Diffractive optical device

A diffractive optical device includes at least one diffractive optical element. The diffractive optical element generates light having a first order and light having a second order from a laser beam input to the diffractive optical element. The diffractive optical element includes a first phase pattern and a second phase pattern. The first phase pattern converts the laser beam into a line beam. The second phase pattern diffracts the laser beam in a short axis direction of the line beam to generate the light having the first order and the light having the second order. A first focal plane of the light having the first order is located at a position different from a second focal plane of the light having the second order on an optical axis of the laser beam.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on PCT filing PCT/JP2020/027097, filed Jul. 10, 2020, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a diffractive optical device.

BACKGROUND ART

Japanese Patent Laying-Open No. 2012-131681 (PTL 1) discloses a diffractive optical element (DOE) that converts a circular laser beam into a line beam. The line beam is used to process a workpiece.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

A diffractive optical device of the present disclosure includes at least one diffractive optical element. The at least one diffractive optical element generates light having a first order and light having a second order from a laser beam input to the at least one diffractive optical element, and superimposes the light having the first order and the light having the second order on each other on an optical axis of the laser beam to cause interference between the light having the first order and the light having the second order, the light having the first order and the light having the second order having diffraction orders different from each other. The at least one diffractive optical element includes a first phase pattern and a second phase pattern. The first phase pattern converts the laser beam into a line beam. The second phase pattern diffracts the laser beam in a short axis direction of the line beam to generate the light having the first order and the light having the second order. A first focal plane of the light having the first order is located at a position different from a second focal plane of the light having the second order on the optical axis of the laser beam.

DETAILED DESCRIPTION

Problem to be Solved by the Present Disclosure

In order to obtain a line beam, a diffractive optical element converges a laser beam more intensely in the short axis direction of the line beam than in the long axis direction of the line beam. When displaced from the focal plane of the diffractive optical element, the line beam is diverged abruptly in the short axis direction of the line beam. Therefore, the focal depth of the line beam is short. When a workpiece is processed using such a line beam having a short focal depth, processing characteristics for the workpiece are greatly varied in response to only a slight change being made in a distance between the diffractive optical element and the workpiece. It is an object of the present disclosure to provide a diffractive optical device by which a line beam having a longer focal depth can be obtained.

Advantageous Effect of the Present Disclosure

According to the diffractive optical device of the present disclosure, a line beam having a longer focal depth can be obtained.

DESCRIPTION OF EMBODIMENTS

(1) A diffractive optical device according to the present disclosure includes at least one diffractive optical element. The at least one diffractive optical element generates light having a first order and light having a second order from a laser beam input to the at least one diffractive optical element, and superimposes the light having the first order and the light having the second order on each other on an optical axis of the laser beam to cause interference between the light having the first order and the light having the second order, the light having the first order and the light having the second order having diffraction orders different from each other. The at least one diffractive optical element includes a first phase pattern and a second phase pattern. The first phase pattern converts the laser beam into a line beam. The second phase pattern diffracts the laser beam in a short axis direction of the line beam to generate the light having the first order and the light having the second order. A first focal plane of the light having the first order is located at a position different from a second focal plane of the light having the second order on the optical axis of the laser beam.

Therefore, a line beam having a longer focal depth can be obtained.

(2) According to the diffractive optical device according to (1), the second phase pattern includes a central phase pattern and peripheral phase patterns disposed on both sides relative to the central phase pattern in the short axis direction of the line beam. The central phase pattern provides a first optical phase to the laser beam. Each of the peripheral phase patterns provides a second optical phase different from the first optical phase to the laser beam. A difference between the first optical phase and the second optical phase is π.

Therefore, a line beam having a longer focal depth can be obtained.

(3) According to the diffractive optical device according to (2), the central phase pattern is uniform in a long axis direction of the line beam. Each of the peripheral phase patterns is uniform in the long axis direction of the line beam.

The second phase pattern generates no diffraction light in the long axis direction of the line beam in which no problem of the focal depth occurs. Therefore, a line beam having a longer focal depth can be obtained without exerting an influence in the long axis direction of the line beam.

(4) According to the diffractive optical device according to any one of (1) to (3), the light having the first order is a +1-order diffraction beam. The light having the second order is a −1-order diffraction beam. When λ represents a wavelength of the laser beam, ω represents a 1/e2beam diameter of the laser beam, P+1represents a first refractive power of the second phase pattern for the +1-order diffraction beam in the short axis direction of the line beam, and P−1represents a second refractive power of the second phase pattern for the −1-order diffraction beam in the short axis direction of the line beam, P+1and P−1are given by the following formula (1) and a coefficient C satisfies the following formula (2):
P+1=−P−1=λC/ω2(1), and
0.0<C≤2.6  (2).

Therefore, the light intensity distribution of the line beam becomes more uniform in the optical axis direction.

(5) According to the diffractive optical device according to (4), the coefficient C satisfies the following formula (3):
1.4≤C≤2.6  (3).

Therefore, a line beam having a longer focal depth can be obtained.

(6) According to the diffractive optical device according to (4) or (5), the coefficient C satisfies the following formula (4):
1.6≤C≤2.1  (4).

Therefore, the light intensity distribution of the line beam becomes more uniform in the short axis direction of the line beam.

(7) According to the diffractive optical device according to any one of (1) to (6), the at least one diffractive optical element is constituted of one diffractive optical element including a light incident surface and a light exit surface. The first phase pattern is formed in one of the light incident surface or the light exit surface. The second phase pattern is formed in one of the light incident surface or the light exit surface.

Therefore, the diffractive optical device can be downsized. Positioning of the diffractive optical device is facilitated. Disturbance in the cross sectional shape of the line beam can be reduced.

