Minghong Lee

Excimer laser-induced temperature field in melting and resolidification of silicon thin films

The liquid/solid interface motion and temperature history during excimer laser annealing of 50-nm-thick Si films on fused quartz substrates are investigated by in situ nanosecond time-resolved electrical conductance, optical reflectance, and transmittance at visible and near-IR wavelengths, combined with 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. Because it is necessary to balance the latent heat across the propagating phase-change interface, the maximum induced temperature in the partial-melting regime remains close to the melting point of amorphous Si. The peak temperature rises in the complete-melting regime, but the nonparticipating nature of the liquid Si/fused quartz interface allows substantial supercooling (>200 K), followed by spontaneous nucleation into fine-grained material. These phase transformations are consistent with the recrysta...

Relationship between fluence gradient and lateral grain growth in spatially controlled excimer laser crystallization of amorphous silicon films

In order to clarify the relationship between excimer laser fluence gradient and the length of lateral grain growth, the excimer laser fluence used for crystallization is modulated by a beam mask. The crystallized 50-nm-thick a-Si films are secco etched in order to observe the lateral grain growth by scanning electron microscope. The fluence distribution across the pattern is measured by a negative photoresist that has a linear relationship between laser fluence and resist thickness after development. This mapped fluence distribution is utilized to deduce the fluence gradient for each laser energy output. It is shown that lateral growth length increases and the directionality of the grains improves as the fluence gradient increases. Lateral growth length as long as 1.5 μm can be driven in a 50-nm-thick a-Si film by a single excimer laser pulse without any substrate heating. Electrical conductance measurement is used to probe the solidification dynamics. The lateral solidification velocity is estimated to b...

Thermal conductivity of amorphous silicon thin films

The thermal conductivity of amorphous silicon thin films is determined by using the non-intrusive, in situ optical transmission measurement as well as by the 3ω method. The temperature dependence of the film complex refractive index is determined by spectroscopic ellipsometry. The acquired transmission signal is fitted with predictions obtained by coupling conductive heat transfer with multi-layer thin film optics in the optical transmission measurement. The results of the two independent methods are in close agreement.

In situ and ex situ diagnostics on melting and resolidification dynamics of amorphous and polycrystalline silicon thin films during excimer laser annealing

The liquid/solid interface motion and the temperature history of 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-infra-red wavelength, and thermal emission measurements. The existence of partial and complete melting regimes are detected. In the partial melting regime, the maximum temperature remains close to the melting point of amorphous Si (a-Si), since the laser energy is consumed by the latent heat of phase-change. In the complete melting regime, substantial supercooling, followed by spontaneous nucleation is observed. These phase transformations are consistent with the recrystallized poly-Si morphologies. It is also found that phase change temperature of poly-Si films is about 140 K higher than that of the a-Si films.

Heat Transfer and Phase Transformations in Laser Annealing of Thin Si Films

Recrystallization of thin amorphous silicon (a-Si) films can yield polysilicon (p-Si) material with functional properties suitable for fabrication of electronic devices, including high definition large area active matrix liquid crystal displays. Pulsed laser-effected melting and recrystallization is exceptionally effective since it avoids damage to the underlying insulator structure. The ensuing phase transformations and ultimately the quality of the produced p-Si material strongly depend on the temperature history. This article presents a review of research aiming to understand the complex nucleation, resolidification and crystal growth phenomena that evolve under severely non-equilibrium conditions. It is shown that elucidation of the fundamental thermodynamic processes enables conception of novel practical thin film crystal growth techniques.

