System and method for drilling in transparent material

Abstract

A method for forming a perforation in a substrate having opposing first and second surfaces may include directing a focused beam of laser pulses into the substrate through the first surface of the substrate and then through the second surface of the substrate. The focused beam of the laser pulse may have a wavelength to which the substrate is at least substantially transparent, and a beam waist of the focused beam of the laser pulse is closer to the second surface than the first surface. The focused beam of laser pulses is characterized by a pulse repetition rate, a peak optical intensity at the substrate, and an average power at the substrate sufficient to: melt a region in the substrate proximate the second surface, thereby creating a melt zone within the substrate; propagating the melting zone towards the first surface; and vaporizing or boiling the material of the substrate and being positioned within the melting zone.

Description

[0001] This application is a divisional application of the invention patent with an application date of October 2, 2019, a priority date of October 8, 2018, and an application number of 201980060712.3.

[0002] CROSS REFERENCE TO RELATED APPLICATIONS

[0003] This application claims the benefit of U.S. Provisional Application No. 62 / 742,694, filed on October 8, 2018, which is incorporated herein by reference in its entirety. Technical Field

[0004] Embodiments of the present invention relate generally to processing of substrates, and more particularly to laser drilling of holes, such as through-holes, in substrates. Background Art

[0005] There is a great deal of interest in thin glass with precisely formed holes for electronic applications. The holes are filled with conductive material and used to conduct electrical signals from one part to another, allowing precise connection of central processing units, memory chips, graphics processing units, or other electronic components. For such applications, substrates with metallized holes in the above components are often referred to as "interposers." Glass has a number of advantageous properties compared to currently used interposer materials (such as fiber-reinforced polymers or silicon). Glass can be formed into large, thin, smooth sheets without the need for polishing, it has higher hardness and higher dimensional stability than organic alternatives, it is a much better electrical insulator than silicon, it has better dimensional (thermal and stiffness) stability than organic options, and it can be adjusted to different thermal expansion coefficients to control stacking deformation in integrated circuits.

[0006] Various hole forming methods can be used to create holes in glass, such as hot pressing, photolithography of glass for photomachines, discharge drilling, powder blasting, and a wide variety of laser drilling processes. With any of these techniques, the challenge is generally about forming holes of sufficient quality (low cracking, appropriate size or roundness) at a sufficiently high hole formation rate (holes / second), which ultimately affects cost. For example, hot pressing of glass has difficulty in forming holes of sufficiently small size (less than or equal to about 100 microns), discharge drilling can be difficult to perform with close hole spacing (i.e., hole-to-hole distances of less than about 50 microns), laser drilling of holes using beam apertures can be slow (e.g., about 1 hole / second), and excimer laser processing and photoprocessable glass can have large initial investment costs.

[0007] Conventional laser drilling processes have been developed to produce particularly high quality holes. For example, a UV nanosecond laser can be used in a percussion drilling process to make pilot holes (each having a diameter of approximately 10 microns) in a glass part using multiple (e.g., hundreds) of laser pulses per hole. The part is then etched with acid to enlarge the pilot holes and achieve the target size. The etched holes are then metallized, a redistribution layer is added to fan out electrical signals, and each part is cut into smaller segments to produce functional inserts. However, laser drilling with a UV nanosecond laser can be a time-consuming process, and with percussion drilling (i.e., one pulse after another at the same location), hundreds of pulses may be required to drill individual holes to the desired depth. Since the investment cost of a precision laser drilling platform can be very large (approaching $1M / machine), the speed of hole formation is a key parameter in the overall insert production cost. In addition, etching glass parts makes it difficult to ensure that different glass parts have substantially the same thickness once etched.

[0008] Another conventional laser drilling process involves a single-shot laser process in which a single laser pulse with a Bessel beam profile is used to modify a glass portion wherever a hole is desired, and the part is subsequently etched out of the hole with acid. Although this process can be used to produce up to 5000 holes per second (200 μs / hole), the process still requires etching, which, as discussed above, makes it difficult to ensure uniform thickness of the glass part in the finished product. In addition, laser pulses with a Bessel beam profile are difficult to deflect quickly using conventional beam steering techniques (e.g., galvanometer mirrors, etc.). Therefore, the time required to form a relatively sparse hole pattern is typically much slower than 5000 holes per second.

[0009] In Karimelahi, Samira, Ladan Abolghasemi and Peter R. Herman. "Rapid micromachining of high aspect ratio holes in fused silica glass by high repetition rate picosecond laser." Applied Physics A114.1 (2014): 91-111, a rigorous study showed that IR and green ultrafast lasers and Gaussian focal spots can be used to produce high aspect ratio holes in glass, but the holes are of poor quality or require a subsequent etching step. Therefore, there is a need for a method of laser drilling materials (such as glass) that minimizes or eliminates the problems mentioned above. Summary of the invention

[0010] A specific example can be characterized as an article comprising a substrate having a plurality of laser-drilled perforations, the plurality of laser-drilled perforations extending continuously from a first surface of the substrate to a second surface of the substrate opposite the first surface of the substrate, wherein: the diameter of the plurality of laser-drilled perforations is 10 μm or greater; the spacing between adjacent etched perforations is 30 μm or greater; the plurality of laser-drilled perforations include an opening in the first surface, an opening in the second surface, and a waist positioned between the opening in the first surface and the opening in the second surface; the diameter of the waist is at least 75% of the diameter of the opening in the first surface or the opening in the second surface; and the difference between the diameter of the opening in the first surface and the diameter of the opening in the second surface is 3.5 μm or less.

