Vacuum microbalance calibration

Method 3 is an absolute method for pressure calibration in contrast to the relative methods 1 and 2. A variant of method 3 is possible by the use of a vacuum microbalance (see e.g. Hiipert [51] and Biefeld [78]). 

This experimental assembly is much more complex than the preceding one. The oxide surfaces are ultrathin alumina films grown on NiAl(l 1 0) single crystals, in the preparation chamber following a standard procedure [16]. The alumina films are characterized in situ by AES and LEED. The metal clusters are prepared by vacuum condensation at RT of a metal atoms beam generated by an electron bombardment evaporator calibrated by a quartz microbalance. Metal atoms condense only on the sample through an aperture placed closed to it. After preparation the sample is transferred in the reaction chamber. The characterization of the metal clusters is based on STM observations of deposits performed in the same conditions in another UHV chamber [16]. 

The high sensitivity, excellent mechanical stability and the controllable thermal influences make the quartz-crystal microbalance a valuable tool. Determinations of the physical film thickness, however, may be more of a problem since the density pf of a thin film, depending on deposition method, chosen parameters and film thickness, is generally different from the density of the bulk material being considered. At film thicknesses below 100 nm, the density discrepancy is greater than with thicker films. This seems to be valid for metals as well as for dielectrics. It is, however, usually possible to reproduce the film density. For the deposition process, this demonstrates that the chosen vacuum and deposition parameters must be carefully controlled and maintained exactly to always attain the same film density. In its manner and after calibration, the geometrical film thickness can be exactly reproduced by quartz-crystal monitoring. 

Uloa-high-vacuum environment is mandatory for HREELS experiments, in order to prevent attenuation/scaitcring of the electron beam, and sample contamination by residual gas. For studies described in this wotk, evsqioration of metal onto the polymer surfoce was p onned in situ, using a Knudsen celt with a very low and well controll evaporation rate, calibrated with a quartz microbalance (typical evaporation rate lA/minute). This is required to grow and characterize stepwise, in the submonolayer coverage range, the metal-polymer interface. 

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