HMDSO Polymer 各种膜应用

II.1.2.1Bioorganic compounds in primitive earth atmosphereII.1.2.2Methane reformationII.1.2.3Diamond depositionII.1.2.4Action of plasmas on organic moleculesII.1.2.5Plasma etchingII.1.2.6Metal depositionII.1.2.7Metal vapour-non organic moleculesII.1.2.8Si-organic compoundsII.1.2.8.1Plasma polymerizationII.1.2.8.2SiO2depositionII.1.2.9Plasma chemistry of airII.1.2.10ConclusionCharacterization of selected systemsII.1.3Characterization of selected systemsMartin SchmidtInstitut f¨u r Physik der Ernst-Moritz-Arndt-Universit¨a t Greifswald,Felix-Hausdorff-Str.6,D-17489Greifswald,Ger-manyThis book is aiming at an introduction into plasma chemistry.Therefore,only a subset of the complete plasma chemistry can be covered.New applications of e.g.thinfilm deposition techniques,surface treatment procedures,and substance transformation processes in the volume controlled by plasma chemical reactions were developed the last years.Process improvement in terms of products determined by specific elementary processes is ongoing intensive development and research as can be seen from literature.Due to this dynamic development in thisfield a data base for plasma chemistry will be outdated rather soon. Therefore,a study of reference journals and data bases like NIST,permanently reflecting improved understanding in thisfield,is necessary for a discussion of new experimental results or the development of adequate models.In the following some selected topics of special interest demonstrating the broad spectrum of applications.II.1.3.1Hydrocarbon plasma chemistryThe plasma chemistry of hydrocarbons is characterized that the important reaction channels are initiated by electron impact.It offers a broad spectrum of processes and even applications beyond standard organic chemistry.The spectrum ranges from complicated processes of the formation of bioorganic compounds in the early earth atmosphere,natural gas reformation, polymeric depositions and creation of higher hydrocarbons to deposition of diamond-like thinfilms and creation of pure carbon as soot or even diamond.One essential starting substance is the methane molecule.The plasma chemistry of methane is often initiated by electron impact dissociation leading to CH3radicals and H ato+ms[1]CH4+e−CH3+H+e−.(II.1)Figure II.1:Reaction scheme of the transformation of C1H x and C2H yspecies in diamond plasma CVD according to[2].M indi-cates the action of the wall.Another pathway is the reaction of this molecule with hydrogen atoms(also generated by electron impact processes)[3]CH4+H CH3+H2.(II.2)The abstraction of further H atoms leading to CH2,CH,and C is possible by H atoms but also by electron collisions,especially in pure methane plasmas.The reverse reaction, the addition of H atom to CH3to form CH4occurs at low temperature[4].H atoms may be generated by dissociative electron collisions of H2molecules,at higher gas temperatures thermal dissociation of hydrogen molecules becomes dominant as studied in thermal plasma chemistry.Here,some processes may be more effective,but the specific production sensitivity of non-thermal plasma chemistry is lost due to generations of new compounds in a cold gaseous environment.A reaction scheme is presented in Fig.II.1for the formation of CH x and C2H y compounds controlled by collisions of hydrocarbon molecules with H atoms[2].Concerning the variety of the processes in a H2-CH4plasma,including the electron impact induced reactions,see also[5].II.1.3.1.1Origin of life on earthOne exciting and still not completely resolved topic is the origin and the development of live on the earth.Thefirst step is the formation of organic molecules,e.g.amino ler [6]observed in a plasma chemical experiment in spark discharges for a mixture of methane, ammonia,hydrogen,water vapour and liquid water under the action of UV radiation the formation of various organic compounds as HCN,amines,aldehydes,acrylonitrile.In the solution,amino acids were formed.The products in the gas phase were generated by the action of free radicals and ions of the gas discharge.The state of the early earth atmosphere is discussed by Abelson[7].An N2–CO–H2atmo-sphere is supposed and HCN and H2O were the principal products of a gas discharge,beside small amounts of CO2and CH4.HCN in aqueous solution can lead to other interesting compounds.Also the plasma chemistry in CH2–H2S[8]and CH4–PH3[9]atmospheres is studied to detect pre-biochemical compounds.Sulphur containing reaction products are CS2and thiols as CH3SH and C2H5SH.The main organo-phosphorus product is CH3PH2.A discussion of the reaction pathways leads to the proposalCH3+PH2+M CH3PH2+M.