Customized Instrumentation

The apparatus consists of an ultrahigh vacuum (UHV) chamber equipped with a sample manipulator, electron spectroscopies for low-energy electron diffraction (LEED)/Auger analysis, an ion sputtering gun for cleaning, electrostatic optics for ion transport, a pulsed molecular beam source with two stages of differentially pumping, a state-selective laser ionization source, and particle detectors with mass and velocity resolution.

A 20 Hz Continuum 682-20/TDL 60 Nd:YAG-pumped dye laser system combined with a UVT-2 autotracker provides tunable UV photons in 6 ns pulses. When the laser is focussed into the molecular beam, n + 1 resonance enhanced multiphoton ionization (REMPI) produces molecular ions in virtually a single rovibrational quantum state. Moreover, plane-polarized laser light can produce an aligned distribution of molecules.

The ions are accelerated by electrostatic lenses and mass-selected with a Wien filter. Just prior to impact with the surface, the ions are decelerated through a Menzinger lens. Because the ions were formed by a 6 ns laser pulse, they arrive at the surface as a spatially and temporally localized "ion packet". Incident and scattered ions are detected by a specially designed Rotatable Two-Dimensional Velocity Image Detector.

The Rotatable Two-dimensional Velocity Image Detector (a.k.a. the "Flyswatter") consists of two parallel plates between which the ions traverse, both on their way toward the surface and again after scattering from the surface. When both plates are at ground potential, the ions pass undeflected through this region; however, at a predetermined delay relative to the laser shot, the repeller plate is pulsed with approximately 1000 V. This imparts an enormous acceleration to the ions in a direction toward the opposite grid plate. The ions pass through the grounded grid plate, after which, they accelerate into a time-of-flight drift tube. As the ions emerge from the drift tube, they are again accelerated into CEMA plates and the resulting secondary electrons strike a phosphor screen. The CEMA plates are pulsed in order to provide mass discrimination. That is, light ions move rapidly down the time-of-flight tube and arrive first at the CEMA plates. A CCD camera records the screen's image, which represents the spatial distribution of the ions at a given point in time. This in turn is converted into a two-dimensional velocity distribution for the ions lying in the scattering plane defined by the pulsed repeller and grounded grid. The entire detector assembly can be rotated throughout the scattering plane, but is normally positioned between the deceleration lens and the surface.

For more information, see:

• Corr, D. and Jacobs, D. C. (1992) "An Ion Detector for Imaging Two- Dimensional Velocity Distributions," Review of Scientific Instruments 63, 1969-1972.
• Martin, J. S., Greeley, J. N., Morris, J. R., Feranchak, B. T., and Jacobs, D. C. (1994) "Scattering state-selected NO⁺ on GaAs(110): The Effect of Translational and Vibrational Energy on NO^- and O^- product formation," Journal of Chemical Physics 100, 6791-6812.
• Maazouz, M., Morris, J. R., and Jacobs, D. C. (2000) "Ion Imaging in Surface Scattering" in Imaging in Chemical Dynamics, edited by A. Suits and R. Continetti, American Chemical Society, 139-150.

The apparatus features a UHV surface analysis chamber coupled to an intense, continuous ion beam source. The resulting high ion beam exposures allow for mass-selective detection of scattered neutrals and ions as well as observation of ion-induced modifications of the surface. The apparatus consists of three differentially-pumped chambers: the source and buffer chambers contain the ion transport optics, and the main scattering chamber houses the surface sample. This design allows the production of incident ion beam currents up to 100 nA, while maintaining a UHV environment (~1E-10 Torr) in the main chamber. The main chamber is further broken into two sections:

• The top half contains routine surface science analysis equipment: Low Energy Electron Diffraction (LEED), Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS), Residual Gas Analysis (RGA), a Kelvin Probe for work function experiments, a sputtering gun for cleaning of the surface, and a load lock for rapid sample transfer.
• The scattering experiments occur in the bottom half of the chamber. Here, we have designed and constructed a novel, triple-differential quadrupole mass spectrometer (QMS). The independently pumped, rotatable detector can detect neutrals and ions of both polarities. It simultaneously resolves the scattering angle, energy, and mass of all scattered products. Another distinguishing characteristic of this chamber's design is that after prolonged ion beam exposure, the sample can be characterized by XPS and spatially-resolved AES to monitor ion beam modification of the surface sample.

For more information, see:

• Quinteros, C. L., Tzvetkov, T. and Jacobs, D. C., (2000) " Eley-Rideal reaction of O⁺ with oxidized Si(100)," Journal of Chemical Physics 113, 5119-5122.
• Quinteros, C. L., Tzvetkov, T., Qin, X., and Jacobs, D. C. (2001) "Reactive Scattering of O⁺ on oxidized Si(001) " Nuclear Instruments and Methods in Physics Research B, 182:187-192.

The deposition chamber features a load lock for rapidly introducing samples into the system. At the far left of the chamber is a z-stage manipulator, which translates the sample on a guide tube. All electrical feedthroughs make contact with the sample through wires running down the bore of the guide tube. The left chamber is equipped for the deposition of insulators using a magnetron-sputtering source. The plasma will be contained within a stainless steel sheath to reduce contamination in the chamber. The left chamber is pumped with a Leybold turbomolecular pump to permit oil-free vacuum with fast pumpdown times.

The left and right chambers are separated by a pneumatic gate valve. The right chamber is independently pumped by a cryopump and is used for evaporating metals from an electron gun evaporation source. The e-gun provides for very efficient evaporation of metals (e.g., Ag, Au, Cu, and Al) at rates approaching 1500 Å per minute. A computer-controlled mask situated between the evaporation source and the substrate allows for precise patterning of metal layers in the fabrication of MIM devices. In both sputter deposition and metal evaporation modes, a quartz microbalance detector provides absolute calibration of the film thickness. The right half of the chamber also contains some simple diagnostics for testing the MIM devices. These include a Faraday cup collector and a retarding field energy analyzer for measuring the emitted electron current and energy distribution, respectively.

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