Patent Application
20200061750
The present disclosure relates in general to processing materials using laser-radiation. The disclosure relates in particular to laser processing workpieces with low surface quality.
Laser material-processing is increasingly used for cutting, drilling, marking, and scribing a wide range of materials, including brittle materials such as glass, ceramics, silicon, and sapphire. Traditional mechanical processing produces unwanted defects, such as micro-cracks that may propagate when the processed brittle material is stressed, thereby degrading and weakening the processed brittle material. Laser-processing of brittle materials using focused beams of laser-radiation produces precise cuts and holes, having high-quality edges and walls, while minimizing the formation of such unwanted defects. Progress in scientific research and manufacturing is leading to laser-processing of an increasing range of brittle materials, while demanding increased laser-processing speed and precision.
Transparent brittle materials interact with focused beams of pulsed laser-radiation through non-linear absorption of the laser-radiation. The pulsed laser-radiation may include a train of individual pulses, or rapid bursts of pulses. Each individual pulse or burst of pulses creates a defect in a workpiece of transparent brittle material at the focus of the beam. An article is cut from the workpiece by translating the focused beam to create a row of defects along a cutting line in the workpiece.
Often the row of defects just weakens the material along the cutting line. To fully separate the article from the rest of the workpiece requires an additional step of applying stress across the cutting line. Applying mechanical stress or thermal stress usually causes separation along the cutting line. Precise and controlled separation has been demonstrated using a laser-beam having a wavelength that is absorbed by the material and relatively high average power. The absorbed laser-power creates a thermal gradient across the cutting line, which causes cracks to propagate between the discrete defects produced by the pulsed laser-radiation, thereby forming a continuous break along the cutting line.
By way of example, a highly focused beam of ultra-short laser-pulses creates a self-guiding “filament” in a glass workpiece. To create a filament, a focused beam of pulsed laser-radiation having a sufficiently high intensity in a material becomes further focused due to non-linear components of the refractive index. Positive feedback between non-linear self-focusing and the high-intensity laser beam creates a plasma. A lower refractive index within the plasma and/or scattering of the focused beam by the plasma causes defocusing. A balance between the focusing and the defocusing sustains the plasma within a filament, which propagates through the glass workpiece and has a diameter much smaller than a diffraction-limited diameter of the focused beam of pulsed laser-radiation.
Propagation of such a filament creates a long slender defect through the workpiece in the form of a void, micro-cracking, or other material modifications. A row of defects is created by translating the focused ultra-short pulsed laser-beam along the cutting line. A carbon dioxide (CO2) laser having wavelengths of around 10 micrometers (μm) can then be used to separate glass, by translating the CO2 laser-beam along the cutting line. Such a laser-cutting process is described in U.S. Pat. Nos. 9,102,007 and 9,296,066, each thereof commonly owned with the present application, and the complete disclosure of each is incorporated herein by reference for all purposes.
Laser material-processing requires a precisely-positioned and tightly-controlled focus of the laser beam. Relatively small variances in material properties (such as normal material inhomogeneities) can cause a loss of focus control. Non-planar material surfaces can defocus a laser beam due to refraction, reducing the intensity of the laser beam at the intended focus. It is possible for the beam intensity to be reduced below a threshold for the intended material processing.
Practitioners of skill in the art use “surface quality” as a measure of these variations. “Surface quality” has two contributions: small-scale surface structure, referred to as “surface roughness” or “surface finish;” and large scale structure, referred to as “surface irregularity” or “surface flatness.”
Small-scale surface structure, having high spatial frequency, causes optical losses. Usually these are scattering losses, which reduce the optical power reaching the processing location after a laser beam is transmitted through the surface. This “surface roughness” or “surface finish” is quantified by Ra (average deviation from a mean plane of the surface) or RRMS (average maximum peak-valley deviation over a prescribed surface area)
Large-scale surface structure, having low spatial frequency, causes wavefront distortion. By way of example, this wavefront distortion prevents a focused laser beam transmitted through the surface from forming a well-defined focus. This “surface irregularity” or “surface flatness” may be quantified by counting interference fringes of a monochromatic test beam when the surface contacts another known flat surface. Therefore, deviation from an ideal flat surface is measured in multiples of the wavelength λ of the test beam.
Existing solutions do not account for laser processing a workpiece when a laser beam is directed through a surface of the workpiece having low surface quality, leading to loss of focus control.
In one aspect, a method is disclosed for laser processing a workpiece having a workpiece surface. The method includes providing a laser beam having a wavelength at which the workpiece is transparent. A cover is provided and spaced apart from the workpiece surface. The cover has a surface proximal to the workpiece surface and a surface distal to the workpiece surface, where the distal surface has a surface quality better than a surface quality of the workpiece surface. A fluid is provided between and in contact with the proximal surface and the workpiece surface. A laser beam is directed through the cover, through the fluid, and through the workpiece surface.
