TWI903099B - Illumination optical systems and laser processing equipment - Google Patents

Abstract

The purpose of this invention is to prevent laser light loss by homogenizing laser light using a lens array with non-fixed lens thickness. An illumination optical system guides laser light toward an irradiation surface, with the z-axis as the optical axis, the x-axis as the direction perpendicular to both the z-axis and y-axis, and the y-axis as the direction perpendicular to both the z-axis and x-axis. The illumination optical system includes a first lens array and a second lens array, each having a plurality of lenses arranged along the z-axis and at least one of the x-axis and y-axis directions, wherein the thickness of the lenses in the first and second lens arrays is non-fixed in at least one direction.

Description

Illumination optical systems and laser processing equipment

This invention relates to an illumination optical system for irradiating a photomask with linear laser light, and a laser processing apparatus having the illumination optical system.

A well-known technique involves ablation (removal of material by melting, evaporation, or other means) a workpiece (such as the resin layer of a printed circuit board) made of non-metallic materials like resin or silicon, using a laser scan through a photomask to create a pattern (e.g., vias) on the workpiece. For applications requiring precision machining, excimer laser (KrF laser, wavelength 248nm) ablation is used.

As an example, the illumination optical system of the aforementioned processing apparatus generates a light beam that makes the irradiated area linear. In order to equalize the luminous flux of the irradiated area (photomask), a lens such as a fly-eye lens is used to homogenize the light. Linear laser light refers to laser light whose beam profile is linear on a plane perpendicular to the optical axis.

In this lighting optical system, because the light source is highly coherent laser light, the illumination of the photomask will be averaged and uniform as long as the wavelengths divided by the fly-eye lenses do not interfere with each other. Generally speaking, the more divisions of the fly-eye lenses (narrowing the spacing between the fly-eye lenses), the higher the uniformity of the illumination. However, because the excimer laser light source has a narrow wavelength band and high spatial coherence, if the spacing between the fly-eye lenses is narrowed, interference fringes will be generated on the photomask. These interference fringes can be avoided by setting an optical path difference along the optical axis of the fly-eye lenses.

For example, in patent document 1, in order to generate the aforementioned optical path difference, a glass plate with a large difference in thickness is arranged parallel to a fly-eye lens to serve as a phase generation unit.

[Prior Patent Documents] Patent Document 1: Japanese Patent Application Publication No. 2016-38456

[Problem to be Solved by the Invention] In the framework of Patent 1, an optical component as a phase generator must be added to the lighting optical system, and the laser light suffers energy loss at this optical component. Since the processing device is a high-throughput laser, the loss caused by the optical path difference component is a significant amount. Furthermore, if there are positioning errors in the optical component and the fly-eye lens, it becomes a further cause of energy loss.

Therefore, the purpose of this invention is to provide an illumination optical system and a laser processing apparatus that reduces energy loss by having the function of generating a phase difference in the fly-eye lens itself, without requiring the positional matching of components.

This invention is an illumination optical system that guides laser light toward an irradiation surface, with the z-axis as the optical axis, the x-axis as the direction perpendicular to the z-axis and y-axis, and the y-axis as the direction perpendicular to the z-axis and x-axis. The illumination optical system includes a first lens array and a second lens array, each having a plurality of lenses arranged along the z-axis and at least one of the x-axis and y-axis directions, wherein the thickness of the lens in the first lens array and the second lens array is not fixed in at least one direction. Furthermore, this invention is an illumination optical system that guides laser light towards an irradiation surface. The system comprises a beam shaping section, a lens array section, and a collimating lens section arranged sequentially along the z-axis, with the z-axis as the optical axis, the x-axis as the direction perpendicular to both the z-axis and y-axis, and the y-axis as the direction perpendicular to both the z-axis and x-axis. The beam shaping section and the collimating lens section consist of a first cylindrical lens that acts as a lens in the x-axis direction and a second cylindrical lens that acts as a lens in the y-axis direction. The lens configuration comprises a first pair consisting of two first cylindrical lens arrays arranged along the z-axis, and a second pair consisting of two second cylindrical lens arrays arranged along the z-axis. The first cylindrical lens array has a lensing function in the x-axis direction, and the second cylindrical lens array has a lensing function in the y-axis direction. The thickness of the first or second cylindrical lens array in the first or second pair is not fixed in at least one direction. Furthermore, the present invention is a laser processing apparatus, comprising: a light source that emits laser light; an illumination optical system that illuminates a photomask with the laser light having a linear cross-section while simultaneously scanning the photomask by a scanning mechanism; a projection optical system that illuminates the workpiece with the laser light passing through the photomask; and a workpiece placed on a table, wherein the workpiece is moved in the x-y direction while being placed on the table, and the illumination optical system having the aforementioned structure.

