Method of forming on a surface of a moving substrate a layer, and vacuum evaporation apparatus for forming vapor deposited layers

Abstract: The thickness distribution of a vapor deposited layer such as an interference filter having deposited on a moving substrate (63) such as a glass faceplate for a projection television tube, is controlled along an axis in the direction of travel of the substrate by employing at least one rotatable dodger (64) to partially shield the substrate as it passes behind the dodger during deposition.

Description: This invention relates to a method of forming on a surface of a moving substrate a layer having a thickness distribution over said surface, the layer being formed by vaporizing a source material in the vicinity of the moving substrate. The invention also relates to a vacuum evaporation apparatus for forming vapor deposited layers from at least one source material on a plurality of moving substrates, the apparatus comprising a vacuum chamber, means within the chamber for heating the source material to vaporization temperature and means for moving the substrates in a path relative to the source material. Such methods and apparatus are used inter alia in the production of cathode ray tubes for projection color television. A method as described in the first paragraph and a vacuum evaporation apparatus as described in the second paragraph are known from the United States Patent Specification U.S. 4,804,884. US Patent 4,804,884 describes a method of forming an interference filter for a cathode ray tube for projection color television having a multilayer interference filter between the glass face panel and the luminescent screen. Such a filter results, among other things, in significantly greater brightness of the luminescent output of the tube. Such interference filters are typically composed of alternating layers of materials having a high and a low index of refraction, respectively. The layers are preferably formed by vapor deposition onto the inner surface of the glass face plate. Mass production of such filters is carried out in a vacuum chamber containing the source materials to be evaporated, means for heating the source materials, and a dome-shaped fixture adapted for holding a multiplicity of glass face plates. The plates are arranged in rows, each row forming a circle around the dome, so that each plate is approximately equidistant with the other plates from the source materials. The dome is rotated during deposition to promote uniform distribution of the deposited material over the plates. The plates pass behind one or more stationary dodgers located between the source materials and the plates. These dodgers are designed to result in increasing thickness of the layers toward the edges of the plates, which has been shown to result in even greater increases in brightness than can be obtained for a uniform thickness distribution. Preferably, the thickness of the layers increases in all directions from the center of the plate. Unfortunately, however, such an arrangement as described above can only be used to control thickness distribution in a direction normal to the direction of movement of the plates. In US patent 4,804,884 a method is further described by which a radial thickness distribution can be achieved. This so called "planetary" method involves rotating each plate about an axis normal to the surface of the plate, the evaporation source being positioned off-axis. However, in addition to adding to the complexity of the apparatus, the rotating plates require significantly more space in the dome than do stationary plates. Thus, the throughput of the apparatus is significantly reduced. Also, in the case of skirted panels, the planetary drive is adequate only for circularly symmetric plates or "discs", and not for rectangular plates. Accordingly, it is an object of the invention to control the thickness distribution of a vapor deposited layer on a moving substrate in the direction of travel of the substrate, without rotation of the substrate about an axis normal to the surface of the substrate being necessary. In accordance with one aspect of the invention, a method as described in the first paragraph is characterized in that at least one dodger positioned between the source material and the substrate is rotated about an axis normal to the direction of travel of the substrate and substantially parallel to the said surface, so that during rotation of the dodger the substrate is partially shielded from and partially exposed to the source material. In accordance with a preferred embodiment of the method, the rotating dodger is used in conjunction with at least one fixed dodger employed to control thickness distribution of the deposited layer in a direction normal to the direction of travel of the substrate. In accordance with another aspect of the invention, an apparatus as described in the second paragraph is characterized in that the apparatus comprises at least one dodger, rotatably mounted between the source material and the path of the substrates, the axis of rotating of the dodger being normal to the direction of travel of the substrates, and substantially parallel to the substrate plane and means for rotating the dodger when a substrate is at least partially behind the dodger, so that during rotation of the dodger the