Analysis of formation mechanism of deposited film in a high-aspect-ratio hole during dry etching using fluorocarbon gas plasmas

In the era of big data, storage devices with a high bit density are required to store more information in a limited volume. To meet these demands, the feature sizes of semiconductor memory, such as 2D-NAND flash, have been miniaturized. However, the downscaling of 2D-NAND has been limited by cell degradation resulting from several factors including signal interference between adjacent cells and data retention. ¹⁾ To overcome this problem, 3D flash memory, in which memory cells are stacked three-dimensionally, has been developed. ²⁾ Nonetheless, a major challenge in manufacturing 3D flash memory is related to the fabrication of high-aspect-ratio (AR) memory holes through a large number of stacked films constructed by SiO₂/polycrystalline Si pairs is required by a dry etching process. ³⁾ Therefore, the key technology in 3D flash memory manufacturing is a high-AR hole-etching process. Usually, selective etching of SiO₂ with a carbon mask is achieved by reactive ion etching (RIE) using fluorocarbon (FC) gases. The corresponding mechanism of the surface reaction has been previously studied. ^4,5) In addition, to improve the selectivity and etch rate, research on etching using various FC gases with different compositions and structures such as CF₄, C₂F₆, C₂F₄, C₃F₆, C₄F₆, C₄F₈, C₅F₈, and C₆F₆ has been conducted. ^6–11) In the RIE process with FC gas, a suitable thickness of amorphous-CF_x (a-CF_x ) film deposited on the mask or sidewalls of the hole by CF_x radicals plays an important role in protecting the structures from ion bombardment, leading to high selectivity and reduction of side etching. Nevertheless, it has been reported that excessive or lack of a-CF_x film deposition leads to various hole profile issues, such as bowing (increased hole diameter near the middle of a hole ^12–14)), distortion, ^15,16) and striation (vertical scratches on the sidewall ¹⁷⁾). Furthermore, the conductivity of a-CF_x films has also been reported, where associated features may induce charging at the bottoms of the high-AR holes during the RIE process. ^18–21) This charge-up phenomenon is reported to infer the twisting of holes. ²²⁾ These reports suggest that controlling the thickness of the a-CF_x film and the AR region where the a-CF_x film is deposited can overcome the various hole shape problems, which occur during the high-aspect-ratio-contact (HARC) process. To control the thickness of the a-CF_x film in the hole, it is necessary to examine the kinetics of various CF_x radicals and control the a-CF_x film thickness for each AR. Izawa et al. reported that a-CF_x films could be formed deeper into the hole by increasing the surface temperature of the sample. ¹²⁾ Similar reports have investigated the relationship between the sample surface temperature and the sticking coefficient of CF_x radical. ^23,24) In addition, it has been reported that the thickness of the a-CF_x film increases with the amount of CF₂ radical. ^25,26) Takahashi et al. showed that C₅F₈ gas, which generates more CF₂ radicals compared to C₃F₆ and C₄F₈, is more easily polymerized on silicon wafers. ²⁷⁾ However, there are few reports on the relationship between FC gas molecular characteristics (structure and composition ratio) and a-CF_x film thickness relative to each AR. In this study, we investigated the characteristics of the a-CF_x film relative to AR, which was formed by several FC gases having different bonds, such as single bonds, double bonds, or benzene rings. We further discussed the effects of the FC gas structure and composition ratio on a-CF_x film formation.

