Characterization of Sio2 Etching Profiles in Pulse-Modulated

materials

Article Characterization of SiO2 Etching Proﬁles in Pulse-Modulated Capacitively Coupled Plasmas

Chulhee Cho 1, Kwangho You 1, Sijun Kim 2, Youngseok Lee 1, Jangjae Lee 1 and Shinjae You 1,3,*

1 Department of Physics, Chungnam National University, 99 Daehak-ro, Daejeon 34134, Korea; [email protected] (C.C.); [email protected] (K.Y.); [email protected] (Y.L.); [email protected] (J.L.) 2 Nanotech, Yongin 28431, Korea; [email protected] 3 Institute of Quantum System (IQS), Chungnam National University, Daejeon 34134, Korea * Correspondence: [email protected]

Abstract: Although pulse-modulated plasma has overcome various problems encountered during the development of the high aspect ratio contact hole etching process, there is still a lack of understanding in terms of precisely how the pulse-modulated plasma solves the issues. In this research, to

gain insight into previously observed phenomena, SiO2 etching characteristics were investigated under various pulsed plasma conditions and analyzed through plasma diagnostics. Speciﬁcally, the disappearance of micro-trenching from the use of pulse-modulated plasma is analyzed via self-bias, and the phenomenon that as power off-time increases, the sidewall angle increases is interpreted via radical species density and self-bias. Further, the change from etching to deposition with decreased

peak power during processing is understood via self-bias and electron density. It is expected that this research will provide an informative window for the optimization of SiO2 etching and for basic

Citation: Cho, C.; You, K.; Kim, S.; processing databases including plasma diagnosis for advanced plasma processing simulators. Lee, Y.; Lee, J.; You, S. Keywords: Characterization of SiO2 Etching silicon dioxide etching; liquid fluorocarbon precursor; global warming potential (GWP); Profiles in Pulse-Modulated etch selectivity; capacitively coupled plasma; pulse-modulated plasma Capacitively Coupled Plasmas. Materials 2021, 14, 5036. https:// doi.org/10.3390/ma14175036 1. Introduction Academic Editors: Shinichi Tashiro As the feature size of semiconductors continues to decrease, and the structure of semi- and Rainer Hippler conductors is altered from two to three dimensions, high-aspect-ratio contact (HARC) hole etching has emerged as one of the most important goals in the semiconductor manufactur- Received: 8 June 2021 ing process. Achieving high-performance HARC hole etching requires high selectivity [1–7], Accepted: 31 August 2021 anisotropic etching [8–12], and reduced charge damage [13–16]. As continuous wave (CW) Published: 3 September 2021 plasma reacts with a consistent ion flux on a wafer surface, it is difficult to attain these aspects with CW plasma. Many researchers subsequently found that pulse-modulated Publisher’s Note: MDPI stays neutral plasma can meet the above requirements [17–22]; despite such findings though, an under- with regard to jurisdictional claims in published maps and institutional affil- standing of the effects of pulse-modulated plasma, namely the detailed plasma parameters iations. and how they influence surface reactions on the wafer, has remained unclear. To clarify the pulse-modulated plasma effects, it is necessary to diagnose the plasma and analyze the key plasma parameters. Etching is dominantly determined by cer- tain key parameters, i.e., radical species density, ion energy, and electron density [23]. Thus, many studies have been conducted to understand the relationship between pulse- Copyright: © 2021 by the authors. modulated plasma (hereafter, pulse plasma) parameters and the SiO etching characteristics. Licensee MDPI, Basel, Switzerland. 2 Samukawa et al. investigated SiO etching characteristics through electron density and This article is an open access article 2 distributed under the terms and self-bias in pulse-time modulated electron cyclotron resonance plasma [24]. Jeon et al. re- conditions of the Creative Commons searched SiO2 HARC profiles in dual-frequency-pulsed capacitively coupled plasma (CCP) Attribution (CC BY) license (https:// via XPS surface analysis and electron temperature [25]. Song and Kushner studied SiO2 creativecommons.org/licenses/by/ etch rates and profiles through average radical fluxes and the electron energy distribution 4.0/). function (EEDF) in pulse-modulated dual frequency CCP simulation [26]. Tokashiki et al.

Materials 2021, 14, 5036. https://doi.org/10.3390/ma14175036 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 13

