[溶接学会論文集 第 33 巻 第 ○ 号 p. 000s-000s（2015)]

Macroscopic and Microscopic Structure Evaluation of Al2O3 Plasma Sprayed Coatings＊

by Jun-ichi Takeuchi**, Ryo Yamasaki** , Kensuke Taguchi** and Yasuo Takahashi***

Alumina (Al2O3) coatings by two types of plasma spraying method (APS: atmospheric plasma spraying and LPS: low pressure plasma spraying) were observed at the view point of coating structure in order to confirm superiority for electric isolation application. A breakdown voltage of dielectric layer is due to interlamellar gap continuous across Al2O3 coatings. The micron size evaluation of this gap was confirmed by macroscopic structure observation. The cross-sectional macroscopic structure was examined by copper electrolytic plating for visualization and by a scanning electron microscopy (SEM). Furthermore, dielectric volume resistivity was dominantly derived from microscopic structure and crystallinity of the coatings. The cross-sectional microscopic structure of the coatings was observed by a field emission SEM (FE-SEM). The crystallographic analyses were performed by electron back scatter diffraction (EBSD). As an evaluation result, there are no differences in macroscopic structures with gap of coatings between two spraying methods. The Al2O3 particles partially melted in spraying and made cracks in solidifying. The gap was derived from the cracking of molten particles and also exhibited the same structures between two spraying methods. Coating electric properties had no significant differences between two spraying methods. In this study, the factors which so significant differences were not observed in electric properties each spraying methods are investigated macroscopically and microscopically.

Key Words: Atmospheric Plasma Spraying, Low Pressure Plasma Spraying, Al2O3 Coatings, Macroscopic structure, Dielectric layer

1. Introduction

Currently electrostatic chucks are used in various fields of semiconductor processing or liquid crystal panel manufacturing, to fix the silicon wafer and glass for performing precision treatment in a vacuum1). Fig.1 schematically illustrates electrostatic chucks. The principle is intended to gain electrostatic adsorption force by applying DC voltage to dielectric layer in Fig.12). In the previous studies3, 4), the authors developed this dielectric layer by using Al2O3 plasma sprayed coatings, to exert a Coulomb’s adsorption force3). In the developing process, the electrical properties of Al2O3 coatings were compared between the atmospheric plasma spraying (APS) and the low pressure plasma spraying (LPS). The APS method is able to spray to various complex works easily. On the other hand, the LPS method is able to produce dense coatings due to high kinetic energy of the sprayed molten particles. Because it was very important to confirm superiority of two methods in terms of economic efficiency, the superiority was evaluated. As a result, so significant differences were not observed in coating electric properties. A dielectric breakdown voltage, dielectric volume resistivity and electric constant of Al2O3 coatings sprayed by each method were very similar4). The difference in two methods was just the spraying atmosphere. In the APS process, it was the normal air pressure and in the LPS method, it was a low

pressure consisting of argon gas. When spraying in low pressure, the spray jet is not affected by the air resistance. Therefore, in the LPS method, the spraying can be done to the substrate by the molten particles with high velocities. As a result, sophisticated and dense coating could be formed. For example, the porosity of Al2O3 coatings sprayed by APS was about 12%, which was just superior to that of LPS coatings (6%) as measured by Amano et al5). In spite of a big difference in porosity of internal coating layer between two spraying methods, each coating electric properties was found to have no significant differences. Therefore, the purpose of this study was clarifying the cause of electrical properties which are not so different between two spraying methods. The structure of Al2O3 coatings sprayed by the two methods were observed macroscopically and microscopically and compared. In order to evaluate the dielectric breakdown voltage, leakage path (vertical gap), the internal Al2O3 coatings was observed macroscopically as a macro size (>50µm) structure combined with copper electrolytic plating by SEM image. Also, in order to evaluate dielectric volume resistivity, micron size (1-2µm) structure of Al2O3 coatings was observed microscopically by FE-SEM and crystallographic was analyzed by EBSD. As a total evaluation result, no big difference will be recognized in macrostructure and microstructure each method.

