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WO2014194179A1 - Traitement au laser utilisant une tache de faisceau allongée astigmatique et utilisant des impulsions ultracourtes et/ou des longueurs d'onde plus longues - Google Patents

Traitement au laser utilisant une tache de faisceau allongée astigmatique et utilisant des impulsions ultracourtes et/ou des longueurs d'onde plus longues Download PDF

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Publication number
WO2014194179A1
WO2014194179A1 PCT/US2014/040192 US2014040192W WO2014194179A1 WO 2014194179 A1 WO2014194179 A1 WO 2014194179A1 US 2014040192 W US2014040192 W US 2014040192W WO 2014194179 A1 WO2014194179 A1 WO 2014194179A1
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WIPO (PCT)
Prior art keywords
astigmatic
axis
beam spot
laser
substrate
Prior art date
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PCT/US2014/040192
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English (en)
Inventor
Jeffrey P. Sercel
Marco MENDES
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IPG Microsystems LLC
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IPG Microsystems LLC
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Filing date
Publication date
Priority claimed from US13/905,352 external-priority patent/US20130256286A1/en
Application filed by IPG Microsystems LLC filed Critical IPG Microsystems LLC
Publication of WO2014194179A1 publication Critical patent/WO2014194179A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/073—Shaping the laser spot
    • B23K26/0736—Shaping the laser spot into an oval shape, e.g. elliptic shape
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/062—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam
    • B23K26/0622—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam by shaping pulses
    • B23K26/0624—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam by shaping pulses using ultrashort pulses, i.e. pulses of 1ns or less
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/36—Removing material
    • B23K26/362—Laser etching
    • B23K26/364—Laser etching for making a groove or trench, e.g. for scribing a break initiation groove
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/36—Removing material
    • B23K26/38—Removing material by boring or cutting
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/36—Removing material
    • B23K26/40—Removing material taking account of the properties of the material involved
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2101/00—Articles made by soldering, welding or cutting
    • B23K2101/36—Electric or electronic devices
    • B23K2101/40—Semiconductor devices
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/08—Non-ferrous metals or alloys
    • B23K2103/10—Aluminium or alloys thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/08—Non-ferrous metals or alloys
    • B23K2103/12—Copper or alloys thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/08—Non-ferrous metals or alloys
    • B23K2103/14—Titanium or alloys thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
    • B23K2103/50—Inorganic material, e.g. metals, not provided for in B23K2103/02 – B23K2103/26

Definitions

  • This invention relates to laser processing, and more particularly, relates to laser processing, such as scribing, using an astigmatic elongated beam spot formed from a solid- state laser producing ultrashort pulses and/or longer wavelengths in the visible or IR ranges.
  • Lasers are commonly used to process or machine a workpiece, for example, by cutting or scribing a substrate or semiconductor wafer.
  • a laser is often used in the process of dicing a semiconductor wafer such that individual devices (or dies) manufactured from the semiconductor wafer are separated from each other.
  • the dies on the wafer are separated by streets and the laser may be used to cut the wafer along the streets.
  • a laser may be used to cut all the way through the wafer, or part way through the wafer with the remaining portion of the wafer separated by breaking the wafer at the point of perforation.
  • LEDs light emitting diodes
  • the individual dies on the wafer correspond to the LEDs.
  • a laser may be focused onto a surface of the substrate or wafer to cause ablation of the material and to effect a partial cut.
  • Laser scribing may be performed on a semiconductor wafer, for example, on the front side of the wafer with the devices formed thereon, referred to as front-side scribing (FSS), or on the back side of the wafer, referred to as back-side scribing (BSS).
  • FSS front-side scribing
  • BSS back-side scribing
  • Existing systems and methods have used an astigmatic elongated beam spot or line beam to perform laser scribing, for example, as described in greater detail in U.S. Patent No. 7,709,768, which is incorporated herein by reference.
  • FIG. 1 is a schematic diagram of a beam delivery system (BDS) with astigmatic focal point optics, according to one embodiment of the present invention.
  • BDS beam delivery system
  • FIG. 2 is a schematic diagram of the BDS shown in FIG. 1 illustrating the sequential modification of the laser beam from the laser to the target.
  • FIG. 3 is a cross-sectional view of a beam, illustrating the formation of two focal points separately in each principal meridian.
  • FIG. 4 is a cross-sectional view of a beam focusing lens in the BDS shown in FIG. 1, illustrating the 'y component' of the highly compressed beam passing through the beam focusing lens.
  • FIG. 5 is a cross-sectional view of a beam focusing lens in the BDS shown in FIG. 1, illustrating the 'x component' of the highly compressed beam passing through the beam focusing lens.
