Plasma–surface interactions in fluorocarbon etching of silicon dioxide
`J. W. Butterbaugh, D. C. Gray, and H. H. Sawin
`
`Citation: Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing,
`Measurement, and Phenomena 9, 1461 (1991); doi: 10.1116/1.585451
`View online: https://doi.org/10.1116/1.585451
`View Table of Contents: http://avs.scitation.org/toc/jvn/9/3
`Published by the American Institute of Physics
`
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`

`Piasma-—surtace interactionsin fluorocarbon etchingof silicon dioxide
`J.W. Butterbaugh
`IBM General Technology Division, Essex Junction, Vermont 05452
`D.C. Gray andH.H. Sawin
`Massachusetts Institute ofTechnology, Department ofChemical Engineering, Cambridge,
`Massachusetts 02139
`
`(Received 1 January 1991; accepted 26 February 1991)
`
`in a fluorocarbon plasma environment were simulated and
`The major species present
`independently controlled using radical and ion beamsin an ultrahigh-vacuurn apparatus. The
`beams used in this study were chosen to determine the importance of CF, radicals in a CF,
`plasma; the beams included F and CF,, with a beam of Ar‘ to simulate energetic ion
`bombardment. Both CF, and F enhance the etching yield of SiO, under energetic Ar’
`bombardment; however, the enhancement with F is twice that seen with CF, at similarfluxes.
`When CF, and F fluxes are used simultaneously, F dominates and the CF, flux haslittle effect on
`the overall etching yield. Combined with previous work on Si substrates,
`these results are
`consistent with qualitative theories for SiO, /Si selectivity in fluorocarbon plasmas. Possivle
`elementary steps in the ion-enhanced etching process are proposed and reduced to a two-
`parameter model which represents the process as ion-enhanced neutral adsorption followed by
`ion-induced reaction to form volatile products.
`
`iL. INTRODUCTION
`
`During plasma-etching processes, the substrate surface is
`subjected to fluxes of several different reactive species. In
`fluorocarbon plasmas, the species striking the substrate sur-
`face include CF, (py = C-3), F, and energetic CPt (x =0-
`3). Knowledgeof the relative importance of these species in
`the plasmaetching of SiO, is necessary to understandselec-
`tivity and anisotropy in S10, etching processes. Selectivity of
`SiO, etching over Si etching in fluorocarbon plasmasis at-
`tributed to carbonaceous film formation on the Si surface,
`while the highly anisotropic nature of SiO, etching is attrib-
`uted to the strongly ion-enhanced nature of the etching pro-
`cess. Previous research into these mechanisms has included
`post-process surface analysis with x-ray photoelectron spec-
`troscopy (APS), Auger electron spectroscopy (AES), and
`ellipsometry. Other research efforts have focused on therel-
`ative importanceof the radical and ionic species by isolating
`these species outside the plasma environment.
`The general conclusion of the post-process surface studies
`is that a carbonaceousfilm forms on the surface of Si while
`little, if any, carbonaceousfilm forms on the surface of Si0,.
`After exposing Si and SiO, surfaces to a C,F, plasma,
`Oshima! found a 20- to 30-A-thick fluorocarbonfilm on the
`Si surfaces and little or no fluorocarbon film on the SiO,
`surfaces. Coburn’ found similar results with AESanalysis of
`Si and SiO, surfaces after exposure to CF, + H, plasmas.
`Coburn found that the C signal on Si surfaces increases and
`the Si signal decreases as the amount of H, in the plasmais
`increased; the C signal is not detectable on SiO, surfaces
`until the level of H, addition is above 17%. Jaso and Oehr-
`lein,’ using XPS andellipsometry, also noted the absence of
`any carbonaceousfilm on SiO, surfaces, although some Si-
`C bonding was detected. Haverlag et al.* studied the SiO,
`surface in a CF, plasma with in situ ellipsometry and again
`found 2 carbonaceouslayer only on the Si after the SiO, was
`
`etched away. The probable explanation for the absence of a
`carbon layer on the 810, surface is the availability of oxygen
`to form volatile carbon-fluorine-oxygen species. Ab initio
`calculations’ show COF, to be very weakly bound onSiO,
`surfaces, making it a likely product of the etching process.
