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Study of TiO2 film growth mechanisms in low-pressure plasma by in situ real-time spectroscopic ellipsometry
Thin Solid Films 447 – 448 (2004) 40–45
Study of TiO2 film growth mechanisms in low-pressure plasma by in situ real-time spectroscopic ellipsometry A. Amassian, P. Desjardins, L. Martinu* Groupe de recherche en physique et technologie des couches minces (GCM), and Department of Engineering Physics, Ecole Polytechnique, P.O. Box 6079, Station Centre-Ville, Montreal, Quebec, Canada H3C 3A7
Abstract We investigate the initial stages of growth of TiO2 films prepared by plasma-enhanced chemical vapor deposition on plasma pre-oxidized c-Si, using in situ real-time spectroscopic ellipsometry. The optical properties of TiO2 films were parameterized from 245 to 1000 nm using the Tauc–Lorentz oscillator and the effective medium approximation. For thin films grown at low substrate ˚ temperature (Tss75 8C) and deposition rate (0.3-r-3.5 Ays), effective refractive index (n550 at ls550 nm) stabilization, ˚ independent of r. When compared at final consistent with island coalescence, was observed for thickness, d, of only 12–15 A, ˚ all films possessed similar optical properties in the visible (n550 s2.30–2.33), with most significant thickness (d;85 A), ˚ grown at rs3.5 Ays ˚ differences in n(l) and k(l) observed in the interband region (l-385 nm). A thicker film (ds700 A) ˚ An additional n550 increase in the order of 0.03 observed for 85-dwas found to exhibit similar growth behavior for d-85 A. ˚ was attributed to refractive index inhomogeneity. 280 A, 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Optical coatings; Plasma-enhanced chemical vapor deposition; In situ ellipsometry; Titanium dioxide
1. Introduction The fabrication of optical interference filters requires accurate control of the deposition process in order to avoid departures from the design. This is especially important for some numerical synthesis techniques such as Flip-Flop w1x, Needle w2x and others w3x, which often ˚ and incorporate ultra-thin films (thickness, d(100 A) thicker, non-quarter wave layers, none of which allow for error compensation w4x. Of particular importance in the latter case is the elimination of refractive index n inhomogeneities as a function of depth z, often encountered in high index materials. TiO2 is frequently used as the high index material in optical filter applications. It is known to exhibit numerous problems due to its susceptibility to form inhomogeneities, n(z) w5x. In optical coatings, TiO2 is usually desired in its amorphous phase w6x, but anatase and rutile TiO2 have also been exploited w6x. TiO2 films have been traditionally grown by electron-beam evapo*Corresponding author. Tel.: q1-514-340-4099; fax: q1-514-3403218. E-mail address: [email protected] (L. Martinu).
ration w7x, sputtering w8x and ion beam assisted deposition w9x. In our laboratory, we particularly focus on amorphous TiO2 (a-TiO2) films prepared by plasma-enhanced chemical vapor deposition (PECVD), and we study the effects of growth conditions on their optical properties w10x. The advantages of PECVD in optical filter fabrication have recently been discussed in a review article w11x. This technique allows one to control over a wide range the optical and mechanical properties in films with compositional w12x or microstructural w13x gradients, and nano-composites w14x. As an example, control of the stoichiometric ratio in TiO2 ySiO2 mixtures is used for the fabrication of graded index (rugate) filters w15x. In multilayer optical filters, optical waveguides and other applications, n inhomogeneities have to be eliminated, as these strongly deteriorate their performance w16x. Such inhomogeneities are believed to strongly depend on film microstructure w17,18x, which is itself determined by the growth mechanisms w19x. Dynamic single-wavelength ellipsometry (SWE) has been used as a non-invasive and non-destructive tool to study the growth kinetics of optically absorbing films,
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.09.019
A. Amassian et al. / Thin Solid Films 447 – 448 (2004) 40–45
such as a-Si:H w20,21x, a-Ge:H w26x, microcrystalline Si w22x, and poly-Si w23x. However, SWE has rather limited ˚ transparent films applicability for very thin (d-100 A) since UV spectral features are usually not accessible w7,24x. In the past decade, in situ real-time spectroscopic ellipsometry (RTSE) has been used for amorphous semiconductor growth studies w25,26x, process monitoring and control of epitaxial semiconductors w27x, multilayers w12,28x, as well as rugate w15x optical filters. RTSE allows one to determine the effective n value and d with sub-monolayer precision w25x even for very thin dielectric films, and can therefore be used to study the initial growth stages of transparent films w29x. However, there are no reports of successful correlation between growth mechanisms and refractive index inhomogeneities in films used in optical filters. In the present work we use RTSE for the study of the initial growth of a-TiO2 films prepared by PECVD onto well-characterized pre-oxidized silicon substrate. We measure the evolution of the optical properties for ˚ and relate it to the film growth mechanisms. d-85 A, 2. Experimental methodology Film growth was performed in a capacitively coupled radio-frequency (RF, 13.56 MHz) plasma system described in detail elsewhere w30x. A negative DC bias, VB, develops on the RF-powered substrate holder (10 cm in diameter) as a function of the RF power and working pressure. Standard c-Si N1 1 1M substrates were cleaned in a Piranha solution (H2SO4:H2O:H2O2s4:1:1) for 5 min, rinsed in DI water, blown-dried with N2, and immediately inserted in the vacuum chamber, which was then pumped down to -10y5 Torr using a turbomolecular pump. As the first step, the substrate surface was plasma pretreated, i.e. plasma cleaned and oxidized. O2 was introduced in the reactor at a flowrate of 60 sccm, and a working pressure of 20 mTorr was adjusted. The plasma was initially ignited at a power level corresponding to VBsy600 V, and it was reduced to VBsy450 V after 3 min in order to set the TiO2 growth conditions and to stabilize the substrate temperature at Tss75 8C as measured by ellipsometry w31,32x (see below). All c-Si structural modifications occur during the first 3 min of pre-treatment, at higher ion energy than TiO2 growth. As the second step, TiCl4 was introduced into the reactor, without stopping the plasma, using a computercontrolled flowmeter, while the O2 flow, the total pressure (20 mTorr) and VBsy450 V were kept constant. We evaluated the effect of deposition rate, r, controlled by the TiCl4 flowrate, on the TiO2 film growth mechanisms. Both the plasma pretreatment and TiO2 deposition processes were monitored in situ using a spectroscopic rotating compensator ellipsometer w33x (RCE, model M-
41
Fig. 1. Imaginary part of the pseudo-dielectric function, N´2M, plotted vs. time for two sample wavelengths, showing the sequence of process steps: O2 plasma pretreatment (Part I A–B) and TiO2 deposition (Part II). Part I: A—plasma switched on in O2 at VBsy600 V; B—plasma power reduced to VBsy450 V; Part II—TiCl4 introduced giving rise to TiO2 film growth.
