Xuchu Zeng

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.

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.

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.

Direct current plasma implantation using a grounded conducting grid

A novel plasma implantation technique performed in a low pressure steady state dc mode utilizing a grounded conducting grid on top of the wafer stage is presented. By numerically simulating the ion paths by the particle-in-cell method, it is observed that the ion paths are optimized for certain implant geometry. In the optimal configuration, the directional angle of the acceleration vector does not depend on the mass and charge state of the ions, and the ratio of the partial differential of the scalar potential φ along the radial and longitudinal directions remains constant for varying applied voltages. The retained dose and impact energy uniformity are totally determined by the ratio of the radius of the wafer stage r, radius of the vacuum chamber R, distance between the wafer stage and the grid H, and thickness of the wafer stage D. The optimal ratio is r:R:H:D=1:4:2.5:2, that is, suggesting a disk shape vacuum chamber, which is quite different from that of a conventional plasma immersion ion implanter....

Damage in hydrogen plasma implanted silicon

The damage and defects created in silicon by hydrogen plasma immersion ion implantation (PIII) are not the same as those generated by conventional beamline ion implantation due to the difference in the ion energy distribution and lack of mass selection in PIII. Defect generation must be well controlled because damage in the implanted and surface zones can easily translate into defects in the silicon-on-insulator structures synthesized by the PIII/wafer bonding/ion-cut process. The defect formation and its change with annealing temperature were investigated experimentally employing channeling Rutherford backscattering spectrometry, secondary ion mass spectrometry, and atomic-force microscopy. We also calculated the damage energy density of the three dominant hydrogen species in the plasma (H+, H2+, and H3+) as well as displacement of silicon atoms in the silicon wafer. H2+ creates the most damage because its damage energy density is very close to the silicon threshold energy. The effects of atmospheric gas...

Improvement of the corrosion property of Cr4Mo4V bearing steel using plasma immersion ion implantation

Abstract The working conditions of aerospace bearings such as engine bearings are quite harsh and prolonging the life span of these components is thus very important to the aerospace industry. Previous results have shown that the main failure mechanism of aerospace bearings is corrosion, and enhancing their corrosion resistance is a key. Cr4Mo4V, which is equivalent to AISI M50 bearing steel, is usually used in aerospace bearings in China. In this study, Cr4Mo4V components are treated in a new generation plasma immersion ion implanter in which ion implantation and sputter deposition can be carried out in the same chamber without breaking vacuum. Three treatment processes involving Cr, Mo, and N are evaluated. Our test results indicate that Cr is the main element enhancing the corrosion resistance and the addition of nitrogen improves the properties further. The non line-of-sight advantage of PIII is important to the processing of complex-shaped samples such as engine bearings.

Plasma-immersion ion implantation of the interior surface of a small cylindrical bore using an auxiliary electrode for finite rise-time voltage pulses

Plasma-immersion ion implantation (PIII) can be used to process the interior surfaces of odd-shape specimens such as a cylindrical bore. The temporal evolution of the plasma sheath in a small cylindrical bore in the presence of a grounded coaxial auxiliary electrode is derived for voltage pulses of different rise times by solving Poissons equation and the equations of ion continuity, and motion numerically using the appropriate boundary conditions. It is found that the maximum ion impact energy and the average impact energy are improved for finite rise-time voltage pulses, and shorter rise times yield better results. Our results allow the selection of a suitable auxiliary electrode radius to improve the average impact energy for a given rise time.

Steady-state direct-current plasma immersion ion implantation using an electron cyclotron resonance plasma source

We have developed a new direct-current (DC) plasma immersion ion implantation (PIII) technique by using a conducting grid positioned between the plasma source and sample chuck. In order to decrease the working gas pressure and increase the plasma density, an electron cyclotron resonance (ECR) plasma source was used in our experiments. In this paper, the experimental parameters and results pertaining to DC-PIII using an ECR plasma source are described. The uniformity of the ion dose and the energy monotonicity are discussed. Our experimental results indicate that DC-PIII is a novel and potentially useful technique for planar sample processing, particularly in microelectronics applications.

In situ sample temperature measurement in plasma immersion ion implantation

Plasma immersion ion implantation (PIII) is an excellent surface modification technique because it is not restricted by the line-of-sight limitation that plagues conventional beamline ion implantation. However, the lack of in situ monitoring has hampered wider acceptance of the technique in industry. It is known that the implantation temperature has a large influence on the surface properties of the treated specimens in addition to the more obvious parameters such as implantation voltage, pulse duration, pulsing frequency, and so on. Direct measurement of the target temperature is complicated by the sample high voltage as well as by interference from the electromagnetic field and plasma. In this article, we present a novel interference-free, in situ temperature measurement technique employing a shielded thermocouple directly attached to the sample stage. Our experiments show that the setup can monitor the target temperature in real time, even under severe arcing conditions. Our results also indicate that ...