S. Duguay

Depth resolution function of the laser assisted tomographic atom probe in the investigation of semiconductors

The investigation of boron delta layers by tomographic atom probe (3DAP) is used to demonstrate that a depth profiling resolution of 0.9 nm (full width at half maximum) can be achieved. Results are compared with measurements provided by secondary ion mass spectrometry. The steepness is found to be below 1 nm/decade. In addition, silicon atomic planes are resolved in the real space demonstrating an in-depth spatial resolution of the 3DAP below 0.2 nm.

Clustering and nearest neighbour distances in atom-probe tomography

The measurement of chemical composition of tiny clusters is a tricky problem in both atom-probe tomography experiments and atomic simulations. A new approach relying on the distribution of the first nearest neighbour (1NN) distances between solute atoms in the 3D space composed of A and B atoms was developed. This new approach, the 1NN method, is shown to be an elegant way to get the composition of tiny B-enriched clusters embedded in a random AB solid solution. The theoretical statistical distributions of first neighbour distances P(r) for both random solid solution and solute-enriched clusters finely dispersed in a depleted matrix are established. It is shown that the most probable distance of P(r) gives directly the phase composition. Applications of this model to both one-phase SiGe alloy and boron-doped silicon containing small clusters indicate that this new approach is quite reliable.

Structural and electrical properties of Ge nanocrystals embedded in SiO2 by ion implantation and annealing

Silicon dioxide (SiO2) on Si layers with embedded germanium nanocrystals (Ge-ncs) were fabricated using Ge+ implantation and subsequent annealing. Transmission electron microscopy and Rutherford backscattering spectrometry have been used to study the Ge redistribution in the SiO2 films as a function of annealing temperature. A monolayer of Ge-ncs near the Si∕SiO2 interface was formed under specific annealing conditions. This layer, with a nc density and mean size measured to be, respectively, 1.1×1012∕cm2 and 5nm, is located at approximately 4nm from the Si∕SiO2 interface. Capacitance–voltage measurements were performed on metal-oxide-semiconductor structures containing such implanted SiO2 layers in order to study their electrical properties. The results indicate a strong memory effect at relatively low programming voltages (<5V) due to the presence of Ge-ncs near the Si∕SiO2 interface.

Advance in multi-hit detection and quantization in atom probe tomography

The preferential retention of high evaporation field chemical species at the sample surface in atom-probe tomography (e.g., boron in silicon or in metallic alloys) leads to correlated field evaporation and pronounced pile-up effects on the detector. The latter severely affects the reliability of concentration measurements of current 3D atom probes leading to an under-estimation of the concentrations of the high-field species. The multi-hit capabilities of the position-sensitive time-resolved detector is shown to play a key role. An innovative method based on Fourier space signal processing of signals supplied by an advance delay-line position-sensitive detector is shown to drastically improve the time resolving power of the detector and consequently its capability to detect multiple events. Results show that up to 30 ions on the same evaporation pulse can be detected and properly positioned. The major impact of this new method on the quantization of chemical composition in materials, particularly in highly-doped Si(B) samples is highlighted.

3D analysis of advanced nano-devices using electron and atom probe tomography.

The structural and chemical properties of advanced nano-devices with a three-dimensional (3D) architecture have been studied at the nanometre scale. An original method has been used to characterize gate-all-around and tri-gate silicon nanowire transistor by combining electron tomography and atom probe tomography (APT). Results show that electron tomography is a well suited method to determine the morphological structure and the dimension variations of devices provided that the atomic number contrast is sufficient but without an absolute chemical identification. APT can map the 3D chemical distribution of the atoms in devices but suffers from strong distortions in the dimensions of the reconstructed volume. These may be corrected using a simple method based on atomic density correction and electron tomography data. Moreover, this combination is particularly useful in helping to understand the evaporation mechanisms and improve APT reconstructions. This paper demonstrated that a full 3D characterization of nano-devices requires the combination of both tomography techniques.

