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Article

Contamination Particles and Plasma Etching Behavior of Atmospheric Plasma Sprayed Y2O3 and YF3 Coatings under NF3 Plasma

by
Je-Boem Song
1,2,†,
Jin-Tae Kim
1,†,
Seong-Geun Oh
2 and
Ju-Young Yun
1,*
1
Materials and Energy Measurement Center, Korea Research Institute of Standards and Science (KRISS), Daejeon 34113, Korea
2
Department of Chemical Engineering, Hanyang University, Seoul 04763, Korea
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this study.
Coatings 2019, 9(2), 102; https://doi.org/10.3390/coatings9020102
Submission received: 26 December 2018 / Revised: 25 January 2019 / Accepted: 1 February 2019 / Published: 7 February 2019
(This article belongs to the Special Issue Surface Plasma Treatments)
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Abstract

:
Yttrium oxide (Y2O3) and yttrium oxyfluoride (YO0.6F2.1) protective coatings were prepared by an atmospheric plasma spraying technique. The coatings were exposed to a NF3 plasma. After the NF3 plasma treatment, the mass loss of the coatings showed that the etching rate of YO0.6F2.1 was larger than that of the Y2O3. X-ray photoelectron spectroscopy revealed that YO0.5F1.9 was present in the Y2O3 coating, whereas YO0.4F2.2 was present in the YO0.6F2.1 coating. Transmission electron microscope analysis conducted on contamination particles generated during the plasma etching showed that both coatings were mainly composed of YFx. The contamination particles estimated by in-situ particle monitoring sensor revealed that the YO0.6F2.1 compared with the Y2O3 coatings produced 65% fewer contamination particles.

1. Introduction

Plasmas are widely used for etching and cleaning in the semiconductor and display industries. Ceramic parts such as electrodes, shower heads, liners, and focusing rings used in these processes are exposed to the plasma. These parts erode and produce contamination particles, which cause serious problems, such as lowering the yield of mass-production [1,2,3,4,5]. In particular, when the dual frequency coupled plasma is applied, the showerhead in the position facing the wafer is heavily etched in a high flux of plasma [6,7,8,9,10]. Corrosion can be minimized with the use of ceramic coatings, which have outstanding plasma resistance. Yttrium oxide (Y2O3) is widely used as a coating material, owing to is low etching rate and low chemical reactivity. Recently, YOF and YF3 coatings have been reported as a new candidate, which can inhibit chemical reactions with fluorine gases, such as CF4, SF6, and NF3. The etching characteristics of fluorocarbon gases, such as CF4 and C2F6, have been widely studied. However, etching with these gases is often accompanied by the formation of an unnecessary fluorocarbon polymer layer; hence, NF3 gas is used as an alternative to fluorocarbon gases. Another advantage is that NF3 is almost fully dissociated in the discharge, which results in a high etching rate [11,12,13,14]. The erosion behaviors of Y2O3, YOF, and YF3 coatings in CF4/O2/Ar plasmas have been reported in previous works [15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34]. However, there have been no studies on the corrosion behavior of the yttrium-based materials or contamination particles generated from them in NF3 plasmas. In this study, we examine and compare the etching behavior and the generation of contamination particles in an NF3 plasma from Y2O3 and YO0.6F2.1 coatings, fabricated by atmospheric plasma spraying (APS).

