MS414 Materials Characterization (재료분석)

Lecture Note 6: SIMS

Byungha ShinDept. of MSE, KAIST

1

2016 Fall Semester

CourseInformationSyllabus1. Overview of various characterization techniques (1 lecture)2. Chemical analysis techniques (9 lectures)

2.1. X-ray Photoelectron Spectroscopy (XPS)2.2. Ultraviolet Photoelectron Spectroscopy (UPS)2.3. Auger Electron Spectroscopy (AES)2.4. X-ray Fluorescence (XRF)

3. Ion beam based techniques (5 lectures)3.1. Rutherford Backscattering Spectrometry (RBS)3.2. Secondary Ion Mass Spectrometry (SIMS)

4. Diffraction and imaging techniques (7 lectures)4.1. Basic diffraction theory4.2. X-ray Diffraction (XRD) & X-ray Reflectometry (XRR)4.3. Scanning Electron Microscopy (SEM) &

Energy Dispersive X-ray Spectroscopy (EDS)4.4. Transmission Electron Microscopy (TEM)

5. Scanning probe techniques (1 lecture)5.1. Scanning Tunneling Microscopy (STM)5.2. Atomic Force Microscopy (AFM)

6. Summary: Examples of real materials characterization (1 lecture)

* Characterization techniques in blue are available at KARA (KAIST analysis center located in W8-1)

Dynamic SIMS Static SIMSTo Mass

Spectrometer Ion Beam

To MassSpectrometer Primary Ion

SIMS (Secondary Ion Mass Spectrometry)• Bombardment of energetic ions (primary ions) à ejection of substrate

atoms and molecules in both neutral and charged (sputtering)• Mass of ejected charged particles (secondary ions) measured

• Faster sputtering rate (0.5 – 5 nm/s)• Material removal• Elemental analysis• Used for depth profiling

• Operate at very low sputtering rate • Ultra surface analysis• Elemental or molecular analysis• Analysis complete before significant fraction of

molecules destroyed (less than 0.1 ML)

Dynamic SIMS

Dynamic SIMS provides depth profile analysis with ppm to ppb detection limits for every element in the periodic table including hydrogen

© Copyright Evans Analytical Group®

Sputtering ProcessInteraction between a charged particle and a solid• Nuclear energy loss

- energy transfer via elastic collisions with the atomic cores- kinematic factor in RBS (light He++, KE=MeV range)- sputtering (heavier ions, KE=keV range; in RBS sputtering yield ~ 10-3)

• Electronic energy loss- inelastic collisions with atomic electrons à electron excitation and ionization- stopping power in RBS

Sputtering Process

Sputter Yield• Sputter yield (# of ejected atoms per incident ion) depends on energy,

incidence angle, bombarding ions, target substrate, and etc.

Energy dependence of the Si sputter yield

Incident ion dependence of the Si sputter yield

Sputter yield of Au vs. ion energy of various noble gas ions

• Comprehensive experimental measurements of sputter yields available at http://dpc.nifs.ac.jp/IPPJ-AM/IPPJ-AM-14.pdf

Sputter yield of Cu vs. incidence angle

© Copyright 2007 Evans Analytical Group®

Sputtering Process in SIMS

Impurity

Primary ion beamO2

+ O- Cs+

200 – 10000eV

} Information depth

Implanted primary ions

Secondary ions

Extraction lens

±200-4500V

Atomic

Molecular

10-10 Torr

±

(Typical mean KE ~ of the order of 10 eV)

Sensitivity of SIMSFactors determining the sensitivity to detect a certain ion or charged fragment • The sensitivity is determined by the ion detection rate in the mass

spectrometer:

• Note: Positive and negative ions are usually collected separately.

: the ion detection rate for element x: the primary ion current (~ 1 nA – 10 µA): the sputter yield for element x (~ 1 – 10): the ionization probability for element x (which varies enormously!) : the fractional concentration of element x in the surface layer: the transmission of the system (0 < h < 1)

𝑅" = 𝐼%𝑌(),"𝑃"±𝜃"𝜂

𝑅"𝐼%𝑌(),"𝑃"±

𝜃"𝜂

ionyield

Ionization ProbabilityFacts about the ionization probability• The fraction of sputtered particles in the ionized state, ion yield, is small

(usually < 1%)• This fraction is strongly dependent on the sputtered particle and on the

sample composition (matrix effects!)• We cannot calculate ion yields (many have tried and failed).• An intuitive picture:

“Competition” for electrons as the sputtered species leaves the surface.

