Download - Introduction to Atom Probe Tomography
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INTRODUCTION TO ATOM PROBE TOMOGRAPHY
Brian P. Gorman [email protected] Department of Metallurgical and Materials Engineering, CSM
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Internal Interface Characterization
‣ Need to know:– Chemical abruptness– Structural roughness (nm spatial resolution)– Grain Boundary and Dopant structure (ppm chemical information in nm
spaces)
‣ How?– SIMS – nm scale chemical profiling in z-direction except with significant
surface roughness, 50nm best resolution x-y, ppb detectability– TEM - Å level spatial resolution, ~1at% best chemical resolution with
EDS, EELS– Atom Probe - Å level spatial resolution, 10ppm chemical resolution, data
needs reconstruction
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Why Atom Probe?
Atom Probe
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Atom Probe Tomography
3-DimensionalReconstructed
Model of Specimen
z is determined from sequence of evaporation events
Data are collected and interpreted
Pulsed High VoltagePulsed Laser
Removes Atoms, 1 at a timeLayer by Layer
2D DetectorDetermines x,y coordinates of atom
Needle-Shaped Specimen
Time of FlightDetermines Atom Type
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Atom Probe Detectability Limits
‣Are there atoms in the field of view?– 100nm diameter FOV is ~100,000atoms / surface
‣Can we detect each atom?– Cross-wire delay line detector has ~50% collection efficiency
– We then capture ~50,000 atoms / surface– Can theoretically detect one atom count above the background, or 1017 to 1018 atoms/cm3
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Detectability Limits
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Local Electrode Atom Probe (LEAP)
‣Advantages of putting the counter electrode within close proximity of the specimen – Wider field of view– Lower extraction voltages– MUCH higher acquisition rates
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FIB / TEM / LEAP Analysis
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LEAP Data Interpretation
STEM APT
‣ FIB prepared Al specimens illustrate Ga phase segregation in STEM-HAADF
‣ LEAP illustrates Ga segregation to GBs
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APT Process ‣Specimen Preparation
– Dependent upon material evaporation field, electrical properties, thermal properties, cost, throughput
‣Field Evaporation / Data Collection– Voltage vs. laser pulsing, laser power, pulse fraction, base
Temperature, flight path
‣Reconstruction and Data Analysis– Need to know evaporation field or tip shape (TEM cross
correlation), many reconstruction correction algorithms, interpret mass spectrum
– Data analysis takes ~3x longer than specimen preparation and collection
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Specimen Preparation
‣Traditionally:– AP primarily used for metallurgical specimens– Electropolishing needle geometries used extensively
‣Currently:– Focused Ion Beam / SEM– In-situ liftout of site-specific areas– FIB used to final polish 100nm specimens
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Atom Probe Specimen Preparation
Deposit a 200 nm thick Pt bar in the FIB1. Dimensions: 2 microns wide, 30 – 40 microns long2. Start with e-beam Pt (~50nm), then switch to ion beam to minimize Ga
damage (~250nm total thickness)
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Liftout – Blanket WafersFirst wedge Second wedge Cantilever
Attach Nanomanipulator Liftout
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FIB Specimen Prep II
‣Site specific sample preparation – 65nm CMOS transistors
Acknowledgement UFL and INTEL
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Si Microtip Arrays
Top-down image of microtip
Side-view of LEAP microtip coupon
Position sample wedge here
Sample wedge
Microtip post
Pt weld
Attach sample to wedge
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Slice Sample and Retract WedgeSlice sample from wedge Remainder of wedge is
retracted
Wedge is aligned to the next microtip and the process is repeated
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Atom Probe Liftout
B. P. Gorman et al., Microsc. Today, 16 (2008) 42.
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Final Sharpening
‣Want: 100 - 200 nm diameter, >50m long specimen
‣Have: ~10m2 specimen on post
‣Annular milling patterns used to remove outer material and leave specimen in the center
Ga+
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Ion-Solid Interaction Considerations: Ga into Si
30keV 0° incidence3.7 sputtered Si / incident Ga
30keV 89° incidence22 sputtered Si / incident Ga
SRIM 2003 Simulations
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Ion-Solid Interaction Considerations: Ga into Si
5keV 89° incidence7.1 sputtered Si / incident Ga
5keV 0° incidence1.4 sputtered Si / incident Ga
SRIM 2003 Simulations
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• Lower FIB energy results in less Ga implantation
• Ga implantation is minimal at 2kV
• K. Thompson, et. al., Ultramic., 107 (2007) 131
FIB Specimen Preparation and Implant Measurement
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Final Preparation of Tip
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FIB / TEM / LEAP Prep
• Specimens milled directly down alpha tilt axis of TEM and in line with AP detector
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Instrumentation for FIB TEM and LEAP
Removable Tip Grid Holder
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APT Designs
‣Straight flight path (LEAP 4000 XSi)– Highest field of view (>200 nm diameter), repetition rate (1
MHz laser pulse), detection efficiency (~57%)– Lower mass resolution
‣Reflectron – energy compensated (LEAP 4000 XHR)– Highest mass resolution– Slightly lower field of view, repetition rate (250 kHz), detection
efficiency (~35%)
‣Laser pulse vs voltage pulse
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Laser Pulsed Local Electrode Atom Probe
Eevap
T
Voltage pulse
Laser pulse
‣Advantages of laser pulsing– Low electrical conductivity materials
– Improved interface transitions