Microelectronic Engineering 84 (2007) 638–645
www.elsevier.com/locate/mee
Etching characteristics and mechanisms of the MgO thin films
in the CF4/Ar inductively coupled plasma
A. Efremov a, J.C. Woo b, G.H. Kim b, C.I. Kim
a
b,*
Department of Electronic Devices and Materials Technology, State University of Chemistry and Technology, 7, F. Engels St., 15300 Ivanovo, Russia
b
School of Electrical and Electronic Engineering, Chung-Ang University, 221, Huksuk-Dong, Dongjak-Gu, Seoul 156-756, Republic of Korea
Received 5 September 2006; received in revised form 5 December 2006; accepted 22 December 2006
Available online 12 January 2007
Abstract
The etching characteristics and mechanisms of MgO thin films in CF4/Ar inductively coupled plasma were investigated. It was found
that the changes in gas mixing ratio as well as in gas pressure result in a non-monotonic behavior of the MgO etch rate. Plasma diagnostics by Langmuir probe indicated the noticeable sensitivity of both electron temperature and density to the variations of the processing parameters. The combination of 0-dimensional plasma model with the model of surface kinetics showed that the reason of the nonmonotonic etch rate is connected with the concurrence of physical and chemical pathways in ion-assisted chemical reaction.
2007 Elsevier B.V. All rights reserved.
Keywords: MgO; Etch rate; Dissociation; Ionization; Sputtering; Desorption; Etch mechanism
1. Introduction
Recently, the development of the microelectronic technology attracts many new materials aimed at substituting
for the conventional ones. Particularly, a great attention
is paid to find advanced dielectrics for the gate structures
in the integral field-effect transistors (FET) used in memory
devices. In this way, the MgO is one of the leading candidates to be used in GaN-based FET. Several researches
reported that MgO thin films keep the advantages of
high-k dielectrics and show a good compatibility with
GaN in thermal expansion coefficient and lattice properties
[1,2]. Secondly, it was found that the MgO can improve the
device characteristics of the Pb(Zr,Ti)O3(PZT)-based ferroelectric random-access memories (FRAMs) being used as
the buffer layer between PZT and SiO2. From several published works [3,4], it can be understood that the PZT/
MgO/SiO2 structure requires lower operating voltage, but
provides larger memory window. And thirdly, the MgO
*
Corresponding author. Tel.: +82 2 820 5334; fax: +82 2 812 9651.
E-mail address: [email protected] (C.I. Kim).
0167-9317/$ - see front matter 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.mee.2006.12.006
thin films are used in the structure of the plasma display
panels (PDP) as a functional layer increasing the device
lifetime [5]. That is why the development of anisotropic
etching process for MgO thin films is an important task
to obtain a small feature size as well as an accurate pattern
transfer.
Until now, there are only three works devoted to the
investigations of etching properties of the MgO thin films
using both fluorine and chlorine-based plasma chemistries
[6–8]. The first shows the dependences of MgO etch rate
on operating conditions for Cl2/Ar, SF6/Ar and CH4/H2/
Ar plasmas, but does not discuss the etch mechanisms as
well as the relationships between process parameters,
plasma chemistry and surface kinetics. The second and
third ones are our works where the etch mechanism of
MgO in both Cl2/Ar and BCl3/Ar plasmas was analyzed
using the combination of modeling and diagnostics tools.
In our opinion, the application of the same algorithm to
analyze the MgO etch characteristics in the fluorine-containing plasma will give a deeper understanding of the etch
mechanisms and provide the future optimization of the
MgO etching process.
A. Efremov et al. / Microelectronic Engineering 84 (2007) 638–645
In this work, we investigated the etch characteristics and
mechanism of MgO thin films in inductively coupled
plasma (ICP) system with CF4/Ar gas chemistry. The
investigation combined the analysis of both etch rate
behaviors versus the main operating parameters and the
influence of input process parameters on the plasma chemistry and surface kinetics. Plasma diagnostics were performed by Langmuir probe measurements.
