Method for adjusting spherical aberration of an objective optical system, objective optical system, and laser processing apparatus

Abstract

To provide a method of adjusting spherical aberration of an objective optical system, which allows for keeping a focal point in a medium at a diffraction limit when the focal point is moved in a depth direction of the medium, an objective optical system, and a laser processing device.SOLUTION: A method of adjusting spherical aberration of an objective optical system (100A, 100B) comprising an objective lens (104) and a diopter adjustment optical system (102A, 102B) located on a side opposite a medium with respect to the objective lens is provided, the method comprising changing the divergence or convergence of a laser beam using the diopter adjustment optical system to change a focal point depth in the medium while maintaining a diffraction limit of the objective optical system.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for adjusting spherical aberration of an objective optical system, an objective optical system, and a laser processing apparatus, and particularly relates to a technique for spherical aberration correction applicable to an objective optical system such as a condenser lens for laser processing or a microscope objective lens that forms a condensing point inside a medium.

Background Art

[0002] In laser processing, there is a method of condensing light not near the surface of the processing object but inside it to generate a laser processing region inside. In order to ensure the condensing performance of the condenser lens for laser processing, it is necessary to narrow the beam to the diffraction limit at the condensing point inside the processing object. Also, in a laser microscope, there are cases where focusing is performed inside the specimen of the observation object for observation. Regarding the resolution of the microscope objective lens, similar to the case of the condenser lens for laser processing, it is necessary to narrow the beam to the diffraction limit at the in-focus point inside the specimen.

[0003] In the condenser lens for laser processing and the microscope objective lens, since the direction of the light rays between the condensing point and the in-focus point is only different, in the following description, the condenser lens and the objective lens may be simply described as the objective lens, and the processing object through which light passes and the specimen of the observation object may be described as the medium or the transparent medium.

[0004] In a laser processing apparatus, the spherical aberration of the light rays converged in the medium changes with the change in the thickness of the medium through which the light passes (the depth of the condensing point). Therefore, when changing the condensing point in the medium, in order to adjust the spherical aberration caused by the change in the thickness of the medium through which the light rays pass, a spherical aberration adjustment mechanism may be provided inside the objective lens. For example, Patent Document 1 discloses a technique for correcting spherical aberration generated by the thickness variation of a cover glass in a microscope objective lens.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

[0006] Incidentally, when the focal point is changed within a medium, spherical aberration occurs not only due to the change in the thickness of the medium through which the light rays pass, but also on the objective lens side. Therefore, in order to ensure the focusing performance or resolution of the objective lens, it is necessary to eliminate the spherical aberration that occurs on the objective lens side.

[0007] The present invention has been made in view of these circumstances, and aims to provide a method for adjusting the spherical aberration of an objective optical system, an objective optical system, and a laser processing apparatus that can maintain the focal point in the medium at the diffraction limit when the focal point is moved in the depth direction of the medium. [Means for solving the problem]

[0008] To achieve the above objective, a first aspect of the present invention provides a method for adjusting the spherical aberration of an objective optical system, comprising an objective lens and a diopter adjustment optical system disposed on the opposite side of the medium from the objective lens, wherein the diopter adjustment optical system changes the divergence or convergence of the laser beam, thereby changing the depth of the focal point in the medium while maintaining the diffraction limit of the objective optical system.

[0009] A second aspect of the present invention is, in the first aspect, the positive power applied to the diopter-adjustable optical system is increased as the distance from the surface of the medium to the focal point within the medium decreases, and the absolute value of the negative power applied to the diopter-adjustable optical system is increased as the distance from the surface of the medium to the focal point within the medium increases.

[0010] A third aspect of the present invention is that, in the first or second aspect, the refractive index of the medium is 1.7 or higher.

[0011] A fourth aspect of the present invention provides an objective optical system comprising an objective lens and a diopter adjustment optical system disposed on the opposite side of the medium from the objective lens, which changes the divergence or convergence of the laser beam and changes the depth of the focal point in the medium while maintaining the diffraction limit.

[0012] A fifth aspect of the present invention relates to an objective optical system, in the fourth aspect, in which the diopter adjustment optical system includes one of a variable focal length lens, a transmissive spatial light modulator, a deformable mirror, and a reflective spatial light modulator.

[0013] In the sixth aspect of the present invention, the objective optical system has an objective lens with a numerical aperture of 0.6 to 0.9, according to the fourth or fifth aspect.

[0014] The objective optical system according to the seventh aspect of the present invention, in any of the fourth to sixth aspects, includes an optical system disposed between the diopter adjustment optical system and the objective lens, which relays the diopter adjustment optical system so that it is conjugate with the objective lens.

[0015] An objective optical system according to the eighth aspect of the present invention includes, in any of the fourth to seventh aspects, a spherical aberration adjustment mechanism that adjusts the spherical aberration of the objective lens by moving some of the lenses in the objective lens in the optical axis direction.

