Characterization of SiO2 Plasma Etching with Perfluorocarbon (C4F8 and C6F6) and Hydrofluorocarbon (CHF3 and C4H2F6) Precursors for the Greenhouse Gas Emissions Reduction

This paper proposes the use of environmentally friendly alternatives, C6F6 and C4H2F6, as perfluorocarbon (PFC) and hydrofluorocarbon (HFC) precursors, respectively, for SiO2 plasma etching, instead of conventional precursors C4F8 and CHF3. The study employs scanning electron microscopy for etch profile analysis and quadrupole mass spectrometry for plasma diagnosis. Ion bombardment energy at the etching conditions is determined through self-bias voltage measurements, while densities of radical species are obtained using quadrupole mass spectroscopy. The obtained results compare the etch performance, including etch rate and selectivity, between C4F8 and C6F6, as well as between CHF3 and C4H2F6. Furthermore, greenhouse gas (GHG) emissions are evaluated using a million metric ton of carbon dioxide equivalent, indicating significantly lower emissions when replacing conventional precursors with the proposed alternatives. The results suggest that a significant GHG emissions reduction can be achieved from the investigated alternatives without a deterioration in SiO2 etching characteristics. This research contributes to the development of alternative precursors for reducing global warming impacts.


Introduction
The semiconductor industry has extensively embraced plasma technology for semiconductor device manufacturing [1,2].In the etching process, plasma has become crucial for nanoscale material processing, enabling precise and deep etch profiles to meet the requirements of scaling down of transistor feature sizes [3].As a results, plasma processing has become indispensable for the production of next-generation semiconductor devices, including high-aspect-ratio contact (HARC) hole.
Plasma etching utilizes various gases or gas mixture tailored to the target materials, such as semiconductors [4,5], metals [6][7][8], and dielectrics [9][10][11][12].For dielectrics, such as SiO 2 and Si 3 N 4 , fluorocarbon-based molecules, such as C 4 F 8 and CHF 3, are widely adopted due to their polymerizing properties, which are particularly advantageous for achieving anisotropic etch profiles.During the etching process, fluorocarbon films passivate the sidewalls of the dielectric trench, preventing them from reacting with reactive etchants.
However, the energetic plasma ion bombardment renders the fluorocarbon film unable to protect the trench bottom, resulting in an anisotropic etch profile [13][14][15].Since anisotropy is a crucial characteristic in semiconductor manufacturing, large quantities of perfluorocarbon (PFC) and hydrofluorocarbon (HFC) precursors, such as C 4 F 8 and CHF 3 , are extensively utilized in the semiconductor industry.
However, the utilization of the PFC and HFC precursors is subject to strict regulations due to their significant greenhouse effect [16,17].Global warming potential (GWP) is a metric used to quantify the relative warming impact of greenhouse gases (GHGs) on the Earth's climate system over a specified time horizon, typically, 20, 100, or 500 years, relatively to that of CO 2 [18].The GWP 100 (GWP over a 100-year time horizon) of C 4 F 8 and CHF 3 is reported as 9540 and 12,400, respectively, indicating that these precursors have a greenhouse effect over 10,000 times stronger than CO 2 [19].Given the substantial global efforts to combat global warming in recent decades [20,21], the emission of these high-GWP gases is facing increasingly stringent restrictions in the industry [22].
In this context, researchers worldwide have been actively engaged in developing alternative plasma etching precursors that process low GWPs while demonstrating comparable etch performance to conventional ones.Sung et al. investigated the etching characteristics of SiO 2 using the C 6 F 6 /Ar/O 2 gas mixture in both capacitively coupled and inductively coupled plasma etching systems, comparing the etch results obtained from these different plasma systems [23].Kim et al. studied SiO 2 etching using C 4 H 3 F 7 O isomers and compared them with C 4 F 8 through X-ray photoelectron spectroscopy, quadrupole mass spectroscopy, and scanning electron microscopy [24].Furthermore, Lim et al. compared the etch results with a low-GWP precursor, C 6 F 12 O, to those with the conventional precursor, CF 4 [25].Although many papers, including the aforementioned ones, have documented the development of alternative low-GWP precursors in the field of semiconductor manufacturing [26][27][28][29][30], there are still numerous potential precursors that need to be investigated regarding their qualification as promising alternatives to conventional high-GPW precursors.
In this work, we propose the use of environmentally friendly alternatives, C 6 F 6 and C 4 H 2 F 6 , as PFC and HFC precursors, respectively, for SiO 2 plasma etching, in lieu of the conventional precursors C 4 F 8 and CHF 3 , which are the most representative etching gases for dielectric etching [3].C 4 F 8 and CHF 3 are selected as the reference precursors for PFC and HFC precursors, respectively, instead of other alternative low-GWP precursors, such as C 4 F 6 (GWP < 1), to excavate additional alternatives that possibly show different etching performance from C 4 F 6 , which would provide a wide range of options to, for example, process engineers.Among various candidates as an alternative low-GWP precursor, the investigation into C 6 F 6 and C 4 H 2 F 6 is reported here.It should also be noted that CHF 3 , an H-containing fluorocarbon precursor, is normally used for nitride etching.To the best of our knowledge, more than one material are etched simultaneously in an etching process and specifically in dielectric etching, etching selectivity of nitride-to-oxide or vice versa is essential.We thus consider that evaluating the etching performance of HFC precursors, such as CHF 3, on SiO 2 is quite necessary.Additionally, we also consider that an HFC precursor should be compared with another HFC precursor, not a perfluorocarbon precursor like C 4 F 8 , to reduce any complexity that might be induced by H atoms included in HFC precursors but not in PFC ones.A low-GWP candidate, C 4 H 2 F 6 , was therefore compared with CHF 3 to determine whether C 4 H 2 F 6 was appropriate as an alternative to CHF 3 or not.
We employ scanning electron microscopy and plasma diagnostics to analyze the etching results.Ion bombardment energy is determined by measuring self-bias voltages, while densities of various radical species are obtained through quadrupole mass spectroscopy.The obtained results include a comprehensive comparison of the etch performance, encompassing etch rate and selectivity, between C 4 F 8 and C 6 F 6 , as well as between CHF 3 and C 4 H 2 F 6 .Furthermore, we evaluate the greenhouse gas emissions using a million metric tons of carbon dioxide equivalent, which demonstrates significantly lower emissions when substituting conventional precursors with the proposed alternatives.The experimental setup and procedure details are provided in the subsequent sections.

