D. Moser

Thermal phonon transport in silicon nanowires and two-dimensional phononic crystal nanostructures

Thermal phonon transport in silicon nanowires (Si NWs) and two-dimensional phononic crystal (2D PnC) nanostructures was investigated by measuring thermal conductivity using a micrometer-scale time-domain thermoreflectance. The impact of nanopatterning on thermal conductivity strongly depends on the geometry, specularity parameter, and thermal phonon mean free path (MFP) distribution. Thermal conductivities for 2D PnC nanostructures were found to be much lower than that for NWs with similar characteristic length and surface-to-volume ratio due to stronger phonon back scattering. In single-crystalline Si, PnC patterning has a stronger impact at 4 K than at room temperature due to a higher specularity parameter and a longer thermal phonon MFP. Nanowire patterning has a stronger impact in polycrystalline Si, where thermal phonon MFP distribution is biased longer by grain boundary scattering.

Electrical and thermal properties of polycrystalline Si thin films with phononic crystal nanopatterning for thermoelectric applications

Electrical and thermal properties of polycrystalline Si thin films with two-dimensional phononic patterning were investigated at room temperature. Electrical and thermal conductivities for the phononic crystal nanostructures with a variety of radii of the circular holes were measured to systematically investigate the impact of the nanopatterning. The concept of phonon-glass and electron-crystal is valid in the investigated electron and phonon transport systems with the neck size of 80 nm. The thermal conductivity is more sensitive than the electrical conductivity to the nanopatterning due to the longer mean free path of the thermal phonons than that of the charge carriers. The values of the figure of merit ZT were 0.065 and 0.035, and the enhancement factors were 2 and 4 for the p-doped and n-doped phononic crystals compared to the unpatterned thin films, respectively, when the characteristic size of the phononic crystal nanostructure is below 100 nm. The greater enhancement factor of ZT for the n-doped s...

Determination of the thermoelectric figure of merit of doped polysilicon thin films by micromachined test structures

This paper reports on the determination of the thermoelectric figure of merit ZT of doped polysilicon (poly-Si) thin films. For the extraction of accurate thermal properties, micromachined membrane test structures for the Seebeck coefficient S and the thermal conductivity κ of poly-Si were developed. The electrical resistivity ρ is measured using van-der-Pauw structures. From the results the temperature dependent value of ZT = S²Tk^-l ρ^-1 is extracted. The material parameters were measured in the temperature range from 90 to 370 K. A ZT of (10.08±0.14)×10^-3 and (10.29±0.12)×10^-3 was obtained at 290 K for n- and p-doped poly-Si thin films. The corresponding material parameters for n-doped material are k = 25.0 Wm^-1K^-1, S = -81 μVK^-1 and ρ = 7.58 μΩm. For p-doped poly-Si the values are found to be κ = 40.6 Wm^-1K^-1, S = 240 μVK^-1 and ρ = 40.1 μΩm.

Fatigue Testing of Polyimide-Based Micro Implants

Due to its excellent chemical and mechanical properties, polyimide is frequently used as substrate material for electrodes in biomedical applications. In addition, highly flexible polyimide ribbon cables are integrated in hybrid micro devices such as silicon-based neural probes to connect these recording and/or stimulation units with external instrumentation. Typically a metallization layer, e.g. gold or platinum, is sandwiched between two polyimide layers with an implant thickness in the range of 10 µm. This paper describes a bending setup constructed to perform fatigue tests with dedicated polyimide specimens in physiological environment. A pivot mounted rod bends the specimens around defined radii of 1, 1.5, 2 or 6 mm at a maximum frequency of 2 Hz. The system concept allows to freely choose angels between 0° and 90° in steps of 0.9°. The applied test method is adapted to t European standard DIN EN 45502-2-1 for pacemaker leads, stipulating the application of mechanical loads by repetitive bending of the specimens. The machine allows the simultaneous testing of up to five samples in a physiological environment. Additionally, online four-wire resistance measurements are performed to detect functional failures during the test. Control specimens are stored at equal conditions without mechanical load. The resistance of all tested samples remained stable longer then the 47,000 cycles demanded in the standard.

