PATENT APPLICATION
`DOCKET NO. MIT-'7581L
`DAVID E. BROOK
`HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
`JAMES M. SMITH
`LEO R. REYNOLDS
`r~~~~~AHAN 6
`TWO MILITIA DRIVE· LEXINGTON, MASSACHUSETTS 02173'4799
`5723 U. s. prc1ELEPHONE: (617) 861'6240· FACSIMILE: (617) 861'9540
`DAVIDJ.BRODY
`
`JOHN W. MEoBURY
`AoMINISlRA11VE DIRECTOR
`
`£IAruwv,. J. FoRGUE
`PATENT IIDMINISnV.TOR
`
`PATENT AGENTS
`
`CAROLYN S. ELMORE
`
`PATENTS. TRADEMARKS.
`COPYRIGHTS AND
`UTIGATION
`
`~"€:~~IIHHlIlII
`~:~~1S~~~AY
`4/03/97
`
`STEVEN G. DAVIS
`CARoL.A. EGNER
`DOREEN M. HOGL.E
`J. GRANT HOIJSTON
`RODNEY D. JOHNSON
`HEL.EN LEE
`EUZABETHW. MATA
`TIMO"lHY J. MEAGHER
`ANTHONY P. ONELLO. JR.
`NINA L. PEARL.MUTTER
`SCOTT D. ROTHENBERGER
`DEIRDRE E. SANDERS
`JAY SHIM
`RICHARD W. WAGNER
`LlsAM. WARREN
`HELEN E. WENDL.ER
`DARRELL L. WONG
`
`Assistant Commissioner for Patents
`Washington, D.C.
`20231
`
`Sir:
`
`:i-::!:~
`
`Transmitted herewith for filing is the patent application of
`~JInventor(s): Joseph B. Bernstein and Zhihui Duan
`
`Title: LASER-INDUCED CUTTING OF METAL INTERCONNECT
`
`Specification, Claims, Abstract of the Disclosure
`[TI __ 7_ sheets of ~:maI/informal drawings. (Figs._1_-_1~4~) ______________________ ___
`
`An assignment of the invention to
`
`Massachusetts Institute of Technology
`
`(2 Assignment Forms)
`
`University of Maryland
`
`D
`
`A verified statement to establish small entity status under 37 C.F.R. 1.9 and
`37 C.F.R. 1.27.
`[TI Executed~» Combined Declaration/Power of Attorney. (2 Declarations)
`D Other:
`
`EXPRESS MAIL Mailing Label No.
`
`EG978842799US
`
`Date of Deposit
`
`April 3, 1997
`
`I hereby certify that this paper or fee is being deposited with the United States Postal
`Service "Express Mail Post Office to Addressee" service under 37 C.F.R. 1.10 on the
`date indicated above and is addressed to the Assistant Commissioner for Patents,
`Washington, D.C.
`20231.
`
`(Typed or
`
`IPR2015-01087 - Ex. 1004
`Micron Technology, Inc., et al., Petitioners
`1
`
`

`
`-2-
`
`The filing fee has been calculated as shown below:
`
`(Col. 1)
`
`(Col. 2)
`
`SMALL ENTITY
`
`FOR
`
`BASIC FEE
`
`TOTAL CLAIMS
`
`INDEP CLAIMS
`
`NO. FILED
`NO. EXTRA
`.......................... . ...........................
`.......................... . ...........................
`.......................... . ...........................
`41 - 20 =
`*
`2 -
`3 =
`o MULTIPLE DEPENDENT CLAIM PRESENTED
`
`FEE
`
`$ 385
`
`RATE
`. .............
`. .............
`. .............
`X $11= $
`
`X $40= $
`
`21
`-0-
`
`*
`
`OTHER THAN A
`SMALL ENTITY
`
`RATE
`. .............
`. .............
`. .............
`X $22=
`
`FEE
`
`$ 770
`
`$ 462
`
`X $80=
`
`$ -0-
`
`*If the difference in Col. 1
`is less than zero, enter "0"
`in Col. 2
`
`+ $130= $
`
`+ $260= $
`
`
`As signment
`
`Fe e
`
`Total:
`
`$
`
`$
`
`
`As signment
`
`Fe e
`
`$ 80
`
`Total:
`
`$1;312
`
`;? I Please charge my Deposit Account No. 08-0380 in the amount of $ ______________ __
`
`A check in the amount of $1,312
`~~~-----------------
`
`to cover the filing fee is enclosed.
