G.E. van Dorssen

Enhancement of reflectivity of multilayer mirrors for soft x-ray projection lithography by temperature optimization and ion bombardment

Abstract In this paper we discuss two techniques to optimize the quality of multilayer x-ray mirrors, namely optimization of the temperature of the substrates during deposition and ion-bombardment of the layers. We produced Mo/Si multilayers applying both methods and present the effect on the near normal incidence reflectivity for λ = 13–14 nm radiation. Furthermore an analysis of the homogeneity of the deposited layers is given.

Synthesis and measurement of normal incidence X-ray multilayer mirrors optimized for a photon energy of 390 eV

Abstract The problems inherent in the fabrication of short period multilayer mirrors are discussed and results of the synthesis of multilayer structures with nanometer period are presented. The shortest period observed is 13 A for WSi and WB 4 C sputtered multilayers. Measurements of near normal incidence reflectivity at λ = 31–32A are presented for WSc multilayers with a period approximately 16 A. The measured reflectivity reaches a maximum of 3.3% and is in good agreement with theoretical modeling after the inclusion of interfacial roughness.

Grazing incidence reflectivity and total electron yield effects in soft x-ray absorption spectroscopy

We report on a study of grazing incidence absorption and reflection spectra of NiO in the region of the Ni 2p edge. The aim is to evaluate the distortion of the near edge spectrum by the critical angle behavior of individual components within the spectrum. This can be used to improve the separation of multiplets and enhance low spectral weight line shapes like charge transfer satellites. The measured spectra have been compared with calculations using an optical model.

EXAFS measurements on the structure of Mo/Si multilayers produced using ion bombardment and increased deposition temperature

Abstract This study focuses on explaining differences in soft X-ray reflectivity observed for Mo/Si multilayers produced by e-beam evaporation with two different additional methods: ion etching of the Si layer and optimization of the substrate temperature during deposition. A 32-period multilayer made with ion etching has a near normal incidence reflectivity of 50%, while the one made at an optimum deposition temperature shows only 41%. The values for the interface roughness or layer thickness errors obtained from the analysis of small angle reflectivity measurements cannot explain this difference, but a different morphology of the Si and Mo layers could. Extended X-ray absorption fine structure (EXAFS) experiments have been carried out at the SiK and the MoK edge to obtain information on the multilayer structure and chemical composition. The results of these EXAFS measurements on the two types of multilayers indicate that both the Si and the Mo have an amorphous structure and that no MoSi compounds are formed at the interfaces. However, it is found that multilayers made with ion etching have a reduced fraction of oxygen (≤ 0.5%) in the Si layer compared to layers produced without etching (4.7%). The latter fraction of oxygen, orginating from the background gas in the deposition system, results in higher absorption of soft X-ray radiation and therefore affects the near normal incidence reflectivity. Although both the Si and Mo layers produced with the three different deposition methods are amorphous, some structural differences have been found using the EXAFS analysis.

Silicon oxide structures measured at the surfaces of silicon and silicate glass: a reflectivity study

Abstract Using a newly developed reflectometer we have measured the reflectivity of crystalline silicon and SiO x coated float glass at the silicon K absorption edge. The reflectometer can be used without the need for ultra high vacuum. The critical angle of total reflection was determined and the reflection as function of the energy was measured below the critical angle. EXAFS analysis of the data reveals the oxygen content in the samples, and the local structure around the silicon absorbers. The measurements show the strength of ReflEXAFS as a technique to study surfaces under “real” conditions.

Core excitons in silicon and silicon oxides

Using a newly developed reflectometer we have measured the reflectivity at the silicon [ital K] edge for different silicon compounds. Combining these measurements with silicon [ital L] edge reflectivity measurements carried out on a different reflectometer we determined the binding energy of core excitons at the [ital L] edge. The results show that it is possible to carry out reflectivity measurements with the resolution necessary for the determination of the exciton parameters. Comparison with literature values shows that a rigid Kramers--Kronig analysis is not needed when the angle of incidence is well below the critical angle of total external reflection.

Characterization of the resolving power of a double multilayer monochromator in the energy range of 600–900 eV

Two different pairs of x‐ray reflecting multilayer coatings have been tested in a double crystal monochromator configuration. The two pairs consisted of Ni (0.4 nm)–C (1.85 nm) and NiSi (0.4 nm)–C (1.85 nm), both with 225 periods. In order to determine the resolution in the energy range of 600–900 eV, the photoelectron yield of several 3d transition metal compounds was recorded as well as the throughput around the Ni L lines. We found for both pairs a resolving power of 220. Simulations taking into account the angular divergence of the x‐ray beam yield a resolution of 250. The difference with the observed value can be due to the nonhomogeneity of the multilayer coating.

Laser Plasmas As X-Ray Sources For Lithographic Imaging Of Submicron Structures

Laser radiation can be used efficiently to generate X rays for lithographic imaging of submicron patterns, e.g. for VLSI device fabrication. Due to their short wavelength and high average power, excimer lasers show much potential for this application. Results are presented of scaling studies for high repetition rate excimer laser application, using the frequency doubled output of a low repetition rate Nd:YAG/Glass laser. Spectral and spatial characteristics of X-ray emission of the laser plasma are shown. The power density in the laser focus was 3x1012 W/cm2. With this source Si X-ray masks with submicron Au absorber profiles are imaged into high sensitivity X-ray photoresist. For the exposures 80 lasershots sufficed to yield high quality submicron structures. Extrapolation of the results to a high power excimer laser reduces the exposure time of the photoresists to several seconds, enabling a wafer throughput at an industrial level.

Figured Multilayer Structures Applied to a Laser Produced Plasma

A prime application of figured multilayer structures as X-ray optics is the combination with a laser produced plasma. As a source of soft X-rays, a laser-plasma has proven its attractive properties: a high instantaneous brightness, good shot-to-shot stability and nearly pointlike dimensions. The combination of laser-plasma and X-ray optics would give improved performance in e.g. plasma diagnostics, rapid detection of low Z elements in solid samples, pulsed laser irradiated materials and lithographic imaging of submicron patterns. In the latter application, a parabolic multilayer structure has the potential to convert the emission of the point source into a large parallel and homogeneous beam to prevent several distortions in the lithographic imaging.