Showing papers on "Silicon nitride published in 2004"

Silicon Nitride and Related Materials

Abstract: Silicon nitride has been researched intensively, largely in response to the challenge to develop internal combustion engines with hot-zone components made entirely from ceramics. The ceramic engine programs have had only partial success, but this research effort has succeeded in generating a degree of understanding of silicon nitride and of its processing and properties, which in many respects is more advanced than of more widely used technical ceramics. This review examines from the historical standpoint the development of silicon nitride and of its processing into a range of high-grade ceramic materials. The development of understanding of microstructure–property relationships in the silicon nitride materials is also surveyed. Because silicon nitride has close relationships with the SiAlON group of materials, it is impossible to discuss the one without some reference to the other, and a brief mention of the development of the SiAlONs is included for completeness.

Quantum confinement effect of silicon nanocrystals in situ grown in silicon nitride films

Abstract: Silicon nanocrystals were in situ grown in a silicon nitride film by plasma-enhanced chemical vapor deposition. The size and structure of silicon nanocrystals were confirmed by high-resolution transmission electron microscopy. Depending on the size, the photoluminescence of silicon nanocrystals can be tuned from the near infrared (1.38eV) to the ultraviolet (3.02eV). The fitted photoluminescence peak energy as E(eV)=1.16+11.8∕d2 is evidence for the quantum confinement effect in silicon nanocrystals. The results demonstrate that the band gap of silicon nanocrystals embedded in silicon nitride matrix was more effectively controlled for a wide range of luminescent wavelengths.

Systems and methods of forming refractory metal nitride layers using disilazanes

Abstract: A method of forming (and apparatus for forming) refractory metal nitride layers (including silicon nitride layers), such as a tantalum (silicon) nitride barrier layer, on a substrate by using a vapor deposition process with a refractory metal precursor compound, a disilazane, and an optional silicon precursor compound.

Silicon nitride nanosieve membrane

Abstract: An array of very uniform cylindrical nanopores with a pore diameter as small as 25 nm has been fabricated in an ultrathin micromachined silicon nitride membrane using focused ion beam (FIB) etching The pore size of this nanosieve membrane was further reduced to below 10 nm by coating it with another silicon nitride layer This nanosieve membrane possesses adequate mechanical strength up to several bars of transmembrane pressure, and it can withstand high temperatures up to 900 C In addition, it is inert to many aggressive chemicals such as hot concentrated potassium hydroxide (KOH), piranha (H2SO4 + H2O2), and nitric acid (HNO3)

Observation of rare-earth segregation in silicon nitride ceramics at subnanometre dimensions

Abstract: Silicon nitride (Si3N4) ceramics are used in numerous applications because of their superior mechanical properties. Their intrinsically brittle nature is a critical issue, but can be overcome by introducing whisker-like microstructural features. However, the formation of such anisotropic grains is very sensitive to the type of cations used as the sintering additives. Understanding the origin of dopant effects, central to the design of high-performance Si3N4 ceramics, has been sought for many years. Here we show direct images of dopant atoms (La) within the nanometre-scale intergranular amorphous films typically found at grain boundaries, using aberration corrected Z-contrast scanning transmission electron microscopy. It is clearly shown that the La atoms preferentially segregate to the amorphous/crystal interfaces. First-principles calculations confirm the strong preference of La for the crystalline surfaces, which is essential for forming elongated grains and a toughened microstructure. Whereas principles of micrometre-scale structural design are currently used to improve the mechanical properties of ceramics, this work represents a step towards the atomic-level structural engineering required for the next generation of ceramics.

Role of fluorocarbon film formation in the etching of silicon, silicon dioxide, silicon nitride, and amorphous hydrogenated silicon carbide

Abstract: The etching of Si, SiO2, Si3N4, and SiCH in fluorocarbon plasmas is accompanied by the formation of a thin steady-state fluorocarbon film at the substrate surface. The thickness of this film and the substrate etch rate have often been related. In the present work, this film has been characterized for a wide range of processing conditions in a high-density plasma reactor. It was found that the thickness of this fluorocarbon film is not necessarily the main parameter controlling the substrate etch rate. When varying the self-bias voltage, for example, we found a weak correlation between the etch rate of the substrate and the fluorocarbon film thickness. Instead, for a wide range of processing conditions, it was found that ion-induced defluorination of the fluorocarbon film plays a major role in the etching process. We therefore suggest that the fluorocarbon film can be an important source of fluorine and is not necessarily an etch-inhibiting film.

