Thermal Rounding Research Articles

Thermal rounding for shape modification of high-performance polyetherketoneketone and reinforced polyetherketoneketone-carbon fiber composite particles

This study presents shape transformation of anisotropic high-performance thermoplastic polyetherketoneketone (PEKK) and carbon fiber reinforced powder composite particles (HT-23) by thermal rounding. The shape transformation is achieved by (partial) melting of the high-temperature thermoplast microparticles. Three different process setups are presented, investigating the impact of the source of heat supply on the resulting shape modification: using a directly heated sheath gas flow, an indirect heat supply through the reactor wall and a combined approach. Regardless of the chosen setup, a modification of the particle shape was observable. The most advantageous shape transformation was observed in the indirect heating approach. In addition, the enhanced shape transformation yields an improved free flow behaviour of the powders, as quantified by ring-shear experiments. Reductions of the unconfined yield strengths of the powders for high consolidation stresses as high as 18 percent for PEKK and 30 percent for the HT-23 are achieved. Thereby, processability of the powder in laser based powder bed fusion is enhanced, extending the range of available (composite) polymer materials.

Particle Residence Time Distribution in a Concurrent Multiphase Flow Reactor: Experiments and Euler-Lagrange Simulations

The present work focuses on investigating the residence time behavior of microparticles in a concurrent downer reactor through experiments and numerical simulations. For the numerical simulations, a three-dimensional multiphase model was developed using the Euler-Lagrange approach. The experiments were performed in a 0.8 m-long steel reactor with gravitational particle injection. The effects of different operating conditions, e.g., the sheath gas velocity on the particle residence time distribution were assessed. An increase in the sheath gas flow rate led to a decrease in the peak residence time, although the maximum residence time increased. Regarding the lowest sheath gas flow rate, the particles’ peak residence time was twice as high compared to the peak residence time within the highest flow rate. The particles’ residence time curves presented a broad distribution coinciding with the size distribution of the powder. The numerical results agreed with the experimental data; thus, this study presents a numerical model for predicting the particle residence time behavior in a concurrent downer reactor. Furthermore, the numerical simulations contributed to a better understanding of the particle residence time behavior inside a concurrent downer reactor which is essential for optimizing thermal rounding processes. Dimensionless correlations for the observed effects are developed.

Thermally activated dynamics of current-driven skyrmions in random media

Based on the Landau-Lifshitz-Gilbert equation, we perform large-scale numerical simulations of the skyrmion dynamics with quench disorder at finite temperatures. Two universal creep behaviors are observed below the depinning transition current, and the creep exponents are accurately determined. The thermal rounding exponents at the depinning transition indicate an anisotropic temperature dependence in the perpendicular and parallel directions of the current. Further, the temperature-current phase diagram of the skyrmion Hall angle is builded, and the thermally activated forces are analyzed. Our results provide comprehensive understanding for the role of temperature in the current-driven skyrmion dynamics.

Yielding in an amorphous solid subject to constant stress at finite temperatures.

Understanding the nature of the yield transition is a long-standing problem in the physics of amorphous solids. Here we use molecular dynamics simulations to study the response of amorphous solids to constant stresses at finite temperatures. We compare amorphous solids that are prepared using fast and slow quenches and show that for thermal systems, the steady-state velocity exhibits a continuous transition from very slow creep to a finite strain rate as a function of the stress. This behavior is observed for both well-annealed and poorly annealed systems. However, the transient dynamics is different in the latter and involves overcoming an energy barrier. Due to the different simulation protocol, the strain rate as a function of stress and temperature follows a scaling relation that is different from the ones that are shown for systems where the strain is controlled. Collapsing the data using this scaling relation allows us to calculate critical exponents for the dynamics close to yield, including an exponent for thermal rounding. We also demonstrate that strain slips due to avalanche events above yield follow standard scaling relations and we extract critical exponents that are comparable to the ones obtained in previous studies that performed simulations of both molecular dynamics and elastoplastic models using strain-rate control.

Thermally rounded depinning of an elastic interface on a washboard potential.

