Secondary Ion Mass Spectrometry Professor Paul K Chu

Secondary Ion Mass Spectrometry (SIMS)

Sputtering by Elastic Collisions Single knock-on < 1keV All secondary ions virtually originate from the uppermost atomic layers Linear cascade 1 keV – 1 MeV sputtering yield proportional to beam energy Spike > 1 MeV High density of recoil atoms

Ion – Solid Interactions

(a) Sputtering event, T=0 Predicted trajectories (b) Sputtering event, T  s Post trajectories - indicated (c) Sputtering event, T  s Sputtering Events with Time

Sputtering / Ion Yields Sputtering 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 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 Al 2 O 3 versus Al + from Al metal

Matrix Effects Absolute secondary ion yields as a function of atomic number, under high vacuum conditions (a) and under oxygen saturation (b): 3keV Ar +, incident angle 60 o, beam density  A/cm -2, pressure Torr  I = I - I CLEAN

Ion Yield Enhancement Relative positive ion yield for 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

Selection of Primary Ions

Positive and Negative Ion Spectra Al alloy Positive ion spectrum Negative ion spectrum GaAs

Instrumentation Ion Sources Ion sources with electron impact ionization - Duoplasmatron: Ar +, O 2 +, 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 7F

Cameca SIMS 1.Cs ion source 2.Duoplasmatron ion source 3.Primary beam mass filter 4.Immersion lens 5.Sample 6.Dynamic emittance matching 7.Transfer lens system 8.Liquid metal source 9.Entrance slit S o electrostatic analyzer 11.Energy slit S 2 12.Intermediate lens o magnetic sector 14.Exit slit S3 15.Projection lenses 16.Projection deflector 17.Channelplate 18.Fluorescence screen 19.Electron multiplier 20.Faraday cup

Magnetic Sector Analyzer High transmission efficiency High mass resolution Imaging Capability R  2000 Capable: R ~ 10 5

Ion Detectors Faraday Cup Secondary electron Multiplier 20 dynodes Current gain 10 7

Quadrupole SIMS

Time-of-Flight (TOF) 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, E P = 900 eV, incident angle = 0 O

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. C 2 H CH 2 N  M = N CO

Common Mass Interferences InterferingAnalyticalRequired  M ionionresolution 28 Si + 32 S Matrix 16 O S ionsSi Fe Ti 28 Si+ 75 As Ti 29 Si+ 75 As Matrix+ 29 Si 30 Si 16 O +75 As primary Hydrates 30 Si 1 H 31 P Al 1 H- 28 Si Fe 1 H+ 55 Mn Sn 1 H+ 121 Sb Hydrocarbons 12 C 2 H Al C 5 H Cu

Boron Implanted Silicon Wafer

Quantitative Analysis I A+ T = j p  A  Y A+ T  f A+ T  C A+ T Primary ion current density Area of analysis Instrumental transmission factor for A + Measured secondary ion current of A+ in the matrix T Secondary ion yield in the matrix T Atomic concentration of A in the matrix T I A+ T = S A+ T  C A+ T Sensitivity factor for A in the matrix T Very difficult to calculate SA+T. It depends on the 1. Element and matrix 2. SIMS instrument 3. System parameters S A+ T Standards are normally used Standard the same matrix Sample Measure I A+ T Use S A+ T from standard Find C A T Measure I A+ T From known C A T Find S A+ T

Quantitative Analysis using Relative Sensitivity Factors

Inherent SIMS Sensitivity Silicon with an atom density of 5  Si atoms/cm 3 Bombarded area of (100  m) 2 = (10 -2 cm) 2 = cm 2 Sputtering rate of 1.0 nm/sec = cm/sec Then, silicon volume removed per second by sputtering is V = cm 2  cm/sec = 5  cm 3 /sec Hence, a number of the removed atoms per second by sputtering is N = 5  cm -3  cm -3 /sec = 5  /sec Assume 1% Secondary ion yield 10% Ion transmission Then, ions detected 5  /sec  ions = 5  10 8 ions/sec If 5 ions/sec is a threshold, then (5 ion/sec)/(5  10 8 ion/sec) = = 10  10 ppb The detection limit is 5  atoms/cm 3

Typical Detection Limits in Silicon Primary Ion Beam O 2 + or Cs + Element Detection Limit Element Detected Ion atom/cm 3 B 11 B + <10 13 P 31 P - <5  As 28 Si 75 As - <10 14 Sb 121 Sb + <5  C 12 C - <5  O 16 O - <5  NSiN - <5  HH - <5  10 17

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.

Fingerprint of polymers Positive mass spectrum from polyethylene, amu Positive mass spectrum from polystyrene, 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

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 (FeS 2 ) 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 S 197 AU

Cross-Sectional Imaging Cross-sectional 27 Al - Image depth profile of SiO 2 capped GaAs/AlGaAs superlattice with a 4 micrometer laser melt strip

TOF-SIMS Analysis of Polymers

TOF-SIMS Surface Analysis of Silicon Wafers

TOF-SMS 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?