Production of corrosion-resistant ceramic members

A process is disclosed for producing a corrosion-resistant ceramic member, which process includes the steps of preparing a sintered body made of a ceramic material containing at least aluminum, immersing the ceramic sintered body in hydrofluoric acid, and forming a film of aluminum fluoride at a surface layer portion of the ceramic sintered body by heating the ceramic sintered body.

BACKGROUND OF THE INVENTION
 Field of the Invention
 The present invention relates to a process for producing
 corrosion-resistant ceramic members. More particularly, the invention
 relates to a process for producing corrosion-resistant ceramic members,
 which are to be favorably used as members in semiconductor-producing
 apparatuses such as heat CVD apparatuses.
 With the increase in memory capacity of super LSIs, the degree of fine
 processing of such super LSIs has been increasing greatly, and processes
 requiring chemical reactions have been expanded. Particularly, in the
 semiconductor-producing apparatuses requiring a super clean state, a
 halogen-based corrosive gas, such as a chlorine-based gas or a
 fluorine-based gas, is used as a deposition gas, an etching gas or a
 cleaning gas.
 In a heating apparatus-in which a semiconductor is heated while being
 contacted with such a corrosive gas, for example, a
 semiconductor-producing apparatus such as a heat CVD apparatus, a
 semiconductor-cleaning gas composed of a halogen-based corrosive gas such
 as ClF.sub.3, NF.sub.3, CF.sub.4, HF or HCl is used after the deposition.
 In the depositing step, another halogen-based corrosive gas such as
 WF.sub.5 or SiH.sub.2 Cl.sub.2 is used as a film-forming gas.
 Each of the members constituting the semiconductor-producing apparatus is
 made of alumina or aluminum nitride. When such members are exposed to and
 contacted with the above halogen-based corrosive gases at a high
 temperature, their surfaces corrode and subsequently, the corroded portion
 peel from the members in the form of particles.
 If such particles are heaped on a substrate placed in the
 semiconductor-producing apparatus, the heaped particles cause insufficient
 insulation or insufficient conductivity resulting in making the
 semiconductor unacceptable.
 In view of the above problem, NGK Insulators, Ltd. disclosed in Japanese
 patent application No. 3-150,932 (filed on May 28, 1991) and Japanese
 patent application No. 4-58,727 (filed on Feb. 13, 1992) that a sintered
 body made of aluminum nitride with a layer of aluminum fluoride on the
 surface thereof exhibits high corrosion resistance against plasmas of the
 above noted halogen-based corrosive gases. That is, even when the aluminum
 nitride sintered body is exposed to the ClF.sub.3 gas or similar gas for
 one hour, the surface state of the sintered body does not change.
 Further, NGK Insulators, Ltd. disclosed in JP-A 5-251,365 that a film of
 aluminum fluoride is formed on a surface of a sintered body made of
 aluminum nitride by using a gas phase growing method.
 Further, it is disclosed in JP-A 7-273,053 that in order to prevent
 corrosion of the surface of an electrostatic chuck for semiconductor
 wafers, the surface is subjected to a surface treatment in which a film of
 aluminum fluoride is formed through preliminarily replacing the surface of
 the electrostatic chuck by fluorine with use of fluorine plasma.
 The aluminum fluoride layer on the surface of the aluminum nitride sintered
 body as described in Japanese patent application No. 3-150,932 and
 Japanese patent application No. 4-58,727 may be formed by using the
 sputtering technique. However, when the sputtering technique is used, only
 the surface of the sintered body opposed to a sputtering target is covered
 with aluminum fluoride. Therefore, if the member to be covered has a
 cylindrical shape, it is difficult to cover a peripheral face of the
 member.
 If the gas phase growing technique as disclosed in JP-A 5-251,365 is used,
 the above-mentioned problem can be prevented, because the raw gas to be
 subjected to a reaction is fed to a peripheral surface of a cylindrical
 member. Further, if the fluorine plasma is used as disclosed in JP-A
 7-273,053, the above-mentioned problem can be also prevented, because the
 fluorine-based gas to generate the plasma is sufficiently fed to the
 peripheral surface of the cylindrical member.
 However, in order to completely cover the member with aluminum fluoride,
 each of the gas phase growing reaction and the plasma processing needs to
 be continuously effected for 50 hours or more in these techniques. Thus,
 clearly, these techniques require a lengthy processing time which leads to
 extremely poor productivity.
 Further, if the gas phase growing reaction or the plasma treatment is
 effected with the fluorine-based gas for a long time period, the chamber
 used for effecting the above reaction or treatment is also corroded,
 thereby increasing the costs of processing.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a new process for
 producing a corrosion-resistant ceramic member by forming a film of
 aluminum fluoride at a surface portion of the ceramic sintered body.
