Abstract:
An optical microscope has an optical axis that coincides with a focused ion beam. The optical microscope can be used to locate target features on a specimen for subsequent operations by the ion beam, thereby eliminating the need for complex and potentially inaccurate registration procedures. The optical microscope can use infrared light so that features on a silicon flip chip are observable through the silicon from the backside. The ion beam can then machine the chip to expose the features for subsequent operations.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to the field of focused ion beam systems and, in particular, to sample observation and registration in such systems. 
     BACKGROUND OF THE INVENTION 
       F ocused  I on  B eam (FIB) systems are widely used in microscopic-scale manufacturing operations because of their ability to image, etch, mill, deposit, and analyze with great precision. Ion columns on FIB systems using gallium  L iquid  M etal  I on  S ource (LMIS), for example, can provide five to seven manometer (10 −9 m) lateral resolution. Because of their versatility and precision, FIB systems have gained universal acceptance in the integrate circuit industry as necessary analytical tools for use in process development, failure analysis, and most recently, defect characterization. 
     During the manufacturing of integrated circuits, multiple copies of an integrated circuit are fabricated on a semiconductor silicon wafer, which is then severed into rectangular dies, each die including a copy of the integrated circuit. A die is typically several hundred microns thick and has electronic circuit elements fabricated on and slightly below its front side surface. The ion beam of a FIB system scans the surface of the integrated circuit in a raster pattern. This raster pattern produces an image of the surface showing the top lines and elements of the circuit. The image is used to navigate the ion beam around the die to locate a specific element or a feature of the circuit. Upon moving the raster pattern to the local area of the feature of interest and increasing the ion beam current, the ion beam will cut into the die and expose circuit features in buried layers. The FIB system can then alter the exposed circuit by cutting conductive traces to break electrical connections or depositing conductive material to provide new electrical connections. FIB systems often include a  S econdary  I on  M ass  S pectrometer (SIMS), which can determine which chemical elements are present in the exposed features. Technology in the semiconductor industry evolves rapidly, however, and existing tools and techniques may be inadequate for use with new integrated circuit designs. 
     In use, a semiconductor die is mounted in a package. The package has metal leads for electrically connecting it to a circuit board on which other electrical components are mounted. The die and the package both have bonding pads for establishing electrical connections between the die and the package leads. The connection between the package leads and the die inherently adds undesirable impedance, that is, resistance to electrical flow, between the package leads and the die. As semiconductor devices operate at higher speeds, lower impedances between the package leads and the active elements on the die are required. At the same time, larger and more complex devices and circuits require an increased number of input/output connections, resulting in larger die size and packages. 
     A response to these needs has been the development of so-called flip-chip or C 4 semiconductor manufacturing technology in which the bond connections are arrayed over the front side of a die and align with an array of corresponding bond connections on the package. The die is then placed front side down in the package, with the bond connections on the die contacting those on the package. The connection length and impedance from the active circuit elements to the package pins are reduced, compared with those of older connection technologies, and the number of connections available between the die and the package is increased.    
     The top layer of the flip-chip die, however, is covered with an array of bond pads, making access to circuit elements from the front surface of the die difficult or impossible, even for unpackaged devices. Thus, conventional analysis and repair tools are often unusable with flip chips. Improved techniques to debug flip-chip devices without damaging them are needed to remove a crucial roadblock in the advancement of flip chip technology and to provide a significant boost in time-to-market for critical IC chips, such as microprocessors. With improved techniques, new circuits can be made and tested within days, not months of receiving a prototype. 
     Some techniques exist for debugging flip-chips. U.S. Pat. No. 5,821,549 to Talbot et al., for example, describe using laser milling to thin the flip-chip from the backside by cutting a series of steps in the substrate. An infrared optical microscope is then used to locate features of interest on the thinned portion of the flip-chip. The edges of the laser drilled steps are used to register the infrared image with a FIB image, and the FIB is then used to accurately mill down to the feature desired. Thus, the operation requires the use of three instruments, a laser, an optical microscope, and a FIB. Registering the infrared image with the FIB image is difficult, time consuming, and the images are subject to misalignment. Thus, to do efficient FIB operations on flip-chip features, there exists a need for a fast and accurate method of locating features through the back surface of the flip-chip. 