(8) According to the diffractive optical device according to (7), the one diffractive optical element includes a phase pattern in which the first phase pattern and the second phase pattern are overlapped with each other. The phase pattern is formed in one of the light incident surface or the light exit surface.

Therefore, the first phase pattern and the second phase pattern can be more precisely positioned with respect to each other. Disturbance in the cross sectional shape of the line beam can be reduced.

(9) According to the diffractive optical device according to any one of (1) to (6), the at least one diffractive optical element is constituted of a first diffractive optical element and a second diffractive optical element each disposed along the optical axis. The first phase pattern is formed in the first diffractive optical element. The second phase pattern is formed in the second diffractive optical element.

Therefore, a line beam having a longer focal depth can be obtained.

(10) According to the diffractive optical device according to (2) or (3), the at least one diffractive optical element is constituted of one diffractive optical element including a light incident surface and a light exit surface. The one diffractive optical element includes a phase pattern in which the first phase pattern and the second phase pattern are overlapped with each other. The phase pattern is formed in one of the light incident surface or the light exit surface. The light having the first order is a +1-order diffraction beam. The light having the second order is a −1-order diffraction beam. When λ represents a wavelength of the laser beam, ω represents a 1/e2beam diameter of the laser beam, P+1represents a first refractive power of the second phase pattern for the +1-order diffraction beam in the short axis direction of the line beam, and P−1represents a second refractive power of the second phase pattern for the −1-order diffraction beam in the short axis direction of the line beam, P+1and P−1are given by the following formula (5) and a coefficient C satisfies the following formula (6):
P+1=−P−1=λC/ω2(5), and
1.6≤C≤2.1  (6).

Therefore, a line beam having a longer focal depth can be obtained. The light intensity distribution of the line beam becomes more uniform in the short axis direction of the line beam without exerting an influence in the long axis direction of the line beam. The first phase pattern and the second phase pattern may be more precisely positioned with respect to each other.

DETAILS OF EMBODIMENTS OF THE PRESENT DISCLOSURE

Details of embodiments will be described below with reference to figures. It should be noted that in the figures, the same or corresponding portions are denoted by the same reference characters, and will not be described repeatedly. At least parts of the configurations of the embodiments described below may be combined appropriately.

First Embodiment

A laser beam irradiation device1of a first embodiment will be described with reference toFIGS.1to18. As shown inFIGS.1and2, laser beam irradiation device1is, for example, a device that irradiates a workpiece30with a line beam20. Workpiece30is processed using line beam20, for example. Workpiece30is, for example, a semiconductor wafer, a glass substrate, a thin film formed on a substrate or the like. Laser beam irradiation device1includes a laser light source5and a diffractive optical device10.

Laser light source5is, for example, a solid-state laser or a gas laser. Examples of the solid-state laser include a fiber laser, a semiconductor laser, and a YAG laser. Examples of the gas laser include a carbon dioxide gas laser. Laser beam6output from laser light source5is input to diffractive optical device10. As shown inFIG.3A, laser beam6has a circular cross sectional shape, for example. The light intensity distribution of laser beam6in the cross section of laser beam6is, for example, Gaussian distribution. In the present specification, the cross section of the light beam (for example, laser beam6or line beam20) refers to a cross section thereof perpendicular to the optical axis of the light beam (optical axis11of laser beam6or optical axis11of diffractive optical device10). The cross sectional shape of the light beam (for example, laser beam6or line beam20) refers to the shape of the light beam in the cross section perpendicular to the optical axis of the light beam (for example, optical axis11of laser beam6or optical axis11of diffractive optical device10). In the present specification, a direction along optical axis11is referred to as “z axis direction”.

Diffractive optical device10can convert laser beam6(seeFIG.3A) into a line beam20(seeFIG.3B) that is more elongated than laser beam6. That is, diffractive optical device10can convert laser beam6into a line beam20having an aspect ratio larger than the aspect ratio of laser beam6. The aspect ratio of a beam represents a degree of elongation of the beam. In the present specification, the long axis direction of line beam20is referred to as “x axis direction”, and the short axis direction of line beam20is referred to as “y axis direction”. Further, diffractive optical device10can attain a long focal depth of line beam20.

As shown inFIGS.1and2, diffractive optical device10includes at least one diffractive optical element (diffractive optical element12). The at least one diffractive optical element is composed of an optical material transparent to laser beam6, such as glass or a transparent resin.

The at least one diffractive optical element (diffractive optical element12) includes a first phase pattern14shown inFIGS.9to12and a second phase pattern15shown inFIG.13. Generally, a phase pattern provides, to a light beam passing through the phase pattern, a phase corresponding to the local thickness of a diffractive optical element in which the phase pattern is formed. The phase pattern changes the wavefront of the light beam to converge or diffract the light beam, for example. Each of first phase pattern14and second phase pattern15is formed by patterning a surface of a plate transparent to laser beam6using, for example, a photolithography process.

In the present embodiment, the at least one diffractive optical element is constituted of one diffractive optical element12including a light incident surface12aand a light exit surface12b. First phase pattern14and second phase pattern15are formed in light exit surface12bof diffractive optical element12. That is, as shown inFIGS.5to8, diffractive optical element12includes a phase pattern13in which first phase pattern14and second phase pattern15are overlapped with each other, and phase pattern13is formed in light exit surface12bof diffractive optical element12.

