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ОСОБЕННОСТИ КИНЕТИКИ РЕАКТИВНО-ИОННОГО ТРАВЛЕНИЯ SiO2 В СМЕСЯХ CF4 + Ar + O2 И C4F8 + Ar + O2
А.М. Ефремов, А.М. Соболев, K.-H. Kwon
Александр Михайлович Ефремов*, Александр Михайлович Соболев
Ивановский государственный химико-технологический университет, Шереметевский просп., 7, Иваново, Российская Федерация, 153000 E-mail: efremov@isuct.ru*, ajlekcandr1213@mail.ru
Kwang-Ho Kwon
Korea University, 208 Seochang-Dong, Chochiwon, Korea, 339-800 E-mail: kwonkh@korea.ac.kr
Проведено исследование влияния соотношения Ar/O2 на характеристики газовой фазы и кинетику травления SiO2 в плазме смесей CF4 + Ar + O2 и C4F8 + Ar + O2 в условиях индукционного ВЧ 13,56 МГц разряда. В качестве постоянных внешних параметров выступали доля фторуглеродного компонента в плазмообразующей смеси (50%), общее давление газа (6 мтор), вкладываемая мощность (700 Вт) и мощность смещения (200 Вт). Было найдено, что полное замещение аргона на кислород в обеих смесях характеризуется немонотонным изменением (с максимумом при ~ 25% Ar + 25% O2) скорости травления SiO2 и монотонным ростом скорости травления фоторезиста при более высоких абсолютных значениях скоростей в плазме на основе CF4. Стационарные концентрации активных частиц определялись при совместном использовании диагностики плазмы зондами Лангмюра и 0-мерном (глобальном) моделировании кинетики плазмохимических процессов. Соответствующие результаты показали, что обе газовые системы характеризуются близкими параметрами электронной и ионной компонент плазмы, но существенно различаются по кинетике нейтральных частиц, особенно в присутствии O2. Последняя особенность обуславливает противоположные изменения как концентрации атомов F, так и эффективной вероятности ионно-стимулированной химической реакции в зависимости от параметра Ar/O2. Взаимосвязи между видом фторуглеродного компонента и кинетикой гетерогенных процессов анализировались с помощью набора параметров, характеризующих газовую фазу (плотностей потоков частиц и их отношений). На основании этого анализа предположено, что рост содержания O2 в смеси CF4 + Ar + O2 со слабой полимеризационной нагрузкой на обрабатываемую поверхность снижает вероятность реакции SiO2 + F через снижение эффективности деструкции оксидных связей и десорбции продуктов травления из-за снижения плотности потока энергии ионов. Напротив, рост содержания O2 в смеси C4F8 + Ar + O2 с высокой полимеризационной нагрузкой способствует росту эффективной вероятности взаимодействия за счет снижения толщины фторуглеродной полимерной пленки и облегчения доступа атомов F к обрабатываемой поверхности.
Ключевые слова: SiO2, травление, полимеризация, поток атомов фтора, поток энергии ионов, эффективная вероятность взаимодействия
FEATURES OF SiO2 REACTIVE-ION ETCHING KINETICS IN CF4 + Ar + O2 AND C4F8 + Ar + O2 GAS MIXTURES
A.M. Efremov, A.M. Sobolev, K.-H. Kwon
Alexander M. Efremov*, Alexander M. Sobolev
Ivanovo State University of Chemistry and Technology, Sheremetevskiy ave., 7, Ivanovo, 153000, Russia E-mail: efremov@isuct.ru*, ajlekcandr1213@mail.ru
Kwang-Ho Kwon
Korea University, 208 Seochang-Dong, Chochiwon, Korea, 339-800
E-mail: kwonkh@korea.ac.kr
The effect of Ar/O2 mixing ratio on gas-phase characteristics and SiO2 etching kinetics in CF4 + Ar + O2 and C4F8 + Ar + O2 plasmas was studied under conditions of 13.56 MHz inductive RF discharge. The constant processing parameters were fraction offluorocarbon component in a feed gas (50%) total gas pressure (6 mTorr), input power (700 W) and bias power (200 W). It was found that the full substitution of Ar for O2 in both gas systems results in non-monotonic (with a maximum at ~ 25% Ar + 25% O2) SiO2 etching rates as well as in monotonically increasing photoresist etching rate with higher absolute values for CF4-containing mixture. The steady-state densities of active species were determined using a combination of plasma diagnostics by Lang-muir probes and 0-dimensional (global) plasma modeling. Corresponding results indicated that both gas systems are characterized by quite close parameters of electron and ion components while exhibit sufficient differences in the kinetics of neutral species, especially in the presence of O2. The latter produces opposite changes in F atom density as well as in effective probability of ion-assisted chemical reaction vs. Ar/O2 mixing ratio. Relationships between type of fluorocarbon component and heterogeneous process kinetics were analyzed through the set of gas-phase-related parameters (fluxes, flux-to-flux ratios) characterizing chemical etching pathways for SiO2 and formation/destruction balance for the fluorocarbon polymer film. It was suggested that the transition toward O2-rich plasma in the low-polymerizing CF4 + Ar + O2 plasma suppresses the effective probability for SiO2 + F reaction through decreasing efficiency for oxide bond breaking and desorption of etching products due to decreasing ion energy flux. Oppositely, an increase in O2 content in the high-polymerizing C4F8 + Ar + O2 mixture lifts up the effective reaction probability by decreasing fluorocarbon film thickness and providing better access of F atoms to the etched surface.
