Horng-Chih Lin

EFFECTS OF SIH4, GEH4, AND B2H6 ON THE NUCLEATION AND DEPOSITION OF POLYCRYSTALLINE SI1-XGEX FILMS

In this study, the effects of different reactants, namely, Sill4, GeH4 and B2H6, on the nucleation and deposition of polycrystalline silicon-germanium (poly-Si~_=Gex) films on oxide surface in an ultrahigh vacuum chemical vapor deposition reactor were explored. The results show that the addition of GeH4 tends to retard the nucleation process of poly films while B2H6 is preferential for adsorbing on the oxide surface. These effects lead to different incubation duration depending on the kind of reactants used. On the deposition of poly-Si~_~.Ge= films, it is observed that the Ge incorporation is only slightly related to the substrate type, but the deposition mode of poly-Si~_~Ge~ films is much different from that of epitaxial growth on St(100). The incorporation of Ge atoms also overcomes the anomalous doping effect encountered in heavily boron-doped poly-Si films and allows extremely low resistivity (below 2 mf~-cm) poly films to be obtained at low temperatures (<-550~

Fabrication and characterization of nanowire transistors with solid-phase crystallized poly-Si channels

The performance of thin-film transistors with a novel poly-Si nanowire channel prepared by solid-phase crystallization is investigated in this paper. As compared with conventional planar devices having self-aligned source/drain, the new devices show an improved on-current per unit width and better control over the short channel effects. The major conduction mechanism of the off-state leakage is identified as the gate-induced drain leakage, and it is closely related to the source/drain implant condition and the unique device structure

Deposition and device application of in situ boron‐doped polycrystalline SiGe films grown at low temperatures

Deposition of in situ boron‐doped polycrystalline silicon‐germanium (poly‐Si1−xGex) films at temperatures below 550 °C was investigated using an ultrahigh‐vacuum chemical‐ vapor‐deposition system. These films with a fine grain structure were obtained for boron concentrations higher than 1021 cm−3. It is attributed to the enhanced nonequilibrium doping effect due to the addition of GeH4 gas during film deposition. Poly‐Si0.56Ge0.44 films with a carrier concentration of 8×1020 cm−3 were achieved at a growth temperature of 500 °C. Such a high activated carrier concentration resulted in a film resistivity less than 2 mΩ cm. Utilizing these characteristics, a novel approach was proposed and demonstrated to fabricate p‐channel polycrystalline silicon thin‐film transistors at process temperatures below 550 °C. These transistors with a maximum field effect mobility up to 28 cm2/V s and an on/off current ratio over 106 were achieved without employing any post‐treatment step, indicating the feasibility of this appr...

Highly selective etching for polysilicon and etch‐induced damage to gate oxide with halogen‐bearing electron‐cyclotron‐resonance plasma

The investigations of polysilicon etching with three halogen‐bearing plasmas (SF6, Cl2, and HBr) in an electron‐cyclotron‐resonance reactor have been made. We examine the etching characteristics which include etching rate, anisotropy, and selectivity based on the discussions of the chemical and electrical properties of the F, Cl, and Br radicals. It was found that the degree of anisotropy strongly depends on the deposition of a silicon halide film on sidewall and on the spontaneous etching properties of three halogen plasma mixtures. The selectivity to oxide is related to the amount of silicon halides that are produced during etching poly‐Si and the electrically polarized level of these products when they adsorb on the oxide surface. We find that the selectivity on oxide is approximately infinite for the HBr system and over 90 for the Cl2 system. In addition, the effects of oxygen addition, microwave power, and rf power are also investigated. It was found that the maximum etching rate and selectivity of S...

Low-Temperature Epitaxial Growth of Silicon and Silicon-Germanium Alloy by Ultrahigh-Vacuum Chemical Vapor Deposition

Low-temperature epitaxy of silicon and silicon-germanium alloy via an ultrahigh-vacuum chemical vapor deposition system was investigated. Bistable conditions were observed for silicon epitaxial growth performed within the temperature range of 550° C to 800° C. The activation energy of the SiGe growth rate was found to decrease as the germanium composition increased. The germanium atomic molar fraction in these epitaxial layers was tightly controlled by a computer-controlled gas source switching system. Si/SiGe superlattice structures of 20-period 5 nm Si/12 nm SiGe layers were grown to demonstrate the excellent controllability of this growth technique.