Materials 2021, 14, 5036 2 of 13 and profiles through average radical fluxes and the electron energy distribution function (EEDF) in pulse-modulated dual frequency CCP simulation [26]. Tokashiki et al. studied SiO2 and poly-Si etch rates and profiles in pulse-modulated inductively coupled plasma studied SiO and poly-Si etch rates and profiles in pulse-modulated inductively coupled through electron2 temperature, electron density, plasma potential, and ion density [27]. plasma through electron temperature, electron density, plasma potential, and ion den- However, as Samukawa et al. and Tokashiki et al. did not consider radical density, the sity [27]. However, as Samukawa et al. and Tokashiki et al. did not consider radical density, chemical etching characteristics could not be fully explained, and otherwise, plasma sim- the chemical etching characteristics could not be fully explained, and otherwise, plasma ulation research should be matched with experimental data. It is important to note that simulation research should be matched with experimental data. It is important to note plasma etching is not simply determined by one or two plasma variables, but rather occurs that plasma etching is not simply determined by one or two plasma variables, but rather as a complex interaction between numerous variables at different levels of influence. occurs as a complex interaction between numerous variables at different levels of influence. Therefore, the combination of the key plasma parameters should be considered to deepen Therefore, the combination of the key plasma parameters should be considered to deepen the understanding of the relationship between plasma and etch profiles. the understanding of the relationship between plasma and etch profiles. In this paper, SiO2 wafers were etched by pulse plasma while changing various ex- In this paper, SiO2 wafers were etched by pulse plasma while changing various externalternal parameters parameters to to know know the the complex complex in interactionsteractions between between the plasma parameters,parameters, namelynamely electronelectron density,density, radicalradical density,density, andand self-bias.self-bias. UsingUsing thesethese plasmaplasma parameters,parameters, etchetch ratesrates andand selectivity selectivity were were analyzed. analyzed. Moreover, Moreover, undesired undesired etch etch profiles profiles such such as tapered as ta- etchingpered etching and micro-trenching and micro-trenching were analyzed were analyzed through through these variables. these variables.

2.2. Experimental Details TheThe experimentsexperiments were were performed performed using using a radio a radio frequency frequency (RF) (RF) CCP CCP source. source. A schematic A sche- outlinematic outline of the of CCP the chamber CCP chamber is shown is shown in Figure in Figure1a. Discharge 1a. Discharge was maintained was maintained between be- 300-mm-diametertween 300-mm-diameter parallel-plate parallel-plate electrodes electr separatedodes separated by 50 mm. by The 50 systemmm. The was system equipped was withequipped a 13.56 with MHz a 13.56 RF frequency MHz RF frequency power generator power generator connected connected to the bottom to the electrode bottom elec- via coaxialtrode via cable. coaxial The cable. RF powerThe RF generator power generator RF5S (Advanced RF5S (Advanced Energy, Energy, Denver, Denver, CO, USA) CO, USA) con- trolledcontrolled the the frequency frequency and and duty duty cycle cycle of of the the pulse pulse modulation modulation power power by by independentlyindependently varyingvarying power on/off time. time. The The 1000 1000 nm nm thick thick SiO SiO2 was2 was deposited deposited on on the the silicon silicon wafers wafers by byhigh-density high-density plasma plasma chemical chemical vaporized vaporized deposition, deposition, and and it was it wasmasked masked with witha 200 anm 200 poly- nm poly-siliconsilicon hole patterned hole patterned layer, with layer, 350 with nm 350diameter nm diameter holes used. holes The used. ratios The of ratiosthe gap of between the gap betweenthe holes theand holes the hole and size the were hole 1:1, size 1.2:1, were 2: 1:1,1 and 1.2:1, 5:1. 2:1This and wafer 5:1. was This cut wafer into wascoupon cut sizes into couponand processed sizes and while processed placed whilein the placedcenter of in the bottom center of electrode. the bottom electrode.

Figure 1. (a) Schematic of the chamber, and (b) diagnostic equipment set-up.

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The diagnostics equipment set-up is shown in Figure1b. The partial pressure of the radicals in the plasma was measured with an RGA-200 quadrupole mass spectrometer (QMS) (Stanford Research Systems, Sunnyvale, CA, USA). It was separated from the vacuum chamber by an oriﬁce with a differential pumping unit system to create a vacuum in the QMS. The base pressure of the QMS chamber was under 10−8 Torr. To obtain radical density in plasma, the QMS signal should be converted. Singh et al. suggested the radical density converted method using appearance potential mass spectrometry (APMS) as follows [28]:

on n A + λ + t(m + )θ(m + ) X = X→X Ar→Ar Ar Ar o f f (1) AAr→Ar+ λX→X+ t(mX+ )θ(mX+ ) nAr

on o f f In this equation, nX is X radical density, nAr is Ar density in the chamber without plasma, A and λ are the slopes of the linear fits of the QMS signal and the cross-section to eliminate electron energy dependence, t(m) is the transmission efficiency of the quadrupole o f f mass filter, and θ(m) is the detection coefficient of the detector. nAr is obtained using the ideal gas law, o f f p = nAr kT (2) where p is chamber pressure, k is the Boltzmann constant, and T is gas temperature. While this method is typically used to obtain absolute radical density, the APMS method requires a high-performance QMS in order to precisely control electron energy. In the current paper, to measure the absolute radical densities in the plasma, the APMS equation was changed [28] based on the quality of the QMS. A variation of this equation was used as follows: on n S + σ + t(m + )θ(m + ) X = X→X Ar→Ar Ar Ar o f f (3) SAr→Ar+ σX→X+ t(mX+ )θ(mX+ ) nAr Here, S is the QMS signal, and σ is the ionization cross-section. As the QMS signals were obtained at 70 eV electron energy, 70 eV ionization cross-sections were used. The Ar reference data for this method was measured at a 20 mTorr chamber pressure. Table1 shows the parameters and symbols in Equations (1)–(3). The radical density was determined by averaging the measurement five times.

