Optical Scanning Apparatus for Photolithography of a Color Cathode Ray Tube Having an Aperture Mask

A concurrently filed application entitled "Optical Scanning Apparatus And Method For Manufacturing Cathode Ray Tubes" bears Ser. No. 699,110 and is filed in the name of John Schlafer, G. Norman Williams and R. F. Wilson. Also, a concurrently filed application entitled "Control System For An Optical Scanning Exposure System For Manufacturing Cathode Ray Tubes" bears Ser. No. 699,045 and is filed in the name of Thomas W. Schultz. Further, a concurrently filed application entitled "Overlap And Overscan Exposure Control System" bears Ser. No. 699,054 and is filed in the name of Mahlon B. Fisher and G. Norman Williams. In addition, a concurrently filed patent application entitled "Exposure Area Control For An Optical Scanning System For Manufacturing Cathode Ray Tubes" bears Ser. No. 699,046 and is filed in the name of Thomas W. Schultz. Lastly, a concurrently filed application entitled "Scanning Rate And Intensity Control For Optical Scanning Apparatus" bears Ser. No. 699,047 and is filed in the name of Thomas W. Schultz.

The present invention relates to a method of and apparatus for photolithographic processing of faceplates intended for color cathode ray tubes having an aperture mask, and more specifically, to exposing photosensitive material with a scanning light beam.

The exposure of this photosensitive material provides a means for delineating the pattern of other material applied to the faceplate for generating, filtering or blocking light or for other functions. In a typical method, phosphor is dusted onto the surface of the photosensitive material, after which the material is selectively exposed. Then, the unexposed photosensitive material is removed from the faceplate by well-known techniques. An important step in this method is the act of exposing the photosensitive material at the proper locations on the faceplate.

Non-scanning methods for exposure of photosensitive material on the inner surface of a faceplate of a cathode ray tube are known. In one known method, the photoresist, such as dichromated polyvinyl alcohol, is exposed by light from an ultraviolet light source, the light passing through an aperture mask registered with the faceplate. The ultraviolet source is a mercury arc lamp whose output is concentrated to pass through a small source aperture and then dispersed to fully illuminate the aperture mask. The proper intensity distribution, which is not necessarily uniform, across the aperture mask is obtained by controlling the intensity distribution at the source aperture and by the insertion of a graded neutral density filter between the source aperture and the aperture mask.

For proper registration of the phosphor pattern on the faceplate with the electron beam landings in the assembled tube, the light rays from the ultraviolet source during photoresist exposure should parallel the electron beam trajectories as they pass through the various apertures in the aperture mask. Due to aberrations in the magnetic deflection process, the apparent location of the electron beam source varies with the deflection angle. Thus, a fixed optical point source alone cannot simulate the deflected electron source over the entire faceplate. To introduce the necessary off-axis correction factors into the optical exposure system, a special aspheric lens is inserted into the system between the light source and aperture mask, with a separate lens being required for each of the three electron gun positions. The contour of each lens is designed such that the light source as seen through the lens from each point on the faceplate has the correct lateral location in the source plane to produce rays passing through the aperture mask with the same angle of incidence as an electron beam through the same aperture in an assembled tube. Design calculations for these lenses are difficult and costly, especially as maximum deflection angles become larger.

In most cases, when a tube design is modified by changing the maximum deflection angle, deflection yoke winding pattern or position, curvature, aperture mask spacing, or certain other parameters, a new lens set and graded neutral density filters are needed. Optimizing the new design may require a trial-and-error procedure which could involve the fabrication of additional lenses and filters.

Various scanning exposure systems are also known. In such a system, a small light beam is scanned over the aperture mask so as to expose the photosensitive material adjacent to the light-transmitting regions or apertures in the mask. For example, a scanning exposure system is described in the British Patent Specification No. 1,257,933. In this patent, a scanned laser beam is used in conjunction with an aperture mask and photosensitive material for delineating phosphor patterns on faceplates for color CRT's. However, this patent does not provide for correction of the inherent discrepancy between electron beam landings and phosphor locations.

