High performance poly-Si TFTs fabricated by continuous-wave laser annealing of metal-induced lateral crystallised silicon films

High performance poly-Si TFTs fabricated

by continuous-wave laser annealing

of metal-induced lateral crystallised

silicon films

C.-P. Chang and Y.S. Wu

In this process, amorphous silicon was first transformed to polycrystal-line silicon (poly-Si) using a metal-induced lateral crystallisation (MILC) process, followed by annealing with a continuous-wave laser lateral (l 532 nm) crystallisation (CLC) with an output power of 3.8 W. MILC-CLC-TFT performed far superior to MILC-TFT. The mobility of the MILC-CLC-TFT was 293 cm2/Vs, which was much higher than that of MILC TFTs (54.8 cm2_{/Vs). In addition,} MILC-CLC TFTs showed better device uniformity and reliability.

Introduction: Low-temperature polycrystalline silicon (LTPS) is very important for device applications such as solar cells and thin-film tran-sistors (TFTs)[1]. Therefore, intensive studies have been carried out to reduce the crystallisation temperature of amorphous silicon (a-Si).

Among many methods, metal-induced lateral crystallisation (MILC) and excimer laser crystallisation (ELC) appear to be very promising methods[2 – 4]. MILC has the merits of low cost and uniform crystalli-sation over a large area. However, not alla-Si film was transformed to crystal Si [2, 3]. The ELC technique appears to be highly promising unfortunately their uniformity is inadequate and the surfaces of their poly-Si films are rough[4]. To improve the uniformity and performance of ELC-TFTs, many methods have been proposed[5 – 7]. The cost of the ELC system, however, is still high.

Recently, continuous-wave (CW) laser lateral crystallisation (CLC) of amorphous Si has been developed for LTPS TFT[8, 9]. Not only are the performances of CLC TFT better, but the manufacturing cost is lower than ELC TFTs. In this Letter, a new manufacturing method using post-annealing of MILC poly-Si TFTs with a CW laser (MILC-CLC) is proposed.

Experiment: The MILC process began with 4-inch quartz wafer sub-strates where wet oxide films of 500 nm were grown. A silane-based undoped amorphous silicon (a-Si) layer with a thickness of 100 nm was deposited using low-pressure chemical vapour deposition (LPCVD). The photoresist was patterned to form the desired Ni lines, and a 20 A˚ -thick Ni film was deposited on the a-Si. The samples were then dipped into acetone for 5 min to remove the photoresist. Samples were subsequently annealed at 5408C for 18 h to form the MILC poly-Si film. The unreacted Ni metal was removed by chemical etching. The MILC poly-Si films were then irradiated by a CW laser (l 532 nm) with various output powers (2.5, 3.8, 5 W) in an air atmosphere to fabricate the MILC-CLC poly-Si. Reactive ion etching (RIE) was employed to form islands of poly-Si regions. Next, a 100 nm-thick oxide layer was deposited as the gate insulator by plasma-enhanced chemical vapour deposition (PECVD). A 200 nm-thick poly-Si film was then deposited for gate electrodes by LPCVD. After defining the gate, self-aligned phosphorous ions were implanted to form the source/drain and gate. The dopant activation was performed at 6008C in N2ambient for 24 h.

Results and discussions: Fig. 1ashows an SEM image of the Secco-etched MILC needle grains. Most of the grains were parallel to each other in the ,111. direction[10]. The width of the needlelike grains was around 50 nm. Among these grains remained some uncrystallised a-Si regions, which had been etched away. To fabricate MILC-CLC poly-Si films, MILC films were irradiated using a CW laser with the scan direction parallel to the needlelike poly-Si grains. When the laser output power was 2.5 W, the sizes and shapes of the needle Si grains were similar to those of MILC poly-Si (Fig. 1a). This is because MILC-CLC films were in the amorphous-melting regime. Only the a-Si regions among Si grains were melted. When the output power reached 3.8 W, the width of the grains increased markedly from 50 nm to3 mm, as shown in Fig. 1b. The large grains were only molten partially and served as the nuclei for growth. The width of these grains markedly increased to3 mm owing to the geometrical coalescence of Si needle grains [11]. The grain boundary between grains disappears, resulting in the sudden development of a much larger grain. This coalescence is an important phenomenon for grains

having a strong preferred orientation (MILC needle grains had a strong preferred orientation ,111.). In this study, the effect of the CW (l¼ 532 nm) laser post-annealing was much better than that of the excimer laser. The width of the Si grains dramatically increased to3 mm (the grains ranged from 2.5 to 4 mm), while that of excimer laser post-annealing was only 600 nm[10]. When the laser power was 5 W, the width of the grains ranged from 3 to 12 mm. The uniformity of the grain size was poor. As a result, device performances were not uniform. To achieve high performance with good uniformity, the CW laser with an output power of 3.8 W was chosen to fabricate MILC-CLC TFTs.