2000FI, J.A. Woollam Co., Inc.), mounted at an angle of incidence near 658. Its spectra spanned over 477 wavelengths from 245 to 1000 nm, and could be acquired in parallel at a rate of up to 20 Hz. The RCE is arguably the most adequate configuration for the study of optical coatings on transparent substrates, mainly because it can measure the ellipsometric angles C and D over their entire range (i.e. 08–908 for C and 08–3608 for D) without loss of sensitivity. This is essential for transparent substrates in the visible and near infrared regions, for which D is either 08 or 1808. The choice of the acquisition rate is dictated by many factors: (i) the dynamic study of near surface effects by RTSE requires good signal-to-noise ratio, as well as (ii) minimum systematic errors, and (iii) adequate time resolution. A good compromise was to perform analyzer two-zone averaging, while using 500 ms integration time at each zone. This resulted in an interval close to 2 s between two successive spectra. Analysis of the ellipsometric data was performed using EASE and WVASE32 softwares (J.A. Woollam Co., Inc.). 3. Results and discussion 3.1. Process steps and dynamic ellipsometry data In order to clearly describe the various process steps, we have plotted, in Fig. 1, the time-evolution of dynamic ellipsometry data during the pretreatment and TiO2 growth experiments. The imaginary part N´2M of the pseudo-dielectric function N´MsN´1 yi´2 M is shown for
A. Amassian et al. / Thin Solid Films 447 – 448 (2004) 40–45
42
sequence indicated in Fig. 1 has the following advantages: (i) TiO2 film growth is initiated on a well-characterized and ‘stabilized’ SiO2 substrate surface (in this respect, the effects of ion bombardment in the initial moments of thin film growth by PECVD would be already accounted for); (ii) the substrate is never taken out of the chamber; (iii) it is possible to determine Ts and adjust the fabrication conditions so that Ts is constant during TiO2 growth; (iv) sample-to-sample reproducibility of the pretreatment effects are exceptionally good (Section 3.2). 3.2. Ellipsometric model for the pretreated substrate (Part I) Fig. 2. Schematic representation of the optical model of c-Si pretreated in O2 plasma (Part I) and followed by TiO2 growth (Part II). TL: top layer; BL: bottom layer.
two distinct wavelengths, l1s400 nm and l2s350 nm, as a function of time in Fig. 1. N´M, obtained by direct inversion of ellipsometric angles C and D w34x, has the advantage of being independent of the angle of incidence. Fig. 1 shows the sensitivity of RTSE to the pretreatment-induced surface modifications (Part I A–B) and to the TiO2 film growth (Part II), as illustrated by important variations in N´2M. However, different wavelengths possess different degrees of sensitivity to surface modifications. This makes the latter uniquely identifiable in RTSE analysis, compared with SWE w29,35x. The two wavelengths, l1 and l2, were chosen in a way that the optical constants of c-Si at l1 are sensitive to Ts, while they are insensitive at l2s350 nm. This enables to discriminate variations in RTSE data due to surface structural modification and those arising from reversible changes, such as Ts cycling. Process steps resulting in RTSE spectral changes may be clearly identified (Fig. 1). The changes in N´2M due to plasma exposure (Fig. 1, Part I) demonstrate that c-Si surface modifications impact the RTSE signal as much as those due to TiO2 growth. This makes studying and accurately modeling c-Si modifications critical pre-requisites to the successful analysis of TiO2 growth by ellipsometry. The
An optical model describing the thin film system formed on c-Si is shown in Fig. 2. Part I represents the structure that yielded the best fit to RTSE data following c-Si pretreatment: it essentially forms a ‘well-defined’ substrate suitable for the study of the TiO2 growth process, completing Part II. Part I consists of the c-Si substrate and two-layers assigned as top layer (TL) and bottom layer (BL), with respective thicknesses dTL and dBL. The TL is SiO2, while the BL is best modeled by the Bruggeman effective medium approximation composed of c-Si, a-Si and SiO2 with respective volume fractions pc-Si, pa-Si and pSiO2. The assumption that the optical constants of c-Si are Ts-dependent allowed us to model temperatureinduced changes in the ellipsometric spectra and to obtain instantaneous Ts values by fitting the RTSE data throughout Part I. Once Ts was stabilized in B (Part I), it was fixed along with the other fit parameters from Part I, and assumed to remain unchanged during TiO2 growth (Part II). In our RTSE analysis, we first determined the substrate’s initial conditions in terms of the thickness of native oxide, dnative, and Ts (Table 1, ‘Initial’). Upon plasma exposure, we continuously fitted the parameters describing the evolution of dTL, dBL, pc-Si, pa-Si, pSiO2 and Ts. In Part I-A (VBsy600 V, see Fig. 1), dTL and dBL reached 90% of their steady-state values after approximately 45 and 10 s, respectively (Table 1, ‘Part I’). Meanwhile, structural transformation (i.e. amorphi-
