Laser Based Machining

Abstract

A direct write laser based machining process wherein a laser beam is controlled to machine a glass material in an interlaced raster scan pattern. An embodiment of machining a glass substrate to form an optical element is described. An ultrashort pulsed laser is used for machining and smoothing fused silica, followed by CO.sub.2 laser polishing. High speed and high quality machining is possible using this approach, which allows efficient use of high laser repetition rates.

Claims

1. A direct write laser based machining process wherein a laser beam is controlled to machine a glass material in an interlaced raster scan pattern.

2. A direct write laser based machining process according to claim 1 wherein the interlaced raster scan pattern is a unidirectional interlaced raster scan pattern wherein, in order, first, second and third scan lines are provided and the third scan line is located between the first and second scan lines.

3. A direct write laser based machining process according to claim 1 wherein the interlaced raster scan pattern is a bidirectional interlaced raster scan pattern wherein, in order, first, second and third scan lines are provided and the third scan line is located between the first and second scan lines.

4. A direct write laser based machining process according to claim 1 wherein the interlaced raster scan pattern is described by passes, i, formed by lines: i, i+n, i+2n, . . . i+kn, where i is the pass number and k and n are integers greater than 1.

5. A direct write laser based machining process according to claim 1 wherein the process includes calculating a desired contour surface from a design for the glass material.

6. A direct write laser based machining process according to claim 5 wherein the process includes: (a) providing a laser beam; (b) providing a computer controlled optical system to direct the laser beam; (c) providing a computer controlled X-Y translation stage; (d) locating the glass material on the translation stage; (e) operating the computer controlled optical system and the translation stage to ablate portions of the glass material in a pattern according to claim 5 and thereby machine the desired contour surface on the glass material.

7. A direct write laser based machining process according to claim 6 wherein the glass material is a glass substrate.

8. A direct write laser based machining process according to claim 7 wherein the glass substrate is selected from a group comprising: fused silica, borosilicate crown glass, titanate high index glass and flint glass.

9. A direct write laser based machining process according to claim 6 wherein the laser beam is provided by an ultrashort pulsed laser.

10. A direct write laser based machining process according to claim 5 wherein the process includes calculating a desired contour surface from an optical design for an optical element.

11. A direct write laser based machining process according to claim 6 wherein the process comprises a machining step and a smoothing step.

12. A direct write laser based machining process according to claim 6 wherein an interlace pitch is between 0.5 and 1.5 times a laser spot diameter.

13. A direct write laser based machining process according to claim 12 wherein the interlace pitch is equal to the laser spot size.

14. A direct write laser based machining process according to claim 11 wherein the process comprises a polishing step.

15. A direct write laser based machining process according to claim 14 wherein the polishing step is performed using a CO.sub.2 laser.

16. A direct write laser based machining process according to claim 1 wherein ablation depths greater than 300 m are machined in the material.

17. A direct write laser based machining process according to claim 16 wherein ablation depths greater than 500 m are machined in the material.

18. A direct write laser based machining process according to claim 17 wherein ablation depths as high as 1000 m are machined in the material.

Description

[0032] Embodiments of the present invention will now be described, by way of example only, with reference to:

[0033] FIG. 1 is a schematic illustration of an interlaced raster scan pattern according to an embodiment of the present invention;

[0034] FIG. 2 is an illustration of a laser based machining process for optical elements according to an embodiment of the present invention;

[0035] FIG. 3 is an illustration of a laser based machining system for optical elements according to an embodiment of the present invention;

[0036] FIGS. 4(a) and 4(b) show (a) ablation depth and (b) roughness against normalised pitch (as per spot diameter) for a substrate surface machined using the method of the present invention;

[0037] FIG. 5 shows ablation depth against fluence for a substrate surface machined using the method of the present invention and that of a standard raster scan; and

[0038] FIGS. 6(a) and 6(b) show (a) ablation depth and (b) roughness against fluence for a substrate surface machined using multiple passes with the method of the present invention.

