4
Heterogeneous Processes

Introduction

In plasma processing, the term "heterogeneous processes" includes all chemical and physical reactions that occur when a flux (or fluxes) of species is (are) incident on a surface and the interaction of the incident species with the species residing on the surface results in either the gasification of the material or the formation of a new material. It has been pointed out in earlier chapters of this report that surface processes are of central importance in plasma processing. The entire purpose of the technology is to modify a surface, to etch or deposit material, or perhaps to clean or treat the surface in some way. A low-pressure plasma turns out to be an extraordinarily powerful medium within which to effect surface chemical and physical modifications with little impact on bulk material, at low cost, and over large areas. However, plasma-surface interactions are among the most complex and least well understood aspects of plasma processing technology. This chapter presents an introduction to the key issues and the current level of understanding of plasma-surface interactions. Several techniques, experimental and computational, are suggested as most promising in order to advance the state of the database and to improve treatments of plasma-surface interactions in tool scale and feature scale models.

Heterogeneous processes that are of interest to plasma processing include the following:

  1. Adsorption of radicals at specific surfaces;
  2. Reactions to form certain intermediate or stable products;
  3. Desorption (etching) or incorporation into a growing film (deposition) of the products formed under 2 above;
  4. Ion enhancement (or suppression) of the above processes;
  5. Sputtering;
  6. Particle and energy reflection;
  7. Ion implantation and production of defects;
  8. Diffusion effects (on the surface, through the reaction layer, and ion-enhanced diffusion effects);
  9. Redeposition of desorbed products on the sidewalls of structures, the walls of the reactor, and elsewhere;
  10. The mutual interaction of etching and deposition processes that occur in parallel, e.g. in fluorocarbon-based silicon dioxide etching;
  11. Surface roughening;
  12. Electron-reduced desorption; and
  13. Electron emission.

The data necessary to characterize or simulate these processes should be available as a function of the relevant parameters, e.g., temperature, crystallinity and coverage of the substrate, kinetic or internal energy, and angle of incidence of the incident species. In many cases, the functional form relating these relevant parameters and the processes listed above are not known. Synergistic effects between ions and neutrals that strike the surface simultaneously are often essential and also need to be characterized.

State of the Database

Very little exists in terms of organized compilations of heterogeneous process data. This may be explained by the fact that the heterogeneous processes relevant to a plasma process are intimately related to the actual application of the plasma process and thus are highly specific. The actual surfaces that are



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--> 4 Heterogeneous Processes Introduction In plasma processing, the term "heterogeneous processes" includes all chemical and physical reactions that occur when a flux (or fluxes) of species is (are) incident on a surface and the interaction of the incident species with the species residing on the surface results in either the gasification of the material or the formation of a new material. It has been pointed out in earlier chapters of this report that surface processes are of central importance in plasma processing. The entire purpose of the technology is to modify a surface, to etch or deposit material, or perhaps to clean or treat the surface in some way. A low-pressure plasma turns out to be an extraordinarily powerful medium within which to effect surface chemical and physical modifications with little impact on bulk material, at low cost, and over large areas. However, plasma-surface interactions are among the most complex and least well understood aspects of plasma processing technology. This chapter presents an introduction to the key issues and the current level of understanding of plasma-surface interactions. Several techniques, experimental and computational, are suggested as most promising in order to advance the state of the database and to improve treatments of plasma-surface interactions in tool scale and feature scale models. Heterogeneous processes that are of interest to plasma processing include the following: Adsorption of radicals at specific surfaces; Reactions to form certain intermediate or stable products; Desorption (etching) or incorporation into a growing film (deposition) of the products formed under 2 above; Ion enhancement (or suppression) of the above processes; Sputtering; Particle and energy reflection; Ion implantation and production of defects; Diffusion effects (on the surface, through the reaction layer, and ion-enhanced diffusion effects); Redeposition of desorbed products on the sidewalls of structures, the walls of the reactor, and elsewhere; The mutual interaction of etching and deposition processes that occur in parallel, e.g. in fluorocarbon-based silicon dioxide etching; Surface roughening; Electron-reduced desorption; and Electron emission. The data necessary to characterize or simulate these processes should be available as a function of the relevant parameters, e.g., temperature, crystallinity and coverage of the substrate, kinetic or internal energy, and angle of incidence of the incident species. In many cases, the functional form relating these relevant parameters and the processes listed above are not known. Synergistic effects between ions and neutrals that strike the surface simultaneously are often essential and also need to be characterized. State of the Database Very little exists in terms of organized compilations of heterogeneous process data. This may be explained by the fact that the heterogeneous processes relevant to a plasma process are intimately related to the actual application of the plasma process and thus are highly specific. The actual surfaces that are

