Thermal Plasma Jet Irradiation Research Articles

Investigation on electrical characteristics of TFTs fabricated with germanium films crystallized by atmospheric-pressure micro thermal plasma jet irradiation

Crystalline germanium (c-Ge) is an attractive material for a thin-film transistor (TFT) channel because of its high carrier mobility and applicability to a low-temperature process. We present the electrical characteristics of c-Ge crystallized by an atmospheric-pressure micro thermal plasma jet (μ-TPJ). The μ-TPJ-crystallized c-Ge showed a maximum Hall mobility of 1070 cm2 V−1 s−1 with a hole concentration of ∼1016 cm−3, enabling us to fabricate a TFT with a field-effect mobility (μ FE) of 196 cm2 V−1 s−1 and an ON/OFF ratio (R ON/OFF) of 1.4 × 104. R ON/OFF and μ FE were dependent on the scanning speed of the TPJ, implying different types of defects were induced in the channel regions. These findings show the possibility of TPJ irradiation as a promising method for making c-Ge TFTs on insulating substrates.

In situ monitoring to visualize temperature distribution in molten silicon region formed under atmospheric pressure thermal plasma jet irradiation

The precise determination of the two-dimensional temperature distribution in a molten silicon region formed by an atmospheric pressure thermal plasma jet was investigated. Our in situ monitoring technique, combining camera observations and non-contact temperature measurements, has revealed the presence of a solid–liquid mixture in the molten silicon and enabled its temperature distribution to be visualized. We discovered several details concerning molten silicon crystallization including a forward supercooling degree of 20 K. Moreover, the temperature distribution in this solid–liquid mixture region was 80 K below the melting point. These findings contribute to the elucidation of the crystallization mechanism.

(Invited) Analysis of a Molten Region on Amorphous Silicon Film By High-Speed Camera and Contactless Temperature Measurement during Atmospheric Pressure Thermal Plasma Jet Annealing

In the production process of thin film transistors (TFTs), many researchers have discussed about crystallization of an amorphous silicon (a-Si) film by rapid thermal annealing for providing the polycrystalline silicon (poly-Si) film at a low temperature. The a-Si film was deposited on the glass substrate at a low temperature and annealed by rapid thermal annealing. It was important to reveal the mechanism of a silicon melt-solidification and grain growth for providing a poly-Si film having high crystallinity. So, they have proposed various measurement methods for finding the important information which is a lattice temperature, time-resolved electrical-conductance and optical-reflectance, and so on [1,2]. We have proposed our own analysis which is a high-speed camera (HSC) observation in real time in microsecond scale and a contactless temperature measurement of a substrate surface during an atmospheric pressure thermal plasma jet (TPJ) annealing [3,4]. We first disclosed leading wave crystallization (LWC) on the a-Si film during TPJ irradiation because of this HSC observation and revealed the two-dimensional temperature distribution for the surface of a glass or quartz substrate. However, we can’t obtain their information at the same time. Here, we demonstrate a temperature distribution of the molten region on a-Si film when we simultaneously use the contactless temperature measurement and the HSC observation in a microsecond scale. Amorphous silicon film with the thickness of 100 nm was formed on a 100-µm-thick flexible glass substrates by plasma-enhanced chemical vapor deposition at 250 °C and was dehydrated in N2 ambient at 450 °C. We carried out the temperature measurement and real-time observation of the molten silicon region by the contactless temperature measurement and the HSC observation when the glass substrate and the prepared film on the substrate was rapidly annealed by TPJ irradiation. In the result of the real-time reflectance by the contactless temperature measurement, we obtained a time resolved temperature on the surface of a glass substrate and a TPJ thermal profile by our own simulation analysis. Also, we got the three-dimensional temperature distribution from the obtained thermal information. We could get a cross-sectional image from the three-dimensional temperature distribution by cutting this distribution across the plane of the 1700 K parallel to a horizontal direction. We obtained isothermal lines from the cross-sectional image. We presumed that these isothermal lines correspond to the molten region on the a-Si film in the HSC image. So, we tried to superimpose the isothermal lines on the HSC image. The black area in the HSC image is the molten silicon region and this area is in the width of ~0.43 mm and length of ~1.13 mm. TPJ irradiation at 1 m/s was scanned from right to left of this image. The crystallization area is located at the rear of TPJ irradiation. The high speed lateral crystallization (HSLC) which is dendrite crystal growth appears in this area. We carried out the overlap of the HSC image and the isothermal lines with the edge of the molten region and the isothermal line of 1700 K and recognized that these images most closely matched as shown in the figure. We found that there is a liquid area even though the temperature is lower than the melting point. Also, it was revealed that this area is supercooling area and supercooling degree is ~100 K. Also, it was obvious that the crystal growth direction didn’t exactly align parallel to the direction of a temperature gradient. In this paper, we clarified that the temperature distribution in the molten region of the a-Si film can be visualized by the proposed method. This is a quite powerful tool to understand the crystalline growth in microsecond time regime. Reference: [1] G.J. Galvin, et al., Phys. Rev. B 27 (1983) 1079. [2] B.C. Larson, et al., Appl. Phys. Lett. 42 (1983) 282. [3] S. Hayashi, et al., Appl. Phys. Lett. 101 (2012) 172111. [4] T. Okada, et al., Jpn. J. Appl. Phys., 45 (2006) 4355. Figure 1