2. Experimental and modeling
2.1. Experimental details
The MgO thin films with a thickness of about 200 nm
were prepared on the Si(1 0 0) substrate by using the sol–
gel method described in Refs. [7–9]. The etch rate of
MgO thin films was measured using the ellipsometry
(L116B-85B, Gaertner Scientific Corp.) for the processing
time of 1 min providing the steady-state etching conditions.
The sample size was about 2 cm2. To measure the etch rate,
we developed the line striping of the photoresist (AZ1512,
positive) with the line/gap ratio of 2 lm/2 lm. The initial
thickness of the photoresist layer was about 1.5 lm.
Experiments were carried out in planar ICP reactor [7,8]
with a working chamber made from stainless steel. The
chamber had a shape of cylinder with an inner radius (r)
of 15 cm. On the top of the chamber, the 24 mm-thick horizontal quartz window separated the working zone and the
4-turn copper coil connected to a rf (13.56 MHz) power
supply. The bottom electrode, normally used as the substrate holder, was made from the anodized Al and connected to another 13.56 MHz RF generator to control
the dc bias voltage. The axial size of the working zone (l)
was 14 cm. The experiments were performed under such
input parameters as: total gas pressure of 1–3 Pa, gas flow
rate of 20 sccm, and input ICP power of 500–900 W. The
CF4/Ar mixing ratios were set by adjusting partial pressures of the components. The etched sample was placed
on the bottom electrode; the temperature of the sample
was stabilized in the range of 30–35 C by using the
water-flow cooling system.
Plasma diagnostics was performed by Langmuir probe
(LP) measurements done with a single, cylindrical, and
rf-compensated probe (ESPION, Hiden Analytical). The
probe was installed through the chamber wall-side view
port, placed at 4 cm above the bottom electrode and centered in the radial direction. For the treatment of I–V
curves aimed at obtaining plasma parameters, we used
the software supplied by the equipment manufacturer.
2.2. Models of plasma chemistry and surface kinetics
To analyze the influence of the process parameters on gas
phase composition, we applied the 0-dimensional (global)
plasma model with a Maxwellian electron energy distribution function (EEDF) and a steady-state approximation
for the volume kinetics [10–12]. The electron temperature
639
and electron density determined by Langmuir probe measurements were used as the model input parameters. To simplify the kinetic scheme, we also took into account some wellknown facts, which were repeatedly mentioned in literature
for the CF4 ICP. First, we assumed CF4, CF3 and F to be
main neutral components of a gas phase ðnCF4 > nCF3 nF
nCFx<3 Þ where CF4 and CF3 are main sources of F atoms
[12–14]. Secondly, we assumed that, in CF4-rich plasma,
dominant positive and negative ions are CFþ
3 and F [15–17].
Volume densities of neutral species were estimated from
the system of kinetic equations
ðk 1 þ k 2 Þne nCF4 þ k 4 ne nCF3 ðk 10 þ k P ÞnF
ðk 1 þ k 3 Þne nCF4 ðk 4 ne þ k 9 þ k P ÞnCF3
ð1Þ
ð2Þ
ð1 dÞn0 T 0 ðnCF4 þ 0:5nCF3 þ 0:5nF ÞT
ð3Þ
where n is the volume-averaged density of corresponding
particles in plasma, k is the rate coefficients for the processes specified in Table 1, kP = 1/sR is the rate coefficient
of pumping loss, sR is the residence time, T is the gas temperature, and d is the Ar fraction in the CF4/Ar mixture.
The lower-case index ‘‘0’’ relates to the state when the plasma is turned off. To describe the surface loss of CF3 and F,
we assumed the Eley–Redeal recombination kinetics with
k9,10 = cD/K 2 [12,18], where c is the recombination probability for corresponding species. According to Refs. [12,19],
we used cF 0.02 and cCF3 0.05 for stainless steel. The
effective diffusion coefficients were calculated as D1 ¼
1/2
1
D1
is the free diffuf þ Din , where Df = (K/3)(8kB T/pm)
sion coefficient, and Din is the inter-diffusion coefficient given by the Chapman–Enskog equation together with
Blanc’s law [10]. The effective diffusion length K was estimated as K2 = (2.405/r)2 + (p/l)2 [10,11].
