Abstract:
A fuse structure in an integrated circuit chip is described that includes an insulated semiconductor substrate; a fuse bank integral to the insulated semiconductor substrate consisting of a plurality of parallel co-planar fuse links; and voids interspersed between each pair of the fuse links, the voids extending beyond a plane defined by the co-planar fuse links. The voids surrounding the spot to be hit by a laser beam during fuse blow operation act as a crack stop to prevent damage to adjacent circuit elements or other fuse links present. By suitably shaping and positioning the voids, a tighter pitch between fuses may be obtained.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This is a Continuation Patent Application of U.S. patent application Ser. No. 09/223,826, filed on Jan. 4, 1999, now abandoned. 
    
    
     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 configurations to contain the extent of damage around a blown fuse. 
     In another aspect of the invention, an integrated circuit is designed to include additional structures characterized by the absence of material, and placed between fuses and parallel to the fuses. The laser fuse link is configured to be set by a laser beam during fabrication of the integrated circuit. 
     In yet another aspect of the invention, these crack stops are positioned immediately surrounding the region of the fuses blown on a fuse link. 
     In one embodiment of the invention, there is provided a fuse structure including: an insulated semiconductor substrate; a fuse bank integral to the insulated semiconductor substrate consisting of a plurality of parallel co-planar fuse links; and voids interspersed between each pair of the fuse links, the voids extending beyond a plane defined by the co-planar fuse links. 
     In another embodiment of the invention, there is provided a fuse structure integral to a semiconductor substrate including: a fuse bank integral to the semiconductor substrate and consisting of a plurality of co-planar fuse links, each of the fuse links being provided with a co-planar area extending beyond the width of the fuse link; and voids positioned within the semiconductor substrate and interspersed between each pair of the fuse links surrounding the co-planar areas. 
     In a third embodiment of the invention there is provided a fuse structure integral to a semiconductor substrate providing support to a plurality of stacked insulated wiring layers including: integral to a first one of the stacked insulated wiring layers, a fuse bank comprising at least two co-planar rows of fuse links; integral to a second one of the stacked insulated wiring layers, a conducting bus positioned in a direction transversal to the direction of at least the two rows of fuse links; and portions of conductive material placed between the at least two rows of fuse links and running in a direction parallel to the bus. 
     In another aspect of the invention, there is provided a method of forming a fuse structure integral to a semiconductor substrate supporting a plurality of stacked insulated wiring layers which includes the steps of: providing to a first one of the stacked insulated wiring layers a fuse bank comprising at least two co-planar rows of fuse links; providing to a second one of the stacked insulated wiring layers, a conducting bus positioned in a direction transversal to the direction of the at least two row of fuse links; and placing portions of conductive material between the at least two rows of fuse links and running in a direction parallel to the bus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects, features and advantages of the present invention will become apparent in the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings. 
     FIG. 1 illustrates a schematic block diagram of a typical DRAM, including a main memory array, a redundancy replacement array, a fuse decoder circuit, fuse latches, and fuses in a fuse array. 
     FIG. 2 a  illustrates a typical arrangement of a plurality of fuse links of the fuse bank in their original unblown state. 
     FIG. 2 b  shows one of the fuse links depicted in FIG. 2 a  blown by a laser beam. 
     FIGS. 3 a  and  3   b , respectively show a top view and a side view of a conventional fuse bank made of a plurality of fuses, wherein additional structures are placed within the fuse bank between the fuses, acting as crack stops when a fuse is blown with the laser. 
     FIG. 4 a  shows air crack stops positioned in the area immediately surrounding the exact spot where the fuse link is blown by the laser, wherein the area surrounding the spot is widened beyond the width of the fuse, in accordance with one aspect of the invention. 
     FIG. 4 b  is a schematic diagram of a laser beam hitting a spot during a fuse blow operation. 
     FIG. 5 shows a cross-sectional view of the fuse structure, wherein cracks-are shown surrounding the fuse ready to be blown. 
     FIGS. 6 a - 6   e  show the various process steps required to manufacture an air crack stop, in accordance with the invention. 
     FIGS. 7 a  and  7   b  respectively show a top and side representation of a second embodiment of the invention, wherein two row of fuses are provided with a crack stop wall separating them in an integrated circuit having a plurality of insulated wiring layers. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In a first embodiment of the invention shown in FIG. 4 a , the crack stop is positioned in the immediate vicinity of the region where the fuse link is to be blown by the laser (depicted by a dark spot, e.g., point  420 ). 
     A plurality of fuses  402 ,  404  and  406  form the fuse bank  400 . Surrounding, e.g., fuse  402 , are depicted two crack stops  410  and  412  centered around point  420 —the target spot for the laser to hit the fuse during the blow operation. Further illustrated, are shown the spots staggered along the length of the respective fuse links, each spot being surrounded by its own crack stop. This configuration is a representation of an instance wherein staggered crack stops can be beneficial in reducing the fuse pitch. Although the crack stop is shown in the shape of a bracket, many other shapes and positions can be envisaged to achieve the same goal. By way of example, the crack stop can be formed by longitudinal portions only without any transverse components. 
