U.S. patent application number 12/712513 was filed with the patent office on 2010-09-02 for system and method for eliminating the structure and edge roughness produced during laser ablation of a material.
This patent application is currently assigned to INTERNATIONAL BUSINESS MACHINES CORPORATION. Invention is credited to Alfred Wagner.
Application Number | 20100219168 12/712513 |
Document ID | / |
Family ID | 39641586 |
Filed Date | 2010-09-02 |
United States Patent
Application |
20100219168 |
Kind Code |
A1 |
Wagner; Alfred |
September 2, 2010 |
SYSTEM AND METHOD FOR ELIMINATING THE STRUCTURE AND EDGE ROUGHNESS
PRODUCED DURING LASER ABLATION OF A MATERIAL
Abstract
The present technology relates generally to laser ablation, and
more particularly pertains to a system and method for eliminating
structure and edge roughness, which is produced during the laser
ablation of a material. Ablation of materials using a femtosecond
laser beam produces a fine scale periodic structure in the ablated
region. The structure consists of residual (i.e. unablated
material) and is always perpendicular to the polarization direction
of the laser beam. By changing the polarization direction during
the ablation process, the structure is averaged over many
directions and thus eliminated. This eliminates structure and edge
roughness in a material caused by the laser ablation of the
material. The method is employed to the repairing of photomasks so
as to cause the optical quality thereof to be improved.
Inventors: |
Wagner; Alfred; (Brewster,
NY) |
Correspondence
Address: |
SCULLY, SCOTT, MURPHY & PRESSER, P.C.
400 GARDEN CITY PLAZA, SUITE 300
GARDEN CITY
NY
11530
US
|
Assignee: |
INTERNATIONAL BUSINESS MACHINES
CORPORATION
Armonk
NY
|
Family ID: |
39641586 |
Appl. No.: |
12/712513 |
Filed: |
February 25, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11624257 |
Jan 18, 2007 |
7732104 |
|
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12712513 |
|
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Current U.S.
Class: |
219/121.67 |
Current CPC
Class: |
B23K 26/0648 20130101;
G02B 27/286 20130101; B23K 2103/50 20180801; G03F 1/72 20130101;
B23K 26/06 20130101; B23K 26/064 20151001; B23K 26/0624 20151001;
B23K 26/0665 20130101 |
Class at
Publication: |
219/121.67 |
International
Class: |
B23K 26/38 20060101
B23K026/38 |
Claims
1. A system of reducing periodic structures formed in a photomask
material during a laser ablation process, the method system: a
laser for producing incident linearly polarized laser light; means
for converting the incident linearly polarized laser light to
circularly or eliptically polarized laser light; and means for
averaging a plurality of polarization directions during the laser
ablation process.
2. The system as claimed in claim 1, wherein the means for
polarizing and averaging comprise a quarter wave plate inserted
into an optical path of the linearly polarized laser light to
produce said circularly or eliptically polarized light.
3. The system as claimed in claim 1, wherein the means for
polarizing comprises a half wave plate inserted into an optical
path of the linearly polarized laser light to alter the direction
of polarization.
4. The system as claimed in claim 1, wherein the means for
averaging comprises means for rotating the half wave plate during
the laser ablation process.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. Ser.
No. 11/624,257, filed Jan. 18, 2007.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to laser ablation,
and more particularly pertains to a system and method for
eliminating structure and edge roughness, which is produced during
the laser ablation of a material.
[0004] Typically, in this particular technology, an ultrashort
pulsed laser beam is utilized to ablate undesired extra material,
which is present in a photomask. The pulsed laser beam is fired in
a programmed spatial pattern, thereby removing the encountered
extra material which causes a defect. However, it is possible that
the process of laser ablation can produce a periodic structure in
the irradiated material, whereby this periodic structure produces a
significant roughness at the edge of the ablated defect, which
degrades the optical quality of the repaired photomask.
