Modification of chemical surface properties by ion beam assisted deposition

Modification of chemical surface properties by ion beam assisted deposition

Nuclear Instruments and Methods in Physics Research B46 (1990) 369-378 369 North-Holland Section VII. Beam assisted deposition MODIFICATION ...

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Nuclear

Instruments

and Methods

in Physics

Research

B46 (1990) 369-378

369

North-Holland

Section

VII. Beam assisted deposition

MODIFICATION OF CHEMICAL SURFACE BY ION BEAM ASSISTED DEPOSITION

PROPERTIES

G.K. WOLF Ph_vsikalische Chemie,

Universitiif Hedelberg,

FRG

Ion beam assisted deposition represents a superior method for controlled modifications of surface layers and production of coatings. In addition. it is a unique tool for studying the influence of ion bombardment parameters on the quality of coatings. The chemical composition and structure of surface films and of the film/substrate interface have a strong influence on many properties of the coated system. It is demonstrated that they change considerably under ion bombardment. often under formation of compound Auger electron depth profiling and Mossbauer spectroscopy were used for these investigations. films or separate “interphases”. Correlations with macroscopic properties were established by adhesion and hardness measurements. As an example for chemical applications the resistance of hard coatings against aqueous corrosion of steel was studied in relation to the composition and stoichiometry of the films. Finally a new approach for ion bombardment and ion beam assisted deposition of powder targets is presented and illustrated by an example for electrochemical applications.

1. Introduction Depositions of thin films consisting of elements or compounds during bombardment with an ion beam find more and more applications for metal finishing, semiconductor processing and fabrication of optical coatings [1,2]. This technique has the great advantage of being extremely flexible because of separate control of the ion beam parameters and the deposition parameters and it works well at low processing temperature (- 200°C). In addition, it is an extremely versatile tool for basic studies on the effect of ion bombardment on the properties of a growing film.

Ion

Reactive --

Beam_____ Assisted

Deposition

It is not possible to distinguish exactly between chemical surface properties which are modified by the ion bombardment and physical ones. There will always be a certain overlap. Therefore this contribution will mainly deal with chemical effects, but some physical ones will also be mentioned. The first main section of the survey describes aspects of the chemical composition of the film/ substrate interface region and its relation to film properties like adhesion and intrinsic stress. The second section is concerned with the chemistry of the compound films, and the relation of their composition to film properties like hardness and corrosion resistance. In the final section

DeDosition

IBAD

Ion

Beam

Assisted

R&l&

gas or reactive layer

interphase

comdound

e.g. Al, Cr, 6, SI, i-C l

evaporation of coatmg material under simultaneous bombardment with rare gas ions

Fig. 1. Schematic 0168-583X/90/$03.50 (North-Holland)

representation

0 Elsevier Science Publishers

layer, e.g. evaporation of coating material under simultaneous bombardment with reactive 10”s or with rare gas ions in atmoshere of backfilled reactive gas molecules

of ion beam assisted deposition

B.V.

interphase

TIN. Cr,N. TIC

techniques

(IBAD).

VII. BEAM ASSISTED

DEPOSITION

370

G.K. Wolf / Modrficatron oJ chemical surface propertres

very recent experiments on powder irradiations will represent an example for future trends in the field.

2. Experimental -1.1. ion beam techniques There are many combinations of atom vaporisation and ion sources described in the literature for performing ion beam assisted deposition [3,4]. Mainly sputtering or evaporation are used for generating the atom vapour. In our lab we use electron beam evaporation as a source for atoms and a duoplasmatron which delivers up to 10 mA beams of ions with a minimum energy of 3 keV and a maximum of 30 keV. The experimental set up is described elsewhere [5,6]. Fig. 1 illustrates schematically the main different modes for running the IBAD process. In order to obtain adherent elemental films on a substrate. a beam of rare gas ions is used for sputter cleaning the surface prior to the evaporation process. Afterwards, the evaporation is started and the film grows under partial or continuous bombardment with ions. The “interphase” developing between film and substrate because of ion beam mixing may be shallow or broad, depending on the ion energy and/or the ion to atom arrival ratio. In the reactive mode a beam of reactive ions like nitrogen. oxygen, carbon may be used for bombarding the growing film. The result is a compound coating on the substrate and a more complicated “interphase” than in the case mentioned above. Compound coatings can also be generated by bombardment with a rare gas beam when the reactive component is introduced into the vacuum chamber in a gaseous form. There is a general discussion on the usefulness of bombardment with medium to high energy ions (E > 2 keV) compared to low energy ones (E = 0.1-2 keV) We think that higher energies are of advantage for modelling a broad overlap region around the interface. This may be beneficial with respect to adhesion and low film stress for coatings on substrates insensitive against radiation damage, like metals and ceramics. For damage sensitive substrates like semiconductors and optical materials, low energy ions should be used. 2.2. Mechanical and chemicul tests The most important ings are: _ adhesion. _ hardness, _ stress. As adhesion tests we The first one is the pull surface of the coating

