KR20130112111A

KR20130112111A - Laser processing method

Info

Publication number: KR20130112111A
Application number: KR1020120034250A
Authority: KR
Inventors: 이동준; 권혁준
Original assignee: 주식회사 이오테크닉스
Priority date: 2012-04-03
Filing date: 2012-04-03
Publication date: 2013-10-14

Abstract

A laser processing method is disclosed. The disclosed laser processing method includes focusing a plurality of laser beams inside a workpiece to form a plurality of focusing points having different depths from the surface of the workpiece, and moving the laser beams along a processing line. And simultaneously forming a plurality of modified regions parallel to each other within the object, wherein the depth of the focusing points is adjusted by at least one of the focusing lenses and the zoom beam expanders.

Description

Laser processing method

The present invention relates to a laser processing method, and more particularly, to a laser processing method for processing a processing object by forming a modified region by focusing laser light inside the processing object.

Conventionally, when cutting a processing object such as a semiconductor wafer or a glass substrate using a laser, the surface of the processing object is irradiated with laser light having a wavelength absorbed by the processing object and cut by absorption of the laser light. The object to be processed is cut by advancing heat melting from the side toward the back surface. However, in this cutting method, the periphery of the area to be cut out of the surface of the object to be cut is also melted. Therefore, when a semiconductor element or the like is formed on the surface of the object to be processed, there is a possibility that the semiconductor element around the cut object is melted when the object is cut.

In recent years, in order to prevent the surface of a process object from being damaged, the method of processing a process object by focusing laser light inside a permeable process object to form a modified area | region is attracting attention. In such laser processing, laser light of ultra-short or ultra-short pulses such as picoseconds or femtoseconds having a high output may be used. Specifically, when a laser beam of high power is focused inside a processing object such as a semiconductor wafer or the like, a modified region due to multiphoton absorption is formed at the converging point. Then, when the crack extends from the modified region thus formed to the surface of the workpiece naturally or by external stress, the workpiece is cut by breaking and thus a dicing process is performed on the workpiece.

On the other hand, when the thickness of the object is thick, it is easy to cut the object only when a plurality of modified regions are formed in the thickness direction of the object. In order to form such a plurality of modified regions, a focus control method or a multi focus method using a birefringent material has been generally used.

1A and 1B illustrate a method of forming a plurality of modified regions inside a workpiece by using a focus adjustment method. First, referring to FIG. 1A, a laser beam 30 of ultra-short or ultra-short pulses emitted from a laser light source (not shown) is focused on the lower part of the object 10 by using the lens 20, and the laser is focused. When the light 30 is moved along the x direction of the line to be cut of the object 10, the first reformed region 41 is formed in the lower portion of the object 10. Next, referring to FIG. 1B, if the laser light 30 is focused on the upper portion of the object to be processed using the lens 20, the laser light 30 is moved along the x direction of the cutting schedule line. The second reformed region 42 is formed over the first reformed region 41.

FIG. 2 is a view for explaining a method of forming a plurality of modified regions 61 and 62 inside a workpiece by using a multi-focus method using a birefringent material. Referring to FIG. 2, when the laser light emitted from the laser light source passes through the predetermined birefringent material 50, first and second laser lights 51 and 52 having different focal lengths are generated to process the object 10. It will focus on the inside. For example, the first laser light 51 is focused on the lower part in the object 10, and the second laser light 52 is focused on the upper part in the object 10. Here, when the laser beams 51 and 52 are moved along the x direction of the cutting target line of the object 10, the lower part of the object 10 by the first and second laser light 51 and 52 and First and second modified regions 61 and 62 are formed at the same time.

An embodiment of the present invention provides a laser processing method for processing a processing object by forming a modified region inside the processing object.

In one aspect of the present invention,

Focusing a plurality of laser beams within the object to form a plurality of light collecting points having different depths from the surface of the object; And

And simultaneously forming a plurality of modified regions parallel to each other inside the object by moving the laser beams along a processing line, wherein the depths of the focusing points are at least one of focusing lenses and a zoom beam expander. A controlled laser processing method is provided.

The converging points may be located further forward with respect to the formation direction of the modified regions as the laser light is located deeper from the surface of the object to which the laser light is incident. The modified regions may be formed by absorption of multiple photons within the object.

