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Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures

Issue 2 • Date Mar 2000

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Displaying Results 1 - 25 of 103
  • Issue Cover

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    Freely Available from IEEE
  • Issue Table of Contents

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  • Fabrication of a silicon based nanometric oscillator with a tip form mass for scanning force microcopy operating in the GHz range

    Page(s): 607 - 611
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    The detectable force resolution of a mechanical oscillator used in scanning force microscopy can be improved by increasing its natural frequency fo and quality factor Q, and by decreasing the spring constant k and the temperature of operation T. For an oscillator having a structure that can be modeled as a concentrated mass-spring model, decreasing the mass of the oscillator is desirable since high fo can then be obtained without increasing the spring constant k. We have developed a novel fabrication technique for fabricating a nanometric oscillator by selective etching of silicon on insulator (SOI) wafers. The oscillator has the form of a tip supported by an elastic neck, and the tip serves as the mass. The tip and the neck length measure approximately 100 nm when fabricated using a separation by implanted oxygen wafer, and are around 1000 nm when fabricated using a bonded SOI wafer. The tips were made of silicon and the necks were made of silicon dioxide. The oscillator could be tailored to have its natural frequency in the range of 0.01–1 GHz and a spring constant between 10-1 and 102N/m. The thin neck, whose diameter is of the order of 10 nm is not brittle and can survive angular bending of around 30°. © 2000 American Vacuum Society. View full abstract»

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  • Fabrication and characterization of nanoresonating devices for mass detection

    Page(s): 612 - 616
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    We report on a novel fabrication process and preliminary characterization of a nanomechanical resonating device, which is to be used for mass detection. The fabrication of the device is based on laser lithography on Al coated SiO2/p++Si/SiO2/Si structures, followed by dry and wet etching. We have fabricated highly doped polysilicon free-hanging cantilevers and anchored drivers for lateral cantilever vibration, where the motion of the cantilever is parallel to the substrate. The cantilevers are actuated electrically by applying an ac voltage between the cantilever and driver. The laterally vibrating cantilever structures are approximately 30–50 μm in length, 1.8 μm in height, and 500 nm in width. The characterization of the resonators was performed by direct observation of the cantilever through an optical microscope. An electrical measuring technique is also presented and discussed. Typical values of resonant frequency and quality factor, at 1 atm, are approximately 500 kHz and 50, respectively. Moreover, a dependence of the resonant frequency on the applied dc voltage between the cantilever and driver has been found. © 2000 American Vacuum Society. View full abstract»

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  • All-photoplastic, soft cantilever cassette probe for scanning force microscopy

    Page(s): 617 - 620
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    Multiple single-lever probes for scanning force microscopy arranged in a cassette design and made of a low-stiffness photoplastic material have been developed and successfully tested by imaging DNA molecules. The new concept consists of a column of a one-dimensional array of cantilevers with integrated tips, the first of which is used for imaging and the others are spares in case the first one becomes degraded. When this happens, the lever is mechanically removed from the chip so that the next spare becomes ready for scanning. The probes are fabricated of a photoplastic material, which allows simple batch fabrication and facilitates realization of specific mechanical properties for cantilevers—such as low stiffness—that are very difficult to achieve with classical silicon technology. The batch fabrication process, based on spin coating and subsequent near-ultraviolet exposure and development steps of the photoplastic SU-8, results in well-controlled and uniform mechanical properties of the probes within the same chip, as well as for different chips over a 4 in. wafer. Successful images of soft, condensed-matter samples were taken using these cassette probes, making consecutive readdressing of the same DNA molecule with two different cantilevers possible. Thus, a worn-out cantilever can be replaced by a new fresh one with only small positional adjustments and without any changes in the operating conditions. This ease of operation was provided by the cassette concept and the excellent uniformity of the mechanical cantilever characteristics. © 2000 American Vacuum Society. View full abstract»

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  • Modeling the hysteresis of a scanning probe microscope