mechanical

properties

of coat-

used three different techniques. test where a stamp fixed at the is pulled off, and the force

necessary to remove the film on the stamp is recorded. The second one is the scratch test where a diamond stylus is moving under load across the surface, and the critical load necessary for spalling off the coating is registered. The third one is the bending test where the coating and substrate are bent around a defined varying radius. The critical radius for failure of adhesion is again recorded. The hardness is measured in the usual way by analysing microscopically the indentation of a diamond under load. We generally use the microhardness test with a Knoop diamond. The film stress can be measured by X-ray diffraction techniques, or by several methods relying on the determination of the curvature of a thin substrate [7]. Chemical degradation of a film/substrate couple mainly takes place: _ by oxidation or - by corrosion. This contribution is restricted to aqueous corrosion experiments. Effects originating because of porosity and poor adhesion of a film can be determined by electrochemical current/potential measurements [8]. During these measurements hydrogen is formed electrolytically in pores at the interface film/substrate. Because of the hydrogen pressure a poorly adhering film spalls off. The corrosion rate of a film/substrate system is determined by measuring the dissolved amount of suitable elements contained in the film or the substrate. This can be done by neutron activation analysis or by polarographic analysis of the substrate. 2.3. Structure and composition of film and interface The elemental composition of the film/interface/ substrate system was analysed by Auger electron spectroscopy combined with sputter depth profiling. The structure of the film was in some cases analysed by X-ray diffraction. In other cases the film/substrate system was studied by kfiissbuuer spectroscopy* using thin substrate foils in order to minimize signals coming from the nonmodified substrate.

3. Adhesion, stress and interface chemistry Important properties of a film/substrate combination like hardness, stress and adhesion rely partly on the morphology and structure of the film and partly on the overlap region around the film/substrate interface, which is often physically and chemically different from the substrate as well as from the film. Therefore one may call this region an “interphase”. Unfortunately, the knowledge of the relationship between the “interphase” structure and chemistry and the film properties is very scarce, mainly because it is very difficult to analyse the thin “interphase” precisely. However, it is possible to

G.K. Wolf / Modificationof chemicul surface properties

concentration

/at%1

concentration 100

371

[at%?

‘~~~

40

20

0 -0

10

5

15

20

25

sputtercharge

30

IJ

35

2

4

[mCl

6

8

10

12 14 16

sputtercharge

-+--a

t-c

+o

18 '20 22

24

20

28 30

[fnC1

*Fe

a. evaporated

b.

IBM 120 keV, 5*E+16/cm2

c.

IBAD 6 keV

t 0

5

10

15

sputtercharge Fig.2. Auger

depth

profiles

20

25

30

[mC1

of 80 nm thick evaporated B layers on steel. (a) As-evaporated: Ar’/ad; (c) IBAD under 6 keV Art bombardment.

report a few experimental results from our lab which at least illustrate the problems and the importance of further relevant studies. 3. I. Interface composition; structure and adhesion

We studied in some detail the boron/iron and boron/steel system by Miissbauer spectroscopy and Auger depth profiling. A comparison was made between ion beam mixing and ion beam assisted deposition. In the first case, a 80 nm thick boron layer was deposited

