Improvement of Plasma Resistance of Anodic Aluminum-Oxide Film in Sulfuric Acid Containing Cerium(IV) Ion

The parts of equipment in a process chamber for semiconductors are protected with an anodic aluminum-oxide (AAO) film to prevent plasma corrosion. We added cerium(IV) ions to sulfuric acid in the anodizing of an AAO film to improve the plasma corrosion resistance, and confirmed that the AAO film thickness increased by up to ~20% when using 3 mM cerium(IV) ions compared with general anodizing. The α-Al2O3 phase increased with increasing cerium(IV) ion concentration. The breakdown voltage and etching rate improved to ~35% and 40%, respectively. The film’s performance regarding the generation of contamination particles reduced by ~50%.


Introduction
Aluminum (Al) is often used as a material in semiconductor equipment parts because it has high ductility and excellent machinability. However, Al is easily corroded when exposed to fluorine plasma, which is used for semiconductor components and displays because it is a highly reactive metal. It is defined as contaminant particles when corroded Al particles fall on the surface.
Recently, as the environment for using semiconductor process equipment becomes increasingly severe and the difficulty of the process becomes very high, the issue of contaminant particles generated in semiconductor process equipment parts is drawing attention [1,2]. As a result, contamination particles are a major cause of decreased process yield [3][4][5].
To resolve this problem, plasma corrosion resistance can be improved by coating processes such as anodizing [6,7], spark-plasma sintering [8], and atmospheric plasma spraying [9]. Anodizing improves the corrosion resistance of aluminum and aluminum alloys by the growth of a porous oxide film. Anodic oxidation is an electrochemical method that is conducted in a solution, such as sulfuric acid, oxalic acid, or chromic acid [10][11][12][13]. Specially, sulfuric acid is used widely in anodizing, as it is inexpensive and yields a relatively thick anodic aluminum oxide (AAO) film that is composed of a self-sealing layer, an outer (porous) layer, and an inner (barrier) layer [14].
Rare earth elements are used in a wide range of applications, such as self-flux alloy coatings [15], aluminum alloys [16], and photocatalysts [17], because they exhibit strong chemical activities.
Cerium, hafnium, and neodymium are used in various anodizing methods, such as surface preparation before anodizing and sealing after anodizing [18][19][20][21]. In particular, many studies associated with anodizing cerium-based aluminum alloy have been reported because of its excellent corrosion inhibiting properties [19,[22][23][24]. Saeedikhani et al. stated that cerium ions in anodizing electrolytes act as cathodic inhibitors to increase the homogeneity and growth rate of the oxide coating [19]. Additionally, M. Kending et al. proved that Ce(Ⅲ) cations adsorb on the anodized aluminum pores, increasing the zeta potential and the corrosion resistance [22]. However, most studies associated with anodizing cerium-based aluminum alloys have only been evaluated from an electrochemical point of view. Therefore, the study of plasma corrosion, which is very important in the semiconductor industry, is exceedingly deficient.
In this study, cerium(Ⅳ) ions were added to a sulfuric acid electrolyte when anodizing. The characteristics of the AAO film were investigated, including the growth rate, crystallinity, and breakdown voltage with cerium(Ⅳ) ion addition. The etching rate and contamination particles were observed when the film was exposed to a CF4/Ar/O2 plasma.

