FUNDAMENTALS OF THE PLASMA INDUCED AND ASSISTED CVD : PLASMA PARAMETERS CONTROLLING THE CHEMICAL EQUILIBRIUM, THE DEPOSITION KINETICS AND THE PROPERTIES OF THE FILMS

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FUNDAMENTALS OF THE PLASMA INDUCED AND ASSISTED CVD : PLASMA PARAMETERS CONTROLLING THE CHEMICAL EQUILIBRIUM,

THE DEPOSITION KINETICS AND THE PROPERTIES OF THE FILMS

S. Vepžek

To cite this version:

S. Vepžek. FUNDAMENTALS OF THE PLASMA INDUCED AND ASSISTED CVD : PLASMA

PARAMETERS CONTROLLING THE CHEMICAL EQUILIBRIUM, THE DEPOSITION KINET-

ICS AND THE PROPERTIES OF THE FILMS. Journal de Physique Colloques, 1989, 50 (C5),

pp.C5-617-C5-635. �10.1051/jphyscol:1989573�. �jpa-00229605�

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JOURNAL DE PHYSIQUE

Colloque C5, suppl6ment au n05, Tome 50, mai 1989

FUNDAMENTALS OF THE PLASMA INDUCED AND ASSISTED CVD : PLASMA PARAMETERS CONTROLLING THE CHEMICAL EQUILIBRIUM, THE DEPOSITION KINETICS AND THE PROPERTIES OF THE FILMS

I n s t i t u t e f o r Chemistry o f Information Recording, ~echriical University Munich, Lichtenbergstrasse 4 , 0-8046 Garching-Miinchen, F.R.G.

Abstract :

Fundamental processes which control the plasma induced and assisted chemical Vapour Deposition of thin films and the crystal growth are.briefly summarized. References to the relevant papers are given in order to enable the reader to get access to the more detailed data. The Partial Chemical Equilibrium which occurs in intense discharges is discussed in connection with the problem of the space charge sheath. The most recent progress achieved in the understanding of the mechanism of plasma induced deposition of amorphous silicon is summarized.

1. INTRODUCTION:

During the last 25 years, the plasma induced and assisted chemical vapour deposition developed from its infancy into a well recognized research field with an increasing number of important industrial applications. Yet, the progress has been achieved rather by the trial and error approach than by well planned experiments based on a tho- roughly designed working hypothesis.

For example, the first diamond crystallites have been prepared in a glow discharge (under conditions far away from those under which the diamond phase is thermodynamically stable) by Schmellenmeier in 1952 incidentally in course of his investigation of the conversion of methane into acetylene [1,2]. The recent explosive growth of the number of publications on the diamond and wdiamond-likeu carbon (more exactly cross-linked amorphous carbon) prepared by means of non-equilibrium discharge plasmas is characterized by a similar degree of empirism.

Similar examples include amorphous silicon and other electronic materials, such as SiO, and Si3N4, wear and corrosion protective coatings

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989573

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C5-618 JOURNAL DE PHYSIQUE

(TiN, Til-xA1,N, TiN1-,C,, AlzO,, etc), optical coatings and others.

Nevertheless, the increasing number of successful applications of plasma induced and assisted CVD and PVD in various branches of industry demonstrates, that the empirical and engineering approach can be extremely fruitful. Many papers have been published on the application of glow discharges for the preparation of electronic materials, optical, decorative, wear and corrosion protective coatings and others (see e.g. [4,30-331) and there are several invited and contributed talks on the particular subjects published in these proceedings.

Therefore, I should like to mention, for illustration of a successful industrial development the very recent commercialization of the hard a-C:H coatings [34]. The applications include dry bearings of high speed motors, fibre leading parts of textile machines, fluid engineering, precision tooling, tools for processing of aluminum, aluminum alloys, wood and plastics and wear, friction and corrosion protec- ting coatings on various materials.

Valws

Fig.1. Schematics of a pro- duction unit for large scale industrial deposition of hard a-C:H protective coa-

tings.

