WO2004021529A1

WO2004021529A1 - A pulsed laser and a method, comprising a diffusing element to equalize the divergence of the laser radiation

Info

Publication number: WO2004021529A1
Application number: PCT/SE2003/001355
Authority: WO
Inventors: Torbjörn Sandström
Original assignee: Micronic Laser Systems Ab
Priority date: 2002-09-02
Filing date: 2003-09-02
Publication date: 2004-03-11
Also published as: EP1543594A1; JP2005537658A; SE0202584D0; AU2003256204A1

Abstract

An aspect of the present invention includes a pulsed laser, comprising a pair of spaced apart mirrors forming a resonant cavity for reflecting a laser radiation, a discharge volume, a housing enclosing at least said discharge volume, wherein said pulsed laser further comprising a diffusing element to equalize a coherence length during a period of time said laser pulses. Other aspects of the present invention are reflected in the detailed description, figures and claims.

Description

A pulsed laser and a method, comprising a diffusing element to equalize the divergence of the laser radiation

TECHNICAL FIELD fOOϋlJ The present invention relates to pulsed lasers, in particular it relates lo a 5 coherence reducing method and device for pulsed lasers.

BACKGROUND OF THE INVENTION

[0002] Excimer lasers use a rare gas such as krypton, xenon, argon or neon, and a halide gas containing a halide. for example F₂ and HCI. as active components. The active components and other gases are contained in a discharge volume provided with 0 laser optics at each end and longitudinally extending lasing electrodes for causing a transverse electrical discharge in the gases. High voltage pulses are applied to the electrodes and cause electrical pulse discharges to excite the rare gas atoms to their mctastable state, which later on causes an emission of photons constituting a laser light. 5 [00031 Pulsed lasers with high M2 number, notably excimer lasers give a time dependent divergence during the pulse. The light becomes more coherent later in the pulse. In certain applications, such as in pattern generation, metrology and inspection, said high coherence at the end of the pulse creates speckle phenomena in an image, which is a problem. 0 SUMMARY OF THE INVENTION

[0004] Accordingly, it is an object of the present invention to provide a method and a device to modify the coherence properties for pulsed lasers, which overcomes or at least reduces the above-mentioned problems. [0005] This object, among others, is according to a first aspect of the invention 5 attained by a pulsed laser, comprising a pair of spaced apart mirrors forming a resonant cavity for reflecting a laser radiation and a region within said resonant cavity in which stimulated emission takes place, wherein said pulsed laser further comprising a diffusing element provided within said resonant cavity to equalize a divergence of said laser radiation during a period of time said laser pulses. 0 [0006] In another embodiment according to the present invention, said diffusing element gives a phase modulation of said laser radiation. [0007] In another embodiment according to the present invention, said diffusing element is integrated with at least one of said mirrors forming the resonant cavity. [0008] In another embodiment according to the present invention, at least one of said mirrors is curved. [0009] In another embodiment according to the present invention, said at least one curved mirror is spherical.

[0010) In another embodiment according to the present invention, said at least one curved mirror is aspherical. [001 1 ] In another embodiment according to the present invention, said at least one mirror comprises a multi-layer of a reflecting coating.

[0012] In another embodiment according to the present invention, said diffusing element only modifies the coherence property of the laser in one direction. [0013] In another embodiment according to the present invention, said diffusing element modifies the coherence properly of the laser in two directions [0014] The invention also relates to a pulsed laser, comprising a pair of spaced apart mirrors forming a resonant cavity for reflecting a laser radiation and a region within said resonant cavity in which stimulated emission takes place, wherein at least one of said mirrors is aspheric to equalize a divergence of said laser radiation during a period of time said laser pulses. [00151 The invention further relates to a pulsed laser, comprising a pair of spaced apart mirrors forming a resonant cavity for reflecting a laser radiation and a region within said resonant cavity in which stimulated emission takes place, wherein at least one of said mirrors being essentially fiat in a region in the vicinity of an optical axis of said laser and a peripheral part of said at least one mirror being sphere shaped to equalize a divergence of said laser radiation during a period of time said laser pulses. [0016] In another embodiment according to the present invention, a diffusing element is provided in said essentially flat region of said at least one of said mirrors for creating a compact laser with high divergence. [0017] In another embodiment according to the present invention, said diffusing element is a separate semitransparent plate arranged within the resonating cavity and having a surface profile providing for a phase modulation of the laser radiation. [0018] The invention also relates to a method for creating a pulsed laser beam with improved coherence characteristics, comprising the actions of, providing a pair of mirrors forming a resonant cavity, providing a region within said resonant cavity in which stimulated emission takes place, providing lasing material into said region within the resonant cavity, providing a diffusing element within the resonant cavity for modifying the coherence property of said laser beam. [0019] In another embodiment according to the present invention, said diffusing element gives a phase modulation of said laser radiation.

