Liquid Metal Alloy Ion Sources Research Articles

Isotopic anomaly in dimer emission from alloy liquid-metal-ion-source mass spectroscopy.

In a pure copper liquid-metal-ion source (LMIS) the three ${\mathrm{Cu}}_{2}^{+}$ isotopes are emitted with intensities close to the natural abundances while for a ${\mathrm{Au}}_{0.5}$${\mathrm{Cu}}_{0.5}$ LMIS we only observe the $^{63}\mathrm{Cu}^{63}$${\mathrm{Cu}}^{+}$ and $^{65}\mathrm{Cu}^{65}$${\mathrm{Cu}}^{+}$ homoisotopic species. A similar phenomenon appears for the emission of ${\mathrm{Ge}}_{2}^{+}$ from pure germanium compared to a ${\mathrm{Au}}_{0.73}$${\mathrm{Ge}}_{0.27}$ alloy. We observe that the emission of the heteroisotopes is strongly reduced in the alloy case. We propose the following interpretation. In the electric-field zone close to the surface the two atoms of the heteroisotopic ion have different trajectories. As a consequence the molecular ${\mathit{M}}_{2}^{+}$ ion (M=Cu or Ge) is deformed and an electronic excitation appears which makes easier the tunneling of the outer electron from ${\mathit{M}}_{2}^{+}$ to the bulk available levels of the liquid metal or alloy. Moreover, the electronic structure of the alloy is such that the tunneling effect is easier than for the metal (for example, the work function is larger for ${\mathrm{Au}}_{0.5}$${\mathrm{Cu}}_{0.5}$ than for Cu). Then, the absence or reduction of the ${\mathrm{Cu}}_{2}^{+}$ or ${\mathrm{Ge}}_{2}^{+}$ heteroisotope intensities in alloy LMIS would be due to the conjunction of two effects: presence of an electronic excitation specific to heteroisotopes and easier tunneling effect for alloys. \textcopyright{} 1996 The American Physical Society.

Cluster ions ejected from an LiMg alloy liquid metal ion source: observation of Mg 2+2 and Mg 2+3

Ions ejected from a liquid metal ion source of an LiMg (10 atom %) alloy have been investigated by using a magnetic mass analyzer. In addition to singly charged homonuclear Li + n ( n ≤ 9) and Mg + n ( n ≤ 4) and heteronuclear Mg m Li + n ( m, n ≤ 2) clusters, doubly charged diatomic and triatomic Mg clusters are observed. Discussion is focused on the observability and the formation mechanism of the doubly charged small Mg clusters. A postionization process is suggested for the formation of the doubly charged clusters.

Mass and energy analyses of an AuSi liquid metal ion source

Mass and energy analyses of ions emitted from an AuSi eutectic alloy liquid metal ion source (LMIS) were carried out using a tandem double-focusing mass spectrometer. In the investigated ion current range between below 1 mu A and 10 mu A a great variety of molecules and clusters were detected. The field strength at the apex of the Taylor cone was estimated from post-ionisation probability to be 20 V nm-1 for silicon ions and 30 V nm-1 for gold ions at 10 mu A emission current. The energy spread of different ion species depends on the charge-to-mass ratio and on the total extracted ion current. For the ion current dependence of the energy spread a nearly two thirds power dependence was obtained. The energy width varies between 10 eV and 100 eV for Au ion species and between 10 eV and 20 eV for Si ion species. The energy deficit decreases with increasing ion current for Au and Si ion species. The authors assume a decrease of the field-dependent activation energy of field evaporation together with growing asymmetry in the energy distributions.

