PAC-MAN SHAPED MAGNETIC TUNNEL JUNCTIONS

FOR SEU-RESISTANT CMOS-BASED MAGNETIC

FLIP FLOPS FOR SPACE APPLICATIONS

by

GAVIN SKY ABO

A THESIS

Submitted in partial fulfillment of the requirements for the degree of Master of Science

in the Department of Electrical and Computer Engineering in the Graduate School of

The University of Alabama

TUSCALOOSA, ALABAMA

2009

Copyright Gavin Sky Abo 2009 ALL RIGHTS RESERVED

ABSTRACT

Pac-man shaped magnetic tunnel junctions are proposed for CMOS-based magnetic flip

flops for space applications. Micromagnetic simulation was performed on single layer elongated

Pac-man shape, modified rectangular shape, and square shaped bilayer for magnetization

process. The experimental effect of CoFe and IrMn thickness on the exchange field and

coercivity of a Co90Fe10/Ir20Mn80 bilayer was studied. Metal mask process was used to fabricate

rectangular shaped MTJ devices and the devices were characterized for magnetoresistance.

In regards to micromagnetic simulation, the lowest coercivity of the elongated Pac-man

element was found at an applied field direction at 45° with respect to the long axis of the

element. Coherent switching of modified rectangular shapes was observed by simulation with a

base height from 0.25 μm to 0.5 μm. In addition, simulation results of a 7 μm square shaped

Ni80Fe20/antiferromagnetic bilayer are in fairly good agreement with experimental results for that

of a 10 μm square shaped Ni80Fe20/Ir20Mn80 bilayer.

Finally, CoFe thickness was found to be dominant in control of coercivity, while a

combined effect of both CoFe and IrMn thickness has a major role in controlling of the exchange

bias for a deposited CoFe/IrMn bilayer. The highest exchange of 87 Oe was achieved for

Co90Fe10(8.5 nm)/Ir20Mn80(17.5 nm). Magnetoresistance of four rectangular shaped MTJ device

was measured to be 21% on average, but average resistance of the devices was 27 Ω. The

resistance does not yet meet the targeted 10 kΩ.

ii

LIST OF ABBREVIATIONS AND SYMBOLS

CoFe Alloy of cobalt and iron

IrMn Alloy of iridium and manganese

Al Aluminum

AlOx Aluminum oxide

A Ampere

AFM Antiferromagnetic

~ Approximately

Ar Argon

CMOS Complementary metal oxide semiconductor

Cu Copper

° Degree

DI Deionized

DOE Design of experiment

α Dimensionless dampening constant

Heff Effective field

ECE Electrical and Computer Engineering

E-beam Electron-beam

EPM-I Elongated Pac-man type I

EPM-II Elongated Pac-man type II

γ Gyromagnetic ratio

iii

hr Hour

ICDD International Centre for Diffraction Data

IBE Ion beam etching

IPA Isopropyl alcohol

K Kelvin

kΩ Kilo-ohm

LLG Landau Lifshitz Gilbert

MMDL Magnetic Materials and Device Laboratory

MTJ Magnetic tunnel junction

M Magnetization

MOKE Magneto-optical Kerr effect

MR Magnetoresistance

MΩ Mega-ohm

MMJ Metal mask junction

m Meter

μm Micro-meter

mA Milli-ampere

mtorr Milli-torr

nm Nano-meter

N2 Nitrogen

OOMMF Object oriented micromagnetic framework

Ω Ohm

O2 Oxygen

iv

PM Pac-man

PM-I Pac-man type I

PM-II Pac-man type II

% Percent

Py Permalloy or Ni80Fe20

PDF Portable Document Format

R Resistance

RA Resistance-area

rpm Revolution per minute

Ms Saturation magnetization

Si Silicon

SiO2 Silicon dioxide

SEU Single event upset

sccm Standard cubic centimeters per minute

SFD Switching field distrobution

Ta Tantalum

TR-SKEM Time-resolved scanning Kerr electron microscope

2D Two-dimensional

uv Ultraviolet

V Volt

W Watt

v

ACKNOWLEDGEMENTS

I would like to thank my advisor Dr. Yang-Ki Hong for his advice and support to perform the

research of this thesis. I would also like to thank my committee members, Dr. Robert Scharstein

and Dr. Byoung-Chul Choi, for their suggestions and guidance for the work of this thesis.

I would like to thank my co-workers of the MMDL (Magnetic Materials and Device

Laboratory, University of Alabama ECE Department) both past and present: Dr. Mun-Hyoun

Park, Dr. Sung-Hoon Gee, Dr. Hongmei Han, James Jabal, Dr. Seok Bae, Jeevan Jalli, Andrew

Lyle, Jae-Jin Lee, Jihoon Park, Kobina Essiam, Ryan Syslo, Nicholas Neveu, Stephanie Mizzell,

and Byron Wong.

I thank my mom Joetta and dad Larry for their love and support in reaching my goals.

My inspiration and perseverance goes to my brother Cole and grandpa Roy. In memory and

dedication to my grandmas, who always wished me the best in my endeavors.

vi

CONTENTS

ABSTRACT............................................................................................................ ii

LIST OF ABBREVIATIONS AND SYMBOLS .................................................. iii

ACKNOWLEDGEMENTS................................................................................... vi

LIST OF TABLES................................................................................................. ix

LIST OF FIGURES .................................................................................................x

1.0 INTRODUCTION .............................................................................................1

1.1 Objectives ..............................................................................................1

1.2 Previous Work .......................................................................................1

1.3 Definition and Notation of Pac-man Shapes..........................................2

2.0 MICROMAGNETIC SIMULATION ...............................................................5

2.1 Simulation of EPM-I for Angular Dependence .....................................5

2.2 Simulation of Modified Rectangles .......................................................6

2.3 Simulation of Square Shaped Py/AFM Bilayer .....................................9

3.0 BILAYER EXPERIMENT..............................................................................12

3.1 Experiment for Bilayer ........................................................................12

3.2 X-ray Diffraction of Bilayer ................................................................14

4.0 METAL MASK JUNCTIONS ........................................................................16

4.1 Experimental ........................................................................................16

4.2 Results..................................................................................................18

vii

5.0 SEMICONDUCTOR-BASED MTJ FABRICATION PROCESS..................20

6.0 CONCLUSION................................................................................................24

6.1 Future Work .........................................................................................24

REFERENCES ......................................................................................................26

Appendix A. A mif File for Angular Field Dependence on Coercivity Simulation

(45°) .......................................................................................................................28

