METHOD FOR STRUCTURING A TRANSPARENT SUBSTRATE WITH A LASER IN A BURST MODE

Abstract

Method for structuring a top surface of a transparent substrate, the substrate being transparent to visible light with a laser source able to emit a laser beam in a burst mode, the burst mode being characterized by a train of pulses comprising sets of laser pulses repeated over time; wherein each of the sets of laser pulses of the train of pulses comprises a first and second pulses with a time interval between them comprised between 5 ns to 50 ns, preferably between 10 ns to 40 ns, and more preferably about 25 ns; wherein each of the first and second pulses has an energy density on the top surface comprised between 0.5 nJ/um.sup.2 and 50 nJ/um.sup.2; and wherein the train of pulses is able to structure the top surface.

Claims

1. A method for structuring a top surface of a transparent substrate, said substrate being transparent to visible light, the method comprising the steps of: (a) providing said transparent substrate with said top surface; (b) providing a laser machining device comprising a laser source able to emit a laser beam in a burst mode, said burst mode being characterized by a train of pulses comprising sets of laser pulses repeated over time; (c) using said laser source for irradiating said transparent substrate from the top surface with a laser beam according to said burst mode; wherein said laser source is a femtosecond laser source; wherein each of said sets of laser pulses of said train of pulses comprises a first and second pulse with a time interval between them between 5 ns to 50 ns; wherein each of said first and second pulses has an energy density on said top surface between 0.5 nJ/um.sup.2 and 50 nJ/um.sup.2; and wherein said train of pulses is configured to structure said top surface.

2. The method according to claim 1 further comprising, before step (c), the additional step of: determining an ablation threshold of said transparent substrate being defined by an energy density per pulse; wherein said first and second pulses have an energy density on said top surface defined relative to said ablation threshold.

3. The method according to claim 2, wherein said transparent substrate has an ablation threshold being defined by an energy density per pulse and wherein said transparent substrate is structured with laser pulses having energy densities lower than the energy density ablation threshold such that said top surface of said transparent substrate is structured with domes.

4. The method according to claim 3, wherein said domes are formed with conservation of the material quantity.

5. The method according to claim 2, wherein said transparent substrate has an ablation threshold being defined by an energy density per pulse and wherein said transparent substrate is structured with laser pulses having energy densities higher than the energy density ablation threshold such that said top surface of said transparent substrate is structured with craters.

6. The method according to claim 5, wherein said craters are formed with ablation of said transparent substrate.

7. The method according to claim 1, wherein said first pulse has an energy density higher than that of said second pulse.

8. The method according to claim 1, wherein said transparent substrate is a glass substrate.

9. The method according to claim 1, wherein said transparent substrate is an alkali-aluminosilicate glass substrate.

10. The method according to claim 1, wherein said laser machining device comprises a deflection device for guiding said laser beam on said top surface of said transparent substrate.

11. The method according to claim 10, wherein said deflection device is a scanner.

12. The method according to claim 1, wherein said laser machining device comprises driving means for positioning said top surface of said transparent substrate relative to said laser beam.

13. The method according to claim 1, wherein said laser machining device comprises focusing means for focusing said laser beam in the vicinity of said top surface of said transparent substrate.

14. The method according to claim 13, wherein said laser beam is focused on said top surface of said transparent substrate.

15. The method according to claim 1, wherein said laser beam is focused below said top surface of said transparent substrate.

16. A structured transparent substrate obtained by a method according to claim 3 and comprising dome structures having: an essentially circular base with a perimeter comprised between 1 um and 10 um, a height comprised between 50 nm and 500 nm, a structure density comprised between 1000 and 20000 domes per mm.sup.2.

17. The structured transparent substrate according to claim 16, wherein it is structured with a laser beam energy density on the substrate surface comprised between 0.5 nJ/um.sup.2/pulse and 3.2 nJ/um.sup.2/pulse such that it is structured with domes.

18. A system for structuring transparent substrates with laser bursts, the system comprising: a transparent substrate having a top surface; a machining device comprising: a laser source, an optical system for obtaining a laser beam from said laser source focused in the vicinity of said transparent substrate top substrate, wherein said laser source is a femtosecond laser source configured for emitting two laser pulses in a burst mode, said burst mode defining a train of a first pulse and a second pulse with a time between said first and second pulse comprised between 5 ns to 50 ns, said first pulse and said second laser pulses having energy densities comprised between 0.5 nJ/um.sup.2 and 50 nJ/um.sup.2, and said train of first and second pulses allowing for structuring said transparent substrate top surface.