Figure 1 shows the schematics of an industrial processing unit which is used for the commercial coating at BERNA-Olten [35]. The ef- fective volume of the reactor available for the deposition is about 1 mS which enables one to coat simultaneously up to several hundred

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parts. A smaller unit for process development and small scale custom dedicated coatings is also available (see [ 3 3 , 3 4 ] for further details).

In the case of thermal CVD one has essentially four parameters to be varied in order to optimize the deposition process: Temperature, pressure, gas composition and the gas flow velocity and its spatial pattern in the reactor which control the uniformity of the deposition rate. These parameters are well accessible to exact measurements and also a meaningful theoretical modelling is possible on the basis of chemical thermodynamics and kinetics since the processes occur under conditions of thermal equilibrium (i.e. Boltzmann energy distribution).

In the case of Plasma CVD at least the following additional experimental Parameters have to be considered: Discharge current density and distribution, electric field strength, excitation frequency and substrate bias. These macroscopic parameters of the discharge determine the fundamental microscopic parameters of the plasma such as the mean electron energy ("electron temperature, Tell) and concentration, n,, and their spatial distribution. The substrate bias together with the total gas pressure, its composition and the geometry of the substrate to be deposited on controls the extent of the bombardment of the surface of the growing films with energetic ions.

Plasma diagnostics necessary for the determination of "Ter1 and n, is very complicated and there is no reliable method available which would allow one to determine the electron energy distribption in reactive plasmas with the desired accuracy. Moreover, the non-equilibrium nature of the plasma with the high population of electronic, vibrational and rotational excited states combined with the lack of knowledge of the corresponding cross sections for inter-states transitions and particle collisions (including chemical reactions) makes theoretical modelling to a very unreliable task (which is usually called the "GI- GOw approach [19]). Indeed, even a simple system, such as the deposition of a-Si from silane, requires for its theoretical modelling several tens-of particle and energy balance equations with the corresponding number of unknown cross sections. Even those cross sections which are relatively well known have an accuracy of about one order of mag-

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nitude and the cross sections for the excitation from vibrationally excited levelk are essentially unknown. For the latter reason non of the theoretical models published so far considers the reactions of the vibrationally excited states although the overhelming part of the electron energy losses is dissipated in the vibrational excitation and momentum transfer interactions. I leave it to the reader to draw his own conclusions as to what knowledge we can gain from such calculati- ons.

In this paper we shall summarize some fundamental principles which enable one to assess the possible progress of a chemical reaction under the conditions of low pressure, non-isothermal plasmas. such approach, although by far not complete and exact, has been very helpful in designing the proper and economical experimental matrix in order to reduce the total number of experiments to be done.

So far we considered only the processes taking place in the gas phase. Turning our attention to the processes occuring at the surface of the growing film one is reminded of the quotation of Wolfgang Pau- li: "The bulk of a crystal has been created by God, the surface by the devilu. Indeed, although a large progress has been achieved in the understanding of the surface and near-surface phenomena occuring under particle bombardment we are only beginning to have a basic idea of the importance of the various processes relevant to plasma CVD. We shall illustrate this point briefly by considering some aspects of the proper choice of the substrate bias during the deposition and its dependence on the discharge parameters. The issue of the surface coverage by chemisorbed fragments of the reactants and its role for the control of deposition and etch rate will be addressed as well.

Examples will be chosen in order to illustrate these principles.

The criterion for the choice of such systems'will be mainly the degree of the understanding of the reactions under consideration. This will naturally exclude some technologically important materials for which such understanding is lacking.

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2. Plasma Induced vs. Plasma Assisted CVD a s Chemical Equilibrium in a Non-Isothermal Plasma.

The difference between the plasma winducedl* and llassistedf* CVD has been discussed in ref. [4] and we refer to this paper for further details.

A necessary (but not sufficient) condition for the thermal CvD reaction to take place in a heterogeneous system described schematically by eq. ( 1 ) is that the chemical equilibrium concentration of the reactants is less than their concentration in the feed gas.