     In integrated circuit operations, as well as in other applications, a FIB operator typically locates a feature of interest by scanning the ion beam over the specimen while using the imaging to view the specimen. A disadvantage of FIB systems is that the focused ion beam incidentally etches and implants gallium ions in the substrate as it is imaging. Thus, there is a need for a fast and accurate method for aligning an ion beam with specific features on any specimen, particularly ones having sensitive surfaces that can be damaged by the focused ion beam, while minimizing undesirable exposure of the specimen to the ion beam. 
     SUMMARY OF THE INVENTION 
     Thus, it is an object of the invention to provide an improved method and apparatus for locating specimen features in a focused ion beam system. 
     It is another object of the invention to provide such a method and apparatus that minimizes exposure of the specimen to the ion beam. 
     It is still another object of the invention to provide a focused ion beam system, having an optical microscope for locating specimen features and provide coaxial alignment between the two. 
     It is a further object of the invention to permit an operator to align a specimen visually and then perform an ion beam operation on the specimen, without moving the specimen to a second instrument or requiring registration with pre-recorded images. 
     It is yet another further object of the invention to provide a focused ion beam system having a coaxial optical microscope for use with infra-red, visible, or other frequencies of light. 
     It is still another object of the invention to provide an improved method and apparatus for using a focused ion beam system to operate on flip-chips. 
     It is a still further object of the invention to provide a method of machining thin-film heads in a focus ion beam system while minimizing the damage to the heads from ion beam imaging. 
     It is yet a further object of the invention to provide a means for optically processing an integrated circuit with a focused ion beam system. 
     The present invention comprises a focused ion beam system that includes an optical microscope having an optical axis that substantially coincides with the axis of the focused ion beam as the axes approach a specimen. The axis of the focused ion beam is defined as a line at the center of the beam when positioned at the point defined as zero deflection. 
     Features on the specimen are located using the image from the optical microscope. When a feature is centered in the image of the optical microscope, the feature will be in the path of the ion beam, when activated. Because the optical image and the FIB image are aligned, the operator can use the optical image to position the ion beam without having to go through a lengthy, less accurate registration procedure that relies on a recorded image and accurate repositioning of a movable specimen stage. With the present invention, the operator uses a live image from the optical microscope to align the focused ion beam with features on the specimen. 
     The optical microscope can use light of different frequencies for different applications. For example, an optical microscope can use infrared light for viewing features through a layer of silicon or visible light frequencies for viewing a surface feature or a feature under a layer of transparent silicon dioxide. The optical microscope can also direct light from a laser or other light source onto a specimen to process the specimen. The scope of optical processing of the sample in conjunction with the ion beam and gas chemistry is large.  O ptical  B eam  I nduced  C urrent, OBIC, is one example of measuring the thickness of material as the ion beam is removing the material. Another example is using a laser beam through the optical microscope to heat the sample in a local area under the ion beam. This heating will enhance the gas chemistry between the ion beam and process gasses. In a preferred embodiment, the optical microscope uses an angled mirror at the end of the ion column to reflect light from a specimen into image-forming optical elements. The mirror includes an aperture hole through which the ion beam travels. The preferred embodiment also includes one or more illumination sources and a camera for recording the image and presenting it on a monitor to an operator. One embodiment allows the operator a choice between dark field illumination, provided by light sources in the vacuum chamber or bright field illumination, provided along the optical axis. 
     Thus, the invention speeds and simplifies locating features on the specimen. Using the optical microscope to locate a feature and position the specimen reduces exposure of the specimen to the scanning focused ion beam, thereby reducing undesirable etching of the specimen, which can damage sensitive specimens, such as thin films. The optical microscope allows rapid positioning of the parts in relation to the ion beam, thus increasing the number of parts that can be machined in a time period and improving the quality of those parts. 
