Preparation and Characterization of Sprayed-Yttrium Oxyfluoride Corrosion Protective Coating for Plasma Process Chambers

This study investigates the microstructure, mechanical and electrical properties of dense yttrium oxyfluoride (YOF) coatings fabricated by the atmospheric plasma spraying technique. Transmission electron microscopy and X-ray diffraction analysis revealed a well crystallized YOF coating with preferred orientations. The YOF coatings were more porous (approximate porosity 0.5%), with higher hardness (290 ± 30 HV), lower electrical resistivity (1016 Ω·cm), and breakdown voltage (5.57 kV), than conventional yttrium-fluoride plasma-protective coating. These results indicate the potential of the YOF coating as a novel antiplasma and corrosion-resistant ceramic.


Introduction
Integrated circuit (IC) technology incorporates various features on single electrical chips [1].The increasing miniaturization of modern ICs has promoted the use of high-density plasma irradiation for silicon wafer processing.However, contaminants generated by the semiconductor plasma processing equipment pose a serious problem in the mass production of ICs [2].Contaminant particles are formed when corrosive fluorocarbon gases (such as CF 4 , CHF 3 , C 4 F 6 , and C 2 F 6 ) bombard and react with the inner wall and inner parts of the chamber, degrading the process reproducibility, causing leakage current in the ICs, and lowering the production yield [3,4].Particle formation during etching or deposition is usually prevented by silicon-based ceramics, which are advantaged by their high hardness, high wear resistance, dielectric strength, and chemical stability [5,6].However, particle contaminants from silicon-based materials are known to react with the fluorine plasma, generating tungsten silicide and polycrystalline silicon [7][8][9].
Aluminum oxide (Al 2 O 3 ) and yttrium oxide (Y 2 O 3 ), with higher plasma-erosion resistance than conventional silicon-based materials, have been adopted as the plasma-facing inner wall materials in the plasma processing equipment of IC production [10][11][12].Unfortunately, the inner-wall Y 2 O 3 coating reacts with the plasma to produce fluorine-containing particles [13].The resulting erosion releases particles that contaminate the wafer.
Substituting the Y 2 O 3 and Al 2 O 3 ceramic materials with yttrium fluoride (YF 3 ) is expected to resolve this problem [14,15].YF 3 reportedly has a high dielectric strength and YF 3 coating exhibits a higher plasma-erosion resistance than Y 2 O 3 coating.Moreover, the standard enthalpy of the metal-oxygen bond is lower in oxidized YF 3 coating (−392 kJ•mol −1 ) than in Y 2 O 3 coating (−318 kJ•mol −1 ), indicating that YF 3 is more chemically stable than Y 2 O 3 .Yttrium oxyfluoride (YOF) was recently detected on a YF 3 coating exposed to fluorocarbon plasma.The oxidized YF 3 effectively suppressed the particle generation [16] (and reference therein), suggesting that YOF coating is a suitable plasma-facing material in plasma processing equipment.Therefore, the present study compares the characteristics of YOF and YF 3 coatings fabricated by atmospheric plasma spraying (APS).By evaluating the microstructure, dielectric, and mechanical properties of the YOF and YF 3 coatings, we evaluated their potentials as protective materials in plasma processing equipment.

