DE102009047098A1 - Optical arrangement for homogenizing a laser pulse - Google Patents

Abstract

Optical arrangement for homogenizing an at least partially coherent light field of a light pulse, a pulsed light source, in particular a laser, preferably an excimer laser, consisting of at least one optical circuit, a beam splitter device being provided in the optical circuit. The beam splitter device separates the light field into two perpendicularly polarized partial beams with a first and a second polarization direction, the first partial beams with the first polarization direction reaching an area to be illuminated without going through the circuit, and the second partial beams with the second polarization direction reaching the optical Circulation, with a polarization rotator being provided in the optical rotation, which rotates the second polarization direction of the second partial beams by a predefinable angle, so that at least some of the second partial beams traverse the optical circulation again with the first polarization direction, and the other through the beam splitter device Part of the second partial beams with the second polarization direction is decoupled from the cycle and superimposed on the first partial beams arrive at the surface to be illuminated with a time delay.

Description

The invention relates to an optical arrangement for homogenizing a laser pulse of a pulse laser light source, in particular of a laser, preferably an excimer laser, in particular for a projection exposure apparatus.

STATE OF THE ART

Pulsed laser light sources, z. B. excimer laser, for UV lithography have a repetition rate of about 1000 to 4000 pulses per second. Each pulse has a pulse length of about 20 to 30 ns. Within each pulse, depending on the gas, the state of the laser, in particular the optical components, and depending on the resonator length, significant modulation of the laser output power occurs over time.

It has been found in practice that a significant disadvantage is that due to the operation of a pulse laser light source to a high Leistungsaufspitzung (peak power) comes, which has a very negative effect on the optical materials, especially glassy materials. Especially with quartz glass and a working wavelength of 193 nm arise as a result of the temporal power distribution non-linear optical effects that damage the material over a designated life of the product. The consequences of this are also transmission losses and an increase in the refractive index in an uncontrolled form.

It is also known that the intensity distribution in the cross section of a coherent light field is generally not homogeneous. This is especially true for the radiation emanating from an excimer laser. The illumination of a surface with a coherent, inhomogeneous laser light beam produces interferences, which manifest themselves in spatially different luminances, and which also vary in different directions of observation because of the thereby changing phase relationships in the interference. This glitch-perceivable disorder is referred to in the art as "speckle". Optical arrangements have been developed which avoid or at least reduce the occurrence of speckle. For this purpose, the coherence of the light beam must be destroyed, so to speak, so that the light beam can no longer interfere with itself and thus produce speckle. This usually succeeds in such a way that a bundle of light is split up and brought together again on paths of different lengths, wherein the path length difference should be of the order of magnitude of the coherence length of the light bundle.

From the EP 0 785 473 A2 a device of the type mentioned above is known, by which the coming of a pulse laser light source light is divided into a plurality of partial beams, which undergo different lengths of circulation paths. In this way, a bundle widening or a division into a plurality of adjacently arranged sub-beams whose coherence is reduced or canceled. These sub-beams are arranged side by side in a lighting device entered.

The disadvantage here, however, that so that multiple optical axes are generated side by side and the lighting device is adjusted accordingly. In addition, the aforementioned device is fixed in their orientation.

In the DE 195 01 521 C1 An arrangement for reducing interference of a coherent light beam by reducing temporal coherence is described. It is proposed to use microstructured phase plates through which the laser beam passes in order to reduce the temporal coherence, wherein in a particular embodiment of the invention, such a phase plate is designed to be reflective with a phase-changing surface structure. When the laser light passes through the microstructured phase plate, the coherence of the laser light is broken.

From the US 7,369,597 B2 a device of the type mentioned above is known, by which the light coming from a pulse laser light source is divided by a beam splitter into two partial beams, wherein a partial beam passes through a fixed circulation path. These sub-beams are superimposed entered into a lighting device.

SUMMARY OF THE INVENTION

The present invention has for its object to provide a device by means of which damage to components that are in the beam path of the pulse laser light source avoided and unwanted Speckle be reduced with minimal loss of efficiency of the pulse laser light source and maximum flexibility.

