Surface Acoustic Waves at Ferromagnetic Piezoelectric Interfaces

Physik-Department
Lehrstuhl E23
Walther-Meißner-Institut für
Tieftemperaturforschung
Bayerische Akademie der
Wissenschaften
Surface Acoustic Waves at Ferromagnetic
Piezoelectric Interfaces
Bachelor's thesis of
Hajo Söde
Prof. Dr. Rudolf Gross
Garching b. München, July 2009
TECHNISCHE UNIVERSITÄT MÜNCHEN
Declaration
I hereby declare that I have written this Bachelor's thesis on my own and have used no
other than the stated sources and aids.
Hiermit erkläre ich, dass ich diese Bachelor Thesis selbstständig verfasst und keine
anderen als die angegebenen Quellen und Hilfsmittel verwendet habe.
Unterschleissheim, 15.07.2009
...................................................................................
(Hajo Söde)
Abstract
The properties of surface acoustic waves (SAWs) propagating at ferromagnetic, piezoelectric interfaces are investigated. Surface acoustic waves are excited on the surface of
a Y-Z LiNbO3 single crystal using interdigital transducers. The preparation of such
samples is described. The influence of the interdigital transducer design and of the substrate material on the delay line transmission is investigated. Furthermore, the interaction of the SAW with a thin nickel film evaporated onto the LiNbO3 substrate is investigated. The magnetic properties of the nickel thin film are characterized with magnetotransport measurements. An external magnetic field is applied in the hall bar plane,
varying in magnitude and orientation. The SAW delay lines show a clear interaction
with the magnetization of the nickel hall bar. The observed results inspire further research on these interactions.
1
Contents
Abstract .................................................................................................. 1
Contents .................................................................................................. 2
List of Figures .......................................................................................... 4
List of Tables ............................................................................................ 6
Notation .................................................................................................. 7
1. Introduction ............................................................................................. 10
2. Theory ...................................................................................................... 12
2.1. Piezoelectricity ...................................................................................... 12
2.2. Rayleigh waves ...................................................................................... 13
2.3. Interdigital transducers ......................................................................... 18
2.4. Magneto transport overview ................................................................ 20
3. Materials .................................................................................................. 22
3.1. Lithium niobate ..................................................................................... 22
3.2. Quartz .................................................................................................... 23
3.3. Nickel ..................................................................................................... 24
4. Experimental techniques .......................................................................... 25
4.1. Vector network analyzer ........................................................................ 25
4.2. Delay line transmission .......................................................................... 26
4.3. Magneto-transport measurement ........................................................ 26
5. Sample preparation .................................................................................. 28
2
5.1. Optical lithography ................................................................................ 28
5.1.1. Lithography preparation ............................................................... 29
5.1.2. Contact and projection lithography ............................................. 30
5.2. DC sputtering ......................................................................................... 31
5.3. Electron beam evaporation ................................................................... 32
5.4. Lift-off .................................................................................................... 32
5.5. Finished sample ..................................................................................... 32
6. Measurements ......................................................................................... 34
6.1. Delay line transmission .......................................................................... 34
6.2. Magneto-transmission .......................................................................... 38
6.2.1. Constant orientation .................................................................... 38
6.2.2. Constant magnitude ..................................................................... 42
6.3. Conclusions ........................................................................................... 44
7. Outlook .................................................................................................... 45
8. List of samples .......................................................................................... 47
Acknowledgments ................................................................................... 49
Bibliography ............................................................................................ 50
3
List of Figures
Figure 1:
Rayleigh wave .................................................................................... 17
Figure 2:
View of an IDT pattern ....................................................................... 19
Figure 3:
Idealized power flow in an IDT pair ................................................... 19
Figure 4:
Measuring apparatus ......................................................................... 26
Figure 5:
Close up of sample in measuring apparatus ..................................... 26
Figure 6:
Four point measurement on a hall bar .............................................. 26
Figure 7:
Completely bonded sample on chip carrier ...................................... 27
Figure 8:
Image reversal process ...................................................................... 29
Figure 9:
First successful 3µm structure ........................................................... 30
Figure 10:
Lift-off process ................................................................................... 32
Figure 11:
Finished substrate with IDTs and hall bar .......................................... 33
Figure 12:
Typical delay line ................................................................................ 34
Figure 13:
Magnitude as function of number of fingers and finger overlap ...... 35
Figure 14:
FWHM as function of number of fingers and finger overlap ............ 35
Figure 15:
Delay line of a 86MHz center frequency sample ............................... 36
Figure 16:
Amplitudes of harmonics ................................................................... 37
Figure 17:
Amplitudes of harmonics ................................................................... 37
Figure 18:
Up sweep and down sweep of SAW magnitude ................................ 39
Figure 19:
Illustration of color code .................................................................... 39
Figure 20:
Effect of the external magnetic field ................................................. 40
Figure 21:
SAW travels at LiNbO3-nickel interface .............................................. 41
4
Figure 22:
Up sweep of SAW phase .................................................................... 41
Figure 23:
Effect of the magnetic field orientation ............................................. 42
Figure 24:
Effect of the magnetic field orientation ............................................. 43
5
List of Tables
Table 1:
Piezoelectric voltage tensor ............................................................... 23
Table 2:
Piezoelectric strain tensor ................................................................. 23
Table 3:
Piezoelectric stress tensor ................................................................. 23
Table 4:
3µm structures on LiNbO3 ................................................................. 47
Table 5:
5µm structures on Quartz .................................................................. 47
Table 6:
5µm structures on LiNbO3 ................................................................. 48
Table 7:
10µm structures on LiNbO3 ............................................................... 48
6
Notation
For those who want some proof that physicists are human, the proof is in the idiocy of all the different units
which they use for measuring energy.
Richard Feynman
A summation convention is assumed throughout this thesis: When an index variable
appears twice in a single term, it implies summing over the index values 1,2,3.
In the whole thesis the SI unit system is used and all electronic symbols used are according to IEC 60617 (DIN EN 60617).
Unless stated otherwise, properties of materials are considered at standard conditions (temperature of 300K, pressure of 100kPa) and measurements with the vector
network analyzer are S21 measurements. SXX measurements are explained in chapter
4.1.
 itself or its components τi.
A vector can be referred to as the vector 
Physical quantities
A●
amplitude of ●th harmonic
b
IDT finger width

B
magnetic induction [T], referred to as magnetic field
c●
phase velocity of ● wave [m/s]

D
electric displacement field [C/m2]
e ●
unit vector in ● direction

E
electrical field [V/m]
f
frequency [Hz]
f0
center frequency
7

H
magnetic field strength [A/m]
Ihb
electrical current trough hall bar [A]
k
wave vector [1/m]

