TEOS helicon reactor

JOURNAL OF

ELSEVIER

Journal of Non-Crystalline Solids 216 (1997) 4 8 - 5 4

In situ ellipsometry and infrared analysis of PECVD SiO 2 films deposited in an O2/TEOS helicon reactor C. Vall~e *, A. Goullet, F. Nicolazo, A. Granier, G. Turban Laboratoire des Plasmas et des Couches Minces, Institut des Mat&iaux de Nantes, Universit~ de Nantes, CNRS, 2 rue de la Houssini~re, 44322 Nantes cedex 03, France

Abstract Silicon dioxide thin films have been deposited at room temperature on silicon substrates in oxygen/tetraethoxysilane (O2/TEOS) helicon diffusion plasmas at low pressure (5 mTorr) and 300 W rf power. The properties of the films have been measured by in and ex situ ellipsometry, ex situ infrared spectroscopy, and chemical etching (p-etch) as a function of the TEOS flow rate (QTEoS)" The growth rate (Vd) is determined in situ using an ultra violet-visible phase modulated spectroscopic ellipsometer (1.5 to 5 eV). Two different kinetic regimes appear: at low TEOS flow rate (QTEoS < 5 s c c m ) Vd increases linearly and no carbon species are detected while the OH content rises strongly. For higher values of QTEOS, Vd saturates at ----11 nm. The change in the kinetics corresponds to the appearance of carbon impurities. The increase in the deposition rate is accompanied by a decrease in the refractive index and an increase in the p-etch rate. The Bruggeman effective medium approximation (BEMA) is used to determine the fraction of voids incorporated in the layer. It is shown that porous films incorporate water when exposed to the atmosphere. Based on this result, an explanation is proposed for the insensitivity of the stretching peak of Si-O-Si to the deposition conditions. Good quality SiO 2 films with optical properties close to that of a thermal oxide can be obtained at low deposition rates (Vd < 5 nm/min). © 1997 Elsevier Science B.V.

1. Introduction Silicon dioxide thin films are widely used in the microelectronics industry as interlayer dielectrics [1]. The main advantage of plasma enhanced chemical vapor deposition (PECVD) over other deposition methods such as thermal chemical vapor deposition (CVD) is that films of good quality can be achieved at low substrate temperature. The most often used gas mixtures are S i H 4 / O 2 or S i H 4 / N 2 0 [2,3]. Silane is an explosive gas at room temperature. Although

* Corresponding author. Tel. + 3 3 - 2 40 37 39 54; fax: +33-2 40 37 39 59; e-mail: [email protected].

silane is widely used as a silicon precursor, TEOS provides better conformality and has the advantage of being non-toxic. This study deals with the use of a high density plasma source, a helicon one, to deposit SiO x films from o x y g e n / T E O S mixtures. In situ spectroscopic eUipsometry (SE) is performed, using a spectroscopic phase modulated ellipsometer (SPME) (ISA Jobin Yvon), during the deposition process to measure the refractive index (n) and the layer thickness ( d ) [4]. Ex situ Fourier transform infrared spectroscopy (FTIR) and chemical etching (p-etch) are carried out to obtain information on the structural properties of the films. The experimental aspects of the study are presented in Section 2,

0022-3093/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S 0 0 2 2 - 3 0 9 3 ( 9 7 ) 0 0 1 7 2 - 5

C. ValiSe et aL / Journal of Non-Crystalline Solids 216 (1997) 48-54

whereas the experimental data are presented and analyzed in Section 3. In Section 4 we correlate the quality of the deposited films with the growth rate.