[0016] A laser processing apparatus according to the ninth aspect of the present invention comprises an objective optical system of any fourth to eighth type, which focuses laser light onto a focal point in a medium. [Effects of the Invention]

[0017] According to the present invention, the depth of the focal point in the medium can be changed while maintaining it at the diffraction limit. [Brief explanation of the drawing]

[0018] [Figure 1] Figure 1 is a side view showing an example of an objective optical system according to one embodiment of the present invention. [Figure 2]FIG. 2 is a side view for explaining an example of changing the depth of the light collection point. [Figure 3] FIG. 3 is a diagram showing an example of spherical aberration caused by changing the depth of the light collection point. [Figure 4] FIG. 4 is a ray diagram when the distance between the objective lens and the medium is changed. [Figure 5] FIG. 5 is a diagram showing an example of spherical aberration that occurs when the distance between the objective lens and the medium is changed. [Figure 6] FIG. 6 is a diagram for explaining an example of adjusting spherical aberration. [Figure 7] FIG. 7 is a diagram showing the adjustment result of spherical aberration adjustment. [Figure 8] FIG. 8 is a graph showing the result of calculating the spherical aberration when the depth of the light collection point is changed in Example 1. [Figure 9] FIG. 9 is a ray diagram of Examples (a1) to (a5) shown in FIG. 8. [Figure 10] FIG. 10 is a cross-sectional view showing the objective lens according to Example 1. [Figure 11] FIG. 11 is a table showing the lens data of the objective lens according to Example 1. [Figure 12] FIG. 12 is a table showing the correspondence between the power given to the diopter conversion element and the distance from the objective lens to the surface of the medium. [Figure 13] FIG. 13 is a graph showing the spherical aberration that occurs when positive and negative powers are given to the diopter conversion element. [Figure 14] FIG. 14 is a ray diagram of Examples (b1) to (b5) shown in FIGS. 12 and 13. [Figure 15] FIG. 15 is a graph showing the adjustment result of the spherical aberration according to Example 1. [Figure 16] FIG. 16 is a ray diagram of Examples (c1) to (c5) shown in FIG. 15. [Figure 17] FIG. 17 is a table showing the stray ratio in Examples (c1) to (c5) shown in FIG. 15. [[ID=^47]] [Figure 18]Figure 18 is a graph showing the results of calculating spherical aberration when the depth of the focal point is changed in Example 2. [Figure 19] Figure 19 shows the ray diagrams for examples (d1) to (d5) shown in Figure 18. [Figure 20] Figure 20 is a cross-sectional view showing the objective lens according to Example 2. [Figure 21] Figure 21 is a table showing the lens data of the objective lens according to Example 2. [Figure 22] Figure 22 is a table showing the correspondence between the power applied to the diopter conversion element and the distance from the objective lens to the surface of the medium. [Figure 23] Figure 23 is a graph showing the spherical aberration that occurs when positive and negative power is applied to a diopter conversion element. [Figure 24] Figure 24 is a ray diagram of the examples (e1) to (e5) shown in Figures 22 and 23. [Figure 25] Figure 25 is a graph showing the results of adjusting spherical aberration according to Example 2. [Figure 26] Figure 26 shows the ray diagrams for examples (f1) to (f5) shown in Figure 25. [Figure 27] Figure 27 is a table showing the Strehl ratios for examples (f1) to (f5) shown in Figure 25. [Figure 28] Figure 28 is a schematic diagram showing a laser processing apparatus according to one embodiment of the present invention. [Figure 29] Figure 29 is a block diagram showing the configuration of the control device. [Modes for carrying out the invention]

[0019] Hereinafter, embodiments of the spherical aberration adjustment method for an objective optical system, the objective optical system, and the laser processing apparatus according to the present invention will be described with reference to the attached drawings.

[0020] [Objective optical system] Figure 1 is a side view showing an example of an objective optical system according to one embodiment of the present invention.

[0021] The objective optical systems (100A and 100B) shown in Figure 1 focus the laser beam LB onto a focal point (F0 to F3) inside the medium (working object for laser processing) W through which the laser beam LB is transmitted.

[0022] The objective optical system 100A shown in Figure 1(a) includes a transmissive diopter conversion element 102A and an objective lens 104. On the other hand, the objective optical system 100B shown in Figure 1(b) includes a reflective diopter conversion element 102B and an objective lens 104. Note that reference numeral 106 in Figure 1(b) indicates a mirror (for example, a total internal reflection mirror) provided for relaying (bending) the optical path.

[0023] In both examples, the diopter conversion elements (102A and 102B) are positioned on the incident light beam side (opposite side of the medium W) of the objective lens 104. By providing positive or negative refractive power, the diopter conversion elements (102A and 102B) can focus or diverge the laser beam LB, thereby changing the degree of convergence or divergence of the laser beam LB. This makes it possible to change the position of the focal point (F0~F3) in the medium W along the optical axis in the depth direction while maintaining the diffraction limit.

[0024] Here, the diopter conversion elements (102A and 102B) are an example of a diopter-adjustable optical system, capable of continuously varying positive and negative power. When the power adjustment function is disabled, the diopter conversion elements (102A and 102B) are optically equivalent to a convex or concave lens.

[0025] As the transmissive diopter conversion element 102A, for example, a variable focal length lens or a transmissive spatial light modulator (LCOS: Liquid Crystal on Silicon) can be used. As the reflective diopter conversion element 102B, for example, a deformable mirror or a reflective spatial light modulator (LCOS) can be used.

[0026] The objective lens 104 focuses the laser beam LB onto a focal point (F0~F3) within the medium W.

