Patent Application
20150165563
In recent years, precision micromachining and its improvement of process development to meet customer demand to reduce the size, weight and material cost of leading-edge devices has led to fast pace growth in high-tech industries in flat panel displays for touch screens, tablets, smartphones and TVs, where ultrafast industrial lasers are becoming important tools for applications requiring high precision.
There are various known ways to cut glasses. In conventional laser glass cutting processes, the separation of glass relies on laser scribing or perforation followed by separation with mechanical force or thermal stress-induced crack propagation. Nearly all current laser cutting techniques exhibit one or more shortcomings, including:
(1) limitations in their ability to perform a free form shaped cut of thin glass on a carrier due to a large heat-affected zone (HAZ) associated with the long laser pulses (nanosecond scale or longer) used for cutting,
(2) production of thermal stress that often results in cracking of the glass surface near the region of laser illumination due to the generation of shock waves and uncontrolled material removal and,
(3) creation of sub-surface damage in the glass that extends hundreds of microns (or more) glass below the surface of the glass, resulting in defect sites at which crack propagation can initiate,
(4) difficulties in controlling the depth of the cut (e.g., to within tens of microns).
The embodiments disclosed herein relate to a method and an apparatus to create small (micron and smaller) “holes” in transparent materials (glass, sapphire, etc) for the purpose of drilling, cutting, separating, perforating, or otherwise processing the materials. More particularly, an ultrashort (i.e., from 10−10 to 10−15 second) pulse laser beam (wavelengths such as, for example, 1064, 532, 355 or 266 nanometers) is focused to an energy density above the threshold needed to create a defect in the region of focus at the surface of or within the transparent material. By repeating the process, a series of laser-induced defects aligned along a predetermined path can be created. By spacing the laser-induced features sufficiently close together, a controlled region of mechanical weakness within the transparent material can be created and the transparent material can be precisely fractured or separated (mechanically or thermally) along the path defined by the series of laser-induced defects. The ultrashort laser pulse(s) may be optionally followed by a carbon dioxide (CO2) laser or other source of thermal stress to effect fully automated separation of a transparent material or part from a substrate sheet, for example.
In certain applications where transparent materials are bonded together to form a stack or layered structure, it is often desirable to selectively “cut” to the boundary of a particular layer without disturbing underlying layers. This may be performed with the addition of a reflective or absorptive (for the desired wavelength) material or layer at the preferred depth of cut. A reflective layer may be formed by depositing a thin material (for example, aluminum, copper, silver, gold, etc). A scattering or reflective layer is preferential as it scatters or reflects the incident energy (as opposed to absorbing and thermally dissipating the incident energy). In this manner, the depth of the cut may be controlled with no damage to the underlying layers. In one application, a transparent material is bonded to a carrier substrate and a reflective or absorptive layer is formed between the transparent material and carrier substrate. The reflective or absorptive layer enables cutting of the transparent material without damage to the underlying carrier substrate, which may then be reused. A carrier substrate is a support layer that is used to provide mechanical rigidity or ease of handling to allow the layers on top of the carrier substrate to be modified, cut, or drilled by one or more laser process steps described herein.
In one embodiment, a method of laser drilling, cutting, separating or otherwise processing a material includes forming a laser beam focal line in a workpiece, the laser beam focal line being formed from a pulsed laser beam, the workpiece comprising a plurality of materials including: a first layer facing the laser beam, the first layer being the material to be laser processed, a second layer, and a beam disruption layer located between the first and second layers. The laser beam focal line generates an induced absorption within the material of the first layer, the induced absorption producing a defect line along the laser beam focal line within the material of the first layer. The beam disruption layer can be, for example, a carrier layer.
In another embodiment, a method of laser processing includes forming a laser beam focal line in a workpiece, the laser beam focal line being formed from a pulsed laser beam, the workpiece including a glass layer and a transparent electrically conductive layer, the laser beam focal line generating an induced absorption within the workpiece, the induced absorption producing a defect line along the laser beam focal line through the transparent electrically conductive layer and into the glass layer.
In yet another embodiment, a method of laser processing includes forming a laser beam focal line in a workpiece, the laser beam focal line being formed from a pulsed laser beam, the workpiece comprising a plurality of glass layers, the workpiece including a transparent protective layer between each of the glass layers, the laser beam focal line generating an induced absorption within the workpiece, the induced absorption producing a defect line along the laser beam focal line within the workpiece.
In still another embodiment, a method of laser processing includes forming a laser beam focal line in a workpiece, the laser beam focal line being formed from a pulsed laser beam, the workpiece including a plurality of glass layers, the workpiece including an air gap between each of the glass layers, the laser beam focal line generating an induced absorption within the workpiece, the induced absorption producing a defect line along the laser beam focal line within the workpiece.
In yet another embodiment, a method of laser processing includes forming a laser beam focal line in a workpiece, the laser beam focal line being formed from a pulsed laser beam. The workpiece has a glass layer, the laser beam focal line generates an induced absorption within the glass layer, and the induced absorption produces a defect line along the laser beam focal line within the glass layer. The method also includes translating the workpiece and the laser beam relative to each other along a contour, thereby forming a plurality of defect lines along the contour, and applying an acid etch process, the acid etch process separating the glass layer along the contour.
Use of acid etching allows for release of complex contours, such as holes or slots or other interior contours inside a larger piece, which can be difficult to do with high speed and high yield with just laser methods. In addition, use of acid etching allows for formation of holes with dimensions that are practical for metallization or other chemical coating. Holes produced by the laser are enlarged in parallel to a target diameter in a parallel process, which may be faster than using a laser to drill out the holes to a large diameter by using further laser exposure.