In order to achieve a sufficiently high deposition rate, the difference of the actual and equilibrium partial pressures of "A" containing reactants has to be larger than about 10e3 atm (the exact value de- pends upon the system). Moreover, the temperature has to be chosen such as to enable the reaction to proceed sufficiently fast towards the near equilibrium gas composition.

For example, the reaction (2) has equilibrium far on the right hand side (equilibrium constant at 500K is about 2*10-8 [ 2 0 ] ) to allow a fast deposition rate even at a silane pressure of 0.3 mbar but, because of a relatively high activation energy of about 50 kcal/mole for the gas phase decomposition, it occurs only if the temperature exceeds about 570°C.

Already a weak glow discharge can ncatalyzen the deposition provi- ding a non-equilibrium excitation of the internal degrees of freedom.

This is an example of "plasma induced" CVD as the desired reaction does not proceed without the plasma. The first .step is the fragmentation into silene and hydrogen, eq. (3) [21,22]. The available data indicate that the decisive reaction channel is probably the electron impact induced excitation of vibrational degrees of freedom [23,24]. The further steps in the composite reaction mechanism will be discussed later in this paper.

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JOURNAL DE PHYSIQUE

The peculiar property of silane as compared with methane is its spontaneous reaction with oxygen (and other oxidants), eq. (4), which occurs even at room temperature whereas methane ignites in air only bove 6000C. The origin of this difference lies in the different electronic structures of silicon and carbon resulting in a negative partial charge on hydrogen in silane but positive in methane [25].

S ~ H , + ²⁰²

---

^> ^si02(s)+ 2H20 (4)

There is no reason to use plasma in the SiH,/02 system. However, even the thermal CVD applied to this system will produce Sio, of poor quality and hardly of any use, unless the deposition will be performed at high temperature in a diluted gas. If silicon dioxide of good electronic quality has to be deposited at a low temperature of S 3500C, plasma induced CVD in the SiH,/NO-system is preferred. NO does not re- act spontaneously with silane and, therefore, the reaction can be induced and well controlled by the discharge [36]. For the deposition of very thin films like gate oxide, in which case the ion bombardment during the deposition causes detrimental damage to the film, one prefer- rably uses the reaction of silane with the postdischarge (wafterglowu) of NO [37]. This is an example of the plasma induced CVD "after the plasma".

The deposition of carbon from methane, as used for the preparation of hard a-C:H films as well as of diamond crystallites is another example of plasma induced CVD. As methane is thermodynamically stable in the typical P CVD temperature range of < 500K, the plasma is used to change the chemical equilibrium, thus decreasing the solubility of carbon in this case:

CH,

---

^> ^C(S) + 2H2 (5

Even a strong dilution of methane with hydrogen (up to 0.1 mol% of

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methane) does not prevent the deposition [26,27]. In an intense glow discharge with a high discharge current density this reaction can be made reversible and carbon films can be prepared by means of chemical transport according to reaction (6) [28,29]:

Here, jl and j 2 mean the low and high current density. Reactions (5) and (6) correspond to quite different discharge regimes:

-

The deposition from methane occurs at a relatively low plasma density. ~ l t h o u g h the mechanism is not sufficiently understood yet, the deposition of hydrogenated carbon (with a typical hydrogen content of several 10 mol% for a-C:H) occurs via CH, fragments formed in the gas phase, their adsorption on the surface, bond breaking of adsorbed species due to ion bombardment and resultant desorption of hydrogen and three dimensional cross linking of the deposited material. (There is a large discussion still going on regarding many details of this mechanism which we can not discuss here).

-

In the high density discharge, on the other hand, even the CH radical which is the most stable species at high temperatures [20] can- not survive and is dissociated by electron impact. Due to the non- isothermal nature of the plasma the steady state temperature of the substrate and of the reactor wall remains relatively low (800-900°C).

As the saturation pressure of carbon is very low under these conditions, deposition occurs (see [28,29] for further details].

Another set of examples of this "high temperature chemistry at low temperatures" are refractory nitrides, such as TiN, AlN, NbN, Si,N4 and others. The chemical equilibrium of the general reaction (7) is far on the side of the gaseous educts and no deposition of the nitride can occur without plasma.