Interface structure and atomic bonding characteristics in silicon nitride ceramics.

Abstract: Direct atomic resolution images have been obtained that illustrate how a range of rare-earth atoms bond to the interface between the intergranular phase and the matrix grains in an advanced silicon nitride ceramic. It has been found that each rare-earth atom bonds to the interface at a different location, depending on atom size, electronic configuration, and the presence of oxygen at the interface. This is the key factor to understanding the origin of the mechanical properties in these ceramics and will enable precise tailoring in the future to critically improve the materials' performance in wide-ranging applications.

Ionic Liquid Lubrication Effects on Ceramics in a Water Environment

Abstract: Ionic liquids were studied to determine their effectiveness as boundary lubricant additives for water. The chemical and tribochemical reactions that govern their behavior were probed to understand lubrication mechanisms. Under water lubricated conditions, silicon nitride ceramics are characterized by a running-in period of high friction, during which time the surface is modified causing a dramatic decrease in friction and wear. Two mechanisms have been proposed to explain the friction and wear behavior. Si3N4 sliding against itself may result in tribochemical reactions that form a hydrated silicon oxide layer on the surface of the sliding contact. This film has been suggested to mediate friction and wear. Others have suggested that tribo-dissolution of SiO2 results in an ultra smooth surface and after a running-in period of high wear, the lubrication mode becomes hydrodynamic. The goal of this study was to examine the effects that ionic liquids have on the friction and wear properties of Si3N4, in particular their effects on the running-in period. Tribological properties were evaluated using pin-on-disk and reciprocating tribometers. The tribological conditions of the tests were selected to produce mixed/hydrodynamic lubrication. The relative lubrication mode between mixed and hydrodynamic was controlled by the initial surface roughness. Solutions containing 2 wt% ionic liquids were produced for testing purposes. Chemical analysis of the sliding surfaces was accomplished with X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The test specimens were 1 in diameter Si3N4 disks sliding against 1/4 in Si3N4 balls. The addition of ionic liquids to water resulted in dramatically reduced running-in periods for silicon nitride from thousands to the hundreds of cycles. Proposed mechanisms include the formation of BFx and PFx films on the surface and creation of an electric double layer of ionic liquid.

Comparison of TiO2 and other dielectric coatings for buried-contact solar cells: A review

Abstract: This paper compares the optical, electronic, physical and chemical properties of dielectric thin films that are commonly used to enhance the performance of bulk silicon photovoltaic devices. The standard buried-contact (BC) solar cell presents a particularly challenging set of criteria, requiring the dielectric film to act as: (i) an anti-reflection (AR) coating; (ii) a film compatible with surface passivation; (iii) a mask for an electroless metal plating step; (iv) a diffusion barrier for achieving a selective emitter; (v) a film with excellent chemical resistance; (vi) a stable layer during high-temperature processing. The dielectric coatings reviewed here include thermally grown silicon dioxide (SiO2), silicon nitride deposited by plasma-enhanced chemical vapour deposition (a-SiNx:H) and low-pressure chemical vapour deposition (Si3N4), silicon oxynitride (SiON), cerium dioxide (CeO2), zinc sulphide (ZnS), and titanium dioxide (TiO2). While TiO2 dielectric coatings exhibit the best optical performance and a simple post-deposition surface passivation sequence has been developed, they require an additional sacrificial diffusion barrier to survive the heavy groove diffusion step. A-SiNx:H affords passivation through its high fixed positive charge density and large hydrogen concentration; however, it is difficult to retain these electronic benefits during lengthy high-temperature processing. Therefore, for the BC solar cell, Si3N4 films would appear to be the best choice of dielectric films common in industrial use. Copyright © 2004 John Wiley & Sons, Ltd.

