Solid Phase Epitaxial Regrowth Process Research Articles

Nanoscale Periodic Modulation of Doping over Large Areas with Block Copolymer Lithography and Ion Implantation

Whenever periodic patterning at the nanoscale over a large surface is required, block-copolymers (BCPs) represent an extremely attractive alternative for lithography1 and nano-templating,2 because of the low cost, if compared to conventional photolithography,3 and the high throughput, if compared to serial lithographic processes, such as electron beam lithography (EBL).4 Despite the relatively simple structure of BCPs, two different linear homopolymers linked to each other at one end by a covalent bond, when annealed above the glass transition temperature, they spontaneously microphase separate generating a variety of periodic nanostructures. The periodicity (L0 ) of the microdomains can be varied in the 10 – 100 nm range by properly adjusting the molecular weight and the interaction parameter of the two blocks.5 In this work, the combination of BCP lithography and ultra-low energy implantation of phosphorus ions at high fluences is investigated to promote a periodic modulation at the nanoscale of the concentration of dopant impurities over the near-surface layer of a silicon substrate.For the fabrication of the masks, two different poly(styrene-b-methyl methacrylate) (PS-b-PMMA) BCPs were used, to obtain two different geometries: the first with out of plane hexagonally packed PMMA cylinders, with an average diameter of 22 nm and center-to-center distance of 35 nm (Figure 1A) , and the second with out of plane lamellae having a periodicity of 28 nm (Figure 1B). The thickness of the BCP films was 35 nm thick. In addition, a graphoepitaxy protocol was implemented to obtain different sets of perfectly aligned 20 μm long parallel lamellae. A mesoporous soft mask can be readily obtianed by selective removal of PMMA cylinders with UV light and acetic acid. Conversely, a different process is required to avoid the collapse of the PS lamellae. Using Sequential Infiltration Synthesis (SIS),6 Al2O3 is incorporate into the PMMA phase of the BCP film. Upon the removal of the organic material by a mild O2 plasma cleaning, a nanostructured Al2O3 layer, that perfectly matches the PMMA pattern, remains on the substrate (Figure 1B).Phosphorus ions were implanted through the soft masks into the Si substrate operating at ultra-low energy (3 keV) to prevent the degradation of the polymeric film and reduce P penetration (10 nm) into the Si substrate. Additionally, high implantation doses are considered, to induce local amorphization of the silicon substrate. In this way Solid Phase Epitaxial Regrowth (SPER) can be exploited to recover the crystallinity of the substrate by thermal treatments at relatively low temperatures.7 During the SPER process the phosphorus atoms are substitutionally incorporated into the silicon lattice preserving their spatial confinement.AFM morphological analysis and ToF-SIMS depth profiling of the mask confirm the capability of the PS and alumina matrix to properly shield the low energy phosphorus ions in the dose range under investigation. AFM and ToF-SIMS characterization of the samples upon removal of the mesoporous template demonstrated that the P ions were effectively implanted into the Si substrate through the mask, leading to localized implantation (Figure 1C - 1E). Raman spectra suggested the presence of a thin layer of amorphous Si in the implanted samples and subsequent recrystallization after the annealing. By specific SPM measurements, it was possible to compare the morphology, the map of surface potential and the map of conductivity of the samples. The analysis of the Si surface after implantation through the mask with cylindrical pores and activation of the P is reported (Figure 1F, 1G). Small variations of the surface potential that perfectly match the implanted regions were obtained, suggesting a local modification of the electrical properties of Si. This result is also supported by conductivity measurements and finite element method (FEM) simulations. Further results will be presented, to investigate the electrical properties of the samples as a function of the implanted P dose and of the geometry of the mask.(1) Feng, H. et al., Nat. Mater. 21, 1426–1433 (2022).(2) Chai, J. & Buriak, J. M., ACS Nano 2, 489–501 (2008).(3) Levinson, H. J., Jpn. J. Appl. Phys. 61, (2022).(4) Yang, X. M. et al., ACS Nano 3, 1844–1858 (2009).(5) Seguini, G. et al. Soft Matter 16, 5525–5533 (2020).(6) Tseng, Y. C. et al. J. Phys. Chem. C 115, 17725–17729 (2011).(7) Luce, F. P. et al., Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms 370, 14–18 (2016). Figure 1

