Surface-wave Plasma Source Research Articles

Prototype of a 2.45 GHz cylindrical ceramic dielectric antenna surface wave plasma source for flood gun

In modern ion implanters, plasma flood gun (PFG) is used to neutralize wafer charge during doping process, preventing the breakdown damage of floating wafers caused by the space charge accumulation. Surface wave (SW) plasma source is a potential choice for PFG, because of its no metal pollution, simple structure, high current intensity and low cost for ion implanters. At Peking University (PKU), a 2.45 GHz surface wave PFG (SW-PFG) with a cylindrical dielectric antenna was designed and tested recently. It aims to obtain plasma at the gas pressure of several 10−3 Pa and below which is strictly required by the ion implanter. A two-dimensional discharge model was established to optimize the cylindrical dielectric antenna structure. Through optimization of operation parameters, the electron current intensity of 88 mA was obtained with RF power 500 W and gas pressure 5.0 × 10−3 Pa in the experiments, which meets the requirement of ion implanters. Further, through optimization of operation parameters, the electron current intensity could reach 140 mA. In addition, by setting the graphite liner to isolate the metal cavity wall and plasma, the improvement in the long-term operation stability has been confirmed, which laid a solid foundation for the industrial application of the SW-PFG.

Study of a linear surface wave plasma source for tin removal in an extreme ultraviolet source

Tin deposition mitigation employs hydrogen radicals and ions, formed in a hydrogen plasma, to interact with tin to form tin hydride (SnH4) in the gaseous state, which is then pumped away. Surface wave plasma (SWP) technology developed at Illinois generates hydrogen radicals and ions, resulting in tin etch rates that are high enough to keep extreme ultraviolet (EUV) lithographic tools clean. An advantage of an SWP antenna is the ability to generate a high density of hydrogen radicals and hydrogen ions directly at the desired etching location. In situ etching of tin enables high availability EUV tools by maintaining high reflectivity of the multilayer mirror of the collector. Additionally, the SWP is characterized with low ion energies and low electron temperature, such that the multilayer mirror does not suffer any damage from sputtering or implantation of hydrogen ions during operation. Here, experiments elucidating the fundamental processes of tin removal are conducted by varying pressure, power, surface temperature, and gas flow rate in order to observe the etch rate behavior. Our results have shown that the presence of hydrogen ions increases etch rates because ion bombardment weakens Sn–Sn bonds, which, in turn, allows for a higher rate of chemical etching by the radicals. The ion bombardment reduces the number of radicals needed to etch a single tin atom to the range of 102–103. The linear SWP antenna yields plasma densities on the order of 1017to1018m−3 and radical densities on the order of 1018to1019m−3, allowing for greater utilization of ion etch enhancement. Etch rates of up to 200 nm/min have been achieved. The surface temperature of the samples is an important factor in the etching process such that the decrease in the surface temperature increases the etch rates and decreases the hydrogen desorption rates. In addition, a kinetic etch model is developed to explain the behavior of etch rates as a function of surface temperature. Furthermore, results from experiments performed in an Illinois NXE:3100 chamber will be discussed.

Study on CF4/O2 plasma resistance of O-ring elastomer materials

The CF4/O2 plasma resistance of the O-ring elastomer samples was investigated using a microwave-excited surface-wave plasma source. The elastomer samples include a fluoroelastomer (FKM) and a perfluoroelastomer (FFKM), both of which are widely used for plasma process equipment. In semiconductor manufacturing, the CF4/O2 plasma was widely used for plasma etching and cleaning processes. To investigate the relation between the plasma resistance of the elastomers and plasma parameters, the Langmuir probe measurement was carried out. The kinetic energy of bombarding ions was increased by applying an rf power to a wafer substrate in some of the experiments. The results suggested that both FKM and FFKM were eroded by the low-energy (less than 10 eV) ion bombardment. The kinetic energy of bombarding ions was also a dominant factor to determine the amount of erosion of both the FKM and the FFKM elastomers. In addition, it is suggested that neutral radical attacks were also a dominant factor to increase the erosion of the elastomer for the FKM elastomer, whereas the stability of the FFKM elastomer against neutral radicals was much better than that of the FKM elastomer.

Compact water-cooled surface wave plasma source for remote plasma cleaning.

This paper describes the design and operation of a compact surface wave plasma source for remote plasma processing [i.e., plasma enhanced chemical vapor deposition chamber cleaning, dry etching (SiO2, Si3N4, and silicon), photoresist stripping (SU-8), and decapsulation of microchips]. In order to get higher radical generation and increased industrial throughput, the source is designed to generate plasma at a high flowrate. The source is designed to be compact so that it can be more beneficial in the case of positioning multiple sources on a large processing chamber for faster radical cleaning with better uniformity. The source can operate from low to high flowrates (i.e., 100 SCCM H2 or 10 slm NF3) and provide high decomposition rates for NF3. The etching rate for SiO2 (higher than 450 nm/min) is achieved with 2.5 kW microwave power and 3-5 slm. The key advantages of the source are compactness, higher microwave coupling due to indirect water-cooling, and thereby high operating flow and decomposition rates.

Development and plasma characterization of an 850 MHz surface-wave plasma source

This work presents a simple design of surface wave plasma (SWP) source based on cylindrical cavity excited by ∼ 850 MHz ultra high frequency (UHF) wave. The cavity equipped with four rectangular slots demonstrates the usefulness of a large aperture coupling for plasma generation using top-wall excitation. The UHF power is coupled to the plasma through an upper dielectric wall placed at the open end of a coaxial transmission line that is short-circuited at the other end to construct a coaxial transmission line resonator. Using high-frequency microwave simulation the structure of the cylindrical cavity and the slots are designed and fabricated. Numerous plasma diagnostic methods are used to study the plasma characteristics and the mechanism of surface wave (SW) plasma generation. The plasma generation capability of the source is investigated at various operating pressures and UHF powers. It is seen that reasonably radial uniform plasmas with a very high plasma density ∼ 1011 cm-3 can be produced by a wide variation of pressures from 10 mTorr to 180 mTorr. Further, the usefulness of this UHF plasma source for large area plasma application is realized.

High deposition rate nanocrystalline and amorphous silicon thin film production via surface wave plasma source

A 900MHz surface wave antenna was used for plasma-enhanced chemical vapor deposition (PECVD) of silicon thin films in an H2+SiH4 discharge, with an emphasis on photovoltaic applications. Gas mixtures of 0.7–10% SiH4 at medium pressure (~100mTorr) were tested with an optimal substrate temperature of 285±15°C, producing nanocrystalline hydrogenated silicon (nc-Si:H) at rates up to 3nm/s, while amorphous films were grown in excess of 10nm/s. A sharp transition from crystalline to amorphous growth was seen as SiH4 flowrate increased, as is characteristic of silane PECVD. Increasing both substrate temperature and source power served to move this transition to higher flowrates, and by extension, higher deposition rates for the crystalline phase. Grain size also increased with substrate temperature, ranging from 10±2nm at 200°C up to 15±3nm at 400°C. Electron spin resonance showed that a-Si:H films grown via SWP were of acceptable defect density (~1016cm−3) and conductivity (~10−8S/cm). Conversely, nc-Si:H films were poor quality (~1018cm−3 defect density, 10−3–10−2S/cm conductivity) due to low hydrogenation and small grain size.

