United States Patent [I91
`Bernstein et al.
`
`US006057221A
`[ i l l Patent Number:
`[45] Date of Patent:
`
`6,057,221
`May 2,2000
`
`[54] LASER-INDUCED CUTTING OF METAL
`INTERCONNECT
`
`[75]
`
`Inventors: Joseph B. Bernstein, Potomac, Md.;
`Zhihui Duan, Erie, Pa.
`
`[73] Assignees: Massachusetts Institute of
`Technology, Cambridge, Mass.;
`University of Maryland, College Park,
`Md.
`
`[21] Appl. No.: 08/825,808
`Apr. 3, 1997
`[22] Filed:
`[51] Int. CL7 ..................................................... HOlL 21/44
`[52] U.S. C1. ........................................ 4381601; 4381132
`[58] Field of Search ...................................... 4381601, 132
`
`~ 5 6 1
`
`References Cited
`U.S. PATENT DOCUMENTS
`511976 Alberts et al. .............................. 15618
`411980 Nicolay .
`311981 Dougherty, Jr. .......................... 427196
`711987 Shiozaki et al. .
`111988 Johnson .................................. 4371173
`511988 Takagi ....................................... 357151
`711989 Coffey et al. ............................. 437119
`811989 Fischer ...................................... 357151
`1211991 Coffey et al. ............................. 357171
`711992 Murakami et al. ..................... 3651200
`1111993 Sun et al. .................................. 372169
`1211994 Batdorf et al. .......................... 4371173
`1111995 Murakami et al. ..................... 3651200
`1211995 Sun ........................................ 372169
`1111996 Yoo et al. ................................. 437160
`
`5,589,706 1211996 Mitwalsky et al. ..................... 2571529
`311997 Lee et al. .
`5,608,257
`5,882,998 1211996 Sur, Jr. et al. .
`
`FOREIGN PATENT DOCUMENTS
`62-162344 711987 Japan .
`
`OTHER PUBLICATIONS
`
`Cohen, S., et al., "Laser-Induced Line Melting and Cutting,"
`ZEEE Transactions on Electron Devices, Nov. 1992, at
`2480-2485.
`Wu, W., "Laser Connection and Disconnection Scheme,"
`IBM Technical Disclosure Bulletin, Jun. 1978, at 268.
`
`Primary Examiner4ichard Booth
`Assistant Examiner-Jonathan Hack
`Attorney, Agent, or Firm4amilton, Brook, Smith &
`Reynolds
`
`ABSTRACT
`[571
`An electrical interconnect includes a substrate having an
`insulating surface upon which is placed an electrically-
`conductive cut-link pad and a pair of electrically-conductive
`lines. The lines are bonded to the cut-link pad and are
`substantially more resistant to heat flow per unit length than
`is the cut-link pad. In a preferred embodiment, the thermal
`resistance per unit length of the cut-link pad is lowered by
`designing the pad such that its width is greater than the width
`of either of the lines. A method for cutting a circuit includes
`directing a laser upon the cut-link pad of an interconnect, as
`described above. The laser is maintained upon the pad until
`the cut-link pad is ablated, severing the circuit.
`
`21 Claims, 6 Drawing Sheets
`
`IPR2015-01087 - Ex. 1003
`Micron Technology, Inc., et al., Petitioners
`1
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`

`
`U.S. Patent
`
`May 2,2000
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`Sheet 1 of 6
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`FIG. I
`PRIOR ART
`
`FIG. 2
`PRIOR ART
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`2
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`

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`U.S. Patent
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`May 2,2000
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`Sheet 2 of 6
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`FIG. 3
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`3
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`U.S. Patent
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`May 2,2000
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`Sheet 3 of 6
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`FIG. 5
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`FIG. 6
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`4
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`U.S. Patent
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`May 2,2000
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`Sheet 4 of 6
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`FIG. 8
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`FIG. 9
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`5
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`U.S. Patent
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`May 2,2000
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`Sheet 5 of 6
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`FIG. 10
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`FIG. II
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`6
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`U.S. Patent
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`May 2,2000
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`Sheet 6 of 6
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`6,057,221
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`Energy
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`Energy
`
`FIG. 12
`
`FIG. 13
`
`FIG. 14
`
`7
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`

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`6,057,221
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`1
`LASER-INDUCED CUTTING OF METAL
`INTERCONNECT
`
`GOVERNMENT SUPPORT
`
`BACKGROUND OF THE INVENTION
`
`35
`
`This invention was made with government support under
`Contract number F19628-95-C-0002, awarded by the United
`States Air Force. The government has certain rights in the
`invention.
