Summary of Thin-Film Transistors

Although the concept of a thin-film field-effect transistor was presented as early as 1935 [42], the first functional thin-film transistor (TFT) was reported by P. K. Weimer in 1961 [43].

After that, TFTs have been intensively researched for possible electronic and display applications. The first active-matrix liquid-crystal display (AMLCD) was composed of CdSe TFTs and nematic liquid crystal [44]. Although there are many successful demonstrations of CdSe TFT LCDs, the industry production was retarded until the report on the feasibility of doping amorphous Si (α-Si) by the glow discharge technique in 1975 was introduced [45].

Since then, α-Si TFT LCDs have become the mainstream for mass-produced AMLCDs for several reasons. First, the characteristics of α-Si TFTs are remarkably well matched to the requirements of liquid-crystal driving, since they have a low off current with good on/off ratios. Second, both the gate insulator and the α-Si layers can be deposited in the same plasma-enhanced chemical vapor deposition (PECVD) system, so that contamination of the critical interface can be avoided. Eventually, α-Si TFTs can be made at low temperatures (250^°C-350^°C), thus allowing the use of inexpensive glass substrates [46].

Even if the α-Si TFTs possess aforementioned advantages, the most serious drawback is the low carrier mobility in the range of 0.5-1.0 cm²/V-s. This makes α-Si TFTs sufficient only for switching devices for each pixel in a display, and cannot meet the desired specifications for high-resolution panels. In addition, the α-Si TFT is not compatible with the CMOS process.

The problem of the low carrier mobility in α-Si TFTs can be got over by introducing polycrystalline silicon (poly-Si) replacing α-Si as a semiconductor layer of TFTs. Besides the higher carrier mobility, there are several advantages of poly-Si TFT LCDs. First, the driver circuitry can be integrated on the display’s substrate to realize the system on panels (SOP).

Thus, the size of the total panel and cost, including drivers and related procedures, are

reduced compared to the α-Si TFT LCDs. Second, the driver contact number of the poly-Si TFT LCD is more than one order of magnitude smaller than that of the α-Si:H TFT LCD.

Third, the poly-Si TFT plate has a smaller pixel size and larger aperture ratio in each pixel than that of the α-Si:H TFT plate. Higher mobility means that the pixel charging can be achieved by a smaller-size TFT, so that it contributes more pixel area for light transmission.

Finally, TFT LCD with self-alignment and COMS process compatibility can be achieved [47][48].

Because the process of high-temperature poly-Si TFTs is as high as 900^°C, the expensive quartz substrates are necessary. Due to the limited profitability of quartz substrate size, most typical applications for high-temperature poly-Si TFT-LCDs are panels for projection displays, because the panel size is limited to small sizes. For low-temperature poly-Si TFTs (LTPS TFTs), the maximum fabrication temperature is below 600^°C and hence, low-cost glass substrates could be employed. This would bring about the production of large-area displays such as monitors and televisions [49]. Therefore, LTPS TFTs have attracted much attention due to their increasing application in high-resolution flat displays such as active-matrix liquid-crystal displays (AMLCDs) [50-54], active-matrix organic light-emitting diode (AMOLED) displays [55-58]. In addition, for the recent reported papers, ZnO is presently attracting much attention due to its possibilities for replacing α-Si that has been widely used as the channel layer in conventional TFTs [32][36][37][38].

Among the many barriers to the low-temperature process of LTPS TFTs, the formation of poly-Si films is the most important issue. There are several methods to fabricate a low-temperature poly-Si film, described as the following:

1.4.1 Solid-Phase Crystallization

Solid-phase crystallization (SPC) is usually performed at 600^°C temperature annealing

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processes are characterized by specific activation energies. For the SPC of α-Si by homogeneous nucleation, the activation energy of grain growth is less than that of nucleation [60]. Thereby, the amount of the nucleation relative to grain growth decreases with reducing annealing temperature. In order to enlarge the grain size, it is desirable to minimize the nucleation/grain growth ratio, and the SPC is typically done at a low temperature [61].

However, the SPC process is time-consuming about 20 hours for the crystallization of the α-Si film to the poly-Si film. Besides, such poly-Si films have a high density of intra-grain defects which lead to decrease the field-effect mobility and increase the threshold voltage of the TFTs [62].

1.4.2 Metal-Induced Lateral Crystallization

With some metals added to the α-Si films, the crystallization temperature can be lowered to below 600^°C, and this phenomenon is known as metal-induced crystallization (MIC) [63].

Metals such as Au, Al, Sb and In, which form eutectics with Si, or metals such as Pd, Ti and Ni, which form silicides with Si, have been added to α-Si films to enhance the nucleation rate.

During the MIC process, metal atoms dissolved in α-Si films may weaken Si bonds and enhance the nucleation of crystalline Si [64]. Some results were reported to be effective in lowering the crystallization temperature down to 500^°C. However, an undesirable metal contamination at the channel region results in the poor electrical properties of the devices. A new method which can reduce metal contamination, called metal-induced lateral crystallization (MILC), has been reported for Pd, where large grains over several tens of microns are achieved [65]. Moreover, many groups have proposed TFTs to be successful in terms of device characteristics and mass productivity with MILC poly-Si, using pure Ni [66][67], Ge [61] and Ni-Co alloys [68].

1.4.3 Excimer Laser Annealing

Excimer laser annealing (ELA) is considered to be the most promising method for the fabrication of LTPS TFTs [69][70][71]. The ELA method is performed by melting α-Si with high-power pulsed laser irradiation. The irradiated α-Si film is then cooled and solidified as a crystal. During the melt-growth period, however, the solidification velocity is too high for the film to form nuclei and to grow sufficiently. For this reason, the grain size of the poly-Si film is not large enough. Besides, non-uniformity of grain size and narrow process window make it difficult to achieve uniform TFTs performance.

To realize this, some techniques such as the bridge method [72], low-temperature substrate heating during laser irradiation [73] and two-step laser crystallization [74] have been proposed to reduce the solidification velocity. In addition, for gaining higher mobility, lateral crystallization is one of the techniques used. The laser beam intensity is spatially modified over the α-Si to control the solid/liquid interface, causing lateral crystallization of the film.

With this technique, the field-effect mobility of TFTs more than 300 cm²/V-s can be achieved [75].