Manfred Reiche

Extended Arrays of Vertically Aligned Sub-10 nm Diameter [100] Si Nanowires by Metal-Assisted Chemical Etching

Large-area high density silicon nanowire (SiNW) arrays were fabricated by metal-assisted chemical etching of silicon, utilizing anodic aluminum oxide (AAO) as a patterning mask of a thin metallic film on a Si (100) substrate. Both the diameter of the pores in the AAO mask and the thickness of the metal film affected the diameter of SiNWs. The diameter of the SiNWs decreased with an increase of thickness of the metal film. Large-area SiNWs with average diameters of 20 nm down to 8 nm and wire densities as high as 10 (10) wires/cm (2) were accomplished. These SiNWs were single crystalline and vertically aligned to the (100) substrate. It was revealed by transmission electron microscopy that the SiNWs were of high crystalline quality and showed a smooth surface.

Hydrophobic silicon wafer bonding

Wafers prepared by an HF dip without a subsequent water rinse were bonded at room temperature and annealed at temperatures up to 1100 °C. Based on substantial differences between bonded hydrophilic and hydrophobic Si wafer pairs in the changes of the interface energy with respect to temperature, secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM), we suggest that hydrogen bonding between Si‐F and H‐Si across two mating wafers is responsible for room temperature bonding of hydrophobic Si wafers. The interface energy of the bonded hydrophobic Si wafer pairs does not change appreciably with time up to 150 °C. This stability of the bonding interface makes reversible room‐temperature hydrophobic wafer bonding attractive for the protection of silicon wafer surfaces.

Fabrication of monodomain alumina pore arrays with an interpore distance smaller than the lattice constant of the imprint stamp

Large-area monodomain porous alumina arrays with an interpore distance of 500 nm are fabricated by imprint lithography. A 4 in. imprint master fully compatible with silicon technology was developed, which allows imprint pressures as low as 5 kN/cm2 for direct imprint on aluminum. Due to the self-ordering phenomenon of porous alumina growth, we were able to reduce the interpore distance of the pore array to 60% of the lattice constant of the master stamp. Three lithographically defined pores are sufficient to guide anodization of a new pore in the center.

Glassy Dynamics in Condensed Isolated Polymer Chains

Polymer Dynamics While free surfaces should allow polymer chains to move faster than in the bulk, the presence of a substrate might slow down the motion if there is an attraction between the two. Tress et al. (p. 1371; see the Perspective by Russell) used dielectric spectroscopy to study “polymer islands” deposited on a substrate from dilute solution, where some islands contained just a few or only one polymer chain. The confinement of the polymer chain to small-surface geometries had virtually no influence on the dynamics of the polymers, aside from the segments in direct contact with the substrate. The glass transition of isolated polymer chains is mainly bulk-like, with altered dynamics only for segments at the substrate. [Also see Perspective by Russell] In the course of miniaturization down to the nanometer scale, much remains unknown concerning how and to what extent the properties of materials are changed. To learn more about the dynamics of condensed isolated polymer chains, we used broadband dielectric spectroscopy and a capacitor with nanostructured electrodes separated by 35 nanometers. We measured the dynamic glass transition of poly(2-vinylpyridine) and found it to be bulk-like; only segments closer than 0.5 nanometer to the substrate were weakly slowed. Our approach paves the way for numerous experiments on the dynamics of isolated molecules.

Wafer bonding for microsystems technologies

In microsystems technologies, frequently complex structures consisting of structured or plain silicon or other wafers have to be joined to one mechanically stable configuration. In many cases, wafer bonding, also termed fusion bonding, allows to achieve this objective. The present overview will introduce the different requirements surfaces have to fulfill for successful bonding especially in the case of silicon wafers. Special emphasis is put on understanding the atomistic reactions at the bonding interface. This understanding has allowed the development of a simple low temperature bonding approach which allows to reach high bonding energies at temperatures as low as 150°C. Implications for pressure sensors will be discussed as well as various thinning approaches and bonding of dissimilar materials.

Sub-20 nm Si/Ge Superlattice Nanowires by Metal-Assisted Etching

An effective and low-cost method to fabricate hexagonally patterned, vertically aligned Si/Ge superlattice nanowires with diameters below 20 nm is presented. By combining the growth of Si/Ge superlattices by molecular beam epitaxy, prepatterning the substrate by anodic aluminum oxide masks, and finally metal-assisted chemical wet etching, this method generates highly ordered hexagonally patterned nanowires. This technique allows the fabrication of nanowires with a high area density of 10(10) wires/cm(2), including the control of their diameter and length.

