METHOD AND APPARATUS FOR FORMING THIN FUNCTIONAL FILM

Abstract: There is provided a method of forming a thin functional film by vacuum vapor deposition in which a functional material is heated from the above thereof so that bumping or splashing of the functional material is not caused upon evaporation of the functional material. Concretely, the functional material (7) is placed in a receiving vessel (8) and the functional material is heated by a heater (9) which is located above the functional material so that bumping or splashing is suppressed, whereby a uniform functional film is produced without a pinhole.

Description: Field of the Invention The present invention relates to a method and apparatus of forming a thin functional film. In the present invention, the thin functional film is intended to mean one of various materials (for example an organic compound) in the form of a thin film attached onto an object by vapor deposition for the purpose of providing one of various functions such as recording information, displaying information, adjusting or changing of an optical property, converting energy and the like. Concretely, the following thin films are exemplified: a thin film as an information recording layer of an organic dye (or pigment) material which is formed on a substrate of an optical recording medium; a thin film of a sensitization material for an photoelectric conversion element which is formed on a substrate of a solar cell panel; and a charge transportation layer of an OPC (optical photo conductivity) material on a photoconductor drum. Nowadays, various functional films are used widely in the fields of an information recording element (such as an optical recording medium and a photoconductor drum), an information displaying element (such as a color filter and a thin electroluminescence panel) as well as an energy conversion element (such as a solar cell) and a medical diagnosis element. Those functional films are all formed by a wet method such as a spin coating method or a dry method such as a vacuum vapor deposition or sputtering. Particularly, a recordable optical disc (CD-R) as one of the optical recording mediums has been recently noted in which an information recording layer is formed on a substrate using an organic dye material since it is convenient in that signals are read out from it using a CD player or a CD-ROM drive. Hitherto, the recording layer has been formed by the wet method in which a solution of the organic dye material is applied onto the substrate by spin-coating followed by drying. In order to produce a satisfactory optical recording medium, experiences based know-how and skills are required since tolerable ranges of production conditions are narrow. On the other hand, the dry method of forming the recording layer has been also attempted in which vacuum vapor deposition is used so as to relatively easily produce a thin functional film having a uniform thickness and uniform optical properties. For example, Japanese Patent Kokoku Publication No. 8-13572 discloses an optical recording medium which is produced by heating a molybdenum made boat containing a phthalocyanine based organic dye material so as to evaporate the material, and depositing the evaporated material onto a substrate surface to form a thin film of the organic dye material as a recording layer on the substrate. Further, an example of an apparatus which carries out such vapor deposition is disclosed in Japanese Patent Kokoku Publication No. 7-62249. FR 2 235 401 and FR 2 232 077 disclose a vacuum evaporation plating method and apparatus wherein a material is heated from the above so as to evaporate it. JP 61-069960 discloses a method and device for forming a protection film by evaporating a target composed of an organic high polymer in vacuum with irradiation of laser beams and depositing it on the surface of a base to be protected. US 2,435,997 discloses an apparatus for vapour coating of large surfaces. Disclosure of the Invention However, the vacuum vapor deposition as described above has a problem in that bumping and splashing are often caused when the organic dye material in the boat is heated so that the formed thin film is likely to have defects due to inclusion of pinholes in the thin film. In order to make the deposition method industrially effective, it is desired to overcome the problem. Such a problem is not limited to the case in which the information recording layer is formed by the vacuum vapor deposition, and it is a common problem in cases in which various materials are evaporated and deposited onto objects for the formation of various thin functional films. It is an object of the present invention to provide a method and an apparatus for forming a uniform thin functional film having fewer defects so as to alleviate and preferably substantially overcome such a problem. It is noted that although the method and the apparatus according to the present invention are explained as examples with reference to a case in which a recording layer as the thin functional film is formed by depositing a dye material onto a substrate for an optical recording medium, such present invention is applicable to the formation of the other various thin functional films. In the present invention which achieves the above mentioned object, a method, in which a material from which a thin functional film is formed (which is also referred to as a "functional