Breakthrough instruments and products: Investigation of atomic layer deposited Al:ZnO layer by spectroscopic ellipsometry from the deep-UV to the mid-IR in one instrument Open Access

Spectroscopic ellipsometry provides a powerful method for characterizing thin film structures by investigating the change in light polarization upon reflection from a sample. As the measurement determines the complex ratio of the p and s polarization states (i.e., the parallel and perpendicular components relative to the plane of incidence, respectively), it does not require any reference sample, and thus the method can be considered an absolute measurement technique. On the other hand, the raw measurement data do not provide the sample parameters of interest, which are the layer thicknesses and optical dispersion of the layer materials. Direct mathematical inversion is only possible in limited cases, so, in general, the layer structure is modeled and the model parameters are fitted in order to match the simulated spectra with the measured ones.¹ 

The polarization change upon reflection can be described by the interference on the layers and the Fresnel equations for the p and s polarization components derived from the boundary conditions of Maxwell’s equations. In Fig. 1(a), we can see a schematic depiction of a polarized reflection from a multilayered structure.

By definition, the sample is illuminated equally by p and s linearly polarized light at a certain angle of incidence. In general, the specularly reflected light becomes elliptically polarized due to a phase shift introduced by the extinction of the materials in the stack structure or the interference of the layers.

In order to obtain the thicknesses and optical properties of the layers from the measured data, the layer structure has to be modeled by parametrically describing the layer dispersions. The appropriate values of the parameters then have to be searched by a regression analysis.

Semilab ellipsometry instruments cover a vast range of capabilities, such as single wavelength ellipsometry, spectroscopic ellipsometry in the range of 190–2500 nm, mid-IR ellipsometry up to 25 µm, polarized photometry measurements, scatterometry, ellipsometric porosimetry, in situ ellipsometry, and automatic systems with measurement spot size smaller than 30 µm for production control. The SE-2000 system [Fig. 1(b)] makes use of the rotating compensator ellipsometry technique, which is the state-of-the-art type of spectroscopic ellipsometry data collection method.

Semilab is using the most advanced rotating compensator layout, where a high-end, super-achromatic quarter wave plate introduces a variable phase shift according to the rotation angle in order to determine the ellipsometric components spectrally.

The unique capabilities of the wide-wavelength-range SE-2000 system will be presented through an investigation of Atomic Layer Deposition (ALD) Al doped ZnO (Al:ZnO) layers.^2,3 The example demonstrates the feasibility of a combined ellipsometric tool with measurement capabilities from the mid-IR to the UV range. In order to perform this investigation test, samples of atomic layer deposited Al:ZnO layers with variations of process parameters on different kinds of substrates were measured and analyzed.

The deposition of the layer structure was performed using a Picosun P-300B batch ALD tool.⁴ The variations in process parameters were to target different Al concentrations in four steps (0%, 2%, 2.5%, 3%) and the deposition temperature also in four steps (200, 225, 250, 275 °C). Three types of layer structures were deposited: type A: Al:ZnO layer on silicon-dioxide on a silicon substrate, type B: Al:ZnO layer on a glass substrate, and type C: Al:ZnO layer on a silicon epitaxial (Si EPI) layer on a silicon substrate. Al:ZnO layers were deposited using diethyl zinc, trimethyl aluminum, and deionized water as precursors.

For the measurement, a Semilab SE-2000 was used equipped with a CCD array-based detector for the range of 250–990 nm and an InGaAs photodiode array detector for the 990–2150-nm range (the whole range denoted as UV-VIS-NIR). An InfraRed Spectroscopic Ellipsometry (IRSE) arm pair mounted on the same tool was available for the 1430–16 700 nm range, making use of a liquid nitrogen cooled mercury cadmium telluride detector. The obtained dopant concentration values were validated with four-point-probe (4PP) measurements using a cylindrical Jandel head and a Keithley 2612B SMU (Source-Measurement Unit). This 4PP setup is also available on the SE-2000 as a metrology option.

In this study, the optical properties of the Al doped ZnO layers were described with the combination of Tauc–Lorentz and Drude dispersion laws⁵ to develop a robust model to fit the experimental measurements.

The Drude model can be used to describe the electrical conduction of quasi-free electrons in metals or carriers in semiconductor materials. The corresponding photon-energy-dependent dielectric function for ellipsometric data analysis can be formulated with the following equations:¹ 

where the parameters E_P and E_Γ are the plasma energy and the broadening related to the scattering frequency.

The electrical properties of conductive thin films can also be obtained from the Drude model based on the above parameters. The sheet resistance of the layer is defined as

where ε₀ is the free-space permittivity and d is the layer thickness.

Both the UV-VIS-NIR and the IRSE measurement were performed at 75° angle of incidence. Goodness-of-fit represented with the coefficient of determination greater than 0.99 was achieved during the regression analyses.

A simultaneous fit approach for the mid-IR and UV-VIS-NIR was applied. A successive, separated, iterative fit for each range would alter the result for the other wavelength range, while the simultaneous fit acted as a mutual constraint for the two ranges.

Figure 2 presents examples of fits to experimental measurements made on samples of each of the three types. The robustness of the model and the necessity of the wide wavelength range measurements can be seen on the multilayer structures, especially on the type C group where there is a Si EPI layer on the substrate.

The ellipsometric model could distinguish the Si EPI from the Al:ZnO layer and also the substrate from the Si EPI layer, while electrical measurements such as 4PP cannot separate the layers because sheet resistance measurements combine the electrical properties of the whole stack.

The parametric fits on the sample set measurements provide dispersion data that show sensitivity for the change in process parameters (Fig. 3). Drude parameters allow us to calculate the electrical properties.

Figure 4 presents color maps of dopant concentration for the Al:ZnO layers as a function of deposition temperature and target al concentration based on data from 16 samples for each type. In between the grid points, the color map is visualized with cubic Shepard interpolation.⁶ 

All three maps in Fig. 4 show that the highest dopant concentration can be obtained using ∼260 °C deposition temperature and ∼2.5% target al concentration.

While the ellipsometric analysis provides reasonable results and the comparison of the Al:ZnO layers in the different layer structure types shows similarity, another measurement technique has to be used to validate the results obtained by ellipsometry. The industrial standard 4PP technique was selected for validation, which can be added to the SE-2000 system as a supplementary measurement technique. From the three sample sets, the type B group was selected for the correlation study because it only has the Al:ZnO layer as a conductive component. Figure 4(d) shows the correlation of the two datasets with an excellent correlation coefficient of 0.9993.

A model was developed for complex layer structures that could provide thickness and optical and electrical properties by fitting both the IR and visible ranges. A further advantage of the ellipsometric measurement is that the layers could be separated and the electrical properties could be determined individually while the applicable electrical measurement types integrate over the layers and the substrate.

The investigation showed that the combined UV-VIS-NIR+mid-IR ellipsometry instrument, which is unique on the market, is a good candidate for industrial scale metrology solution to determine layer thickness and optical and even electrical properties of Al:ZnO layers in a nondestructive way.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.