Materials and Methods
The spraying materials were commercially pure YOF powders (25-50 µm, 99.99%, Shin-Etsu Chemical, Tokyo, Japan) and YF 3 powders (25-50 µm, 99.99%, Shin-Etsu Chemical, Tokyo, Japan).The YOF and YF 3 coatings were fabricated by APS with an F4-MB plasma gun (Oerlikon Metco, Pfäffikon, Switzerland).Prior to APS, the alloy aluminum (A6061) substrate (of area and thickness 400 mm 2 and 20 mm, respectively) was sandblasted with SiO 2 and cleaned in acetone.The stand-off distance was adjusted to 10 cm.The Ar and H 2 gas cylinders were opened by initiating the air compressor (HAUG, St. Gallen, Switzerland).The Ar and H 2 flow rates, system voltage, gun movement rate, and feed rate were set to 45 L/min, 6 L/min, 50 V, 10 cm/s, and 15 g/min, respectively.Previously, we reported uniform plasma spreading at 15 kW; therefore we set the plasma power to 15 kW in the following work [17].
The YOF and YF 3 spraying parameters are shown in Table 1.The surface morphology, microstructure, and elemental analysis of the coating samples were conducted using scanning electron microscopy (SEM, S-3000H, Hitachi, Tokyo, Japan), energy dispersive X-ray diffraction (EDX, Bruker Nano GmbH, Berlin, Germany), and transmission electron microcopy (TEM, 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 Al Kα X-ray source at a passing energy of 20 eV with a spot size of 650 µm.The chemical-composition depth profile of the sample was obtained by etching the surface with focused argon-ion sputtering (Thermo Fisher Scientific K-Alpha, East Grinstead, UK).To estimate the contributions of bonding with fluorine elements, the photoelectron spectrum resulting from the core energy levels of Yttrium 3d states was deconvoluted by a fitting software program (Thermo Fisher Scientific, Waltham, MA, USA).The porosity of the coating was evaluated from the optical image by metallographic optical microscope.The bond strengths of the samples were measured in a pulling test (Cytec Fiberite FM1000, Havre de Grace, MD, USA) according to the ASTM D412-98a standard [18].The Vickers microhardness was estimated using the Vickers indentation method based on ASTM C1327-99 [19].The dielectric voltage was measured following the standard protocol of ASTM D419-09 [20].