Processing Chamber Setup
The schematic diagram of the plasma chamber used for SiO 2 etching is presented in Figure 1.Radiofrequency (RF) power of 13.56 MHz is applied to the electrode, which has a diameter of 208 mm covered with a 2 mm-thick ceramic plate, through an impedance matching box.While being injected into the chamber via a 1/4" SUS tube, the flow rate of Ar is controlled by a mass flow rate controller (MFC) (1179A611522K, MKS Instruments Inc., Andover, MA, USA) and that of PFC and HFC precursors is controlled by the different model of an MFC (M3030VA, Line-Tech, Daejeon, Republic of Korea).The investigated PFC precursors are octafluorocyclobutane (C 4 F 8 ) and hexafluorobenzene (C 6 F 6 ), while the investigated HFC precursors are trifluoromethane (CHF 3 ) and hexafluoro-isobutylene (C 4 H 2 F 6 ).The specific material information on the investigated precursors is summarized in Table 1.when substituting conventional precursors with the proposed alternatives.The experimental setup and procedure details are provided in the subsequent sections.

Processing Chamber Setup
The schematic diagram of the plasma chamber used for SiO2 etching is presented in Figure 1.Radiofrequency (RF) power of 13.56 MHz is applied to the electrode, which has a diameter of 208 mm covered with a 2 mm-thick ceramic plate, through an impedance matching box.While being injected into the chamber via a 1/4" SUS tube, the flow rate of Ar is controlled by a mass flow rate controller (MFC) (1179A611522K, MKS Instruments Inc., Andover, MA, USA) and that of PFC and HFC precursors is controlled by the different model of an MFC (M3030VA, Line-Tech, Daejeon, Republic of Korea).The investigated PFC precursors are octafluorocyclobutane (C4F8) and hexafluorobenzene (C6F6), while the investigated HFC precursors are trifluoromethane (CHF3) and hexafluoro-isobutylene (C4H2F6).The specific material information on the investigated precursors is summarized in Table 1.The flow rate of Ar and a fluorocarbon precursor are fixed at 20 sccm and 10 sccm, respectively.It should be noted that the canisters containing C6F6 and C4H2F6 are heated to 130 °C in order to vaporize them, and are in the liquid phase at room temperature.The RF power applied ranges from 300 W to 500 W and the pressure varies from 10 mTorr to 30 mTorr (Table 2).When the RF power varies, the pressure is regulated by a throttle valve; when the pressure changes, the RF power is fixed at 400 W. The chamber is evacuated by a turbomolecular pump, with an oil rotary pump serving as a backing pump, to achieve a base pressure below 10 −4 Torr.The flow rate of Ar and a fluorocarbon precursor are fixed at 20 sccm and 10 sccm, respectively.It should be noted that the canisters containing C 6 F 6 and C 4 H 2 F 6 are heated to 130 • C in order to vaporize them, and are in the liquid phase at room temperature.The RF power applied ranges from 300 W to 500 W and the pressure varies from 10 mTorr to 30 mTorr (Table 2).When the RF power varies, the pressure is regulated by a throttle valve; when the pressure changes, the RF power is fixed at 400 W. The chamber is evacuated by a turbomolecular pump, with an oil rotary pump serving as a backing pump, to achieve a base pressure below 10 −4 Torr.

Plasma Diagnosis
We utilized a high-voltage (HV) probe (P5100, Tektronix, Inc., Beaverton, OR, USA) and a residual gas analyzer (RGA) (SRS-200, Stanford Research Systems, Sunnyvale, CA, USA) to analyze the SiO 2 etching results obtained with different precursors.The HV probe is connected to the transmission line responsible for RF input power positioned between the electrode and the matching box.This measures the self-bias voltage of the electrode, providing an estimation of the ion bombarding energy during plasma processing [32].RGAs are widely employed plasma diagnostics tools that offer information about gas-phase species in plasmas [33][34][35][36].With an RGA, gas-phase atoms or molecules ionized by the collision with 70 eV energetic electrons and then filtered based on their mass as they pass through a quadrupole mass filter operating in a cross-linked manner [37].This filtering process enables the RGA to generate a signal intensity profile as a function of atomic mass [38].The RGA, located on a chamber port, has a separate vacuum space from the main chamber via an orifice with the diameter of approximately 100 µm, which helps in maintaining the RGA pressure at approximately two orders of magnitude lower than the main chamber pressure [39].The orifice is considered to directly contact to the plasma diagnosed as the chamber wall.This dedicated vacuum system allows gas-phase atoms and molecules to traverse the RGA without experiencing collisions with background gas, thereby increasing data reliability.
It should be noted that the impact of electrons of 70 eV would result in a measurement error from the dissociative ionization reaction of a parent molecule [40]; specifically, if C 4 F 8 collides with 70 eV electrons, CF 2 + ions can be generated and they are almost indistinguishable from the CF 2 + ions generated in plasma in QMS analysis.Reducing the noise from dissociative ionization will be addressed in future work.

