Appl. Phys. A 69 [Suppl.], S295–S298 (1999) / Digital Object Identifier (DOI) 10.1007/s003399900211
Applied Physics A Materials Science & Processing Springer-Verlag 1999
Thermal transport in melting and recrystallization of amorphous and polycrystalline Si thin films C.P. Grigoropoulos1,∗ , S. Moon1 , M. Lee1 , M. Hatano1,2 , K. Suzuki3 1 Department of Mechanical Engineering, University of California, Berkeley, 2 Central Research Laboratory, Hitachi Ltd., Tokyo 185-8601, Japan 3 Electron Tube & Devices Division, Hitachi Ltd., Mobara 297, Japan
CA 94720-1740, USA
Received: 21 July 1999/Accepted: 31 August 1999/Published online: 28 December 1999
Abstract. The liquid–solid interface motion and temperature history of thin Si films during excimer laser annealing are observed by in situ experiments combining time-resolved (∼ 1 ns) electrical conductance, optical reflectance/transmittance at visible and near-IR wavelengths and thermal emission measurements. For laser energy densities causing partial melting, the maximum temperature remains close to the melting point of amorphous silicon (a-Si). When complete melting occurs, substantial supercooling is observed, followed by spontaneous nucleation. These phase transformations are consistent with the recrystallized poly-Si morphologies. It is also found that the melting of poly-Si occurs close to the melting point of crystalline silicon. This temperature is higher than the melting point of a-Si by about 100–150 K. PACS: 42.62.Cf; 81.05.Ge; 64.70.Dv
Excimer laser crystallization is an efficient technology for obtaining high-performance poly-Si TFTs for advanced flat panel display applications. In order to improve both the device performance and uniformity, high-quality poly-Si films with controlled grain size and location are required. To accomplish this objective, several methods [1–5] have been developed utilizing spatially selective melting and lateral temperature modulation. A melt-mediated transformation scenario has been proposed [6–8] suggesting that the recrystallized Si morphology is determined by complex phase transformations. However, the evolution of these melting and resolidification phenomena has not been experimentally verified by direct temperature measurements. Optical diagnostic methods are appropriate for nonintrusively monitoring the melting and recrystallization phenomena. Since the optical properties depend on temperature and state of phase, the reflectivity and transmissivity are good ∗ Corresponding
author. (Fax: 1-510/642-6163, E-mail:
[email protected]) COLA’99 – 5th International Conference on Laser Ablation, July 19–23, 1999 in Göttingen, Germany
probing indicators of the laser annealing process. The analysis of time-resolved reflectivity data during the laser heating of silicon and germanium was used to determine the onset of melting and the melting duration [9]. For understanding the solidification mechanism, it is crucial to quantify the transient temperature field. In situ experiments combining time-resolved (∼ 1 ns) electrical conductance and optical reflectance and transmittance at visible and near-IR wavelengths, in conjunction with thermal emission measurements [10], were conducted to analyze the temperature history and the dynamics of melting and resolidification in thin amorphous- and polycrystalline-Si films. A time-resolved electrical conductance measurement was utilized for obtaining the melting duration, the melt depth, and the solid–liquid interface velocity. Spectrallyresolved pyrometry based on Planck’s blackbody radiation intensity distribution enabled measurement of the transient temperature during the phase transition process. Hence, it was possible to identify the origin of the recrystallized material morphologies that critically depend on the applied laser energy density. Furthermore, the difference in the melting and resolidification behavior for the amorphous compared to the polycrystalline films was quantified. 1 Experiments A schematic of the experimental system is shown in Fig. 1. A detailed description of the experimental procedure is given in [11]. The sample consisted of a 50-nm-thick amorphous silicon (a-Si) film deposited onto a fused quartz substrate by LPCVD. A pulsed KrF excimer laser (λ = 248 nm, FWHM = 25 ns) was utilized for heating the sample. The time-resolved electrical conductance measurement [12, 13] was applied in order to obtain the melt depth transients. The temperature history of the phase-change process was obtained by measuring the thermal emission signals based on Planck’s blackbody radiation intensity distribution law. The thermal emission signal is collected by an InGaAs photodetector with a nanosecond response time. In order to enhance the temperature measurement accuracy, signals at
S296
Fig. 2. Transient thermal emission, electrical conductance, transmissivity (λ = 633 nm) plotted along with the laser beam profile. The laser fluence F = 262 mJ/cm2
Fig. 1. Schematic of the in situ diagnostic probes
four different wavelengths (1.2, 1.4, 1.5, 1.6 µm) were detected. In addition, the front transmissivity and reflectivity were measured to obtain the absorptivity, which is equal to the emissivity at the 1.52-µm wavelength of the near-IR HeNe laser. In terms of optical diagnostics, transient reflectivity and transmissivity were measured to determine the melt duration [9]. A continuous wave HeNe laser (633 nm) was used as the probing light source; the signals were focused onto the fast Si PN photodiode with a nanosecond response time. 2 Results and discussion 2.1 Melting and recrystallization process of a- Si thin films Transient traces of thermal emission, electrical conductance, transmissivity, and the laser profile are shown in Fig. 2 for the laser fluence F = 262 mJ/cm2 . The conductance signal rises at A due to melting of the a-Si surface and rapidly increases to a peak value that corresponds to near-full