3020 lecture 10 - eds wds fib

7/22/2019 3020 Lecture 10 - Eds Wds Fib

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Energy Dispersive X-ray Spectroscopy

EDS, EDX, EDAX, etc.

Detect X-rays emitted by incident electrons Determine compositional information at high spatial

resolution


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Bremsstrahlung X-rays

Incident e- undergo

Coulombic repulsion with

charged atomic species inthe sample, e- energy is

lost

Conservation of energy and

momentum requires thatphotons are produced

X-ray photons due to

braking (Bremsstrahlung)

gives rise to an energycontinuum

( )

E

EEZiI opcontinuum

=

Gives rise to background in EDS

measurements, must be removedbefore quantification


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Inelastic Scattering - X-ray emission

After the incoming e- beam excites inner shell e- in the sample, theexcited e- decay back to a ground state

In order to obey energy conservation, they emit an X-ray The X-ray energy is dependent upon the elements present within the sample

Can quantify composition by measuring these energies (EDX)

May also emit an Auger e-

Most X-rays generated are re-absorbed by the sample (will discusslater in quantitative analysis)

X-ray energies are labeled by the type of transition (see graphprevious page):

decay of L to K shells: K decay of M to K shells: K decay of M to L shells: L etc.

K1 = K to LII

K3 = K to LI, but is not an allowed transition (see table in book CD)


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Inelastic Scattering Characteristic X-ray

Formation

Bohr model - electronsaround an atom willequilibrate in certain energylevels around an atom

Energy levels are dependentupon the nuclear mass andthe number of electrons


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More Characteristic X-rays

Excited e- decay time ~ 1ps

The probability of a transition is given by:

Incident electron energy (in practice, we want an accelerating voltage 1.5xthe energy of the highest X-ray peak)

Critical Ionization Energy (Ec) a certain E is required in order to excite a

characteristic X-ray (greater than X-ray E) in general, as Z decreases, Ec

decreases - table of all Ec available in table on book CD Cross section for ionization

Fluorescence Yield

Ratio of X-rays produced to the number of shell ionizations

Increases with Z, resets with each X-ray line (K, L, M)

)log(1051.62

20 Uc

UE

bnQ s

c

ss=


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X-ray Detection

How detect the energy

dispersion of the emitted

X-rays?

Similar to the p-n BSE

detector

X-rays hit a

semiconductor, produceelectron-hole pairs

The number of pairs

produced is proportional to

the energy of the X-rays Si(Li) detector diffusion

produced p-n junction that

has intrinsic region ~mm

range under reverse bias

Si(Li) detectors: must be kept cold (77K) to

stop E field induced diffusion and reduce thermal

noise

Other detectors: HPGe, proportional counters,

microcalorimeters, silicon drift detectors


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X-ray detection

The number of e-

/hopairs created

equals the X-rayenergy divided by the

energy needed per

charge (3.8eV/charge

in Si(Li) detectors)

ex: for a 5keV X-

ray, 1316 carriers are

produced, which

results in a charge of2x10-16C for most

detectors

How do we measure

such a small charge?

EDS Windows: block stray light, avoid oil and ice

contamination, sometimes seal vacuum

Window Materials: Be (old systems), polymer films (TW,UTW, SUTW, most common in new systems), windowless


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X-ray detection

The charge is

converted to voltage

by the FET

The voltage pulse is

then shaped and

processed by a set of

amplifiers

Problem: want to

average out noise

(need many pulses),but this must happen

before another X-ray

comes into the

detector and charges

pile up


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X-ray pulse processing

Solution: use 2

amplifiers

Fast amplifier -

determines when a

pulse is coming in or

not, and if there are 2

overlapping pulses -

discriminator andpile-up inspector

decide whether to

reject or accept the

pulse

Slow amplifier -

actually does the

pulse shaping so the

peak height can be

converted into X-rayenergy


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Time Constants and Dead Time

