Current Understanding and Modeling of B Diffusion and Activation Anomalies in Preamorphized Ultra-Shallow Junctions

Abstract

The formation of ultra-shallow junctions (USJs) for future integrated circuit technologies requires preamorphization and high dose boron doping to achieve high activation levels and abrupt profiles. To achieve the challenging targets set out in the semiconductor roadmap, it is crucial to reach a much better understanding of the basic physical processes taking place during USJ processing. In this paper we review current understanding of dopant-defect interactions during thermal processing of device structures – interactions which are at the heart of the dopant diffusion and activation anomalies seen in USJs. First, we recall the formation and thermal evolution of End of Range (EOR) defects upon annealing of preamorphized implants (PAI). It is shown that various types of extended defect can be formed: clusters, {113} defects and dislocation loops. During annealing, these defects exchange Si interstitial atoms and evolve following an Ostwald ripening mechanism. We review progress in developing models based on these concepts, which can accurately predict EOR defect evolution and interstitial transport between the defect layer and the surface. Based on this physically based defect modelling approach, combined with fully coupled multi-stream modelling of dopant diffusion, one can perform highly predictive simulations of boron diffusion and de/re-activation in Ge-PAI boron USJs. Agreement between simulations and experimental data is found over a wide range of experimental conditions, clearly indicating that the driving mechanism that degrades boron junction depth and activation is the dissolution of the interstitial defect band. Finally, we briefly outline some promising methods, such as co-implants and/or vacancy engineering, for further down-scaling of source-drain resistance and junction depth.

Boron deactivation in preamorphized silicon on insulator: Efficiency of the buried oxide as an interstitial sink

“…3, we have chosen to compare the number of interstitials reaching the surface with the measured deactivation, using vertical scales differing by a factor of 2, corresponding to the usually observed deactivation of about two B atoms by one interstitial. 16,17 The agreement is good for all data points except the first ͑remaining crystal thickness of 37 nm͒ for which the EOR band slightly overlapped the B implant distribution. The slightly low value for this point could be a result of direct deactivation by implanted interstitials prior to annealing.…”

“…As this EOR band evolves, it releases self-interstitials, causing transient enhanced diffusion ͑TED͒ and boron-interstitial cluster ͑BIC͒ formation which manifests itself as electrical deactivation. 4 In SOI, a proportion of the emitted self-interstitials may migrate to nearby sinks such as the silicon/buried oxide ͑BOX͒ interface, thus reducing the amount of B deactivation that occurs. 5 This letter quantifies the role of the BOX and shows the dramatic advantages-in terms of reduced diffusion and deactivation-that can be achieved by exploiting the positioning of the EOR defect band within the silicon top layer in SOI.…”

“…At low PAI energies, the characteristic deactivation and reactivation of B are observed, arising from the formation and subsequent dissolution of BICs. 4 However, as the PAI energy is increased, bringing the EOR defect band toward the BOX interface, the amount of deactivation decreases very dramatically.…”

Diffusion and activation of ultrashallow B implants in silicon on insulator: End-of-range defect dissolution and the buried Si∕SiO2 interface

“…During higher temperature annealing these defects undergo a series of transitions from self-interstitial clusters to ͕113͖ defects to dislocation loops depending on the initial PAI and subsequent conditions. 3 These defects dissolve during annealing and the silicon self-interstitials so released migrate to nearby sinks such as the silicon surface. 3 Transient enhanced diffusion and B electrical deactivation result from the interstitials that diffuse towards the silicon surface during annealing.…”

“…3 These defects dissolve during annealing and the silicon self-interstitials so released migrate to nearby sinks such as the silicon surface. 3 Transient enhanced diffusion and B electrical deactivation result from the interstitials that diffuse towards the silicon surface during annealing. 3,4 In the absence of sinks within the silicon, all of the released interstitials diffuse towards the surface.…”