Patent Description:
Substrates have been used as an interposer disposed between electrical components (e.g., printed circuit boards, integrated circuits, and the like). Such substrates have holes that may be filled by an electroplating process wherein electrically conductive material (e.g., copper) is deposited on the sidewalls of the hole and continuously built up until the hole is hermetically sealed. This process results in metallized through-substrate vias that provide a path through the interposer for electrical signals to pass between opposite sides of the interposer.

Small-diameter vias conducive to providing electrical connections through substrates may be formed by a laser-damage-and-etch process. In this process, a damage track is initially formed in the substrate by using a laser to modify the material along the damage track. An etching solution is then applied to the substrate. The substrate is thinned by the etching solution. Because the etching rate of the material is faster at the damage track than at non-damaged regions, the damage track is preferentially etched so that a hole is opened through the substrate. Downstream processes, such as metallization processes, may require uniformly round holes (i.e., holes having low circularity) and relatively smooth interior surface.

Document <CIT> ( describing the preamble of claims <NUM> and <NUM>). discloses a method for removing brittle-hard material which is transparent to laser radiation by means of laser radiation. <CIT> provides additional prior art. <CIT> discloses a method showing the features of the preamble of claim <NUM> and an article showing the features of the preamble of claim <NUM>.

In a first aspect of the present invention, a method of processing a substrate having a first surface and a second surface is defined in claim <NUM>, and includes applying an exit material to the second surface of the substrate, wherein a difference between a refractive index of the exit material and a refractive index of the substrate is <NUM> or less, and focusing a pulsed laser beam into a quasi-non-diffracting beam directed into the substrate such that the quasi-non-diffracting beam enters the substrate through the first surface. The substrate is transparent to at least one wavelength of the pulsed laser beam. The quasi-non-diffracting beam generates an induced absorption within the substrate that produces a damage track within the substrate.

In a second embodiment, the method of the first embodiment wherein the difference is <NUM> or less.

In a third embodiment, the method of the first or second embodiments, wherein the substrate is made from one of glass, glass-ceramic and ceramic.

In a fourth embodiment, the method of any preceding embodiment, wherein a location that the quasi-non-diffracting beam exits the exit material is <NUM> or more away from the second surface of the substrate in a direction parallel to the quasi-non-diffracting beam.

In a fifth embodiment, the method of any preceding embodiment, wherein the exit material is applied to the second surface such that there is a reflection of less than <NUM>% at a predetermined region surrounding the damage track.

In a sixth embodiment, the method of any preceding embodiment, wherein a diameter of the predetermined region is <NUM>.

In a seventh embodiment, the method of any preceding embodiment, wherein the exit material includes at least two layers.

In an eighth embodiment, the method of any preceding embodiment, wherein the exit material is a polymer.

In a ninth embodiment, the method of any one of the first through seventh embodiments, wherein the exit material is an anti-reflective coating.

In a tenth embodiment, the method of any one of the first through seventh embodiments, wherein the exit material is water.

In an eleventh embodiment, the method of the tenth embodiment further including a supporting substrate attached to the substrate such that the water is disposed between the supporting substrate and the second surface of the substrate.

In a twelfth embodiment, the method of any one of the first through seventh embodiments, wherein the exit material is a silicone layer.

In a thirteenth embodiment, the method of the twelfth embodiment, further including a polyester substrate, wherein the silicone layer is disposed between the polyester substrate and the second surface of the substrate.

In a fourteenth embodiment, the method of any one of the first through seventh embodiments, wherein the exit material is a photoresist polymer material applied to the second surface of the substrate.

In a fifteenth embodiment, the method of any preceding embodiment, wherein the quasi-non-diffracting beam is a Gauss-Bessel beam.

In a sixteenth embodiment, the method any of the first through fourteenth embodiments, wherein the quasi-non-diffracting beam is an Airy beam.

In a seventeenth embodiment, the method of any preceding embodiment, wherein the quasi-non-diffracting beam has a beam waist, and the quasi-non-diffracting beam defines a laser beam focal line having a first end point and a second endpoint each defined by locations where the quasi-non-diffracting beam has propagated a distance from the beam waist equal to a Rayleigh range of the quasi-non-diffracting beam.

In an eighteenth embodiment, the method of any preceding embodiment, wherein the pulsed laser beam includes a burst further including a plurality of pulses.

In a nineteenth embodiment, the method of the eighteenth embodiment, wherein a pulse width of each pulse of the plurality of pulses is within a range of 100fsec to 10psec, including endpoints.

In a twentieth embodiment, the method of any preceding embodiment, further including etching the substrate in an etching solution to produce a hole having a diameter or <NUM> or more by enlarging the damage track in the substrate.

In a twenty-first embodiment, the method of the twentieth embodiment, further including coating interior surfaces of the hole with an electrically conductive material to provide electrical conductivity between a top and a bottom of the hole.

In a twenty-second embodiment, the method of the twentieth embodiment, wherein a difference between an average surface roughness of interior surfaces of the hole from a waist of the hole to the first surface and an average surface roughness of interior surfaces of the hole from the waist of the hole to the second surface <NUM> Ra or less.

In a twenty-third embodiment, the method of any one of the first through sixteenth embodiments and eighteenth through twenty-second embodiments, wherein the quasi-non-diffracting beam defines a laser beam focal line having a first endpoint and a second endpoint each defined by locations where the quasi-non-diffracting beam has propagated a distance from a beam waist equal to a Rayleigh range, the first endpoint is closer to the first surface of the substrate than the second surface, the second endpoint is closer to the second surface of the substrate than the first surface, and the second endpoint is outside of the substrate such that a distance between the second endpoint and the second surface is <NUM> or less.

In a twenty-fourth embodiment, the method of the twenty-third embodiment, wherein the second endpoint is outside of the substrate such that a distance between the second endpoint and the second surface is <NUM> or less.

In a second aspect of the present invention, an article is defined in claim <NUM>, and includes a substrate having a first surface, a second surface, and at least one damage track extending within the substrate. The article further includes an exit material disposed on at least one of the first surface and the second surface, wherein an interface is defined between the exit material and the at least one of the first surface and the second surface, and a difference between a refractive index of the exit material and a refractive index of the substrate is <NUM> or less, such that a reflectivity of the interface is <NUM>% or less at a wavelength within a range of <NUM> to <NUM>, including endpoints.