Theoretical and experimental investigations of surface-waves plasma column and microwave plasma source for applications in reconfigurable plasma antenna

In this paper, we have investigated plasma column excited by surface-waves using monopole plasma antenna. Since the plasma frequency has an important role in conductivity of the plasma column and antenna radiation characteristics, determining the distribution electron density and plasma frequency in the inhomogeneous plasma column axis is particularly important. For this purpose, the plasma column is modelled by fluid equations for high-frequency discharges. Because of significant advantages such as high power handling capability at 2.45 GHz, the surfaguide launcher is chosen from the other conventional launchers to propagate the surface wave along plasma column. The surfaguide is experimentally excited an argon gas-filled tube at a pressure of 0.2 torr as a surface-wave plasma source. In this work, because of safety considerations about high power radiation, plasma frequency measurements was impractical using conventional methods. Therefore, a new system is designed consists of a solid-state VCO, amplifier, circulator, attenuator and two double-ridged horn antennas. The plasma frequency is obtained close to 5.7 GHz, which is consistent with theoretical concepts. © 2017 The Authors. Microwave and Optical Technology Letters. Microwave Opt Technol Lett 59:806–811, 2017

Measurements and modeling of the impact of weak magnetic fields on the plasma properties of a planar slot antenna driven plasma source

The radial line slot antenna plasma source is a type of surface wave plasma source driven by a planar slot antenna. Microwave power is transmitted through a slot antenna structure and dielectric window to a plasma characterized by a generation zone adjacent to the window and a diffusion zone that contacts a substrate. The diffusion zone is characterized by a very low electron temperature. This renders the source useful for soft etch applications and thin film deposition processes requiring low ion energy. Another property of the diffusion zone is that the plasma density tends to decrease from the axis to the walls under the action of ambipolar diffusion at distances far from where the plasma is generated. A previous simulation study [Yoshikawa and. Ventzek, J. Vac. Sci. Technol. A 31, 031306 (2013)] predicted that the anisotropy in transport parameters due to weak static magnetic fields less than 50 G could be leveraged to manipulate the plasma profile in the radial direction. These simulations motivated experimental tests in which weak magnetic fields were applied to a radial line slot antenna source. Plasma absorption probe measurements of electron density and etch rate showed that the magnetic fields remote from the wafer were able to manipulate both parameters. A summary of these results is presented in this paper. Argon plasma simulation trends are compared with experimental plasma and etch rate measurements. A test of the impact of magnetic fields on charge up damage showed no perceptible negative effect.

Experimental Investigation of Surface Wave Plasma Excited by a Cylindrical Dielectric Rod

An improved surface wave plasma source equipped with a cylindrical quartz rod has been developed, which has great potential in processing inner wall of cylindrical workpieces. A cylindrical quartz rod not only excites the plasma around the rod, but also guides surface wave plasma along the rod. The distributions of plasma density and plasma temperature under different incident microwave powers and pressures are diagnosed by a Langmuir probe. The electron density near the rod is around the order of 1011cm−3. When the incident power is 450 W, the length of surface wave plasma column can reach up to 420 mm at 20 Pa.

Particle-in-cell simulation on surface-wave discharge process influenced by gas pressure and negative-biased voltage along ion sheath layer

Due to surface electromagnetic waves propagating along the dielectric-plasma interface, the application of surface-wave plasma (SWP) is limited in view of the fact that it is very difficult to realize metal sputtering by using negative-biased voltage in traditional SWP sources. Recently, this problem is overcome by a type of SWP source based on the guided wave in ion sheath layer driven by negative-biased voltage. And the plasma heating mechanism is originated from gas discharges excited by the local-enhanced electric field of surface plasmon polariton (SPP). However, the best discharge condition is not obtained because the influence factors affecting the discharge process studied is not clear. In this paper, the discharge mechanism of SWP ionization process influenced by gas pressure and negative-biased voltage along the ion sheath layer is investigated. The simulation method is by means of combining particle-in-cell (PIC) with Monte Carlo collision (MCC). Simulated results suggest that the values of negative-biased voltage and gas pressure can influence the thickness of ion sheath layer, the excitation of SPP, and the spatio-temporal conversion of wave mode, which further induces the different discharge performances. Moreover, the discussed analysis states that a better discharge performance can be obtained when approximately a negative-biased voltage of -200 V and a gas pressure of 40 Pa applied.

