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    • Abstract: utilizing transmission electron microscopy (TEM) and focused ion beam ... A program which simulates electron diffraction contrast from FEM stress ...

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DISSERTATION ABSTRACT
A new nanoscale technique for measurement of stresses has been developed for
use in crystalline structures of arbitrary complexity. This has been accomplished by
utilizing transmission electron microscopy (TEM) and focused ion beam (FIB)
technology in conjunction with an ensemble of computer programs which include finite
element modeling (FEM), electron diffraction strain contrast simulation, and image
manipulation. Stresses in semiconductor devices have been measured quantitatively with
a spatial resolution on the order of nanometers (nm) and a relative sensitivity on the order
of tens of Mega Pascals (MPa). This technique has been applied to Si/GeSi/Si
heterojunction bipolar transistor (HBT) device structures and InAs quantum dots
embedded in a GaAs matrix.
The stress measurement process consists of first obtaining a sample by FIB micro-
machining which results in a thin cross sectional membrane of known geometry through
the selected device structure, a HBT. An experimental image obtained via TEM is then
used to build the geometry of a finite element structure with ANSYS software. FEM is
then used to obtain a simulated stress field throughout the HBT of the actual thin
membrane structure, where the intrinsic stresses of polycrystalline and amorphous
materials contained within the device structure were determined by wafer curvature
measurements. A program which simulates electron diffraction contrast from FEM stress
field data by the application of the dynamic electron diffraction Howie-Whelan Equations
is then used to create a simulated TEM image. The two images, experimental and
simulated, are then normalized to each other such that their intensities and gray levels fall
along the same dynamic range. Subtraction of one image from the other on a pixel by
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pixel basis can then be performed, which creates a difference map of changes in
grayscale. Once the relative stress sensitivity per gray level is determined, it becomes
possible to quantify the stresses in the experimental structure by using the simulated
device as a starting point. Areas of higher stress within the device structure have been
quantified. These include single crystalline regions in close proximity to tungsten (W)
vias, material immediately adjacent to thin thermal SiO 2 layers, and oxide trenches.
Understanding the nature of stress relaxation due to surface relaxation in the thin
film FIB membrane structures is quantified as this can cause stress relaxation on the order
of 30-50% compared to the bulk device structure. Additionally, to verify the overall
accuracy of the described quantitative procedure, the strain fields due to lattice mismatch
strain around InAs quantum dots embedded in a GaAs matrix are quantified and
compared to an independently derived analytical solution for the identical structure.
Finally, initial implementation of a portable multidimensional minimization and
interpolation algorithm is discussed that independently varies the stress levels in different
materials composing the structure. This serves to improve the stress quantification
algorithm by finding the global minimum for matching of experimental and simulated
image intensities.
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ACKNOWLEDGEMENTS
Ø First and foremost I must thank my mentor, advisor, and friend during this endeavor:
Dr. Robert Hull (a.k.a. the Devil’s advocate). I could not have asked for a nicer or
more intelligent guide for my graduate experience and not a week has gone by where
I have not considered myself truly lucky to tutored by him at this stage of my career.
Robert, you taught me to attempt to think and understand, and I thank you.
Ø My family (particularly my parents) deserve a place here as well. For without them, I
would not be here today and certainly would not have my interest or predisposition to
science. And I would certainly not have my stubbornness and tenacity.
Ø To those truly wonderful people that I count among my friends—particularly my
close ones upon whom I base my self and I’m certain you know who you are—my
thanks for putting up with my complaints during the bad times (of which there were
few), sharing in my elation during the good, and providing vacation spots throughout
the U.S. and beyond.
Ø I must then thank the people at IBM (K. Schonenberg in particular), and the company
itself (for funding this project through a research grant and fellowship award),
without whose help I would not have been able to accomplish this work.
Ø Koen Janssens and Leo Zhigilei deserve an acknowledgement for their help and
patience as a programming gurus.
Ø Finally I would like to thank everyone at the Materials Science Department at the
University of Virginia, because all of you have touched my life and made my
experience here extraordinary.
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Statements which reach to the core of my being:
Happiness was just around the corner, a corner they never turned. And the source of it all
was the human mind.
Dan Milman, The Way of the Peaceful Warrior
Try, fail, and learn.
Do things for your own approval and no one else’s.
My friendships are the foundation upon which I build my self.