Materials and Methods
We prepared aluminum 6061 alloy (Alfa Aesar, Haverhill, Massachusetts, USA, Table. 1) with a square sample of 40 mm  40 mm  1 mm and a circle sample of 76 ø  3 mm. Impurities on the aluminum-alloy surface were removed by sonication in ethanol (Duksan, Ansan, Korea, 99.8%) for 30 min. The aluminum alloy was electropolished for 120 s at 25 V, and 300 s at 20 V in 300 mL of perchloric acid (Duksan, Ansan, Korea, 70%) and 1200 mL of ethanol (Duksan, Ansan, Korea, 99.8%) to remove native surface oxide. Then, the aluminum alloy was rinsed in deionized water for 60 s. The thickness of the anodized sample before and after plasma etching was investigated and repeated five times per sample using an eddy-current thickness tester (ISOSCOPE FMP 10, Helmut Fisher, Sindelfingen, Germany).
The anodized sample morphology was studied by field-emission scanning electron microscopy (FE-SEM, S-4800, HITACHI, Tokyo Japan). The crystal phase was analyzed using a high-resolution X-ray diffractometer (XRD, SmartLab, RIGAKU, Tokyo, Japan) with Cu Kα radiation at 45 kV and 200 mA. The AAO film composition analysis was performed by X-ray photoelectron spectroscopy (MultiLab 2000, Thermo Scientific, Waltham, Massachusetts, USA) with Al Kα (1486.7 eV) and a total instrumental resolution of ~1 eV.
Inductively coupled plasma-reactive-ion etching (ICP-RIE) was used to study the corrosive properties of the plasma (Figure 1a). Table 2 shows the plasma-etching conditions. Additionally, in situ contamination particles that resulted from exposure to the plasma were studied using an in-situ particle-monitoring system (ISPM, INFICON, Heidiland, Switzerland) that was mounted on a capacitive coupled-plasma(CCP) particle evaluation system (Figure 1b). A light-scattering sensor ISPM was attached to the exhaust line to measure the contamination particles based on the principles of laser light scattering [25]. In situ contaminant particles were measured when exposed to plasma for 2 h at 400 W in a 120 mTorr chamber with a gas flow, as given in Table 2. Insulator properties of the sample after exposure to plasma were measured using a breakdown-voltage measuring instrument (TOS 9200, KIKUSUI, Yokohama, Japan). We defined the breakdown voltage of the sample when 4 mA was energized with an increasing voltage to 15 V/s.  Figure 1. Schematic diagram of equipment: (a) ICP-RIE etching system, (b) CCP particle evaluation system. On the other hand, the growth rate of the AAO film increased with the addition of cerium(Ⅳ) below 3 mM, compared with the non-Ce sample. The growth rate of the AAO film was improved by ~20% for the 3 mM-Ce compared with the non-Ce. In general, the growth rate was proportional to the current density [26]. Cerium(Ⅳ) addition, with a high oxidation potential in the sulfuric acid electrolyte, increased the current density [20], which led to an increase in growth rate. However, the current density and the growth rate of the AAO film decreased to 15.02 A/dm 2 and 415 nm/min, respectively, for the 4 mM-Ce. Cerium can be present as ions such as Ce 3+ , Ce 4+ , and Ce(OH)2 2+ in sulfuric acid electrolyte, and it also can be present as salts such as Ce(OH)3, Ce2O3, Ce(OH)4, and CeO2 [27]. It is speculated that cerium(Ⅳ) ions react with other materials in the anodizing baths, such as sulfuric acid and H2O, when 4 mM or more cerium(Ⅳ) ions are added, forming cerium species excluding Ce 3+ . The formed Ce species might drop the current density of anodizing solution. Therefore, we performed an additional analysis for a cerium(Ⅳ) ion addition of 3 mM or less.  Figure 3 shows the XRD patterns of the AAO film with cerium(Ⅳ) ion addition. The AAO films consisted mainly of α-Al2O3 and γ-Al2O3 [28,29]. The weak diffraction peak of Al also appeared because the Al peak originated from the substrate. The full widths at half maximum (FWHMs) of diffraction peaks α-Al2O3 and γ-Al2O3 were almost unchanged when the amount of cerium(Ⅳ) ion addition was changed. The α-Al2O3 phase is usually in a thermodynamically stable state at all temperatures, whereas the γ-Al2O3 phase is thermodynamically metastable. Generally, the γ-Al2O3 phase is more easily formed than the α-Al2O3 phase, and is transformed into the α-Al2O3 phase at high temperature above 1300 °C. The α-Al2O3/γ-Al2O3 peak ratio according to the amount of cerium(Ⅳ) ions added to the sulfuric acid electrolyte is shown in Table 3. The ratio of the α-Al2O3 phase in the AAO film increased with cerium(Ⅳ) ion addition without changes in surface integrity features. Table 3. X-ray diffraction peak ratio of α-Al2O3 and γ-Al2O3 according to Ce ion.