In a preferred embodiment, the article of the twenty-eighth embodiment, wherein the reflectivity of the interface is <NUM>% or less at a wavelength of <NUM> ±<NUM>, <NUM> ±<NUM>, and <NUM> ±<NUM>.

In a further preferred embodiment, the article of the thirtieth embodiment, wherein the difference is <NUM> or less.

Additional features and advantages of the embodiments described herein will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments described herein, including the detailed description which follows, the claims, as well as the appended drawings.

The embodiments set forth in the drawings are illustrative and exemplary in nature and are not intended to limit the subject matter defined by the claims.

Referring generally to the figures, embodiments of the present disclosure are generally related to methods for forming holes in substrates. Particularly, embodiments described herein employ a laser-damage-and-etch process to form one or more damage tracks within the substrate, and then subsequently etch the substrate such that one or more holes are formed within the substrate. In some embodiments, the substrate is a glass-based substrate, such as glass and glass-ceramics. Such glasses may be, for example, Corning Eagle XG® glass, Corning Willow® Glass, Coming code <NUM> glass, Corning code <NUM> glass, Corning Lotus™ NXT glass, or high purity fused silica. In embodiments, pulsed, quasi-non-diffracting laser beams are applied through a substrate to form the one or more damage tracks through the substrate. An etching solution is then applied to the substrate to open up the one or more damage tracks into one or more through holes. However, as described in more detail below, Fresnel reflections of the pulsed quasi-non-diffracting laser beam at the exit surface of the substrate and back into the bulk of the substrate may introduce undesirable microcracks and/or voids extending laterally from the one or more damage tracks at a location closer to the exit surface than the entrance surface. These microcracks and/or voids may create undesirable defects within the holes following the etching process, such as hole walls having a high surface roughness, and high circularity values, as described in more detail below.

Electrically non-conducting substrates, such as silicon glass, ceramic, glass-ceramic, sapphire, and the like, may be used as an interposer disposed between electrical components (e.g., printed circuit boards, integrated circuits, and the like). Metallized through-substrate vias (TSVs) provide a path through the interposer for electrical signals to pass between opposite sides of the interposer. These substrates may also be used as a redistribution layer in an electronics assembly. As an example and not a limitation, glass-based substrates such as glass and glass-ceramics may have desirable electrical properties in high-frequency applications, such as low electrical loss at high frequencies. Further, such glass-based materials have excellent thermal dimensional stability due to a low coefficient of thermal expansion (CTE).

In some embodiments, the substrates described herein may be fabricated from any material that is transparent to at least one wavelength of a laser beam used to form the at least one damage track. As used herein, "transparent" means that the material has an optical loss, such as absorption or scattering, of less than about <NUM>% per mm of material depth, such as less than about <NUM>% per mm of material depth for the specified pulsed laser wavelength, or such as less than about <NUM>% per mm of material depth for the specified pulsed laser wavelength. The absorption of the substrate may be measured using a spectrophotometer, such as a Cary <NUM> sold by Agilent Technologies of Santa Clara, CA. Example substrate materials include, but are not limited to borosilicate glass, soda-lime glass, aluminosilicate glass, alkali aluminosilicate glass, alkaline earth aluminosilicate glass, alkaline earth boro-aluminosilicate glass, fused silica, crystalline materials such as sapphire, silicon, gallium arsenide, glass-ceramic, or silicon materials or combinations thereof.

The holes formed within the substrate may be filled by an electroplating process wherein electrically conductive material (e.g., copper) is coated on the interior surfaces of the hole and continuously built up until the via is hermetically sealed. It should be understood that any process that successfully metallizes the holes to form the TSVs may be utilized. However, roughness caused by the microcracks and/or voids described above may lead to non-uniform metal coatings inside of the hole, which may result in inadequate electrical conductivity or mechanical reliability issues.

The present invention suppresses the formation of microcracks and/or voids during the laser damage process by use of an exit material applied to an exit surface of the substrate. This exit material, which may be one or more layers of material, reduces the back reflections of the laser back into the bulk of the substrate. Damage tracks formed with an exit material applied to the exit surface of the substrate are shown to have fewer microcracks and/or voids along the damage track than substrates without an exit material applied to the exit surface. Thus, the resulting holes following the chemically etching process have a smoother surface than holes formed by a laser process that does not employ an exit material applied to the exit surface of the substrate.

Some embodiments of methods for forming holes in substrates are described in detail below.

Referring now to <FIG>, an example substrate <NUM> having a plurality of damage tracks <NUM> formed therein is schematically illustrated. The substrate <NUM> may be fabricated from any suitable material that is transparent to at least one wavelength of a laser beam used to form the damage tracks <NUM> as described in more detail below. The substrate <NUM> may have any suitable thickness depending on the end-application, including, but not limited to <NUM> to <NUM>, including endpoints. In some embodiments, the thickness of the substrate <NUM> is within a range of <NUM> to <NUM>, including endpoints. The damage tracks <NUM> are formed within a bulk of the substrate <NUM> between an entrance surface <NUM> (i.e., a first surface) and an exit surface <NUM> (i.e., a second surface). Damage tracks <NUM> are lines formed within a bulk of the substrate <NUM> having a substrate material that is modified by laser-induced multi-photon absorption, as described in more detail below. The damage tracks <NUM> may be a narrow hole that extends through the substrate <NUM>, or may be a non-continuous channel that is interrupted by substrate material.

It is noted that when the damage tracks <NUM> are formed completely from the entrance surface <NUM> to the exit surface <NUM>, through-holes disposed entirely through the substrate <NUM> will be formed after etching, such as the through-holes <NUM> depicted in <FIG>. When the damage tracks <NUM> do not reach either the entrance surface <NUM> or the exit surface <NUM>, blind holes may be formed after etching. The damage tracks <NUM> are formed by application of a quasi-non-diffracting laser beam through the bulk of the substrate <NUM>, as described in detail below and schematically illustrated by <FIG>.

