Plasma-based Ion Implantation Process Research Articles

Nonuniform plasma diffusion and multi-pulse effect in plasma-based ion implantation

The nonuniform plasma caused by plasma diffusion and the incomplete plasma recovery during short pulse-off time have great influences on the sheath dynamics and the implantation efficiency in plasma-based ion implantation (PBII). In this paper, a magnetized plasma diffusion fluid model is established to describe the plasma diffusion in PBII process. Together with a magnetized sheath fluid model, the full pulse period including sheath dynamics during pulse-on time and plasma recovery during pulse-off time can be described, and the models are verified to be accurate by comparing with experimentally measured electron density profiles. The influence of process parameters on sheath dynamics, the influence of incomplete plasma recovery under multi-pulse bias on implantation efficiency are investigated by solving the presented models with considering plasma diffusion. It is found that the variations of process parameters which accelerate the plasma diffusion reduce the steady-state sheath thickness and increase the ion implantation current, and vice versa. Change the pulse frequency from 1kHz to 100kHz under typical PBII process parameters significantly increases the average ion implantation current density, and the limiting factor which affects the implantation efficiency is converted from duty cycle to plasma diffusion. Increasing the plasma density and decreasing the transverse magnetic field are the effective methods to improve the implantation efficiency as well. The results reported here help to provide a theoretical guidance for the parameters optimization in PBII process.

Ballistic-Mode Plasma-Based Ion Implantation for Surface-Resistivity Modification of Polyimide Film

Plasma-based ion implantation (PBII) is the well-established technique for material surface modification. In this paper, we described the ballistic-mode PBII process in which repetitive high-voltage pulses are applied to the grid a few centimeters from a polyimide (PI) film target. The high-voltage pulse applied to the grid has the peak values of 30 kV, width of 2.5 μs, rise time of 1.5 μs, and fall time of 0.5 or 40 μs. In this process, ions propagate ballistically from a grid to the PI film and modify the surface resistivity of the PI film. The efficiency of surface-resistivity modification depends on the fall time of the pulse applied to the grid. The ballistic-mode PBII process affects the surface characteristics of PI to a depth of 90 nm.

Plasma-based ion implantation and deposition: a review of physics, technology, and applications

After pioneering work in the 1980s, plasma-based ion implantation (PBII) and plasma-based ion implantation and deposition (PBIID) can now be considered mature technologies for surface modification and thin film deposition. This review starts by looking at the historical development and recalling the basic ideas of PBII. Advantages and disadvantages are compared to conventional ion beam implantation and physical vapor deposition for PBII and PBIID, respectively, followed by a summary of the physics of sheath dynamics, plasma and pulse specifications, plasma diagnostics, and process modeling. The review moves on to technology considerations for plasma sources and process reactors. PBII surface modification and PBIID coatings are applied in a wide range of situations. They include the by-now traditional tribological applications of reducing wear and corrosion through the formation of hard, tough, smooth, low-friction, and chemically inert phases and coatings, e.g., for engine components. PBII has become viable for the formation of shallow junctions and other applications in microelectronics. More recently, the rapidly growing field of biomaterial synthesis makes use of PBII and PBIID to alter surfaces of or produce coatings on surgical implants and other biomedical devices. With limitations, also nonconducting materials such as plastic sheets can be treated. The major interest in PBII processing originates from its flexibility in ion energy (from a few electron volts up to about 100 keV), and the capability to efficiently treat, or deposit on, large areas, and (within limits) to process nonflat, three-dimensional workpieces, including forming and modifying metastable phases and nanostructures.

Study on the treatment capability of a plasma-based ion implanting trench-shaped target

The treatment capability of a plasma based ion implanting two-dimensional trench-shaped target has been discussed through particle-in-cell simulation. By analysing time-dependent expansion of the plasma sheath and the ion implantation parameters during a high voltage pulse, we found that the sidewall of the trench could not be implanted effectively. Trenches with the same depth and three different widths were simulated in this paper. Simulation results show that the plasma sheath conforms to the target better in the case of a wider trench, but it tends to expand as a plane for the majority of the pulse. The ions accelerated in such a sheath would impinge vertically on the upper surface and the bottom of the trench with the converging at the convex corner and diverging at the concave corner, but neglecting the sidewall. After the short initial stage of the pulse when the sheath is conformal to the target well (about 1/10 of the pulse duration), the ions impact the sidewall of the trench with grazing angle, lowest ion impact current, and lowest ion impact energy. All the above three factors would induce a severe sputtering effect, reduce ion implantation depth and decrease retained dose. Increasing the width of the trench would alleviate this effect, but to a slight extent. The plasma based ion implantation process in trench cases is similar to ion beam implantation with a large beam and would not get the effective implantation for all the surfaces of the trench.