     The invention is particularly suitable for performing focused ion beam operations on flip chips. Using infrared light, it is possible to look through 100 μm or more of silicon, thereby allowing an operator to locate, from the backside of a thinned die, features that can then be exposed by ion beam milling for other FIB operations. 
     Additional objects, advantages and novel features of the invention will become apparent from the detailed description and drawings of the invention. 
     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 which form the subject of the claims of the invention. 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is perspective view of a focused ion beam system having a coaxial optical microscope in accordance with the present invention; 
     FIG. 2A is a partial cross-sectional view of the coaxial optical microscope apparatus of FIG  1 ; 
     FIG. 2B is a detail view of a portion of the coaxial optical microscope apparatus of FIG. 2A; 
     FIG. 3 is an enlarged cross-sectional view showing the mirror assembly of the apparatus of FIG. 2A; 
     FIG. 4 is an enlarged cross-sectional view showing the mirror assembly of the apparatus of FIG. 2A, the section being taken normal to that of FIG. 3; 
     FIG. 5 is a perspective view showing the orientation relationship between the specimen and an illumination source; 
     FIG. 6 is similar to FIG.  3  and illustrates the light path in the system of FIG.  1 . 
     FIG. 7 is an enlarged partial cross-sectional view showing the illumination assembly of FIG. 2A; 
     FIG. 8A is a enlarged partial cross-sectional showing the mounting of the lens assembly of FIG. 2A; FIG. 8B shows a portion of the lens assembly of FIG. 8A further enlarged. 
     FIG. 9 is a flow chart showing a method of performing focused ion beam operations on a flip-chip. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     General features of a preferred embodiment of a focused ion beam system having a coaxial optical microscope according to the present invention are illustrated in the following figures. 
     FIG. 1 shows a focused ion beam system  10  of the present invention comprising a focused ion beam column  12  and an optical microscope system  14  for observing a specimen  16  maintained in a vacuum chamber  24 . The details of focused ion beam column  12  are known in the art and focused ion beam columns are commercially available, for example, from FEI Company, Hillsboro, Oreg., the assignee of the present invention. Generally, column  12  focuses ions into a sub-micron ion beam  26 . Ions are emitted from a liquid metal ion source and directed into ion beam  26  toward the specimen  16 , which typically comprises a semiconductor device positioned on a movable X-Y stage  28  within vacuum chamber  24 . A main flange  30 , is attached to and supported by the wall of the vacuum chamber  24 . Most of the components of optical microscope system  14  are supported by the main flange  30 . 
     FIG. 2A is a partial cross-sectional view of the system of FIG. 1, showing the relationship between the components thereof. FIG. 2 shows that system  10  includes a lens tube  32  mounted on the main flange  30  and supporting a camera  40 , an illumination assembly  42  and a lens assembly  44 , which comprising an movable objective lens  46  and a fixed lens  48 . FIGS. 2A and 2B also shows a mirror assembly  56  supported by the ion beam column  12 . Mirror assembly  56  collects the light from specimen  16  and reflects it into lens assembly  44 , which includes optical elements for focusing the light to form an image on the image plane of camera  40 . Mirror assembly  56  is electrically grounded to minimize electric fields that would tend to affect the path of ions in ion beam  26 . 
     Illumination assembly  42  provides a source of illumination for bright field viewing. Camera  40  is preferably a commercially available CCD camera that detects light incident on an array of a charge-coupled devices. Camera  40  is selected to be sensitive to the frequency of light used to illuminate the sample  16  to form the image on camera  40 . The output from camera  40  is displayed on a monitor (not shown) for viewing by an operator, who then adjusts the movable stage  28  to align a desired feature with the optical axis of optical system  14 . Once aligned with the optical axis of optical system  14 , the feature will also be aligned with axis of the focused ion beam  26 . Camera  40  can be positioned at different locations within lens tube  32  to provide the desired field of view. For example, in one embodiment, the zoom range of the camera  40  is from 800 μm to 250 μm. 
