Focused ion beam system

Details Focused ion beam system Focused ion beam system

2001-10-17

2004-03-23

Lee, John R. (Department: 2881)

Radiant energy

Inspection of solids or liquids by charged particles

Positive ion probe or microscope type

C250S306000, C250S307000, C250S398000, C250S42300F, C250S424000, C250S3960ML, C250S492300

Reexamination Certificate

active

06710338

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The invention relates to FIB (Focused Ion Beam) systems and in particular to FIB optical columns.
BACKGROUND OF THE INVENTION
FIB systems, while once used primarily by highly skilled technicians in laboratories, are being used more and more in the high volume manufacturing of products such as semiconductors and disk drives. Most focused ion beam systems are still designed as multipurpose machines that are suitable for a broad range of applications. While such systems are very versatile, they are relatively expensive to manufacture. With the migration of FIB systems from the laboratory to the production floor, it is important to make FIB systems easier to use and to improve their performance. Improved performance means being able to process material more efficiently and increasing resolution, that is, being able to focus the beam to a smaller spot to perform finer operations.
Part of making FIB systems more suitable for production involves reducing the cost and size of the optical column used to generate and focus the ion beam. Minimizing the column optical length reduces ion interactions within the beam. These interactions increase the beam diameter and reduce resolution. Minimizing the column width facilitates combining a FIB in a vacuum chamber with a second beam, such as an electron beam for a scanning electron microscope (“SEM”). Such systems allow a work piece to be inspected using an electron beam after the work piece is machined using the focused ion beam. In such dual beam systems, the FIB column preferably penetrates deeply into the vacuum chamber and consequently should have a narrow profile to clear the SEM column and work piece.
A part of the ion column that includes the ion source, and often a first lens, is referred to as an “ion gun.” An ion gun typically includes an emitter, from which the ions are emitted, an extractor that provides a high voltage to assist in extracting ions from the emitter, an extractor aperture that helps to initially define the beam diameter, and a suppressor around the emitter that provides fine control of the emitter emission current. Because air molecules would interfere with the ions in the beam, the entire path of the ion beam is contained in a system vacuum chamber that maintains a high or an ultrahigh vacuum. The ion gun is often contained within its own separately sealable gun vacuum chamber so that the emitter will not be contaminated when the system vacuum chamber is opened to insert or remove a work piece. The mechanism for actuating the vacuum isolation valve that seals the gun vacuum chamber typically extends out of the system vacuum chamber.
An ion column also typically includes a beam aperture that is positioned after the ion gun and that further refines the beam diameter. Many FIB systems use an automatic variable aperture (AVA). An AVA typically includes a thin sheet of metal having multiple small holes of various sizes to form a line of apertures. A stage moves the aperture strip to position a hole of the desired diameter in the path of the beam. The mechanism for moving the aperture strip typically extends outside of the system vacuum chamber.
FIG. 1
is a partial cross-sectional drawing of a typical FIB column assembly
100
. The column uses an ultrahigh vacuum gun chamber
102
, which is composed of welded stainless steel surrounded by a magnetic shield
104
composed of a mumetal, that is, a metal that reduces transmission of magnetic fields into the column. Ion optical elements that carry a high voltage, such as a gun lens
110
, a final lens
112
, and deflector plates
114
are typically metallic elements that are supported and electrically isolated by complex alumina and machineable glass dielectric elements, such as a high voltage insulator
116
. The metal optical elements are typically screwed or brazed to the dielectric elements, but may also be glued to the dielectrics using, for example, an epoxy.
Electrical and mechanical connections, or “feedthroughs” from inside to outside high vacuum chamber
102
must be sealed so that air does not leak into the vacuum chamber through the feedthroughs. There are known techniques for use in ultrahigh vacuum (UHV) systems, in which brazed-metal-to-ceramic elements are employed for high-voltage isolation and vacuum sealing.
FIG. 1
shows high voltage electrical feedthroughs
120
.
FIG. 1
also shows mechanical feedthroughs for driving the automatic variable aperture and for positioning the emitter. The emitter is aligned within the gun chamber
102
by using four knobs
122
external to the gun chamber
102
, which are coupled through differential screws (not shown) to an emitter/suppressor assembly by vacuum bellows and high voltage isolation. Such an arrangement is complex and expensive. The automatic variable aperture is moved by a drive mechanism
124
that is outside of the system vacuum chamber and a mechanical connection extends from the external drive mechanism to the aperture plate itself.
An ion pump port
130
connects to an ion pump for creating the ultra high vacuum within vacuum chamber
102
, while a gun chamber vacuum isolation valve mechanism
132
allows vacuum chamber
102
to be vacuum isolated so that, for example, a system vacuum chamber in which the work piece is positioned and into which column
100
is inserted, can be opened to atmosphere without contaminating the elements in vacuum chamber
102
. Vacuum isolation valve mechanism
132
extends through a wall of the system vacuum chamber. Column assembly
100
is mounted onto a system vacuum chamber using mounting flange
140
.
In a conventional ion column design as described above, the overall, in-vacuum surface area is large. Because gases tend to adsorb onto surfaces in the vacuum chamber and the gas molecules desorb over time, a large surface area tends to increase the vacuum pumping requirements.
SUMMARY OF THE INVENTION
An object of the invention is to provide a high performance FIB optical column design of a less complex design.
In accordance with the present invention, a portion of an ion optical column is formed using a dielectric bushing to support metallic optical elements, electrically isolate them, and form a vacuum chamber around those elements. In particular, the dielectric bushing is suitable for forming an ion gun vacuum chamber in which are contained an emitter assembly and other optical elements, the gun vacuum chamber preferably being vacuum sealable separately from the system vacuum chamber.
In another aspect of the invention, a compact ion column includes, within the system vacuum chamber, an automated variable aperture drive mechanism and a gun chamber vacuum isolation valve activation mechanism. Including these mechanisms within the vacuum chamber facilitates the design of multi-beam systems by eliminating mechanical feedthroughs that would interfere with the placement of other components in the vacuum chamber.
It will be understood that the invention includes more than one novel aspect. Different embodiments can be constructed for different purposes using any one of, or combination of, the different aspects of the invention, and not all the advantages of the invention are, therefore, necessarily achieved by every embodiment that is within the scope of the attached claims.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.


REFERENCES:
patent: 4698236 (1987-10-01), Kellogg et al.
patent:

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