Reactive ion etching of Si by Cl and Cl2 ions

Reactive ion etching of Si by Cl and Cl2 ions: Molecular dynamics simulations with comparisons to experiment D. E. Hanson, J. D. Kress, and A. F. Voter Citation: Journal of Vacuum Science & Technology A 17, 1510 (1999); doi: 10.1116/1.581844 View online: http://dx.doi.org/10.1116/1.581844 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/17/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Molecular dynamics simulations of Cl + etching on a Si(100) surface J. Appl. Phys. 107, 113305 (2010); 10.1063/1.3361038 Molecular dynamics simulations of Si etching with energetic F + : Sensitivity of results to the interatomic potential J. Appl. Phys. 88, 3734 (2000); 10.1063/1.1288701 Study of the impact of the time-delay effect on the critical dimension of a tungsten silicide/polysilicon gate after reactive ion etching J. Vac. Sci. Technol. A 18, 1173 (2000); 10.1116/1.582320 An interatomic potential for reactive ion etching of Si by Cl ions J. Chem. Phys. 110, 5983 (1999); 10.1063/1.478499 Molecular dynamics simulation of reactive ion etching of Si by energetic Cl ions J. Appl. Phys. 82, 3552 (1997); 10.1063/1.365674

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Reactive ion etching of Si by Cl and Cl2 ions: Molecular dynamics simulations with comparisons to experiment D. E. Hanson,a) J. D. Kress, and A. F. Voter Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545

~Received 14 October 1998; accepted 18 January 1999! We present results of molecular dynamics simulations of reactive ion etching ~RIE! of a reconstructed Si~100!~231! surface. The existing Stillinger–Weber interatomic potential for Si/Cl of Feil et al. has been modified by correcting the Si–Si bond strength for a SiCln moiety bound to a Si surface and the Si–Cl bond strength in SiClm molecules. This potential has been used to study RIE of Si by Cl and Cl2 ions. The calculated properties such as the Si yield, product stoichiometry, stoichiometry of the chlorosilyl surface, and Cl content of the chlorosilyl layer are in reasonable agreement with experiment. The dissociative chemisorption probability of Cl2 on Si~100!~231! as a function of energy has been simulated and the results are in reasonable agreement with experiment. © 1999 American Vacuum Society. @S0734-2101~99!06104-7#

I. INTRODUCTION Reactive ion etching ~RIE! of silicon and other surfaces is an important material processing technique that is widely employed by the semiconductor industry1,2 in the fabrication of integrated circuits. Since quantitative experimental characterization of the process is difficult, molecular dynamics ~MD! simulations have been used3–9 to predict important performance parameters, such as the etch rate or yield, the stoichiometry of the desorbed material, and the equilibrium halogen content of the surface layer. Such results, suitably parametrized, can be used as fundamental input to device feature scale topography simulations.10 MD simulation of sputtering was pioneered by Harrison,11 with simulations of Ar1 sputtering of Si presented by Clary and co-workers12 and by Schoolcraft et al.13 Stillinger and Weber ~SW!14–17 developed classical two- and three-body interaction potentials for the Si/F system. The etching of Si by 3 eV F atoms9 and by thermal F atoms5 have been simulated by MD. Feil et al.3 determined the appropriate parameter values to describe Si/Cl systems with a SW potential. Barone and Graves have performed MD simulations of various processes: RIE of Si/F by Ar1, 6 the direct etching of Si by F1, 7 and by Cl1, 7,18 and RIE of Si/Cl by Ar1. 18 They reported a number of properties, including the Si yield and the Cl content of the chlorosilyl layer. A deficiency of the SW/Feil ~SW/F! potential for Si/Cl1 systems is the constant strength of the Si–Si bond ~2.17 eV!, independent of the type and number of neighboring bonded atoms. We believe that this is especially important in the study of RIE of Si by halogen atoms because a candidate desorption molecule is bound to the surface by a Si–Si bond. Another shortcoming of the SW/F potential is the constant Si–Cl bond strength, independent of the number of Cl bonds to the Si atom. We know from ab initio calculations19 and experiment20 that the bond energy is not constant due to the rehybridization of the s and p orbitals on the Si atom. The discrepancy between the calculated and observed atomization energy is greatest for a!

