Determination of Film Thickness

Having shown that the effective optical properties of CuPc and DMPTI both depend on the layer thickness and on the substrate material and possibly also on the preparation conditions, it is clear that ellipsometry will not be suited to determining absolute film thicknesses to 1 nm precision, particularly not in a multi-layer system on a glass sub-strate where reflectivity is low. Ellipsometry is, however, very efficient for checking the homogeneity of the evaporation process or the slope of a gradient profile.

The arrangement of the evaporation cells in the evaporation chamber is of great import-ance for the production of homogeneous films. For small distimport-ances between the evap-oration cell and the substrate, a homogeneous film cannot be obtained without special evaporation protocols. For a first test of the film homogeneity, we have evaporated films from the five different sources (three effusion cells and two crucibles) onto glass substrates and have measured the film thickness at 5×9 spots with a profilometer. The results and the geometric arrangement of the evaporation cells are shown in Figure3.13. The sub-stances (tris(2-phenylpyridine) iridium (Ir(ppy)₃), aluminium and lithium fluoride) were chosen arbitrarily from the set of substances that are commonly used in the evaporation chamber. We see that none of the films are of constant height. Instead, they show a shape that can be fairly well approximated by a gradient. The direction of the gradients

3.4. Ellipsometry 51

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Figure 3.12: ∆ and Ψ of a 67 nm thick DMPTI film on silicon and on glass as measured (black) and as predicted by piecewise LOM fits (red and blue). A and B show the predic-tions of the piecewise LOM fit on silicon for silicon and glass substrate, C and D show the predictions of the piecewise LOM fit on glass for glass and silicon substrate.

can be understood from the position of the evaporation sources. Secondly, we see from the scattering of the values that the accuracy of the profilometer results is in the order of 5–10 nm. To overcome this accuracy problem and to replace the tedious profilometer measurements we decided to check whether ellipsometry would be a useful tool for this task. Knowing the difficulties with glass substrates, we performed another evaporation

52 Chapter 3. Combinatorial Techniques

Figure 3.13: Profilometer data of films evaporated from the 5 different evaporation sources.

All films show a slight gradient due to the position of the evaporation source, the gradient of Q3 being the largest.

FilmThickness[nm]

Figure 3.14: Fit for the thickness of a homogeneously evaporated CuPc film onto a silicon substrate.

experiment with CuPc on silicon, and automated the ellipsometer to measure the ∆ and Ψ spectra on an array of 8×8 spots. A subsequent automated fit of the layer thickness with the optical constants taken from the results described in the previous section leads to a map of thickness values. This is displayed in Figure3.14 together with the mean square error. Although we have said that we should not expect an absolute accuracy on the nm scale, the fitting results suggest that the relative accuracy is indeed as precise as that. We observe a continuous increase in the layer thickness along the diagonal of the

3.4. Ellipsometry 53

Figure 3.15: Fit for the thickness of a CuPc film on a silicon substrate that has been evaporated with a continuous gradient.

sample with values ranging from 40 nm to 60 nm, the mean value being 54.85 nm and the standard deviation being 1.92 nm. Realising that the profile is mainly a linear gradient, we have adapted our evaporation protocol in such a way that we rotate the sample around 180 degrees after the first half of the evaporation time. This procedure eliminates any linear component in the layer profile and leads to nearly flat films.

In the same way, we have investigated a CuPc layer that has been evaporated as a gradient onto a silicon wafer by a moving mask. Details about the evaporation process can be found in [Sch01]. Figure3.15 shows the expected gradient, which is precisely mapped out by the fitting routine. A second glance reveals the influence of the sample position which leads to a slight gradient along the lines of nominally constant thickness. In this case, the variation is small compared with the intended gradient. Depending on whether the device properties are strongly correlated with the thickness, the second gradient may be neglected. In cases where the gradient is non-negligible, measurements like the one presented can be used to calibrate the layer thickness at each particular spot on the sample. The geometry of our current setup does not, unfortunately, allow us to correct for the unintended gradient by rotation of the sample, since the mask can only move in one direction, due to space limitations. Having been successful in determining the thickness of a CuPc layer on silicon, we tried to map out a bilayer system of a DMPTI gradient and a CuPc gradient on glass as it is used in our photovoltaic cells. For this purpose, we selected the wavelength of 405 nm and used the n and k values of CuPc and DMPTI determined by a global fit to two samples of different layer thickness (see Table3.1). The fitting results give the correct direction of the gradient (Figure3.16), but the accuracy is far less than for the single-layer measurements presented above and the starting parameters had to be chosen appropriately for the fitting algorithm to converge.

We even achieved a reasonable convergence of a fit over the whole wavelength range by using two LOM models for CuPc and DMPTI (Figure3.17), but again not without much fine-tuning in the fitting algorithm.

We conclude that ellipsometry is able, with some restrictions, to approximately determine layer thicknesses in a bilayer system of CuPc and DMPTI, but due to the large parameter space and due to the problems of anisotropy, reflectivity etc., which were discussed in the

54 Chapter 3. Combinatorial Techniques

Figure 3.16: Fits for the layer thickness of DMPTI and CuPc that were evaporated onto a glass substrate with perpendicular gradients. The fits were performed at a wavelength of 450 nm with optical properties that were determined by simultaneously fitting two samples of different film thickness for each of the substances.

FilmThickness[nm]

Figure 3.17: Fit for the layer thickness of DMPTI and CuPc on the basis of two Lorentz-oscillator models.

previous chapter, we should talk of verification rather than of determination. As in the near future the acquisition of a combinatorial flash lamp UV/Vis spectrometer will give us another method for layer thickness determination, we have not put any more effort into the optimisation of combinatorial ellipsometry. Preliminary results of the combinatorial UV/Vis method are discussed in the following section.