[0011] Another specific example can be characterized as a method for forming a through hole in a substrate, wherein the method includes: providing a substrate having a first surface and a second surface opposite the first surface; and directing a focused beam of laser pulses at a position into the substrate through the first surface of the substrate and subsequently through the second surface of the substrate during drilling of a duration less than 100 μs, wherein the focused beam of laser pulses has a wavelength, the substrate is at least substantially transparent to the wavelength, wherein an optical intensity of the focused beam of laser pulses at the substrate is less than an optical breakdown intensity of the substrate, wherein the focused beam of laser pulses has a Gaussian energy distribution, wherein the focused beam of laser pulses has a pulse repetition rate greater than 5 MHz, and wherein laser pulses within the focused beam of laser pulses have a pulse width greater than 200 fs.

[0012] Another embodiment features a method for forming a through-hole in a substrate, wherein the method includes: providing a substrate having a thickness in a range of 30 μm to 150 μm and having a first surface and a second surface opposite the first surface; and directing a focused beam of laser pulses into the substrate through the first surface of the substrate and then through the second surface of the substrate, wherein the focused beam of laser pulses has a wavelength, and the substrate is at least substantially transparent to the wavelength; wherein an optical intensity of the focused beam of laser pulses at the substrate is less than an optical breakdown strength of the substrate; wherein the focused beam of laser pulses has a Gaussian energy distribution; wherein the focused beam of laser pulses has a pulse repetition rate, a peak optical intensity at the substrate, and an average power that drives a cumulative heating effect at the substrate; and wherein laser pulses within the focused beam of laser pulses have a pulse width, wherein the peak optical intensity, pulse repetition rate, average power, and pulse width are selected so that the through-hole is formed in less than 40 μs.

[0013] Yet another embodiment features a method for forming a through hole in a substrate, wherein the method includes: providing a substrate having a first surface and a second surface opposite the first surface; and directing a focused beam of laser pulses into the substrate through the first surface of the substrate and subsequently through the second surface of the substrate, wherein the focused beam of laser pulses has a wavelength, the substrate is at least substantially transparent to the wavelength, and wherein a beam waist of the focused beam of laser pulses is closer to the second surface of the substrate than to the first surface of the substrate, and wherein the focused beam of laser pulses is characterized by a pulse repetition rate, a peak optical intensity at the substrate, and an average power at the substrate, which is sufficient to: melt a region of the substrate near the second surface, thereby creating a molten zone within the substrate; propagate the molten zone toward the first surface; and evaporate or boil material of the substrate and be positioned within the molten zone. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 An exemplary laser processing system for performing a laser drilling process is shown.

[0015] Figure 2 and Figure 3 A photograph showing a through-hole drilled through an exemplary glass substrate using a laser drilling process according to one embodiment.

[0016] Figure 4 A graph showing results of an experiment in which a glass substrate was subjected to a laser drilling process according to some embodiments.

[0017] Figures 5 to 8 is a photograph of a cross-sectional glass substrate processed to form perforations according to specific examples discussed herein. DETAILED DESCRIPTION

[0018] Example embodiments are described herein with reference to the accompanying drawings. Unless otherwise expressly stated, in the drawings, the size, position, etc. of components, features, elements, etc., and any distances therebetween are not necessarily drawn to scale, but are exaggerated for clarity. In the drawings, the same element symbols refer to the same elements throughout. Therefore, the same or similar element symbols may be described with reference to other drawings, even if the element symbols are neither mentioned nor described in the corresponding drawings. In addition, even elements not indicated by figure numerals may be described with reference to other drawings.

[0019] The terms used herein are for the purpose of describing specific examples only, and are not intended to be limiting. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meanings as generally understood by those skilled in the art. As used herein, unless the context clearly indicates otherwise, the singular forms "a / an" and "the" are intended to also include plural forms. It should be recognized that the term "comprise / comprising" specifies the presence of stated features, wholes, steps, operations, elements and / or components when used in this specification, but does not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, components and / or groups thereof. Unless otherwise specified, when describing a value range, the value range includes both the upper and lower limits of the range and any sub-ranges therebetween. Unless otherwise indicated, terms such as "first", "second" and the like are only used to distinguish one element from another. For example, a node may be referred to as a "first node", and similarly, another node may be referred to as a "second node", or vice versa.

