Thickogram Analysis #
overlayer film and substrate are different (for example Rh on a Si substrate) [1].
- Any uniform surface contamination is not important (i.e. advantageous carbon can be
neglected) - Instrumental factors (which may be unknown), would be common to overlayer and
substrate and so cancel out - Works over a range of film thicknesses, from small to large
- Applicable to a wide range of kinetic energies above ca.500 eV
- Simple equation and graphical method with error of ±10% based on accuracy of
attenuation lengths obtained by calculation
The Thickogram itself is a graphical method, with minimal calculations required, however the
emission angle for analysis must be between 0o and 60o (i.e. Take-off angle of 90o to 30o), with
emission angles around 45o considered to be the most accurate.
Figure 1 shows the graphical Thickogram together with the values A, B and C values required
to plot to calculate the film thickness (t), where:
Io and Is = Integrated peak area, where o = overlayer and s = substrate
So and Ss = Relative Sensitivity Factor (RSF) of peak
Ex = Kinetic Energy peak
θ = Emission angle (Zero for a 90° take off angle, so COS(θ) = 1)
λo = Attenuation length of photoelectrons (from the substrate) in the overlayer
1) Calculate A and add point to Thickogram
A = (Is/Io) / (Is/Is)
2) Calculate B and add point to Thickogram
B = E0 / Es
3) ) Draw a line from A to B; point C is found at the crossover on the curve
4) Thickness (t) is calculated as:
t = C . (λo Cos(θ) )
The standard Thickogram equation assumes that the surfaces studied with are flat. Whilst most XPS quantification takes this simplistic view and with reasonable accuracy, Shard has sought to improve such quantification through introducing correction factors to provide overlayer thicknesses on topographic samples of known geometry, such as spherical or cylindrical particles [2].
Their concept is simple; analysis is performed with the sample normal directed towards the XPS analyser, the equivalent planar thickness is calculated from the Thickogram and the Topofactor applied to the result to provide the actual thickness.
1
Cumpson, Peter J. “The Thickogram: a method for easy film thickness measurement in XPS.” Surface and Interface Analysis: An International Journal devoted to the development and application of techniques for the analysis of surfaces, interfaces and thin films 29.6 (2000): 403-406.
2
Shard, A. G., J. Wang, and S. J. Spencer. “XPS topofactors: determining overlayer thickness on particles and fibres.” Surface and Interface Analysis: An International Journal devoted to the development and application of techniques for the analysis of surfaces, interfaces and thin films 41.7 (2009): 541-548.
