U.S. patent application number 10/058363 was filed with the patent office on 2002-08-01 for vertical fuse structure for integrated circuits and a method of disconnecting the same.
This patent application is currently assigned to CLEAR LOGIC, INC.. Invention is credited to Huggins, Alan H., MacPherson, John, Schmidley, Richard J..
Application Number | 20020100958 10/058363 |
Document ID | / |
Family ID | 22859603 |
Filed Date | 2002-08-01 |
United States Patent
Application |
20020100958 |
Kind Code |
A1 |
MacPherson, John ; et
al. |
August 1, 2002 |
Vertical fuse structure for integrated circuits and a method of
disconnecting the same
Abstract
A vertical fuse structure and methods for customization of
integrated circuits include a substantially vertically-oriented
interconnect structure or "fuse" which provides for a more densely
packed and thus smaller programmable integrated circuit. In a
preferred embodiment, a vertical interconnect structure is
fabricated by forming a first interconnect layer, forming an
insulating layer over the first interconnect layer in which
substantially vertically-oriented vias are patterned in contact
with the first interconnect layer, filling the vias with a
conductive plug, and forming a second interconnect layer over the
insulating layer in contact with the conductive plug. The vertical
interconnect structure is preferably disconnected by forming a
narrow, substantially vertical disconnect cavity through the second
interconnect layer and a portion of the conductive plug, thereby
removing the connection between the second interconnect layer and
the plug.
Inventors: |
MacPherson, John; (Fremont,
CA) ; Huggins, Alan H.; (Gilroy, CA) ;
Schmidley, Richard J.; (San Jose, CA) |
Correspondence
Address: |
David A. Jakopin
PILLSBURY WINTHROP LLP
1600 Tysons Boulevard
McLean
VA
22012
US
|
Assignee: |
CLEAR LOGIC, INC.
|
Family ID: |
22859603 |
Appl. No.: |
10/058363 |
Filed: |
January 28, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10058363 |
Jan 28, 2002 |
|
|
|
09229040 |
Jan 12, 1999 |
|
|
|
Current U.S.
Class: |
257/529 ;
257/E23.149; 438/132 |
Current CPC
Class: |
H01L 2924/0002 20130101;
H01L 2924/0002 20130101; H01L 2924/00 20130101; H01L 23/5256
20130101 |
Class at
Publication: |
257/529 ;
438/132 |
International
Class: |
H01L 021/82; H01L
029/00 |
Claims
We claim:
1. A method for disconnecting a circuit comprising: providing on a
substrate a first conductive layer, an insulating layer disposed
over the first conductive layer, a second conductive layer disposed
over the insulating layer and the first conductive layer, and a
conductive link connecting the first conductive layer to the second
conductive layer, the conductive link having an outer surface
peripheral area where the conductive link electrically connects to
the second conductive layer; and forming a disconnect cavity
through a portion of the second conductive layer and a portion of
the conductive link such that electrical connection between the
second conductive layer and the conductive link is removed, the
disconnect cavity having a larger outer surface peripheral area
than the outer surface peripheral area of the conductive link.
2. A method according to claim 1, wherein the outer surface
peripheral area of the disconnect cavity is in part determined
based upon possible alignment errors that exist when forming the
disconnect cavity.
3. A method according to claim 1, wherein the step of providing is
preceded by disposing a photoresist layer on the upper surface of
the second conductive layer and providing for a disconnect hole in
the photoresist layer.
4. A method according to claim 3, wherein the step of forming is
accomplished by etching away the portion of the second conductive
layer and the portion of the conductive link using the disconnect
hole as a guide.
5. A method according to claim 4, wherein the disconnect hole
exposes only the second conductive layer.
6. A method according to claim 1, wherein the step forming is
accomplished by irradiation.
7. A method according to claim 1, further comprising the step of
sealing the disconnect cavity.
8. A method according to claim 1 wherein the step of providing
provides the conductive link as a plug in a via.
9. A method according to claim 8 wherein the step of providing
provides a plug that has substantially vertical sidewalls, and the
step of forming provides the disconnect cavity having substantially
vertical sidewalls.
