Application of laser processing for improved oxides grown from polysilicon

Laser annealing techniques were successfully incorporated into standard MOS processing to improve the quality of oxides grown over polysilicon. Polysilicon films (5000 Å thick) deposited over 1000-Å SiO2, grown over     

Improvement of polysilicon oxide by growing on polished polysiliconfilm

This letter first reports the characteristics of polyoxide that was thermally grown on polished polysilicon film. Compared to conventional polyoxide, polyoxide grown on polished polysilicon film exhibits lower leakage current, higher dielectric breakdown field, larger effective barrier height, and higher charge-to-breakdown. This simple and well-operated process provides a very promising option for the interpolysilicon oxide     

Structural characterization of oxide layers thermally grown on 3C-SiC films

The oxidation of 3C-SiC films deposited on off-oriented Si(001) substrates by reactive magnetron sputtering has been studied. The oxidation was carried out using dry conditions at a temperature of 1200°C. The composition of the oxide layer was investigated by Auger electron spectroscopy (AES). The oxide layer was found to contain no C except for the region very close to the interface, and the stoichiometry was found to be close to that of SiO₂. Cross-sectional transmis-sion electron microscopy (XTEM) showed the oxide layer to be completely amorphous, dense, and homogeneous with a uniform thickness. High-resolution XTEM imaging showed an atomically sharp SiO₂/SiC interface.     

Determination of the electrical properties of thermally grown ultrathin nitride films

The aim of this work is to obtain the electrical properties of a leaky 2 nm ultrathin thermally grown nitride film using the classical HF C(V) measurements. The substrate Si (1 0 0) surface cleaned in UHV is nitrided in a low pressure of nitric oxide (NO) gas at 1050°C. This film is characterized by Auger electron spectroscopy (AES), fourier transform infra red spectroscopy (FTIR), transmission electron microscopy (TEM) and electron energy loss spectrum (EELS). The deposition of a self-assembled insulating monolayer of organic molecules (octadecyltrichlorosilane) on the nitride gives rise to a metal–insulator–semiconductor (MIS) structure, which permits to obtain the electrical properties of the ultrathin nitride film.     

Highly reliable ECR N2O-plasma polyoxide grown on heavily phosphorus-doped polysilicon films

Reliability of polyoxide grown by electron cyclotron resonance (ECR) N₂O-plasma on heavily phosphorus-doped polysilicon has been investigated for the interpoly dielectrics (IPDs) of nonvolatile memories (NVMs). ECR N₂O-plasma polyoxide grown on polysilicon with phosphorus of 1 × 10²¹ cm⁻³ exhibits a significantly high breakdown field of 10 MV/cm and low electron trapping rate of 0.5 V, which are regardless of phosphorus concentration. The improvements are attributed to the smooth polyoxide/polysilicon interface, low phosphorus concentration, and nitrogen-rich layer with strong silicon-nitrogen bonds at the polyoxide/polysilicon interface.     

A quantitative model for the conduction in oxides thermally grown from polycrystalline silicon

Oxides thermally grown from polycrystalline silicon are known to conduct much higher currents than oxides grown on monocrystalline material, which has led to their application in floating-gate electrically erasable programmable READ-only memories (EEPROM) devices (so-called textured devices) for the purpose of electrical programming and erasing of the memory device. This increased conductivity has been previously explained qualitatively by field enhancement due to the surface roughness of the polysilicon-polyoxide interfaces, but a quantitative model that could explain and predict the true injection current behavior was never proposed. In this paper, a new model is introduced, which is able to explain all of the experimental observations of conduction in polyoxides, including trapping phenomena. The new model is verified by comparison with two types of capacitor measurements. Since electron trapping in the oxide is included in the model it can also be used to investigate the degradation behavior of the textured-type floating-gate EEPROM cells. In all cases it is found that the nonuniformity of the polyoxide current strongly influences its behavior and that more specifically for EEPROM devices this nonuniformity puts severe limits on the number of program ERASE cycles that are possible.     

Topography induced preferential crystallization of thermally grown silicon dioxide films

We have investigated the crystallization of the oxide layer that grows on a deposited silicon film in a high temperature furnace. The growth of large SiO₂ crystal grains can be controlled by interfacial stress or surface topography. When the Si film is deposited on topographically patterned surfaces, the SiO₂ grains are nucleated along the edges and extremities of the relief structure. A competition in which faster growing grains terminate slower growing grains results in an average growth direction perpendicular to the edges. Single crystal grains of α-cristobalite up to hundreds of microns in length can be grown in this fashion.     

