: Rayleigh wave generation methods with (a) Wedge and (b) Comb transducer [23]
The non-dispersive acoustic waves were used in the wedge delay line. In that, a set of transducers is fabricated on both inclined faces of quartz crystal with variable spacing.
These transducers generate waves that travel horizontally; at high frequencies, the stronger waves are generated where transducers are placed closely, so the acoustic path length and hence the delay varies with frequency.
Figure 2.2: Wedge delay line [23]
2.2.2 Mortley’s model
Wilfrid S. Mortley demonstrated a similar kind of device using interleaved electrodes as the transducers. In this instead of wedging, the arrangement was done on the plane surface so now it is planar and the transducers generate surface waves instead of bulk waves. This was suggested by Mortley and Rowen independently during 1963 and became pioneers of the planar SAW transducers. Later on, surface wave devices became more advance and now the waves can only propagate in one direction which simplifies its behavior and the process of fabrication.
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Figure 2.3: Planar SAW pulse compressor [23]
2.2.3 Constant pitch model
In 1965, R. M. White and F. W. Voltmer demonstrated the constant pitch interdigital transducer that was generating and receiving the waves on a quartz substrate. These IDTs were consisted simply of interleaved metal electrodes connected alternatively to the bus bars. In order to act like a half-space, the substrate need to be only few wavelengths thick because the wave has a small depth of penetration.
Figure 2.4: Interdigital SAW transducer by White and Voltmer, 1965 [23]
2.2.4 Dispersive IDT model
Sooner in 1969, R. H. Tancrell came up with first result for a dispersive interdigital
SAW device, with both transducers are dispersive as we can see in the figure 2.5. This device had a lithium niobate substrate with center frequency around 60MHz, bandwidth of
20MHz and dispersion time of 1 sec. He also proposed that the electrode overlaps could be varied, that technique later called as apodization, to provide weighting.
These developments were the beginning of the modern SAW world. The tedious wedge or comb transducer structures were replaced by IDTs that could be easily fabricated by processes like photolithography or electron beam lithography (EBL). The important
14 step was the use of a piezo- electric materials like quartz and lithium niobate in which electric and elastic fields are coupled. Hence, it plays vital role in the transduction process, converting electric signals in to acoustic waves and vice-versa at the receiver end.
Figure 2.5: Interdigital pulse compressor [23]
2.3 Modern SAW device model
The Surface Acoustic Wave devices are belonging to Microelectromechanical systems (MEMS) which uses the modulation of the surface acoustic waves to sense or determine a physical phenomenon. In this the sensor convert an input electrical signal into the mechanical wave which can be easily influenced by the physical phenomenon. Basically, it works on the principle of piezoelectric effect.
The basic structure of SAW device includes a piezoelectric substrate, inter digital transducers (IDT), one on the input side and other on the output side. The region between these IDTs, through which the surface acoustic wave will propagate, is known as Delay Line.
This region is called delay line because the wave travelling at this point are mechanical waves and are travelling at much lower speed than usual which leads into considerable delay.
Figure 2.6: Schematic of Surface Acoustic Wave Device [27]
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As we can see in the figure 2.6, there are various regions in the SAW device that contributes in its performance and those are:
2.3.1 Aperture (a)
The aperture can also be called as length of a finger of the SAW device. Its function is very analogues to that of optical lenses. The focus of the mechanical waves is depending on the width of the aperture with respect to the wavelength used. Wider the aperture more focused will be the mechanical waves. Generally, the aperture a is preferred at around 50
λ. [26]
2.3.2 Acoustic Wavelength (λ)
In the above figure, the part referred as pitch is also called as acoustic wavelength.
