ENHANCED-WETTING, BORON-BASED LIQUID-METAL ION SOURCE AND METHOD

Abstract: A binary, boron-based alloy as a source for field-emission-type, ion-beam generating devices, wherein boron predominates in the alloy, preferably with a presence of about 60 atomic percent. The other constituent in the alloy is selected from the group of elements consisting of nickel, palladium and platinum. Predominance of boron in these alloys, during operation, promotes combining of boron with trace impurities of carbon in the alloys to form B4C and thus to promote wetting of an associated carbon support substrate.

Description: ENHANCED-WETTING, BORON-BASED LIQUID-METAL ION SOURCE AND METHOD Background and Summary of the Invention This invention pertains to a high-wetting-capability, liquid-metal ion source for use in field-emission-type, ionbeam generating devices employing carbon (graphite) substrates, and to a substrate wetting method employing the source. More particularly, it relates to such a source which takes the form of a binary alloy including a predominance of boron, which alloy employs a sufficient enrichment of boron to promote significant substrate wetting. In the recent past, there has been an extensive and increasing interest in the employment of so-called focused ion beams. Such beams are used for various purposes, such as for micromachining of certain materials, and for implantation of ions in the making of semiconductor devices. One of many elements which offers an important source of ions for such purposes is boron. However, implementing a successful focused beam of boron ions has presented a number of problems. For example, boron-based ion sources are extremely corrosive to conventional, metallic, point-source support substrates which are employed in ion-beam generating devices. As a consequence, they yield an operational situation in which practical substrate lifetime expectancy is extremely short -like just a few hours. Carbon, typically in the form of graphite, offers a "material" solution to the corrosion problem when it is employed as such a support substrate, but frustration has existed over the poor ability of known boronbased ion sources to wet such a substrate well enough to function properly as a practical source. A general object of the present invention, in this setting, is to obviate these difficulties and frustrations of the past by offering a unique, enriched boron, boron-based source, and a related method, which can be used successfully with a carbon substrate, with the "wetting difficulty" substantially resolved. These and other advantages which are offered and attained by the invention will become more apparent as one reads the description which now follows. Detailed Description of the Invention Acknowledging the difficulty of employing a boronbased metal ion source, vis-a-vis corrosion of conventional metallic substrates in ion-beam generating devices, the present invention focuses attention on resolving the difficulty of employing such a source with a carbon (graphite) substrate which resists corrosion, but which also resists, in the case of many conventional source materials, the necessary wetting. The invention, in essence, is founded on the discovery that the non-wetting culprit takes the form of free carbon impurities which exist in the various, usual, boronbased alloys that are employed for liquid-metal ion sources. More particularly, the anti-wetting difficulty appears to spring from the fact that free carbon, the impurity carbon, in conventional alloy materials forms a resistive outer shell over the source material when it is in its plural-phase (liquid/solid) state, which shell tends to cause the source material to remain as a non-spreading, non-wetting bead. Surprisingly, we have discovered that it is possible, using only a binary alloy, and without resulting to substrate pretreatment techniques, as by boron powder coating, to disable the anti-wetting tendency of free carbon by employing an enriched supply of boron in the alloy to combine with, and thus capture, this carbon in the form of the compound B4C. This discovery has led to the development of three new and very successful ion-source alloys, which are binary in nature, and which combine nickel, palladium and platinum, selectively, with boron. More particularly, in each binary alloy, boron forms a predominant member of the alloy -- having at least more than a 50-atomic-percentage presence. While boron concentrations in these alloys as low as about 51% display a significant ability to combine with carbon in the form of B4C, the most preferred forms of the alloys are those in which boron is present with about a 60-atomicpercentage presence. The mechanism which occurs during use of these alloys in a focused ion beam operation, following application of the respective alloys to a graphite substrate, and heating of the applied alloy to a plural-phase (liquid/solid) state, involves the combining of excess boron with the free carbon impurities to capture these impurities in the form of B4C. This creates an alloy condition which promotes spreading of the alloy, and wetting of the substrate, so that the heated source can function well as an ion-beam source. As was mentioned above, while boron content in excess of 50-atomic-percent offers significant improvement, when the boron content centers at around 60-atomic-percent, the best results seem to be achieved. Accordingly, while a preferred embodiment of, and a method of practicing, the invention have been described, variations and modifications apparent to those skilled in the art may be employed without departing from the spirit of the invention.

Claims: IT IS CLAIMED AND DESIRED TO SECURE AS LETTERS PATENT: 1. A boron(B)-based liquid-metal ion source for use in field-emission-type, ion-beam generating devices, said source comprising a binary alloy with the formulation (X)a(B)b, wherein a equals less than 50-atomic-percent, b equals (100 - a)-atomic- percent, and X is selected from the group of elements consisting of nickel, palladium and platinum. 2. The source of claim 1, wherein b is preferably about 60-atomic-percent. 3. In combination with a field-emission-type, ionbeam, carbon-support substrate, a boron(B)-based liquid-metal ion source comprising a binary alloy with the formulation (X) a (B)b, wherein a equals less than 50-atomic-percent, b equals (100 - ¯)-atomic-percent, and X is selected from the group of elements consisting of nickel, palladium and platinum. 4. The combination of claim 3, wherein b is preferably about 60-atomic-percent. 5. A carbon-capturing method of promoting wetting of a field-emission-type, ion-beam, carbon substrate by a boron-based, metal ion source which includes trace impurities of carbon, said method comprising applying to such a substrate a binary metal alloy including a predominance of boron, heating the substrate-applied alloy to place it in a heated, plural-phase state, during said heating, and with the alloy in such a state, capturing trace impurities of carbon in the form of B4C and, by said capturing and B4C forming, creating a condition in the alloy which enhances its ability to spread and wet the substrate. 6. The method of claim 5, wherein the alloy takes the form of boron combined with an element selected from the group of elements consisting of nickel, palladium and platinum. 7. The method of claims 5 or 6, wherein boron makes up preferably about 60 atomic percent of the alloy.

Inventors: BELL ANTHONY E, BOZACK MICHAEL J, CLARK WILLIAM M JR, STORMS EDMUND K, SWANSON LYNWOOD W, UTLAUT MARK W, BELL, ANTHONY, E., BOZACK, MICHAEL, J., CLARK, WILLIAM, M., JR., STORMS, EDMUND, K., SWANSON, LYNWOOD, W., UTLAUT, MARK, W.
Applicants: BELL ANTHONY E, BOZACK MICHAEL J, CLARK WILLIAM M JR, HUGHES AIRCRAFT CO, OREGON GRADUATE CENTER, SWANSON LYNWOOD W, UTLAUT MARK W, BELL, ANTHONY, E., BOZACK, MICHAEL, J., CLARK, WILLIAM, M., JR., HUGHES AIRCRAFT COMPANY, SWANSON, LYNWOOD, W., UTLAUT, MARK, W.
Application year: 1988
Main classification: H01J27
IPC codes: H01J27, H01J37