Patent Application
20190233321
This disclosure pertains to methods for processing transparent dielectrics with a laser. More particularly, this disclosure pertains to methods for forming features in transparent dielectrics that use a laser to remove material. Most particularly, this disclosure pertains to methods for forming holes in transparent dielectrics.
Precision machining of materials is needed for many applications. Precision machining allows for the formation of miniature features in materials. Such features include holes, slots, channels, grooves, and chamfers. Traditional techniques for precision machining involve mechanical methods (e.g. cutting, sawing, drilling, and scoring) or chemical methods (e.g. etching).
Adaptation of traditional techniques to more demanding applications, however, has proven to be challenging. There is increasing demand for machining finer features and for forming features in a wider variety of materials. There is currently great interest in the precision machining of hard dielectric materials and in forming high aspect ratio features with a high degree of precision. CNC machining, for example, has challenges in drilling holes with a diameter smaller than 100-200 μm in glass, especially when the aspect ratio exceeds 10-20.
Laser ablation in air (dry ablation) is an alternative process for machining hard materials and has been demonstrated in a range of glasses and crystals. In dry laser ablation, a high intensity laser is directed to the surface of a material and the energy of the laser is sufficient to break bonds and release matter from the surface. Thermal effects associated with dry laser ablation, however, are disadvantageous for many applications. Thermal effects often lead to surface damage (e.g. oxidation, melting, cracking and stresses) and other effects that limit the resolution of machining. Thermal effects also lead to surface roughness, irregularities or non-uniformities in the dimensions of features formed by dry laser ablation, and to reduced mechanical strength of separated parts due to induced surface flaws. Much of the matter released by the laser also remains as debris on the surface.
Liquid-assisted laser micromachining is an alternative technique designed to overcome the limitations of dry laser ablation. In liquid-assisted laser micromachining, the working surface of the material is placed in contact with a liquid. The presence of the liquid increases the rate of heat removal from the material to minimize deleterious thermal effects. The liquid also provides a medium for sweeping away debris formed by the laser and prevents re-deposition of released matter on the surface. Shorter processing times are also possible for liquid-assisted laser micromachining relative to dry laser ablation.
Water is the most common liquid used for liquid-assisted laser micromachining. Water has a high thermal conductivity and efficiently removes heat from the surface. While water-assisted laser micromachining leads to faster ablation, it also leads to high roughness due to aggressive forces associated with cavitation bubbles formed in the process. Smooth surfaces are required for many applications. In photonic devices, for example, it is common to insert an optical fiber into a hole of a transparent glass or crystal substrate. Control of the radius of the hole to within a tolerance better than 0.25 μm is needed to permit precise positioning of an optical fiber. If the roughness of the inside surface of the hole exceeds 0.25 μm, an optical fiber cannot be inserted in the hole without damaging (e.g. scratching or breaking) the fiber. If the hole diameter is increased to avoid damage, the fiber fits loosely in the hole and is susceptible to motion or misalignment in practical applications.
There is a need for improved micromachining processes for forming fine features in hard transparent materials. In particular, there is a need for a micromachining process for forming high aspect ratio features with smooth interior surfaces as well as features with precise dimensions and shapes.
The present disclosure provides a method for processing transparent dielectric materials. The method is an improved liquid-assisted laser micromachining process for forming holes and other features in high hardness transparent dielectrics. The method includes placing a transparent dielectric in direct contact with a liquid and directing a laser to the interface of the transparent dielectric and the liquid. The liquid contains a fluorinated compound. The laser has sufficient intensity to release material from the transparent dielectric. The liquid is selected to have low surface tension. The low surface tension minimizes the force of the cavitation micro-bubbles in the liquid on the transparent dielectric during ablation and provides holes and other features with smooth surfaces and good dimensional control.
The present disclosure extends to:
A method of processing a transparent dielectric material, comprising:
focusing a laser beam to a focal point in a liquid, the liquid directly contacting a working surface of a transparent dielectric material, the liquid comprising a fluorinated compound.