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TABLE OF CONTENTS
DISSERTATION ABSTRACT...................................................................................................................................I
ACKNOWLEDGEMENTS.......................................................................................................................................III
TABLE OF CONTENTS ............................................................................................................................................ V
LIST OF FIGURES................................................................................................................................................. XIII
LIST OF TABLES ..................................................................................................................................................XXII
LIST OF SYMBOLS............................................................................................................................................ XXIII
CHAPTER 1: INTRODUCTION..............................................................................................................................1
1.1 INTRODUCTION OF PROJECT ............................................................................................................................... 1
1.1.1 INTRODUCTION.................................................................................................................................................. 1
1.1.2 M OTIVATION AND BRIEF BACKGROUND ........................................................................................................ 2
1.1.3 SIGNIFICANCE OF THIS WORK.......................................................................................................................... 4
1.1.4 CURRENT LIMITS OF STRAIN CHARACTERIZATION ....................................................................................... 5
1.2 INTRODUCTION TO THE PRIMARY EXPERIMENTAL SYSTEM STUDIED: GESI/SI HETEROJUNCTION
BIPOLAR TRANSISTORS .............................................................................................................................................. 8
1.2.1 INTRODUCTION.................................................................................................................................................. 8
1.2.2.1 THE BIPOLAR JUNCTION TRANSISTOR ......................................................................................................... 9
1.2.2.2 THE HETEROJUNCTION BIPOLAR TRANSISTOR ......................................................................................... 10
1.2.3 F RONT END HBT FABRICATION PROCESS .................................................................................................... 13
1.2.3.1 OXIDE GROWTH ........................................................................................................................................... 13
1.2.3.2 SI GE HETEROEPITAXY ................................................................................................................................ 15
1.2.4 EMITTER STRUCTURE ..................................................................................................................................... 17
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1.2.5 BACKEND PROCESSING ................................................................................................................................... 17
1.3 CONCLUSIONS ..................................................................................................................................................... 19
CHAPTER 2: EXPERIMENTAL TECHNIQUES ............................................................................................21
2.1 FOCUSED ION BEAMS......................................................................................................................................... 21
2.1.1 INTRODUCTION................................................................................................................................................ 21
2.1.2 P RINCIPLES OF OPERATION ............................................................................................................................ 23
2.1.2.1 LIQUID METAL ION SOURCE (LMIS).......................................................................................................... 23
2.1.2.2 SUPPRESSOR/EXTRACTOR ASSEMBLY ....................................................................................................... 24
2.1.2.3 LENS 1 AND THE BEAM DEFINING APERTURE .......................................................................................... 25
2.1.2.4 LENS 2 AND THE DEFLECTION OCTUPOLE................................................................................................. 26
2.1.2.5 IMAGE GENERATION .................................................................................................................................... 26
2.1.3 FIB APPLICATIONS.......................................................................................................................................... 28
2.1.3.1 M ATERIAL REMOVAL .................................................................................................................................. 28
2.1.3.2 DEPOSITION .................................................................................................................................................. 29
2.1.4 FIB SAMPLE PREPARATION ............................................................................................................................ 30
2.2 TRANSMISSION ELECTRON M ICROSCOPY....................................................................................................... 35
2.2.1 INTRODUCTION................................................................................................................................................ 35
2.2.2 DIFFRACTION................................................................................................................................................... 36
2.2.2.1 ELASTIC SCATTERING.................................................................................................................................. 36
2.2.2.2 INELASTIC SCATTERING45 ........................................................................................................................... 37