After the damage tracks <NUM> are formed, the substrate <NUM> is then subjected to a chemical etchant. Etchants are not limited by the present disclosure. Typical etchants that may be used include, but are not limited to hydrogen fluoride acid mixtures, and also basic solutions such as potassium hydroxide and sodium hydroxide. The damage tracks <NUM> are regions within the bulk of the substrate <NUM> having been damaged by the laser beam. The etch rate of the damage tracks <NUM> is greater than the etch rate of non-damaged regions of the substrate <NUM>. The increased etch rate of the damage tracks <NUM> allow holes <NUM> to open up at the damage tracks <NUM> during etching, as schematically shown in <FIG>. Although the holes <NUM> are illustrated in <FIG> as being substantially cylindrical, embodiments are not limited thereto. Holes <NUM> formed by the laser-damage-and-etch techniques described herein may have an hourglass shape such that they have a hole waist with a diameter that is smaller than a diameter of the hole openings at the entrance surface <NUM> and the exit surface <NUM>. As an example and not a limitation, a diameter of the hole openings of the holes <NUM> may be between <NUM> and <NUM>, including endpoints. However, other hole-opening diameters may be formed. Example laser and etching conditions to form damage tracks and resulting holes in the substrates are described in <CIT>.

Following the etching process, in some embodiments the holes <NUM> are coated or otherwise filled with an electrically conductive material in a metallization process to provide electrical conductivity between a top and a bottom of the holes <NUM>. The electrically conductive material is not limited by this disclosure. Any known or yet-to-be-developed process for metalizing the holes <NUM> may be utilized (e.g., electroplating).

<FIG> schematically illustrates an example optical system <NUM> used to form a pulsed quasi-non-diffracting beam 122C through the substrate <NUM> to form one or more damage tracks <NUM>. The quasi-non-diffracting beam 122C may form a focal line <NUM> through the substrate <NUM>. Directing the pulsed quasi-non-diffracting beam 122C into the substrate <NUM> generates an induced absorption within the substrate <NUM> and deposits enough energy to break chemical bonds in the substrate <NUM> to form the damage tracks <NUM>. The optical system <NUM> may include any optical components capable of producing the quasi-non-diffracting beams 122C described herein. In the embodiment illustrated by <FIG>, the optical system <NUM> includes an axicon <NUM> (i.e., a conical lens), a collimating lens <NUM>, and a focusing lens <NUM>. A pulsed laser beam <NUM> from a laser source (not shown) passes through the axicon <NUM>, which creates a primary quasi-non-diffracting beam 122A of the pulsed laser beam <NUM>. The primary quasi-non-diffracting beam 122A diverges to form a ring beam 122B that is received by the collimating lens <NUM>. The collimating lens <NUM> and the focusing lens <NUM> act as a telescope that relays and de-magnifies the primary quasi-non-diffracting beam 122A such that an imaged quasi-non-diffracting beam 122C is provided through the substrate <NUM>. The imaged quasi-non-diffracting beam 122C provides a beam spot on the entrance surface (i.e., the entrance surface <NUM>) of the substrate <NUM>. The telescope may be employed because it projects the primary quasi-non-diffracting beam 122A to a comfortable working distance away from the optical surfaces of the optical system <NUM>, and also allows for the ability to more easily control the size of the focal line <NUM> defined by the quasi-non-diffracting beam 122C.

As a non-limiting example, the pulsed laser beam <NUM> may have a wavelength within a range from <NUM> to <NUM>, including endpoints, for example, without limitation, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>. The laser source is operated to produce a burst of a plurality of pulses having a pulse width. In the examples described herein, each burst includes twenty pulses. However, it should be understood that more or fewer pulses may be provided per burst. The pulse width of the pulses may be within a range of 100fsec to 10psec, including endpoints. As described in more detail below, pulse widths in the femtosecond range may be desirable to reduce coherent back reflections that may create microcracks.

The pulsed laser beam can have an average laser burst energy measured, at the substrate, greater than 40µJ per mm thickness of the substrate. The average laser burst energy used can be as high as 2500µJ per mm of thickness of substrate, for example <NUM>-2000µJ/mm, <NUM>-1750µJ/mm, or <NUM>-1500µJ/mm. This average laser energy can also be referred to as an average, per-burst, linear energy density, or an average energy per laser burst per mm thickness of substrate. As stated above, additional laser parameters to form damage tracks within substrates to create etched holes are described in <CIT>.

The cross sectional profile of an example quasi-non-diffracting beam 122C can be described by a Bessel function, and hence such laser beams are frequently referred to as Bessel beams. In a non-limiting example, the quasi-non-diffracting beam has a wavelength of about <NUM> and a numerical aperture of about <NUM>, which provides a core at the center of the Bessel beam having a diameter of about <NUM>. The intensity of the laser beam in this core spot can be maintained over lengths of hundreds of microns, which is much longer than the diffraction limited Rayleigh range of a typical Gaussian profile beam of equivalent spot size (i.e., only a few microns).

Such an optical system <NUM> as shown in <FIG> can be thought of as mapping the radial (i.e., lateral) intensity distribution of the input pulsed laser beam <NUM> to an intensity distribution along the optical axis to form a focal line. With a typical Gaussian beam from a laser illuminating this optical system <NUM>, the actual intensity along the optical axis will take the form as shown in <FIG>. The length of the focal line that is produced is proportional to the diameter of the pulsed laser beam <NUM> sent into the axicon <NUM>. Such a quasi-non-diffracting beam is known as a Gauss-Bessel beam.

It is noted that the pulsed laser beam <NUM> used to illuminate the optical system <NUM> need not have a Gaussian profile, and additionally one need not use an axicon <NUM> to form the quasi-non-diffracting beam 122C. Thus, it is possible to form different energy distributions along the optical axis, where the intensity may take the form of a "top hat" profile, or other profile shape. As shown in <FIG>, this provides the ability to more uniformly distribute the energy through the depth of the substrate <NUM>, or to tailor the energy distribution so that certain regions of the substrate <NUM> receive more or less energy than others in a deterministic manner. The creation of such optics is described in U. Patent Publication <CIT>.

As stated above, the length of the quasi-non-diffracting beam 122C is determined by its Rayleigh range. Particularly, the quasi-non-diffracting beam 122C defines a laser beam focal line <NUM> having a first end point and a second end point each defined by locations where the quasi-non-diffracting beam has propagated a distance from the beam waist equal to a Rayleigh range of the quasi-non-diffracting beam. A detailed description of the formation of quasi-non-diffracting beams and determining their length, including a generalization of the description of such beams to asymmetric (such as non-axisymmetric) beam cross sectional profiles, is provided in <CIT> and Dutch Patent Application No. <CIT>.