The focusing lenses may be provided on an optical path of laser beams on the object to be processed. Here, the depth of the focusing points may be adjusted by changing at least one of the structure and the position of the focusing lenses.

The zoom beam expanders may be provided on an optical path of laser beams on the focusing lenses. Here, the depth of the focusing points can be adjusted by varying the distance between the lenses inside the zoom beam expanders. In this case, the distance between the focusing lenses and the workpiece may be maintained the same.

After forming the reformed regions, the method may further include separating the object to be processed along the processing line by expanding internal cracks generated from the reformed areas to the surface of the object.

The object to be processed may include a material that is transparent to the laser lights.

In another aspect of the present invention,

In the laser processing method of sequentially forming a plurality of modified regions having different depths inside the object to be processed by focusing the laser light inside the object to form a condensing point, and then moving a plurality of times along the expected processing line.

The modified regions are provided by a laser processing method formed by repeatedly moving the laser light in a zigzag form along the processing line.

Each of the modified regions may maintain a constant depth when the laser light moves in one direction. Here, the modified regions may be formed in order of decreasing depth from the surface of the object to which the laser light is incident.

Each of the modified regions may change in depth as the laser beam moves in one direction. Here, each of the modified regions may have a smaller depth from the surface of the object to which the laser light is incident as the laser light moves in one direction.

According to embodiments of the present invention, by using a focusing lens or a zoom beam expander, a plurality of modified regions may be simultaneously formed in the object by placing focus points having different depths inside the object. In addition, by focusing the laser light inside the object to be processed to form a focusing point, the plurality of modified regions having different depths may be sequentially formed in the object to be processed by moving in a zigzag form along a line to be processed. Therefore, a plurality of modified regions of different depths can be formed by only one laser scan.

1A and 1B illustrate a method of forming a plurality of modified regions inside a workpiece by using a focus adjustment method.
FIG. 2 is a view for explaining a method of forming a plurality of modified regions inside a workpiece by using a multi-focus method using a birefringent material.
3 is a view illustrating a laser processing method according to an exemplary embodiment of the present invention.
4 is a view illustrating a laser processing method according to an exemplary embodiment of the present invention.
5 is a view illustrating a laser processing method according to an exemplary embodiment of the present invention.
6 is a view for explaining a laser processing method according to an exemplary embodiment of the present invention.
7 is a view for explaining a laser processing method according to an exemplary embodiment of the present invention.
8 exemplarily illustrates dicing of a silicon wafer using a laser processing method according to the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, like reference numerals refer to like elements, and the size and thickness of each element may be exaggerated for clarity of explanation.

The laser processing method according to the exemplary embodiments described below processes the object by irradiating a plurality of laser beams inside the object to form a plurality of modified regions in the thickness direction of the object. The modified regions may be formed by absorbing multiple photons while the laser beams of high power are focused inside the object to be processed. Each of the laser beams may be, for example, an ultrashort or ultrashort pulsed laser beam having a pulse width of 1 ns (nano second) or less to form a modified region by multi-photon absorption. The peak power density can be, for example, approximately 1 × 10 ⁸ W / cm ² or more. However, it is not necessarily limited thereto. In the following embodiments, a case in which three modified regions are formed in the substrate along the thickness direction of the substrate using three laser beams will be described as an example. However, the present exemplary embodiment is not limited thereto, and two modified regions or four or more modified regions may be formed in the thickness direction of the workpiece.

3 is a view illustrating a laser processing method according to an exemplary embodiment of the present invention.

