Volume 17, Issue 4, July 1999
- regular articles
- brief reports and comments
- rapid communications
- shop notes
- papers from the 26th conference on the physics and chemistry of semiconductor interfaces
- nitride epitaxy, photonic probes, and band offsets
- magnetic interfaces and photon scattering
- piezoelectric effects, minigaps, defect centers, and devices
- superlattices, iii-v growth, and characterization
- si/so(2) and si-si(3)n(4) interfaces: defect properties and novel structures
- novel growth, nano-machining, and characterization
Index of content:
- REGULAR ARTICLES
17(1999); http://dx.doi.org/10.1116/1.590751View Description Hide Description
We have investigated a force sensing technique for high-resolution scanning force microscopy which uses a piezoelectricquartz resonatoroscillating perpendicular to the direction of the sample surface at a frequency of 1 MHz. The achievement of true atomic resolution on single-wall carbon nanotube surfaces illustrates the promises of this new technique which is especially well suited for ultrahigh vacuum or low-temperature conditions. The force sensitivity and the mechanical amplitude of the sensoroscillation were characterized semiquantitatively.
Surface structure characterization of DNA oligomer on Cu(111) surface using low temperature scanning tunneling microscopy17(1999); http://dx.doi.org/10.1116/1.590752View Description Hide Description
The surface structures of DNA oligomers, pAAAAAAATTTTTTT, deposited on Cu(111) surface have been characterized at liquid nitrogen temperature using a scanning tunneling microscope. Four different types of adsorbed structures have been observed in DNA oligomers; (i) an isolated whole molecule, (ii) a shortened molecule, (iii) a cluster, and (iv) a double helix. The internal structures of the oligomers also have been resolved.
Microstructure and electrical properties of Sb nanocrystals formed in thin, thermally grown layers by low-energy ion implantation17(1999); http://dx.doi.org/10.1116/1.590753View Description Hide Description
We have formed Sb nanocrystals in thin, thermally grown layers using low-energy ion implantation which was followed by thermal annealing. These Sb nanocrystals showed good uniformity with respect to size and position. Both the narrow as-implanted profile and the compressive strain that exists near the interface are considered to contribute to the uniformity. We found that it was possible to control the size of the Sb nanocrystal by changing the dose of the implanted Sb. The characteristics of the diodes which had Sb nanocrystals with an average diameter of 3.3 nm showed good reproducibility. Moreover, these diodes showed a Coulomb blockade region around 0 V and a Coulomb staircase at 4.2 K. Identical characteristics were obtained for all the samples measured, indicating an excellent reproducibility. Almost the same Coulomb blockade region as that at 4.2 K was observed up to a temperature as high as 100 K even for a diode which had larger Sb nanocrystals (with an average diameter of 5.6 nm). The technique introduced here offers the possibility of developing practical Si-based single-electron devices.
17(1999); http://dx.doi.org/10.1116/1.590754View Description Hide Description
A metalnanostructure was fabricated on a Si(111) surface in ultrahigh vacuum. The stages in the fabrication were H passivation of the surface,Pb evaporation, depassivation of selected areas with the tip of a scanning tunneling microscope, and heating. Using this procedure, a nanowire and two contact pads to connect it to external probes were fabricated. At each stage of the process, the surface was imaged with scanning tunneling microscopy.
Mechanical properties, stress evolution and high-temperature thermal stability of nanolayered Mo–Si–N/SiC thin films17(1999); http://dx.doi.org/10.1116/1.590755View Description Hide Description
A study of the microstructure, thermal stability,nanoindentationmechanical properties, and residual stress evolution of nanolayered Mo–Si–N/SiC thin films as a function of vacuum annealing time and temperature is reported. Multilayers of Mo–Si–N and SiC were deposited by magnetron sputtering from planar and SiC targets onto single crystal silicon wafers. The relative amount of both components was varied (12.5–50 vol. % of SiC) while keeping the bilayer thickness constant (12 nm), or the bilayer thickness was varied (6–24 nm) with constant Mo–Si–N to SiC ratio (25 vol. % of SiC).Mechanical properties were measured by nanoindentation on as-deposited films and filmsannealed in vacuum at 500 and 900 °C. Microstructure and thermal stability were examined by cross-sectional transmission electron microscopy, glancing angle x-ray diffraction and nuclear resonance broadening. Stress evolution induced by thermal annealing was determined by measuring optically the change in curvature of coated silicon beams. In the as-deposited state, all films exhibited an amorphous microstructure. At 900 °C SiC still remained amorphous, but Mo–Si–N had developed a microstructure where nanocrystals of were embedded in an amorphous matrix. The interface between Mo–Si–N and SiC was indirectly shown to be stable at least up to 41 h annealing at 1075 °C in vacuum. The potential of Mo–Si–N as a barrier layer against intermixing between nanolayered and SiC at 900 °C has been demonstrated. Hardness, modulus and residual stress followed the volume fraction rule of mixture of both constituents of the nanolayered Mo–Si–N/SiC structure. Consequently, by optimizing the volume fraction of the constituents, zero residual stress on a silicon substrate is possible after annealing.