According to the invention, this object is achieved by an optical arrangement for homogenizing an at least partially coherent light field of a light pulse of a pulsed light source, in particular a laser, preferably an excimer laser, according to claim 1. For this purpose, at least one optical circulation is provided, wherein a beam splitter device is provided in the optical circulation. The beam splitter device separates the light field into two sub-beams polarized perpendicular to one another with a first and a second polarization direction, the first partial beams with the first polarization direction without passing through to reach a surface to be illuminated, and the second partial beams with the second polarization direction through the optical circulation. In the optical circulation, a polarization rotator is provided, which rotates the second polarization direction of the second partial beams by a predefinable angle, so that at least a part of the second partial beams pass through the optical circulation with the first polarization direction again. By means of the beam splitter device, the other part of the second partial beams with the second polarization direction is coupled out of the circulation and, with a time shift, superimposed to reach the first partial beams to the surface to be illuminated.

Advantageously, the second partial beams pass through the optical circulation several times.

As a result, the light pulse is extended in time and thus reduces the power of the light pulse.

In one embodiment, in each revolution, the angle is changed such that a predetermined intensity of the second partial beams is coupled out by the beam splitter device and, with a time offset superimposed to the first partial beams, reach the surface to be illuminated. As a result, the pulse shape and width can be influenced in a targeted manner.

Particularly advantageously, the optical circulation on such a length that the difference of the optical path lengths of the paths of the partial beams is greater than the temporal coherence in the light field. This reduces the coherence in the laser light emerging from the array, thus avoiding or at least reducing the occurrence of speckle.

In a further embodiment, reflective components form the optical circulation, which are designed as mirrors.

In a further advantageous embodiment, reflective components form the optical circulation which are formed as prisms. This reduces the transmission loss in circulation.

If the second partial beams are coupled into the prisms at the Brewster angle, the transmission losses can be further reduced.

In one embodiment, the beam splitter device is designed as a polarizing beam splitter.

Advantageously, the coherence of the light field at the surface to be illuminated is reduced by the time-shifted superposition of the partial beams and, due to the path length difference.

The peak power of the light field at the surface to be illuminated is particularly advantageously reduced by the time-shifted multiple superimposition of the partial beams.

In a further advantageous embodiment, the polarization rotator is designed as a Pockels cell. Thus, the polarization direction can be changed very quickly.

Advantageously, the polarization rotator ( 14 . 24 A non-linear optical crystal selected from the following group of crystals: beta-barium borate (BBO), potassium dihydrogen phosphate (KDP), deuterated potassium dihydrogen phosphate (DKDP), or lithium triborate (LiB3O5, LBO). This has the advantage that a broad wavelength range of the light can be influenced in its polarization with the lowest possible losses.

A further embodiment has a polarization-adjusting element between the light source and the beam splitter device for adjusting the polarization of the light field. This has the advantage that the polarization direction of the light entering the optical arrangement can be preset in a targeted manner.

Further advantageous embodiments and developments emerge from the remaining dependent claims and from the embodiments described in principle below with reference to the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

It shows:

1 an embodiment of the invention with beam splitter and polarization rotator with mirrors in the detour;

2 An embodiment of the invention with beam splitter and polarization rotator with deflecting prisms in the detour;

3 Schematic representation of a deflection prism

4 Diagram showing the angular rotation of a polarization rotator per revolution

DETAILED DESCRIPTION

According to the embodiment according to 1 meets a ray of light 10 a pulsed laser light source, e.g. B. an Eximer laser on a first beam splitter device 15 in the form of a known polarization beam splitter, for example a polarizing beam splitter cube. A polarizing beam splitter splits unpolarized light into two mutually orthogonally polarized beams. One partial beam is transmitted, the other is reflected. The transmitted partial beam 10b is polarized parallel to the plane of incidence of the beam splitter cube (p-polarization), the reflected partial beam 10a is polarized perpendicular to the plane of incidence (s-polarization). Other elements which divide polarized radiation into two partial beams with mutually perpendicular polarization directions are also conceivable.