Substrate Bias Effects on the Structure of the Film by a Hybrid PVD and Plasma-Based Ion Implantation Process

This paper descirbes the characteristics of gold films prepared by a hybrid plasma based an ion implantation/deposition (PBIID) system. The surface morphology and structure of the film were affected by the voltage applied to the target. With increasing negative voltage, the surface became thinner with a lesser number of nuclei. The grain structure varied from the continuous film at 0 kV to the channel at -1 kV, and further to the islands (mounds) at -5 kV. The ions in the sheath are believed to play an important role in the deposition of the film.

Plasma-based ion implantation: a valuable technology for the elaboration of innovative materials and nanostructured thin films

Plasma-based ion implantation (PBII), invented in 1987, can now be considered as a mature technology for thin film modification. After a brief recapitulation of the principle and physics of PBII, its advantages and disadvantages, as compared to conventional ion beam implantation, are listed and discussed. The elaboration of thin films and the modification of their functional properties by PBII have already been achieved in many fields, such as microelectronics (plasma doping/PLAD), biomaterials (surgical implants, bio- and blood-compatible materials), plastics (grafting, surface adhesion) and metallurgy (hard coatings, tribology), to name a few. The major advantages of PBII processing lie, on the one hand, in its flexibility in terms of ion implantation energy (from 0 to 100 keV) and operating conditions (plasma density, collisional or non-collisional ion sheath), and, on the other hand, in the easy transferrability of processes from the laboratory to industry. The possibility of modifying the composition and physical nature of the films, or of drastically changing their physical properties over several orders of magnitude makes this technology very attractive for the elaboration of innovative materials, including metastable materials, and the realization of micro- or nanostructures. A review of the state of the art in these domains is presented and illustrated through a few selected examples. The perspectives opened up by PBII processing, as well as its limitations, are discussed.

A study of interface and adhesion of c-BN film on Si(100) modified by nitrogen plasma based ion implantation technique

Cubic boron nitride (c-BN) films were deposited on Si substrate with poor adhesion using magnetically enhanced active reaction evaporation (ME-ARE). An attempt has been made to enhance the adhesion strength between c-BN film and substrate by nitrogen plasma based ion implantation (PBII) into c-BN film. Nitrogen ion doses range from 5×10 16 to 1×10 17 ions cm −2 at an implant voltage of 50 kV. The nitrogen ion implanted c-BN films were analyzed using FTIR, scratch test, and XPS to investigate the change of structure, adhesion strength of c-BN film, and interfacial mixing between the initial turbostratic BN (t-BN) film layer and substrate caused by nitrogen ion implantation. FTIR spectra showed little change of c-BN phase content in the films under the above implantation conditions but XPS depth elemental profile of N + -implanted boron nitride films displayed a mixed layer consisting of elements from film and substrate formed at interface. A highly optimized dynamic Monte Carlo program TAMIX was used to simulate the PBII process in a good agreement with above measured depth elemental profile. The scratch test showed that the adhesion strength evaluated in terms of the critical load of N + -implanted c-BN film was 1.4 times higher than that of as deposited c-BM film.

Metal plasma source for PBII using arc-like discharge with hot cathode

A plasma source using arc-like discharge for plasma-based ion implantation (PBII) has been developed. Molten metals such as titanium are used as cathode in this plasma source. The utilization of the arc-like discharge in PBII systems is reported for the first time. The plasma source with the arc-like discharge provides a new promising method to enable macroparticle-free process without magnetic filter. The characteristics of the arc-like discharge using the hot cathode and application for a plasma source of PBII process are reported. The implanted dose distribution on a substrate with spherical shape is measured using Rutherford back scattering (RBS).

Metal plasma source for PBII using arc-like discharge with hot cathode

A plasma source using arc-like discharge for plasma-based ion implantation (PBII) has been developed. Molten metals such as titanium are used as cathode in this plasma source. The utilization of the arc-like discharge in PBII systems is reported for the first time. The plasma source with the arc-like discharge provides a new promising method to enable macroparticle-free process without magnetic filter. The characteristics of the arc-like discharge using the hot cathode and application for a plasma source of PBII process are reported. The implanted dose distribution on a substrate with spherical shape is measured using Rutherford back scattering (RBS).

