Ultrashort Voltage Pulses Research Articles

Laser writing of memristive logic gates and crossbar arrays

Memristor-based logic circuits are gaining a lot of attention due to the potential for high logic density hardware and novel in-memory computing applications. Readily available methods for fabricating of memristor logic structures that are suitable for integration with conventional computer hardware are a growing need. This work presents a direct laser writing process capable of rapidly fabricating memristor logic circuits by laser irradiation of metal salt precursor solutions. Planar memristor patterns are fabricated, and their I–V response is characterized. Boolean logic gates are fabricated from planar memristor pairs that exhibit low programming voltages and rapid switching. Cu/Cu2O/Cu and Ag/Cu2O/Cu memristors are also fabricated in crossbar arrays, showing the ability to be programmed to multiple resistance states through ultrashort voltage pulses. The devices also show the potential to have high endurance and nonvolatile resistance state retention.

Ultra-fast switching memristors based on two-dimensional materials

The ability to scale two-dimensional (2D) material thickness down to a single monolayer presents a promising opportunity to realize high-speed energy-efficient memristors. Here, we report an ultra-fast memristor fabricated using atomically thin sheets of 2D hexagonal Boron Nitride, exhibiting the shortest observed switching speed (120 ps) among 2D memristors and low switching energy (2pJ). Furthermore, we study the switching dynamics of these memristors using ultra-short (120ps-3ns) voltage pulses, a frequency range that is highly relevant in the context of modern complementary metal oxide semiconductor (CMOS) circuits. We employ statistical analysis of transient characteristics to gain insights into the memristor switching mechanism. Cycling endurance data confirms the ultra-fast switching capability of these memristors, making them attractive for next generation computing, storage, and Radio-Frequency (RF) circuit applications.

Micromachining of predesigned perpendicular copper micropillar array by scanning electrochemical microscopy

Because the electrochemical reactions can occur gently at normal temperature and pressure, electrochemical micromachining (ECMM) is an important nontraditional precision machining technique free of tool wear, heating effect, residual stress, surface and subsurface damage, comparing with the ultra-precision milling, electrical discharging machining, laser beam machining, etc. Although scanning electrochemical microscopy (SECM) has been proven powerful in ECMM, it is difficult to fabricate the predesigned perpendicular microstructures because of the inhomogenous potential distribution, the complex electrode reaction kinetics and the interfacial mass transfer processes. Here we succeeded in fabricating the copper micropillar arrays by combining the SECM direct-writing mode and the ultrashort voltage pulse (USVP) modulations. The machining accuracy and efficiency are affected by multiple technical parameters, including the amplitude, the period and the duty cycle of the USVP modulations, the components and concentration of the electrolyte solution, the tip-substrate distance, the motion mode of the tool electrode, etc. By employing multiple parameters central composite experimental designs, we succeed in optimizing the technical parameters and avoiding the complexities of electrode processes. The results demonstrate the powerful capability of SECM in the controllable micromachining of the perpendicular metallic microstructures.

Theoretical analysis of micro/nano electrochemical machining with ultra-short voltage pulses

The effect of pulse-on and pulse-off times on the efficiency of micro/nano ECM with ultra-short voltage pulses is theoretically analyzed. A new approximate analytical equation for the time of charging electrical double layer (EDL) is obtained. The equation takes into account all main parameters of the process. It enables one to determine the pulse-on time that provides a high localization of Faradaic process in the zones with small interelectrode gap and a high productivity of ECM process. The results of numerical calculations agree well with the data calculated by the proposed approximate analytical equation. The numerical calculations of the pulse-off time are performed. The effect of reverse voltage pulse imposed in the pauses on the EDL relaxation time is analyzed. The amplitude of reverse voltage pulse is limited by the condition of the absence of the tool-electrode wear. The proposed algorithm for calculating the pulse-off time and amplitude of reverse voltage pulse provides the required high coefficient of metal dissolution localization in the zone of small interelectrode gaps at a given pulse-on time.

Justification in Selection of Voltage Pulse Durations at the Electrochemical Dimensional Micromachining with Nano- and Microsecond Pulses

The work is devoted to justifying the selection of a rational range of voltage pulse duration on the basis of comparing the time of the anodic dissolution start with the time of the electric double layer’s charging during electrochemical dimensional micromachining. The range of short circuit times as the limit of the maximum voltage pulse duration is justified. The simulation of dimensional electrochemical micromachining using ultrashort voltage pulses without taking into account the changes in the properties of the interelectrode environment is performed on the assumption that the anodic dissolution is determined by the Butler–Volmer equation. The zones of the current density, which have varying influence on the process of the dimensional electrochemical micromachining, are revealed. An approach to the selection of an effective voltage pulse duration based on the analysis of the character of the time zones of the process is proposed.