(II.3) The radicals CH3and PH2are generated by reaction of hydrogen atoms with the CH4and PH3molecules.These experiments show the formation of organic compounds in plasma chemical experi-ments,but a detailed understanding of the processes is still lacking.II.1.3.1.2Methane conversionA very important topic is the methane conversion.Methane is a dominant part of natural gas.Other sources are petroleum processing off-gas and biogas.It is an important energy carrier and initial compound of chemical industry but also a dangerous greenhouse gas.Besides classical chemical procedures plasma chemical methods are investigated for methane transformation into higher hydrocarbons.The principle process scheme(Fig.II.1)shows the formation of ethane,ethylene and acetylene.An investigation of methane conversion in a pulsed microwave discharge(p=30mbar)yields a selectivity of acetylene generation near70percent with an energy input of10eV/molecule.Here the methane dissociation is initiated by electron impact.The generated H atoms provide the source for H abstraction of the methane molecule[10].The conversion of a CH4/CO2mixture into higher hydrocarbons or synthesis gas(CO/H2) in a hybrid catalytic plasma reactor is reviewed by[11].The chemical reactions are initiated by electron impact dissociation of CO2and CH4generating CO and O as well as CH3and H,respectively.An important research topic is the direct conversion of methane and carbon dioxide to methanol[12,13].The investigation of the reaction products of a methane-CO2 mixture in an atmospheric pressure dielectric barrier discharge shows a small concentration of methanol,but a lot of other pure and oxygenated hydrocarbons.A carbon chain growth is supposed to occur mainly by the reactionC n H2n+2+CH4C n+1H2n+4+H2.(II.4) According to the practical application an essential problem is the bad selectivity of the plasma processes,as it is shown in the diagram in Fig.II.2[13]and pointed out by[14]. Products of the methane conversion include syngas(H2+CO),gaseous products as ethylene, acetylene,and propylene,liquid hydrocarbons,plasma polymers and oxygenates.II.1.3.1.3Plasma polymerizationPlasma polymerization is a process of thinfilm deposition on electrodes,walls,or on a substrate under the action of plasmas.The plasma contains small or higher concentrations of organic molecules.The term polymerization is misleading because this material is not a polymer consisting of equal components,but it is a highly cross linked,brittle material with good dielectric properties,pinhole free,low solubility,chemical inertness and good adhesion to the surface.A broad spectrum of organic compounds was applied for plasma polymerization.In contrary to chemical polymerization these starting compounds in the feed gas can be free of double or multiple bonds or cyclic structures.In classical chemistry these substances cannot be used as monomers for polymerization.A reaction scheme of plasma polymerization can be given as follows Fig.II.3[15,16].The starting gas is activated in the plasma by electron collisions or by collisions with other energetic plasma components as H atoms.Ionic or neutral radicals are created.The target surface is activated e.g.by ion bombardment.The radicals diffuse(neutral)orflow(ionic) to the surface where they are bonded to the surface.The starting material can also move directly to the surface where a plasma induced polymerization is possible.The generationFigure II.2:Dominant chemical processes in atmospheric pressure DBDin a(CH4/CO2mixture).DME dimethyl ether CH3-O-CH3,MF methyl formate CH3-O-CH=O,ETO ethylene oxide H2C O CH2,PO propylene oxide H2CO CH CH3,Ac acetone CH3-C O-CH3[13].of non-polymer forming gas as H2may be initiated in the interaction of the plasma with the starting gas,during thefilm formation process or by the plasma interaction with the plasma polymer.An interesting feed gas for plasma polymerization is the vapor of the silicon organic com-pound hexamethyldisiloxane(CH3)3SiOSi(CH3)3).This compound is important because it can change the properties of the resulting polymers starting with nearly organic,silicone-like properties[17]using the pure compound under mild plasma conditions up to nearly nonorganic properties under the action of higher O2admixtures for SiO2-like[18]thinfilm.Figure II.3:Reaction scheme of plasma polymerization[15,16].The activation of the HMDSO molecule by electron impact ionization results in the scission of the Si-C and Si-O bond generating(CH3)3SiOSi(CH3)+2ions and CH3radicals or Si(CH3)+3 ions and OSi(CH3)3radicals,respectively[19].These products are observed also in ion molecule reactions of HMDSO molecules with Ar+ions[20].The gas phase ion chemistry is studied by mass spectrometric methods[21–23].The formation of oligomeric species with masses221,309,295,369up to383amu by reactions of fragment ions with neutral HMDSO molecules is reported.It is supposed,that the thinfilm deposition is essentially determined by the ions arriving to the surface[21,22,24].II.1.3.1.4Thinfilm deposition of metal compoundsA method of thinfilm deposition of simple metal compounds is the plasma enhanced chemical vapor deposition with metal organic starting gases.The advantage of this method is e.g. the low substrate temperature und is therefore useful for temperature sensitive materials [25].A study of deposition of TiN using Tetrakis(diethylamine)titanium(TDEAT)shows the importance of H-radicals in the H2plasma for the stripping of TDEAT.The formation of TiN requires N2addition to the process[26].The deposition of thinfilms of pure metals or simple metal compounds is possible by sput-tering in inert gas low pressure discharges and by reactive sputter deposition.An example is the deposition of TiNfilms.The nitride formation is a surface process of the fresh deposited Ti with the plasma activated nitrogen.II.1.3.1.5Diamond depositionPlasma assisted chemical vapour deposition enables the formation of diamond at moderate temperature and low pressure with hydrocarbons as starting compounds on non-diamond substrates.This allows the extended industrial use of the outstanding properties of the diamond,as the extreme hardness,high thermal conductivity,broad optical transparency (deep UV to far IR),wide band gap(5.4eV).In gas phase chemistry of a CH4/H2plasma the CH4molecule is activated which leads to carbon deposition with sp3(diamond)or sp2(graphite)bonding.The hydrogen atoms generated in the plasma etch the deposited carbon producing volatile compounds CH n(n=1−4).Because the etch rate of graphite is approximately100times higher than for diamond[27]diamond remains on the substrate. For the diamond synthesis the principle growth species are CH3,C2and H[28].In the con-ventional H2rich plasma in H2/CH4the CH3radical is responsible for the diamond growth. In H2poor plasmas,as in Ar/H2/CH4gas mixtures C2controls the diamond deposition.A variety of reactions and transformation processes in a CH4/H2plasma is illustrated by the reaction scheme Fig.II.1.The radicals C2,CH,H,CH3are observed by tuneable infrared diode laser and emission spectroscopy,respectively[2,29].The action of the H atoms is manifold for the diamond deposition process.Besides the already mentioned etching of the graphite phase and the importance for CH3generation theH atoms essentially control the chemistry of the diamond surface during the growth phase[28].Apart from the CH4/H2gas mixture also other gases were used for diamond deposition. Admixtures like CO2,O2,CO also N2and inert gases Ar,Xe or as carbon carrier e.g. chloro-andfluoro-carbons are successfully applied in deposition experiments.Bachmann[27] investigated the diamond deposition area in the C/H/O triangle phase diagram by analysis of hundreds of diamond CVD experiments with various C/H/O containing gas mixtures. Diamond growths near the“CO-line(C/O=1).Hotfilament chemical vapour deposition process and microwave plasma CVD with low or higher energy input were the most successfully applied methods for diamond deposition [28].High substrate temperatures(typically>700C)ensure good diamond quality.For various industrially important applications as deposition on microelectronic substrates lower substrate temperatures are necessary.The decrease of the deposition rate with decreas-ing substrate temperature could be compensated using other starting gases as halogenated compounds like C2H5Cl[30]or CO[31]with H2admixtures.The low temperature diamond deposition is investigated by Tsugawa[32]by a non-thermal microwave plasma with substrate temperatures from100–500C.The plasma was sustained by surface waves excited by an array of two sets of eight coaxial linear antennas powered by 2.45GHz,at a maximum of20kW.The feed gas consists of H2,CH4,CO2(∼100:1:1). The CO2admixture enhances the etching of the non-diamond depositions,especially at lowtemperatures.The discharge operates at a pressure of100Pa.Data are reported of plasma parameter(n e=1011cm−3,T e=1.5eV)measured by Langmuir probes at a typical position of the substrate.A deposition rate of the nano-diamond coating of about50nm/h was observed.The diamond single crystal growth in a high microwave power density plasma,which will be nearly a thermal plasma,is discussed by Archard[33].For microwave power densities of about100Wcm−3gas pressures near200mbar the plasma has the role to heat the gas.For gas temperatures of the gas of1000-2000C and of the substrate near1000C,H2is thermally dissociated and CH3is generated by CH4+H CH3+H2.A growth rate is observed e.g. increasing from2μm/h up to16μm/h for CH4admixtures in H2of2–8%at a power density of100W/cm3.A more detailed discussion of the plasma assisted diamond deposition is given in chapter xx.The deposition of well-ordered nanostructures as nanotips and nanotubes, nanowalls,ultrananocristalline diamond is also observed under plasma conditions using as source material mixtures of carbon-carrier gases as hydrocarbon,fluorcarbons etc.