Acousto-optic deflector applications in laser processing of dielectric or other materials

• US 8,404,998 B2
• Filed: 05/28/2010
• Issued: 03/26/2013
• Est. Priority Date: 05/28/2009
• Status: Active Grant

• Current Assignee: Electro Scientific Industries Incorporated (MKS Instruments Incorporated)
• Original Assignee: Electro Scientific Industries Incorporated (MKS Instruments Incorporated)
• Inventors: Unrath, Mark A., Jordens, William J., Ismail, James, Matsumoto, Hisashi, Lineburg, Brian J.
• Primary Examiner(s): Coleman, William D
• Application Number:
  US12/790,082
• Publication Number:
  US 20100301023A1
• Time in Patent Office: 1,033 Days
• Field of Search: 219/121.6, 219/121.67
• US Class Current: 219/121.67
• CPC Class Codes: B23K 2101/40 Semiconductor devices
• CPC Class Codes: B23K 2103/50 Inorganic material, e.g. metals, not provided for in B23K2103/02 – B23K2103/26
• CPC Class Codes: B23K 26/032 using optical means
• CPC Class Codes: B23K 26/046 Automatically focusing the laser beam
• CPC Class Codes: B23K 26/0648 comprising lenses
• CPC Class Codes: B23K 26/0665 by beam condensation on the workpiece, e.g. for focusing
• CPC Class Codes: B23K 26/08 Devices involving relative movement between laser beam and workpiece
• CPC Class Codes: B23K 26/082 Scanning systems, i.e. devices involving movement of the laser beam relative to the laser head
• CPC Class Codes: B23K 26/40 taking account of the properties of the material involved

Abstract

A laser processing system for micromachining a workpiece includes a laser source to generate laser pulses for processing a feature in a workpiece, a galvanometer-driven (galvo) subsystem to impart a first relative movement of a laser beam spot position along a processing trajectory with respect to the surface of the workpiece, and an acousto-optic deflector (AOD) subsystem to effectively widen a laser beam spot along a direction perpendicular to the processing trajectory. The AOD subsystem may include a combination of AODs and electro-optic deflectors. The AOD subsystem may vary an intensity profile of laser pulses as a function of deflection position along a dither direction to selectively shape the feature in the dither direction. The shaping may be used to intersect features on the workpiece. The AOD subsystem may also provide rastering, galvo error position correction, power modulation, and/or through-the-lens viewing of and alignment to the workpiece.

Claims

1. A method for dithering a laser beam to process features in a workpiece, the method comprising:

imparting, using a first positioning system, first relative movement of a laser beam spot position along a processing trajectory;

imparting, using a second positioning system, second relative movement of the laser beam spot position along a dither direction relative to the processing trajectory, wherein the second relative movement is superimposed on the first relative movement, and wherein the second relative movement is at a substantially higher velocity than that of the first relative movement;

amplitude modulating, using the second positioning system, a laser beam based on a specific feature characteristic at one or more corresponding target locations on the workpiece, wherein the specific feature characteristic comprises a selected shape of a cross-section of a portion of a trench cut in a direction of the processing trajectory along a surface of the workpiece by the laser beam; and

emitting the laser beam to the workpiece during the second relative movement to effectively widen a laser beam spot at the workpiece in the dither direction.

2. The method of claim 1, further comprising:

controlling one or more acousto-optic deflectors to impart the second relative movement of the laser beam spot position along the dither direction and to amplitude modulate the laser beam.

3. The method of claim 1, further comprising:

controlling one or more electro-optic deflectors to impart the second relative movement of the laser beam spot position along the dither direction; and

controlling an acousto-optic modulator to amplitude modulate the laser beam.

4. The method of claim 1, further comprising:

controlling the second positioning system to overlap, at least partially, two or more laser pulses of the laser beam to remove selected amounts of target material from the workpiece based on a selected shape of a feature along the dither direction.

5. The method of claim 1, wherein the amplitude modulation varies a fluence profile provided by the laser beam to the workpiece to selectively shape the cross-section of the portion of the trench in the dither direction.

6. The method of claim 1, wherein imparting, using the second positioning system, the second relative movement of the laser beam spot position further comprises:

moving the laser beam spot position back and forth between parallel portions of a first feature and a second feature such that emitting the laser beam to the workpiece further comprises processing the parallel portions in a single pass along the processing trajectory.

