DE10204141A1 - Bandwidth narrowing module - Google Patents

Abstract

The invention relates to a bandwidth narrowing module for a light source, particularly a laser, with a reflection grating (5). Preferred s-polarization is set for the light falling on the reflection grating by means of polarizing means (6). The inventive module can be used for narrowing the bandwidth of lasers, for example.

Description

The invention relates to a module for bandwidth narrowing a light source, in particular a laser, with a reflection grating. Such modules are used, for example, for excimer lasers Reducing the bandwidth of the emitted laser light is used represent the rear resonator termination of the laser. As Reflection grating is used e.g. B. an Echelle grating in a so-called Littrow Configuration used to the bandwidth of the emitted Reduce laser radiation by wavelength selective reflection. The module typically includes a beam expansion system that is between the laser resonator and the reflection grating is arranged and made several prism bodies connected in series can, and a z. B. to tune the wavelength rotatably arranged mirror.

A modified one disclosed in US Pat. No. 5,917,849 Bandwidth narrowing modules of this type are between the Beam expansion system and the pivoting mirror additionally a polarizing beam splitter with associated Reflection mirror and a polarization rotating element in the form of a λ / 4 plate brought in. The from the gain medium, the gas discharge chamber, coming light is due to the oblique position of the Chamber exit window below or near the Brewster angle and after passing through the prism beam expansion system due to the strong Polarization dependence of the reflection losses on the prisms highly p- or TM-polarized, with any remaining s- or TE- polarized light components reflected by the polarizing beam splitter while allowing p-polarized light to pass through. The λ / 4 plate changes the p-polarized light into circularly polarized light, which is then transmitted through the rotating mirror on the Echelle grating in Littrow configuration falls and is reflected back. The λ / 4 plate changes this for the first time back-reflected, circularly polarized light into s-polarized light emitted by the polarizing beam splitter to the associated mirror and reflected back is, so that it again under the λ / 4 plate Conversion to circularly polarized light falls on the Echelle grating. That from the λ / 4 plate reflects light reflected there again in p- polarized light converted by the polarizing beam splitter can pass through and runs back into the gas discharge chamber. It also states that the reflectance of Echelle gratings is polarization dependent with a difference between p- and s- Polarization of about 10%, which leads to some disturbance of the circular polarization state leads. As a result, a small one Proportion of light reflected only once by the Echelle grating Beam splitter passes through, the efficiency doubling However, reflection is reduced by a maximum of 10%, which is given by the double reflection improved bandwidth narrowing accepted becomes.

The invention has the technical problem of providing a Bandwidth narrowing module of the type mentioned at the beginning, while maintaining the optical properties of the module, in particular a high degree of polarization for an assigned laser, increased efficiency, reduced thermal loads and a allows longer life of the reflection grating.

The invention solves this problem by providing a Bandwidth narrowing module with the features of claim 1. This Module contains polarization means for setting one Preferred polarization for light incident on the reflection grating. This Polarization means thus cause that on the reflection grating incident light is essentially s-polarized. As usual, labeled here s or TE polarization is a polarization perpendicular to the input and Plane of the light, while the perpendicular polarization with p- or TM polarization is called.

Theoretical calculations and practical observations show that the light absorption of reflection gratings, such as in particular Echelle grids of the type used here, without further measures such as z. B. a protective layer for s-polarized light, d. H. with an Echelle Grid for light polarized parallel to the grating grooves, clearly is less than for p-polarized light as it is strong due to the polarization-dependent transmission of the used here Beam expansion system emerges from the laser resonator. At the same time and as The immediate consequence is a significantly higher one for s-polarized light Diffraction efficiency e.g. B. for high diffraction orders of Echelle gratings in Littrow configuration compared to p-polarized light. All in all results in the use of said s-polarizing Polarization means consequently in an improved bandwidth narrowing due to the Reduction of the thermal loads on the reflection grating, one associated increase in its lifespan and in a Improve the efficiency of the entire bandwidth narrowing module.