Nonvolatile semiconductor memory device

Abstract: PROBLEM TO BE SOLVED: To provide a reliable nonvolatile semiconductor storage device free from occurrence of interference between adjacent cells. SOLUTION: As for the ONO film of a slit 205 on an element separation area 202, a silicon nitride film is cut off at its center. Since the ends of the cut off silicon nitride films are covered with a silicon oxide film formed thereon, electrons are trapped in the silicon nitride film. Thus, even when the electrons are spread and drifted in the silicon nitride film, the electrons never reach the adjacent cells. COPYRIGHT: (C)2004,JPO

Fabrication of a carbon nanotube-embedded silicon nitride membrane for studies of nanometer-scale mass transport

Abstract: Membranes consisting of multiwall carbon nanotubes embedded in a silicon nitride matrix were fabricated for fluid mechanics studies on the nanometer scale. Characterization by tracer diffusion and scanning electron microscopy suggests that the membrane is free of large voids. An upper limit to the diffusive flux of D2O of 2.4 × 10-8 mol/m2 s was determined, indicating extremely slow transport through the membranes. By contrast, hydrodynamic calculations of water flow across a nanotube membrane of similar specifications predict a much higher molar flux of 1.91 mol/m2 s, suggesting that the nanotubes used in the membrane have a “bamboo” morphology. The carbon nanotube membranes were then used to make nanoporous silicon nitride membranes, which were fabricated by sacrificial removal of the carbon. Nitrogen flow measurements on these structures give a membrane permeance of 4.7 × 10-4 mol/m2 s Pa at a pore density of 4 × 1010 cm-2. Using a Knudsen diffusion model, the average pore size of this membrane is esti...

A novel strain enhanced CMOS architecture using selectively deposited high tensile and high compressive silicon nitride films

Abstract: A novel CMOS architecture utilizing tensile/compressive silicon nitride capping layers to induce tensile/compressive strain in NMOSFET/PMOSFET channel regions was developed. NMOSFET device delivers 1.05mA//spl mu/m on-current for 70nA//spl mu/m off-current at IV drain voltage. PMOS device exhibits peak 66% increase of linear drain current and 55% increase of saturation current. It was shown that drain current improvements both for N- and PMOSFETs strongly correlate with channel doping levels. Therefore, advanced methods of shallow and low resistance junction formation are required for maintaining low channel doping concentration and efficiently utilizing channel strain at sub-40nm gate length.

Semiconductor device and method of manufacturing the same

Abstract: The present invention provides a semiconductor device capable of preventing deterioration in carrier mobility of a semiconductor layer, which is a quality of the interface between the semiconductor layer and an insulating layer, and a method of manufacturing the semiconductor device. In the semiconductor device, an interface layer is provided between a semiconductor layer made of active polycrystalline silicon and an insulating layer made of silicon oxide. The nitrogen element in silicon nitride diffuses into the semiconductor layer made of active polycrystalline silicon to compensate for lattice strain of the active polycrystalline silicon film, to satisfy the desired quality of the interface between the semiconductor layer and the insulating layer.

Young's modulus of silicon nitride used in scanning force microscope cantilevers

Abstract: The Young’s modulus and Poisson’s ratio of high-quality silicon nitride films with 800 nm thickness, grown on silicon substrates by low-pressure chemical vapor deposition, were determined by measuring the dispersion of laser-induced surface acoustic waves. The Young’s modulus was also measured by mechanical tuning of commercially available silicon nitride cantilevers, manufactured from the same material, using the tapping mode of a scanning force microscope. For this experiment, an expression for the oscillation frequencies of two-media beam systems is derived. Both methods yield a Young’s modulus of 280–290 GPa for amorphous silicon nitride, which is substantially higher than previously reported (E=146 GPa). For Poisson’s ratio, a value of ν=0.20 was obtained. These values are relevant for the determination of the spring constant of the cantilever and the effective tip–sample stiffness.

Fabrication and structural characterization of self-supporting electrolyte membranes for a micro solid-oxide fuel cell

Abstract: Micromachined fuel cells are among a class of microscale devices being explored for portable power generation In this paper, we report processing and geometric design criteria for the fabrication of free-standing electrolyte membranes for microscale solid-oxide fuel cells Submicron, dense, nanocrystalline yttria-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC) films were deposited onto silicon nitride membranes using electron-beam evaporation and sputter deposition Selective silicon nitride removal leads to free-standing, square, electrolyte membranes with side dimensions as large as 1025 μm for YSZ and 525 μm for GDC, with high processing yields for YSZ Residual stresses are tensile (+85 to +235 MPa) and compressive (–865 to -155 MPa) in as-deposited evaporated and sputtered films, respectively Tensile evaporated films faul via brittle fracture during annealing at temperatures below 773 K; thermal limitations are dependent on the film thickness to membrane size aspect ratio Sputtered films with compressive residual stresses show superior mechanical and thermal stability than evaporated films Sputtered 1025-μm membranes survive annealing at 773 K, which leads to the generation of tensile stresses and brittle fracture at elevated temperatures (923 K)