(b) ion beam

mixing with 5 x 10’”

on thin iron or steel substrates and subsequently bombarded with 120 keV argon ions. In the second case, 80 nm boron was deposited under continuous bombardment with 6 keV Ar+ ions on an oxide-free substrate. Fig. 2 shows the Auger depth profiles of the resulting specimens together with an as-evaporated sample. It is interesting to note that the sample bombarded with 120 keV argon exhibits a substantial broadening of the interface indicated by the low and high concentration tails of the iron and boron signal, respectively. The 6 keV IBAD treatment, however. leads only to a negligiVII. BEAM ASSISTED

DEPOSITION

G. K. Wolf / Modificatron

372

Subspectra

:

Fe~,..,B.(mognetic) -----

FeC,_,B.(non-magnetic)

Difference

+

Fe,,_,,Be.

is a-Fe

100

99 98 97 96 95

95 94 9

100

of chemical surf&e properties

paramagnetic amorphous iron boride. However, the weak signal indicates that only a portion of the boron atoms in the Fe/B-overlap region have reacted chemically under formation of iron boride. This is different from previously studied [9.10] implanted boron in iron, which reacts completely. The spectra of the IBAD B/Fe system in fig. 4 contain no clear indication of compound formation in the interface region. Nevertheless, a very small reaction zone of a few monolayers might exist because the sensitivity of the method allows only the detection of concentrations of more than 1 at.%. The results obtained up to now show clearly that the extent of the overlap region depends on the ion energy and on the thermodynamics of the system. If silicon is substituted for boron in the IBAD apparatus under the same general conditions, in contrast to the boron case a rather broad Si/Fe-overlap zone is generated [9,11]. Probably the degree of mixing is only partly caused by collisional phenomena, which are independent of the thermodynamics of the system, and partly by thermal spike and radiation enhanced diffusion reactions which are not [12]. It is very interesting to compare these microscopic properties with the macroscopic ones like adhesion. The results of the scratch test are displayed in fig. 5. The

99

Subspectra

98

--

Fe,,.,,B,(non-mogneilc)

Difference

t

Fe,,_.,Be,

IS a-Fe

97 96 95

-6

-4

-2

0

2

II

6

VELOCITY(mm/s) spectra of 80 nm boron layers on iron Fig. 3 Miissbauer substrates as a function of the Ar’ ion mixing dose (120 keV). (a) As evaporated a-Fe.

ble broadening. The interface region contains mainly carbon impurities, which are a constituent of the steel substrate. The carbon content of the interface region is comparable for both samples. On the other hand, the oxygen contamination of the IBAD sample is smaller than that of the mixed one. Because of the generally low signal it is difficult to see the difference in fig. 2. The related MGssbauer spectra are shown in figs. 3 and 4. In the as-evaporated sample only the characteristic pattern of magnetic a-Fe is seen together with a small quadrupole doublet arising from an iron contamination in the beryllium backing. The ion beam mixed samples show, in addition, broad lines caused by

Fig. 4. MKssbauer spectra of 80 nm boron layers on iron substrates deposited with and without 6 keV Ar+ bombardment. (a) As evaporated a-Fe.

G. K. Wolf / Modification

2-

I

0 I

2

3

4

measurements of 80 nm boron layers on steel with the scratch test. (1) As-evaporated; (2) ion beam mixed with 1 ~lO’~Ar+/cm* (120 keV); (3) ion beam mixed wth 1 x 10” Ar+ cm’; (4) evaporated under 6 keV Ar’ bombardment. Fig. 5. Adhesion

critical load for delamination of the boron layers. which is proportional to the adhesion, increases substantially with the mixing dose. However, the best adhesion occurs for the film produced under 6 keV Ar+ bombardment. Obviously, the key parameters for adhesion of boron layers on steel are neither the extension of the overlap region around the interface nor the degree of iron boride formation in the “interphase”. Probably an oxygen-free interface and a little bit of boride formation across a few monolayers are already sufficient for excellent adhesion. These ideas were verified by a completely different system, where the overlayer and the substrate have no chemical affinity to each other, namely silver films on AI,O, substrates. 100 nm of silver was deposited under continuous ion bombardment on Al,O, ceramics. In order to obtain a high degree of interface mixing Xe ions were used for bombardment, and the ion energy was varied. Fig. 6 shows the result of the pull-off test. The bombardment improves the adhesion, and there is a direct correlation of the ion energy which affects the broadness of the interface region with adhesion. The conclusions from the present status of the investigations are: ~ if there is chemical affinity between surface film and substrate a clean (e.g. oxygen-free) interface and a small overlap region are sufficient for good adhesion; _ if there is little or no chemical affinity, physical linking together with a broad overlap region is necessary for good adhesion. 3.2. Modelling