`
`Authorization is hereby granted to charge payment of the following fees
`associated with this communication or credit any overpayment to Deposit Account
`No. 08-0380. Two duplicate copies of this letter are enclosed.
`
`Any additional filing fees required under 37 CFR 1.16.
`
`Any patent application processing fees under 37 CFR 1.17.
`
`Authorization is hereby granted to charge payment of the following fees
`during pendency of this application or credit any overpayment to Deposit Account
`No. 08-0380. Two duplicate copies of this letter are enclosed.
`
`EJ
`EJ
`
`Any patent application processing fees under 37 CFR 1.17.
`
`Any filing fees under 37 CFR 1.16 for presentation of extra claims.
`
`Respectfully submitted,
`
`HAMILTON, BROOK, SMITH & REYNOLDS
`
`egistration No.
`Attorney for Applicant(s)
`(617) 861-6240
`
`

`
`· _ .. -,," .
`1
`
`DAVID E. BROOK
`JANES M. SMITH
`LEo R. REYNOLDS
`PATRICIA GRANAHAN
`JOHN L. DUPRE
`DAVID J. BRODY
`MARY Lou WAKIIoCURA
`THOMAS O. HOOVER
`AUCE O. CARROLl.
`N. SCOTT PIERCE
`
`ANNEJ. COLUNS
`ROBERT T. CONWAY
`STEVEN G. DAVIS
`CAROL.A.EGNER
`DoREEN M. HOGLE
`J. GRANT HOUSTON
`RODNEY D. JOHNSON
`HEl.ENLEE
`EuzABETH W. MATA
`TIMOTHY J. MEAGHER
`ANTHONY P. ONELl.O. JR.
`NINA L. PEARLMUTTER
`SCo". D. ROTHENBERGER
`DE1I\DRE E. SANDERS
`JAY SHIM
`RICHARD W. WAGNER
`LISA M. WARREN
`HELEN E. WENDLER
`DARRELl. L. WONG
`
`PATENT APPLICATION
`DOCKET NO.~M~I~T-~7~5~8~lL~ ______ __
`HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
`TWO MILITIA DRIVE· LEXINGTON, MASSACHUSETTS 02173'4799
`TELEPHONE: (617) 861'6240· FACSIMILE: (617) 861·9540
`
`MUNROE H. 1iAMn..7ON
`(1906'1984)
`
`OF COUNSEl.
`
`RlCHAADAWISE
`SUsAN G. L GI..cNsKY
`
`JOHN W. MEosuRY
`AoMINIsmA1M! DIRECTOR
`BARBARAJ. FoRGuE
`PATENT .hDMINISTRATOR
`
`PATENT AGENTS
`
`CARoL.YN S. ELMORE
`
`PATENTS. TRADEMARKS.
`COPYRIGHTS AND
`unGATION
`
`Assistant Commissioner for Patents
`Washington, D.C.
`20231
`
`Sir:
`
`Transmitted herewith for filing is the patent application of
`Inventor(s): Joseph B. Bernstein and Zhihui Duan
`
`Title: LASER-INDUCED CUTTING OF METAL INTERCONNECT
`
`Specification, Claims, Abstract of the Disclosure
`IX] __ 7 _ sheets of ~/ informal drawings. (Figs .---'1_-..;.;1...;4..:..) __ ___ ____ ___ _
`
`An assignment of the invention to
`
`Massachusetts Institute of Technology
`
`(2 Assignment Forms)
`
`University of Maryland
`
`o
`
`A verified statement to establish small entity status under 37 C.F.R. 1.9 and
`37 C.F.R. 1.27.
`IX] Executed~ Combined Declaration/Power of Attorney. (2)Declarations)
`o Other:
`
`EXPRESS MAIL Mailing Label No.
`
`EG978842799US
`
`Date of Deposit
`
`April 3, 1997
`
`I hereby certify that this paper or fee is being deposited with the United States Postal
`Service "Express Mail Post Office to Addressee" service under 37 C.F.R. 1.10 on the
`date indicated above and is addressed to the Assistant Commissioner for Patents,
`Washington, D.C.
`20231.
`
`(Typed or
`
`

`
`--
`.
`..•.. "'.--..... _ ..... _ .... "-' ····{':'-m~<P'? t i r '"" ...... ~..:.."'~. ______ _
`
`~
`
`-2-
`
`The filing fee has been calculated as shown below:
`
`(Col. 1)
`
`(Col. 2)
`
`SMALL ENTITY
`
`FOR
`
`BASIC FEE
`
`NO. FILED
`NO. EXTRA
`.......................... ............................