INTERPRETATION OF OPTICAL DIAGNOSTICS FOR THE ANALYSIS OF LASER CRYSTALLIZATION OF AMORPHOUS SILICON FILMS

The focus of this investigation is on optical diagnostics of melting and solidification during excimer laser crystallization of a-Si. Although optical techniques are effective for probing the laser annealing process, the analysis of the results is not straightforward due to temperature variation of the optical properties, phase change, and interference effects. These effects are clarified by analyzing the front and back, s- and p-polarized reflectivity traces at the lambda=633-nm and 1,520-nm laser wavelengths. Ellipsometry was utilized to measure the temperature-dependent spectral optical properties of the deposited 50-nm a-Si films. Spectral optical properties of laser-annealed poly-silicon were also measured at room temperature. These optical properties were used in the optical models constructed for the melting and solidification processes. Comparison with the experimental results unveiled evidence of spontaneous volumetric nucleation for laser fluences sufficiently high to impart complete melting of ...The focus of this investigation is on optical diagnostics of melting and solidification during excimer laser crystallization of a-Si. Although optical techniques are effective for probing the laser annealing process, the analysis of the results is not straightforward due to temperature variation of the optical properties, phase change, and interference effects. These effects are clarified by analyzing the front and back, s- and p-polarized reflectivity traces at the lambda=633-nm and 1,520-nm laser wavelengths. Ellipsometry was utilized to measure the temperature-dependent spectral optical properties of the deposited 50-nm a-Si films. Spectral optical properties of laser-annealed poly-silicon were also measured at room temperature. These optical properties were used in the optical models constructed for the melting and solidification processes. Comparison with the experimental results unveiled evidence of spontaneous volumetric nucleation for laser fluences sufficiently high to impart complete melting of the a-Si film.

Chapter 2 Heat Transfer and Phase Transformations in Laser Melting and Recrystallization of Amorphous Thin Si Films

Publisher Summary This chapter presents the basic crystallization mechanisms, including the explosive crystallization and interface kinetics. A variety of recrystallization phenomena are developed during the pulsed laser annealing of amorphous silicon (a-Si) samples. The thermal history in excimer laser crystallization melting and recrystallization of thin a-Si films has been experimentally probed by an array of in situ diagnostics. The situ experiments combining time-resolved optical probes and electrical conductance measurements Are presented and data revealing the nucleation process are examined. The chapter introduces a new double-laser recrystallization experiment enhancing the lateral growth length. Motivated by the results of the previous study, the double-laser recrystallization technique is developed in order to improve grain size, uniformity, and hence device performance. The evolution of the resolidification process that leads to lateral grain growth is visualized using high-speed laser flash photography. Issues related to the possibility and role of explosive crystallization in thin films clearly remain open to investigation and new experimental probes have to be developed to unveil with precision the temporal sequence of nucleation at high quenching rates and accurately quantify the microscale temperature field across a rapidly advancing solid–liquid interface.

Transient temperature measurement of amorphous silicon thin films during excimer laser annealing

The excimer laser annealing of amorphous silicon thin films has been investigated via optical diagnostics. Amorphous silicon films of 50 nm thickness are used in laser annealing. To obtain the transient temperature variation in the laser annealing process, the thermal emission and near- IR optical properties are measured. The front transmissivity and reflectivity are measured to obtain the emissivity at the 1.52 micrometers wavelength of the probe IRHeNe laser. Significant undercooling of the liquid silicon is observed during the cooling stage. The emissivity is almost constants during the melting period, but increases during the melting and solidification transformations.

High-resolution laser flash photography for probing the solidification of the new double laser recrystallization process

A new double laser recrystallization technique that can produce lateral grains of tens of micrometers is presented. A nanosecond laser (excimer or Nd:YLF laser) and a pulse modulated ArPLU laser are used in the experiment. The effect of different parameters on lateral grain growth is investigated. These parameters include the time delay between the two lasers, the excimer laser fluence, the ArPLU laser power and the pulse duration. This process has wide process window and is insensitive to both the excimer laser fluence and the ArPLU laser power fluctuations. Preheating and melting of the a-Si film with the ArPLU laser before firing the excimer laser is found to be necessary for inducting lateral grain growth. The transient excimer laser irradiation is believed to generate nucleation sites for initiating the subsequent lateral grain growth. The solidification dynamics of the process is probed by high spatial and temporal resolution laser flash photography. A lateral solidification velocity of about 10m/s observed.

High resolution laser flash photography for probing the new double laser recrystallization process

Summary form only given. Laser recrystallization has been demonstrated as an efficient technology for obtaining poly-Si TFTs for advanced flat panel display applications. In order to improve grain size, uniformity and hence device performance, the double laser recrystallization technique, which is a process that can produce ultra-large direction- and location-controlled lateral poly-Si grains, is developed. The technique uses a modulated CW Ar/sup +/ laser in conjunction with a superposed nanosecond laser pulse to achieve lateral grains. This process has a wide fluence process window and is insensitive to the power fluctuation of both lasers. The results show preheating and melting of the a-Si film with the Ar/sup +/ laser before firing the nanosecond laser is necessary for inducing lateral grain growth. The transient nanosecond laser irradiation is believed to generate nucleation sites for initiating the subsequent lateral grain growth. In order to clarify the ultralarge grain formation mechanism, high spatial and temporal resolution laser flash photography is used to probe the solidification process.