Method for making marks in a transparent material by using a laser

#This application is based on Japanese Patent Applications No. 9-277921 filed on Sep. 26, 1997, No. 9-306043 filed on Nov. 7, 1997, No. 10-18284 filed on Jan. 16, 1998, No. 10-90605 filed on Mar. 20, 1998, and No. 10-243439 filed on Aug. 28, 1998, the entire contents of which are incorporated herein by reference.

#a) Field of the Invention

#The present invention relates to a method for making marks in a transparent material, and more particularly, to a method for making marks in a transparent material using laser beam.

#FIG. 1 is an oblique view of a device for making marks used in an embodiment of the present invention;

#FIG. 2 is a sectional view of an object to be marked;

#FIG. 3A and FIG. 3B are diagrams illustrating laser beam propagation in a case where a generic convex lens is used and a case where an fθ lens is used, respectively;

#Before describing an embodiment of the present invention, an evaluation test carried out by the present inventors will be explained. Namely, the fourth harmonic (wavelength 262 nm) of an Nd:YLF laser was illuminated onto the inner portion of a glass substrate using a convex lens having a focal length of 100 mm. The fourth harmonic laser beam had a diameter of 3 mm and the energy was 0.4 mJ/pulse. In this case, it was discovered that a crack approximately 100 μm wide was formed inside the glass substrate in the planar direction of the substrate, and starting from this crack, a further crack approximately 500 μm long was formed in the thickness direction of the glass substrate.

#It was also discovered that when making marks in a glass substrate of 1-2 mm thickness, cracks also appeared on the surface of the substrate as well as inside the substrate. When cracks appeared in the surface of the substrate, the mechanical strength of the substrate was lowered, and particles were dispersed from the substrate.

#The occurrence of cracks in the surface of the substrate is thought to be due to the following reasons. Firstly, cracks are generated on the basis of indentations and minute fractures already present on the surface of the glass substrate. Secondly, since dirt adhering to the surface of the substrate absorbs laser beams, the amount of laser energy absorbed at the substrate surface is greater than expected. In order to make marks on the inner portion of the substrate without causing cracks to occur on the surface of the substrate, it is thought to be necessary to exercise more precise control of the position of the focal point of the laser beam in the depth direction.