2.2.2.3 BRAGG DIFFRACTION .................................................................................................................................. 38
2.2.2.4 DIFFRACTION VECTOR ................................................................................................................................ 39
2.2.2.5 DEVIATION PARAMETER............................................................................................................................. 40
2.2.2.6 STRUCTURE FACTOR.................................................................................................................................... 40
2.2.2.7 EXTINCTION DISTANCE ............................................................................................................................... 41
2.2.2.8 M EASUREMENT OF DEVIATION PARAMETER............................................................................................ 42
2.2.2.9 THE TWO BEAM CONDITION ....................................................................................................................... 43
2.2.2.10 KINEMATICAL DIFFRACTION.................................................................................................................... 45
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2.2.2.11 DYNAMICAL DIFFRACTION....................................................................................................................... 45
2.2.3 IMAGE CONTRAST MECHANISMS ................................................................................................................... 47
2.2.3.1 DIFFRACTION CONTRAST ............................................................................................................................ 47
2.2.3.2 PHASE CONTRAST ........................................................................................................................................ 50
2.2.3.3 A BSORPTION CONTRAST ............................................................................................................................. 51
2.2.4 THE TRANSMISSION ELECTRON MICROSCOPE ............................................................................................. 52
2.2.4.1 THE ELECTRON GUN .................................................................................................................................... 52
2.2.4.2 W EHNELT CUP .............................................................................................................................................. 52
2.2.4.3 CONDENSER LENSES .................................................................................................................................... 53
2.2.4.4 SPECIMEN REGION ....................................................................................................................................... 53
2.2.4.5 OBJECTIVE LENS........................................................................................................................................... 53
2.2.4.6 OBJECTIVE APERTURE ................................................................................................................................. 55
2.2.4.7 INTERMEDIATE AND PROJECTOR LENS ASSEMBLY .................................................................................. 55
2.2.4.8 SELECTED AREA APERTURE ........................................................................................................................ 55
2.2.4.9 VIEWING CHAMBER..................................................................................................................................... 56
2.2.4.10 SLOW SCAN CAMERA ................................................................................................................................ 56
2.2.5 EXPERIMENTAL DETAILS CONCERNING TEM TECHNIQUES EMPLOYED ................................................. 56
2.2.5.1 A LIGNMENT ROUTINE FOR AN FIB/TEM SAMPLE CONTAINING A SEMICONDUCTOR DEVICE
STRUCTURE ................................................................................................................................................................ 57
2.2.5.2 IMAGE RECORDING VIA THE SSC............................................................................................................... 59
2.2.5.3 THICKNESS DETERMINATION OF SPECIMENS............................................................................................ 60
2.2.6 CONVENTIONAL TEM CROSS SECTION SAMPLE PREPARATION ............................................................... 62
2.3 CONCLUSIONS ..................................................................................................................................................... 63
CHAPTER 3: SIMULATION AND MODELING TECHNIQUES ..............................................................64
3.1 FINITE ELEMENT ANALYSIS.............................................................................................................................. 64
3.1.1 INTRODUCTION................................................................................................................................................ 64
3.1.2 P RINCIPLES OF THE FINITE ELEMENT METHOD ............................................................................................ 65
3.1.2.1 TENSOR NOMENCLATURE ........................................................................................................................... 66
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3.1.2.2 THE STIFFNESS MATRIX .............................................................................................................................. 66
3.1.2.2.1 P RINCIPLE OF VIRTUAL WORK ................................................................................................................ 69
3.1.3 FINITE ELEMENT MODEL CONSTRUCTION .................................................................................................... 72
3.1.3.1 EXPERIMENTAL IMAGE ............................................................................................................................... 72
3.1.3.2 KEY POINTS................................................................................................................................................... 73
