CC BY
36DOI: 10.6060/ivkkt.20216407.6390
УДК: 537.525
ПАРАМЕТРЫ ПЛАЗМЫ И КОНЦЕНТРАЦИИ АКТИВНЫХ ЧАСТИЦ В СМЕСЯХ ФТОРУГЛЕРОДНЫХ ГАЗОВ С АРГОНОМ И КИСЛОРОДОМ
А.М. Ефремов, В.Б. Бетелин, К.А. Медников, K.-H. Kwon
Александр Михайлович Ефремов*
Ивановский государственный химико-технологический университет, Шереметевский просп., 7, Иваново, Российская Федерация, 153000 E-mail: amefremov@mail.ru*
Владимир Борисович Бетелин, Константин Александрович Медников
ФГУ ФНЦ НИИСИ РАН, Нахимовский просп., 36, к.1, Москва, Российская Федерация, 117218 E-mail: betelin@niisi.msk.ru, k.a.mednikov@gmail.com
Kwang-Ho Kwon
Korea University, 208 Seochang-Dong, Chochiwon, Korea, 339-800 E-mail: kwonkh@korea.ac.kr
Проведено сравнительное исследование электрофизических параметров плазмы и стационарного состава газовой фазы в плазме индукционного ВЧ 13б56 МГц разряда в смесях CF4 + O2 + Ar, CHF3 + O2 + Ar и C4F8 + O2 + Ar. В качестве постоянных внешних параметров выступали доля фторуглеродного компонента (50%), общее давление газа (6 мтор), вкладываемая мощность (700 Вт) и мощность смещения (200 Вт). Схема исследования включала диагностику плазмы зондами Лангмюра и 0-мерное (глобальное) моделирование кинетики плазмохимических процессов. Найдено, что полимеризационная способность бескислородных (50% Ar) и кислородсодержащих (50% O2) смесей согласуется с отношением z/x в исходной молекуле CHyFz. Замещение аргона на кислород приводит к однотипным изменениям параметров электронной и ионной компонент плазмы (температуры электронов, концентраций заряженных частиц и энергии ионной бомбардировки), всегда снижает концентрации полимеробразующихрадикалов и толщину полимерной пленки, но оказывает различное влияние на кинетику атомов фтора. Увеличение доли кислорода в смесях 50% CF4 + O2 + Ar и 50% CHF3 + O2 + Ar приводит к монотонному росту концентрации атомов фтора. Механизмы этих явлений связаны с увеличением скорости генерации атомов и снижением частоты их гибели, соответственно. Добавление кислорода в системе 50% C4F8 + O2 + Ar снижает скорость генерации атомов фтора, но не приводит к заметным изменениям частот их гибели. Это соответствует монотонному снижению концентрации атомов фтора при увеличении содержания кислорода в смеси. В результате, стационарная концентрация атомов фтора в условиях 50% O2 увеличивается в ряду C4F8 - CHF3 - CF4.
Ключевые слова: фторуглеродные газы, плазма, параметры, активные частицы, ионизация, диссоциация, травление, полимеризация
Для цитирования:
Ефремов А.М., Бетелин В.Б., Медников К.А., Kwon K.-H. Параметры плазмы и концентрации активных частиц в смесях фторуглеродных газов с аргоном и кислородом. Изв. вузов. Химия и хим. технология. 2021. Т. 64. Вып. 7. С. 46-53 For citation:
Efremov A.M., Betelin V.B., Mednikov K.A., Kwon K.-H. Plasma parameters and densities of active species in mixtures of fluorocarbon gases with argon and oxygen. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2021. V. 64. N 7. P. 46-53
PLASMA PARAMETERS AND DENSITIES OF ACTIVE SPECIES IN MIXTURES OF FLUOROCARBON GASES WITH ARGON AND OXYGEN
A.M. Efremov, V.B. Betelin, K.A. Mednikov, K.-H. Kwon
Alexander M. Efremov*
Ivanovo State University of Chemistry and Technology, Sheremetevskiy ave., 7, Ivanovo, 153000, Russia
E-mail: efremov@isuct.ru *
Vladimir B. Betelin, Konstantin A. Mednikov
SRISA RAS, Nakhimovsky ave., 36, bld. 1, Moscow, 117218, Russia
E-mail: betelin@niisi.msk.ru, k.a.mednikov@gmail.com
Kwang-Ho Kwon
Korea University, 208 Seochang-Dong, Chochiwon, Korea, 339-800
E-mail: kwonkh@korea.ac.kr
