ICMCTF2010 Session HP: Symposium H Poster Session

In the experiment, the CoSi₂ thin film deposited by sputtering in O₂ ambient has been studied to fabricate the Co nanocrystals memory device. Based on the difference of the oxidation free energy between the Co and Si elements, a selective oxidation process during the thermal annealing enhance the formation of cobalt nanocrystals embedded in silicon oxide. The charge-storage ability of the formed Co nanocrystals has also been studied by fabricating the capacitance structure. In addition, more factors such as annealing temperature and oxygen flow rate are discussed for the nanocrystals formation. It is found that the Co nanocrystals show a larger size and lower density at high temperature annealing. The size and density distribution of nanocrystals affect the performance of the memory device. In the study of oxygen content, the XPS analyses have confirmed that the compositions of nanocrystals were changed by different oxygen flow rate during sputtering process. An inappropriate oxygen flow causes the in-complete oxidation or over-oxidation, which degenerate the charge storage ability of the Co nanocrystals device.

High power impulse magnetron sputtering of zirconium was investigated at a high average target power density in a period, being approximately 100Wcm^-2. The depositions were performed using an unbalanced circular magnetron with a directly water-cooled planar zirconium target of 100mm diameter. The repetition frequency was 500Hz at duty cycles ranging from 4 to 10% and an argon pressure of 1Pa. Time evolutions of the discharge characteristics were measured to provide information on absorption of energy in the discharge plasma and on transfer of arising ions to a substrate (located 100mm from the target) at an average target power density in a pulse up to 2.22kWcm^-2. Time-averaged mass spectroscopy was performed at the substrate position. High fractions (21-32%) of doubly charged zirconium ions were found in total ion fluxes onto the substrate at decreasing fractions (from 23 to 3%) of singly charged zirconium ions when the average target power density in a pulse increased from 0.97 to 2.22kWcm^-2. It was shown that ion energy distributions are extended to high energies (up to 100eV relative to ground potential) under these conditions. The rise in the values of the average target power density in a pulse resulted in a decrease of the deposition rate of films from 590 to 440nm/min.

Strain engineering has been widely adopted to extend scaling limits and the device performance of MOSFETs. To sustain further enhancement in device performance, new materials and novel device structures are needed. The high electron mobility of III–V compound semiconductor makes them an attractive candidate as MOSFET channel material for high-speed applications beyond the sub 32 nm technology node. National University of Singapore (NUS) research group had reported the first demonstration of a strained InGaAs with In mole fraction of 0.53 channel n-MOSFET featuring in situ doped InGaAs with In mole fraction of 0.4 source/drain (S/D) stressors (Please refer to a conference paper by Prof. Yee-Chia Yeo at Symposium on VLSI Technology Digest, pp. 244-245, 2009). To realize the full potential of the III–V compound-based MOSFETs, channel-strain engineering will be an important direction. The effect of strain on electron transport in many III–V compound-based semiconductors, e.g., gallium arsenide (GaAs) and indium gallium arsenide (InGaAs), is need to further study. However, the integration of the process-induced strain in III–V compound-based MOSFETs to exploit the effect of local stressors, such as the lattice-mismatched S/D for mobility enhancement, is also needed to explore. In this work, we report the results of electron mobility calculations and device simulations for novel InGaAs with In mole fraction of 0.53 NMOSFETs with InGaAs S/D stressors using In mole fraction of 0.4 under various geometry structures. The lattice mismatch between InGaAs alloy S/D and InGaAs channel is exploited to induce tensile strain in the channel for mobility enhancement. The stress distribution in the InGaAs channel regions in NMOSFETs with various lengths and widths were studied using 3D ANSYS simulations. The mobility enhancement was found to be dominated by the tensile stress along the transport direction and compressive stress along the growth direction in wide width devices. Stress along the width direction was found to have the least effect on the drain current in wide length and width cases. Impact of width on performance improvements such as the drive current gain was also analyzed using 3D TCAD simulations.