Investigation on Crack Suppression by Thermal-Plasma-Jet Crystallization of Amorphous Silicon Films on Flexible Glass Substrate

Introduction.Ultra-thin glass substrate is attracting much attention as flexible substrates for curved display applications. In fabrication of thin-film transistors (TFTs) on flexible glass, crystallization of amorphous Si (a-Si) is one of the key process technologies [1-3]. Rapid-thermal-annealing (RTA) in microsecond regime is necessary to form high crystallinity Si without thermal damage to glass. We have proposed the application of atmospheric pressure DC ark discharge thermal-plasma-jet (TPJ) to the crystallization of a-Si films [4]. Because of its simple structure and atmospheric pressure process, TPJ enables low cost crystallization compared with excimer laser annealing (ELA) technique. High speed lateral crystallization (HSLC) is induced by sweeping a molten region at a speed as high as 4000 mm/s. However, residual tensile stress is generated because of the densification of glass surface after RTA process. As a result, significant number of cracks are generated on the surface of glass substrate in cases of sever annealing conditions. In this study, we applied µ-TPJ crystallization of a-Si films on ultra-thin glass substrate and investigated the effect of glass thickness on residual stress and crack generation. Experimental.After RCA cleaning and HF treatment of 100-µm-thick flexible glass and 500-µm-thick conventional glass substrates, 100-nm-thick a-Si films were deposited by plasma-enhanced chemical vapor deposition (PECVD) at a substrate temperature of 250 °C. Dehydrogenation was carried at 450 °C in N2 ambient for 1 h. TPJ irradiation set up is shown in Fig. 1. µ-TPJ was generated by DC arc discharge under atmospheric pressure with supplying power (P) of 1.7 kW between electrodes. The Ar gas flow rate (f) was 1.0 L/min. µ-TPJ was generated by blowing out the thermal plasma through an orifice with its diameter (ϕ) of 600 µm. The distances between the plasma source and glass substrates (d) were fixed at 1.0 mm and the substrates were linearly moved by a motion stage in front of the µ-TPJ with scanning speed (v) of 2200 mm/s. For the measurement of temperature profile in the glass substrate, transient reflectivity during TPJ annealing was measured by irradiating bare conventional glass substrate with a He-Ne laser (632.8nm) from the back surface. Details of the non-contact temperature measurement are described in Ref [5]. Results and discussion.The microscope images of crystallized Si films on each substrate are shown in Fig. 2. Many cracks were observed in conventional glass substrate. On the other hand, no crack was observed in flexible glass substrate, although both glass substrates were annealed at the same time. Crystallization was conducted under various µ-TPJ irradiation conditions, and almost no cracks were found in flexible glass and it showed significant resistance against RTA. This result suggests that the tensile stress at glass surface is significantly reduced in the case of ultra-thin flexible glass. Previous study calculated residual stress profile in glass substrate on the basis of multilayered beam theory [6]. To obtain stress profile, we measured transient temperature profile during TPJ irradiation by optical probe (Fig .3). We assumed surface densification of glass when the temperature exceeded softening point (1258 K). We conducted residual stress analysis of glass substrate as the residual stress is shown in Fig. 4 [6]. Strong tensile stress is introduced in glass surface due to the shrinkage by RTA. At the same time, compressive stress is introduced inside of glass to compensate the tensile stress at the surface. It should be noted that the depth of stress compensation is in the range of ~400 µm as shown in Fig. 4, which is larger than the thickness of flexible glass. It is expected that the back side of thin glass substrate is free to relax the stress and plays a roll in suppression of residual stress in the surface of glass. Conclusions.We have crystallized a-Si films on flexible glass substrate by µ-TPJ without crack generation. Flexible glass substrate is useful to relieve surface tensile stress and as the result, crack generation is significantly suppressed. Acknowledgments. A part of this work was supported by Research Institute for Nanodevice and Bio Systems (RNBS) Hiroshima University, and JSPS KAKENHI Grant Number (B)(16H0433400).