17(1999); http://dx.doi.org/10.1116/1.590756View Description Hide Description
Optical interferometry has been applied to determine the displacement of Si beams. Clamped-clamped Si beams and cantilevered beams were fabricated with short and long B diffusion processes and characterized. Measurements of beam bending for released Si structures with length varying from 50 to 1000 m, width varying from 5 to 15 m, and thickness varying from 6 to 37 m were obtained. By taking advantage of an etch-diffusion process, thicker beams can be fabricated which have less bending due to stress gradients. A 6.0-m-thick cantilevered beam had a deflection of 11.2 m due to stress gradients, while a 36.7-m-thick beam had a deflection of only 0.3 m. Beams fabricated using a dissolved wafer process with a 12 h B diffusion were found to bend the same amount as those fabricated with a 4 h diffusion. This indicates that bending in doped Si beams not only depends on the gradients in the B concentrations, it could also be related to the distribution of dislocations. Using the deep-etch shallow-diffusion process, resonating elements that are 20 m long, 4 m wide, and 28 m thick were found to be perfectly flat without any bending.
In situ ellipsometric study of the formation process of metalorganic vapor-phase epitaxy-grown quantum dots17(1999); http://dx.doi.org/10.1116/1.590757View Description Hide Description
Our in situellipsometry study of metalorganic vapor-phase epitaxy-grown quantum dot(QD) structures showed that the Δ–Ψ trajectory of the ellipsometric signal in Stranski–Krastanow QDgrowth sharply differs from that in layer-by-layer growth. When QD formation starts, Δ rapidly decreases, inflecting the Δ–Ψ trajectory. This indicates increased scattering loss, deduced from increased surface roughness produced in the transition from two- to three-dimensional surface morphology.Ex situatomic force microscopy and photoluminescence results correspond well to the ellipsometric signal at the start of QD formation. Based on these results, we discuss growth-dependentQD formation processes such as QD formation onset and growth mode transition.
High temperature reaction of nitric oxide with Si surfaces: Formation of Si nanopillars through nitride masking and oxygen etching17(1999); http://dx.doi.org/10.1116/1.590758View Description Hide Description
In this article, we report the fabrication of Si nanopillars by surface reaction with nitric oxide (NO) at high temperature. NO was leaked onto clean Si surfaces, at a temperature of ∼850 °C, in ultrahigh vacuum chamber and examined in situ by x-ray and ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, low energy electron diffraction, and ex situ by atomic force microscopy. NO molecules dissociate on the surface and nitrogen atoms thus produced form nitride islands. These islands act as protective masks for the etching of Si by the oxygen atoms, through the desorption of SiO species. Occurrences of these two simultaneous processes results in the formation of nanometer sized Si pillars (typical basewidth 100–150 nm and height 5–15 nm) capped by silicon nitride. These pillar structures are separated by clean Si areas as shown by the clear presence of two domains. We also show that the height of these Si pillars can be increased by ex situ chemical etching.
Low energy electron beam decomposition of metalorganic precursors with a scanning tunneling microscope at ambient atmosphere17(1999); http://dx.doi.org/10.1116/1.590759View Description Hide Description
Electron beam induced decomposition of metalorganic precursor substances with a scanning tunneling microscope is a convenient way to create nanometer-sized structures. Up to now, the application was limited due to the necessity of vacuum conditions. In this contribution, we report experiments to form metallic nanostructures by the decomposition of dimethylgold(III)-trifluoro-acetylacetonate and cyclopentadienylplatin(IV)-trimethyl on graphite and indium–tin–oxide coatedglass substrates at ambient atmosphere. Small hillocks with diameters down to 30 nm and heights of 10 nm and wire-shaped structures were produced. The amount of the deposited material depends on the energy and the dose of the electrons. A threshold voltage was found for both materials.