M
magnetization [A/m]
N
number of finger pairs
P
power [W]
s ik
charge voltage coupling factor
u

displacement vector
u ik =

1 ∂u i ∂u K

2 ∂ xk ∂ xi

strain tensor
Ul
longitudinal hall bar voltage [V]
Ut
transverse hall bar voltage [V]
W
IDT overlap of fingers
η
metallization ratio

 and the direction of Ihb [deg]
angle between M
0
wavelength [m]
 ik
stress tensor [N/m2]

electric potential [V]

penetration depth [m]

radial frequency [rad/s]
Abbreviations
Al
aluminum
Au
gold
AMR
anisotropic magnetoresistance
Cr
chromium
DC
direct current
FWHM
full width at half magnitude
IDT
interdigital transducer
IR
image reversal
8
l
longitudinal
LiNbO3
lithium niobate
Ni
nickel
RF
radio frequency
SAW
surface acoustic wave
SMA
subminiature version a
t
transverse
VNA
vector network analyzer
Material values
d ik
piezoelectric strain tensor [C/N]
eik
piezoelectric stress tensor [C/m2]
E
Young's modulus [N/m2]
g ik
piezoelectric voltage tensor [m2/C]
Hc
coercivity [A/m]
RA
anomalous Hall coefficient [m3/C]
RH
Hall coefficient [m3/C]

permittivity [F/m]
iklm
elastic modulus tensor [N/m2]
0
vacuum permeability [1.2566×10-6 H/m] [5]

Poisson coefficient

mass density [kg/m3]
9
1. Introduction
The history of science teems with examples of discoveries
which attracted little notice at the time, but afterwards
have taken root downwards and borne much fruit upwards.
Lord Rayleigh
In 1885 Lord Rayleigh1 described an acoustic wave phenomenon which today plays an
important role in electronic devices such as band-pass filters and in seismology. He described a surface acoustic wave (SAW), today known as a Rayleigh wave, which is a
wave that propagates along the surface of an isotropic solid half-space. This gives the
possibility of measuring the wave within its propagation path. Requirement for this is
the reliable generation and detection of the SAW. This is achieved by so called interdigital transducers (IDTs), periodically spaced gratings of thin metallic films on the surface
of the substrate. First attempts to apply SAWs in electronic devices were made in the
1960s. Since then they have consequently found their way into many branches of electronics, including bandpass filtering for communication systems, such as the omnipresent cellular phones.
Goals of this thesis are the realization of micro-structures, consisting of aluminum IDT
pairs and nickel hall bars, on lithium niobate (LiNbO3) and quartz single-crystals substrates. The motivation is to quantify the effects of the IDT design and the substrate
material on the quality of the SAW delay lines. Furthermore, the interactions of a SAW
with a ferromagnetic nickel film deposited in its propagation path shall be studied as a
function of an external magnetic field.
This thesis is structured as follows: The theoretical foundations of surface waves and
their excitation as well as piezoelectric and ferromagnetic effects are summarized in
1 Lord Rayleigh, On waves propagated along the plane surface of an elastic solid, Proc. Lond. math.
Soc., No 17, p. 4–11, 1885
10
chapter 2. An overview of the relevant properties of the materials used is given in
chapter 3 along with the expected qualitative results. The measuring techniques and
setup are presented in chapter 5. Chapter 4 describes in detail the realization of metallic micro-structures using optical lithography, sputter and evaporation processes and
the lift-off technique. An optimized procedure and the parameters that gave the best
results are proposed. The results of the measurements and the conclusions drawn are
presented in chapter 6. The thesis concludes with a discussion of the results obtained
and gives an overview of the open questions and an outlook on further research.
11
2. Theory
Everything should be made as simple as possible, but not
simpler.
Albert Einstein
2.1 Piezoelectricity
Piezoelectricity is the phenomenon that couples elastic strains and stresses to electric
fields and displacements and manifests itself in two effects. First, the direct piezoelectric effect where the application of a force results in charge separation and thus electric
polarization within the material. Second, the converse piezoelectric effect where an external electric field results in a deformation of the piezoelectric material. The effect
only occurs in an anisotropic material whose internal structure lacks a center of symmetry. Only piezoelectric insulators will be considered here.
In a homogeneous piezoelectric insulator, the stress tensor  ik at each point depends
on the electric field 
E in addition to the strain tensor u ik. These quantities are assumed
to be small so we can neglect higher order terms and take the relationship to be linear
[11]:
T
 ik =
iklm u lm− e
ikl E l


elastic
(2,1)
conversepiezo.
Di = eikl u kl  ik E k
 ,
direct piezo.
(2,2)
dielectric
where iklm is the elastic modulus tensor, eikl the piezoelectric tensor, D i the electric
displacement and  ik the permittivity.
The propagation of an acoustic wave in a piezoelectric material can be described by
the general equation of motion [26]
12
2

∂ ui
∂ ik
∂t
∂ xk
=
2
(2,3)
It is convenient to express the electric field in terms of the electric potential . Since
electromagnetic disturbances travel much faster than mechanic ones, a quasi-static approximation can be used, so that
E i=
−∂
.
∂ xi
(2,4)
Substituting (2,1) and (2,4) in the general equation of motion (2,3) yields

∂2 ui
∂t
=e ikl
2
∂2 ul
∂2 
.
iklm
∂ xk ∂ xl
∂ x k ∂ xm
(2,5)
Equation (2,5) determines the motion of the wave if appropriate boundary conditions
are specified.
When dealing with piezoelectric materials, usually the piezoelectric strain tensor d ik
or the piezoelectric voltage tensor g ik is given. Then (2,1) and (2,2) become (subscripts
have been dropped for clarity)2, [22],
u=s d T E
D=d  E
u=s gT D
E=−g   D .
2.1. Rayleigh waves
An acoustic wave in a solid material involves changes in the relative positions of the
atoms. The changes in position from the equilibrium position is described by a dis-
u . Displacement can also be described in terms of strains. With
placement vector 
strains, internal forces arise which tend to return the body to its equilibrium state.
2 In this case E is a component Ei of the electrical field 
E and not the Young's modulus E as in the rest
of this thesis.
13
These forces are called stresses. Acoustic waves propagate with stresses obeying
Hooke's law.
One form of elastic waves is those that propagate on the surface of the body and decay exponentially inside the bulk, away from the surface. Such waves are called
Rayleigh waves.
We are looking for solutions of the wave equation [23],
∂2 u 2
−c  u=0
∂t 2
(2,6)
with u being a component of ul or ut and c=c l , ct the according velocity, that comply
with such boundary conditions.
Let the surface of the body be the xy-plane. The inner body can be identified with z <
0. The general solution to the wave equation is
u=const e
ikx− t z
,
(2,7)
from which the dispersion relation can be obtained
 =c  k −
2
2
2
2

[
1
]
2 2
⇒ = k − 2 ,
c
2
(2,8)
 characterizes the penetration depth of the wave.
Inside the body longitudinal and transverse waves propagate independently from
each other. However, as waves are reflected on a surface, boundary conditions mix lon-
u must be some
gitudinal and transverse components. Hence, the displacement vector 
linear combination of the vectors ul and ut . In order to determine this linear combination we use boundary conditions. On a free surface with the normal vector n the conez , this implies
ditions must satisfy  ik nk =0. Because n∥
 xz = yz = zz=0
and from that
u xz =0,
u yz =0,
  u xx uyy   1−  u zz =0.
14
(2,9)
Since there is no y dependence,
u yz =