Important stuff from all the pulse

processing

Time Constant- time allowed for

the pulse processor to evaluate themagnitude of the pulse

short time constant, more counts per

second, but with decreased energy

resolution

long time constant gives better

resolution, but smaller cout rate

Dead Time - period of time between

when a pulse is processed and the

detector shuts off to when thedetector can measure another pulse

(closely related to the time constant)


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EDS Spectra

Bremsstrahlung background

Characteristic peaks illustrating the presence ofindividual elements


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Analyzing EDS Data

What we know:

How X-rays are generated How the X-rays are measured in a detector

How the pulse processor turns the detector signal into

X-ray energies

What we can do with this info:

Analyze what elements are in the samples Quantify how much of each element is in the sample


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Qualitative EDS

Qualitative EDS - find out what elements are present pick highest peak intensity and highest energy first

match each of the peak lines with the energies of the X-rays

Today - all systems are computerized use software to scan through the element lists and determine what is present by

which peaks line up with what is in your spectrum careful of SUM PEAKS!

*ALL measurable peaks* (K, L, M) must match with the elements you think are

there - just one line ID does not mean it is correct

Problems - many elements have peaks that overlap with otherelements

KMnKCr

KCrKV

KVKTi

K

K

Al, P, S, Cl

SrMOs

LineInterferes withLineElementNote: Dont

forget what

you coated your

samples with!

Au/Pd can

overlap with many

elements


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Si Escape Peaks

It is probable that an incoming X-ray will

ionize the Si within the detector material,

producing electron/hole pairs, Auger

electrons, and characteristic Si X-rays

Some of these characteristic Si K X-rays

can escape from the detector

This shows itself as an extra peak within

the EDS spectra with an energy equal tothe characteristic X-ray energy minus the

Si K X-ray energy (1.74eV)

The probability of forming escape peaks

increases as the energy of the incoming X-ray approaches 1.74eV (see graph)

Si escape peaks will not occur for X-ray

energies less than the Si K ionization

energy (1.838ev)


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Detector Resolution

Energy spread of the detector is

dependent upon several factors:

natural line width of the characteristicX-rays

electronic noise (cool with LN2)

pulse processing

abruptness of the p-i-n junction in theSi(Li)

Full width at half maximum

(FWHM) is determined at optimal

conditions and standardized for agiven composition (Mn), time

constant, and dead time

Typical ideal resolutions ~137-

140eV

If the FWHM is known at oneenergy, it can be determined at

other energies by:

FWHM= [2.5(EEref) + FWHMref2]1/ 2


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Quantitative Analysis

Following qualitative analysis (what elements are present),

we may be able to quantitatively determine whatcomposition is present

What controls X-ray intensity?

Composition (number of X-rays at a certain energy being emittedfrom the sample)

X-rays produced = (ionizations/e-)*(X-rays/ionization)*(atoms/mole)

*(moles/g)(g/cm3

)(cm) Detection process (how many X-rays emitted get measured)

X-rays detected = (X-rays/ionization)*(fraction emitted towards

detector)*(detector efficiency)*(dead time)*(signal processing

efficiency) etc..


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More Quantitative Analysis

Best way to elminate all of those efficiency factors, etc.is to do Standards Basedanalyses

take a sample of known composition (preferrably close to onethat you are trying to measure) and collect spectra using goodcollection statistics (30% dead time, long collection times,etc.) and standard beam conditions (the same gun bias, spot

size, collection area, etc. you are going to use on your sample) calculate the composition of your sample from:

ZAF = correction for Z (atomic #), A (absorption) and F(fluorescence)

Ci

Cst

ZAF(Ii)

ZAF(Ist)


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ZAF Correction Factors

Z - larger atoms, more BSE (with relation to standard)

A - X-rays must escape the surface without being reabsorbed

dependent upon the (z) distribution of x-ray production with depth into sample

F - fluorescence - X-ray energy from one element is enough to fluoresce an X-rayfrom a neighboring atom, typically zero for


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More ZAF

(z) distribution is dependent

upon accelerating voltage!


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Special Topics - Quant EDS of Thin

Films Thin Film Correction

Model with Monte Carlo

Need to know substrate

comp.

