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A Simple Ion Flux Estimation in a Low Pressure R.F.
Plasma (13.56 MHz)
I. Grenier, V. Massereau, A. Celerier, J. Machet
To cite this version:
I. Grenier, V. Massereau, A. Celerier, J. Machet. A Simple Ion Flux Estimation in a Low Pres- sure R.F. Plasma (13.56 MHz). Journal de Physique III, EDP Sciences, 1997, 7 (4), pp.937-950.
�10.1051/jp3:1997166�. �jpa-00249624�
A Simple Ion Flux Estimation in a Low Pressure R.F. Plasma
(13.56 MHz)
1. Grenier (*), V. Massereau (*), A. Celerier and J. Machet
L.M.C.T.S (**), Facult4 des Sciences, Umversit4 de Limoges, 123 avenue Albert Thomas, 87060 Cedex, France
(Received 9 April 1996, revised 25 September1996, accepted 10 January 1997)
PACS.52.70.Gw Radio-frequency and microwave measurements
PACS 52 80 Vp Discharge in vacuum
Abstract. A new application of the sputtering rate measurement is given in this paper In
fact, by measuring the sputtering rate of different materials fixed on the radio frequency (r.f.)
biased electrode, it is possible to determine easily ion flux that falls onto this biased electrode This study is realized in a low pressure (0.4 Pa) argon planar r.f. discharge system (13.56 MHz).
This sputtering method is interesting to have informations about the deposition process in
physical vapour deposition. In order to demonstrate the validity of this method, experiments have been carried out in two reactors, each one with different geometrical parameters and the
results obtained have been compared and confirmed using the Child-Langmuir law. The ion flux
increases as a function of the incident r-f- power (0-300 W). The values obtained range from
10~~ to 10~9
ions m~~ s~~. These results in
an argon plasma are applied to estimate incident
ion flux in a nitrogen atmosphere Finally, we show that it is possible to evaluate the incident
ion flux by measuring the sputtering rate when the plasma is densified using either an auxiliary hot cathode discharge or an additional magnetic field These experimental cases correspond r~spectively to r f. triode ion plating or r-f magnetron sputtering.
1. Introduction
In physical vapour deposition ii-e., in sputtering, in ion plating. .), energetic ion bombardment of substrates or targets is very important [Ii. These ions are created by an auxiliary ion source
or by an electrical discharge id-c-, r-f- or microwave discharges are used). In fact, resputtering during deposition, presumably due to ion bombardment has an important influence on film
properties. It results in a well recognized improvement in film quality (improvement of the
layer adhesion, decrease of the film porosity. .). As a consequence, energetic ion bombardment of growing films is usually encouraged in vacuum deposition. Thus, it is necessary to control
this parameter. If these ions are created in d c. discharges, it is generally easy to determine ion fluxes that fall onto the cathode directly measuring the electric dc current flowing to the
electrode. But if they are created in r-f- plasmas, it is more difficult. The aim of this paper is to progress in the characterization of the deposition process in Physical Vapour Deposition (P V-D.) applications.
(*) Author for correspondence (**)URA 320
@ Les (ditions de Physique 1997
In fact, we will present a new and easy method to measure the ion flux that falls onto the radio-frequency jr-f- biased cathode in low-pressure argon planar r.f. discharge system (13.56 MHz). It consists of measuring the sputtering rate of different coatings fixed on the r.f. biased electrode. The use of this method is easy comparatively to the realization of
an electrostatic analyzer to measure the ion current. However, it is only available in low pressure rare gases plasmas (< I Pa). The sputtering method heavily relies on the knowledge
of sputtering yield.
Furthermore, the incident ion flux determination will be carried out in two different reactors
(described in the following paragraph).
The study is realized in an r-f- discharge created in an argon atmosphere at a total pressure equal to 0.4 Pa and an incident r-f- power ranged from 0 to 300 W.
Moreover~ in physical vapour deposition, it is interesting to create nitride layers like chromium
nitride, boron nitride... So, we will evaluate the incident ion fluxes in a nitrogen plasma, and compare these values with those obtained in an argon atmosphere.
In P-V-D- techniques, it is often interesting to have important ion fluxes; that is why, plasma densification setups are usually used. We will show that it is possible to evaluate the incident ion flux when the plasma is densified.
