Reactive ion beam etching of silicon with a new plasma
ion source operated with CF4 : SiO2 over Si selectivity
and Si surface modification
C. Lejeune, J.P. Grandchamp, J.P. Gilles, E. Collard, P. Scheiblin
To cite this version:
C. Lejeune, J.P. Grandchamp, J.P. Gilles, E. Collard, P. Scheiblin. Reactive ion beam etching of silicon with a new plasma ion source operated with CF4 : SiO2 over Si selectivity and Si surface modification. Revue de Physique Appliquee, 1989, 24 (3), pp.295-308.
<10.1051/rphysap:01989002403029500>. <jpa-00246051>
HAL Id: jpa-00246051
https://hal.archives-ouvertes.fr/jpa-00246051
Submitted on 1 Jan 1989
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
Revue
24
Phys. Appl.
Classification
Physics Abstracts
81.60
61.80J
-
-
(1989)
295-308
MARS
1989,1
295
81.60C
Reactive ion beam etching of silicon with a new plasma ion source
operated with CF4 : SiO2 over Si selectivity and Si surface modification
C.
Lejeune,
J. P.
Grandchamp,
J. P. Gilles, E. Collard and P. Scheiblin
Institut d’Electronique Fondamentale, Université Paris XI et C.N.R.S.
91405 Orsay, Cedex, France
(Reçu
le 23
juin 1988,
révisé le 21
juillet 1988, accepté
le 26
(Unité
associée N°
22),
Bâtiment 220,
septembre 1988)
Résumé.
Nous présentons des résultats de gravure sous un faisceau d’ions réactifs délivré par un nouveau
de
canon
à ions
la Source d’Ions Reflex Electrostatique Maxi-SIRE - alimenté en gaz
type
Ils
la
et le silicium monocristallin et démontrent que les conditions d’irradiation
concernent
silice
CF4 pur.
peuvent être optimisées de façon à définir un procédé de gravure à la fois très sélectif et anisotrope de la silice
vis-à-vis du silicium et qui n’entraîne pas d’altérations irrémédiables du silicium sous-jacent ; les facteurs de
qualité en font une alternative très valable aux procédés actuels sous plasmas CHF3 ou CF4/H2, pour lesquels
les dommages induits par l’hydrogène sont bien connus. Pour le procédé proposé, avec des ions de 500 eV sous
incidence normale, les faits essentiels sont : i) une sélectivité SiO2/Si de 19 est obtenue pour l’opération de la
décharge de source au voisinage de sa pression minimale de fonctionnement, ce qui entraîne une très forte
fragmentation des neutres injectés ; ii) les vitesses de gravure associées à cette sélectivité sont respectivement
de 130 nm/min et 7 nm/min pour SiO2 et Si, résultats normalisés à une densité de courant d’ions de
1 mA cm-2 ; iii) la couche de blocage fluorocarbonée qui se forme sur le silicium et assure l’atténuation de son
attaque, peut être enlevée par un simple bain de 60 s dans l’acide fluorhydrique concentré (50 %) ; iv) ce
traitement laisse un silicium propre dont les qualités électriques ne sont que faiblement altérées vis-à-vis de
celles d’un échantillon témoin ; la procédure standard de guérison des dommages, c’est-à-dire un traitement en
plasma oxygène suivi d’un recuit lent sous azote, semble donc pouvoir dans ces conditions conduire à de très
bons résultats. Des informations concernant les cinétiques et les mécanismes de croissance de la couche de
résidu et d’évolution des dégâts superficiels du silicium ont été obtenues grace à des mesures ellipsométriques,
des mesures de caractéristiques électriques de contacts métal-silicium et des spectres d’analyse Auger (en
surface et en profondeur). Les résultats sont rapportés et discutés en mettant en avant les effets associés à la
dose d’irradiation par les ions et à la pression de fonctionnement du canon à ions.
2014
2014
Reactive Ion Beam Etching is obtained from a new specific ion gun, the Electrostatic Reflex Ion
Abstract.
Source (Maxi-ERIS), which is operated with pure CF4 gas. The reported results concern both silicon dioxide
and single-crystal silicon. They show that the operation of the source discharge down to its minimum pressure
which implies an extensive fragmentation of the injected neutrals, provides a very convenient process for
selective etching of SiO2 over Si, a basic problem in semiconductor technology. From the characteristic
performances which are achieved, this process appears as a fair alternative solution to the standard reactive ion
etching process with CF4/H2 or CHF3 (in a plasma environment). It is known that these latter ones lead to
deep lying modifications of the Si single-crystal, which are attributed to hydrogen-induced extended defects.
For the proposed RIBE process with a 500 eV beam at normal incidence the main features are : i) selectivity
SiO2/Si : 19/1 ; ii) etch rates : 130 nm/min and 7 nm/min, respectively for SiO2 and Si, data normalized to a
1 mA cm-2 current density ; iii) the blocking carbonaceous film which is formed over the silicon and insures
the slow-down of the etch rate may be removed by a simple dip for 60 s in concentrated hydrofluoric acid
(50 %) ; iv) such a post-etching treatment without further plasma oxidation or thermal annealing - leaves
a clean Si substrate, the electrical properties of which are only slightly altered as compared to a control sample.
Informations about the kinetics and mechanisms of the formation of both the overlayer and the near-surface
damage are obtained from ellipsometry, Auger electron spectroscopy, Auger sputter profiling and metalsilicon contact electrical measurements. They are reported and discussed with a special emphasis on the effect
of both the ion exposure dose and the operation pressure of the ion gun.
2014
2014
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01989002403029500
296
1. Introduction.
Selective etching of Si02 over Si is a basic problem in
semiconductor technology. In a pure CF4 discharge
excited with RF power, Si and Si02 are etched at
very similar rates. Therefore anisotropic and selective etching of silicon dioxide over silicon is generally
achieved by the combination of an ion activated
process (Reactive Ion Etching - RIE) in a plasma
environment and the presence of hydrogen either in
the etchant molecule or in the gas mixture [1, 2].
Unfortunately, hydrogen atoms and ions have been
shown to be responsible of damage and contamination of the silicon crystal down to depths as large
as 30-50 nm [3-5]. They are very difficult to cure and
alternative processes are required. Reactive Ion
Beam Etching (RIBE) using pure halocarbon gases
may provide a solution [6-10]. However, the selectivity has to be further increased and, furthermore
the compatibility of the process with the VLSI circuit
requirements has to be investigated. Therefore the
contamination and damage of the silicon near-surface have to be analyzed first to determine their
effects and their origin and second in order to find a
solution which may restore the surface to a device-
quality
state.