10. A method according to claim 1, wherein an etching
characteristic of the insulating layer is matched to an etching
characteristic of the conductive link.
11. A method according to claim 1 wherein the step of forming
substantially completely removes the plug and maintains the
integrity of the first conductive layer.
12. An interconnect apparatus for a fusible link array in an
integrated circuit comprising: a first signal line; a conductive
interconnect; a second signal line; insulating material disposed
between the first signal line and the conductive interconnect and
the second signal line and the conductive interconnect; a first
plug disposed within a first portion of the insulating material
that electrically connects the first signal line to the conductive
interconnect; and a second plug disposed within a second portion of
the insulating material different from the first portion which
originally electrically connected the conductive interconnect to
the second signal line, and wherein subsequently a portion of the
second plug has been removed, thereby preventing an electrical
connection between the conductive interconnect and the second
signal line.
13. An apparatus according to claim 12 wherein the first signal
line and the second signal line are disposed on a same
semiconductor layer.
14. An apparatus according to claim 12 wherein the first signal
line and the second signal line are disposed on a different
semiconductor layer.
15. An apparatus according to claim 12 further including: a third
signal line; insulating material disposed between the third signal
line and the conductive interconnect; a third plug disposed within
a third portion of the insulating material that electrically
connects the third signal line to the conductive interconnect.
16. An apparatus according to claim 15, wherein a portion of the
third plug has been removed, thereby preventing an electrical
connection between the conductive interconnect and the third signal
line.
17. An apparatus according to claim 12 wherein the first and second
plugs are disposed in vias formed in the insulator, wherein the
electrical disconnection is caused by a disconnect cavity, and the
first and second plugs and the disconnect cavity have substantially
vertical sidewalls.
18. An apparatus according to claim 12 wherein an etching
characteristic of the insulating material is matched to an etching
characteristic of the second plug.
19. An apparatus according to claim 12 wherein substantially
completely all of the second plug is removed to prevent the
electrical connection between the conductive interconnect and the
second signal line.
20. An interconnect apparatus for a fusible link array in an
integrated circuit comprising: an input line; an output line that
crosses the input line at a predetermined position, the input line
and the output line disposed on different semiconductor layers; an
insulating layer disposed between the input line and the output
line; a plug disposed at the predetermined position and within a
first portion of the insulating material between the input line and
the output line, the plug originally providing an electrically
connection of the input line to the output line, and wherein
subsequently a portion of the plug has been removed, thereby
preventing the electrical connection between the input line and the
output line.
21. An apparatus according to claim 12 wherein substantially
completely all of the plug is removed to prevent the electrical
connection between the input line and the output line.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to the manufacture of semiconductor
devices. More specifically, it relates to semiconductor devices
containing fuses that are disposed in a compact interconnect
structure and a method for disconnecting them.
[0003] 2. Background of the Related Art
[0004] Custom electronic devices which are made from standard
components suffer from several disadvantages. Since several
components are required to implement such a custom device, more
circuit board space is required than if a custom or semi-custom
integrated circuit had been used in place of the standard
components. This makes the overall size of the device larger and
more expensive. Furthermore, the assembly process is longer, more
costly, and prone to reworks or scrap, since more components are
used. Further, a larger number of components usually requires more
power consumption, which means a larger, heavier, and more
expensive power supply. Therefore, the overall size, weight, and
cost of the resulting custom device may make it unappealing to the
consumer or not competitive when compared to a similar product
offered by another company. Therefore, custom or
application-specific integrated circuits (ASICs) are frequently
used to implement new circuit designs in the place of standard
components.
[0005] There are several types of ASICs which are available,
depending on size, power, and programmability requirements, and
volume of devices used. Fully custom devices offer the lowest cost
and least amount of power consumption, but are only economical in
very large quantities because of the costs of a custom mask set and
engineering design time. Semi-custom devices such as gate arrays
require a smaller number of custom masks and design time as
compared to fully custom devices, but have a larger die size, cost
more to produce, and are typically used when needed quantities are
not quite as large. Similarities exist in custom and semi-custom
devices in that both have relatively long lead times to produce
prototypes and production volumes, and the designs are expensive
and time consuming to change. Since the non-recurring costs are so
large, it is not economical to produce them in small quantities.