Addition sf hydrogen shifts the equilibrium more towards the ni-

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tride, eq. (8), but high temperatures of 900-10000C are still necessary for deposition to take place.

It has been demonstrated already twenty years ago, that the chemical equilibrium of reaction (7) can be shifted far to the right hand

,

side and that growth of single crystals of various nitrides (TiN, AlN, SiJN4,..) can be facilitated by the use of an intense glow discharge [29,38,39]. The reaction of metal chlorides with atomic hydrogen and nitrogen, which are available in the intense discharge at high concentrations, is strongly exothermic as illustrated by the data for AICls/AIN given in Fig. 2 [38]. However, gradients of plasma density which occur in the vicinity of solid surfaces and in particular in shadowed regions can,result in a local shift of the equilibrium back to the chloride. Thus, deposition and etching regions can be found which are separated only by small dista~ces of a few mm. Nevertheless, a careful design of the experiment avoias these problems (see [29,40]

for further details).

Fig.2. Equilibrium Constant in the A1C13/A1N -system vs. Temperature for various degrees of dissociation

(from [38])

5 AlN(s)+3Cl@J = A1 C&@) + 7/2 /1C (sl

--..a

A ! N ( ~ +3a(4 = AlC13@) + Nlgl '''...L AlNfs) + 3 / 2 c Y ' U 3 / S ) + N (g)

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A similar shift of the chemical equilibrium by the high internal energy of the plasma occurs also in the MeC1,1+N2+H2/ MeN(s)+HCl -system. Figure 3 illustrates this point [4]. The thermally driven deposition of TiN, TiN1-,C, and Tic occurs only at temperatures above about 1'200K.

TI Cl, ⁺CHL = Ti C ( s i ⁺LHCI

TI Cl, ⁺

t

^N,⁺^2H2 ⁼^{T i N}( s ) + L H C I

Ti Cl, ⁺

-$?-

N~ ⁺xCHL ⁺211-xIH2 = T I C x N l r ) + 4HCI l - X

EQUILIBRIUM 20

- - - - n

1800

Fig.3. Temperature dependence of the Gibbs free energy of CVD reactions used for the deposition of TiN, TiN1-,C, and Tic [4]

Already in a weak discharge the shift is easily achieved at a temperature below 400°C [41-441. Li Shizhi et all [41,42] have developed this technique for the commercial coating of cutting tools at a temperature of about 5000C. Lowering the deposition temperature resulted in an increase of the chlorine content in the deposit above the tolerable level of 1-2 at%. More recently, Patscheider et a1 [44,45] succeeded to deposit TiN with a low chlorine content of < 2 at% at temperatures as low as 300°C. The apparatus used in the work of Patscheider et a1 is shown in Fig. 4. The key problem in achieving the deposition of good quality films is a high plasma density within the space charge layer in the vicinity of the substrate and of the workpiece. This problem is associated with the general problem of the space charge sheat and substrate bias to be discussed in the following section.

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Fig.4. Schematics of the apparatus used ~y J.Patscheider et a1 for the plasma induced CVD of TiN at low temperatures [44,45]

3. Control of substrate Bias and its Effect on the Properties of the Deposit.

The substrate bias is the difference between the electric potential of the substrate and that of the surrounding, unperturbed plasma.

Because of the non-isothermal nature of the glow discharge plasma characterized by a high electron energy ("T," of 30'000 to 100'000K) and a low kinetic temperature of the atoms, radicals, moLecules and ions (300 to lTOOOK), the diffusivity of the electrons is orders of magni- tude higher than that of the ions. Therefore, any surface in contact with the plasma will charge negatively with respect to the plasma in order to compensate for the different diffusivities of electron and ion, and to assure equal fluxes of positively and negatively charged

~articles.

If an external voltage is applied to an electrically conducting substrate, its potential can be driven several hundred Volts negative,

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but only about 20 Volts positive with respect to the plasma. The reason for this difference is the large negative cathode fall (which is necessary for the continuity of the discharge current at the cathode due to secondary emission and avalanche amplification of the electrons) as compared to the small anode fall of the electric potential.

