Mirko Fraschke

Impact of Temperature on the Resistive Switching Behavior of Embedded

Back-end-of-line integrated 1 × μm2 TiN/HfO2/Ti/TiN MIM memory devices in a 0.25- μm complementary metal-oxide-semiconductor technology were built to investigate the conduction mechanism and the resistive switching behavior as a function of temperature. The temperature-dependent I- V characteristics in fresh devices are attributed to the Poole-Frenkel mechanism with an extracted trap energy level at φ ≈ 0.2 eV below the HfO2 conduction band. The trap level is associated with positively charged oxygen vacancies. The electroformed memory cells show a stable bipolar switching behavior in the temperature range from 213-413 K. The off -state current increases with temperature, whereas the on-state current can be described by a weak metallic behavior. Furthermore, the results suggest that the I-V cycling not only induces significant changes in the electrical properties of the MIM memory devices, i.e., the increase in the off-state current, but also stronger temperature dependence. The temperature effect on the on-state and off-state characteristics is modeled within the framework of the quantum point-contact model for dielectric breakdown using an effective temperature-dependent confinement potential.

\hbox{HfO}_{2}

We demonstrate tunable Schottky barrier height and record photo-responsivity in a new-concept device made of a single-layer CVD graphene transferred onto a matrix of nanotips patterned on n-type Si wafer. The original layout, where nano-sized graphene/Si heterojunctions alternate to graphene areas exposed to the electric field of the Si substrate, which acts both as diode cathode and transistor gate, results in a two-terminal barristor with single-bias control of the Schottky barrier. The nanotip patterning favors light absorption, and the enhancement of the electric field at the tip apex improves photo-charge separation and enables internal gain by impact ionization. These features render the device a photodetector with responsivity (3 A/W for white LED light at 3 mW/cm2 intensity) almost an order of magnitude higher than commercial photodiodes. We extensively characterize the voltage and the temperature dependence of the device parameters and prove that the multi-junction approach does not add extra-inhomogeneity to the Schottky barrier height distribution. This work represents a significant advance in the realization of graphene/Si Schottky devices for optoelectronic applications.

-Based RRAM Devices

Filament-type HfO2-based RRAM has been considered as one of the most promising candidates for future non-volatile memories. Further improvement of the stability, particularly at the “OFF” state, of such devices is mainly hindered by resistance variation induced by the uncontrolled oxygen vacancies distribution and filament growth in HfO2 films. We report highly stable endurance of TiN/Ti/HfO2/Si-tip RRAM devices using a CMOS compatible nanotip method. Simulations indicate that the nanotip bottom electrode provides a local confinement for the electrical field and ionic current density; thus a nano-confinement for the oxygen vacancy distribution and nano-filament location is created by this approach. Conductive atomic force microscopy measurements confirm that the filaments form only on the nanotip region. Resistance switching by using pulses shows highly stable endurance for both ON and OFF modes, thanks to the geometric confinement of the conductive path and filament only above the nanotip. This nano-engineering approach opens a new pathway to realize forming-free RRAM devices with improved stability and reliability.

Tunable Schottky barrier and high responsivity in graphene/Si-nanotip optoelectronic device

Integrated AlN/SiO2/Si (1 0 0) delay lines for Rayleigh surface acoustic waves (SAWs) with resonant frequencies up to 3.4 GHz were fabricated using a new CMOS compatible concept. Different thicknesses of textured AlN films with wurtzite structure were deposited on tungsten-based interdigital transducers embedded in a SiO2 layer by reactive pulse dc-sputtering at a temperature of 200 °C. Rocking curves of the films indicate c-axis (0 0 0 1) oriented, textured piezoelectric AlN films with a full-width at half-maximum of 1.88°. The determined propagation loss and coupling factor K2 of these SAW devices are 0.07 dB/λ and 0.78%, respectively. Different Rayleigh modes with acoustic velocities up to 5770 m s−1 are observed. By varying the wavelength, number of fingers as well as the length (i.e., the separation between the transducers) of the delay lines, the impact of several physical parameters on the frequency responses (S11, S21) was studied. The influence of the AlN thickness and of the orientation of the delay lines on the silicon (1 0 0) wafers was also investigated. Finite element method simulations were applied to model the resonant frequencies, giving resonant frequencies in reasonable agreement with the experimental data.