The comparative study of plasma electro-physical parameters and steady-state gas phase compositions in CF4 + O2 + Ar, CHF3 + O2 + Ar and C4F8 + O2 + Ar gas mixtures was carried out under the condition of 13.56 MHz inductive RF discharge. Constant processing parameters were fluorocarbon component fraction in a feed gas (50%), total gas pressure (6 mTorr), input power (700 W) and bias power (200 W). The investigation scheme included plasma diagnostics by Lang-muir probes and 0-dimensional (global) modeling ofplasma chemistry. It was found that polymerizing ability in both non-oxygenated (50% Ar) and oxygenated (50% O2) gas systems correlates with the x/z ratio in the original CxHyFz molecule. The substitution of Ar for O2 causes similar changes in electrons- and ions-related plasma parameters (electron temperature, plasma density, ion bombardment energy), always suppresses densities ofpolymerizing radicals and polymer film thickness, but has the different impact on the F atom kinetics. An increase in O2 fraction in 50% CF4 + O2 + Ar and 50% CHF3 + O2 + Ar gas mixtures results in monotonically increasing F atom densities. Mechanisms of these phenomena are increasing F atom formation rate and decreasing the F atom decay frequency, respectively. The addition of oxygen to 50% C4F8 + O2 + Ar gas mixture lowers the F atom formation rate, but does not result in sufficient changes in their decay frequency. This corresponds to monotonically decreasing F atom density toward O2-rich plasmas. As a result, the steady-state density of F atoms in gas systems with 50% O2 increases in the sequence of C4F8 - CHF3 - CF4.
Key words: fluorocarbon gases, plasma, parameters, active species, ionization, dissociation, etching,
polymerization
INTRODUCTION
Fluorocarbon gases with a general formula of CxHyFz are frequently used for the reactive-ion etching (RIE) of silicon and silicon-based materials [1-3]. This process represents the critical part of the photolithography circle because it determines patterning quality and thus, whole device dimension and performance. Among the fluorocarbon gas family, the CF4 is characterized by the highest z/x ratio and provides the domination of etching over the surface polymerization process under the typical RIE conditions [4]. This allows one to obtain high absolute etching rates, but results in both nearly isotropic etching profiles for Si (due to the spontaneous chemical reaction between silicon and fluorine atoms) and low SiO2/Si selectivity [3, 4]. Oppositely, more polymerizing fluorocarbons with z/x < 3
(for example, CHF3 and C4F8) allow one to obtain the anisotropic high aspect ratio etching of Si (due to the passivation of side walls by the fluorocarbon polymer film [2, 4]) as well as provide the much higher etching selectivity over the SiO2 (due to the much lower polymer film thickness on the oxygen-containing surface [4]). At the same time, the negative issues are the decrease in absolute Si and SiO2 etching rates and an increase in etching residues. It is known also that all flu-orocarbons are frequently combined with Ar or O2 with the aims of accelerating the physical etching pathway, increasing the F atoms yield and suppressing surface polymerization [2, 3]. Therefore, the choice of an appropriate fluorocarbon gas, additive components and their mixing ratios is a powerful tool to adjust output characteristics of RIE process for a given type of etched material. The mandatory condition for the use
of this tool is the understanding of relationships between processing conditions, internal plasma parameters and steady-state densities of plasma active species.