The thermal rounding of the depinning transition of an elastic interface sliding on a washboard potential is studied through analytic arguments and very accurate numerical simulations. We confirm the standard view that well below the depinning threshold the average velocity can be calculated considering thermally activated nucleation of defects. However, we find that the straightforward extension of this analysis to near or above the depinning threshold does not fully describe the physics of the thermally assisted motion. In particular, we find that exactly at the depinning point the average velocity does not follow a pure power law of the temperature as naively expected by the analogy with standard phase transitions but presents subtle logarithmic corrections. We explain the physical mechanisms behind these corrections and argue that they are nonpeculiar collective effects which may also apply to the case of interfaces sliding on uncorrelated disordered landscapes.

Thermal rounding of micron-sized polymer particles in a downer reactor: direct vs indirect heating

Purpose The purpose of this paper is to investigate the effect of different heating approaches during thermal rounding of polymer powders on powder bulk properties such as particle size, shape and flowability, as well as on the yield of process. Design/methodology/approach This study focuses on the rounding of commercial high-density polyethylene polymer particles in two different downer reactor designs using heated walls (indirect heating) and preheated carrier gas (direct heating). Powder bulk properties of the product obtained from both designs are characterized and compared. Findings Particle rounding with direct heating leads to a considerable increase in process yield and a reduction in powder agglomeration compared to the design with indirect heating. This subsequently leads to higher powder flowability. In terms of shape, indirect heating yields not only particles with higher sphericity but also entails substantial agglomeration of the rounded particles. Originality/value Shape modification via thermal rounding is the decisive step for the success of a top-down process chain for selective laser sintering powders with excellent flowability, starting with polymer particles from comminution. This report provides new information on the influence of the heating mode (direct/indirect) on the performance of the rounding process and particle properties.

Spherical Polybutylene Terephthalate (PBT)-Polycarbonate (PC) Blend Particles by Mechanical Alloying and Thermal Rounding.

In this study, the feasibility of co-grinding and the subsequent thermal rounding to produce spherical polymer blend particles for selective laser sintering (SLS) is demonstrated for polybutylene terephthalate (PBT) and polycarbonate (PC). The polymers are jointly comminuted in a planetary ball mill, and the obtained product particles are rounded in a heated downer reactor. The size distribution of PBT–PC composite particles is characterized with laser diffraction particle sizing, while the shape and morphology are investigated via scanning electron microscopy (SEM). A thorough investigation and characterization of the polymer intermixing in single particles is achieved via staining techniques and Raman microscopy. Furthermore, polarized light microscopy on thin film cuts enables the visualization of polymer mixing inside the particles. Trans-esterification between PBT and PC during the process steps is investigated via vibrational spectroscopy and differential scanning calorimetry (DSC). In this way, a new process route for the production of novel polymer blend particle systems for SLS is developed and carefully analyzed.

Numerical simulations of critical dynamics in anisotropic magnetic films with the stochastic Landau-Lifshitz-Gilbert equation.

With the stochastic Landau-Lifshitz-Gilbert (sLLG) equation, critical dynamic behaviors far from equilibrium or stationary around the order-disorder and pinning-depinning phase transitions in anisotropic magnetic films are investigated. From the dynamic relaxation with and without an external field, the Curie temperature and critical exponents of the order-disorder phase transition are accurately determined. For the pinning-depinning phase transition induced by quenched disorder, the nonstationary creep motion of domain wall activated by finite temperatures is simulated, and the thermal rounding exponent is extracted. The results show that the dynamic universality class of the sLLG equation is different from those of the Monte Carlo dynamics and quenched Edwards-Wilkinson equation, and it may lead to alternative understanding of experiments. The dynamic approach shows its great efficiency for the sLLG equation.