`
`2
`circuitry is cut by a laser, the high resistance of a fuse is not
`required to produce the needed influx of thermal energy. In
`this context, the thermal energy needed to melt the conduc-
`tive material is supplied by an external source, i.e., the laser.
`s As the present inventors have recognized, the use of the laser
`frees the designer from the necessity of using a high resis-
`tance segment to generate the heat necessary to cut the
`circuit. Although intuition might further suggest that a
`fuse-shaved cut-link of thin width could be severed with
`10 greater precision and efficiency than an otherwise compa-
`rable cut-link of greater width, the present inventors have
`The use of lasers to cut integrated circuits is well-known
`recognized that this notion is generally false,
`in the art. In existing methods, a laser is directed onto a
`To the contrary, an electrical interconnect which may be
`cut-link segment of the circuit. The laser supplies sufficient
`cut with greater success and with improved efficiency
`heat to vaporize the cut-link, thereby severing the cut-link. 15 includes a cut-link pad in which the thermal resistance per
`In many cases such as in a
`the circuit is
`unit length is lower, rather than higher, than the connected
`coated with a passivation layer to protect the circuit from
`lines, In a preferred embodiment, the thermal resistance is
`oxidation. To allow the cut-link to vaporize, the passivation
`lowered by adopting a form that is the inverse of the
`traditional '(dog bone" design, me form of this new design
`layer is usually etched away from the site above the cut-link
`the cut-1ink
`in advance
`the
`20 is such that the width of the cut-li& pad is
`cutting.
`greater than the width of the lines.
`of purposes
`Lasers are used to cut circuits for a
`The electrical interconnect of this invention includes an
`including the avoidance of defects, the replacement of
`insulating substrate upon which a pair of electrically-
`defective circuits with redundant circuits, and the program-
`conductive lines are bonded to a cut-link pad, wherein the
`ming of circuits. The practices used to achieve each of these 25 cut-linkpad has
`less thermal resistance per unit
`objectives are described, below.
`length, i.e., along the axis of the lines, than each of the lines
`Lasers have been used to avoid defects by circumventing
`to which it is bonded, The cut-li& pad may lie in the same
`a short through laser-induced cuts. This practice is described
`plane as the lines, or the pad may lie on a plane which is
`in U.S. Pat. No. 4,259,367, issued to Dohert~, Jr. The circuit
`merely intersected by the lines, as in a via structure where
`is repaired by using a laser to cut the lines connected to the 30 the lines, or vias, extend from the underside of the pad into
`short to thereby isolate the short. Patch lines may then be
`the substrate to a lower level of circuitry. Alternatively, the
`used to reestablish severed connections.
`cut-link pad may be bonded both to a line on the same plane
`The use of redundant circuitry has become an industry
`as the pad and also to a via extending from the pad deeper
`standard for memory chip fabrication. Redundant circuitry
`into the substrate.
`in a chip is typically supplied in the form of identical
`In a preferred embodiment of the electrical interconnect,
`segments of circuitry occupying remote areas on the chip. If
`the thermal resistance of the pad is decreased, relative to the
`a segment is flawed, that segment is disconnected and
`lines, by increasing the width of the pad such that the pad's
`replaced by activating its redundant counterpart elsewhere
`width is greater than that of the lines to which it is con-
`on the chip.
`40 nected. Benefit is provided in an interconnect where the
`cut-link pad is just 10 percent greater than the width of each
`Finally, circuits may be programmed by cutting lines from
`of the lines. Benefits rise in significance when the pad has a
`a generic, repeating pattern to disconnect unwanted or
`width that is at least 25 percent greater than the width of each
`unnecessary segments and thereby produce the desired pat-
`of the lines. The full advantage of this embodiment,
`tern through a process of reduction. Circuitry produced by
`this method may extend across a plurality of levels as well. 45 however, is usually not realized until the width of the pad is
`One example of multi-level circuit design by laser-induced
`at least 50 percent greater than the width of each of the lines.
`cutting is disclosed in U.S. Pat. No. 4,720,470, issued to
`In a further preferred embodiment, the width of the pad is
`approximately equal to its length, i.e., the distance between
`Johnson.