substrates are at least partially shielded from and partially exposed to the source material. In a preferred embodiment of the apparatus, the means for moving the substrates comprises a rotatably mounted dome-shaped fixture, the fixture adapted for holding the substrates in one or more circular rows and for moving the substrates in these circular rows above the source material. The rotatable dodger may be used in conjunction with at least one fixed dodger mounted between the source material and the path of the substrates, the dodgers being spaced apart from one another so that they shield the substrates at different times during the substrates' travel about the source material. Brief Description of the Drawings Fig. 1 is a diagrammatic representation of the positional relationship between source material, substrates and fixed dodger in a prior art vapor deposition apparatus; Fig. 2 is a diagrammatic representation of the relationship between substrate and dodger as viewed from the source in the apparatus of Fig. 1; Fig. 3 is a diagrammatic representation of the relationship between a substrate and a rotatable dodger as viewed from the source in accordance with the invention; Fig. 4 is an end view of a substrate and rotatable dodger and one embodiment of a mechanical means for rotating the dodger synchronously with the passage of the substrate behind the dodger; Fig. 5 is an end view similar to that of Fig. 4 showing another embodiment of mechanical means for rotating the dodger synchronously with the passage of the substrate; and Fig. 6 is a diagrammatic representation of a vapor deposition apparatus of the invention showing computer controlled means for rotating the rotatable dodger. Fig. 7 is a graphical representation of typical thickness distributions along the main axis achieved by an arrangement as shown in Fig. 5. Description of the Preferred Embodiments Fig. 1 shows the positional relationship between source material, substrates and fixed dodger in a prior art vapor deposition apparatus. Dome-shaped fixture 1 holds substrates 2 and 3 in positions which are approximately equidistant from source materials 7 and 8. Fixture 1 rotates about an axis 4. Fixed dodgers and 6 shield substrates 2 and 3 respectively from the path of vapor from the source materials each time the rotating fixture brings the substrate behind the dodger. In the application envisioned for the invention, the substrates are glass face panels for cathode ray tubes for projection television, and the source materials are usually oxides of silicon and titanium or tantalum, which are deposited on the inner surface of the substrates in alternate layers. Oxidation is later completed to form an interference filter of alternating layers of SiO₂ or Ta₂O₃ having low and high indices of refraction, respectively. In this application, the ability to control the thickness distribution of the filter layers across the face plate enables the optimization of certain operating characteristics of the projection tubes such as lumen output to the projection optics. A certain degree of control over thickness distribution is possible using fixed dodgers such as dodgers 5 and 6 shown in Fig. 1. Fig. 2 shows the relationship between dodger 5 and substrate 3 as viewed from the position of the source materials. Substrate 3 is a glass face plate which is rectangular in shape. The long axis of the face plate is conventionally referred to as the major axis which in this Figure corresponds to the Y axis. The short axis is the minor axis and here corresponds to the X axis, which represents the circular path of travel of the substrate caused by the rotation of fixture 1, as viewed in the plane of the circle. Dodger 6 has a width W which varies as a function of distance Y from the X axis. As the substrate is moved behind the dodger, the substrate is shielded from the evaporation source and less material is deposited on the substrate than would otherwise be the case. If the dodger had approximately straight sides parallel to the Y axis, all of the substrate would be shielded for the same proportion of the time and the distribution of the deposited layer would not change. However, since the dodger width varies as a function of Y, the substrate exposure time also varies along the Y axis and the thickness of the deposit also changes. It is not possible to control thickness along the minor axis with static dodgers of any geometry. In accordance with the invention, thickness along the minor axis is controlled using a rotating dodger. Fig. 3 shows an arrangement as viewed from the position of the source materials in which dodger 30 is mounted between the source and the substrate 2 to rotate along its longitudinal axis, parallel to the Y axis of the substrate. While Fig. 3 shows the dodger as having a constant, width, W, the width can vary to fine tune the final thickness distribution. The dodger is normally in an open position, normal to the substrate surface, allowing essentially unobstructed access of the vapor from the source to the substrate. As shown in Fig. 3, as the leading edge of substrate 2 passes by dodger 30, the dodger rotates to a closed position (Fig. 3b) to shield the central part of the substrate behind the dodger from the vapor. As the trailing edge of the substrate 30b approaches the dodger, the dodger again rotates