MODELING ALUMINUM ETCH CHEMISTRY IN HIGH DENSITY PLASMAS

E. Meekst, P. HoS, R. Buss$ Sandia National Laboratories

tLivermore, California 9455 1-0969 $Albuquerque, New Mexico 87185

We have assembled a chemical reaction mechanism that describes the BCl,/Cl,/Ar plasma etch of Al metallization layers. The reaction set for gas- phase and surface processes was derived either from literature data or estimated from data on related systems. A well-mixed reactor model was used to develop the mechanism and test it against experimental measurements of plasma species and etch-rates in processing reactors. Finally, use of reduced chemistry mechanisms me demonstrated in 2-D simulations for a complex reactor geometry.

INTRODUCTION

In the definition of metal lines for microelectronics circuitry, high plasma-density reactors are used to etch perpendicularly into material stacks containing aluminum or aluminum- alloy layers. Predictable etching of the aluminum conductor is essential to the resulting circuit performance and reliability. Typical gas flows in metal-etch reactors are mixtures of BCl, and C1, with other additives, such as Ar. While it is well known that molecular chlorine chemically etches aluminum, the roles of ion bombardment and boron trichloride radical fluxes on the etch process are less well understood. Modeling of the competing chemical mechanisms is one way to improve our understanding of these processes. Once these models are sufficiently validated against experimental data, they may be used directly in process development and reactor design. In addition, the models may eventually be incorporated into model-based control strategies.

PLASMA MODELS

For our investigation and testing of the chemistry mechanisms that control the plasma etching behavior, we employ a well mixed reactor model, AURORA, that has been described in detail previously (1, 2) . This model assumes the plasma is essentially rate- limited by chemical kinetic processes and not by transport effects. Comparisons of this model with experimental measurements and with 2-D plasma simulation results reveal that the well mixed assumptions are reasonable for simulating low-pressure (tens of millitorr) plasma reactors. The model uses Chemkin and Surface Chemkin software and formalisms for describing the homogeneous and heterogeneous reaction kinetics (3,4).

Our 2-D inductively coupled plasma model, PROTEUS, is based on the inductively coupled plasma transport model, INDUCT (5), but also includes neutral convective flow and diffusion, as well as complex surface chemistry on multiple reactor surfaces. The

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liabili- ty or responsibility for the accuracy, completeness, or usefulness of any information, appa- ratus, product, or process disdased, or represents that its use would not infringe privately owned rights. Reference herein to any specific commeraal product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessar- ily state or reflect those of the United States Government or any agency thereof.

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DISCLAIMER

Portions of this document may be illegible electronic image products. Images are produced from the best available original document.

neutral reacting-flow portion of the simulator is based on the CURRENT model developed by Evans, et al. (6). Simulations using this iteratively coupled 2-D plasma- fI ow model allow investigation of wafer uniformity dependencies on reactor geometry, flow rates, power deposition, and pressure.

GAS-PHASE CHEMISTRY

Both our well mixed and 2-D fluid plasma models require temperature-dependent reaction-rate coefficients for the plasma chemistry. For electron-impact reactions, these rate coefficients are generated by integrating electron-impact cross sections with Maxwellian electron-energy distribution functions. This approach has been described previously (1, 7). The most important gas-phase reactions are electron-impact dissociation and ionization, although vibrational excitation and dissociative attachment also play significant roles in determining the plasma behavior.

For BCl,, we employ the dissociation and vibrational excitation cross sections determined by Nagpal and Garscadden (S), and the ionization cross sections of Jiao, et al. (9). For dissociative attachment, forming C1-, we use the attachment cross section of Buchel’Nikova (10) and neglect the contribution of direct attachment to BCl,.

Using these BCl, cross sections, we find that the gas dissociates easily in high-density- plasma conditions, forming large amounts of BCl (xc3) fragments and chlorine. It was therefore necessary to estimate rates for electron-impact dissociation, ionization, and vibrational excitation of the BCl, fragments. For ionization of BCI, and BCl, we used the formulas provided by Jiao, et al. for BCl, ionization, substituted appropriate ionization thresholds, then scaled down the resulting cross-sections according to molecule size. The ionization potentials for BCl, and BCl were estimated using lowest orbital energies determined by electronic structure calculations (11). Similarly for BCI, and BCI dissociation, we estimate reaction energies from heats of formation, modify BC1, cross sections using these thresholds, and scale the resulting cross sections. For vibrational excitations, we estimate energy losses from calculated vibrational frequencies for BC1 and BCI, (1 1) and use the cross sections of BCl, vibrational excitations that correspond most closely in energy. While these estimates are imperfect, they allow us to investigate the sensitivity of the models to the reactions used and to prioritize the need for cross section measurement and calculation.

