CaF2 polycrystalline body, focus ring, plasma processing apparatus, and method for producing CaF2 polycrystalline body

Abstract

A CaF.sub.2 polycrystalline body which is a polycrystalline body constituted of CaF.sub.2 and of which an average grain size of crystalline grains is 200 m or more, and a method for producing a CaF.sub.2 polycrystalline body, the method including a process of introducing a compact which is obtained by using a CaF.sub.2 powder raw material into a vacuum sintering furnace and sintering at a temperature of not higher than 1400 C. for six hours or more, thereby obtaining the CaF.sub.2 polycrystalline body.

Claims

1. A CaF.sub.2 polycrystalline body which is a polycrystalline body constituted of CaF.sub.2 and of which an average grain size of crystalline grains is 200 m or more.

2. The CaF.sub.2 polycrystalline body according to claim 1, wherein a relative density is 94.0% or more.

3. A focus ring which is constituted of the CaF.sub.2 polycrystalline body according to claim 1.

4. A plasma processing apparatus which is provided with the focus ring according to claim 3.

5. A method for producing the CaF.sub.2 polycrystalline body according to claim 1, the method including a process of introducing a compact which is obtained by using a CaF.sub.2 powder raw material into a vacuum sintering furnace and sintering at a temperature of not higher than 1400 C. for six hours or more, thereby obtaining the CaF.sub.2 polycrystalline body.

6. A focus ring which is constituted of the CaF.sub.2 polycrystalline body according to claim 2.

7. A plasma processing apparatus which is provided with the focus ring according to claim 6.

8. A method for producing the CaF.sub.2 polycrystalline body according to claim 2, the method including a process of introducing a compact which is obtained by using a CaF.sub.2 powder raw material into a vacuum sintering furnace and sintering at a temperature of not higher than 1400 C. for six hours or more, thereby obtaining the CaF.sub.2 polycrystalline body.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a relationship diagram between a crystalline grain size (average grain size of crystalline grains) and an etching rate, of a polycrystalline body according to Examples and Comparative Examples.

(2) FIG. 2 is a relationship diagram between sintering time and a crystalline grain size (average grain size of crystalline grains), of a polycrystalline body according to Examples and Comparative Examples.

(3) FIG. 3 is a SEM (Scanning Electron Microscope) image (50-fold) of a polycrystalline body sample according to Example 2.

(4) FIG. 4 is a SEM (Scanning Electron Microscope) image (1000-fold) of a polycrystalline body sample according to Comparative Example 1.

(5) FIG. 5A is a schematic diagram showing a top view of a focus ring which is constituted of CaF.sub.2 polycrystalline body according to an embodiment.

(6) FIG. 5B is a schematic diagram of a focus ring which is constituted of CaF.sub.2 polycrystalline body according to an embodiment and is a diagram showing a cross-section view of FIG. 5A along the line X1-X2.

(7) FIG. 6 is a schematic diagram of a plasma processing apparatus which is provided with a focus ring constituted of CaF.sub.2 polycrystalline body according to an embodiment.

DESCRIPTION OF THE EMBODIMENTS

(8) Hereinafter, embodiments of the present invention will be described in detail. Note that, these embodiments are specifically described for better understanding of the scope of the present invention, and the present invention is not limited thereto, unless otherwise specified.

(9) A CaF.sub.2 polycrystalline body according to the present embodiment is a polycrystalline body which is constituted of CaF.sub.2, and the average grain size of crystalline grains is 200 m or more.

(10) In the CaF.sub.2 polycrystalline body according to the present embodiment, the relative density can be 94.0% or more and is preferably 99.0% or more in order to have superior process-ability. FIG. 3 is a SEM (Scanning Electron Microscope) image of a CaF.sub.2 polycrystalline body with a relative density of 94.0% or more (99.69%), and FIG. 4 is a SEM (Scanning Electron Microscope) image of a CaF.sub.2 polycrystalline body with a relative density of less than 94.0% (93.71%).

(11) The relative density of the CaF.sub.2 polycrystalline body can be obtained by measuring the density of the CaF.sub.2 polycrystalline body using Archimedes' Principle and expressing the ratio of the density of the CaF.sub.2 polycrystalline body to the density of a CaF.sub.2 single crystal as a percentage.

(12) As is shown in FIG. 3, neither vacancies nor residual particles are observed in the CaF.sub.2 polycrystalline body according to the present embodiment. On the other hand, as is shown in FIG. 4, in the case that the relative density is less than 94.0%, vacancies and residual particles are observed. The CaF.sub.2 polycrystalline body, due to such vacancies or residual particles, absorbs cutting oil or water at the time of processing.

