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Introduction to Atom Probe Tomography. Brian P. Gorman bgorman@mines.edu Department of Metallurgical and Materials Engineering, CSM. Internal Interface Characterization. Need to know: Chemical abruptness Structural roughness (nm spatial resolution)

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introduction to atom probe tomography

Introduction to Atom Probe Tomography

Brian P. Gorman bgorman@mines.edu

Department of Metallurgical and Materials Engineering, CSM

internal interface characterization
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
why atom probe
Why Atom Probe?

Atom

Probe

atom probe tomography

Pulsed High VoltagePulsed Laser

Removes Atoms, 1 at a time

Layer by Layer

Atom Probe Tomography

2D Detector

Determines x,y coordinates of atom

Data are collected and interpreted

Needle-Shaped Specimen

3-Dimensional

Reconstructed

Model of Specimen

z is determined from sequence of evaporation events

Time of Flight

Determines Atom Type

atom probe detectability limits
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
local electrode atom probe leap
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
leap data interpretation
LEAP Data Interpretation

STEM APT

  • FIB prepared Al specimens illustrate Ga phase segregation in STEM-HAADF
  • LEAP illustrates Ga segregation to GBs
apt process
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
specimen preparation
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
atom probe specimen preparation
Atom Probe Specimen Preparation
  • Deposit a 200 nm thick Pt bar in the FIB
    • Dimensions: 2 microns wide, 30 – 40 microns long
    • Start with e-beam Pt (~50nm), then switch to ion beam to minimize Ga damage (~250nm total thickness)
liftout blanket wafers
Liftout – Blanket Wafers

Second wedge

First wedge

Cantilever

Attach Nanomanipulator

Liftout

fib specimen prep ii
FIB Specimen Prep II
  • Site specific sample preparation – 65nm CMOS transistors

Acknowledgement UFL and INTEL

si microtip arrays
Si Microtip Arrays

Position sample wedge here

Sample wedge

Pt weld

Microtip post

Top-down image of microtip

Side-view of LEAP microtip coupon

Attach sample to wedge

slice sample and retract wedge
Slice Sample and Retract Wedge

Slice sample from wedge

Remainder of wedge is retracted

Wedge is aligned to the next microtip and the process is repeated

atom probe liftout
Atom Probe Liftout

B. P. Gorman et al., Microsc. Today, 16 (2008) 42.

final sharpening
Final Sharpening
  • Want: 100 - 200 nm diameter, >50m long specimen
  • Have: ~10m2 specimen on post
  • Annular milling patterns used to remove outer material and leave specimen in the center

Ga+

ion solid interaction considerations ga into si
Ion-Solid Interaction Considerations: Ga into Si

SRIM 2003 Simulations

30keV 0° incidence

3.7 sputtered Si / incident Ga

30keV 89° incidence

22 sputtered Si / incident Ga

ion solid interaction considerations ga into si1
Ion-Solid Interaction Considerations: Ga into Si

SRIM 2003 Simulations

5keV 0° incidence

1.4 sputtered Si / incident Ga

5keV 89° incidence

7.1 sputtered Si / incident Ga

fib specimen preparation and implant measurement
FIB Specimen Preparation and Implant Measurement
  • Lower FIB energy results in less Ga implantation
  • Ga implantation is minimal at 2kV
  • K. Thompson, et. al., Ultramic., 107 (2007) 131
fib tem leap prep
FIB / TEM / LEAP Prep
  • Specimens milled directly down alpha tilt axis of TEM and in line with AP detector
instrumentation for fib tem and leap
Instrumentation for FIB TEM and LEAP

Removable Tip Grid Holder

apt designs
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
laser pulsed local electrode atom probe
Laser Pulsed Local Electrode Atom Probe
  • Advantages of laser pulsing
    • Low electrical conductivity materials
    • Improved interface transitions

Voltage pulse

Laser pulse

Eevap

T