19. The system according to claim 18, wherein said machining device comprises additionally: deflecting means for deflecting said focused laser beam from a first spot to a second spot on said top surface.

20. The system according to claim 18, wherein said machining device comprises additionally: driving means for moving said transparent substrate relative to said deflecting means.

Description

DESCRIPTION OF THE DRAWINGS

[0087] The foregoing aspects and many of the attendant advantages of the claimed subject matter will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

[0088] FIG. 1 shows a schematic representation of two trains of pulses in an Energy versus Time diagram;

[0089] FIGS. 2a and 3a show the surface topology of a substrate area after structuring according to the first aspect of the disclosure with energy intensities lower than the ablation threshold of the substrate;

[0090] FIGS. 2b and 3b show the profile of each features structured according to the first aspect of the disclosure with energy intensities lower than the ablation threshold of the substrate obtained for FIG. 2a and FIG. 3a respectively;

[0091] FIG. 4 shows the surface topology of a large substrate area after structuring according to the first aspect of the disclosure with energy intensities lower than the ablation threshold of the substrate;

[0092] FIG. 5a shows the surface topology of a substrate area after structuring according to the first aspect of the disclosure with energy intensities higher than the ablation threshold of the substrate;

[0093] FIG. 5b shows the profile of each features structured according to the first aspect of the disclosure obtained for FIG. 5a;

[0094] FIG. 6 shows a matrix of craters with increasing numbers of pulses and pulse energies for determining the ablation threshold according to a D.sup.2 method;

[0095] FIGS. 7a and 7b show in FIG. 7a the displacement of the sample in the direction of the Y and Z axes for the implementation of the D-Scan method and in FIG. 7b the ablation lobes resulting from the implementation of the experimental protocol of FIG. 7a; and

[0096] FIGS. 8a, 8b, 8c, 8d, 8e, and 8f show several embodiments of pulse or pulses within the meaning of the present disclosure.

[0097] The drawings in the figures are not necessarily to scale. Generally, similar elements are designated by similar reference signs in the figures. The presence of reference numbers in the drawings is not to be considered limiting, even when such numbers are also included in the claims.

DETAILED DESCRIPTION

[0098] The detailed description set forth below in connection with the appended drawings, where like numerals reference like elements, is intended as a description of various embodiments of the disclosed subject matter and is not intended to represent the only embodiments. Each embodiment described in this disclosure is provided merely as an example or illustration and should not be construed as preferred or advantageous over other embodiments. The illustrative examples provided herein are not intended to be exhaustive or to limit the claimed subject matter to the precise forms disclosed.

[0099] FIG. 1 shows an exemplary embodiment of the laser in burst mode. FIG. 1 shows a diagram with a first pulse 1 having a maximum energy E.sub.peak, a rise in energy starting from no energy to E.sub.peak, and a decrease in energy starting from E.sub.peak to no energy. A second pulse 2 is shown having a maximum energy E.sub.peak, a rise in energy starting from no energy to E.sub.peak, and a decrease in energy starting from E.sub.peak to no energy. The time between the first pulse 1 and the second pulse 2 is shown by the interval 3. The interval 3 can be defined from the rise from no energy to a pulse energy as shown on the right-hand side of FIG. 1. The interval 3 can be defined from the decay from a pulse energy to no energy as shown on the left-hand side of FIG. 1. The point showing a rise of the pulse energy from no energy or the point showing a decay of energy to no energy might be defined regarding a defined level of energy such as an energy baseline. The train 5 of the first 1 and second 2 pulses can be reproduced after a time interval 4. For example, the time interval 4 corresponds to the time necessary to move a deflection element from a first position to a second position for structuring another position on a substrate. For example, the time interval 4 corresponds to the time for moving the substrate relative to the laser machining device. FIG. 1 typically shows a scan for testing the effect of the burst mode at various energies, for example to performed on a glass substrate.