On the presence of aluminum in thermally grown oxides on 6H-siliconcarbide [power MOSFETs]

N-type and P-type 6H-Silicon carbide wafers, along with P-type silicon control wafers were oxidized in dry oxygen at 1275°C for 45 min. Capacitance-voltage measurements on the SiO₂ films formed on the N-type 6H-SiC and the silicon control wafers yielded near ideal characteristics while the SiO₂ films on P-type 6H-SiC revealed high effective charge and interface state densities. Secondary Ion Mass Spectroscopy performed on the oxides showed significant levels of aluminum and sodium in SiO₂ on both N-type and P-type 6H-SiC but not on the silicon control wafers indicating that neither element was introduced during processing. The presence of aluminum in oxides on both types suggests that it is not solely responsible for the increased effective oxide charge and interface state densities commonly observed in oxides formed on P-type 6H-SiC     

Performance of thin-film transistors on polysilicon films grown bylow-pressure chemical vapor deposition at various pressures

Defect properties of undoped low-pressure chemical-vapor-deposited (LPCVD) polysilicon films have been investigated by capacitance techniques on a simple metal-oxide-semiconductor (MOS) capacitor structure. The results show that the effective density of bulk and interface trap states is almost independent of the deposition pressure. After reducing the polysilicon film thickness by etching, although the grain size decreases due to the columnar mode of growth at low pressures, the trap states density reduces significantly. This finding could be explained by the hypothesis that, during the growth of the material, impurities are segregated at the film surface by fast diffusion through the grain boundaries. The transport properties of 0.5-μm-thick polysilicon films deposited at a pressure ranging from 100 to 0.5 mtorr were evaluated from measurements on thin-film transistors (TFTs). The results demonstrate that at high pressures the grain boundaries and at low pressures the polysilicon-SiO₂ interface roughness scattering are the main factors in determining the transistor performance     

Depth profiles of refractive index in thermally grown and LPCVD oxide films on silicon

Thin and thick silicon oxide films are grown by thermal oxidation of silicon in dry oxygen or by LPCVD from a mixture of monosilane with oxygen or nitrous oxide. Depth profiles of refractive index are determined in these films by monochromatic ellipsometry, with the measurements interpreted by solving an inverse problem within a multilayer model. The differences between the depth profiles are found to be consistent with well-known mechanisms of film growth under the process conditions applied.     

Characterization of charge trapping and high-field endurance for15-nm thermally nitrided oxides

Device-quality gate oxides have been nitrided using both rapid thermal processing and conventional furnace treatment. Charge trapping and high-field endurance including breakdown field and time-dependent dielectric breakdown, are investigated in detail. It is found that proper nitridation can eliminate positive charge accumulation in oxides, increase charge to breakdown, suppress high-field injection-induced interface state generation, and decrease the dependence of the breakdown field on the gate area as a result of the reduced density of microdefects. Experimental results show that although both the density and capture cross-section of the bulk and interface traps increased by nitridation, the combined effects of bulk and interface traps induced by high-field injection can improve the stability of the flatband voltage. For lightly nitrided oxides, the trap generation rate is greatly decreased as compared with the as-grown oxide. Not only are the density and capture cross-section of the traps affected by nitridation, but also the locations of the trapped-charge centroids are changed. The experimental results for postnitridation annealing suggest that these property modifications most likely result from nitridation-induced structural changes rather than hydrogenation alone     

Polyoxides grown on n+ polysilicon

The polarity asymmetry on the electrical characteristics of the oxides grown on n⁺ polysilicon (polyoxides) was investigated in terms of the oxidation process, the doping level of the lower polysilicon layer, the oxidation temperature and the oxide thickness. It was found that the thin polyoxide prepared by using a low-temperature wafer loading and N₂ pre-annealing process, has a smoother polyoxide/polysilicon interface and exhibits a lower oxide tunneling current, a higher dielectric breakdown field when the top electrode is positively biased, a lower electron trapping rate and a larger charge-to-breakdown than does the normal polyoxide. The polarity asymmetry is also strongly dependent on the doping level of the lower polysilicon layer, the oxidation temperature and the oxide thickness. It was found that only the thinner polyoxides (⩽240 Å) grown on the heavily-doped polysilicon film (30 Ω/sq) by using the higher-temperature oxidation process (⩾950°C) conduct a less oxide tunneling current when the top electrode is positively biased     

低温多晶硅薄膜制备技术应用进展

系统介绍了金属诱导横向晶化法、准分子激光晶化法、触媒化学气相沉积法(Cat-CVD)以及电感耦合等离子体化学气相沉积法(ICP-CVD)制备低温多晶硅薄膜的原理及进展。对不同制备工艺的优势和不足进行了比较,重点讨论了Cat-CVD和ICP-CVD在实用化中需克服的技术问题。对上述制备方法的应用前景作了评述和展望。     

Characterization of liquid-phase epitaxially grown HgCdTe films by magnetoresistance measurements

In this paper, we demonstrate that measurements of the magnetoresistance can be used as a valuable alternative to conventional characterization tools to study transport properties of advanced semiconducting materials, structures, or devices. We have measured magnetoresistance on two different systems, namely, three liquid-phase epitaxially grown HgCdTe films and two GaAs-based high-electron-mobility-transistor (HEMT) structures. The results are analyzed by using a two-carrier model as a reference in the context of the reduced-conductivity-tensor scheme. The HEMT data are in quantitative agreement with the two-carrier model, but the HgCdTe data exhibit appreciable deviations from the model. The observed deviations strongly indicate a mobility spread and material complexity in the HgCdTe samples which are probably associated with inhomogeneities and the resulting anomalous electrical behavior.     