2. Experimental

The disc-like substrates were made of Al alloy 6061 and had a diameter of 76 mm and thickness of 3 mm. The substrates were then coated with Y2O3 and YO0.6F2.1 by atmospheric plasma spraying (APS) [23,24,25,26,27,28,29,30,31], with the use of a plasma spray system (Mettech’s Axial III, Northwest Mettech Corp., North Vancouver, BC, Canada), where the Y2O3 and YF3 were in a powder form (99.99%, D50 = 30 µm, Shin-Etsu, Tokyo, Japan). The sprayed coatings of Y2O3 and YO0.6F2.1 were respectively 110 and 70 µm thick. The APS coating was performed as follows; the Ar, N2, and H2 at flow rates were 80, 80, and 20 L/min, respectively, were used to generate a plasma arc and the plasma arc current was 230 A.
Figure 1 shows a schematic diagram of the capacitively coupled plasma system. A specimen was placed in the upper electrode, as shown in Figure 1. The NF3 gas was used for plasma generation and was supplied through a showerhead with a mass flow controller. Magnets were inserted in the upper electrode to enhance the plasma density. A dry pump and turbo pump were used in the vacuum system and the working pressure of the experiment was 26.6 Pa. The power was set to be 13.56 MHz (Sizer Generator, Advanced Energy, Fort Collins, CO, USA), and an impedance matching network (Navigator, Advanced Energy, Fort Collins, CO, USA) was used to deliver the maximum power. The RF power applied to the plasma was 400 W. Before and after the NF3 plasma etching, the surface morphology and composition of the Y2O3 and YO0.6F2.1 coatings were analyzed by the field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Tokyo, Japan) and X-ray photoelectron spectroscopy (XPS) (Monochromatic Al-Kα, AXIS-NOVA, Manchester, UK), respectively. The mass of the specimen was measured before and after the plasma etching test using a XP205 analytical balance (Mettler Toledo, Greifensee, Switzerland). After plasma etching was performed for 10 min, the specimen was taken out and the mass loss was measured. This procedure was repeated until the accumulated plasma exposure time was 60 min.
The contamination particles produced from the Y2O3 and YO0.6F2.1 coatings were measured in real time according to the NF3 plasma exposure time. A light scattering sensor ISPM (Stiletto, In Situ Particle Monitor, Inficon, Heidiland, Switzerland) was attached to the exhaust line to measure the amount and size of the contamination particles. This system was capable of measuring contamination particles in real time as they passed through the exhaust pipe. The minimum measurable particle size was ~0.2 µm. The sensor was based on the principles of laser light scattering, and more details of its working principles can be found in previous reports [35,36]. The contamination particles generated during the plasma etching were collected on a TEM grid and observed for shape and composition under a transmission electron microscope (TEM, Taitan 300 K, Renton, WA, USA). As shown in Figure 1, the TEM grid was placed at the bottom of the chamber.

3. Results and Discussion

Figure 2 shows FE-SEM images of the surface of Y2O3 and YO0.6F2.1 coatings before and after exposure to NF3 plasma; Figure 2a,b for Y2O3 and Figure 2c,d for YO0.6F2.1. Before NF3 plasma etching, the surfaces of the Y2O3 and YO0.6F2.1 coatings had a similar rough surface. After plasma etching, the Y2O3 coating showed more cavities than the YO0.6F2.1 coating, as denoted in Figure 2b. This result is consistent with a recent report [29]. Figure 3 shows the mass loss of Y2O3 and YO0.6F2.1 vs. the NF3 plasma etching time. The YO0.6F2.1 coating was etched more than the Y2O3 coating. The etch rates of the Y2O3 and YO0.6F2.1 coatings were ~29 and 117 nm/min/m2, respectively. After the plasma exposure of the coatings, the amounts of Y (Yttrium), O (Oxygen) and F (Fluorine) were measured by XPS analysis. The results are presented in Table 1. Compared to a before etching specimen, the Y2O3 coating on the electrode after plasma exposure had less O, but more F. This result not surprising and is consistent with previous studies [29,30,31].
Figure 4 shows the XPS spectra for yttrium in Y2O3 and YO0.6F2.1 before and after the plasma treatment. We assigned dash lines the peaks in the XPS spectra to the cations of Y3d5/2 and Y3d3/2. The two peaks had a difference of 2.05 eV with an intensity ratio of 3:2 in their binding energy, which is consistent with the figure provided by national institute of standards and technology (NIST) [37]. In the case of pristine Y2O3, the Y3d5/2 peak positions were 157.35 and 156 eV, and the Y3d3/2 peak positions were 159.4 and 158.05 eV. When Y2O3 was exposed to the NF3 plasma, XPS analysis revealed binding energies of 158.65 eV for Y3d5/2 and 160.7 eV for Y3d3/2, which indicated binding of yttrium to fluorine and formation of Y–F bonds. Figure 4a consists of three Y–O peaks located at 159.4, 158.05, and 156 eV. As shown in Figure 4c, the peak shifted to higher energy could be attributed to the Y–F bond, which is possibly attributed to the different electronegativity of fluorine and oxygen atoms. When the oxygen atoms around the cations are replaced by fluorine atoms, more electrons transferred to fluorine. Therefore, the electron density around the cation decreases and the binding energy is enhanced [20]. This result indicates that the surface of Y2O3 reacted with fluorine radicals and was composed of YO0.5F1.9. However, the YO0.6F2.1 coating showed less change in the composition after the plasma treatment.
Figure 5 shows the real-time concentration of accumulated contamination particles generated from the Y2O3 and YO0.6F2.1 coatings during the NF3 plasma treatment of 60 min. The YO0.6F2.1 coating produced fewer contamination particles than did the Y2O3 coating; the concentration of particles measuring over 0.2 µm from the YO0.6F2.1 coating was less than 65% that of the Y2O3 coating. Figure 6 shows the distribution of the sum of contamination particles from Figure 5. Most contamination particles had sizes falling in the range of 0.2 to 0.5 µm. The etching rate of YO0.6F2.1 was higher than that of Y2O3 in NF3 plasma. However, YO0.6F2.1 produced less contamination particle than Y2O3. This can be explained as follows. The boiling temperature of Y2O3 and YF3 are 4570 and 2500 K, respectively. In addition, the sublimation enthalpies of Y2O3 are also higher than that of YF3. Hence, Y2O3 is more stable and more difficult to vaporize than YF3. Therefore, its sputtering yield by ion bombardment may be lower for the Y2O3 than for the YF3 containing a relatively large amount of oxygen. This is consistent with the result of Reference 17 and 30, where the etching rate differences depend on the bias voltage [17,30]. On the surface of Y2O3, YOxFy layer and volatile NOx are formed by the chemical reaction with the fluorine radical. On the other hand, YOxFy layer and NOx can be formed less on the surface of YO0.6F2.1 because Y–F bond already exists. Also, the less oxygen on the coating surface, the smaller the chemical reaction. Thus, the Y2O3 surface provides a more appropriate environment for the growth of YOxFy (or YF3) contamination particles, and, therefore, more contamination particles from Y2O3 are generated compared to YO0.6F2.1 [28,29]. Furthermore, in the case of YO0.6F2.1 surface, physical etching is more likely to occur than chemical etching by fluorine radical. YO0.6F2.1 is relatively inadequate to grow YF3 contamination particle compared to Y2O3.
Figure 7 shows TEM images of particles that detached from the coatings during plasma etching. The particles were of various sizes, and the selected particle was approximately 500 nm in size. The particles that fell off the Y2O3 and YO0.6F2.1 coatings had irregular shapes and crystalline structures. In addition to observing particle shapes by TEM, we used energy dispersive X-ray spectroscopy (EDS) to examine their composition. These results are listed in Table 2. The contamination particles derived from Y2O3 and YO0.6F2.1 contained almost no oxygen and their chemical composition was most likely YFx, and rather close to YF3, which is consistent with Reference [29,30,31].