Solid phase recrystallization induced by multi-pulse nanosecond laser annealing

The solid phase recrystallization of partially amorphized SOI structures using a pulsed Ultra-Violet Nanosecond Laser Annealing (UV-NLA) is reported. It is shown that combining UV-NLA process with a heating chuck at low temperature (450 °C) results in the perfect recrystallization of the amorphized portion of the silicon layer. Actually, sub-melt multi-pulse UV-NLA enables obtaining the total crystal recovery and a very high dopant activation rate, comparable to melt laser annealing approaches or high temperature rapid thermal annealing, with a very low overall thermal budget. This approach is a great alternative to traditional solid phase epitaxial regrowth process, which reach it limits, in terms of recrystallization rate, at temperatures lower than 500 °C. The process presented here is thus suitable for integration paths in which the thermal budget must be limited as for example 3D sequential integration. Besides the high activation rate, another benefit of the solid phase recrystallization, compared to an equivalent laser anneal in the melt regime, is the almost unchanged surface roughness. Furthermore, the use of an in-situ metrology based on Time Resolved Reflectometry (TRR) enables the monitoring of the crystalline seed thickness evolution during UV-NLA and the estimation of the recrystallization rate.

Molecular dynamics modeling of solid phase epitaxial regrowth

Solid phase epitaxial regrowth (SPER) is of great technological importance in semiconductor device fabrication. A better understanding and accurately modeling of its behavior are vital to the design of fabrication processes and the improvement of the device performance. In this paper, SPER was modeled by molecular dynamics (MD) with Tersoff potential. Extensive MD simulations were conducted to study the dependence of SPER rate on temperature, growth orientation, pressure, and uniaxial stress. The simulation data were fitted to empirical formula, and the results were compared with experimental data. It was concluded that MD with Tersoff potential can qualitatively describe the SPER process. For a more quantitatively accurate model, larger simulation systems and a better interatomic potential are needed.

Characterization of the low temperature activated N +/P junction formed by implant into silicide method

Shallow junction formation and low thermal budget control are important for advanced device manufacturing. Implant into silicide (IIS) method is a candidate to achieve both requirements. In this work we show that the high activation ability of the implant into nickel silicide method at low activated temperature is strongly related to the solid phase epitaxial regrowth (SPER) process. The SIMS, capacitance–voltage ( C– V), four points probe (FPP), and current–voltage ( I– V) measurements are combined to demonstrate that the SPER process of the IIS method is starting from the silicide/silicon (M/S) interface. The best N +/P interface is formed when SPER is complete. After SPER process finished, additional thermal budget may cause junction performance degradation at the temperature higher than 550 °C.

Effect of stress on the evolution of mask-edge defects in ion-implanted silicon

In an effort to extend the scaling of advanced complementary metal-oxide semiconductor devices, strain has been incorporated to enhance carrier mobility. In this study, the effect of strain on the solid-phase epitaxial regrowth process of patterned wafers was explored. Implants of 1×1015∕cm2 Si+ with an energy of 40keV were introduced into these patterned regions forming a continuous amorphous layer. After implantation, the oxide and nitride masks were removed and the wafers stressed uniaxially between 0 and 250MPa. The wafers were subsequently annealed under stress to crystallize the amorphous layer, which was monitored using transmission electron microscopy. It was found that without stress, defects formed at the mask edge where the vertical and lateral epitaxial regrowth fronts meet. These defects were threading dislocations which form at the amorphous-crystalline interface and propagate to the surface. Tensile stress levels of as little as 50MPa were found to begin to suppress the formation of mask-edge defects by altering the shape of the corner of the regrowing amorphous layer at the mask edge. Tensile stress appears to retard the lateral recrystallization velocity, creating the obtuse corner geometry necessary to prevent the occurrence of a pinch point at the corner and thereby suppressing defect formation. The evidence suggests that the half-loop threading dislocation nucleates at the corner. The role of varying the stress on the formation of mask-edge defects will be discussed.

Fluorine-enhanced boron diffusion in germanium-preamorphized silicon

Silicon wafers were preamorphized with 60 keV Ge+ or 70 keV Si+ at a dose of 1×1015 atoms∕cm2. F+ was then implanted into some samples at 6 keV at doses ranging from 1×1014 to 5×1015 atoms∕cm2, followed by B+11 implants at 500 eV, 1×1015atoms∕cm2. Secondary-ion-mass spectrometry confirmed that fluorine enhances boron motion in germanium-preamorphized materials in the absence of annealing. The magnitude of boron diffusion scales with increasing fluorine dose. Boron motion in as-implanted samples occurs when fluorine is concentrated above 1×1020atoms∕cm3. Boron atoms are mobile in as-implanted, amorphous material at concentrations up to 1×1019atoms∕cm3. Fluorine directly influences boron motion only prior to activation annealing. During the solid-phase epitaxial regrowth process, fluorine does not directly influence boron motion, it simply alters the recrystallization rate of the silicon substrate. Boron atoms can diffuse in germanium-amorphized silicon during recrystallization at elevated temperatures without the assistance of additional dopants. Mobile boron concentrations up to 1×1020atoms∕cm3 are observed during annealing of germanium-preamorphized wafers.

