Ni metal-induced crystallization (MIC) / Ni metal-induced lateral

Solid phase crystallization of α-Si needed a high temperature and longer annealing time for furnace annealing process. In the NIC/NILC method, the annealing time and temperature could be reduced, and the grain size of NILC poly-Si films uniformly over large area could be obtained [16]-[19]. In 2000, Sharp Corp. and SEL (Semiconductor Energy Lab.) propose the CGSi (Continuous Grain Silicon) technique to fabricate the 60 inch HDTV rear projector [20]. When thin Ni is deposited on α-Si and annealed, Ni disilicide (NiSi2) forms [21]. The nickel disilicide is cubic with CaF2 structure and has a very close lattice parameter match to c-Si (-0.4%), the lattice constant of NiSi2, 5.406A, is nearly equal to that of Si, 5.430A. The disilicide is actually the species that mediates the transformation of α-Si to c-Si. As shown in Fig. 1-6, the c-Si formed below the Ni-pad is called NIC and the lateral growth is called NILC.

Fig. 1-6 Schematic illustration of the Ni-metal induced Crystallization (MIC) and Ni-metal induced lateral crystallization (MILC)

The silicide mediate growth of silicon occurs in three stages. In the first stage, precipitation and growth of NiSi2 occur in the temperature range of 325~400°C. In the second phase, crystalline Si nucleates on one or more the eight {111} faces of the octahedral NiSi2, as shown in Fig. 1-7. Finally, in the third phase, c-Si growth proceeds with a NiSi2 precipitate at the planar advancing growth front.

As shown in Fig. 1-7, for <110>-oriented precipitates, four of the {111} planes exhibit surface normal within the planes of the film, which makes extensive growth possible. On the other hand, the <100>- and <111>-oriented precipitates exhibits {111} planes normal that intersect the upper and lower surface of α-Si films.

Fig. 1-7 Schematic representation of favorable precipitate orientations for long-range growth of epitaxial Si within the plane of the α-Si.

The driving force for the migration of NiSi2 precipitates is reduction in the free energy associated with the transformation of meta-stable α-Si to stable c-Si. An equilibrium free-energy diagram is provided for explanation, as shown in Fig. 1-8 [21]. It is well known that the α-Si has a higher free energy than c-Si. In the case of Ni silicide mediated crystallization, the free energy difference between Ni and Si atoms at the NiSi2/α-Si and NiSi2/c-Si interface acts as the driving force for Ni diffusion [21]. The free energy of the Ni atom is lower at the NiSi2/α-Si interface than at the NiSi2/c-Si interface, whereas the free energy of the Si atom is lower at the NiSi2/c-Si interface. Therefore, with the dissociative model [21], the NiSi2 layer dissociates to provide free Si for epitaxial growth of c-Si at the c-Si/NiSi2 interface by the diffusion of Ni atoms. The Ni atoms diffuse to α-Si following by formation of a fresh NiSi2/α-Si interface. Repetition of this process results in migration of NiSi2 precipitates through α-Si and growth of needlelike Si. Fig.

1-1Fig. 1-9 shows a schematic representation of a possible growth process incorporating

the formation of intermediate thin layer of c-Si on the leading edge of migrating NiSi2

precipitate [21]. As a result of this growth mechanism, NILC poly-Si films demonstrate a needlelike microstructure, with each needle grain attribute to c-Si growth from an individual disilicide precipitate.

Fig. 1-8 Schematic equilibrium molar free-energy diagram for NiSi2 in contact with α-Si

In addition to Ni, other metals have been investigated as far as their effectiveness in enhancing Si crystal growth. These include Au [22], Al [23] and Sb [24] which form eutectic with Si, and Pd [25], [26], which forms silicide with Si.

Fig. 1-9 Schematic representation of a possible growth mechanism involoving the formation of a thin layer of c-Si at the NiSi2/α-Si interface

As a result, Ni remains the undisputed metal of choice for silicide-assisted crystallization. It should be noted that traces of NiSi2 also remain within the c-Si that is left behind after the growth phase. This would have presented an insurmountable obstacle had it not been for the existence of an efficient gettering process [27], [28]. This process utilizes the implantation of phosphorous, followed by low-temperature annealing to generate electrically inactive compounds. Previous studies have demonstrated the

effectiveness of the gettering process in removing the remaining silicide in the film after Si crystallization [20], [29]. In practice, the necessity to maintain a low processing temperature poses certain limitations on the quality of the poly-Si microstructure. One way to boost the poly-Si quality is by combining MILC with laser annealing process to produce high quality and good uniformity poly-Si films to realize the system-on-panel technology [30]-[33].

In this thesis, we will focus on the Ni Metal induced lateral crystallization method. To produce high performance LTPS TFTs by combined MILC and CLC method. Moreover, discuss the growth mechanism of MILC and utilized fluorine ion implantation and CF4

plasma etching treatment to fabricate the LTPS TFT. And develop a simple method to reduce the nickel impurity and passivate the trap-state density within the MILC polycrystalline silicon films by drive-in method.

1.3. Electrical properties of Ni metal-induced lateral