Recent applications of secondary neutral mass spectrometry for quantitative analysis of homogeneous and structured samples

Nuclear Instruments and Methods in Physics Research B33 (1988) 918-925 North-Holland, Amsterdam

918

RECENT APPLICATIONS OF SECONDARY NEUTRAL FOR QUANTITATIYE ANALYSIS OF HOMOGENEOUS

MASS SPECTROMETRY AND STRUCTURED SAMPLES

Hans OECHSNER Technische Physik,

Univ. Kaiserslautern,

D-6750 Kaiserslautern,

FRG

The main advantage of secondary neutral mass spectrometry SNMS compared to its sister method SIMS is its high quantificability in consequence of the separation of the emission and the ionization process of the analyzed particles. Electron gas SNMS using electron impact postionization by the electron component of a resonantly excited low pressure high frequency plasma is at present the most developed analytical technique of this type. The characteristic properties and the potentialities of SNMS are demonstrated by comparative SNMS-SIMS measurements, and for the analysis of homogeneous multielement samples. Detection sensitivities down to 100 ppb are obtained with SNMS in such cases. The direct bombardment mode of SNMS is applied to depth profile analysis of a Ta-Si multilayer system. For bombarding- energies - in the order of lo2 eV a depth resolution down to the fundamental limits set by the low energy ion-surface interaction itself becomes possible.

1. Introduction Mass spectrometric identification of atoms and molecules being removed from a solid surface by controlled ion bombardment has proved to be an extremely sensitive method for the compositional analysis of solid surfaces in particular when combined with recent single particle counting techniques. Hence, mass spectrometry of positively or negatively charged secondary ions, being immediately available for such purposes, has rapidly developed to a widely employed analytical technique with highly sophisticated instrumentation (for a recent overview see ref. [l]). It is well known, however, that the quantification of secondary ion mass spectrometry SIMS suffers strongly from so-called matrix effects in secondary ion formation. The strong dependence on the chemical and electronic surface conditions often causes the ionization probability during particle ejection to vary by orders of magnitude for different matrices. Another drawback for the quantitation of SIMS is the high selectivity in secondary ion formation for certain elements or certain molecular species as, e.g., SiaO+ from an SiO,-sample [2]. Since only little progress has been made towards a quantitative understanding of such effects, the quantitative determination of the surface composition of an unknown sample is mostly not possible with SIMS. Apart from several exceptions, as for instance alkali halides [3], secondary ions contribute only very little to the total particle flux from an ion bombarded surface. Even for oxide surfaces the secondary ion yield Y * exceeds seldom 10W2 of the total sputtering yield Yi,, [4-71. Therefore, mass spectrometric signals determined by the ejected neutral particles are expected to be not subjected to matrix influences, even when the formation 0168-583X/88/$03.50 Q Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

probability of the secondary ions varies largely due to changes of the actual surface composition. Secondary (or sputtered) neutral mass spectrometry SNMS [8], however, requires an effective and non-disturbing postionization of sputtered neutrals to such an extent that the mass spectrometric signals of postionized particles become comparable or even larger than the respective SIMS signals. The separation of particle ejection and (post-)ionization is the most important difference between SNMS and SIMS. Among the presently employed methods for the postionization of sputtered neutrals, electron impact ionization in a hot, spatially expanded electron gas of high density is the most elaborated technique. After a brief description of this experimental approach, for which the acronym SNMS has been introduced at first [9], some examples of recent applications of SNMS to the analysis of homogeneous and of structured samples will be given. In the latter case, the potentialites of sputter depth profiling in conjunction with the so-called direct bombardment mode of electron gas SNMS [8] will be discussed.

2. Experimental performance of electron gas SNMS Postionization of sputtered neutrals is in principle possible with an electron beam crossing the flux of sputter removed surface particles [lo]. However, only moderate postionization factors being at best in the order of 10e4 are obtained with corresponding arrangements [11,12]. This has to be attributed mainly to the short traveling time of the sputtered neutrals with a mean energy in the order of lo-15 eV [13] in the electron beam volume.