Sample Preparation
We prepared SiO 2 coupon wafers with a masking layer of amorphous carbon (ACL) that has bar-type deep-trench patterns.Figure 2 presents the cross-sectional view of the mask pattern before etching with a width of 170 nm, a depth of 1500 nm, and a pitch of 530 nm.This cross-sectional image of the not-etched SiO 2 sample serves as a reference for comparing the images of the samples after etching.All the images presented in this paper were acquired using a scanning electron microscope (SEM) (SU7000, Hitachi, Ltd., Tokyo, Japan).

SiO2 Etching with PFC Precursors (C4F8 and C6F6)
Figure 3 illustrates the etch results obtained with different PFC precursors (C4F8 and C6F6) at varying RF power levels ranging from 300 W to 500 W, while maintaining a fixed pressure of 20 mTorr.The resulting etch rates and selectivity for the C4F8 and C6F6 cases are summarized in the right-most column of Figure 3.The etch rate of ACL is calculated by measuring the difference between the lengths from the ACL-SiO2 border line to the top surface of ACL before and after etching, divided by the etching duration of 10 min.Similarly, the etch rate of SiO2 is determined by dividing the etch depth, defined as the length between the border line and the minimum position of the etched SiO2 trenches, by the etching duration of 10 min.The etch selectivity is then obtained by dividing the SiO2 etch rate by the ACL etch rate.Regarding the SiO 2 etch rate, there is no significant difference observed between C 4 F 8 and C 6 F 6 , although the difference becomes more pronounced with increasing RF power.It is noteworthy that, despite the significantly higher GWP 100 of C 4 F 8 (9540) compared with C 6 F 6 (7) by over 1000 times [23], the similar etch rate implies that adopting the low-GWP alternative precursor can result in a substantial reduction in the greenhouse effect without compromising etch throughput.
As depicted in Figure 3, it can be observed that the SiO 2 and ACL etch rates with C 4 F 8 increase almost linearly with increasing RF power, leading to a nearly constant selectivity.On the other hand, with C 6 F 6 , it is notable that the ACL etch rate abruptly increases with increasing RF power while the SiO 2 etch rate shows a slight linear increase.Consequently, this rapid increase in ACL etch rate causes the selectivity to drop below 1.Such low selectivity with C 6 F 6 can be attributed to the mask faceting, which worsens with higher RF power.Comparing the etch profiles obtained with C 4 F 8 and C 6 F 6 , it is evident that mask faceting is more pronounced with C 6 F 6 than with C 4 F 8 .For both C 4 F 8 and C 6 F 6 cases, it is commonly observed that, as RF power increases, mask faceting tends to deteriorate.This deterioration results in bowing of the mask, referring to a concave etch profile caused by the lateral, as well as vertical etching of the trench sidewall due to ion bombardment on the facet [3].According to material sputtering theory, the sputtering yield is highest at incident angles ranging from 40 • to 60 • , rather than 90 • [41].This behavior is clearly reflected in the etch results shown in Figure 3. Comparing the transition of mask height and faceting, it is observed that, while the mask height barely changes, the mask faceting evolves rapidly with increasing RF power.Mask faceting induces mask bowing, which eventually affects the SiO 2 etch profile as the concave mask profile expands across the mask-SiO 2 border.Such mask faceting occurs more intensely with C 6 F 6 than with C 4 F 8 , as presented in Figure 3. Notably, an increase in RF power from 400 W to 500 W resulted in a significant rise in the mask etch rate.This could be attributed to the left and right mask facets meeting during etching, forming a spear-like mask head.As discussed earlier, ion bombardment is more effective on a tilted surface compared with a flat surface, and thus the spear-like mask head created by the joint of the left and right facets could lead to the rapid mask etching.
To analyze the differences in etching behavior between C 4 F 8 and C 6 F 6 precursors, we investigated changes in ion bombardment energy and radical density.In a capacitively coupled plasma etching system, such as the one used in this work, where the samples are loaded on the bottom electrode, also as in this work, the powered electrode (bottom electrode) voltage primarily consists of a negative DC component that accelerates the bombarding ions, allowing them to reach even the bottom of deep trenches [41].Therefore, it is crucial to measure the self-bias voltage of the powered electrode to determine the ion bombardment energy under specific plasma conditions.We employed a high-voltage probe to measure the self-bias voltage on the powered electrode.
Figure 4a shows the measured self-bias voltage (left y-axis) and the CFx/F (x = 1, 2, 3) ratio (right y-axis) as a function of applied RF power using C 4 F 8 and C 6 F 6 .The CFx/F ratios are calculated by using the FC radical signals represented in Figure 4b where CFx refers to the sum of the CF, CF 2 , and CF 3 signal and is divided by the F signal at different RF power conditions.Note that the parameter, a CFx/F ratio, is adopted to interpret the chaotic contribution of numerous radical species to etch results in a simple manner.More detailed analysis of the measured radical densities will be followed separately.It can be observed that, as the RF power increases, the absolute value of the self-bias voltage increases, indicating an increase in ion bombardment energy.Notably, there is little difference between the self-bias voltages with C 4 F 8 and C 6 F 6 , suggesting that the ion bombardment energy is very similar for both precursors.Furthermore, the absolute value of a self-bias voltage is known to be proportional to the plasma electron density.In CCP at a constant RF power, the dominant current path in the bulk plasma is the conduction current by plasma electrons so that, if the electron density decreases, the voltage between plasma and the electrode increases to meet the constant input power, a product of current and voltage [41].In this context, the almost identical self-bias voltages at different RF powers may indicate that the ion density arriving at the etched surface is also very similar.In this case, the etch results are primarily influenced by the density of radical species [42].As shown in Figure 4a, the CFx/F ratios at different RF powers are higher with C 4 F 8 than C 6 F 6 , which indicates that polymerization occurs more with C 4 F 8 than C 6 F 6 .This might lead to the better passivation of trench sidewalls during etching with C 4 F 8 , as shown in Figure 3.
In plasma etching, the synergetic effect between ions and radicals plays a significant role [43].Therefore, measuring the radical composition is just as essential as investigating the ion bombardment energy.Radicals are generated through the dissociation of parent molecules by electron impact in plasma etching.Complex molecules like C 4 F 8 undergo dissociation into various species with a wide range of molecular masses, including light molecules, such as CF and CF 2 , as well as heavy molecules, such as C 2 F 3 and C 3 F 5 [44].This wide range of species has led to the extensive use of mass spectroscopy for investigating FC plasma chemistry [38].
As depicted in Figure 4b, while low-mass radicals, such as F, CF, and CF 2 , show no significant dependence on the power variation, a notable difference in high-mass radical signal between C 4 F 8 and C 6 F 6 is observed.Several high-mass species, such as C 2 F 4 , C 2 F 5 , and C 3 F 5 , exhibit higher densities with C 4 F 8 compared with C 6 F 6 , primarily due to the dominant dissociation pathways of C 4 F 8 in plasma [44].Apart from these radicals, highmass species are observed to be predominantly generated through the dissociation of C 6 F 6 .This may contribute to intense faceting of the ACL mask during etching with C 6 F 6 by inducing the creation of high-mass ions that could transfer larger momentum to the ACL mask compared with low-mass species ions.The identification of the existence of such high-mass ions, however, is not conducted in this work due to the unverified ion measurement ability of the QMS equipment used.The modification of the equipment for reliability ion density measurement will be addressed in future work.arriving at the etched surface is also very similar.In this case, the etch results are primarily influenced by the density of radical species [42].As shown in Figure 4a, the CFx/F ratios at different RF powers are higher with C4F8 than C6F6, which indicates that polymerization occurs more with C4F8 than C6F6.This might