[Invention Effects] According to at least one embodiment, the present invention prevents interference by varying the thickness of the fly-eye lens itself, thereby preventing energy loss of laser light. The effects described herein are not limited thereto, and may also include some effects described herein or effects different in nature.

The embodiments of the present invention will now be described with reference to the figures. The embodiments described below are preferred examples of the present invention, but the content of the present invention is not limited to these embodiments.

Figure 1 is a schematic diagram showing an example of a processing apparatus, such as a laser processing apparatus, that can be used with the present invention. The laser processing apparatus has a laser light source 11. The laser light source 11 is, for example, an excimer laser light source that pulses KrF excimer laser light with a wavelength of 248 nm. The laser light is provided to a linear laser scanning mechanism 12.

The linear laser scanning mechanism 12 has an illumination optical system that shapes the laser beam into a rectangular (linear) shape; and a scanning mechanism (linear motion mechanism) for the laser beam LB scanning mask 13.

A photomask pattern is formed on photomask 13, corresponding to a processing pattern formed on the workpiece (hereinafter, appropriately referred to as substrate W) by ablation. That is, a pattern is formed by drawing a light-shielding film (e.g., Cr film) that blocks the KrF excimer laser onto a substrate (e.g., quartz glass) that transmits KrF excimer laser light. The processing pattern includes vias, non-vias, and trenches for wiring patterns. After the processing pattern is formed by ablation, a conductor such as copper is filled in.

Laser light LB passes through photomask 13 and enters projection optics system 14. Laser light emitted from projection optics system 14 illuminates the surface of substrate W. Projection optics system 14 has a focal plane on the photomask surface and the surface of substrate W. Substrate W is a resin substrate, such as epoxy resin, on which a copper wiring layer is formed and an insulating layer is formed.

A plurality of patterned areas WA are provided on the substrate W and are fixed on a mounting table 15 for holding the workpiece. The mounting table 15 can be moved in a 2-dimensional direction and the position of each patterned area WA relative to the photomask 13 can be determined by rotation. Furthermore, in order to process the workpiece area on the entire substrate W, the mounting table 15 moves the substrate W stepwise in the scanning direction.

An embodiment of the laser processing apparatus will be described with reference to Figure 2. The laser processing apparatus is mounted to a base 21 and an upper frame 22 that constitute a support. The upper frame 22 is fixed to the base 21. The base 21 and the upper frame 22 are made of a material with high rigidity and vibration damping properties.

The upper frame 22 fixes a linear laser scanning mechanism consisting of a scanning mechanism 16 and an illumination optical system 17, a photomask platform 18 (the support part of the photomask) on which the photomask 13 is mounted, and a projection optical system 14. A tabletop 15 is fixed on the base 21. That is, the scanning mechanism 16, the illumination optical system 17, the photomask platform 18, the projection optical system 14, and the tabletop 15 are positioned to satisfy a predetermined optical relationship (the relationship of correct incident laser light on the illumination optical system 17). After positioning, due to the scanning action of the illumination optical system 17 and the positional change of the tabletop 15, vibrations are caused, etc., so that the base 21 and the upper frame 22 will move together in the event of shaking. The incident position and incident angle of the laser light of the illumination optical system 17 are corrected by the beam position correction unit 27.

The laser source 11 is housed in a frame 24 that is separately disposed from the base 21 and the upper frame 22. The laser source 11 pulses a KrF excimer laser (referred to as laser light) L1 with a wavelength of 248 nm. The laser light L1 and the guiding laser light (not shown) are incident on the beam position correction unit (also referred to as the beam steering mechanism) 27.

