Christopher C. Umbach

Surface self-diffusion on Si from the evolution of periodic atomic step arrays

Abstract The development, during annealing, of periodic atomic step arrays associated with etched grating structures on Si(001) has been monitored by optical methods and by STM. The grating amplitudes decay exponentially with time in the temperature range from 800 to 1100°C with characteristic decay constants that scale approximately as the inverse fourth power of the grating period; this indicates the dominance of surface diffusion as the mass transport mechanism. The activation energy for Si surface self diffusion is found to be ~2.3 eV. Various detailed atomic mechanisms of diffusion are possible; comparing with other relevant data and calculations, our experimental value is consistent with an adatom transfer process. The details of the atomic step morphologies as determined by STM for these grating structures are described. The interactions of the atomic steps during decay are related to the curvature dependent driving forces for mass transfer.

Fabrication of arrays of large step‐free regions on Si(001)

In this letter we describe a method for producing large areas of Si(001) surfaces which are (i) free of atomic steps and (ii) arranged in regular patterns on the wafer. The first step is the fabrication of a two‐dimensional grating structure using e‐beam lithography and reactive ion etching. This grating is then annealed within the appropriate temperature window in ultrahigh vacuum to produce the desired array of (001) step‐free regions. We illustrate the success of the method through the use of low‐energy electron microscopy for a few repeat spacings on test structures each extending over a 3×3 mm2 area. Alternative processing steps are discussed as well as application to submicron device technology.

Atomic step distributions on annealed periodic Si(001) gratings

One-dimensional (1D) and two-dimensional (2D) periodic square-wave gratings fabricated on Si(001) with repeat spacings of 4 and 5 μm and amplitudes of 0.1–0.25 μm were annealed in ultrahigh vacuum at temperatures between 900 and 1200 °C. The surface profiles of the gratings were determined by atomic force microscopy in air after cooling to room temperature. The microscopic structures of the steps and terraces of the gratings were investigated in situ using low-energy electron microscopy. The morphology of the gratings changes with annealing temperature. At 950 °C, both 1D and 2D gratings have surface profiles resembling truncated sinusoids, with large flat regions at the grating extrema. Between 1050 and 1100 °C, the flats at the minima of the 2D gratings expand whereas the flats at the maxima shrink. This asymmetry does not occur for the 1D gratings in this temperature range. At 1200 °C, the profiles of both the 1D and 2D gratings become sinusoidal, with no flats at the extrema. The different morphologies and their temperature dependence can be explained on the basis of the thermodynamics of the surface and the role of surface diffusion and evaporation.One-dimensional (1D) and two-dimensional (2D) periodic square-wave gratings fabricated on Si(001) with repeat spacings of 4 and 5 μm and amplitudes of 0.1–0.25 μm were annealed in ultrahigh vacuum at temperatures between 900 and 1200 °C. The surface profiles of the gratings were determined by atomic force microscopy in air after cooling to room temperature. The microscopic structures of the steps and terraces of the gratings were investigated in situ using low-energy electron microscopy. The morphology of the gratings changes with annealing temperature. At 950 °C, both 1D and 2D gratings have surface profiles resembling truncated sinusoids, with large flat regions at the grating extrema. Between 1050 and 1100 °C, the flats at the minima of the 2D gratings expand whereas the flats at the maxima shrink. This asymmetry does not occur for the 1D gratings in this temperature range. At 1200 °C, the profiles of both the 1D and 2D gratings become sinusoidal, with no flats at the extrema. The different morphologie...

Scanning tunneling microscopy of one‐dimensional periodic corrugated silicon surfaces

A scanning tunneling microscope has been used to study atomic step arrays associated with periodically corrugated grating structures on Si(001). These gratings have repeat spacings of 1.5 and 2.0 μm, and amplitudes between 65 and 750 A. The change in morphology of the gratings with annealing is described. The widths of the (001) terraces at the tops and bottoms of the gratings are observed to be as much as four to five times larger than those for perfect sinusoids. This is consistent with a surface tension singularity at the (001) orientation. The relative areas of the (2×1) and (1×2) dimer domains at the extrema indicate that the gratings may produce additional stresses in the surface.

