SPECIAL TOPIC—Several problems in plasma physics and material treatment
气体放电产生的等离子体是集成电路制备不可或缺的关键技术, 利用等离子体中活性粒子赋予的独特的物理和化学特性, 可为超大规模集成电路制备提供具有定向性、选择性和纳米级精细性的绿色先进加工技术, 大规模应用于其沉积、刻蚀、封装、清洗等工艺制程. 在材料表面改性、新材料制备、生物灭菌消毒、等离子体隐身、医疗器具及人造器官的清洗、臭氧生成、新型光源、废弃物处理等领域也具有极其重要的应用前景, 其低温加工的特性使其成为柔性可穿戴智能材料和器件最合适的加工技术之一.
等离子体材料处理技术与等离子体和材料的相互作用密切相关. 不同的等离子体源及其放电设计具有不同的等离子体温度、密度、活性种类和结构特性, 适应不同种类的材料处理; 不同种类的材料处理对等离子体特性也有不同的要求, 并且反过来影响等离子体放电的过程和特性. 因此需要深入研究等离子体放电技术、等离子体特性诊断、等离子体材料处理的机理等, 特别是针对各种应用需求建立其与等离子体特性的关联与机制, 促进等离子体材料处理技术在精细加工、新材料制备等各领域的应用. 研究方法包括理论数值模拟、诊断和实验研究.
希望本专题能促进等离子体物理及其材料处理技术的进步, 以及与各交叉研究领域的交流, 扩大国内等离子体物理及其材料处理研究的影响, 推动该领域研究的蓬勃发展.
2021, 70 (9): 095205.
doi: 10.7498/aps.70.20202181
Abstract +
Atmospheric pressure plasma jet has received increasing attention due to its wide potential applications such as in material processing and surface modification. This paper presents the characteristics of titanium oxide (TiO2) thin films deposited by using atmospheric pressure corona plasma jet based on a needle-plate configuration. The influences of corona polarity and operating parameters on the properties of TiO2 films are investigated. The characteristics of positive and negative corona discharge, the developing process and the emission spectrum of the plasma jet are tested, and the TiO2 films prepared under different conditions are measured and analyzed. The results show that the TiO2 film prepared by negative corona plasma has a more uniform surface, and the Ti content in TiO2 film is higher than that by the positive corona plasma. The adhesion force is higher than 4.7 N/cm and the surface resistance of the film is less than 1010 Ω. The deposition of the TiO2 film is closely related to the nucleation mechanism of the precursor in the plasma jet and/or the interface between jet and substrate. These results will provide useful reference for preparing uniform and functional oxide film materials by atmospheric pressure plasma jet.
2021, 70 (9): 095210.
doi: 10.7498/aps.70.20201676
Abstract +
The rapid development of social economy leads the output of solid waste to increase rapidly. The traditional treatment methods, such as landfilling, incineration and composting, are not only inefficient, but also have many limitations, such as secondary pollution and waste of resources. Therefore, it is urgent to explore new solid waste treatment technology. Due to its high efficiency, environmental protection and high energy conversion, the plasma gasification technology has been applied to the harmless treatment of solid waste. This article introduces the background and significance of plasma gasification technology in solid waste treatment, and summarizes the application of plasma gasification technology to different solid waste treatments, the technical level and research progress of plasma gasification of solid waste in the world are described in detail, and the existing problems in the current application of plasma gasification of solid waste are emphatically analyzed. It is pointed out that plasma gasification technology is an effective way to treat solid waste.