`.......................... ............................
`.......................... ............................
`41 - 20 =
`*
`21
`2 -
`3 =
`-0-
`o MULTIPLE DEPENDENT CLAIM PRESENTED
`
`TOTAL CLAIMS
`
`INDEP CLAIMS
`
`*
`
`*If the difference in Col. 1
`is less than zero, enter "0"
`in Col. 2
`
`FEE
`
`$ 385
`
`RATE
`. .............
`. .............
`. .............
`X $11= $
`
`X $40= $
`
`+ $130= $
`
`As
`signment
`Fe
`e
`
`Total:
`
`$
`
`$
`
`OTHER THAN A
`SMALL ENTITY
`
`FEE
`
`RATE
`::::::::::::::
`$ 770
`$22= $ 462
`X
`
`X
`
`$80= $ -0-
`
`+ $260= $
`
`
`As signment
`e
`Fe
`
`$ 80
`
`Total:
`
`$1,312
`
`::::.~::?
`
`~~ I Please charge my Deposit Account No. 08-0380 in the amount of $ ______________ __
`~ X I A check in the amount of $-=1:...;,'-"3::..;1::..;2:::.-________________ _ to cover the filing fee is enclosed.
`~
`
`Authorization is hereby granted to charge payment of the following fees
`associated with this communication or credit any overpayment to Deposit Account
`No. 08-0380. Two duplicate copies of this letter are enclosed.
`
`Any additional filing fees required under 37 CFR 1.16.
`
`Any patent application processing fees under 37 CFR 1.17.
`
`Authorization is hereby granted to charge payment of the following fees
`during pendency of this application or credit any overpayment to Deposit Account
`No. 08-0380. Two duplicate copies of this letter are enclosed.
`
`E1
`E1
`
`Any patent application processing fees under 37 CFR 1.17.
`
`Any filing fees under 37 CFR 1.16 for presentation of extra claims.
`
`Respectfully submitted,
`
`HAMILTON, BROOK, SMITH & REYNOLDS
`
`egistration No.
`Attorney for Applicant(s)
`(617) 861-6240
`
`

`
`•
`
`• -
`
`• . . : - ',.-.--.--..:. .• ..!~, •••
`
`PATENT APPLICATION
`DOCKET NO. MIT~7581L
`----~~~----------
`HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
`MUNROE 1-1. HMm..roN
`(I9O&1984)
`TWO MILITIA DRIVE· LEXINGTON, MASSACHUSETTS 02173'4799
`TELEPHONE: (617) 861'6240' FACSIMILE: (617) 861'9540
`
`OF COUNSEL
`R!aiARo A. WISE
`SlJs.\N G.1.. Gi..oYsKY
`
`JOHN W. MEo8URY
`AoI.uNIs't'RATI\£ DIRECTOR
`BARBARA J. FoRGuE
`PATENT I\OI.IlNls'mATOR
`
`PATENT AGENTS
`
`CARoLYN S. ELMORE
`
`PATENTS. TRADEMARKS.
`COPYRIGHTS AND
`UTIGATION
`
`DAVID E. BROOK
`JAMES M. SMITH
`LEO R REYNOLDS
`PATRIClAGRANAHAN
`JOHN1..DuP~
`DAVIDJ. BRODY
`MARY Lou WhKlMURA
`THOWAS O. HOOVER
`AUCE O. CARROLL
`N. SCOTT PIERCE
`
`ANNEJ. COLUNS
`RoBERTT. CONWAY
`STEVEN G. DAVIS
`CARoL A. EGNER
`DOREEN M. HOGLE
`J. GRANT HOUSTON
`RoONEY D. JOHNSON
`HELEN LEE
`EuzAsETHW. MATA
`TIMOTHY J. MEAGHER
`ANTHONY P. ONELLO. JR.
`NINA 1.. PEARLMUTTER
`SCOTT D. ROTHENBERGER
`DElRORE E. SANDERS
`JAYSHlM
`RICHARD W. WAGNER
`LISA M. WARREN
`HELEN E. WENDLER
`DARRELL L WONG
`
`Assistant Commissioner for Patents
`Washington, D.C.
`20231
`
`Sir:
`
`Transmitted herewith for filing is the patent application of
`Inventor(s}: Joseph B. Bernstein and Zhihui Duan
`
`Title: LASER-INDUCED CUTTING OF METAL INTERCONNECT
`
`Specification, Claims, Abstract of the Disclosure
`DO __ 7_ sheets of ~/informal drawings.