[34–39], see also[40].Fullerenes are generated successfully in thermal plasmas[34].II.1.3.2Deposition of silicon solar cellsThe development of alternative energy sources to fossil fuels is an important task for science and technology in the21st century.Photovoltaic is a promising candidate in future renewable energy technology.Thin Si-films are expected to be successful material for effective solar cells[41,42].Beside other methods plasma enhanced chemical vapor deposition is widely used for generation of amorphous silicon(a-Si:H)and microcrystalline(c-Si:H)films.Feed gas for plasma assisted silicon deposition are mainly monosilane SiH4or SiH4/H2mixtures. Inside the plasma silane is dissociated by electron impact into SiH3,SiH2,SiH,Si,H2and H,and H2in H atoms.Also SiH+x,SiH−x and H+x ions are generated.Secondary ion molecule reactions and reactions between neutral species have to be taken into account,too.Data on silane plasma chemistry are reviewed in[43].SiH3reaching the surface reacts to SiH4with bonded hydrogen atoms and generates dangling bonds or recombines with another SiH3to Si2H6.SiH4and Si2H6are desorbed from the surface.An on the surface diffusing SiH3radical reacts of with the site of dangling bond by formation of a Si-Si bond,the Sifilm is growing[41].The formation ofμc-Si:H is enhanced by high hydrogen dilution and low RF power conditions.The high optical absorption coefficient for sunlight of a-Si:H allows the application offilms with a thickness lower1μm for solar cells.The low temperature plasma enhanced deposition process(150–300C)enables the deposition on temperature sensitive substrates as polymer foils.The fabrication of homogenous large-area a-Si:Hfilms with high deposition rate is impor-tant for applications from an economical viewpoint.Parallel plate rf-reactors operating at 13.56MHz are usually used.Deposition rates of0.2-0.3nm/s are observed.Rates of2nm/s were achieved by increased operating frequency(70MHz).Table II.1:Etching gases for various materials[45–48].Material Etching gasesSilicon CF4/O2,CF2/Cl2,CF3/Cl,SF6/O2/Cl2,Cl2/H2/C2F2/CCl4,C2ClF5/O2,SiF4/O2,NF3,CCl4,C2ClF5/SF6,C2F6/CF3Cl,Br2,CF3Cl/Br2,HBrSiO2CF4/H2,C2F6,C3F8,C4F8,C4F6,C5F8,CHF3/O2Si3N4CF4/O2/H2,C2F6,C3F8,CHF3,NF3,CHF3/O2,CH3F,SF6Organics,Polymers O2,CF4/O2,SF6/O2Silicides CF4/O2,NF3,SF6/Cl2,CF4/Cl2Al BCl3,BCl3/Cl2,CCl4Cl2/BCl3,SiCl4/Cl2Cr Cl2,CCl4Cl2Cu Cl2/ArMo,Nb,Ta,Ti,W CF4/O2,SF6/O2,NF3/H2Au C2Cl2F4,Cl2,CClF3GaAs BCl3/Ar,Cl2/O2/H2,CCl2/F2/O2/Ar/He,CCl4,PCl3,HCl,Br2,COCl2,SiCl4InP CH4/H2,C2H6/H2,Cl2/Ar,Cl2/O2,HBr,CF3Br,Br2,IBr,HI,CH3I,I2,IClNiFeCo Ar/Cl2,N2/Cl2,H2/Cl2The deposition rate ofμc-Si:H could be increased by application of a narrow gap discharge at higher pressures[44].II.1.3.3Plasma etchingPlasma etching is a fundamental technology for patterning in chip production microelectronic industry.For this process a non-reactive gas is fed into the plasma where it is activated. The interaction of this activated gas with a solid substrate generates in a chemical reaction a volatile compound which contains atoms of the substrate.Gases for etching of various ma-terials are presented in Tab.II.1.An example is the silicon etching by afluorine compound as CF4.The plasma activation leads to generation offluorine atoms by electron impact dissociation of the CF4molecule.CF4+e−CF3+F+e−.(II.5) Thefluorine atom reacts with silicon and produces volatile SiF4Si+4F→SiF4.(II.6) This is only the gross reaction.A detailed model of the silicon etching by a CF4plasma is given by Standaert[50],see also[49]and Fig.II.4.Fluorine atoms F as are adsorbed onto bare silicon sites and the silicon is passivated by chemisorption(=Si-F).Activation energyFigure II.4:Reaction scheme of etching of Si by in a CF4plasma througha polymerfilm[49].for desorption of the etch product SiF n is transferred from the plasma to the surface by ion bombardment(I+).Because of the existence offluorocarbon radicals(CF n)a polymerfilm is deposited on the silicon surface.Now thefluorine atoms are adsorbed of the polymer surface (F s).They diffuse through the polymerfilm and are adsorbed by the silicon sites.Ions provide the desorption energy for the etch products,which diffuse through the polymer into the gas phase.Thefluorine atoms generated inside the plasma can also etch