The Rayleigh range corresponds to the distance (relative to the position of the beam waist as defined in Section <NUM> of ISO <NUM>-<NUM>:<NUM>(E)) over which the variance of the laser beam doubles (relative to the variance at the position of the beam waist) and is a measure of the divergence of the cross sectional area of the laser beam. The Rayleigh range can also be observed as the distance along the beam axis at which the peak optical intensity observed in a cross sectional profile of the beam decays to one half of its value observed in a cross sectional profile of the beam at the beam waist location (location of maximum intensity). The quasi-non-diffracting beam defines a laser beam focal line having a first end point and a second end point. The first and second end points of a quasi-non-diffracting beam are defined as the locations where the quasi-non-diffracting beam has propagated a distance from the beam waist equal to a Rayleigh range of the quasi-non-diffracting beam. Laser beams with large Rayleigh ranges have low divergence and expand more slowly with distance in the beam propagation direction than laser beams with small Rayleigh ranges.

Beam cross section is characterized by shape and dimensions. The dimensions of the beam cross section are characterized by a spot size of the beam. For a Gaussian beam, spot size is frequently defined as the radial extent at which the intensity of the beam decreases to <NUM>/e<NUM> of its maximum value. The maximum intensity of a Gaussian beam occurs at the center (x = <NUM> and y = <NUM> (Cartesian) or r = <NUM> (cylindrical)) of the intensity distribution and radial extent used to determine spot size is measured relative to the center.

Beams with Gaussian intensity profiles may be less preferred for laser processing to form damage tracks <NUM> because, when focused to small enough spot sizes (such as spot sizes in the range of microns, such as about <NUM>-<NUM> or about <NUM>-<NUM>) to enable available laser pulse energies to modify materials such as glass, they are highly diffracting and diverge significantly over short propagation distances. To achieve low divergence, it is desirable to control or optimize the intensity distribution of the pulsed laser beam to reduce diffraction. Pulsed laser beams may be non-diffracting or weakly diffracting. Weakly diffracting laser beams include quasi-non-diffracting laser beams. Representative weakly diffracting laser beams include Bessel beams, Gauss-Bessel beams, Airy beams, Weber beams, and Mathieu beams.

Non-diffracting or quasi-non-diffracting beams generally have complicated intensity profiles, such as those that decrease non-monotonically vs. radius. By analogy to a Gaussian beam, an effective spot size wo,eff can be defined for any beam, even non-axisymmetric beams, as the shortest radial distance, in any direction, from the radial position of the maximum intensity (r = <NUM>) at which the intensity decreases to <NUM>/e<NUM> of the maximum intensity. Further, for axisymmetric beams wo,eff is the radial distance from the radial position of the maximum intensity (r = <NUM>) at which the intensity decreases to <NUM>/e<NUM> of the maximum intensity. A criterion for Rayleigh range based on the effective spot size wo,eff for axisymmetric beams can be specified as non-diffracting or quasi-non-diffracting beams for forming damage regions in Equation (<NUM>), below: <MAT> where FD is a dimensionless divergence factor having a value of at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, at least <NUM>, in the range from <NUM> to <NUM>, in the range from <NUM> to <NUM>, in the range from <NUM> to <NUM>. For a non-diffracting or quasi-non-diffracting beam the distance, ZR in Equation (<NUM>), over which the effective beam size doubles, is FD times the distance expected if a typical Gaussian beam profile were used. The dimensionless divergence factor FD provides a criterion for determining whether or not a laser beam is quasi-non-diffracting. As used herein, the pulsed laser beam <NUM> is considered quasi-non-diffracting if the characteristics of the laser beam satisfy Equation (<NUM>) with a value of FD ≥ <NUM>. As the value of FD increases, the pulsed laser beam <NUM> approaches a more nearly perfectly non-diffracting state.

Referring now to <FIG>, a pulsed quasi-non-diffracting beam 122C may create microcracks <NUM> along the damage tracks <NUM> closer to the exit surface (i.e., the exit surface <NUM>) than the entrance surface (i.e., the entrance surface <NUM>). The microcracks <NUM> may include crack lines or voids within the substrate <NUM> extending within and from the damage tracks <NUM>. As discussed above, these microcracks <NUM> may lead to undesirable effects after etching the holes <NUM>, such as high average interior surface roughness (e.g., Ra greater than <NUM>) and high circularity values (e.g. > <NUM> microns) Circularity is described in detail below with reference to <FIG> and <FIG>.

<FIG> is an optical microscope digital image of damage tracks <NUM> formed within a substrate <NUM> fabricated from <NUM> alkaline earth boro-aluminosilicate glass manufactured and sold under the trade name EAGLE XG® by Corning, Inc. of Coming, New York. A quasi-non-diffracting beam 122C having a focal line <NUM> length of about <NUM> was pulsed at <NUM> pulses per burst and provided 120µJ of energy per burst. The wavelength of the laser was <NUM>, the pulse width was <NUM> psec, with 20nsec between each pulse. One burst was applied per damage track (one burst per hole). As shown in <FIG>, small microcracks <NUM> are clustered near the exit surface (i.e., exit surface <NUM>) of the substrate <NUM>.

To illustrate how energy per burst affects the formation of microcracks <NUM>, several substrates having a thickness and composition as the example illustrated by <FIG> were processed using the same laser conditions as the example illustrate by <FIG> except that the energy per burst was varied. <FIG> illustrate digital images of damage tracks <NUM> resulting from an energy per burst as indicated in each figure. At energies per burst of 120µJ and above, microcracks <NUM> are clustered near the exit surface <NUM>. As the energy per pulse is increased further, the microcracks <NUM> become stronger and the range of depths over which they are present increases. In all cases, the microcracks <NUM> tend to be clustered or most numerous near the exit surface <NUM>. It is noted that for low energies (≤110µJ/burst), no microcracks <NUM> are observed in the corresponding images. However, only a small number of damage tracks <NUM> are shown in each image. Inspection of larger numbers of damage tracks <NUM> almost always reveals the presence of microcracks <NUM> near the exit surface <NUM>, although on a small fraction of the holes (e.g., <NUM>% or even <NUM>%). However, even a small fraction of holes with higher roughness may be of concern for metallization processes. Furthermore, for energies per burst at the lower end of this regime, the damage tracks <NUM> are often too weak to allow for reliable and thorough etchant penetration to form well-opened holes, resulting in numerous holes with tight waists to the final hourglass hole profile.

