Structural and Fluorine Plasma Etching Behavior of Sputter-Deposition Yttrium Fluoride Film

Abstract

Yttrium fluoride (YF₃) films were grown on sapphire substrate by a radio frequency magnetron using a commercial ceramic target in a vacuum chamber. The structure, composition, and plasma etching behavior of the films were systematically investigated. The YF₃ film was deposited at a working pressure of 5 mTorr and an RF power of 150 W. The substrate-heating temperature was increased from 400 to 700 °C in increments of 100 °C. High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction results confirmed an orthorhombic YF₃ structure was obtained at a substrate temperature of 700 °C for 2 h. X-ray photoelectron spectroscopy revealed a strongly fluorinated bond (Y–F bond) on the etched surface of the YF₃ films. HRTEM analysis also revealed that the YF₃ films became yttrium-oxyfluorinated after exposure to fluorocarbon plasma. The etching depth was three times lower on YF₃ film than on Al₂O₃ plate. These results showed that the YF₃ films have excellent erosion resistance properties compared to Al₂O₃ plates.

1. Introduction

Silicon-oxide series of ceramics, such as SiO₂ and Al₂O₃ coatings, are valued for their hardness, high wear resistance, dielectric strength, high corrosion resistance, and chemical stability. They have been extensively used as plasma-resistant materials in plasma etching equipment and in the deposition thin-film processing semiconductor industry [1,2]. They are also popular shield materials that protect the interior ceramic parts, chamber windows, cover baffles, rings, and reactor chamber walls of plasma equipment from corrosive fluorocarbon gases (e.g., C₂F₆, CF₄, CHF₃, and C₂F₆) [3]. However, on the inner walls of the processing chamber, these oxide materials easily interact with fluorine-based plasma, causing significant erosion and particle generation [4]. As integrated circuits continue to downscale to the nanoscopic level, particle contaminants in their wafers are becoming increasingly worrisome, because they short-current the integration circuit and reduce mass-production yield [5]. Yttrium oxide (Y₂O₃) promises to replace SiO₂ and Al₂O₃ as the material of plasma-facing inner walls, owing to its much higher chemical stability and reduced erosion rate [6]_. Nevertheless, as pointed out by mass-production factories, Y₂O₃ inner wall coatings contain pores and crack defects that release particles into the plasma by flake-off [7]. Yttrium fluoride (YF₃) coatings have recently attracted substantial attention because their high plasma erosion resistance prevents the generation of fluoride particles on the chamber wall surface, reducing particulate contamination [8]. Moreover, YF₃ reportedly has a high dielectric strength [9]. Thus, YF₃ coating is a new plasma-facing material. Researchers have fabricated protective coatings in chamber walls using a plasma spray technique. However, as the spray method rapidly forms thick films with porous structures and rough surfaces, it might introduce critical particle impurities during the semiconductor plasma deposition/etching process [10]. Vacuum coating techniques such as magnetron sputtering can address these problems.

To our knowledge, sputtered YF₃ films and their erosion behavior under fluorocarbon plasma etching had not previously been investigated. Herein, we investigate the composition, structure, erosion behavior, and surface morphology changes of YF₃ films on sapphire substrate, prepared by magnetron sputtering.

2. Materials and Methods

The sputter material was YF₃ ceramic target (99.99% purity, 2 inch diameter, 3 mm thickness). YF₃ thin films were deposited on c-plane sapphire substrate (single crystal of Al₂O₃) at room temperature by radio frequency magnetron sputtering in a vacuum chamber. The substrates were cleaned before deposition, first in acetone and alcohol, and then by ultrasonic cleansing in de-ionized (DI) water for 30 min. They were blow-dried in nitrogen gas. The sputtering gas was high-purity argon (99.995%), maintained at a constant flow rate (~100 sccm). The sputtering process was performed under a base chamber pressure of approximately 1.5 × 10⁻⁵ torr, with turbo molecular and oil diffusion pumps. The plasma generation was activated by RF power at 13.56 MHz. The target–substrate distance was 15 cm. To ensure uniform film thickness, the substrate holder was rotated at 20 rpm during the deposition process. Substrate heating temperature was varied from 400 to 700 °C in steps of 100 °C. The sputter deposition conditions of the YF₃ films are detailed in

Table 1. Experimental parameters of sputter deposition conditions of YF₃ film.

Parameters	Conditions
RF power (W)	150
Duration (hours)	2
Substrate temperature (°C)	400–700
Substrate rotation (rpm)	20
Working pressure (mTorr)	5
Substrate to target distance (cm)	15
Argon gas flow rate (sccm)	100

. The plasma etching behaviors of all samples were performed using an inductively-coupled plasma etcher. The etching gases mixed were CF₄ and O₂. The etching conditions are shown in

Table 2. Plasma etching conditions.

Parameters	Condition
RF power (W)	800
RF power, bias (W)	100
Pressure (mTorr)	3
CF4 (sccm)	25
O2 (sccm)	5
Etching time (min)	10, 20, 30

.