  • FIG. 6 is a cross-sectional view of the BDS shown in FIG. 1, illustrating the formation of two separated focal points in one principal meridian.
  • FIG. 7 is a cross-sectional view of the BDS shown in FIG. 1, illustrating the formation of two separated focal points in the other principal meridian.
  • FIGS. 8 and 9 are cross-sectional views of the BDS shown in FIG. 1, illustrating the flexibility of adjusting processing parameters in the BDS.
  • An adjustable astigmatic elongated beam spot may be formed from a laser beam having ultrashort laser pulses and/or longer wavelengths, consistent with embodiments described herein, to machine substrates made of a variety of different materials.
  • the laser beam may be generated with pulses having a pulse duration of less than 1 ns and/or having a wavelength greater than 400 nm.
  • the laser beam is modified to produce an astigmatic beam that is collimated in a first axis and converging in a second axis.
  • the astigmatic beam is focused to form the astigmatic elongated beam spot on a substrate, which is focused on the substrate in the first axis and defocused in the second axis.
  • the astigmatic elongated beam spot may be adjusted in length to provide an energy density sufficient for a single ultrashort pulse to cause cold ablation of at least a portion of the substrate material.
  • the adjustable astigmatic elongated beam spot allows the energy density to be adjusted to avoid losing the benefit of using ultrashort pulses for ablation, as described in greater detail below.
  • laser machining and “laser processing” refer to any act of using laser energy to alter a workpiece and “scribing” refers to the act of machining or processing a workpiece by scanning the laser across the workpiece. Machining or processing may include, without limitation, ablation of the material at a surface of the workpiece and/or crystal damage of the material inside the workpiece. Scribing may include a series of ablations or crystal-damaged regions and does not require a continuous line of ablation or crystal damage.
  • cold ablation refers to the ablation or removal of material caused by absorption of laser energy while also removing heat through the ejection of ablated materials.
  • Laser induced photonic ablation may occur when atoms of a material with a defined bandgap are excited into higher quantum states through the absorption of energy.
  • energy of a single photon meets or exceeds the bandgap of the target material (quantum absorption energy)
  • laser energy can be absorbed, the exposed material is vaporized, and heat and debris are carried away in the plasma in a cold ablation process.
  • material bandgap exceeds the energy of a single photon (e.g., at longer wavelengths)
  • multiphoton absorption may be required for cold ablation. Multiphoton absorption is a non-linear intensity dependent process, and thus shorter pulses provide a more efficient process.
  • Ultrashort laser pulses with high photonic energy may provide an advantage in achieving multiphoton absorption.
  • the benefits of using ultrashort laser pulses to achieve cold ablation may be eliminated, however, when the energy density (J/cm 2 ) or the average power (W) used are too high above an optimum value.
  • the energy density (J/cm 2 ) or the average power (W) used are too high above an optimum value.
  • W average power
  • a fraction of pulse energy may be converted to heat and remain in the material. Excess heat accumulation may result in melting and/or other heat damage. This heat may accumulate when excess energy is locally applied to the material, for example, by using an energy density above an optimum process and material dependent value. In one example, the energy density should be maintained below 5 J/cm 2 for a 10 ps pulse to avoid undesirable heat accumulation.
  • This heat may also accumulate when ultrashort laser pulses are applied at higher repetition rates (e.g., at 100 kHz and greater). Higher repetition rates may also cause interaction of the laser pulse with the debris plume from a prior pulse, sometimes referred to as plasma shielding, which may cause material removal to be less effective. Although increased scanning speeds may be one way to dissipate heat from high-repetition-rate lasers, accuracy may be sacrificed at higher scanning speeds.
  • an astigmatic elongated beam spot may improve laser processing speeds with lower repetition rates and lower part- movement speeds, thereby reducing localized heating because the energy is distributed over a larger area as well as overcoming the plasma-shielding problem.
  • Adjusting the length of the astigmatic elongated beam spot allows optimal use of the energy density with the available power to provide minimal heat accumulation while spreading the available energy over a large area to achieve the desired throughput.
  • using ultrashort laser pulses facilitates the multiphoton absorption needed for cold ablation with higher wavelengths and the variable astigmatic elongated beam spot enables higher processing speeds without losing the cold ablation benefits of the ultrashort pulses.
  • the variable astigmatic elongated beam spot allows use of the full range of pulse energy available out of any laser (and particularly ultrashort pulses) because the size of the beam spot may be optimized to match the optimum process fluence.