3.1.3.2 LINES.............................................................................................................................................................. 74
3.1.3.3 A REAS............................................................................................................................................................ 75
3.1.3.4 VOLUMES ...................................................................................................................................................... 76
3.1.4 FINITE ELEMENT MODEL MESHING ............................................................................................................... 76
3.1.4.1 ELEMENT TYPES........................................................................................................................................... 76
3.1.4.2 ELEMENT SHAPE CHECKING ....................................................................................................................... 77
3.1.4.3 ELEMENT MESH DENSITY............................................................................................................................ 78
3.1.5 M ATERIAL PARAMETERS................................................................................................................................ 78
3.1.5.1 COEFFICIENT OF THERMAL EXPANSION.................................................................................................... 82
3.1.5.2 INTRINSIC STRESS MEASUREMENTS.......................................................................................................... 82
3.1.6.1 FINITE ELEMENT MODEL BOUNDARY CONDITIONS.................................................................................. 84
3.1.6.2 EXPERIMENTAL STRUCTURE ...................................................................................................................... 84
3.1.6.3 SAINT VENANT ’S PRINCIPLE 75&54 .............................................................................................................. 84
3.1.6.4 STRUCTURAL SYMMETRY54 ........................................................................................................................ 85
3.1.6.5 BOUNDARY CONDITION CONCLUSIONS..................................................................................................... 86
3.1.7 A PPLYING LOADS AND SOLVING THE MODEL .............................................................................................. 87
3.1.8 RESULTS OF THE FEM .................................................................................................................................... 87
3.1.9 OUTPUT FILES FOR THE SIMCON PROGRAM.............................................................................................. 87
3.1.10 M ODEL ACCURACY....................................................................................................................................... 88
3.1.11 H-CONVERGENCE VERSUS P -CONVERGENCE ............................................................................................. 88
3.1.12 HARDWARE .................................................................................................................................................... 89
3.2 ELECTRON DIFFRACTION CONTRAST SIMULATION ...................................................................................... 90
3.2.1 INTRODUCTION TO SIMCON......................................................................................................................... 90
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3.1.2 THE SIMCON APPROACH ............................................................................................................................. 91
3.2.2 SIMCON MODIFICATIONS (A PPENDIX A) .................................................................................................. 91
3.2.3 OPERATION OF THE SIMCON P ROGRAM .................................................................................................... 92
3.2.3.1 SIMCON_FE_MM ..................................................................................................................................... 94
3.2.3.1.1 IMAGEIN ..................................................................................................................................................... 94
3.2.3.1.2 COORDINATE_ SYSTEMS ........................................................................................................................... 97
3.2.3.1.3 FE_ FILE_ DATA .......................................................................................................................................... 98
3.2.3.1.4 EXECUTION OF THE SIMCON ROUTINE ............................................................................................... 99
3.2.3.2 POST PROCESSOR.......................................................................................................................................... 99
3.3 IMAGE COMPARISON........................................................................................................................................101
3.3.1 IMAGE COMPARISON SOFTWARE .................................................................................................................101
3.3.2 P REPROCESSING.............................................................................................................................................101
3.3.2.1 EXPERIMENTAL PREPROCESSING .............................................................................................................101
3.3.2.2 SIMULATION PREPROCESSING ..................................................................................................................103
3.3.2.3 P REPROCESSING RESULTS........................................................................................................................104
3.3.3 AVS IMAGE ALIGNMENT .............................................................................................................................107
CHAPTER 4: QUANTIFICATION OF STRESSES IN SI/SIGE/SI HETEROJUNCTION
BIPOLAR TRANSISTORS ................................................................................................................................... 110
4.1 EXPERIMENTAL PROCEDURES ........................................................................................................................110