[0027] In this embodiment, the depth of the focal point (F0 to F3) in the medium W can be changed by the diopter conversion elements (102A and 102B). As shown in Figure 1(a), the focal point when the power of diopter conversion element 102A is set to 0 is defined as F0. When the diopter conversion element 102A has the power of a concave lens on its incident side, the focal point F1 is located further (deeper) from the surface Wa of the medium W than the focal point F0 when the power of diopter conversion element 102A is set to 0. On the other hand, when the diopter conversion element 102A has the power of a convex lens on its incident side, the focal point F2 is located closer (shallower) from the surface Wa of the medium W than the focal point F0 when the power of diopter conversion element 102A is set to 0.

[0028] As described above, changing the depth of the focal point (F0~F3) within the medium W causes spherical aberration. The objective lens 104 is designed to satisfy the sinusoidal condition and minimize spherical aberration and coma aberration. When using such an objective lens 104, changing the position of the emission point of the incident light beam in order to shift the focal point (F0~F3) in the direction of the optical axis causes spherical aberration.

[0029] In this embodiment, the spherical aberration generated in the medium W as the depth of the focal point (F0 to F3) changes is canceled out with the spherical aberration generated in the objective lens 104, thereby achieving focal point movement within the medium W while maintaining focal point performance up to the diffraction limit.

[0030] In Figure 1, no optical elements are placed between the diopter conversion elements (102A and 102B) and the objective lens 104, but the present invention is not limited to this. An optical system for relaying the laser beam LB (relay optical system) may be provided between the diopter conversion elements (102A and 102B) and the objective lens 104. In this case, the diopter conversion elements (102A and 102B) and the pupil of the objective lens 104 should be optically conjugate.

[0031] (Changing the depth of the focal point) Next, we will explain how to change the depth of the focal point, using the case of an objective optical system 100A equipped with a transmissive diopter conversion element 102A as an example. Figure 2 is a side view illustrating an example of changing the depth of the focal point.

[0032] As shown in Figure 2(a), when the power of the diopter conversion element 102A is set to 0, the laser beam LB travels in a straight line without being bent by the diopter conversion element 102A. Here, the objective lens 104 is designed to focus the light at a focal point F0 at a predetermined depth position in the medium W with spherical aberration corrected.

[0033] In Figure 2(a), the distance WD from the objective lens 104 to the surface Wa of the medium W is defined as WD = WD1, and the distance d from the surface Wa of the medium W to the focal point F0 (depth of the focal point) is defined as d = d1.

[0034] As shown in Figure 2(b), when the diopter conversion element 102A has the power of a concave lens on the incident side, the laser beam LB is bent by the diopter conversion element 102A. In this case, in order for the distance d from the surface Wa of the medium W to the focal point F0 to be d1, the distance WD from the objective lens 104 to the surface Wa of the medium W must be made longer than WD1, WD2 (WD2 > WD1).

[0035] As described above, when the diopter conversion element 102A has the power of a concave lens on the incident side, negative spherical aberration occurs on the objective lens 104 side, as shown in Figure 3.

[0036] (Correction of spherical aberration) Next, we will explain how to adjust for spherical aberration.

[0037] Figure 4(a) is a ray diagram when using objective lens 104 (see Figure 2(a)), which is designed so that spherical aberration is minimized at a distance d1 from the surface Wa of the medium W to the focal point F0.

[0038] As shown in Figure 4(b), when the objective lens 104 is brought closer to the medium W, the distance d2 from the surface Wa of the medium W to the focal point F becomes longer than d1.

[0039] As described above, when the objective lens 104 is brought close to the medium W, positive spherical aberration occurs on the objective lens 104 side, as shown in Figure 5.

[0040] In this embodiment, the spherical aberration of the objective optical system 100A is adjusted by canceling out the negative spherical aberration shown in Figure 3 and the positive spherical aberration shown in Figure 5. Figure 6 is a diagram illustrating an example of spherical aberration adjustment.

[0041] In the example shown in Figure 6, the diopter conversion element 102A has the power of a concave lens on the incident side. In this case, in order for the distance d from the surface Wa of the medium W to the focal point F0 to be d1, the distance WD from the objective lens 104 to the surface Wa of the medium W needs to be longer than WD1 (see Figure 2(b)). However, in the example shown in Figure 6, the distance WD from the objective lens 104 to the surface Wa of the medium W is constant at WD1, and the diopter conversion element 102A is adjusted so that the depth d of the focal point F0 in the medium W is d=d1.

[0042] Figure 7 shows the results of the adjustment for spherical aberration. As shown in Figure 7, it can be seen that the spherical aberration caused by changing the depth of the focal point by giving power to the diopter conversion element 102A (Figure 3) and the spherical aberration caused by bringing the objective lens 104 closer to the medium W (Figure 5) cancel each other out.

[0043] As described above, according to this embodiment, by canceling out the spherical aberration generated in the medium W and the spherical aberration generated in the objective lens 104, it is possible to achieve a shift in the focal point within the medium W while maintaining the focusing performance up to the diffraction limit.

[0044] Here, when the refractive index of the medium W is 1.7 or higher, the spherical aberration generated by the depth of the focal point in the medium W and the spherical aberration generated by the objective lens 104 cancel each other out, ensuring a light-gathering performance that can be described as aberration-free, with a Strehr ratio of 0.8 or higher (see Examples 1 and 2).

