Organic field ionization source

[0002] This application is a continuation of U.S. patent application Ser. No. 09/386,976, filed Aug. 31, 1999, now U.S. Pat. No. 6,265,722.

[0003] The present invention relates to ion beam devices and, more particularly, to a field ionization source that is specially configured to minimize contamination of an ion beam target.

[0004] Liquid metal ion guns are used routinely to generate focused ion beams for analysis of a substrate. Liquid metal ion sources are typically very bright and are well-suited for applications requiring sharp focus of the ion beam and maintenance of a suitable ion fluence. Typically, the ion fluence of a liquid metal ion gun is of the same order as that of a field emission source of a conventional scanning electron microscope. However, liquid metal ion guns typically use gallium, indium, or gold.

[0005] The present invention embodies the recognition that gallium, indium, gold, and other conventional materials are not well suited for all ion source applications. For example, where an ion beam is to be used to analyze a semiconductor target, the use of gallium, indium, or gold as a sputter ion source material may lead to unacceptable contamination of the substrate. Accordingly, there is a need for a field ionization source that does not embody the disadvantages of the conventional types of ion sources, particularly in the context of focused ion beam defect analysis systems.

[0006] This need is met by the present invention wherein an organic field ionization source is provided and is arranged to be suitable for use in focused ion beam defect analysis systems.

[0007] In accordance with one embodiment of the present invention, an organic field ionization source is provided comprising: an ionization needle, an extraction electrode, a voltage source, and a heated reservoir. The ionization needle defines a tip. The extraction electrode defines an extraction aperture therein and is positioned such that the extraction aperture is disposed proximate the tip of the ionization needle. The voltage source is arranged to maintain the tip of the ionization needle at a high potential relative to the extraction electrode. The heated reservoir contains an organic ion source material therein in contact with the ionization needle. The heated reservoir is arranged to maintain a temperature of the organic ion source material at a magnitude sufficient to encourage capillary flow of the organic ion source material from the heated reservoir along the ionization needle to the tip of the needle. The high potential, the extraction electrode, the tip of the ionization needle, and the organic ion source material are selected and arranged such that the organic material is ionized at the tip of the needle and such that organic ions are drawn through the extraction aperture from the tip of the needle.

[0008] The organic ion source material preferably comprises coronene, phenylalanine, a vacuum grease, a diffusion pump oil, or another organic material with a boiling point of at least from about 250° C. to about 450° C.

[0009] In accordance with another embodiment of the present invention, a focused ion beam defect analysis system is provided comprising an organic field ionization source, a target, and an electrode assembly. The organic field ionization source comprises an ionization needle, an extraction electrode, a voltage source, and a heated reservoir. The ionization needle defines a tip. The extraction electrode defines an extraction aperture therein and is positioned such that the extraction aperture is disposed proximate the tip of the ionization needle. The voltage source is arranged to maintain the tip of the ionization needle at a high potential relative to the extraction electrode. The heated reservoir contains an organic ion source material therein in contact with the ionization needle and is arranged to maintain a temperature of the organic ion source material at a magnitude sufficient to encourage capillary flow of the organic ion source material from the heated reservoir along the ionization needle to the tip of the needle. The high potential, the extraction electrode, the tip of the ionization needle, and the organic ion source material are selected and arranged such that the organic material is ionized at the tip of the needle and such that organic ions are drawn through the extraction aperture from the tip of the needle. The target is subject to analysis by the organic ions and the electrode assembly is arranged along the path of the ions and is configured to direct the organic ions to the target.

[0010] The organic ions may comprise fragments of the organic ion source material and the electrode assembly may include a filter arranged to cause selected ones of the fragments to be directed away from the target. The filter may comprise a Wein filter, a magnetic sector, or a time-of-flight system. The electrode assembly may include an ion accelerating electrode, an ion beam focusing electrode, ion beam scanning electrode, and a ion beam deflecting electrode.

[0011] In accordance with yet another embodiment of the present invention, a focused ion beam defect analysis system is provided comprising an organic field ionization source, a target, an electrode assembly, and a particle analyzer. The organic field ionization source is arranged to ionize organic ion source material. The electrode assembly is arranged along the path of the ions and is configured to direct the organic ions to the target. The target is subject to analysis by the organic ions and the particle analyzer is arranged to detect particles ejected from the target in response to the incidence of the organic ions on the target.

[0012] Accordingly, it is an object of the present invention to provide a field ionization source that is specially configured to minimize contamination of an ion beam target so as to be suitable for use in focused ion beam defect analysis systems, focused ion beam milling systems, or other types of focused ion beam systems. Other objects of the present invention will be apparent in light of the description of the invention embodied herein.