1 ∂u y ∂u z 1 ∂ u y

=
=0
2 ∂z ∂y
2 ∂z
and with (2,7) follows
u y =0 .
(2,10)
Thus, the displacement vector lies in the xz-plane.
The transverse part ut of the wave satisfies
∂u tx
∂x

∂ utz
∂z
(2,11)
=0.
Substituting (2,7) into the equation above one gets the equation
iku tx t utz =0,
which defines the ratio u tx /utz . With constant At we can write
u tx= A t t eikx  z−i  t ,
u tz =−ikAt e ikx z−i t .
t
t
(2,12)
Analogously for the longitudinal part ul of the wave:
∂u lx ∂u lz
−
=0 ⇒ ikulz −l u lx =0,
∂z ∂x
u lx = Al k eikx  z−i  t ,
u lz =−iAl l e ikx z−i  t .
l
l
(2,13)
where Al is a constant. Since u x =u lx utx is in phase quadrature (out of phase) with
u z =ulz u tz, the motion of individual atoms are elliptical.
Now the relations (2,9) are used again. Using the definition of u ik (see notation
chapter) and c l and c t as defined in the wave equation (2,6), the first and second relation can be rewritten as
∂u x
∂x

∂u z
∂z
=0,
c 2l
∂u z
∂z
= c2l −2 c 2t 
∂u x
∂x
(2,14)
By substituting equations (2,12) and (2,13) into the first of the equations (2,9) becomes
15
A t  k 22l 2 Al k l =0.
(2,15)
Now substituting equations (2,12) and (2,13) into the second of the equations (2,9), di2
viding by c t and substituting
2
2
l
k − =
2
c 2l
=k −
2
2
t
c 2t
c ,
(2,16)
2
l
which derives from (2,8), the second equation (2,9) becomes analogous to (2,15)
A l  k t 2 At k t =0.
2
2
(2,17)
Squaring the latter equation and substituting l and t using again (2,8), it yields the
constraint for consistency of the homogenous equations (2,15) and (2,17),

4
   
2
2
2 k − 2 =16 k 4 k 2− 2
ct
cl
2
k 2−
2
.
c2t
(2,18)
Obviously this equation gives a linear relation between ω and k
=c t k.
(2,19)
The proportionality factor was chosen as c t  because a factor k8 cancels out on both
sides of (2,18) giving the following equation for 
6
4
2
   
 −8  8  3−2
c2t
c2l
−16 1−
c2t
c2l
=0.
(2,20)
So  only depends on the ratio c t /cl , which is a material constant, and only depends on
the Poisson coefficient .
Due to (2,8) and because l and t are real and positive by their definition,  must be
real, positive and smaller than 1. Equation (2,20) has exactly one solution that satisfies
all constraints [23]. When going from (2,18) to (2,20) the solution =0 is lost. In this
case (2,8) gives k=l =t and (2,19) then implies =0 and so fulfills the constraints.
But with (2,15), (2,17) and k=l =t one gets the relation A t =−Al . Using this in the
u =0 , thus there
displacements (2,12) and (2,13) yields the total displacement ul ut =
16
is no motion at all.
The constraints on  also imply that Rayleigh waves propagate slower than volume
waves
c Rayleigh=ct ct c l .
The ratio of the amplitudes of the longitudinal and the transverse part can be written
in terms of  [23]
At
Al
=
2−2
2  1−2
.
The conditions (2,9) imply a strain tensor of the form


xx xy 0
u ik = xy yy 0 ,
0 0  zz
therefore a Rayleigh wave is a coupled shear and compressional wave.
When dealing with anisotropic materials, it is essential to specify the orientation. The
internal structure of a crystal can be referred to a set of axes denoted by the symbols X,
Y and Z. These directions are defined by convention in relation to the crystal lattice
[19]. The surface normal and the wave propagation direction must be defined in relation to these axes, e.g. Y-X lithium niobate indicates that z is in the crystal Y direction
and x is in the crystal X direction. Hence, material constants have to be specified in relation to the crystal axes.
Fig. 1: Rayleigh wave on the surface of a substrate with time dependent
elliptical displacement
Picture taken from www.geo.brown.edu
17
In the following an expression for the electric potential that is accompanying the SAW
is derived. A piezoelectric material requires an electrical boundary condition at the surface when dealing with surface waves. In general, there are two cases. Firstly the so
called free-surface condition, with an insulating vacuum above the surface. Secondly
the so called metalized condition where the surface is covered with a thin layer of conducting metal, which does not affect the mechanical conditions but shorts the x and y
components of the electric field. In the following, only the free-surface condition will
be discussed.
To the displacements (2,12) and (2,13) corresponds an electric potential . According
to relation (2,4),  can be written as
= f  z  e ikx −i  t .
For the free surface condition the potential must satisfy Laplace's equation ∇ 2 =0.
Therefore f (z) must be of the form e±kz and because the potential vanishes at infinity,
 has to be
=0 e−∣k∣z e ikx−i t ,
(2,21)
with the constant  0, which is set by (2,1).
The anisotropy of the material has a significant effect on the solutions obtained
above. In general the solutions depend on all three displacement components, so that
the motion actually does have a y component. However, the electric field is always confined to the xz plane [12].
2.2. Interdigital transducer
An IDT is a simple transducer in the sense that no transition from one material to another is required and therefore transmission is not affected by scattering effects. It basically converts electrical energy into SAWs and vice versa. An IDT pattern can easily be
realized with optical lithography if the smallest structure size is ≥ 1µm.
18
Fig. 2: Sketch of an IDT pattern. The number of finger pairs is N = 5.
Figure 2 shows an IDT that generates a wave in two directions. The number of finger
pairs is N = 5 and the overlap of the fingers is W. In this case the gaps between the fingers are of the same size as the width b of the fingers and the distance between two
fingers of the same polarity is the center wavelength 0 = 4b. Excitation with a RF electrical field frequency that corresponds to the center wavelength or higher oddnumbered harmonics result in SAW generation. Hereby the the metallization ratio η =
2b/0, influences the excitation of the harmonics as described in more detail in chapter
6.2.1.
A usual SAW device consists of two transducers, the input IDT for generating and the
output IDT for detecting waves. An IDT generates waves in two directions, so the insertion loss is about 3dB. The same applies for an output IDT in which acoustic energy is
converted back into electrical energy but the second half of the energy is re-emitted as
acoustic waves. A second reflection at the input IDT leads to interferences with the
main SAW signal. This kind of corruption of the signal is called triple transit interfer-
Fig. 3: Idealized power flow in an IDT pair, assuming 100%
conversion of input power into SAW and electrical potential
wave power and vice versa.
19
ence. Besides the generation of SAWs, bulk waves are generated, especially when not
exciting with an odd harmonic.
The simplest theoretical model for the generation of an SAW by an IDT is the delta
function model. This is to give a small introduction to IDTs. For an extensive discussion
of the delta function model and other IDT models the book of Morgan and Paige [26] is
recommended. The delta function model is based on the assumption that an IDT is a
superposition of single wave sources. Assuming that a voltage U i is applied to the ith
finger pair, it induces a charge q k in the kth finger pair, which is, only taking linear effects into account, proportional to U i with some factor s ik . If the voltages are applied
simultaneously the total charge amounts to the superposition of all induced single
charges
q i =s ik U k .
As described in chapter 2.1, the piezoelectric material strains when charges are applied
to its surface. Positive and negative charges induce strain in different ways, either
stretching or compression. Since a RF voltage is applied dilation and compression alternate generating acoustic waves.
Since an IDT not only excites SAWs, s ik can be split up into different components:
s ik =
S
sik
surface waves
B
 
