Lateral Crystallization Of Amorphous Silicon Research Articles

Sequential lateral crystallization of amorphous silicon on glass by blue laser annealing for high mobility thin-film transistors

We report the thickness effect of amorphous silicon for the polycrystalline silicon (poly-Si) layer laterally crystallized by blue laser annealing (BLA) using 50 μs melting time. The grain size is much larger and full width at half-maximum of Raman intensity is lower compared to those of the poly-Si by excimer laser annealing. It is found that the average width of lateral grain is wider than 3 μm and full width of half maximum of Raman intensity for the BLA poly-Si with an optimum thickness of 90 nm is 3.32 cm−1. The p-type poly-Si thin-film transistor with 90 nm exhibits field-effect mobility of 161.91 ± 6.14 cm2/Vs and subthreshold swing of 227 ± 7 mV/dec.

Inclusions in Si whiskers grown by Ni metal induced lateral crystallization

The formation of Nickel-di-silicide inclusions in silicon whiskers grown during low temperature Ni Metal Induced Lateral Crystallization of amorphous Silicon was studied by High Resolution Transmission Electron Microscopy, Scanning Transmission Electron Microscopy, and Electron Energy Loss Spectroscopy. The heat treatment of the samples lasted for 11 + 11 days at 413 °C for the first 11 days and 442 °C for the rest of the time. The size of the inclusions ranges from just a few atoms to 15–20 nm. It was shown that the NiSi2 inclusions have the form of tetrahedrons, which are bound by {111} coherent interfaces with the Si matrix. These inclusions are homogeneously distributed along the whiskers, and the Ni percentage incorporated in these is 0.035 at. %. The tetrahedral inclusions are formed by trapping NiSi2 clusters at the Si/NiSi2 interface during whisker growth.

Structural characterization of nanostructures grown by Ni metal induced lateral crystallization of amorphous-Si

The nickel metal induced lateral crystallization of amorphous silicon is studied by transmission electron microscopy in the range of temperatures from 413 to 521 °C. The structural characteristics of the whiskers grown at 413 °C are compared to the grains grown at 600 °C, where both Metal Induced Lateral Crystallization (MILC) and Solid Phase Crystallization (SPC) are involved. At 413 °C, long whiskers are formed at any crystallographic direction almost free of defects. In contrary, whiskers grown by MILC around 600 °C are crystallized along the ⟨111⟩ directions. These differences are attributed to the low crystallization rate and suppression of the SPC process. The activation energy of the pure MILC was measured in the order of 2 eV. The effect of Ni on the crystallization rate is studied by in-situ heating experiments inside the microscope. The role of contamination that can inhibit MILC is discussed. The cases of MILC process under limited Ni and unlimited Ni source were studied and compared to in-situ annealing experiments. The crystallization rate is strongly influenced by the neighbouring Ni sources; this long-range interaction is attributed to the requirement of a critical Ni concentration in amorphous silicon before the initiation of the MILC process. The long-range interaction can enhance crystallization along a certain direction. The transition from MILC to SPC and the change of the crystallization mode due to the lack of Ni are discussed. The beneficial effect of long annealing at 413 °C is also discussed.

Effect of an Oxide Cap Layer and Fluorine Implantation on the Metal-Induced Lateral Crystallization of Amorphous Silicon

In this work, we investigate the effect of oxide cap layer on the metal-induced lateral crystallization (MILC) of amorphous silicon. The MILC is characterized at temperatures in the range 550 to 428°C using Nomarski optical microscopy and Raman spectroscopy. It is shown that better lateral crystallization is obtained when the oxide cap layer is omitted, with the crystallization length increasing by 33% for a 15 hour anneal at 550°C. A smaller increase of about 10% is seen at lower temperatures between 525°C and 475°C and no increase is seen below 450°C. It is also shown that the detrimental effect of the oxide cap layer can be dramatically reduced by giving samples a fluorine implant prior to the MILC anneal. Raman spectroscopy shows that random grain growth is significantly less for unimplanted samples without an oxide cap and also for fluorine implanted samples both with and without an oxide cap. The crystallization length improvement for samples without an oxide cap layer is explained by the elimination of random grain crystallization at the interface between the amorphous silicon and the oxide cap layer.