material") is placed preferably in an evaporation vessel and heated under a reduced pressure (preferably at a highly vacuum pressure) so as to evaporate the material, and the evaporated material is deposited onto an object such as a substrate so that a thin functional film is formed thereon, is characterized in that the functional material is heated and evaporated from its above by a heating means, whereby the thin functional film is formed on the substrate, wherein the heating which heats from the above comprises a plate type electric heater having a plurality of through holes. According to the present invention, a thin functional film having a high quality is formed with greatly reduced defects such as pinholes. That is, the present invention provides a method for forming a thin functional film by evaporating a functional material under a reduced pressure condition and depositing the evaporated material onto an object, which is characterized in that the functional material is placed preferably in an evaporation vessel and heated from a position above the functional material so that the functional material is evaporated. In addition, the present invention provides an apparatus for forming a thin functional film by evaporating a functional material under a reduced pressure condition and depositing the evaporated material on to an object, which is characterized in that the apparatus comprises an evaporation vessel which receives the functional material from which the thin functional film is formed, and a heating means which heats the functional material from a position above the functional material placed in the evaporation vessel, wherein the heating means which heats from the above comprises a plate type electric heater having a plurality of through holes. Brief Description of the Drawings Fig. 1 schematically shows an apparatus for forming a functional film according to the first embodiment of the present invention; Fig. 2 schematically shows forms of an upper heater according to the first embodiment of the present invention; and Fig. 3 schematically shows a cross-sectional view of an optical recording medium along a direction which is perpendicular to a substrate of the medium. In the drawings, the reference numbers mean the following members: 1...base plate, 2...vacuum chamber, 3...vacuum pump, 4...substrate, 5...film thickness monitor, 6...shutter, 7...functional material, 8...receiving vessel, 9...upper heater, 10...supplemental heater, ... 11...lead wire, 12... electrode insert terminal, 17...optical recording layer, 18...reflecting layer, 19...protection layer. Detailed Description of the Invention The present invention essentially resides in that upon the formation of the thin functional film, heating the functional material which is to be formed into the thin functional film is carried out from the above of the functional material, whereby across the functional material and, for example, the functional material contained in the evaporation vessel, a temperature of its upper portion is higher than a temperature of its portion which is located below said upper portion or its inside portion. By continuing such heating, the temperature of the upper portion becomes the highest, so that the functional material is evaporated from its upper portion successively, and thereby the functional material evaporates without bumping or splashing. Contrary to the above, in the conventional method in which an evaporation vessel itself is heated, the functional material in the evaporation vessel is so heated that a temperature of the inside portion or the lower portion of the functional material becomes higher than a temperature of the upper portion, whereby the evaporation of the material occurs preferentially in the inside or the lower portion which is at the higher temperature compared to the upper portion. As a result, the evaporated material (namely, evaporated molecules) passes upward through the upper portion of the functional material which is at the lower temperature, which causes bumping and splashing. Upon the formation of the thin functional film according to the present invention, heating may be carried out under any proper conditions (a heating temperature, and an operation pressure during heating) depending on the thin functional film to be formed, and thus depending on the functional material. Generally, heating is carried out under a reduced pressure condition, and usually under a highly vacuum pressure condition. For example, the production of an optical recording medium (such as CD-R, DVD-R or the like) may be carried out, for example, at a temperature between 100 °C and 500 °C under an operation pressure of not more than 10 -3 torr (concretely 250 °C and 10 -4 torr) 1 torr 133,3 Pa. Further, the production of a photoconductor drum may be carried out, for example, at a temperature between 100 °C and 500 °C under an operation pressure between 10 -3 torr and 10 -7 torr. The material from which the thin functional film is formed, that it, the functional material is selected depending on the intended thin functional film. For example, for the production of the optical recording medium, the functional material may be an organic dye material such as a metal containing azo based dye material, a phthalocyanine based dye material or the like. Further, for the production of the photoconductor drum, the functional material may be a phthalocyanine. The functional