MgO-based Ceramics with Excellent Corrosion
Resistance against Plasma Etching
プラズマエッチング耐性に優れる MgO 系セラミックス
Kota TSUTSUMI

堤

Shinzo MITOMI幸太

味冨Yuji KASASHIMA*晋三

笠嶋悠司 *Fumihiko UESUGI*

上杉

文彦 *

*National Institute of Advanced Industrial Science and Technology (AIST), Advanced Manufacturing
Research Institute, 807-1 shuku-machi, Tosu, Saga, 841-0052, Japan
Key words; corrosion resistance, plasma etching, component parts used in a chamber of plasma etchingequipment, MgO, composite ceramics, semiconductor
キーワード；耐腐食性、プラズマエッチング、プラズマエッチング装置用部材、MgO、複合セラミックス、半導体
AbstractIn plasma etching process, materials having higher corrosion resistance are required. We developed anadvanced MgO-based ceramics with excellent corrosion resistance and high mechanical characteristics.
Further, we also succeeded in lowering electric resistance to the order of 100 ohm centimeter without
deterioration of its excellent corrosion resistance. These are particularly suitable for component parts
used in plasma etching equipment.
プラズマエッチングプロセスでは、より高い耐食性を有する材料が求められている。我々は、優れた耐食性と機械特性を有する MgO 系複合セラミックスを開発した。また、耐腐食性を劣化させることなく、電気抵抗を 100
Ω cm まで低下させることにも成功した。これら材料はプラズマエッチング装置内で用いられる部品に適している。

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Nippon Tungsten Review Vol.40 (2016)

1. Introduction
With the shrinkage of semiconductor chip size, the requirement on corrosion resistance of materials
used in a plasma etching process becomes strict. In plasma etching process, not only a semiconductor
wafer but also component parts inside a chamber of plasma etching equipment are exposed to halogenbased corrosive reactive plasma. In the case of selecting materials having low corrosion resistance,component parts are corroded and change in their shapes in a short time. Finally, it becomes impossible
to keep their shapes as designed and their performances deteriorate. The reaction between the corrosive
reactive plasma and the materials can generate particles [1, 2]. The particles which fall on the siliconwafer cause killer defect such as pattern defect and short circuits. The change in shape of the component
parts and particles could be factors of the reduction in the production yield in a semiconductor
manufacturing process. Therefore, high corrosion resistance material is required.
Currently, yttrium oxide (Y2O3) is drawing attention because of its excellent corrosion resistance ascompared to aluminum alloy and aluminum oxide (Al2O3) which have been used as conventional
materials [2, 3]. However, Y2O3 has difficulty in obtaining a dense sintered body and it has low
mechanical strength (bending strength and hardness). Further, materials which contain a large amount of
rare-earth element such as Y2O3 are costly as compared to other ceramic materials.
In the previous study, we found that magnesium oxide (MgO) has excellent corrosion resistance against
CF4 plasma. Depending on the corrosive conditions, MgO has higher corrosion resistance than that ofY2O3. Further, MgO is extremely low in cost. Therefore, adopting MgO-based ceramics can contribute tothe improvement of corrosion resistance and cost reduction. On the other hand, MgO is not suitable forcomponent parts in semiconductor manufacturing equipment due to its poor mechanical characteristics.
MgO has a hardness of about 550 Hv in Vickers hardness and a bending strength of about 250 MPa,
even in the form of a dense sintered body. In order to use MgO-based ceramic material for various
component parts, it is necessary to improve mechanical characteristics.
We succeeded in enhancement of MgO mechanical characteristics without deterioration of corrosion
resistance by adding the phases of MgAl2O4 and YAlO3.
Further, component parts for semiconductor manufacturing equipment are required to have a low
electrical resistivity in some cases. Thus, we developed a MgO-MgAl2O4-YAlO3-based material which has
an electrical resistivity of the order of 100 ohm centimeter and excellent corrosion resistance.