Ion Yield• Ion yields vary over many orders of magnitude from element to

element (this is the a major problem with SIMS).• Example ion yields for different elements sputtered from a material X

• High ionization potential à low positive ion yield (atoms “love” to stay/become neutral)

• High electron affinity results in a high negative ion yield (atoms “love” to pick up an electron from the surface)

Li

Ne

Ar

NbMo Th

U

Hg

Bi

Pb

Tl

W

Hf

Yb

Cs

Sn

InZr

YSr

Sb

TeI

XeKr

BrSe

As

Ge

Ga

MnCrTiV

CaSc

K

Al

Na

P

SiBeB

Au

Pt

Ta

ErHo

DyTbEu

Sm

Nd

Ce

La

Ba

Cd

Ag

Pd

Rh

Rb

Zn

Cu

NiCoFe

ClS

Mg

F

O

N

C

He

H

1.E-29

1.E-28

1.E-27

1.E-26

1.E-25

1.E-24

1.E-23

1.E-22

1.E-21

1.E-20

0 10 20 30 40 50 60 70 80 90 100

Atomic Number

Rel

ativ

e Io

n Yi

eld

• Secondary ion yields can change by 6 orders of magnitude or more!• Ion yields depend strongly on the analysis element.

Positive Ion Yields in Silicon

1E0

1E11E1

1E2

1E3

1E4

1E5

1E6

1E7

1E8

1E9

Ion Yield

Matrix Effects• Table of secondary ion yields from clean and oxidized metal surfaces

• Large increase in the secondary ion yield due to the presence of oxygen• This is a chemical effect: compare the M–M bond with M+–O- bond

chargedividedequallyuponscission

formationofionsismorelikely

Matrix EffectsExample: the effects of oxygen on the Si signal

Secondary ion yield of Si in SiO2 is much higher than in the Si!

• Ion yields depend just as strongly on the sample matrix.

• Arsenic implanted into a Si wafer shows the familiar implant profile shape.

• The same implant into SiO2 on Si looks dramatically different.

• The reason? The ion yield for arsenic in oxide is much (larger? smaller?)

SiO2

Matrix Effects

© Copyright Evans Analytical Group®

Methods to Reduce Matrix EffectsEmploy oxygen ion beams• Oxygen increases the yield of positive secondary ions (chemical effect)• If oxygen could ionize all of the sputtered particles matrix effects would

be reduced (but unfortunately, this doesn’t happen).• Ionization efficiencies can increase by over 4 orders of magnitudes!Supply a partial pressure of O2 in the analysis chamber• After reacting with the sample, the oxygen increases the positive ion

yields.Employ cesium ion beams• Cesium (Cs+) bombardment increases the yield of negative ions• Implantation of Cs tends to reduce the work function of materials (this

makes it easier for electrons to escape the material and “hop” on the atom).

Detect the atoms and not the ions • Use Secondary Neutral Mass Spectrometry• In this technique one tries to post-ionize sputtered neutrals before the

mass spectrometer

Primary Beam Choice for Best Ion YieldIn general, choose the beam that gives you the highest secondary ion yield• For the elements in yellow, O- or O2

+ ion beams are best• For the elements in green, Cs+ beams are best ?

Primary Beam Choice for Best Ion Yield

(Better Negative Yield)(Better Positive Yield)

Quantitative Analysis using StandardsThe use of SIMS standards• As we have seen, matrix effects make quantitative analysis difficult• The ionization probability of a particle strongly depends on its chemical

environment• When dilute concentrations are to be measured, standard can be used.