Referring to FIG. 3, the object to be processed 110 according to the present exemplary embodiment may include a material through which the laser lights 131, 132, and 133 may pass. For example, the object to be processed 110 may include a silicon wafer, a glass substrate, a sapphire substrate, or the like. However, the present invention is not limited thereto. A plurality of focusing lenses, for example, first, second, and third focusing lenses 151, 152, and 153 are provided on the optical path above the object to be processed 110. The first, second, and third focusing lenses 151, 152, and 153 focus the first, second, and third laser beams 131, 132, and 133, respectively, at first, second, and third positions at predetermined positions within the object 110. Three condensing points P1, P2, and P3 are formed. The first, second and third focusing lenses 151, 152 and 153 may be provided at the same height on the upper portion of the object to be processed 110. That is, as shown in FIG. 3, the first, second and third focusing lenses 151, 152, and 153 are all spaced apart by the same distance d from the surface of the object 110 to which the laser lights 131, 132, and 133 are incident. Can be prepared. In this case, when the structures of the first, second and third focusing lenses 151, 152 and 153 are changed, the first, second and third condensing points P1, P2 and P3 may be formed in the object 110. It may be formed at different depths along the thickness direction of the object to be processed (110). For example, when the shapes of the first, second, and third focusing lenses 151, 152, 153 are adjusted, the first, second, and third laser lights 131, 132, 133 are focused at different depths in the object to be processed 110. The first, second, and third light collecting points P1, P2, and P3 may be formed. Here, the first focusing point P1 may be formed at the deepest depth from the surface of the object 110 by the structure of the first focusing lens 151, and the third focusing point P3 may be the third focusing point. The structure of the lens 153 may be formed at the lowest depth from the surface of the object to be processed 110.

In the present exemplary embodiment, the condensing points P1, P2, and P3 are located deeper from the surface of the object 110 to which the laser lights 131, 132, and 133 are incident. (The x direction in FIG. 3) can be located further forward. That is, the first condensing point P1 formed at the deepest position is positioned at the frontmost side with respect to the x direction, and the third condensing point P3 formed at the lowest position is located at the rearmost position with respect to the x direction. Can be located.

Next, in the state where condensing points P1, P2, and P3 having different depths are formed inside the object 110, for example, the first, second, and third laser beams 131, 132, and 133 may be processed. Accordingly, when moved in one direction, for example, the x direction, the first, second and third modified regions 141, 142, and 143 may be simultaneously formed in parallel with each other along the x direction. Here, the processing schedule line is a line scheduled for laser processing on the object to be processed 110, and means a line set on the object to be processed so that the laser lights 131, 132, and 133 move. In this case, the front end of the first reformed region 141 formed at the deepest position is formed in front of the x direction, and the front end of the third reformed region 143 formed at the lowest position is formed from the x direction. It is formed on the back. This is to prevent scattering of laser light generated due to the modified region formed at a higher position inside the object 110. Meanwhile, the first, second, and third reformed regions 141, 142, and 143 may be formed by moving the object to be processed 110 in the -x direction along a processing line, and the laser beams 131, 132, and 133 to be processed. It may be formed by moving all of the (110) relatively.

As described above, a plurality of modified regions 141, 142, and 143 having different depths may be formed inside the object 110 by adjusting the structure of each of the focusing lenses 151, 152, and 153. When the cracks generated from the modified regions 141, 142, and 143 extend toward the surface of the object 100 by a natural or external stress, the object 110 is separated by a breaking by the breaking line. Can be.

4 is a view illustrating a laser processing method according to an exemplary embodiment of the present invention. Hereinafter, a description will be given focusing on differences from the above-described embodiment.

Referring to FIG. 4, a plurality of focusing lenses, for example, first, second, and third focusing lenses 151 ′, 152 ′, and 153 ′, are provided on the optical path above the object 110. The first, second and third focusing lenses 151 ′, 152 ′ and 153 ′ focus the first, second and third laser beams 131, 132 and 133 at predetermined positions inside the object 110, respectively. . The first, second and third focusing lenses 151 ′, 152 ′, and 153 ′ may all have the same structure. In this case, when the positions of the first, second and third focusing lenses 151 ′, 152 ′ and 153 ′ are adjusted, the first, second and third condensing points may be disposed in the object 110. P1, P2, and P3 may be formed at different depths along the thickness direction of the object to be processed 110. For example, since the distance d1 between the first focusing lens 151 ′ and the object 110 is closest to each other, the first light collecting point P1 may be formed at the deepest depth from the surface of the object 110 to be processed. The third condensing point P3 may be formed at the lowest depth from the surface of the object 110 because the third focusing lens 153 ′ and the distance d3 are farthest from the object 110. . In FIG. 4, d2 represents a distance between the second focusing lens 152 ′ and the object to be processed 110.

In the present exemplary embodiment, the converging points P1, P2, and P3 are located deeper from the surface of the object 110 to which the laser beams 131, 132, and 133 are incident (FIG. 1). X-direction at 4). That is, the first condensing point P1 formed at the deepest position is positioned at the frontmost side with respect to the x direction, and the third condensing point P3 formed at the lowest position is located at the rearmost position with respect to the x direction. Can be located.

