1 MgO MTJ biosensors for immunomagnetic lateral-flow detection Ricardo Jorge Penelas Janeiro Dissertação para obtenção do Grau de Mestre em Engenharia Física Tecnológica Júri Presidente: Prof. Dr. João Carlos Carvalho de Sá Seixas Orientador: Prof. Dr. Susana Isabel Pinheiro Cardoso de Freitas Vogal: Prof. Dr. Paulo Jorge Peixeiro de Freitas Outubro de 2010
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MgO MTJ biosensors for immunomagnetic lateral-flow
detection
Ricardo Jorge Penelas Janeiro
Dissertação para obtenção do Grau de Mestre em
Engenharia Física Tecnológica
Júri
Presidente: Prof. Dr. João Carlos Carvalho de Sá Seixas
Orientador: Prof. Dr. Susana Isabel Pinheiro Cardoso de Freitas
Vogal: Prof. Dr. Paulo Jorge Peixeiro de Freitas
Outubro de 2010
ii
iii
Acknowledgement
Com chegada ao fim da tese de mestrado, etapa final do curso, impõe-se o dever e felicidade
de agradecer a todos os que permitiram empreender toda esta jornada académica. Dever porque sem
todos eles quem sabe o que poderia ter acontecido pelo caminho ardiloso que é a vida de um jovem
universitário; Dever porque de todos eles o meu ser se formou, por causa da interacção com cada um
se moldou. Alegria porque poder agradecer significa não só o concluir de um ciclo, como a existência
de a quem agradecer, sinal de um caminho não percorrido isoladamente.
Começo por agradecer à instituição que permitiu a realização deste trabalho, o INESC-MN,
que me proporcionou o meu primeiro contacto com a Ciência: a verdadeira ciência crivada de
dificuldades, imprevistos e cujos resultados nem sempre premeiam o esforço dispendido, mas que é
recompensadora quando é atingido um objectivo. Agradeço ao Professor Paulo Freitas e à
Professora Susana Freitas pela oportunidade de trabalhar no INESC-MN e pela orientação científica
e experimental proporcionada. Um agradecimento também ao Dr. Ricardo Ferreira pela paciência e
disponibilidade com encarou todas as infindáveis questões. Obrigado também ao resto da equipa
científica do INES-MN, dos seus engenheiros aos alunos pela disponibilidade e ajudas prestadas
sempre que solicitados. Um especial obrigado aos alunos de mestrado de curso de física pelos
excepcionais momentos de companheirismo (Cláudio, Raquel, Zita).
Prosseguindo na enumeração daqueles que marcaram a etapa que agora termina é
imperioso referir o conjunto de amigos com quem partilhei diariamente a experiência académica no
IST (made me a man). Desde o primeiro grupo com que travei conhecimento / já conhecia – Mike,
Ana, Romão – às mais recentes crianças que incautamente entraram neste curso – Patos e Amigos
Lmta – passando pelo mais experiente e já Mestre José Gustavo Rebelo, e pelo caríssimo D. Diogo
Capelo, até aquela que pertence a uma classe só dela - Raquel (obrigado coisa :) ) o meu muito
obrigado por tudo sendo que muitas vezes o proporcionarem uma gargalhada foi o suficiente e ideal.
Agradeço a toda a minha família por todo o apoio e confiança depositada em mim e nas
minhas capacidades (pobres ingénuos :p). Um muito obrigado aos meus pais, elementos
fundamentais e sempre presentes em qualquer momento.
Não me escudando na minha confessa e evidente inaptidão para a “escrita livre e pessoal”
desde já as minhas desculpas às inúmeras pessoas que aqui ficaram por referir.
Obrigado Mãe, Obrigado Pai, Obrigado Raquel =)
Ricardo Janeiro
Dezembro de 2010
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Resumo
Nos tempos que correm, é evidente o potencial das nanotecnologias, as quais são capazes
de fornecer os meios necessários para a detecção e rastreamento de patogéneos na forma de
sensores de tamanho micrométrico muito sensíveis.
O objectivo deste trabalho é a fabricação de sensores magnetoresistivos, com resposta linear
capazes de executarem testes de reconhecimento através da detecção de nanopartículas
funcionalizadas, magneticamente polarizadas e previamente ligadas a biomoléculas. Os sensores
magnetoresistivos usados foram Junções de Efeito de Tunel, as quais apresentam por norma um
valor elevado de magnetoresistência. Neste trabalho junções individuais foram ligadas em série,
formando vectores de 360 elementos individuais. Tal foi feito com o propósito de ganhar na
detectividade dos dispositivos.
