TECHNICAL FIELD

[0001] The present invention relates in general to an aperture stop assembly, and more particularly, to an aperture stop which is capable to handle high power laser beams without changing its shape or dimension.

BACKGROUND OF THE INVENTION

[0002] With ordinary light sources, light emitted by the source is usually of low energy. In optical lithography or metrology an excimer laser may be used as a light source, and typically the radiation output firam the laser is passed a radiation beam- scrambling illuminator to distribute the light intensity uniform over a particular area, e.g. an SLM area. The energy from high power laser beam sources may be too high to use a simple blocking aperture device in order to set the beam shape. Such simple blocking aperture may be heated up and for that reason change its dimension and/or form. Another problem with a simple blocking aperture is that, due to the heating up effect from the high power laser beam, material from the aperture stop may start to ablate and redeposit onto other components of the optical system in which the aperture stop may be arranged. Other prior art aperture stops may often be a bulky construction comprising of several separate parts, which is a problem. [0003] Hence, there is a need in the art for a compact yet effective solution that is capable to handle the power that is cut off from a laser beam.

SUMMARY OF THE INVENTION

[0004] Accordingly it is an object of the present invention to provide an aperture stop assembly for high power laser beams, which overcomes or at least reduces at least one of the above mentioned problems. [0005] This object, among others, is according to a first aspect of the invention attained by an aperture stop assembly for use in an electromagnetic radiation system, comprising: an aperture stop made of an at least partly transparent material to an applied electromagnetic radiation, at least a part of said aperture stop is formed at its border to an aperture to deflect the electromagnetic radiation into said at least partly transparent material. [0006] A further object of the invention is to provide a method for shaping high power electromagnetic radiation beams, which overcomes or at least reduces one of the above mentioned problems. [0007] According to a second aspect of the invention there is provided a method for blocking at least a part of a beam of electromagnetic radiation, comprising the actions of: generating abeam of electromagnetic radiation, impinging said beam of electromagnetic radiation onto a beam aperture assembly, blocking at least a portion of said beam of electromagnetic radiation in a material which is at least partly transparent to the applied electromagnetic radiation, defining an aperture stop , by deflecting said portion through at least one total internal reflection. [0008] Other aspects of the present invention are reflected in the detailed description, figures and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 depicts a side view of a laser according to prior art which, may benefit from the present invention. [0010] Figure 2 depicts a cross-section of an example embodiment of the inventive aperture stop assembly. [0011] Figure 3 depicts a top view of a simple blocking aperture according to prior art.

DETAILED DESCRIPTION

[0012] The following detailed description is made with reference to the ft gures. 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. [0013] In the following detailed description reference is made to optically transparent material. It should be understood that the following description shiould not be limited to only optically transparent material. Those of ordinary skill in the art will recognize that the material chosen is dependent upon the applied electromagn.etic radiation, i.e., the material to be used should at least be partly transparent to the applied electromagnetic radiation. [0014] Fig 3 illustrates a top view of a simple blocking aperture 300 according to prior art. Said simple blocking aperture comprising an aperture 310 and an aperture stop 320. The aperture stop 320 is made of an opaque material to the applied electromagnetic radiation, for instance a metal. The aperture 300 is here illustrated to be rectangular. The rectangular aperture may form a coherent radiation beam originating from a source of radiation, which may be an excimer laser. [0015] Figure 2 illustrates a cross section of an example embodiment of an aperture stop assembly 200 according to the present invention. Said aperture stop assembly 200 comprises an optically transparent material 230, aperture stop 210, a beam dump 220, an aperture 260, a housing 270. [0016] The aperture stop assembly 200 as illustrated in figure 2 may work as follows. A laser beam 240, 250a, 250b coming from the right in figure 2 may impinge onto the aperture stop assembly 200. An inner part of said laser beam 240 may pass the aperture 260 undisturbed. An outer part of said laser beam 250a, 250b may hit a prism formed aperture stop 210. Said prism formed aperture stop 210 may be made of an optically transparent material, for instance glass, quarts or fused silica or any other optically transparent material. The material chosen depends on the wavelength used in the electromagnetic radiation beam to be shaped or filtered by the aperture stop assembly 200. An angle of the prism formed aperture stop 210 may be chosen so that there may be total internal reflection. Said angle may be 45 degrees. Any design of the aperture stop may be used which fulfills the requirement of total internal reflection. [0017] In a normal laser application, the thickness of the transparent material 230 may be 5-lOmm. A length perpendicular from the beam may be adopted so that the beam dump 220 may have enough space for it's cooling. The beam dump 220 and the rest of the housing 270 may be made of metal. A simple form of beam dump 220 is a razor-blade stack, but any beam dump 220 absorbing a high percentage of the beam energy may be used. The beam dump 220 may be cooled by any gas or liquid. The aperture 260 may take any form, one or two dimensional. Example embodiments of a two dimensional apertures 260 are circular, square, elliptic, star formed etc. Since the material chosen for the aperture stop 210 is optically transparent, there is very little absorption and thus only a small heat deposition in the aperture stop 210. For that reason said aperture stop 210 may maintain its form and shape very well. The light is not absorbed at the aperture stop 210. In figure 2, a part of said beam of radiation is trapped in a beam dump 220 where the excess energy is dissipated as heat. In order to reduce back reflections from the aperture stop Z lO, said aperture stop 210 may be at least partly coated with an anti reflective layer. The aperture stop assembly 200 could be used for any kind of electromagnetic radiation by a proper selection of materials. [0018] The radiation source may be, as mentioned above, an excimer lasex at any wavelength, such as UV, DUV, EUV etc. In particular this invention is suitable for use in pattern generators and metrology and inspection systems that use a uniform illumination by an excimer laser or other electromagnetic radiation sources with a defined shape. [0019] The inventive aperture stop assembly 200 could be used to shape any beam of electromagnetic radiation. The inventive aperture stop assembly could also be used as a Fourier aperture assembly. In such a case, one or a plurality of diffractions orders may pass a Fourier aperture 260 and one or a plurality of diffraction orders may be blocked by a Fourier aperture stop 210. [0020] Figure 1 illustrates a prior art transversally excited laser, for example an excimer laser. Said laser comprises a first mirror 110 and a second mirror 12O, 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 110, 120 is partially reflecting for allowing a beam of radiation created within the resonant cavity to be emitted. The other mirror may be totally reflecting. The housing may be transparent to the emitted wavelength at an end where said partially reflecting mirror is arranged. [0021] While the present invention is disclosed by reference to the 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. [0022] We claim as follows: