Study by real time ellipsometry of the growth of amorphous and microcrystalline silicon thin films combining glow discharge decomposition and UV light irradiation

Thin Solid Films, 233 (1993) 281 285

281

Study by real time ellipsometry of the growth of amorphous and microcrystalline silicon thin films combining glow discharge decomposition and UV light irradiation N. Layadi, P. Roca i Cabarrocas, V. Y a k o v l e v * a n d B. D r r v i l l o n Laboratoire de Physique des Interfaces et des' Couches Minces (UPR 258 du CNRS), Ecole Polytechnique, 91128 Palaiseau (France)

Abstract We present here the application of spectroscopic phase modulated ellipsometry (SPME) to the study of the growth of amorphous and microcrystalline silicon thin films combining plasma-enhanced chemical vapor deposition (PECVD) and excimer laser irradiation. Our results show that laser fluence is a critical parameter in UV-assisted deposition. When we increase the laser fluence we observe a gradual transition from hydrogenated amorphous silicon (a-Si:H) to an a-Si:H with greater roughness and porosity, then to a very dense a-Si:H, after that to a dense microcrystalline silicon (p.c-Si) material, and finally to a porous microcrystalline material. The crystallization, after and during plasma deposition, is characterized by the evolution of the imaginary part of the pseudo-dielectric function which allows us to identify the threshold of crystallization in the two cases. Moreover, the substrate temperature is found to activate the crystallization process in UV-assisted PECVD.

1. Introduction Hydrogenated amorphous silicon (a-Si:H) is one of the semiconductor materials widely studied owing to its applications in optoelectronic devices. A number of studies have also been devoted to microcrystalline silicon (~tc-Si) thin films which have intermediate properties between those of amorphous and crystalline silicon and are used in many technological applications. The most widely used method for deposition of these materials is the plasma-enhanced, chemical vapor deposition (PECVD) technique [1-6]. In this work, we combine PECVD with excimer laser irradiation of the growing surface. In the following we refer to this as UV-assisted deposition [7, 8]. The UV photons generated by the laser can modify the chemical reactions in the plasma phase and on the growing surface because they are able to break Si-Si and S i - H bonds and increase the surface temperature. Thus the deposition of new materials can be expected, in particular microcrystalline and polycrystalline silicon films. In addition, we investigate the combination of the two techniques to study the kinetic growth of these materials, and also for more fundamental understanding of plasma discharge complex mechanisms and their relationship with the growing processes. A detailed analysis of the film deposition needs a real time probe, which should not perturb either the reactive *Present address: Instruments SA Inc, 60lsen Avenue, Edison, NJ 08820, USA.

0040-6090/93/$6.00

plasma or the probed surface and can identify the various possible stages of growth. Among the various optical methods, spectroscopic phase modulated ellipsometry (SPME) appears to be a suitable technique to investigate the simultaneous interaction of a laser with a surface exposed to plasma. Compared with other ellipsometric techniques, the phase modulation uses a high frequency of about 50 kHz provided by a photoelastic modulator coupled with a fast acquisition system [9]. A time resolution of 1-5 ms can be achieved. Thus SPME is extremely well adapted to real time applications, in particular to sophisticated deposition techniques such as kinetic investigations between two laser pulses. In the present study, the kinetic growth and the morphological properties, deduced from the real time ellipsometric analysis, of thin standard a - S i : H films deposited by PECVD at 250 °C substrate temperature are compared with those of films deposited under the same plasma conditions but exposed to excimer laser irradiation during growth. The effects of the laser energy and of the substrate temperature are illustrated. The crystallization behavior during and after plasma deposition is discussed.

2. Experimental details Hydrogenated amorphous silicon films were deposited in a conventional r.f. (13.56MHz) glow discharge plasma reactor by the decomposition of a

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ElsevierSequoia. All rights reserved