`Several studies in the literature have isolated radical spe-
`cies present in the discharge and have measured their reacti-
`vity towards SiO,.° 73 Flamm et a/.° measured the reactivity
`of atomic fluorine with SiO, and found that F will spontan-
`eously etch SiO, at a rate similar to that found in CF, barrel
`etchers, where the SiO, surface is subjected to little ion borm-
`bardment. Riching rates in parailel-plate or reactive ion
`etching (RIE) reactors, where the SiO, receives consider-
`able energetic ion bombardment, are much higher than can
`be accounted for by spontaneous reaction with F.
`Thereactivity of CF, with SiO, has been studied by sever-
`al researchers.’"' Selarnogin ef a/.° reported that SiO, does
`not etch in the presence of CF, without simultaneous sur-
`face irradiation. Langan et al.’ performed XPS analysis of
`SiO, surfaces exposed to CF, radicals and found that CF,
`adsorbs on the 8:0, surface, but does not dissociate or react
`with the surface. After bombarding the surface with energet-
`ic ions, the CF, adsorption probability increased, but there
`wasstill negligible dissociation. These studies clearly show
`that CF, does not spontaneously etch 810,; CF, will only
`etch S10, when the surface is under simultaneous energy
`irradiation (e.g., ion bombardment).
`Thereactivity of CF, towards SiO, has also been studied
`by several researchers.°!! 1° These studies reported that the
`reactivity of CF, toward SiO, is below experimental detec-
`tion limits, placing the reaction probability at less than
`10~ ©. Joyce et al.'* found that CF, will adsorb on SiO,, but
`will undergo very limited dissociation. XPS analysis, after
`exposure to CF,, showed no evidence of O-F bonding and
`some evidence ofboth C-Si and C-O bonding.'? McFeely et
`ai.’ used XPS to examine SiO, surfaces exposed to CF, and
`
`1467
`
`J. Vac. Scl. Technol. B 8 (3), May/Jun 1991
`
`9734-241X/91 /031461-10201.06
`
`© 19974 American Vacuum Society
`
`4461
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`

`1462
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`Butterbaugh, Gray, and Sawin: Plasma-—surfaceinteractions in FC, etching of SIO,
`
`1462
`
`also determined that CF, adsorbs, but not dissociatively. In
`addition, McFeely concluded that adsorbed CF, is mainly
`bound to O on the surface through C-O bonding. These
`studies show that CF,, like CF,, does not spontaneously
`etch SiO,.
`In addition to the flux of neutral! radical species, there is
`significant ion bombardment ofthe SiO, surface in a paral-
`lel-plate plasma reactor. Ions in a CF, plasma are mainly
`CF;', CFj, and CF. Several studies'*"* demonstrate
`that these molecular ions, which supply chemical reactants
`to the surface as well as energy, remove a larger number of
`SiO, molecules than a noble gas ion like Ar”. The etching
`yield (number of SiO, molecules removed per incoming
`ion} has been measured at 0.2 for 500 eV Ar* and 0.5 for
`500 eV CF;.'* It has also been shown that the energetic ion —
`bombardment can inducethe reaction of species already ad-
`sorbed on the surface. The SiO, etching yield of Art is
`enhanced when the surface is simuitaneously exposed to a
`neutral source of F, such as XeF,.'*19 Because XeF, disso-
`ciatively chemisorbs leaving a monolayer of F on the SiO,
`surface,'? XeF, has been used to simulate the flux of atomic
`fluorine in the plasma environment. Loudiana et al.’° found
`an enhancement of the 500 eV Ar* etching yield of about
`3.6% with simultaneous XeF, exposure.
`it has also been suggested that the etching yield of SiO, is
`enhanced by the adsorption of neutral CF, species. In stud-
`ies using CF,", the SiO,
`is also exposed to CF, species,
`which are produced along with the ions. Mayer and Bark-
`er*° attributed the pressure dependenceofthe etchingrate in
`reactive ion beam etching to the adsorption of neutral spe-
`cies. The pressure dependenceflattened out at higher pres-
`sure, indicating a saturation or a shift from an adsorption-
`limited to an ion fiux limited etching process.
