Hirotaka Kaku

Application of Plasma Jet Crystallization Technique to Fabrication of Thin-Film Transistor

The crystallization of a-Si films on glass substrates using the plasma jet crystallization (PJC) technique and its application to thin-film transistor fabrication were studied. Amorphous Si (a-Si) films deposited by plasma-enhanced chemical vapor deposition (PECVD) of 50% SiH4 diluted with H2 were crystallized by thermal plasma jet under the power of 1.6 to 2.6 kW input to the plasma source and the substrate scan speed of 170 to 1000 mm/s. The crystallinity of the films was improved by treating the films at a higher input power for a longer duration. Thin-film transistors (TFTs) fabricated using the crystallized films showed good electrical characteristics. By increasing the input power from 1.86 to 2.29 kW in the crystallization, the average field-effect mobility was increased from 42 to 61 cm2V-1s-1, and the threshold voltage was decreased from 4.0 to 3.4 V. These results indicate that the PJC technique is a very promising low-temperature process technology.

Analysis of Transient Temperature Profile During Thermal Plasma Jet Annealing of Si Films on Quartz Substrate

The transient temperature profile during thermal plasma jet annealing has been investigated by optical reflectivity measurements. The transient reflectivity measured during the annealing shows oscillation, which originates from the changes in the refractive indices of a Si film and a quartz substrate with temperature. By analyzing the oscillation, we have successfully obtained the temperature profile during the annealing with a time resolution of milliseconds. As a result of such analysis when the power input to a plasma source is 2.2 kW, the surface temperature has been observed to increase from 1300 to 1560 K by decreasing the scan speed from 1000 to 500 mm/s.

Crystallization of Si in Millisecond Time Domain Induced by Thermal Plasma Jet Irradiation

Investigations on the temperature profiles and formation of crystalline Si in rapid thermal annealing induced by thermal plasma jet (TPJ) irradiation have been reviewed. Substrate surface temperature during annealing has been measured by an optical probe method which has an accuracy of 30 K and a time resolution of millisecond. By changing the annealing conditions such as scan speed (v), plasma–substrate gap (d) and Ar gas flow rate ( f), maximum surface temperature (Tmax) is controlled in the ranges of 960 to 1860 K with a typical annealing duration (ta) of ~3 ms. On the basis of temperature measurement and in-situ reflectivity measurement techniques, the phase transformation of amorphous Si (a-Si) films has been investigated. When the a-Si films are heated to a temperature higher than the melting point, solid phase crystallization (SPC) followed by melting and resolidification of the films have been observed. It was found that SPC temperature increased from 1096 to 1284 K with decreasing crystallization time from 1.4 to 0.12 ms. Thin-film transistors (TFTs) fabricated using the SPC films show good electrical characteristics with an average field effect mobility of 61 cm2 V-1 s-1 and a threshold voltage of 3.4 V. By annealing SiOx films at temperatures higher than 1430 K using a TPJ, the precipitation of nanocrystalline Si with a size ranging from 10 to 250 nm has been observed.

In-situ Measurement of Temperature Variation in Si Wafer during Millisecond Rapid Thermal Annealing Induced by Thermal Plasma Jet Irradiation

An in-situ measurement technique for the temperature profile of a Si wafer during millisecond rapid thermal annealing has been developed. By analyzing the oscillation observed in transient reflectivity of the Si wafer during annealing, we obtain a transient temperature profile with a millisecond time resolution. Since this measurement is based on optical interference, a highly sensitive temperature measurement with an accuracy of 2 K is expected. Using this measurement technique, we controlled Si wafer surface temperature during thermal plasma jet irradiation with the heating and cooling rates in the order of 104–105 K/s.

Characterization of Microcrystalline Silicon Thin Film Transistors Fabricated by Thermal Plasma Jet Crystallization Technique

The electrical characteristics of thin-film transistors (TFTs) fabricated by thermal plasma jet (TPJ)-crystallized microcrystalline Si (µc-Si) films have been investigated. Amorphous Si (a-Si) films were crystallized with the TPJ under the scanning speed (v) of 350 to 550 mm/s, and µc-Si TFTs were successfully fabricated with a 300 °C process. By reducing v, µFE increases from 3.2 to 17.1 cm2 V-1 s-1, and Vth and S decrease from 9.2 to 5.2 V and 1.3 to 0.6 V/decade, respectively. The variations of µFE, Vth, and S were kept within small values of 1.06 (±4.4%), 0.14 (±1.1%), and 0.04 (±4.0%), respectively. The µc-Si is formed with ~20-nm-sized randomly oriented small grains, and this isotropic nature results in very small variation of TFT performance. With decreasing v, the fraction of nano sized grains and disordered bonds at the grain boundary decreases, which results in improved TFT performance.

Formation of Low-Defect-Concentration Polycrystalline Silicon Films by Thermal Plasma Jet Crystallization Technique

Defect concentration in polycrystalline silicon (poly-Si) films formed by thermal plasma jet (TPJ) annealing and excimer laser annealing (ELA) has been investigated on basis of the electrical property and spin density (Ns). Phosphorus-doped Si films with an average concentration of 4.3 ×1017 cm-3 and crystallized by TPJ annealing showed electrical conductivity (σ) values of 2.0 ×10-3–7.8 ×10-2 S/cm, whereas ELA Si films show much lower σ values of (1.6–4.5) ×10-6 S/cm regardless of irradiated laser energy density. Ns values in TPJ annealed Si films were (2.3–4.5) ×1017 cm-3, which are roughly one order of magnitude lower than those of ELA films. These results indicate that dangling bonds in crystallized films are the predominant traps and they strongly govern the electrical property. TPJ crystallization offers the possibility of fabricating poly-Si films with a low defect concentration presumably owing to the much lower cooling rate (~105 K/s) during crystalline growth than that of ELA (~1010 K/s). By treating TPJ annealed films with hydrogen plasma for 10 min at 250 °C, a defect density as low as 5.0 ×1016 cm-3 is achieved.