As shown inFIG.4, the at least one diffractive optical element (diffractive optical element12) generates light21having a first order and light22having a second order from laser beam6input to the at least one diffractive optical element (diffractive optical element12), and superimposes light21having the first order and light22having the second order on each other on optical axis11of laser beam6to cause interference between light21having the first order and light22having the second order. Light21having the first order and light22having the second order have diffraction orders different from each other. A first focal plane23of light21having the first order is located at a position different from a second focal plane24of light22having the second order on optical axis11of laser beam6.

First phase pattern14converts laser beam6into line beam20(seeFIG.3B) that is more elongated than laser beam6. First phase pattern14converges laser beam6in the short axis direction (y axis direction) of line beam20. First phase pattern14has a positive refractive power in the short axis direction (y axis direction) of line beam20.

Second phase pattern15diffracts laser beam6in the short axis direction (y axis direction) of line beam20to generate light21having the first order and light22having the second order. Light21having the first order is, for example, a +1-order diffraction beam. Light22having the second order is, for example, a −1-order diffraction beam.

Specifically, as shown inFIG.13, second phase pattern15includes a central phase pattern15aand peripheral phase patterns15b. Peripheral phase patterns15bare disposed on both sides relative to central phase pattern15ain the short axis direction (y axis direction) of line beam20. Central phase pattern15aextends over a width d with respect to y=0. For example, central phase pattern15aextends between a first line defined by y=d/2 and a second line defined by y=−d/2. Central phase pattern15aprovides a first optical phase to the laser beam input to second phase pattern15. Each of peripheral phase patterns15bprovides a second optical phase different from the first optical phase to the laser beam input to second phase pattern15. A difference between the first optical phase and the second optical phase is, for example, π.

Second phase pattern15may be uniform in the long axis direction (x axis direction) of line beam20. Specifically, central phase pattern15amay be uniform in the long axis direction (x axis direction) of line beam20, and peripheral phase pattern15bmay be uniform in the long axis direction (x axis direction) of line beam20. Therefore, second phase pattern15may diffract laser beam6only in the short axis direction (y axis direction) of line beam20.

The first refractive power of second phase pattern15for light21having the first order in the short axis direction (y axis direction) of line beam20is different from the second refractive power of second phase pattern15for light22having the second order in the short axis direction (y axis direction) of line beam20. For example, the first refractive power may be greater than the second refractive power. Therefore, first focal plane23of light21having the first order is located at a position different from second focal plane24of light22having the second order on optical axis11of laser beam6.

For example, second phase pattern15has a positive refractive power for light21having the first order (for example, the +1-order diffraction beam) in the short axis direction (y axis direction) of line beam20. Second phase pattern15has a negative refractive power for light22having the second order (for example, the −1-order diffraction beam) in the short axis direction (y axis direction) of line beam20. Due to the positive refractive power of first phase pattern14and the positive refractive power of second phase pattern15for light21having the first order, first focal plane23of light21having the first order is located close to diffractive optical device10(or the at least one diffractive optical element (diffractive optical element12)) relative to focal plane25on optical axis11of laser beam6as shown inFIG.4. On the other hand, due to the positive refractive power of first phase pattern14and the negative refractive power of second phase pattern15for light22having the second order, second focal plane24of light22having the second order is located distant away from diffractive optical device10(or the at least one diffractive optical element (diffractive optical element12)) relative to focal plane25on optical axis11of laser beam6as shown inFIG.4.

It should be noted that in the present specification, focal plane25is defined as a plane in which the length of a line beam formed only by first phase pattern14in the short axis direction (y axis direction) is minimum as in a below-described Comparative Example, among planes perpendicular to optical axis11(z axis). In the present specification, focal plane25may be referred to as “focal plane25of diffractive optical device10”. As shown inFIG.4, focal plane25of diffractive optical device10is a plane defined by z=0. Focal plane25of diffractive optical device10is located on a surface of workpiece30or is located inside workpiece30, for example.

Thus, first focal plane23of light21having the first order is located at a position different from second focal plane24of light22having the second order. Light21having the first order and light22having the second order are superimposed on each other on optical axis11of laser beam6(optical axis11of diffractive optical device10) to cause interference therebetween. Therefore, the focal depth of line beam20can be made long.

For example, as shown inFIG.16, focal depth Dzof line beam20can be made long. In the present specification, focal depth Dzis defined as a length thereof on optical axis11(z axis), in which the optical axis direction relative light intensity of line beam20is more than or equal to 0.5. The optical axis direction relative light intensity of line beam20is obtained by dividing the light intensity of line beam20on optical axis11(z axis) by the maximum light intensity of line beam20on optical axis11(z axis).

The optical axis direction relative light intensity of line beam20is preferably more than or equal to 0.5 between a first position P1and a second position P2in an optical axis direction relative light intensity profile of line beam20. In the present specification, the optical axis direction relative light intensity profile refers to a distribution of optical axis direction relative light intensity on the optical axis (z axis). Therefore, the light intensity in a region irradiated with line beam20becomes more uniform. A variation in processing of workpiece30in the region irradiated with line beam20can be reduced. First position P1in the optical axis direction relative light intensity profile of line beam20is a position on optical axis11at which the optical axis direction relative light intensity of line beam20is 0.5 in the optical axis direction relative light intensity profile of line beam20, and is a position furthest away from diffractive optical device10(or laser light source5). Second position P2in the optical axis direction relative light intensity profile of line beam20is a position on optical axis11at which the optical axis direction relative light intensity of line beam20is 0.5 in the optical axis direction relative light intensity profile of line beam20, and is a position closest to diffractive optical device10(or laser light source5).