Table 1. Symbols representing the parameters used in Equations (1)–(3).

Parameters Symbols Linear ﬁtting of QMS signal A Linear ﬁtting of ionization cross section λ X radical Density nX Transmission probability t Detection probability θ Chamber pressure p Boltzmann constant k Gas temperature T QMS signal S Ionization cross section σ

Here, Radical species C, CF, CF2, ... , etc., were measured, and they were substituted as in Table2. When the electron energy is set at 70 eV, the dissociative ionization of C 4F8 in the QMS generate the gross error of radical density due to the high electron energy. To investigate the dissociative ionization effect on the gross error, we measured electron energy distribution in the QMS ionizer via an electric probe [29,30]. By sweeping the probe bias voltage from −12 to +80 V, the electron current was measured and the second derivative of the current–voltage curve, which is proportional to the electron energy distribution, was evaluated. As a result, it is found that the electrons mostly existed in the energy range of Materials 2021, 14, 5036 4 of 13

0~15 eV, or 30~50 eV and there are two depletion regions between the two groups and over the electron energy of 50 eV when with 75 eV setting. The first depletion region in 15~30 eV energy is believed to be made by the ionization process of molecular gases in ionizer, more ionization creates a higher depletion of electrons. The second depletion region over 50 eV is believed to be made by the escape of electrons which can overcome the potential barrier of the ionizer. Therefore, ionization mostly occurs in the first depletion region. Moreover, in the cross-section of dissociative ionization, and direct ionization above the first depletion electron energy region, the direct ionization is higher than dissociative ionization [31]. Therefore, the estimated radical density is highly affected by direct ionization.

Table 2. Labels to be displayed in Figures 4a, 6a and 8a substituting each radical species to labels.

Radical Species Labels C a F b CF c CF2 d CF3 e C2F3 f CF4 g C2F4 h C3F4 i C3F5 j C4F6 k Ar l

The electron density of the plasma was measured via a cutoff probe, which measures electron density with a simple method using microwaves [32]. The electron density was measured by averaging 50 times to get accuracy. Self-bias was measured with a high- voltage probe connected to an oscilloscope (Tektronix, Beaverton, OR, USA). The 100:1 high-voltage probe was connected to the bottom electrode. The voltage waveform is obtained using the high-voltage probe. Self-bias is the DC offset of this waveform. Etch profiles were obtained with a scanning electron microscope (SEM) (Topcon, Tokyo, Japan). The coupon wafer was cut into cross-sections and images were obtained through SEM. Since the depth of the etched hole is visible in the cross-sectional images, the etch rate can be obtained by dividing the depth of the hole by the processing time. The poly-Si mask etch depth is obtained by subtracting poly-Si height after the etching from 200 nm, which is the thickness height of the mask before the etching. As the SiO2 etch rate, mask etch rate is obtained by dividing mask etch depth to process time. The SiO2–mask selectivity was calculated by dividing the SiO2 etch rate by the mask etch rate. In this paper, etch profiles were obtained by varying three external parameters, namely power, pulse on-time, and pulse off-time, where the pulse change is explained through pulse on/off time, not frequency and duty cycle. To analyze the effect on pulsed plasma by directly controlling the discharge on time and discharge off time of the plasma, rather than the frequency and duty cycle of the plasma, the experiment was conducted by controlling the RF power on/off time. The on/off control scheme is shown in Figure2. In the first experiment, the pulse-modulated RF power applied to discharge the plasma was varied from 100 to 400 W, while the other parameters were kept constant. In the second experiment, the plasma pulse on- and off-time were varied together while the power was kept constant. In the third experiment, only the pulse off-time was changed while the other parameters remained constant. In all experiments, the benchmarked conditions, which have high SiO2 etch rate and selectivity, were as follows: 400 W power, 2 ms on- time, 2 ms off-time, 1:1 C4F8/Ar ratio, and 20 mTorr pressure. The total on-time in all experiments was set to 10 min, and accordingly, the processing time was changed for each experimental condition. Materials 2021, 14, x FOR PEER REVIEW 5 of 13

power was kept constant. In the third experiment, only the pulse off-time was changed while the other parameters remained constant. In all experiments, the benchmarked con- Materials 2021, 14, x FOR PEER REVIEW 5 of 13 ditions, which have high SiO2 etch rate and selectivity, were as follows: 400 W power, 2 ms on-time, 2 ms off-time, 1:1 C4F8/Ar ratio, and 20 mTorr pressure. The total on-time in

Materials 2021, 14, 5036 allpower experiments was kept constant. was set In to the 10 third min, experiment, and accord onlyingly, the pulse the off-time processing was changed time5 of 13 was changed for eachwhile experimental the other parameters condition. remained constant. In all experiments, the benchmarked conditions, which have high SiO2 etch rate and selectivity, were as follows: 400 W power, 2 ms on-time, 2 ms off-time, 1:1 C4F8/Ar ratio, and 20 mTorr pressure. The total on-time in all experiments was set to 10 min, and accordingly, the processing time was changed for each experimental condition.

