Metal wire fuse structure with cavity

An integrated circuit has primary devices and redundant devices being selective substituted for the primary devices through at least one fuse. The fuse includes a first layer having at least one fuse link region, a second layer over the first layer and cavities in the second layer above the fuse link region.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention generally relates to the fabrication of integrated
 circuits. More particularly, the present invention relates to improved
 techniques for increasing circuit density and/or reducing substrate damage
 in an integrated circuit employing fusible links.
 2. Description of Related Art
 Semiconductor integrated circuits (IC) and their manufacturing techniques
 are well known. In a typical integrated circuit, a large number of
 semiconductor devices may be fabricated on a silicon substrate. To achieve
 the desired functionality, a plurality of conductors are typically
 provided to couple selected devices together.
 In some integrated circuits, conductive links are coupled to fuses, which
 may be cut or blown after fabrication using lasers or electrical power. In
 a dynamic random access memory (DRAM) circuit, for example, fuses may be
 employed during manufacturing to protect some of the transistors' gate
 stacks from an inadvertent built-up of charges. Once 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.
 Alternatively, the fuse links may be used to re-route the conductive lines
 and hence modify the functionality of the circuit.
 Laser fusible links are generally metal lines that can be explosively fused
 open by application of laser energy. The laser energy causes a portion of
 the link material to vaporize and a portion to melt. Typically, the
 fusible link is thin and is made of aluminum or polysilicon or it may be
 made of the same metals as the chip conductors. In operation, a short
 pulse of laser energy in predetermined arcs (spot) is impinged upon the
 link.
 Electrically fusible links comprise metal lines that can be fused open by
 application of electrical power which induces a portion of the link
 material to vaporize, melt or otherwise be caused to form an electrical
 discontinuity or "open". Typically, the electrically fusible link is
 formed of a metallic conductor (such as aluminum) or a polysilicon. In
 operation, a short pulse of electrical power is applied to induce the fuse
 to open.
 Since every link is not necessarily blown, it is important to ensure that
 adjacent fuses are not blown by reflected light or thermal energy. Two
 methods are currently used to ensure that only the desired fuses are blown
 and that adjacent fuses are not inadvertently blown. The first method
 simply spaces the fuses two or three spot diameters apart. The second
 method builds metal walls between the adjacent fuses. Both those methods
 result in large fuse pitches and significant use of chip area.
 In cases where the fusible links: are built from the same material as the
 chip conductors; become thicker; are made of composite layers including
 layers of refractory metals (tungsten and various silicides); or are
 comprised of highly reflective metals (copper/aluminum), blowing the fuses
 with lasers becomes more difficult.
 The increasing speed requirements of logic chips are the driving force
 behind these fusible link materials. More commonly, fuses may be employed
 to set the enable bit and the address bit of a redundant array element in
 a DRAM circuit.
 FIG. 1 illustrates a typical dynamic random access memory integrated
 circuit, having a main memory array 102. To facilitate replacement of a
 defective main array element within the main memory array 102, a redundant
 array 104 is provided as shown. A plurality of fuses in a fuse array 106
 are coupled to the redundant array 104 via a fuse latch array 108 and a
 fuse decoder circuit 110. To replace a defective main memory array
 element, individual fuses in the fuse array 106 may be blown or cut to set
 their values to either a "1" or a "0" as required by the decoder circuit.
 During operation, the values of the fuses in the fuse array 106 are
 typically loaded into the fuse latch array 108 upon power up. These values
 are then decoded by the fuse decoder circuit 110 during run time, thereby
 facilitating the replacement of specific failed main memory array elements
 with specific redundant elements of the redundant 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's sake.
 As mentioned above, the fuse links within fuse array 106 may be selectively
 blown or cut with a laser beam or electrical power. Once blown the fuse
 changes from a highly conductive state to a highly resistive (i.e.,
 non-conductive) state. A blown fuse inhibits current from flowing through
 and represents an open circuit to the current path. With reference to FIG.
 2A, fuse links 202, 204, 206, and 208 of the fuse array element 106 are
 shown in their unblown (i.e., conductive) state. In FIG. 2B, a laser beam
 or electrical power has been employed to cut or blow one fuse link 204,
 thereby inhibiting the flow of current therethrough.
 It has been found that the use of a laser beam to set, cut or blow a fuse
 link may render the area under the fuse link or adjacent fusible links
 vulnerable to laser-induced damage, mainly due to the absorption of laser
 energy during the fuse setting operation. Because of the possibility of
 laser-induced damage, the areas underlying the fuse links are typically
 devoid of semiconductor devices (e.g., transistors) and the fuses are
 spaced far apart in conventional systems.
