WO2003065520A1

WO2003065520A1 - Bandwidth narrowing module

Info

Publication number: WO2003065520A1
Application number: PCT/EP2003/000781
Authority: WO
Inventors: Bernd Kleemann; Jeffrey Erxmeyer; Johannes Kraus; Klaus Heidemann
Original assignee: Carl Zeiss Laser Optics Gmbh
Priority date: 2002-01-29
Filing date: 2003-01-27
Publication date: 2003-08-07
Also published as: DE10204141A1

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

Description Bandwidth single gun module

The invention relates to a module for bandwidth narrowing for a light source, in particular a laser, with a reflection grating. Such modules are used, for example, for excimer lasers to reduce the bandwidth of the emitted laser light, and they represent the rear resonator termination of the laser. As a reflection grating e.g. an Echelle grating in a so-called Littrow configuration is used to reduce the bandwidth of the emitted laser radiation by wavelength-selective reflection. The module usually includes a beam expansion system, which is arranged between the laser resonator and the reflection grating and can be constructed from a plurality of prism bodies connected in series, and a e.g. to tune the wavelength of the rotating mirror.

In a modified bandwidth narrowing module of this type disclosed in US Pat. No. 5,917,849, a polarizing beam splitter with associated reflection mirror and a polarization rotating element in the form of a λ / 4 plate are additionally inserted between the beam expansion system and the pivotably arranged mirror. The light coming from the amplification medium, the gas discharge chamber, is highly p- or TM-polarized due to the inclined position of the chamber exit windows at 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, with any remaining ones , s or TE polarized light components are reflected by the polarizing beam splitter while transmitting p-polarized light. The λ / 4 plate converts the p-polarized light into circularly polarized light, which then falls over the rotating mirror onto the Echelle grating in Littrow configuration and is reflected back. The λ / 4 plate converts the first time back-reflected, circularly polarized light into s-polarized light, which is reflected from the polarizing beam splitter to the associated mirror and back, so that it is again passed through the λ / 4 plate with conversion into circular polarized light falls on the Echelle grating. The light reflected again from there is now converted by the λ / 4 plate into p-polarized light, which can pass through the polarizing beam splitter and runs back into the gas discharge chamber. It also states that the degree of reflection of Echelle gratings is polarization-dependent with a difference between p and s polarization of approximately 10%, which leads to a certain disturbance of the circular polarization state. As a result, a small proportion of light reflected only once at the Echelle grating passes through the beam splitter, but the efficiency of the double reflection is reduced by at most 10%, which is accepted in view of the bandwidth narrowing which is improved by the double reflection ,

The invention is based on the technical problem of providing a bandwidth narrowing module of the type mentioned at the outset which, while maintaining the optical properties of the module, in particular a high degree of polarization for an associated laser, an increased efficiency, reduced thermal loads and an increased service life of the Reflection grating allows.

The invention solves this problem by providing a bandwidth narrowing module with the features of claim 1. This

Module contains polarization means for setting an preferred polarization for light incident on the reflection grating. This polar sation means thus cause that the light striking the reflection grating is essentially s-polarized. As usual, here s or TE polarization denotes a polarization perpendicular to the plane of incidence and output of the light, while the perpendicular polarization is designated as p or TM polarization.

Theoretical calculations and practical observations show that the light absorption of reflection gratings, such as in particular Echelle gratings of the type used here, without further measures, such as a protective layer for s-polarized light, i.e. in an Echelle grating for light polarized parallel to the grating furrows, is significantly lower than for p-polarized light, as it emerges from the laser resonator due to the strongly polarization-dependent transmission of the beam expansion system used here. At the same time and as a direct consequence, a significantly higher diffraction efficiency is shown for s-polarized light e.g. for high diffraction orders of Echelle gratings in Littrow configuration compared to p-polarized light. Overall, the use of the said s-polarizing polarization means consequently results in an improved bandwidth narrowing due to the reduction in the thermal loads on the reflection grating, a concomitant increase in its service life and an improvement in the efficiency of the overall bandwidth narrowing module.

In a further development of the invention according to claim 2, which in particular avoids the need to rotate the radiation emerging from the laser resonator as a whole, the polarization means simply comprise a λ / 2 plate which 90 ° the direction of polarization of the p-polarized light coming from an amplification medium ° rotates so that the light is essentially s-polarized. At the same time, the λ / 2 plate converts the s-polarized light reflected by the reflection grating back into p-polarized light before it re-enters the gain medium. A module developed according to claim 3 includes a beam expansion unit with one or more successive beam expansion elements. The polarization means are arranged at any point between the foremost beam expansion element in the light beam path and the reflection grating.

In a further embodiment of this measure, at least two beam expansion elements are provided, and the polarization means are located somewhere between the first and last beam expansion elements in the direction of light incidence, e.g. in the case of three beam expansion elements, between the second and third beam expansion elements. With this positioning, the polarization means are consequently located in the partially expanded light beam with the result that on the one hand their spatial dimension can be kept smaller than the cross section of the fully expanded light beam and on the other hand the power density of the light beam and thus the thermal load for the polarization means relative to the area of the not yet expanded light beam is significantly reduced.

In a further embodiment, the beam expansion element or elements arranged behind the polarization means are provided with an anti-reflective coating which is optimized for s-polarized light. At the same time, the high reflection losses for s-polarized light components on the beam expansion element or elements located in front of the polarization means ensure a high degree of polarization, e.g. in a laser resonator reflected back, bandwidth narrowed light.