Another scanning exposure system is described in the U.S. patent to Geenen et al., No. 3,876,425. In this system, the effective light beam source is actually translated about a source plane to provide correlation between phosphor locations and electron beam landing locations. Such beam translation eliminates the need for the aspheric lens which is necessary in the non-scanning exposure system. In the system described in the Geenen patent, the effective light beam source is the center of a mirror which deflects the beam toward the faceplate. An optical system always insures that the beam from the actual source is always directed to the center of the scanning mirror. The scanning mirror is carried by an assembly that rotates the mirror about two orthogonal axes to provide scanning and that translates the mirror along two orthogonal axes to provide movement of the center of the mirror in the source plane. The optical system includes a plurality of mirrors, bearing assemblies and a telescoping member.

This system has limitations which render it less than suitable for use in a production environment for cathode ray tubes. First, a scanning exposure system must have accurate optical alignment characteristics, i.e., the ability to repeatably position the light beam at a predetermined point on the faceplate. The machanical and optical system described is of such a complicated nature that it is doubtful that such alignment characteristics could be obtained. More specifically, the large number of rotating parts and simultaneously rotating and translating parts of the system could result in misalignment with continued use as is necessary in a production environment. Furthermore, the feature of actually translating the effective light beam source in a plane adds complexity to the electronic system which is necessary to control the scanning and mirror translation functions. More specifically, each time the mirror is translated, the beam, if not corrected by the scanning function, would impinge upon other than the desired faceplate location. Thus, the translating and scanning functions are interdependent.

An object of the present invention is to provide an optical scanning exposure system for use in the manufacture of color cathode ray tubes in which the scanning light beam undergoes substantially no translational motion at the faceplate when angle of incidence corrections are being generated.

Another object of the invention is the provision of a scanning exposure system capable of precise, repeatable alignment characteristics.

An additional object of the present invention is to provide a scanning exposure system which is capable of exposing the photosensitive material on the faceplate of a cathode ray tube in a time interval as low as a half minute so that the system speed is compatible with introduction into a CRT production assembly line.

In another object of the invention, the angle of incidence corrections and the beam scanning controls are not interdependent thereby reducing the complexity of the electronic control system for the optical scanning exposure system.

According to the present invention, there is provided an optical scanning apparatus for use in manufacturing cathode ray tubes. Such tubes include tubes in color television receivers, monitoring tubes and monoscopes in electrostatic printing devices. The apparatus exposes a layer of photosensitive material on an inner surface of a tube faceplate by scanning a light beam over an array of light-transmitting apertures in a mask disposed adjacent to the material on the faceplate. In the apparatus, a light source is provided for creating a light beam having a wavelength spectrum which exposes the photosensitive material. A device is disposed in the path of the light beam for deflecting the beam through an angle which is related to a predetermined angle of incidence that an electron beam has at each point on the aperture mask as it passes through the transparent regions of the mask in an operating cathode ray tube. Further, an optical device images the point of deflection of the light beam substantially onto the faceplate. Accordingly, the light beam may be made to impinge on the mask with the same angle of incidence as the electron beam in an operating tube. This proper angle of incidence is obtained substantially without translation of the light beam at the faceplate. Lastly, a device scans the deflected light beam over the aperture mask in a predetermined fashion to expose the photosensitive material adjacent to all light-transmitting regions on the mask. The beam deflecting device and the scanning device are adapted to operate in synchronism to provide the light beam with the proper angle of incidence for each light-transmitting region on the mask.