Acid etching creates a stronger part than use of the laser only, by blunting any micro-cracks or damage that may be caused by prolonged exposure to the laser.
In still another embodiment, a method of laser processing includes forming a laser beam focal line in a workpiece, the laser beam focal line being formed from a pulsed laser beam. The workpiece has a glass layer, the laser beam focal line generates an induced absorption within the workpiece, and the induced absorption produces a defect line along the laser beam focal line within the workpiece. The method also includes translating the workpiece and the laser beam relative to each other along a closed contour, thereby forming a plurality of defect lines along the closed contour, and applying an acid etch process, the acid etch process facilitating removal of a portion of the glass layer circumscribed by the closed contour.
In yet another embodiment, a method of laser processing includes forming a laser beam focal line in a workpiece, the laser beam focal line being formed from a pulsed laser beam, the workpiece having a glass layer, the laser beam focal line generating an induced absorption within the workpiece, the induced absorption producing a defect line along the laser beam focal line within the workpiece, translating the workpiece and the laser beam relative to each other along a contour, thereby forming a plurality of defect lines along the contour, and directing an infrared laser beam along the contour. The infrared laser beam can be produced by a carbon dioxide (CO2) laser or other infrared laser.
Laser cutting of thin glasses in accordance with the present disclosure has advantages that include minimization or prevention of crack creation at or near the region of ablation and the ability to perform free form cuts of arbitrary shape. It is important that edge cracking and residual edge stress are avoided in parts separated from glass substrates for applications such as flat panel displays because parts have a pronounced propensity to break from an edge, even when stress is applied to the center. The high peak power of ultrafast lasers combined with tailored beam delivery in the method described herein can avoid these problems because the present method is a “cold” ablation technique that cuts without a deleterious heat effect. Laser cutting by ultrafast lasers according to the present method produces essentially no residual stress in the glass.
The present embodiments further extend to:
A method of laser processing comprising:
The present embodiments further extend to:
A method of laser processing comprising:
The present embodiments further extend to:
A method of laser processing comprising:
forming a laser beam focal line in a workpiece, the laser beam focal line being formed from a pulsed laser beam, the workpiece comprising a plurality of glass layers, the workpiece including a transparent protective layer between each of the glass layers, the laser beam focal line generating an induced absorption within the workpiece, the induced absorption producing a defect line along the laser beam focal line within the workpiece.
The present embodiments further extend to:
A method of laser processing comprising:
The present embodiments further extend to:
A method of laser processing comprising:
The present embodiments further extend to:
A method of laser processing comprising:
The present embodiments further extend to:
A method of laser processing comprising:
The present embodiments further extend to:
A method of forming a perforation comprising:
(i) providing a multilayer structure, the multilayer structure including a beam disruption element disposed on a carrier and a first layer disposed on the beam disruption element;
(ii) focusing a laser beam with wavelength λ on a first portion of the first layer, the first layer being transparent to the wavelength λ, the focusing forming a region of high laser intensity within the first layer, the high laser intensity being sufficient to effect nonlinear absorption within the region of high laser intensity, the beam disruption element preventing occurrence of nonlinear absorption in the carrier material or other layer disposed on the side of the beam disruption element opposite the first layer, the nonlinear absorption enabling transfer of energy from the laser beam to the first layer within the region of high intensity, the transfer of energy causing creation of a first perforation in the first layer in the region of high laser intensity, the first perforation extending in the direction of propagation of the laser beam;
(iii) focusing the laser beam on a second portion of the first layer; and
(iv) repeating step (ii) to form a second perforation in the second portion of the substrate, the second perforation extending in the direction of propagation of the laser beam, the beam disruption element preventing occurrence of nonlinear absorption in the carrier material or other layer disposed on the side of the beam disruption element opposite the first layer during the formation of the second perforation.
The foregoing will be apparent from the following more particular description of the example embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the representative embodiments.
FIG. 3B-1-3B-4 are an illustration of various possibilities to process the substrate by forming the laser beam focal line at different positions within the transparent material relative to the substrate.
A description of example embodiments follows.
The embodiment described herein relates to a method and apparatus for optically producing high precision cuts in or through transparent materials. Sub-surface damage may be limited to the order of 100 μm in depth or less, or 75 μm in depth or less, or 60 μm in depth or less, or 50 μm in depth or less, and the cuts may produce only low debris. Cutting of a transparent material with a laser in accordance with the present disclosure may also be referred to herein as drilling or laser drilling or laser processing. Within the context of the present disclosure, a material is substantially transparent to the laser wavelength when the absorption is less than about 10%, preferably less than about 1% per mm of material depth at this wavelength.
In accordance with methods described below, in a single pass, a laser can be used to create highly controlled full line perforation through the material, with extremely little (<75 μm, often <50 μm) subsurface damage and debris generation. This is in contrast to the typical use of spot-focused laser to ablate material, where multiple passes are often necessary to completely perforate the glass thickness, large amounts of debris are formed from the ablation process, and more extensive sub-surface damage (>100 μm) and edge chipping occur. As used herein, subsurface damage refers to the maximum size (e.g. length, width, diameter) of structural imperfections in the perimeter surface of the part separated from the substrate or material subjected to laser processing in accordance with the present disclosure. Since the structural imperfections extend from the perimeter surface, subsurface damage may also be regarded as the maximum depth from the perimeter surface in which damage from laser processing in accordance with the present disclosure occurs. The perimeter surface of the separated part may be referred to herein as the edge or the edge surface of the separated part. The structural imperfections may be cracks or voids and represent points of mechanical weakness that promote fracture or failure of the part separated from the substrate or material. By minimizing the size of subsurface damage, the present method improves the structural integrity and mechanical strength of separated parts.