Basically, it is determined by the distance between two consecutive fingers connected to the same polarity. It can also be calculated by using the mathematical formula containing phase velocity and the center frequency. [26]
(2.1) � = � 2.3.3 Metallization Thickness (h)
The minimum requirement for the thickness of the metal so that IDTs to generate a surface acoustic wave is given by, [26]
h (0.01) (2.2)
≥ λ 2.3.4 Number of fingers in IDT
The number of fingers in the IDT helps in determining the quality factor of the device. The quantity called relative bandwidth f / f has an inverse relation with the number of periods in the IDT. Ideally this number (N) should be around 50 at the start. [26]
(2.3) � =
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2.4 Working Principle of SAW
An alternating electrical signal given at the input, generates alternating polarity between the fingers of the IDT. Between two adjacent sets of fingers, polarity of the fingers will be switched. As result of this, the direction of the electric field between two fingers will alternate between adjacent sets of fingers. This results in the formation of alternating regions of expansion and compression between the fingers by piezoelectric effect generating a mechanical wave at the surface known as a surface acoustic wave. These wave travel across the piezoelectric substrate and gets converted back in to the electrical signal.
Now this electrical signal reflects or shows the changes occurred in the mechanical wave due to the influence of any physical quantity such as temperature, strain, stress etc while its journey across the substrate.
2.5 Types of SAW devices
2.5.1 Resonator SAW devices
This kind of SAW devices generally comes with high quality factor (Q) around or greater than 10,000. Its basic structure consists of two reflectors one on each side and the
IDT in the middle. These resonators can be excited by the radio frequency signal. Once the signal gets its excitation level, it is possible to evaluate the decaying response of the resonator. Basically, there are two types of SAW devices that are categorized based on number of ports and they are,
2.5.2 One Port Resonator
The figure 2.7 represents the structure of one-port SAW resonator. It consists of an interdigital transducer in the center and two reflectors on either side of the device. This is the most general form of one-port SAW arrangement.
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Where figure 2.8 is the equivalent circuit representation of a one-port SAW device at the resonance. The purpose of introducing this circuitry is to get better understanding of the working of this device. Also, figure 2.9 shows the simulation schematic of one-port SAW device using tool called ADS.
Figure 2.7: One Port SAW Resonator [12]
Figure 2.8: One-port SAW equivalent at resonance [22]
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Figure 2.9: ADS schematic of one-port device
Figure 2.10: S-parameter response of one-port SAW schematic
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Figure 2.11: Phase response of one-port SAW schematic
2.5.3 Two Port Resonator
A two-port SAW resonator is a device in which, two IDTs are placed on side by side with some definite distance between them and a reflector is placed on outer edges of these
IDTs.
Figure 2.13 shows idealized lumped element equivalent circuit in the vicinity of resonance. Basically, it’s a circuit that gives frequency response like SAW resonator. This consist of series of inductance, capacitance and resistance (LCR) branch shunted by a capacitor. Shunt capacitance C0 represents the IDT capacitance, while elements L1, C1 and
R1 are equivalent motional parameters.
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Figure 2.12: Two Port SAW Resonator [12]
Figure 2.13: Two-port SAW equivalent at resonance [22]
For |Γ| = 1, the elements are approximated by the following expressions,
(2.4) −| | _ = { {| | }
� (2.5) � � = { 4 | | }
(2.6) � = � �
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Where,
f0=resonant frequency
Γ = coefficient of reflection
R0 = radiation resistance
C0 = static capacitance
Leff = total effective cavity length and is given by
(2.7)
� = � + �� L = cavity length
Lp = penetration distance
R1, L1, C1 = equivalent RLC parameters for series resonance [22]
Figure 2.14: ADS schematic of two-port device
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Figure 2.15: S-parameter response of two-port SAW schematic
Figure 2.16: Phase response of two-port SAW schematic
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2.5.4 Reflective Delay Line Device
RDL SAW device contain acoustical reflectors in the surface of the substrate. These reflectors are fixed within a range of the IDT so as to generate delayed reflections of the signal transmitted by the transmitting unit and received in the IDT with the help of antenna.
The working principle of the RDL-SAW device is similar to that of the SAW resonator. In the simple words, input electrical signal received by the antenna associated with IDT, generates the surface acoustic wave on the substrate that travels towards the reflectors and reflected to the IDT. Here, surface acoustic wave is converted back in to the electrical signal.