     The configuration shown can be modified by having the exact spot selected for each fuse link to be advantageously surrounded by an area extending beyond the boundaries defining the width of the fuse link, as shown by square  425 . This extension has the advantage of absorbing an added amount of the energy emitted by the laser beam during the blowing operation. Its drawback, though, is that it may interfere with obtaining a closer pitch than that would have been possible in the absence of such metal extensions  425 . 
     Shown in FIG. 4 b  is a schematic diagram of a laser beam  450  hitting spot  420  of fuse link  402 . Each fuse link is shown being provided with the aforementioned area surrounding the spot. For simplicity sake, no crack stops are shown. 
     In conventional designs, fuse links placed in a row inside a fuse bank cannot be reliably used when fuse pitches (i.e., distance between adjacent fuses) fall below approximately 3 μm. This is because lasers that are typically used for blowing fuses have a wavelength of the order of 1-1.3 μm. As a result, the smallest spot that can be focused is greater than 2-2.6 μm. This, coupled with the uncertainty associated with the positioning of the substrate relative to the laser spot renders the blowing of fuses an unreliable operation. For fuse pitches less than 3 μm, the probability of damaging a neighboring fuse increases as the pitch decreases. The introduction of a metallic crack stop between fuse links as described by Gilmour et al. will also not work at these tight pitches, since the crack stop itself will be ablated by the laser, causing damage to fuses or circuit elements next to it. In the invention herein described, since the crack stop is made by removing material, there is no possibility of damage by the laser beam to the crack stop. This makes it possible to pack fuses even down to a pitch of 2.2 μm without damaging neighboring fuses during fuse blow. Moreover, the crack stop still performs the function of stopping cracks from damaging neighboring fuses. 
     FIG. 5 shows a cross-sectional view of the structure. The fuse  502  is shown embedded in a substrate with two crack stops  504  and  506 , preferably made of air, in order to minimize the drawbacks found in the prior art and which were described previously. Cracks  508  are depicted arrested by crack stops  504  and  506 . 
     Referring now to FIGS. 6 a - 6   e , a description follows of the various steps applicable to the manufacturing of an air crack stop. A dielectric or passivation film C 1  is initially deposited on top of a silicon substrate (not shown). This is followed by a metal (M 1 ) deposition. The fuse element M 1  can be formed using standard etch and lithographic means. Subsequently, a second dielectric or passivation film C 2  is deposited. Chemical-mechanical polishing (CMP) can advantageously be performed, if so desired or required. The dielectric C 2  is etched to make contact to other metal structures present on level M 1 . 
     Referring now to FIG. 6 b , a modification of the mask allows for the formation of the air crack stop trench as indicated. Normally, this is followed by a final metallization level M 2 , as shown in FIG. 6 c , wherein the vias are filled with a blanket deposition of conductive material. This means that the trenches will be refilled with metal M 2 . However, since M 2  will be patterned and etched it is preferable to remove metal M 2  from the air crack stop trenches. It is also possible not to remove metal M 2  from the trenches by modifying the M 2  mask in an appropriate manner. Using this approach, one can form metal crack stop trenches instead of air crack stop trenches. 
     The final process of chip manufacturing, illustrated in FIG. 6 d , consists of a passivation step using a dielectric. and a buffer coating in order to prevent the chip from scratching or humidity uptake. In case of the air crack stop, the application of passivation again fills the air crack stop trenches with material. However, as in the case of metal M 2 , this material is removed because of the existence of the very last etch step that is used to open windows, i.e., air crack stop trench, which is required in order to blow fuses by way of a laser beam (FIG. 6 e ). 
     Referring now to FIGS. 7 a  and  7   b , respectively showing a top and side representation of a second embodiment of the invention, two row of fuses are shown provided with a crack stop wall separating them. 
     A fuse bank  700  is provided with fuses  702 ,  704  and  706  in one row, and fuses  708 ,  710 , and  712  in a second row within the same bank  700 . The crack stop is shown with reference to numeral  720 . When the rows share a common electrical bus  430  set to separate them (a top and a bottom strip), the crack stop is inhibited from running from the top surface down to the substrate because of the requirement to electrically connect the two rows. FIG. 7 b  illustrates the cross-sectional view of FIG. 7 a  taken at a location depicted by numeral  775 , where fuse  702  is shown connected to fuse  708  through the bottom layer  430  used as a common electrode. The crack stop  720  is preferably made up of only the layers above  730  and which are not in contact with them. The crack stop (e.g.,  720 ) can be designed to have one of its ends make electrical contact with the bus  430  or, if so desired, it can be kept separate from the bus altogether. The metal segments referenced globally by numeral  750  join common electrical bus  730  to fuses  704  and  710 . By way of example, referring to FIG. 6 d , those segments are C 1 , M 1 , M 2  and C 2 . 
     Whereas the invention has been described with respect to several embodiments, it will be apparent to those skilled in the art that these embodiments are intended only as illustrative examples only and that various changes and modifications may be incorporated without departing from the scope of the invention as set forth by the following claims.