Consequently, there is a need to provide a system and method which
will prevent or eliminate this edge roughness, and thereby to
resultingly produce a repaired photomask with improved optical
quality.
[0005] 2. Discussion of the Prior Art
[0006] In the current state-of-the-technology, a number of
publications are known which disclose and teach the application of
equipment and methods, which are required in order to remove
defects encountered in lithographic masks. To that effect, an
ulstrashort pulsed laser beam may be utilized to ablate undesired
extra material in a programmed spatial pattern, thereby removing
the encountered defects. The foregoing aspects are disclosed in
Grenon, et al., U.S. Pat. Nos. 6,190,836; 6,165,649; 6,156,461;
6,090,507; and Haight, et al., U.S. Pat. No. 6,333,485.
[0007] Furthermore, as known, laser ablation can produce a periodic
structure in the irradiated material, thereby resulting in a
significant degree of roughness at the edge of the ablated defect,
which degrades the optical quality of the repaired photomask. This
aspect is discussed in various publications, such as, for instance,
the following articles: "Laser Induced Periodic Surface Structure:
Experiments on Ge, Si, Al, and Brass", Young, Preston, vsn Driel,
and Sipe, Physical Review B, Vol. 27, No. 2, pgs. 1155-1172 (1983);
"Ultraviolet Laser Induced Periodic Surface Structures", Clark and
Emmony, Physical Review B, Vol. 40, No. 4, pgs 2031-2041 (1989);
"Femtosecond Laser Induced Periodic Surface Structure on Diamond
Film", Wu, Ma, Fang, Liao, Yu, Chen, Wang, Applied Physics Letters,
Vol. 82, No. 11, pgs 1703-1705 (2003); and "Self Organixed
Nanogratings in Glass Irradiated by Ultrashort Light Pulses",
Shimotsuma, Kazansky, Qui, Hirao, Physical Review Letters, Vol. 19,
No. 24, pgs 247205-1 to 4 (2003).
SUMMARY OF THE INVENTION
[0008] Ablation of materials using a femtosecond laser beam
produces a fine scale periodic structure in the ablated region. The
structure consists of residual (i.e. unablated material) and is
always perpendicular to the polarization direction of the laser
beam. By changing the polarization direction during the ablation
process, the structure is averaged over many directions and thus
eliminated.
[0009] Accordingly, it is an object of the present invention to
provide a method of eliminating structure and edge roughness in a
material caused by the laser ablation of the material.
[0010] Another object of the invention resides in imparting the
method as described in an application to the repairing of
photomasks so as to cause the optical quality thereof to be
improved.
[0011] Yet another object is to provide a system of eliminating
structure and edge roughness imparted to a material, such as a
photomask, during laser ablation of the material.
[0012] The foregoing and other objects, aspects, features, and
advantages of the invention will become more readily apparent from
the following description and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] In the drawings, like reference characters generally refer
to the same parts throughout the different views. Also, the
drawings are not necessarily to scale, emphasis instead generally
being placed upon illustrating the principles of the invention;
wherein:
[0014] FIG. 1 is an illustrative picture of a repaired photomask in
which a defect was removed by femtosecond laser ablation;
[0015] FIG. 2 shows illustrative pictures of periodic structures in
ablated lines as a function of the direction of polarization of the
femtosecond laser beam;
[0016] FIG. 3 is an illustrative block diagram of a system for
rotating the polarization of a laser beam to average the direction
of the ablation structure, and thereby eliminate it, according to
one embodiment of the invention; and
[0017] FIG. 4 is an illustrative picture of a line of ablated
material in which the edge roughness (ablation structure) has been
eliminated, according to one embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Defects are commonly encountered during the fabrication of a
photomask, whereby these defects generally consist of extra
(unwanted) material that must be removed in order to create a
perfect photomask. Femtosecond lasers can be used to ablate this
extra material, thus removing the defect. Hereby, the placement and
spatial sharpness of the edge of the repaired region is critical to
producing a high quality photomask. Anything that detracts from the
placement and spatial sharpness of the repaired edge must be
avoided.