[0020] Unless otherwise indicated, the terms "about", "approximately", etc. mean that the amount, size, formula, parameter and other quantities and characteristics are not and need not be exact, but may be approximate and / or larger or smaller as necessary, thereby reflecting tolerances, conversion factors, rounding, measurement errors and the like, as well as other factors known to those skilled in the art. Spatially relative terms such as "below", "beneath", "lower", "above", and "upper" and the like may be used herein to describe the relationship of one element or feature to another element or feature as illustrated in the accompanying drawings. It should be recognized that the spatially relative terms are intended to cover different orientations other than the orientation depicted in the accompanying drawings. For example, if the object in the accompanying drawings is turned over, the elements described as "below" or "below" other elements or features will be oriented "above" other elements or features. Therefore, the exemplary term "below" may cover both orientations above and below. Objects may be oriented in other ways (e.g., rotated 90 degrees or in other orientations), and the spatially relative descriptors used herein may be interpreted accordingly.

[0021] It should be understood that many different forms, embodiments and combinations are possible without departing from the spirit and teachings of the present invention, and therefore, the present invention should not be considered limited to the example embodiments set forth herein. Rather, examples and embodiments are provided so that this invention will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art.

[0022] In the specific examples discussed herein, a laser drilling process for drilling a hole in a substrate includes directing a beam of laser pulses into the substrate such that the beam of laser pulses is transmitted into the substrate through a first surface of the substrate and propagates within the substrate toward a second surface of the substrate opposite the first surface. Generally speaking, the beam of laser pulses is focused to produce a beam waist positioned at or near the second surface (i.e., positioned inside or outside the substrate).

[0023] In some embodiments, the substrate is provided as an article made of glass (e.g., borosilicate glass, aluminoborosilicate glass, aluminosilicate glass, alkali lead silica glass, alkali alkaline earth silica glass, germanium oxide glass, fused silica, etc.), synthetic quartz, glass ceramic (e.g., lithium aluminosilicate), ceramic, sapphire, semiconductor material (e.g., Si, SiGe, GaAs, GaN, SiC, etc.), or the like, or any combination thereof. Thus, the article may be provided as a glass insert, a silicon insert, a semiconductor wafer, a semiconductor die, a substrate (e.g., for a display screen, for wafer-level packaging or other semiconductor packaging, for display panel-level packaging, for MEMS or sensor devices, etc.), or the like, or any combination thereof. In some specific examples, the substrate is provided as an article (e.g., having a thickness ranging from 30 μm (or so) to 150 μm (or so). However, it should be understood that the thickness of the substrate may be less than 30 μm or greater than 150 μm, depending on the specific article implemented by the substrate.

[0024] In general, the beam of laser pulses directed to the substrate has a wavelength to which the substrate is at least substantially transparent. As used herein, a substrate is considered to be "substantially transparent" to a wavelength of a laser pulse beam when the substrate absorbs less than about 20% (e.g., less than about 15%, less than about 10%, less than about 5%, less than about 1%) of the laser pulse beam per millimeter of material depth at this wavelength. Thus, depending on the material forming the substrate, the laser pulse beam may have a wavelength in the ultraviolet, visible, or infrared region of the electromagnetic spectrum. For example, in some embodiments, if the substrate is provided as a glass article, quartz, or sapphire, the wavelength of the laser pulse beam may be in the range of 300 nm (or thereabouts) to 1064 nm (or thereabouts).

[0025] As will be discussed in more detail below, the laser drilling process may be characterized by parameters including the following: pulse wavelength, pulse width, pulse energy, pulse repetition rate, peak optical intensity at the substrate, drilling period (i.e., the time period during which the laser pulse is directed to a specific location at the substrate to form a through-hole), and average power of the laser pulse beam at the substrate. Generally speaking, the laser pulse beam is ultimately directed to the substrate during the laser drilling process with a Gaussian energy profile. Using a laser pulse beam with a Gaussian energy profile allows the laser pulse beam to be rapidly deflected using conventional beam steering techniques (e.g., galvanometer mirrors, fast steering mirrors, rotating polygon mirrors, acousto-optical deflectors, electro-optical deflectors, etc.) to form a through-hole in the substrate. Depending on the desired hole diameter, percussion drilling may be employed (e.g., to form holes with diameters in the range of 1 μm to 100 μm, such as 10 μm, 20 μm, 30 μm, 50 μm, etc.) or a beam may be deflected (e.g., as described above) to form holes of similar or larger diameters. Because the beam can be deflected rapidly, it is also possible to form even relatively sparse perforation patterns at a higher throughput than using conventional laser drilling processes. Finally, the laser drilling processes described herein do not require an acid etching process to obtain perforations with acceptable diameters or surface morphology.

[0026] The laser drilling process may also be characterized by a parameter referred to herein as "z-range". The z-range is the range of distances between the second surface of the substrate and the beam waist of the laser pulse beam that creates a perforation in the substrate during the laser drilling process. The inventors have discovered that the z-range depends on at least some of the other aforementioned parameters of the laser drilling. For example, at a given pulse wavelength, pulse width, and pulse repetition rate, and above a threshold average power during drilling and at the substrate, it has been discovered that the variation of the z-range during the laser drilling process depends on the average power during drilling and at the substrate. It is important to determine the values ​​of laser drilling process parameters (e.g., average power during drilling and at the substrate for a given pulse wavelength, pulse width, and pulse repetition rate) for obtaining a sufficiently large z-range because substrates typically do not have a completely uniform thickness (i.e., there may be thickness variations of approximately 10 μm or more) and / or are not completely flat when subjected to the laser drilling process (i.e., there may be some ripples of approximately 10 μm or more in the substrate), and the thickness of one substrate may be different from the thickness of another substrate. Therefore, if the z-range does not accommodate variations in substrate thickness or flatness, the laser drilling process will not be able to reliably form through-holes.

