WO2007108589A1

WO2007108589A1 - An adjustable laser beam delivery system and method for forming the same

Info

Publication number: WO2007108589A1
Application number: PCT/KR2007/000458
Authority: WO
Inventors: Beng So Ryu
Original assignee: QMC Co Ltd
Current assignee: QMC Co Ltd
Priority date: 2006-03-20
Filing date: 2007-01-26
Publication date: 2007-09-27
Anticipated expiration: 2008-09-20
Also published as: KR100588459B1; TWI305504B; TW200735989A

Abstract

This invention is related to an adjustable BDS with which the laser processing parameters can be readily changed without exchanging an optical component. The adjustable beam delivery system for a laser cutting according to the present invention comprises: a spherical concave lens to diverge a raw laser beam from a laser; a lateral anamorphic convex lens to adjust a length and a width of a focused beam spot by correcting a lateral divergence of said laser beam; a vertical anamorphic convex lens to adjust a numerical aperture by correcting a vertical divergence of said laser beam; and a focusing lens to focus said laser beam corrected independently in both said lateral anamorphic convex lens and said vertical anamorphic convex lens.

Description

AN ADJUSTABLE LASER BEAM DELIVERY SYSTEM AND METHOD FOR FORMING THE SAME

Technical Field

[1] This invention relates to a system and method for forming a beam delivery system

(BDS) for laser scribing and cutting, wherein the BDS has adjustable functions, including a numerical aperture, a target irradiance and a target irradiance for optimized processing conditions. Background Art

[2] Solid state lasers have been used for various material processing applications, including laser cutting and scribing. Recent developments in ultraviolet (UV) solid state lasers have broadened applications of the lasers in semiconductor material processing, especially for laser cutting of wafer substrates for chip separation. There are various types of the substrates requiring the chip separation, such as silicon wafers, compound semiconductor wafers, ceramic substrates and metal substrates. Important parameters of a laser beam for cutting these substrates are focusability (or focusing resolution), depth of focus, and target irradiance (or peak power density in W/cm ). These processing parameters are substantially different from one type of substrate to another, requiring different configuration of laser optics in a laser beam delivery system (BDS).

[3] For example, a thin silicon wafer with a small chip-size (usually in hundreds of micro-meters) requires a tightly focused (or highly resolved) beam spot having a short depth of focus with an optimum irradiance. These parameters results in narrow- width and shallow-depth laser scribing lines on the silicon wafer, which is subsequently broken into small individual chips.

[4] In case of metal substrates, the required processing parameters are different. Since the laser scribe-and-break technique used for the thin silicon wafer is not applicable to the metal substrates due to high ductility of them, the laser beam needs to cut through an entire thickness of the metal substrates. A higher depth of focus is necessary for the deep penetration in the metal cutting. Additionally, a proper adjustment is required for the BDS, because threshold irradiance is generally higher in metal substrates than that of a silicon substrate. Disclosure of Invention Technical Problem

[5] A conventional way to change the laser cutting parameters, such as focusability, depth of focus and target irradiance, is to exchange optical components in a BDS. However, the exchange process sometimes results in a substantially different configuration and requires a lengthy re-alignment of the BDS.

Technical Solution

[6] The present invention provides a method for forming an adjustable beam delivery system for a laser cutting, said method comprising the steps of: generating a raw laser beam; diverging said raw laser beam by a spherical concave lens; correcting said diverging laser beam independently in a lateral direction and in a vertical direction, wherein said lateral direction of said diverging laser beam is corrected by a lateral anamorphic convex lens and said vertical direction of said diverging laser beam is corrected by a vertical anamorphic convex lens; varying a distance between said spherical concave lens and said vertical anamorphic convex lens to change a numerical aperture of a focusing lens, and a distance between said spherical concave lens and said lateral anamorphic convex lens to change a length and a width of a focused beam spot; and adjusting a depth of focus and a resolution of focus by said variation of said numerical aperture, and adjusting an irradiance by said variation of said length and width of said focused beam spot.