Figure 2. Pulse-modulated power scheme. FigureFigure 2.2.Pulse-modulated Pulse-modulated power power scheme. scheme. 3.3. Results

3. ResultsFigureFigure 3 3shows shows the the etch etch proﬁles, profiles, SiO SiO22 etchetch rates,rates, andand SiOSiO22 toto SiSi selectivityselectivity withwith increasingincreasing pulsedpulsed RFRF power.power. DepositionDeposition occursoccurs inin thethe 100100 WW condition.condition. EtchEtch raterate cannotcannot Figure 3 shows the etch profiles, SiO2 etch rates, and SiO2 to Si selectivity with bebe expressedexpressed here,here, soso ** isis used.used. In order to equalizeequalize the sumsum ofof thethe totaltotal powerpower inin thethe increasingetchingetching process,process, pulsed experimentsexperiments RF power. were Deposition performedperformed by occurs increasingincreasing in thethe processingproc 100essing W condition. timetime asas thethe Etch rate cannot bepowerpower expressed decreased.decreased. here, so * is used. In order to equalize the sum of the total power in the etching process, experiments were performed by increasing the processing time as the power decreased.

FigureFigure 3.3. SiO2 etching profile, proﬁle, etch etch rate, rate, and and selectivity selectivity of of SiO SiO2/Si2/Si at atincreasing increasing power. power. * Etch rate cannot be expressed.

Figure 3. SiO2 etching profile, etch rate, and selectivity of SiO2/Si at increasing power.

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At high power, the etch rate and selectivity appeared to be similar to other research At high power, the etch rate and selectivity appeared to be similar to other research [1,24], [1,24], and anisotropic etching occurred. However, as the power decreased, the etch rate and anisotropic etching occurred. However, as the power decreased, the etch rate decreased, decreased, the selectivity increased, and the etch profile became isotropic. At low RF the selectivity increased, and the etch profile became isotropic. At low RF power, deposition power, deposition occurred, despite the etching process. occurred, despite the etching process. These etching characteristics were analyzed by plasma diagnostic data. Figure 4 These etching characteristics were analyzed by plasma diagnostic data. Figure4 shows shows the plasma diagnostic data for radical densities, the absolute value of self-bias, and the plasma diagnostic data for radical densities, the absolute value of self-bias, and the the electron density as functions of RF power. As the RF power increased, the electron electron density as functions of RF power. As the RF power increased, the electron density densityand absolute and absolute self-bias, self-bias, which can which represent can represent the ion bombardment the ion bombardment energy, also energy, increased. also increased.In addition, In it addition, was confirmed it was thatconfirmed chemical that reactions chemical in thereactions plasma in werethe plasma increased were by increasedthe increase by in the electron increase density, in electron so that thedensi densityty, so of that the highthe densi massty molecules of the high decreased mass moleculeswhile that decreased of the low while mass moleculesthat of the increased. low mass Asmolecules most of increased. the low mass As most molecules of the have low massa higher molecules fluorine/carbon have a higher ratio (F/C fluorine/carbon ratio) than theratio high (F/C mass ratio) molecules, than the the high low mass molecules,molecules arethe low expected mass tomolecules deposit are fewer expected polymer to deposit layers than fewer the polymer high mass layers molecules. than the highWith mass these molecules. results, the With etch these characteristics results, the shown etch characteristics in Figure3 can shown be analyzed. in Figure The3 can ion be analyzed.bombardment The ion energy bombardment is high at energy high power, is high soat high that thepower, polymer so that layers the polymer deposited layers by x y depositedCxFy radicals by C canF beradicals sufficiently can be removed, sufficiently in additionremoved, to in the addition high density to the ofhigh the density low mass of themolecules low mass decreasing molecules the decreasing formation the of formation the polymer of the layers. polymer On layers. the other On the hand, other at hand, low atRF low power, RF thepower, ion bombardmentthe ion bombardment energy becomes energy insufficientbecomes insufficient to remove to C xremoveFy polymers. CxFy polymers.Additionally, Additionally, the increase the in increase polymers in withpolymers low F/Cwith ratios low F/C such ratios as C 3suchF4 and as CC33F45 andcan Ccontribute3F5 can contribute to the formation to the formation of the polymer of the po layers.lymer layers. Therefore, Therefore, the process the process changes changes from fromdeposition deposition to etching to etching with increasingwith increasing RF power. RF power.

Figure 4. Diagnostic datadata asas aa functionfunction of of input input power: power: (a ()a radical) radical densities, densities, (b )(b self-bias) self-bias and and averaged aver- agedself-bias, self-bias, and (andc) electron (c) electron density. density. The The parameters parameters for thesefor these experiments experiments were were a CW a CW RF RF power, a power,pressure a ofpressure 20 mTorr, of 20 and mTorr, a C4F and8/Ar a gasC4F ratio8/Ar gas of1:1. ratio of 1:1.