In the drawings:

FIG. 1 is a block diagram of an optical scanning exposure system including an optical scanning apparatus according to the invention and an electrical control for the apparatus;

FIG. 2 is a diagram illustrating the various optical components of an embodiment of the optical scanning apparatus;

FIG. 3 is a perspective view of an illustrative embodiment of an optical scanning apparatus including both optical and mechanical components according to the present invention;

FIG. 4a is a perspective view of the optical path in a preferred embodiment of an optical scanning apparatus according to the invention;

FIG. 4b is a plan view of the embodiment shown in FIG. 4a showing the disposition of mechanical components; and

FIG. 4c is a side elevational view of the embodiment shown in FIG. 4a showing the disposition of mechanical components.

In an exemplary embodiment of the present invention, as illustrated in block diagram form in FIG. 1, an optical scanning exposure system, represented generally by the reference numeral 10, is utilized in the manufacture of color cathode ray tubes. The exposure system 10 exposes a layer of photosensitive material (not shown) on an inner surface 12 of a faceplate 13 for a cathode ray tube. The exposure is accomplished by scanning a light beam 14 over an array of light-transmitting apertures, in a mask 16 disposed adjacent to the faceplate. The exposure system 10 includes an optical scanning apparatus, represented generally by the reference numeral 18, and an electrical control system 20. The apparatus 18 includes the necessary mechanical and optical components which perform the actual scanning of the beam 14 on the faceplate 13, while the electrical control system 20 generates the command signals for the apparatus 18. The optical scanning apparatus 18 includes a light source 22, beam forming optics 23, a beam deflector 24, an imaging optics assembly 26, and a scanner 28. The light source 22 emits light 11 which has a wavelength spectrum which exposes the photosensitive material. Preferably, the light source 22 is a laser light source. In the exemplary embodiment, the laser is an argon-ion laser. The beam forming optics 23 modifies light 11 from the source 22 to obtain the proper beam 14 diameter and divergence angle at the aperture mask 16. The beam deflector 24 is positioned in the path of the light beam and deflects the light beam through an angle which is related to a predetermined angle of incidence that an electron beam has at each point on the aperture mask as it passes through the transparent regions of the mask in an operating tube. The imaging optics 26 receives the deflected light beam 14 and images the point of deflection of the light beam substantially onto the faceplate 13. In this manner, the light beam 14 may be made to impinge on the mask with the same angle of incidence as the electron beam in an operating tube. By this arrangement, the proper angle of incidence is obtained substantially without translation of the light beam at the faceplate. The deflected light beam is scanned over the aperture mask in a predetermined fashion to expose the photosensitive material adjacent to all light-transmitting regions on the mask. The scanner 28 and the beam deflector 25 operate in synchronism via the electrical control 20 so that the light beam has the proper angle of incidence for each light-transmitting region on the mask. Before initiation of an exposure sequence, the electrical control 20 is provided with, and stores, predetermined information on the deflection which must be applied to the beam 14 to obtain the proper angle of incidence for each position on the aperture mask. Beam position information 30 fed from the scanner to the electrical control causes signals 32, generated from this stored information, to be applied to the beam deflector 24 in the correct sequence. This scan position information is also used by the electrical control in generating the scan signals 34.

Referring now to FIG. 2, there is illustrated in detail a preferred embodiment of the various optical components comprising the optical scanning apparatus 18 shown in FIG. 1. The scanner 28 comprises a light reflecting surface, such as a mirror M1, which is rotatable about first and second scanning axes 38 and 40, which are orthogonal with respect to each other. The beam deflector 24 comprises a pair of rotatable mirrors M2 and M3. Each mirror is rotatable about one of a pair of orthogonal axes 42 and 44. While the beam deflector is shown as being a pair of mirrors, it is to be understood that the invention should not be limited since any combination of beam deflecting elements may be used so long as the required function is obtained. The imaging optics 26 includes first and second optical focusing elements L1 and L2 in the path of the deflected light beam 14. The focusing elements L1 and L2 are separated by a distance measured along the beam path equal to the sum of the focal lengths f.sub.1 and f.sub.2, respectively, of the focusing elements L1 and L2. Also, the first focusing element L1 is separated from the beam deflector 24 by a distance measured along the beam path equal to the focal length of f.sub.1 of the first focusing element L1. The second focusing element L2 is separated from the faceplate by a distance measured along the beam path substantially equal to the focal length f.sub.2 of the second focusing element L2. In the example of FIG. 2, both focusing elements L1 and L2 are double-convex converging lenses. However, it is to be understood that any combination of focusing elements, such as mirrors with either concave or convex surfaces or lenses with either concave or convex surfaces may be used so long as such combination of focusing elements images the deflected light beam onto the vicinity of the faceplate 13. Also, as illustrated in FIG. 2. f.sub.2 is greater than f.sub.1 to provide magnification of the cross-sectional area of the light beam. Preferably, the beam area at the mask is greater than the size of the apertures, thereby to simultaneously direct light through a plurality of apertures.