High-field ion sources

High electric fields may be used to create ions either by field evaporation, as in the case of liquid metal ion sources (LMISs) or by field ionization, as in the case of gas-phase field ionization sources (GFISs). LMISs have now reached a mature state of development and have found an increasing number of focused beam applications, ranging from submicron mask repair to maskless ion implantation. They may be fabricated from either pure elements, or from alloys, provided that the element or alloy vapor pressure is low enough to avoid excessive evaporation. In the case of alloys, it is necessary to employ a mass filter, typically a Wien filter, to remove unwanted ion species from the ion beam. Angular intensities, at the threshold of source operation, range from 1 μA/cm2 for alloy LMIS ion species to 20 μA/cm2 for elemental LMISs; under the same operating conditions, the ion energy spreads are ∼5 eV. Recent advances in the development of GFISs raise the possibility that GFISs may be nearing the stage when they also might be used in fine focus applications. By special shaping of the metal emitter in the apex region and by cooling the emitter to ∼20 K; it has been found possible to cause confinement of the ion emission to ∼0.5° half-angle, thereby increasing angular intensities of beams of He+ or H+2 to values similar to those obtained from LMISs.

Mass spectra of AuGe alloy liquid metal ion sources

Measurements are reported on the mass spectrum of a AuGe alloy liquid metal ion source with various alloy compositions and values of total source current. To obtain a high fraction of Ge ions, it is advantageous to use an alloy composition with a higher germanium content than that of the eutectic composition. It is shown that the fraction of Ge 2+ ions varies within a small range depending on the total source current, whereas the Ge + fraction increases with increasing current. A strong isotopic effect for the Ge + 2 fraction could be observed which may be explained by charge transfer reactions. The temperature range of the coexistence of the solid and liquid phase of the investigated alloys was determined by the method of differential thermal analysis (DTA).

Focused ion beam system with a four stage E× B mass filter

A focused ion beam (FIB) optical system which incorporates a four stage E× B mass filter has been developed for an alloy liquid metal ion source (LMIS). The mass filter is designed to have a high mass resolution ( M ΔM ) and to be achromatic. Its preliminary operation is demonstrated using a NiAs alloy LMIS. From the spectrum of Ni 2+ isotopes, the mass filter is confirmed to have an M ΔM value of over 63 up to an accelerating voltage of 100 kV. A scanning ion muscope image taken under mass filtering shows a 0.1–0.2 μm image resolution.

Energy distributions of liquid metal alloy ion sources

The energy distributions of liquid metal ion sources using Au–Si–Be, Pd–Ni–Si–Be–B, Al–Si, and Pt–Si alloys have been measured. Profiles of singly charged ion species which are strongly dependent on alloy systems typically exhibit an abrupt rise at the high-energy position and gradual decay at the low-energy position in the profiles. This gradual decay, which appears rather like a subsidiary shoulder, may be caused by free space field ionization in addition to field evaporation. Profiles also have long tails at both the high- and low-energy positions. These tails can be explained by charge transfer collision between ions and neutral atoms coming from the outer space. The intensity of the tails is discussed on the basis of the charge transfer collision process.

Mass spectra of Au-Si alloy liquid metal ion sources

Measurements are reported on the mass spectrum of an Au-Si alloy liquid metal ion source with various alloy compositions and values of total source current. To obtain a high fraction of Si ions, it is best to use an alloy composition with a higher silicon content than that of the eutectic composition. It is shown that the fraction of Si2+ ions is independent of the total source current, whereas the Si+ fraction increases with increasing current. The eutectic composition and the temperature of the solidus line were determined to be (18.0+or-0.5) atomic per cent and (362+or-4) degrees C, respectively.

Ion formation in alloy liquid-metal-ion sources

Mass and energy analyses are carried out for ions emitted from liquid-metal-ion sources (LMISs) using Cu-P base, Pt-P, and Ni-B-Si alloys. A strong matrix effect of about 10−1 and 1 is observed on the intensity ratio of P++/P+ for Cu-P and Pt-P alloys, respectively. The electric field at the ion emitting surface is estimated to be 27–30 V/nm from the post-ionization model. A difference of only 10% in the field strength between these alloys is responsible for this matrix effect. The Ni-B-Si alloy LMIS, on the other hand, does not present an identical field strength for each component. The relationships of [ΔE(M++)/2≤ΔE(M+) <ΔE(M++)] for the energy width and [Ep(M+)≥Ep(M++)/2] for their most probable energy are observed in the energy analysis of mass-analyzed ions. Most M++ ions are formed in the post-ionization process on field-evaporated M+ ions.