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In Figure5, the etch proﬁles, SiO etch rates, and SiO to Si selectivity are shown In Figure 5, the etch profiles, SiO2 etch2 rates, and SiO2 to Si2 selectivity are shown as asfunctions functions of pulse of pulse on/off on/off times, times, whichwhich were set were to CW, set to2 ms, CW, and 2 ms,4 ms. and For 4 an ms. equal For total an equal totalpower power in all inexperiments, all experiments, the etch the time etch timeincreased increased from from10 to 1020 tomin 20 when min when the power the power changedchanged from from CW CW to to pulsed. pulsed. In In the the CW CW mode, mode, the the micro-trench micro-trench phenomenon phenomenon occurred, occurred, butbut it was absent absent in in the the pulse pulse plasma plasma mode. mode. When When the thepower power changed changed from from CW to CW pulsed, to pulsed, thethe etch etch rate rate decreased decreased from from 90.2 90.2 to 39.85 to 39.85 nm/min, nm/min, and the and selectivity the selectivity increased increased from 8.05 from 8.05to 9.8. to 9.8.

FigureFigure 5. SiOSiO22 etchingetching profile, proﬁle, etch etch rate, rate, and and selectivity selectivity of SiO of SiO2/Si 2at/Si increasing at increasing on/off on/off times. times.

Figure 6 showsshows the the radical radical densities, densities, self-bias self-bias,, and electron and electron density density as functions as functions of the of thepulse pulse on/off on/off time, time,which which ranged ranged from 2 fromto 16 2ms to. The 16 ms. radical The densities, radical densities, self-bias, and self-bias, elec- and electrontron density density did not did obviously not obviously change change when whenthe pulse the pulseon-time on-time increased. increased. Referring Referring to toFigure Figure 5, 5the, the etch etch rate rate of the of thepulse pulse plasma plasma was washalf that half of that the of CW the plasma. CW plasma. The cause The of cause ofthe the different different etch etch characteristics characteristics between between CW plasma CW plasmaand pulse and plasma pulse was plasma that the was pulse that the pulseplasma plasma had half had the halfpulse the on-time pulse within on-time the withinsame period, the same meaning period, that meaningthe ion bombard- that the ion bombardmentment time of the time pulse of plasma the pulse was plasma half that was of the half CW that plasma. of the It CW should plasma. be noted It should that be notedions may that not ions exist may during not existthe off-time during considering the off-time their considering short lifetimes, their while short radicals lifetimes, can while radicalsexist during can existthe off-time during thedue off-time to their due long to lifetimes their long [18]; lifetimes hence, [18 radicals]; hence, formed radicals pas- formed sivation layers on the SiO2 and poly-Si surface during off-time. As the passivation layers passivation layers on the SiO2 and poly-Si surface during off-time. As the passivation layers protectingprotecting the wafer were the same with bothboth plasma types, the reduced ion ion bombard- bombardment withment thewith pulse the plasmapulse plasma resulted resulted in decreased in decreased surface etchingsurface comparedetching compared to CW plasma. to CW Thus, pulse-modulatedplasma. Thus, pulse-modulated plasma had higher plasma selectivity had higher than selectivity CW plasma. than CW plasma. In Figure7, the etch proﬁles, SiO 2 etch rates, and SiO2 to Si selectivity are shown as functions of pulse off-time, which varied from 2 to 11.3 ms. The bottom of the etched feature was narrower than the top feature at the 11.3 ms off-time, giving a triangular etch proﬁle. As the off-time decreased, the bottom feature width increased, leading to anisotropic etching. Additionally, the etch rate increased from 4.3 to 45.05 nm/min, and the selectivity decreased from 10.98 to 9.59.

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Figure 6. Diagnostic data as a function of on/off time: (a) radical densities, (b) self-bias and averaged self-bias, and (c) electron density. The parameters for these experiments were a RF power of 400 W, a pressure of 20 mTorr, a C4F8/Ar gas ratio of 1:1, and a pulse off-time equal to the on-time.

In Figure 7, the etch profiles, SiO2 etch rates, and SiO2 to Si selectivity are shown as functions of pulse off-time, which varied from 2 to 11.3 ms. The bottom of the etched feature was narrower than the top feature at the 11.3 ms off-time, giving a triangular etch Figureprofile. 6. AsDiagnostic the off-time data as decr a functioneased, of the on/off bottom time: feature (a) radical width densities, increased, (b) self-bias leading and to averaged aniso- Figure 6. Diagnostic data as a function of on/off time: (a) radical densities, (b) self-bias and aver- self-bias,tropic etching. and (c) electronAdditionally, density. Thethe parametersetch rate increased for these experiments from 4.3 to were 45.05 a RF nm/min, power of 400and W, the a aged self-bias, and (c) electron density. The parameters for these experiments were a RF power of pressureselectivity of 20decreased mTorr, aC from4F8/Ar 10.98 gas to ratio 9.59. of 1:1, and a pulse off-time equal to the on-time. 400 W, a pressure of 20 mTorr, a C4F8/Ar gas ratio of 1:1, and a pulse off-time equal to the on-time.