The optical scanning apparatus 18 may, although not necessary, include additional structural features which may advantageously be utilized. For example, it may be desirable that the beam from the light source 22 be well collimated or that its diameter be modified before entering the remainder of the optical system. In the preferred embodiments, the light source is a laser light source and to improve collimation of the beam or to change its diameter, it may be desirable to insert a telecentric lens system, such as lenses L3 and L4, in the path of the light beam 14 immediately as it leaves the light source 22. These lenses L3 and L4 are separated by substantially a distance equal to the sum of their focal lengths, f.sub.3 and f.sub.4, respectively; in the exemplary embodiment, the lenses are double-convex, converging lenses but it is to be understood that other combinations of focusing elements could also be used. Additionally, in the preferred embodiment, it may be desired to make the light beam appear to be originating from a point source as seen from the aperture mask. To accomplish this feature, a diverging lens L5 may be inserted into the path of the light beam 14 at a location to the left of the beam deflector 24. Lastly, it may be desirable with certain types of aperture masks, such as the slot aperture type, to obtain more beam divergence in one axis than in its orthogonal axis. This feature may be obtained by placing a lens L6 in the path of the light beam, the lens L6 being a cylindrical or toroidal diverging lens.

FIG. 3 is an illustrative embodiment of the combination of mechanical and optical components which comprise the optical scanning apparatus 18 previously mentioned with reference to FIGS. 1 and 2. The purpose of the mechanical components is to provide the means for implementing the rotating capability of the scanner 28 and the beam deflector 24. The scanner 28 includes the mirror M1 for steering the light beam 14 from the imaging optics 26 through a sequence of angles defined with respect to two intersecting scan axes 38 and 40, these axes being orthogonal with respect to each other. With respect to scanning about scan axis 40, the scanner includes a cradle assembly, represented by the reference numeral 60, having a base 62 and a component support platform 64 rotatably coupled to the base 62 and a drive assembly for rotating the support platform 64 with respect to the base 62. The scan axis 40 is the axis of rotation of the cradle assembly 60. In order that the beam position on the mirror M1, and thus the origin of scanning action in the scanner 28, remain invariant with respect to rotation about scan axis 40, it is important that the light beam 14 be coaxial with the scan axis 40 as the beam enters the cradle assembly. In rotatably coupling the support platform 64 to the base 62, a pair of cradle support flanges 66 and 68 are rigidly affixed to the base 62 on opposite sides of the platform 64. Flanges 66 and 68 have shafts 70 and 72, respectively, rotatably mounted thereon. The shaft 70 is formed with a central aperture, permitting the beam 14 to enter the cradle 60 along its axis. The shaft 70 is rigidly affixed to a support flange 78 which, in turn, is rigidly affixed to one side of the platform 64. Likewise, a support flange 80 is rigidly affixed to the other side of the platform 64 and to the shaft 72. The drive assembly for rotating the support platform 64 about the scan axis 40 includes a motor 82 affixed at 84 to the base 62 and a speed reduction mechanism, illustrated here as a gear train, comprising gears 86 and 88, coupling the output of the motor 82 to the shaft 72. One advantage of the speed reduction mechanism is in reducing the inertia load on the motor. This load is caused by the necessity of rotating not only the platform 64, but also all the components which are mounted on the platform. The speed reduction mechanism also allows finer control of the platform by the motor.