lead to the better passivation of trench sidewalls during etching with C4F8, as shown in Figure 3.In plasma etching, the synergetic effect between ions and radicals plays a significant role [43].Therefore, measuring the radical composition is just as essential as investigating the ion bombardment energy.Radicals are generated through the dissociation of parent Figure 5 illustrates the etch results obtained with different PFC precursors (C 4 F 8 and C 6 F 6 ) at varying pressure ranging from 10 mTorr to 30 mTorr, while maintaining a fixed RF power of 400 W. The resulting etch rate and selectivity for the C 4 F 8 and C 6 F 6 cases are summarized in the right-most column of Figure 5.When etching with C 4 F 8 , a notable trend is observed where the SiO 2 etch rate increases with increasing pressure, while the ACL etch rate decreases.As a result, there is a significant increase in selectivity.Conversely, when using C 6 F 6 , the SiO 2 etch rate initially rises with increasing pressure but eventually starts to decrease at higher pressure, while the ACL etch rate shows a linear increase.Consequently, this leads to a decrease in selectivity.These findings suggest that the optimal SiO 2 etching window for C 6 F 6 exists at lower pressures than that of C 4 F 8 , as the etch rate and selectivity with C 6 F 6 can be comparable to those achieved with C 4 F 8 .
C6F6) at varying pressure ranging from 10 mTorr to 30 mTorr, while maintaining a fixed RF power of 400 W. The resulting etch rate and selectivity for the C4F8 and C6F6 cases are summarized in the right-most column of Figure 5.When etching with C4F8, a notable trend is observed where the SiO2 etch rate increases with increasing pressure, while the ACL etch rate decreases.As a result, there is a significant increase in selectivity.Conversely, when using C6F6, the SiO2 etch rate initially rises with increasing pressure but eventually starts to decrease at higher pressure, while the ACL etch rate shows a linear increase.Consequently, this leads to a decrease in selectivity.These findings suggest that the optimal SiO2 etching window for C6F6 exists at lower pressures than that of C4F8, as the etch rate and selectivity with C6F6 can be comparable to those achieved with C4F8.Regarding mask faceting, the etch profiles of the mask with both C4F8 and C6F6 show a relatively constant extent of faceting as the pressure increases.This is in contrast to the changes observed with variations in RF power, as depicted in Figure 3.This characteristic Regarding mask faceting, the etch profiles of the mask with both C 4 F 8 and C 6 F 6 show a relatively constant extent of faceting as the pressure increases.This is in contrast to the changes observed with variations in RF power, as depicted in Figure 3.This characteristic of mask faceting is advantageous for SiO 2 etching, as it induces rapid etching of the mask and leads to bowing in the etch profile of both the mask and the substrate.
Figure 6a illustrates the relationship between the measured self-bias voltage (left y-axis) and the CFx/F (x = 1, 2, 3) ratio (right y-axis) and pressure with C 4 F 8 and C 6 F 6 .As the pressure increases, the self-bias voltage also increases, indicating a decrease in ion bombardment energy.With the change in pressure from 10 mTorr to 20 mTorr, the selfbias voltages show minimal difference between C 4 F 8 and C 6 F 6 .However, as the pressure continues to rise, the disparity in self-bias voltage between C 4 F 8 than C 6 F 6 becomes more pronounced, with C 4 F 8 exhibiting a larger increase.Since the absolute value of the self-bias voltage with C 4 F 8 at 30 mTorr is lower than that with C 6 F 6 , the ion density and energy reaching the substrate are considered to be less, compared with the C 6 F 6 case.This finding will aid in the interpretation of the etching discrepancy observed with C 4 F 8 and C 6 F 6, along with measurements of radical species.
Figure 6b demonstrates the changes in radical species signal as pressure increases for C 4 F 8 and C 6 F 6 .As the pressure increases, the F signal is found to increase for both cases.However, the SiO 2 etch rate with C 6 F 6 does not follow the F signal trend regarding the pressure change, which implies that the contribution of radical species to etching should be addressed through comprehensive consideration of various radical species.There is a significant disparity in high-mass radical signal between C 4 F 8 and C 6 F 6 , as observed in Figure 4b.Additionally, the dependence of radical species signal on pressure is much substantial compared with its dependence on RF power.
Considering the slight decrease in self-bias voltage and the simultaneous rapid increase in radical species signal with varying pressure, it is expected that the SiO 2 etch rate would decrease as the deposition rate increases more than the etch rate [42].However, contrary to this expectation, Figure 5 reveals that the SiO 2 etch rate with C 4 F 8 actually increases as the pressure rises.This anomalous behavior can be attributed to the increasing difficulty of radical transport toward the trench bottom as the trench's aspect ratio rises.Since a SiO 2 etch rate increases when the depositing FC film is very thin and eventually decreases and saturates when the FC film becomes significantly thick [45], the monotonic increase in the SiO 2 etch rate with C 4 F 8 depicted in Figure 5 appears to be caused by the insufficient radical supply [46].
Figure 6a illustrates the relationship between the measured self-bias vo axis) and the CFx/F (x = 1, 2, 3) ratio (right y-axis) and pressure with C4F8 and pressure increases, the self-bias voltage also increases, indicating a decrease bardment energy.With the change in pressure from 10 mTorr to 20 mTorr, voltages show minimal difference between C4F8 and C6F6.However, as the p tinues to rise, the disparity in self-bias voltage between C4F8 than C6F6 becom nounced, with C4F8 exhibiting a larger increase.Since the absolute value of voltage with C4F8 at 30 mTorr is lower than that with C6F6, the ion density reaching the substrate are considered to be less, compared with the C6F6 case.will aid in the interpretation of the etching discrepancy observed with C4F8 an with measurements of radical species.Overall, a comparison of ACL-masked SiO 2 etch results with C 4 F 8 and C 6 F 6 under variations in RF power and pressure reveals that C 6 F 6 exhibits superior etch rate and selectivity.This finding is even more remarkable considering that C 6 F 6 has a GWP 100 over 1000 times lower than that of C 4 F 8 .While C 6 F 6 demonstrates a nearly identical self-bias voltage to that of C 4 F 8 , indicating similar ion bombardment energy, the plasma radical chemistry significantly differs.However, ACL etching with C 6 F 6 experiences mask faceting, leading to rapid mask etching and decreased selectivity.Thus, it is necessary to identify a more optimal etching window for C 6 F 6 to enhance performance for both ACL and SiO 2 etching processes.
3.2.SiO 2 Etching with HFC Precursors (CHF 3 and C 4 H 2 F 6 ) Figure 7 presents the etch results obtained with different HFC precursors (CHF 3 and C 4 H 2 F 6 ) at varying RF power levels from 300 W to 500 W, while maintaining a fixed pressure of 20 mTorr.The resulting etch rates and selectivity values for C 4 F 8 and C 6 F 6 are summarized in the right-most column of Figure 7.It is commonly observed that as RF power increases, mask faceting tends to deteriorate, as evidenced in the etch results with C 4 F 8 and C 6 F 6 .While the use of CHF 3 produces nearly vertical ACL etch profiles, the utilization of C 4 H 2 F 6 at an RF power of 500 W results in a distorted ACL etch profile.This distortion appears to be induced by the rough mask surface, which causes random ion scattering.However, when the RF power is set below 500 W, sharp and vertical trench etching can be achieved without any defects, such as bowing.Regarding the etch rate and selectivity, CHF 3 exhibits a SiO 2 etch rate lower than that of ACL.As RF power increases, this trend is reversed, leading to a selectivity slightly above unity.On the other hand, with C 4 H 2 F 6 , both the SiO 2 and ACL etch rates gradually increase as RF power increases.The etch rate of SiO 2 remains slightly higher than that of ACL, while the selectivity remains above unity, even at an RF power of 500 W when the mask sidewall is distorted, possibly due to random ion scattering.Although the SiO 2 etch rate with C 4 H 2 F 6 is slightly lower than that with CHF 3 , the use of C 4 H 2 F 6 yields a comparable etch selectivity with that of CHF 3 .Additionally, with minor optimization of the processing pressure, such as decreasing the pressure as demonstrated in Figure 7b, a fine etch profile could be achieved.