Asynchronous Optical Sampling for Laser-Based Vector Network Analysis on Coplanar Waveguides

We employ asynchronous optical sampling to realize a laser-based vector network analyzer for the measurement of complex electrical reflection coefficients. This sampling method utilizes two separate femtosecond lasers for the generation and detection of ultrashort voltage pulses on a waveguide structure and does not require any optical delay lines. The scheme allows us to increase the measurement epoch, enabling a frequency resolution of 76 MHz while simultaneously covering a broad frequency range up to approximately 500GHz. Our analysis comprises a direct comparison with measurements from a conventional synchronous optical sampling technique. We obtain a very good agreement between the two measurement methods. However, asynchronous sampling reveals a four times higher frequency resolution and shows a reduced residual systematic error. In addition, we investigate different laser stabilization methods utilizing laser-pulse-cross-correlation measurements. Our analysis reveals a temporal jitter of better than 46 fs with the possibility to use one free-running laser oscillator.

Investigation of Electrochemical Nanostructuring with Ultrashort Pulses by Using Nanoscale Electrode

Background: Electrochemical machining (ECM) is a non-traditional machining method for the metal material based on the principle of anode electrochemical dissolution which has been used in micro/nano fabrication with advantages as not influenced by materials intensity and hardness, no residual stress and no heat treatment born on the surface of the workpiece. Several researches and applications have shown that the surface quality can be improved effectively during the electrochemical machining by using ultrashort pulse power supply. Method: This paper presents a potential of electrochemical machining at the nanometer scale. First, a transient charging double layer mathematical model is developed to describe electrochemical nanostructuring of metallic materials with ultrashort (nanosecond) voltage pulses. And then, by using finite element method (FEM), the analysis model of electrochemical interface between poles is established to give a more realistic analysis of the comparison of transient currents at different separations between the tool and workpiece. Second, a nanoscale electrode is an essential tool in electrochemical nanostructuring. In this paper, electrodes with diameters of several ten to hundred nanometers are successfully prepared by the liquid membrane electrochemical etching. Finally, by using the nanometer scale electrodes above and the ultrashort pulse power supply, several nanostructures with physical dimension of several hundred nanometers are fabricated on nickelbased superalloys. Results: Using the optimal machining parameters, a tool electrode with 230 nm in diameter is obtained from the initial tungsten rod radius of 100 μm. By using 0.05 M H2SO4 solution, the pulse generator with 1μs in period, 100 ns in pulse on-time and 4 V in voltage, a micro/nano groove with the depth of 150 nm and maximum entrance width of 3 μm is obtained. Conclusion: Nanoscale electrodes with diameters of several ten to hundred nanometers is obtained successfully demonstrating that the liquid membrane electrochemical etching is a very effective method to fabricate nanoscale electrode. Several nanostructures with physical dimension of several hundred nanometers can be fabricated successfully demonstrating that ECM with ultrashort pulses is a highly promising nanostructuring technology.

Fabrication of Taper Free Micro-Holes Utilizing a Combined Rotating Helical Electrode and Short Voltage Pulse by ECM.

Fabrication of the injection nozzle micro-hole on the aero engine is a difficult problem in today’s manufacturing industry. In addition to the size requirements, the nozzle micro-hole also requires no burr, no taper and no heat-affected zone. To solve the above problem, an ultra-short voltage pulse and a high-speed rotating helical electrode were used in electrochemical drilling (ECD) process. Firstly, a theoretical model of ECD with ultra-short voltage pulse was established to investigate the effects of many predominant parameters on machining accuracy, and the effect of rotating helical electrode on the gap flow field was analyzed. Secondly, sets of experiments were carried out to investigate the effects of many key parameters on machining accuracy and efficiency. Finally, the optimized parameters were applied to machine micro holes on 500 μm thickness of GH4169 plate, and micro-holes with the diameter of 186 μm with no taper were machined at the feed rate of 1.2 μm/s. It is proved that the proposed ECD process for fabricating micro-holes with no taper has a huge potential and broad application prospects.

