Low-temperature poly-Si (LTPS) thin films formed by excimer laser annealing (ELA) are used as the channel material for thin film transistors (TFTs), which have an application as switching devices in flat panel displays. It is well known that one of the major problems in TFT manufacturing is the prominent ridges that form on LTPS thin films after ELA due to volume expansion by crystallization, which in turn induces gate leakage current in the TFTs. In this presentation, we report on the use of additional laser irradiation to reduce the height of the ridges and resulting changes in the electrical properties of LTPS-TFTs.
Ultra-large scale integrated circuits (ULSIs) have been continually scaled down according to Moore’s law. This can improve their power consumption and operation frequency but not the RC delay of their interconnections; to this end, super low dielectric constant films are required. We propose a novel method to fabricate porous SiO2 films with a super low dielectric constant by F2 laser deposition. In this method, a quartz target is evaporated by F2 laser ablation in vacuum-chamber-controlled Ar partial pressure. The evaporated SiO2 molecules are agglomerated in the vacuum, and the size of the SiO2 nanoparticles are controlled by the Ar partial pressure. Porous SiO2 films are formed on a Si-receiving substrate, which is placed in front of the quartz target. The pulse duration of the F2 laser was approximately 20 ns, and the repetition rate of laser shots was 100 Hz. The base pressure of the vacuum chamber was 5 × 10−3 Pa. Then, Ar gas was introduced into the vacuum chamber through a mass flow controller to control the Ar partial pressure. The dominant size of the SiO2 nanoparticles decreased from 1.5–2.0 nm to 1.0–1.5 nm with the Ar partial pressure decreasing from 20 Pa to 4.5 Pa. In addition, the relative dielectric constant k of the porous SiO2 film formed at an Ar partial pressure of 4.5 Pa was 2.8, which is lower than that of thermal SiO2 (k = 4.0). In addition, the leakage current of the nanoporous SiO2 film was almost equal to that of the thermal SiO2 film. From these results, we conclude that nanoporous SiO2 films with a super low dielectric constant can be formed by F2 laser deposition.
We propose low-temperature and high-concentration doping of 4H-silicon carbide (4H-SiC)(0001) by KrF excimer laser irradiation of source films on a 4H-SiC substrate, in which a dopant atom is included. In n-type doping, a SiNx film with a thickness of 100 nm was deposited on an n-type 4H-SiC(0001) substrate by chemical vapor deposition. A gas supply nozzle for ambient environment control was installed to prevent oxidation of the SiC surface. High-concentration nitrogen doping (~1 × 1021/cm3 at the surface) was achieved by laser ablation of the SiNx film. Al/Ti electrodes were formed on the doped area at a room temperature, and a contact resistance of 2.2 × 10-5 Ω・cm2 was obtained, which is sufficiently small for the backside contact resistance of Schottky barrier diodes. In p-type doping, an Al film with a thickness of 240 nm was deposited on a 4H-SiC substrate by sputtering deposition. After laser irradiation of the Al film in ambient Ar, high-concentration Al doping (~1 × 1021/cm3 at the surface) was achieved. Al/Ti electrodes were formed on the doped area at a low temperature of 600 °C, and a contact resistance 1.9 × 10-4 Ω・cm2 was obtained. We conclude that low-temperature and high-concentration doping of 4H-SiC for low contact resistance can be achieved by laser ablation of the source films on the 4H-SiC substrate.
We have proposed that germanium-tin (GeSn) particles with high substitutional Sn concentrations can be synthesized by pulsed laser deposition (PLD) in ambient Ar at low pressure (~100 Pa). In this method, a Ge0.9Sn0.1 target is ablated by KrF excimer laser irradiation. At low Ar pressure (~100 Pa), the agglomeration of Ge and Sn atoms occurs easily in ambient Ar, and the agglomerated particles are rapidly cooled by collision with Ar atoms. An Si-receiving substrate was placed in front of the target. Various GeSn particles from several 100 nm to approximately 20 μm with spherical, disk, and irregular shapes were deposited on the Si-receiving substrate. In Raman spectra, the Ge-Ge vibration peaks of all the particles were shifted to lower wavenumbers compared with those of the Ge(100) crystal. The Raman peak position reportedly shifts to lower wavenumbers with increased substitutional Sn concentration in crystalline Ge. Thus, GeSn crystal particles with over 10% substituted Sn atoms can be synthesized by low-pressure PLD.
The electrical properties of poly-Si thin films doped using KrF excimer laser irradiation with a phosphoric-acid coating were investigated. After laser doping, the mobility, carrier concentration, activation ratio, and contact resistivity of the poly-Si were found to be 61 cm2 /Vs, 1.5×1018 cm-3 , 18.1 %, and 8.5 × 10−5 Ω⋅cm2 , respectively. Additionally, the operation of a bottom gate transistor fabricated using laser doping was realized and is described herein.
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