Gas Mixture Plasma Research Articles

Low-temperature Cu/SiO2 hybrid bonding based on Ar/H2 plasma and citric acid cooperative activation for multi-functional chip integration

Cu/SiO2 hybrid bonding is a crucial technique for three-dimensional (3D) integration, which can greatly shorten the interconnected spacing by metal-electrode (Cu) and insulator (SiO2) hybrid interfaces without requiring microbumps. By this bumpless hybrid bonding, the vertical stack of devices with ultrahigh density can be accomplished to meet the demand of high-performance artificial intelligence (AI) chips. To improve the compatibility for multi-functional chip integration, low-temperature bonding conditions are essential. Moreover, it is a great challenge to achieve oxide-free Cu/Cu bonding interface as well as ensure SiO2/SiO2 bonding with sufficient hydroxylated groups. Therefore, we proposed a two-step cooperative activation process as argon and hydrogen gas mixture (Ar/H2) plasma activation followed by citric acid treatment, which makes the Cu oxide reduction and SiO2 hydroxylation simultaneously. The low-cost Cu/SiO2 hybrid bonding was successfully obtained at 200 °C in ambient air. There were nearly neither oxides at Cu/Cu bonding nor residual carbon at SiO2/SiO2 bonding interfaces. More interestingly, the bonding pairs not only survived in the aging tests in the range of 150–350 °C, but the bonding area and strength were further improved. Consequently, this two-step cooperative activation process has great potential for low-temperature Cu/SiO2 hybrid bonding with high-temperature reliability.

Tribological Behaviour of Low Temperature Plasma Assisted Carburized 316 L Steel Produced by L-PBF Technique

Austenitic stainless steels produced by Laser Powder Bed Fusion (L-PBF) are interesting materials because of their excellent corrosion resistance. Due to their relatively low hardness, the tribological response of these materials is poor, which limits their use in applications where control of wear degradation is important. Nevertheless, low-temperature plasma-assisted carburisation is an interesting process for improving the wear resistance of austenitic stainless steels, as has been observed for wrought materials. In fact, the increase in hardness is guaranteed by a surface layer of expanded austenite (S-phase) with a thin top layer of amorphous carbon. In this work, AISI 316 L, produced by the L-PBF technique, was carburised using 5 different plasma gas mixtures (by varying the CH4/H2 ratio) at 475°C for 7 hours. The samples obtained were then subjected to a detailed microstructural characterisation in order to obtain information on surface modification. The morphological features of the surface were examined by SEM observations in top view and in cross-section. The tribological performance was evaluated by pin-on-flat tests (alumina sphere as counter-material) with 2 different applied loads and a stroke length of 5 mm. Friction coefficient, wear rate (stylus profilometer) and wear mechanisms (SEM) were also evaluated. Preliminary results show an increase in wear resistance of all plasma treated materials compared to the untreated material. The improved tribological performance was discussed in relation to the abrupt increase in surface hardness.

Role of Oxygen in Amorphous Carbon Hard Mask Plasma Etching.

In the current and next-generation Si-based semiconductor manufacturing processes, amorphous carbon layer (ACL) hard masks are garnering considerable attention for high-aspect-ratio (HAR) etching due to their outstanding physical properties. However, a current limitation is the lack of research on the etching characteristics of ACL hard masks under plasma etching conditions. Given the significant impact of hard mask etching on device quality and performance, a deeper understanding of the etching characteristics of ACL is necessary. This study aims to investigate the role of oxygen in the etching characteristics of an ACL hard mask in a complex gas mixture plasma etching process. Our results show that a small change of oxygen concentration (3.5-6.5%) can significantly alter the etch rate and profile of the ACL hard mask. Through our comprehensive plasma diagnostics and wafer-processing results, we have also proven a detailed mechanism for the role of the oxygen gas. This research provides a solution for achieving an outstanding etch profile in ACL hard masks with sub-micron scale and emphasizes the importance of controlling the oxygen concentration to optimize the plasma conditions for the desired etching characteristics.