As shown inFIG.17, the light intensity peak of line beam20on the short axis (y axis) of line beam20in focal plane25of diffractive optical device10is flattened. That is, the relative light intensity profile of line beam20on the short axis (y axis) of line beam20in focal plane25of diffractive optical device10(hereinafter, referred to as “short axis direction relative light intensity profile of line beam20”) has a flat top shape. Therefore, the light intensity in the region irradiated with line beam20becomes more uniform. The variation in processing of workpiece30in the region irradiated with line beam20can be reduced.

In the present specification, the short axis direction relative light intensity profile of line beam20refers to a distribution of short axis direction relative light intensity on the short axis (y axis) of line beam20in focal plane25. The short axis direction relative light intensity of line beam20is obtained by dividing the light intensity of line beam20on the short axis (y axis) of line beam20in focal plane25by the maximum light intensity of line beam20on the short axis (y axis) of line beam20in focal plane25.

The short axis direction relative light intensity profile of line beam20with a flat top shape means that a ratio W1/W2of a 0.9 peak width W1(seeFIG.17) of the short axis direction relative light intensity profile of line beam20to a 1/e2peak width W2(seeFIG.17) in the short axis direction relative light intensity profile of line beam20is more than or equal to 0.400, and that the short axis direction relative light intensity profile of line beam20is more than or equal to 0.9 between a third position P3(seeFIG.17) and a fourth position P4(seeFIG.17) in the short axis direction relative light intensity profile of line beam20.

0.9 peak width W1of the short axis direction relative light intensity profile of line beam20is defined as a length thereof on the short axis (y axis) in focal plane25, at which the short axis direction relative light intensity of line beam20is more than or equal to 0.9. 1/e2peak width W2of the short axis direction relative light intensity profile of line beam20is defined as a length thereof on the short axis (y axis) in focal plane25, at which the short axis direction relative light intensity of line beam20is more than or equal to 1/e2. Third position P3in the short axis direction relative light intensity profile of line beam20is a position at which the short axis direction relative light intensity of line beam20is 0.9 in the short axis direction relative light intensity profile of line beam20, and is a position furthest away from optical axis11in the +y axis direction. Fourth position P4in the short axis direction relative light intensity profile of line beam20is a position at which the short axis direction relative light intensity of line beam20is 0.9 in the short axis direction relative light intensity profile of line beam20, and is a position furthest away from optical axis11in the −y axis direction.

Diffractive optical device10of the present embodiment can reduce the diameter of laser beam6required to obtain the short axis direction relative light intensity profile of line beam20having a small short axis direction width close to the diffraction limit and having a flat top shape. Therefore, diffractive optical device10can be downsized. The cost of diffractive optical device10can be reduced. First phase pattern14and second phase pattern15can be formed with higher precision.

As shown inFIG.18, the light intensity peak of line beam20on the long axis (x axis) of line beam20in focal plane25of diffractive optical device10is flattened. That is, the relative light intensity profile of line beam20on the long axis (x axis) of line beam20(hereinafter, referred to as “long axis direction relative light intensity profile of line beam20”) in focal plane25of diffractive optical device10has a flat top shape. For example, a ratio W3/W4of a 0.9 peak width W3of the long axis direction relative light intensity profile of line beam20to a 1/e2peak width W4of the long axis direction relative light intensity profile of line beam20is more than or equal to 0.400.

In the present specification, the long axis direction relative light intensity profile of line beam20refers to a distribution of long axis relative light intensity on the long axis (x axis) of line beam20in focal plane25. The long axis relative light intensity of line beam20is obtained by dividing the light intensity of line beam20on the long axis (x axis) of line beam20in focal plane25by the maximum light intensity of line beam20on the long axis (x axis) of line beam20in focal plane25. 0.9 peak width W3of the long axis direction relative light intensity profile of line beam20is defined as a length thereof on the long axis (x axis) in focal plane25, at which the long axis relative light intensity of line beam20is more than or equal to 0.9. 1/e2peak width W4of the long axis direction relative light intensity profile of line beam20is defined as a length thereof on the long axis (x axis) in focal plane25, at which the long axis relative light intensity of line beam20is more than or equal to 1/e2.

An aspect ratio of line beam20is defined in focal plane25. Specifically, the aspect ratio of line beam20is defined as a ratio W4/W2of 1/e2peak width W4(seeFIG.18) of the long axis direction relative light intensity profile of line beam20to 1/e2peak width W2(seeFIG.17) of the short axis direction relative light intensity profile of line beam20. An aspect ratio of laser beam6is also defined in the same manner as aspect ratio W4/W2of line beam20. For example, when laser beam6has a circular shape, the aspect ratio of laser beam6is 1.0.

In one example of the present embodiment, laser beam6has a circular cross sectional shape, the light intensity distribution of laser beam6in the cross section thereof is Gaussian distribution, laser beam6has a wavelength λ, light21having the first order is a +1-order diffraction beam, and light22having the second order is a −1-order diffraction beam. The 1/e2beam diameter of laser beam6is defined as ω. The 1/e2beam diameter of laser beam6is a diameter of laser beam6with which the relative light intensity of laser beam6in the cross section thereof is 1/e2. The relative light intensity of laser beam6in the cross section thereof is obtained by dividing the light intensity of laser beam6in the cross section of laser beam6by the maximum light intensity of laser beam6in the cross section of laser beam6(the light intensity of laser beam6at the center of the cross section of laser beam6).

