Secondary Ion Mass Spectrometry

Professor Paul K Chu

Secondary Ion Mass Spectrometry (SIMS)

Sputtering by Elastic Collisions

Single knock-on< 1keVAll secondary ionsvirtually originate fromthe uppermost atomiclayers

Linear cascade1 keV – 1 MeVsputtering yieldproportionalto beam energy

Spike > 1 MeVHigh density of recoil atoms

Ion – Solid Interactions

(a) Sputtering event, T=0

Predicted trajectories

(b) Sputtering event, T10-13 s

Post trajectories - indicated

(c) Sputtering event, T 10-10s

Sputtering Events with Time

Simulated Trajectories

Computer simulation: Displacement of Cu atoms due to the impact of 4 keV argon ions

(a) Trajectories within the entire volume of collision cascade for 10 incident particles

(b,c) Transport of target atoms out of and into the designated layer (20 incident particles)

(d) Trajectories of sputtered atoms (50 incident particles)

Sputtering YieldSputtering yield is the average number of

sputtered particles per incident ion.

• In the linear cascade regime, the sputtering yield is proportional to ion beam energy.

• Sputtering yield depends on a) atomic number, b) Displacement energy, c) Matrix of solid.

Ion sputtering yield is the average number of ions emitted per incident primary ion.

Many factors affect the ion yield. The most obvious are

• Intrinsic tendency to be ionized

Positive ion : Ionization potential (IP)

Negative ion: Electron affinity (EA)

• Matrix effects

Al+ from Al2O3 versus Al+ from Al metal

Secondary ion cluster spectrum from Ar ion bombardment of Al. Note that the ordinate is in a log scale. Predominant species are Al+ ions; Al2

+ and Al3+

are also abundant

Matrix Effects

Absolute secondary ion yields as a function of atomicnumber, under high vacuum conditions (a) and under oxygen saturation (b): 3keV Ar+, incident angle 60o, beam density 10-3 A/cm-2, pressure 10-10 Torr

I = I - ICLEAN

Ion Yield Enhancement

Relative positive ion yield for 13.5 13.5 keV normal incident O-; -compound was used, B.D; Barely detectable

Relative negative ion yield for 16.5 keV Cs+, normal incidence

Enhancement by O- Enhancement by Cs+

Ion Yield versus Ionization Potential and Electron Affinity

(a) Positive relative ion yield of various certified elements (M+/Fe+) in NBS 661 stainless steel reference material versus ionization potential

(b) Negative relative ion yields of various certified elements (M-/Fe-) in NBS 661 stainless steel reference material versus electron affinity

Secondary Ion YieldThe variability in ionization efficiency leads to different analysis conditions for elements as indicated on the periodic table.

Selection of Primary Ions

Positive and Negative Ion Spectra

Al alloyPositive ion spectrum

Negative ion spectrum

GaAs

Positive and Negative Ion SpectraPositive and negative spectra are complementary and useful in searching for traces of chemical elements and their complexes.

Instrumentation Ion Sources• Ion sources with electron impact ionization - Duoplasmatron: Ar+,

O2+, O-

• Ion sources with surface ionization - Cs+ ion sources• Ion sources with field emission - Ga+ liquid metal ion sources

Mass Analyzers• Magnetic sector analyzer• Quadrupole mass analyzer• Time of flight analyzer

Ion Detectors• Faraday cup• Dynode electron multiplier

SIMS CAMECA 6F

Cameca SIMS1. Cs ion source2. Duoplasmatron ion source3. Primary beam mass filter4. Immersion lens5. Sample6. Dynamic emittance matching7. Transfer lens system8. Liquid metal source9. Entrance slit S1

10. 90o electrostatic analyzer11. Energy slit S2

12. Intermediate lens 113. 90o magnetic sector 14. Exit slit S315. Projection lenses16. Projection deflector17. Channelplate18. Fluorescence screen19. Electron multiplier20. Faraday cup

Magnetic Sector Analyzer

High transmission efficiencyHigh mass resolutionImaging Capability

R 2000Capable: R ~ 105

Ion Detectors

Faraday Cup

Secondary electronMultiplier20 dynodesCurrent gain 107

Quadrupole SIMS

Energy Distribution of Sputtered Particles

Energy distribution of neutral particles of some elemental polycrystalline targets emitted in the direction of the surface normal: Ar+ ions, EP = 900 eV, incident angle = 0 O

Voltage Offset Technique

Voltage Offset Technique

Mass Resolution• Several definitions of mass resolution (R).

• R - capability of a mass spectrometer to differentiate between masses.

M - mass difference between two adjacent peaks that are just resolved M - nominal mass of the first peak or mean mass of two peaks.

• Resolution is also defined as the full width at half maximum (FWHM) of a peak.