Additionally, sample glass substrates having the same thickness and composition as described above with respect to the sample shown in <FIG> were laser-processed to evaluate the effect of the location of the laser beam focal line defined by the quasi-non-diffracting beam on the formation of microcracks <NUM>. The laser beam parameters were the same as described above with respect to <FIG>. <FIG> illustrate the resulting damage tracks <NUM> in substrates when shifting the distance between the substrate and the optics used to create the quasi-non-diffracting beam and resulting focal line.

<FIG> is an image of damage tracks within a glass substrate formed by a quasi-non-diffracting beam with a position of the quasi-non-diffracting beam offset from a nominal position by -<NUM>. <FIG> is an image of damage tracks with a position of the quasi-non-diffracting beam offset from a nominal position by -<NUM>. <FIG> is an image of damage tracks with a position of the quasi-non-diffracting beam offset from a nominal position by - <NUM>. <FIG> is an image of damage tracks with a position of the quasi-non-diffracting beam offset from a nominal position by +<NUM>. <FIG> is an image of damage tracks with a position of the quasi-non-diffracting beam offset from a nominal position by +<NUM>. <FIG> is an image of damage tracks with a position of the quasi-non-diffracting beam offset from a nominal position by +<NUM>.

It is noted that the reference or nominal zero position of the focal line is such that the focal line is centered upon the thickness of the substrate, so that approximately equal amounts of the focal line extend both above and below the substrate. <FIG> illustrate that moving a position of the optics within a range of -<NUM> to +<NUM> does not significantly change a position of the microcracks <NUM> along the damage tracks <NUM>. The microcracks <NUM> remain clustered near the exit surface of the substrate, and do not move along with the shifted position of the optics. Only for ±<NUM> changes in focal position are changes in the position of the microcracks <NUM> observed, in which cases the focal line is now moved so severely that the majority of the length of the focal line is located outside of the substrate.

The examples illustrated by <FIG> and <FIG> illustrate that the microcracks are in an approximately fixed location with respect to the exit surface of the substrate, and grow in extent as the energy per burst is increased. Without being bound by theory, the microcracks may be formed by Fresnel reflections from the exit surface (i.e., exit surface <NUM>) of the substrate that focus-fold some fraction of light back onto the focal line itself.

Referring to <FIG>, a pulsed laser beam <NUM> is conditioned to form a quasi-non-diffracting beam 122C that creates a focal line <NUM> within a bulk of the substrate <NUM> to form a damage track <NUM> as described above. A portion <NUM> of the pulsed laser beam <NUM> is back reflected at the interface between the material of the substrate <NUM> and the atmosphere by Fresnel reflection. This portion <NUM> of the pulsed laser beam <NUM> may form another focal line within the substrate <NUM> having a length that is proportional to a length of the original focal line <NUM> that extends beneath the substrate (i.e., outside of the exit surface <NUM>).

Because the pulsed laser beam <NUM> has a short pulse width (e.g., about 7psec to about 11psec), the coherence length is on the order of millimeters, and thus the portion <NUM> of light that is back-reflected is also coherent with the input pulsed laser beam <NUM>. This coherence reflection interferes with the original focal line <NUM>. Thus, there may be significantly more intensity enhancement than would arise with incoherent reflections. This enhanced intensity proximate the exit surface of the substrate <NUM> may cause the microcracks along the damage tracks <NUM>.

Referring to <FIG>, some embodiments according to the present invention mitigate the Fresnel reflections and the formation of undesirable microcracks and/or voids by applying an exit material <NUM> to the exit surface <NUM> of the substrate <NUM> prior to laser processing. An exit material <NUM> having a refractive index that reduces the Fresnel reflection at the exit surface <NUM> is chosen. According to the present invention, the refractive index of the exit material <NUM> closely matches the refractive index of the substrate <NUM> such that a difference between the refractive index of the exit material <NUM> and the refractive index of the substrate <NUM> is <NUM> or less, <NUM> or less, or <NUM> or less. Such a reduction in refractive index difference leads to a reduction of the expected reflection intensity at near normal incidence from a typical air to glass interface observed value of <NUM>%, to an reduced reflection intensity <NUM>% or less, <NUM>% or less, or <NUM>% or less at an interface between the exit surface <NUM> and the exit material <NUM> when measured at a wavelength within a range of <NUM> to <NUM>, including endpoints. In a non-limiting example, the reflectivity of the interface is <NUM>% or less at a wavelength of <NUM> ±<NUM>, <NUM> ±<NUM>, and <NUM> ±<NUM>. Refractive index can change based on wavelength and other parameters. "Refractive index" as used herein and according to the present invention refers to the refractive index of a material at the peak wavelength of the laser, and under the conditions at which the material, such as the material of substrate <NUM> and exit material <NUM>, are exposed to the laser. Related parameters such as reflection intensity are also for the peak wavelength of the laser under the conditions at which substrate <NUM> and exit material <NUM> are exposed to the laser.

As shown in <FIG>, the focal line <NUM> defined by the quasi-non-diffractive beam 122C extends below the interface between the exit surface <NUM> of the substrate <NUM> and the exit material <NUM> such that any Fresnel reflection of the pulsed laser beam <NUM> occurs outside of the bulk material of the substrate <NUM>. Accordingly, the presence of a coherent reflected focal line within the bulk material of the substrate <NUM> is reduced or eliminated. It is noted that thicker exit material <NUM> may be preferred. As shown in <FIG>, the portion of the focal line <NUM> that is below the substrate is effectively "folded back" on the original focal line <NUM> produced by the quasi-non-diffracting beam 122C. If the exit material <NUM> at the exit surface <NUM> is thin, there may still be a reflection from the bottom surface <NUM> of the coating, which may cause a similar problem created by any reflection from the exit surface <NUM> of the substrate <NUM>. Thus, the exit material <NUM> should be thick enough so that the focal line exits the bottom surface <NUM> at a distance great enough to avoid Fresnel reflections into the substrate <NUM>. As non-limiting examples, the exit material <NUM> has a thickness such that the quasi-non-diffracting beam 122C exits the exit material <NUM> at a distance of <NUM> or more from the exit surface <NUM>, <NUM> or more from the exit surface <NUM>, <NUM> or more from the exit surface <NUM>, <NUM> or more from the exit surface <NUM>, <NUM> or more from the exit surface <NUM>.

