Patent Application
20070281183
Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements numbered alike in several figures, in which:
(Structure 1) A film formation method comprising the step of forming a film of a carbide via a thermal CVD employing a hydrogen, and a chloride or a hydrocarbon used as a raw material gas, wherein a gas flow rate of the hydrogen is 3-8 times larger than a gas flow rate of the chloride or the hydrocarbon.
After considerable effort during intensive studies, the inventors have found out that a carbide film having a dense texture structure, in which defects such as holes are reduced, is successfully formed via the thermal CVD. By applying this to a die to mold an optical element and so forth, a die hardly generating cutting chips and cracks can be obtained during a cutting process of the molding transfer surface. Incidentally, the above-described gas flow rate (amount of gas flow per unit time) of the hydrogen is preferably 3-6 times larger than a gas flow rate of the chloride or the hydrocarbon used as a raw material gas, and more preferably 3-5 times larger than a gas flow rate of the chloride or the hydrocarbon used as a raw material gas.
Next, the present invention will be described in detail.
The basic chemical reaction of a thermal CVD treatment is shown in Formula (1) employing silicon carbide as an example, but the significant point herein is that silicon tetrachloride is directly reduced with hydrogen gas rather than methane gas to obtain silicon. Specifically, as shown in Formula (2), two moles for a mole ratio of hydrogen gas with respect to one mole of each raw material gas remain unchanged before and after the reaction, but hydrogen gas called a carrier gas is also employed to surely reduce silicon.
SiCl4+2H2+CH4→Si+4HCL+CH4→SiC+4HCL+2H2 (2)
Silicon carbide having 50 mol % of silicon and 50 mol % of carbon in composition formed via the thermal CVD is brownish yellow, and transparent in the case of the thin film. The silicon carbide becomes black-opaque by increasing only a few percent of carbon content. Each silicon carbide formed via the thermal CVD is completely black-opaque, and there is a slightly larger amount of carbon content than that of silicon in composition. In other words, it is understood that silicon content is slightly insufficient in composition.
Focusing on hydrogen gas as a carrier gas, the inventors conducted film formation of silicon carbide via the thermal CVD by varying the composition. The film formation was conducted under three conditions such as 1 mole of hydrogen gas, 2 moles (conventional) and 3 moles (that is, 1, 2 and 3 times in gas flow rate) with respect to 1 mole of each raw material gas, without changing gas flow rate of the raw material gas (silicone tetrachloride gas and methane gas). SEM micrographs (a magnification of 50 times) of the film surface formed under the conditions via the thermal CVD are shown in
When hydrogen gas is reduced to 1 mole, a fine roughened structure on the surface, as shown in
On the other hand, in the case of hydrogen gas being set to 3 moles, generation of concave portions after processing the molding transfer surface was largely reduced as shown in
The inventors have found out from the above-described results that free carbons increase in the case of reducing hydrogen gas as a carrier gas, so that concave portions are generated and increased during the molding transfer surface process. As disclosed in Patent Document 1, it was commonly known that hydrogen gas employed for the thermal CVD was set to 2 moles, but it was found out for the first time from the studies done by inventors that generation of concave portions was possible to be largely inhibited by setting hydrogen gas to at least 3 moles. However, a level of up to 8 moles is preferable in view of practical use, since dilution of the total raw material gas causes a decline of reaction speed in the case of too much increase of hydrogen gas, resulting in the low speed of film formation.
(Structure 2) The film formation method of Structure 1, wherein the carbide is silicon carbide. Since silicon carbide exhibits high heat resistance together with high hardness, it is preferable as a die material of an optical element formed from glass as a material. Titanium carbide, tantalum carbide and so forth are also usable as the die material.
(Structure 3) A die formed by the film formation method of Structure 1 or 2, wherein when cutting a surface of a substrate on which the film of the carbide is formed to form a molding transfer surface, the number of cutting chips per unit area of the cut surface is at most 1000/mm2. Thus, an optical element having a high precision optical surface can be molded. Herein, the number of cutting chips means the number of concave portions having a maximum span size of at least 1 μm.