[0045] Here, the Strehr ratio refers to the ratio of the light-gathering ratio in an aberration-free optical system to the light-gathering ratio in an aberration-free optical system, where the light-gathering ratio at the image plane is set to 100%. Generally, a Strehr ratio of 80% is called the diffraction limit, and if the Strehr ratio exceeds 80%, the objective optical system 100A is considered to have sufficient light-gathering performance.

[0046] Furthermore, when the refractive index of the medium is 3 or greater, the depth of the focal point can be changed without changing the distance between the objective lens 104 and the medium W. Autofocus measures the reflection of the surface Wa of the medium W, but when the refractive index of the medium is 3 or greater, there is no need to change the distance between the objective lens 104 and the medium W, so the depth of the focal point within the medium W can be changed without changing the autofocus conditions.

[0047] Furthermore, as the objective lens 104, it is also possible to use an objective lens with a corrective ring, which is an objective lens with a spherical aberration adjustment mechanism (see, for example, Patent Document 1). In an objective lens with a corrective ring, spherical aberration can be adjusted by adjusting the interplanar spacing by moving at least one group of lenses among the multiple lenses constituting the objective lens 104 along the optical axis. This allows adjustment of the depth to which spherical aberration is corrected within the medium W, thereby expanding the range from near the surface to deeper areas.

[0048] Here, when the focal length of the objective lens 104 is f, the numerical aperture is NA (Numerical Aperture), and the refractive index (absolute refractive index) of the medium W is n, it is preferable that the following condition (1) is satisfied. If the value of f·NA / n is less than 0.1, a sufficient NA cannot be secured, and if it is greater than 1.4, the design of the objective lens 104 becomes difficult.

[0049]

number

[0050] Furthermore, it is preferable that the following condition (2) is satisfied.

[0051]

number

[0052] By satisfying the above conditions, it is possible to design an objective lens 104 that allows for an appropriate distance between the objective lens 104 and the medium W.

[0053] Furthermore, the numerical aperture (NA) of the objective lens 104 is preferably 0.6 or more and less than 0.9. This numerical range for the numerical aperture is based on the following reasons. Specifically, if the numerical aperture of the objective lens 104 is less than 0.6, it becomes impossible to focus the laser beam LB and sufficiently increase the energy density, making it difficult to obtain the energy necessary for processing within the medium W. On the other hand, if the numerical aperture of the objective lens 104 is 0.9 or more, the optical design difficulties of the objective lens 104 make it impossible to set a long distance WD to the surface Wa of the medium W. The required distance WD is approximately 1 mm or more.

[0054] [Example 1] In Example 1, the wavelength of the laser light LB is 1064 nm, the numerical aperture (NA) of the objective lens 104 is 0.65, and the refractive index of the medium W is 3.55. The objective lens 104 used is designed so that spherical aberration is minimized when the depth d of the focal point in the medium W is d = 0.5 mm.

[0055] Figure 8 is a graph showing the results of calculating spherical aberration when the depth d of the focal point is changed in Example 1. Figure 9 is the ray diagram for Examples (a1) to (a5) shown in Figure 8.

[0056] As shown in Figure 8, negative spherical aberration occurs when the depth d of the focal point is shorter than 0.5 mm (example (a3)) (examples (a1) and (a2)), and conversely, positive spherical aberration occurs when the depth d of the focal point is greater than 0.5 mm (examples (a4) and (a5)).

[0057] Figure 10 is a cross-sectional view showing the objective lens according to Example 1. The objective lens 104 according to Example 1 has a wavelength of 1064 nm, an NA of 0.65, and a focal length of 3.6 mm.

[0058] As shown in Figure 10, the objective lens 104 includes five lenses 104A to 104E. The lenses included in the objective lens 104 are designated 104A to 104E in order from the incident side (upstream side) of the laser beam LB.

[0059] Furthermore, the diopter conversion element 102A is positioned at a distance L1 of 8.4 mm from the uppermost surface of the objective lens 104 (surface S1 of lens 104A).

[0060] Figure 11 shows the lens data for the objective lens 104 according to Example 1. The table in Figure 11 shows the radii of curvature of surfaces S1 to S10 of the five lenses 104A to 104E, the spacing between the next surface downstream (surface spacing), and the refractive indices of lenses 104A to 104E.

[0061] The objective lens 104 according to Example 1 is designed so that when parallel laser light LB is incident on it, spherical aberration is minimized at a position 0.5 mm from the surface Wa of the medium W with a refractive index of 3.55.

[0062] Figure 12 is a table showing the change in the distance WD from the objective lens 104 to the surface Wa of the medium W when positive and negative power is applied to the diopter conversion element 102A, so that the depth d of the focal point is d = 0.5 mm. Figure 13 is a graph showing the spherical aberration that occurs when positive and negative power is applied to the diopter conversion element 102A, and Figure 14 is a ray diagram of examples (b1) to (b5) shown in Figures 12 and 13.

[0063] In example (b1), a positive power of +7.1D is applied to the diopter conversion element 102A. Converting the positive power of +7.1D to focal length yields the following:

[0064]

number

[0065] In other words, in example (b1), the diopter conversion element 102A becomes a positive lens with a focal length of approximately 141 mm. In this case, as shown in Figure 13 (example (b1)), spherical aberration appears on the positive side. Furthermore, as shown in Figure 12, the distance WD from the objective lens 104 to the surface Wa of the medium W, which is required to set the depth d of the focal point to d = 0.5 mm, is shorter at 2.75 mm compared to example (b3) (WD = 2.85 mm) where no power is supplied to the diopter conversion element 102A.

