Small-spot XPS

Fitzpatrick et al. [41] used small-spot XPS to determine the failure mechanism of adhesively bonded, phosphated hot-dipped galvanized steel (HDGS) upon exposure to a humid environment. Substrates were prepared by applying a phosphate conversion coating and then a chromate rinse to HDGS. Lap joints were prepared from substrates having dimensions of 110 x 20 x 1.2 mm using a polybutadiene (PBD) adhesive with a bond line thickness of 250 p,m. The Joints were exposed to 95% RH at 35 C for 12 months and then pulled to failure. 

The irradiating X-ray beam cannot be focussed upon and scanned across the specimen surface as is possible with an electron beam. Practical methods of small-spot XPS imaging rely on restriction of the source size or the analysed area. By using a focussing crystal monochromator for the X-rays, beam sizes of less than 10 pm may be achieved. This must in turn correspond with the acceptance area and alignment on the sample of the electron spectrometer, which involves the use of an electron lens of low aberration. The practically achievable spatial resolution is rarely better than 100 pm. A spatial resolution value of 200 pm might be regarded as typical, and it must also be remembered that areas of up to several millimetres in diameter can readily be analysed. 

ODA presents a different beam induced effect. Small spot XPS analysis across the sample does not show any... 

Fig. 1.10 Compositional depth profile through a PVdF (LHS)/ polyurethane (RHS) interface for a two-coat paint system acquired using ULAM preparation and 15 p,m small-spot XPS [23].

Fig- 23 Distribution of ions from the defect to the delamination front as measured by means of small spot XPS after the removal of the organic coating [75], (Reprinted with permission.)... 

Obviously, a large potential difference between the active metal surface in the defect (ca. —O.SVshe) and the intact zinc-polymer interface is observed. Between these areas, a steep potential increase marks the location of the delamination front. The potential maps indicate that, as for polymer-coated iron, a cathodic reaction leads to the delamination of the coating. Fiirbeth and Stratmann proved the cathodic mechanism by small spot XPS analysis of the delaminated surface [83]. While no chloride was detected in the delaminated region, the amount of sodium decreased from a high value near the defect to a small value at the front of the delamination. The distribution of sodium ions was in total agreement with the potential maps. 

Figure 5 Small spot XPS system using a monochromated focused x-ray source, demagnification retardation lens and a parallel data acquisition system, (after reference 13).

Finally, instrumentation trends today include the reduction of the analysis area to that of semiconductor device dimensions through small spot XPS and imaging photoemission microscopy. 

An almost equally valuable ability of AES/SAM is the ease by which sputter-depth profiles can be obtained. These and other methods to derive depth distribution information will be discussed in Section 3.3. In this procedure, the surface is slowly milled away by inert ion bombardment and AES is used to monitor the composition as a function of time. Although this can also be done with small spot XPS, AES is more commonly used because of its smaller analysis area which permits a smaller sputtered area and, hence, a faster sputter rate, its faster data-acquisition rate, and its compatibility with the higher pressures in the analysis chambers needed for sputtering with older models of ion guns. 

Financial support for this effort was received from the National Science Foundation through grant No. CTS-9122575. The efforts of William A. Epling and John T. Wolan with regard to figure and manuscript preparation are appreciated, Numerous comments and contributions by John C. Riviere are deeply appreciated. particularly with regard to small-spot XPS. 

More interesting is the case where the instrument is able to resolve the detail on the specimen surface. Scanning the electron beam (in scanning Auger microscopy (SAM)) or the sample position (in small-spot XPS) then allows the acquisition of maps of signal intensity versus position. In the newer XPS... 

Depth profile obtained by small spot XPS and ultra-low angle microtomy at the interface region of a PVdF coating applied to a polyurethane primer (Reproduced from Hinder and Watts (2004), with permission]... 

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