(13) In addition, in the case that the relative density is less than 94.0%, because the contact surface in contact with a corrosive material is increased, the corrosion durability is also negatively affected.

(14) In the CaF.sub.2 polycrystalline body according to the present embodiment, the average grain size of the crystalline grains (crystalline grain size) is 200 m or more. In the present embodiment, it is clarified that corrosion durability is enhanced with the increase of the crystalline grain size and that, when the crystalline grain size becomes 200 m or more, the corrosion durability reaches a region of saturation.

(15) It is considered that the reason why corrosion durability is enhanced with the increase of the crystalline grain size is that the interface which is easy to be etched is reduced with the increase of the crystalline grain size. This is because the crystal interface is preferentially etched. In addition, it is understood that corrosion durability of the CaF.sub.2 polycrystalline body of the present embodiment is close to that of CaF.sub.2 single crystal. Thus, the CaF.sub.2 polycrystalline body of the present embodiment combines an advantage of polycrystalline body which does not have cleavability with an advantage of single crystal body which has superior corrosion durability.

(16) As is described above, the CaF.sub.2 polycrystalline body of the present embodiment has excellent process-ability and excellent corrosion durability. Note that, CaF.sub.2 which constitutes the above-described polycrystalline body may include an impurity element as much as the crystalline quality and corrosion durability of the CaF.sub.2 polycrystalline body are not negatively affected.

(17) A method for producing a CaF.sub.2 polycrystalline body of the present embodiment has a process of introducing a compact which is obtained by using a CaF.sub.2 powder raw material into a vacuum sintering furnace and sintering at a temperature of not higher than 1400 C. for six hours or more, thereby obtaining the CaF.sub.2 polycrystalline body.

(18) The particle size (median size) of the CaF.sub.2 powder raw material can be 3 m or less and is preferably 0.5 m or less. In the case that the size of the CaF.sub.2 powder raw material is large, it is possible to crush it into pieces by ball milling or the like in advance of use.

(19) By using the above-described CaF.sub.2 powder raw material, forming is performed by using a CIP (Cold Isostatic Press) method, a casting method, a vibration filling method, or the like.

(20) The CIP method is a method in which the CaF.sub.2 powder raw material is preliminarily formed by mold press, and the preliminary compact is vacuum-packed and then is set in a CIP apparatus where pressure holding is performed at a predetermined pressure (for example, 50 to 200 MPa) and for a predetermined length of time (for example, 1 to 10 minutes), thereby preparing a compact. For example, pressure holding may be performed at a pressure of 100 MPa for 1 minute to prepare the compact.

(21) The casting method is a method in which slurry that is prepared by mixing the CaF.sub.2 powder raw material and water is cast into a plaster mold and, for example, is maintained stationary at room temperature for 48 hours or more to obtain a compact, and then the compact is removed from the plaster mold and is dried in a drying furnace at a predetermined temperature (for example, 30 to 90 C.) for a predetermined length of time (for example, 3 to 240 hours). For example, the compact may be dried at a condition of 80 C. and 48 hours.

(22) The vibration filling method is a method in which the CaF.sub.2 raw material powder is filled into a carbon mold and then set in a vibrating machine where, for example, vibration filling is performed at predetermined amplitude (for example, 1 to 6 mm), at a predetermined vibration frequency (for example, 20 to 120 Hz), and for predetermined vibration time (for example, 5 to 120 sec), thereby preparing a compact. For example, vibration filling may be performed at a condition of amplitude of 4.6 mm, a vibration frequency of 40 Hz, and vibration time of 60 sec.

(23) A compact which is formed by the above-described forming method is introduced into a vacuum sintering furnace and sintered. The sinter process is performed in a vacuum atmosphere, and the degree of vacuum can be 10 Pa or less for the purpose of densification and prevention of oxidation of CaF.sub.2.

(24) In the sinter process, the compact is sintered for 6 hours or more at a temperature of 1400 C. or less, which is a temperature that is not higher than the melting point of CaF.sub.2. When the sintering temperature is higher and the sintering time is longer, the crystalline grain size of the prepared polycrystalline body tends to become larger. However, there are disadvantages such as the decrease of weight of the polycrystalline body due to volatilization of the compact and the increase of production lead time. Therefore, the sintering temperature is 1400 C. or less and is preferably 1250 to 1350 C. In addition, the sintering time is 6 hours or more and is preferably 24 hours or less.