Effect of doping on the temperature coefficient of resistance of polysilicon films

For doped polysilicon films, an experimental investigation is presented into the variation of temperature coefficient of resistance (TCR) with formation conditions, the dopants being rare-earth elements (Eu, Gd, and Yb), oxygen, germanium, and implanted boron or phosphorus. The sign and magnitude of the TCR are shown to be governed by the dopant concentrations and the conditions of postimplantation annealing. Common and specific features are identified in the behavior of TCR for undoped films and for doped ones differing in dopant and doping technique. A negative TCR is observed in undoped films and in ones doped with a rare-earth element or germanium. B or P ion implantation into films predoped with Ge is shown to give a positive TCR if the implant concentration exceeds 10¹⁸ atoms/cm³. With any of the dopants employed, raising the dopant concentration makes possible the transition from a negative to a positive TCR.     

Subthreshold properties of TFTs with laser-crystallized laterally grown polysilicon layers

Trap-density analysis of laterally grown polysilicon films formed by continuous-wave laser revealed the main factor that controls the subthreshold property of low-temperature polysilicon thin-film transistors. In the low-current region, traps at the gate oxide/polysilicon interface are charged and the consequent insensitiveness of polysilicon surface potential to gate bias dominates the subthreshold property. In the higher current region, that is, close to the threshold voltage, a transport mechanism in which carriers are scattered at the grain boundaries becomes the dominant factor governing the subthreshold property.     

Boron diffusion and activation in polysilicon multilayer films for P MOS structure: Characterization and modeling

This work deals with in situ boron diffusion and activation in multilayer films: polysilicon (Poly1)/amorphous silicon (Poly2). These films are deposited by LPCVD technique. However, several heat treatments were carried in order to determine the optimal annealing conditions to suppress boron penetration from the gate to the substrate through the gate oxide in MOS structure. The boron concentration is monitored by secondary ion mass spectrometry (SIMS). To investigate SIMS profiles we proposed a model of boron diffusion into these multilayer structures. It is important to note that the parameter values of the studied films such as the diffusion coefficient, the activation percentage of boron as well as the acceleration rate of boron diffusion are deduced from adjustment of simulated profiles with experimental profiles. From these results, we inferred that the boron is electrically active and its distribution does not reach the oxide layer and consequently, the Poly2 may reduce the boron diffusion in optimal annealing conditions.     

Characterization of CuGaSe2 thin films grown by MOCVD

CuGaSe₂ thin films have been grown by metalorganic chemical vapor deposition (MOCVD), from three organometallic precursors. Samples of about 1-2 μm thick are codeposited onto Pyrex and Mo-coated soda lime glass. A large range of compositions was investigated and characterized. Stoichiometric CuGaSe₂ thin films are single-phased and their optical bandgap is about 1.68 eV. The features of the films are presented in relation with their composition. XRD spectra always exhibit a preferential orientation along the (112) plane. Secondary phases have been observed: Cu₂Se for Cu-rich films, CuGa₃Se₅ for Ca-rich films. Observation of the morphology reveals larger polyhedral grains for Cu-rich films becoming platelet-shaped and tilted for Ga-rich compounds. The optical properties are also sensitive to the compositional changes and related to the eventual presence of binary phases. The gap increases with the Ga-content. The CuGa₃Se₅, phase exhibit a gap of about 1.85 eV. All the samples have a p-type conductivity     

SIMS analysis of nitrided oxides grown on 4H-SiC

This paper shows for the first time, physical evidence of nitrogen incorporation at the oxide-SiC interface as a result of post-oxidation annealing in nitric oxide (NO). Using secondary ion mass spectroscopy (SIMS) analysis, the location and shape of the nitrogen profile is seen to be almost identical to that found in oxide-silicon interfaces. Close examination of oxygen and carbon SIMS profiles and atomic force microscope scans also indicate a sharper interface when annealing is done using NO compared to inert gases such as N₂ or argon, possibly due to the removal of carbon clusters which form at the interface during oxidation. As in the case of silicon, NO annealing shows great promise as a processing step in the production of device quality gate oxides on SiC.     

Optical dispersion analysis within the IR range of thermally grown and TEOS deposited SiO2 films

SiO₂ thin films, with thickness ranging between approximately 13 and 95 nm, have been thermally grown at 950°C in dry oxygen and chemically vapor deposited at low pressures (0.3 Torr) by decomposition of tetraethylorthosilicate (TEOS) at 710°C, on Si (100) substrates. Dispersion analysis was performed on Fourier transform infrared (FTIR) transmission spectra of these films within the range 900–1400 cm⁻¹. It was found that the spectra were best described within this range, by four Lorentz oscillators located near 1060, 1089, 1165 and 1220 cm⁻¹ almost independent of film thickness. The polarization of the oscillators (proportional to their strength) was found to increase slightly, and their widths to decrease, with film thickness. From the study of the FTIR spectra obtained at room temperature, it was suggested that at this temperature, a considerable number of Si–O–Si angles in these SiO₂ films are distributed in a way expected at higher temperatures and that the distribution of the Si–O–Si angles depends on the thermal history of the film and the method of growth.