Ultrathin, dual-sided silicon neural microprobes realized using BCB bonding and aluminum sacrificial etching

This paper presents an innovative fabrication process for dual-sided silicon-based microprobe arrays using (i) temporary wafer bonding applying B-staged bisbenzo-cyclobutene (BCB), (ii) wafer grinding, (iii) deep reactive ion etching (DRIE), and (iv) the electrochemical removal of a sacrificial aluminum layer. The dual-sided microprobes comprise aligned electrodes on the front and rear of 120-μm-wide and only 50-μm-thick probe shafts. The temporary BCB bonding to a glass substrate is compatible with process temperatures up to 300°C and with DRIE. Furthermore, dual-side mask alignment is enabled by the high optical transparency of both the glass substrate and the BCB bonding layer. Even at this exploratory stage, probes realized using this process sequence have exhibited a yield of functional electrodes of better than 96% after probe assembly. Initial in vivo electrophysiology recordings in a rat brain have demonstrated an satisfactory probe performance.

Fabrication of microfluidic neural probes with in-channel electrodes

This paper reports on the design and fabrication of a neural probe for simultaneous recording of neural activity and localized drug delivery. In comparison to common microfluidic probes, additional electrodes facing the fluidic channel are integrated in the polyimide-based membrane covering the microchannels. The novel fabrication process applies two-sided deep reactive ion etching (DRIE) of silicon and the wafer-level transfer of the thin membrane cover onto patterned silicon substrates. The process to transfer the membrane uses low-temperature adhesive bonding. It allows integrated electrodes to be exposed either to the surrounding neural tissue or the fluidic channels. The electrical functionality of the probes was verified using electrochemical impedance spectroscopy of both electrode types. The fluidic function was demonstrated by injecting a dyed fluid through the integrated microchannels.

Microstructure for the determination of the Seebeck coefficients of doped polysilicon thin films

This paper reports on microstructures used for the determination of the Seebeck coefficient of doped polycrystalline silicon thin film. The design and fabrication of planar and micromachined, membrane based, test structures and the measurement of the Seebeck coefficients of n-and p-doped polysilicon layers in the temperature range from 300 K to 350 K are described. The Seebeck coefficients of the n-and p-doped material at 300 K are −87.5±3.7 μV/K and 247.3±17.5 μV/K with respective temperature dependences of −0.261±0.011 μV/K2 and 0.205±0.036 μV/K2 in the temperature range from 300 K to 350 K.

Compact multifunctional test structure to measure the in-plane thermoelectric figure of merit ZT of thin films

In response to recently renewed interest in thermoelectrics, this paper reports a novel compact, multifunctional test structure to measure the in-plane thermoelectric figure of merit ZT of thin films. All material parameters contributing to ZT = S²Tκ^-1ρ^-1 are determined on a single sample with dimensions of about 500×500 μm². These are the Seebeck coefficient S, the thermal conductivity κ, and the electrical resistivity ρ. The method can be applied to thin films deposited at high temperature (T), such as poly-Si, and at low T, such as metal layers. We report the temperature-dependent ZT of n-doped poly-Si from 300 to 380 K, as well as the application of the device to Al thin films.

Thermoelectric Properties Investigation of Single nanowires by utilizing a Thermoelectric Nanowire Characterization Platform

We demonstrate the design and fabrication of a novel micromachined Thermoelectric Nanowire Characterization Platform (TNCP) which is utilized to characterize the thermoelectric properties of various nanowires. Single nanowire is assembled onto the pre-fabricated TNCP by means of dielectrophoresis (DEP) in combination with a water droplet evaporation scheme. After assembly, a reliable ohmic contact is generated between the bismuth telluride (Bi2Te3) nanowire and the underlying electrodes by means of scanning electron microscope (SEM) focused electron beam-induced deposition (EBID). Finally, the electrical conductivity and Seebeck coefficient of Silver (Ag) and Bi2Te3 nanowires are investigated and presented in this paper.

CMOS-based force sensor with overload protection and improved assembly tolerance

This paper reports on a novel CMOS-based force sensor with integrated overload protection and improved assembly tolerance. The device consists of an anodically bonded silicon-glass stack comprising an n-type Wheatstone bridge as the piezoresistive stress-sensing element. Narrow trenches defining cross or seesaw shaped structures are micromachined into the surface of the Si chip. Depending on the desired measurement range of the sensor, these etched structures are either suspended via two hinges over a cavity etched into the glass substrate or bonded directly to the glass chip. A two-point force bridge is used to split the load to be detected into two components acting on the free ends of the cross or seesaw structure. This sensor concept provides an overload protection and an increased assembly tolerance with regard to the force application point. Twelve low and one high force device have been fabricated and characterized for loads up to 10 N and 30 N, respectively.