The A Leak Methodology

3 The O2 Leak Methodology As covered in Section 4.2.1.1, SIMS requires a vacuum of better than 1 X 10 Torr (that at which the mean free path of an ion equates to 1 m, which equates to the typical path length of a primary or secondary ion column). More typically, a vacuum of better than 1 x 10 Torr is used to control the adsorption of gas phase molecules. As an example, a monolayer of Oxygen will form on Silicon in Is when in the 10 Torr range. 

Such analysis should, however, be carried out with extreme care, because  

Not only does the secondary ion yield vary with the O2 leak pressure used, but so too does the sputter rate (a decrease in sputter rates is commonly observed) 

Higher chamber pressures will result in increased neutralization of the primary ion beam. This, in turn, can result in a loss of depth resolution (as is discussed in Section 5.3.2.4, this will occur if the neutral population were to strike an area away from that of the primary ion beam) 

The adsorption of Oxygen onto the internal surfaces of the analysis chamber that also occurs during the implementation of O2 leak conditions will require extended pump-down times if UHV conditions are required in subsequent analysis. Such analysis should, therefore, be carefully planned in accordance with any other analysis required. 

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