Effect of compressive stress on nickel-induced lateral crystallization of amorphous silicon thin films

The present study describes a method enabling the metal-induced lateral crystallization (MILC) of amorphous silicon (a-Si) films. Glass substrates coated with a-Si films were contacted with ground nickel sheets under compressive stresses ranging from 3.7 to 265.8MPa at 550°C for 1h. Subsequently, the nickel sheet and stress were removed and the specimens were annealed at 550°C for 1 to 4h. The experimental results indicate that the extent of MILC decreased when the preliminary compressive stress was increased, while all specimens exhibited the same rate of lateral crystallization during annealing. The present study indicates that, by applying an appropriate compressive stress (~4MPa), an effective method to reduce the residual Ni content in polycrystalline silicon (poly-Si) can be obtained.

Mechanism of Aluminum Induced Lateral Crystallization of Amorphous Silicon

The aluminium-induced lateral crystallization (AILC) of amorphous silicon (a-Si) on a glass substrate has been investigated. By means of a photoresist-based process, Al islands (100 nm) were thermally evaporated using 15 V, 3.5 A, and 25 V, 5.6 A on the a-Si layer (100 nm), which was deposited on a glass substrate. SEM examinations indicated that the Al islands exhibited smooth and crystalline-grain morphology. Annealing processes were carried out at 748 and 823 K for various times. After annealing, AILC could be clearly observed in the crystalline-grain sample, but not in the smooth sample. TEM analyses showed that the mechanism of AILC resulted from two layer exchange processes. First, the Al islands exchanged with the underlying a-Si layer vertically during AIC, and then the generation of Al particles accompanying AIC caused a lateral layer exchange with the remaining a-Si layer with further annealing.

Roughness Reduction in Polycrystalline Silicon Thin Films Formed by Continuous-Wave Laser Lateral Crystallization with Cap SiO2 Thin Films

In continuous-wave (cw) laser lateral crystallization of amorphous silicon (a-Si) thin films, the effects of cap SiO2 thin films were investigated. The thickness of the cap SiO2 film was 10 nm, which is sufficiently thin to exclude anti reflection effect. The cap SiO2 thin films suppressed the generation of voids during the cw laser crystallization, and the available crystallization condition to form lateral-crystallized polycrystalline silicon (poly-Si) thin films was expanded. By using the cap SiO2 thin films, the surface of the lateral-crystallized poly-Si thin films became smooth, and an average roughness of 1.3 nm was achieved.

Effect of Ni Thickness on Off-State Currents of Poly-Si TFTs Using Ni-Induced Lateral Crystallization of Amorphous Silicon

We have studied the effect of metal thickness for the metal-induced lateral crystallization (MILC) of a-Si on the off-state currents of poly-Si thin-film transistors (TFTs). It is found that they decrease with decreasing the Ni thickness for MILC. The Ni-MILC poly-Si TFT with the Ni area density of 1.4 X 10 14 cm -2 exhibited a field-effect mobility of 53.5 cm 2 /Vs and minimum leakage current of 0.44 pA/μm at V ds = -10 V. However, the off-state current is 6.6 pA/μm when the Ni density is 9.2 X 10 15 cm -2 , corresponding to the average Ni thickness of 10 A. It is concluded that the average Ni thickness for MILC should be less than 0.79 A to have low off-state currents from our experimental data.

Increased Lateral Crystallization Width during Nickel Induced Lateral Crystallization of Amorphous Silicon Using Fluorine Implantation

This paper reports a study of the effect of fluorine implantation on the nickel-induced lateral crystallization of amorphous silicon. To distinguish the effects of the fluorine and the implantation damage, the fluorine implant is either made directly into the or into the buffer oxide below the . For a anneal at 500°C, both types of fluorine implant give a 65% increase in the lateral crystallization width, a five times reduction in the density of nickel silicide precipitates, and an improved grain texture. In contrast, for anneals at 550 and 600°C, both types of fluorine implant give 29% and 85% reductions in the lateral crystallization width, respectively. The identical results obtained for fluorine implantation into the and the buffer oxide indicates that the effects observed are due to chemical effects of the fluorine rather than implantation damage in the . The increased crystallization width at 500°C is explained by the suppression of random crystallization at the bottom interface. The reduced crystallization widths at 550 and 600°C are attributed to the diffusion and activation of fluorine and the formation of bonds making the more resistant to silicide-mediated phase transformation.