C2H4+ 28.0313

CH2N+ 28.0187 M = 0.0126

N2+ 28.0061

CO+ 27.9949

1000,11000

M

MRthenMandMIf

2220

0126.0

0187.280313.2821

R

Common Mass Interferences Interfering Analytical Required M

ion ion resolution

28Si+ 32S+ 960 0.0146

Matrix 16O2+ 32S+ 1800 0.0178

ions Si2+ 56Fe+ 2960 0.0189

46Ti28Si+ 75As+ 10940 0.0069 46Ti29Si+ 75As+ 10500 0.0091

Matrix+ 29Si30Si16O+75As+ 3190 0.0235 primary Hydrates 30Si1H 31P+ 3950 0.0078

27Al1H- 28Si- 2300 0.012054Fe1H+ 55Mn+ 6290 0.0087120Sn1H+ 121Sb+ 19250 0.0062

Hydrocarbons 12C2H3+ 27Al+ 640 0.0420

12C5H3+ 63Cu+ 670 0.0939

Boron Implanted Silicon Wafer

Quantitative Analysis

IA+T = jp A YA+

T fA+T CA+

T

Primaryion current density

Area ofanalysis

Instrumentaltransmission factor for A+

Measured secondary ion current of A+in the matrix T

Secondary ion yieldin the matrix T

Atomic concentration of Ain the matrix T

IA+T = SA+

T CA+T Sensitivity factor for A in the

matrix T

Very difficult to calculate SA+T. It depends on the1. Element and matrix2. SIMS instrument3. System parameters

SA+T

Standards are normally used

Standard the same matrix

SampleMeasure IA+

T

Use SA+T from standard

Find CAT

Measure IA+T

From known CAT

Find SA+T

Quantitative Analysis usingRelative Sensitivity Factors

Detection Limits (Sensitivity)

Inherent SIMS Sensitivity• Silicon with an atom density of 51022 Si atoms/cm3

• Bombarded area of (100 m)2 = (10-2 cm)2 = 10-4 cm2

• Sputtering rate of 1.0 nm/sec = 10-7 cm/sec• Then, silicon volume removed per second by sputtering is V = 10-4 cm2 10-7 cm/sec = 5 10-11 cm3/sec• Hence, a number of the removed atoms per second by sputtering is N = 5 1022 cm-3 10-11 cm-3/sec = 5 1011/sec Assume • 1% Secondary ion yield• 10% Ion transmission• Then, ions detected 5 1011/sec 10-3 ions = 5 108 ions/sec• If 5 ions/sec is a threshold, then • (5 ion/sec)/(5 108 ion/sec) = 10-8 = 10 10 ppb • The detection limit is 5 1014 atoms/cm3

Typical Detection Limits in Silicon

Primary Ion Beam O2+ or Cs+

Element Detection Limit Element Detected Ion atom/cm3

B 11B+ <1013 P 31P-/31P+ <51014

As 28Si75As - <1014

Sb 121Sb+ <51013

C 12C- <51015

O 16O- <51016

N SiN- <51015

H H- <51017

Common Modes of Analysis

• The bulk analysis mode is used to detect trace-level components, while sacrificing both depth and lateral resolution.

• The mass scan mode is used to survey the entire mass spectrum within a certain volume of the specimen.

• The depth profiling mode is use to measure the concentration of pre-selected elements as a function of depth from the surface.

• The imaging mode is used to determine the lateral distribution of pre-selected elements. In certain circumstances, an imaging depth profile combining both depth profiling and imaging can be obtained.

Mass Spectrum

Fingerprint of polymers

Positive mass spectrumfrom polyethylene, 0 - 200 amu

Positive mass spectrumfrom polystyrene,0 - 200 amu

Sometimes both positive and negative spectra are needed

Positive mass spectrum from polyphenylene sulfide,0 – 200 ammu No indication of SIt looks like polyethylene

Negative mass spectrumfrom polyphenylene sulfide,0 –250 amu

Dynamic SIMS – Depth Profiling

Factors Affecting Depth Resolution

CRATER EFFECT

The shape of the depth profile

can be affected by

a) Redeposition by sputtering

from the crater wall onto

the analysis area

b) Direct sputtering from the

crater wall

Crater Effect

(a)

(b)

(a) Ions sputtered from a selected central area (using a physical aperture or electronic gating) of the crater are passed into the mass spectrometer.

(b) The beam is usually swept over a large area of the sample and signal detected from the central portion of the sweep. This avoids crater edge effects.

The analyzed area is usually required to be at least a factor of 3 3 smaller than the scanned area.

Crater Side-Wall Contribution

Crater Bottom Flatness

Effects of Reducing Primary Ion Energy

Effects of Reducing Primary Ion Energy

Effects of Primary Angle of Incidence

Crater Bottom Roughening

Sample Rotation

• Stable Primary Ion Gun

• Mass Analyzer with High Stability

• Low Noise Electronics and Highly Stable Detector

• Consistent Secondary Ion Extraction

Requirements for High Precision SIMS Analysis

High Precision Depth Profiling

Typical Applications in Semiconductor Industry

Energy Contamination in Ion Implanted Materials

P-N Junction in Silicon

Gate Oxide Breakdown

Imaging

The example (microbeam) images show a pyrite (FeS2) grain from a sample of gold ore with gold located in the rims of the pyrite grains. The image numerical scales and associated colors represent different ranges of secondary ion intensities per pixel.

Some instruments simultaneously produce high mass resolution and high lateral resolution. However, the SIMS analyst must trade high sensitivity for high lateral resolution because focusing the primary beam to smaller diameters also reduces beam intensity. High lateral resolution is required for mapping chemical elements.

34 S197 AU

Cross-Sectional Imaging

Cross-sectional 27Al- Image depth profile of SiO2 capped GaAs/AlGaAs superlattice with a 4 micrometer laser melt strip

Dynamic SIMS versus Static SIMS

Time-of-Flight (TOF) SIMS

TOF-SIMS Analysis of Polymers

Surface Analysis of Silicon Wafers

Characterization of Hard Disk Lubricants

Characterization of Hard Disk Lubricants

Sample Tutorial Questions

• What are matrix effects?• What is the difference between ion yield

and sputtering yield?• When are oxygen and cesium ions used as

primary ions?• Why is the primary ion rastered when

acquiring a depth profile?• How can depth resolution be improved?• How are mass interferences separated?