UDK 533.9 Review article/Pregledni članek ISSN 1580-2949 MTAEC9, 46(1)7(2012) HETEROGENEOUS SURFACE RECOMBINATION OF NEUTRAL NITROGEN ATOMS HETEROGENA POVRŠINSKA REKOMBINACIJA DUŠIKOVIH NEVTRALNIH ATOMOV Alenka Vesel Center of Excellence for Polymer Materials and Technologies, Tehnološki park 24, 1000 Ljubljana, Slovenia alenka.vesel@ijs.si Prejem rokopisa - received: 2011-06-14; sprejem za objavo - accepted for publication: 2011-07-18 An overview of data reported in literature on heterogeneous surface recombination of nitrogen atoms is presented. The data are often scattered for over an order of magnitude depending on experimental technique and perhaps also on surface finish. As a general rule the recombination coefficient is rather small for materials that do not chemisorb nitrogen atoms, and it is rather large for some metals that are known as catalysts. Values as low as 5x10-8 have been reported for Pyrex glass, although a typical value of the recombination coefficient for glasses is rather 1x10-5. The similar order of magnitude is found for very stable polymers such as polytetrafluoroethylene (teflon), although larger values are typical for some other polymers. The recombination coefficient for metals is typically 10-2 but can be as large as 0.3 for selected catalysts. Keywords: nitrogen plasma, neutral nitrogen atoms, surface recombination, recombination probability Podajamo pregled podatkov iz literature o heterogeni površinski rekombinaciji dušikovih atomov. Obstoječi podatki se pogosto razlikujejo za več kot velikostni red odvisno od eksperimentalne tehnike in verjetno tudi od predobdelave površine. V splošnem je rekombinacijski koeficient precej majhen za materiale, ki ne kemisorbirajo dušikovih atomov in precej velik za nekatere materiale, ki so znani kot katalizatorji. Zelo majhne vrednosti okoli 5x10-8 so bile objavljene za steklo Pyrex, čeprav je bolj značilna vrednost rekombinacijskega koeficienta za steklo okoli 1x10-5. Podoben velikostni red velja tudi za polimere kot je politetrafluoroetilen (teflon), medtem ko so za ostale polimere značilne precej višje vrednosti. Rekombinacijski koeficient za covine je tipično okoli 10-2, vendar je lahko tudi precej višji okoli 0.3 za nekatere katalitične materiale. Ključne besede: dušikova plazma, nevtralni atomi dušika, površinska rekombinacija, verjetnost za rekombinacijo 1 INTRODUCTION Nitrogen plasma has attracted much attention in the past decades not only due to application for surface modifications of various materials1-2 but also due to some specific characteristics of excited particles found in non-equilibrium nitrogen plasma.3 Low pressure non-equilibrium gaseous plasma is always characterized by a rather high average energy of electrons and a rather low kinetic temperature of all other particles.4-6 The simplest example of such plasma is created in a noble gas. Free electrons are accelerated in an appropriate electric field, typically next to the powered electrode and enter plasma volume with a kinetic energy often exceeding 100 eV. In plasma volume they suffer from elastic collisions with slow electrons. These collisions assure for a rapid thermalization of the fast electrons: they effectively loose their kinetic energy, while plasma electrons gain it. According to the basic roles of statistical mechanics such intensive energy exchange always leads to a Maxwellian distribution of particles over their kinetic energy. Plasma is therefore rich with electrons with a Max-wellian energy distribution function. The high energy tail of the function is exponential so the electrons may gain practically any kinetic energy well above their average kinetic energy. Number of electrons with very high kinetic energy (in the high energy tail) are capable of ionizing any atom or molecule. In the case of noble gas such direct ionization may occur since there is a lack of low energy excited states of atoms. For instance, the first excited state of helium atom is at 20.2 eV7 while the ionization energy is 24.6 eV.8 Similar considerations apply for many other atomic plasmas, but fail completely in the case of nitrogen plasma. Namely, nitrogen plasma has some unique characteristics due to many different excited states that may or may not be metastable. The unique properties of these excited states make characterization and understanding of nitrogen plasma rather difficult, let alone interaction of such plasma with solid materials. 