Abstract
We present the design of a novel collector mirror for laser produced plasma (LPP) light sources to be used in extreme ultraviolet (EUV) lithography. The design prevents undesired infrared (IR) drive laser light, reflected from the plasma, from reaching the exit of the light source. This results in a strong purification of the EUV light, while the reflected IR light becomes refocused into the plasma for enhancing the IR-to-EUV conversion. The dual advantage of EUV purification and conversion enhancement is achieved by incorporating an IR Fresnel zone plate pattern into the EUV reflective multilayer coating of the collector mirror. Calculations using Fresnel-Kirchhoff's diffraction theory for a typical collector design show that the IR light at the EUV exit is suppressed by four orders of magnitude. Simultaneously, 37% of the reflected IR light is refocused back the plasma.
Highlights
Electronic devices are becoming more powerful and more energy efficient, the key to this progress being the ongoing miniaturization in the semiconductor industry
The novel collector is based on a binary Fresnel zone pattern incorporated into the surface of a standard ellipsoidal collector
The design prevents undesired IR drive laser light, reflected from the plasma, from reaching to the exit of the light source. This results in a strong purification of the usable extreme ultraviolet (EUV) light
Summary
Electronic devices are becoming more powerful and more energy efficient, the key to this progress being the ongoing miniaturization in the semiconductor industry. In the secondary focus an aperture matching the size of the EUV beam forms the exit of the source towards the lithography optical system This standard collector approach can collect substantial EUV powers [3,4], there are two clear disadvantages. The second problem is that the reflected drive laser radiation is directed into the most undesired location, the intermediate focus, from where it can exit together with EUV radiation This is highly undesired because the high power of the IR radiation causes heating of the photoresist materials used in lithography process. Only the EUV radiation purified from the IR radiation leaves the exit while the reflected IR light is refocused into the plasma
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