Study on High-Mobility Low-Temperature Polycrystalline Silicon and Silicon-Germanium Thin Film Transistors Fabricated by Excimer Laser Crystallization
|關鍵字:||薄膜電晶體;準分子雷射結晶;複晶矽鍺;橫向晶粒成長;鍺偏析;thin film transistor;excimer laser crystallization;poly-SiGe;lateral grain growth;Ge segregation|
雖然利用鍺摻雜技術可以有效的提升複晶矽薄膜電晶體的載子移動率，但是對於元件的均勻性卻沒有辦法改善。因此我們提出了一個新穎的側向結晶方式，在想要的區域上控制晶粒的橫向成長，如此一來，除了可以提高元件之載子移動率外，還可以進一步的改善元件的變異性。此一結構為利用傳統間隙壁製程，在局部微小區域產生兩種厚度不同的非晶矽薄膜，當準分子雷射照射在此一結構上使較薄的區域完全熔融時，晶粒便會以這些非晶矽間隙壁為結晶起始點，做橫向成長。由實驗的結果發現，當我們適當的選擇間隙壁的高度，可以形成2.5um長之長型晶粒。此外，由於在此結晶方法中，間隙壁是作為結晶的起始點，因此適當的安排間隙壁與元件通道的相關位置，我們將可以在通道中除去所有垂直於電流方向的晶粒邊界，更進一步的改善元件的驅動能力。以通道長度為2um的元件為例，以此結晶方法做出的元件其載子移動率可以到達280 cm2/V-s，而傳統的元件只有128 cm2/V-s.
In this thesis, based on the excimer laser crystallization (ELC) technique, the influences of laser crystallization parameters on the electrical characteristics and uniformity of the low-temperature polycrystalline silicon thin film transistors (LTPS TFTs) have been investigated. From the two perspectives of channel material and crystallization structure, several approaches have been proposed to further enhance the performance of LTPS TFTs respectively. In addition, the electrical characteristic and long-term reliability of the low-temperature gate oxide have been also improved by using short-time plasma treatments. First, LTPS TFTs have been fabricated with excimer laser crystallization and the effects of several laser process parameters on the device electrical characteristics are investigated in detail. According to the material analysis results, it is observed that the resulted grain size and crystallinity of the ELC poly-Si thin film are greatly related to the applied laser energy density. Large grains can only be produced in a narrow laser process window. Meanwhile, in this narrow laser density window, the non-uniform distribution of grain size can be easily found due to the instability of the excimer laser itself. This leads to a large variation in the electrical characteristics of ELC LTPS TFTs, especially when the device dimension is comparable with the grain size of the poly-Si thin film. In addition, although better crystallinity can be produced with ELC, the surface roughness of the ELC poly-Si thin film is more apparent compared to those crystallized with other techniques. The enormous surface roughness makes the thickness of gate oxide unable to decrease with the device dimension. The thicker gate oxide directly reduces the driving ability of the LTPS TFTs. Thus, in this work, short-time plasma treatment is adopted to improve the electrical characteristics and reliability of the thin gate oxide. From the experimental results, the electrical characteristics and reliability of thin gate oxide can be ameliorated effectively after short-time NH3 plasma treatment. In order to further enhance the driving ability of the LTPS TFTs, another material is tried to replace the poly-Si thin film in the channel layer. At present, polycrystalline silicon-germanium (poly-Si1-xGex) is an excellent candidate as an alternative to poly-Si for the channel active layer due to the high carrier mobility of Ge atom. Thus, poly-Si1-xGex thin film is adopted as the device channel material in this work. The mechanism of amorphous Si1-xGex (a-Si1-xGex) thin film deposition and excimer laser crystallization of a-Si1-xGex thin films are investigated in detail. To overcome the nucleation problem of the deposition of a-Si1-xGex thin film on SiO2, a thin seed Si layer is pre-deposited on SiO2 surface by exposing the SiO2 substrate to SiH4 precursor for a short time. The thin Si film deposition (Si flash technique) on oxide is intended to provide adequate nucleation sites on the oxide surface for subsequent growth of a-Si1-xGex thin film. Besides, the experimental results demonstrate that a-Si1-xGex thin films can be deposited at lower temperature by low-pressure chemical vapor deposition (LPCVD) than a-Si thin films owing to the better catalytic effect of Ge atoms. As the GeH4 to SiH4 gas flow ratio increases, the deposition rate of a-Si1-xGex thin films is enhanced. According to the material analysis results, a-Si1-xGex thin film can be effectively converted to polycrystalline phase after excimer laser irradiation. However, due to the difference in the melting point of Si and Ge atoms, Ge segregation occurs seriously at film surface and grain boundary during ELC process. Furthermore, ELC poly-Si1-xGex thin films exhibit worse crystallinity in comparison with ELC poly-Si counterparts. As the result, TFTs fabricated by direct laser crystallization