The exit material <NUM> should be intimately applied to the exit surface <NUM> of the substrate <NUM> to ensure that there are substantially no gaps or air bubbles between the exit material <NUM> and the exit surface <NUM>. For example, if the interface between the exit material <NUM> and the exit surface is <NUM>% air bubbles, then the exit material <NUM> may provide a <NUM>% decrease in effectiveness. If an air bubble is present at a position where the quasi-non-diffracting beam passes, Fresnel reflections may occur and create microcracks at the resulting damage track <NUM>. Thus, the exit material <NUM> should be applied to the exit surface <NUM> such that the reflectivity at the interface where a hole is desired is <NUM>% or less. In some embodiments, the reflectivity at the interface is <NUM>% or less within a predetermined region of a damage track <NUM> to ensure prevention of microcracks. As an example and not a limitation, the predetermined region may have a diameter of <NUM> and surround the damage track <NUM>. In other words, no air gaps or other interfering materials should be present between the exit material <NUM> and the exit surface <NUM> within a predetermined region surrounding the damage track <NUM>.

The exit material <NUM> may be made from a single layer of material, or of multiple, stacked layers. <FIG> schematically illustrates an exit material <NUM> comprising a layer of water <NUM> (or other material) disposed between the exit surface <NUM> of the substrate <NUM> and an additional substrate <NUM> (i.e., a supporting substrate). As an example and not a limitation, the additional substrate <NUM> may be fabricated from the same material as the substrate <NUM>. In the case where the substrate <NUM> is fabricated from glass (e.g., a <NUM> thick glass substrate) having a refractive index of about <NUM>, a difference in refractive index between the glass and water <NUM> having a refractive index of about <NUM> is about <NUM>, which is smaller than the refractive index difference between glass and air, which is about <NUM>. During laser processing, there may be minimal Fresnel reflections at the interface between the substrate <NUM> and the water <NUM> because of the closely matched refractive indices.

The exit material <NUM> may be any material(s) having a refractive index that closely matches the refractive index of the material of the substrate <NUM>. Other materials include, but are not limited to, polymers (e.g., polyethylene film), glass-based materials, optical glue (e.g., NOA <NUM> sold by Norland Products, Inc. of Cranbury, NJ), blue photoresist (e.g., D15133640 21x100 MX5015 CS1 <NUM> sold by E. du Pont de Nemours and Company of Wilmington, DE), silicone layer on polyester substrate (e.g., PF-<NUM>-X0 and PF-<NUM>-X4 PF Film sold by Gel-Pak® of Hayward, CA), anti-reflective coatings (e.g., coating code UV sold by Thor Labs of Newton, NJ), and combinations thereof. Of the exit materials evaluated, the following non-limiting material systems showed to be good candidates for microcrack mitigation:.

It should be understood that the exit material <NUM> is not limited to the materials described above, and other materials having a difference of a refractive indices with respect to the substrate <NUM> that is less than or equal to <NUM> may be utilized.

To illustrate the effect of an exit material comprising water disposed between an exit surface of a glass substrate and an additional glass substrate, <NUM> thick EAGLE XG® glass substrates were laser-processed by a quasi-non-diffracting beam having a focal line length of about <NUM> that was pulsed at <NUM> pulses per burst and provided 130µJ of energy per burst. The wavelength of the laser was <NUM>, the pulse width was <NUM> psec, with <NUM> nsec between each pulse. One burst was applied per hole. A first sample glass substrate did not include an exit material applied to the exit surface <NUM> and was used as a baseline (<FIG>). A second sample glass substrate had water between an exit surface <NUM> and an additional <NUM> thick EAGLE XG® glass substrate (<FIG>). A third sample glass substrate had a <NUM> thick photoresist polymer layer (D15133640 21x100 MX5015 CS1 <NUM>) applied to the exit surface <NUM> (<FIG>). Microcracks <NUM> were observed in the first, baseline glass substrate shown in <FIG>. However, no microcracks <NUM> were observed in the second and third sample glass substrates as shown in <FIG> and <FIG>. Thus, the water and the photoresist polymer layer both prevented the formation of microcracks <NUM> in the damage tracks <NUM>.

As stated above, microcracks <NUM> present within the damage track <NUM> may cause the resulting etched holes to have a rough surface and a poor (high) circularity. Circularity is defined as a maximum diameter of a hole minus a minimum diameter of a hole determined from an image taken from either then entrance surface <NUM> or the exit surface <NUM> of the substrate <NUM>. <FIG> illustrate a top view of the exit surface <NUM> and a cross sectional side view, respectively, of a <NUM> thick EAGLE XG® glass substrate <NUM> having holes formed without an exit material applied to the exit surface <NUM>. Damage tracks <NUM> (not shown in <FIG>) were first formed using a quasi-non-diffracting beam having a focal line length of about <NUM> that was pulsed at <NUM> pulses per burst and provided 160µJ of energy per burst. The wavelength of the laser was <NUM>, the pulse width was <NUM> psec, with <NUM> nsec between each pulse. One burst was applied per hole. The glass substrate was then chemically etched to form a plurality of holes <NUM> that were opened from the damage tracks <NUM>. The etchant was a <NUM> hydrofluoric acid and <NUM> nitric acid solution. Other etchant solutions may be used as well, such as different concentrations of hydrofluoric acid, different concentrations of mineral acids such as hydrochloric acid or nitric acid, or etching with basic solutions instead such as potassium hydroxide or sodium hydroxide.

In the top view of <FIG>, the dark circle is the diameter of the exit hole <NUM> at the exit surface <NUM> of the glass substrate <NUM>. The post-etch holes <NUM> show elliptical shapes. Holes 140A and 140B particularly have elliptical shapes, and thus high circularity, which may be undesirable in down-stream processes such as hole metallization to form TSVs. In the cross sectional side view of <FIG>, the holes <NUM> have an hourglass shape having an entrance segment <NUM>, a waist segment <NUM> and an exit segment <NUM>. The waist segment <NUM> is narrower than the entrance segment <NUM> and the exit segment <NUM>. Because of the presence of microcracks <NUM> closer to the exit surface <NUM> than the entrance surface <NUM> in the damage tracks <NUM> prior to etching, the walls of the exit segment <NUM> are scalloped in texture, resulting in rough interior surfaces. As described above, rough interior surfaces may negatively impact later metallization processes to fill the hole <NUM> with electrically conductive material. From <FIG>, the interior surface of the exit segment <NUM> appears to be rougher than that of the entrance segment <NUM>.