For molecular and atomic chlorine, the electron-impact cross section set has been described previously (1, 12). Two modifications of this former set were made, as recommended by Morgan (13): 1) substitution of the C4 ionization cross section of Kurepa and Belic by that of Stevie and Vasile (14), and 2) addition of the C1 metastable formation cross section determined by Griffin, et al. (15) for production of C1*. The 4s excitation of Ganas (16) is retained to provide electron energy loss due to excitation of the doublet state. The argon cross sections are those employed previously for simulating high-density plasma deposition of oxide in SiH,/O,/Ar plasmas (7).

In addition to the electron-impact reactions, we include ion-ion mutual neutralization, some measured and estimated charge-exchange reactions, some radical abstraction reactions, and an estimated argon-metastable-impact chlorine dissociation reaction. The neutralization of C1- with all positive ions is assumed to have the same rate coefficient, in accordance with the formulas provided by Smirnov (17).

SURFACE CHEMISTRY

The Surface-Chemkin formalism allows us to specify a variety of surface species as well as treating adsorption, reaction between surface species, and desorption either as lumped or separate processes. Different sets of reactions can also be included for different materials. This flexibility is needed to model systems that include both surfaces that are actively being etched (i.e. wafers) as well as surfaces that are not being etched (i.e. walls) but still facilitate radical recombination or ion neutralization.

On aluminum surfaces, chlorine is known (1 8-20) to spontaneously react at the surface with high efficiency. In our reaction mechanism, these reactions form AlCl and AlCl, surface species, which undergo further reactions on the surface to form an AlC1, surface species. AlC1, is then desorbed from the surface via either thermal or ion-enhanced evaporation. Under low chlorine flux conditions, the A1 etch rate is limited by the arrival of C1 at the Al surface. At relatively high chlorine pressures, the removal of etch products from the surface via the evaporation of AlCl, becomes rate limiting. The thermal evaporation rate is determined from the vapor pressure. Ion-enhanced evaporation was included in order to match etch rates reported in plasma etching systems (21-23). Although Park (19) and Efremow (20) reported that the addition of ion bombardment did not significantly increase Al etch rates, it appears that these experiments were done under conditions where thermal evaporation adequately removed AlC1, from the surface, so ion bombardment would not effect the AlC1, removal rate. Under typical plasma reactor conditions, it appears that ion-enhanced AlCl, evaporation is needed to explain the observed etch rates.

For non-etching surfaces near the plasma (Le. walls), we include reactions of neutral radicals (Cl, Al, B, BCl and BC1,) at chlorinated and non-chlorinated surface sites with a variety of probabilities, as well as the neutralization of positive ions (Cl+, Cl;, BC1+, BC4' BCl,', and Ar') and the quenching of metastables (Ar* and Cl') with unit probability.

We carried out experimental measurements using freshly deposited aluminum films that confirmed the literature reports that C1, spontaneously etches Al. We also experimentally showed that BCl, does not etch A1 in the absence of plasma excitation, and that neither Cl, nor BCl, spontaneously attacks aluminum oxide.

RESULTS

Hebner, et al. (24) have performed many experimental measurements of negative ion and electron densities in an inductively coupled GEC Reference Cell at Sandia. The experiments provide an opportunity to test and validate the gas-phase plasma mechanisms used in the models under conditions similar to industrial high-density-plasma (HDP) systems. The reactor contains no aluminum wafer and has stainless-steel walls. The aluminum-etch portion of the chemistry was thus omitted in modeling the GEC-cell data.

For the AURORA model, the geometric complexity of the GEC Reference Cell required the definition of two materials with different sets of surface reactions. The first material is defined as the portion of the reactor walls that are in contact with neutral species and radicals. For this material we estimate a surface area of - 2000 cm’, since it contains much of the open space surrounding the inter-electrode region. The second material is defined as the surfaces in direct contact with the ions, electrons, and metastable species. For this we use a surface area of 3 15 cm2, or roughly twice the cathode area of the GEC cell. The plasma volume is taken to be the inter-electrode region of 867 cm3.

Figure 1 shows comparisons of predicted electron and C1- densities with line-integrated measurements taken near the center of the plasma, for 300W input power and 20 mTorr pressure. The plasma power was estimated as 80% of the input power, according to cwrentholtage measurements made by Hebner, et al. (24). Results in Fig. 1 show reasonable quantitative agreement as composition is varied for BCl,/Cl, and CldAr mixtures. The open symbols illustrate the spread of the experimental data taken on different days. The electron density shows little sensitivity to the percent C1, in BCI,, since BCI, rapidly dissociates to form chlorine.