Head and Media Design for Curvature Reduction in Heat-Assisted Magnetic Recording

Recording curvature in magnetic data-storage technology has long been one of the significant challenges impacting on recording performance. Despite curvature occurrence in the conventional recording techniques such as perpendicular magnetic recording (PMR), heat-assisted magnetic recording (HAMR) is demonstrated to induce much more severe curvature than PMR. HAMR curvature could cause poor bit error rate and limits the maximum areal density capacity. Here we have theoretically predicted and demonstrated various approaches for curvature reduction from the aspect of either altering the near-field transducer head design or recording medium design. Optical and thermal modeling have indicated that by utilizing a crown-shape peg to change the thermal source profile and compensate for thermal expansion and rounding effect, it could potentially improve curvature figure of merit (FOM) and achieve curvature reduction by ~45%. In terms of the recording media design, by altering the heat sink and internal layer media material or geometry, it could also achieve curvature cancellation of ~40% with increased thermal gradient. The combined approach from both HAMR head and media perspectives with balanced recording FOMs, could potentially realize significant curvature reduction to be of similar or better recording curvature level to PMR.

Creep and thermal rounding close to the elastic depinning threshold.

We study the slow stochastic dynamics near the depinning threshold of an elastic interface in a random medium by solving a particularly suited model of hopping interacting particles that belongs to the quenched-Edwards-Wilkinson depinning universality class. The model allows us to compare the cases of uniformly activated and Arrhenius activated hops. In the former case, the velocity accurately follows a standard scaling law of the force and noise intensity with the analog of the thermal rounding exponent satisfying a modified "hyperscaling" relation. For the Arrhenius activation, we find, both numerically and analytically, that the standard scaling form fails for any value of the thermal rounding exponent. We propose an alternative scaling incorporating logarithmic corrections that appropriately fits the numerical results. We argue that this anomalous scaling is related to the strong correlation between activated hops that, alternated with deterministic depinning-like avalanches, occur below the depinning threshold. We rationalize the spatiotemporal patterns by making an analogy of the present model in the near-threshold creep regime with some well-known models with extremal dynamics, particularly the Bak-Sneppen model.

Thermal Rounding Kinetics and Demonstration of Steep and Flat Facets Succession in Selective Si-Based Epitaxy

Si1-xGex(SiGe) alloys are integrated in more and more complex structures, and their applications are numerous. Initially, epitaxy process was applied for SiGe channels in 32nm technologies and beyond [1]. In the 20nm technology generation, SiGe epitaxial growths are used for raised source/drain (RSD) in Fully Depleted Silicon-On-Insulator (FD-SOI) p-Metal Oxide Semiconductors (p-MOS). However, since the scaling of the transistors’ dimensions is reaching physical, material, and technological limitations [2], new challenges in Si-based technologies have to be defined. Up to now, one of the most promising studies overcoming major device issues such as parasitic capacitance [3], is the morphology’s control of SiGe epitaxial growths. This is the reason why the morphological behavior of epitaxial SiGe is of great interest for performance improvements. In SiGe epitaxies, surface morphology depends on different parameters such as deposited thickness, Ge content... But, one of the most important parameters for the morphological change is the thermal budget brought to the film during or after epitaxial growth. The consequence of a thermal budget on an epitaxial layer is the modification of the morphology to reduce the surface energy leading to thermal rounding. Finally, the transistors’ dimensions being very small, the morphology is even more affected and sensitive to any process conditions (temperature, pressure), and extensive studies on SiGe morphology are needed. For this work, patterned 300mm Si(001) bulk wafers are used as substrates. All the layers were selectively grown in a Rapid Thermal Chemical Vapor Deposition reactor using dichlorosilane, germane, and HCl chemistry at low pressure and relatively low temperature. The purge and unloading steps were adjusted to freeze the as-deposited morphology. This way, the observed morphology is the closest as possible to the one during the deposition. The first part consists of a quantitative study of the thermal rounding’s kinetics in Si1-xGex (x=0.25) epitaxies. To do so, as-deposited SiGe layers are annealed at different thermal budgets. Then, the morphology evolution during annealing is studied as a function of temperature and time. The results of post annealed SiGe epitaxies are reported in Fig.1. It is observed that an annealing temperature increase of 25°C (from 650°C to 725°C) considerably impacts the morphology which evolves progressively passing through different steps. First, the edges begin to round and retract. In this step, the as-grown facets and their corresponding overgrowths fade away completely. Then, the thermal rounding gives rise to a formation of rounded ridges all around the pattern. Finally, the corner ridges approach towards each other until they collapse to form a single large dome. The rounding kinetics is quantified by measuring the displaced volume of matter. In the final paper, an extensive study will be given in which the effect of H2pressure on the kinetics will be presented. It will be shown that the pressure has a real influence on the rounding effect due to an increase of the hydrogen coverage slowing the Si diffusion at the surface. In many MOS applications, the SiGe epitaxy has to be capped typically with a Si (or SiGe) layer to maintain good performances. In this second part, the morphology behavior of such caps is studied. Si and SiGe layers of different thicknesses, growth rates and Ge contents were grown on a rounded SiGe layer, prepared as explained in the first part. It is shown that both Si and SiGe behave differently. The SiGe cap decreases the surface undulation amplitudes more efficiently than Si. Indeed, in large patterns, the 16 nm thick SiGe cap surface is smoother (RRMS=3.57nm) than the surface of Si cap (RRMS=7.37nm) (see Fig.2). In small patterns, a regular faceted morphology is observed for thick SiGe cap which is not the case for Si cap, showing once again that SiGe cap is more efficient (Fig.3). On the other hand, for thin Si and SiGe caps, the top surface presents domes with a succession of flat and steep facets. Those facets are <113> facets for the steep ones, and <11h> with h being 9 or even more, for flat facets (Fig.4). To summarize, firstly, the thermal rounding has been quantified and shown to be very sensitive to moderate thermal budgets (e.g.700°C). Secondly, Si and SiGe caps deposited on such a rounded surface have been compared. It has been reported that thin caps present successive terraces rather than regular faceted morphology. Keywords: SiGe, Si, Epitaxy, Rounding, Capping. [1] Weber,O Symp. VLSI Techno Dig Tech Pap, 2004, p. 42. [2] Haron,N. IEEE, 2008; pp 98–103. [3] Cheng,K. Symp. VLSI Tech., 2009, pp 212-213. Figure 1