The beam position correction unit 27 is a mechanism for real-time positioning (position and incident angle) of the laser beam L1. Using the beam position correction unit 27, regardless of the tilt of the base 21 and upper frame 22 of the laser processing apparatus, the laser beam L1 is adjusted to always be incident on the illumination optical system 17 at the correct position and angle. Furthermore, the wavelength of the guiding laser beam is, for example, 400nm to 700nm. The mirror included in the beam position correction unit 27 has two reflective films that reflect the laser beam L1 with different wavelengths and the guiding laser beam with different wavelengths. A beam shaping unit is provided in the beam position correction unit 27 to allow each laser beam to be incident on each reflective film.

Laser light L1 emitted from beam position correction unit 27 is reflected by mirror 28 and enters illumination optical system 17. Illumination optical system 17 homogenizes the intensity distribution of light emitted from laser source and forms linear processing laser light. Illumination optical system 17 has a lens array (also called fly-eye lens array) for forming linear laser light. The lens array is a lens array with a plurality of convex lenses arranged in the direction of laser light magnification. Linear laser light LB from illumination optical system 17 illuminates photomask 13. Specific examples of illumination optical system 17 will be described later.

The scanning mechanism 16 is part of the illumination optical system 17, enabling the entire illumination optical system 17 to move. The scanning mechanism 16 moves the laser beam LB relative to the photomask 13, and the photomask 13 and the substrate W, which are respectively fixed on the photomask platform 18 and the table 15, are scanned by the laser beam.

Figure 3 shows the relationship between the size of the laser beam LB and the photomask 13. For example, the length × width of the laser beam LB is (100 × 0.1 (mm)) or (35 × 0.3 (mm)). The length direction and the perpendicular width direction of the laser beam LB are the scanning directions.

A blocking film (chromium film, aluminum film, etc.) is formed on the photomask 13 on a substrate (such as quartz glass) through which the KrF excimer laser light passes to block the KrF excimer laser light, thereby drawing the photomask pattern. The photomask 13 can be drawn with a pattern that repeats on the substrate W, or with a pattern that covers the entire substrate W.

The photomask platform 18 has an xyθ platform that holds the photomask 13 and can position the photomask. There is also a camera (not shown) that reads the alignment marks set on the photomask 13 to position the photomask 13.

Laser light passing through photomask 13 is incident on projection optical system 14. Projection optical system 14 is a projection optical system with focal points on the surface of photomask 13 and the surface of substrate W, which projects the light passing through photomask 13 onto substrate W. Here, projection optical system 14 is configured as a scaled-down projection optical system (e.g., 1/4).

The mounting table 15 is fixed to the substrate W by vacuum adsorption or the like, and the substrate W is positioned relative to the photomask 13 by moving and rotating the table in the x-y directions. Furthermore, it can move stepwise along the scanning direction, allowing for ablation processing on the entire substrate W. An alignment camera (not shown) is provided next to the mounting table 15, which photographs the alignment marks placed on the substrate W. A focus adjustment mechanism (Z-mechanism) may also be provided.

The substrate W (workpiece), such as an organic substrate for printed circuit boards, has a laser-processed layer formed on its surface. This layer can be formed from materials such as resin films or metal foils, which can be processed by laser light to form through-holes, etc. Through-holes or wiring patterns are formed using a laser processing machine, and in subsequent steps, conductors such as copper are filled into the processed areas.

Figure 4 shows an example of a magnified display substrate W. The substrate W is a multi-beveled substrate, and pattern areas WA corresponding to the pattern on the photomask 13 are repeatedly arranged in an 8×8 matrix on the substrate W. In Figure 4, the horizontal direction is the sub-stepping direction, and the vertical direction is the main stepping direction. When a pattern area WA is scanned, the next pattern area is then scanned. The scanning direction (arrow) shown in the figure is an example.

In addition, in one embodiment of the invention, a conveying mechanism (not shown) is provided to place or remove the workpiece from the loading table. For example, a SCARA robotic arm can be used. Also, an air-conditioned room (not shown) is provided, containing a frame that encloses the processing apparatus and laser light source.

In one embodiment of the present invention described above, a control device (not shown) is provided for controlling the entire device. The control device performs control of the laser light source 11, control of each part of the drive unit, alignment of the photomask and the substrate W, management of production information, and management of the inventory.