Várias amostras foram processadas e magnetoresistências da ordem de 70% foram
conseguidas, assim como sensibilidades de 1.24%/Oe na região linear.
Palavras chave: Sensores magnetoresistivos de efeito de túnel, junções de efeito de túnel
de MgO, ruído, fluxo lateral, séries de junções de efeito de túnel.
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Abstract
In the modern times is clear the potential of the nanotechnologies, which are capable of
providing the necessary means for the detection and screening of pathogens in the form of very
sensitive micron size sensors.
The aim of this work is the fabrication of magnetoresistive (MR) sensors with linear response,
capable of performing tests of recognition by the detection of functionalized nanoparticles magnetically
polarized which had been previously linked to biomolecules. The magnetoresistive sensors used were
Magnetic Tunnel Junctions which present a high value of magnetoresistance. In addition, in this work
individual Magnetic Tunnel Junctions were connected in series, forming arrays of three hundred and
sixty individual elements. Such was done with the purpose of gain in the devices’ detectivity.
Several samples were patterned and magnetoresistances of the order of 70% were achieved,
as well as sensitivities of 1.24%/Oe in the linear range.
Acknowledgement ................................................................................................................................... iii
Abstract................................................................................................................................................... vii
Figure List ................................................................................................................................................ xi
Tables List ............................................................................................................................................. xiii
A. Appendix ........................................................................................................................................ 67
B. Appendix ........................................................................................................................................ 69
Run Sheet for MTJs Array fabrication: Influenza_ 2010 ................................................................... 69
Figure 5.5: Picture of the ellipsometer. .................................................................................................. 46
Figure 6.1: Standard MTJ stack used ................................................................................................... 47
Figure 6.2: Resulting photoresist after the 1st lithography revelation. ................................................... 48
Figure 6.3: Bottom contact definition: resulting structure after the ion milling. ...................................... 48
Figure 6.4: Picture of bottom contact. ................................................................................................... 49
Figure 6.5: Left - Wet bench’s picture: ultrasound machine and hot bath; Right - microstrip solution. . 49
Figure 6.6: Pillar junction definition. Left - After the 2nd
lithography; Right – After the 2nd
etch ............ 50
Figure 6.7: Picture of the pillar junction. ................................................................................................ 50
Figure 6.8: MTJ pillar and bottom contact insulation ............................................................................. 51
Figure 6.9: Top contact definition ......................................................................................................... 51
Figure 6.10: Picture of the top electrode. .............................................................................................. 52
Figure 6.11: Sample patterned and mounted on a chip-carrier............................................................. 52
Figure 7.1: Close view of one MTJ series with the 4 contacts identified as a, b, c and d. .................... 53
Figure 7.2: ZarMTJ1 transfer curve ....................................................................................................... 53
Figure 7.3: ZarMTJ2 transfer curve. ...................................................................................................... 54
Figure 7.4: ZarMTJ3 transfer curve ....................................................................................................... 54
Figure 7.5: ZarMTJ5 transfer curve. ...................................................................................................... 54
Figure 7.6: ZarMTJ1 and ZarMTJ3; measurement for 360 elements. .................................................. 55
Figure 7.7: The RxA of the sensors in ZarMTJ1 and ZarMTJ2 samples. ............................................. 55
Figure 7.8: ZarMTJ1 - TMR vs number of junctions. ............................................................................. 56
Figure 7.9: Transfer curve between contacts of two independent series. ............................................. 57
Figure 7.10: N2#1 transfer curve. .......................................................................................................... 58
Figure 7.11: N2#2 transfer curve. .......................................................................................................... 58
Figure 7.12: N2#4 transfer curve. .......................................................................................................... 58
Figure 7.13: N2#1 - TMR vs number of junctions ................................................................................. 59
Figure 7.14: TMR vs Vbias ...................................................................................................................... 60
Figure 7.15: TMR of a MTJ decreasing with the applied voltage .......................................................... 61
Figure 7.16: Graph - Response of one sensor under magneto nanoparticles excitation. ..................... 62
Figure .A.1: Mask used .......................................................................................................................... 67
Figure A.2: Top: Series detail ................................................................................................................ 68
Figure A.3: 3D scheme of MTJ’s connected in serious ......................................................................... 68
xiii
Tables List
Table 4.1: Deposition setpoints of Nordiko 2000. ................................................................................. 31
Table 4.2: Standard conditions operation in the used modules ............................................................ 33
Table 4.3: Deposition conditions of UHV2. ............................................................................................ 34
Table 4.4: Ion milling used set values for assist gun of both IBD systems. .......................................... 35
Table 7.1: Variations in the stack of samples with Si substrates. ......................................................... 53
Table 7.2: Variations in the stack of samples with glass substrates. .................................................... 58
xiv
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1. Introduction
The last century saw the advent of non-classical science, beyond the limits of the Newtian
physics and Maxwell’s electromagnetism. These scientific progresses were instrumental in the
development of several technological areas, having allowed, among others, major advances in
electronics, whose importance has assumed a dominant role nearly in all the areas of human
intervention. In this context of technological progression a pre-requisite for the success of any system:
it’s contained area and volume. From this requirement born microelectronics. driving the need for a
continued miniaturization; Currently, it is no longer just the need of space that dictate the importance
of micro / nanotechnologies, which has extended from their traditional applications’ area in computer
systems and automated control systems, to the most recent biosciences. A family of small-scale
devices that is particularly interesting is the magnetoresistive family, whose applications include
sensors for read heads in hard disk drives, memories, biosensors and diagnostic techniques using
brain mapping.