In section 2, the experimental apparatus and procedures are described. A specific ion gun has been
developed for RIBE which can be operated with
fluorocarbon gases without lifetime problems and
with reduced contamination. It has a new ionization
chamber : the Electrostatic Reflex Ion Source
(ERIS) and a plasma bridge neutralizer. For the
RIBE procedure, the neutrals are injected into the
source discharge chamber. For the present ion gun
the pressure within this latter chamber may be as low
as 1 x 10-4 mbar whereas the corresponding preswithin the interaction chamber is 1 x
sure
10- 5 mbar. The composition of the ion beam is online recorded ; it will be hereafter referred to as a
« CF+x ion beam ». The gun also delivers a flow of
reactive neutrals and radicals which may affect the
ion beam-sample interactions.
In section 3, we first report the variations of the
etch rates and the resulting Si02 over Si selectivity as
functions of the gun operation pressure ; they are
discussed briefly from a comparison to the associated
variation of the ion beam composition. Then we
report results of measurements concerning the irradiation of unmasked Si single-crystal ; they were
devoted to improve the understanding of the
mechanisms which insure : 1) the growth and thickness limitation of the CF-carbonaceous overlayer ;
2) the Si etch rate slow-down, and 3) the Si nearsurface damage and contamination. Both the transient and steady states have been analyzed using the
following complementary diagnostics : 1) on-line
variation of the SiF4 partial pressure ; 2) ellipsomet-
(0394 2013 03C8) variations ; 3) Auger sputter
profiles (ASP), (100 eV Ar+ sputter beam) ; 4) electrical evaluation of Metal-Si contacts (Mercury-Silicon probe diode). Both as-etched and wet-cleaned
ric parameter
samples as obtained after a 60 s dip in HF 50 %,
have been compared. Such a simple post-etching
treatment has been chosen
instead of the more
standard oxygen plasma treatment followed by a
in order to minimize the
concentrated HF dip
consumption of the underlying silicon substrate.
The influence of the CFx ion beam energy and
incidence has been studied, but in this paper we
report and discuss mainly the influence of the gun
operation pressure and the exposure dose effects.
-
-
2.
Expérimental set-up
and procedures.
A schematic diagram of the RIBE apparatus is
shown in figure 1.
2.1 THE ION GUN : ELECTROSTATIC REFLEX ION
A specific ion gun is
SOURCE (MAXI-ERIS).
The
source discharge is a
used, the Maxi-ERIS [11].
three-electrode structure, with a hot tantalum
cathode and a small graphite anode which are both
located within the cylindrical source chamber
(250 mm diameter). This latter is negatively biased
with respect to the hot cathode and thereby insures
the electrostatic containment (reflex effect) of the
primary ionizing electrons which have been initially
accelerated from the cathode. This gun has been
especially developed in order to satisfy the various
requirements of ion-beam assisted processes
either deposition or etching. In particular a steadystate operation may be reached for the operation
with fluorocarbon gases which generally leads to the
deposition of insulating films on the chamber walls
or electrodes [12]. Because of the small anode the
temperature of which may be far above room
temperature no deposition occurs on the anode
which conversely is slowly etched. Its material must
be chosen according to the chemically reactive
plasma environment. It has to be pointed out that
standard ion sources using a magnetic confinement
and a DC potential excitation have a very short
lifetime in relation with the extinction of the discharge [8, 13]. With fluorocarbon gases such as
CF4, the hot cathode tantalum wire has been shown
to be passivated after about one hour of operation
due to the formation of tantalum carbide. The beam
contamination is then strongly reduced and for
typical operation conditions as such reported in this
paper the cathode lifetime is about 100 h [14]. The
extraction optics is a three grid system constructed of
stainless steel as the ionization chamber. It may
deliver a 7 cm diameter beam in the energy range
0.2-2 keV at current densities of up to 1 mA cm- 2,
as measured on the sample holder located 15 cm
-
-
297
Fig. 1. - Schematic diagram of the RIBE apparatus.
discharge chamber. For a CAIBE process, gases are
For
also
downstream the optics. Graphite may also be used
as a base material for the grids, but not molybdenum
which is etched and thus leads to beam and substrate
contamination. The positive ion beam current is
electron compensated on the target in order to allow
the irradiation of insulating material (Si02 ). The
electrons are delivered by an auxiliary discharge
which has the same structure but a much smaller
chamber : the Medium-ERIS [15]. It is fed with
argon ; a plasma bridge insures the coupling between
the neutralizer discharge plasma and the ion-beam
plasma.
The interaction chamber is pumped by a liquidnitrogen-trapped diffusion pump (7001/s for Argon).
The base pressure (pi) in this chamber is 5 x
10-7 mbar and the operation pressure range is
1 x 10- 5 mbar up to 10 x 10- 5 mbar during typical
RIBE experiments, the neutrals being injected into
the source chamber. This pressure, recorded both by
a capacitance manometer and a Penning ionization
gauge, will be used as a characteristic parameter of
the RIBE process. The pressure (ps ) within the
source chamber - also recorded by a capacitance
manometer - is an order of magnitude higher. The
REVUE DE PHYSIQUE
APPLIQUÉE. - T. 24,
N’ 3, MARS 1989
a
RIBE process the neutral gas is injected into the
into the interaction chamber.
source
injected directly
ion beam composition is on-line recorded with a
magnetic-mass-spectrometer (MMS). The composition of the stable neutrals present within the
interaction chamber is recorded with a quadrupole
mass spectrometer (QMS) ; this latter is located
within an independent chamber differentially
pumped and connected to the main chamber
through a 2 mm diameter hole. Because of the DC
excitation (80-130 V) of the hot cathode discharge
linked to the efficient electrostatic containment, the
primary electrons have very large dissociation and
ionization yields. Therefore, the injected neutrals
may be extensively fragmented both as CFy radicals
and CF+x ions through stepwise processes. The
fragmentation is more extensive as the discharge
voltage and current increase, and as the injected
neutral flow decreases. The variation of this latter
implies the variation of the pressures in both the
ionization and interaction chambers. In figure 2 are
shown typical ion beam and neutral phase composition spectra for two values of the CF4 flow rate,
which correspond to the extreme values of the
pressure range of interest for RIBE with the present
ion gun. The spectra of the neutral phase (Figs. 2a,
21
298
Ion beam and neutral composition for two values of the CF4 gas flow rate,
chamber. a) and b) : neutrals and ions for the resulting highest pressure HP
interaction chamber. c) and d) : the corresponding spectra for the lowest pressure LP
current density was respectively 0.5 and 0.25 mA cm- 2 for HP and LP.