Example: P in Si• The chemical environment of all P atoms is the same!• You can buy a reference (standard) sample with a known concentration of P in Si• Using this sample, you can determine the relative sensitivity factor (RSPP):

• The concentration of P in an unknown sample can now be determined using:

ISi,R and IP,R are secondary ion currents from Si and P in referenceCSi,R and CP,R are the conc. of Si and P in the reference

Relative Sensitivity Factors• RSFs for an oxygen ion beam, a Si matrix, and detection of positive

secondary ions

• Low RSFs mean a high sensitivity ( )

Wilson,Int.J.MassSpectrometry.IonProc.,143,43(1995)

ingeneral,lowerRSH(highersensitivity)

Relative Sensitivity Factors• RSFs for a cesium ion beam, a Si matrix, and detection of negative

secondary ions

• Modest concentrations of high sensitivity elements can saturate the detector

ingeneral,lowerRSH(highersensitivity)

SIMS Analysis: Example 1Profiling dopants in semiconductors• One of major applications of SIMS (extremely sensitive and quantitative)• Example: Analyzing a P implant in Si using Cs+ primary ions

• The raw data can then be converted to a concentration profile

SIMS Analysis: Example 1Converting sputter time to depth in the sample• One needs to measure the crater depth with a profiler• In our example (P in Si), profilometry gave a 740 nm deep crater

à sputter rate is R = 740 nm / 150 sec ~ 5 nm/s

Converting ion counts to concentration• The RSF for P is used to convert ion counts into consideration

The RSF value for the Phosphorous implant is 1.07X1023 atoms/cm3

The matrix current (ISi) is 2.2X108 counts/sec

𝐶),( =𝐼)𝐼(0𝑅𝑆𝐹

𝐶),( =𝐼)𝐼(0𝑅𝑆𝐹 =

𝐼)2.2 5 108 ×1.07 5 10

;< ≈ 𝐼)×5 5 10?@

SIMS Analysis: Example 1Converting the x-axis and y-axis• x-axis: Depth = Sputter rate x time = 5 nm/s x time• y-axis: Concentration = 𝐶𝑃,𝑆 ≈ 𝐼𝑃×5 ∙ 1014

00 0.20 0.40 0.60 0.80 1.0010

14

1015

1016

1017

1018

1019

1020

1021

DEPTH(µm)

As,B

,XeCO

NCEN

TRAT

ION(atoms/cc)

As

B

Xe

00 50 100 150 200 250 3000.1

1

10

100

103

104

105

106

CYCLES

SECO

NDAR

YIONINTENS

ITY(cts/sec)

As-

B+

Xe+

ProcessedDataRawData

SIMS Analysis: Example 2Effect of elements: different ion yield depending on elements analyzed

Order of increasing ion yield among Xe+, As-, B+? Consistent with Slide 11?

Things to Keep in Mind When Doing SIMSDepth scale• Depth axis is determined from a crater depth• Sputtering can lead to rough crater bottom (limits depth resolution)• Example: in polycrystalline materials rough crater bottoms usually

form because different crystallographic directions have different sputter rates.

Surface mixing• As the sputtering progresses, knock-on diffusion and cascade effects

cause mixing

• The mixing cause buried interfaces to become diffuse

SIMS Analysis: Example 3Copper Diffusion Barrier Evaluation using SIMS Depth Profiling

• Is copper diffusing into the substrate? SIMS depth profiling might be able to tell, but there are problems.

• Polycrystalline materials such as copper will roughen during profiling à depth resolutionis severely degraded

Rough crater bottomCross-section of SIMS crater bottom showing roughening

SIMS Analysis: Example 3Copper Diffusion Barrier Evaluation using SIMS Depth Profiling

• Additionally, the primary ion beam pushes the top material (copper) deeper

• The result? Copper appears diffused even if it is not!

The Solution?

*

SIMS Analysis: Example 3• Flip the sample over. • Polish the backside until there is less than 1 mm of Si left.

1E+15

1E+16

1E+17

1E+18

1E+19

1E+20

1E+21

1E+22

0 0.5 1 1.5 2

DEPTH (microns)

CO

NC

ENTR

ATIO

N (a

tom

s/cc

)

1E+02

1E+03

1E+04

1E+05

1E+06

1E+07

1E+08

1E+09

SEC

ON

DAR

Y IO

N C

OU

NTS

Si->

Cu (repeat)Cu

<---------SiO2-------->

Profiles from the backside reveal that Cu diffusion is real.

Thinningtohere

Assignment ProblemSIMS spectrum of a Si wafer• Problem results from the fact that SiH and P almost have the same

mass!

