N. E. B. Cowern

TRANSIENT DIFFUSION OF ION-IMPLANTED B IN SI : DOSE, TIME, AND MATRIX DEPENDENCE OF ATOMIC AND ELECTRICAL PROFILES

The time evolution of B diffusion and electrical activation after ion implantation and annealing at 800 and 900 °C is studied using secondary‐ion mass spectrometry and spreading‐resistance profiling. The time evolution at 800 °C is observed in both crystalline and post‐amorphized samples. Amorphized samples show near‐normal concentration enhanced diffusion. Crystalline samples show anomalous transient diffusion, with a rapidly diffusing low‐concentration region and a static peak region above a critical concentration Cenh=3.5×1018 cm−3. The peak region above Cenh is shown to be electrically inactive. The static, inactive B is released over a period of many hours, compared with the transient diffusion enhancement which relaxes to near‐normal within 30 min. The time evolution of B diffusion at 900 °C is observed as a function of implantation dose. A critical concentration for transient diffusion, Cenh=8×1018 cm−2, independent of dose, is observed at this temperature. The transient diffusion enhancement in th...

Mechanisms of implant damage annealing and transient enhanced diffusion in Si

Interactions between self‐interstitials (I) and {113} interstitial defects during annealing of Si implant damage have been studied. At low damage levels diffusion is ultrafast, driven by I released direct from the ion collision cascade. At higher damage levels, free I are quenched by nucleation of {113} defects. We show that the transient enhanced diffusion seen in most previous studies arises from the subsequent dissolution of the {113} defects.

Role of self- and boron-interstitial clusters in transient enhanced diffusion in silicon

We investigate the nucleation and evolution of boron-interstitial clusters (BIC), driven by high interstitial supersaturations, S(t), during Si implant damage annealing. The BICs are “fabricated” in a narrow band by overlapping the Si implant damage tail with a lightly doped B buried layer. The BIC band is found to be a net sink for interstitials at supersaturations S(t)>104. Our results suggest that silicon self-interstitial defects are the primary source of interstitials driving transient enhanced diffusion, and that BICs act as a secondary “buffer” for the interstitial supersaturation.

Transient enhanced diffusion in preamorphized silicon: the role of the surface

Abstract Experiments on the depth dependence of transient enhanced diffusion (TED) of boron during rapid thermal annealing of Ge-preamorphized layers reveal a linear decrease in the diffusion enhancement between the end-of-range (EOR) defect band and the surface. This behavior, which indicates a quasi-steady-state distribution of excess interstitials, emitted from the EOR band and absorbed at the surface, is observed for annealing times as short as 1 s at 900°C. Using an etching procedure we vary the distance xEOR from the EOR band to the surface in the range 80–175 nm, and observe how this influences the interstitial supersaturation, s(x). The supersaturations at the EOR band and the surface remain unchanged, while the gradient ds/dx, and thus the flux to the surface, varies inversely with xEOR. This confirms the validity of earlier modelling of EOR defect evolution in terms of Ostwald ripening, and provides conclusive evidence that the surface is the dominant sink for interstitials during TED.

On the optimization of SiGe-base bipolar transistors

Extensive computer simulations of NPN SiGe-base bipolar transistors were performed to examine the effect of the Ge profile in the electrical characteristics. It is shown that extra charge storage in the emitter-base (E-B) junction, caused by the Ge profile, affects the device performance considerably. In addition, it is shown that an abrupt Ge profile in the middle of the base region is optimal for a given critical layer thickness of approximately 600A.

Interstitial traps and diffusion in epitaxial silicon films

Oxidation‐enhanced diffusion in molecular beam epitaxially grown epitaxial silicon films decreases rapidly with depth due to trapping of injected interstitials at microscopic defects. Apparently inconsistent data on trapping kinetics, recently reported in the literature, are resolved by analyzing the time evolution of the interstitial distribution CI(x,t). The analysis enables characterization of trap size and trap concentration in the parts‐per‐billion range.

Ultrafast interstitial injection during transient enhanced diffusion of boron in silicon

The injection of interstitials during annealing of nonamorphizing Si implants has been monitored by means of sharp boron‐doped marker layers grown by reduced pressure chemical vapor deposition. The boron diffusivity enhancement measured during the initial annealing stages (t<15 s) at 700 °C is at least an order of magnitude larger than the enhancement during subsequent annealing. The high supersaturation of interstitials during the early stages of the anneal induces immobilization of boron down to concentrations of ≊1×1017 cm−3, consistent with interstitial‐driven boron clustering. The ultrafast diffusion sets lower limits for the silicon and boron interstitial diffusivities at 700 °C of 2×10−10 cm2 s−1 and 2×10−13 cm2 s−1, respectively.

Is there an effect of the proximity of a “free-surface” on the formation of End-Of-Range defects?

Abstract We have studied the effect of the proximity of the wafer surface on the formation of End-Of-Range defects. These experiments are aimed at elucidating the behavior, upon annealing, of the Si self-interstitial supersaturation responsible for transient enhanced diffusion of boron in pre-amorphized silicon wafers. By implanting with Ge at constant energy while carefully etching away increasing thicknesses of the amorphous layer the nucleation and growth of End-Of-Range defects have been studied by transmission electron microscopy. Clearly, no influence in the loop population can be shown even when using state-of-the-art “quantitative” electron microscopy. These results are explained by considering that the c a interface is a diffusion barrier for the Si self-interstitial atoms during the nucleation stage, i.e., when the supersaturation is high. Only after the solid phase epitaxial regrowth, i.e., during the coalescence of the loops when the supersaturation is already low, the surface can interact with the loops. However, this interaction is not measurable through the observation of extended defects and this leads to simplifying assumptions for the simulation of Transient Enhanced or Retarded Diffusion in pre-amorphized Si wafers.

Dislocation formation and B transient diffusion in C coimplanted Si

Suppression of dislocation formation and boron transient diffusion by carbon coimplantation is studied by means of transmission electron microscopy, secondary‐ion‐mass spectrometry, photoluminescence spectroscopy, and high‐resolution x‐ray diffraction. It is shown that both the effects are due to the formation of C‐related damage which acts as a trap for Si interstitials. Quantitative simulations indicate that this damage is probably formed by coprecipitation of Si and C atoms in Si1.15C complexes. These complexes also deteriorate the electrical properties of the implanted layer. They dissolve at annealing temperatures higher than 900 °C. When this occurs, the effect of C is reduced and both B transient diffusion and dislocations, as well as the recovery of the electrical properties, are observed.

Shallow boron junctions and preamorphization for deep submicron silicon technology

In this study, shallow p+‐n junction diodes were formed by implanting BF+2 ions into single‐crystal silicon or silicon preamorphized by either Si or Ge implantation. BF+2 implantation at energies of 20 or 25 keV and a dose of 1×1015 cm−2 was followed by furnace annealing at 600 °C in nitrogen ambient. Most samples received a further nitrogen‐ambient anneal at 850 °C, with various periods of time. Secondary ion mass spectroscopy was used to measure the B profiles. Cross‐sectional transmission electron microscopy was used to study the amorphous layers and the defects remaining after annealing. Electrical characterization of the diodes is described. In preamorphized samples, the residual defect density decreases, and the defect band located at the original amorphous‐crystalline interface becomes sharper, as the mass of the amorphizing ion species is increased. Ideal low‐leakage shallow junctions can be made following either Si or Ge preamorphization and furnace annealing, without removing all the defects ind...