Calibration of secondary neutral and secondary ion mass spectrometry: A comparativestudyJ. Tümpner, R. Wilsch, and A. Benninghoven Citation: Journal of Vacuum Science & Technology A 5, 1186 (1987); doi: 10.1116/1.574636 View online: http://dx.doi.org/10.1116/1.574636 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/5/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Comparative ion yields by secondary ion mass spectrometry from microelectronic films J. Vac. Sci. Technol. A 19, 1134 (2001); 10.1116/1.1361037 Sputtering investigation of boron nitride with secondary ion and secondary neutral mass spectrometry J. Vac. Sci. Technol. A 15, 243 (1997); 10.1116/1.580519 A scanned microfocused neutral beam for use in secondary ion mass spectrometry J. Vac. Sci. Technol. A 4, 1888 (1986); 10.1116/1.573741 High sensitivity quasisimultaneous secondary neutral, secondary ion, and residual gas mass spectrometry by anew electron impact postionizer J. Vac. Sci. Technol. A 3, 2035 (1985); 10.1116/1.572921 Performance of a new ion optics for quasisimultaneous secondary ion, secondary neutral, and residual gas massspectrometry J. Vac. Sci. Technol. A 3, 2007 (1985); 10.1116/1.572917

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Calibration of secondary neutral and secondary ion mass spectrometry: A comparative study

J. TOmpner, R. Wilsch, and A. Benninghoven Physikalisches Institut, Universitiit Munster, Federal Republic a/Germany

(Received 26 September 1986; accepted 19 January 1987)

The sensitivity and the dynamic range of a quadrupole based mass spectrometer for the quasisimultaneous analysis of secondary ions and electron beam postionized sputtered neutrals has been improved. Counting rates larger than 106 counts/s of postionized neutrals with a dynamic range of six orders of magnitude have been obtained for iron samples bombarded by 10-/-LA, Ar+, 5 keY. A comparison of secondary ion mass spectrometry (SIMS) and secondary neutral mass spectroscopy (SNMS) results considering target preparation, sensitivity, reproducibility, accuracy of quantification, and total time of analysis will be presented for 13 steel targets alloyed with up to 15 different elements. Concentration ranges between 8 ppm and 1 % have been covered. From the experimental results we found that the detection limit is in the range of 10-4 (oxygen) of a monolayer on a target area of 4 mm2

, when the spectrometer is tuned to a distinct mass. This allows quantitative surface and interface analysis in a concentration range below Auger electron spectroscopy.

I. INTRODUCTION

Although secondary ion mass spectroscopy (SIMS) has been developed to a widely used and extremely sensitive tool for surface analysis, depth profiling, and bulk analysis, the conversion of the measured signal intensity into the corre­sponding concentration of the investigated element is still a problem. In recent years mathematical conversion routines based on physical models 1-3 as well as the calibration of data utilizing a well-defined set of standards4

,5 have been applied. Usually, calibration methods give better accuracy than mod­els, but the pronounced matrix dependence of secondary ion (SI) signals demands calibration curves of high point den­sity.

um the matrix dependence of SNMS is reduced to that re­sulting from transmission and postionization changes due to the variation of angular and/or energy distribution with ma­trix composition, as long as the total emitted secondary ion flux can be neglected.

In secondary neutral mass spectroscopy (SNMS) post­ionized, sputtered neutrals are analyzed. In sputter equilibri-

Because electron-beam (e-beam) SNMS is a relatively new analytical technique and because of the pronounced ma­trix effects in SIMS, available data on calibration and com­parative SIMS/SNMS studies6 are only a few. As a part of our program on quantification we present some calibration measurements on multi component steel targets. Contrary to common use we did not determine relative sensitivity factors by only examining one standard sample, but analyzed 13 different targets covering concentrations ranging from 8 ppm to 1 %, We present information on overall instrument

TABLE I. SNMS calibration: Selected isotopes, concentration ranges covered by the standards, correlation coefficients, and detection and quantification limits arc summarized for 16 elements.