H. Oechsner / Recent applications of SNMS

919

/Multiplier

/ UZY

-----Quadrupole Mass Spectrometer

EleCtrOn Multiplter

Ion

Ht

Plosmo

,Hf-Plasma

b Electrical Diaphragm

Fig. 1. Schematic diagrams of SNMS arrangements for a) the direct bombardment mode (DBM) and the separate bombardment mode (SBM), b) the external bombardment mode (EBM). With SBM and EBM comparative SIMS-measurements can be performed in situ.

To achieve considerably higher postionization factors, the density ne of postionizing electrons has to be raised above the n,-values in appropriate electron beams (n, = 10’ cm -3 in an electron beam of 100 eV and 1 mA cme2), and the ionizing volume has to be expanded as well. Both requirements are met by employing a dense electron gas of suffiently high temperature. Such an electron gas is produced by ionizing noble gas atoms at very low pressures with an electrodynamic effect, the so-called electron cyclotron wave resonance [14]. For argon at a working pressure of a few lo-” mbar electron densities ne of 10’“-lO1’ cmm3 are achieved at electron temperatures T, corresponding to 15 eV. Such high ne values become possible since the electron space charge is compensated by the positive noble gas ions in the generated low pressure high density plasma. For a traveling path of about 5 cm postionization factors (Y’ in the order of a few 10m2 are achieved for most of the elements [15]. Due to the low pressure and the nature of the working gas, interfering interactions between sputtered particles and the working gas become negligible. Two different types of SNMS arrangements are schematically shown in fig. 1. The apparatus in fig. la can be operated in the so-called direct bombardment mode DBM, in which noble gas ions from the SNMS plasma are accelerated onto the sample by means of a simple ion optics mounted on the sample head [8]. Under appropriate operation conditions the sample is

bombarded normally with such a high lateral homogeneity that the bombarding current densities of 1 to a few mA cmm2 differ less than 10m5 across a diameter of 8-10 mm. As an essential condition for high resolution sputter depth profiling (see section 4) the bombarding ion energy can be varied down to a few 10 eV. In the separate bombardment mode SBM, which can also be performed with the arrangement in fig. la, the sample is bombarded with a rastered ion beam traversing the SNMS plasma. In the case of insulating samples positive surface charging by the primary ion beam is automatically compensated by an equal retardation current of plasma electrons onto the floating sample [16]. While in SBM the flux of postionized sputtered neutrals can be superimposed by a small contribution of positive secondary ions, charged particles are completely prevented from entering the detection system in the arrangment of fig. lb showing the external bombardment mode EBM of SNMS [17]. In this mode the sample is bombarded with an “external” ion gun and separated from the SNMS plasma by an electrical diaphragm which permits, e.g., only a precisely controlled compensation current of plasma electrons to reach an insulating sample, but prevents any charged particles from the sample from entering the postionization chamber [18,19]. Both types of SNMS apparatus shown in fig. 1 meet UHV conditions with base pressures of 10-9-10-1’ mbar. The DBM-SBM arrangement in figure la is XIV. ION BEAM TECHNOLOGY

920

H. Oechsner / Recent applications of SNMS

56Cu/44Ni SIMS

(L6at%

SNMS

Ni. SLat%Cu)

55vIri

Arf 3 keV

4,’

Ar+, 1 keV

A= lo-‘cm2

‘*Ni

Ip= 3~10-~A

63 cu

A=O,5 ;3c”

cm2

I = 3.1 mA

l

C’ .-

C

6ONi' /

+

X

l-4

1

I

,

I

I

I

I

5xlO3cps

T

2

R

4xlO5cps ‘%U

,

5L 58 62 66

m/e

Fig. 2. Comparativein situ SIMS- and SNMS-measurementsfor a Cu-Ni-alloy [S]. The Mn+-peak obtained with SIMS corresponds to an Mn-impurityof about 100 ppm.