In fact, in biosciences context, which are currently showing themselves as a scientific area
with major scientific and social impact, it’s clear the potential of the magnetoresistive (MR) sensors, as
a way of providing the necessary means for the detection and screening of health threatening
pathogens. Was in order an application under the biorecognition and detection that this work was
developed, using the advantages of a particular type of magnetoresistive sensors: the magnetic tunnel
junctions (MTJ’s) sensors – high spatial resolution provided by their small size, and higher sensitivities
and output values comparing with the others MR sensors.
1.1. Thesis’s framework – motivation
Done in the INESC-MN facilities, with special emphasis in the INESC clean-room, the work
done in this thesis is integrated in an international project which has, as ultimate goal, to develop a
new diagnostic tool for influenza virus detection based on an immunochromatographic assay with
functionalized magnetic nanoparticles as markers of the virus and ultra sensitive field transducers for
their detection and quantification.
Actually, the present methods for human influenza virus based on cell cultures have a
response time frame of some days, too long to help physicians to make clinically relevant decisions
during the first days of the disease, when the treatment will be more effective. There are already some
immnunochromatography based tests, which yield a response in less than 30 minutes, but they suffer
from low sensitivity (only 50 to 70% of true influenza cases are detected).
2
These rapid immunochromatographic tests are fast assays based on the immunological
capture of a ligand attached to an active microparticle when it flows over a solid support where an anti-
ligand has been immobilized.
The chosen approach for this project takes advantage of the exploitation of magnetic
properties of magnetic nanoparticles as active particle, which will be detected and quantified by ultra-
sensitive magnetic sensors. The first developments in field sensors lead to inductive readers, in which
an electrical response is induced in some coils placed around the field source: if a magnetic flux
variation through the coils exists, an electromotive force is created, like stated in Faraday’s law. The
coils method has a low sensitivity so in the project the coils are replaced by magnetoresistance based
sensors (magnetoresistive sensors).
Resuming, the final goal of the project is the creation of an integrated platform including a
system that enables the processing of samples allegedly virus contaminated and the system of
detection and quantification.
The working principle is like follows: first the testing sample containing the virus is put in
contact with magnetic nanoparticles functionalized with a specific antibody to the particular virus
strain. The fluid is then put in contact with a porous membrane strip; the fluid sample passes through
the membrane until the virus attached to the magnetic markers are recognized by membrane
immobilized antibodies; finally the presence of a particular virus strain is detected through the sensing
of the magnetic fringe field created by the bound magnetic nanoparticles, using a magnetic sensor. Is
worthy to note that this concept can be used to detect other virus than influenza one.
Figure 1.1: Lateral flow recognition. a) the fluid sample is put into contact with magnetic particles; b)virus is attached to nanoparticles antivirus; c) fluid sample migrates through the membrane strip.
The magnetic field and magnetic properties of materials play a major role in the behaviour of
devices that were fabricated and studied during this thesis. Therefore this first section will introduce
and remember some basic concepts and definitions.
2.1. Magnetic field and Magnetization
A magnetic field is a field of force that arises from two sources: electric currents (generically
electric charge moving) and materials with an intrinsic nonzero magnetic moment (magnetic
materials). In fact, the magnetism of the matter is due to the movement of electrons from atoms, which
are responsible by the microscopic magnetization currents postulated by Ampère before knowing the
structure of matter, so we can see the common ground in these two sources of field: it results from
electric charge movement.
Because of the nonexistence of magnetic charges every magnetic field source has a dipolar
form (the first term appearing in the dipolar treatment of a distribution of magnetic sources is the
dipolar), and so, magnetic materials, being a magnetic field source, have a magnetic moment
associated just like current loops. Through the dipolar moment is possible to define the magnetization
of a material:
m is the magnetic moment of the material and V its volume.
