microwave plasma deposition of amorphous silicon oxide thin films

J O U R N A L OF

NON-CRYS E ELSEVIER

Journal of Non-Crystalline Solids 187 (1995) 7(~74

Dual-mode radio frequency/microwave plasma deposition of amorphous silicon oxide thin films R. Etemadi*, C. Godet, M. Kildemo, J.E. Bour6e, R. Brenot, B. Dr6villon Laboratoire de Physique des Interfaces et des Couches Minces (UPR 0258 CNRS), Ecole Polytechnique, 91128 Palaiseau cbdex, France

Abstract

A new dual-plasma (surface wave-coupled microwave and capacitively-coupled radio frequency) plasma enhanced chemical vapor deposition reactor for high growth rate deposition of amorphous insulating alloys has been developed. A high degree of flexibility for thin film material synthesis is expected, because the energy of the ion bombardment can be controlled independently of the microwave plasma chemistry. In situ spectroscopic ellipsometry is used for the optimization of the dual-mode plasma deposition of hydrogenated silicon oxides a-SiO x:H (with 0 < x < 2) providing monitoring of the index of refraction and deposition rate. A new procedure for the real-time calculation of both parameters is reported. The growth rate of nearly stoichiometric oxides increases as a function of the oxygen flow rate with a maximum value of 33 As -1 using a 315 W microwave power.

1. Introduction For large area thin film deposition and surface treatment by plasma enhanced chemical vapor deposition (PECVD) techniques, different plasma frequencies are available and the actual choice is dictated by the type of substrate and the material to be grown [1]. The influence of the plasma frequency on the electron energy distribution function (EEDF) has been modelled only for a few rare gases I-2]. The E E D F determines the production of primary species (radicals, ions and excited species), which may then react with other gas phase species or recombine on the walls. The plasma frequency also has a major influence on the energy of positive * Corresponding author. Tel: +33-1 69 33 36 83. Telefax: +331 69 33 30 06.

ion b o m b a r d m e n t which is of the order of a few eV for microwave (MW) and ECR plasmas but reaches tens of eV at the smaller electrode in radio frequency (RF) plasmas. We expect that a single-vessel multi-plasma system can combine the advantages of different plasma frequencies at a manageable level of complexity. In this paper, we describe a dual-frequency (RF at 13.56 M H z and M W at 2.45 GHz) plasma reactor with coupling of the microwave power from a waveguide to a quartz tube. The substrate is located at the end of the tube in the M W postdischarge region. In contrast to some dual mode M W / R F reactors where the substrate is simultaneously exposed to M W and RF plasmas [3], in our case the two plasmas are localized at different places. We will'show that the M W and RF plasmas are not strictly independent and they do interact

0022-3093/95/$09.50 ~ 1995 Elsevier Science B.V. All rights reserved SSD1 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 1 1 4 - X

R. Etemadi et al. / Journal o f Non-Crystalline Sofids 187 (1995) 70 74

in the MW post-discharge region, close to the substrate location, providing a complex system with many experimental parameters. To investigate the interaction of the RF and MW plasmas during the growth, in situ diagnostics have been implemented for the characterization of the gas phase using optical emission spectroscopy (OES), and the film surface using UV-visible spectroscopic ellipsometry (SE). Some characteristic properties of the dualplasma using helium-argon gas mixtures have been reported in previous studies [4,5]. In this study, a low substrate temperature (70°C) was chosen for the deposition of a-SiOx: H thin films as functional coatings on polymers which are unable to sustain temperatures above IO0°C [6]. 2. Experimental

The dual-plasma system has been discussed in detail elsewhere [4,7]. In summary, helium, argon and oxygen are injected at the extremity of a quartz tube. Microwave power is fed to the discharge via an impedance matching network and a field applicator. In the small quartz tube, surface-wave propagation takes place in the direction of the bellshaped termination of the quartz tube. Silane is injected in the MW post-discharge, close to the substrate. The RF plasma is located downstream from the tube between the 200 cm 2 RF-polarized substrate holder and the grounded surfaces (reactor walls and the plane of the silane injector grid). Optical characterizations of the amorphous films have been performed in situ with a UV-visible phasemodulated ellipsometer (Instruments S.A) and ex situ using conventional IR transmission, IR Fourier transform phase modulated ellipsometry (FTPME) [8], and energy-dispersive X-ray analysis (EDX) [9]. 3. Results and discussions

3.1. Growth monitoring by ellipsometry By simulating the (~u, A) ellipsometric real-time trajectory with a fixed refractive index model at 3.8 eV, a homogeneous growth model provides the deposition rate (Rd) and the complex refractive index of the film (Fig. 1). This trajectory is