(Invited) Industrial Applications of Si-Based Epitaxy in Nanoelectronics

Si1-x-yGexCy epitaxial depositions (epi) on silicon wafers are of great interest as they allow dramatic performance improvements or even new devices fabrication. About 20 years ago, an actual breakthrough in SiGe epitaxy allowed an important extension of processes in terms of materials, chemistry and temperature range. Consequently, very different epitaxies are currently used in a variety of industrial technologies Historically, a number of epi techniques that have been developed to a more or less advanced degree: MBE, UHV-CVD, PE-CVD... Firstly, in this lecture, these techniques will be reviewed and benchmarked for industrial usage, thus their advantages/drawbacks, listed in table1, will be discussed. It appears that RTCVD presenting more advantages than competitors is indeed the most developed/adopted one, and that batch LPCVD is a serious challenger for depositions simple enough and for mass production (1). In a second part, important epi applications will be presented. The list is not exhaustive but based on the applications developed at STMicroelectronics for BiCMOS, Imagers, Photonics and CMOS technologies. Then, very different depositions will be reviewed, with thicknesses ranging from 3-6nm for CMOS to 3-6µm for Imagers, and from pure Si to pure Ge. A particular attention will be paid on the heterojunction bipolar transistors (HBTs) fabrication and on the problematic of epi on ultra-thin (3-5nm) SOI films, 2 items we have worked extensively on. Historically, HBTs first have used SiGe and advanced epi with a tremendous success. But the SiGeC:B base epi, especially when selective, is very demanding in terms of process control (complex stack and very thin elementary films) and in terms of structural quality (interface, defects and C incorporation). On the other hand, in advanced fully-depleted planar CMOS technologies, ultra-thin (3-6nm) SOI active zones are very sensitive to any non-oxidizing thermal budget, they require then special precautions before/during the epi process. The following part presents the challenges of advanced epi, namely: patterns, loading effect, limited thermal budget, selectivity, deposition control, uniformity, structural quality, morphology, defectivity, cost… And in industrial applications, contrarily to pure research activity, all of them have to be fulfilled at the same time; thus the process corresponds to a trade-off. Here, we will focus on the main difficulty of the RTCVD technique, i.e. the temperature control, and on the “small size effects” that becomes dominant in current (~14nm) and in future CMOS nodes.In current RTCVD single wafer systems, heating is radiative and there is no solution to measure accurately the wafer temperature with the desired accuracy, better than 1K. Thermocouples or pyrometers only measure the susceptor temperature. As the susceptor/wafer thermal coupling is not perfect, any vertical heat flux will gives rise to a temperature gradient and then to an offset between the set-point (susceptor) and the process temperature (wafer). The net heat balance varies with different parameters such as the bottom/top power ratio and the wafer optical properties, and the coupling depends on the wafer curvature and on the gas nature/pressure. Usually, the bottom heating is predominant and the vertical T gradient negative. Fig.1 reports a typical T offset under H2 and its variation as a function of the pressure. It is observed that the offset and its variation, are huge compared to the process requirements. They have then to be controlled and taken into account. On the other hand, in advanced nodes the lateral size of epi is small (10-30nm) and the thickness often very similar, epi have to be considered as 3D objects. In such a case, the crystal orientation plays a crucial role, and the epi morphology is very sensitive to small thermal budgets. In 3D objects, the epi lateral overgrowth (ELO) is significant and depends on the orientation via the faceting mechanism. As an illustration, Fig.2 displays active areas before epi and after epi in patterns orientated along <110> and <100>. It appears that <100> ELO is much more extended than the <110> one. This has some important consequences we have recently reported (2-3). Firstly, it changes the epi morphology. Secondly, it leads to new effects such as “anisotropic loading effect” and “time-non-linear deposition kinetics”. Furthermore, the small size of epi makes the surface energy dominant (larger than the giga-pascal stress of SiGe) and induces a rapid thermal rounding and even morphological instabilities more intense and rapid that Stranski-Krastanov ones. Both phenomena are illustrated in Fig.3. In the final part and as a conclusion, we will summary and discuss the recent and future trends in epi; and among the new processes, cyclic deposition illustrated in Fig.4 appears as the most promising. Figure 1