A study of interface and adhesion of c-BN film on Si(1 0 0) modified by nitrogen plasma based ion implantation technique

Cubic boron nitride (c-BN) films were deposited on Si substrate with poor adhesion using magnetically enhanced active reaction evaporation (ME-ARE). An attempt has been made to enhance the adhesion strength between c-BN film and substrate by nitrogen plasma based ion implantation (PBII) into c-BN film. Nitrogen ion doses range from 5×10 16 to 1×10 17 ions cm −2 at an implant voltage of 50 kV. The nitrogen ion implanted c-BN films were analyzed using FTIR, scratch test, and XPS to investigate the change of structure, adhesion strength of c-BN film, and interfacial mixing between the initial turbostratic BN (t-BN) film layer and substrate caused by nitrogen ion implantation. FTIR spectra showed little change of c-BN phase content in the films under the above implantation conditions but XPS depth elemental profile of N +-implanted boron nitride films displayed a mixed layer consisting of elements from film and substrate formed at interface. A highly optimized dynamic Monte Carlo program TAMIX was used to simulate the PBII process in a good agreement with above measured depth elemental profile. The scratch test showed that the adhesion strength evaluated in terms of the critical load of N +-implanted c-BN film was 1.4 times higher than that of as deposited c-BM film.

Positive pulse bias method in plasma-based ion implantation

A novel scheme of the plasma-based ion implantation (PBII), the positive pulse bias (PPB) method has been proposed, in which a workpiece to be treated is at the ground potential while the plasma surrounding it is biased to a positive high voltage; thus the ion sheath same as the one in the conventional negative pulse bias (NPB) method is formed on the workpiece. Principal merit of the PPB method over the NPB method is that it possesses a mechanism of suppressing X-rays from the wall by impact of secondary electrons and it enables to manipulate the workpiece during PBII processing. The latter merit will serve to realize an efficient continuous processing system. The principle of the PPB method is discussed based on the asymmetric double probe theory and results of the proof-of-principle experiment are presented.

Plasma based ion implantation technology for high-temperature oxidation-resistant surface coatings

Ir–Re alloy surface coatings were performed by means of plasma based ion implantation (PBII) technology, using a coaxial type arc-vacuum metal-plasma source, in order to prevent surface oxidation of the engineering tools (WC alloys) at high temperature. The energy controllability of the PBII process allows implantation of high-energy ions, to improve adhesion properties, and subsequent deposition at low temperature for producing very pure metal and alloys. The properties of the Ir–Re alloy produced with the PBII technology (PBII-film) were studied in comparison with films prepared by a conventional sputtering method (SP-films), which needs a Cr interlayer for assuring good adhesion. Both types of coatings were heat treated at 570 °C for approximately 170 h, in nitrogen atmosphere, and analyzed by Scanning Electron Microscopy, Auger Electron Microscopy and cross sectional Transmission Electron Microscopy. The results showed that the heat treatment did not alter the surface of the PBII-films. On the contrary, the SP-film surface became very rough after heat treatment. Because the Cr diffused from interlayer to the surface and oxidized. Therefore, PBII-films were also more resistant to scratch tests.

Effect of nitrogen plasma-based ion implantation on joint prosthetic material

In total hip arthroplasty (THA), Ultra High Molecular Weight Polyethylene (UHMWPE) wear debris generated at the articular surface has been recognized as a long-term cause of loosening and failure of artificial hip joints due to osteolysis. The technique of ion implantation has been used to improve wear resistance of the metal femoral head. The Plasma-Based Ion Implantation (PBII) technique is more suitable for complex shaped implants such as femoral head and femoral prosthesis due to three-dimensional ion implantation effects. The effect of pulse voltage and cooling of the substrate of the PBII process for Co–Cr-based materials were examined in terms of wear resistance by the pin-on-disc wear test. The wear resistance of Co–Cr alloy with high nitrogen-ion implantation dose was superior to the untreated Co–Cr alloy. Their corrosion resistance has also been examined with an anodic polarization measurement. High dose and cooling of PBII process proved to be effective in enhancing corrosion resistance of Co–Cr alloy. In this study, it was confirmed that the improvement of wear and corrosion resistance of Co–Cr alloy modified by PBII, as well as high pulse voltage and cooling of the substrate for PBII were the most effective treatments.

