Abstract:
A fuse structure and a method for operating the same. The fuse structure operating method includes providing a structure. The structure includes (a) an electrically conductive layer and (b) N electrically conductive regions hanging over without touching the electrically conductive layer. N is a positive integer and N is greater than 1. The N electrically conductive regions are electrically connected together. The structure operating method further includes causing a first electrically conductive region of the N electrically conductive regions to touch the electrically conductive layer without causing the remaining N−1 electrically conductive regions to touch the electrically conductive layer.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates to electric fuses, and more specifically, to electric fuses using CNTs (carbon nanotubes). 
   2. Related Art 
   Fuses are commonly employed to record information about chips and to redirect memory inquiries away from defective elements. Historically, lasers were employed to program metal fuses by blowing (melting) them. However, blown fuses cannot be un-blown and thus the information stored by the fuse structure cannot be erased. Therefore, there is a need for a fuse structure (and a method for operating the same) in which the information can be programmed more than once. 
   SUMMARY OF THE INVENTION 
   The present invention provides a structure operating method, comprising providing a structure which includes (a) an electrically conductive layer, (b) a first dielectric region and a second dielectric region on top of the electrically conductive layer, (c) N electrically conductive regions resting on the first and second dielectric regions and hanging over without touching the electrically conductive layer, wherein N is a positive integer and N is greater than 1, wherein the N electrically conductive regions are electrically connected together, wherein N electrically conductive region segments of the N electrically conductive regions have different lengths, wherein the N electrically conductive region segments are not in direct physical contact with the first and second dielectric regions and the electrically conductive layer; and causing a first electrically conductive region segment of the N electrically conductive region segments to touch the electrically conductive layer without causing the remaining N−1 electrically conductive region segments to touch the electrically conductive layer, wherein the first electrically conductive region segment is the longest electrically conductive region segment of the N electrically conductive region segments. 
   The present invention provides a structure operating method, comprising providing a structure which includes (a) an electrically conductive layer, (b) N electrically conductive regions hanging over without touching the electrically conductive layer, wherein N is a positive integer and N is greater than 1, wherein the N electrically conductive regions are electrically connected together; and causing a first electrically conductive region of the N electrically conductive regions to touch the electrically conductive layer without causing the remaining N−1 electrically conductive regions to touch the electrically conductive layer. 
   The present invention provides a semiconductor fuse structure (and a method for operating the same) in which the fuse structure can be erased and programmed more than once. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A-7D  illustrate a fabrication process for forming a substrate structure, in accordance with embodiments of the present invention. 
       FIGS. 8-10  describe a method for operating of the structure of  FIG. 7C , in accordance with embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A-7D  illustrate a fabrication process for forming a substrate structure  100 , in accordance with embodiments of the present invention. More specifically, with reference to  FIG. 1A , in one embodiment, the fabrication process of the substrate structure  100  starts with a conductive layer  110 . In one embodiment, the conductive layer  110  can be formed on top of a semiconductor (e.g., silicon, germanium, . . . ) substrate (not shown) which is omitted for simplicity. Illustratively, the conductive layer  110  can comprise copper, aluminum, tungsten, or any other electrically conductive material. 
   Next, with reference to  FIG. 1B , in one embodiment, a dielectric layer  120  is formed on top of the conductive layer  110 . Illustratively, the dielectric layer  120  comprises silicon dioxide. In one embodiment, the dielectric layer  120  can be formed by CVD (Chemical Vapor Deposition). 
   Next, with reference to  FIG. 1C , in one embodiment, a trench  205  is formed in the dielectric layer  120  of the structure  100  of  FIG. 1B  such that a top surface  112  of the conductive layer  110  is not exposed to the surrounding ambient via the trench  205 . Illustratively, the trench  205  can be formed by photo-lithography and then etching. 
     FIG. 1D  illustrates a cross-section view of the structure  100  of  FIG. 1C  along a plane defined by a line  1 D. As can be seen in  FIG. 1D , the conductive layer  110  is not exposed at the bottom  205   a  of the trench  205 . 
   Next, with reference to  FIG. 2A , in one embodiment, a catalyst region  210  is formed in the trench  205 . Illustratively, the catalyst region  210  comprises iron or nickel. In one embodiment, the catalyst region  210  can be formed by CVD of iron followed by a CMP (Chemical Mechanical Polishing) step until a top surface  122  of the dielectric layer  120  is exposed to the surrounding ambient. 
     FIG. 2B  illustrates a cross-section view of the structure  100  of  FIG. 2A  along a plane defined by a line  2 B. 