Is the light linearly polarized and hits the beam 10 for example, with its polarization directions below 45 ° (in 1 thus 45 ° to the plane of the drawing, with the double arrows and crossed-over double points as a mixture of both polarization directions indicated) on the beam splitter device 15 , so pass 50% of the total beam as a linearly polarized partial beam 10b unhindered and lossless the beam splitter device 15 in the direction of a surface to be illuminated 12 with a polarization direction of 0 ° after the beam splitter device 15 ,

In the example, the polarization direction is then parallel to the plane of the drawing (p-polarized), indicated by the two parallel double arrows. The other 50% of the beam are reflected and take their path as a partial beam 10a via an optical circulation (hereinafter referred to as the circulation path), which is formed by beam-deflecting components, between which the partial beam 10a can go through a certain distance. In the following, this is referred to briefly as the circulation path. In the present embodiment, four mirrors are formed 11a -D as beam deflecting components a circulation path, by the partial beam 10a each deflect 90 ° and forward to the next mirror. partial beam 10a is after the beam splitter device 15 linearly polarized with a polarization direction of 90 °, ie in the example perpendicular to the plane (s-polarized). This is indicated by the crossed off colons. The length of the circulation path in the arrangement shown above of the beam splitter device 15 over the mirrors 11a -D back to the beam splitter device 15 is sized so that the pulse can be fully absorbed.

In the case of, for example, 20 ns of pulse, the circulation distance would thus have to be 6 m (20 × 10 ^-9 s × 3 × 10 ⁸ m / s = 6 m), distributed over the four distances between the mirrors 11a d.

The beam splitter device 15 acts on its back also as a beam splitter. When the s-polarized sub-beam 10a without changing its polarization at the beam splitter device 15 arrives, he is at the back of the beam splitter device 15 reflected. Without further action, therefore, the now s-polarized sub-beam would 10a the circulation path in the direction of the surface to be illuminated 12 temporally offset to the partial beam 10b leave. The partial beam 10a has traversed a larger path after passing through the circulation path than the partial beam 10b , Is the difference of the paths of the two partial beams greater than the temporal coherence length of the laser pulse in the incident light beam 10 this would already mean a reduction in coherence and peak power due to the "smearing" of the laser pulse. However, such a structure is still very inflexible, since there is no further influence on the orbital period, the pulse shape and difference of the ways to reduce the coherence.

According to the invention in the embodiment according to 1 in the arm of the circulation path in front of the beam splitter device 15 a polarization rotator 14 with a control unit 13 introduced into the beam path, which can rotate the polarization direction of a light beam targeted by an angle α. This may be, for example, a Pockels cell of nonlinear optical crystal, as for example from the WO 2005/085955 A2 is known. A Pockels cell rotates the polarization direction of a radiation passing through the cell when an electrical voltage is applied by the control unit 13 by an angle that is proportional to the applied voltage. In one embodiment, the nonlinear optical crystal is transparent to light in a wavelength range below 200 nm. Crystals which are transparent below this wavelength are particularly suitable for use in microlithography, in which the production of very fine semiconductor structures is performed with illumination light having wavelengths of less than 200 nm, in particular 193 nm or 157 nm. In a further development, the non-linear optical crystal consists essentially of beta-barium borate (BBO), potassium dihydrogen phosphate (KDP), deuterated potassium dihydrogen phosphate (DKDP) or lithium triborate (LiB3O5, LBO). Crystals made of these materials are transparent even at wavelengths smaller than 200 nm. KDP and DKDP have a transmission range from about 190 nm to about 1500 nm. In the case of LBO, the transmission range is from about 160 nm to about 2600 nm. Therefore, applications in the visible or infrared range are also possible. The Pockels cell must have a fast switching time because, for example, after a pulse has circulated, the polarization direction must be switched. This can be in the range of a few ns. In the present example <20 ns.

Before the circulating partial beam 10a to the polarization rotator 14 passes, becomes the polarization rotator 14 switched so that it turns the incoming linearly polarized light by an angle α.

If the first passage S ₁ through the polarization rotator 14 set angle α = 90 ° and would be the polarization rotator 14 set angle α immediately after the passage of the pulse set back to zero, the coupled light remains in the arrangement shown, without the intensity would be coupled out, since now the partial beam 10a is p-polarized by the 90 ° rotation, and the beam splitter device 15 can happen unhindered.