Fig. 2.
discharge
-
2c), clearly shows that heavy fluorocarbon molecules
such as C2F6, C3F6, C4F6, C3F8 and C4F8 are also
synthetized. The existence of these species may be
imputed to the presence of the large amount of
unsaturated radicals such as CF and CF2 both within
the source chamber and the ion beam chamber. A
discussion about the plasma chemistry which governs
the overall behaviour of the ion gun would be
interesting but is nevertheless out of the scope of the
present paper. It must be pointed out that these
spectra are associated to the steady-state operation
of the gun, that means conditions for which both the
thermal equilibrium and the chamber conditioning
steady state are reached. Starting from a clean vessel
the latter requires about 2 h [14]. From figure 2 it is
clearly seen how the pressure is a sensitive parameter
in order to modify and/or control the ion beam and
neutral flow composition, although this latter is
rather difficult to be determined with the present
experimental set-up.
2.2 SAMPLE EXPOSURE AND POST-ETCHING TREATThe samples to be exposed to the ion beam
MENT.
were introduced via a load-lock system. They were
stuck with a carbon paste on the water-cooled
substrate holder ; this latter is driven by a motor
system which allows the choice of both the sample
-
injected into the source
9 x 10- 5 mbar within the
1 x 10- 5 mbar. The beam
as
=
=
and orientation within the beam cross-section. A small Faraday cup is included in the sample
holder to measure the irradiation current density.
The emission or the subsequent formation of stable
neutrals associated to the beam sample exposure are
on-line recorded by the QMS - in particular the
position
SiF4 partial pressure.
Masked samples with HPR 204 resist, post-backed
at 110 °C, were used for etch rate measurements.
They were exposed for a given period of time so that
a
step of about 150-200 nm is obtained, in order
to
reduce the uncertainties associated to the profilometer measurement and to the transient variation of the
silicon etch rate (Sect. 3.2a). Thermal silicon dioxide
(600 nm) grown on silicon substrate and Si singlecrystal were used for the present results. The Si
substrates were n-type, (100)-oriented crystals with
resistivity ranging between 4.0 and 6.0 ilcm. For the
analysis of the CF+x/Si interaction kinetics and
mechanisms, unmasked samples were considered. A
standard organic cleaning was the only pre-exposure
treatment.
After the RIBE exposure, the wafers were dipped
into absolute ethanol as soon as they were taken out
of the load-lock chamber. This procedure has been
shown to reduce the sample contamination and
oxidation in particular for those having the thinnest
299
carbonaceous overlayer. They will be subsequently
called as-etched samples. It has already been shown
that 02 plasma post-etching treatments are efficient
for the removal of C, F-films grown on silicon [4,
16]. However a silicon dioxide layer is also produced,
thereby consuming the underneath silicon substrate
up to 2-3 nm depth. Thus after a HF dip, the nearsurface of the silicon substrate is also removed and it
is no longer possible to evaluate the damage and
contamination of the very interesting interfacial
layer. In order to preserve the integrity of this layer,
different post-RIBE wet procedures were evaluated
concerning their ability to remove the carbonaceous
overlayer. A 60 s dip in concentrated hydrofluoric
acid (50 %) was shown to be efficient from ellipsometric, Auger and electrical characterizations and
for standard exposure conditions, as discussed in
section 3. One may expect that such a wet procedure
did not remove the underneath silicon as far as this
latter had not suffered important oxidation. They
will be called wet-cleaned samples. They were also
maintained in absolute ethanol until they were
characterized. For samples which were left at room
air after etching, the time for the HF dip which was
required to remove the overlayer decreases ; after a
week, 30 s were effective.
2.3 SAMPLE CHARACTERIZATIONS.
A manual
Ellipsometric measurements.
Research
T436
was
used
to
determine
the
Rudolph
The
ellipsometric angles.
polarizer-compensatorsample-analyzer configuration (PCSA) was chosen.
2.3.1
-
and w were measured in all four zones at a 70°
angle of incidence and at a 546.1 nm wavelength
d
[17]..
Auger Electron Spectroscopy (AES) ; Auger
Sputter Profiling (ASP).
Auger Electron Spectrometry was performed from nonderivative spectra
collected in the EN(E) mode. They were quantitatively exploited by the use of the peak to background
ratio (Px/B), hereafter referred to as the Auger
ratio. The advantages of the method have already
2.3.2
-
recorded with this device which can be used
and instantaneously to form a diode.
3.
Expérimental
locally
results.
3.1 ETCH RATES AND Si02/Si SELECTIVITY. - The
dependence of the etch rates and Si02/Si selectivity
is shown in
versus the operation pressure
pi
rate
have
been
etch
values
normalized
The
3.
figure
to a 1 mA cm- 2 current density, as it is usual in
RIBE data in order to make easier the comparison
between various experiments. Of course this presentation of the results has only a practical meaning if
the ion gun is able to deliver a current density up to
this value and if the beam composition is not
affected by the necessarily linked modification of the
source parameters. Generally, for a low operation
pressure, the beam composition is strongly modified
as the discharge current is increased and a nonlinear
behaviour of the Si etch rate is observed, with a
deficit as compared to a linear extrapolation [10].
Our own experiments confirm this feature. Conversely the Si02 etch rate is not as much affected, so
that higher selectivity might be achieved with a
further increase of the beam current density.
For the present work the Si etch rate of
7 nmlmin/mAlcm 2 corresponds more physically to
an effective sputtering yield of 0.1 Si atoms/impinging CFx ion (500 eV ions at normal incidence). This
value is smaller than the 0.2-0.3 values reported in
the literature for beams extracted at about the same
energy from more standard source discharges operated with CF4 [6-8]. However it must be pointed out
that, for these latter experiments, the operation
pressures within the ion sources and process chambers (respectively about 10-3 and 10-4 mbar) are
higher than those of the present device. The 0.1
-
-
been discussed [18]. The electron gun delivered a
3 keV, 0.5 J..LA beam, and the CMA aperture angle
was 42.18°. For ASP, a low energy ion gun has been
developed in order to reduce the knock-on spreading
effect ; it delivered a 100 eV-50 03BCA Ar+ ion beam in
a 7 mm diameter spot.
2.3.3 Electrical evaluation : Mercury Probe-Silicon
Diode.
A mercury-silicon contacting device was
used in order to analyze the Schottky-barrier diode
as established between mercury and silicon : it is the
Mercury Probe-Silicon Diode [19]. The Hg-Si contact to be tested had a 1 mm2 area ; the samples were
stuck backside on a metal electrode with a silver
paste which insured a large area contact (100 mm2).
I-V ; C-V ; 1/C2-V and G-V characteristics were
-
3.