A novel thin-film transistor with self-aligned field induced drain

In this letter, a novel thin-film transistor with a self-aligned field-induced-drain (SAFID) structure is reported for the first time. The new SAFID TFT features a self-aligned sidewall spacer located on top of the drain offset region to set its effective length, and a bottom gate (or field plate) situated under the drain offset region to electrically induce the field-induced-drain (FID). So, unlike the conventional off-set-gated TFTs with their effective FID length set by two separate photolithographic masking layers, the new SAFID is totally immune to photomasking misalignment errors, while enjoying the low off-state leakage as well as high turn-on characteristics inherent in the FID structure. Polycrystalline silicon TFTs with the new SAFID structure have been successfully fabricated with significant improvement in the on/off current ratio.

Temperature-accelerated dielectric breakdown in ultrathin gate oxides

Temperature-accelerated effects on dielectric breakdown of ultrathin gate oxide with thickness ranging from 8.7 to 2.5 nm are investigated and analyzed. Although superior reliability for ultrathin gate oxide at room temperature has been reported in recent literatures, a strong temperature-accelerated degradation of oxide reliability is observed in this study. Experimental results show that both charge-to-breakdown (Qbd) and breakdown field (Ebd) characteristics are greatly aggravated for ultrathin oxide at elevated temperature. The Arrhenius plot also confirms that the activation energies of Qbd and Ebd increase significantly as oxide thickness decreases, explaining the higher sensitivity to temperature for thinner oxides.

Post-soft-breakdown characteristics of deep submicron NMOSFETs with ultrathin gate oxide

The impacts of soft-breakdown (SBD) on the characteristics of deep sub-micron NMOSFETs were investigated. It is shown that the BD location plays a crucial role in the post-BD switching function of the device. When BD occurs at the channel, the turn-on behavior of the drain current would not be significantly affected, which is in strong contrast to the case of ED at the drain, Nevertheless, significant increase in gate current is observed in the off-state when the gate voltage is more negative than -1 V. Its origin is identified to be due to the action of two parasitic bipolar transistors formed after SBD occurrence at the channel.

Fabrication of p‐channel polycrystalline Si1−xGex thin‐film transistors by ultrahigh vacuum chemical vapor deposition

In this report, we present a novel approach for fabricating polycrystalline silicon‐germanium thin‐film transistors at low temperatures (≤550 °C). A bottom gate configuration is used for this approach, and an i‐Si1−xGex/i‐Si/p+‐Si1−yGey multilayer is deposited sequentially on the gate oxide using an ultra‐high vacuum chemical vapor deposition technique. The i‐Si1−xGex serves as the channel while the i‐Si is used as a buffer layer for allowing p+‐Si1−yGey to be etched selectively on. p‐channel thin‐film transistors with a field‐effect mobility of 13 cm2/V s were achieved using this method, which is superior to those grown by low pressure chemical vapor deposition. Our results indicate that the deposition of poly‐Si1−xGex/poly‐Si multilayer structure would be a promising way for polycrystalline thin‐film device applications.

EFFECT OF BORON DOPING ON THE STRUCTURAL-PROPERTIES OF POLYCRYSTALLINE SILICON FILMS GROWN AT REDUCED PRESSURES

The deposition and properties of in situ boron‐doped polycrystalline silicon (poly‐Si) films grown at 550 °C were investigated using an ultrahigh vacuum chemical vapor deposition system. It is observed that, if the doping level is high enough, the boron incorporation would significantly reduce the deposition rate, impede the grain growth, and degrade the crystallinity of the poly‐Si films. We also found that the preferential adsorption of boron atoms on the SiO2 surface at the initial stage of deposition shortened the incubation time of deposition. The property of trapping states at the grain boundary is also examined, and a density of about 4.7×1012 cm−2 is obtained for poly‐Si films with a doping level less than 2.2×1019 cm−3.