The type of motor 82 which is employed depends upon the nature of the scanning sequence. In the exemplary embodiment, the scanning sequence is similar to that of an electron beam in an operating cathode ray tube displaying a NTSC-type television signal. In both situations, the sequence comprises a vertically descending series of horizontal scan lines. The difference is that the electron beam scan is interlaced and is unidirectional from left to right as viewed from the outer surface of the faceplate, whereas the light beam scan is not interlaced and is bidirectional. In this type of light beam scan sequence, the line scan velocity is greater than the average scan velocity orthogonal to the lines, i.e., the frame scan velocity. Accordingly, the scan axis 40 of the cradle controls the frame scanning, and it has been found to be advantageous to use a stepper motor for the motor 82. Thus, a raster is generated as the line scanner, described below, sweeps the laser light beam back and forth across the aperture mask in a zig-zag motion, and the frame scan cradle steps down some fraction of a line at the end of each line. Other types of scan patterns may be utilized; for example, the pattern may be spiral and in such case, a continuous motion motor, such as a dc servo motor, may be more desirable for the motor 82.

As previously stated, the scanner 28 also includes a device for steering the light beam from the imaging optics 26 through a sequence of angles about the line scan axis 38. In FIG. 3, this device includes the mirror M1 and a motor 90 whose output shaft 92 is rigidly affixed to the mirror M1. Preferably, the motor 90 is a dc servo motor. There are several features of the mirror M1. First, the intersection point of the scan axes 38 and 40 is at the mirror reflecting surface so that the origin of scan for both axes is coincident. Further, the reflecting planar surface of the mirror M1 is at an angle of 45.degree. with respect to the scan axis 38.

The dc servo motor 90 and the stepper motor include angular shaft encoders 94 and 96, respectively, for providing a scan position signal which is sent to the electrical control 20 via the line 30 as previously shown in FIG. 1. Also, each motor receives a command signal from the electrical control 20; these signals are shown as being carried over the line 34 in FIG. 1.

The beam deflector 24 in FIG.s 1 and 2 is shown on FIG. 3 as mirrors M2 and M3 and their associated rotational drive mechanisms 100 and 102. The rotational axes 42 and 44 of the mirrors M2 and M3, respectively, are orthogonal and closely spaced to bring the origin of beam deflection for each axis in near coincidence. This feature of bringing the origin of beam deflection in near coincidence could also be accomplished without actually physically mounting the mirrors close to one another. For example, if desirable, the mirrors could be spaced apart and a focusing element, such as a lens, could be used to focus one mirror deflection point onto the other mirror. The mirrors M2 and M3 are rotated by galvanometer-type motors 100 and 102, respectively; these galvanometers, as is well known, provide an angular rotation of the output shaft proportional to a current input. The galvanometers in the example should provide an output shaft rotation in the range of .+-.15.degree..

While many arrangements of the basic optical components of the scanning apparatus 18 are possible within the spirit and scope of the invention, a particularly compact and convenient embodiment is shown in FIGS. 4a, 4b, and 4c, representing perspective, top and side views, respectively. In this embodiment, the light path is folded by mirrors M4 and M5 and right angle prisms P1 and P2. An advantage of prisms for folding the optical path is that beam deviation is produced by total internal reflection at the glass-air interface rather than from a specially coated surface, as in standard mirrors. Reflection efficiency can be nearly 100% and is not subject to deterioration as readily as standard mirror surfaces.

