Spectroscopic ellipsometry application to optimize parameters of ITO films grown by reactive dual magnetron sputtering

Reactive magnetron sputtering provides possibility of high-quality tin doped indium oxide In2O3:Sn (ITO) film deposition on a large substrates at low temperature. Although resistivity and transmittance of ITO films prepared by this technique greatly depend on the deposition parameters [1], which have to be optimized. Unique combination of the ITO properties is controlled by its electronic band structure parameters [2]; the latter can be characterized by optical methods. That is why, this work is focused on application of spectroscopic ellipsometry (SE) for probing of ITO layers.

The ITO films were grown by sputtering of metallic In (90%) - Sn (10%) alloy targets in atmosphere of Ar and O2 mixture by using pulsed dual magnetron system. Base pressure, Ar/O2 ratio and magnetron pulse duration have been varied during the deposition. The films were prepared on Si (100) covered with 500 nm SiO2 and soda lime glass substrates. Spectroscopic ellipsometry was applied within the wavelength range 300-1700 nm in combination with optical transmittance measurements. The spectral dependences of ellipsometric parameters were acquired in autoretarder mode. SE data were fitted simultaneously with transmittance data by using the model of rough graded layer [3] on substrate.

Such approach yielded ITO film thickness and roughness values, which were corroborated by profilometry and atomic force microscopy respectively. A significant variation in film deposition rate was observed only for experiments with different magnetron pulse duration. This parameter is minimal at 50 µs magnetron on-time, than it reaches maximum at 125 µs followed by further decrease at longer pulse durations. Such behavior can be related to changes in plasma density with variation of pulse length that was obtained by Langmuir probe measurements. The film lowest resistivity at high transmittance was achieved at pulse length 100-150 µs. The ITO optical constants were parameterized in Drude-Lorentz approach that permitted to obtain free electron parameters (plasma, wp, and relaxation, wt, frequencies) and interband transition characteristics. Vertical grading of optical constants through the film was quantified. During variation of base pressure, Ar/O2 ratio and pulse duration the minimum values of wt/wp^2 were reached at optimal deposition parameters that provided the lowest resistivity (3.2 mOhm·cm) of the film along with the high transmittance (0.84 for 180 nm thickness). For non optimal deposition parameters the behavior of wt/wp2 ratio qualitatively resembled resistivity dependence. Only in the vicinity of optimal deposition parameters resistivity is proportional to the ratio wt/wp^2 that points on the applicability of free electron approach in this case. For non optimal deposition parameters mechanisms of electric resistivity are more complex and require more detailed investigation of the film structure.

[1] R.B.H. Tahar, T. Ban, Y. Ohya, Y. Takahashi, J. Appl. Phys., 83 (1998) 2631
[2] O.N. Mryasov, A.J. Freeman, Phys. Rev. B, 64 (2001) 233111.
[3] R.A. Synowicki, Thin Solid Films 313-314 (1998) 394.