MiniSIMS Secondary Ion Mass Spectrometer

1. MiniSIMS Secondary Ion Mass Spectrometer Dr Clive Jones Millbrook Instruments Limited Blackburn Technology Centre, England www.millbrook-instruments.com Depth Profiling 101

2. Contents • Depth profiling overview • Sputter rate • Calibration • Depth resolution • Detection limit • Noise • Reproducibility

3. DepthProfilingOverview • Continuously sputter sample to make a crater • Peak switch spectrometer through a selected list of up to 10 masses • Acquire data for the selected number of scans at each mass • Record raw data as counts per second versus etch time • Convert raw data to concentration versus depth

4. Depth Profiling Overview Dopant isotope matrix Matrix isotope concentration  cps dopant depth etch time Rawdata Processed data

5. Depth Profiling Overview MiniSims Depth Profile As Si/100

6. Contents • Depth profiling overview • Sputter rate • Calibration • Depth resolution • Detection limit • Noise • Reproducibility

7. Sputter rate Sputter rate is a function of crater size1 Sputter rate = K / D2 K is a matrix dependant constant2 200 microns D is the lateral crater dimension 1.5 nm/min Si matrix 100 microns D 6 nm/min Si matrix 1For a given primary beam incident angle 2sputter rate also proportional to beam current but this fixed on MiniSIMS

8. Sputter Rate Sputter rate is a function of angle of incidence Sputter rate can be increased by using an angled stub

9. Contents • Depth profiling overview • Sputter rate • Calibration • Depth resolution • Detection limit • Noise • Reproducibility

10. Calibration Concentration = (I/M) . RSF I = Impurity secondary ion counts per second M = Matrix secondary ion counts per second RSF = Relative sensitivity factor of impurity for that matrix

11. Calibration RSF1 calculation from profile of implant of known dose Concentration = (I/M) . RSF Sputter rate = S Depth sputtered in dt = S.dt Dose in one slice = (I/M). RSF. S.dt M = matrix counts Total dose = (I/M). RSF.S .dt I(t) = impurity counts at time t counts RSF .S .dt . I = dt M time M . dose Therefore, RSF = S.dt . I 1 relative sensitivity factor

12. Calibration How to calculate an RSF from a profile of known concentration Concentration = (I/M) . RSF M = matrix counts counts I = impurity counts at peak M . P Then RSF = I time Where P is the known concentration of the implant at its peak Also please note that the sputter rate is not required for this calculation

13. Calibration Presence of oxygen at the surface enhances the positive ion yield Surface ion yield transient region Si+ Counts O+ Time

14. Calibration The surface ion yield transient can distort an impurity profile This may be corrected to some extent during data reduction matrix impurity Data normalized to matrix profile Raw Data The normalized profile is closer to the true distribution

15. Contents • Depth profiling overview • Sputter rate • Calibration • Depth resolution • Detection limit • Noise • Reproducibility

16. Depth Resolution How ion beam mixing affects depth resolution1 buried layer gaussian  counts counts exponential depth depth actual ideal This effect can be reduced by using by using an angled sample stub 1 exaggerated for illustration

17. Depth Resolution Depth resolution improves with increasing angle of incidence

18. Depth Resolution Using angled stub changes angle of incidence of primary beam Orientation of stub important for good secondary ion yields Using an angled stub leads to better depth resolution and a faster sputter rate

19. Depth Resolution Sputter rate is a function of angle of incidence Be aware of sputter rate

20. Depth Resolution Illustration of consequence of sputtering too quickly  concentration concentration depth depth Remedy – slow down sputter rate or reduce number of masses per cycle

21. Depth resolution Gate Illustration of the need for gating Beam size poor gating concentration  concentration depth depth good gating Without gating, some ions from the crater wall will be counted

22. Depth resolution Gating ensures the monitored ions come only from the crater bottom 200 microns 100 microns Indicates size of 10 micron beam Indicates size of 25% gate

23. Depth resolution Choosing the appropriate gate size 100 microns 50% gate 200 microns 50% gate Poor crater edge rejection Good crater edge rejection Indicates size of 10 micron beam

24. Depth resolution Smaller craters may need smaller gate size to preserve depth resolution 100 microns 100 microns 10% gate 50% gate Good Poor Indicates size of 10 micron beam

25. Depth resolution For deep profiles (microns) the crater bottom may become rounded1 100 microns 200 microns gate gate Larger raster size gives better depth resolution because the curvature is less 1Exaggerated for illustration

26. Contents • Depth profiling overview • Sputter rate • Calibration • Depth resolution • Detection limit • Noise • Reproducibility

27. Detection Limit • Count rate limitations • Background from “residual vacuum” species • Background from sample doping • High surface concentration (surface sputtering) • Interferences from primary beam, matrix and residual vacuum species

28. Count rate limit possible solutions • Increase integration (scan) time • Sputter faster • Use oblique ion bombardment

29. Count rate limit - possible solutions Effect of scan duration on data quality Illustrating reduced noise with longer scan time Longer scan time Shorter scan time

30. Count rate limit - possible solutions Effect of scan time on data quality Illustrating improved dynamic range with longer scan time Shorter scan time Longer scan time

31. Count rate limit - possible solutions Effect of scan time on data quality 500px and 200px single scan profiles of same sample 500px is 13.93s/scan; 200px is 2.19s/scan; ratio = 6.36 Longer scan times  less noise + better dynamic range But note that longer scan times also result in less data density

32. High surface or near surface concentration • If there is a peak in impurity e.g. from an ion implant or surface contamination, then the gate needs to be small enough to reject sputtered crater sidewall ions. • Use smaller gate • For high concentration at surface do a two stage profile. Profile through top surface with large raster, continue with smaller raster

33. High surface or near surface concentration Typical ion implant profile illustrating the need for gating Gate Beam size concentration Dynamic Range Without gating With gating depth Without gating, some ions from the crater wall will be counted

34. Interferences Residual vacuum possible solutions • Bakesample and stub • Pump down overnight • Background subtract • Choose a different isotope

35. Interferences • Use sloping stub to reduce level of Ga in sample • Monitor different isotopes, dimers or doubly charged species Primary beam and matrix interference – possible solutions

36. Sample doping issue • Check whether there is real doping or residual vacuum problem • Run the analysis with faster and slower scan times • The unknown / matrix ratio will remain unchanged if the unknown is sample doping

37. Contents • Depth profiling overview • Sputter rate • Calibration • Depth resolution • Detection limit • Noise • Reproducibility

38. Noise Statistical counting noise is proportional to n1/2

39. Noise CPS Noise is inversely related to scans per cycle

40. Contents • Depth profiling overview • Sputter rate • Calibration • Depth resolution • Detection limit • Noise • Reproducibility

41. Guidelines for obtaining the best reproducibility • Use same sample stubeach time with same orientation • Use exactly the same analysis position coordinates each time • Use the integrated peak counts rather than the peak height for detailed comparison of spectra

42. Guidelines for obtaining the best reproducibility • Use exactly the same vacuum conditions each time (either pump down for a given length of time before analysis, or wait until the pressure reading reaches a certain value) • Use the same raster and gate conditions each time • Make sure that the peaks used are at count rates in the linear range of the channeltron. A good rule of thumb would be <100,000 cps.

43. Guidelines for obtaining the best reproducibility Make sure you select the exact mass