similar to a commercially available SNMS system *. The arrangements for SBM and EBM can be used for comparative in situ SIMS measurements when the SNMS plasma is switched off, and the electrical diapraghm in EBM is opened for positive or negative secondary ions [8,16-201. An example of such a direct comparison is presented in fig. 2 [18]. While the heights of the SNMS peaks describe almost quantitatively the composition of the investigated CuNi alloy, the SIMS intensities of Ni are by a factor of about 2 above that of Cu. The highest SIMS peak, however, refers to manganese which is not detectable in the SNMS spectrum for constant registration conditions. By increasing the registration sensitivity for SNMS, Mn is found as an impurity with a concentration of about 100 ppm. The resulting large SIMS signal, and to a lesser extent the higher Ni+ peaks demonstrate the magnitude of selectivity effects in SIMS. A comparison of the signal intensities in fig. 2 demonstrate that under identical

* INA 3 from Leybold AG, KMn.

experimental conditions sensitive than SIMS.

electron gas SNMS

is more

3. Quantitative SNMS analysis of homogeneous samples The basic equation for an SNMS signal Z(X’) of a species X removed from the sample surface reads [8,17,201 Z(XO) = z,Y,cKjl?j0,(1-a:

-a;).

(1) Zp denotes the bombarding ion current, Y, the partial sputtering yield (or the formation probability) of X, ai the postionization probability, n: a geometry, transmission and amplification factor for X. a,’ describes the probability that the particular species X is positively or negatively charged during its ejection, or that the particles X are emitted as secondary ions with the respective secondary ion yields Y,* = a,’ Y,. In in the vast majority of all cases Y,* is much less than Y,, or a,’ << 1. Even for oxides, where a,’ does not exceed 10m2 for many cases [7], the term in parenthesis in eq. (1) can be taken as unity.

921

Ii. Oechmer / Recent applications ofSNiWS The experimental condition in the respective SNMS equipments can be chosen such that postionization occurs, when those particles being ejected in excited states have returned to their ground states. Then at is a particle specific apparatus constant for identical operation of the SNMS plasma. Hence, eIf can be combined with the quantity ~0, from eq. (1) to a detection (or sensitivity) factor D, of a species X, and the SNMS signal for X is given by the simple relation Z(XO> = Z,Y,Z&_

(2)

It has to be mentioned that 110,may vary for different sputtered particles X in a different way, when the bombarding energy E, of the primary ions is changed. Such influences have recently been shown to exist for very low bombarding energies around 100 eV, but are within the experimental error at E, 2 1 keV under normal ion bombardment, where the angular distribution of the sputtered particles agree in general sufficiently well with a cosine distribution [Zl]. Such energy dependent variations of D, which might be different for samples of different composition should not be mixed up with the large matrix and selectivity effects in secondary ion formation. When the sputtered particle flux contains a negligible fraction of molecules, the partial sputtering yield under stationary conditions is given by r, = cxYtot9

(3)

with c, being the atomic concentration of X in the sample and &,, the total sputtering yield for the respective bombarding conditions. Combining eqs. (2) and (3), the ratio of the atomic concentrations of two elements i and j is given by 3

Ci

_--z(e) 4 1(X;) Di.

_

According to this equation the concentration ratios of two components i and j are inversely proportional of the corresponding relative sensitivity factors D,/Di. If all D’s are referred to the same reference particle R and the condition xc, = 1 is taken into account additionally, the concentration c, of a component X in an atomically sputtered sample is given by

Z( x*)/D;I

cx =C(1(X:)/D:“)

’

(5)

with DF’= L&/D,. All SNMS signals Z entering eq. (5) must be taken under identical conditions, i.e. for the same YtQtand the same bombarding current ZP. Relative detection or sensitivity factors can be readily obtained from the SNMS spectra of nonelemental samples of well know composition. As already done in eq. (5), the individual D-factors may always be normalized to the factor D, of a reference element R. In order