Figure 2.17: Reflective Delay Line Device [25]
The impulse response of the RDL SAW device is shown in the following equation
[16],
(2.8)
ℎ � = ℎ � � ∗ Σ �� � − Where,
= combined electro-acoustic response of the IDT
ℎ � � = the time delay to the kth reflector and k ranges from 1 to N
24
ai = the reflection coefficient of the reflector
(t)= Dirac’s delta
Assuming the constant acoustic velocity of the wave, the time delay of each reflector is given by following equation and the delays vary according to the position of the reflector from the IDT. [16]
(2.9) =
Where,
L = distance of the reflector from the IDT, v = SAW velocity
It’s always been interesting to watch the change in the response of a sensor in accordance with various physical parameters. The relation between time delay and change in the physical quantity is given by the following equation [16],
(2.10)
= + ∗ � = + Where,
T0 = delay without get affected by physical quantity x,
STx = relative sensitivity between time and physical quantity x
To obtain the impulse response of the RDL device affected by the physical quantity can be given as [16]
(2.11)
ℎ � = ℎ � � ∗ Σ �� (� − + ∗ � )
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whereas, change in time delay (∆T ) can be given by the difference between change of affected time delay due to physical quantity and the original time delay i.e. [16]
(2.12)
Δ = − = ∗ ∗ �
2.6 Materials for SAW devices
The characteristics of the surface acoustic wave can be modified by changes in the surface properties of the device substrate such as changing the spacing between the fingers of IDTs, length of the substrate or using different type of the piezoelectric material for the substrate.
The materials generally used for making substrate of SAW sensors are Quartz
(SiO2), Lithium Niobate (LiNbO3), Lithium Tantalate (LiTaO3), and Lanthanum Gallium
Silicate (LGS) also known as Langasite, Silicon Carbide (SiC), Zinc oxide (ZnO), Aluminum
Nitride (AlN), Lead Zirconium Titanate (PZT) and Polyvinylidene fluoride (PVDF).
26
2.6.1 Properties of commonly used piezoelectric SAW substrates
Table 2.1: Properties of SAW piezoelectric materials [26]
Material Orientation Phase K2 TCD
Velocity % ppm/0C
Cut Prop m/s
Quartz ST X 3158 0.1 0
370 Y 900X 5094 0.1 0
LiNbO3 Y Z 3488 4.1 94
410Y X 4750 15.8 69
1280Y X 3980 5.5 75
0 LiTaO3 36 Y Z 4220 6.6 30
Where,
Orientation cut: It indicates the crystalline orientation of the substrate surface to normal
Orientation Prop: Indicates the crystalline orientation of the wave propagation direction
K2: Electro-mechanical coupling coefficient
TCD: Temperature Coefficient of Delay
2.6.2 Quartz (SiO2)
Quartz is a mineral composed of silicon and oxygen. It is a second most abundant mineral in earth’s crust behind feldspar. Quartz crystals known to have piezoelectric properties; they develop an electric potential upon the application of mechanical stress. One of the most common piezoelectric uses of quartz is crystal oscillator. Now a day it also
27 become popular as a surface acoustic wave sensor. The resonant frequency of a quartz crystal oscillator is changed by mechanically loading it, and this fact is used for very accurate measurement of very small changes in the quartz crystal microbalance and in thin-film thickness monitors.
Quartz crystals are asymmetrical, and found in two different forms, -quartz and high temperature beta-quartz. Alpha-quartz belongs to the trigonal crystal system where as beta-quartz belongs to the hexagonal system. Both -quartz and -quartz are examples of chiral crystal structures composed of achiral building blocks.
(a) Alpha-quartz (b) beta-quartz
Figure 2.18: Crystalline structure of (a) alpha-quartz, (b) beta-quartz [21]
As we know, quartz belongs to trigonal crystal system. Three silicon dioxide (SiO2) molecules build each unit cell. In the unit cell, there are three silicon and six oxygen atoms.
Oxygen is being lumped in pairs. Each silicon atom carries four positive charges and a pair of oxygen carries four negative charges (two charges per atom). Therefore, a quartz crystal is electrically neutral under the unstressed condition. When any external force or stress is applied, the lattice becomes deformed.