of a-Si1-xGex thin film will reveal a degraded performance because of these process related issues, such as worse crystallinity and Ge segregation. In order to improve the problems of Ge segregation and worse crystallinity, two novel modified processes are proposed to fabrication high-performance poly-Si1-xGex TFTs. In the first approach, a-Si1-xGex thin film of a 500Å thickness is deposited on oxidized silicon wafer and subjected to the first ELC process. After the first laser irradiation, another 500Å-thick a-Si thin film is deposited on top of the ELC poly-Si1-xGex thin film. Next, the second excimer laser irradiation is performed to crystallize the upper a-Si thin film. The Ge atoms diffuse upward into the Si layer during laser irradiation and naturally formed poly-Si1-xGex thin film in the end. Because of the less shot numbers and rapid vertical solidification velocity during the second ELC, the Ge segregation can be suppressed effectively. Although the device performances are improved by introducing a Si capping layer, the worse crystallinity of ELC poly-Si1-xGex active layer still limit the electrical properties of the devices. Thus, the second modified process is provided to solve the problem of worse crystallinity while still suppress the Ge segregation. In this method, a-Si thin film is used as the starting layer and exposed to the first excimer laser irradiation, in which the laser parameters are optimized. Next, a thin Si1-xGex layer is deposited on the poly-Si thin film and subjected to the second laser irradiation. The Ge atoms diffuse downward into the poly-Si layer during the second laser crystallization and naturally form poly-Si1-xGex thin film in the end. Because of the better crystallinty of ELC poly-Si thin film, the two issues of crystallinity and Ge segregation for ELC poly-Si1-xGex thin film can be entirely improved. Since the good crystallinity and carrier mobility enhancement by Ge incorporation, Ge-doped ELC poly-Si1-xGex TFTs exhibit excellent carrier mobility and high driving current in short channel devices. The mobility and drain current of the Ge-doped ELC poly-Si0.91Ge0.09 TFTs with W / L = 2 um / 2um areenhanced by 41% and 52% than those of the conventional ELC poly-Si counterparts. Although the field-effect mobility of poly-Si TFTs can be effectively enhanced by incorporation of Ge atoms in the channel region, the uniformity of device performance is still an unsolved problem. A novel lateral crystallization method, in which the large and uniform longitudinal grains can be artificially produced in the desired local region, is proposed to improve the carrier mobility as well as the device uniformity. In this method, a-Si thin film with two kinds of thicknesses in a local region is fabricated by using conventional spacer process. As a proper laser fluence is applied on the a-Si thin film, in which the thin part of a-Si is completely melted, the grains will grow laterally from the un-melting a-Si spacer seeds in the thick part of the a-Si towards the thin region where the Si is completely melted. From the SEM analysis, large longitudinal grains about 2.5 um long can be acquired when the thickness of a-Si spacer is suitably adjusted. On the other hand, the lateral grain growth starting from the a-Si spacer seed can progress along the opposite direction. Hence, when the channel region is designed to arrange at the spacer region, the boundary perpendicular to the current flow in the channel region is reduced, which further enhance the driving ability of TFT devices. Take the dimension of W = L = 2 um for example, poly-Si TFT with field effect mobility of about 288 cm2/V-s can be achieved by using this a-Si spacer method while the mobility of the conventional counterpart is about 128 cm2/V-s. Except for producing single longitudinal grain in the channel region, periodic lateral longitudinal grains can also be fabricated with the a-Si spacer structure. Owing to the tiny width of the a-Si spacer (< 1000Å), the a-Si spacers, i.e. the nucleation seeds, can be arranged periodically inside the channel region. Consequently, under suitable spacer distance and ELC conditions, periodic lateral grain growth can be acquired at the channel region of large-dimension device. This helps to improve the driving ability of the large dimension TFTs. Thus, the proposed crystallization technique become useful in the field of system on panel (SOP) because the circuits on a single panel need varied sizes of TFT devices for different applications.
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