Next, in the state where condensing points P1, P2, and P3 having different depths are formed inside the object 110, for example, the first, second, and third laser beams 131, 132, and 133 may be processed. Accordingly, when moved in one direction, for example, the x direction, the first, second and third modified regions 141, 142, and 143 may be simultaneously formed in parallel with each other along the x direction. In this case, the front end of the first reformed region 141 formed at the deepest position is formed in front of the x direction, and the front end of the third reformed region 143 formed at the lowest position is formed from the x direction. It is formed on the back. Meanwhile, the first, second, and third reformed regions 141, 142, and 143 may be formed by moving the object to be processed 110 in the -x direction along a processing line, and the laser beams 131, 132, and 133 to be processed. It may be formed by moving all of the (110).

As described above, the plurality of modified regions 141, 142, and 143 having different depths may be formed in the object 110 by adjusting the positions of the focusing lenses 151 ′, 152 ′, and 153 ′. When the cracks generated from the reformed regions 141, 142, and 143 extend toward the surface of the object 110 by natural or external stress, the object 110 may be separated along a process line. On the other hand, in the above embodiments by modifying the structure of the focusing lenses (151, 152, 153) or by changing the position of the focusing lenses (151 ', 152', 153 ') modification of different depths inside the object 110 to be processed. The case of forming the regions 141, 142, and 143 has been described. However, the modified regions 141, 142, and 143 having different depths may be formed inside the object 110 by changing both the structures and positions of the focus lenses 151, 152, 153 and 151 ′, 152 ′, 153 ′.

5 is a view illustrating a laser processing method according to an exemplary embodiment of the present invention. Hereinafter, a description will be given focusing on differences from the above-described embodiments.

Referring to FIG. 5, a plurality of focusing lenses, for example, first, second, and third focusing lenses 251, 252, and 253 are provided on an optical path above the object 110. The first, second, and third focusing lenses 251, 252, and 253 focus the first, second, and third laser lights 231, 232, 233, respectively, so that the first, second, and third condensing points P1 are inside the object to be processed. , P2, P3). The first, second and third focusing lenses 251, 252, and 253 may all have the same structure, and may be provided to be spaced apart by the same distance d from the surface of the processing object 110.

In this embodiment, a plurality of Zoom Beam Expanding Telescopes, for example, first, second, and third zoom beam expanders 271, 272, 273 are provided on each of the focusing lenses 251, 252, 253. Here, when the distance between the lenses provided in each of the first, second and third zoom beam expanders 271, 272, 273 is adjusted, the first, second and third condensing points P1, P2 and P3 may be formed at different depths along the thickness direction of the object to be processed 110. For example, when the distance between the lenses in the first zoom beam expander 271 is minimized, the first condensing point P1 may be formed at the deepest depth from the surface of the object 110, and the third zoom beam expander When the distance between the lenses in 273 is maximized, the third condensing point P3 may be formed at the lowest depth from the surface of the object 110 to be processed.

In the present embodiment, the converging points P1, P2, and P3 are located deeper from the surface of the object 110 to which the laser beams are incident in the forming direction of the modified regions 141, 142, and 143 (FIG. 5). In the x direction). That is, the first condensing point P1 formed at the deepest position is positioned at the frontmost side with respect to the x direction, and the third condensing point P3 formed at the lowest position is located at the rearmost position with respect to the x direction. Can be located.

Next, in the state where condensing points P1, P2, and P3 having different depths are formed in the object 110, for example, the first, second, and third laser beams 231, 232, 233 may be processed. Accordingly, when moved in one direction, for example, the x direction, the first, second and third modified regions 141, 142, and 143 may be simultaneously formed in parallel with each other along the x direction. In this case, the front end of the first reformed region 141 formed at the deepest position is formed in front of the x direction, and the front end of the third reformed region 143 formed at the lowest position is formed from the x direction. It is formed on the back. Meanwhile, the first, second, and third modified regions 141, 142, and 143 may be formed by moving the object to be processed 110 in the -x direction along a machining line, and the laser beams 231, 232, and 233 to be processed. It may be formed by moving all of the (110).