Two-dimensional dopant profiling of patterned Si wafers using phase imaging tapping mode atomic force microscopy with applied biases17(1999); http://dx.doi.org/10.1116/1.590760View Description Hide Description
Tapping mode atomic force microscopy with applied bias was used to spatially resolve areas of different doping type on Si wafers patterned with photolithography and subsequent ion implantation. The application of a direct current bias between cantilever and sample during the measurement produces Coulomb (electrostatic) forces, whose magnitude depends on the spatial variation of the doping density. This effect was utilized to detect areas of different doping type by monitoring the phase angle between the driving frequency and the cantilever response while scanning areas of different doping density. In this article we present a series of measurements at various bias voltages demonstrating that the observed phase contrast between differently doped areas is directly connected to the bias induced surface potential (band bending) present on these areas. To investigate the contrast mechanism quantitatively, we also measured deflection (force), amplitude and phase versus distance curves for a typical cantilever with an applied bias on a goldthin film. This allowed correlation between phase contrast observed and the actual Coulomb force measured.
Fabrication of a microcavity structure with a polyimide thin film prepared by vacuum deposition polymerization17(1999); http://dx.doi.org/10.1116/1.590761View Description Hide Description
A microcavitystructure has been fabricated in which a polyimide layer with thickness on the order of the wavelength of light was sandwiched in between dielectricmirrors by using vacuum depositionpolymerization (VDP). We deposited a 330-nm-thick polyimide layer through a polymerization reaction between two mutually reactive monomers evaporated onto a dielectricmirror consisting of multistacks. In the middle of the VDP process we deposited tris(8-hydroxyquinoline) aluminum (ALQ) so that a sublayer 30%-doped with ALQ was inserted within the polyimide layer. Upon the polyimide layer we coated a dielectricmirror consisting of multistacks. The mirrors were designed to have a broad high reflection band (stop band) with a reflectivity maximum of ∼90% at 530 nm. Fabrication of a microcavity was considered to be successful based on its optical properties. Within the stop band a sharp transmission band with a width of 8 nm appeared due to Fabry–Perot resonance. Correspondingly, ALQ photoluminescence became narrowed by the same width. We characterized the cavity resonance modes using a model considering penetration of the optical field into the dielectric layers.
17(1999); http://dx.doi.org/10.1116/1.590762View Description Hide Description
We have studied low voltage (1–2 kV) electron beam lithography processes in PMMA and compared them to conventional high voltage processing. We looked at the deposited metal after liftoff as well as directly imaging resist profiles by atomic force microscopy. As expected, the proximity effects were greatly reduced. The forward scattering was found to increase at low voltage. The study of developed resist profiles showed that linewidth versus dose has a single Gaussian functional form, proving that forward scattering plays the major role in line broadening. The effective Gaussian linewidth is 60 nm at 1 kV in a 50 nm resist layer. Modeling of the lithographic process showed a significant increase in resolution and process latitude for thinner resists.
17(1999); http://dx.doi.org/10.1116/1.590763View Description Hide Description
The minimization of nanoscale roughness in patterned images has become a priority for the process of photolithography in the production of microprocessors. In order to probe the molecular basis for surface roughness, the development of photoresist has been simulated through application of the critical-ionization model to a three-dimensional molecular lattice representation of the polymer matrix. The model was adapted to describe chemically amplified photoresists of the sort now commonly used in microlithography. Simulations of the dependence of the dissolution rate and surface roughness on the degree of polymerization, polydispersity, and fractional deprotection agree with experimental results. Changes in surface roughness are shown to correlate with the length of the experimentally observed induction period. Model predictions for the effect of void fraction and developer concentration on roughness are also presented. Observations of differences in the effect of developer concentration on top-surface and sidewall roughness are explained by a critical development time predicted by the simulation.
Relations between the solubility speed and the electrical conductivity of phenol novolak polymer solutions17(1999); http://dx.doi.org/10.1116/1.590764View Description Hide Description
Relations between the solubility speed of phenol novolak resins which are typical photoresist base polymers, with the ac-electrical conductivity of their polymer solutions were studied. Strong correlations between these quantities were found. These relations were found to be affected by the concentration of the solution and the measured temperature. It is suggested to use such relations to predict the solubility speed of photosensitive novolak polymers by measuring their ac-electrical conductivity instead of measuring the solubility speed directly.