Until now, there were many works dealt with investigations of plasma parameters and gas-phase compositions in CF4 + Ar/O2 [5-8, 10], CHF3 + Ar/O2 [9-11] and C4F8 + Ar/O2 [7, 12-15] plasmas. In fact, results of these researches allowed one a) to figure out key gas-phase processes determining kinetics of fluorine atoms and polymerizing radicals; b) to compose kinetic schemes (sets of reactions with corresponding rate coefficients) for the adequate description of plasma chemistry in the presence of oxygen; and c) to understand basic responses of both etching and polymerization kinetics to changes in processing parameters (input power, pressure and gas mixing ratios). In addition, our previous studies [8, 11, 14-16] suggested an advanced research scheme as a combination of etching experiments, plasma diagnostics by Lang-muir probes and plasma modeling. Such a method clearly demonstrates how processing parameters do effect on gas-phase plasma characteristics as well as allows one to analyze etching mechanisms with model-predicted fluxes of plasma active species. The problem is that the most of existing data for various gas chemistries were obtained at different processing conditions and/or in different types of plasma reactors. As such, in many cases it is impossible to compare directly the features of gas-phase plasma characteristics even for widely used fluorocarbon-based gas mixtures and thus, to evaluate their etching performances in respect to the given treated material. Such situation retards both optimization of reactive-ion etching technologies and the overall progress in the electronic device fabrication field.
The idea of this work was to carry out the comparative study of plasma electro-physical parameters and steady-state gas phase compositions in CF4 + O2 + Ar, CHF3 + O2 + Ar and C4F8 + O2 + Ar gas mixtures under one and the same operating conditions. Corresponding fluorocarbon gases provide a continuous decrease of z/x ratio in the sequence of CF4 - CHF3 - C4F8 that causes sufficient differences in their polymerizing abilities and fluorine atom densities [16]. Accordingly, the main goals were 1) to compare how the change in O2/Ar mixing ratio does influence electrons- and ions related plasma parameters; 2) to analyze differences in densities of fluorine atoms and polymerizing radicals in the presence of oxygen; and 3) to suggest features of etching and polymerization kinetics based on gasphase plasma characteristics.
EXPERIMENTAL AND MODELING DETAILS
Experiments were performed in the planar (with top-side flat coil) inductively coupled plasma (ICP) reactor described in our previous works [14-16]. Plasma was excited using the 13.56 MHz power supply while another 12.56 MHz rf generator biased the bottom electrode. The latter allowed one to adjust the ion bombardment energy through the negative dc bias voltage (-Udc). An actual -Udc value was measured by highvoltage probe (AMN-CTR, Youngsin Eng.). Constant processing parameters were total gas flow rate (q = 40 sccm), gas pressure (p = 6 mTorr), input power (Wmp = 700 W) and bias power (Wdc = 200 W). The variable parameter was the O2/Ar mixing ratio in CF4 + O2 + Ar, CHF3 + + O2 + Ar and C4F8 + O2 + Ar gas mixtures with fixed 50% fraction of fluorocarbon component. Accordingly, an increase in O2 fraction in a feed gas, y(O2), from 0-50% corresponded to the full substitution of Ar for O2.
In order to obtain the data on electro-physical plasma parameters, such as electron temperature (Te) and ion current density (J+), we used plasma diagnostics by the double Langmuir probe (DLP2000, Plasmart Inc.). The probe installation and measurement details have been described in Refs. [8, 11]. The treatment of raw I-V curves was based on well-known statements of the double probe theory for low pressure plasmas [17].
In order to analyze the influence of O2/Ar mixing ratio on kinetics and densities of plasma active species, we used a simplified 0-dimensional (global) model. Detailed information on model assumptions and algorithm may be found in Refs. [7, 8, 15, 18]. Kinetic schemes (sets of chemical reactions with corresponding rate coefficients) were taken from published works that dealt with the modeling of CF4 + Ar/O2 [7, 8], CHF3 + Ar/O2 [11] and C4F8 + Ar/O2 [7, 15] plasmas. As input parameters, the model used experimental data on Te and J+. The latter yielded the total density of positive ions n+ as well as the electron density ne assuming ne « n+. The low electronegativity of low pressure CF4, CHF3, C4F8 and O2 plasmas has been confirmed in earlier works [5, 6, 9, 13, 19]. The neutral gas temperature (Tgas) was approximated by the typical (for given set of processing conditions, reactor type and geometry) value of ~ 600 K, as have been done in Refs. [10, 11, 14]. The output model parameters were volume-averaged steady-state densities of plasma active species and their fluxes to the etched surface.