     FIG. 3 shows a detailed cross-sectional view of mirror assembly  56  of FIG.  2 A. FIG. 4 shows detailed cross-sectional view of mirror assembly  56  taken along lines B—B of FIG.  2 A and FIG.  3 . FIGS. 3 and 4 shows that mirror assembly  56  includes a mirror  60  on the end of ion column  12 . Mirror  60  includes an aperture  62  for passing the ion beam  26  to the sample  16 . Aperture  62  is sufficiently large to pass ion beam  26  at maximum deflection without undue defocusing due to any incidental electrostatic field associated with mirror assembly  56 , yet sufficiently small to minimize loss of light from specimen  16 . Mirror  60  can be a polished portion of the ion objective lens itself or it can be a separate part that is attached to the ion objective lens. Mirror  60  can be planar or curved. Mirror assembly  56  also includes a light shield  64  that prevents stray light from entering objective lens  46 . 
     Mirror assembly  56  includes two light emitting diodes (LED)  70   a  and  70   b  that illuminate the specimen  16  when the operator selects dark field illumination. In a preferred embodiment for flip chip repair, LEDs  70   a  and  70   b  emit strongly in infrared frequencies. Suitable LEDs are commercially available, for example, from American Bright Optoelectronics Corp. as Model BIR-BM17J4, which have an emission peak at a wavelength of 940 nm and an emission angle of 20 degrees. 
     FIG. 5 shows the orientation of LEDs  70  with respect to each other and specimen  16 . LEDs  70   a  and  70   b  are preferably separated by a rotational angle of ninety degrees on mirror assembly  56  so that each LED  70  illuminates specimen  16  at right angles to the other LED  70 . FIG. 6, a cross-sectional view similar to FIG. 3, shows light  75   a  from both LEDs  70  is incident at approximately 45 degrees from the surface of specimen  16 . Lines on the integrate circuits are typically laid out on a rectangular grid pattern  72 , and specimen  16  is preferably mounted in system  10  so that light from the LEDs  70   a  and  70   b  is incident perpendicular to the edges of the rectangular grid. Thus, one LED  70  illuminates primarily the vertical lines of specimen  16  and the other LED  70  illuminates the horizontal lines of specimen  16 . LEDs  70  are positioned in relation to mirror  60  so that the light  75   b  reflected from the specimen is not collected by mirror  60 . Only light  75   c  scattered by the rectangular grid pattern  72  on specimen  16  is directed by mirror  60  into the optical system  14  to form an image in camera  40 , thereby providing a dark field image of sample  16 &#39;s rectangular grid pattern  72 . 
     The light from the LED&#39;s is typically exiting the LED at a diversion angle of 20 degrees. To focus the light at the surface of the sample  72 , focussing optics  73  can be placed between the LED and the sample  16 . This can be achieved by using standard optical components, by having the light exit through small apertures or by placing the center of the LED&#39;s elimination point in, or close to the focal point of an elliptical mirror. The other focal point of the ellipse can then be close to or beyond the sample surface. Other sources of focused light, such as fiber optics or laser generated light, can also be used. Using a focused ray of light, as the illumination for the dark field mode will generate back scattered light off the sample surface edges  72 , without having reflected light entering the objective lens  46  and on to the camera  40 . 
     Mirror  60  has a gold-coated reflecting surface that reflects approximately 99% of the incident infrared light into objective lens  46  which focuses the infrared light. Suitable lenses for infrared light are commercially available, such as the Mitutoyo Model 378-823 M Plan NIR 10X, which has a numerical aperture of 0.26, a working distance of 30.5 mm, a focal length of 20 mm, a resolving power of 1.1 um, and a depth of focus of 4.1 um. 