A description of the experimental setups and the theory for the incident ion flux determina- tion by measuring the sputtering rate will be presented in the first part.
Then, the experimental results obtained in an argon plasma will be given. In the two reactors, these results will be compared and discussed. Finally, two applications of this ion flux determination will be given:
in a nitrogen atmosphere, using two densification setups.
2. Experimental Setups
The first vacuum reactor [3] (called reactor I) enables the realization of insulating coatings both on insulating or conducting substrates by ion plating. Metal is evaporated using an
electron beam gun in the presence of an electrical discharge created in an argon or a nitrogen atmosphere.
The second vacuum reactor [4] (called reactor 2) permits the realization of metallic coatings either on insulating substrates or on conducting substrates using a d.c. sputtering from a
metallic target
In these two vacuum reactors, the same residual total pressure (10~~ Pa) is obtained using
two diffusion oil pumps The pressure m the chamber is controlled by a MKS Baratron gauge during the deposition process.
The two deposition reactors characteristics are summarized m Table I and a schematical
representation is given m Figure I.
As can be seen in Table I, both the reactors have a cylindrical geometry but they present:
on the one hand, different distances between the biased electrode and the evaporation
source,
and on the other hand, different cathode diameters.
Moreover, the cathode bearing the substrates is biased by a r.f. generator. The two reactors show a dissymmetry between the r.f. biased electrode (cathode) and the grounded electrode
(anode). In this case, we have a capacitive r.f. discharge which is dominated by d-c- self bias, ii, of the r-f- electrode. This d-c- self bias appears because a capacity is located between the r-f- generator and the cathode. Thus, when the stationary state is reached, the number
Table I. Characteristics of the two uac1lilm chambers.
Reactor I Reactor 2
cylindrical chamber diameter 450 770
D (mm)
distance evaporation 170 120
source-substrate h (mm)
diameter of the cathode
bearing the substrates 100 300
L (mm)
r.f. generator and matching
network
work frequency (MHz) 13.56 13.56
maximum r-f- power (W) 1000 600
D
i' ~
' '
catllode
screen
staidess steel mile
h glow region
Evaporation
source or
metallic target
to pump
Fig. 1 Schematical representation of the experimental setup and experimental setup to measure
the average sheath thickness.
of electrons and ions that fall onto the cathode is equal. As a result, there appears under the biased electrode, a sheath that is also called cathodic dark space [5].
A capacitive r.f. discharge is used to enhance the bombardment of the cathode through this sheath with enough energetic ions 100 eV to 1500 eV), whereas ions with low energy (10 eV
to 30 eV) fall onto the grounded vacuum chamber.
Moreover, the d-c- self bias ((( depends on the experimental setup configuration and on the r-f- power dissipated in the discharge as indicated in Figure 2. It can be seen that for
a same incident r.f. power, the measured d-c- self biases are different in the two reactors.
>~-
~$
.~
~$
~
fl3 15 800 1i
j 4°°
. Reactor I
$
o Reactor 2
0
0 70 140 210 280 350
Incident rf power (W)
Fig 2 Absolute d c. self bias value evolution versus incident
r f power
These results can be explained by the ratio difference between the biased surface (Ai) and the
grounded surface (A2). Indeed, the ,42/Ai ratios for reactor I and for reactor 2 are respectively equal to 51 and 15. We can see that this ratio is higher in the first reactor than in the second
one. These different A2/,41 ratios will induce different electrical parameters in the reactors [5]
3. Determination of the Ion Flux that Falls Onto the r.f. Biased Electrode
The ion flux is determined by measuring the sputtering rate of different metallic coatings and the obtained results will be confirmed using the Child,Langmuir law.
3.I. INCIDENT ION FLUX DETERMINATION BY MEASURING THE SPUTTERING RATE. In
high frequency r.f. glow discharges (13.56 MHz), the argon ions cannot follow r.f. excitation because of their high mass and their low mobility. In these conditions, argon ions just see the d.c self bias ( as in a d.c discharge. So, the sputtering of the growing films due to ionic bombardment is controlled by the d.c. self bias. In low pressure discharge (0.4 Pa), positive ions, created in the glow space, cross the dark space without making inelastic collisions and
they sputter the substrates fixed onto the cathode.
Furthermore, due to the weak pressure, the probability that the sputtered atoms return on
the substrates is very low.