Electrolyte α-Al 2 O 3 /γ-Al 2 O 3
Non-Ce 4.14 1 mM-Ce 4.54 2 mM-Ce 6.04 3 mM-Ce 6.64 In the study of M. Kending et al., cerium ions in anodizing baths adsorbed on the surface within the pores of the AAO film and increased the zeta potential. Additionally, cerium ions were able to reversibly adsorb at the site, which could fix the proton on the surface of the AAO film [22].
The contribution of cerium ions to chemisorption during anodizing is as follows [30]: In this case, the cerium ions that are adsorbed on surface of AAO film did not exist in the Al2O3 crystal-lattice phase, but on the surface, and it supported α-Al2O3 phase nucleation [31]. Because of the reaction of the cerium ions, the α-Al2O3 phase increased with the addition of cerium(Ⅳ) ions.
The 3 mM-Ce was analyzed using XPS spectra to confirm the presence of cerium ions on the surface (Figure 4). However, the cerium peak did not exist in the XPS analysis results as expected. It can be expected that cerium acts as an accelerating agent, but exists on the surface below the analyzable amount in the XPS resolution.  to be distributed irregularly on the surface after single-step anodizing because of the field-assisted dissolution of the oxide [32]. The pores were not visible because they were covered in a self-sealing layer. The pores became visible after exposure to the CF4/Ar/O2 plasma because a self-sealing layer was etched by the physicochemical reaction with Ar ions and fluorine radicals ( Figure 6). It was confirmed that the pore size increased gradually depending on the amount of cerium(Ⅳ) ions added (Figure 6b-d). Al transferred the stress to the cell wall region to reduce the film growth stress that occurred during the anodizing reaction, and the transferred stress resulted in the porous structure of the AAO film due to the high plasticity of the anodic alumina [33]. When the current density increased, the AAO film growth rate increased and it led to increased AAO film growth stress. The pore diameter increased as the current density increased. When cerium(Ⅳ) ions below 3 mM were added, the applied current density was increased ( Figure 2). As a result, the pore size of the AAO film was increased. It appeared that the Al-O atoms behaved differently when the cerium ions affected the formation of the Al2O3 phase [34].   Figure 7 shows the etching rate for the CF4/Ar/O2 plasma, which is important for the reliability of semiconductor equipment parts. Fluorine radicals generated from CF4 promote the chemical reaction, and Ar ions that become separated from Ar gas cause physical damage [8]. Therefore, the CF4/Ar/O2 plasma caused chemical and physical damage to all components that contained the sample in the chamber.