 Even when there are no active devices beneath the fusible link or other
 closely spaced fusible links, the substrate itself may also experience
 some degree of laser-induced damage. This is because silicon, which is the
 typical substrate material, absorbs the laser energy readily, particularly
 when short wavelength lasers are employed. For this reason, lasers having
 relatively long wavelengths such as infrared lasers have been employed in
 conventional systems for the fuse setting operation.
 Even though infrared lasers are helpful in minimizing laser-induced damage
 to the underlying substrate, the use of lasers having relatively long
 wavelengths involves certain undesirable compromises. By way of example,
 the relatively long wavelength of the infrared laser forms a relatively
 large spot on the substrate during the fuse setting operation, which
 limits how closely the fuse links can be packed next to one another. For
 infrared lasers having a wavelength of, for example, about 1 micron, the
 spot created on the substrate may be two times the wavelength or about 2
 to 2.5 microns wide.
 The disadvantages associated with lasers having relatively long wavelengths
 is illustrated with reference to FIGS. 3A and 3B below. FIG. 3A is a
 cross-sectional view of a portion of the fuse array 106, including fuse
 links 202, 204, 206, and 208. In FIG. 3A, fuse links 202, 204, 206, and
 208 are shown encapsulated within a passivation layer 302. A substrate 304
 underlies the fuse links as shown. It should be noted that the
 illustration of FIG. 3A is highly simplified to facilitate illustration,
 and the fuse array 106 may include other conventional layers and/or
 components as is known.
 In FIG. 3B, fuse link 204 of FIG. 3A has been blown or cut using a laser
 beam. In place of fuse link 204, a void 310 exists, whose diameter C is
 roughly twice the wavelength of the laser beam employed. The diameter C of
 the laser spot places a lower limit on the minimum fuse pitch 312 between
 adjacent fuse links. If the fuses are placed too closely together for a
 given laser wavelength, an adjacent fuse link may be inadvertently blown
 or cut, rendering the IC defective.
 Using a laser with a shorter wavelength would reduce the diameter C of the
 laser spot and concomitantly the minimum fuse pitch. However, a shorter
 wavelength laser substantially increases the likelihood of underlying
 substrate damage in conventional systems since the silicon substrate
 absorbs laser energy from shorter wavelength lasers much more readily. If
 a shorter wavelength laser is employed to set the fuse links of the
 conventional system's fuse array 106, excessive substrate damage in area
 320 may result, possibly leading to integrated circuit defects and
 failure.
 In view of the foregoing, there is a conventional need for improved
 techniques for fabricating integrated circuits having laser and or
 electrically fusible links. More particularly, there is a conventional
 need for improved laser and/or electrical fuse link structures and methods
 therefor, which reduce the amount of damage caused when the fuse element
 blows.
 SUMMARY OF THE INVENTION
 It is, therefore, an object of the present invention to provide a structure
 and method for a dynamic random access memory integrated circuit which
 includes a main memory array having main memory array elements, a
 redundant memory array, coupled to the main memory array, including
 redundant memory array elements, at least one laser fuse link selectively
 substituting at least one of the redundant memory array elements for
 defective ones of the main memory elements upon application of laser
 energy and at least one cavity portion positioned between the laser fuse
 link and a source of the laser energy
 The laser fuse link can include a first conductive layer and a second
 conductive layer above the first conductive layer, the cavity portion is
 within the second conductive layer. The laser fuse link can also include a
 fuse window allowing the laser energy to reach the fuse link, the cavity
 is between the fuse link and the fuse window. The cavity directs energy
 and fuse material from the fuse link toward the fuse window. The dynamic
 random access memory includes conductive islands within the cavity. The
 conductive islands concentrate laser energy on the fuse link.
 The inventive integrated circuit includes primary devices and redundant
 devices being selectively substituted for the primary devices through at
 least one fuse. The fuse also includes a first layer having at least one
 fuse link region, a second layer over the first layer and a cavity in the
 second layer positioned with respect to the fuse link region to direct
 fuse material away from adjacent devices within the integrated circuit.
 The first layer includes a conductive layer and the second layer also
 includes a conductive layer. A fuse window allows laser energy to reach
 the fuse link region. The cavity is between the fuse link region and the
 fuse window. The cavity directs energy and fuse material from the fuse
 link region toward the fuse window. The integrated circuit also includes
 conductive islands within the cavity. The conductive islands concentrate
 laser energy on the fuse link region.
