MgO MTJ sensors in Wheatstone bridge for
magnetometer devices
Cláudio Aquino dos Santos
Dissertação para obtenção do Grau de Mestre em
Mestrado Integrado em Engenharia Física Tecnológica
Júri
Presidente: Prof. Dr. João Carlos Carvalho de Sá Seixas
Orientador: Prof. Dr. Paulo Jorge Peixeiro de Freitas
Vogais: Prof. Dr. Susana Isabel Pinheiro Cardoso de Freitas
Outubro 2010
2
Abstract
For the past twenty years, a huge effort has been made to understand the magnetism in
ultrathin films. The motivation behind the continuous research in this area is tied to the
countless applications of magneto resistive sensors, such as, navigation systems, read heads
of hard disks drives, biomolecular detection, among others. For this purpose we rely on several
fabrication techniques, for instance, lithography, sputtering and ion beam systems. Until 1988,
the research on magnetic properties was restricted to the enhancement of magnetic moment
and perpendicular anisotropy. In 1991, the discovery of giant magnetoresistance effect changed
the focus of research to magneotransport phenomena. This tendency was accelerated by the
discovery of tunnel magnetoresistance, seven years later. A new field of science and
technology emerged: spin-electronics.
A sensor consists of two ferromagnetic layers which are separated by an insulator, nonmagnetic layer. In angle sensing elements one ferromagnetic layer is fixed in its magnetization
direction whereas the second one is free to follow any external in-plane field direction
The objective of this thesis is to fabricate a Wheatstone bridge though series of magnetic tunnel
junctions with a linear response sensitive to an external magnetic field.
Keywords: Wheatstone bridge, Magnetoresistive sensors, Linear response
Resumo
Nos últimos vinte anos, fez se um esforço enorme para perceber o magnetismo em
filmes ultrafinos. A motivação por detrás da pesquisa continua nesta área prende-se nas
inúmeras aplicações de sensores magneto resistivos, tais como, sistemas de navigação,
cabeças de leitura para discos rígidos, deteção biomolecular, entre outras. Para este propósito,
confiamos em diversas tecnicas de fabricação, como por exemplo, litografia, sputtering e
sistemas de ion beam. Até 1988, a pesquisa a nível de propriedades magnéticas estava restrita
à melhoria do momento magnético e à anisotropia perpendicular. Em 1991, a descoberta do
efeito de magneto resistência gigante mudou o foco de pesquisa para o fenomeno de
magnetotransporte. Esta tendência foi acelerada pela descoberta pela magnetoresistência de
efeito de túnel, sete anos depois. Uma nova área da ciência e tecnologia emerge: electrónica
de spins.
Um sensor consiste em duas camadas ferromagnéticas separadas por uma camada
não magnetica, um isolador. No caso dos elementos sensíveis ao ângulo, uma camada
ferromagnética está presa à sua direção de magnetização, enquanto que a segunda é livre de
seguir qualquer direção de um campo externo no plano.
O objectivo desta tese é de fabricar uma ponte de Wheatstone atravês de series de
junções de efeito de túnel com resposta linear sensível a um campo magnético externo.
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4
Índice
I)
Theoretical Basis........................................................................................................... 11
1.1)
Different Types of Magnetic Materials ...................................................................... 11
1.1.1)
Diamagnetic ..................................................................................................... 11
1.1.2)
Paramagnetic................................................................................................... 11
1.1.3)
Ferromagnetic.................................................................................................. 12
1.1.4)
Antiferromagnetic ............................................................................................. 12
1.1.5)
Ferrimagnetic ................................................................................................... 13
1.2)
Magnetism ............................................................................................................... 14
1.2.1)
Magnetocrystalline Anisotropy .......................................................................... 14
1.2.2)
Demagnetizing Field ........................................................................................ 14
1.2.3)
Exchange Interaction........................................................................................ 15
1.2.4)
Coupling Forces ............................................................................................... 16
1.2.4.1)
Néel Coupling........................................................................................... 16
1.2.4.2)
RKKY Coupling ........................................................................................ 16
1.2.5)
1.3)
Hysteresis loop ................................................................................................ 16
Magnetoresistance .................................................................................................. 17
1.3.1)
Anisotropic Magnetoresistance (AMR) .............................................................. 17
1.3.2)
Giant Magnetoresistance (GMR) ...................................................................... 18
1.3.3)
Tunneling Magnetoresistance (TMR) ................................................................ 20
Wheatstone Bridges formed by Series of MTJ’s.......................................................... 24
II)
2.1)
Wheatstone Bridge .................................................................................................. 24
2.2)
Wheatstone bridge assembly ................................................................................... 24
2.3)
Why series over single MTJ’s? ................................................................................. 26
III)
Experimental Equipment and Process ......................................................................... 29
3.1)
Equipment Used ...................................................................................................... 29
5
3.1.1)
3.1.1.1)
Nordiko 2000 ............................................................................................ 29
3.1.1.2)
Nordiko 7000 ............................................................................................ 30
3.1.1.3)
Ultra High Vacuum II (UHV2) .................................................................... 30
3.1.2)
Ion Beam Systems ........................................................................................... 31
3.1.2.1)
Nordiko 3000 ............................................................................................ 32
3.1.2.2)
Nordiko 3600 ............................................................................................ 33
3.1.3)
Pattern transfer system .................................................................................... 33
3.1.3.1)
Direct Write Lithography ........................................................................... 33
3.1.3.2)
Lift off ....................................................................................................... 34
3.1.4)
3.2)
Sputtering Systems .......................................................................................... 29
Measurement Equipment ................................................................................. 35
3.1.4.1)
Magnetic Thermal Annealing .................................................................... 35
3.1.4.2)
Manual Measurement Setup ..................................................................... 35
3.1.4.3)
Profilometer.............................................................................................. 36
3.1.4.4)
Ellipsometer ............................................................................................. 36
3.1.4.5)
Vibrating Sample Magnetometer ............................................................... 37
Microfabrication Process .......................................................................................... 38
IV) Experimental Data ......................................................................................................... 49
V)
Conclusion .................................................................................................................... 55
References ............................................................................................................................ 57
Appendix A: Run sheet ......................................................................................................... 59
Appendix B: Mask ................................................................................................................. 67
APPENDIX C: Fotos of the process ..................................................................................... 68
6
List of Figures
Fig 1.1 - Diamagnetism phenomenon ...................................................................................... 11
Fig 1.2 - Paramagnetism phenomenon ................................................................................... 11
Fig 1.3 - Ferromagnetic materials in the absence of magnetic field .......................................... 12
Fig 1.4 - Antiferromagnetism phenomenon ............................................................................. 12
Fig 1.5 - Ferrimagnetic phenomenon ...................................................................................... 13
Fig 1.6 - Magnetization and demagnetizating field of the same structure depending on the
orientation ....................................................................................................................... 14
Fig 1.7 - Identical structure formed by 1, 2 or 4 domains .......................................................... 15
Fig 1.9 - RKKY coupling in function of the barrier thickness ..................................................... 16
Fig 1.10 – Hysteresis loop of a ferromagnetic material............................................................. 17
Fig 1.10 - Transfer curve of a GMR multilayer measured at INESC-MN. The red arrows
represent the magnetization of different layers depending on the applied magnetic field .. 18
Fig 1.11 - Spin Valve structure. Blue arrows represent the magnetization of the layers ............ 19