sik
.
bulk waves
2.3. Magneto-transport overview
One goal of this thesis is the investigation of the interaction of SAWs with a ferromagnetic thin film deposited within its propagation path. In order to determine the magnetic properties of this thin film, DC transport measurements are carried out. In this
section an overview over magneto-transport is given. A more comprehensive version
can be found in the Diploma thesis of Matthias Althammer [2], for further reading on
various topics of magnetism, including magneto-transport, we refer to the book of
Robert M. White [32].
20
In magneto-transport measurements a constant DC current Ihb is send trough a hall
bar. The longitudinal voltage drop Ul parallel and the transverse voltage drop Ut perpendicular to Ihb are measured. A transverse voltage can e.g. be caused by the Hall effect.
Spin-orbit coupling in ferromagnetic materials also results in electrical resistivities
parallel and perpendicular to the magnetization. So the resistivity is dependent on the
angle between the magnetization and the direction of Ihb. This anisotropy of the resistivity is called anisotropic magnetoresistance (AMR). For the AMR dependence on the
 and the direction of Ihb holds the equation [2]
angle  between M
U AMR ∝ constcos 2   .
21
3. Materials
The nature of matter, or body considered in general, consists not in its being something which is hard or heavy or
colored, or which affects the senses in any way, but simply
in its being something which is extended in length,
breadth and depth.
Rene Descartes
We introduced the physical principles of piezoelectric materials. All relevant properties
of the materials used in this thesis will be discusses here.
Piezoelectricity can only be present if the material is anisotropic, therefore its properties vary with direction of the internal structures. Hence, it is necessary to discuss the
relevant properties for all crystal orientations. Important properties are the surface
acoustic wave velocity, piezoelectric coupling, temperature effects, and the level of (unwanted) bulk-wave generation. Due to the anisotropic material properties, it is also essential to quote the orientation when specifying a material. For example, 36°Y-X lithium tantalate (LiTaO3), means that the surface normal makes an angle of 36° with the
crystal Y axis, and the surface wave propagates in the crystal X direction. These axes as
well as the crystal Z axis are defined by convention [19].
3.1. Lithium niobate
Lithium niobate (LiNbO3) was mainly used as piezoelectric substrate for the samples.
LiNbO3 was chosen because of its high electromechanical coupling factor of 4.6%, e.g.
compered to quartz with a coupling factor of 0.11% [9]. Commercially available LiNbO3
is grown using the Czochralski process and sold as wafers. The crystal is widely applied
in filter devices and resonators and used as ultra sonic transducer due to its high electromechanical coupling factor, stable physical and chemical properties [12].
22
The results reported for LiNbO3's piezoelectric values vary significantly in different
publications. Though, the preferred set is that of Smith and Welsh [29] shown in Table
1 - 3. Less ambiguous is the SAW velocity with 3485m/s in the Z direction [1].
The isotypic lithium tantalate (LiTaO3) would have been an option as substrate, too,
since it has similar properties as LiNbO3.
g33
g22
g31
g33
91.8
27.6
-3.3
23.6
Tab. 1: Piezoelectric voltage tensor [10-3 m2/C] [29]
d15
d22
d31
d33
69.2
20.8
-0.85
6.0
Tab. 2: Piezoelectric strain tensor [10-12 C/N] [29]
e15
e22
e31
e33
3.76
2.43
0.23
1.33
Tab. 3: Piezoelectric stress tensor [C/m2] [29]
3.2. Quartz
We also used quartz (SiO2) as a substrate for a few samples to compare the SAW properties on LiNbO3 and SiO2. A quartz single crystal is grown by hydrothermal synthesis
method in autoclaves. Due to its piezoelectric properties, low thermal expansion, good
mechanical parameters and excellent optical characteristics, quartz is widely used in
laser optics, optical fiber communications, and X-ray optics, etc.
Our wafer was a ST-X quartz wafer. This is a 42.7° rotated Y-cut, with SAW propagation along the X axis. Quartz's piezoelectric strain tensor is d22 = 2.3×10-12C/N; d33 =
72.7×10-12C/N [3] and its SAW velocity is 3159m/s [1].
23
3.3. Nickel
Nickel was used as the ferromagnetic material for the hall bars. Although it is one of
only three ferromagnetic elements at room temperature, its main areas of application
do not use this property, e.g. rechargeable batteries and coinage. Nickel has a Curie
temperature of 627,15K [31] which is well above room temperature. Nickel crystallizes
in the face centered cubic structure and has a mass density of 8908kg/m 3. Its magnetostrictive constants are 4.9×10-5 in <100> direction and 3.0×10-5 in <111> direction [33].
24
4. Experimental techniques
Natural science does not simply describe and explain
nature; it is part of the interplay between nature and ourselves.
Werner Heisenberg
4.1. Vector network analyzer
A network analyzer is used for the accurate RF measurement of the ratios of the reflected electric RF signal to the incident signal, and the transmitted signal to the incident
signal. A complete characterization of devices involves measurement of the RF magnitude as well as the phase and is accomplished by a vector network analyzer (VNA).
At high frequencies, it is very hard to measure total voltage and current at the device
ports. One cannot simply connect a voltmeter or current probe and get accurate measurements due to the impedance of the probes themselves. That is why scattering or Sparameters were introduced. S-parameters relate to measurements such as gain, loss,
and reflection coefficient. They are defined in terms of voltage traveling waves, which
are relatively easy to measure. The numbering convention for S-parameters is that the
first number following the “S” is the port where the signal arrives, and the second
number is the port where the signal is applied. So, S21 is a measurement of the transmission from port 1 to port 2. When the numbers are the same (e.g., S11), it indicates
a reflection measurement, as the input and output ports are the same.
For all measurements a two port Rohde&Schwarz ZVA24 VNA was used. It has a frequency range from 10MHz to 24GHz and in our frequency range an error of less than
1dB (<0.2dB over -30dB and <1dB under -30dB). Its dynamic range is typically 105dB
(10MHz-100MHz) and 120dB (100MHz-700MHz) and its power output is typically
between -40dBm and 18dBm (10MHz to 13GHz) [28].
25
4.2. Delay line transmission
Measuring the transmission of a SAW delay line can easily be done by using a VNA. The