[0020] In another embodiment according to the present invention, said diffusing element is integrated with at least one of said mirrors forming the resonant cavity. [0021] In another embodiment according to the present invention, said at least one mirror comprises a multi-layer of a reflecting coating.

[0022] In another embodiment according to the present invention, said diffusing element only modifies the coherence property of the laser in one direction. T0023J In another embodiment according to the present invention, said diffusing element modifies the coherence property of the laser in two directions [0024] In another embodiment according to the present invention, at least one of the mirrors is curved

[0025] In another embodiment according to the present invention, at least one of said mirrors being essentially fiat in a region in the vicinity of an optical axis of said laser and a peripheral part of said at least one mirror being sphere shaped. [0026] In another embodiment according to the present invention, said diffusing element is provided in said essentially flat region of said at least one of said mirrors for creating a compact laser with high divergence.

[0027] In another embodiment according to the present invention, said curved mirror is spherical. [00281 i another embodiment according to the present invention, said curved mirror is aspherical

[00291 Further characteristics of the invention, and advantages thereof, will be evident from the detailed description of preferred embodiments of the present invention given hereinafter and the accompanying figures 1-8. which are given by way of illustration only, and thus are not limitative of the present invention.

BRI EF DESCRIPTION OF THE DRAWINGS

[0030] Figure I depicts in a side view a prior art excimer laser. [0031] Figure 2a depicts a first wave of photons in an excimer laser pulse.

[0032] Figure 2b depicts a second wave of photons in an excimer laser pulse.

[0033] Figure 2c depicts a third wave of photons in an excimer laser pulse.

[0034] Figure 3 depicts a third wave of photons in an excimer laser having a first embodiment of diffusing mirrors according to the present invention.

[0035] Figure 4a depicts a first embodiment of a phase surface according to the present invention.

[0036] Figure 4b depicts a second embodiment of a phase surface according to the present invention. [0037] Figure 4c depicts a third embodiment of a phase surface according to the present invention.

[0038] Figure 4d depicts a fourth embodiment of a phase surface according to the present invention.

[0039] Figure 4e depicts a fifth embodiment of a phase surface according to the present invention.

[0040] Figure 5a depicts a first method of creating a phase surface according to the present invention.

[0041] Figure 5b depicts a second method of creating a phase surface according to the present invention. [0042] Figure 5c depicts a third method of creating a phase surface according to the present invention.

[0043] Figure 5d depicts a fourth method of creating a phase surface according to the present invention.

[0044] Figure 6 depicts another embodiment of arranging the phase surface. [0045] Figure 7a-h depict different embodiment of phase patterns according to the invention.

[0046] Figure 8 depicts a prior art stable hemispherical resonator.

[0047] Figure 9 depicts a prior art stable resonator.

[0048] Figure 10 depicts a prior art flat-flat resonator. [0049] Figure 1 1 depicts an embodiment of a resonator according to the present invention.

[0050] Figure 12 depicts another embodiment of a resonator according to the present invention. [0051] Figure 13 depicts the geometry of an inventive embodiment according to the present invention.

[0052]

DETAILED DESCRIPTION [0053] The following detailed description is made with reference to the figures. Preferred embodiments are described to illustrate the present invention, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows. [0054] Further, the preferred embodiments are described with reference lo an excimer laser. It will be obvious to one ordinary skill in the art that there may be situations when sources other than excimer lasers will be equally applicable; for example optically or electrically pumped gas. liquid or solid state lasers such as Nd.YAG-lasers. dye-lasers, copper-vapour-lasers. ruby lasers, garnet lasers, CO₂ lasers, free-electron lasers. Ti-sapphire lasers, semiconductor lasers etc. Additional lasers can be found in "Lasers" by Siegman. University Science Books, or in Journal of Quantum Electronics.

