1 Secondary Ion Mass Spectrometry (SIMS) Thomas Sky
Depth (µm) 2 Characterization of solar cells 0,0 1E16 1E17 1E18 1E19 1E20 0,2 0,4 0,6 0,8 1,0 1,2 P Concentration (cm -3 ) Characterization Optimization of processing Trouble shooting
Atomic concentration (cm -3 ) Characterization of device structure 10 21 10 20 Si 07 Ge 0.3 10 19 10 18 10 17 P B As 10 16 10 15 10 14 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Depth (um) 3
Assignment of electrically active defects Assignment of an impurity to an electrically active deep level defect in ZnO SIMS (impurities) DLTS (electrically active defects) Quemener et al., Appl. Phys. Lett.,102, 232102 (2013)
5 Outline Basic principle and characteristic features Physical processes Sputtering Ionization SIMS instrumentation Types of mass spectrometers Measurement modes: Mass spectra, Depth profiling, Ion imaging
6 Outline Basic principle and characteristic features Physical processes Sputtering Ionization SIMS instrumentation Types of mass spectrometers Measurement modes: Mass spectra, Depth profiling, Ion imaging
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Atomic concentration (cm -3 ) Counts/sec 8 Secondary Ion Mass Spectrometry O Zn 10000 O 2 1000 100 Li Na K Cr ZnO 10 ZnO 2 1 0 20 40 60 80 100 Mass (AMU) 10 21 10 20 Si 07 Ge 0.3 10 19 10 18 10 17 P B As 10 16 10 15 10 14 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Depth (um)
Intensity (counts/sec) 9 SIMS Basic principle 10000 Mass spectrometer 1000 100 Secondary beam 10 1 0 100 200 300 400 500 600 Sputter time (sec)
1 Characteristic features Quantitative chemical analysis High detection sensitivity 10 16 10 12 atoms/cm 3 (ppm-ppb) Can measure H Large dynamic range > 5 orders of magnitude Very high depth resolution Resolution < 20 Å can be obtained Ion microscopy Lateral resolution < 0.5 µm (NanoSIMS ~ 60nm) But, Limited to concentration <1-5% Samples must be vacuum compatible Samples must be partially conductive Destructive technique
11 Outline Basic principle and characteristic features Physical processes Sputtering Ionization SIMS instrumentation Types of mass spectrometers Measurement modes: Mass spectra, Depth profiling, Ion imaging
Intensity (counts/sec) 12 SIMS Basic principle 10000 Mass spectrometer 1000 100 Secondary beam 10 1 0 100 200 300 400 500 600 Sputter time (sec)
Ion solid interaction Matrix atom Impurity atom Primary ion Primary beam Secondary ions are accelerated by an applied sample voltage Energy is transferred from the energetic primary ions to atoms in the sample. Some of these receive enough energy to escape the sample. 13
Sputtering Sputtering Yield: number of sputtered atoms per incoming ion Sputtering yield: S K E it i E i Sn U0 Eit Sputtering is a multiple collision process involving a cascade of moving target atoms, this cascade may extend over a considerable region inside the target. 0.5ln 1 / Sn 383 E K Nuclear stopping cross-section: it it 2/3 2/3 2 1 M M Z Z Z Z 1/ 32.5 kev 3/8 5/ 6 Z Z 3 for 0.05 Z Z 5 i t i t i t i t t i M i, Z i : Ion mass and atomic number M t, Z t : Target mass and atomic number U 0 : Surface escape barrier in ev E i : Ion energy Sigmund P. Theory of Sputtering, Phys. Rev. 184(2), 383 (1969) 14
Normalized ion yield Sputtering Dependence of ion Dependence of target on sputtering yield: (Si 1-x Ge x ) 3 2 1 0 20 40 60 80 100 Ge content (%) Sputtering of polycrystalline Fe surface The erosion rate is different for the different grains: Sputtering yield vary with the crystal orientation 15
Depth (µm) 16 Sputtering Example of sputtering yield: Current: 200 na Sputtering time: 700 sec 0,0-0,2-0,4-0,6-0,8-1,0 200 µm 0 100 200 300 400 500 Width (µm)
Depth (µm) 17 Sputtering Example of sputtering yield: Current: 200 na Sputtering time: 700 sec 0,0-0,2-0,4-0,6-0,8-1,0 200 µm 0 100 200 300 400 500 Width (µm) Material removed: 1 200 200 µ 3 = 4 10-8 cm 3 2 10 15 atoms Incoming ions: 200 10-9 A 6.24 10 18 ions/c 700 sec = 9x10 14 ions Sputtering Yield = 2.2 atoms/ion