2 CHARACTERISTICS OF LOW-PRESSURE NON-EQUILIBRIUM NITROGEN PLASMA Nitrogen molecules are characterized by a variety of different excited states. Most of these excited states are metastable with a typical radiative lifetime exceeding 1 ms. The dissociation energy of a nitrogen molecule is much higher than for many other molecules including oxygen, hydrogen and alike. The ionization energy is also rather high at about 14.5 eV.9 Some excited states of nitrogen molecules are presented in Refs. 2 and 3. The richness of excited states causes many channels for electron energy loss at inelastic collisions with nitrogen molecules. Unlike for the case of noble gases where the channels are limited, a collision with a fast electron from a high energy tail is likely to cause excitation of such a molecule rather than dissociation or even ionization. Electrons with a lower kinetic energy are likely to excite vibrationaly excited states. Nitrogen molecules are famous for many vibrational excited states. Unlike in many other gases where superelastic collisions between vibrationaly excited molecules and other particles cause substational cooling of the vibrational states, the coupling between vibrational and translational states for nitrogen molecules is extremely poor. A consequence of that is that nitrogen molecules are found in highly vibrationaly excited states even in a plasma created by a weak electrical discharge.910 The high excitation energy of electronically excited states as well as rich population of vibrationaly excited states of nitrogen molecules in gaseous plasma allow for interesting inelastic collisions that may lead to either dissociation or even ionization of nitrogen molecules. Such multistep processes, that involve firstly excitations by electron impact and then redistribution of the potential energy finally causing dissociation or ionization, make any predictions about the behaviour of nitrogen plasma very difficult. Until recently, not many methods have appeared for determination of the basic plasma parameters such as dissociation fraction in nonequilibrium plasma. Application of optical absorption techniques i.e. TALIF,11 NO titration,12 and catalytic probes13,14 definitely help understanding interaction between nitrogen plasma and solid materials. Reactive particles of particular importance are neutral nitrogen atoms in the ground state. In nitrogen plasma they are usually found at the kinetic temperature close to the temperature of plasma facing components. In many technologically important nitrogen plasmas they actually represent the most important reactants. The interaction between neutral nitrogen atoms and solid materials can be either chemical or physical. Chemical interaction stands for a chemical reaction between a nitrogen atom and a solid material atom. Nitrogen atoms can either bond onto the surface forming a sort of a nitride, or pick an atom from the surface forming a radical that can leave the surface. A typical example of the latter mechanism is etching of organic materials or hydrogenated carbon by neutral nitrogen atoms.15 The consequence of the chemical reactions is a loss of neutral nitrogen atoms. The loss rate definitely depends on the intensity of the chemical reactions. In many practical cases however, chemical reactions are not predominant mechanism of neutral nitrogen atom loss. In many cases, the major mechanism is a physical reaction which is often called heterogeneous surface