Recently, metallic nanocrystals have been widely investigated for high programming/erasing (P/E) efficiency and low power consumption nonvolatile memory application [1-2]. Metallic nanocrystal nonvolatile memory has many advantages, such as smaller operating voltage, a wide range of available work functions, and larger charge capacity. In addition, for possibility breaking the scaling limits of thickness of the tunnel oxide, multi-stacked tunneling layer has been also extensively studied [3-5] . Multi-stacked tunneling layer provided asymmetric tunnel barrier (ATB) and it could enhance the P/E efficiency. In this work, iridium-nanocrystals (Ir-NCs) lying on SiO₂/Si₃N₄ (3-nm/3-nm) tunnel layer stacks have been demonstrated and compared its characteristics with SiO₂ (6-nm) as tunnel oxide in the application of nonvolatile memory. Two type of metal-insulator-semiconductor (MIS) capacitor stacks of Al/SiO₂/Ir-NCs/Si₃N₄/SiO₂/P-Si/Al and Al/SiO₂/Ir-NCs/SiO₂/P-Si/Al were analyzed by scanning electron microscope (SEM), high resolution transmission electron microscopy (HRTEM), capacitance-voltage (C-V) measurement, and current-voltage (I-V) measurement, respectively. SEM images showed the mean size of Ir-NCs and density was approximately 5 nm and 1 × 10¹² cm^-2, respectively. Due to the bi-stacked tunneling layer improving P/E efficiency, significant hysteresis window of 11.6 V have been observed with a sweep voltage of +/- 10 V. The hysteresis window was larger than the single tunneling layer. And then, data storage decay percentage of the bi-stack (SiO₂/Si₃N₄) after 10⁴ sec was only less 5 percentages than the single (SiO₂). Furthermore, the endurance of these devices shows excellent characteristics by P/E voltage stressing at +/- 9 V, 100 msec. In summary, Ir-NCs nonvolatile memory with bi-stacked tunneling layer provided high P/E efficiency, sufficient data retention and good device endurance.

References:

[1] J.J. Lee et al : Appl. Phys. Lett. 86 (2005) 103505.

[2] S. S. Yim et al : Appl. Phys. Lett. 89 (2006) 093115.

[3] C. C. Wang et al : Appl. Phys. Lett. 91 (2007) 202110.

[4] Konstantin K. Likharev : Appl. Phys. Lett. 73 (1998) 152137.

[5] T. H. Hou et al : Appl. Phys. Lett. 92 (2008) 153109.

X-ray tubes using cold cathodes as electron source attract great attention in a wide range of applications, especially in x-ray radiography such as diagnostic medical imaging and embedded-type radiation brachytherapy, because they have a lot of advantages, including possibility of miniaturization, fast response, low power consumption, and low fabrication cost, over the x-ray systems using conventional thermionic cathodes. Carbon nanotubes (CNTs) have been considered to be one of the most promising materials for point-type cold electron sources due to their mechanical and chemical stability as well as superior electron emission properties. In order to achieve high resolution x-ray images, CNT-based emitters must have a large current capacity and, at the same time, ensure a stable prolonged-operation of current emission. Furthermore, x-ray images may strongly depend on the emission performance of CNT-based electron sources. Recently, coating of CNTs’ surface with wide bandgap materials has been attempted to obtain a stable and uniform emission current from CNT-emitters. However, this manner may cause the increase of turn-on voltage for electron emission and also the decrease of tunnelling probability.