4.1.1 INTRODUCTION..............................................................................................................................................110
4.1.2 THE QUANTIFICATION PROCEDURE AS APPLIED TO THE SHALLOW ISOLATION TRENCH GEOMETRY 111
4.1.3 RESULTS OF DIFFERENT TWO BEAM CONDITIONS.....................................................................................119
4.1.4 VARYING THE DEVIATION PARAMETER, W , FOR A CONSTANT FEM MODEL .......................................122
4.1.5 FRESNEL CONTRAST 46,86&87 .........................................................................................................................124
4.1.5.1 EXPERIMENTAL ANALYSIS........................................................................................................................125
4.2 RESULTS.............................................................................................................................................................127
4.2.1 VARYING FEM LOADS TO OBTAIN THE BEST IMAGE MATCH .................................................................127
4.2.1.1 ORIGIN OF THE OBSERVED STRESS DISTRIBUTION IN THE SHALLOW ISOLATION TRENCH...............133
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4.2.1.2 STRESSES CORRESPONDING TO GEOMETRICALLY SQUARE AND ELLIPTICAL ISOLATION TRENCH
GEOMETRIES.............................................................................................................................................................135
4.2.2 COLLECTOR CONTACT ANALYSIS................................................................................................................136
4.2.3 QUANTIFICATION OF STRESSES IN STRUCTURES WITH MULTIPLE SINGLE CRYSTAL MATERIALS (HBT
EMITTER/ BASE STRUCTURE )—AN EXPLORATION OF THE LIMITS OF OUR QUANTITATIVE TECHNIQUES.....141
4.2.3.1 QUANTITATIVE ANALYSIS.........................................................................................................................142
4.2.3.2 BASE STRESS VARIATION..........................................................................................................................147
4.2.3.3 SUMMARY OF BASE -EMITTER ANALYSIS; IMPLICATIONS FOR STRESS QUANTIFICATION OF VERY
COMPLEX STRUCTURES...........................................................................................................................................148
4.3 STRESS IN THE EXPERIMENTAL SAMPLE VERSUS THE BULK STRUCTURE ..............................................152
4.3.1 SURFACE STRESS RELAXATION IN THE EXPERIMENTAL SAMPLE ............................................................152
4.3.2 DIFFERENT FEM MODELS USED .................................................................................................................153
4.3.3 M ETHOD OF QUERYING RESULTS OF FEM.................................................................................................157
4.3.4 EXAMINATION OF MODEL A ........................................................................................................................159
4.3.5 EFFECT OF CHANGING THE THICKNESS OF THE CROSS SECTIONAL MEMBRANE (M ODEL B)..............160
4.3.6 EFFECT OF MODEL GEOMETRY AND APPLIED BOUNDARY CONDITIONS.................................................162
4.3.7 EFFECT OF MODEL GEOMETRICAL EXTENT UPON STRESS ........................................................................163
4.3.9 RELATING QUANTIFIED STRESSES TO THE REAL STRUCTURE .................................................................166
4.4 CONCLUSIONS ...................................................................................................................................................168
4.4.1 GENERAL STATEMENTS................................................................................................................................168
4.4.2 RESULTS.........................................................................................................................................................168
4.4.3 EXPERIMENTAL AND MODELING ISSUES.....................................................................................................170
4.4.4 SURFACE RELAXATION.................................................................................................................................171
CHAPTER 5: QUANTIFICATION OF LATTICE MISMATCH STRAINS IN
NANOSTRUCTURED SYSTEMS ...................................................................................................................... 172
5.1 INTRODUCTION .................................................................................................................................................172
5.2 FINITE ELEMENT MODELING OF THE QD GEOMETRY..................................................................................174
5.3 TRANSMISSION ELECTRON MICROSCOPY OF QUANTUM DOTS...................................................................176
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5.3.1 TEM PARAMETERS .......................................................................................................................................176
5.3.2 EFFECT OF QD POSITION WITHIN THE THICKNESS OF THE SAMPLE .......................................................177
5.3.3 EXPERIMENTAL ANALYSIS ...........................................................................................................................182
5.4 QUANTIFICATION OF STRAIN AROUND QD ...................................................................................................182
5.4.1 STOLERU ANALYTICAL SOLUTION ..............................................................................................................183
5.4.2 QUANTIFICATION OF STRAINS VIA THE METHOD DESCRIBED IN THIS DISSERTATION .........................184
5.5 EXPERIMENTAL EVIDENCE THAT THE QD COMPOSITION MAY NOT BE UNIFORM ...................................187
5.6 CONCLUSIONS ...................................................................................................................................................188
CHAPTER 6: GENERAL CONCLUSIONS AND FUTURE WORK....................................................... 190
6.1 GENERAL CONCLUSIONS..................................................................................................................................190
6.2 FUTURE WORK ..................................................................................................................................................195