ability
Key words: SiO2, etching, polymerization, fluorine atom flux, ion energy flux, effective reaction prob-
Для цитирования:
Ефремов А.М., Соболев А.М., Kwon K.-H. Особенности кинетики реактивно-ионного травления SiO2 в смесях CF4 + Ar + O2 и C4F8 + Ar + O2. Изв. вузов. Химия и хим. технология. 2020. Т. 63. Вып. 9. С. 21-27 For citation:
Efremov A.M., Sobolev A.M., Kwon K.-H. Features of SiO2 reactive-ion etching kinetics in CF4 + Ar + O2 and C4F8 + Ar + O2 gas mixtures. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. [Russ. J. Chem. & Chem. Tech.]. 2020. V. 63. N 9. P. 21-27
INTRODUCTION 1) In all fluorocarbon-based gas chemistries,
Recently, silicon and silicon-based materials the dominant contribution to the chemical etching of (SiO2, Si3N4, SiC) still keep the leading position in ^ is ¡?У F at°ms [5-7]. ^ much weaker
mode2rn e3le4ctronic device technology. Particularly, chemical effect from CFx radicals due to the ion-
SiO2 is widely used as the gate dielectric in field- induced dissociation of their adsorbed states has no
effect structures as well as frequently plays the roles principal influence on the overan etching kinetics
of inter-element insulation or final passivation [1-3]. [13] .
Since most of these applications assume the precise 2) The patterning of SiO2 is possible under
patterning of SiO2 layers in fluorocarbon (CxFy) gas the conditions of reactive-ion etching process and replasmas [3, 4], the optimization of corresponding dry quires the ion bombardment energy of ~ 200-300 eV.
etching processes is a key problem for achieving ad- The reason is that the spontaneous reaction SiO2 + F
vanced device characteristics. For this purpose, one is thermodynamically Prohibited, as follows from the
must clearly understood the interconnections between comparison of Si-° (~ 799 kJ/mol) and Si-F (~ 552
processing conditions, plasma chemistry and heteroge- kJ/mol) bond dissociation energies [14]. Therefore, neous kinetics the role of ion bombardment includes a) the destruc-
Until now, there were enough works reported tion of ^ bonds to produce the chemisorption sites on etching characteristics and mechanisms for SiO2 in for F atoms; and b) the desorption heterogeneous re-fluorocarbon gas plasmas [3-13]. Results of these action products which appear in a form of tte nonworks allow one to conclude that: saturated low volatile SiFx layer [5, 6 8].
3) In high-polymerizing fluorocarbon gases (y/x < 3 in the CxFy molecule), the SiO2 etching process is affected by the fluorocarbon (FC) polymer film grown at the plasma/etched surface interface [57]. Since the thick (> 10 nm) FC film limits the transport of F atoms to the target material as well as reduces the effective ion bombardment energy, the SiO2 etching kinetics strongly depends on the film thickness and thus, on the polymer deposition/decomposition dynamics [6-8].