Figure 2.19(b) shows a compressing force which shifts atoms in a crystal in such a way that a positive charge is built up at the silicon side and a negative charge at the oxygen side. Thus, the lattice develops an electric charge along the Y-axis which is a result of a
28 deformation. These simple figures 2.19 illustrate that the crystalline material can develop an electric charge on its surface in response to a mechanical deformation.
Figure 2.19: (a) Molecular structure of SiO2, (b) Atoms with compressed force, (c) Atoms with expanding force [22]
2.6.3 Lithium Niobate(LiNbO3)
Lithium Niobate is a ferroelectric material suitable for a variety of applications. Its versatility is made possible by the excellent electro-optic, nonlinear, and piezoelectric properties of the intrinsic material. It is one of the most thoroughly characterized electro- optic materials, and crystal growing techniques consistently produce large crystals of high perfection.
Single crystals of lithium Niobate can be grown using Czochralski process. After crystal is grown, it is sliced in to wafer of different orientation like Z-cut, X-cut, Y-cut, and cuts with rotated angles of previous axes. Lithium niobate and its close relative lithium tantalate are ferroelectric crystals which possess high curie temperatures. These crystals exhibit excellent piezoelectric coupling coefficients, making them attractive for ultrasonic device applications. After they are grown in to single crystals, they get poled into single domain at the Curie temperature this results into a uniform, highly consistent piezoelectric transducer single crystal.
29
Lithium niobate possesses several useful cuts which are frequently used in transducer applications. Z-cut and 360 rotated Y-cut. The most commonly used shear mode cuts are X-cut and 1630 rotated Y-cut.
Lithium niobate holds very large electro-mechanical coupling coefficients several times larger than quartz and very low acoustic losses. Because of its high Curie temperature of1142 0C, it can be utilized as a high temperature acoustic transducer (Such as an accelerometer for jet aircraft)
Figure 2.20: Crystalline structure of Lithium Niobate [9]
Acoustic wave delay lines and acousto-optic modulators, filters now frequently using lithium niobate for both shear and compressional wave generator because of its high efficiency, broader bandwidth capability, low dielectric constant for all orientations and reliability.
Compared to quartz, even lithium tantalate has a much larger electro-mechanical coupling and zero temperature coefficient cuts of resonant frequency and it provides acoustic resonator filters of broader bandwidth. That’s the reason why it is so much popular in the telecom industry.
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2.6.4 Physical Characteristics
Table 2.2 explains, some of the very important characteristics such as melting point, mechanical hardness, density, thermal conductivity etc. As we can see in the table 2.2, the thermal conductivity of the lithium niobate is higher than quartz; hence become good conductor of heat. This property helps lithium niobate to become a better temperature sensor than quartz.
Table 2.2: Characteristics of Lithium Niobate and Quartz [20] [29]
Properties Lithium Niobate Quartz
Melting point 1250 ◦C 1670 ◦C
Lattice constant AH = 5.151A◦ CH = 13.866A◦ AH = 4.9133A◦ CH = 5.4053A◦
Density 4.65 g/cm3 2.65g/cm3
Mechanical 5 Mohs 7 Mohs
hardness
Thermal 0.01 Cal/s cm ◦C 0.003 Cal/s cm ◦C
conductivity
Pyroelectric −8.3 × 10−5C/◦C/m2 -
coefficient
6 6 Thermal expansion αa = 15 × 10− ◦C, αa = 14 × 10− ◦C,
6 6 αc = 7.5 × 10− ◦C αc = 7.64 × 10− ◦C
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CHAPTER 3
WAFER PROCESSING AND MAPPING
In this chapter, we are going to see the mapping and the characterization of the lithium niobate wafer and its comparison with existing quartz wafer that designed and fabricated in the UCCS facility.
3.1 What is wafer?
A wafer, also known as disc, is a thin, glossy slice of a silicon rod that is cut in to the disc of specific diameters. Most wafers are made of silicon. The main advantage of using silicon is that it is abundantly present in the earth’s crust. In this thesis lithium niobate wafers are used but the actual process during this section is explained by referring silicon, just for the sake understanding.