recombination. Nitrogen atoms are chemically rather reactive and can chemisorb on surfaces of different materials. If such chemisorption occurs, another atom from the gas phase may adsorb on the same site. The abundance of chemi-sorbed atoms can quickly cause formation of a molecule which does not fill strong chemisorption bonds but is rather desorbed from the surface immediately after being formed. This is so called Eley-Rideal mechanism of heterogeneous surface recombination. The other mechanism is the so called Langmuir-Hinshelwood which postpones surface migration of nitrogen atoms until they found a suitable site where they recombine and leave the surface of the solid material as a molecule. Whatever the mechanism is the recombination probability is often expressed in the terms of recombination coefficient which has been defined as the ratio between the number of atoms recombining on a surface area in a unit time and the total flux of atoms on the surface from the gas phase. Since the loss of atoms by a surface recombination is often the most important mechanism for draining atoms from plasma many authors attempted to measure it. 3 SELECTED RESULTS FOR NITROGEN ATOM RECOMBINATION COEFFICIENTS The literature on recombination of neutral nitrogen atoms is scattered among different journals and also different periods. The techniques used for quantification of the recombination probabilities heavily depend on a sort of model predicting the behaviour of atoms near a surface. As mentioned earlier nitrogen atoms are usually not produced by direct impact dissociation of a molecule in a ground state but also at other collisions. In practice it means that the atoms can be also produced away from glowing discharge in the region called an afterglow. In fact, measurements in the discharge itself are usually too complicated and often not very reliable so the majority of recombination coefficients was measured under afterglow conditions. The recombination coefficients for two types of glasses often used in construction of vacuum systems as well as plasma reactors are presented in Table 1. A quick look at the Table 1 reveals that the recombination coefficient is very low. This means that nitrogen atoms practically do not interact with glasses. The explanation has been given implicitly in the upper text and will be only stressed again here. The luck of chemisorption states on chemically very inert glasses prevent substan-tional sticking of nitrogen atoms on the surface so the probability of recombination is extremely low. A detailed consideration of Table 1 indicates a rather large scattering of the experimental results. The smallest value of 5x10-8 was found by Brennenen16 while on the other hand Gordiets17 reported values of the order of 10-4 and even larger for a more complex experiment. The discrepancy of the results is therefore over four orders of magnitude and may be explained by particularities of the experimental set-ups, surface finish of the materials and possibly also the purity of materials used at particular experiment. Table 1: Recombination coefficients for borosilicate glass (Pyrex) and quartz glass as obtained by different authors Tabela 1: Rekombinacijski koeficienti za borosilikatno steklo (Pyrex) in kvar~no steklo Reference Material Recombination coefficient Temperature (K) Pressure (Pa) Method Brennen16 Pyrex 5x10-8 294 66.5-1330 Pa afterglow intensity decay in a static system Wentink18 Pyrex 3x10-5 300 >40 Pa Pt heat resistance thermometer Ricard19 Pyrex 10-5 300 399 Pa afterglow intensity measurements Sancier20 Pyrex 5x10-5 300 2.4 Pa difussion method + measurements of the luminescence and the heat of lumophors excited by N atoms Marshall21 Pyrex 3x10-4 300 66.5-266 Pa electron spin resonance measurements Yamashita22 Pyrex 3.2x10-6 300 80-560 Pa static system + mass spectrometer Mavroyannis23 Pyrex 7.5x10-5 / 333 Pa flow system + NO titration Young24 Pyrex 1.7x10-5 / 133-1596 Pa flow system+ afterglow