chip carrier with the sample fixed with double-faced adhesive tape on it is directly connected to the VNA with coaxial cables with SMA connectors. To record the data, the
VNA is connected to a computer with a standard network cable . A LabVIEW 3 program,
written by Mathias Weiler, automatically operates the VNA after entering the desired
settings for center frequency, sweep range and number of measuring points.
4.3. Magneto transport measurement
Fig. 4: Measuring apparatus: 2D magnet,
gauss- and multimeter, VNA (from left to right)
Fig. 5: Close-up of sample fixed between 2D
magnet and hall probes
For magneto-transport measurements the magnetic thin film is structured into a hall
bar. An example is shown in Fig. 6. The the hall bar is bonded in such a way that a four
point measurement can be done. The sample is placed in the center of the 2D magnet,
consisting of four coils and iron yoke, such that the magnetic field 
B is always in the
Fig. 6: Four point measurement on a hall bar
3 LabView is a registered trademark of National Instruments
26
sample plane. The orientation of the external magnetic field with respect to the direction of the SAW propagation is proportional integral (PI-) controlled by two Lake Shore
421 gaussmeters that adjust the current trough the coils depending on the field measured with hall probes. A combination of a Keithley 2400 source-meter and a Keithley
2010 multimeter is used for the transport measurement.
All adjustments of the field magnitude and orientation as well as the VNA and multimeters are controlled automatically by a LabVIEW program (by Mathias Weiler). The
maximum applicable magnetic field magnitude was 100mT in the plane. This setup
now allows simultaneous determination of the SAW transmission and magneto-transport properties.
Fig. 7: Completely bonded sample on chip carrier. Bonds to the top
and bottom bond pads are from the hall bar the other from the two
IDTs.
27
5. Sample preparation
Let us watch well our beginnings, and results will manage
themselves.
Alexander Clark
In the previous chapters the motivation and theory of SAWs on LiNbO 3 or quartz substrates were addressed. Now the technological realization of the IDT and hall bar structures on the substrates is discussed. The single-crystal wafers are cut into rectangles of
5mm×6mm on which the SAWs should propagate along the short side (Z direction). The
preparation of the IDTs require structure sizes down to 1µm. For structures of that size
the technique of optical lithography is an established method. This chapter deals with
the basics of the lithography process and describes details of the steps in the order of
their use. Simultaneously, an optimized process is presented as an important result of
this thesis.
The presented, optimized sample preparation is valid for LiNbO3 substrates only. Other substrates may have different parameters for the temperature of the image reversal
process and the adhesion of the IDTs, and hall bar metals may be different on other
substrates. Because we use different metals for IDTs and hall bars we need to do the
lithography process twice. First, the IDTs are prepared and in a second process the hall
bars. This sequence was chosen due to the very good adhesion of the aluminum IDTs
and to do reference measurements without hall bars.
5.1. Optical lithography
Using the technique of optical lithography, a pattern can be transferred from a so called
mask onto a thin layer of photoresist by exposing mask and resist to UV light. A developer dissolves the resist depending on the exposure and the substrate is sub-
28
sequently uncovered partially.
One distinguishes between positive resist, negative resist and image reversal resist.
The positive resist dissolves in the exposed regions during development, whereas negative resist resolves in the regions that were not exposed. An image reversal resist behaves like a positive resist after initial exposure, but with a reversal bake (heating the
resist on a hotplate) the exposed areas are cross-linked, while the unexposed area remains photo-active. Exposing the resist without mask to UV light again (flood exposure)
renders the resist, which was not exposed in the first step, soluble in developer. After
developing, the areas exposed in the first step remain with an undercut. A schematic illustration of the reversal process is depicted in Fig. 8.
Fig. 8: Image reversal process. (1) UV exposure with mask. (2) The resist would now behave like a positive
resist. (3) Exposed area cross-linked by image reversal bake, the unexposed area remains photoactive. (4)
Flood exposure. (5) Cross-linked area stays insoluble. (6) Structure after developing.
Independent from the resist that was used, the development process leaves the
structure uncovered. If a sufficient thin metal film is evaporated or sputtered onto the
resist pattern (chapters 5.2. and 5.3.) it will not be continuous at the photoresist edge.
Upon resist removal with acetone in ultrasound, the parts of the metal film on top of
the resist will be lifted off and removed with the resist.
5.1.1. Lithography preparation
The basis of a successful result is a clean substrate. Cleaning can be done mechanically
with an acetone moisturized tissue or in an ultra sonic bath, in which the substrate has
to be in a beaker filled with acetone.
Coating the substrate with photoresist on a spin coater is the next step. The
photoresist must be spun on the substrate, rotation speed and the viscosity of the used
resist define the thickness. The AZ 5214E4 resist we used has a thickness of about
4 AZ is a registered trademark of Clariant AG.
29
1.4µm at 4000rpm and 1.14µm at 6000rpm [27]. For structures with sizes of 5µm or
larger, spinning 40s at 4000rpm, gives a thickness of 1.4µm, while for smaller structures spinning 60s at 6000rpm, gives a thickness of 1.14µm. After the coating there is a
well of resist at the edges of the substrate, which have to be removed mechanically,
with e.g. a razor blade or a toothpick. Afterwards a prebake on a hotplate at 110°C for
50s is done to harden the resist.
5.1.2. Contact and projection lithography
A Karl Süss MJB 3 mask aligner is utilized for the IDT structures, using contact lithography. As the name indicates the mask is in contact with the sample, which makes a
lens system unnecessary. The mask consists of chromium structures on a glass plate.
Because of the direct contact of mask and substrate the pattern on the mask must have
the desired size, since no demagnification is possible.
Fig. 9: First successful 3µm structure (LNOB 55)
Masks for the mask aligner have to be designed with AutoCAD5. At the Walter Schottky Institut the masks are written by a laser beam on chromium plated glass plates
covered by photoresist. Depending on the complexity of the structures writing can take