Appendix B. A mif File for the Modified Recthagle Shape Simulation (0.250) μm

................................................................................................................................29

Appendix C. Simulation Parameter for Py/AFM Bilayers ....................................30

Appendix D. RA Product Calculation for Metal Mask and PM MTJ ...................31

Appendix E. List of Publications for Gavin S. Abo ..............................................32

Appendix F. List of Presentation for Gavin S. Abo...............................................34

viii

LIST OF TABLES

Table I. Generated and collected data for bilayer DOE.........................................13

ix

LIST OF FIGURES

Figure 1. Definition of PM-I, PM-II, EPM-I, and EPM-II. ....................................3

Figure 2. Angular field dependence on coercivity of EPM-I. .................................6

Figure 3. Modified rectanglar shapes for studying end shape variation. ................6

Figure 4. Simulated hysteresis loop and definition of modified rectangular shape.8

Figure 5. Base height dependence on coercivity for modified rectangular shapes..8

Figure 6. (a) Experimental MOKE hysteresis loop of 10 μm square shaped

Py/IrMn bilayer by (Choi, 2009). (b) Simulated hysteresis loops of 0.5 μm to

7 μm square shaped Py/AFM bilayer. ...................................................................10

Figure 7. Snapshots of magnetization for the kinks of the simulated hysteresis

loops. .....................................................................................................................11

Figure 8. Hysteresis loop of one run for the DOE. ...............................................13

Figure 9. Pareto chart of bilayer DOE. .................................................................14

Figure 10. XRD spectra of blanket bilayer of CoFe-IrMn. ...................................15

Figure 11. Deposition process for the metal mask junctions. ...............................18

Figure 12. MR curves of four metal mask MTJ. ...................................................19

Figure 13. Semiconductor-based fabrication process of a magnetic tunnel

junction. .................................................................................................................20

Figure 14. Patterned EPM-I resist by e-beam lithography. ..................................22

Figure 15. Sketch of a magnetoelectric random access memory cell. (Bibes, 2008)

................................................................................................................................25

x

1.0 INTRODUCTION

In section 1.1, the research project objectives are given. In section 1.2, the previous work

for this project is summarized. The definition of Pac-man shapes is given in section 1.3.

Micromagnetic simulation of single layer elongated Pac-man shape, modified rectangular

shape, and square shaped bilayer for magnetization process is given in section 2.0. Bilayer

experiment and characterization is reported in section 3.0. The fabrication and characterization

of rectangular shaped MTJ with metal masks is described in section 4.0. The semiconductor-

based fabrication process of Pac-man shaped MTJ is described in section 5.0. Outcomes and

future work are recited in section 6.0.

1.1 Objectives

The objectives of this thesis are to fabricate and characterize MTJ with MR

greater than 15% and R of 10 kΩ.

0.720 1801.4E( PM )

1.2 Previous Work

The research work that ended up in establishment of this project was the study of sub-micron

patterned magnetic elements. The result of the study on magnetic elements with linear

(rectangular, ellipse shapes) and circular (disk, ring shapes) magnetization modes led to the

development of the Pac-man1 shaped element (Park, 2005).

1”Pac-man” is a registered trademark of Namco Corporation

1

A circuit design for CMOS-based magnetic flip flops that are SEU (single event upset)

resistant to radiation for space applications was laid out (Hass, 2007), which included the Pac-

man shaped MTJ.

In order to realize the circuit design, further study of the Pac-man shaped element and

MTJ (magnetic tunnel junction) by simulation, fabrication, and characterization was needed.

Therefore, micromagnetic simulations focusing on magnetization dynamics for an ultrafast

switching Pac-man shaped element was studied (Jabal, 2008). Whereas, fabrication and

characterization of PM (Pac-man) shaped MTJ was performed (Han, 2008). The continuation of

this dissertation work for approaching the objectives in section 1.1 is the content of this thesis.

Of note, during the previous work that focused on magnetic field switching of the

magnetic element in MTJ, a great interest was also taken in spin-transfer torque switching in

spin-valve junction. Thus, simulation and fabrication of Pac-man shaped and nanopillar spin-

valves were investigated (Andy, 2009).

1.3 Definition and Notation of Pac-man Shapes

There are four different PM shapes (Park, 2005). They are PM-I, PM-II, EPM(elongated Pac-

man)-I, and EPM-II as defined in Figure 1. The Pac-man’s slot angle is the angle between the

two lines drawn from the center (or slot end) of an outer-diameter circle to form the two slot

heads. The left-side PM-I and PM-II are shown with a slot angle of 45°. The right-side PM-I

and PM-II are shown with a slot angle of 180° before (solid line) and after (dashed line)

elongation. The lines drawn to form the slot angle are the slot lines for PM-I. On the other hand,

the slot lines for PM-II are defined as two lines from the slot heads that connect tangent to an

2

Figure 1. Definition of PM-I, PM-II, EPM-I, and EPM-II.

imaginary inner-circle. For EPM (EPM-I or EPM-II), it can be convenient to describe its long

and short lateral dimension as a length and width, respectively.

The general notation used to describe the PM-I, PM-II, EPM-I, and EPM-II shapes (Park,

2005) is

ODID anglexE( PM ) ,

where “x” is the elongation ratio with E denoting elongation, “OD” is the outer-diameter in μm,

“ID” is the imaginary inner-diameter in μm, and “angle” is the slot angle of the PM in degrees.

The “xE” and brackets are dropped if the PM is not elongated. If ID = 0, then the notation

describes a PM-I, else a non-zero ID describes a PM-II. The length of the elongated element is

the outer-diameter multiplied by the elongation ratio, while the width is one-half the outer-

diameter. The PM is elongated along the length direction to provide shape anisotropy.