The present disclosure extends to:
A method of processing a transparent dielectric material, comprising:
directing a laser beam to a transparent dielectric material in direct contact with a liquid, the laser beam passing through the transparent dielectric material and entering the liquid, the liquid comprising a fluorinated compound.
The present disclosure extends to:
A transparent dielectric material comprising a hole, the hole having a circular cross-section with a diameter, the diameter having a variability of 0.5 μm or less.
Additional features and advantages 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 as described in the written description and claims hereof, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understand the nature and character of the claims.
The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings are illustrative of selected aspects of the present description, and together with the specification serve to explain principles and operation of methods, products, and compositions embraced by the present description. Features shown in the drawing are illustrative of selected embodiments of the present description and are not necessarily depicted in proper scale.
While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the written description, it is believed that the specification will be better understood from the following written description when taken in conjunction with the accompanying drawings, wherein:
The embodiments set forth in the drawings are illustrative in nature and not intended to be limiting of the scope of the detailed description or claims. Whenever possible, the same reference numeral will be used throughout the drawings to refer to the same or like feature.
The present disclosure is provided as an enabling teaching and can be understood more readily by reference to the following description, drawings, examples, and claims. To this end, those skilled in the relevant art will recognize and appreciate that many changes can be made to the various aspects of the embodiments described herein, while still obtaining the beneficial results. It will also be apparent that some of the desired benefits of the present embodiments can be obtained by selecting some of the features without utilizing other features. Accordingly, those who work in the art will recognize that many modifications and adaptations are possible and can even be desirable in certain circumstances and are a part of the present disclosure. Therefore, it is to be understood that this disclosure is not limited to the specific compositions, articles, devices, and methods disclosed unless otherwise specified. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting.
In this specification and in the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings:
“Include,” “includes,” or like terms means encompassing but not limited to, that is, inclusive and not exclusive.
The indefinite article “a” or “an” and its corresponding definite article “the” as used herein means at least one, or one or more, unless specified otherwise.
As used herein, contact refers to direct contact or indirect contact. Direct contact refers to contact in the absence of an intervening material and indirect contact refers to contact through one or more intervening materials. Elements in direct contact touch each other. Elements in indirect contact do not touch each other, but are otherwise joined to each other through one or more intervening elements. Elements in contact may be rigidly or non-rigidly joined. Contacting refers to placing two elements in direct or indirect contact. Elements in direct (indirect) contact may be said to directly (indirectly) contact each other.
As used herein, a transparent dielectric material is “transparent” to a wavelength of light if the internal transmission of light at the wavelength is greater than 80%. Preferably, the internal transmission is greater than 90%, or greater than 95%. As used herein, internal transmission refers to transmission exclusive of reflection losses. It is understood that internal transmission depends on the path length of light through the material (e.g. thickness of the material). As used herein, “transparent” is in reference to the dimensions of the transparent dielectric material subjected to a liquid-assisted micromachining process. It is recognized that as the thickness (optical path length) increases, internal transmission decreases. A dielectric material is transparent for purposes of the present disclosure if it has a thickness (optical path length) such that internal transmission is as defined herein.
As used herein, “working surface” refers to a surface of a transparent dielectric material in contact with a liquid in a liquid-assisted laser micromachining process.
Reference will now be made in detail to illustrative embodiments of the present description.
