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Industrial Francis applications of low temperature plasma physics F Chent University of California Los Angeles California 900241594 Received14 November 1994 accepted6 February 1995 The applicationof plasmaphysics to the manufacturingand processingof materials may be the new frontier of our discipline Already partially ionized discharges are used in industry and the performanceof plasmashas a large commercialand technologicalimpact However the scienceof low temperatureplasmasis not as well developedas that of high temperature collisionlessplasmas In this paperseveralmajor areasof applicationare describedand examplesof forefront problemsin each are given The underlying thesis is that gas dischargeshave evolved beyond a black art and that intellectually challenging problems with elegant solutions can be found 0 I995 American institute of Physics 1 INTRODUCTION During the past four decades the science of hightemperatureand collisionless plasmashas grown explosively fueled by the challenging problemsin magneticfusion inertial fusion and space plasma physics As funds for basic researchin fusion and spaceplasmasdwindle it is fortunate that a new application of plasma physics has loomed large within the past five years that of the use of lowtemperature partially ionized plasmasin manufacturingand materialsprocessing Indeed this aspect of plasma physics may ultimately be the one with the greatestimpact on our everydaylives Although industrial applicationshave drawn great interest in plasma physics the number of paperspublished monthly on plasma relatedtopics in semiconductor processingalone far exceedsthe numberin fusion researchat its peak the field has not benefitedfrom the expertiseof the cadreof physicists who have honed their skills in the classical areasof plasma physics Gas dischargesare viewed by them as being an empirical discipline devoid of elegance and beset with unnecessarycomplications The purpose of this paperis to show that intellectually challengingproblems can be found in low temperatureplasmaphysics and that the complicationsof high collisionality and multiple speciesmay be no more complicated and resistantto treatmentthan say instabilities in toroidal magnetic fields The subject is very broad and with due apologiesto all the scientistsworking in this field we must limit our coverageto a few representative examplesin each case The succeedingsectionswilI discuss semiconductorprocessing flat panel displays ion implantation plasma polymerization and coating thermal plasmas and basic physics of low temperatureplasmas II SEMICONDUCTOR A Physical mechanisms PROCESSING in etching The productionof integratedcircuits consistsof repeated stepsof deposition masking etching and stripping to form and connectcircuit elementslike transistorsand capacitors Hundredsof chips can be made simultaneouslyon a silicon Paper 9RV Bull Am Phys Sot 39 1749 1994 ted speaker 2164 Phys Plasmas 2 6 June 1995 wafer which is typically 4 8 in in diameternow and lo 12 in in the near future To put some five million transistorson a Pentium chip for instance the individual elementshave to be below 0 5 um in size and moving toward 0 25 pm Such resolution cannotbe achievedwithout a plasma All computers and other electronic devicesof the future will dependon plasma processing yet at the moment very few plasma physicists have been involved The plasma is neededfor etching in at least three ways 1 it producesthe atomic species usually Cl or F which does the etching 2 it preparesthe substratesurfaceso that the etchantspeciescan be more effective and 3 it provides the directionality that allows the etching to proceed in a straight line The plasmadoes not always have to touch the surface to perform its functions The symbiosis between a plasma and an etching gas was demonstratedin the classic 1979 experimentof Coburn and Winters Fig 11 in which they showedthat the etch rate of fluorine in an argon plasma was over an order of magnitudelarger than with either the gas or the plasma alone In addition to the etch rate the plasma also provides profile control the ability to etch a trench with straight sidewalls Purely chemical etching would undercutthe mask and producea trench with rounded corners By acceleratingions through a sheath one can make them impinge on the mask and substrate at right angles therefore affecting only the surface at the bottom of the trench not the sides This is known as anisotropic etching However isotropic chemical etching is still present to degrade the trench profile Fig 2 By a fortunate accident someof the etch productsform a plastic polymer that deposits on the trench walls and protects them from the chemical etchant unless they are cleanedby a flux of energeticions Only by carefully balancing this passivation mechanism and the plasma enhanced etch rate can one produce a square trench profile2q3 Pig 3 Four types of materialsneedto be etchedin Ultra LargeScale Integration ULSI processing silicon monocrystalline or polycrystalline doped or undoped dielectrics usually Si02 or SiN metals usually aluminum tungsten or molybdenum and photoresist Each of theseinvolves different chemistries and different groups of experts The processesthat follow are not necessarilythoseused in any actual production line but will serve to give the flavor of what is 1070 664 95 2 6 2164 V 6 00 0 1995 American Institute of Physics Downloaded 27 Nov 2001 to 128 97 88 10 Redistribution subject to AIP license or copyright see http ojps aip org pop popcr jsp h Ion Beam XeF Gas 4 54 3I3 38 RESIST l 32 I 1 1 0 L 0 100 200 300 400 500 6CKL 700 800 900 L Time set FIG 1 Evidence for the catalytic effect of plasma on chemical etching reprinted with the permission of the American Institute of Physics Ref I OXIDE involved 4Silicon can be etched by either fluorine or chlorine In chlorine etching the plasmafirst dissociatesCl into Cl atoms Thesereact with Si to form SiCl and SiCl Si 2 Cl SiCl Sic 2 C1 SiC14 Here SiCl is a gasand can be pumpedout In addition SiClz polymerizes to form SiC12 the passivation agent If needed the amountof SiC12can be increasedby addinghydrogen SiC1 2 H SiClz 2 HCl Oxide is also etchedby F and Cl but more slowly than silicon is For instance with fluorine we could use a CF4 plasma CF4 2 F CF monomer ai r Si02 4 F iSiF4 2 0 gases 1 Si02 2 CF GF4 2 CO gases r I Meanwhile the silicon is being etchedfaster by the reaction Si 4 F SiF lsotropif2 etch Noions Directional etch ions no passivdoo Vertical etch With

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