`(Figs.~1~-...;;1;,.;4~) ____________ _
`
`An assignment of the invention to __ M_a __ s_s_a_c_h_u_s __ e_t_t_s __ I_n __ s_t_i_t_u_t_e ___ o_f __ T_e_c_h __ n_o_l_o~g~y~ ______ _
`
`(2 Assignment Forms)
`
`University of Maryland
`
`o
`
`A verified statement to establish small entity status under 37 C.F.R. 1.9 and
`37 C.F.R. 1.27.
`DO Executed~ Combined Declaration/Power of Attorney. (2,'Declarations)
`o Other:
`
`EXPRESS MAIL Mailing Label No.
`
`EG978842799US
`
`Date of, Deposit
`
`April 3, 1997
`
`I hereby certify that this paper or fee is being deposited with the united States Postal
`Service
`"Express Mail Post Office to Addressee" service under 37 C.F.R. 1.10 on the
`date indicated above and is addressed to the Assistant Commissioner for Patents,
`Washington, D.C.
`20231.
`
`(Typed or
`
`

`
`.. "
`_~·w.~ __ •• ~~ .• ~."";...~~u;!:~ .. ..:..s:!!:"...:~..;.:.~_~ ____ ..
`
`.
`
`-2-
`
`The filing fee has been calculated as shown below:
`
`(Col. 1)
`
`(Col. 2)
`
`SMALL ENTITY
`
`OTHER THAN A
`SMALL ENTITY
`
`FOR
`
`BASIC FEE
`
`TOTAL CLAIMS
`
`NO. FILED
`NO. EXTRA
`.......................... ............................
`.......................... ............................
`.......................... . ...........................
`41 - 20 =
`*
`21
`2 -
`INDEP CLAIMS
`-0-
`3 =
`o MULTIPLE DEPENDENT CLAIM PRESENTED
`
`*
`
`*If the difference in Col. 1
`is less than zero, enter "0"
`in Col. 2
`
`FEE
`
`385
`
`RATE
`. .............
`. .............
`..............
`X $11=
`
`X $40=
`
`$
`
`$
`
`$
`
`+ $130=
`
`$
`
`signment
`As
`
`Fe e
`
`Total:
`
`$
`
`$
`
`FEE
`
`RATE
`..............
`..............
`. .............
`X $22= $ 462
`
`$ 770
`
`X $80= $ -0-
`
`+ $260= $
`
`
`As signment
`Fe
`e
`
`$ 80
`
`Total:
`
`$1.312
`
`"~D::~:
`
`:1,,~ M
`
`;0: ~
`;:::: ~
`
`Please charge my Deposit Account No. 08-0380 in the amount of $ ______________ __
`
`A check in the amount of $~1~,~3~1~2~ ________________ _ to cover the filing fee is enclosed.
`
`:~;
`
`Authorization is hereby granted to charge payment of the following fees
`associated with this communication or credit any overpayment to Deposit Account
`No. 08-0380. Two duplicate copies of this letter are enclosed.
`
`~~lgJ
`"~~
`
`Any additional filing fees required under 37 CFR 1.16.
`
`Any patent application processing fees under 37 CFR 1.17.
`
`Authorization is hereby granted to charge payment of the following fees
`during pendency of this application or credit any overpayment to Deposit Account
`No. 08-0380. Two duplicate copies of this letter are enclosed.
`
`Any patent application processing fees under 37 CFR 1.17.
`
`Any filing fees under 37 CFR 1.16 for presentation of extra claims.
`
`Respectfully submitted,
`
`HAMILTON, BROOK, SMITH & REYNOLDS
`
`egistration No.
`Attorney for Applicant(s)
`(617) 861-6240
`
`

`
`MIT7581L.APL
`RJS5
`03/20/97
`
`PATENT APPLICATION
`Docket No.: MIT-7581L
`
`-1-
`
`LASER-INDUCED CUTTING OF METAL INTERCONNECT
`
`--
`
`'tI
`>-i o
`
`10
`
`15
`
`BACKGROUND OF THE INVENTION
`The use of lasers to cut integrated circuits is well(cid:173)
`known in the art.
`In existing methods, a laser is directed
`5 onto a cut-link segment of the circuit. The laser supplies
`sufficient heat to vaporize the cut-link, thereby severing
`the cut-link.