RESULTS AND DISCUSSION
Features of electrons- and ions-related plasma parameters for CF4 + Ar, CHF3 + Ar and C4F8 + Ar
plasmas were subjects of detailed analysis in our previous studies [10, 14, 16, 18]. That is why the below discussion deals only with effects obtained during the substitution of argon for oxygen. Experiments indicated that the substitution of Ar for O2 in all three gas mixtures causes similar changes in electron temperature, plasma density and negative dc bias voltage on the lower electrode (Table). Corresponding results may be briefly explained as follows:
- A decrease in Te toward O2-rich plasmas is caused by an increase in the electron energy loss due to increasing fraction of molecular components in a gas phase (Fig. 1). The reason is that the first excitation potential for Ar atom of ~ 11.6 eV (in fact, the starting point in the corresponding electron energy loss spectrum) is much higher than that for O2 (~ 0.16 eV for the vibration excitation R1: O2(V=0) + e ^ O2(V>0) + e). In addition, O2 provides the low-threshold excitation of metastable states in R2: O2 + e ^ O2(a:A) + e (82 = 0.98 eV) and R3: O2 + e ^ O2(b!E) + e (83 = 1.64 eV). As a result, the almost continuous energy loss spectrum from ~ 0.2 eV takes place.
- A decrease in plasma density with increasing O2 content in a feed gas is provided by the simultaneous action of two mechanisms. These are a) decreasing ionization rate coefficients, according to the behavior of Te (since 8iz « 12-15 eV > (3/2)Te where 8iz is the threshold energy for ionization, and (3/2)Te is the mean electron energy); and b) increasing densities of electronegative species due to both O2 itself and oxygen-containing reaction products. The latter accelerates losses of positive ions and electrons through the ion-ion recombination and dissociative attachment, respectively. Ion current densities and ion fluxes follow the behavior of n+ and also exhibit decreasing tendencies toward O2-rich plasmas.
- An increase in negative dc bias is evidently connected with the decreasing ion flux. This is because the lower ion flux provides the weaker compensation for the excess negative charge produced by corresponding power supply under the condition of Wdc = const.
When analyzing kinetics of neutral species, it was found that all three gas mixtures exhibit sufficient dissimilarities in respect to fluorine atom kinetics. Such situation is provided by specific electron-impact dissociation mechanisms for original fluorocarbon molecules that pre-determine different first-step dissociation products and their interaction pathways with other species, including oxygen atoms and molecules.
In the CF4 + Ar plasma, dominant fluorine-containing components are original CF4 molecules, CF3 radicals and F itself [5, 6, 18]. The formation of F atoms is mainly provided by R4: CF4 + e ^ CF3+ + F + 2e
and R5: CFx + e ^ CFx-1 + F + e for x = 3, 4. The decay of atomic species is due to their heterogeneous recombination in R6: F + F(s.) ^ F2 and R7: F + CFx(s.) ^ ^ CFx+i, where index "(s.)" points out on the surface-bonded particle. The substitution of Ar for O2 rapidly reduces densities of CFx radicals (due to the decomposition of these species in R8: CFx + O/O(1D) ^ CFx-iO + F, R9: CF3 + CFO ^ CF4 + CO and R10: CF3 + CFO ^ CF2O+ + CF2), but introduces new formation pathways for F atoms. Among latters, most important are electron-impact reactions R11: CFO + e ^ CO + F + e, R12: CF2O+ + e ^ CFO + F + e and R13: FO + e ^ F + O + e. High rate of R11 is provided by the fast formation of CFO species in R12 and R14: CO + F ^ CFO while the same effect for R12 is due to R10, R15: 2CFO ^ CF2O + CO and R16: CFO + F ^ CF2O. In addition, formation kinetics of fluorine atom in O2-rich plasmas is noticeably affected by atom-molecular reactions R17: FO + O/O(1D) ^ F + O2, R18: 2FO ^ 2F + O2 and R19: CFO + O ^ CO2 + F. As a result, the substitution of Ar for O2 leads to a continuous increase in the F atom formation rate and thus, the F atom density (Fig. 1(a)).
