Ablative production device and method for a periodic line structure on a workpiece

Abstract

The invention relates to an ablative production device and method for a periodic line structure on a workpiece. The device comprises a pulsed laser for generating ablative light, a phase mask arranged in the beam path of the ablative light, imaging optics arranged on an optical axis and a holder to hold the workpiece in an image plane. The phase mask produces a plurality of equidistant parallel lines in an object plane by interference and suppresses an order of diffraction parallel to the optical axis. The optical axis is perpendicular to the object plane. The imaging optics comprises a cylindrical lens, which is aligned in parallel to the lines and is designed to image the object plane into the image plane.

Claims

1. A device for the ablative production of a periodic line structure on a workpiece, comprising: a pulsed laser for generating an ablative light; a phase mask, which is arranged in the beam path of the ablative light and is formed to produce a plurality of equidistant parallel lines in an object plane by interference and to suppress an order of diffraction parallel to an optical axis, wherein the optical axis is perpendicular to the object plane; an imaging optics arranged on the optical axis with a cylindrical lens, which is aligned parallel to the lines and is formed to image the object plane into an image plane; and a holder, which is formed to arrange the workpiece in the image plane.

2. The device according to claim 1, wherein the non-suppressed orders of diffraction enclose discrete angles with the optical axis and the cylindrical lens has no acylindrical correction.

3. The device according to claim 1, wherein the non-suppressed orders of diffraction and the imaging optics lie symmetrically to the optical axis.

4. The device according to claim 1, wherein the phase mask is formed to produce the plurality of equidistant parallel lines in the object plane over a first width, and wherein the imaging optics is arranged on the optical axis, so that non-suppressed orders of diffraction in the imaging optics are spaced from the optical axis over a second width corresponding to the first width.

5. The device according to claim 1, wherein the workpiece is arranged movably in the image plane perpendicular to the lines.

6. The device according to claim 1, wherein the holder is formed to move the workpiece parallel to the image plane and perpendicular to the imaged equidistant lines at a steady feed rate.

7. The device according to claim 6, wherein a repetition rate, r, of the pulsed laser and the feed rate, v, of the holder are synchronized, so that
v=r.Math.b.Math.n for an integer n, wherein b is a periodicity of the imaged equidistant lines.

8. The device according to claim 6, wherein a feed between two consecutive pulses is an integer fraction of a width on an image side.

9. The device according to claim 1, wherein the imaging optics reduces parallel to the equidistant parallel lines.

10. The device according to claim 1, further comprising: an amplitude mask arranged between the imaging optics and the image plane.

11. A method for the ablative production of a periodic line structure on a workpiece, comprising: generation of an ablative light by means of a pulsed laser; arrangement of a phase mask in the beam path of the ablative light to produce a plurality of equidistant parallel lines in an object plane and to suppress an order of diffraction parallel to an optical axis by interference, wherein an optical axis is perpendicular to the object plane; imaging of the object plane into an image plane by means of a cylindrical lens, which is arranged on the optical axis and aligned parallel to the lines; and arrangement of the workpiece in the image plane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features of the technique are described below on the basis of exemplary embodiments with reference to the enclosed drawings, wherein:

(2) FIG. 1 shows a schematic block diagram of a device for producing a periodic line structure on a workpiece;

(3) FIG. 2 shows schematically a distribution of the output of the light emanating from a phase mask in respect of angle and distance to the optical axis;

(4) FIG. 3 shows schematically an exemplary embodiment of the device in FIG. 1;

(5) FIG. 4 shows a first configuration of a phase mask for the device in FIGS. 1 and 3 or the distribution in FIG. 2; and

(6) FIG. 5 shows a second configuration of a phase mask for the device in FIGS. 1 and 3 or the distribution in FIG. 2.

DETAILED DESCRIPTION

(7) FIG. 1 shows a device generally designated by the reference sign 100 for the production of a periodic line structure. The device 100 comprises a pulsed laser 109 for generating ablative light 110, a phase mask 102 arranged in the beam path of the ablative light 110 with an object plane 103, an imaging optics 104 arranged on an optical axis 101, and a holder 106, in order to arrange a workpiece 108 in an image plane 107 of the imaging optics 104.

(8) The phase mask 102 produces a first line structure in the object plane 103 by interference. The object plane 103 is perpendicular to the optical axis 101. The object plane is in the near field of the phase mask 102 on the side of the phase mask 102 facing away from the laser 109.

(9) The object plane 103 can be spaced a Talbot length or half a Talbot length (or an integer multiple of the half or whole Talbot length) from the plane of the phase mask.

(10) In the far field of the phase mask 102 the interference suppresses an order of diffraction parallel to the optical axis 101. Emanating from the phase mask 102 (at least substantially) are two beams 116 and 118. The beams 116 and 118 run symmetrically to the optical axis 101.

