Hitoshi Kuribayashi

Shape transformation of silicon trenches during hydrogen annealing

Shape transformation of silicon trenches during annealing at high temperatures in a hydrogen ambient was investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). By SEM observation of the trench profiles, we found that the rate of shape transformation increases with decreasing hydrogen pressure. Performing the simulation based on a continuum surface model, we show that the shape transformation during annealing in a hydrogen ambient is due to surface self-diffusion. By quantitative comparison of the results between the experiment and simulation, we estimated the diffusion coefficients. The obtained activation energy for surface diffusion under a hydrogen pressure of 40 Torr was much higher than that measured under ultrahigh-vacuum conditions. Furthermore, it was found by AFM observation of the trench sidewall surfaces that, during the thermal treatment, the large roughness of the as-etched trench sidewall surface decreased significantly due to surface self-diffusion of silicon atoms, resulting structures with atomically flat terraces and steps.

Void shape evolution and formation of silicon-on-nothing structures during hydrogen annealing of hole arrays on Si(001)

We have studied the structural evolution of voids in the Si substrate and the formation of silicon-on-nothing (SON) structures upon spontaneous reshaping of square arrays of cylindrical holes on Si(001) substrates by hydrogen annealing. Vertically elongated voids covered with {111}, {100}, {110}, and {113} facets are initially formed by the closure of the hole inlets. This step is followed by volume preserving shape changes of the faceted voids in the bulk Si. In situations where the hole-hole separation is sufficiently small, void coalescence occurs due to the shape changes of individual voids, leading to the formation of a SON structure. Until void coalescence, the shapes of individual voids change without being affected by the adjacent voids. Numerical simulations of the shape change of a completely faceted void via solely surface diffusion have been performed and have reproduced the observed shape change.

Investigation of Shape Transformation of Silicon Trenches during Hydrogen Annealing

We have investigated the corner rounding of a micron-sized silicon trench by annealing under hydrogen pressure of 40–760 Torr in the temperature range of 1000 to 1100°C, and have obtained plots showing the relationship between the curvature of the trench corner and annealing time for various annealing conditions. It was found that the evolution of the curvature of the trench corner follows a time scaling law, expressed by t-1/4. This finding strongly suggests that the shape transformation is attributable to the self-diffusion of the silicon surface under the experimental conditions studied. The surface self-diffusion coefficient in the case of hydrogen pressure of 40 Torr and a temperature of 1000°C was estimated to be approximately 2 ×106 nm2/s.

Hydrogen pressure dependence of trench corner rounding during hydrogen annealing

We have investigated the rounding of micron-sized trenches fabricated on Si(001) substrates during annealing in hydrogen ambient in a temperature range of 1000 to 1100 °C, especially the effect of hydrogen pressure on the rate of rounding. Observing the profiles of the trenches annealed under hydrogen pressures from 10 up to 760 Torr by scanning electron microscopy, we have found that the rate of corner rounding decreases with increasing hydrogen pressure. It was also found that these rates of corner rounding are smaller than that during annealing in atmospheric argon ambient. This result suggests that adsorbed hydrogen suppresses the surface self-diffusion, by which the corner rounding occurs. We present the contour map of corner curvature in the process parameter space of hydrogen-pressure versus annealing temperature for an annealing time of 3 min.

Variation of Chemical Vapor Deposited SiO2 Density Due to Generation and Shrinkage of Open Space During Thermal Annealing

Chemical vapor deposited (CVD) SiO2 using tetraethoxysilane has a low density compared with thermally grown SiO2, and the as-deposited film contains impurities. In addition, compressive stress exists in the Si–O–Si network. In this work, CVD SiO2 films annealed with various thermal budgets were evaluated using ellipsometry. Positron annihilation spectroscopy indicated that the desorption of residual impurities by thermal annealing generates open spaces in the films. Subsequent annealing shrinks the open spaces and relaxes the compressive stress with reconstruction of the Si–O–Si network. Consequently, the refractive index of CVD SiO2 shows turn-around characteristics with increasing annealing temperature.

Shape evolution of high aspect ratio holes on Si(001) during hydrogen annealing

We study the dynamics of void formation through the shape evolution of high-aspect-ratio cylindrical holes in Si(001) substrates under hydrogen annealing. We compare the observed evolution of these holes with numerical simulations based on the continuum theory for surface-diffusion-driven shape evolution. We find that a strong morphological instability arises near the hole opening, regardless of the presence of anisotropy in surface energy. The observed shape evolution of high-aspect-ratio holes during hydrogen annealing is understood as a surface-diffusion-driven evolution subject to the stability of the facets which form the vertical sidewall.