     Mirror assembly  56  prevents any undesirable light  75   b  from entering objective lens  46 . The interior of mirror assembly  56  is coated with carbon die  77  to prevent light scattered from the interior surfaces of assembly  56  from entering into the objective lens  46 . None of the light  75   a  from the LEDs  70  that is specularly reflected enters objective lens  46 , except the light  75   c  that is specularly reflected off the edge of the rectangular grid pattern  72  on specimen  16 . Thus, the image formed by objective lens  46  and lens  48  on camera  40 , is a dark field image created by the light scattered from the rectangular grid pattern  72  on specimen  16 . 
     FIG. 7 shows an alternative source of illumination, illumination assembly  42 , which is used for bright field illumination. Illumination assembly  42  includes a LED  84 , which can be the same model as LEDs  70   a  and  70   b  of FIG.  3 . The direction of the illumination from the infrared LED  84  can be adjusted by moving it in and out and by rotating the beam-splitter housing  86  until the best illumination is observed. The infrared light emitted from LED  84  is transmitted through an acrylic light-pipe  89  having a face  89   a  through which the light exits light-pipe  89 . The surface finish of  89   a  is a white beat blast finish on the end of the acrylic light pipe  89 . Emitting from the surface of  89   a  is a defused scatter light pattern  83 . The light continues through four apertures  87  and only the center of the defused scattered light  83   a  strikes the beam-splitter  88 , which reflects a portion of the incident light  83   b  through the lens assembly  44  along the optical axis  90  of the optical microscope system  10  to illuminate specimen  16 . The remaining, unreflected portion of the incident light  83   c  is transmitted through beam splitter  88  and absorbed by a heat absorbing filter  92 , which is tilted so that the light reflected from its surface will not be reflected off the beam splitter  88  into camera  40 . The beam splitter is sufficiently reflective to provide adequate illumination of specimen  16 , yet sufficiently transmissive to allow passage of sufficient light back from specimen  16  to form an image at camera  40 . 
     Applicant has found that a suitable beam splitter is a Melles Griot Pellicle Beamsplitter Model 03 BPL001/01, which reflects approximately fifteen percent of the 940 nm light incident from LED  84  towards specimen  16 . Heat absorbing filter  92  can be, for example, a Melles Griot Model No. 03 FHA 005, which transmits less than one percent of the incident infrared radiation at 940 nm. 
     Light reflected from specimen  16  is reflected by mirror  60  into lens system  44 , through beam splitter  88 , which passes about ninety percent of the infrared light to camera  40 . 
     FIGS. 8A and 8B show an adjustment mechanism  94  by which the position of the objective lens  46  (FIG.  3 ), located near specimen  16  within vacuum chamber  24 , is adjusted by a motor  118  from the outside of the vacuum chamber  24 . Adjustment mechanism  94  is mounted to main flange  30 . Objective lens  46  is mounted on an objective lens tube  96 , which is guided toward or away from specimen  16  by a guiding flange  98 . To move objective lens  46 , an operator rotates a knob  100  or operates the motor  118 , which rotates a lead screw  104 , causing linear motion of a nut  102  threaded onto the lead screw  104 . Nut  102  is attached to objective lens tube  96 , and it, together with objective lens  46 , moves towards or away from specimen  16 . The lead screw  104  is clamped together with the knob  100  by the set screw  119   a.  The set screw  119   b  clamps the motor shaft to the knob  100  by applying pressure on the motor shaft through the Teflon safety pin  120 . If the load on the motor gets too high the Teflon will slip on the motor shaft. Leaving the set screw  119   b  loose, the knob can be turned by hand and provide a manual override of the focus. Fixed lens  48  is mounted within adjustment mechanism  94  and is supported between O-rings  106   a  and  106   b . O-ring  106   a  seals vacuum chamber  24 , while O-ring  106   b  provides pressure to maintain fixed lens  48  in position. O-rings  108  seals between lead screw  104  and guiding flange  98  to allow motion of knob  100  to be transferred into vacuum chamber  24 . Guiding flange  98  is attached to the main flange  30  by three screws  110 . An O-ring  112  positioned between guiding flange  98  and main flange  30  provides a vacuum seal and spaces guiding flange  98  slightly away from main flange  30 . By adjusting the tension on the screws  110 , the optical axis of optical system  14  can be adjusted into alignment with the ion beam column  12 . Deflecting the ion beam to coincide with the optical axis and setting that deflection as zero deflection performs final alignment. O-ring  114  seals main flange  30  to the wall of vacuum chamber  24 . 
