M.J. Vasile

Scanning probe tips formed by focused ion beams

Probe tips for scanning tunneling microscopy have been sharpened using focused ion beam milling. Reproducible tips were formed on polycrystalline W and Pt‐Ir shanks, but this technique is not limited to these materials. The tips were found to have cone angles of 12±3° and radii of curvature as sharp as 4 nm. Focused ion beam machining allows precise control of the final shape of the tips which is important in metrology measurements of various nanostructure devices.

Electron impact ionization cross sections of F2 and Cl2

Total ionization cross sections for the production of positive ions by electron impact ionization of F2 and Cl2 have been measured over the energy range 12 to 100 eV. The technique involves the ionization of a modulated molecular beam with cross sections determined by measurement of a calibrant gas. Mass spectrometric measurements insured that impurities did not contribute to the measured ion currents. The 70 eV ionization cross section of F2 was found to be 1.1×10−16 cm2, and the 70 eV ionization cross section of Cl2 was 7.1×10−16 cm2.

Ionic and neutral products of an RF discharge in methane

Abstract The production of ionic and neutral species from a 13.56 MHz discharge in methane has been monitored at pressures from 0.1 to 1 torr at flow rates which allow a relatively long residence time in the discharge chamber. The objective was to determine the significance of ionic species in glow discharge induced polymerization. The following were found in the discharge: H2+, H3+, CH3+, CH4+, CH5+, C2H2+, C2H4+, C2H5+, C2H6+, C3H3+, C3H5+, and C3H7+ and undifferentiated C4+, C5+, C6+, and C7+ ions; CH2, CH3, C2H3, C2H5, and C3H7 radicals; and H2, C2H2, C2H4, C2H6, C3H4, and C3H8 neutral compounds. Electron impact on methane is believed to produce the C1 ions and radicals, and these react with methane to produce the C2 ions and radicals, respectively. Known ion—molecule reactions can account for the production of all the C2 and C3 species, while the higher homologues can be generated by condensation reactions between C2 ions and C2 neutrals, and between C3 ions and C1 and C2 neutrals.

The chemistry of radiofrequency discharges: Acetylene and mixtures of acetylene with helium, argon and xenon

Abstract The acetylene discharge is characterized by the formation of a visible precipitate indicating a high rate of conversion to non-volatile products. The principal ionic condensation reaction can be summarized by the reaction sequence: C2Hx+ + C2H2 → C4Hy+ + C2H2 → C6Hz+ + C2H2, etc. Axially sampled ions are characteristic of more energetic ionization processes than radially sampled ions; the latter result more from ion-molecule condensation reactions than the former. Non-ionic gaseous products found were hydrogen and diacetylene. The ion chemistry closely parallels that observed by high pressure mass spectrometry. The degree of stabilization and subsequent break-down of chemical intermediates formed from an ion and an acetylene molecule largely determine the ion chemistry in the discharge. The greater the state of excitation of the reacting ion, the more unsaturated the product ion is likely to be. Discharges of mixtures of acetylene with helium, argon or xenon, respectively, show no evidence of the rare gas altering the chemistry by the exchange of potential energy from either long-lived metastable states or charge exchange. Only in the case of the axially sampled xenon-acetylene mixture is there any evidence of an “excess energy” reaction occurring with acetylene ions. Isotropic scattering from xenon allows a gain of about 6 eV in random kinetic energy from the oscillating electric field in the RF sheath. In argon the gain is only 3.5 eV. In helium the random energy gained from the RF field is much smaller and the trajectory of ions across the sheaths must be nearly linear. The ions sampled in either direction from a helium mixture are more representative of plasma products than sheath products. Thus helium behaves as a true diluent in that it does not participate measurably in the ion chemistry either electronically or kinetically.

Mass-spectrometric sampling of the ionic and neutral species present in different regions of an RF discharge in methane

Abstract The ionic and neutral products of RF discharges in methane produced by capacitive and inductive coupling have been examined. The configurations studied were (1) axial sampling through an internal RF capacitor electrode; (2) axial sampling through an electrically grounded internal capacitor electrode; (3) radial sampling of a capacitively coupled discharge through an electrically floating electrode; and (4) axial sampling of an inductively coupled discharge through a grounded electrode. Major differences in the ions observed occur between configuratons (1) and (3), above. In the former, the dominant ions found are C + , CH + , CH 2 + , CH 3 + C 2 H 2 + , and C 2 H 3 + , with very little abundance of ions containing more than three carbon atoms. In the latter, the ions CH 3 + , CH 5 + , C 2 H 3 + , and C 2 H 5 + are dominant, with a significant fraction of the total comprising C 4 to C 6 ions. The dissimilarity observed between these two cases clearly can be attributed to the difference in energy of the electrons which cause the ionization and the difference in kinetic energy of the primary ions which undergo ion—molecule reactions to yield secondary ions. Ions containing two or more carbon atoms have been shown to result from ion—molecule reactions rather than from primary ionization of C 2 neutrals. The neutral gas composition is also function of position in the discharge and is clearly related to the energy available at any particular sampling point.

