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There are many processes known for fabricating thin films/1, 2.Among them the group of physical vapor deposition processes comprising evaporation, sputtering and ion plating has received special attention.Especially evaporation but also the other PVD techniques are widely used to deposit various single and multilayer coatings for optical and electrical thin film applications/3,4/.A large number of parameters is important in obtaining the required film properties in a reproducible manner when depositing thin films by such processes.Amongst the many are the film thickness, the condensation rate,the substrate temperature,as well as the qualitative and the quantitative composition of the residual gas of primary importance.First of all the film thickness is a dimension which enters in practically all equations used to characterize a thin film. However,when discussing film thickness,definitions are required since there one has to distinguish between various types of thicknesses e.g.geometrical thickness,mass thickness and optical thickness.The geometrical thickness,often also called physical thickness,is defined as the step height between the substrate surface and the film surface.This step height multiplied by the refractive index of the film is termed the optical thickness and is expressed generally in integer multiples of fractional parts of a desired wavelength.The mass thickness finally is defined as the film mass per unit area obtained by weighing.Knowing the density and the optical data of a thin film its mass thickness can be converted into the corresponding geometrical as well as optical thickness.However,with ultrathin films ranging between a few and several atomic or molecular "layers"the concept of a film thickness may become senseless since often no closed film exists of such minor deposits.Although film thickness is a length,the measurement of it can,obviously,not be accomplished with conventional methods for length determinations but requires special methods.The great efforts made to overcome this problem led to a remarkable number of different,often highly sophisticated film thickness measuring methods reviewed in various articles such ase.g./5,6/.With some of the methods,it is possible to carry out measurement under vacuum during and after the film formation other determinations have to be undertaken outside the deposition chamber only after the film has been produced.Many of the methods cannot be employed for all film substances,and there are varying limits as regards the range of thickness and measuring accuracy.Furthermore, with these methods the film to be measured is often specially prepared or dissolved during measurement and therefore becomes useless for additional investigations or applications.If only those methods which can be employed during the film deposition are considered,then the very large number of methods is considerably reduced.Insitu measurements,however,are highly desired since many basic investigations and practically all industrial applications require a precise knowledge of thefilm thickness at any instant to enable termination of the deposition process at the predetermined right moment.Apartfrom few exceptions in practical film deposition only optical measuring units andmass determination monitors are used.
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