Plasma Implantation Research Articles

Plasma-Sheath Expansion During Plasma Immersion Ion Implantation of Insulating Materials

Due to the high throughout and capability of treating objects with regular geometries, plasma immersion ion implantation (PIII) has large industrial potential. PIII has mainly been applied to metals and semiconductors because that of insulating materials is not as straightforward. In this work, two-dimensional (2-D) numerical simulation on plasma implantation of insulating samples is carried out. The simulation is conducted on an insulating sample 100 mm in diameter placed on a cylindrical target holder with the dimension of /spl phi/150 mm/spl times/ 50 mm biased to the high negative bias. The voltage reduction induced by the capacitance effect decreases both the ion implantation energy and incident dose. Lateral nonuniformity in the ion dose may result due to variation in the surface potential and reduction of the incident dose is also observed due to the reduced bias voltage. Our results suggest that the nonuniformity and dose reduction are not determined exclusively by the variation in the surface potential. The effects are worsened by the self-response (distortion) of the plasma sheath to the surface potential. For instance, when the surface potential changes from V=1/5 to V=5/5 (normalized to applied potential), the incident dose increases by 183% at t=8 /spl mu/s, while the sheath thickness changes by only 47%. Our preliminary experimental results corroborate the distortion of plasma sheath as predicted by our 2-D model. In order to increase the ion energy, mesh-assisted PIII should be employed.

Implantation dynamics of plasma implantation into insulating strips

Plasma immersion ion implantation (PIII) of insulating materials is quite challenging because of surface charging and capacitance effects. In this paper, we conduct a two-dimensional plasma–sheath simulation of PIII into insulating strips that have practical industrial applications. Our results reveal distortion of the plasma sheath above the parallel strips. Consequently, there exist horizontal components of the ion velocity affecting the implantation results in terms of both the incident ion dose and the penetration depth. The strip dimension is also found to exert a considerable influence on the implantation dynamics. When the strip is wider, the ion dose in the insulating strip is higher but the difference in the vertical velocity is quite small. This illustrates the importance of sample placement in the implantation process.

Characterization of B–C–N hybrid prepared by ion implantation

Ion implantation method is applied to synthesize B–C–N hybrids and their electronic structures are characterized by x-ray photoelectron spectroscopy. A boron nitride film is deposited on a graphite target by borazine plasma implantation. At the interface between the BN film and the graphite, a variety of bonding combinations including B–N, B–C, and C–N are observed. This proved that B–C–N hybrids are formed by this method.

Control of implantation area in direct-current plasma immersion ion implantation (DC-PIII)

In plasma immersion ion implantation (PIII) of planar samples such as silicon wafers in the PIII-ion-cut as well as separation by plasma implantation of oxygen (SPIMOX) processes, the only important ions are the ones arriving at the top surface. Ions implanted into the other surfaces are, in fact, undesirable as they reduce the efficiency of the power supply and plasma source and give rise to metallic contamination. We have demonstrated direct-current PIII (DC-PIII) by using a grounded grid to separate the vacuum chamber for planar sample implantation. The advantages include lower equipment cost, higher power and time efficiency, larger impact energy, and last but not least, smaller instrument footprint. In this paper, we investigate the control of the implantation area by adjusting the radius of the extraction hole, the distance between the conducting grid and the sample, and the radius of the wafer stage. Theoretical simulation is conducted using particle-in-cell and experiments are also carried out. Our results indicate that the implanted area increases with the radius of the extraction hole and wafer stage, but decreases with a larger distance between the grid and sample. The effects of the extraction hole radius G/sub r/ are the largest, followed by the placement of the sample to the conducting grid H. The wafer stage poses the least influence in this respect, but a proper wafer stage dimension improves the lateral implant dose and incident angle homogeneity. Our simulation and experimental results suggest optimal ratios of these parameters for each wafer size.

Blood compatibility of titanium oxides with various crystal structure and element doping.

