Depth profiling by ion microprobe with high mass resolution1

International Journal of Mass Spectrometry Eisevier Scientific Publishing Company,

@

DEPTH PROFILING RESOLUTION *

F. DEGREVE, Centre (First

received

BY ION MICROPROBE

R. FIGARET

de Recherches 7 March

and Zen Physics, 29 (1979) 351-361 Amsterdam - Printed in The Netherlands

WITH HIGH MASS

and P. LATY

de l’illuminium 1978;

351

in revised

Pkhiney, form

38340 7 July

-

Voreppe

(France)

1978)

ABSTRACT Instrumental improvements were made to a commercial Ion Microprobe AEI, TM20 in order to obtain “true” depth pr&iles. A new optics assembly provides an increase by a factor of 10 in the secondary ion transmission when working at high mass resolution @Z/&Z = 5000). An electronic aperture acting along the ion path before the detector allows analysis of the central portion of the crater whatever the type of the ion detector: photoplate or electrical detection_ Typical examples of metaiiurgical samples analysis by SIMS illustrate the importance “true” depth profiles. Simultaneous depth profiles of high mass resolution for obtaining on industrial aiuminium by recording mass spectra on photoplates and “surface” migration of Be in Al-Mg-Zn aiioy measured by electrical detection are presented. The advantages of high mass resolution are briefly discussed with respect to vacuum conditions, dynamic range in concentration, energy window adjustment, and information for quantitative SIMS models.

INTRODUCTION Numerous properties of industrial metals and alloys depend on surface composition and on variation of concentration near the oxide-metal interface. With this objective, we have critically evaluated the possibilities of a modified commercial Ion Microprobe AEI, lM20, for depth profiling e\ements over shallow depths (G.1000 A) in metallurgical samples. The factors that influence an elemental depth concentration profile have been widely discussed in the literature [l-3]. Figure 1, taken from Werner [3], illustrates the situation well. (1) Crater-edge contribution and redeposition effects are related to the sputtering and ion optics extraction. (2) Mass interference depends on the quality of the mass spectrometer. The necessity of high mass resolution has already been reported [4--6] for

Paper presented at the International (SIMS) and Ion Microprobes, Miinster l

Conference on Secondary (West Germany), September

Ion Mass Spectroscopy 19-23, 1977.

352

contribution

memory and

/or

effect prepeok

bombardment

Fig. 1. ref. 3.

Depth

distribution

(schematic)

:

from

time

obtained

(a.u.)

from

different

disturbing

effects,

after

analysis of a complex matrix, but little attention has been paid up to now [7,8] to its influence on depth profiles_ It is the purpose of this paper to illustrate the fundamental importance of high mass resolution in depth profiling trace elements in pure and complex matrices_ In a first step, some instrumental improvements intended to attenuate the spurious effects mentioned in (1) above will be described. ION

MICROPROBE

AND

INSTRUMENTAL

IMPROVEMENTS

The basic instrument is outlined in Fig. 2. The primary ion beam is produced by a duoplasmatron, mass-filtered and can be focussed in a lo-pm spot on the sample by two electrostatic lenses. After extraction, the secSecondary

ion,

BeOIl deflect plores

Fig. 2. Schematic

representation

of the AEI

ion microprobe

(ref.

9).

353

‘2C2Hf

(29.0391) _

2’AlHz+ 2BSiH+ %i+ 58Ni2+

Ar+-

(28.9611) 7);

I;

1

Ill1

Al

I 1

L

L

m/t

29

Fig. 3. Typical high mass resolution spectrum plate by Ar* sputtering of aluminium.

(M/&M

= 4000

at 10%)

29

obtained

on photo-

ondary icns are analysed by a double focussing, Mattauch-Herzog-type mass spectrograph. Detection is performed by photoplate or by electrica.l detection [lo]. In order to -provide a good transmission at high mass resolution (M/AM = 400G) (Fig. 3), a new optics assembly was set between the sample and the

electronic

aperture

Fig. 4. Schematic representation of the new ion extraction optics between sample and mass spectrometer entrance slit to achieve high transmission at high mass resolution. The electronic aperture acts before the detector (black points: secondary ions are rejected; open points: ions allowed to pass).

boeroded

x

resultant (>o

eroded

ion

dzplh

density

bollom

crsler)