[0100] For example, an energy density on the substrate surface can be determined as follows for a laser beam diameter on the substrate surface of 17 um (beam area of a pulse diameter of 17 um227 um.sup.2) and an energy per pulse deposited on a substrate being comprised between 0.2 J and 3.0 J:

TABLE-US-00001 energy per pulse energy density per pulse 0.2 uJ 0.9 nJ/um.sup.2/pulse 0.4 uJ 1.8 nJ/um.sup.2/pulse 0.6 uJ 2.6 nJ/um.sup.2/pulse 0.8 uJ 3.5 nJ/um.sup.2/pulse 1.0 uJ 4.4 nJ/um.sup.2/pulse 1.2 uJ 5.3 nJ/um.sup.2/pulse 1.5 uJ 6.6 nJ/um.sup.2/pulse 2.0 uJ 8.8 nJ/um.sup.2/pulse 2.5 uJ 11.0 nJ/um.sup.2/pulse 3.0 uJ 13.2 nJ/um.sup.2/pulse

[0101] For example, the energy density per pulse is applied with a laser emitting around 1030 nm.

[0102] FIG. 2a shows a topology image of a substrate area of about 18 um per 18 um where four structuring were performed by applying for each structure a train of two pulses as shown in FIG. 1. Each train of two pulses was applied with the same experimental conditions. Each dome was a series of only one set of pulses, the set of pulses having an energy of 1.21 uJ, the set of pulse comprising the first and the second pulse 17 um spot. FIGS. 2a and 2b were measured with a white light interferometer.

[0103] On FIG. 2a, the height of the transparent substrate is shown by the homogenous color or greyscale and the structures are shown by the four circular shapes. These structures are domes for which the height regarding the surface of the substrate is shown on the scale on the right-hand side of FIG. 2a. The center of each structure corresponds to a height of about 160 nm to 180 nm. It can be seen that the structuring results obtained are very reproducible, both regarding the shape of the structures and the height at the center of the structures. FIG. 2b shows four height profiles of the four structures shown on FIG. 2a. The results shown on FIG. 2b corroborate the good reproducibility of the structures obtained with structuring the substrate with the method of the disclosure. The profiles of each four structures show profiles having similar heights and widths. On FIG. 2b, maximal heights are comprised between 0.11 um and 0.15 um and the width of the structures at their bases is of about 3.5 um.

[0104] FIG. 3a shows a topology image of a substrate area of about 18 um per 18 um where four structures were fabricated by applying for each structure a train of two pulses as shown in FIG. 1. Each train of two pulses was applied with the same experimental conditions. Each train of two pulses having an energy of 1.21 uJ, the set of pulse comprising the first and the second pulse 17 um spot. FIG. 3a was measured with a white light interferometer.

[0105] FIGS. 3a and 3b show similar results than already described for FIGS. 2a and 2b but obtained with different structuring parameters. The domes on FIG. 3a show height of about 70 nm to 90 nm. The structures on FIG. 3a are very similar to each other. Their corresponding profiles shown on FIG. 3b gives maximal heights that are comprised between 0.065 um and 0.080 um and the width of the structures at their bases is of about 2.5 um.

[0106] FIGS. 2a, 2b, 3a and 3b show that with the structuring method of the disclosure and more particularly when pulse energy densities are below the ablation threshold of the substrate, a good control of the formed structures is obtained. The structuring method parameter allows control of the height and width as shown in FIGS. 2a, 2b, 3a and 3b with a very good reproducibility of the formed structures.

[0107] FIG. 4 shows another example of a surface structured by the method of the disclosure. FIG. 4 shows a substrate surface area of about 0.05 mm0.15 mm. The domes on FIG. 4 show maximum height for most of the structures of about 400 nm to 500 nm.

[0108] FIG. 5a shows a topology image of a substrate area of about 30 um per 40 um where four structures were fabricated by applying for each structure a train of two pulses as shown in FIG. 1. Each train of two pulses was applied with the same experimental conditions. Each train of two pulses having an energy of 1.55 uJ, the set of pulse comprising the first and the second pulse 17 um spot. FIG. 5a was measured with a white light interferometer.

[0109] Characterization of the structured substrate is preferably carried out with white light interferometer, confocal microscope, or profilometer.

[0110] Determining the Ablation Threshold by the D.sup.2 Method.

[0111] The determination of the ablation threshold by the D.sup.2 method is the most accepted and the most used to determine the ablation threshold of all types of materials. This method is used for Gaussian beam energy profiles. This method has the advantage of offering the possibility of estimating the radius of the Gaussian beam w on the surface of the material and the incubation coefficient.