Thus, it is possible to create microscopic (i.e., <2 μm and >100 nm in diameter, and in some embodiments <0.5 μm and >100 nm) elongated defect lines (also referred to herein as perforations or damage tracks) in transparent material using one or more high energy pulses or one or more bursts of high energy pulses. The perforations represent regions of the substrate material modified by the laser. The laser-induced modifications disrupt the structure of the substrate material and constitute sites of mechanical weakness. Structural disruptions include compaction, melting, dislodging of material, rearrangements, and bond scission. The perforations extend into the interior of the substrate material and have a cross-sectional shape consistent with the cross-sectional shape of the laser (generally circular). The average diameter of the perforations may be in the range from 0.1 μm to 50 μm, or in the range from 1 μm to 20 μm, or in the range from 2 μm to 10 μm, or in the range from 0.1 μm to 5 μm. In some embodiments, the perforation is a “through hole”, which is a hole or an open channel that extends from the top to the bottom of the substrate material. In some embodiments, the perforation may not be a continuously open channel and may include sections of solid material dislodged from the substrate material by the laser. The dislodged material blocks or partially blocks the space defined by the perforation. One or more open channels (unblocked regions) may be dispersed between sections of dislodged material. The diameter of the open channels is may be <1000 nm, or <500 nm, or <400 nm, or <300 nm or in the range from 10 nm to 750 nm, or in the range from 100 nm to 500 nm. The disrupted or modified area (e.g, compacted, melted, or otherwise changed) of the material surrounding the holes in the embodiments disclosed herein, preferably has diameter of <50 μm (e.g, <10 μm).
The individual perforations can be created at rates of several hundred kilohertz (several hundred thousand perforations per second, for example). Thus, with relative motion between the laser source and the material these perforations can be placed adjacent to one another (spatial separation varying from sub-micron to several or even tens of microns as desired). This spatial separation is selected in order to facilitate cutting.
In addition, through judicious selection of optics, selective cutting of individual layers of stacked transparent materials can be achieved. Micromachining and selective cutting of a stack of transparent materials is accomplished with precise control of the depth of cut through selection of an appropriate laser source and wavelength along with beam delivery optics, and the placement of a beam disruption element at the boundary of a desired layer. The beam disruption element may be a layer of material or an interface. The beam disruption element may be referred to herein as a laser beam disruption element, disruption element or the like. Embodiments of the beam disruption element may be referred to herein as a beam disruption layer, laser beam disruption layer, disruption layer, beam disruption interface, laser beam disruption interface, disruption interface, or the like.
The beam disruption element reflects, absorbs, scatters, defocuses or otherwise interferes with an incident laser beam to inhibit or prevent the laser beam from damaging or otherwise modifying underlying layers in the stack. In one embodiment, the beam disruption element underlies the layer of transparent material in which laser drilling will occur. As used herein, the beam disruption element underlies the transparent material when placement of the beam disruption element is such that the laser beam must pass through the transparent material before encountering the beam disruption element. The beam disruption element may underlie and be directly adjacent to the transparent layer in which laser drilling will occur. Stacked materials can be micromachined or cut with high selectivity by inserting a layer or modifying the interface such that a contrast of optical properties exists between different layers of the stack. By making the interface between materials in the stack more reflective, absorbing, defocusing, and/or scattering at the laser wavelengths of interest, cutting can be confined to one portion or layer of the stack.
The wavelength of the laser is selected so that the material within the stack to be laser processed (drilled, cut, ablated, damaged or otherwise appreciably modified by the laser) is transparent to the laser wavelength. In one embodiment, the material to be processed by the laser is transparent to the laser wavelength if it absorbs less than 10% of the intensity of the laser wavelength per mm of thickness of the material. In another embodiment, the material to be processed by the laser is transparent to the laser wavelength if it absorbs less than 5% of the intensity of the laser wavelength per mm of thickness of the material. In still another, the material to be processed by the laser is transparent to the laser wavelength if it absorbs less than 2% of the intensity of the laser wavelength per mm of thickness of the material. In yet another embodiment, the material to be processed by the laser is transparent to the laser wavelength if it absorbs less than 1% of the intensity of the laser wavelength per mm of thickness of the material.
The selection of the laser source is further predicated on the ability to induce multi-photon absorption (MPA) in the transparent material. MPA is the simultaneous absorption of multiple photons of identical or different frequencies in order to excite a material from a lower energy state (usually the ground state) to a higher energy state (excited state). The excited state may be an excited electronic state or an ionized state. The energy difference between the higher and lower energy states of the material is equal to the sum of the energies of the two or more photons. MPA is a nonlinear process that is generally several orders of magnitude weaker than linear absorption. It differs from linear absorption in that the strength of MPA depends on the square or higher power of the light intensity, thus making it a nonlinear optical process. At ordinary light intensities, MPA is negligible. If the light intensity (energy density) is extremely high, such as in the region of focus of a laser source (particularly a pulsed laser source), MPA becomes appreciable and leads to measurable effects in the material within the region where the energy density of the light source is sufficiently high. Within the focal region, the energy density may be sufficiently high to result in ionization.
At the atomic level, the ionization of individual atoms has discrete energy requirements. Several elements commonly used in glass (e.g., Si, Na, K) have relatively low ionization energies (˜5 eV). Without the phenomenon of MPA, a wavelength of about 248 nm would be required to create linear ionization at ˜5 eV. With MPA, ionization or excitation between states separated in energy by ˜5 eV can be accomplished with wavelengths longer than 248 nm. For example, photons with a wavelength of 532 nm have an energy of ˜2.33 eV, so two photons with wavelength 532 nm can induce a transition between states separated in energy by ˜4.66 eV in two-photon absorption (TPA), for example. Thus, atoms and bonds can be selectively excited or ionized in the regions of a material where the energy density of the laser beam is sufficiently high to induce nonlinear TPA of a laser wavelength having half the required excitation energy, for example.