 The method for forming an integrated circuit fuse structure includes
 forming a fuse link layer, a second layer over the fuse link layer and at
 least one cavity in the second layer such that the cavity is positioned
 with respect to the fuse link region to direct fuse material away from
 adjacent devices within the integrated circuit. The second layer includes
 deposition processes and the second layer includes damascene processes. An
 insulating layer is formed over the second layer, wherein the fuse link
 layer includes a conductive layer and the second layer is a conductive
 layer. A fuse window over the second layer allows laser energy to reach
 the fuse link layer, wherein the cavity is between the fuse link layer and
 the fuse. The cavity directs energy and fuse material from the fuse link
 layer toward the fuse window. Conductive islands are formed within the
 cavity. The conductive islands concentrate laser energy on the fuse link.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
 In order to avoid damage to surrounding structures when a fuse link blows,
 a cavity may be formed next to the fuse link to absorb some of the energy
 and the vaporized fuse material expelled during the fuse clearing process.
 The invention includes a uniquely and conveniently formed and uniquely
 located cavity which directs the energy and material from the clearing
 fuse away from structures which might otherwise be damaged.
 More specifically, referring to FIGS. 4A-4D, a first embodiment of the
 inventive fuse structure/process is illustrated. FIG. 4A illustrates a
 cross section of a metal wire stack (e.g. a first conductive "R1"
 structure) formed by conventional deposition techniques, photolithography
 and plasma etching such as sputtering. The wire stack preferably includes
 a dielectric substrate 450, a first thin conductive (Ti) under layer 402
 that is 5 nm-50 nm thick, a second conductive (AlCu alloy) layer 401 that
 is 10 nm-1000 nm thick. In this structure the AlCu alloy layer 401 acts as
 the fusible element or fuse link.
 As would be known by one ordinarily skilled in the art given this
 disclosure, the inventive structure could be formed using any number of
 similar structures and materials. For example, the conductive layer 402
 could be a barrier metal layer formed by a damascene process with a liner
 underneath. For example, the conductive layer and liner could be formed
 using deposition and chemical mechanical planarization (CMP) on a
 dielectric substrate. The substrate would preferably be patterned into the
 shape of a fuse structure using lithography and etching. In such a
 damascene structure, the contract would be formed in a portion of the
 barrier layer which is undercut to form a cavity in a similar manner to
 the cavity 410 shown in FIG. 4C.
 An interlayer dielectric (ILD) layer 400 (e.g., a second contact "C2"
 layer), such as silicon dioxide, is deposited over the patterned metal
 stack to a thickness of 10 nm-1000 nm. The C2 layer 400 is transparent to
 the laser energy which will be applied to the fuse material 402. The ILD
 layer 400 is patterned using conventional lithography and dry etching
 techniques, such as reactive ion etching (RIE) using gases such as
 CF.sub.4, CHF.sub.3, C.sub.4 F.sub.8, CO, Ar, O.sub.2 and N.sub.2 to open
 regions 403 of the C2 contact 400 to match the layout design.
 As shown in FIG. 4B, the second conductive "R2" structure, in the form of a
 similar metal wire stack formed from of an AlCu layer 405 that is 10
 nm-1000 nm thick, and a TiN layer 404 that is 10 nm-50 nm thick, is
 deposited and patterned, using selective deposition and etching
 techniques, such as lithography and reactive ion etching (RIE) using gases
 such as BCl.sub.3, HCl, Cl.sub.2, He, N.sub.2, and Ar.
 The regions of the TiN layer 401 exposed by the openings 403 in the C2
 contact 400 are then etched using an isotropic process, such as a dry etch
 (containing fluorine) at a temperature of 50.degree. C. to 300.degree. C.
 Such an etch only affects the TiN layer 401 and does not significantly
 alter the inter layer dielectric 400 or the fuse material 402. As shown in
 FIG. 4C, this etching process undercuts the TiN and forms a cavity 410 and
 islands 406 of TiN between the C2 patterns 400. The TiN islands 406 are
 allowed to remain in the cavity 410 to help the absorption of the
 subsequent optical energy in the case of a laser fuse blow and to provide
 mechanical support of the ILD 400.
 In FIG. 4D, a terminal via (TV) dielectric 407, such as silicon dioxide, is
 deposited and patterned to form fuse windows 408 and bond pad windows 409
 using conventional techniques. When the fuse is blown, laser energy is
 directed through the fuse window 408 or excessive voltage/current is
 applied to the fuse element 402 which melts/vaporizes that area of the
 AlCu alloy to open the fuse.