of film stress under ion bombardment

The above mentioned considerations with respect to adhesion did not take into account the influence of film stress on adhesion. There is no doubt that films under

313

ofchemrcal surfuce properties

high tensile or compressive stress fail; however, there are no detailed quantitative investigations on the correlation. In the following we want to show that IBAD is the ideal tool for performing such investigations because one is able to model any desirable stress state, even though the corresponding experiments have not been completed yet. In fig. 7, the results of stress measurements with boron films are plotted as a function of the bombardment parameters. 200 nm thick films were condensed on thin iron foils under bombardment with different fluences of 6 keV argon ions. The stress was determined in terms of the degree of bending of the substrates. The nonbombarded films exhibit tensile stress. Increasing the ion/atom ratio (= ratio of ions/atoms arriving on the substrate) shifts the stress to the compressivc regime. Especially interesting is the “zerostress” region which requires, in the boron case, an ion/atom ratio of only 2 : 100. The “zero-stress” region is dependent on the composition of the film and the energy of the bombarding ions. This was verified in further experiments with carbon films and chromium films on iron [3,13]. All results obtained are put together in the diagram shown in fig. 8. Here the ion/atom ratio necessary for “zero stress” is plotted as a function of the ion energy [14]. The dashed lines correspond to overall energies of 100, 10 and 1 eV. respectively, deposited for every

X = failure of glue Fig. 6. Adhesion measurements of 100 nm silver layers on AI,O,. (1) As-evaporated: (2) evaporated under 6 keV Xe+ bombardment: (3) evaporated under 20 keV Xe+ bombardment. VII. BEAM ASSISTED

DEPOSITION

374

G.K. Wolf / Modification

of chemical surfuce properties

In conclusion. there are two parameters, the ion energy and the ion/atom ratio, which can be used to produce a desired film stress state or even “zero-stress”. Experiments in order to correlate the stress state with adhesion are in preparation.

4. Film chemistry, pound films

Fig. 7. Intrinsic stress of IBAD boron films on iron foils in relation to the 6 keV Ar+/atom arrival ratio, measured in terms of the substrate bending.

atom. Our values for chromium on iron, where the ion energy was varied over a broad range, fall exactly on the Em’ line corresponding to 100 eV/atom. Therefore, the overall energy transferred to the growing film is the key parameter for the modeling of the film stress for this system. Carbon and boron films which have covalent bonds require less energy deposition for “zero stress” than the metal chromium. The older data of Hirsch and Varga [15] for germanium and of Hoffman and Gaerttner [16] for chromium are also included in the diagram. There is a good qualitative agreement. However, we cannot confirm the Em0 5 dependence found by the latter authors. arriving

Fig. 8. Ion/atom arrival ratio for “zero stress” as a function ion energy for various coating systems.

of

generation

of stoichiometric

com-

Compound films with well defined stoichiometry are mainly applied as insulators for electronic purposes or as hard coatings for improved wear resistance of metals. In the latter case, the most important parameter is the hardness, which is mainly determined by the stoichiometric composition of the film. It is therefore important to control the composition precisely. The IBAD process offers two possibilities to do so (see also fig. 1): (1) The condensing film is bombarded with reactive ions and the film composition is determined by the ion/atom arrival ratio under consideration of the sputtering effect. (2) The film is bombarded with inert ions in an atmosphere containing the reactive component (typical gas pressure lop2 Pa). The film composition is determined by the partial pressure of the reactive gas and by the ion/atom arrival ratio. The following examples illustrate the results obtainable. 4.1. Hardness