    Page(s): 621 - 625
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    Most scanning probe microscopes use piezoelectric actuators in open loop configurations. Therefore a major problem related to these instruments is the image distortion due to the hysteresis effect of the piezo. In order to eliminate the distortions, cost effective software control based on a model for hysteresis can be applied to the scanner. We describe a new rate-independent model for the hysteresis of a piezo scanner. Two reference standards were used to determine the accuracy of the model; a one-dimensional grating with a period of 3.0 μm and a two-dimensional grating with 200 nm pitch. The structures were scanned for different scan ranges varying from 5 V peak to peak to 440 V peak to peak, so that 99% of the scanners’ full motion range was covered. A least-squares fit of the experiments to the hysteresis model provided standard deviations per scan range of around 0.2%. This represents an uncertainty of 1 pixel. Since our model is based on a differential equation, it is flexible even to simulate arbitrary experimental conditions such as a sudden change in the offset. © 2000 American Vacuum Society. View full abstract»

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  • High resolution sampling electrostatic force microscopy using pulse width modulation technique

    Page(s): 626 - 631
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    This article presents a sampling electrostatic force microscopy technique for high frequency integrated circuit internal measurement that utilizes a pulse width modulation method. The electrostatic force microscope is a class of scanning probe microscopes that is capable of providing noninvasive measurements of the electrical signals inside operating integrated circuits with very high spatial resolution. With this technique, the internal circuit voltage is determined by sensing the local electrostatic force on a miniature probe cantilever that is closely positioned above the circuit test point. The ability to extract repetitive voltage wave forms using direct force measurement is typically limited by the mechanical response of the probe. To overcome the measurement bandwidth limitation of the electrostatic force microscope previous methods have employed a modulated high bandwidth sampling pulse approach. The temporal resolution of these pulse sampling methods is limited by the pulse width. This article presents a pulse width modulation method. The resolution of the pulse width modulation technique is not dependent on the pulse width, but rather on the modulation depth. This enhances measurement resolution several times more than previous methods. The current instrument has a voltage sensitivity of 10 mVrms/Hz1/2. © 2000 American Vacuum Society. View full abstract»

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  • Point contact current–voltage measurements on individual organic semiconductor grains by conducting probe atomic force microscopy

    Page(s): 632 - 635
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    Conducting probe atomic force microscopy (CP-AFM) was used to make point contact current–voltage (I–V) measurements on individual microscopic grains of the organic semiconductor sexithiophene (6T). The 6T grains ranged from 1 to 6 molecules (2–14 nm) in thickness, 1–2 μm in length and width, and were deposited by thermal evaporation onto SiO2 substrates previously patterned with 200 nm wide Au wires. Au-coated AFM probes were used to image the substrates in air to identify individual 6T grains which grew in contact with a wire. The same probes were used to record the I–V characteristics of single grains. Analysis of the differential resistance as a function of probe wire separation yielded typical grain resistivities of 100 Ω cm and contact resistances of ∼100 MΩ. Over the 0–3 V range probed, the shape of the I–V curves can be attributed to a combination of the nonlinear I–V characteristics of the Au-6T junctions and the ohmic response of the grain. In general, we have shown that CP-AFM is a reliable method for correlating electrical transport properties with microscopic morphology in organic semiconductors. © 2000 American Vacuum Society. View full abstract»

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  • Topography of skeletal muscle ryanodine receptors studied by atomic force microscopy

    Page(s): 636 - 638
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    Tapping mode atomic force microscopy (AFM) was used to investigate the topography of isolated ryanodine receptors of rabbit skeletal muscle (RyR1) both in air and in a buffer. In air, images fourfold symmetric in appearance were obtained. Two different configurations could be distinguished in the AFM topography of single RyR1s, depending on whether there was a center protrusion. In the buffer, even though square images of RyR1s were discerned, the detailed topography of RyR1 appeared different from that in air. Possible reasons for this are discussed. © 2000 American Vacuum Society. View full abstract»

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  • Search of optimum bias voltage for oxide patterning on Si using scanning tunneling microscopy in air