Table 1 Absolute and corrected values of relative SNMS detection factors Element X

DT’ = q/n,

D.$U,x??l;‘/2

C

0.15 1.02

0.48

Si P

0.52

1.00

1.61

S

0.69

1.28

Cr

0.95

0.89

Fe

1

1.04

CO

0.79

0.81

Ni CU MO

0.56 0.53 1.10

0.56 0.52 0.83

to make the resulting relative sensitivity factors 0,“’ = D,/D, sufficiently independent of the already mentioned influences at low bombarding energies, the D:” should be determined for E,-values around 1 keV. Of course, such an E, must also be applied for an accurate determination of the composition of a specimen. Examples for corresponding relative sensitivity factors D$ using Fe as reference element are given in table 1. The differences between the individual DFt-values are supposed to be mainly due to the variations of the postionization probability at of a sputtered species X during its flight through the postionizing plasma. aI is a convolution of the constant velocity distribution f(u,) of the plasma electrons, the elemental ionization functions Qi( u,) and, via the dwelling time, the velocity distribution N,(u,) of the sputtered particles X, i.e. by a: - ne

J/

et(u,)f(u,>u,N,(v,)u,_’

doe d-s-

(6)

In a first approximation, the influence of Q: and of the dwelling time of sputtered particles X with an atomic mass m, should possibly be taken into account by putting uz or Dx proportional U;‘m;‘/*(U~ ionization potential of X). The values of D?’ being normalized accordingly are also given in table 1. It is found that such reduced values of 0,“’ differ from an average value around 1 by not more than approximately f 50%. Hence, the quantification of SNMS according to eq. (5) becomes possible within a factor of 2 when the relative detection factors DJD, are replaced by the corresponding products Uxiim;I/*. For such limited accuracy it is sufficient in principle, when the corresponding correction is applied for particles with particularly high or low values of U;’ and M x. The SNMS spectrum of an Fe-sample containing a large number of different constituents at very low concentrations (NBS standard 1261a) is shown in fig. 3. This measurement was performed with the direct bombardment mode DBM of SNMS [S]. While the signal intensity of the main 56Fe isotope amounts to 10’ XIV. ION BEAM TECHNOLOGY

H. Oechsner / Recent applications of SNMS

922 I

NBS-Standard

1261 a

Ug=lkeV A? Ig=2,6mA Recording Time lOs/amu

2aSi I

I

'

10

20

30

1

60

50

60

70

80

90

100

110

120

130

1LO

lb

160

170

180 s

emu

Fig. 3. SNMS-spectrum of an Fe-sample with a large nmnber of other low concentration constituents (NBS Standard 1261a). The measurements have been performed with the direct bombardment mode of SNMS. Part of the spectntm has been recorded with

improvedparticlecountingelectronics.

cps, a mass independent background of 10 cps is found with a noise in the order of also 10 cps. With an improved registration kit (see inset in fig. 3) the mass independent background can be eliminated completely and the noise is reduced to about l-2 cps. In view of the small molecular contributions in fig. 3, the conditions for the evaluation procedure assuming atomic sputtering are well fulfilled. Since the exact relative detection factors were not available for all sample constituents, the simplified evaluation procedure discussed above was applied. Identical relative detection were assumed except for those elements factors l3JD, of small atomic mass and/or high ionization potential, i.e. H, C, N, and 0. The atomic concentrations c, obtained from the SNMS-spectrum in fig. 3 by this simplified evaluation are plotted in fig. 4 versus the c,-values quoted in the certification of the investigated standard sample. The SNMS results agree well with the nominal composition across the entire concentration range down to 1 ppm. This reveals quite clearly the low selectivity of SNMS and demonstrates that compositional analysis with a mostly sufficient degree of accuracy is achieved by very simple evaluation of the SNMS signals.