`In many cases such as in a typical
`microchip, the circuit is coated with a passivation layer
`to protect the circuit from oxidation. To allow the cut-
`link to vaporize, the passivation layer is usually etched
`away from the site above the cut-link to expose the cut(cid:173)
`link to the atmosphere in advance of cutting.
`Lasers are used to cut circuits for a variety of
`purposes including the avoidance of defects, the
`replacement of defective circuits with redundant circuits,
`and the programming of circuits. The practices used to
`achieve each of these objectives are described, below.
`Lasers have been used to avoid defects by
`circumventing a short through laser-induced cuts. This
`20 practice is described in U.S. Patent No. 4,259,367, issued
`to Doherty, Jr. The circuit is repaired by using a laser
`to cut the lines connected to the short to thereby isolate
`the short. Patch lines may then be used to reestablish
`severed connections.
`The use of redundant circuitry has become an industry
`standard for memory chip fabrication. Redundant circuitry
`in a chip is typically supplied in the form of identical
`segments of circuitry occupying remote areas on the chip.
`If a segment is flawed, that segment is disconnected and
`replaced by activating its redundant counterpart elsewhere
`on the chip.
`
`25
`
`30
`
`

`
`-2-
`
`Finally, circuits may be programmed by cutting lines
`from a generic, repeating pattern to disconnect unwanted or
`unnecessary segments and thereby produce the desired
`pattern through a process of reduction. Circuitry produced
`5 by this method may extend across a plurality of levels as
`well. One example of multi-level circuit design by laser(cid:173)
`induced cutting is disclosed in u.s. Patent No. 4,720,470,
`issued to Johnson.
`Typically, the cut-link has taken one of two forms.
`10 First, the cut-link is often an undistinguished segment of
`a line in the circuit, where the width of the cut-link is
`equal to the width of the lines to which it is conductively
`coupled. A cut-link exhibiting these characteristics is
`illustrated in Figure 1 and disclosed, for example, in the
`following u.s. patents: No. 4,853,758, issued to Fischer,
`and No. 5,589,706, issued to Mitwalsky et al. The second
`common form is that of a
`IIdog bone, It which is illustrated
`in Figure 2. This design is disclosed, for example, in the
`following u.s. patents: No. 4,748,491, issued to Takagi,
`and No. 5,374,590, issued to Batdorf et al.
`In this
`design, the cut-link 20 is narrower than the lines 21, 22
`to which it is connected.
`
`20
`
`15
`
`DISCLOSURE OF THE INVENTION
`Whereas the design of earlier cut-links mirrors the
`25 narrowed Itdog-bone lt of conventional fuse design, a
`preferred embodiment of this invention rejects this model
`and, instead, widens the segment where the circuit is to be
`severed. Where 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 conductive material is supplied
`by an external source, i.e., the laser. As the present
`inventors have recognized, the use of the laser frees the
`designer from the necessity of using a high resistance
`
`30
`
`

`
`-3-
`
`5
`
`15
`
`segment to generate the heat necessary to cut the circuit.
`Although intuition might further suggest that a fuse-shaped
`cut-link of thin width could be severed with greater
`precision and efficiency than an otherwise comparable cut-
`link of greater width, the present inventors have
`recognized that this notion is generally false.
`To the contrary, an electrical interconnect which may
`be cut with greater success and with improved efficiency
`includes a cut-link pad in which the thermal resistance per
`10 unit length is lower, rather than higher, than the
`connected lines.
`In a preferred embodiment, the thermal
`resistance is lowered by adopting a form that is the
`inverse of the traditional IIdog bone ll design. The form of
`this new design is such that the width of the cut-link pad
`is substantially greater than the width of the lines.
`The electrical interconnect of this invention includes
`an insulating substrate upon which a pair of electrically(cid:173)
`conductive lines are bonded to a cut-link pad, wherein the
`cut-link pad has substantially less thermal resistance per
`unit length, i.e., along the axis of the lines, than each
`of the lines to which it is bonded. The cut-link pad may
`lie in the same plane as the lines, or the pad may lie on a
`plane which is merely intersected by the lines, as in a via
`structure where the lines, or vias, extend from the
`25 underside of the pad into the substrate to a lower level of
`circuitry. Alternatively, the cut-link pad may be bonded
`both to a line on the same plane as the pad and also to a
`via extending from the pad deeper into the substrate.