Fig. 1 A schematic illustration of electrostatic chucks．

* Received: 2014.11.28 ** Tocalo Co.,Ltd. ,Kobe, Japan

*** Member, Joining and Welding Research Institute (JWRI), Osaka University, Osaka, Japan

［溶接学会論文集　第 33 巻　第 2 号　p. 30s-33s （2015）］

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2. Cross-sectional macroscopic structure of Al2O3 coatings

sprayed by APS and LPS

2.1 Experimental procedure of evaluation by SEM The cross-sectional macroscopic structures of the Al2O3

coatings sprayed by the two spraying methods were observed. Test coating specimens were APS and LPS sprayed 50µm thick onto Al plates by using pure Al2O3 powder which has a specification shown in Table 1. The used spraying parameters are presented in Table 2. In order to evaluate the interlamellar gap as general through-path inside of the Al2O3 coatings, macroscopic observation was carried out. In that stage, in order to clarify interlamellar gap, internal pore were given by a copper plating process. The copper plating process was carried out by sulfuric acid with copper sulfate in solution for impregnating copper into the internal coating structure. The copper plating conditions were similar to those reported by Adachi et al6). The concentration of the copper plating solution is shown in Table 3. The cross-sectional metallographic test specimens were prepared by cutting, mounting in epoxy resin, polishing by SiC emery paper and finally by lapping with 0.03µm size alumina powder suspensions. Evaluation of the cross-sectional macroscopic structures was carried out by a scanning electron microscopy (SEM) and interlamellar gap ratio was investigated using SEM images and 2D image analysis software (Win ROOF).

2.2 Results of evaluation by SEM Fig. 2 presents the typical macroscopic structures of the Al2O3

coatings at 250µm by SEM (BEI). The black spots in the coatings are the pore. The APS coating has higher porosity than the LPS coating. As a result of measuring porosity using by image analysis, 10% was obtain for the APS coating and 4% for the LPS coating. It was confirmed that the LPS method was able to produce dense coatings due to high kinetic energy of the sprayed molten particles, as was also described by Amano et.al5).

Table１ Specifications of Al2O3 powder.

Al2O3

Impurity

>99.99%

Si<90 ppm , Fe<50 ppm , Na<20 ppm Table 2 Spraying parameters.

Method APS LPSGun　Type F4，Oerlikon Metco F4VB，Oerlikon MetcoAtmosphere Air, (1013hPa) Argon, (180hPa)

Plasma gas Argon, Hydrogen Argon, Hydrogen

Input power 42ｋW 37ｋW

Fig. 3 presents the cross-sectional macroscopic structures of copper plated Al2O3 coatings as a SEM observation. The copper plating seems as white part to be located in the interlamellar gap and in the pore between each Al2O3 particles. The interlamellar gap of the latter coating is the leakage path of electrical conductivity. It was found from the observations of the copper plated macroscopic structures that there were no clear differences between the two method coatings. A dielectric breakdown voltage is affected by the leakage path as interlamellar gap leading to the coating and the substrate continuously. Any leakage path difference was not observed in Fig. 3. In order to compare the interlamellar gap area ratio of Al2O3

particles quantitatively, these gap between the particles except the pore are colored with green color, as can been seen in Fig. 4. This is to determine the area occupied by the interlamellar gap in total measuring area (enclosed by yellow broken line in Fig.4). Fig. 4 is a SEM image obtained by 2D image analysis and shows

a test result of area ratio between interlamellar gap area and measuring total area in Table 4.

Table 3 Concentration of the copper plating solution used.

CuSO4・5H2O 100 (g/l)

H2SO4 150 (g/l)CuCl2 50 (mg/l)Plating brightener KuppelightNSA-A (Nihon Kagaku Sangyo Co.,Ltd.)Plating brightener Kuppelight

NSA-AB (Nihon Kagaku Sangyo Co.,Ltd.)

2 (ml/l)

4 (ml/l)

(a) (b)

Fig. 2 Typycal macroscopic SEM image of Al2O3 coatings sprayed by

APS (a) and LPS (b).

Fig. 3 SEM image of copper plated Al2O3 coatings sprayed by APS (a)

and LPS (b). White part in the coating is copper plating.

