Excimer Laser-Induced Melting and Resolidification Dynamics of Silicon Thin Films

Transcription

1 Journal of the Korean Physical Society, Vol. 39, December 2001, pp. S419 S424 Excimer Laser-Induced Melting and Resolidification Dynamics of Silicon Thin Films Mutsuko Hatano Hitachi Research Laboratory, Hitachi Ltd., Tokyo , Japan Seungjae Moon, Minghong Lee and Costas P. Grigoropoulos Department of Mechanical Engineering, University of California, Berkeley, CA 94720, U.S.A Kenkichi Suzuki Electron Tube & Devices Division, Hithchi Ltd., Mobara 297, Japan (Received 1 November 2000, in final form 12 September 2001) Excimer laser crystallization is an efficient technology for obtaining high-performance polycrystalline Si thin film transistors for advanced flat panel display applications. The liquid-solid interface motion and the temperature history of thin Si films during excimer laser annealing are observed by in-situ experiments combining time-resolved ( 1 ns) electrical conductance, optical reflectivity/transmissivity at visible and near-ir wavelength, and thermal emission measurements. The temperature response, melt propagation and evolution of the recrystallization process are fundamentally different in the partial-melting and the complete-melting regimes. In the partial-melting regime, the maximum temperature remains close to the melting point of amorphous Si, since the laser energy is consumed on the latent heat of phase-change. The peak temperature rises in the complete-melting regime, but substantial supercooling (200K), followed by spontaneous nucleation into fine-grained material is observed. Lateral crystal growth is induced by fluence distribution using a mask projection system leading to spatial control of partial/complete melting. Growth length is limited by the triggering of spontaneous nucleation in supercooled liquid Si and lateral solidification velocity is measured to be about 7.0 m/s. I. INTRODUCTION Polycrystalline silicon (poly-si) films are the key material of thin film transistors (TFTs) in advanced information displays, such as super-high-definition active matrix liquid crystal displays (AMLCD) and organic light emitting diode (OLED) displays. Low temperature process poly-si technologies have realized mass production of AMLCD with integrated driver circuits [1]. However, technological break through is needed to realize the system in displays and IC replacing high speed and low power consumption circuits. Pulse laser crystallization of amorphous silicon(a-si) films -usually effected by nanosecond, ultraviolet (UV) excimer laser radiation - has emerged as a promising fabrication method for obtaining poly-si TFTs. In order to improve both performance and uniformity of TFTs, high-quality poly-si films with controlled grain size and location are required. To accomplish this objective, sev- mutsuko-crl.hatano@c-net3.crl.hitachi.co.jp eral methods [2 7] have been devised utilizing spatially selective melting and lateral temperature modulation. A melt-mediated transformation scenario has been proposed [6 8] suggestion that the recrystallized Si morphology is determined by several complex phase transformations. However, the evolution of these melting and resolidification phenomena has not been experimentally verified by direct measurements. Therefore, it was necessary to clarify the dynamics of melting and resolidification which determine the recrystallized Si morphologies. In this paper, the temperature history and the phase transformations under non-equilibrium conditions in thin amorphous and poly-si films (50nm) have been observed by in-situ diagnostics combining time-recolved ( 1ns) electrical conductance, optical reflectance and transmittance, in conjuction with thermal emission measurements. Lateral crystal growth has been induced by fluence distribution using a mask projection system leading to spatial control of partial/complete melting. Electrical conductance measurement was also performed to probe the solidification dynamics and infer the lateral -S419-