[0066] In example (b3), the power of the diopter conversion element 102A is zero, and converting zero power to focal length results in infinity. In this case, it is equivalent to having parallel light incident on the objective lens 104. As shown in Figure 13 (example (b3)), spherical aberration is minimized. Also, as mentioned above, in example (b3), WD = 2.85 (see Figure 12).

[0067] In example (b5), a negative power of -8.4D is applied to the diopter conversion element 102A. Converting the negative power of -8.4D to focal length yields the following:

[0068]

number

[0069] In other words, in example (b5), the diopter conversion element 102A becomes a negative lens with a focal length of approximately 119 mm. In this case, as shown in Figure 13 (example (b5)), spherical aberration appears on the negative side. And, as shown in Figure 12, the distance WD becomes longer at 2.95 mm compared to example (b3) (WD = 2.85 mm).

[0070] As described above, in the example shown in Figure 12, the distance WD decreases as the power applied to the diopter conversion element 102A increases.

[0071] Figure 15 is a graph showing the adjustment results for spherical aberration according to Example 1, and Figure 16 is a ray diagram of Examples (c1) to (c5) shown in Figure 15.

[0072] In the example shown in Figure 15, the shallower the depth of the focal point (the shorter the distance from the surface of the medium W) compared to the zero-power example (c3) of the diopter conversion element 102A, the greater the positive power supplied to the diopter conversion element 102A. On the other hand, the deeper the depth of the focal point (the longer the distance from the surface of the medium W) compared to example (c3), the greater the absolute value of the negative power supplied to the diopter conversion element 102A. As a result, as shown in Figure 15, the spherical aberration generated in the medium W shown in Figure 8 and the spherical aberration generated in the objective lens 104 shown in Figure 13 cancel each other out.

[0073] Note that while the distance WD was varied in Figure 13, in Figure 15, the position d of the focal point within the medium W was calculated by fixing the WD to 2.85 mm when the power of the diopter conversion element 102A was set to zero.

[0074] As shown in Figure 16, in example (c1), WD = 2.85 mm and d = 0.145 mm, and in example (c3), the power of the diopter conversion element 102A is zero, WD = 2.85 mm and d = 0.5 mm. And in example (c5), WD = 2.85 mm and d = 0.855 mm.

[0075] Figure 17 is a table showing the Strehl ratios in Examples (c1) to (c5). In Example 1, in all of Examples (c1) to (c5), the spherical aberration is canceled out and reduced, resulting in a Strehl ratio of 0.99. In other words, in all of Examples (c1) to (c5), the objective optical system 100A exceeds the diffraction limit of 0.8, indicating that it has sufficient light-gathering performance.

[0076] [Example 2] In Example 2, the wavelength of the laser beam LB is 1064 nm, the NA of the objective lens 104 is 0.83, and the refractive index of the medium W is 3.55. An objective lens is used that is designed to minimize spherical aberration when the depth d of the focal point in the medium W is d = 0.5 mm.

[0077] Figure 18 is a graph showing the results of calculating spherical aberration when the depth d of the focal point is changed in Example 2. Figure 19 is the ray diagram for Examples (d1) to (d5) shown in Figure 18.

[0078] As shown in Figure 18, negative spherical aberration occurs when the depth d of the focal point is less than 0.5 mm (e.g., (d3)), (e.g., (d1) and (d2)), and conversely, positive spherical aberration occurs when it is greater than 0.5 mm (e.g., (d4) and (d5)).

[0079] Figure 20 is a cross-sectional view showing the objective lens according to Example 2. The objective lens 104 according to Example 2 has a wavelength of 1064 nm, an NA of 0.83, and a focal length of 1.8 mm.

[0080] As shown in Figure 20, the objective lens 104 includes six lenses 104A to 104F. The lenses included in the objective lens 104 are designated 104A to 104F in order from the incident side (upstream side) of the laser beam LB.

[0081] Furthermore, the diopter conversion element 102A is positioned at a distance L1 of 8.4 mm from the uppermost surface of the objective lens 104 (surface S1 of lens 104A).

[0082] Figure 21 shows the lens data for the objective lens 104 according to Example 2. The table in Figure 21 shows the radius of curvature of surfaces S1 to S12 of the six lenses 104A to 104F, the spacing between the next surface downstream (surface spacing), and the refractive indices of lenses 104A to 104F.

[0083] The objective lens 104 according to Example 2 is designed so that when parallel laser light LB is incident on it, spherical aberration is minimized at a position 0.5 mm from the surface Wa of the medium W with a refractive index of 3.55.

[0084] Figure 22 is a table showing the change in the distance WD from the objective lens 104 to the surface Wa of the medium W when positive and negative power is applied to the diopter conversion element 102A, so that the depth d of the focal point is d = 0.5 mm. Figure 23 is a graph showing the spherical aberration that occurs when positive and negative power is applied to the diopter conversion element 102A, and Figure 24 is a ray diagram of examples (e1) to (e5) shown in Figures 22 and 23.

[0085] In example (e1), a positive power of +15.6D is applied to the diopter conversion element 102A. Converting the positive power of +15.6D to focal length yields the following equation.

