Silicon etching in Cl2 environment

Applied Surface Science 253 (2006) 1581–1583 www.elsevier.com/locate/apsusc

Silicon etching in Cl2 environment R. Knizikevicˇius * Department of Physics, Kaunas University of Technology, 73 K. Donelaicˇio St., LT-44029 Kaunas, Lithuania Received 6 January 2006; received in revised form 17 February 2006; accepted 20 February 2006 Available online 31 March 2006

Abstract The ion-beam-assisted etching of silicon in Cl2 environment is considered. The theoretically calculated dependences of silicon etching rate on the flux of Cl2 molecules at different ion current densities are compared with experimentally measured. The composition of the adsorbed layer is determined. It is found that SiCl2 molecules prevail in the adsorbed layer. The reciprocal of relative concentration of SiCl2 molecules in the adsorbed layer linearly depends on the ion-to-neutral flux ratio. # 2006 Elsevier B.V. All rights reserved. PACS: 81.65.Cf; 82.20.Nk Keywords: Cl2; Silicon; Ion-beam-assisted etching

1. Introduction The reactive ion etching of silicon in chlorine-based plasmas is widely used in integrated circuit manufacture. Ion bombardment assists in achieving the etching anisotropy due to the synergism of chemical etching and ion-beam etching processes. In the absence of ion bombardment, Cl2 molecules reaching the surface of single-crystal silicon at room temperature saturate it to one monolayer. The probabilities of three scattering channels (two atom adsorption, single atom abstraction and unreactive scattering) for incident Cl2 molecules depend on the surface coverage. Two atom adsorption is the dominant channel in the limit of zero coverage and decays monotonically to zero. Single atom abstraction is the minor channel at low coverage but increases to a maximum at 0.5 ML coverage before decaying to zero. Unreactive scattering is dominant channel at high coverage. These adsorption mechanisms are measured experimentally [1– 3] and calculated theoretically [4]. Surface studies of Si(1 0 0) under these conditions show the stable SiCl configuration. In the presence of ion bombardment, the formation of SiCl2 molecules takes place, and these molecules are the major etching product [5–7]. Time-of-flight distributions indicate that

* Tel.: +370 37 300325; fax: +370 37 350737. E-mail address: [email protected]. 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.040

under 100 eV Ar+ ion bombardment the species are not sputtered by a collision-cascade mechanism. The spectra can be fitted by Maxwell–Boltzmann distributions at a high (>2000 K) temperature. At the incident ion energy of 250 eV, the time-of-flight spectra of the sputtered species change from Maxwell–Boltzmann-like into spectra expected for a collision-cascade mechanism [8,9]. Chemically enhanced physical sputtering becomes prevailing mechanism. Concentration of SiCl2 molecules in the adsorbed layer depends on the ion and neutral fluxes. At constant flux of Cl2 molecules, concentration of SiCl2 molecules at first decreases linearly with the increase of ion current density, later it reaches a minimum value. Concentration decreases faster at lower flux of Cl2 molecules. At constant ion current density, concentration of SiCl2 molecules at first increases linearly with the increase of the flux of Cl2 molecules, later it reaches a maximum value. Concentration increases faster at lower ion current density [5,10]. Concentration of Cl2 molecules in the adsorbed layer is low due to the low desorption activation energy. In this work, Ar+ ion-beam-assisted etching of silicon in a Cl2 environment is considered. The dependences of concentration of SiCl2 molecules in the adsorbed layer on the ion and neutral fluxes are calculated. The obtained theoretical results are compared with experimental measurements. It is found that the reciprocal of relative concentration of SiCl2 molecules in the adsorbed layer linearly depends on the ion-to-neutral flux ratio. Concentration of Cl2 molecules in the adsorbed layer is negligible.