Multilayer Metallization Systems of Submicron Integrated Circuits

The creation of a multilevel system of interconnections in submicron integrated circuits makes it possible to reduce the electrical resistance of conductive tracks, parasitic capacitance between conductors, and increase the speed of microelectronic devices. It is proposed to form a transverse profile of the current-carrying tracks of a multilayer metallization system in the form of an isosceles trapezoid with angles at the lower base equal to 75–85 degrees. Etching of an aluminum-based alloy film is carried out in a plasma gas mixture of BCl3, Cl2, and N2 at the pressure of 150–250 mTorr and power density of 1.6–2.2 W/cm2, with the following component content, vol.%: BCl3 – 50–65; Cl2 – 25–35; N2 – the rest.

Wet Cleaning of Molybdenum for Nano Interconnects

The effect of various chemical solutions on the etch characteristics of molybdenum and on the removal of post-etch residues formed between Mo lines was investigated. In contrast to diluted SC1 mixtures, it was found that diluted HF solution (0.05%) and formulated semi-aqueous alkaline mixtures are highly compatible with Mo. The high etching rate measured for SC1 is attributed to the presence of the oxidizer in the mixture. Post-etch residue removal assessment performed on a semi-damascene structure with metal pitch of 32 nm showed that cleaning using diluted HF did not completely removal the residues. On the contrary, a clean using FOTOPUR R-2403, a semi-aqueous formulated mixture, resulted in efficient removal of the residues. The effect of oxygen in the plasma gas mixture and subsequent wet cleaning on the thickness of Mooxides was also performed on blanket Mo

Plasma Carburizing of Laser Powder Bed Fusion Manufactured 316 L Steel for Enhancing the Surface Hardness

Austenitic stainless steels produced by laser powder bed fusion (L-PBF) are quite interesting materials owing to their specific microstructure consisting of dendrite walls built of dislocations pinned by many nano-oxides that involves significant strengthening without loss of ductility. In this work, different plasma treatments were performed to harden the surface of 316 L steel manufactured by L-PBF. The samples were characterized by X-ray diffraction (XRD), Raman spectroscopy (RS), light microscopy (LM) and micro-hardness tests. The experimental results show that all the plasma treatments enhance the hardness of the surface because a C-enriched layer of austenite (S-phase) forms with a thickness up to 25 μm. The plasma gas mixture, consisting of 2.5% (CH4) + 97.5% (H2), resulted in being the most effective and produced a surface hardness (547 ± 27 HV) more than double with respect to that of the untreated material. The treatment temperature was 475 °C, which represents a good compromise between the necessity to avoid the precipitation of M23C6 carbides and the compatibility of treatment time with the industrial practice. Moreover, it has been observed that a 2 μm-thick over-layer of amorphous C forms on the sample surface. The hardness of such over-layer, which depends on the specific treatment and is related to the degree of topological disorder, is generally greater than that of S-phase. The work demonstrates that plasma carburizing is quite effective in hardening the surface of 316 L steel manufactured by L-PBF and further improves its mechanical properties, which are basically superior to those of the same material prepared by conventional processes.

(Invited) Low-Temperature Plasma-ALD of Wide and Ultra-Wide Bandgap Semiconductors: Accomplishments and Hurdles to Overcome