  • linear machining speed may be determined as follows:
  • pulse spacing beam length/total shots per location. Increasing the beam length thus increases the number of shots per location for a given pulse spacing. In other words, the longer beam allows an increased overlap (i.e., to achieve a desired depth of cut), which allows for increased cutting speeds while maintaining optimum fluence.
  • the astigmatic elongated beam spot allows for generating narrower kerfs than those created by simply focusing the beam to a standard circular spot using traditional optical methods. Because diffraction limited focusing depends on wavelength, the astigmatic elongated beam spot facilitates the ability to achieve narrower kerfs at the longer
  • a beam delivery system (BDS) 10 capable of generating a variable astigmatic elongated beam spot.
  • the variable astigmatic elongated beam spot may be used to cut or machine a substrate made of various types of materials.
  • the BDS 10 improves the productivity of LED die separation by forming a highly-resolved adjustable astigmatic elongated beam spot, which maximizes scribing speed and minimizes consumption of scribing-related real estate on a wafer.
  • the BDS 10 can also be used in other scribing or cutting applications.
  • a solid-state laser 12, preferably diode pumped, generates a raw laser beam.
  • the raw laser beam may be a pulsed laser beam with ultrashort pulses, i.e., a pulse duration less than 1 nanosecond (ns), providing a peak power that causes multiphoton absorption.
  • the ultrashort pulse duration may be in any possible laser pulse duration range less than 1 ns, such as a range less than 10 picosecond (ps), a range less than 1 ps, or a range less than 1 femtosecond (fs).
  • the laser beam may also have any possible laser wavelength including, without limitation, a wavelength in the UV range of about 100 nm to 380 nm (e.g., a 157 nm laser, a 266 nm laser, a 315 nm, or a 355 nm laser), a wavelength in the visible range of about 380 nm to 750 nm (e.g., a 515 nm or 532 nm green laser), a wavelength in the near IR range of about 0.75 ⁇ to 1.3 ⁇ (e.g., a 1.01 ⁇ laser, a 1.03 ⁇ , or a 1.07 ⁇ laser), a wavelength in the mid IR range of 1.3 ⁇ to 5 ⁇ , and a wavelength in the far IR range of over 5 ⁇ .
  • a wavelength in the UV range of about 100 nm to 380 nm
  • a wavelength in the visible range of about 380 nm to 750 nm e.g., a
  • an ultrafast laser may be capable of producing the raw laser beam at different wavelengths (e.g., about .35 ⁇ , .5 ⁇ , 1 ⁇ , 1.3 ⁇ , 1.5 ⁇ , 2 ⁇ or any increments therebetween) and at different ultrashort pulse durations (e.g., less than about 10 ps, 1 ps, 1 fs, or any increments therebetween).
  • An example of an ultrafast laser includes one of the TruMicro series 5000 picosecond lasers available from TRUMPF.
  • the laser may also provide a pulse energy in a range of about 1 ⁇ ] to 1000 ⁇ at repetition rates in a range of about 10 to 1000 kHz.
  • the laser may be a fiber laser such as the type available from IPG Photonics.
  • the raw laser beam is usually in TEMoo mode with Gaussian distribution and is enlarged by a beam-expanding telescope (BET) 14.
  • the exemplary embodiment of the BET 14 is composed of the spherical plano-concave lens 16 and spherical plano-convex lens 18.
  • D c f sx +f sv
  • D c is a collimation distance.
  • Combinations of f sx and f sv can be used to satisfy designed values of the magnification M and the collimation distance D c .
  • the expanded beam is reflected by the 100% mirror 20a and then directed to the beam shaping iris 22.
  • the beam shaping iris 22 symmetrically crops out the low intensity edges of the beam in a Gaussian profile, leaving a high intensity portion passing through the iris 22.
  • the beam is then directed to the center of a variable anamorphic lens system 24.
  • the exemplary variable anamorphic lens system 24 is composed of a cylindrical plano-concave lens 26 and a cylindrical plano-convex lens 28.
  • the incident beam is asymmetrically modified in one of the two principal meridians, which appears in the horizontal direction in FIG. 1.
  • the degree of convergence or combined focal length (f as ) of the anamorphic system 24 is governed by the distance D, and it is generally expressed by the two lens principle: Namely, the larger the distance D, the shorter the focal length f as .
  • the degree of convergence increases in only one principal meridian of the collimated incident beam.
  • One principal meridian of the incident beam loses its collimation and converges after the variable anamorphic lens system 24; however, the other principal meridian is not affected and keeps its beam collimation. Consequently, the size of the beam after the variable anamorphic lens system 24 is changed in only one principal meridian by adjusting the distance between the two lenses in the anamorphic system 24.