[0027] After drilling, the through-holes may be coated and / or filled with a conductive material or the like. For example, the through-holes may be formed in a substrate such as a glass insert and coated and / or filled with a conductive material (e.g., copper, aluminum, gold, silver, lead, tin, indium tin oxide, or the like, or any combination or alloy thereof). The process used to metallize the interior of the through-holes may be, for example, electroplating, electroless plating, physical vapor deposition, or other evaporative coating methods. The through-holes may also be coated with catalytic materials such as platinum, palladium, titanium dioxide, or other materials that promote chemical reactions within the holes. Alternatively, the through-holes may be coated with other chemical functionalizations to change the surface wetting properties or allow attachment of biomolecules for biochemical analysis. Such chemical functionalizations may be silanization of the glass surface of the through-holes, and / or additional attachment of specific proteins, antibodies, or other biologically specific molecules designed to promote attachment of biomolecules for the desired application.

[0028] In general, the beam of laser pulses may be characterized by parameters such as pulse repetition rate, pulse energy, and pulse width. The pulse energy and pulse width are selected such that when the beam of laser pulses is focused, the beam waist is spaced apart from the second surface of the substrate by an offset distance, and each laser pulse in the focused beam of laser pulses illuminates an area of ​​the substrate (i.e., a spot size corresponding to the laser pulse at the area of ​​the substrate) with a peak optical intensity that is less than the optical breakdown intensity of the substrate. In addition, the pulse repetition rate of the beam of laser pulses is selected to be sufficiently high (e.g., taking into account the peak optical intensity of each laser pulse illuminating the area of ​​the substrate) so that the illuminated area of ​​the substrate can accumulate heat during an initial time period (e.g., also referred to herein as an "incubation period"). During the incubation period, heating of the substrate is primarily initiated by multiphoton absorption by the substrate material. During the incubation period, absorption of the laser pulse may also occur at point defects in the substrate, which may generate defect states residing between the conduction energy band and the valence energy band for light absorption.

[0029] At the end of the incubation period, the irradiated region of the substrate (e.g., which may be located at or near the second surface of the substrate, at or near the first surface of the substrate, or a combination thereof) has accumulated heat sufficient to locally increase the substrate temperature and reduce its band gap energy, enabling a transition to a state where the irradiated region of the substrate may melt and ultimately evaporate. It is suspected that this is due to thermal ionization, i.e., the band gap of the substrate material is reduced by the gradual covering of the band gap by the electronic and vibrational states of the molecules at high temperatures. It is possible that impact ionization and cascade ionization may also occur in the high temperature region of the substrate, which may result in an increase in carrier density that may also contribute to enhanced laser absorption therein. The substrate region that melts during the laser drilling process may be characterized as a "melt zone." A significant increase in laser pulse absorptivity at the edge or boundary or interface of the high temperature region in the substrate rapidly causes the melt zone to grow gradually toward the first surface (i.e., in a direction opposite to the propagation direction of the laser pulse beam from the scanning lens 110). In addition, at extremely high temperatures within the substrate (i.e., at temperatures greater than 3000° C.), it has been shown that impact ionization and cascade ionization can also be seeded by free electrons from thermal ionization without the contribution of multiphoton absorption, resulting in a significant increase in the absorption of the laser pulse. Gradually increasing the temperature in the molten zone beyond the evaporation / boiling point of the substrate material will eventually generate a large vapor pressure that can push the molten substrate material out of the hole (i.e., at least mostly through the second surface) to produce a through-hole. Therefore, immediately after the incubation period, the focused beam of laser pulses can be maintained at the irradiated area of ​​the substrate for an additional period of time (e.g., also referred to herein as the "removal phase") to heat, melt and evaporate the irradiated area of ​​the substrate (i.e., through the entire thickness of the substrate) to form a through-hole. The entire time period from the start of the incubation period to the end of the removal phase is referred to herein as the "drilling period."

[0030] In view of the above, it should be apparent that the laser drilling process described herein is highly dependent on the heat accumulation dynamics of the substrate (i.e., on being irradiated by a focused beam of laser pulses). Thus, the beam of laser pulses may be generated and focused in any manner known in the art to produce a focused beam of laser pulses, wherein each laser pulse in the focused beam of laser pulses has a peak power and peak optical intensity at the substrate sufficient to heat (but not ablate, crack, or otherwise damage) the substrate. The laser pulse beam is further generated in any manner known in the art to produce laser pulses with sufficient peak power at a pulse repetition rate sufficient to prevent the substrate from substantially cooling between laser pulses (e.g., sufficient to enable the substrate to accumulate heat during the incubation period and continue to accumulate heat during the removal phase, as discussed above). However, the peak power, peak optical intensity, and pulse repetition rate of the focused beam of laser pulses should also be selected to prevent the substrate from accumulating heat too quickly, which can lead to undesirable phenomena such as cracking in areas of the substrate not irradiated by the laser pulse beam and ejection of molten material over an undesirably large area on the exterior of the substrate.