Table
Electrons- and ions-related plasma parameters in 50% CF4 + O2 + Ar, 50% CHF3 + O2 + Ar and 50% C4F8 +
+ O2 + Ar plasmas Таблица. Параметры электронной и ионной компоненты плазмы в смесях 50% CF4 + O2 + Ar, 50% CHF3 + O2 + Ar и 50% C4F8 + O2 + Ar
CF4 + O2 + Ar CHF3 + O2 + Ar C4F8 + O2 + Ar
y(O2), % Te, eV n+, 1010 cm-3 -Udc, V Te, eV n+, 1010 cm-3 -Udc, V Te, eV n+, 1010 cm-3 -Udc, V
0 3.6 4.9 215 4.8 6.2 190 4.8 4.4 212
50 3.4 3.2 250 3.0 3.0 254 3.1 3.7 269
In the CHF3 + Ar plasma, main fluorine-containing species are HF, CHF3 and CFx (x = 1-3) [9-11, 20]. The high density of HF [20, 21] is provided by two mechanisms, such as a) the direct formation of these species in R20: CHF3 + e ^ HF + CF2 + e; and 2) the high efficiency of gas-phase reactions R21: CHFx + F^ ^ CFx + HF, R22: CHFx + H ^ CHFx-1 + HF and R23: CFx + H ^ CFx-1+ HF. Accordingly, main formation channels for F atoms are R24: HF + e ^ H + F + e and R5 for x = 2, 3. Another important feature is that the contribution of R21 to the total decay rate for F atoms exceeds those for R6 and R7. The substitution of Ar for O2 retards the electron-impact dissociation kinetics (because of sufficient falls in both Te and ne) as well as strongly suppresses all F atom formation pathways which do work in the CHF3 + Ar plasma. The last effect
is caused by a) fast losses of CFx and CHFx radicals due to their effective conversion into CFxO species in R8, R25: CHFx + O — CFxO + H and R26: CHFx + O — — CFx-iO + HF; and b) the decreasing rate of R24 due to the tenfold fall in k24ne (88.5-8.3 s-1 at 0-50% O2). Another remarkable differences in respect to previous gas system are the lower production rates for O and O(1d) in R27: O2 + e — 2O + e and R28: O2 + e — —>O + O(1D) + e as well as the faster decay of oxygen atoms R8, R25 and R26. In fact, this limits the formation of CFxO and FO species in both gas-phase and
heterogeneous reactions and thus, lowers contributions of R11-R13 and R17-R19 to the F atom formation kinetics. All these result in a monotonie decrease in the total F atom formation rate toward O2-rich plasmas (by ~ 2.3 times at 0-50% O2). At the same time, rapidly decreasing densities of CHFx and CFx radicals reduce the effective decay frequency for F atoms in both heterogeneous (R6, R7) and gas-phase (R21) reactions. Since the last tendency appears to be faster compared with a change in the fluorine atom formation rate, the monotonic increase in [F] takes place (Fig. 1(b)).
10"
a) CF. + O, + Ar
1013
10"
101'
b) CHF + O + Ar
101
0 10 20 30 40 50
10"
1014
c) C4F., + O2 + Ar
1013
0 10 20 30 40 50
10"
0 10 20 30 40 50
O2 fraction in a feed gas [%]
Fig. 1. Steady-state densities of neutral species in CF4 + O2 + Ar (a), CHF3 + O2 + Ar (b) and C4F8 + O2 + Ar (c) plasmas. Dashed lines
indicate the oxygen-containing components Рис. 1. Стационарные концентрации нейтральных частиц в плазме CF4 + O2 + Ar (а), CHF3 + O2 + Ar (b) and C4F8 + O2 + Ar (c).