`Typically, the cut-link has taken one of two forms. First,
`the lines.
`the cut-link is often an undistinguished segment of a line in so
`In an alternative embodiment, the thermal resistance of
`the circuit, where the width of the cut-link is equal to the
`the pad is decreased, relative to the lines, by comprising the
`width of the lines to which it is conductively coupled. A
`cut-link pad of a material having lower thermal conductivity
`cut-link exhibiting these characteristics is illustrated in FIG.
`than the material comprising the lines. In another alternative
`1 and disclosed, for example, in the following U.S. Pat.
`embodiment, the per-unit-length thermal resistance of the
`Nos.: No. 4,853,758, issued to Fischer, and No. 5,589,706, 55 pad is decreased both by comprising the cut-link pad of a
`issued to Mitwalsky et al. The second common form is that
`material having greater thermal conductivity than the mate-
`of a "dog bone," which is illustrated in FIG. 2. This design
`rial of the lines and by providing the pad with a width greater
`is disclosed, for example, in the following U.S. Pat. Nos.:
`than that of the lines.
`No. 4,748,491, issued to Takagi, and No. 5,374,590, issued
`In another preferred embodiment, the surface of the
`to Batdorf et al. In this design, the cut-link 20 is narrower 60 substrate upon which the circuit lies is comprised of a silicon
`than the lines 21, 22 to which it is connected.
`oxide, and the circuit is covered by a passivation layer which
`prevents oxidation of the circuit yet which can be removed
`DISCLOSURE OF THE INVENTION
`from the site of the cut-link pad upon heating the pad with
`the laser. To promote the desired fracture of the passivation
`Whereas the design of earlier cut-links mirrors the nar-
`rowed "dog-bone" of conventional fuse design, a preferred 65 layer, the passivation layer is preferably comprised of a
`embodiment of this invention rejects this model and, instead,
`material, such as silicon nitride, that is harder than the
`widens the segment where the circuit is to be severed. Where
`substrate.
`
`8
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`6,057,221
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`-,
`
`,
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`3
`4
`of the substrate, where the width of the pad is greater than
`In the method of this invention a laser is directed upon a
`cut-link pad, as described above, and the laser is maintained
`the width of each of the lines.
`there until the conductive link between the lines is broken.
`FIG, 7 is a cross-sectiona~ illustration of a hexagonally-
`Preferably, the laser beam covers the entirety of the cut-link
`shaped cut-link pad from a perspective normal to the plane
`pad when the laser is directed upon the pad.
`5 of the substrate.
`The metallic cut-link pads of the present invention can be
`FIG. 8 is a cross-sectional illustration of an electrical
`more efficiently and more effectively ablated for the follow-
`the plane
`the
`a perspective
`ing reasons, First, the structures of the present invention
`substrate, where the width of each of the lines is equal to the
`retain thermal energy at the site of the cut-link more effec-
`width of the cut-link pad, except in the vicinity of the
`tively than the cut-links of the prior art by restricting
`juncture between the line and the pad, where the width of the
`dissipative heat transfer from the cut-li*
`into the connected
`line
`lines. Second, the shapes of preferred structures produce
`FIG. 9 is a cross-sectional illustration similar to FIG. 8,
`increased stress at the perimeter of the cut-link pad, thereby
`except that the cut-link pad is in the form of a hexagon,
`generating a more forceful fracture of the surrounding
`rather than a square.
`material. Third, the shapes of preferred structures create
`stress concentration points that will produce fractures radi-
`FIG, 10 is a side-view, through the substrate, of a multi-
`sting outwardly from the site of the cut-link pad, thereby
`level via structure from a perspective within the plane of the
`improving the likelihood that the fracture will be clean, i.e.,
`substrate.
`will not create inter-linked fracture passages through which
`FIG, 11 is a view of the multi-level circuit of FIG, 10 from
`escaping metal may form a "short" defeating the attempt to 20 underneath the via structure,
`cleanly sever the circuit. Fourth, where the cut-link pad
`l2 is a chart
`the
`a
`more closely approximates the size and shape of the laser
`cumulative probability function measuring the likelihood
`beam, the cut-link absorbs a greater portion of the laser's
`that a circuit
`be cut Over a range of energy levels.
`energy, thereby producing a more efficient transfer of energy
`FIG. 13 is a chart illustrating the approximate shape of a
`and a reduced danger of damaging the surrounding material.