100μm 100μm

(a) (b)

10μm 10μm

31s溶 接 学 会 論 文 集 第 33 巻（2015）第 2号

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As a result, the interlamellar gap area ratio of 9.02% was obtained for APS coating and 10.2% for LPS coating. They gained almost a similar value. This result shows that there was no relationship between coating porosity and electric leakage path.

In order to evaluate the difference of the vertical gap in the direction perpendicular to coating layer, a line analysis in three horizontal directions (along red color lines) was carried out on the cross-sectional macro-SEM images, as can be seen in Fig. 5. The point number of vertical gap (copper plating) crossing those red lines was counted on the SEM image with green directing arrow. It was investigated by using three macro-SEM images for each coating. This test result is shown in Table 5. 201 points were as total numbers for the APS coating, and 223 points were counted for the LPS coating. It did not appear a large difference between these results. This mechanism is considered as follows; melted Al2O3 powder is flying to substrate by plasma jet. When it reaches to the surface of substrate, the molten Al2O3 particles are cooling down quickly. This cooling speed is approximately equal between two spraying methods, and then it takes same micro cracks (vertical gap) occurring. There are no significant differences between the APS and LPS coatings at point of view macroscopic structure and therefore the dielectric breakdown voltage is the same.

Fig. 4 Explanation exsample for SEM image of copper plated Al2O3

coating sprayed by APS. Green line shows interlamellar gap as

leakage path except the pore.

Table 4 Measurement datum of interlamellar gap area ratio by 2D

image analysis software (Win ROOF).

Sample Interlamellar Total measuring

Number gap area areaAPS-1 138 8.28%APS-2 169 10.13%

APS-3 145 8.65%LPS-1 134 9.40%LPS-2 153 10.80%

LPS-3 146 10.30%

Average9.02%

Average10.20%

1671

1413

Ratio of interlamellar gap area/total measuring area

Table 5 Test results of counting vertical gap point at APS and LPS

Al2O3 coating.

Method TotalAPS 22 21 17 24 20 24 26 23 24 201

LPS 23 20 23 27 25 27 28 29 21 223

Vertical gap crossing poit number

3. Cross-sectional microscopic structure of Al2O3 coatings sprayed by APS and LPS

3.1 Experimental procedure of evaluation by FE-SEM and EBSD

The cross-sectional microscopic structure of the Al2O3 coatings sprayed by two methods was compared. Test coating specimens were sprayed in the thickness of 50μm onto the one side of the aluminum substrate, using pure Al2O3 powder, the specification of which is shown in Table 1. The spraying conditions are shown in Table 2. The cross-sectional metallographic test specimens were cut and mounted in epoxy resin and then polished by SiC emery paper. After that, they were lapped by a cross-section polisher JEOL SM-0910. Evaluation of the cross-sectional microscopic structures was carried out by FE-SEM (Hitachi S-400). The crystalline phase evaluation was carried out by EBSD (TSL OIM). The measuring condition was as follows; the accelerating voltage was 20 kV, the evaluation zone was 50 x 26 microns, and the step length was 0.2µm with the spot diameter of 0.2 nm.

3.2 Results of evaluation by FE-SEM The evaluation results by FE-SEM are shown in Fig. 6.

Morphology of each coating was similar structure. It was recognized that bond layers (surrounded by red broken line) are different gap such as interlamellar gap and pore in high magnification image. These bond layers are observed in the boundary of the laminated particle 2-3μm thickness in a direction substantially perpendicular to the stacking direction of the coating. The bond layers must be made from molten particles jointed each other during cooling down at spraying. These thicknesses of bond layer were the same between two spraying methods. This phenomenon suggests that the cooling speed at joining each molten particles is the same, when the APS and LPS spraying.

Fig. 5 Explanation exsample of counting vertical gap points using by

SEM image. This coating was coated by APS.