Thermal Rounding Kinetics and Demonstration of Steep and Flat Facets Succession in Selective Si-Based Epitaxy

In the first part of this work, a quantitative study of rounding kinetics in Si1-xGex (SiGe) epitaxies on patterned wafers is shown. A perfect screening of different patterns’ morphological evolution during annealing is performed by AFM. This allows the kinetics to be quantified by measuring the displaced volume of matter during the annealing. Two energies are extracted: 7.1eV between 650°C and 675°C and 1.2eV between 675°C and 700°C. The second part deals with the behavior of Si-based caps deposited on a non-flat surface. Different caps (Si vs SiGe) of different thicknesses were used. Both caps do not behave in the same way since SiGe caps are more efficient to flatten the rounded surface and obtain a “standard” morphology as if it was deposited on a perfectly flat surface. This is not the case for Si which makes a succession of terraces when deposited on such a surface.

SiGe channel deposition by batch epitaxy

Batch epitaxy has been introduced for high volume manufacturing of SiGe channels in order to reduce the cost for this epitaxial process by a factor of 3. Beside cost, SiGe channel deposition by batch epitaxy offers many benefits for manufacturing. The stability of the process and the reduced variability of the SiGe thickness greatly improve the variation of VT. The batch epitaxy process does not show a pattern loading effect for SiGe thickness reducing the complexity for manufacturing significantly. However, since the tool concept is very different to that of the widely used single wafer tools, there are some tool specific issues that need to be managed. The wafer backside is critical for batch epitaxy. A nitride backside facing the front side of the wafer results in a clear degradation of the uniformity and a change of the morphology of the SiGe channel compared to that facing a Si backside. The thermal rounding is more pronounced for the channels deposited in a batch tool for both large and narrow width devices. The device parameters of the large width device are not affected by thermal rounding but the performance of the narrow width device is clearly degraded. The thin SiGe layer at the edge of the channel driven by thermal rounding affects the VT and thus the effective device width. An in-situ etching before SiGe deposition to avoid thermal rounding was not feasible due to defects issues which were induced by the wafer backside. Finally a thermal rounding of the Si by an aggressive H2 bake before SiGe deposition improves the SiGe channel uniformity and recovers the performance degradation of the narrow width device partly.