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a closed loop, making a half tour for a thickness of approximately 2/4 and returning towards the initial values of (7j, A) at approximately 2/2. The procedure described above to calculate Ro and the refractive index, normally performed off-line (after data acquisition) can be replaced by a real-time inversion procedure giving rapid access to the physical properties. An on-line system performing inversion of the ellipsometric equations in real time [10] has been developed. From a single set of measurements (~u, A) at a chosen wavelength we extract two physical properties (refractive index and thickness or deposition rate) in the case of a transparent film or a film with known absorption. Specifically, we use a method developed by Charlot and Maruani [11] for the inversion of the standard equation describing the optical structure and measurement with respect to the refractive index of the film n and its corresponding thickness (Fig. 2). The discrepancies at the beginning of the deposition could be due to noise in the measurement of the first point and possibly to the influence of a thin film of higher void fraction on the silicon of film surface. The measured thickness is interactively used to stop the film growth. Thickness and refractive index are also deduced from the spectrocopic measurement and modelling of the experimental pseudo-dielectric function, assuming a harmonic oscillator behavior [12]. The results are coherent with the on-line method.

RI Etemadi et al. /Journal o f Non-Crystalline Solids 187 (1995) 70-74

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3.2 Film properties Using a constant M W power of 315 W at 80 m T o r r and a H e - A r mixture (300-125 sccm), the influence of the oxygen flow rate and the RF power on the deposition rate and optical properties of a-SiOx: H films has been investigated. F o r a-SiO~: H deposition, a variable flow rate of 0 2 up to 60 sccm is injected with the helium-argon mixture (300-125 sccm) into the M W discharge. Since oxygen and argon have similar ionization potentials and E E D F [2], the M W plasma characteristics are expected to remain relatively unaffected by the 02 flow rate. The silane flow rate is held constant at 6.5 sccm and the oxygen to silane ratio R is varied from 0.08 to 2.3. The radial homogeneity of the oxide deposition rate is rather good. The thickness (measured using step-profiler) is greatest at the center and typically 15% lower at 7 cm from the center. With the M W plasma only, a sharp increase of the deposition rate as a function of R has been measured (Fig. 3(a)), from 1.3 A s - 1 at R = 0.08 up to 5 A s - 1 at R = 2.3. Although the influence of

Fig. 3. Refractive index (continuous line) and deposition rate (dashed line) at a constant 6.5 sccm silane flow rate as a function of (a) the oxygen to silane ratio (R) without R F power, (b) the R F power. Lines are d r a w n as guides for the eye.

excited He (He*) species on the silane decomposition cannot be completely ruled out, this may reveal a Penning reaction of He* with 0 2 and a dissociative reaction of oxygen radicals with silane molecules, following some of the reactions which may produce silyl and silanol radicals [13]: O + SiHa---~SiH3 + OH,

(1)

O + S i H 3 ~ S i H O H + H.

(2)

It is observed that the relative effect of adding a 40 W RF plasma to the M W (315 W) plasma is an increase of the growth rate by a factor of 2, independent of R. In the oxygen depleted regime (R = 0.15) the stoichiometry changes from x ~ 2 to lower (Si-rich) values, which causes an increase of the refractive index, while in the oxygen excess regime

R. Etemadi et al. / Journal o f Non-Crystalline Solids 187 (1995) 70 74

(R = 2.3) the stoichiometry is unaffected (Figs. 3(a) and (b)). The main effect of the RF plasma seems to be the enhancement of the electronic dissociation of Sill4 to produce Sill3 radicals which are transformed to silanol radicals through reaction (2). The influence on the deposition rate of the contribution of the RF and M W plasmas is more an interactive effect than an adding effect (Rd, RF+MW > Rd, RF -~- Ro. Mw) - This behaviour reveals the beneficial effects of the dual-plasma mode in the case of insulating thin film deposition. In this high growth rate regime, Ra saturates as a function of the 0 2 flow rate. As the full incorporation of injected silane would correspond to a 65 ,A s-1 deposition rate, this indicates a depletion of silane and a high sticking coefficient of the silicon-related deposition precursors on the growing a-SiOx: H (x ~ 2) surface. The high growth rate (Rd ~ 33 A s- 1) obtained for silicon oxide films in our reactor compares favorably to other dualplasma systems [3,14] which have demonstrated growth rates of 2 < Ro _< 20 A s- 1, and to an RF plasma reactor 1-15] with Rd reaching 15 As-1. Analysis of IR spectra from either ellipsometric or transmission measurements reveals a decrease of the stretching frequency v of the S i - O bonds as the RF power is increased (Fig. 4). The shift to lower frequency can be attributed to the densification of the film (revealed by the increase of n), induced by positive ion bombardment of the film surface [15].