Particle-in-cell/Monte Carlo collision simulation of the ionization process of surface-wave plasma discharges resonantly excited by surface plasmon polaritons

Although surface-wave plasma (SWP) sources have many industrial applications, the ionization process for SWP discharges is not yet well understood. The resonant excitation of surface plasmon polaritons (SPPs) has recently been proposed to produce SWP efficiently, and this work presents a numerical study of the mechanism to produce SWP sources. Specifically, SWP resonantly excited by SPPs at low pressure (0.25 Torr) are modeled using a two-dimensional in the working space and three-dimensional in the velocity space particle-in-cell with the Monte Carlo collision method. Simulation results are sampled at different time steps, in which the detailed information about the distribution of electrons and electromagnetic fields is obtained. Results show that the mode conversion between surface waves of SPPs and electron plasma waves (EPWs) occurs efficiently at the location where the plasma density is higher than 3.57 × 1017 m−3. Due to the effect of the locally enhanced electric field of SPPs, the mode conversion between the surface waves of SPPs and EPWs is very strong, which plays a significant role in efficiently heating SWP to the overdense state.

Development of a large-area planar surface-wave plasma source with a cavity launcher driven by a 915 MHz UHF wave

A large-area planar surface-wave plasma (SWP) source driven by a 915 MHz ultrahigh frequency (UHF) wave was developed. To avoid using large, thick dielectric plates as vacuum windows, we propose a cavity launcher consisting of a cylindrical cavity with several small quartz discs at the bottom. Three types of launchers with quartz discs located at different positions were tested to compare their plasma production efficiencies and spatial distributions of electron density. With the optimum launcher, large-area plasma discharges with a radial uniformity within ±10% were obtained in a radius of about 25–30 cm in Ar gas at 8 Pa for incident power in the range 0.5–2.5 kW. The maximum electron density and temperature were approximately (0.95–1.1) × 1011 cm−3 and 1.9–2.0 eV, respectively, as measured by a Langmuir probe located 24 cm below the bottom of the cavity launcher. Using an Ar/NH3 SWP with the optimum launcher, we demonstrated large-area amino-group surface modification of polyurethane sheets. Experimental results indicated that a uniform amino-group modification was achieved over a radius of approximately 40 cm, which is slightly larger than the radial uniformity of the electron density distribution.

PIC/MCC simulation of the ionization process of SWP influenced by gas pressure and SPP

A surface-wave plasma (SWP) source based on surface plasmon polariton (SPP) has fine performances such as high density, low temperature, high production, and so on. It is applied in electronic device micro or nano processing, material modification, etc. Because the ionization of SWP heated by SPP is difficult to describe by theoretical analysis and experimental measurement, the ionization process of producing uniform stable SWP source is not yet well understood. The method in this paper is a numerical simulation of SWP discharges. The electromagnetic energy coupling mechanism of ionization process, influenced by gas pressure, which is studied by combining particle-in-cell (PIC) simulation of reciprocity between plasma and electromagnetic wave with Monte Carlo Collide (MCC) method in merit of dealing with particle collision. Simulated results suggest that the efficient production of SWP is induced by locally enhanced electric field of SPP, and the gas pressure influences the ionization process of SWP by altering the appearance time of wave-mode resonant conversion. Results of this paper show the ionization process of SWP discharge maintained by SPP, and further provide some advices for designing the parameter optimization of next generation meter-scale SWP source.

Mode changes of surface wave on the interface between dielectric rod and microwave plasma with increasing plasma density

A surface-wave plasma source with a cylindrical dielectric waveguide has been developed in our laboratory. A Teflon rod was used as waveguide where the microwave could excite the plasma around it. The transmitting modes of the surface wave on the interface between a dielectric rod and plasma changed with increasing microwave power. It was observed that the cutoff frequency of the surface wave decreased with the increase of the plasma density. And the transmitting modes of the surface wave changed from HE11 mode under no plasma or low plasma density to the degenerate mode of TM01 and TE01 with plasma density 8.9 × 1011 cm−3.