2 -S420- Journal of the Korean Physical Society, Vol. 39, December 2001 as the probing light source; the signals are focused onto the fast silicon PN photo diode with nanosecond response time. The reflectivity increases and the transmissivity decreases during the melting of the surface, since liquid- Si has higher reflectivity than that of solid-si. The diagnostics are conducted simultaneously, with the exception of the emission and conductance signals, which are measured separately due to spurious contribution from the contact Al film in samples used for the conductance measurement. Lateral crystal growth is enforced through a mask projection system with a 2:1 demagnification. A beam mask consisting 20 µm spacing is insert and then fluence gradient is induced. The fluence distribution is precisely quantified by a sensitive UV photo-resist. Electrical conductance measurement is also performanced to probe the solidification dynamics and infer the lateral solidification velocity. Fig. 1. Schematic of the in-situ diagnostic probes. III. RESULTS AND DISCUSSION solidification velocity. 1. Melting and resolidification dynamics in vertical direction II. EXPERIMENTS A schematic of the experimental system is shown in Fig. 1. A detailed description of the experimental procedure is given in [11,12]. The sample consists of a 50 nm-thick a-si film deposited onto a fused quartz substrate by LPCVD. A pulsed KrF excimer laser (λ = 248 nm, FWHM = 25 ns) is utilized for heating the sample. The time-resolved electrical conductance measurements [13] is applied in order to obtain the melt duration, the melt depth, and the solid-liquid interface velocity. The molten Si produces an abrupt rise in the conduction electron density so that the electrical conductivity reaches values typical of liquid materials. The electrical conductivity of molten Si (75Ω 1 cm 1 ) is much higher than that of solid Si (0.3 Ω 1 cm 1 ) [14] and consequently the total conductance of the Si is drastically increased due to the presence of a molten layer. Spectrally-resolved pyrometry based on Planck s blackbody radiation intensity distribution enables measurements of the transient temperature during th phase transition processes [15]. The thermal emission signal is collected by an InGaAs photodetector with nanosecond response time. In order to obtain accurate temperature, four different wavelengths (1.2, 1.4, 1.5, 1.6 µm) signals are detected. In addition, the front transmissivity and reflectivity are measured to obtain the emissivity at the 1.52 µm wavelength of a HeNe laser. In terms of optical diagnostics, transient reflectivity and transmissivity are measured to determine the melt duration. A continuous wave HeNe laser (633nm) is used Dependence of maximum temperature obtained by the emission signals, melt depth estamated by the conductance signals, and average grain size measured by SEM analysis are shown in Fig. 2. The conductance signal is essentially representative of the volume fraction of the liquid phase. For convenience, the melt depth is used as an indicator of the liquid-si volume. The temperature is integrated over the absorption depth at the near-ir wavelength. The threshold fluence for surface melting, F t = 155 mj/cm 2 and for complete melting, F c = 262 mj/cm 2. In the partial melting regime, F a <F<F c, the melt depth increases with laser fluence. Since the absorbed laser energy in excess of the level needed for surface melting is consumed by the latent heat of phasechange from solid to liquid-si, the maximum temperature remains nearly constant. The constancy of the measured value at about 1510K is subject to the condition that the melting depth exceeds the absorption depth in liquid-silicon, which in the near-ir wavelength range is about 20 nm. This is the reason why the measured temperature rises when the fluence Ft<F<Fa = 179 mj/cm 2. In the complete melting regime, F>F c, the temperature increases with fluence, since the excess laser energy beyond F c is used to heat the liquid-si, consequently raising the peak temperature. The grain size strongly depends on fluence and therefore on the temperature history and the solid-liquid interface velocity. Accordingly, the grain size variation follows closely the regimes defined via both the conductance and temperature measurements. In the low fluence range that corresponds to the partial melting regime, SEM shows a gradual increase in grain size with fluence. In the high