7. The method of claim 1, wherein the specific feature characteristic comprises a selective shape of a first feature in a direction of the processing trajectory, the method further comprising:

varying, using the second positioning system, a fluence profile to create a sloped portion of the first feature in the direction of the processing trajectory, wherein an amount of slope for the sloped portion of the first feature in the direction of the processing trajectory is based on a desired depth within an intersection area corresponding to an intersection of the first feature with a second feature; and

processing the second feature in the workpiece with a respective series of laser pulses, the second feature intersecting the sloped portion of the first feature at the intersection area.

8. The method of claim 1, further comprising:

generating a raster pattern of spot amplitudes defining spot locations and power amplitudes of laser pulses for combining fluences of a first feature with fluences of an intersecting portion of a second feature; and

applying a plurality of laser pulses to the workpiece according to the raster pattern of spot amplitudes.

9. The method of claim 1, further comprising:

sensing a positioning error in the first positioning system;

filtering the positioning error to match a phase and gain response over a desired frequency range and to remove noise; and

applying the filtered positioning system to the second positioning system to correct for the positioning error.

10. The method of claim 1, wherein the second positioning system comprises an acousto-optic device (AOD), the method further comprising:

selecting a maximum power of the laser beam based on a diffraction efficiency of the AOD based on a desired deflection range in the dither direction provided by the second positioning system.

11. The method of claim 1, further comprising:

while maintaining a constant velocity of the first positioning system, using the second positioning system to change a velocity of the laser beam spot position relative to the workpiece as a selected feature being processed changes widths to thereby provide different dosages to different portions of the selected feature having different widths.

12. The method of claim 1, further comprising:

shaping an intensity profile of the laser beam.

13. The method of claim 1, further comprising:

slanting the laser beam at an angle with respect to a surface of the workpiece.

14. The method of claim 5, wherein selectively shaping the cross-section of the portion of the trench in the dither direction comprises sloping a sidewall of the trench, the method further comprising:

selecting an amount of slope for the sidewall of the trench based on a desired depth at an intersection area corresponding to an intersection of the trench with an intersecting trench; and

processing the intersecting trench in the workpiece with the laser beam, the intersecting trench intersecting the sloped sidewall of the trench at the intersection area.

15. The method of claim 14, further comprising:

controlling the second positioning system to slope a portion of the intersecting trench corresponding to the intersection area based on the desired depth of the intersection area produced when the sloped sidewall of the trench is combined with the sloping portion of the intersecting trench.

16. The method of claim 6, further comprising:

adjusting effective beam processing velocities between the parallel portions to account for turns along the processing trajectory that result in different path lengths corresponding to the respective parallel portions; and

modulating the power of the laser beam, using the second positioning system, based on the adjusted effective beam processing velocities to maintain desired processing dosages for each of the parallel portions.

17. The method of claim 6, wherein the first feature and the second feature further include diverging portions that are not parallel to each other, and wherein a transition region defines a change from the parallel portions to the diverging portions, the method further comprising:

in the transition region, selectively shaping the parallel portion of the second feature along the processing direction;

while continuing to process the first feature along the processing trajectory, terminating processing of the second feature in the transition region; and

after processing the diverging portion of the first feature, processing the diverging portion of the second feature along a diverging processing trajectory so as to intersect the selectively shaped parallel portion of the second feature in the transition region, wherein the diverging portion of the second feature is selectively shaped in the transition region in the direction of the diverging processing trajectory such that the intersection of the parallel portion and the diverging portion of the second feature corresponds to a desired depth of the second feature in the processing region.

18. The method of claim 7, wherein sloping the portion of the first feature in the direction of the processing trajectory comprises:

sloping the first feature from a first depth to a second depth as the first feature enters an intersection area corresponding to an intersection of the first feature with a second feature;

sloping the first feature from the second depth back to the first depth as the first feature exits the intersection area, wherein the second depth is about half the first depth; and

processing the second feature with a respective series of laser pulses such that the second feature is shaped so as to slope from the first depth to the second depth upon entering the intersection and slopes from the second depth to the first depth upon exiting the intersection area.

19. The method of claim 8, wherein generating the raster pattern of spot amplitudes comprises:

setting up a candidate raster grid corresponding to an intersection area;

for each spot in the raster grid, calculating a fluence profile over a raster field to generate a respective influence function;

combining the influence functions into an influence function matrix;

calculating a pseudo-inverse of the influence function matrix; and

calculating the spot amplitudes at the respective grid points using the pseudo-inverse of the influence function matrix.

