On mechanisms of argon addition influence on etching rate in chlorine plasma

Thin Solid Films 435 (2003) 232–237

On mechanisms of argon addition influence on etching rate in chlorine plasma A.M. Efremova,b, Dong-Pyo Kima, Chang-Il Kima,* a School of Electrical and Electronic Engineering, Chung-Ang University, 221 Huksuk-Dong, Dongjak-Gu, Seoul 156-756, South Korea Department of Microelectronic Devices and Materials Technology, Ivanovo State University of Chemistry and Technology, F. Engels st., 7, Ivanovo 153460, Russia

b

Abstract Parameters of Cl2 yAr plasma were investigated aimed to understand the mechanism of Ar addition influence on etching rate acceleration. Analysis was carried out on the base of combination of experimental methods and plasma modelling. It was found that the addition of Ar to chlorine under a constant total pressure condition cause changes in plasma electro-physical properties (EEDF, mean electron energy) due to the ‘transparency’ effect. Direct electron impact dissociation of Cl2 molecules was found as the main source of chlorine atoms while the contributions of dissociative attachment and stepwise dissociation involving Ar metastable atoms are negligible. It was supposed that the main reason of etching rate increasing in Cl2 yAr mixture plasma is connected with simultaneous action of Ar on volume chemistry and the heterogeneous stage of etching process. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Chlorine plasma; Electron impact; Rate coefficient; Ionization; Dissociation

1. Introduction Plasma of Cl2 yAr mixtures is widely used in microelectronic technology for dry etching of metals and semiconductors. Primary aims of Ar addition were plasma stabilization in low-pressure region and reducing of chlorine content in wasted gases aimed to pumping equipment defense and environmental protection. Nevertheless, first applications of Cl2 yAr plasma for etching purposes showed that dilution of Cl2 by Ar up to 50– 60% under the constant general pressure conditions not only does not lead to etching rate reduction, but sometimes causes an etching rate increase. As a result, the dependence of the etching rate on Ar content in Cl2 yAr mixture plasma for wide scale of materials (Pt, Cu, Si, GaAs, GaP, GaSb and AlGaAs) is characterized by extreme behavior with a maximum corresponding to 30–70% of argon addition. This effect is stable for observation for various etching systems such as bulk plasma reactors as well as RIE and ICP reactors w1–4x. Although the effect of etching rate increasing in Cl2 y Ar mixtures is known more than 10 years, the reasonable explanation of the mechanism of this phenomena are *Corresponding author. E-mail address: [email protected] (C.-I. Kim).

absent. For the most probable reasons some authors proposed increasing of physical sputtering contribution w1,3x or the appearance of additional channels of chlorine molecules dissociation due to interactions with Ar metastable atoms w2x. Generally speaking, these proposals are rather viable but not confirmed by analysis of plasma chemistry of both volume and heterogeneous. The aim of the present work was the analysis of possible mechanisms of the influence of Cl2 yAr mixture content on the etching rate behavior. 2. Experimental and modeling Experiments were performed in quartz discharge cell with excitation of DC or RF (13.56 MHz) capacitive discharges under such conditions as: input power density 0.1–0.4 Wycm3, total pressure of Cl2 yAr mixture 100 Pa and gas flow rate of 2 sccm. It is well known that in a high-pressure region, when frequency of collisions exceeds frequency of plasma excitation, basic characteristics of DC and RF discharges are very similar under the equal pressure and absorbed power conditions. Neutral gas temperature was measured by the method of two thermocouples of various diameters. Volume densities of neutral ground state particles (chlorine

0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090Ž03.00330-4

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Table 1 Kinetic scheme for charged and neutral particles formation and decay in Cl2yAr plasma Process

N

Scheme

Threshold or rate coefficient

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13

Cl2qe™Clq 2 q2e Clqe™Clqq2e Cl2qe™ClqClqq2e Arqe™Arqq2e y Cl2qe™Cly 2 ™ClqCl Clyqe™Clq2e Cl2qe™Cl*2V ( )qe Cl2qe™Cl*2(B3P)™ClqClqe Cl2qe™Cl*2(21P)qe Cl2qe™Cl*2(21S)qe Clqe™Cl*(4s–5d)qe Arqe™Ar*(1p1, 3p0 –3p2)qe Cl2qe™ClqClqe