of IBA D compound

films

Chromium nitride was produced by bombarding evaporated chromium during condensation with a 12 keV N: beam (method 1). The substrates were steel and glass. The hardness of the resulting films on glass substrates was measured with a Knoop microindenter. The results are shown in fig. 9 as a function of the applied load. The values are very much dependent on the stoichiometric composition. Chromium nitride B contains only lo-15 at.% nitrogen. It is already much harder than chromium metal. Chromium nitride A contains 30 at.% nitrogen corresponding exactly to the composition Cr,N. Its hardness is considerably higher than that of sample B. Titanium nitride was generated using method 2. By properly adjusting the nitrogen pressure in the vacuum chamber and the Ar+/Ti ratio, one is able to obtain stoichiometric TIN with superior hardness values, as shown in fig. 10. TIN was also produced by Hubler et al. with a similar technique and by Hayashi et al. [17] and Sato et al. [18] using a nitrogen beam. The latter technique was also successfully applied in our lab [19]. All groups found that films with a Ti/N ratio well above or well below 1 were much less hard than stoichiometric TiN [20]. With the same technique, well adhering TIC coatings are also available. For TIC the

G.K. Wolf / Modification of chemical surfuce properties

-_ Fig. 9. Knoop microhardness of IBAD chromium nitride coatings on glass with different nitrogen content as a function of the applied load. had to be filled with ethylene. Again, by adjusting the argon ion beam to the appropriate value, TiC with a 1 : 1 ratio could be produced. This coating is even harder than TiN (see fig. 10).

I :

375

( 1



I

Fig. 11. Auger depth profile of a graded steel with Ti intermediate

11

Fj

IBAD TIC coating layer.

on

chamber

4.2. Corrosion protection

Hard coatings necessarily good

hardness

‘3

by IBAD compound films

are often porous against corrosion.

and therefore not The ability of a

[HK’IOOO!

L

2t 1'

‘2

I

.___

0

---_ic_

A.

_---__-----------x

__

50

L

.--

i

-

. ..L_

imo

150

100

-

--

2’50

Load ImNl iM- TIN

+

TIC

-X--

silbslrare

Fig. 10. Knoop microhardness of IBAD titanium nitride and carbide coatings on gIass as a function of the applied toad.

coating for corrosion protection depends, generally speaking, on three properties: _ the coating itself must be corrosion-resistant; _ the porosity must be low; _ the adherence has to be very good. Therefore. the TIN and TIC films mentioned in section 4.1 were, in addition to their hardness, separately optimized with respect to their performance in aqueous solutions (buffered acetate solution, pH = 5.6). The hard coatings mentioned above already showed a good protective power when subjected to electrochemical potential multisweeps. However. it turned out that samples with an intermediate pure titanium film followed by the nitride or carbide layer worked even better. This can easily be done with IBAD by first evaporating titanium under bombardment with the argon beam under good vacuum conditions, and introducing the reactive gas later on. The Auger depth profile of such a layered TiC coating is shown in fig. 11. The 1 : 1 TIC near the surface changes gradually into pure Ti, which is directly upon the iron substrate. The oxygen contamination of such coatings is very low too. Fig. 12 contains the results of multisweep experiments with this coating in comparison with a TiN film without intermediate Ti. The critical current density (=I the highest measured dissolution current of the steel substrate) for every potential sweep is plotted against the number of cycles. The dissolution takes place via growing pores or pinholes in the film, and the film adhesion is lowered from sweep to sweep because of the pressure of the hydrogen formed in the pores during corrosion. VII. BEAM ASSISTED

DEPOSITION

G.K. Wolf / Modi’icution

376

ofchemrcal surf&vproperties

chambe (stain1 teflon

r

es*

s

beari

beam

ing

eccentric

Fig.

I

15

-+

l/l

+,

,,w..,

I,‘,,

,‘hi

Xl

Fig 12. Critical current density for the dissolution of low alloy steel with different IBAD coatings as a function of the number of voltage sweep cycles. (1) Uncoated; (2) 3 nrn TIN; (3) 3 pm TIC with intermediate Ti layer.