282

N, Layadi et al. / Growth ~/ a-Si and lu'-Si

mixture of silane and helium gases with flow rates of 15 and 22 standard cm 3 min-~ respectively at a substrate temperature of 250 °C, the temperature of the substrate surface being calibrated with a flat Pt-100 resistor attached to the glass surface. The total pressure in the reactor was 412 m T o r r and the r.f. power density was 88 m W cm 2. The deposition rate was estimated at 2 3 :~ s t. 1500& thick chromium films evaporated onto glass (Corning 7059) at room temperature and plain Corning glass were used as substrates. The UV source was an XeC1 excimer laser (£ = 308 nm, pulse duration 40 ns); the laser beam was focused on the surface of the sample, the irradiated area being a rectangular section of 1.3 cm 2. The laser fluence measured on the surface with a joulemeter was increased from 15 to 190 mJ cm -z per pulse with a repetition rate of 1 Hz. The growth kinetics and the optical properties of the films were monitored by a fast UV-visible phase modulated ellipsometer described elsewhere [9]. All measurements were carried out at an incidence angle of 76 ~'. Ellipsometric data (q/, A) in the spectral range 1.7 5 eV with an interval of 0.05 eV were recorded. To cover this range, a full spectroscopic acquisition requires a time of about 70 s. Real-time data acquisition was performed at a fixed photon energy of 3.5 eV with a kinetic period of 10ms (duration between two consecutive measurements). In order to obtain a clear idea of the effect of the laser on hydrogen incorporation, we analyzed a set of films, about 400 & thick, deposited at 250 °C onto Corning glass, by an IR phase modulated ellipsometer based on a Fourier transform principle [10]. Two sets of experiments were performed separately. On the one hand we studied the effect of the UV irradiation on a-Si:H films after their deposition. On the other hand we used the UV irradiation during plasma deposition to modify the growth of the film. Before presenting the results, we briefly recall that spectroellipsometric data were computed according to the effective medium theory which allows the dielectric response of a heterogeneous material to be described from the dielectric functions of its constituents [ 11]. The experimental ellipsometric angles (q/, A) measured as a function of energy were converted to the pseudo-dielectric function (~:) of the deposited film using the equation for a single interface [12]. The film morphology was characterized by the evolution of the imaginary part (e2). A decrease in the (~2) spectrum together with a shift towards lower energies is characteristic of an increase in the surface roughness. However, an increase in (e2 .... ) reveals an increase in density of the film [13, 14]. In addition, the appearance of a peak in the ( % ) spectrum at about 4.2 eV corresponds to optical transitions in crystalline silicon and is a sign of the

presence of a microcrystalline material [15, 16]. It is remarked that for microcrystalline materials the size effects were not included in our analysis, so that the results of the effective medium theory approach should be treated with caution [17].

3. Results and discussion Figure l shows the evolution of the imaginary part of the pseudo-dielectric function (~'2~ of a standard a-Si:H film 5000& thick deposited at 250 C onto a glass substrate. After the deposition we cooled the sample down to room temperature while keeping a vacuum of 10 6 m b a r in order to reduce oxidation of the a-Si:H surface and then exposed it to laser pulses of increasing fluence. As shown in Fig. 1, one laser pulse of 15 mJ cm -2 induces an increase in the surface roughness and film porosity, revealed by a decrease in (~:2) with a shift towards lower energies. As we increase the laser fluence above 15 mJ cm 2 we obtain a gradual increase in (e2) which indicates densification of the a-Si:H film. [t is at 145 m J c m 2 that we obtain a drastic change in the (~:2) spectrum which presents a broad shoulder at about 4.2 eV and indicates the presence of a dense microcrystalline silicon material. According to simulation of the pseudo-dielectric function, this material consists of a mixture of about 40% crystalline phase embedded in a dense a-Si:H phase. The threshold for crystallization found is in agreement with previous studies using other characterization techniques [18, 19]. It should be noted that after this pulse-induced crystallization, a very slight modification was observed in (e2) spectra even after 100 pulses at 190 mJ cm 2 Thus spectroellipsometry clearly shows that a single shot laser irradiation ( 145 mJ cm 2) is enough to crystallize an a-Si:H film. Figure 2 shows the (e2) spectra for a set of films deposited at 250 °C onto chromium substrates. The 30 : After 1 pulse .,--. 25 • 145 mJ/em 2-~,~''" ", e,I 20

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standard film (Std., in Fig. 2) is deposited without UV (e2max,-~23 at about 3.5eV). Using different laser fluences in the UV-assisted deposition we obtain four kinds of material. (1) At 15 mJ cm -2 we observe a change in the magnitude for the (e2) peak and a shift in the peak position towards lower energies. The material deposited (material 1) has a surface roughness of 35 A and a bulk void fraction of 8%. Both values are larger than those of standard a-Si:H films. (2) Between 80 and 145 mJcm -2 a dense a-Si:H (material 2) is deposited, as revealed by an increase in e2max; the densification increases considerably with incident laser fluence ( / 3 2 m a x ' 3 0 at about 3.5eV, at 145 mJ cm-2). (3) When we increase the laser fiuence between 165 and 180 mJ cm -2, broad shoulders near 3.4 and 4.2 eV appear (material 3) indicating crystallization of the films. The material deposited under 180mJ cm -2 contains about 30% crystalline phase according to simulation using the effective medium theory. Interestingly, our films have a higher density and smaller surface roughness than the gc-Si films obtained from the r.f. glow discharge decomposition of silane-hydrogen mixtures [3]. (4) As the laser fiuence is increased slightly at 185 mJcm -2 a porous microcrystalline silicon is deposited (material 4). A low value of (e2) (approximately 6) is obtained over the whole energy spectrum corresponding to a large density deficit (Fig. 2). It should be noticed that the transition between material 3 and material 4 is very sharp and thus difficult to control. We find that the threshold of crystallization in the UV-assisted deposition is higher than the value required for crystallization after deposition. This may be attributed to the fact that in UV-assisted deposition photochemical reactions in the plasma phase or near the illuminated surface could take place and absorb a fraction