Titanium oxides are known to be good hemocompatible, therefore they are suggested as coatings for blood contacting implants. But little is known about the influence of physical characteristics like crystal structure, roughness and electronic state on the activation of blood platelets and the blood clotting cascade. Titanium oxide films were produced by metal plasma deposition and implantation in the form of rutile, crystalline and nanocrystalline anatase + brookite and amorphous TiO2. The redox potential was reduced by implantation of chromium ions, the Fermi level of the semiconductive oxide was shifted by ion implantation of the electron donor phosphorous. Hemocompatibility was determined by measuring the adhesion of blood platelets, their P-selectine expression, and of the blood clotting time on these samples. The crystalline titanium oxides had a slightly higher activation of the clotting cascade but lower platelet adhesion than nanocrystalline and amorphous titanium oxides. The surface roughness below 50 nm had no obvious effect. Both, implantation of phosphorous or chromium ions, strongly reduced the activation of the clotting cascade, but only the phosphorous implanted surface also showed a reduced platelet activation, whereas platelet adhesion and activation was strongly increased on the chromium implanted surfaces. Phosphorous doping of rutile TiO2 can increase its hemocompatibility, both concerning blood platelets and blood clotting cascade, but the biochemical mechanism has to be worked out.

Current control for magnetized plasma in direct-current plasma-immersion ion implantation

A method to control the ion current in direct-current plasma-immersion ion implantation (PIII) is reported for low-pressure magnetized inductively coupled plasma. The ion current can be conveniently adjusted by applying bias voltage to the conducting grid that separates plasma formation and implantation (ion acceleration) zones without the need to alter the rf input power, gas flux, or other operating conditions. The ion current that diminishes with an increase in grid bias in magnetized plasmas can be varied from 48 to 1 mA by increasing the grid voltage from 0 to 70 V at −50 kV sample bias and 0.5 mTorr hydrogen pressure. High implantation voltage and monoenergetic immersion implantation can now be achieved by controlling the ion current without varying the macroscopic plasma parameters. The experimental results and interpretation of the effects are presented in this letter. This technique is very attractive for PIII of planar samples that require on-the-fly adjustment of the implantation current at high implantation voltage but low substrate temperature. In some applications such as hydrogen PIII-ion cut, it may obviate the need for complicated sample cooling devices that must work at high voltage.

Enhancement of process efficacy using seed plasma in pulsed high-voltage glow-discharge plasma implantation

Pulsed high-voltage glow-discharge plasma implantation is a practical and effective surface modification technique, although the factors influencing the mechanism are still not well understood. In this Letter, we describe the use of seed plasma to alter the discharge characteristics and improve the efficiency of the process. The seed plasma is produced by an ionization gauge that is a common vacuum measurement device in plasma ion implanters. The ignition behavior of the glow discharge in the presence of seed plasma was experimentally investigated. The seed plasma induces early igniting and the impact diminishes with increasing applied voltage or working pressure. Early triggering is favorable with respect to the mitigation of arcing and the enhancement of the processing and electrical efficacy, and the phenomenon may convey useful information pertaining to the mechanism of the complicated pulsed high-voltage glow-discharge process.

High temperature plasma based ionic implantation of titanium alloys and silicon

Plasma based ionic implantation (PBII) of refractory materials is an alternative technique to conventional beam line implantation which appears to be very promising in the field of aeronautics, biomaterials and semiconductor electronics. In order to monitor sample temperature independently of the plasma discharge and of the pulsed high voltage conditions, we have developed a new thermally assisted PBII set-up. The thermally assisted plasma immersion implantation reactor (TAPIIR) which enables plasma implantation in the 0.5–60 keV range at controlled temperature between 200 and 1000 °C. Thermochemical treatments like nitriding of titanium and silicon were studied with a separated control of implantation and diffusion parameters. This paper describes implantations made in TAPIIR at elevated temperatures (500–900 °C) on titanium. The new results are presented and discussed by considering transport mechanisms during implantation at high temperature.

Conformal ion implantation

Using a 40–50 kV system we have made measurements of how small the sheath can be in plasma implantation with the objective of finding the limits of the process. We have made measurements of sheath size in plasma implantation using the probe technique developed at the University of Wisconsin [M.M. Shamim et al., J. Appl. Phys. 70 (1991)]. We also developed a tritium tracer technique for rapidly measuring the relative implanted dose. The tritium tracer technique is particularly attractive because the amount of tritium required is extremely small (below the activity level which requires regulatory approval) and it can be obtained from a source such as a display tube. In the measurements, we find that implantation with sheaths of distance 1.5–3 cm at voltages of 40–50 kV is similar to operation in other parameter regimes. The only limits occur in the range (100 kV/cm) where vacuum breakdown of the substrate is anticipated in any case.