[0086]

number

[0087] In other words, in example (e1), the diopter conversion element 102A becomes a positive lens with a focal length of approximately 64 mm. In this case, as shown in Figure 23 (example (e1)), spherical aberration appears on the positive side. Furthermore, as shown in Figure 22, the distance WD from the objective lens 104 to the surface Wa of the medium W, which is required to set the depth d of the focal point to d = 0.5 mm, is shorter at 1.44 mm compared to example (e3) (WD = 1.5 mm) where no power is applied to the diopter conversion element 102A.

[0088] In example (e3), the power of the diopter conversion element 102A is zero, and converting zero power to focal length results in infinity. In this case, it is equivalent to directing parallel light into the objective lens 104. As shown in Figure 13 (example (e3)), spherical aberration is minimized. Also, as mentioned above, in example (e3), WD = 1.5 (see Figure 22).

[0089] In example (e5), a negative power of -22.7D is applied to the diopter conversion element 102A. Converting the negative power of -22.7D to focal length yields the following:

[0090]

number

[0091] In other words, in example (e5), the diopter conversion element 102A becomes a negative lens with a focal length of approximately 44 mm. In this case, as shown in Figure 23 (example (e5)), spherical aberration appears on the negative side. And, as shown in Figure 22, the distance WD becomes longer at 1.56 mm compared to example (e3) (WD=1.5 mm).

[0092] As described above, in the example shown in Figure 22, the distance WD decreases as the power applied to the diopter conversion element 102A increases.

[0093] Figure 25 is a graph showing the adjustment results for spherical aberration in Example 2, and Figure 26 is a ray diagram of Examples (f1) to (f5) shown in Figure 25.

[0094] In the example shown in Figure 25, the shallower the depth of the focal point (the shorter the distance from the surface of the medium W), the greater the positive power supplied to the diopter conversion element 102A compared to the example where the power of the diopter conversion element 102A is zero (f3). On the other hand, the deeper the depth of the focal point (the longer the distance from the surface of the medium W), the greater the absolute value of the negative power supplied to the diopter conversion element 102A. As a result, as shown in Figure 25, the spherical aberration occurring in the transparent medium shown in Figure 18 and the spherical aberration occurring in the objective lens 104 shown in Figure 23 cancel each other out.

[0095] Note that while the distance WD was varied in Figure 23, in Figure 25, the position d of the focal point within the medium W was calculated with WD fixed at 1.5 mm when the power of the diopter conversion element 102A was set to zero.

[0096] As shown in Figure 26, in example (f1), WD = 1.5 mm and d = 0.287 mm, and in example (f3), the power of the diopter conversion element 102A is zero, WD = 1.5 mm and d = 0.5 mm. And in example (f5), WD = 1.5 mm and d = 0.713 mm.

[0097] Figure 27 is a table showing the Strehr ratios in Examples (f1) to (f5). In Example 2, in all of Examples (f1) to (f5), spherical aberration is canceled out and reduced, and the Strehr ratio is 0.9 or higher in all cases. In other words, in all of Examples (f1) to (f5), the objective optical system 100A exceeds the diffraction limit of 0.8, indicating that it has sufficient light-gathering performance.

[0098] Furthermore, according to the above embodiment, there is no need for a mechanism (for example, a mechanical movement mechanism) to move the light emission position of the laser light source LB.

[0099] In the method of changing the depth d of the focal point by moving the light emission position of the light source, the amount of movement of the focal point follows the imaging formula for a single lens shown below.

[0100]

number

[0101] Here, the objective lens is assumed to be an ideal thin lens with a thickness of 0, and its focal length is f, the distance from the light source to the objective lens is a, and the distance from the objective lens to the point of convergence is b.

[0102] For example, in Patent Document 1, the configuration is such that a is greater than b. Therefore, changing b requires a large change in a. For example, when f = 4 mm, if a = 200 mm in the initial state, then b = 4.08 mm. If the light source is moved 50 mm from the initial state to a = 150 mm, then b = 4.11 mm. In this case, the difference of b, 0.03 mm, corresponds to the amount of movement of the focal point.

[0103] As described above, in the method of moving the emission position of the light source, the amount of movement of the focal point is small compared to the amount of movement of the light source. That is, the larger the distance the focal point is moved, the larger the amount of movement of the light source becomes, and the mechanism for moving the light source may become large-scale. In the above embodiment, since there is no need to provide a mechanism for moving the emission position of the light source of the laser beam LB in order to change the depth d of the focal point, the device can be simplified.

[0104] [Laser processing equipment] Next, an example of a laser processing apparatus equipped with the objective optical systems (100A, 100B) according to the above embodiment will be described with reference to Figures 28 and 29.

[0105] (Configuration of laser processing equipment) Figure 28 is a schematic diagram showing a laser processing apparatus according to one embodiment of the present invention. As shown in Figure 28, the laser processing apparatus 10 of this embodiment comprises a stage 12, a processing apparatus body (optical system unit) 20, a processing lens 26, and a control device 50. In this embodiment, the processing apparatus body 20 and the control device 50 are configured separately, but the configuration is not limited to this, and the processing apparatus body 20 may include part or all of the control device 50.

