B. Y. Tang

Principles and characteristics of a new generation plasma immersion ion implanter

A new generation multipurpose plasma immersion ion implanter (PIII) was custom designed, constructed, and installed in the City University of Hong Kong. The system is designed for general R&D applications in metallurgy, tribology, surface modification, and fabrication of novel materials. Using the new rf ion source in conjunction with the internal antenna system, the plasma density achieves excellent uniformity both laterally and axially. The system also incorporates two metal sources, including four metal arc sources and a sputtering electrode, so that multiple metal deposition and implantation steps can be performed in succession in the same equipment without exposing the samples to air. This capability can be critical to the study of surface properties and corrosion resistance. This article describes the design objectives, the novel features, and the characteristics of this new generation PIII equipment.

Third-generation plasma immersion ion implanter for biomedical materials and research

A third generation plasma immersion ion implanter dedicated to biomedical materials and research has been designed and constructed. The distinct improvement over first and second generation multipurpose plasma immersion ion implantation equipment is that hybrid and combination techniques utilizing metal and gas plasmas, sputter deposition, and ion beam enhanced deposition can be effectively conducted in the same machine. The machine consists of four sets of high-efficiency metal arc plasma sources with magnetic filters, a custom designed high voltage modulator for operation up to 60 kV, a separate high-frequency, low-voltage power supply for hybrid treatment processes, special rotating sample stage for samples with an irregular shape, and other advanced features. The machine has been installed at Southwest Jiaotong University and operated reliably for 6 months. This article describes the design principles and performances of the machine as well as pertinent biomedical applications.

Radio-frequency plasma nitriding and nitrogen plasma immersion ion implantation of Ti-6A1-4V alloy

Abstract Nitrogen ion implantation improves the wear resistance of Ti-6A1-4V alloys by forming a hard TiN superficial passivation layer. However, the thickness of the layer formed by traditional ion implantation is typically 100–200 nm and may not be adequate for many industrial applications. We propose to use radio-frequency (RF) plasma nitriding and nitrogen plasma immersion ion implantation (PIII) to increase the layer thickness. By using a newly designed inductively coupled RF plasma source and applying a series of negative high voltage pulses to the Ti-6A1-4V samples, RF plasma nitriding and nitrogen PIII can be achieved. Our process yields a substantially thicker modified layer exhibiting more superior wear resistance characteristics, as demonstrated by data from micro-hardness testing, pin-on-disc wear testing, scanning electron microscopy (SEM), as well as Auger electron spectroscopy (AES). The performance of our newly developed inductively coupled RF plasma source which is responsible for the success of the experiments is also described.

Special modulator for high frequency, low-voltage plasma immersion ion implantation

Plasma immersion ion implantation is a burgeoning surface modification technique and not limited by the line-of-sight restriction plaguing conventional beam-line ion implantation. It is therefore an excellent technique to treat interior surfaces as well as components of a complex shape. To enhance the implant uniformity and increase the thickness of the modified layer, we are using a high frequency, low-voltage process to achieve high temperature and dose rate to increase the thickness of the modified layer. The low voltage conditions also lead to a thinner sheath more favorable to conformal implantation. In this article, we will describe our special modulator consisting of a single ended forward converter with a step-up transformer. The modulator is designed to operate from 5 to 35 kHz and the output voltage is adjustable to an upper ceiling of 5000 V that is deliberately chosen to be our voltage limit for the present experiments. We will also present experimental data on SS304 stainless steel materials ...

Improving the plasma immersion ion implantation impact energy inside a cylindrical bore by using an auxiliary electrode

We propose a method to improve the impact energy of ions implanted into the interior sidewalls of cylindrical specimens during plasma immersion ion implantation. Our idea is based on a zero potential conductive auxiliary electrode positioned at the axis of the implanted cylindrical bore. We calculate the structure of the ion‐matrix sheath in an infinitely long cylindrical bore with an auxiliary electrode and analyze the dependence of the radius of the auxiliary electrode on the electric field in the bore. Our results show that the auxiliary electrode improves significantly the distributions of the potential and the electric field inside the cylindrical bore. In addition, because the auxiliary electrode improves the potential drop from axis to sidewalls of the bore and introduces an electric field component which does not vary when the ions are implanted into the sidewalls, the impact energy can be improved in a cylindrical bore during plasma immersion ion implantation.