[0039] Reference is initially made to FIG. 2 of the drawings which illustrates a process, generally indicated by reference numeral 10, for creating an optical element according to an embodiment of a present invention. While an optical element is described, the process may be applied to any glass material in order to achieve surface contouring and provide drilling, milling, texturing and engraving effects on the glass material. The process is particularly suited to materials with low thermal conductivity such as glass, plastic, ceramic and silicon.

[0040] In the first step 12, an optical design is created for the optical element and from this the contour shape of the optical element 16 is calculated. A laser program is generated. In known laser micromachining applications, pulsed lasers are used to remove a controlled amount of material with each pulse. In particular, by varying the location and energy of each delivered pulse, a net shape can be generated.

[0041] The laser program is input to a second step 34 which is an ultrashort pulsed laser based machining step. The second step 34 provides high speed machining of the optical element to create the desired shape. This machining is achieved using an interlaced machining technique wherein a raster scan is performed in which the scanned lines are not performed in sequence but are interlaced. This interlaced scan is as illustrated in FIG. 1.

[0042] A smoothing step is the third step 36 wherein the beam from the picosecond laser is defocussed and used to smooth out ridges created in the second step 34. A fourth step 38 is the final step, wherein a CO.sub.2 laser is used to perform polishing of the optical element. A shallow melt pool is generated, causing material to re flow and reduce the roughness of the optical element.

[0043] The laser program of the first step 12 is input to a laser micro-machining system 14. Reference is now made to FIG. 3 of the drawings which illustrates the components of a laser micro-machining system 14, for creating a micro-optical element 16. A laser 18 is arranged before a computer 28 controlled optical assembly 20. The assembly 20 directs the output laser beam 22 to a substrate 24 upon which the desired shape will be machined. The beam 22 is translated on the substrate 24 either by using scanning optics or by translating the stage 26 or a combination of both.

[0044] The substrate 24 is fused silica (typically a piece of flat, parallel-sided fused silica 1 mm thick for elements around 5 mm in diameter). The substrate 24 may be any glass but is preferably fused silica (Corning HPFS 7980), N-BK7 (Schott Borosilicate crown glass), STiH53 (Ohara Titanate high index glass) or NLaF21(Schott Lanthanum flint glass). The substrate 24 is mounted upon an XY translation stage 26, which is computer 28 controlled to move in steps of 100 nm in the two dimensions. A focussing lens 30 mounted on a computer controlled Z stage 32, focuses the beam 22 onto the substrate, a required depth to ablate the silica. For the first step 12, the laser 18 is a diode pumped picosecond laser that provides pulses of <10 ps, ideally 6 ps, at a maximum repetition rate of 400 kHz. The laser has an output at 1030 nm with an average power of 50 W. Second and harmonic outputs at 515 nm (average power 25 W) and 343 nm (average power 15 W) are also available for use. The computer 28 moves the stages 26,32 in a raster configuration so that controlled ablation, by pulse-by-pulse laser writing at a selected pitch, of the silica 24 is achieved to create the desired shape. Those skilled in the art will recognise that the optical system 20 could be based around a Galvo-scanner with the substrate 24 being held in position so that the raster is created by a pass formed via the Galvo-scanner. Where a CW laser, such as a wavelength stabilised CO.sub.2 laser, is used the optical system 20 may include an acousto-optic modulator to create pulses if desired. Such a laser micro-machining system is described in U.S. Pat. No. 9,089,927 to the present Applicant's and the contents of which are incorporated herein by reference. In the present invention, the polishing step 38 uses a CO.sub.2 laser in CW mode.

[0045] Typically the laser beam 18 spot size on the substrate 24 corresponds to a Gaussian beam waist such that the spot profile at the surface to be machined is circular Gaussian. The measured focal spot diameters (measured at 1/e.sup.2) are 35.5 m at 1030 nm (M.sup.2 <1.3), 20 m at 515 nm (M.sup.2 <1.4) and 21 m for 343 nm (M.sup.2 <2.1). The higher M.sup.2 at 343 nm is due to the degradation of the third harmonic generator crystal.

