Denis Buttard

Toward two-dimensional self-organization of nanostructures using wafer bonding and nanopatterned silicon surfaces

The structure of ultrathin silicon layers obtained by molecular hydrophobic bonding is investigated. The twist and tilt angles between the two crystals are accurately controlled. The buried Si|Si interface is observed by transmission electron microscopy and by grazing incidence X-ray techniques. For low twist angle values (/spl psi/<5/spl deg/) plane view observations reveal well-defined dislocation networks. Cross-section observations give evidence that the dislocation networks are localized at the bonding interfacial plane with no threading dislocation. Grazing incidence small angle X-ray scattering measurements confirm the good quality of the bonding interface as well as the quality of the dislocation networks. Grazing incidence X-ray diffraction is also used and shows the long-range order of the periodic strain field in the silicon layer. It shows, especially, the interaction between the dislocations. X-ray reflectivity was employed and estimated that the interfacial thickness (i.e., thickness of the bonding) lower than 1 nm decreases when the twist angle increases. The nanopatterned surface is then investigated by scanning tunneling microscopy and X-ray methods. To validate these substrates for long-range order self-organization, the growth of Si and Ge quantum dots is finally achieved.

Highly organised and dense vertical silicon nanowire arrays grown in porous alumina template on silicon wafers

In this work, nanoimprint lithography combined with standard anodization etching is used to make perfectly organised triangular arrays of vertical cylindrical alumina nanopores onto standard <100>−oriented silicon wafers. Both the pore diameter and the period of alumina porous array are well controlled and can be tuned: the periods vary from 80 to 460 nm, and the diameters vary from 15 nm to any required diameter. These porous thin layers are then successfully used as templates for the guided epitaxial growth of organised mono-crystalline silicon nanowire arrays in a chemical vapour deposition chamber. We report the densities of silicon nanowires up to 9 × 109 cm−2 organised in highly regular arrays with excellent diameter distribution. All process steps are demonstrated on surfaces up to 2 × 2 cm2. Specific emphasis was made to select techniques compatible with microelectronic fabrication standards, adaptable to large surface samples and with a reasonable cost. Achievements made in the quality of the porous alumina array, therefore on the silicon nanowire array, widen the number of potential applications for this technology, such as optical detectors or biological sensors.

Nanometric patterning with ultrathin twist bonded silicon wafers

Abstract Ultrathin films of silicon bonded on 4-inch (001) silicon wafers have been obtained by combining a direct hydrophobic silicon bonding technique with a layer transfer. The strain field produced by the dislocation network localized at the bonded interface is a good candidate to induce a long-range order growth of nanostructure. To be able to make this new kind of substrate, knowledge of the dislocation strain field extension is essential. Grazing incidence X-ray diffraction allows us to measure its spatial extension through the diffraction peak satellites due to different dislocation networks. The exponential decay of these peaks were measured and compared. We found that the decrease of the strain field extension is almost two times lower for the screw dislocation network than for the ‘mixed’ dislocations one. The film thickness control is then two times more critical for the screw dislocations.

Gold colloidal nanoparticle electrodeposition on a silicon surface in a uniform electric field

The electrodeposition of gold colloidal nanoparticles on a silicon wafer in a uniform electric field is investigated using scanning electron microscopy and homemade electrochemical cells. Dense and uniform distributions of particles are obtained with no aggregation. The evolution of surface particle density is analyzed in relation to several parameters: applied voltage, electric field, exchanged charge. Electrical, chemical, and electrohydrodynamical parameters are taken into account in describing the electromigration process.

Grazing incidence X-ray studies of twist-bonded Si/Si and Si/SiO2 interfaces

Twist-bonded Si/Si (0 0 1) and Si/SiO2 interfaces have been investigated by grazing incidence X-ray scattering methods. For Si/Si (0 0 1) bonding, conventional X-ray reflectivity reveals the good quality of the interfaces in terms of flatness and roughness. In-plane grazing incidence diffraction measurements around the (2 2 0) reflection show satellite peaks close to the substrate and the layer diffraction peaks. These sharp satellites are produced by a periodic displacement resulting from a very regular buried dislocation network. The Si/SiO2 bonding has been studied with X-ray reflectivity within a transmission geometry. The analysis of the data shows the high quality of both bonded Si/SiO2 and thermal oxide SiO2/Si interfaces.