As described above, the plurality of modified regions 141, 142, and 143 having different depths are formed in the object 110 by adjusting the distance between the lenses inside the zoom beam expanders 271, 272, 273 provided on the focusing lenses 251, 252, 253. can do. In addition, when the cracks generated from the modified regions 141, 142, and 143 extend toward the surface of the object 110 by a natural or external stress, the object 110 may be separated along a process line. Meanwhile, in the above embodiments, modified regions having different depths inside the object 110 by changing any one of the focusing lenses 151, 152, 153, 151 ′, 152 ′, 153 ′ and the zoom beam expanders 271, 272, 273. The case of forming (141, 142, 143) has been described. However, the focus lenses 151, 152, 153, 151 ′, 152 ′, 153 ′ and the zoom beam expanders 271, 272, 273 may be changed to form modified regions 141, 142, 143 having different depths inside the object 110. It may be.

Next, the laser processing method according to the exemplary embodiments described below forms a plurality of modified regions in the thickness direction of the object by irradiating and moving one laser light inside the object. On the other hand, it is also possible to form a plurality of modified regions using a plurality of laser lights. The object to be processed may include a material that is transparent to the laser beam as described above, and the modified regions may be formed by multiphoton absorption while the laser beams of high power are focused inside the object to be processed. Each of the laser beams may be, for example, an ultrashort or ultrashort pulsed laser beam having a pulse width of 1 ns (nano second) or less to form a modified region by multi-photon absorption. The peak power density can be, for example, approximately 1 × 10 ⁸ W / cm ² or more. However, it is not necessarily limited thereto.

6 is a view for explaining a laser processing method according to an exemplary embodiment of the present invention.

Referring to FIG. 6, the laser beam 330 is focused on the inside of the object 210 to form a focusing point, and then the laser light 330 is moved a plurality of times along the scheduled line. A plurality of modified regions having different depths in the thickness direction of the object to be processed 210, for example, the first, second, third and fourth modified regions 241, 242, 243 and 244 are sequentially formed. The modified regions 241, 242, 243, and 244 may be formed by repeatedly moving the laser light 330 in a zigzag form along a processing line.

Specifically, the laser light 330 is first focused at the deepest position from the surface of the object 210 into which the laser light 330 is incident in the object 210. The first modified region 241 is formed by moving the laser light 330 in one direction, for example, the x direction. In this case, the first reformed region 241 is formed while maintaining a constant height inside the object 210. In addition, the laser beam 330 is raised from the outside of the object 210 by a predetermined distance in the -y direction, and then the inside of the object 210 is moved in the -x direction again. To form. In this case, the second reformed region 242 is formed while maintaining a constant height inside the object 210. The second reformed region 242 has a lower depth from the surface of the object 210 than the first reformed region 241. Next, the third modified region 243 is formed by raising the laser light 330 outside the object 210 by a predetermined distance in the -y direction and then moving the inside of the object 210 in the x direction. do. In this case, the third reformed region 243 is formed while maintaining a constant height inside the object 210. The third reformed region 243 has a lower depth from the surface of the object 210 than the second reformed region 242.

When the laser light 330 is repeatedly moved in a zigzag form in the object 210 while repeating the above process, a plurality of modified regions 241, 242, 243, and 244 having different depths are formed in the object 210. Will be formed. Accordingly, a plurality of modified regions 241, 242, 243, and 244 having different depths may be formed in the object 210 by only one laser scan.

7 is a view for explaining a laser processing method according to an exemplary embodiment of the present invention.

Referring to FIG. 7, the laser beam 430 is focused inside the object 210 to form a focusing point, and then the laser light 430 is moved a plurality of times along the processing line. A plurality of modified regions, for example, first, second, third, fourth and fifth modified regions 341, 342, 343, 344 and 345 having different depths in the thickness direction of the object 210 are sequentially formed. The modified regions 341, 342, 343, 344 and 345 may be formed by repeatedly moving the laser light 430 in a zigzag form along a processing line.