 The specific materials and solutions mentioned above are merely exemplary
 and, as would be known by one ordinarily skilled and the art given this
 disclosure, any number of similar materials may be used to form the
 structure shown in FIG. 4D. Principally, the decision as to which
 materials are utilized depends on the specific requirements of the
 integrated circuit device being manufactured.
 The void formed on the undercut evacuated region 410 of the layer 401
 serves to collect material displaced from layer 402 during the fuse blow
 process and hence localize ablative damage to adjoining regions in the
 case of a laser fuse blow or localize damage induced by the vaporization
 of the fuse in the case of an electrically blown fuse.
 FIG. 5 is a flowchart illustrating the foregoing embodiment of the
 invention. More specifically, in item 501 the fuse layer 402 is formed. In
 item 502, a functional layer, such as the alloy layer 401 is formed over
 the fuse layer 402. In item 503 a laser transparent material, such as the
 silicon dioxide 400, is formed above the functional layer 401.
 Regardless of the material compositions selected for a given integrated
 circuit, one feature of the invention (which can be included in any
 material embodiment of the invention) is that the cavity is positioned
 between the laser window 408 and the fuse material 402. By locating the
 cavity in this matter, the surface of the fuse material 402 which
 initially receives the laser energy is thermally isolated. Therefore, the
 laser energy will be more concentrated on this surface of the fuse
 material 402 and cause a more reliable and faster vaporization/melting of
 the fuse material 402.
 To the contrary, conventional cavities are formed below the fuse material
 (e.g., on a side opposite from the direction of the laser energy).
 Therefore, with conventional structures, the fuse link surface which first
 receives the laser energy is in contract with an adjacent layer of
 material and this surface of the fuse material is not truly thermally
 isolated as the fuse link in the invention is.
 Additionally, whether the fuse link is opened using laser energy or
 excessive voltage/current, by providing the cavity in a direction toward
 the laser window 408, the energy and material being expelled by the fuse
 opening process is directed toward the dielectric material 400 and away
 from integrated circuit devices which may exist below the fuse material,
 such as those devices discussed above with respect to FIG. 3B. Therefore,
 with the invention, any damage which occurs as a result of the fuse
 opening process (such as cracking or melting) is primarily limited to the
 contact layer 400. Thus, the chances of damaging integrated circuit
 devices below the titanium layer 411 is substantially reduced when
 compared to fuse structures which include a cavity in a direction toward
 underlying integrated circuit devices.
 Regardless of the materials selected to form the fuse material and
 surrounding structures, another important feature of the invention is the
 utilization of an existing layer in the formation of the cavity. More
 specifically, the copper/aluminum alloy 401 in which the cavity 410 is
 formed, is a layer which is utilized to perform a useful function in a
 different portion of the integrated circuit device. By carefully selecting
 reactive agents during the reactive ion etching, such as a previously
 existing layer (such as the alloy 401 in the present structure) is
 utilized with the invention to reduce the number of processing steps
 required to implement the invention.
 Additionally, as mentioned above, the islands 406 are allowed to remain in
 the cavity 410 to help the fuse material 402 absorb the laser energy. In
 other words, the islands 406 within the cavity 410 concentrate the laser
 energy transmitted through the laser window 408 and increase the energy
 absorption within the fuse material 402 at the point of the island 406.
 The functional layer 401 is overetched using, for example the isotropic
 etching process described above, to form the cavity 410 and the islands
 406 in item 504. In item 505 additional processing continues to form
 layers such as the insulating layer 407 and the additional conductive
 layer 404, 405.
 The processing then continues depending upon whether the fuse will be
 opened electrically or with laser energy, in item 506. If the fuse is to
 be opened with laser energy, a fuse window 408 is formed over the laser
 transparent material 400, as shown in item 507. Then, in item 508, laser
 energy is applied to open the fuse 402. To the contrary, if the fuse is to
 be opened electrically, excessive voltage/current is applied to the fuse
 402 which causes it to open. The process ends at item 510.
 Another benefit of this invention is that the inventive fuse structure does
 not add any additional deposition steps to the conventional process
 because the fuse uses the existing metal wire stack structure which is
 used elsewhere on a topical integrated circuit chip as an interconnect
 structure.
 While the invention has been described in terms of preferred embodiments,
 those skilled in the art will recognize that the invention can be
 practiced with modification within the spirit and scope of the appended
 claims.