Method of cleaving a brittle plate and device for carrying out the method

Abstract: A method and a device for cleaving a plate of a brittle material, such as, for example, glass, by means of a radiation beam repeatedly moving over the plate. The radiation beam is repeatedly passed over a desired track until the plate has been cleft along a desired line of rupture of the plate.

Description: The invention relates to a method of cleaving a brittle plate, in which an emitted radiation beam originating from one source of radiation and focused to be incident on a rotatable mirror generates a series of successive single deflected radiation beams, whereby the said deflected beams pass over a desired track of the plate successively such that only one deflected beam passes over said track at any one time, the said deflected beams produce thermomechanical stresses in the plate and cause, starting from a crack initiator, a crack to be formed along a desired line of rupture. A method of the kind mentioned in the opening paragraph and a device for carrying out this method are known from patent application FR-A-2 228 040. This application discloses a method of cleaving a sheet of glass by passing two laser beams over the same track, the beams being focused at points which are spaced apart. To obtain two laser beams an emitted laser beam originating from a laser source is focused to be incident on mirrors to split the emitted laser beam into two deflected laser beams. For passing the deflected laser beams over the desired track the mirrors are pivotably driven. The disadvantage of this known method is that for obtaining two deflected laser beams at the same time and focusing these beams at different points of the track it requires a number of optical focusing systems and a control system for controlling the optical focusing systems in relation to each other. This makes the method not only complicated but also expensive. Moreover more optical focusing systems result in a higher loss of energy. Another method related in more general terms to the kind mentioned in the opening paragraph and a device for carrying out such a method are known from US 3,543,979. In this known method, a crack initiation is provided in a side of a glass plate, whereupon a laser beam is moved along a desired line of rupture. By means of the laser beam, thermal energy is produced in the plate, which energy produces thermomechanical stresses, as a result of which, starting from the crack initiation, a crack is formed. The crack follows the laser beam at a certain distance along the desired line of rupture until the laser beam has reached an end of the line of rupture. Due to the thermomechanical stresses in the plate, the crack then propagates further to the end of the desired line of rupture. This method has the disadvantage that the speed at which the crack propagates through the plate is limited in that the energy is produced in a comparatively small part of the track. Further, the quantity of energy produced in the plate per unit time must not be excessively large because the plate then locally softens, melts or burns. The invention has for its object to provide a method of cleaving a brittle plate, as indicated above, by means of radiation beams which are generated one after the other in order to be passed along the desired line of rupture at a high speed. In the method according to the invention, this object is achieved in that the said rotatable mirror comprises a number of mirror surfaces constituting the polygonal sides of a regular polygon disk which is rotated about its axis of revolution such that the said series of successive single deflected radiation beams are generated by deflection of the focused emitted radiation beam incident upon the rotating polygonal mirror. This is a very simple and reliable manner to generate radiation beams on after the other which pass along the same line of rupture. The thermal energy is produced in the plate along the whole track substantially at the same time, as a result of which thermomechanical stresses are produced in the plate, which cause a crack to be formed along the desired line of rupture. The speed at which the crack propagates along the desired line of rupture depends inter alia upon the quantity of thermal energy produced in the plate per unit time. This quantity of energy can be considerable, because the thermal energy is distributed over the whole track. As a result, the overall quantity of energy required for cleaving the plate is produced in the plate in a shorter time and a higher cleaving speed is attained. The diameter of the radiation beam can be small so that the crack extends more accurately along the desired line of rupture. It should be noted that it is known from US-A-3 587 956 to produce power at the same time over the whole track. By the method described in this Patent Specification, stresses causing the formation of a crack along a desired line of rupture are produced in a plate along a desired track with the aid of a heated wire. However, this method suffers from a number of limitations; the distance between the wire and the track should be substantially constant along the whole track in order to guarantee a satisfactory heat transfer; the shape of the track can be modified only with difficulty and the surface of the plate heated by the wire is comparatively large, as a result of which the desired line of rupture is not followed accurately. An embodiment of the method according to the invention in which the plate is cleft