A. Kloidt

Enhancement of the reflectivity of Mo/Si multilayer x-ray mirrors by thermal treatment

Thermal treatment of a Mo/Si multilayer stack enhances its reflectivity in the soft x‐ray region. The multilayer x‐ray mirrors are fabricated by electron beam evaporation in ultrahigh vacuum. In situ measurement of the reflectivity during the deposition allows thickness control and an observation of changes in quality of the boundaries. By heating the substrates during deposition we obtain a smoothing of the interfaces. This leads to x‐ray mirrors with peak reflectivity around 50% for normal incident radiation of wavelengths between 130 and 140 A.

Thermal stability of Mo/Si multilayer soft-X-ray mirrors fabricated by electron-beam evaporation

Mo/Si multilayers are fabricated by electron-beam evaporation in UHV at different temperatures (30° C, 150° C, 200° C) during deposition. After completion their thermal stability is tested by baking them at temperatures (Tbak) between 200° C and 800° C in steps of 50° C or 100° C. After each baking step the multilayers are characterized by small angle CuKα-X-ray diffraction. Additionally, the normal incidence soft-X-ray reflectivity for wavelengths between 11 nm and 19 nm is determined after baking at 500° C. Furthermore, the layer structure of the multilayers is investigated by means of Rutherford Backscattering Spectroscopy (RBS) and sputter/Auger Electron Spectroscopy (AES) technique. While the reflectivity turns out to be highest for a deposition temperature of 150° C, the thermal stability of the multilayer increases with deposition temperature. The multilayer deposited at 200° C stands even a 20 min 500° C baking without considerable changes in the reflectivity behaviour.

Electron-beam-deposited Mo/Si and MoxSiy/Si multilayer x-ray mirrors and gratings

For the wavelength region above the Si- L edge normal incidence, soft x-ray mirrors are produced with peak reflectivities close to 60%. The multilayer systems consist of molybdenum and silicon and are fabricated by electron beam evaporation in ultrahigh vacuum. A smoothing of the boundaries, and thereby a drastic enhancement of the reflectivity, is obtained by thermal treatment of the multilayer systems during growth. The thermal stability of the multilayer stacks could be improved considerably up to 850° C by mixing Mo and Si in the absorber layers and producing thus Mo x Si y /Si multilayers with x and y denoting the amounts of Mo and Si in the absorber layer, respectively. First attempts are reported to produce mirrors with a bilayer thickness of 2.6 nm. An improvement in the quality of these interfaces can be obtained by bombardment with Ar + ions. We report on normal incidence reflectivity measurements of the mirrors with synchrotron radiation and finally on the normal incidence diffraction efficiencies of a Mo/Si multilayer coated grating, for which values of 5.5% are achieved for the + 1st and - 1st diffraction orders.

Smoothing of interfaces in ultrathin Mo/Si multilayers by ion bombardment

Mo/Si multilayers with a bilayer thickness of 2.6 nm are produced by electron beam evaporation in ultrahigh vacuum for soft X-ray optical applications. High reflectivities resulting from constructive interference in the stack are limited by the optical constants of the materials and by the quality of the interfaces. Smoothing of the boundaries is obtained by bombardment of the deposited layers with Ar+ ions. The smoothness of the interfaces is controlled during the deposition by in situ measurement of the reflectivity for the C K radiation of the stack and after completion of the stack by means of a grazing X-ray reflection set-up with Cu Kα radiation. The soft X-ray reflectivity is measured with a laser-induced plasma light source.

Mo0.5Si0.5/Si multilayer soft x‐ray mirrors, high thermal stability, and normal incidence reflectivity

Multilayer soft x‐ray mirrors with an absorber consisting of the mixture Mo0.5Si0.5 have been fabricated by electron‐beam evaporation in UHV. This has been done to get soft x‐ray normal incidence mirrors for 80–100 eV photon energy with enhanced thermal stability and still high reflectivity. The thermal stability is studied by baking them at temperatures between 600 and 950 °C. The results were compared with multilayers of pure Mo and Si, which were also fabricated by electron‐beam evaporation. After each baking step the x‐ray mirrors are characterized by small angle CuKα x‐ray diffraction. The reflectivity of the first‐order Bragg peak is nearly constant up to 20 min baking at 900 °C. Further we present the normal incidence soft x‐ray reflectivity for wavelengths between 12 and 18 nm of a Mo0.5Si0.5/Si mirror with 12 double layers (N=12) and of a Mo0.5Si0.5/Si mirror as deposited with 33 double layers (N=33). With the latter a reflectivity of 46% is achieved.