High Efficiency Activation of Phosphorus Atoms in 4H-SiC by Atmospheric Pressure Thermal Plasma Jet Annealing

In this study, the application of atmospheric pressure thermal plasma jet (TPJ) annealing for impurity activation in 4H–SiC is reported. The activation of phosphorus atoms implanted at 300°C in 4H–SiC by TPJ irradiation and analysis of its crystallinity are investigated. At the maximum annealing temperature of 1630°C, the minimum resistivity value is 3.1 mΩ·cm and the maximum carrier concentration value is 2.0 × 1020 cm−3. Orientation analysis suggests that the sample implanted at 300°C was recrystallized to a 4H–SiC(0001) structure after 1630°C annealing. Furthermore, a significant increase in the carrier concentration was observed with the increasing cooling rate during the activation annealing process. Rapid cooling may suppress the impurity deactivation. These results suggest that short-time high-temperature TPJ irradiation annealing is effective for P dopant activation in 4H–SiC.

Investigations on crack generation mechanism and crack reduction by buffer layer insertion in thermal-plasma-jet crystallization of amorphous silicon films on glass substrate

The crack generation mechanism and the effect of crack reduction by buffer SiO2 layer insertion in thermal-plasma-jet (TPJ) crystallization of an amorphous silicon film on a glass substrate have been investigated. The crack generation was clearly observed 13.7 s after TPJ irradiation using a high-speed camera, which indicates that cracks are generated not during heating, but during cooling. From the measurement and simulation of substrate deformations, it was clarified that the substrate deformed convexly during heating and it consequently deformed concavely after cooling owing to the substrate surface densification. This result indicated that the tensile stress generated by the concave deformation is the origin of cracks. The deposition of the buffer SiO2 layer generated compressive stress, which minimizes accumulation of tensile stress after TPJ annealing. The number of cracks in unit length significantly decreased owing to the decrease in tensile stress with the increase in the thickness of the buffer SiO2 layer.

Improvement in Characteristic Variability of TFTs Using Grain Growth Control by Micro Thermal Plasma Jet Irradiation on a-Si Strips

Grain growth from molten silicon during micro-thermal-plasma-jet ( $\mu$ -TPJ) irradiation has been controlled by applying amorphous-silicon (a-Si) strips. The grain boundaries (GBs) in strip pattern with the width of ${{1}}~\mu{{m}}$ are significantly reduced compared with the case without strip pattern. In strip pattern, random GBs were almost excluded and most of the strips consist of $\sum 3$ coincidence site lattices (CSLs) GBs or in some cases single grains. TFTs with strip pattern showed threshold voltage $({V}_{\rm th})$ , field effect mobility $(\mu_{\rm FE})$ and sub-threshold swing value (S) of ${{1.8}} \pm {{0.10}}~{{V}}$ , ${{303}} \pm {{24}}~{{cm}}^{2}/{{V}}\cdot{{s}}$ and ${{240}}\pm {{17}}~{{mV/dec}}$ , respectively. In the case of P-type TFTs, they were $-{{1.8}}\pm {{0.22}}~{{V}}$ , ${{98}}\pm {{7}}~{{cm}}^{2}/{{V}}\cdot{{s}}$ , ${{285}} \pm {{17}}~{{mV/dec}}$ , respectively. TFTs fabricated with the proposed pattern showed high performance and the variability reduced to $1/3$ compared with the case without strip pattern. These characteristics and small variation with proposed patterns enabled us to operate CMOS circuits. The 8-bits shift register fabricated with proposed pattern was operated at a supply voltage of 5 V clock frequency of 4 MHz.

Formation of High-Density Pt Nanodots on SiO2Induced by Millisecond Rapid Thermal Annealing Using Thermal Plasma Jet for Floating Gate Memory

We formed nanometer-scale Pt dots on SiO2 by thermal plasma jet (TPJ) irradiation and demonstrated the feasibility of TPJ millisecond annealing for controlling the areal density of the dots. 2–10-nm-thick Pt/quartz was exposed to an Ar plasma jet at peak temperatures of 455–695 °C for a duration of 1–2 ms. When the Pt/quartz was annealed by TPJ irradiation without any extra heating, the Pt nanodot density was controlled in the range of 7.8 ×1010 to 2.0 ×1011 cm-2 by changing the annealing temperature and the number of irradiations. These results imply that ultra-rapid annealing using a TPJ plays an important role in enhancing the surface migration of Pt atoms in order to reduce the interface energy of Pt/quartz.