2021, 70 (9): 095213.
doi: 10.7498/aps.70.20210546
Abstract +
The capacitive CF4/Ar discharges driven by a dual frequency source based on the electrical asymmetry effect (EAE) are studied by using a one-dimensional fluid coupled with Monte-Carlo (MC) model and a two-dimensional trench model. The effects, induced by varying the relative gap distance, on self-bias voltage, electronegativity, ion flux, neutral flux and other plasma characteristics are systematically discussed. In this asymmetric discharge, as the gap distance increases, the absolute value of the self-bias voltage and electronegativity decrease. Meanwhile, the plasma density and absorption power increase accordingly because the effective discharge area expands but the boundary loss is still limited. In addition, both $ \mathrm{\alpha } $ mode and drift-ambipolar (DA) mode can play their important roles in the discharges with different gap distances, though DA mode is weakened in large gap discharge. Owing to the fact that the self-bias is larger and electronegativity is stronger for the case of smaller gap distance, the sheath expansion electric field at the powered electrode and the bulk electric field heat the electrons, leading the ionization rate to greatly increase near the collapse of the sheath at the grounded electrode. Besides, at the larger gap distance, the maximum value of the ionization rate decreases due to the reduction of electrons with relatively high-energy, and the ionization rate near the grounded electrode is reduced evidently. Moreover, with the increase of the gap distance, the maximum ion energy decreases and the ion energy distribution width becomes smaller due to the reduction of the self-bias voltage. Meanwhile, the etching rate increases a lot since the neutral flux increases significantly near the powered electrode. However, as the gap distance increases to 5 cm, the etching rate stops increasing and the trench width at the bottom becomes narrow because the neutral flux increases greatly compared with ion flux, forming a thick layer of polymer. So, besides separately controlling the ion energy and flux, optimizing the synergistic effect of ion flux and neutral group flux to adjust the etching rate and improve the etching morphology is also an interesting topic in the asymmetric CF4/Ar discharges.
2021, 70 (9): 095206.
doi: 10.7498/aps.70.20202247
Abstract +
Inductively coupled plasmas have been widely used in the etch process due to the high plasma density, simple reactor geometry, etc. Since the plasma characteristics are difficult to understand only via experiments, the numerical study seems to be a valuable and effective tool, which could help us to gain an in-depth insight into the plasma properties and the underlying mechanisms. During the past few years, various models have been employed to investigate inductive discharges, such as global model, fluid model, fluid/Monte Carlo collision hybrid model, biased sheath model, particle-in-cell/Monte Carlo collision hybrid model, etc. Since the plasma parameters are volume averaged in the global model, which effectively reduces the computational burden, it is usually used to study the reactive gas discharges with a complex chemistry set. In order to obtain the spatial distribution, a two-dimensional or three-dimensional fluid model is necessary. However, in the fluid model, the electron energy distribution function is assumed to be Maxwellian, which is invalid under special discharge conditions. For instance, strong electric field and low pressure may result in non-Maxwellian distributions, such as bi-Maxwellian distribution, two-temperature distribution, etc. Therefore, a fluid/Monte Carlo collision hybrid model is adopted to take the electron kinetics into account. Besides, a separate biased sheath model is necessary to study the influence of the sheath on the plasma properties self-consistently. The particle-in-cell/Monte Carlo collision hybrid model is a fully kinetic method based on the first-principles, which could be used to investigate the non-local and non-thermal equilibrium phenomena. In conclusion, the numerical investigation of inductively coupled plasmas has a significant importance for plasma process optimization.
2021, 70 (9): 095214.
doi: 10.7498/aps.70.20210473
Abstract +
Capacitively coupled plasma (CCP) has gain wide attention due to its important applications in industry. The researches of CCP mainly focus on the discharge characteristics and plasma parameters under different discharge conditions to obtain a good understanding of the discharge, find good methods of controlling the charged particle properties, and improve the process performance and efficiency. The controlling of plasma parameters is based on the following three aspects: gas, chamber, and power source. Changing these discharge conditions can directly influence the sheath dynamics and the charged particle heating process, which can further influence the electron and ion distribution functions, the plasma uniformity, and the production of neutral particles, etc. Based on a review of the recent years’ researches of CCP, the electron heating dynamics and several common methods of controlling the plasma parameters, i.e. voltage waveform tailoring, realistic secondary electron emission, and magnetized capacitively coupled plasma are introduced and discussed in detail in this work.