PBII processing of dielectric layers: physical aspects limitations and experimental results

Processing of dielectric layers using a plasma-based ion implantation (PBII) technique has general implications in terms of plasma specifications and pulse characteristics. In particular, the different aspects of the processing of dielectric layers are discussed as functions of plasma density, pulse duration, and layer characteristics (thickness and permittivity). Clearly, severe limitations (true implantation energy, arcing) may appear for high-density plasmas as well as for long pulse durations when processing dielectric layers with thicknesses in the millimeter range. Typical examples of ion implantation in dielectric materials are presented, e.g. oxygen ion implantation in polymer sheets (for hydrophilic or adhesion treatments) and nitrogen implantation of polysilicon films on glass. The experimental results demonstrate the feasibility of processing dielectric layers with the PBII technique, but with severe limitations resulting from the process itself.

Nano-indentation testing for plasma-based ion-implanted surface of plastics

We have developed a technique to modify the surface of plastics to improve the hardness. Carbon ions were implanted with 20 and 10 keV energy into the plastic surface using the conventional ion-implantation and the plasma-based ion-implantation methods. The modified surface was loaded to 1 mN and unloaded using the diamond tip of the nano-indentation equipment. We found that the deformation of the modified surface was elastic deformation. According to the surface Young's modulus FEM simulation of plastics, surface Young's modulus was improved from 1.8 to 25 GPa using the plasma-based ion-implantation process.

Effect of pulse waveform on plasma sheath expansion in plasma-based ion implantation

In this paper, the effect of the pulse waveform on plasma sheath evolution around a diamond-shaped target has been simulated using fluid dynamic model in the context of plasma-based ion implantation (PBII). The implanted parameters of ions such as ion-implanted energy, impact angle and impact current have also been studied under different pulse shapes. Understandably, the longer pulse rise time would result in the lower ion impact energy, and less sheath expanding extent. By comparing the sheath structure under different pulse rise time, we found that long pulse rise time would enhance the conformation of the sheath to the target at the earlier stage of the pulse and would reduce the tendency of the ion depletion in the plasma sheath. Accordingly increase the ion impact current at the later stage of the pulse, which is quite important for the PBII process, when the ions have been accelerated adequately.

Ion implantation by plasma immersion: interest, limitations and perspectives

Plasma-based ion implantation (PBII) is a recent method to implant ions into materials for modifying surface properties. Negative high voltage pulses are applied to the substrate to extract ions from the plasma and accelerate them directly onto the substrate surface. The main advantages of PBII over ion beam implantation are its simplicity for processing large surfaces or three-dimensional objects and the possibility of preparing surfaces in situ prior to the implantation process. The two general specifications required for PBII are low pressure, large size plasmas and high voltage–high current pulse generators. Multipolar discharges excited by distributed electron cyclotron resonance, which can operate with reactive gases and can be easily scaled up, are thus well adapted to the PBII process. In order to produce the high voltage–high current pulses with rise and fall times of the order of the inverse ion plasma frequency necessary for PBII processing, generators using pulse transformers, where the voltage at the primary is provided by transistor switches and where the energy is stored at a low voltage level, have been developed. Then, before discussing the various limitations of the process, PBII in plasmas is illustrated through few selected examples.

Distributed electron cyclotron resonance plasma immersion for large area ion implantation (invited)

Plasma-based ion implantation (PBII) is a recent method to implant ions into materials for modifying surface properties. Negative high voltage pulses are applied to the substrate to extract ions from the plasma and accelerate them directly onto the substrate surface. The main advantages of PBII over ion beam implantation are its simplicity for processing large surfaces or three-dimensional objects, and the possibility of preparing surfaces at low ion bombardment energy prior to the implantation process. However, in contrast to conventional ion implantation, the PBII process does not apply mass selection. Due to the wide ion sheath expansion (a few tens of cm), large volumes of plasma are mandatory around the substrate. Multipolar discharges, which produce a peripheral ionization facing the substrate and can be easily scaled up, are well adapted to the PBII process and thus widely used. However, hot filaments to sustain plasmas of reactive gases in multipolar magnetic field structures are progressively phased out in favor of distributed electron cyclotron resonance (DECR) plasma sources. The principle, the design, and the performances of DECR plasmas are presented. Then, PBII in DECR plasmas is illustrated through two selected examples.