MiniSIMS Secondary Ion Mass Spectrometer

MiniSIMS

Secondary Ion Mass Spectrometer

Dr Clive Jones

Millbrook Instruments Limited

Blackburn Technology Centre, England

www.millbrook-instruments.com

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 data Processed data

cps

etch timeco

ncen

trat

ion

depth

Dopant isotope

Matrix isotopematrix

dopant

Depth Profiling OverviewDepth Profiling Overview

AsSi/100

As Implant 60 keV 1e16 at/cm2

1E+18

1E+19

1E+20

1E+21

1E+22

0 500 1000 1500 2000

Sputtered Depth (Angstroms))

Con

cent

ratio

n (a

t/cc)

Depth Profiling OverviewDepth Profiling OverviewMiniSims Depth Profile

Contents


• Sputter rate

• Calibration


• Detection limit

• Noise

• Reproducibility

6 nm/minSi matrix

1.5 nm/minSi matrix

100 microns

200 microns

D

Sputter rate = K / D2

K is a matrix dependant constant2

D is the lateral crater dimension

2sputter rate also proportional to beam current but this fixed on MiniSIMS

Sputter rateSputter rate is a function of crater size1

1For a given primary beam incident angle

Sputter RateSputter rate is a function of angle of incidence

Sputter rate can be increased by using an angled stub

Ga 6keV in Si - Results of Detailed SRIM Calculations

0

2

4

6

8

10

12

14

16

18

0 10 20 30 40 50 60 70 80 90

Angle from Normal (Degrees)

Spu

tte

r Y

ield

Si i

n a

tom

s/io

n

Contents


• Sputter rate

• Calibration


• 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

coun

ts

Concentration = (I/M) . RSF

M = matrix countsDose 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

=

RSF1 calculation from profile of implant of known dose

1 relative sensitivity factor

Calibration

I = impurity counts at peak

time

coun

ts M = matrix countsConcentration = (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+

Cou

nts

Time

Surface ion yield transient region

Presence of oxygen at the surface enhances the positive ion yield

Calibration

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


• Sputter rate

• Calibration


• Detection limit

• Noise

• Reproducibility

coun

ts

depth depthco

unts

exponential

gaussian

buried layer

How ion beam mixing affects depth resolution1

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

Detailed SRIM calculations for 6kV Ga ion bombardment of Si

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80

Degrees from normal

Ga R

ang

e in

Angst

rom

s

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

T U R B O

Quadrupole

Sample

Sloping stub

Best analysis area

Direct away from quad / turbo

Depth ResolutionSputter rate is a function of angle of incidence

Be aware of sputter rate

Ga 6keV in Si - Results of Detailed SRIM Calculations

0

2

4

6

8

10

12

14

16

18

0 10 20 30 40 50 60 70 80 90

Angle from Normal (Degrees)

Spu

tter

Yie

ld S

i in

atom

s/io

n

conc

entr

atio

n

depth

conc

entr

atio

n

depth

Depth Resolution

Illustration of consequence of sputtering too quickly

Remedy – slow down sputter rate or reduce number of masses per cycle

conc

entr

atio

n

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 depth

conc

entr

atio

n

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 rejection Good crater edge rejection

Depth resolution


100 microns50% gate

200 microns50% gate

Choosing the appropriate gate size

100 microns

Good


Depth resolution

Smaller craters may need smaller gate size to preserve depth resolution

100 microns

Poor

10% gate 50% gate

For deep profiles (microns) the crater bottom may become rounded1

gate

gate

100 microns 200 microns

1Exaggerated for illustration

Larger raster size gives better depth resolution because the curvature is less

Depth resolution

Contents


• Sputter rate

• Calibration


• 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 solutionsCount rate limit - possible solutions

1

10

100

1000

10000

100000

0 1000 2000 3000 4000 5000 6000

Etch Time (s)

Inte

nsi

ty

Effect of scan time on data quality

Longer scan time

Shorter scan time

Illustrating improved dynamic range with longer scan time


1

10

100

1000

10000

0 1000 2000 3000 4000 5000 6000

Etch Time (s)

Inte

nsi

ty (

cps)

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


1

10

100

1000

0 200 400 600 800

Etch Time (s)

Inte

ns

ity

(cp

s)

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

conc

entr

atio

n

depth

With gating

Without gating

Dyn

amic

Ra

nge

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 concentrationHigh surface or near surface concentration

Residual vacuum possible solutions

• Bake sample and stub

• Pump down overnight

• Background subtract

• Choose a different isotope

Interferences

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


• Sputter rate

• Calibration


• Detection limit

• Noise

• Reproducibility

Statistical counting noise is proportional to n1/2

Counts

(n)

Noise

(root n)

Relative noise

10 3.2 32%

100 10 10%

1000 32 3.2%

10000 100 1.0%

Noise

Size

of

gate

Number of

scans

Actual analysis

time

1 count in units of

cps

10% s 2.19s

10

4.57

s

25% s 2.19s

4

1.83

s

50% s 2.19s

2

0.91

s

CPS Noise is inversely related to scans per cycle

Noise

Contents


• Sputter rate

• Calibration


• 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


• 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.


Make sure you select the exact mass

MiniSIMS Secondary Ion Mass Spectrometer

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