MPA can result in a local reconfiguration and separation of the excited atoms or bonds from adjacent atoms or bonds. The resulting modification in the bonding or configuration can result in non-thermal ablation and removal of matter from the region of the material in which MPA occurs. This removal of matter creates a structural defect (e.g. a defect line, damage line, or “perforation”) that mechanically weakens the material and renders it more susceptible to cracking or fracturing upon application of mechanical or thermal stress. By controlling the placement of perforations, a contour or path along which cracking occurs can be precisely defined and precise micromachining of the material can be accomplished. The contour defined by a series of perforations may be regarded as a fault line and corresponds to a region of structural weakness in the material. In one embodiment, micromachining includes separation of a part from the material processed by the laser, where the part has a precisely defined shape or perimeter determined by a closed contour of perforations formed through MPA effects induced by the laser. As used herein, the term closed contour refers to a perforation path formed by the laser line, where the path intersects with itself at some location. An internal contour is a path formed where the resulting shape is entirely surrounded by an outer portion of material.
The laser is an ultrashort pulsed laser (pulse durations on the order tens of picoseconds or shorter) and can be operated in pulse mode or burst mode. In pulse mode, a series of nominally identical single pulses is emitted from the laser and directed to the workpiece. In pulse mode, the repetition rate of the laser is determined by the spacing in time between the pulses. In burst mode, bursts of pulses are emitted from the laser, where each burst includes two or more pulses (of equal or different amplitude). In burst mode, pulses within a burst are separated by a first time interval (which defines a pulse repetition rate for the burst) and the bursts are separated by a second time interval (which defines a burst repetition rate), where the second time interval is typically much longer than the first time interval. As used herein (whether in the context of pulse mode or burst mode), time interval refers to the time difference between corresponding parts of a pulse or burst (e.g. leading edge-to-leading edge, peak-to-peak, or trailing edge-to-trailing edge). Pulse and burst repetition rates are controlled by the design of the laser and can typically be adjusted, within limits, by adjusting operating conditions of the laser. Typical pulse and burst repetition rates are in the kHz to MHz range.
The laser pulse duration (in pulse mode or for pulses within a burst in burst mode) may be 10−10 s or less, or 10−11 s or less, or 10−12 s or less, or 10−13 s or less. In the exemplary embodiments described herein, the laser pulse duration is greater than 10−15.
The perforations may be spaced apart and precisely positioned by controlling the velocity of a substrate or stack relative to the laser through control of the motion of the laser and/or the substrate or stack. As an example, in a thin transparent substrate moving at 200 mm/sec exposed to a 100 kHz series of pulses (or bursts of pulses), the individual pulses would be spaced 2 microns apart to create a series of perforations separated by 2 microns. This defect line (perforation) spacing is sufficiently close to allow for mechanical or thermal separation along the contour defined by the series of perforations. Distance between adjacent defect lines along the direction of the fault lines can, for example, be in range from 0.25 μm to 50 μm, or in the range from 0.50 μm to about 20 μm, or in the range from 0.50 μm to about 15 μm, or in the range from 0.50 μm to 10 μm, or in the range from 0.50 μm to 3.0 μm or in the range from 3.0 μm to 10 μm.
In some cases, a fault line created along a contour defined by a series of perforations or defect lines is not enough to separate the part spontaneously, and a secondary step may be necessary. If so desired, a second laser can be used to create thermal stress to separate it, for example. In the case of low stress glass such as Corning Eagle XG or Corning glass code 2318 before it has undergone chemical strengthening from ion-exchange, separation can be achieved, after the creation of a fault line, by application of mechanical force or by using a thermal source (e.g., an infrared laser, for example a CO2 laser) to create thermal stress and force a part to separate from a substrate. Another option is to have the CO2 laser only start the separation and then finish the separation manually. The optional CO2 laser separation can be achieved, for example, with a defocused continuous wave (cw) laser emitting at 10.6 μm and with power adjusted by controlling its duty cycle. Focus change (i.e., extent of defocusing up to and including focused spot size) is used to vary the induced thermal stress by varying the spot size. Defocused laser beams include those laser beams that produce a spot size larger than a minimum, diffraction-limited spot size on the order of the size of the laser wavelength. For example, defocused spot sizes (1/e2 diameter) of 2 to 12 mm, or about 7 mm, 2 mm and 20 mm can be used for CO2 lasers, for example, whose diffraction-limited spot size is much smaller given the emission wavelength of 10.6 μm.
Acid etching can be used, for example, to separate a workpiece having a glass layer, for example. In one embodiment, for example, the acid used can be 10% HF/15% HNO3 by volume. The parts can be etched for 53 minutes at a temperature of 24-25° C. to enlarge the diameter of the holes formed via MPA with the laser to ˜100 μm, for example. The laser-perforated parts can be immersed in this acid bath, and ultrasonic agitation at a combination of 40 kHz and 80 kHz frequencies, for example, can used to facilitate penetration of fluid and fluid exchange in the holes. In addition, manual agitation of the part within the ultrasonic field can be made to prevent standing wave patterns from the ultrasonic field from creating “hot spots” or cavitation related damage on the part. The acid composition and etch rate can be intentionally designed to slowly etch the part—a material removal rate of only 1.9 μm/minute, for example. An etch rate of less than about 2 μm/minute, for example, allows acid to fully penetrate the narrow holes and agitation to exchange fresh fluid and remove dissolved material from the holes which are very narrow when initially formed by the laser. Once the acid penetrates the holes, and the holes enlarge to a size which connects them to an adjacent hole, then the perforated contour will separate from the remainder of the substrate. For example, this allows an interior feature such as a hole or a slot to be dropped out from a larger part, or a window to be dropped out from a larger “frame” containing it.