Figure 8a depicts the measured self-bias voltage (left y-axis) and the CFx/F (x = 1, 2, 3) ratio (right y-axis) as a function of applied RF power using CHF 3 and C 4 H 2 F 6 .As the RF power increases, both CHF 3 and C 4 H 2 F 6 show an increase in the absolute value of the self-bias voltages, indicating an increase in the ion bombardment energy.Notably, the absolute value of the self-bias voltages with CHF 3 at different RF powers is approximately twice that with C 4 H 2 F 6 , indicating more intense ion bombardment during etching with CHF 3 compared with C 4 H 2 F 6 .This higher ion energy can be advantageous for HARC etching, where sufficient ion energy is required to reach the trench bottom.Etching with C 4 H 2 F 6 would therefore require additional biasing to enhance the ion bombardment in order to match that achieved with CHF 3 .
Materials 2023, 16, x FOR PEER REVIEW have relatively low reaction rates.However, this particular behavior does not appear a significant impact on ACL-masked SiO2 etching.Despite the prevalence of carb radicals in the process, the use of C4H2F6, as shown in Figure 7, does not demonstr stantial advantages in terms of ACL mask protection or SiO2 trench profiles.Nevertheless, it may be meaningful to focus on the difference in radical comp between CHF3 and C4H2F6, particularly in relation to the major dissociation frac CHF3, rather than solely considering the evident difference in plasma chemistry b the two gases.This is because the major species resulting from CHF3 dissociatio Figure 8b presents the signal of various radical species resulting from the dissociation of CHF 3 and C 4 H 2 F 6 , which varies with increasing RF power.As the RF power increases, CHF 3 and C 4 H 2 F 6 show different trends of radical signal variation species by species.Specifically, F from CHF 3 decreases with increasing RF power, while that from C 4 H 2 F 6 increases slightly.Those trends, however, do not follow the etch rate trend that changes with an increase in RF power, which might be attributed to a complex contribution of other FC radicals to the etch rate.When comparing CHF 3 and C 4 H 2 F 6 , it is noteworthy that C 4 H 2 F 6 yields carbon-rich radical species that contain multiple carbon atoms in their chemical structure, while CHF 3 generates them to a lesser extent.This difference is expected since the generation of such carbon-rich radicals from CHF 3 dissociation requires additional recombination processes between carbon-containing species such as CF 2 and CHF 2 , which have relatively low reaction rates.However, this particular behavior does not appear to have a significant impact on ACL-masked SiO 2 etching.Despite the prevalence of carbonrich radicals in the process, the use of C 4 H 2 F 6 , as shown in Figure 7, does not demonstrate substantial advantages in terms of ACL mask protection or SiO 2 trench profiles.
Nevertheless, it may be meaningful to focus on the difference in radical compositions between CHF 3 and C 4 H 2 F 6 , particularly in relation to the major dissociation fractions of CHF 3 , rather than solely considering the evident difference in plasma chemistry between the two gases.This is because the major species resulting from CHF 3 dissociation could play a critical role in determining the etch results.In Figure 8b, it can be observed that C 4 H 2 F 6 dissociation yields comparable densities for most major species compared with CHF 3 fractions.However, the densities of C 2 H 2 and CHF 2 are significantly lower with C 4 H 2 F 6 than with CHF 3 .Furthermore, it is found that the densities of major species are barely affected by RF power variation, suggesting the need for another control parameter to manipulate plasma conditions.
Figure 9 illustrates the etch results obtained with different HFC precursors (CHF 3 and C 4 H 2 F 6 ) at varying pressures from 10 mTorr to 30 mTorr, with a fixed RF power of 400 W. The resulting etch rate and selectivity for the CHF 3 and C 4 H 2 F 6 cases are summarized in the right-most column of Figure 9. play a critical role in determining the etch results.In Figure 8b, it can be observed that C4H2F6 dissociation yields comparable densities for most major species compared with CHF3 fractions.However, the densities of C2H2 and CHF2 are significantly lower with C4H2F6 than with CHF3.Furthermore, it is found that the densities of major species are barely affected by RF power variation, suggesting the need for another control parameter to manipulate plasma conditions.Figure 9 illustrates the etch results obtained with different HFC precursors (CHF3 and C4H2F6) at varying pressures from 10 mTorr to 30 mTorr, with a fixed RF power of 400 W. The resulting etch rate and selectivity for the CHF3 and C4H2F6 cases are summarized in the right-most column of Figure 9.When etching with CHF3, an interesting trend is observed.The ACL etch rate initially decreases and then rebounds as the mask faceting reduces, while the SiO2 etch rate monotonically increases.This behavior leads to a minimum selectivity at the intermediate pressure of 20 mTorr.In contrast, when using C4H2F6, ACL etching continuously increases with enhanced mask faceting as the pressure rises, resulting in significant bowing at the mask sidewall.At the same time, SiO2 etch rate shows minimal change with increasing pressure, leading to a substantial decrease in etch selectivity.
These findings highlight the importance of pressure control in achieving desirable etch results.With CHF3, an optimal pressure range can be identified to balance ACL etch rate and SiO2 etch rate, thereby maximizing selectivity.On the other hand, C4H2F6 exhibits challenges in maintaining selectivity as pressure increases, leading to distorted mask sidewalls and reduced etch performance.
Overall, understanding the pressure-dependent behavior of different HFC precursors When etching with CHF 3 , an interesting trend is observed.The ACL etch rate initially decreases and then rebounds as the mask faceting reduces, while the SiO 2 etch rate monotonically increases.This behavior leads to a minimum selectivity at the intermediate pressure of 20 mTorr.In contrast, when using C 4 H 2 F 6 , ACL etching continuously increases with enhanced mask faceting as the pressure rises, resulting in significant bowing at the mask sidewall.At the same time, SiO 2 etch rate shows minimal change with increasing pressure, leading to a substantial decrease in etch selectivity.
These findings highlight the importance of pressure control in achieving desirable etch results.With CHF 3 , an optimal pressure range can be identified to balance ACL etch rate and SiO 2 etch rate, thereby maximizing selectivity.On the other hand, C 4 H 2 F 6 exhibits challenges in maintaining selectivity as pressure increases, leading to distorted mask sidewalls and reduced etch performance.
Overall, understanding the pressure-dependent behavior of different HFC precursors is crucial for optimizing the etching process and achieving the desired etch profiles and selectivity.
Figure 10a illustrates the measured self-bias voltage (left y-axis) and the CFx/F (x = 1, 2, 3) ratio (right y-axis) as a function of pressure for CHF 3 and C 4 H 2 F 6 .It is observed that with increasing pressure, both CHF 3 and C 4 H 2 F 6 show no significant change in the self-bias voltages.However, it is important to note that CHF 3 consistently provides higher absolute self-bias voltages compared with C 4 H 2 F 6 across a wide range of pressures.This indicates the ion bombardment during etching is stronger with CHF 3 than with C 4 H 2 F 6 owing to higher ion density and energy.This difference in ion bombardment strength may be linked to the higher SiO 2 etch rate observed with CHF 3 compared with C 4 H 2 F 6 .
In Figure 10b, the signal of various radical species resulting from the dissociation of CHF 3 and C 4 H 2 F 6 is presented as a function increasing pressure.As previously observed in Figure 8b, carbon-rich radical species containing multiple carbon atoms in their chemical structure are significantly observed with C 4 H 2 F 6 , while they are barely present with CHF 3 .Such carbon-rich species are believed to react with ACL, enhancing its etching rather than passivating the ACL surface.Additionally, the key etchants of FC plasmas, CF 2 and CHF 2 , are observed to exhibit reversed relative densities depending on the precursor used.In the case of CHF 3, the CF 2 signal is lower than that of CHF 2 , whereas CF 2 exceeds CHF 2 with C 4 H 2 F 6 .This reversal in CF 2 and CHF 2 densities between the two precursors would be useful in selective etching of SiO 2 over Si 3 N 4 [47].

