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)– 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?

Atom Probe

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

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

Detectability Limits

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

FIB / TEM / LEAP Analysis

LEAP Data Interpretation

STEM APT

‣ FIB prepared Al specimens illustrate Ga phase segregation in STEM-HAADF

‣ LEAP illustrates Ga segregation to GBs

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

‣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

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)

Liftout – Blanket WafersFirst wedge Second wedge Cantilever

Attach Nanomanipulator Liftout

FIB Specimen Prep II

‣Site specific sample preparation – 65nm CMOS transistors

Acknowledgement UFL and INTEL

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

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

Atom Probe Liftout

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

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+

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

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

• 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

Final Preparation of Tip

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

Removable Tip Grid Holder

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

Eevap

T

Voltage pulse

Laser pulse

‣Advantages of laser pulsing– Low electrical conductivity materials

– Improved interface transitions