 The present invention relates to the process for producing a
 corrosion-resistant ceramic member, comprising the steps of preparing a
 sintered body made of a ceramic material containing at least aluminum,
 immersing the ceramic sintered body in hydrofluoric acid, and forming a
 film of aluminum fluoride at a surface layer portion of the ceramic
 sintered body by heating the ceramic sintered body.
 According to the corrosion-resistant ceramic member-producing process of
 the present invention, the aluminum fluoride film can be formed over the
 entire surface portion of the ceramic sintered body containing at least
 aluminum by directly immersing the sintered body in hydrofluoric acid and
 effecting the following reaction through heating the sintered body.
EQU Al.sub.2 O.sub.3 +6HF.fwdarw.2AlF.sub.3 +3H.sub.2 O
 Further, according to the process of the present invention, since a film of
 aluminum fluoride having high corrosion resistance can be formed at the
 surface portion of the ceramic sintered body in a relatively short time
 period, about 5 to 20 hours, the process of the present invention enables
 the corrosion-resistant ceramic members to be produced at a low cost and
 high rate of production.
 The present invention a process for producing a corrosion-resistant ceramic
 member, comprising the steps of preparing a first sintered body made of a
 ceramic material containing at least aluminum, immersing the ceramic
 sintered body in hydrofluoric acid, forming a first film of aluminum
 fluoride at a surface layer portion of the ceramic sintered body by
 heating the ceramic sintered body, separately preparing at least one
 second sintered body made of a ceramic material containing at least
 aluminum, generating a gaseous product containing aluminum fluoride
 through corroding a surface of said at least one second ceramic sintered
 body by contacting the second ceramic sintered body with a fluorine-based
 corrosive gas, and forming a second film of aluminum fluoride by
 precipitating the gaseous product on the first aluminum fluoride film of
 the first ceramic sintered body.
 These and other objects, features and advantages of the invention will be
 apparent upon reading of the following description of the invention when
 taken in conjunction with the attached drawing, with the understanding
 that some modifications, variations and changes could be easily made by
 the skilled person in the art to which the invention pertains.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention will be explained in more detail with reference to
 specific examples of the present invention.
 Since a film of aluminum fluoride needs to be formed at a surface portion
 of the ceramic sintered body, aluminum must be contained in the ceramic
 sintered body to be used in the present invention. As specific examples,
 alumina (Al.sub.2 O.sub.3), aluminum nitride (AlN), mullite (3Al.sub.2
 O.sub.3 --2SiO.sub.2) and spinel (MgO--Al.sub.2 O.sub.3) may be used, for
 example, as a material for the sintered body.
 (0012)
 The sintered body is produced by mixing a ceramic powder as a raw material
 preferably with a binder, by means of ball milling or the like, obtaining
 a pressed body by a uniaxial pressing, and pressureless sintering or hot
 press sintering the pressed body. The sintered body is treated, that is
 surface-treated e.g., ground, polished or sand blasted, in order that the
 sintered body may then be subjected to a reaction with hydrofluoric acid.
 A film of aluminum fluoride is formed in the ceramic sintered body
 according to the present invention by using a reacting container shown in
 FIG. 1, in the following manner.
 A given amount of hydrofluoric acid is placed in a Teflon container 1, a
 ceramic sintered body made of alumina, for example, is placed therein.
 Thereafter, the Teflon container 1 is covered with a Teflon lid 2 to seal
 the container.
 Next, the Teflon container 1 is placed in a stainless holding container 3
 via a lower plate 7 for the stainless holding container, and a stainless
 lid 4 is screwed to the stainless holding container 3.
 Next, a tightening bolt 5 provided in the stainless lid 4 is screwed
 inwardly to seal the Teflon lid 2 to the Teflon container 1 via the upper
 plate 6 for the stainless holding container.
 The sintered body is heated to a given temperature by placing the stainless
 holding container 3 in a drying unit, for example. At that time, HF gas
 permeating the Teflon container is released to the outside through a gas
 escape hole 8.
 The concentration of hydrofluoric acid is not particularly limited, but the
 concentration is preferably 1 to 50 wt % so as to form a uniform film of
 aluminum fluoride around the ceramic sintered body.
 In order to form a film of aluminum fluoride that does not peel or crack
 even if the sintered body is subjected to a heating/cooling test wherein
 it is heated to a given temperature and then left to be cooled to room
 temperature, the concentration of hydrofluoric acid is preferably 1 to 40
 wt %. Further, in order to form a film of aluminum fluoride that does not
 peel or crack even if the sintered body is subjected to a heating/rapid
 cooling test where it is heated to a given temperature, and then cooled in
 water, the concentration of hydrofluoric acid is preferably 5 to 30 wt %.