Si02 and Si etch rates as functions of the neutral
pressure in the interaction chamber and resulting the
Si02/Si etch selectivity (steady-state values). Data are
normalized to an ion-current density of 1 mA cm - 2. Beam
500 eV, and incidence 0
0°.
energy W+
Fig.
-
=
=
300
sputtering yield value compares to the values reported for the operation of such standard ion sources
but operated with CHF3 [16]. This different behaviour may be attributed to the extensive fragmentation of the injected neutrals and/or to the low
operation pressure which both may be achieved with
the Maxi-ERIS ion gun. The pressure variation
implies the variation of the composition of the beam,
but also the variation of both the composition and
flux of the neutrals and radicals which are emitted
out of the ionization chamber, as already mentioned.
As far as the ion beam is concerned, its composition
dependence versus pressure is shown in figure 4,
where are plotted the variations of the peak height
ratio of the main ions. The reference is CF3’ , the
most abundant ion in the usual RIE plasma environment. The high selectivity values, in the range 10-20,
are achieved for the lowest part of the pressure
variation range. The comparison between figure 2
and figure 4 shows that such high values are associated to the more extensive fragmentation of the
neutrals and to a large increase of both the
C+ and F+ monoatomic ions, the dominant ion
being then CF+ . Values reported for CFx bombardment of silicon show that the etch yield decreases as
far as the F/C ratio decreases [20]. A value of 0.1 at
500 eV was reported for the sputtering yield of
silicon by CF+ ions ; it is in agreement with the value
derived from this work. However as shown in
figure 2, the ion spectrum is so rich that this comparison has only a qualitative meaning.
of the
CFx
ion beam
composition
figure 3.
The source discharge voltage is 120 V. The discharge
current is adjusted (about 2 A) in order to provide a
0.5 mA cm-2). Yet
constant beam current density (j+
for the lowest pressure value j + decreases to
0.25 mA cm- 2 because of the pressure limitation of the
discharge current.
Fig.
4.
versus
-
Dependence
pressure
pi to be associated to the data in
=
What about the contribution of the neutrals and
radicals in the present RIBE process ? As already
known, the slow-down of the Si etch rate can be
attributed to the formation of a C, F-blocking
overlayer [2, 5]. This latter can develop in RIBE
processes with CF4, as will be discussed later on, but
not in the more classical Reactive Ion Etching (RIE)
with pure CF4 in a plasma environment because of
the role played by the dense population of reactive
neutrals. In typical RIE conditions the neutral to ion
flow density ratio range is 10- 2-10- 3,the ion flow
density having about the same value as in the present
RIBE experiment (0.25-0.5 mA cm- 2). On the other
hand, for this latter this ratio has a much smaller
value. If we consider
as a first estimation
only
the stable neutrals which are thermalized within the
interaction chamber (as they are recorded by the
QMS), the following values are obtained :
-
Low pressure
operation :
High
operation :
pressure
-
Two comments must be added. Firstly, as shown
in figure 2a-c, a large variety of fluorocarbon molecules are present in the interaction chamber and
their respective sticking coefficient and further on
influence may be quite different. Secondly, radicals
and heavy molecules are ejected from the ion source
and impinge straight on the sample. They contribute
to an increase of the above values of the ratio. A
more accurate estimation would require the analysis
of the nature and flow of the neutral species which
are emitted outward the ion source. As far as
concerns the RIBE process by itself, experiments
have been performed in order to estimate the effects
of these particles :1) Similar measurements as those
reported above but done in a farther cross section of
the beam, located 37 cm downstream the grids [21] ;
2) Influence of the electrons delivered by the neutralizer discharge and 3) Exposure of the samples to
the ion source plasma when the optics bias potentials
were turned off. Data will be reported in a further
paper.
The present results show that a Si02/Si etch rate
selectivity up to 20 may be achieved for the RIBE
mode and the operation of the gun at the lowest
pressure (1 x 10- 5 mbar). In a practical process
higher values might also be expected with the
301
present apparatus from the use of two modifications
to the present primitive RIBE experimental process.
First, if the operation pressure is varied du ring the
etching of the silicon dioxide film of given thickness ;
it is possible to begin at the highest value and then
lower the pressure as far as the etching proceeds
towards the silicon substrate. The mean value of the
silicon dioxide etch rate will then be higher, because
for the pressure range which is considered in figure 3,
it varies by a factor of 2. The etch time of the oxide
film will then reduced, and if the mean value of the
silicon dioxide etch rate is used to define a concept
of « effective selectivity », a higher value of this
latter might be expected from a convenient monitoring of the RIBE pressure. It must be noticed that
whatever is the pressure in this range the etching
remains anisotropic and that the operation pressure
of an ion source is easily monitored. Second, from
the direct injection of fluorocarbon gases within the
interaction chamber known as the CAIBE mode for
Chemically Aided Ion Beam Etching. Our measurements show that starting from the lowest pressure
value as reported in figure 3 (1 x 10-5 mbar) without
additional gases (RIBE procedure), the injection of
CF4 up to 10 x 10- 5 mbar, does not modify the Si
etch rate but conversely increases that of Si02 by a
factor of 1.3, and thereby involves a selectivity value
of about 25.
3.2 CARBONACEOUS OVERLAYER GROWTH
sidered, in order to demonstrate the influence of
both the ion dose and neutral pressure. For these
measurements the characteristic time of the vacuum
equipment was estimated to be about T 2 s ; it
affects mainly the signal increase which would be
very sharp in the limit of a vanishing characteristic
time. The sample introduction speed and the ion
beam density profile have also to be considered for a
more accurate quantitative analysis of these graphs.
Experiment with RCA clean silicon demonstrates
that the effect of the native oxide layer does not
affect significantly the characteristic features of the
above graphs, as they are discussed now. The
maximum Ro of each curve corresponds to the silicon
etching. Then the silicon removal rate is decreasing
as a result of the carbonaceous overlayer growth on
the Si surface. A critical dose Dl does appear which
may be attributed to the establishment of the steady
state of the overlayer thickness and structure, the
etch rate (Rs) being then constant. The resulting
attenuation factor Rs/ Ro of the etch rate is clearly
seen ; it varies from 0.45 to 0.15, for the considered
pressure variation range. The lower the attenuation
factor, the higher the dose DI, a feature that will be
further imputed to the increase of the overlayer
thickness (see Fig. 9). Here again Dl presents a
sharper increase in the lowest part of the pressure
range ; typical values are :
=
AND
CLEANING.
3.2.1 SiF4 partial pressure.
The on-line variation
of the SiF4 partial pressure (SiF’ peak height) as a
function of the Si irradiation ion dose is shown in
figure 5. Three values of the pressure were con-
On-line variation of the SiF4 partial pressure
in the QMS) as a function of the Si
irradiation dose. Parameter : the gas flow rate and the
resulting pressure p; within the interaction chamber.