SNMS promises quantitation with very high precision when exact relative sensitivity factors are available. This is confirmed by the results in table 2. For the particular SNMS equipment employed in the present investigations, the relative detection factors &‘D, given in table 1 were available from previous calibration measurements. Because of the small concentrations of Table 2 Quantitative homogeneous Mass

‘*C *sSi 31P 3% s*Cr s’Fe sac0 60Ni 63CU 9sMo

SNMS analysis of isotope wncentration in a multielement sample (NBS Standard 1261a) Isotope con~ntration certified

by SNMS

1.77x1o-z 4.15 x 10s 2.8 x1o-4 2.47~10-~ 6.11 x IO-’ 2.06x10-2 3.0 x10-4 5.0 x10-’ 2.49 x 1O-4 2.61 x~O-~

1.71 x lo-* 4.05 x10-3 2.35 x 1O-4 2.37 x 1O-4 6.05 x10-s 2.06x10-* 5.2 x~O-~ 7.3 x10-3 3.13x10-4 3.75x10-4

H. Oechsner / Receni applications of SNMS

all other constituents except Fe, eq. (5) can be applied although the set of the DX.J’-values is not complete. The atomic concentrations obtained by SNMS are compared with the nominal values in the certified sample for 10 different isotopes. In most cases the agreement is excellent, especially in view of the relatively small concentrations. The maximum signal-to-noise ratio of several lo6 taken from the SNMS spectrum in fig. 1 indicates the high detection sensitivity obtained with the DBM of SNMS. Therefore, the ppm-range, yielding SNMS signals in the order of 10 cps, should be safely accessible. A corresponding example is given in fig. 5. The SNMS peak of 16 cps at mass 209 corresponds to a nominal concentration of 1 ppm Bi in the investigated sample, while the signal at mass 203 to 205 corresponds most probably to a previously not detected thalhum concentration of 300 ppb in the sample. Moreover, a small SNMS peak corresponding to a nominal Pb concentration of 67 ppb becomes also clearly detectable in fig. 5. Hence, SNMS provides a detection sensitivity in the order of 100 ppb for the surface near region of a solid sample.

NBS-Standard

1’ ‘I’ ‘I’ ‘:’

12610

‘!’ I” “’ ‘1

16’

100 certlfled

IO'

102

cx m ohmic%

Fig. 4. Sample composition obtained from the SNMS-spectrum in fig. 3 compared with the nominal composition from the NBS certificate. Only a simplified evaluation procedure for the SNMS signals has been applied using a first order correction of the relative detection factors only for such elements with low mass and high ionization potential (for details see text and table 1).

923 209 BI

Us = l.5keV I, = 2,smA

/

1: -5cps

I/

206-208

Fig. 5. SNMS-peaks for constituents with very low concentrations in the investigated standard (NBS Standard 1261a). The peak at mass 209 corresponds to a nominal Bi-concentration of 1 ppm, at mass 206-208 to a Pb-concentration of only 67 ppb. The additional SNMS-peak at mass 203-205 refers presumably to a Tl-impurity of about 300 ppb which is not included in the sample certificate.

4. High resolution depth profiling with low energy SNMS As already mentioned in sect. 2, extremely high lateral homogeneity of the bombarding ion current and simultaneously very low bombarding energies can be established with the direct bombardment mode of SNMS [8]. It has been shown experimentally that under normal ion bombardment at energies around lo2 eV sputter induced collisional mixing is limited to the uppermost 2 atomic layers of the sample [22]. Such a depth interval, on the other hand, corresponds to the lower limit of the information depth for surface analysis involving bombardment induced particle ejection [23,24]. Hence, optimum depth resolution is expected for laterally extremely homogeneous sputter removal at bombarding ion energies in the order of lo2 eV. Corresponding results obtained with the DBM of SNMS are shown in figs. 6-8. The SNMS sputter depth profile in fig. 6 describes the diffusion determined profile of the P dopant in a test structure for the fabrication of integrated circuits [25]. The roofed plateaus correspond each to a depth interval of 200 nm and describe the P-profile in a TaSi, (left-hand side) and a polycrystalline sputter deposited Si (right-hand side) sublayer. The P-source was initially located at the interface between these two layers. In this figure again a comparison between SNMS and SIMS is XIV. ION BEAM TECHNOLOGY