III-nitride and III-oxide based wide (GaN, InGaN, AlGaN) and ultra-wide bandgap (Ga2O3, AlN, BN) semiconductors play critical roles in high-power, high-frequency, and harsh-environment electronic components utilized in wireless communications, power grids, homeland security, and space applications. While GaN-based wide bandgap (WBG) devices have reached a certain degree of maturity, the emerging ultra-wide bandgap (UWBG) alloys are still in their early stage of technology development. Conventional chemical vapor deposition (CVD) based crystal growth processes for these materials are carried out at elevated substrate temperatures, reaching over 1000 °C. In an effort to substantially reduce this synthesis temperature, plasma-assisted CVD techniques are of particular interest. Owing to its unique features including pinhole-free synthesis, precision film thickness control, large-area uniformity, and ultimate 3D conformality, plasma-assisted atomic layer deposition (plasma-ALD) offers significant opportunities for electronic device layers at reduced substrate temperatures.Relatively limited number of research groups have targeted this challenging opportunity by using different types of plasma sources integrated into various plasma-ALD reactors. In addition, different metal precursors and plasma gas mixtures have been utilized and the resulting material properties are studied on different substrates. While mostly polycrystalline WBG and UWBG films are reported, recently several groups have reported epitaxial relationship of plasma-ALD grown III-nitride and Ga2O3 films on sapphire and SiC substrates, which hints the potential for success. However, there still exist critical technology barriers for the adoption of plasma-ALD as an alternative low-temperature materials synthesis technique for device layers. Reliable electron mobility values are lacking and atomic layer doping efforts are barely investigated.In this presentation, we would like to provide and updated overview of the field including the latest reports and achievements. In addition, we would like to present our own efforts and recent advances in developing crystalline III-nitride and Ga2O3 films by using the hollow-cathode plasma (HCP) source, which demonstrated its advantages for reducing the oxygen impurities in III-nitride layers. By incorporating a multi-wavelength ellipsometer for in-situ monitoring, unit-cycle growth behavior of WBG and UWBG films were analyzed and correlated with the obtained material properties. Moreover, real-time in-situ monitoring allowed the realization of the saturation curve experiments on a single sample, without the need for breaking the vacuum and using multiple samples necessitating individual ex-situ thickness measurements. The growth parameters studied include the substrate temperature, rf-power applied to the remote HCP source, and plasma gas mixtures (N2/H2/Ar and O2/Ar with different flow ratios for III-nitrides and Ga2O3, respectively). Structural, chemical, optical, and electrical properties of the grown WBG and UWBG films have been characterized and correlated with the growth conditions. One of the main results obtained for these films is the different behavior each binary compound shows for similar synthesis parameter variations. A roadmap towards overcoming the current limitations will be discussed with an outlook for potential device applications of these low-temperature plasma-ALD grown films.

Versatilely tuned vertical silicon nanowire arrays by cryogenic reactive ion etching as a lithium-ion battery anode

Production of high-aspect-ratio silicon (Si) nanowire-based anode for lithium ion batteries is challenging particularly in terms of controlling wire property and geometry to improve the battery performance. This report demonstrates tunable optimization of inductively coupled plasma reactive ion etching (ICP-RIE) at cryogenic temperature to fabricate vertically-aligned silicon nanowire array anodes with high verticality, controllable morphology, and good homogeneity. Three different materials [i.e., photoresist, chromium (Cr), and silicon dioxide (SiO2)] were employed as masks during the subsequent photolithography and cryogenic ICP-RIE processes to investigate their effects on the resulting nanowire structures. Silicon nanowire arrays with a high aspect ratio of up to 22 can be achieved by tuning several etching parameters [i.e., temperature, oxygen/sulfur hexafluoride (O2/SF6) gas mixture ratio, chamber pressure, plasma density, and ion energy]. Higher compressive stress was revealed for longer Si wires by means of Raman spectroscopy. Moreover, an anisotropy of lattice stress was found at the top and sidewall of Si nanowire, indicating compressive and tensile stresses, respectively. From electrochemical characterization, half-cell battery integrating ICP-RIE-based silicon nanowire anode exhibits a capacity of 0.25 mAh cm−2 with 16.67% capacity fading until 20 cycles, which has to be improved for application in future energy storage devices.

FEATURES OF PLASMA CHEMICAL ETCHING OF LITHIUM TANTALATE SUBSTRATE (LiTaO3)

The results of researches of plasma chemical treatment of lithium monocrystalline tantalate (LiTaO3) from gas type, bias voltage (energy of chemically active ions) and from current of additional bias generator are given. A closed-loop electron drift plasma chemical reactor and gas mixtures containing Ar, Ar + ClС4, and Ar + SF6 were used for the experiments. It was found that the etching rate of LiTaO3 for the discharge in the gas mixture Ar + CCl4 is 14 times higher than all other mixtures that were used. It is shown that the proposed idea and approaches of LiTaO3 processing can be effectively applied for the production of optical systems with a minimum core thickness of about 2…3 μm.