(25) By the above-described production method, a CaF.sub.2 polycrystalline body having a relative density of 94.0% or more and an average grain size of crystalline grains of 200 m or more is obtained. In order to additionally improve corrosion durability of the obtained CaF.sub.2 polycrystalline body, mirror polishing is performed such that the surface roughness (Ra) can be 1 m or less, preferably 0.5 m or less, and more preferably 0.1 m or less.

(26) The CaF.sub.2 polycrystalline body of the present embodiment is excellent in corrosion durability and process-ability and therefore is effectively used as an inner wall member of a semiconductor manufacture apparatus such as a plasma processing apparatus or a focus ring. The focus ring is a member which is provided at the circumference of a processed object in order to reduce the non-uniformity of etching rate of the processed object which arises from non-uniform plasma distribution when the processed object is plasma-etched in a plasma processing apparatus.

(27) Hereinafter, a focus ring which is constituted of CaF.sub.2 polycrystalline body of the present embodiment and a plasma processing apparatus having the focus ring are described with reference to FIGS. 5A, 5B, and FIG. 6.

(28) FIGS. 5A and 5B are schematic diagrams of the focus ring which is constituted of CaF.sub.2 polycrystalline body according to the present embodiment. FIG. 5A shows a top view of the focus ring, and FIG. 5B shows a cross-section view of the focus ring of FIG. 5A along the line X1-X2.

(29) A focus ring 1 which is shown in FIGS. 5A and 5B is formed in a circular shape and is in a shape having a step portion 2 at an inner circumferential portion of the focus ring 1. The focus ring 1 having such a configuration is used such that the inner circumferential portion surrounds the processed object.

(30) FIG. 6 is a schematic diagram of a plasma processing apparatus having the focus ring 1 which is shown in FIGS. 5A and 5B.

(31) A plasma processing apparatus 3 shown in FIG. 6 is provided with a top electrode 7 and a bottom electrode 8 within a chamber 4 having a gas supply port 5 and a gas discharge port 6. In addition, the top surface of the bottom electrode 8 is provided with the focus ring 1 shown in FIGS. 5A and 5B and an electrostatic chuck 9 so as to support a processed object 10. The electrostatic chuck 9 is arranged to be surrounded by the focus ring 1, and by placing the processed object 10 on the electrostatic chuck 9, the circumference of the processed object 10 is surrounded by the focus ring 1.

(32) In order to plasma-etch the processed object 10 by using the plasma processing apparatus 3 of such a configuration, firstly, etching gas is supplied from the gas supply port 5 while the chamber 4 is evacuated, and a radiofrequency voltage is applied to the top electrode 7 and the bottom electrode 8. Thereby, the etching gas is made to be plasma by a radiofrequency electric field which is formed between the top electrode 7 and the bottom electrode 8. Then, etching of the processed object 10 is performed by this plasma.

(33) During the plasma etching of the processed object 10 is performed as described above, the focus ring 1 is also exposed to the plasma as is the case with the processed object 10. For this reason, it is required to use a material having corrosion durability as the material of the focus ring 1. As is described above, the CaF.sub.2 polycrystalline body of the present embodiment has excellent corrosion durability and therefore can be suitably used as the material of the focus ring 1 which is exposed to plasma as is shown in FIGS. 5A and 5B and FIG. 6.

Example 1

(34) Hereinafter, Examples of the present invention will be specifically described. However, the present invention is not limited to the Examples described below.

Preparation of Sample

Reference Example

(35) By using the method disclosed in Japanese Patent Publication No. 4158252, CaF.sub.2 powder raw material was melted and solidified to obtain an ingot of CaF.sub.2 single crystal. Then, cutting and grinding process of the ingot were performed by using a cutting machine and a grinding machine, and test pieces having a sample size of 20 mm20 mm2 mm were cut out. Note that, at the time of the cutting and grinding process, cutting oil or water was used in order to facilitate the process. Then, both surfaces of the sample were mirror-polished, and the surface roughness (Ra) was made to be 0.1 m or less. Thus, CaF.sub.2 single crystal of Reference Example having crystal orientation of <111> directed toward the top surface of the sample surface was obtained.