3

The shape was chosen for the MTJ due to possibly narrow SFD (Park,

2003), single domain probability of 90%, and lower coercivity than EPM-II with the same

footprint (Park, 2004).

0.720 1801.4E( PM )

4

2.0 MICROMAGNETIC SIMULATION

Micromagnetic simulation was performed on single layer shape, modified

rectangular shape of CoFe, and on square shaped Py/AFM bilayer. The OOMMF 1.2a3

simulator (Donahue, 1999) is used for the single layer 2D simulations of sections 2.1 and 2.2.

The LLG Micromagnetics Simulator (Scheinfein, 2009) is used for the bilayer simulations of

section 2.3. All simulations were at 0 K.

0.720 1801.4E( PM )

Both simulators solve the Landau-Lifshitz equation by numerical integration, which is

given below.

(eff effM M H M M H

s

ddt M

γ αγ= − × − × × ) , (1)

where is the magnetization [A , M /m] sM is the saturation magnetization [A , is the

effective field [A ,

/m] effH

/m] γ is the gyromagnetic ratio [m/(A seconds)]⋅ , and α is the dimensionless

dampening constant.

2.1 Simulation of EPM-I for Angular Dependence

Single layer CoFe of shape was simulated for angular field dependence on

coercivity, as shown in Figure 2. The simulation parameters for angular field of 45° are given in

Appendix A. All parameters were kept the same for the other simulations, except the field

values for changing the applied field direction.

0.720 1801.4E( PM )

The lowest coercivity was found when the uniform magnetic field was applied at 45° to

the element. The switching is coherent for angle less than or equal to 45° and becomes non-

5

coherent for angle greater than 45°. The hysteresis loops becomes asymmetric somewhere

greater than 75° due to a transverse domain wall forming in the long axis of the element.

Figure 2. Angular field dependence on coercivity of EPM-I.

2.2 Simulation of Modified Rectangles

To study the effect of end shape variation, micromagnetic simulation was performed by

modifying the single layer rectangular shape as shown in Figure 3.

6

Figure 3. Modified rectangular shapes for studying end shape variation.

The definition used to modify the rectangular shape is shown on the right in Figure 4. The base

height is varied from 0 μm to 0.5 μm, which changes the shape from a rectangle to diamond as

was shown in Figure 3. The simulated hysteresis loops for these CoFe elements are shown on

the left in Figure 4, where the applied field was set at 45° to the element. The parameters used

for the simulation are given in Appendix B.

The base height dependence on cocercivity is shown in Figure 5 for the modified

rectangular elements. The modified rectangles were designed with the same footprint as the

shape for comparison. The shape has a higher coercivity than

the modified rectangular shapes. The switching of the modified rectangular shape is found to be

non-coherent as indicated by the hysteresis loops of Figure 4 for a base height less than 0.25 μm.

From 0.25-0.5 μm, the switching is found to be coherent.

0.720 1801.4E( PM ) 0.72

0 1801.4E( PM )

7

Figure 4. Simulated hysteresis loop and definition of modified rectangular shape.

Figure 5. Base height dependence on coercivity for modified rectangular shapes.

8

2.3 Simulation of Square Shaped Py/AFM Bilayer

Experimental hysteresis loop is shown in Figure 6 (a) of a 10 μm square shaped Ni80Fe20(12

nm)/Ir20Mn80(5 nm) bilayer (Choi, 2009). The bilayer was field cooled at 50 Oe to obtain an

exchange field of 12 Oe. The inset in Figure 6 (a) shows a measured snapshot of the

magnetization by TR-SKEM at an external field of 0 Oe.

Figure 6 (b) shows the simulated hysteresis loops of 0.5 μm to 7 μm square elements for

an exchange field of 12 Oe. The coercivity decreases as its square shape gets larger. The inset

in Figure 6 (b) shows the snapshot of the 7 μm square at an external field of 0 Oe, which is

similar to that of the 10 μm experimental results.

For all simulations, a 12 nm thick square Py layer was used. The exchange field is then

modeled by adding a uniform pinning field of 12 Oe to the Py layer. The parameters used for the

simulation are given in Appendix C.

9

Figure 6. (a) Experimental MOKE hysteresis loop of 10 μm square shaped Py/IrMn bilayer

(Choi, 2009). (b) Simulated hysteresis loops of 0.5 μm to 7 μm square shaped Py/AFM bilayer.

Figure 7 shows snapshots of the circled kinks in Figure 6, which show the vortex

configuration except for the 3 μm square shaped bilayer where an S-state is seen.

10

Figure 7. Snapshots of magnetization for the kinks of the simulated hysteresis loops.

11

3.0 BILAYER EXPERIMENT

3.1 Experiment for Bilayer

Minitab 15 software was used to determine the effect of CoFe and IrMn thickness on exchange

bias and coercivity of a Co90Fe10/Ir20Mn80 bilayer. The Minitab software was setup for a 2-level

factorial design with 2 factors, two center points, a random base generator of 1, CoFe thicknesses

from 5 to 12 nm, and IrMn thickness from 15 to 20 nm with other settings default.

The bilayer films were deposited on oxidized Si substrates (18 mm x 18 mm) in a

magnetron sputtering system under the same film conditions described in section 4.1. The

blanket bilayer films were characterized by a B-H loop tracer for their magnetic properties. The

exchange field and coercivity were extracted by calculation using linear interpolation.

One of the hysteresis loops obtained from B-H loop tracer measurement is shown in

Figure 8.

12

Figure 8. Hysteresis loop of one run for the DOE.

The exchange field and coercivity results are summarized in Table I. The highest exchange field

of 87 Oe was observed for a Co90Fe10(8.5 nm)/Ir20Mn80(17.5 nm) bilayer.

Table I. Generated and collected data for bilayer DOE.

13

The Pareto chart is shown in Figure 9. The results indicate that the dominate effect on

the exchange field is a combined effect of both the CoFe and IrMn thickness. The dominant

effect on coercivity is determined to be CoFe thickness.