In this study, we present x-ray imaging data obtained from cold cathodes using gallium-doped zinc oxide (GZO) coated CNT-emitters. Multi-walled CNTs were directly grown on conical-type (250 μm in diameter) metal-tip substrates at 700°C by inductively coupled plasma-chemical vapor deposition (ICP-CVD). GZO films were deposited on the grown CNTs at room temperature using a pulsed laser deposition (PLD) technique. Field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) were used to monitor the variation in morphologies and microstructures of CNTs before and after GZO-coating. Energy-dispersive x-ray spectroscopy (EDX) was used to extract the relative atomic content in GZO films. Effects of GZO-coating on the electron-emission performances of fabricated CNT-emitters were examined by characterizing turn-on fields for electron-mission, maximum emission currents, and field enhancement factors. The CNT-emitters with a triode in structure were installed in the x-ray tube which was composed of gate-electrode meshes and electrostatic lens inside. The x-ray tube was housed in a vacuum chamber pumped down to 3×10^-7 Torr. X-rays radiated from the anode (Cu) was extracted passing through a beryllium (Be) window where the specimen was in contact with. The relationship between the electron-emission performance of GZO-coated CNT-emitters and the x-ray imaging data was also elucidated.

Recently, one dimensional (1-D) tin oxide nanostructures, including nanowires, nanorods and nanobelts, have attracted considerable attention because of their application in constructing nanoscale devices. In order to enhance and modulate the various properties, SnO₂ nanostructures are frequently doped with appreciate elements, such as indium (In) and antimony (Sb). In this present work, Sb-doped SnO₂ nanowires were synthesized on an Al₂O₃ substrate using a thermal evaporation. It is found that both the diameter and Sb concentration of the nanowires increases with increasing growth time (0.5-4 hr). The morphology and microstructure of Sb-doped SnO₂ nanowires were characterized by a field emission scanning electron microscope (FESEM) and a transmission electron microscope (TEM). The composition and chemical bonding nature were investigated by x-ray photoelectron spectroscopy (XPS). The photo-sensing measurements were carried out using a volt-amperometric technique, which shows that both the reaction and response time of the nanowires decrease with increasing growth time.

Different process of fabricated Indium layers was by Thermal Evaporation, Sputter, and electrodeposited to make high-quality CuInSe₂. The great method was found in this work. First, the precursor layer of copper was sputtered onto the Mo-coated glass. The same thickness of indium layer through different process was deposited on the precursor layer of copper. Then, the layer of selenium was electrodeposited on the indium layers, and annealed by rapid thermal processing (RTP). The method of rapid thermal processing reduces selenium dissipation. We deposit CIS respectively to control ratio, and to fabricate the superior structural properties of these selenized films were also clearly reflected by X-ray diffraction. Field-Emission-Scanning electron microscopy indicates that the ordered copper indium diselenide thin-film are entirely filled and the structure of the molybdenum thin-film. X-ray diffraction result shows that the copper indium diselenide thin-films are crystalline and have the highly preferential orientation. Energy dispersive spectrometer analysis observation shows the composition atomic ratio of copper indium diselenide. Mobility of the CIS film was indicated by Hall Measurement.

Many of the technologically important metal oxide thin films (e.g. Al₂O₃, TiO₂, etc.) are sputter deposited from metal targets in a reactive gas atmosphere, usually argon and oxygen, i.e. using reactive magnetron sputtering to ensure industrially relevant coating deposition rates. High Power Impulse Magnetron Sputtering (HIPIMS) is known to provide a highly ionised flux of sputtered material and is thought to provide enhanced coating structure-property relationship control (through self-species plasma/ion assistance), as compared to DC or mid-frequency AC/pulse-DC magnetron sputtering. It is therefore desirable to use HIPIMS in a reactive deposition mode in order to apply its advantages in metal oxide thin film fabrication.

This paper reports on the deposition process and structural and compositional properties of titanium dioxide thin films produced by reactive HIPIMS. A new plasma emission monitoring based reactive sputtering process control system (developed specifically to aid stable and reproducible reactive HIPIMS processes) was used for coating deposition. Coating morphology and structure were investigated by field emission gun scanning electron microscopy, atomic force microscopy and X-ray diffraction. Coating chemical composition was investigated by electron probe micro-analysis and glow discharge optical emission spectroscopy, while their mechanical (hardness and elastic modulus) properties were investigated by nano-indentation testing. Processing parameters are correlated to the film properties.