Plasma-surface modification of biomaterials

Plasma-surface modification (PSM) is an effective and economical surface treatment technique for many materials and of growing interests in biomedical engineering. This article reviews the various common plasma techniques and experimental methods as applied to biomedical materials research, such as plasma sputtering and etching, plasma implantation, plasma deposition, plasma polymerization, laser plasma deposition, plasma spraying, and so on. The unique advantage of plasma modification is that the surface properties and biocompatibility can be enhanced selectively while the bulk attributes of the materials remain unchanged. Existing materials can, thus, be used and needs for new classes of materials may be obviated thereby shortening the time to develop novel and better biomedical devices. Recent work has spurred a number of very interesting applications in the biomedical field. This review article concentrates upon the current status of these techniques, new applications, and achievements pertaining to biomedical materials research. Examples described include hard tissue replacements, blood contacting prostheses, ophthalmic devices, and other products.

Two-dimensional sample temperature modeling in separation by plasma implantation of oxygen (SPIMOX) process

Plasma immersion ion implantation (PIII) offers high throughput and efficiency in the synthesis of silicon-on-insulator (SOI) materials. In the separation by plasma implantation of oxygen (SPIMOX) process, the spatial and time variation of the sample temperature must be known and well controlled to ensure uniform buried oxide and silicon overlying layer thicknesses over the entire silicon wafer. In this paper, we describe a two-dimensional model and derive the temperature distribution on the silicon wafer with respect to time and other process parameters. Our results show laterally nonuniform heating by the incoming ions and the local temperature is influenced more by the sample voltage and thermal irradiation coefficient of the target than the pulse duration and plasma density. The model provides a simple and quick means to determine whether external heating will be needed to maintain the sample temperature at 600/spl deg/C during the SPIMOX process.

Trapping and Detrapping of H in Si: Impact on Diffusion Properties and Solar Cell Processing

AbstractInfluence of trapping and detrapping on the diffusion behavior of H in Si is investigated using both experiment and theory. Experimental H (or D) diffusion profiles, produced by plasma and ion implantation processes, are fitted with a theoretical model. This model includes three kinds of traps – stationary, process-induced, and mobile. Excellent correlation between theory and experiment is observed. Best–fit parameters provide an insight into the trapping mechanisms. We also show how some of the problems resulting from trapping can be circumvented by suitable process conditions.

Novel approaches for low cost fabrication of SOI

Two trials for low cost manufacture of silicon-on-insulator (SOI) wafers were implemented. Low dose separation by implantation of oxygen (SIMOX) procedure has been conducted on a beam-line ion implanter with mass analyzer. The energy dependence of the formed SOI structure was studied at varied implant dosages. The integrity of the buried oxide (BOX) layer was examined by transmission electron microscopy (TEM) and the threading dislocation in the top silicon layer was evaluated by Secco technique. The results indicated that not only the implanted oxygen dose but also the oxygen ion energy plays an important role in the formation of SOI structure with good crystallinity of top silicon, sharp Si/SiO2 interfaces and highly integrated BOX layer free of silicon inclusion. For separation by plasma implantation of oxygen (SPIMOX) approach, water plasma, rather than oxygen plasma, was employed to avoid oxygen spread in the implant depth profile. The SPIMOX process using water plasma was carried out on a beam-line ion implanter without mass selector to simulate the plasma implantation procedure. Cross-sectional TEM study revealed that uniform BOX layer was formed under single crystal silicon superficial layer with the present approach. The interfaces between silicon superficial layer, BOX layer and bulk silicon were smooth and sharp. An implant dose window has been identified for fabricating the desirable SOI structure.

Formation of silicon on insulator using separation by implantation of oxygen with water plasma

Separation by plasma implantation of oxygen (SPIMOX) was developed to fabricate silicon-on-insulator (SOI) wafers based on separation by implantation of oxygen technology, however, it suffers from the coexistence of O+ and O2+ ions in the oxygen plasma, which gives rise to nonuniformity of the formed buried oxide layer. In the present work, water plasma was used as a source of oxygen (H2O+, HO+, and O+) for achieving small spread in the oxygen implant depth profile. The feasibility of SPIMOX with water plasma to form SOI wafers was explored at an ion implanter without mass separator. Cross-sectional transmission electron microscopy revealed that a uniform buried oxide layer was formed under a single crystal silicon overlayer with the present process. The interfaces between the silicon overlayer, buried oxide layer, and bulk silicon were smooth and sharp. An implant dose window has been identified for producing a desirable SOI structure.

