Abstract:
A fuse structure with aggravated electromigration effect is disclosed, which comprises an anode area overlaying a first plurality of contacts that are coupled to a positively high voltage during a programming of the fuse structure, a cathode area overlaying a second plurality of contacts that are coupled to a complementary low voltage during a programming of the fuse structure, and a fuse link area having a first and second end, wherein the first end contacts the anode area at a predetermined distance to the nearest of the first plurality of contacts, and the second end contacts the cathode area at the predetermined distance to the nearest of the second plurality of contacts, wherein the cathode area is smaller than the anode area for the aggravating electromigration effect.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to fuse structure in integrated circuits (ICs), and more particularly to electrical fuse structures. 
         [0002]    Fuses in an IC are convenient nonvolatile memories for permanently storing information such as “chip-ID”, etc. An electrical fuse is a fuse that can be programmed by applying excessive current or long stress time. One semiconductor material for making such electrical fuse is silicided polysilicon. After stressing the silicided polysilicon material by applying a moderately high current density, typically about 600 mA/um 2 , for a certain period of time, its resistance may rise due to electromigration (EM). The EM is a phenomenon that electrons in an electrical field impacting fixed ions in the fuse, which creates voids and eventually opens a circuit after a prolonged stress. The initial low resistance and the after-stress high resistance may be used to represent two different logic states, commonly known as HIGH and LOW. 
         [0003]    In addition to EM, there are two other fuse programming mechanisms, i.e., silicide agglomeration and rupture. The silicide agglomeration happens when the fuse temperature is higher than 850° C., which is beyond the silicide formation temperature. The rupture is physically breaking a fuse when the temperature gradient causing different thermal expansion in different parts of the fuse that causes the break. 
         [0004]    For an electrical fuse that has initial resistance of 100 ohm, after an EM programming, its after-stress resistance may range from 500 to 10K ohm. If the same fuse is programmed by silicide agglomeration, its final resistance may reach 100K to 1M ohm. If the fuse is simply ruptured after programming, its final resistance may be more than 10M ohm. 
         [0005]    Structures of electrical fuses also affect their programming effectiveness.  FIGS. 1A and 1B  shows two conventional electrical fuse structures  100  and  150 , respectively. Referring to  FIG. 1A , the electrical fuse structure  100  has a rectangular anode  102  and a rectangular cathode  112  which is linked to the anode  102  by a fuse link  122 . Both the anode  102  and the cathode  112  substantially overlay their respective contacts  134  to utilize contact current density capacities. The anode  102  and the cathode  112  are symmetrical in size. Referring to  FIG. 1B , similarly, the electrical fuse structure  150  also has an anode  152 , a cathode  162  connected by a fuse link  172 . The top and bottom parts of the electrical fuse structure  150  are also symmetrical. A problem with symmetrical structure or larger cathode structure is that the EM effect does not receive a boost as the cathode would have an ample supply of electrons. Lesser EM effect means lesser resistance differentiation between a before and after programming. Even though the fuse link  172  of the electrical fuse structure  150  is tapered toward the middle, there is no reported boost on the EM effect from the tapering. 
         [0006]    Kothandaraman, et al. in “Electrically Programmable Fuse Using Electromigration in Silicides”, IEEE Elec. Dev. Lett. Vol. 23, No. 9, September 2002, pp. 523-525, proposed a structure using small anode and large cathode. This structure actually resists the EM effect. The rationale of this structure is to suppress the EM effect such that the rupture could happen at a higher programming voltage that results in a higher resistance state. Alavi et al. in “A PROM Element Based on Salicide Agglomeration of Poly Fuses in a CMOS Logic Process,” IEDM 1997, pp. 855-858, designed a symmetrical fuse structure for electrical fuses, which provides no aggravation to the EM effect. Kalnitsky, et al. in “CoSi2 integrated fuses on poly silicon for low voltage 0.18 um CMOS applications,” IEEE IEDM 1999, pp. 765-768, reported another electrical fuse using EM effect, but it is still based on symmetrical structure. 
         [0007]    As such, what is desired is an electrical fuse structure that can aggravate the EM effect which makes a fuse structure easier to be programmed and has a larger resistance differentiation between a before and after programming. 