The problem is that the most of existing works have the pure experimental nature and thus, did not analyze the relationships between heterogeneous effects and gas-phase plasma characteristics. That is why the basic principles to control the SiO2 etching process by plasma-related operating parameters are still not clear enough. Another negative issue is that studies of different gas systems were performed for different processing conditions and reactor types. Such situation limits the possibility to compare corresponding results and thus, to select the gas mixture for achieving the desirable combination of etching rate, anisotropy and selectivity in the given etching process.
In our previous work, we have performed the comparative study of plasma chemistry in CF4 + Ar + +O2 and C4F8 + Ar + O2 gas systems [15]. Thought some reasonable data on steady-state plasma parameters and gas-phase composition were obtained, these were not applied for the analysis of Si and/or SiO2 etching process. Therefore, since interconnections between gas-phase plasma parameters and heterogeneous kinetics in given gas systems are not still clear, differences in Si and SiO2 etching mechanisms are not completely understood. Such situation retards the optimization of plasma etching technology for silicon-based materials. From the others side, results of Refs. [10, 15, 16] clearly indicated that Ar/O2 mixing ratio at constant fraction of fluorocarbon component is a powerful tool to adjust densities of F atoms and polymerizing radicals. Accordingly, the idea of current work was to perform the comparative study of SiO2 etching kinetics in CF4 + Ar + O2 and C4F8 + Ar + O2 plasmas with various Ar/O2 mixing ratios as well as to analyze the relationships between gas-phase plasma parameters and heterogeneous reaction pathways determining the features of SiO2 etching mechanisms.
EXPERIMENTAL AND MODELING DETAILS
Experiments with both CF4 + Ar + O2 and C4F8 + Ar + O2 gas mixtures were performed in the planar inductively coupled plasma (ICP) reactor described in our previous works [15, 16]. Plasma was excited using a 13.56 MHz radio-frequency (RF)
power supply. Another RF power supply was connected to the chuck electrode in order to control the ion bombardment energy (si) through the negative DC bias voltage (-Udc). The chuck electrode was equipped by the water-flow cooling system that allowed one to maintain its temperature at -17 °C. In all experimental series, the constant parameters were gas pressure (p = 6 mTorr), total gas flow rate (q = 40 sccm), input power (W = 900 W) and bias power (Wdc= 200 W). The variable parameter was the Ar/O2 mixing ratio. The initial composition of each gas mixture was set by adjusting partial flow rates for corresponding gases. Particularly, content of CF4 or C4F8 was always 50% while the remaining half was represented by various fractions of Ar and O2. Accordingly, an increase in O2 fraction y(O2) from 0-50% corresponded to the full substitution of Ar for O2.
Etched samples (parts of Si wafer with the 500 nm-thick SiO2 layer) had the size of about 2x2 cm and was partially masked by the photoresist AZ1512 (positive). The etching rates for SiO2 and photoresist (PR) were determines as R = Ah/x, where Ah is the etched depth determined using surface profiler Alpha-Step 500 (Tencor), and x = 2 min is the processing time. In preliminary experiments, it was found that, under the given set of processing conditions, both etching processes are featured by the steady-state kinetic regime with the negligible influence of reaction products on gas-phase plasma parameters.
Plasma diagnostic was performed by double Langmuir probe (DLP2000, Plasmart Inc.). The probe setup had the built-in tip cleaning system for measurements in polymerizing plasmas. The treatment of voltage-current curves with using the well-known statements of probe theory [5, 17] provided information on electron temperature (Te) and ion current density (J+). In order to minimize the effect of fluoro-carbon polymer film on diagnostics results, the probe tip was additionally cleaned in 50% Ar + 50% O2 plasma for 1 min before and after each measurement. As a result, there were no sufficient differences between data points recorded under one and the same experimental conditions.
Plasma modeling was represented by 0-di-mensional (global) kinetic model operated with volume-averaged plasma characteristics. Kinetic schemes (reaction sets with corresponding rate coefficients), modeling approaches and algorithm were taken from our previous works [15, 16, 18]. Input model parameters were experimental data on Te and J+. As output parameters, the model yielded steady-state densities of plasma active species and their fluxes on the etched surface.
