Method and apparatus for determining focus position of a laser

Details Method and apparatus for determining focus position of a laser Method and apparatus for determining focus position of a laser

1999-08-11

2001-10-16

Elve, M. Alexandra (Department: 1725)

Electric heating

Metal heating

By arc

C219S121820, C219S121600, C219S121750, C219S121730, C219S121740

Reexamination Certificate

active

06303903

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to positioning a surface of a workpiece for laser machining and, in particular, to using a an emission from a plasma formed by a laser to adjust the position of the surface of the workpiece.
BACKGROUND OF THE INVENTION
Lasers are scanned for a variety of purposes including drilling holes in or otherwise machining a workpiece. For example, a laser may be used to machine glass or silicon wafers to form waveguides or microoptical structures. Laser machining is often performed by focusing a laser beam to a focal point. The focal point is a point of heightened power density which may advantageously be used for machining a workpiece. In order to use the focal point for precise machining applications, it may be necessary to precisely position the workpiece relative to the location of the focal point.
Ultrafast lasers with pulse duration in the picosecond range and femtosecond range may be used for machining a workpiece. Ultrafast lasers may be used for precise ablation with little or no heat affected zone. They are especially suited for machining microscopic features in delicate materials such as thin films and brittle ceramics, which are otherwise difficult to machine. Because of their small effective work area, however, the focal point of ultrafast lasers are desirably accurately positioned relative to the workpiece.
The exact location of the focal point of a focused laser may be difficult to ascertain, however, due to deviations in the optical path. These deviations may be caused by abnormalities in mirrors used to direct the laser beam and in the focusing lens and by variations in the operating characteristics of the laser source. Thus, it is desirable to accurately adjust the position of the focal point with respect to the workpiece independent of variations in the laser source and the optical path.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for positioning a surface of a workpiece for laser machining. A laser beam is focused to a focal point to form a plasma generating a plasma emission. The position of the focal point is determined and the surface of the workpiece is then moved to the focal point.
According to an aspect of the present invention, the laser beam is diverted away from the focal point before the surface is moved to the focal point.
According to another aspect of the present invention, an imaging system is oriented to receive the plasma emission and generate a plasma image, the imaging system is adjusted to focus the plasma image, the workpiece is positioned for an image of the surface of the workpiece to be directed to the imaging system to generate a workpiece surface image, and the workpiece is actuated to focus the image of the surface of the workpiece.
According to another aspect of the present invention, the laser beam is an ultrashort pulse laser beam.
According to another aspect of the present invention, the plasma emission is filtered to narrow the spectral bandwidth of the plasma image.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.


REFERENCES:
patent: 5744780 (1998-04-01), Chang et al.
patent: 6057525 (2000-05-01), Chang et al.

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