Development of Liquid‐Metal‐Ion Sources for Focused‐Ion‐Beam Applications

Abstract

Prototype liquid-metal-ion sources (LMIS) have been developed for focused-ion-beam (FIB) applications. The investigation has included commonly used ions such as Ga and In, and others such as B, P, and As, which are dopants in silicon semiconductors. With the addition of new results, the present paper reviews LMIS structure, favorable source materials, LMIS characteristics, and the FIB properties connected with LMIS characteristics.Liquid metal ion sources (LMIS) have attracted increasing interest due to their potential advantages in focusedion-beam (FIB) technology applications such as maskless ion implantation, microfabrication, and microscopies (1, 2). The variety of ion species has been greatly increased by using alloys, as well as pure elements, as the LMIS source material. Prototype LMIS were developed by the authors for ion species ranging from commonly used ions such as gallium (Ga) and indium (In), to others such as boron (B), phosphorus (P), and arsenic (As), which are dopants for silicon. Ion emission of the latter elements is difficult because of several problems associated both with the strong corrosive effect of B on the ion source metal electrode at high temperatures, and the high vapor pressure of P and As. This paper will review LMIS structure, favorable source materials, LMIS characteristics, and an FIB profile related to LMIS characteristics. LMIS StructureThe movable-needle type of LMIS (3) is shown schematically in Fig. 1. It has a ribbon heater with a small aperture to allow the needle emitter to pass through. A small amount of source material is placed on the heater and melted. The needle, which is supported so as to allow precise movement along its axis, is mechanically controlled from outside of the vacuum chamber during LMIS operation.The needle movement plays two important roles. First, before operating the fresh LMIS, the virgin needle is passed into the molten metal to achieve complete wetting. Then, it is pushed through the metal and toward the extractor, keeping the needle apex covered with the molten metal film. In addition to wetting, this approach is useful for recovering the accidentally broken metal film.Second, positioning the needle is useful for lessening ion current drift. Such drift is governed by the ratio of the source material loss at the needle apex to the material inflow along the needle from the source reservoir. This inflow is inversely proportional to several factors such as the viscosity of the molten material, the distance (L) from the heater to the needle apex, and the radius of the curvature at the needle apex (4). The source material loss at the needle apex, on the other hand, is approximately proportional to the total emission flux. Therefore, the L value should be optimized for each LMIS and its operating conditions. After the initial optimization, however, the needle is seldom moved during FIB operation. Due to its flexibility, this movable-needle type of LMIS is widely applicable for many combinations of source and needle materials.The movable-needle type o...

Transmission electron microscope specimen preparation of Zn powders using the focused ion beam lift-out technique

“…In the FIB, Ga is field evaporated from a liquid metal ion source (LMIS), [32] postionized, and accelerated down the column to a kinetic energy between 20 and 30 keV. In the FIB, Ga is field evaporated from a liquid metal ion source (LMIS), [32] postionized, and accelerated down the column to a kinetic energy between 20 and 30 keV.…”

Comprehensive study of focused ion beam induced lateral damage in silicon by scanning probe microscopy techniques

“…These include primary beam enlargement due to aberrations, 23 coulomb interaction between Ga ions inside the optical column, 12 or spreading due to non-zero transverse velocity which is due to the energy spread of the Ga ions and the Coulomb interaction between the Ga ions. 9,[19][20][21]24 These effects, contributing to the formation of the primary beam tails, are strongly linked to the FIB aperture or the focus quality of the beam. They can be already excluded as the main reason for the detected damage extension by the previously discussed experimental results about damage dependence on ion current and beam defocus.…”

“…In addition to the core part, the primary beams show always large tails, with a current density of 3 or more orders of magnitude lower than the maximum current density, and slowly decaying with the distance from the beam center 8 (quantified values for the tail extension are usually around 0.5 -1 µm). These tails originate mainly from both, the velocity or energy distribution of the ions due to space charge effects and related Coulomb interactions especially in the region close to the LMIS apex, 9,[19][20][21] and the focusing properties of the electro-optical column, which involves the introduction of spherical aberrations on the focused beam. 13 The exact shape and extension of these current profiles is varying with the focusing conditions and the beam apertures, but usually they are modeled by exponential functions.…”

“…These phenomena include, together with all the possible causes of primary beam tails formation, other particle interactions effects like interaction between Ga ions, or interactions involving residual gas particles in the FIB vacuum chamber like scattering of the ion beam and residual gas molecules. 7,9,21 Depending on the chamber pressure, these spreading effects are usually much more significant than the contribution of the beam tails, as they can lead to a broadening effect of several micrometers. 7…”