Stress investigation of PECVD dielectric layers for advanced optical MEMS

Abstract: Detailed stress investigations of silicon nitride and silicon dioxide defined by PECVD are presented. The spatial variation of the stress of the dielectric material is evaluated by both, laser induced diffraction imaging method (macroscopically averaged stress) and by a large number of laterally distributed MEMS structures on the wafer (microscopically detected stress). By changing the duty cycle of two different plasma excitation frequencies during the deposition, the stress of silicon nitride (deposited at 300 °C) is controlled in a wide range from +850 MPa (compressive) to −300 MPa (tensile). A similar dependence is also observed for silicon nitride deposited at 60 °C. In contrast, silicon dioxide shows in both cases no strong frequency dependence. The microscopically detecting involves a low cost MEMS technology based on photoresist as sacrificial layer. Applying this technology, differently shaped Fabry–Perot filter membranes with various stress values are implemented. The cavity length of the filters and the radii of curvatures of the upper DBR membranes are also varied. Concave, convex and flat membranes with radii of curvature of −0.31 mm, 0.19 mm and −184.71 mm, respectively, are produced.

Low temperature fabrication of immersion capacitive micromachined ultrasonic transducers on silicon and dielectric substrates

Abstract: A maximum processing temperature of 250/spl deg/C is used to fabricate capacitive micromachined ultrasonic transducers (CMUTs) on silicon and quartz substrates for immersion applications. Fabrication on silicon provides a means for electronics integration via post-complementary metal oxide semiconductor (CMOS) processing without sacrificing device performance. Fabrication on quartz reduces parasitic capacitance and allows the use of optical displacement detection methods for CMUTs. The simple, low-temperature process uses metals both as the sacrificial layer for improved dimensional control, and as the bottom electrode for good electrical conductivity and optical reflectivity. This, combined with local sealing of the vacuum cavity by plasma-enhanced chemical-vapor deposition of silicon nitride, provides excellent control of lateral and vertical dimensions of the CMUTs for optimal device performance. In this paper, the fabrication process is described in detail, including process recipes and material characterization results. The CMUTs fabricated for intravascular ultrasound (IVUS) imaging in the 10-20 MHz range and interdigital CMUTs for microfluidic applications in the 5-20 MHz range are presented as device examples. Intra-array and wafer-to-wafer process uniformity is evaluated via electrical impedance measurements on 64-element ring annular IVUS imaging arrays fabricated on silicon and quartz wafers. The resonance frequency in air and collapse voltage variations are measured to be within 1% and 5%, respectively, for both cases. Acoustic pressure and pulse echo measurements also have been performed on 128 /spl mu/m/spl times/32 /spl mu/m IVUS array elements in water, which reveal a performance suitable for forward-looking IVUS imaging at about 16 MHz.

Comparison of tensile and bulge tests for thin-film silicon nitride

Abstract: The mechanical properties of thin-film, low-pressure chemical vapor deposited silicon nitride were measured in uniaxial tension and by a bulge test method suitable for wafer-level testing. This research compares the two approaches and presents additional data on silicon nitride. The common property from the two test methods is the Young's modulus. Tensile tests performed at the Johns Hopkins University provided a value of 257±5 GPa. Bulge tests conducted by Exponent, Inc., an engineering and scientific consulting firm, yielded a value of 258±1 GPa. It is concluded that this bulge test is a valid wafer-level test method. These tensile results, when added to earlier results, yield the following properties for low-stress silicon nitride: Young's modulus =255±5 GPa, Poisson ratio=0.23±0.02, and fracture strength=5.87±0.62 GPa.