Examples 1 to 4, Comparative Examples 1 to 5

(36) CaF.sub.2 powder raw material having a median size of 32.7 m was prepared. Crushing process by ball milling was applied to this material, and raw materials having a median size of 2.4 m, 2.7 m, and 3.1 m were prepared. In addition, by using the method disclosed in Japanese Unexamined Patent Application, First Publication No. 2006-206359, CaF.sub.2 fine particles having a median size of 0.5 m were prepared. The CaF.sub.2 fine particles were produced by reacting an aqueous solution of calcium acetate and an aqueous solution of hydrogen fluoride to obtain a solution in which fine particles of calcium fluoride were suspended, loading the solution in which fine particles of calcium fluoride were suspended into a closed vessel, performing heating at a temperature of not lower than 100 C. and not higher than 300 C., and drying the heated suspension of fine particles of calcium fluoride at a temperature of not lower than room temperature and not higher than 70 C.

(37) By using these materials, forming was performed.

(38) The forming was performed by using any one of the CIP (Cold Isostatic Press) method, the casting method, and the vibration filling method, as the forming method.

(39) In the CIP method, powder was preliminarily formed by mold press, and the preliminary compact was vacuum-packed. Then, the preliminary compact was set in a CIP apparatus, and pressure holding was performed at a pressure of 100 MPa for 1 minute. Thus, a compact was prepared.

(40) In the casting method, CaF.sub.2 raw material and water were mixed and thereby slurry was prepared. The obtained slurry was cast into a plaster mold and was maintained stationary at room temperature for 48 hours or more to obtain a compact. Then, the obtained compact was removed from the plaster mold and was loaded in a drying furnace, where the compact was dried at a temperature of 80 C. for 48 hours. Thus, the compact was prepared.

(41) In the vibration filling method, CaF.sub.2 raw material powder was filled into a carbon mold and then was set in a vibrating machine where vibration filling was performed at amplitude of 4.6 mm and a vibration frequency of 40 Hz for vibration time of 60 sec. Thereby, a compact was prepared.

(42) The compact which was formed by using the above-described forming method was introduced into a vacuum sintering furnace and was sintered at a condition shown in Table 1.

(43) The obtained polycrystalline body was cut into samples having a sample size of 20 mm20 mm2 mm as is the case with the above-described Reference Example. Then, both surfaces of the sample were mirror-polished, and the surface roughness (Ra) was made to be 0.1 m or less. Thus, CaF.sub.2 poly crystal of Examples 1 to 4 and Comparative Examples 1 to 5 was prepared.

(44) Following evaluations were made using CaF.sub.2 single crystal of Reference Example and CaF.sub.2 poly crystal of Examples 1 to 4 and Comparative Examples 1 to 5, which were prepared as described above. Results were shown in Table 1.

Bulk Density, Relative Density

(45) Density measurements were performed by Archimedes' Principle. Electric balance AUX320 provided by Shimadzu Corporation was used as the measurement machine. The percentage relative to the density of 3.18 g/cm.sup.3 of CaF.sub.2 single crystal of Reference Example was defined as the relative density.

Crystalline Grain Size

(46) Measurements of crystalline grain size of the polycrystalline body were performed by observing crystalline grains using scanning electron microscope (SEM). As the pretreatment for measurements, the sample was dipped in 60% perchloric solution (Wako Pure Chemical Industries, Ltd.) at room temperature for 24 hours. Then, the sample was rinsed by pure water and dried, and PtPd coating was applied to the sample. The SEM observation was performed in a reflected electron image mode and was performed at 50-fold or 1000-fold magnification, depending on the size of the crystalline grain size. Any three fields of view were observed in one sample. The long axis and the short axis of crystalline grains within each field of view were measured, and the average was made to be crystalline grain size (average grain size). The measurement of the axis conformed to JISR1670, Measurement method of grain size of fine ceramics. Note that, regarding Reference Example, because it was formed as single crystal, the column of crystalline grain size in Table 1 was filled with No grain boundary.

Etching Rate, Durability

(47) The prepared polished sample was dipped in 60% perchloric solution at a temperature of 80 C. for 1 hour. The thickness of the sample before and after dipping was measured, and the difference between the thickness of the sample before dipping and the thickness of the sample after dipping was divided by process time. The result of calculation was made to be etching rate. For the thickness measurement of the sample, Digital micrometer ME50HA (provided by NIKON) was used.

(48) The sample having an etching rate of 10 m/hr or less was judged as Good, while the sample having an etching rate exceeding 10 m/hr was judged as No-good.

Process-Ability

(49) When a test piece having a sample size of 20 mm20 mm2 mm was cut out, a determination of Good was made in the case that absorption of cutting oil or water was not observed, while a determination of No-good was made in the case that absorption of cutting oil or water was observed.