2. Experiment The helicon plasma diffusion reactor has been described in earlier publications [2,5]. Briefly, it is made of a source attached to a diffusion chamber where a (100) silicon substrate is positioned (Fig. 1). This arrangement allows control of the energy of the positive ions impinging onto the surface of the substrate by biasing it. During our experiments, the silicon substrate is grounded and not heated. The plasma is generated in a source made of a glass tube, 15 cm long and 25 cm in diameter, surrounded by a helicon antenna and a coil inducing a static axial magnetic field. The stainless steel diffusion chamber is 30 cm in diameter and length. The oxygen gas is introduced at the top of the helicon source. TEOS is heated at 40°C, and its vapor is injected into the diffusion chamber, close to the wall. The deposition

conditions are: a constant total gas flow rate (oxygen + TEOS) of 37 sccm, which corresponds to a pressure of 5 mTorr before plasma ignition, 300 W rf power (13.56 MHz) and a magnetic field equal to 60 G at the center of the helicon source. In this study, films deposited for TEOS flow rates ranging from 1 to 12 sccm have been analyzed. Since the total gas flow rate is fixed, the corresponding oxygen to TEOS flow rate ratio, denoted R, ranges from 36 to 2. The SPME is mounted on the diffusion chamber. It is operated either in the spectroscopic mode (in the 1.5-5 eV range) or the kinetic mode (at four wavelengths). FTIR spectra of the SiO~ films are obtained with a spectrometer (Nicolet 20 SXC) equipped with a HgCdTe detector. Normalized absorbances were obtained by converting transmission spectra to absorbance and performing a baseline correction. The films were etched in a 3:2:60 HF:HNO3:H20 solution to measure etch rates: the denser the film the slower it is etched. The p-etch rate was determined from the thickness changes, measured by SE, as a function of the etching time.

½ PyrexTube (0= 15cm)

49

"~

HELICON SOURCE optical fiber

Monochromator Yranslatin 9 Substrate HoMer Fig. 1. Experimental setup of the deposition helicon reactor.

C. Vall~e et al. / Journal of Non-Crystalline Solids 216 (1997) 48-54

50

3. Results and data analyzes

140

R=17.6

R=6

I

I

In order to obtain a reliable description of the growth rate and quality of the deposited films, two complementary measurements have been performed: first, real time acquisition during the growth, second, in situ and ex situ spectroscopic measurements. SE provides the ellipsometric ratio p = (tan q S ) e x p ( j A ) where A and ~ are the conventional ellipsometric angles. The three parameters of the film n, k and d (with 1V = n - j k ) are calculated from the measurement of p [6]. 3.1.1. Kinetic mode For a transparent film ( k = 0) deposited on a semi-infinite absorbing substrate, the A = f ( ~ ) curve is cyclic. Thus, by monitoring the real time trajectory, a = f ( ~ ) , it is possible to measure the transparency of the deposited film. Furthermore, the A = f ( ~ ) curve was used to control the thickness of the films, since the thickness of one cycle of the real time trajectory can be determined from the formula:

A

d = A/(2~/n 2 - sin2~b0 ) (where is the wavelength, 4, 0 the angle of incidence) [6]. An example of a A = f ( g t ) real time trajectory is given in Fig. 2. All

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3.1. Ellipsometry

3 0 0

R=2

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30

40

50

60

70

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Psi (degrees) Fig. 2. Real time trajectory A = f ( ~ ) during the growth of SiOx on c-Si recorded at 3 eV for QTEOS= 3.5 sccm (R = 10): simulation: full curve, experimental data: (o): 0< t < 1300 s, (×): 1300 < t < 2600 s. The experimental data are measured each 20 s and the closed circle corresponds to the ellipsometric angles at t = 0. The corresponding growth rate and refractive index are 86 ,~/min and 1.44, respectively.

E

60 40

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i

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TEOS flow rate (sccm) Fig. 3. Variations o f the deposition rate Va ( [ ] ) and normalized p-etch rate ( 0 ) versus the T E O S flow rate. The p-etch rate of the thermal oxide is 1.6 A / r a i n .

