Radiant energy – Ion generation – Field ionization type
Reexamination Certificate
2001-04-04
2004-07-27
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Ion generation
Field ionization type
C250S424000
Reexamination Certificate
active
06768119
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the production of beams of ions for applications such as electrical propulsion or ion milling and writing, as well as to the production of beams of charged particles with nanometer sizes.
BACKGROUND OF THE INVENTION
Beams of charged particles in moderate or high vacuum are useful in many applications. Cluster beams of metals have been used for depositing thin films with special properties, exploiting the fact that the kinetic energy per cluster molecule may be controlled in the range below 1 eV. Closely related considerations have led to the use of beams of glycerol clusters for surface cleaning, such as in U.S. Pat. Nos. 5,796,111 and 6,033,484. Beams of charged colloids are useful also in electrical propulsion. Similarly, beams of atomic or molecular ions have been used in applications such as surface treatment (ion implantation), surface writing, etching or milling, high resolution lithography for microelectronic and other purposes. Also for sputtering and secondary ion mass spectrometry, and electrical propulsion among others. As discussed in U.S. Pat. No. 6,137,110, focused ion beams play a very useful role in the semiconductor industry by greatly reducing the time and costs associated to integrated circuit development and repair.
Ion beams may be generated in a variety of ways: by producing ions and extracting them from gas or plasma sources; from positively charged liquid metal surfaces by field evaporation from the tip of a Taylor cone; from solid surfaces, for instance by laser ablation ionization; etc. Charged beams of metal clusters have been produced by electron bombardment of neutral clusters formed by evaporation-condensation. Unfortunately, these methods are able to yield only relatively restricted classes of ionic species.
Gas sources may form ion beams only of gaseous or volatile substances.
Liquid metal ion sources (LMIS) have been described by P. D. Prewett and G. L. R. Mair in the book
Focused ion beams from LMIS
(Wiley, N.Y.; 1991). They are based on charging the surface of a molten metal held in a vacuum to a high voltage with respect to a neighboring extractor electrode. Within a suitable range of voltage differences, the meniscus of molten metal forms a so-called Taylor cone, whose sharp apex region emits predominantly metal ions. Such liquid metal ion sources (LMIS) have high brilliance and can be focused into submicron spots. But they can deliver only ions of metals and the few substances soluble in them, with exclusion of all negative ions. Liquid metal ion sources are generally operated in the low-current regime, where they emit predominantly individual metal ions. Although LMIS can also be run under conditions where larger ion clusters, nanometer drops and considerably larger drops are emitted together with neutral atoms and clusters, such regimes involve quite complex distributions of mass over charge ratios m/q, and are generally far less suitable than the pure ionic regime for most applications.
Laser sources involve high energy deposition on a surface. Although they may produce large and moderately charged clusters as well as small atomic and molecular species, they are unsuitable to generate beams of intact labile substances. Furthermore, in the regime where they would produce large clusters, they would have a wide distribution of charge over mass q/m and would be mixed with neutrals, being therefore unfit for applications such as electrical propulsion.
Existing ion beam sources are therefore limited to m/q values smaller than 200 Dalton, such as Cs
+
(133 Dalton) or Au
+
(197 Dalton) in liquid metal ion sources, or Xe
+
(131 Dalton) in plasma sources. They are also severely restricted in chemical composition, limiting the possibilities to combine chemical attack with purely physical erosion in applications such as etching. Hence, no chemically assisted focused ion beam analog exists to the etching processes commonly performed with the assistance of masks. Because chemical composition and mass/charge are key parameters determining the properties and breadth of applications of ion beams, there is a need to introduce new ion sources able to cover a much wider range of ionic chemical composition and m/q than previously available. To do so is one of the objectives of this invention.
Electrostatic atomization via Taylor cones of organic electrolytes (rather than liquid metals) has provided a related but different scheme to produce charged beams of small drops (rather than atomic ions) in a vacuum environment. In these systems, a liquid of low volatility and electrical conductivity K is supplied at a flow rate, Q (cm
3
/s) to the tip of a capillary tube. A voltage difference V=V
1
−V
2
is established between this tip and a neighbouring electrode, generally called the extractor. Within a suitable range of values of Q and V (where Q depends on K), the liquid meniscus takes the form of a cone, whose tip ejects a steady microjet, which in turn breaks into drops. This structure is often referred to as a cone-jet, following the classification discussed by M. Cloupeau in the Journal of Aerosol Science, Volume 25, pages 1143-1157, 1994. The spray of drops streaming from the end of the jet is generally referred to as an electrospray, and this terminology will be used here also. When the cone-jet forms in a vacuum environment, the charged drops produced are accelerated towards the extractor by the voltage difference V, and pass through an orifice in the extractor forming a relatively narrow beam. These beams of charged drops have proven useful in applications such as electrical propulsion and surface cleaning. This approach has been successful mainly with glycerol electrolytes, having high dielectric constant and a relatively small volatility. Furthermore, Taylor cones of glycerol held in vacuum in a multicone highly stressed regime under high voltage are capable also of producing ions of dissolved substances, and this feature has found applications in mass spectrometry, as explained by K. D. Cook in Mass Spectrometry Reviews, volume 5, pages 467-519, 1986. However, the electrospraying technique to produce beams of small drops and ions in vacuo has been limited by the relatively large size and associated large m/q of the drops, and by the relatively small concomitant ion currents. Also by the wide range of energies and solvation states in which the ions are produced in this highly stressed regime. In other words, Taylor cones of glycerol are ineffective as ion sources, with poor ion intensities, and wide energy and m/q distributions. As sources of drops, electrosprays of glycerol tend to produce drop diameters of several hundred nanometers, rather than the more desirable range of tens of nanometers. As a result, there is an inaccessible gap of many orders of magnitude in m/q space between values typical of ions (~100 Dalton), and those accessible with electrosprayed glycerol drops (~10
6
Dalton). This forbidden region includes the range of greatest practical interest for a number of applications, including electrical propulsion. Hence, a further object of this invention is to narrow down this m/q gap both on its upper and lower ends, by developing means to produce beams of much heavier ions as well as much smaller drops than previously possible.
It is known that the m/z ratio of electrospray drops can be decreased by reducing drop size, which in turn requires increasing the electrical conductivity K of the electrosprayed liquid. Furthermore, an article by J. J. Perel, et al. in the AIAA Journal, 7, 507-511 (1969) has revealed that Taylor cones of sulfuric acid with K values in excess of 10 S/m tend to yield dominantly ions rather than drops, in a regime presenting some analogies with the behaviour of liquid metal ion sources. Hence, those skilled in the art must have known for years that electrospraying solutions with electrical conductivities K larger than those available in glycerol electrolytes would have considerable advantages from the point of view of producing both ion
de la Mora Juan F.
Gamero-Castano Manuel
Martinez-Sanchez Manuel
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