melting. The signal begins decreasing at C and gradually returns to the initial value at D. The oscillations appearing on the signal are due to electrical noise. The emission signal starts increasing after a delay of 4 ns with respect to A. The delay is related to the response time and sensitivity of the InGaAs detector. The emission signal attains maximum value at B with a 20-ns delay with respect to the peak laser intensity. The maximum temperature, melt depth and average grain size depend on laser fluence as shown in Fig. 3. The conductance signal is essentially representative of the melt depth. The temperature is integrated over the absorption depth at the near-IR wavelength. The threshold fluence for surface melting is Ft = 155 mJ/cm2 and for complete melting, Fc = 262 mJ/cm2 . In the partial melting region, 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 phase-change from solid a-Si to
Fig. 3. Dependence of maximum temperature, melt depth, and average grain size on laser fluence
liquid, the maximum temperature remains nearly constant at about 1510 K. This is subject to the condition that the melt depth exceeds the absorption depth in liquid Si, which in the near-IR wavelength range is about 20 nm. A fluence of approximately Fa = 179 mJ/cm2 is needed for the maximum melt depth to reach this value. Consequently, the measured temperature rises when the fluence Ft < F < Fa . In the complete melting region, F > Fc , the temperature increases with fluence since the excess laser energy beyond Fc 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 is consistent with the results of both the conductance and temperature measurements. In the low fluence range that corresponds to partial melting, scanning electron microscopy shows a gradual increase in average grain size with fluence. In the high fluence range, where complete melting is induced, a dramatic reversal of the microstructural trend is observed. This phenomenon is related to supercooling which triggers spontaneous nucleation [8]. Substantially enlarged grain size
S297
is obtained for ‘near complete’ melting, i.e. in the transition zone from partial to complete melting [1]. The transient temperature and melt depth data elucidate these phenomena, which have important repercussions for practical applications. 2.1.1 Partial melting region. Figure 4 presents the experimental results for a laser fluence of F = 229 mJ/cm2 , which induces partial melting. The transient variation of the optical properties measured at the IR HeNe laser wavelength of 1.52 µm (reflectivity, transmissivity, emissivity), together with the emission signal, are shown in Fig. 4a. Upon melting, the film reflectivity increases and the transmissivity decreases. The front reflectivity measurement was carried out without the aperture in order to evaluate the effect of scattering by the evolving surface morphology. However, no appreciable scattering was detected as a result of melting and solidification. The measured emissivity data are used in the temperature calculation. Figure 4b shows the transient temperature and melt depth. Since the maximum measured temperature at t = 22 ns is 1535 K, i.e. lower than the melting point of crystalline Si (1685 K), it can be inferred that the liquid Si exists in supercooled state. In the partial-melting region, where the maximum temperature is still lower than the equilibrium crystalline silicon melting point, crystallization may originate from unmelted silicon seeds. Explosive crystallization, which is a self-sustained process [14], yields fine grain material underneath larger grain poly-Si.
2.1.2 Complete melting region. As the radiant laser energy increases, the silicon layer becomes completely molten and the duration of melting is prolonged. This is clearly observed in the reflectivity and transmissivity traces shown in Fig. 5. Two bumps, aligned with the melting and crystallization transitions are shown in the emissivity curve displayed in Fig. 5a. The behavior of the measured optical properties can be explained by utilizing thin film optics and invoking the effective medium theory concept as detailed in [11]. The existence of the flat region indicates that the optical properties of liquid silicon do not depend strongly on temperature. The constant emissivity value of about 0.15 is in agreement with the value of 0.156 obtained from thin film optics calculations using the measured data. The flat region (1) of the transient melt depth trace indicates that the film is completely molten across the entire 50 nm thickness. The molten Si cools very rapidly, > 1010 K/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 60–70 ns, which exactly coincides with the end of full melting deduced by the transient conductance signal. Therefore, the temperature dip is interpreted as preceding the onset of spontaneous nucleation in supercooled liquid Si. The supercooling prior to nucleation is about 230 K. The temperature during this dip corresponds to the nucleation temperature, i.e. is substantially lower than the melting point. Upon solidification, latent heat is released, increasing the temperature of the film (re-
Fig. 4a,b. Transient front reflectivity, transmissivity, emissivity (λ = 1.52 µm) and emission signal (λ = 1.52 µm) at the angle of 45◦ (a). Temperature and melt depth histories. The laser fluence F = 229 mJ/cm2 lies in the partial melting region (b)
Fig. 5a,b. Transient front reflectivity, transmissivity, emissivity (λ = 1.52 µm) and emission signal (λ = 1.52 µm) at the angle of 45◦ (a). Temperature and melt depth histories (b). The laser fluence F = 365 mJ/cm2 generates complete melting and liquid superheat
S298
gion 2). Following this temperature rebound, growth of the solid continues as heat is conducted into the substrate (region 3). The rate of nucleation increases in the deeply supercooled liquid [8] and spontaneous nucleation results in a fine-grain structure.