11.5 eV 13.5 eV 12.0 eV 15.76 eV 0.00 eV 3.4 eV 0.069 eV 3.4 eV 5.25 eV 8.25 eV 8.9–12.4 eV 11.6–13.6 eV 3.4 eV

R14 R15 R16

y Clq 2 qCl ™ClqClqCl ClqqCly™ClqCl ArqqCly™ArqCl

;5=10y8 cm3 ys ;5=10y8 cm3 ys ;3=10y8 cm3 ys

R17 R18 R19 R20

Cl2qClqCl™Cl2qCl2 ClqClqCl™Cl2qCl ArqClqCl™Cl2qCl Ar*(3p0 –3p2)qCl2™ClqClqAr

;2=10y32 cm6 ys ;3=10y33 cm6 ys ;2=10y33 cm6 ys (2–5)=10y10 cm3 ys

R21 R22 R23 R24 R25

Cl(g)qCl(s)™Cl2(s)™Cl2(g) Clq 2 ™wall Clq™wall Arq™wall e™wall

Electron impact reactions:

Charged volume reactions:

Neutrals volume reactions:

Heterogeneous reactions:

atoms and molecules) were controlled by absorption spectroscopy. Absorption spectroscopy measurements were carried out using a Hg-lamp (333 nm) and an Aglamp with a hollow cathode (324 nm). Working wavelengths were chosen as maximally close to the maximum of light absorption cross-section for Cl2 molecules (330 nm). Electric field strength in DC discharge was measured using double Langmuir probes in compensational scheme while effective electric field for RF discharge was estimated on the base of absorbed power balance equation. Plasma modeling algorithm was represented by zerodimensional (global) plasma model operating with a terms of average plasma parameters. Electron gas subsystem was described by stationary Boltzmann kinetic equation without taking into account electron–electron collisions and second-order impacts (energy transmission from heavy particles to electrons). Boltzmann equation was solved by direct numerical integration technique using the modified Sherman method and the cross-section set from w5,6x. As an output parameter from Boltzmann equation we obtained the electron energy distribution function (EEDF), which allows the calculation of such characteristics of electron gas as

1–100 sy1

mean electron energy and drift rate, reduced diffusion coefficient and mobility, rate coefficient of electron impact processes. Subsystems of charged and neutral particles were described by mass and movement continuity equations w7,8x in quasi-stationary approximation. Kinetic scheme of processes taken into account for plasma modeling is specified in Table 1. 3. Results and discussion For our analysis we selected condition of bulk plasma etching reactor, which showed etching rate increasing effect for such materials as Cu, GaAs and Si w2,4,9x. It is evidently clear that under these conditions etching process is supported only by chemical mechanism while the role of ion bombardment include only the effect of activation of surface chemistry but not physical sputtering of the main material. Preliminary analysis of the mechanisms for etching rate increasing according to the data of w10,11x allow us to select at least three probable reasons. The first reason is connected with the appearance of additional channel of Cl2 molecules dissociation due to the interaction with argon metastable atoms. Such a process is wholly

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234

Fig. 1. Electron energy distribution function: (1) pure chlorine plasma; (2) 50% Cl2y50% Ar.

Fig. 2. Rate coefficient of electron impact processes as function of Cl2yAr mixing ratio: (1) ionization of Cl (R2); (2) dissociation of Cl2 (R13); (3) dissociative attachment (R5); (4) ionization of Ar (R4); (5) ionization of Cl2 (R1); and (6) dissociative ionization (R3).