Compared to uncoated steel, the critical current density was reduced initially by a factor of lo3 by means of the layered coating. After 50 sweeps it was still lower by a factor of 100. The pure TIN coating did not at all perform as well. The given examples show that by proper combinations of multilayers, and with accurate control of stoichiometry and ion energy, rather different properties of a film like adhesion, hardness and porosity can be optimized.

5. Future trends: powder irradiations There exist a number of reasons for ion bombardment of powder targets. _ In isolating targets the heat conduction is poor and powder targets may dissipate the thermal energy faster. _ For many applications, powders are a necessity: catalysts, for example, and the base materials for ceramics production are very often used in the form of powders. _ One could speculate that very fine powders may undergo interesting and unexpected transformations when bombarded with energetic ions. _ A single fine grain hit by an energetic ion may obtain a short-term very high temperature. There have been a number of earlier attempts for powder irradiations, starting with the ideas of Dearna-

13.

“Vibratory conveyer” target bombardment of powder

bearing

arrangement targets.

for

ion

ley to bombard on a sloping conduit [21], and of Freeman to magnetically bend the beam and bombard a powder horizontally [22], and continuing with different constructions tested in South Africa [23,24]. None of these attempts have been very successfully applied for reasons which are not completely understood. Recently, we tried a new approach to this old problem, especially because of our continuous interest in the modification of catalysts. Our instrumental approach went in two directions: The first one uses the principle of the vibratory conveyer as shown in fig. 13. The powder is kept in continuous motion on a diagonally vibrating metal strip driven by an electromotor. The strip is contained in a stainless steel chamber in which the ion beam can enter. The chamber is isolated and acts as a Faraday cup for measuring the beam current. Test experiments were performed with CrzO, powder using a gold ion beam. By analyzing the gold content of the powder by neutron activation analysis it could be proved that practically all gold ions have been implanted in the powder. The second approach relies on the principle of the concrete-mixer (fig. 14). The powder is contained in a revolving drum, where it forms a powder curtain which is hit by the ion beam in the same way as

haft

be

rotating

chamber

glasstube

Fig.

14. “Concrete-mixer” bardment

target arrangement of powder targets.

for ion bom-

311

G.K. Wolf / Modification of chemical surface properties

I-

1 I ,!i !,I’i

pure GC

I ’

GC

+ I.@ld

8 Ar+

\I’

strate material for electrochemical sensors. For this purpose a high double layer charging current masks any small signals. The reduction of the current by ion bombardment is already well known for flat carbon samples [25]. Catalytically active metals like Pt are difficult to implant directly in substrate powders because the surface concentration of Pt, which is the most important parameter for catalysis, is very low. This is caused by the high surface area of the powder and the ion energy leading to penetration of the bombarding particles well below the surface. Here the “sputter + bombardment” approach gives much better results.

6. Conclusions 1

200

400

600

800

1000

1200

E (mV NHE) Fig. 15. Current potential diagram in 1N H,S04 of unbombarded glassy carbon (GC) compared to GC bombarded with 1.6X10’XAr’ionsof60keV.

a solid thin film. The first concept is for smaller quantitities of powders, the second one may be scaled up easily for larger powder quantities. A number of experiments have been performed with Cr,03, Al,O,, WC and glassy carbon. Some problems arising include: - charging of the powder, - sintering of powder, _ extensive beam heating, especially with small, nonheat-conducting powders. The first two problems can be solved by appropriate constructive measures, the third one only by defocussing or by reducing the beam power. By introducing a sputter target in the powder apparatus a simple version of ion beam assisted deposition may be performed too. A part of the ion beam is used for sputtering the desired element onto the powder surface; the other part for ion beam mixing of the sputtered surface atoms. Fig. 15 shows an example for applications, the bombardment of glassy carbon with argon ions in order to modify its electrochemical properties. The cyclic current potential diagram was taken in H,SO, before and after bombardment. The virgin sample shows the usual behaviour: a rather large double layer charging current, an oxidation reaction around a potential of +700 mV and CO, formation above +1200 mV. In the case of the bombarded sample the double layer charging current is greatly reduced and the first oxidation reaction suppressed because of structural changes in the surface region. This is an important finding for practical applications because glassy carbon may be used as a sub-