First refractive power P+1of second phase pattern15for the +1-order diffraction beam in the short axis direction (y axis direction) of line beam20and second refractive power P−1of second phase pattern15for the −1-order diffraction beam in the short axis direction (y axis direction) of line beam20are given by the following formula (1):
P+1=−P−1=λC/ω2(1)

Coefficient C may satisfy the below-described formula (2). Therefore, the optical axis direction relative light intensity of line beam20can be more than or equal to 0.5 between first position P1and second position P2in the optical axis direction relative light intensity profile of line beam20(seeFIG.19). The light intensity in the region irradiated with line beam20becomes more uniform. The variation in processing of workpiece30in the region irradiated with line beam20can be reduced.
0.0<C≤2.6  (2)

Coefficient C may satisfy the below-described formula (3). Therefore, the focal depth of diffractive optical device10of the present embodiment (see, for example,FIGS.5to8) is 1.5 times or more as large as the focal depth of the diffractive optical device of the Comparative Example (seeFIGS.9to12) which does not include second phase pattern15(seeFIG.19). Even when laser beam6is more intensely converged in the short axis direction (y axis direction) of line beam20by using diffractive optical device10, focal depth Dzof line beam20can be made further longer.
1.4≤C≤2.6  (3)

Coefficient C may satisfy the below-described formula (4). Therefore, with diffractive optical device10of the present embodiment, the short axis direction relative light intensity profile of line beam20with a flat top shape can be obtained. The light intensity in the region irradiated with line beam20becomes more uniform. The variation in processing of workpiece30in the region irradiated with line beam20can be reduced.
1.6≤C≤2.1  (4)

In the above one example of the present embodiment, width d of central phase pattern15ais given by the following formula (5):

EXAMPLES

Examples 1 to 11, each of which is a specific example of the present embodiment, will be described in comparison to a Comparative Example. In each of Examples 1 to 11 and the Comparative Example, laser beam6has a circular cross sectional shape, the aspect ratio of laser beam6is 1.000, and 1/e2beam diameter ω of laser beam6is 3.0 mm. The light intensity distribution of laser beam6in the cross section of laser beam6is Gaussian distribution. Wavelength λ of laser beam6is 1070 nm. A focal distance f of diffractive optical device10is 250 mm. Focal distance f is a distance between diffractive optical device10and focal plane25in the optical axis direction (z axis direction). As shown in each ofFIGS.18,24,34,44,54,64,74,84,94,104,114, and124, 0.9 peak width W3of the long axis direction relative light intensity profile of line beam20is 1.00 mm, and 1/e2peak width W4of the long axis direction relative light intensity profile of line beam20is 1.35 mm.

Comparative Example

A diffractive optical element of the Comparative Example includes only a phase pattern shown inFIGS.9to12. That is, the phase pattern of the Comparative Example is constituted of only a first phase pattern14of Example 1, and does not include a second phase pattern15of Example 1 shown inFIG.13. Therefore, in the diffractive optical element of the Comparative Example, width d of central phase pattern15ais regarded as being infinite. Since the diffractive optical element of the Comparative Example does not generate light21having the first order (for example, the +1-order diffraction beam) and light22having the second order (for example, the −1-order diffraction beam), coefficient C is regarded as zero.

A line beam20having a defocus profile shown inFIGS.20and21is obtained by the diffractive optical element of the Comparative Example. Referring toFIG.22, focal depth Dzof line beam20is 33.0 mm. The relative light intensity on optical axis11in focal plane25(z=0) is 1.000. The optical axis direction relative light intensity of line beam20is more than or equal to 0.5 between first position P1and second position P2in the optical axis direction relative light intensity profile of line beam20.

Referring toFIG.23, 0.9 peak width W1of the short axis direction relative light intensity profile of line beam20is 0.026 mm, and 1/e2peak width W2of the short axis direction relative light intensity profile of line beam20is 0.114 mm. Aspect ratio W4/W2of line beam20is 11.84. Ratio W1/W2is 0.228, which is less than 0.400. Therefore, the short axis direction relative light intensity of line beam20does not have a profile flat top shape.

A diffractive optical element12of Example 1 includes phase pattern13shown inFIGS.5to8. Phase pattern13of the present example is a phase pattern in which first phase pattern14shown inFIGS.9to12and second phase pattern15shown inFIG.13are overlapped with each other. Central phase pattern15aof second phase pattern15provides a first optical phase of π to laser beam6. Each of peripheral phase patterns15bof second phase pattern15provides a second optical phase of 0 to laser beam6. Since diffractive optical element12of the present example includes second phase pattern15, diffractive optical element12generates a +1-order diffraction beam as light21having the first order and generates a −1-order diffraction beam as light22having the second order. In the present example, width d of central phase pattern15ais 3.22 mm, and coefficient C is 1.74.

A line beam20having a defocus profile shown inFIGS.14and15is obtained by diffractive optical element12of the present example. Referring toFIG.16, focal depth Dzof line beam20is 60.4 mm. The relative light intensity on optical axis11in focal plane25(z=0) is 0.758. The optical axis direction relative light intensity of line beam20is more than or equal to 0.5 between first position P1and second position P2in the optical axis direction relative light intensity profile of line beam20.

Referring toFIG.17, 0.9 peak width W1of the short axis direction relative light intensity profile of line beam20is 0.075 mm, and 1/e2peak width W2of the short axis direction relative light intensity profile of line beam20is 0.163 mm. Aspect ratio W4/W2of line beam20is 8.28. Ratio W1/W2is 0.460, which is more than or equal to 0.400. The short axis direction relative light intensity profile of line beam20is more than or equal to 0.9 between third position P3and fourth position P4in the short axis direction relative light intensity profile of line beam20. Therefore, the short axis direction relative light intensity profile of line beam20has a flat top shape.

A diffractive optical element12of Example 2 includes a phase pattern13shown inFIGS.25to28. Phase pattern13of the present example is a phase pattern in which first phase pattern14shown inFIGS.9to12and a second phase pattern15shown inFIG.29are overlapped with each other. Second phase pattern15of the present example is similar to second phase pattern15of Example 1, but width d of central phase pattern15aof the present example is 5.07 mm. In the present example, coefficient C is 0.70.

