Applied Science and Convergence Technology 2021; 30(3): 92-94
Published online May 31, 2021
Copyright © The Korean Vacuum Society.
Ju-Hong Cha, Sang-Woo Kim, and Ho-Jun Lee∗
Department of Electrical Engineering, Pusan National University, Busan 46241, Republic of Korea
Correspondence to:E-mail: firstname.lastname@example.org
In high current implantation, improving the ion extractability is a decisive factor contributing to the equipment efficiency and process cost reduction. A direct current filament ion source and inductively coupled plasma ion source were prepared to observe the degree of plasma extraction according to the ion source characteristics. The plasma parameters used to optimize beam extraction for the filament and RF discharges were 600 W, Ar 2 × 10−5 Torr, and Ø200 × 250 mm discharge space. In the thermionic ion source, the ion density, electron temperature, and Bohm current density were 5 × 109 1/cm3, ≥ 6 eV, and ≈ 3.5 A/m2, respectively. In the RF ion source, the ion density, electron temperature, and Bohm current density were approximately 5 × 1010 1/cm3, ≈ 3 ∼ 4 eV, and ≈ 25 A/m2, respectively. Optimization and analysis of ion extraction according to the plasma and accelerator parameters were calculated using IBSimu. The beam emittance, which is the most important parameter of the ion beam quality, was measured as the beam envelope onto the phase space by an ellipse. A change in the plasma meniscus shape was observed according to the plasma parameters in the ion source, and the emittance of the extracted ion beam was calculated according to the electrode gap distance and aperture structure.
Keywords: High current ion implant, Inductively coupled plasma, Hot cathode filament discharge, Ion extraction
In high current low energy ion-implantation equipment used for S/D contact or poly-Si gate doping, the long-term operational stability of the ion beam source and the ion beam current extracted from the ion source are the main factors affecting the process productivity. In high current ion implant equipment used in ion implantation for semiconductor processing, thermionic discharged ion sources are commonly used for plasma generation. Thermionic discharged ion sources are based on electron emission from a hot cathode to produce the desired plasma density through ionization of the background gas. The process is suitable for use in low-pressure (1 × 10−5 Torr) processes owing to the electron-emitting discharge mechanism. The use of a filament is essential for cathode heating, but corrosion of the filament due to heating leads to a decrease in their lifetime and performance. As the development of a high performance and long life ion source is required, studies on RF inductively coupled plasma (ICP) ion sources, which can be driven at low pressures, are being conducted actively . The ICP plasma source is driven by the induced electric field generated by the time-varying magnetic field, and relatively high ionization efficiency is achieved because of the low energy losses in the sheath and wall at the power coupling mechanism. In order to improve the productivity compared to the previously used ion source, ion sources with plasma properties higher than electron density 5 × 1010 1/cm3 and bohm current density 25 A/m2 are required. For the increase of ion beam efficiency, structural optimization in the form of emittance smaller than ‘10’ is also required. The ion source lifetime using the RF ion sources (≈ 1000 hr) can be improved two fold compared to that of filament-type ion sources (≈ 500 hr). To increase the ion beam current extracted from the ion source, two methods that increase the electrical power applied during discharge in the ion source and optimize the beam extraction in the accelerator are needed . On the other hand, the increase in applied power is limited because of the trade-off relationship with contamination inside the chamber. Therefore, it is essential to optimize ion beam extraction to prevent chamber contamination problems and increase the extractability of ions . In this study, the plasma parameters used to optimize beam extraction for filament and RF discharges were based on 600 W (applied power), pressure of 2 × 10−5 Torr (Ar gas), and discharge space of Ø200 × 250 mm. Extraction was based on the three-electrode system consisting of a plasma electrode, extraction electrode, and ground electrode, which are commonly used in high current ion implantation equipment . The optimization and analysis of ion extraction according to the plasma and accelerator parameters were calculated using IBSimu (Ion Beam Simulation). IBSimu is an optical computer simulation used for the optimization and design of ion-source extraction systems. This code can also be used to analyze the negative