[7] Also, the present invention provides an adjustable beam delivery system for a laser cutting comprising: a spherical concave lens to diverge a raw laser beam from a laser; a lateral anamorphic convex lens to adjust a length and a width of a focused beam spot by correcting a lateral divergence of said laser beam; a vertical anamorphic convex lens to adjust a numerical aperture by correcting a vertical divergence of said laser beam; and a focusing lens to focus said laser beam corrected independently in both said lateral anamorphic convex lens and said vertical anamorphic convex lens.

[8] Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. Advantageous Effects

[9] It is an advantage of the present invention that the laser processing parameters can be readily changed without exchanging an optical component by use of an adjustable BDS. Brief Description of the Drawings

[10] These and other features and advantages of the present invention will be better understood by reading the following detailed description, taken together with the drawings wherein:

[11] Figure 1 is a schematic ray-tracing diagram of the adjustable BDS in a front view. [12] Figure 2 is a schematic ray-tracing diagram of the adjustable BDS in a top view.

[13] Figure 3, in accordance with FlG IA, is a schematic ray-tracing diagram with a cross-sections of laser beam on a focusing lens in two different adjustment position of the first cylindrical lens. [14] Figure 4, in accordance with FlG IB, is a schematic ray-tracing diagram with a top views of a focused laser beam on a target in two different adjustment positions of the second cylindrical lens. [15] Figure 5 is an exemplary embodiment of the adjustable laser BDS in a laser cutting system.

Best Mode for Carrying Out the Invention [16] Reference will now be made in detail to an embodiment of the present invention, example of which is illustrated in the accompanying drawings. [17] Underlying laser physics of this invention is explained as follows. Commonly, a solid state laser beam has a Gaussian distribution of laser irradiance. In a far-field imaging of the laser beam, a numerical aperture of a focusing lens is expressed by: [18]

NJtEk nπi

[19] where NA is a numerical aperture of a focusing lens, λ is a wavelength of a laser beam, n is a refractive index of the focusing lens and d is a minimum spot size of a focused laser beam. The minimum spot size is usually measured by "full width at e^" maximum" of the Gaussian distribution. A theoretical limit of the minimum spot size of the focused laser beam is close to 4/π times the laser wave length λ. A perfect Gaussian laser beam (or TEM ) might meet this theoretical limit, all practical laser beams, however, exceed the limit. A ratio between the practical limit and the theoretical limit is referred to as M . The numerical aperture can be expressed by an approximated relationship:

[20]

NAz °-

2/

[21] where D is a diameter of an incident laser beam to the focusing lens, and/ is a focal length of the focusing lens. Considering this relationship and M , the practical minimum spot size of the focused laser beam can be expressed as:

[22] d=- nτiD

[23] In a practical application of this equation, the wavelength λ and the M are usually constants, fixed by type and performance of a laser, respectively. Also, the focal length /and the refractive index n are usually fixed value by a focusing lens used in a BDS. Then, this relationship means that the minimum spot size of the focused beam is controlled by the diameter of the incident laser beam. When the diameter of the incident laser beam at the focusing is changing, a depth of focus z is changing accordingly, by the following relationship: [24]

[25] Thus, a controlled change of a diameter of an incident laser beam at a focusing lens can give proper adjustments of a minimum spot size (or resolution of a focused beam) and a depth of focus. This adjustment capability is one of the main advantages of this invention.

[26] Another aspect of adjustment capability in this invention is the irradiance (or peak power density). The irradiance is generally denoted by W/tf or J/(sec-U ). The time (sec) in the irradiance unit is a duration of a laser pulse, which is usually a fixed value, determined by type and design of a laser. Then, a controlling factor of adjusting the irradiance is essentially a laser energy density (J/tf ) of a focused laser beam. In laser cutting applications, an optimum laser energy density based on material properties of a target material is important to achieve high efficiency of the cutting. For example, too low energy density causes a lack of proper ablation resulting in an improper laser cutting process. Furthermore, too high laser energy density causes overflow of the laser energy, causing excessive thermal effects in the wake of a laser cut, such as melting and recasting. Most of solid state lasers in present time have capabilities to adjust the output laser energy in Joule. To achieve an optimum value of the laser energy density, the output laser energy needs to be adjusted to a lower value. However, limiting the output laser energy usually results in reduction of an average power output of the laser. In a mass-production, this is not a proper way to achieve a desired laser energy density, because the limitation in the average power out put affects productivity of the laser. A better way to achieve an optimum laser energy density is adjusting the size of a laser beam spot while keeping the laser energy output for a maximum average power. Higher speed of a laser cutting gained by utilizing the maximum power output gives higher productivity of a laser. This adjustment capability in a focused laser beam spot is another advantage of this invention.

