The novel post treatment technology, ELA, was introduced for low temperature recrystallization of BST films in this investigation. Although the excimer laser annealing is well known for recrystallizing the amorphous silicon, its applications to ferroelectric films is very recent [67]. Some of the reports depicted the crystallinity of the film can be greatly enhanced by post ELA treatment, but the electric properties very hardly reveal remarkable improvement. In another words, the process window of ELA on ferroelectric films is too narrow to be easily controlled, as reported by J. Gottmann et al [68]. The general background and recrystallization mechanisms are introduced below.
2-3.1 General features of Excimer Laser Annealing
Excimer laser is the most powerful UV light source, and that has been widely applied on the semiconductor industry, such as lithography, thin film fabrication and post annealing [69 – 71]. Table 2-3 shows that the various wavelengths, between 157~351 nm, can be obtained using different laser gas, and all excimer lasers are pulsed laser modes. In conventional technologies, amorphous ferroelectric films can be furnace annealed as high as 600oC above for a long period, reaching the solid phase crystallization. Besides, rapid thermal annealing (RTA) is also widely applied on thin film treatment, but it will induce large thermal stress in ferroelectric films. A novel technology, excimer laser annealing (ELA), was introduced to enhance the crystallinity of the BST films in this study, and a CMOS compatible process was also performed by ELA due to its low thermal budget and small thermally induced stress.
ELA behaves direct energy processing that the surface can be heated by laser beam.
The first demonstrations of the utility of directed energy processing came from the annealing of implantation damage using lasers. For the roughly compared ELA with conventional furnace annealing (FA) and rapid thermal annealing (RTA), the large thermal budget and the entirely heating of FA result in the inter-diffusion and damages of under-layer structures, besides, too large thermal stress of RTA is not suitable in deep
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Table 2-3 Different excimer laser gases and corresponding wavelengths
submicron even smaller design rules. ELA technology, strictly speaking, is not a real low temperature process, higher than 700oC in the top of laser absorption region, but this high-temperature absorption region is very shallow, 20-nm only, and the sustained time is as short as 10~100 nanosec. Hence, the under-layer substrate would not be damaged after ELA treatment.
ELA technology is widely used for preparing poly-Si thin film in current semiconductor industry. Although excimer laser may be an extremely powerful implement to achieve low temperature integrated process, but up to now, very few reports investigate the ELA mechanism for ferroelectric films. Therefore, the qualitative analyses below are all referred from the research about poly-Si.
(1) The effect of laser energy density: Fig. 2-18 shows that the average grain size is plotted as a function of the laser energy fluence, Elaser [72, 73]. Samples prepared at room temperature and with low laser energy fluence were composed of small grains.
With increasing laser energy density the average grain size increases and eventually reaches a maximum value. But when the energy density above maximum value, the grain size decreases to a constant value. According to the above, the optimal energy Figure 2-18 Average grain size of laser crystallized poly crystalline silicon films as a function of the laser energy fluence [72].
density is limited in a narrow region.
(2) The effect of substrate temperature:
Fig. 2-19 shows the time dependence of Si-layer melt front profiles under excimer laser irradiation [74]. It is assumed the whole Si layer was completely melted. As shown in this figure, the melting duration of the Si-layer is prolonged with increasing substrate temperature. The slope of the melt depth verses time graph decreases with increased substrate temperature during excimer laser annealing.
(3) The effect of number of shots: Fig. 2-20 shows the transition of grain size distribution with the above lateral grain growth as revealed in secco-etched SEM images [75 – 79]. As shown in this figure, poly-si films annealed using two shots exhibited almost lognormally distribution grains with an average grain size of about 200nm. The distribution of 64-shot-annealed poly-Si films, however, shifted to bimodal. This distribution seemed to occur during the process of lateral grain growth. Finally, 128-shots-annealed poly-Si films again exhibited almost monomodal grain size distribution with an average grain size of about 1.5μm. This result clearly shows that, as the number of laser shots was increased, the average grain size is increased, and it was also found that dramatic lateral grain growth occurred.
Figure 2-19 Time dependence of Si-layer melt front profiles under excimer laser irradiation [85].
In addition to the above, other variable process parameters, annealing ambient, laser duration, type of annealed films and substrate, should affect the re-crystallized process. Of course, the physical parameters, melting point, latent heat, thermal conductivity, density, specific heat, absorption coefficient, of annealed films must be serious considered first.
2-3.2 Mechanisms of Excimer Laser Annealing (ELA)
The directed energy processing has emerged for the processing and modification of the surface layers of semiconductors. And the directed energy sources such as lasers are used to heat the surface. The unique temporal and spatial control exercised over the heat flow by these beams allows formation of quite novel structures and alloys. For example, surface layers can be recrystallized in exceedingly short times. The dimensions of the layers that can be modified by the incident beams are just those required by Si integrated circuit technology [80 – 84]. The very short laser irradiating duration of ELA suppress the inter-diffusion and damages of the films bulk and under-layer structures.
The coupling of lasers to materials is very sensitive to the wavelengths and the states of the material. About the absorption mechanisms of laser radiation in semiconductor,
Figure 2-20 The transition of grain size distribution using various numbers of shots to perform excimer laser annealing [75].
there are three regions must be contributed. One is the direct excitation of lattice vibrations in materials. This happens while the photon energy (hν) of light well below the band-gap energy (Eg). The other state is the excitation of free or nearly free carriers by absorption light. And this form occurs as hν<Eg; such carriers will always be present as a result of finite temperatures and/or doping. Finally, the third way of absorption energy of light is to create electron-hole pairs by light with hν>Eg. [82, 83].
The Eg of BST is about 3.6eV. for 248nm light, hν=5eV > Eg. We know it should be an efficient approach to BST films by 248nm light.
The thermal theory of ELA for poly-Si films is thoroughly developed in the past decade, but very few investigations reported the ELA thermal theory on ferroelectric films. As discussed below, the mechanisms of the thermal conduction will refer to the theory of Si-ELA. The facility of rapidly heating and cooling surface layers without heating the bulk depends on the pulse duration timeτand the coupling depths of the heat source. The cooling or quench rates of the surface layers using these pulsed sources will be in the range 109~1014℃/sec. Longer irradiation times can be achieved by the use of continuous sources with scanned spots. The fastest irradiation times will be 10-7~10-8sec, with cooling rates less than 109℃/sec. In the case for 248nm applied on BST film for post annealing, the α-1<(2Dτ)1/2 where α-1, D and τ are absorption distance, heat diffusivity and pulse duration time. The absorbed energy is simply the laser power density multiplied by the pulse length Iτminus the reflected energy IRτ.
The average temperature rise in this (2Dτ)1/2–thick layer is therefore given by the following equations [71, 85, 86]
∆T=(1-R)Iτ/ρC(2Dτ)1/2 (2-19)
ρ, C are density and specific heat. For more accuracy, temperature dependence of the material constants for each layer and latent heat during the phase change were taken into account. Three-dimensional nonlinear equation is used for solving the temperature distribution, solidification velocity during laser irradiation.
z Q
However, the thermal properties of ELA processing in ferroelectric films exhibit large differences from that in Si films due to the different absorption constant, heat capacitance, and thermal conduction mechanisms. Therefore, the processing model must be modified, as investigated in this thesis.