and positive ion extraction, but the most significant advantage is the relatively accurate calculation of the ion extraction boundary of the plasma source and space charge dominated ion beam transport. Analytical scaling and numerical calculations of ion extraction from plasma are difficult because of the nonlinearity by the plasma into Poisson’s equation. As an alternative, a simple method for approximating based on Kovaleski’s hypothesis that the sheath edge from an unmagnetized plasma coincides with a critical electric field magnitude . The plasma sheath surface over the aperture is called the meniscus, whose shape and surface area are dependent on the plasma density and temperature. The beam optics depends on the plasma density and electron temperature of the plasma source for the given electrode geometry. The extraction surface area and topology can be determined using
The beam emittance is one of the important parameters among the optical characteristics of the beam. With practical accelerators, the important measurement in the ion beam quality is the volume of the beam envelope. In the case where forces acting on the particles are linear, the phase space of the particle is considered by the normalized momenta (
In Eqs. (1)-(3), ε is the two dimensional transverse emittance, and α, β, and γ are the Twiss parameters, which are used to describe the emittance ellipse.
Figure 2 presents the ion beam trajectory composed of three different electrodes on a two-dimensional cylindrical symmetry geometry. The first electrode terminates the plasma surface, and the plasma electrode plays an important role in the optical characteristics of the beam. The shape of the emission aperture and plasma meniscus is determined here. The second extraction electrode has a negative potential compared to the plasma potential that extracts and accelerates the ion beams. The gap distance between the first and second electrodes, electric field direction formed by the inclined angle, and applied DC voltage of the extraction electrode help determine the ion beam optics. In this simulation, the simulation parameters were set to Ar+ ion at a 13 kV extraction voltage, gap distance of 33 mm, particle trajectory number of 50000, mesh size of 0.05 mm, and iteration number of 15.
Figure 3 presents the result of a ion-beam extraction simulation for the DC filament discharge ion source and RF ion source. In Fig. 3(a), where the ion beam is extracted from the filament discharge source, a concave meniscus appears, and the beam proceeds in the converging direction. In Fig. 3(b), the ion beam extracted from the RF ion source proceeds as parallel, and a slightly concave meniscus shape is achieved . Because electric potential difference of RF aperture wall is much thinner than the filament-type ion source, a higher quality beam, which proceeds in the direction normal to the equipotential plane, can be achieved. In the filament-discharge ion source with a relatively low electron density, the depth of electric potential difference is thick, so the plasma meniscus shape formed on the aperture surface is critical to the ion beam optics.
Figure 4 shows the variation of the emittance versus the gap distance between the plasma electrode and the extraction electrode without a change in the other conditions. The beam emittance, ε, is the product of
In the trajectory composed of three different electrodes on a two dimensional cylindrical symmetry geometry, the simulation of the beam transport of a 13 keV Ar+ beam was conducted using IBSimu. The plasma sheath surface over the aperture called the meniscus is dependent on the plasma density and temperature. The ion beam extracted from the filament-discharge source results in the formation of a concave meniscus, and the beam proceeds in the converging direction. The ion beam extracted from the RF ion source has a slightly concave meniscus shape. A higher quality beam can be achieved because the sheath depth at the RF aperture wall is much thinner than the filament ion source, and the meniscus shape is slightly concave at the source aperture. When using RF ion sources, the change in the ion beam properties according to the structure of the plasma electrode is relatively small, and the beam extractability described by the position and structure of the secondary electrode is more important. On the other hand, the plasma meniscus on the aperture surface has a major effect on the beam property extracted from the DC filament ion source. Therefore, optimization of the aperture geometry is most important for generating a high quality beam discharge.
This research was supported by Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (MOTIE)(P0012451, The Competency Development Program for Industry Specialist).