`In the present work, the fluxes to a SiO, surface in a flu-
`orocarbon plasma environment are simulated with three
`controlled beamsin an ultrahigh-vacuum (UHV) appara-
`tus. In the literature, the primary etchant in a CF, plasma
`has been identified as atomic fluorine, although it has been
`suggested that CF, and CF, may be active participants in
`the etching reaction;”'”* a beam of atomic fluorine (F) was
`used as the primary etchant in the UHV apparatus. There
`are several fluorocarbon radical species present in the CF,
`plasma, inchiding CF,, CF,, and CF; a beam of CF, was
`used to represent the effect of fluorocarbon radicals. The
`ionic species ina CF, plasma are mainly CF," , in addition to
`less fluorinated ions. Due to difficulties in making a clean,
`high-flux beam of CF; , a beam ofAr‘ ions was used in the
`UHVapparatus as the source of energetic ions. This substi-
`tution has been madeinpast studies ofion-enhanced etching
`of SiO, with XeF, to decouple the effects of ion inertia and
`ion chemistry.'°'"?3?4 Tu et al.’> studied SiO, sputtering
`with CF," ions and Ar’ ions in the presence of XeF, and
`found that the CF," ions were twice aseffective as Ar? in
`etching SiO, , but that the effectiveness of both ions showed
`the same dependence on the fiux of XeF,. Winters”? found
`that the products from the sputtering of SiO, in the presence
`of XeF, were relatively independentofthe type of ion (Ar *
`or CF; ) used to bombard the surface.
`
`J. Vac. Sci. Technol. B, Vol. 9, No. 3, May/Jun 1997
`
`This paper describes an investigation ofthe individual and
`combinedeffects of CF, and F on the Ar? ion sputtering
`yield of SiO, . The flux levels in these experiments are repre-
`sentative of the flux levels in the plasma environment. Sim-
`ple models involving the ion-enhanced adsorption of CF,
`and F were derived from a muchlargerset ofpossible surface
`reactions and ion enhancement mechanisms. These simple,
`two-parameter models capture the majorfeatures of the ion-
`enhanced etching of SiO,.
`
`I. EXPERIMENTAL
`
`The experimental apparatus used for these studies has
`been describedin detail elsewhere.’ A schematic ofthe sys-
`tem is provided in Fig. 1. The SiO, sample can be simulta-
`neously exposed to three separately controlled beams. The
`beams used in this work include an Ar * ion beam generated
`in a Kaufmanion source, a difluorocarbene (CF, ) radical
`beam generated by pyrolysis of hexafluoropropylene oxide
`(HFPO), and an atomic fluorine (F) beam generated by a
`microwave discharge ofF, and Xe. The sample temperature
`is measured and controlled with a thermocouple and stain-
`less-steel heating filament. The etching rate of SiO, filmsis
`measured by laser interferometry at a 45° angle with the sur-
`face [Fig. 1(a) ]. The sample is mountedto a transfer assem-
`bly, which insures alignmentof the sample to the beams and
`
`Profile
`
`
`
`*" Photediode
`
`Film
`
`
`Detector
`
`a” a
`~~. f He-Ne Laser
`‘ex
`
`
`Neutral Fiux
`Probe
`
`Ditferential
`Capacitance
`Manometer
`
`Rotary Motion
`Feedthrough
`
`Fic. |. The experimental apparatus. The profile view showsthelaser inter-
`ferometry and line-of-sight mass spectrometer. The beam plane view shows
`the beams and the two flux probes (retarding grid analyzer and pitot tube)
`which can be rotated into the sample position.
`
`

`

`1483
`
`Butterbaugh, Gray, and Sawin: Plasma-surface interactions in FC, etching of SiO,
`
`1463
`
`CF; , with the commonly used ionizer electron energy of 70
`eV. Since CF,’ is a product of the electron impact of HFPO
`and CF, CFO,as well as CF,, at high ionizer energies, it is
`advantageous to choose an electron energy which ionizes the
`CF, radical, but which producesaslittle dissociation as pos-
`sible of higher molecular weight species. Therefore, the ion-
`izer electron energy was held at 20 eV (the minimum for the
`mass spectrometer) for the HFPO pyrolysis characteriza-
`tion. The appearance potential for the process
`
`(2)
`CF, +e” -CF, +2e°
`has been estimated at 13.3 eV.?’ Appearance potentials for
`the dissociative processes are unknown, but are expected to
`be several eV larger.