Figure 9. Pareto chart of bilayer DOE.

3.2 X-ray Diffraction of Bilayer

Figure 10 shows the XRD spectra obtained from the bilayer of Co90Fe10(8.5 nm)/Ir20Mn80(17.5

nm).

The β-Ta (200) peak at 33° indicates good ordering with the Si substrate. The Ta peak is

a combination of different Ta orientations around 34°. These peaks correspond well to known

results (Liu, 2003).

The IrMn (111) peak is important for obtaining exchange bias, and it is observed at 41°.

There is also an IrMn (200) peak that correlates closely with an article (Peng, 2007). A Si (400)

peak at 69.2° is commonly reported in published literature. An error peak due to reflection is the

Si (400) peak at 61.7°, which is reported (Akazawaa, 2004).

14

The peak at 65.9° is associated with CoFe (200) (Kanak, 2009). The peak at 68.8° is

indexed to IrMn (220) with ICDD card for PDF # 03-065-4062, while Ta (211) at 69.4° is

closely matched from ICDD card for PDF # 00-004-0788.

Figure 10. XRD spectra of blanket bilayer of CoFe-IrMn.

There are a couple of unknown peaks at 116.6° and 117.1°. These might be different

orientations of IrMn or Ta. The ICDD card, PDF # 03-065-4062, for IrMn shows (223) at

116.330°. The ICDD card, PDF # 00-004-0788, for Ta (321) is 121.349°. This could also be

error peaks from the system near the end of the scan due to the obtuse angle near 120°.

15

4.0 METAL MASK JUNCTIONS

Metal mask junctions (MMJs) were fabricated and characterized for magnetoresistance. The

rectangular 300 μm x 150 μm MMJs were fabricated based on reported results (Sato, 1997).

4.1 Experimental

A Si wafer having a layer of thermally grown SiO2 was scribed and broken into 18 mm x 18 mm

square substrates. The substrates were placed in an acetone filled beaker. Then, the beaker was

placed into an ultrasonic bath for 15 minutes. The substrates were transferred to an ethanol filled

beaker, and the beaker was submerged in an ultrasonic bath for 15 minutes. Finally, the

substrates were placed in a fresh beaker of ethanol for storage and use. This solvent cleaning

process removes particles and contamination generated during the cutting.

The deposition process of the MMJs is given by Figure 11. A metal mask with a 300 μm

wide line is applied to a cleaned substrate. The substrate is placed on a substrate holder with

permanent magnets with an in-plane field of 350-510 Oe, and the line of the mask is oriented

perpendicular to the applied field. DC magnetron sputtering is used to deposit all films. The

Ta(5 nm)/Co90Fe10 (13 nm) films are deposited. The vacuum is then broken to remove the 300

μm metal mask and a circular mask is applied to the junction before deposition of 2 nm thick Al

and RF plasma oxidation of the Al. The vacuum is broken a second time to apply a 150 μm

metal mask parallel to the applied field for the final deposition of Co90Fe10(6 nm)/Ir20Mn80(14

nm)/Ta(5 nm). The MTJ stacking is based on reported results (Song, 2005).

16

The working pressure and Ar gas flow rate for deposited films were 1 mtorr and 12 sccm,

respectively. The base pressure was less than 1 x 10-7 torr. All films were deposited with 40 W,

except Al with 30 W, from 2” targets. The distance between the target and substrate center was

fixed at roughly 152 mm, where the angle of the guns are all fixed at 10° from horizontal. The

substrate holder was biased with 4 W of RF power with 12 sccm of Ar and 4 sccm of O2 for 25

seconds for the plasma oxidation process. This was also performed at a process pressure of 50

mtorr. The substrate holder was rotating at 20 rpm for all depositions and for plasma oxidation.

MR curves are obtained from an MR Tester by sourcing current and measuring voltage

( / ) in a sweeping magnetic field. The MR [measured sourcedR V I= max min min( ) /R R R= − ] was first

defined (Julliere, 1975).

17

Figure 11. Deposition process for the metal mask junctions.

4.2 Results

The measured MR curves of four 300 μm x 150 μm junction are shown in Figure 12. They have

an average MR of 21%, which meets the objective of MR greater than 15%. However, the

average resistance of the four junction is 27 Ω rather than the needed 10 kΩ for impedance

matching. Correspondingly, the targeted RA product of the junction is 450 MΩμm2, but the

average RA product was 1.2 MΩμm2.

18

Figure 12. MR curves of four metal mask MTJ.

19

5.0 SEMICONDUCTOR-BASED MTJ FABRICATION PROCESS

The semiconductor-based MTJ fabrication process is shown in Figure 13.

Figure 13. Semiconductor-based fabrication process of a magnetic tunnel junction.

20

A Si/SiO2 wafer is cut into 18 mm x 18 mm square substrates. The substrates are cleaned by the

solvent process of section 4.1. The blanket MTJ stack of Ta(20 nm)/Co90Fe10(13 nm)/AlOx(2

nm)/Co90Fe10(6 nm)/Ir80Mn20(14 nm)/Ta(5 nm) is deposited by DC magnetron sputtering using

the same film deposition parameters in section 4.1. A positive 1818 photoresist is spin coated for

30 seconds at 2,500 rpm. The ~2.5 μm positive photoresist is soft baked for 30 minutes at 100°C

in an oven. The substrate and photomask with our electrode pad design are oriented so that the

bottom electrode on the mask and the set magnetic anisotropy direction in the sample are

parallel. The photoresist is then exposed for 6 seconds using a mask aligner.

The 1818 photoresist is developed in DI water for 60 seconds, Microposit Developer

Concentrate for 75 seconds, and DI water 90 seconds followed by drying with N2 gas. The

photoresist is hard baked for 30 minutes at 115°C. A 2 minute plasma ash at 300 W and 1 torr is

used to remove residual resist.

The electrode pad is formed by IBE using a 90° etch with recipe 7 (40 V discharge

voltage, 40 mA beam current, 250 V beam voltage, 100 V accelerator voltage, 4 mA neutralizer

current), 10 sccm Ar, 20 rpm platen rotation, and base pressure of approximately 3 x 10-7 torr.