Poly and Nano-crystalline High Electron Mobility Thin Film Transistors on Plastic Substrates for Large Area Applications

AbstractThin-film transistors (TFT) of poly and nano crystalline silicon have been made at temperature as low as 170°C on flexible PET (polyethylene terephthalate) substrates.The crystallization of the silicon film has been achieved using external mechanical stress assisted by a plasma hydrogenation technique. The formation of TFT is possible by means of a lateral crystallization of amorphous silicon under the channel region. High mobility TFTs with an electron mobility of 25cm2/Vs and an on/off ratio of 2000 have been obtained. Scanning electron microscopy, X-ray diffraction analysis and optical microscopy have been used to examine the crystallinity of the layer. In addition we report the deposition of high quality low-temperature silicon-oxide layers on PET substrates using an RF-plasma enhanced chemical vapor deposition unit with direct introduction of oxygen gas into the chamber and its reaction with Silane. Infrared spectroscopy was used to examine the quality of the oxide layer.

Lateral crystallization of amorphous silicon by germanium seeding

This paper investigates the time and temperature dependence of amorphous silicon lateral crystallization when polycrystalline germanium is used as a seed. Dramatically different crystallization behaviour is observed for heavy and light crystallization anneals. For a heavy anneal of 40 h at 550 °C increased crystallization of the amorphous silicon is seen in all areas beneath the germanium seed compared with areas without any germanium, as has been reported previously. In contrast for light anneals at 500 °C crystallization of the amorphous silicon only occurs around the perimeter of the germanium seed. The perimeter lateral crystallization is reasonably uniform, reaching a distance of 500 nm after a 60 h anneal at 500 °C. Transmission electron microscopy (TEM) shows that the crystallized material is polycrystalline and made up of grains with various orientations. This different behaviour for short and long anneal times suggests that two different mechanisms are taking place; we speculate that the perimeter crystallization is due to stress.

In situ spectromicroscopic study of nickel induced lateral crystallization of amorphous silicon thin film using SPESM

Scanning photoelectron spectromicroscopy (SPESM) has been used to study nickel metal induced lateral crystallization (Ni-MILC) of amorphous silicon (a-Si) thin films, produced by in situ annealing of vacuum deposited Ni patterned films on a-Si. The spatial variations in the chemical composition of the Ni-MILC of a-Si were directly imaged. High-resolution photoemission spectra of both Si 2p and Ni 3p core levels and valence band were used to evaluate morphological changes and chemical interactions. Our direct spectromicroscopic characterization clearly shows that the Ni-MILC process in UHV leads to the lower crystallization temperature and a faster crystallization speed of a-Si, and a poly-Si film with high-crystallinity can be obtained. A unified mechanism for the enhanced growth rate of the high-crystallinity poly-Si film produced by Ni-MILC in UHV is proposed.

Effects of oxygen in Ni films on Ni-induced lateral crystallization of amorphous silicon films at various temperatures

Effects of oxygen in Ni films on the Ni-induced lateral crystallization (NILC) of amorphous silicon (a-Si) films at various temperatures have been investigated. It was found that oxygen in Ni films retarded the nucleation of polycrystalline silicon (poly-Si) from a-Si, but had little effect on the growth rate of poly-Si. This is because that needed an incubation period to be reduced to Ni metal for the subsequent mediated crystallization of a-Si.

Geometric effect of nickel source on low-temperature polycrystalline silicon TFTs by metal-induced lateral crystallization

Low-temperature polycrystalline silicon thin-film transistors were fabricated by nickel-induced lateral crystallization of amorphous silicon. Line and oval-shaped nickel source patterns were compared. The oval-shaped nickel source was found to render better device performance, including lower leakage current and higher on/off current ratio. The observation is interpreted by the crystallization and nickel diffusion behavior. The oval-shaped nickel source introduces less nickel in the channels, which is the physical mechanism responsible for the improved performance.