In a development of the invention according to claim 2, which in particular the need to rotate the out of the laser resonator avoids radiation emitted, contain the polarizing agent simply a λ / 2 plate, which shows the direction of polarization of one Amplification medium coming, p-polarized light rotates by 90 °, so that the light is essentially s-polarized. At the same time, the λ / 2 plate converts that reflected by the reflection grating s-polarized light before re-entering the gain medium back into p-polarized light.

A module developed according to claim 3 includes one Beam expansion unit with one or more successive Beam expansion elements. The polarization means are anywhere between the foremost in the light beam path Beam expansion element and the reflection grating arranged.

In a further embodiment of this measure, according to claim 4 at least two beam expansion elements are provided, and the Polarizing agents are somewhere between the in Light incidence direction first and last beam expansion element, for. B. at three Beam expansion elements between the second and third Beam expansion element. With this positioning are the Polarization means consequently in the partially expanded light beam with the consequence that on the one hand, their spatial dimension is smaller than the cross section of the full expanded light beam can be kept and on the other hand the Power density of the light beam and thus the thermal load for the polarization means versus the area of the not yet expanded light beam is significantly reduced.

In a further embodiment, the one or more are according to claim 5 the beam expansion elements arranged with the polarizing means provided with an anti-reflective coating for s-polarized light is optimized. At the same time, the high reflection losses ensure s-polarized light components on or in front of the polarization means lying beam expansion elements a high degree of polarization of the z. B. reflected back into a laser resonator, bandwidth restricted light.

An advantageous embodiment of the invention is in the drawings illustrated and described below. Here show:

Fig. 1 is a schematic side view of a group consisting of bandwidth narrowing module, gain medium and the output mirror laser resonator,

FIG. 2 is a graph illustrating the diffraction efficiency of an Echelle grating that can be used in the module of FIG. 1 and increased for s-polarized light compared to p-polarized light

Fig. 3 is a graph illustrating the polarized s-by light compared to p-polarized light of the usable lower absorption in the module of FIG. 1 echelle grating.

The bandwidth narrowing module shown in Fig. 1 represents the rear resonator termination of a laser, e.g. B. an excimer laser, and has the function of reducing the bandwidth of the emitted laser radiation by wavelength-selective reflection. An important area of application is UV radiation-emitting excimer lasers for lithography systems for wafer structuring.

The bandwidth narrowing module comprises, in the light beam path one after the other after an amplification or gas discharge chamber 1, a beam expansion unit with three successively arranged, suitably arranged prisms 3 a, 3 b, 3 c, a mirror 4, not shown, conventionally arranged, and a Echelle mirror. Lattice 5 over construction in Littrow configuration. Furthermore, the bandwidth narrowing module includes a λ / 2 plate 6 arranged between the second prism 3 b and the third prism 3 c in the direction of light incidence.

Since the cross section of the laser beam coming from the gas discharge chamber 1 has no rotational symmetry, but rather an elongated rectangular profile, there is a preferred orientation with regard to the alignment of the grating and beam expansion and with respect to the inclined position of the chamber window 2 at or near the Brewster angle. Optimal bandwidth narrowing is achieved if the direction of the beam expansion is parallel, that of the grating furrows perpendicular to the short axis of the beam profile. The required size of the chamber window is also minimal if the axis of rotation of the inclined position is parallel to the long axis of the beam profile. From the orientation of the inclined position of the chamber windows and the orientation of the prism beam expansion, the p-polarization results as the preferred polarization to minimize losses due to reflection. If this polarization, which is favorable for the chamber and the beam expansion, is to be maintained, the s-polarization which is more favorable for the operation of the grating can be achieved by introducing a polarization rotator near the grating.

