MiniSIMS Secondary Ion Mass Spectrometer Dr Clive Jones Millbrook Instruments Limited Blackburn Technology Centre, England Depth Profiling 101

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

Depth Profiling Overview 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

 Raw dataProcessed data cps etch time concentration depth Dopant isotope Matrix isotope matrix dopant Depth Profiling Overview

As Si/100 Depth Profiling Overview MiniSims Depth Profile

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

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

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

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

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

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

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

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

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

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

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

Depth Resolution Depth resolution improves with increasing angle of incidence

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

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

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

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

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

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

100 microns Good Indicates size of 10 micron beam Depth resolution Smaller craters may need smaller gate size to preserve depth resolution 100 microns Poor 10% gate50% gate

For deep profiles (microns) the crater bottom may become rounded 1 gate 100 microns200 microns 1 Exaggerated for illustration Larger raster size gives better depth resolution because the curvature is less Depth resolution

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

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

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

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

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

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 Effect of scan time on data quality Count rate limit - possible solutions

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

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

Residual vacuum possible solutions Bake sample and stub Pump down overnight Background subtract Choose a different isotope 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

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

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

Statistical counting noise is proportional to n 1/2 Counts (n) Noise (root n) Relative noise % % % % Noise

Size of gate Number of scans Actual analysis time 1 count in units of cps 10%s2.19s s 25%s2.19s s 50%s2.19s s CPS Noise is inversely related to scans per cycle Noise

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

Guidelines for obtaining the best reproducibility Use same sample stub each 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

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.

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