[0031] In one embodiment, a laser pulse as generated, focused, and delivered to a substrate to perform the laser drilling process described herein may be characterized as having a 30 MHz pulse repetition rate and a peak power at the substrate of about 78 kW (e.g., as measured by pulse energy and pulse width, as known in the art) at 1.27×10 11 W / cm 2The substrate is irradiated at a peak optical intensity (e.g., as measured by peak power and spot size at the substrate, as known in the art) of about (or below) 30 MHz. It is recognized that a beam of laser pulses may have a pulse repetition rate greater than or less than 30 MHz, and that the peak power of a beam of laser pulses at the substrate may be maintained (e.g., by adjusting the pulse width or pulse energy of the laser pulses within the beam of laser pulses) or otherwise adjusted as needed to compensate for changes in the pulse repetition rate. Likewise, the peak optical intensity at the substrate may be maintained (e.g., by adjusting the pulse width, pulse energy, or spot size at the second surface of the substrate within the beam of laser pulses) or otherwise adjusted as needed to compensate for changes in the pulse repetition rate, as appropriate. During drilling, the pulse repetition rate, peak power, pulse width, or pulse energy of the laser pulses within the beam of laser pulses may be constant or variable. Generally speaking, a laser operable to generate a beam of laser pulses having a sufficiently high pulse repetition frequency and peak power to implement the laser drilling process is a QCW laser capable of generating laser pulses having a pulse width in the range of 200 fs (or thereabouts) to 50 ps (or thereabouts). For example, the pulse width of the laser pulse may be 200 fs, 400 fs, 800 fs, 1 ps, 2 ps, 5 ps, 10 ps, ​​20 ps, ​​25 ps, 30 ps, ​​40 ps, ​​45 ps, 50 ps, ​​or any value between any of these values.

[0032] Therefore, to implement the laser drilling process described herein, the pulse repetition rate of the laser pulses may be in the range of 5 MHz (or so) to 5 GHz (or so), subject to the proviso that the peak power of the laser pulses and the peak optical intensity at the substrate as appropriate are adjusted to compensate for the variation in the pulse repetition rate. In one embodiment, the pulse repetition rate may be in the range of 5 MHz (or so) to 500 MHz (or so). In other embodiments, the pulse repetition rate may be in the range of 25 MHz (or so) to 40 MHz (or so), in the range of 150 MHz (or so) to 180 MHz (or so).

[0033] Depending on one or more factors, such as the thickness of the substrate, the pulse repetition rate of a beam of laser pulses, the pulse energy of the laser pulses in the beam of laser pulses, the pulse width of the laser pulses in the beam of laser pulses, the peak power at the substrate, the optical intensity at the substrate, the drilling period may be in the range of 5 μs (or so) to 120 μs (or so). In some embodiments, the drilling period may advantageously be less than or equal to 50 μs (or so), less than or equal to 30 μs (or so), less than or equal to 20 μs (or so), less than or equal to 15 μs (or so), less than or equal to 10 μs (or so), etc. or between any of these values.

[0034] When the beam waist of the focused beam of laser pulses is outside the substrate near the second surface, absorption of laser energy occurs at the interface of the second surface of the substrate and the surrounding environment (i.e., air). This interface helps to lower the ablation threshold to begin coupling the laser energy in the beam of laser pulses into the substrate. The relatively high peak intensity helps to eliminate the need for an absorptive coating at the second surface of the substrate. According to experiments performed by the inventors, there appears to be an initial latency period in which the focused beam of laser pulses does not damage the substrate.

[0035] In some embodiments, the first surface of the substrate is cleaned to remove (or reduce the amount of) contaminants on the first surface that will absorb energy within the beam of the laser pulse. In experiments performed by the present inventors, it has been found that when a beam of laser energy is transmitted through a portion of the first surface that has not been cleaned, any through-holes formed in the substrate tend to have relatively large openings at the first surface and relatively small openings at the second surface. When a beam of laser energy is transmitted through a portion of the first surface that has been suitably cleaned, any through-holes formed in the substrate tend to have openings at the first surface and the second surface that have approximately the same diameter (e.g., within a range of 2 μm to 5 μm of each other).