Geometric conductive filament confinement by nanotips for resistive switching of HfO2-RRAM devices with high performance

A CMOS compatible AlN/SiO2/Si3N4/Si(100) surface acoustic wave (SAW) device has been fabricated and will be compared with standard AlN/SiO2-based devices. The presented filter demonstrates high potential for CMOS integrated high-frequency SAW devices. The filter insertion loss could be improved to -12.8 dB. The device exhibits high crosstalk suppression of -50 dB on a standard Si-substrate (10 Ωcm). X-ray diffraction, (scanning) transmission electron microscopy, and energy dispersive X-ray spectroscopy studies correlate the signal quality with c -axis orientation of aluminum nitride films on interdigitated transducer finger electrodes. Finite-element method simulations are in good agreement with the electric measurements and show typical Rayleigh particle displacement.

Monolithic integrated SAW filter based on AlN for high-frequency applications

Graphene based electron field emitter arrays consisting of cone-shaped silicon tips, a thin Al2O3 tunnel barrier, and graphene top electrode are fabricated. Due to the monolayered graphene top electrode, the electrons are able to tunnel through the Al2O3 layer and emit into the vacuum. The temperature behavior of the tunnel leakage current as well as the emission current is characterized.

AlN/SiO 2 /Si 3 N 4 /Si(100)-Based CMOS Compatible Surface Acoustic Wave Filter With −12.8-dB Minimum Insertion Loss

We investigate the use of perfluorodecyltrichlorosilane-based self-assembled monolayer as seeding layer for chemical vapour deposition of HfO2 on large area CVD graphene. The deposition and evolution of the FDTS-based seed layer is investigated by X-ray photoelectron spectroscopy, Auger electron spectroscopy, and transmission electron microscopy. Crystalline quality of graphene transferred from Cu is monitored during formation of the seed layer as well as the HfO2 growth using Raman spectroscopy. We demonstrate that FDTS-based seed layer significantly improves nucleation of HfO2 layers so that graphene can be coated in a conformal way with HfO2 layers as thin as 10 nm. Proof-of-concept experiments on 200 mm wafers presented here validate applicability of the proposed approach to wafer scale graphene device fabrication.

Graphene based electron field emitter

We introduce an approach that combines a 3” InP-DHBT transferred-substrate process with a SiGe-BiCMOS process. First, silicon and InP wafers are processed separately in different fabs. The silicon wafer runs through the complete 0.25 μm BiCMOS production process with five metal layers aluminum/tungsten back-end-of-line using silicon dioxide as dielectric. The processing was adapted for the following wafer bond process by planarization of the topmost metal level. This process flow was improved by using a SiN CMP stop layer on top of the metal layer stack, comparable to trench fill planarization. In that way a low surface topography was reached, this guarantees proper bonding results. Different mm-wave circuits operating at frequencies up to 246 GHz were produced to demonstrate the capability of the process flow.

Perfluorodecyltrichlorosilane-based seed-layer for improved chemical vapour deposition of ultrathin hafnium dioxide films on graphene.

Summary The generation of surface acoustic waves using a process, which is compatible with standard BEOL fabrication was demonstrated. In this process, the inter-digitated acoustic transducers are embedded in a silicon dioxide matrix, which is subsequently coated with the piezoelectric material ZnO. The delay lines with sub-μm wavelength fabricated by using this process achieve resonance frequencies in the range of 1 to 4 GHz, thus opening the door for the monolithic integration of acoustic devices in the (Bi)CMOS technology. In addition, the reliable bipolar resistive switching characteristics of TiN/HfO 2 /Ti/TiN based MIM diodes processed by a BiCMOS technology compatible process flow is demonstrated. The voltages, which are required to switch the devise from the On into the Off state and vise versa are + 1 V respectively - 1 V. The resistance ratio R OFF / R ON is in the range of 5. Acknowledgments This work was supported by the grant from German BMBF (grant No. 13N9891). ECS Transactions, 33 (6) 823-829 (2010)828

Silicon nitride stop layer in back-end-of-line planarization for wafer bonding application

In this paper, wafer level transfer of graphene on to a dielectric substrate is demonstrated based on SiO2-SiO2 fusion bonding and de-bonding processes. The developed technique allows to transfer graphene on 200 mm wafer without any contamination; thus CMOS compatible. The experimental data verifies the successful transfer of the graphene on to another substrate with high quality and a yield value of 98% with average 3.5 kΩ/□ sheet resistance. To the best of authors knowledge, it is the first time demonstration of the graphene transfer based on SiO2-SiO2 fusion bonding and de-bonding process on 200 mm wafer level which would allow a complete integration of graphene material into a CMOS line and opens the way for new devices based on graphene material.