Other non-custom devices, such as programmable array logic (PALs),
field-programmable gate arrays (FPGAs), and programmable logic
devices (PLDs), are fabricated as unprogrammed "blanks" which are
programmed by the end user as packaged units or after installation
onto a circuit board. These non-custom devices have a lower cost
basis for smaller quantities, since no custom masks are required.
The lead time to produce prototypes and production quantities is
short, since the programming is performed near the end of the
manufacturing cycle. They are also useful for designs which are
expected to undergo revisions, since virtually no programmed
material needs to be inventoried. However, these non-custom devices
have the drawback of requiring a relatively large amount of die
area dedicated to circuitry to perform the programming, and to
signal paths to provide flexibility in routing, therefore they are
more expensive to fabricate and don't achieve the same programmable
capacity as custom devices and gate arrays. In some cases, they
also use more power and are slower.
[0006] Programming of the non custom programmable devices is
sometimes accomplished through the use of disconnectable fuses. In
this specification, the word fuse will be used to refer to fuses,
anti-fuses, disconnection points, disconnectable links, or any
combination of these terms. In certain types of programmable
devices using electrically disconnectable or connectable fuses, the
fuses in the device which define how the circuit is configured are
accessed through the I/O pins on the device package. For techniques
which require joining conductive regions together (for example,
shorting a P/N junction), an excessive amount of current relative
to normal operating conditions is passed through the junction,
shorting it and allowing current to pass freely. To disconnect a
conductive line, a fuse element in an undesired conduction path is
subjected to enough current to heat it to its melting point,
causing structural breakdown and creating a disconnection. Both of
these methods require a relatively large amount of current to
program the device. The transistors for generating these large
currents in conventional MOS devices require large channel widths.
Furthermore, a certain amount of heat insulation area is required
around the fuses to prevent thermal damage to neighboring
circuitry. This makes it difficult to achieve high device packing
densities using these programming methods. Other devices have fuses
which are disconnected through the use of a laser or other radiant
energy beam device (hereinafter referred to as a "laser"). For
these devices, the laser is used to disconnect the fuses near the
end of or after the conclusion of the fabrication process. These
laser programmable devices have a smaller die size than the
electrically programmable devices, and don't require expensive
precision custom masks and long lead times like the custom and
semi-custom devices. The laser programmable devices are also
economical to produce in smaller quantities compared to custom
devices.
[0007] In addition to using fuses for the customization of an
integrated circuit to give it specific circuit or electrical
characteristics, fuses have also been used to (1) repair
non-functional devices through the selective deletion of defective
portions of the circuitry, or by substituting functional redundant
circuitry for the defective portions of the circuitry; and (2) mark
the device for identification of characteristics in a manner that
is readable visually or electrically, for example serialization of
the integrated circuit, or how the device has been configured by
the laser.
[0008] FIG. 1 shows an array of fuses that can be disconnected by a
laser. Fuse body 2 on fuse 1 is irradiated by laser beam 7, which
has an energy distribution that is approximately radial Gaussian in
nature. This energy distribution results in an effective laser spot
size 6, that is the area of the beam which has an energy sufficient
to disrupt active circuit elements. This disruption can be physical
damage which causes the device to be non-functional, or it could
cause performance degradation, such as silicon crystal dislocation
which causes current leakage. The two characteristics which define
the area requirements for a fuse, also called the fuse cell, are
the pitch of the fuses in a group, and the length of the fuse.
These two dimensions have a direct bearing on how much die area the
fuse cell occupies, and thus the overall die size. The pitch 5 of
the fuse cell is the distance from the center of one fuse to the
center of the nearest neighboring fuse. This dimension is
controlled by the requirement that the spot size 6 not disrupt any
other fuses, and is conventionally calculated by adding the
diameter of the spot size 6 and two times the maximum expected
alignment error in the placement of the spot. The length 8 of the
fuse is the sum of the length of fuse body 2 and fuse terminals 3.
