Dixon T. K. Kwok

Mechanical and biological characteristics of diamond-like carbon coated poly aryl-ether-ether-ketone

Poly aryl-ether-ether-ketone (PEEK) is an alternative to metal alloys in orthopedic applications. Although the polymer provides many significant advantages such as excellent mechanical properties and non-toxicity, it suffers from insufficient elasticity and biocompatibility. Since the elastic modulus of diamond-like carbon (DLC) is closer to that of cortical bone than PEEK, the DLC/PEEK combination is expected to enhance the stability and surface properties of PEEK in bone replacements. In this work, PEEK is coated with diamond-like carbon (DLC) by plasma immersion ion implantation and deposition (PIII&D) to enhance the surface properties. X-ray photoelectron spectrometry (XPS), Raman spectroscopy, and Fourier transform infrared (FTIR) spectroscopy demonstrate successful deposition of the DLC film on PEEK without an obvious interface due to energetic ion bombardment. Atomic force microscopy (AFM) and contact angle measurements indicate changes in the surface roughness and hydrophilicity, and nanoindentation measurements reveal improved surface hardness on the DLC/PEEK. Cell viability assay, scanning electron microscopy (SEM), and real-time PCR analysis show that osteoblast attachment, proliferation, and differentiation are better on DLC/PEEK than PEEK. DLC/PEEK produced by PIII&D combines the advantages of DLC and PEEK and is more suitable for bone or cartilage replacements.

Osteoblast behavior on polytetrafluoroethylene modified by long pulse, high frequency oxygen plasma immersion ion implantation

Polytetrafluoroethylene (PTFE) is a commonly used medical polymer due to its biological stability and other attractive properties such as high hardness and wear resistance. However, the low surface energy and lack of functional groups to interact with the cellular environment have severely limited its applications in bone or cartilage replacements. Plasma immersion ion implantation (PIII) is a proven effective surface modification technique. However, when conducted on polymeric substrates, conventional PIII experiments typically employ a low pulsing frequency and short pulse duration in order to avoid sample overheating, charging, and plasma sheath extension. In this paper, a long pulse, high frequency O(2) PIII process is described to modify PTFE substrates by implementing a shielded grid in the PIII equipment without these aforementioned adverse effects. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and contact angle measurements are carried out to reveal the surface effects of PTFE after long pulse, high frequency O(2) PIII and the results are compared to those obtained from conventional short pulse, low frequency O(2) PIII, O(2) plasma immersion, and the untreated control samples. Our results show that less oxygen-containing, rougher, and more hydrophobic surfaces are produced on PTFE after long pulse, high frequency O(2) PIII compared to the other 2 treatments. Cell viability assay, ALP activity test, and real-time PCR analysis are also performed to investigate the osteoblast behavior. It is clear that all 3 surface modification techniques promote osteoblast adhesion and proliferation on the PTFE substrates. Improvements on the ALP, OPN, and ON expression of the seeded osteoblasts are also obvious. However, among these treatments, only long pulse, high frequency O(2) PIII can promote the OCN expression of osteoblasts when the incubation time is 12 days. Our data unequivocally disclose that the long pulse, high frequency O(2) PIII technique is better than the other two types of traditional plasma treatment in the development of PTFE for bone or cartilage repair.

Particle-in-cell and Monte Carlo simulation of the hydrogen plasma immersion ion implantation process

Hydrogen plasma immersion ion implantation into a 200-mm-diam silicon wafer placed on top of a cylindrical stage has been numerically simulated by the particle-in-cell (PIC) and transport-and-mixing-from-ion-irradiation (TAMIX) methods. The PIC simulation is conducted based on the plasma comprising three hydrogen species H+, H2+, and H3+ in a ratio determined by secondary ion mass spectrometry. The local sputtering losses and retained doses are calculated by the Monte Carlo code TAMIX. The combined effect of the three species results in a maximum retained dose variation of 11.6% along the radial direction of the wafer, although the implanted dose variation derived by PIC is higher at 21.5%. Our results suggest that the retained dose variations due to off-normal incident ions can partially compensate for variations in incident dose dictated by plasma sheath conditions. The depth profile becomes shallower toward the edge of the wafer. Our results indicate that it is about 34% shallower at the edge, but with...

Tailoring of Mesenchymal Stem Cells Behavior on Plasma-Modified Polytetrafluoroethylene

By altering the surface properties of polytetrafluoroethylene (PTFE) substrates using a special PIII technique, mesenchymal stem cells (MSCs) proliferation and osteogenesis can be promoted in culture without osteogenic supplements. The structures are created intrinsically in the PTFE for no risk of materials delamination. Large-scale features and locally different functions can also be readily produced on the same substrate by this technique.