up to a week. An etching process finally reveals the mask.
The UV exposure times for IDTs with the mask aligner are 5.5s for a 1.4µm thick
photoresist or 6.1s for the 1.14µm thickness. The most critical parameter of the IR-process is that the reversal-bake temperature must be kept constant within ±1°C to maintain a consistent process. We found 119°C for 120s to the optimized settings, that were
5 AutoCAD is a registered trademark of Autodesk, Inc
30
obtained from a series of measurements. Subsequently, a flood exposure is done for at
least 50s with the mask aligner, but without the mask. This step is uncritical as long as
sufficient radiation energy is applied. However, if expose for more than three minutes
the resist gets porous.
For the development the substrate is dipped in AZ 726 MIF developer for 45s and development is stopped in water. If the structures do not look as expected, the resist can
easily be removed with acetone.
In the projection system, which is used for the hall bar structures, a negative film
mask is placed between lamp, aperture and microscope. UV light of wavelength 400nm
is sent trough the mask and the microscope. Here the pattern of the mask is demagnified by a factor between 1.6 and 50. We used a demagnification factor of 10 for our
hall bars, and projected on the coated substrate. The main advantages of this system
are quick and simple operation and that cheap foil masks can be used. The hall bar
must be centered between the IDTs, it is essential that the hall bar is focused. Exposing
time with the UV filter is 40s in total. AZ developer is used for the development. Development should be stopped after 21s in water. In the projection system, AZ 5214E is
used as a positive resist.
5.2. DC sputtering
DC sputtering is a vacuum process used to deposit thin films on substrates. A high
voltage is applied across a low-pressure gas, in our apparatus we use 5×10-3kPa and argon to create a plasma. This plasma consists of electrons and gas ions in a high-energy
state. Because of their charge, the ions hit the source material and cause atoms from
that target to be ejected and bond with the substrate.
We use DC sputtering to deposit a 3nm chromium layer as an adhesive prior to the
aluminum evaporation of the IDT structures.
31
5.3. Electron beam evaporation
Electron beam evaporation is a high vacuum thermal evaporation process. The source
material is heated using an electron beam to temperatures near the boiling point. The
vaporized material travels trough the vacuum chamber to the cooler substrate. Trough
condensation the vapor forms a thin layer that grows with time. The deposition control
is done via a crystal oscillator film thickness mounter. Process pressures are in the
range of 10-6kPa. This is mainly because the pressure must be below the gas pressure of
the evaporating material. The other reason is that the mean free path must be much
longer than the distance between source and substrate, to avoid unwanted oxidation.
For the IDTs, we used 70nm aluminum on top of the 3nm chromium. The hall bars are
evaporated with 50nm nickel.
5.4. Lift-Off
This is the last step of the preparation of the substrate. At the lift-off process the remaining resist is lifted off the substrate including the part of the metal film on top of
the resist.
The substrate is put into a glass of acetone at 70°C until the metal film gets bumpy.
For Ni this takes only a few minutes, Al, however, needs about 15 minutes. With ultra
sonic pulses the metal film is carefully lifted off. This is done until no visible metal is left
except the structures.
Fig. 10: Lift-off process. Undercut allows lift-off of evaporated film.
5.5. Finished sample
Figure 11 shows a magnified photograph of a sample after its complete preparation.
The black spots on the IDT contacts result from ripping of the wire bonds before preparation of the hall bar.
32
Fig. 11: Finished sample with Al IDTs and Ni hall bar (LNOB 53)
33
6. Measurements
No phenomenon is a physical phenomenon until it is an
observed phenomenon.
John A. Wheeler
6.1. Delay line transmission
Fig. 12: Typical delay line. Marked are the maximum and half maximum magnitude, FWHM as well as the center frequency. LNOB 55
In this section the influence of the IDT design on the characteristics of the acoustic
delay line formed by a pair of identical IDTs on LiNbO3 substrates is investigated. Figure
13 shows the dependence of the maximum magnitude of the first harmonic on the finger overlap W and the number of finger pairs N. It is observed that a higher W and N
increase the transmission trough the delay line. The full width at half magnitude
(FWHM) of our delay lines is shown in Fig. 14 as a function of W and N. The FWHM is
decreased for higher W and N, but stays constant for W ≥ 100µm and N ≥ 10.
34
The center frequency f 0 = c/0 of the b = 5µm IDTs is 170.1 ± 0.4MHz, which gives a
phase velocity cSAW = 3402 ± 8m/s in good agreement to the literature value [1]. The
main characteristic of the delay line is the maximum of the magnitude in relation to the
stimulus at the first harmonic excitation which is excited at f 0 . A second quality characteristic is the full width at half magnitude (FWHM).
Fig. 13: Maximum magnitude as function of the number of fingers and the finger
overlap. Green indicates less attenuation.
Fig. 14: Full width at half magnitude as function of the number of fingers and the
finger overlap. Green indicates a better FWHM. Note that W and N are the other
way round than in Fig. 13.
35
Generally it thus can be seen that a longer finger overlap and an increasing number
of finger pairs improve the delay line transmission. Low N generates more bulk waves,
many papers (e.g. [7], [12]) indicate that for N ≤ 5 the generation of bulk waves increases drastically. The SAW has a width of W when generated, but on propagation it
diverges. Thus the wider the initial wave the greater the percentage of the SAW that
can be detected with the output IDT.
When evaluating the delay lines an interesting observation was made. The first harmonic has the highest excitation. But in contrast to prior assumptions, that every odd
harmonic, i.e. every second harmonic, is excited, only every fourth harmonic (1 st, 5th,
9th, etc.) can be clearly observed.
Fig. 15: Delay line characteristics of a 86MHz center frequency sample. All clearly
identifiable harmonics are labeled. LNOB 71
The reason for this phenomenon is the metallization ratio, the ratio of b and the distance between the centers of two neighboring fingers, in our case η = 0.5. Odd
numbered harmonics are excited with different amplitudes. A relation between the excited amplitudes of the harmonics and η can be derived theoretically [11]. The result is
 
