This invention relates to a method and system for the preparation of specimens for analysis by electron microscopy, and more particularly to a method and system for the milling of specimens using an ion beam.
Ion beam milling systems are used for the preparation of specimens whose internal and surface structures are then analyzed using transmission electron microscopy (TEM) or scanning electron microscopy (SEM). Many techniques have been developed which have included the use of mechanical cutting, grinding, and/or polishing operations in combination with ion beam bombardment. In many instances, such techniques have required that the specimen be repeatedly moved from one apparatus to another (either different preparation devices or observation devices) and/or be moved or realigned while mounted in a cuffing, grinding, or milling device.
Transmission electron microscopy is an important technique for studying the detailed microstructure of many materials. Improvements in the design and operation of electron microscopes have generated considerable interest in obtaining atomic resolution images of various materials. However, the preparation of specimens for atomic resolution transmission electron microscopy is demanding, requiring a final specimen which is very thin (i.e.,  less than 50 nm) and free from artifacts. Typically, specimen preparation involves initial slicing, sectioning, trepanning, and/or grinding operations to produce a relatively thin (100-200 xcexcm) disk of approximately 3 mm diameter.
Ion beam milling systems have been used to prepare specimens of various materials including ceramics, semiconductors, metals, and combinations thereof for atomic resolution transmission electron microscopy. In such ion beam milling systems, such as the system disclosed in commonly-assigned U.S. Pat. No. 5,009,743, to Swann, specimens are mounted on holders and placed in the path of one or more ion beams. The ion beams gradually remove atoms from the surface of the specimen until a small perforation is formed in the center of the specimen. Generally, the area immediately around the perforation is then thin enough (i.e.,  less than 50 nm) for atomic resolution analysis by a transmission electron microscope.
Swann et al, U.S. Pat. No. 5,472,566, provides a specimen holder adapted to permit the simultaneous two-sided ion beam milling of a specimen at very low angles of beam incidence, down to 0xc2x0, from both sides of the specimen through the use of one or more support arms to secure the specimen. This produces both rapid milling as well as reducing artifacts to provide high quality specimens for transmission electron microscopy analysis.
In such ion beam milling systems, the ion milled specimens have to be removed from the specimen holders and loaded in a TEM holder for imaging. Clearly, loading and unloading of the specimen from the holders increases the potential of damage to fragile specimens. Additionally, if the specimen is not completely electron transparent upon initial TEM examination, it must be returned to the ion mill for further thinning. Multiple milling steps may be required, depending upon thickness requirements at the area of interest in the specimen.
Slope cutting by directing an ion beam at a masking screen located at or near a specimen surface is also known. The masking screen protects a portion of the specimen, while the remainder of the specimen is milled by the ion beam to reveal desired profiles. This technique is capable of providing cross-sectional profiles of layered structures, the surfaces of which can then be studied by scanning electron microscopy. For example, Hauffe, East German published Patent Application No. 201,538, teaches an apparatus and method for preparing specimens for SEM analysis by providing an ion gun inside the specimen chamber of a scanning electron microscope. A mask is positioned adjacent the sample and is used, in conjunction with the ion gun, to remove material from the specimen.
Double slope cutting to produce a very thin film of a specimen for TEM analysis has also been developed. Yoshioka et al, U.S. Pat. No. 5,907,157, describe a method and apparatus for preparing a specimen adapted for electron microscopy which includes an evacuated specimen-processing chamber. A specimen having a surface to be processed is placed inside the processing chamber and a beam-blocking member is placed close to the processed surface so as to block a part of an etching beam. A first etching step is performed by directing the beam at the specimen via the blocking member. Then, the specimen and the blocking member are moved relative to each other to expose another portion of the specimen. A second etching step is then performed by directing the beam at the specimen via the blocking member. As a result, the specimen is thinned and can be observed with an electron microscope. However, the requirement that the specimen and blocking member be moved relative to one another to perform the second etching step complicates the process and introduces the potential for errors as the relative positions of the two must be controlled to within an accuracy on the order of a few hundred nanometers (10xe2x88x929 meters) or less.