material may be in any form. Before being heated, the material is usually in the form of powder consisting of fine particles, but it may be in a lump form. The object onto which the thin functional film is formed may be any object depending on the application of the thin functional film. In the case of the production of the optical recording medium, the object may be a plastic (for example a polycarbonate) substrate which is permeable to the light. Further, in the case of the production of the photoconductor drum, the object may be an aluminum substrate. It is noted that the thin functional film is intended to mean a film which is relatively thin and which exhibits a predetermined function, and its thickness is not particularly limited. For example, the thickness of the thin functional film may be in the range between about 0.01 µm and about 10 µm. The evaporation vessel in which the functional material is placed may be made of any material and in any structure as far as it receives the functional material and the material is heated from its above. For example, the vessel may be in a boat form, a crucible form or the like which is made of a metal or a ceramic. Generally, an upper (or top) portion of the vessel is at least partly open and most preferably substantially completely open. It is preferable that the vessel does not disturb movement (for example going up) of the material evaporated through heating (namely, the evaporated molecules) from the vessel toward the object. The means to heat the functional material is not particularly limited as far as it heats the functional material from a position above the material. Concretely, an electric heater which is located above the evaporation vessel, a photo energy supplying means (such as a light beam source) which is so located that it heats the upper portion of the evaporation vessel, and a heating source which irradiates radiant energy are exemplified, and these means may be combined. Further, a supplemental (or auxiliary) heating means which preheats the functional material up to a temperature at which no evaporation of the functional material occurs may be combined with the heating means which heats the functional material from its above. Such a supplemental heating means may be any one conventionally used, and it may be, for example, means which heats the evaporation vessel itself. Concrete embodiments of the present invention will be explained hereinafter with reference to the accompanying drawings. Figs. 1 schematically shows an apparatus for the formation of the thin functional film according to the present invention, Fig. 2 schematically shows various heating means, and Fig. 3 schematically shows in a cross-sectional view, a standard structure of the optical recording medium produced in Examples. Embodiment 1 Fig. 1 is a schematic view of a first embodiment of the apparatus for the production of the thin functional film according to the present invention when seeing from its side. The apparatus is for forming the thin functional film, which functions as a recording layer, on the substrate for an optical recording medium. The apparatus comprises a base plate (1) and a vacuum chamber (2) in the form of a bell jar made of a transparent glass placed on the plate. The vacuum chamber (2) is evacuated by a vacuum pump (3) such as a diffusion pump, a cryo-pump, a turbo molecular pump or the like so that the inside of the vacuum chamber is kept at a highly vacuum pressure. A substrate (4) onto which the thin functional film is to be formed, a monitor (5) which detects a thickness of the deposited film and a shutter (6) which interrupts the movement of the evaporated molecules toward the substrate (shown in a closed state) are provided in the upper portion of the vacuum chamber (2). An evaporation vessel (8) which contains the functional material, an upper heating means (9) and a supplemental heating means (10) are provided in the lower portion of the vacuum chamber (2). A distance between the substrate (4) and the evaporation vessel (8) is about 20 to 30 cm, and a distance between the substrate (4) and the shutter (6) is about 3 to 5 cm. The upper heating means (9) and the supplemental heating means (10) may be electrically heating means. As clearly seen from the drawing, the upper heating means (9) is placed above the functional material (7), so that it heats preferentially the upper portion of the functional material and a portion which is below the upper portion is heated by heat conduction from the upper portion of the heated functional material. The thickness monitor (5) is a conventional one of a quartz oscillator type. The substrate (4) is made of a plastic material such as a polycarbonate, a polyolefin, a polymethyl methacrylate or the like. The functional material (7) is an organic dye (or pigment) material such as a phthalocyanine, naphthalocyanine, squarylium, croconium, pyrylium, naphthoquinone, anthraquinone, xanthene, triphenylmethane, azulenium, tetrahydrocholine, phenanthrene, triphenothiazine, polymethine, azo based dye material. The shape of the evaporation vessel (8) is not particularly limited as far as the evaporated molecules are able to leave the evaporation vessel toward the substrate (4) which is placed above the vessel. It is generally preferable that at least a portion of a top of the vessel is open (for example, a boat form). The evaporation vessel (8) may be made of a metal