Efficient n-type doping of Si nanocrystals embedded in SiO2 by ion beam synthesis

It is shown that co-implantation, with overlapping projected ranges of Si and P or As, followed by a single thermal annealing step is an efficient way to form doped Si nanocrystals (Si-ncs) embedded in SiO2 with diameters of a few nanometers. Atom probe tomography is used to image directly the spatial distribution of the various species at the atomic scale, evidencing that the P and As atoms are efficiently introduced inside the Si nanocrystals. In addition, we report on the influence of the dopant doses on the Si-ncs related photoluminescence as well as on the I(V) characteristics of MOS structures including these Si-ncs.

Direct imaging of boron segregation to extended defects in silicon

Silicon was implanted with a high boron dose (5×1015 at. cm−2) at 30 keV and further annealed at 950 °C for 30 s. The sample was analyzed using transmission electron microscopy (TEM) and atom probe tomography (APT). TEM images revealed the presence of a high density of dislocation loops (∼1011/cm−2) distributed around the projected range of implanted atoms. APT reconstructions showed local enrichment of boron in the form of loops that were interpreted as Cottrell atmosphere. Boron enriched rods, interpreted as the {113} defects, were also observed. Segregation energies of boron atoms to these defects were estimated to be ∼0.35 eV.Silicon was implanted with a high boron dose (5×1015 at. cm−2) at 30 keV and further annealed at 950 °C for 30 s. The sample was analyzed using transmission electron microscopy (TEM) and atom probe tomography (APT). TEM images revealed the presence of a high density of dislocation loops (∼1011/cm−2) distributed around the projected range of implanted atoms. APT reconstructions showed local enrichment of boron in the form of loops that were interpreted as Cottrell atmosphere. Boron enriched rods, interpreted as the {113} defects, were also observed. Segregation energies of boron atoms to these defects were estimated to be ∼0.35 eV.

Atomic-scale redistribution of dopants in polycrystalline silicon layers

80 nm thick polycrystalline silicon (poly-Si) layers implanted with As, P, and C were subjected to spike heating (1000 °C, 1.5 s) or laser anneal (1300 °C, 0.25 ms) and analyzed by atom probe tomography. A strong interfacial segregation of dopants to the grain boundaries (GBs) was revealed in the spike annealed samples. The heterogeneous precipitation of C to the GBs was observed, as well as the clustering of C in the interior of the grains. Theses clusters are also rich in As and P. Their shapes (loop, rod) strongly suggest that these clusters are the result of dopant segregation to extended defects. Nanometer size oxygen clusters were also observed. They originate from the recoil of oxygen atoms during the implantation process through the oxide layer. Laser annealed samples showed a lower segregation excess of dopants to GBs. Consequently, the dopant concentration inside grains was found larger compared to the spike annealed sample. The lower segregation rate at GB is explained by the larger temperature...

Atomic scale study of boron interstitial clusters in ion-implanted silicon

Monocrystalline silicon was implanted with boron (32 keV, 1.3×1015 at. cm−2), post-annealed (740°, 10 min, N2) and further analyzed at the atomic scale by atom probe tomography. A comparison between the as-implanted and annealed samples demonstrated the presence of large B–Si clusters after annealing which were associated with the well-known boron interstitial clusters. The cluster density (up to 5×1017 cm−3) and the number of B atoms per cluster (up to 50) were found to vary with the boron concentration. Only 8% of the B atoms were found trapped in those clusters, suggesting the presence of a majority of very small B–Si aggregates in correlation with simulations.

Retention in metal-oxide-semiconductor structures with two embedded self-aligned Ge-nanocrystal layers

Structural and electrical characterization has been carried out on metal–oxide–semiconductor (MOS) structures with a silicon dioxide (SiO2) layer containing a germanium nanocrystals (Ge-ncs) floating gate. Ge-nc layers were embedded in SiO2 by ion implantation with subsequent annealing. Structural analysis proved the presence of two self-aligned nanocrystal layers within the SiO2 host material. The electrical results indicate a strong memory effect due to the presence of a near-interface Ge-nc layer. Few volts memory windows can easily be obtained at relatively low programming voltages (<6 V). For comparison, operating voltages used in current FLASH technology are about 12 V. Despite its promising structural properties, retention times extracted from capacitance measurements and scanning Kelvin microscopy were found to be too low (~105 s) to comply with the non-volatility industry requirements (<10 years required).