  • the anamorphic BDS 10 deliberately introduces astigmatism to produce focal points separated in two principal meridians, i.e. vertical and horizontal.
  • a series of anamorphic lenses in different focal lengths or convergences is preferred to provide a variable astigmatic beam spot
  • the variable anamorphic lens system can be replaced by a single anamorphic lens for a fixed convergence.
  • the beam is reflected by another 100% mirror 20b, and then directed to the center of a beam focusing lens 30.
  • the exemplary beam focusing lens 30 is an aberration corrected spherical multi-element lens having a focal length range between about +20mm to +100mm. In one embodiment of the BDS 10, an edge- contact doublet with +50mm focal length is used.
  • a substrate 32 such as a semiconductor wafer.
  • the substrate 32 is translated by computer controlled x-y motion stages 34 for scribing. In semiconductor scribing applications where the
  • the semiconductor wafer contains square or rectangular dies, the semiconductor wafer can be rotated 90 degrees by a rotary stage 36 for scribing in both the x direction and the y direction.
  • a minimum beam waist diameter (w 0 ) of a Gaussian beam can be expressed by: ⁇ is a wavelength of an incident laser beam, f is a focal length of a beam focusing lens, ⁇ is the circular constant, and w; is a diameter of the incident beam.
  • the minimum beam waist diameter (w 0 ) or a size focused spot is inversely proportional to the incident beam diameter (w;).
  • the BET 14 anamorphically increases the incident beam diameter (w which is focused by the multi- element beam focusing lens 30, resulting in a minimized beam waist diameter and yielding a highly-resolved focal beam spot.
  • This provides a sharply focused scribing beam spot capable of providing about 5 ⁇ or less scribing kerf width on a semiconductor wafer. Consequently, the minimized scribing kerf width significantly reduces consumption of real estate on a wafer by scribing, which allows more dies on a wafer and improves productivity.
  • variable anamorphic lens system 24 results in two separate focal points in each principal meridian of the incident beam.
  • the flexibility of changing beam convergence from the variable anamorphic lens system 24 provides an instant modification of a laser energy density on a target semiconductor wafer. Since the optimum laser energy density is determined by light absorption properties of the particular target semiconductor wafer, the variable anamorphic lens system 24 can provide an instant adaptation to the optimum processing condition determined by various types of semiconductor wafers.
  • anamorphic BDS 10 can use different components to create the astigmatic focal beam spot or the anamorphic BDS 10 can include additional components to provide further modification of the beam.
  • a bi -prism 38 or a set of bi-prisms can be inserted between the anamorphic lens system 24 and the BET 14.
  • the bi-prism equally divides the expanded and collimated beam from the BET 14, then crosses the two divided beams over to produce an inversion of half Gaussian profile.
  • the distance between the two divided beams can be adjusted by changing the distance between the set of bi-prisms.
  • the bi-prism 38 divides the Gaussian beam by half circles and inverts the two divided half circles. A superimposition of these two circles creates superimposition of the edges of Gaussian profiles in weak intensity. This inversion of a Gaussian profile and intensity redistribution creates a homogeneous beam profile and eliminates certain drawbacks of a Gaussian intensity profile.
  • the BDS 10 can include an array of anamorphic lens systems 24 used to create small segments of separated astigmatic 'beamlets', similar to a dotted line.
  • the astigmatic beamlets allow an effective escape of laser-induced plasma, which positively alters scribing results.
  • the distance between the lenses in the array of anamorphic lens systems controls the length of each segment of the beamlets.
  • the distance among the segments of the beamlets can be controlled by introducing a cylindrical plano-convex lens in front of the array of anamorphic lens systems.
  • the BDS 10 may include a high speed galvanometer followed by a focusing element such as an f-theta lens.
  • the galvanometer allows the astigmatic elongated beam spot to be scanned across a workpiece or substrate in one or more axes without moving the workpiece.
  • the f-theta lens allows the scanning beam from the galvanometer to be focused onto a flat surface of the substrate or workpiece without moving the lens. Other scan lenses may also be used.
  • the profile of raw beam 50 from the laser generally has about 0.5mm to 3 mm of diameter in a Gaussian distribution.
  • the raw beam 50 is expanded by the BET 14 and the expanded beam 52 is about 2.5 times larger in diameter.
  • the expanded beam 52 is passed through the beam shaping iris 22 for edge cropping and the expanded and edge- cropped beam 54 is directed to the center of the anamorphic lens system 24.
  • the anamorphic lens system 24 modifies the expanded and edge-cropped beam 54 in only one principle meridian, resulting in a slightly compressed beam shape 56.