Figure 7. SiO2 etching proﬁle, etch rate, and selectivity of SiO2/Si at decreasing off-times.

In Figure8, the radical densities, electron density, and self-bias are shown with an off-time ranging from 2 to 18 ms. Here, the on-time was ﬁxed to 2 ms. The C, F, CF2, and CF3 radical densities increased, while the C2F4 and C3F5 radical densities decreased with decreasing off-time. The cause of these results was that the low mass radicals combined to form high mass radicals during the plasma power off-time. These high mass products

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Figure 7. SiO2 etching profile, etch rate, and selectivity of SiO2/Si at decreasing off-times.

In Figure 8, the radical densities, electron density, and self-bias are shown with an off-time ranging from 2 to 18 ms. Here, the on-time was fixed to 2 ms. The C, F, CF2, and CF3 radical densities increased, while the C2F4 and C3F5 radical densities decreased with Materials 2021, 14, 5036 decreasing off-time. The cause of these results was that the low mass radicals9 ofcombined 13 to form high mass radicals during the plasma power off-time. These high mass products enhanced the passivation layer formation. The self-bias did not change much when the off-timeenhanced decreased, the passivation while layer the formation.time-averaged The self-bias self-bias did increased not change rapidly much whenas the the off-time decreased.off-time decreased, The self-bias while is the obtained time-averaged as the self-biasVdc of the increased on-time rapidly voltage as wave the off-time form; on the dc otherdecreased. hand, The the self-bias averaged is obtainedself-bias as is theobtained Vdc of theas the on-time V of voltage the total wave voltage form; onwave the form. Therefore,other hand, a thedecreasing averaged averaged self-bias is self-bias obtained do ases the not V dcleadof theto decreasing total voltage ion wave bombardment form. energy.Therefore, These a decreasing results show averaged that self-biasthe energy does of not the lead sputtered to decreasing ions was ion the bombardment same, but as the sputteringenergy. These time results increased show thatwith the decreasing energy of theoff-time, sputtered the ions etch was rate the increased same, but asand the selectivitythe sputtering decreased. time increased In the case with of decreasing electron off-time,density, it the affected etch rate the increased ion flux and on the the wafer surface.selectivity The decreased. electron Indensity the case increased of electron with density, the decreasing it affected theoff-time, ion ﬂux representing on the wafer an ion fluxsurface. increase. The electron Therefore, density both increased the ion with sputte the decreasingring rate and off-time, the representingchemical etching an ion of the ﬂux increase. Therefore, both the ion sputtering rate and the chemical etching of the radicals increased as the off-time decreased. radicals increased as the off-time decreased.

Figure 8. Diagnostic data as a function of off-time: (a) radical densities, (b) self-bias and averaged Figure 8. Diagnostic data as a function of off-time: (a) radical densities, (b) self-bias and averaged self-bias,self-bias, andand ((cc)) electron electron density. density. The The parameters parameters for for this this experiment experiment were were a RF powera RF power of 400 of W, 400 a W, a pressure of 20 mTorr, a C F /Ar gas ratio of 1:1, and a pulse on-time of 2 ms. pressure of 20 mTorr, a 4C48F8/Ar gas ratio of 1:1, and a pulse on-time of 2 ms. In the plasma process, not only are the etch rate and the selectivity important but so is theIn shape the plasma of the trench. process, As not can beonly seen are in the the etch top-left rate panel and the of Figure selectivity5, the edgeimportant of the but so isbottom the shape feature ofwas the etchedtrench. lower As ca thann be the seen center in the of the top-left bottom panel feature of inFigure the CW 5, condition.the edge of the bottomThis phenomenon feature was is etched called micro-trenchinglower than the center [33], and of the its cause bottom has feature been reported in the CW [34 ].condition. It is also known that pulse-modulated plasma reduces the micro-trench effect [34]; Figure9 represents this mechanism. Positive ions are accelerated in the plasma sheath, anisotropi- cally toward the substrate, while electrons isotropically move in the plasma sheath. As a result, the bottom of the trench is positively charged because of the anisotropic positive ions, and the side of the trench is negatively charged from the isotropic electrons. Charge Materials 2021, 14, x FOR PEER REVIEW 10 of 13