Results and Discussion
Figure 1a,b show the surface and a cross-sectional SEM image, respectively, of the YOF coating sprayed at a plasma power of 15 kW.The surface microstructure exhibits fully molten granules and relatively flat morphologies (Figure 1a).The YOF coating layers (average thickness = 200 µm) are dense with very poor porosity (Figure 1b).Because the thermal expansions of the YOF coating (14.1 × 10 −6 /K) and Al substrate (23 × 10 −6 /K) are very similar, no cracks were observed in the YOF sample.These results indicate a better crystalline and denser microstructure of the YOF than the YF 3 coating sample.Density defects density such as pores, cavities and cracks significantly affect the antiplasma-erosion resistance of the material [21].Hence, the YOF coating is eminently applicable to plasma equipment.Density defects density such as pores, cavities and cracks significantly affect the antiplasma-erosion resistance of the material [21].Hence, the YOF coating is eminently applicable to plasma equipment.Figure 2 shows typical XRD patterns of the YOF sample coated under a plasma power of 15 kW.The highest diffraction peaks in Figure 2a are assignable to the (012) and ( 110) planes of a stable rhombohedral phase, and are exactly matched to the JCPD data (No.71-2100) [22].In Figure 2b, the XRD results show the (020) plane of the orthorhombic YF3 phase and the (400) plane of the cubic Y2O3 phase, which is in agreement with the JCPD data (No.74-0911) [17].The compositions of the two coating samples were determined by EDX.The EDX spectra are shown in Figure 3, and the obtained data are included in Table 2.In Figure 3a, The F, Y, and O contents in the YOF coating are 26.02at.%, 36.01 at.%, and 25.27 at.%, respectively, confirming that YOF was the major phase in the samples.The YF3 coating contained mainly F and Y atoms (64.62 at.% and 27.01 at.%, respectively), with minor amounts of oxygen (3.93 at.%; see Figure 3b).As discussed above, the YOF coating contained oxygen and fluorine elements, and both the oxide and fluoride materials were chemically stable during the plasma etching/deposition process [23].110) planes of a stable rhombohedral phase, and are exactly matched to the JCPD data (No.71-2100) [22].In Figure 2b, the XRD results show the (020) plane of the orthorhombic YF 3 phase and the (400) plane of the cubic Y 2 O 3 phase, which is in agreement with the JCPD data (No.74-0911) [17].Density defects density such as pores, cavities and cracks significantly affect the antiplasma-erosion resistance of the material [21].Hence, the YOF coating is eminently applicable to plasma equipment.Figure 2 shows typical XRD patterns of the YOF sample coated under a plasma power of 15 kW.The highest diffraction peaks in Figure 2a are assignable to the (012) and (110) planes of a stable rhombohedral phase, and are exactly matched to the JCPD data (No.71-2100) [22].In Figure 2b, the XRD results show the (020) plane of the orthorhombic YF3 phase and the (400) plane of the cubic Y2O3 phase, which is in agreement with the JCPD data (No.74-0911) [17].The compositions of the two coating samples were determined by EDX.The EDX spectra are shown in Figure 3, and the obtained data are included in Table 2.In Figure 3a, The F, Y, and O contents in the YOF coating are 26.02at.%, 36.01 at.%, and 25.27 at.%, respectively, confirming that YOF was the major phase in the samples.The YF3 coating contained mainly F and Y atoms (64.62 at.% and 27.01 at.%, respectively), with minor amounts of oxygen (3.93 at.%; see Figure 3b).As discussed above, the YOF coating contained oxygen and fluorine elements, and both the oxide and fluoride materials were chemically stable during the plasma etching/deposition process [23].The compositions of the two coating samples were determined by EDX.The EDX spectra are shown in Figure 3, and the obtained data are included in Table 2.In Figure 3a, The F, Y, and O contents in the YOF coating are 26.02at.%, 36.01 at.%, and 25.27 at.%, respectively, confirming that YOF was the major phase in the samples.The YF 3 coating contained mainly F and Y atoms (64.62 at.% and 27.01 at.%, respectively), with minor amounts of oxygen (3.93 at.%; see Figure 3b).As discussed above, the YOF coating contained oxygen and fluorine elements, and both the oxide and fluoride materials were chemically stable during the plasma etching/deposition process [23].The crystal structure of these coatings was revealed by TEM. Figure 4 show cross-sectional TEM micrographs of the YOF and YF3 coatings, respectively.The insets show the selected area electron diffraction (SAED) patterns of the two coating samples, which reveal their crystallographic relationships.The regularly arranged diffraction rings in the SAED patterns suggest a polycrystalline structure for both coatings, consistent with the XRD result.In the inset of Figure 4a, the diffraction rings derive from the (012) and (110) crystal facets, consistent with the rhombohedral phase of the YOF structure.In the inset of Figure 4b, they derive from the (111) and (012) crystal facets, consistent with the orthorhombic phase of the YF3 structure.The calculated porosity volume fractions of the YF3 and Y2O3 coatings obtained at various plasma-spray powers are shown in Figure 5.The YOF and YF3 coatings were least porous (1.0% and 0.5%, respectively) under a plasma-spray power of 21 kW.Therefore, the YOF and YF3 