    Page(s): 639 - 643
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    Nanometer-scale oxide patterns were fabricated on H-passivated Si using a scanning tunneling microscopy (STM) in air. We found that the optimum bias voltage to generate clean and uniform oxide patterns depends on the composition of the tip material rather than on the tip head sharpness. For tungsten tips, oxide patterns with the desired geometrical features can be obtained at bias voltages ranging from -0.8 to -1.2 V, while for platinum–iridium tips, the bias voltages lie between -1.5 and -2.5 V at a fixed tunneling current of 2.0 nA. These biases correspond to the working voltage generating the oxide pattern with the lowest apparent depth. Beyond these voltage ranges, tip scratching on the sample surface or field-induced mass transfer from the tip might occur, as evidenced by tip wearing and the contamination of debris of tip material in the vicinity of the patterns. On the other hand, the tip head sharpness affects the width and the height of line patterns. When extremely fine oxide lines were desired, a sharp tip has to be used for STM patterning. © 2000 American Vacuum Society. View full abstract»

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  • Optical emission spectroscopy of microscopic gas discharges using a high-pressure scanning tunneling microscope

    Page(s): 644 - 647
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    A high-pressure scanning tunneling microscope (STM) unit was used to produce electrical discharges on a mesoscopic scale. Piezoelectric positioning elements allow to adjust the electrode gap with the precision needed. Optical emission spectroscopy was performed in high-pressure environments of helium and argon in a range of 5–20 bar. Spectra were recorded at different gas pressures and discharge currents. The increase of gas pressure changed line intensities and caused broadening of spectral lines of helium and argon. Increasing the discharge current yielded a higher intensity of the emitted light. The use of different electrode materials had no significant influence on the spectra. Electrical gas discharges on the microscopic scale can be used not only as a lithographic tool, but also as a compact light source of high luminance with emission of well-defined line spectra. © 2000 American Vacuum Society. View full abstract»

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  • Atomic force microscope tip sharpening and evaluation by electric field confinement using a metal grid close to the tip

    Page(s): 648 - 652
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    By setting a metal grid close to a Si tip on an atomic force microscope (AFM) cantilever and applying voltage between the grid and the tip, electrons were field emitted from the tip apex, where a high electric field was induced confining the metal grid. The pattern of field-emitted electrons was projected at high magnification by a shadow of the grid onto the phosphor screen of a field emission microscope (FEM). The FEM image shows the tip radius and the cleanliness. The confined electric field was also used to sharpen the tip by a thermal-field (TF) treatment, in which the tip is heated under a high electric field. The change in the FEM images before and after the treatment is reported, and the faceting of the Si tip apex by the treatment is discussed. The noncontact-AFM image of Si(111) 7×7 with atomic resolution was observed with the TF treated tip immediately after the tip came near to the sample. © 2000 American Vacuum Society. View full abstract»

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  • Atom-by-atom analysis of diamond, graphite, and vitreous carbon by the scanning atom probe

    Page(s): 653 - 660
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    Various artificial diamonds, graphite and vitreous carbon were atom-by-atom mass analyzed by a scanning atom probe (SCAP). Carbon clusters were detected in all specimens and the abundance of the clusters was found to be high for graphite and vitreous carbon. Nearly 90% of the carbon atoms field evaporated from 99.99% pure graphite were detected as clusters up to C11. After soaking in acid (H2SO4:HNO3:H2O=1:1:1) for 15 min no clusters were detected in the high-pressure high-temperature (HPHT) diamond which exhibited mass peaks of the clusters up to C6. The number of clusters formed by odd numbers of carbon atoms was larger than that formed by even numbers. The hydrogen concentration in chemically vapor deposited diamonds appeared to be related to the percentage of hydrogen in the mixed reaction gas while growing the diamonds. Few hydrogen ions were detected in the HPHT diamond and vitreous carbon. The hydrogen and oxygen distributions in the 99.8% graphite were rather uniform while those in the 99.99% graphite were localized. The present results suggest that the mass analysis by the SCAP may shed light on even the binding state of the analyzed materials. © 2000 American Vacuum Society. View full abstract»

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  • Microprocess for fabricating carbon-nanotube probes of a scanning probe microscope

    Page(s): 661 - 664
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    We have developed microprocesses to make carbon-nanotube probes for a scanning probe microscope. The processes contain electric-field induced transportation, welding and fixation by electron-beam carbon deposition and are performed in a scanning electron microscope equipped with two individual manipulable stages. Using the nanotube probes produced, a fine structure of helical and twinned deoxyribonucleic acid and an abrupt height transition with high fidelity in a 4.7 GB digital versatile disk are imaged with tapping-mode atomic force microscopy in air. © 2000 American Vacuum Society. View full abstract»