The partial beam 10a goes through the circulation route a second time. This is indicated by the two double arrows in round brackets. After a further or several up to n passes S ₂ to S _n-1 , the last time before arrival of the sub-beam 10a the polarization rotator 14 switched again so that it rotates the polarization direction by 90 °. This will be partial beam 10a in the last round S _n again s-polarized, at the beam splitter device 15 reflected and decoupled. This is indicated by the crossed-over colon in curly brackets. This allows the s-polarized partial beam 10a the circulation path in the direction of the surface to be illuminated 12 after the partial beam 10b temporally by n times the time compared to the partial beam 10b left displaced. This allows almost any delay of the sub-beam 10a can be achieved as multiples of the orbital period in the mirror 11a -D formed orbit. Even if the pulse z. B. of 20 ns to be stretched by 50 times to 1 μs that has no effect on the space of the proposed arrangement, but only the number of cycles increases. Thus, the cycle time of the pulse is controllable.

Dodges through the polarization rotator 14 set angle α from 90 °, has the partial beam 10a an s-polarized and a p-polarized component with respect to the beam splitter device 15 , The component of the sub-beam 10a which is relative to the divisional plane of the beam splitter device 15 s-polarized, is decoupled. The component of the sub-beam 10a which is relative to the divisional plane of the beam splitter device 15 is p-polarized is by the beam splitter device 15 passed through and again passes through the circulation route. The next time through the polarization rotator 14 The polarization is again rotated by an angle α _n , but not necessarily coincide with the rotation angle α from the first round. After reaching the beam splitter again 15 In turn, the s component is coupled out and the p component is transmitted. By choosing the angle α _n can be adjusted, which intensity is to be coupled out of the device according to the invention. So it is possible, for example, each the same intensity component of the sub-beam 10a to decouple as s-polarized light if the angle α _n is chosen to be the same every time. As the total intensity of the circulating light is getting smaller and smaller, the decoupled intensity becomes smaller and smaller. By skillful choice of the angles α _n one can achieve that the intensity of the decoupled light is in each case the same. If, for example, the procedure described is not started with a light which is linearly polarized at 45 °, but with an s-polarized light beam 10 , the light beam is 100% as a partial beam 10a coupled into the circulation route. In 4 It is shown how the angle α _{n of} the linear polarization must be changed from round to round when the pulse expansion factor 10 be and the temporal intensity course of the stretched pulse should be constant. Plotted is the angle α _n over the number n of cycles in the circulation path.

In the last (nth) round, the orientation of the linear polarization is finally rotated by 90 ° (≈ 1.57 rad), so that then all the remaining light from the beam splitter device 15 is decoupled.

The course of the angle α _{n to be set} varies from circulation to circulation. In the example, the values are based on the assumption that one-tenth of the total intensity is extracted per revolution, ie the orientation of the linear polarization must each be rotated such that the square of the decoupled components corresponds to 10% of the initial intensity.

The number of mirrors ( 11a -D) is only an example. Any other number and spatial arrangement of the mirrors that can form an optical circuit is possible. The mirrors are in the example designed as a plane mirror. Also conceivable are other mirror shapes, for example as concave mirrors with a parabolically curved surface, even in combination of different shapes.

To the direction of polarization of the beam splitter device 15 meeting the radiation beam 10 to be able to adjust specifically, in the beam path of the beam 10 in front of the beam splitter device 15 a polarization adjusting element 17 be provided. This polarization adjusting element 17 For example, a polarization rotator analogous to the polarization rotator 14 be that of an already existing polarization of the beam 10 rotates or a polarizer, for example, generates unpolarized light linearly polarized light with a certain angle. The adjustment of the polarization can by a static element, such as a polarizing filter or by a variable element, for example a Pockels cell. Any other suitable element for influencing the polarization and polarization direction is also conceivable.

In the 5a In the following, the principle of the time-shifted overlay is explained in more detail.