3 Excimer Laser-Induced Melting and Resolidification Dynamics Mutsuko Hatano et al. -S421- fluence range, which corresponds to the complete melting regime, a dramatic reversal of the micro-structural trend is observed, even only with a slight increase in fluence. This phenomenon is related to supercooling which is followed by homogeneous nucleation [13]. In the near complete melting regime, i.e. in the transition zone from the partial melting to the complete melting regimes, substantially enlarged grain size is obtained [2]. The transient temperature and melt depth data exhibited in Fig. 3 help elucidate these phenomena. Figure 3(a) shows the transient temperature and melt depth for a laser fluence of F = 229 mj/cm 2, which induces partial melting in the a-si films. Since the maximum temperature observed at t = 23 ns is 1510 K, i.e. lower than the equilibrium melting point of crystalline-si (1685 K), it can be inferred that the liquid-si exists in a supercooled state. Some structures in both the emission and conductance signal can be observed during the resolidification process before the melt depth reaches 20 nm. These are probably related to release and consumption of the latent heat caused by explosive crystallization, which is a self-sustained process [16]. Figure 3(b) shows the results in the complete melting regime. Substantial supercooling, followed by homogeneous nucleation is observed in both the thermal emission and conductance. These signals consist of (1)-(3). The flat region (1) indicates that the film is completely molten over the 50 nm thickness. On the other hand, the molten-si cools very rapidly, >10 10K/s, as shown in the transient temperature signal. Due to lack of nucleation sites, the liquid-si is supercooled until sufficient nucleation sites are formed. Accordingly, the transient temperature exhibits a dip in the neighborhood of ns and this point exactly coincides with the end of the full melting exhibited in the conductance. Therefore, the temperature dip is interpreted as preceeding the on- Fig. 2. Dependence of maximum temperature, melt depth, and average grain size on laser fluence. Fig. 3. Transient temperature and melt depth. (a) The laser fluence, F = 229 mj/cm2 lies in the partial melting regime. (b) The laser fluence, F = 365 mj/cm 2 generates complete melting. set of homogeneous nucleation in supercooled liquid-si. The supercooling prior to nucleation is about 230K. The temperature during this dip corresponds to the nucleation temperature, i.e. is much lower than the melting point. Upon the initiation of solidification, latent heat is released, increasing the temperature of the film up to the melting point(region(2)). Following this recalescence, growth of the solid continuous under steady-state conditions as heat is conducted into the substrate(region(3)). It was found that the rate of nucleation increases in the deeply supercooled liquid [13] and that homogeneous nucleation results in a fine-grain structure. Temperature measurement, in-situ optical probing and electrical conductance measurements have been conducted on poly-si films as starting materials fabricated by excimer laser crystallization (average grain size =120 nm). Comparison of maximum melt depth for a-si and poly-si dependence on the laser fluence is shown in Fig. 4(a). The threshold fluences of surface melting, Ft and complete melting, Fc, for the a-si films are lower than for the poly-si films. Moreover, the melt depth in the poly- Si films is smaller than for a-si films at the same fluence. These effects are mainly caused by the difference in melting point and thermal conductivity between amorphous and polycrystalline materials. Figure 4(b) shows comparison of measured maximum temperature for a-si and poly-si dependence on the laser fluence. The tempera-

4 -S422- Journal of the Korean Physical Society, Vol. 39, December 2001 Fig. 6. In situ electrical conductance signals for both lateral grain growth (with mask) and conventional grain growth (maskless). Fig. 4. Comparison of a-si and poly-si. All specimens are 50 nm-thick. (a) Maximum melt depth dependence on laser fluence. (b) Maximum temperature dependence on laser fluence. ture for the poly-si films are lower than for the a-si films in the under melting regime. It is mainly caused by the thermal conductivity difference. On the other hand, in the partial melting regime, the temperature of the poly- Fig. 5. Dependence of lateral grain growth length on fluence gradient for 50 nm a-si films. Si films exhibits a plateau at about 1650 K, which is 140 K higher than that of the a-si films. It is noted that the equilibrium melting temperature of crystalline silicon is at 1685K. Since the absorbed laser energy in excess of the level needed for surface melting is consumed by the latent heat of phase-change from solid poly-si to liquid, the maximum temperature remains nearly constant. around the melting point of the poly-si films. 2. Melting and resolidification dynamics in lateral direction The dependence of lateral growth length on fluence gradient is shown in Fig. 5. The lateral growth length is almost constant at about 500 nm for fluence gradients below 80 mj/cm 2 µm but increase rapidly as the laser fluence gradient increases further. The directionality of the lateral grains is also improved by increasing the fluence gradient as shown in SEM pictures. Figure 6 shows an electrical conductance, together with the signal corresponding to maskless laser illumination (conventional laser crystallization technique). In according to the previously reported work on large area laser annealing [11], the latter trace shows that solidification begins at A and ends at B, taking about 35 ns to complete. On the other hand, a substantially longer solidification time is observed by using a masked laser beam. In this case, solidification begins at point a and concludes its course at c. Changes in slope on the conductances signal are noticed at point b. At time between a and b and due to the latent heat release, the interface is at the highest temperature T i, while the bulk liquid temperature is at a supercooling temperature, T s >T n, where T n is the temperature for spontaneous nucleation. At b, the spontaneous nucleation occurs in the liquid. At the time c, the liquid zone is completely extingushed. It is therefore inferred

