Broad Argon Beam for Post FIB Clean-Up -...

CONFIDENTIAL (presentation title here)

Broad Argon Beam for Post FIB Clean-Up

Mike Hassel Shearer1 & Vikstram Hakan2

1 Gatan Inc. 2 Oxford Instruments


Broad Argon Beam Tools Gatan

JEOL

Technoorg Linda

Fischione

Leica


Sample Preparation and EM

FIB + 300 V Argon Polish

Argon Polish 8Kev + 1 KeV

Argon Polish 6 KeV

Titan TEM Image of Gate + EELS Data later in presentation

SEM Image FOV 500 micron in diameter ready for EBSD

SEM Image FOV 1mm x 600 microns voids as small as 4nm visible . Key analysis in Fracking


Some of the Tools Used for Sample Prep.


Or Even These $$$$$ ! Useful ? Not Always

CONFIDENTIAL (presentation title here) 6

Ion Beam Sample Preparation Broad Ion Beam milling

Suitable for all types of sample materials o Semiconductors, Ceramics, Metals and Geological materials

Requires mechanical pre-preparation of samples o Dimpling technique

o Wedge technique (Tri-Pod / T-Tool) Semiconductors only

Focused ion beam (FIB) technique Expensive, but advantage is precision

Suited primarily to semiconductors for precision cross sections

Can produce considerable ion beam damage (Amorphization layer)

Samples generally not suitable for HREM without additional cleanup

Broad Ion Beam milling + FIB Best of Both Worlds

o Positional Precision of FIB

o Minimal Damage from low energy (~100-300Volt) Argon


Factors Influencing Ion Milling

Sputtering rate and artifact formation play a role in ion milling

specimens

Sputtering is the removal of atoms from the surface of a sample by

incident energetic particles.

Sputtering occurs by momentum transfer from energetic particles to

specimen atoms.

Sputtering rates depend on several factors:

Flux and velocity of the incident particles (Gun voltage)

Angle of incidence

Relative masses of the specimen atoms and incident particles.

Cohesive energy of the specimen atoms

Chemical interactions between incident particles and the specimen


Sputtering Yield Calculated average sputtering in atoms per ion

Yield decreases with energy

A maximum occurs between 45 and 10 degree beam angle

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0

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0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Sp

utt

erin

g Y

ield

[at

om

s/io

n]

Ion Energy [keV]

Sputtering Yield of Si for various angles (Ar)

90

45

10

8

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0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Sp

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erin

g Y

ield

[ato

ms/

ion

]

Ion Energy [keV]

Sputtering Yield of Si at 4 degree beam angle

Neon

Argon

Xenon

Yield increases with ion mass, so highest for Xenon


Factors Influencing Sample Outcome

Instrument related factors

Ion beam voltage --> Low voltage milling is new direction (100eV – 1.0keV)

Ion beam current --> Output of the ion gun affects Sputter rate

Milling angle (Typically 1° – 10°)

o Low angle milling (< 3°) improves sample quality

o Increases milling times

Specimen holder

o Poor design can restrict low angle milling.

o Poor design can add to re-deposition

o Poor design can damage fragile specimens

Sample related factors

Type of sample (Micro-structural features, Heat sensitive materials)

Method of pre-preparation (Electro-polished, Dimpled)

Starting surface must be free of Mechanical damage (Scratches)

Starting thickness (Low angle milling requires thin samples < 70 µm)

Sample mounting


FIB Technique

Primary vendors of Focused Ion Beam instruments

FEI, Hitachi, JEOL, TESCAN, Zeiss

Two basic model types

Single beam and dual beam

o Single beam permits milling or viewing

o Dual beam permits simultaneous milling and viewing

Liquid Gallium metal source

<5nm resolution

Beam energies from 1keV to 50keV

Beam currents from 1pA to 10,000pA

Image with either Secondary Electrons (SED) or Secondary Ions


FIB Technique

Advantages

Precise cutting of site specific areas

In-situ milling/sectioning and imaging

Precision deposition of metal layers

Disadvantages

High initial cost

Considerable Gallium induced surface damage (20-60nm)