`In a preferred embodiment of the electrical
`interconnect, the thermal resistance of the pad is
`decreased, relative to the lines, by increasing the width
`of the pad such that the pad's width is greater than that
`of the lines to which it is connected. Benefit is provided
`in an interconnect where the cut-link pad is just 10
`35 percent greater than the width of each of the lines.
`
`20
`
`30
`
`

`
`-4-
`
`5
`
`Benefits rise in significance when the pad has a width that
`is at least 25 percent greater than the width of each of
`the lines. The full advantage of this embodiment r however,
`is usually not realized until the width of the pad is at
`least 50 percent greater than the width of each of the
`lines.
`In a further preferred embodiment, the width of the
`pad is approximately equal to its length, i.e., the
`distance between the lines.
`In an alternative embodiment, the thermal resistance
`10 of the pad is decreased, relative to the lines, by
`comprising the cut-link pad of a material having lower
`thermal conductivity than the material comprising the
`In another alternative embodiment, the per-unit-
`lines.
`length thermal resistance of the pad is decreased both by
`15 comprising the cut-link pad of a material having greater
`thermal conductivity than the material of the lines and by
`providing the pad with a width greater than that of the
`lines.
`In another preferred embodiment, the surface of the
`substrate upon which the circuit lies is comprised of a
`silicon oxide, and the circuit is covered by a passivation
`layer which prevents oxidation of the circuit yet which can
`be removed from the site of the cut-link pad upon heating
`the pad with the laser. To promote the desired fracture of
`the passivation layer, the passivation layer is preferably
`comprised of a material, such as silicon nitride, that is
`harder than the substrate.
`In the method of this invention a laser is directed
`upon a cut-link pad, as described above, and the laser is
`30 maintained there until the conductive link between the
`lines is broken. Preferably, the laser beam covers the
`entirety of the cut-link pad when the laser is directed
`upon the pad.
`The metallic cut-link pads of the present invention
`35 can be more efficiently and more effectively ablated for
`
`20
`
`25
`
`

`
`-5-
`
`S
`
`the following reasons. First, the structures of the
`present invention retain thermal energy at the site of the
`cut-link more effectively than the cut-links of the prior
`art by restricting dissipative heat transfer from the cut-
`link into the connected lines. Second, the shapes of
`preferred structures produce increased stress at the
`perimeter of the cut-link pad, thereby generating a more
`forceful fracture of the surrounding material. Third, the
`shapes of preferred structures create stress concentration
`10 points that will produce fractures radiating outwardly from
`the site of the cut-link pad, thereby improving the
`likelihood that the fracture will be clean, i.e., will not
`create inter-linked fracture passages through which
`escaping metal may form a
`11 short 11 defeating the attempt to
`15 cleanly sever the circuit. Fourth, where the cut-link pad
`more closely approximates the size and shape of the laser
`beam, the cut-link absorbs a greater portion of the laser's
`energy, thereby producing a more efficient transfer of
`energy and a reduced danger of damaging the surrounding
`20 material. Fifth, where a passivative coating comprised of
`a brittle material is used, the fracture is biased toward
`the passivative coating, and, hence, toward the surface of
`a chip for efficient removal of the passivative coating and
`non-damaging expUlsion of the metal comprising the cut-link
`25 pad. Sixth, by trapping the heat within the confines of
`the cut-link pad and limiting its escape through the
`connected lines, the site at which the cut develops is
`confined within this narrow region and damage to other
`parts of the circuit is minimized.
`Each of these
`30 phenomena will be discussed in greater detail and with
`greater clarity in the context of describing the structure
`and methods of several preferred embodiments of this
`invention.
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`BRIEF DESCRIPTION OF THE DRAWINGS
`The 'foregoing and other objects, features and
`advantages of the invention will be apparent from the
`following, more particular description of preferred
`embodiments of the invention, as illustrated in the
`accompanying drawings in which like reference characters
`refer to the same parts throughout the different views.
`The drawings are not necessarily drawn to scale, emphasis
`instead being placed upon illustrating the principles of
`the invention.
`Figure 1 is a cross-sectional illustration of a cut(cid:173)
`link pad of the prior art, from a perspective normal to the
`plane of the substrate, where the width of the pad is equal
`to the width of the lines.
`Figure 2 is a cross-sectional illustration of a cut-
`link pad of the prior art, from a perspective normal to the
`plane of the substrate, where the interconnect has the
`shape of a dog bone.
`Figure 3 is a cross-sectional illustration of a cut(cid:173)
`link pad in the form of a square, from a perspective normal
`to the plane of the substrate, where the width of the pad
`is greater than the width of each of the lines.