[0112] Fluence (or energy density) in a plane perpendicular to the axis of the laser beam is given by:

[00001]$\begin{array}{cc}F\left(r\right)=F_0.Math.\exp \left(\frac{-2.Math.r^2}{{}^2}\right),& \left(1\right)\end{array}$

[0113] where r is the distance to the axis of the beam, .sup.2 the radius of the beam at the surface of the material at e.sup.2 of the maximum intensity. If the beam is focused on the surface, =.sub.0. F.sub.0 is the maximum fluence (at r=0) and is calculated and given by:

[00002]$\begin{array}{cc}{F}_0=\frac{2.Math.E_P}{.Math.{}^2}.& \left(2\right).\end{array}$

[0114] The diameter of an ablation crater produced with one or more laser pulses can be described according to the maximum fluence:

[00003]$\begin{array}{cc}{D}^2=2.Math.{}^2.Math.\ln \left(\frac{F_0}{F_{t.Math.h}}\right),& \left(3\right)\end{array}$

[0115] where F.sub.th is the ablation threshold. Since F.sub.0 increases linearly with E.sub.P, the radius of the beam can be estimated from the graph of crater diameters D.sup.2 as a function of the logarithm of the pulse energy, using equation 3. The radius w can be calculated from the slope of a curve defined by the experimental results:

[00004]$\begin{array}{cc}=\sqrt{\frac{\mathrm{slope}}{2}}.& \left(4\right)\end{array}$

[0116] The ablation threshold can be calculated by extrapolating the crater diameter at power 2 to a crater diameter equal to zero in a graph representing crater diameter at power 2 (D.sup.2) as a function of the logarithm of the energy per pulse.

[0117] Generally, the ablation threshold decreases by increasing the number of incident pulses for the same point on the surface of the material to be machined. This behavior is attributed to the increase in radiation absorptivity caused by the accumulation of defects created at each impulse, even at fluences lower than the threshold fluence. Such an increase in radiation absorptivity can be characterized by a factor or incubation coefficient, expressed by the following equations (5) and (6):

F.sub.th(N)=F.sub.th(1)N.sup.S1 (5)

[0118] When F.sub.th(N) is the fluence of the ablation threshold for N pulses, F.sub.th(1) is the fluence of the ablation threshold for a single pulse and S is the incubation coefficient. S=1 means that there is no incubation, i.e. the ablation threshold is independent of the number of pulses. For most materials S<1, the ablation threshold decreases with the increase in the number of pulses.

[0119] In general, the incubation or decrease of the threshold fluence is more pronounced during the first 100 pulses for a large majority of materials and in particular for the materials most commonly used.

[0120] To apply the D.sup.2 method to find the ablation threshold, the incubation coefficient and, if necessary, the beam radius, we need to produce a crater matrix with increasing numbers of pulses and pulse energies, as illustrated in FIG. 6. To have sufficient experimental points, it is necessary to have at least 20 energy values, and several pulse number values less than 100 pulses. The diameter of each crater must be measured from images obtained with an optical or electronic microscope.

[0121] Determination of the Ablation Threshold by the Diagonal Scan Method.

[0122] The Diagonal-Scan or D-Scan method is a geometric method for determining the value of the ablation threshold. The D-Scan method is an alternative method to the D.sup.2 method, it has the advantage of being more efficient and faster in terms of time of experiments and numbers of measurements to be made. The disadvantage of this method with respect to the D.sup.2 is related to the method for determining the beam radius, a value that is always important for measurements and calculation of the parameters.

[0123] The Diagonal-Scan method consists in moving the sample diagonally with respect to the focal point of the beam using different beam energies and a variable number of pulses, while changing the speed of displacement (FIGS. 7a and 7b). The experiments making it possible to implement the D-Scan method provide for the determination of the ablated surface. The ablated surface corresponds to the points for which the power of the radiation has exceeded a critical value defined by:

P.sub.crit=1/2e.sub.0.sup.2I.sub.th, (7)

[0124] where I.sub.th is the ablation threshold in terms of intensity, given by:

[00005]$\begin{array}{cc}{I}_{\mathrm{th}}=\frac{P_0}{e.Math..Math.{}_{\max }^2},& \left(8\right)\end{array}$

[0125] where P.sub.0 is the maximum pulse power and .sub.max is the maximum width of the ablated region (FIGS. 7a and 7b). The ablation threshold is given by:

F.sub.th=I.sub.th.sub.p, (10)

[0126] where .sub.p is the pulse duration.