R. Knizikevicˇius / Applied Surface Science 253 (2006) 1581–1583

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2. Model The comparison of theoretically calculated dependences of silicon etching rate on the flux of Cl2 molecules at different ion current densities with experimentally measured [10] is used to evaluate the composition of the adsorbed layer. During the experiment chemical vapour-deposited polycrystalline silicon films were etched in a Cl2 environment in the presence of Ar+ ion bombardment, while surface coverage was measured on single-crystal silicon substrates (1 0 0). Cl2 molecules from the gas phase adsorbed on the surface and reacted with Si atoms: Cl2 ðgÞ ! Cl2 ðadsÞ;

(1.1)

SiðsÞ þ Cl2 ðadsÞ ! SiCl2 ðadsÞ:

(1.2)

The adsorption process is characterized by adsorption constant a = sIn/C, where s is the sticking coefficient, In = n(kT/2pm)1/2 is the flux of Cl2 molecules, n is the concentration of Cl2 molecules in the gas phase, k the Boltzmann constant, T the temperature, m the atomic mass of Cl2 molecules, and C is the concentration of surface atoms (C = 1.36  1019 m2). The reaction process is characterized by reaction rate constant kr = gI0/C, where g is the activation constant and I0 is the ion flux. Reaction defined by Eq. (1.2) is a resultant reaction, which includes the dissociative chemisorption of Cl2 molecules on the surface and formation of SiCl2 as the final reaction product Si(1 0 0) + Cl(ads) + Cl(g) ! Si(1 0 0)  Si + SiCl2(ads) [11,12]. The components of the adsorbed layer desorb and are sputtered by incident ions. Incident ions also sputter Si atoms. The frequency probability of removal of ith component consists of frequency probabilities of desorption vi,d and sputtering vi,s:   Ei;d Yi I0 ; (2) vi ¼ vi;d þ vi;s ¼ n0 exp þ kT C where n0 is the frequency of oscillation of atoms in the solid, Ei,d the desorption activation energy of ith type molecules, and Yi is the sputtering yield of ith component. Let us assume that the adsorbed layer is one-monolayer thickness. Cl2 and SiCl2 molecules exist in the adsorbed layer with relative concentrations c1 = [Cl2]/C and c2 = [SiCl2]/C, respectively. Si atoms exist on the surface with relative concentration c3 = 1. The following system of equations includes rate expressions of processes mentioned earlier and describes the kinetics of component concentrations in the adsorbed layer: dc1 ¼ að1  QÞ  c1 ðkr þ v1 Þ; dt

dc2 ¼ kr c1  v2 c2 ; dt

Fig. 1. Experimental [10] (points) and theoretical (curves) dependences of silicon etching rate on the flux of Cl2 molecules at different ion current densities. The incident ion energy is 600 eV. The etching rate is calculated with error of 2 nm/min.

The etching rate is proportional to the sum of removal rates of formed SiCl2 molecules and Si atoms. According to Eq. (4.2) the etching rate at the steady-state regime is equal to Vst ¼ h0 ðv2 c2;st þ v3 Þ ¼

h0 akr v2 þ h0 v 3 ; akr þ v2 ða þ kr þ v1 Þ

(5)

˚ is the thickness of a monolayer. where h0 = 2.72 A 3. Results and discussion The comparison of theoretically calculated dependences of silicon etching rate on the flux of Cl2 molecules at different ion current densities with experimentally measured [10] is used to evaluate the composition of the adsorbed layer. The experimental and theoretical calculated using Eq. (5) dependences are shown in Fig. 1. The following values of constants are used:

(3)

where Q = c1 + c2 is the surface coverage. The steady-state concentrations of components are equal to av2 c1;st ¼ ; (4.1) akr þ v2 ða þ kr þ v1 Þ

c2;st

akr : ¼ akr þ v2 ða þ kr þ v1 Þ

(4.2)

Fig. 2. Experimental [10] (points) and theoretical (curve) dependences of concentration of SiCl2 molecules on the ion current density at the flux of Cl2 molecules of 1.5  1016 cm2 s1. The incident ion energy is 600 eV. The concentration of SiCl2 molecules is calculated with error of 4%.