Secondary intensity (arb. unit) 28 Si 4 Energy distribution of secondary ions 100000 10000 28 Si 5kV 1000 28 Si 3 28 Si 2 100 10 0 20 40 60 80 100 Energy (ev) Accelerated further (5kV) by an external field in SIMS 18
19 Ionization Ion yield/ionization efficiency : The fraction of sputtered ions that becomes ionized Ion yield can generally not be predicted theoretically Ion yield can vary by several orders of magnitude depending on element and chemistry of the sputtered surface Oxygen on the surface will increase positive ion yield Cesium on the surface will increase negative ion yield 5kV
20 Ionization Positive Ion Yield exp C Negative Ion Yield exp C E i A / v / v C ± : Constants v: velocity perpendicular to surface : work function Positive secondary Negative secondary (O) (Cs) E i A
Secondary Intensity(cps) Ionization 10 7 10 6 10 5 10 4 10 3 10 2 10 1 10 0 Mass spectrum of ZnO, Zn peaks 64 Zn (48.6%) 66 Zn 68 (27.9%) Zn (18.8%) 67 Zn (4.1%) 64 66 68 70 M/q (AMU) Positive mode Negative mode 70 Zn (0.6%) 21
Normalized P - - yield Ionization 1.1 Phosphorus in Si 1-x Ge x 1 0.9 0.8 0.7 0.6 0.5 0.4 0 20 40 60 80 100 Ge concentration (%) Limits the quantification procedure: SIMS is mainly a tool for measuring small concentrations in a given matrix 22
23 General Yield Measured intensity I t for a specific target atom I I Y C T t P t t I P : Primary ion current Y : Sputtering yield (number of sputtered particles per impinging primary ion) [C t ]: Concentration of species t t : Secondary ion formation and survival probability (ionization efficiency) T: Instrument transmission function t is highly dependent on species and matrix
24 General Yield Measured intensity I t for a specific target atom I I Y C T t P t t C RSF I t t From calibration I P : Primary ion current Y : Sputtering yield (number of sputtered particles per impinging primary ion) [C t ]: Concentration of species t t : Secondary ion formation and survival probability (ionization efficiency) T: Instrument transmission function
25 Outline Basic principle and characteristic features Physical processes Sputtering Ionization SIMS instrumentation Types of mass spectrometers Measurement modes: Mass spectra, Depth profiling, Ion imaging
26 The SIMS instrument Focused ion beam Mass Spectrometer
Types of SIMS instrument Instruments are usually classified by the type of mass spectrometer: Quadrupole Low impact energy Time of Flight Simultaneous detection of many elements High transmission Measures large molecules Magnetic Sector High mass resolution High transmission Low detection limit MiNaLab/NICE since 2012 UiO-MiNaLab since 2004
Quadrupole SIMS 28
Time Of Flight-SIMS (TOF-SIMS) Time of Flight SIMS 29
TOF-SIMS Analysis beam Sputter beam Analysis is performed by a short pulse length and small spot size ion beam Sputtering is achieved by a beam of reactive species(e.g. O 2 or Cs) Sputter beam and analysis beam conditions are optimized independently! Analysis gun Extraction Spectrum Sputter gun
33 Magnetic sector - mass spectrometer Electrostatic sector analyser Magnetic sector analyser qe 0 mv r e 2 qvb mv r m 2 Centripetal force: F mv r r r Lorenz force: F qe q v B 2 m q r B 2 r m e E 0
Intensity (counts/sec) Counts/sec 34 Secondary ion mass spectrometry 10000 1000 100 Mass spectrum 10 1 0 20 40 60 80 100 Mass (AMU) 10000 1000 100 10 Depth profile 1 0 100 200 300 400 500 600 Sputter time (sec) m q r B 2 r m e E 0 20 µm Ion image
Magnetic sector vs. TOF-SIMS Superior detection limit (~ppb) Better depth limit (>50um) Better mass resolution Faster depth profiling Better lateral resolution (<130nm) Simultaneous mass spectra Measures large molecules (<10 000 amu) Better for insulating samples Can provide molecular information