A line beam20having a defocus profile shown inFIGS.30and31is obtained by diffractive optical element12of the present example. Referring toFIG.32, focal depth Dzof line beam20is 34.2 mm. The relative light intensity on optical axis11in focal plane25(z=0) is 0.999. The optical axis direction relative light intensity of line beam20is more than or equal to 0.5 between first position P1and second position P2in the optical axis direction relative light intensity profile of line beam20.

Referring toFIG.33, 0.9 peak width W1of the short axis direction relative light intensity profile of line beam20is 0.029 mm, and 1/e2peak width W2of the short axis direction relative light intensity profile of line beam20is 0.119 mm. Aspect ratio W4/W2of line beam20is 11.34. Ratio W1/W2is 0.244, which is less than 0.400. Therefore, the short axis direction relative light intensity profile of line beam20does not have a flat top shape.

A diffractive optical element12of Example 3 includes a phase pattern13shown inFIGS.35to38. Phase pattern13of the present example is a phase pattern in which first phase pattern14shown inFIGS.9to12and a second phase pattern15shown inFIG.39are overlapped with each other. Second phase pattern15of the present example is similar to second phase pattern15of Example 1, but width d of central phase pattern15aof the present example is 4.24 mm. In the present example, coefficient C is 1.00.

A line beam20having a defocus profile shown inFIGS.40and41is obtained by diffractive optical element12of the present example. Referring toFIG.42, focal depth Dzof line beam20is 40.6 mm. The relative light intensity on optical axis11in focal plane25(z=0) is 0.974. The optical axis direction relative light intensity of line beam20is more than or equal to 0.5 between first position P1and second position P2in the optical axis direction relative light intensity profile of line beam20.

Referring toFIG.43, 0.9 peak width W1of the short axis direction relative light intensity profile of line beam20is 0.035 mm, and 1/e2peak width W2of the short axis direction relative light intensity profile of line beam20is 0.130 mm. Aspect ratio W4/W2of line beam20is 10.38. Ratio W1/W2is 0.269, which is less than 0.400. Therefore, the short axis direction relative light intensity profile of line beam20does not have a flat top shape.

A diffractive optical element12of Example 4 includes a phase pattern13shown inFIGS.45to48. Phase pattern13of the present example is a phase pattern in which first phase pattern14shown inFIGS.9to12and a second phase pattern15shown inFIG.49are overlapped with each other. Second phase pattern15of the present example is similar to second phase pattern15of Example 1, but width d of central phase pattern15aof the present example is 3.72 mm. In the present example, coefficient C is 1.30.

A line beam20having a defocus profile shown inFIGS.50and51is obtained by diffractive optical element12of the present example. Referring toFIG.52, focal depth Dzof line beam20is 48.4 mm. The relative light intensity on optical axis11in focal plane25(z=0) is 0.897. The optical axis direction relative light intensity of line beam20is more than or equal to 0.5 between first position P1and second position P2in the optical axis direction relative light intensity profile of line beam20.

Referring toFIG.53, 0.9 peak width W1of the short axis direction relative light intensity profile of line beam20is 0.045 mm, and 1/e2peak width W2of the short axis direction relative light intensity profile of line beam20is 0.143 mm. Aspect ratio W4/W2of line beam20is 9.44. Ratio W1/W2is 0.315, which is less than 0.400. Therefore, the short axis direction relative light intensity profile of line beam20does not have a flat top shape.

A diffractive optical element12of Example 5 includes a phase pattern13shown inFIGS.55to58. Phase pattern13of the present example is a phase pattern in which first phase pattern14shown inFIGS.9to12and a second phase pattern15shown inFIG.59are overlapped with each other. Second phase pattern15of the present example is similar to second phase pattern15of Example 1, but width d of central phase pattern15aof the present example is 3.59 mm. In the present example, coefficient C is 1.40.

A line beam20having a defocus profile shown inFIGS.60and61is obtained by diffractive optical element12of the present example. Referring toFIG.62, focal depth Dzof line beam20is 51.4 mm. The relative light intensity on optical axis11in focal plane25(z=0) is 0.863. The optical axis direction relative light intensity of line beam20is more than or equal to 0.5 between first position P1and second position P2in the optical axis direction relative light intensity profile of line beam20.

Referring toFIG.63, 0.9 peak width W1of the short axis direction relative light intensity profile of line beam20is 0.051 mm, and 1/e2peak width W2of the short axis direction relative light intensity profile of line beam20is 0.148 mm. Aspect ratio W4/W2of line beam20is 9.12. Ratio W1/W2is 0.345, which is less than 0.400. Therefore, the short axis direction relative light intensity profile of line beam20does not have a flat top shape.

A diffractive optical element12of Example 6 includes a phase pattern13shown inFIGS.65to68. Phase pattern13of the present example is a phase pattern in which first phase pattern14shown inFIGS.9to12and a second phase pattern15shown inFIG.69are overlapped with each other. Second phase pattern15of the present example is similar to second phase pattern15of Example 1, but width d of central phase pattern15aof the present example is 3.46 mm. In the present example, coefficient C is 1.50.

A line beam20having a defocus profile shown inFIGS.70and71is obtained by diffractive optical element12of the present example. Referring toFIG.72, focal depth Dzof line beam20is 54.1 mm. The relative light intensity on optical axis11in focal plane25(z=0) is 0.831. The optical axis direction relative light intensity of line beam20is more than or equal to 0.5 between first position P1and second position P2in the optical axis direction relative light intensity profile of line beam20.