In one aspect, a laser processing apparatus includes a cover, a fluid dispenser, and a laser system. The cover can be spaced apart from a workpiece's surface and includes a surface proximal to the workpiece surface and a surface distal to the workpiece surface. The distal surface has a surface quality better than a surface quality of the workpiece surface. The fluid dispenser is configured to introduce fluid between and in contact with the proximal surface and the workpiece surface. The laser system is configured to direct a laser beam through the cover, through the fluid, and through the workpiece surface, and the laser beam has a wavelength at which the workpiece is transparent.
Methods and apparatuses described herein position a cover and fluid between a laser system and a workpiece surface, with the cover including a distal surface having a surface quality better than a surface quality of the workpiece surface. Embodiments described herein reduce ray scattering due to refraction through a surface having low surface quality, thereby increasing control of the position and size of a laser beam's focus inside or outside a workpiece. They enable formation of a focus with an intended intensity distribution when a converging beam of laser radiation must traverse a surface having low surface quality. Methods and apparatuses described herein may be advantageous where, for example, a workpiece inherently includes low surface quality (e.g., drawn glass or unpolished glass), other processing steps reduce surface quality of a workpiece's surface (e.g., microfabrication of semi-conductor devices), and in any laser process that requires a tightly controlled laser focus.
Herein, “focus” refers to tight foci and to elongated foci, which are both used in laser material-processing. A tight focus can be formed by focusing optics having relatively short focal lengths that cause minimal aberration of the laser beam. An elongated focus can be formed by focusing optics that deliberately cause aberration of the laser beam. By way of example, an elongated focus can be created by filling the clear aperture of a focusing lens having spherical aberration. Alternatively, an aspheric focusing lens may be configured to form an elongated focus having a uniform intensity distribution along the optical axis, as described in U.S. patent application Ser. No. 15/352,385 (U.S. Patent Publication No. 2018/0133837), which is commonly owned with the present application. An elongated focus has advantages in laser-cutting, because the focused laser-radiation is distributed to favor creation of long defects that extend through the full thickness of the workpiece.
Turning now to the drawings, where like features are designated by like reference numerals,
The cross-section in photograph 100 is in a plane traversed by the optical axis of the pulsed laser beam. Dark regions 104 represent areas where the laser beam formed defects that weakened the material. Clear regions 106 represent incomplete or interrupted defects, i.e., where the material was not weakened. As can be seen in photograph 100, unprocessed regions where there are no defects 106 are frequent and irregular. Although the workpiece in
The inventors posited that the low surface quality of surface 102 of the workpiece adversely affected the laser system's (not shown) ability to produce a precisely-positioned and tightly-controlled focus, thereby forming the incomplete or interrupted defects.
The improved results depicted in
Apparatus 200 further includes an optional beam-steering optic 226, an optional beam-conditioning optic 228, and a focusing lens 230.
Focusing lens 230 could be a single-element lens as depicted or a multi-element lens assembly. Workpiece 212A is depicted being translated with respect to a stationary focused beam of pulsed laser-radiation 214. Alternatively, galvanometer-actuated mirrors could be included in beam-conditioning optic 228 and a flat-field objective lens used for focusing lens 230, thereby enabling focused beam of pulsed laser-radiation 214 to be translated with respect to a stationary workpiece 212A.
In laser filament processing, for example, the scattering may reduce the intensity of the laser beam at the intended focus 232B below a threshold for non-linear self-focusing, preventing formation of filaments. When filaments do form, the anomalous intensity distribution along optical axis 234 may lead to the creation of incomplete and irregular defects 220B. Under such conditions, laser filament processing would produce frequent and irregular unprocessed regions, like those shown in photograph 100 of
Embodiments disclosed herein can produce the superior laser processing of
Distal surface 304A of cover 302 has a surface quality better than surface 213B of workpiece 212B. As used herein, a first surface has a better surface quality than a second surface when the first surface's surface roughness is lower than the second surface's surface roughness and/or the first surface's surface irregularity is lower than the second surface's surface irregularity. In some embodiments, distal surface 304A has an optical quality equivalent to a surface having roughness of less than 20 Å (Angstroms) and/or irregularity of less than λ/4, where λ is the wavelength of the laser beam. In some embodiments, distal surface 304A has an optical quality equivalent to roughness of less than 5 Å and/or irregularity of less than λ/20. As used herein, “surface quality” refers to those areas of a workpiece where the laser beam is incident on a surface of the workpiece.