In the embodiment shown in
In one embodiment, the beam disruption element is positioned immediately below the layer of the stack in which modification via two-(or multi-)photon absorption will occur. Such a configuration is shown in
The beam disruption element has different optical properties than the material to be cut. For example, the beam disruption element may be a defocusing element, a scattering element, a translucent element, a diffracting element, an absorbing element, or a reflective element. A defocusing element is an interface or a layer comprising a material that prevents the laser light from forming the laser beam focal line on or below the defocusing element. The defocusing element may be comprised of a material or interface with refractive index inhomogeneities that scatter or perturb the wavefront of the optical beam. A translucent element is an interface or layer of material that allows light to pass through, but only after scattering or attenuating the laser beam to lower the energy density sufficiently to prevent formation of a laser beam focal line in portions of the stack on the side of the translucent element that are remote from the laser beam. In one embodiment, the translucent element effects scattering or deviating of at least 10% of the light rays of the laser beam.
More specifically, the reflectivity, absorptivity, defocusing, diffractivity, attenuation, and/or scattering of the disruption element can be employed to create a barrier or impediment to the laser radiation. The laser beam disruption element can be created by several means. If the optical properties of the overall stack system are not of a concern, then one or more thin films can be deposited as a beam disruption layer(s) between the desired two layers of the stack, where the one or more thin films absorb, scatter, defocus, attenuate, reflects, diffracts, and/or dissipates more of the laser radiation than the layer immediately above it to protect layers below the beam disruption layer(s) from receiving excessive energy density from the laser source. If the optical properties of the entire stack system do matter, the beam disruption element can be implemented as a notch filter. This can be done by several methods:
It is not necessary that the absorption, reflection, diffraction, scattering, attenuation, defocusing etc. of the laser beam by the beam disruption element be complete. It is only necessary that the effect of the beam disruption element on the laser beam is sufficient to reduce the energy density or intensity of the focused laser beam to a level below the threshold required for cutting, ablation, perforating etc. of the layers in the stack protected by (underlying) the beam disruption element. In one embodiment, the beam disruption element reduces the energy density or intensity of the focused laser beam to a level below the threshold needed to induce two-(or multi-)photon absorption. The beam disruption layer or beam disruption interface may be configured to absorb, reflect, diffract, or scatter the laser beam, where the absorption, reflection, diffraction, or scattering are sufficient to reduce the energy density or intensity of the laser beam transmitted to the carrier (or other underlying layer) to a level below the level needed to induce nonlinear absorption in the carrier or underlying layer.
Turning to
Layer 1 is the layer of a multilayer stack in which internal modifications by laser processing and two-(or multi-)photon absorption is to occur. Layer 1 is a component of a larger multilayer workpiece (the balance of which is not shown), which typically includes a substrate or carrier upon which a multilayer stack is formed. Layer 1 is the layer within the multilayer stack in which holes, cuts, or other features are to be formed through two-(or multi-)photon absorption assisted ablation or modification as described herein. In
As
As
Representative optical assemblies 6, which can be applied to generate the focal line 2b, as well as a representative optical setup, in which these optical assemblies can be applied, are described below. All assemblies or setups are based on the description above so that identical references are used for identical components or features or those which are equal in their function. Therefore only the differences are described below.
To ensure high quality (regarding breaking strength, geometric precision, roughness and avoidance of re-machining requirements) of the surface of separation after cracking along the contour defined by the series of perforations, the individual focal lines used to form the perforations that define the contour of cracking should be generated using the optical assembly described below (hereinafter, the optical assembly is alternatively also referred to as laser optics). The roughness of the separated surface is determined primarily by the spot size or the spot diameter of the focal line. Roughness of a surface can be characterized, for example, by an Ra surface roughness parameter defined by the ASME B46.1 standard. As described in ASME B46.1, Ra is the arithmetic average of the absolute values of the surface profile height deviations from the mean line, recorded within the evaluation length. In alternative terms, Ra is the average of a set of absolute height deviations of individual features (peaks and valleys) of the surface relative to the mean.
In order to achieve a small spot size of, for example, 0.5 μm to 2 μm for a given wavelength λ of the laser 3 that interacts with the material of layer 1, certain requirements must usually be imposed on the numerical aperture of laser optics 6. These requirements are met by laser optics 6 described below. In order to achieve the required numerical aperture, the optics must, on the one hand, dispose of the required opening for a given focal length, according to the known Abbé formulae (N.A.=n sin (theta), n: refractive index of the material to be processed, theta: half the aperture angle; and theta=arctan(DL/2f); DL: aperture diameter, f: focal length). On the other hand, the laser beam must illuminate the optics up to the required aperture, which is typically achieved by means of beam widening using widening telescopes between the laser and focusing optics.
The spot size should not vary too strongly for the purpose of a uniform interaction along the focal line. This can, for example, be ensured (see the embodiment below) by illuminating the focusing optics only in a small, circular area so that the beam opening and thus the percentage of the numerical aperture only vary slightly.
