Material Characterization of Femtosecond Laser Crystallized Poly-Si

Scanning electron microscopy (SEM) pictures of annealed areas (Fig. 2-6) with obvious

uniformity of poly-Si grains. Pictures from (a) to (d) in Fig. 2-6 represent steady-state multi-shot samples irradiated with 20 laser pulses of 50 fs with different energy densities.

The average grain size is relatively small (smaller than 50 nm) and almost independent on the energy density. It should be mentioned that in comparison to linear ELA for non-linear steady-state FLA, small grains (<50 nm) might probably be due to the fact that fast cooling (or laser-energy turn-off) associated with short pulses causes random recrystallization from more nucleation sites [5].

The situation is dramatically changed when we introduce scanning. In our second experiment we scan the laser beam along the sample with the speed of 2.5-25 mm/sec (10-100 laser-shots per unit area, or, equivalently, an overlapping of 90-99%). Pictures from (e) to (h) in Fig.2-6 represent line-scan FLA samples irradiated with 95% interpulse overlapping of 50 fs laser pulses with different energy densities. As shown in Fig. 2-7, the average grain sizes of FLA pc-Si films are plotted as a function of laser energy density (denoted as EL) and pulse duration for both steady-state and line-scan FLA. Herein, annealing parameters for line-scan (steady-state) FLA were overlapping ~95 % (20 laser-shots).

For line-scan FLA using 125 fs pulses, the grain sizes of the crystallized a-Si films initially increased, and then saturates, before finally declining as the laser fluence was increased from 35 mJ/cm² to 61 mJ/cm². The maximum average grain size was around 200 nm when a-Si films were irradiated at 50 mJ/cm². Despite the difference between the mechanism of photoexcitation-melting using FLA (non-linear annealing) [6-10] and that using nanosecond or longer pulse laser annealing (linear annealing), such as ELA, lateral elongating still dominates the growth of grains in FLA pc-Si films.

With reference to SLG phenomena [5, 13-14], laser fluence (grain sizes) in line-scan FLA can be reasonably divided into three major regimes of partial-melting (small grains), near-complete-melting (largest grains), and complete-melting (fine grains), which fit well

the trends plotted in Fig. 2-7. For line-scan FLA using short pulses with 50 fs duration, the trend in average grain sizes of FLA pc-Si films versus EL is similar to that obtained with 125 fs pulses, but the maximum average grain size (800 nm) of crystallized films, and the optimal EL of crystallization (47 mJ/cm²) for FLA using 50 fs pulses are markly better than those for FLA using 125 fs pulses. The increase in the efficiency of nonlinear photo-energy absorption [15-16] with the peak power of the laser pulses during infrared FLA, is responsible for the dependence of the grain sizes of line-scan FLA pc-Si films on the duration of pulses, and the laser fluence.

It is remarkable that the maximum of average grain size (~800 nm) equals the wavelength used. This phenomenon is also observed in ELA: in multiple pulse irradiations [15]. Typically, this is related to the interference effects at the surface due to reflection of the hillocks that are formed at the grain boundaries. But in FLA we do not observed such kind of phenomena in the steady-state multiple shots experiment. Only scanning leads to grain elongating. Therefore, we may conclude that we observed the SLS(sequential lateral solidification)-like mechanism (will be discussed later).

The examination of grain sizes of line-scan FLA pc-Si films crystallized at various overlapping, at E_Lvalues of 47 mJ/cm² and 38 mJ/cm² for 50 fs pulses, and 50 mJ/cm² for 125 fs pulses, is shown in Fig. 2-8. The required laser-shots is lower for FLA using shorter pulses and higher E_L. Typically, 10 to 100 laser-shots are required to perform laser linear annealing [5, 13, 17]. But again we should mention that increasing of the pulse numbers or shots in the steady-state FLA does not result in enlarging of crystallized grains (Fig. 2-8).

On the other hand, the data in Fig. 2-7 and 2-8 for line-scan FLA clearly demonstrates the high effectiveness of scanning and non-linear photo-absorption in crystallizing amorphous silicon. Assuming a Gaussian shape of the laser beam, for the steady-state mode, each illuminated point is repeatedly shined by the same energy. The first few laser pulses

transform the amorphous material into the poly-phase with large grain distribution. The last pulses (with the same energy) cannot significantly change that distribution.

In the line-scan mode, each illuminated point is also repeatedly shined. But the shined energy for the considered area is not the same from pulse to pulse. It follows a Gaussian sequence. That is possible reason why smaller grains have possibilities in gaining larger energy, therefore, resulting in grain-re-growing into larger grains. Thus, suggested SLS-like mechanism, assisted with the feature of low melting-energy for small grains obtained with the steady-state FLA, significantly enlarges the grain-size of line-scan FLA pc-Si films.

Moreover, for ultra-short laser pulses the melting thickness is determined by non-linear absorption skin depth rather than heat penetration length due to thermal conduction [10], and thus more insensitive to laser-fluence in comparison with that for linear annealing.

Besides the proposed SLS-like mechanism, this melting-depth thinning mechanism during FLA is also responsible for the significant process window in laser fluence (45-60 mJ/cm²) for line-scan FLA using 50 fs pulses.

The RMS roughness measured by the atomic force microscopy (AFM) of all FLA pc-Si films is below 4.5 nm. The peak-to-peak roughness is about 26 nm for line-scan FLA samples. Figures 2-9 (b) and (c) represent the AFM images of line-scan and steady-state FLA pc-Si films respectively. The crystalline fraction in such films exceeds 98 %, as calculated from their Raman spectra in Fig. 2-9 (a), in which a sharp peak at 519.5 cm^-1, and the absence of a broad peak at 480 cm^–1 associated with amorphous phase, implies the high crystallinity of the FLA pc-Si films [16]. Diffraction spots on the TEM selected area diffraction patterns of FLA pc-Si irradiated with 50 fs pulses, presented in Fig. 2-10, reveal that those films are highly crystalline. The corresponding XRD spectrum in Fig. 2-11 shows that the preferred orientation of FLA pc-Si is <111>, same as the orientation of those annealed by other types of lasers [21, 22].

The maximum grain size of FLA pc-Si films increases twice (400 to 800 nm) as the substrate temperature increases from room temperature to 400 °C (see Fig. 2-12). James S.

Im et al. observed similar results in ELA pc-Si films, explaining them by a longer time for lateral growth (larger grains) due to higher substrate temperatures (or lower quenching rates [18]). Conventional laser thermal annealing makes the grain sizes of annealed films relatively insensitive to the thermal energy from the substrate temperature, because laser thermal annealing “heats” the lattice to a melting temperature far above the substrate temperature. This implies that the substrate temperature more strongly influences grain growth in films crystallized by FLA using shorter pulses than by ELA using longer pulses, and by laser thermal annealing, thus explaining the trend observed on Fig. 2-12.

2.4.2 Electrical Characterization of TFTs Fabricated by Femtosecond