One of the key tasks in terms of characterization of microelectronic devices is the non-destructive measurement of electrical properties of the interfaces between dielectric materials and semiconductors. For example, ITRS 2013 [1] mentions that the measurement techniques of local interfacial electrical properties are among the difficult challenges in terms of metrology. This is not only critical in microelectronics, but also in other fields: for example in photovoltaics, the electrical quality of the passivation layer of the semiconductor is crucial in order to obtain high yield solar cells. In this context, a characterization tool based on the second harmonic generation (SHG) [2], answers positively to most of the metrology requirements: it is contactless, non-destructive, sensitive and can provide a high spatial resolution down to micrometer size.

The working principle of the method is to illuminate a sample with a high intensity laser beam of wavelength λ and to measure the generated second harmonic signal (at wavelength λ/2). If an internal electrical field is present in the material, the SHG signal is partially given by the electrical field-induced SHG (EFISH) [3]. For centrosymmetric media (as Si, SiO₂ etc.), the interest of the SHG lays in its extremely high sensitivity to the dielectric-semiconductor interface, since the electric field in such a stack of materials relates to the defects in the oxide and at the interface itself. For instance the SHG measurements proved to be interesting to detect radiation induced defects in the buried oxide of silicon-on-insulator wafers [4], or metal contamination on such wafers [5]. The presence of traps induced at the interface Si/SiO₂ due to boron were also probed by SHG [6]. Other studies show charging dynamics at Al₂O₃ –Si interfaces measured by SHG [7].

We show our recent results on the characterization of dielectric materials deposited (or grown) on silicon. The SHG measurement tool used is the new inspection system Harmonic F1x from FemtoMetrix [8]. A pulsed femtosecond laser of 780nm wavelength is shined upon the sample. The second harmonic generated beam from the sample is detected at 390nm by a photomultiplier, coupled with a photon counter. We have used this equipment on various dielectric materials deposited (or grown) on silicon wafers. Figure 1a shows an example of the SHG evolution in time obtained on Al₂O₃ thin films grown by Atomic Layer deposition on Si substrates, as-deposited and after an annealing step. The annealing is known to increase the density of negative fixed charges in the alumina layer. The electric field at the interface between the alumina layer and the Si is reinforced and gives a higher SHG signal after annealing. Additionally, we can map samples in order to locally estimate spatial variations of the SHG signal (see an example in Figure 1b: Al₂O₃ on Si (100) after annealing).

Different materials, as well as thermal processes will be discussed and compared in the full paper. The SHG results will be validated by comparison with other well-known techniques, such as the photoconductance decay for lifetime measurements.

[1]          http://www.itrs.net/Links/2013ITRS/Home2013.htm. (2013).

[2]          J. J. H. Gielis, P. M. Gevers, I. M. P. Aarts, M. C. M. van de Sanden, and W. M. M. Kessels, "Optical second-harmonic generation in thin film systems," Journal of Vacuum Science & Technology A, vol. 26, pp. 1519-1537, 2008.

[3]          P. N. Butcher and D. Cotter, The Elements of Nonlinear Optics: Cambridge University Press, 1991.

[4]          B. Jun, R. D. Schrimpf, D. M. Fleetwood, Y. V. White, R. Pasternak, S. N. Rashkeev, et al., "Charge trapping in irradiated SOI wafers measured by second harmonic generation," IEEE Transactions on Nuclear Science, vol. 51, pp. 3231-3237, 2004.

[5]          M. L. Alles, R. Pasternak, X. Lu, N. H. Tolk, R. D. Schrimpf, D. M. Fleetwood, et al., "Second Harmonic Generation for Noninvasive Metrology of Silicon-on-Insulator Wafers," Semiconductor Manufacturing, IEEE Transactions on, vol. 20, pp. 107-113, 2007.

[6]          H. Park, J. Qi, Y. Xu, K. Varga, S. M. Weiss, B. R. Rogers, et al., "Boron induced charge traps near the interface of Si/SiO2 probed by second harmonic generation," Physica Status Solidi (b), vol. 247, pp. 1997-2001, 2010.

[7]          J. J. H. Gielis, B. Hoex, M. C. M. van de Sanden, and W. M. M. Kessels, "Negative charge and charging dynamics in Al2O3 films on Si characterized by second-harmonic generation," Journal of Applied Physics, vol. 104, p. 073701, 2008.

[8]          http://femtometrix.com.