[0055] The invention relates to a method to equalize variations in coherence during a lifetime of a pulse, for example in laser pulses in an excimer laser. Such a laser is useful when patterning a workpiece using a spatial light modulator (SLM). [0056] Figure 1 shows a prior art transversally excited laser, for example an excimer laser. Said laser comprises a first mirror 1 10 and a second mirror together forming a resonant cavity 170. The laser further comprising a first electrode 130 and a second electrode 140 together forming a discharge volume 160. A housing 150 encloses said discharge volume 160 and said resonant cavity 170. One of the mirrors 1 10 or 120 is partially reflecting for allowing a beam of radiation created within the resonant cavity to be emitted. The other mirror is totally refiecting. The housing is transparent for the emitted wavelength in an end where said partially reflecting mirror is arranged. [0057] Figure 2a illustrates a first wave 180 of photons created within the laser 100. Said first wave 180 is emitted from the laser without any internal reflection from the electrodes 130. 140 and/or the mirrors 1 10 and 120. Said first wave of photons can therefore be said to be relatively divergent due to the fact that said radiation is allowed to be emitted just at the end of the discharge volume, where said end is close to the partially reflecting mirror.

T0058] Figure 2b illustrates a second wave 182 of photons created within the laser 100. Since this illustration comprises elements similar to those in the figure 2a embodiment, a detailed description of such elements will be omitted here only for the sake of simplicity, by assigning the same reference numerals to the corresponding elements. The same applies to figure 2c. Said second wave 182 is emitted from the laser after being reflected once from the mirror being totally reflecting. Said second wave of photons is less divergent, or more coherent compared to the first wave of photons, because of geometrical truncation during the second pass through the electrode area.

[0059] Figure 2c illustrates a third wave 184 of photons created within the laser 100. Said third wave 184 is emitted from the laser being reflected once at the partially reflecting mirror and once at the totally reflecting mirror. Said third wave of photons can therefore be said to be less divergent than said second wave and far even less divergent than said first wave of photons.

[0060] Figure 3 illustrates a first embodiment according to the present invention for creating waves of photons having equalized divergence. In this inventive embodiment the mirrors 1 10. 120 forming the resonant cavity 170 are provided with diffusing surfaces 190, 192. Said diffusing surface 190. 192 counteracts a narrowing of the beam at the end of the pulse and keeps a constant radiation pattern regardless of the number of roundtrips. Here both mirrors 1 10, 120 are depicted to be provided with said diffusing surface 190. 192, however only one of them may have said diffusing surface. Said diffusing surface 190, 192 may be equal or non-equal for said mirrors 1 10, 120.