[0046] In the present invention the raster configuration is not the standard line by line pattern but an interlaced unidirectional raster scan as illustrated with the aid of FIG. 1. This process employs step and scan pitches that are identical to the prior art raster scan process. However, for the first pass across the machined area, lines 1, 1+n, 1+2n, etc are machined, where n is an integer. A second machining pass is then made, where lines 2, 2+n, 2+3n etc are machined. FIG. 1 demonstrates an example of an n=3 raster scan 40. Other numbers are available. In this way, each consecutive line is spaced apart and parallel to the preceding line, with the space being filled with further lines, the separation of which is uniform and a multiple of the line thickness i.e. laser spot size. The lines may be overlapped to reduce the ridges which will be present between neighbouring lines. The entire surface of the substrate 24 is machined to provide a complete interlaced raster scan 40. While three passes are described, it will be realised that a general interlaced raster scan pattern is described by passes, i, formed by lines: i, i+n, i+2n, . . . , i+kn, where i is the pass number and k and n are integers greater than 1. The scan may be unidirectional, i.e. each line is created in the same direction, say, left to right. Alternatively, the scan may be bidirectional, allowing lines to be machined in either direction, which can speed up the process.

[0047] The process uses a special form of raster scan wherein multiple passes, five in FIG. 1, are used to obtain the final step direction overlap. Unlike the prior art sequential scan wherein the subsequent scanned lines are in sequence, the scanning technique of the present invention provides the final surface by skipping a set number of lines and machining them in subsequent passes. FIG. 1 shows an example of a surface machined using 15 lines machined in the interlaced scan technique using a 3 y interlace for simplicity with y being the distance between the centre on adjacent lines in the step direction. In the sequential scan technique commonly used, these lines would be machined in sequence from 1 to 15 using one raster or pass. In the proposed technique however, the surface is machined by the first pass with a raster pitch of 3 y giving lines 1, 4, 7, 10 and 13. After this, the laser spot returns to a position y from the first line and performs the second pass with a raster pitch of 3 y giving lines 2, 5, 8, 11 and 14. After this, the laser spot returns to a position 2 y from the first line machined and performs the third pass with a raster pitch of 3y giving lines 3, 6, 9, 12 and 15. Although this example shows a 3 y interlaced scan for simplicity, this process can have the form of n y where n is an integer and n>1. The process will then be performed in n number of passes. Additionally in this example a unidirectional scan is used for simplicity, nevertheless, the process can be described for both unidirectional and bidirectional scan technique in both the step and scan direction.

[0048] By separating consecutive lines so that they are not touching overheating of the substrate 24 surface is avoided. Although ultrafast or ultrashort pulsed lasers reduce thermal effects, they are not completely eliminated. The contribution of thermal accumulation in ultrashort pulsed laser processing is well-documented, and can indeed be exploited to improve efficiency for processes like welding. However thermal accumulation is a particular issue when machining glass e.g. in comparison with metals, due to their low thermal conductivity. This can lead to the formation of filamentary strands of glass, caused by the rapid solidification of molten jets of glass being ejected from the surface. Use of a lower pulse overlap provides a solution to avoid these filaments however the surface produced is rough and the process is unpredictable at lower pulse energy. The present invention gives a solution by use of the interlaced scan to provide sufficient time for the dissipation of the heat energy allowing a high effective overlap to be used, thus providing relatively smooth and predictable machining results with an R.sub.a in the range of 0.6 m1.2 m.

