1. Field of the Invention
The present invention relates to a color selection electrode (color selection mask) having a finer pitch of apertures, a method of producing the same, and a cathode ray tube using the same.
2. Description of the Related Art
Picture tubes, called Braun tubes or CRTs (cathode ray tubes), are being widely used as television receivers or computer displays. FIG. 8 is a cross-sectional view of a cathode ray tube. A container or tube comprises a panel 11, a funnel 12 and a neck 13, and the inside of the container is under vacuum. In the neck part, an electron gun 14 is positioned which generates an electron beam 15. On the inside surface of the panel 11, a phosphor screen 16 is formed. Inside the container, a color selection electrode 17 is placed before the phosphor screen 16. A color picture tube has a color selection mask and a three-color (red, green, and blue) phosphor screen corresponding to the individual apertures on the color selection mask. The different color phosphors selectively emit light for color selection by the angle of incidence of an electron beam emitted from the electron gun to the color selection mask.
The color selection masks in actual use may be roughly classified according to the shape of the perforations into shadow masks having circular perforations and aperture grilles having slit-shaped (rectangular) openings, but are often also known generically as xe2x80x9cshadow masksxe2x80x9d.
For general television use, aperture grilles with slits of a width of about 200 to 250 xcexcm arranged at a pitch of about 0.5 to 1.0 mm are often used, for example, in the Trinitron system of SONY CORPORATION.
In the case of a shadow mask, dot-shaped circular perforations of a diameter of about 200 to 250 xcexcm are arranged regularly at about a 0.6 mm pitch. The smaller the pitch of the perforations, the higher the resolution, so as a color selection mask for a high definition cathode ray tube, a shadow mask with a pitch reduced to about 0.2 mm is used.
The ordinary method of manufacturing a color selection mask will be explained below.
First, a resist is coated on an iron-alloy plate of about 100 to 300 xcexcm thickness to form a resist film. Next, the resist is exposed and developed in accordance with the pattern of the color selection mask for resist patterning. The resist is then used as a mask for etching the iron-alloy plate to open the required apertures, then the resist is removed.
After the color selection mask is perforated by the etching, it is worked to shape it to a curved surface having a predetermined radius of curvation. Next, it is heated at 400 to 500xc2x0 C. and oxidized by reactive gas to blacken the surface. The blackening is to prevent random reflection.
The color selection mask manufactured by the above method is placed about 10 mm before the phosphor screen on the electron gun side.
In the conventional method of manufacturing a color selection mask explained above, the mask is only perforated by the etching method. The general limit in fine working in the case of the etching method is about the same as the thickness of the worked plate in the case of the pitch of perforations and about 15% of the designated dimension in the case of the working precision.
Accordingly, when manufacturing a mask for a high definition cathode ray tube by the conventional method of manufacturing a color selection mask, the limit of fining of the pitch of the mask perforations becomes about the same as the thickness of the iron-alloy plate, that is, about 0.2 mm.
When manufacturing a mask for a high definition cathode ray tube by the above conventional method, since the limit of fining of the pitch of the mask perforations becomes about the thickness of the iron-alloy plate of the mask, finer mask perforations are generally formed by the method of making thinner the thickness of the iron-alloy plate being worked.
If just making a thin film, instead of the conventional method of perforating an iron-alloy plate of a predetermined thickness by etching, it would be possible to make fine perforations precisely by forming a metal layer on portions other than the mask perforations by electrofineforming.
Japanese Unexamined Patent Publication (Kokai) No. 60-30038 discloses a shadow mask of a 100 xcexcm thickness of the iron-alloy plate and about a 0.2 mm pitch of perforations manufactured only by electrofineforming without perforation by etching.
However, the mask apertures of the color selection mask must be tapered in cross-section so as not to obstruct a passing electron beam, when forming a color selection mask only by electrofineforming, it is difficult to give the fine mask apertures a tapered cross-sectional shape in a mask for a high definition cathode ray tube with a pitch of apertures narrower than the shadow mask described in the above Japanese Unexamined Patent Publication (Kokai) No. 60-30038. As shown in FIG. 9, the narrower the taper width (shown by xe2x80x9cAxe2x80x9d in FIG. 9) of the mask apertures and the closer the side walls of the mask apertures to perpendicular, the easier the occurrence of scattered electrons (halation) due to the random reflection of the electron beam at the inside surfaces of the mask perforations.
