SAW PARAMETERS ANALYSIS AND EQUIVALENT CIRCUIT OF SAW DELAY LINE

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Thickness Shear Mode (TSM) resonator

The Thickness Shear Mode (TSM) Resonator widely referred to as a quartz crystal microbalance (QCM), is the best-known, oldest and simplest acoustic wave device. As shown in Figure 1.2, the TSM typically consists of a thin disk of AT-cut quartz with parallel circular electrodes patterned on both sides. The application of a voltage between these electrodes results in a shear deformation of the crystal.
This device is known as a resonator because the crystal resonates as electromechanical standing waves are created. The displacement is maximized at the crystal faces, making the device sensitive to surface interactions. The TSM resonator was originally used to measure metal deposition rates in vacuum systems where it was commonly used in an oscillator circuit. The oscillation frequency tracks the crystal resonance and indicates mass accumulation on the device surface. In the late 1960s, the TSM resonator was shown to operate as a vapour sensor.
The TSM features simplicity of manufacture, ability to withstand harsh environments, temperature stability and good sensitivity to additional mass deposited on the crystal surface. Because of its shear wave propagation component, the TSM resonator is also capable of detecting and measuring liquids, making it a good candidate for a biosensor. Unfortunately, these devices have the lowest mass sensitivity of the sensors examined here. Typical TSM resonators operate between 5 and 30 MHz. Making very thin devices that operate at higher frequencies can increase the mass sensitivity, but thinning the sensors beyond the normal range results in fragile devices that are difficult to manufacture and handle. Recent work has been done to form high-frequency TSM resonators using piezoelectric films and bulk silicon micromachining techniques.

Surface Acoustic Wave Device

The stress-free boundary imposed by the surface of a crystal gives rise to a unique acoustic mode whose propagation is confined to the surface and therefore is known as a surface acoustic wave (SAW). In 1887, Lord Rayleigh discovered the surface acoustic wave mode of propagation and in his classic paper predicted the properties of these waves [1]. The theoretical aspect of acoustic wave was written by Viktorov [1]. Named for their discoverer, Rayleigh waves have a longitudinal and a vertical shear component that can couple with a medium in contact with the device’s surface (see Figure 1.3). The surface deformation is thus elliptic. Such coupling strongly affects the amplitude and velocity of the wave. This feature enables SAW sensors to directly sense mass and mechanical properties. The surface motion also allows the devices to be used as microactuators. The wave has a velocity that can be ~5 orders of magnitude less than the corresponding electromagnetic wave, making Rayleigh surface waves among the slowest to propagate in solids. The wave amplitudes are typically ~10 Å and the wavelengths range from 1 to 100 microns in sensors applications.
Figure 1.3 Rayleigh waves move vertically in a direction normal to the surface plane of a surface acoustic wave (SAW) sensor. SAW waves are very sensitive to surface changes, but do not work well for most liquid sensing applications
Figure 1.4 details the deformation field caused by a SAW propagating along the Z-axis and the associated distribution of potential energy. Because Rayleigh waves have virtually all their acoustic energy confined within one wavelength under the surface, SAW sensors have the highest sensitivity of the acoustic sensors reviewed.
Figure 1.4 The wave energy is confined to within one wavelength from the surface of a SAW sensor. This characteristic yields a sensor that is very sensitive to interactions with the surface
One disadvantage of these devices is that Rayleigh waves are surface-normal waves, making them poorly suited for liquid sensing. When a SAW sensor is contacted by a liquid, the resulting compressional waves cause an excessive attenuation of the surface wave.

SAW excitation and detection

Viktorov presented some methods for the acoustic wave excitation [1]. And, useful method was discovered by R.M.White of the University of California at Berkeley [3] in which surface acoustic wave could be excited and detected by lithographically pattern interdigital electrodes (or InterDigital Transducer IDT) on the surface of piezoelectric crystals (see Figure 1.5). This discovery has led to widespread use of SAW devices in a number of applications such as frequency filters, delay lines, resonators, convolvers, correlators.

SAW perturbation mechanisms

When SAW devices are used for sensors or thin-film characterization, the measured responses arise from perturbation in wave propagation characteristics, specifically wave velocity and attenuation, resulting from interactions between the SAW and a surface layer. Because a SAW propagating in a piezoelectric medium generates both mechanical deformation and an electrical potential, both mechanical and electrical coupling between the SAW and surface film are possible. Therefore, a number of interactions between surface waves and a surface film have been found that give rise to velocity and attenuation responses.
SAW-film interactions that arise from mechanical coupling between the wave and film include mass loading caused by the translation of the surface mass by SAW surface displacement, elastic and viscoeleastic effects caused by SAW-induced deformation of a surface film.
SAW-film interactions that arise from electrical coupling between the wave and film include acoustoelectric interactions between electric fields generated by the SAW and charge carriers in a conductive film. Some new interactions are being discovered all the time.