of the UV light, thus resulting in a reduction in intensity of the UV light impinging on the irradiated surface [20]. We mention that as the laser fluence increases, the hydrogen content ( S i - H band) diminishes as determined by Fourier transform IR ellipsometry [10]. Figure 3 shows the real time (~k, d) trajectories recorded at 3.5 eV, with a kinetic period of l0 ms. They correspond to the growth of some of the films mentioned in Fig. 2. For laser fluences corresponding to the deposition of materials 1 and 2, no drastic effect of UV light is observed and the trajectory converges to a stable final point which corresponds to the pseudo-dielectric function of the deposited film. The experimental trajectories can be simulated by nucleation (revealed by the initial decrease in A, if one assumes that the microstructural development is columnar during the initial stage of deposition) followed by homogeneous growth of the bulk material beneath a surface roughness which is a consequence of incomplete coalescence of the initial nuclei [5, 6]. The deposition rate is slightly increased for films deposited with laser assistance, as previously reported [7]. At high laser fluence where gc-Si films are deposited, the kinetic trajectory is completely different both during the initial stage and in the long term where the final point is not stable. This behavior suggests that the growth of this material is much more complex and cannot be described by the simple models used for a-Si:H deposition. This is more clearly shown in Fig. 4, where we again plot on another scale the real time examination of the growth of the film deposited with a laser fluence of 185mJcms 2 (material 4). The first point characterizes the semi-infinite substrate (t = 0 s). The final point collected corresponds to laser pulse 200 and happens at t = 200 s. This real time monitoring of the kinetic growth between laser pulses (done here for the first time, to our knowledge) seems very interesting for investigation of the crystallization process in the area of laser applications. In particular, we clearly

284

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observe the presence of sharp variations of ~b and A, which could be related to relaxation of the film after the laser shot and may be related to thermal transients. These features deserve more detailed studies which will be undertaken in the future. The long term behavior of the experimental trajectories lac-Si deposited at high laser fluence (material 4) is characterized by an increase in <~2) independent of the energy (Fig. 5). The spectroellipsometric measurements were recorded 2, 4, and 7 min after the beginning of the UV-assisted deposition which correspond approximately to thicknesses of 360, 720, and 1260/~ respectively. This behavior is a sign of both the evolution of the surface of the film and the densification of the bulk, because a stable surface would result in convergence of the ellipsometric trajectory, which is not the case (see Figs. 3, 4). The effect of the substrate temperature on the UVassisted growth is illustrated in Fig. 6, which shows the evolution of
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Fig. 6. Energy dependence of the imaginary part of the pseudo-dielectric function for a-Si:H film deposited onto chromium at 250 "C without UV, and for films deposited with UV-assisted growth ( 135 mJ cm 2), onto chromium at 250 and 350 C .

and 350 °C. We find that under the plasma conditions and laser parameters corresponding to a densified film at 250 °C (material 2), the film deposited at 350 °C is already crystallized (material 4), which reveals the importance of the substrate temperature in the activation of the laser crystallization process in UV-assisted plasma deposition.

4. Summary and conclusion

The growth and the morphological properties of hydrogenated amorphous silicon films, prepared by r.f. glow discharge, have been compared with those of films prepared under the same plasma conditions but exposed to UV irradiation during growth. Briefly, as a result of use of real time spectroellipsometry, the following conclusions may be drawn from the present study. Four kinds of material were identified as the laser fluence is increased: (1) an a-Si:H material with more significant surface roughness and bulk porosity than the a-Si:H deposited without UV for a laser fluence of 15 mJ cm 2; (2) a dense a-Si:H material between 80 and 145 mJ cm-2; (3) a dense microcrystalline material between 165 and 180 mJ cm 2; (4) a porous microcrystalline material at 185 mJ cm -2. The crystallizations during and after plasma deposition were compared. The threshold of crystallization of a-Si:H films during growth was found to be higher than after deposition• The substrate temperature was found to activate the crystallization process in UV-assisted growth. The present work illustrates the ability of in situ spectroeUipsometry to monitor the growth processes in a complex system where PECVD and excimer laser irradiation are coupled. It confirms that spectroellipsometry is a very sensitive probe for studies of the surface, interface, and film morphology•

N. Layadi et al. / Growth of a-Si and i~c-Si

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