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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?