(Structure 4) A method of manufacturing a die, comprising the steps of forming a film of a carbide on a surface of a substrate via a thermal CVD, with controlling that a hydrogen gas flow rate is 3-8 times larger than a gas flow rate of a chloride or a hydrocarbon used as a raw material gas; and forming a molding transfer surface by ductile-mode-cutting the surface of the substrate on which the film of the carbide is formed employing a diamond tool.
As described above, when ductile-mode-cutting the foregoing substrate surface on which a film of carbide is formed employing a diamond tool, reduced concave portions result in the higher precision mirror surface. That is, according to the present invention, a hydrogen gas flow rate is 3-8 times larger than a gas flow rate of a chloride or a hydrocarbon used as a raw material, and a film of a carbide is formed on the substrate surface via the thermal CVD to inhibit generation of concave portions, whereby a die having a high precision molding transfer surface can be produced.
(Structure 5) The method of Structure 4, further comprising the step of molding-transferring from the molding transfer surface to an optical surface of an optical element, wherein the die is employed to mold the optical element.
(Structure 6) The method of Structure 4 or 5, wherein the carbide is silicon carbide.
(Structure 7) The method of any one of Structures 4-6, further comprising the step of forming a release film on the molding transfer surface. Thus, the optical element can easily be released from the die after molding.
While the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims.
Next, embodiments of the present invention will be described referring to figures.
In
Cylindrical adiabatic supporting member 5 is placed in the center of chamber 1. Cylindrical carbon heater 6 is placed in the inner circumference of cylindrical adiabatic supporting member 5, and base 7 on which die material 10 is placed is further situated medially thereto. A lot of holes are formed in the portions of base 7 on which die material 10 is placed, and it is designed that the mixed gas flows upward from the bottom of chamber 1 with no interruption.
The mixed gas which passes through die material 10 flows around outward from the upper portion of supporting member 5, and passes through between supporting member 5 and the inner wall to flow into exhaust passage 8. Passage 8 is connected to exhaust displacement pump P via valve V and condenser 9. In addition, pipe 3 for the condenser in which a cooling medium flows is twisted around the outer circumference of chamber 1.
In
Die material 10 was produced with silicon carbide by sintering, engaged masking member 11, and was placed on base 7 as shown in
A glass lens was molded employing die 10, and an optical surface molding-transferred from molding transfer surface 10a is observed by a stereomicroscope. The results are shown in
Under the similar conditions in Example 1, 5 moles of hydrogen gas as a carrier gas was mixed with 1 mole of silicon tetrachloride gas and 1 mole of methane gas to form a film on molding transfer surface 10a via the thermal CVD. An optical element was similarly molded employing die material 10, and an aspheric optical surface was molding-transferred from molding transfer surface 10a, but no generation of protrusions corresponding to concave portions was obtained, resulting in an excellent surface. Molding results of optical elements were also excellent, except that the film formation speed dropped from 100 μm/hr to 65 μm/hr.
Under the similar conditions in Example 1, 8 moles of hydrogen gas as a carrier gas was mixed with 1 mole of silicon tetrachloride gas and 1 mole of methane gas to form a film on molding transfer surface 10a via the thermal CVD. An optical element was similarly molded employing die material 10, and an aspheric optical surface was molding-transferred from molding transfer surface 10a, but no generation of protrusions corresponding to concave portions was obtained, resulting in an excellent surface. Molding results of optical elements were also excellent, except that the film formation speed dropped from 100 μm/hr to 20 μm/hr.
The present invention can provide a film formation method with which a layer having reduced defects, a die obtained by the film formation method, and a method of manufacturing the die.
| Number | Date | Country | Kind |
|---|---|---|---|
| JP2006-152187 | May 2006 | JP | national |