the deposited films were transparent except for R = 2 where k is less than 0.01. Fig. 3 shows the evolution of the growth rate determined from the real time trajectory versus the TEOS flow rate (QTEoS)" It can be seen that it first increases linearly with the TEOS flow rate and then saturates at a value around 11 n m / m i n . The change of kinetics appears when QTEOS is > 4.5 sccm, which corresponds to R = 6. Comparable results were recently reported by Deshmukh and Aydil [7,8] under similar deposition conditions. 3.1.2. Spectroscopic mode Spectroscopic measurements have been performed to determine the refractive index, in situ, just after deposition (under vacuum) and ex situ one month later, n is determined by fitting the ellipsometric parameters qt and A, assuming a Lorentz oscillator behavior of the dielectric function of the SiO x layer. The in situ and ex situ indices determined from this model are denoted n~n and nexL. The indices niLn and L at 1.96 eV are plotted versus QTEOS in Fig. 4. nex W e remark: (i) niLn and nex L decrease as QTEOS increases, except for R = 2, (ii) there is some discrepancy between n~n and nexL, (iii) when R = 17.6, e coincide and are very close to the nign and nex refractive index of thermal oxide (nox = 1.456 at L 1.96 eV [9]). The decrease of niLn and nex a s QTEOS is increased indicates the presence of defects such as microporosity. For R = 2, the increase in n is assumed to be due to the incorporation of carbon in the

C. Vail& et a l . / Journal of Non-C~stalline Solids 216 (1997) 48-54 1,5 1,48

R=17.6

R=6

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R=2

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3.2. Infrared absorption spectroscopy

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TEOS flow rate (sccm) Fig. 4. V a r i a t i o n s o f the r e f r a c t i v e i n d i c e s at 1.96 e V ( f r o m s p e c t r o s c o p i c m e a s u r e m e n t s ) : ( ~ ) : n~n, (~1'): nexL, ( O ) : niBn a n d the f r a c t i o n o f v o i d s ( + ) as f u n c t i o n s o f the T E O S f l o w rate. T h e a r r o w indicates the r e f r a c t i v e i n d e x o f t h e r m a l oxide.

L film. The difference observed between n~n and nex can be explained by the presence of voids: a high void fraction favors the incorporation of water when exposed to the atmosphere, this incorporation increase the refractive index. At small TEOS flow rate (R = 36), the values obtained for n are not reliable due to the non-uniformity of the sample, which leads to an overestimate of the refractive index. The Bruggeman effective medium approximation (BEMA) [10], has been used to verify our hypothesis of void incorporation. Each film is assumed to be a homogeneous mixture of voids and amorphous SiO 2. An expression for the BEMA is: E i f i ( g i - (o° ) ) / ( • i + 2 ( e ) ) = 0, where ( e ) is the composite dielectric function and e i and f~ are the dielectric function and volume fraction respectively for the ith component. The index of refraction, n~,, is calculated using the following equation: niBn= f v o i d -k (1 - - f v o i d ) X 1.456, where fvo~d denotes the fraction of voids resulting from Bruggeman analysis of spectroscopic measurements [l 1]. The values of niBn and fvoid, corresponding to in situ measurements, are plotted versus QTEOS in Fig. 4. There is a good agreement ( < 1%) between the values of n obtained using BEMA or the oscillator model, except for R = 2 (film with carbon incorporation) and for R = 36 (no voids incorporated). The enhancement of the void fractions with QTEOS is a consequence of the increasing growth rate, as will be discussed in Section 4.

The nature of the bonding groups in the films has been analyzed by FTIR absorption spectroscopy. Fig. 5 presents two typical spectra recorded for R = 2 and R = 17.6. Both spectra exhibit the three characteristic bands of SiO 2 near 1072, 800 and 450 c m - 1. For R = 17.6, the band at 3650 cm -l is the only additional component observed and is assigned to O - H stretches of isolated SiOH groups [12]. As QTEOS increases, absorption bands appear at 950 and 3450 cm - l , due to S i - O H and O - H vibration of associated SiOH species and water, respectively [12]. The presence of water is confirmed by a weak bending vibration of H 2 0 near 1650 cm -1 [7]. When R reaches 4, carbon species are revealed with C = O absorption at 1700 cm -l [13]. Finally, when R equals 2, C - H bands are detected at 1350, 1420 c m - I (alkyl groups) and 2870, 2930, 2950 cm 1 (ethoxy groups) [8]. There is no evidence for a S i - H absorption band at 2260 cm i [14].

