(Invited) Second Harmonic Generation: Non-Linear Optics for Characterization of Electrical Properties of Dielectric-on-Semiconductor Interfaces

Abstract

Dielectric layers are deposited on semiconductor materials for many applications, either in the active stacks of the devices (e.g. the gate dielectric in a metal-oxide-semiconductor transistor) or as passivation solutions (e.g. in image sensors [1], solar cells [2] etc.). The progress at the material and technology levels must be accompanied by the development of new characterization tools. One of the challenges in terms of characterization is to obtain the information on the electrical quality of the dielectric-on-semiconductor interface during the fabrication flow, non-destructively, just after the corresponding deposition step (if possible) and without needing to fully fabricate a particular test structure. At wafer level, a typical electrical evaluation method is the Corona characterization of semiconductors [3] that provides the interface state density (Dit) and the “total” charge responding in the structure. Its main drawback is the charging of the surface during the measurement. Optical methods (like the photoconductance decay or the photoluminescence) are non-destructive, but they are directly related to material and interface quality through the carrier lifetime [4] and they don’t allow a simple separation between fixed charge in the oxide (Qox) and Dit.However optical-based methods are still a recommended strategy, since they ensure non-destructive measurements. In this context, the second harmonic generation (SHG) is a good option, because it can be sensitive to the “static” electric field induced between layers. In the SHG, the surface of the sample is irradiated with a femtosecond laser and a second harmonic wave is then generated and detected. In general, the SHG contains both bulk and surface contributions, but for centrosymmetric materials (such as silicon, silicon dioxide, alumina, etc.) in the dipolar approximation, the interface signal is dominant and it is related to the symmetry breaking due to both the interface itself and to the “static” electric field [5]. The SHG has already been used for dielectric characterization using various modalities: SHG versus power, time, wavelength, etc. [6], [7], [8].In this paper, we focus on the analysis of the interface electric field, which is related to Qox and Dit. If trapping/detrapping phenomena occur during the illumination of the sample, this field can actually depend on time. The value of the SHG (proportional to the square of the field) will then be related to Qox and its evolution in time should correlate to Dit. The technique is promising but it triggers two questions: (1) how to separate optical phenomena (absorption, interferences) specific to multi-layer structures and access electrical field only and (2) how to actually extract Qox and Dit. During the presentation we explain recent results in these two areas, using simulation results obtained with our “home-made” code and a wide panel of samples for the experiments. Alternative electrical characterization performed through capacitance versus voltage measurements gives hints for future calibration.AcknowledgementsThis work was supported by Region Rhône Alpes (ARC6 program), the French National Research Agency within the framework of the OXYGENE project (ANR-17-CE05-0034) and French National Plan Nano2022, within the IPCEI Nanoelectronics for Europe program.[1] J. L. Regolini, D. Benoit, and P. Morin, "Passivation issues in active pixel CMOS image sensors," Microelectronics Reliability, vol. 47, pp. 739-742, 2007.[2] A. G. Aberle, "Surface passivation of crystalline silicon solar cells: a review," Progress in Photovoltaics: Research and Applications, vol. 8, pp. 473-487, 2000.[3] M. Wilson, J. Lagowski, L. Jastrzebski, et al., "COCOS (corona oxide characterization of semiconductor) non-contact metrology for gate dielectrics," AIP Conference Proceedings, vol. 550, pp. 220-225, 2001.[4] D. K. Schroder, "Carrier lifetimes in silicon," Electron Devices, IEEE Transactions on, vol. 44, pp. 160-170, 1997.[5] J. E. Sipe, D. J. Moss, and H. M. van Driel, "Phenomenological theory of optical second- and third-harmonic generation from cubic centrosymmetric crystals," Physical Review B, vol. 35, pp. 1129-1141, 1987.[6] J. Price, M. Lei, P. S. Lysaght, et al., "Charge trapping defects in Si/SiO2/Hf(1−x)SixO2 film stacks characterized by spectroscopic second-harmonic generation," Journal of Vacuum Science & Technology B, vol. 29, p. 04D101, 2011.[7] N. M. Terlinden, G. Dingemans, V. Vandalon, et al., "Influence of the SiO2 interlayer thickness on the density and polarity of charges in Si/SiO2/Al2O3 stacks as studied by optical second-harmonic generation," Journal of Applied Physics, vol. 115, p. 033708, 2014.[8] H. Park, J. Qi, Y. Xu, 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.