  • the highly compressed beam 57 passes through the beam focusing lens 30 to form the astigmatic elongated beam spot 58. Since the highly compressed beam 57 has converging beam characteristics in one principal meridian and collimated beam characteristics in the other, focal points are formed separately in each principal meridian after the beam focusing lens 30.
  • the three-dimensional diagram in FIG. 3 illustrates in greater detail the formation of the two focal points separately in each principal meridian when the highly compressed beam 57 passes through the beam focusing lens (not shown). Since the highly compressed beam 57 in one principal meridian (hereinafter the 'y component') has converging characteristics, the y component exhibits the short distance focal point 60. In contrast, since the other meridian (hereinafter the 'x component') has collimating beam characteristics, the x component exhibits the long distance focal point 62. Combination of the x and y components results in the astigmatic beam spot 58.
  • FIG. 4 shows the y component of the highly compressed beam 57, which passes through the beam focusing lens 30 and results in the focal point 60. After the focal point 60, the beam diverges and creates the astigmatic side of the astigmatic elongated beam spot 58.
  • FIG. 5 shows the x component of the highly compressed beam 57, which passes through the beam focusing lens 30 and results in the focal point 62.
  • the collimated x component of the highly compressed beam 57 is sharply focused at the focal point 60, which creates the sharply focused side of the astigmatic elongated beam spot 58.
  • FIGS. 6 and 7 illustrate further the formation of two separated focal points 60, 62 in each principal meridian.
  • the schematic beam tracings in FIGS. 6 and 7 include two- dimensional layouts of the BDS 10 shown in FIG. 1 excluding the 100% mirrors 20a, 20b and the beam shaping iris 22 for simplicity.
  • the raw beam from the solid-state laser 12 is expanded by the BET 14 and then collimated.
  • the variable anamorphic lens system 24 modifies the collimated beam in this principle meridian, resulting in convergence of the beam.
  • the converging beam is focused by the beam focusing lens 30. Due to its
  • the beam forms the focal point 60, shorter than the nominal focal length of the beam focusing lens 30.
  • the beam tracing in FIG. 6 is analogous to the view of the y component in FIG. 4.
  • the expanded and collimated beam from BET 14 is not affected by the variable anamorphic lens system 24 in this principal meridian.
  • the collimation of the beam can be maintained in this meridian after the variable anamorphic lens system 24.
  • the collimated beam is focused at the focal point 62, which is formed at a nominal focal length of the beam focusing lens 30.
  • the beam tracing in FIG. 7 is analogous to the view of the x component in FIG. 5.
  • the BET 14 increases the incident beam diameter, which is focused by the multi-element beam focusing lens 30, resulting in minimized a beam waist diameter and yielding a highly- resolved elongated beam spot.
  • the target substrate 32 e.g., a semiconductor wafer
  • the combination of these two separated focal points 60, 62 generates an astigmatic elongated beam spot having one side with a defocused and compressed circumference and the other side with a sharply focused and short circumference.
  • the astigmatic elongated beam spot is directed at the substrate and applied with a set of parameters (e.g., wavelength, energy density, pulse repetition rate, beam size) depending upon the material being scribed.
  • the astigmatic elongated beam spot can be used for scribing semiconductor wafers, for example, in wafer separation or dicing applications.
  • the wafer can be moved or translated in at least one cutting direction under the focused laser beam to create one or more laser scribing cuts.
  • a plurality of scribing cuts can be created by moving the wafer in an x direction and then by moving the wafer in a y direction after rotating the wafer 90 degrees.
  • the astigmatic beam spot is generally insensitive to polarization factors because the wafer is rotated to provide the cuts in the x and y directions.
  • the semiconductor wafer can be separated along the scribing cuts to form the dies using techniques known to those skilled in the art.
  • the astigmatic elongated beam spot provides an advantage in scribing applications by enabling faster scribing speeds.
  • the pulse repetition rate r p depends on the type of laser that is used. Solid state lasers with a few pulses per second to over 10 5 pulses per second are
  • the number of pulses n ⁇ j is a material processing parameter, which is determined by material properties of the target wafer and a desired cut depth. Given the pulse repetition rate r p and the number of pulses n ⁇ j, the beam length l is a controlling factor to determine the speed of the cut.
  • the focused astigmatic elongated beam spot formed according to the method described above increases the beam length l b resulting in higher scribing speeds.
  • variable anamorphic lens system 24 also provides greater flexibility to adjust processing parameters for achieving an optimum condition.
  • processing parameters should preferably be adjusted for optimum conditions based on material properties of a target.