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  • Field emission properties of carbon nanotubes

    Page(s): 665 - 678
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    We have investigated the field emission properties of nanotube thin films deposited by a plasma enhanced chemical vapor deposition process from 2% CH4 in H2 atmosphere. Depending on the deposition of the metallic catalyst [Fe(NO3)3 in an ethanol solution or sputtered Ni] the nanotube films showed a nested or continuous dense distribution of tubes. The films consisted of multiwalled nanotubes (MWNTs) with diameters ranging from 40 down to 5 nm, with a large fraction of the tubes having open ends. The nanotube thin film emitters showed a turn-on field of less than 2 V μm-1 for an emission current of 1 nA. An emission site density of 10 000 emitters per cm-2 is achieved at fields around 4 V μm-1. The emission spots, observed on a phosphorous screen, show various irregular structures, which we attribute to open ended tubes. A combined measurement of the field emitted electron energy distribution (FEED) and the current-voltage characteristic allowed us to determine the work function at the field emission site. In the case of the MWNT thin films and arc discharge grown MWNTs we found work function values around 5 eV, which agree well with the global work function of 4.85 eV we determined by photoelectron spectroscopy. From the shape of the FEED peaks we can conclude that the field emission originates from continuum states at the Fermi energy, indicating the metallic character of the emission site. In the case of single-walled nanotubes we found significantly lower work function values of around 3.7 eV compared to those of MWNTs. We attribute this to a size dependent electrostatic effect of the image potential, which lowers the work function for small (≪5 nm) structures. ©- - 2000 American Vacuum Society. View full abstract»

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  • Beam steering by integrated electrodes for brighter field-emitter displays

    Page(s): 679 - 682
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    Field-emitter displays need to become more luminous. It is shown in this article by means of simulations and experiments that adding a second gate on top of the first one permits focusing and deflecting the beams emitted by microtips. A setup combining integrated focusing and deflection to increase the current density received by a pixel without increasing the current density emitted by the cathode is then described. While the row addressing power is increased, the luminous efficiency is not significantly affected and can even be enhanced. View full abstract»

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  • Stability of low-temperature amorphous silicon thin film transistors formed on glass and transparent plastic substrates

    Page(s): 683 - 689
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    This article describes the formation of amorphous silicon thin film transistors (TFTs) on glass and flexible transparent plastic substrates using rf plasma enhanced chemical vapor deposition and a maximum processing temperature of 110 °C. Silane diluted with hydrogen was used for the preparation of the amorphous silicon, and SiH4/NH3/N2 or SiH4/NH3/N2/H2 mixtures were used for the deposition of the silicon nitride gate dielectric. The amorphous silicon nitride layers were characterized by transmission infrared spectroscopy and current-voltage measurements; the plastic substrates were 10 mil thick (0.25 mm) polyethylene terephthalate sheets. Transistors formed using the same process on glass and plastic showed linear mobilities ranging from 0.1 to 0.5 cm2/V s with ION/IOFF ratios≥107. To characterize the stability of the transistors on glass, n- and p-channel transconductances were measured before and after bias stressing. Devices formed at 110 °C show evidence of charge trapping near the a-Si/SiNx interface and the creation of dangling-bond defects. The defect dynamics are consistent with the defect pool model. Under +10 and +25 V bias stress, the rates of creation of low energy defects are only moderately larger than those for high temperature devices; the devices show markedly higher rates of defect creation under higher positive bias. Current-voltage analysis of low temperature dielectrics shows very low leakage, but positive bias s- - tress shows a significantly higher electron trapping rate near the a-Si:H/SiNx interface, indicating problems with low temperature dielectric formation. The magnitude of the rates of defect creation and trapping in these nonoptimized devices suggests that amorphous silicon TFTs with stability approaching that of typical large area active matrix electronic devices could be formed at low temperatures compatible with transparent flexible polymeric substrates. © 2000 American Vacuum Society. View full abstract»