`Figure 2 is a plot of the ratio of the CF, peak to the CF,
`peak versus temperature. The ratio was plotted to eliminate
`noise due to variations in the flow rate and base pressure
`during the series of readings. The ratio starts increasing at
`about 300 K, which corresponds to the minimum tempera-
`ture at which Knickelbeinet a/.”° could detect CF, radicals
`by LIF. Theratio of CF, to CF, has a sigmoidal shape witha
`knee at about 750 K. This saturation is at a significantly
`higher temperature than that detected by Knickelbein e¢ al.
`(600 K),’° attributable to a different hot-zone residence
`time and to the qualitative nature of the mass spectrometric
`analysis. The pyrolysis dependence on the HFPO flow rate
`was also investigated at 730 K with no significant change in
`the CF, to CF, signal ratio over a range of 0.2-0.6 scom.
`The qualitative results of the mass spectrometric charac-
`terization ofthe pyrolysis ofHFPOin this apparatus indicat-
`ed that pyrolysis was occurring with an efficiency close to
`100%, based on the work of Knickelbein et 2/7 The tem-
`perature of the HFPO pyrolysis tube was held at 740-760 K
`
`provides electrical connection to the thermocouple and
`stainiess-steel heating filament.
`A Faraday cup with retarding grids measures the ion flux
`and energy distribution at the sample position. Withthe
`sample removed, the cup is rotated into place on a rotary
`feedthrough, as shown in Fig. 1(6). The flux of neutral spe-
`cies is measured with a pitot tube that is also moved into the
`sample position on a rotary feedthrough. The pressure dif-
`ferential in the pitot tube is measured with a differential bar-
`atron.
`
`A quadrupole mass spectrometer is mounted on the upper
`flange and can be used for analysis of the beamsorfor analy-
`sis of etching products from the surface of the sample. The
`ionizer is equipped with a line-of-sight shield such that the
`signal from background gas is minimized.
`The chamberis pumped with an 8 in. cryogenic pump and
`the load lock is pumped with a turbomolecular pump. The
`base pressure in the chamberis typically around 5x 107 °
`Torr after 5-10 h of pumping; the chamber is frequently
`broughtup to atmospheric pressure to replace the filaments
`in the ion source. The gases used are research purity
`( > 99.9995 9%) Ar, research purity ( > 99.99%) Xe, techni-
`cal grade ( > 97.0%} F,, and HFPO (98.83%). Gas flow
`rates are set with needle valves, based on pitot tube measure-
`ments. The SiO, samples are 2-u-thick sputtered quartz
`films on Si substrates.
`The Ar? and F sources were previously described in de-
`tail.> A monoenergetic beam of Ar? ions is created with a
`Kaufman ion source with a 1 cm’ nominal beam cross sec-
`tion. The F beam is created by the microwave discharge dis-
`sociation of F, in an alumina tube. The F, is mixed with
`about 2 sccm of Xe to maintain stability of the discharge.
`Mass spectrometric analysis indicates that dissociation of F,
`in this sourceis close to 100%.
`The CF, beam was created from the pyrolysis of hexa-
`fluoropropylene oxide (HFPO)*° in a resistively heated Py-
`rex tube. Knickelbein er ai.7° provided evidence that HFPO
`is completely dissociated at temperatures above 590 K in a
`pyrolysis tube with a residence time of 10 ms according to
`the reaction:
`
`CF,-CF-CF, ~CF,-CF = 0+ CF,.
`\7
`oO
`
`()
`
`
`
`intensityRatioCF3/CF3(ArbitraryUnits)
`
`Operating Region
`
`
`
`
`
`
`
`Knickeibein et a/. monitored the extent of this reaction via
`laser-induced flucrescence (LIF) detection of CF,. Since
`the pyrolysis tube used in this workts of different dimensions
`(straight, 3 mm inner diameter} than that used by Knickel-
`bein ef al. (nozzle geometry), it was important to character-
`ize the cracking of HFPO as a function of tube temperature.