Fig 1.12 - Resistor model ........................................................................................................ 20
Fig 1.13 - Magnetic Tunnel Junction structure. Blue arrows represent the magnetization ........ 21
Fig 1.14 - Tunneling magnetoresistance effect ........................................................................ 22
Fig 2.1 - Wheatstone bridge .................................................................................................... 24
Fig 2.2 - Magnetization of one row of series after the annealing represented by blue arrows ... 25
Fig 2.3 - Two legs of the Wheatstone bridge with opposite magnetizations .............................. 25
Fig 2.4 – Linear transfer curves ............................................................................................... 25
Fig 3.1 - Magnetron Sputtering deposition schematic .............................................................. 29
Fig 3.2 - Illustration of the Z configuration inside the IBD chamber ........................................... 31
Fig 3.3 - User interface and main chamber of Nordiko 3000 system........................................ 32
Fig 3.4 - loadlock and main chamber of Nordiko 3600 ............................................................. 33
Fig 3.5 - DWL system present at INESC-MN ........................................................................... 34
Fig 3.6 - Lift off process ........................................................................................................... 34
Fig 3.7 - Older annealing setup present in the characterization room ....................................... 35
Fig 3.8 - Profilometer present inside the clean room at INESC-MN. ......................................... 36
Fig 3.9 - Ellipsometer present inside the clean room at INESC-MN.......................................... 37
Fig 3.10 - VSM setup present in the characterization room at INESC-MN. ............................... 37
Fig 3.11 - Different layers forming the stack deposited in Nordiko 2000 with respective thickness
in
................................................................................................................................. 38
Fig 3.12 - Stack after deposition of TiW(N) .............................................................................. 39
Fig 3.13 - First layer exposed defining the bottom electrode .................................................... 40
Fig 3.14 - Sample ready to be exposed ................................................................................... 40
Fig 3.15 - Sample after exposure ............................................................................................ 41
Fig 3.14 - Sample after revelation............................................................................................ 41
Fig 3.15 - Sample after first ion milling and resist strip ............................................................. 41
7
Fig 3.16 - Picture of the 2nd layer only..................................................................................... 42
Fig 3.17 - Picture of the first two layers .................................................................................... 42
Fig 3.18 - Sample after 2nd exposure ....................................................................................... 42
Fig 3.19 - Layers to remove for the second etch ..................................................................... 43
Fig 3.20 - Etch with 60º and 30º ............................................................................................. 44
nd
Fig 3.21 - Sample after the 2
etch ......................................................................................... 44
Fig 3.22 - Zoom in of the sample after the 2nd etch ................................................................. 44
Fig 3.23 - Oxide deposition represented by the white blocks and the transparent layer. ........... 45
Fig 3.24 - Sample after lift off .................................................................................................. 45
rd
Fig 3.25 - Picture of the 3 layer............................................................................................. 46
Fig 3.26 - Picture of all layers .................................................................................................. 46
Fig 3.27 - Top view of the sample after 3rd exposure................................................................ 46
Fig 3.28 - Side view of the sample after 3rd exposure............................................................... 46
Fig 3.29 - Sample after the deposition of Al (green layer) ........................................................ 47
Fig 3.30 – Top view of the final structure ................................................................................. 47
Fig 3.31 - Side view of the final structure ................................................................................ 47
Fig 4.1 – Mask used during the process .................................................................................. 49
Fig 4.2 – Distribution of the two different pillar’s dimensions. In yellow 2
blue 2
by 30
by 20
, and in
.......................................................................................................... 49
Fig 4.3 – Transfer curves obtained by the different segments forming the series...................... 52
8
List of Tables
Table 3.1 - Sequence 39 (Xiohong) ......................................................................................... 38
Table 3.2 - Sequence 40 (Filipe) ............................................................................................. 39
Table 3.3 - Sequence 16: Soft Etch and TiW(N) deposition conditions .................................... 39
Table 3.4 – Parameters of Nordiko 3600 for the first ion milling ............................................... 41
Table 3.5 - Parameters used for the 2nd etch .......................................................................... 44
Table 3.6 - Oxide deposition parameters ................................................................................. 45
Table 3.7 – Parameters used for the 3 steps, soft etch, Al deposition, TiW(N) deposition......... 47
9
10
I)
Theoretical Basis
1.1)
1.1.1)
Diamagnetic
In the presence of an external magnetic field, the individual magnetic moments of the
material will present a magnetization opposite to the external magnetic field. This magnetization
is due to the fact that the magnetic permeability is less than 1, therefore having a magnetic
susceptibility being less than 0; the individual magnetic moments are repelled by the magnetic
field. In the absence of such magnetic field, individual magnetic moments of the atoms do not
present a magnetization. Thus, the total net magnetization is always null.
Fig 1.1 - Diamagnetism phenomenon
1.1.2)
Paramagnetic
In the absence of a magnetic field, the atoms of these materials present randomly
orientated magnetic moments, but the total magnetization of those materials is kept null. In
presence of an external magnetic field, all individual magnetic moments will align according to
the direction of the magnetic field, since the magnetic susceptibility of those materials is small
and positive.
Fig 1.2 - Paramagnetism phenomenon
11
1.1.3)
Ferromagnetic
This kind of material exhibits a large permanent magnetization, even in the absence of a
magnetic field. In the presence of an external magnetic field, individual magnetic moments tend
to align in a parallel way to the applied field. Typically, the magnetic susceptibility of such
materials is positive and very large.
Fig 1.3 - Ferromagnetic materials in the absence of magnetic field
1.1.4)
Antiferromagnetic
In the case of antiferromagnetic materials, individual magnetic moments of atoms
constituting each layer have alternate magnetizations in opposite directions. This antiparallel
alignement is tied to quantum mechanical exchange forces. Above a certain temperature,
depending on the material, antiferromagnetic materials become paramagnetic; this temperature
is called Néel temperature. In the absence of an external field, the net magnetization of this kind
of material is null. The magnetic susceptibility is small and positive
Fig 1.4 - Antiferromagnetism phenomenon
12
1.1.5)
Ferrimagnetic
Like
antiferromagnetic
materials,
ferromagnetic
materials
exhibit
alternate
magnetization on each atomic layer; however the opposing moments are unequal inducing a
net magnetization in the material. This is due to the fact that those materials are made by
different types of magnetic ions.
Fig 1.5 - Ferrimagnetic phenomenon
13
1.2)
Magnetism
1.2.1)
Magnetocrystalline Anisotropy
Since all materials used in this work are polycrystalline, it is important to understand
how those materials work. One of the most important properties of this kind of material is the
magnetic anisotropy due to their different magnetic behavior along different crystallographic
directions.
Polycrystalline materials possess two different directions of magnetization, the easy axis
and the hard axis. The easy axis correspond to the direction in which spin are more easily align,
contrary the perpendicular direction defined as the hard axis, being the direction in which spins
have more difficulties to align. Thus, when a field is applied to a material in the easy direction
the dipoles will more easily align as this corresponds to their natural alignment, meaning that
there will always be the need to apply stronger fields in the hard axis direction to reach the
saturation.
1.2.2)
Demagnetizing Field
Another important property is the shape anisotropy. Whenever a structure is
magnetized, there is always the creation of a demagnetizing field. In order to understand better
this demagnetizing field let’s have a look at the following picture.
Fig 1.6 - Magnetization and demagnetizating field of the same structure depending on the orientation
We can see the presence of charges in the surface of the structure due to the
magnetization of the structure, but also the presence of another magnetic field with the opposite
direction called demagnetizing field. This field depends on the shape of the structure and has
magnetostatic energy associated with it that increases as the distance between poles
decreases, as we can see from the picture with two identical structures rotated 90º from one
another.