Figure 1 shows a schematic of our experimental setup. Plasma was formed in a dual-frequency (3.2/100 MHz) capacitively coupled plasma (CCP) reactor, as shown in Fig. 1 (a). The input power of the high frequency was set to 300 W, whereas that of the low frequency was set to 0 W to prevent overheating of the sample due to ion bombardment. The temperatures of the upper electrode, sidewall, and bottom electrode were maintained at 100 °C, 80 °C, and 20 °C, respectively. The FC gases used in this study were hexafluoro-1,3-butadiene (C₄F₆), octafluorocyclobutane (C₄F₈), hexafluorobenzene (C₆F₆), tetrafluoroethylene (C₂F₄) and tetradecafluoromethylcyclohexane (C₇F₁₄). A gas mixture of FC (flow rate: 39 sccm) and Ar (flow rate: 94 sccm) was injected into the reactor from the upper electrode through a showerhead. The gas pressure was maintained at approximately 2 Pa. The molecular structure of FC is shown in Fig. 1 (b). C₄F₈ and C₇F₁₄ are cyclic, C₂F₄ contains a double bond, C₄F₆ has two double bonds, and C₆F₆ contains a benzene ring. The atomic composition ratios of fluorine to carbon (F/C) of C₄F₈, C₇F₁₄, and C₂F₄ were 2, and the F/C ratios of C₄F₆ and C₆F₆ were 1.5 and 1, respectively. Aluminum nitride (AlN) plates with rectangular grooves (0.4 × 5 × 50 mm³) were placed on the Si substrate, as shown in Fig. 1(c). Following plasma exposure, the Si surface was analyzed by X-ray photoelectron spectroscopy (XPS). The samples were exposed to a mixed gas plasma consisting of FC gas and Ar for 1 min. Under our experimental process conditions, the temperature of the substrate was confirmed to be below 40 °C. A gap was made between the AlN plate and Si substrate. The migration behavior of the CF_x radicals generated in the plasma can be investigated by measuring the thickness of the formed a-CF_x film in the lateral direction. This is mainly because the AlN plate blocks the ions above this gap region, as shown in Fig. 1(d). As shown in Fig. 1(e), the a-CF_x film thickness was measured using either scanning electron microscopy (SEM) or XPS. The film thickness was calculated from XPS measurements using Eq. (1): ^28,29)

$$\begin{eqnarray}&&\displaystyle \frac{I}{{I}_{0}}=\exp \left(-\displaystyle \frac{d}{\lambda \,\cos \,\phi }\right),\end{eqnarray} \tag{ 1 }$$

where d is the a-CF_x thickness, λ is the mean free path of the photoelectrons in the a-CF_x film (approximately 2.5 nm), and ϕ is the photoelectron emission angle (45°). I₀ and I are the Si 2p spectral intensities of the Si substrates before and after the plasma treatment, respectively. In addition, the degree of dissociation by the electron impact with an energy of 70 eV for FC gases was measured by a quadrupole mass spectrometer (QMS) to reveal the influence of the structure and composition of the FC gas molecule on the a-CF_x film deposition.

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Figure 2 shows the AR dependencies of the a-CF_x film thickness deposited by various FC gas plasmas in the (a) low-AR region up to 5 and (b) higher AR region up to 40. The thickness of the a-CF_x film formed in the low-AR region increased in the following order: C₆F₆, C₄F₆, C₇F₁₄, C₂F₄, and C₄F₈. The a-CF_x films formed by C₄F₆ or C₆F₆ plasma were noticeably thicker than those formed by the other gases. Nonetheless, there was no significant difference in the thicknesses of the a-CF_x films deposited with C₂F₄, C₄F₈, and C₇F₁₄ gases. On the other hand, in the high-AR region, the a-CF_x films deposited with C₂F₄, C₄F₈, and C₇F₁₄ plasmas were thicker than those with C₄F₆ and C₆F₆ plasmas, as shown in Fig. 2(b). From the results observed above, in the low-AR region, thick a-CF_x films are deposited by plasma exposure to a gas with multiple double bonds (C=C) or benzene rings and a small F/C ratio, such as C₄F₆ and C₆F₆. Conversely, in the high-AR region, relatively thick a-CF_x films were deposited by plasma exposure to a gas with fewer unsaturated bonds in the FC gas molecules and a high F/C ratio, such as C₂F₄, C₄F₈, and C₇F₁₄.

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Figure 3 shows the fragmentation of the FC gases after an electron impact of 70 eV, as detected by QMS. The major fragment species are shown in Table I. FC gases with double bonds or benzene rings, such as C₂F₄, C₄F₆, and C₆F₆, are hardly decomposed. The larger the number of π bonds in the double bond or benzene ring structure, the less decomposition is observed. Conversely, in the case of gases without double bonds, such as C₄F₈ and C₇F₁₄, no parent ion species are detected. These results suggest that the double bonds included in the molecule help maintain their structures against electron attack. The F/C ratio of the majority of fragments is strongly influenced by the F/C ratio of the parent gas. For example, C-rich fragments like C₆F₆ ⁺ and C₅F₃ ⁺ are mainly observed in the case of using C-rich C₆F₆ gas. In contrast, F-rich fragments, such as C₃F₅ ⁺, are mainly detected using F-rich C₄F₈ and C₇F₁₄ gases.