Anal. Concentration Correlation Detection Quantification Element mass range coefficient limit limit

amu qf; % % Carbon 12 0.0008- 0.61 0.998 0.00031 0.0031 Aluminum 27 0.007 - 0.27 0.927 0.0003 0.0023 Silicon 29 O.Og - 3.21 0,996 0.0018 0.018 Phosphorus 31 0.007 - 0.094 0.955 0.000 10 0.0010 Sulfur 32 0.006 - 0.11 0.982 0.00013 0.0013 Titanium 48 0.005 - 0.13 0.984 0.00016 0.0016 Vanadium 51 0.005 ~ .. 0.19 0.966 0.00012 0.0012 Chromium 52 0.005 . 0.53 0.955 O'(X)O 21 0.0021 Manganese 55 0.00 10- 1.26 0.992 0.00063 0.0063 Iron 56 96.2 -99.99 0.00025 0.0025 Nickel 60 0.005 - 0.120 0.961 0.00080 0.0080 Copper 63 0.010 - 0.185 0.993 0.00038 0.0038 Arsenic 75 0.005 - 0.03 0.910 D.OOO 15 0.0015 Zirconium 90 0.006 - D.OR 1 0.980 0.000 28 0.0028 Niobium 93 0.005 - 0.140 0.993 0.00020 0.0020 Molybdenum 98 0.003 - 0.194 0.937 0.00070 0.0070

1186 J. Vac. Sci. Technol. A 5 (4), Jul/Aug 1987 0734-2101/87/041186-05$01.00 © 1987 American Vacuum Society li86

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1187 TOmpner, Wilsch, and Benninghoven: Calibration of SNMS/SIMS: A comparative study 1187

TABLE n. PSI calibration: Selected isotopes, concentration ranges covered by tl1C standards, correlation coefficients, and detection and quantification limits arc summarized for elements with correlation coefficients bcttcr than 0.9.

Anal. Concentration Element mass range

amu % Carbon 12 0.0008- 0.61 Aluminum 27 0.007 - 0.27 Silicon 28 0.08 - 3.21 Phosphorus 31 0.007 _. 0.094

Sulfur 32 0.006 - n.11 Titanium 48 0.005 - 0.13 Vanadium 51 0.005 - 0.19 Chromium 52 (WOS _. 0.53

Manganese 55 0.0010- 1.26 Iron 56 96.2 ·-99.99 Nickel 60 0.005 - 0.120 Copper 63 0.010 - 0.185 Arsenic 75 0.005 0.03 Zirconium 90 0.006 _. 0.081

Niobium 93 0.005 - 0.140 Molyhdenum 98 D.Om .. 0.194

stability and reproducibility as well as on detection and quantification limits.

The instrument stability and reproducibility will be as­sessed by the standard deviation of repeatedly measured cali­bration points [Figs. 3 (a)-3 (d) ], the quality of calibration by the correlation of all measured signals to the established calibration curve (Tables I and II). The total time of analy­sis will be specified considering sample introduction, sample preparation, and the analysis itself. The influence of residual gas pressure and target preparation will also be discussed. Finally, the usefulness of SIMS and SNMS for routine quan­titative bulk analysis will be demonstrated.

It INSTRUMENTATION

The principle of our e-beam SNMS instrument has al­ready been described. 7

•8 Secondary particles emitted from

the target pass a deflector for SI suppression in the SNMS mode, the ionizer, a system of apertures for residual gas dis­crimination, an immersion lens, and an energy filter before being mass separated in a quadrupole mass analyzer. Detec­tion is performed with a secondary electron multiplier in the counting mode. Switching between analysis of secondary ions, secondary neutrals, and residual gas is done within se­conds by applying a suitable set of voltages to the elements of the optics. Recent improvements in ionization efficiency and background suppression result in a dynamic range that ex­ceeds six decades for all three modes of analysis. In the SNMS mode of operation a maximum useful yield of 3 X 10 8 is achieved. Contrary to plasma SNMS9 sputtering and postionization are completely decoupled, Le., primary ion current and energy as well as electron-beam current and energy can be selected independently. Primary ion currents between 20 nA and 20 I.LA are produced by a "Finkelnstein"­type ion gun. The primary ion energy can be varied between 1 and 10 keY. For primary ion currents of 10 f1A and ener­gies exceeding 5 keY the beam diameter is smaller than 0.5

J. Vec. Sci. Techno!. A, Vol. 5, No.4, Jui/Aug 1987

•..•......... -.-........... -... -.-.-.-.. -.-..• -•.. -.•.•.• ;".-. ..••.•.•. ' .•.•.•.•••••••.•••••••••.••••.•••.•.•....• ,..y ............................... " •.•. , .•.. -.-.