Table 1 Optical fit parameters before and after O2 pretreatment of c-Si for three consecutive experiments Sample
1 2 3
Initial
Part I
˚ dnative (A) "0.2
Ts (8C) "2
TL ˚ dTL (A) "0.8
BL ˚ dBL (A) "2
pa-Si (%) "6
pc-Si (%) "6
pSiO2 (%) "2
24.0 24.7 23.6
33 37 37
59.9 60.7 59.6
33 37 34
53 54 56
20 19 20
27 28 25
Stabilization Ts (8C) "2 72 75 74
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zation and oxidation) of c-Si during the first 5 s of exposure resulted in pc-Si, pa-Si and pSiO2 changing from their initial 99, 0 and 1% values to 20, 55 and 25%, respectively. This proportion did not change significantly with further exposure or with a change of VB to y450 V (Fig. 1, Part I-B). It took 240 s for Ts to reach 103 8C at VBsy600 V, while it dropped to 75 8C for VBsy450 V (Table 1). The two-layer model was necessary, because energetic ion-bombardment (;300–700 eV) w36x by oxidizing species from the plasma (Oq and Oq 2 ) results in both a significant damage of the Si crystal lattice w37–39x, as well as oxide formation due to the affinity of Si and O atoms. Such energetic ions penetrate several nanometers into c-Si and amorphize the structure through successive collisions with the crystalline network, displacing Si atoms, and eventually bonding with neighboring Si. As a result of collisions, a partially amorphous and oxidized interfacial layer is formed between the TL and c-Si, identified as the BL. As oxidation occurs (TL), the interface (BL) progresses into the substrate. (In fact, there is a gradient, but the RTSE data are well modeled by a two-layer model.) RTSE analysis of Part I resulted in the final values summarized in Table 1. Very good agreement was obtained for oxidation of three nominally identical samples at VBsy600 V (Table 1). This confirms an excellent reproducibility for both the fabrication and characterization of Part I. This structure was then used as a reference platform for the study of TiO2 film growth presented in the next section. 3.3. Initial growth stages of TiO2 films (Part II) After having determined the characteristics of the substrate exposed to O2 plasma pre-treatment (Table 1, ‘After Part I’), we now concentrate on the growth of TiO2 (Fig. 1, Part II). In modeling the RTSE data, the TiO2 film was assumed to be a homogeneous mixture of bulk-like a-TiO2 and voids. This model is based on the assumption that thin films in their initial growth process, such as nucleation, island growth and coalescence, can be optically represented as a mixture of dense material and void w21,24,25x. The analysis can be carried out without making any assumption on the geometry of TiO2 clusters andyor pores, since the feature sizes are much smaller than l. In order to determine the optical constants of TiO2, we first analyzed thicker films (Fig. 3), i.e. films at the end of the growth experiment. The optical constants of TiO2 were parameterized using the Tauc–Lorentz oscillator (TLO) w40x, which is known to adequately reproduce the line-shape of optical constants of amorphous materials in the interband and the transparent regions. For l-385 nm, the optical constants of a-TiO2 are expected to exhibit strong
43
Fig. 3. Dispersion curves, n(l) and k(l), for TiO2 films with ˚ and;700 A. ˚ d;85 A
spectral features in n(l) and k(l), where n and k increase dramatically to n;3–3.6 (ls315 nm) and k;1–1.7 (ls265 nm), and adopt a bell shaped form. The presence of such features in both n and k of the film and the substrate over the probed ellipsometric spectrum are essential to the accurate and precise computation of both n and d from RTSE data for thin films ˚ in their initial growth stages (typically 0-d-120 A). Thus, extending the RTSE spectrum sufficiently below the TiO2 optical gap enables to decorrelate n and d, and to significantly enhance the accuracy of optical thin film ˚ The transparent part of the metrology for d-100 A. spectrum does not contribute very much to this decorrelation, but it yields useful optical constants in the visible and near infrared. The complex refractive index, N(l)sn(l)yik(l) of the growing film was computed by a linear combination of TLO parameterized optical constants, NTLO(l), with a void fraction, pvoid (ns1), as N(l)s(1ypvoid)NTLO(l)qpvoid
(1)
The TiO2 thickness d and n550 (n(l) at ls550 nm, ks0) are plotted as a function of deposition time in Fig. 4 for three values of r, namely 0.3, 1.4 and ˚ 3.3 Ays. Fig. 4a indicates that d increases nearly linearly with time, except during the initial 10 s of growth. This behavior is attributed to the TiCl4 flow reaching steady state (Fig. 1, Part II), which should not be an issue in standard PECVD deposition, where plasma ignition follows flow stabilization. The time evolution of n550 is more complex; at first, it increases rapidly from n550;1.1 to 2.1, with a ratedependent slope (Fig. 4b); then, n550 slowly tends towards saturation (;2.3). The error bars represent one standard deviation; they are determined from the quality of the fit, including parameter cross-correlation and signal-to-noise ratio.