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73

The position of the stretching frequencies are invariant for the same R, even if the 0 2 and Sill4 flow rates are increased by a factor 2 (at constant R). It is also interesting to note that vR=0.15 < VR= 2.3. For R = 0.15, the presence of Si-Si bonding groups induces a low frequency shift of the Si-O stretching mode [16] in these silicon suboxide films (SiOx, x < 2), which are rich in Si-Si and Si-H bonds. In these conditions, at a high RF power density, it is possible to obtain silicon-rich material with x < 1. In contrast, for R = 2.3, the resulting oxygen-rich films show SiOH bonding between 3300 and 3700 c m - 1, but no Si-Si or S i - H bonds. Complementary stoichiometry measurements by energy dispersive X-ray analysis at low electron voltage (3.6 keV) confirm the high oxygen contents in these films [O]/[Si] ,~ 2.3. This quantitative analysis of [O]/[Si] ratio has been performed by taking into account this ratio measured from the spectrum of silica obtained under similar analysis conditions.

4. Summary and conclusion

A dual-plasma reactor (surface wave-coupled MW and capacitively-coupled RF) with in situ monitoring by ellipsometry has been developed for the deposition of insulating thin films. H e - A r - O 2 mixtures have been used for the microwave plasma, while silane is injected in the M W post-discharge. Both plasmas are not strictly independent because interesting interactive effects are observed in the a-SiOx: H deposition rate. The growth rate of a-SiOx: H increases as a function of the oxygen flow rate and the power injected to the plasma from 1.3 ,~s 1 with the single-RF plasma and low oxygen flow up to 33 ~, s-1 using the double-mode plasma and sufficient oxygen to obtain stoichiometric material. From our results, we deduce: (1) a strong dissociation of silane by oxygen-related species produced in the MW plasma; (2) a high sticking coefficient of the silicon-related deposition precursors; (3) and enhanced dissociation of silane when the RF plasma is added to the MW plasma. From these preliminary results, it can be inferred that this reactor is well adapted for the deposition of insulating thin

74

R. Etemadi et al. /Journal of Non-Crystalline Solids 187 (1995) 70-74

films. F u t u r e studies will a i m at i n c r e a s i n g t h e M W a n d R F p o w e r , p o r o s i t y c h a r a c t e r i z a t i o n , a n d electronic properties assessment. T h e a u t h o r s a r e g r a t e f u l to J. P e r r i n for v a l u a b l e suggestions and discussions concerning plasma physics. T h e y w o u l d like to t h a n k J. H u c , J.Y. P a r e y , G. Rose, J.-C. R o s t a i n g a n d F. C o e u r e t for helpful discussions a n d technical assistance. T h i s p r o ject has been g r a n t e d by the M i n i s t r y for Research.

References [1] M. Moisan, C. Barbeau, R. Claude, C.M. Ferreira, J. Margot, J. Paraszczak, A.B. S~., G. Sauv+ and M.R. Wertheimer, J. Vac. Sci. Technol. B9 (1991) 8. I-2] C.M. Ferreira, L.L. Alves, M. Pinheiro and A.B. S/t, IEEE Trans. Plasma Science 19 (1991) 229. [3] J.M. Klemberg-Sapieha, O.M. KiJttle, L. Martinu and M.R. Wertheimer, Thin Solid Films 193&194 (1990) 965. [4] R. Etemadi, O. Leroy, B. Dr~villon and C. Godet, Mater. Res. Soc. Symp. Proc., San Francisco, Apr. 1994, p. 109.

[5] R. Etemadi, C. Godet, O. Leroy, P. Morin and B. Dr6vilIon, in: Proc 12th Photovoltaic Solar Energy Conferences, Amsterdam, Apr. 1994, p. 342. [6] J.C. Rostaing, F. Coeuret, B. Dr6villon, R. Etemadi, C. Godet, J. Hue, J.Y. Parey and V.A. Yakovlev, Thin Solid Films 236 (1993) 58. [7] R. Etemadi et al., submitted to J. Vac. Sci. Technol. A. [8] A. Canillas, E. Pascual and B. Dr6villon, Rev. Sci. Instrum. 64 (1993) 2153. [9] J.C. Russ, Fundamentals of Energy-dispersive X-ray Analysis (Butterworths, Boston, MA, 1984). [-10] I. Fan Wu, J.B. Dottellis and M. Dagenais, J. Vac. Sci. Technol. A11 (1993) 2398. [11] D. Chariot and A. Maruani, Appl. Opt. 24 (20) (Oct 1953) p. 3368. [12] B. Dr6villon, Prog. Cryst. Growth Charact. Mater 27 (1993) 1. [13] M.J. Kushner, J. Appl. Phys. 74 (1993) 6538. [14] K.A. Buckle, K. Pastor, C. Constantine and D. Johnson, J. Vac. Sci. Technol. B10 (1992) 1133. [15] C. Charles, G. Giroult-Matlakowski, R.W. Boswell, A. Goullet, G. Turban and C. Cardinaud, J. Vac. Sci. Technol. A11 (1993) 2954. [16] G. Lucovsky, D.V.Tsu, S.S. Kim, R.J. Markunas and G.G. Fountain, Appl. Surf. Sci. 39 (1989) 33.