Development of Decontamination Treatment Techniques for Dry Powder Foods by Atmospheric-Pressure Nonequilibrium DC Pulse Discharge Plasma Jet

Dry powder food ingredients imported to Japan contain large amounts of viable bacteria and coliform bacteria, and we need a simple, low-cost, dry nonthermal decontamination method without spoiling nutrients, color, fragrance, and flavor. In this study, it is shown that the decontamination performance against viable bacteria and coliform bacteria is proportional to the plasma irradiation time when OH and O3 radicals are incident on the dry powder food ingredients placed in an atmospheric-pressure nonequilibrium DC pulse discharge Ar + O2 mixture gas plasma jet. Our study revealed that there is a correlation between the plasma irradiation time and DC pulse frequency increase and the decontamination effect on the general bacterial count.

Steam and plasma bubble evolution on laser treating of a sample being in liquid

An object of investigation is pointed - the metal articles being in water and exposed to pulse laser treating. The purpose of research was the investigation of the processes occurring near the metal target treated by pulse laser radiation with flux density ∼106 W/cm–2. The results of the experimental investigation of the pulse laser treating of a led target being in water are described. The process of steam and gas plume generation on the surface of the irradiated metal was investigated experimentally. The features of steam and gas plume form and dimensions evolution on different stages of the process (even after completion of laser treating of the material) were investigated. It is shown that when using GOR-100M operating in free oscillating regime (pulse duration 1.2 ms, laser radiation flux density ∼106 W/cm–2), the form of a crater developed on the irradiated target surface being in water essentially differs from the topography of the crater developed on the analogical target surrounded by air at normal pressure (105 Pa). It is pointed in the conclusions that the substantial difference of the forms of crater surfaces developed as a result of processing of the identical targets being in water or air by laser pulses with the identical parameters , determines by principally different character of plasma and steam and gas mixture flow in the mentioned cases.

Surface morphology of nanostructured zinc oxide materials obtained by plasma-enhanced chemical vapor deposition

Zinc oxide nanostructures have been obtained by plasma-enhanced chemical vapor deposition technique. High-purity elemental zinc was used as the precursor. RF (40MHz) inductively coupled plasma discharge was used for the initiation of the plasma-chemical reactions in the gas phase. Hydrogen-oxygen (H2-O2) composition of various ratios was used as the source plasma gas mixture. The optical emission spectroscopy was used as the powerful tool to determine the intermediate excited particles and assume the possible mechanism of the plasma-chemical reactions. The paper also investigates the influence of the substrate’s materials, its temperature, and the power of the heat input into the discharge. The properties of the sprayed materials were characterized by the method of atomic force microscopy.

Reactive ion etching process of SiO2 film using on-site synthesized C2F4 from CF4

C2F4 is a potential etching gas for high aspect ratio etching of SiO2 films owing to its high etch rate. However, it is difficult to fill C2F4 to a gas cylinder with high pressure for mass production due to its high reactivity. To overcome this problem, we developed an etching system, where the on-site synthesized C2F4 from CF4 using a VHF plasma was supplied directly into the etching reactor. It was demonstrated that SiO2 etching using synthesized-C2F4/O2/Ar gas mixture plasma was carried out successfully and the etch rate became 2.7 times higher than that of CF4/O2/Ar gas mixture plasma.