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--> present in the plasma environment are a result of the response of the initial surface to the incident species profile. It is likely that certain specific surfaces may be produced only in the plasma environment. Because of this characteristic it is not possible to draw on databases that were developed for other technological fields, as is possible in the area of electron impact processes, for example. If information is available, e.g. thermochemical data for solids in the 1985 JANAF thermochemical tables,1 its use may be questionable, at least for direct ion-surface interactions, because of departures from thermodynamic equilibrium due to ion bombardment. An important review of the fluorine-silicon system and to a lesser extent of other systems has been published by Winters and Coburn.2 They present their survey as a summary of "the status of this virtually unexplored field of surface chemistry." The fluorine-silicon model system can serve as a prototype of the complexities encountered in plasma-surface chemistry. Not all information available on the important key applications has been reviewed in a similar fashion. Surface processes related to poly-Si etching have at least been studied by several investigators using C12 and HBr gases.3 For SiO2 etching in high-density fluorocarbon plasmas many studies exist, but little surface chemistry work has been performed.4 The surface processes related to silicon dioxide PECVD have been studied by attenuated-total reflectance IR.5 Techniques for Improving the Database Approach A two-fold experimental approach appears most promising in improving the database in this field. First, detailed measurements on actual plasma processing systems need to be made. Second, controlled investigations of the different surface science aspects of the plasma etching or deposition process need to be made in an ultrahigh-vacuum (UHV) apparatus using well-controlled and well-characterized beams of different plasma species at the relevant energies. These can interact with the substrate one at a time, two at a time, and so on. The measurements performed under each approach are listed in Table 4.1. TABLE 4.1 Goals of a Two-Fold Experimental Approach Including Measurements in Actual Plasma Processing Systems and in Ultrahigh-Vacuum (UHV) Reactive Beams Plasma Studies UHV Beam Studies Measurement of incident species flux decomposition Production of well-characterized and "clean" beams of       Realistic ions and Measurement of loss rates of species     Realistic neutral radicals Characterization of surfaces in situ     With realistic energies Characterization of products Measurement of interactions with pristine and realistic surfaces Characterization of effects associated with three-dimensional structures Determination of       Etching or deposition rates, Verification of technological figures of merit     Reaction probabilities,       Products and their energy content, and       Composition of the surface reaction layer Measurements On Realistic Plasma Reactors Incident Flux and Desorbing Flux Analysis Regarding the incident flux, we would like to know the identities and energies of species incident on the surface; how these species interact with the surface and with each other; the importance of angular effects; scaling with process parameters; and reaction probabilities.

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--> Figure 4.1 (a) Schematic illustration of the IRIS apparatus. Plasma species are directed into a high-vacuum chamber where they impact a surface. The dye laser beam illuminates both incident and reflected species; (b) illustration of the laser-induced fluorescence measurement of both incident and reflected species. (Reprinted, by permission, from E.R. Fisher, P. Ho, W.G. Breiland, and R.J. Buss, J. Phys. Chem. 96:9855 (1992). Copyright © 1992 by the American Chemical Society.) For desorbing species, the issues are the identifies and energies of dominant etching products and other desorbing species; the importance of these species for the composition of the plasma, i.e. "recycling"; and the identity of species that redeposit on the surface or the walls of the reactor. Techniques traditionally used for gas phase diagnostics have been adopted to characterize the interaction of radicals with specific surfaces. An example mentioned in Chapter 3 is the IRIS (imaging of radicals interacting with a surface) technique.6 The principle of the technique is shown in Figure 4.1. In Figure 4.1a, a plasma chamber is shown in contact with a high-vacuum chamber where the substrate is located. Figure 4.1b illustrates how the molecular beam is directed to the surface and is partly reflected. A dye laser is used to excite specific radicals in the incident and reflected beams and the resulting fluorescence is measured. An example of data obtained on the interaction of NH radicals from a NH3 plasma is shown in Figure 4.2. By performing the measurement with and without a surface and taking the difference of the fluorescence data, it is possible to measure the intensity of scattered NH and thus determine the reactivity of NH with that surface. Mass spectrometry, threshold ionization mass spectrometry, and Fourier transform infrared spectroscopy are other important tools for plasma sampling that are very helpful for incident flux analysis. The products and product energy distributions for many of the systems of current interest, e.g. etching of Si3N4 in a downstream plasma, are still unknown. Work is needed to apply the above techniques to obtain information on the products that are evolved from the surfaces. Condition of the Surface The state, or condition, of the surface exposed to a plasma is crucial to a fundamental understanding of heterogeneous processes. Issues of importance include the identity and coverage of species adsorbed or absorbed at the surface; adsorption, diffusion, and reaction mechanisms; the identities of species incorporated into growing films; the role of ion bombardment; and the proper parameter dependence of these quantities, i.e. the dependence on substrate temperature, gas mixture, and so on.