Correlation coefficient

0.966

0.975

0.941 0.96\ 0.996

0.960 0.933

0.973

Detection limit

q~

0.00017

O'(X)4

0.000 02 0.000 O(XJ 6 0.000003

O.OO() 1 0.0003

0.00001

Quantification limit

% 0.002 <)

0.04

0.000 2 0.000 006 0.00003

0.001 0.003

0.000 1

mm full width at half maximum (FWHM). Ion gun and mass spectrometer as well as data aquisition are computer controlled. Up to 18 targets can be introduced into the analy­sis chamber on the tip of a rod simultaneously. A vacuu~ lock facilitates the replacement of the targets while the anal­ysis chamber is still evacuated. The rod itself is mounted on a xyz manipulator. The base pressurein the instrument is 10-9

mbar. With the ion gun running at maximum current the working pressure increases to 7 X 10 --7 mbar.

III. MEASUREMENTS

We have investigated 13 different multicomponent iron alloys containing up to 16 different elements. A concentra­tion range between 8 ppm and 1 % has been covered. The standards have been produced by alloying pure iron with the different elements. They have been characterized by wet chemistry and additionally checked by different optical spectroscopies (e.g., atom absorption spectroscopy, infrared spectroscopy, and x-ray fluorescence spectroscopy).

Before acquiring spectra the targets have been cleaned by sputtering until the influence of signal drift dropped below the statistics of the peak itself. All spectra cover a mass range of 1-130 amu. The response time of the rate meter was 0.25 s. The targets were bombarded with lO-p,A, Ar-'-, 5 keY. When running spectra the primary ion beam was focused to the point of maximum spectrometer acceptance. In SNMS mode a postionizing electron current of 1 rnA was used. All spectra were repeated at least ten times.

IV. RESULTS

Secondary ions (SI) and secondary neutrals (SN) need essentially different presputter dose densities to guarantee stable signals. The necessary presputtering for SI varied between 7 X 10-- 2 As/cm2 for iron and 2 X 10- 1 As/cm2 for aluminum. Therefore, we needed pres putter times between 700 and 2000 s at a primary ion current of lO-IlA raster

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1188 Tumpner, Wilsch, and Bennlnghoven: Calibration of SNMS/SIMS: A comparative study 1188

,.-... 108

(f)

CL () 107

.0 10s

rn c Q) 105 ...... C

104

10J

10 2

10'

10°

a 20 40 60 80 100

... i

t

120

FIG. I. Positive secondary ion spectrum of a sputter cleaned low alloy steel target (Ref. 10): primary ions; Ar' , 10 flA, 5 keY.

atomic mass units

scanned over 10 mm2• For SN the most critical element with

respect to target preparation was carbon with a presputter dose of7.5 X 10 3 As/cm2. For the prebombardment condi­tions mentioned above the presputter times in SNMS varied between 8 s for silicon and 75 s for carbon.

Examples of typical SIMS and SNMS spectra are shown in Figs. 1 and 2, respectively. With a primary ion current of 10 flA a maximum count rate of 2 X 107 counts/s is reached in SIMS. The maximum peak intensity of the SNMS spec­trum is 6 X 106 counts/so The spectra do not show statistical background noise. For both spectra the dynamic range is limited by the dead time of the detection system. SNMS as well as SIMS spectra show molecular ion peaks. Compared with the corresponding intensities of the emitted atoms these are slightly lower for SN than for S1. In contrast to SNMS there are still MeO- and MeOH-type molecular ions (e.g., TiO, FeO, FeOH, etc.) detected in SIMS.

...-... 107

Fe (f)

CL 106 ()

SNMS

SI

]:' i 0 3

P en c

104 Q) ......

A I s

"I Nt Fee FeS'

C

103 As

102

10 1

10°

0 20 40 60 80

In Figs. 3(a)-3(d) examples of SI and SN calibration curves are presented. There exists a simple linear relation­ship between normalized intensities and concentrations. The results ofSNMS and SIMS quantification for all investigated elements are summarized in Tables I and II. Information about the selected isotopes, the concentration range covered by the standards, the correlation coefficients, and the detec­tion and quantification limits are presented. The detection and the quantification limits were defined to be the concen­trations giving a minimum of 10 or 100 counts/s, respective­ly, ata primary ion current of lO-pA, Ar+, 5 keY, an integra­tion time of 1 s/amu and for SNMS a postionizing electron current of 1 rnA. In most cases the concentrations of the standards did not allow the direct measurement of detection and quantification limits. Therefore, they were extrapolated from the calibration curves.