44
A. Amassian et al. / Thin Solid Films 447 – 448 (2004) 40–45
Fig. 4. Evolution of (a) d vs. time and (b) n550 vs. time for three ˚ (dots and circles), rs1.4 Ays ˚ (narrow deposition rates: rs0.3 Ays ˚ (solid line and cross). Tss75 8C dots and diamonds), and rs3.3 Ays in all three experiments.
Since n550 converged to the same value at similar final thickness, we conclude that n550 depends on d rather than on time, and that it must be controlled by the film microstructural evolution. In order to point out this relationship, we plotted n550 as a function of d in Fig. 5. This dependence appears identical for all r values, and one can distinguish three regimes, each of which can be related to different growth stages: I: 0˚ rapid increase of n550, compatible with nucled-15 A; ˚ ation and 3D island growth w27x; II: 15-d-50 A; slower increase of n550 towards an asymptotic value, in line with optical effects of densification through island coalescence and pore filling w25x; and finally, III: ˚ n550 remains almost constant with d, suggestd)50 A; ing that TiO2 already forms a continuous layer w7,26x.
The average refractive index (at film center, zsdy2) was 2.365, and was very similar to the effective index ˚ n550s reported in the previous paragraph for d)280 A, 2.36. Since n550 simply represents the effective refractive index of the overall film, we believe that the calculated ˚ is mostly symptomatic of increase of n550 for d)100 A an inhomogeneous refractive index increase, i.e. n does not increase homogeneously throughout the full film depth contrary to what is suggested by the increase of n550. In fact, it is improbable that at Tss75 8C, parts of ˚ below the growing film’s the film buried 100–200 A surface could undergo densification or rearrangement, since ion bombardment does not penetrate TiO2 to that depth (density, rTiO2;3.0–3.8 gycm3)rSi;2.2 gycm3). Rather, we expect that only the near surface region has incrementally higher index. There is strong evidence to suggest that n(z) is dependent on ion bombardment. We have shown previously w29x that the slope of n(z) of PECVD TiO2 films grown on pretreated c-Si is strongly affected by VB: films grown at NVBN-200 V exhibited descending n(z) profile, lower index (n-2.3, at ls550 nm) and high ˚ films grown at NVBN) surface roughness (ds)100 A); 200 V exhibited ascending n(z) profile, higher index ˚ Similar (n)2.3) and low surface roughness (ds-20 A). dependences of n(z) slope on ion bombardment have been observed for ZrO2 films deposited by IAD w24x, where n(z) is also descending, and ascending for, respectively, low and high ion-bombardment. 4. Conclusion The initial growth of TiO2 by PECVD on pre-oxidized c-Si substrate was analyzed by in situ RTSE for 0˚ We were able to decorrelate n and d in this d(85 A. thickness range because: (i) the measurement spectrum was broad enough to include a sufficiently large portion of the interband transitions region, where n and k exhibit
3.4. Inhomogeneity of TiO2 films ˚ was deposited at rs A thicker TiO2 film (d;700 A) ˚ 3.5 Ays by increasing the TiCl4 flowrate and using otherwise identical conditions (Section 2). The initial growth was found to behave similarly to what we reported in Fig. 5. In this case, however, n550 continued ˚ from 2.33 to increase slightly in regime III (d)50 A) ˚ and from 2.35 to 2.36 for to 2.35 for 80-d-180 A, ˚ We obtained the approximate inhomo180-d-280 A. geneity profile as a function of depth, n(z), from the ˚ by ellipsometric spectrum at final thickness (d;700 A) assuming a linear gradient w41x. It was found that n(z) increased from 2.33 (film bottom, zs0) to 2.40 (film top, zsd), which translates into a 3% ascending slope.
˚ grown at rs0.3 Fig. 5. Plot of n550 vs. d for TiO2 films (d;85 A) ˚ (open circles), rs1.4 Ays ˚ (diamonds), and rs3.3 Ays ˚ (crosses). Ays Three regions of the film growth mechanism are identified (see text for detail).