Effects of Tensile Stress on Growth of Ni–Metal-Induced Lateral Crystallization of Amorphous Silicon

The Ni–metal-induced lateral crystallization (NILC) of amorphous silicon (a-Si) has been used to fabricate low-temperature polycrystalline silicon thin-film transistors. Three stages have been identified in the NILC process: (1) the formation of NiSi2 precipitates, (2) the nucleation of crystalline Si (c-Si) on NiSi2 precipitates, and (3) the subsequent migration of NiSi2 precipitates and growth of c-Si. In this study, a bending fixture was used to investigate the effects of tensile stress on the growth of NILC. It was found that tensile stress did not enhance NiSi2 formation and c-Si nucleation stages, but enhanced the c-Si growth stage. It was also found that compressive stress did not change NILC rate.

Mechanism of field-aided lateral crystallization of amorphous silicon

The crystallization mechanism of the amorphous silicon using the field-aided lateral crystallization process was studied. The crystallization initiates at the negatively biased side and progresses towards the positively biased side with an enhanced velocity. In addition, the crystallization velocity increases monotonically as the applied voltage increases to a point, and then decreases beyond the critical voltage point. The current level measurement during the heat treatment of simple test patterns showed that the resistivity of the Ni-free amorphous silicon was reduced by 4 orders of magnitude at 510°C. The current density under the bias of 100V is in the range of 100A∕cm2, which is enough to cause the electromigration at that temperature. Using the Nernst-Einstein equation, we prove that the process is governed by competition between the field-assisted diffusion and electromigration.

Dopant effect on in situ doped metal-induced lateral crystallization of amorphous silicon films

Dopant (boron or phosphorus) effect on metal-induced lateral crystallization (MILC) of amorphous silicon (a-Si) films has been systematically studied by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The MILC was realized by depositing a-Si film onto an oxidized Si wafer and then annealing in a N 2 atmosphere. The doped amorphous films have been developed by plasma-enhanced chemical vapor deposition (PECVD). Various dopant gases and annealing times were investigated to study dopant effects on MILC films. It has been found that the un-doped poly-Si film has a MILC length larger than that of samples doped with boron or phosphorus. The phosphorus-doped films exhibit larger grain size as compared with un-doped and boron-doped films. Larger MILC length is obtained by increasing annealing time, but the MILC rate is decreased. In addition, nickel-silicides were found in the doped samples by XRD. Physical mechanisms for these experimental results are proposed.

TCAD Modeling of Metal Induced Lateral Crystallization of Amorphous Silicon

AbstractIn our previous publications [1, 2] nickel diffusion and spreading resistance probe (SRP) measurements for quality control of metal induced lateral crystallization (MILC) of amorphous silicon (a-Si) were studied. Now we present TCAD modeling and an explanation of experimental results. By using ISE TCAD the Ni concentration distributions were calculated and compared with results obtained by experiments using SIMS analysis.

P-1: Peripherally Crystallized Polycrystalline Silicon (PCP) for Thin-Film Transistors

Metal-induced peripheral crystallization is proposed and demonstrated as an alternative scheme based on the general technique of metal-induced lateral crystallization of amorphous silicon. Compared to the scheme of unilateral crystallization, peripheral crystallization requires a shorter process time and reduced masking step. Compared to the scheme of self-aligned bilateral crystallization, it largely retains the better material and device properties of unilateral crystallization.

Nickel induced lateral crystallization behavior of amorphous silicon films

Nickel induced lateral crystallization of amorphous silicon with and without electric field has been studied. Dendritic silicon growth behavior is observed, with crystallites of a few hundred nanometers in width and up to a few microns in length. The behavior can be understood from the preferential epitaxial growth of silicon from the (1 1 1) facets of the NiSi 2 precipitate, which forms during the early stage of the annealing process. The dendritic growth fronts are different with and without electric field in the nickel induced lateral crystallization process. Electric field is found to be beneficial in increasing the lateral crystallization rate and improving the film crystallinity. Joule heating plays an important role as well to enhance the lateral crystallization.