While residual gas pressure and composition strongly in-

Fe.

100 120

FIG. 2. Secondary neutral spectrum of a sputter cleaned low alloy steel target (Ref. 10). Primary iOlls: Ar+, 10 flA, 5 keY; electron beam: 1 rnA, 40 eV.

atomic mass units

J. Vac. Sci. Technol. A, Vol. 5, No.4, Jul/Aug 1987

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1189

.."..">:X 1 0-2

'" C

" Z.O <:

" > 1.5

~

" '" L

1.0

0.5

0.0

0.0

(a)

0.00

(c)

TOmpner, Wilsch, and Benninghoven: Calibration of SNMS/SIMS: A comparative study

SIMS s iIi con

1.0

o 10

2.0 3.0

concentration [ I J

0.20

concentration

0.30

% 1

1? .,C·' 1 if)

"-<l) 5 0

c

~ 4 0 I

J.O -:

o. co

(b)

] "0-~ j.' c

" 6 > '

0.00

(d)

SIMS su i fur

0.02

SNMS copper

0.05

1189

O. C4 O. 06 o. 08 C. 10 0.12

concentration [ % ]

i , -----+-0.10 (j _ 15 0<20

concentrction [ % J

FIG. 3. Examples of calibration lines of S1 and SN. They have been fitted to the data points using the method ofleast mean square: C a), (b) SI calibration plots

of silicon and sulfur; (el, Cd) SN calibration plots of aluminum and copper.

>. ....., +1----~-----L-----L----~----~-----L-----L--·

.., .\./-: ... \ CIl

1.0 c (j)

~\t ' .. +-' :'\-:.. c '..,

0.8

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N

0.6 0 E '-0 FeO c 0.4

0.2

0.0 0.0 1.0 2.0

J. Vac. Sci. Technol. A, Vol. 5, No.4, JuliAIl9 1987

SNMS

Target:

5ig. MaXimum

cpa

0 7900

Fe 252900

FeD 7880 ._--

3.0 4.0

ron

Integra' i

counts : .-----J

34000

--67150

5.0 X10 '5

FIG. 4. Normalized secondary neutral signals 0, Fe. FeO detected during sput­ter removal of oxygen from Fe. Target preparation: 240 L 0, on sputter cleaned Fe; sputter condition: Ar+. 4.7 ,tA, 5 ke V rastcred over 3 mm X 3 mm; elec­tron beam: i rnA. 40 eV.

primary ion fliJX (cm- 2)

" ••••• n •.•.•• ~ •••• ; ••••• ;-.>; •••••••• ~ ••••••••••• _._ •••• ;:.:._.~ ... ;:.:.: ••• :.:.:.:.:.: ••••• -••• -.~.-.-.-••••• -•.••• -•• -•••• -•••••••••••• ~.~ ••••••• ~ .••••• ;- .•• "'-;v_-=-= •.•. _._._ -;0.'

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1190 Tiimpner, Wilsch, and Benninghoven: Calibration of SNMS/SIMS: A comparative study 1190

>- 1 .0 +-'

en c Q)

+' 0.8 c

"U Cll N 0.6 Slg.

0 O' E L Fe' 0 0.4 c FeO' ----

0.2

0.0 0.0

SIMS

Target: iron

MaxImum Integral I

cps counts J 81000 721~OO ~

8120000 - .---1 136000 683000

, ----

FIG. 5. Normalized positive secondary ion signals 0+, Fc t, PeO+ detected during removal of oxygen from Pe. Tar­get preparation: 240 L O2 on sputter cleaned Fe; sputter condition: Ar+, 4.7 /-LA, 5 ke V rastered over 3 mm X 3 mm.

primary ion flux (cm- 2)

fiuence absolute and relative signal intensities of SIMS spec­tra, they have virtually no influence on SNMS spectra. We have been able to quantify SN data successfully with the analysis chamber flooded up to 2x 10-6 mbar air. Even for elements, which have been expected to be sensitive to insuffi­cient residual gas suppression and/or residual gas adsorp­tion like sulfur (32 amu) and silicon (28 amu), the results obtained agree excellently with those measured under ultra­high (UHV) conditions.