According to
Lens 7 is centered on the central beam and is designed as a non-corrected, bi-convex focusing lens in the form of a common, spherically cut lens. The spherical aberration of such a lens may be advantageous. As an alternative, aspheres or multi-lens systems deviating from ideally corrected systems, which do not form an ideal focal point but a distinct, elongated focal line of a defined length, can also be used (i.e., lenses or systems which do not have a single focal point). The zones of the lens thus focus along a focal line 2b, subject to the distance from the lens center. The diameter of aperture 8 across the beam direction is approximately 90% of the diameter of the beam bundle (defined by the distance required for the intensity of the beam to decrease to 1/e2 of the peak intensity) and approximately 75% of the diameter of the lens 7 of the optical assembly 6. The focal line 2b of a non-aberration-corrected spherical lens 7 generated by blocking out the beam bundles in the center is thus used.
One potential disadvantage of this type of a focal line formed by lens 7 and the system shown in
FIG. 3B-1-4 show (not only for the optical assembly in
In the case shown in
It is particularly advantageous to position the focal line 2b in such a way that at least one of surfaces 1a, 1b is covered by the focal line, so that the section of induced nonlinear absorption 2c starts at least on one surface of the layer or material to be processed. In this way it is possible to achieve virtually ideal cuts while avoiding ablation, feathering and particulation at the surface.
However, the depicted layout is subject to the following restrictions: Since the region of focal line 2b formed by axicon 9 begins within the axicon 9, a significant part of the laser energy is not focused into the section of induced absorption 2c of focal line 2b, which is located within the material, in the situation where there is a separation between axicon 9 and the material to be processed. Furthermore, length l of focal line 2b is related to the beam diameter through the refractive indices and cone angles of axicon 9. This is why, in the case of relatively thin materials (several millimeters), the total focal line is much longer than the thickness of the material to be processed, having the effect that much of the laser energy is not focused into the material.
For this reason, it may be desirable to use an optical assembly 6 that includes both an axicon and a focusing lens.
It is therefore advantageous if the focal line is formed at a certain distance from the laser optics, and if the greater part of the laser radiation is focused up to a desired end of the focal line. As described, this can be achieved by illuminating a primarily focusing element 11 (lens) only circularly (annularly) over a particular outer radial region, which, on the one hand, serves to realize the required numerical aperture and thus the required spot size, and, on the other hand, however, the circle of diffusion diminishes in intensity after the required focal line 2b over a very short distance in the center of the spot, as a basically circular spot is formed. In this way, the formation of defect lines is stopped within a short distance in the required substrate depth. A combination of axicon 10 and focusing lens 11 meets this requirement. The axicon acts in two different ways: due to the axicon 10, a usually round laser spot is sent to the focusing lens 11 in the form of a ring, and the asphericity of axicon 10 has the effect that a focal line is formed beyond the focal plane of the lens instead of a focal point in the focal plane. The length l of focal line 2b can be adjusted via the beam diameter on the axicon. The numerical aperture along the focal line, on the other hand, can be adjusted via the distance Z1 (axicon-lens separation) and via the cone angle of the axicon. In this way, the entire laser energy can be concentrated in the focal line.
If the formation of the defect line is intended to continue to the back side of the layer or material to be processed, the circular (annular) illumination still has the advantage that (1) the laser power is used optimally in the sense that most of the laser light remains concentrated in the required length of the focal line, and (2) it is possible to achieve a uniform spot size along the focal line—and thus a uniform separation process along the perforations produced by the focal lines—due to the circularly illuminated zone in conjunction with the desired aberration set by means of the other optical functions.
Instead of the plano-convex lens depicted in
In order to generate very short focal lines 2b using the combination of an axicon and a lens depicted in
As shown in
The optical assembly 6 depicted in
In the depicted example it is possible to achieve a length of the focal line 1 of less than 0.5 mm using a typical laser beam diameter of 2 mm, a focusing lens 11 with a focal length f=25 mm, a collimating lens with a focal length f′=150 mm, and choosing distances Z1a=Z1b=140 mm and Z2=15 mm.
More specifically, as illustrated in
The required energy to modify the material can be described in terms of the burst energy—the energy contained within a burst (each burst 500 contains a series of pulses 500A), or in terms of the energy contained within a single laser pulse (many of which may comprise a burst). For these applications, the energy per burst (per millimeter of the material to be cut) can be from 10-2500 μJ, or from 20-1500 μJ, or from 25-750 μJ, or from 40-2500 μJ, or from 100-1500 μJ, or from 200-1250 μJ, or from 250-1500 μJ, or from 250-750 μJ. The energy of an individual pulse within the burst will be less, and the exact individual laser pulse energy will depend on the number of pulses 500A within the burst 500 and the rate of decay (e.g, exponential decay rate) of the laser pulses with time as shown in
The use of lasers capable of generating such pulse bursts is advantageous for cutting or modifying transparent materials, for example glass. In contrast with the use of single pulses spaced apart in time by the repetition rate of a single-pulsed laser, the use of a burst pulse sequence that spreads the laser energy over a rapid sequence of pulses within burst 500 allows access to larger timescales of high intensity interaction with the material than is possible with single-pulse lasers. While a single-pulse can be expanded in time, conservation of energy dictates that as this is done, the intensity within the pulse must drop as roughly one over the pulse width. Hence if a 10 psec single pulse is expanded to a 10 nsec pulse, the intensity drops by roughly three orders of magnitude. Such a reduction can reduce the optical intensity to the point where non-linear absorption is no longer significant and the light-material interaction is no longer strong enough to allow for cutting. In contrast, with a burst pulse laser, the intensity during each pulse or sub-pulse 500A within the burst 500 can remain very high—for example three pulses 500A with pulse duration Td 10 psec that are spaced apart in time by a separation Tp of approximately 10 nsec still allows the intensity within each pulse to be approximately three times higher than that of a single 10 psec pulse, while the laser is allowed to interact with the material over a timescale that is three orders of magnitude larger. This adjustment of multiple pulses 500A within a burst thus allows manipulation of timescale of the laser-material interaction in ways that can facilitate greater or lesser light interaction with a pre-existing plasma plume, greater or lesser light-material interaction with atoms and molecules that have been pre-excited by an initial or previous laser pulse, and greater or lesser heating effects within the material that can promote the controlled growth of defect lines (perforations). The amount of burst energy required to modify the material will depend on the substrate material composition and the length of the line focus used to interact with the substrate. The longer the interaction region, the more the energy is spread out, and the higher the burst energy that will be required.)