NCTU SEI 15.0kV ×10,000

a b

1mm WD 7.8mm NCTU SEI 15.0kV ×10,000 1mm WD 7.9mm

Fig. 1 SEM images of MILC and MILC-CLC poly-Si grains a MILC b MILC-CLC 10–12 –15 –10 –5 0 5 10 15 10–11 10–10 10–9 10–8 10–7 10–6 10–5 10–4 10–3 drain current IDS , A gate voltage V_GS

,

Fig. 2 Typical IDS-VGStransfer characteristics and field-effect motilities of MILC and MILC-CLC TFTs

Fig. 2 shows the transfer characteristics and field-effect mobility against the gate voltage of MILC-CLC and MILC TFTs. The measured and extracted key device parameters are summarised inTable 1. MILC-CLC TFTs exhibited field-effect mobility reaching 293 cm2/Vs, which was much higher than that of MILC TFTs. The subthreshold slope (SS) and VTHof the MILC-CLC TFTs were 0.39 V/dec. and 24.54 V, which

were superior to 1.42 V/dec. and 2.24 V of the MILC TFTs. The ON/ OFF current ratios of the MILC-CLC and MILC poly-Si TFTs were 6.69  107and 0.18  106at VDS¼ 5 V, respectively.

Table 1: Device characteristics of MILC and MILC-CLC TFTs

Device parameters MILC-CLC MILC Field-effect mobility (cm2

/Vs) 293 54.8 Subthreshold slope (V/dec.) 0.39 1.42 Threshold voltage VTH(V) 24.54 2.24 ON/OFF current ratio 6.69  107

0.18  106

As mentioned earlier, many intragrain defects and a-Si regions remained among MILC poly-Si grains. These defects trap charge car-riers and degrade electric performance. MILC-CLC TFTs do not have these problems because, as presented in Fig. 1b, the width Si of MILC-CLC grains dramatically increased to 3 mm. Most of these geo-metrical coalescence grains and their boundaries are parallel to the drain current (Ids), reducing the impedance to carrier flow and thereby

reducing the threshold voltage and greatly increasing the mobility and Ion/Ioffcurrent ratio.

Twenty MILC-CLC TFTs were measured inmFEand VTHto

investi-gate device uniformity. The standard deviations of themFEand VTHare

7.46 and 0.165, respectively. As a result, small standard deviations of MILC-CLC TFTs indicate a fine uniformity owing to the CW laser annealing. The other important issue of MILC-CLC poly-Si TFTs is their reliability, which was examined under hot-carrier stress (HCS).

Fig. 3shows the field-effect mobility and threshold voltage variation against stress time, and the stress condition is VD,stress¼ 15 V,

VG,stress¼ VGS- VTH¼ 15 V for varied time duration. It is found that

MILC-CLC TFTs also had a good reliability.

–5.0 0 1000 2000 3000 4000 5000 –4.9 –4.8 –4.7 –4.6 –4.5 –4.4 V_D,stress=15 V, V_G,stress = V_GS-V_TH=15 V measured at V_DS=0.1 V stress time, s VTH ,V 250 300 350 400 mFE , cm 2/Vs

Fig. 3 Field-effect mobility and threshold voltage variation examined under hot-carrier stress

Conclusions: A high-performance LTPS TFT fabricated by MILC-CLC was investigated. In this process, amorphous silicon was first trans-formed to poly-Si using an MILC method, and then annealed using a continuous-wave laser. Laser-annealing with an output power of 3.8 W greatly increased the width of the needle grains from 50 nm to 3 mm by geometrical coalescence. MILC-CLC-TFT markedly outper-formed that of the MILC-TFT because the MILC-CLC poly-Si film had much larger grains and fewer intragrain defects than the MILC poly-Si film. The MILC-CLC-TFT has a lower threshold voltage, smaller subthreshold slope and a higher on/off current ratio than the MILC-TFT. The mobility of the MILC-CLC-TFT was 293 cm2/Vs, which was much higher than that of MILC TFTs (54.8 cm2/Vs). Besides, MILC-CLC TFTs showed better device uniformity and reliability.

Acknowledgments: This project was funded by the Sino American Silicon Products Incorporation and the National Science Council of

the Republic of China under grant no. 95-2221-E009-087-MY3. Technical supports from the National Nano Device Laboratory, Center for Nano Science and Technology and the Nano Facility Center of the National Chiao Tung University are acknowledged.

#_{The Institution of Engineering and Technology 2008} 7 June 2008

Electronics Letters online no: 20081620 doi: 10.1049/el:20081620

C.-P. Chang and Y.S. Wu (Department of Material Science and Engineering, National Chiao Tung University, 1001 Ta-Hsueh Road, Hsinchu, Taiwan, Republic of China)

E-mail: [email protected] References

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