to an open position (Fig. 3c) A typical mechanical arrangement for achieving the rotation of the dodger synchronously with the passage of the substrate is shown in Fig. 4. Dodger 30 is attached to a cam 31 having four protruding fingers 31a through d. Studs 32 and 33 protrude downward from fixture 40. As the fixture 40 carries substrate 2 past dodger 30, fingers of cam 31 successively engage studs 32 and 33. Each time a cam finger engages a stud, the dodger is rotated 90 degrees. Another embodiment of a mechanical arrangement of rotating the dodger is shown in Fig. 5. In this arrangement, the dodger 50 is held in a normally open position by spring 51. As fixture 52 brings substrate 53 past dodger 50, cam 54 engages stud 55 and rotates dodger 50 into a closed position. When stud 55 disengages cam 54, spring 51 rotates dodger 50 back to an open position. Other arrangements will become apparent to those skilled in the art. By contouring the studs and the cams, the speed of opening and closing the dodger can be controlled, thereby controlling the distribution profile in the area between the center and the edge of the substrate. A typical thickness distribution along the minor axis achieved by such an arrangement is shown graphically in Fig. 7. The thickness of the deposited layer is plotted in terms of percent deviation from center thickness along the Y axis, versus distance in inches (1 inch ≙ 2.54 cm) along the minor axis, here the X axis. Three curves are presented, all of which represent thickness distributions obtained on 20 layer SiO₂/TiO₂ filtering in a 1100 mm vacuum coater containing 24 substrates in two rows. The collet rotated at 30 RPM and coating time was about 1.5 minutes per layer for SiO₂ and 3 minutes per layer for TiO₂. Curve 1 represents the thickness distribution obtained without the use of a rotating dodger, while curves 2 and 3 represent the thickness distributions obtained when using two rotating dodgers 2.54 cm (1'') wide, mechanically driven with a system similar to the one pictured in Figure 5. A suitable apparatus 60 for carrying out the method of the invention is shown schematically in Figure 6. The vacuum enclosure 61 contains the rotating collet 62. The collet is supported by a shaft 73 and rotated by a motor 74 located outside the vacuum chamber 61. Collet 62 holds at least one substrate 63 having a surface exposed to the interior of the enclosure. Rotating dodger 64 is mounted between substrate 63 and sources 69 and 70. The sources are heated to evaporation temperatures by separate heaters 71 and 72, typically using electron heating means. The apparatus includes more for rotating the dodger comprising motor 65 connected to dodger 64 by drive coupling 67. The motor may be located inside or outside the vacuum chamber 61. Both the motor 65 and the heaters 71 and 72 are controlled by computer 68. One advantage of this arrangement is that the dodger can be rotated at any one of a range of speeds selected by inputting appropriate commands into the computer. Thickness distribution can be controlled by controlling the rate at which the dodger is rotated, or in systems where many resolutions per layer are made, by controlling the amount of time the dodger is held open or closed. Further advantages include a generally smoother rotation of the dodger than can be obtained using mechanical means and the ability to automatically control the heaters in accordance with a desired schedule. As will be appreciated, there is a limit to the amount of correction to the thickness distribution of the deposited layer which can be obtained from a single dodger. For the conditions described with reference to Fig. 7, it is seen that the use of a dodger in accordance with the invention results in a maximum thickness at the edge of the substrate about 1.7 percent greater than the thickness obtained without such a dodger. Of course, for greater corrections than can be obtained with a single dodger, multiple rotatable dodgers may be used. Other means can be used to synchronize the motion of the dodger and the motion of the collet. The dodger can be driven by a belt, chain or gear train from the collet, collet shaft or collet drive motor. It is desired that the layer thickness decrease from center to edge of the substrate, the dodger can be normally closed instead of normally open, and rotate to the open position at the central region of the substrate, instead of close. The rotatable dodger described herein can be used in conjunction with a fixed dodger of the prior art. With such a combination, the thickness distribution can be made a nearly arbitrary function of X and Y by making one or more of the following adjustments: (a) changing the shape of the fixed dodger; (b) changing the width of the rotatable dodger along its length; (c) changing the shape of the cams and studs that drive the dodger or otherwise controlling the rates at which the dodger opens and closes or the amount of time the dodgers are held open or closed; (d) changing the number of fixed and/or rotatable dodgers in the system. The invention has been described in terms of a limited number of embodiments. Other embodiments and variations of embodiments will become apparent to those skilled in the art, and these are intended to be encompassed within the scope of this description and the appended claims.