Referring toFIG.73, 0.9 peak width W1of the short axis direction relative light intensity profile of line beam20is 0.057 mm, and 1/e2peak width W2of the short axis direction relative light intensity profile of line beam20is 0.153 mm. Aspect ratio W4/W2of line beam20is 8.82. Ratio W1/W2is 0.373, which is less than 0.400. Therefore, the short axis direction relative light intensity profile of line beam20does not have a flat top shape.

A diffractive optical element12of Example 7 includes a phase pattern13shown inFIGS.75to78. Phase pattern13of the present example is a phase pattern in which first phase pattern14shown inFIGS.9to12and a second phase pattern15shown inFIG.79are overlapped with each other. Second phase pattern15of the present example is similar to second phase pattern15of Example 1, but width d of central phase pattern15aof the present example is 3.35 mm. In the present example, coefficient C is 1.60.

A line beam20having a defocus profile shown inFIGS.80and81is obtained by diffractive optical element12of the present example. Referring toFIG.82, focal depth Dzof line beam20is 56.4 mm. The relative light intensity on optical axis11in focal plane25(z=0) is 0.804. The optical axis direction relative light intensity of line beam20is more than or equal to 0.5 between first position P1and second position P2in the optical axis direction relative light intensity profile of line beam20.

Referring toFIG.83, 0.9 peak width W1of the short axis direction relative light intensity profile of line beam20is 0.063 mm, and 1/e2peak width W2of the short axis direction relative light intensity profile of line beam20is 0.157 mm. Aspect ratio W4/W2of line beam20is 8.60. Ratio W1/W2is 0.401, which is more than or equal to 0.400. The short axis direction relative light intensity profile of line beam20is more than or equal to 0.9 between third position P3and fourth position P4in the short axis direction relative light intensity profile of line beam20. Therefore, the short axis direction relative light intensity profile of line beam20has a flat top shape.

A diffractive optical element12of Example 8 includes a phase pattern13shown inFIGS.85to88. Phase pattern13of the present example is a phase pattern in which first phase pattern14shown inFIGS.9to12and a second phase pattern15shown inFIG.89are overlapped with each other. Second phase pattern15of the present example is similar to second phase pattern15of Example 1, but width d of central phase pattern15aof the present example is 2.93 mm. In the present example, coefficient C is 2.10.

A line beam20having a defocus profile shown inFIGS.90and91is obtained by diffractive optical element12of the present example. Referring toFIG.92, focal depth Dzof line beam20is 71.3 mm. The relative light intensity on optical axis11in focal plane25(z=0) is 0.640. The optical axis direction relative light intensity of line beam20is more than or equal to 0.5 between first position P1and second position P2in the optical axis direction relative light intensity profile of line beam20.

Referring toFIG.93, 0.9 peak width W1of the short axis direction relative light intensity profile of line beam20is 0.098 mm, and 1/e2peak width W2of the short axis direction relative light intensity profile of line beam20is 0.179 mm. Aspect ratio W4/W2of line beam20is 7.54. Ratio W1/W2is 0.547, which is more than or equal to 0.400. The short axis direction relative light intensity profile of line beam20is more than or equal to 0.9 between third position P3and fourth position P4in the short axis direction relative light intensity profile of line beam20. Therefore, the short axis direction relative light intensity profile of line beam20has a flat top shape.

A diffractive optical element12of Example 9 includes a phase pattern13shown inFIGS.95to98. Phase pattern13of the present example is a phase pattern in which first phase pattern14shown inFIGS.9to12and a second phase pattern15shown inFIG.99are overlapped with each other. Second phase pattern15of the present example is similar to second phase pattern15of Example 1, but width d of central phase pattern15aof the present example is 2.86 mm. In the present example, coefficient C is 2.20.

A line beam20having a defocus profile shown inFIGS.100and101is obtained by diffractive optical element12of the present example. Referring toFIG.102, focal depth DZof line beam20is 73.4 mm. The relative light intensity on optical axis11in focal plane25(z=0) is 0.619. The optical axis direction relative light intensity of line beam20is more than or equal to 0.5 between first position P1and second position P2in the optical axis direction relative light intensity profile of line beam20.

Referring toFIG.103, 0.9 peak width W1of the short axis direction relative light intensity profile of line beam20is 0.091 mm, and 1/e2peak width W2of the short axis direction relative light intensity profile of line beam20is 0.182 mm. Aspect ratio W4/W2of line beam20is 7.42. In the short axis direction relative light intensity profile of line beam20, there is a portion in which the optical axis direction relative light intensity of line beam20is less than 0.9 between third position P3and fourth position P4in the short axis direction relative light intensity profile of line beam20. For example, in the short axis direction relative light intensity profile of line beam20, the short axis direction relative light intensity of line beam20on optical axis11(y=0) is 0.895. Therefore, the short axis direction relative light intensity profile of line beam20does not have a flat top shape.

A diffractive optical element12of Example 10 includes a phase pattern13shown inFIGS.105to108. Phase pattern13of the present example is a phase pattern in which first phase pattern14shown inFIGS.9to12and a second phase pattern15shown inFIG.109are overlapped with each other. Second phase pattern15of the present example is similar to second phase pattern15of Example 1, but width d of central phase pattern15aof the present example is 2.64 mm. In the present example, coefficient C is 2.60.

A line beam20having a defocus profile shown inFIGS.110and111is obtained by diffractive optical element12of the present example. Referring toFIG.112, focal depth DZof line beam20is 85.0 mm. The relative light intensity on optical axis11in focal plane25(z=0) is 0.515. The optical axis direction relative light intensity of line beam20is more than or equal to 0.5 between first position P1and second position P2in the optical axis direction relative light intensity profile of line beam20.