(50) TABLE-US-00001 TABLE 1 MEDIAN SIZE OF RAW MATERIAL SINTERING SINTERING POWDER FORMING TEMPERATURE TIME SAMPLE No. EXAMPLE STRUCTURE (m) METHOD ( C.) (hr) 1 REFERENCE SINGLE EXAMPLE CRYSTAL 2 EXAMPLE 1 POLY 0.5 CIP 1250 12 CRYSTAL 3 EXAMPLE 2 POLY 0.5 CIP 1250 6 CRYSTAL 4 EXAMPLE 3 POLY 2.7 CASTING 1350 6 CRYSTAL 5 EXAMPLE 4 POLY 2.7 CASTING 1350 24 CRYSTAL 6 COMPARATIVE POLY 32.7 VIBRATION 1250 12 EXAMPLE 1 CRYSTAL FILLING 7 COMPARATIVE POLY 3.1 CIP 1250 3 EXAMPLE 2 CRYSTAL 8 COMPARATIVE POLY 2.4 CIP 1250 3 EXAMPLE 3 CRYSTAL 9 COMPARATIVE POLY 2.7 CASTING 1250 3 EXAMPLE 4 CRYSTAL 10 COMPARATIVE POLY 2.7 CASITNG 1250 3 EXAMPLE 5 CRYSTAL BULK RELATIVE CRYSTALLINE ETCHING DENSITY DENSITY GRAIN SIZE RATE SAMPLE No. (g/cm.sup.3) (%) (m) (m/hr) PROCESS-ABILITY DURABILITY 1 3.18 100.00 NO GRAIN 4.5 BOUNDARY 2 3.18 100.00 360.8 8.5 GOOD GOOD 3 3.17 99.69 252.3 9.0 GOOD GOOD 4 3.15 99.06 420.8 10 GOOD GOOD 5 3.17 99.69 705.3 7 GOOD GOOD 6 2.98 93.71 233.3 7.5 NO-GOOD GOOD 7 3.14 98.74 92.7 17.5 GOOD NO-GOOD 8 3.17 99.69 123.7 26.5 GOOD NO-GOOD 9 3.15 99.06 91.0 39.5 GOOD NO-GOOD 10 3.10 97.48 88.0 24.2 GOOD NO-GOOD

(51) As is shown in Table 1, in the CaF.sub.2 polycrystalline body of Comparative Example 1 having a relative density of less than 94.0%, grinding (cutting) oil, water, or the like was absorbed at the time of process, and thus a difficulty was found in the process-ability.

(52) In addition, in the CaF.sub.2 polycrystalline body of Comparative Examples 2 to 5 having a crystalline grain size (average grain size of crystalline grains) of the polycrystalline body of less than 200 m, etching rates exceeded 10 m/hr, and thus inferior durability was confirmed.

(53) Moreover, on the basis of the results shown in Table 1, the relation between the crystalline grain size (average grain size of crystalline grains) and the etching rate with respect to the polycrystalline body was shown in FIG. 1. It was found from FIG. 1 that, as the crystalline grain size was increased, the etching rate was reduced and corrosion durability was enhanced. It was generally known that the crystal interface was preferentially etched. Therefore, it was considered that the reason for the results in FIG. 1 was that the interface which was easy to be etched was reduced with the increase of the crystalline grain size. In the present invention, it was found that corrosion durability was in a saturation range when the crystalline grain size was 200 m or more. It was estimated that, because the etching rate of single crystal was 4.5 m/hr, the fitted curve became close to this value.

(54) Next, on the basis of the results shown in Table 1, the relation between the sintering time and the crystalline grain size (average grain size of crystalline grains) with respect to the polycrystalline body was shown in FIG. 2. As is shown in FIG. 2, the crystalline grain size depended on the sintering time, and the crystalline grain size tended to be increased with the increase of the sintering time. It was confirmed that crystalline grain size of 200 m or more was able to be achieved by sintering for 6 hours or more.

(55) From the above-identified results, according to the present invention, it is clearly possible to provide a CaF.sub.2 polycrystalline body having excellent corrosion durability and excellent process-ability and a method for producing the same.

Availability for Industrial Application

(56) According to the present invention, it is possible to provide a CaF.sub.2 polycrystalline body of which corrosion durability and process-ability are excellent and a method for producing the same. By using the CaF.sub.2 polycrystalline body of the present invention, durability of a member which is used at a condition of being exposed to plasma, such as a focus ring, can be improved.