The two types of sources contributing to the magnetic field can be related by the following
expression:
, µ0 free space permeability
with H taking into consideration the free currents influence, while B considers all the currents ( free
and magnetization currents; so B takes into account the magnetization of the medium).
The influence of the H field over the magnetization of magnetic materials is the information
contained in magnetic susceptibility:
4
2.2. Magnetic Materials Classification
Diamagnetism
The atoms of diamegnetic materials do not present a permanent magnetic moment in absence
of applied magnetic field, since they don’t have unpaired electrons on their completely filled orbital
shells. However, when a magnetic field is applied a small intensity response occurs: these materials
have a negative small susceptibility, so they tend to align the magnetic moment of their atoms against
the applied field.
The diamagnetic behaviour is present in every material, but due to its small magnitude is often
negligible or covered by stronger magnetic effects.
Paramagnetism
In the absence of a magnetic field, the atoms of a paramagnetic material have a permanent
magnetic moment due to the unpaired electrons on their partially filled shell. Since these atoms, as
magnetic dipoles, poorly interact with one another they get a random orientation due to thermal
agitation. In the presence of an external magnetic field the moments will gradually align along with it
as the intensity of the field increases. So paramagnetic materials have a positive, but small
susceptibility.
So, for both diamagnetic and paragmetic materials, when a magnetic field is applied a
magnetization proportional to that field is produced:
and so the magnetic field is:
Figure 2.1: Diamagnetic material: Left - In absent of applied field; Right - Under applied field.
Figure 2.2: Paramagnetic material in field absence (left) and under its influence (right).
5
Diamagnetic materials:
- ;
- is thermally independent;
- has a very small absolute value;
Paramagnetic materials:
- ;
- is generally small (however greater than the absolute values of diamagnetic materials): for
low temperatures when the thermal agitation decreases significantly paramagnetic materials
exhibit higher values.
Ferromagnetism
Ferromagnetic materials exhibit a large permanent magnetization even when a magnetic field
is not present. Like in paramagnetism, the atoms of ferromagnetic materials have unpaired electrons.
The main difference between these two types of magnetism lies on the stronger interaction existing
between ferromagnetic atoms – the exchange interaction. If a magnetic field rises, the individual
magnetic moments tend to align with the field, so the susceptibility is positive and typically large.
Above a certain temperature, the Curie temperature, ferromagnetic materials behave like
paramagnetic ones.
Figure 2.3: Ferromagnetic material – moments align without an applied field.
6
Antiferromagnetism
In antiferromagnetic materials the exchange interaction between their atoms results in
individual magnetic moments ordered in such way that moments of consecutive atoms in the lattice
are aligned anti-parallel. Since in these two sub-lattices the moments have an equal magnetic
magnitude, the net magnetic moment in the absence o field is zero. This kind of materials has a small
and positive susceptibility.
Antiferromagnetic materials behave like paramagnetic ones above the Néel
temperature (it is the analogous of the Curie temperature in ferromagnetism); each
ferromagnetic also has a characteristic blocking temperature above which the strong
interlayer exchange interaction vanishes and an antiferromagnetic layer loses its “ability” to
pin a ferromagnetic layer.
Ferrimagnetism
The magnetic moments exhibit alternate orientation like in ferromagnetism. However because
there is more than one type of magnetic ion in the material, not all magnetic moments have the same
absolute value. This means that they have a non null net magnetization.
Figure 2.4: Antiferromagnetic material – antiparallel alignment of the moments; no net magnetization.
Figure 2.5: Ferrimagnetic material
7
2.3. Micromagnetism for ferromagnetic material
Ferromagnetic and antiferromagnetic materials are key elements in the behaviour of the
devices focused by thesis. So to better understand their properties it is a good approach to make
some considerations about the interactions and the associated energies that occur in or with these
materials.
2.3.1. Exchange energy
The spontaneous ordering of ferromagnetic atomic moments is mainly due to the exchange
interaction of their atoms, which is independent of the total magnetic moment of the sample.
According to the theoretical atomic dipoles model for ferromagnets, the main difference
between ferromagnets and paramagnets is that the each permanent dipole of the ferromagnets
interact strongly with their nearest neighbours dipoles. The interaction energy between two sequenced
atoms is described by the Heisenberg model:
is the coupling constant between the spins of the neighbour atoms, and is usually considered
constant through a material and Si and Sj are the spins of each neighbour.
The total exchange energy of a material implies sum over all pairs of nearest neighbours;
generalizing to the continue approximation the exchange energy of entire crystalline lattice is
a is the lattice constant.