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Next, the chemical bonding states of the a-CF_x film were analyzed using XPS to understand the AR dependence of the a-CF_x film characteristics. Figures 4(a)–4(e) show the C 1 s spectra of the deposited a-CF_x films at AR = 0, 9, 20, 30, and 40. As shown in Figs. 4(a)–4(e), a thick a-CF_x film with four peaks corresponding to C–CF_x , C–F, C–F₂, and C–F₃ bonds was observed in the low-AR region. The attribution for each peak was confirmed in previous reports. ^30,31) As the AR increased and the a-CF_x film became thinner, signals originating from the contamination peak under the a-CF_x film were observed. Figure 4(f) shows a comparison of the areal ratios of C–CF_x , C–F, C–F₂, and C–F₃ in the C1s spectra at AR = 0. The composition of the a-CF_x films was strongly influenced by the F/C ratio of the parent gas. The a-CF_x films deposited by F-rich C₄F₈, C₇F₁₄, and C₂F₄ gases have more F-rich components, such as C–F₂ and C–F₃. As the composition of the parent gas becomes C-rich, such as C₄F₆ and C₆F₆, C-rich C–CF_x and C–F components increase. A very small C–F₃ peak was observed in the a-CF_x film formed by C₆F₆ even at AR = 0. This is explained by the fact that the C₆F₆ gas is composed solely of CF groups, where it is difficult to form a CF₃ group with two F atoms bonded to the CF group. This suggests that the composition ratio of CF_x radicals produced in the plasma is greatly influenced by the composition ratio of the parent FC gas. For a-CF_x films deposited by F-rich gases such as C₄F₈, C₇F₁₄, or C₂F₄, as AR increases from 0 to 30, the peak intensities originating from the C–CF_x and C–F bonds decrease significantly, whereas the peak intensities of the C–F₂ and C–F₃ bonds decrease slightly, as shown in Figs. 4(a)–4(c). Therefore, the atomic composition ratio of the a-CF_x films deposited in the high-AR region gradually became F-rich. This suggests that only F-rich radicals with high C–F₂ and C–F₃ contents can reach the high-AR region. This can be attributed to the difference in the sticking coefficients between the C-rich CF_x radicals and F-rich CF_x radicals. The sticking coefficient of F-rich CF_x radicals such as CF₂ is reported to be lower than that of C-rich radicals; ¹²⁾ hence, F-rich CF_x radicals can reach high-AR regions. On the other hand, no C–F₂ and C–F₃ peaks were observed in the a-CF_x film deposited by C₄F₆ or C₆F₆ plasmas in the high-AR regions (e.g., AR = 30), as shown in Figs. 4(d) and 4(e). This may be because F-rich gases such as C₄F₈, C₇F₁₄, and C₂F₄, F-rich fragments with high C–F₂ and C–F₃ contents are unlikely to be generated in the plasma. C₄F₆ and C₆F₆ gases are not easily decomposed, and few F-rich fragments are generated compared to C₄F₈, C₇F₁₄, and C₂F₄, as shown in Table I. For example, in the case of C₄F₆ gas, the amount of F-rich fragments (e.g., F/C > 1.5), such as CF₃ ⁺ and C₃F₅ ⁺, is approximately 2%. In C₆F₆ gas, no F-rich fragments were measured. Although the C₄F₆ molecule has two CF₂ groups, the gas is not easily decomposed because of its double bond, and the amount of CF₂ radicals produced in the plasma is thought to be low, as shown in Fig. 3 (d).

