"Nanolithography and Transistor Degradation by Hot Electrons:" J.W. LYDING, Karl Hess, Department of Electrical and Computer Engineering and Beckman Institute, University of Illinois at Urbana-Champaign, IL 61801; I.C. Kizilyalli, Lucent Technologies Bell Laboratories, 9333 S. John Young Parkway, Orlando, FL 32819
It has been shown recently that hydrogen passivated silicon can be used to create patterns with nanometer feature sizes. The selective nanometer scale desorption of hydrogen is accomplished by scanning tunneling microscopy under ultrahigh vacuum conditions. Later experiments of this kind showed that deuterium is much more difficult to remove from silicon surfaces than hydrogen. In fact, fifty to one hundred times as many tunneling electrons are needed for the removal of deuterium as compared to hydrogen, demonstrating an unexpected and giant isotope effect.
This effect in nanolithography suggested that hot electron damage to silicon-silicon dioxide interfaces might also show an isotope effect; this damage had been attributed to the removal of the hydrogen that passivates the dangling bonds at the interface. Our detailed experiments on hot electron degradation of complementary metal-oxide-semiconductor technology indeed showed a giant isotope effect in transistor aging: a factor of 10 - 50 increase in transistor "lifetimes".
This unexpected connection between conventional silicon technology and research on naolithography demonstrates that the atomistic understanding of processes at silicon interfaces (as is currently being developed by use of scanning tunneling microscopy) offers new and important viewpoints of considerable relevance for current generation devices.
"Nano-Scale Patterning of Hydrogen-Passivated Silicon Surfaces Using a Scanning Near-Field Optical Microscope:" S. MADSEN, DME-Danish Micro Engineering A/S, Herlev, Denmark; M. Mullenborn, K. Birkelund, F. Grey, Mikroelektronik Centret, Danmarks Tekniske Universitet, Lyngby, Denmark
We present a novel technique in which a reflection scanning near-field optical microscope(SNOM)is used for direct writing of nano-scale patterns into hydrogen-passivated silicon surfaces. The reflection SNOM is operated as a combined shear-force/near-field microscope enabling simultaneous imaging of topographical and optical properties with subwavelength resolution. The samples consist of a thermally oxidized silicon substrate with a thin amorphous silicon layer on top. The emitted light from an uncoated fiber probe leads to optically induced hydrogen desorption resulting in a local oxidation of the thin amorphous silicon layer. Subsequent wet etching in KOH is applied to remove the unoxidized regions. An atomic force microscope (AFM) operating in contact mode is used for surface characterization after etching. With this direct writing technique, optically-induced structures have been transferred into the amorphous silicon layer with a resolution of ~100nm.
The main advantage of the presented technique is its general applicability. Beside high resolution optical imaging (<50nm) and patterning of hydrogen-passivated silicon surfaces, the SNOM can also be used for transferring nano-scale patterns into ordinary photo resist with a typical resolution of 50-100nm. The SNOM thereby overcomes the far-field diffraction limit of conventional photolithography.
"Scanning Probe Nanolithography:" STEVEN KONSEK, Physics Department, University of Washington, Box 351560, Seattle, WA 98195-1560; Thomas P. Pearsall, Materials Science and Engineering Department, University of Washington, Box 2120, Seattle, WA 98195-2120
We are developing the use of scanning probe microscopy to create nanometer-scale structures on electronic materials. The Scanning Tunneling Microscope(STM)and the Atomic Force Microscope(AFM)are used to write oxide lines with a width of 20nm and a height of 2nm at both positive and negative tip biases. Serving as etch masks, these patterns define structures in the substrate. Our work has been twofold. First we have studied the use of the STM and AFM to modify hydrogen-passivated silicon surfaces. Second, we extend our work to nanolithography.
We are studying the physical mechanism of STM/AFM-stimulated oxide deposition on hydrogen passivated Si(100)and Si(111)surfaces both in air and in an evacuated, 10e-6 torr, SEM chamber. Our results indicate that the electric field strength, rather that the tunneling current, is the key parameter. We report threshold measurements at 10e-6 torr of surface modification between -5 and -6 volts. Furthermore, we have extended this to positive biases. In doing so, these "medium vacuum" results are fully consistent with neither ultra-high vacuum nor air experiments. Surface modification in air may occur by depassivation of the H-terminated surface, followed by oxidation. However, successful direct writing under vacuum conditions suggests that selective depassivation can be followed by decoration with carbon. These results are exceedingly promising for future work in direct writing with metalorganic precursors.