Fig. 5. Micro-roughness of the crater bottom depends on the overlapping assuming a gaussian density distribution in the bombarding spot.

ratio d/A

entrance slit (Fig. 4). It consists of an Einzel lens followed by a doublet of electrostatic quadrupole lenses which astigmatically focusses [ 11,121 the secondary ion beam on the narrow entrance slit (lo-20 pm)_ At a resolving power of 5000, transmission is increased by a factor of 10. A flat-bottomed crater (typically 250 pm square) is obtained by raster scanning the primary beam on the surface. The contributions from the craters’ edges (black points) are eliminated by an electronic aperture which acts along the ion path before the electrostatic analyser by biasing a rapid beam suppress electrode assembly. The electronic aperture does not gate the detector as in the former designs [ 13,141. On a raster line, the rejected edges represent l/Sth on each side. Thus, the central portion (open points = 56% aperture) can be mass-analysed whatever the type of the ion detector: photoplate or electron multiplier, associated with a d.c. amplifier or an ion counter. The flatness of the crater bottom depends on the uniformity of the primary beam density, that is on a correct overlapping of spots [ 153 (Fig. 5). By assuming a gaussian density distribution in the spot [ 16 ] and by summation of the adjacent rastered spots, the resultant density may be computed versus the “overlapping ratio” d/A (d = spot diameter, A = distance between two spots) and may be optimized according to the information required: depth profile or ion micrograph. This computer program will be published in the near future. A similar computation has already been described 1171 for a continuous rastering, but not for a digital (point-to-point) rastering. The vacuum in the sample chamber is maintained by a turbo-molecular pump and a liquid nitrogen cold finger giving an ultimate pressure of 5 X lo-* torr. These are not ultra-high vacuum conditions, but rather provide a hydrocarbon-free vacuum.

355 RESULTS: DEPTH PROFILES AT HIGH MASS RESOLUTION Analysis

of pure aluminium

with a photoplate

A survey analysis of an industrial polycrystalline aluminium sample was performed by recording on a photoplate the spectrum corresponding to successive layers of 30 A (exposure time 20 s at a mean erosion rate of 1.5 J% s-‘) starting from the surface. The blackening B (optical density) of the lines identified by the exact mass was recorded (Fig. 6 for m/z 28) and by a computer program converted into relative ion intensities PM by the Hull relationship [ 18,191. The intensities normalized to the intensity of a characteristic matrix line (I’Al,) are plotted versus time in order to establish the depth profiles of every atomic or molecular ion simultaneously detected on the plate. Figure 7 shows the results at m/z 28 for each component (Si and AlH) of the resolved doublet and for the simulated profile (I‘Si + I*AlH) that would have been obtained without a high mass resolution. Other typical examples for m/z 44 and 56 are given in Figs. S-11. The more striking features appearing after collection and interpretation

07+-Al

ml+=28

J

n

4r u-20

25-15

O-30

37-t7

L

1,

J

50-70

75-95

100-120

125-M

t 5 erosion time 150-170 175-195 ZOO-220 225-245 250.270 2754!l5 X10-320

75-105

112-147

150-180

107-717

225-255 762-292 300-330 337-367

375.405 W-442 deplh

Fig. 6. At 30-A layers

nominal mass m/z versus erosion time.

28, blackening

of

a resolved

doublet

9-

450-480

Z(A)

corresponding

to

Al

Oz”-

.___ _________

Si +AIH _ AlH+

.l\_ 0

100

200

300

a00 erosion

C27.98931

500 time

600 (S)

Fig. 7. At nominal mass m/z 28, converted depth profiles of each doublet. Dotted lines show the profile that would have been obtained

component of the without high maSS

resolution.

m/z

. o-70

25-45

50-70

O-30

37-67

75-105

75-95

100-120

112-742 150-10

02+ -

-44

1

Al

Jl

IX-l45

150-170

175-195 xto-220

107-217

225-255

262-292

3w-330

225-245 250-270 275-295 3oD-320 erosion time t(5) 337-367

375-405 412-402 _450-480

w

*

100 I1

I

all

300 I1

4al

estimated PI0 600 11

depth (3) 700 600 9cul InlO 11 I1

357

1

----

AlOH+Ca

+SiO

10-3).