One-step electrodeposition is the most well-known technique to form CuInSe₂ (CIS) for solar cell applications. However, the composition of CIS films is hard to control in a ternary solution. In this study, the deposition potential and electrolyte concentration were confined in a narrow range to control the atom proportion of CIS films. Two-step electrodeposition of CuSe_χ and InSe_χ precursors provide another route to form CIS film, which was able to control the deposition amount of copper (Cu) and indium (In) without considering the huge variation of their reduction potentials. For CuSe_χ deposition, the reduction potential of Cu is smaller than that of selenium (Se) such that the deposition amount of Cu is more than that of Se at the same electrolyte concentration. Increasing the deposition potential and the concentration of SeO₂ raised the percentage of Se. Higher electrolyte concentration also led to a smooth and dense CuSe_χ film. The sheet resistance of CuSe_χ film decreased and the micro-structure became smoother after thermal annealing. The existence of CuSe leads to easier formation of InSe and low-resistance CuSe_χ films could decrease the terminal effect to improve the uniformity of subsequently InSe_χ electrodeposition. The deposition potential ranged from –0.8V to –0.9V. The content of In would be too low when the deposition potential was under –0.8V, and when the potential was larger than –0.9V, electrolysis occurred and produced mass SeO₂ film on the surface. The ratio of Cu/In was verified by energy dispersive spectrometer (EDS) and easily controlled by electrodeposition coulomb in each step. After two-step electrodeposition, rapid thermal annealing was employed to form CuInSe₂. The treatment was carried out at 550℃ for 70S. The formation of CuInSe₂ was verified by X-ray diffraction.

In this study, the nanosized SnO₂ powder was synthesized by a novel thermal decomposition method and electrophoretic deposited on a patterned Al₂O₃ substrate for the gas sensing property measurement. The phases and microstructure of the powders were characterized by the XRD, SEM, TEM and BET. Tin oleate complex and 1-octadecanol were used as the precursors, which were thermally decomposed and assembled as octahedron hollow or solid Sn₆O₄ (OH)₄ agglomerate powder at the temperature of 200^oC in N₂ atmosphere. After heat treatment at 700^oC for 1h in air, the powder was transformed to crystalline SnO₂ nanoparticle in a size of 50 nm. The SnO₂ electrodes with promising gas sensing properties were prepared by electrophoretic deposition using an isopropyl alcohol colloid. The nitrogen-adsorption and desorption isotherms and the relatively gas sensing properties of CO gas were characterized and discussed in details.

High-Power Impulse Magnetron Sputtering (HiPIMS) is a plasma-based thin film growth technique. Short (several µs) high-power (several kW/cm²) pulses are delivered to the plasma allowing a high ionization degree of the sputtered metal atoms. As a consequence, ion bombardment assists film growth. However, it is commonly found that the deposition rate is lower in HiPIMS than in conventional dc magnetron sputtering (dcMS) processes. This would indicate a different plasma-surface interaction mechanism during HiPIMS.

In our study, plasma composition was characterized by Time-Resolved Optical Emission Spectroscopy (TR-OES) and the amount of material deposited was determined by X-Ray Fluorescence (XRF). HiPIMS and dcMS discharges were operated with a titanium target in argon or neon with a working pressure set to 5 or 10 mTorr.