Drilling of microholes using electrochemical machining

Few studies have been reported on the drilling of microholes with a diameter of approximately 10 μm or less by electrochemical machining (ECM). The ECM of such holes was therefore attempted using ultrashort voltage pulses. Stainless-steel sheets were drilled using cemented tungsten carbide tool electrodes. As a result, a microhole of 4.5 μm entrance diameter and 4 μm exit diameter was successfully drilled in a 5-μm-thick sheet with a pulse width of 60 ns using a tool electrode 3 μm in diameter. This is the smallest-diameter hole drilled by ECM, to the best of our knowledge. A microhole of 5.5 μm entrance diameter and 4.5 μm exit diameter was also drilled in an 8-μm-thick sheet. Furthermore, the machining properties were investigated. The effect of the tool electrode shape, pulse width, pulse frequency, high-level and low-level voltages, and electrolyte concentration on the drilling speed and hole diameter was examined. The results suggested that a semicylindrical tool electrode, long pulse width, high pulse frequency, high low-level voltage, and high electrolyte concentration were preferable for high-speed drilling without widening the lateral gap between the tool electrode and hole.

The possibility of multi-layer nanofabrication via atomic force microscope-based pulse electrochemical nanopatterning

Pulse electrochemical nanopatterning, a non-contact scanning probe lithography process using ultrashort voltage pulses, is based primarily on an electrochemical machining process using localized electrochemical oxidation between a sharp tool tip and the sample surface. In this study, nanoscale oxide patterns were formed on silicon Si (100) wafer surfaces via electrochemical surface nanopatterning, by supplying external pulsed currents through non-contact atomic force microscopy. Nanoscale oxide width and height were controlled by modulating the applied pulse duration. Additionally, protruding nanoscale oxides were removed completely by simple chemical etching, showing a depressed pattern on the sample substrate surface. Nanoscale two-dimensional oxides, prepared by a localized electrochemical reaction, can be defined easily by controlling physical and electrical variables, before proceeding further to a layer-by-layer nanofabrication process.

Micromagnetic simulation of electric-field-assisted magnetization switching in perpendicular magnetic tunnel junction

The feasibility of a voltage assisted unipolar switching in perpendicular magnetic tunnel junction (MTJ) has been studied using a micromagnetic simulation. Assuming a linear modulation of anisotropy field with voltage, both parallel (P) to anti-parallel (AP) and AP to P switchings were observed by application of unipolar voltage pulse without external magnetic field assistance. In latter case, the final P state can only be achieved with an ultrashort voltage pulse which vanishes before spin transfer torque (STT) becomes dominant to restore the initial AP state. In addition, it was found that the larger change in anisotropy field is required for the MTJ with smaller diameter.

Drilling of Microholes by Electrochemical Machining Using Ultrashort Voltage Pulses

Drilling of Microholes by Electrochemical Machining Using Ultrashort Voltage Pulses

(Invited) Compliance-Free Pulse Forming of Filamentary RRAM

Despite the overwhelming effort to improve the efficacy of resistive random access memory (RRAM), the underlying physics governing RRAM operation have proven elusive. Consequently, substantial effort has been spent to empirically develop transition metal oxide (filamentary) RRAM structures which exhibit reasonable behavior [1,2]. A survey of the recent literature almost universally indicates that the remaining glaring issues center around device variability as well as endurance. Perhaps the only consensus in the RRAM community is that these issues are linked to the forming process [2,3]. The forming process is an initial conditioning step in which voltage is applied across the dielectric stack to form, or break-down, the oxide via the formation of a resistive filament [2,3]. The forming process is thought to greatly determine the subsequent behavior of the device [2,3]. Thus, the vast majority of this presentation will detail our recent efforts to bring the forming process under control and the resulting improvements in RRAM viability in HfO2-based devices. The vast majority of efforts to control the forming process all involve the inclusion of an additional series resistor element to limit the current that flows through the RRAM during forming (compliance). Our efforts show that even the most careful series resistor implementation still invokes a serious forming variability due to the unavoidable parasitic capacitance [4]. These parasitics introduce a current overshoot which greatly alters the ability to terminate forming (i.e. a large current continues to flow through the filament for some uncontrollable time before the compliance element can clamp) [4]. This introduces a relatively large uncertainty in the forming energy ((forming voltage) x (current) x (time)) and consequent variability in the filament [5]. This pitfall can be avoided by removing all current compliance elements, and their associated parasitics, and utilizing ultrashort voltage pulses to induce forming [5,6]. In this approach, the forming process can occur uncontrolled at any point in time during the voltage pulse. However, utilization of the ultrashort pulse width minimizes the timing uncertainty and bring the forming energy under control [5]. One can, of course, invoke multi-pulse approaches to further tune the forming energy to the specific RRAM device stack [5]. Using this approach, we show for nominally identical RRAM devices, varying the forming energy using ~100 ps pulses can greatly alter the variability as measured via the endurance window [5]. In extreme cases, we show that a change in forming energy can effectively open up an endurance window in a device which was unusable using more traditional forming methods [5]. In addition, we show (figure 1) further improvement in endurance when applying multiple pulse forming schemes to “tune” the forming energy to the desired value and consequent endurance [5]. In summary, we describe a new compliance-free RRAM forming technique that greatly enhances the energy control of the forming process and improves the device endurance characteristics proportionately. [1] Y. Shimeng, et al., IEDM, pp. 26.1.1 (2012) [2] B. Butcher, et al., IIRW, pp. 146 (2011) [3] A. Kalantarian, IRPS, pp. 6C.4.1 (2012) [4] P. Shrestha, et al., IIRW, pp. 55 (2013) [5] P. Shrestha, et al., IRPS, pp. MY.10.1 (2014) [6] P. Shrestha, et al., VLSI-TSA, pp. TR82 (2014) Figure 1