Evaluation of the Greenhouse Effect
Overall, our results demonstrate that the conventional FC gases of C 4 F 8 and CHF 3 exhibit a more vertical etch profile and a higher SiO 2 etch rate across different power and pressure conditions.On the other hand, the alternative gases of C 6 F 6 and C 4 H 2 F 6 exhibit improved etch selectivity.However, when evaluating the etch performance of low GWP gases, it is crucial to assess the reduction in GHG emissions achieved by their implementation.
To quantify the environmental impact, we employ the million metric tons of carbon dioxide equivalents (MMTCDE) metric, which allows for a comparison of GHG emissions based on their GWP.This metric is particularly useful when a GHG is emitted in significantly smaller quantities compared with the reference amount of 1 ton.The carbon dioxide equivalent for a gas is derived as below.
In Equation ( 1), Q refers to the mass of a GHG emitted, measured in kg.To calculate Q for each PFC or HFC gas, we multiply the flow rate of the FC gases used for etching (10 sccm) by the processing time (10 min) to obtain the volumetric amount of the gas used.This value is then converted to mass in kg, taking into account the atomic mass unit of the gas.It should be noted that, in calculating the MMTCDE, we employ the mass of mother gas species, such as C 4 F 8 and CHF 3 , rather than their fractions, including CF and CF 2 , that are observed by the RGA.This is based on a presumption that such reactive species would have relatively short lifetimes and thus have less affect global warming at the atmosphere compared with the chemically stable non-reactive molecules.
Figure 11 illustrates a comparison of the estimated MMTCDEs for the conventional gases (C 4 F 8 and CHF 3 ) and the alternative gases (C 6 F 6 and C 4 H 2 F 6 ).The MMTCDE comparisons under the other experimental conditions show a nearly identical result so that we introduce only the result under etching conditions of 20 mTorr pressure and 400 W RF power for succinctness.Notably, the MMTCDEs associated with the alternative gases are approximately three orders of magnitude lower than those with the conventional gases, despite the etch results showing little difference.This observation suggests that the investigated alternative gases, C 6 F 6 and C 4 H 2 F 6 , hold significant promise as candidates for reducing GHG emissions in the semiconductor industry.
Materials 2023, 16, x FOR PEER REVIEW 14 bombardment during etching is stronger with CHF3 than with C4H2F6 owing to highe density and energy.This difference in ion bombardment strength may be linked t higher SiO2 etch rate observed with CHF3 compared with C4H2F6.In Figure 10b, the signal of various radical species resulting from the dissociati CHF3 and C4H2F6 is presented as a function of increasing pressure.As previously obse in Figure 8b, carbon-rich radical species containing multiple carbon atoms in their c ical structure are significantly observed with C4H2F6, while they are barely present CHF3.Such carbon-rich species are believed to react with ACL, enhancing its etc It should be noted, however, that the above MMTCDE evaluation may not be sufficient since it is conducted only in the processing chamber, not at the exhaust line of the chamber.Even though low-GWP precursors such as C 6 F 6 and C 4 H 2 F 6 , which are investigated in this work, are utilized in plasma processes, undesired high-GWP molecules such as C 2 F 6 with 11,000 GWP can be created and exhausted into the atmosphere, which may reduce the benefits from the low-GWP precursors employment.Additionally, the MMTCDE evaluation with QMS could also include a certain level of inaccuracy due to the dissociative ionization by high-energy electron impacts.Thus, evaluating MMTCDE at the exhaust line of the vacuum chamber with non-invasive diagnostic tools would lead to more rigorous analysis on the global warming effects.

