Abstract:
A fuse for semiconductor devices, in accordance with the present invention, includes a cathode including a first dopant type, and an anode including a second dopant type where the second dopant type is opposite the first dopant type. A fuse link connects the cathode and the anode and includes the second dopant type. The fuse link and the cathode form a junction therebetween, and the junction is configured to be reverse biased relative to a cathode potential and an anode potential. A conductive layer is formed across the junction such that current flowing at the junction is diverted into the conductive layer to enhance material migration to program the fuse.

Description:
BACKGROUND 
     1. Technical Field 
     This disclosure relates to fuses for semiconductor devices and more particularly, to electrical fuses with characteristics to enhance the efficiency of fuse programming. 
     2. Description of the Related Art 
     In semiconductor devices, fuses are employed in a variety of applications. For example, fuses are employed to enable redundant elements to be employed in the case of failures encountered on the semiconductor device. Some of these fuses are electrically programmable, i.e.; they are programmed by applying electric voltage or current. These electrically programmable fuses may be fabricated to include poly-silicide. Poly-silicide includes polycrystalline silicon and an overlayer of silicide, such as a metal silicide. The electrically programmable fuses typically include an appropriately shaped polysilicon layer that is silicided to obtain a poly-silicon/metal silicide stack structure. 
     Referring to FIG. 1, a layout (shape) of a fuse  10  is shown. Fuse  10  includes a fuse link  12 , an anode  14  and a cathode  16 . Current crowding takes place around a location  18  where the fuse link  12  abuts the cathode  16 , when a bias is applied to set or program the fuse. When a poly-silicide fuse is programmed, the cathode is negatively biased and the anode is positively biased. The current crowding initiates electro-migration effects at the fuse link  12  resulting in further current crowding and finally for appropriate bias conditions, the poly-silicide line melts or the silicide agglomerates to result in an open circuit or a high resistance state (i.e., the fuse gets programmed) at the location  18 . The effect of material migration due to, for example, electro-migration can be increased at the cathode-fuse link junction by increasing the ratio of L cathode  to L fuse , as this encourages current crowding. In typical layouts, the thickness of the fuse link  12 , the anode  14  and the cathode  16  are the same thickness because they are formed on the same level. Therefore, the lengths of L cathode  and L fuse  are determinative of the effective cross-sectional area of the fuse link/anode intersection. A polysilicon layer  20  and a silicide layer  22  as shown in FIG. 2 are provided at a uniform thickness for the fuse link  12 , the anode  14  and the cathode  16 . A nitride capping layer  24  is also provided over layers  20  and  22 . Typical electrically programmable fuses require current flow and voltage levels at an appropriate level for a requisite amount of time to program the fuse. 
     Therefore, a need exists for an apparatus and method to initiate and aid mass transport processes near a fuse link/cathode intersection to reduce the programming current, voltage and time. These reductions are desirable for the electrical fuse technology to minimize energy consumption and the cost of programming fuses. 
     SUMMARY OF THE INVENTION 
     A fuse for semiconductor devices, in accordance with the present invention, includes a cathode including a first dopant type, and an anode including a second dopant type where the second dopant type is opposite the first dopant type. A fuse link connects the cathode and the anode and includes the second dopant type. The fuse link and the cathode form a junction therebetween, and the junction is configured to be reverse biased relative to a cathode potential and an anode potential. A conductive layer is formed across the junction such that current flowing at the junction is diverted into the conductive layer to enhance material migration to program the fuse. 
     Another fuse for semiconductor devices includes a conductive pattern formed on a substrate. The conductive pattern forms a cathode on a first end portion. A fuse link connects the cathode and an anode, and the anode is formed on a second end portion of the conductive pattern. The fuse link and the cathode form a junction therebetween. A conductive layer is formed across the junction such that when the fuse is electrically active the junction is reverse biased and material migration is enhanced to program the fuse. 
     Yet, another fuse for semiconductor devices includes a polysilicon pattern formed on a substrate. The polysilicon pattern forms a p-doped cathode on a first end portion. An n-doped fuse link connects the cathode and an anode. The anode is formed on a second end portion of the polysilicon pattern. A reverse biased junction is formed at the interface between the cathode and the fuse link. A silicide material is formed across the junction, such that, when the fuse is electrically active, current is diverted from the reverse biased junction into the silicide material to enhance electromigration and program the fuse. 