We also present our work in nanolithography. The surface modifications previously described serve as a negative resist etch mask for KOH, which selectively etches Si over SiO2. Hence, they yield solely Si structures when the oxide is removed and the surface repassivated. We demonstrate, via STM, AFM, and SEM viewing, patterns of Si lines with linewidths of 20nm, heights of 12nm, and center-to-center spacing of 30nm.
Scanning probe lithography as a method for creating nanometer-scale structures is important in two respects. First, we have demonstrated that the resolution of scanning probe lithography is equal to or superior to that of competing technologies. And second, our work will, because of the availability of scanning probe microscopes, help to enable widespread access to nanolithographic technology.
"Surface Modification of Niobium (Nb) by Atomic Force Microscope (AFM) Nano-Oxidation Process:" JIN-ICHI SHIRAKASHI, Masami Ishii, Kazuhiko Matsumoto, Electrotechnical Laboratory (ETL), 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305, Japan; Naruhisa Miura, Makoto Konagai, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152, Japan
Nanometer-scale surface modification of niobium (Nb) thin films deposited on SiO2/Si substrates was investigated by an atomic force microscope (AFM) nano-oxidation process for the first time. Successful fabrication of the modified structure was performed by applying a negative tip bias voltage between the metal-coated conductive cantilevel and the sample. By varying the applied voltage from about -7V to above -20V, the size of the modified structure was controlled ranging from about 17nm to 90nm in width and about 0.5nm to 5nm in height. From Auger electron spectroscopy (AES) analysis, it was revealed that the structure consists of Nb and a large amount of oxygen (O), suggesting the formation of Nb oxides. As is well known in the case of oxidation of titanium (Ti) and chromium (Cr) by the AFM-based oxidation process, the mechanism of oxidation of Nb may also appear to be tip-induced anodization, i.e., water and/or oxygen-containing species adsorbed on the sample surface could electrochemically react with Nb by applying the bias voltage between the cantilevel and the sample.
This technique, with the possibility of fabricating ultra-small tunnel junction devices such as SETs, was initially applied to the fabrication of planar-type metal/insulator/metal (MIM) diodes. Current-voltage (I-V) characteristics of a Nb/Nb oxide/Nb MIM diode were measured at 300 K. From this result, non-linear I-V characteristics were clearly observed, suggesting that Nb oxides formed by the AFM nano-oxidation process act as a barrier material for the electron. Furthermore, from the temperature dependence of the I-V characteristics, the barrier height (ØB) and relative dielectric constant ([[epsilon]]) were determined for the Nb/Nb oxide system as 133 and 64 meV, respectively.
"Nanolithography Using Self-Assembled Monolayers of Undecylenic Acid On Silicon Dioxide: Electron Beam Patterning and Pattern Development by CEVE:" T.K. WHIDDEN, Min Pan, M.N. Kozicki, Center for Solid State Electronics Research, Arizona State University, Tempe, AZ 85287-5706
Previously, we reported on the action of electron-beam induced patterns of cross-linked hydrocarbons on silicon dioxide as catalysts in the HF vapor etching of the oxide film (1-3). We have employed both STM-and SEM-based electron-beam exposures to fix hydrocarbon patterns on silicon dioxide surfaces and subsequently performed high temperature HF vapor etching to produce these masks. The masks exhibit feature sizes as small as 3-5nm (1) and are of utility in nanometer scale patterning of the underlying substrate (2). Various patterns of lines, boxes and grids have been produced in the silicon dioxide layers by SEM based electron beam lithography (2,3). Combinations of the technique with reactive ion etching and conventional metal deposition and silicidation have yielded a process for the formation of 12-20nm wide lines of cobalt silicide on silicon (4). Evidence for the activity of COOH chemical groups as promoters of localized HF etching of silicon dioxide has been obtained and self-assembled monolayers of such acids have been prepared and shown to be active in the CEVE process (5). Preliminary patterning studies of monolayers of myristic and undecylenic acids have shown good resolution, yielding linewidths below 100nm (5). In this work we describe the systematic evaluation of electron beam parameters in a patterning process for self-assembled monolayers of undecylenic acid on silicon dioxide. Etch rates of the patterned oxide are correlated against such parameters as total electron beam dose, accelerating voltage and beam current, and the results interpreted in terms of cross-linking mechanisms within the monolayer. Nanoscale patterning of the oxide film is evaluated. Characterization studies of the monolayer before and after electron beam exposure will be discussed.