`The pyrolysis of HFPO was characterized by monitoring the
`mass spectrum of the products flowing out of the tube. The
`chamber pressure, due to HFPO flow, was 3x 107° Torr
`(chamberbase pressure was less than 8x 107 ® Torr), which
`corresponded to an HFPO flow rate of about 0.2 sccm and a
`
`
`450 500=5550 650=700 750=8800600
`10 ms residence time in the hot zone of the pyrolysis tube.
`Quantitative mass spectrometric characterization of
`HFPOpyrolysis would require much effort due to its corm-
`plicated cracking pattern, which includes CF +, CF;*, and
`
`
`
`
`
`J. Vac. Sci. Technol. 8, Vat. 9, No. 3, May/Jun 1991
`
`Temperature (K}
`
`Fic, 2. Mass spectrometric intensity ratio ofCF,” toCF,' during the pyro-
`lysis of HFPO. The mass spectrometer ionizer energy was 20 eV.
`
`

`

`1464
`
`Butterbaugh, Gray, and Sawin: Piasma_surfaceinteractions in FC, etching of SiO,
`
`1464
`
`0.6 0.5
`
`
`
`
`
`EtchingYield(Si02/Ar*)
`
`0.3 §
`
`0.2 f
`
`
`0 5 1 6 20 2
`Flux Ratio (CF,/Ar™)
`
`Fic. 4. The etching of SiO, with Ar+ ions in the presence of CF, at ion
`energies of 150, 250, and 350 eV. The error bars indicate estimated uncer-
`tainty in instrumentreadings. Filled symbols indicate baseline sputtering in
`a “dirty” system. Thesolid lines are derived from the two-parameter model
`of Eq. (9).
`
`molecules leaving the surface per incident Ar* ion) is plot-
`ted against the CF, -to-Ar + flux ratio for ion energies of 150,
`250, and 350 eV. The yields with a clean system and no CF,
`flux are in reasonable agreement with previously published
`values.'”'® After the system was exposed to HFPO,the ap-
`parentbaseline physical sputtering rates were higher than in
`a well-cleaned system due to residual fluorocarbon conta-
`mination in the ion gun (filled symbols in Fig. 4). Etching of
`the SiO, was not observed with a CF, flux in the absence of
`ion bombardment. Previous studies have shown that CF,
`adsorbs to some extent on the surface of both virgin and
`sputtered SiO,, but does not etch the SiO, surface’? (see
`discussion above). We saw no indications of polymer film
`deposition on the surface by the CF,. Evidencefor the lack
`of a polymerfilm was that the SiO, etching rate immediately
`returned to its baseline level after the CF, flux was terminat-
`ed.
`It is evident in Fig. 4 that the Ar* etching yield is en-
`hanced bythe flux of CF, on the surface; at an ion energy of
`150 eV, the saturated yield with CF, is over three times the
`physical sputtering yield. The yields tend toward an asymp-
`totic value as the ratio of CF, to Ar* increases; i.e., increas-
`ing amounts of CF, have less effect on the yield. It appears
`that saturation of the yield occurs at lower flux ratios for
`higher energy ions. This last feature is counter-intuitive in
`that one would expect higher energy ions to be moreefficient
`at removing products from the surface, causing saturation to
`occurat higher flux ratios. An explanation of this behavioris
`that the CF, sticking probability increases with ion energy.
`The more damagedsurface, expected with higher energy ion
`bombardment, may offer more adsorptionsites and increase
`
`for all studies in which a CF, flux was used.
`One must also be concerned with the effect of the
`CF, CFO and any unreacted HFPOon the etching reaction.
`In the case of HFPO,the etching of SiO, by Ar* ions was
`monitored in the presence of various flux levels of room-
`temperature HFPO (Fig. 3). The physical sputtering rate
`was unaffected by the impingement of HFPO,so it was rea-
`sonable to assume that the HFPO had a negligible sticking
`probability on SiO,. This was not surprising as Winters”®
`determined that the sticking probability ofmolecular species
`like CF, , CF, H, and CF, Clis less than 10~ 7. It was expect-
`ed that the sticking probability of CF, CFO wasalso negligi-
`ble since it is a stable molecular species. Thus, it was assumed
`that the only species in the pyrolyzed HFPO beam that can
`affect the etching of the SiO, was the CF, radical.