A. Amassian et al. / Thin Solid Films 447 – 448 (2004) 40–45
strong spectral features; (ii) the RCE configuration of the ellipsometer provided high quality D(l) data even near 08 or 1808; (iii) the c-Si substrate was pretreated in ion-bombardment conditions similar to those at the beginning of PECVD growth in order to modify its surface to saturation, thus eliminating the surface damage effects of initial growth and simplifying the growth model considerably; (iv) c-Si substrate temperature was preset during pretreatment (Fig. 1, Part I-B) and calculated from RTSE data (Table 1). Refractive index evolution as a function of film thickness for ˚ is compatible with thin film nucleation, 0-d-85 A coalescence, and continuous film growth. For thicker films, additional increase of the effective index, n550, ˚ results in a slight ascending slope in n, for d)80 A very much in line with previous findings w37x, where inhomogeneity profile of PECVD TiO2 films was related to bias conditions. Acknowledgments ´ The authors thank Mr Stephane Larouche, Dr Oleg Zabeida, and Mr Blaine Johs (J.A. Woollam Co., Inc.) for invaluable discussions, and Mr Gilles Jalbert for expert technical assistance. This work was supported by the NSERC and CFI programs of Canada. P.D. acknowledges the support from the Canada Research Chair program. References w1 x w2 x w3 x w4 x w5 x
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Study of TiO2 film growth mechanisms in low-pressure plasma by in situ real-time spectroscopic ellipsometry A. Amassian, P. Desjardins, L. Martinu* Groupe de recherche en physique et technologie des couches minces (GCM), and Department of Engineering Physics, Ecole Polytechnique, P.O. Box 6079, Station Centre-Ville, Montreal, Quebec, Canada H3C 3A7
Abstract We investigate the initial stages of growth of TiO2 films prepared by plasma-enhanced chemical vapor deposition on plasma pre-oxidized c-Si, using in situ real-time spectroscopic ellipsometry. The optical properties of TiO2 films were parameterized from 245 to 1000 nm using the Tauc–Lorentz oscillator and the effective medium approximation. For thin films grown at low substrate ˚ temperature (Tss75 8C) and deposition rate (0.3-r-3.5 Ays), effective refractive index (n550 at ls550 nm) stabilization, ˚ independent of r. When compared at final consistent with island coalescence, was observed for thickness, d, of only 12–15 A, ˚ all films possessed similar optical properties in the visible (n550 s2.30–2.33), with most significant thickness (d;85 A), ˚ grown at rs3.5 Ays ˚ differences in n(l) and k(l) observed in the interband region (l-385 nm). A thicker film (ds700 A) ˚ An additional n550 increase in the order of 0.03 observed for 85-dwas found to exhibit similar growth behavior for d-85 A. ˚ was attributed to refractive index inhomogeneity. 280 A, 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Optical coatings; Plasma-enhanced chemical vapor deposition; In situ ellipsometry; Titanium dioxide
1. Introduction The fabrication of optical interference filters requires accurate control of the deposition process in order to avoid departures from the design. This is especially important for some numerical synthesis techniques such as Flip-Flop w1x, Needle w2x and others w3x, which often ˚ and incorporate ultra-thin films (thickness, d(100 A) thicker, non-quarter wave layers, none of which allow for error compensation w4x. Of particular importance in the latter case is the elimination of refractive index n inhomogeneities as a function of depth z, often encountered in high index materials. TiO2 is frequently used as the high index material in optical filter applications. It is known to exhibit numerous problems due to its susceptibility to form inhomogeneities, n(z) w5x. In optical coatings, TiO2 is usually desired in its amorphous phase w6x, but anatase and rutile TiO2 have also been exploited w6x. TiO2 films have been traditionally grown by electron-beam evapo*Corresponding author. Tel.: q1-514-340-4099; fax: q1-514-3403218. E-mail address: [email protected] (L. Martinu).
ration w7x, sputtering w8x and ion beam assisted deposition w9x. In our laboratory, we particularly focus on amorphous TiO2 (a-TiO2) films prepared by plasma-enhanced chemical vapor deposition (PECVD), and we study the effects of growth conditions on their optical properties w10x. The advantages of PECVD in optical filter fabrication have recently been discussed in a review article w11x. This technique allows one to control over a wide range the optical and mechanical properties in films with compositional w12x or microstructural w13x gradients, and nano-composites w14x. As an example, control of the stoichiometric ratio in TiO2 ySiO2 mixtures is used for the fabrication of graded index (rugate) filters w15x. In multilayer optical filters, optical waveguides and other applications, n inhomogeneities have to be eliminated, as these strongly deteriorate their performance w16x. Such inhomogeneities are believed to strongly depend on film microstructure w17,18x, which is itself determined by the growth mechanisms w19x. Dynamic single-wavelength ellipsometry (SWE) has been used as a non-invasive and non-destructive tool to study the growth kinetics of optically absorbing films,
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.09.019
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such as a-Si:H w20,21x, a-Ge:H w26x, microcrystalline Si w22x, and poly-Si w23x. However, SWE has rather limited ˚ transparent films applicability for very thin (d-100 A) since UV spectral features are usually not accessible w7,24x. In the past decade, in situ real-time spectroscopic ellipsometry (RTSE) has been used for amorphous semiconductor growth studies w25,26x, process monitoring and control of epitaxial semiconductors w27x, multilayers w12,28x, as well as rugate w15x optical filters. RTSE allows one to determine the effective n value and d with sub-monolayer precision w25x even for very thin dielectric films, and can therefore be used to study the initial growth stages of transparent films w29x. However, there are no reports of successful correlation between growth mechanisms and refractive index inhomogeneities in films used in optical filters. In the present work we use RTSE for the study of the initial growth of a-TiO2 films prepared by PECVD onto well-characterized pre-oxidized silicon substrate. We measure the evolution of the optical properties for ˚ and relate it to the film growth mechanisms. d-85 A, 2. Experimental methodology Film growth was performed in a capacitively coupled radio-frequency (RF, 13.56 MHz) plasma system described in detail elsewhere w30x. A negative DC bias, VB, develops on the RF-powered substrate holder (10 cm in diameter) as a function of the RF power and working pressure. Standard c-Si N1 1 1M substrates were cleaned in a Piranha solution (H2SO4:H2O:H2O2s4:1:1) for 5 min, rinsed in DI water, blown-dried with N2, and immediately inserted in the vacuum chamber, which was then pumped down to -10y5 Torr using a turbomolecular pump. As the first step, the substrate surface was plasma pretreated, i.e. plasma cleaned and oxidized. O2 was introduced in the reactor at a flowrate of 60 sccm, and a working pressure of 20 mTorr was adjusted. The plasma was initially ignited at a power level corresponding to VBsy600 V, and it was reduced to VBsy450 V after 3 min in order to set the TiO2 growth conditions and to stabilize the substrate temperature at Tss75 8C as measured by ellipsometry w31,32x (see below). All c-Si structural modifications occur during the first 3 min of pre-treatment, at higher ion energy than TiO2 growth. As the second step, TiCl4 was introduced into the reactor, without stopping the plasma, using a computercontrolled flowmeter, while the O2 flow, the total pressure (20 mTorr) and VBsy450 V were kept constant. We evaluated the effect of deposition rate, r, controlled by the TiCl4 flowrate, on the TiO2 film growth mechanisms. Both the plasma pretreatment and TiO2 deposition processes were monitored in situ using a spectroscopic rotating compensator ellipsometer w33x (RCE, model M-
41
Fig. 1. Imaginary part of the pseudo-dielectric function, N´2M, plotted vs. time for two sample wavelengths, showing the sequence of process steps: O2 plasma pretreatment (Part I A–B) and TiO2 deposition (Part II). Part I: A—plasma switched on in O2 at VBsy600 V; B—plasma power reduced to VBsy450 V; Part II—TiCl4 introduced giving rise to TiO2 film growth.