SIMS Instrumentation

Extraction

Quadrupole

SIMS Mass SpectrometersQuadrupole mass spectrometer

• Light ions more readily responding to AC bias; heavy ions responding to DC bias• Heavy ions are focused to (defocused from) the center axis in x-z plane (y-z plane) • Only allows analysis of secondary ions of a particular mass at a time• Adjusting AC and DC components à choice of a mass

Quadrupole

M+DMMM-DM

MagneticSector

ü Fast peak switching• Multi-element profiles

üEasier charge compensation• Insulating samples

üEasier low primary beam energy• Better depth resolution

O Low mass resolution• Molecular interferences

O Lower transmission• Poorer detection limit

üHigh mass resolution• Reduce interferences

üHigh transmission• Better detection limit

O Slow peak switching• Fewer elements per profile

O Low primary beam energy more difficult• Poorer depth resolution

SIMS Mass Spectrometers

MagneticSector(Cameca) QuadrupoleMassFilter(PHI)

QuadrupoleMassAnalyzer

ElectronMultiplierDetector

90° ElectrostaticAnalyzer

SampleViewingMicroscope

ElectronGun

SampleManipulator

IonPump

IonSource

IonSource

ElectrostaticAnalyzer

MagneticAnalyzer

EnergySlit

ProjectorLens

ElectronMultiplier FaradayCup

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SIMS Instrumentation

Instrumentation and Capabilities

Mass Separation Manufacturer Strengths Weaknesses

Magnetic sector Cameca

• High transmission (~40%)• High mass resolution (M/DM

~ 10,000)

• Slow peak switching (magnet hysteresis effect)

Quadrupole mass filter PHI, Atomika

• Low primary beam energy (down to 100 eV)

• Effective charge compensation for electrically insulating samples

• Fast peak switching

• Low mass resolution (M/DM ~ 200)

• Low transmission (~1%)

• Strengths–Excellent detection sensitivity for dopants, impurities and known

contaminants–Depth profiles of layered structures–Lateral distribution with good resolution–Can detect all elements and isotopes, including H

• Limitations–Destructive–Element specific (poor survey technique): Difficult to find unknown

contaminants–No chemical information–Limited surface information

Strengths and Limitations

SIMS provides the best depth profiling detection limits available.

Time of Flight (TOF)-SIMS

TOF-SIMS is a very surface sensitive technique providing full elemental and molecular analysis with excellent detection limits.

© Copyright Evans Analytical Group®

Ion Induced Desorption

Ejected Species: Atoms, Molecules, Clusters, Ions/Neutrals (+/-)

Static SIMS

• Ultra surface analysis• Elemental or molecular analysis• Analysis complete before significant

fraction of molecules destroyed• Mass spectrometer: quadrupole, TOF*

* TOF SIMS can be also combined with dynamic SIMS

TOF SIMS: Basic Principles

Sample

VAccel

Pulsed Primary Beam

Measure spectrum in flight time:

Convert time axis to mass:

Detector

VDEEFG𝑞 = EJKLFMKE =12 𝑚𝑣

;

𝑡 = 𝑘𝑚?/;

𝑚 = 𝑎𝑡; + 𝑏

Time between pulses long enough so that the slower heavy ions of the first pulse are overtaken by the faster light ions of the second pulse.

Example of TOF-SIMS: Silicon WaferSi+

SiOH+

SiO2-

HSiO3-

O2-

HSi2O5-

Si-

C8H15O-

Positive ion spectrum

Negative ion spectrum

Strengths and Limitations

• Strengths–Can provide specific molecular information on thin (submonolayer)

organic films/contaminants–Survey analysis allows more complete characterization of a

surface–Excellent detection limits (ppm) for most elements–Probe size ~0.2 µm for imaging–Can analyze insulators and conductors

• Limitations–Usually not quantitative–For some samples, organic information can be limited–UHV technique (though cold stage can be used)–At times, too surface sensitive

TOF-SIMS Technical Data Table

Quantitative Limited Destructive No

Detection Limits 107 – 1011 at/cm2 Lateral resolution(Probe size) 0.2 mm

Chemical Bonding Yes Analytical Depth 1 – 5 monolayers