Abstract:
A fuse structure and method for fabricating same are disclosed. The fuse structure is designed for opening by conventional laser energy application. The fuse structure is characterized by an absence of high stress areas in the surrounding substrate thereby resulting in higher fabrication yields due to lower occurrence of substrate fracturing or other damage occasioned by the opening of the fuse.

Description:
TECHNICAL FIELD 
     The present invention is generally related to microchips and their fabrication. More particularly, the invention relates to fusible structures and fabrication processing related to their integration with microchips. 
     BACKGROUND OF THE INVENTION 
     Fuse structures are commonly fabricated as part of microchips. Various objectives are accomplished in the application of such fuses. For example, yield management can be improved through redundant structure designs that provide for the selective disabling of defective portions of a circuit in favor of faultless similar portions. Memory structures are but one well known example of such usage wherein columns, rows or entire arrays of memory elements may be fabricated in complete or partial redundancy and wherein faultless structures are substituted for defective structures by strategically placed and severed fuse structures. 
     Fuse structures also find utility in allowing for flexibility in circuit design or on-chip programmability wherein microchips may be tailored for particular applications by removal, addition, substitution and the like of distinct structures. 
     Microchip protection during fabrication handling and processing is yet another example of the utility of such fuse structures. In dynamic random access memory, for example, charge accumulation at sensitive structures may be prevented during fabrication processing by providing preferential charge paths through such fuse structures. The fuse structures are severed when fabrication is substantially complete. 
     The use of such fuse structures is gaining even more importance as system-on-chip manufacturers integrate ever more chip sub-systems and memory devices together in a single monolithic chip design. 
     Such microchip fuses are routinely fabricated from metallic or polysilicon materials. Any conductive material may be employed. Generally, a narrowed or neck section of a conductive line is provided as a fusible portion of the line whereby severing or opening of the fusible portion is accomplished by applying a controlled pulse of laser energy. The laser energy superheats the fusible portion and vaporizes the material that forms the fuse and leaves a crater in the surrounding area. Opening a fuse in this manner is, although on a small scale, a very violent procedure. Laser blowing of fuses is known to harbor potential for damaging adjacent circuit structures or substrates including, but not limited to, other fuses which may not be desirably opened. Damage to the microchip, including to laterally and vertically adjacent structures, may be caused by reflected or absorbed laser energy. Structural damage to the microchip substrate is known to occur due to absorbed laser energy and localized stresses. For this reason, adjacent fuses or other circuit structures are spaced outside of the lateral area of influence of the laser and are generally not located below a fuse structure. This of course limits the placement density of fuses and adjacent structures. Lasers having relatively long wavelengths such as infrared lasers may be selected to avoid absorption induced damage since conventional substrate material (e.g. silicon) absorbs shorter wavelengths more readily. However, adoption of longer wavelength lasers also necessitates sparse layouts of fuses and adjacent structures since longer wavelengths have inherently less controllability with respect to spot focus of the laser energy. In other words the area of application over which the longer wavelength laser can be controlled is larger than the area over which a shorter wavelength laser can be controlled. 
     Various attempts have been made at decreasing the spacing between fuses and adjacent structures including other fuses. U.S. Pat. No. 5,420,455 for example discloses laterally placed barriers characterized by high melting point, non-frangible materials. These barriers, it is taught, are not substantially affected by reflected laser energy and present a physical barrier to the adjacent vaporized material thus limiting expansion of the crater therebeyond. Another type of barrier strategy is disclosed in U.S. Pat. No. 6,297,541 wherein a blocking layer is placed vertically beneath the fuse structure. This blocking layer is also described to be a material that is not substantially affected by applied laser energy. Both of these proposed solutions require additional processing steps, require additional lateral or vertical space, and may do nothing to alleviate localized stresses in the microchip substrate. 