36 ion sources Instrumentation electrostatic sector analyser magnetic sector analyser sample chamber detectors
Counts/sec Mass spectrum O Zn 10000 O 2 1000 100 Li Na K Cr ZnO m q Br 2 m E r 0 e 10 ZnO 2 1 0 20 40 60 80 100 Mass (AMU) Mass spectrum of a ZnO-sample with traces of Li, Na, K, and Cr. 37
Secondary intensity (cps) 38 Isotopic abundance 10 6 10 5 91.6% 8.4% 10 4 10 3 10 2 10 1 10 0 11,5 12,0 12,5 13,0 13,5 Mass (AMU) Mass spectrum of graphite
39 Mass interference Several ions/ionic molecules have similar mass to charge ratio: 10 B - 30 Si 3+ = 10 amu 75 As - 29 Si 30 Si 16 O = 75 amu 31 P - 30 Si 1 H = 31 amu The measured SIMS intensity is the sum of the intensity from each elements
40 Mass interference Several ions/ionic molecules have similar mass to charge ratio: 10 B - 30 Si 3+ m q r B 2 r m e E 0
Secondary intensity (cps) 41 Isotopic abundance 10 6 10 5 91.6% 8.4% 10 4 10 3 10 2 10 1 10 0 11,5 12,0 12,5 13,0 13,5 Mass (AMU) Mass spectrum of graphite
42 Mass interference Several ions/ionic molecules have similar mass to charge ratio: 10 B - 30 Si 3+ Monitor 11 B 75 As - 29 Si 30 Si 16 O m q r B 2 r m e E 0
Secondary beam log (ion intensity) 43 Energy selection E 0 electrostatic sector analyser r e Energy selection slit Increasing kinetic energy 75 As qe 0 mv r e 2 29 Si 30 Si 16 O 0 50 100 Ejection energy (ev)
44 Mass interference Several ions/ionic molecules have similar mass to charge ratio: 10 B - 30 Si 3+ Monitor 11 B 75 As - 29 Si 30 Si 16 O Energy selection 31 P - 30 Si 1 H
Intensity (counts/sec) 45 High mass resolution magnetic sector analyser Discriminating between 31 P and 30 Si 1 H: r m B M( 31 P) = 30.973761 M( 30 Si 1 H) =30.98160 Exit slit 10000 31 P 1000 30 Si 1 H qvb mv r m 2 100 30,85 30,90 30,95 31,00 31,05 31,10 Mass (AMU)
46 Mass interference Several ions/ionic molecules have similar mass to charge ratio: 10 B - 30 Si 3+ Monitor 11 B 75 As - 29 Si 30 Si 16 O Energy selection 31 P - 30 Si 1 H High mass resolution
Secondary intensity (cps) Secondary intensity (cps) Secondary intensity (cps) 10 6 10 5 10 4 10 3 10 2 10 1 10 0 Isotopes 98.9% 10 6 10 5 10 4 11,5 12,0 12,5 13,0 13,5 10 3 Mass (AMU) 10 5 91.6% 8.4% 10 2 10 4 10 1 10 3 High mass 1.1% resolution 10 0 10 2 13 C 12 11,5 12,0 12,5 13,0 C 1 H 13,5 10 1 Mass (AMU) Mass spectrum of graphite 10 0 12,96 13,00 13,04 Mass (AMU) 47
Depth resolution O 2+ /Cs + Limited by Surface roughness Sputter rate For large sputter rates or many elements Sputter rate/ measurement cycle Primary beam energy 10-15keV O 2+ /Cs + 5-10 nm 500 ev O + 2 ~2 nm First 2-10nm gives an artificial signal (Dynamic SIMS)
Intensity (counts/sec) Depth (µm) 50 Calibration of depth profiles Depth calibration Raw phosphorus profile 0,0-0,2 100 10-0,4-0,6-0,8-1,0 1 0 100 200 300 400 500 Width (µm) 0 100 200 300 400 500 600 700 Sputter time (sec) Sputter time: 700 sec Depth: 9310 Å Erosion rate: 13,3 Å/sec
Intensity (counts/sec) Intensity (counts/sec) Calibration of depth profiles Concentration calibration Raw phosphorus profile 100 10 1 0 100 200 300 400 500 600 700 Sputter time (sec) Ion implanted sample: P dose 1e15 P/cm 2 10000 1000 100 10 1 0,1 0 100 200 300 400 500 600 Sputter time (sec) sensitivity factor: Relate the intensity to atomic concentration I t I P Y C t T S: Sensitivity factor t 1 C t S S Dose I x dx Sensitivity factor: 1 count/sec = 3,4 10 15 P/cm 3 51
Intensity (counts/sec) 52 Calibration of depth profiles Raw phosphorus profile Calibrated phosphorus profile 100 10 1 0 100 200 300 400 500 600 700 Sputter time (sec) Erosion rate: 13,3 Å/sec 1E17 1E16 1E15 1E14 0,0 0,2 0,4 0,6 0,8 P concentration (cm -3 )1E18 Depth (µm) Sensitivity factor: 1 count/sec = 3,4 10 15 P/cm 3
Ion imaging Intensity recorded as a function of primary beam position Primary beam Secondary beam to detector Distribution of given atoms at the surface Sample Surface 53
Ion imaging 54
Summary 55