2. Characteristics of“NMA”(MgO-MgAl2O4-YAlO3)
A MgO-MgAl2O4-YAlO3 composite sintered body (hereinafter referred to as NMA) can be obtained byconventional powder metallurgical method. Characteristics of NMA can be controlled by changing theamount of MgAl2O4 and YAlO3. The adjustable range of characteristics of NMA is shown in Table 1. As acomparative material, conventional Y2O3 is also shown in Table 1. The bending strength and hardness(Vickers hardness) of NMA are higher than those of Y2O3.

Nippon Tungsten Review Vol.40 (2016)–2–Table 1 Characteristics of NMA
NMA

Y2O3

Measurement condition

Bending strength

250 ～ 550MPa

100 ～ 200MPa

3-points bend method

Vickers hardness

Hv:550 ～ 1400

Hv:700 ～ 800

Load: 1kgf

Electrical resistance＞ 1015 Ω cm
（or 100 Ω cm）＞ 1015 Ω cm

Room temperature

Thermal conductivity

15 ～ 40W/(m/K)

10 ～ 15W/(m/K)

Laser flash method

Heat expansion
coefficient

6 ～ 10×10-6/K

7 ～ 8×10-6/K

373 K

Permittivity

9 ～ 1210 ～ 13

13.56MHz※ All values are representative ones.
As an example, the microstructure and mechanical characteristics of MgO-21%MgAl2O4-15%YAlO3(hereinafter referred to as NMA-1) are described below. A SEM image of fracture cross-section of NMA-1
sintered body is shown in Fig. 1(a). As a comparative example, MgO single phase sintered body which
was obtained by the same manufacturing process is also shown in Fig. 1(b).

(a)

(b)
Fig. 1 Microstructures of (a) NMA-1 and (b)MgO–3–

Nippon Tungsten Review Vol.40 (2016)

By compounding MgO with MgAl2O4 and YAlO3, the grain growth is suppressed during the sintering
process. Consequently, the grain size of NMA becomes smaller as compared to that of single-phase MgO.
Fig.2 shows a mirrored surface of NMA-1. Whitish parts in this image are YAlO3 grains. Uniform anddensified microstructures are confirmable.

10μm

Fig. 2 Microstructure on a mirrored surface of NMA-1
Mechanical characteristics of NMA-1 and single-phase MgO sintered body are shown in Table 2. The
bending strength and hardness (Vickers hardness) of NMA-1 are higher than those of MgO due to shrunkgrains, densified sintered body and added MgAl2O4 and YAlO3 which have high hardness.
Table 2 Mechanical characteristics of NMA-1
NMA-1

MgO

Measurement condition

Bendingstrength

554MPa250MPa3-points bend method

Vickershardness

Hv: 929

Hv: 550

Load: 1kgf
※ All values are central ones.

3. Corrosion resistance against plasma etching of“NMA”
3-1. Experimental
The composition of measurement samples is illustrated in Table 3. The measurement samples except
Si, SiC (CVD) and Al Alloy were obtained by the conventional powder metallurgical method. Each ofthem was formed into a shape having a diameter of 30 mm and a thickness of 4 mm. A part of sample
was masked by a masking tape. A schematic diagram of above sample is shown in Fig. 3.

Nippon Tungsten Review Vol.40 (2016)–4–Table 3 Composition of measurement samples
Sample nameComposition

Si waferSi

SiC (CVD)

SiC

Al Alloy

Al, Mg（2 ～ 3%）,etc（＜ 1.5%）

NPA-03

Al2O3、SiO2（＜ 0.5%）

Y2O3

Y2O3

NMA-1
( ＞ 1014 Ω cm)
NMA-2(6 Ω cm)

MgO, MgAl2O4(21%), YAlO3(15%)
MgO, MgAl2O4(21%), YAlO3(15%), C(3%)

Fig. 3 Schematic diagram of measurement sample
Each of the samples was etched by plasma etching under conditions shown in Table 4. The plasmaetching treatment in this work was carried out with parallel plate reactive ion etching equipment ofNational Institute of Advanced Industrial Science and Technology (AIST). CF4 was used as an etching
corrosive gas. A pressure of the CF4 was 10 Pa. A total etching time was 130 minutes.

–5–Nippon Tungsten Review Vol.40 (2016)Table 4 Conditions of plasma etching
RF Power

1000 WEtching Gas

CF4Gas pressure

10 PaGas Flux60 sccm

Cathode Temp.