5a shows greatly simplified the course of the power of a laser pulse, for example, as an s-polarized light beam 10 in 1 to the beam splitter device 15 meets. The pulse is completely coupled into the circulation path and after each revolution in the present example, a quarter of the power by appropriate choice of the angle α _n on the polarization rotator 14 decoupled. In 5b It is shown how the four decoupled pulses successively overlap over time. The pulses arrive at the surface to be illuminated 12 as a pulse, resulting from the four superimposed pulses 5b composed. This is exemplary in 5c shown. The resulting pulse is not only stretched in time, but the overall performance is reduced. In addition, the coherence of the laser pulse is reduced if the optical path length of the circulation path is greater than the coherence length of the light of the laser pulse.

Thus, the pulse of the pulsed laser light source is smoothed. At the same time, the peak power of the pulse is reduced and thus damage to optical elements is avoided. In the beam splitter 15 the splitting takes place, as already described, so that different radiation components pass through the circulation differently and are combined with each other after decoupling, whereby due to the multiple splitting and the optical path length difference, changes in the wavefronts of the individual radiation components result in a reduction of coherence in the radiation component result from the arrangement emerging laser light. This avoids or at least reduces the occurrence of speckle.

In the 2 and 3 In the following, a further embodiment of the invention is described.

The mirror 10a -D from the embodiment according to 1 lead to undesirable transmission losses of the deflected partial beam, since in each reflection at a mirror due to a reflectivity, which can be a maximum of about 95% -98%, losses occur.

In 2 an embodiment is shown in which the beam deflection of prisms 21a and 21b is trained. A polarized light beam 20 meets a beam splitter device 25 , There is light bundle 20 split into two sub-beams 20a and 20b , The transmitted partial beam 20b is after the beam splitter device 25 p-polarized, the reflected partial beam 20a s-polarized. partial beam 20a meets the entrance surface of the prism 21b ideally below the Brewster angle of the interface between the material of the prism 21a . 21b and the adjacent atmosphere, such as air or vacuum. Is that incident light 20a completely p-polarized, the losses are ideally even zero. partial beam 20a is in the prism 21b so reflected that he came out of the prism 21b on the second prism 21a meets. There will be partial beam 20a reflected again and then passes through a polarization rotator 24 with a control device 23 and from there back to the beam splitter device 25 , For operation is on the analogous description of the embodiment according to 1 referenced, except that instead of the deflection mirror 10a -D now the prisms 21a . 21b serve as a deflection device for forming the detour route. In the prism 21a . 21b Thus, the light is deflected by utilizing the total reflection and leaves the prism 21a . 21b back under the Brewster angle. Theoretically, the transmission loss in such a beam deflection is zero. Although the loss per deflection prism will not reach the theoretically possible value due to the finite surface quality, it is in the range of ≤0.5%. Again, again a polarization-adjusting element 27 for adjusting the polarization direction of the light beam 20 between the light source and the beam splitter device 25 be provided.

Based on 3 is an advantageous embodiment of a Umlenkprismas 21 explained in more detail. The prism 21 has four side surfaces, wherein the side surfaces include four angles a, b, b 'and c. The angles b and b 'are the same in the example, so that an incident partial beam 20a and a failing sub-beam 20ae parallel to each other. partial beam 20ai meets at an angle Θ _B to the surface of the prism 21 on and is at an angle Θ _{B '} in the prism 21 broken into it. After two total reflections, the partial beam occurs 20ae back from the prism 21 and then runs parallel in the opposite direction to the incident partial beam 20ai continue in the detour route.

In the example, the refractive index of the prism material is n _P = 1.5 and that of the surrounding atmosphere n _L = 1. The angles are then calculated as follows: a = 180 ° - 2 · Θ _B b = 45 ° + 0.5 * (Θ _B + Θ _{B '} ) c = 90 ° + (Θ _B - Θ _{B '} ) Θ _B = arctan (n): Brewster angle Θ _{B '} = arcsin (1 / n · sin · (Θ _B ))

With n = n _P / n _L

This yields: Θ _B = 56.31 °, Θ _{B '} = 33.69 °, a = 67.38, b = 90 °, c = 112.62

Of course, other embodiments and number of prisms for beam deflection are conceivable that redirect light rays with minimal losses and thus form a circulation path. Other forms of optical elements for beam deflection, which operate on the principle of total reflection at interfaces are suitable for implementing a detour path according to the invention.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0785473 A2 [0005]
• DE 19501521 C1 [0007]
• US 7369597 B2 [0008]
• WO 2005/085955 A2 [0035]