*Work Supported by the Advanced Research Projects Agency
1. T.K. Whidden, J. Allgair, J.M. Ryan, M.N. Kozicki and D.K. Ferry, J.
Electrochem. Soc., 142, 1199(1995).
2. Thomas K. Whidden, John Allgair, Angela Jenkins-Gray and Michael N. Kozicki, J. Vac. Sci. Technol.,B13(3)(1995).
3. T.K. Whidden, J. Allgair, A. Jenkins-Gray, M.N. Kozicki and D.K. Ferry, Jpn. J. Appl. Phys., 34(8B), 4420(1995).
4. J. Allgair, T.K. Whidden, M.N. Kozicki and D.K. Ferry, to be published in the J. Vac. Sci. Technol.
5. T. K. Whidden, A. Jenkins-Gray, M. Pan and M.N. Kozicki, submitted to J. Electrochem. Soc.
"Focusing A Thermal Atomic Beam to Nanometer Resolution Using a Laser:" GREGORY TIMP, Robert Behringer, Vasant Natarajan, AT&T Bell Labs, 600 Mountain Ave., Murray Hill, NJ 07974-0636
A thermal flux of neutral atoms can be focused using a laser that is nearly resonant with an atomic transition. Essentially, this capability develops from the interaction between an inhomogeneous intensity in the laser beam, and the dipole moment induced in the atom by the light. This interaction is used to focus atoms in a way that is superficially similar to how a gradient-index optical lens focuses light: by reshaping the trajectories through refraction in an inhomogeneous medium.
Lately, the atom-optical performance of a Gaussian standing wave laser beam (SW) has been scrutinized. To evaluate its performance, the laser beam, with an intensity that varies according to: I(x,z)=I cos2(2[[pi]]/[[lambda]]) exp[-2z2/[[sigma]]2] for z<zf, is interposed between a diverging atomic flux propagating along the z-direction and a substrate surface located at z=zf. When the laser frequency is detuned from the atomic resonance, the interaction between the induced dipole and the inhomogeneous intensity of the SW effectively gives rise to a series of practically identical potential wells, regularly spaced every [[lambda]]/2 along the x-direction. If the interval of time required for an atom to transit the SW in the z-direction coincides with the time required for the atom to reach a potential minimum along the x-direction, then the atomic flux is focused onto the substrate into a regular array of lines with a period of [[lambda]]/2.
As for any optical system, the parameters used to characterize the performance are resolution and contrast. So far, we have observed that a thermal sodium flux can be focused using a laser beam, subject to the above mentioned timing criterion, onto a clean silicon substrate with 13nm resolution and 10:1 contrast, producing about 100,000 lines in less than 1 minute. Thus atom-optics is poised on the threshold of a frontier, beyond the scope of conventional optics where the resolution is limited by the wavelength, and beyond the scope of conventional high resolution lithography which is throughput limited. This unprecedented performance was achieved by using a Gaussian SW with l/2=294.5nm and a waist of [[sigma]]=34 um, detuned from the sodium D2 resonance by 1.7GHz, in conjunction with a sub-Doppler optical cooling scheme to reduce the angular divergence of the sodium flux. Within the range of the experiments, we observe that the resolution is limited by the divergence of the atomic flux and the focal length of the atom-optical lens, in substantial agreement with semi-classical numerical simulations which account for the effects of the atomic velocity distribution, but ignore the velocity dependence of the interaction. However, preliminary data at extremely short focal lengths reveal a discrepancy with this simplistic model.