[0036] Removing material from a substrate during a laser drilling process thus appears to involve a combination of melting and ablation of the substrate material, and despite the fact that the beam of laser pulses has a relatively low average power, the material removal rate is surprisingly high compared to conventional laser drilling processes. In fact, the laser drilling process described herein provides a significant increase in hole formation rate compared to conventional laser drilling processes. Experiments performed by the inventors on optically clear glass substrates about 100 μm thick (i.e., the laser drilling process described herein was performed according to the percussion drilling technique) indicate that through-holes having a diameter of about 10 μm can be reliably formed within 15 μs (or thereabouts). Experiments performed by the inventors on optically clear glass substrates about 50 μm thick (i.e., the laser drilling process described herein was performed according to the percussion drilling technique) indicate that through-holes having a diameter of about 10 μm can be formed in only 5 μs (or thereabouts) and can be reliably formed within 15 μs (or thereabouts). In contrast to the laser drilling process in the specific example described herein, conventional laser drilling processes in optically transparent glass substrates using laser pulses having a pulse width of less than 10 picoseconds at the transmission wavelength rely on nonlinear absorption, and the time required to form a through-hole is often several orders of magnitude slower than the time required to form a through-hole according to the laser drilling process described herein.

[0037] Figure 1 An exemplary laser processing system for implementing the laser drilling process described in the above embodiments is described. Figure 1The laser processing system 100 includes a laser 102 , a beam expander 104 , a beam adjuster 106 , a beam scanner 108 , a scanning lens 110 , and a stage 112 .

[0038] The laser 102 may be provided as any suitable laser capable of generating (e.g., a QCW laser capable of generating) a beam of laser pulses having the above-mentioned characteristics. For example, the laser 102 may be configured to generate a beam of laser pulses at an average power of 100 W (or so), a wavelength of 515 nm (or so, for example, 532 nm), a pulse width in the range of 20 ps (or so) to 40 ps (or so), and a pulse repetition rate in the range of 25 MHz (or so) to 40 MHz (or so). In one specific example, the pulse width of the laser pulse is 28 ps (or so), and the pulse repetition rate is 30 MHz (or so).

[0039] The beam expander 104 may be provided as any suitable device capable of expanding an incident beam from a first beam size (e.g., a diameter of 2 mm or more) to a second beam size (e.g., a diameter of 4.5 mm or more). In one specific example, the beam expander may be provided as a motorized variable beam expander.

[0040] The beam adjuster 106 may be any suitable device capable of selectively attenuating (e.g., partially, completely, or not at all) the energy of the laser beam. In one specific example, the beam adjuster 106 may be an acousto-optic (AO) modulator (AOM) or an AO deflector (AOD).

[0041] The beam scanner 108 may be provided as any suitable device (or devices) capable of deflecting an incident beam of laser pulses (e.g., within a one-dimensional scanning field, within a two-dimensional scanning field, etc.). Thus, the beam scanner 108 may include one or more galvanometer mirrors, one or more fast steering mirrors, one or more deformable mirrors, one or more AODs, a rotating polygon mirror, or the like, or any combination thereof.

[0042] The scanning lens 110 may be provided as any suitable device (or devices) capable of focusing an incident beam of laser pulses so as to produce a focused beam of laser pulses. In one specific example, the scanning lens 110 is provided as a telecentric scanning lens. In another specific example, the scanning lens 110 has a focal length of 100 mm (or so) and a numerical aperture (NA) of about 0.5 or less. In one specific example, the scanning lens 110 may focus the beam of laser pulses to produce a beam waist (e.g., at the focal plane of the scanning lens 110) having a diameter in the range of 10 μm (or so) to 13 μm (or so), for example, in the range of 11 μm to 12 μm.

[0043] The stage 112 may be provided as any suitable device (or devices) capable of supporting and moving the aforementioned substrate (e.g., identified at 114). In one embodiment, the stage 112 is configured to linearly move the substrate 114 along one axis, along two axes, or the like, or any combination thereof. Typically, the substrate 114 may be characterized as including a first surface (e.g., which faces the scan lens 110) and a second surface, which is opposite the first surface (e.g., the second surface faces away from the scan lens 110). The stage 112 is configured to support the substrate 114 so that the second surface of the substrate 114 does not contact an underlying structure, such as the stage 112. As used herein, the first surface of the substrate 114 is also referred to as the "front side" of the substrate 114, and the second surface of the substrate 114 is also referred to as the "back side" of the substrate 114.

[0044] In one embodiment, the stage 112 supports the substrate 114 so that the focal plane of the scan lens 110 is positioned outside the substrate 114 (i.e., below the second surface of the substrate 114). The distance between the second surface of the substrate 114 and the focal plane of the scan lens 110 (i.e., the aforementioned "offset distance") is set so that the focused beam of the laser pulses illuminates an area of ​​the substrate at a peak intensity that is less than the optical resolution intensity of the substrate, while still creating a through hole in the substrate, as discussed above. It will be appreciated that the offset distance may be experimentally determined or may be calculated (e.g., based on parameters of the laser drilling process, based on the material of the substrate, or the like, or any combination thereof).

[0045] System 100 may further include one or more other components to polarize, reflect, expand, focus, etc., the beam of laser pulses so that they propagate from laser 102 to scan lens 110 (e.g., along the dashed lines, such as Figure 1106). Examples of such components include a half-wave plate 116, a pair of relay lenses 118, and a plurality of fold mirrors (e.g., fold mirror 120). In one specific example, the pair of relay lenses are disposed and configured to amplify the beam of the laser pulse output from the beam adjuster 106 (e.g., to double the beam size of the beam of the laser pulse as output from the beam adjuster 106). Thus, if the beam of the laser pulse output from the beam adjuster 106 has the aforementioned second beam size of 4.5 mm (or so), the beam of the laser pulse output from the pair of relay lenses may be characterized as having a third beam size of 9 mm (or so).