(Invited) “Small Size” Effects in Si-Based Epitaxies for Advanced CMOS Technologies

Most of the Small Size Effects that take place in epitaxies used in advanced CMOS are reviewed. The most common one is the faceting that is a consequence of different deposition kinetics of the dense crystal planes. On the other hand, when considering small areas, the deposition rate is not a constant and the effective thickness depends on the pattern size and orientation. Thus, the concepts of “time-nonlinear kinetics” or “anisotropic loading effect” have been proposed. These epitaxies are also very sensitive to temperature. Thermal rounding takes place at temperatures as low as 600-625°C. Moreover, with higher thermal budgets, surface energy is capable, as a function of boundary conditions, to create instabilities such as Plateau-Rayleight ones or on the contrary to stabilize the Stranski-Krastanov ones. Finally, the importance of these mechanisms is illustrated in two CMOS applications: thermal rounding and anisotropic loading in SiGe channels and faceting in elevated sources/drains.

(Invited) “Small Size” Effects in Si-Based Epitaxies for Advanced CMOS Technologies

Epitaxy (epi) is widely used in recent and advanced CMOS technologies for SiGe channels, SiGe stressors or raised sources/drains… and the device performances rely on the morphology and structure of these epitaxial layers. As deposition takes place in very small (10-40 nm) areas, we will see that physical mechanisms such as faceting, thermal rounding and agglomeration that strongly influence the final morphology cannot be ignored. Faceting. The first noticeable feature of epi or selective epi (SEG) is faceting [1]. This morphology has been demonstrated to be due mainly to kinetic considerations and not to equilibrium ones such as in Wulff construction [2]. The concept is that some (low index) planes present a smooth growth and the extension of these facets is determined by the relative deposition kinetics of these planes and by the configuration. Fig.1 illustrates the concept in the case of Si SEG between two walls. On the left, the SEM cross-section gives the epi morphology evolution owing to SiGe markers introduced during deposition. On the right, we have plotted the possible growth of different planes, and the transitory and the final morphologies are determined by the most limiting ones, i.e. {311} at high T (800°C). Anisotropic Loading Effects. When faceted SEG is realized in very small active areas, then with a significant vertical/horizontal aspect ratio, we have to introduce a new concept that has been named Anisotropic Loading Effects [3]. Indeed, considering as an example the case of <100> and <110>-oriented lines on (100) wafers, a similar deposition (same nominal thickness) gives rise to different morphologies and to different amounts of deposited material. The situation is illustrated in Fig.2. Depositions in <100>-oriented lines are limited by {100} facets and exhibit an important overgrowth whereas <110>-oriented ones are limited by {111} facets and with a very small overgrowth. Clearly, the amount of deposited material is not the same and if we define an effective thickness as the ratio between the deposited volume and the active area surface, <100>-oriented lines exhibit a growth rate much higher that the <110>-ones. Simple calculations with a <111> to <100> GR ratio of 0.32 led to the results given in Fig.3. Thermal rounding In first part, we mentioned that faceted epi does not correspond to equilibrium. This is illustrated in Fig. 4 where a SiGe SEG has been submitted to a moderate thermal budget: 30 sec at 637°C. Whereas the as-deposited morphology was fully faceted, like in Fig.2a, it is observed that the final morphology is very rounded; then, even very moderate thermal budgets can induce very important morphology changes. Indeed, very small objects are very sensitive to thermal rounding. Pattern-induced stability/instability Thermal rounding takes place in any small pattern and combines with other mechanisms such as SK instabilities in SiGe [4]. Taking the example of narrow lines on (100) wafers, we also discovered that this rounding is capable either to stabilize SK instabilities, either to induce new instabilities such as the Plateau-Rayleigh ones, and also that the morphology stability depends on the crystalline orientation. As an illustration, Fig. 5 shows AFM pictures of Si (a) and SiGe (b) epi in narrow lines; note that Si lines are instable whereas some SiGe lines are stable. CMOS SiGe Channel In recent “gate first” technologies (32-28 nm), SiGe is often used in <100>-aligned pMOS channels for threshold voltage (VT) and performances (hole mobility), and the epi has to be precisely controlled. In this context, we found that the as-deposited morphology was not suitable and that a certain epi rounding, as given in Fig.4, was able to optimize the control of VT, to improve the driven current (effective width increase) and the technology yield (no parasitic stringer present) CMOS elevated Sources/Drains FD-SOI technologies rely on ultra-thin channels and require raised sources/drains (RSD) to get a proper silicidation. However, this extra deposition also forms with the spacer and the gate electrode a parasitic capacitor that limits the commutation speed of the transistor. This effect can be reduced by changing the RSD epi morphology. Indeed, Fig.6 reports possible morphologies, and as indicated schematically, faceting (b) may be used to decrease this parasitic capacitor, as compared to a non-faceted one (a), and to produce more performing devices.In the full paper, physics and integration will be detailed.