Parameter study on the electrical resistivity of plasma sprayed TiOx/Cr2O3 coatings

Thermally sprayed coatings based on TiOx/Cr2O3 gain increased attention for the application as resistive heating elements. The higher resistivity of ceramic compared to metallic coatings improves the geometric flexibility of the coating as no long or complex conductor paths are required to achieve sufficient resistances. The main objective of this work is to understand the influence of the process parameters on plasma sprayed TiOx/Cr2O3 coatings. Therefore, a design of experiment method is conducted to investigate the effects on the coatings’ resistivity. Initially, the feedstock of TiOx with a weight proportion of 20 % Cr2O3 is characterized. A three cathode-plasma spray gun with an Ar/H2 plasma gas mixture is used to apply the coatings. The current, the amount of H2 in the plasma gas mixture as well as the stand-off distance of the spray gun are varied according to a circumscribed central composite (CCC) design. These three factors are found to be significant in terms of influencing the coatings’ resistivity and a corresponding regression model is established. Furthermore, a significant phase change of the powder due to the spraying process is detected by means of XRD.

Unified gas-kinetic wave-particle methods IV: multi-species gas mixture and plasma transport

In this paper, we extend the unified gas-kinetic wave-particle (UGKWP) methods to the multi-species gas mixture and multiscale plasma transport. The construction of the scheme is based on the direct modeling on the mesh size and time step scales, and the local cell’s Knudsen number determines the flow physics. The proposed scheme has the multiscale and asymptotic complexity diminishing properties. The multiscale property means that according to the cell’s Knudsen number the scheme can capture the non-equilibrium flow physics when the cell size is on the kinetic mean free path scale, and preserve the asymptotic Euler, Navier-Stokes, and magnetohydrodynamics (MHD) when the cell size is on the hydrodynamic scale and is much larger than the particle mean free path. The asymptotic complexity diminishing property means that the total degrees of freedom of the scheme reduce automatically with the decreasing of the cell’s Knudsen number. In the continuum regime, the scheme automatically degenerates from a kinetic solver to a hydrodynamic solver. In the UGKWP, the evolution of microscopic velocity distribution is coupled with the evolution of macroscopic variables, and the particle evolution as well as the macroscopic fluxes is modeled from a time accumulating solution of kinetic scale particle transport and collision up to a time step scale. For plasma transport, the current scheme provides a smooth transition from particle-in-cell (PIC) method in the rarefied regime to the magnetohydrodynamic solver in the continuum regime. In the continuum limit, the cell size and time step of the UGKWP method are not restricted by the particle mean free path and mean collision time. In the highly magnetized regime, the cell size and time step are not restricted by the Debye length and plasma cyclotron period. The multiscale and asymptotic complexity diminishing properties of the scheme are verified by numerical tests in multiple flow regimes.

Investigation of Physicochemical Properties of Plasma Activated Water and its Bactericidal Efficacy

Presently, the world is using different types of disinfectants for various applications such as disinfecting instrument surfaces in the medical field, sanitizers in airplanes, wash-rooms, etc. These disinfectants carry various harmful chemicals that harm our ecosystem. Hence in search of more eco-friendly disinfectants, our research leads us to explore plasma activated water (PAW). PAW can be produced with the help of plasma interaction with water. This interaction changes the physicochemical properties of water and makes it reactive to kill pathogens. PAW can be used for a wide variety of applications such as food preservation, fast germination, sterilization agents, etc. In the present work, PAW has been formed using non-thermal plasma and different gas or gas mixture as plasma forming gas. Its physicochemical properties have been studied to identify pH value, oxidation reduction potential, and chemical species formed after plasma interaction with a water surface. Further, the study has been carried out to determine the disinfectant ability of PAW for two types of pathogens named Staphylococcus aureus and Pseudomonas aeruginosa. Different bactericidal methods have been adopted to confirm the results. SEM, fluorescence microscopy, and release of nucleic acids and proteins from raptured cell membrane of pathogens clearly show bactericidal efficacy of PAW. The important finding of the present work is the effective killing of pathogens by PAW in a short span of interaction time (≤ 10 s). The PAW has a strong possibility of using it as a disinfectant to kill a wide variety of pathogens and micro-organisms.

