D. Giubertoni

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.

Depth profile characterization of ultra shallow junction implants

A need for analysis techniques, complementary to secondary ion mass spectrometry (SIMS), for depth profiling dopants in silicon for ultra shallow junction (USJ) applications in CMOS technologies has recently emerged following the difficulties SIMS is facing there. Grazing incidence X-ray fluorescence (GIXRF) analysis in the soft X-ray range is a high-potential tool for this purpose. It provides excellent conditions for the excitation of the B-K and the As-Liii,ii shells. The X-ray standing wave (XSW) field associated with GIXRF on flat samples is used here as a tunable sensor to obtain information about the implantation profile because the in-depth changes of the XSW intensity are dependent on the angle of incidence. This technique is very sensitive to near-surface layers and is therefore well suited for the analysis of USJ distributions. Si wafers implanted with either arsenic or boron at different fluences and implantation energies were used to compare SIMS with synchrotron radiation-induced GIXRF analysis. GIXRF measurements were carried out at the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the electron storage ring BESSY II using monochromatized undulator radiation of well-known radiant power and spectral purity. The use of an absolutely calibrated energy-dispersive detector for the acquisition of the B-Kα and As-Lα fluorescence radiation enabled the absolute determination of the total retained dose. The concentration profile was obtained by ab initio calculation and comparison with the angular measurements of the X-ray fluorescence.

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.

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 ...

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.

Grazing incidence x-ray fluorescence and secondary ion mass spectrometry combined approach for the characterization of ultrashallow arsenic distribution in silicon

Dopant depth profiling and dose determination are essential for ultrashallow junction technology development. However they pose a challenge to the widely used dynamic secondary ion mass spectroscopy (SIMS) technique that suffers uncertainties due to an initial transient width comparable to the dopant depth distribution. In this work the authors report on the application of grazing incidence x-ray fluorescence (GIXRF) for arsenic in silicon dose and profile determination and its combination with SIMS in order to try to overcome the limitations of the latter in the topmost few nanometers. A polynomial variation of the sputtering rate is supposed in the first sputtering stage of the SIMS analysis and the parameters that regulate the magnitude of such correction are determined by a least square fitting of the angle dependent fluorescence signal. The total retained fluence was also measured by instrumental neutron activation analysis and synchrotron radiation soft x-ray GIXRF. The comparison among the total re...

Quantitative depth profiling of boron and arsenic ultra low energy implants by pulsed rf-GD-ToFMS

In very recent years particular effort is being devoted to the development of radiofrequency (rf) pulsed glow discharges (GDs) coupled to time of flight mass spectrometry (ToFMS) for depth profile qualitative analysis with nanometre depth resolution of technological materials. As such technique does not require sampling at ultra-high vacuum conditions it facilitates a comparatively high sample throughput, related to the reference technique secondary ion mass spectrometry (SIMS). In this work, pulsed rf-GD-ToFMS is investigated for the fast and sensitive characterization of boron and arsenic ultra low energy (ULE) implants on silicon. The possibility of using a simple multi-matrix calibration procedure is demonstrated for the first time for quantification of this type of samples and the validation of the proposed procedure has been carried out through the successful analysis of a multilayered sample with single and couple 11B delta markers. Results obtained with the proposed methodology for boron and arsenic ULE implants, prepared under different ion doses and ion energy conditions, have proved to be in good agreement with those achieved by using complementary techniques including SIMS and grazing incidence X-ray fluorescence. Thus, although further investigations are necessary for more critical evaluation of depth resolution, the work carried out demonstrates that rf-GD-ToFMS can be an advantageous tool for the analytical characterization of boron and arsenic ULE implants on silicon.