14
However in order to minimize the total magnetic energy, the magnetostatic energy must
also be minimized. To do so, the ferromagnetic material divides itself into magnetic domains to
reduce the demagnetizing field.
Fig 1.7 - Identical structure formed by 1, 2 or 4 domains
1.2.3)
Exchange Interaction
This interaction is of extreme importance for every magneto resistive sensors, since this
interaction is in charge of defining the magnetization of the pinned layer acting like a reference
layer. This exchange interaction is independent of the direction of the total magnetic moment of
the sample. In fact, the magnetic moments interact strongly between each other even in the
absence of an applied field. Therefore this interaction forces the magnetic dipole of an atom to
align with the magnetic dipole of its neighbor, having a smoothing effect on the dipoles
orientation. The dipoles are aligned in a parallel or antiparallel way depending on the material
being ferromagnetic or antiferromagnetic. Finally it is important to notice that the exchange
interaction will only dominate in short ranges.
Fig 1.8 - Exchange interaction between the antiferromagnetic layer pinning the ferromagnetic
one
15
1.2.4)
Coupling Forces
1.2.4.1) Néel Coupling
Néel coupling is present in almost every structures composed by two ferromagnetic
layers and a non magnetic layer, for instance, spin valves or magnetic tunnel junctions. This
effect is caused by the magnetostatic interactions between free poles and both
ferromagnetic/non magnetic barrier interface, due to the non equal thickness of the layers along
the substrate, shifting the magnetic response of the free layer and having repercussion in the
magnetoresistance.
1.2.4.2) RKKY Coupling
The Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, is an exchange interaction
between both ferromagnetic layers. This coupling has an oscillatory behavior since the coupling
oscillates from ferromagnetic to antiferromagnetic depending on the distance between both
ferromagnetic layers, in other words, depending on the thickness of the non metallic barrier.
This phenomenon is illustrated in the following picture.
Fig 1.9 - RKKY coupling in function of the barrier thickness
1.2.5)
Hysteresis loop
A hysteresis loop occurs for ferromagnetic materials when an external applied magnetic field is
varied and the direction is reversed. When the external magnetic field is increased, the domain
walls of the materials begin to align with the magnetization direction of the magnetic field until
each domain is completely align with the field, this corresponds to the saturation magnetization
(Ms) of the material. When the field is decreased until zero, some domains will have remanent
magnetization (Mr), others will rotate back to their easy direction. Finally, the magnetization can
be brought back to zero when a strong magnetic field is applied in the opposite direction of the
material’s magnetization, thus demagnetizing the material, this field is called coercive field,
represented by Hc.
16
Fig 1.10 – Hysteresis loop of a ferromagnetic material
1.3)
Magnetoresistance
1.3.1)
Anisotropic Magnetoresistance (AMR)
The anisotropic magnetorestiance is an effect occurring when we observe a change in the
electrical resistance of a material depending on the direction of an applied magnetic field. The
AMR is maximum when the direction of the electrical current is parallel to the direction of the
magnetic field, and minimum when the electrical current is perpendicular to the direction of the
magnetic field.
In order to measure the AMR we need to obtain the values of the resistance in both cases: in
parallel direction and in perpendicular direction. For this purpose we need to apply a strong
magnetic field that will saturate the magnetization in both directions allowing the measurement
of the resistivity of the material, obtaining therefore the parallel and the perpendicular resistivity.
The anisotropic magnetoresistance is given by the following expression:
Where
and
are respectively the resistance in parallel and antiparallel direction.
This effect is tied to the anisotropic scattering of electrons, caused by spin orbit coupling.
Typically, AMR vary between 2 and 6%.
17
1.3.2)
Giant Magnetoresistance (GMR)
The giant magnetoresistance [3] was first discovered by two physicists, Albert Fert [4]
and Peter Grunberg [5] in 1988. This discovery was so important, that they were awarded the
Nobel Prize in physics in 2007. The studied structure was a thin film composed of two
ferromagnetic materials and a non magnetic conductive material: Fe/Cr/Fe. The effect observed
was a change in the electrical resistance depending on the magnetization direction of the
ferromagnetic layers, whether there alignment is parallel or antiparallel [6] [7]. In the absence of
an external magnetic field, the magnetization directions of successive ferromagnetic layers tend
to be in opposite direction. This can be explained by the presence of an interlayer coupling
exchange favoring the antiparallel alignment of consecutive ferromagnetic layers, increasing the
electrical resistance of the structure. In the presence of an external magnetic field, layers having
magnetization in the opposite direction of the field will rotate until being parallel with the field. In
this case, all layers will have the same magnetization direction, causing a decrease of the
electrical resistance.
Fig 1.10 - Transfer curve of a GMR multilayer measured at INESC-MN. The red arrows represent the
magnetization of different layers depending on the applied magnetic field
One well known application of the GMR is the Spin Valve structure introduced in 1991.
This structure consists in four layers, two ferromagnetic layers separated by a layer of a non
magnetic conductive material, and an antiferromagnetic layer on the bottom. Comparing to the
previous structure, we notice the introduction of an antiferromagnetic layer. This layer will allow
the pinning of the magnetization of the lower ferromagnetic layer thanks to the exchange
coupling. With the presence of an external magnetic field, the other ferromagnetic layer will be
free to rotate, and since the exchange coupling forces are strong enough, they will not be
affected by the applied field, allowing having a ferromagnetic layer with a pinned magnetization,
and another one having a magnetization depending on the applied field, called free layer. With
this kind of configuration the current flow will be parallel to the plane of the layers, as we can
see from the following figure.
18
Fig 1.11 - Spin Valve structure. Blue arrows represent the magnetization of the layers
With this structure we can define two states: the lower resistance state and the higher
resistance state. As we saw earlier, the lower resistance state is achieved when both
ferromagnetic layers have parallel magnetization, in opposition to the higher resistance state
achieved when those magnetizations are antiparallel. In the Spin Valve case, the
magnetoresistance is given by:
Typically, values of GMR vary between 4% and 10%.
To have a better understanding of this effect, we can approach this structure to the
resistor model as we can see from the following picture. The ferromagnetic layers are
represented by the resistances.
19
Fig 1.12 - Resistor model
In the case of the antiparallel configuration, both orientations of the spins scatter at the
same rate in the two ferromagnetic layers. One of the orientations will have a higher scattering
rate in one of the ferromagnetic/nonmagnetic interfaces and a smaller one in the other and vice
versa for the other orientation. In the resistance model this is illustrated by smaller and bigger
resistances for the same spin, depending on the ferromagnetic layer. Ultimately the current
flowing through the spin valve will have the same contribution from electrons with spin up and
spin down.
With the parallel configuration, both ferromagnetic layers have the same orientation of
the magnetization. Therefore the scattering of the electrons of a certain spin orientation will
increase whereas for the other orientation it will strongly decrease. Therefore for the spins
which are strongly scattered the contribution of these electrons to the current will be small and
for the spin’s orientation which is weakly scattered the electron contribution to the current flow
will be large. As we can see in the resistance model, for one of the spin orientations, both the
resistances are now small and, for the other, both the resistances are now big which translates
in a larger current flow of one of the spin orientations.
1.3.3)
Tunneling Magnetoresistance (TMR)
The tunneling effect takes place in a structure very similar to the previous one, in this
case those structures are called magnetic tunnel junctions. Both are composed by two
ferromagnetic layers separated by a non magnetic layer, but contrary to spin valves, the non
conductive layer is an insulator and the current flow is perpendicular to the plane of the layers.