along the desired line of rupture, which extends from one side of the plate to another side of the plate, is characterized in that the series of successive deflected radiation beams pass over the desired track from one side of the plate to a certain distance from the other side of the plate, the crack cleaving the plate being completed within a part of the plate kept free from the deflected radiation beams. Thus, it is prevented that in this part of the plate thermomechanical stresses are built up, which can cause an undesired crack to be formed from an unevenness in this part of the plate or which make it difficult for the crack to propagate as far as the side of the plate. A further embodiment of the method according to the invention is characterized in that first deflected radiation beams pass over a first desired track of a first glass plate belonging to a number of liquid crystal display devices (LCDs) and second deflected radiation beams pass over a second desired track of a second glass plate belonging to the liquid crystal display devices until the plates have been cleft along a first and a second desired line of rupture, respectively, which are located between two adjacent liquid crystal display devices. The glass plates of the liquid crystal display devices are arranged in parallel and are interconnected. By the method according to the invention, it is possible to split up a number of interconnected liquid crystal display devices in a simple manner into separate liquid crystal display devices. Another embodiment of the method according to the invention is characterized in that the first and second lines of rupture are spaced apart when viewed perpendicular the plane formed by the surface of the plate. These parts are provided with connections of the liquid crystal display device, which in this manner become freely accessible. The device suitable for carrying out the method according to the invention comprises a laser, a device for focusing the emitted laser beam originating from the laser and a rotatable mirror for deflecting the laser beam incident on the mirror such that a series of successive single deflected beams may be generated for passing over a track or desired line of rupture is characterized in that the rotatable mirror comprises a regular polygon disk rotatable about its axis of revolution provided with a number of mirror surfaces on the polygonal sides of the disk. Preferably the end part of the desired line of rupture opposite the end having the crack initiator is screened by a shield. As a result, a trajectory over which the radiation beam is passed need not be adapted when a part of the plate should be kept free from the radiation beam. It has been found in practice that preferably a part of the desired line of rupture should always be kept free from the radiation beam. The extent of radiation screening by the shield is determined by experiments in such a manner that the crack extends as far as the side of the plate and the initiation of an undesired crack from the part of the plate kept free is avoided. The invention will be described more fully with reference to the drawings, in which: Fig. 1 shows diagrammatically a device suitable for carrying out the method according to the invention, Fig. 2 shows diagrammatically another device suitable for carrying out the method according to the invention, Fig. 3 shows diagrammatically a part of a device according to the invention, and Fig. 4 shows a number of interconnected liquid crystal display devices, which are split up into separate liquid crystal display devices. Fig. 1 shows a device 1 which is suitable for cleaving a plate 5 along a desired line of rupture 7. The device 1 comprises a laser 9, a laser beam narrowing element 11 and a disk 15 rotatable about an axis 13. The axis 13 of the disk 15 perpendicularly crosses the emitted laser beam 19 originating from the laser 9. A section of the disk 15 perpendicular to the axis 13 has the shape of a regular polygon. The disk 15 is provided with a number of mirror surfaces 17, which constitutes the sides of the polygon. A normal vector n of the mirror surface 17 is perpendicular to the axis 13 of the disk 15. The laser beam 19 is reduced by the laser beam narrowing element 11 to the desired diameter and is then deflected by a mirror surface 17 away from the disk 15 in the direction of the plate 5 to be cleft. The disk 15 is driven by an electric motor (not shown) and rotates about the axis 13 in the direction of the arrow 21. By reflection of the laser beam 19 on one of the rotating mirror surfaces 17, a track 23 is described by the deflected laser beam 19' over the plate 5 to be cleft. When the disk 15 rotates, the deflected laser beam 19' is passed repeatedly over a track 23 of the plate 5. By the deflected laser beam 19', in the plate 5 thermomechanical stresses are produced, which cause a crack to be formed, starting from a crack initiation 25. This beam 19' is passed over the track 23 until the crack has propagated along the whole desired line of rupture 7. The track 23 over which the deflected laser beam 19' is passed should be defined so that the thermomechanical stresses are symmetrical around the line of rupture, as a result of which the plate 5 is cleft along the desired line of rupture 7. Especially when a plate is split up into two parts having different dimensions, the track will not coincide with the desired line of rupture. For determining the correct track, use may be made of the method described in Netherlands Patent