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--> Figure 4.2 Spatial profile of measured fluorescence from an IRIS apparatus using NH radicals impacting and reflecting from a substrate. The "scattered NH" profile is the difference between the signal with the substrate in place ("surface in") and the signal with no substrate ("beam"). (Reprinted, by permission, from E.R. Fisher, P. Ho, W.G. Breiland, and R.J. Buss, J. Phys. Chem. 96:9855 (1992). Copyright © 1992 by the American Chemical Society.) Most conventional surface science techniques cannot be employed because of the relatively high pressure of the glow discharge. However, several in situ real-time diagnostics have been employed in the study of surfaces in contact with glow discharges. These are listed in Chapter 3. In many cases we do not know what the surface reactions are. We measure an overall reaction rate constant for an overall etching reaction. This may be all that is needed in etching. However, in deposition processes, rates of different reactions affect the film composition, and we may need more details on the reaction pathways and reaction rate coefficients for those pathways. Technology Additional diagnostics are needed to identify adsorbates, to describe their bonding to the surface, and to determine their concentrations—and to make similar measurements on resists and the sidewalls. The presence of other materials, such as photoresist in etching applications, is known to alter surface kinetics in some cases. Coupling between surface chemistry and chemical species transport in submicron features of complex three-dimensional shape may be important. For instance, sidewall passivation layers are often important for microelectronic applications. In many cases we do not know the composition of these layers or the mechanism of their formation. The formation rate of these films shows a strong temperature dependence. Examples of measurements on three-dimensional structures have been described.7 Ultrahigh-Vacuum Approach Using Mass and Energy Selected Reactive Beams Winters and Coburn, and the FOM group, did excellent work using this approach8 by employing inert gas ion bombardment and chemical etchants like XeF2 and C12. In the future, the major emphasis should be on the ionic and neutral species that typically interact with surfaces under realistic plasma processing conditions. Particle Beams A common problem in surface studies with beams of neutral and/or ionic species is controlling and/or characterizing beam composition and energy. Development of well characterized and controllable sources that can produce pure radical and ion beams at the relevant energies for use in such studies is an important goal to pursue. A distinction must also be made between neutral and charged energetic species, and it will be interesting to examine the differences in their behavior. It is important to distinguish between those processes that form the adlayer, which involve both neutral and ion species, and those that are important in desorbing the adlayer. There is also recent data in UHV