Referring toFIG.113, 0.9 peak width W1of the short axis direction relative light intensity profile of line beam20is 0.065 mm, and 1/e2peak width W2of the short axis direction relative light intensity profile of line beam20is 0.195 mm. Aspect ratio W4/W2of line beam20is 6.92. In the short axis direction relative light intensity profile of line beam20, there is a portion in which the optical axis direction relative light intensity of line beam20is less than 0.9 between third position P3and fourth position P4in the short axis direction relative light intensity profile of line beam20. For example, in the short axis direction relative light intensity profile of line beam20, the short axis direction relative light intensity of line beam20on optical axis11(y=0) is 0.748. Therefore, the short axis direction relative light intensity profile of line beam20does not have a flat top shape.

A diffractive optical element12of Example 11 includes a phase pattern13shown inFIGS.115to118. Phase pattern13of the present example is a phase pattern in which first phase pattern14shown inFIGS.9to12and a second phase pattern15shown inFIG.119are overlapped with each other. Second phase pattern15of the present example is similar to second phase pattern15of Example 1, but width d of central phase pattern15aof the present example is 2.58 mm. In the present example, coefficient C is 2.70.

A line beam20having a defocus profile shown inFIGS.120and121is obtained by diffractive optical element12of the present example. Referring toFIG.122, focal depth DZof line beam20is 85.7 mm. In the optical axis direction relative light intensity profile of line beam20, there is a portion in which the optical axis direction relative light intensity of line beam20is less than 0.5 between first position P1and second position P2in the optical axis direction relative light intensity profile of line beam20. For example, the relative light intensity on optical axis11in focal plane25(z=0) is 0.487.

Referring toFIG.123, 0.9 peak width W1of the short axis direction relative light intensity profile of line beam20is 0.061 mm, and 1/e2peak width W2of the short axis direction relative light intensity profile of line beam20is 0.198 mm. Aspect ratio W4/W2of line beam20is 6.82. In the short axis direction relative light intensity profile of line beam20, there is a portion in which the optical axis direction relative light intensity of line beam20is less than 0.9 between third position P3and fourth position P4in the short axis direction relative light intensity profile of line beam20. For example, in the short axis direction relative light intensity profile of line beam20, the short axis direction relative light intensity of line beam20on optical axis11(y=0) is 0.703. Therefore, the short axis direction relative light intensity profile of line beam20does not have a flat top shape.

FIG.19shows a change in focal depth DZwith respect to coefficient C and a change in the relative light intensity on the optical axis at z=0 with respect to coefficient C in each of Examples 1 to 11 and the Comparative Example. Table 1 shows the numerical values of the parameters of each of Examples 1 to 5 and the Comparative Example. Table 2 shows the numerical values of the parameters of each of Examples 6 to 11.

Modifications of the present embodiment will be described. In a first modification of the present embodiment, first phase pattern14and second phase pattern15may be formed in light incident surface12aof diffractive optical element12. That is, diffractive optical element12may include phase pattern13(seeFIGS.5to8) in which first phase pattern14and second phase pattern15are overlapped with each other, and phase pattern13may be formed in light incident surface12aof diffractive optical element12. In a second modification of the present embodiment, first phase pattern14may be formed in light incident surface12aof diffractive optical element12, and second phase pattern15may be formed in light exit surface12bof diffractive optical element12. In a third modification of the present embodiment, first phase pattern14may be formed in light exit surface12bof diffractive optical element12, and second phase pattern15may be formed in light incident surface12aof diffractive optical element12.

Second Embodiment

A laser beam irradiation device1bof a second embodiment will be described with reference toFIGS.125and126. Laser beam irradiation device1bof the present embodiment has a configuration similar to that of laser beam irradiation device1of the first embodiment, but is mainly different therefrom in terms of the configuration of diffractive optical device10.

Diffractive optical device10of the present embodiment includes a first diffractive optical element (diffractive optical element12) and a second diffractive optical element (diffractive optical element17). Diffractive optical element12and diffractive optical element17are disposed along optical axis11of diffractive optical device10. Diffractive optical element12is disposed on the light incident side of laser beam6relative to diffractive optical element17. First phase pattern14is formed in diffractive optical element12, and second phase pattern15is formed in diffractive optical element17. Particularly, first phase pattern14is formed in one of light incident surface12aor light exit surface12bof diffractive optical element12. Second phase pattern15is formed in one of light incident surface17aor light exit surface17bof diffractive optical element17.

In a modification of the present embodiment, first phase pattern14may be formed in diffractive optical element17, and second phase pattern15may be formed in diffractive optical element12. Particularly, first phase pattern14is formed in one of light incident surface17aor light exit surface17bof diffractive optical element17. Second phase pattern15is formed in one of light incident surface12aor light exit surface12bof diffractive optical element12.

The first and second embodiments and modifications thereof disclosed herein are illustrative and non-restrictive in any respect. The scope of the present invention is defined by the terms of the claims, rather than the first and second embodiments and modifications thereof described above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

REFERENCE SIGNS LIST

1,1b: laser beam irradiation device;5: laser light source;6: laser beam;10: diffractive optical device;11: optical axis;12,17: diffractive optical element;12a,17a: light incident surface;12b,17b: light exit surface;13: phase pattern;14: first phase pattern;15: second phase pattern;15a: central phase pattern;15b: peripheral phase pattern;20: line beam;21: light having first order;22: light having second order;23: first focal plane;24: second focal plane;25: focal plane;30: workpiece.