Pinning-dependent field-driven domain wall dynamics and thermal scaling in an ultrathin Pt/Co/Pt magnetic film.

Magnetic-field-driven domain wall motion in an ultrathin Pt/Co(0.45 nm)/Pt ferromagnetic film with perpendicular anisotropy is studied over a wide temperature range. Three different pinning dependent dynamical regimes are clearly identified: the creep, the thermally assisted flux flow, and the depinning, as well as their corresponding crossovers. The wall elastic energy and microscopic parameters characterizing the pinning are determined. Both the extracted thermal rounding exponent at the depinning transition, ψ=0.15, and the Larkin length crossover exponent, ϕ=0.24, fit well with the numerical predictions.

Si-Passivation of Epitaxial SiGe: Kinetics and Impact on Morphology

In this work a process was developed to passivate epitaxial SiGe on Si (001) substrates by a very thin Si layer. The Si-passivation was characterized in terms of deposition kinetics and of impact on the SiGe morphology during annealing. Different passivation durations allowed determining the time-dependence of the deposited thickness. A non-linear behavior was found resulting in a non-constant growth rate which decreases rapidly during the first seconds of passivation. Concerning the morphology of annealed SiGe, we showed that it can be strongly influenced by Si-passivation. The undulations appearing due to strain relaxation decrease in amplitude with increasing passivation duration until disappearance. In contrast, the morphology observed at the pattern edge, caused by thermal rounding, was not stabilized with similar passivation and annealing conditions.

Morphology evolution of epitaxial SiGe and Si in patterns

The morphology of Si and Si0.73Ge0.27 (SiGe), deposited in 〈100〉 and 〈110〉 oriented patterns on (001) Si wafers, has been studied after an annealing step. Thin films of 19nm have been realized by selective epitaxy in patterns of varying size and have subsequently been annealed in the epitaxy chamber. It has been shown that the morphology depends on both the size and orientation of the patterns. At the edges and corners of the patterns, thermal rounding of the faceted as-deposited morphology occurs during annealing, leading to a certain local thickening of the film. The influence of this effect on the morphology has been found to increase with decreasing pattern size. Due to the additional contribution of strain relaxation the effect is enhanced by a factor of 2 in the case of SiGe. In large patterns, anisotropic SK-like strain relaxation of SiGe along 〈100〉 directions results in smooth 〈100〉 oriented edges and undulated 〈110〉 oriented edges. In small patterns, the morphology results in ridges when the pattern orientation is parallel to the direction of strain relaxation and in faceted islands when the pattern orientation is turned by 45°. In the case of very narrow lines, the anisotropic loading effect during deposition controls the morphology after annealing. The result is the formation of instabilities in 〈100〉 oriented lines, for both Si and SiGe.

Thermal rounding of the depinning transition in ultrathin Pt/Co/Pt films

We perform a scaling analysis of the mean velocity of extended magnetic domain walls driven in ultrathin Pt/Co/Pt ferromagnetic films with perpendicular anisotropy, as a function of the applied external field for different film-thicknesses. We find that the scaling of the experimental data around the thermally rounded depinning transition is consistent with the universal depinning exponents theoretically expected for elastic interfaces described by the one-dimensional quenched Edwards-Wilkinson equation. In particular, values for the depinning exponent $\beta$ and thermal rounding exponent $\psi$ are tested and the present analysis of the experimental data is compatible with $\beta=0.33$ and $\psi=0.2$, in agreement with numerical simulations.