W+
500 eV ; 0
0° ; Si(100) ; electron-compensated
ion beam. The given time scale is related to the high and
medium pressure (0.5 mA cm-2 ion current density).
Fig.
5.
-
(SiF’ peak height
=
=
state relative values of the etch rate
from these graphs have been comderived
(R,)
etch
rate values as measured from
to
the
pared
data.
They are proportional. From this
profilometer
result it may be assumed that when the overlayer
steady state is reached, whatever is the pressure,
similar mechanisms are implied in the removal of the
underneath silicon. In the lack of a better information, if we assume that for the transient period of
the overlayer formation the etch rate variation is
also given by the graphs in figure 5, it is possible to
obtain a crude estimation of the silicon thickness
el which is removed for an exposure dose Di: A
value of about 4 nm is obtained, almost independent
of the operation pressure. Similar graphs to those in
figure 5 are also obtained when other RIBE process
parameters are modified, such as the incidence, the
energy or the nature of the injected neutrals. An
important feature which must be emphasized is that
for the present RIBE experiments the overlayer
thickness increases with increasing the energy, in the
studied energy range (250-1 500 eV) ; see reference
[22] for the preliminary results related to the energy
influence and for the operation with CHF3.
The data shown in figure 5 imply a time dependence of silicon etch rate. Conversely, for the
The
steady
as
302
present experimental conditions there is no blocking
overlayer grown on the silicon dioxide, the etch rate
of which does not depend on the exposure time,
excepted for a slight initial increase imputed to the
variation of the surface temperature. In such cases
for which a blocking overlayer formation is required
to insure the etch rate slow-down of the underneath
substrate and thereby to provide for the film-tosubstrate selectivity, this latter selectivity concept
has to be more precisely defined and used. In
particular, the values estimated from long time
exposure, i, e. the « steady-state selectivity » values
as those reported in figure 3, are not sufficient to
determine the optimum overetch time for a given
process. The time dependence of the etch rate and
the resulting removed thickness during the initial
transient period of the silicon exposure to reactive
species have to be considered [2].
3.2.2 Ellipsometric data and post-RIBE cleaning.
In figure 6a is shown the variation of d as a function
of the ion dose for the same three pressures as in
figure 5. The initial decrease of à may be attributed
to the thickness increase of a thin overlayer grown
on the substrate. However the critical dose Dl, as
defined previously in figure 5, now corresponds
approximately to a minimum value of d. Beyond
Dl a slight increase of à is observed up to a higher
dose D2 beyond which a plateau is reached. The
value of D2 is about twice that of Di. Such a
variation of d versus the ion dose has been attributed
to the final reorganization of the near-surface of the
silicon substrate, once the overlayer thickness is
-
constant and as long as the Si interface regresses,
because two phenomena interfere for the determination of the ellipsometric parameters : the substrate
damage (crystal defects and atom incorporation) and
the overlayer growth. The first amorphization step
which requires a dose of about 2 x 1015 cm- 2 [20]
cannot be seen in the present exposure scale. The
saturation value of à depends on the properties of
both layers. Yet for low layer thicknesses, a quantitative analysis is not accurate [17]. Nevertheless the
above variations are significative of the sequential
effects of the reactive ion bombardment as will be
discussed later on. In order to investigate more
precisely for the contribution of the overlayer in the
d values, the measurements have been done on wetcleaned samples. The data are plotted in figure 6b
for the samples processed at the lowest pressure.
They show that :
i) for the lowest dose in the scale, say D DI, the
HF dip has no effect on 0394. Times higher than 60 s
have been tested without effect. The substrate is not
able to be cleaned by this wet procedure. The AES
spectrum of the wet-cleaned sample surface is shown
in figure 7a ; it demonstrates the presence of a large
amount of both carbon and fluorine, but the
SILVV peak is nevertheless seen. The AES spectrum
of the as-etched sample is about the same as that for
the wet-cleaned sample. It may be assumed that
carbon and fluorine are incorporated within the
silicon lattice, as discussed by Chuang et al. [23] ;
AES spectra
7.
peak over background EN(E)
mode - for as-etched and wet-cleaned samples processed
at the lowest pressure : a) Wet-cleaned after a small
exposure dose (1.2 x 1016 cm-2) ; b) and c) Wet-cleaned
and as-etched after exposure for the critical dose D2
1.5 1017 cm-2, as defined in figure 6.
Fig.
Fig. 6. a) Variation of the ellipsometer angle à as a
function of the Si irradiation dose. The parameters are the
same as in figure 5 ; b) Variation of .L1 for both as-etched
and wet-cleaned samples processed at the lowest pressure .
value LP.
-
-
-
=
303
ii)
for D &#x3E;
Dl,
the wet
cleaning
treatment leads
to an increase of d which is approximately constant
and equal to 17°. In figure 7b and 7c are shown the
AES spectra of wet-cleaned and as-etched samples
which both have received the same CFx ion dose
D2 (1.5 x 1017 cm- 2). The as-etched spectrum is
typical of Si with its C, F-blocking overlayer. The
oxygen peak is not intense (Po/B
0.02) and the
is
not
seen.
The
wet-cleaned
eV)
peak
SiLVV, (89
to
spectrum
corresponds
sample
typically the case of
silicon contamined in room air, during the transfer
time to the Auger diagnostic chamber ; Po/B
0.08 corresponds approximately to one Si02 monolayer. The carbon peak is very small (PCIB
0.03), and no residual fluorine signal is seen : the
substrate seems really « clean », from the AES point
of view. The d value of the etched + wet-cleaned
silicon sample for D &#x3E; D2, is approximately equal to
that of a silicon with its native oxide. Although the
ellipsometric measurements were done under a dry
nitrogen flow, a native oxide overlayer does exist.
Furthermore, as shown in reference [24], a silicon
damaged under Ar+ ion bombardment may have,
when the saturation is reached and according to the
light wavelength, the same d value as that of a
substrate cleaned in an ultra-high vacuum. Therefore
a more sensitive characterization must be done in
order to further investigate the residual near-surface
=
=
=
damage (Sect. 3.4).
Fig. 8.
Auger Sputter Profiles (peak over background
EN(E) mode), through the C, F-overlayer and the Si
-
near-surface. The parameter is the pressure the values of
which are those given in figure 3 and referred to as HP,
MP and LP ; Ar+ sputter-beam : 100 eV ; 0.13 mA cm- 2.
The given overlayer thickness values are estimated from
the C- and Si-signal variations.
3.3 OVERLAYER
(ASP).