Errors large if thickness of

film unknown High spatial resolution if no

substrate (STEM)

Can also use back to bulkanalysis - use low kV such

that interaction volume is

similar to film thickness


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Special Topics - Quant EDS of Particles

Particle Analyses

Particle size vs. interaction

volume Alters take off angles

Measure many particles tolook for inhomogeneities /

irregularities in spectra


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Special Topics - Quant EDS

Surface Roughness Artifacts

Minimize by tilting sample towards

detector

Low Z Analyses

Use lowest possible U to avoid

large A factors and deal with

detection efficiency problems

Use windowless detectors (can

make results suspect)

z corrections important

Quantitative analysis very difficult


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EDS Summary

Measure composition qualitatively using software, making sure alllines match up

Measure standard composition and ZAF correction factors

Measure sample, quantitatively determine composition usingcorrection factors

Quantitative musts:

choose standards and conditions carefully be aware of ZAF corrections - if ZAF moves significantly away from 1, bad data

Good things about EDS Fast spectra collection

Easy to operate Small, low cost system

Bad things about EDS Spectral artifacts

Poor resolution


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WDS Intro

Wavelength Dispersive Spectroscopy (WDS)

Used in electron probe microanalysis (EPMA)

Use crystal of known crystallography and takeadvantage of Braggs law to increase resolution


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WDS Analysis System


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WDS Crystals


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WDS Analysis

Advantages

High resolution if take off angle remains same Low Z analysis

Lower background

Disadvantages Slow (need many counts)

Large detector can limit imaging resolution

Mechanical movements needed


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EDS / WDS Sampling

How determine composition of your sample? BSE imaging can illustrate possible compositional changes in the

specimen Composition from a specific position: spot mode

Composition from set of specific positions: multipoint spectrumcollection

Composition from an area: area scan mode Composition from a boundary: line scan mode

Composition from an array of specific positions: mapping(spectral imaging, dot mapping, X-ray mapping, etc)

Allows for determination of phase distributions

Allows for determination of minor phases

Quantitative mapping possible


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EDS Mapping - Setup

Qualitative mapping

Max count rate using high beam currents, analytical WD, tilt sample towards detector

Small time constant, low dwell times at each pixel

Ex: 1000cps, 512 x 512 bitmap, 30% deadtime

1 sec dwell time: 375,000 sec, 6240 min, 104 hours!

Better solutions: decrease dwell time (lower total counts) or use smaller energy range for

analysis

Ex: 0.02sec dwell time: 7,500 sec, 125 min, 2 hours

Long scan times: need drift correction

Quantitative mapping Need sufficient counts and energy resolution to do background subtraction, ZAF

corrections

Usually sacrifice spatial resolution to prevent long scans


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Data Recall (all images courtesy EDAX)

Produce line

profiles from

spectral images

Recall EDS

data from

each pixel(voxel)

Produce

element

maps


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EDS Mapping

Digital Dot Mapping (spectra at each pixel)

Window around energy region of interest

Displaying data:

Grayscale - intensity of peaks given grayscale value

(need for scaling the data)

Color - intensity again scaled to give bright / dark areas,different colors given to different elements

Modern EDS systems allow for determination and display of

composition at each pixel or along line profiles


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Quant Map - ZAF Corrected

Pb M

S K

Pb M ZAFPb L ZAF

BSE S K ZAF


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Live Spectral Mapping

Frame averaging gives gradually better EDS data at each pixel,

displays live maps either elementally or as overlay


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Electron Energy Loss Spectroscopy

EELS

Typically used in the TEM / STEM

Can be done in parallel and serial modes Measures the amount of energy lost by an electron after

it passes through a sample of some thickness


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EELS

Bend electron beam around hemispherical

analyzer to separate e- of different energies in

space Can also select a certain window of e- energies to

image with (energy filtered TEM, EFTEM or GIF)

Make maps of composition without STEM


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EELS

Information gathered from EELS

Characteristic energy loss gives elemental composition,

complementary to EDS Zero loss peak intensity gives sample thickness if mean

free path length is known

Plasmon losses give information about majority carriermobility

Ability to map electronic conductivities, etc.