<FIG> illustrate a top view of the exit surface <NUM> and a cross sectional side view, respectively, of a <NUM> thick EAGLE XG® glass substrate <NUM> having holes formed a PF-<NUM>-X4 material applied to the exit surface <NUM>. The laser parameters were the same as described with respect to <FIG>. <FIG> shows that the resulting holes <NUM>' have a circularity that is improved over the circularity of the holes <NUM> shown in <FIG>. Further, the walls entrance segments <NUM>', the waist segments <NUM>' and the exit segments <NUM>' of the holes <NUM>' (particularly the walls of the exit segment <NUM>') are smooth, and do not possess the scalloped shape of the holes <NUM> depicted in <FIG>. This illustrates that the process window of laser energy over which smooth and non-microcracked holes are made is expanded by the use of an exit material. From <FIG>, the interior surface of the exit segment <NUM>' appears to be just as smooth as that of the entrance segment <NUM>'. Due to the mitigation of microcracks, some embodiments described herein result in an average surface roughness of interior surfaces of the hole from a waist of the hole to the entrance surface (i.e., entrance segment <NUM>') and an average surface roughness of interior surfaces of the hole from the waist of the hole to the exit surface (i.e., exit segment <NUM>') is less than <NUM> Ra.

Surface roughness may be measured by forming holes near an edge of a substrate. To measure surface roughness, a side-profile image is taken of the hole near the edge. An edge detection algorithm is performed on the image of the hole to determine the edges of the hole and the bulk of the substrate. The image processing program ImageJ converts an <NUM> bit image of the side profile of the hole into a binary figure using the "minimum method" within ImageJ. Subsequently, an edge detection algorithm is used where every row in the image is scanned until a transition from <NUM> to <NUM> in the intensity scale is detected (which corresponds to the edge of the hole). Using a least squares minimization fitting routine, the detected edge is fitted to a polynomial curve, typically a second degree polynomial of the kind: y=ax<NUM>+bx+c, where y is the distance from the horizontal axis to the detected edge, x is the location on the horizontal axis corresponding to the depth in the substrate, and a, b and c are constants calculated during the fitting routine. Next, the intrinsic curvature is removed by subtracting the fitted polynomial curve from the detected edge data, and the residuals are calculated to yield a straightened roughness profile. Various statistical roughness parameters may be extracted from the straightened roughness profile, such as, without limitation, Ra, Rq, Rz, highest peak, lowest valley, top diameter, bottom diameter, and waist percentage. Additional information regarding calculating surface roughness Ra of interior walls of holes is provided in U. Patent Publication No. <CIT>.

To determine the improvement of hole quality over a large array of holes of a substrate having an exit material applied to the exit surface, three <NUM> thick EAGLE XG® glass substrates were processed to form <NUM>,<NUM> holes in each by the laser-damage-and-etch process described above. Particularly, <NUM>,<NUM> damage tracks were first formed using a quasi-non-diffracting beam having a focal line length of about <NUM> that was pulsed at <NUM> pulses per burst and provided 130µJ of energy per burst. The wavelength of the laser was <NUM>, the pulse width was <NUM> psec, with <NUM> nsec between each pulse. One burst was applied per hole. A baseline glass substrate did not have any material applied to the exit surface. A second glass substrate had water disposed between the exit surface and an additional glass substrate. A third glass substrate had a photoresist polymer (D15133640 21x100 MX5015 CS1 <NUM> sold by E. du Pont de Nemours and Company) applied to its exit surface.

The glass substrate was then chemically etched to form a plurality of holes that were opened from the damage tracks. The etchant was a <NUM> hydrofluoric and <NUM> nitric etchant solution. The holes had a diameter of approximately <NUM>. After etching, the holes were measured with a VHX-<NUM> microscope sold by Keyence Corp. of America of Itasca, IL to characterize the diameter and the circularity of the holes. As stated above, the circularity is the maximum diameter of the hole minus the minimum diameter of the hole measured at either the entrance surface or the exit surface. It is noted that because the microscope optics have a depth of field, and do not simply measure the hole diameter at exactly the entrance and exit surfaces of the glass substrates, the measurements may pick up irregularities in the depth of the glass, such as those that may be caused by an internal defect such as a bowed shape in the wall of a hole.

Histograms showing the statistics for the entrance surface diameter, the exit surface diameter, the entrance surface circularity and the exit surface circularity for the baseline glass substrate without an exit material are shown in <FIG>. Particularly, <FIG> graphically depicts a histogram of entrance surface diameters of the holes, <FIG> graphically depicts a histogram of exit surface diameters of the holes, <FIG> graphically depicts a histogram of entrance surface circularities of the holes, and <FIG> graphically depicts a histogram of exit surface circularities of the holes.

The diameter distributions of both the entrance surface and the exit surface are very similar, with averages close to <NUM>-<NUM>. However, the circularity histograms are different. The entrance surface circularities shown in <FIG> are lower, indicating rounder holes. However, the exit surface circularities shown in <FIG> shift distinctly away from zero, indicating that the holes are less round near or at the exit surface of the glass substrate. This is a signature of imperfections or roughness inside of the hole but only near the exit surface.

<FIG> depict the same measurements as <FIG> for the glass substrate having a photoresist polymer applied to the exit surface. Particularly, <FIG> graphically depicts a histogram of entrance surface diameters of the holes, <FIG> graphically depicts a histogram of exit surface diameters of the holes, <FIG> graphically depicts a histogram of entrance surface circularities of the holes, and <FIG> graphically depicts a histogram of exit surface circularities of the holes.

The exit surface circularity histogram shown by <FIG> is distinctly different from <FIG>, thereby showing more holes with near-zero circularity. However, a fraction of holes have a higher circularity (i.e., less round) as indicated by the tail of the distribution. The holes with higher circularity may be the result of the photoresist polymer not being consistent or spatially uniform, thereby causing some locations to be improved and other not leading to the bi-modal histogram.

