AVS1997 Session NS-TuA: Novel Measurements at Nanoscales

Tuesday, October 21, 1997 2:00 PM in Room K

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		NS-TuA-1 Atom Species Identification in STM with Time-of-Flight Technique U. Weierstall, J.C.H. Spence, U. Knipping (Arizona State University) An STM has been constructed which allows atomic clusters of interest to be transferred into a time-of-flight spectrometer for species identification¹. Atoms are first transferred onto the tip, using a small voltage pulse. The sample is then removed, and these atoms ejected into a time-of-flight (TOF) analyzer for mass identification². The time of flight analyzer is similar to the imaging atom probe design by Panitz³. A 75mm flat Chevron MCP is used as TOF-detector and for field-ion and field emission imaging of the tip. The output is led to a digital oscilloscope, whose trace is triggered by the tip pulse. Scope, triggering and H.V. are under Labview control, spectra are transferred into the PC. The transfer rate has been increased to 20 spectra/sec, therefore the available time between pulses for atom diffusion on the tip is reduced. Initial testing of the instrument was done with a Si (111) sample. By applying a 5V, 10ms pulse on the 77 surface, a pit was created. Afterwards the TOF spectrum of the STM tip showed Si+ and Si2+ peaks. Recent improvements of the instrument include a tip heating station, an upper stage for FIM and FEM examination of the tip with large angular view, and a UHV-prechamber for sample heating equipped with an evaporator and thickness monitor. We grow Ge on Si (111) 77 at substrate temperatures of 400-500C. Ge nucleates preferentially at step edges and coverage over 2ML results in a completely 5*5 reconstructed layer ⁴^,⁵. By pulsing on a Ge-island and subsequent TOF analysis, a Ge2+ peak was found in the spectrum. Supported by NSF award DMR9526100. ¹J. C. H. Spence, U. Weierstall and W. Lo, J. Vac. Sci. Technol. B 14(3) (1996) 1587 ²M. Miller and D. A. Smith, Atom Probe Microanalysis (Materials Research Society) 1989 ³J. A. Panitz, Rev. Sci. Instr. 44 1034 (1973) ⁴U. Koehler, O. Jusko, G. Pietsch, B. Mueller and M. Henzler, Surface Science 248 (1991) 321 ⁵A. W. Denier van der Gon, R. M. Tromp and M. C. Reuter, Thin Solid Films, 236 (1993) 140
2:20 PM		NS-TuA-2 Detecting Electron Interference Fringes on a Quantum Wedge D.M. Chen, I.B. Altfeder (Rowland Institute for Science); K.A. Matveev (Duke University) We have successfully fabricated a Pb quantum wedge on a stepped Si(111) surface. Using a low temperature STM we reveal a set of quantized energy spectra of the electrons confined in the nano-scale wedge. Selective detection of the highest occupied quantum state in the wedge gives rise to a discrete fringe pattern in the STM images, which manifests the change of the interference condition for the Fermi electrons as the thickness of the wedge varies. From the quantized tunneling spectra, the number of the metal layers can be determined accurately. Together, these measurements open a new possibility for probing nondestructively the buried interface. I.B. Altfeder, K. A. Matveev, and D. M. Chen, Phys. Rev. Lett. 78, 2815(1997).
2:40 PM		NS-TuA-3 Development and Application of a Dual-Probe Scanning Tunneling Microscope for Nanoscale Investigations of Materials H. Grube, M. Allgeier, J.J. Boland (University of North Carolina, Chapel Hill) Scanning tunneling microscopy has evolved into a valuable tool for the study of the structural and electronic properties of semiconductor and metal surfaces, as well as enabling fabrication of novel nanoscopic electronic devices. However, the single probe geometry of STM limits its application to local and static measurements of the local density of states (LDOS) ¹. Incorporation of a second electrically and mechanically independent STM tip within 100 nm of the first is expected to enable measurements of surface properties that conventional STM cannot perform ²^,³^,⁴. To this end our lab has completed construction of one of the first dual probe STMs in which tips can be placed 10-100nm apart. Tips suitable for operation in DP-STM must have very high aspect ratios so that their apices will not physically interfere when brought into a volume of space less than 100 nm³. This has been achieved by ion milling Pt-Ir tips to a cone angle of less than 10 degrees and a tip radius of less than 5 nm. Each tip is mounted on an independent tube scanner with independent piezo drivers, feedback loops and preamps. The DP-STM has five degrees of freedom for coarse approach, achieved through the use of modified commercial inertial sliders. These five degrees of freedom allow for the precise positioning of the two tips on nanometer distances. In this DP-STM configuration it is possible to pass current between tips through the sample (that is, measure a transconductance signal) thereby probing the transport properties of the medium or nanoscale device. Our DP-STM has been characterized by using each tip to scan its local surface environment and then overlaying the images obtained to determine the inter-tip separation. The design is currently being optimized with respect to noise, mechanical stability and software control to facilitate separation of the transconductance signal from primary tunneling current and other interference. ¹G. Binnig et al., Phys. Rev. Lett., 49 (1), 57 (1982) ²Q. Niu, M.C. Chang and C.K. Shih, Phys. Rev., B 51 (8), 5502 (1995) ³J.M. Beyers and M.E. Flatte, Phys. Rev. Lett., 74 (2), 306 (1995) ⁴J.M. Beyers and M.E. Flatte, J. Phys. Chem. Solids., 56 (12), 1701 (1995)