The essentially p-polarized light coming from the amplification chamber 1 undergoes a partial widening through the first prism 3 a and the second prism 3 b, after which it is rotated by 90 ° in its polarization by the λ / 2 plate, ie by p- is converted into s-polarized light. The light which is largely s-polarized in this way is then expanded by the last prism 3 c to the full cross-section, with which it is incident on the Echelle grating 5 at a suitable large Littrow angle via the mirror 4 . The back-reflected light from the Echelle grating 5 with s preferred polarization then passes via the mirror 4 and the third prism 3 c back to the λ / 2 plate 6 , from which it is converted back into light with p-polarization, after which the essentially p - Polarized and by the action of the Echelle grating 5 bandwidth narrowed light is coupled via the second prism 3 b and the first prism 3 a again for amplification in the discharge chamber 1 . At the front of the discharge chamber 1 , a corresponding laser beam 9 emerges via a likewise obliquely designed exit surface 7 , it being guided via a conventional coupler 9 on the output side.

The placement of the λ / 2 plate 6 in the partially widened light beam between the second prism 3 b and the third prism 3 c has the advantage over a positioning at a point further forward in the direction of the discharge chamber 1 that the power density of the laser beam and thus the thermal Load for the λ / 2 plate 6 is reduced accordingly. On the other hand, by this positioning compared to arranging the λ / 2 plate 6 between the third prism 3 c and the Echelle grating 5 , ie in the fully expanded light beam, the dimension of the λ / 2 plate 6 can be kept correspondingly small, for example in the order of 25 mm × 25 mm with square dimensions.

As an alternative to the positioning shown, the λ / 2 plate can, depending on the application, also in the region that has not yet been partially expanded between the first prism 3 a and the second prism 3 b or in the region that has not yet expanded between the discharge chamber 1 and the first prism 3 a be arranged if the higher power density there is no problem, in which case the λ / 2 plate 6 can be dimensioned even smaller. Further alternatively, the λ / 2 plate can be arranged in the fully expanded light beam between the third prism 3 c and the mirror 4 or between the mirror 4 and the Echelle grating 5 if the power density in the only partially expanded light beam is a problem and therefore the necessary larger dimension of the λ / 2 plate is accepted.

The third prism 3 c is preferably provided with an anti-reflective coating, not explicitly shown, which is optimized for s-polarization, ie for the one coming from the λ / 2 plate 6 and the one reflected back from the Echelle grating 5 , each essentially s polarized light which crosses this third prism 3 c on the way there and back. This minimizes the loss of light through the third prism 3 c. On the other hand, the two upstream prisms 3 a, 3 b are retained in their conventional design, which provides comparatively high reflection losses for s-polarized light. This leads to the desired effect that these two prisms 3 a, 3 b reflect any interfering polarized light components that may be contained in the reflected light emerging from the λ / 2 plate 6 from the actual main beam path, so that the same does not reflect in the amplification chamber 1 are coupled in, which ensures the achievement of a high degree of polarization of the radiation emitted by the laser 9 .

In the above-mentioned alternative positioning of the λ / 2 plate 6 , the prism or prisms located in the beam path between the λ / 2 plate 6 and the Echelle grating 5 are each provided with the anti-reflective coating optimized for s-polarization, while the or the prisms located between the λ / 2 plate 6 and the discharge chamber 1 take over the task of the suppression of any s-polarized radiation portions by reflect away.

As described, the use of the λ / 2 plate 6 causes the light with s preferred polarization to strike the Echelle grating 5 . This has significant advantages over conventional arrangements in which light with p-polarization or in any case with a noticeable proportion of p-polarized radiation is incident on the reflection grating, based on the one hand that the diffraction efficiency of Echelle gratings for s-polarized light is clear is higher than for p-polarized light, especially at suitable angles of incidence, and secondly the absorption of Echelle gratings for s-polarized light is significantly lower than for p-polarized light. This is shown in the diagrams of FIGS. 2 and 3 by way of example for a typical grid, as is suitable for operation in a bandwidth narrowing module of the type shown in FIG. 1. It is assumed that an Al-Echelle grating with d = 7648 nm for a laser light wavelength of λ = 248.4 nm in a configuration with a large Littrow angle of 77 ° for blaze profiles with an apex angle of 90 ° in the -60. Diffraction order is used. Such high diffraction orders are z. B. used in Echelle grids with a large Littrow angle within bandwidth narrowing modules as the last propagating diffraction order. At large Littrow angles, the small facet of the triangular profile of the Echelle grating serves as a blaze flank.