<FIG> depict the same measurements as <FIG> for the part having water applied to the exit surface. Particularly, <FIG> graphically depicts a histogram of entrance surface diameters of the holes, <FIG> graphically depicts a histogram of exit surface diameters of the holes, <FIG> graphically depicts a histogram of entrance surface circularities of the holes, and <FIG> graphically depicts a histogram of exit surface circularities of the holes.

In this case, the exit surface circularity histogram shown by <FIG> is demonstrably improved over the exit surface circularity histogram shown by <FIG>. The single peak of the histogram of <FIG> is very close to zero, indicating that the holes are much smoother and rounder than those of the baseline glass sample.

<FIG> graphically shows a comparison of <NUM>,<NUM> exit surface circularity averages for three baseline glass samples, two glass samples having the photoresist polymer applied, and two glass samples having water applied. Holes were fabricated as described above with respect to the glass substrate illustrated by <FIG>, <FIG> and <FIG>. <FIG> illustrates that while both exit materials work to improve the quality of the holes, the water yields more consistent results than the photoresist polymer material. This may suggest that the photoresist polymer material is not attached to the exit surface uniformly at all locations where the holes were fabricated.

Accordingly, material having a refractive index close to that of the substrate may be applied to the exit surface of the substrate to reduce Fresnel reflections of the quasi-non-diffracting beam and thus reduce the formation of microcracks and/or voids along the damage track proximate the exit surface. Minimizing the formation of microcracks and/or voids create high quality holes having low circularity and smooth interior walls having a roughness of less than <NUM> Ra.

In some cases not covered by the present invention, there are other ways to minimize the impact of Fresnel side reflections in addition to, or in lieu of, applying an exit material to the exit surface. For example, the optical system <NUM> may be optimized to form a quasi-non-diffracting beam 122C that produces a shorted focal line <NUM> such that very little or none of the focal line <NUM> extends below the exit surface <NUM> of the substrate <NUM> (see <FIG>). This may be achieved by using optics that create sharp cut-offs in the beam intensity along the optical axis, such as shown by the waxicon example of <FIG>. Alternatively, the cut-off may be accomplished by positioning a hard aperture such as an iris in the beam path in front of the axicon <NUM> as shown in <FIG>. By vignetting the outermost rays in the input pulsed laser beam <NUM>, the tail (i.e., end) of the focal line <NUM> will be cut off, creating a sharp cut-off at the tail of the focal line <NUM>.

Regardless of the method used to generate the sharp cut-off in the tail of the focal line <NUM>, if the tail of the focal line is made to extend just barely beyond the bottom of the substrate (e.g., <NUM> or less, <NUM> or less, <NUM> or less, <NUM> or less, or <NUM> or less), then very little energy can be reflected back into the substrate <NUM> to cause the intensity enhancement and resulting microcracks. In some embodiments, precise focus control of the focal line <NUM> should be made with respect to the substrate <NUM>. Focus stage compensation schemes may be employed with minimal impact on process time.

Another way to minimize back reflections with coherency is to use shorter pulse width laser beams. Particularly, the pulse widths of the laser pulses may be such that the coherence time of the pulses is much less than the thickness of the substrate <NUM>. As a non-limiting example, when the pulse width is about 10psec, the coherence length is on the order of millimeters. This means that the back reflections coherently (either constructively or destructively) interfere with the original focal line <NUM>, creating strong interference effects. However, if femtosecond pulse lasers are used, the coherence time and length may be greatly reduced, and the coherent interaction may be reduced or eliminated. The back reflection may still occur, but the magnitude of interference effects that could create the microcracks may be reduced.

It should now be understood that embodiments described herein provide methods of fabricating holes in a substrate by a laser-damage-and-etch process wherein the holes have a relatively low circularity and relatively smooth interior wall surfaces. Holes with such characteristics are desirable for downstream processes, such as metallization processes to fabricate interposers or redistribution layers. Particularly, the methods described herein employ quasi-non-diffracting beams to produce damage tracks that are then chemically etched to open holes at the damage tracks. Microcracks along the damage tracks proximate an exit surface of the substrate that result in high circularity and rough interior surfaces are mitigated by reducing the effects of Fresnel reflections of the quasi-non-diffracting beam back into the substrate. In some embodiments, an exit material having a refractive index similar to the substrate is applied to the exit surface of the substrate to minimize Fresnel reflections of the quasi-non-diffracting beam back into the substrate. In some embodiments, the laser focal line of the quasi-non-diffracting beam is focused into the substrate or otherwise prepared such that it does not significantly extend below the exit surface of the substrate. In some embodiments, the pulse width of the pulsed laser beam is short (e.g., within the femtosecond range) to minimize coherent back reflections of the quasi-non-diffracting beam back into the substrate.

Thus, embodiments described herein eliminate or significantly reduce the small microcracks present near the exit surface of the substrate without reducing the pulse energy of the laser beam. By allowing higher laser pulse energies to be used without creating microcracks, the exit material applied to the exit surface of the substrate enables stronger damage tracks to be formed, and hence holes with more open (i.e., wider) waists post-etch. Further, the embodiments described herein lead to larger process tolerances for laser burst (or pulse) energy during the laser damage step. By reducing or eliminating the exit surface back reflections, high laser energies may be used without causing microcrack formation. Therefore the processes described herein may be more stable as small changes in laser energy no longer have a significant impact on damage track formation, thereby increasing yield.

Claim 1:
A method of processing a substrate (<NUM>) comprising a first surface (<NUM>) and a second surface (<NUM>), the method comprising:
applying an exit material (<NUM>) to the second surface (<NUM>) of the substrate (<NUM>); and
focusing a pulsed laser beam (<NUM>) into a quasi-non-diffracting beam (122C) directed into the substrate (<NUM>) such that the quasi-non-diffracting beam (122C) enters the substrate (<NUM>) through the first surface (<NUM>), the quasi-non-diffracting beam (122C) generating an induced absorption within the substrate (<NUM>), the induced absorption producing a damage track (<NUM>) within the substrate (<NUM>), wherein the substrate (<NUM>) is transparent to at least one wavelength of the pulsed laser beam (<NUM>),
characterized in that a difference between a refractive index of the exit material (<NUM>) and a refractive index of the substrate (<NUM>) is <NUM> or less, wherein a refractive index refers to the refractive index of a material at the peak wavelength of the laser and under the conditions at which the material, such as the material of substrate (<NUM>) and exit material (<NUM>) are exposed to the laser.