Mitro et al, U.S. Pat. No. 5,922,179, teaches an apparatus and process for etching and coating specimens in a single vacuum chamber, to minimize handling and transfer. The apparatus includes a sealed chamber and a vacuum pump for forming and maintaining a vacuum in the chamber, a first ion gun positioned in the chamber to etch a specimen, a sputtering target in the chamber, and at least one additional ion gun positioned in the chamber to cause material from the target to be directed onto the specimen.
Techniques have also been developed in the preparation of specimens for scanning electron microscopy in which the specimen may be observed by a SEM during preparation thereof. For example, Grxc3xcnewald, U.S. Pat. No. 5,986,264, teaches an ion beam milling system using two ion guns for use in the preparation of samples to be used in both SEM and TEM analysis. The system uses an SEM as a high resolution imaging device to observe the progress of the ion milling of a sample and to determine when a proper sample thickness has been achieved. Also, as the sample stage can be tilted, the system is also taught to be useful for slope cutting of specimens.
The inadequacy in localized thinning of TEM specimens with broad ion beams, coupled with the ability of focused ion beam (FIB) to micro-machine materials, has led to the development of FIB milling systems. The first FIB based instrument (Precision Ion Milling System (PIMS)), became commercially available in 1984 from Gatan, Inc. Since the introduction of this instrument, several other commercial FIB systems have become available. These machines are capable of both imaging and milling selected areas within TEM and SEM specimens with submicron resolutions. The PIMS system is a gas ion source (argon) instrument, whereas the newer generation FIB systems employ liquid metal (gallium) ion sources having superior milling and imaging capabilities.
Specimens are normally milled in gallium based FIB systems with higher energy ions (compared to the PIMS), which may produce a thick amorphous surface artifact (damaged layer). This damaged layer should be removed to produce high quality specimens. Post-FIB milling in broad ion beam systems for final milling and cleaning of such specimens is a common practice. Specifically, FIB milled specimens have been re-milled and cleaned in broad ion beam machines using the specimen holder described in the above-mentioned U.S. Pat. No. 5,472,566 to improve specimen quality. However, again, this requires moving the specimen among various specimen preparation systems and the final TEM or SEM.
Others have reported the use of a wire shadow technique for TEM specimen preparation. Senz et al, Ultramicroscopy 70 (1997) pp. 23-28, and Langer, Elektronenoptische Untersuchung des Werkstoffverhaltesn in mechanisch belasteten Mikrobauteilen (1996) teach gluing a wire onto a specimen surface and exposing the surface to ion milling by a Gatan Duo-Mill. The non-shadowed areas of the specimen are removed, and a thin area is produced in the wire shadow.
However, a need still exists in this art for simplified methods and techniques for specimen preparation for both SEM and TEM analysis.
The present invention provides a method and system for the preparation of specimens for analysis by electron microscopy which utilizes ion beam milling, but without the need for multiple positioning, cutting, and/or handling steps. The system and process are especially useful for the preparation for analysis by either TEM or SEM of semiconductors, metals, alloys, ceramics, and other inorganic materials.
In accordance with one aspect of the present invention, a system and process are provided for the preparation of specimens for analysis by transmission electron microscopy. The system includes a specimen processing chamber, at least two ion beam generators, and a specimen support or holder. Preferably, the system also includes the ability to view the progress of the milling operation and may include an imaging device such as a light microscope which permits monitoring of the area of interest on a specimen as the specimen is milled.
Specimens are prepared by initially forming, cutting, and/or polishing the specimen into a piece having a preferred maximum length of about 3 mm, a width of from about 500-1000 microns, and a thickness of from about 20-150 microns. The edge (thickness dimension and surface of interest for analysis) of the specimen should be relatively flat. A preferred shape for the specimen is a generally semi-circular disc having a radius of about 3 mm and a thickness of from about 20-150 microns, although other shapes may be utilized.
An ion beam masking member is then secured to the edge (thickness dimension and surface of interest for analysis) of the specimen using a suitable adhesive such as an epoxy glue or using a suitable mechanical attachment mechanism. The ion beam masking member is preferably a fiber having a diameter of less than about 100 microns, preferably less than about 80 microns, and most preferably less than about 35 microns. The fiber preferably has a length which approximates the length of the edge of the specimen, although the length of the fiber need only be as long as the area of interest on the specimen. Fibers having diameters down to about 7 microns have been found to be suitable for use. Alternatively, the ion beam masking member may have a non-circular cross-section including square, rectangular, elliptical, or other geometric shape. Preferably, the ion beam masking member comprises an amorphous or single crystal material such as carbon (either amorphous, graphitic, or diamond), silicon carbide, or sapphire. In one embodiment of the invention, the ion beam masking member may be pre-milled prior to securing it to the specimen through ion beam exposure to reduce its diameter or cross-section. This permits a more precise mounting and placement of the masking member on the surface of the specimen relative to features of interest on or in the specimen.