[27] An exemplary embodiment of the invention is described in detail as follows. Nevertheless, present invention can have various other embodiments than the one described in detail herein. The scope of the present invention is expressly not limited expect as specified in the accompanying claims. For clear description and comprehension, components in diagrams are schematically presented and not shown to precise scale.

[28] Referring to Figure 1, a front view of a schematic ray-tracing diagram of a multi- functionally adjustable beam delivery system (BDS) is shown with associated optical components. Laser 2 can be any solid state laser, preferably with a Gaussian profile and with a wavelength in UV range. Laser 2 emits a laser beam 2A which is passing through a spherical plano-concave lens 4 with a focal length of -50 mm. A diverging beam after the spherical plano-concave lens 4 is corrected to have a much less divergence by the first cylindrical plano-convex lens 6 with a focal length of 150 mm. A curvature of the first cylindrical plano-convex lens 6 is placed in a vertical direction, such that first cylindrical plano-convex lens 6 can also be called a vertical anamorphic lens.The laser beam after the first cylindrical plano-convex lens 6 is slightly diverging towards up to a focusing lens 10 with a focal length of 40 mm. A curvature of the second cylindrical plano-convex lens 8 with a focal length of 250 mm is placed in a lateral direction, such that the second cylindrical plano-convex lens 8 can also be called a lateral anamorphic lens. With this curvature configuration, a vertical component of the diverging beam after the first cylindrical plano-convex lens 6 is passing through the second cylindrical plano-convex lens 8 as a plain window. Namely, the vertical component of the diverging beam after the first cylindrical planoconvex lens 6 is not affected by the second cylindrical plano-convex lens 8. The diverging beam in the vertical direction is finally focused on a target 12 by the focusing lens 10.

[29] As above, the lateral anamorphic convex lens 8 may have a longer focal length than that of said vertical anamorphic convex lens 6. Also, the absolute value of a focal length of said spherical concave lens 4 may be shorter than focal lengths of both said vertical anamorphic convex lens 6 and said lateral anamorphic convex lens 8. The lateral anamorphic convex lens 8 may be located between said spherical concave lens 4 and said vertical anamorphic convex lens 6. Or, the vertical anamorphic convex lens 6 may be located between said spherical concave lens 4 and said lateral anamorphic convex lens 8.

[30] Referring to Figure 2, a top view of a schematic ray-tracing diagram of the multi- functionally adjustable BDS is shown with associated optical components. Laser 2 emits the laser beam 2A which is passing through the spherical plano-concave lens 4. A curvature of the first cylindrical plano-convex lens 6 is placed in a vertical direction, such that a lateral component of the diverging beam after the spherical plano-concave lens 4 is passing through the first cylindrical plano-convex lens 6 as a plain window. Namely, the lateral component of the diverging beam after the spherical plano-concave lens 4 is not affected by the first cylindrical plano-convex lens 6. After the first cylindrical plano-convex lens 6, the diverging beam iscorrected to have a slight convergence by the second cylindrical plano-convex lens 8 with the curvature in the lateral direction. Laser beam 2A after the second cylindrical plano-convex lens 8 is slightly converging towards to the focusing lens lO.The converging beam in the lateral direction is finally focused before the target 12 by the focusing lens 10. [31] Referring to Figure 3 in accordance with Figure 1 in the front view, the first cylindrical plano-convex lens 6 is in a converging position 6A. A distanced ) between the spherical plano-concave lens 4 and the first cylindrical plano-convex lens 6 in the converging position 6 A can be expressed by: [32]