     It is possible to use the entire light spectrum, including ultraviolet, visible, and infrared for viewing different materials. Different lenses are typically required for different frequencies. Optical elements used with the invention can vary from simple glass lenses to high end scanning confocal microscope optics. Such optical elements are typically commercially available. Illumination normal to the surface is ineffective in some instances and that dark field illumination, that is, the image is formed by scattered, as opposed to specularly reflected, light is sometimes preferable. Because dark field illumination requires forming an image with less light than bright field illumination, the number of optical elements in the optics train is minimized to reduce light loss. 
     When visible light is used with the optical microscope, the operator can see through transparent layers, such as silicon dioxide, that would be opaque to the focused ion beam. When the optical microscope uses infrared light, the operator can see through approximately 100 microns or more of silicon. This is particularly advantageous for focused ion beam milling on the IC chips using flip-chip technology, in which failure analysis must be performed through silicon from the backside of the chip. 
     FIG. 9 is a flow chart showing the steps in a process for focused ion beam operations on a flip-chip. Step  136  shows that the backside of a flip chip die is thinned, preferably by grinding, to a thickness of approximately 100 μm. Step  138  shows that the die is placed on movable stage  28  in vacuum chamber  24 , which is then evacuated. Step  144  shows that the operator uses features of the die identifiable through the optical system  14 , to navigate around the die and locate the target circuit feature. Optical system  14  preferably uses dark field illumination and 940 nm infrared light. Upon locating the target feature, step  146  shows that the ion beam is then used to mill through the silicon to expose the target feature. The cross-sectional area of the milled hole will vary with the application, but holes of 10 μm by 10 μm, 20 μm by 20 μm, or 50 μm by 50 μm are typical. The operator can view the milling as it progresses using both optical and ion beam imaging. Because the resolution of the infrared optical system is typically about one or two microns, which is much lower than that of the ion beam, ion imaging may be required for final alignment before operating on the circuit feature. Step  148  shows that the operator can then perform a desired operation on the exposed feature. 
     The invention is also particularly useful with parts having sensitive surfaces, such as thin film coating. Such parts can be positioned for ion beam milling much more quickly using an optical microscope than with using focused ion beam imaging, and the sensitive surface is not damaged by photons. Such films are used, for example, on write heads for disk drives. An optical system  14  for use in machining such parts will typically use visible light and bright field illumination. 
     The invention is not limited to gallium liquid metal ion FIBs, but is also applicable to FIBs using other ion sources, such as a duoplasmatron ion source, and primary ions of other species, such as oxygen, argon, nitrogen, etc. The nature of the focused ion beam and the optical system used in practicing the invention also depends upon the nature of the specimen under investigation, the desired ion beam energy and the type of analysis. Other types optical systems and illumination systems can also be used. 
     It will be understood that when applicants describe the optical axis and the ion beam axis as coinciding or coaxial, it is meant that the optical axis is generally, not necessarily exactly, parallel to the ion beam as it approaches the specimen and that the optical axis at the specimen is near, but necessary at exactly at, the impact point of the ion beam at zero deflection. Moreover, it will be understood that the ion beam scan area is typically a subset of the optical image, and that the areas are not necessarily equal in extent, due to the different resolutions and fields of view of the optical microscope and the ion beam system. 
     The embodiments described above are merely illustrative and skilled persons can make variations on them without departing from the scope of the invention, which is defined by the following claims. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.