2000FI, J.A. Woollam Co., Inc.), mounted at an angle of incidence near 658. Its spectra spanned over 477 wavelengths from 245 to 1000 nm, and could be acquired in parallel at a rate of up to 20 Hz. The RCE is arguably the most adequate configuration for the study of optical coatings on transparent substrates, mainly because it can measure the ellipsometric angles C and D over their entire range (i.e. 08–908 for C and 08–3608 for D) without loss of sensitivity. This is essential for transparent substrates in the visible and near infrared regions, for which D is either 08 or 1808. The choice of the acquisition rate is dictated by many factors: (i) the dynamic study of near surface effects by RTSE requires good signal-to-noise ratio, as well as (ii) minimum systematic errors, and (iii) adequate time resolution. A good compromise was to perform analyzer two-zone averaging, while using 500 ms integration time at each zone. This resulted in an interval close to 2 s between two successive spectra. Analysis of the ellipsometric data was performed using EASE and WVASE32 softwares (J.A. Woollam Co., Inc.). 3. Results and discussion 3.1. Process steps and dynamic ellipsometry data In order to clearly describe the various process steps, we have plotted, in Fig. 1, the time-evolution of dynamic ellipsometry data during the pretreatment and TiO2 growth experiments. The imaginary part N´2M of the pseudo-dielectric function N´MsN´1 yi´2 M is shown for
A. Amassian et al. / Thin Solid Films 447 – 448 (2004) 40–45
42
sequence indicated in Fig. 1 has the following advantages: (i) TiO2 film growth is initiated on a well-characterized and ‘stabilized’ SiO2 substrate surface (in this respect, the effects of ion bombardment in the initial moments of thin film growth by PECVD would be already accounted for); (ii) the substrate is never taken out of the chamber; (iii) it is possible to determine Ts and adjust the fabrication conditions so that Ts is constant during TiO2 growth; (iv) sample-to-sample reproducibility of the pretreatment effects are exceptionally good (Section 3.2). 3.2. Ellipsometric model for the pretreated substrate (Part I) Fig. 2. Schematic representation of the optical model of c-Si pretreated in O2 plasma (Part I) and followed by TiO2 growth (Part II). TL: top layer; BL: bottom layer.
two distinct wavelengths, l1s400 nm and l2s350 nm, as a function of time in Fig. 1. N´M, obtained by direct inversion of ellipsometric angles C and D w34x, has the advantage of being independent of the angle of incidence. Fig. 1 shows the sensitivity of RTSE to the pretreatment-induced surface modifications (Part I A–B) and to the TiO2 film growth (Part II), as illustrated by important variations in N´2M. However, different wavelengths possess different degrees of sensitivity to surface modifications. This makes the latter uniquely identifiable in RTSE analysis, compared with SWE w29,35x. The two wavelengths, l1 and l2, were chosen in a way that the optical constants of c-Si at l1 are sensitive to Ts, while they are insensitive at l2s350 nm. This enables to discriminate variations in RTSE data due to surface structural modification and those arising from reversible changes, such as Ts cycling. Process steps resulting in RTSE spectral changes may be clearly identified (Fig. 1). The changes in N´2M due to plasma exposure (Fig. 1, Part I) demonstrate that c-Si surface modifications impact the RTSE signal as much as those due to TiO2 growth. This makes studying and accurately modeling c-Si modifications critical pre-requisites to the successful analysis of TiO2 growth by ellipsometry. The
An optical model describing the thin film system formed on c-Si is shown in Fig. 2. Part I represents the structure that yielded the best fit to RTSE data following c-Si pretreatment: it essentially forms a ‘well-defined’ substrate suitable for the study of the TiO2 growth process, completing Part II. Part I consists of the c-Si substrate and two-layers assigned as top layer (TL) and bottom layer (BL), with respective thicknesses dTL and dBL. The TL is SiO2, while the BL is best modeled by the Bruggeman effective medium approximation composed of c-Si, a-Si and SiO2 with respective volume fractions pc-Si, pa-Si and pSiO2. The assumption that the optical constants of c-Si are Ts-dependent allowed us to model temperatureinduced changes in the ellipsometric spectra and to obtain instantaneous Ts values by fitting the RTSE data throughout Part I. Once Ts was stabilized in B (Part I), it was fixed along with the other fit parameters from Part I, and assumed to remain unchanged during TiO2 growth (Part II). In our RTSE analysis, we first determined the substrate’s initial conditions in terms of the thickness of native oxide, dnative, and Ts (Table 1, ‘Initial’). Upon plasma exposure, we continuously fitted the parameters describing the evolution of dTL, dBL, pc-Si, pa-Si, pSiO2 and Ts. In Part I-A (VBsy600 V, see Fig. 1), dTL and dBL reached 90% of their steady-state values after approximately 45 and 10 s, respectively (Table 1, ‘Part I’). Meanwhile, structural transformation (i.e. amorphi-