Scanning probe metrology

The high resolution, three‐dimensional data provided by scanning probe microscopes makes them strong candidates for use in metrology. Many technological applications, such as integrated circuit metrology, require measurement precision to better than 10 nm. To achieve this level of performance, the nonlinear behavior of the piezoceramic actuators and of the probe–sample interaction must be carefully controlled. Positioning of the actuators can be addressed in a straightforward manner. The probe–sample interaction is, on the other hand, a difficult problem, which depends on the topography being measured.

Mass-spectrometric ion sampling from reactive plasmas I. Apparatus for argon and reactive discharges

An apparatus has been constructed to sample the ionic and neutral species that emerge from r.f. discharges in which chemical reactions are taking place. The results for pure argon show that the sheath potential (∼ 50 V) between the sampling orifice and the plasma is too large to be supported by the plasma electron temperature (1.9eV ≤ Te ≤ 2.3 eV, as determined by a floating double probe experiment). Most ions are sampled after at least one collision in the sheat. A 1 % mixture of vinyltrimethylsilane (VTMS) in argon yielded ions (ArH+, H3+, CH3+, C2H2+ and C2H3+) characteristic of elementary ion—molecule reactions or dissociative charge exchange rather than primary electron impact. The most probable sources of ArH+ are Ar+ + H2 → ArH+ + H H2+ + Ar → ArH+ + H Ar+ + VTMS → ArH+ + (VTMS—H). Reactions of VTMS in the r.f. discharge appear to be very efficient.

The monomeric character of xenon hexafluoride vapor. The mass spectroscopy of noble gas binary fluorides and xenon oxide tetrafluoride

Positive ion mass spectra have been obtained for KrF2, XeF2, XeF4, XeF6, and XeOF4 using 60 or 70 V electron impact ionization and a molecular beam source designed especially for reactive fluorides. Flight time distribution measurements have been made for neutral XeF6. Both these and the mass spectrometric fragmentation pattern indicate monomeric species in the saturated vapor. Associated (dimer) molecules can be obtained by isentropic expansion of XeF6; these are detected as dimer ions at one part in 104 of the monomeric ions in the mass spectrum. Electric deflection measurements on XeF6 vaporizing from inlets at temperatures from −19 to 40 °C showed no focusing effect.

Experimental tests of the steady‐state model for oxygen reactive ion etching of silicon‐containing polymers

A steady‐state model based on a silicon material balance has been proposed to predict the oxygen reactive‐ion‐etching resistance of organosilicon polymers. This model assumes that the rate determining step is sputtering of SiO2 film that forms on the surface of the organosilicon polymer. It predicts the etching rate of organosilicon polymers relative to the sputtering rate of SiO2 , based on the mass density of silicon in the polymer. The steady‐state etching rate of a silyl novolac polymer is accurately predicted by the model over a wide range of etching conditions. Silyl methacrylates etch at the predicted rate under high‐bombardment‐energy conditions typical of trilevel processing, but exceed the predicted rate under low‐bombardment‐energy conditions. Surface analysis shows that the SiO2 film thickness continues to increase with time under these conditions, invalidating the steady‐state approximation. Low silicon content (4.1 wt. %) polymers do not etch according to the model but form highly porous oxi...

The chemistry of the radiofrequency ethane discharge

Abstract The ionic and neutral species produced in an RF discharge in ethane have been examined by mass spectrometry. The most energetic processes occur in the sheath between the bulk plasma and the RF electrode. Primary reactions are observed in this region, while secondary and less energetically induced reactions are observed near the reactor walls. The major ions found near the RF electrode contain less than four carbon atoms and consist of CH3+, C2H2+, C2H3+, C2H5+, C3H2+, C3H3+, and C3H5+. The ions found near the wall contain as many as six carbon atoms but consist mainly of C2H4+, C2H5+, C3H3+, C3H5+, C3H7+, and C4H5+ through C4H9+. When sufficient energy is available to the electrons the major reaction channel for ethane is, C2H6 → C2H5 → C2H5+. When less energetic electrons are available, channel C2H6 → C2H4 → C2H4+ dominates. The ion chemistry observed in a given region of the discharge is consistent with the energetics of that region and known ion—molecule processes. The neutral composition is also dependent upon position in the discharge. The ratio of hydrogen produced to ethane consumed is a function of reaction conditions (increasing with increasing plasma density) but is always greatest in the vicinity of the RF electrode.