Thermal grease is applied to the back of the substrate before all IBE steps.

A 3 minute plasma ash at 300 W and 1 torr is used to break the resist crust. The

photoresist is lifted off by ultrasonic in acetone for 20 minutes. A 2 minute plasma ash at 300 W

and 1 torr is used to remove residual resist. The Ma-N 2405 e-beam resist is spin coated at 4500

rpm for 40 seconds to obtain ~380 nm thickness. A hotplate is used to soft bake the resist for 60

seconds at set temperature of 90°C (surface ~75°C). E-beam lithography is used to pattern an

array of 8 or 64 Pac-man shaped elements on top of the electrode pad. The e-beam resist is than

developed for 30 seconds in MF-CD-26, 20 seconds in IPA, and 20 seconds in DI water. Figure

21

14 shows the patterned e-beam resist of , which demonstrates its successful

patterning.

0.720 1801.4E( PM )

Figure 14. Patterned EPM-I resist by e-beam lithography.

A hard bake of the e-beam resist is done in an oven for 10 minutes at 100°C. The MTJ

stack is ion beam etched with the same condition as the electrode pad. A metal mask with

circular cut-outs is applied to the substrate (similar to how the circle metal mask was used for the

Al deposition). Then, RF magnetron sputtering is used to deposit 93 nm thick SiO2 at 200 W (2”

target) with the same process parameters as the other sputtered films.

A 2 minute plasma ash at 300 W and 1 torr is used to break the resist crust. The e-beam

resist is removed by ultrasonic in acetone for 20 minutes, followed by rubbing with a q-tip in

acetone, ultrasonic for 5 minutes, and a 2 minute plasma ash at 300 W and 1 torr. A dehydration

bake in an oven for 1 hr at 115°C is performed, which is critical for adhesion of the 5214-IR

photoresist. The 5214-IR photoresist is spin coated at 2,500 rpm. After 60 seconds, the ~1.7 μm

22

photoresist is placed into an oven at 100°C for 10 minutes to soft bake the resist. The electrode

pad mask again oriented with the sample for forming the top electrode. The uv light is applied

for 2.5 seconds for exposure. The resist is then placed in an oven at 100°C for 10 minutes for a

reversal bake. A flood exposure is performed for 7 seconds. The photoresist is developed in DI

water 60 seconds, MF-319 developer 60 seconds, and DI water 90 seconds. A hard bake is

performed for 30 minutes at 115°C in an oven.

A 2 minute plasma ash at 200 W and 1 torr is used to cleanup the residual resist. E-beam

evaporation is used to deposit Cu at 0.1 nm/s for the 120 nm thick top electrode. The lift-off of

the photoresist is performed by ultrasonic for 20 minutes in acetone. A 2 minute plasma ash at

300 W and 1 torr is used to remove residual resist. The sample is finished and is ready for MR

and RA product characterization, but no signal was observed due to shorting of the electrodes.

The cause of the shorting is still under investigation. Assuming a constant RA product of 1.2

MΩμm2, a junction area of 120 μm2 is needed to meet a resistance of 10 kΩ.

23

7.0 CONCULSION

Micromagnetic simulation results showed the lowest coercivity at an applied field

direction at 45° with respect to the long axis of the shape. Coherent switching of

modified rectangular shapes was observed with a base height from 0.25 μm to 0.5 μm.

Simulation results of a 7 μm square shaped Ni

0.720 1801.4E( PM )

80Fe20/AFM bilayer are in fairly good agreement

with experimental results for that of a 10 μm square shaped Ni80Fe20/Ir20Mn80 bilayer.

Experimentally, CoFe thickness was found to mainly control coercivity, while a

combined effect of both CoFe and IrMn thickness is dominant on the exchange bias for a

Co90Fe10/Ir20Mn80 bilayer. The highest exchange of 87 Oe was achieved for Co90Fe10(8.5

nm)/Ir20Mn90(17.5 nm). Magnetoresistance of four rectangle MTJ device was measured to be

21% on average, but average resistance of the devices was 27 Ω. The resistance does not yet

meet the targeted 10 kΩ.

7.1 Future Work

Future work will be to develop a MERAM (magnetoelectric random access memory) similar to

that shown in Figure 15 (Bibes, 2008).

24

Figure 15. Sketch of a magnetoelectric random access memory cell. (Bibes, 2008)

25

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Bibes, M. & Barthelemy, A. (2008). Multiferroics: Towards a Magnetoelectric Memory. Nature

Materials, 7, 425-426.

Choi, B. C. (2009). Unpublished data.

Donahue, M. J., & Porter, D. G. (1999). OOMMF User's Guide, Version 1.0. National Institute

of Standards and Technology, Gaithersburg, MD, NISTIR 6376.

Han, H. (2008). Fabrication and Characterization of Pac-man shaped magnetic tunneling

junctions. Dissertation Abstracts International, 70, 1. (UMI No. 3347535).

Hass, K. J. (2007). Radiation-tolerant Embedded Memory using Magnetic Tunnel

Junctions. Dissertation Abstracts International, 68, 1. (UMI No. 3265567).

Jabal, J. F. (2008). Micromagnetic Simulation of Ni80Fe20 Pac-man Shaped Elements for

Magnetic Random Access Memory Applications. Unpublished master’s thesis,

University of Idaho - Moscow.

Julliere, M. (1975). Tunneling Between Ferromagnetic Films. Phys. Lett., 54A, 225-226.

Kanak, J., Wisniowski, P., Zaleski, A., Powroznik, W., Stobiecki, Cao, J., et al. (2009, October

22). Crystallization of CoFeB Electrodes in Magnetic Tunnel Junctions. Retrieved

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Poster_Spinwork_JK.jpg

26

Liu, H. R., Ren, T. L., Qu, B. J., Liu, L. T., Ku, W. J., & Li W. The Optimization of Ta Buffer

Layer in Magnetron Sputtering IrMn Top Spinvalve. Thin Solid Films, 441, 111-114.