3.2.1. Incorporation of water The water incorporated into the film may have two origins: first, as suggested before, the film takes up water when left in the air; second, the water accumulated on the top of the layer as a reaction product during growth, will exceed a critical value for surface coverage, and then some water will be incorporated into the film. This latter mechanism

R=2

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0.2 ~~ . . . . ~4. . . . i . . . . ~ , , 500 1000 1500 2000

,~ . . . . ~ , , ~ , 2500 3000 3500

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wavenumber (cm-l) Fig. 5. T r a n s m i s s i o n F T I R s p e c t r a f o r R = 17.6 a n d R = 2. Both s a m p l e s are 5 5 0 0 ~, thick.

C. Vall£e et al. / Journal of Non-CrystaUine Solids 216 (1997) 48-54

52

was proposed by Gruska and Wandel [15] in the case of remote PECVD SiO x films deposited at low temperature from N 2 - N 2 0 / S i H 4 mixtures. They demonstrated that the SE data were best fitted by a multi-layer structure involving different amounts of SiO 2 and water. To relate more precisely the properties of the layer to the deposition conditions, the integrated normalized absorbance of OH bands (A(OH) between 3000 and 3800 cm -1) and SiOSi stretching band (~¢(SiOSi) between 950 and 1300 cm -1) have been calculated and plotted in Fig. 6. ~¢'(SiOSi) decreases (less than a factor 2) with increasing TEOS flow rate while ~¢(OH) first increases, reaching a m a x i m u m when R = 4 and then decreases. This effect suggests that the macroscopic density of the film decreases as R increases.

3.2.2. Position of the stretching peak Following K i r k ' s interpretation [16], the wavenumber at maximum absorbance of amorphous (a) SiO 2 is assigned to TO~ mode. It is generally denoted as the frequency of the stretching band (v~) of a-SiO 2. It is known to be sensitive to the structural atomic arrangement of the layer: ~,~ depends on the thickness, the stoichiometry and the density of the SiO x film [16-18]. In this study the thickness of the films analyzed by F U R was fixed at 550 nm to avoid the shifts in v5 due to the thickness dependent effect [17,18]. The stoichiometry of the film, S, defined as the ratio of atomic oxygen to silicon was estimated by energy

3 l0 s

R=17.6 I •

R=5.8 I

R=4 I Q

R=2 I

~-.'~

8 105

9

~ ~ 6105

dispersive spectrometry (EDS). At small TEOS flow rate (R > 6), S is equal to 2 ( + 0 . 1 ) At higher QTEOS (R < 6) suboxide films (SiO x) are obtained: 1 < x < 2. In any case, the stretching frequency is found experimentally to be independent of R and equal to 1072 cm -1, which is less than the frequency measured and predicted for a thermal oxide of same thickness ( = 1090 cm-1). Indeed, even when substoichiometric films are deposited (x = 1), v~ is not shifted towards smaller wavenumbers as already observed [19]. In addition, when near-stoichiometric films are deposited (R > 6), variations of v~ can be expected due to variations in film density [17]. Indeed, in the case of thermal oxide u, shifts towards smaller wavenumbers as it is densified. This effect can be applied to PECVD SiO 2 stoichiometric films to determine the fluctuations in the density of a-SiO 2. In the case of films containing voids the density deduced from IR data (Pa-SiO,) overestimates the macroscopic density (Psio2) as compared to the one deduced from refractive index data, because v~ is not directly sensitive to the content of voids while the measured refractive index of the film, as well as the p-etch, depend on the fraction of voids. Although there is no absolute correlation between void fraction and Pa-SiO~ (consequently vs), an increase in void fraction is not inconsistent with an increase in Pa- sio: [11]. This behavior would shift t,~ towards smaller wavenumbers. This shift is not detected in our case by IR measurements. On the other hand, the presence of water shifts v, towards larger wavenumbers [12,20], so that both effects may cancel each other. This cancellation is consistent with the fact that the presence of water increases with the fraction of voids. We conclude that, in this case, ~'s is not a good indicator of the structural changes occuring in the films.