   Next, with reference to  FIG. 3A , in one embodiment, a dielectric layer  310  is formed on top of the structure  100  of  FIG. 2A . Illustratively, the dielectric layer  310  can be formed by CVD of silicon dioxide on top of the structure  100  of  FIG. 2A . It should be noted that the dielectric layer  310  and the dielectric layer  120  can be collectively referred to as a dielectric layer  120 + 310 . 
     FIG. 3B  illustrates a cross-section view of the structure  100  of  FIG. 3A  along a plane defined by a line  3 B. As can be seen in  FIG. 3B , the catalyst region  210  is buried inside the dielectric layer  120 + 310 . 
   Next, with reference to  FIG. 3C , in one embodiment, a trench  405  is formed in the dielectric layer  120 + 310  such that the top surface  112  of the conductive layer  110  is exposed to the surrounding ambient via the trench  405 . More specifically, in one embodiment, when going in a direction  420 , the width of the trench  405  in a direction  421  increases (wherein the direction  421  is essentially perpendicular to the direction  420 ). For instance, a width  412  is smaller than a width  414 . In one embodiment, the trench  405  can be formed by photo-lithography and then etching. As a result of the etching the dielectric layer  120 + 310 , what remains of the dielectric layer  120 + 310  is dielectric regions  310   a  and  310   b  as shown in  FIG. 3C . 
   Next, with reference to  FIG. 4A , in one embodiment, a sacrificial layer  410  is formed in the trench  405 . Illustratively, the sacrificial layer  410  can be formed by CVD of germanium followed by a CMP step until a top surface  312  of the dielectric regions  310   a  and  310   b  is exposed to the surrounding ambient. 
     FIG. 4B  illustrates a cross-section view of the structure  100  of  FIG. 4A  along a plane defined by a line  4 B. 
     FIG. 4C  illustrates a top-down view of the structure  100  of  FIG. 4A . 
   Next, with reference to  FIG. 5A , in one embodiment, holes  510   a - 510   i  are formed in the dielectric region  310   a . More specifically, in one embodiment, the holes  510   a - 510   i  are formed such that the catalyst region  210  is exposed to the surrounding ambient via the holes  510   a - 510   i . In one embodiment, the holes  510   a - 510   i  can be formed by photo-lithography and then etching. 
     FIG. 5B  illustrates a cross-section view of the structure  100  of  FIG. 5A  along a plane defined by a line  5 B. As can be seen in  FIG. 5B , the catalyst region  210  is exposed to the surrounding ambient via the hole  510   a.    
   Next, with reference to  FIG. 6A , in one embodiment, CNTs (carbon nanotubes)  610   a - 610   i  are formed on top of the structure  100  of  FIG. 5A . Illustratively, the CNTs  610   a - 610   i  are a molecular form of carbon. In one embodiment, the CNTs  610   a - 610   i  are formed by placing the structure  100  of  FIG. 5A  in a plasma environment (not shown) which includes chemicals such as methane or alcohol, at an appropriate temperature (typically 600-900° C.). As a result, CNTs grow up from the catalyst region  210  at bottom of the holes  510   a - 510   i  to, and beyond, the top of the holes  510   a - 510   i . At the same time, reactants flow in the direction  421  from the dielectric region  310   a  to the dielectric region  310   b . As a result, CNTs  610   a - 610   i  grow in the direction  421  from the dielectric region  310   a  to the dielectric region  310   b  across a top surface  412  of the sacrificial layer  410 . In one embodiment, the CNTs  610   a - 610   i  have the same length, though having the same length is not required. 
     FIG. 6B  illustrates a cross-section view of the structure  100  of  FIG. 6A  along a plane defined by a line  6 B. 
     FIG. 6C  illustrates a top-down view of the structure  100  of  FIG. 6A . In one embodiment, with reference to  FIG. 6C , the CNTs  610   a - 610   i  have the same length. It should be noted that CNT segments  610   a ′- 610   i ′ of the CNTs  610   a - 610   i  which are directly above the sacrificial layer  410  have different lengths. In one embodiment, when going in a direction  630 , the lengths of the CNT segments  610   a ′- 610   i ′ increases. For instance, the length of the CNT segment  610   a ′ is shorter than the length of the CNT segment  610   b′.    
   Next, with reference to  FIG. 7A , contact holes  710  and  720  are formed in the dielectric regions  310   a  and  310   b , respectively. More specifically, the contact hole  710  is formed in the dielectric region  310   a  such that the catalyst region  210  is exposed to the surrounding ambient via the contact hole  710 . In one embodiment, the contact hole  720  is formed in the dielectric region  310   b  such that the conductive layer  110  is exposed to the surrounding ambient via the contact hole  720 . Illustratively, the contact holes  710  and  720  can be simultaneously formed by photo-lithography and then etching. 