AND
NEAR-SURFACE
ASP
IN-DEPTH
used to estimate the
thickness and the composition profile of the carbonaceous overlayer. In-depth profile scans are
shown in figure 8, for as-etched samples having
received an exposure dose higher than D2, and for
the three typical pressures already considered. Of
course the method may involve uncertainties, in
particular concerning the fluorine which desorbs
under electron impact [23, 25] and may chemically
react under the ion bombardment. The F-profiles in
figure 8 correspond to the steady state of the electron
induced desorption. A further paper will report
more details on AES, ASP measurements and the
comparison of the results with XPS data. We comment the main features of the in-depth analysis
(ASP) in relation with the present address.
As the sputtering of the residue proceeds the Sisignal grows and the CKLL signature changes beyond
a critical dose from a graphitic shape to a shape
which is suggestive of the presence of Si-C. In order
to clarify the discussion, this dose was chosen to
define a conventional C, F-film/Si interface (1). A
question now arise : do the Si-C bonds exist within
the silicon near-surface or are they induced by the
Ar+ sputter ion bombardment (100 eV ions) ? The
answer to this question is important ; first in order to
determine the appropriate post-etching treatment to
restore the silicon crystal to a device quality state [4]
and, second because Si-C bonds have been identified
from XPS data on silicon exposed to CF4/H2 RIE,
for conditions of selective etching of Si02 over Si [2,
3]. As shown in figure 7b the AES spectrum of a
wet-cleaned Si surface does not show incorporated
carbon. Therefore it may be assumed that the C, Foverlayer/silicon interface is relatively abrupt, at
least for the irradiation conditions involving the
formation of a rather thick carbonaceous overlayer
(low pressure operation). The exponential decrease
of the carbon signal in the graphs of figure 8, as
observed for the low pressure values, may be
considered as a corroboration of this feature. Such a
variation is in fact predicted when an atomic layer is
sputtered, if the removal rate of the monolayer is
assumed to be proportional to the surface coverage
as a consequence of the statistical nature of the
sputtering process [26]. Recent XPS data on Si
samples exposed to RIBE with CHF3 in the present
apparatus have been reported by Cardinaud et al.
[22]. No Si-C bonds were detected even for 1500 eV
ions in the Si2p detailed peak as obtained from a
monochromatized radiation. It may be expected that
the same behaviour is also valid for RIBE with
ANALYSIS
-
was
CF4.
The lower the RIBE processing pressure, the
higher the sputter dose required to remove the
major fraction of the C, F-overlayer. For the lowest
pressure a plateau is clearly seen in the carbon
304
profile with about the
same Auger ratio PC/B ~
the energy, incidence and gas
(CF4 or CHF3). It corresponds to the bulk of the
carbonaceous layer grown under the ion bombard-
0.75, whatever
are
ment.
Whatever is the pressure, the fluorine profile
shows two regions which correspond either to a
difference of concentration or to a difference in the
bond strength of the fluorine with the surrounding
atoms. The inner region, near the film-Si interface
may be imputed to the direct incorporation into the
carbonaceous overlayer of the fluorine atoms which
are produced by the dissociation of the impinging
fluorocarbon molecular ions such as CF+x. Conversely the steep fluorine decrease which is seen at
the topmost part of the carbonaceous film may be
imputed to the formation of a mixed layer. This
latter would be formed under the bombardment of
reactive neutrals, neutralizing electrons and energetic ions. The synergetic effects of the irradiation of
this mixed layer with the three types of particles
determine the balance between the deposition and
the removal of the carbon at the interface between
the C, F-film and the gas phase. On the other side of
the C, F-film, and beyond the interface I, the
fluorine steeply decreases within the silicon. This
feature may be partly attributed to the Ar+ activated
etching of the Si in the presence of fluorine. It has
been shown that the chemical enhancement of
sputtering is very high at low ion energy [27].
Anyhow, no more fluorine is seen in the AES
spectrum of a wet-cleaned sample which has not
been bombarded by Ar+ (Fig. 7b). We may assume
that, as well as the carbon atoms, the fluorine atoms
are not incorporated deeply into the silicon lattice.
For the rather low pressure, the Si signal shows an
exponential increase here again, as that which might
be expected from the existence of a sharp interface I.
Assuming this fact, this portion of the graphs has
been used to determine the required dose to sputter
a thickness equal to the electron escape depth
(SiLVV electrons mean free path through graphite
was taken as : A e ~ 3.8 Â). The overlayer thickness
values given in figure 8 were then derived, assuming
a constant sputter erosion through the entire overlayer. The comparison of the slopes of the SiLVV
signal increase, in figure 8, demonstrates that the
sputtering yield of the blocking overlayer depends
on the process pressure. The associated variation of
the composition of the incoming ion and neutral
flows may be supposed to affect the physico-chemical nature of the carbonaceous residue layer. XPS
measurement would give further information about
the chemical bonds and thickness of this film [22].
Whereas at the interface 1, the (PSi/B)I Auger
ratio is independent of the pressure and is equal to
0.4, the (PC/B)I Auger ratio increases as the
pressure increases and is respectively 0.2, 0.3 and
to be compared to 0.75 corresponding to
the bulk carbonaceous film. This increase of the
carbon percentage at the interface may be explained
from the decrease of the overlayer thickness which
as a consequence leads to a deeper incorporation of
the carbon atoms into the silicon lattice, associated
to a less sharp interface between the overlayer and
the silicon. These features are corroborated by the
following results dealing with the electrical evaluation of the residual contamination and damage of
the silicon.
0.4, values
3.4 NEAR-SURFACE SILICON DAMAGE : CONTACT
ELECTRICAL EVALUATION. - The current-voltage
characteristics of Hg/Si contacts of RIBE-exposed
Si are shown in figure 9 for wet-cleaned samples.
Both forward and reverse 1 V characteristics are
given for three exposure doses and compared to
those of a control sample which also has received the
wet cleaning treatment, but without beam exposure.
The dose effect on the near-surface damage is clearly
demonstrated.
* D
D2 : The I-V, C-V and G-V characteristics
of the contact are almost unchanged, in the limit of
the data uncertainty. A fair Schottky contact is
and forward characteristics for
samples for increasing
values of the exposure dose. Low pressure RIBE process ;
W+
500 eV ; 0 0° ; (n-type (100) Si ; 4-6 Hem. Mercury-Probe Silicon Diode).
Fig. 9. 2013
Hg/Si
=
I-V
reverse
contacts on wet-cleaned
=
305
obtained with a barrier height OB
0.65-0.70, and
an ideality factor n
1.2-1.4 ; the given ranges
correspond to measurement uncertainty and dispersion of the results for various samples which have
received the same overall treatment. For the control
samples : OB 0.7-0.74 and n 1.05-1.2. The barrier height values and the good quality of the
contacts were confirmed from the linearity of the
1/C2-V characteristics.