Acoustic Plate Mode (APM) devices

These devices utilize a shear-horizontal (SH) acoustic plate mode (APM), which has been developed for sensing in liquids. SH modes have particle displacement predominantly parallel to the device surface and normal to the direction of the propagation. The absence of a surface-normal component of displacement allows each SH plate mode to propagate in contact with a liquid without coupling excessive amounts of acoustic energy into the liquid. By comparison, when surface acoustic waves are propagated at a solid-liquid interface, the surface-normal displacement radiates compressional waves into the liquid and severely attenuates the wave.
These devices use a thin piezoelectric substrate, or plate, functioning as an acoustic waveguide that confines the energy between the upper and lower surfaces of the plate (see Figure 1.6). This is in contrast to the SAW, for which nearly all the acoustic energy is concentrated within one wavelength of the surface. As a result, both surfaces undergo displacement, so detection can occur on either side. This is an important advantage, as one side contains the interdigital transducers that must be isolated from conducting fluids or gases, while the other side can be used as the sensor. Figure 1.6 In the shear-horizontal acoustic plate mode (SH-APM) sensor, the waves travel between the top and bottom surfaces of the plate, allowing sensing on either side
Although being more sensitive to mass loading than the TSM resonator, SH-APM sensors are less sensitive than surface wave sensors. There are two reasons: the first is that the sensitivity to mass loading and other perturbations depends on the thickness of the substrate, with sensitivity increasing as the device is thinned. The minimum thickness is constrained by manufacturing processes. Second, the energy of the wave is not maximized at the surface, which reduces sensitivity.

Flexural Plate Wave (FPW) or Lamb wave device

A sensor concept similar to SAW sensors but employing Lamb waves was first presented by Stuart W.Wenzel, Richard M.White in 1988 [14]. In a flexural plate wave (FPW) or Lamb wave device (see Figure 1.7), an acoustic wave is excited in a thinned membrane with a thickness small compared to the propagation wavelength. As with the other acoustic sensors mentioned above, the FPW device can sense quantities that cause its phase velocity to change.
A unique feature of FPW is that it can be dimensioned so that its phase velocity is lower than that of most liquids, which lie in the range from 900 to about 1500 m/s. When the FPW device contacts or is immersed in such a liquid, a slow mode of propagation exists in which there is no radiation from the plate. Thus, the FPW device functions well in a liquid environment and is therefore a good candidate for biosensing and chemical sensing in liquid [1].
Figure 1.7 Schematic of a flexural plate wave device. The side view shows the different layers and membrane movement. Interdigital electrodes are used for actuation

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Comparation between four sensors

Figure 1.8 shows the side views and cross sections of four devices, in which lower diagrams in each column illustrate the wave motion; double-headed arrows indicate directions of surface particle displacements and shaded areas illustrate the wave motion or indicate the depth of wave penetration in the plate.
Table 1.1 summarizes qualitatively the characteristics of four sensors discussed above.

SAW sensor and the application pressure sensor in this research

SAW sensors offer many new applications. Because physical, chemical quantities can be measured from remote locations without the need of a separate power supply, SAW sensors have some advantages as follows:
+ They can be placed on moving, or rotating parts, for instance, in tire pressure [4],[5],[6],
+ used in hazardous environments such as high voltage plants, contaminated areas, strong radioactive areas, high vacuum process chambers, extreme heat, where the use of conventional sensors with wire connection is impossible, dangerous for human, complicated or expensive,
+ besides, because SAW sensors can operate at high frequencies (GHz range), the system based on them can be well protected from the electromagnetic interference that occurs in the vicinity of industrial equipment such as high voltage line.
Figure 1.9 shows the influences to SAW sensors and applications. The environmental parameters (such as temperature, pressure, humidity, mass loading . . .) are converted directly to a change in frequency, phase, or delay time of SAW sensor.
Figure 1.9 Environmental influences to the SAW sensors
Among these applications, the pressure sensor plays a key part of many systems, both commercial and industrial. Our work focuses on this kind of sensor.
SAW pressure sensors could be divided into two types:
+ One port IDT,
+ Two port IDT.

SAW pressure sensor with one port IDT

The SAW pressure sensor using one port IDT has been studied in literature [4]-[9]. In general, there are two types as follows:
+ Without external sensor circuit (see Figure 1.10)
Figure 1.10 SAW wireless pressure sensor with one broadband reflective delay line
The interrogator transmits an RF-impulse which is received by the antenna of the wireless sensor. The interdigital transducer transforms the RF-impulse to a surface acoustic wave. Reflectors are placed in the propagation path of the SAW at which small parts of the SAW is reflected. The impulses are reflected back to the transducer where they excite an RF-impulse train which is detected by the interrogator. The sensor signal is determined by evaluating the phase shifts ∆φi of the reflected impulses.
The reflective delay line requires at least three reflectors [7]. The first reflector serves as a reference reflector. This makes the sensor response independent on the distance between sensor and interrogator. For optimal reflector configuration, the first and second reflectors divide the delay line into stretched and compressed sections. The third one is placed at the end of the propagation path.
The temperature correction of the pressure signal could be obtained by arranging the electrically loaded reflector equidistantly between two reference reflectors in a second track [6].

