S. B. McLane

Gas‐Surface Interactions and Field‐Ion Microscopy of Nonrefractory Metals

The performance of the helium field‐ion microscope depends critically upon accommodating He atoms of 0.15‐eV kinetic energy to the specimen tip. The small accommodation coefficient requires the field‐trapped He atom to make several hundred contacts with the cold tip surface. The hopping He atoms diffuse preferentially to tip regions where the high local field permits ionization before full accommodation is reached. Improved accommodation is achieved with the provision of an intermediate collision partner in the form of adsorbed neon or, preferably, hydrogen or deuterium. Now a high‐resolution He ion image is obtained at 70% of the field used before. As the addition of hydrogen promotes field evaporation, its partial pressure must be carefully controlled to achieve image stability of the nonrefractory metals. Low‐field evaporation by the hydrogen reaction permits easy conditioning of the tip surface of the nonrefractory transition metals so that artifacts caused by yielding to He evaporation field stress a...

Pulsed‐laser time‐of‐flight atom‐probe field ion microscope

A linear‐type pulsed‐laser time‐of‐flight atom probe has been developed. With a flight path length of ∼200 and ∼425 cm, the mass resolution is comparable to the energy focused time‐of‐flight atom probe of much more elaborate mechanical and electrical design. The isotopes of Mo, Xe, and W are completely separated to the roots of the mass lines. We also discuss how the mass resolution of the pulsed‐laser atom probe can be further improved and where this instrument will be useful.

Surface segregation of a Pt–Au alloy: An atom‐probe field ion microscope investigation

An absolute composition depth profile of a dilute gold alloy of platinum is obtained with a true atomic layer depth resolution. At 600±20 °C, the top {001} layer Au concentration is found to be as high as 99±20% for a sample with a bulk gold concentration of 4.1±0.7%. The Au concentration decreases monotonically into the bulk, with a characteristic depth of about three atomic layers.

Atom-probe fim investigation of surface segregation in Ni-Cu, stainless steel 410 and Pt-Au alloys

Abstract An absolute composition depth profile of a Ni-5%Cu alloy was obtained with a single atomic layer spacial resolution. The first layer Cu concentration is found to be 54.1 ± 4.7% on the (111) plane at 550 ± 50°C. The near surface layers are slightly depleted with Cu, and the Cu concentration returns to bulk value in about 5 atomic layers. A comparison with existing quasichemical theories of surface segregation is made. It is found that the regular solution model without surface relaxation does not give a close fit to our data. Although the ideal solution model gives a very good fit for the first layer Cu concentration, it does not account for the observed slight Cu depletion in the near surface layers. When surface relaxation of bond en thalpy is taken into consideration, a Cu depletion in the second atomic layer is predicted for an ideal solution model. Similarly a Cu depletion in the near surface layers is predicted for a regular solution model. For the ideal solutions case, a relaxation parameter δ = 0.022 gives the best fit while for the regular solution model, a δ = 0.17 gives the best fit. A comparison with results obtained from other macroscopic surface techniques on the same system are also made. The segregation of Cr in stainless steel 410 was also discussed. A binary regular solution model was found to fit the data better than an ideal solution model. However, it is also pointed out that a ternary alloy solution model is needed for a more accurate comparison. A preliminary study on Pt-5%Au indicates that Au segregates to the first atomic layer on the (111) plane.

Field Ion Microscopy with an External Image Intensifier

The exposure time of field ion micrographs at normalized pressure and voltage represents a figure of merit for comparison of various intensifier systems. External image intensification with a commercial RCA tube reduces the exposure time by a factor of more than 104, allowing recording of helium ion images in 1/1000 sec and standard speed motion pictures of neon ion images. Fast field evaporating metals can now be observed dynamically, and gold and copper photographs are shown as examples. The intrinsic noise of the image intensifier is not disturbing. A flicker effect is observed in the helium ion image of a stable tungsten surface at 21 °K in the presence of a small amount of water vapor. The fluctuations depend upon the local field strength; they disappear with increasing field, increasing temperature, or with the addition of a trace of neon to the imaging gas. The flicker effect is probably due to desorption of water molecules by the impact of imaging gas atoms.

Methods for a precision measurement of ionic masses and appearance energies using the pulsed‐laser time‐of‐flight atom probe

An attempt has been made to use the pulsed‐laser time‐of‐flight atom probe for a precision measurement of ionic masses and the critical energy deficits, or the appearance energies, of field emitted ions. For this purpose methods for determining accurately the flight‐path constant and the time‐delay constant of the atom probe have been devised. Using 300‐ps laser pulses and a digital electronic timer of 1‐ns resolution and a flight path of slightly over 4 m, ionic masses of pulsed‐laser field desorbed gaseous ions can be measured with an accuracy of better than 0.0005 to 0.003 amu for ions of low to intermediate masses. For multiply charged field evaporated heavy metal ions, the accuracy is not as good due mainly to the very limited number of ions which can be collected at a given low emitter voltage. The accuracy of measuring the critical energy deficit depends on the emitter voltage, and the mass and the charge of the ions. It ranges from ∼0.3 eV for singly charged, heavy gas ions at a low emitter voltag...

Interpretation of Pt3Co FIM images as investigated with an atom probe

Cobalt atoms in ordered Pt–Co alloys are not imaged in the field ion microscope, although in the lattice the Co atoms occupy sites equivalent to those of the Pt atoms. Using the atom‐probe FIM, we show that the invisibility of the Co atoms in Pt3Co is caused by selective field ionization of the image gas above Pt atoms only, rather than by the absence of Co atoms in the surface by preferential field evaporation.

ToF atom-probe fim study of LaB6 single crystals

Abstract The field evaporation mass spectra of LaB 6 are obtained with time-of-flight atom-probe field ion microscope in vacuum and in the presence of hydrogen, nitrogen and oxygen. The spectra are consistent with a preferential chemisorption of these gases on boron sites.

Aiming performance of the atom probe

We evaluate the aiming performance of our artifact‐free energy focused atom‐probe FIM employing bright contrast rhenium atoms in tungsten alloys as target atoms. The aiming efficiency averaged over the entire surface is 42% with the maximum of 60% on the (011) plane limited by the efficiency of the dual channel plate detector. We conclude that aiming through a 4.7×10−4 sr angular aperture is nearly perfect, and that aiming errors due to ion trajectory deviation as seen by multilayer field desorption patterns are minimal. Aiming capability is further established by the identification of bright impurity atoms in rhodium specimens as due to 0.1% platinum. Depth profiling of dilute constituents which do not provide visibility contrast is facilitated when the base metals such as V, Fe, Ni, W, or Pt have one or more low‐abundance natural isotopes which may be employed for counting and evaluating the statistical errors. This is demonstrated with the 192Pt isotope for the case of Pt–0.1% Mo.

Method for an accurate calibration of the flight‐time‐focused time‐of‐flight atom probe

A method is devised to determine accurately the flight‐path constant, the time‐delay constant, and the pulse factor for the flight‐time‐focused time‐of‐flight atom probe field ion microscope. Using these constants, masses of ions can be determined routinely to the accuracy of ∼0.005 to 0.06 u for ions of mass 1–100 u with our present system. This system has a flight‐path length of ∼250 cm. It is equipped with a time digital converter of 1‐ns time resolution and supporting measuring instruments of very modest specifications. The accuracy of the system can be further improved by having better supporting instruments. The principle of this calibration method can be easily adopted for calibrating other types of mass spectrometers.