As the rays of beam 214 pass through distal surface 304A, a difference between the refractive indices of the gas and cover 302 causes the rays to refract. Because distal surface 304A of cover 302 has a better surface quality than surface 213B of workpiece 212B, the rays will refract more predictably than if the rays passed through surface 213B depicted in
Fluid 306 can flow to occupy the troughs of the rough surface of workpiece 212B, resulting in a cover/fluid/workpiece arrangement with distal surface 304A serving as the interface of the gas and the cover/fluid/workpiece arrangement for incident laser beam 214. Fluid 306 is selected such that the difference between the refractive indices of fluid 306 and workpiece 212B is smaller than the difference between the refractive indices of the gas and workpiece 212B. This selection reduces refraction as the laser beam passes through surface 213B, thereby reducing undesirable scattering of the rays. Both cover 302 and fluid 306 are selected to be transparent at the wavelength of laser beam 214. Ray scattering can be further reduced, as explained below.
To minimize reflective losses through the cover/fluid/workpiece arrangement, it is preferable to select a cover that has a refractive index less than or equal to the refractive index of the workpiece. The fluid would be preferably selected to have a refractive index that is between the refractive index of the workpiece and the refractive index of the cover. To further minimize reflective losses, one or both of the proximal and distal surfaces of the cover may have an antireflection coating.
In some embodiments, fluid 306 has a refractive index matching the refractive index of workpiece 212B. As used herein, a refractive index matches another refractive index when they are less than 10% different from one another. In some embodiments, a fluid's refractive index is less than 3% different from a workpiece's refractive index. Matching the refractive indices of fluid 306 and workpiece 212B reduces or eliminates refraction at surface 213B of workpiece 212B. In some embodiments, cover 302 has a refractive index matching the refractive indices of both fluid 306 and workpiece 212B. After passing through distal surface 304A, the rays would pass through cover 302, fluid 306, and workpiece 212B without changing direction due to the constant (or near constant) refractive index.
In some embodiments, the cover's thickness is chosen so that the cover is sufficiently resilient to prevent warping or changes in position. In some embodiments, the cover thickness and fluid thickness are chosen to minimize the distance between the cover's distal surface and the workpiece surface. Minimizing the distance between the cover's distal surface and the workpiece surface maximizes an effective working distance of the laser system. Specifically, here, the working distance between focusing lens 230 (depicted in
In some embodiments, cover 302 is made of a glass. In some embodiments, a cover is made of soda lime glass. By way of example, the cover used to capture
As depicted in
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In some embodiments, fluid 306 includes a liquid, a gel, a malleable polymer, or a conformable solid. In some embodiments, fluid 306 is an oil. Exemplary oils to match a workpiece made of Gorilla® Glass having a refractive index of about 1.51 at 1064 nanometers include IM01-immersion oil/IM02-immersion oil (refractive index 1.48-1.482), glycerin (refractive index 1.46), and Olympus immersion oil (refractive index 1.51). In some embodiments, the cover is a transparent foil (e.g., PVC) and the fluid is an adhesive having a refractive index matching a workpiece's refractive index.
In some embodiments, laser beam 214 has a wavelength at which workpiece 212B is transparent. As used herein, an object is “transparent” to a laser beam when all or a portion of the laser beam's power incident on an object's surface is transmitted to a location below the object's surface. For example, an object is transparent to a laser beam when 40% of incident laser power is transmitted to a location below the object's surface or an object is transparent to a laser beam when 70% of incident laser power is transmitted to a location below the object's surface. For example, a workpiece is transparent when at least 40% of incident laser power is transmitted to the location of a focus.
In some embodiments, the laser system is configured to form a focus 232A at a location inside workpiece 212B. In some embodiments, the laser system is configured to direct the laser beam 214 through a second opposite surface of the workpiece and form a focus outside the workpiece. For example, below the lower surface (in the orientation depicted in
In some embodiments, an apparatus includes a translation stage configured to move the workpiece relative to the laser beam and the fluid dispenser is configured to introduce fluid between the cover and workpiece while the workpiece moves relative to the laser beam (see
In some embodiments, the apparatus includes a fluid removal system configured to remove fluid 306 from workpiece 212B after the laser beam 214 has processed workpiece 212B. In such embodiments, a volatile index matched fluid may be used for efficient and complete fluid removal.
In some embodiments, the laser system is configured to focus the laser beam to form a filament and thereby create a defect 220A in workpiece 212B. In some embodiments, laser processing apparatus 300 is used for other laser processes, such as stealth dicing (e.g., processing of silicon at a wavelength of about 1 μm). Laser processing apparatus 300 may be advantageous in, for example, any laser material processing requiring good beam integrity, particularly high intensity and/or fine control of beam parameters.