`~ i f t h , where a passivative coating comprised of a brittle 25 cumulative probability function measuring the likelihood
`that the integrated circuit will be damaged over the same
`material is used, the fracture is biased toward the passivative
`12.
`in
`coating. and. hence. toward the surface of a chia for efficient
`range
`levels
`FIG. 14 is a chart illustrating the approximate shape of the
`removal of the passivative coating and non-damaging expul-
`sion of the metal comprising the cut-link pad. Sixth, by 30 combined cumulative probability that a circuit will be cut
`trapping the heat within the confines of the cut-link pad and
`without otherwise damaging the circuit over the same range
`of energy levels charted in FIGS. 12 and 13.
`limiting its escape through the connected lines, the site at
`-.
`which the cut de;elops isconfined within this narrow region
`and damage to other parts of the circuit is minimized. Each
`
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS
`with greater clarity in the context of describing the structure
`Acut-link shaped in the form of a square pad is illustrated
`and methods of several preferred embodiments of this
`in FIG. 3. The cut-link pad 20, is bonded to a pair of lines
`invention.
`21 and 22, each positioned on the substrate of a chip or
`circuit board within a single plane. The pad and the lines are
`BRIEF DESCRIPTION OF THE DRAWINGS
`all electrically conductive, whereas that part of the substrate
`The
`and
`features and advantages 40 that contracts the metal is dielectric. Although this: embodi-
`of the invention will be apparent from the following, more
`ment could just as fairly be described as a single line
`particular description of preferred embodiments of the
`including a cut-link site, the embodiment is herein broken
`invention, as illustrated in the accompanying drawings in
`down into narrower constituents to provide greater clarity
`which like reference characters refer to the same parts 45 when d i f f e r e n t embodiments a r e discussed and
`throughout the different views. The drawings are not nec-
`differentiated, below,
`can be seen by juxtaposing the
`instead being placed up0n
`essaril~ drawn
`cut-link of this figure with those illustrated in FIGS, 1 and
`illustrating the principles of the invention.
`2, the cut-link pad of FIG. 3 has a much greater width in
`FIG. 1 is a cross-sectional illustration of a cut-link pad of
`relation to the width of each of the attached lines, ~h~
`the prior art, from a perspective normal to the plane of the
`reasons for widening the cut-link are explained below.
`substrate, where the width of the pad is equal to the width
`First, ablation, or rupture, of the cut-link requires an
`of the lines.
`accumulation of thermal energy within the cut-link. During
`FIG. 2 is a cross-sectional illustration of a cut-link pad of
`cutting, the accumulation of thermal energy within the link
`the prior art, from a perspective normal to the plane of the
`can be roughly measured as the difference between the
`substrate, where the interconnect has the shape of a dog 5s energy supplied by the laser and the heat conductively
`bone.
`transferred out of and away from the cut-link. Therefore, the
`FIG. 3 is a cross-sectional illustration of a cut-link pad in
`cut-link can be ablated from its site most
`by
`the form of a square, from a perspective normal to the plane
`maximizing the absorption of laser energy and minimizing
`of the substrate, where the width of the pad is greater than
`the transfer of that energy, in the form of heat, away from the
`the width of each of the lines.
`60 cut-link.
`FIG. 4 is a perspective view of the cut-link pad and the
`~h~ am0,nt of laser energy absorbed by the cut-link is
`lines shown in FIG. 3.
`maximized by providing a cut-link shaped in a form
`FIG. 5 is a cross-sectional illustration of a cut-link pad,
`approximating that of the laser beam spot 24. Although the
`from a perspective within the axis of the lines, illustrating
`shape of the beam spot is Gaussian, i.e., no clear delineation
`the fractures which typically form during cutting.
`65 exists for identifying the outer edge of the spot; the position
`FIG. 6 is a cross-sectional illustration of a cut-link pad in
`of the edge may be approximated using, for example, the
`full-width half-max method. Using the full-width half-max
`the form of a square, from a perspective normal to the plane
`
`of these phenomena will be discussed in greater detail and .,
`
`d d
`
`9
`
`

`
`method, the edge of the spot is designated as the boundary
`created by a fracture and the accompanying flow of metal
`at which the intensity of the laser drops to half of the
`linking the two lines must cover a greater distance to
`maximum intensity. When cutting, the cut-link should ide-
`circumscribe the two lines must cover a greater distance to
`ally be entirely within the beam spot as determined above.
`circumscribe the void and create a short. Further, cracks
`Absorption across the entire surface promotes uniform heat-
`5 generated in the horizontal plane on which the circuit lies
`ing of the pad. An infrared laser typically produces beams
`will typically emanate from the corners where stress is
`concentrated. As is shown in FIG. 5, cracks 32a and 32b
`having a minimum diameter of about two microns. Further,
`emanating from the edges 36 of a square-shaped pad 20 will
`the tolerance for beam positioning error is typically about
`typically be directed outward at an angle of 135" from the
`0.2 to 0.5 microns. Accordingly, a pad designed to have a
`lo sides of the pad 20 mitigating the likelihood that cracks from
`preferable length and width of two to three microns can
`absorb the bulk of the energy transmitted by the laser while
`opposite sides of the pad will cross to create a passage for
`remaining entirely within the beam spot.
`forming a short.
`Transfer of heat away from the cut-link is minimized by
`Designing the cut-link pad to approximate the size and
`shape of the laser beam spot also provides an additional
`creating a comparatively-lower thermal resistance per unit
`benefit. Less energy misses the cut-link pad resulting in less
`length within the pad than without. Because the top layer of
`energy being absorbed into the surrounding dielectric mate-
`the substrate and any passivative coating are insulators, the
`rial which reduces the likelihood that this surrounding
`primary mode through which heat is transferred from the
`material will be damaged by thermal stresses.
`pad is through the connected lines. Accordingly, the direc-
`tion of heat transfer out of the pad can be approximated as
`Both the lines and the cut-links 20 in a circuit are coated
`being along axis A-A', shown in FIG. 3. Along axis A-A', 20 with a passivation layer 30 to protect the circuit from
`oxidation as a result of exposure to air. For the cut-link 20
`thermal resistance is a function of the heat transfer coeffi-
`cient of the conducting material and the cross-sectional area
`to be ablated, fractures 32 must be created within the
`passivation layer 30, which allow the section of passivation
`of the conducting material measured in the plane normal to
`layer 30 covering the cut-link 20 to be blasted away from the
`the axis. The pad and the lines are preferably comprised of
`a material with high thermal conductivity and high thermal 25 chip, thereby exposing the cut-link 20 to air and allowing it
`expansion, such as aluminum and copper. In such a case,
`to vaporize. Preferably, a hard, brittle material, such as
`silicon nitride is used as the passivation layer 30 because
`where the composition of the pad is identical to the com-
`position of each of the lines, a differential in thermal
`increasing hardness and brittleness is correlated with
`resistance is due primarily to a difference in cross-sectional
`increasing susceptibility to fracture. When a laser pulse is
`area. The dimensions of the cross-section normal to the A-A' 30 incident on the cut-link pad 20, the cut-link pad 20 is heated
`axis are illustrated in FIG. 4. As shown, the cross-section of
`and expands. The passivation layer 30 can not be heated as
`quickly as the metal lines because the passivation layer 30
`the line (the product of w and h) is much smaller than the
`cross-section of the pad (the product of w' and h').
`can not absorb energy from the laser beam directly and can
`Accordingly, the thermal resistance along the A-A' axis will
`not conduct heat from the metal quickly. As a result, stress
`be much greater in each of the lines than in the pad. This 35 develops within the passivation layer 30 due to the differ-
`ence in temperature between the metal 20 and the passiva-
`differential reduces the loss of heat into and through the line
`tive coating 30 as well as the mismatch of the thermal
`and thereby allows the temperature of the pad to quickly
`rise.
`expansion coefficients of the two materials. As the metal 20
`Moreover, the difference in thermal resistance produces a
`expands with increasing temperature, greater tensile stress is
`sharp temperature differential between the pad and the lines 40 placed on the passivative coating 30, which is why choosing
`to create stress at the interface which promotes a clean
`a metal having a high thermal expansion coefficient is
`severance at the interface between the pad and the line. The
`important. When the stress is sufficiently great, the passiva-
`tive coating 30 is blasted from the area above the cut-link20,
`difference in thermal resistance also creates a boundary
`and the cut-link 20 vaporizes into the open space above it.
`limiting the area within which fracture will occur. I.e.,
`fracture will not initiate from a comparatively cool region 45
`Typically, the substrate 34 of a chip includes a silicon
`beyond the area where heat is concentrated. The differential
`wafer base upon which a dielectric material, such as a silicon
`on each side of the pad thereby ensures that fracture will
`oxide, is layered. The circuit is then imprinted onto the
`commence from the pad, rather than from a line, providing
`dielectric material. Because silicon nitride is more brittle
`greater cutting accuracy and minimizing damage to the
`than silicon oxide, fracture initiation is biased toward the
`surrounding circuitry.
`so silicon nitride. By promoting fracture through the passiva-
`tion layer 30, rather than through the substrate 34, the metal
`Means other than increasing the width of the pad exist for
`creating a differential in thermal resistance at the interface of
`is more likely to be completely ablated from the chip, and
`the pad and the line. For example, in one embodiment, the
`the likelihood of forming a short through the substrate is
`pad may be fabricated from a material having greater
`reduced. Though not as brittle as silicon nitride, silicon
`thermal conductivity than the material from which the lines 5s oxide is also sufficiently brittle to be sued as the passivation
`layer 30.
`are comprised. In another alternative embodiment the pad
`may be both wider than the lines and comprised of a material
`Increasing the width of the cut-link also improves the
`having greater thermal conductivity than the lines.
`fracture mechanics during ablation. The fracture initiates
`In addition to improving the absorption and retention of
`and grows as a result of thermal stress at the interface of the
`thermal energy from the laser, creating a roughly square- 60 cut-link and the surrounding material. The thermal stress, in
`shaped pad reduces the likelihood of producing a short
`turn, is a function of the energy density at the surface of the
`circumventing the cut. As noted, the metal from the cut site
`pad. The lines of maximum stress, from which fractures
`typically propagate, are along the edge 36 of the pad 20
`escapes through fractures in the passivation layer. After or
`forming the perimeter of the surface 38 facing the passiva-
`during a cut, the circuit will be shorted if metal migrates
`through the fractures to provide a continuous link from one 65 tion layer 30. The energy density along the maximum stress
`lines 36 is governed by the ratio of the energy absorbed to
`line to the other. Because a comparatively large void is left
`after the ablation of a wide, square cut-link, the passage
`the length of the perimeter.
`
`10
`
`

`
`Ep=k*S/P
`
`more precipitous temperature drop across this interface
`Making the approximating assumption that the energy
`thereby fueling the conduction of heat out of the pad 20.
`density of the laser beam is uniform, the energy absorbed by
`Another advantage of the patterns illustrated in FIGS. 8
`a square surface 38 will be the produce of k and S, where k
`and 9 is that the fracture along the interface of the pad 20 and
`is a constant and S represents the area of surface 38. The
`energy density, E,, along the perimeter, P, of the surface 38 s the line 21 or 22 is enhanced. As noted above, fractures
`across the interface will typically emanate along a line
`can be expressed'by the function:
`substantially parallel to the edge 36 of the pad 20. In the
`pattern shown in FIG. 3, the edge of the line meets the edge
`of the pad perpendicularly. Accordingly, the stress along the
`As the energy density increases, the stresses increase and the 10 edge of the line is perpendicular to the direction in which the
`cracks grow. Energy density increases as a result of increas-
`interfacial fracture will propagate and will provide little or
`ing the surface dimensions of the pad. As the dimensions
`no impetus toward initiating or driving this fracture. On the
`other hand, the edge 51 of the line 21 in FIG. 8 slopes toward
`increase, the area of the square, S (S=l *w), increases as a
`square function while the length of the perimeter, P [P=2 *
`the edge 36 of the pad 20. In proportion to the component
`(l+w)], increases linearly. Accordingly, S will increase at a IS of this slope which parallels the edge 36 of the pad 20, the
`stress along the edge 51 of the line 21 will supplement the
`quadratic rate while P increases at a linear rate. Because the
`stress along the edge 36 of the pad 20 to initiate and drive
`numerator of the energy-density function increases with size
`the fracture across the interface between the pad 20 and the
`at a rate faster than the denominator, a fracture driven by a
`line 21.
`large, square-shaped cut-link will propagate through the
`passivation layer with greater energy than a fracture driven 20
`In contrast to the embodiments thus described, wherein
`by a small or narrow cut-link.
