Heji Huang

Molybdenum (Mo), owing to its excellent high-temperature resistance and low sputtering yield, is widely used in advanced equipment such as extreme ultraviolet (EUV) lithography systems. To meet the demand for regulating the surface wettability of Mo, this study employs micromilling to fabricate microgroove arrays on Mo surfaces to enhance their hydrophobicity. First, comparative micromilling experiments are conducted under flood cooling and minimum quantity cooling lubrication (MQCL) conditions. The effects of process parameters, including axial depth of cut (a_p), spindle speed (n), and feed per tooth (f_z), on burr height (H) are investigated under the two cooling/lubrication conditions. The results show that the burr heights obtained under flood cooling are generally lower than those obtained under MQCL. Subsequently, the machining parameters under flood cooling are evaluated using an orthogonal experimental design. A mathematical model relating the microgroove array geometry to the contact angle is then established based on Gibbs free energy to guide the design of surface microgroove structures. Finally, microgroove arrays are fabricated on Mo surfaces using the preferred machining parameters. The results demonstrate that the microgroove arrays effectively enhance the water hydrophobicity of the Mo surface. The static water contact angle increases from 62.45 ± 0.25° for the untreated Mo surface to a maximum of 128.50 ± 0.12°, thereby achieving a transition from hydrophilic to hydrophobic behavior.

A hydrodynamic model is developed in this study for extreme ultraviolet (EUV) nanolithography to investigate the tin transport in a hydrogen buffering environment, where hydrogen is often adopted to avoid tin contamination of the optical reflective mirror from high-energy tin plasma and debris. The combined diffusion coefficient method is first employed in an EUV lithography chamber to describe the tin-hydrogen diffusion under the concentration, temperature, and pressure gradients. The results show that injecting bulk hydrogen gas can effectively control the tin mass fraction near the mirror down to 10−3–10−5. Two or more vortices are generated within the flow field by the interactions between multiple hydrogen streams and their interactions with the walls, which on an excessively large scale, can alter the transport processes of tin, exacerbating tin contamination on the mirror. Concentration diffusion dominates the diffusion processes of the tin majority near the central axis, while pressure diffusion is the key for tin deposition on the mirror and outer walls, and temperature diffusion has a relatively small influence near the vane wall. By optimizing the three hydrogen inlet fluxes, the tin-hydrogen flow field transitions from three vortices to two vortices far from the center with reduced vortex scale, thus reducing the tin mass fraction on the mirror significantly below 5 × 10−6. Our work provides a deeper flow-diffusion mechanism of tin under buffering hydrogen, contributing to strategies for reducing tin contamination in EUV lithography.

Molybdenum (Mo), owing to its excellent properties, is widely used as a plasma-facing material and is recognized as a typical difficult-to-machine material. Achieving high-quality, low-damage machining is essential for ensuring the service reliability of Mo components. However, studies on the milling of Mo remain limited. Therefore, this study investigates a high-quality, low-damage milling technique for Mo based on analyses of milling force, machined surface roughness, and white layer formation. First, the effects of machining parameters, including radial depth of cut (a_e), spindle speed (n), and feed per tooth (f_z), on the responses, namely milling force (F) and surface roughness (R_a), were investigated. The relationships between milling force, surface roughness, and white layer formation were analyzed. Subsequently, the response surface methodology (RSM) was employed to reveal the influence mechanisms of the machining parameters and their interactions on the response variables. Finally, a Kriging surrogate model integrated with the Non-dominated Sorting Genetic Algorithm II (NSGA-II) was adopted to identify the optimal machining parameter combination for high-quality, low-damage milling. The results indicate that the milling force and white-layer thickness exhibit consistent increasing trends with increasing feed per tooth under the investigated conditions, demonstrating that controlling the milling force is an effective approach for achieving high-quality, low-damage milling of Mo. For the simultaneous minimization of milling force and surface roughness, the optimal machining parameters were determined to be a radial depth of cut of 0.2101 mm, a spindle speed of 10,090.7 rpm, and a feed per tooth of 0.01 mm/z. These findings provide valuable process parameter guidance for the precision machining of Mo components.

Iodine has a high atomic mass, low ionization threshold, abundant reserves, and low cost, making it a promising propellant for RF ion thrusters with significant development potential and application prospects. Two-dimensional axisymmetric PIC-MCC numerical simulation results for a low-power iodine RF ion thruster indicate that I + ions dominate the discharge with a large radial gradient, whereas I 2 + ions exhibit a lower but radially uniform concentration. Comparative analysis at RF power 35 W demonstrates that iodine achieves higher plasma density and beam current than xenon, which is attributed to the higher electron-neutral collision frequency of iodine, which mitigates electron inertia effects and weakens negative power absorption. Studies on the iodine thruster at different RF powers (screen grid voltage 1800 V) indicate that as power increases from 28 W to 47.5 W, the mass utilization efficiency and thrust efficiency improve from 36.13% and 24% to 58.49% and 29.6%, respectively, while power efficiency decreases slightly. Changing the grid voltage affects the state of the grid sheath layer. As the grid voltage increases from 1000 V to 1800 V (RF power 41.5 W), the sheath transitions from under-focused to over-focused, and the efficiencies of the thruster exhibit a linear increase trend.