(Invited) Compliance-Free Pulse Forming of Filamentary RRAM

Despite the overwhelming effort to improve the efficacy of resistive random access memory (RRAM), the underlying physics governing RRAM operation have proven elusive. A survey of the recent literature almost universally indicates that the remaining glaring issues center around variability as well as endurance. The initial filamentary “forming” process is often linked to these problems. This work details our recent efforts to bring the forming process under control and the resulting improvements in RRAM viability in hafnia-based devices. We track the forming process via a “forming energy” metric that allows for filament optimization. By removal of all current compliance elements, and their associated parasitics, a targeted forming energy is achieved using ultrashort voltage pulses. By tailoring the forming energy, we show remarkable endurance window control.

Discussion of ultrashort voltage pulses electrochemical micromachining: a review

One of the most effective way of electrochemical machining (ECM) accuracy increase is to carry out process with application of voltage pulses. In one of the variants of ECM, ultra short (nono- or picosecond range) voltage pulses are applied. It gives possibility to achieve high localization of electrochemical dissolution process and allows to machine microparts with accuracy less than 0.01 mm. However, because of the process principles, this variant of ECM has number of limitations which stop its wider application in micromanufacturing industry. Based on the literature review, the physical principles of ultrashort voltage pulses electrochemical machining were presented and anodic dissolution localization issues were discussed. Also, differences between other electrochemical machining variants were underlined. It gave possibility to identify limitations and future perspectives of industrial applications.

超短電圧パルスを用いた電解加工による微細穴あけ加工

超短電圧パルスを用いた電解加工による微細穴あけ加工

Research on Multiple Wires Electrochemical Micromachining with Ultra-Short Voltage Pulses

Wire electrochemical micromachining (WECMM) with ultra-short voltage pulses has presented its great potential in the fabrication of micro structures in difficult to cut materials because of its tiny etch resolution and good surface integrity. However, this micromachining method has not been well applied as its current machining efficiency is unsuitable for mass production. In this paper, multiple wires electrochemical micromachining (MWECMM) is proposed to increase the productivity in machining arrayed micro structures. The principle of MWECMM has been analyzed to study the influence of ultrashort voltage pulse and the wire electrode amount on the machining process. Results indicated that the charging potential is significantly influenced by voltage pulse on time and the wire electrode amount. As the wire electrode amount increased, the maximum charged potential and the potential charging rate was obviously decreased. Besides, the actual applied voltage between the electrodes decreased because of the pulse generator's output characteristics. Thus, larger voltage amplitude and pulse on time are demanded to obtain a stable MWECMM process with more wire electrodes. In this paper, the optimal wire electrode amount was three when the voltage pulse on time was 80ns and the voltage amplitude was 8V. And the machining efficiency using three wire electrodes was enhanced to be twice that using a wire electrode. Finally, an arrayed tree-like microstructure was successfully fabricated by MWECMM using three wire electrodes simultaneously.

Complex microshaping of bulk metallic glass surfaces by electrochemical means

Abstract

Time-Domain Optoelectronic Vector Network Analysis on Coplanar Waveguides

The development of a one-port laser-based vector network analyzer (VNA) is described. Measurements are carried out in the time domain using a femtosecond laser for the generation and detection of ultrashort voltage pulses on a coplanar waveguide (CPW). The accurate separation of voltage signals propagating in forward and backward directions, being one of the key tasks of vector network analysis, is achieved by measuring voltage pulses at different positions on the CPW. This separation technique even works in the case of temporally overlapping forward and backward propagating signals. With this laser-based VNA, a frequency spacing of 500 MHz, a measurement bandwidth of approximately 500 GHz, and a dynamic range of more than 40 dB are obtained. As an application example, the laser-based VNA is used to realize an ultra-broadband voltage pulse standard.