6.2.1 M ULTI-MATERIAL MINIMIZATION AND INTERPOLATION ALGORITHM ...................................................195
6.2.2 A PPLICATION OF THE MINIMIZATION AND INTERPOLATION ALGORITHM .............................................197
6.2.3 COMPARISON OF QUANTIFIED STRESS TO CBED RESULTS.....................................................................202
6.3 OTHER AVENUES FOR FUTURE WORK ............................................................................................................203
APPENDIX A: MODIFICATIONS TO SIMCON SOURCE CODE......................................................... 205
A.1 ANSYS_ PREPROCESSOR_ MM_3D................................................................................................................205
A.2 DETERMINE_ MAT _PAR.F.................................................................................................................................205
A.3 DETERMINE_ ZONE.F ........................................................................................................................................207
A.4 GETRUNCOPY ...................................................................................................................................................211
A.5 PREFIELD_SOLID92_10.F ..............................................................................................................................215
A.6 PREFIELD_STIF2.F...........................................................................................................................................232
A.7 RESUME.............................................................................................................................................................234
A.8 SAVE RESULTS..................................................................................................................................................237
A.9 SIMCON..............................................................................................................................................................239
A.10 SIMCON.COMPILE ...........................................................................................................................................242
A.11 SIMCON.DECLARATIONS ...............................................................................................................................244
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A.12 SIMCON.EDCI................................................................................................................................................249
A.13 SIMCON.INPUT ................................................................................................................................................251
A.14 SIMCON.NAMELISTS.......................................................................................................................................254
A.15 SIMCON_ FE_MM.COMPILE .LIBRARY ...........................................................................................................255
A.16 SIMCON_ FE_MM_3D.....................................................................................................................................256
A.17 SSIMCON .........................................................................................................................................................257
APPENDIX B: MULTI-MATERIAL MINIMIZATION AND INTERPOLATION SOURCE CODE
........................................................................................................................................................................................ 259
APPENDIX C: SUMMARY OF EXPERIMENTAL AND SIMULATION CONDITIONS............... 272
REFERENCES .......................................................................................................................................................... 274
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LIST OF FIGURES
FIGURE 1: F LOW CHART OF THE STRESS QUANTIFICATION PROCESS DESCRIBED IN CHAPTER
4
FIGURE 2: S CHEMATIC BJT (A) AND HBT (B) ENERGY BAND DIAGRAMS HIGHLIGHTING
THE DIFFERENCE BETWEEN THE TWO TYPES OF TRANSISTORS. AFTER REFERENCE 20
FIGURE 3: BJT SCHEMATIC DEPICTION (A) AND A HBT (400) TEM/FIB CROSS SECTION
(B)
FIGURE 4: HBT FABRICATION SCHEMATIC: (A) ISOLATION TRENCH FABIRCATION, (B) BASE
CONTACT , (C) EMITTER PEDESTAL, (D) BACKEND PROCESSING
FIGURE 5: SI GE BASE FACET SCHEMATIC (CROSS SECTIONAL VIEW )
FIGURE 6: FIB SCHEMATIC DIAGRAM SHOWING PRINCIPLE ION COLUMN FEATURES 36
FIGURE 7: S CHEMATIC OF FIB LMIS SHOWING RESERVOIR AND WETTED NEEDLE AND TIP
FIGURE 8: FIB SECONDARY ELECTRON IMAGE (A) AND SECONDARY ION IMAGE (B) OF THE
SAME AREA ON A SPECIMEN
FIGURE 9: FIB/TEM SAMPLE SCHEMATIC SHOWING BOTH IN SITU TEM AND FIB BEAM
ORIENTATIONS
FIGURE 10: FIB MILLING SCHEMATIC (½ OF A MIRROR IMAGE—MIRROR PLANE IS X- Y) FOR
TEM SAMPLE PREPARATION
FIGURE 11: S ECTION 2.2 FLOW CHART SHOWING PROGRESSION OF THE REMAINING
CHAPTER
FIGURE 12: BRAGG'S LAW GEOMETRY FOR TEM THIN FOIL SPECIMENS
FIGURE 13: DIFFRACTION VECTOR AND THE EWALD SPHERE CONSTRUCTION
xiv
FIGURE 14: DEVIATION PARAMETER SHOWING DEVIATION FROM EXACT BRAGG CONDITION
FIGURE 15: (400) TWO BEAM DIFFRACTION PATTERNS OF SI APPROXIMATELY 280 NM
THICK. COMPARISON OF PATTERN WITHOUT INTENSITY LIMITING OBJECTIVE
APERTURE (A) TO INTENSITY ARISING JUST FROM THE TRANSMITTED AND DIFFRACTED
BEAMS (B) INDICATES ~85% OF ELECTRON INTENSITY IS CONTAINED WITHIN THE TWO
BEAMS
FIGURE 16: INTENSITY IN DIFFRACTED BEAM AS A FUNCTION OF DEVIATION PARAMETER46
FIGURE 17: CONVENTIONAL (A) AND FIB (B) SAMPLE GEOMETRIES IN CROSS SECTION
FIGURE 18: TEM SCHEMATIC OF THE JEOL 2000 FX USED DURING THIS STUDY47
FIGURE 19: S CHEMATIC VIEW OF FIB SPECIMEN AT LOW MAGNIFICATION IN THE TEM TO
ILLUSTRATE LOCATING THE THIN MEMBRANE
FIGURE 20: DEFINITION OF THE COMPONENTS OF THE STRESS TENSOR
FIGURE 21: 2D FEM MODEL ( LEFT) AND CORRESPONDING (400) TEM IMAGE (RIGHT )
FIGURE 22: FEM KEY POINTS (A), LINES (B), AND AREAS (C) SHOWING DEVELOPMENT OF
FEM MODEL CONSTRUCTION FOR THE SHALLOW ISOLATION TRENCH GEOMETRY
FIGURE 23: FEM ELEMENTS FORMING A MESH OF THE SHALLOW ISOLATION TRENCH
STRUCTURE DESCRIBED IN FIGURE 21
FIGURE 24: (400) TEM IMAGE SHOWING REPEATING HBT STRU


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