Claims: A method of forming on a surface of a moving substrate a layer having a thickness distribution over said surface, the layer being formed by vaporizing a source material in the vicinity of the moving substrate, characterized in that at least one dodger positioned between the source material and the substrate is rotated about an axis normal to the direction of travel of the substrate and substantially parallel to the said surface, so that during rotation of the dodger the substrate is partially shielded from and partially exposed to the source material. The method as claimed in Claim 1, characterized in that the rotating dodger is used in conjunction with at least one fixed dodger employed to control thickness distribution of the deposited in a direction normal to the direction of travel of the substrate. The method as claimed in Claim 1 or 2, characterized in that the sides of the rotatable dodger normal to the direction of travel are parallel to each other and to the axis of rotation of the dodger. A vacuum evaporation apparatus for forming vapor deposited layers from at least one source material on a plurality of moving substrates, the apparatus comprising a vacuum chamber, means within the chamber for heating the source material to vaporization temperature and means for moving the substrates in a path relative to the source material, characterized in that the apparatus comprises at least one dodger, rotatably mounted between the source material and the path of the substrates, the axis of rotating of the dodger being normal to the direction of travel of the substrates, and substantially parallel to the substrate plane and means for rotating the dodger when a substrate is at least partially behind the dodger, so that during rotation of the dodger the substrates are at least partially shielded from and partially exposed to the source material. The apparatus as claimed in Claim 4, characterized in that the means for moving the substrates comprises a rotatably mounted dome-shaped fixture, the fixture adapted for holding the substrates in one or more circular rows and for moving the substrates in these circular rows above the source material. The apparatus as claimed in Claim 4 or 5, characterized in that at least one fixed dodger is mounted in the vacuum chamber between the source material location and the path of the substrates, the dodger being spaced apart from the rotatable dodger so that the dodgers shield the substrates at different times during the substrates' travel about the source material. The apparatus as claimed in Claim 4, 5 or 6, characterized in that the means for rotating the dodger comprises a cam attached to the dodger and at least one stud attached to and extending from the means for moving the substrates, so that as the substrates move past the dodger, the stud engages the cam and rotates the dodger. The apparatus as claimed in Claim 4, 5, 6 or 7, characterized in that the means for rotating the dodger comprises a motor, drive means connecting the dodger and the motor, and means for controlling the motor. The apparatus as claimed in Claim 8, characterized in that the motor is mounted inside the vacuum chamber. The apparatus as claimed in Claim 8, characterized in that the motor is outside the vacuum chamber. The apparatus as claimed in Claim 8, characterized in that the means for controlling the motor also controls the means for heating the source materials.

Inventors: BRENNESHOLTZ MATTHEW SCOTT, PATT PAUL, BRENNESHOLTZ, MATTHEW SCOTT, PATT, PAUL
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1990
Main classification: C23C14
IPC codes: C23C14