Hole forming method and laser processing apparatus

A hole forming method of forming a laser processed hole in a workpiece configured by bonding a first member formed of a first material and a second member formed of a second material, said laser processed hole extending from said first member to said second member, said hole forming method comprising:

• a minimum shot number setting step of applying a pulsed laser beam to an arbitrary area of said workpiece from the side of said first member to form said laser processed hole, counting the number of shots of said pulsed laser beam, measuring a material-inherent spectral wavelength of plasma generated by the application of said pulsed laser beam, and setting as a minimum value the number of shots of said pulsed laser beam at the time the spectral wavelength of said plasma has changed from the spectral wavelength inherent in said first material to the spectral wavelength inherent in said second material;
• a maximum shot number setting step of setting as a maximum value the number of shots of said pulsed laser beam at the time the spectral wavelength of said plasma has completely changed from the spectral wavelength inherent in said first material to the spectral wavelength inherent in said second material;
• and
• a hole forming step of applying said pulsed laser beam to a predetermined position on said workpiece from the side of said first member to form said laser processed hole, counting the number of shots of said pulsed laser beam, measuring the material-inherent spectral wavelength of said plasma generated by the application of said pulsed laser beam, and stopping the application of said pulsed laser beam in the case that the number of shots of said pulsed laser beam has reached said minimum value and the spectral wavelength of said plasma has changed from the spectral wavelength inherent in said first material to the spectral wavelength inherent in said second material, whereas continuing the application of said pulsed laser beam until the number of shots of said pulsed laser beam reaches said maximum value in the case that the spectral wavelength of said plasma has not changed from the spectral wavelength inherent in said first material to the spectral wavelength inherent in said second material even after the number of shots of said pulsed laser beam has reached said minimum value.