`CF, fluxes of up to 5x 10'*/cm?s were obtainable; at
`higher fluxes back-mixing of fluorocarbon species into the
`Kaufman ion source affected the quality of the data. This
`flux level compares very well with the typical CF, flux strik-
`ing the surface in a CF, plasma. LIF measurements” indi-
`cate that the CF, concentration in a CF, plasmais on the
`order of 10'°/cm', which corresponds to a surface flux of
`about 10'*/cm’ s at 300 K.F fluxes of up to 2 10'’/cm? s
`(corresponding to 1.4 10'/cm? in the plasma environ-
`ment) and Ar* fluxes of up to 3x 10'°/em’s at 250 eV
`(corresponding to 0.5 mA/cm’in the plasma environment)
`were obtainable.
`
`Ill, RESULTS AND DISCUSSION
`
`In the first series ofexperiments, the ion-enhanced etching
`ofSiO, in the presence ofCF, was examined. The results are
`shownin Fig. 4, where the etching yield (the numberof SiO,
`
`0.3 Etching
`
`
`
`Yield(Si02/Ar*)
`
`So
`
`0 605
`
`1 45 2 25
`HFPO Flux (10'icm?~s)
`
`Fic. 3. Dependence of the etching yield on the flux of room-temperature
`HFPO showing that HFPO has negligible interaction with the ion-bom-
`barded surface.
`
`J. Vac. Sci. Technol. B, Vol. 8, No. 3, May/Jun 1991
`
`
`
`

`

`1465
`
`Butterbaugh, Gray, and Sawin: Plasma-surtaceinteractions in FC, etching of SiO,
`
`1465
`
`(SiQo/Ar*)
`
`
`
`EtchingYield
`
`
`
`1
`
`3
`2
`fon Flux (10"7cm?-s)
`
`4
`
`the sticking probability of the CF, radical.
`The CF, radical interaction with SiO, vs Si surfaces (Fig.
`5) illustrates the importance of fluorocarbon radicals in de-
`termining SiO,:Si selectivity. CF, enhances the Ar* sput-
`tering yield of SiO, while suppressing that of Si, due to the
`formation of fluorocarbon surface films. Therefore, the se-
`lectivity of SIO, sputtering to Si sputtering increases with
`increasing CF, flux. This selectivity effect is quantified in
`Fig. 5 for an Ar* ion energy level of 250 eV and substrate
`temperature of 80°C, and is likely to become more pro-
`nounced with decreasing ion energy and substrate tempera-
`ture. Formation of passivating fluorocarbon films onsilicon
`is also enhanced through the use of carbonaceous, rather
`thaninert ions.
`Figure 4 assumesthat the etching yield is dependent only
`on the neutral-to-ion flux ratio and not on the absolute neu-
`tral or ion flux level; surface kinetic modeling presented be-
`low also predicts this dependence. This assumption was in-
`vestigated by varying the fluxes of CF, and Ar? while
`holding the ratio constant (Fig. 6). It was found that any
`dependence of the yield on the absolute flux level is within
`the experimental uncertainty of these data over a 2.5x
`change in absolute flux, validating the use ofthis ratio as a
`dimensionless system parameter. It was also found that the
`yield is independent of the substrate temperature over a
`range of 350-475 K, as expected, since thermal etching of
`SiO, by CF, has not been detected.