A defect line or a hole is formed in the material when a single burst of pulses strikes essentially the same location on the glass. That is, multiple laser pulses within a single burst can produce a single defect line or a hole location in the glass. Of course, if the glass is translated (for example by a constantly moving stage) or the beam is moved relative to the glass, the individual pulses within the burst cannot be at exactly the same spatial location on the glass. However, they are well within 1 μm of one another—i. e., they strike the glass at essentially the same location. For example, they may strike the glass at a spacing sp where 0<sp≦500 nm from one another. For example, when a glass location is hit with a burst of 20 pulses the individual pulses within the burst strike the glass within 250 nm of each other. Thus, in some embodiments 1 nm<sp<250 nm. In in some embodiments 1 nm<sp<100 nm.
In general, the higher the available laser power, the faster the material can be cut with the above process. The process(s) disclosed herein can cut glass at a cutting speed of 0.25 m/sec, or faster. A cut speed (or cutting speed) is the rate the laser beam moves relative to the surface of the substrate material (e.g., glass) while creating multiple defect lines holes. High cut speeds, such as, for example 400 mm/sec, 500 mm/sec, 750 mm/sec, 1 m/sec, 1.2 m/sec, 1.5 m/sec, or 2 m/sec, or even 3.4 m/sec to 4 m/sec are often desired in order to minimize capital investment for manufacturing, and to optimize equipment utilization rate. The laser power is equal to the burst energy multiplied by the burst repetition frequency (rate) of the laser. In general, to cut glass materials at high cutting speeds, the defect lines are typically spaced apart by 1-25 μm, in some embodiments the spacing is preferably 3 μm or larger—for example 3-12 μm, or for example 5-10 μm.
For example, to achieve a linear cutting speed of 300 mm/sec, 3 μm hole pitch corresponds to a pulse burst laser with at least 100 kHz burst repetition rate. For a 600 mm/sec cutting speed, a 3 μm pitch corresponds to a burst-pulsed laser with at least 200 kHz burst repetition rate. A pulse burst laser that produces at least 40 μJ/burst at 200 kHz, and cuts at a 600 mm/s cutting speed needs to have a laser power of at least 8 Watts. Higher cut speeds require accordingly higher laser powers.
For example, a 0.4 m/sec cut speed at 3 μm pitch and 40 μJ/burst would require at least a 5 W laser, a 0.5 m/sec cut speed at 3 μm pitch and 40 μJ/burst would require at least a 6 W laser. Thus, preferably the laser power of the pulse burst picosecond laser is 6 W or higher, more preferably at least 8 W or higher, and even more preferably at least 10 W or higher. For example, in order to achieve a 0.4 m/sec cut speed at 4 μm pitch (defect line spacing, or damage tracks spacing) and 100 μJ/burst, one would require at least a 10 W laser, and to achieve a 0.5 m/sec cut speed at 4 μm pitch and 100 μJ/burst, one would require at least a 12 W laser. For example, a to achieve a cut speed of 1 m/sec at 3 μm pitch and 40 μJ/burst, one would require at least a 13 W laser. Also, for example, 1 m/sec cut speed at 4 μm pitch and 400 μJ/burst would require at least a 100 W laser.
The optimal pitch between defect lines (damage tracks) and the exact burst energy is material dependent and can be determined empirically. However, it should be noted that raising the laser pulse energy or making the damage tracks at a closer pitch are not conditions that always make the substrate material separate better or with improved edge quality. A pitch that is too small (for example <0.1 micron, or in some exemplary embodiments <1 μm, or in other embodiments <2 μm) between defect lines (damage tracks) can sometimes inhibit the formation of nearby subsequent defect lines (damage tracks), and often can inhibit the separation of the material around the perforated contour. An increase in unwanted micro cracking within the glass may also result if the pitch is too small. A pitch that is too long (e.g. >50 μm, and in some glasses >25 μm or even >20 μm) may result in “uncontrolled microcracking”—i.e., where instead of propagating from defect line to defect line along the intended contour, the microcracks propagate along a different path, and cause the glass to crack in a different (undesirable) direction away from the intended contour. This may ultimately lower the strength of the separated part since the residual microcracks constitute flaws that weaken the glass. A burst energy for forming defect lines that is too high (e.g., >2500 μJ/burst, and in some embodiments >500 μJ/burst) can cause “healing” or re-melting of previously formed defect lines, which may inhibit separation of the glass. Accordingly, it is preferred that the burst energy be <2500 μJ/burst, for example, <500 μJ/burst. Also, using a burst energy that is too high can cause formation of microcracks that are extremely large and create structural imperfections that can reduce the edge strength of the part after separation. A burst energy that is too low (e.g. <40 μJ/burst) may result in no appreciable formation of defect lines within the glass, and hence may necessitate especially high separation force or result in a complete inability to separate along the perforated contour.