[0049] The interlaced scan process takes advantage of the surface angle with respect to the beam. In the prior art sequential scan process, as the step pitch is small, the beam impinges on the wall of the previously machined surface. Hence, the fluence is reduced by the distorted beam and due to the angle of impinging, the debris is pushed in the direction opposite to the wall where the laser is incident on the material. In the interlaced scan technique of the present invention, due to the larger pitch used, the laser impinges on a fresh flat surface instead of the wall from the previous machining pass, hence has better absorption characteristics and the debris is better ejected away from the machined region. The interlaced scan technique also temporally separates out the machining of the region thereby reducing the thermal accumulation.

[0050] In the process, we define the laser spot as the individual spot made by each laser pulse with a laser spot diameter or size. The step pitch or interlace pitch is the distance between two subsequent scanned lines during laser machining and is the step distance determined by n. The scan pitch is the distance between two individual spots produced by subsequent laser pulses is the scan pitch. This can be set by adjusting the scan speed of the laser considering the pulse repetition rate used. The scan direction overlap is the scan pitch described in terms of overlap percentage considering the spot diameter.

[0051] In the second step 34, the interlaced machining can use parameters similar for those used in conventional raster scanning as illustrated in the following table:

TABLE-US-00001 Overlap Pitch (um) Scan speed (mm/sec) (%) 1030 nm 515 nm 343 nm 1030 nm 515 nm 343 nm 98.75 0.44375 0.25 0.25 8.875 5 5 97.5 0.8875 0.5 0.5 17.75 10 10 95 1.775 1 1 35.5 20 20 90 3.55 2 2 71 40 40

[0052] The key parameters of scan overlap and step overlap are chosen in order to provide a direct comparison. However, higher repetition rate may be used with the interlacing technique, whereas in this case it is limited to 20 kHz, to prevent the ejection of glass filaments and related debris for the prior art conventional raster scanning.

[0053] The process 10 uses an ultrashort pulsed laser (a picosecond laser at 6 ps pulses) for machining with the interlaced unidirectional raster scan method as the second step 34 to create the desired optical surface shape, whilst avoiding overheating of the workpiece surface. The as-machined region has a somewhat large roughness (R.sub.a typically 1.5 m), including clearly visible ridges.

[0054] The interlace technique was tested for different step sizes in comparison with the laser spot size to determine the optimum step size for maximum depth with minimum roughness. Results are plotted in FIGS. 4(a) Ablation depth (m) 42 and (b) Roughness (m) 44 as a function of normalized interlace pitch 46, for 1030 nm, 515 nm and 343 nm with two different overlaps (97.5% and 95%). In each case the repetition rate is maintained at 20 kHz to avoid filaments with low values of n. The results show maximum depths achievable at the longer 1030 nm wavelength with the greater overlap 97.5%. Step roughness is fairly constant to pitch size for all wavelengths. The results also show that the ablation depth is maximized when the interlace pitch is roughly equal to the spot size. The roughness is reasonably constant up to this value; for larger values of n, surface grooves are introduced, thereby significantly increasing the R.sub.a value. Hence the optimum value of interlace pitch is taken to be the laser spot size.

[0055] The results in the following section use a constant interlace scan with 101.8 m pitch. Though the machining results are optimum for interlace pitch equal to the spot diameter (201.8 m for 1030 nm), these parameter maps have been created at 101.8 m. To provide a direct comparison with the standard raster scan process, results are plotted in FIG. 5 for 1030 nm, using a spot size of 35 m, 98.75% overlap for a 20 kHz repetition rate. Here the ablation depth (m) 42 against fluence (J/cm.sup.2) 48 is plotted for a normal raster scan 50 and the interlaced raster scan 52. For the prior art raster scan 50, depths in the range 57 to 274 m were achievable. However, for the interlaced raster scan of the present invention increased depths in the range of 240 to 660 m were realised. This illustrates that greater ranges of ablation depths are available using the present invention. For the normal scan raster 50, a roughness of 0.7 to 1.6 m was recorded as compared to 1.1 to 1.5 m for the interlaced raster scan 52. The surface roughness increase for the interlaced raster scan 52 machining can be attributed to ridges formed due to low step direction overlap.

