Stephan Müllender

Short period La/B and LaN/B multilayer mirrors for ~6.8 nm wavelength.

In the first part of this article we experimentally show that contrast between the very thin layers of La and B enables close to theoretical reflectance. The reflectivity at 6.8 nm wavelength was measured from La/B multilayer mirrors with period thicknesses ranging from 3.5 to 7.2 nm at the appropriate angle for constructive interference. The difference between the measured reflectance and the reflectance calculated for a perfect multilayer structure decreases with increasing multilayer period. The reflectance of the multilayer with the largest period approaches the theoretical value, showing that the optical contrast between the very thin layers of these structures allows to experimentally access close to theoretical reflectance. In the second part of the article we discuss the structure of La/B and LaN/B multilayers. This set of multilayers is probed by hard X-rays (λ = 0.154 nm) and EUV radiation (λ = 6.8 nm). The structure is reconstructed based on a simultaneous fit of the grazing incidence hard X-ray reflectivity and the EUV reflectivity curves. The reflectivity analysis of the La/B and LaN/B multilayer mirrors shows that the lower reflectance of La/B mirrors compared to LaN/B mirrors can be explained by the presence of 5% of La atoms in the B layer and 63% of B in La layer. After multi-parametrical optimization of the LaN/B system, including the nitridation of La, the highest near normal incidence reflectivity of 57.3% at 6.6 nm wavelength has been measured from a multilayer mirror, containing 175 bi-layers. This is the highest value reported so far.

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.

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

Improved contrast and reflectivity of multilayer reflective optics for wavelengths beyond the extreme UV

We present a computational and experimental study on interface passivation of B4C/La multilayers for photolithography at wavelengths beyond 13.5 nm. We successfully applied N-plasma treatment to form interface-localized BN and LaN layers, preventing LaB6 and LaC2 interlayer formation and increasing the optical contrast. Experiments suggest an improvement of absolute reflection by up to 20% for 200 period multilayers, with a best-so-far result of 41.5 % at near-normal incidence of 6.7 nm.

Coatings for next generation lithography

The latest generation of 193nm immersion lithography optics, with a numerical aperture (NA) of 1.35 and ultra pure water as immersion fluid serves the 45nm node on the ITRS roadmap. The potential solutions for the next step, the 32nm node, as presented in December 2007 by the ITRS are: 193nm double patterning / exposure, 193nm with 2nd generation fluid and EUVL. The performance of such next generation lithography optics is increasingly driven by the coating performance. For 193nm the performance of the antireflection and high reflection coatings is driven by the increasing NA, which requires the control of polarisation effects and transmission uniformity over light incidence angles. For EUV only high reflection coatings are needed and the NA is comparatively small. But the performance is limited by higher absorption and lower refractive index contrasts of the applicable coating materials at 13.5nm with respect to 193nm. In this talk we discuss and compare the different requirements and challenges in coating material, design, process, lifetime and accuracy for next generation lithography optics.

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.

Characterization of large off-axis EUV mirrors with high accuracy reflectometry at PTB

CZ SMT AG produced large off-axis EUV mirrors as they are used e.g. in ASMLs alpha demo tools, the predecessor for Extreme Ultraviolet Lithography (EUVL) production tools by ASML. The coating development and a large part of the actual coatings were done by the FOM-Institute. The Physikalisch-Technische Bundesanstalt (PTB) operates an EUV reflectometry facility at the electron storage ring BESSY II for at-wavelength metrology of full-size EUVL optics with a weight of up to 50 kg and a diameter of 550 mm. Critical issues for EUVL mirrors are the high reflectivity close to the theoretical limit, the matching of the period to the operating wavelength of the stepper (13.5 nm) and the imaging properties of the EUV optics. The full multilayer stack needs to be controlled laterally to such extend that the initial sub-nanometre surface figure of the substrate is preserved. The so-called added figure error should not exceed 100 pm in order to ensure faultless imaging at 13.5 nm wavelength. Here, we discuss representative results obtained at large off-axis EUV mirrors. We especially discuss the challenges of measurements at higher local angles of incidence according to the optical design and the accuracy needed in sample alignment for measurement of the coating profiles. PTB has shown excellent reproducibility for measurements of the near normal incidence reflectance of flat homogeneous mirrors over several years. For large off-axis EUV mirrors, measurements have to be done at angles significantly off normal, which dramatically increases the influence of angular alignment errors of the sample on the measured peak wavelength. Furthermore, according to the optical design, these optics have gradients of the coating thickness which require exact knowledge of the measurement position in the mirror coordinates. Extensive studies were done to estimate and validate the uncertainties connected to the sample alignment. Our results clearly show that it is possible to meet and verify the tight specifications for the lateral coating profiles of EUV multilayer mirrors. The non-correctable added figure error is significantly better than required and the overall reflectance of the coatings with a special protective capping layer is 65%.

High reflectance multilayers for EUVL HVM-projection optics

Reported is a summary of multilayer deposition results by FOM on three elements of the projection optics of the ASML Extreme UV Lithography HVM tools. 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 68%, with a maximum value of 69.6% and a non-correctable figure error added by the full multilayer stack of better than 35 picometer. The results are compared to the earlier coatings of the EUVL Process Development Tool.

Enhanced performance of EUV multilayer coatings

Reported is a summary of the development of EUV Mo/Si multilayer coating technology. Though the results are developed for application in Extreme Ultraviolet Lithography, they are of a broader relevance including optics for astronomy. The coating process used consists of electron beam evaporation in combination with low energy ion beam smoothening. The radiation hardness of these coatings is discussed and methods to reduce the multilayer induced substrate stress. The reflectance of the coatings, which are covered with a special protective capping layer, is typically around 65%, while the non correctable figure error added by the full multilayer stack is controlled to better than 15 picometer.

Smoothing properties of single and multilayer coatings: a method to smoothen substrates

In this work we present the smoothing properties of our ion beam smoothened multilayers and, based on the same technology, the extreme smoothing properties in the high and near mid spatial frequency range of a single-material smoothing layer. Coating results of high reflectance multilayers both on rough substrates and on substrates smoothened with silicon bufferlayers are discussed.