  • the overflow of laser energy density can result in detrimental thermal damage to the target, and the lack of laser energy density can cause improper ablation or other undesired results.
  • the energy density of an ultrashort pulse with higher irradiance may need to be reduced to avoid losing the cold ablation benefits.
  • the variable anamorphic lens system 24 allows the energy density to be adjusted as needed depending on the pulse duration and other parameters such as laser power, wavelength, and material absorption properties.
  • FIGS. 8 and 9 show the flexibility of adjusting processing parameters of the BDS in this invention.
  • the lenses 26, 28 of the variable anamorphic lens system 24 are placed close together, which results in low convergence of the collimated incident beam. This low convergence forms the focal point 60 at a relatively further distance from the beam focusing lens 30. Consequently, the length of the beam spot 58 is relatively shorter and the energy density is increased.
  • the lenses 26, 28 of the variable anamorphic lens system 24 are placed further apart, which results in high convergence of the collimated incident beam.
  • This increased convergence introduces astigmatism and forms the focal point 60 at a relatively shorter distance from the beam focusing lens 30. Consequently, the length of the beam spot 58 is relatively longer and the energy density is decreased.
  • the astigmatic focal beam spot can be used to scribe a sapphire substrate used for blue LEDs.
  • Optimum processing of a sapphire substrate for blue LEDs generally requires an energy density of about 10 J/cm 2 .
  • blue LED wafers are generally designed to have about a 50 ⁇ gap among the individual die for separation, the optimum laser beam size is preferably less than about 20 ⁇ for laser scribing.
  • the conventional beam focusing at a 15 ⁇ diameter results in laser energy density of 34 J/cm 2 .
  • the energy density on target has to be adjusted by reducing the power output of the laser for optimum processing to avoid an overflow.
  • the laser power output cannot be fully utilized to maximize the scribing speed or productivity.
  • the preferred embodiment of the BDS 10 can adjust the size of the compressed beam spot to maintain the optimum laser energy density for 10 J/cm 2 without reducing the power output from the laser.
  • the size of the astigmatic elongated beam spot can be adjusted to have about 150 ⁇ in the astigmatic axis and about 5 ⁇ in the focused axis. Since the astigmatic axis is lined up in the scribing translation direction, this increase in beam length proportionally increases the scribing speed as discussed above.
  • the astigmatic beam spot can provide processing speeds that are about 10 times faster than that of conventional beam focusing.
  • the astigmatic focal beam spot can be used to scribe a sapphire substrate by coupling with one or more GaN layers on the sapphire substrate (e.g., about 4-7 ⁇ over the sapphire substrate) instead of coupling directly with sapphire.
  • the lower bandgap of GaN provides more efficient coupling with the incident laser beam, requiring only about 5 J/cm 2 for the laser energy density.
  • the ablation through the sapphire substrate is much easier than direct coupling with the sapphire.
  • the size of the astigmatic elongated beam spot can be adjusted to have about 300 ⁇ in the astigmatic axis and about 5 ⁇ in the focused axis.
  • the processing speed can be 20 times faster than the conventional far field imaging or spot focusing techniques.
  • the minimized spot size in the focused axis also significantly reduces the scribing kerf width, which subsequently reduces consumption of a wafer real estate. Furthermore, by reducing total removed material volume, the narrow scribing cuts reduce collateral material damage and ablation-generated debris.
  • a sapphire based LED wafer may be scribed with the astigmatic focal beam spot from the BDS 10 using a 266nm DPSS laser with on target power of about 1.8 Watt at 50kHz.
  • the size of the astigmatic elongated beam spot may be adjusted to have about 180 ⁇ in the astigmatic axis and about 5 ⁇ in the focused axis to provide a cut width of about 5 ⁇ .
  • the BDS 10 is capable of scribing speeds of greater than 50 mm/sec.
  • the laser cut forms a sharp V-shaped groove, which facilitates well controlled fracturing after the scribing.
  • the variable astigmatic elongated beam spot from the adjustable BDS 10 utilizes the maximum power output from the laser, which directly increases the processing speeds.
  • front side scribing can be used to decrease the street width and increase fracture yield, thereby increasing usable die per wafer.
  • the astigmatic elongated beam spot can also be used advantageously to scribe other types of semiconductor wafers.
  • the astigmatic elongated beam spot readily adjusts its laser energy density for an optimum value, based on the target material absorption properties, such as bandgap energy and surface roughness.
  • a silicon wafer may be scribed with the astigmatic focal beam spot from the BDS 10 using a 266nm DPSS laser with on target power of about 1.8 Watt at 50kHz.