Applications 8901143 and 8901144 in the name of N.V. Philips' Gloeilampenfabrieken. The speed at which the crack is formed in the plate 5 inter alia depends upon the power of the laser beam passing over the plate and upon the size of the crack initiator 25. The crack may be an unevenness already present in the plate or a scratch provided in the plate by means of a sharp object. When a comparatively small crack initiator has been provided in the plate, the time passing before sufficient thermomechanical stresses have been built up around the crack initiator for starting the crack is comparatively long. In this time, stresses are also built up in the remaining part of the plate. When the crack starts, the crack is displaced at a comparatively high speed through the plate. When a comparatively large crack initiator has been provided in the plate, the time passing before the crack starts is shorter, while smaller thermomechanical stresses are built up in the plate and the crack is displaced at a lower speed through the plate. Due to the lower speed, the crack follows the desired line of rupture more accurately. When cleaving a plate of calcarious glass (soda-lime), use was made of a CO 2 laser having a wavelength of 10.6 µm and a power applied to the plate of 400 W. The laser beam had a diameter of 2 mm at the area of the plate. A disk with twelve mirror surfaces was rotated at a speed of 200 rev/sec so that the frequency at which the laser beam was passed over the plate was 2400 Hz(/sec). The plate having dimensions of 350 x 320 x 1 mm was provided with a crack initiation at the centre of a side; the laser beam was passed along a desired line of rupture located symmetrically in the plate. The crack propagated through the plate within 5 seconds. Fig. 2 shows another device 2 suitable for carrying out the method according to the invention. The parts corresponding to Fig. 1 are provided with the same reference numerals. An emitted laser beam 19 originating from a laser 9 is passed via a beam narrowing element 11, a disk 15 provided with mirror surfaces 17 and a fixedly arranged curved mirror 27 over the plate to be cleft. The curved mirror 27 has a concave mirror side facing the plate and the disk 15. A normal vector n of the curved mirror surfaces encloses an angle of less than 90° with the laser beam, which is deflected by the disk 15 on the curved mirror 27 and encloses an angle of less than 90° with the surface of the plate 5. By the curved mirror 27, the deflected laser beam 19' is passed along the curved track 23 on the plate 5. The plate 5 is cleft along a line of rupture 7 located asymmetrically in the plate. The track 23 along which the thermal energy is produced in the plate is defined so that the thermomechanical stresses are symmetrical along the desired line of rupture 7 in order to avoid deviations of the crack with respect to the desired line of rupture. Fig. 3 shows a device 3 according to the invention, in which the deflected laser beam 19' is prevented by a shield 29 from being passed over the whole plate 5 to be cleft. The shield 29, for example an anodized aluminium plate, is connected via a support 31 to a frame 33. The plate 5 which has to be cleft along a desired line of rupture, is located on the frame 33. The deflected laser beam 19' is passed repeatedly over the plate 5, but it is not passed over a part of the plate in order to avoid an undesired start of the crack from an unevenness, such as, for example, a scratch or an inhomogeneity in that part of the plate. Thus, it further becomes simpler for the crack to propagate to the side 35. The distance S over which the laser beam is prevented from being passed over the plate is determined by experiments. Fig. 4 shows first and second plates 41, 43, which belong to a number of interconnected liquid crystal display devices 45. First and second defleted radiation beams 47, 49 are passed over the first and second plates 41 and 43, respectively, along first and second desired tracks 51 and 53, respectively. This may be effected, for example, by two devices 1 arranged on either side of the liquid crystal display devices 45 or by splitting a laser beam originating from a laser into two beams and causing these beams to pass via drums arranged on either side of the liquid crystal display devices 45 along the desired tracks 51, 53. The plates 41, 43 are cleft along the desired lines of rupture 55 and 57, respectively, which are located in a projection on one of the plates at a certain relative distance. As a result, connections 59 become freely accessible. In order to cleave brittle plates in a controlled and reproducible manner, it is recommendable to provide a crack initiator in the plate instead of using an unevenness already present in the plate as crack initiation. Instead of a laser, other radiation sources, such as infrared sources and halogen lamps, may also be used. For cleaving a plate, a radiation source should be chosen having a wavelength that is fully absorbed by the plate to be cleft. The curvature of the curved mirror 27 depends upon the desired track along which the radiation beam is passed. Besides flat plates, also objects having a curved surface may be cleft by the method according to the invention. However, the radiation beam must be capable of passing along the desired track. The material of the plate to be cleft should be brittle. This means that it should have a comparatively low K IC value. As examples of brittle materials are mentioned glass, ceramic material and silicon.