If the magnetization varies too rapidly in a short range the exchange energy will be very high,
so in order to keep it in a minimum value the magnetic dipoles have a preference to remain aligned
with each other.
2.3.2. Magnetocrystalline Anisotropy – Anisotropic Energy
Crystalline materials are magnetically anisotropic because there is a preferential direction for
orientation of the dipoles along the main crystallographic axis of the structure. This means that exist a
sort of internal field which forces the net magnetization to align with certain axis of the crystalline
structure, defining the so called easy axis or easy direction.
The magnetocrystalline anisotropy energy, Ek, is the work needed to rotate the sample
magnetization to a certain direction out of the easy axis direction, and is calculated as follows:
8
where K is an energy density, and α is the angle between the magnetization and the easy axis.
Is clear to see that this energy will have a minimum for α=0, which means that the
minimization of this energy implies the magnetization to prefer align with the easy axis. This
contributes to the “memory effect” of ferromagnets (after the magnetic field get off the material will
tend to align their dipole with the easy axis).
2.3.3. Shape anisotropy – Demagnetizing Energy
The demagnetizing energy corresponds to the interaction between the magnetization of the
material and the demagnetizing field, Hd. His expression follows below:
In order to understand the appearance of the demagnetizing field a good strategy could be to
establish the analogy with the electric field. A polarized material creates a distribution of electric
charges at the surfaces whose normal has the same direction of the polarization vector P. That
charges are responsible for the rise of an electric field/electrostatic potential (Poisson’s equation). With
the magnetization a similar phenomenon happens: on the surfaces, at the material boundaries (those
that are perpendicular to some magnetization component) the magnetization is no longer continuous,
and “magnetic charges” appear. These charges are the sources of the demagnetizing field that will
oppose to the normal magnetization of the material; this means that this energy tries to become the
magnetization parallel to “charged” surfaces.
Minimizing the demagnetizing energy corresponds to rotate the magnetic atomic dipoles of the
sample, so that the number of charges created on surfaces is minimal. That makes the material
become divided into different magnetic domains oriented in reverse directions. This way, the magnetic
charges formed by one domain will cancel the charges of neighbour domains, reducing Hd and Ed.
Figure 2.6: Magnetization and demagnetizing field: the field is greater the closer are the “magnetic charges”.
9
If the demagnetizing energy was the only one ruling the behaviour of ferromagnetic materials
they would divide themselves in as many domains as necessary to completely eliminate the
demagnetizing field resulting in a zero total magnetization.
However, indeed, the exchange energy avoids this to happen, because it has the inverse
effect: its minimization cause the atomic moments of neighbours to align with each others. So, it is the
balance of these two energies is responsible by the creation of magnetic domains separated by
domain walls, which are regions where the orientation of the magnetic dipoles that belong to them is
not constant, rotating from the orientation in the first domain to the orientation of the second (Figure
2.8: Domain WallFigure 2.8). The thickness of these domain walls is also influenced by the anisotropy
energy: this energy forces the moments to align by the easy axis, so it would “prefer” no walls at all,
but the exchange energy would prefer a wall so big as the material so that neighbour dipoles would be
as parallel as possible; so anisotropy energy will try to create a thin wall and exchange energy will try
to enlarge it.
It is important to note that wasn’t written that there are magnetic monopoles. Isolated magnetic
charges doesn’t exist ( ), so that charges referred above always appears in pairs constituting a
dipole. In fact if one tries to split the north pole from the south pole of a magnet will end with two
smaller magnets, each with one north and one south pole.
Figure 2.7: a) single domain; b) 2 domains; c) 4 domains.
Figure 2.8: Domain Wall
10
2.3.4. Zeeman Energy
There is one last energy term that describes the magnetic properties of a ferromagnet: the
Zeeman energy. The Zeeman energy is related to the interaction between the magnetization of a
material and the external applied field (Ha) and is given by:
This energy represents the amount of work necessary to rotate the magnetization by a certain
angle with respect to the applied field, and would be a minimal value when the magnetization became
align along the magnetic field direction.
2.3.5. Interlayer coupling forces
2.3.5.1. Néel Coupling
It is right to say that the Néel coupling is present in every ferromagnetic interlayer
discontinuity, because it is associated with roughness of the ferromagnetic interfaces (it is reasonable
to say that exist always some level of roughness in an interface). The magnetostatic interactions
between the “magnetic charges” at the ferromagnetic interface cause a ferromagnetic coupling.
