References cited in this section

27. J.W. Rabalais, in Surf. Sci., Vol 299/300, 1994, p 219

28. L.C. Feldmann, in Surf Sci., Vol 299/300, 1994, p 233

29. A. Benninghoven, F.G. Rüdenauer, and H.W. Werner, Secondary Ion Mass Spectrometry, Wiley, 1987

30. A. Wucher and H. Oechsner, in Fresenius Z. Anal. Chem., Vol 333, 1989, p 470

31. N. Jakubowski and D. Stuewer, in J. Anal. At. Spectr., Vol 7, 1992, p 1951

32. A. Bengtson, A. Eklund, M. Lundholm, and A. Sarie, in J. Anal. At. Spectr., Vol 5, 1990, p 563

33. A. Turos and O. Meyer, in Nucl. Inst. Meth. Phys. Res., Vol B4, 1984, p 92

34. Th. Enders, M. Rilli, and H.D. Carstanjen, in Nucl. Inst. Meth. Phys. Res., Vol B64, 1992, p 817 Surface Analysis Applications

Surface Adsorbates and Contamination. The most obvious application of surface analysis methods is the study and control of the surface composition prior to further treatment, as in the production of thin-film structures in microelectronics (Ref 35) (see Fig. 8). Trace elements on flat, larger surfaces, for example silicon wafers, can be investigated with total reflection x-ray fluorescence spectroscopy at glancing incidence angle (Ref 7) with high sensitivity. Different chemical species can be observed. For example, the detection of residues after etching of a silicon surface with a (CHF3 + O2) plasma in semiconductor fabrication is enabled by the carbon 1s XPS spectrum, which allows the recognition of different peaks attributable to several compounds. Further Ar+ ion etching removes the fluorine-containing compounds (Ref 35). Any kind of surface contamination can be detected. For example, friction and wear often is accompanied by transfer of materials from one part to the other, which can be monitored by surface analysis.

Thin Film and Interface Analysis. Analysis of internal interfaces with surface analysis methods can be principally performed in either of two ways: in-situ fracture of a sample along the interface (e.g., coating-substrate) and subsequent surface analysis of one or both parts (Ref 36, 37, 38), or by depth profiling through the interface (Ref 39, 40). A special method is angle lapping of the sample and taking a line scan across the interfacial region (Ref 3).

Coating-substrate interfaces are often prone to accumulation of impurities (e.g., by segregation or oxidation) that lead to a change in adhesive properties. These can be disclosed by a scratch test, by which a part of the interface is exposed due to peeling of the coating. For example, sulfur segregation was shown to have a detrimental effect on the adhesion of thin films, as determined by an in-situ scratch test in an AES instrument (Ref 41).

Whereas sputter depth profiling is a destructive method, nondestructive methods make use of the known energy-range relation of ions (RBS) or electrons (AES, XPS) in solids. A particularly important nondestructive method to disclose the in-depth distribution of composition is angular resolved XPS, which is, however, restricted to a probing depth of 2 to 3 times the electron attenuation length (i.e., to <5 nm for conventional XPS). By tilting the sample around an axis perpendicular to the analyzer axis and away from the latter, relatively more and more intensity from the species in the top layer is obtained as compared to deeper layers. An example is given in Fig. 10 for the determination of the thickness of an oxide layer (Al2O3) on aluminum. The peak area Iox of the Al-2p peak indicating Al2O3 (75.7 eV) increases relative to that of pure aluminum (72.3 eV, Imet) with increasing takeoff angle 9. The thickness d of the Al2O3 layer is given by (Ref 42, 43, 44):

where 1 is the electron attenuation length (2.0 nm) and k is a sensitivity correction factor of the order of unity. Figure 10(b) shows a plot of the ratio Iox/Imet from Fig. 10(a) as a function of the emission angle 9= 90° - j. Using a more general formulation of the angular dependence I( j) being a Laplace transform of I(1/1), more detailed layer profiles can be revealed, as shown by Bussing and Holloway (Ref 45) for the altered layer of sputtered Ga-As surfaces. Recently, multilayer samples were successfully studied by using grazing incidence XPS, which combines the low penetration depth of x-rays at j <3° incidence angle to the surface (and its variation with j) with all the XPS features (Ref 46).

Fig. 10 Angle-resolved x-ray photoelectron spectroscopy spectrum of a 2.3 nm thick Al2O3 layer on aluminum. (a) Al-2p peak as a function of the takeoff angle j. (b) Ratio of the peak areas of Al2O3 and aluminum as a function of the emission angle 6 = 90° - j . Source: Ref 47

Fig. 10 Angle-resolved x-ray photoelectron spectroscopy spectrum of a 2.3 nm thick Al2O3 layer on aluminum. (a) Al-2p peak as a function of the takeoff angle j. (b) Ratio of the peak areas of Al2O3 and aluminum as a function of the emission angle 6 = 90° - j . Source: Ref 47

Most common and straightforward is depth profiling by ion sputtering in combination with a surface analysis method. This technique discloses the elemental distribution as a function of the sputtered depth, including the composition of interfaces. Important is the attainment of a high depth resolution (Ref 3, 39, 47), which can be achieved by a rastered beam of Ar+ ions with less than 3 keV energy, a small analyzed area, and a high angle of incidence (e.g., 70°) of the ion beam (Ref 39). Sample rotation during profiling has been shown to give optimum results with respect to high depth resolution (Ref 40, 48). With this technique, a depth resolution of Az = 6 nm was achieved for Ni-Cr multilayer thin films (even at a sputtered depth of about 0.5 pm) employing AES and SIMS profiling in a study involving four laboratories (Ref 48).

SIMS depth profiling is particularly useful for determining dopant profiles (e.g., implantation profiles) in semiconductors or detecting impurity segregation at interfaces. The profile obtained from less than a monolayer ("delta layer") of aluminum in Ga-As is shown in Fig. 11 (Ref 49, 50), demonstrating the advantageous sensitivity and high depth resolution of SIMS. Other examples of thin-film depth analyses are given in previous sections of this article.

0 0

Post a comment