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SiCl2, where the predicted energy is higher than experiment by 0.73 eV. A proper description of this effect is important to ensure the correct population distribution of SiCln molecules bound to the surface during a RIE simulation. We have modified the SW/F potential to correct these deficiencies21 by: ~1! adding an embedding term to correct the Si–Cl bond energy for rehybridization and ~2! adding a four-body term to account for the effect of neighboring Cl atoms on the strength of the Si–Si bond. The embedding term is similar to that used in the embedded atom method ~EAM!22 potentials, and the four-body term is similar to the BOC potential introduced by Brenner23 to account for p bonding in hydrocarbon molecules. In this article, we present results of MD simulations using this new potential for Cl1 and Cl1 2 etching of a Si~001!~231! surface. II. SIMULATION METHOD The details of our MD simulation methods are given elsewhere.24 The simulations started with a 12312 38 (x,y,z) atom cell ~1152 total Si atoms!, periodic in the two dimensions ~x,y! parallel to the surface, with an exposed ~001!~231! face on which the Cl atoms impacted at normal incidence. 1 monolayer ~ML! of ion flux is equal to 72 atoms. The topmost layer contained 72 atoms ~1 ML! and had a surface area of 1061 Å2. The bottom two layers of the cell, 144 atoms, were rigidly fixed at all times. The initial temperature of the cell was 300 K. Impact ions, assumed to be neutralized by a fast Auger process25 before interacting with the surface, were directed normal to the ~001! surface at the desired energy at randomly chosen x,y coordinates. In this article, all references to ‘‘impact ion’’ are to be interpreted as a neutral atom. To simulate the neutral background flux consistent with some experiments, we also launched three or more thermal-energy (T5300 K) Cl atoms towards the surface after each energetic impact atom. The equations of motion were numerically integrated using the leapfrog Verlet method26 for a total of 0.5 ps for each impact and the simulation cell was checked for desorbed moieties at regular intervals. For the first 40 fs of each impact event, when in-

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Hanson, Kress, and Voter: Reactive ion etching of Si by Cl and Cl2 ions

FIG. 1. Representative distribution of desorption event times ~every 0.025 ps! from MD simulations of 50 eV Cl1 impacts ~taken at an ion exposure of 5 ML!. The total integration time for each impact of 0.5 ps was not sufficient to allow the desorption algorithm to detect all of the desorbing moieties.

dividual atom velocities were highest, a time step of 0.2 fs was used; for the remaining 460 fs, the time step was 0.8 fs. We chose the total integration time of 0.5 ps based on the transit time for a ‘‘shock wave’’ to travel from the top surface of the simulation cell to the fixed bottom layer and back.24 Although we have seen no evidence in our MD simulations to suggest that a reflected shock wave influences the desorption process, we thought it prudent to eliminate or reduce such extraneous complications at the outset. Figure 1 shows a representative histogram of desorption times after impact for a 50 eV Cl impact event. The distribution peaks at about 0.2 ps and appears to extend somewhat beyond our integration time limit. From visual inspections of impact/ desorption events, we believe that desorption generally occurs promptly ~,200 fs! after impact while the impact energy is localized. The long delays seen in the histogram are due to an artifact of the desorption algorithm which requires a candidate desorption molecule to have no neighbor atoms. For desorbing molecules with very low velocities, this can take several hundred fs to achieve. This does not introduce appreciable error into our simulations, since the initial state for subsequent ion impacts used the final state of the previous impact, thus allowing most of the desorbing molecules, not yet detected by the desorption algorithm, to be correctly counted. Because each energetic ion impact event raised the temperature of the cell, the cell was returned to ;300 K between impacts by employing a Berendsen thermostat27 with a coupling constant of 2.031014 s21 and integrating for 100 fs. No thermostat was in effect during the 0.5 ps after each impact. III. RESULTS AND DISCUSSION The cumulative Si yield as a function of Cl ion fluence up to 13 ML of 25, 50, and 100 eV ions at normal impact is shown in Fig. 2. We chose this fluence limit based on experimental evidence28 which suggests that the RIE process achieves steady state within ;12 ML of ion fluence. These