 The temperature at which the sintered body is heated according to the
 method of the present invention is not particularly limited as long as a
 film of aluminum fluoride is formed over the surface of the ceramic
 sintered body. However, in order to form a film of aluminum fluoride in a
 shorter time period and to form such a film of aluminum fluoride that does
 not peel or crack even if the sintered body is subjected to a
 heating/cooling test, the heating is effected preferably at a temperature
 of 100.degree. C. to 250.degree. C. Further, in order to form a film of
 aluminum fluoride that does not peel or crack even if the sintered body is
 subjected to a heating/rapid cooling test, the heating is preferably
 effected at 130.degree. C. to 200.degree. C.
 The thickness of the aluminum fluoride thus formed is preferably 0.5 .mu.m
 to 100 .mu.m to afford sufficient corrosion resistance upon the ceramic
 sintered body as a substrate, more preferably the thickness is 1 .mu.m to
 50 .mu.m.
 Further, in order to remove excess hydrofluoric acid attached to the
 sintered body after the treatment and to improve adhesion of the aluminum
 fluoride film to the sintered body, the ceramic sintered body formed with
 the aluminum fluoride as mentioned above is preferably heated at
 200.degree. C. to 500.degree. C. in air.
 The time period required for the formation of a film of aluminum fluoride
 having a thickness of 1 .mu.m to 50 .mu.m under the above-mentioned
 condition is 5 to 20 hours, thus making the treating time period shorter
 and increasing productivity, when compared with the conventional gas phase
 growing technique and the plasma treating technique. Further, since only
 simple heating devices such as the Teflon container and the drying unit
 are required in the method of the present invention, the treatment cost
 can be decreased further.
 The corrosion-resistant ceramic member produced according to the method of
 the present invention can be effectively used as a substrate for a member
 which is to be exposed to temperatures as high as 400.degree. C. to
 500.degree. C. Examples include a member for a semiconductor-producing
 apparatus such as a heat CVD apparatus, a susceptor to be heated through
 heating with an infrared lamp, a ceramic heater for heating
 semiconductors, a susceptor placed on a heating surface of the ceramic
 heater, a susceptor in which an electrostatically chucking electrode is
 buried, a susceptor in which an electrostatically chucking electrode and a
 resistive heating element are buried, a susceptor in which a high
 frequency plasma generating electrode is buried, or a susceptor in which a
 high frequency plasma generating electrode and a resistive heating element
 are buried.
 Members normally, exposed to such high temperatures are generally largely
 corroded with a fluorine-based corrosive gas in particular. A film of
 aluminum fluoride formed in the surface portion of the ceramic sintered
 body made of alumina, aluminum nitride or the like in a thickness of 0.5
 to 100 .mu.m according to the method of the present invention acts as a
 barrier layer against the above fluorine-based corrosive gas and prevents
 corrosion of the ceramic sintered body. As a result, the formation of
 particles which leads to defects of the semiconductors can be prevented.
 The corrosion-resistant ceramic member produced according to the method of
 the present invention can also be effectively used as substrate for a
 member which is to be exposed to a relatively low temperature of
 200.degree. C. to 400.degree. C., such as a shower plate.
 As mentioned above, a member exposed to a fluoine-based corrosive gas at a
 high temperature is corroded at its surface portion, and a gaseous product
 of a fluoride is generated. If the gaseous product floats in a vacuum
 chamber in which the semiconductor-producing apparatus is placed, and
 reaches the above member at the relatively lower temperature, the gaseous
 product is deposited on the surface of the member.
 Although the member at such a relatively lower temperature is also made of
 a ceramic sintered body of such as alumina or aluminum nitride similar to
 the high temperature members, the coefficient of thermal expansion and the
 crystalline structure differ between the ceramic sintered body and the
 fluoride formed through the deposition of the gaseous product. Adhesion
 between the ceramic sintered body and the deposited fluoride is low, so
 that the fluoride peels in the form of particles. Such particles cause the
 defects of the semiconductors as described in connection with the prior
 art.
 In this case, if a film of aluminum fluoride is preliminarily formed in a
 surface portion of the above ceramic sintered body constituting the above
 low temperature member by heating the sintered body immersed in
 hydrofluoric acid under the above-mentioned condition and the above
 gaseous fluoride product is deposited on the surface of the ceramic
 sintered body, the deposited fluoride will not peel, because the
 preliminarily formed film and the deposited film are both made of
 fluorides and their coefficients of thermal expansion and the crystalline
 structures are similar.
 Since the ceramic sintered body used in the present invention contains
 aluminum, a gaseous product of aluminum fluoride is generated when the
 ceramic sintered body is exposed to the fluorine-based corrosive gas.