or a ceramic, and it is required to be heat resistant at a temperature of about 500 °C and generate no adverse gas (i.e. impurity gas or outgas) during heating. When a ceramic for example of which thermal conductivity is lower than that of a metal is used, thermally effective deposition is possible. The upper heating means (9) may be a heater which heats electrically and which is made of a metal having a high melting point such as tantalum, molybdenum, tungsten or the like, and it is preferable that the material for the heater has a large thermal emissivity. The heater may have any suitable shape. Concretely, the heaters shown in Fig. 2 may be used. Fig. 2A (not covered by the present invention) is a usual heater of a wire type, Fig. 2B (not covered by the present invention) is a spiral type heater in which the wire type heater is coiled into the spiral form, Fig. 2C is a net (or mesh) type heater which has a plurality of large openings and Fig. 2D is a perforated heater which has a plurality (preferably a large number) of pores through a plate. Among them, the plate type porous heater D is good in the evaporation efficiency. The supplemental heating means (10) comprises a heater of a Nichrome wire type, and it is so designed that no outgas is generated during heating. The supplemental heater may be used supplementarily to preheat the functional material up to a temperature just before the evaporation of the material begins, or to suppress temperature decrease of the functional material after the evaporation has started. Each heater is connected to electrode insert terminals 12a or 12b through lead wires 11a or 11b. The terminals are connected to a power source (direct current or alternating current) for heating outside the apparatus. Using the apparatus for the formation of the thin functional film as described above, a thin functional film forming method is carried out as described below. First, an organic dye material as the functional material (7) is chaged into the evaporation vessel (8), the substrate (4) is placed in the upper portion of the vacuum chamber, and the shutter (6) is closed as shown. The vacuum chamber (2) is placed on the base plate (1) and evacuated to a pressure of for example 1×10 -5 torr. Then, current is passed through the upper heater (9) and the organic dye material is heated from its above. When the current is increased, the evaporation of the organic dye material starts from its top portion. Thereafter, the shutter (6) is opened and the thin film is formed on the substrate (4). Upon the formation, a thickness of the deposited film is detected by the thickness monitor (5), and when the thin functional film having a predetermined thickness has been produced, the shutter is closed. An opening period of the shutter is for example about one minute. Upon heating of the organic dye material, when the supplemental heater is used so as to heat the organic dye material within such an extent that no evaporation occurs before heating by using the upper heater (9), a period until the evaporation begins is shortened and the evaporation rate is increased upon heating by means of the upper heater (9), so that more efficient deposition is possible in a shorter period. In a case in which the organic dye material is in the form of fine particles, when the organic dye material is heated from its lower portion by using only the supplemental heater (10), heat conduction from the organic dye material in the lower portion to the organic dye material particles which are located above the former particles is degraded because of the presence of vacuum spaces between the particles. As a result, there occurs a temperature difference between in the upper portion and in the lower portion of the organic dye material. Therefore, since the evaporated molecules at a higher temperature enter the upper portion of the organic dye material at a lower temperature, bumping and splashing are likely to occur. Industrial Applicability of the Invention As described above, according to the present invention, by heating the functional material from its above upon the formation of the thin functional film on the object, bumping and splashing of the functional material are suppressed so that the uniform thin functional film is formed on the object without flying splashing matters and their trace which cause the defects of the thin film. Examples Next, the present invention will be further explained concretely by examples in which the thin functional is formed from the organic dye material on the substrate for the optical recording medium. In the examples, an optical recording layer made of a phthalocyanine based functional material and having a thickness of 0.15 µm was formed on a polycarbonate substrate having a thickness of 1.2 mm (an evaporation starting temperature of the functional material was 300 °C, and a vacuum pressure was 5×10 -5 torr), and then a gold reflecting layer having a thickness of 0.1 µm was formed on the recording layer by sputtering. Thereafter, the substrate was taken out of the vacuum chamber and a protection layer (4) made of a ultra-violet curable resin having a thickness of 5 µm was formed on the reflecting layer by the spin coating method so that the optical recording medium (CD-R) was obtained. Fig. 3 schematically shows a cross-sectional view of the opticat recording medium thus obtained. The optical recording medium comprises the optical