Volume densities of charged species were calculated
from the combination of quasi-neutrality condition for
bulk plasma with the kinetic equations for both negative
and positive ions. The first was written as ð1 þ bÞne nCFþ3 þ nArþ , where
b¼
nF k 3 nCF4
ne
k 7 ðnCFþ3 þ nArþ Þ
ð4Þ
follows from the F balance determined by R3 and R7. In
Eq. (4), we did not take into account the heterogeneous decay of F assuming the quasi-neutrality condition for the
chamber wall to be written as Ce C CFþ3 þ CArþ , where
C is the flux of particles. The partial fractions of CFþ
3
and Ar+ inside the total density of positive ions were estimated according the equations
þ
tCF3
k 5 nCF3 ne ¼
þ k 7 nF nCFþ3 ; k 6 nAr ne
dc
tArþ
¼
þ k 7 nF nArþ
ð5Þ
dc
where dc = 0.5rl/(rhl + lhr). Top
determine
the ion Bohm velocities
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi
t we used the expression t ¼ eT e ð1 þ bs Þ=mi ð1 þ bs cT Þ [20],
where Te is the electron temperature in eV, cT = Te/Ti, and
bs = b[exp((1 + bs)(cT 1)/2(1 + bscT))]1 is the relative
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A. Efremov et al. / Microelectronic Engineering 84 (2007) 638–645
Table 1
Reaction set for simulation of CF4/Ar plasma
Process
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
CF4 + e ! CF3 + F + e
CF4 þ e ! CFþ
3 þ F þ 2e
CF4 + e ! CF3 + F
CF3 + e ! CF2 + F + e
CF3 þ e ! CFþ
3 þ 2e
Ar + e ! Ar+ + 2e
þ
F þ nþ ðCFþ
3 ; Ar Þ ! n þ F
+
CFþ
,
Ar
!
wall
3
CF3 ! wall
F ! wall
eth, eV
Rate coefficient
5.6
15.9
3.0
3.8
9.2
15.8
–
–
–
–
1:190 1016 T e1:309 expð1:446 105 =T e Þ
1:159 1011 T e0:765 expð1:993 105 =T e Þ
2:369 108 T 0:489
expð5:876 104 =T e Þ
e
1.3 · 1010
1:4 1011 T e0:6481 expð1:133 105 =T e Þ
7.93 · 108exp(2.23 · 105/Te)
1.0 · 107
t/dc
cCF3 D CF3 =K2
cFDF/K2
Note: For R1–R6, Te is in ‘‘K’’ and rate coefficients are in ‘‘cm3/s’’. The fitting expressions for rate coefficients as functions of electron temperature are
taken from Refs. [10,17].
density of negative ions at the plasma sheath edge [20]. For
the simplicity, we assumed the temperatures of all kinds of
ions to be equal and dependent only on gas pressure following an empirical correlation from Refs. [18,21]:
Ti T + (0.5-T)/P, where T is expressed in ‘‘eV’’ and P
is in ‘‘mTorr’’. The correction factors hl and hr for the radial and axial sheath sizes are given by the low pressure diffusion theory described in Refs. [10,20].
To investigate the relationships between the composition of gas phase and surface kinetics, we used the simplified phenomenological model based on the theory of
active surface sites [22–24]. Model makes the following
assumptions: (1) Only F atoms are effective in the chemical
reaction with the etched material; (2) Only Ar+ ions are
effective in sputtering of the substrate; (3) Both CFþ
3 and
Ar+ are effective in ion-stimulated desorption of reaction
products; (4) Sputtering of the main material is possible
only when the incident ion impacts the clean surface, and
(5) The etch mechanism is not seriously affected by the fluorocarbon polymerization on the surface. The last point
means, in fact, the approximation of thin steady-state
(non-reactive) [25,26] fluorocarbon layer that does not limit
the transport of F atoms to the etched surface as well as
does not result in the sufficient energy loss for ions. When
making the last assumption, we also accounted for the fact
that, since the sticking coefficients and sputtering yields for
CFx to/from MgO are not known, the full and accurate
description of surface kinetics (i.e. with fluorocarbon layer,
as it was done in Refs. [27,28] for C4F8/Si and Ar–C2F6/Si)
is not possible. In other words, in our case we see no reasons to increase the uncertainty of the model with involving
more free variables.