Submicrometer FET gate fabrication using resistless and focused ion beam techniques

We report using a focused ion beam lithography, without the use of organic resists, to produce submicrometer gates. Refractory metal gates of WTi, with a minimum length near 0.1 μm, and p-type polysilicon gates, with lengths down to 0.5 μm, were fabricated. The gate lithography utilizes focused beams of Si++ or B+ ions produced from Au–Si or B–Pt alloy liquid metal ion sources and a three-lens mass-separating focusing column. We found that using proper implant and subsequent etch conditions causes the gate material to behave as a negative resist thus enabling gate definition. NMOS FET’s were fabricated with the WTi and p-type Si gates using these techniques and these devices had gate oxide thicknesses of 38 and ∼13 nm. The gate lengths varied between 2.0 and 0.15 μm. Transconductances as large as 180 mS/mm were obtained.

100 keV focused ion beam system with a E×B mass filter for maskless ion implantation

Various liquid metal alloy ion sources and a 100 keV mass separated focused ion beam system have been fabricated and their basic characteristics have been measured. These are mass spectra, energy spread and angular current intensity for ion sources, and focusing characteristics of the system. It was observed that Be–Si–Au ternary alloy ion sources produce doubly charged Be and Si ions and the importance of these ion sources is demonstrated by fabricating a GaAs JFET using a maskless ion implantation technique and by PMMA resist exposure.

Liquid metal alloy ion sources for B, Sb, and Si

Liquid metal alloy ion sources for B, Sb, and Si

B, Sb and Si Liquid Metal Alloy Ion Sources for Submicron Fabrication

B, Sb and Si liquid metal alloy ion sources have been fabricated and basic characteristics such as current-voltage relations, angular current intensity, energy distribution and mass spectra have been measured. Characteristics of ion beam focusing systems are also discussed. It was observed that the energy distribution width (FWHM) of various ion species obtained for Sb alloy sources decrease with increasing the charge to mass ratio of the ion beam. It was also observed that the mass spectra varied slightly with a total ion source current. These ion sources could be operated for more than 10 h without any change in mass spectra.

Liquid metal alloy ion sources for B, Sb, and Si

B, Sb, and Si ion sources using liquid metal alloys have been fabricated and basic characteristics such as current–voltage relations, angular current intensity, energy distribution, and mass spectra have been measured. Singly charged B ions which amount to 33% of the total ions and have an angular current intensity of ∠40 μA/sr at a total energy spread (FWHM) of ∠20 eV were obtained using B–Ni–Pt alloys. Singly charged Si and Sb ions with an angular current intensity of ∠11 μA/sr and ∠1.4 μA/sr at an energy spread of ∠20 eV were obtained by using Si–Au and Sb–Pb–Au alloys, respectively. These ion sources could be operated for more than 10 h without any changes in mass spectra.

FET fabrication using maskless ion implantation

Focused ion beams from Au–Si, Au–Be, and B–Pt liquid–metal–alloy ion sources have been used to implant GaAs and Si. An Al stopping layer on the wafers was used to trap the Au and Pt ions. Hall mobilities consistent with those in bulk materials have been obtained for B-doped Si and Be-doped GaAs. In addition, a 2000-Å-diam Au–Si focused ion beam was used to implant the doped regions of GaAs metal-semiconductor gate field-effect transistors. The 140-keV Si++ beam component was deflected under computer control to implant 8×50 μm active channel regions and 16×50 μm contact regions. The devices were metalized using conventional lithography. DC electrical characteristics of the 1.5-μm-gate-length devices are comparable to those of conventionally processed devices of identical geometry.