M. Bersani

Transient enhanced diffusion of arsenic in silicon

The transient enhanced diffusion (TED) of As in silicon samples implanted at 35 keV with dose 5×1015 cm−2 has been investigated in the temperature range between 750 and 1030 °C by comparing experimental and simulated profiles. For temperatures higher than 900 °C the phenomenon is of modest entity and vanishes after a few seconds, whereas at lower temperatures diffusivity enhancements of some order of magnitude have been observed. The anomalous shift of the junction depth, evaluated at 2×1018 cm−3, is about 12 nm at 900 °C and increases up to 45 nm at 750 °C. It has been verified that the two are the contributions, that generate the interstitial excess responsible for the TED: (i) the implantation damage and (ii) the aggregation in clusters of the As atoms. From an experiment that allows us to separate the two contributions, we estimate that about one third of the TED observed in the first 20 min of annealing at 800 °C is due to the defects produced by clustering. The influence of clustering on the shape o...

Deactivation of ultrashallow boron implants in preamorphized silicon after nonmelt laser annealing with multiple scans

Electrical activation and redistribution of 500eV boron implants in preamorphized silicon after nonmelt laser annealing at 1150°C and isochronal rapid thermal postannealing are reported. Under the thermal conditions used for a nonmelt laser at 1150°C, a substantial residue of end-of-range defects remained after one laser scan but these were mainly dissolved within ten scans. The authors find dramatic boron deactivation and transient enhanced diffusion after postannealing the one-scan samples, but very little in the five- and ten-scan samples. The results show that end-of-range defect removal during nonmelt laser annealing is an achievable method for the stabilization of highly activated boron profiles in preamorphized silicon.

Arsenic uphill diffusion during shallow junction formation

The behavior during annealing of low-energy As-implanted Si have been investigated by comparing secondary ion mass spectrometry (SIMS) and simulated profiles. Z-contrast scanning transmission electron microscopy (STEM) imaging has also been used to determine the As local distribution in proximity of the sample surface. The implants have been performed with energies between 1 and 10keV both through a thermally grown 11nm thick oxide and without any oxide mask. SIMS and STEM profiles show, after short annealing at 800–1000°C, an As pileup in the first nanometers of the Si matrix in proximity of the SiO2∕Si interface. We demonstrate that this phenomenon can be explained with a “Fickian” standard diffusion by assuming the presence of unspecified “dopant traps” near the SiO2∕Si interface that cause a drastic reduction of the dopant able to diffuse inside the bulk. We have also verified that removing before annealing the superficial 4nm of Si does not eliminate the As pileup. Different mechanisms proposed in li...

Vacancy-engineering implants for high boron activation in silicon on insulator

The formation of boron interstitial clusters is a key limiting factor for the fabrication of highly conductive ultrashallow doped regions in future silicon-based device technology. Optimized vacancy engineering strongly reduces boron clustering, enabling low-temperature electrical activation to levels rivalling what can be achieved with conventional preamorphization and solid-phase epitaxial regrowth. An optimized 160keV silicon implant in a 55∕145nm silicon-on-insulator structure enables stable activation of a 500eV boron implant to a concentration ∼5×1020cm−3.

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

The fabrication of preamorphized p-type ultrashallow junctions in silicon-on-insulator (SOI) has been investigated. Electrical and structural measurements after annealing show that boron deactivation and transient enhanced diffusion are reduced in SOI compared to bulk wafers. The reduction is strongest when the end-of-range defects of the preamorphizing implant are located deep within the silicon overlayer of the SOI silicon substrate. Results reveal a very substantial increase in the dissolution rate of the end-of-range defect band. A key player in this effect is the buried Si∕SiO2 interface, which acts as an efficient sink for interstitials competing with the silicon surface.

Real-time observation and optimization of tungsten atomic layer deposition process cycle

In the search for a chemical sensing strategy to monitor atomic layer deposition (ALD) processes suitable for real-time application in wafer manufacturing, we have applied downstream mass spectrometry sampling to study process dynamics during ALD cycles for tungsten deposition from WF6 and SiH4. The ALD reactor has UHV cleanliness conditions and incorporated a minireactor chamber to simulate the small reaction volume anticipated for manufacturing tools to achieve adequate throughput. Mass spectrometry revealed essential surface reaction dynamics through real-time signals associated with by-product generation as well as reactant introduction and depletion for each ALD half-cycle. These were then used to optimize process cycle time and to study the effect of process recipe changes on film growth. The reaction by-products were clearly observed as H2 from SiH4 exposure and SiF4 from WF6 exposure. For each of the two half-cycles, rapid increase of by-product leds to steady-state adsorption/reaction conditions,...

Diffusion and electrical activation of indium in silicon

In this work we investigate the diffusion and the electrical activation of In atoms implanted into silicon with energies ranging from 40 to 360 keV and doses of 5×1012 and 5×1013 In/cm2 during rapid thermal processing. Our investigation shows a clear dependence of In outdiffusion and electrical activation on the implant depth. For a fixed dose, the electrical activation was found to increase with the implant energy. We propose that the data can be explained by considering the balance between the local In concentration and the C background. The occurrence of coupling between the C present in the substrate and the implanted In, depending on the C/In ratio, may in fact give rise to significant formation of C–In complexes. Such complexes play a role in the enhanced electrical activation due to the shallower level they introduce into the Si band gap (Ev+0.111 eV), with respect to the rather deep level (Ev +0.156 eV) of In alone [R. Baron et al., Appl. Phys. Lett. 30, 594 (1977); R. Baron et al., ibid. 34, 257 ...

Effects of boron-interstitial silicon clusters on interstitial supersaturation during postimplantation annealing

Boron marker-layer structures have been used to investigate the effects of B doping on the evolution of the implantation damage and of the associated transient enhanced diffusion. The samples were damaged by Si implants at different doses in the range 2×1013–1×1014 cm−2 and annealed at 740 °C for times between 2 s and 4 h. The values of interstitial supersaturation, from the beginning of the annealing up to the complete damage recovery, have been determined for the different Si doses for a given B doping level. Damage removal has been followed by double crystal x-ray diffraction. Our results confirm that the formation of boron-interstitial silicon clusters traps a relevant fraction of the interstitials produced by the implantation. This trapping action gives rise to a strong reduction of the interstitial supersaturation, prevents the interstitial clusters from being transformed in {113} defects and modifies the time evolution of the transient enhanced diffusion. X-ray analyses indicate also that the size ...

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

Preamorphization of ultrashallow implanted boron in silicon on insulator is optimized to produce an abrupt boxlike doping profile with negligible electrical deactivation and significantly reduced transient enhanced diffusion. The effect is achieved by positioning the as-implanted amorphous/crystalline interface close to the buried oxide interface to minimize interstitials while leaving a single-crystal seed to support solid-phase epitaxy. Results support the idea that the interface between the Si overlayer and the buried oxide is an efficient interstitial sink.

Electrical activation of B in the presence of boron-interstitials clusters

Boron marker-layer structures have been used to analyze the evolution of boron-interstitial clusters (BICs) formed during transient enhanced diffusion. Our approach is based on the measure of B activation by spreading resistance profiling after annealing of Si implantation damage. We investigated a wide range of implant conditions in terms of defect densities below and above the amorphization threshold of Si. We found a common behavior of BICs in terms of trapping and release processes of B atoms. The BICs density as a function of time for different concentration ratios of I and B has been determined.