Typical exemplary cutting rates (speeds) enabled by this process are, for example, 0.25 m/sec and higher. In some embodiments, the cutting rates are at least 300 mm/sec. In some embodiments, the cutting rates are at least 400 mm/sec, for example, 500 mm/sec to 2000 mm/sec, or higher. In some embodiments the picosecond (ps) laser utilizes pulse bursts to produce defect lines with periodicity between 0.5 μm and 13 μm, e.g. between 0.5 and 3 μm. In some embodiments, the pulsed laser has laser power of 10 W-100 W and the material and/or the laser beam are translated relative to one another at a rate of at least 0.25 m/sec; for example, at the rate of 0.25 m/sec to 0.35 m/sec, or 0.4 m/sec to 5 m/sec. Preferably, each pulse burst of the pulsed laser beam has an average laser energy measured at the workpiece greater than 40 μJ per burst per mm thickness of workpiece. Preferably, each pulse burst of the pulsed laser beam has an average laser energy measured at the workpiece greater of less than 2500 μJ per burst per mm thickness of workpiece, and preferably less than about 2000 μJ per burst per mm thickness of workpiece, and in some embodiments less than 1500 μJ per burst per mm thickness of workpiece; for example, not more than 500 μJ per burst per mm thickness of workpiece.
We discovered that much higher (5 to 10 times higher) volumetric pulse energy density (μJ/μm3) is required for perforating alkaline earth boroaluminosilicate glasses with low or no alkali content. This can be achieved, for example, by utilizing pulse burst lasers, preferably with at least 2 pulses per burst and providing volumetric energy densities within the alkaline earth boroaluminosilicate glasses (with low or no alkali) of about 0.05 μJ/μm3 or higher, e.g., at least 0.1 μJ/μm3, for example 0.1-0.5 μJ/μm3.
Accordingly, it is preferable that the laser produces pulse bursts with at least 2 pulses per burst. For example, in some embodiments the pulsed laser has a power of 10 W-150 W (e.g., 10 W-100 W) and produces pulse bursts with at least 2 pulses per burst (e.g., 2-25 pulses per burst). In some embodiments the pulsed laser has a power of 25 W-60 W, and produces pulse bursts with at least 2-25 pulses per burst, and periodicity or distance between the adjacent defect lines produced by the laser bursts is 2-10 μm. In some embodiments, the pulsed laser has a power of 10 W-100 W, produces pulse bursts with at least 2 pulses per burst, and the workpiece and the laser beam are translated relative to one another at a rate of at least 0.25 m/sec. In some embodiments the workpiece and/or the laser beam are translated relative to one another at a rate of at least 0.4 m/sec.
For example, for cutting 0.7 mm thick non-ion exchanged Corning code 2319 or code 2320 Gorilla® glass, it is observed that pitches of 3-7 μm can work well, with pulse burst energies of about 150-250 μJ/burst, and burst pulse numbers that range from 2-15, and preferably with pitches of 3-5 μm and burst pulse numbers (number of pulses per burst) of 2-5.
At 1 m/sec cut speeds, the cutting of Eagle XG® glass typically requires utilization of laser powers of 15-84 W, with 30-45 W often being sufficient. In general, across a variety of glass and other transparent materials, applicants discovered that laser powers between 10 W and 100 W are preferred to achieve cutting speeds from 0.2-1 m/sec, with laser powers of 25-60 W being sufficient (or optimum) for many glasses. For cutting speeds of 0.4 m/sec to 5 m/sec, laser powers should preferably be 10 W-150 W, with burst energy of 40-750 μJ/burst, 2-25 bursts per pulse (depending on the material that is cut), and defect line separation (pitch) of 3 to 15 μm, or 3-10 μm. The use of picosecond pulse burst lasers would be preferable for these cutting speeds because they generate high power and the required number of pulses per burst. Thus, according to some exemplary embodiments, the pulsed laser produces 10 W-100 W of power, for example 25 W to 60 W, and produces pulse bursts at least 2-25 pulses per burst and the distance between the defect lines is 2-15 μm; and the laser beam and/or workpiece are translated relative to one another at a rate of at least 0.25 m/sec, in some embodiments at least 0.4 m/sec, for example 0.5 m/sec to 5 m/sec, or faster.
In some of the embodiments described herein, the thickness of the air gap is between 50 μm and 5 mm, or between 50 μm and 2 mm, or between 200 μm and 2 mm.
Exemplary beam disruption layers include polyethylene plastic sheeting (e.g., Visqueen, commercially available from British Polythene Industries Limited). Transparent layers, as shown in
Embodiment methods have the advantage that substantially transparent materials such as glass, plastic, and rubber can be perforated and cut. The perforation can be through multiple laminate layers or selected layers of a laminate workpiece. Very unique product shapes and features can be produced, and embodiments can even be used to cut a formed 3D shape, with the laser beam oriented at a normal to a 3D surface of the laminate workpiece to perforate all layers, for example. Selected layers can also be perforated and/or weakened to allow for controlled breakage, such as for automotive windshields or other safety glass applications. Laminate layers of glass, plastic, and/or rubber with layer thicknesses of 0.1 mm to 1 mm, for example, can be cut at high speed for manufacturing, with very high accuracy and with very good edge quality. The disclosed laser processes can even eliminate a need for any edge finishing, which has significant cost advantages.
The relevant teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.
While exemplary embodiments have been described herein, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope encompassed by the appended claims.
This application claims the benefit of U.S. Provisional Application No. 61/917,092 filed on Dec. 17, 2013 as well as the benefit of U.S. Provisional Application No. 62/022,896 filed on Jul. 10, 2014, the entire disclosures of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61917092 | Dec 2013 | US | |
| 62022896 | Jul 2014 | US |