4. Conclusions

We exposed Y2O3 and YO0.6F2.1 coatings prepared by atmospheric plasma spraying (APS) to NF3 plasma. Both coatings had rough surfaces in the pristine state, and no differences were observed between the two. When subjected to a NF3 plasma treatment, the Y2O3 coating showed many defects, and cavities formed in the coatings whereas the YO0.6F2.1 coating did form any cavities. We estimated the etching rates of Y2O3 and YO0.6F2.1 coatings from the mass loss to be ~29 and 117 nm/min/m2, respectively. During etching, the surface of Y2O3 reacted with fluorine radicals to form particles composed of YOxFy and YFx. However, particles from the YO0.6F2.1 coating showed almost no change in composition. Fewer contamination particles over 0.2 μm size were generated for the YO0.6F2.1 coating than for the Y2O3 coating. The particles produced from both coatings had irregular shapes, mainly consisting of YFx, with a composition close to YF3. These results indicate that the fluorine radicals replaced oxygen at the Y2O3 surface such that YFx particles were formed. The YO0.6F2.1 coating did not provide conditions suitable for YFx particles to grow. This study demonstrates that the YO0.6F2.1 coating might be used by the semiconductor industry as a candidate material to reduce contamination particles over 0.2 μm size.

Author Contributions

Conceptualization, J.-B.S. and J.-T.K.; Methodology, J.-B.S.; Investigation, J.-B.S.; Writing—Original Draft Preparation, J.-B.S. and J.-T.K.; Writing—Review and Editing, S.-G.O. and J.-Y.Y.; Supervision, S.-G.O. and J.-Y.Y.