[33] where d is the converging distance at the converging position 6A,/ is a focal length of the spherical plano-concave lens 4, and/ cl is a focal length of the first cylindrical plano-convex lens 6. In this exemplary embodiment, the distance between the spherical plano-concave lens 4 and the first cylindrical plano-convex lens 6 in the converging position 6A is about 130 mm. In this distance, from a top view 200A of the focusing lens 10, a cross sectional height 106A of the converging beam just before the focusing lens 10 is about 6 mm. Consequently, the converging position 6A of the first cylindrical plano-convex lens 6 results in a smaller numerical aperture. And, the smaller numerical aperture results in a higher depth of focus and a lower resolution of focus. In contrast, when the first cylindrical plano-convex lens 6 is in a diverging position 6B, a distanced ) between the spherical plano-concave lens 4 and the first d cylindrical plano-convex lens 6 in the diverging position 6B can be expressed by: [34]

[35] where d is the diverging distance at the diverging position 6B. In this exemplary d embodiment, the diverging distance between the spherical plano-concave lens 4 and the first cylindrical plano-convex lens 6 is about 70 mm. In this distance, from a top view 200B of the focusing lens 10, a cross sectional height 106B of the diverging beam just before the focusing lens 10 is about 15 mm. Consequently, the diverging position 6B of the first cylindrical plano-convex lens 6 results in a larger numerical aperture. And, the larger numerical aperture results in a shallower depth of focus and a higher resolution of focus. As above, the variation of said distance between said spherical concave lens and said vertical anamorphic convex lens may change one of divergence, convergence and collimation of said laser beam in the vertical direction. [36] Referring to Figure 4 in accordance with Figure 2 in the top view, the second cylindrical plano-convex lens 8 is in a converging position 8A. A distanced ) between the spherical plano-concave lens 4 and the second cylindrical plano-convex lens 8 in the converging position 8 A can be expressed by: [37]

[38] where/ c2 is a focal length of the second cylindrical plano-convex lens 6. In this exemplary embodiment, the distance between the spherical plano-concave lens 4 and the second cylindrical plano-convex lens 8 in the converging position 8A is about 220 mm. In this distance, from a top view 202A of the target 12, a beam spot length 108A of the converging beam on the target 12 is about 500 D. Consequently, the converging position 8A of the second cylindrical plano-convex lens 8 results in a longer and narrower beam spot on the target 12. And, the longer and narrower beam spot on the target 12 results in a higher irradiation. In contrast, when the second cylindrical planoconvex lens 8 is in a diverging position 8B, a distance^ d ) between the spherical plano-concave lens 4 and the second cylindrical plano-convex lens 8 in the diverging position 8B can be denoted by: [39]

[40] In this exemplary embodiment, the diverging distance between the spherical planoconcave lens 4 and the second cylindrical plano-convex lens 8 is about 170 mm. In this distance, from a top view 202B of the target 12, a beam spot length 108B of the diverging beam on the target 12 is about 100 D. Consequently, the diverging position 8B of the second cylindrical plano-convex lens 8 results in a shorter and wider beam spot on the target 12. And, the shorter and wider beam spot on the target 12 results in a lower irradiation. The variation of said distance between said spherical concave lens and said lateral anamorphic convex lens changes one of divergence, convergence and collimation of said laser beam in the lateral direction.

[41] In the above exemplary embodiment, curvature directions of the first cylindrical plano-convex lens 6 and the second cylindrical plano-convex lens 8, respectively a vertical direction and a lateral direction,are interchangeable. For example, the curvature of the first cylindrical plano-convex lens 6 can be placed with a lateral direction, and then the curvature of the following second cylindrical plano-convex lens 8 can be placed in a vertical direction.

