Secondary ion mass spectrometry (SIMS) Lasse Vines 1
Secondary ion mass spectrometry O Zn 10000 O 2 Counts/sec 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 Atomic concentration (cm -3 ) 10 19 10 18 10 17 10 16 10 15 P B As 10 14 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Depth (um) 2
Chemical identification Impurity identification Isotope ratio 64 Zn 66 Zn 68 Zn 67 Zn 70 Zn 3
Characterization of device structure 10 21 Atomic concentration (cm -3 ) 10 20 10 19 10 18 10 17 10 16 10 15 10 14 Si 07 Ge 0.3 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 P B As Depth (um) 4
Characterization of solar cell 0,0 1E16 1E17 1E18 1E19 1E20 0,2 Depth (µm) 0,4 0,6 0,8 1,0 1,2 Characterization Optimization of processing Trouble shooting P Concentration (cm -3 ) 5
Ion imaging Na distribution 6
Outline Characteristic features Comparison with other techniques Physical processes Sputtering Ionization SIMS instrumentation Types of mass spectrometers Measurement modes: Mass spectra, Depth profiling, Ion imaging Examples of applications Diffusion studies Material development 7
Characteristic features Quantitative chemical analysis High detection sensitivity 10 16 10 13 atoms/cm 3 (ppm-ppb) Can measure H Large dynamic range > 5 orders of magnitude Very high depth resolution Resolution of 20 Å can be obtained Ion microscopy Lateral resolution < 0.5 µm But, Limited to concentration <5% Samples must be vacuum compatible Samples must be partially conductive Destructive technique 8
Comparison to other techniques 9
Outline Characteristic features Comparison with other techniques Physical processes Sputtering Ionization SIMS instrumentation Types of mass spectrometers Measurement modes: Mass spectra, Depth profiling, Ion imaging Examples of applications Diffusion studies Material development 10
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. 11
Sputtering 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. Sputtering Yield: number of sputtered atoms per incoming ion Sigmund P. Theory of Sputtering, Phys. Rev. 184(2), 383 (1969) 0.5ln1 / Sn 383 E K Nuclear stopping cross-section: it it 2/3 2/3 1/ 2 1 M M Z Z Z Z 32.5 kev 3/8 5/6 Z Z 3 for 0.05 Z Z 5 i t i t 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 i t i t t i 12
Sputtering Dependence of ion 13
Sputtering Example of dependence of target on sputtering yield: (Si 1-x Ge x ) 3 Normalized ion yield 2 1 0 20 40 60 80 100 Ge content (%) 14
Sputtering Example of sputtering of polycrystalline Fe surface The erosion rate is different for the different grains: Sputtering yield vary with the crystal orientation 15
Sputtering Example of sputtering yield: Current: 200 na Sputtering time: 700 sec Depth (µm) 0,0-0,2-0,4-0,6-0,8-1,0 0 100 200 300 400 500 200 µm Width (µm) Material removed: 1200200 µ 3 = 410-8 cm 3 210 15 atoms Incoming ions: 20010-9 A 6.2410 18 ions/c 700 sec = 9x10 14 ions Sputtering Yield = 2.2 atoms/ion 16
Sputtering Secondary ions Energy distribution of secondary ions Secondary intensity (arb. unit) 100000 10000 1000 100 10 28 Si 4 28 Si 28 Si 3 28 Si 2 0 20 40 60 80 100 Energy (ev) 17
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 18
Positive Ion Yield exp C Negative Ion Yield exp C Ionization E i A / / v v C ± : Constants v: velocity perpendicular to surface : work function Negative secondary Positive secondary (Cs) (O) E i A 19
Secondary Intensity(cps) 10 7 10 6 10 5 10 4 10 3 10 2 10 1 10 0 Ionization Mass spectrum of ZnO, Zn peaks. 64 Zn (48.6%) 66 Zn (27.9%) 67 Zn (4.1%) 68 Zn (18.8%) 64 66 68 70 M/q (AMU) Positive mode Negative mode 70 Zn (0.6%) 20
Ionization 1.1 Phosphorus in Si 1-x Ge x 1 Normalized P - - yield 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 21
General Yield Measured intensity I t for a specific target atom I I YC 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 22