`Figure 4 is a perspective view of the cut-link pad and
`the lines shown in Figure 3.
`Figure 5 is a cross-sectional illustration of the cut(cid:173)
`link pad, from a perspective within the axis of the lines,
`illustrating the fractures which typically form during
`cutting.
`Figure 6 is a cross-sectional illustration of a cut-
`link pad in the form of a square, from a perspective normal
`to the plane of the substrate, where the width of the pad
`is greater than the width of each of the lines.
`Figure 7 is a cross-sectional illustration of a
`hexagonally-shaped cut-link pad from a perspective normal
`to the plane of the substrate.
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`Figure 8 is a cross-sectional illustration of an
`electrical interconnect, from a perspective normal to the
`plane of the substrate, where the width of each of the
`lines is equal to the width of the cut-link pad, except in
`the vicinity of the juncture between the line and the pad,
`where the width of the line narrows.
`Figure 9 is a cross-sectional illustration similar to
`Figure 8, except that the cut-link pad is in the form of a
`hexagon, rather than a square.
`Figure 10 is a side-view, through the substrate, of a
`multi-level via structure from a perspective within the
`plane of the substrate.
`Figure 11 is a view of the multi-level circuit of
`Figure 10 from underneath the via structure.
`Figure 12 is a chart illustrating the approximate
`shape of a cumulative probability function measuring the
`likelihood that a circuit will be cut over a range of
`energy levels.
`Figure 13 is a chart illustrating the approximate
`shape of a cumulative probability function measuring the
`likelihood that the integrated circuit will be damaged over
`the same range of energy levels depicted in Figure 12.
`Figure 14 is a chart illustrating the approximate
`shape of the combined cumulative probability that a circuit
`25 will be cut without otherwise damaging the circuit over the
`same range of energy levels charted in Figures 12 and 13.
`
`15
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`20
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`DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
`A cut-link shaped in the form of a square pad is
`illustrated in Figure 3. The cut-link pad 20, is bonded to
`a pair of lines 21 and 22, each positioned on the substrate
`of a chip or circuit board within a single plane. The pad
`and the lines are all electrically conductive, whereas that
`part of the substrate that contacts the metal is
`dielectric. Although this embodiment could just as fairly
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`be described as a single line including a cut-link site l
`the embodiment is herein broken down into narrower
`constituents to provide greater clarity when different
`embodiments are discussed and differentiated l below. As
`can be seen by juxtaposing the cut-link of this figure with
`those illustrated in Figures 1 and 21 the cut-link pad of
`Figure 3 has a much greater width in relation to the width
`of each of the attached lines. The reasons for widening
`the cut-link are explained below.
`First l ablation l or rupture 1 of the cut-link requires
`an accumulation of thermal energy within the cut-link.
`the accumulation of thermal energy within
`During cutting J
`the link can be roughly measured as the difference between
`the energy supplied by the laser and the heat conductively
`transferred out of and away from the cut-link. Therefore 1
`the cut-link can be ablated from its site most efficiently
`by maximizing the absorption of laser energy and minimizing
`the transfer of that energy 1
`in the form of heat l away from
`the cut-link.
`The amount of laser energy absorbed by the cut-link is
`maximized by providing a cut-link shaped in a form
`approximating that of the laser beam spot 24. Although the
`i.e' l no clear
`shape of the beam spot is Gaussian l
`delineation exists for identifying the outer edge of the
`spot; the position of the edge may be approximated using l
`for example 1
`the full-width half-max method. Using the
`full-width half-max method l
`the edge of the spot is
`designated as the boundary at which the intensity of the
`laser drops to half of the maximum intensity. When
`30 cutting l
`the cut-link should ideally be entirely within the
`beam spot as determined above. Absorption across the
`entire surface promotes uniform heating of the pad. An
`infrared laser typically produces beams having a minimum
`the tolerance for
`diameter of about two microns. Further l
`35 beam positioning error is typically about 0.2 to 0.5
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`20
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`microns. Accordingly, a pad designed to have a preferable
`length and width of two to three microns can absorb the
`bulk of the energy transmitted by the laser while remaining
`entirely within the beam spot.
`Transfer of heat away from the cut-link is minimized
`by creating a comparatively-lower thermal resistance per
`unit length within the pad than without. Because the top
`layer of the substrate and any passivative coating are
`insulators, the primary mode through which heat is
`transferred from the pad is through the connected lines.