Formation of High-Density Pt Nanodots on SiO2 Induced by Millisecond Rapid Thermal Annealing Using Thermal Plasma Jet for Floating Gate Memory

We formed nanometer-scale Pt dots on SiO2 by thermal plasma jet (TPJ) irradiation and demonstrated the feasibility of TPJ millisecond annealing for controlling the areal density of the dots. 2–10-nm-thick Pt/quartz was exposed to an Ar plasma jet at peak temperatures of 455–695 °C for a duration of 1–2 ms. When the Pt/quartz was annealed by TPJ irradiation without any extra heating, the Pt nanodot density was controlled in the range of 7.8 ×1010 to 2.0 ×1011 cm-2 by changing the annealing temperature and the number of irradiations. These results imply that ultra-rapid annealing using a TPJ plays an important role in enhancing the surface migration of Pt atoms in order to reduce the interface energy of Pt/quartz.

Application of Thermal Plasma Jet Irradiation to Crystallization and Gate Insulator Improvement for High-Performance Thin-Film Transistor Fabrication

Application of Thermal Plasma Jet Irradiation to Crystallization and Gate Insulator Improvement for High-Performance Thin-Film Transistor Fabrication

Application of Thermal Plasma Jet Irradiation to Crystallization and Gate Insulator Improvement for High-Performance Thin-Film Transistor Fabrication

Large grains with a maximum length of ∼60 µm were grown by high speed scanning (∼4000 mm/s) of a molten region in amorphous silicon (a-Si) films formed by micro-thermal-plasma-jet (µ-TPJ) irradiation. By reducing the TPJ nozzle diameter and increasing the spacing between anode and cathode, the power density transferred to a-Si film surface increased to as high as 53 kW/cm2, which enabled melting and lateral solidification in the microsecond time domain. The a-Si transformed to crystalline through solid-phase crystallization, followed by melting and recrystallization induced by the movement of the molten region with the maximum size of ∼483 µm in width and ∼990 µm in length. The laterally crystallized Si films show anisotropic large grains and a high crystalline volume fraction of ∼100% and preferential surface orientation of (111) plane. Thin-film transistors (TFTs) fabricated by solid-phase-crystallized microcrystalline Si (µc-Si) show a small field effect mobility (µFE) of ∼2 cm2 V-1 s-1 with small variation less than 1%, while the high-speed lateral-crystallization (HSLC) Si film shows a very high µFE of 350 cm2 V-1 s-1. We improved the bulk bond network of the low-temperature-deposited gate SiO2 films by TPJ-induced millisecond annealing. By combining TPJ annealing and postmetallization annealing (PMA), a high-quality SiO2/Si interface with a density of interface states (Dit) of 3.0 ×1010 cm-2 eV-1 is obtained. In addition, we found that the improvement in the bulk bond network of SiO2 is quite effective to improve the stress immunity of µc-Si TFTs. TFTs fabricated with TPJ-annealed gate SiO2 films show much smaller on-current degradation and threshold voltage shift after DC bias stress compared with untreated TFTs. Not only the threshold voltage (Vth) shift under high-gate-field stress condition, but also on-current degradation under drain avalanche hot carrier (DAHC) generation condition are markedly suppressed. This improvement is attributed to the reduction of Si–OH bonds and relaxation of the bulk chemical bond network of SiO2 induced by TPJ annealing.

Activation of B and As in Ultrashallow Junction During Millisecond Annealing Induced by Thermal Plasma Jet Irradiation

We have investigated effects of annealing temperature and heating and cooling rates during millisecond annealing on the activation of B and As in the Si lattice. In the case of As+-implanted samples, efficient dopant activation was observed at a temperature higher than 1000 K, while it was observed at a temperature higher than 1400 K in the case of B-implanted samples. The sheet resistance (RS) of B-implanted samples monotonically decreases with temperature, and no significant dependence on heating rate (Rh) or cooling rate (Rc) is observed. On the other hand, As+-implanted samples show significant dependence of RS on Rh and Rc. We have performed thermal plasma jet (TPJ) annealing on an As2+-implanted sample, and obtained an ultrashallow junction (USJ) with a junction depth (Xj) of 11.9 nm and a RS of 1095 Ω/sq. B USJ is also obtained with a Xj of 23.5 nm and a RS of 392 Ω/sq. Precise control of Rh and Rc in addition to annealing temperature is quite important for achieving highly efficient doping in USJ.