     FIGS. 1A and 1B illustrate prior art fuse structure  10  and damage that occurs to surrounding microchip structure from localized stresses. Fuse structure  10  may typically include a lower substrate layer  11  comprising silicon including circuit structure, metalization or other materials, functionality or purposes. Typically located on top of lower substrate layer  11  is a passivation layer  13  such as silicon dioxide or other suitable material of choice. Fuse  15  is a conductor such as metals including alloys or polysilicon and is typically a narrowed or necked portion of a conductor line. Controlled laser energy  17  is applied to the fuse and the laser energy superheats the fusible portion and vaporizes the fuse material and leaves a crater in the surrounding area as can be seen in FIG.  1 B. Localized stresses at the relatively sharp, acute intersections of the sidewalls and bottom of the channel defining the fuse  15  may result in cracking  19  of the surrounding passivation layer  13 . 
     SUMMARY OF THE INVENTION 
     Therefore, it is one object of the present invention to provide a microchip fuse structure that when opened by laser energy is resistant to damaging adjacent microchip structures. 
     It is a further object of the present invention to provide a microchip fuse structure that when opened by laser energy is resistant to damaging adjacent microchip structures without the addition of intervening barriers between the fuse and adjacent structures. 
     It is a further object of the present invention to provide for the above objects of the present invention by implementing conventional microchip fabrication materials and steps. 
     These objects and advantages of the present invention are realized by a fuse structure comprising an elongate fusible region at least partially defined by a substrate surface that is characterized by the absence of acute transitions. Preferably, the fuse structure is at least partially defined by a substrate surface that is substantially curvilinear in axial cross-section. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIGS. 1A and 1B illustrate sectional views through conventional fuse structure before and after opening of the fuse by laser energy; 
     FIG. 2A illustrates a sectional view taken through section line  2 A— 2 A of FIG. 2C of one embodiment of a fuse structure in accordance with the present invention before opening of the fuse by laser energy; 
     FIG. 2B illustrates the sectional view of the fuse structure of FIG. 2A after opening of the fuse by laser energy; 
     FIG. 2C illustrates a plan view of one embodiment of a fuse structure in accordance with the present invention before opening of the fuse by laser energy; 
     FIG. 3 illustrates a sectional view of an alternate embodiment of a fuse structure in accordance with the present invention before opening of the fuse by laser energy; and, 
     FIGS. 4A-4E illustrate sectional views representing various exemplary steps in a fuse structure manufacturing process in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference first to the embodiment of the invention illustrated in FIGS. 2A-2C, a fuse structure  20  includes an elongate fuse  25  which may optionally be a narrowed or necked region of a wider conductor. Fuse  25  is defined by substrate  23  which may comprise any suitable material conventionally utilized in microchip fabrication. Generally, preferred material for substrate  23  includes insulators and dielectrics such as conventional glasses and passivation materials. Substrate  23  may actually comprise several layers of the same or compatible materials. Substrate  23  may be adjacent or on top of a lower substrate layer  21  which may comprise any suitable material conventionally utilized in microchip fabrication. Silicon is a conventional substrate material for substrate  21  and may include active doped regions comprising circuit elements. Insulators and dielectrics such as conventional glasses and passivation materials may also provide substrate  21  and may include conductive traces and plugs. Reference to either of the substrates  23  and  21  is understood to include materials or structures that make up either or both of the substrates  23  and  21 . Additionally, reference to substrates or substrate material is understood to mean either or both of the substrates  23  and  21 . 
     With reference specifically to FIG. 2A and a fuse structure according to the present invention prior to opening, substrate  23  has formed therein an elongate channel  24  defining the elongate fuse  25 . Channel  24  is characterized by a curvilinear cross section. Channel  24  is arcuate in cross section and may be semicircular. 
     With reference now to FIG. 2B, the fuse structure described with reference to FIG. 2A is illustrated after opening by application of laser energy. Opening of the fuse is accomplished by conventional application of controlled laser energy  27  to fuse  25 . The absence of regions of concentrated stresses (e.g. corners in a conventional trench) results in an undamaged channel  24  and absence of cracks and fissures extending into the substrates. 