In these conditions, the sputtered thickness could be related to the ion flux that falls onto the r-f- biased cathode. In order to calculate this ion flux, the multicharged ions (Ar+, Ar++...)
will not be taken into account.
Thus, the sputtered mass, mp, by time unit and surface unit is given by :
mp = np x m II)
where
m is the mass of a sputtered atom,
np the number of sputtered particles by time unit and surface unit.
We can also write:
mp = p x up (2)
where p is the sputtered material density and up the sputtering rate.
From these two relations ii) and (2), we obtain:
pxup mp=~.
m
If we call the sputtering yield of the considered material, S, and if we suppose that all the ions have the same energy, we can write:
np=Sxn+
where n+ is the number of ions that fall onto a surface unit during a time unit.
Finally, we obtain:
~+
=
P X "P
m x S
The values p, m and S are already known, so by measuring the sputtering rate, up, it is possible
to determine n+- However, S depends on the ion energy given by the dc self bias voltage.
3.2. ION FLUX DETERMINATION USING THE CHILD-LANGMUIR LAW. The ion current
injected into the sheath from the glow region is given by the simplest form of the model proposed by Koenig [6] who used the Child-Langmuir law to relate the average sheath thickness d to the d-c- self biased potential across the sheath. Indeed, the total ion flux through a r f.
sheath can be approximated by the flux through a d.c. sheath [7]. Finally, the incident ion flux is given by the following expression [2]:
~ ~ l/2 j_~ ~3/2
+ a
~ 9~~ qilli d~ '
where
eo is the vacuum permitivity,
q the electron absolute charge,
mi the ion mass,
Va the cathode d.c. self bias voltage,
d the average sheath thickness (the measure of the sheath thickness is described in Sect. 4).
4. Experimental Results and Discussion
In order to determine the ion flux from the Child-Langmmr law, it is necessary to measure the average sheath thickness Furthermore, in both calculation methods, we suppose that the ions that fall onto the r-f- biased electrode have the same energy So it is important to evaluate the ion energy dispersion
4 1. SHEATH THICKNESS AND INCIDENT R-F- POWER. The sheath thickness measurement
is done experimentally only looking at the different zones m the discharge through a glass window. In order to measure this thickness, a stainless steel rule that has been previously graduated in millimeters, is fixed on the grounded screen surrounding the cathode (Fig, I).
In fact, we do not obtain an absolute sheath thickness value. The sheath is in continuous
movement. It is zero when electrons reach the cathode and the thickness has a maximum value
30
~~
]~
# ~~
~
l~ 18
~
g 14 . Reactor I
~f O Reactor2
10
0 70 140 210 280 350
Incident rf power (W)
Fig. 3. Average sheath thickness evolution as a function of incident r.f. power.
when the instantaneous bias voltage reaches its minimum In fact, only an average thickness is measured due to the high frequency (13.56 MHz).
As can be seen in Figure 3, by increasing the incident r.f. power (0-300 W), the r.f. sheath increases. The r.f, sheath, in these conditions, ranges from 13 to 23 mni. These values are comparable with those given in the literature [5].
4.2. ION ENERGY AND ION ENERGY DISPERSION. In low pressure r-f- discharges, the
sheath thickness, between 10 and 25 mm following the experimental conditions (pressure, r-f- power. ), stays lower than the mean free path of charge exhange for an argon atom. According
to Hasted [8], this mean free path of charge exhange between an argon ion and an argon atom
is equal to 40 mm. In these conditions, we can presume that the energy of the ions that fall onto the cathode is practically equal to the acquired energy in the sheath voltage. Moreover,
the calculation of the ion energy dispersion has been obtained from the theoretical model given by Coburn and Kay [9]. For example, for an r-f- bias amplitude and a plasma potential,
respectively equal to -500 eV and 19 V and an average sheath thickness equal to 16.5 mm, the ion energy ranges from 450 to 550 eV Thus, the ion energy dispersion is weak. In conclusion,
the dc self bias lion energy) and the sputtering yield are known in our case.
4.3. INCIDENT R-F- POWER INFLUENCE ON THE ION FLux. In order to estimate the
ion flux by measuring the sputtering rate, different titanium layers have been sputtered. The
sputtering yield depends on the ion energy that falls onto the cathode therefore it also depends
on the r.f. power. The values for titanium under argon ion bombardment at different energies
are given by Andersen [10].