       SUMMARY 
       [0008]    In view of the foregoing, the present invention provides a fuse structure with an aggravated electromigration effect. In one aspect of the invention, the fuse structure comprises an anode area overlaying a first plurality of contacts that are coupled to a positively high voltage during a programming of the fuse structure, a cathode area overlaying a second plurality of contacts that are coupled to a complementary low voltage during a programming of the fuse structure, and a fuse link area having a first and second end, wherein the first end contacts the anode area at a predetermined distance to the nearest of the first plurality of contacts, and the second end contacts the cathode area at the predetermined distance to the nearest of the second plurality of contacts, wherein the cathode area is smaller than the anode area for aggravating electromigration effects. 
         [0009]    According to another aspect of the present invention, a reverse biased PN junction is formed in the body of the fuse link area to shun current to the surface of the fuse structure for further aggravating the EM effect. 
         [0010]    According to yet another aspect of the present invention, the cathode area overlaying the second plurality contacts by a smaller distance than specified by a predetermined design rule for restricting current density at the second plurality of contacts, and therefore, aggravating the EM effect at the same time. 
         [0011]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIGS. 1A and 1B  illustrate conventional electrical fuse structures. 
           [0013]      FIG. 2  illustrates an electrical fuse structure according to a first embodiment of the present invention. 
           [0014]      FIG. 3  is a sectional view of the electrical fuse structure having a reverse biased PN juncture according to a second embodiment of the present invention. 
           [0015]      FIGS. 4A and 4B  are top views of electrical fuse structures each having a reverse biased PN junction according to the second embodiment of the present invention. 
           [0016]      FIG. 5A  illustrates a compact electrical fuse structure according to a third embodiment of the present invention. 
           [0017]      FIG. 5B  illustrates another compact electrical fuse structure having a reverse biased PN junction according to a combination of the second and third embodiment of the present invention. 
           [0018]      FIG. 5C  illustrates yet another compact electrical fuse structure having a reverse biased PN junction at a cathode-and-fuse-link interface according to a combination of the second and third embodiment of the present invention. 
       
    
    
     DESCRIPTION 
       [0019]    The following will provide a detailed description of a fuse structure that provides greater resistance differentiation between before and after programming through aggravating electromigration (EM) effects in the fuse structure. The EM effects from which the present invention benefits include a cathode depletion effect and a reservoir effect. The cathode depletion effect refers to a phenomenon that during a programming, the cathode area is more prone to have voids than the anode area. The reservoir effect refers to a phenomenon that the larger the cathode area the more resistant the fuse structure to the EM stress. 
         [0020]      FIG. 2  illustrates an electrical fuse structure  200  according to a first embodiment of the present invention. The electrical fuse structure  200  comprises an anode area  202 , a cathode area  212  and a fuse link area  222  connecting the anode area  202  and cathode area  212 . Contacts  234  and  244  make connections to the anode area  202  and cathode area  212 , respectively. During a programming of the fuse structure  200 , a current  224  flows from the anode area  202  to the cathode area  212  causing a resistance of the fuse structure  200  to rise due to the EM effect. Under a given current  224  and a certain stress time, the more severe the EM effect the bigger the resistance differentiation between the before and after programming. Referring to  FIG. 2 , the cathode area  212  is therefore made smaller than the anode area  202  to aggravate the EM effect according to the first embodiment of the present invention. 
         [0021]    Although electrical fuse structure  200  is commonly made of polysilicon material, one having skills in the art would appreciate other materials, such as silicided polysilicon and diffusion or a combination of them, may also be used. Besides, the electrical fuse structure  200  is not limited to be on top of a field oxide. The underneath material may be thin gate oxide, as a programming voltage of such electrical fuse structure  200  is low enough not to cause damage to the gate oxide. 
         [0022]      FIG. 3  is a sectional view of the electrical fuse structure  300  having a reverse biased PN juncture in a fuse link area according to a second embodiment of the present invention. Here the electrical fuse structure  300  is made of silicided polysilicon, i.e., a silicide layer  330  is formed on top of a polysilicon layer  320 . Prior to the silicide process, the polysilicon  320  is implanted with N-type ions such as arsenic (As) in an area  323  which is coupled to an anode  302 . The polysilicon  320  is implanted with P-type ions such as boron (B) in an area  327  which is coupled to a cathode  312 . Therefore, the polysilicon  320  has a reverse biased PN junction during programming, which will shun the majority of the programming current to the silicide layer  330 . Large currents in turn will more severely stress the fuse structure  300 , and cause the resistance of the fuse structure  300  to arise more due to the EM effect. 