Lyle, A. (2009). Fabrication and Simulation of Deep Sub-micron Spin Valve Elements: Pac-man

and Nanopillar. Unpublished master’s thesis, University of Alabama - Tuscaloosa.

Park, M. H. (2005). Electron-beam Patterned Sub-micron Magnetic Elements and

Switching Mechanisms. Dissertation Abstracts International, 66, 3359. (UMI No.

3178908).

Park, M. H., Hong, Y. K., Gee, S. H., Erikson, D. W., Choi, B. C. (2003). Magnetization

Configuration and Switching Behavior of Submicron NiFe Elements: Pac-man Shape.

Appl. Phys. Lett., 83, 79-81.

Park, M. H., Hong, Y. K., Gee, S. H., Erikson, D. W., Tanaka, T. (2004). Effect of Shape

Anisotropy on Switching Behaviors of Pac-man NiFe Submicron Elements. J. Appl.

Phys., 95, 7019-7021.

Peng, T. Y., Chen, S. Y., Lo, C. K., Yao, Y. D. (2007). Enhancement of Exchange Field in

CoFe/IrMn by Os/Cu Buffer Layer. J. Appl. Phys., 101, 09E514.

Sato, M. & Kobayashi, K. (1997). Spin-valve-like Properties of Ferromagnetic Tunnel

Junctions. Jpn. J. Appl. Phys., 36, L200-L201.

Scheinfein, M. R. (2009, October 23). LLG Micromagnetic SimulatorTM. Retrieved

October 23, 2009, from http://llgmicro.home.mindspring.com

Song, J. O., Lee, S. R., and Shin, H. J. (2005). Band Structure Modification of Al Oxide by Ti-

alloying and Magnetoresistance Behavior of Magnetic Tunnel Junctions with Ti-alloyed

Al Oxide Barrier. Appl. Phys. Lett., 86, 252501.

27

Appendix A. A mif File for the Angular Field Dependence on Coercivity Simulation (45°)

Ms: 1680000 A: 1.9999999999999999e-11 K1: 56000 Edge K1: 0 Anisotropy Type: uniaxial Anisotropy Dir1: 0 1 0 Anisotropy Dir2: 0 1 0 Anisotropy Initialization: Constant Do Precess: 1 Gyratio: 38300 Damp Coef: 0.017999999999999999 Demag Type: ConstMag Part Width: 3.5999999999999999e-07 Part Height: 9.9999999999999995e-07 Part Thickness: 1e-08 Cell Size: 1e-08 Part Shape: Mask /home/gavin/epm1l36wcofer03/epm1l36wcofer03.ppm Init Mag: Uniform 90 90 Field Range: 0.1 0.1 0 -0.1 -0.1 0 250 Field Range: -0.1 -0.1 0 0.1 0.1 0 250 Default Control Point Spec: -torque 1e-4 Field Type: Uniform Base Output Filename: /home/gavin/epm1l36wcofer03/out/epm1l36wcofer03 Magnetization Output Format: binary 4 Total Field Output Format: binary 4 Data Table Output Format: %.16g Randomizer Seed: 0 Min Time Step: 0 Max Time Step: 0 User Comment: 1 um x 0.36 um elongated Pac-man of 10 nm thickness

28

Appendix B. A mif File for the Modified Rectangle Shape Simulation (0.250 μm)

# MIF 1.1 # Material Name:CoFe Ms:1680e3 A:2e-11 K1:5.6e4 Anisotropy Type:uniaxial Anisotropy Dir1:0 1 0 Anisotropy Init:Constant Do Precess:1 Gyratio:3.83e4 Damp Coef:0.018 Demag Type:ConstMag Part Width:360e-9 Part Height:1000e-9 Part Thickness:10e-9 Cell Size:10e-9 Part Shape:Mask /home/gavin/rectp250/rectp250.ppm Init Mag:Uniform 90 90 Default Control Point Spec:-torque 1e-4 Field Range: 0.1 0.1 0 -0.1 -0.1 0 250 Field Range: -0.1 -0.1 0 0.1 0.1 0 250 Field Type:Uniform Base Output Filename: /home/gavin/rectp250/out/rectp250 User Comment: rectangle pointed

29

Appendix C. Simulation Parameter for Py/AFM Bilayers

• Initial Magnetization: Vortex Z-Plane • Predictor Corrector with α=0.01 • Disabled Iterations-max to change stopping condition to convergence stopping condition

of 1e-4 • Layer 1: Permalloy

– Ms = 800 emu/cm3 – A = 1.05 μerg/cm – Ku2 = 1000 erg/cm3 – Rho = 15 μohm-cm – Uniaxial Anisotropy – Ainterlayer(1->0)= Ainterlayer(1->2)= 0

• Layer 2: Vacuum (IrMn) except for 7 μm simulation – Rho = 999,999 μohm-cm

• Uniform Hysteresis Loop – Max field 250 Oe with 27 points (i.e., sweep 250 to -250 Oe in ~19.2 Oe steps)

• X = 0.5, 1, 2, 3, 5, or 7 μm, Y = 0.5, 1, 2, 3, 5, or 7 μm, Z = 17 nm • Nx = 50, 100, 200, 300, 500, or 700; Ny = 50, 100, 200, 300, 500, or 700; Nz = 2 or 1

for 7 μm simulation (i.e., cell size 10 nm x 10 nm x 8.5 nm [Z/Nz]) • Pinning Hx = 12 to layer 1

30

Appendix D. RA Product Calculation for Metal Mask and PM MTJ

RA R A= ×

Goal resistance: 10R k= Ω Metal mask junction area: 2300 150 45A m m k mμ μ μ= × =

2 210 45 450RA R A k k m M mμ μ= × = Ω× = Ω PM MTJ area: 21 0.36 0.36A m m mμ μ μ≈ × =

2 210 0.36 3.6RA R A k m k mμ μ= × = Ω× = Ω

Metal metal mask average RA product: 21.2 M mμΩ

221.2 120

10RA M mA mR k

μ μΩ= = =

Ω

31

Appendix E. List of Publications for Gavin S. Abo

1. Bae, S., Hong, Y. K., Lee, J. J., Jalli, J., Abo, G. S., Lyle A., et al. (2009). High Q Ni-Zn-Cu

Ferrite Inductor for On-Chip Power Module. IEEE Transactions on Magnetics, 45,

4773-4776.