`In the next series of experiments, the ion-enhanced etch-
`ing of SiO, in the presence of F was examined. The results of
`these experiments are shown in Fig. 7 in which the etching
`yield is plotted against the F-to-Ar* fiux ratio. At low ion
`energy (150 eV), the yield,in the presence ofF, shows much
`the same dependence on the flux ratio as the yield in the
`presence of CF,. There is an immediate enhancementof the
`
`Fic. 6, Dependence of the etching yield on the absolute flux level at a con-
`stant flux ratio. Circles indicate data taken for F/Ar* ata flux ratio of 7
`and an ion energy of 250 eV. Squares indicate data taken for CF, /Ar* ata
`flux ratio of 3 and anion energy of 250 eV. Theerror bars indicate estimated
`uncertainty in instrumentreadings. |
`
`yield as the F flux is increased from zero; however, this en-
`hancement is almost twice that seen with CF,. The begin-
`ning of saturation is clearly seen for 150 eV ion bombard-
`ment, but experimental limitations precluded the attainment
`ofsufficiently high flux ratios to see the onset of saturation at
`250 and 350 eV. It has been shown that SiO, is etched spon-
`
`(SiO2/Ar™)
`
`
`
`EtchingYield
`
`~ 8@ 400 «120~=~C«*
`o 2 40—
`Flux Ratio (FiArt)
`
`ions in the presence of F at ion
`Fic. 7. The etching of SIO, with Ar‘
`energies of 150, 250, and 350 eV. The error bars indicate estimated uncer-
`tainty in instrument readings. Filled symbols indicate baseline sputtering in
`a “dirty” systern. Thesolid linesare fits to the two-parameter model of Eg.
`(8). Only data with R> 15 were used in fitting the model.
`
`
`
`
`
`
`
`EtchingYield(SV/Ar*orSiQg/Ar*)
`
`(Si02/Si)
`
`Selectivity
`
`Fiux Ratio (CFa/Art;
`
`Fic. 5. Comparison ofthe effect ofCF, on the etching of SiO, and Si. As the
`CF,flux increases,the selectivity ofSiO, over Si increases. Si data are taken
`from Ref. 25.
`
`J. Vac. Sel. Technol. 8, Vol. 9, No. 3, May/Jun 1991
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`Buiterbaugh, Gray, and Sawin: Plasma-—surface interactions in FC, etching of SiO,
`
`:
`
`1466
`
`taneously by F,° but the contribution to the yield at these
`fluxes is iess than 0.001.
`As with CF,, it is found that any dependenceof the yield
`on the absolute flux level ofF or Ar * is within the uncertain-
`ty of the experimental measurements (Fig. 6). The ion-en-
`hanced reaction of F is also independent of substrate tem-
`perature over the range of 350-475 K. The activation energy
`for the spontaneous etching of SiO, by F has been measured
`at about 0.16 eV,° but the contribution of spontaneous etch-
`ing to the ion-enhanced etchingrate is less than 1% at the F
`fluxes used.
`The nextstep in this series of experiments was to examine
`the combined effects of CF, and F fluxes on the etching
`yield. The experimental results are shown in Fig. 8 in the
`form of a three-dimensiona! surface with the yield plotted as
`a function of the CF,-to-Ar? fiux ratio and the F-to-Ar*
`flux ratio. At the F flux ratios studied. here (> 5), the CF,
`hadlittle effect on the yield. It appears that the ion-enhanced
`etching yield of F with SiO, is unaffected by the flux of CF,.
`This is consistent with plasma process observations, that
`SiO, etching in F containing discharges is relatively unaf-
`fected by increases in fluorocarbon radicals.°° These results
`also indicate that CF, is not a limiting reagentto the etching
`of SiO, and is not necessary for the removal of oxygen. This
`is consistent with the experimental work of Donnelly etal.”
`in which SiO, films were etched in both CF, + O, and
`NF, -++ Ar plasmas. Donnelly found that the SiO, etching
`rate correlated with the F concentration and was insensitive
`to the source gas (CF, or NF, ). The work of McFeely ez
`ai.? suggests that adsorbed CF, species may preferentially
`
`
`
`EtchingYield
`
`(SiOa/Ar”)
`
`bond to oxygen at the surface which would keep it from
`interfering with F adsorption on Si adsorption sites. The
`reactivity of CF, species to the surface oxygen probably
`modifies the distribution of O-containing etch products.
`The majordifference between these beam simulation stud-
`ies and a real plasma environmentis that the ions in the real
`plasma are molecular, mainly CF; . Tu et al.'° studied the
`chemical etching of SiO, with Ar? and CF,’ ions in the
`presence of XeF, . They found that the CF,’ ion has an etch-
`ing yield about twice that of Ar*, but that the dependence
`on XeF, flux was the same.It is expected that the trends in
`the yield with CF, and F under Ar * bombardmentwill be
`similar under CF; bombardment, but with a2 enhance-
`ment, putting the measured yields of the beam experiments
`in reasonable agreement with yields measured in our labora-
`tory single-wafer plasma etcher.
