Dept. of Applied Physics
Oregon Graduate Center
Liquid metal ion (LMI) sources are currently the subject of active investigation due to their potential application in microcircuit fabrication, focused beam technology, and surface analytical instruments. A LMI source is defined as a low volatility liquid metal film which flows to the apex of a solid needle support structure. Subsequent application of a high electric field deforms the liquid and results in ion emission. Considerable interest has been shown in development of LMI sources capable of producing a variety of technologically important ions. For implantation of silicon, for example, B is the preferred p-type dopant, while As and P are the preferred n-type dopants. It has been difficult to construct long-lived ion sources based upon these species because B possesses a high melting point and reacts strongly with most refractory metal supports, while As and P have high vapor pressures. Further, little is known about the surface behavior of high temperature liquid metal alloys. To overcome these difficulties, the material and thermochemical properties of liquid metal alloy surfaces have been studied. A number of successful contact systems have been identified for B, while the development of a LMI source of As has been completely solved. To lower the chemical reactivity of B alloys, it has been necessary to utilize nonmetallic support structures, but then problems with wettability arise because wetting is dependent upon sufficient chemical reaction between alloy and substrate. The wettability of B-based alloys to nonmetallic substrates is governed by surface segregation of low-level, low surface tension impurities within the alloys. At melting, the molten alloy surface possesses a large concentration of segregated material (e.g., C and N) which inhibits reaction between alloy and substrate. This results in a poorly-wetted droplet of alloy with a large contact angle. Coating the substrate with a material having a high affinity to carbon (e.g., B or Si) acts to tie up the segregated material and promote wetting. When purified in this way, the alloy subsequently wets the virgin substrate. Suppressing the high vapor pressure of As has been accomplished by constructing a liquid compound with a low (i.e., strongly negative) Gibbs free energy of formation. For a given arsenic compound, AsX[subscript n][superscript(1)], where X is a low volatility element, the equilibrium condition between vapor and liquid is AsX[subscript n][superscript(1)]= n X(g) + As(g). The equilibrium constant for this reaction is K = P (As) pn(X) and the Gibbs free energy is dG = -RT in K. When dG is strongly negative, K and P (As) will typically be smaller than the case of an ideal solution (no compound formation) of the same elemental constituents. Since the vapor pressure of As at the relatively low temperature of 600 C is nearly 1 atmosphere, it is necessary to lower the vapor pressure by over 10 orders of magnitude. This technique has been successful because the increased stability of the alloy results in a situation where it is energetically more favorable for As to exist in the liquid than in the gaseous state.
Bozack, Michael J., "Surface phenomena in liquid metal alloys with application to development of a liquid metal ion source of B and As [arsenic]" (1985). Scholar Archive. 72.