A method and apparatus for machining transparent dielectric materials is described. The method uses a laser to machine features in transparent dielectric materials. Features include holes, channels, grooves, chamfers, and slots. The features extend through the thickness of the transparent dielectric material or through a portion of the thickness (e.g. halfway) of the transparent dielectric material. The features have smooth interior surfaces. The features are formed by using a laser to remove portions of the transparent dielectric material. The preferred mechanism of removal includes laser ablation and material removal through acoustic shock generated by cavitating bubbles. To avoid heating and melting of the transparent dielectric material, linear absorption of the laser by the transparent dielectric material is minimized and ablation is instead effected by non-linear optical absorption. Non-linear optical absorption occurs in transparent materials when the intensity of the laser exceeds a threshold. The intensity of the laser can be controlled by adjusting the power of the laser and/or focusing the laser. Non-linear optical absorption is a multiphoton absorption process that has an absorption coefficient that increases with increasing intensity. The high intensity and tight focusing of the laser lead to strong non-linear absorption in a highly localized region of the material. The conditions can be controlled to provide absorbed energy that is sufficiently high to directly evaporate a portion of the transparent dielectric material without proceeding through a melting transition. Thermal effects during laser micromachining are further minimized when using pulsed lasers with pulse duration less than about 100 ps, or less than about 50 ps, or less than about 25 ps. Ablation through non-linear absorption provides a mechanism for removing material from the transparent dielectric material and enables patterning or formation of fine features in the transparent dielectric material.
As material is dry-ablated from the transparent dielectric material, debris forms and accumulates on the surface or within features. When forming holes, for example, debris accumulates within the hole. The debris is difficult to remove and can interfere with the ablation process by, for example, scattering the laser beam and preventing attainment of the localized intensity needed for non-linear absorption. To aid removal of debris, laser micromachining can be performed in the presence of a liquid. The working surface of the transparent dielectric substrate is placed in contact with a liquid and as ablation occurs, the liquid displaces debris from the working surface to prevent accumulation of debris and to provide holes and other features free of clogs. The liquid also removes heat from the working surface.
Water is currently the preferred liquid for liquid-assisted laser micromachining and high ablation rates have been reported for water-assisted laser micromachining processes. In the course of work supporting this disclosure, however, limitations associated with using water as a medium for liquid-assisted laser micromachining have been identified. When forming a hole in transparent dielectric materials in a water-assisted laser micromachining process, it has been discovered that although debris is removed, the interior surface of the hole is rough and non-uniform. Rough surfaces are undesirable because they interfere with insertion of objects into the hole and promote cracking or other damage of the surrounding material.
While not wishing to be bound by theory, it is believed that cavitation phenomena that occur in liquid-assisted laser micromachining lead to acoustic-shock forces at the working surface that cause damage or roughness of the working surface or surfaces of features formed in the working surface. In liquid-assisted laser micromachining, the focal point of the laser is positioned at the interface of the liquid with the working surface of the transparent dielectric material or in the liquid at a position near the interface of the liquid with the working surface of the transparent dielectric material (e.g. in the liquid within about 10 μm of the working surface). The intensity of the laser at and near the focal point needs to be sufficiently high to enable non-linear optical absorption in the transparent dielectric material. The necessary intensity leads to optical breakdown and formation of a plasma in the liquid. During the laser pulse, the plasma expands rapidly. When the laser pulse terminates, the plasma relaxes and cools. Relaxation of the plasma is accompanied by rapid release of energy to the liquid and formation of a cavitation bubble in the liquid. Due to pressure gradients associated with cavitation, the cavitation bubble migrates to the surface of the transparent dielectric material. The bubble is unstable and collapses. Upon collapse of the cavitation bubble, a shock wave develops and a high-speed liquid jet forms. The shock wave from the collapsing bubble causes increased roughness, while the liquid jet acts to quickly remove material.
When water is used as the liquid medium, the effects of cavitation forces on the working surface of a transparent dielectric material are strong due to high surface tension of water and the surface roughness of features formed in water-assisted laser micromachining is high because acoustic shock force increases with the surface tension of the liquid. The present description provides liquids for liquid-assisted laser micromachining that minimize cavitation forces and produce features in transparent dielectric materials that have smooth surfaces with low roughness.
Preferred liquids are liquids that contain a fluorinated compound. Liquids with fluorinated compounds are preferred because they exhibit low surface tension. A liquid containing a fluorinated compound is also referred to herein as a fluorinated liquid or a fluorine-containing liquid. In one embodiment, the fluorinated liquid includes two or more fluorinated compounds. In another embodiment, the fluorinated liquid includes a fluorinated compound and a non-fluorinated compound. In still another embodiment, the fluorinated liquid lacks a non-fluorinated compound.