Electrochemical Micromachining of Bulk Metallic Glasses

Owing to unique structural and functional properties bulk metallic glasses (BMG) are very promising materials. Their outstanding durability aligned with high mechanical strength and/or soft magnetic properties make them suited for micro­-electro­mechanical systems (MEMS), diminutive spring-mass systems, micronsized yokes in positioning systems, magnetic cores or medical gadgets. For shaping the required micro-sized devices thermo-mechanical forming is so far applied, but this is only applicable to selected glassy alloys with appropriate thermal characteristics. An upcoming alternative which allows operating at room temperature is electrochemical micromachining (ECMM). As new technique voltage pulsed electrochemical micromachining with a micro-tool electrode is introduced and a specially designed four-electrode setup for machining glassy plates was developed. The multi-component and reactive nature of metallic glasses demands a strict control of the local electrochemical dissolution process based on the choice of a suitable electrolyte and the identification of the effects of the ideal ultra-short pulse voltage parameters. First studies focused on Zr-based BMGs. Their high susceptibility for pitting corrosion limits the choice of electrolytes to halide-free ones. The standard electrolyte for industrial ECM is a concentrated (250 g/l) sodium nitrate solution. When conducting machining experiments with ultra-short voltage pulses in the transpassive regime, it could be demonstrated that the electrochemical dissolution of the glassy alloy can be focused on the microscale. But machined structures were irregularly shaped and covered with corrosion products and no clear dependence from the machining parameters was given. The underlying reaction mechanism and the role of the amorphous alloy structure and of the alloy constituents are discussed [1,2]. Recent studies were conducted on selected Fe-based glasses. Surface structuring of the soft magnetic glassy alloy Fe65.5Cr4Mo4Ga4P12C5B5.5 (at.-%) using an electrolyte based on a methanolic sulfuric acid solution was successful. The transpassive dissolution process was utilized for a defined, localized material removal. By applying nanosecond pulses between a work piece and a tool electrode, micro-holes of high aspect ratio and depth of up to 100 µm can be machined. But methanolic solutions are highly toxic and flammable and thus, undesirable in industrial mass production. Water-based electrolytes are the better choice, but the BMG is forming a passive layer which is very stable throughout the machining process. The addition of an oxidizing agent (iron sulphate) is successfully introduced to reduce this passivation process. Further, the Fe(III) ion reduction dominates the W-tool cathode reactivity and thus, suppresses disturbing hydrogen evolution. The influence of the applied oxidizing agents is analyzed and robust parameters for a fast and repeatable forming process without corrosion products are evaluated. The impact of the crucial process parameters as voltage, heading velocity or pulse-pause-ratio on the surface structuring is discussed. Optimum hole and line micro-structures were machined on BMG surfaces in 0.1 M sulphuric acid solution with 0.1 M iron sulphate at parameters: pulse on/off voltage = 6.0/0.5 V, pulse period/length = 750/75 ns, machining velocity = 0.1 µm/s (deep hole) and pulse on/off voltage = 6.0/1.0 V, pulse period/length = 2400/300 ns, machining velocity = 0.05 µm/s (line) [3-5]. This work is funded by the German Research Foundation (DFG) under project numbers GE 1106/10 and 1106/11. [1] J. A. Koza, R. Sueptitz, M. Uhlemann, L. Schultz, A. Gebert: Electrochemical micromachining of a Zr-based bulk metallic glass using a micro-tool e lectrode technique, Intermetallics 19 (2011) 437-444 [2] A. Gebert, P.F. Gostin, R. Sueptitz, S. Oswald, S. Abdi, M. Uhlemann, J. Eckert: Polarization studies of Zr-based bulk metallic glasses for electrochemical machining , Journal of the Electrochemical Society 161 (4) (2014) E66-E73 [3] R. Sueptitz, K. Tschulik, C. Becker, M. Stoica, M. Uhlemann, J. Eckert, A. Gebert: Micro-patterning of Fe-based bulk metallic glass surfaces by pulsed electrochemical micro-machining (ECMM), Journal of Materials Research 27 (2012) 3033-3040 [4] R. Sueptitz, P. Dunne, K. Tschulik, M. Uhlemann, J. Eckert, A. Gebert : Electrochemical micromachining of passive electrodes, Electrochimica Acta 109 (2013) 562-569 [5] S. Horn, A. Freidank, M. Uhlemann, M. Stoica, J. Eckert, A. Gebert, Electrochemical micromachining of passive Fe-based bulk metallic glasses in aqueous solutions, Proceedings INSECT 2014, Saarbrücken, Germany (in press)