     FIG. 3 illustrates an alternate embodiment of a fuse structure according to the present invention. The sectional view of FIG. 3 is similar to that of the fuse structure of FIG.  2 A. Fuse structure  30  includes substrate  31  and  33 . Fuse  35  is defined in or by the substrate including a pair of side surfaces  32  and a bottom surface  36 . Regions  37  provide smooth, curvilinear transitions between the bottom surface  36  and respective ones of the pair of sidewalls  32 . Such embodiment may be considered to have a lower portion and upper portion wherein the lower portion is partially defined by a substantially curvilinear cross section at the corners. As bottom surface  36  becomes smaller and regions  37  become closer together, a single curvilinear transition region joins the pair of side surfaces  32 . Such a single transition region may be substantially arcuate or semicircular. Such an embodiment may be considered to be similar to the embodiment of FIG. 2A with additional side surfaces continuing at the top of the semicircular channel. Such embodiment may be considered to have a lower portion and upper portion wherein the lower portion is characterized by a substantially curvilinear cross section. Additionally, the pair of side surfaces  32  may be inclined or sloped away from the center to form a V-shaped channel having a curvilinear transition at the valley. 
     Turning now to FIGS. 4A-4E an exemplary novel process for yielding a fuse structure according to the present invention is illustrated. In FIG. 4A a lower substrate, for example silicon with active structures or a metallic layer, has deposited thereupon a substrate layer  43 ′ such as a suitable insulator, dielectric or glass. The present example contemplates the deposition of silicon dioxide such as by conventional chemical vapor deposition. 
     FIG. 4B illustrates trench  44  formation in layer  43 ′ by conventional dry etch processing steps including resist deposition and patterning by masking, exposing, developing and any of a variety of dry/plasma etch processes. Dry etching yield trench  44  having substantially vertical sidewalls and regular, horizontal bottom surface. Also characteristic of the dry etch process are sharp angled intersections of the bottom surface with each of the vertical sidewalls. 
     Following the dry etch of trench  44  are additional wet etching steps to ease the corners of the trench or as is shown in the section of FIG. 4C to additionally run each of the bottom to sidewall transitions into each other at the bottom surface resulting in a substantially semicircular trench. Wet etching is known to be isotropic in nature. Such isotropic results are generally undesirable when forming microchip structures but are utilized in the present process to achieve the desired results of eased transitions, arcuate or semicircular trenches. Exemplary wet etching techniques include us of hydrofluoric (HF) acids, buffered oxide etcher (BOE) to name a few non-exhaustive examples. 
     Next, as represented by FIG. 4D, the trench  44  is filled with a conductor to form fuse  45 . Generally this will include sputtering a barrier layer of, for example, tantalum or tantalum-nitride, followed by sputtering of a seed layer of the bulk material used for the fill of the trench. Copper is becoming a preferred material for conductors in microchip fabrication and is a preferred material for bulk fill in the present embodiment. Bulk filling of copper is preferably accomplished by copper electroplating. Other conductors may be used for the bulk fill including alloys. Other processes may also be used for the bulk fill including conventional sputtering. Since bulk fill of the trench  44  to establish the fuse  45  will result in overfill, the process is followed up with a planarization step, preferably chemical mechanical processing. 
     Finally, as represented in FIG. 4E, the upper substrate layer  43  containing the fuse  45  is completed by depositing a cap layer  43 ″. A suitable insulator, dielectric or glass compatible or identical to layer  43 ′ is selected. The present example contemplates the deposition of silicon dioxide such as by conventional chemical vapor deposition. 
     The previously described steps with respect to FIGS. 4A-4E may be described as a modified damascene process wherein additional wet etching steps are applied to the trench formed in preceding dry etch steps. 
     The invention has been described with respect to certain preferred embodiments intended to be taken by way of example and not by way of limitation. Certain alternative implementations and modifications may be apparent to one exercising ordinary skill in the art. Therefore, the scope of invention as disclosed herein is to be limited only with respect to the appended claims. 
     The invention in which an exclusive property or privilege is claimed are defined as follows: