Self-aligning tool for hands-free cross-sectioning of an integrated circuit

A tool is provided for cross-sectioning an integrated circuit in a hands-free mode of operation. The tool comprises an enclosure or cage having a passageway through which a sample--such as an integrated circuit, which may be encased in epoxy or some other substance--is brought into contact with an abrasive surface such as a milling disk. A wall or plate of the enclosure is adjustable in order to accommodate a variety of sample sizes, including both dies and packages. During the grinding or polishing of the sample the rotation of the milling disk helps stabilize the sample against the adjustable wall and one or more other walls of the enclosure. The enclosure is situated at a selectable position along a frame or guide that is mounted above the milling disk. Releasable connectors are used to secure the enclosure in a selected position yet allow it to be relocated as the milling disk becomes worn. The stability of the cross-sectioning tool allows a sample to be cross-sectioned without constant attention by a human operator, and the sample can be removed, examined and replaced without adjusting the enclosure, adjustable wall or the frame. The force of gravity keeps the sample in contact with the milling disk as the sample is ground, and may be augmented by a small weight. Because the arrangement of the enclosure and adjustable wall need not be changed to examine the sample or move it to an unworn section of the milling disk, the sample easily re-aligns itself each time it is returned to the enclosure.

BACKGROUND
 This invention relates to the field of failure analysis of integrated
 circuits and semi-conductors. More particularly, a self-aligning tool is
 provided for cross-sectioning an integrated circuit in a hands-free mode
 of operation.
 When an integrated circuit (IC) fails to operate, or fails to operate as
 expected, it may be subjected to failure analysis in order to determine
 the cause of the failure. For example, when a newly designed circuit is
 first manufactured, it is usually tested to ensure that it operates
 correctly. If it does not, operational testing of the part may be able to
 identify a symptom of the fault or the general area in which the fault may
 lie, but may not be able to determine the exact cause of the failure
 (e.g., bad soldering, an open or shorted line). Therefore, one goal of
 failure analysis is to determine the cause of the part's failure by
 examining one or more areas or suspected points of failure.
 Thus, failure analysis often requires locating a specific element (e.g.,
 junction, line) or area of an IC for close examination (e.g., under a
 powerful microscope). Typically, a cross-section of the IC is taken at the
 suspected area in order to gain an unobstructed view. Because of the
 microscopic level of detail involved (e.g., sub-micron units of measure in
 some cases), however, it is essential that just enough of the IC is
 removed to expose the desired element, but no more.
 If a relatively large portion of the circuit must be removed little
 precision is required at first--i.e., the part may be sawn or cut roughly
 in order to remove most of the extraneous material. However, in the latter
 stages of the cross-sectioning effort great care and precision are
 required in order to uncover the desired area without introducing any new
 fractures or other errors. Many failure analysis operations employ an
 abrasive surface, such as a milling disk (e.g., a circular piece of
 sandpaper), to grind away the edge of the IC to uncover the suspected
 fault.
 One tool for cross-sectioning an IC with a milling disk is a hand-held
 tool. A die may be secured to this tool, which is then guided by the
 operator to grind away the unneeded portion of the IC. The die can be
 removed from the tool for examination (e.g., to determine if the area of
 interest has been uncovered yet).
 This tool is deficient in several respects. First, it requires continuous
 human operation and attention regardless of how much of the IC needs to be
 ground away in order to reach the target area. Also, the angle at which
 the die is cross-sectioned depends upon the angle at which the operator
 holds the tool. Because there is no mechanism for ensuring that the same
 angle is maintained throughout the cross-sectioning, the operator may
 inadvertently remove too much of the IC. In addition, this tool can only
 be used with IC dies, not packages. Finally, this tool is relatively
 expensive to use due to the need for constant human attendance.
 Another tool for cross-sectioning an IC by grinding an edge with a milling
 disk is more automated than the tool described above, but still suffers
 from significant shortcomings. This tool, which can accommodate dies but
 not packages, includes a mechanical arm to hold an IC for grinding. The IC
 must be encased in wax, however, in order to be held by the arm. In
 addition, the arm must be continually adjusted to ensure that the IC is
 placed in contact with the abrasive surface. The IC must be removed from
 the arm to be examined, thus requiring the wax to be removed (e.g.,
 melted) prior to examination, and then re-applied if further grinding is
 necessary. Repeatedly mounting the IC in wax makes it very easy to
 misalign the IC in between grinding evolutions, and the effort required to
 repeatedly prepare the IC for examination and then grinding adds a
 significant amount of time to a failure analysis operation.
 Thus, what is needed is a cross-sectioning tool that can operate without a
 human operator's continuous participation and which facilitates easy
 re-alignment of an IC that is being cross-sectioned so as to maintain the
 same angle of cross-sectioning. Also, the tool should be able to handle
 ICs of various sizes, including both dies and packages.
 SUMMARY
 In one embodiment of the invention a tool is provided for facilitating the
 cross-sectioning of an integrated circuit (IC). In this embodiment the
 tool is capable of operation without continuous human activity and accepts
 ICs of various sizes. Further, the tool is self-aligning so that after an
 IC is removed from the tool, examined, and replaced, the IC returns to
 substantially the same alignment that it had prior to its removal.
 In this embodiment the tool includes an enclosure, such as a carriage or
 cage, which partially or fully defines a passage. At one end of the
 passage an IC may be inserted or removed. The other end of the passage
 opens upon an abrasive surface that is used to grind or polish the IC. The
 enclosure includes two or more fixed walls to help stabilize the circuit,
 and an adjustable wall allows the tool to accommodate ICs of various sizes
 (e.g., dies and packages). The adjustable wall is, in one embodiment of
 the invention, continuously movable along tracks or guides that form part
 of the enclosure. Further, the adjustable wall is configured to be
 releasably secured in a selected positions along the tracks so as to
 stabilize an IC and help the IC maintain a particular orientation (e.g.,
 angle) to the abrasive surface.
 The tool also includes a frame comprising a plurality of rails or guides
 along which the enclosure may be relocated. In one embodiment of the
 invention the enclosure can be positioned at virtually any location along
 the frame. Further, the enclosure is releasably securable in a selected
 location, similar to the semi-permanent manner in which the adjustable
 wall may be located within the enclosure.
 During a cross-sectioning operation the frame provides a base or support
 for the enclosure, which stabilizes the IC for grinding and polishing as
 it is in contact with the abrasive surface. In one embodiment of the
 invention the force of gravity is sufficient to maintain contact between
 the IC and the abrasive surface. However, in an alternative embodiment a
 small weight or force may be applied to the IC.
 Because the enclosure may be selectably positioned at various locations
 along the frame, as one portion of the abrasive surface becomes worn, the
 IC can be relocated to a portion that is less worn. In various embodiments
 of the invention different types of releasable connectors, such as screws
 and wing nuts, are used to movably secure the adjustable wall within the
 enclosure and the enclosure within the frame.

DETAILED DESCRIPTION
 The following description is presented to enable any person skilled in the
 art to make and use the invention, and is provided in the context of
 particular applications of the invention and their requirements. Various
 modifications to the disclosed embodiments will be readily apparent to
 those skilled in the art and the general principles defined herein may be
 applied to other embodiments and applications without departing from the
 spirit and scope of the present invention. Thus, the present invention is
 not intended to be limited to the embodiments shown, but is to be accorded
 the widest scope consistent with the principles and features disclosed
 herein.
 In particular, various materials that may be used in manufacturing
 embodiments of the invention are identified below, as are a variety of
 means for coupling elements of these embodiments. Other construction
 materials and coupling means possessing similar properties are also
 suitable and are envisioned within the scope of the invention.
 In one embodiment of the invention a self-aligning hands-free tool is
 provided for facilitating the cross-sectioning of an integrated circuit
 (IC). The tool is designed to accept ICs of various sizes, including dies
 (e.g., individual chips) as well as packages (e.g., an IC assembled with
 connectors for insertion in a socket). In this embodiment the IC is
 brought into contact with an abrasive surface (e.g., a milling disk) that
 grinds off portions of the circuit to uncover an area or element to be
 examined. Illustratively, the milling disk is circular in shape, and
 different disks having a range of coarseness may be used during the
 cross-sectioning. A milling disk may be rotated at a constant or varying
 rate, depending upon how much of the IC needs to be removed to uncover the
 target area. In alternative embodiments of the invention abrasive means
 having other configurations may be applied to the IC. For example, a belt
 sander or abrasive material moving in a non-circular pattern may be
 employed. In yet another alternative embodiment, the tool may be
 configured to rub an IC against a stationary abrasive.
 FIG. 1 depicts one embodiment of cross-section tool 100 mounted on a cover
 ring of a milling disk. FIG. 2 is a perspective view of cross-section tool
 100, demonstrating one configuration in which its components may be
 assembled.
 Cross-section tool 100 in this embodiment includes frame 102, adjustable
 carriage 104 and adjustable wall 106. Sample 108 comprises an IC to be
 cross-sectioned. Sample 108 may, for example, constitute an IC encased in
 epoxy, plastic or other material (e.g. a fast-curing acrylic).
 Particularly with small samples, enclosure within epoxy yields a more
 manageably sized object for fitting in the cross-section tool. In
 addition, mounting an IC in epoxy or other stabilizing material promotes
 edge retention by resisting damage to the edge of the IC (e.g.,
 splintering, cracking) during cross-sectioning.
 In FIG. 1, cover ring 110 provides a convenient mounting surface for tool
 100 above milling disk 112. Other arrangements for positioning tool 100
 above the abrading surface are similarly envisioned. Illustratively, the
 milling disk rotates in an anti-clockwise direction, as viewed from above,
 at a variable or fixed rate.
 Coordinate axes are depicted in FIG. 1 for reference purposes. In the
 illustrated coordinate system the length of frame 102 and carriage 104
 extend parallel to the y-axis (e.g., across a diameter of cover ring 110),
 their width along the x-axis and sample 108 is inserted and removed
 parallel to the z-axis. Note that frame 102 may be positioned across a
 diameter of the cover ring or milling disk (e.g., over the center of the
 disk) or may be mounted off-center.
 During a cross-sectioning operation, carriage 104 may be fixed in position
 at any location along the length of frame 102 using fasteners such as
 screw 170 and wing nut 172. Carriage 104 may, however, bring the full
 surface of milling disk 112 to bear on sample 108 just by being
 relocatable along one radius of the disk.
 For grinding and polishing, sample 108 is inserted into carriage 104 and
 wall 106 is positioned to hold the sample in place yet allow it to be
 easily removed and replaced. Wall 106 is adjustable in order to
 accommodate samples of various dimensions and is fixed in a position by
 fasteners such as screws and wing nuts. While the sample is being ground
 or polished by milling disk 112, no manual assistance is required from a
 human operator to hold the sample in place. Further, the configuration of
 carriage 104 and wall 106 facilitates the self re-alignment of sample 108
 as it is re-inserted in tool 100 after being examined.
 In this embodiment, frame 102 includes tracks, rails or guides 120, 122 and
 cross-members 124, 126. The configuration of tracks 120, 122 allow
 carriage 104 to be temporarily or permanently secured at a position of
 choice along frame 102. In this embodiment the lengths of frame 102 and
 tracks 120, 122 are determined by the diameter of cover ring 110, but in
 alternative embodiments frame 102 may be adjustable in length and/or
 width. Tracks 120, 122 are continuous open slits in this embodiment,
 thereby allowing carriage 104 to be secured at virtually any location, but
 may alternatively be configured to limit the locations at which the
 carriage may be positioned. For example, a track may alternatively
 comprise one or more discrete apertures or indentations.
 Adjustable carriage 104 includes walls 140, 142, tracks 144, 146 and
 cross-member 148. Tracks 144, 146 perform similar roles to those of tracks
 120, 122 of frame 102 in that they allow adjustable wall 106 to be
 situated parallel to wall 142 at virtually any location along tracks 144,
 146. Also similar to tracks 120, 122, tracks 144, 146 may comprise any
 configuration of apertures, slits, indentations, obstructions and the
 like, suitable for securing the adjustable wall. The embodiment depicted
 in FIGS. 1 and 2 demonstrate that tracks 144, 146 may be configured to
 allow adjustable wall 106 to be continuously movable along the entire
 length of the apertures forming the tracks. In other words, in the
 illustrated embodiment the releasable connectors securing wall 106 to
 tracks 144, 146 need merely be loosened, not removed, in order to allow
 the adjustable wall to be relocated along the tracks (e.g., to accommodate
 samples of varying sizes).
 Thus, adjustable wall 106 snugly ensconces sample 108 in carriage 104 and
 helps stabilize the sample during grinding and polishing. The adjustable
 wall also facilitates re-alignment of the sample whenever it is returned
 to the tool (e.g., after the cross-sectioned edge is examined). In
 particular, adjustable wall 106 helps the sample maintain a particular
 orientation (e.g., angle) to the abrasive surface, which is important in
 producing a clear cross-section of the sample.
 The orientation of frame 102 to cover ring 110 may be chosen arbitrarily in
 the embodiment of FIGS. 1 and 2 as long as carriage 104 can be positioned
 in the frame so that the rotation of milling disk 112 will bring a portion
 of its abrasive surface into contact with sample 108. However, mounting
 the frame astride the center of the milling disk ensures the availability
 of the full surface of the disk for grinding or polishing. Then, as the
 surface of milling disk 112 becomes worn (i.e., in a circular pattern),
 carriage 104 and sample 108 may be relocated along frame 102 to a location
 above a portion of the milling disk that is less worn.
 For most samples, the force of gravity will be sufficient to keep the
 sample in contact with the milling disk as the encased IC is ground and
 polished. However, the amount of plastic than an IC is encased in may be
 augmented, both to facilitate handling and to lend enough weight to the
 sample to ensure continuous, hands-free operation. A short IC, for
 example, may be encased in a tall plastic envelope so that the sample will
 extend above carriage 104 for easy grasping. Another method of ensuring
 adequate force on a sample is to place a small object or weight on top of
 the sample after it is lodged in carriage 104. Adding weight to a sample
 may also accelerate the grinding or polishing process.
 It can be seen in FIGS. 1 and 2 that the rotation of milling disk 112
 forces sample 108 into the alcove formed by adjustable wall 106 and walls
 140, 142 of carriage 104. Thus, in the presently described embodiment, in
 which carriage 104 includes just two walls, the carriage is primarily
 functional along one-half of the diameter of milling disk 112 (i.e., from
 cross-member 124 to one-half the length of frame 102. To use carriage 104
 in the other half of frame 102 the carriage is removed from frame 102 and
 rotated 180 degrees before being re-inserted in the frame. However,
 because of the rotational nature of milling disk 112, carriage 104
 generally only needs to be positioned along one-half the length of frame
 102. In alternative embodiments of the invention walls may be added under
 track 146 and/or cross-member 148 so that carriage 104 may be used in any
 position along frame 102 without being re-oriented. This alternative
 embodiment is particular suited for use with a non-circular abrading
 surface, such as a belt sander, in which maximum use of the surface
 requires greater flexibility in the positioning of carriage 104 and sample
 108.
 In FIGS. 1 and 2, adjustable wall 106 is secured to carriage 104 and
 carriage 104 is secured to frame 102 at multiple points by fastening
 means. Illustratively, the fastening means may include screw 170 and wing
 nut 172. One skilled in the art will appreciate that the use of releasable
 connectors such as these facilitates the relocation of wall 106 or
 carriage 104 yet allows for secure placement of these components to
 provide stability as an IC is cross-sectioned. For example, screw and wing
 nut combinations at locations 160, 162 are easily loosened so that wall
 106 may be slid along tracks 144, 146 and then tightened in a new position
 (e.g., for a new sample) without having to detach the connector
 combinations.
 Various other combinations or configurations of fasteners, connectors and
 anchors may be employed to secure carriage 104 and/or adjustable wall 106
 as long as the combination enables a movable yet positionally stable
 platform for sample 108. Some combinations may include clamp or vise-like
 arrangements, hooks, prongs, etc. For example, in one alternative
 embodiment of the invention a track may consist of a series of threaded
 apertures suitable for receiving a screw. In this embodiment wall 106 may
 be secured to tracks 144, 146 by screws introduced downward at locations
 160, 162. In another alternative embodiment one or more of the tracks
 depicted in FIGS. 1 and 2 may comprise a series of apertures or
 indentations configured to receive a gear or wheel. In this alternative
 embodiment the gear or wheel may be rotated to reposition carriage 104
 and/or wall 106 and may include locking means for securing the carriage or
 adjustable wall at a particular position. Other suitable means for
 securely positioning carriage 104 and adjustable wall 106, while allowing
 their repositioning as needed, will be apparent to one of skill in the
 art, and embodiments of the invention are not limited to any particular
 means.
 As with carriage 104 and wall 106, frame 102 may be secured to cover ring
 110 via any suitable stable means, such as screws, bolts, nails, clamps,
 glue, etc. In one alternative embodiment of the invention frame 102 may be
 affixed to, or included as part of, the cover ring by the manufacturer of
 the cover ring. Further, cross-members 124, 126 may be omitted in one or
 more embodiments of frame 102, particularly where tracks 120, 122 are
 permanently mounted or affixed to cover ring 110 with sufficient stability
 to support carriage 104.
 In the illustrated embodiment of the invention milling disk 112 comprises
 an abrasive surface or film of sandpaper, emery or the like. Suitable
 milling disks include, but are not limited to, silicon carbide grinding
 paper, aluminum oxide microfinishing film, lapping film and polishing
 cloths used in combination with powders such as alumia.
 Disks and abrasives of varying coarseness may be used during different
 stages of the cross-sectioning of an IC. For example, a very coarse disk
 may be used until the cross-sectioning begins to approach an area of the
 IC that is to be examined. Then, one or more finer-grained disks may be
 used as the area becomes closer to exposure. Finally, when the area is
 exposed or is close to exposure a very fine-grained polishing surface
 (e.g., a polishing cloth with alumia powder) may be used to finish the
 cross-sectioning and give a smooth and clean finish to the area in order
 to prepare it for examination under a microscope.
 Typically, when cross-sectioning an IC with a milling disk the sample is
 repeatedly ground or polished and then examined to determine if additional
 grinding or polishing is required. The amount of material ground from the
 IC between examinations may vary greatly over the course of the
 cross-sectioning. In addition, the rate of rotation of a milling disk is
 typically adjusted during the cross-sectioning so as to provide fine
 control over the amount of the IC that is removed between examinations.
 For example, the closer the area of interest is to being exposed, the less
 material needs to be removed. Thus, depending on the coarseness of milling
 disk 112 very few rotations, or even less than one rotation, may be
 applied between one examination and the next. In particular, during the
 polishing phase or the end of the grinding phase, the amount of material
 that is to be removed may be measured in microns or even less than a
 micron.
 Frame 102, adjustable carriage 104 and adjustable wall 106 may be
 constructed from a variety of materials with sufficient rigidity. The
 construction material may be metallic (e.g., stainless steel, aluminum) or
 non-metallic (e.g. plastic, graphite or other composite) that provides
 these characteristics and that does not easily shed flakes, chips,
 splinters that could damage sample 108. A rigid material may be preferred
 in order to lend stability to tool 100 and sample 108 and thus facilitate
 the grinding and polishing of an optimal cross-section. Advantageously,
 the interior surfaces of carriage 104 and wall 106 are relatively smooth
 so that sample 108 may be moved freely in the z-direction during
 cross-sectioning and may be easily extracted from and re-inserted into
 tool 100 without repositioning adjustable wall 106.
 The foregoing descriptions of embodiments of the invention have been
 presented for purposes of illustration and description only. They are not
 intended to be exhaustive or to limit the invention to the forms
 disclosed. Many modifications and variations will be apparent to
 practitioners skilled in the art. Accordingly, the above disclosure is not
 intended to limit the invention; the scope of the invention is defined by
 the appended claims.