The examples shown below illustrate that fluorinated liquids are excellent liquids to use when forming features with liquid-assisted laser micromachining. Features formed when using a fluorinated liquid as the liquid medium in liquid-assisted laser micromachining have smooth surfaces with low roughness. Fluorinated compounds include fluorinated alkanes, fluorinated alcohols, and fluorinated amines. Representative fluorinated compounds include the Fluorinert™ series of liquids (e.g. FC-70, FC-40, FC-770), fluorinated hexane, fluorinated octane, perfluorodecalin, and fluorinated trialkylamines,. The degree of fluorination of the fluorinated compound ranges from monofluorinated to fully fluorinated. The concentration of fluorine in the fluorinated compound is 30 wt % or greater, or 40 wt % or greater, or 50 wt % or greater, or 60 wt % or greater, or 70 wt % or greater, or in the range from 30 wt %-80 wt %, or in the range from 40 wt %-70 wt %, where wt % refers to percent by weight and is computed as the ratio of the mass of fluorine per formula unit of the fluorinated compound to the total mass per formula unit of the fluorinated compound. By way of example, Fluorinert™ FC-70 has the molecular formula C15F33N and a molecular weight 821 g/mol. The concentration of fluorine in Fluorinert™ FC-70 is 76.4 wt % ((33)(19 g/mol)/821 g/mol).
Fluorinated liquids with high boiling points are preferred. Due to the intensity of laser light in the liquid needed to induce optical breakdown and plasma formation through non-linear optical absorption, heating of the liquid occurs. If the liquid has a low boiling point, the heating can be sufficient to induce boiling. Boiling leads to formation of bubbles. If bubbles increase to dimensions above the feature size (e.g. diameter of a hole) and stick to the feature (e.g. opening of a hole), they can block access of the liquid to the feature and prevent the liquid from removing debris. The inability of liquid to access recessed portions of a feature also precludes wetting of interior surfaces by the liquid, thus preventing removal of heat from interior surfaces by the liquid. It is therefore desirable to minimize bubble formation by avoiding boiling. The boiling point of the fluorinated liquid is preferably 100° C. or 125° C. or greater, or 150° C. or greater, or 175° C. or greater, or 200° C. or greater, or in the range from 100° C.-225° C., or in the range from 125° C.-200° C.
Fluorinated liquids with low surface tension are preferred. It is believed that low surface tension reduces the magnitude of cavitation forces to facilitate formation of features with low surface roughness. The surface tension of the fluorinated liquid at 25° C. is preferably 70 dynes/cm or less, or 55 dynes/cm or less, or 40 dynes/cm or less, or 30 dynes/cm or less, or 20 dynes/cm or less, or in the range from 10 dynes/cm-70 dynes/cm, or in the range from 10 dynes/cm-50 dynes/cm, or in the range from 10 dynes/cm-30 dynes/cm.
The wavelength of the laser can be any wavelength at which the dielectric material is transparent. Typical laser wavelengths for common transparent dielectric materials are in the UV, visible, or infrared portions of the electromagnetic spectrum. Representative laser wavelengths include wavelengths in the range from 325 nm-1700 nm, or in the range from 400 nm-1500 nm, or in the range from 500 nm-1250 nm, or in the range from 700 nm-1100 nm.
Laser pulse durations over a range extending from the femtosecond (fs) regime to the picosecond (ps) regime to the nanosecond (ns) regime are used. Representative pulse durations include pulse durations in the range from 1 fs-100 ns, or in the range from 5 fs-10 ns, or in the range from 10 fs-1 ns, or in the range from 100 fs-100 ps, or in the range from 1 ps-10 ps. In some aspects, shorter laser pulses are preferable to longer laser pulses. While not wishing to be bound by theory, it is believed that surface roughness is higher when longer laser pulses are used because longer laser pulses have higher threshold pulse energies for ablation and lead to ablation of larger pieces of matter from the working surface than shorter laser pulses.