intensity measurements Gordiets25 Pyrex 2x10-4 <400 266 Pa kinetic model + actinometry + LIF1 Lefevre26 Pyrex 2x10-4 1500 Pa numerical procedure of hydrodynamic model and kinetic model + OES2 and NO titration Oinuma27 Pyrex 6.8x10-5 323 atmospheric pressure two-dimensional numerical model Marshall28 Quartz 8.3x10-4 300 399-1729 Pa flow system + ESR3 measurements Marshall28 Quartz 6.9x10-4 598 399-1729 Pa flow system + ESR measurements Marshall28 Quartz 9.6x10-4 779 399-1729 Pa flow system + ESR measurements Marshall28 Quartz 1.51x10-3 995 399-1729 Pa flow system + ESR measurements Marshall28 Quartz 2,04x10-3 1224 399-1729 Pa flow system + ESR measurements 29 Evenson Quartz 0.7x10-5 / 400 Pa flow system + ESR measurements Evenson29 Quartz 5.5x10-4 / 400 Pa flow system + ESR measurements Belmonte30 Quartz 9.3x10-5 Exp(-3700/RT) 300-823 1500 Pa Ar/N2=1000/50 sccm numerical procedure of hydrodynamic model + OES and NO titration Kim32 silica ~10-4 27 Pa smith diffusion method + thermocouple probe Adams31 Si 2.6x10-3 ~ 300 133 Pa two-dimensional model (Chantry equation) + TALIF1 calibrated by NO titration Adams31 Si 1.6x10-3 ~ 300 399 Pa Adams31 Si 5x10-5 ~ 300 665 Pa Herron33 Pyrex contaminated with water 1.6x10-5 195-450 400 Pa flow system + NO titration Table 2: Recombination coefficients for polymer PTFE, plastic foil (the authors give no information on the type of the foil) and ceramics BN Tabela 2: Rekombinacijska koeficienta za polimer PTFE, plasti~no folijo (avtorji niso podali informacije o vrsti folije) in keramiko BN Reference Material Recombination coefficient Temperature (K) Pressure (Pa) Method Ricard19 Plastic foil 3x10-4 300 399 Pa afterglow intensity measurements Ricard19 Teflon 10-5 300 399 Pa afterglow intensity measurements Young24 Teflon 2.9x10-5 / 133-1596 Pa flow system+ afterglow intensity measurements Evenson29 Teflon 0.2x10-6 / 40 Pa resonant cavity, pure nitrogen plasma Evenson29 Teflon 2.5x10-5 / 40 Pa resonant cavity, nitrogen plasma with oxygen impurities Oinuma27 BN 4.8x10-5 323 atmospheric pressure two-dimensional numerical model Adams31 BN 2x10-4 300 665 Pa two-dimensional model (Chantry equation) + TALIF calibrated by NO titration A similar behaviour as for glasses is found for some polymers. PTFE (teflon) has been known for decades as a very inert material. Not surprisingly, the recombination coefficient for nitrogen atoms has been found as low as for glasses. Table 2 summarizes the recombination coefficients for two types of polymers. Again, a rather low recombination coefficient of 10-4 is due to a lack of adsorption sites for neutral nitrogen atoms on such materials. Similarly low coefficients were also found for 1 (TA)LIF - (Two Atom) Laser Induced Fluorescence 2OES - Optical Emission Spectroscopy 3ESR - Electron Spin Resonance some ceramics. The value for boron nitride is added to Table 2. The coefficient is somehow higher than for glasses or Teflon and this fact can be explained by existence of irregularities on the surface of otherwise well bounded material. Moderately large recombination coefficients are found for many metals including silver (Ag), aluminium (Al), iron (Fe), and molybdenum (Mo). The recombination coefficients for these materials are presented in Table 3: The recombination coefficients for metals such as Ag, Al, Cu, Fe, Mo and for stainless steel. Data for Mo were estimated from the graph y(J) which was published in a relevant reference. Tabela 3: Rekombinacijski koeficienti za kovine Ag, Al, Cu, Fe, Mo ter za nerjavno jeklo. Podatki za Mo so bili od~itani iz grafa y(T), ki je bil objavljen v ustrezni literaturi ozna~eni v tabeli. Reference Material Recombination coefficient Temperature (K) Pressure (Pa) Method Hartunian34 Ag <10-2 300 13-133 Pa catalytic probe + NO titration Ricard19 Al 6X10-4 300 399 Pa afterglow intensity measurements Hartunian34 Al <10-2 300 13-133 Pa catalytic probe + NO titration Adams31 Al 2.8x10-3 300 133 Pa two-dimensional