3:00 PM		NS-TuA-4 Femtosecond Laser Induced Imaging in Scanning Tunneling Microscopy Y.M. Chang, L. Lauhon, W. Ho (Cornell University) Atomically resolved tunneling current images, induced by femtosecond laser irradiation of the tunnel junction in an STM, are obtained. The Ti-sapphire laser, 25 fs, 90 MHz, 800 nm, and 100 mW, was focused (50 µm spot size) on the air junction between a W tip and a highly oriented pyrolytic graphite (HOPG) sample. The laser, chopped at 2 KHz, induced a modulation in the tunneling current. The modulated current was detected by phase sensitive lock-in amplifier and used to construct femtosecond laser induced images. Comparisons to topographical STM images of HOPG were made. Images from laser induced tunneling current clearly distinguished between the inequivant carbon sites, and identified the hexagonal carbon structure of the top layer. The modulated tunneling current was measured as a function of the laser polarization, intensity, and modulation frequency. These results could not be fully explained by the photothermal expansion of the tip and sample. Unlike CW laser photothermal modulation of the tip and surface, the transient photo-carrier excitation (~ 10¹⁷ - 10¹⁸ / cm³) due to femtosecond laser pulses can significantly perturb the electronic local density of states near the Fermi level. The thermal relaxation and recombination of photoexcited electron and hole pairs thus affect the tunneling current. These experimental results demonstrate the potential combination of optical pump-and-probe experiments with the STM. By varying the delay time between the pump and probe pulses, it is possible to atomically time-resolve the ultrafast surface carrier dynamics by measuring the changes in the laser induced tunneling current. This novel technique can apply to metal, semimetal, and semiconductor surfaces in both ambient and ultrahigh vacuum environments.
3:20 PM		NS-TuA-5 A Combined AFM/STM/FIM System in UHV A. Stalder, G. Cross, A. Schirmeisen, P. Grütter (McGill University, Canada); U. Dürig (IBM Research Division, Switzerland) We present a unique UHV surface science system which features simultaneous spectroscopy of force and tunneling current in a tip-sample configuration while the tip is characterized by field ion microscopy (FIM). We will show the preparation of atomically defined W(111) and W(110) tips as observed by TEM and FIM. Furthermore we explain a force detection set-up which is capable of measuring the tip sample force directly and the force gradient at the same time. Simultaneously we can measure the I-V characteristics of this tunneling junction. A fast switching mechanism between the spectroscopy mode and the FIM operation allows us to monitor the structure of the tip and possible changes it undergoes due to approaching and probing the surface. Experimental results confirm the performance and capabilities of this machine.
3:40 PM		NS-TuA-6 Holographic Imaging of Macromolecules A. Gölzhäuser, B. Jäger, B. Völkel, M. Zharnikov (Universität Heidelberg, Germany); H.J. Kreuzer (Dalhousie University, Canada); M. Grunze (Universität Heidelberg, Germany) In a projection microscope¹ the coherent electron beam from a point source can be utilized to generate holograms of macromolecular entities. The holograms are formed by the interference between the unscattered part of the electron wave and the part scattered by the atoms in the molecule, i. e. the holographic reference and object waves. Structural information on the object can then be obtained by numerical reconstruction of the hologram². In our experimental setup, the point source is realised by an atomically sharp field emitter that can be positioned to close proximity to the sample. Magnified in-line holograms are recorded with a multi channel plate detector 15 cm away from the sample. To prepare adequate samples of chainlike macromolecules, a new class of substrates, thin microstructured silicon membranes, were used. The molecular chains 'bridge' 100 nm wide gaps across these membranes. By approaching the source to such an opening in the substrate only the object molecule is exposed to the electron wave and the unwanted interaction of the electrons with the supporting structure is minimized. Holograms of DNA, PcPS (phtalocyanato polysiloxane) and other long chain polymers were recorded and analysed for distinct differences between the molecular entities. The mechanisms of image formation and uncertainties relating to the interpretation will be discussed. ¹H. W. Fink , W. Stocker, H. Schmid, Phys. Rev. Lett. 65, 1204 (1990) ²H. J. Kreuzer et al., Ultramicroscopy 45, 381-403 (1992)