As can be observed, the light beam passes through or is reflected from the following components in sequence:

diverging lens L5

rotatable deflection mirror M2

rotatable deflection mirror M3

fixed mirror M4 (90.degree. deviation)

focusing lens L1

prism P1

prism P2

fixed mirror M5

focusing lens L2

scan mirror M1

This embodiment may be easily adapted to use with faceplates for different size cathode ray tubes by changing the focal length of lens L2 and re-establishing the distance between lens L1 and lens L2 at f.sub.1 plus f.sub.2 by moving prism P2 along a track assembly 104 in the direction of arrow 106. The focal length f.sub.2 of lens L2 is chosen such that the center of rotation of the scan mirror M1 is located in a position with respect to the faceplate substantially equivalent to the position of the origin of electron beam deflection in an operating CRT when the faceplate is at a distance substantially f.sub.2 from lens L2.

The details of the electrical control 20, shown in FIG. 1, do not comprise a part of the present invention. As stated previously, the function of the control is to generate the scan control signals while at the same time to supply the proper current values to the galvanometers at each scan position. This function can be obtained by numerous types of apparatus depending upon the degree of automation desired. In an unautomated case, the control for each galvanometer may merely be a variable current source, and the control for the scanning motors may be a suitable, indexable electrical power source. In operation, the scan motors are indexed to the proper scan position, and then the motors are stopped; for each position, the variable current sources are adjusted for the proper values. Then, the laser is turned on and after exposure, the laser is turned off. This operation is repeated in sequence until the faceplate is exposed. For more automated operation, a properly programmed, general purpose computer may be used as the electrical control. The computer stores the angle of incidence adjustments (i.e., the values of current for each scan position) in a memory and outputs the proper current values and scan position signals.

One of two techniques may be used for establishing the proper angle of incidence adjustments for each scan position. The first is an empirical process requiring the exposure of a sample faceplate by another system such as that utilizing the aspheric lens and graded neutral density filter. The exposed sample faceplate is then mounted into position in the scanning apparatus of the present invention. The scanning mirrors are then indexed to the various scan positions, and at each scan position, the proper amount of current is applied to the galvanometers so that the light beam landing is coincident with the exposed material. These current values comprise the angle of incidence adjustments for processing other faceplates of the same type as the sample. In another method, it is possible to derive an equation relating the current to the galvanometers to the scan mirror angular position. The equation relating current to scan mirror angle is a function of the geometry of the optical scanning apparatus 18 and of a set of data which defines the effective position (X.sub.p, Y.sub.p) of the electron beam in a deflection plane for each faceplate location (X, Y, Z). This equation may be implemented by a suitable programmed, general purpose computer or by a special purpose computer forming the electrical control 20.

Preferably, the electrical control 20 is implemented in the manner described in the concurrently filed patent application entitled "Control System For An Optical Scanning Exposure System For Manufacturing Cathode Ray Tubes" bearing Attorney's Docket No. D-8561 and being filed in the name of Thomas W. Schultz. In this control system, there is provided a memory storage device for storing information representative of the proper angle of incidence of a light beam at a matrix of positional locations on the faceplate of the cathode ray tube and of the rate of scan of the light beam from one positional location to the next. The encoder provides horizontal and vertical light beam scan position information to the storage device. A scan rate device, responsive to the scan rate and position information in the memory storage provides signals for controlling the rate of light beam scanning. Further, an angle of incidence control device, responsive to angle of incidence and position information from the memory storage, provides electrical signals for the galvanometers which control the angle of incidence deflecting mirrors.

The embodiments of the present invention are intended to be merely exemplary and those skilled in the art shall be able to make numerous variations and modifications of them without departing from the spirit and scope of the present invention. For example, it is not necessary that the scan function and the imaging function be separate components; for example, a rotatable, curved reflective surface may serve the same functions of the focusing element f.sub.2 and the scan mirror M1. Also, it is possible to have continuous unidirectional rotation about the scan axis 38 instead of bidirectional rotation. All such variations and modifications are intended to be within the scope of the present invention as defined by the appended claims.