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FIG. 2. Cumulative Si yield vs ion fluence in monolayers ~ML! for Cl1 RIE of Si~001!~231!. For 25 and 50 eV impacts, three background Cl atoms at thermal (T5300 K) were directed toward the surface for each ion. For 100 eV impacts, six background Cl atoms were used.

experiments ~laser induced fluorescence measurements in a helical resonator etch reactor! showed that the surface concentration of SiCl reached equilibrium in less than the time between laser pulses, 200 ms, which corresponded to about 12 ML of ion fluence. The 50 eV yield may be compared to a recent experimental value29,30 of 0.39 at 55 eV. The average of the simulation over the last 12 ML of ion fluence is 0.3960.17. To match the conditions in this experiment, each ion impact event should have been followed for the ;0.25 ms time interval between impacts, a task well beyond our computational capability. However, we did check for weakly bound SiCln surface moieties at the end of each impact and removed any that were bound to the surface by less than 0.3 eV, as suggested by Barone and Graves.7 The occurrence of such weakly bound species was quite rare and they did not significantly influence the calculated yields. We also simulated etching of Si~001!~231! by Cl1 2 at 50 eV ~25 eV for each Cl atom! obtaining a yield of 0.51 which is within the statistical uncertainty of our result for Cl1. Several measurements have been reported to which this can be compared. Balooch et al.31 performed molecular beam experiments with a saturating background flux of neutral Cl2 and obtained yields of 0.2 and 0.4 at 40 and 60 eV, respectively. Cheng et al.32 performed in situ measurements of the Si etch rate in a helical resonator reactor over a range of background pressures that included neutral: ion ratios of ;2–200. From the measured current density, they computed a yield for 50 eV Cl1 2 of 0.38. The value we obtain from MD simulations is in reasonable agreement with both of these experiments. The stoichiometry of the desorbed molecules, for impact energies of 50 and 100 eV, is shown in Fig. 3. At both energies, SiCl2 is the predominate species. Szabo et al.33 have studied the product stoichiometry resulting from thermal etching of Si~001! by Cl2 using thermal desorption spectroscopy. They reported seeing predominantly SiCl4 at temperatures below ;500 K and exclusively SiCl2 above 500 K. Oostra et al.34 recorded the product stoichiometry for Ar1

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Hanson, Kress, and Voter: Reactive ion etching of Si by Cl and Cl2 ions

FIG. 3. Average product stoichiometry for MD simulation of Cl1 RIE of Si~001!~231! at 50 eV ~dark bars! and 100 eV ~light bars!. The category ‘‘other’’ was comprised principally of Si2Cl, Si2Cl2, Si3Cl, and Si3Cl2.

etching of Si~100! in the presence of Cl2. Since the SiClx cracking fractions were not determined in their experiment, it is difficult to draw quantitative conclusions about the relative product stoichiometry. For 75 eV ions, they observed peaks for all of the SiCln (n51 – 4) molecules, with SiCl1 being the highest, presumably originating from the cracking of SiCl2. The prevalence of SiCl2 as the major desorption product in our simulations is consistent with these experimental results. For 50 eV Cl ion bombardment of Si~100!~231!, we find that the relative surface concentrations for SiCl, SiCl2, and SiCl3 normalized to SiCl are 1.0:0.29:0.03, respectively, with uncertainties of about 610%. These results may be compared with experimental data from x-ray photoelectron spectroscopy ~XPS! measurements taken for 40 eV Cl ion etching in a plasma reactor35 of: 1.0:0.34:0.087. The simulation results are in good agreement with this experiment. A previous MD simulation7 reported values of 1.0:0.14:0.008.

FIG. 4. Cl content of the chlorosilyl layer in monolayers ~ML! as a function of ion energy: ~open triangles! from the MD simulations of Cl1 RIE of Si~001!~231! at 25, 50, and 100 eV; ~solid triangles! experimental data from Layadi et al. ~Ref. 35!.

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FIG. 5. Probability for Cl2 dissociative chemisorption on a pristine Si~001!~231! surface: ~open triangles! MD simulations, ~solid triangles! experimental data ~Ref. 36!.

