Laser processing method and circuit board manufacturing method

A circuit board manufacturing method comprising:

• radiating ultraviolet pulse laser light onto a circuit board having a stacked structure in which a first conductor layer, a first insulating layer, and a first sacrificial layer are stacked on a principal surface of the circuit board in the presented order, the pulse laser light radiated from a side facing the first sacrificial layer, to start processing of a hole extending from a surface of the first sacrificial layer toward the first conductor layer using laser ablation, and changing a laser ablation processing mode in the first sacrificial layer with progress of the laser ablation to form a first through hole in the first sacrificial layer;
• radiating the pulse laser light onto the first insulating layer through the first through hole to form a first via hole in the first insulating layer;
• and
• removing the first sacrificial layer after the formation of the first via hole,
• wherein a laser processing apparatus including a mask having a mask pattern corresponding to the first via hole and a projection optical system configured to project an image of the mask onto the circuit board, the mask and the projection optical system disposed on an optical path of the pulse laser light, is used to form the first via hole by transferring the image of the mask via the projection optical system to radiate the pulse laser light onto the circuit board,
• a position where the pulse laser light radiated via the projection optical system forms the image is between a surface of the first sacrificial layer and an interface between the first sacrificial layer and the first insulating layer, and
• the mask has a light transmitting diameter across which the mask pattern transmits light corrected based on a target hole diameter to be formed in the first insulating layer, an amount of shift of a processed hole diameter according to a thickness of the first sacrificial layer, and a magnification of the projection optical system.