R. Knizikevicˇius / Applied Surface Science 253 (2006) 1581–1583

Fig. 3. Experimental [10] (points) and theoretical (curves) dependences of concentration of SiCl2 molecules on the flux of Cl2 molecules at different ion current densities. The incident ion energy is 600 eV. The concentration of SiCl2 molecules is calculated with error of 4%.

v1,d = v2,d = v3,d = 0 s1, Y2 = 1.1  0.2. The sputtering yield of Si atoms Y3 increased from 0.34 at 0.20 mA cm2 to 0.53 at 0.30 mA cm2 due to the contamination of the surface [13]. Terms akr and v1v2 in Eq. (4.2) are of the same order only, therefore it was impossible to determine the sputtering yield of Cl2 molecules Y1, sticking coefficient s, and activation constant g separately. The following value of constant is used A = Y1Y2/ sg = 24  4. The determined value of the sticking coefficient of Cl2 molecules in experiment [10] is equal to s = 0.3. It is important to note that the etching process is completely iondriven. SiCl2 molecules prevail in the adsorbed layer. The concentration of Cl2 molecules does not exceed 1%. The experimental and theoretical calculated using Eq. (4.2) dependences of concentration of SiCl2 molecules on the ion current density at constant flux of Cl2 molecules are shown in Fig. 2. According to the proposed model, the composition of the adsorbed layer does not depend on the sputtering yield of Si atoms. The same results are measured experimentally [10]. The experimental and theoretical calculated using Eq. (4.2) dependences of concentration of SiCl2 molecules on the flux of Cl2 molecules at different ion current densities are shown in Fig. 3. It is observed that concentration of SiCl2 molecules in the adsorbed layer decreases with the increase of ion current density. Eq. (4.2) can be expressed as 1 AI0 ¼1þ : c2 In

(6)

The fitted experimental dependence of the reciprocal of relative concentration of SiCl2 molecules on the ion-to-neutral flux ratio is shown in Fig. 4. The following value of constant is found A = 16.7  0.7. The concentration of SiCl2 molecules is calculated with error of 4%. The worst measurement errors in

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Fig. 4. The fitted experimental dependence of the reciprocal of relative concentration of SiCl2 molecules on the ion-to-neutral flux ratio. Experimental data are the same as in Fig. 3.

the experiment [10] were expected at low ion and neutral fluxes. 4. Conclusions 1. Silicon etching in Cl2 environment is completely ion-driven at the incident ion energy of 600 eV. 2. The reciprocal of relative concentration of SiCl2 molecules in the adsorbed layer linearly depends on the ion-to-neutral flux ratio. The concentration of Cl2 molecules does not exceed 1%. References [1] H.N. Waltenburg, J.T. Yates, Chem. Rev. 95 (1995) 1589. [2] I. Lyubinetsky, Z. Dohna´lek, W.J. Choyke, J.T. Yates, Phys. Rev. B 58 (1998) 7950. [3] M.R. Tate, D. Gosalvez-Blanco, D.P. Pullman, A.A. Tsekouras, Y.L. Li, J.J. Yang, K.B. Laughlin, S.C. Eckman, M.F. Bertino, S.T. Ceyer, J. Chem. Phys. 111 (1999) 3679. [4] M.R. Tate, D.P. Pullman, Y.L. Li, D. Gosalvez-Blanco, A.A. Tsekouras, S.T. Ceyer, J. Chem. Phys. 112 (2000) 5190. [5] J. Dieleman, F.H.M. Sanders, P.C. Zalm, Nucl. Instr. Meth. B 7/8 (1985) 809. [6] G.A. de Wijs, A. de Vita, A. Selloni, Phys. Rev. B 57 (1998) 10021. [7] T. Halicioglu, Surf. Sci. 445 (2000) L53. [8] J. van Zwol, J. van Laar, A.W. Kolfschoten, J. Dieleman, J. Vac. Sci. Technol. B 5 (1987) 1410. [9] D.J. Oostra, A. Haring, R.P. van Ingen, A.E. de Vries, J. Appl. Phys. 64 (1988) 315. [10] R.A. Barker, T.M. Mayer, W.C. Pearson, J. Vac. Sci. Technol. B 1 (1983) 37. [11] T. Halicioglu, D. Srivastava, Surf. Sci. 437 (1999) L773. [12] S.P. Walch, Surf. Sci. 496 (2002) 271. [13] D.C. Gray, H.H. Sawin, J.W. Butterbaugh, J. Vac. Sci. Technol. A 9 (1991) 779.