The Cl content of the surface, as predicted by the MD simulations at 25, 50, and 100 eV, is shown in Fig. 4 with the experimental data of Layadi et al.35 As discussed above, the surface Cl is predominately in the form of SiCl with about 30% occurring as SiCl2. For each species, the Si atom makes bonds with other surface and subsurface Si atoms. The density of Cl atoms in the chlorosilyl layer is determined mostly by the total surface area available, i.e., the surface roughness. A consequence of the relatively small simulation cell is that surface features ~peaks and valleys! can grow to sizes comparable to the dimensions of the periodic boundaries well before steady state roughness is achieved. Once a feature has propagated across the simulation cell, it becomes periodic, i.e., infinitely long. We believe this is the reason that the Cl content continues to increase after the yield has achieved steady state. Nevertheless, the predicted values at 13 ML of ion fluence are in reasonably good agreement with the reactor experiments. Cl2 is known to undergo dissociative chemisorption when it impacts a pristine Si surface. Sullivan et al.36 performed ultrahigh vacuum ~UHV! measurements of the dissociative sticking probability of Cl2 on Si~111! and Si~001!~231! using the method of King and Wells.37 We have simulated this process and find that our results are in reasonable agreement with the experiment, as shown in Fig. 5. For these simulations, the orientation of the incident Cl2 molecule was averaged over all possible angles at zero rotational energy. Our analysis of the process indicates that the orientation of the Cl2 molecule with respect to the surface is crucial; the Cl2 dissociates only if the Cl–Cl bond is approximately normal to the surface at the instant of impact, due to an energy barrier imposed by the three-body terms in the potential. In contrast, ab initio studies of the dissociation of Cl2 on Si~111!38 suggest that the most likely precursor state is actually for the axis of the Cl2 molecule to be parallel to the surface. The Si yield has been shown to depend strongly on the ratio R of the background flux ~either neutral Cl atoms or Cl2! to the ion flux. Levinson et al.39 have observed changes in the Si yield for values of R up to 100 for etching by Ar1

J. Vac. Sci. Technol. A, Vol. 17, No. 4, Jul/Aug 1999

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ion beams in the presence of Cl or Cl2, and Chang and Sawin29 report similar results for Cl1/~Cl, Cl2!. At 50 eV, the yield increases sharply to 1 for values of R up to 50, and then more slowly to ;3 for R of several hundred, never reaching saturation. The MD simulations do not exhibit this behavior. For both Ar1 and Cl1, with a Cl neutral flux, the predicted yield saturates roughly when the total flux of Cl atoms, both ions and neutrals, is sufficient to provide the Cl for the product molecules. Since the product is principally SiCl2, this is approximately two times the yield. The saturated yield at 50 eV, as discussed above, is 0.3960.17, so the Cl ion alone, without any background Cl flux, is sufficient to satisfy the stoichiometric requirements. IV. CONCLUSIONS Using a modified form of the SW potential, we have performed MD simulations of RIE of a Si~001!~231! surface for Cl and Cl2 ions, and for the dissociative chemisorption of Cl2. The computed etch yields, product and surface stoichiometry, and dissociation probability were compared with representative experimental data. The agreement between simulation results and experimental data for these properties is noteworthy and encouraging. The MD results, suitably parameterized, should be useful as fundamental input to feature scale models for topographic simulation of integrated circuits. ACKNOWLEDGMENTS The authors gratefully acknowledge useful discussions with Josh Levinson, Eric Shaqfeh, Jeff Hay, and Steve Valone. This work was supported by a Cooperative Research and Development Agreement ~CRADA! ‘‘Center for Semiconductor Modeling and Simulation’’ between the U.S. Department of Energy ~DOE! and the Semiconductor Research Corporation. The work at Los Alamos National Laboratory was carried out under the auspices of the U.S. Department of Energy under Contract No. W-7405-ENG-36. D. B. Graves, Science 22, 31 ~1994!. H. Winters and J. Coburn, Surf. Sci. Rep. , 163 ~1992!. 3 H. Feil, J. Dieleman, and B. Garrison, J. Appl. Phys. 74, 1303 ~1993!. 4 P. Weakliem and E. Carter, J. Chem. Phys. 98, 737 ~1993!. 5 P. Weakliem, C. Wu, and E. Carter, Phys. Rev. Lett. 69, 200 ~1992!. 1 2

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