2021, 70 (9): 097201.
doi: 10.7498/aps.70.20201809
Abstract +
A variety of carbon-based thin films are prepared by self-developed helicon wave plasma chemical vapor deposition (HMHX, HWP-CVD) through changing the parameters of plasma discharge. The Ar/CH4 plasma discharge is diagnosed in situ by Langmuir probe, emission spectroscopy and mass spectrometry. The carbon thin films are characterized by scanning electron microscopy (SEM) and Raman spectroscopy (Raman). The results show that under the given parameters, the plasma discharge modes are all helicon wave discharge modes. Under a given CH4 flow rate, the energy distribution in the plasma is enough to dissociate the methane molecules and form carbon free radicals. The preparation of different carbon-based films is realized by adjusting the CH4 fluence. The research result shows that when the plasma is rich in CH and H radicals, it is suitable for growing diamond-like carbon films. When the plasma is rich in C2 radicals and less H, it is favorable for growing vertical graphene nanosheets. According to the results of plasma diagnosis and material characterization, the decomposition mechanism of methane molecules under the action of Ar helicon wave plasma (HWP) is proposed, and the growth model of carbon-based materials is established, the feasibility of Ar/CH4-HWP in the preparation of carbon-based nanomaterials is verified, which provides a reference for preparing the carbon-based materials by HWP-CVD technology.
2021, 70 (9): 095208.
doi: 10.7498/aps.70.20202078
Abstract +
Plasma contains highly reactive species, including electrons, ions, radicals, photons, etc., which are critical for catalyzing or directly participating in chemical reactions. Plasma is a highly efficient tool in chemical synthesis and material modification, since it can make the chemical reactions that are difficult or even impossible to occur under thermal equilibrium conditions take place and accelerate through its catalysis. The chemical reactivity of graphene under conventional conditions is low, which means that the reaction of graphene requires high temperature, high pressure and/or strong acid or alkali, thereby restricting the synthesis and modification of novel graphene-derived materials. Plasma-assisted graphene reaction can trigger a series of chemical reactions, such as reduction, oxidation, defect repair, doping, grafting, epitaxial growth and cross-linking of graphene, under ambient temperature and pressure without any corrosive conditions. It provides great potentials for the functional modification of graphene and the synthesis of graphene composites, which deserve further exploration. Over the past decade, a number of studies of graphene synthesis and modification by using plasma with distinctive characteristics have been reported. However, most of reports focused on the presentation of technical routes and corresponding results, and the research on chemical reaction kinetics is still far from being fully addressed. In this review, we make a comprehensive discussion about these reports by mainly summarizing and discussing some of the representative results, in order to promote further research in the relevant fields.
2021, 70 (9): 095212.
doi: 10.7498/aps.70.20210585
Abstract +
The ultra-durable, self-healing superhydrophobic polyester fabric based on ZnO@PDMS nanoparticle composite coating (PET-g-PDMS@ZnO fiber) is prepared by low-temperature plasma polymerization. The influences of the preparation process on the superhydrophobicity, self-healing property, durability and stability of the coating are studied. The results show that the water contact angle on the surface of PET-g-PDMS@ZnO fabric can reach 162.7°, and the sliding angle is 7.5°. After 300 washing cycles and 1300 rubbing cycles, the superhydrophobic property is still maintained. Both plasma method and heating method are used to repair the worn coating, and it is found that the repair effect of plasma is obvious, while the heating repair is effective only in the case of small loads. Moreover, scanning electron microscope, nanoindentation and X-ray photoelectron spectroscopy measurement are used to explore the self-healing mechanism. This research provides the theoretical and technical support for the development and application of plasma technology in the preparation of superhydrophobic fabrics.
2021, 70 (9): 095203.
doi: 10.7498/aps.70.20202233
Abstract +
Plasma technology plays an important role in preparing and processing materials nowadays. This review focuses on the applications of non-thermal plasma (NTP) in the surface treatment and functionalization of materials, including the plasma sources for generating plasmas, NTP techniques and specific application fields. The plasma sources include inductively coupled plasma, capacitively coupled plasma, electron cyclotron resonance plasma, surface wave plasma, helicon wave plasma, atmospheric pressure plasma jet, and dielectric barrier discharge plasma. The NTP techniques for material surface treatment and functionalization include plasma surface grafting and polymerization, plasma enhanced chemical vapor deposition, plasma assisted atomic layer deposition, plasma enhanced reactive ion etching, and plasma assisted atomic layer etching. Specific applications of plasma surface treatment and functionalization cover hydrophilic/hydrophobic surface modification, surface micro-nano processing, biological tissue surface treatment, and catalyst surfaces treatment. Finally, the application prospects and development trends of NTP technology for material surface treatment and functionalization are proposed.
