Ultrathin SiO₂ on Si. VII. Angular accuracy in XPS and an accurate attenuation length

Abstract

Following an earlier study of the uncertainties for defining thicknesses by angle-resolved XPS, one of the major instrumental uncertainties has been evaluated that limits both precision and accuracy. For analysis of the thicknesses of SiO 2 on Si, certain angles of emission have been recommended, but an error of 1• in these angles leads directly to an error in the thickness of ∼1% from this contribution alone. This is significant since the total uncertainty required in the (International technology roadmap for semiconductors) ITRS is only 1%. In many instruments, to reduce sample-stage vibration and for other reasons, the angle and other setting adjustments are engineered with backlash. This, combined with the manufacturer's tolerances, can lead to angular errors above 1 • . We report here a device using a laser and reflectors, fixed to the sample mount, that allow the angle of emission to be set to a precision better than 0.1• and, furthermore, a method to set the zero angle of emission to 0.1 • . Using this geometrical device as well as by measurements of intensities in XPS, it is deduced that the data in our earlier report for the CCQM (Consultative Committee for Amount of Substance) intercomparison were for angles 1.89 • ± 0.15• too high. Consequently, by a re-analysis of all of that data, we find that our recommended attenuation length data for the Si 2p photoelectrons in thermal SiO 2 using Mg or Al Ka X-rays should be increased to 2.996 nm and 3.485 nm, respectively, an increase of 1.2% on the originally calculated values. These values now have standard uncertainties of 0.54% instead of the 20% of the TPP-2M calculations. This leads to an improved accuracy in the measurement of ultrathin thermal oxides on silicon by XPS, of better than 1% for thicknesses greater than 1.5 nm but less than 8 nm.  Crown

Dependence of calculated electron effective attenuation lengths on transport mean free paths obtained from two atomic potentials

“…3 SESSA has recently been used to calculate EALs for thin films of SiO 2 on Si for XPS by Mg K˛X rays and the measurement conditions considered by Seah and Spencer. 29 These calculations (to be reported separately) show that the SiO 2 EALs vary with film thickness to a greater extent than expected from use of SRD 82. 3 This thickness dependence of the EAL is expected to be important in thin-film metrology by XPS.…”

“…Further analysis of  tr throughout the periodic table will be reported elsewhere. 28 Seah and Spencer 29 reported EALs for Si 2p photoelectrons in silicon dioxide on the basis of an extensive analysis of thickness measurements made by XPS and other methods. Their EAL values were (29.96 š 0.16)Å and (34.85 š 0.19)Å for Si 2p photoelectrons excited by Mg and Al K˛X rays, respectively, and where the stated uncertainties represent standard uncertainties.…”

Remarks on Some Reference Materials for Applications in Elastic Peak Electron Spectroscopy

“…The experimental geometry was carefully checked by a laser beam technique and reflector fixed to the sample holder, in a way similar to that described recently by Seah and Spencer. 20 The electron elastic-backscattering intensities were recorded at primary-electron kinetic energies of 200, 350, 500, 650, 800, 1000, 1250, and 1500 eV. The typical width of the elastic peak reached about 0.5 eV (FWHM).…”

Uncertainty in Quantification of Binary Alloy Films and Thickness Measurement of nm Oxide Films

“…In the thickness measurement of nm SiO 2 film on Si (100), reference geometry (RG) is recommended to minimize the diffraction effect of the substrate. As shown in Figure 10, Although the electron emission angle could be controlled within 0.1 % accuracy by using a laser beam, [18] it is not easily applicable unless suitable windows are available in the instrument. The emission angle scale in most instruments is of undefined accuracy and may exhibit errors as high as 5% without calibration.…”

“…Figure 11. Concept of crystal axis method [18] Two methods to determine surface normal were suggested. The crystal axis method (CAM); utilizes an electron diffraction effect of a Si (100) wafer as shown in Figure 11.…”