Table 1 Optical fit parameters before and after O2 pretreatment of c-Si for three consecutive experiments Sample
1 2 3
Initial
Part I
˚ dnative (A) "0.2
Ts (8C) "2
TL ˚ dTL (A) "0.8
BL ˚ dBL (A) "2
pa-Si (%) "6
pc-Si (%) "6
pSiO2 (%) "2
24.0 24.7 23.6
33 37 37
59.9 60.7 59.6
33 37 34
53 54 56
20 19 20
27 28 25
Stabilization Ts (8C) "2 72 75 74
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zation and oxidation) of c-Si during the first 5 s of exposure resulted in pc-Si, pa-Si and pSiO2 changing from their initial 99, 0 and 1% values to 20, 55 and 25%, respectively. This proportion did not change significantly with further exposure or with a change of VB to y450 V (Fig. 1, Part I-B). It took 240 s for Ts to reach 103 8C at VBsy600 V, while it dropped to 75 8C for VBsy450 V (Table 1). The two-layer model was necessary, because energetic ion-bombardment (;300–700 eV) w36x by oxidizing species from the plasma (Oq and Oq 2 ) results in both a significant damage of the Si crystal lattice w37–39x, as well as oxide formation due to the affinity of Si and O atoms. Such energetic ions penetrate several nanometers into c-Si and amorphize the structure through successive collisions with the crystalline network, displacing Si atoms, and eventually bonding with neighboring Si. As a result of collisions, a partially amorphous and oxidized interfacial layer is formed between the TL and c-Si, identified as the BL. As oxidation occurs (TL), the interface (BL) progresses into the substrate. (In fact, there is a gradient, but the RTSE data are well modeled by a two-layer model.) RTSE analysis of Part I resulted in the final values summarized in Table 1. Very good agreement was obtained for oxidation of three nominally identical samples at VBsy600 V (Table 1). This confirms an excellent reproducibility for both the fabrication and characterization of Part I. This structure was then used as a reference platform for the study of TiO2 film growth presented in the next section. 3.3. Initial growth stages of TiO2 films (Part II) After having determined the characteristics of the substrate exposed to O2 plasma pre-treatment (Table 1, ‘After Part I’), we now concentrate on the growth of TiO2 (Fig. 1, Part II). In modeling the RTSE data, the TiO2 film was assumed to be a homogeneous mixture of bulk-like a-TiO2 and voids. This model is based on the assumption that thin films in their initial growth process, such as nucleation, island growth and coalescence, can be optically represented as a mixture of dense material and void w21,24,25x. The analysis can be carried out without making any assumption on the geometry of TiO2 clusters andyor pores, since the feature sizes are much smaller than l. In order to determine the optical constants of TiO2, we first analyzed thicker films (Fig. 3), i.e. films at the end of the growth experiment. The optical constants of TiO2 were parameterized using the Tauc–Lorentz oscillator (TLO) w40x, which is known to adequately reproduce the line-shape of optical constants of amorphous materials in the interband and the transparent regions. For l-385 nm, the optical constants of a-TiO2 are expected to exhibit strong
43
Fig. 3. Dispersion curves, n(l) and k(l), for TiO2 films with ˚ and;700 A. ˚ d;85 A
spectral features in n(l) and k(l), where n and k increase dramatically to n;3–3.6 (ls315 nm) and k;1–1.7 (ls265 nm), and adopt a bell shaped form. The presence of such features in both n and k of the film and the substrate over the probed ellipsometric spectrum are essential to the accurate and precise computation of both n and d from RTSE data for thin films ˚ in their initial growth stages (typically 0-d-120 A). Thus, extending the RTSE spectrum sufficiently below the TiO2 optical gap enables to decorrelate n and d, and to significantly enhance the accuracy of optical thin film ˚ The transparent part of the metrology for d-100 A. spectrum does not contribute very much to this decorrelation, but it yields useful optical constants in the visible and near infrared. The complex refractive index, N(l)sn(l)yik(l) of the growing film was computed by a linear combination of TLO parameterized optical constants, NTLO(l), with a void fraction, pvoid (ns1), as N(l)s(1ypvoid)NTLO(l)qpvoid
(1)
The TiO2 thickness d and n550 (n(l) at ls550 nm, ks0) are plotted as a function of deposition time in Fig. 4 for three values of r, namely 0.3, 1.4 and ˚ 3.3 Ays. Fig. 4a indicates that d increases nearly linearly with time, except during the initial 10 s of growth. This behavior is attributed to the TiCl4 flow reaching steady state (Fig. 1, Part II), which should not be an issue in standard PECVD deposition, where plasma ignition follows flow stabilization. The time evolution of n550 is more complex; at first, it increases rapidly from n550;1.1 to 2.1, with a ratedependent slope (Fig. 4b); then, n550 slowly tends towards saturation (;2.3). The error bars represent one standard deviation; they are determined from the quality of the fit, including parameter cross-correlation and signal-to-noise ratio.