Basic assumptions for the analysis of heterogeneous kinetics were also the same with those used in our previous works [15, 16, 19-21]:
1) Any chemical etching pathway (interactions of F atoms with the SiO2 surface as well as O atoms with the fluorocarbon polymer film) has the rate of yR,XrX [5, 20-23], where TX (X = F or O) is the flux of corresponding atomic species with the gasphase density [X], and yR,X is the effective reaction probability. Generally, the constant surface temperature allows one to assume yR,X « const [5] as well as to associate changes in the chemical etching pathway with changes in rX only.
2) Any physical etching pathway (the breaking of Si-O bonds, the destruction of the fluorocarbon polymer film as well as the ion-stimulated desorption of reaction products) has the rate of YSr+ [21-24], where YS is the ion-type-averaged sputtering yield, and r+ « J+/e is the ion flux. According to Refs. [15, 16], one can suggest YS ~ (Misi)12, where Mi is the effective ion molar mass, si = e(-Uf - Udc) is the ion bombardment energy, and Uf is the floating potential. As such, the change in the physical etching pathway may be traced by the parameter (Misi )1/2r characterizing the ion energy flux.
3) The formation of the fluorocarbon polymer film is provided by CFx (x = 1, 2) radicals as well as appears to be slower in fluorine-rich plasmas [6-9]. As such, the polymer deposition rate is characterized by the rpol/rF ratio, where rpol is the total flux of polymerizing radicals. Accordingly, the change in fluo-rocarbon polymer film thickness due to destruction by ion bombardment and etching by oxygen atoms may be traced by the parameters rpol/(Misi
)1/2r+rF and
rpol/rOrF, respectively.
RESULTS AND DISCUSSION
From Fig. 1(a), it can be seen that SiO2 etching rates in oxygen-free CF4 + Ar and C4F8 + Ar plasmas are quite close with typical values ~ 100 nm/min. An increase in O2 fraction in both gas systems (i.e. the substitution of Ar for O2) results in similar non-monotonic R = f(y(O2)) functions which exhibit maximums at ~ 25% O2. The main differences between two gas systems are only in quantitative scale and may be summarized as follows. First, under the condition of y(O2) > 0 the CF4-based gas system provides the systematically higher SiO2 etching rates compared with the C4F8-based one. And secondly, the substitution of Ar with O2 has the noticeably stronger effect on the SiO2 etching process in CF4 + Ar + O2 plasma. Particularly, the change in y(O2) from 0-25% causes the almost twofold (102-200 nm/min) increase in SiO2 etching rate in CF4 + Ar + O2 plasma while the corresponding effect
in C4F8 + Ar + O2 plasma is only 1.3 times (97-124 nm/min). As such, the principal
questions for understanding of SiO2 etching mechanism in given gas systems are: 1) what mechanism does produce the maximum on SiO2 etching rate as function of y(O2) and 2) what are the reasons providing the quantitative differences in SiO2 etching rates for CF4- and C4F8-based gas chemistries under one and the same operating conditions?
■gi.6
y(O2), % y(O2). %
Fig. 1. Parameters characterizing the SiO2 etching kinetics in CF4 +Ar + O2 (solid lines) and C4F8 + Ar + O2 (dashed lines) plasmas. In Fig. a): 1 - SiO2 etching rate; 2 - etching selectivity over the
photoresist. In Fig. b): effective reaction probability (yR,F) Рис. 1. Параметры, характеризующие кинетику травления SiO2 в плазме CF4 + Ar + O2 (сплошная линия) и C4F8 + Ar + O2 (пунктир). На рис. a): 1 - скорость травления SiO2; 2 -селективность травления по отношению к фоторезисту. На рис. б): эффективная вероятность взаимодействия (yR,F)
From previously published works, it can be understood that SiO2 etching rates shown in Fig. 1(a) may be expressed as Rphys + Rchem [21-23], where Rphys is the rate of physical sputtering and Rchem is the rate of ion-assisted chemical reaction between SiO2 and F atoms. The experiments showed that, under conditions of given study, the SiO2 etching rate in Ar plasma is ~ 5 nm/min. As such, the condition Rphys << Rchem « R takes place for both gas mixtures. Then, taking in mind that R^hem = yR FrF, one can suggest that the quantitative differences in SiO2 etching rates for CF4- and C4F8-based gas mixtures mentioned in Fig. 1(a) may be connected with differences in both gas-phase plasma characteristics (through [F] and rF) and the heterogeneous kinetics (through yR,F). Accordingly, the reasons which may cause the non-mono-tonic changes of SiO2 etching rate vs. the O2 fraction in a feed gas are: 1) the maximum on the rF = f(y(O2)) curve due to the change in formation/decay balance for F atoms in the presence of oxygen; and 2) monotonic, but opposite tendencies for both rF and yRF. Particularly, even at constant surface temperature, yRF may be dependent on the fraction of "active" surface which provides the chemisorption of F atoms. Obviously, the latter is sensitive to the intensity of ion
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bombardment (through the Si-O bong breaking, desorption of reaction products and physical destruction of fluorocarbon polymer film) and O atom flux (through the chemical destruction of fluorocarbon polymer film). Therefore, the adequate interpretation of SiO2 etching mechanism in CF4 + Ar + O2 and C4F8 + Ar + O2 plasmas requires the study of plasma parameters and gas phase composition.