Fabrication and characterization of Si-based soft x-ray mirrors

The fabrication, by electron beam evaporation, of Mo/Si and Ta/Si mutilayers designed as soft-X-ray mirrors is described. The mirrors were characterized using surface analytical methods (RBS and sputtering in combination with AES), Cu-k(alpha) reflection, and soft-X-ray optical methods, and their soft-X-ray optical properties were correlated with microstructural characteristics. A comparison of in situ C-k reflectivity curves with calculations disclosed the existence of roughnesses at the interfaces, which can not be completely described by multiplying the reflected amplitude at each interface by a Debye-Waller factor. It was found that heating of Ta/Si samples induces a considerable change (up to and above 10 percent) in the d-spacing of multilayers, while the reflected amplitude is only reduced to two thirds of its original value.

Fabrication, thermal stability, and reflectivity measurements of Mo/Si multilayers as x-ray mirrors and other optical components

For the wavelength region above the Si-L edge normal incidence soft X-ray mirrors are produced with peak reflectivities around 55 percent. The Mo/Si multilayer systems are fabricated by electron beam evaporation in ultrahigh vacuum. Analysis of the quality of the stack is made by using an in situ monitoring system measuring the reflection of the C-K line and ex situ grazing X-ray reflection of the Cu-K-alpha line. A smoothing of the boundaries and thereby a drastic enhancement of the reflectivity can be obtained by thermal treatment of the multilayer system during growth. The microstructure of the multilayer systems is investigated by means of Rutherford Backscattering spectroscopy and Sputter/AES technique. Baking the final stack after deposition up to 900 C is applied to study the thermal stability of the soft X-ray mirror. Near normal incidence mirrors even for short wavelengths, e.g., the water window (2.4 - 4.4 nm), are produced with a Mo/Si bilayer thickness of 2.6 nm. An improvement in the quality of the interfaces for such ultrathin multilayer systems can be obtained by bombardment of the deposited layers with Ar(+) ions as well as by thermal treatment of the multilayer system and mixing of Mo and Si in the absorber layer during the deposition run. We report on reflectivity measurements of the mirrors and their behavior as polarizers and analyzers and on the diffraction efficiencies of laterally structured multilayer systems as gratings.

Large-area soft x-ray projection lithography using multilayer mirrors structured by RIE

SXPL (soft X-ray projection lithography) is one of the most promising applications of X-ray reflecting optics using multilayer mirrors. Within our collaboration, such multilayer mirrors were fabricated, characterized, laterally structured and then used as reflection masks in a projecting lithography procedure. Mo/Si-multilayer mirrors were produced by electron beam evaporation in UHV under thermal treatment with an in-situ X-ray controlled thickness in the region of 2d equals 14 nm. The reflectivities measured at normal incidence reached up to 54%. Various surface analysis techniques have been applied in order to characterize and optimize the X-ray mirrors. The multilayers were patterned by reactive ion etching (RIE) with CF4, using a photoresist as the etch mask, thus producing X-ray reflection masks. The masks were tested in the synchrotron radiation laboratory of the electron accelerator ELSA at the Physikalisches Institut of Bonn University. A double crystal X-ray monochromator was modified so as to allow about 0.5 cm2 of the reflection mask to be illuminated by white synchrotron radiation. The reflected patterns were projected (with an energy of 100 eV) onto the resist (Hoechst AZ PF 514), which was mounted at an average distance of about 7 mm. In the first test-experiments, structure sizes down to 8 micrometers were nicely reproduced over the whole of the exposed area. Smaller structures were distorted by Fresnel-diffraction. The theoretically calculated diffraction images agree very well with the observed images.