"Fabrication of Silicon Nanopillars Containing Poly-Si/Insulator Multilayer Structures:" H. FUKUDA, J.L. Hoyt, M.A. McCord, R.F.W. Pease, Solid State Electronics Laboratory, Stanford University, Stanford, CA 94305
A number of quantum functional devices such as the single electron transistor require fabrication of three-dimensional structures with dimension control on the order of a few nanometers. It has been demonstrated that nanometer-diameter vertical silicon wires can be formed by laterally oxidizing Si pillar structures [H. Liu, et al., Appl. Phys. Lett. 64(1994) l383]. In this work, the nanopillar fabrication technique has been applied to structures containing multiple polysilicon/insulator layers, with the goal of fabricating vertically-stacked, ultra-small tunneling junctions.
The multilayer structures were formed by repeated deposition of polysilicon followed by rapid thermal oxidation or nitridation. The polysilicon thickness was varied from 10 to 20nm and the insulator thicknesses ranged from 1.5 to 2.5nm. Pillars with diameters ranging from 50 to 150nm were patterned by electron-beam exposure followed by Cr lift-off and reactive ion etching using NF3. Oxidation in a dry oxygen ambient was performed at approximately 900C, which has previously been shown to yield self-limiting oxidation of single-crystal silicon pillars.
Direct side-view imaging of the nanopillars by transmission electron microscopy shows the vertical polysilicon core wire surrounded by oxide, and vertically-stacked Si nano-islands connected to the wire through thin insulator layers. In one case, the diameter of the polysilicon core after oxidation is 15nm, and the island diameter and height are approximately 15 and 10nm, respectively. Clear lattice fringes were observed within the grains for both the silicon islands and wires. In most cases, the outer surface of the polysilicon pillar remained smooth during the oxidation process, in spite of the grain structure. The lateral oxidation rate of the polysilicon was, on average, equal to that of the single-crystal structure. The lateral oxidation rate of the polysilicon was, on average, equal to that of the single-crystal material. The thickness of the thin oxide interlayers increases during the lateral oxidation process. This is most likely due to diffusion of oxygen along the thin oxide interlayer. This process was suppressed for structures containing nitride interlayers, though some birds-beak-like features were observed. To obtain final structures with thin insulating layers which may be suitable for tunneling junctions, it is important to reduce the diffusion of oxygen along the thin insulator layers, and nitride appears to be a much more promising interlayer material than oxide.
In summary, our method allows the fabrication of very small three-dimensional structures using conventional silicon processing equipment. Fabrication of contacts to the top of pillars to examine the electrical properties of these structures is a future challenge.
"Nanofabricated Break Junctions:" C. ZHOU, C.J. Muller, M.R. Deshpande, J.W. Sleight, M.A. Reed, Center for Microelectronic Materials and Structures, Yale University, PO Box 208284, New Haven, CT 06520-8284
Conventional mechanically-controllable break junctions (MCB) are a unique tool for the investigation of electrical and mechanical properties of matter at atomic dimensions. This system has a technical potential to be used as an ultrasensitive displacement transducer. Previously work on MCB (a thin wire anchored on a beam with the contact area of the break adjusted by an external piezo) has demonstrated atomic resolution, although greater stability attainable by scaling down the dimensions of the devices is desirable.
We have therefore fabricated and demonstrated a novel micromachined MCB one hundred times smaller than previously achieved. An e-beam-defined one micron wide Au stripe with a central neck of 100nm x 100nm is deposited onto an oxidized Si wafer. CF4/O2 plasma etching is utilized to etch through a small window in the SiO2 layer around the central neck. Wet etching of the exposed Si area using pyrocatechol-ethylenediamine mixture gives two suspended bridges of metal/SiO2 connected by the 100nm wide metal neck. Stability measurements done at room temperature in the tunneling regime infer an electrode stability within 3pm in a 1KHz bandwidth. In the point contact regime, the conductance shows plateaus near multiples of the fundamental conductance unit. A fully integrated version of break junction, in which the contact size is controlled by applying a potential with a gate electrode instead of using an external piezo, is in progress.