Claims: A method of cleaving a brittle plate (5,41,43), in which an emitted radiation beam (19) originating from one source of radiation (9) and focused to be incident on a rotatable mirror (17) generates a series of successive single deflected radiation beams (19',47,49), whereby the said deflected beams pass over a desired track (23) of the plate successively such that only one deflected beam passes over said track at any one time, the said deflected beams produce thermomechanical stresses in the plate and cause, starting from a crack initiator (25), a crack to be formed along a desired line of rupture (7,55,57), characterized in that the said rotatable mirror comprises a number of mirror surfaces (17) constituting the polygonal sides of a regular polygon disk (15) which is rotated (21) about its axis of revolution (13) such that the said series of successive single deflected radiation beams (19',47,49) are generated by deflection of the focused emitted radiation beam incident upon the rotating polygonal mirror. A method as claimed in Claim 1, in which the plate (5) is cleft along the desired line of rupture (7), which extends from one side of the plate to another side of the plate, characterized in that the series of successive deflected radiation beams (19') pass over the desired track (23) from one side of the plate to a certain distance (S) from the other side of the plate, the crack cleaving the plate being completed within a part of the plate kept free from the deflected radiation beams (19'). A method as claimed in Claim 1, characterized in that first deflected radiation beams (47) pass over a first desired track (51) of a first glass plate (41) belonging to a number of liquid crystal display devices (45) and second deflected radiation beams (49) pass over a second desired track (53) of a second glass plate (43) belonging to the liquid crystal display devices (45) until the plates have been cleft along a first and a second desired line of rupture (55,57), respectively, which are located between two adjacent liquid crystal display devices. A method as claimed in Claim 3, characterized in that the first and the second lines of rupture (55,57) are spaced apart when viewed perpendicular the plane formed by the surface of the plate (41,43). A device suitable for carrying out the method claimed in Claim 1, comprising a laser (9), a device (11) for focusing the emitted laser beam (19) originating from the laser and a rotatable mirror (17) for deflecting the laser beam incident on the mirror such that a series of successive single deflected beams (19') may be generated for passing over a track (23) or desired line of rupture (7), characterized in that the rotatable mirror comprises a regular polygon disk (15) rotatable (21) about its axis of revolution (13) provided with a number of mirror surfaces (17) on the polygonal sides of the disk. A device as claimed in Claim 5, characterized in that the end part of the desired line of rupture (7) opposite the end having the crack initiator (25) is screened by a shield (29).

Inventors: DEKKER JACOBUS NICOLAAS, FIERKENS IRENEUS HENRICUS THEO, GENNIP THEODORUS JOHANNES VAN, VERKADE GERRIT CORNELIS, ZONNEVELD MAARTEN HARM, DEKKER, JACOBUS NICOLAAS, FIERKENS, IRENEUS HENRICUS THEODORUS, VAN GENNIP, THEODORUS JOHANNES MARIA JACOBUS, VERKADE, GERRIT CORNELIS, ZONNEVELD, MAARTEN HARM
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: C03B33
IPC codes: C03B33, G02F1, B23K26, B28D1