Effect of CF[sub 4] Plasma on Properties and Reliability of Metal-Induced Lateral Crystallization Silicon Transistors

Abstract

In this study, a CF 4 plasma etching treatment was applied to metal-induced lateral crystallization ͑MILC͒ polycrystalline silicon thin film transistors ͑poly-Si TFTs͒. It was found that the electrical properties and reliability of the MILC poly-Si TFTs were improved by the treatment. The minimum off-state currents were also reduced. This is because this etching method involves not only passivating the trap states but also etching away the Ni-related defects on the top surface of MILC poly-Si.In recent years, low temperature polycrystalline silicon ͑poly-Si͒ thin film transistors ͑TFTs͒ have attracted considerable interest for their use in active-matrix liquid crystal displays because they exhibit good electrical properties and can be integrated into peripheral circuits on inexpensive glass substrates. 1 Because poly-Si TFTs require glass substrates, intensive studies have been carried out to reduce the crystallization temperature of amorphous silicon ͑␣-Si͒ films.Many techniques for crystallizing ␣-Si have been proposed. Among them, solid-phase crystallization ͑SPC͒ is a well-established poly-Si formation technique which offers several advantages including smoother surfaces, superior uniformity, and batch process in furnace annealing. 2,3 The major drawback of SPC is that the ␣-Si films need to be annealed for 24-48 h at 600°C, a temperature higher than the strain temperature of a normal glass substrate. Moreover, the process is time-consuming. Furthermore, the resultant lower field effect mobility limits the development of SPC poly-Si TFTs.Compared with SPC, both Ni-metal-induced crystallization ͑Ni-MIC͒ and Ni-metal-induced lateral crystallization ͑Ni-MILC͒ involve a lower thermal budget. In MIC and MILC, a thin Ni metal layer is selectively deposited on the top of the ␣-Si film, followed by annealing at a temperature ͑ϳ500°C͒ much lower than 600°C. 4-9 Three stages have been identified in the process: ͑i͒ the formation of NiSi 2 precipitates, ͑ii͒ the nucleation of crystalline silicon ͑c-Si͒ on NiSi 2 precipitates, and ͑iii͒ the subsequent migration of NiSi 2 precipitates and growth of c-Si. 10-12 The poly-Si formed below the metal film is called "MIC," while that formed outside the metal coverage is called "MILC." Crystalline Si nucleates on one or more of the eight ͕111͖ faces of octahedral NiSi 2 . The crystallization of needlelike Si grains proceeds via the migration of nickel silicides through ␣-Si. MILC has been proposed as a better alternative to MIC, which yields poly-Si films with reduced Ni incorporation and large needlelike grains. 10,11 Unfortunately, the MILC poly-Si grain boundaries trap Ni and NiSi 2 precipitates, which increase the leakage current and shift the threshold voltage. [13][14][15][16][17] This problem can be solved using the Nigettering method. 18 However, the crystal quality of a Ni-gettering poly-Si film was poorer than that of the conventional MILC poly-Si film.To eliminate the trap states of the poly-Si film, a hydrogen plasma treatment process has been utilized to improve the ...

Improvement in performance and reliability with CF4plasma pretreatment on the buffer oxide layer for low-temperature polysilicon thin-film transistor

“…It is well known that dissociation of CF 4 gas into fluorine ions by RF discharge is a commonly used etching process. It was reported [13,17] that the CF 4 plasma can induce damage at the interface of gate oxide/poly-Si to result in poor device electrical characteristics and reliability, resulting from plasma induced poly-Si surface roughness. To avoid CF 4 plasma-induced damage on the poly-Si film, the CF 4 plasma pretreatment is conducted on the buffer oxide layer before the deposition of the poly-Si channel layer.…”

Effects of crystallinity of silicon channels formed by two metal-induced lateral crystallization methods on the cell current distribution in NAND-type 3D flash memory

“…1) Therefore, cell conductance degradation owing to the increasing number of WL stacks is a critical issue. State-of-the-art metal-induced lateral crystallization (MILC) techniques 2,3) are some of the most promising techniques in 3D flash memory with ultrahigh (>300) WL stacks; [4][5][6] this technique is called "single MILC" in this study. Meanwhile, "Regional MILC," a unique technique, is another MILC candidate.…”

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

“…4 In most published work, the Ni is defined either by a lift-off process [5][6][7][8][9] or by Ni deposition in a window opened in a cap layer, which is normally deposited silicon dioxide.…”

“…1 For MILC, nickel reacts with amorphous silicon (α-Si) to form nickel disilicide and the lateral transport of Ni induces crystallization of the adjacent amorphous silicon, 2,3 creating a crystallized region with low Ni contamination suitable for high performance transistors. 4 In most published work, the Ni is defined either by a lift-off process [5][6][7][8][9] or by Ni deposition in a window opened in a cap layer, which is normally deposited silicon dioxide. [10][11][12][13][14][15] So far, no work has been reported that compares the effect of these two different Ni definition techniques on the lateral crystallization.…”