The Neel energy is given by:
2.3.5.2. RKKY Coupling
Ruderman-Kittel-Kasuya-Yodsida coupling or indirect oscillatory exchange interaction is an
exchange interaction between two ferromagnetic layers separated by a non magnetic spacer. The
wave functions of the atoms of the two ferromagnetic layers interact with each other in a way mediated
by the spacer, whose thickness is responsible for the oscillatory character of the interaction between a
ferromagnetic and an anti-ferromagnetic coupling.
Figure 2.9: Néel coupling.
11
2.4. Magnetoresistance
Magnetoresistance is the property of a material to change the value of its electrical resistance
when the value of the applied magnetic field changes. The magnitude of this effect can be expressed
numerically as follows and is presented as a percentage:
2.4.1. Anisotropic Magnetoresistance (AMR)
The AMR effect consists in the change of electrical resistivity with the orientation of
magnetization in respect with the direction of the electrical current in the material. Thus, the resistivity
of a magnetic material depends on the angle between the magnetization and the current direction and
can be described by the following equation:
For most materials the resistance is maximized when the current is parallel to the
magnetization and minimized when it is perpendicular.
To measure the amplitude of this effect (to know how much the resistivity varies) is necessary
measure the maximum and the minimum, so it is necessary to saturate the magnetization in parallel
and perpendicular directions. With these two values it’s possible to calculate the anisotropic
magnetoresistivity ratio:
Typically AMR is somewhere between 2 a 6%.
Figure 2.10: Oscillatory character of RKKY coupling with the spacer thickness. The strength of the ferromagnetic and anti-ferromagnetic coupling decreases with the spacer thickness.
12
2.4.2. Giant Magnetoresistance (GMR)
Giant magnetoresistance is an effect observed in thin film structures composed of alternating
ferromagnetic and non magnetic layers. The observed effect is a significant change in the electrical
resistance of such structures depending on whether the magnetization of adjacent ferromagnetic
layers are parallel or anti-parallel aligned. The overall resistance is low for parallel alignment and high
for anti-parallel alignment. Therefore the intensity of the GMR effect is given by:
Multilayer GMR
In this structure two or more ferromagnetic layers are separated by a non-ferromagnetic
spacer (FM/AFM/FM structure - e.g. Fe/Cr/Fe – the first GMR sample). To a certain thickness of the
spacer, the RKKY coupling between the adjacent ferromagnetic layers becomes anti-ferromagnetic,
making it, in the absence of a magnetic field, energetically preferable for the magnetizations of
adjacent ferromagnetic layers to align anti-parallel. When an external filed is applied and reaches a
certain value which makes energetically favourable to break the inter layer coupling the ferromagnets
will align parallel to each other and parallel with the field. This rotation and alignment of the
ferromagnetic layers are followed by a drop in the electrical resistance of the structure. In the
multilayer configuration with 10 or more stacks MRs of about 10% were achieved.
Figure 2.11: Transfer curve of a GMR multilayer. The red arrows represent the magnetization of each layer to different applied fields.
line ac(#240) ad(#360) ac(#240) ad(#360) ac(#240) ad(#360)
6 58.9 44.62 117.50 158.60 117.00 92.10
This non expected kind of behaviour can be verified in other series, in other dies and even in
other samples. Because of the decreasing of the resistance relatively to what was expected be this
effect seems like the one obtained with resistors mounted in parallel. In fact this behaviour warned that
there might be some leak of current to the substrates. Subsequent measurements confirmed that
there were current leaks through the oxide that is supposed to insulate the Si substrate. Was then
determined that all the series were this way connected with each others. It was even possible to get
some transfer curves making the contact through pads that should be isolated one from the other.
Figure 7.9: Transfer curve between contacts of two independent series.
Every 1st generation samples had this current leak problem, which obviously affects the
performance of the magnetic tunnel junctions, which were deposited over a silica substrate with 800Å
of SiO2.
2st Generation – Glass substrate
The second sample generation was processed over a glass substrate and the conduction
problem disappeared. Meanwhile, was discovered by the team of INESC researchers that the
deposition rates in the Nordiko2000 system didn’t correspond to the known values. So in order to
regain the control of the thickness of deposited films the deposition times were recalibrated. This new
glass substrate samples were already deposited with the new deposition times.