Specifically, FIG. 2 shows the profile of the diffraction efficiency η for -60. Diffraction order depending on the blaze facet angle over an angular range from 74 ° to 84 ° on the one hand for s-polarized light, ie TE polarization, according to the upper characteristic curve with the circularly marked data points and on the other hand in comparison for p-polarized light, ie TM polarization, according to the lower characteristic with the triangularly marked data points. The diffraction efficiency for s-polarized light in the -60 can be seen for blaze facet angles above 77 ° in this special example. Diffraction order in absolute terms about 15% higher than for p-polarized light.

For the same example with the same parameters, FIG. 3 illustrates the course of the absorption a as a function of the blaze facet angle on the one hand for s-polarized light, ie TE absorption, according to the lower characteristic curve with the data points marked in a circle and on the other hand in comparison for p-polarized light, ie TM absorption, according to the upper characteristic with the triangularly marked data points. As can be seen from this, the absorption for s-polarized light is less than half as large as that for p-polarized light over the entire range of blaze facet angles from 74 ° to 84 °.

Since in the module of FIG. 1 the light strikes the Echelle grating 5 in essentially s-polarized manner by using the λ / 2 plate 6 , a correspondingly low absorption and high diffraction efficiency is achieved. The low absorption means reduced thermal problems, which among other things could negatively influence the wavelength resolution of the module, and an increased service life of the Echelle grating 5 .

It goes without saying that the advantages mentioned above apply not only to the shown and the above-mentioned alternative implementations, but also to further implementations of the bandwidth narrowing module according to the invention. Such further implementations can e.g. B. include the use of other conventional polarization means instead of the λ / 2 plate 6 , which are only to be designed so that they ensure that the light is incident s-polarized on the Echelle grating 5 . In other alternative embodiments, instead of the shown different beam expansion units of conventional type can be used, e.g. B. those with only two or those with more than three prisms and / or those with other optical components, for. B. lenses, in addition to or instead of the respective prism.

It also goes without saying that the bandwidth narrowing module not only as shown, for a laser, but also for other light sources is usable, where the task arises, the bandwidth of a emitted light beam appropriately constrict. As a reflection grating not only Echelle grids, but also others depending on the application conventional reflection grille types depending on the respective Use case.

Claims (5)

1. bandwidth narrowing module for a light source, in particular a laser, with
a reflection grating ( 5 ),
marked by
Polarization means ( 6 ) for setting a preferred polarization for light incident on the reflection grating ( 5 ). 2. bandwidth narrowing module according to claim 1, further characterized in that the polarizing means include a λ / 2 plate ( 6 ) which is arranged between a gain medium ( 1 ) of the light source and the reflection grating ( 5 ). 3. bandwidth narrowing module according to claim 1 or 2, further characterized in that a beam expansion unit with one or more successive beam expansion elements ( 3 a, 3 b, 3 c) is provided and the polarization means ( 6 ) in the area between the foremost beam expansion element ( 3 a) and the reflection grating ( 5 ) are arranged. 4. bandwidth narrowing module according to claim 3, further characterized in that the polarization means ( 6 ) in the area behind a foremost and in front of a rearmost of a plurality of beam expansion elements ( 3 a, 3 b, 3 c) are arranged. 5. bandwidth narrowing module according to claim 4, further characterized in that the one or more between the polarization means ( 6 ) and the reflection grating ( 5 ) located beam expansion elements ( 3 c) are provided with an anti-reflective coating optimized for s-polarization.