Specifically, the laser light 430 is first focused at the deepest position from the surface of the object 210 to which the laser light 430 is incident in the object 210. The first modified region 341 is formed by moving the laser light 430 in the x-direction and rising in the -y direction, for example. In this case, the first modified region 341 formed in the x-direction gradually decreases in depth from the surface of the object 210 to be processed. The second modified region 342 is formed by moving the laser light 430 in the -x direction and raising the -y direction at one end of the object 210. In this case, the second reformed region 342 formed gradually decreases in depth from the surface of the object 210 along the -x direction. In addition, the second reformed region 342 has a lower depth from the surface of the object 210 than the first reformed region 341. When the laser light 430 is repeatedly moved in a zigzag form in the processing object 210 while repeating the above process, a plurality of modified regions 341, 342, 343, 344 and 345 having different depths are formed in the processing object 210. Will be formed. Therefore, a plurality of modified regions 341, 342, 343, 344 and 345 having different depths may be formed in the object 210 by only one laser scan.

FIG. 8 exemplarily illustrates dicing a silicon wafer using a laser processing method according to exemplary embodiments of the present invention described above. Referring to FIG. 8, a plurality of cutting scheduled lines are formed on the silicon wafer in parallel with each of the processing direction 1 and the processing direction 2. Here, a plurality of devices may be stacked on the silicon wafer in the area bounded by the cutting lines. First, at least one modified region is formed in the silicon wafer along the thickness direction by moving at least one laser light along cutting lines to be formed parallel to the processing direction 1. Next, at least one modified region is formed in the silicon wafer along the thickness direction by moving the at least one laser light along cutting lines to be formed parallel to the processing direction 2. Then, when the crack is extended from the at least one modified region formed inside the silicon wafer to the silicon wafer surface naturally or by external stress, the silicon wafer is divided into a plurality of chips by breaking.

While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined by the appended claims.

110,210 ... Object to be processed 131,231 ... First laser beam
132,232 ... 2nd laser light 133,233 ... 3rd laser light
141 ... first reformed area 142 ... second reformed area
143 ... Third modified area 151,151 ', 251 ... First focusing lens
152,152 ', 252 ... Second Focusing Lens
153,153 ', 253 ... Third Focusing Lens
330,430 ... laser light P1 ... first condensing point
P2 ... 2nd condensing point
P3 ... Third Condensing Point

Claims

Focusing a plurality of laser beams within the object to form a plurality of light collecting points having different depths from the surface of the object; And
And simultaneously forming a plurality of modified regions parallel to each other inside the object to be processed by moving the laser beams along a processing line.
And a depth of the focusing points is controlled by at least one of focusing lenses and zoom beam expanders.

The method of claim 1,
And the converging points are located further forward with respect to the direction in which the modified regions are formed, the deeper a position is located from the surface of the object to which the laser light is incident.

The method of claim 1,
And the modified regions are formed by absorption of multiple photons within the object.

The method of claim 1,
And the focusing lenses are provided on an optical path of laser beams on the object to be processed.

5. The method of claim 4,
And the depth of the focusing points is adjusted by changing at least one of the structure and position of the focusing lenses.

5. The method of claim 4,
And the zoom beam expanders are provided on an optical path of laser beams on the focusing lenses.

The method according to claim 6,
And the depth of the focusing points is adjusted by varying the distance between the lenses inside the zoom beam expanders.

The method of claim 7, wherein
The distance between the focusing lens and the object is maintained the same laser processing method.

The method of claim 1,
And forming the modified regions, and separating the object to be processed along the scheduled line by expanding internal cracks generated from the modified regions to the surface of the object.

The method of claim 1,
The processing object is a laser processing method comprising a material that is transparent to the laser light.

In the laser processing method of sequentially forming a plurality of modified regions having different depths inside the object to be processed by focusing the laser light inside the object to form a condensing point, and then moving a plurality of times along the expected processing line.
The modified regions are formed by repeatedly moving the laser light in a zigzag form along the processing line.

The method of claim 11,
Each of the modified regions maintain a constant depth when the laser light moves in one direction.

13. The method of claim 12,
And the modified regions are formed in order of decreasing depth from the surface of the object to which the laser light is incident.

The method of claim 11,
Each of the modified regions is changed in depth as the laser light moves in one direction.

15. The method of claim 14,
And each of the modified regions is gradually decreased in depth from the surface of the object to which the laser light is incident as the laser light moves in one direction.

The method of claim 11,
The processing object is a laser processing method comprising a material that is transparent to the laser light.

The method of claim 11,
And the modified regions are formed by multiphoton absorption inside the object.