Laser processing apparatus

A laser processing apparatus comprising:

• a chuck table having a holding surface for holding a platelike workpiece;
• processing laser beam applying means for applying a processing laser beam to the upper surface of said workpiece held on said chuck table, said processing laser beam applying means including processing laser beam oscillating means for oscillating said processing laser beam and focusing means for focusing said processing laser beam oscillated by said processing laser beam oscillating means;
• feeding means for relatively moving said chuck table and said processing laser beam applying means in a feeding direction;
• focal position adjusting means for changing the focal position of said processing laser beam to be focused by said focusing means;
• height detecting means for detecting the height of said workpiece held on said chuck table;
• and
• control means for controlling said focal position adjusting means according to a detection signal from said height detecting means;
• said height detecting means including
• detecting laser beam oscillating means for oscillating a detecting laser beam having a wavelength different from the wavelength of said processing laser beam;
• annular spot forming means for forming the spot shape of said detecting laser beam oscillated by said detecting laser beam oscillating means into an annular spot shape;
• a detecting laser beam applying path for leading said detecting laser beam having said annular spot shape from said annular spot forming means to said focusing means;
• a detecting laser beam reflecting path for leading reflected light obtained by reflecting said detecting laser beam applied through said detecting laser beam applying path and said focusing means to said workpiece held on said chuck table;
• a pinhole mask provided in said detecting laser beam reflecting path for passing the reflected light reflected on the upper surface of said workpiece, but blocking the reflected light reflected on the lower surface of said workpiece;
• and
• reflected light analyzing means for analyzing the reflected light passed through said pinhole mask and transmitting the result of analyzation to said control means;
• said focusing means including
• an objective lens for focusing said processing laser beam and said detecting laser beam;
• and
• a window lens for focusing said detecting laser beam having said annular spot shape without focusing said processing laser beam.