Пунктирными линиями выделены кислородсодержащие компоненты
In the C4F8 + Ar plasma, the gas phase is mostly composed by fluorocarbon components CFx (x = 1, 2, 3) and C2Fx (x = 3, 4) (Fig. 1(c)) [12-14]. These particles appear as the first-step dissociation products of original C4F8 molecules in R29: C4F8 + e— — 2C2F4 + e and R30: C4F8 + e — C3F6 + CF2 + e as well as result from the further decomposition of corresponding reaction products through R5 for x = 2, R31: C3F6 + e — C2F4 + CF2 + e, R32: C2F4 + e — 2CF2 + e and R33: C2F4 + e — C2F3 + F + e. The main source of F atoms is given by R5 for x = 1-3 while their decay in addition to R6 and R7 is noticeably contributed by R34: C2F4 + F — CF2 + CF3. The substitution of Ar for O2 also reduces the efficiency of R5 (due to the simultaneous decrease in Te and ne) as well as introduces new pathways for the decomposition of CFx radicals in
a form of R8-R10. At the same time, a decrease in [CFx] appears to be much slower compared with CF4-and CHF3-based plasmas. The reason is the effective loss of O2 molecules in R35: CF + O2 ^ CFO + O and R36: C + O2 ^ CO + O that limits formation rates for O and O(1D) atoms through R27 and R28. The lack of oxygen atoms reduces the significance of R8, R11-R13 and R17-R19 in respect to production of F atoms while the total F atom formation rate exhibit the monotonic decrease toward O2-rich plasmas. Accordingly, the same behavior is also for the F atoms density, as shown in Fig. 1(c).
In order to understand how above differences in gas-phase plasma characteristics do influence the reactive-ion etching kinetics, one can use the phenome-nological approach developed in earlier works [7, 8,
10, 11]. The latter is based on several experimental studies [22-26] and may be formulated as follows:
- The rate of physical sputtering Rphys for both target surface and fluorocarbon polymer film may be expressed as Ysr+, where Ys is the process yield, and r+ « J+/e is the ion flux. When taking in mind that Ys ~ (MiSi)1/2 for the given ion mass, the efficiency of the physical etching pathway in different gas systems with Mi « const may be compared using the the parameter Si1/2r+.
- The rate of chemical reaction Rchem between F atoms and target surface may be expressed as YrTf, where Tf is the flux of F atoms with the gas-phase density [F], and yr is the effective reaction probability. The similar formula works also for the chemical interaction of O atoms with polymer surface. The sufficient difference between these two cases is that the reaction of F atoms appears on the polymer/etched surfaced interface and may have the ion-assisted nature. That is why the corresponding yr may depend not only on surface temperature, but be also sensitive to any factor influencing the access of etchant species to desorption sites. In general, these are the ion bombardment intensity and the polymer film thickness.
- The growth of polymer film is provided by CHxFy (x + y < 2) radicals as well as appears to be slower in fluorine-rich plasmas. As such, the polymer deposition rate is characterized by the Tpol/TF ratio, where Tpol is the total flux of polymerizing radicals, while the change in fluorocarbon polymer film thickness due to physical and chemical decomposition pathways may be traced by parameters rpol/Si1/2r+rF and rpol/roTF, respectively.
Data of Figs. 1, 2 and 3 allow one to summarize features which may influence heterogeneous process kinetics in given gas systems. First, the substitution of Ar for O2 in all there gas mixtures introduces identical gas-phase reaction mechanisms to reduce densities of polymerizing radicals, but has specific impacts on the formation-decay balance for F atoms. As a result, an increase in y(O2) up to 50% does not change the basic rule concerning the correlation between the polymerizing ability and the z/x ratio in original CxHyFz molecules, but disturbs the difference in F atom densities compared with non-oxygenated binary mixtures with Ar [16]. Particularly, the rapid fall of both [F] and Tf in the C4F8 + O2 + Ar plasma produces the more than 10 times gap compared with the CF4 - based gas system as well as leads to lowest values of these parameters among other O2-rich mixtures. Another principal effect is that even the formally similar changes of F atom density in CF4 + O2 + Ar and
CHF3 + O2 + Ar plasmas are caused by different reasons. These are changes in F atom formation or decay kinetics, respectively. second, the substitution of Ar for O2 always suppresses the polymer deposition rate through increasing gap between densities of F atoms and polymerizing radicals. The stronger effect for CF4+ O2 + Ar and CHF3 + O2 + Ar plasmas (see Fig. 2(b)) is because of increasing density of F atoms and opposite changes in Tpol and Tpol toward higher y(O2) values.