5.00E+04
6.00E+04
7.00E+04
8.00E+04
9.00E+04
-180 -140 -100 -60 -20 20 60 100 140 180
R (
Oh
m)
H(Oe)
ZarMTJ1 line 12 pad a & line 13 pad dTMR = 36.7%
58
7500
7700
7900
8100
8300
8500
-150 -50 50 150
R (
kO
hm
)
H (Oe)
N2#4
TMR = 3.8%Sensibility = 0.033%/OeVbias = 1.82 V
4.00E+04
4.50E+04
5.00E+04
5.50E+04
6.00E+04
-150 -50 50 150
R (O
hm
)
H (Oe)
N2#1
TMR = 39.5%Sensibility = 0.711%/OeVbias = 1.79 V
16000
18000
20000
22000
24000
-150 -50 50 150
R (k
Oh
m)
H (Oe)
N2#2
TMR =32.0%Sensibility=0.1723%/OeVbias = 1.80 V
N2#1 N2#2 N2#3 N2#4
CoFeB thickness (Å) 15.5 30 100 60
MgO thickness (Å) 12 12 12 12
Table 7.2: Variations in the stack of samples with glass substrates.
Figure 7.10: N2#1 transfer curve.
Figure 7.11: N2#2 transfer curve.
Figure 7.12: N2#4 transfer curve.
59
All these samples were patterned at the same time, so they could be compared with each
other. The sample N2#3 (100 Å CoFeB) was not working at the end of the process. Comparing with
the 1st generation, the sample with 15.5Å of CoFeB showed an improved magnetoristance although
the decrease of the sensibility and the increase of the coercivity. For the sample N2#2 was expected
higher TMR values than N2#1 sample. The sample N2#4 has marginal TMR. The sample with higher
TMR, N2#1 sample, was chosen to further studies.
Once more a study of the behavior of a series based on the number of elements that
compose it was conducted. The results of the sample N2#1 are shown below:
Figure 7.13: N2#1 - TMR vs number of junctions
The TMR values for 120, 240 and 360 individual elements are almost the same, close to what
would be expected from perfect series concerning the likeness of every individual junction. Since
during the patterning process of both ZarMTJ1 and N2#1 nothing occurred that could explain a lesser
uniformity of properly working individual junctions through a series, again can be concluded that was
the conduction path through the substrate in the case of the first generation samples that played a
major influence in the unexpected behavior of ZarMTJ1 (and all the other 1st generation samples), and
not an unidentified cause in the manufacturing process.
0
10
20
30
40
50
0 5 10 15 20
TMR
(%)
Line
360 junctions
240 junctions
120 junctions
sample N2#1Ibias = 40 µA
60
0
10
20
30
40
50
60
70
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
TMR
(%)
V(V)
N2#1line 18 ab (#120)
A study of TMR vs Vbias was conducted (this study required a remake of the program that
controls the manual transport measurement setup).
0
10
20
30
40
50
60
70
-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35
TMR
(%)
V(V)
N2#1line 18 ab (#120)
-35
-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
30
35
-0.003 -0.002 -0.001 0 0.001 0.002 0.003V (V
)
I (A)
H = 140 Oe
H = -140 Oe
N2#1line 18 ab (#120)
Figure 7.14: Top – Curves V vs I for maximum and minimum resistance saturation fields; Center – TMR vs I; Bottom – Close look of the near zero bias current.
61
It can be seen that the TMR value has a plateau for currents in the vicinity of I=0 (V=0). In fact
it is possible to apply about 1.5V to the series without any concern about the decrease of TMR. If the
current keeps increasing the TMR starts to decrease. With the used voltmeter wasn’t possible to
measure higher voltages (it has a maximum range limited to the interval between -30 V and 30 V).
Therefore we were not able to determined the value of V1/2 .
This last result shows some of the greatest advantages of using arrays of MTJs:
- with relatively small bias currents high output voltages are achieved;
- the higher robustness of a MTJ array which tolerate a very high potential drop
between its terminals without disruption and with just a small TMR drop.
Regarding the robustness it can be better understand when compared with the result for an
individual MTJ:
Figure 7.15: TMR of a MTJ decreasing with the applied voltage.
The figure above[17]
shows a decreasing of 50% in TMR for an individual junction biased with
350 mV, which is substantially less voltage than what was applied to the MTJ array, which supported
30V without suffering a 50% TMR loss. This feature makes the MTJs series particularly useful and
usable since they support any electrostatic discharge that could occur in their handling, and is justified
by the “distribution” of the total potential drop in the series by each of its individual elements.
62
M
H
H
Functionalized stripe
Nanoparticle
While the work of characterization of the sensors was being done, one set of sensors was sent
to another partner institution in the project with the goal of proceeds real applications measurements.
Thus, sensors of the first generation (ZarMTJ1), were sent. The wheel device already explained was
used.
Figure 7.16: Graph - Response of one sensor under magneto nanoparticles excitation. Scheme - Functionalized stripe with nanoparticles attached, and a vertically magnetized nanoparticle.