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--> studies that indicate that reaction probabilities on the surface can be a function of the flux. This may be the case if, for example, the flux is so high that the surface does not get a chance to relax to an equilibrium state. Most UHV experiments are done under low-flux conditions where this effect may not be seen often. Sticking Coefficients The "sticking coefficient" S (or more precisely, the "loss coefficient") is a lumped parameter that describes the loss rate of a certain species on the surface. The sticking coefficient does not say anything about how that species is lost at the surface. There may be many reaction pathways that result in the loss of the incident species. The most important parameter affecting S is probably the coverage, and, in particular, this involves the determination of the coverage that would be encountered in a plasma processing system. However, the effective surface coverage probably depends on the composition and structure of the surface reaction layer. For example, a Si surface may be covered with F atoms alone, or the fluorine may be present as part of a fluorocarbon film layer. The total F coverage (in atoms/cm2) may be the same in the two cases, but it is probable that effective sticking coefficients would differ. Sticking coefficient dependence on temperature is in general unknown, although evidence suggests that in high density plasma reactors etching silicon dioxide, surface temperature can play a key role in polymer deposition. Sticking coefficients also probably depend on the species incident energy. Energetic species may impact surfaces either due to ion bombardment or through the formation of fast neutrals via charge exchange near the surface. For energetic impacting species, additional unknown factors include the angle of impact and the number and composition of atoms in molecular species. In addition, surface roughness (corrugation) effects may enhance trapping of the incident reactive species. The underlying mechanisms can be very complex and are in general not known. For instance, F2 can stick at a crystalline Si surface via either dissociative or abstractive chemisorption. These processes have been shown to depend on incident angle, flux, translational energy, and (for molecular species) rovibrational energy. Pure beams, both atomic and molecular, must be studied as a function of these parameters before the mixed beam effect can be understood. A distinction between thermal and nonthermal reaction pathways must be made here. Surface temperature affects Langmuir-Hinshelwood channels whereas nonthermal channels may not exhibit a temperature dependence.9 Most studies have focused on thermal channels. Synergistic Effects Understanding of synergistic effects, e.g. the ion/neutral synergism or the enhancement of polymer removal by O atoms in the presence of F atoms (a few percent), relates to the essence of "reactive ion etching." The effects of non-ground state species, in particular metastables or electronically excited species, need to be evaluated. For instance, O(1D) atoms appear to be much more reactive than O(3P) atoms (possibly by one or two orders of magnitude). The role of site occupation by other species is unknown. We would like to know if the ions are both reagent and energy source and how that affects the process. Ultraviolet (UV) photons may play a role in promoting surface reactions, either directly by photolytic processes or indirectly by molecular excitation. The nature of the internal excitation in promoting a reaction is not known. Substrate Temperature Dependence Tachi's data on cryogenic etching10 are a good survey of several materials of interest to the microelectronics industry, e.g. Si, SiO2, Si3N4, photoresist, tungsten, tungsten silicide, and aluminum. In these data all effects are convoluted, but they can be used as a basis against which to compare other work. It is not clear whether low-temperature etching results in better feature profile anisotropy because of the reduction of surface reaction rate coefficients or because ions scatter less from cold feature sidewalls,

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--> including possibly frozen sidewall layers. The work of Szabo and Engel on the etching of Si in C12 environments is noteworthy. These authors report surprising phenomena at 77 K and just above that.11 Angle Dependence Winters and Coburn's work12 showed that the angular dependence of the etching rate in plasma etching environments is very small and does not show the characteristic angular peak at about 60° from normal that is observed for physical sputtering. Often, the angular distributions encountered in practical systems are distributed narrowly about the normal. The dependence of etching rate on angle may be secondary compared to other effects. On the other hand, anisotropy requirements are becoming so stringent that even small effects should be considered. Surface corrugation or roughness can mask angular dependencies. The reaction mechanism, especially for nonthermal pathways, may depend strongly on the angle of incidence. On the other hand, Barklund and Blom showed a very pronounced angular dependence of the nitride etching rate in CHF3/O2.13 This was not seen for SiO2. They explained it by the angular dependence of the fluorocarbon passivation layer produced by the CHF3/O2 plasma. Computer Simulations A methodology is required to link the fundamental studies of carefully prepared surfaces to real conditions found in reactors in terms of etching rates, selectivities, anisotropies, etc. This can be done by (a) molecular dynamics (MD) simulations, (b) Monte Carlo simulations, and (c) statistical analysis (multivariate analysis) that relates input or intermediate process conditions to etching rates and other variables. For (c) to be of use the correlation need not be made to the input conditions but to intermediate conditions, such as densities of neutrals and ions, bias voltages that develop, and so on. This would permit sensitivity analysis of different conditions. Molecular dynamics calculations appear useful.14 Currently, they are applicable primarily to the silicon halides because of the availability of relatively good interatomic potential energy surfaces. Application to other systems such as SiO2 and photoresist requires determining realistic potentials for those systems. A large barrier to the use of these techniques may also be the disparity between the time scales used in molecular dynamics (picoseconds) and the time scales associated with neutral and ion fluxes (microseconds to seconds) or product desorption rates. These techniques should be used in conjunction with other techniques borrowed from the vast literature on statistical mechanics (e.g. Metropolis Monte Carlo, lattice gas models, and so on). In principle, combinations of these techniques could be used together with surface diagnostic experiments to shed light on surface reaction pathways and rates. Ultimately, information from the MD calculations could be combined with Monte Carlo or continuum methods for predicting profile evolution. Findings Processes occurring at surfaces exposed to plasmas are, in general, poorly understood. Even the proper variables characterizing the state of the surface (the state variables) are not fully known. Knowledge of the dependence on the state variables of the rates of chemical and physical processes is correspondingly sketchy. Experimental diagnostics and modeling of plasma-surface processes based on first principles are rudimentary and require much development. Surfaces exposed to plasmas are often strongly modified by the intense bombardment by ions, photons, and radicals. Therefore, not only are the chemical and physical processes themselves strongly perturbed by plasma exposure, but in addition, the surfaces on which the processes take place axe unconventional in their structure and composition.