Enhanced boron activation in silicon by high ramp-up rate solid phase epitaxial regrowth

We investigate the influence of thermal conditions during solid phase epitaxial regrowth (SPER) on the electrical activation level of boron in preamorphized silicon, both with respect to heating ramp rates and the use of low temperature preanneals. Enhancement of electrically active boron concentration by 36% is observed for activation with the fastest ramp rate (487°C∕s) compared to the slowest one (1°C∕s). An important clustering pathway occurs within the amorphous silicon phase (during low temperature preanneal) prior to completion of the SPER process. In these junctions boron deactivation during isochronal post-annealing is almost independent on the maximum boron activation level.

Athermal germanium migration in strained silicon layers during junction formation with solid-phase epitaxial regrowth

The formation of a thin strained Si layer on top of a strain-relaxed SiGe buffer is a recent approach to improve the drive current of complementary metal-oxide-semiconductor devices by inducing strain within the transistor channel. At the same time, advanced process technologies require junction formation processes with minimal diffusion and very high dopant activation. Solid-phase epitaxial regrowth is a low temperature process based on preamorphization and subsequent regrowth leading to highly activated and shallow junctions. In this letter, we investigate the stability of the thin strained Si layer, during solid-phase epitaxial regrowth process by monitoring the Ge redistribution∕strain after the preamorphization step (without any anneal) and after the thermal regrowth process.

Solid Phase Recrystallization and Strain Relaxation in Ion-Implanted Strained Si on SiGe Heterostructures

AbstractThe relaxation process of strained silicon films on silicon-rich relaxed SiGe alloys has been studied. Experimental structures were grown via Molecular Beam Epitaxy (MBE) growth techniques and contain a strained silicon capping layer approximately 50 nm thick. The relaxed SiGe alloy compositions range from 0 to 30 at.% germanium. A 12 keV Si+ implant at a dose of 1×1015 atoms/cm2 was used to generate an amorphous layer ∼30 nm thick, which was confined within the strained silicon capping layer. Upon annealing at 500 °C, it was found that the solid phase epitaxial regrowth process of the amorphous silicon breaks down for high strain levels and regrowth related defects were observed in the regrown layer. In addition, high-resolution X-Ray diffraction results indicate a reduction in strain for the silicon capping layer. This study addresses the critical strain regime necessary for the breakdown of solid phase epitaxial recrystallization in silicon.

Matrix study of material improvement of SOS by DSPEG

Optimization of the double solid phase epitaxial regrowth process to improve SOS material is investigated. In this study, the ion implantation fluence used to form an amorphous silicon layer is the variable of interest. The energy of the ions is either 55 or 120 keV. One ion energy is used for the shallow implantation and a second higher ion energy is used for the deep implantations. The defect density at any depth in the regrown silicon layer depends on both the deep and shallow implantations. This result was not anticipated using a simple heuristic model described below. The functional dependency of the matrix results are displayed in three dimensional surface plots. The structural defects in the epitaxially regrown Si layer consist of dislocation loops and lines without any clearly discernible residual stacking faults or microtwins for large fluences. Autodoping by Al occurs even when the deposited damage energy is substantially below the reported autodoping threshold of 5 X 10 21 keV/cm 3.

Pulsed ruby and CW, Nd-YAG laser annealing of Bi implanted Si single crystals investigated by channeling

Abstract The channeling Rutherford back scattering technique (RBS) has been used to investigate pulsed Q-switched ruby laser and CW Nd-YAG laser annealing of ⟨111⟩ Si single crystals implanted with 40-keV Bi to a dose of 1 × 1014/cm2. The pulsed laser annealing completely removed implantation induced lattice damage and left high impurity substitutionality. However, redistribution of Bi atoms occurred in the annealing process; the percentage of impurities, which segregated at the surface reached 30% and 60% for a single pulse and overlapped pulse, respectively. This implies that a surface melting process occurred. The same sample was also annealed by a CW Nd-YAG laser. Backscattering spectra indicated that recrystallization occurred by means of a thermal solid-phase epitaxial regrowth process, which under appropriate conditions optimized almost completely the recovery of the lattice damage, leaving the Bi atoms mainly in substitutional sites (∼90%) without impurity redistribution.