Benefits of Hydrogen in a Segmented-Anode Plasma Torch in Suspension Plasma Spraying

Suspension plasma spraying (SPS) enables the production of various coating microstructures with unique mechanical and thermal properties. Aeronautical manufacturers have been working for fifty years to improve the thermal barrier coating (TBC) performances in gas turbines. Commercial plasma torches with a segmented anode that are characterized by stable plasma jets should enable a better control of the TBC microstructure. The addition of diatomic gases such as hydrogen in the plasma-forming gas affects the plasma jet formation and causes some instabilities. However, it enhances the thermal conductivity of the gas flow, the plasma mass enthalpy and the heat transfer to particles. This study aims to characterise and describe the coating microstructure changes of yttria-stabilised zirconia when gradually adding hydrogen with argon into the plasma gas mixture. The effect of hydrogen is weighted out due to the gas mass enthalpy, mean velocity at the nozzle exit and “hot zone” length of the plasma jet. The coating microstructures, which depend on these plasma jet parameters, will be mapped from feathery and porous to dense and cracked deposits depending on the spraying conditions.

水中で発生させたHe とO2 の混合ガスの大気圧低温プラズマによるホルムアルデヒド水溶液の分解

水中で発生させたHe とO2 の混合ガスの大気圧低温プラズマによるホルムアルデヒド水溶液の分解

Control of ion energy during plasma enhanced atomic layer deposition: A new strategy for the modulation of TiN growth delay on SiO2

Deposition of titanium nitride thin films by plasma enhanced atomic layer deposition has been realized on thermal silicon oxide substrates in an inductively coupled plasma reactor. The plasma step involves a H2 (40 sccm)/N2 (5 sccm)/Ar (10 sccm) gas mixture, and growth has been followed by in situ ellipsometric measurements. A tunable substrate bias voltage has been applied in the vicinity of the substrate to modulate plasma-ion energy and investigate its impact on the growth mechanism. We have observed that an increase in the applied bias power leads to a gradual TiN nucleation delay of up to 30 cycles at 80 W radio frequency bias power. An increase in the H2 content of the plasma gas mixture shows that hydrogen species from the plasma can significantly deactivate the SiO2 substrate, thanks to reduction reactions induced by H3+, Ar+, and ArH+ ions leading to the formation of Si–H surface bonds. A nitrogen-rich plasma gas mixture results in N atom incorporation on the substrate surface, which in turn favors subsequent TiN growth. The combination of hydrogen-rich plasma chemistry with a high applied substrate bias power leads to a TiN growth delay larger than 50 cycles. These results provide a valuable implementation for the development of area-selective deposition processes.

Changes in the Coating Composition Due to APS Process Conditions for Al2O3-Cr2O3-TiO2 Ternary Powder Blends

Thermally sprayed coatings from the single oxides and binary compositions of the Al2O3-Cr2O3-TiO2 system show multifunctional properties. Ternary compositions are promising for further improvement in their performance. The stability of the composition during coating formation is an important issue for blended feedstock powders in order to obtain the desired properties. This work focuses on the compositional changes of a ternary blend of Al2O3, Cr2O3 and TiOx powders of equal content by mass in a conventional atmospheric plasma spraying (APS) process using an Ar/H2 plasma gas mixture. By increasing the argon flow rate at constant hydrogen flow rate, the total plasma gas flow rate and the Ar/H2 ratio were varied. For the highest argon flow rate, this resulted in an average particle velocity of 140% and an average particle temperature of 90% of the initial values, respectively. Coating composition and microstructure were studied by optical microscopy, SEM, including EDS analyses, and XRD. In addition, the coating hardness and electrical impedance were also measured. Differences in the “difficulty of melting factor” (DMF) and the thermal diffusivity of the three oxides appear to be responsible for the dramatic change of the coating composition with an increasing argon flow rate. For the highest argon flow rate applied, besides TiO2, the coating contains only 8 wt.% Al2O3, while the Cr2O3 content remained almost constant. At the same time, the change of the Ar/H2 ratio resulted in the formation of stoichiometric TiO2 in the coating by oxidation of TiOx in the feedstock powder. Moreover, a small content of titanium was found in the Cr2O3 splats, showing that there are only limited interactions between the large oxide powder particles. Thus, the study has shown that stability of the chemical composition during spraying of ternary powder blends is strongly influenced by the process conditions.