An advantageous embodiment of the invention is illustrated in the drawings and is described below. Here show: 1 is a schematic side view of a laser resonator consisting of bandwidth narrowing module, gain medium and coupling-out mirror,

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 lower absorption of the Echelle grating that can be used in the module of FIG. 1 for s-polarized light compared to p-polarized light.

The bandwidth narrowing module shown in Fig. 1 provides the rear resonator termination of a laser, e.g. of 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 in the light beam path, one behind the other after an amplification or gas discharge chamber 1, comprises a beam expansion unit with three successively arranged, suitably arranged prisms 3a, 3b, 3c, a mirror 4, not shown, which is conventionally arranged, and an echelle Grid 5 over-the-counter design in Littrow configuration. Furthermore, the bandwidth narrowing module includes a λ / 2 plate 6 arranged in the light incidence direction between the second prism 3b and the third prism 3c.

Since the cross section of the laser beam coming from the gas discharge chamber 1 is not a rotational symmetry, but rather an elongated Nes rectangular profile, there is a preferred orientation with respect to the alignment of the grating and beam expansion and with respect to the inclination 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 lattice 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 for minimizing losses due to reflection. If this polarization, which is favorable for chamber and 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 expansion through the first prism 3a and the second prism 3b, after which it is rotated by 90 ° in its polarization, ie from p- is converted into s-polarized light. The light which is largely s-polarized in this way is then expanded by the last prism 3c 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 light with s-preferential polarization which is reflected back by the Echelle grating 5 then passes via the mirror 4 and the third prism 3c to the λ / 2 plate 6, from which it is converted back into light with p-polarization, after which the essentially p- polarized light, which is narrowed in bandwidth by the action of the Echelle grating 5, is coupled back into the discharge chamber 1 via the second prism 3b and the first prism 3a for the purpose of amplification. At the front of the discharge chamber 1, a corresponding laser beam 9 emerges via an exit surface 7, which is also inclined, whereby it is routed via a conventional output-side coupler 9.

The placement of the λ / 2 plate 6 in the partially widened light beam between the second prism 3b and the third prism 3c has the advantage over 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, this positioning can prevent the λ / 2 plate 6 from being arranged between the third prism 3c and the Echelle grating 5, i. 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 2525 mm with square dimensions.

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

The third prism 3c is preferably provided with an anti-reflective coating, which is not explicitly shown, and 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, essentially each s-polarized light, which - o -

ches through this third prism 3c on the way there and back. This minimizes the loss of light through the third prism 3c. On the other hand, the two upstream prisms 3a, 3b 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 3a, 3b reflect any interfering s-polarized light components, which 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 be coupled into the amplification chamber 1, which ensures that a high degree of polarization of the radiation 9 emitted by the laser is achieved.

In the above-mentioned alternative positions of the λ / 2 plate 6, the prism or prisms lying 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 lying between the λ / 2 plate 6 and the discharge chamber 1 take on the task of masking out any s-polarized radiation components by reflecting away.

As described, the use of the λ / 2 plate 6 causes the light with s preferential 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 falls on the reflection grating, which is 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 on the other hand 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 grating, as is used for operation in a bandwidth Constriction module of the type shown in Fig. 1 is suitable. It is assumed that an Al-Echelle grating with d = 7648nm for a laser light wavelength of λ = 248.4nm in a configuration with a large Littrow angle of 77 ° for blaze profiles with an apex angle of 90 ° in the - 60th Diffraction order is used. Such high diffraction orders are used, for example, in Echelle gratings with a large Littrow angle within bandwidth narrowing modules as the last propagating order. At large Littrow angles, the small facet of the triangular profile of the Echelle grating serves as a blaze flank.

2 shows the course of the diffraction efficiency η for the -60. Diffraction order depending on the blaze facet angle over an angle range of 74 ° to 84 ° on the one hand for s-polarized light, i.e. TE polarization, according to the upper characteristic with the circularly marked data points and on the other hand in comparison for p-polarized light, i.e. TM polarization, according to the lower characteristic with the triangularly marked data points. In this particular example, the diffraction efficiency for s-polarized light in the -60 can be seen for blaze facet angles above 77 °. 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 is essentially s-polarized and strikes the Echelle grating 5 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. 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 on the Echelle grating 5 essentially s-polarized. In further alternative embodiments, different types of beam expansion units of conventional type can be used instead of the shown, e.g. those with only two or those with more than three prisms and / or those with other optical components, e.g. Lenses, in addition to or instead of the respective prism.

Furthermore, it goes without saying that the bandwidth narrowing module can be used not only for a laser, as shown, but also for other light sources which have the task of suitably narrowing the bandwidth of an emitted light beam. Not only Echelle gratings can be used as reflection gratings, but depending on the application also other conventional reflection grating types can be used depending on the respective application.

Claims

claims

1. bandwidth narrowing module for a light source, in particular a laser, with a reflection grating (5), characterized 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 polarization means include a λ / 2 plate (6) which is arranged between an amplification medium (1) of the light source and the reflection grating (5).

3. The 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 (3a, 3b, 3c) is provided and the polarization means (6) in the area between the foremost beam expansion element (3a). and the reflection grating (5) are arranged.

4. bandwidth narrowing module according to claim 3, further characterized in that the polarization means (6) are arranged in the area behind a foremost and in front of a rearmost of a plurality of beam expansion elements (3a, 3b, 3c).

5. The bandwidth narrowing module according to claim 4, further characterized in that the beam expansion element or elements (3c) located between the polarization means (6) and the reflection grating (5) are provided with an anti-reflective coating optimized for s-polarization.