Effect of chemical composition of SiOx films on rapid formation of Si nanocrystals induced by thermal plasma jet irradiation

AbstractSi nanocrystal floating gate MOS capacitors were formed on p‐Si (100) wafers by thermal plasma jet (TPJ) annealing of SiO2/SiOx /SiO2/Si(100) stacked structure. The chemical composition of SiOx layer was controlled by changing the SiH4, He, and O2 gas flow ratio during plasma enhanced chemical vapour deposition. The MOS capacitors showed clear hysteresis in capacitance‐voltage (CV) characteristics after TPJ annealing. The hysteresis width shows maximum value when initial composition x =1.7, which shows the maximum photoluminescence (PL) intensity. The maximum hysteresis width of 6.8 V was observed with gate voltage swept between 20 and ‐20 V in x = 1.7 sample. The result means 7.4 × 1012 cm‐2 carriers are injected to or emitted from Si nanocrystals. The duration of 1 V shift in flatband voltage was ∼0.1 ms with 20 V pulse, and charged carriers were stably maintained for ∼2 hours. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Millisecond Rapid Thermal Annealing of Si Wafer Induced by High-Power-Density Thermal Plasma Jet Irradiation and Its Application to Ultrashallow Junction Formation

We have investigated the generation of high-power-density thermal plasma jet (TPJ) as a heat source for the annealing of a Si wafer surface in a millisecond period and the formation of an ultrashallow junction. The power density of DC arc discharge thermal plasma jet markedly increased from 11.0 to 32.3 kW/cm2 with increasing spacing between an anode and a cathode (ES) from 1.0 to 3.0 mm. The increase in TPJ power density with ES was mainly due to the increase in plasma temperature from about 16000 to 23000 K. By applying this high-power-density TPJ, the Si wafer was heated by more than 700 K within 10 ms. Using this annealing technique, we demonstrated the dopant activation of an arsenic-implanted Si wafer and successfully obtained a low sheet resistance of 262 Ω/sq at an annealing temperature as high as 1207 K without a significantly enhanced diffusion of the implantation profile.

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

An in-situ mnitoring 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 sutface temperature during thermal plasma jet irradiation with the heating and cooling rates in the order of 10^4 to 10^5 K/s.

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.

Electrical Characteristics of Lightly-Doped Si Films Crystallized by Thermal Plasma Jet Irradiation

50-nm-thick amorphous Si films doped with 4.3 x 10^17 cm-3 phosphorus atoms were crystallized by thermal plasma jet (TPJ) irradiation. The electrical conductivity (sigma) of the crystallized films ranges from 1.2 x 10^-5 - 1.7 x 10^-2 S/cm, while Si films crystallized by excimer laser annealing (ELA) show much smaller sigma of 1.6 - 4.5 x 10^-6 S/cm regardless of laser fluence. This result can be interpreted that the defect density in the TPJ crystallized films is much smaller than that in ELA Si films. By treating the films with hydrogen plasma for 60 s at 250oC, sigma of TPJ crystallized films increased to 1.4 x 10^-2 - 1.1 S/cm, while it was 3.0 x 10^-6 - 6.5 x 10^-3 S/cm in the case of films crystallized by ELA. This result indicates that a short period hydrogenation is effective to improve the electrical characteristics of TPJ crystallized Si films.

Rapid nitriding of pure iron by thermal plasma jet irradiation

Pure iron samples were exposed to thermal plasma jet and nitrided for 15 min at a temperature ranging from 545 °C to 742 °C. For comparison purposes, conventional ion nitriding was also performed using glow discharge plasma at 580 °C for 8 h. For conventional plasma nitriding, a top compound layer and a bottom diffusion zone are observed. In contrast, an additional “transition zone” is observed between the compound layer and the diffusion zone for the thermal plasma jet treatment when the process temperature is 667 °C or above. This “transition zone” is quite thick (a few tens of micrometers), determined by the treatment temperature, and has unique features in the phase formation and nitrogen distribution. At 706 °C, the content of γ′phase, as well as the content of nitrogen and the hardness, reaches the maximum. Compared with the traditional plasma nitriding process, the reaction speed of thermal plasma irradiation is much higher. A nitrided case depth over a few tens of micrometers is obtained in plasma jet nitriding for only 15 min versus a depth of only a few micrometers in conventional plasma nitriding for 8 h. The formation of the “transition zone” and the mechanism for the high nitriding speed are discussed in the paper. It is believed that this technology can be applied at atmospheric pressure in field without the requirement for a vacuum system. Hence, this technology may be advantageous for practical applications.