E. Louis

Optical element for full spectral purity from IR-generated EUV light sources

Laser produced plasma (LLP) sources are generally considered attractive for high power EUV production in next generation lithography equipment. Such plasmas are most efficiently excited by the relatively long, infrared wavelengths of CO2-lasers, but a significant part of the rotational-vibrational excitation lines of the CO2 radiation will be backscattered by the plasmas critical density surface and consequently will be present as parasitic radiation in the spectrum of such sources. Since most optical elements in the EUV collecting and imaging train have a high reflection coefficient for IR radiation, undesirable heating phenomena at the resist level are likely to occur. In this study a completely new principle is employed to obtain full separation of EUV and IR radiation from the source by a single optical component. While the application of a transmission filter would come at the expense of EUV throughput, this technique potentially enables wavelength separation without loosing reflectance compared to a conventional Mo/Si multilayer coated element. As a result this method provides full spectral purity from the source without loss in EUV throughput. Detailed calculations on the principal of functioning are presented.

Spectral-purity-enhancing layer for multilayer mirrors

We demonstrate, both theoretically and experimentally, that special spectral-purity-enhancing multilayer mirror systems can be designed and fabricated to substantially reduce the level of out-of-band radiation expected in an extreme ultraviolet lithographic tool. A first proof of principle of applying such spectral-purity-enhancement layers showed reduced out-of-band reflectance by a factor of five, while the in-band reflectance is only 4.5% (absolute) less than for a standard capped multilayer.

Wavelength separation from extreme ultraviolet mirrors using phaseshift reflection

A generic design and fabrication scheme of Mo/Si multilayer-grating phaseshift reflector systems is reported. Close to optimized extreme ultraviolet (EUV, λ=13.5 nm) reflectance values up to 64% are demonstrated, while the diffractive properties can be exploited in spectral filtering applications. The results can contribute to a wavelength-unspecific solution for the suppression of λ>100 nm out-of-band radiation in EUV lithography.

Enhanced reflectance of interface engineered Mo/Si multilayers produced by thermal particle deposition

A new deposition technique that builds on the thermal particle characteristics typical for e-beam deposition is described. This technique applies magnetron sputtering in a special scheme where these characteristics of the e-beam deposition method are achieved. The method was used for interface engineering of Mo/Si multilayers, with different barrier layer materials being tested. Composition of the barrier layers formed was studied using XPS. Results are shown on the general example of a Mo/B4C/Si/B4C system. The ultra-thin reflectance enhancement B4C barriers can be deposited with low added stress, resulting in a multilayer stress as low as about -150 MPa. The best interface engineered multilayers reflect 70.5% at 13.3 nm and 70.15% at 13.5 nm. These results were achieved with 50 period multilayers terminated with a standard Si layer.

Infrared diffractive filtering for extreme ultraviolet multilayer Bragg reflectors

We report on the development of a hybrid mirror realized by integrating an EUV-reflecting multilayer coating with a lamellar grating substrate. This hybrid mirror acts as an efficient Bragg reflector for extreme ultraviolet (EUV) radiation at a given wavelength while simultaneously providing spectral-selective suppression of the specular reflectance for unwanted longer-wavelength radiation due to the grating phase-shift resonance. The test structures, designed to suppress infrared (IR) radiation, were fabricated by masked deposition of a Si grating substrate followed by coating of the grating with a Mo/Si multilayer. To give the proof of principle, we developed such a hybrid mirror for the specific case of reflecting 13.5 nm radiation while suppressing 10 μm light, resulting in 61% reflectance at the wavelength of 13.5 nm together with the 70 × suppression rate of the specular reflection at the wavelength of 10 μm, but the considered filtering principle can be used for a variety of applications that are based on utilization of broadband radiation sources.

Surface morphology of Kr+-polished amorphous Si layers

The surface morphology of low-energy Kr+-polished amorphous Si layers is studied by topographical methods as a function of initial substrate roughness. An analysis in terms of power spectral densities reveals that for spatial frequencies 2×10−2–2×10−3 nm−1, the layers that are deposited and subsequently ion polished reduce the initial substrate roughness to a rms value of 0.1 nm at the surface. In this system, the observed dominant term in linear surface relaxation, proportional to the spatial frequency, is likely to be caused by the combined processes of (a) ion-induced viscous flow and (b) annihilation of (subsurface) free volume during the ion-polishing treatment. Correspondingly, a modification of the generally assumed boundary conditions, which imply strict surface confinement of the ion-induced viscous flow mechanism, is proposed. Data on surface morphology are in agreement with the optical response in extreme ultraviolet from a full Mo/Si multilayered system deposited onto the modified substrates

Multilayer optics with spectral purity layers for the EUV wavelength range

Reported are the first calculations and experimental results of the deposition of EUV multilayer coatings that actively suppress the reflectance in the VUV wavelength range. In the undesired 100-200 nm band a factor of five reduction was achieved for one single optical element, while only a minor loss of 4.5% reflectance for λ = 13.5 nm, the operating wavelength of EUVL, was found.

Multilayer mirror with enhanced spectral selectivity for the next generation extreme ultraviolet lithography

We have demonstrated a hybrid extreme ultraviolet (EUV) multilayer mirror for 6.x nm radiation that provides selective suppression for infrared (IR) radiation. The mirror consists of an IR-transparent LaN∕B multilayer stack which is used as EUV-reflective coating and antireflective (AR) coating to suppress IR. The AR coating can be optimized to suppress CO2 laser radiation at the wavelength of 10.6 μm, which is of interest for application in next-generation EUV lithography systems.

Damage studies of multilayer optics for XUV free electron lasers

We exposed standard Mo/Si multilayer coatings, optimized for 13.5 nm radiation to the intense femtosecond XUV radiation at the FLASH free electron laser facility at intensities below and above the multilayer ablation threshold. The interaction process was studied in-situ with reflectometry and time resolved optical microscopy, and ex-situ with optical microscopy (Nomarski), atomic force microscopy and high resolution transmission electron microscopy. From analysis of the size of the observed craters as a function of the pulse energy the threshold for irreversible damage of the multilayer could be determined to be 45 mJ/cm2. The damage occurs on a longer time scale than the XUV pulse and even above the damage threshold XUV reflectance has been observed showing no measurable loss up to a power density of 1013 W/cm2. A first explanation of the physics mechanism leading to damage is given.

Multilayer coatings for the EUVL Process Development Tool

Reported is a summary of the coating of three elements of the illuminator and three of the projection optics of the EUVL Process Development Tool. The coating process used is e-beam evaporation in combination with low energy ion beam smoothening. The reflectance of the coatings, which are covered with a special protective capping layer, is typically around 65% and the non correctable figure error that is added by the full multilayer stack is controlled to better than 15 picometer.