20
The conduction between both ferromagnetic layers though the insulator material can only be
possible if the insulator layer is thin enough to allow the tunneling effect to take place.
Fig 1.13 - Magnetic Tunnel Junction structure. Blue arrows represent the magnetization
The pinning of the bottom ferromagnetic layer is also achieved by the introduction of an
antiferromagnetic layer, while the other ferromagnetic layer is free to rotate depending on the
external applied magnetic field as we saw earlier in the case of spin valves. In the same way,
when both ferromagnetic layers have parallel magnetization, the MTJ’s resistance is low, but
when their magnetization is antiparallel, the MTJ’s resistance is high. There are several
mechanisms to set the magnetic layer. During our work we have used a synthetic
antiferromagnetic (SAF) structure to set the ferromagnetic layer into its pinned state since it
provides a stronger pinning than having just an antiferromagnetic material.
The discovery of the TMR dates back to 1975, when Julière [8] proposed the first model
to explain the tunneling magnetoresistance. His structure was composed by two ferromagnetic
layers, iron and cobalt, separated by a layer of germanium acting as the barrier. He assumed
that the electron’s spin is conserved during the tunneling though the insulator layer. Based on
this assumption, each electron leaving a ferromagnetic layer and going towards the other one,
will fill the states corresponding to their spin in the other ferromagnetic layer, i.e., spin up
electrons will fill spin up states and spin down electrons will fill spin down states, so the
tunneling of spin up and spin down electrons are two independent processes, so the
conductance occurs in two independent spin channels. Therefore, if both ferromagnetic layers
have parallel magnetizations, the majority states tunnel to fill the majority states and minority
spins tunnel to fill minority states. However, if both ferromagnetic layers have antiparallel
magnetizations, we have the majority spins tunneling to fill minority states and vice versa.
21
Fig 1.14 - Tunneling magnetoresistance effect
The tunneling magnetoresistance is given by the following formula:
Where
stands for the maximum resistance, i.e., the resistance measured when the
magnetization between ferromagnetic layers is antiparallel, and
for the minimum
resistance measured when the magnetization is parallel. Typically, TMR ratios can go up to
70% at room temperature when using AlOx barriers and 604% for MgO barriers [9].
22
23
II)
Wheatstone Bridges formed by Series of MTJ’s
2.1)
Wheatstone Bridge
The Wheatstone bridge is a measuring device first invented by Samuel Hunter Christie in
1833, improved and popularized by Sir Charles Wheatstone in 1843. A Wheatstone bridge is
composed by two voltage dividers, both connected to the same input, as shown in the picture.
The output of the circuit is taken from the voltage divider’s output.
Fig 2.1 - Wheatstone bridge
Using such Wheatstone bridge design, each resistance will correspond to an array of MTJ’s,
increasing the sensitivity of TMR sensor when compared to a magnetosensitive sensor
consisting in only one single patterned MTJ element.
2.2)
Wheatstone bridge assembly
For practical applications, the sensor element will be in a Wheatstone bridge arrangement in
order to obtain an offset-free signal. In this case, we need at least two opposite directions of
reference layer for the two half bridges, in order to obtain a signal from the bridge in a
homogeneous field. In other words, for the same applied magnetic field, two sensors will be in
the minimum resistance state and the other two in the maximum resistance state,
corresponding respectively to the parallel state and to the antiparallel state. The main
fabrication problem is the setting of different pinning directions on a local scale. Currently
various methods are used to create different pinning directions: i) manifold deposition in an
applied magnetic field with intervening lithography, ii) local heating in an applied magnetic field,
iii) inhomogeneous applied field during deposition. All of these techniques are rather complex
and time consuming. As an alternative approach, we need to anneal the sample to obtain the
same magnetization direction for all series in the sample, as we can see from the following
figure.
24
Fig 2.2 - Magnetization of one row of series after the annealing represented by blue arrows
The next step is to cut the sample between the second and the third series, with a dicing saw, in
order to obtain half bridges, and place the second half upside down under the other one. This
step is crucial for the obtention of a Wheatstone bridge, defining two opposite magnetization
directions.
Fig 2.3 - Two legs of the Wheatstone bridge with opposite magnetizations
The last step is to perform is to glue series to a chip carrier and connect the series’ pads to the
pins of the chip carrier, this last procedure is called wire bonding. This way we can connect both
half bridges and obtain Wheatstone bridge. This step is not as easy as it sounds, as the
wirebonding can destroy sensors, leading to the loss of the signal.
Regarding expected results we should measure transfer curves with the transport measurement
setup, later explained in chapter 3.
Fig 2.4 – Linear transfer curves
25
The first transfer curve is the kind of transfer curve obtained for the top two series due to the
fact that they have the same magnetization direction, in this case for a negative magnetic field,
the resistance state is low, contrary to positive magnetic field giving a high resistance state. The
exact opposite is observed for the bottom serie, since they have opposite magnetization
directions.
2.3)
Why series over single MTJ’s?
The minimum detectable field of magnetoresistive sensors is limited by their intrinsic noise.
Magnetization fluctuations are one of the crucial noise sources and are related to the
magnetization alignment at the antiferromagnetic-ferromagnetic interface. This minimum
detectable field is called detectivity of a sensor, which is given by:
Where
is the output noise of the sensor and
represents the sensor sensitivity.
To determine the detectivity of a sensor we need to apply an external magnetic field, and a bias
voltage V passing through each sensor, since in our case we use an array of MTJ’s.
The voltage noise of a single sensor is given by:
The first term is the white noise and the second one the flicker noise, where
charge,
the biasing current of the sensor,
through the sensor,
the resistance of the sensor,
the Boltzmann constant,
the area of the sensor, and
the temperature,
is the electron
the voltage passing
the Hooge like parameter,
the frequency.
If we now consider a device composed by an array of N single MTJ’s sensors, the noise
spectral density will be N times the noise spectral of a single sensor:
The respective device sensitivity is given by:
26
Therefore, the device detectivity is given by:
Assuming that
, the detectivity becomes:
As we can see from the last equation, the detectivity of an array of MTJ’s decreases as we
increase the number of sensors in the array. This is why we choose a series of MTJ’s over
single MTJ.
27
28
III)
Experimental Equipment and Process
3.1)
Equipment Used
3.1.1)
Sputtering Systems
Sputtering deposition systems [10] are used for the deposition of material on to the
substrate. This method is based on the transfer of momentum between the highly energetic ions
forming the plasma, and the atoms of the target. This interaction occurs in a vacuum chamber,
where we controllably introduce an inert gas and apply a negative voltage on the target. This
negative voltage will accelerate ions present in the gas towards the target, causing the
ionization of neutral atoms before reaching the target, initiating a cascade process inducing the
ignition of the plasma. Permanent magnets present in the magnetron will confine electrons
generated by the cascade process in long paths before reaching the target in order to collide
with atom’s gas, favoring plasma ignition even with lower concentration atoms.
Fig 3.1 - Magnetron Sputtering deposition schematic
The whole process described above is only valid for conductive targets, called DC
sputtering. In the case of non conductive targets, if we use the same system, we would have an
accumulation of positive charges in the target. Using a RF voltage this problem would be
overcome.
3.1.1.1) Nordiko 2000
This machine is a magnetron sputtering system installed in the grey area of the clean
room. This equipment can support up to six different targets, all water cooled to avoid over
heating. The samples are introduced in the loadlock, which is isolated from the deposition
chamber by a hydraulic hatch preventing drastic changes of pressure in the main chamber. A
robot arm will be in charge of transportation between these two chambers. Once the sample is
loaded to the deposition chamber, a permanent magnet array will apply 30 Oe during the
deposition, defining the easy axis. Usually samples are processed with pressures near
Torr.