The surface morphology, microstructure, and elemental analysis of these coating samples were analyzed by scanning electron microscopy (SEM, S-3000H, Hitachi, Tokyo, Japan) coupled with energy dispersive X-ray diffraction (EDX), atomic force microscopy (AFM, DI-3100, Veeco, New York, NY, USA), and high-resolution transmission electron microcopy (HRTEM, H-600, Hitachi, Tokyo, Japan). The sample compositions were examined by X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, ULVAC-PHI, Kanagawa, Japan) using a monochromatic Cu Kα X-ray source (λ = 1.541874 Å) at a passing energy of 20 eV with a spot size of 650 μm. After XPS, the sample surface was etched using focused argon-ion sputtering to investigate the chemical compositional depth profile (Thermo Scientific K-Alpha). Finally, the photoelectron spectrum resulting from the core energy levels of yttrium 3D states was deconvoluted by a fitting software program (Thermo Fisher Scientific, Inc., Waltham, MA, USA) to estimate the contributions from bonding with fluorine elements.

3. Results and Discussion

Figure 1 depicts the XRD scan patterns of YF₃ films deposited on Al₂O₃ substrate at 400, 500, 600, and 700 °C. All YF₃ samples were polycrystalline, and their structural orientations depended on the substrate temperature in the deposition process. The (020) plane of the orthorhombic YF₃ phase was formed at substrate temperatures above 500 °C, which is in agreement with the reported data (JCPDS card files No. 74-0911) [11]. Moreover, the preferential orientation of the orthorhombic YF₃ crystal structure differed in films fabricated at different temperatures. This might be attributed to the high kinetic energy imparted to YF₃ molecules at high substrate temperatures, which enables them to migrate and rearrange on the substrate surface. YF₃ samples fabricated at higher temperatures also showed a weak peak of the cubic Y₂O₃ phase (400) plane (JCPDS card files No. 79-1716), indicating contamination by oxygen atoms during the sputtering process [9]. As substrate temperature increases, Y₂O₃ formation might be favored by the higher instability of fluoride in the YF₃ crystal lattice. Unstable F ions in the lattice can be gradually replaced by environmental oxygen atoms. Therefore, the number of oxygen atoms in the crystal lattice increases with increasing temperature, forming Y₂O₃ (see Figure 1). When YF₃ film was deposited at a lower substrate temperature, an amorphous structure was formed. Amorphous film usually has a loose structure, resulting in greater thickness and a higher growth rate. In contrast, with increasing substrate temperature, the YF₃ film became crystalline, and its crystal was relatively ordered. This indicates that a denser structure was found in the YF₃ film grown at higher substrate temperatures, leading to its smaller thickness and lower growth rate. In TEM analysis, the YF₃ films exhibited nanocrystalline grains with an average size of 5–20 nm (Figure 2). The regular ring-like electron diffraction pattern of a selected area (Figure 2, inset) implies a polycrystalline structure of YF₃ film, consistent with the XRD result.

Figure 3a,b show the XPS survey spectra in the 0–1200 eV range of the YF₃ films before and after CF₄/O₂ plasma etching, respectively. To collect the atomic signals, all samples were bombarded by argon ions for 2 min. The XPS peaks in the as-grown YF₃ thin film (Figure 3a) correspond to Y, F, O, and C elements. The Y and F elements existed in the original YF₃ film, and an O1s peak arose from oxygen contamination during the sputter deposition process. This result is consistent with previous reports [12]. After exposure to CF₄/O₂ plasma, the spectrum of the YF₃ film exhibited an enhanced F1s peak and an additional C1s peak. The former feature is attributed to the high fluorine content of YF₃, and the latter indicates the formation of a carbon-polymer surface layer under fluorocarbon plasma etching (Figure 3b). Figure 4a,b plot the compositions of the YF₃ films as functions of sputtering time before and after exposure to the CF₄/O₂ plasma, respectively. Before exposure to the plasma, the maximum F content was 37% and the minimum O content was 32% (Figure 4a). This result indicates a two-phase mixture of YF₃ and Y₂O₃ in the film, consistent with a previous study that reported a deficiency of fluorine atoms during the sputter deposition process [13]. The maximum percentage of F atoms in the etched sample was 44.69%, implying a fluorination layer on the YF₃ specimen (Figure 4b). Similar results were observed in a previous study of surface fluorination by CF₄/O₂ plasma etching [14]. The carbon content on the YF₃ surface decreased abruptly with sputtering time, indicating a very thin carbon polymer layer on the etched surface. This is because of interactions between the fluorocarbon plasma and Si-based materials, which generate carbon polymer and a SiF_xO_y reaction layer [15].

