CHARACTERISATION OF THE QUALITY OF SPUTTERED AND PCVD METAL/Si MULTILAYERS
M. Löhmann, J. Bläsing, F. Klabunde and T. P. Drüsedau
Otto-von-Guericke-University Magdeburg, Faculty of Natural Science, Department of Solid State Physics, Postfach 4120, D-39106 Magdeburg
Keywords: multilayer, X--ray reflectivity, diffuse scattering
Multilayer structures made of two materials, one with a relatively large value of the diffractive index decrement d and a second element with an relatively small value of d, are used as devices in UV and soft X-ray optics. The performance of the multilayer mirrors is limited by the ability to deposit layers of the correct thickness and low roughness and diffusion.
The presented work deals with a comparison of two different vacuum deposition techniques used for preparation of multilayers. The multilayers investigated consist of metal (Molybdenum) prepared by dc magnetron sputtering (MSP) at an Ar pressure of 0.5 Pa and Si. The Si--layers were deposited using either (MSP) with a partial Ar pressure of 0.5 Pa and a partial H pressure of 0.06 Pa and a power of 100 W or PCVD with a partial SiH4 pressure of 6.4 Pa and a power of 2 Watt respectively. The multilayers consisted of 60 bilayers, which were deposited on a silica substrate with similar layer thicknesses of ~ 11 A for the molybdenum and ~ 40 A for the silicon layer.
The structure of both materials was found to be amorphous by X-ray diffraction under grazing incidence using an angle of incidence of 1o in thin film attachment (Cu Ka). The crystallization threshold of the molybdenum embedded in silicon was determined by a thickness variation and is around 50 A. The molybdenum layer thickness is with 11 A far below.
X--ray reflectometry was measured using a four slit geometry with an resolution of 0.04o in symmetric set-up (Co-Ka). The data were forward simulated by computational code following transfer matrices formalism. Parameters like thickness, d and the rms-roughness were obtained.
The thicknesses of the the MSP-Mo/Si systems were dMo=10.6 A and dSi=38.4A with roughnesses of sMo=5.6A and sSi=16A. The multilayer prepared by MSP has got an higher reflectivity for the first order Bragg peak (52 % reflectivity) than the one prepared by the combination of MSP/PCVD which reaches an reflectivity value of 2% at the first order. The MSP/PCVD multilayer shows a wide broadening of the bragg peaks. It can be a thickness variation concluded from the analysis of the reflectometry data of approximately +- 10 % . This effect is due to a poorer reproducibility of the PCVD deposition process, probably an effect due to plasma instabilities during short deposition times.
Furthermore some other multilayer systems were extensively (especially W/Si) investigated in reciprocal space. Qualitatively different behaviour was found for the Bragg sheets as well as for the Bragg likes for different metal/Si systems and preparation techniques.