At high energy resolution, can get information about

bandgap Need specialized electron source


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FIB

Focused Ion Beam

Controlled milling / machining using ion beams


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Ion - Solid Interactions

Elastic / Inelastic collisions

Sputtering: Elastic collisions,

momentum transferred fromincident ions into the targetatoms, forming secondary ions(SI)

aka backsputtering

SIs can be measured by its ownplate detector, gives rise todifferent contrast than SEs, similarto BSE

Inelastic scattering: producesSE, phonons, plasmons, etc. Nuclear

Electronic


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Ion - Solid Interaction Ranges

R = range

Rp = projected range

Xs = projected rangealong a vector normal

to the surface

Rr = radial range


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Collision Cascades

Collision cascade: moving sea of particles within a solidunder ion bombardment

3 regimes: I. Single knock-on (M1M2, Eo is high)

Majority of atoms within the spike volume move during thecollision cascade


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More Ion - Solid Interactions

Modeling energy loss in amorphous solids Universal screening function leads to the stopping power of the

target material Take into account both conservation of momentum and the

interatomic potential

Best for MSE scientists to model with SRIM calculations (MonteCarlo, www.srim.org)

Ion Implantation Flux - time rate of flow of energy (ions / cm2 / s)

Fluence - number of particles incident on a specific area (ions /cm2) during a certain time interval

Dose - quantity of ions absorbed by a medium (ions / cm2)

Beam Current / Current Density - time rate of flow (Amps (C/s) orAmps / cm2)


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More Ion - Solid Interactions

Sputtering - sputteringyield = # ejected particles

per incident ion Depends on target,incident energy, angle ofincidence

In FIB, sputter yield variesbetween 0.1 and 100

Ejected ion energiestypically between 2-5eV

Sputter yield tables


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Dual beam:

FEGSEM

FIB

EDS

Omniprobe

EBSD

Mounted directlyunder FIB column

Nova 200 Nanolab


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The FIB Instrument

Similar to SEM

optically Accelerate ions into a

solid sample - but

why? Attachments:

GIS

Liftout / manipulator Dual Beam

EDS, EBSD


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FIB Column

Optically similar to SEM

Consists of:

Gun (next slide)

Condenser Lens

Beam defining aperture (changes spot size and beam current

alone, no real change in condenser setup)

Beam blanker - Faraday cage which brings beam out of optic

axis and into bulk material

Objective lens - changes focal length

Scan coils

Stigmators


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FIB Sources

Liquid Metal Ion System (LMIS)

heat Ga metal above melting temperature

Ga flows to a W tip with radius ~2-5m

use field emission to form 2-5nm Ga tip (Taylorcone)

extract Ga+ ions and accelerate them down the

column

Ga flow continuously replenishes source

i


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FIB Imaging

SE detection

same as SEM (E-T detector)

spatial resolution limited by large interaction volume, aberrations (largeenergy spread for incident beam)

SI detection

can image insulators!!! - as long as charging is not too severe

similar to BSE - only line of sight ions hit the detector Topographical contrast

surface roughness introduces contrast into the electron image

Channeling contrast

ion channeling into individual grains at different orientations givesdifferent backscatter yields, thus contrast

FIB I C l Ch li C


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FIB Ion Column Channeling Contrast

FIB I i P bl


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FIB Imaging - Problems

Ion beam can damage / alter the surface you are

trying to image

Sputtering can take the surface of interest away

Beam induced grain growth

Secondary Ga phase formation

FIB Milli


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FIB Milling

Effects of Z, crystal orientation, incidence angle

Damage (amorphization, theatre curtain)

effects of accelerating voltage, beam current, angle of

incidence

Redeposition

Grain Growth

Ga secondary phase formation

FIB Milli Th t C t i Eff t


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FIB Milling - Theatre Curtain Effect

FIB Milli R d iti


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FIB Milling - Redeposition

FIB Milling Effects of Orientation


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FIB Milling - Effects of Orientation

FIB Milling Parameters


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FIB Milling - Parameters

Can mill using patterns Rectangle - mill top to bottom, reverse, or side to side

Cross section (swimming pool)