4:00 PM		NS-TuA-7 Independent Detection of Lateral and Vertical Forces with a Novel Dual-axial Piezoresistive AFM Cantilever B.W. Chui (Stanford University); H.J. Mamin, B.D. Terris, D. Rugar (IBM Almaden Research Center); T.W. Kenny (Stanford University) We have fabricated a novel dual-axial AFM cantilever with independent piezoresistive sensors for simultaneous detection of lateral and vertical forces. The micromachined single-crystal silicon cantilever consists of a flat triangular probe (100 µm long, 1.3 µm thick) which is connected to a base by four high-aspect-ratio ribs (1.3 µm wide, 110 µm long, 5 µm tall). The ribs give the structure lateral compliance and have embedded piezoresistors which serve as lateral deflection sensors. The flat triangular probe provides vertical compliance and has a separate set of piezoresistors to serve as vertical deflection sensors. The cantilever structure is designed so that forces at the probe tip are mechanically resolved into orthogonal components that excite the lateral and vertical deflection modes of the cantilever, thereby producing synchronized x-axis and z-axis output signals. A novel ion implant technique is used to fabricate the piezoresistive sensors on the cantilever. The sensors have measured sensitivities ΔR/R on the order of 10^-7 per Å. Using two Wheatstone-bridge circuits for dual-channel signal processing, x-axis and z-axis AFM images can be obtained in parallel with the cantilever during a single scan. A high degree of correlation has been observed between the x-axis and z-axis images. This demonstrates the potential utility of the dual-axial cantilever in nanotribology and other applications that require a compact, efficient system for orthogonal force detection.¹ ¹This work is supported by the NSF CAREER Program and an IBM Cooperative Fellowship.
4:20 PM		NS-TuA-8 Scanning Joule Expansion Microscopy at Nanometer Scales J. Varesi (University of California, Santa Barbara); A. Majumdar (University of California, Berkeley) We have developed a new technique - Scanning Joule expansion microscopy (SJEM) - that can simultaneously image surface topography and material expansion due to Joule heating with nanometer- scale spatial resolution. Expansions on the order of 1-10 pm have been detected with observed spatial resolution of 25 nm. Such a technique allows one to locate regions of power dissipation and thermal stresses in semiconductor devices and interconnects. Since thermal expansion depends on expansion coefficeint, temperature rise, and surface topography, one can use this technique for localized material characterization or temperature mapping. By coating the surface with a polymer of high expansion coefficient, we demonstrate that temperature distributions can be directly mapped out with nanometer-scale spatial resolution. This is a major advantage over other high-resolution thermal imaging techniques which require fabrication of temperature sensors such as thermocouples and diodes on probe tips. Instead, SJEM uses commercially-available cantilever probes that are used in atomic force microscopes.
4:40 PM		NS-TuA-9 Construction of an Ultrahigh Vacuum, Low Temperature Atomic Force Microscope D.E. Muzzall, D.N. Futaba, S. Chiang (University of California, Davis) We are currently constructing a variable temperature atomic force microscope (AFM) that will operate from 4K to 400 K in ultrahigh vacuum (UHV). The entire AFM stage is cooled using a mechanical heat switch. Radiation shields at 4.2K and 77K have movable doors and surround the stage. Samples are prepared and cantilevers replaced in situ by transferring them to an adjacent surface analysis chamber, which is equipped with X-ray photoemission spectroscopy and low energy electron diffraction. Both piezoresistive and optically-detected cantilevers are supported and exchangeable with a pincer-grip wobble stick. The macroscopic approach is implemented with an inertial walker. The AFM is vibrationally isolated from the chamber by a double spring stage and magnetic eddy-current damping. Preliminary measurements using the instrument will be presented.
5:00 PM		NS-TuA-10 Experimental Test of Scanned Probe Tip Reconstruction S. Dongmo, J.S. Villarrubia, S.N. Jones, T.B. Renegar, M.T. Postek, J.F. Song (National Institute of Standards & Technology) Dimensional measurements from a scanned probe microscope image are complicated by distortions caused by the non-vanishing size of the probe tip. It is important to know the shape and size of the tip in order to accomplish a restoration of the sample surface. The tip shape may be deduced from images of "tip characterizer" specimens. We have previously shown that the tip geometry may in principle be reconstructed in a "blind" technique from the experimental image alone, without independent knowledge of the characterizer geometry. This method promises substantial advantages over methods requiring a known characterizer, but there has not yet been an experimental test of its efficacy. Such a test requires a comparison of the reconstructed tip shape to an actual shape given by some other reliable method (e.g. SEM). This is difficult at the small size scale of the usual STM and AFM tips, since other methods also have probe/specimen interaction issues at that scale. Accordingly, we have performed our initial tests by blind reconstruction of stylus tip shapes from profiler images of a random roughness specimen. At the size scales of diamond stylus tips (0.5 µm to 2 µm) SEM metrology issues are more manageable, allowing us to compare SEM profiles of the tip to the reconstruction results. They are in good agreement, even in the case of a double tip.