Chemical treatment of glass substrates

Abstract The effects of cleaning on the near-surface composition of Corning Code 1737 glass was studied. Cleaned glass substrates were found to develop a thin silica-rich surface layer approximately 6 nm thick formed by chemical leaching. The presence of this layer was verified by X-ray photoelectron spectroscopy, scanning transmission electron microscopy and X-ray scattering. Atomic force microscopy measurements of the glass before and after cleaning showed no increase in the surface roughness. Top-gated polycrystalline silicon thin film transistors fabricated on cleaned substrates had leakage currents, similar to those on substrates with deposited SiO2 layers.

Scanning tunneling microscopy studies of phase separation on Si(001) surfaces with periodic step density

Using photolithography and ultrahigh vacuum annealing, we have produced atomic step arrays on Si(001), where the number of steps per unit length normal to the [110] direction changes periodically. These arrays afford the opportunity to study step phenomena that depend on average step density. A scanning tunneling microscope was used to image the detailed structure of the arrays. A transition in step height—from primarily monatomic to primarily biatomic steps—as a function of surface orientation was observed. Sharp changes in local slope associated with this transition were also observed. The connection with the equilibrium crystal shape is discussed.

Characterization of large‐area arrays of nanoscale Si tips fabricated using thermal oxidation and wet etching of Si pillars

Two‐dimensional periodic arrays containing 108 Si pillars with heights of 600–700 nm, widths of 100 nm, and repeat spacings of 300 nm have been fabricated using electron beam lithography on Si(001) substrates. These pillars have subsequently undergone wet oxidation at 800 °C and etching in hydrofluoric acid to produce an array of sharp tips with a height of ∼4000 A. The x‐ray diffraction from this array appears to be dominated by scattering from the bases of the tips. Correlated variations in tip shape, observed with scanning electron microscopy, produce a modulated diffuse background in the diffracted x‐ray intensity. These observations demonstrate the feasibility of using high‐resolution x‐ray diffraction for studying defects in large‐area arrays of periodic structures.

Fabrication of bi‐periodic sinusoidal structures on silicon

We illustrate a method for fabricating bi‐periodic quasi‐sinusoidal structures that are difficult to produce by conventional microfabrication techniques. By annealing bi‐periodic square wave structures in ultra‐high vacuum (UHV), the surface profile becomes quasi‐sinusoidal due to mass transport processes. The method is illustrated by the creation of 4 and 6 μm period structures on Si(001) through annealing at 1100 °C in UHV. Surface profiles are measured by atomic force microscopy (AFM). The structures produced may have applications in electronic or optical devices.

Pairing of curved atomic steps at the extrema of periodic gratings on Si(001)

Scanning tunneling microscopy has been used to study the shape of curved steps at the extrema of periodic step arrays produced from gratings on Si(001). These steps are created by the overlap of the step density associated with the gratings with a background step density in the perpendicular direction; the contour of such a step crosses from one side of an extremum to the other. A pairing of the crossing steps is explained as arising from the energetics of the steps and the reconstructed surface. Steps on gratings along both [110] and [010] show a lengthening of the low energy A‐type portions of crossing step contours and a shortening of the high energy B‐type portions; this results in the steps approaching to within 200 to 400 A of one another. This pairing can be attributed to a balancing of the decrease in step energy as low‐energy portions are formed with an increase in repulsive energy as the steps approach each other. The observed step shapes are consistent with model calculations based on the curre...

Annealing instabilities in small fabricated structures

Abstract When small fabricated structures are annealed under conditions where significant mass transport can occur, details of the shape can be altered as the material tries to reduce its total surface free-energy. Fluctuations in local surface curvature can lead to instabilities under the right geometrical conditions. In this Letter we describe the development of inhomogeneities in thin columns and ridges which have been fabricated on Si(001) wafer surfaces. We believe these instabilities to be closely related to the “Rayleigh” instability that causes such diverse phenomena as the break-up of fine columns of fluid into drops or the break-up of cylindrical second-phase material in a solid matrix.