Silicon solar cells and methods of fabrication

Abstract: Devices, solar cell structures, and methods of fabrication thereof, are disclosed. Briefly described, one exemplary embodiment of the device, among others, includes: a co-fired p-type silicon substrate, wherein the bulk lifetime is about 20 to 125 μs; an n+ layer formed on the top-side of the p-silicon substrate; a silicon nitride anti-reflective (AR) layer positioned on the top-side of the n+ layer; a plurality of Ag contacts positioned on portions of the silicon nitride AR layer, wherein the Ag contacts are in electronic communication with the n+-type emitter layer; an uniform Al back-surface field (BSF or p+) layer positioned on the back-side of the p-silicon substrate on the opposite side of the p-type silicon substrate as the n+ layer; and an Al contact layer positioned on the back-side of the Al BSF layer. The device has a fill factor (FF) of about 0.75 to 0.85, an open circuit voltage (VOC) of about 600 to 650 mV, and a short circuit current (JSC) of about 28 to 36 mA/cm2.

Role of nitrogen in the crystallization of silicon nitride-doped chalcogenide glasses

Abstract: Glasses in the Ge-S, Ge-As-Se, and Ge-As-Se-Te systems, doped with Si 3 N 4 , were melted after sealing under reduced pressure, and their crystallization behavior was examined using differential thermal analysis/differential scanning calorimetry and X-ray diffractometry. The effect of Si 3 N 4 doping on the suppression of the crystallization of chalcogenide glasses was confirmed for all three systems and is attributed to the increased crosslinking upon substitution of the chalcogens by nitrogen atoms, presumably forming structural units that are similar to Ge 3 N 4 .

Cvd method for forming silicon nitride film

Abstract: A CVD method for forming a silicon nitride film includes exhausting a process chamber ( 8 ) that accommodates a target substrate (W), and supplying a silane family gas (HCD) and ammonia gas (NH 3 ) into the process chamber, thereby forming a silicon nitride film on the target substrate by CVD. Said forming a silicon nitride film on the target substrate alternately includes a first period of performing supply of the silane family gas (HCD) into the process chamber ( 8 ), and a second period of stopping supply of the silane family gas.

Fabrication and Microstructure of Silicon Nitride/Boron Nitride Nanocomposites

Abstract: A chemical process for fabrication of Si 3 N 4 /BN nanocomposite was devised to improve the mechanical properties. Si 3 N 4 /BN nanocomposites containing 0 to 30 vol% hexagonal BN (h-BN) were successfully fabricated by hot-pressing α-Si 3 N 4 powders, on which turbostratic BN (t-BN) with a disordered layer structure was partly coated. The t-BN coating on α-Si 3 N 4 particles was prepared by reducing and heating α-Si 3 N 4 particles covered with a mixture of boric acid and urea. TEM observations of this nanocomposite revealed that the nanosized hexagonal BN (h-BN) particles were homogeneously dispersed within Si 3 N 4 grains as well as at grain boundaries. As expected from the rules of composites, Young's modulus of both micro-and nanocomposites decreased with an increase in h-BN content, while the fracture strength of the nanocomposites prepared in this work was significantly improved, compared with the conventional microcomposites.

Over-erase phenomenon in SONOS-type flash memory and its minimization using a hafnium oxide charge storage Layer

Abstract: The over-erase phenomenon in the polysilicon-oxide-silicon nitride-oxide-silicon (SONOS) memory structure is minimized by using hafnium oxide or hafnium aluminum oxide to replace silicon nitride as the charge storage layer (the resulting structures are termed SOHOS devices, where the "H" denotes the high dielectric constant material instead of silicon nitride). Unlike SONOS devices, SOHOS structures show a reduced over-erase phenomenon and self-limiting charge storage behavior under both erase and program operations. These are attributed to the differences in band offset and the crystallinity of the charge storage layer.

Machinability of silicon nitride/boron nitride nanocomposites

Abstract: The machinability and deformation mechanism of Si 3 N 4 /BN nanocomposites were investigated in the present work. The fracture strength of Si 3 N 4 /BN microcomposites remarkably decreased with increased hexagonal graphitic boron nitride (h-BN) content, although machinability was somewhat improved. However, the nanocomposites fabricated using the chemical method simultaneously had high fracture strength and good machinability. Hertzian contact tests were performed to clarify the deformation behavior by mechanical shock. As a result of this test, the damage of the monolithic Si 3 N 4 and Si 3 N 4 /BN microcomposites indicated a classical Hertzian cone fracture and many large cracks, whereas the damage observed in the nanocomposites appeared to be quasi-plastic deformation.