Ultradense and planarized antireflective vertical silicon nanowire array using a bottom-up technique

The production and characterization of ultradense, planarized, and organized silicon nanowire arrays with good crystalline and optical properties are reported. First, alumina templates are used to grow silicon nanowires whose height, diameter, and density are easily controlled by adjusting the structural parameters of the template. Then, post-processing using standard microelectronic techniques enables the production of high-density silicon nanowire matrices featuring a remarkably flat overall surface. Different geometries are then possible for various applications. Structural analysis using synchrotron X-ray diffraction reveals the good crystallinity of the nanowires and their long-range periodicity resulting from their high-density organization. Transmission electron microscopy also shows that the nanowires can grow on nonpreferential substrate, enabling the use of this technique with universal substrates. The good geometry control of the array also results in a strong optical absorption which is interesting for their use in nanowire-based optical sensors or similar devices.

Sub-10 nm Silicon Nanopillar Fabrication Using Fast and Brushless Thermal Assembly of PS-b-PDMS Diblock Copolymer.

A new approach to obtaining spherical nanodomains using polystyrene-block-polydimethylsiloxane (PS-b-PDMS) is proposed. To reduce drastically the process time, we blended a copolymer with cylindrical morphology with a PS homopolymer. Adding PS homopolymer into a low-molar-mass cylindrical morphology PS-b-PDMS system drives it toward a spherical morphology. Besides, by controlling the as-spun state, spherical PDMS nanodomains could be kept and thermally arranged. This PS-homopolymer addition allows not only an efficient, purely thermal arrangement process of spheres but also the ability to work directly on nontreated silicon substrates. Indeed, as shown by STEM measurements, no PS brush surface treatment was necessary in our study to avoid a PDMS wetting layer at the interface with the Si substrate. Our approach was compared to a sphere-forming diblock copolymer, which needs a longer thermal annealing. Furthermore, GISAXS measurements provided complete information on PDMS sphere features. Excellent long-range order spherical microdomains were therefore produced on flat surfaces and inside graphoepitaxy trenches with a period of 21 nm, as were in-plane spheres with a diameter of 8 nm with a 15 min thermal annealing. Finally, direct plasma-etching transfer into the silicon substrate was demonstrated, and 20 nm high silicon nanopillars were obtained, which are very promising results for various nanopatterning applications.

Grazing incidence x-ray scattering investigation of Si surface patterned with buried dislocation networks

Investigation of a surface patterned by buried dislocation networks is performed with grazing incidence x-ray scattering (GIXS). It is shown that surface long-range undulations lead to a azimuth-dependent diffusion spot, the scattering vector of which is mainly parallel to the x-ray propagation direction. This unusual scattering direction with GIXS is explained by the scales of the scattering objects. A geometrical model is proposed to extract the periodicity of the surface from GIXS.

Dislocation Networks Strain Fields Induced By Si Wafer Bonding.

Buried dislocation superlattices are obtained by bonding ultra-thin single crystal Si (001) films on Si (001) wafers. The twist of two Si wafers induces a regular square grid of dissociated screw dislocations and the tilt a 1-D array of mixed dislocation. The Burgers vector is a/2 for both types of dislocation. The atomic displacements and deformations of pure screw and edge dislocations are calculated with an isotropic elasticity approximation taking into account the free surface and the thickness of the upper crystal. It is shown by these calculations that the elastic strain field propagates up to the surface, and quantitative arguments are given to choose the network period / film thickness ratio.

X-ray reflectivity of silicon on insulator wafers

Abstract The ability of X-ray reflectivity to analyse different silicon on insulator structures is underlined. The standard geometry with first reflection occurring at the surface gives information about the thickness, roughness, and density of the layers. Deeply buried interfaces, i.e. in between thick wafers, are analysed with a non-standard geometry (the first reflection occurs at the buried interface) and with a high-energy radiation. These two methods are, respectively, illustrated by the reflectivity measurements of (SiO2/Si/SiO2|bulk Si) and (bulk Si/thermal SiO2|native SiO2/bulk Si) bonded structures, and are explained in the framework of kinematic theory of X-ray reflectivity.