2. Lee, J. J., Hong, Y. K., Bae, S., Jalli, J., Abo, G. S., Seonug W. M., et al. (2009). Broadband

NixZn0.8-xCu0.2Fe2O4 Electromagnetic Absorber for 1 GHz Application. IEEE

Transactions on Magnetics, 45, 4230-4233.

3. Bae, S., Hong, Y. K., Lee, J. J., Jalli, J., Abo, G. S., Sung W. M., et al. (2009). Co2Z

Hexaferrite T-DMB Antenna for Mobile Phone Applications. IEEE Transactions on

Magnetics, 45, 4199-4202.

4. Jalli, J., Hong, Y. K., Bae, S., Abo, G. S., Lee, J. J., Sur, J. C., et al. (2009). Conversion of

Nano-Sized Spherical Magnetite to Spherical Barium Ferrite Nanoparticles for High

Density Particulate Recording Media. IEEE Transactions on Magnetics, 45, 3590-3593.

5. Bae, S., Hong, Y. K., Lee, J. J., Jalli, J., Abo, G. S., Lyle, A., et al. (2009). New Synthetic

Route of Z-Type (Ba3Co2Fe24O41) Hexaferrite Particles. IEEE Transactions on

Magnetics, 45, 2557-2560.

6. Lyle, A., Hong, Y. K., Choi, B. C., Abo, G. S., Park, M. H., Gee, S. H., et al. (2009). Spin-

Polarized Current Switching of Co/Cu/Py Elongated Pac-Man Spin-Valve. IEEE

Transactions on Magnetics, 45, 2367-2370.

7. Bae, S., Hong, Y. K., Lee, J. J., Jalli, J., Abo, G. S., Lyle, A., et al. (2009). Low Loss Z-type

32

Barium Ferrite (Co2Z) for Terrestrial Digital Multimedia Broadcasting Antenna

Application. J. Appl. Phys., 105, 07A515.

8. Lee, J. J., Bae, S., Hong, Y. K., Jalli, J., Abo, G. S., Seong, W. M., et al. (2009). Novel Ni-

Mn-Co Ferrite for Gigahertz Chip Devices. J. Appl. Phys., 105, 07A514.

9. Jalli, J., Hong, Y. K., Bae, S., Lee, J. J., Abo, G. S., Lyle, A., et al. (2009). Growth and

Characterization of 144 μm Thick Barium Ferrite Single Crystalline Film for Microwave

Device Application. J. Appl. Phys., 105, 07A511.

10. Jalli, J., Hong, Y. K., Gee, S. H., Bae, S., Lee, J. J., Sur, J. C., et al. (2008). Magnetic and

Microwave Properties of Sm-doped SrFe12O19 Single Crystals. IEEE Transactions on

Magnetics, 44, 2978-2981.

11. Bae, S., Hong, Y. K., Lee, J. J., Abo, G. S., Jalli, J., Lyle, A., et al. (2008). Optimized

Design of Low Voltage High Current Ferrite Planar Inductor for 10 MHz On-chip Power

Module. Journal of Magnetics, 13, 37-42.

12. Han, H., Hong, Y. K., Park, M. H., Choi, B. C., Gee, S. H., Jabal, J. F., et al. (2005).

Interaction Effect on Switching Behaviors of Paired “Pac-man” Array. IEEE

Transactions on Magnetics, 41, 4341-4343.

33

Appendix F. List of Presentations for Gavin S. Abo

1. Lee, J. J., Hong, Y. K., Bae, S., Jalli, J., Abo, G. S., Seoung, W., et al. (2009, May).

Broadband NixZn0.8-xCu0.2Fe2O4 Electromagnetic Absorber for 1 GHz Application.

Presented at the IEEE International Magnetics Conference, Sacramento, CA.

2. Kum, J., Seong, W., Kim, G., Park, S., Ahn, W., Bae, S., et al. (2009, May). Low Loss

Ba2-xSrxCo2Fe12O22 Y-type Hexaferrite for 1 ~ 3 GHz Applications. Unpresented at the

IEEE International Magnetics Conference, Sacramento, CA.

3. Jalli, J., Hong, Y. K., Bae, S., Lee, J. J., Abo, G. S., Gee, S. H., et al. (2009, May). Conversion

of Nano-sized Spherical Magnetite (S-Mag) to Spherical Barium Ferrite (S-BaFe)

Nanoparticles for High Density Particulate Recording Media. Presented at the IEEE

International Magnetics Conference, Sacramento, CA.

4. Bae, S., Hong, Y. K., Lee, J. J., Jalli, J., Abo, G. S., Kim, B., et al. (2009, May). High Q

Ferrite Film Inductor for On-chip Power Module. Presented at the IEEE International

Magnetics Conference, Sacramento, CA.

5. Lyle, A., Hong, Y. K., Choi, B. C., Abo, G. S., Jalli, J., Bae, S., et al. (2009, May). Spin-

Polarized Current Switching of Co/Cu/Py Pac-man Type II. Presented at the IEEE

International Magnetics Conference, Sacramento, CA.

6. Bae, S., Hong, Y. K., Lee, J. J., Jalli, J., Abo, G. S., Sung, W., et al. (2009, May). Co2Z

Hexaferrite T-DMB Antenna for Mobile Phone. Presented at the IEEE International

Magnetics Conference, Sacramento, CA.

7. Lyle, A., Hong, Y. K., Choi, B. C., Abo, G. S., Park, M. H., Gee, S. H., et al. (2008,

34

December). Spin-Polarized Current Switching of Co/Cu/Py Elongated Pac-man Spin-

Valve. Presented at the Asian Magnetics Conference, Busan, Korea.

8. Bae, S., Hong, Y. K., Lee, J. J., Jalli, J., Abo, G. S., Lyle, A., et al. (2008, December). New

Synthetic Route of Single-phase Z-type (Ba3Co2Fe24O41) Hexaferrite Particles. Presented

at the Asian Magnetics Conference, Busan, Korea.