Concluding Remarks
In this study, we have explored the etch performance of C 6 F 6 and C 4 H 2 F 6 as alternatives to the conventional etch gases, C 4 F 8 and CHF 3 , respectively, with the goal of reducing GHG emissions during plasma etching.Given the significantly lower GWP of the alternative gases, it is encouraging to investigate whether they can deliver comparable etch performance to the conventional gases.
To evaluate their performance, we conducted etching experiments using gas mixtures of different FC gases under various conditions of RF power and pressure.The obtained results were carefully analyzed using SEM images.Specifically, we compared the etch results of C 4 F 8 with those of C 6 F 6 , and the etch results of CHF 3 with those of C 4 H 2 F 6 .
Our findings indicate that the alternative gases demonstrated comparable etch performance to the conventional gases across various conditions.This suggests that the alternative gases hold promise for utilization in the next-generation etch processes with reduced GHG emissions.Importantly, our MMTCDE estimation revealed that replacing the conventional gases with the alternative gases could result in GHG emissions reduction by several orders of magnitude.
The outcomes of this study are expected to contribute to the wider adoption of low-GWP gases in semiconductor manufacturing.By incorporating these alternative gases into the etch processes, the industry can effectively reduce its environmental impact while maintaining the desired etch performance.Further research and development efforts should be directed towards optimizing the use of low-GWP gases and promoting their integration into commercial semiconductor fabrication processes.