3.1.1 Ingot- the foundation for wafers
Once silicon is extracted from sand, it needs to be purified before it gets used. First, it is heated until it melts into a highly pure liquid which then solidified in to elongated cylinder like structure called ingot.
The popular Czochralski process uses a small piece of solid silicon which is placed in molten silicon and then slowly pulled in rotations as the liquid grows into a cylindrical ingot. This is how we end up with the round shape discs or wafer.
Before it completely cools down, the cone shaped ends of the ingot are cut off while the body is sliced into thin wafers of uniform thickness with sharp diamond saw blades.
This explains why an ingots diameter would ultimately determine the size of a wafer. In
32 the beginning of the semiconductor industry, wafers used to be only three inches in diameter. Since then, wafers have been growing and results in more number of chips and higher productivity. The largest wafer diameter used in semiconductor fabrication today is 11.8 inches, or 300 mm.
Figure 3.1: Ends cut off from elongated cylindrical structure or ingot [7]
3.1.2 The lapping and polishing process
In this stage, sliced wafers need to be prepped before getting into the production line. Abrasive chemicals and machines being used to polish the uneven surface of the wafer to get a mirror- smooth finish. The flawless surface allows the circuit patterns to print better on the wafer during the lithography process.
In the figure 3.2, we can see the different sizes of polished wafers ready to undergo lithography process.
33
Figure 3.2: Polished wafers ready for process [7]
3.2 Wafer processing
The sensors are either made traditionally by Photolithographic process or by using more advanced Electron Beam Lithography (EBL). Manufacturing begins with carefully polishing and cleaning the piezoelectric substrate. Metal, usually aluminum but in this thesis, we have used Platinum, is then deposited uniformly onto the substrate. The device is spin coated with a negative photoresist (S-1805) and baked it to harden for 30minutes. It is then exposed to the ultraviolet light through a mask with the areas corresponding to the areas to be metallized for contacts on the final device. The exposed areas undergo a chemical change that allows them to be removed with a developing solution. At last, the remaining photoresist is removed. The pattern of the metal remaining on the device is called an interdigital transducer (IDT).
34
Figure 3.3: Inter Digital Transducer [18]
Figure 3.4: Microscopic image of actual IDT
The process described above is summarized in the following flowchart.
35
Figure 3.5: Flowchart of a fabrication process [15]
36
As a product of above processing, we will get a wafer ready for scribing and these scribed devices can be further packaged in the different packing styles to use in the applications they are required in.
Following is the real wafer that has been gone through all the processes mentioned in the above flowchart.
Figure 3.6: Actual Lithium Niobate wafer after post processing
3.3 Knowing the wafer
1. Chip: It is a tiny piece of lithium niobate with electronic circuit patterns printed on it.
2. Scribe lines: These are thin, non-operational spaces between the operational pieces, where a diamond saw can safely cut the wafer without damaging the circuit patterns.
3. Test Element Group (TEG): It is a prototype pattern that reveals the actual physical characteristics of a chip like transistors, diodes, capacitors, resistors and other small circuit elements so that it can be tested to see whether it works properly.
37
4. Edge Die: Dies around the edge of a wafer is responsible for production loss; larger the wafers lower is the loss.
5. Flat Zone: One edge of a wafer that is cut flat to help identify the wafers orientation and type.
Figure 3.7: Wafer with labelled sections [7]
3.3.1 Wafer nomenclature
The devices on the actual wafer are named in the specific manner as, the whole wafer is divided into the four quadrants i.e. north, south, east and west. The alphabets like A, B,
C, D and E were used along with the series number 35 and 36. The series number and the alphabets contribute in to the name of the device and the quadrant where it is placed helps in locating the device. For example, the device 35CE3N3 should be in the first quadrant
(in the north-east quadrant), third row from the diameter of the wafer with series 35.