[0019] Referring to FIG. 1, there is represented a picture of a
repaired photomask 10 in which a defect was removed by femtosecond
laser ablation of a repair site 12. Laser ablation typically
produces a highly periodic structure 14, which degrades the optical
quality of the repaired edge 16. This periodic structure 14 can be
traced to the polarization of the laser beam, which is used to
ablate the material.
[0020] Referring to FIG. 2, there are illustrated images of
periodic structures 20, 22, 24, 26, 28 and 30 formed in an ablated
line as a function of the direction of polarization 32, 34 and 36
of the femtosecond laser beam. These periodic structures 20, 22,
24, 26, 28 and 30 are always oriented perpendicular to the
polarization directions 32, 34 and 36 of the laser beam. If
multiple polarization directions are used during the ablation
sequence, the resulting structure consists of an average over these
directions of the periodic structure formed by any individual
polarization direction. Thus, by performing the ablation using a
series of polarization directions, the periodic structure is
minimized or eliminated by means of averaging.
[0021] Referring to FIG. 3, there is shown a block diagram of a
system for rotating the polarization of a laser beam to average the
direction of the ablation structure, and thereby eliminated. The
system includes a 1/4 waveplate 40, a focusing lens 42, and a
photomask 44. Linearly polarized light 46, which is pulsed from a
femtolaser 48 passes through the 1/4 waveplate 40 and is turned
into circularly (or eliptically) polarized light 50. The circularly
polarized light 50 passes through the focusing lens 42 and is
incident on the photomask 44. In this case, the polarization
direction of the incident laser light would continuously change
direction during each laser pulse. By way of example, for 100
femtosecond, 266 nm laser pulses, which are employed for mask
repair, the polarization direction would rotate a full 360 degrees
through approximately one hundred times, thus averaging the
periodic structure over all directions many times.
[0022] Since ablation occurs only over the portion of each laser
pulse in which the laser amplitude exceeds the threshold for
ablation, the effective number of polarization direction cycles
will be considerably less than one hundred. At the limit which only
the peak of the laser pulse ablates material (a situation which
results in the highest spatial resolution), the effective
polarization direction would be nearly identical for each laser if
the amplitude of each laser pulse was nearly identical. This is
undesirable since it reduces the amount of averaging over each
polarization direction. Therefore, it is also advantageous if there
is some pulse to pulse variation in the amplitude of laser pulses,
and if multiple laser pulses overlap spatially. This variation will
help to randomize the polarization directions from one laser pulse
to the next.
[0023] Referring to FIG. 4, there is illustrated an image of a line
of ablated material 60 in which the edge roughness (ablation
structure) has been eliminated. In this case, a quarter wave plate
was inserted into the laser path just prior to the laser beam
entering the final focusing lens. There was approximately a 5%
pulse to pulse variation in the amplitude of each laser pulse. The
resulting ablation does not evidence any of periodic structure, and
thus the edges of the ablated region are very smooth.
[0024] An alternative method of averaging over many polarization
directions involves a rotating half wave plate. By mechanically
rotating a half wave plate during an ablation, the polarization
direction also rotates, thus averaging the periodic ablation
structure. For example, the repaired region could be scanned
repeatedly with the half wave plate rotated by 90 degrees between
each scan. This would produce an average of two periodic structures
oriented at 90 degrees to each other.
[0025] Another method of averaging over many polarization
directions involves inserting a Pockell Cell in the path of the
laser beam. By applying a voltage to the Pockell Cell, the
polarization direction can be rotated to any desired angle. If the
applied voltage is varied as the laser beam is scanned, averaging
over any desired number of polarization directions can be
achieved.
[0026] Variations, modifications, and other implementations of what
is described herein may occur to those of ordinary skill in the art
without departing from the spirit and scope of the invention.
Accordingly, the invention is not to be defined only by the
preceding illustrative description.
* * * * *