Electric heating – Metal heating – By arc
Reexamination Certificate
1999-05-10
2001-09-04
Dunn, Tom (Department: 1725)
Electric heating
Metal heating
By arc
C219S121680
Reexamination Certificate
active
06285002
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for three-dimensional micro machining using a modulated ultra short laser pulse having different beam profiles.
BACKGROUND AND PRIOR ART
Laser beam machining is a machining method widely used for cutting, welding, etc. Although it is a convincing tool for micro machining, the continuous beam laser machining leads to a heat affected zone, and hence it is not suitable for many applications such as in the biomedical field. The continuous laser beam adversely affects the peripheral area adjacent to the focused area of the laser beam and hence cannot produce features with very high precision and control. Ultra short laser pulses find an attractive advantage over the continuous laser beam due to the reduced effect of the heat affected zone adjacent to the focused point of the laser beam. A smaller pulse rate has less influence on the periphery of the focused beam. U.S. Pat. No. 5,656,186 by Mourou et al. teaches that reducing the pulsing cycle to the range of femto seconds results in a spot size of the machined area which is smaller than the spot size of the laser beam, since the maximum intensity is at the very center of the beam waist.
An ultra short laser pulse finds wide application in transparent material such as in optical storage devices, since the damage to the adjoining area is minimized. Methods to apply ultra short laser pulses for data storage can be carried out by directly focusing the ultra short laser pulse inside the transparent material as disclosed in U.S. Pat. No. 5,761,111 by Glezer, or by splitting the laser beam and directing the separate laser beams on to a target point as in U.S. Pat. No. 5,786,560 by Tatah et al.
Although an ultra short laser pulse is an efficient tool for micro machining precise movement of the laser beam and the resolution of the beam scanning are important factors which limit the finishing and feature size. Since the beam is focused to a very small spot size and the machined surface reflects the shape of the beam profile, filtering and processing the beam are important factors that need careful consideration. Also the ultra short laser pulse needs to be modulated so that the number of pulses striking each of the target points can be controlled and a very small material removal can be achieved. Also a better finish of the machined feature can be obtained. The shape of the beam can be modified to meet a specific application such as for machining grating lines which employs an elliptical shaped beam instead of a circular shaped beam.
The present invention takes into consideration all the above mentioned factors, such as the resolution of beam scanning, modulation of the ultra short laser pulse, filtering of the laser beam and modification of the beam shape.
SUMMARY OF THE INVENTION
The first preferred embodiment of the present invention employs an ultra short laser which generates a laser pulse in the range of a femtosecond as in U.S. Pat. No. 5,656,186 by Mourou et al. for three dimensional machining applications. The disclosed invention includes a beam spatial filter to filter the ultra short laser pulse profile, which in turn improves the quality of the machining surface. The filtered beam is made to pass through a diaphragm or a slot to further enhance the beam quality.
The disclosed invention includes a laser beam modulation mechanism by using a combination of acousto optic modulators, depending on the requirements. With this ultra short laser pulse modulation, the ultra short laser pulse is prevented from striking the target during the non-machining time and vice versa.
The system of the present invention is not restricted to the modulation of a laser beam striking a surface during a machining period and non machining period, but it also controls the number of ultra short laser pulses from the source striking the target surface. By controlling the number of pulses from the ultra short laser pulse source striking the target, it is possible to machine a very small part of the material. Thus, it is possible to make features smaller than the spot size of the laser beam. That is, it is possible to ablate a hole in the target which is smaller than the spot size of the laser beam. In order to have an efficient use of laser beam energy, the system works with the operation of beam modulation acousto optic modulators for fine machining and the beam modulating acousto optic modulators switched off for rough machining.
The disclosed invention comprises a non-mechanical scanning means which employs acousto optic deflectors for scanning. Two acousto optic deflectors are used to scan the ultra short laser pulse in the X-axis and the Y-axis. The scanning beam passes through a scan lens for focusing the beam on to the target surface. The present invention includes a technique of scanning the beam along the concave or convex profile by moving the scan lens laterally. The work piece or target is positioned on a X-axis and Y-axis stage for coarse or rough movement. A Z-axis piezo electric stage is used for precision Z-axis movement, and it also is connected to an auto focusing mechanism. A camera is installed in the optical layout to monitor the machining operation of the work surface. The work piezo table is enclosed in a chamber with inlet and out for reactive gas in order to do enhanced machining operations with assisted gas. The present invention can also preferably do the process of etching or deposition reaction. The system can work also without the gas chamber.
The disclosed second embodiment of the present invention employs an elliptical beam for machining rather than circular beam as in the current known laser machining systems. The elliptical beam is obtained by using a combination of two cylindrical lens of specified appropriate focal length as required for the application. Some of the potential applications of the disclosed invention include three dimensional micro machining for in use elliptical and a spherical micro lens fabrication, optical data storage, fine grating lines, etc. The grating lines can preferably be drawn using the second embodiment of the present invention, and the length and width of the grating line can be determined by the selecting the appropriate focal length of the cylindrical lens.
The disclosed third embodiment of the present invention involves rotation of the work piece held in a chuck by a precision spindle and scanning the pulsed and modulated laser beam in a plane parallel to the axis of rotation of the workpiece. The system applies either circular or elliptical beam for the machining operation depending upon the requirement.
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Ngoi Bryan Kok Ann
Venkatakrishnan Krishnan
Burns Doane Swecker & Mathis L.L.P.
Dunn Tom
Stoner Kiley
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