The optical system in the laser processing apparatus described above is represented by a block diagram as shown in Figure 5. The corresponding parts in Figure 5 are labeled with the same reference symbols as those in Figures 1 and 2. Laser light from the laser source 11 is supplied to the beam forming unit 30. Laser light from the beam forming unit 30 is supplied to the beam position correction unit 27. The beam position correction unit 27 adjusts the laser light so that it always enters the illumination optical system 17 at the correct position and angle. As described above, the beam forming unit 30 forms laser light by directing the laser light from the laser source 11 and the guided laser light onto a reflective film different from a mirror surface.

The illumination optical system 17 has a structure in which a beam shaping section 31, a lens array section 32 serving as a light intensity homogenizing section, and a collimating lens section 33 are arranged sequentially along the optical axis. The beam shaping section 31 forms a rectangular laser beam with a predetermined length and width. The lens array section 32 makes the distribution of the laser beam uniform and linear. The lens array section 32 consists of a first pair 34 and a second pair 35. The first pair 34 consists of two first cylindrical lens arrays (labeled SLA in FIG. 5) 36a and 36b arranged along the optical axis. The second pair 35 consists of two second cylindrical lens arrays 37a and 37b arranged along the optical axis.

Laser light from lens array 32 is collimated into nearly parallel light by collimating lens 33. Laser light from collimating lens 33 of illumination optical system 17 illuminates photomask 13. Laser light passing through photomask 13 enters projection optical system 14. Projection optical system 14 projects the light passing through photomask 13 onto substrate W.

An example of the illumination optical system 17 will be explained with reference to Figure 6. The direction parallel to the optical axis of the illumination optical system 17 is defined as the z-axis, the direction perpendicular to both the z-axis and y-axis is defined as the x-axis, and the direction perpendicular to both the z-axis and x-axis is defined as the y-axis. In other words, the axes perpendicular to and orthogonal to the z-axis are the x-axis and y-axis. Figure 6A is a side view of the illumination optical system 17, and Figure 6B is a top view of the illumination optical system 17. Furthermore, the width direction of the linear laser beam is defined as the x-axis direction, and the length direction of the linear laser beam is defined as the y-axis direction.

In the side view of Figure 6A, the cylindrical lens 31a, cylindrical lens arrays 36a, 36b, and cylindrical lens 33a, indicated by thick lines, are elements that have a lensing effect in the x-axis direction. These elements with lensing effects are extracted and shown in Figure 6C. Also, in the side view of Figure 6B, the cylindrical lens 31b, cylindrical lens arrays 37a, 37b, and cylindrical lens 33b, indicated by thick lines, are elements that have a lensing effect in the y-axis direction. These elements with lensing effects are extracted and shown in Figure 6D.

The beam shaping section 31 has a structure in which cylindrical lenses 31a, which act as lenses in the x-axis direction (in other words, have refractive index in the x-axis direction), and cylindrical lenses 31b, which act as lenses in the y-axis direction (in other words, have refractive index in the y-axis direction), are arranged sequentially along the z-axis. When laser light from a light source is incident on the cylindrical lens 31a, laser light is generated, which amplifies from the cylindrical lens 31a in the x-axis direction (width direction). Similarly, when laser light is incident on the cylindrical lens 31b, laser light is generated, which amplifies from the cylindrical lens 31b in the y-axis direction (length direction). The laser light from the cylindrical lens 31b exits from the beam shaping section 31. The beam shaping section 31, in conjunction with the cylindrical lens array section 32, amplifies the laser light by adjusting the size of the incident surface, and ensures that the laser light is incident on the cylindrical lens array in parallel. Furthermore, the laser light incident on the fly-eye lens exhibits an intensity distribution with a Gaussian curve or similar characteristics.

Laser light emitted from the beam shaping section 31 enters the first pair 34 of the cylindrical lens array 36a on the light source side of the lens array section 32. A cylindrical lens array 36b is arranged parallel to the cylindrical lens array 36a along the z-axis. The cylindrical lens arrays 36a and 36b are cylindrical lenses (convex lenses) with a plurality of small diameters arranged along the x-axis. The incident-side lens surface of the cylindrical lens array 36a is convex, and the exit-side lens surface is flat. The incident-side lens surface of the cylindrical lens array 36b is flat, and the exit-side lens surface is convex. Cylindrical lens arrays 36a and 36b homogenize the laser light.