Funding

This research was funded by the R&D Convergence Program of National Research Council of Science and Technology (NST) of the Republic of Korea (NST, CAP-16-04-KRISS) and Korea Research Institute of Standards and Science (KRISS-2018-GP2018-0011).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of capacitively coupled plasma (CCP) etching system.
Figure 1. Schematic diagram of capacitively coupled plasma (CCP) etching system.
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Figure 2. FE-SEM images of the surface of Y2O3 and YO0.6F2.1 coatings before and after exposure to NF3 plasma; (a) Y2O3 and (c) YO0.6F2.1 before etching, (b) Y2O3 and (d) YO0.6F2.1 after etching.
Figure 2. FE-SEM images of the surface of Y2O3 and YO0.6F2.1 coatings before and after exposure to NF3 plasma; (a) Y2O3 and (c) YO0.6F2.1 before etching, (b) Y2O3 and (d) YO0.6F2.1 after etching.
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Figure 3. Mass loss owing to NF3 plasma etching: Y2O3 and YO0.6F2.1 coatings.
Figure 3. Mass loss owing to NF3 plasma etching: Y2O3 and YO0.6F2.1 coatings.
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Figure 4. Peak positions of the XPS spectra of the surface; (a) Y2O3 and (b) YO0.6F2.1 before etching, (c) Y2O3 and (d) YO0.6F2.1 after etching.
Figure 4. Peak positions of the XPS spectra of the surface; (a) Y2O3 and (b) YO0.6F2.1 before etching, (c) Y2O3 and (d) YO0.6F2.1 after etching.
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Figure 5. Real-time detection of accumulated contamination particle concentration, over 0.2 μm size, generated from Y2O3 and YO0.6F2.1 during the 60 min NF3 plasma treatment.
Figure 5. Real-time detection of accumulated contamination particle concentration, over 0.2 μm size, generated from Y2O3 and YO0.6F2.1 during the 60 min NF3 plasma treatment.
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Figure 6. Size distribution of contamination particles, over 0.2 μm size, generated from Y2O3 and YO0.6F2.1 during the 60 min NF3 plasma treatment.
Figure 6. Size distribution of contamination particles, over 0.2 μm size, generated from Y2O3 and YO0.6F2.1 during the 60 min NF3 plasma treatment.
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Figure 7. TEM images of contamination particles; (a,b) generated from Y2O3 and (c,d) generated in YO0.6F2.1.
Figure 7. TEM images of contamination particles; (a,b) generated from Y2O3 and (c,d) generated in YO0.6F2.1.
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Table
Table 1. XPS analysis results of Y2O3 and YO0.6F2.1 coatings before and after exposure to the NF3 plasma.
Table 1. XPS analysis results of Y2O3 and YO0.6F2.1 coatings before and after exposure to the NF3 plasma.
Compound Content (at.%)Y2O3 CoatingYO0.6F2.1 Coating
Before EtchingAfter EtchingBefore EtchingAfter Etching
Yttrium (Y3d)28.329.226.927.3
Oxygen (O1s)70.315.016.811.8
Fluorine (F1s)1.455.856.360.9
Table
Table 2. EDS analysis results of contamination particles generated in Y2O3 and YO0.6F2.1 coatings after exposure to the NF3 plasma.
Table 2. EDS analysis results of contamination particles generated in Y2O3 and YO0.6F2.1 coatings after exposure to the NF3 plasma.
Compound Content (wt.%)Particle Generated
in Y2O3 Coating		Particle Generated
in YO0.6F2.1 Coating
Yttrium63.561.8
Oxygen0.80.6
Fluorine35.737.6

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Song, J.-B.; Kim, J.-T.; Oh, S.-G.; Yun, J.-Y. Contamination Particles and Plasma Etching Behavior of Atmospheric Plasma Sprayed Y2O3 and YF3 Coatings under NF3 Plasma. Coatings 2019, 9, 102. https://doi.org/10.3390/coatings9020102

AMA Style

Song J-B, Kim J-T, Oh S-G, Yun J-Y. Contamination Particles and Plasma Etching Behavior of Atmospheric Plasma Sprayed Y2O3 and YF3 Coatings under NF3 Plasma. Coatings. 2019; 9(2):102. https://doi.org/10.3390/coatings9020102

Chicago/Turabian Style

Song, Je-Boem, Jin-Tae Kim, Seong-Geun Oh, and Ju-Young Yun. 2019. "Contamination Particles and Plasma Etching Behavior of Atmospheric Plasma Sprayed Y2O3 and YF3 Coatings under NF3 Plasma" Coatings 9, no. 2: 102. https://doi.org/10.3390/coatings9020102

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