Although the model does well under nominal conditions of power and pressure, we find some disagreement in the predicted and observed trends as conditions are changed away from these nominal values. Figure 2 shows examples of this disagreement, where the measured electron density is much less sensitive to pressure than the model predicts, and the negative ion density does not have the correct trend with power. We have found that the pressure dependence of the electron density depends on the effective surface- recombination probability of atomic chlorine. However, if the surface recombination coefficient is lowered to improve agreement in Fig. 2(b), then the ability of the model to capture the trend of increasing Cl- density with power (Fig. 2(a)) becomes worse. The set of electron-impact cross sections used in these simulations may contain flaws that contribute to these effects. There may also be two-dimensional effects not captured with the AURORA model, which we are currently investigating using the PROTEUS model.

In addition to the GEC reference-cell data, we have compared the plasma chemistry model with mass spectrometry experiments performed on a inductively coupled laboratory plasma. The experiments were run for pure chlorine only and mass-spectrometric measurements provide the ratio of atomic to molecular chlorine in the plasma. As shown

in Figure 3, the model predicts well the dependence of chlorine dissociation as a function of power and pressure. We note that the model results are strongly dependent on the effective surface recombination coefficient used for the quartz walls. For the comparisons shown in Fig. 3, we used a reaction probability for C1 on chlorinated surface sites of 0.1.

To develop and test the etch chemistry mechanism, we have compared model predictions of aluminum etch rate with literature data over a very wide range of conditions. As shown in Fig. 4, etch simulations with the AURORA code agree with experimental data over a wide region of parameter space. The experimental data ranges from molecular beam experiments, modeled as well stirred reactors with very small residence times, to plasma reactors at pressures of 2-2000 mTorr (19-23). For the plasma reactors, ion-enhanced desorption was included in addition to thermal desorption of AlC1,.

The simulation results presented so far, which include a detailed description of the chemistry but a simplified description of species transport in the plasma, allow evaluation of the coupling between plasma and surface reactions during the mechanism development. These well stirred reactor simulations are often limited, however, in their ability to capture plasma behavior in systems where the species gradients are large. The GEC cell is an example where geometric complexity causes difficulties for the well mixed reactor approach. For this reason, we have also performed some preliminary studies of the GEC reference cell using a 2-D simulator. To do these calculations cost-effectively, however, we use reduced reaction mechanisms. As a first approximation, we limited the mechanism to the following species: C1, Cl,, BCl,, BCl,, e-, Cl-, Cl+, CJ+, BCI;, and BCl;, and eliminated any reactions involving other species. For the simulation conditions, we used 300 W input power, 10 sccm total flow, and 20 mTorr pressure, with 66% C1, in BC1,. With the 2-D simulator and the reduced chemistry, we found that the peak plasma density predicted is over 10" /cm3, which is much higher than either the AURORA model predictions with the full chemistry or the experimental data. This is due to BC1, not being allowed to dissociate in the reduced model, which causes an unreasonably high concentration of BCl,' to form in the plasma. Further work is therefore necessary and underway to reduce the chemistry more carefully using sensitivity analysis and the AURORA code.

CONCLUSIONS

Using both a well mixed reactor model and a 2-D plasma-flow model, we have assembled and tested plasma-chemistry and surface-etch mechanisms for BCl,/ClJAr etching of aluminum. Comparisons with experimental data show reasonable quantitative agreement for nominal conditions representative of HDP reactors. In some cases the model was not able to reproduce observed trends as power and pressure were moved away from the nominal conditions. The use of a surface recombination coefficient of 0.1 for atomic chlorine on chlorinated quartz surfaces gave good agreement between model and experiment for chlorine dissociation fractions over a range of powers and pressures. For aluminum etch rates, the model compares well with data available in the literature.

The 2-D plasma-flow simulations overpredict the ion density using a reduced chemistry mechanism, demonstrating the need to be able to handle more complex chemistry in 2-D models and to proceed very carefully in mechanism reduction schemes.

ACKNOWLEDGMENTS

We thank G. A. Hebner and C. B. Fleddermann for access to their experimental data, and thank A. Ting, S. J. Choi, R. Veerasingam, M. E. Riley, and J. Shon for helpful technical discussions. This work was supported by a CRADA with SEMATECH. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.

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