17(1999); http://dx.doi.org/10.1116/1.590841View Description Hide Description
I describe the development of low-stress W/Cr bilayer films, for use as SCALPEL® mask scattering layers. These films are produced by dc magnetron sputtering in argon, and consist of 25–50-nm-thick W layers deposited onto 5–10-nm-thick Cr layers. X-ray reflectance analysis is used to measure the thicknesses of the individual W and Cr layers with subangstrom precision; surface and interface roughnesses,film densities, and also the thickness of the tungsten–oxide overlayer which forms after exposure to air are determined by this technique as well. Film stress, which is measured using the wafer curvature technique, is controlled by adjusting the deposition conditions such that the Cr layers are in tension while the W layers are in compression (and thus have high density and low surface roughness), so that the net stress in the bilayer is balanced near zero. I present data that illustrates how the net stress in these films varies with argon pressure, background pressure (i.e., partial pressure of residual gases present in the vacuum chamber), and Cr layer thickness. I also show how the stress depends on the composition of the substrate: i.e., stresses measured in filmsdeposited onto Si wafers are systematically higher (by several hundred MPa) than the stresses measured for the same filmsdeposited onto silicon–nitride-coated Si wafers. I discuss the implication of these results with regard to the production of high-quality SCALPEL® mask blanks for sub 0.12 μm lithography.
17(1999); http://dx.doi.org/10.1116/1.590765View Description Hide Description
A structure integrating a sharp field emission tip inside a coaxial structure with an overall diameter as small as 60 μm is described. It can emit nA electron current with a minimum kinetic energy of 50 eV. It is demonstrated that this structure behaves like an electron gun and is able to produce a low-energy, divergent and highly coherent electron beam at distances as small as 100 μm from the tip.
17(1999); http://dx.doi.org/10.1116/1.590766View Description Hide Description
An electron gun for an accelerating voltage of 100 kV to be operated under space charge limited condition is designed, and its emittance and brightness are calculated. To obtain low brightness, a Pierce-type electron gun with a Wehnelt electrode and a control anode is adopted. Choosing a low value of the ratio between anode and cathode curvatures reduces the electric field strength at the cathode. A maximum emittance of 1.8 mm mrad and a minimum brightness of 350 are obtained under the above conditions with a cathode diameter of 1.66 mm. The radius of the Wehnelt aperture and its axial position are optimized. The effects of deviations of the shapes of the Wehnelt and control electrodes from concentric sphere are studied. It is found that in the space between the anode and the control anode the electrodes must be spherical up to an angle of at least whereas in the space between the Wehnelt electrode and the control anode the electrodes must be spherical up to an angle of at least
17(1999); http://dx.doi.org/10.1116/1.590767View Description Hide Description
Paraxial electron/ion trajectories in rotationally symmetric fields are analyzed in the Picht space to derive analytical forms of thin lens characteristics such as focal length F, magnification M, and chromatic and spherical aberration coefficients and Dimensionless forms of and which are determined only by the normalized lens-potential distribution, are derived to evaluate the lens. Here, T is the lens thickness and is the distance from the object to the lens. As to bipotential or unipotential lenses composed of electrodes facing each other symmetrically, it is found that the accelerating mode at and the decelerating mode at have relationships of and where and are the 1st and 2nd electrode potentials, respectively. Under constant lens fields, the known M-dependent and expressions, i.e., and are analytically derived also in the present approach.
X-ray photoelectron spectroscopy analyses of oxide-masked organic polymers etched in high density plasmas using gas mixtures17(1999); http://dx.doi.org/10.1116/1.590768View Description Hide Description
Top surface imaging processes are considered as attractive schemes for future lithographic processes. A major issue associated with these schemes is the dry development step which implies the obtention of anisotropic resist profiles. This anisotropy is achieve by adding to gas mixtures. In this work, oxide-masked organic resist features are etched in high density plasma sources using gas mixtures. The chemical constituents present on the tops, sidewalls, and bottoms of the etched features are determined by quasi in situx-ray photoelectron spectroscopy(XPS).XPSanalyses show that when using gas mixtures, a thin passivation layer is formed on the sidewalls of the polymer features. This layer is mainly formed by sulfur atoms present on the polymer surface in elemental state or bonded to carbon atoms. This film is thick enough to minimize the spontaneous etching reaction of oxygen atoms with the organic resist. Samples are also analyzed after air exposure in order to determine the air-induced modification of the reactive layers formed during the dry development step. XPSanalyses show that the sulfur-based passivation layer formed during the etch process interact with water moisture to form sulfur-based acids.
Fabrication and characterization of chromium based single-electron transistors with evaporated chromium oxide barrier tunnel junctions17(1999); http://dx.doi.org/10.1116/1.590769View Description Hide Description
We fabricated chromium based single-electron transistors comprising small-area tunnel junctions with an evaporated chromium oxide barrier. The transistors are fabricated using e-beam lithography with a bilayer resist and two-angle shadow evaporation. We describe the fabrication process and discuss the device characteristics.