"On the Formation and Properties of the Sb2Te3 Microtubes:" D.K. GASKILL, O. Glembocki, E. Dobisz, Naval Research Laboratory, Code 6861, Washington, DC 20375; T.J. Groshens, R.W. Gedrige, Jr., Naval Air Warfare Center Research and Technology Division, China Lake, CA 93555
Conducting and semiconducting materials deposited in microchannel and nanochannel glass have been proposed for a wide array of applications such as photonic bandgap media, 3-dimensionally defined nanostructures, and small definition lithography masks. In this work a novel approach to coat the channel walls or fill the channels of these special glasses with semiconducting Sb2Te3 is described. The films were deposited at room temperature and slightly elevated temperatures via a novel N,N-dimethylaminotrimethylsilane (Me3SiNMe2) elimination reaction using volatile Sb(NMe2)3 and (Me3Si)2Te sources. Stoichiometric films were obtained irrespective of excess Sb or Te mole fraction. Previous work has shown that room temperature deposited films are amorphous-like (from x-ray measurements) with p-type conductivity and hole density of order 1018cm-3. Best film quality is obtained at reduced (<100 torr) pressures. Deposition experiments in microchannel glass resulted in the deposits conformal to the channel walls, i.e., microtubes of Sb2Te3, demonstrating that surface reactions determine film growth. The initial growth on the walls was highly uniform but films 1um or more thick displayed a granular texture with grain size scale length of about 1um. This morphology variation is indicative of limited mobility of adatoms during growth. Raman spectroscopy of the as-deposited films (on the surface or in the tubes) reveal two dominant phonon-modes. As the intensity of the Raman pump beam is increased, several additional phonon modes become obvious at a critical intensity, demonstrating laser annealing of the films. The complexity of the phonon spectra with respect to that of bulk samples implies the annealed films, and by extrapolation the as-deposited films, consist of more than one Sb2Te3 phase. Experiments to test the applicability of these microtubes to uv lithography will be described.
"Fabrication and Optical Characterization of GaN-Nanostructures:" H. ZULL, J. Müller, J. Koeth, F. Kieseling, A. Forchel, Technische Physik, University of Würzburg, Germany
III-V nitride materials are attracting a great deal of attention because they permit the realization of visible and near UV light emitting diodes (LEDs) and semiconductor lasers due to their large direct bandgap energies. Their chemical stability, high thermal conductivity and high melting temperature also make them suitable for high temperature electronic and photonic device applications. For the fabrication of optoelectronic devices (e.g. laser diodes, waveguides, distributed feedback(DFB)gratings) appropriate patterning and etching techniques must be developed.
We have grown GaN/AlN heterostructures by electron cyclotron resonance-MBE (ECR-MBE) on sapphire substrates. A 30nm thick AlN buffer layer was deposited at a temperature of Tg,AlN=500deg.C. The GaN epilayer was grown at Tg.GaN=650deg.C with a nitrogen flow rate of IN2=2 sccm and an ECR-power of 65 W. Using modified ECR-aperture sizes we obtained very flat surfaces suitable for high resolution patterning.
For the fabrication of sub-100nm GaN structures the samples were patterned with 30nm thick Cr masks. The masks were defined by high resolution electron beam lithography in 100nm thick PMMA and a lift-off process. The mask patterns were transferred into the GaN by ECR-enhanced reactive ion etching (RIE) using Cl2/Ar (1:9). The etch conditions, including the plasma self-bias voltage, chamber pressure, microwave input power, and gas mixture, were optimized with respect to the etch rates and the surface morphology. The process provides etch rates of up to 100nm/min and results in mirror like surfaces.
We have obtained wire structures with grating periods down to 80nm and wire widths of <30nm. This period corresponds to that of a first order DFB grating for GaN room temperature emission. The patterns were etched completely through the GaN and AlN layers down to the sapphire substrates. SEM micrographs of the etched structures show steep sidewalls and excellent homogeneity over the wafer.
Etched gratings with wire widths between 1um and 26nm were investigated by
photoluminescence using an Ar+ laser([[lambda]]=335nm) as the excitation
sources. The wire patterns show intense excitonic photoluminescence and no
degradation of the signal even at lateral dimensions down to 30nm. This
indicates that the ion-induced damage of the employed etching process is small.
Most remarkably, we observe a maximum of the luminescence intensity for wire
widths of about 80nm which exceeds the emission intensity of large
two-dimensional structures. We assign this maximum to the combined influence of
optical excitation and non -radiative recombination via the etched sidewalls.
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