coatings at this power were very dense, as observed in the SEM morphologies [17].Note that the porosity was 0.5%-1.0%lower in the YOF coating than in the YF3 coating.As the plasma power increases, the granule velocity and deposition temperature also increase.These increases might explain the decreased porosity with better interface bonding under higher plasma powers [24].The crystal structure of these coatings was revealed by TEM. Figure 4 show cross-sectional TEM micrographs of the YOF and YF 3 coatings, respectively.The insets show the selected area electron diffraction (SAED) patterns of the two coating samples, which reveal their crystallographic relationships.The regularly arranged diffraction rings in the SAED patterns suggest a polycrystalline structure for both coatings, consistent with the XRD result.In the inset of Figure 4a, the diffraction rings derive from the (012) and (110) crystal facets, consistent with the rhombohedral phase of the YOF structure.In the inset of Figure 4b, they derive from the (111) and (012) crystal facets, consistent with the orthorhombic phase of the YF 3 structure.The crystal structure of these coatings was revealed by TEM. Figure 4 show cross-sectional TEM micrographs of the YOF and YF3 coatings, respectively.The insets show the selected area electron diffraction (SAED) patterns of the two coating samples, which reveal their crystallographic relationships.The regularly arranged diffraction rings in the SAED patterns suggest a polycrystalline structure for both coatings, consistent with the XRD result.In the inset of Figure 4a, the diffraction rings derive from the (012) and (110) crystal facets, consistent with the rhombohedral phase of the YOF structure.In the inset of Figure 4b, they derive from the (111) and (012) crystal facets, consistent with the orthorhombic phase of the YF3 structure.The calculated porosity volume fractions of the YF3 and Y2O3 coatings obtained at various plasma-spray powers are shown in Figure 5.The YOF and YF3 coatings were least porous (1.0% and 0.5%, respectively) under a plasma-spray power of 21 kW.Therefore, the YOF and YF3 coatings at this power were very dense, as observed in the SEM morphologies [17].Note that the porosity was 0.5%-1.0%lower in the YOF coating than in the YF3 coating.As the plasma power increases, the granule velocity and deposition temperature also increase.These increases might explain the decreased porosity with better interface bonding under higher plasma powers [24].The calculated porosity volume fractions of the YF 3 and Y 2 O 3 coatings obtained at various plasma-spray powers are shown in Figure 5.The YOF and YF 3 coatings were least porous (1.0% and 0.5%, respectively) under a plasma-spray power of 21 kW.Therefore, the YOF and YF 3 coatings at this power were very dense, as observed in the SEM morphologies [17].Note that the porosity was 0.5%-1.0%lower in the YOF coating than in the YF 3 coating.As the plasma power increases, the granule velocity and deposition temperature also increase.These increases might explain the decreased porosity with better interface bonding under higher plasma powers [24].Figure 6 presents the Y3d XPS spectra of the YOF and YF3 coating samples.The curve-fitted YOF XPS spectrum deconvolutes into four peaks (Figure 6a).The Y3d splits into two peaks (Y3d5/2 and Y3d3/2), implying two bonding sources of the cations.The intensity ratio and peak shift in binding energy (~3:2 and 2 eV, respectively), in accord with the XPS data reported in the literature [25].The peaks at higher binding energy (159 and 161.1 eV) are attributable to the Y-F bonds in the YOF coating sample, whereas the lower peaks (at 157 and 159 eV) correspond to the Y-O bonds.The energy difference is due to the higher electronegativity of fluorine (4.0) than of oxygen (3.5) [26].The XPS spectrum of the YF3 coating also exhibits two doublets (Figure 6b), one representing the high binding energy of the Y-F bonds (peaking at 159 and 161.1 eV), the other corresponding to the low binding energy of the Y-O bonds (peaking at 157.6 and 159.7 eV).As clarified in the TEM observations, the oxyflouride layer protects the coating of the chamber sidewall from further erosion by the fluorocarbon plasma, thereby reducing the particle contamination [16].Therefore, it is believed that the YOF coating is more chemically stable to CF4/O2 plasma than the YF3 coating.To meet the plasma resistance requirements, the YOF coating must also exhibit appropriate mechanical and electrical properties.The properties of YOF and YF3 are compared in Table 3.The Vickers' hardness values of the YOF and YF3 coatings were estimated as 260-320 and 235-285 HV, respectively.The YOF coating is harder than other yttrium-based plasma-resistant materials (YF3 and Y2O3) [17].The microstructure, residual stress, grain size and crystal phase configuration significantly affects the hardness of oxide ceramics [27].The higher hardness in the YOF coating might be attributed to the larger number of Y-O bonds and higher bond strength in YOF than in YF3.Meanwhile, YOF with its high concentration of oxygen vacancies can maintain its hardness under operation at high ambient temperatures.The dense microstructure of YOF also contributes to its hardness value.Figure 6 presents the Y3d XPS spectra of the YOF and YF 3 coating samples.The curve-fitted YOF XPS spectrum deconvolutes into four peaks (Figure 