During this experiment the particles where vertically magnetized, as shown in the previous
figure. The wheel was not rolling freely but fixed and was pulled over the sensor When the
nanoparticles are trapped in the stripe they have a certain tendency to accumulate themselves at the
edge of the stripes. Thus, the boundaries of the stripes have a higher concentration of the
nanoparticles, and will create a higher magnetic field in these regions of the stripes. Therefore, when
the stripe is pulled over the sensors, considering this nom-uniformity in the vertically magnetized
particles concentration over the stripe, two spikes appears close to the boundaries of the stripe (Figure
7.16). Therefore the shape of the curve is concordant with the prediction made in Chapter 3.
Stripe width
63
8. Conclusions and Future work
During this thesis several samples of MTJs connected in series were patterned. TMRs off about
70% were achieved as well as samples with sensibilities of 1.2%/Oe. It was proved the great
robustness of these devices from which we can get a Voltage output of the order of some tenth of
Volts without occur their disruption.
Being an ongoing work it still requires a fully noise characterization (Sv vs #junctions , Sv vs
Vbias) and the pattern of new samples with improved TMR singnal and Sensibility.
64
65
Bibliography
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novel devices,Wiley-VCH, 2nd edition, 2005. [4] A. Fert, et al., Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices, Phys. Rev.
Letter 61(21): 2472-2475, 1988. [5] B. Dieny, et al., Giant magnetoresistance in soft ferromagnetic multilayers, Phys. Rev. B 43(1):
1297-1300, 1991. [6] M. Julliere, Tunneling between ferromagnetic films, Phys. Lett. A 54: 225, 1975. [7] P. Grunberg, et al., Enhanced Magnetoresistance in layered magnetic structures with
antiferromagnetic interlayer exchange, Phys. Rev. B 39(7): 4828-4830, 1989. [8] J. C. Slonczewski, Conductance and exchange coupling of two ferromagnets separated by a
tunneling barrier, Phys. Rev. B 39: 6995, 1989. [9] D. Wang, et al., Spin dependent tunneling junctions with reduce Neel coupling, J.Appl.Phys.,
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Superlattices,Phys. Rev. Letter 67: 1602, 1991. [11] P.Wisniowski, et al., Effect of free layer thickness and shape anisotropy on the transfer curves
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[15] Jianguo Wang, Low-resistance tunnel junctions for read head applications, PhD thesis,
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67
A. Appendix
Figure .A.1: Mask use – Blue lines: top contacts; Yellow lines: define the shape of the contact pathways in the last passivation layer; in the lower right corner: two individual junctions, with a
reference role for comparison.
68
148µm
956µm
Figure A.2: Top: Series detail – Green: bottom contacts; Red: pillar junctions; Blue: top contacts Bottom: The entire array of MTJs, with intermediate contacts;.
Figure A.3: 3D scheme of MTJ’s connected in serious: the current coming from a top contact goes through a junction, in a descending direction until reach the bottom contact; flows in the bottom contact reaching the other junction and crosses it in the upper direction; now the current flows again in a top contact until the next junction and so on. The top contact, the bottom contact and the pillar junction correspond to the blue lines, green lines and red lines in the previous figure
69
B. Appendix
Run Sheet for MTJs Array fabrication: Influenza_ 2010
Process Start: / /2010 Process Complete: / /2010
Substrate: Si wafer / 800 Å SiO2 : ~1 inch2
Machine: Nordiko 2000
Magnetic Tunnel Junction Structure:
Bottom electrode: Ta 50 Å / Ru 180 Å / Ta 30 Å / PtMn 200 Å / CoFe 20 Å / Ru 9 Å / CoFeB 30 Å
Oxide Barrier: MgO x Å
Top electrode: CoFeB y Å / Ru 50 Å / Ta 50
Total height: 619 + x + y Å
Calibration samples: VSM
Top electrode for 2nd etch (glass substrate)
Machine sequences:
Seq. 3: Pre-sputtering of all targets x 2;
Seq. 38: Xianghiong
Seq. 39: FILIPE
Caracterization: VSM after annealed
STEP 1 Tunnel Junction Deposition
70
Step 1: Read Values
B.P:8.6x10-8 Torr
Seq. 3: pre sputtering of all targets x2
B.P:8.4x10-8 Torr
Sequence:
(#/name) Function : Read Values
38/Xianghiong
F18, Ta 50 40 mA | 331V | 10 W | 9.7 sccm | 4.6 mTorr | S4T3 | 100%
F7, Ru 180 40 mA | 302V | 10 W | 7.8 sccm | 5.1 mTorr | S4T2 | 100%
F6, Ta 30 40 mA | 329V | 10 W | 9.8 sccm | 4.6 mTorr | S4T3 | 100%