5 Excimer Laser-Induced Melting and Resolidification Dynamics Mutsuko Hatano et al. -S423- Fig. 7. Schematics of lateral temperature distribution indicating the evolution of the crystal growth process. that the lateral growth occurs in the time interval c-a. Utilizing the estimates lateral solidification time, it can be inferred that the lateral solidification velocity is about 7 m/s. To interpret these conductance signals, a qualitative solidification model is proposed as depicted by the simplified one-dimensional temperature profiles shown in Fig. 7. Figure 7(a) represents the temperature distribution at times between a adn b (as defined in Fig. 6) when the phase boundary advances towards the liquid region. The interface ia at the highest temperature Ti, because of the release of latent heat due to lateral solidification at the interface. The bulk liquid temperature is at a supercooling temperature Ts below the melting point but liquid supercooling temperature Ts drops below the spontaneous nucleation temperature Tn (Fig. 7(b)). At this point, spontaneous nucleation begins in the bulk liquid triggering solidification. Howerer, the interface is still at the highest temperature Ti due to continuous release of latent heat at the interface by lateral solidification. As time proceeds beyond point b, a second solid-liquid interface is produced because of spontaneous nucleation and solidification in the bulk liquid (Fig. 7 (c)). Due to latent heat release at the second interface, the temperature is raised to Ti 2. The resulting temperature distribution is shown in Fig. 7(c) with lateral grains on the left due to lateral solidification, fine grains on the right due to spontaneous nucleation, confining the liquid silicon pool. Fianlly, the two interfaces meet when the solidification ends at time c. The final grain microstructure arrangement is shown in Fig. 7(d) and SEM picture in Fig. 5. Lateral grain 1, which grows from left to right in Fig. 11(c) and from bottom to top in Fig. 5, takes place over time interval c-a. Fine grains are produced from time b to c. And the lateral grain 2, which grows from right to left in Fig. 11d and from top to bottom on Fig. 5, takes place over a time interval of less than c-b. It is evident that lateral growth is limited by spontaneous nucleation in the bulk liquid. If spontaneous nucleation could be suppressed or delayed, the lateral growth would continue to a longer distance, hence producing longer lateral growth. In the case of high fluence gradient, the temperature increases rapidly from the partially molten region toward the completely molten region. Higher local temperature in the completely molten region implies correspondingly longer time to reach the deep supercooling required for spontaneous nucleation. Therefore the increase in nucleation time, i. e. the time elapsed from the beginning of the lateral growth till the inception of spontaneous nucleation, is a crucial parameter for lateral growth. The success of producing long lateral grains for the 50 nm-thick a-si films by imposing high fluence gradient is mainly attributed to this effect. IV. CONCLUSIONS In-situ diagnostics combining electrical conductance, optical reflectivity/transmissivity, and thermal emission are effective for investigating the temperature history and melt-resolidification dynamics of thin Si films. The measured maximum temperature and the melt depth delineated the conditions, for obtaining enhanced grain size in the near-complete melting regime. Supercooling, followed by homogeneous nucleation and recrystallization to fine-grain poly-si material could be observed in the complete melting regime. Poly-Si was observed to melt at temperature close to the melting point of crystalline silicon. In contrast, the measured melting temperature of a-si was lower by about 140 K. Lateral crystal growth was induced by fluence distribution using a mask projection system leading to spatial control of partial/complete melting. Growth length was limited by the triggering of spontaneous nucleation in supercooled liquid Si and lateral solidification velocity is measured to be about 7.0 m/s. REFERENCES [1] N. Ibaraki, Digest of technical papers of international symposium for information display for Information Displays 99, 172 (1999). [2] H. J. Kim and J. S. Im, MRS Symp. Proc. 697, 401 (1996). [3] M. Matsumura and Chang-Ho Oh, J. Thin Film Solid 337, 123 (1999). [4] J. Jang, J. Oh, S. Kim, Y. Choi, S Yoon and C. Kim, Nature 395, 481 (1998). [5] R. Ishihara and M. Matsumura, Jpn. J. Appl. Phys. 36, 6167 (1998). [6] T. Samejima, K.O zaki and N. Andoh, Appl. Phys. A 71, 1 (2000). [7] G. Nebel, S. Christainsen, H. Strunk and M. Stutzmann, Phys. Stat. Sol. (a) 166, 667 (1998). [8] V. V. Gupta, H. J. Song and J. S. Im, Appl. Phys. Lett. 71, 99 (1997).

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