`
`iV. MODELING OF [ON-ENHANCED SURFACE
`KINETICS
`
`Comprehensive models ofthe ion-enhanced surface kinet-
`ics described above require complete knowledge of product
`distributions, surface bonding states, and radical sticking co-
`efficients in the presence of energetic ion bombardment. The
`continuous mixing of the surface adsorbates into the near-
`surface bulk region complicates the kinetic treatmentof this
`problem beyond the scopeof classical treatises on gas-solid
`reactions at well-defined surfaces. A rigorous consideration
`of ion-induced surface reaction pathways suggests applica-
`tion of Monte Carlo type models to this problem to capture
`the random natureofthis process,” sinceit is unlikely that a
`simple analytical theory ofchemical sputtering will be found
`to parallel Sigmund’s classical treatment of physical sputter-
`ing.** Nevertheless, a few researchers have attempted to
`model ion-enhanced surface kinetics on the basis of steady-
`state yield data as a function cf neutral-to-ion flux ratios.
`Gerlach-Meyer™ first modeled ion-enhancedetchingofsili-
`con by XeF, and Cl, with noble gas ions of various masses,
`using yield information and neutral-to-ion flux ratios as we
`present here. Although many physical interpretations given
`to this model have since been invalidated, we follow a similar
`mathematical development that reveals great similarity be-
`tween the effect of increasing ion energy and the effect of
`increasing ion mass modeled by Gerlach-Meyer.
`Winters and Coburn** state that the basic steps for a
`chemical etching process paraliel those in many heterogen-
`eous catalysis mechanisms. These steps include (1) adsorp-
`tion ofreactive gas species on the surface, (2) chemical reac-
`tion to produce a product molecule, and (3) desorption of
`the product molecule into the gas phase, liberating a clean
`adsorption site. Depending on the specific etchant-substrate
`system, ion bombardment may enhance (or impede) any or
`all of these steps. At steadystate, all steps proceed at the
`same rate, but one is rate-limiting; our data indicate both
`neutral adsorption limited and ion flux limited etching re-
`gimes as a function of the neutral-to-ion flux ratio and the
`ion energy. The ion-enhanced etching of Si and SiO, by
`XeF, has been studied extensively; several proposed ion-
`enhanced etching models based on these studies have been
`
`Fic. 8. The etching of SiO, with Ar‘ ions in the presence of CF, and F at
`an ion energy of 250 eV. The data points indicate experimental data used to
`constructthis surface.
`
`J. Vac. Sci. Technol. B, Vol. 9, No. 3, May/Jun 1991
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`Butterbaugh, Gray, and Sawin: Pilasma—suriace interactions in FC, etching of SiO,
`
`1467
`
`summarized by Coburn and Winters,*® who present data
`which discredit a number of them. Serious controversy has
`evolved concerning the suggestion that ion bombardment
`enhances the adsorption of fluorine, which is subsequently
`involved in a rapid thermally driven reaction with the da-
`magedlattice.” Winters’’ has shown that XeF, does not
`spontaneously etch an ion-damaged lattice at significantly
`increased rates and that continuous ion mixingofthe surface
`adsorbates into thelattice is necessary to accelerate the reac-
`tion.
`We have constructed a simplified mechanistic model that
`decouples radical adsorption/recombination effects from
`the ion-driven reaction and mixing steps. The exact ion-en-
`hanced chemical pathway is unknown and involves multiple
`surface layers, where SiF, -type species becomefully fluorin-
`ated as the near-surface region is “mixed” by the ballistic
`effect of the ions. We have represented this process with gio-
`bal reaction steps which attempt to capture the correct over-
`all stoichiometry, but should not be taken as elementary sur-
`face-reaction steps. The correct functional dependence of
`the steady-state etching yields on neutral-to-ion flux ratios is
`well represented when the ion-enhanced reaction rates are
`assumed to be first order in ion flux and fluorine adsorbate
`concentration, as shown below.
`In modeling the Art /F etching of SiO, , weinitially con-
`sidered seven potentially important reaction steps (Table E).
`The “*” indicates an unoccupied adsorptionsite in this mod-
`el. We have implicitly assumed that SIF, and O, are the
`primaryproducts res