     In alternate embodiments, the cathode, anode and fuse link preferably include polysilicon. The conductive layer preferably includes a silicide. The first dopant type may include a p type dopant and the second dopant type includes an n type dopant. The cathode preferably has a larger cross-sectional area than the fuse link. The cathode and the fuse link may provide an interface of the junction, which is substantially perpendicular to a current flow direction through the fuse link. The polysilicon of the cathode is preferably p-doped, and the polysilicon of the fuse link is preferably n-doped. 
     These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     This disclosure will present in detail the following description of preferred embodiments with reference to the following figures wherein: 
     FIG. 1 is a top view of a fuse structure in accordance with the prior art; 
     FIG. 2 is a cross-sectional view of a fuse link for the fuse structure of FIG. 1; 
     FIG. 3 is a top view of a reverse biased fuse structure for enhancing fuse programming in accordance with the present invention; and 
     FIG. 4 is a cross-sectional view of at a junction taken at section line  4 — 4  of FIG. 3 in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention provides an apparatus and a method to initiate and aid material migration near a fuse link/cathode intersection to perform a fusing operation. In accordance with the invention, methods are provided to enhance the current crowding at the fuse link by creating a reverse biased p+/n+ junction in a fuse. In one embodiment, a polysilicon layer is formed with a silicide layer thereon. The polysilicon is doped to form the reverse biased p+/n+ junction. This provides increased current at the junction. The enhanced current crowding will result in faster initiation of the electromigration in the fuse link. This would aid in faster programming of the fuse (lower programming voltage/current/time). The reverse biased junction along with shape of the fuse layout may also be employed in localizing the fuse blow. 
     In one embodiment, the intersection includes a cathode material, which is doped differently from the anode/fuse link material, such that material migration is enhanced to provide a break or increased resistance in the fuse link in less time and with less severe current and/or voltage levels. In one embodiment, a silicided layer is employed adjacent to the anode, the fuse link, and the cathode. This structure advantageously reduces the programming current, voltage and time by increasing current density in the silicided layer, as will be explained below. 
     One embodiment of the present invention provides a silicide layer, which exacerbates the material migration process further by providing a less resistive path than the reverse biased junction. This increases the current density in the silicide, thus initiating the voids in the cathode-fuse link junction at a faster rate. The higher conductivity of the silicide also provides current crowding in the vertical direction at the fuse link and cathode junction further aiding electron migration. This abetting of the material migration process advantageously permits lower programming voltage/current and shorter programming time. In one embodiment, voltages and/or current are reduced by a factor of at least two and the programming time is reduced by a factor of about 100. Although described in terms of silicide and polysilicon materials, these materials should not be construed as limiting the present invention. Instead, these materials are employed to illustrate the present invention in one application. 
     Referring now in specific detail to the drawings in which like reference numerals identify similar or identical elements throughout the several views, and initially to FIG. 3, a fuse structure or fuse  100  is shown in accordance with the present invention. Fuse  100  includes an anode  102  and a cathode  104  for providing current flow across a fuse link  106  that connects anode  102  to cathode  104 . Anode  102 , cathode  104  and fuse link  106  are preferably disposed on a substrate  101  (see, e.g., FIG.  3 ), which may include a semiconductor substrate, diffusion regions, metal lines, dielectric layer(s) and other components. In one illustrative embodiment, when fuse  100  is programmed, cathode  104  is negatively biased and anode  102  is positively biased. However, cathode  104  advantageously includes p+ dopants while fuse link  106  and anode  102  include n+ dopants. This configuration provides a reverse bias through region  110  between cathode  104  and fuse link  106 . 
     A reduction in cross-sectional area (which is proportional to the width in this embodiment) from L cathode  to L fuse  results in additional current crowding in a region  110 . The increased current density will result in material migration (or electro-migration) in region  110 . The material migration causes voids to form at the corners of fuse link  106  and cathode  104 , and further increases the current density. This ultimately results in fuse link  106  melting thereby causing an open in the fuse circuit or a material segregation thereby causing an increased resistance in the fuse. Cathode  104  and anode  102  preferably include polysilicon, which may be appropriately doped to cause the reverse biasing in accordance with the present invention. In a preferred embodiment, a conductive layer, e.g., a silicide layer, (not shown in FIG. 3) is formed in contact with cathode  104  and fuse link  106  in region  110 . 