2 106 []

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2 10 5

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,,I,,,I,,,L,,,L,, 4

6

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TEOS flow rate (sccm) Fig. 6. Integrated absorbance of Si-O-Si stretching peak (0) and OH band ([]) as a function of the TEOS flow rate.

3.2.3. P-etch measurement The chemical etch rates of the deposited films and thermal SiO 2 have been measured. This method is sensitive to the microstructural defects since the etch rate is affected by porosity and density of the oxide. The p-etch rate of the deposited films normalized to the one of a thermal dioxide is plotted as a function of the QTEOS in Fig. 3. The smallest p-etch rate is obtained at small TEOS flow rate (R = 17.6) and is

C. Vallge et al. / Journal of Non-C~stalline Solids 216 (1997) 48-54

equal to three times the p-etch rate of the thermal dioxide, The p-etch rate increases with QTEOS,which is in agreement with the simultaneous increase in the void fraction [2,7,8].

4. Discussion

The deposition rates deduced from ellipsometry data have two different kinetics associated with two ratio ranges (Fig. 3). For large R (R > 6) the deposition rate increases linearly with QTEOS; no carbon species are detected by FFIR measurements and the integrated absorbance of OH band (~¢(OH), 30003800 cm ~) increases with QTEOS' For small R (R < 6) the deposition rate saturates; the change of slope corresponds to the appearance of carbon related peaks in the FTIR spectra while ~¢'(OH) decreases. The increase in the deposition rate with QTEOS is accompanied by an increase in the p-etch rate which varies similarly to the measured void fraction. These two kinetics are discussed below on the basis of a simple deposition scheme. 4.1. Large ratio range (R >__6)

Since the deposition conditions correspond to low pressure ( < 10 mTorr) and low temperature ( < 100°C) the deposition mechanism is supposed to be dominated by mobile surface species, resulting from the TEOS fragmentation. According to the kinetic scheme proposed by Stout and Kushner [21], the precursors contributing mainly to the film growth are -=Si-OR (with R = C2H5). Two reactions are then occuring at the surface of the growing film. First, elimination of carbon from the film results from the oxidation reaction: O + (~Si-OR)f--* ( ~ S i - O H ) f + reaction products.

(1)

Second, silanol species created from Eq. (1) or directly created by the TEOS fragmentation, diffuse on the surface to propagate the SiO 2 network and eliminate water [13]: (~Si-OH)f + (~Si-OH)f ~ (~Si-O-Si~)f + (H20)g.

(2)

53

Since in the large ratio range no carbon is detected in the films, Eq. (1) is always occuring whatever the TEOS flow rate. It means that the oxygen atom concentration in the plasma is large enough to react with all TEOS a n d / o r fragments of TEOS on the surface. This reaction is consistent with the evolution of the growth rate as a function of QTEOS: the linear relationship observed shows that the rate limiting step for the growth rate is the availability of TEOS. When QTEOS is less than 2.5 sccm (R >_ 17) FTIR spectra show that no impurities related bonds are detected except a small signal of the SiO-H band at 3650 cm 1. The refractive indices are close to that of a thermal oxide ( ~ 1%) and no voids are detected from SE measurements. Moreover the p-etch rate obtained in that range is similar to that of most stoichiometric and reliable PECVD deposited oxides. These results indicate that good quality films are deposited at low deposition rate. As QTEOS increases (6 < R < 17) the deposition rate increases and the deposition process is then limited by surface reaction (Eq. (2)). Consequently, SiOH species are buried in the film and their concentration increases with the growth rate, which is in agreement with the decrease in S i - O - S i integrated absorbance. This process is consistent with the presence of voids measured in the film as well as the variations of the p-etch rates.