Table of contents :

Introduction
Chapter 1 BACKGROUND
1.1 Acoustic wave devices
1.1.1 Thickness Shear Mode (TSM) resonator
1.1.2 Surface Acoustic Wave Device
1.1.2.1 SAW excitation and detection
1.1.2.2 SAW perturbation mechanisms
1.1.3 Acoustic Plate Mode (APM) devices
1.1.4 Flexural Plate Wave (FPW) or Lamb wave device
1.1.5 Comparation between four sensors
1.1.6 SAW sensor and the application pressure sensor in this research
1.1.6.1 SAW pressure sensor with one port IDT
1.1.6.2 SAW pressure sensor with two ports IDT
1.2 Piezoelectric materials. Aluminum Nitride (AlN) and its applications in SAW devices
1.2.1 Piezoelectric materials and the choice of AlN
1.2.2 General Information of AlN
1.3 Modelling SAW devices
1.3.1 Why Equivalent Circuit model is chosen?
1.3.2 The Finite Element Model (FEM)
1.4 Micromachining process, the choice of surface micromachining
1.5 Conclusion
Chapter 2 SAW PARAMETERS ANALYSIS AND EQUIVALENT CIRCUIT OF SAW DELAY LINE
2.1 Introduction
2.2 Calculation of SAW properties
2.2.1 Wave velocity, coupling factor in AlN/Si structure
2.2.2 Wave velocity, coupling factor in AlN/SiO2/Si structure
2.2.3 Wave velocity, coupling factor in AlN/Mo/Si structure
2.3 Equivalent circuit for SAW delay line based on Mason model
2.3.1 Equivalent circuit for IDT including N periodic sections
2.3.2 Equivalent circuit for propagation path
2.3.3 Equivalent circuit for SAW delay line
2.4 Equivalent Circuit for IDT Based On The Coupling-Of-Mode Theory
2.4.1 COM equation for particle velocities
2.4.2 Equivalent circuit for IDT based on COM theory
2.4.3 Equivalent circuit for propagation path based on COM theory
2.4.4 Equivalent circuit for SAW delay line based on COM theory
2.5 Comparison of Equivalent circuit of SAW device based on Mason model and COM thoery
2.6 Conclusion
Chapter 3 DESIGN OF SAW PRESSURE SENSOR DEVICES
3.1 Introduction
3.2 Temperature compensated structure for SAW device
3.2.1 Temperature dependence of Si, SiO2, AlN properties
3.2.2 Temperature Coefficient of Frequency (TCF) and temperature compensated structure for SAW sensor
3.3 Pressure dependence of frequency and phase in saw delay line
3.3.1 Mechanical analysis of membrane under pressure
3.3.2 Pressure-dependence of frequency by pressure dependence of AlN elastic properties
3.3.3 Pressure-dependence of frequency by delay line
3.3.4 Pressure-dependence of phase shift
3.4 Conclusion
Chapter 4 FABRICATION PROCESS
4.1 General description
4.2 Masks designed
4.2.1 Trench, counter masque, hole, and PSG layers
4.2.2 Metal AlCu and polyimide layers
4.3 Creating the stop wall of etching SiO2- Trench
4.4 Non-selective epitaxy
4.5 COUNTER MASK lithography, etching Si and CMP process
4.6 Etching holes
4.7 HF Etching of the sacrificial layer
4.8 PSG (Phospho Silicate Glass)
4.9 Depositing AlN as the piezoelectric layer and its properties
4.9.1 Influence of substrate roughness on crytal quality of AlN
4.9.2 Dependence of FWHM of AlN on AlN thickness
4.9.3 Dependence of FWHM of AlN on using bottom Mo layer
4.9.4 AlN at high temperature
4.10 Metal layer AlCu for IDT and probes
4.11 Polyimide as absorber
4.12 Conclusion
Chapter 5 CHARACTERIZATION OF SAW DEVICE
5.1 Parametric tests
5.1.1 The Square Resistance: Van Der Pauw
5.1.2 Isolation and continuity
5.1.3 Measuring under etching
5.1.4 Mask for parametric test
5.1.5 Parametric characterization
5.2 Experimental setup
5.3 Experimetal results
5.3.1 Propagation losses measurement
5.3.2 Piezoelectric coupling factor extraction
5.3.3 Comparison between experiment and simulation
5.3.4 Effect of Mo layer on performance of AlN/Si SAW device
5.3.5 Effect of thin Polyimide film
5.3.6 Device under pressure
5.3.6.1 Phase shift
5.3.6.2 Frequency shift
5.4 Conclusion
CONCLUSION and PERSPECTIVE

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