Claims (14)

Optical arrangement for homogenizing an at least partially coherent light field (US Pat. 10 . 20 ) of a light pulse of a pulsed light source, in particular of a laser, preferably of an excimer laser, consisting of at least one optical circulation, wherein in the optical circulation a beam splitting device ( 15 . 25 ), characterized in that the beam splitting device ( 15 . 25 ) the light field ( 10 . 20 ) in two mutually perpendicular polarized partial beams ( 10a . 20a . 10b . 20b ) with a first and a second polarization direction, wherein the first partial beams ( 10b . 20b ) with the first polarization direction without passing through to a surface to be illuminated ( 12 . 22 ), and the second partial beams ( 10a . 20a ) pass through the optical circulation with the second polarization direction, and that in the optical circulation a polarization rotator ( 14 . 24 ) is provided, the second polarization direction of the second partial beams ( 10a . 20a ) rotates by a predefinable angle α, α _n , so that at least a part of the second partial beams ( 10a . 20a ) again through the optical circulation with the first polarization direction, and that by the beam splitter device ( 15 . 25 ) the other part of the second partial beams ( 10b . 20b ) is decoupled from the circulation with the second polarization direction and is superimposed over time with the first partial beams ( 10b . 20b ) to the surface to be illuminated ( 12 . 22 ) reach. Optical arrangement according to claim 1, characterized in that the second partial beams ( 10b . 20b ) go through the optical circulation several times. Optical arrangement according to claim 2, characterized in that in each revolution the angle α _{n is} changed such that a predetermined intensity of the second partial beams ( 10b . 20b ) by the beam splitting device ( 15 . 25 ) is coupled out and overlaid in time to the first partial beams ( 10b . 20b ) to the surface to be illuminated ( 12 . 22 ) reach. Optical arrangement according to claim 1 to 3, characterized in that the optical circulation has a length such that the difference of the optical path lengths of the paths of the partial beams ( 10a . 20a . 10b . 20b ) is greater than the temporal coherence in the light field ( 10 . 20 ). Optical arrangement according to Claims 1 to 4, characterized in that reflective components ( 11a -d, 21a B) form the optical circulation. Optical arrangement according to claim 5, characterized in that the reflective components as mirrors ( 11a -D) are formed. Optical arrangement according to Claim 5, characterized in that the reflective components are in the form of prisms ( 11a -D) are formed. Optical arrangement according to claim 6, characterized in that the second partial beams ( 10b . 20b ) at the Brewster angle into the prisms ( 11a -D) are coupled. Optical arrangement according to one of the preceding claims, characterized in that the beam splitting device ( 15 . 25 ) is formed as a polarizing beam splitter. Optical arrangement according to one of the preceding claims, characterized in that the partial beams ( 10a . 20a . 10b . 20b ) are superimposed with time offset and the path length difference is set such that the coherence of the light field ( 10b ) on the surface to be illuminated ( 12 . 22 ) is reduced. Optical arrangement according to one of the preceding claims, characterized in that the partial beams ( 10a . 20a . 10b . 20b ) time-shifted are superimposed several times such that the peak power of the light field ( 10b ) on the surface to be illuminated ( 12 . 22 ) is reduced. Optical arrangement according to one of the preceding claims, characterized in that the polarization rotator ( 14 . 24 ) is designed as a Pockels cell. Optical arrangement according to claim 12, characterized in that the polarization rotator ( 14 . 24 ) has a non-linear optical crystal selected from the following group of crystals: beta-barium borate (BBO), potassium dihydrogen phosphate (KDP), deuterated potassium dihydrogen phosphate (DKDP) or lithium triborate (LiB3O5, LBO) Optical arrangement according to one of the preceding claims, characterized in that between the light source and the beam splitting device ( 15 . 25 ) a polarization adjusting element ( 17 . 27 ) is arranged to adjust the polarization of the light field ( 10 . 20 ).

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