possible since the average energy of any argon metastable atom Ar(3P0,3P1,3P2 ) 11.8 eV sufficiently more than the Cl2-dissociation threshold. Second mechanism may be caused by an increase of the Cl2 direct electron impact dissociation rate due to the change of plasma electro-physical properties. Two simultaneous channels may support this mechanism. One channel represents the well-known ‘transparency’ effect, which leads to the growth of electrons mean energy and hence to increasing the rate coefficient of electron impact dissociation of chlorine molecules. Other channel is connected with the increase of electron density due to the decrease of dissociative attachment efficiency. Third reason is the influence of Cl2 yAr mixture content on surface reaction probabilities through the intensification of surface bombardment by energetically active species. This influence may be connected with the increase of heterogeneous reaction probability due to the cleaning of surface-active centers from reaction products. As for energetically active species, any species with energies more than the reaction products desorption energy should be taken into account. For Cl2 yAr plasma under the conditions of bulk etching reactor, these species are positive ions, Ar metastable atoms and near-UV photons emitted by Cl2 molecules (256 nm, 307 nm). Fig. 1 represents comparison of electron energy distribution functions (EEDF) corresponding to pure chlorine plasma and to Cl2 yAr mixture with equal content

of both components. The data for Fig. 1 show that addition of Ar leads to EEDF deformation, which causes increases of middle-energy electrons fraction (4–12 eV) but decreases in high-energy electrons fraction. Generally speaking, this effect is rather predictable and called the ‘transparency’ effect. The reason is that argon is characterized by elementary processes with higher threshold energies in comparison with chlorine atoms and molecules. Calculations show also that the ‘transparency’ effect is brighter at argon concentration scopes up to 50%. Therefore, addition of Ar to chlorine up to 50% leads to rapid increasing of mean electron energy while further addition of Ar causes saturation region. Modelling data concerning mean electron energy and electron drift rate are represented by Table 2. Analysis of influence of Ar on plasma electro-physical properties allows to assume that rate coefficients of electron impact processes are sensitive to variations of Cl2 yAr mixing ratio. Nevertheless, this sensitivity should be different for high-threshold and low-threshold reactions. For low-threshold processes (less than 12 eV), addition of Ar leads to increasing of rate coefficient while for high-threshold processes (more than 12 eV) an opposite situation should be obtained. This assumption is confirmed by data in Fig. 2, which represents rate coefficients of electron impact processes as function of Cl2 yAr mixing ratio. Data in Fig. 2 allow to draw conclusions concerning

Table 2 Electrons mean energy and drift rate Ary(Cl2qAr)

0

20

40

60

80

100

N´M, eV Ve, cmys

4.40 1.07=107

4.71 1.77=107

5.28 1.78=107

5.71 1.87=107

5.72 1.92=107

5.70 1.90=107

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mechanisms of active species formation and decay in electron impact processes. First, it is evidently clear that rate coefficients of dissociative attachment (R5) and dissociative ionization (R3) are approximately more than 1 order of magnitude less than the rate coefficient of dissociation of Cl2 molecules by direct electron impact (R13). Such differences are connected with corresponding differences in threshold energies and cross-sections. It means that direct electron impact dissociation may be considered as the main source of chlorine atoms while the influence of reactions R3 and R5 on neutrals particles kinetics is negligible. Note, this conclusion is in good correlation with the results of w10,11x concerning pure chlorine plasma and mixtures of chlorine with molecular gases N2 and O2. Second, the same conclusion may be made concerning the contributions of direct ionization and dissociative ionization to formation of electrons and positive ions. Taking into account sufficient differences between rate coefficients R1, R2, R4 and R5, direct electron impact ionization may be considered as a dominant mechanism of charged particles formation in plasma volume. As for stepwise dissociation mechanism (R20), its maximal contribution to chlorine atoms generation rate in plasma volume may be estimated using ‘upper-side’ evaluation. Assuming that all metastable Ar atoms formed in discharge are deactivated in reaction R20, upper limit of the rate of this process is determined by the rate of metastable atoms excitation (R12). Table 3 illustrates comparison of rate coefficients of direct electron impact dissociation of Cl2 molecules and excitation of Ar (3P0,3P1,3P2). It is evident that rate coefficient of R12 is approximately 2 orders of magnitude less than rate coefficient of R13 due to significant differences in threshold energies and cross-sections. The same differences are typical for the rates of corresponding processes, which are represented in Table 3. Data in Table 3 show that even in frameworks of ‘upper-side’ evaluation, contribution of R20 to total generation rate of chlorine atoms become comparable with direct electron impact dissociation only when Ar content in Cl2 yAr mixture exceeds 90%. Therefore, taking into account that in any real case, Ar metastable atoms have additional several channels of deactivation except R20 w10x, stepwise dissociation of Cl2 molecules seems to be ineffective. Thus, the formation of chlorine atoms in Cl2 yAr plasma under investigated conditions is com-

