In order to laser-scribe a brittle substrate such as a semiconductor wafer and break it, a glass substrate or a ceramic substrate, a method for forming a median crack (or vertical crack) is used where a laser beam and a brittle substrate are relatively moved along a laser-scribe line to be formed with median cracks on the substrate so that the substrate is heated while a refrigerant is supplied to the heated portion on the substrate.
In accordance with such a method for forming a median crack by means of a laser beam, a median crack (blind crack) is formed due to a difference between the compressive stress that is created around the heated portion and the tensile stress that is created around the cooled portion.
Patent Document 1 discloses a method for laser-scribing a substrate having predetermined properties and for breaking it, where cooling conditions and laser beam parameters which relate to the rate of laser-scribing and breaking of the substrate are appropriately selected when a median crack is formed by irradiating the substrate with the beam, and thereby, a blind crack having a required depth is formed in the substrate.
Patent Document 1:Japanese Patent No. 3027768
Patent Document 2 discloses a cutting apparatus and a cutting method for cutting a brittle material where a laser beam is partially blocked within a region irradiated with the laser beam that is formed on the surface of the brittle material, that is to say within a beam spot, using a light blocking plate, and thereby, a region where the laser beam is blocked is formed in such a manner that thermal stress is caused by a temperature gradient in the vicinity of the border between the region irradiated by the laser beam and the above described region where the laser beam is blocked, and thus, the brittle material is cut. This document discloses that a brittle material is irradiated with a laser beam of which the energy intensity is discontinuous so as to be cut.
Patent Document 2: Japanese Unexamined Patent Publication 2001-212683
It is preferable for a crack to be formed as deep as possible in the perpendicular direction in order for the division of a brittle material to be carried out easily, and in order for the quality of the brittle material on the broken surfaces to be excellent after the division in a breaking process, where the brittle material is divided by applying force to the left and right of the formed crack after the surface of the brittle material is inscribed by radiation from a laser.
Thermal strain within a substrate that may hinder the formation of such an excellent median crack is described in the following.
FIGS. 21 to 23 are cross sectional diagrams of a brittle substrate for illustrating thermal strain that is created within the brittle substrate when the brittle surface of the brittle substrate is scanned and irradiated with a laser beam so that a median crack is formed in the brittle substrate. In the figures, the laser beam continuously moves outward from the paper.
As shown in FIG. 21, the compressive stress as shown by broken line arrows in the figure is created in a portion 100 that is heated by radiation from a laser beam. Subsequently, as shown in FIG. 22, when a coolant is supplied to the vicinity of portion 100 that has been heated by radiation from the laser beam so as to form a cooled spot 120, the tensile stress is created as shown by solid arrows in the figure.
As a result of this, as shown in FIG. 23, median cracks 130 extending to the thickness-wise direction of the brittle substrate is formed in the direction perpendicular to the tensile stress in accordance with the difference between the compressive stress and the tensile stress.
However, even when cooled spot 120 is formed, a sufficient difference in stress for forming a crack is limited in a surface portion of the brittle substrate. When the sufficient difference in stress for forming a crack, that is to say a difference in temperature, disappears between the heat that spreads to the thickness-wise direction of the brittle substrate from cooled spot 120 and the heat that spreads to the thickness-wise direction of the brittle substrate from heated portion 100, excessive heat that has spread to the thickness-wise direction of the brittle substrate from heated portion 100 is considered to remain in compress-stressed region 150 of a high temperature. Compress-stressed region 150 of a high temperature is defined as relative thermal strain within the brittle substrate.
As shown in FIG. 23, compress-stressed region 150 of a high temperature prevents median crack 130 from extending straight to the thickness-wise direction of the brittle substrate, and therefore, the depth of the crack to the thickness-wise direction of the brittle substrate is limited to approximately 20% to 40% of the thickness of the brittle substrate, in the case where the surface of a brittle substrate is scanned with a laser beam at a practical rate at a mass production site.
FIG. 24 is a photograph taken in the direction along one of the broken surfaces of a brittle substrate showing the broken surface that was exposed as a result of breaking after the brittle substrate was broken along the formed median crack.
FIG. 24 shows the state where the formed median crack has been bent in the middle. This is caused by the above described compress-stressed region 150 of a high temperature (compress-stressed region 150 of a high temperature is added and indicated by a broken line in the photograph for the purpose of illustration).
As shown in FIGS. 23 and 24, compress-stressed region 150 of a high temperature that remains prevents the formation of a deep, straight median crack, and as a result, the quality of the broken surfaces of the brittle substrate which are formed as a result of breaking is lowered. Concretely speaking, a phenomenon where the broken surfaces, including the locally broken surfaces, are formed diagonally instead of perpendicular to the surface of the brittle substrate occurs and this is referred to as “chipping.”
In the above described Patent Document 1 and Patent Document 2, the vicinity of a prearranged broken line (prearranged line where a median crack is formed) is continuously irradiated with a beam spot which is not interrupted in the direction of its progress, and therefore, compress-stressed region of a high temperature remains inside a glass plate.
The present invention is provided in view of the described problem, and an object of the invention is to provide a method for forming a median crack and an apparatus for forming a median crack where the formation of a deep, straight median crack is possible, and an excellently broken surface of a brittle substrate can be gained as a result of breaking. Here, the term “substrate” in this invention means a substrate (brittle substrate) formed of a single or a number of substrates made of a brittle material.