29
3.1.1.2) Nordiko 7000
The Nordiko 7000 [11] is an automated system installed in the grey area of the clean room,
having the user’s interface inside the clean room. This machine is composed by four process
modules, a dealer and a loadlock. The dealer allows the transportation of the wafers between
the loadlock and the desired process modules. The function of each module is the following:

Module 1: flash annealing, which wasn’t used in this project.

Module 2: Soft etch. This etch is performed before the both deposition due to the
natural oxidation of materials and ensuring a better contact between the stack and the
material being deposited

Module 3: Deposition of TiW(N). This layer prevents the sample from being damaged
(physically and chemically), and since this material is very dark, it will work as an antireflective layer during lithography.

Module 4: Aluminum deposition. This material is used for the top contacts.
In this thesis we only used modules 2, 3 and 4.
3.1.1.3) Ultra High Vacuum II (UHV2)
The UHVII [11] is a manual sputtering deposition system, used to deposit
. The
role of this material is to prevent current flows between the bottom and the top electrode,
ensuring that any current flow will pass through the barrier, isolating both electrodes from one
another. This system is only composed by one chamber, meaning that every time a sample has
to be introduced or removed, the chamber needs to be vented. In order to reach pressures near
Torr, at least 10 hours are necessary to pump the chamber and start the deposition. Argon
gas will ignite the plasma, powered by a RF source. The deposition rate can change up to 10%
between the center and the edge to the table.
30
3.1.2)
Ion Beam Systems
Ion beam deposition systems [13] are used for the deposition of thin films and also for
ion milling in a non selective way. IBD systems are composed by two ion beam guns, one used
for deposition directly aiming targets, the deposition gun and the other one used for ion milling
directly aiming the substrate, the assist gun. Besides those guns we also have a substrate table
and a target holder assembled in a Z configuration as we can see from this figure:
Fig 3.2 - Illustration of the Z configuration inside the IBD chamber
The main difference between sputtering systems and IBD is the fact that the plasma (Ar
or Xe) is not created inside the chamber; in fact, it is created inside the gun through a RF
antenna. Two electrodes grids are located inside the gun in order to confine ions in to a beam
configuration. The huge advantage offered by IBD over sputtering systems is the possibility to
achieve lower pressures, typically
against
.
The deposition process is the same as we saw earlier in sputtering systems, based on
the momentum transfer between ions and the atoms of the target, with the only difference that
in this case, ions are accelerated outside the chamber. Regarding the milling process, the
principle is the same, but in this case the ion beam is directed towards the substrate removing
material from the sample and not the target. Due to the high power used, considerable heating
occurs, so, in order to prevent permanent damages, machines are equipped with a water
cooling system. The system is also equipped with two neutralizers, whose role is to avoid the
accumulation of charges in case of using insulating materials, otherwise the accumulation of
charges would be such that the accumulated ions would start repelling the incoming ions
blocking them from the target or the substrate.
The target holder can have up to six different targets thanks to his hexagonal shape, having
only one target facing the deposition gun at a time. During deposition a shield is placed in order
to prevent contamination by material of other targets.
31
The substrate table is a rotating table allowing the substrate to either be facing the target or the
assist gun but also to change the angle between the sample and the target (or gun). The table
is equipped with magnets providing a magnetic field of 40 Oe defining the easy axis of the
sample and rotating itself with the intention of having a better uniformity not only in the case of
deposition but also for ion milling. In order to protect the sample while all parameters are
stabilizing, a shutter is located in front of the sample openning when process conditions are
reached.
The clean room is equipped with two IBD systems, Nordiko 3000 and Nordiko 3600. In this work
we only used Nordiko 3000.
3.1.2.1) Nordiko 3000
Nordiko 3000 [10] [11] [12] is an automated IBD system installed inside the clean room.
This machine is composed by a loadlock, a chamber and a robotic arm is charge of the
transportation between one another. As mentioned earlier, the chamber is equipped with two
ion beam guns, a target holder, a substrate table and two neutralizers. The magnetic field in
charge of defining the easy axis is achieved by a permanent magnet array placed on the
substrate table, creating a magnetic field of 40Oe. Usually the the working pressure inside the
chamber is in the order of
Torr. This equipment was only used for ion milling.
Fig 3.3 - User interface and main chamber of Nordiko 3000 system
32
3.1.2.2) Nordiko 3600
This machine is composed by the same component of the previous one. The main
differences between those equipments lies in the size of the chamber, this one is considerably
larger, and also in the size of the wafer supported, in this case we are able to process wafers up
to 8 inches. This system was only used for ion milling.
Fig 3.4 - loadlock and main chamber of Nordiko 3600
3.1.3)
Pattern transfer system
One of the first tasks for the elaboration of a process starts by drawing desired patterns
in AutoCAD. A pattern transfer system will be in charge of transferring the AutoCAD patterns on
to the photo resist. For this purpose, INESC-MN has two pattern transfers devices, a laser
lithography equipment having a resolution in the
range, and ebeam lithography equipment
having a resolution in the sub micron range. In this work we only used the laser lithography
equipment, due to the fact that the device processed is in the order of microns
3.1.3.1) Direct Write Lithography
The DWL [10] is equipped with a 442nm wavelength HeCd laser, having a resolution up
to 1
. The mask that we want to expose is transferred to the laser hard drive in stripes of 200
microns, due to the fact that the laser can only expose one stripe at a time. The exposure is
done by exposing the photo resist to the laser light, since the photo resist used is positive, once
developed it will disappear remaining only the photo resist that wasn’t exposed, contrary to the
negative one which would allow to remove the photo resist that wasn’t exposed to the light.
33
Fig 3.5 - DWL system present at INESC-MN
3.1.3.2) Lift off
This process consists in depositing material on the top of previously exposed photo
resist. The sample is then introduced in a crystallizer in the presence of micro strip. This
substance degrades photo resist, removing it and consequently removing the material on top of
it. The degrading action of the micro strip towards photo resist can be enhanced by heating the
micro strip; typically the hot bath is near 60ºC, and also by ultrasounds. The following image
illustrates the lift off process
Fig 3.6 - Lift off process
34
3.1.4)
Measurement Equipment
3.1.4.1) Magnetic Thermal Annealing
An annealing setup is needed to optimize material properties. At INESC-MN we can find
two systems, an older and a newer setup. Since only one of them was used for this work, we
will focus on the older one, denoting the fact that the main difference between them is the
possibility to apply the magnetic field during the heating with the older setup instead of only
being able to apply it during the cooling for the newer one. This system is composed by a
copper block, where the samples are placed, and to improve thermal conductivity grease is
used. The heat source is a halogen lamp (100W, 12V) located inside the copper block. To
achieve vacuum during the annealing process, a removable glass chamber is placed, involving
the sample, the copper block and the lamp, and with the help of a mechanic and a turbo pump
we are able to reach approximately
Torr. Those components are located between two
large electromagnetic coils being able to create a magnetic field up to 5kOe.
Fig 3.7 - Older annealing setup present in the characterization room
3.1.4.2) Manual Measurement Setup
After the annealing of the sample is done, we can now use this system to obtain transfer
curves of the device, in other words, to see how the electrical resistance varies in function of the
applied magnetic field along the easy axis of the sample. For this purpose, the sample is placed
between two coils creating the magnetic field, having a range of -140 Oe to 140 Oe with a
resolution of 0.1 Oe. Two micro positioning probes are connected to the bottom and the top pad
to apply current through junctions and also to measure the voltage between the two electrodes.
