Determining the morphology of islands during their growth on a substrate is a very important task in the control of the fabrication of nano-objects. We have developed the recent technique of Grazing-Incidence Small-Angle X-ray scattering (GISAXS) to analyse the morphology of growing particles in situ, in ultra high vacuum (UHV), and in real time, starting from the very beginning of the growth. We have chosen metal/oxide systems which are known to grow in three dimensions, and for which the use of imaging techniques is difficult, because the AFM tip often moves the metal clusters, and because STM can not be used on an insulating substrate. The growth of Pt, Pd and Ag on MgO(001) surfaces was investigated at different temperatures.

A fully-dedicated experimental setup has been built on the ID32 beamline without a window before the sample, thus avoiding any background scattering. The beam was defined by two pairs of motorised slits working in the secondary vacuum. A hollow diode measuring the scattering by the beam-defining slits was used as a monitor, followed by a fast (1 ms of closure time) beam shutter.A 1-metre long differential pumping pipe was installed between this beam-defining line, with a pressure of 5 x 10-8 mbar, and the UHV chamber (base pressure 10-10 mbar) mounted on a six circle diffractometer. The UHV chamber was hooked to the entrance line by a CF38 bellow, allowing for a small rotation of the chamber around the vertical axis, used to define the angle of incidence of the X-ray beam with respect to the vertical sample surface. A 0.8 metre-long cone was connected to an exit pipe through a CF38 bellow, and ended by a 100 mm diameter beryllium window placed just in front of a 16-bit X-ray CCD detector. The cone and the detector were hooked to the goniometer detector arm, thus allowing precise vertical and horizontal alignment of the camera. A motorised tantalum beam-stop with a T-shape was introduced between the exit Be window and the 2-D detector.

Figure 47 shows a typical GISAXS pattern obtained on a 18 Å-thick Ag deposit. The lateral extension of the scattering yields the average lateral dimension of the islands; the perpendicular extension, and in particular the location of the second-order scattering peak yields the average height; while the location of the interference peaks parallel to the surface yields the average separation between the islands.

Figure 48 shows the evolution of the GISAXS during room-temperature growth of Ag on MgO(001), from 0.5 Å up to 50 Å equivalent thickness. As growth proceeds, all the scattering become more and more concentrated toward the origin of the reciprocal space, which corresponds to a continuous increase of all dimensions in real space. A detailed analysis shows that the growth proceed via nucleation, growth and coalescence at the same time, and finally percolation of the islands.

The pictures are quantitatively very precise, thus enabling the deduction of the average lateral size, height and separation between islands, and even the width of the distributions of these parameters. As an example, Figure 49 shows a cut of the scattered intensity perpendicular to the surface, together with a fit without and with a distribution of islands heights. Clearly, the minima can only be quantitatively reproduced by introduction of a finite distribution.

Authors
G. Renaud (a), M. Noblet (a), A. Barbier (a), C. Revenant (a), O. Ulrich (a), Y. Borensztein (b), R. Lazzari (c), J. Jupille (c), C. Henry (d).

(a) CEA-Grenoble (France)
(b) Laboratoire d'Optique des Solides, Université P. et M. Curie, Paris (France)
(c) Laboratoire CNRS, Saint-Gobain (France)
(d) CRMC2, Marseille (France)