Improvement of Dielectric Properties on Deposited SiO2 Caused by Stress Relaxation with Thermal Annealing

The relationship between the electrical and structural characteristics of tetraethylorthosilicate (TEOS) SiO2 has been investigated to reveal the mechanism of the improvement of the dielectric properties with thermal annealing. Stress relaxation in TEOS–SiO2 caused by thermal annealing was observed as a blue shift of the infrared absorption spectral peak in a Fourier transform infrared attenuated total reflection (FTIR-ATR) spectrum. It was concluded that the stress relaxation increased the band gap of TEOS–SiO2, resulting in suppression of the leakage current. Additionally, thermal desorption spectroscopy (TDS) performed to investigate the phenomena of film densification by thermal annealing.

Electronic Energy Level of Off-Center Substitutional Nitrogen in Silicon: Determination by Electron Spin Resonance Measurements

It is demonstrated that a deep level of a paramagnetic off-center substitutional N can be determined by electron spin resonance (ESR) measurements of the peak-to-peak derivative linewidth ΔHmsl over a wide temperature region from 150 to 550 K. The temperature dependence of ΔHmsl observed above ~330 K is interpreted in terms of one type of motional broadening. According to this model, we obtain an energy level of 0.33±0.02 eV below the conduction band and the electron capture cross section σ0 of 1.0×10-15 cm2, which are in agreement with one of the levels obtained by deep-level transient spectroscopy (DLTS).

Suppression of Leakage Current of TEOS-SiO2 with Bandgap Increasing by High Temperature Annealing

Tetraethylorthosilicate (Si(OC2H5)4) (TEOS) SiO2 has been used for the internal isolation dielectric layers in integrated circuits, a gate dielectric in poly crystalline-silicon thin-film transistors (poly-Si TFTs). Recently, TEOS-SiO2 has been also an attractive material as an electrical insulator for high-power metal-oxidesemiconductor (MOS) device, three-dimensional LSI and so on, because TEOS-SiO2 has superior coverage, selectivity of substrate, arbitrary properties of film thickness and deposition temperature. However, the dielectric properties of TEOS-SiO2 are inferior compared with thermally grown SiO2. In this work, we reduce the leakage current of TEOS-SiO2 by high temperature annealing, and explain the decreases of leakage current with stress relaxation in the films. The TEOS-SiO2 films of 100 nm thickness were deposited by thermal chemical vapor deposition (CVD) at 680°C on single-crystalline P-type (100) silicon substrates. Then, all the samples were annealed in H2O ambient at 800°C for 5 minutes. For improvement of dielectric properties, the films were additionally annealed at 6001200°C for 60 minutes in Ar ambient. The thermal SiO2 films of 100 nm thickness were also grown in O2 ambient at 1100°C for comparison. The thickness and the refractive index of the films were measured with an ellipsometer. Finally, Al electrodes were deposited by vacuum evaporation in order to form the MOS capacitor structures. Figure 1 shows the thickness and the refractive index of the TEOS-SiO2 films annealed at various temperatures. It is clear that the thickness decreases as annealing temperature increases. Additionally, at above 900°C, the refractive index increases as the annealing temperature increases. Then, it approaches to the value for the thermally grown SiO2 (n = 1.462). The results infer that as-grown TEOS-SiO2 is less dense than thermal oxide, and that additional annealing give rise to densification. Fig. 2 shows the current densities vs. electric field (J-E) characteristics of the TEOS-SiO2 films annealed at various temperatures. It was found that the leakage current decreased as the annealing temperature increased. It is well known that the leakage current of thermally grown SiO2 is characterized as Fowler-Nordheim (F-N) tunneling described by following equation,

Nano-scale morphology and hydrogenation of Si surfaces in the early phase of hydrogen annealing

In this study we experimentally observed the atomic-level morphology of Si surface in the early phase of hydrogen annealing, in order to investigate the progress of the flattening of trench sidewall surface. We performed both AFM on trench sidewall (011) surface and UHV-STM on Si (001) substrate surface. The AFM image of the trench sidewall surface within 0.5 min of H2 annealing clearly shows the evanescence of chemical Si dioxide formed by RCA cleaning process, and in the void area without chemical oxide the appearance of atomic steps is observed. The observed Si substrate surface morphology by means of UHV-STM shows that the exposed Si surfaces reconstruct to the 2×1 dimer structure, being mono-hydrogen terminated and stabilized. From these experimental results, we can make an early phase model of hydrogen annealing, in which the evanescence of chemical Si dioxide is followed by formation and growth of voids without chemical oxide, where the exposed Si surfaces reconstruct to the 2×1 mono-hydrogen dimer structure, growing to atomic steps and terraces characteristic to the surface orientation.