[0013] The following detailed description of the preferred embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:

[0014] FIG. 1 is a schematic illustration of a focused ion beam defect analysis system including an organic field ionization source according to the present invention.

[0015] Referring now to FIG. 1, a focused ion beam defect analysis system 10 and an organic field ionization source 20 according to the present invention is illustrated in detail. As will be appreciated by those practicing the present invention, the organic field ionization source 20 is arranged to operate in a vacuum. The focused ion beam defect analysis system 10 comprises the organic field ionization source 20, a target 40 subject to analysis, an electrode assembly 50, and a particle analyzer 60. The organic field ionization source 20 includes an ionization needle 22 defining a tip 26. The ionization source 20 further includes an extraction electrode assembly 28 incorporating an extractor 30 and a suppressor 32, each defining an extraction aperture 34 therein. Additionally, the ionization source 20 includes a voltage source 36 and a heated reservoir 38 containing an organic ion source material therein.

[0016] The extraction electrode assembly 28 is positioned such that the extraction aperture 34 is disposed proximate the tip 26 of the ionization needle 22. The voltage source 36 is arranged to maintain the ionization needle 22 and, more particularly, the tip 26 of the ionization needle 22, at a high positive potential relative to the extraction electrode assembly 28.

[0017] The heated reservoir 38 contains the organic ion source material in contact with the ionization needle 22 and is arranged to maintain the temperature of the organic ion source material at a magnitude sufficient to encourage capillary flow of the organic ion source material from the heated reservoir 38 along the ionization needle 22 to the tip 26 of the needle 22. The magnitude of the high potential and the relative positions of the extraction electrode assembly 28, the extraction aperture 34 and the tip of the ionization needle 22 are selected such that the organic ion source material is ionized at the tip of 26 of the needle 22 and such that organic ions are drawn through the extraction aperture 34 from the tip 26 of the needle 22. The specific selection and arrangement of these elements and the specific operating parameters utilized according to the present invention will vary depending on the design preferences of those practicing the present invention and the nature of the organic ion source material utilized. It is noted that a suitable voltage source 36 will be capable of generating a potential difference of about 1000 V to about 50,000 V between the ionization needle 22 and the extraction electrode assembly 28. The spacing between the tip 26 of the needle 22 and the extraction aperture 34 is typically about 0.1 mm to about 5.0 mm. The temperature imposed upon the organic ion source material by the heated reservoir 38 is selected to be sufficient to place the material in a low viscosity liquid phase. It is contemplated by the present invention that suitable operating parameters may be determined without undue experimentation by those of ordinary skill in the art of focused ion beam generation. Any one of a number of commercially available reservoir and needle arrangements may be suitable for use in the present invention. For example, a suitable heated reservoir 38 and needle 22 arrangement is available as a two lens ion column from the FEI Company of Hillsboro, Oreg. under the product identifier 2LI.

[0018] The target 40 typically comprises a semiconductor substrate but may comprise any substrate suitable for analysis by organic ions. According to one embodiment of the present invention, the organic ions drawn through the extraction aperture 34 and focused by the electrode assembly 50 cause ejection of particles from the target 40. The particle analyzer 60 is arranged to detect the ejected particles and provide an output indicative of the nature of the ejected particles. This output is utilized to characterize and analyze the target 40.

[0019] The electrode assembly 50 is arranged along the path of the organic ions drawn through the extraction aperture 34 and is configured to direct the organic ions to the target 40. The specific nature and design of the electrode assembly 50 is beyond the scope of the present invention. However, it is noted that the design of the electrode assembly 50 may be derived or taken directly from existing focused ion beam system designs. In fact, it is noted that the organic field ionization source 20 may be employed in a commercially available focused ion beam system, such as a two lens ion column available from the FEI Company of Hillsboro, Oreg. under the product identifier the FEI Company of Hillsboro, Oreg. under the product identifier 2LI.

[0020] Typically, the organic ions drawn through the extraction aperture 34 comprise fragments of the organic ion source material. In which case, the electrode assembly 50 may include a filter, e.g., a Wein filter, a magnetic sector, or time-of-flight device, arranged to cause selected ones of the fragments to be directed away from the target 40. The electrode assembly 50 also typically includes an ion accelerating electrode, an ion beam focusing electrode, an ion beam scanning electrode, and ion beam deflecting electrode to enhance and direct the incidence of organic ions on the target 40.

[0021] Suitable organic ion source materials comprises coronene, phenylalanine, or any one of a number of vacuum greases or diffusion pump oils. Preferably, the organic ion source material comprises an organic material with a boiling point of at least about 250° C. to about 450° C. These types of organic materials are particularly well-suited for used in focused ion beam defect analysis systems because they are less likely that conventional liquid metal ion sources to cause deleterious contamination of the target under examination.

[0022] Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.