[0056] For glass materials in particular, the high scan direction overlap allows the machining process to be more predictable by lowering the ablation threshold fluence thereby avoiding uncertain machining outcomes. Depending on the material and end result, lower scan overlap can be used. Scan overlap is not applicable to CW lasers due to the continuous nature of the output.

[0057] Reference is now made to FIGS. 6(a) and 6(b) which show results for multiple pass machining using the interlaced process, in order to provide a route to the generation of deeper structures. These use 1030 nm, a spot size of 35 m, 98.75% overlap for a 20 kHz repetition rate. FIG. 6(a) plots the ablation depth (m) 42 against fluence (J/cm.sup.2) 48 and FIG. 6(b) plots step roughness R.sub.a (m) 44 against fluence (J/cm.sup.2) 48 for one to five passes. This shows that ablation depths towards 1000 m can be obtained while the surface roughness R.sub.a is around 0.8 m. It should be noted that the z-axis was not refocused in between scans, so the fluence reduces for subsequent passes, which reduces the removal efficiency somewhat, whilst having the positive effect of reducing the surface roughness. Hence considering the picosecond laser used, 1030 nm provides the best machining results. The interlacing scan strategy allows the use of high repetition rates while supressing the formation of filaments. Hence high repetition rates can be used along with multiple passes to get deeper structures.

[0058] The second step 34 has shown the use of the interlaced scan to provide sufficient time for the dissipation of the heat energy allowing a high effective overlap to be used, thus providing relatively smooth and predictable machining results with an R.sub.a in the range of 0.6 m-1.2 m. This surface roughness R.sub.a is however inadequate for micro-optical elements.

[0059] An embodiment of the present invention therefore introduces a post-process smoothing step 36. This can be considered as a third stage of the process 10 flow being a smoothing pass, where the beam 22 of the ultrashort pulsed laser 18 is defocused to provide a spot on the machined surface of 75 m and a sequential raster process (i.e. that of the prior art) is used with 1.8 m step with the scan direction orthogonal to the interlace scan direction. This reduces the ridges formed by the interlaced machining process 34. Using 42 J pulse energy and z=focus+1 mm, produces a fluence level which causes the ablation of a small layer of the surface and the larger spot provides higher overlap thereby creating an overall smoothing effect. Hence the smoothing process can be carried out using a range of parameters (spot diameter, pulse energy, scan speed and repetition rate). However, since it does not induce melting and reflowing, the surface after the smoothing process is still optically rough, i.e. R.sub.a of 0.6 to 0.4 m, and so unsuitable for optical applications.

[0060] A final stage provides the fourth step 38 in an embodiment of the process 10. This uses a CO.sub.2 laser polishing process as is known in the art to provide an optically-smooth finish. The surface of the material is melted using a CO.sub.2 laser beam with care taken to keep the surface temperature below 2700 C. (vaporisation temperature). The melt pool depth, width and temperature are carefully controlled to ensure that molten glass flows under surface tension thereby eliminating the unwanted high frequency components while maintaining the surface form (i.e. the as-designed lower frequency components). An R.sub.a better than 100 nm is obtained in this step 38.

[0061] It is noted that the polishing process is only available for fused silica and is unsuitable for materials with a higher CTE.

[0062] The principal advantage of the present invention is that it provides a laser based machining process using an interlacing raster scan which increases the ablation efficiency and reduces the thermal accumulation in machining glass materials.

[0063] A further advantage of an embodiment of the present invention is that it provides a laser based machining process using an interlacing raster scan which increases the efficiency of machining optical elements.

[0064] A yet advantage of an embodiment of the present invention is that it provides a laser based machining process using an interlacing raster scan which provides increased depth control in machining optical elements.

[0065] It will be appreciated by those skilled in the art that modifications may be made to invention herein described without departing from the scope thereof. For example, other laser systems may be used to machine alternative materials with different dimensions. While the process described has been applied for ultrashort pulsed lasers it can be extended to other pulsed or CW lasers.