60 ℃

Anode Temp.

30 ℃Etching Time

130minAfter etching, the etched amount was measured and specifically, the masking tape was peeled from themeasurement sample, and a step height between the etched surface and the masked (un-etched) surface
was measured as illustrated in Fig. 4. The measured step height was considered as an etched amount(corroded amount). The step height was measured by a contour measuring instrument (SURFCOM 2800,
Tokyo Seimitsu Co., Ltd).

Fig. 4 Schematic diagram of a measurement method for etched amount3-2. Results of the evaluation on resistance to corrosion
Each etching amount of the measurement samples is shown in Fig. 5. This result shows that ourdeveloped materials, NMA-1 and NMA-2, have higher corrosion resistance as compared to the othersamples. Each etched amount of NMA-1 and NMA-2 is as small as 1/5 of or less than that of Al2O3.
Further, both NMA-1 and NMA-2 have corrosion resistance superior to that of Y2O3 drawing attention as
excellent corrosion resistance material.

Nippon Tungsten Review Vol.40 (2016)–6–Fig. 5 Etching amount of the samples
Fig. 6 shows each surface microstructure of (a) Al2O3 and (b) NMA-1 after etching. The left side ofimages is a non-etched surface which was covered with masking tape. The right side of these is anetched surface. Obviously, the surface of Al2O3 becomes rough by etching treatment. On the other hand,the deterioration of surface roughness of NMA-1 is suppressed due to its excellent corrosion resistance.

100μm

100μm

(a)(b)

Fig. 6 Surface microstructures of (a) Al2O3 and (b) NMA-1 after plasma etching–7–

Nippon Tungsten Review Vol.40 (2016)

4. Application

NMA, a corrosion resistance MgO-based ceramics developed by our company, is particularly suitablefor component parts exposed to corrosive reactive plasma. For example, it may be used for component
parts for semiconductor manufacturing equipment, such as ring parts (Fig. 7), and other chamber parts.
These materials allow the component parts to have improved resistance to corrosion. In addition,
problems in a semiconductor manufacturing process such as a change in shape of the component partsand generation of particles may be solved.
NMA has significantly high mechanical characteristics as compared to conventional MgO and Y2O3.
Thus, it can be used for component parts requiring high hardness and bending strength.
NMA which has an electrical resistivity of the order of 100 ohm centimeter is usable as component
parts which need an adjustment of an electrical resistivity. Further, this conductive material can suppress
micro-arc discharge in a plasma etching equipment, and generation of particles caused by the micro-arc
discharge.

Fig. 7 Ring parts made of NMA

5. Conclusions

1) Our company’s “NMA” has excellent corrosion resistance to plasma etching using CF4 gas.
2) The etched amount of NMA is as small as 1/5 of or less than that of Al2O3. Further, NMA shows
corrosion resistance superior to that of Y2O3.
3) The deterioration of surface roughness of NMA by plasma etching is suppressed due to its excellent
corrosion resistance.
4) The bending strength and hardness (Vickers hardness) of NMA are higher than those of MgO andY2O3.
5) We also developed NMA which has an electrical resistivity of the order of 100 ohm centimeter.
6) NMA is particularly suitable for component parts used in plasma etching equipment.
7) NMA can contribute to the improvement of production yield in a semiconductor manufacturing
process, performance and quality of a semiconductor device.

Nippon Tungsten Review Vol.40 (2016)–8–

References[1] N. Ito, T. Moriya, F. Uesugi, M. Matsumoto, S. Liu, and Y. Kitayama, Jpn. J. Appl. Phys., Part 1 47,3630 (2008).
[2] K. Miwa, T. Sawai, M. Aoyama, F. Inoue, A. Oikawa, and K. Imaoka, Proceedings of the IEEE,
International Symposium on Semiconductor, Manufacturing, Paper No. PO-O-210, 479 (2007)[3] K. Miwa, N. Takada, K. Sasaki, J. Vac. Sci. Technol. A 27(4), 831 (2009)–9–Nippon Tungsten Review Vol.40 (2016)