TR-OES analysis shows a decrease of the argon signal at the end of the pulse ; apparently indicating the rarefaction of the argon gas during the pulse. In contrast, neon signal only presents a weak saturation for long pulses. TR-OES results would suggest that gas heating (induced by the sputtering wind) and the gas rarefaction thereof has a smaller amplitude during HiPIMS in neon. On the other hand, Ti⁺ signal strongly increases during the pulse in both cases, demonstrating the high ionization rate of the sputtered material. Strong argon rarefaction and the increased number of metal ions would allow the sputtering process to evolve from argon-sputtering to metal-sputtering i.e. self-sputtering. In the case of titanium, self-sputtering is less efficient than argon sputtering. This could be an explanation to the decrease of the deposition rate, as observed by XRF. In neon, the gas rarefaction would be less pronounced and metal ions would contribute to the sputtering process to a smaller extent. The reduction of the deposition rate in neon, as compared to the dcMS discharge, is therefore less pronounced than in argon.

The corrosion behavior in artificial seawater of different HP-HVOF cermet coatings applied on low alloyed steel was studied. 5 conditions, associated to modifications of the composition of the powder or deposition parameters were evaluated. The degradation mechanisms were studied during extended immersion tests using conventional electrochemical measurement and electrochemical impedance spectroscopy.

The extended immersion tests reveal that these thermal sprayed coatings present a cathodic behavior compared to steel. During the first hours of immersion, the electrolyte infiltrates the defects of the coatings, which then result to the local degradation of the substrate accelerated by the galvanic coupling with the cermet coating. Optical observations and Raman and FTIR analyses reveal the formation of calcium carbonates like aragonite on the cermet surface, very close to the appearance of local anodic sites. The cross-sectioned view reveal the infiltration of the corrosive solution, and the depth penetration of the degradation of steel substrate probably due to the acidification of the anodic sites.

Deposition of patterned nanostructures by atmospheric pressure plasma enhanced chemical vapour deposition (PECVD) is today of fundamental importance in material science research. A breakthrough has recently been done by using micro-plasmas [1]. These results combine the benefits of a plasma environment together with the advantages of self-organization. Observation of self-organization phenomena under plasma conditions is still limited, and even less is known about self-organization promoted by atmospheric microplasmas.

Recently, we have developed a small-scale remote plasma at atmospheric pressure based on a resonant microwave cavity. By controlling the hydrodynamics of the gas, we have shown that it is possible to extract through a tinny hole in the cavity a straight beam of active species (~500 µm in radius) over a length of 10 cm . By mixing this beam of atoms with a flux of HDMSO, we could grow SiO_x coatings on stainless steel substrates. Deposition rate is about 80 nm/s.

Under specific conditions, self-organized nanodots are obtained. Their diameter is close to 200 nm. Depending on time, these particles get assembled into threads, separated one from another by an almost constant distance, and reminding a Volmer-Weber nucleation step followed by coarsening.

In this work, we describe the influence of the main experimental parameters on the synthesis of these patterned structures. Image analysis is essentially used to determine the distributions in size and distance of the nanostructures. A specific analysis of thermal fluxes is also carried out to try to determine the influence of the temperature gradients on the growth. Results are based on simulations of rotational spectra of the OH (306.4nm—A²Σ⁺ ; v = 0–X²Π ; v’ = 0) transition and infra-red camera measurements.

[1] D. Mariotti, V. Švrček and D.-G. Kim, Appl. Phys. Lett. 91, 183111 (2007)

Polymethylsilsesquioxane (PMSQ) is well-known abrasion-resistant coating material as protective coatings for the relatively soft resin, which is particularly desirable in the hard coatings of optically transparent materials such as plastic window glazing, anti-abrasion coating of mobile phone or portable display screens, etc. as well as other applications like oxidation barrier of metal. However, PMSQ films exhibited poor adhesion to polymer substrates. In this study, PMSQ coatings on PMMA, PC, PC/ABS alloy substrates were studied by grafting the substrate with trimethyl ethoxysilane (TMES) after surface functional treatments with oxygen plasma or chromic acid. The polymer surfaces after various surface treatments were also characterized by contact angle measurements and ATR-FTIR in order to optimize the treatment time. The improvement of adhesion and hardness was examined respectively by cross cut test and pencil hardness test.