Wafer bonding of silicon wafers covered with various surface layers

Studies dealing with the bonding behavior of silicon wafers coated with thermal oxide, plasma-enhanced chemical vapor deposition (PE-CVD) oxide, PE-CVD oxynitride, PE-CVD nitride and low-pressure (LP) CVD nitride are presented. The PE-CVD layers require a chemo-mechanical polishing (CMP) before bonding to reduce the surface roughness. The bonding energies of the wafer pairs with different interface layers are similar to those of bonded hydrophilic silicon wafers. Infrared microscopy of patterned wafer pairs reveals interfaces which are almost free of bubbles. Presumably, the gas, which usually generates such interface bubbles, diffuses into the interface cavities. The tensile strengths of patterned wafer pairs including a PE-CVD oxide or a PE-CVD oxynitride interface layer are about twice as high as for patterned hydrophilic silicon wafer pairs.

Segmental and chain dynamics in nanometric layers of poly(cis-1,4-isoprene) as studied by broadband dielectric spectroscopy and temperature-modulated calorimetry

Segmental and chain dynamics in nanometric (7–400 nm) layers of poly (cis-1,4-isoprene) (PI) are analyzed by Broadband Dielectric Spectroscopy (BDS) and temperature-modulated AC calorimetry. While for the segmental mode, taking place at the length scale of 2–3 polymer segments and corresponding to the dynamic glass transition, no dependence on the layer thickness and molecular weight is found, the normal mode, reflecting fluctuations of the end-to-end vector of the chain, shows pronounced effects: (i) it strongly varies in its relaxation strength with the layer thickness; (ii) for polymers having a molecular weight Mw comparable to M*, the critical molecular weight marking the onset of reptation dynamics, the mean spectral position does not change with the thickness, (iii) in contrast, polymers with Mw > M* are found to be severely influenced in their relaxation strength and the mean spectral position of the normal mode relaxation, and (iv) it is proven that the concentration of the polymer solution out of which the layers are prepared by spincoating has a hitherto unrecognized impact on the chain dynamics in (one-dimensional) nanometric confinement. These results prove that the dynamic glass transition in thin layers of PI is not influenced by nanometric confinement, while the chain dynamics are altered in a manifold of ways due to interactions with the surface of the underlying substrate.

Characteristics of Germanium-on-Insulators Fabricated by Wafer Bonding and Hydrogen-Induced Layer Splitting

There is considerable interest in germanium-on-insulator (GeOI) because of its advantages in terms of device performance and compatibility with silicon processing. In this paper, fabricating GeOI by hydrogen-induced layer splitting and wafer bonding is discussed. Hydrogen in germanium exists in molecular form and is prone to outdiffusion, resulting in a storage-time dependence of blistering. In contrast to the case of silicon, little effect of substrate doping on blistering is observed in germanium. Hydrogen implantation in germanium creates both {100}- and {111}-type microcracks. These two types of platelets are located in the same region for (111)-oriented wafers, but in different zones for (100) samples. This variation in distribution explains the smoother splitting of (111) surfaces than that of (100) surfaces. Hydrogen implantation also introduces a significant concentration of charged vacancies, which affect dopant diffusion in the transferred germanium film. Boron, with a negligible Fermi-level dependence, shows an identical diffusion profile to that of bulk germanium. In contrast, phosphorus diffusion is enhanced in the fabricated GeOI layers. These results also shed light on the understanding of dopant diffusion mechanisms in germanium.

Size-Controlled Si Nanocrystals for Photonic and Electronic Applications

A new approach for the fabrication of ordered Si quantum dots fully compat ible with normal Si technology is presented. The preparation of SiO/SiO 2 superlattices represents a simple and efficient method for fabricating highly luminescent Si nanocryst als and allows independent control of size, size distribution, and density. The Si nanocrystals can be arrange d to a specific depth and for a specific number of layers with a nanometer adjustment. The density of the Si nanocrystals is in the range of 10 /cm. TEM and XRD investigations confirm control of the upper limit of the nanocrystal size to an average size of below 2.5 nm with a full w idth at half maximum of 0.6 nm. We report on TEM images showing early states of phase separati on in SiO/SiO2 superlattices and combine these results with IR and PL investigations. Three differe nt states of phase separation are distinguished and correlated to specific luminescence and infrared fe atures. Photoluminescence experiments after crystallization show a size-dependent blue shif t of the luminescence from 950 to 750 nm and a luminescence intensity comparable to porous Si. The nearly s ize-independent PL intensity observed in our SiO/SiO 2 superlattices indicates the achievement of independent control of crystal size and number. In addition, PECVD preparation of amorphous SiO/ SiO2 superlattices is reported which shows a similar size dependent luminescence after crystalliz ation.