Results from 2 years of operation of a plasma implantation facility

In 1995, North Star Research Corporation and Empire Hard Chrome developed a plan to use ‘Pre-owned’ equipment from North Star to develop a user facility at Empire which would ease the transition from wet chrome plating to plasma implantation. Over the last 2 years, more than 6000 parts, and 400 batches of parts have been treated. The equipment has operated for more than 2000 h without failure after initial shake-down. Practical issues in facility operation will be discussed, including cooling, scheduling, and marketing.

Modeling of energy distributions for plasma implantation

Plasma immersion ion implantation (PIII) has significant advantages over conventional implantation in high dose and low energy implant applications. One potential drawback is the poly-energetic nature of pulsed PIII implantation. The contribution of low energy ions to the total implant dose has been computed for pulsed PIII. An analytical approach allows the extraction of the scaling of the dose of the low energy with the implant parameters. The developed models allow the engineering of the rise times, fall times, total pulse time, pulsing frequency, and plasma ion density to minimize the implant energy spread, while maximizing the dose rate.

Production of focused electron beams in a plasma implantation system

Rapid heating/quenching using plasma extracted electrons is a unique and very cost effective method to with which we can create new non-equilibrium, nanocrystaline, and amorphous materials. We recently demonstrated that short pulse (<2 μs) electron beam irradiation is a viable and inexpensive means of providing rapid heat pulses for surface material processing. North Star Research laboratory experiments using our plasma ion implantation equipment have indicated a very efficient delivery of an energy flux to a target surface (up to 70% using a plasma electron beam). This technique offers extremely low cost per processing watt when compared to other heat pulse generation techniques such as lasers or ion beams. Preliminary experiments conducted with aluminum and plastic substrates proved that our new method produced exposures of 1–2 J/cm 2.

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 majority of the ions implanted during the plateau region of the voltage pulse as well as a low-energy one encompassing ions implanted during the finite rise time of the voltage pulses. The lowest-energy component is attributed to a small initial expanding sheath obeying the Child-Langmuir law. Our model can also deal with broadening of the energy spectra due to molecular ions such as N2+ or O2+, in which case each implanted atom only carries a fraction (in this case, half) of the total acceleration energy.

Low-temperature plasma-assisted nitriding

Plasma-assisted nitriding is an attractive surface treatment for metallurgical surface modification to improve wear, hardness and fatigue resistance of ferrous and non-ferrous materials. For this purpose, ion nitriding by a d.c. glow discharge is generally efficient for numerous materials. However, for some metals and alloys, the processing temperature, dominated by the discharge parameters, is too high and cannot be controlled independently from the plasma reactivity. This paper reviews the following solutions for low-temperature plasma-assisted nitriding: pulsed d.c. discharge, thermionically assisted d.c. triode arrangements, plasma implantation, electron cyclotron resonance systems and thermionic arc discharges. We focus on metallurgical results obtained by these techniques on austenitic stainless steel and aluminium.

Direct temperature monitoring for semiconductors in plasma immersion ion implantation

In situ temperature monitoring is extremely important in plasma immersion ion implantation (PIII) of semiconductors. For instance, the silicon wafer must be heated to 600 °C or higher in separation by plasma implantation of oxygen, and in the PIII/ion-cut process, the wafer temperature must remain below 300 °C throughout the experiment. In this article, we present a thermocouple-based direct temperature measurement system for planar samples such as silicon wafers. In order to ensure reliable high-voltage operation and overall electrical isolation, the thermocouple assembly and wires are integrated into the sample chuck and feedthrough. Hydrogen plasma immersion ion implantation is performed in silicon to demonstrate the effectiveness and reliability of the device. Our experimental results indicate that instrumental parameters such as implantation voltage, pulse duration, and pulsing frequency affect the sample temperature to a different extent. The measured temperature rise is higher than that predicted by a theoretical model based on the Child–Langmuir law. The discrepancy is attributed to the finite-sample size and the nonplanar, conformal plasma sheath.