Outline Characteristic features Comparison with other techniques Physical processes Sputtering Ionization SIMS instrumentation Types of mass spectrometers Measurement modes: Mass spectra, Depth profiling, Ion imaging Examples of applications Diffusion studies Material development 23
The SIMS instrument Focused ion beam Mass Spectrometer 24
The SIMS instrument Instruments are usually classified by the type of mass spectrometer: Time of Flight Simultaneous detection of many elements High transmission Measures large molecules Quadrupole Low impact energy Magnetic Sector High mass resolution High transmission Low detection limit 25
Time of Flight SIMS 26
Quadrupole SIMS 27
ion source Primary beam Magnetic sector - mass electrostatic sector analyser E 0 Secondary beam r e spectrometer magnetic sector analyser r m B detector Electrostatic sector analyser qe 0 mv r e 2 Magnetic sector analyser qvb mv r m 2 sample Lorenz force: F qe qv B Centripetal force: F mv r 2 r r m q r B 2 r e m E 0 28
Secondary ion mass spectrometry ion source Primary beam electrostatic sector analyser E 0 Secondary beam sample m q r e Br 2 m E r 0 e magnetic sector analyser r m B detector Counts/sec 10000 1000 100 10 Intensity (counts/sec) 10000 1000 100 1 0 20 40 60 80 100 Mass (AMU) 10 Ion image Depth profile 1 0 100 200 300 400 500 600 Sputter time (sec) Mass spectrum 20 µm 29
sourcesinstrumentation ion electrostatic sector analyser magnetic sector analyser sample chamber detectors 30
Mass spectrum O Zn 10000 O 2 Counts/sec 1000 100 Li Na K Cr ZnO 10 1 0 20 40 60 80 100 Mass (AMU) ZnO 2 Mass spectrum of a ZnO-sample with traces of Li, Na, K, and Cr. 31
Mass Interference Secondary intensity (cps) 10 6 10 5 10 4 10 3 10 2 10 1 91.6% 8.4% 10 0 11,5 12,0 12,5 13,0 13,5 Mass (AMU) Mass spectrum of graphite 32
Mass interference Several ions/ionic molecules have similar mass to charge ratios: 10 B - 30 Si 3+ Monitor 11 B 75 As - 29 Si 30 Si 16 O 33
Energy selection E 0 electrostatic sector analyser r e Energy selection slit Increasing kinetic energy Secondary beam qe 0 mv r e 2 log (ion intensity) 75 As 29 Si 30 Si 16 O 0 50 100 Ejection energy (ev) 34
Mass interference Several ions/ionic molecules have similar mass to charge ratios: 10 B - 30 Si 3+ Monitor 11 B 75 As - 29 Si 30 Si 16 O Energy selection 31 P - 30 Si 1 H 35
High mass resolution magnetic sector analyser r m B Discriminating between 31 P and 30 Si 1 H: M( 31 P) = 30.973761 M( 30 Si 1 H) =30.98160 Exit slit 10000 qvb mv r m 2 Intensity (counts/sec) 31 P 1000 30 Si 1 H 100 30,85 30,90 30,95 31,00 31,05 31,10 Mass (AMU) 36
Mass interference Several ions/ionic molecules have similar mass to charge ratios: 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 37
SIMS depth profiling Primary beam Intensity (counts/sec) 10000 1000 100 10 1 0 100 200 300 400 500 600 Sputter time (sec) 38
Calibration of depth profiles Depth calibration Raw phosphorus profile 0,0 Intensity (counts/sec) 100 10 1 0 100 200 300 400 500 600 700 Sputter time (sec) Depth (µm) -0,2-0,4-0,6-0,8-1,0 0 100 200 300 400 500 Width (µm) Sputter time: 700 sec Depth: 9310 Å Erosion rate: 13,3 Å/sec 39
Calibration of depth profiles Concentration calibration Raw phosphorus profile Intensity (counts/sec) I t 100 10 1 0 100 200 300 400 500 600 700 I P Sputter time (sec) Y C t T t 1 C t S S: Sensitivity factor Intensity (counts/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 S Dose I dx x Sensitivity factor: 1 count/sec = 3,410 15 P/cm 3 40
Calibration of depth profiles Raw phosphorus profile Calibrated phosphorus profile Intensity (counts/sec) 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 Sensitivity factor: 1 count/sec = 3,410 15 P/cm 3 Depth (µm) 41
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 42
43 Ion imaging