P n 2 cos 2
A 2n1 ∝
 
 ]

 1−  −1
2
 [

K 1−cos
 1−
2
,
(6,1)
36
where A● is the amplitude of the ●th harmonic, Pn is the nth Legendre polynomial and K
is the complete elliptic integral of the first kind. A different derivation with a slightly different result can be found in [6], which does not change the conclusions, though. In
Fig. 16 and 17 the amplitude excitation (6,1) is plotted for various harmonics. The plots
that for η = 0.5, as in our samples, only every fourth harmonic is excited. This result is
more accurate for larger structures b ≥ 5µm. The delay line characteristics in Fig. 15 are
from a sample with b = 10µm. Structures of smaller sizes also show small 3rd, 7th, 11th,
etc. harmonics. This might result from the undercut of the photoresist. On the upper
surface of the resist, the uncovered fingers have the width of the structure of the mask,
but they widen towards the substrate. Sputtering and evaporation work with some
kind of vapor so that the Al fingers get wider than intended. But wider fingers with constant 0 result in η > 0.5. The widening of the fingers is of an absolute value, therefore
for dividing is relatively smaller than for a 3µm finger. Hence, η is less affected in larger
structures.
Fig. 17: Amplitudes of the harmonics up to 21 as
function of the metallization ratio. Only those
excited at a metallization ratio of 0.5 are drawn.
Fig. 16: Amplitudes of the harmonics up to 19 as
function of the metallization ratio. Only those
not excited at a metallization ratio of 0.5 are
drawn.
Due to time constraints, only a short comparison between the different substrates
could be done. Samples with quartz substrates have a much larger attenuation of
about 40dB as samples with the same IDTs on LiNbO3 substrates (Tab. 5 and 6). The dif2
2
ference in the coupling factor of both materials (K LiNbO = 4.6% and K quartz = 0.11% [9])
3
would only explain a difference of a factor 42 or 16dB.
37
6.2. Magneto-transmission
We now turn to the discussion of the SAW transmission on magnetic fields. The group
of Feng et al. [13] observed attenuation of SAWs by a nickel hall bar in a magnetic field.
They found that the attenuation curve as a function of the magnetic field is strictly similar to the low-frequency susceptibility curves.
A delay line with a nickel hall bar within the propagation path is exposed to an inplane magnetic field. The SAW is excited ±2MHz around f 0 . For each magnetic field
point, S21 magnitude and phase as well as the longitudinal voltage Ul and the transverse voltage Ut are recorded by means of the VNA. The mean value of all measuring
point within this frequency range for each magnitude and orientation set are depicted
in Fig. 12, 15 and the following graphs. Two types of measurement were done, one
where the magnitude of the external field is swept from 50mT to -50mT and backwards. The resulting relative change of S21 magnitude for such a sweep is shown in
 applied at a constant orientation of 45°, where the 0° direction is parallel
Fig. 18 for H
to the direction of the SAW propagation. Such field sweeps were performed for several
magnetic field orientations in the sample plane and are named constant orientation in
the following.
In the second measurement type the orientation of the external magnetic field is rotated stepwise from 0° to 360°, while the magnitude is kept constant for a single 360°
rotation. Again this is done for several magnitudes and these sweeps are named constant magnitude in the following. For the magneto-transport the longitudinal voltage
Ul and the transverse voltage Ut with a constant current Ihb = 2mA across the hall bar
are measured. The hall bars had a length of 400µm and a width of 200µm.
6.2.1. Constant orientation
In Fig. 18 the SAW transmission magnitude is illustrated, measured simultaneously
with the resistivities Ul/Ihb and Ut/Ihb. the different colors correspond to the up and
down sweep. In order to illustrate the hysteresis [15], the difference of up and down
sweep is taken, normalized and color-coded as Fig. 19 exemplifies. Here, hardly any
38
changes occur at high magnetic fields. But for small fields abrupt switches of the SAW
transmission can be clearly observed, with the magnetic field magnitude at which the
switches occur depending on the sweep direction. The same effect is also observed for
the SAW phase. Hence, the external field effects the SAW in its propagation.
Fig. 18: Up and down sweep of relative SAW magnitude as a function of the
magnitude of the magnetic field. S21 measurement on LNOB 67 at 172MHz
and H orientation 45°.
Fig. 19: Illustration of color code. Up sweep minus down sweep, S21 magnitude measurement on LNOB 67 at 172MHz and H orientation 45°.
The hysteresis in Fig. 18 looks similar to the hysteresis curves of the resistivity measurements, which are directly connected to the switching of the magnetization in the
39
hall bar [15], also called magneto-transport hysteresis (MTH). In Fig. 20 the dependencies of the magneto-transport voltages Ul and Ut as well as the SAW magnitude and
 are illustrated. It can be observed that
phase on the magnitude and orientation of H
the switching occurs simultaneously at the same points and with the same dynamics
for all four properties. So, it can be reasoned that the SAW interacts with the magnetization in the ferromagnetic hall bar.
We now discuss the mechanism for producing the interaction between SAW and
magnetizationd. Because the SAW travels either on the free LiNbO3 surface or along
the LiNbO3-nickel interface, see Fig. 21, it induces 0 -periodic strains within the nickel
hall bar when passing it. Due to magnetostriction, these strains induce oscillations in
the magnetization. Around the coercivity Hc the domains in the ferromagnetic film
switch and the excitement of oscillations is corrupted. Once again due to the magnetostrictive effect, the switching magnetic domains cause strains in the nickel and there-
Fig. 20: Effect of an external magnetic field on DC magneto-transport and SAW magneto-transmission at
f0 = 172MHz on sample LNOB 67: (a) Ul , (b) Ut , (c) S21 magnitude and (d) S21 phase.
40
fore on the LiNbO3 surface, disturbing the mechanical SAW. The Young's modulus E is
the isotropic equivalent to the elastic modulus tensor iklm, so (2,1) for nickel looks like
=E⋅u ,
(6,2)
taking into account that nickel is not a piezoelectric material. Assuming that the 50nm
thick nickel film is clamped to the LiNbO3 surface, internal stresses in the nickel film do
not result in strains. So E must change when internal stresses arise, Fig. 21 illustrates
this effect. When such stresses are caused by magnetostriction this is often called the
ΔE-effect [5]. Nickel's Young's modulus E rises from 2.08×1011N/m2 to 2.23×1011N/m2
Fig. 21: SAW travels at LiNbO3-nickel interface. Arrows represent the magnetic dipoles. (a) No net
magnetization, nickel is unstressed. (b) Saturation magnetization, nickel is stressed but not strained.
[5] or by 6.7% when a demagnetized film is changed to saturation magnetization.
Therefore the nickel is stiffer when fully magnetized, absorbing less energy from the
SAW and attenuating it less than in the demagnetized state at Hc (Fig. 21). Since E is
connected to the wave velocity by c= E /  [5], the SAW is faster when the nickel film
is in the saturation state, cmagnet. = 1582.2m/s to cdemagnet. = 1528.1m/s. The hall bar on
the LNOB 67 sample has a width of 600µm, so the phase velocity difference leads to a
Fig. 22: Up sweep of phase as a function of the magnitude of the
magnetic field. S21 measurement on LNOB 67 at 172MHz and H
orientation 45°, same measurement as shown in Fig. 18.
41
propagation time difference of 1.4×10-8s and a theoretical phase shift of just under
3.8°, with 0 = 20µm. The experimentally observed phase shifts between a high magnetic field and the coercive field Hc are around 2°, Fig. 22. The difference results from
the fact that at Hc there is zero net magnetization, but the magnetic dipoles are not
totally unaligned.
Another interaction could be due to the fact that the strain in the hall bar strains the
LiNbO3 on its surface. Taking the largest strain component of nickel for magnetostrictive effects, u11 = 9.06×10-5 [13] the change in the phase velocity cSAW is less that 0.2m/s.
Because cSAW has different values under the hall bar and with a free surface, reflexions
occur. But the reflection coefficient