1685 K. Hence, it is verified that nanosecond laser-heated a-Si melts at a temperature of about 100–150 K lower than crystalline Si. Calorimetric studies of thick a-Si layers prepared by high energy implantation yielded a melting temperature of 1420 K [15]. The present study provided direct temperature measurement during the pulsed laser annealing process.
2.2 Recrystallization of poly- Si versus a- Si films Poly-Si samples, fabricated by excimer laser crystallization (average grain size of 120 nm), were used. The melting behavior for initial material a-Si was compared to poly-Si by examining the melting duration and melt depth. The threshold fluences for surface melting, Ft , and complete melting, Fc , are lower for the a-Si film than for the poly-Si film. Both the melt duration and depth in the poly-Si film are smaller than their counterparts for a-Si at the same fluence. These effects are mainly caused by the difference in melting point and thermal conductivity. Spectroscopic ellipsometry measurement of the film complex refractive index at room temperature showed that at λ = 248 nm the absorptivity of a-Si (α = 0.376) is close to the absorptivity of poly-Si (α = 0.367). Figure 6 compares the measured peak temperature values for a-Si and poly-Si. The poly-Si melting temperature exhibits a plateau slightly below 1700 K. It is noted that the equilibrium melting temperature of crystalline silicon, Tm =
Fig. 6. Comparison of the measured maximum temperatures for a-Si and poly-Si as functions of the KrF excimer laser fluence
3 Conclusions In situ diagnostics combining electrical conductance, optical reflectance/transmittance, and thermal emission are effective for investigating the temperature history and melt– resolidification dynamics of 50-nm-thick Si films. The measured maximum temperature and the melt depth unveiled the conditions for obtaining enhanced grain size in the nearcomplete melting region. Supercooling, followed by spontaneous nucleation and recrystallization to fine- grain poly-Si material could be observed in the complete melting region. It was found that poly- Si melts at a temperature close to the melting point of crystalline silicon. In contrast, the measured melting temperature of a-Si was lower by 100–150 K.
References 1. J.S. Im, R.S. Sposili, M.A. Crowder: Appl. Phys. Lett. 70, 3434 (1997) 2. H.J. Kim, J.S. Im: MRS Symp. Proc. 697, 401 (1996) 3. G. Aighmayre, D. Toet, M. Mulato, P.V. Santos, A. Spangerberg, S. Christiansen, M. Albrecht, H.P. Strunk: Phys. Status Solidi A 166, 659 (1998) 4. R. Ishihara, M. Matsumura: Jpn. J. Appl. Phys. 36, 6167 (1998) 5. G. Groos, M. Stutzmann: J. Non-Cryst. Solids 227, 938 (1998) 6. V.V. Gupta, H.J. Song, J.S. Im: Appl. Phys. Lett. 71, 99 (1997) 7. R.F. Wood, G.A. Geist: Phys. Rev. Lett. 57, 873 (1986) 8. S.R. Stiffler, M.O. Thompson: Phys. Rev. Lett. 60, 2519 (1988) 9. E.E. Jellison, D.H. Lowndes, D.N. Mashburn, R.F. Wood: Phys. Rev. B 26, 2111 (1986) 10. X. Xu, R.E. Russo, C.P. Grigoropoulos: Appl. Phys. A 62, 51 (1996) 11. M. Hatano, S.J. Moon, M.H. Lee, K. Suzuki, C.P. Grigoropoulos: J. Appl. Phys, to appear January 2000 12. G.J. Galvin, M.O. Thompson, J.W. Mayer, R.B. Hammond, N. Paulter, P.S. Peercy: Phys. Rev. Lett. 48, 33 (1982) 13. V.M. Galzov, S.N. Chizhevskays, N.N. Glagoleva: Liquid Semiconductors (Plenum, New York 1969) 14. K. Murakami, O. Eryu, K. Takita, K. Masuda: Phys. Rev. Lett. 59, 2519 (1987) 15. E.P. Donovan, F. Spaepen, D. Turnbull, J.M. Poate, D.C. Jacobson: J. Appl. Phys. 57, 1795 (1985)