Cleaning cross section (one line at a time progressively towardsendpoint)

Circle or donut (mill top to bottom, reverse, side to side, or annular)

Can also mill using bitmap (24-bit RGB)

R: not used G: 0 - 255 (0 = blanking, 255 = no blanking)

B: dwell time (0 = 100ns, 255 = determined by user)

Only works in 100ns steps

If user defines 500ns dwell time, only 5 milling levels allowed If user defines 500s dwell time, 5000 levels allowed (better

depth control

Can ultimately define depth by changing the number of passes

More FIB Milling Parameters


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More FIB Milling Parameters

Computer analyzes milling data pixel by pixel

Max number of points = 1Million

Pitch (x,y) = size of mill set in UI / #pixels in image

Pattern resolution = HFW / pixels in length

Overlap = resolution / spot size (given by selected beam

current aperture)

Decreasing overlap (necessary at low beam

currents) changes milling rate and possibly

structure

FIB TEM Specimen Preparation


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FIB TEM Specimen Preparation

Geometries

H bar or Swimming Pools

Wedge polishing / FIB finishing

Omniprobe Wedge FIB and total liftout technique

Plan-view

In-situ and Ex-situ liftout Artifacts

Ga implantation / contamination - reduce with low kV

cleaning Theatre curtain / Amorphization - prevent with Pt

Spotting

FIB TEM Specimen Prep


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FIB TEM Specimen Prep

In-situ Liftout


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In-situ Liftout

Dual Beam FIB


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Dual Beam FIB

Allows for 3-Dreconstructionsfrom serial sections

IMOD (U.Coloradosoftware)

AMIRA(Mercurysoftware,tgs.com)

EDS - TEM prep

EBSD - TEM prep

X-C SEMpreparation

FIB SIMS

FIB Auger

Courtesy: IMOD

Other Ion Systems in the FIB


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Other Ion Systems in the FIB

Gas Injection Systems (GIS), i.e., Deposition (e-beam, i-beam)

Allow for deposition / etching of structures using gases other than the Ga beam

precursor chemical is heated and injected into the beam path of the electrons / ions via along needle

precursor is converted into metallic species

Methylcyclopentadienyl Pt trimethyl - (CH3)3(CH3C5H4)Pt

Tungsten Hexacarbonyl, W(CO)6

Trimethylamine Alane, (CH3)3NAlH3

TEOS, TMOS + H2O - SiO2 Phenanthrene, pyrene, napthalene for C deposition

On roadmap: Au, Pd, Co, Fe

Enhanced Etching chemistries

gaseous chemical is injected into the Ga beam path, enhances the etch rate by reacting

with the target material Halogens used most often (XeF2, Cl2, Br2, I2) for Si, SiO2, Al, W

also H2O for PMMA and polyimide

Delineation etch (2,2,2-trifluroroacetamide) removes insulator, leaving Si intact

CuRx - reduces Cu channeling / preferential sputtering

Selective C mill - removes organics and plastics, leaving inorganics intact

GIS Deposition with e- and ions


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GIS Deposition with e and ions

e- beam deposition

1. Lower dep rate (~0.2)

2. No Ga contamination

3. No surface damage

4. C and porosity content

increased5. Smaller structures

possible

Ion Beam Deposition

1. High dep rate

2. Ga contamination

3. Residual surface

damage

4. Effects of beam taillimit minimum feature

size

3-D Nanofabrication


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3 D Nanofabrication

Use GIS

deposition to

produce nanoscale3-D structures

Usefulness:

connect lines in IC

devices

lithography mask

repair

MEMS

Milling cap layers

3-D Nanofabrication


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3 Na o ab ca o

3-D Etches


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Useful for

variable etchingrate materials

and devices

FIBIX


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X-ray spectroscopy induced by Ga beam

No Bremsstrahlung X-rays, greatly reduced background

and higher resolution at low Es

Very sensitive at low Es

No high E X-rays detectable

NeednA to get decent spectra

Sample will not last long in spot mode at these high

beam currents

3020 lecture 10 - eds wds fib

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