  • the size of the astigmatic elongated beam spot may be adjusted to have about 170 ⁇ in the astigmatic axis and about 5 ⁇ in the focused axis to produce 75 ⁇ deep scribing with a speed at about 40 mm/sec.
  • a GaP wafer may be scribed using a 266nm DPSS laser with on target power of about 1.8 Watt at 50kHz.
  • the size of the astigmatic elongated beam spot may be adjusted to have about 300 ⁇ in the astigmatic axis and 5 ⁇ in the focused axis to produce a 65 ⁇ deep scribing with a speed at about 100 mm/sec. Similar results may be achieved in other compound semiconductor wafers such as GaAs, InP and Ge.
  • Other semiconductor materials such as cadmium or bismuth telluride can also be scribed/machine with high speed high quality by using an astigmatic elongated beam spot and ultrashort pulses.
  • a 532 nm 10 ps laser can be used to form an astigmatic elongated beam spot 600 microns long by 20 microns wide to produce a 500 microns deep scribe with high speed (e.g., 2 meters/sec) multiple passes using 3 W average power at 200 kHz.
  • the throughput can be roughly doubled by adjusting the beam size using a 1200 microns long beam at 6 W and 200 Khz. If higher pulse energy is available, the throughput can further be increased by correspondingly increasing the beam length, while keeping an optimum fluence.
  • substrates that can be scribed include, but are not limited to, InP, Alumina, glass, and polymers.
  • the systems and methods described herein may also be used to scribe or process ceramic materials including, but not limited to, silicon nitride, silicon carbide, aluminum nitride, or ceramic phosphors used for light conversion in LEDs.
  • the astigmatic focal beam spot can also be used advantageously to scribe or machine metal films, such as molybdenum. Due to high thermal conductivity, laser cutting of metal films using conventional techniques has shown extensive heat affected zones along the wake of the laser cut. With the application of the astigmatic elongated beam spot, the 5 ⁇ beam width in the focused axis significantly reduces a laser cutting kerf width, which subsequently reduces heat affected zones, collateral material damage and ablation-generated debris. The size of the astigmatic elongated beam spot was adjusted to have about 200 ⁇ in the astigmatic axis and about 5 ⁇ in the focused axis.
  • metal can also be cut including, but not limited to, aluminum, titanium or copper. These metals may having varying thicknesses, for example, including several hundreds of microns thick down to very thin films such as those used as metallization layers for contacts on solar cells.
  • the astigmatic elongated beam spot can also be used to scribe other shapes or to perform other types of machining or cutting applications. Operating parameters other than those given in the above examples are also contemplated for scribing LED wafers.
  • surface protection can be provided on the substrate by using a water soluble protective coating.
  • the preferred composition of the protective coating comprises at least one surfactant in a water-soluble liquid glycerin and can be any kind of generic liquid detergent that satisfies this compositional requirement.
  • the surfactant in the liquid glycerin forms a thin protective layer due to its high wetability. After the thin film layer is dried off, the glycerin effectively endures heat from the laser induced plasma, while preventing laser generated debris from adhering on the surface.
  • the thin film of liquid detergent is easily removed by cleaning with pressurized water.
  • the preferred embodiment of the present invention provides advantages over conventional systems using patterned laser projection and conventional systems using far field imaging. Unlike simple far field imaging, the present invention provides greater flexibility for modifying the laser beam by using the anamorphic BDS to produce the astigmatic elongated beam spot. Unlike conventional patterned laser projection, the anamorphic BDS delivers substantially the entire beam from a laser resonator to a target, thus maintaining very high beam utilization. The formation of the astigmatic elongated beam spot also allows the laser beam to have excellent characteristics in both the optimum intensity and the beam waist diameter.
  • variable anamorphic lens system enables an adjustable uniplanar compression of a laser beam, which results in a variable focal beam spot for prompt adjustments of the optimum laser intensity.
  • the formation of the astigmatic elongated beam spot results in numerous advantages on separation of various semiconductor wafers, including fast scribing speeds, narrow scribing kerf width, reduced laser debris, and reduced collateral damage.
  • the variable astigmatic elongated beam spot enables longer wavelength lasers with ultrashort pulses to be used for cold ablation with desired processing speeds and with minimal melting or heat damage.
  • a method for forming an astigmatic elongated beam spot for machining a substrate.
