Coating processes – Spray coating utilizing flame or plasma heat
Reexamination Certificate
2001-07-31
2004-02-03
Green, Anthony J. (Department: 1755)
Coating processes
Spray coating utilizing flame or plasma heat
C106S286100, C106S287180, C106S287270, C427S447000, C428S469000, C428S696000
Reexamination Certificate
active
06685991
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a novel material and method for the formation of a thermal-spray coating layer of a rare earth fluoride or, more particularly, to an efficient method for the formation of a highly corrosion-resistant thermal-spray coating layer of a rare earth fluoride on the surface of a substrate.
The method of so-called thermal spray coating utilizing a gas flame or plasma flame is a well established technology for the formation of a coating layer having high heat resistance, abrasion resistance and corrosion resistance on the surface of a variety of substrates such as articles made from metals, concrete, ceramics and the like, in which a powder to form the coating layer is ejected or sprayed as being carried by a flame at the substrate surface so that the particles are melted in the flame and deposited on the substrate surface where the melt is solidified to form a coating layer solidified by subsequent cooling.
While oxide powders of aluminum, chromium and the like are employed as the conventional material of the thermal-spray coating layers, these oxide materials are not always quite satisfactory to withstand the plasma atmosphere of a halogen-containing etching gas at an elevated temperature frequently encountered in the manufacturing process of semiconductor devices in respect of the corrosion resistance.
Namely, it is very conventional that the manufacturing process of semiconductor devices involves a step of plasma etching using a halogen-containing gas. Examples of the halogen-containing or fluorine- and/or chlorine-containing gases used for plasma etching include SF
6
, CF
4
, CHF
3
, ClF
3
, HF, Cl
2
, BCl
3
and HCl either singly or as a mixture of two kinds or more.
It is known that a thermal-spray coating layer having particularly high resistance against corrosion in a plasma atmosphere of these halogen-containing gases can be obtained by using particles of aluminum fluoride as the material of coating among the above mentioned metal oxide powders. While the aluminum fluoride powder used in the prior art for the thermal-spray coating is prepared by pulverizing or grinding coarse particles of aluminum fluoride and used as such, such a powder is generally not satisfactory for the purpose due to the poor flowability sometimes to cause clogging of the feed tubes of the powder and the spray nozzles greatly decreasing the smoothness of the thermal-spray coating procedure or greatly decreasing the quality of the thermal-spray coating layer.
SUMMARY OF THE INVENTION
In view of the above described problems and disadvantages in the material and method of thermal-spray coating using particles of aluminum fluoride as the thermal spray powder, the present invention has an object to provide an improvement in the thermal-spray coating method with particles of a rare earth fluoride as the thermal spray powder by which the thermal-spray coating works can be conducted very smoothly and efficiently with high stability to give high productivity of the process and high quality of the thermal-spray coating layer.
Thus, the present invention provides a material and method for the formation of a highly corrosion-resistant coating layer of a rare earth fluoride on the surface of a substrate by thermal-spray coating, the method comprising the step of:
spraying particles of a rare earth fluoride at the substrate surface as being carried by a flame or, in particular, plasma flame to deposit a melt of the particles on the substrate surface forming a layer, in which the rare earth fluoride particles have a globular particle configuration having an average diameter in the range from 20 to 200 &mgr;m and are prepared by granulation of primary particles of the rare earth fluoride having an average particle diameter not exceeding 10 &mgr;m.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As is described above, the most characteristic feature of the inventive method consists in the use of granules of a rare earth fluoride, as a novel material for thermal spray coating, having a specified average particle diameter as prepared by granulation of primary particles of the rare earth fluoride having a specified average particle diameter. The average particle diameter of the rare earth fluoride granules should be in the range from 20 to 200 &mgr;m. When the average particle diameter of the rare earth fluoride granules is too small, a substantial portion of the rare earth fluoride forming the granules may eventually be evaporated in the flame during the thermal-spray coating procedure resulting in a decrease in the utilizability of the thermal-spray powder while, when the average diameter of the granules is too large, large granules cannot be melted completely to the core in the flame resulting in a decrease in the quality of the thermal-spray coating layer of the rare earth fluoride.
The inventors have further continued investigation on the physical properties required for the rare earth fluoride granules and arrived at an unexpected discovery that the granules should have a powder compression factor S not exceeding 30%. The term of “powder compression factor” S here implied is defined by the equation
Powder compression factor S(%)=(&tgr;
T−&tgr;A
)/&tgr;
T×
100,
in which &tgr; A is the bulk density of the granules and &tgr;T is the tapped density of the granules.
Although the thermal-spray powder used according to the invention consists of granules obtained by granulation of primary particles of a rare earth fluoride, it is desirable that the granules have a relatively low porosity or small void spaces. This requirement is important to ensure good resistance of the granules against cracking in handling and to decrease emission of a gas occluded in the void spaces of the granules by the thermal spraying to cause melting of the granules.
In order to ensure little emission of gases from the granules melted in the flame, it is desirable that the content of water or hydroxyl groups in the rare earth fluoride particles should be as low as possible since water or hydroxyl groups may be an emission source of gases and may react with the rare earth fluoride to form corrosive hydrogen fluoride gas heavily affecting the quality of the thermal-spray coating layers of the rare earth fluoride. In this regard, the content of water and hydroxyl groups in the rare earth fluoride granules should not exceed 1% by weight.
The primary particles of a rare earth fluoride, from which the granules are prepared by using a suitable granulator machine such as a spray-drying granulator, can be prepared by mechanical grinding and particle size classification and should have an average particle diameter not exceeding 10 &mgr;m or, preferably, not exceeding 5 &mgr;m or, more preferably, not exceeding 3 &mgr;m. This requirement is important in order to obtain rare earth fluoride granules of a globular particle configuration having good flowability and suitable for the thermal-spray coating according to the present invention.
The rare earth elements forming the rare earth fluoride include yttrium and the elements having an atomic number in the range from 57 to 71, of which yttrium, cerium and ytterbium are preferable, though not particularly limitative thereto. These rare earth elements can be used either singly or as a combination of two kinds or more according to need. When two kinds or more of the rare earth elements are combined in the fluoride granules, it is optional that the granules are prepared by granulation of a blend of primary particles of the two kinds or more of different rare earth fluorides or the primary particles per se are prepared to include two kinds or more of the rare earth elements.
The procedure for the preparation of the rare earth fluoride granules from the primary particles is rather conventional. Namely, the primary particles of the rare earth fluoride are dispersed in water, an alcohol of 1 to 4 carbon atoms in a molecule, toluene, hexane and the like or a mixture thereof as a dispersion medium to give a slurry of 10 to 40% solid concentra
Maeda Takao
Wataya Kazuhiro
Green Anthony J.
Shin-Etsu Chemical Co. , Ltd.
Wenderoth Lind & Ponack LLP
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