In-situ Monitoring of Si Wafer Temperature during Millisecond Rapid Thermal Annealing

INTRODUCTION. For the formation of ultra-shallow junction, a millisecond rapid thermal processing such as flash lamp annealing (FLA) [1] and laser spike annealing (LSA) [2] has been developed. Substitutional doping of impurities to Si lattice with a concentration beyond equilibrium soluble limit has to be achieved without significant diffusion of dopant atoms. Accordingly, precise control of heating and cooling rates in millisecond or even shorter rapid thermal annealing (RTA) is indispensable. However, there is no practical way that allows us to measure Si wafer temperature with such a high time resolution. In this work, we have developed a new temperature measurement technique using an optical probe. Experimental results on the in-situ monitoring of transient temperature evolution in millisecond RTA are demonstrated. EXPERIMENTAL. Millisecond RTA was performed by irradiating 0.7-mm-thick n-type Si (100) double side polished wafers with thermal plasma jet (TPJ). The W cathode and the water-cooled Cu anode separated 1 mm from each other were connected to a power supply. Arc discharge was generated by supplying DC power (p) of 2.22 kW, where the DC voltage was 14.8 V with a constant discharge current of 150 A, between the electrodes under an Ar gas flow rate of (f) 7.0 L/min. The TPJ was formed by blowing out the arc plasma through an orifice of 2 mm in diameter. The substrate was linearly moved by a motion stage in front of the TPJ with scanning speed (v) ranging from 500 to 2000 mm/s. The distance between the plasma source and the substrate (d) was set at 0.9 mm. For the measurement of temperature profile in the Si wafer during the TPJ annealing, an optical probe was used. The transient reflectivity was measured by irradiating the Si substrate with an infrared laser ( = 1310 nm) from the backside of the substrate and detecting the reflected light intensity by a photodiode. The optics and the substrate was set on a motion stage, and moved together. RESULTS AND DISCUSSION. An example of observed transient reflectivity waveform under v of 500 mm/s is shown in Fig. 1. A large number of clear oscillations in reflectivity were observed during the TPJ irradiation. These oscillations in the transient reflectivity are due to the interference of the incident light multiply reflected between the top and bottom surfaces of the wafer. Since the refractive index of Si (nSi) changes with temperature, the optical thickness of Si substrate during the annealing changes in accordance with heat diffusion and this induces the oscillation. We extracted the optical thickness variation from the number of peaks as shown by the open diamonds in Fig.2. It should be noted that the discontinuity in oscillation observed around 14 ms in Fig.1 is the indication of optical thickness turning over from increase to decrease. By simulating the change in optical thickness based on heat diffusion and optical interference analysis, we can precisely reproduce transient optical thickness variations as shown by the dashed line in Fig.2. Here, we use the reported thermal conductivity [3], and the temperature dependence of refractive index nSi = 3.50 + 1.77 × 10 × T ( C) was obtained from ex-situ measurement using same optics. Additionally, we noticed the amplitude modulation. This was caused by the lateral temperature gradient within the probe laser spot. From the analysis, we obtained the surface temperature of Si wafer with millisecond time resolution as shown in Fig. 2. Under this annealing condition, the wafer surface reached 993 K with maximum heating (Rh) and cooling rates (Rc) of 1.71 × 10 and 0.67 × 10 K/s, respectively. We have confirmed that this technique can measure the change of surface temperature under different RTA condition as summarized in Table.1. Moreover, the fitting accuracy suggests a high sensitivity of ~ 1 K by this method. These results indicate that the transient temperature profile and temperature distribution of Si substrate are obtained very accurately by this newly developed technique. CONCLUSIONS. Present temperature measurement way is a very powerful method for the further development of millisecond and even shorter RTA process.

Melting and solidification of microcrystalline si films induced by semiconductor diode laser irradiation

Rapid thermal annealing of microcrystalline Si (µc-Si) films induced by cw semiconductor diode laser (SDL) irradiation has been investigated. Owing to the higher absorption coefficient of µc-Si than that of amorphous Si (a-Si), 1.2-µm-thick µc-Si films are melted and recrystallized within 3 ms, whereas no phase transformation of a-Si films is observed under the same annealing condition. The annealed Si films show a high crystalline volume fraction of 97% and [111] preferential orientation. Characteristic triangle surface structures aligned to the laser scanning direction, which suggests that the lateral solidification from molten Si is observed.

Characterization of Sb-Doped Fully-Silicided NiSi/SiO/sub 2//Si MOS Structure

X-ray photoelectron spectroscopy (XPS) measurement of Sb-doped fully-silicided (FUSI) NiSi/SiO2 interface has been carried out to evaluate location of Sb pileup and to discuss its role for workfunction shift. The XPS result revealed Sb encroachment into SiO2. Workfunction characterization by XPS implied that NiSi workfunction was identical to its original value without Sb pileup located inside the gate oxide. The impact of predoping on silicidation reaction was also investigated.