Investigation of dose uniformity on the inner races of bearings treated by plasma immersion ion implantation

Plasma immersion ion implantation (PIII) is an effective technique for the surface modification of industrial components possessing an irregular shape. We have recently used PIII to treat a real industrial ball bearing to enhance the surface properties of the race surface on which the balls roll. The implantation dose uniformity along the groove is assessed using theoretical simulation and experiments. The two sets of results agree very well, showing larger doses near the center. However, the highest dose is not observed at the bottom or center of the groove, but rather offset toward the side close to the sample platen when the bearing is placed horizontally. The minimum dose is observed near the edge or corner of the groove and our model indicates that it is due to the more glancing ion incidence as a result of the evolution of the ion sheath near the corner. The dose nonuniformity along the groove surface is about 40% based on our experimental data.

Medium-temperature plasma immersion-ion implantation of austenitic stainless steel

Conventional elevated-temperature plasma immersion-ion implantation (PIII) is usually conducted at 350°C, or above, to achieve a thick modified layer for practical engineering applications. In this paper, we focus on medium-temperature PIII treatment of SS304 stainless steel. Two experimental protocols: high frequency, low voltage (LV); and high voltage (HV), low frequency are evaluated. The samples are characterized by Auger electron spectroscopy, glancing angle X-ray diffraction (XRD), corrosion test, pin-on-disk friction and wear test, and so on, to determine the composition, phase structure, as well as the tribological properties of the modified layer. Our results indicate that PIII at 300°C not only improves the mechanical properties, but also the corrosion resistance. Comparison of the wear tracks shows that 300°C-PIII results in an 11-fold improvement in the surface-wear resistance. A procedure involving high implantation flux at LV is more favorable to the formation of a thick modified layer with a higher nitrogen concentration.

PLASMA IMMERSION ION IMPLANTATION OF THE INTERIOR SURFACE OF A LARGE CYLINDRICAL BORE USING AN AUXILIARY ELECTRODE

A model utilizing cold, unmagnetized, and collisionless fluid ions as well as Boltzmann electrons is used to comprehensively investigate the sheath expansion into a translationally invariant large bore in the presence of an auxiliary electrode during plasma immersion ion implantation (PIII) of a cylindrical bore sample. The governing equation of ion continuity, ion motion, and Poisson’s equation are solved by using a numerical finite difference method for different cylindrical bore radii, auxiliary electrode radii, and voltage rise times. The ion density and ion impact energy at the cylindrical inner surface, as well as the ion energy distribution, maximum ion impact energy, and average ion impact energy for the various cases are obtained. Our results show a dramatic improvement in the impact energy when an auxiliary electrode is used and the recommended normalized auxiliary electrode radius is in the range of 0.1–0.3.

Effects of the auxiliary electrode radius during plasma immersion ion implantation of a small cylindrical bore

The temporal evolution of the plasma sheath in a small cylindrical bore in the presence of an auxiliary electrode is determined for different electrode radii. The ion density, velocity, flux, dose, ion energy distribution, and average impact energy are calculated by solving Poisson’s Equation and the equations of ion motion and continuity using finite difference methods. The particle-in-cell method is also used to confirm the validity of the data. Our results indicate that more ions will attain high impact energy when the auxiliary electrode radius is increased, but the dose will decrease. Our results suggest that the normalized auxiliary electrode radius should range from 0.10 to 0.30 in order to maximize the dose and produce a larger number of ions with higher impact energy.

Accurate determination of pulsed current waveform in plasma immersion ion implantation processes

This article reports on the measurement of the ion current in plasma immersion ion implantation. Our simulation results indicate that the total current peaks at the end of rise time of the applied voltage. However, our experimental data acquired using a Rogowski coil and digital oscillator show the highest current at the beginning of the voltage pulse. The discrepancy can be explained by a displacement current attributable to the changing voltage, sheath capacitance, circuit loading effects, as well as secondary electron emission.