Due to the small dimensions of the devices, a microscope is also used to perform all
connections between the probes and the junctions.
35
3.1.4.3) Profilometer
The profilometer is used to study the topography of a sample and to measure
thicknesses though a piezo-resistive sensor. This sensor will detect any changes in the
topography of the sample. First of all, a calibration needs to be performed in order to define a
reference; usually this calibration is done by sweeping the sensor in an area where the height is
constant. To calculate thicknesses, the standard procedure is to draw a line with an ink pencil
on the top of the substrate, we deposit the stack that we want to measure, then with a swab
imbedded in acetone, we remove the ink under the stack, like a lift off process, and we are then
able to measure the thickness of the stack. This machine has a resolution of 5 Å vertically and
can be used to measure thicknesses higher than 400 Å.
Fig 3.8 - Profilometer present inside the clean room at INESC-MN.
3.1.4.4) Ellipsometer
The ellipsometer is used to measure the index of refraction of thin transparent films and
film thicknesses. A monochromatic beam light with a wavelength of 632.8 nm focus on the
sample’s surface, and though the polarization of the incident and reflected beams, the machine
returns the values of the refraction index and the thickness of the film. This equipment is mostly
used to determine the thickness of the deposited oxide, in our case we only used
.
36
Fig 3.9 - Ellipsometer present inside the clean room at INESC-MN
3.1.4.5) Vibrating Sample Magnetometer
The VSM [10] [11] system is used to check if there is no problem with the magnetic
properties of the deposited stack by measuring the magnetic moment. Since this measurement
requires a small piece of unpatterned material, we use a calibration sample deposited at the
same time of sample that we are processing. The calibration sample is glued to a quartz rod
that will vibrate horizontally at 200 Hz, causing a variation of the magnetic flux, inducing a
current in the coils that will allow us to obtain the magnetization of the sample
Fig 3.10 - VSM setup present in the characterization room at INESC-MN.
37
3.2)
Microfabrication Process
In order to simplify all illustrations in this section, we only represented 20 pillars out of 480,
since the pattern is the same for all pillars.
1)
Stack deposition
In this first step, the whole stack is deposited on top of a substrate glass. For this deposition we
used Nordiko 2000.
Fig 3.11 - Different layers forming the stack deposited in Nordiko 2000 with respective thickness in
For the deposition of the stack we used to different sequences in Nordiko 2000. Obviously it
was deposited from the bottom to the top.
The first one deposit all layers until reaching the MgO layer, without depositing it, and
performing an etch of the MgO target preventing possible contamination from other targets. The
parameters used were the following:
Current
Voltage
Power
Gas flux
Pressure
Target
Separation
(mA)
(V)
(W)
(sccm)
(mT)
Number
(%)
1’36’’
40
343
10
9.8
4.7
S4T3
100
Ru
6’59’’
40
313
10
7.8
5.2
S4T2
100
6
Ta
57’’
40
338
10
9.7
4.7
S4T3
100
5
PtMn
4’26’’
30
294
-
8.9
5.1
S4T1
100
9
CoFe
1’02’’
7.8
5.1
S4T6
100
3
Ru
21’’
40
303
10
7.7
5.3
S4T2
100
47
CoFeB
52’’
30
398
10
8.7
5.2
S4T4
100
9.4
5.1
S4T2
100
Function
Material
Time
18
Ta
7
94
MgO
clean
2’30’’
F34R0B287
F149R0B284
Table 3.1 - Sequence 39 (Xiohong)
38
Current
Voltage
Power
Gas flux
Pressure
Target
Separation
(mA)
(V)
(W)
(sccm)
(mT)
Number
(%)
9.6
18.2
S4T5
50
10
8.8
5.2
S4T4
100
312
10
7.8
5.1
S4T2
100
338
10
9.8
4.6
S4T3
100
Function
Material
Time
51
MgO
1’03’’
50
CoFeB
2’53’’
30
398
4
Ru
2’05’’
40
18
Ta
1’36’’
40
F129R0B251
Table 3.2 - Sequence 40 (Filipe)
2)
Passivation layer
Here, we deposit 150
of TiW(N) acting like a protective layer, preventing the stack from
physical and chemical damages, but also to work as an anti-reflective layer during lithography.
Fig 3.12 - Stack after deposition of TiW(N)
The parameters used in the Nordiko 7000 were the following
Power
Module
Function
Time
DC
(KW)
Power
Power
Voltage
Current
Gas
Pressure
RF1
RF2
(V)
(A)
Flux
(mT)
50.2
3.1
2
9
1’
-
F60R10B120
F40R2
-
-
3
19
27’’
0.5K
-
-
427
1.2
50.5
10.6
3.0
Table 3.3 - Sequence 16: Soft Etch and TiW(N) deposition conditions
39
3)
Bottom contact definition
In this step, we define the shape of the bottom electrode using the DWL system. The
sample is coated with 1.5µm of photo resist before the exposure, and then the DWL was used
to imprint the bottom electrode shape on to the photo-resist. This picture shows the shape of the
bottom electrode from the AutoCAD file.
Fig 3.13 - First layer exposed defining the bottom electrode
The following pictures illustrate the lithography process:
Fig 3.14 - Sample ready to be exposed
40
Fig 3.15 - Sample after exposure
Fig 3.14 - Sample after revelation
Now that we have the desired pattern for the bottom electrode, it’s time to perform an etch until
reaching the substrate in order to transfer the pattern on to the stack.
Fig 3.15 - Sample after first ion milling and resist strip
The ion milling was performed by Nordiko 3600 with a pan angle of 60º with the following
parameters:
RF Power
198
724.3
104.4
344.8
2.3
Ar flux (sccm)
Rotation (%)
60
30
Table 3.4 – Parameters of Nordiko 3600 for the first ion milling
4)
Pillar definition
For this step, we also used the DWL system since the dimension of the pillars are
and
. The following picture was taken from the AutoCAD file used for the exposure;
the pillars are represented by the yellow squares.
41
Fig 3.16 - Picture of the 2
nd
layer only
Fig 3.17 - Picture of the first two layers
After the second exposure we had photoresist covering areas defining the pillars.
Fig 3.18 - Sample after 2
nd
exposure
42
After the exposure we need to perform an etch in order to transfer the pattern to the
stack.
Fig 3.19 - Layers to remove for the second etch
This step represents the most important step in all the fabrication process and yet, it is the most
difficult part of the process. Two kinds of problems can occur. The first one is the etch doesn’t
reach the barrier, in this case the tunneling effect will be lost and we will have direct conduction
between the bottom and the top electrodes. The second one is over etch, leaving a thin layer for
the bottom electrode leading to a huge loss of signal from the device. To be prevented from
such problems, calibration samples are used. Those samples have the exact stack and
thickness of the total layers that we want to remove. Once the calibration sample turns
transparent, we know that we have removed the material from the sample.
During the ion milling, two angles were used: 60º and 30º. The first one is used until we reach
the MgO barrier, minimizing the shadow effect. When it comes to remove the barrier, we cannot
use the 60º angle due to the possibility of redeposition of material in the lateral part of the
barrier causing a short circuit allowing current to flow from one electrode to the other without
passing though the barrier. To overcome this issue we used a 30º angle that will minimize the
redeposition of material, ensuring that the current will have to pass though the barrier. Notice
that the angles 60º and 30º are only valid for Nordiko 3000, since in Nordiko 3600 the
equivalent angles are 70º and 40º due to an offset of the assist gun.
