Radiant energy – Inspection of solids or liquids by charged particles – Electron microscope type
Reexamination Certificate
1999-10-07
2001-10-09
Berman, Jack (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron microscope type
C250S310000, C250S442110, C250S307000
Reexamination Certificate
active
06300631
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of focused ion beam milling, and more particularly, this invention relates to a method of focused ion beam milling a cut electron transparent membrane used in transmission electron microscopy.
BACKGROUND OF THE INVENTION
Membranes used in transmission electron microscopy (TEM) are prepared by a focused ion beam (FIB) that are routinely used to prepare site specific TEM specimens of semiconductor devices. A site is prepared for milling with a focused ion beam and trenches are milled parallel to a 1-2 millimeter thick platinum (Pt) film that has been deposited on the surface. When the film is approximately 300 nanometers thick, the focused ion beam cuts are made to partially detach the section from the wafer. The wafer is tilted to 60°, exposing the base and sides of the film. The focused ion beam cuts are then made at the base and sides to produce a “flag on a pole” configuration. The final “release” cuts are usually made at a 0° tilt.
The wafer is usually removed from the focused ion beam and placed under an optical microscope having a long working distance objective lense and a micromanipulator with coarse and fine adjustments. A glass pipette is usually attached to the micromanipulator to remove the TEM film from the wafer. The pipette is usually positioned over the film and lowered to contact it. The electrostatic forces attract the film to the tip of the pipette, which is then raised from the wafer surface. The microscope stage is then moved to bring a TEM grid into view. The pipette with the dangling film is then lowered and the TEM film dropped on the grid.
A major drawback of this process is that once a sample is mounted on the TEM grid, it cannot be further thinned if the membrane is too thick for electron transmission. Thus, a second sample must be prepared, and if the sample is prepared from a specific site on the sample, such as the semiconductor, the specimen is lost. Some uses of an argon mill to further thin the mounted sample have been attempted, but the carbon film typically used on the grid is destroyed and the sample is then lost in the vacuum because it cannot be retained on the grid.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method of thinning a cut electron transparent membrane used in transmission electron microscopy that is mounted on a transmission electron microscope grid.
The method of the present invention allows the thinning of a cut electron transparent membrane used in transmission electron microscopy when it is mounted on the transmission electron microscope grid. The method comprises the step of cutting an electron transparent thin film membrane from a sample that is positioned within a focused ion beam device by focusing an ion beam in predetermined directions onto the sample to mill the sample and cut the electron transparent thin film membrane from the sample. The thin film membrane is lifted-out from the sample and then mounted onto a transmission electron microscope grid having a mesh surface and carbon coating. This grid is then mounted in a vertical orientation onto a support stage of a focused ion beam device such that the vertical orientation of the membrane is parallel with the incident focused ion beam produced by the focused ion beam device. The support stage is tilted a predetermined amount to orient the membrane at an angle incident to the focused ion beam. The focused ion beam is then focused at a glancing angle into a predetermined portion of the membrane for thinning the membrane by cutting a thin portion from the membrane.
In one aspect of the present invention, the method comprises the step of tilting the support stage such that the focused ion beam is about 5° to about 15° normal with a vertical orientation of the thin film membrane. The membrane can be mounted on the grid with the carbon coating face down opposite the membrane. The method can also comprise the step of mounting the membrane on a transmission electron microscope grid having a copper mesh surface. This grid can comprise a 400 mesh transmission electron microscope grid.
In still another aspect of the present invention, the method can comprise the step of focusing an ion beam at a lower voltage and current during the thinning step than the voltage and current used when milling the membranes from the sample, such that only the carbon coating adjacent the area that is milled is destroyed to aid in retaining the membrane to the transmission electron microscope grid. The step of milling the membrane in the sample can be at a current of about one microampere. The step of thinning the membrane after removal from the sample can be at a current of about 10 to about 100 picoamperes.
The method can also comprise the step of milling the membrane in the sample to a thickness of about 0.1 to about 0.2 micrometer thickness. The step of thinning the membrane can be about removing 100 nanometers of material. The step of lifting-out the electron transparent thin film membrane can comprise the step of lifting-out from the sample with a charged glass rod.
REFERENCES:
patent: 5270552 (1993-12-01), Ohnishi et al.
patent: 5783830 (1998-07-01), Hirose et al.
patent: 5799104 (1998-08-01), Nakamura et al.
patent: 5894058 (1999-04-01), Hatakeyama et al.
patent: 5986264 (1999-11-01), Grunewald
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Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Berman Jack
Fernandez Kalimah
Lucent Technologies - Inc.
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