H. Oechsner / Recent applications of SNMS

924

-SNMS

(250eV

Ar’)

--SIMS

(12 keV 0;)

/

f

1! i

;

.-N

0.5

0

E

s

: 0

depth

--c

Fig. 6. Comparative SNMS- and SIMS-profiles of the P-distribution in a microelectronic test structure. The total width of the profiles corresponds to a depth of 400 nm (from refs. [25] and [26].

included. There is obviously almost no simularity between the SNMS profile and a SIMS profile obtained under bombardment with 12 keV 0: [26]. The prominent feature of the SIMS profile is a strong pile-up of the P+-signal which has to be ascribed to enhanced secondary ion formation at the interface between an SiO, overlayer and the tantalum silicide layer. While for relatively wide structures as those in fig. 6 the requirements to depth resolution are not too severe, detailed depth information becomes continuously important with decreasing layer thickness. Thin multilayer structures are often deposited by sequential sputtering from two different targets. The SNMS depth profile of such a multilayer system with a total thickness of 2000 A, consisting of Ta/Si double layers of 200 A thickness is shown in fig. 7. After the deposition of the outermost Ta-sublayer an additional thin Si cover had been deposited upon the multilayer structure. The high depth resolution obtained with the direct bombardment mode of SNMS for normal bombardment with Ar+-ions of 200 eV is demonstrated by the large variation of the Si-signals of the individual Si-sublayers. Even structural details, as the small shoulder at the first Si-Ta interface before the substrate, are reproducible by SNMS-depth profiles measured for the same layer system at different positions across the backing Si-wafer (see arrow in fig.

I 1.li :

:

Fig. 7. SNMS-sputter depth profile of Si in a Ta-Si multilayer structure produced by sequential sputter deposition of Ta and Si on a (100) Si-wafer. Nominal thickness of the individual Ta-Si sublayers 200 A. Sputter profiling has been performed with normally incident Ar+-ions of 200 eV at a bombarding current density of 1,s mA cm-* (sample by courtesy of R. v. Criegem and H. Rehme, Munich).

can be ascribed to a delayed sputter removal of the high mass species Ta from the low mass Si underlayer, the light component Si is expected to be rapidly removed from a high mass Ta-backing. One of the sharper Si + Ta-interfaces is shown in detail in fig. 8. The 84-to-168 interface width is found to be only in the order of 15 A. Such a small interface width agrees well with the depth of an atomic microroughness being induced even at ideally plane, defect free surfaces due to the statistical character of the

: Y

.E

i_

“CJ

7).

The SNMS-profile in fig. 7 reveals clearly two different experimental interface widths, namely a broader for the Ta --* Si-transition and a much sharper for the the Si --) Ta-interface, when proceeding from the surface of the layer system down to the interface with the Si-substrate. The different behaviour is presumably due to the different sputter behaviour when crossing both types of interfaces: While the broadening of the Ta-Si-interface

Fig. 8. Enlarged representation of the Si- and Ta-SNMS profiles across an Si-Ta interface in the multilayer structure of fig. 7. The experimental 84-m-16% transition widths correspond to 15 A for Si and 14 A for Ta when constant removal velocity across the layer system is assumed.