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--> References 1. M.W. Chase, Jr., C.A. Davies, J.R. Downey, Jr., D.J. Frurip, R.A. McDonald, and A.N. Syverud, JANAF Thermochemical Tables, 3rd edn., J. Phys. Chem. Ref. Data, suppl. 1 (1985). 2. H.F. Winters and J.W. Coburn, "Surface Science Aspects of Etching Reactions," Surf. Sci. Rep. 14:161 (1992). 3. K. Ono, T. Oomori, M. Tuda, and K. Namba, J. Vac. Sci. Technol. A 10:1071 (1992); C.C. Cheng, K.V. Guinn, V.M. Donnelly, and I.P. Herman, "In Situ Pulsed Laser-Induced Desorption Studies of the Silicon Chloride Layer During Silicon Etching in High Density Plasmas of C12/O2," J. Vac. Sci. Technol. A 12:2630 (1994); M. Hayerlag, G.S. Oehrlein, and D. Vender, "Sidewall Passivation During the Etching of poly-Si in an Electron-Cyclotron-Resonance-Plasma of HBr," J. Vac. Sci. Technol. B 12:96 (1994). 4. H.F. Winters and J.W. Coburn, "Surface Science Aspects of Etching Reactions," Surf. Sci. Rep. 14:161 (1992); J.W. Butterbaugh, D.C. Gray, and H.H. Sawin, J. Vac. Sci. Technol. B 9:1461 (1991); G.S. Oehrlein, Y. Zhang, D. Vender, and O. Joubert, "Fluorocarbon High Density Plasmas II: Silicon Dioxide and Silicon Etching Using CF4 and CHF3," J. Vac. Sci. Technol. A 12:333 (1994); S. Samukawa and K. Terada, J. Vac. Sci. Technol. B 12:3300 (1994); M.J. Goeckner, M.A. Henderson, J.A. Meyer, and R.A. Breun, J. Vac. Sci. Technol. A 12:3120 (1994). 5. S.M. Hart and E.S. Aydil, "Study of Surface Reactions During Plasma Enhanced Chemical Vapor Deposition of SiO2 from SiH4, O2, and Ar Plasma," J. Vac. Sci. Technol. A 14:2062 (1996). 6. E.R. Fisher, P. Ho, W.G. Breiland, and R.J. Buss, J. Phys. Chem . 96:9855 (1992). 7. G.S. Oehrlein, J.F. Rembetski, and E.H. Payne, J. Vac. Sci. Technol. B 8:1199 (1990). 8. H.F. Winters and J.W. Coburn, "Surface Science Aspects of Etching Reactions," Surf. Sci. Rep. 14:161 (1992). 9. K.P. Giapis, T.A. Moore, and T.K. Minton, "Hyperthermal Neutral Beam Etching," J. Vac. Sci. Technol. A 13:959 (1995). 10. S. Tachi, K. Tsujimoto, and S. Okudaira, "Low-Temperature Reactive Ion Etching and Microwave Plasma Etching of Silicon," Appl. Phys. Lett. 52:616 (1988). 11. A. Szabo and T. Engel, J. Vac. Sci. Technol. A 12:648 (1994). 12. H.F. Winters and J.W. Coburn, "Surface Science Aspects of Etching Reactions," Surf. Sci. Rep. 14:161 (1992). 13. A.M. Barklund and H.O. Blom, J. Vac. Sci. Technol. A 11:1226 (1993). 14. B.J. Garrison, "Molecular Dynamics Simulation of Surface Reactions," Chemical Society Reviews 21:155 (1992); H. Feil, J. Dieleman, and B.J. Garrison, J. Appl. Phys. 74:1303 (1993); M.E. Barone and D.B. Graves, J. Appl. Phys. 77:1263 (1995).

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