Chapter 1 Introduction
1.6 Organization of thesis
In Chap. 2, the grain size of FLA-processed poly-Si is firstly analyzed in order to obtain the annealing parameter of FLA. The characteristics of FLA-processed poly-Si are also analyzed by various techniques. The electrical characteristics of FLA-processed TFTs are also measured and applied for the calculation of defect state density.
In Chap. 3, the results of dopant profile engineering by FLA will be shown. The activation parameter which is affected by various sample preparation parameters will also be shown. We will discuss the mechanisms of dopant diffusion due to ion implantation with different ions and different activation parameters. Our results show that FLA has a high potential for future application on ultra-shallow junction activation. Not only the ultra-shallow formation in silicon substrate is important, bulk germanium (Ge) gains the attention for its significantly high carrier mobility. Our preliminary results will be shown in this chapter.
The results of FLA-processed poly-Si analyzed by THz-TDS will be shown in chapter 4. Poly-Si in two different grain sizes can be easily distinguished by OPTP technique. The mobility can also be obtained by fitting the results of THz-TDS measurements with the Drude model. The reason that the increase of mobility for large grain size poly-Si measured by OPTP will be discussed.
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Figures
Fig. 1-1 Grain size and corresponding poly-Si TFT mobility vs. laser fluence for conventional ELA process [29].
Fig. 1-2 SLG growth initiates from the seeds, which survived in the melting process, at the Si-SiO2 interface [29].
Fig. 1-3 SIMS profiles of a 20 keV BF2 2×1015/cm2 before and after annealing. The annealing process was performed by RTA or ELA [43].
Fig. 1-4 Comparison of boron concentration profiles after laser annealing with successive pulses at 1 J/cm2.
103 106 109 1012 1015 1018 1021 kilo mega giga tera peta exa zetta
Visible x-ray Microwaves
Photonics Electronics
γ -ray Thz
Frequency (Hz) 1THz ~ 1ps ~ 300μ m ~ 4.1meV
Fig. 1-5 Electromagnetic spectrum of THz.
Fig. 1-6 The wavelength dependence of linear absorption coefficient for silicon.
Chapter 2
Femtosecond Laser Crystallization of Amorphous Silicon Thin Films and Characterizations of Crystallized Polycrystalline Silicon Thin Film Transistors
2.1 Introduction
With the demand of larger display area and pixel density of thin film transistor liquid crystal display (TFT-LCD), high mobility TFTs are required for pixel driver of TFT-LCD in order to shorten the charging time of pixel electrodes. Low temperature poly silicon (LTPS) technology has been studied for the purpose integration of drivers at the periphery of active matrix liquid crystal display [1]. Because of its better crystalline quality than amorphous silicon (a-Si), especially the grain size, poly-Si attracted a great attention for last two decades [2-5].
Unlike annealing and activation using continuous-wave (CW) and long pulse lasers, nonlinear photo-energy absorption and non-equilibrium thermodynamics are expected to dominate the interactions between the intense femtosecond laser pulses and irradiated transparent materials [6-11]. Such a non-linear process provides precise and low-threshold fluence associated with femtosecond laser-ablation [8-11]. Due to non-linear photon absorption, the melting of crystal silicon takes place in very short time scale, after 100-800 fs depending on fluence [8]. The purpose of this work is to develop a new approach of using ultrafast laser for recrystallization of a-Si. The quality of poly-Si will be analyzed by SEM and AFM et al. and further examined by measuring the electrical characteristics of FLA-processed TFTs. Good transistor characteristics, as confirmed by measurements of
electrical parameters and grain trap-state densities were obtained for a wide process window of annealing laser fluences.
Firstly, general features of femtosecond laser will be discussed in this chapter.
Secondly, the experimental setup of laser annealing will be shown. This will be followed by sample preparation of a-Si and TFT processing for poly-Si. The analyzing results of poly-Si annealed by FLA by several different instruments will be shown in Sec. 2.4.1. The electrical characteristics of FLA-processed TFTs will be shown in Sec. 2.4.2.
2.2 General Features of Femtosecond Lasers
The invention of the laser in 1960 stimulated the development of optical physics and gave rise to many rising research fields. One of these rising research fields was ultrafast optics, which was raised and developed in mid-1960s with the production of nanosecond (10-9 s) pulses by the first mode-locked laser. Nowadays, a lot of progresses of ultrafast optics lead to some practical lasers which can produce pulses in the femtosecond (10-15 s) time scale. Early in the 1990s, Ti:Sapphire femtosecond lasers became commercially available and ultrafast optoelectronics began to spread all over the world. In this section, our femtosecond laser system will be briefly introduced.