[0061] Figure 4a-e illustrates four types of phase surfaces 410 having diffusing characteristics. Said phase surfaces are provided at a flat substrate 400, which could be a mirror or a separate plate. Figure 4a illustrates a continuous periodic or non- periodic grating 410, which could be of 1 -dimensional or 2-dimensioι.al nature. Figure 4b illustrates a continuous phase profile giving a spherical or an aspherical surface plus a diffusion of the light. The aspherical surface may be rotational symmetric, may have different symmetry in two different directions being orthogonal to each other or may be non-symmetric. Figure 4c illustrates a phase surface built up from flat portions. Said flat portions could be arranged in a regular symmetrical or non-symmetrical fashion. For example, flat portions being arranged adjacent to each other could have different phases throughout the surface. The flat portions could also be in states randomly distributed over the surface. The phases could be two. three or more states. Said states of the flat portions refer to a distance from a top surface of said flat portion to a virtual surface within or outside the mirror. Figure 4d illustrates a kinoform surface. The kinoform surface is in this embodiment built up of flat portions. The phase surfaces shown in figure 4a-d are all built up from a multi-layer reflecting coating. The coherence properties of the laser are here shown to be modified by introducing a small amount of light scattering inside the resonator. Since said phase pattern and the depth of said pattern can easily adjust the amount and angular characteristics of the scattering, the laser can be tailored to a specific application. In figure 4e the phase surfaces are arranged to be a non-regular pattern. [0062] Figure 5a-d illustrates how to create said phase surface. Figure 5a depicts a side view of a substrate 500. which has a figured surface 510. On top of said figured surface 5 10 a multi-layer of reflecting coating 520 is deposited. The figured surface 510 may be ion etched or polished to the desired shape. The multi-layer 520 is for example created by evaporation of one or a plurality of different optical materials as is well known in the art. In figure 5b it is depicted that a profiled layer 530 may be deposited on top of a flat substrate 505. i.e., first said profiled layer 530 is deposited on the substrate and after that, said multi-layer coating 520 is deposited on said profiled layer. The profiled layer 530 may be created by evaporation of a first layer through one or a sequence of mechanical masks. In this way it is possible to create a kinoform pattern giving a controlled amount of light scattering and a surface that is optimized for energy extraction and coherence. In figure 5c it is illustrated that portions of flat surfaces 550 may be evaporated or plasma etched before said multilayer coating 525 is deposited. In figure 5d it is illustrated that the multi-layer coating 525 may be arranged with a further coating 560, giving a flat top surface. |0063] Figure 6 illustrates another embodiment of the present invention, where one of the diffusing mirrors I 10. 120 is replaced with a flat mirror 610 and a diffusing plate 620. The illustrated embodiment has a Brewster window 630 and the min or 610 and diffusing plate 620 outside the discharging volume 160. This is considered to be equivalent to the embodiment illustrated in figure 3. Likewise, the plate 620 and Brewster window 630 can be combined into the same component, namely a Brewster window with a surface profile that gives a phase modulation. [0064] Figure 7a-f illustrates top views of different embodiments of phase maps of the surfaces of the diffusing mirror or said di ffusing plate. Figure 7a shows a circular symmetrical pattern, figure 7b shows an elliptical symmetrical pattern, figure 7c shows a rectangular symmetrical pattern, figure 7d shows a square shaped asymmetrical pattern, figure 7e illustrates a rectangular asymmetrical pattern and figure 7f shows a hexagonal pattern. Likewise is random pattern possible; see figure 7g and 7h, though of course they can be described in a matrix representation. One- dimensional gratings can be used where coherence need only be modified in one direction.

[0065] In figure 8 it is depicted a prior art stable hemispherical resonator 870 supporting a single transversal mode. Said resonator is built up from curved mirrors 810. 820. In figure 9 it is depicted a stable resonator with curved mirrors 910. 920 supporting many transversal nodes. The difference between the embodiment illustrated in figure 8 and 9 lies in the size of the mirrors forming the resonating cavity. In figure 10 it is illustrated a resonating cavity 1070 built up from flat mirrors 1010. 1020. In said cavity the modes are constrained by geometrical constraints of stops and electrodes. [00661 Figure I I illustrates a laser with scattering or di ffusing min ors according to the invention. Planar mirrors 1 1 10. 1 120 and a diffusing layer 1 190. 1 192 attached on top of said mirrors 1 1 10. 1 120 will increase losses since some light is diffracted outside the open area of the resonating cavity. [0067] In figure 12 the resonating cavity is somewhat modified with curved mirrors 1210, 1220 (shown aspheric). which will direct light that would be scattered outside the resonating cavity with planar mirrors into it. Said curved mirrors are provided with a light scattering surface 1290, 1292. The mirror curvature improves extraction efficiency and increases divergence also without the scattering surface. US patent application with publication number 2002/0012376 and US provisional application with number 60/332,386 illustrates spherical mirrors.

[0068] Figure 13 illustrates a further embodiment according to the present invention. Here the resonator geometry is designed for high extraction efficiency and high divergence. A peripheral part of the mirrors 1310. 1320 in the resonator is closer to a sphere while a center part, close to an optical axis 1395, is significantly flatter. In one embodiment the center curvature is between flat and hemispherical and light scattering structures 1390 provided near the center of at least one mirror. The efficient extraction makes it possible to make the discharge region 1360 shorter, i.e.. a shorter discharge volume, thereby creating a smaller laser with high divergence.