recording layer (17) of the phthalocyanine based material on the polycarbonate substrate (3) having a spiral groove thereon, and the reflecting layer of gold (18) as well as the protection layer (19) of the UV curable resin on the recording layer (17). (Example 1) Using the apparatus as shown in Fig. 1 and passing a direct current of 70 A through the upper heater (9), the phthalocyanine based dye material placed in the evaporation vessel (8) (of a boat type, molybdenum made) was heated from its above without using the supplemental heater (10), so that the dye material was evaporated and deposited onto the substrate, whereby the thin film of the dye material was formed. Thirty optical recording mediums were totally prepared. (Comparative Example) Example 1 was repeated so as to form the thin film of the dye material on the substrate except that the upper heater (9) was not used, but only the supplemental heater (10) was used to heat the phthalocyanine based dye material while applying a power of 150 W thereto. Thirty optical recording mediums were totally prepared. With respect to each of the optical recording mediums produced as described above, a bit error rate was measured, and the number of pinholes having a size of not smaller than 30 µm was counted, and then the number of discs of which pinhole number was more than ten (which is a standard) was counted. (1) Bit error rate The bit error rate is the number of byte error per one second. Error occurring frequency of one byte corresponds to a bit error rate of 5.6×10 -6 when constituting with 176400 bytes/sec. (2) Pinhole The number of pinholes are counted visually while irradiating a light from a back side of the disc. As a result, the optical recording mediums obtained in Example 1 had an averaged bit error rate of 2.0×10 -5 and the number of discs of which pinhole number were above the standard was one. The optical recording mediums obtained in Example 2 had an averaged bit error rate of 1.0×10 -5 and the number of discs of which pinhole number were above the standard was zero. The optical recording mediums obtained in Comparative Example had an averaged bit error rate of 22.0×10 -5 and the number of discs of which pinhole number were above the standard was sixteen. Based on the above results, when the phthalocyanine based dye material was heated from its above and evaporated so as to form the thin film of the dye material on the substarate, the uniform optical recording medium is obtained of which thin film has fewer pinholes, so that qualities such as a bit error rate are improved. The present invention has been explained particularly and concretely with reference to the production of the optical recording medium comprising the recording layer of the organic dye material as the thin functional film on the plastic substrate, but the present invention is similarly applicable to cases in which other thin functional film is formed on other object.

Claims: A method for forming a thin functional film on an object by heating and evaporating a functional material which forms the thin functional film under a reduced pressure wherein the functional material is heated from its above by a heating means so as to evaporate it, characterized in that the heating means which heats from the above comprises a plate type electric heater having a plurality of through holes. The method according to claim 1 characterized in that the functional material is placed in an evaporation vessel, and the functional material placed in the evaporation vessel is heated from its above so as to evaporate it. The method according to claim 1 or 2 characterized in that the evaporation vessel is separately and supplementally heated to a temperature below an evaporation temperature of the functional material. An apparatus for forming a thin functional film on an object by heating and evaporating a functional material which forms the thin functional film under a reduced pressure wherein the apparatus comprises an evaporation vessel which receives the functional material, and a heating means which heats the functional material received in the evaporation vessel from its above, characterized in that the heating means which heats from the above comprises a plate type electric heater having a plurality of through holes. The apparatus according to claim 4 characterized in that the heating means which heats the functional material from its above is an electric heater which is located above the evaporation vessel. The apparatus according to any one of claims 4 to 5 characterized in that the apparatus further comprises a supplemental heating means which heats the evaporation vessel to a temperature below an evaporation temperature of the functional material. The method according to any one of claims 1 to 3 characterized in that the object is a substrate for an optical recording medium, the functional material is an organic dye material and the thin functional film is a recording layer. The apparatus according to any one of claims 4 to 6 characterized in that the object is a substrate for an optical recording medium, the functional material is an organic dye material and the thin functional film is a recording layer.

Inventors: KAMIYAMA TOSHIBUMI, KUNIEDA TOSHIAKI, OKAZAKI SADAYUKI, KAMIYAMA, TOSHIBUMI, KUNIEDA, TOSHIAKI, OKAZAKI, SADAYUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1998
Main classification: C23C14
IPC codes: C23C14, G11B7