38
3.3.2 Wafer characteristics
Table 3.1: Physical characteristics of the lithium niobate wafer
Diameter 100 mm
Thickness 0.50 mm
Orientation 42◦45’
3.4 Calibrating the Network Analyzer
Before going to any measurements or characterization process the calibration of the network analyzer must be done to insure the precise readings or results during the testing.
The process of calibrating the network analyzer with a single microwave probe contains de- embedding process that points out the issues like parasitic impedance between the two adjacent ground contacts and the RF contact. The following procedure explains more about the calibration process:
1. The network analyzer should be in the close vicinity of the stationery micro-probe station.
2. Secure one end of the microprobe to port 1 of the network analyser and the other end goes to the micro-probe fastener at micro-probe station.
3. Take the impedance standard substrate (ISS) wafer and place it on the viewing platform of the micro-probe station. Make sure initially, while placing the ISS wafer on the
39 platform probe tip should be held high enough that it avoids any contact with the ISS wafer.
4. Once ISS wafer placed securely on the viewing platform, activate the vacuum to hold the wafer in the place. This helps us to ensure proper contact integrity between the micro-probe tip and the wafer.
5. Now, with the help of microscope locate the short and load gold imprints on the wafer. This is very important step in order to perform the single port calibration of the network analyzer.
6. The calibration used here is called as SLOT, which stands for Short-Open-Load
Through. Now, properly position the micro-probe for calibration.
7. Turn on the network analyzer. Once the network analyzer boot up completely, select the calibration pull down menu from top of the screen. Select the Calibration wizard.
A new window will pop up, asking the user to select the type of calibration. Select Unguided
Calibration (Response, 1-port, 2-port); Use Mechanical Standards and click next.
8. At the same time on the other screen, within the Cal Type Selection menu, choose
1-port SLOT. Click on the View/Select Cal Kit button. A new window will appear presenting various calibration kits. By selecting class SLOT, scroll down and select kit ID
20 (named as 85031B or APC7 Calibration Kit.)
9. The calibration set up procedure is now complete. A short, secondary short and load measurement must be conducted on port-1 of the network analyzer for the result of the calibration to be valid.
10. With all measurements performed, save the calibration under specific name according to user. In this thesis, it is saved under the name of One Port SAW. This calibration is now available in the calibration set on the network analyzer and can be
40 accessed at any time by the user for characterization purpose. Finally, by clicking finish calibration process in the wizard comes to an end. [17]
3.5 Wafer mapping
In this process, we have covered the entire wafer and found out the working devices based on their S11- parameter. Machine being used during this testing method were micromanipulator and the PNA network analyzer. Below is the picture that shows the SAW device status on the wafer at the time of testing. During this thesis, we have used two lithium niobate wafers which were newly designed and fabricated in the UCCS lab facilities for the research purpose.
Figure 3.8: Lithium Niobate wafer under testing
Following figure 3.9 and figure 3.10 are the actual mapped representation of the lithium niobate wafers that indicates the status of the SAW devices on the wafer during the time of testing. Around 600 SAW devices combined on both wafers undergone the testing.
41
Figure 3.9: Mapped wafer-1
Figure 3.10: Mapped wafer-2
As we can see in the figure 3.9 and figure 3.10,
Black region: Represents the devices on the wafer that are not working or in other words showing some undesirable response.
42
Golden region: It shows the well working devices on the wafer which can be scribed and used as a SAW sensor in various applications.
43
CHAPTER 4
CHARACTERIZATON AND COMPARISON
4.1 SAW device specification
In this section, we are going to discuss the characteristics of the lithium niobate surface acoustic wave devices.
Table 4.1: Specification of Lithium Niobate devices
Piezoelectric material Lithium Niobate
Device orientation 90◦
Device length 3030.98 µm
Device width 3060.35 µm
Length of a finger 145.26 µm
Width of a finger 5.98 µm
Distance between consecutive 1.18 µm fingers
SAW wavelength 14.32 µm
SAW velocity 9952.4 m/s
Center frequency 695 M Hz
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Table 4.2: Specification of Quartz devices [15]
Piezoelectric material ST-X Quartz
Device orientation 90◦
Device length 3096 µm
Length of finger 172 µm
Width of a finger 1.8 µm
SAW wavelength 7.2 µm
SAW velocity 3139 m/s
Center frequency 436 M Hz
4.2 Frequency response of temperatures
The following two plots are the actual response recorded for the quartz and lithium niobate devices during their testing. These responses shown the shifts in the center frequency of the device as temperature of the device goes on increasing.