Within the assumptions mentioned above, total etch rate
of the substrate (Re) may be represented as the combination of chemical etching (Rch) and physical sputtering by
the energetic ions (Rsp):
Re ¼ Rch þ Rsp ¼ ð1 hÞms0 CF þ ð1 hÞY s C
Arþ
ð6Þ
where h is the fraction of the surface covered by reaction
products, m is the stoicheometric coefficient for reaction
products (for example, m = 0.5 for MgF2), s0 is the sticking
probability of F atoms for the clean surface, CF 0.25nF
(8kBT/pmF)1/2, Ys is the sputtering yield, and CArþ hl tArþ nArþ . Neglecting thermal (spontaneous) desorption
of the etch products, the steady-state balance for h can
be written as
X
CF ms0 ð1 hÞ ¼ h
Y d;i Cþ;i
ð7Þ
which gives
ms0 CF
P
and
ms0 CF þ Y d;i Cþ;i
ms0 CF
P
Re ¼ ðms0 CF þ Ys CArþ Þ 1 ms0 CF þ Yd;i Cþ;i
h¼
ð8Þ
In Eqs. (7) and (8), Yd,i and C+,i are the partial yields of
ion-stimulated desorption and fluxes for each kind of positive ion. Similar to the results reported in Refs. [29,30], we
assumed both Ys and Yd to be a function of square root of
ion energy:
1=2
Y s ¼ AðE1=2 E0;s Þ;
1=2
Y d ¼ BðE1=2 E0;d Þ
ð9Þ
where A and B depend on type of ion and sputtered material, E 0.5Te + eUf + eUdc is the incident ion energy (the
sum of ion acceleration energy in the plasma sheath and the
negative dc bias voltage Udc, applied to the substrate) while
E0,s and E0,d are the threshold energies for sputtering and
ion-stimulated desorption.
3. Results and discussion
It is well known that, for any plasma etching process,
the etch mechanism is influenced not only by the processing
parameters, but also depends strongly on the types of both
chemically active species and surface atoms determining
the volatility of reaction products. That is why, before analyzing the MgO etch results, let’s account for some peculiarities of this system pre-determining the etch
mechanism. From Refs. [6,31], it can be seen that the magnesium fluorides are very low volatile compounds (the
melting and boiling point for MgF2 are 1263 C and
2227 C, respectively), so that the thermal desorption of
etch products, and thus, the spontaneous etching can be
neglected. In this situation, the MgO etch mechanism in
A. Efremov et al. / Microelectronic Engineering 84 (2007) 638–645
the CF4-containing plasma can be assumed as a purely ionassisted process where the role of ion bombardment
includes three main effects: (1) sputtering of the main material; (2) sputtering (ion-stimulated desorption) of the etch
products to provide the access for the F atoms to the
etched surface; and (3) destruction of the Mg–O bonds to
provide the chemical interaction of F atoms with Mg.
The last pathway, however, is not expected to be the limiting stage of the whole process even in the CF4-rich plasma.
This conclusion follows from the fact that the Mg–F bond
is stronger than the Mg–O one (461.9 kJ/mol and
363.2 kJ/mol, respectively [31]), so that direct interaction
between MgO and F atoms has no threshold and seems to
be possible even at room temperature.
Figs. 1 and 2 illustrate the influence of the CF4/Ar mixing ratio and operating parameters on the MgO etch rate.
It was found that an increase of an input rf power as well as
of dc bias voltage within the ranges of 500–900 W and 50
to 250 V, respectively, causes a near-to-linear increase of
MgO etch rate both in Ar-rich and CF4-rich plasmas.