20. A laser processing system for micromachining a workpiece, the system comprising:

a laser source to generate a laser beam for processing a feature in a surface of the workpiece;

a first positioner to impart a first relative movement of a laser beam spot position along a processing trajectory with respect to the surface of the workpiece; and

a second positioner comprising an acousto-optic deflector (AOD) subsystem to effectively widen a laser beam spot along a direction perpendicular to the processing trajectory, wherein the AOD subsystem is configured to vary an intensity profile of the laser beam as a function of deflection position along the direction perpendicular to the processing trajectory to thereby selectively shape a portion of the feature in the direction perpendicular to the processing trajectory.

21. The system of claim 20, wherein the first positioner is selected from the group comprising a galvanometer-driven (galvo) subsystem and a fast-steering mirror (FSM) subsystem.

22. The system of claim 20, wherein the AOD subsystem comprises:

a first AOD driven by a first radio frequency (RF) signal; and

a second AOD driven by a second RF signal,

wherein the first AOD and the second AOD use chirping to effectively widen the laser beam spot by applying a frequency ramp to the first RF signal and the second RF signal to defocus the laser beam.

23. The system of claim 20, wherein the AOD subsystem uses dithering to effectively widen the laser beam spot by imparting a second relative movement of the laser beam spot position along the perpendicular direction, wherein the second relative movement is superimposed on the first relative movement, and wherein the second relative movement is at a substantially higher velocity than that of the first relative movement.

24. The system of claim 20, wherein, to selectively shape the feature in the direction perpendicular to the processing trajectory, the AOD subsystem adjusts the average fluence of the laser beam based on the laser beam spot position along the direction perpendicular to the processing trajectory.

25. The system of claim 20, wherein the AOD subsystem comprises:

an electro-optic deflector to deflect the laser beam in at least one of the processing trajectory and the direction perpendicular to the processing trajectory; and

an acousto-optic modulator to modulate the power amplitude of the laser beam.

26. The system of claim 20, further comprising:

a sensor to generate a positioning error signal of the galvo subsystem; and

a filter to filter the positioning error signal, the filter substantially matching both phase and gain of a transfer function between galvo error to beam position over a selected frequency range, while filtering high frequency sensor noise,

wherein the AOD subsystem deflects the laser beam based on the filtered error signal.

27. The system of claim 20, further comprising:

a low pass filter to receive a commanded beam position signal, the low pass filter removing portions of the commanded beam position signal that are beyond a bandwidth of the galvo subsystem, the low pass filter outputting a galvo command for controlling deflection provided by the galvo subsystem;

a delay element to delay the commanded beam position signal based on an amount of delay introduced to the galvo command signal by the low pass filter; and

an element for subtracting the galvo command signal output by the low pass filter from the output of the delay element to generate an AOD command signal for controlling deflection provided by the AOD subsystem.

28. The system of claim 20, further comprising:

a shaping element for shaping an intensity profile of the laser beam; and

a relay lens positioned to receive the laser beam from the AOD subsystem and to center the laser beam on an aperture of the shaping element independent of a deflection angle provided by the AOD subsystem to the laser beam.

29. The system of claim 20, further comprising:

a scan lens for focusing the laser beam at the laser beam spot position; and

a relay lens positioned to receive the laser beam from the AOD subsystem at a selected location with respect to the scan lens so as to slant the laser beam with respect to a surface of the workpiece.

30. The system of claim 24, wherein the AOD subsystem further adjusts the average fluence the power amplitudes of the laser beam based on deflection positions along the processing trajectory to selectively shape the feature along the processing trajectory.

31. The system of claim 28, wherein the shaping element comprises a diffractive optical element.

32. A system for dithering a laser beam to process features in a workpiece with a series of laser pulses, the system comprising:

means for imparting first relative movement of a laser beam spot position along a processing trajectory;

means for imparting second relative movement of the laser beam spot position along a dither direction relative to the processing trajectory, wherein the second relative movement is superimposed on the first relative movement, and wherein the second relative movement is at a substantially higher velocity than that of the first relative movement;

means for amplitude modulating a laser beam based on a specific feature characteristic at one or more corresponding target locations on the workpiece, wherein the specific feature characteristic comprises a selected shape of a cross-section of a portion of a trench cut in a direction of the processing trajectory along a surface of the workpiece by the laser beam; and

means for emitting the laser beam during the second relative movement to effectively widen a laser beam spot at the workpiece in the dither direction.