Koichi Sudoh

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.

Scaling of Si/SiO2 interface roughness

The spatial scaling law of the Si/SiO2 interface roughness was investigated with atomic force microscopy. Scaling behavior was observed on smaller scales, where the root‐mean‐square (RMS) roughness increased as a power of the scale of observation. When viewed as a fractal geometry, such a structure is characterized as a self‐affine fractal. On larger scales, the roughness was no more dependent on the scale, showing the (macroscopic) RMS roughness in the conventional sense. The observed structure (self‐affine fractal with a finite‐length cut‐off) is consistent with the prediction of the theory of kinetic roughening in a far‐from‐equilibrium growth, where the fluctuation on smaller scales evolves into roughness on larger scales. Statistical description of the Si/SiO2 interface roughness was also given in terms of autocorrelation function and power spectral density. It was found that the autocorrelation function of the Si/SiO2 interface roughness is well approximated by an exponential form rather than a Gaus...

Focused Ion Beam Induced Surface Damage Effect on the Mechanical Properties of Silicon Nanowires

In this paper, the effect of surface damage induced by focused ion beam (FIB) fabrication on the mechanical properties of silicon (Si) nanowires (NWs) was investigated. Uniaxial tensile testing of the NWs was performed using a reusable on-chip tensile test device with 1000 pairs of comb structures working as an electrostatic force actuator, a capacitive displacement sensor, and a force sensor. Si NWs were made from silicon-on-nothing (SON) membranes that were produced by deep reactive ion etching hole fabrication and ultrahigh vacuum annealing. Micro probe manipulation and film deposition functions in a FIB system were used to bond SON membranes to the devices sample stage and then to directly fabricate Si NWs on the device. All the NWs showed brittle fracture in ambient air. The Youngs modulus of 57 nm-wide NW was 107.4 GPa, which was increased to 144.2 GPa with increasing the width to 221 nm. The fracture strength ranged from 3.9 GPa to 7.3 GPa. By assuming the thickness of FIB-induced damage layer, the Youngs modulus of the layer was estimated to be 96.2 GPa, which was in good agreement with the literature value for amorphous Si.

Electron Tunneling Through an Al2O3 Thin Film on NiAl(110) in Scanning Tunneling Microscopy

We calculate electron tunneling for an Al2O3/NiAl(110) structure in scanning tunneling microscopy (STM) by employing a simple and exactly solvable one-dimensional model. The calculation reproduces the essential features of the apparent height of an Al2O3 thin film vs. sample bias curve for an Al2O3(0.5 nm)/NiAl(110) structure studied by Hansen et al. [Surf. Sci. 475 (2001) 96] for the appropriate electron affinity of the Al2O3 film, about 1.35 eV. Origins of fine structures found in the calculated transmission probability vs. sample bias curves are discussed.

Application of Silicon on Nothing Structure for Developing a Novel Capacitive Absolute Pressure Sensor

In the field of silicon on nothing (SON) structure , micrometer-thick monocrystalline layers suspended over their parent wafer were produced by high-temperature annealing of specific arrays of trenches. Those trenches reorganize into one single void and leave a thin overlayer on top. Since this method may be an easy way of synchronous fabricating high-quality silicon films and vacuum void, this paper investigates its potential applications for a pressure sensor. A capacitive absolute pressure sensor whose pressure sensitive membrane is formed by the SON structure was fabricated and evaluated. The radius and thickness of the sensitive membrane are 100 and 1.7-μm, respectively. The average sensitivity of the sensor array with 15 diaphragms is 2.88 fF/kPa. This novel fabrication process enables to easily form a high vacuum cavity without hermetical sealing process such as anodic bonding technology, to achieve an excellent long-term stability and reliability, in particular, and to easily integrate detection circuits with the sensor.

Roughening of the Si/SiO2 interface during SC1-chemical treatment studied by scanning tunneling microscopy

The Si/SiO2 interface morphology is observed with subnanometer resolution by an ultrahigh vacuum scanning tunneling microscope (STM). We analyze the roughness of the Si/SiO2 interface for a chemical oxide film formed by a wet chemical process (NH4OH/H2O2/H2O treatment). The oxide film is selectively removed by irradiating a field emission electron beam extracted from a STM tip at a temperature of 300–350 °C. We find that during the chemical process the roughness of the Si/SiO2 interface increases with the treatment time.

Fluctuations of a Single Step and Surface Height on Vicinal Surfaces

Fluctuation of a single step on the vicinal surface in thermal equilibrium is investigated by Monte-Carlo simulation for the ASOS (absolute solid-on-solid) model. The relation W 2 ( y ) = G ( y )/ρ 2 (at large y ) between the squared step width W 2 ( y ) and the height-height correlation function G ( y ), which had been predicted by the capillary wave theory for a vicinal surface with the step density of ρ [T. Yamamoto et al. : J. Phys. Soc. Jpn. 63 (1994) 915], is confirmed by the simulation.