The length 8 of the fuse 1 is controlled by the need to isolate the
thermal energy transmitted to the fuse by the laser from
interconnect lines 4 attached to the fuse. Another influence to the
area requirements of the fuse is whether circuitry can be routed
underneath or in close proximity to the fuse. Most design rules
specify that all of the area underneath of the fuse, and a certain
area around it, be free of active circuitry to protect it from
damage. Some designs provide for a barrier at another level between
the fuses and the active circuitry and thus utilize some of the
area.
[0009] As is also well known in the prior art, laser type fuses can
alternatively be disconnectable by photolithographic techniques
combined with etching to remove a section of the fuse, thus forming
a disconnection. Photoresist layer 21 in FIG. 2 is patterned to
make a hole 22 in the photoresist over the fuse 24. Well-known
etching techniques are then used to etch through the fuse 24,
completing the disconnection. As can be seen from FIG. 2, the
disconnection can be made cleanly and thoroughly without a risk of
damage to any underlying circuitry 25, which would not typically be
routed under the fuse, but is shown here to illustrate that such
routing can take place if desired. FIG. 3 illustrates that a
smaller disconnection hole 31 on fuse 32 permits a smaller pitch 5.
Since the thermal isolation requirements are not needed when using
photolithographic techniques, the length of the fuse can also be
shortened. The above-mentioned methods of disconnecting fuses can
be used, in varying degrees, to reduce required fuse cell area and
thus permit a smaller fuse cell and an advantageously smaller die.
Accordingly, it is desirable to have a fuse design that makes most
efficient use of these disconnection methods in order to increase
the packing density of the fuses and produce a smaller die size,
and a method of disconnecting that allows for such increased
packing density.
SUMMARY OF THE INVENTION
[0010] It is an object of this invention to decrease the amount of
space required for a fuse, or an array of fuses which form a
specific circuit such as an AND array, thus reducing the overall
size of the integrated circuit.
[0011] It is another object of the present invention to provide a
method of disconnecting fuses that allows reduction in the overall
size of the integrated circuit The present invention achieves the
above objects, among others, by providing a vertical fuse structure
and method for customization of integrated circuits containing such
a fuse structure that provides for a more densely packed and thus
smaller programmable integrated circuit. In a preferred embodiment,
the interconnect structure is fabricated by forming a first
interconnect layer, forming an insulating layer over the first
interconnect layer in which substantially vertically-oriented vias
are patterned in contact with the first interconnect layer, filling
the vias with a conductive plug, and forming a second interconnect
layer over the insulating layer in contact with the conductive
plug. The vertical interconnect structure is preferably
disconnected by forming a narrow, substantially vertical disconnect
cavity through the second interconnect layer and a portion of the
conductive plug, thereby removing the connection between the second
interconnect layer and the plug.
[0012] Thus, the area required for the link is only as large as the
cross-sectional area of the plug at the point of disconnection.
Since the cross-sectional diameter of the plug is typically smaller
than the width of the line connecting to it, the disconnection can
be made without breaking the continuity of the line. This feature
has benefits in optimizing the layout of the design to provide
packing densities that would not be achievable with conventional
fuse designs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The above and other objects, features, and advantages of the
present invention are further described in the detailed description
which follows, with reference to the drawings by way of
non-limiting exemplary embodiments of the present invention,
wherein like reference numerals represent similar parts of the
present invention throughout several views and wherein:
[0014] FIG. 1 is a top plan view of a conventional fuse array
within an integrated circuit device that illustrates the use of a
laser to disconnect a conventional fuse within the fuse array;
[0015] FIG. 2 is a side elevation cutaway view of a conventional
fuse structure within an integrated circuit device that illustrates
the use of photolithographic techniques combined with etching to
disconnect a conventional fuse;
[0016] FIG. 3 is a top plan view of a conventional fuse array
illustrating the ability to design smaller dimension fuses due to
the use of photolithographic techniques combined with etching to
disconnect a conventional fuse within the fuse array;
[0017] FIG. 4a is a top plan view showing the construction of a
preferred disconnect interconnect structure or "fuse" according to
the invention.