100

0

200

300

Fig. 9. At nz/z 44, depth low resolution profile.

r

400 500 er0sir.n time

profiles

600 (5)

700

of the resolved

triplet

components

and of the simulated

225-245

%e+

(55.9349)

O-20

25-45

Cdl-70

75-s

Kal-120

KS-145

l5o-l70

175-195

0~30

37-61

75 -105

112-142

150-180

187-217

225-255

262-292

200-220 erosion

time

250-270

t (5)

3011-330 937-367 37=-405 rlenlh 7 (11

* *

358

10-3 ;

1

100

Al

I

300

L

400 erosnon

,

3Xl time

L

600 (s)

L

700

Fig. 11. At m/z 56, depth profiles of the resolved doublet ulated low resolution profile.

of the information

present

on the plate

components

and of the sim-

are:

(i) the complexity of the spectrum decreases drastically from the surface to the bulk; (ii) at almost any detectable nominal mass, a resolvable multiplet is present in the outermost layers; (iii) the depth profiles of several atomic impurities like Si, Ca, Fe determined without high mass resolution would be completely wrong and misinter-

preted. Looking in more detail at Figs. 6-11, even more obvious. Vacuum

the advantages of high resolution are

conditions

At nominal masses 24 and 44, molecular ions AIH’ and AlOH* resulting horn interaction of the sample with residual water vapour are characterized by a roughly constant intensity, which generally becomes much larger than the intensity of the element (Ca+, Si+) to be profiled as the depth increases_

359

At low resolution, an apparent plateau in concentration (attributed to bulk concentration?) would appear. Thus at high resolution, ultra-high vacuum conditions are not required for depth profiling elements whose isotopes would be interfered with by a spurious molecular ion. “True”profile

and dynamic

range in concentration

When the “true” profile is obtained, the dynamic range in concentration is increased by several orders of magnitude (Figs. 7, 9, 11). Moreover, the “true” profile sometimes exhibits a surprising shape. For instance, the profile of two species characteristic of the sample Fe’ and CaO’ are quite different; Fe’ concentration is constant within the first 200 R and then decreases, while CaO’ concentration decreases continuously. Energy

window

and “prepeaks”

With certain low resolution mass spectrometers, it is possible to discriminate atomic and molecular ions by adjusting the energy window according to their different widths in the energy distribution curves: low energy discrimination (LED [20,21]). However, this method is inefficient for discriminating between several molecular ions. Again, high mass resolution is the only way to solve the problem (see, for instance, SiO*, AlOH’ in Figs. 8, 9). On the other hand, “prepeaks” [3] are not detectable in the spectrum using a double focussing mass spectrometer. rrTrue” information

for quantitative

SIMS

models

Considerable efforts have been made to obtain quantitative analysis by SIMS from phenomenological and physical models [ 221. The first step is obviously to deal with unambiguous information. High mass resolution eliminates the need for delicate deconvolution ,of non-resolved spectra [ 231 before checking the validity of a given model. Yhrface” tion

migration

of Be in Al-Mg-Zn

alloy measured

by electrical defec-

It has been observed that addition of 20 pg g-’ Be in an Al-Mg-Zn alloy leads to particular surface properties after thermal oxidation at 530°C. Conventional analysis techniques failed to point out a possible migration of this light impurity towards the surface. To solve the question by SIMS, it is necessary, first, to resolve a possible interference between the trace element ‘Be’ and a polycharged ion of the matrix “A13+. Figure 12 shows a profile of the peaks obtained by scanning a narrow magnetic field [24] range and by integrating the signal with an ion counter for 1 s every 2 s. Figure 13 shows the Be depth profiles obtained by argon sputtering while flooding polycrystalline samples with an oxygen jet. This maintains essentially constant sputter and ionisation yields on each side of the oxide--metal interface; the time scale may be thus be converted to a depth scale in a first

1

Ar+,

with

AI-rflg-Zn

(7075)

thermal

oxidation

eslimaled

depth

(8)

Fig. 12. At m/z 9, mass spectrum obtained by magnetic scanning using a ion counter. Fig. 13. Influence of the thermal oxidation on Be depth profiles in Al-Mg-Zn

alloy.

approximation. The oxide-metal interfaces are localized in separate experiMoreover, ments by the oxygen 160* profile, but without oxygen flooding. the Be* profiles observed by Ar+ sputtering without 0, flooding are quite similar, in the first hundred A, to those obtained with O2 flooding. This would indicate that the surface is highly oxidized after annealing. It can be also observed that a thermal oxidation leads to: (i) an increase in the oxide thickness by a factor of 5; (ii) a migration of Be to the oxide-metal interface; (iii) an enrichment in Be by a factor of 100. In conclusion, the formation of a compact beryllium oxide layer could explain the particular surface properties. CONCLUSION