44
A. Amassian et al. / Thin Solid Films 447 – 448 (2004) 40–45
Fig. 4. Evolution of (a) d vs. time and (b) n550 vs. time for three ˚ (dots and circles), rs1.4 Ays ˚ (narrow deposition rates: rs0.3 Ays ˚ (solid line and cross). Tss75 8C dots and diamonds), and rs3.3 Ays in all three experiments.
Since n550 converged to the same value at similar final thickness, we conclude that n550 depends on d rather than on time, and that it must be controlled by the film microstructural evolution. In order to point out this relationship, we plotted n550 as a function of d in Fig. 5. This dependence appears identical for all r values, and one can distinguish three regimes, each of which can be related to different growth stages: I: 0˚ rapid increase of n550, compatible with nucled-15 A; ˚ ation and 3D island growth w27x; II: 15-d-50 A; slower increase of n550 towards an asymptotic value, in line with optical effects of densification through island coalescence and pore filling w25x; and finally, III: ˚ n550 remains almost constant with d, suggestd)50 A; ing that TiO2 already forms a continuous layer w7,26x.
The average refractive index (at film center, zsdy2) was 2.365, and was very similar to the effective index ˚ n550s reported in the previous paragraph for d)280 A, 2.36. Since n550 simply represents the effective refractive index of the overall film, we believe that the calculated ˚ is mostly symptomatic of increase of n550 for d)100 A an inhomogeneous refractive index increase, i.e. n does not increase homogeneously throughout the full film depth contrary to what is suggested by the increase of n550. In fact, it is improbable that at Tss75 8C, parts of ˚ below the growing film’s the film buried 100–200 A surface could undergo densification or rearrangement, since ion bombardment does not penetrate TiO2 to that depth (density, rTiO2;3.0–3.8 gycm3)rSi;2.2 gycm3). Rather, we expect that only the near surface region has incrementally higher index. There is strong evidence to suggest that n(z) is dependent on ion bombardment. We have shown previously w29x that the slope of n(z) of PECVD TiO2 films grown on pretreated c-Si is strongly affected by VB: films grown at NVBN-200 V exhibited descending n(z) profile, lower index (n-2.3, at ls550 nm) and high ˚ films grown at NVBN) surface roughness (ds)100 A); 200 V exhibited ascending n(z) profile, higher index ˚ Similar (n)2.3) and low surface roughness (ds-20 A). dependences of n(z) slope on ion bombardment have been observed for ZrO2 films deposited by IAD w24x, where n(z) is also descending, and ascending for, respectively, low and high ion-bombardment. 4. Conclusion The initial growth of TiO2 by PECVD on pre-oxidized c-Si substrate was analyzed by in situ RTSE for 0˚ We were able to decorrelate n and d in this d(85 A. thickness range because: (i) the measurement spectrum was broad enough to include a sufficiently large portion of the interband transitions region, where n and k exhibit
3.4. Inhomogeneity of TiO2 films ˚ was deposited at rs A thicker TiO2 film (d;700 A) ˚ 3.5 Ays by increasing the TiCl4 flowrate and using otherwise identical conditions (Section 2). The initial growth was found to behave similarly to what we reported in Fig. 5. In this case, however, n550 continued ˚ from 2.33 to increase slightly in regime III (d)50 A) ˚ and from 2.35 to 2.36 for to 2.35 for 80-d-180 A, ˚ We obtained the approximate inhomo180-d-280 A. geneity profile as a function of depth, n(z), from the ˚ by ellipsometric spectrum at final thickness (d;700 A) assuming a linear gradient w41x. It was found that n(z) increased from 2.33 (film bottom, zs0) to 2.40 (film top, zsd), which translates into a 3% ascending slope.
˚ grown at rs0.3 Fig. 5. Plot of n550 vs. d for TiO2 films (d;85 A) ˚ (open circles), rs1.4 Ays ˚ (diamonds), and rs3.3 Ays ˚ (crosses). Ays Three regions of the film growth mechanism are identified (see text for detail).
A. Amassian et al. / Thin Solid Films 447 – 448 (2004) 40–45
strong spectral features; (ii) the RCE configuration of the ellipsometer provided high quality D(l) data even near 08 or 1808; (iii) the c-Si substrate was pretreated in ion-bombardment conditions similar to those at the beginning of PECVD growth in order to modify its surface to saturation, thus eliminating the surface damage effects of initial growth and simplifying the growth model considerably; (iv) c-Si substrate temperature was preset during pretreatment (Fig. 1, Part I-B) and calculated from RTSE data (Table 1). Refractive index evolution as a function of film thickness for ˚ is compatible with thin film nucleation, 0-d-85 A coalescence, and continuous film growth. For thicker films, additional increase of the effective index, n550, ˚ results in a slight ascending slope in n, for d)80 A very much in line with previous findings w37x, where inhomogeneity profile of PECVD TiO2 films was related to bias conditions. Acknowledgments ´ The authors thank Mr Stephane Larouche, Dr Oleg Zabeida, and Mr Blaine Johs (J.A. Woollam Co., Inc.) for invaluable discussions, and Mr Gilles Jalbert for expert technical assistance. This work was supported by the NSERC and CFI programs of Canada. P.D. acknowledges the support from the Canada Research Chair program. References w1 x w2 x w3 x w4 x w5 x
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