On the other hand, in the one-side etching method of etching from the side of the color selection mask which becomes the phosphor screen side, openings having the predetermined precision are formed by etching with slight over etching. As a result, especially when making the iron-alloy plate thin, the taper width A shown in FIG. 9 tends to become narrower.
Even if the taper width A becomes narrower, not that much of a disadvantage arises in a color selection mask having a pitch of apertures of 0.6 mm or more, but in a color selection mask having a pitch of apertures of 0.4 mm or less, if the taper width A becomes narrow, the above-mentioned halation may occur.
When making the iron-alloy plate of the mask thin, as explained above, not only does it become difficult to finely taper the cross-sectional shape of the mask apertures, but the disadvantage also arises in the mechanical strength of the mask member. It is also necessary to make the iron-alloy plate of the mask maintain tension when welding the color selection mask to the picture tube frame.
According to the conventional method of manufacture, there is a certain limit to thinning the color selection mask in order to avoid uneven etching when making the mask apertures and improve the uniformity. It is difficult to make the thickness less than 80 xcexcm.
An object of the present invention is to provide a color selection electrode (a color selection mask) with a finer pitch of apertures free from limitations due to the thickness of the metal substrate.
Another object of the present invention is to provide a method producing the above color selection electrode.
Still another object of the present invention is to provide a CRT using the color selection electrode.
According to the present invention, there is provided a method of producing a color selection electrode used for a cathode ray tube including an electron gun and a phosphor screen, the color selection electrode being positioned between the electron gun and the phosphor screen and having apertures through which an electron beam generated from the electron gun selectively passes to the phosphor screen, the method comprising the steps of: depositing a first resist on both surfaces of a metal substrate, a first plane of the substrate facing to the electron gun and a second plane of the substrate facing to the phosphor screen, and patterning the same by photolithography; electrodepositing a metal on portions of the first plane of the substrate where the first resist is not formed to form an electrodeposited metal layer; depositing a second resist on the entire surface of the substrate which is directed to the first plane to cover the first resist and the electrodeposited metal layer; using the first resist as a mask and etching the second plane of the substrate; and removing the first resist and the second resist.
By using electrofineforming to form an electrodeposited metal layer, it is possible to form mask apertures of a tapered cross-sectional shape at a high precision (i.e., a precision of xc2x15% or better) at the electron gun side of the mask (i.e., at the small diameter side of the mask apertures). By this, it is possible to manufacture a mask for a superhigh definition cathode ray tube with about a 15 xcexcm diameter of openings on the electron gun side small diameter side and about a 0.06 mm pitch of apertures and possible to improve the resolution of the picture tube.
Further, if using electrofineforming, since it is possible to form an electrodeposited metal layer of a high hardness (Hv 450 to 550), it is possible to increase the mechanical strength of the mask member. Accordingly, it is possible to secure tension when welding the color selection mask to the picture tube frame with the thickness of the iron-alloy plate.
The thickness of the iron-alloy plate can be made thinner to the limit of the secondary shaping for preventing strain in the mask perforations in the secondary shaping for making the mask a curved surface, and the like. According to the method of manufacture of the present invention, for example, it is possible to make the thickness of an aperture grille thinner to about 30 xcexcm.
Further, working the method of manufacture of the present invention is not accompanied with massive investment in new facilities; it is sufficient to just change part of the conventional color selection mask manufacturing equipment. For example, in the case of a two-side etching manufacturing apparatus, it is sufficient to change the first etching chamber to an electrofineforming chamber.
The method of producing a color selection electrode used for a cathode ray tube of the present invention preferably is characterized in that in the step of forming the electrodeposited metal layer, a metal is not electrodeposited on the second plane of the substrate.
The method of producing a color selection mask used for a cathode ray tube of the present invention preferably is characterized in that in the step of forming the electrodeposited metal layer, the second plane of the substrate is insulated by adhesion of a laminate film.
