Patent ID: 7714249

Claim:
A laser beam processing machine comprising a chuck table having a workpiece holding surface for holding a plate-like workpiece and a laser beam application means for applying a laser beam to the top surface of the workpiece held on the chuck table, the laser beam application means comprising a processing laser beam oscillation means for oscillating a processing laser beam and a condenser for converging the processing laser beam oscillated by the processing laser beam oscillation means, wherein the laser beam processing machine further comprises a focal point position adjusting means which is interposed between the processing laser beam oscillation means and the condenser and shifts the position of the focal point of the processing laser beam converged by the condenser, a height position detection means for detecting the height position of the top surface of the workpiece held on the chuck table, and a control means for controlling the focal point position adjusting means based on a detection signal from the height position detection means; the height position detection means comprises a detection laser beam oscillation means for oscillating a detection laser beam having a wavelength different from the wavelength of the processing laser beam oscillated by the processing laser beam oscillation means, a dichroic half mirror which is interposed between the processing laser beam oscillation means and the focal point position adjusting means, allows the processing laser beam oscillated from the processing laser beam oscillation means to pass through and deflects a detection laser beam oscillated from the detection laser beam oscillation means toward the focal point position adjusting means, a first splitter which is interposed between the dichroic half mirror and the detection laser beam oscillation means, allows the detection laser beam oscillated from the detection laser beam oscillation means and deflects reflected light deflected by the dichroic half mirror, a band pass filter which allows only reflected light corresponding to the wavelength of the detection laser beam to pass through out of the reflected light deflected by the first splitter, a second splitter which splits the reflected light passing through the band pass filter into a first route and a second route, a first light receiving device for receiving the reflected light split into the first route by the second splitter, a second light receiving device for receiving the reflected light split into the second route by the second splitter, and a light receiving area restricting means which is arranged in the second route and restricts the reception area of the reflected light received by the second light receiving device; and the control means calculates the ratio of the quantity of light received by the first light receiving device to the quantity of light received by the second light receiving device and controls the focal point position adjusting means so that the ratio becomes a predetermined value.