[0018] FIG. 4b is a cross-sectional view of the interconnect
structure of FIG. 4a;
[0019] FIGS. 5-7 illustrate a method of disconnecting the
interconnect structure of FIGS. 4a and 4b according to the present
invention;
[0020] FIG. 8 illustrates dimensions needed for fuses when
conventional fuses are disconnected using laser or
photolithographic techniques;
[0021] FIG. 9 illustrates dimensions needed for fuses when
conventional fuses are disconnected using a photolithographic
technique;
[0022] FIG. 10 illustrates dimensions needed for fuses when using
the structure and disconnection techniques according to the present
invention;
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] FIGS. 4a and 4b show respectively a top view and
cross-sectional view, the cross-section of FIG. 4b taken along the
line A-A' from FIG. 4a, which shows an interconnect structure
deposited on a substrate of active circuitry. The layout in FIG. 4a
is commonly used for interconnecting two conductive layers in an
integrated circuit. Substrate 41 is initially formed and underlies
first interconnect layer 42, which may be polysilicon, tungsten
silicide, aluminum, titanium, tungsten, or other alloys or
combinations of these materials. For simplicity, substrate 41 is
shown as a monolithic layer, but it should be recognized that the
substrate 41 could include various types of circuit elements. For
example, circuit elements or active circuitry such as transistors,
diodes, resistors, capacitors, other interconnect layers or
insulating layers, or even a P/N-well in an N/P-type semiconductor
substrate could be used instead of the substrate 41. An insulating
material such as doped or undoped silicon dioxide 43 is formed over
first interconnect layer 42 and patterned to form via 44. A filler
material for via 44, such as tungsten plug 45, is used to fill the
via and provide a vertical electrical connection to a second
interconnect layer 46.
[0024] It is recognized by the present invention that this
structure provides a disconnectable conductive path between the
first interconnect layer 42 and the second interconnect layer 46
through plug 45. A method of disconnecting this conductive path
will now be described with reference to FIGS. 5-7. As shown in FIG.
5 at the time that the circuit is ready for customization or
repair, a layer of photoresist 51 is applied. A disconnect hole 52
is formed in the photoresist 51 using well-known photolithographic
techniques. It is advantageous to design the top surface width of
the second interconnect layer 46 to be large enough so that the
disconnect hole 52 does not overlap its side edge and thereby
expose any top surface portion of insulating material 43 prior to
initiating the etch that will be subsequently described. This will
prevent first interconnect layer 42 from being subsequently and
undesirably etched. The minimum top surface width of the disconnect
hole 52 is determined by adding the nominal diameter for the via
44, the maximum sizing error for the via 44, and the maximum
registration error for the placement of disconnect 52 over via
44.
[0025] As shown in FIG. 6, the exposed portion of second
interconnect layer 46 is completely etched and the entire top
surface of plug 45A is etched to result in a disconnect cavity 61
by a process which removes second interconnect layer 46 and the
plug 45 at a rate which is substantially higher than that of
photoresist 51. As shown, if a stub 45B of the plug 45 remains
after the etch is complete, a complete electrical disconnection
will have been achieved. It is most desirable, however, to remove
the entire plug 45, thereby not leaving any stub 45B, while
ensuring that first interconnect layer 42 is not damaged. By
removing the entire plug 45, there will not remain the stub 45B,
which could, at the conclusion of the etch, break off and cause
shorting or contamination problems. For example, a plug which is
formed from tungsten can be further processed with hydrogen
peroxide to dissolve the plug while leaving the remainder of the
circuit unaffected. Thereafter, as shown by FIG. 7, the photoresist
51 is removed and a passivation layer 71 of silicon nitride or
other suitable material is used to seal the disconnect cavity 61
and provide a moisture barrier and physical protection to the
remainder of the die.