Our femtosecond later system is shown as the block diagram in Fig. 2-1. We use the Ti:Sapphire laser as the seeding laser which is then injected into the Ti:Sapphire regenerative amplifier (Spitfire, Spectra-Physics) for amplification.
The pump laser for the Ti:Sapphire laser (Spectra Physics Tsunami) is a 5W frequency doubled diode-pumped Nd:YVO4 laser (Millennia Vs, Spectra-Physics) with a wavelength λ=532 nm. The Ti:Sapphire laser provides an output train of intense 35 fs pulses with wavelengths ranging from 770nm to 830 nm. The pulse repetition rate is ~82 MHz and the output power can reach 0.5 W.
The pump laser for the amplification process in Spitfire is a Q-switched Nd:YVO4
laser. This Nd:YVO4 laser delivers a high average power output of 20 W at 527 nm. The chirped-pulse amplification diagram of Spitfire is shown in Fig. 2-2. The Spitfire amplifies the seeding pulses by a million times from pulse energy of 6 nJ to 2 mJ. The repetition rate is 1 kHz and the output average power is 2 W. The properties of the Ti:Sapphire regenerative amplifier are shown in Table 2-1.
2.3 Experiment setup and procedures
2.3.1 The Setup of Femtosecond Laser Crystallization and Sample
Preparation
Silane-based a-Si films (100 nm) were deposited on oxidized (500 nm) silicon wafers by low-pressure chemical vapor deposition (LPCVD) at 550 °C. An infrared femtosecond Ti:Sapphire laser system (TSUNAMI, and SPIT FIRE, from Spectra-Physics company) with a wavelength of 800 nm, a repetition rate of 1 kHz, a pulse duration in the range of 50-125 fs, and an output energy per pulse of ~0.5 mJ, was guided to crystallize amorphous silicon as shown in Fig. 2-3. All samples were placed in a vacuum chamber, and a heater increased the substrate temperature up to 400 °C (see Fig. 2-4). A two-axis transition stage was used for the line-scan FLA as shown in Fig. 2-5.
First, we made a steady-state multiple shots FLA experiment (overlapping 100% with 20 laser shots). The pulse train from the mentioned laser system was focused in the strip spot (3 mm×250 μm). We changed the fluence (38-63 mJ/cm2) and the number of shots (5-100). For comparison, we also processed the line-scan FLA with different laser fluence (28-70 mJ/cm2) and interpulse overlapping (10-99%), which corresponds to the number of laser shots from 10-99.
2.3.2 Fabrication of Femtosecond Laser Crystallized Poly-Si TFTs
Amorphous Si layers of 100 nm were deposited by low pressure chemical vapor deposition (LPCVD) at 550 °C on 500 nm-SiO2-coated silicon wafers. The active layers for the TFTs were crystallized by line-scanning irradiation of twenty ultrafast (~ 50 fs) near-infrared (λ = 800 nm) laser pulses with fluences of 34-50 mJ/cm2 (or total fluences of 20 laser pulses about 0.68-1.0 J/cm2). The beam spot size was 8 mm ×110 μm. During the scanning process, the overlapping of neighboring pulses was fixed at 95%. FLA was conducted on a substrate heated at 400 °C in a vacuum chamber. FLA-crystallized layers were then defined into active regions for transistors with channel length (L)/ channel width (W) of 2μm/2μm, 3μm/3μm, 5μm/5μm, and 10μm/10μm. A SiO2 gate dielectric layer of 50 nm and polycrystalline silicon gate layer of 150 nm were then grown by LPCVD and patterned for self-aligned phosphorous implantation with dosage of 5×1015 cm-2, and energy of 53 keV. After thermal activation and metal connection were performed, n-type transistors were completed.
For comparison, TFTs with channels crystallized by furnace annealing (solid phase crystallization, SPC) in nitrogen ambient at 600 °C for 24 hours were also processed on the same run. The transfer characteristics (drain current Id versus gate voltage Vg) of the devices were measured at a drain voltage Vd = 0.1 V, to extract electrical parameters. Grain trap-state densities, nGT, for all TFTs were also examined using the field-effect conductance method [12].
2.4 Results and Discussions
2.4.1 Material Characterization of Femtosecond Laser Crystallized Poly-Si Thin Films
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 ELvalues 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 EL. 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
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