43
Fig 3.20 - Etch with 60º and 30º
After perfoming the etch we obtained the following:
Fig 3.21 - Sample after the 2nd etch
Fig 3.22 - Zoom in of the sample after the 2
nd
etch
The ion milling was performed by Nordiko 3000 with a pan angle of 70º and 40º with the
following functions and parameters:
junction_etch_angle_70and40:

etch gun stab

junction etch (70º/350’’)

end_etch

etch gun stab 1

junction etch angle (40º/230’’)

end etch
RF Power
Ar flux (sccm)
(W)
54
488
26.6
193.5
Table 3.5 - Parameters used for the 2
1.6
nd
7.9
etch
44
5)
Oxide deposition
In this step we deposit 800Å of
to isolated bottom electrodes from top electrodes,
avoiding short circuits, ensuring the current flow to pass though the barrier in order to have the
tunneling effect.
Fig 3.23 - Oxide deposition represented by the white blocks and the transparent layer.
6)
Oxide lift off
Since we have oxide on top of the pillars due to the oxide deposition, we need to remove it so
we can access it.
Fig 3.24 - Sample after lift off
RF Power
Deposition
(W)
time
200
69’09’’
Pressure
Thickness
800
( )
Ar flux (sccm)
(mT)
3.0
45
Table 3.6 - Oxide deposition parameters
7)
Contact leads
In this step we need to define the shape of the top contacts using the DWL system and the
respective shape drawn in the AutoCAD file.
45
rd
Fig 3.25 - Picture of the 3 layer
Fig 3.26 - Picture of all layers
Unlike the first two exposures, where we used inverted masks, we wanted to have photo resist
inside the patterns, in this case we want the exact opposite, so we need the photo- resist to be
everywhere outside the patterns since the next step will be the metallization followed by a lift off.
Fig 3.27 - Top view of the sample after 3rd exposure
Fig 3.28 - Side view of the sample after 3rd exposure
46
After the exposure we need to deposit Aluminum on top of the sample to obtain the top contact.
Fig 3.29 - Sample after the deposition of Al (green layer)
The parameters used for the Al deposition were the following:
Module
DC Power
RF1 Power
RF2 Power
Current (A)
(KW)
Gas Flux
Pressure
(sccm)
(mT)
2
-
F60R1B126
F39R1
-
50.2
3.0
4
2
-
-
5.1
50.3
3.0
3
0.5
-
-
1.2
50.3
3.0
Table 3.7 – Parameters used for the 3 steps, soft etch, Al deposition, TiW(N) deposition
Fig 3.30 – Top view of the final structure
Fig 3.31 - Side view of the final structure
47
48
IV)
Experimental Data
First of all, a mask had to be created in AutoCAD in order to obtain the desired patterns
for our devices. During this work we changed the mask three times, and the final result was the
following.
Fig 4.1 – Mask used during the process
Our samples were composed by eight columns each one of them having twelve rows of MTJ’s
series, a test zone formed by eighteen single tunnel junctions and a series of MTJ to measure
the convergence of the TMR.
Regarding the sensors, we used two different dimensions for the pillars, 2
by 30
and
2
by 20 . For each series we had five top contacts, having 120 sensors between them as
we can see from this picture:
Fig 4.2 – Distribution of the two different pillar’s dimensions. In yellow 2
30
by 20
, and in blue 2
by
49
Since every series has 5 top electrodes, we performed a measurement between 2 successive
pads in order to obtain the TMR of smaller segments. The measurements were done with a
magnetic field with a range of -140 Oe and 140 Oe. Between -140 Oe and -20 Oe we used a
step of 40 Oe and between -20 Oe and 20 Oe we used a step of 2 Oe to have a clear idea of
the behavior of the sensor in the linear zone, and from 20 Oe to 140, again, we used a step of
40 Oe, since the focus of interest is the linear zone.
50
51
Fig 4.3 – Transfer curves obtained by the different segments forming the series
As we can see from figure 4.3, we observe that different segments have variations of TMR due
to their independence .The first measurement was made for the entire series giving a TMR of
29.6%. Since the TMR of the entire series is given by the average TMR of all junctions, if we
have one segment with a poor TMR and one with a huge TMR, the lower TMR segment will
have a huge impact in the TMR of the whole series. In this particular case we can verify that the
TMR of the whole series is given by the average TMR of all junctions by adding the contribuition
of each one of the segments and dividing by the number of segment as we can see from the
following calculation:
We can see from the first measurement that the TMR of the entire series is 29,6%, and by our
calculation we obtained 29,1%. The difference observed is due to the rounding of TMR but also
to the variation of resistance when a field is applied, since we can obtain small deviations of
TMR when measuring the same device.
From a transfer curve we can obtain the sensitivity of a sensor, as we can see from the
following explanation:
52
Fig 4.4 – Linear range in a linear response
Both fields
and
are given by the intersection of the tendency curve of
the linear zone with both constant curves representing Rmax and Rmin. The linear range is given
by:
The sensitivity of a series response is given by:
As we saw in the first chapter, the TMR is obtain by the following expression:
53
54
V)
Conclusion
The aim of this work was to study Wheatstone bridges composed by series of magnetic tunnel
junctions with a linear response. In order to obtain linear sensors with low coercivity a MgO
barrier of 12 Å The crucial step for the achievement of a Wheatstone bridge revealed itself to be
quite easy since we manage to obtain opposite magnetization direction by dicing the sample
and placing half bridge upside down another one, this way we can use half bridge as a
reference. The main issue was tie to the wire bonding, due to the possibility of seriously
damaging sensors leading to the loss of the signal. In this work some test structures were
introduced and tested to improve this control, either for optical and electrical inspection,
showing to be very useful for the analysis of the problem. This makes the inspection more
effective, but brings major constraints in terms of time spent to control the process.
Nonetheless, these developments are major advances and a cautious procedure may avoid
damaging the samples irreversibly. Unfortunately, time didn’t allow more tests, nor to study the
control of this process.
In an overall analysis, this work allowed me to obtain several skills either in terms of the
principles behind the operation of a magnetic tunnel junction, and all the constraints related to it,
but above all it provided a number of experimental techniques.
55
56
References
[1] B. M. Moskowitz, 1991, Hitchhiker's Guide to Magnetism, Environmental Magnetism
Workshop, Institute for Rock Magnetism.
[2] S. O. Kasap, 2002, Principles of Electronic Materials and Devices, (McGrawHill), 2nd edition
[3] R. Waser, 2005, Nanoelectronics and Information Technology, (Wiley-VCH), 2nd edition.
[4] A. Fert, et al, 1988, “Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices”,
Phys. Rev. Lett. 61, 2472.
[5]P. Grunberg et al, 1989, “Enhanced Magnetoresistance in layered magnetic structures with
antiferromagnetic interlayer exchange”, Phys. Rev. B 39, 4828.
[6] A. Fert et al, 1976, “Electric resistivity of ferromagnetic nickel and iron based alloys”, J. Phys.
F 6, 849.
[7] Ricardo Ferreira, 2008, Ion Beam Deposited Magnetic Spin Tunnel Junctions targeting HDD
Read Heads, Non-volatile Memories and Magnetic Field Sensor Applications, PhD Thesis, IST.
[8] M. Julliere, 1975, “Tunneling between ferromagnetic films”. Phys. Lett. 54 A, 225.
