Production of vertical nanowire resonators by cryogenic-ICP–DRIE

Silicon resonant sensors with large surface area-to-volume ratios provide high weighing sensitivity. This fact implies the possibility for detection of slight mass changes [i.e. by attached nanoparticles (NPs)]. Vertical silicon nanowire (SiNW) resonators are therefore suitable for exposure assessment or airborne NPs. SiNW arrays are top-down fabricated by nanolithography and subsequent inductively coupled plasma reactive ion etching at cryogenic temperature. Nanolithography is performed by conventional UV-lithography and nanoimprint for even smaller structures. Wire diameters are further reduced by multiple thermal oxidations and oxide stripping at times. Parameter effects of cryogenic dry etching are studied for SiNW arrays.     

Fabrication and Characterization of Nanopillars for Silicon-Based Thermoelectrics

Si-based nanopillars of various sizes were fabricated by lateral structuring using anisotropic etching and thermal oxidation. We obtained pillars of diameter <500 nm, about 25 μm in height, with an aspect ratio of more than 50. The distance between pillars was varied from 500 nm to 10 μm. Besides the fabrication and structural characterization of silicon nanopillars, implementation of adequate metrology for measuring single pillars is described. Commercial tungsten probes, self-made gold probes, and piezoresistive silicon cantilever probes were used for measurements of nanopillars in a scanning electron microscope (SEM) equipped with nanomanipulators.     

Anelastic properties of aluminium thin films on silicon cantilevers

Thin films (thickness 40 to 250 nm) of Al on microstructurized Si substrates have been investigated by the vibrating-reed technique (typical frequencies 100 Hz to 10 kHz) with strain amplitudes in the range of 10⁻⁷ to 10⁻⁴ and for temperatures up to 850 K. The combined evaluation of flexural and torsional vibrations permits to separate the complex shear modulus and biaxial modulus of the thin layer, which helps to identify the damping mechanisms. For Al thin films with thickness <200 nm, in addition to the well-known damping peak due to grain boundary sliding (peak temperature about 370 K), a further maximum of damping has been observed around 600 K, the nature of which is discussed.     

Cleaning of structured templates from nanoparticle accumulation using silicone

Polydimethylsiloxan (PDMS) turned out to be a simple and cost efficient material for the removal of nanoparticles from patterned surfaces. After molding the particle-laden surface using liquid silicone, surface cleaning is realized by curing the PDMS comprising the encapsulated particles and subsequent removal. The method is proven for silicon, SiO₂ and gold surfaces occupied by carbon and Polytetrafluorethylen (PTFE or Teflon) particles. Samples up to 2?inch wafers were successfully cleaned. The effect of PDMS on the surface energy is verified by contact angle measurements showing a clear change in wetting for H₂O. This effect is abolished by oxygen plasma and HF-Dip.     

Automatic counting of etch Pits in InP

A computer algorithm for automatic EPD-counting (etch-pit density) with an optical microscope is presented. Several dislocation etchants proposed in the literature to reveal structural defects on InP were employed and improved. Their reliability for automatic counting was proven. For a considerable number of samples exhibiting an EPD between 10⁴ and 10⁶ cm⁻² it is shown that the automatically counted number of etch pits agrees with the visually determined value within less than ±30%. Using a modified H₃PO₄:HBr etchant good results for automatically determined EPDs beyond 10⁷ cm⁻² were obtained on InP layers epitaxially grown on Si substrates.     

Thermoelectric Coolers with Sintered Silver Interconnects

The fabrication and performance of a sintered Peltier cooler (SPC) based on bismuth telluride with sintered silver interconnects are described. Miniature SPC modules with a footprint of 20 mm² were assembled using pick-and-place pressure-assisted silver sintering at low pressure (5.5 N/mm²) and moderate temperature (250°C to 270°C). A modified flip-chip bonder combined with screen/stencil printing for paste transfer was used for the pick-and-place process, enabling high positioning accuracy, easy handling of the tiny bismuth telluride pellets, and immediate visual process control. A specific contact resistance of (1.4 ± 0.1) × 10^?5 Ω cm² was found, which is in the range of values reported for high-temperature solder interconnects of bismuth telluride pellets. The realized SPCs were evaluated from room temperature to 300°C, considerably outperforming the operating temperature range of standard commercial Peltier coolers. Temperature cycling capability was investigated from 100°C to 235°C over more than 200 h, i.e., 850 cycles, during which no degradation of module resistance or cooling performance occurred.     