The basic properties of CF4 + Ar + O2 and C4F8 + Ar + O2 plasmas have been discussed in detail in Refs. [15, 16]. As such, it seems to be reasonable just to summarize their principal features which are important for the purpose of given study at given process conditions. Particularly, one must underline several effects taking place during the substitution of Ar with O2 at constant fraction of fluorocarbon component:
1) Both gas mixtures demonstrate similar effect of y(O2) on electron temperature and ion current density (Tab. 1). A decrease in Te (3.6-3.4 eV for CF4 + O2 + Ar and 4.8-3.1 eV for C4F8 + O2 + Ar at 0-50% O2) is connected with increasing electron energy loss in low-threshold excitations (vibrational, electronic) for O2 itself and molecular products of plasma chemical reactions. The decreasing tendency for J+ is produced by corresponding changes in ion
densities n+ (4.2- 101O-3.2-101(0 cm-3 for CF4 + O2 + Ar and 4.21010-3.71010 cm-3 for C4F8 + O2 + Ar at 0-50% O2). The reasons are that a) decreasing Te lowers ionization rate coefficients for all types of neutral species; and b) an increase in y(O2) enriches the gas phase by oxygen-containing electronegative species that accelerate decay for both positive ions and electrons through the ion-ion recombination and dissociative attachment, respectively.
Table
Electro-physical plasma parameters
y(O2), % CF4 + Ar + O2 C4 F8 + Ar + O2
Te, eV J+' 2 mA/cm -Udc, V Te, eV ^ 2 mA/cm -Udc, V
0 3.60 1.10 137 4.81 1.21 145
25 3.53 1.01 149 4.01 1.08 167
50 3.40 0.95 153 3.12 0.91 177
polymerizing radicals CFx (x = 1, 2), but by opposite changes in corresponding values for F atoms (Fig. 2b). The monotonie increase in F atom density in CF4
13 cm-3 at
+ O2 + Ar plasma ([F] = 5.6-10-5.4-10 0-50% O2) is provided by the effective formation of these species in R1: CFx + O ^ CFx-1O + F, R2: CFx + + O(1D) ^ CFx-1O + F and R3: F2 + O ^ FO + F (with a consequent electron-impact dissociation of fluorine-containing reaction products) that overcom-pensate the decreasing rate of R4: CFx + e ^ CFx-1 + + F + e. The monotonic decrease in F atom density in C4F8 + O2 + Ar plasma ([F] = 8.0-1012-8.6-1011 cm-3 at 0-50% O2) results from the same change in the rate of R4 with the almost negligible contribution from the side of R1-R3. The reason is that the formation rates for both O and O(1D) in R5: O2 + e ^ ^ 2O + e, R6: O2 + e ^ O + O(1D) + e and R7: O + e ^ ^ O(1D) + e are sufficiently limited by the decay of O2 molecules through R8: C + O2 ^ CO + O. In addition, the low efficiency of R1-R3 provides also the much weaker fall in the density of polymerizing radicals compared with CF4-based gas system.