 Thus, in this case, since the film formed on the surface of the ceramic
 sintered body and the deposited fluoride are both aluminum fluoride,
 adhesion between the two is extremely high.
 Due to this high adhesion the deposited aluminum fluoride will not peel but
 instead, will remain on the aluminum fluoride film formed on the surface
 of the ceramic sintered body. This means that an additional film of
 aluminum fluoride is formed on the existing film of aluminum fluoride on
 the sintered body.
 Experiment
 The present invention will now be explained in more detail with reference
 to examples.
 Alumina powder (particle size: 0.5 .mu.m) was used as the ceramic material.
 This raw material powder was uniaxially pressed under pressure of 200
 kgf/cm.sup.2, thereby obtaining a pressed body having a rectangular
 parallel piped shape, and a sintered body of 60 mm in length, 60 mm in
 width and 20 mm in thickness was produced by firing and sintering the
 pressed body at 1600.degree. C. for 4 hours. A test piece having 10 mm in
 length, 10 mm width and 5 mm in thickness was cut out from the thus
 obtained sintered body, and both its surfaces, 10 mm.times.10 mm, were
 ground at Ra.congruent.0.5 .mu.m.
 The test piece was placed in the reaction Teflon container 1 shown in FIG.
 1 together with 20 cc of hydrofluoric acid having a concentration given in
 Table 1, and this Teflon container 1 was placed and held in the stainless
 holding container 3 in the same procedure mentioned above.
 Next, the stainless holding container 3 was placed in a drying unit
 (manufactured by Yamato Kagakusha Co., Ltd.), and heated to a temperature
 given in Table 1, thereby treating the alumina sintered body with
 hydrofluoric acid. The heating time period was 15 hours.
 Thereafter, the treated test piece was left at room temperature, and cooled
 to 30.degree. C. or less, and then taken out.
 The surface of the alumina sintered body after the heat treatment in
 hydrofluoric acid was observed by SEM and analyzed by X rays, in order to
 determine whether or not a film of aluminum fluoride was formed. Results
 are shown in Table 1.
 TABLE 1
 80.degree. C. 100.degree. C. 130.degree. C. 150.degree. C.
 200.degree. C. 250.degree. C. 300.degree. C.
 1% not etched etched granular .largecircle. .largecircle. .DELTA.
 HF changed
 5% not etched etched .circleincircle. .circleincircle.
 .largecircle. .DELTA.
 HF changed
 15% not etched .circleincircle. .circleincircle. .circleincircle.
 .largecircle. .DELTA.
 HF changed
 30% not .largecircle. .circleincircle. .circleincircle.
 .largecircle. .largecircle. .DELTA.
 HF changed
 40% not needled needled .largecircle. .largecircle. .DELTA.
 .DELTA.
 HF changed
 50% not needled needled needled .DELTA. .DELTA. .DELTA.
 HF changed
 Not changed: The microstructure was not changed.
 Etched: The surface was corroded.
 Granular: Granular AlF.sub.3 particles were deposited, and a film of
 AlF.sub.3 was not formed.
 Needled: Needle-shaped AlF.sub.3 particles were deposited, and a film of
 AlF.sub.3 was not formed.
 .circleincircle.: After the test piece was heated at 500.degree. C. in air,
 the AlF.sub.3 film was not peeled or cracked through rapid cooling by
 falling it into water.
 .largecircle.: After the test piece was heated at 500.degree. C. in air,
 the AlF.sub.3 film was not peeled or cracked through being left to be
 cooled at room temperature.
 .DELTA. After the test piece was heated at 500.degree. C. in air, the
 AlF.sub.3 film was peeled and/or cracked through being left to be cooled
 at room temperature.
 The peeling and/or cracking of the AlF.sub.3 film were confirmed by the SEM
 observation.
 As shown in Table 1, according to the corrosion-resistant ceramic
 member-producing process of the present invention, a film of aluminum
 fluoride can be formed in the surface portion of the ceramic sintered
 body.
 Further, if the heating temperature is 130 to 200.degree. C., a film of
 aluminum fluoride can be formed on the ceramic sintered body without
 peeling or cracking even using the heating/rapid cooling test, when the
 concentration of hydrofluoride acid is 5 to 30 wt %.
 As mentioned above, according to the method of the present invention, since
 a film of aluminum fluoride is formed at the surface portion of the
 ceramic sintered body by directly immersing the ceramic sintered body
 containing at least aluminum in hydrofluoric acid and heating it, the
 present invention can provide a method for producing, with high
 productivity, a corrosion-resistant ceramic member the surface of which
 will not be corroded even if the ceramic member is exposed to a
 halogen-based corrosive gas.