Ion beam assisted deposition represents a superior method for controlled modification of surface layers and production of coatings. In addition, it is a unique tool for studying the influence of ion bombardment parameters on the quality of coatings. It has been demonstrated that the subsurface and interface structure of a film/substrate combination changes considerably under ion bombardment, often under formation of a separate “ interphase”. These changes are partly responsible for improvement of adhesion and other properties. Modifications of the coatings themselves included preparation of films with well defined stress state and compound films with controlled stoichiometry and hardness. Finally, a new approach for ion bombardment and ion beam assisted deposition of powder targets was presented. The author would like to thank M. Barth, W. Ensinger, G. Frech, M. Hans. A. SchrGer, and R. Spiegel who performed most of the investigations mentioned, and BMFT, Bonn, who supported parts of the work under contracts nos. 13 N 5352/9 and 13 N 5428/7.

References [II G.K.

Hubler, Proc. SM*IB 88, Riva del Garda, Italy (1988) Mater. Sci. Eng. All5 (1989) 181. [21 W. Ensinger and G.K. Wolf. ibid. [31 G.K. Wolf, M. Barth and W. Ensinger. Nucl. Instr. and Meth. B37/38 (1989) 682. J.J. Cuomo, R.J. Gambino and H.R. [41 J.M.E. Harper, Kaufman, Nucl. Instr. and Meth. B7/8 (1985) 886. [51 G.K. Wolf, K. Zucholl M. Barth and W. Ensinger, Nucl. Instr. and Meth. B21 (1987) 570. [61 W. Ensinger, M. Barth and G.K. Wolf, Nucl. Instr. and Meth. B32 (1988) 104. der Eigenspannungen (VEB I71 H.-D. Tietz, Grundlagen Verlag fir Grundstoffindustrie, Leipzig, 1983). VII. BEAM ASSISTED

DEPOSITION

378

G. K. Wolf / Modification of chemrcal surface properties

[8] D. MacDonald, Transient Techniques in Electrochemistry, (Plenum, New York, 1977). [9] G.K. Wolf, Structure-Property Relationships in SurfaceModified Ceramics, II Ciocco, Italy (1988) in press. [lo] M. Hans, Diplomarbeit, Heidelberg (1987). [ll] M. Barth, W. Ensinger and G.K. Wolf, to be published. [12] W.L. Johnson, Y.T. Cheng, M. Van Rossum. M.-A. Nicolet. Nucl. Instr. and Meth. B7/8 (1985) 657. [13] M. Barth and G.K. Wolf, to be published. [14] J.J. Cuomo and S. Rossnagel Nucl. Instr. and Meth. B19/20 (1987) 963. [15] E.H. Hirsch and I.K. Varga, Thin Solid Films 69 (1980) 99. [16] D.W. Hoffman and M.R. Gaerttner. J. Vat. Sci. Technol. 17 (1980) 725. [17] K. Hayashi K. Sigiyama. K. Fukutani and H. Kittako. Proc. SM’IB 88. Riva de1 Garda, Italy (198X) Mater. Sci. Eng. All5 (1989) 349.

[18] T. Sato, K. Ohata, N. Asahi, Y Ono, Y. Oka and I. Hashimoto. J. Vat. Sci. Technol. f L4 (1986) 7X4. [19] G.K. Wolf, Proc. LEIB-5, Guild brd, UK (1989) to be published. [20] M. Satou, K. Fujii, M. Kiuchi and F. Fujimoto, Nucl. Instr. and Meth. B39 (1989) 166. [21] G. Dearnaley, J.A. Cairns and F .J. Brook, Brit. Patent Application no. 41930/72 (1972). [22] J.H. Freeman and W. Temple, Rzdiat. Eff. 28 (1976) 85. [23] U. v.Wimmersperg, J.F. Prins and T.E. Derry, Nucl. Instr. and Meth. 197 (1982) 597. [24] J.P.F. Sellschop, J.F. Prins and U. v.Wimmersperg, private communication (1984). [25] K. Takahashi. K. Yoshida and M. Iwaki, Nucl. Instr. and Meth. B7/8 (1985) 526.