Secondary ion mass spectrometry (SIMS)

Transcription

1 Secondary ion mass spectrometry (SIMS) Lasse Vines 1

2 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 ) 2

3 Characterization of device structure Example: Integrated circuits operational amplifier 741 op-amp 3

4 Characterization of device structure Atomic concentration (cm -3 ) Si 07 Ge P B As Depth (um) 4

5 Secondary ion mass spectrometry O Zn O 2 Counts/sec Li Na K Cr ZnO 10 ZnO Mass (AMU) Atomic concentration (cm -3 ) Si 07 Ge 0.3 P B As Depth (um) 5

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 in semiconductors Identification of surface contamination 6

7 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 in semiconductors Identification of surface contamination 7

8 Characteristic features Quantitative chemical analysis High detection sensitivity atoms/cm 3 (ppm-ppb) Large dynamic range > 5 orders of magnitude Very high depth resolution Resolution of 20 Å can be obtained Ion microscopy Lateral resolution < 0.5 µ 8

9 Comparison to other techniques 9

10 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 in semiconductors Identification of surface contamination 10

11 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

12 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) { } 3/8 ( ξ ) = 0.5ln( 1+ ξ ) ξ + ( ξ / ) 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] 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

13 Sputtering Dependence of ion 13

14 Sputtering Example of dependence of target on sputtering yield: (Si 1-x Ge x ) 3 Normalized ion yield Ge content (%) 14

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, µm Width (µm) Material removed: µ 3 = cm atoms Incoming ions: A ions/c 700 sec = 9x10 14 ions Sputtering Yield = 2.2 atoms/ion 15

16 Sputtering Example of sputtering of polycrystalline Fe surface The erosion rate is different for the different grains: Sputtering yield vary with the crystal orientation 16

17 Sputtering Secondary ions Energy distribution of secondary ions Secondary intensity (arb. unit) Si 4 28 Si 28 Si 3 28 Si Energy (ev) 17

18 Ionization Ion yield: 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

19 Positive Ion Yield exp Negative Ion Yield exp Ionization + ( C ( Ei ϕ) / v) C ( ϕ A) / v ( ) C ± : Constants v: velocity perpendicular to surface ϕ: work function Negative secondary Positive secondary (Cs) (O) E i A 19

20 Secondary Intensity(cps) Ionization Mass spectrum of ZnO, Zn peaks. 64 Zn (48.6%) 66 Zn (27.9%) 67 Zn (4.1%) 68 Zn (18.8%) M/q (AMU) Positive mode Negative mode 70 Zn (0.6%) 20

21 Ionization Phosphorus in Si 1-x Ge x Normalized P - - yield Ge concentration (%) 21

22 General Yield Measured intensity I t for a specific target atom I t = I P Y [ C ] γ T t t I P : Primary ion current Y : Sputtering yield (number of sputtered particles per impinging primary ion) [C t ]: Concentration of species t γ t : Secondary ion formation and survival probability (ionization efficiency) T: Instrument transmission function γ t is highly dependent on species and matrix 22

23 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 in semiconductors Identification of surface contamination 23

24 The SIMS instrument 24

25 The SIMS instrument Instruments are usually classified by the type of mass spectrometer: Time of Flight Simultaneous detection of many elements High transmission Quadrupole Low impact energy Magnetic Sector High mass resolution High transmission Low detection limit 25

26 Time of Flight SIMS 26

27 Quadrupole SIMS 27

28 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 + q( v B) Centripetal force: F = mv r 2 r r m = q ( r B ) 2 r e m E 0 28

29 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 Intensity (counts/sec) Mass (AMU) 10 Ion image Depth profile Sputter time (sec) Mass spectrum 20 µm 29

30 sourcesinstrumentation ion electrostatic sector analyser magnetic sector analyser sample chamber detectors 30

31 Mass Interference Secondary intensity (cps) % 8.4% ,5 12,0 12,5 13,0 13,5 Mass (AMU) Mass spectrum of graphite 31

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 32

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 Ejection energy (ev) 33

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 34

35 High mass resolution magnetic sector analyser r m B Discriminating between 31 P and 30 Si 1 H: M( 31 P) = M( 30 Si 1 H) = Exit slit qvb = mv r m 2 Intensity (counts/sec) 31 P ΔΜ/Μ = Si 1 H ,85 30,90 30,95 31,00 31,05 31,10 Mass (AMU) 35

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 36

37 Secondary intensity (cps) Secondary intensity (cps) % ,5 12,0 12,5 13,0 13, Mass (AMU) Secondary intensity (cps) % Isotopes High mass 1.1% resolution C 12 11,5 12,0 12,5 13,0 C 1 H 13,5 Mass (AMU) 8.4% Mass spectrum of graphite 12,96 13,00 13,04 Mass (AMU) 37

38 Mass spectrum O Zn O 2 Counts/sec Li Na K Cr ZnO Mass (AMU) ZnO 2 Mass spectrum of a ZnO-sample with traces of Li, Na, K, and Cr. 38

39 SIMS depth profiling Primary beam Intensity (counts/sec) Sputter time (sec) 39

40 Calibration of depth profiles Depth calibration Raw phosphorus profile 0,0 Intensity (counts/sec) Sputter time (sec) Depth (µm) -0,2-0,4-0,6-0,8-1, Width (µm) Sputter time: 700 sec Depth: 9310 Å Erosion rate: 13,3 Å/sec 40

41 Calibration of depth profiles Concentration calibration Raw phosphorus profile Intensity (counts/sec) I t = I P Sputter time (sec) Y [ C ] γ T t t = ( )[ ] 1 C t S S: Sensitivity factor Intensity (counts/sec) Ion implanted sample: P dose 1e15 P/cm , Sputter time (sec) sensitivity factor: Relate the intensity to atomic concentration S = Dose I dx ( x) Sensitivity factor: 1 count/sec = 3, P/cm 3 41

42 Calibration of depth profiles Raw phosphorus profile Calibrated phosphorus profile Intensity (counts/sec) 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, P/cm 3 Depth (µm) 42

43 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 43

44 Examples of Applications Depth profiling Dopant diffusion in semiconductors Isotope enriched superstructures Ion imaging Characterizing contacts on sample surface Impurity after processing Mass spectrum N doping of MOCVD grown ZnO 44

45 Radiation enhanced diffusion of B in Si Fick model: D B = cm 2 /s Irradiation 2 MeV H + 1 Hour, 580 C B concentration (cm -3 ) Depth (µm) Lévêque et al. J. Appl. Phys. 89, 5400 (2001) 45

46 Isotope enriched superstructures As in-diffusion in a Ge isotope enriched superstructure 46

47 Ion imaging 200 µm Si Cu Na 30 µm 47

48 48 Ion imaging

49 N doping of MOCVD grown ZnO Main impurities: C, Al, Si, and Ca Profiles of contamination Ca is introduced through interface and from the surface New optimized samples have reduced contamination Al still present Intentional uniform N doping achieved ZnO Al 2 O 3 49