Laser beam 214 passes through cover 302, through fluid 306, and into workpiece 212B. In the embodiment depicted in
Fluid dispenser 404 incorporates the cover 302, at least one fluid supply line 402, and at least one fluid reservoir (not shown). The shape of the bottom surface of the dispenser that includes the cover could be round, rectangular, or any shape suitable for the application. In some embodiments, fluid dispenser 404 is either a part of the laser-processing head or is attached to the head. Fluid is dispensed by a pump, capillary action, and/or gravity. For a pump embodiment, the pumps (not shown) may include an adjustable pump speed that is varied in combination with the translation speed (of the workpiece) to create a desired fluid feed between the cover and the workpiece. A fluid film may remain on the workpiece after laser processing. As described above with respect to
Laser beam 214 passes through cover 302, through fluid 306, and into workpiece 212B. In the embodiment depicted in
As shown in
Cover 602 includes a distal surface 604A upon which laser beam 214 is incident normally and proximal surface 604B that is parallel to surface 213B of workpiece 212B. Distal surface 604A and proximal surface 604B are thus mutually inclined and cover 602 has a wedge or prism shape. In other embodiments, the cover may have a different shape, provided that the distal and proximal surface of the workpiece are mutually inclined. Again, a fluid 606 is between and in contact with proximal surface 604B and surface 213B. For example, a variety of proximal surface-distal surface relative inclinations (including a parallel arrangement) are available so that a cover can be moved to accommodate a variety of cuts on a workpiece.
In some embodiments of the method, directing the laser beam includes focusing the laser beam and forming a defect in the workpiece.
In some embodiments, the fluid has a refractive index that is between a refractive index of a gas above the cover and a refractive index of the workpiece. In some embodiments, the fluid has a refractive index that is between a refractive index of the cover and a refractive index of the workpiece. In some embodiments of the method, the fluid has a refractive index matching the refractive index of the workpiece.
In some embodiments of the method, the distal surface has a lower surface roughness than a surface roughness of the workpiece surface. In some embodiments of the method, the distal surface has a surface roughness of less than 20 Å.
In some embodiments of the method, the distal surface has a lower surface irregularity than a surface irregularity of the workpiece surface. In some embodiments of the method, the laser beam has a wavelength λ and the distal surface has a surface irregularity of less than λ/4.
In some embodiments of the method, the proximal surface is parallel to the workpiece surface.
In some embodiments of the method, an optical axis of the laser beam is incident normally onto the workpiece surface.
In some embodiments of the method, wherein an optical axis of the laser beam is incident onto the workpiece surface at a non-normal angle.
In some embodiments of the method, the proximal surface and distal surface are parallel. In some embodiments of the method, the proximal surface and distal surface are mutually inclined. In some embodiments of the method, the proximal surface is parallel to the workpiece surface, and directing the laser beam comprises focusing the laser beam to produce a defect in the workpiece and oblique to the workpiece surface.
In some embodiments of the method, the distal surface has a convex shape.
In some embodiments of the method, the cover is glass. In some embodiments of the method, the fluid is an oil. In some embodiments of the method, the cover is a foil and the fluid is an adhesive having a refractive index matching a refractive index of the workpiece.
In some embodiments of the method, directing the laser beam comprises focusing the laser beam at a location inside the workpiece. In some embodiments of the method, directing the laser beam further comprises directing the laser beam through a second surface of the workpiece and focusing the laser beam at a location outside the workpiece.
Some embodiments of the method further comprise repeatedly directing the laser beam while moving the workpiece relative to the laser beam and adding fluid between the proximal surface and workpiece surface.
Some embodiments of the method further comprise removing the fluid after the directing the laser beam.
In some embodiments, focused beam of pulsed laser-radiation 214 converges to a focus that is elongated along optical axis 234, as discussed above. Referring to
In some embodiments, the cover surfaces may be non-planar. Although cover 302 is depicted in
As discussed herein above, in some embodiments, the laser processing apparatus is configured to direct laser beam 214 through surface 213B and through a second opposite surface of workpiece 212B. In these embodiments, a second cover may be spaced apart from the second surface, and having a fluid filling the space thereinbetweeen. This arrangement allows a focus to be formed outside a workpiece with opposing surfaces that both have low surface quality. An external focus is favorable in some applications. For example, to form a defect that extends to a surface may require an elongated focus that traverses the surface.
Some embodiments include an additional step of exposing a workpiece to a beam of laser-radiation generated by a source of laser-radiation different from laser-source 218 of
The present invention is described above with reference to preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted herein. Rather, the invention is limited only by the claims appended hereto. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.