4) The oxygen-free C4F8 + Ar gas system provides the much thicker FC polymer film compared with CF4 + Ar one [6, 7]. Such situation results from the sufficient difference in polymer deposition rates (rpol/rp = 6.4 for C4F8 + Ar vs. 0.1 for CF4 + Ar) at quite close polymer destruction rates by ion bombardment ((MiSi)1/2r+ = 7.21017 eV1/2cm"2s-1 for № + Ar vs. 6.11017 eV1/2cm"2s_1). An increase in y(O2) reduces the thickness of the FC polymer film in both gas mixtures, as follows from the rapidly decreasing
rpol/rOrF ratios (2.8 10-18 cm2s for C4F8 + Ar and
-18
2) Both gas mixtures exhibit quite similar changes in ion flux and ion bombardment energy (Fig. 2a). A weak increase in negative dc bias at constant Wdc (-Udc = 137-153 V for CF4 + O2 + Ar and 145177 V for C4F8 + O2 + Ar at 0-50% O2) and thus, in ion bombardment energy (si = 159-173 eV and 175196 eV for CF4- and C4F8-based gas mixtures, respectively) does not compensate the decrease in r+. Accordingly, the monotonic decrease in the parameter (Misi)12r+ (6.11017-4.91017 eV1/2cmV for CF4 + + O2 + Ar and 7.3-1017-5.4-1017 eV1/2cmV for C4F8 + + O2 + Ar at 0-50% O2) suggests that the substitution of Ar for O2 suppresses the physical etching pathway.
3) Both gas mixtures are characterized by similar decreasing trends for densities and fluxes of
1.9-10 cm2s at 5-50% O2). The latter implies also that the 50% C4F8 + 50% O2 plasma keeps the much higher polymerization ability compared with corresponding CF4-based gas mixture.
The comparison of Figs. 1(a) and 2 allows one to formulate two principal features of the SiO2 etching mechanisms in CF4 + Ar + O2 and Cf8 + Ar + O2 plasmas. First, in both gas mixtures, the nonmonotonic etching rates contradict with monotonic changes of both rF and (Misi)1/2r+. And secondly, the similar changes in etching rates correspond to opposite behaviors of rF. Obviously, the last fact also provides the opposite changes in effective reaction probabilities yR,F = R/rF, as shown in Fig. 1(b). Particularly, it can be seen that the substitution of Ar for O2 in CF4 + Ar + O2 gas mixture results in monotonically decreasing yRF in the range of (3.9-0.52)-10-2 for 0-50% O2. When neglecting the influence of FC polymer film on the SiO2 etching kinetics in the low-polymerizing CF4-based gas system (that assumes, in fact, the case of non-continuous of very thin continuous FC film), the behavior of yR,F may be associated with the decrease in ion energy flux (Fig. 3). Obviously, this effect appears through decreasing rates of
both oxide bonds breaking and ion-stimulated desorption of reaction products that lowers the fraction of free adsorption sites for F atoms. At the same time, the substitution of Ar with O2 in C4F8 + Ar + O2 gas mixture causes monotonically increasing yRF in the range of (2.0-20) • 10"2 for 0-50% O2. This contradicts with the change in ion energy flux, but shows an agreement with the behavior of rpol/TOrF ratio (Fig. 3) which traces the change in the FC film thickness. As the polymer layer retards the access of F atoms to the etched surface, one should reasonably assume that the decreasing FC film thickness accelerates the etching process through increasing effective F atom flux on the FC film/etched surface interface. In addition, it is important to mention that the shape of the yR,F = = f(rpol/rOrF) curve shown in Fig. 3 as well as the tendency itself are very close to those obtained for reaction probabilities and etching yields of Si and SiO2 in high-polymerizing fluorocarbon gas plasmas [6, 8]. Therefore, based on above data, one can conclude that 1) the SiO2 etching mechanism in both gas systems is represented by ion-assisted chemical reaction in the neutral-flux-limited regime; and 2) in both cases, the non-monotonic SiO2 etching rate as a function of O2 fraction in a feed gas results from opposite changes in fluorine atom flux and the effective probability of their reaction with the SiO2 surface. However, the mechanisms influencing the effective reaction probabilities are different and depend, in general, on the polymerizing ability of the fluorocarbon component.
Finally, we would like to comment the SiO2/PR selectivity issue which seems to be important for etching process optimization. It was found that the etching rate of PR in both gas systems increases monotonical-ly toward O2-rich plasmas (260-412 nm/min for C4F8 + Ar + O2 and 240-520 nm/min for CF4 + Ar + O2 at 0-50% O2). All these are in good agreements with both tendencies and quantitative differences for oxygen atom fluxes, as shown in Fig. 2b. The ratios of SiO2-to-PR etching rate have the quite close values (0.37-0.22 for C4F8 + Ar + O2 and 0.43-0.25 for CF4 + Ar + O2 at 0-50% O2) and exhibit the only weak decrease in the range of 0-25% O2 (Fig. 1a). As such, the obtaining of maximum SiO2 etching rates in 25% O2 + 25% Ar gas mixtures is not escorted by principal losses in SiO2/PR etching selectivity.