Fundamentals of Slot Antenna Designing for Application in Surface Wave Plasma Sources

Different aspects of slot antenna designing for application in large-scale flat surface wave plasma (SWP) sources are discussed. Finite difference time domain simulation method is applied to investigate radiation characteristics of different slot antenna structures. The effects of slot width and location, and waveguide wall thickness on antenna radiation are studied. Using the transmission line theory, optimum thicknesses for waveguide walls are offered. The vertical, longitudinal, and compound structures of slot antenna arrays are analyzed, and their efficiencies are compared for producing SWPs.

The current status of surface wave plasma source development

In this paper, a flat-type surface wave plasma (SWP) source generated by microwave discharg is introduced systematically. The principle of the surface wave plasma is analyzed and the energy absorption mechanism of the surface wave plasma discharge is explored. A novel wave-mode converter composed of the single-mode resonator array, sub-wavelength diffraction grating and a new type of slot antenna array is introduced. The research findings, such as the mechanism of the generation, the realization, the characteristics of plasma parameters and the numerical simulation of the new SWP sources are beneficial to industrial applications, will promote the effectiveness of the microelectronics industry and obtain a new breakthrough.

Particle-in-cell investigation on the resonant absorption of a plasma surface wave

The resonant absorption of a plasma surface wave is supposed to be an important and efficient mechanism of power deposition for a surface wave plasma source. In this paper, by using the particle-in-cell method and Monte Carlo simulation, the resonance absorption mechanism is investigated. Simulation results demonstrate the existence of surface wave resonance and show the high efficiency of heating electrons. The positions of resonant points, the resonance width and the spatio-temporal evolution of the resonant electric field are presented, which accord well with the theoretical results. The paper also discusses the effect of pressure on the resonance electric field and the plasma density.

Numerical simulation of microwave power absorption of large-scale surface-wave plasma source

A full-size three-dimensional model of large-scale rectangular surface-wave plasma (SWP) source was built, the power deposition problems of SWP source based on collision mechanism were investigated through numerical simulations. The microwave reflectivity curves varying with plasma parameters were obtained, and the influence of different antenna arrays on power deposition is analyzed. The results show that the power deposition of uniformly discharged SWP source mainly depends on plasma property, and too big or too small plasma density is unfavorable to the energy absorption. In the range of working gas pressure, SWP source can achieve effective power deposition only through collision mechanism, and the absorption rate of microwave can reach more than 80%, which agrees with the existing experimental result. The results also show that compact and intensive surface wave is more favorable to the absorption of microwave.

Three-Dimensional Modeling of Argon Discharge Characteristics of a Large-Scale Rectangular Surface-Wave Plasma Source

A three-dimensional fluid model for surface-wave plasma (SWP), to investigate the discharge characteristics of a rectangular SWP source working in a steady state, was presented. The simulation is performed for different gas pressures in argon and different deposited powers. The results showed that there is a peak of plasma density at a distance of 2 cm to 3 cm from the plasma-quartz interface whose position depends mainly on the gas pressure but not the deposited power. The spatial distributions of plasma parameters and their dependence on the gas pressure and deposited power are also presented and discussed. Using this model a good agreement between the simulation results and the available experimental data is obtained.

Effect of device parameters on propagation of plasma surface wave excited by slot antenna

The plasma surface wave propagation in a rectangular structured device is simulated using the finite-difference time-domain method. The effect of device parameters on the propagation of plasma surface wave excited by slot antenna array is investigated. The parameters considered include the relative permittivity and thickness of dielectric slab, and the air gap on the top of dielectric slab. Appropriate parameters for the relative permittivity and thickness of dielectric slab are obtained through simulations, and it is found that the existence of air gap would greatly weaken the excitation of surface wave. The simulation results provide a good reference for the design optimization of large-scale rectangular surface-wave plasma source.