[0046] Figure 2 and Figure 3 is a photograph illustrating drilling a hole through a 50 μm thick glass substrate (i.e., AF manufactured by SCHOTT). The through-holes of the glass substrate (formed substrate) are obtained from the first surface and the second surface of the glass substrate, respectively, using the aforementioned laser processing system 100 and implementing the laser drilling process described above. The glass substrate is uncoated because it does not have any absorptive coating formed on any surface, and the average power of the beam of the laser pulse at the substrate (i.e., after having been reflected, transmitted or diffracted by the aforementioned components of the laser processing system 100) is in the range of 66 W (or so) to 67 W (or so). The diameter of the through-hole at the first surface (such as Figure 2 ) is in the range of 9.5 μm to 10.1 μm; and the diameter of the through-holes at the second surface (as shown in Figure 3 ) is 9 μm.

[0047] Figure 4 The following is a graph illustrating the results of an experiment in which a 50 μm thick glass substrate (i.e., AF manufactured by SCHOTT) was used. The uncoated substrate formed by the above-described laser drilling process was subjected to the laser drilling process described above. In the experiments, the average power of the beam of laser pulses at the substrate was varied between 25 W (or so) and 67 W (or so), and the drilling period at each location in the substrate where a hole was attempted to be formed was varied between 5 μs and 120 μs at 5 μs intervals. The points in the graph represent actual data points obtained from each experiment, and the boundaries of the contour lines were derived from the experimentally obtained data using the statistical analysis software JPM produced by SAS INSTITUTE.

[0048] like Figure 4As shown in , experiments performed by the inventors indicate that at an average power of 25 W and a drilling cycle of 120 μs, the laser drilling process does not damage the substrate at all and thus does not form any through-holes. The same is true at average power / drilling cycle pairs of 25 W / 40 μs, 30 W / 40 μs, 35 W / 40 μs, 40 W / 30 μs, 40 W / 20 μs, 40 W / 10 μs, 50 W / 10 μs. At an average power of 67 W and a drilling cycle of 5 μs, through-holes are formed in the substrate with a z-offset of 0.05 mm, but no through-holes are formed at a z-offset greater than 0.05 mm. Therefore, at an average power of 66 W (or so) to 67 W (or so) and a drilling cycle of 5 μs, the z-range of the laser drilling process is 0.05 mm. It is believed that the z-range of the laser drilling process can be extended by increasing the average power at the substrate and / or by increasing the drilling cycle at each drilling location, e.g. Figure 4 . For example, by increasing the drilling cycle from 40 μs to 120 μs (at an average power of 30 W), a through-hole can be formed at a z-offset of 0.075 mm. Similarly, at an average power of 40 W and a drilling cycle of 40 μs, a through-hole is formed in the substrate with a z-offset of 0.075 mm (but no through-hole is formed at a z-offset greater than 0.075 mm). Importantly, it has been found that when the average power at the workpiece is in the range of 66 W (or so) to 67 W (or so), the through-hole formed at a drilling cycle of 15 μs can be formed at a z-offset of at least 0.2 mm (or so). Therefore, at an average power of 66 W (or so) to 67 W (or so) and a drilling cycle of 15 μs, the z-range of the laser drilling process is 0.2 mm. In many cases, this z-range ideally accommodates variations in substrate thickness or flatness, thereby ensuring that through-holes can be quickly and reliably formed through the substrate.

[0049] Figures 5 to 8 is a cross-sectional glass substrate (i.e., AF manufactured by SCHOTT The glass substrate is 50 μm thick (formed) Figure 5 and Figure 7 ) or 100μm thick (in Figure 6 and Figure 8 In the case of ), the glass substrate is processed (e.g., using the laser processing system 100 and the laser drilling process described above) to form through-holes, wherein each hole is formed within a drilling period of 15 μs. The glass substrate does not have any absorptive coating formed on any surface, and the average power of the beam of the laser pulse at the substrate (i.e., after having been reflected, transmitted, or diffracted by the aforementioned components of the laser processing system 100) is in the range of 67 W (or thereabouts) to 69 W (or thereabouts). Figures 5 to 8The uneven edge quality at the cross-section plane is attributed to the fragility of the glass material to the mechanical grinding method used to cross-section the substrate. Therefore, before cutting the cross section, Figures 5 to 8 The diameter identified in may be smaller than the diameter annotated above it. Figures 5 to 8 The main purpose of is to illustrate the straightness of the sidewalls of the through-holes and the uniformity of the diameter of the through-holes through the entire thickness of the substrate.