[0069] The aspheric shape in figure 13 improves extraction efficiency and increases divergence in itself, and divergence is further improved by the diffusion of the mirrors. In one embodiment said aspheric curvature is formed in said multi-layer coating on top of a flat substrate. In another embodiment said substrate is aspheric without any diffusion layer. In yet a further embodiment said substrate is aspheric with a diffusion coating applied on top of it. Only one of the mirrors needs to be aspheric. One may be aspheric without coating and the olher flat with a diffusion coating. Both mirrors may be flat and covered with a diffusion coating. As an alternative embodiment one mirror is flat without any coating and the other is aspheric without any coating. Yet another embodiment is where both minors are aspheric one with a diffusion coating and the other uncoated. Further alternative embodiment may be apparent by combining different features from individual embodiments as described above. [0070J While the present invention is disclosed by reference to ihc preferred embodiments and examples detailed above, it is understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art. which modifications and combinations will be within the spirit of the invention and the scope of the following claims.

Claims

1. A pulsed laser, comprising a pair of spaced apart mirrors forming a resonant cavity for reflecting a laser radiation and a region within said resonant cavity in which stimulated emission takes place, wherein said pulsed laser further comprising a diffusing element provided within said resonant cavity to equalize a divergence of said laser radiation during a period of time said laser pulses.

2. The laser according to claim 2, wherein said diffusing element gives a phase modulation of said laser radiation.

3. The laser according to claim 2. wherein said diffusing element is integrated with at least one of said mirrors forming the resonant cavity.

4. The laser according to claim 2, wherein at least one of said mirrors is curved.

5. The laser according to claim 4. wherein said at least one curved mirror is spherical.

6. The laser according to claim 5. wherein said at least one curved mirror is aspherical.

7. The laser according to claim 2, wherein said at least one mirror comprises a multilayer of a reflecting coating.

8. The laser according to claim 2, wherein the coherence property of the laser is modified by said diffusing element only in one direction.

9. The laser according to claim 2, wherein the coherence property of the laser is modified by said diffusing element in two directions.

10. A pulsed laser, comprising a pair of spaced apart mirrors forming a resonant cavity for reflecting a laser radiation and a region within said resonant cavity in which stimulated emission takes place, wherein at least one of said mirrors is aspheric to equalize a divergence of said laser radiation during a period of time said laser pulses.

1 1. A pulsed laser, comprising a pair of spaced apart mirrors forming a resonant cavity for reflecting a laser radiation and a region within said resonant cavity in which stimulated emission takes place, wherein at least one of said mirrors being essentially flat in a region in the vicinity of an optical axis of said laser and a peripheral part of said at least one mirror being sphere shaped to equalize a divergence of said laser radiation during a period of time said laser pulses.

12. The laser according to claim 1 1 , wherein a diffusing element is provided in said essentially fiat region of said at least one of said mirrors for creating a compact laser with high divergence.

13. The laser according to claim 2, wherein said diffusing element is at least one separate semitransparent plate airanged within the resonating cavity and having a surface profile providing for a phase modulation of the laser radiation.

14. A method for creating a pulsed laser beam with improved coherence characteristics, comprising the actions of.

- providing a pair of mirrors forming a resonant cavity,

- providing a region within said resonant cavity in which stimulated emission takes place,

- providing lasing material into said region within the resonant cavity.

- providing a diffusing element within the resonant cavity for modifying the coherence property of said laser beam.

15. The method according to claim 14. wherein said laser radiation is phase modulated by said diffusing element.

16. The method according to claim 14, wherein said diffusing element is integrated with at least one of said miιτors forming the resonant cavity.

17. The method according to claim 16, wherein said at least one mirror comprises a multi-layer of a re flee ting co ting.

1 18. The method according to claim 14. wherein the coherence property of the

2 laser is modified by said diffusing element only in one direction.

1 19. The method according to claim 14. wherein the coherence property of the

2 laser is modified by said diffusing element in two directions.

1 20. The method according to claim 14. wherein at least one of the mirrors is

2 curved.

1 21. The method according to claim 14. wherein at least one of said mirrors being

2 essentially flat in a region in the vicinity of an optical axis of said laser and an

3 peripheral part of said at least one mirror being sphere shaped.

22. The method according to claim 21. wherein said diffusing element is provided

? in said essentially flat region of said at least one of said mirrors for creating a compact

--- laser with high divergence.

23. The method according to claim 20. wherein said curved mirror is spherical.

24. The method according to claim 20. wherein said curved mirror is aspherical.