[0026] It is desirable to form the vias 44 with straight sidewalls
as opposed to a sloped profile so that the diameter at the top of
the via is maintained at the minimum dimensions allowed by the
design rules. This permits the greatest packing density of the
fuses. It is further desirable to use a material for the plug 45
which is different in composition than that used for the second
interconnect layer 46, such that the etch selectivity of the second
interconnect layer 46 to the plug 45 is approximately equal to the
selectivity of the second interconnect layer 46 and the underlying
insulating layer 43. If the material for the plug 45 is
substantially less selective than the insulating material used to
create insulating layer 43, the etchant can penetrate all of the
plug material and damage the first interconnect layer 42. If the
plug material is substantially more selective, the stump 45B of
plug material may remain at the conclusion of the etch which could
break off and cause shorting or contamination problems as
previously mentioned. It is, however, within the scope of the
present invention to use a disconnect cavity 61 that has larger,
non-critical dimensions, such that non-precision materials and
equipment can be used for the photomask, to create the patterns on
the photomask, and to transfer the disconnect hole pattern from the
mask to the wafer. This requires increasing the size of the second
interconnect layer pattern so that the disconnect hole 52 will fall
on the second interconnect layer 46 and not misalign such that it
overlaps its side edge, as has been previously discussed.
Alternatively, the disconnect hole 52 can be made to a minimum size
such that the packing density of the fuses is increased, reducing
the overall size of the die.
[0027] While the disconnection method according to the present
invention was described above using a photolithographic technique
that etched away material necessary to obtain a complete
disconnection, the present invention can also be implemented using
a laser removal technique, in which the laser spot is directed so
as to remove the plug 45, and the material from the second
interconnect layer 46 disposed above. When using a laser removal
technique, photoresist layer 51 is not needed, but the laser spot
will need to be directed to the appropriate location on the
semiconductor device. It should be noted, however, that the use of
a photolithographic disconnection technique is preferred, since
when a laser removal method is used, the thermal energy that is
created at the plug 45 is higher than that which is desired, which
has the potential for adversely affecting other interconnects.
[0028] Using the interconnect method of the present invention as
described above allows a substantial increase in the packing
density of fuses in a circuit. This will now be explained with
reference to FIGS. 8-10, in which FIG. 8 illustrates the area
needed for disconnecting fuses using conventional laser or photo
lithographic techniques, FIG. 9 illustrates the area needed for
disconnecting fuses using only photolithographic technique, and
FIG. 10, illustrates the area needed for disconnecting fuses using
the described techniques of the present invention.
[0029] In FIG. 8, a fuse structure using a conventional fuse design
is shown. As shown, the disconnectable links are designed so that
the link runs parallel to the plane of the die. This requires some
amount of lateral space to be dedicated to this link beyond that
required to make connections to other circuitry. Further, the
disconnection hole must be no smaller than the width of the link
itself, which in turn can be no smaller than the minimum design
rule width for that interconnect layer. Thus, a signal line 81 is
connected to signal lines 84 and 85 through conventional fuses 82
and 83, respectively. Signal line 84 may carry a "high" logic state
signal, while line 85 may carry a "low" logic state. During the
fuse configuration process, fuse 82 would be disconnected if the
transmission of a "low" logic state from line 85 is desired.
Alternatively, fuse 83 would be disconnected if the transmission of
a "high" logic state from line 84 is desired. One width dimension
86 required for this fuse structure is the sum of the fuse pitch
and the width of a single fuse. In this example, the following
drawn dimensions are given:
1 Fuse pitch: 7.0 .mu.m Fuse width: 3.5 .mu.m
[0030] for a total of 10.50 .mu.m. The length dimension 87 required
for this fuse structure is the length of the fuse, which is 14.00
.mu.m in this example. Thus, the area required to form this fuse
structure using a conventional fuse design is dimension 86
multiplied by dimension 87, or 147.0 .mu.m.sup.2. The fuse
structure illustrated in FIG. 8 takes into account fuse rules that
thus allow these fuses to be disconnected using either a laser or
conventional photolithographic masking and etch processes.