`the pad and the lines all lie within the same plane of a chip
`Of course, fracture will initiate from an edge of the outer
`or circuit board, the invention may also be embodied in
`surface because stress is concentrated at the edges. Use of a
`multi-layered chips. Such an embodiment is illustrated in
`wider cut-link promotes fracture 32a from an edge 36 of the
`FIGS. 10 and 11. As before, the orientation of the pad 20 is
`upper surface of the cut-link rather than fracture 32b from 25 parallel to the plane of the chip. However, in place of the
`laterally-extending lines, the lines take the form of vias 21a
`the lower surface facing toward the silicon wafer. After the
`fracture 32a has propagated through the passivation layer
`and 22a extending downward from the surface 52 of the pad
`30, this segment of the passivation layer 30 can be "blown
`20 away from the laser and into the substrate. These vias 21a
`and 22a provide a conductive link between levels of the
`off' by the escaping metal, as desired. In contrast, fracture
`32b commencing from the lower side can damage intercon- 30 integrated circuit. Typically, the vias 21a and 22a have a
`nects existing on lower levels of a chip, and the metal
`smaller cross-section than the lines of the single-level
`flowing through a fracture directed toward the silicon has no
`embodiments. Where half-micron technology is used in
`standard silicon processing, the via 21a or 22a preferably
`outlet and is therefore more likely to produce a short.
`Therefore, the likelihood of producing a clean severance of
`has a width of about a half micrometer across its cross-
`an interconnect is vromoted bv the wider cut-links because 35 section. The vias 21a and 22a are conductivelv couvled to
`the pad 20, which forms a conductive cap stretching across
`fracture is biased toward the passivation layer and away
`and conductively bonded to the upper end of each via 21a,
`from the silicon wafer.
`22a. At their ends opposite the pad 20, the vias 21a and 22a
`Although a preferred shape of the cut-link pad is square,
`are conductively bonded to lines 53, 54 extending laterally
`other relatively compact shapes approximating the size and
`shape of the laser beam spot may also be used. FIG. 7 40 in a layer beneath the pad 20. By removing the pad 20
`illustrates a hexagonally-shaped pad of this invention. As
`through laser ablation, the conductive link is broken.
`When the laser strikes the pad 20, heat migrates down-
`before, the width, w, of the pad is approximately equal to the
`length, 1, of the pad. Although this hexagonal design is more
`ward away from surface. During the first few nanoseconds
`after the laser strikes, the temperature at the bottom 52 of the
`compact and provides a better approximation of the laser
`beam shape 24, as compared to a square pad, the crack 46 45 pad is much lower than the temperature at the top surface 38.
`As a result, less heat is dissipated through the vias 21a, 22a
`must propagate across a longer distance to cross the inter-
`face between the line 21 and the pad 20. As the energy
`than if they were laterally connected, and the efficiency with
`density builds along the edges 42 and 44, the pad 20 is
`which the cut is performed is thereby improved.
`severed from the line 21 when cracks 46, which parallel the
`In either a single-layer or multi-layer embodiment of the
`edges 42, 44, branch across the interface.
`so electrical interconnect, the method for ablating the cut-link
`Although each of the single-plane embodiments thus far
`pad is substantially identical. First, a laser beam having an
`described includes lines having widths uniformly narrower
`intensity on the order of 10 Joules per square centimeter is
`than the width of the cut-link pad, the width of the lines need
`directed onto the site of the pad. When embodied in a chip,
`not be uniform to provide benefit. FIGS. 8 and 9 illustrate
`the pad is typically coated with a passivation layer. The
`embodiments wherein the cut-link and the lines 21 and 22 ss beam must be able to pass through the passivation layer,
`possess equal widths over much of their lengths. The width
`which is preferably silicon nitride, and be absorbed by the
`of each line 21,22 narrows only as it approaches and where
`metal of the cut-link pad. The wavelength of the laser beam
`it meets the pad 20. Because the juncture of the line 21, 22
`is determined by what is transmittable through the passiva-
`and the pad 20 is narrow, thermal resistance along the A-A'
`tion layer. Each material has an upper limit as to the size of
`axis increases at the juncture relative to the resistance within 60 the wavelength that it is capable of absorbing. For silicon
`the pad, as in the previous embodiments. However, the
`nitride, that limit is about a half micrometer. Therefore, an
`overall ability to restrict heat flow out of the pad 20 may be
`infrared laser producing a beam with a wavelength of about
`somewhat compromised if the width beyond the bottlene