For TEM analysis, once the specimen has been mounted onto a support or holder and installed in the processing chamber, the specimen and ion beam masking member are exposed to two or more ion beams to mill and thin exposed areas on the edge of the specimen. Generally, the surface of interest on the specimen will be positioned perpendicular or nearly perpendicular to the direction of the ion beams. Preferably, an ion beam having a higher intensity in the center of the beam is utilized. That is, for an ion beam having a generally circular cross-section, the intensity at or near the center of the circle is higher than at the edges.
The ion beams will mill both the ion beam masking member as well as the specimen surface causing a necking or thinning of the masking member as milling is continued. Likewise, the masked area of the specimen beneath the ion beam masking member will also be thinned as milling continues because of the thinning of the masking member. The protected area beneath the masking member may be monitored using a suitable imaging device such as an optical or scanning electron microscope which is arranged generally perpendicular to the axis of the ion beams and which provides real time information concerning the progress of the milling process. Alternatively, a light emitting diode and photodetector may be arranged to provide an indication of when the specimen has been sufficiently thinned to become optically transparent. Additionally, if the ion beam masking member is electrically conductive, its electrical resistance may be measured and monitored such that a change in resistance is measured when the milling process causes the member to be severed (i.e., a gap forms).
Once the specimen has been thinned sufficiently to be electron transparent (i.e., less than about 200 nanometers, and preferably less than about 100 nanometers), it is removed from the processing chamber and is ready for TEM analysis. During milling, the specimen and ion beam masking member are positioned in tandem so that there is no relative movement between them. This avoids the need to stop the milling operation and accurately reposition the ion beam masking member prior to the resumption of milling as in some prior art processes.
In an alternative embodiment, the present invention provides a system and process for the preparation of specimens for analysis by a scanning electron microscope. In one form, the system includes a specimen processing chamber, at least one ion beam generator, and a specimen support or holder. Preferably, a second ion beam generator is also included. Generally, a specimen will be prepared so that it has a length of from about 4-8 mm, preferably about 6 mm; a height of from about 6 to about 12 mm, preferably about 8 mm; and a thickness of from about 250-750 microns, preferably about 500 microns.
Preferably, the system also includes the ability to view the progress of the milling operation, in real time, and may include an imaging device such as a light microscope which permits monitoring of the area of interest on a specimen as the specimen is milled. Preferably, the system also includes a sputtering target which may be positioned so that one or both of the ion beam generators can sputter material from the target onto the specimen to coat its surface. Thus, once the specimen is etched and coated, it can be moved to an SEM for analysis.
In a preferred form, the specimen is secured to a rotatable specimen stage for milling. After milling has been completed, the specimen stage is rotated (e.g., 90xc2x0) to a position where the surface of the specimen is etched (cleaned). The specimen stage is then rotated back to its original position for sputter coating using one of the ion beam generators and a sputtering target.
In yet another embodiment of the present invention, the present invention provides a system and process for the preparation of specimens for analysis by a scanning electron microscope in a processing chamber which also includes an SEM. The SEM is arranged such that the specimen milling and preparation process is observable with the SEM being utilized as a high resolution imaging device. The ability to observe the milling process in real time permits precise positioning of the specimen and monitoring of the process. Further, the milled specimen can be analyzed directly in the chamber without movement to a separate apparatus.
Accordingly, it is a feature of the present invention to provide a method and system for the preparation of specimens for analysis by electron microscopy which utilizes ion beam milling, but without the need for multiple cutting, positioning, and/or handling steps. It is another feature of the invention to provide a system and process which is adaptable for specimen preparation for both TEM and SEM analysis. These, and other features and advantages of the present invention, will become apparent from the following detailed description, the accompanying drawings, and the appended claims.