  • the method includes: generating a laser beam with pulses having a pulse duration of less than 1 ns; modifying the laser beam to produce an astigmatic beam that is collimated in a first axis and converging in a second axis; and focusing the astigmatic beam to form an astigmatic elongated beam spot on a substrate, the focused astigmatic beam having a first focal point in the first axis and a second focal point in the second axis, the second focal point being separate from the first focal point such that the astigmatic elongated beam spot is focused on the substrate in the first axis and defocused in the second axis, the astigmatic elongated beam spot having a width along the first axis and a length along the second axis, the width being less than the length such that the astigmatic elongated beam spot is narrower in the first
  • the method includes: generating a laser beam having a wavelength greater than 400 nm; modifying the laser beam to produce an astigmatic beam that is collimated in a first axis and converging in a second axis; and focusing the astigmatic beam to form an astigmatic elongated beam spot on a substrate, the focused astigmatic beam having a first focal point in the first axis and a second focal point in the second axis, the second focal point being separate from the first focal point such that the astigmatic elongated beam spot is focused on the substrate in the first axis and defocused in the second axis, the astigmatic elongated beam spot having a width along the first axis and a length along the second axis, the width being less than the length such that the astigmatic elongated beam spot is narrower in the first axis and wider in the second axis.

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  • Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Mechanical Engineering (AREA)
  • Laser Beam Processing (AREA)

Abstract

Une tache de faisceau allongée astigmatique peut être formée à partir d'un faisceau laser ayant des impulsions laser ultracourtes et/ou des longueurs d'onde plus longues pour usiner des substrats faits d'une variété de matériaux différents. Le faisceau laser peut être généré avec des impulsions ayant une durée d'impulsion de moins de 1 ns et/ou ayant une longueur d'onde plus grande que 400 nm. Le faisceau laser est modifié pour produire un faisceau astigmatique qui est collimaté dans un premier axe et convergent dans un second axe. Le faisceau astigmatique est focalisé pour former la tache de faisceau allongé astigmatique sur un substrat, qui est focalisée sur le substrat dans le premier axe et défocalisée dans le second axe. La tache de faisceau allongé astigmatique peut être allongée en longueur pour fournir une densité d'énergie suffisante pour qu'une seule impulsion ultracourte provoque une ablation à froid d'au moins une partie du matériau de substrat.
PCT/US2014/040192 2013-05-30 2014-05-30 Traitement au laser utilisant une tache de faisceau allongée astigmatique et utilisant des impulsions ultracourtes et/ou des longueurs d'onde plus longues Ceased WO2014194179A1 (fr)

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US13/905,352 US20130256286A1 (en) 2009-12-07 2013-05-30 Laser processing using an astigmatic elongated beam spot and using ultrashort pulses and/or longer wavelengths
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KR101855307B1 (ko) * 2015-10-07 2018-05-09 주식회사 이오테크닉스 레이저 마킹 시스템 및 이를 이용한 레이저 마킹 방법
JP7043124B2 (ja) * 2017-09-22 2022-03-29 株式会社ディスコ ウェーハの加工方法
JP7353171B2 (ja) * 2019-12-26 2023-09-29 株式会社ディスコ レーザー加工装置
TWI836971B (zh) * 2023-04-27 2024-03-21 佳世達科技股份有限公司 雷射投影設備

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US20020041418A1 (en) * 1999-04-27 2002-04-11 Timothy Fillion Method and apparatus for shaping a laser-beam intensity profile by dithering
US20090169871A1 (en) * 2006-02-23 2009-07-02 Reijo Lappalainen Method for Producing High-Quality Surfaces and a Product Having a High-Quality Surface
US20100025387A1 (en) * 2005-09-08 2010-02-04 Imra America, Inc. Transparent material processing with an ultrashort pulse laser
US20100301027A1 (en) * 2003-02-19 2010-12-02 J. P. Sercel Associates Inc. System and method for cutting using a variable astigmatic focal beam spot
US20120234807A1 (en) * 2009-12-07 2012-09-20 J.P. Sercel Associates Inc. Laser scribing with extended depth affectation into a workplace

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Publication number Priority date Publication date Assignee Title
US20020041418A1 (en) * 1999-04-27 2002-04-11 Timothy Fillion Method and apparatus for shaping a laser-beam intensity profile by dithering
US20100301027A1 (en) * 2003-02-19 2010-12-02 J. P. Sercel Associates Inc. System and method for cutting using a variable astigmatic focal beam spot
US20100025387A1 (en) * 2005-09-08 2010-02-04 Imra America, Inc. Transparent material processing with an ultrashort pulse laser
US20090169871A1 (en) * 2006-02-23 2009-07-02 Reijo Lappalainen Method for Producing High-Quality Surfaces and a Product Having a High-Quality Surface
US20120234807A1 (en) * 2009-12-07 2012-09-20 J.P. Sercel Associates Inc. Laser scribing with extended depth affectation into a workplace

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