[0106] Stage 12 is used to hold the workpiece by suction. Stage 12 is configured to be movable in the X and θ directions by a stage drive mechanism 28 (see Figure 29). The stage drive mechanism 28 can be composed of various mechanisms, such as a ball screw mechanism or a linear motor mechanism. The operation of the stage drive mechanism 28 is controlled by a control device 50 (movement control unit 54). In Figure 28, the three directions X, Y, and Z are orthogonal to each other, of which the X and Y directions are horizontal, and the Z direction is vertical. The θ direction is a rotational direction with the vertical axis (Z axis) as the axis of rotation.

[0107] In this embodiment, a semiconductor wafer (hereinafter referred to as "wafer") W, such as a silicon wafer, is used as the workpiece. The wafer W is divided into multiple regions by cutting lines arranged in a grid pattern, and various devices constituting a semiconductor chip are formed in each of these divided regions. In this embodiment, the case in which wafer W is used as the workpiece is described, but the present invention is not limited to this, and for example, glass substrates, piezoelectric ceramic substrates, glass substrates, etc. can also be used.

[0108] A wafer W has a backgrind tape (hereinafter referred to as BG tape) with adhesive material attached to the surface (device surface) on which the device is formed, and is placed on the stage 12 with the back surface facing upwards. There are no particular restrictions on the thickness of the wafer W, but it is typically 700 μm or more, and more typically 700 μm to 800 μm.

[0109] Alternatively, the wafer W may be placed on the stage 12 with a dicing tape having an adhesive material attached to one side, and the wafer W integrated with the frame via this dicing tape.

[0110] The processing apparatus body 20 comprises a housing 21, a laser light source 22, a spatial light modulator 24, a relay optical system 30, a beam expander 32, and a λ / 2 wave plate 34.

[0111] Inside the housing 21 are a laser light source 22, a spatial light modulator 24, a relay optical system 30, a beam expander 32, and a λ / 2 wave plate 34. The laser light source 22 may also be located outside the housing 21 (for example, on the top or side of the housing 21). A processing lens 26 is detachably attached to the bottom of the housing 21.

[0112] The processing apparatus body 20 is configured to be movable in the Y and Z directions by a body drive mechanism 29 (see Figure 2). The body drive mechanism 29 can be composed of various mechanisms, such as a ball screw mechanism or a linear motor mechanism. The operation of the body drive mechanism 29 is controlled by a control device 50 (movement control unit 54). This allows the processing apparatus body 20 to be moved in the Y direction and also moved in the Z direction according to the processing position on the wafer W (the position where the laser processing area is formed). Therefore, the position of the focal point of the laser beam L focused by the processing lens 26 can be changed to form the laser processing area at a desired position on the wafer W.

[0113] The laser light source (IR laser light source) 22 emits laser light L for processing to form a laser processing area inside the wafer W. The emission operation of the laser light L by the laser light source 22 is controlled by the control device 50 (laser control unit 56). The conditions for the laser light L are, for example, a semiconductor laser-pumped Nd:YAG (Yttrium Aluminum Garnet) laser, a wavelength of 1.1 μm, and a laser light spot cross-section of 3.14 × 10⁻¹⁴. -8 cm 2 The oscillation mode is Q-switched pulse, the repetition frequency is 80-200 kHz, the pulse width is 180-370 ns, and the output power is 8 W.

[0114] The spatial light modulator 24 is a phase-modulation type spatial light modulator that has a light modulation surface consisting of multiple pixels (micro-modulation elements) arranged in two dimensions, and modulates the phase of light incident on the light modulation surface pixel by pixel. The spatial light modulator 24 is positioned at a position optically conjugate to the lens pupil (exit pupil) 26a of the processed lens 26. Based on a predetermined modulation pattern set by the spatial light modulator control unit 58 (described later), the spatial light modulator 24 modulates the phase of light incident on the light modulation surface pixel by pixel and emits the modulated light in a predetermined direction. For example, a reflective liquid crystal on silicon (LCOS) spatial light modulator (SLM) is used as the spatial light modulator 24. The operation of the spatial light modulator 24 and the modulation pattern presented by the spatial light modulator 24 are controlled by the control device 50 (spatial light modulator control unit 58). The modulation pattern may be a pattern (two-dimensional information) in which control values ​​(phase change amounts) corresponding to each of the multiple pixels constituting the optical modulation surface of the spatial light modulator 24 are distributed in two dimensions, or it may be something like coefficient information when the modulation within the modulation region (optical modulation surface) is expressed as a certain function.

[0115] The processing lens 26 is an objective lens (focusing optical system) that focuses the laser beam L into the wafer W. The numerical aperture (NA) of this processing lens 26 is, for example, 0.65.

[0116] The relay optical system 30 is provided in the optical path of the laser beam L between the spatial light modulator 24 and the processing lens 26. The relay optical system 30 has at least two lenses 30a and 30b (hereinafter referred to as "first lens 30a" and "second lens 30b"). The relay optical system 30 constitutes an afocal optical system (a bilaterally telecentric optical system) and projects the laser beam L modulated by the spatial light modulator 24 onto the processing lens 26. This relay optical system 30 is a bilaterally telecentric reduction optical system, and its projection magnification (hereinafter simply referred to as "magnification") is less than 1 (for example, 0.66).

[0117] The beam expander 32 expands the laser light L emitted from the laser light source 22 to a beam diameter suitable for the spatial light modulator 24. The λ / 2 wave plate 34 adjusts the polarization plane of the laser light incident on the spatial light modulator 24.