These facts may be confidently attributed to the wellknown relations between input power and volume densities
of active species both in CF4 and Ar plasma as well as
between the dc bias and ion bombardment energy. Similar
results concerning the influence of input power and dc bias
on the MgO etch rate were obtained by Baik et al. [6] for
both SF6/Ar and Cl2/Ar plasmas. The change in CF4/Ar
mixing ratio at constant gas pressure (1–3 Pa) and input
power (500–900 W) results in a non-monotonic behavior
of the MgO etch rate, as it shown in Fig. 2. Similar effect
has been found in our previous work for the etching of
MgO in the Cl2/Ar ICP [7]. The position of the maximum
on the X-axis seems to be low sensitive to variations of the
input power or the dc bias voltage, but depends clearly on
gas pressure. Particularly, as the gas pressure increases
Fig. 1. MgO etch rate as a function of input power (1) and negative DC
bias voltage (2): (1) 50% CF4/50% Ar, 2 Pa, 150 V; (2) 700 W, 2 Pa,
50% CF4/50% Ar. The solid lines are to guide the eye only.
641
Fig. 2. MgO etch rate as a function of CF4/Ar mixing ratio (1) and gas
pressure (2): (1) 700 W, 2 Pa, 150 V; (2) 700 W, 50% CF4/50% Ar,
150 V. The solid lines are to guide the eye only.
from 1–3 Pa, the etch rate maximum appears more clear
and is shifted from 40% to 60% CF4 with a near-to-constant ratios to the etch rates in pure gases. It was found also
that the influence of gas pressure on the MgO etch rate is
different for the CF4-rich and Ar-rich plasmas. When the
CF4 mixing ratio exceeds 40% and up to pure CF4 plasma,
a change in gas pressure results in clear maximum of the
etch rate, as it shown in Fig. 2 for 50% CF4/50% Ar. However, as the mixture composition shifts toward pure Ar, the
maximum appears to be less clear and, finally, disappears
at 10% CF4/90% Ar. Accordingly, when the MgO is etched
in pure Ar plasma, an increase in gas pressure is escorted
by decreasing etch rate as it generally expected from
decreasing both ion flux and energy.
From the data of Figs. 1 and 2, it can be seen that the
effects of both rf power and dc bias voltage on the MgO etch
rates do not make a surprise, show a good agreement with
published data (not only for MgO, but also for numerous
material forming the low volatile etch products) and can
be clearly associated with expected changes of plasma
parameters in the framework of the ion-assisted etch mechanism. At the same time, the non-monotonic effects of both
gas pressure and CF4/Ar mixing ratio can be caused by several reasons, for example, by the passivation of the surface
by low volatile etch products and/or by the fluorocarbon
polymer layer [25]. Therefore, the situation is not quite clear
and requires the simultaneous analysis of plasma chemistry
and surface kinetics. Assuming both non-monotonic effects
shown in Fig. 2 to be caused by the similar mechanisms,
for further investigation we selected gas mixing ratio as the
main parameter. Another reason for such choice is that,
for the CF4/Ar plasma, the change in gas mixing ratio
reflects the transition between chemical and physical etch
pathways and thus, provides better illustration for the etch
mechanism peculiarities.
