(Invited) White-Light-Induced Annihilation of Percolation Paths in SiO2 and High-k Dielectrics - Prospect for Gate Oxide Reliability Rejuvenation and Optical-Enabled Functions in CMOS Integrated Circuits

Ultra-thin SiO₂ and high-k dielectrics have been shown to exhibit electrical-stress-induced soft breakdown (SBD).¹ Typically, SBD is an irreversible process, as observed from electrical measurement, and would evolve towards the catastrophic hard breakdown upon continued stressing.² In this talk, we present our recent observation that white light can restore the insulating property of SiO₂ and HfO₂ dielectrics which have suffered soft breakdown. The finding suggests the possibility of (1) rejuvenating soft-breakdown MOSFET gate oxides via light illumination for reliability extension; (2) electrical-cum-light enabled memory or switch function based on the SiO₂ or HfO₂ dielectric, whose relatively large bandgap has previously precluded it from optically-stimulated applications. 

The impact of white light illumination (from a LED light source) on the electrical conduction through an oxide percolation path is demonstrated using an ultra-high vacuum conductive atomic force microscope or CAFM (Fig. 1). Fig. 2(a) shows the sequence of experimental steps. The samples tested include conventional SiO₂/p-Si, HfO₂/p-Si gate stacks as well as the HfO₂/TiN/Ti/p-Si resistive memory structure. Formation of the oxide percolation path was achieved by constant-voltage stressing, supplied via an external parameter analyzer connected to the diamond-coated Si CAFM probe. The breakdown process was arrested when the current increase reached a preset current compliance (cc), which was varied from 0.5 to 500 nA (corresponding to 2×10³ to 2 A/cm² based on the estimated area (~26 nm²) of the probe-oxide contact region³). Fig. 2(b) shows a typical current evolution towards SBD for the SiO₂/p-Si sample subjected to negative-voltage stressing.

From the current-voltage curves depicted in Fig. 2(c), it is evident that that the insulating property of SBD SiO₂ (cc = 0.5 nA) can be partially restored after a 10-minute illumination. A further 15-minute illumination nearly restores the oxide to the pre-stress state. Similar results are obtained on the SBD HfO₂/p-Si sample subjected to constant positive-voltage (Fig. 3(a)) and negative-voltage (Fig. 3(b)) stressing.

As depicted in Fig. 4(a), white light can also cause the HfO₂/TiN/Ti/p-Si resistive memory structure, programmed to the low-resistance state by a prior forming step, to reset back to the high-resistance state, indicating that upon illumination, the conducting filament is annihilated. The effect of white light is shown to be similar to that achieved by a bipolar electrical reset (Fig. 4(b)). By adjusting the light exposure period, intermediate levels between the high and low states can be achieved (not shown).

Fig. 5 shows a possible explanation for the observed light-induced SBD oxide restoration effect. At the instant of oxide breakdown, Joule-heating leads to the migration of dislodged lattice oxygen ions away from the percolation path. When the stress is interrupted quickly, most of the oxygen ions are believed to remain at interstitial sites surrounding the percolation path. Back migration towards the vacancy-filled percolation path would require overcoming an energy barrier (~0.3-0.6 eV)⁴ and thus could not occur without an external excitation. It is believed that white light (with photon energy ranging from 1.8-3 eV) is able to excite these interstitial oxygen ions thus facilitating their back migration towards the percolation path. Subsequent recombination with the vacancies there thus restores the insulating property of the SBD oxide.

References: [1] J. S. Suehle, IEEE Trans. Electron Dev., vol. 49, no. 6, pp. 958-971, 2002; [2] F. Monsieur et al., in Proc. Int. Reliab. Phys. Symp., 2002, pp. 45-54; [3] B. Cappella and G. Dietler, Surf. Sci. Rep., vol. 34, no. 1-3, pp. 1-3, 5-104, 1999; [4] K. P. McKenna and A. Shluger, Microelectron. Eng., vol. 86, no. 7-9, pp. 1751-1755, 2009.