Analysis of Gas Incorporation in Thin Films
Authors: Kay Eric 
Published: July 2002

Numerous experimental examples in the literature indicate that the presence of certain gases in the environment in which the film grows can significantly influence the nucleation and growth process. This is often reflected in the crystallographic structure and the resultant physical properties of the film. More recently direct experimental evidence has been reported to show that in many cases gases are actually “trapped” in the growing film and as such undoubtedly contribute to the observed behavior of the film. It is the purpose of this paper to examine the conditions under which such “permanent gas trapping” should be anticipated and to examine the trapping mechanisms which are involved. It is assumed throughout that a gas particle can only be trapped in a film if it is first sorbed by one of several sorption mechanisms on the surface of the growing film. Several gas analysis methods will be discussed which have recently been shown to be reasonably quantitative, in some cases on the ppb level. Inert gases in their electronic ground state can be trapped predictably and to significant degrees in a growing film if the deposition takes place below ∼20 K. So, for example, 1 at.% of Xe will be found in a film analyzed at room temperature, if the film was deposited in 10−6 Torr Xe environment at a metal deposition rate of 100 monolayers/sec on a substrate held at 20 K. This result is consistent with a condensation coefficient of unity at 20 K and no significant subsequent diffusion of gas as the film is warmed up to room temperature for analysis. If the deposition is performed at room temperature where physical adsorption of inert gases is negligible, no inert gas is found regardless of the relative amounts of inert gas to that of the condensible species arriving at the substrate. Experimental evidence will be given to show that inert gases can be trapped during film growth at deposition temperatures well above room temperature to concentration levels as high as 20 at.% provided the gas particle has sufficient kinetic energy on arrival at the growing film surface, i.e., by low energy (several hundred eV) ion implantation. This situation can be quite prevalent in film growth in a discharge environment particularly as encountered in the dc bias sputtering or rf sputtering modes. Evidence will also be provided to demonstrate that inert gas trapping is in general more pronounced in sputtered oxide films than in sputtered metal films. This is shown to be due to a greater sticking probability as a function of inert gas ion energy on oxygenated surfaces as anticipated from our earlier work. Reactive molecular gases are sorbed most commonly by normal chemisorption at exothermic sites. Predictable amounts of gas are actually found in films in those cases where sticking probabilities on pre-existing surfaces as a function of coverage, surface temperature, etc., are known. In many cases normal chemisorption only populates a fraction of the total number of sites available for sorption by the time saturation sets in. Other sorption mechanisms, particularly endothermic adsorption, as well as low energy ion implantation can cause population of some or all the remaining sites. These two sorption mechanisms are frequently encountered in reactive sputtering situations and will be described. Several nondestructive methods of gas analyses will be described with emphasis on x-ray fluorescence and infrared spectroscopy. Experimental evidence will be provided to show that simultaneous electron microprobe x-ray fluorescence together with low resolution (2000 Å) scanning electron microscopy can be readily performed by monitoring the characteristic x-radiation and secondary electron emission simultaneously. Spatial distribution of a particular gas species and its relation to topographical features can be determined in this fashion. Direct correlation between substrate topography and gas concentration profiles in sputtered thin films are of particular interest and will be discussed. For example, 0.1 at.% of argon in Ni films can be seen by this technique. Among the destructive but very high sensitivity gas analysis schemes, laser-induced flash evaporation followed by conventional high resolution mass spectrometry is considered most appropriate for spatial distribution gas analysis in thin films and will be described. Inert gases and N2 can be detected to a ppb. Analyses of other molecular gases present a variety of as yet unresolved problems which reduce the sensitivity considerably.