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A. Efremov et al. / Microelectronic Engineering 84 (2007) 638–645
Fig. 3 represents electron temperature (Te) and electron
density (ne) given by Langmuir probe measurements. It can
be seen that, as the CF4 content in CF4/Ar plasma
increases, Te also increases from 2.7–3.9 eV while ne shows
a decrease in the range of 2.6 · 1011–7.5 · 1010 cm3. Relatively weak changes of Te found by experiments are caused
by the quite close ionization potentials for CF4 (15.9 eV)
and Ar (15.8 eV). Accordingly, when the gas mixing ratio
is varied, the deformation of EEDF appears to be insignificant. Similar conclusion was reported by Choi et al. [32]
based on the experimental investigations of the EEDF
for CF4/Ar ICP. Also, in our opinion, the behavior of Te
confirms the applicability of the assumption nCF4 >
n CFx<4 used in for plasma modeling (see Section 2.2). The
reason is that CFx<4 radicals have ionization potentials
of about 10 eV [17], which is noticeably lower compared
with those for both CF4 and Ar. That is why, we expect
that a predominance of CFx will result in an opposite
response for Te with the change of gas mixing ratio. From
the behavior of Te, we can also conclude that volume density of F atoms is not too high and, at least, the rule
nCF4 > nF works. The ionization potential for F atoms is
noticeably higher than one for Ar (17.42 eV and 15.8 eV,
respectively), so that high volume density of F atoms can
provide more significant deviations of Te than it was shown
in Fig. 3. As for the behavior of ne, the decreasing tendency
of this parameter looks reasonable [12] and could be associated with three main factors, such as a near-to-constant
total ionization frequency, increasing electron diffusion
coefficient (due to increasing Te and plasma electronegativity) and increasing rate of dissociative attachment.
Fig. 4 illustrates the influence of the gas mixing ratio on
the volume densities of neutral and charged particles in
CF4/Ar plasma. As the CF4 mixing ratio increases, an
increase in the rate coefficients for R1–R4 is overcompensated by decreasing ne, so that an effective formation rate
for F atoms given by the RHS of Eq. (1) changes almost
linearly. At the same time, the recombination rate coefficient
for fluorine atoms decreases monotonically in the range
of 181–57 s1 due to the corresponding behavior of the
effective diffusion coefficient. The last decreases more than
twice (4.8 · 104-1.5 · 104 cm2/s) because of the sufficient
differences in masses and sizes for the dominant species
in the Ar-rich and CF4-rich plasmas. As a result, the F
atom density follows the general behavior of the CF4 dissociation rate, but increases faster in the CF4-rich plasma.
Olthoff and Wang [15] have reported similar results on
the CF4 dissociation kinetics in the CF4/Ar ICP. The modeling results show also that, as the gas composition is
shifted from pure Ar to pure CF4, the total density of positive ions nCFþ3 þ nArþ follows the behavior of ne and
decreases monotonically from 2.5 · 1011–1.06 · 1011 cm3.
Correspondingly, the density of F is lower than ne, so
that, even in pure CF4 plasma, the relative density of negative ions b ¼ nF =ne does not exceed 0.5. This fact is in
good agreement with repeatedly reported conclusions that,
at low pressures, electronegative plasmas become low electronegative [10,11].
The results presented above show that a non-monotonic
behavior of MgO etch rate cannot be directly caused by the
variations in the gas phase composition. This conclusion is
also confirmed by the data on fluxes of active species shown
in Fig. 5. Therefore, considering the opposite tendencies for
CF and C+ with a change in gas mixing ratio, we assume the
reason of the etch rate maximum must be linked to the surface kinetics through the ‘‘superposition’’ of chemical and
physical etch pathways. However, before applying the model
of surface kinetics to analyze the situation in detail, we would
like to mention about the numerical values of the kinetic
coefficients involved in Eqs. (6)–(9). From Refs. [33,34], it
can be understood that, for MgO, Es,0 = 50–60 eV while
the sputtering yield by Ar+ ions is 0.36 atom/ion for
E = 600 eV. Accordingly, this gives the coefficient A in Eq.
(9) 0.02 and Ys 0.10–0.23 for E = 100–300 eV. For pure
Ar plasma h = 0, so that the sputtering rate Y s CArþ for
Udc = 150 V was found to be 1.42 · 1015 cm2 s1.
Fig. 3. Electron temperature (1) and electron density (2) as functions of
CF4/Ar mixing ratio at 700 W, 2 Pa. The solid lines are to guide the eye
only.
Fig. 4. Model-predicted densities of active species at 700 W, 2 Pa: (1) – F
+
atoms; (2) – total density of positive ions; (3) – F; (4) – CFþ
3 ; (5) – Ar .