6a).The Y3d splits into two peaks (Y3d 5/2 and Y3d 3/2 ), implying two bonding sources of the cations.The intensity ratio and peak shift in binding energy (~3:2 and 2 eV, respectively), in accord with the XPS data reported in the literature [25].The peaks at higher binding energy (159 and 161.1 eV) are attributable to the Y-F bonds in the YOF coating sample, whereas the lower peaks (at 157 and 159 eV) correspond to the Y-O bonds.The energy difference is due to the higher electronegativity of fluorine (4.0) than of oxygen (3.5) [26].The XPS spectrum of the YF 3 coating also exhibits two doublets (Figure 6b), one representing the high binding energy of the Y-F bonds (peaking at 159 and 161.1 eV), the other corresponding to the low binding energy of the Y-O bonds (peaking at 157.6 and 159.7 eV).As clarified in the TEM observations, the oxyflouride layer protects the coating of the chamber sidewall from further erosion by the fluorocarbon plasma, thereby reducing the particle contamination [16].Therefore, it is believed that the YOF coating is more chemically stable to CF 4 /O 2 plasma than the YF 3 coating.Figure 6 presents the Y3d XPS spectra of the YOF and YF3 coating samples.The curve-fitted YOF XPS spectrum deconvolutes into four peaks (Figure 6a).The Y3d splits into two peaks (Y3d5/2 and Y3d3/2), implying two bonding sources of the cations.The intensity ratio and peak shift in binding energy (~3:2 and 2 eV, respectively), in accord with the XPS data reported in the literature [25].The peaks at higher binding energy (159 and 161.1 eV) are attributable to the Y-F bonds in the YOF coating sample, whereas the lower peaks (at 157 and 159 eV) correspond to the Y-O bonds.The energy difference is due to the higher electronegativity of fluorine (4.0) than of oxygen (3.5) [26].The XPS spectrum of the YF3 coating also exhibits two doublets (Figure 6b), one representing the high binding energy of the Y-F bonds (peaking at 159 and 161.1 eV), the other corresponding to the low binding energy of the Y-O bonds (peaking at 157.6 and 159.7 eV).As clarified in the TEM observations, the oxyflouride layer protects the coating of the chamber sidewall from further erosion by the fluorocarbon plasma, thereby reducing the particle contamination [16].Therefore, it is believed that the YOF coating is more chemically stable to CF4/O2 plasma than the YF3 coating.To meet the plasma resistance requirements, the YOF coating must also exhibit appropriate mechanical and electrical properties.The properties of YOF and YF3 are compared in Table 3.The Vickers' hardness values of the YOF and YF3 coatings were estimated as 260-320 and 235-285 HV, respectively.The YOF coating is harder than other yttrium-based plasma-resistant materials (YF3 and Y2O3) [17].The microstructure, residual stress, grain size and crystal phase configuration significantly affects the hardness of oxide ceramics [27].The higher hardness in the YOF coating might be attributed to the larger number of Y-O bonds and higher bond strength in YOF than in YF3.Meanwhile, YOF with its high concentration of oxygen vacancies can maintain its hardness under operation at high ambient temperatures.The dense microstructure of YOF also contributes to its hardness value.To meet the plasma resistance requirements, the YOF coating must also exhibit appropriate mechanical and electrical properties.The properties of YOF and YF 3 are compared in Table 3.The Vickers' hardness values of the YOF and YF 3 coatings were estimated as 260-320 and 235-285 HV, respectively.The YOF coating is harder than other yttrium-based plasma-resistant materials (YF 3 and Y 2 O 3 ) [17].The microstructure, residual stress, grain size and crystal phase configuration significantly affects the hardness of oxide ceramics [27].The higher hardness in the YOF coating might be attributed to the larger number of Y-O bonds and higher bond strength in YOF than in YF 3 .Meanwhile, YOF with its high concentration of oxygen vacancies can maintain its hardness under operation at high ambient temperatures.The dense microstructure of YOF also contributes to its hardness value.The adhesion strengths of the YOF and YF 3 coatings were 7.35 and 8.56 MPa, respectively.This result, which is consistent with the XPS results, might reflect the different types of chemical bonds in the YOF and YF 3 coatings.The dielectric strengths of the YOF and YF 3 coatings were 24.67 and 22.65 kV/mm, respectively.However, the dielectric strength rapidly decreases with increasing porosity [28].Therefore, the higher dielectric strength of the YOF than the YF 3 coating can be explained by the dense crystalline structure and low film porosity of the former, as mentioned in the SEM results.This means that the YOF coating prevents electron conductivity during high-voltage operation.Furthermore, the YOF coating exhibited superior insulating capability, with an electrical resistivity of 1 × 10 16 Ω•cm and a breakdown voltage of 5.57 kV, implying that YOF is an effective material for insulating applications.Moreover, owing to their large electrical resistivity and high corrosion resistance, YOF coatings meet the requirements of ceramic electrostatic chucks [29,30] (Coulomb type: Electrical resistivity > 10 14 Ω•cm, Johnsen-Rahbek type: Electrical resistivity > 10 10 Ω•cm).Overall, the mechanical and thermal properties of YOF coating are higher than those of the YF 3 coating.