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Figure 5 shows a schematic model of the relationship between the atomic composition ratio (F/C) and the structure of the FC gases on a-CF_x deposition. In the C–F bond, it is estimated that electrons are attracted near the F atom because of their higher electron affinity compared to that of the C atom. Therefore, F is relatively electron-rich, whereas C is relatively electron poor. F-rich CF_x radicals are less reactive to nucleophilic attack, ³²⁾ leading to a low sticking coefficient because most molecules are covered with electron-rich F, and the unpaired electron on C is strongly hindered. When there are many F-rich radicals in the plasma, after the CF_x radicals are attached to the substrate, the newly approaching CF_x radicals cannot approach the same location because of electrostatic repulsion, as shown in Fig. 5(a). Therefore, a thin CF_x film was deposited in the high-AR region. In comparison, the unpaired electron on C of the C-rich CF_x radicals was less hindered, leading to a high sticking coefficient and reactivity to another unpaired electron on the substrate. After the C-rich radical attaches to the substrate, the newly approaching CF_x radicals easily approach and react in the same place due to the absence of electrostatic repulsion, as in the case of F-rich radicals, as shown in Fig. 5(a). Therefore, the a-CF_x film is thickly deposited in the low-AR region and not so much in the high-AR region because most of the C-rich CF_x radicals are consumed in the low-AR region. As mentioned above, the F-rich CF_x radicals generated in C₄F₈, C₇F₁₄, and C₂F₄ plasmas reach the higher AR region and form a-CF_x films, as shown in Fig. 5(b). In contrast, when gases with F/C ratios lower than 2, such as C₄F₆ and C₆F₆, are used, many C-rich CF_x radicals with high sticking coefficients are generated in the plasma, and thick a-CF_x films are deposited in the low-AR region. It follows that the C-rich CF_x radicals did not reach the higher AR region and a thin a-CF_x film was thus observed. FC gases with two double bonds, such as C₄F₆, generate radicals with two unpaired electrons and one double bond, which promotes further polymerization compared to FC gases with one double bond. ³³⁾ Therefore, a-CF_x films deposited in the low-AR region by C₄F₆ plasma exposure are considerably thicker than those deposited by C₄F₈, C₇F₁₄, and C₂F₄ plasma exposures. C₂F₄ gas has one double bond, but its high F/C ratio prevents the formation of thick a-CF_x films because of the large electrostatic repulsion between the F-rich CF_x radicals. In addition, a single double bond in the structure cannot promote further polymerization. The a-CF_x film deposited using the C₆F₆ plasma was significantly thicker than that deposited using the C₄F₆ plasma. This could be attributed to the molecular structure of C₆F₆. Molecules with benzene rings stack face-to-face owing to their π-π interaction. ³⁴⁾ This interaction causes radicals with benzene ring structures to cluster in the low-AR region, which may result in significantly thicker a-CF_x film formation in regions where AR is approximately 0. As described above, the AR dependence of a-CF_x film deposition is affected by two factors: (1) magnitude of electrostatic repulsion derived from the F/C ratio of the CF_x radicals, and (2) multiple double bonds or benzene rings in the FC gas molecules. To form a-CF_x films in high-AR regions, it is important to increase the F/C ratio of the FC gas and generate F-rich CF_x radicals in the plasma. Conversely, by lowering the F/C ratio, including multiple double bonds or benzene rings in the FC gas, and generating C-rich CF_x radicals in the plasma, a-CF_x films can be deposited only in the low-AR region. To form thicker a-CF_x films, it is effective to either use FC gases with two or more double bonds, which consequently lowers the F/C ratio and promotes polymerization, or FC gases with benzene rings to promote stacking via π-π interactions.

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In summary, the effects of the composition ratio and structure of the FC gas on the deposition of a-CF_x films were investigated in a high-aspect-hole structure. When the F/C ratio of the FC gas was 2 or higher, F-rich CF_x radicals with low sticking coefficients were easily generated in the plasma, and a-CF_x films were deposited up to the high-AR region. However, as the F/C ratio decreased, C-rich CF_x radicals with high sticking coefficients were formed in the low-AR region, whereas thin a-CF_x films were deposited in the high-AR region. The multiple double bonds in the FC gas molecules promote polymerization, resulting in a thicker a-CF_x film that can be deposited in the low-AR region. Furthermore, FC gas with a benzene ring structure creates a thicker a-CF_x film in the region where AR is approximately 0, which may be attributed to the molecular assembly owing to the π-π interaction of benzene rings. These process controls of a-CF_x film formation inside the patterns can lead to high etching performance.