50 nm-thick a-Si layers were deposited on quartz glass substrate by molecular beam epitaxy (MBE) at room temperature. Then, 60 nm-thick nickel films deposition by thermal evaporation and successive 150 nm-thick tungsten films deposition by sputtering at room temperature were performed on regions which were patterned using lift-off process (Structure A). We also prepared samples without nickel films (Structure B). The a-Si films were then crystallized in the solid phase using microwave plasma heating. Input microwave power is 1000 W, hydrogen gas flow rate is 5 sccm, treatment time is 60 s, and pressure is 30 Pa.

For device fabrication, 100 nm-thick a-Si layers were deposited on quartz glass substrate by MBE at room temperature. Then, samples with structure A and B were prepared. tungsten and nickel were patterned only inside the source and drain regions, and the pattern sizes are smaller than source and drain regions. The microwave plasma heating method was used to perform crystallization. Input microwave power is 1000 W, hydrogen gas flow rate is 5 sccm, treatment time is 60 s, and pressure is 30 Pa. The sample with the structure B was heated for 120 s in addition to heating time of 60 s.

Then, source and drain regions, which were larger than the metal patterned regions, were implanted with arsenic atoms at a dose of 1.0 × 10¹⁵ cm² and an energy of 50 KeV. Gate SiO₂ layers 150 nm thick were deposited by plasma enhanced chemical vapor deposition (PECVD) using tetraethyl orthosilicate (TEOS) at temperature of 450˚C. Then, the contact holes were patterned by wet etching using hydrofluoric acid. Finally, aluminum films deposited by thermal evaporation and patterned by wet etching to form the contact pad. Sintering temperature was 350˚C in mixture of nitrogen and hydrogen gases.

To investigate the crystallinity of Si layers, we used optical microscope and Raman scattering spectroscope. The optical micrograph of the sample with the structure B after heat treatment is shown in Figure 1. The color of a-Si film was red and that of crystallized silicon film was yellow. The poly-Si aria was obtained near tungsten region. The length of the poly-Si aria was about 2 mm.

The crystallinity of the sample with the structure B after plasma heating was investigated by microscope Raman spectroscopy. Figure 2(a) shows the spectrum of the yellow aria in Figure 1. The spectrum has a Si-Si TO-LO phonon band centered at 520 cm⁻¹, showing the presence of Si crystalline phase [6]. Figure 2(b) shows the spectrum of the red aria in Figure 1. The spectrum has broad peak centered at 480 cm⁻¹, it’s characteristic of a-Si.

The optical micrograph of the sample with the structure A after plasma heating was shown in Figure 3. Even though heat treatment was performed for the same period of time as the sample with structure B, whole area of the sample with structure A (2 mm × 15 mm) was crystallized.

The length of polycrystallized region in Figure 3. was larger than that in Figure 1. It is thought to be attributed to metal induced lateral crystallization

(MILC) by nickel [7] [8] [9].

Figure 4 and Figure 5 show I_d-V_d characteristic of the sample with structure B (without Ni) which was heated 60 s and 120 s respectively. Device dimensions are 100 µm of a channel length and 150 µm of a channel width. The gate voltage was changed from 0 V to 40 V with 5 V increment.

Drain current of the 120 s heat treated sample with structure B was twice larger than those of the 60 s heat treated samples.

Effective electron mobilities were derived using the split C-V method. The mobilities of the 60 s and 120 s heat treated samples with structure B are 30 cm²/Vs and 60 cm²/Vs, respectively. By elongating the plasma heating time, it was thought to obtain higher electron mobility.

Figure 6 shows I_d-V_d characteristic of the 60 s heat treated sample with structure A (with Ni). The electron mobility is 55 cm²/Vs. Despite the same plasma heating time, the mobility of sample A is about twice as high as that of the sample with structure B.