1. Field of the Invention
The present invention relates to an optical device wafer processing method and a laser processing apparatus for dividing an optical device wafer into individual optical devices along a plurality of crossing streets formed on the front side of the optical device wafer, the optical device wafer being composed of a substrate and an optical device layer formed on the front side of the substrate, the individual optical devices being respectively formed in a plurality of regions partitioned by the streets.
2. Description of the Related Art
In an optical device fabrication process, an optical device layer of a gallium nitride compound semiconductor is formed on the front side of a substantially disk-shaped substrate such as a sapphire substrate and a silicon carbide substrate, and this optical device layer is partitioned by a plurality of crossing streets into a plurality of regions where optical devices such as light emitting diodes (LEDs) and laser diodes (LDs) are respectively formed, thus constituting an optical device wafer. The optical device wafer is cut along the streets to thereby divide the regions where the optical devices are formed from each other, thus obtaining the individual optical devices.
Cutting of the optical device wafer along the streets is usually performed by using a cutting apparatus called a dicing saw. This cutting apparatus includes a chuck table for holding a workpiece, cutting means for cutting the workpiece held on the chuck table, and feeding means for relatively moving the chuck table and the cutting means. The cutting means includes a rotating spindle, a cutting blade mounted on the rotating spindle, and a driving mechanism for rotationally driving the rotating spindle. The cutting blade is composed of a disk-shaped base and an annular cutting edge mounted on a side surface of the base along the outer circumference thereof. The cutting edge is formed by fixing diamond abrasive grains having a grain size of about 3 μm to the base by electroforming so that the thickness of the cutting blade becomes about 20 μm, for example.
However, the substrate of the optical device wafer, such as a sapphire substrate and a silicon carbide substrate, has high Mohs hardness, so that cutting of the substrate by the cutting blade is not always easy. Accordingly, the depth of cut by the cutting blade cannot be set large, so that a cutting step must be performed plural times to cut the optical device wafer, causing a reduction in productivity.
As a method of dividing an optical device wafer along the streets, a laser processing method using a pulsed laser beam for performing ablation to the wafer has been proposed to solve the above problem. In this laser processing method, the pulsed laser beam is applied to the wafer along the streets to thereby form a laser processed groove on the wafer along each street as a break start point. Thereafter, an external force is applied to the wafer along each street where the laser processed groove is formed as the break start point, thereby breaking the wafer along each street (see Japanese Patent Laid-open No. Hei 10-305420, for example).