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  • Effect of post-nitride-passivation processing on the long-term stability of polysilicon integrated circuit resistors

    Page(s): 690 - 694
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    Results from 24 h experiments show that heat treatments of nitride-passivated polysilicon resistors at temperatures below that of the contact anneal, 450 °C, have a decisive influence on the stability of the resistors. In stress tests at 150 °C, it is shown that for heat treatments at 230 °C, the amount of loosely bound hydrogen in the grain boundaries and, consequently, the resistance drift is reduced by more than a factor of 2. For heat treatments above 270 °C, the drift is larger due to a reoccupation of released dangling-bond sites in the temperature range 230–270 °C during cool down. This is also the temperature range in which the largest reduction in the resistance drift is found. The heat treatments did not affect the mechanisms involved in the drift. There are no indications that hydrogen is released from the nitride at the highest temperatures, the reoccupation being independent of temperature. Possible mechanisms are discussed and the results are evaluated in terms of a previously introduced model for the time-dependent loss of loosely bound hydrogen. © 2000 American Vacuum Society. View full abstract»

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  • Reduction of sidewall defect induced leakage currents by the use of nitrided field oxides in silicon selective epitaxial growth isolation for advanced ultralarge scale integration

    Page(s): 695 - 699
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    Defects in the near sidewall region in selective epitaxial growth of silicon have prevented its widespread use as a viable dielectric isolation technology. The main cause of these defects has been demonstrated to be thermal stress due to mismatch in the coefficient of thermal expansion between silicon and silicon dioxide. This article presents the detailed electrical characterization of these sidewall defects using P+/N junction diodes fabricated using silicon dioxide and thermally nitrided silicon dioxide as the field insulator. It is shown that the use of field oxide which was nitrided at 1100 °C for 60 min in ammonia gas ambient reduced the reverse saturation current density in the diodes by a factor of 6 and also improved the forward recombination and ideality factors when compared to standard thermal field oxide isolated diodes. The improvement of the sidewall quality was attributed to a reduction in thermal stress due to the modification of the coefficient of thermal expansion of nitrided silicon oxide. © 2000 American Vacuum Society. View full abstract»

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  • Active corner engineering in the process integration for shallow trench isolation

    Page(s): 700 - 705
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    The electrical characteristics of metal–oxide–semiconductor field effect transistor devices with shallow trench isolation (STI) have been studied to evaluate the active corner shaping and the trench-fill dielectric densification techniques. The suppression of corner parasitic transistor effects was observed in the two different corner shaping schemes used. In the first approach, an undercut of pad oxide below the nitride mask defining the active region facilitated corner oxidation during liner oxide growth. In the second approach, a high temperature post-chemical mechanical polishing (CMP) oxidation created a rounded corner, forming a minibird’s beak under the nitride mask edge. Cross-sectional transmission electron microscopy shows that, while the post-CMP oxidation “rounds” the corner, the pad oxide undercut produces a concave corner profile. Though both approaches improved the subthreshold characteristics of the transistor, the leakage current of field-edge-intensive diodes became very high for post-CMP oxidation. The leakage was also strongly influenced by the annealing ambient during densification of STI gap-fill dielectric. An oxidizing ambient resulted in high leakage current whereas a nonoxidizing ambient resulted in low levels of leakage current. The excessive leakage is attributed to the silicon defects generated along the STI edge as a result of stress exerted by the gap-fill oxide. Densification in nonoxidizing ambient also helped improve the subthreshold characteristics of the transistors with STI fabricated using the pad oxide undercut scheme. © 2000 American Vacuum Society. View full abstract»

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  • Apparent depths of B and Ge deltas in Si as measured by secondary ion mass spectrometry