235

Fig. 3. Density of chlorine atoms and dissociation rate of Cl2 molecules as function of Cl2yAr mixing ratio: points, experiment; line, modeling.

pletely determined by direct electron impact dissociation of Cl2 molecules. Calculations of volume density of Ar metastable atoms using rate coefficients data from w12x shows that the maximal value corresponded to pure Ar plasma does not exceed 2=1011 cmy3. It confirms our conclusion that these particles are not effective in stepwise dissociation of Cl2 molecules. Moreover, this fact allows us to assume that Ar metastable atoms are not effective in heterogeneous processes due to the low value of flux to the etched surface. Fig. 3 illustrates comparison of experimental and modelling results concerning chlorine atom density in plasma volume. Modelling data were obtained on the base of simultaneous solution of balance kinetic and transport equations for chlorine atoms and molecules: RFsRDq

D n nq l2 tres

(1)

where RF and RD: total formation and decay rates in chemical reactions; D: diffusion coefficient; n: volume density; tres: residence time. As for chlorine atoms, total formation rate was assumed as R13 while total decay rate was represented by the sum of R17–R19 and R21. Data in Fig. 3 show good correlation between experimental and modeling results. It is important to note that

Table 3 Comparison of direct and stepwise dissociation of Cl2 molecules Ary(Cl2qAr) 3

k12, cm ys k13, cm3 ys R12, cmy3 sy1 R13, cmy3 sy1

0 – 5.80=10y9 – 3.80=1016

20

40 y10

1.05=10 6.34=10y9 3.76=1014 5.87=1016

60 y11

8.28=10 6.95=10y9 6.48=1014 4.24=1016

80 y11

8.09=10 7.82=10y9 1.10=1015 2.60=1016

100 y11

8.01=10 9.00=10y9 2.11=1015 0.80=1016

7.80=10y11 – 4.72=1015 –

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Fig. 4. Densities of charged particles as function of Cl2yAr mixing ratio: (1) relative density of negative ions; (2) density of positive ions; (3) density of electrons.

both experimental and modeling results show the constancy of chlorine atoms density up to 50–60% of Ar content in Cl2 yAr mixture. This fact may be explained by acceleration of direct electron impact dissociation rate (Fig. 3) due to changes of electro-physical properties of plasma. Similar results and conclusions were obtained in w10x. Analysis of all results mentioned above allow us to conclude that influence of Ar addition on volume kinetics in Cl2 yAr plasma may explain only the constancy of etching rate but not extreme behavior with maximum. Fig. 4 represents data concerning the influence of Cl2 yAr mixing ratio on mass content of charged particles. These data were obtained during self-consistent plasma modelling using balance kinetic and transport equations for electrons, positive ions and negative ions in quasi-stationary approximation: De ne l2

(2)

Dq nq l2

(3)

n0neŽk1yCl2qk2yClqk4yAr.sn0nek5yCl2q

n0neŽk1yCl2qk2yClqk4yAr.sk14nqnyq n0nek5yCl2sk14nqny

right-hand sides represent total rate of decay. In Eq. (3) for the balance for negative ions, we did not take into account the heterogeneous decay of these species. This assumption is connected with the presence of double electric layer and negative charges on the reactor walls under plasma conditions. So we are able to propose that decay mechanism of negative ions is limited only by volume reactions (R14–R16) while for average volume densities of charged particles a quasi-neutrality equation may be applied: nqsnyqne. Fig. 4 shows that increasing the Ar content in Cl2 yAr mixture leads to decreasing of both positive and negative ion volume densities while density of electrons increase. As for positive ions, this effect is caused by decreasing of total ionization rate due to high-threshold ionization of Ar atoms. Effects for electrons and negative ions are connected with the decrease of the rate of dissociative attachment process, which simultaneously, play the roles of single source of negative ions and a channel of electrons decay. Fig. 5 represents the fluxes of positive ions and internal UV irradiation as a function of Cl2 yAr mixing ratio. Fluxes of positive ions were evaluated from diffusion decay rate while flux of UV photons was calculated on the base of excitation rate using the method described in w13x. Data show that total flux of positive ions and UV photons are characterized by similar behavior with a maximum at approximately 20% of Ar addition. At the same time, flux of UV photons remains practically constant up to 40% of Ar addition while flux of Arq ions increase more than 1 order of magnitude in Ar content scopes 10–90%. Therefore, addition of Ar to Cl2 yAr mixture leads to relative intensification of UV irradiation of etched surface and to absolute intensification of Arq ion bombardment.