A. Efremov et al. / Microelectronic Engineering 84 (2007) 638–645
Fig. 5. Model-predicted fluxes of active species at 700 W, 2 Pa: (1) – F
atoms; (2) – total flux of positive ions; (3) – CF/C+.
Multiplying the last value by a factor of 6 · 108M/qNa(q=
3.56 g/cm3 is MgO density, M = 40.3 is MgO molar mass
and Na = 6.02 · 1023), we obtain 16.1 nm/min versus
15.1 nm/min given by the experiment. From this fact, we
can conclude that the model provides a quite accurate
description of the sputter etch pathway. The parameters s0,
B and E0,d are not known exactly and may be evaluated only
using an indirect literature data for well-studied systems,
particularly for CF4/SiO2, CF4/Si, F2/Si and C2F6/Si
[23,35–37]. The corresponding ranges are 0.1–0.3, 0.1–1.0
and 5–15 eV for s0, B and E0,d, respectively. Although the
+
CFþ
3 ion has a higher mass compared with Ar , we expect
+
Y d;Arþ > Y d;CFþ3 (i.e. that Ar ions are more effective in ionstimulated desorption) with Y d;Arþ =Y d;CFþ3 5 [38,39]. The
reasons are high fragmentation possibility as well as high
adhesive coefficients of CFþ
x ions for metal-containing surfaces. Particularly, the last point means that these ions can
be easily neutralized on the surface by either forming a polymer layer or returning back to the gas phase as neutral reactants [35,40]. Also, we understand that the parameters s0, B
and E0,d depend on a large number of factors such as the temperature of the surface, the stoichiometry of the reaction
products, and the type of incident ion and surface atom,
which cannot be taken into account accurately. That is
why, we fixed E0,d = 12 eV, but used s0 and B as the model
parameters to obtain the best correlation with experiments.
Figs. 6 and 7 illustrate the influence of gas mixing ratio
on surface kinetics. It was found that an increase of CF4
content in the CF4/Ar plasma escorted by the opposite
behaviors of CF and C+ shifts the fraction of free surface
toward lower values (h = 0.025–0.92 for 10–90% CF4 at
BArþ ¼ 0:25), and the rate of chemical reaction Rch exhibits
a maximum at 50–60% CF4 due to the concurrence of
increasing CF and decreasing fraction of free surface
(1 h) accessible for the adsorption of F atoms. At the
same time, the rate of physical sputtering changes monotonically and stops to contribute total etch rate when the
Ar content falls below 70%. In this situation, total etch rate
keeps a non-monotonic behavior determined by Rch, and
643
Fig. 6. Model-predicted rates for ion-assisted chemical reaction and
physical sputtering at 700 W, 2 Pa: (1) – Rsp; (2) – Rch; (3) –
Re = Rsp + Rch. Model parameters are m = 2, s0 = 0.1, and BArþ ¼ 0:25:
Fig. 7. The influence of the CF4/Ar mixing ratio on efficiency of ion
stimulated desorption and fraction of fluorinated surface: (1) – BArþ ¼ 1:0;
(2) – BArþ ¼ 0:1 at m = 2 and s0 = 0.1.
the height and position of maximum along X-axis depend
on the properties of etched surface through the parameters
m, s0 and Yd. For example, an increase in BArþ up to 1
(Y d;Arþ 9:5) increases the efficiency of ion stimulated
desorption of reaction products, provides favorable conditions for chemical reaction (h = 6 · 103–0.75 for 10–90%
CF4, see Fig. 7, curve 1), decreases the contribution of
Rsp and shifts the maximum on both Rch and Re toward
CF4-rich plasma. A decrease in BArþ down to 0.1
(Yd 0.95) as well as an increase in s0 causes an opposite
effects (h = 0.06–0.97 for 10–90% CF4, see Fig. 7, curve
2) and etch rate response, but the Rch/Rsp ratio also exceeds
1 starting from 20% CF4. Therefore, it can be clearly seen
that the experimentally obtained non-monotonic effect of
CF4/Ar mixing ratio on the MgO etch rate (see Fig. 2)
can be explained by the concurrence of chemical and physical
etch pathways in ion-assisted chemical reaction even without taking into account the influence of the fluorocarbon
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A. Efremov et al. / Microelectronic Engineering 84 (2007) 638–645
polymer layer. Moreover, the accounting for the non-reactive (passivating) fluorocarbon layer does not change the
situation principally. From Refs. [25,28], it can be understood that the steady-state thickness of the fluorocarbon
layer on both Si and SiO2 is directly proportional to the
CFx flux while the etch yield is inversely proportional to
the thickness. Applying this situation to our case, one
can expect a near-to-linear contribution to h with increasing CF4 mixing ratio that affects only the absolute values
of Rsp and Rch, but does not affect their relative behaviors.