Laser light emitted from the first pair 34 is incident on the source side of the cylindrical lens array 37a of the second pair 35 in the lens array section 32. A cylindrical lens array 37b is arranged parallel to the cylindrical lens array 37a along the z-axis. The cylindrical lens arrays 37a and 37b are cylindrical lenses (convex lenses) with a plurality of small diameters arranged along the y-axis. The cylindrical lens arrays 37a and 37b homogenize the laser light.

Laser light emitted from the second pair 35 of cylindrical lens array 37b in lens array section 32 is incident on the first cylindrical lens 33a of collimating lens section 33. Cylindrical lens 33a acts as a lens in the x-axis direction. The second cylindrical lens 33b is arranged parallel to cylindrical lens 33a. Cylindrical lens 33b acts as a lens in the y-axis direction. Collimating lens section 33 makes the segmented laser light parallel, overlapping and homogenizing it on the illumination surface.

In one embodiment of the invention, the lens thickness of one of the first and second cylindrical lens arrays included in the first pair 34 and/or the second pair 35 of the lens array section 32 is not fixed in at least one direction. Figure 7 shows an example where the lens thickness of the cylindrical lens array 36b of one of the first pair 34 is not fixed. The cylindrical lens arrays 36a and 36b are, for example, cylindrical lens arrays with 5 minor diameters arranged in the x-direction.

To ensure that the lens surfaces of the cylindrical lens array 36b have a height difference of ΔT when viewed from the side, the lens thicknesses are different. ΔT is set to a value that will not produce interference fringes (e.g., ΔT is approximately 1 mm). By creating an optical path difference in this cylindrical lens array 36b, interference fringes can be prevented on the exit side of the cylindrical lens array 36b.

Furthermore, the beam profile of an excimer laser typically forms a rectangle with a width-to-length ratio of 1:2, 1:5, etc., and the spatial interference is not isotropic, especially since the shorter side of the beam profile is higher than the longer side. Therefore, interference fringes tend to occur along the shorter side of the beam profile. In this case, when the spatial interference of the laser light is not equidirectional, the thickness of the lens becomes variable in the direction of the highest spatial interference.

Furthermore, when the thickness of the lenses is the same, and interference fringes appear bright or dark in the x-axis direction on the irradiated surface, as in the example of Figure 7, the thickness of the lenses in the x-axis direction of the cylindrical lens array 36b is varied. Also, when the thickness of the lenses is the same, and interference fringes appear bright or dark in the y-axis direction on the irradiated surface, the thickness of the lenses in the y-axis direction of the cylindrical lens array 37b is not fixed. Furthermore, when the thickness of the lenses is the same, and interference fringes appear bright or dark in both the x-axis and y-axis directions on the irradiated surface, the thickness of the lenses in both the x-axis direction of the cylindrical lens array 36b and the y-axis direction of the cylindrical lens array 37b is not fixed.

In one embodiment of the present invention described above, because the thickness of the cylindrical lens array itself is different, the energy loss of laser light can be reduced compared with a structure that has other optical components.

Furthermore, in one embodiment of the invention described above, the thickness of the lenses in the lens array varies, resulting in a configuration where the thicknesses of the lenses differ from one another, creating a height difference on the lens surfaces when viewed from the side. The invention is not limited to this configuration; for example, the thickness can vary in a predetermined stepwise manner in one direction when viewed from the side, or lenses of different thicknesses can be arranged randomly. It is permissible if each lens constituting the lens array has a different thickness from other lenses adjacent in at least one direction.

The above describes one embodiment of the present invention in detail. However, the present invention is not limited to the above embodiment and various modifications can be made according to the technical concept of the present invention. For example, a dual-directional lens array with lenses arranged in the x-axis and y-axis directions can be used. Furthermore, the present invention is not limited to a structure with two pairs of lenses; it can also be used to construct a structure with a single pair of lens arrays. Moreover, the structures, methods, steps, shapes, materials, and values exemplified in the above embodiments are merely examples, and different structures, methods, steps, shapes, materials, and values can be used as needed.