As a result, it is possible to prevent the formation of the electrodeposited metal layer on the surface of the phosphor screen side in the step of forming an electrodeposited metal layer on the surface of the electron gun side. Consequently, it is possible to improve the etchability of the etching of the phosphor screen side performed in the following step.
The method of producing a color selection electrode used for a cathode ray tube of the present invention preferably is characterized in that in the step of forming the electrodeposited metal layer, a metal is simultaneously electrodeposited on the second plane of the substrate as well.
As a result, since it is possible to eliminate the film forming and peeling step compared with the case of forming an insulating laminate film on the phosphor screen side, it is possible to improve the productivity.
The method of producing a color selection electrode used for a cathode ray tube of the present invention preferably is characterized in that the substrate and the electrodeposited metal layer are different types of metals.
A shadow mask normally has a transmittance of an electron beam of 15 to 20%. The remaining 80 to 85% of the electron beam strikes the shadow mask and flows inside the shadow mask as an electric current, therefore generating Joule""s heat and causing the temperature of the shadow mask to rise. Accordingly, the shadow mask thermally expands during use, the relative positional relationship among the shadow mask, apertures, and phosphors changes, and, in extreme cases, the color purity and color uniformity (e.g., the white uniformity) on the screen fall. This phenomenon is called the doming effect.
Conventionally, to prevent doming of a color selection mask, Invar (an alloy of iron and nickel) is used as a low expansion material, a bimetal is used for the mask holding portion to correct the thermal expansion, or the mask surface is coated with various materials to raise the rigidity of the mask.
In the method of producing of the present invention, a metal layer having the above function is formed as an electrodeposited metal layer on the small diameter side of the mask apertures. Accordingly, the doming phenomenon is prevented and it is possible to make the mask thinner compared with the case of further coating a functional film on a mask formed with mask perforations with tapered cross-sections.
According to the method of producing of the present invention, by suitably selecting the types of the metals combined, it is possible to reduce not only the doming effect caused by thermal expansion, but also the AG vibration due to the intrinsic vibration, drift of the electron beam due to the magnetic characteristics, and halation due to the secondary electrons.
As examples of metals combined, mention may be made of an Fe layer and Fe-(35 to 45%)Ni layer, Cr layer or Cr alloy and Fe-(35 to 45%)Ni layer, and so forth.
According to the present invention, there is also provided a color selection electrode used for a cathode ray tube including an electron gun and a phosphor screen, the color selection electrode being positioned between the electron gun and the phosphor screen and having apertures through which an electron beam generated from the electron gun selectively passes to the phosphor screen, the color selection electrode comprising: a metal substrate; an electrodeposited metal layer formed on a first plane of the substrate, facing the electron gun; a plurality of first apertures formed on the electrodeposited metal layer; and a plurality of second apertures formed on a second plane of the substrate, facing the phosphor screen, the second apertures having sectional areas larger than the first apertures and being connected to the corresponding first apertures.
The color selection electrode used for a cathode ray tube of the present invention preferably is characterized in that the sectional areas of the first apertures is constant, the sectional areas of the second apertures on the second plane is larger than the areas of the same on the first plane, and the second apertures are tapered in cross-section.
The color selection electrode used for a cathode ray tube of the present invention preferably is characterized in that the substrate comprises an iron-nickel alloy or a chromium alloy.
The color selection electrode used for a cathode ray tube of the present invention preferably is characterized in that the electrodeposited metal layer comprises ferrite, nickel, chromium or gold.
The color selection electrode used for a cathode ray tube of the present invention preferably is characterized in that the diameter of the first apertures are within 15 xcexcm, and the apertures are arranged at a pitch of within 0.06 mm.
According to the present invention, there is further provided a cathode ray tube comprising an electron gun, a phosphor screen, and a color selection electrode placed between the electron gun and phosphor screen, and having apertures through which electron beam generated from the electron gun selectively passes to the phosphor screen, wherein, in the color selection electrode, the cathode ray tube comprising: a metal substrate; an electrodeposited metal layer formed on a first plane of the substrate, facing the electron gun; a plurality of first apertures formed on the electrodeposited metal layer; and a plurality of second apertures formed on a second plane of the substrate, facing the phosphor screen, the second apertures having sectional areas larger than the first apertures and being connected to the corresponding first apertures.
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