Requires high operator skill

Relatively slow processing compared to standard ion milling


Ion Milling Artifacts

Amorphization Layer Formation

Sufficient atom displacement within the collision cascade to lose long-range order

Surface Oxidation / Contamination

On cleaved Si surface in air, always 1.0 – 1.5 nm of amorphous SiO2 present (J.P. McCaffrey et al., Ultramicroscopy 87 (2001), 97-104)

Re-deposition of Sputtered Material

Especially in milling of confined spaces, FIB Milling of trenches gives considerable re-deposition

Any factor increasing sputtering yield, increases re-deposition (S. Rajsiri et al., M&M 8 (2002), 50-51)


0

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0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Lay

er T

hic

knes

s [A

]

Energy [keV]

Amorphization Layer Thickness of Si at 4 degree beam angle

Neon

Argon

Xenon

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s [A

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Energy [keV]

Amorphization Layer Thickness of Si (Xe)

90

10

6

4

2

Amorphization Layer Thickness

Layer Thickness decreases with beam energy

Layer Thickness decreases with beam angle

However, effects are small for low energy and angle

13

The lightest ion has the thickest layer

Effect highest for high beam energy


Why Improvements in Sample Prep are Needed ?

Significant Improvements in TEM Performance and Analysis

Corrected TEM’s

Resolution improvements in EELS

o Drives the reduction in the damage layer on a 20-40 nm thick sample to less than 1-2 nm

Emerging Improvements in SEM Performance and Analysis

SEM Accelerating Voltages < 100eV

Sensitivity of EBSD and CL to surface damage

o Drives the reduction in the damage layer of the surface to less than 1-2 nm

Sample Prep can limit the performance of a very expensive microscope TEM or SEM !


POST- FIB Clean- Up of TEM Lamella

Time ~< 2 minutes

Keys are : X,Y Stage and Low Energy Performance


Argon ion milling of FIB lift-out samples

Application Note

Note: Very nice application note explaining the geometries involved. Can not find on their website anymore !

Thank You


Issues : 1. Damage from FIB Prep. 2.Re- sputter from welding


Beam Orientation to Lamella on Side of Finger


Beam Orientation to Lamella on Front of Finger


Key Points

Ion Beam is angled with respect to the front edge of the lamella to minimize re-sputtering from the “finger”

Think billiards/pool/snooker

In general the angle to the surface is high ~ 7 degrees

The area of interest of the lamella is centered to the beam and center of rotation

This permits the sample to the beam to be rocked +- ~ 5-7 degrees to reduce channeling and improve uniformity

No Need to go to low voltage on the FIB as this is time consuming and manual.

Customers stop Fibbing at 30 KeV, 10 KeV and 5 KeV


1. Sample prepared in FIB, final milling at 5 keV

2. Sample imaged in TEM

3. Sample was milled in PIPS II:

I. 2 min at 0.1 keV, -80 ⁰ C, ±7⁰ milling angle, Stationary milling mode

4. Sample imaged in TEM

Imaging was done in a FEI TF20 at 200 KV and spot 1, using a Gatan Orius camera.

Work Flow


After FIB After 120 s in PIPS II

Thickness maps Contrast settings are similar for both maps.


After FIB After 120 s in PIPS II


After 120 s in PIPS II


Milling conditions for the clean up:

• Stationary milling mode

• Polish Time : 90 seconds

• Each gun milled separately at 100 eV

• Left gun milled at +7 and right gun milled at -7 degrees

• Temperature: -80° C

The x-y stage in PIPS II made it possible to bring the sample to the rotation center where the guns were aligned.

Imaging in TEM:

• Imaging was done in a FEI TF20 at 200 KV and spot 3, using a single tilt holder and Orius camera.


Area 1 Note : Post FIB Images from Customer’s Titan : Post PIPS Images in Gatan’s TF-20

After FIB After 90 S in PIPS II, 100 eV


Area 1

After FIB After 90 S in PIPS II, 100 eV


Question Which TEM Is Better ? Titan or TF20?

After FIB Titan After 90 S in PIPS II, 100 eV TF-20


What Happens if FIB Only is used ?


And FIB + Broad Ion Beam ?

Thank You

Broad Argon Beam for Post FIB Clean-Up -...

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