4.2. Small ratio range (R < 6)

In this range, Fig. 3 shows that the deposition rate saturates at 11 n m / m i n as QTEOS increases. As the total flow rate is kept constant, the 0 2 partial pressure decreases and thus the availability of O atoms probably limits the deposition process [8]. This limitation is confirmed by actinometric studies [5]: as the TEOS flow rate increases (above 2 sccm) the atomic oxygen concentration is reduced. In these conditions, Eq. (1) becomes limited so that ethoxy groups are directly incorporated in the growing film. This incorporation is demonstrated by FTIR results: for R < 4 the presence of carbonyl group ( ~ C = O , which is supposed to be an intermediate reaction product [13]) at 1700 cm ~ is detected and finally, for R _< 2 C - H absorptions are detected. The appearance of carbon impurities is in agreement with the decrease of OH concentration as shown in Fig. 6. Furthermore, the

54

C. Vall~e et al. / Journal of Non-Crystalline Solids 216 (1997) 48-54

presence of carbon in the layer for R = 2 explains the enhancement of the refractive index value.

5. Conclusion For low TEOS flow rate (R > 17), good quality films are obtained at small deposition rate (Vd < 5 nm/min). The deposition rate can be increased provided the process is limited by the availability of TEOS. For small values of oxygen to TEOS flow rate ratio (R < 6) the growth rate saturates. In this

case, the presence of voids in the layer is clearly demonstrated. From in situ and ex situ ellipsometry results, it seems that porous films take up water when exposed to air.

Acknowledgements The authors would like to thank A. Barreau (Centre de MicroCaract~risation de l'Institut des Mat~riaux de Nantes) for the analysis of the films by energy dispersive spectrometry.

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[3] Y. Nishimito, N. Tokumasu, K. Maeda, Jpn. J, Appl. Phys. 34 (1995) 762. [4] B. Drevillon, J. Perrin, R. Marbit, A. Violet, J.L. Dalby, Rev. Sci. Instrum. 53 (1992) 969. [5] F. Nicolazo, A. Goullet, A. Granier, C. Vallee, G. Turban, B. Grolleau, Proc. PSE'96, Garmisch-Partenkirchen, Germany, Sept. 9-13, 1996, p. 322; to appear in Surface Coatings and Technology. [6] R.M.A. Azzam, N.M. Bashara, Ellipsometry and Polarized Light (North-Holland, Amsterdam, 1977). [7] S.C. Deshmukh, E.S. Aydil, J. Vac. Sci. Technol. AI3 (1995) 2355. [8] S.C. Deshmukh, E.S. Aydil. J. Vac. Sci. Technol. B14 (1996) 738. [9] E.D. Palik, Handbook of Optical Constants of Solids (Academic Press, New York, 1985). [10] D.E. Aspnes, A.A. Studna, Appl. Opt. 14 (1975) 220. [11] R.A.B. Devine, J. Electron. Mater. 19 (1990) 1299. [12] J.A. Theil, D.V. Tsu, M.W. Watkins, S.S. Kim, G. Lucovsky, J. Vac. Sci. Technol. A8 (1990) 1374. [13] N. Selamoglu, J.A. Mucha, D.E. Ibbotson, D.L. Flamm, J. Vac. Sci. Technol. B7 (1989) 1345. [14] L. He, Y. Kurata, T. Inokuma, S. Hasegawa, Appl. Phys. Lett. 63 (1993) 162. [15] B. Gruska, K. Wandel, Thin Solid Films 233 (1993) 240. [16] C.T. Kirk, Phys. Rev. B38 (1988) 1255. [17] C. Martinet, R.A.B. Devine, Appl. Phys. Lett. 67 (1995) 2696. [18] R. Ossikovski, B. Drevillon, M. Firon, J. Opt. Soc. Am. AI2 (1995) 1797. [19] P.G. Pai, S.S. Chao, Y. Takagi, G. Lucovsky, J. Vac. Sci. Technol. A4 (1986) 689. [20] I. Montero, L. Galan, O. NajmL J.M. Albello, Phys. Rev. B50 (1994) 4881. [21] P.J. Stout, M.J. Kushner, J. Vac. Sci. Technol. A l l (1993) 2562.