Active solid-state devices (e.g. – transistors – solid-state diode – Integrated circuit structure with electrically isolated... – Passive components in ics
Reexamination Certificate
1999-01-04
2002-11-26
Thomas, Tom (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Integrated circuit structure with electrically isolated...
Passive components in ics
Reexamination Certificate
active
06486526
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the fabrication of integrated circuits and, more particularly, to a method for protecting fuses from damage while at the same time increasing the fuse density (i.e., the number of fuses per unit area) when employing a laser beam to blow selected fuses forming a fuse bank.
BACKGROUND OF THE INVENTION
Semiconductor integrated circuits (IC) and their manufacturing techniques are well known in the art. In typical integrated circuits, large number of semiconductor devices are fabricated on a silicon substrate. To achieve the desired functionality, a plurality of conductors are typically employed for coupling selected devices together. In some integrated circuits, some of the conductive links or wires may be coupled to fuses, which may be selectively programmed (i.e., blown) after fabrication using lasers. By way of example, in a dynamic random access memory (DRAM), fuses may be employed during manufacturing to protect from destruction some of the gate stacks of the transistors from inadvertent built-up charges. Once the fabrication of the IC is substantially complete, the fuses may be blown or cut to permit the DRAM circuit to function as if the protective current paths never existed. More commonly, fuses may be employed to set an enable bit and address bits of a redundant array element in a DRAM circuit or for repairing defects found in the DRAM by appropriate replacement of defective elements with redundancy replacement elements present within or without the chip.
To facilitate discussion,
FIG. 1
illustrates a typical dynamic random access memory (DRAM) integrated circuit, including a main memory array
102
. To allow the replacement of a defective main array element within main memory array
102
, a redundancy replacement array
104
is provided as shown. A plurality of fuses in fuse array
106
are coupled to redundancy array
104
via a fuse latch array
108
and a fuse decoder circuit
110
. In order to replace a defective main memory array element, individual fuses in fuse array
106
may be blown or cut by setting their values to either a binary 1 or 0 as dictated by the decoder circuit. During this operation, the values of the fuses in fuse array
106
are typically loaded into the fuse latch array
108
when power is turned on. These values are then decoded by fuse decoder circuit
110
during run time, thereby facilitating the replacement of selected defective memory array elements with specific redundancy elements which are part of the redundancy array
104
. Techniques for replacing failed main memory array elements with redundant array elements are well known in the art and will not be discussed in great detail here for brevity sake.
As previously mentioned, the fuse links within fuse array
106
may be selectively blown or programmed with a laser beam. Once blown, the fuse changes from a highly conductive state to a highly resistive (i.e., non-conductive) state, since a programmed fuse inhibits current from flowing through it and represents an open circuit to the path taken by the current.
With reference to
FIG. 2
a
, a fuse bank
200
is shown having a plurality of fuse links, such as
202
,
204
,
206
, and
208
, (represented in
FIG. 1
as fuse array
106
), are shown in their original unblown, i.e., conductive state.
In
FIG. 2
b
, a laser beam has been employed to cut or blow fuse link
204
, thereby inhibiting the flow of current therethrough. If the fuses are placed too closely together for a given laser wavelength and spot size, i.e., by deleting the fuse, an adjacent fuse link may inadvertently be blown or cut, rendering the IC defective or, at best, a possibility exists of causing damage to neighboring fuses during the fuse blow process. This is because the damage zone around a blown fuse is typically larger than the fuse itself in view of a number of factors, such as the laser spot size, thereby damaging the passivation layer over the fuses which extend outward from the fuses. Clearly many factors enter in determining the severity of the damage caused by laser beam, e.g., the amount of energy carried by the beam, its wavelength and the diameter of the beam.
In
FIG. 3
a
, there is shown a typical fuse bank
300
consisting of fuses
302
,
304
, and
306
. Additional structures, e.g.,
308
and
310
, are placed within the fuse bank and between the fuses. These structures, formed by barrier material, typically tungsten or molybdenum, act as crack stops when fuse
304
is blown by a laser beam (not shown). Further exhibited are cracks
320
propagating from fuse
304
and stopping at crack stop
308
and
310
.
In another view of the same structure (
FIG. 3
b
), a top view of the above structure built on a substrate
350
is illustrated. Only two fuses, i.e.,
302
and
304
, are exhibited. The crack stop is made up of several layers
330
through
340
which coincide with the several levels of wiring used in the fabrication process of the semiconductor chip. Once again, the cracks
320
are arrested by the crack stop, preferably made of portions of refractory metal.
Several other methods of protecting fuses have been advanced for protecting fuse elements to ensure that the fuse remains unaffected when the fuse blows open. In one example described in U.S. Pat. Nos. 5,420 455 and 5,523,253, both issued to Gilmour et al. on Mar. 31, 1994 and May 30, 1995, respectively, a deposition of metal having a melting point higher than the melting point of the fuses is interspersed between the fuses. Such metals include tungsten and molybdenum. These barriers are resistant to cracking and are designed to prevent any cracks from propagating to other adjacent fuses in the immediate neighborhood. The bodies of barrier material are positioned such that they extend from the top surface of the layer containing the fuse links to approximately half-way down to the mid-point of the thickness of the fuse link.
The structure described by Gilmour et al. suffers from a major drawback, in that it requires specialized metals, oftentimes undesired ones due to manufacturing process considerations and/or they suffer from unwanted characteristics which may adversely affect the integrity of the circuits making the integrated chip. Another drawback resides in the added requirement of process alterations necessitated by the presence of certain refractory metals which tend to increase the cost of the product, rendering them uneconomical. Yet a further drawback resides in the presence of conductive paths provided by the presence of portions of barrier metal which may, under certain circumstances, alter the electrical characteristics of the circuits forming the IC chip. Finally, the introduction of a metallic crack stop between fuse links as described by Gilmour et al. will also not work at tight pitches, since the crack stop itself will be ablated by the laser, causing damage to fuses or circuit elements next to it.
OBJECTS OF THE INVENTION
Accordingly, it is an object of the invention to provide an improved fuse structure and method for fabricating integrated circuits having laser fuse links.
It is another object of the invention to provide an improved fuse structure and a method therefor, which advantageously allows more fuses to be concentrated into a given space by reducing the distance (pitch) between adjacent fuses.
It is still another object to prevent either the energy inherent of a laser beam or the damage caused by the fuse blowing, to reach or affect any adjacent fuse links or circuit elements in the immediate vicinity of the fuse link being programmed.
It is a further object to protect fuse links which span over several wiring layers when at least some of the fuse links within the structure are programmed by a laser beam.
SUMMARY OF THE INVENTION
In one aspect of the invention, the damage done to areas surrounding fuses packed closely together is limited by creating material discontinuities in the form of voids between fuses which will act as crack arresting structures. These “crack stops” can then be used in various configurat
Brintzinger Axel C.
Kiewra Edward W.
Narayan Chandrasekhar
Radens Carl J.
International Business Machines - Corporation
Owens Douglas W.
Schnurmann H. Daniel
Thomas Tom
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