During testing, the random devices from wafer viz; 35CE3N3, 35DE4N5, 35CW6S6 and 35DW5S4 were set to different temperatures. One by one starting from the room temperature (21 ◦C) up to 100 ◦C. All readings were taken by using micromanipulator and
PNA network analyzer.
The plot in the figure 4.1 of the lithium niobate device shows upward progression
(or positive frequency shift) which means its frequency goes on increasing as the temperature increases. In this case, the average frequency shift is 192.8817 kHz. This indicates that for every 10◦C rise in temperature the frequency is shifting by around 193 kHz.
45
Figure 4.1: Frequency response of Lithium Niobate sensors for varying temperature
As we can see in the plot of figure 4.2 of quartz device, it shows downward progression means, the frequency is continuously falling for temperature increase. This drop in the frequency can be called negative frequency shift. In this case, this frequency shift noted to be 30.0589 kHz and which can be rounded around 30 kHz. This means, the quartz SAW device shifts its frequency by 30 kHz for every 10◦C rise in temperature.
Figure 4.2: Frequency response of Quartz sensor for varying temperatures
46
From above comparison, we can say that the sensitivity of the lithium niobate device is almost six times as much as that of the quartz device.
4.3 Quality factor
It is a dimensionless physical quantity that describes how under-damped a resonator is, and characterizes a resonators bandwidth as compared to its Center frequency. The higher value of the quality factor indicates lower rate of energy loss compared to the one stored in the resonator and slower rate of damping. In other words, resonators with high quality factor have low damping rate which results into more sustained oscillations. [19]
Mathematically it is explained as ratio of center frequency (resonant frequency) to the bandwidth of a resonator,
(4.1) �� = � In the context of resonators, generally there are two ways in which we can define the quality factor. Both of them become almost equivalent at higher values of Q. One way is already given in the above equation in terms of ratio of center frequency to the bandwidth at the resonating frequency.
The second way of defining the Q factor is the ratio of energy store in the oscillating resonator to the energy dissipated per cycle during its damping. [28]
(4.2) = � ∗ � � � � = �� ∗ � In the electrical systems, the stored energy is basically energy stored in lossless inductors and the capacitors. The lost energy is the actual work done by an external conservative force per cycle to maintain the amplitude.
47
The figure 4.3 and figure 4.4 shows the relation between the Q-factor and temperature of the SAW devices.
Figure 4.3: Quality factor vs Temperature for Lithium Niobate
In the figure 4.3 devices used here are 35CE3N3, 35CW6S6, 35DE4N5. Here the bandwidths of the devices were taken around -7 to -10 dB. The reason behind this is, the resonating curve becomes very narrow towards its peak and hence makes it very difficult to locate the point around the -3 dB region. Here, the quality factor goes on decreasing gradually with the rise in temperature which means the oscillations of the resonator starts damping with respect to the amount of decrease.
48
Figure 4.4: Quality factor vs Temperature for Quartz
The figure 4.4 shows the response in terms of quality factor of the quartz devices
37N1E5 and 36N1E5. We can clearly see that the tendency of the plot is to decrease with temperature rise with some crest and trough. The magnitude of the quality factor of the quartz is lower than that of lithium niobate.
4.4 Bandwidth
In this section, we are going to see the effect of changing temperature on the bandwidth of lithium niobate and quartz SAW devices at the resonance.
In the figure 4.5 we can see that, the responses for the device 35CW6S6 and 35CE3N3 shows almost same behavior with different values. Its behavior can be described as, increase in temperature leads to linear rise in the bandwidth and later it tries to get settle with the constant value. This increment is varied from device to device. But, in case of third device i.e. 35DE4N5, its response is quite different than other two devices, it starts with stable progression and turn out in to unstable response.