In this paper, a new attempt for improving CO2 decomposition in a dielectric barrier discharge (DBD) reactor is proposed from the aspect of regulating the inlet CO2 gas temperature and the plasma discharge zone temperature. Meanwhile, a comparative study is conducted under three cases of no cooling, the use of gas precooling, and the combination of gas precooling and bare electrode cooling. The experimental results show that the introduction of gas precooling can nearly double CO2 conversion, while the introduction of bare electrode cooling in the presence of gas precooling only leads to an increase of about 10% in CO2 conversion. With the combined use of gas precooling and bare electrode cooling, the highest CO2 conversion of 47% is obtained at a CO2 flow rate of 20 sccm. Further analysis of discharge characteristics indicates that gas precooling results in a considerable enhancement of the reduced electric field in plasma filaments, while the bare electrode cooling enhances the reduced electric field slightly. It is inferred that the two kinds of cooling could enhance the electron impact dissociation in plasma filaments and suppress the three-body recombination process in the entire discharge zone, which leads to different degrees of improvement on CO2 decomposition. The work provides a reference for controlling the temperature of chemical reactions in atmospheric DBD.

The anodic arc of vacuum discharge can generate ion beams with relatively high density, which exhibits unique advantages and broad application prospects in the field of material processing. However, the generation of traditional anodic arcs is often constrained by conditions such as kA-level high current, sub-millimeter short gap, sub-millimeter anode size, and low-melting-point materials like aluminum. This study proposes an effective formation method for vacuum anodic arcs: by combining the pre-ionization effect of triggering with the discharge of the main circuit, an anodic arc for high-melting-point metals like titanium is realized. The results indicate that with triggering-induced pre-ionization, a reasonable combination of titanium anode parameters—including a main circuit breakdown voltage of 8–15 kV, an electrode gap of 4–10 mm, and an anode size of 1–3 mm—achieves a microsecond-level ( μ s) pulsed anodic arc. Under the same trigger current and anode size, the anodic arc intensity increases significantly with the increase in electrode gap; with the same electrode gap and anode size, it increases significantly as the trigger current decreases; and when the trigger current and electrode gap remain unchanged, it weakens rapidly with the increase in anode size. Studies on the influence of discharge and structural parameters on the anodic arc indicate that the power density deposited on the anode during the high-voltage discharge stage is the key parameter determining arc generation. Based on the experimental results, the order of magnitude of the power density required is estimated to be 10 ¹⁰ W m ⁻² . This study can provide a reference for the design of vacuum anodic arcs devices.

The pinch-off process of liquid bridges in metal droplet-on-demand printing directly determines printing precision and structural quality. Numerical simulations have investigated the regulation mechanism of the liquid bridge necking process by the positive and negative pressure pulses, namely, the “push–pull effect,” generated by electromagnetic actuation on the molten metal. The results show that the positive pressure pulse dominates the initial necking of the liquid bridge through axial stretching effect, while the negative pressure pulse significantly accelerates the necking process of the liquid bridge through a dual strengthening effect, namely, axial reverse stretching and radial shearing effect. The increase in the negative pulse peak causes the upward movement of the pinch-off position and the dynamic evolution of the morphology from inverted cone, cylinder to positive cone. The scaling law of necking velocity under bidirectional drive clarifies the competitive mechanism of surface tension suppressing necking in the early stage and negative pulse-induced curvature instability triggering self-accelerated breakup and the analysis of axial and radial Weber numbers reveals the competition mechanism between inertial force and capillary force. The droplet generation time, velocity, and size can be controlled by the “pulse intensity-duration” collaborative optimization method, which provides a theoretical basis for high-precision electronic printing and microscale metal 3D printing.

The optimization design and accurate performance prediction of a radio frequency ion thruster require coupled analysis of the discharge chamber and grided acceleration. In this numerical simulation study, the three-dimensional grid structure is reduced to an axisymmetric model through the equivalent transmittance method, enabling self-consistent simulation of the overall structure of the thruster. A particle-in-cell Monte Carlo collisions (PIC-MCC) method is implemented to investigate a 45 W xenon RF ion thruster operation, and obtain the spatial distributions of the magnetic field, the electric field, the potential, and the plasma parameters in the discharge chamber and the plume region. This approach conveniently correlates the upstream plasma generation process and downstream beam acceleration. Through combined theoretical analysis and numerical simulation, the electron energy absorption mechanism is elucidated, i.e. the RF coil generates the axial magnetic field through the current of a certain frequency, and further induces the azimuthal electric field, which causes the absorbed power to oscillate at a two-fold frequency due to the phase difference between the induced electric field and the current. The change of the absorbed energy of the electrons from the time-varying electromagnetic field causes the periodic change of the ionization rate and potential. Comparative analysis of the ion velocity and ion energy flux distribution before the screen grid and after the acceleration grid reveals that increasing RF power from 18 W to 45 W enhances discharge chamber plasma density while the degree of ionization at the grid exit rises from 15.37% to 32.83%. Concurrently, the grid sheath potential increases, improving the extracted ion beam occupancy ratio and focusing effect and ultimately enhances the thrust and specific impulse. The mass utilization and thrust efficiency of the thruster increase linearly with RF power.