    Page(s): 706 - 712
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    The apparent depths of B and Ge deltas in Si were measured with secondary ion mass spectrometry using 1–4 keV O2+ beams at oblique incidence (40°–80° with respect to the surface normal). The real depths of the Ge deltas were obtained via calibration against transmission electron microscopy (TEM). The measured centroids of the Ge delta peaks were 0.5±0.4 nm shallower than the real (TEM) depths, independent of angle and energy. For B there was a clear angular dependency of the centroid position, but the energy dependency was virtually absent (viz., differential shift ≪0.2 nm/keV/ion). Repeated analyses during a 2.5 year period showed a reproducibility in Ge delta peak position of 0.04 nm. Surprisingly, the slow but continuous growth of the native surface oxide had no effect on the apparent Ge delta depths. The profile shift of B towards the surface was attributed to the high sputter rate during the surface transient and to B outdiffusion under oxygen bombardment. It was concluded that decreasing the beam energy did not reduce the B shift; the positive effect of a thinner surface transient layer was nullified by a larger (equilibrium) oxygen concentration. In contrast, indiffusion of Ge compensated at all energies and angles the (apparent) Ge shift due to the transient high sputter rate. © 2000 American Vacuum Society. View full abstract»

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  • Fabrication of dielectric hollow submicrometric pipes

    Page(s): 713 - 716
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    A simple technique to obtain an array of hollow submicrometric titanium oxide pipes is described. Initially, a submicrometric structure is formed in a positive photoresist by holographic exposure, followed by the deposition of the titanium oxide film and the removal of the photoresist by an appropriate solvent. These structures may be useful as waveguides for x-ray and neutron capillary optics. © 2000 American Vacuum Society. View full abstract»

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  • Effect of lateral dimensional scaling on the thermal stability of thin CoSi2 layers reacted on polycrystalline silicon

    Page(s): 717 - 720
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    The thermal stability of patterned cobalt silicide layers grown on polycrystalline silicon was studied by electrical and morphological analyses in the temperature range of 850–1000 °C. The strip degradation upon annealing resulted in a faster resistance increase than that in blanket films, and occurred from preferential agglomeration of the grains located at the strip edges. An activation energy for this process of 3.5 eV was obtained by the increase in resistance and attributed to the growth of silicon grains from the substrate that protruded into the silicide layer. © 2000 American Vacuum Society. View full abstract»

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  • Role of the substrate in the C49–C54 transformation of TiSi2

    Page(s): 721 - 728
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    In this work we report the results of the crystallographic and morphological characterization of TiSi2 films grown on a patterned and on a blanket Si substrate. We show that the C49 films grown on crystalline silicon exhibit a rougher surface with respect to that grown on polycrystalline substrate. The different surface morphology is maintained after transformation to the C54 phase. This latter is always (040) textured in the case of patterned films, whereas in blanket films texturing occurs only on a polysilicon substrate. The C49 phase displays (200) texturing when the film is grown on single crystalline Si(100), while random orientation of the grains is detected in the other cases. The experimental indications are associated with semiempirical total energy estimations of the surface energies for the two competing phases. From this comparison it turns out that, in the case of texturing, the preferential surfaces facing the substrate are the most energetic. © 2000 American Vacuum Society. View full abstract»

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  • Investigation of the mechanism for Ti/Al ohmic contact on etched n-GaN surfaces

    Page(s): 729 - 732
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    In this study, the mechanism for ohmic contact of Ti/Al bilayer formation on as-grown, etched and postetch annealed GaN surfaces were investigated. A nonalloyed Ti/Al ohmic contact to etched GaN surface, with postetch annealing prior to metal deposition, was obtained. The specific contact resistance of 2.3×10-4 Ω cm2 was obtained. The nonalloyed ohmic contact may be attributed to the postetch annealing which generates nitrogen vacancies that result in a heavily n-type surface thereby forming a tunneling junction. On the other hand, the nonalloyed Ti/Al contact on as-grown and as-etched GaN surfaces exhibits non-ohmic behavior. After alloying at 500 °C for 5 min, Ti/Al contacts on as-grown, as-etched and postetched annealing GaN surfaces have specific contact resistances around 9.8×10-5, 1×10-4, and 7.2×10-5 Ω cm2, respectively. Nonalloyed Ti/Al ohmic contacts would be especially useful for fabricating high breakdown, recessed-gate field effect transistors on GaN since the moderate postannealing condition converts only the near surface layer to heavily n type. © 2000 American Vacuum Society. View full abstract»

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Aims & Scope

The Journal of Vacuum Science and Technology B is devoted to reports of original research, review articles, and Critical Review articles.

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Editor
Gary E. McGuire
International Technology Center