We checked the surface sensitivity ofSNMS by analyzing submonolayer oxygen coverages on different metal surfaces. They were prepared by flooding the surface of a sputter cleaned metal with oxygen. While removing this oxygen lay­er by sputtering we measured the SI and SN signals of oxy­gen, metal and metal oxide versus primary ion fluence. In Figs. 4 and 5 the normalized SN and SI intensities are pre­sented. Oechsner et at.9 assume that the SN intensity of iron oxide N FeO (co) varies with oxygen concentration Co as NFeO(CO)-COCFe=CO(l-co)' Therefore, the maxi­mum of the FeO signal in SNMS should correspond to a surface coverage of50%. On the basis of this assumption we estimated the oxygen surface sensitivity of our SNMS appa­ratus. From the maximum signal intensities orO and FeO we calculated a minimum detectable oxygen surface concentra­tion of7 X 10 -4 and 6 X 10-4, respectively.

v. CONCLUSION

For the experimental conditions applied in our investiga­tion SIMS is, in general, more sensitive than e-beam SNMS (Tables I and II). Differences in useful yields are below a decade for most of the elements. For carbon and silicon e­beam SNMS nearly reaches and for phosphorus it exceeds the sensitivity of SIMS. Quantification ofSN is much easier than it is for SI. In SNMS the influence of surface contami­nations can be eliminated by presputter times between 8 and 70 s. For SIMS even in favorable cases a minimum presput­ter time of 10 min is mandatory. In all cases the correlation coefficients-characterizing the quality of quantification-

J. Vac. Sci. Technol. A, Vol. 5, No.4, Jul/Aug 1987

in SNMS are better than in SIMS. The performance of our apparatus allows a quantitative bulk analysis down to a few ppm within seconds. Even at high background pressures SN data are easily quantified. Monolayer analysis in the range of 10 -4 is also possible.

For routine analysis e-beam SNMS offers some very inter­esting features. Because of the complete separation of sput­tering and postionizing parameters, different primary ion species may be used to optimize SIMS performance with unchanged SNMS performance. The postionizer is UHV compatible. On the other hand, the stability of postioniza­tion is also maintained at high background pressures ( < 10 6 mbar O2, air). Therefore e-beam SNMS may be operated simultaneously with XPS, AES, and other UHV surface analytical techniques, but it also works at high back­ground pressures allowing fast sample change and large sample throughput. The electron energy of the postionizer is adjustable. Therefore, it may be adjusted to the maximum of the total ionization cross section for each element under in­vestigation.

I C. A. Anderson and J. R. Hinthorne, Anal. Chern. 4-5, 1421 (197 3). 2p. G. Ruedenauer and W. Steiger, Vacuum 26,537 (1976). 'J. M. Schroeer, J. Vac. Sci. Techno!' 14, 343 (1977). 4G. H. Morrison and J. D. Gangei, Report 2. Jpn.-U.S. Joint Seminar SIMS 23,19i8.

5H. Kobayashi, Report 2. Jpn.-U.S. Joint Seminar SIMS 30, 1978. "J. Dittmann, F. Leiber, J. Tiimpner, and A. Bcnninghoven, in SIMS 5. Springer Series in Chemical Physics 44, edited by A. Benninghoven et al., (Springer, Berlin, 1986), p. 105.

7n. Lipinsky, R. Jede, O. Ganschow, and A. Bcnninghoven, J. Vac. Sci. Techno!. A 3, 2007 (1985).

"D. Lipinsky, R. Jede, J. Tiimpner, O. Ganschow, and A. Benninghoven, J. Vac. Sci. Techno!. A 3, 2035 (1985).

°H. Oechsner, in SIMS 3, Springer Series in Chemical Physics 19, edited by A. Benninghoven et al. (Springer, Berlin, 1983), p.l06.

l"Thc spectra presented in Figs. I and 2 demonstrate the actual spectrom­eter performance (Sept. 1986). They have been taken with an increased spectrometer resolution setting compared to the earlier measurements the calibration plots are based on.

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