`AccordinglYI the direction of heat transfer out of the pad
`can be approximated as being along axis A-A' I
`shown in
`Figure 3. Along axis A-A'I thermal resistance is a
`function of the heat transfer coefficient of the conducting
`15 material and the cross-sectional area of the conducting
`material measured in the plane normal to the axis. The pad
`and the lines are preferably comprised of a material with
`high thermal conductivity and high thermal expansion l such
`as aluminum and copper.
`In such a case, where the
`20 composition of the pad is identical to the composition of
`each of the lines, a differential in thermal resistance is
`due primarily to a difference in cross-sectional area. The
`dimensions of the cross-section normal to the A-AI axis are
`illustrated in Figure 4. As shown, the cross-section of
`the line (the product of wand h) is much smaller than the
`cross-section of the pad (the product of w' and h').
`Accordingly, the thermal resistance along the A-A' axis
`will be much greater in each of the lines than in the pad.
`This differential reduces the loss of heat into and through
`the line and thereby allows the temperature of the pad to
`quickly rise.
`the difference in thermal resistance
`Moreover I
`produces a sharp temperature differential between the pad
`and the lines to create stress at the interface which
`35 promotes a clean severance at the interface between the pad
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`25
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`and the line. The difference in thermal resistance also
`creates a boundary limiting the area within which fracture
`will occur.
`I.e., fracture will not initiate from a
`comparatively cool region beyond the area where heat is
`5 concentrated. The differential on each side of the pad
`thereby ensures that fracture will commence from the pad,
`rather than from a line, providing greater cutting accuracy
`and minimizing damage to the surrounding circuitry.
`Means other than increasing the width of the pad exist
`for creating a differential in thermal resistance at the
`interface of the pad and the line. For example, in one
`embodiment, the pad may be fabricated from a material
`having greater thermal conductivity than the material from
`which the lines are comprised.
`In another alternative
`embodiment, the pad may be both wider than the lines and
`comprised of a material having greater thermal conductivity
`than the lines.
`In addition to improving the absorption and retention
`of thermal energy from the laser, creating a roughly
`square-shaped pad reduces the likelihood of producing a
`short circumventing the cut. As noted, the metal from the
`cut site escapes through fractures in the passivation
`layer. After or during a cut, the circuit will be shorted
`if metal migrates through the fractures to provide a
`25 continuous link from one line to the other. Because a
`comparatively large void is left after the ablation of a
`wide, square cut-link, the passage created by a fracture
`and the accompanying flow of metal linking the two lines
`must cover a greater distance to circumscribe the void and
`30 create a short. Further, cracks generated in the
`horizontal plane on which the circuit lies will typically
`emanate from the corners where stress is concentrated. As
`is shown in Figure 5, cracks 32a and 32b emanating from the
`edges 36 of a square-shaped pad 20 will typically be
`35 directed outward at an angle of 135 0
`from the sides of the
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`pad 20 mitigating the likelihood that cracks from opposite
`sides of the pad will cross to create a passage for forming
`a short.
`Designing the cut-link pad to approximate the size and
`shape of the laser beam spot also provides an additional
`benefit. Less energy misses the cut-link pad resulting in
`less energy being absorbed into the surrounding dielectric
`material which reduces the likelihood that this surrounding
`material will be damaged by thermal stresses.
`Both the lines and the cut-links 20 in a circuit are
`coated with a passivation layer 30 to protect the circuit
`from oxidation as a result of exposure to air. For the
`cut-link 20 to be ablated, fractures 32 must be created
`within the passivation layer 30, which allow the section of
`15 passivation layer 30 covering the cut-link 20 to be blasted
`away from the chip, thereby exposing the cut-link 20 to air
`and allowing it to vaporize. Preferably, a hard, brittle
`material, such as silicon nitride is used as the
`passivation layer 30 because increasing hardness and
`brittleness is correlated with increasing susceptibility to
`fracture. When a laser pulse is incident on the cut-link
`pad 20, the cut-link pad 20 is heated and expands. The
`passivation layer 30 can not be heated as quickly as the
`metal lines because the passivation layer 30 can not absorb
`25 energy from the laser beam directly and can not conduct
`heat from the metal quickly. As a result, stress develops
`within the passivation layer 30 due to the difference in
`temperature between the metal 20 and the passivative
`coating 30 as well as the mismatch of the thermal expansion
`30 coefficients of the two materials. As the metal 20 expands
`with increasing temperature, greater tensile stress i