=
=
=
=
* D
D1: The contacts now appear very poor
and worse as the dose exposure decreases. They
have now a MIS tunnel diode behaviour, as that
demonstrated in figure 10c, i.e. 1) almost the same
reverse and forward I V characteristics ; 2) a capacitance plateau in the C-V characteristics and for
direct bias voltage and 3) a conductance peak in the
G-V characteristics. The value of the maximum of
this latter increases and its position is shifted towards
more negative bias voltage as the exposure dose
decreases.
The degradation of the Hg/Si contact, as discussed
above in relation with the dose variation in the lower
pressure range, was also observed when starting
from the standard exposure (low energy, typically
500 eV and low pressure : 1 x 10-5 mbar), either
the energy or the pressure were increased. The
pressure effect is demonstrated in figures 10a and
10c, where 1 V, C-V and G-V are shown for the two
extreme pressures as previously considered. The
energy effect is pointed out in figure 10b, corresponding to the exposure to a 1 000 eV CFx ion
beam and for the low pressure value. It clearly
appears from these graphs that the degradation
resulting from this energy increase is less than that
which is involved by the pressure increase. In this
latter situation, the overlayer thickness is strongly
reduced, as shown in figure 8 whereas the energy
increase implies the enlargement of the overlayer
thickness, as mentioned previously. Thus the energy
increase has not such a deleterious effect as it is
generally admitted, because of the protecting action
insured by the residue overlayer.
Very few data have been reported about the
electrical evaluation of silicon after RIBE since the
work of Gildenblat et al. [28] which emphasized the
contamination by tungsten issued from both the
source and neutralization filament. In the present
work, the plasma bridge neutralizer avoids the
immersion of a hot filament within the beam and the
use of tantalum within the ionization chamber
strongly reduced this source of metal contamination
as already mentioned. However a more sensitive
evaluation of the induced interface states will be
examined.
4. Discussion and conclusion.
10.
I V, C-V and G-V for Hg/Si contacts on wetcleaned samples having received the « optimum exposure »
dose D2, but with different RIBE process parameters of
the CF+ beam : a) low pressure (LP) and 500 eV ; b) low
pressure (LP) and 1000 eV ; c) high pressure (HP) and
500 eV.
Fig.
-
We have shown that RIBE as obtained from a
specific ion source, the Maxi-ERIS, operated with
the injection of pure CF4 gas may provide a very
selective etching of Si02 over Si. The injected
neutrals have to be extensively fragmented in order
to deliver ions with a low F/C ratio, such as
CF+ and C+ and also probably the associated
insaturated radicals such as CF2 and CF. This
situation is achieved for the lowest range of the gun
operation pressure : 10-4 mbar in the source
chamber and 10- 5 mbar in the interaction chamber.
Of course, for a well-collimated ion beam the
etching is also anisotropic and the Si02 etch rates are
very convenient. Further on, the problem of the
residue overlayer and the silicon near-surface contamination and damage has been analyzed in order
to test the compatibility of the process with the VLSI
circuit requirements. As a first approach, singlecrystal silicon samples were exposed to the CFx
beam ; pressure and irradiation dose effects were
mainly considered. The following points have to be
emphasized. They concern both the optimization of
the RIBE process and the mechanisms which deter-
306
mine the properties of the carbonaceous blocking
overlayer and of the underneath silicon.
(i) The C, F-fluorocarbon overlayer thickness
reaches a steady state beyond a critical dose
DI. Both the film thickness and Dl increase as far as
fragmentation of the neutrals is more extensive. The
ellipsometric data and the SiF4 partial pressure
variations show that the C, F-film thickness insures
the slow-down of the Si etch rate (both transient and
steady states). The greater the thickness, the
smaller the etch rate. The lower etch rate attenuation factor is 0.15 for the lowest operation pressure
of the present ion gun operated in the RIBE mode ;
it is associated to a 2.5 nm layer thickness, the steady
state of which was reached for Di
6 x 1016 cm-2.
The silicon removal still proceeds through the carbonaceous overlayer at a rate of 7 nm/min/mA/cm 2.
=
(ii) For doses D &#x3E; Dl, the overlayer may be
removed by a simple dip in concentrated HF for
60 s, leaving a « clean silicon » substrate. This result
implies that a rather sharp interface is then formed
between the carbonaceous blocking overlayer and
the silicon substrate. It may be assumed that a
competition between incorporation and removal
mechanisms insures the self-limiting growth of the
C, F-overlayer. It is known from implantation-sputtering models that the initially broad interface becomes more sharp as the steady state of the topmost
layer is reached [29]. However the silicon nearsurface was modified up to a critical dose D2, as
demonstrated both by ellipsometry and contact
electrical evaluation of as-cleaned samples. For
given beam parameters this latter critical dose also
corresponds to the least degradation of the electrical
properties of Hg/Si contacts. Considering now the
influence of the RIBE pressure, the best Hg/Si
Schottky contacts were achieved for the thickest
C, F-overlayers and corresponding optimum dose D
about or higher than D2. For D D2 and the lower
pressure range, no more fluorine neither carbon are
seen on as-cleaned samples. One may therefore
conclude that the fluorocarbon film which grows
under the CFx ion beam bombardment insures,
when the steady states of the C, F-overlayer and of
the Si near surface are reached, both : 1) The slowdown of the Si etch rate and, 2) The protection of
the Si crystal against the ion irradiation damage.
Both features are important in order to optimize a
selective Si02/Si RIBE process (source operation
parameters and overetch time).
Fig.
11.
-
Schematic anatomy of the evolution of both
the
overlayer and Si single-crystal near-surface damaged
regions as far as the Si interface is removed by etching
ion beam : a) saturation of the
mechanism under the CFx
bombardment at a dose about 2 x 1015 cm-2, for which the
overlayer has a negligible thickness. b) dose D1; the
steady state of the overlayer is reached. Conversely the
near-surface silicon region will be still modified as far as
the C, F-film/Si interface will move downstream towards
the initially created a-Si/c-Si interface. c) Dose
D2 ; the
steady state of both regions is reached. The conditions for
a minimum of the damage as a function of the ion dose are
satisfied.
upon exposure to
CF+x
amorphization-incorporation
=
=
(iii) Conversely for doses D Dl, the silicon
substrate cannot be cleaned by the previous wet
The exposure dose effects as they result from the
treatment. Carbon and fluorine are incorporated
within the silicon near-surface ; they are seen from’ graphs and discussions reported in this paper are
AES spectra. Very poor Hg/Si contacts are achieved summarized in figure 11. A schematic anatomy of
the different regions which have been distinguished
which however are improved as the dose increases
from Auger in-depth analysis
for as-etched samfrom low values up to Dl.