Materials 2021, 14, x FOR PEER REVIEW 10 of 13

This phenomenon is called micro-trenching [33], and its cause has been reported [34]. It is Thisalso phenomenon known that pulse-modulatis called micro-trenchinged plasma [33]reduces, and theits causemicro-tr hasench been effect reported [34]; [34]. Figure It is 9 alsorepresents known thatthis pulse-modulatmechanism. edPositive plasma ions reduces are theaccelerated micro-trench in theeffect plasma [34]; Figure sheath, 9 representsanisotropically this towardmechanism. the substrate, Positive whileions electronsare accelerated isotropically in the move plasma in the sheath, plasma anisotropicallysheath. As a result,toward thethe bottomsubstrate, of whilethe tren electronsch is positivelyisotropically charged move becausein the plasma of the Materials 2021, 14, 5036 10 of 13 sheath.anisotropic As a positiveresult, theions, bottom and the of sidethe trenof thech trenchis positively is negatively charged charged because from of the the anisotropicisotropic electrons. positive Chargeions, and differences the side between of the trenchthe sides is andnegatively bottom chargedof the trench from create the isotropican electric electrons. field, which Charge bends differences the ions towardbetween the the side sides walls, and resultingbottom of in the micro-trenching. trench create andifferencesIn electric the etch field, profiles between which from the bends sides 2 and the and 4 msions bottom pulse toward ofplas the thema trench side in Figure walls, create 5, resulting anno electric micro-trenching in field,micro-trenching. which appears. bends IntheIt the has ions etch been toward profiles reported the from sidethat 2 and the walls, 4ions ms resulting pulselosing plas directionality inma micro-trenching. in Figure during 5, no micro-trenchingthe In thepower etch off-time profiles appears. causes from It2the has and disappearance been 4 ms reported pulse plasma of that micro-trenching the in ions Figure losing5, no [34]; directionality micro-trenching the time-varying during appears. self-biasthe power It hasdata off-time been in the reported causespresent thethatwork disappearance the verified ions losing this. of directionality micro-trenching during [34]; the the power time-varying off-time self-bias causes the data disappearance in the present of workmicro-trenching verified this. [34 ]; the time-varying self-bias data in the present work verified this.

Figure 9. Micro-trench etching mechanism with continuous wave plasma and pulse-modulated plasma. FigureFigure 9. 9. Micro-trenchMicro-trench etching mechanismmechanism with with continuous continuous wave wave plasma plasma and and pulse-modulated pulse-modulatedplasma. plasma. Figure 10 showsshows thethe time-varyingtime-varying inputinput voltagevoltage ofof thethe poweredpowered electrodeelectrode inin pulsepulse plasmaplasmaFigure asas a a10 functionfunction shows ofofthe off-time.off-time. time-varying TheThe VVdcdc input of the voltage data is theof the self-bias. powered The ionselectrode are accelerated in pulse by the V dc;; since since V Vdc isis almost almost zero zero during during pulse pulse off-time off-time,, ions do not accelerate duringduring thethe plasma asdc a functiondc of off-time. The Vdc of the data is the self-bias. The ions are accelerated off-time. Therefore, the positive ions move isotropicallyisotropically likelike electrons,electrons, whichwhich reducesreduces thethe by the Vdc; since Vdc is almost zero during pulse off-time, ions do not accelerate during the micro-trenching phenomenon [34]. off-time.micro-trenching Therefore, phenomenon the positive [34]. ions move isotropically like electrons, which reduces the micro-trenching phenomenon [34].

Figure 10. Input voltagevoltage ofof thethe poweredpowered electrodeelectrode in in pulse-modulated pulse-modulated plasma plasma at at increasing increasing off-time. off- time. FigureReferring 10. Input voltage to Figures of the4 poweredand8, the electrode averaged in pulse-modulated self-bias from plasma 100 W at power increasing with off- 2 ms time. off-timeReferring is −160 to V, Figures while that4 and from 8, the 400 averaged W power self-bias with 11 from ms off-time100 W power is −120 with V, showing2 ms off- that ion acceleration is lower at 400 W. Therefore, this condition should produce less etching timeReferring is −160 V, to while Figures that 4 andfrom 8, 400 the W averaged power with self-bias 11 ms from off -time100 W is power −120 V, with showing 2 ms off- that thanion acceleration the 100 W condition. is lower at However, 400 W. Therefore, Figure4 also this shows condition that theshould 400 Wproduce condition less is etching more timedeeply is − etched160 V, while because that the from self-bias 400 W value, power which with means11 ms theoff-time on-time is − self-bias120 V, showing only, is that high ionat −acceleration675 V. The is shape lower of at the 400 etch W. profileTherefore, is not this anisotropic, condition should but a triangle. produce Onless the etching other hand, the 100 W condition shows deposition rather than etching. Figure 11 shows the mechanism of triangular etching and deposition. A passivation layer deposits on the entire wafer surface due to radicals. As the radicals survive during power off-time because of their long lifetime, the thickness of the passivation layer increases under the 11 ms off-time condition. During the on-time, ions remove the passivation layer with high ion bombardment energy. Since the accelerating ions move straight, though, the sides of the passivation layer are not sufficiently removed, and deposition is repeated during the next Materials 2021, 14, x FOR PEER REVIEW 11 of 13