Method of cvd for forming silicon nitride film on substrate

Abstract: A method of CVD for forming a silicon nitride film on substrate (W), comprising the step of accommodating substrate (W) in treating vessel (8) and heating the same to a treating temperature and the step of feeding a treating gas containing hexaethylaminodisilane gas and ammonia gas onto the substrate (W) heated to a treating temperature so as to deposit a silicon nitride film on the substrate (W).

Electronic color charts for dielectric films on silicon

Abstract: This paper presents the calculation of the perceived color of dielectric films on silicon. A procedure is shown for computing the perceived color for an arbitrary light source, light incident angle, and film thickness. The calculated color is converted into RGB parameters that can be displayed on a color monitor, resulting in the generation of electronic color charts for dielectric films. This paper shows generated electronic color charts for both silicon dioxide and silicon nitride films on silicon.

Comparative investigation of the biocompatibility of various silicon nitride ceramic qualities in vitro.

Abstract: There is a controversy about the biocompatibility of silicon nitride ceramics contained in the literature, which appears to be related to a factor of the individual chemical composition of different qualities of silicon nitride ceramics and of the different surface properties. This study attempts to investigate the cytotoxicity of different qualities of industrial silicon nitride ceramics applying an L929-cell culture model in a direct contact assay combined with a cell viability assessment. Five different qualities of industrial standard silicon nitride ceramics were chosen for in vitro testing. The chemical composition was determined by EDS analysis. Different biomedically approved aluminium oxide qualities, a titanium alloy, glass and polyvinylchloride (PVC) served as control materials. L929 mice fibroblasts were incubated directly on the materials for 24 h, stained with bisbenzimide and propidium iodine for double fluorochromasia viability testing, and evaluated by inversion-fluorescence microscopy to control cell morphology, viability and cell counts compared to empty well values. Scanning electron microscopy was applied to additionally investigate cell morphology. There was no observation of cytotoxic effects on the silicon nitride ceramic samples; moreover cell morphology remained the same as on aluminium oxide and titanium. Viability testing revealed the presence of avital cells exclusively on PVC, which served as a negative control. Cell counts on all polished surfaces showed significantly higher numbers, whereas some rough surface samples showed significantly lower numbers. We conclude that silicon nitride ceramics show no cytotoxic effects and should be considered for biomedical application owing to its favourable physiochemical properties, especially its superior resistance to mechanical stress, which would be useful for compression loaded conditions. Polished surfaces would appear to promote advanced biocompatibility.

Influence of Yttria–Alumina Content on Sintering Behavior and Microstructure of Silicon Nitride Ceramics

Abstract: The present study investigates the influence of the content of Y2O3–Al2O3 sintering additive on the sintering behavior and microstructure of Si3N4 ceramics. The Y2O3:Al2O3 ratio was fixed at 5:2, and sintering was conducted at temperatures of 1300°–1900°C. Increased sintering-additive content enhanced densification via particle rearrangement; however, phase transformation and grain growth were unaffected by additive content. After phase transformation was almost complete, a substantial decrease in density was identified, which resulted from the impingement of rodlike β-Si3N4 grain growth. Phase transformation and grain growth were concluded to occur through a solution–reprecipitation mechanism that was controlled by the interfacial reaction.

Target poisoning during reactive magnetron sputtering: Part I: the influence of ion implantation

Abstract: Gettering plays a minor role during reactive sputtering of silicon in a nitrogen/argon mixture. However, an abrupt increase of the target voltage as a function of the nitrogen mole fraction is noticed which is not expected from the classical models explaining reactive magnetron sputtering. To explain the target voltage behaviour during DC magnetron sputtering of silicon in an argon/nitrogen mixture, a model is proposed which is based on the reactive ion implantation into the subsurface region of the silicon target. The model calculates the concentration of the nitrogen ions implanted into the target and assumes three possible pathways for these implanted ions. A first pathway is the chemical reaction between the implanted nitrogen ions and the target material to form silicon nitride. The implanted nitrogen can also remain in the target as non-reacted nitrogen atoms. Or, the nitrogen atoms can recombine in the target and diffuse from the target. The compound formation results in a decrease of the target surface recession speed or target erosion rate. As the surface concentration of the implanted ions is inversely proportional to the surface recession speed, an avalanche situation becomes possible. This abrupt transition in recession speed is accompanied with a sudden increase of the concentration of non-reacted nitrogen atoms in the target. In this way, the abrupt target voltage change, noticed at a given mole fraction of nitrogen in the plasma, can be understood.