the polymerfilm. This model illustrates the diversity of the etch process.Only a complex treatment of the various volume and surface reactions leads to an understanding of this process.The sidewall protection by the thin polymericfilm is important for the anisotropy of trench etching. Fundamental starting processes of activating the etching gases are the electron-molecule collisions.A critical review of data for a lot offluorine and chlorine containing gases is given by Christophorou and Olthoff[51].II.1.3.4Air,nitrogen,and oxygen plasma chemistryChemical reactions in a non-thermal oxygen,nitrogen or air plasma are initiated by electron impact.The main processes are dissociation of molecules with the generation of the reactive atoms(O,N)[52,53],the formation of excited atoms and molecules as well as ions.The formation of negative ions is important mainly for the electronegative gas oxygen.The dissociative attachment of electrons generates negative atomic ions and oxygen atoms also. This process is important in the plasma because its threshold is essential lower,especially for excited molecules[54],than electron impact dissociation and dissociative ionization of O2.The excited states may be short lived or electronically excited metastable states.The rate coefficients for heavy particle reactions of ground state or electronically excited14CHAPTER II.FUNDAMENTALS,SOURCES AND DIAGNOSTICS species can differ by orders of magnitudeN+O2→NO+O(II.7) with k=7.7·10−17cm3mol−1s−1[55]andN(2D)+O2→NO+O(II.8) with k=5·10−12cm3mol−1s−1[56].The rate coefficient of the second reaction is much larger because the activation energy of this reaction is lowered by the potential energy of the excited reaction partner[57].Figure II.5:Diagram of primary chemical reactions in an air plasmainduced by electron impact[58].The air plasma chemistry is of interest because it is responsible for the producing of N x O y compounds,which have a key role in global environmental problems like acid rain.A scheme Fig.II.5of the plasma chemical reactions in dry air demonstrates the complexity of the processes[58].The plasma chemistry in air and in pure oxygen has following the important applications.II.1.SPECIFICS OF NON-THERMAL PLASMA CHEMISTRY15 II.1.3.4.1Ozone generationOzone a result of the three body collision processO+O2+M→O3+M→O3+M,(II.9) where M is a third collision partner as O2,O,also O3or N2.Oxygen atoms are generated by dissociative electron impact.The Ozone formation is reduced by competitive reactions like recombination of two O atoms to O2or reactions of O atoms with ozone molecules O+ O3+M→2O2+M[59].In chapter xxx!!!!!!!!!!!!!!!!the ozone synthesis is discussed in detail.II.1.3.4.2Plasma ashingThe interaction of an oxygen plasma with hydrocarbon compounds leads to CO2and H2O.In microelectronic industry the photoresist mask is removed(stripped)by an oxygen plasma. Damage of the semiconductor material by high energy ions must be avoided by low ion energies and highfluxes of neutral radicals i.e.oxygen atoms to the resist surface.The plasma ashing procedure is used also for quantitative analysis of lignite for separation of organic and non-organic components[60].This method is successfully applied for the preparation of samples for electron microscopic investigations[61].The advantage of plasma ashing is the low temperature of the process,because the active particles are the oxygen atoms.The reaction temperature is low as40C,molecular oxygen need reaction temperatures higher than600C[62].The formation of the high reactive ozone must be taken into account in atmospheric pressure discharges.Oxygen plasma procedures are also used for ashing of organicfilters in asbestos analytic.Such plasmas are applicable to cleaning of metallic surfaces of organic substances as grease or oil[63].Hazardous gaseous organic moleculesmay be destroyed by reactive species like N∗2,(A3S+u),N∗2(B3P g),O∗2(a1D g),O(1D),O(3P),H.OH,and N best into CO2or H2O[64].II.1.3.5ConclusionsSelected examples of the various possibilities of plasma chemical processes were presented. Most applications concern surface processes,e.g.thinfilm deposition,etching and cleaning, and to a lower extent volume processes,like voc‘s destruction.Only one volume process for the generation of larger amounts of material has reached technical maturity,namely the ozone synthesis.Some processes and procedures are specific for plasma processing.Examples are the micro patterning in microelectronics,or the deposition of plasma polymers on various substrats.The aim of future technical developments must be to enhance the selectivity and energy efficiency of plasma chemical processes,research may lead to new materials with exciting properties.。