model (Chantry equation) + TALIF calibrated by NO titration Adams31 Al 1.8x10-3 300 399 Pa Adams31 Al 1.0x10-3 300 665 Pa Oinuma27 Al 1.8x10-4 323 atmospheric pressure two-dimensional numerical model Ricard19 Cu 2x10-3 300 399 Pa afterglow intensity measurements Hartunian34 Cu 7x10-2 300 13-133 Pa catalytic probe + NO titration Lefevre35 Fe foil with Fe2O3 layer 8.2x10-2Exp (-11400/RT) 300-473 1500 Pa Ar/N2=1000/50 sccm numerical procedure of hydrodynamic model + OES + NO titration Belmonte36 Fe2N1-x 1x10-2 823 900 Pa Ar/N2=60%/40% numerical procedure of hydrodynamic model + OES + NO titration Belmonte30 nitrided Fe foil 6.7x10-4 300 1500 Pa Ar/N2=1000/50 sccm numerical procedure of hydrodynamic model + OES + NO titration Mozetic13 nitrided Fe foil 0.21 400 100 Pa N2=600 sccm Ar=200-3000 sccm catalytic probe Lefevre37 Fe2O3 58.1xExp (-26940/RT) 300-330 1500 Pa Ar with 4.7% N2 numerical procedure of hydrodynamic model + OES + NO titration Hays38 Mo sputtered on Pyrex 4.4x10-3 283 665 Pa intensity measurements during plasma decay (continuity equation) Hays38 Mo sputtered on Pyrex 4.8x10-3 307 665 Pa Hays38 Mo sputtered on Pyrex 4.9x10-3 317 665 Pa Hays38 Mo sputtered on Pyrex 5.7x10-3 407 665 Pa Hays38 Mo sputtered on Pyrex 7.4x10-3 482 665 Pa Hays38 Mo sputtered on Pyrex 8.4x10-3 557 665 Pa Hays38 Mo sputtered on quartz 6.11x10-3 300 13.3 Pa intensity measurements during plasma decay (continuity equation) Hays38 Mo sputtered on quartz 5.58x10-3 300 27.9 Pa Hays38 Mo sputtered on quartz 2.73x10-3 300 105 Pa Hays38 Mo sputtered on quartz 2.19x10-3 300 154.3 Pa Hays38 Mo sputtered on quartz 2.11x10-3 300 240 Pa Hays38 Mo sputtered on quartz 4.52x10-3 300 45.2 Pa Hays38 Mo sputtered on quartz 3.68x10-3 300 67.8 Pa Adams31 Stainless steel 7.5x10-3 ~ 300 133 Pa two-dimensional model (Chantry equation) + TALIF calibrated by NO titration Adams31 Stainless steel 6.3x10-3 ~ 300 399 Pa Adams31 Stainless steel 4.8x10-3 ~ 300 665 Pa Singh39 Stainless steel 7x10-2 330 173-532 Pa steady state plasma model + NMS4 and EEDF5 measurements Oinuma27 Stainless steel 1.8x10-3 323 atmospheric pressure Two-dimensional numerical model 4NMS - Neutral Mass Spectrometry 5EEDF - Electron Energy Distribution Function Table 4: The recombination coefficients for catalytic materials such as Co, Ir, Pd, Pt and W. The recombination coefficient is temperature independent for Pd and Pt. For other materials the recombination coefficient is temperature dependent and it was estimated from the graph y(J) which was published in a relevant reference. Tabela 4: Rekombinacijski koeficienti za kataliti~ne materiale kot so Co, Ir, Pd, Pt in W. Rekombinacijski koeficient ni odvisen od temperature za materiala Pd in Pt. Za ostale materiale je temperaturno odvisen in je bil od~itan iz grafa y(T), ki je bil objavljen v ustrezni literaturi ozna~eni v tabeli. Reference Material Recombination coefficient Temperature (K) Pressure (Pa) Method Halpern40 Co 0.43 418 133 Pa flow system + NO titration Halpern40 Co 0.51 446 133 Pa flow system + NO titration Halpern40 Co 0.55 556 133 Pa flow system + NO titration Halpern40 Co 0.54 605 133 Pa flow system + NO titration Halpern40 Co 0.54 656 133 Pa flow system + NO titration Halpern40 Co 0.55 676 133 Pa flow system + NO titration Halpern40 Co 0.54 791 133 Pa flow system + NO titration Halpern40 Co 0.49 882 133 Pa flow system + NO titration Halpern40 Co 0.43 938 133 Pa flow system + NO titration Halpern40 Co 0.41 987 133 Pa flow system + NO titration Halpern40 Ir 0.32 1115 133 Pa flow system + NO titration Halpern40 Ir 0.26 1146 133 Pa flow system + NO titration Halpern40 Ir 0.34 1202 133 Pa flow system + NO titration Halpern40 Ir 0.27 1246 133 Pa flow system + NO titration Halpern40 Ir 0.27 1302 133 Pa flow system + NO titration Halpern40 Ir 0.37 1400 133 Pa flow system + NO titration Halpern40 Ir 0.38 1490 133 Pa flow system + NO titration Halpern40 Ir 0.37 1517 133 Pa flow system + NO titration Halpern40 