[9] S. Ikeda et al, 2008, “Tunnel magnetoresistance of 604% at 300 K by suppression of Ta
[10] Ricardo Ferreira, 2008, Ion Beam Deposited Magnetic Spin Tunnel Junctions targeting
HDD Read Heads, Non-volatile Memories and Magnetic Field Sensor Applications, PhD Thesis,
IST.
[11] Susana Freitas, 2001, Dual-Stripe GMR and Tunnel Junction Read Heads and Ion Beam
Deposition and Oxidation of Tunnel Junction, PhD Thesis, IST.
[12] S. Cardoso et al, 1999, “Ion Beam Deposition and Oxidation of Spin-Dependent Tunnel
Junctions”, IEEE Transactions on Magnetics, 35, 2952.
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junctions”, J. Appl. Phys. 103, 07A905.
57
58
Appendix A: Run sheet
Process Start : ___ / ___ / ___
STEP 0
Process Finish : ___ / ___ / ___
Tunnel Junction Deposition
Date: ___ / ___ / __
Machine: Nordiko 2000
Target RxA =
Comments: tunnel junction structure:
bottom electrode:
top electrode:
TiWN2: 150 Å
Total height:
Calibration samples: VSM
Top electrode for 2nd etch (glass substrate)
Caracterization: VSM as deposited and after annealed
STEP 1
1st Exposure – Main Pillar Definition
Date: ___ / ___ / ___
Coating PR: Vapor Prime 30 min (Recipe - 0)
coat 1.5 μm PR (Recipe 6/2)
Machine: DWL
Mask:
seriesL1i
Easy
Axis
Map: Amsion
m
t
m
m
X,Y
59
Energy :
Power : _120mW
Focus : ______
Develop : Recipe 6/2
Development time : 1 min
Optical Inspection:
Sample
STEP 2
Comments
1st Ion Milling – Total Structure Etch
Date: ___ / ___ / ___
Machine: N3600
A (etch rate: ________A/s  time:________s)
Total thickness to etch:
Base Pressure (Torr): ______
T Cryo (K): ______
Standard Etching Recipe (Junction Etch) :
Etch junction 60°
Junction_etch
Assist Gun:
subst.pan
Assist Gun
105 mA +750V/-350V 12sccm Ar; Assist Neut ; 30% subst.rot ,60º
Power
(W)
V+ (V)
I+ (mA)
V- (V)
I- (mA)
Ar Flux
Pan
Rotation
(sccm)
(deg)
(%)
Read Values
60
STEP 3
Resist Strip
Date: ___ / ___ / ___
Hot Micro-Strip + Ultrasonic
Rinse with IPA + DI water + dry compressed air
Started:_____________
Stoped:_____________
Total Time in Hot Micro-Strip : __________
Ultrassonic Time : ________________
Optical Inspection:
Sample
STEP 4
Comments
2nd Exposure – Top Electrode and Junction Definition
Date: ___ / ___ /
___
Coating PR: Vapor Prime 30 min (Recipe - 0)
coat 1.5 μm PR (Recipe 6/2)
Machine: DWL
Mask:
seriesL2i
Map: Amsion
Alignment mark position: X= 500 , Y= 8500
Easy
Axis
m
t
m
m
Energy :
X,Y
Power : _120mW
Focus : _____
Develop : Recipe 6/2
Development time : 1 min
Optical Inspection:
Sample
Comments
61
STEP 5
2nd Ion Milling – Top Electrode and Junction Definition
Date: ___ / ___ / ___
Machine: N3600
287 A / 290 A (etch rate: ________A/s  time:________s)
Base Pressure (Torr): ______
T Cryo (K): ______
Standard Etching Recipe (Etch Junction Top electrode) :
Junction etch 30°
Junction_etch
Assist Gun: 105mA +750V/-350V 12sccm Ar; Assist Neut ; 30% subst.rot 30º subst.pan
Calibration Sample
Wafer
Assist Gun
samples
Power
(W)
Structure
Etching Turn
V+ (V)
Time
I+ (mA)
V- (V)
Effect
I- (mA)
Ar Flux
Pan
Rotation
(sccm)
(deg)
(%)
Read Values
Optical Inspection:
Sample
Comments
62
STEP 6
Insulating Layer Deposition
Date: ___ / ___ / ___
Responsible: Fernando
Machine: UHV2
Al2O3 thickness
Deposition
Ar gas flow
Pressure
Power Source
Time
500 A
Comments:
STEP 7
Oxide Lift-Off
Date: ___ / ___ / ___
Hot μ-strip + ultrasonic
Rinse with IPA + DI water + dry compressed air
Started:_____________
Stoped:_____________
Total Time in hot μ-strip : _______________________________
________________
Ultrasonic Time :
Optical inspection:
STEP 8
3rd Exposure – Contact
Date: ___ / ___ / ___
Coating PR: Vapor Prime 30 min (Recipe - 0)
coat 1.5 μm PR (Recipe 6/2)
Machine: DWL
Mask:
seriesL3ni
Map: Series
Alignment mark position : X= 500 , Y= 8500
Easy
Axis
m
t
m
m
X,Y
63
Energy :
Power : _120mW
Focus : _____
Develop : Recipe 6/2
Development time : 1 min
Optical Inspection:
Sample
STEP 9
Comments
Contact Leads Deposition
Date: ___ / ___ /___
1200 A Al
Machine: Nordiko 7000
Seq.48 (svpad) –
Run#
Run#
Run#
mod.2 – f.9
(1’ soft sputter etch) P=60W/40W, p=3mTorr, 50 sccm Ar
mod.4 – f.1
(1500A Al, 18’’)
mod 3 – f.19
(150A TiW, 27’’) P=0.5 kW, 3mTorr, 50sccm Ar + 10 sccm N 2
P=2 kW, 3mTorr, 50 sccm Ar
Power1
Readings – Module 2
Power2
Gas flux
Pressure
Power
Readings – Module 4
Voltage
Current
Gas flux
Pressure
Power
Readings – Module 3
Voltage
Current
Gas flux
Pressure
64
STEP 10
Al lift-off
Date: ___ / ___ / ___
Hot μ-strip + ultrasonic
Rinse with IPA + DI water + dry compressed air
Started:_____________
Stoped:_____________
Total Time in hot μ-strip : _______________________________
Ultrasonic Time :
________________
Optical inspection:
STEP 11
4th Exposure – junction top contact
Date: ___ / ___ / ___
Coating PR: Vapor Prime 30 min (Recipe - 0)
coat 1.5 μm PR (Recipe 6/2)
Machine: DWL
Mask:
Map: Amsion
Alignment mark position : X= 500 , Y= 8500
Easy
Axis
m
t
m
m
X,Y
Energy :
Power : _120mW
Focus : _____
Develop : Recipe 6/2
Development time : 1 min
Optical Inspection:
STEP 12
Insulating Layer Deposition
Date: ___ / ___ /___
65
Responsible: Fernando
Machine: UHV2
Deposition
Al2O3 thickness
Ar gas flow
Pressure
Power Source
Time
1000 A
Comments:
STEP 13
Oxide lift-off
Date: ___ / ___ /
___
Hot μ-strip + ultrasonic
Rinse with IPA + DI water + dry compressed air
Started:_____________
Stoped:_____________
Total Time in hot μ-strip : _______________________________
Ultrasonic Time :
________________
Optical inspection:
66
Appendix B: Mask
67
APPENDIX C: Fotos of the process
Fig - Sample after first exposure
Fig – Sample after second exposure. On the left we have a series, and on the right a single
junction for tests
Fig – Sample after oxide deposition
68
Fig – Final structure, after metallization
Fig – Zoom in of the final structure
Fig – Final test zone
69
70