         [0023]    Although the silicided polysilicon is used to illustrate the second embodiment of the present invention, one having skills in the arts would recognize that the principle of the present invention may be applied to other structures, such as silicide over silicon and anti-fuse structure, as long as a reverse biased PN junction can be formed underneath a layer which is subject to EM effects. 
         [0024]      FIGS. 4A and 4B  are top views of electrical fuse structures  400  and  450  each having a reverse biased PN junction according to the second embodiment of the present invention. The fuse structures  400  and  450  both have an anode area  402 , a cathode area  412  and a fuse link area  422 . Built on top of the first embodiment of the present invention, the cathode area  412  is smaller than the anode area  402 . Referring to  FIG. 4A , the reverse biased PN junction is formed at a location  424 , which is close to a middle section of the fuse link area  422  reflecting the fuse structure  300  shown in  FIG. 3 . Referring to  FIG. 4B , the reverse biased PN junction is formed instead at a location  454  which is approximately an interface of the fuse link area  422  and the cathode area  412 . In such a way, the fuse structure  450  also benefits from the cathode depletion effect, which makes the fuse programming even easier than in the case where the reverse biased PN junction is at the middle of the fuse link area. 
         [0025]      FIG. 5A  illustrates a compact electrical fuse structure  500  according to a third embodiment of the present invention. More compact fuse structures are always preferred. But certain design rules, such as an anode area  502  or a cathode area  512  should overlay their respective contacts  504  and  514  by a certain amount to fully utilize current density of the contacts  504  and  514 . Then the anode  502  and cathode  512  would be doted line enclosed areas  532  and  542 , respectively, which are larger than the shaded anode area  502  and cathode area  512 , respectively. According to the third embodiment of the present invention, the fuse structure  500  is a narrow strip that occupies less space than conventional, design rule abiding fuse structures. By reducing a terminal, i.e., the anode  502  or cathode  512 , overlaying contacts  504  or  514 , the current density at the contacts  504  or  514  may be restricted, which makes the contacts  504  or  514  also prone to the EM effect. This should be avoided in normal circuits, but is desirable in fuse applications, as the more severe the EM effect, the easier the fuse to be programmed and the larger the resistance differentiation between a before and after programming. In this case, the contact EM effect adds to the fuse link EM effect. A large resistance differentiation may be realized on this fuse structure  500 . 
         [0026]      FIG. 5B  illustrates another compact electrical fuse structure  550  having a reverse biased PN junction according to a combination of the second and third embodiment of the present invention. Apparently, if the fuse structure  550  is made of silicided polysilicon, a PN junction may be formed in the polysilicon. Referring to  FIG. 5B , an anode area  552  and a fuse link area  572  are implanted with N-type ions, and a cathode area  562  is implanted with P-type ions, then a reverse biased PN junction is formed at an interface location  576  which is near the cathode area  562 . Therefore, the reverse biased PN junction serves to shun current to the silicide as well as to cause a cathode depletion effect, both of which intensify the EM effect in the fuse structure. 
         [0027]      FIG. 5C  illustrates yet another compact electrical fuse structure  580  having a reverse biased PN junction according to the combination of the second and third embodiment of the present invention. The fuse structure  580  differs from the fuse structure  550  only in that the reverse biased PN junction interface in the fuse structure  580  is created at a location  586  approximate to a middle section of the fuse link area  582 . Similarly, the reverse biased PN junction shuns current to the silicide, which provides boosts to the EM effect on the electrical fuse structure  580 . 
         [0028]    Beside the aforementioned functionality advantages, the present invention may also be a cost down solution for anyone needing a fuse in an IC, as the poly fuse structure may be fabricated in a normal logic process without employing any additional mask. 
         [0029]    Although the silicide on top of the polysilicon is described as embodiments of the present invention, one having skills in the arts would appreciate the bottom polysilicon layer may be replaced by other materials, such as diffusion, as long as a PN junction can be formed therein. Forming the top silicide layer may also be substituted by other processes as long as the top layer is subject to the EM effect. In another aspect, the layer subject to the EM effect may be at the bottom and the layer with reverse biased PN junction may be on the top. 
         [0030]    The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
         [0031]    Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.