H. Oechsner / Recent applications of SNMS

sputtering process itself [24]. Taking into account the information depth of l-2 atomic distances obtained by low energy SNMS, the experimental transition widths in fig. 8 are readily referred to such residual influences. This gives evidence that the variation of the SNMS signals within a distance of lo-20 A as shown in fig. 8 refer to sputter depth profiling across an atomically sharp interface. The error bars in fig. 8 refer to small differences between the behaviour of both the Ta- and the Si-signals at the individual Si-Ta-interfaces along the investigated layer system. It is found that the experimental transition width at these interfaces varies between 12 and 16 A across the entire thickness of the multilayer system. This demonstrates the extremely high depth resolution obtained with low energy SNMS and vice versa the high quality of the layer system with evidently atomically sharp interfaces at least between the Si- and the Ta-sublayers. The author wants to thank K.H. Mtiller for many helpful discussions, and M. Kopnarski and P. Stetzenback for the careful and skilful performance of the SNMS-measurements. Thanks are also due to R. v. Criegern and H. Rehme, Munich, for supplying the high quality multilayer samples.

References [l] A. Berm&&oven, F.G. Riidenauer, and H.W. Werner, Secondary Ion Mass Spectrometry (Wiley, New York, 1987). [Z] K. Wittmaack, Surf. Sci. 68 (1977) 118. [3] A.B. CampeIl III and C.B. Cooper, J. Appl. Phys. 43 (1972) 863. [4] H.E. Beske, Z. Naturforsch. 22a (1967) 459. [S] A. Berminghoven, Surf. Sci. 53 (1975) 596. [6] H.A. Storms, K-1. Brown and J.D. Stein, Analyt. Chem. 49 (1977) 2023.

925

]71 H.W. Werner, in: Electron and Ion Spectroscopy of Solids, eds. L. Fiermans, J. Vennik and W. Dekeyser (Plenum, New York, London 1978) p. 324. PI H. Oechsner, in: Thin Film and Depth Profile Analysis, Topics in Current Physics, vol. 37, ed. H. Oechsner (Springer, Berlin, Heidelberg, New York, Tokyo, 1984) p. 63. 191 H. Oechsner, Appl. Phys. 8 (1975) 185. [101 R.E. Honig, J. Appl. Phys. 29 (1958) 549. 1111 H. Gnaser, J. Fleischhauer and W.D. Hofer, Appl. Phys. A37 (1985) 211. [=I D. Lipinsky, R. Jede, 0. Ganschow, A. Benninghoven, J. Vat. Sci. Technol. A3 (1985) 2007. 1131 H. Oechsner, Z. Physik 238 (1970) 433. 1141 H. Oechsner, Plasma Phys. 16 (1974) 835. I151 T. Halden, Thesis Univ. Kaiserslautem (1984). WI K.H. Miiller, K. Seifert and M. Wihners, J. Vat. Sci. Technol. A3 (1985) 1367. 1171 H. Oechsner, W. Rhhe and E. Stumpe, Surf. Sci. 85 (1979) 289. WI H. Oechsner and E. Stumpe, Proc. 4th Int. Conf. on Solid Surfaces and 3rd ECOSS, Cannes (1981) eds. D.A. Degras and M. Costa, vol. II, p. 1234. 1191 J.F. Geiger, H. Oechsner and H. Paulus, Proc. IPAT 87 (CEP, Edinburgh, 1987) p. 390. PO1 K.H. MiiIler and H. Oechsner, Mikrochim. Acta Suppl. 10 (1983) 51. WI A. Wucher, F. Novak and W. Reuter, J. Vat. Sci. Technol. in press. P21 J. BarteIla and H. Oechsner, Surface Sci. 126 (1983) 581. 1231 J.P. Biersack and W. E&stein, Appl. Phys. A34 (1984) 73. 1241 H. Oechsner, in: Festkorperprobleme (Adv. in Solid State Physics) Vol. XXIV, ed. P. Grosse (Vieweg, Braunschweig, 1984) p. 269. M. Kopnarski and H. Oechsner, v51 P. Beckmann, Mikrochimica Acta Suppl. 11 (1985) 79. WI R. v. Criegem, T. Hilhner, V. Huber, H. Oppolzer and I. Weitzel, Fresenius Z. Anal. Chemie 319 (1984) 861.

XIV. ION BEAM TECHNOLOGY