Figure 1 .
Figure 1.Schematic of the experimental setup.

Figure 1 .
Figure 1.Schematic of the experimental setup.

Figure 2 .
Figure 2. Reference SEM image of an un-etched SiO 2 coupon wafer.

3.
Figure 3 illustrates the etch results obtained with different PFC precursors (C 4 F 8 and C 6 F 6 ) at varying RF power levels ranging from 300 W to 500 W, while maintaining a fixed

Figure 2 .
Figure 2. Reference SEM image of an un-etched SiO2 coupon wafer.

Figure 3 .
Figure 3. SEM images of the etch profile evolution with C4F8 and C6F6 at varying RF power from 300 W to 500 W. The pressure is maintained at 20 mTorr by controlling the Ar flow rate.The etch rate and selectivity with each precursor are plotted in the right-most column.

Figure 3 .
Figure 3. SEM images of the etch profile evolution with C 4 F 8 and C 6 F 6 at varying RF power from 300 W to 500 W. The pressure is maintained at 20 mTorr by controlling the Ar flow rate.The etch rate and selectivity with each precursor are plotted in the right-most column.

Figure 4 .
Figure 4. (a) Self-bias voltage and (b) radical species signal at varying RF power with PFC gas mixtures of C4F8 and C6F6.The pressure is maintained at 20 mTorr.

Figure 4 .
Figure 4. (a) Self-bias voltage and (b) radical species signal at varying RF power with PFC gas mixtures of C 4 F 8 and C 6 F 6 .The pressure is maintained at 20 mTorr.

Figure 5 .
Figure 5. SEM images of the etch profile evolution with C4F8 and C6F6 at varying pressure from 10 mTorr to 30 mTorr.The RF power is maintained at 400 W. The etch rate and selectivity with each precursor are plotted in the right-most column.

Figure 5 .
Figure 5. SEM images of the etch profile evolution with C 4 F 8 and C 6 F 6 at varying pressure from 10 mTorr to 30 mTorr.The RF power is maintained at 400 W. The etch rate and selectivity with each precursor are plotted in the right-most column.

Figure 6 .
Figure 6.(a) Self-bias voltage and (b) radical species signal at varying pressure with tures of C4F8 and C6F6.The RF power is maintained at 400 W.

Figure 6 .
Figure 6.(a) Self-bias voltage and (b) radical species signal at varying pressure with PFC gas mixtures of C 4 F 8 and C 6 F 6 .The RF power is maintained at 400 W.

Figure 7 .
Figure 7. SEM images of the etch profile evolution with CHF 3 and C 4 H 2 F 6 at varying RF power from 300 W to 500 W. The pressure is maintained at 20 mTorr by controlling the Ar flow rate.The etch rate and selectivity with each precursor are plotted in the right-most column.

Figure 8 .
Figure 8.(a) Self-bias voltage and (b) radical species signal at varying RF power with HFC tures of CHF3 and C4H2F6.The pressure is maintained at 20 mTorr.

Figure 8 .
Figure 8.(a) Self-bias voltage and (b) radical species signal at varying RF power with HFC gas mixtures of CHF 3 and C 4 H 2 F 6 .The pressure is maintained at 20 mTorr.

Figure 9 .
Figure 9. SEM images of the etch profile evolution with CHF3 and C4H2F6 at varying pressure from 10 mTorr to 30 mTorr.The RF power is maintained at 400 W. The etch rate and selectivity with each precursor are plotted in the right-most column.

Figure 9 .
Figure 9. SEM images of the etch profile evolution with CHF 3 and C 4 H 2 F 6 at varying pressure from 10 mTorr to 30 mTorr.The RF power is maintained at 400 W. The etch rate and selectivity with each precursor are plotted in the right-most column.

Figure 10 .
Figure 10.(a) Self-bias voltage and (b) radical species signal at varying pressure with HFC gas tures of CHF3 and C4H2F6.The RF power is maintained at 400 W.

Figure 10 .
Figure 10.(a) Self-bias voltage and (b) radical species signal at varying pressure with HFC gas mixtures of CHF 3 and C 4 H 2 F 6 .The RF power is maintained at 400 W.

Figure 11 .
Figure 11.MMTCDE of the investigated etching gases during the etch process under the condition of 400 W RF power, 20 mTorr pressure, and 10 min duration.

Table 1 .
The atmospheric lifetime and GWP100 of the investigated PFC and HFC precursors.

Table 2 .
The experimental conditions for the examined etching processes.