2021, 70 (9): 095201.
doi: 10.7498/aps.70.20201790
Abstract +
Based on the Stark broadening method and the imaging method, the electron densities of the plasma generated at different pulse frequencies, gap distances and inner diameters of the electrodes are diagnosed. The experimental results indicate that reducing the pulse frequency, shortening the gap distance between the electrodes, and using thinner diameter electrode are all in favor of enhancing the electron density. With the help of the global model, we perform the numerical simulation to explore the factors that influence the variation of the electron density. According to the simulations results, we find that the reduced discharge volume results in the increase of electron density with the increase of pulse frequency. When the gap distance between the electrodes is reduced, although the increased absorbed power and the reduced discharge volume both have an effect on the electron density, the reduced discharge volume plays a decisive role in these two factors. Moreover, using a thinner inner diameter electrode can also reduce the discharge volume, which is of benefit to obtaining the plasma with high electron density.
2021, 70 (9): 095211.
doi: 10.7498/aps.70.20202248
Abstract +
Optical emission spectroscopy (OES) has been widely applied to plasma etching, material processing, development of plasma equipment and technology, as well as plasma propulsion. The collisional-radiative model used in OES is affected by the deviation of fundamental data such as collision cross sections, thus leading to the error in diagnostic results. In this work, a novel method is developed based on feedforward neural network for OES. By comparing the error characteristics of the new method with those of the traditional least-square diagnostic method, it is found that the neural network diagnosis method can reduce the transmission of basic data deviation to the diagnosis results by identifying the characteristics of the spectral vector. This is confirmed by the experimental results. Finally, the mechanism of the neural network algorithm against fundamental data deviation is analyzed. This method also has a good application prospect in plasma parameter online monitoring, imaging monitoring and mass data processing.
2021, 70 (9): 099401.
doi: 10.7498/aps.70.20210077
Abstract +
2021, 70 (9): 098104.
doi: 10.7498/aps.70.20202245
Abstract +
With the shrinkage of chip feature sizes, porous materials are widely used in microelectronics. However, they are facing severe challenges in plasma etching, as the reactive radicals can diffuse into the interior of material and damage the material, which is called plasma induced damage. In this paper, we review two kinds of etching processes based on low chuck temperature, i.e. cryogenic etching. By lowering the chuck temperature, either the etching by-products or the precursor gas can condense in the porous material, and thus preventing the radicals from diffusing and protect the material from being damaged by plasma. The technology of cryogenic filling inside the porous material is simple but effective, which allows it to have a good application prospect.
2021, 70 (9): 095202.
doi: 10.7498/aps.70.20202246
Abstract +
Atmospheric pressure plasma plume generated by pulsed discharge is studied by experimental diagnostics and numerical simulations. It is found that the plasma plume is generated in the rising phase of pulse voltage, in which a plasma bullet propagates toward the ground electrode at a speed on the order of 104 m/s. It is also found that the electric field in the vicinity of the plasma bullet reaches 106 V/m, indicating that the formation of plasma bullet can be attributed to the localized enhanced electric field, which will be enhanced near to the grounded electrode. The spatiotemporal evolution of electron density in the discharge reveals that the residual electron density remains after the plasma bullet has passed through, which explains the tailing phenomenon of plasma bullet. The enhanced electron generation rate at the head of plasma bullet corresponds to the localized enhanced electric field, which explains the generation mechanism of plasma bullet. This study of the characteristics and mechanism of plasma bullet provides a theoretical basis for developing the atmospheric plasma plume generated by pulsed discharge.