1,8 1,6
S 14
c-P 1
>
2 " 1,2
ci
1,0
0,8
\
0 10 20 30 40 O fraction in a feed gas, %
a
50
10"
10*'
10"
10
20
30
40
50
O fraction in a feed gas, % b
Fig. 2. Gas-phase-related parameters characterizing the ion bombardment intensity (a) and the polymer deposition rate (b) in CF4 + O2 + Ar (1), CHF3 + O2 + Ar (2) and C4F8 + O2 + Ar (3) plasmas Рис. 2. Параметры газовой фазы, характеризующие интенсивность ионной бомбардировки поверхности (а) и скорость осаждения полимера (b) в плазме CF4 + O2 + Ar (1), CHF3 + O2 + Ar (2) и C4F8 + O2 + Ar (3)
And thirdly, the substitution of Ar for O2 in all three gas systems results in decreasing polymer film thickness. Again, the stronger change of hpol for CF4 + +O2 + Ar and CHF3 + O2 + Ar plasmas is due to the simultaneous acceleration in physical (Fig. 3(a)) and chemical (Fig. 3(b)) polymer decomposition pathways.
2
0
In the C4F8 + O2 + Ar plasma, the decrease in Si1/2r+ (Fig. 2(a)) has almost the same slope with the change in rpol/rF ratio (Fig. 2(a)). Such situation causes the nearly constant efficiency for sputter etching of polymer film in the range of 0-50% O2.
10"
> 10"1
U
+
сч"4
w-1
Г1^ 10-1
10 20 30 40 50 O fraction in a feed gas, %
a
10'
u
о u
10'
0 10 20 30 40 50 O fraction in a feed gas, %
b
Fig. 3. Gas-phase-related parameters characterizing the change in polymer film thickness due to physical (a) and chemical (b) etching pathways in CF4 + O2 + Ar (1), CHF3 + O2 + Ar (2) and
C4F8 + O2 + Ar (3) plasmas Рис. 3. Параметры газовой фазы, характеризующие изменение толщины полимерной пленки за счет физического (а) и химического (б) механизмов травления в плазме CF4 + O2 + Ar (1), CHF3 + O2 + Ar (2) и C4F8 + O2 + Ar (3)
From above data, it can be suggested that the C4F8 + O2 + Ar gas system under the condition of y(O2) > y(Ar) represents the worse source of etchant species as well as is featured by highest polymer deposition rate and polymer film thickness. Therefore, one can expect the lowest silicon etching rate together with an advanced etching profile. The latter is due to both weaker spontaneous etching (because of lower Г/Г+ ratio) and better passivation of sidewalls.
CONCLUSIONS
In this work, we investigated how the O2/Ar ratio in CF4 + O2 + Ar, CHF3 + O2 + Ar and C4F8 + O2+ + Ar gas mixtures does influence plasma parameters and steady-state gas phase compositions under the condition of 13.56 MHz inductive RF discharge. It was shown that the transition toward O2-rich plasmas a) causes similar changes in electrons- and ions-related plasma parameters (electron temperature, plasma density, ion bombardment energy); b) always suppresses densities of polymerizing radicals and reduces the polymer film thickness; and c) has the different impact on the F atom kinetics. As a result, the presence of oxygen does not disturb the correlation between the polymerizing ability and the z/x ratio in original CxHyFz molecules while the increasing F atom density in the sequence of C4F8 - CHF3 - CF4 in at 50% O2 contradicts with that for non-oxygenated plasmas.
The publication was carried out within the framework of the state assignment of the Federal State Institution «Scientific Research Institute for System Analysis of the Russian Academy of Sciences» (fundamental research) on subject No. 0580-2021-0006 "Fundamental and applied research in the field of lithography limits in semiconductor technologies as well as physical and chemical etching processes for 3D nanometer dielectric structures for the development of critical technologies for the production of ECB. Investigations and developments of both models and constructions for microelectronic elements in the extended temperature range (from -60С to +300С)".
Публикация выполнена в рамках государственного задания ФГУ ФНЦ НИИСИ РАН (проведение фундаментальных научных исследований) по теме № 0580-2021-0006 «Фундаментальные и прикладные исследования в области литографических пределов полупроводниковых технологий и физико-химических процессов травления 3D нано-метровых диэлектрических структур для развития критических технологий производства ЭКБ. Исследование и построение моделей и конструкций элементов микроэлектроники в расширенном диапазоне температур (от -60С до +300С)».
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Поступила в редакцию 24.02.2021 Принята к опубликованию 01.04.2021
Received 24.02.2021 Accepted 01.04.2021