Crossing point number

Crossing point example 10μm

22

21

17

10μm

32s 研究論文　　TAKEUCHI et al.: Macroscopic and Microscopic Structure Evaluation of Al2O3 Plasma Sprayed Coatings

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3.3 Results of evaluation by EBSD The evaluation results of the crystalline phase of the Al2O3

coating by EBSD are shown in Fig.7. According to the Image Quality Map in upper part of Fig.7, excellent crystallinity part is indicated by white color. On the other hand, poor crystallinity or irregularities part are indicated by dark color. Both of APS and LPS coatings consist in independent 10μm size white grains and dark color zone. It was considered, during spraying molten Al2O3 particles were cooling down rapidly and crystallinity was deteriorated. Inverse Pole Figure Map (normal direction) was mapped to identify Al2O3 α-phase and γ-phase based on the Image Quality Map as shown in lower part of Fig. 7. White color was identified Al2O3 α-phase and dark color zone was identified Al2O3 γ-phase and others. Al2O3 α-phase part was not flat shape. This indicates that un-melted particles exist in the coatings. Almost all APS coatings were formed the γ-phase although a little part was α-phase. In contrast, the LPS coating often showed poor crystallinity. There are no large significant differences between the APS and LPS coatings from the view point of crystallography. The bond layers indicated by FE-SEM observation (Fig.6) were investigated in detail. The bond layer was found to be a collection of ambiguous crystalline fine particles at APS and LPS coatings and the crystalline phase were almost same.

(a) (b)

Fig.6 FE-SEM image of Al2O3 coating sprayed by APS (a) and LPS (b).

(a) (b)

Inve

rse

Pole

Fig

ure

Map

Spray ingMethod

ImageQuality

Map

γ-Al2O3

α-Al2O3

Fig.7 Results of EBSD analyzed Al2O3 coating by APS (a) and LPS (b).

4. Conclusions

(1) From SEM evaluation results of macroscopic structure of copper plated Al2O3 coatings by APS and LPS method, it was found that the interlamellar gap area ratio except the pore was almost equal between APS and LPS coatings. Also, according to measurement of the vertical gap number, the leakage paths in the direction perpendicular to the coating layers were almost the same between two spraying methods. Therefore, it was endorsed that the dielectric breakdown voltage was approximately equal between APS and LPS coatings, that is, existence status of leakage paths was almost the same.

(2) From FE-SEM evaluation results of microscopic structure of Al2O3 coatings by APS and LPS method, it was found that microscopic structures did not differ between two spraying methods. The bond layers with the thickness of 2-3 µm was observed in both coatings. The bond layers were made of Al2O3 particles bonded with each other.

(3) From EBSD analysis it was suggested that both coatings have not clear crystalline structure. The structures were not so large different from the view point of crystallography. Crystalline phases were mainly γ-Al2O3 .

(4) The macroscopic and microscopic structures of the Al2O3

coatings sprayed by the APS and LPS method were almost similar. It is concluded that sprayed coatings with similar electrical properties can be obtained whichever the APS or LPS method is used.

Acknowledgements

The authors would like to express thanks to Professor Petri Vuoristo at Tampere University of Technology in Finland for the advice in this study.

References

1) T.Iguchi, T.Tachikawa, Y.Kayamoto：Journal of the vacuum society of Japan, Vol.45, No.8 (2002), pp.633-636

2) Field, J., "Electrostatic Wafer Clamping for next-Generation Manufacturing," Solid State Technology, 37 (9) (1994), pp.91-98

3) J.Takeuchi, R.Yamasaki, K.Tani and Y. Takahashi：Proc. of 18th Symposium on “Micro joining and Assembly Technology in Electronics”31 Jan.-1st Feb, Yokohama , Japan (2012), pp.421-424

4) J.Takeuchi, R.Yamasaki, K.Tani, Y.Takahashi：Transactions of the JSME (in Japanese) Vol. 77, No. 779 (2011-7), pp.367-374

5) K.Amano, Y.Kataoka “Mechanical properties of Plasma sprayed Alumina coatings”: Reports of Industrial Research Institute Aichi Prefectural Government, No. 26 (1990-11), pp.49-58

6) M.Adachi, T.Takabatake, A.Ohmori, M.Kremer :Proc of ITSC 2008, 2nd-4th June, Maastricht, Netherland (2008)

Bond layer

33s溶 接 学 会 論 文 集 第 33 巻（2015）第 2号