[0118] Although not shown in the diagram, the processing apparatus body 20 is also equipped with an alignment optical system for aligning with the wafer W, and an autofocus unit for maintaining a constant distance (working distance) between the wafer W and the processing lens 26.

[0119] The control device 50 is implemented by a general-purpose computer, such as a personal computer or a microcomputer.

[0120] The control unit 50 includes a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), and an input / output interface. In the control unit 50, various programs, such as control programs stored in ROM, are loaded into RAM, and the CPU executes the programs loaded into RAM. This realizes the functions of each part of the control unit 50 shown in Figure 2, and various arithmetic and control processes are executed via the input / output interface.

[0121] Figure 29 is a block diagram showing the configuration of the control device 50. As shown in Figure 29, the control device 50 functions as a main control unit 52, a motion control unit 54, a laser control unit 56, a spatial light modulator control unit 58, and a memory unit 60.

[0122] The main control unit 52 comprehensively controls each component of the control device 50 (including the movement control unit 54, laser control unit 56, spatial light modulator control unit 58, and memory unit 60).

[0123] The movement control unit 54 controls the relative movement between the stage 12 and the processing device body 20. The movement control unit 54 outputs control signals to the stage drive mechanism 28 to control the movement of the stage 12 in the X and θ directions, and outputs control signals to the main body drive mechanism 29 to control the movement of the processing device body 20 in the Y and Z directions.

[0124] The laser control unit 56 controls the emission of laser light L. The laser control unit 56 outputs control signals to the laser light source 22 that control the wavelength, pulse width, intensity, emission timing, and repetition frequency of the laser light L.

[0125] The spatial light modulator control unit 58 outputs a control signal to the spatial light modulator 24 that controls the operation of the spatial light modulator 24. In other words, the spatial light modulator control unit 58 controls the spatial light modulator 24 to present a predetermined modulation pattern.

[0126] The memory unit 60 is composed of external memory (e.g., hard disk or flexible disk) or internal memory (e.g., RAM or ROM made of semiconductor memory) provided in the control device 50.

[0127] The objective optical system (100A, 100B) according to this embodiment can be used as the processing lens 26 of the laser processing apparatus 10 described above. This makes it possible to ensure focusing performance regardless of the depth of the focusing point when forming laser processing regions at different depth positions within the wafer W as the medium W, and to ensure the processing accuracy of the laser processing region.

[0128] In the above embodiments, an example in which the objective optical system (100A, 100B) is applied to a laser processing apparatus 10 has been described, but the present invention is not limited thereto. The objective optical system (100A and 100B) according to the above embodiments can also be applied to microscope objective lenses. In this case, it becomes possible to ensure resolution regardless of the depth of focus in the specimen. [Explanation of Symbols]

[0129] 100A, 100B…Objective optical system; 102A, 102B…Diopter conversion element; 104…Objective lens; 106…Mirror

Claims

1. A method for adjusting the spherical aberration of an objective optical system comprising an objective lens and a diopter adjustment optical system disposed on the opposite side of the medium from the objective lens, By adding power to the aforementioned diopter-adjusting optical system, the depth of the laser beam's focusing point can be changed. By canceling out the spherical aberration generated in the medium due to the change in the depth of the focal point and the spherical aberration caused by the change in the distance from the objective lens to the surface of the medium, the diffraction limit of the objective optical system is maintained. The focal length f and numerical aperture NA of the objective lens are such that, with respect to the refractive index n of the medium, A method for adjusting the spherical aberration of an objective optical system that satisfies the following conditions.

2. A method for adjusting the spherical aberration of an objective optical system according to claim 1, wherein the positive power applied to the diopter adjustment optical system is increased as the distance from the surface of the medium to the focal point within the medium decreases, and the absolute value of the negative power applied to the diopter adjustment optical system is increased as the distance from the surface of the medium to the focal point within the medium increases.

3. A method for adjusting the spherical aberration of an objective optical system according to claim 1 or 2, wherein the refractive index of the medium is 1.7 or greater.

4. The objective lens, A diopter adjustment optical system positioned on the opposite side of the medium from the objective lens, the diopter adjustment optical system which changes the depth of the laser beam's focusing point by applying power, Equipped with, By canceling out the spherical aberration generated in the medium due to the change in the depth of the focal point and the spherical aberration caused by the change in the distance from the objective lens to the surface of the medium, the diffraction limit of the objective optical system is maintained. The focal length f and numerical aperture NA of the objective lens are such that, with respect to the refractive index n of the medium, An objective optical system that satisfies the following conditions.

5. The objective optical system according to claim 4, wherein the diopter adjustment optical system includes one of a variable focal length lens, a transmissive spatial light modulator, a deformable mirror, and a reflective spatial light modulator.

6. The objective optical system according to claim 4 or 5, wherein the numerical aperture of the objective lens is 0.6 to 0.

9.

7. The objective optical system according to any one of claims 4 to 6, comprising an optical system disposed between the diopter adjustment optical system and the objective lens, the optical system relaying the diopter adjustment optical system so that it is conjugate to the objective lens.

8. The objective optical system according to any one of claims 4 to 7, further comprising a spherical aberration adjustment mechanism that adjusts the spherical aberration of the objective lens by moving some of the lenses within the objective lens in the optical axis direction.

9. A laser processing apparatus comprising an objective optical system according to any one of claims 4 to 8, the objective optical system for focusing the laser light to a focal point in the medium.

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