R=
c magnet. −c demagnet.
c magnet. c demagnet.
turns out to be only about 5×10-5. Hence, only about 1% of the attenuation attained
experimentally can be attributed to such reflections.
6.2.2. Constant magnitude
The rotation of the magnetic field strength with constant magnitude shows a surprising
effect. The previously discussed experiments with changing magnetic field magnitude
Fig. 23: Effect of magnetic field orientation on DC magneto-transport and SAW magneto-transmission at
f0 = 172MHz and µ0H = 5mT on sample LNOB 67. (a) magneto-transport (b): magneto-transmission..
42
yield in analogous changes in the DC magneto-transport and the SAW magneto-transmission. On the contrary, changing the orientation of the external magnetic field, does
not result in analogous changes in the magneto-transport and the magneto-transmission. Instead the SAW transmission shows a four-fold periodicity in its magnitude and
phase, while the magneto-transport shows a two-fold symmetry, see Fig. 23 and 24.
Reason might lie in the high-frequency susceptibility to which the energy dissipation
of the SAW is proportional to [13]. Current research at the institute is aimed to evaluate this part.
Fig. 24: Effect of magnetic field orientation on DC magneto-transport and SAW magneto-transmission at
f0 = 172MHz and µ0H from 5mT to 50mT on sample LNOB 67. U l , Ut , S21 magnitude and phase are color
coded as in Fig. 23.
43
6.3. Conclusions
Our investigation of surface acoustic waves lead to the following results. Good SAW
transmission means little attenuation and small FWHM. These properties mainly depend on the number of finger pairs and the finger overlap of the IDTs. For the tested
structures it was shown that an IDT with more and longer fingers give better delay lines
(Fig. 13 and 14). For further research, applicable structures must have at least 10 finger
pairs and a finger overlap of 250µm. Samples with quartz substrates have an attenuation that is about 40dB larger as samples with the same IDT structure on LiNbO3 substrates (Tab. 5 and 6).
SAW attenuation peaks were observed that depend on the direction of the magnetic
field sweep. DC magneto-transport was measured simultaneously as a function of the
magnetic field. The graphs bore a striking resemblance to the SAW magneto-transmission graphs (Fig. 20). Therewith the results of Feng et al. [13] have been reproduced.
Analogous measurements with rotating a magnetic field with constant magnitude were
performed. In contrast to the measurements with a constant magnetic field orientation, the SAW magneto-transmission and DC magneto-transport graphs now show discrepancies (Fig. 23 and 24). The SAW magneto-transmission graphs feature a four-fold
symmetry, compared to the two-fold symmetry of the DC magneto-transport graphs.
This behavior can not be explained by a direct magnetostrictive effect and needs further investigation.
44
7. Outlook
Science cannot solve the ultimate mystery of nature. And
that is because, in the last analysis, we ourselves are a
part of the mystery that we are trying to solve.
Max Planck
In this thesis, two mayor topics have been studied. First, the requirements of IDTs for
good SAW transmission, as well as the preparation of such micro-structures. Second,
the SAW attenuation of samples with a ferromagnetic thin films deposited in the SAW
propagation path in an magnetic field.
Our research on the interdigital transducer structures has shown how to improve surface acoustic waves on LiNbO3 substrates. Preparation procedures have been optimized, such that IDTs with structure sizes down to 3µm can be reproducibly fabricated
within one day. The preparation of samples with reproducible SAW delay lines is the
basis for any research involving SAWs. The hitherto research on the structure design is
limited to the number of finger pairs, the finger overlap and the differences between
single and dual IDT finger pairs as well as differences resulting from the used substrates, lithium niobate and quartz. So it still remains to be explored how the SAW
delay lines are affected by using two different IDTs as input and output IDT. Also the
study of lithium tantalate (LiTaO3), which is isotypic to LiNbO3, would be of interest.
Due to time constraints, it was not possible to work on this topic within the scope of
this thesis. But with the obtained results a good picture of the optimal IDT design parameters can be drawn: N ≥ 10, b ≥ 250µm.
The magneto-transmission measurements have revealed interesting characteristics. It
is shown that a SAW interacts with the magnetization when propagating at the interface of a ferromagnetic thin film and a piezoelectric substrate. The origin for this interaction certainly is linked to magnetostrictive effects of the nickel hall bar. In this area
45
more measurements at different frequencies, either using samples with different center frequencies or at higher harmonics, would give more information on the interaction
between SAW and magnetization. In addition, the magnetic anisotropy should be
probed e.g. by ferromagnetic resonance to directly measure the high frequency susceptibility.
First measurements with a magnetic field of constant magnitude rotated in the
sample plane showed a different angular dependency for magneto-transmission and
magneto-transport, more precisely a qualitatively different periodicity. Until submission of this thesis we were not able to explain this difference, but high frequency susceptibility appears as a promising mechanism.
In conclusion, it was found that SAWs can be manipulated by an external magnetic
field. This might be technologically relevant when dealing with band pass filters. Here
the attenuation of a narrow frequency spectrum could be controlled.
46
8. List of samples
Here all samples that have been prepared with the mask aligner are listed. Listed are
the dimensions of the IDTs, the maximum magnitude and FWHM of the first harmonic.
If not stated otherwise the IDTs have single finger pairs and are made of aluminum. If a
sample has a hall bar it is stated in the miscellaneous column.
Sample
Nx bx W
Mag [dB]
FWHM [MHz]
LNOB 55
10 x 3 x 450
-18
11.5
-18.2
12.3
Misc
10 x 3 x 20
LNOB 56
10 x 3 x 450
10 x 3 x 20
LNOB 58
LNOB 70
15 x 3 x 250
no fingers
15 x 3 x 100
no fingers
10 x 3 x 450
no fingers
10 x 3 x 20
no fingers
Tab. 4: 3µm structures on LiNbO3
Sample
Nxbx W
QRZ 63
20 x 5 x 450
Mag [dB]
FWHM [MHz]
-51.7
6.0
-52.1
12.2
Misc
20 x 5 x 20
QRZ 64
20 x 5 x 450
20 x 5 x 20
QRZ 52
20 x 5 x 250
20 x 5 x 100
QRZ 67
20 x 5 x 250
90° rotated; Au fingers
90° rotated; Au fingers
-60.2
9.3
20 x 5 x 20
Au fingers
Au fingers
Tab. 5: 5µm structures on Quartz
47
Sample
Nx bx W
Mag [dB]
FWHM [MHz]
LNOB 51
10 x 5 x 250
-23.5
7.0
10 x 5 x 100
-35.5
12.3
10 x 5 x 450
-20.1
8.7
10 x 5 x 20
-47.1
27.0
20 x 5 x 250
-13.7
4.9
20 x 5 x 100
-25.1
4.0
20 x 5 x 450
-14.0
5.8
90° rotated
20 x 5 x 20
-46.8
15.9
90° rotated
10 x 5 x 450
-21.1
8.6
Ni hall bar
4 x 5 x 250
-42.4
96.5
4 x 5 x 100
-45.1
81.6
4 x 5 x 450
-44.5
92.0
4 x 5 x 20
-48.6
56.0
20 x 5 x 450
-10.3
4.9
20 x 5 x 20
-44.9
14.3
10 x 5 x 250
-46.9
9.1
dual IDTs
10 x 5 x 100
-40.8
5.0
dual IDTs
4 x 5 x 250
-41.8
46.1
dual IDTs
4 x 5 x 100
-50.2
36.9
dual IDTs
LNOB 52
LNOB 53
LNOB 54
LNOB 59
Misc
10 x 5 x 20
LNOB 65
LNOB 66
LNOB 67
LNOB 68
LNOB 69
LNOB 73
Ni hall bar
20 x 5 x 250
broken
20 x 5 x 100
broken
Tab. 6: 5µm structures on LiNbO3
Sample
Nxbx W
Mag [dB]
FWHM [MHz]
LNOB 63
10 x 10 x 450
-24.6
5.4
10 x 10 x 20
LNOB 71
Misc
no signal
10 x 10 x 450
-24.5
5.9
10 x 10 x 20
-50.2
36.9
Tab. 7: 10µm structures on LiNbO3
48
Acknowledgments
At this point I would like to express my thanks to the entire magnetism and spintronics
group for being such an open and helpful working group that I always enjoyed working
with.
In particular I would like to thank...
Prof. Dr. Rudolf Gross for giving me the opportunity to compose the first Bachelor's
thesis at the Walther-Meißner-Institute;
Dr. Sebastian T. B. Gönnenwein for his ideas, comments and the great help proofreading the thesis;
Mathias Weiler for introducing me to the topic, experimental facilities and his
LabVIEW programs and for supporting me the entire three month;
Dr. Peter Spiess, Deutsche Bahn AG, for his third party perspective;
My parents for their never-ending encouragement and ongoing support.
49
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