[42] Referring to Figure 5, an exemplary embodiment of the adjustable laser BDS in a laser cutting system is shown. Three identical 45 degree turning mirrors, the first 16A, the second 16B and the third 16C are employed to properly direct the beam through the BDS components 4, 6, 8, and 10 to the target 12 on motion stages 14. Target 12 on the motion stages 14 is moved to be cut by the focused laser beam. A top view 204 of a schematic ray-tracing of the BDS components between the second turning mirror 16B to the third turning mirror 16C is also shown. In this configuration, the first 16A and second 16B turning mirrors are conveniently used to direct the laser beam 2A to a center of BDS components between the second 16B and third 16C turning mirrors. [43] It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

[I] A method for forming an adjustable beam delivery system for a laser cutting, said method comprising the steps of: generating a raw laser beam; diverging said raw laser beam by a spherical concave lens; correcting said diverging laser beam independently in a lateral direction and in a vertical direction, wherein said lateral direction of said diverging laser beam is corrected by a lateral anamorphic convex lens and said vertical direction of said diverging laser beam is corrected by a vertical anamorphic convex lens; varying a distance between said spherical concave lens and said vertical anamorphic convex lens to change a numerical aperture of a focusing lens, and a distance between said spherical concave lens and said lateral anamorphic convex lens to change a length and a width of a focused beam spot; and adjusting a depth of focus and a resolution of focus by said variation of said numerical aperture, and adjusting an irradiance by said variation of said length and width of said focused beam spot.

[2] The method of claim 1, further comprising directing said focused beam spot to a substrate, wherein said substrate is moved by at least one motion stage to achieve at least a partial cut in said substrate.

[3] The method of claim 1, wherein said raw beam has a Gaussian profile.

[4] The method of claim 3, wherein said raw beam is from a solid state laser.

[5] The method of claim 4, wherein said raw beam is in an ultraviolet range.

[6] The method of claim 1, wherein said lateral anamorphic convex lens has a longer focal length than that of said vertical anamorphic convex lens

[7] The method of claim 1, wherein an absolute value of a focal length of said spherical concave lens is shorter than focal lengths of both said vertical anamorphic convex lens and said lateral anamorphic convex lens.

[8] The method of claim 1, wherein said variation of said distance between said spherical concave lens and said lateral anamorphic convex lens changes one of divergence, convergence and collimation of said laser beam.

[9] The method of claim 1, wherein said variation of said distance between said spherical concave lens and said vertical anamorphic convex lens changes one of divergence, convergence and collimation of said laser beam.

[10] The method of claim 1, wherein said lateral anamorphic convex lens is located between said spherical concave lens and said vertical anamorphic convex lens.

[II] The method of claim 1, wherein said vertical anamorphic convex lens is located between said spherical concave lens and said lateral anamorphic convex lens. [12] An adjustable beam delivery system for a laser cutting comprising: a spherical concave lens to diverge a raw laser beam from a laser; a lateral anamorphic convex lens to adjust a length and a width of a focused beam spot by correcting a lateral divergence of said laser beam; a vertical anamorphic convex lens to adjust a numerical aperture by correcting a vertical divergence of said laser beam; and a focusing lens to focus said laser beam corrected independently in both said lateral anamorphic convex lens and said vertical anamorphic convex lens. [13] The adjustable beam delivery system of claim 12, wherein said raw beam has a

Gaussian profile. [14] The adjustable beam delivery system of claim 13, wherein said raw beam is from a solid state laser. [15] The adjustable beam delivery system of claim 14, wherein said raw beam is in an ultraviolet range. [16] The adjustable beam delivery system of claim 12, wherein said lateral anamorphic convex lens has a longer focal length than that of said vertical anamorphic convex lens. [17] The adjustable beam delivery system of claim 12, wherein an absolute value of a focal length of said spherical concave lens is shorter than focal lengths of both said vertical anamorphic convex lens and said lateral anamorphic convex lens. [18] The adjustable beam delivery system of claim 12, wherein said variation of said distance between said spherical concave lens and said lateral anamorphic convex lens changes one of divergence, convergence and collimation of said laser beam. [19] The adjustable beam delivery system of claim 12, wherein said variation of said distance between said spherical concave lens and said vertical anamorphic convex lens changes one of divergence, convergence and collimation of said laser beam. [20] The adjustable beam delivery system of claim 12, wherein said lateral anamorphic convex lens is located between said spherical concave lens and said vertical anamorphic convex lens. [21] The adjustable beam delivery system of claim 12, wherein said vertical anamorphic convex lens is located between said spherical concave lens and said lateral anamorphic convex lens.