[0050] From the foregoing, it will be appreciated that the beam of laser pulses may be scanned (e.g., by beam scanner 108) to form a plurality of through-holes. In the laser drilling processes described herein, the minimum inter-hole spacing (i.e., the distance between nearest neighbor holes) will typically be limited by the diameter of the "sputtering zone" around the opening of each through-hole at the second surface of the substrate. Typically, the sputtering zone is the area around the opening at the second surface where molten material is ejected from the substrate during formation of the through-holes. In some embodiments, the laser drilling processes described above produce a sputtering zone having a diameter of about 30 μm. Thus, the inter-hole spacing may be at least 30 μm (or so).

[0051] The foregoing describes embodiments and examples of the present invention and should not be construed as limiting thereof. Although several specific embodiments and examples have been described with reference to the accompanying drawings, it will be readily apparent to those skilled in the art that many modifications to the disclosed embodiments and examples and other embodiments are possible without significantly departing from the novel teachings and advantages of the present invention.

[0052] For example, although through-holes having openings of about 10 μm in diameter at the first and second surfaces of a substrate having a thickness of 50 μm or 100 μm have been described, it will be appreciated that through-holes having openings of similar size may be formed in substrates having a thickness of less than 50 μm, between 50 μm and 100 μm, or greater than 100 μm. In addition, through-holes having openings of smaller diameters at the first and second surfaces may be obtained by using a scanning lens 110 having an NA higher than 0.5.

[0053] In another example, while the laser system 100 has been described above as including a laser 102 operable to generate a beam of laser pulses having a wavelength of 515 nm (or thereabouts), it will be appreciated that the laser drilling process described in the above embodiments may be achieved using laser pulses having wavelengths other than 515 nm (or thereabouts), depending on one or more other factors such as average power at the substrate, pulse width, pulse repetition rate, the material forming the substrate, and the like. For example, the aforementioned laser drilling process may be performed using a beam of laser pulses having a wavelength in the near-infrared region of the electromagnetic spectrum (e.g., 1030 nm, 1064 nm, or the like), with the proviso that the beam of laser pulses has a pulse repetition rate as discussed above, and also with the proviso that the average power of the beam of laser pulses at the substrate is 100 W (or thereabouts) or greater (e.g., 110 W, 120 W, 130 W, or the like, or between any of such values). At wavelengths in the near infrared region, the laser pulses in the beam of laser pulses may have pulse widths as discussed above. However, it should be noted that the choice of pulse width may affect the duration of the drilling cycle. It has generally been found that, at a given average power and pulse repetition rate, a beam of laser pulses with a relatively short pulse width (e.g., 10 ps or so) will be able to form a perforation in a shorter drilling cycle than a beam of laser pulses with a relatively long pulse width (e.g., 30 ps or so).

[0054] In another example, while the laser drilling process has been described above as eliminating the need for an absorptive coating at the second surface of the substrate, it will be appreciated that the second surface of the substrate may be coated or otherwise treated with a material that is more absorptive to the beam of laser pulses than the substrate or that alters the absorptivity of the substrate (e.g., at or near the second surface thereof) to the laser pulses.

[0055] In another example, after the laser drilling process has been performed to form the through-holes, the substrate may be subjected to a thermal annealing process as appropriate. In this case, the thermal annealing may be used to reduce the internal stress present in the substrate due to the solidification of the molten material ejected into the sputtering region during the laser drilling process.

[0056] Therefore, all such modifications are intended to be included within the scope of the present invention as defined in the claims. For example, unless such combinations are mutually exclusive, one of ordinary skill in the art will understand that the subject matter of any sentence, paragraph, example, or specific example may be combined with some or all of the subject matter of other sentences, paragraphs, examples, or specific examples. The scope of the present invention should therefore be determined by the following claims, and equivalents of the claims are included in the scope of the present invention.

Claims

1. A method for forming a through hole in a substrate, the method comprising: providing a substrate having a thickness in a range equal to or greater than 150 μm and having a first surface and a second surface opposite to the first surface; as well as directing a focused beam of laser pulses into the substrate through the first surface of the substrate and then through the second surface of the substrate, wherein the focused beam of laser pulses has a wavelength, and the substrate is at least substantially transparent to the wavelength; wherein the optical intensity of the focused beam of laser pulses at the substrate is less than the optical breakdown intensity of the substrate; wherein said focused beam of laser pulses has a Gaussian energy distribution; wherein the focused beam of laser pulses has a pulse repetition rate, a peak optical intensity at the substrate, and an average power that drives a cumulative heating effect at the substrate; and wherein the laser pulses within said focused beam of laser pulses have a pulse width, The peak optical intensity, pulse repetition rate, average power and pulse width are selected so that the perforation is formed in less than or equal to 120 μs.

2. The method of claim 1, wherein the peak optical intensity, pulse repetition rate, average power, and pulse width are selected so that the perforation is formed in less than 110 μs.

3. The method of claim 1, wherein the peak optical intensity, pulse repetition rate, average power, and pulse width are selected so that the perforation is formed in less than 100 μs.

4. The method of claim 1, wherein the peak optical intensity, pulse repetition rate, average power, and pulse width are selected so that the perforation is formed in less than 90 μs.

5. The method of claim 1, wherein the peak optical intensity, pulse repetition rate, average power and pulse width are selected so that the perforation is formed in the range of 40 μs to 80 μs.

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