Figure 5 shows the XPS spectra of yttrium atoms of the YF₃ film after CF₄/O₂ plasma etching. In the curve-fitted XPS spectra of the YF₃ film, two peaks represent two bonding sources for Y cations, that is, the Y3d peak splits into a doublet (Y3d_5/2 and Y3d_3/2 electrons) with a binding-energy separation of 2 eV. The two yttrium bonding sources are consistent with the XPS standard [16]. The Y3d_5/2 and Y3d_3/2 peaks in the spectrum of the as-deposited YF₃ film (Figure 5a) deconvolute into four peaks. The two peaks located at higher binding energies (159.4 and 161.4 eV) correspond to Y–F bonding in the YF₃ film, whereas those at lower binding energies (157 and 159 eV) are ascribed to Y–O bonding. In the XPS spectra of etched YF₃ film surface, the peaks are more intense than in the un-etched specimen (Figure 5b). Again, the strong doublet at higher binding energies (159.6 and 161.6 eV) and the weak doublet at lower binding energies (157 and 159 eV) correspond to Y–F and Y–O bonding, respectively. The higher XPS binding energy of the Y–F bonding can be attributed to the higher electronegativity of fluorine atoms (4.0) than oxygen atoms (3.5) [17]. Consequently, more electrons are transferred to fluorine, decreasing the electron density around the cation and enhancing the binding energy. Meanwhile, the intensity ratio of Y–F to Y–O bonds was estimated as 2.1 on etched YF₃ films, indicating strong chemical interaction between the YF₃ films and the fluorocarbon plasma.

To evaluate the plasma erosion resistance of as-deposited YF₃ films, the etching rate of the YF₃-coated Al₂O₃ substrate was measured after exposure to the CF₄/O₂ plasma for different etching times. The etching rate of bare Al₂O₃ crystal was used as the reference. Figure 6 shows that the etching depths of both samples linearly increased with increasing etching time. After 30 min, the etching depth reached 120 mm in the sapphire, but only 38 mm in YF₃. The erosion resistance of the YF₃ film to CF₄/O₂ plasma etching was more than three times higher than that of sapphire crystal. This is attributed to the chemical stability of YF₃ in a chemical environment dominated by fluorocarbon plasma. The surface morphologies and roughness of the YF₃ films etched for different lengths of time were measured by AFM measurements, with a scanning area of 5 m². The surface roughness results are shown in Figure 7. The etching time of the CF₄/O₂ plasma did not remarkably influence the surface roughness of the YF₃ film.

Figure 8a,b show the surface microstructures of YF₃ and Al₂O₃ samples, respectively, after the etching experiment. The micrographs were acquired by optical microscopy (OM). The right and left images in each panel show the un-etched and etched surfaces, respectively. The step height of the Al₂O₃ sample changed after CF₄/O₂ plasma etching. These images confirm the hardness and stronger CF₄/O₂ corrosion resistance of the YF₃ films compared to conventional Al₂O₃ substrate.

AlF₃ is a typical fluoride of Al₂O₃ materials [4]. The boiling temperature of YF₃ (2230 °C) is higher than the sublimation temperature of AlF₃ (1275 °C). Hence, YF₃ is more stable and more difficult to vaporize than AlF₃. Therefore, its fluorinated layer can be removed by a physical sputtering process. The sputtering yields of the films are inversely proportional to sublimation enthalpies of their compounds [18]. The sublimation enthalpy of YF₃ (481 ± 21 kJ/mol) is higher than that of AlF₃ (301 ± 4 kJ/mol). Moreover, the enthalpy of formation of the metal–oxygen bond is lower in YF₃ (−392 kJ mol⁻¹) than in Al₂O₃ (−279 kJ mol⁻¹). Therefore, YF₃ is chemically more stable than Al₂O₃ [19]. In particular, YF₃ is extremely stable under the fluorocarbon plasma etching process.

Figure 9 shows a cross-sectional HRTEM image of the YF₃ film after CF₄/O₂ plasma exposure. Continuous and nearly-complete lattice fringes were observed near the surface, indicating that the YF₃ ceramic crystalline lattice was not distorted by the fluorine plasma irradiation. Furthermore, the d-spacings of the near surface of film were 3.131 Å (blue dotted circle) and 2.621 Å (red dotted circle), very close to the d-spacings of the altered layers of yttrium oxyflouride (YOF): 3.147 Å (006) and 2.698 Å (104) [11]. The thin YOF layer formed on the YF₃ surface plays a protective role, suppressing the particle generation during CF₄/O₂ plasma etching process. The reaction and formation of an altered YOF layer has been reported previously [14]. Therefore, YF₃ film is a very attractive plasma-corrosion-resistant material that produces fewer contamination particles during the semiconductor fabrication process.

4. Conclusions

YF₃ films were successfully deposited through radio frequency magnetron sputtering on sapphire substrates at different temperatures. HRTEM and XRD results revealed polycrystalline YF₃ films with an orthorhombic structure. XPS results confirmed the superior chemical stability of YF₃ film after fluorocarbon plasma treatment. The robustness of YF₃ film was confirmed to be more than that of Al₂O₃ film after the plasma exposure. The YF₃ film is expected to provide a more protective barrier than Al₂O₃ or quartz plates in the fluorine plasma etching process.