11: Laser light source 12: Linear laser scanning mechanism 13: Photomask 14: Projection optical system 15: Tabletop 16: Scanning mechanism 17: Illumination optical system 18: Photomask platform 21: Base 22: Upper frame 24: Frame 27: Beam position correction unit 28: Mirror surface 30, 31: Beam shaping unit 31a, 31b: Cylindrical lens 32: Lens array 33: Collimating lens 33a, 33b: Cylindrical lens 34: First pair 35: Second pair 36a, 36b: First cylindrical lens array 37a, 37b: Second cylindrical lens array L1: Laser beam LB: Laser beam W: Workpiece (substrate) WA: Pattern area

Figure 1 is a schematic structural diagram showing the laser processing apparatus capable of using the present invention. Figure 2 is a front view of an embodiment of the present invention. Figure 3 is a plan view showing the relationship between the photomask and the linear beam in an embodiment of the present invention. Figure 4 is an enlarged plan view of an example of a substrate used in an embodiment of the present invention. Figure 5 is a block diagram showing an optical system in an embodiment of the present invention. Figure 6A is a side view of an example of the structure of an illumination optical system. Figure 6B is a top view of an example of the structure of an illumination optical system. Figure 6C is a side view of an example of the structure of an illumination optical system (with a portion omitted). Figure 6D is a top view of an example of the structure of an illumination optical system (with a portion omitted). Figure 7 is an enlarged side view of a portion of the architecture of one embodiment of the invention.

31a: Cylindrical lens

34: The first pair

36a: First cylindrical lens array

36b: First cylindrical lens array

Claims (5)

An illumination optical system guides laser light toward an irradiation surface with the z-axis as the optical axis, the x-axis as the direction perpendicular to the z-axis and y-axis, and the y-axis as the direction perpendicular to the z-axis and x-axis. The illumination optical system includes a first lens array and a second lens array arranged along the z-axis, and each having a plurality of lenses arranged along at least one direction of the x-axis and y-axis. The lenses have varying thicknesses in the direction in which the light beams emitted from each lens of the second lens array interfere. The illumination optical system of claim 1, wherein: when the thickness of the lens is fixed, when interference fringes of brightness and darkness are generated in the x-axis direction on the illumination surface, the lens has lenses with alternating thicknesses in the x-axis direction; when interference fringes of brightness and darkness are generated in the y-axis direction, the lens has lenses with alternating thicknesses in the y-axis direction; and when interference fringes of brightness and darkness are generated in both the x-axis and y-axis directions, the lens has lenses with alternating thicknesses in both the x-axis and y-axis directions. The illumination optical system of claim 1 or 2, wherein: the first and second lens arrays are cylindrical lens arrays. An illumination optical system guides laser light toward an irradiation surface, wherein: with the z-axis as the optical axis, the x-axis as the direction perpendicular to both the z-axis and y-axis, and the y-axis as the direction perpendicular to both the z-axis and x-axis, a beam shaping section, a lens array section, and a collimating lens section are sequentially arranged along the z-axis. The beam shaping section and the collimating lens section are composed of a first cylindrical lens that acts as a lens in the x-axis direction and a second cylindrical lens that acts as a lens in the y-axis direction. The lens array section consists of... The system comprises a first pair consisting of two first cylindrical lens arrays arranged along the z-axis and a second pair consisting of two second cylindrical lens arrays arranged along the z-axis. The first cylindrical lens array has a lensing function in the x-axis direction, and the second cylindrical lens array has a lensing function in the y-axis direction. The center thickness of the plurality of cylindrical lenses constituting the first pair of the first cylindrical lens array or the second pair of the second cylindrical lens array is not fixed in at least one direction. A laser processing apparatus includes: a light source that emits laser light; an illumination optical system as described in claim 1, which illuminates a photomask with the laser light having a linear cross-section while simultaneously scanning the photomask by a scanning mechanism; a projection optical system that illuminates a workpiece with the laser light passing through the photomask; and a workpiece placed on a table, which moves the workpiece in the x-y direction while placing the workpiece.