Investigation of Plasma-Resistance Characteristics
AlF3 is formed on the Al2O3 surface when Al2O3 reacts with fluorine plasma, and is formed with a thickness of a few nanometers by the reaction shown in Equation (2) [35]: Al2O3 + CF4 → 2AlF3 + CO2 (2) The formed AlF3 has a high boiling temperature of 1275 °C, which means that the AlF3 has a high binding energy and is stable to chemical reaction [36]. Therefore, AlF3 is physically etched only because it is chemically stable.
The etching reaction of AAO film with the CF4/Ar/O2 plasma consists of two steps. A chemical reaction occurs when Al2O3 reacts with the CF4 plasma to form AlF3 (Equation (2)), and then a physical reaction occurs in which AlF3 reacts with the Ar plasma.
It appears from Figure 3 and Table 3 that the AAO film with cerium(Ⅳ) ion addition had a high ratio of α-Al2O3 peak. It is widely known that α-Al2O3 is the most chemically and physically stable phase because it has a corundum structure in which oxygen anions are stacked in an ideal order on a dense plane [37]. Because of its structural characteristics, it was expected that the etching rate of the AAO film with cerium(Ⅳ) ion addition would decrease. Consequentially, we confirmed that the etching rate of the AAO film decreased by 40% with the addition of 3 mM cerium(Ⅳ) ions in a CF4/Ar/O2 plasma. The input power increased because of an increased plasma density and a reduction in the line width during semiconductor etching. In this case, the insulation characteristic of the AAO film was important because insulator-film breakdown occurs easily because of the high voltage when a high power is applied. Figure 8a shows the measurement results of the breakdown voltage according to the amount of cerium(Ⅳ) ions added. The breakdown voltage often depends on the insulator-film thickness [38][39][40]. For the same time conditions, 3 mM-Ce had the highest breakdown voltage of 1.01 kV, which is ~35% higher than the non-Ce because of the highest AAO film thickness. Additionally, the breakdown voltage was measured after exposure to the CF4/Ar/O2 plasma in order to observe the insulation characteristics after plasma treatment (Figure 8b). The sample breakdown voltage with added cerium(Ⅳ) ions was still higher than in the non-Ce sample. In particular, the 3 mM-Ce (0.77 kV) that was exposed to CF4/Ar/O2 had a higher breakdown voltage than the non-Ce before (0.74 kV) and after (0.63 kV) plasma treatment. This occurred because of the thickness and dense structural characteristics of the 3 mM-Ce with a high growth rate and resistance to etching. The generation of contamination particles is important for the reliability of the anodizing coating for parts in a process chamber for semiconductors. Figure 9 shows a number of contamination particles that were generated upon exposure to the CF4/Ar/O2 plasma. The 3 mM-Ce was compared with the non-Ce because it is an optimized process condition with the best properties. Figure 9a shows the number of in-situ-generated contamination particles according to the exposure time to the CF4/Ar/O2 plasma. In the initial reaction, the contamination particles did not appear to be generated from the non-Ce or 3 mM-Ce. This was because of the structure of the AAO Etch rate (m/h) film. As mentioned previously, AAO film is composed of a self-sealing layer, an outer (porous) layer, and an inner (barrier) layer. The self-sealing layer reacts preferentially with the CF4/Ar/O4 plasma. The self-sealing layer is traditionally known as a corrosion-inhibitor layer because it has fewer reactive sites than the outer layer, with a porous structure [41]. When reacting with CF4/Ar/O2 plasma, few reaction sites exist, and the etching reaction is slow. Therefore, the number of generated contamination particles is low. After all the self-sealing layers have been etched, the outer layer and the CF4/Ar/O2 plasma react. Figure 9b shows that the amount of generated contamination particles decreased by ~50 % when 3 mM cerium(Ⅳ) ions were added compared with the non-Ce case. The 3 mM-Ce sample had a high ratio of α-Al2O3 peak and was more physically stable than non-Ce. Hence, the sample that contained 3 mM cerium(Ⅳ) ions yielded fewer contamination particles.
(a) (b) Figure 9. Generated contamination particles with exposure to CF4/Ar/O2 plasma: (a) in-situ generated contamination particles, (b) total number of generated contamination particles after 2 h.

Discussion
Cerium(Ⅳ) ion addition to sulfuric-acid electrolyte affected the characteristics of the AAO film, such as the thickness growth rate, breakdown voltage, and etching-gas plasma corrosion. The thickness growth rate increased by 20% and the AAO film grew at a high ratio of α-Al2O3 peak when 3 mM of cerium(Ⅳ) ions was added. The physical properties of the AAO film with a high ratio of α-Al2O3 peak improved, and led to an improvement in breakdown voltage and plasma-etching resistance of 25% and 40%, respectively. In particular, the generation of contamination particles was reduced by 50%. As a result, the AAO film with cerium(Ⅳ) ion addition has a high reliability for application as an anodic oxidation coating film in semiconductor production equipment.

Conflicts of Interest:
The authors declare no conflict of interest.