From the two metallurgical studies presented here, the advantages of a high mass resolution when analyzing an unknown sample can be summarized as follows: (1) “True” depth profiles are obtained. (2) Maximum dynamic range in concentration. (3) For elements interfering with molecular ions resulting from matrixresidual vacuum interactions, ultra-high vacuum conditions are not required. (4) Adjustment of the energy window is not required. (5) Prepeaks are absent.

361

(6) Unambiguous

information is directly available for quantitative models. In this first stage of investigation into metallurgical applications, we are convinced that high m%;s resolution is a necessary technique. ACKNOWLEDGEMENTS

The authors wish to thank C. le Bars and M. Cuinat and mechanical designs and improvements_

for the electronic

REFERENCES 1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

23 24

J.A. McHugh,

in K.F.J. Heinrich and D.E. Newbury (Eds.), Secondary Ion Mass Nat. Bur. Stand. (U.S.), Spec. Publ. No. 427, 1975, p. 179. R.E. Honig, Adv. Mass Spectrom., 6 (1974) 337. H-W. Werner, Acta Electron. 19, 1 (1976) 53. R. Hernandez, P. Lanusse and G. Slodzian, CR. Acad. Sci., Ser. B, 271 (1970) 1033. D.K. Bakaie, B.N. Colby and C.A. Evans, Jr., Anal. Chem., 47 (1975) 1532. S.J.B. Reed, J-VP. Long, J.N. Coles and D.M. Astill, Int. J. Mass Spectrom. Ion Phys., 22 (1976) 333. R. Hemandez, P. Lanusse, G. Slodzian and G. Vidal, Rech. Aerosp., 6 (1972) 313. R. Castaing, in XVI Colloq. Spectrosc. Int. Heidelberg, 1971, p. 29. C.A. Evans, Jr., Anal. Chem., 44 (1972) 67A. A.E. Banner and B.P. Stimpson, Vacuum, 24 (1974) 511. C.F. Giese, Rev. Sci. Instrum., 30 (1959) 260. C.S. Lu and H.E. Carr, Rev. Sci. Instrum., 33 (1962) 823. W-0. Hofer, H. Liebl, G. Roos and G. Staudenmaier, Int. J. Mass Spectrom. Ion Phys., 19 (1976) 327. P. Williams and C.A. Evans, Jr., Int. J. Mass Spectrom. Ion Phys., 22 (1976) 327. T.A. Whatley, C.B. Slack and E. Davidson, Proc. 6th Int. Conf. X-Ray Optics and Microanalysis, University of Tokyo Press, Tokyo, 1972, p_ 409. H. Liebl, J. Vat. Sci. Technol., 12 (1975) 385. T.A. Whatley, D.J. Comaford, J. Colby and P. Miller, in R.S. Carbonara and J-R. Cuthill (Eds.), Surface Analysis Techniques for Metailurgicai Applications, ASTM Spec. Techn. Pubi. no. 596,1976, p_ 114. C.W. Huii, 10th Ann. Conf.’ Mass Spectrometry and Allied Topics, New Orleans, 1962. F. Degreve and D. Champetier de Ribes, Int. J. Mass Spectrom. Ion Phys., 4 (1970) 125. R.F. Herzog, W.P. Poschenrieder and F.G. Satkewicz, NASA Report CR, 1967, p. 683. J.M. Morabito and R.K. Lewis, in A.W. Czanderna (Ed.), Methods of Surface Anaiysis, Elsevier, Amsterdam, 1975, p. 280. F.G. Riidenaucr, Present State of Quantitative Elemental SIMS Analysis, the Proceedings of International Conference on Secondary Ion Mass Spectroscopy (SIMS) and Ion Microprobes, Miinster, 1977. F. Pregesbauer and F-G. Riidenauer, Proc. 7th Int. Vat. Congr. and 3rd Int. Conf. Solid Surfaces, Vienna, 1977, p. 2539. F. Degreve, R. Figaret, J.J. le Goux and L. Caiavri&, Int. J. Mass Spectrom. Ion Phys., 14 (1974) 183. Spectrometry,