(4)

where n0: is the total density of neutral particles determined by gas pressure and temperature; ne: is the electron density; nq: is the total density of positive ions q (Clq and Arq); ny: is the density of negative ions 2 , Cl y (Cl ); y: mole fractions of chlorine molecules and chlorine and argon atoms in plasma volume; D: is the diffusion coefficient; k: the rate coefficients of processes specified in Table 1. In each of the Eq. (1)–Eq. (3) left-hand sides represent total rate of formation while

Fig. 5. Fluxes of positive ions and UV photons as function of Cl2yAr mixing ratio: (1) total flux of positive ions; (2) flux of Arq; (3) UV photons.

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This fact, probably forms the second reason for etching rate increasing due to ion and radiation stimulated desorption of reaction products. As it evidently clear from the results analyzed above, both volume chemical and surface effects of Ar addition to Cl2 yAr mixture cannot explain separately the presence of maximum on etching rate dependence on Ar content at 40–60% of Ar. Nevertheless, we assume that simultaneous action of both mechanisms may result on etching rate increasing. In that case, presence of maximum may be caused by the concurrence between increasing of heterogeneous reaction probability due to ion and radiation stimulated desorption of reaction products and decreasing of volume density of chlorine atoms. 4. Conclusion Kinetic parameters and mass content of Cl2 yAr plasma as a function of Ar mixing ratio were investigated aimed to understand mechanism of Ar addition influence on etching rate acceleration. It was found that addition of Ar to chlorine under constant total pressure condition causes changes in plasma electro-physical properties (EEDF, mean electron energy) due to the ‘transparency’ effect. Direct electron impact dissociation of Cl2 molecules was found as the main source of chlorine atoms while the contributions of dissociative attachment and stepwise dissociation involving Ar metastable atoms are negligible. It was supposed that the main reason of etching rate increasing in Cl2 yAr mixture plasma is the

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concurrence between increasing of heterogeneous reaction probability due to ion and radiation stimulated desorption of reaction products and decreasing of volume density of chlorine atoms. References w1x Y.B. Hahn, J.W. Lee, J. Vac. Sci. Technol. B 17 (1999) 334. w2x E.M. Vrublevsky, A.V. Gusev, A.G. Zhidkov, High Energy Chem. 24 (1990) 356. w3x T. Shibano, K. Nakamura, T. Takenaga, J. Vac. Sci. Technol. A 17 (1999) 799. w4x N.L. Ovchinnikov, V.I. Svettsov, A.M. Efremov, Russ. Microelectron. 28 (1999) 16. w5x A.P. Kupriyanovskaya, V.V. Rybkin, J.A. Sokolova, A.N. Trostin, Compilation of Elementary Processes Cross-section Data for Calculations of Kinetic Coefficients Non-equilibrium Sustems, VINITI, Cherkassy, 1990 (in Russian). w6x A.M. Efremov, V.I. Svettsov, High Energy Chem. 29 (1995) 433. w7x S.C. Deshmukh, D.J. Economou, J. Vac. Sci. Technol. B 11 (1993) 206. w8x B. Ramamurthi, D.J. Economou, J. Vac. Sci. Technol. A 20 (2002) 467. w9x A.M. Efremov, V.I. Svettsov, High Energy Chem. 24 (1995) 330. w10x A.M. Efremov, V.I. Svettsov, D.V. Sitanov, High Energy Chem. 32 (1998) 123. w11x A.M. Efremov, K.H. Kwon, J. Semicond. Sci. Technol. 1 (2001) 197. w12x Physical values, in: A. Kikkoin (Ed.), Energoatomizdat, Moscow1991 (in Russian). w13x D.I. Balashov, Yu.V. Kirillov, A.M. Efremov, High Energy Chem. 32 (1998) 267.