(11) The imaging optics 104 images the object plane 103 into the image plane 107 by means of the positive refractive power of a cylindrical lens. The imaging optics 104 optionally comprises other optical elements, for example each with positive refractive power. By using a doublet with two cylindrical lenses, with equal combined refractive power, imaging errors can be reduced in comparison with refractive power corresponding to a single cylindrical lens. The doublet can be asymmetrical. The cylindrical lenses can each be aligned with a convex side to the phase mask 102.

(12) The object plane 103, the imaging optics 104 and the image plane 107 are arranged in relation to one another so that the first line structure is imaged to a reduced second line structure. The second line structure is created in the image plane 107 by interference of the two beams 116 and 118.

(13) The ablative light can be monochromatic. The ablative light can comprise ultraviolet light. The pulsed laser 109 can generate ultraviolet light. The pulsed laser 109 can be an excimer laser, for example an argon fluoride laser. A wavelength of the light can be in the range from 126 nm to 351 nm. The wavelength of the light can be approximately 193 nm.

(14) The first line structure and the second line structure each contain equidistant parallel lines, i.e. maxima of the intensity of the light. The lines are perpendicular to the drawing sheet in FIG. 1 and define a longitudinal direction. The longitudinal direction and the optical axis 101 span a plane of symmetry. With reference to the plane of symmetry, an angle and a distance s are defined for each of the beams 116 and 118, e.g. in a plane perpendicular to the optical axis in the case of imaging optics 104. Angle and/or distance s can have opposed signs on opposite sides of the plane of symmetry.

(15) The imaging optics 104 can be formed to receive a coherence and/or a relative phase position of the two beams 116 and 118. An optical path length of the imaging optics 104 can (for example, due to uncorrected optical elements, e.g. on account of the cylindrical lens) be a function of the angle of the beams 116 and 118 falling on the imaging optics.

(16) The phase mask 102 can distribute the ablative light (or at least a majority of this) at discrete angles, e.g. at two defined angles corresponding to the two beams 116 and 118. The imaging optics 104 can (e.g. on account of the illumination through the phase mask and/or on account of one or more apertures) only be used for the discrete angles. An influence of the angular dependence of the optical path length that is disadvantageous for sharpness and/or contrast can be avoided by this. A correction of the imaging optics, e.g. in respect of the angular dependence, can be omitted.

(17) The two beams 116 and 118 can be symmetrical to the plane of symmetry. The two angles of the beams 116 and 118 can have the same value. The two beams 116 and 118 can thereby cover the same optical path length between phase mask 102 and workpiece 106 for a high-contrast interference.

(18) FIG. 2 shows schematically an output distribution 200 of the ablative light in a plane perpendicular to the optical axis 101 before or in the imaging optics 104. The distribution 200 of the output of the light is represented schematically with reference to the angle to the plane of symmetry and the distance s to the plane of symmetry. The distribution 200 shows a discretised distribution of the output with reference to the angle to the plane of symmetry.

(19) An order of diffraction defines an acute angle >0 for the output 202 of the first beam 116. Another order of diffraction defines an acute angle <0 for the output 204 of the first beam 118. No paraxial light 206 gets into the imaging optics 104 (at least relative to the output of the off-axis light 202 and 204).

(20) The cylindrical lens can (e.g. in the plane of symmetry) have a circular cross section. The cylindrical lens is not necessarily corrected (e.g. with reference to geometric imaging errors). Due to the discrete angular distribution 200, a corrective acylinder can be dispensed with in the imaging optics 104.

(21) At least in exemplary embodiments, an optical path length of the imaging optics can (at least substantially) be a function only of the angle of the beams falling on the imaging optics relative to the optical axis or to the plane of symmetry. The optical path length of the imaging optics can be (at least substantially) independent of the distance s to the optical axis 101. Light of the phase mask can thereby be imaged over a width X, which overlaps with the optical axis 101.

(22) Alternatively or in addition, the width X can be small, for example the width X can be small in relation to the object width G. Alternatively or in addition, the beams 116 and 118 cannot overlap in the imaging optics 104, so that the width X is small in relation to the distance s to the plane of symmetry, as shown schematically for the output distribution 200. The demand on the imaging optics 104 can be reduced further by this. For example, lighter or cheaper cylindrical lenses can be used in the imaging optics 104.

(23) FIG. 3 shows a schematic sectional view of an exemplary embodiment of the device 100. The plane of the sectional view shown in FIG. 3 is parallel to the optical axis 101 and perpendicular to the imaged equidistant lines 113.

(24) In the exemplary embodiment of the device 100 shown in FIG. 3, the phase mask 102 produces the first line structure 112 in the object plane 103. Regardless of whether the first line structure 112 is present at all, the beams 116 and 118 are formed to create the ablative line structure by superimposition in the image plane 107.