Conclusions
A dense, low-porosity (0.5%-1.0%)YOF coating was deposited on an Al substrate by the APS method.The mechanical and electrical properties of the YOF coating were excellent, and comparable to those of YF 3 .These results demonstrated that YOF is a promising material for the inner walls and electrostatic chucks, and will be expected to prevent the particle contaminations of mass-production semiconductor-plasma processing equipment.

Figure 1 .
Figure 1.Surface (a) and cross-sectional SEM images (b) of yttrium oxyfluoride (YOF) coated at a plasma spray power of 15 kW.

Figure 1 .
Figure 1.Surface (a) and cross-sectional SEM images (b) of yttrium oxyfluoride (YOF) coated at a plasma spray power of 15 kW.

Figure 2
Figure2shows typical XRD patterns of the YOF sample coated under a plasma power of 15 kW.The highest diffraction peaks in Figure2aare assignable to the (012) and (110) planes of a stable rhombohedral phase, and are exactly matched to the JCPD data (No.71-2100)[22].In Figure2b, the XRD results show the (020) plane of the orthorhombic YF 3 phase and the (400) plane of the cubic Y 2 O 3 phase, which is in agreement with the JCPD data (No.74-0911)[17].

Figure 1 .
Figure 1.Surface (a) and cross-sectional SEM images (b) of yttrium oxyfluoride (YOF) coated at a plasma spray power of 15 kW.

Figure 3 .
Figure 3. Energy dispersive X-ray diffraction (EDX) results of (a) YOF and (b) YF3 coated under a plasma spray power of 15 kW.

Figure 4 .
Figure 4. Cross-sectional transmission electron microcopy (TEM) images of (a) YOF and (b) YF3 coated under a plasma spray power of 15 kW.

Figure 3 .
Figure 3. Energy dispersive X-ray diffraction (EDX) results of (a) YOF and (b) YF 3 coated under a plasma spray power of 15 kW.

Figure 4 .
Figure 4. Cross-sectional transmission electron microcopy (TEM) images of (a) YOF and (b) YF3 coated under a plasma spray power of 15 kW.

Figure 4 .
Figure 4. Cross-sectional transmission electron microcopy (TEM) images of (a) YOF and (b) YF 3 coated under a plasma spray power of 15 kW.

Figure 5 .
Figure 5. Porosities of the YOF and YF3 coatings obtained under different plasma-spray powers.

Figure 6 .
Figure 6.Variations of chemical compositions measured by X-ray photoelectron spectroscopy (XPS) with the sputtering times of (a) YOF and (b) YF3 coatings.

Figure 5 .
Figure 5. Porosities of the YOF and YF 3 coatings obtained under different plasma-spray powers.

Figure 6 .
Figure 6.Variations of chemical compositions measured by X-ray photoelectron spectroscopy (XPS) with the sputtering times of (a) YOF and (b) YF3 coatings.

Figure 6 .
Figure 6.Variations of chemical compositions measured by X-ray photoelectron spectroscopy (XPS) with the sputtering times of (a) YOF and (b) YF 3 coatings.

Table 2 .
Compositions of YOF and YF3 obtained by energy dispersive X-ray diffraction (EDX).

Table 2 .
Compositions of YOF and YF 3 obtained by energy dispersive X-ray diffraction (EDX).

Table 2 .
Compositions of YOF and YF3 obtained by energy dispersive X-ray diffraction (EDX).

Table 3 .
Mechanical and electrical properties of YOF and YF 3 .