It is important to note that the model describes also the
effect of other input parameters on the MgO etch rate. For
example, from the Langmuir probe measurements, we have
found that Te is low sensitive to the change of input power
while ne increases linearly. In this situation, the parameters
Ys and Yd are assumed to be the constant, and the change
in the MgO etch rate results only from the changes in CF
and C+. Accordingly, with the increasing input power,
the model gives an increase in both CF and C+, but a
decrease in CF/C+. That is why the fraction of free surface
(1 h) increases and the MgO etch rate increases almost
linearly, exactly as it was mentioned by the experiment.
Also, with increasing gas pressure for the CF4-rich plasma,
the flux of fluorine atoms CF increases, but the values of
C+, Ys and Yd change oppositely due to the corresponding
changes of both Te, ion mean free path and ion energy
[20,13]. Therefore, taking into account that Rsp Rch,
the situation is similar to that for the effect of gas mixing
ratio, where the total etch rate maximum is produced by
the combination of increasing CF and decreasing fraction
of free surface (1 h).
For the model parameters of Fig. 6, the model-predicted
etch rates given by Eq. (9) are 1.42 · 1015 cm2 s1 for pure
Ar, 2.44 · 1015 cm2 s1 in maximum at 50% Ar/50% CF4
and 6.03 · 1014 cm2 s1 for pure CF4. Again, multiplying
these values by a factor of 6 · 108M/qNa, we obtain
16.1 nm/min, 27.6 nm/min and 6.8 nm/min (Fig. 8). Taking into account an outstanding agreement between model
and experiment, we can conclude that the set of assumptions for surface kinetics described in Section 2 provides
a near-to-adequate description of the MgO etch mechanism in the CF4/Ar plasma. This may be connected with
the low thickness (low passivating ability) of the fluorocarbon layer due to low volume density of CF2 radicals
ðnCF4 : nCF2 : nF 1 : 0:05 : 0:8 in pure CF4 plasma [12])
as well as due to high sputtering yield of poly-CFx on the
biased substrate. Additionally, the model simply accounts
for numerous experimental data demonstrating the possibilities of non-monotonic etch rate behavior for the system
with monotonic changes of fluxes of active species.
4. Conclusion
In this work, we investigated the effects of operating
parameters and gas mixing ratio on both etch characteristics and mechanism of MgO thin films in CF4/Ar inductively coupled plasma. It was found that an increase in
input power as well as in negative dc bias voltage causes
a near-to-linear growth of the MgO etch rate while the
effects of gas mixing ratio and gas pressure are non-monotonic. The combination of 0-dimensional plasma model
with plasma diagnostics by Langmuir probe showed the
monotonic changes for densities and fluxes of active species
responsible both chemical and physical etch pathways. The
model of surface kinetics showed the possibility of nonmonotonic rate for ion-assisted chemical reaction due to
an opposite behaviors of the flux of chemical active species
and the fraction of free surface acceptable for chemical
reaction. Also, the model demonstrated an outstanding
agreement with the experiments.
Acknowledgement
This work was supported by Korea Research Foundation and The Korean Federation of Science and Technology Grant No. 1052S-3-5 founded by Korea Government
(MOEHRD, Basic Research Promotion Fund).
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Fig. 8. Comparison of model-predicted (1) and experimentally measured
(2) MgO etch rates. Model parameters are the same with Fig. 6.
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