CONCLUSIONS
In this work, we investigated the possibility of Ar/O2 mixing ratio in CF4 + Ar + O2 and C4F8 + Ar + O2 gas mixtures to adjust steady-state plasma parameters and SiO2 etching kinetics. It was found that the CF4-based gas system provides higher etching rates for both SiO2 and photoresist, and the substitution of Ar with O2 at constant fraction of any fluorocarbon component results in the non-monotonic (with a maximum at ~ 25% Ar + 25% O2) SiO2 etching rates.
0 10 20 30 40 50 0 10 20 30 40 50
Y(O2). % y(O2). %
Fig. 2. Fluxes of active species on the etched surface in CF4 + Ar + O2 (solid lines) and C4F8 + Ar + O2 (dashed lines) plasmas. In Fig. a): 1 - total positive ion flux; 2 - negative dc bias. In Fig. b): "F" - fluorine atoms; "pol" - polymerizing radicals (CF2 + CF);
"O" - oxygen atoms Рис. 2. Плотности потоков активных частиц на обрабатываемую поверхность в плазме CF4 + Ar + O2 (сплошная линия) и C4F8 + Ar + O2 (пунктир). На рис. a): 1 - суммарная плотность потока положительных ионов; 2 - отрицательное смещение. На рис. б): "F" - атомы фтора; "pol" - полимеробразующие радикалы (CF2 + CF); "O" - атомы кислорода
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N1/V ia17 w1/2 -2 -1 (MSj) Г+. 10 eV cm s
Fig. 3. Effective SiO2 + F reaction probability in CF4 + Ar + O2 plasma (solid line) as a function of the parameter (Misi)1/2r+ characterizing the ion energy flux on the etched surface. Effective SiO2 + F reaction probability in C4F8 + Ar + O2 plasma (dashed line) as a function of Г^/ГрТз
flux ratio characterizing the fluorocarbon polymer film thickness Рис. 3. Эффективная вероятность взаимодействия SiO2 + F в плазме CF4 + Ar + O2 (сплошная линия) как функция параметра (Misi)1/2r+, характеризующего плотность потока энергии ионов на обрабатываемую поверхность. Эффективная вероятность взаимодействия SiO2 + F в плазме C4F8 + Ar + O2 (пунктир) как функция отношения плотностей потоков Г^/ТрЛз, характеризующего толщину фторуглеродной полимерной пленки на обрабатываемой поверхности
The comparative study of gas-phase plasma characteristics indicated such principal features as 1) quite
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close parameters of electron and ion components in both gas systems; 2) the much higher polymerizing ability for C4F8-based plasma due to higher formation rates and densities of CFx (x = 1, 2) radicals; and 3) opposite behaviors of F atom density with increasing O2 content in a feed gas. The last phenomenon is produced by differences in CFx + O ^ CFx-1O + + F reaction kinetics due to various loss rates and densities of oxygen atoms. The model-based analysis of SiO2 etching mechanism in a framework of ionassisted chemical reaction yielded also the opposite behaviors for effective probabilities for the heterogeneous SiO2 + F reaction. It was suggested that the transition toward O2-rich plasma in the low-polymerizing CF4 + Ar + O2 plasma reduces the effective reaction through decreasing efficiency for oxide bond breaking and desorption of etching products due to decreasing ion energy flux. In spite of this, the substitution of Ar with O2 in the high-polymerizing C4F8 + Ar + O2 mixture increases the effective reaction probability by lowering the fluorocarbon film thickness and providing better access of F atoms to the etched surface. The last suggestion is confirmed by the typical correlation (the shape of the curve and the tendency itself) between effective reaction probability and the fluorocarbon film thickness traced by the flux-to-flux ratio for polymerizing radicals, F atoms and O atoms.
The reported study was funded by RFBR, project number 19-07-00804А.
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Поступила в редакцию (Received) 20.01.2020 Принята к опубликованию (Accepted) 20.03.2020