Mechanical spectroscopy of thin layers of PPV polymer on Si substrates

The vibrating reed technique with electro“static” excitation and optical detection has been applied to investigate thin layers of poly-phenylene-vinylene, deposited by spin coating onto microfabricated Si cantilevers, during temperature cycling programs between 90 and 540 K at a rate of 1 K/min. From the vibration frequencies the Young’s modulus of the film can be estimated to be about 10 MPa at room temperature in the precursor phase (if prepared from a solution in toluene), which increases by conversion to the conjugate bonded polymer to about 50 MPa. The temperature dependence of internal friction reveals the processes of γ relaxations (crankshaft motion of side branches in the precursor) and β-relaxation (movements of a few monomer blocks in the polymer chain), as well as peaks indicating the structural transformations during conversion, and possibly a glass transition in the amorphous precursor phase. After conversion only the β-relaxation persists.     

Finite element modeling and experimental proof of NEMS-based silicon pillar resonators for nanoparticle mass sensing applications

The potential use of nanoelectromechanical systems (NEMS) created in silicon nanopillars (SiNPLs) is investigated in this work as a new generation of aerosol nanoparticle (NP)-detecting device. The sensor structures are created and simulated using a finite element modeling (FEM) tool of COMSOL Multiphysics 4.3b to study the resonant characteristics and the sensitivity of the SiNPL for femtogram NP mass detection in 3-D structures. The SiNPL arrays use a piezoelectric stack for resonance excitation. To achieve an optimal structure and to investigate the etching effect on the fabricated resonators, SiNPLs with different designs of meshes, sidewall profiles, heights, and diameters are simulated and analyzed. To validate the FEM results, fabricated SiNPLs with a high aspect ratio of approximately 60 are used and characterized in resonant frequency measurements where their results agree well with those simulated by FEM. Furthermore, the deflection of a SiNPL can be enhanced by increasing the applied piezoactuator voltage. By depositing different NPs [i.e., gold (Au), silver (Ag), titanium dioxide (TiO₂), silicon dioxide (SiO₂), and carbon black NPs] on the SiNPLs, the decrease of the resonant frequency is clearly shown confirming their potential to be used as airborne NP mass sensor with femtogram resolution level. A coupling concept of the SiNPL arrays with piezoresistive cantilever resonator in terms of the mass loading effect is also studied concerning the possibility of obtaining electrical readout signal from the resonant sensors.     

Thermoelectric Properties of High-Doped Silicon from Room Temperature to 900 K

Silicon is investigated as a low-cost, Earth-abundant thermoelectric material for high-temperature applications up to 900 K. For the calculation of module design the Seebeck coefficient and the electrical as well as thermal properties of silicon in the high-temperature range are of great importance. In this study, we evaluate the thermoelectric properties of low-, medium-, and high-doped silicon from room temperature to 900 K. In so doing, the Seebeck coefficient, the electrical and thermal conductivities, as well as the resulting figure of merit ZT of silicon are determined.     

Silicon cantilever sensor for micro-/nanoscale dimension and force metrology

A piezoresistive silicon cantilever-type tactile sensor was described as well as its application for dimensional metrology with high-aspect-ratio micro components and as a transferable force standard in the micro-to-nano Newton range. As an example for micro-/nanoscale tactile probing metrology the novel cantilever sensor was used for surface scanning with calibrated groove and roughness artifacts. Micro-/nano-Newton force metrology using the novel cantilever sensor was addressed with calibration procedures which were developed for low-force stylus instruments as well as for glass micro pipettes designed for the manipulation of isolated living cells.