[0127] To apply the D-Scan method to find the ablation threshold and the incubation coefficient, it is necessary to produce scans as shown in FIGS. 7a and 7b, with different values of energy and speed of movement to have measurements for different numbers of superimposed pulses. The number of pulses superimposed on the points corresponding to the larger part of the lobes is given by:

[00006]$\begin{array}{cc}N=\sqrt{}.Math.\frac{f{}_{\max }}{v_y},& \left(11\right)\end{array}$

[0128] where f is the pulse frequency and v.sub.y is the speed in the direction of the y axis.

[0129] FIG. 6 shows a crater matrix to be machined in order to be able to implement the determination of the ablation threshold by the D.sup.2 method. FIG. 6 depicts a preferred embodiment for producing a crater matrix with the laser machining machine of the disclosure. The matrix shown in FIG. 6 shows a number of pulses for each of the columns between 10 and 110 and beam energies of between 10% and 100% of a maximum beam value that can be provided by the laser machining machine. of the disclosure. The realization of such a matrix of craters by respecting the beam energies and the numbers of pulses indicated followed by the topological and or dimensional characterization of the craters of this matrix thus makes it possible to define an ablation threshold, an incubation coefficient or a beam radius. Preferably the production of the crater matrix should be performed with increasing numbers of pulses and pulse energies, as illustrated by the arrows in FIG. 6. To have sufficient experimental points, it is necessary to have at least twenty values. The diameter of each crater can be measured from images obtained with an optical or electronic microscope or a profilometer.

[0130] FIG. 7a describes the relative displacement of the sample in the direction of the Y and Z axes for the implementation of the D-Scan method and in FIG. 7b the ablation lobes resulting from the implementation of the experimental protocol of FIG. 7a.

[0131] FIGS. 8a to 8f shows a pulse or pulses which should be understood within the meaning of the present disclosure. FIGS. 8a, 8b and 8c show a single pulse which has a large dip in energy in its center and showing two maxima around the dip. FIG. 8a shows two maxima having a same maximum energy density. FIG. 8b shows two maxima, a first maximum 11 having an energy density lower than a second maximum 22. FIG. 8c shows two maxima, a first maximum 11 having an energy density higher than a second maximum 22. FIG. 8d shows pulses not being a Gaussian but having a flat top. FIG. 8e shows a single pulse having a very jagged structure over its duration. FIG. 8f shows two pulses with two maxima, a first pulse 1 having a maximum energy density lower than a second pulse 2. Two pulses with two maxima could also have a first pulse 1 having a maximum energy density higher than a second pulse 2.

[0132] According to a second aspect, the inventors suggest a system for structuring transparent substrate. The system comprises optical modules for shaping the laser beam; a beam expander and preferably an attenuator.

[0133] The present disclosure has been described with reference to specific embodiments, the purpose of which is purely illustrative, and they are not to be considered limiting in any way. In general, the present disclosure is not limited to the examples illustrated and/or described in the preceding text. Use of the verbs comprise, include, consist of, or any other variation thereof, including the conjugated forms thereof, shall not be construed in any way to exclude the presence of elements other than those stated. Use of the indefinite article, a or an, or the definite article the to introduce an element does not preclude the presence of a plurality of such elements. The reference numbers cited in the claims are not limiting of the scope thereof.

[0134] In summary, the disclosure may also be described as follows.

[0135] Method for structuring a top surface of a transparent substrate, the substrate being transparent to visible light with a laser source able to emit a laser beam in a burst mode, the burst mode being characterized by a train of pulses comprising sets 5 of laser pulses repeated over time;

[0136] wherein

[0137] each of the sets 5 of laser pulses of the train of pulses comprises a first 1 and second 2 pulses with a time interval 3 between them comprised between 5 ns to 50 ns, preferably between 10 ns to 40 ns, and more preferably about 25 ns; wherein

[0138] each of the first 1 and second 2 pulses has an energy density on the top surface comprised between 0.5 nJ/um.sup.2 and 50 nJ/um.sup.2; and wherein

[0139] the train of pulses 5 is able to structure the top surface.

[0140] The principles, representative embodiments, and modes of operation of the present disclosure have been described in the foregoing description. However, aspects of the present disclosure which are intended to be protected are not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. It will be appreciated that variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present disclosure. Accordingly, it is expressly intended that all such variations, changes, and equivalents fall within the spirit and scope of the present disclosure, as claimed.