than the 100 W condition. However, Figure 4 also shows that the 400 W condition is more deeply etched because the self-bias value, which means the on-time self-bias only, is high at −675 V. The shape of the etch proﬁle is not anisotropic, but a triangle. On the other hand, the 100 W condition shows deposition rather than etching. Figure 11 shows the mechanism of triangular etching and deposition. A passivation layer deposits on the entire wafer surface due to radicals. As the radicals survive during power off-time because of their long lifetime, the thickness of the passivation layer increases under the 11 ms off-time condition. During the on-time, ions remove the passivation layer with high ion bombard- Materials 2021, 14, 5036 ment energy. Since the accelerating ions move straight, though, the sides11 of of 13the passivation layer are not sufficiently removed, and deposition is repeated during the next power off-time. As a result, the side walls gradually narrow to form an etched triangle. In thepower 100 off-time.W condition, As a result,the passivation the side walls layer gradually is also formed, narrow to but form as anthe etched off-time triangle. is short, the thicknessIn the 100 of W the condition, layer is the smaller. passivation Then layerduring is also the formed,power on-time, but as the the off-time accelerating is short, ions do notthe remove thickness the of passivation the layer is layer smaller. because Then duringthe self-bias the power is weak on-time, at almost the accelerating−200 V. Therefore, althoughions do not the remove averaged the passivationself-bias is higher, layer because when the the self-bias self-bias is is weak weak at during almost −on-time,200 V. etch- ingTherefore, does not although occur, but the averagedrather deposition. self-bias is higher, when the self-bias is weak during on-time, etching does not occur, but rather deposition.

Figure 11. Etch profile mechanism in two different conditions: (a) RF power of 400 W and off-time Figureof 11 ms 11. giving Etch profile a triangular mechanism etch profile, in two and different (b) RF power conditions: of 100 W (a and) RF off-time power ofof 2400 ms W giving and aoff-time ofdeposition 11 ms giving profile. a triangular etch profile, and (b) RF power of 100 W and off-time of 2 ms giving a deposition profile. 4. Conclusions 4. ConclusionsTo interpret pulse-modulated plasma etching, a SiO2 wafer with a poly-Si patterned mask was etched under various plasma conditions while the key plasma parameters—radical speciesTo density,interpret self-bias, pulse-modulated and electron plasma density—were etching, measured. a SiO2 wafer The chemical with a poly-Si properties patterned maskof the was plasma etched were interpretedunder various by the plasma radical densities,conditions while while the physicalthe key propertiesplasma parameters— of the radicalplasma species were interpreted density, byself-bias, self-bias and electronelectron density. density—were Additionally, measured. using all plasmaThe chemical propertiesparameters, of the the shapes plasma of the were etch interpreted profiles were by investigated. the radical Results densities, showed while that whenthe physical propertiesthe RF power of the increased, plasma the were SiO interpreted2 etch rate also by increasedself-bias and because electron of the density. increasing Additionally, self- usingbias. Additionally,all plasma parameters, as the power on/offthe shapes time increased,of the etch the profiles micro-trenching were investigated. phenomenon Results disappeared because negative ions neutralized the wafer surface. When the power off-time showed that when the RF power increased, the SiO2 etch rate also increased because of increased, the etch rate decreased since the high mass radicals increased and the averaged theself-bias increasing decreased. self-bias. It was alsoAdditionally, found that aas triangular the power etch profileon/off occurred time increased, due to the the low micro- trenching phenomenon disappeared because negative ions neutralized the wafer surface. Vdc during the power off-time and the high Vdc during the power on-time. WhenAs the a fundamentalpower off-time study increased, to understand the etch pulse rate plasma decreased characteristics since the high and etchingmass radicals increasedproperties, and the resultsthe averaged here can self-bias be examined decreased. to clarify It how was plasma also found parameters that a change triangular in etch pulse-modulated plasma and to analyze SiO2 etching results using the parameters. This fundamental understanding will help to interpret more complex experimental plasma etching properties.

Author Contributions: Data curation, C.C.; Formal analysis, C.C., S.K., Y.L., J.L. and S.Y.; Project

administration, C.C.; Supervision, K.Y. and S.Y.; Writing—original draft, C.C.; Writing—review & editing K.Y., S.K., Y.L., J.L. and S.Y. All authors have read and agreed to the published version of the manuscript. Materials 2021, 14, 5036 12 of 13

Funding: This research was supported by a National Research Council of Science & Technology (NST) grant from the Korean government (MSIP) (No. CAP-17-02-NFRI), by the Industrial Strategic Technology Development Program-Next Generation Semiconductor R&D (20010412, Development of Metal Oxide Carbon Layer Strip Process and Commercial Equipment for EUV Mask) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea), by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the MOTIE of the Republic of Korea (No. 20172010105910), by the MOTIE (20009818, 20010420) and KSRC (Korea Semiconductor Research Consortium) support program for the development of future semiconductor devices, by a Korea Institute for Advancement of Technology (KIAT) grant funded by the Korean government (MOTIE) (P0008458, The Competency Development Program for Industry Specialist), and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF- 2020R1A6A1A03047771). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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