Transparent dielectric materials include glasses and crystals. Glasses include oxide glasses and non-oxide glasses. Preferred glasses are silica glasses, including alkali silica glasses and alkaline earth silica glasses. Glasses include glasses strengthened by ion exchange or thermal tempering. Crystals include oxide crystals, such as metal oxides, and non-oxide crystals.
Features include holes, slots, channels, grooves and chamfers. Multiple features can be formed in a transparent dielectric material through relative motion of the laser and the transparent dielectric material. The laser and/or transparent dielectric material can be mounted on an XYZ positioning stage and relative motion of the laser and transparent dielectric material can be controlled to form features of various types at selected positions of the transparent dielectric material.
A representative laser system for liquid-assisted laser micromachining is shown in
In the liquid-assisted laser micromachining technique, formation of a feature begins at the working surface 15 and continues toward the interior of the transparent dielectric material. In one embodiment, formation of a feature includes ablation of the transparent dielectric material. In the embodiment of
In one aspect, relative motion of the focal point of the laser and the transparent dielectric material forms a helical pathway for ablation. In
In the embodiment of
In an alternative embodiment, relative to the direction of beam propagation, the working surface is further from the laser source than the initial focal point and micromachining occurs by moving the focal point of the laser toward the working surface in the direction of beam propagation. Stated alternatively, micromachining occurs by moving the focal point of the laser in the liquid toward the working surface, then through the working surface, and ultimately away from the working surface into the interior of the transparent dielectric material to form a feature. In this embodiment, the working surface corresponds to the surface of incidence of the laser beam to the transparent dielectric material. This embodiment can be visualized with respect to the embodiment of
Feature shapes other than holes can be similarly fabricated by controlling the position of the focal point of the laser beam and defining a trajectory of relative motion of the laser beam and transparent dielectric material needed to form a pattern of two or more ablated regions having a shape consistent with a desired feature. Cross-sectional shapes of features include circular, elliptical, round, square, and rectangular. Features extend through the entire thickness of the transparent dielectric material or through a fraction of the thickness of the transparent dielectric material. Grooves, channels, recesses, chamfers, holes and slots having arbitrary cross-sectional shapes can be formed through proper control of the relative motion of the focal point of the laser beam and transparent dielectric material to directly remove matter by ablation or to form patterns of ablated regions that permit separation to provide features of a particular shape and size.
The results shown in
The reduction in roughness and enhanced uniformity in cross-sectional dimensions obtained by the present method is surprising and counterintuitive because water has a much higher thermal conductivity than the fluorinated liquids contemplated herein. The thermal conductivity of water at 20° C. is 0.6 W/m-K, while the thermal conductivity of Fluorinert™ FC-70 at 20° C. is 0.070 W/m-K. Other fluorinated liquids contemplated herein also have much lower thermal conductivity than water. One would accordingly expect more efficient heat transfer during ablation when using water as the liquid-assist medium relative to using Fluorinert™ FC-70 or other fluorinated liquid as the liquid-assist medium. Since thermal effects are known to deteriorate surface quality in laser micromachining processes, one would expect improved surface quality in materials processed by liquid-assisted laser micromachining using water as the liquid-assist medium relative to using Fluorinert™ FC-70 or other fluorinated liquid as the liquid-assist medium.
The RMS (root-mean-square) roughness of feature surfaces formed by liquid-assisted laser micromachining when using a fluorinated liquid as the liquid-assist medium is estimated to be ˜0.5 μm or less. When using water as the liquid-assist medium, the expected RMS roughness is ˜1 μm.