9. Lyle, A., Hong, Y. K., Choi, B. C., Abo, G. S., Han, H., Jalli, J., et al. (2008, November).

Spin-Polarized Current Stimulation of 100 nm Dual Vortex Co/Cu/Py Spin Valve

Nanopillars. Presented at the 53rd Annual Conference on Magnetism and Magnetic

Materials, Austin, TX.

10. Bae, S., Hong, Y. K., Lee, J. J., Jalli, J., Abo, G. S., Lyle, A., et al. (2008, November). Low

Loss Z-type Barium Ferrite (Co2Z) for T-DMB Antenna Application. Presented at the

53rd Annual Conference on Magnetism and Magnetic Materials, Austin, TX.

11. Bae, S., Hong, Y. K., Lee, J. J., Abo, G. S., Jalli, J., Lyle, A., et al. (2008, October).

Optimized Design of Low Voltage High Current Ferrite Planar Inductor for 10 MHz On-

chip Power Module. Presented at the Global KMS International Conference, Jeju, Korea.

12. Bae, S., Hong, Y. K., Lee, J. J., Abo, G. S., Jalli, J., Lyle, A., et al. (2008, October).

Miniature IL Chip Antenna for T-DMB Applications. Presented at the Global KMS

International Conference, Jeju, Korea.

13. Jalli, J., Hong, Y. K., Bae, S., Lee, J. J., Kothakonda, M., Abo, G. S., et al. (2008,

November). Growth and Characterization of 144 μm Thick Barium Ferrite Single

Crystalline Film for Microwave Device Application. Presented at the 53rd Annual

Conference on Magnetism and Magnetic Materials, Austin, TX.

14. Bae, S., Hong, Y. K., Lee, J. J., Abo, G. S., Jalli, J., Lyle, A., et al. (2008, May). High Q and

35

High Current NiZnCu Ferrite Inductor for On-chip Power Module. Presented at the

IEEE International Magnetics Conference, Madrid, Spain.

15. Lyle, A., Hong, Y. K., Choi, B. C., Rudge, J., Abo, G. S., Gee, S. H., et al. (2008, May).

Fabrication of 8x8 Array of Spin Valve Pillars and MR Characterization. Presented at

the IEEE International Magnetics Conference, Madrid, Spain.

16. Bae, S., Hong, Y. K., Lee, J. J., Jalli, J., Abo, G. S., Lyle, A., et al. (2008, May).

Optimization of Design Parameters for Ferrite Inductor for 10 MHz On-chip Power

Module. Presented at the IEEE International Magnetics Conference, Madrid, Spain.

17. Jalli, J., Hong, Y. K., Gee, S. H., Lee, J. J., Bae, S., Abo, G. S., et al. (2008, May).

Ferrimagnetic Ba0.5Sr1.5Zn2Fe12O22 (Zn-Y) Single Crystal Barium Ferrites. Presented at

the IEEE International Magnetics Conference, Madrid, Spain.

18. Jalli, J., Hong, Y. K., Gee, S. H., Lee, J. J., Bae, S., Abo, G. S., et al. (2008, May). Magnetic

and Microwave Properties Sm-doped SrFe12O19 Single Crystals. Presented at the IEEE

International Magnetics Conference, Madrid, Spain.

19. Jalli, J., Hong, Y. K., Gee, S. H., Abo, G. S., Han, H., Lyle, A., et al. (2007, November).

Microwave and Magnetic Properties of Mn Substituted Zn-Y Type Barium Ferrite Single

Crystals. Presented at the 52nd Annual Conference on Magnetism and Magnetic

Materials, Tampa, FL.

20. Jalli, J., Hong, Y. K., Gee, S. H., Juan, C. C., Han, H., Abo, G. S., et al. (2007, January).

Observation of Magnetic Domain Patterns in Bulk Barium Ferrite Single Crystals.

Presented at the 10th Joint Magnetism and Magnetic Materials/IEEE International

Magnetics Conference, Baltimore, MD.

21. Jabal, J. F., Hong, Y. K., Choi, B. C., Han, H., Gee, S. H., Abo, G. S., et al. (2006, May).

36

Lateral Dimension Dependence of Pac-man Shaped Ni80Fe20 Elements on Magnetization

Reversal. Presented at IEEE International Magnetics Conference, San Diego, CA.

22. Jabal, J. F., Hong, Y. K., Choi, B. C., Han, H., Abo, G. S., Gee, S. H., et al. (2006, April).

Suppression of Magnetization Ringing in Submicron Pac-man Shaped Ni80Fe20 Thin Film

Elements. Presented at the 12th Biennial IEEE Conference on Electromagnetic Field

Computation, Miami, FL.

23. Jabal, J. F., Hong, Y. K., Han, H., Gee, S. H., Zhao, B., Choi, B. C., et al. (2005, October). A

Study of Magnetic Element Shapes for MRAM Switching Behaviors. Presented at the

12th NASA Symposium on VLSI Design, Cour d’Alene, ID.

24. Hong, Y. K., Han, H., Jabal, J. F., Gee, S. H., Abo, G. S., & Lyle, A (2005, August).

Magnetization Process in E-beam Patterned Magnetic Submicron Element. Presented at

2005 US-Korea Conference on Science and Technology, Irvine, CA.

25. Jabal, J. F., Hong, Y. K., Han, H., Gee, S. H., Park, M. H., Choi, B. C., et al. (2005, June).

Modeling of Hysteresis Curve and Magnetization Configuration of Deep-submicron

Ni80Fe20 Elements with Various Shapes. Presented at the International Workshop on Next

Generation HDD Technology and Korean Magnetic Society 2005 Summer Conference,

Jeju, Korea.

26. Han, H., Hong, Y. K., Park, M. H., Choi, B. C., Gee, S. H., Jabal, J. F., et al. (2005, April).

Paired Interaction Effect on Switching Behaviors of Patterned “Pac-man” Array.

Presented at the IEEE International Magnetics Conference, Nagoya, Japan.

37