     Referring to FIG. 4, a cross-sectional view of region  110  is shown. The present invention advantageously exacerbates fuse programming. In the illustrative structure shown in FIG. 4, cathode  104  is formed from polysilicon and is doped with p+ dopants. p+ may include for example, boron, aluminum or gallium. Fuse link  106  and anode  102  are doped with n+ dopants, which may include, for example, phosphorus, arsenic or antimony. The p+/n+ junction is formed at the meeting point of fuse link  106  and cathode  104 . Dopant concentration for cathode  104  preferably include between about 10 15  and about 10 22  atoms/cm 3 . Dopant concentration for anode  102  and fuse link 106 preferably includes between about 10 15  and about 10 22  atoms/cm 3 . 
     A conductive layer  103  is formed in electrical contact with region  110 . Conductive layer  103  may be formed locally at the cathode/fuse link junction or may extend over a larger area (e.g., along the length of cathode, fuse link, and/or anode). Conductive layer  103  preferably includes a silicide, such as, for example, cobalt silicide or other material such as tungsten silicide, nickel silicide or metals, such as aluminum. When cathode  104  is negatively biased and anode  102  is positively biased, p+/n+ junction  111  is reverse biased. Hence, no current can flow through the doped polysilicon. As indicated by arrows A, the electrons, which carry the current, will crowd in conductive layer  103  above the reverse biased junction. It is to be understood that conductive layer may be formed on top of, below or on a lateral side of junction  111 . 
     In addition, lateral current crowding due the large ratio of L cathode  to L fuse  is enhanced by vertical current crowding due to the reverse biased junction  111 . Most of the electrons that carry the current will be crowded in the silicide in fuse link  106  at the junction. Advantageously, this current crowding in the vertical direction at a substantially perpendicular interface  107  adds to the crowding due the shape of the large cathode connected to the narrow fuse link  106 , and further amplifies the material migration effect at this junction. The fuse programming therefore takes place faster and at a lower voltage/current for fuse  100 . This means that the fuse can advantageously be programmed at a lower voltage/current and in a shorter time than prior art structures. 
     It is preferable, to ensure current crowding will result in a localized fusing action and quicker fuse programming, that the resistance of conductive layer  103  (e.g., silicide) and doped polysilicon of cathode  104  and fuse link  106  in region  110  are comparable. This can be ensured by the use of silicides for layer  103  with lower resistivity and by appropriately choosing the thickness of the silicide ( 103 ) and the polysilicon layers (cathode  104 , fuse link  106  and anode  102 ). Silicide resistivity may be about 5 to 10 ohms per square. The resistivity of the polysilicon can be more than one order of magnitude different. In one illustrative embodiment, for a cobalt silicide (layer  103 ) and polysilicon layer (cathode  104 , fuse link  106  and anode  102 ) combination, the silicide may include a thickness of between about 20 nm to about 50 nm and the polysilicon may include a thickness of between about 100 nm and about 200 nm. In other embodiments, vertical current crowding is enhanced by increasing the polysilicon (e.g., cathode  104 , fuse link  106  and anode  102 ) thickness and decreasing the conductive layer  103  thickness. The for the cobalt silicide/polysilicon combination, the silicide may be, for example, reduced in thickness to 5 nm to 10 nm, while the polysilicon thickness is increased to say, 500 nm. In such a situation, the vertical current crowding will be increased. Other thicknesses and materials may be employed as well. 
     The reverse biased junction is used to initiate a fuse blowing process faster. The reverse bias in the poly-silicon will show a higher resistance for the programmed state if the silicide is opened up but the poly-silicon is not fully opened up. In any event, a larger difference in fuse resistance between the unprogrammed state and programmed is expected. The fuse of the present invention combines the advantage of both lateral and vertical current crowding into the fuse link from the cathode. This fuse also localizes the fuse blowing location. 
     The present invention may be extended to any junction formed with a reverse biased interface or junction with a conductive path in contact with the junction. In addition, different geometry may be employed for the cathode, anode and fuse link. For example, the p/n junction may be formed between the fuse link and the anode. Different levels of doping may be employed on the cathode and fuse link, for example, n+/p, n/p+, etc. 
     Having described preferred embodiments electrical fuses employing reverse biasing to enhance programming (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.