~ h u s , if a positive potential applied to the substrate holder is in- creased above the value of the anode fall, the plasma potential will simply follow this increase and the bias will remain nearly constant (see e.g. [46-491 for further details). Therefore, the correct determination of the substrate bias requires the knowledge of the plasma potential. Measurement of the voltage difference of the substrate with respect to the ground can yield erroneous data.

The substrate bias determines, together with other plasma parameters, the impact energy of ions arriving at the surface of the growing film. This ion bombardment has important effects on the chemical and physical processes occuring during the growth and, finally, on the properties of the deposited films.

This is a very complicated issue as the ion bombardment can have beneficial as well as detrimental effects on the properties of the growing film, depending on the nature of the material and its desired applications. Let us mention just a few examples:

-

The deposition of a-C:H films from gaseous hydrocarbons, which are usually mixed with argon and hydrogen, at a negative substrate potential at which the ion impact energy is less than about 50 eV yields polymer-like films whereas dense, hard and transparent material with a low electric conductivity and low coefficient of friction is formed when the ion energy exceeds the threshold value for displacement damage and the resultant cross linking [3-61 (of about 1 53 eV for H+ - - >

C(s) [ 7 - 8 1 ) . Thus, the bombardment with energetic ions is necessary for the formation of the desirable phase.

-

On the other hand, significant degradation of the electronic properties of the growing semiconductor films occurs when the ion energy exceeds the displacement damage threshold [g-111 (e.g. 11.7 eV for H+

- - > Si(s) [12]).

-

There are many publications dealing with a variety of materials including metals [13], silicon nitride [14,15], optical coatings con-

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sisting of alternating layers of various oxides [16], silicon [17,18]

and others, in which the beneficial effect of ion bombardment at low energies on the densification of the films and on the the concomitant improved stability against gas incorporation has been documented.

The limited space available for this paper does not allow me to discuss this issue in more detail. Therefore I have limited myself to the discussion of the most important aspects and trends. The reader should search further information in the quoted papers.

4 . The Mechanism of Plasma Induced Deposition of Silicon from

Silane

As the reader who arrived at this point in the paper may have be- come frustrated by the large number of phenomena whose understanding seems to be hopelessly "hidden behind the moonll, I should like to fi- nish this paper with one example in which a deep understanding of the kinetics and mechanism has been achieved and utilized for an improved control of the deposition process.

The deposition of amorphous, a-Si, polycrystalline and epitaxial crystalline silicon, c-Si, from silane is probably the best studied system. Yet, only recently some understanding into the mechanism of thermal CVD has been achieved. The crucial steps in the composite mechanism of thermal CVD are the following [50-521:

The insertion of silene, SiHz into monosilane and higher silanes is very fast [55,56] and it occurs with a rate constant close to the molecular collision limit. Thus, the rate limiting step of the formation of higher silanes is reaction (3). Di- and trisilane represent in this mechanism the reactive intermediates from which the solid silicon is formed. Monosilane does not contribute significantly to the deposition rate as its reactive sticking coefficient is, under the typical

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conditions, very low as compared with those of SizHs and SiaHe [52,54,67]. The reactive sticking coefficient of trisilane is larger than that of disilane. Therefore, the trisilane can contribute compa- rably to the deposition rate although its concentration is much less.

The conditions under which monosilane can dominate the deposition rate via a direct decomposition on the surface have not been clarified yet.

Possibly, this might occur only under conditions of high dilution, but more investigation into this problem is necessary.

In order to avoid the polymerization, eq. (ll), and the resulting homogeneous nucleation the concentration of disilane and trisilane must not increase above a certain critical value. Thus, the control of the high deposition rate of good quality films requires balancing the rate of the formation, eq. (3), and decomposition, eq. (10), of the di- and trisilane.

The values of the reactive sticking coefficients of di- and trisilane, which determine their decomposition rate at the surface, (eq.lO), depend on the conditions of the surface of the growing film.