The importance of bias pulse rise time for determining shallow implanted dose in plasma immersion ion implantation

The composition of the low-energy ions arising from the rise and fall time periods of the voltage pulse in plasma immersion ion implantation ~PIII! are simulated by particle-in-cell ~PIC! modeling. It is shown that more than 70% of the low-energy ions with an energy corresponding to less than half of the applied voltage come from the short rise time period. Although the fall time period is typically 30 times longer than the rise time, less than 25% of the low-energy ions originate from it. Based on the PIC results, the depth profile of the implanted ions is derived using the Monte-Carlo code SRIM2000 @J. F. Ziegler, The Stopping and Range of Ions in Solids~Pergamon, New York, 1985!#. The low-energy ions are found to be implanted to a much shallower depth than ions introduced during the fall time period the concentration profile which decays more sharply into the bulk. These results indicate that the most effective way to reduce or increase the surface concentration is by adjusting the rise time of the PIII voltage pulse. This will require a power supply capable of fast rise times and good matching between power supply and load.

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....

Ion dose uniformity for planar sample plasma immersion ion implantation

In spite of recent progress on plasma immersion ion implantation (PIII) in semiconductor processing, for example, formation of silicon on insulator and shallow junctions, ion dose, and energy uniformity remains a major concern. We have recently discovered that the sample stage (chuck) design can impact ion uniformity significantly. Using a theoretical model, we have investigated three different chuck designs and conclude that insulators on the stage can alter the adjacent electric field and ion trajectories. Even though the conventional stage design incorporating a quartz shroud reduces the load on the power supply and contamination, it yields ion dose and energy nonuniformity unacceptable to the semiconductor industry. Thus, for semiconductor applications, the stage should be made of a conductor, preferably silicon or silicon coated materials and free of quartz.

Energy distribution and depth profile in BF3 plasma doping

Abstract Plasma doping (PD) is intrinsically different from beam-line doping (BD), as there is no mass filtering and the ion impact angle depends on the target geometry and plasma conditions. There are several ways to alter the impact energy of the incident ions and consequently the dopant depth profile when a BF 3 plasma is used, because the plasma consists of ion species with different masses, compositions, and charge states. The rise and fall times of the sample voltage pulse also contribute to the overall energy distribution, since a long rise or fall time will increase the low-energy component. In this work, a particle-in-cell model is employed to simulate BF 3 PD into silicon under typical plasma doping conditions. The energy distributions of the implanted B and F, as well as the effects of the rise/fall time and other factors are discussed. The PD profiles are compared to BD depth profiles acquired by the TRIM simulation code. Our results reveal that the plasma conditions and pulse shape can be altered to obtain the desired B depth profile. In addition, PD gives rise to a low-energy surface component that is larger for a longer rise and fall time.

Modeling of incident particle energy distribution in plasma immersion ion implantation

Plasma immersion ion implantation is an effective surface modification technique. Unlike conventional beam-line ion implantation, it features ion acceleration/implantation through a plasma sheath in a pulsed mode and non-line-of-sight operation. Consequently, the shape of the sample voltage pulse, especially the finite rise time due to capacitance effects of the hardware, has a large influence on the energy spectra of the incident ions. In this article, we present a simple and effective analytical model to predict and calculate the energy distribution of the incident ions. The validity of the model is corroborated experimentally. Our results indicate that the ion energy distribution is determined by the ratio of the total pulse duration to the sample voltage rise time but independent of the plasma composition, ion species, and implantation voltage, subsequently leading to the simple analytical expressions. The ion energy spectrum has basically two superimposed components, a high-energy one for the majorit...

A new algorithm for charge deposition for multiple-grid method for PIC simulations in r-z cylindrical coordinates

A mesh of nodes used in particle-in-cell simulations may be refined in some regions to obtain better local spatial resolution without adding excessive computational cost. Each refinement can be seen as a finer grid in a multiple-grid system. The standard bilinear weighting method in two-dimensional (r-z) cylindrical coordinates using a multiple-grid system is detailed and an inadequacy in the method is presented. A new algorithm of particle to node weighting in a multiple-grid system is then described and compared to the standard bilinear weighting method in two-dimensional (r-z) cylindrical coordinates. This description pays particular attention to the nodes near the interface between grids of differing cell-size. The volume weighting method used in this study is geometrically similar to the standard bilinear PIC method in Cartesian coordinates, but weights in proportion to the volumes of revolution of the areas about the z-axis, rather than the areas themselves. As the volume weighting method has the axisymmetric geometry built in, it is a natural system to use in this particular case. A full particle-in-cell (PIC) simulation of metal plasma-immersion ion implantation and deposition (MePIIID) of a flat stage using three grids of cell size: 0.5x0.5, 1x1 and 2x2mm; is performed and results are compared to those obtained from a single-grid simulation with the same parameters. The multiple grid method compares very well to the single-grid simulation.