The decreased surface roughness obtained by using a fluorinated liquid as liquid-assist medium provides transparent dielectric materials having features with more uniform dimensions. When forming features having a cross-section with a linear dimension, the variability in the linear dimension attributable to surface roughness is 1.0 μm or less, or 0.8 μm or less, or 0.6 μm or less, or 0.4 μm or less, or in the range from 0.2 μm-1.0 μm, or in the range from 0.2 μm-0.8 μm, or in the range from 0.2 μm-0.6 μm, or in the range from 0.2 μm-0.5 μm. As used herein, variability in linear dimension refers to the difference between the maximum and minimum value of the linear dimension. It should be noted that other process limitations (e.g. stability of the laser, precision of positioning of the laser) may contribute to variability in linear dimensions independent of the contribution from surface roughness. Linear dimensions include length, width, height, depth, and diameter.
When forming circular features (e.g. circular holes) or approximately circular features (e.g. rounded or elliptical holes), for example, the variability in diameter attributable to RMS surface roughness is 1.0 μm or less, or 0.8 μm, or less, or 0.6 μm or less, or 0.5 μm or less, or 0.4 μm or less, or in the range from 0.2 μm-1.0 μm, or in the range from 0.2 μm-0.8 μm, or in the range from 0.2 μm-0.6 μm, or in the range from 0.2 μm-0.5 μm. As used herein, variability in diameter refers to the difference between the maximum diameter and minimum diameter mean square deviation of a circular or approximately circular feature from an ideal circle.
In addition to forming features with low roughness and low variability in cross-sectional dimensions, the present method provides features with minimal damage and defects. In particular, chipping on the surface of the transparent dielectric material adjacent to the feature is minimized. Irrespective of the cross-sectional shape of the feature formed by the liquid-assisted laser micromachining methods described herein, chips adjacent the feature have a longest linear dimension less than 15 μm, or less than 10 μm, or less than 5.0 μm, or in the range from 1.0 μm-15 μm, or in the range from 3.0 μm-12 μm, or in the range from 5.0 μm-10 μm.
In one aspect, features having high aspect ratio are formed by liquid-assisted laser micromachining using a fluorinated liquid-assist medium. As used herein, aspect ratio of a feature refers to the ratio of a linear dimension of the feature normal to the incident surface to the smallest linear dimension of the feature orthogonal to the linear dimension of the feature normal to the incident surface. For a hole with a circular cross-section, the aspect ratio corresponds to the ratio of the depth of the hole (a dimension normal to the incident surface) to the diameter of the hole (a dimension orthogonal to the depth of the hole). For a hole with a square cross-section, the aspect ratio corresponds to the ratio of the depth of the hole (a dimension normal to the incident surface) to the side length of the hole (a dimension orthogonal to the depth of the hole). For a hole with a rectangular cross-section, the aspect ratio corresponds to the ratio of the depth of the hole (a dimension normal to the incident surface) to the smaller of the side length or side width of the hole (a dimension orthogonal to the depth of the hole). For a hole with an elliptical cross-section, the aspect ratio corresponds to the ratio of the depth of the hole (a dimension normal to the incident surface) to the length of the minor axis of the hole (a dimension orthogonal to the depth of the hole).
In some embodiments, the aspect ratio is 2:1 or greater, or 4:1 or greater, or 6:1 or greater, or 8:1 or greater, or 10:1 or greater, or in the range from 2:1-20:1, or in the range from 3:1-15:1, or in the range from 4:1-10:1.
The present disclosure encompasses transparent dielectric materials having features with roughness, variability in linear dimension, and/or aspect ratio described herein. Products formed by liquid-assisted laser micromachining of transparent dielectric materials using a fluorinated liquid are within the scope of the present disclosure. In one aspect, the feature extends through a thickness of the transparent dielectric material and the product further includes an optical fiber inserted in the feature.
Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or description that the steps are to be limited to a specific order, it is no way intended that any particular order be inferred.
It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the illustrated embodiments. Since modifications, combinations, sub-combinations and variations of the disclosed embodiments that incorporate the spirit and substance of the illustrated embodiments may occur to persons skilled in the art, the description should be construed to include everything within the scope of the appended claims and their equivalents.
This application claims the benefit of priority to U.S. Provisional Application Ser. No. 62/622,265 filed on Jan. 26, 2018, the content of which is relied upon and incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62622265 | Jan 2018 | US |