With decreasing surface coverage by chemisorbed hydrogen the values of the reactive sticking coefficients increase [52,54]. Thus, the rate determining step of the growth (under conditions of a sufficient sup- ply of di- and trisilane) is the dehydrogenation of the surface of the growing film.

Let us now turn our attention towards the plasma induced CVD process in this system. During the last four to five years several wor- kers have promoted the idea that the deposition of a-Si:H occurs pre- dominantly via the SiH, radical (e.g. [57-621). A critical reading of these papers, however, reveals that there is no fair evidence for the validity of this hypothesis. Moreover, the data of Robertson and Gal- lagher on the supposed concentration of the SiH3 radical [57] together with the data of Perrin and Broekhuizen [62] and Knights, et a1 [63- 661 on the phenomenological value of the sticking coefficient of the non-identified species show, that the maximum possible contribution of SiHs to the deposition rate is about 0.02 Angstrijms/sec., i.e. quite

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negligible as compared with the generally observed deposition rates of 210 Angstroms/sec [22].

These questions have been discussed in our recent papers [12,21- 241 to which we refer for further details. The reader will find there also much experimental evidence which supports the view that the mechanism of plasma induced CVD of a-Si:H from silane is very similar to that discussed above for thermal CVD. Here, I shall summarize only the most crucial facts.

The first step of the plasma induced decomposition of silane is the electron impact induced fragmentation into silene and hydrogen, eq. (3) [21,22]. Let us recall that the largest part of the energy lost by electrons in the interactions with heavy species is dissipated into the vibrational excitation of SiH4 and momentum transfer. Thus, the first step, eq. (3) occurs probably via vibrational excitation

[23,24], but details are not known yet.

The most crucial data in suport of the mechanism via the di- and trisilane are shown in Figs. 5 and 6. [24,68]. Figure 5 shows the dependence of the silane (Fig. 5a), disilane and trisilane (Fig. 5b) concentrations and of the deposition rate (Fig. c) on the dwell time in a weak discharge. The broken lines show the calculated data from the mechanism based on reactions (3),(9) and (10) with only one nume- rical fitting parameter, the reactive sticking probability for the decomposition of di- and trisilane. The excellent fit was obtained with one constant value of the reactive sticking coefficient for disilane of about 5.10-=, which is in reasonable agreement with the data of Ga- tes [52,53] and Buss et a1 [54] (see [24] for further details).

Figure 6 shows the proportionality of the a-Si:H deposition rate with the concentration of disilane for a weak (Fig. 5) and a somewhat more intense discharge. One notices the perfect correlation between the deposition rate and the concentration of disilane which strongly supports the suggested mechanism.

The excellent correlation is encouraging, but it is not necessari- ly the exclusive evidence. A mechanism of a composite reaction can not be proved unambiguously, it can be only disproved. Thus, the 'SiH3"

story has been disproved by these data and the suggested mechanism of the deposition of a-Si:H via di- and trisilane should be considered as

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a working hypothesis which can quantitatively and in a selfconsistent way explain the available experimental data. Moreover, this mechanism opens the way for novel future studies into the details of the mechanism. The understanding of the rate controlling steps also enables one to increase the deposition rate of a good quality films (see [69] for further details).

1

ion

(10-"1 4

Fig.5. Dependence of silane concentration (Fig. a), of the di- and trisilane concentrations (Fig.b) and of the deposition rate (Fig.c) on the dwell time of feeded silane in the discharge with a dicharge current density of 0.06 mA/cmz. The points and full curves are the measured data, the broken lines are the results of the theoretical calcula- tions based on the mechanism discussed in the text (From [24,68]).

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Fig.6. Proportionality of the measured a-Si:H deposition rate and the concentration of disilane for the small (0.06 mA/cm2, see Fig. 5) and a higher (7.5 mA/cmZ) discharge current density (From [24]).

ACKNOWLEDGEMENT

I should like to thank my wife Maritza who has been patiently cleaning up and packing my office and apparatuses thus preparing for the move to Munich whereas I have been, under the stress of a bad con- science, writing this paper.

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