49
Figure 4.5: Plot of Bandwidth vs Temperature for Lithium Niobate
Figure 4.6: Plot of Bandwidth vs Temperature for Quartz
Figure 4.6 represents the bandwidth response of 37N1E6 and 36N1E5 SAW quartz devices against the increasing temperature. As we can see the device 37N1E6 shows pretty stable response at the beginning which start fluctuating at higher temperature.
50
4.5 Parameter Characterization
S parameters: It is referred as scattering parameters which are differ from others in the way that, S-parameters do not use open or short circuit conditions to characterize the circuit or the network, instead it uses principle of load matching. Terminating the network with matched load is much easier to use at high frequencies.
The electrical properties such as inductance, capacitance, resistance, reflection coefficient, return loss, reactance etc. can be expressed in terms of S-parameters. This parameter is most suitable for microwave or radio frequency applications because these areas mainly focus on the signal power and energy rather than voltages or currents. S- parameters greatly depends on the frequency of operation of the device. Hence frequency must be mentioned in the S-parameter measurement along with characteristic impedance and system impedance. In case of one port devices like one port SAW resonators, the parameter called S11 describes the insertion loss. Following is the comparison of the temperature against S11 parameter for lithium niobate and quartz devices.
Figure 4.7: Plot of S11 parameter vs Temperature of Lithium Niobate
51
Figure 4.8: Plot of S11 parameter vs Temperature of Quartz
If we closely observe the plots from figure 4.7 and figure 4.8, we can see
that the S11 response of the quartz is stable for changing temperature as
compared to the lithium niobate. This means, lithium niobate devices are
prone to more insertion loss than quartz and their magnitudes for lithium
niobate are in the range of -20 to -45 dB and -15 to -25 dB in case of quartz.
(a) S11 parameter of Lithium Niobate (b) S11 parameter of Quartz
Figure 4.9: S-parameter responses of oscilloscope
52
4.6 Temperature Coefficient of Frequency
The thermal stability of a resonator is defined by the temperature coefficient of resonant frequency. It shows a gradual change of the resonant frequency with the varying temperature. It can be calculated as [8],
(4.3) Δ = Δ� Where,
f0 = the resonant frequency at reference temperature (here, we are considering room temperature as our reference)
∆f0 = the change in the resonance frequency.
∆T = the temperature difference between working temperature and the reference temperature [14]
In this thesis, the actual theoretically calculated values of TCF for lithium niobate and quartz are 45.0678 x 10-6 ppm/0C and 2.55 x 10-6 ppm/0C respectively.
53
CHAPTER 5
CONCLUSION AND FUTURE WORK
5.1 Conclusion
The main goal of this thesis was to study the newly fabricated lithium niobate SAW wafer and characterized it. Further, it’s been decided to compare its characteristics with the pre-existing quartz SAW wafer. To achieve this goal, it was necessary to gain the knowledge about the surface acoustic wave devices, its history, its development phases, working principle, different characteristics and the various materials and methods used for its fabrication. In this thesis, the focus of the study is, understanding the effect of the various or changing temperatures on the characteristics of the lithium niobate SAW devices.
After comparing different parameters between quartz and lithium niobate SAW devices, we came to conclusion that the lithium niobate device is more sensitive and more responsive to the temperature changes and hence can be best suited as a temperature sensor in various applications.
The other conclusion drawn from the response and results from the quartz devices is that, it can be a good sensing material or device in the application where temperature should not affect the working of the sensors.
5.2 Future work
As we look forward in to the future, sensors are going to be of utmost importance.
This research represents only the first step towards the application of lithium niobate as
54
wireless temperature sensors. There is a lot of work and research need to be done towards its effective and reliable implementations along with affordability. One can start its research by using the results obtained in this thesis as a base. As this thesis study includes comparative results of number of parameters of a surface acoustic wave devices, it can be a real guide for the further research.
As world is marching towards IoT (Internet of Things) era, it is highly important and strongly recommended to find various ways of implementation of a lithium niobate as a temperature sensor and if possible to sense other physical quantities as well.
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