-
307
is proposed for conditions corresponding to
ples
three particular irradiation doses : D2 and Dl, as
defined above, and a much smaller dose of about
2 x 1015 cm- 2. This latter dose leads to the initial
saturation of the single-crystal damage before the
C, F-film has begun to develop. This damage extends
to a depth 03940a which may be estimated to be about
¿12 (Rp + 2 Qp) K, from the data reported in reference [24] ;
Rp is the projected range and ce the
longitudinal range straggling ; K is a factor the value
of which is between 1 and 2 according to the nature
and energy of the impinging ions, and the nature and
crystallinity of the solid. If one considers the low
pressure operation of the gun, the F+ and C+
monoatomic ions have an important intensity and
they enter the silicon with the full energy. These ions
will thereby determine the damage depth. For
500 eV C+ ions impinging on an amorphous silicon
target, the following values have been reported :
Rp 3 nm and ap = 1.5 nm [3]. The initial damage
depth will be about 6 x K nm, i.e. a larger value than
4 nm which is removed after an
the thickness el
exposure dose Dl.
As far as the silicon removal in the presence of the
C, F-blocking overlayer is concerned, it may be
assumed to occur by an ion-assisted mechanism [16,
27]. The energy deposition of the incoming ions is
shared between the C, F-film and the Si-substrate.
-
=
=
=
Both carbon of the overlayer and silicon are removed, the carbon removal being however compensated by its incorporation. The Si slow-down etch ’
rate is then determined by the relative values of the
blocking overlayer thickness and the ion energy
deposition profile. As long as the overlayer thickness
increases at a given energy, due to variations of
either the ion exposure dose or the beam composition, the thickness of « silicon reacted layer »
decreases and the Si etch rate is reduced ; the C, Ffilm/Si interface is sharper and the silicon damage is
minimized.
Acknowledgments.
It is
a
Pagnod,
pleasure to acknowledge D. Bouchier, P.
F. Meyer and A. Bosseboeuf for helpful
discussions on surface characterization data. We
would like to thank F. Fort and C. Mardirossian for
their expert assistance with the experimental program. This work was supported by the French
Ministry of Research through its Silicium Integrated
Circuit Program (GCIS) and by the CNRS (Research
Group GRECO 57). It is now also supported by the
Centre National d’Etudes des Télécommunications
under
Nos.
grant
(CNET)
87 3B 067 00 790 9245 CNS. The authors gratefully
acknowledge these Organizations.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
HEINECKE R. A. H., Solid State Electron. 18 (1975)
1146.
OEHRLEIN G. S. and WILLIAMS H. L., J. Appl. Phys.
62 (1987) 662.
OEHRLEIN G. S., TROMP R. M., TSANG J. C., LEE
Y. H. and PETRILLO E. J., J. Electrochem. Soc.
132 (1985) 1441.
MU X. C. and FONASH S. J., OEHRLEIN G. S.,
CHAKRAVARTI S. N., PARKS C. and KELLER J.,
J. Appl. Phys. 59 (1986) 2958.
OEHRLEIN G. S., SCILLA G. J. and SCHWU-JEN JENG,
Appl. Phys. Lett. 52 (1988) 907.
HARPER J. M. E., CUOMO J. J., LEARY P. A.,
SUMMA G. M., KAUFMAN H. R. and BRESNOCK
F. J., J. Electrochem. Soc. 128 (1981) 1077.
MAYER T. M. and BARKER R. A., J. Vac. Sci.
Technol. 21 (1982) 757.
HEATH B. A., J. Electrochem. Soc. 129 (1982) 396.
MIYAMURA M., TSUKAKUSHI O. and KOMIYA S., J.
Vac. Sci. Technol. 20 (1982) 986.
CHEN W., WALPITA L. M., SUN C. C. and CHANG
W. S. C., J. Vac. Sci. Technol. B 4 (1986) 701.
LEJEUNE C., C.R. Acad. Sci. Paris 296 (1983) 1381.
LEJEUNE C., GRANDCHAMP J. P., KESSI O. and
GILLES J. P., Vacuum 36 (1986) 851.
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
WICKER T. E. and MANTEI T. D., J. Appl. Phys. 57
(1985) 1638.
LEJEUNE C. et al. (to be published).
LEJEUNE C., GRANDCHAMP J. P., KESSI O., Vacuum
36 (1986) 857.
VITKAVAGE D. J. and MAYER T. M., J. Vac. Sci.
Technol. B 4 (1986) 1283.
AZZAM R. M. A. and BASHARA N. M., Ellipsometry
and polarized light (North-Holland, Amsterdam)
1977.
LANGERON J. P., MINEL L., VIGNES J. L., BOUQUET
S., PELLERIN F., LORANG G., AILLOUD P. and
LE HÉRICY J., Surf. Sci. 138 (1984) 610.
SEVERIN P. J. and POODT G. J., J. Electrochem. Soc.
119 (1972) 1384.
MIYAKE K., TACHI S., YAGI K. and TOKUYAMA T.,
J. Appl. Phys. 53 (1982) 3214.
GRANDCHAMP J. P., GILLES J. P. et LEJEUNE C., Le
Vide, les Couches Minces Sup. 233 (1986) 55.
CARDINAUD Ch., TURBAN G. and GROLLEAU B.,
GRANDCHAMP J. P., LEJEUNE C., SCHEIBLIN P.
and COLLARD E., E-MRS 1988, Conf. Proc.,
Strasbourg May 31-June 2, to be published in
Appl. Surf. Sci.
308
[23] CHUANG T. J., WINTERS H. F. and COBURN J. W.,
Appl. Surf. Sci. 2 (1978) 514.
[24] HOLTSLAG A. H. M., SLAGER U. C. and VAN
SILFHOUT A., Surf. Sci. 152/153 (1985) 1079.
[25] TUPPEN C. G., HECKINGBOTTOM R., GILL M.,
HESLOP C. and DAVIES G. J., Surf. Interf. Anal.
6 (1984) 267.
[26]
[27]
[28]
[29]
SEAH M. P., SANZ J. M. and HOFMANN S., Thin
Solid Films 81 (1981) 239.
OOSTRA D. J., Phd Thesis FOM Amsterdam (1987).
GILDENBLAT G., HEATH B. A. and KATZ W., J.
Appl. Phys. 54 (1983) 1855.
TODOROV S. S. and FOSSUM E. R., Appl. Phys. Lett.
52 (1988) 48.