[0031] As is shown in FIG. 3, the dimensions of the fuse can be
reduced substantially if the fuse is disconnected using only by a
photolithographic process. FIG. 9 shows the same fuse structure
with the dimensions optimized for a photolithographic process. The
equivalent elements of the conventional fuse structure shown in
FIG. 8 are numbered the same in FIG. 9 with the addition of the
suffix "a". The length dimension 87a of the fuse structure is the
sum of the fuse body 94, plus four times the distance 91 from the
edge of the via 95 to the edge of the upper conductive layer 92,
plus two times the diameter of via 95. In this example, the
following drawn dimensions are given:
2 Fuse body 94: 2.4 .mu.m Distance 91: 0.8 .mu.m Via 95: 1.2
.mu.m
[0032] for a total of 8.0 .mu.m The width dimension 86a is the sum
of minimum space 96 between features on the second interconnect
layer 92, plus four times the distance 93 from the edge of via 95
to the edge of the upper conductive layer 92, plus two times the
diameter of via 95. In this example, the following drawn dimensions
are given:
3 Minimum space 96: 1.8 .mu.m Distance 93: 0.8 .mu.m Via 95: 1.2
.mu.m
[0033] for a total of 7.4 .mu.m. Thus, the area required to form
this fuse structure using a fuse design optimized for a masking
process is dimension 86a multiplied by dimension 87a or 59.2
.mu.m.sup.2.
[0034] Alternatively, as in FIG. 10, the same fuse circuit is
formed using the present invention. The equivalent elements of the
conventional fuse structure shown in FIG. 8 are numbered the same
in FIG. 10 with the suffix "b". It is noted that because
disconnection is made at the plug, as previously described, that
the layout of the circuit can be efficiently modified, and,
therefore, the number of plugs 45 needed can be reduced. As
modified, the dimension 86b of the fuse structure would be the
diameter of the fuse 82b plus two times the distance 101 from the
edge of the fuse 82b to the edge of the upper conductive layer 102.
In this example, the following drawn measurements are given:
4 Fuse diameter 82b: 1.2 .mu.m Distance 101: 0.8 .mu.m
[0035] for a total of 2.8 .mu.m. The other dimension 87b is the sum
of three times the diameter of the fuse 82b, plus six times the
distance 103 from the edge of the fuse 82b to the edge of the upper
conductive layer 102, plus two times the minimum space 104 between
signal lines 81b and 84b. In this example, the following drawn
dimensions are given:
5 Fuse diameter 82b: 1.2 .mu.m Distance 102: 0.8 .mu.m Minimum
space 104: 1.8 .mu.m
[0036] for a total of 12.0 .mu.m. Thus, the area required to form
this fuse structure using a fuse design of the present invention is
dimension 86b multiplied by dimension 87b or 33.6 .mu.m.sup.2. The
area required to implement the fuse design of the present invention
is less than one-quarter of the area using a conventional fuse
design, and less than 60% of the area using a fuse design optimized
for a photolithographic process.
[0037] It is also a feature of the present invention that a
disconnection can be made to a fuse within the array rather than at
the ends as described above, without disturbing the continuity or
current-carrying capability of the second interconnect layer in a
way that would be disruptive to the circuit. For example, in the
previous example shown in FIG. 10, the input signal lines 84b and
85b bracket output line 81b. However, it may become advantageous to
lay out the array such that lines 81b and 84b are the input lines
and line 85b is the output line. In this case, the fuse 105 for
line 81b can be disconnected while maintaining the connection
between line 84b and 85b through second interconnect layer 102. It
may, however, be necessary to increase dimension 86b to accomplish
this, depending on the design rules that are used.
[0038] While the present invention has been described herein with
reference to particular embodiments thereof, a latitude of
modification, various changes and substitutions are intended in the
foregoing disclosure, and it will be appreciated that in some
instances some features of the invention will be employed without a
corresponding use of other features without departing from the
spirit and scope of the invention as set forth.
* * * * *