We have represented, in Figures 4 (reactor I and 5 (reactor 2), the ion flux evolution, evalu-
ated either by measuring the sputtering rate or using the Child-Langmuir law,
uersils incident r-f- power. The values obtained range between 1.5 x 10~~ ions m~~ s~~ and
4.5 x 10~~ ions m~~ s~~. The
ion flux increa8es with the r.f. power and the results obtained,
5
. n+ from sputterulg rate
O n+ from average sheath thickness
~~ ~
~
~ C£ f
O
~ 3
~
~
~ C£
~ ~ ff
<
Reactor i
0 70 140 210 280 350
llicident rf power (W)
Fig. 4. Comparison of ion fluxes determined using the Child-Langmuir law or by measuring the sputtering rate as a function of incident r-f- power (reactor 1).
either by measuring the sputtering rate or by measuring the average sheath thickness, are comparable.
4.4. ANALYSIS OF THE RESULTS AND DIscussIoN. Through our results, we can see that
in the two reactors, the average sheath thickness increases with the incident r.f. power. In
fact, the d.c. self bias ill) directly depends on the incident r-f- power. When the d,c. negative self bias increases, the electrons are repelled more and more in the plasma. As a consequence the cathodic dark space which presents a positive space charge, becomes larger. Furthermore, for the same r-f- power, the average sheath thickness is higher in the first reactor than in the
second one because the d-c- self bias Va is greater.
As far as ions are concerned, Figures 4 and 5 show that the ion flux regularly increases with the r-f- power in reactor I whereas it reaches a limiting value, around 2.6 x 10~~ ion8 m~~ s~~,
in reactor 2. So, we can advance the following hypothesis: when the r-f- power increases in
the second reactor, the plasma expands, so the dissipated power per volume unit is always the
same. The r.f. power increase leads only to a slight plasma densification-
In fact, the free volume between the two electrodes is more important in reactor 2
(56 x 10~~ m~) than in reactor 1 (27 x 10~~ m~). In these conditions, the plasma densifi-
cation and the ion fluxes are higher in reactor I than in reactor 2 because the plasma expands less in the first reactor.
For each reactor, the estimated ion fluxes are about 10~~ ions m~~ s~~. We see that the values obtained by measuring the sputtering rate are slightly higher than those determined using the Child-Langmuir law. However, taking into account both the error on the measurements of the average sheath thickness and of the sputtering rate, we can conclude that the two methods give identical results. These values are comparable with those given in the literature. For example,
Catherine [III with an electrostatic analyser obtained ionic flux ranging between 10~~ and 10~9 ions m~~ s~~ following the experimental conditions
4
fl~s
fi
I 3 .9
~O CO
li~
.9C ~
g Reactor2
~
. n+ from sputtering rate
O n+ nom average sheath thickness
0 70 140 210 280 350
Incident rf power (W)
Fig. 5 Comparison of ion fluxes determined using the Child-Langmuir law or by measuring the
sputtering as a function of incident r-f- power (reactor 2)
Table II. Compar~son between argon ion jlilxes that fall onto different matertals (reactor 1).
Sputtered materials Titanium Silicon
Sputtering yield at 600 eV [12] 0.58 0.53 Ion flux (10~~ ions m~~ s~~) 1.6 ~ 0.5 IA + 0.4
This study has been carried out merely for titanium layers. Howe(~er, the sputtering yield plays an important part in the relation giving the ion flux by measuring the sputtering rate
(Sect. 3.I). For a same argon ion energy, this sputtering yield depends on the sputtered material. That is why, in the two reactors, we have compared the ion fluxes that fall onto conductive or insulating materials.
In reactor I, using the relation given in Section 3.I, we have compared the ion flux that falls onto titanium and silicon samples, fixed on the r.f. biased electrode and sputtered simulta-
neously. The results obtained, for an argon bombardment at 600 eV [12], are summarized in Table II.
Table III. Comparison between argon ion jlilxes that fall onto different materials (reactor 2).
Sputtered materials Copper Aluminium Chromium Silicon
Sputtering yield at 500 eV 2.06 1.04 1.15 0.46
[10,12]
Ion flux (10~~ ions m~~ s~~) 2.6 + 0.2 2.7 + 0.3 2.2 + 0.4 2.3 + 0.3
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