(25) FIG. 3 is schematically for the benefit of clarity. For example, the beam progression can have an intermediate focus (not shown in FIG. 3), e.g. between an imaging plane 105 and the image plane 107. Alternatively, the intermediate focus with convergent beams 116 and 118 can be between the object plane 103 and the imaging plane 105.

(26) In the exemplary embodiment shown in FIG. 3, the two beams 116 and 118 are each (at least approximately) collimated beams, for example in that the beams 116 and 118 correspond to defined orders of diffraction, which are emitted by the phase mask at a defined diffraction angle to the optical axis. The width X of the beams 116 and 118 at reference sign 114-2 can substantially correspond to the width X illuminated by the laser 109 at reference sign 114-1.

(27) With a small divergence of the beams 116 and 118, the width X at reference sign 114-2 can be greater than the illuminated width X at reference sign 114-1. For example, in the imaging plane 105 (or directly in front of the imaging optics 104) the width X can be a maximum of 10% greater than the width X in the object plane 103 (or directly behind the phase mask 102).

(28) The beams 116 and 118 enclose with the optical axis 101 on the input side of the imaging optics 104 (at least approximately) a defined angle + or . The imaging of the beams 116 and 118 by means of the imaging optics 104 utilises only one (at least approximately) discrete angular range. In addition, with a progression of the beams 116 and 118 symmetrical to the optical axis and an arrangement of the imaging optics 104 symmetrical to the optical axis 101, the imaged angles are (at least approximately) of equal value.

(29) Imaging with only a single angular value can improve the interference of the imaged beams and/or reduce demands on the imaging optics 104. For example, a particularly sharp and/or high-contrast line structure can be made possible, although with a single cylindrical lens a distortion of a wavefront of the beams 116 and 118 would be expected.

(30) The line structure can have a rectangular profile in the image plane 107 trans-verse to the equidistant lines. A high sharpness of the imaging can correspond to steep flanks of the rectangular profile. Alternatively, the line structure can have a sinusoidal intensity distribution in the image plane 107 transverse to the equidistant lines.

(31) With a high contrast, intensity minima of the line structure can be substantially radiation-free, so that in the valleys of the intensity distribution the workpiece 108 remains unprocessed.

(32) FIG. 3 shows an exemplary embodiment with beams 116 and 118 separated in the imaging optics. A distance W of the beams 116 and 118 from the plane of symmetry can be great in relation to the width X of the beams 116 and 118.

(33) FIGS. 4 and 5 show a section 300 of the device 100 for exemplary embodiments of the phase mask 102. The imaging optics 104 comprises a cylindrical lens aligned parallel to the lines 112. A radius of the cylindrical lens is large in relation to the distance W of the beams 116 and 118 from the plane of symmetry in the imaging plane 105. The cylindrical lens 104 is not necessarily acylindrically corrected.

(34) FIG. 4 shows a first configuration of the phase mask 102, in which the beams 116 and 118 correspond to the first two orders of diffraction +1 and 1. The first configuration can facilitate a symmetrical distribution of the output of the ablative light 110 to the two beams 116 and 118.

(35) FIG. 5 shows a second configuration of the phase mask 102, in which the beam 116 corresponds to the zeroed order of diffraction and the beam 118 to a first order of diffraction. The second configuration can facilitate a complete suppression of paraxial light without dissipative apertures.

(36) The cylindrical lens 104 permits extended processing of the workpiece 108 in the longitudinal direction. A processing region is further expanded by moving the workpiece by means of the holder 106. The holder 106 moves the workpiece 108 continuously in the image plane 107 with uninterrupted pulsed operation of the laser 109. Alternatively or in addition, the holder rotates the workpiece, for example to process a curved surface. The rotation takes place about the instantaneous normal intersection point of the current processing region.

(37) For example, the workpiece is moved continuously in the transverse direction, so that with each pulse of the laser 109 in an overlapping region of processing with a region processed by the preceding pulse, the parallel lines imaged are congruent. Since the product of pulse duration and feed rate is small in comparison with the width of the imaged lines, time-consuming start-up and deceleration processes can be avoided.

(38) By using the phase mask 104, a fluence of the laser 109 can be utilised almost com-pletely for processing the workpiece 108.

(39) The workpiece 108 can comprise glass. The technique can be used for identification or surface processing of hobs with glass ceramic, spectacle lenses or primary packagings.

(40) In each of the exemplary embodiments an amplitude mask can be arranged between workpiece 108 and imaging optics 104, e.g. in the image plane 107. The amplitude mask can facilitate a representation composed of the line structure. The representation can be a graphic, a logo, a pictogram or a machine-readable code. The machine-readable can be one-dimensionally structured (e.g. as a barcode) or can be two-dimensionally structured (e.g. as a QR code).