Ir 0.38 1591 133 Pa flow system + NO titration Halpern40 Ir 0.40 1678 133 Pa flow system + NO titration Halpern40 Ir 0.46 1729 133 Pa flow system + NO titration Halpern40 Ir 0.45 1779 133 Pa flow system + NO titration Halpern40 Ir 0.47 1828 133 Pa flow system + NO titration Halpern40 Ir 0.41 1872 133 Pa flow system + NO titration Halpern40 Ir 0.45 1921 133 Pa flow system + NO titration Halpern40 Ir 0.44 1946 133 Pa flow system + NO titration Halpern40 Ir 0.48 2020 133 Pa flow system + NO titration Halpern40 Pd 0.3 500-1100 133 Pa flow system + NO titration Halpern40 Pt 0.31 500-1600 133 Pa flow system + NO titration Halpern40 W 0.21 1614 133 Pa flow system + NO titration Halpern40 W 0.24 1684 133 Pa flow system + NO titration Halpern40 W 0.29 1759 133 Pa flow system + NO titration Halpern40 W 0.32 1821 133 Pa flow system + NO titration Halpern40 W 0.36 1896 133 Pa flow system + NO titration Halpern40 W 0.40 1960 133 Pa flow system + NO titration Halpern40 W 0.43 2023 133 Pa flow system + NO titration Halpern40 W 0.48 2091 133 Pa flow system + NO titration Halpern40 W 0.53 2161 133 Pa flow system + NO titration Halpern40 W 0.57 2235 133 Pa flow system + NO titration Halpern40 W 0.58 2315 133 Pa flow system + NO titration Halpern40 W 0.68 2402 133 Pa flow system + NO titration Halpern40 W 0.74 2500 133 Pa flow system + NO titration Table 3. One should read the results for these particular materials with some precaution. These materials are known to form native oxide films. The oxide films can be removed prior to experiments with neutral nitrogen atoms but it is rather difficult to keep oxygen free surface in plasma reactors due to the existence of residual atmosphere, especially water vapour. The dissociation energy of water vapour is much lower than for nitrogen molecule so any water that appears in a nitrogen plasma reactor is likely to be dissociated to highly oxidative radicals such as O and OH. These radicals may stick on surfaces even though a lot of precautions were taken in order to assure appropriate cleanliness of the material surfaces. The highest recombination coefficients are found for catalytic materials. The results for these particular materials are summarized in Table 4. These materials are known for their good chemisorption abilities and transformation of a variety of radicals to stable molecules even at low temperatures. Not surprisingly they are also good catalysts for neutral nitrogen atoms. The recombination coefficient can be as high as 0.4 at room temperature. 4 CONCLUSIONS An overview of recombination coefficients was presented. The data summarized in this paper are useful for understanding the interaction between nitrogen plasma and solid materials. The first consideration goes for the right choice of the materials used for plasma facing components. More than obvious, catalytic materials should be avoided if a high density of nitrogen atoms in plasma is required. The best material for construction of nitrogen plasma reactors is the one with the lowest recombination coefficient, i.e. Pyrex. Just next to Pyrex is quartz glass. Both materials are high vacuum materials and thus very useful for construction of even large reactors where the purity of processing gas is important. From the recombination point of view polymers are often suitable but in practice they are usually avoided because they are not vacuum materials. If, for any reason glass should be avoided and only metals can be used, the recommended material is aluminium covered by a thin film of alumina (oxide). The oxide film should be compact and rather smooth in order to assure for low recombination coefficient and high stability of surface properties even after prolonged application. Other materials except some ceramics are not suitable as plasma facing components in nitrogen reactors due to a rather high recombination coefficient or rather unpredictable behaviour. 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