   Next, in one embodiment, the sacrificial layer  410  ( FIG. 6A ) is removed to expose the top surface  112  of the conductive layer  110  to the surrounding ambient. Illustratively, the sacrificial layer  410  can be removed by wet etching. It should be noted that after the removal of the sacrificial layer  410 , the CNTs  610   a - 610   i  hang above without touching the conductive layer  110 . 
     FIG. 7B  illustrates a cross-section view of the structure  100  of  FIG. 7A  along a plane defined by a line  7 B. As can be seen in  FIG. 7B , the catalyst region  210  is exposed to the surrounding ambient via the contact hole  710  and the conductive layer  110  is exposed to the surrounding ambient via the contact hole  720 . 
   Next, with reference to  FIG. 7C , in one embodiment, contact regions  712  and  722  are formed in the contact holes  710  and  720 , respectively. Illustratively, the contact regions  712  and  714  can be formed by CVD of tungsten followed by a RIE (reactive ion etching) step until top surfaces  714  and  724  of the dielectric regions  310   a  and  310   b  are exposed to the surrounding ambient, respectively. In one embodiment, the contact regions  712  and  724  provide electrical access to the catalyst region  210  and the conductive layer  110 , respectively. 
     FIG. 7D  illustrates a cross-section view of the structure  100  of  FIG. 7C  along a plane defined by a line  7 D. 
   In summary, the CNTs  610   a - 610   i  are electrically connected together via the catalyst region  210 , and the CNTs  610   a - 610   i  are electrically insulated from the conductive layer  110  by the dielectric regions  310   a  and  310   b . It should be noted that the lengths of the CNT segments  610   a ′- 610   i ′ are different. 
   In the embodiments described above, the contact regions  712  and  722  are formed after the formation of the CNTs  610   a - 610   i . Alternatively, the contact regions  712  and  722  are formed before the formation of the holes  510   a - 510   i  (from where the CNTs  610   a - 610   i  are subsequently grown). 
   In one embodiment, the structure  100  of  FIG. 7C  is in either a first state or a second state depending on whether the catalyst region  210  is electrically connected to the conductive layer  110 . More specifically, the structure  100  of  FIG. 7C  is in the first state if at least one of the CNTs  610   a - 610   i  electrically couples the catalyst region  210  to the conductive layer  110 . In contrast, the structure  100  of  FIG. 7C  is in the second state if none of the CNTs  610   a - 610   i  electrically couples the catalyst region  210  to the conductive layer  110 . 
     FIGS. 8-10  describe a method of operation of the structure  100  of  FIG. 7C , in accordance with embodiments of the present invention. 
     FIG. 8  illustrates the method for bringing the structure  100  of  FIG. 7C  from the second state to the first state, in accordance with embodiments of the present invention. More specifically, in one embodiment, a voltage V 1  is applied between the catalyst region  210  and the conductive layer  110  via a resistor  810 . As a result, between CNTs  610   a - 610   i  and the conductive layer  110  appears the voltage V 1 . This is because the CNTs  610   a - 610   i  are electrically connected to the catalyst region  210 . As a result of the voltage V 1  between the CNTs  610   a - 610   i  and the conductive layer  110 , the CNTs  610   a - 610   i  are attracted towards the conductive layer  110 . Because the CNT segment  610   i ′ is the longest among the CNT segments  610   a ′- 610   i ′, the CNT segment  610   i ′ is the first one to touch the conductive layer  110 . As soon as the CNT segment  610   i ′ touches the conductive layer  110 , the CNTs  610   a - 610   i  and the conductive layer  110  have the same voltage. Therefore, the CNT segments  610   a ′- 610   h ′ are no longer pulled toward the conductive layer  110 . In one embodiment, the resistor  810  is selected such that the current flowing from catalyst region  210  through the CNT  610   i  to conductive layer  110  is not high enough to blow (burn through) the CNT  610   i  after the CNT segment  610   i ′ comes into direct physical contact with the conductive layer  110 . After that, in one embodiment, the voltage V 1  is removed from the catalyst region  210  and the conductive layer  110 . In one embodiment, after the removal of the voltage V 1 , the CNT  610   i  remains in direct physical contact with the conductive layer  110 . As a result, one of the CNTs  610   a - 610   i  electrically couples the catalyst region  210  to the conductive layer  110 . In other words, the structure  100  is in the first state. 
     FIG. 9  illustrates the method for bringing the structure  100  of  FIG. 8  from the first state back to the second state, in accordance with embodiments of the present invention. More specifically, in one embodiment, a voltage V 2  is applied between the catalyst region  210  and the conductive layer  110 . As a result, there is a current flowing from the catalyst region  210  through the CNT  610   i  to the conductive layer  110 . In one embodiment, the voltage V 2  is high enough such that the resulting current flowing through the CNT  610   i  is strong enough to blow the CNT  610   i  but the voltage V 2  is not high enough to cause the remaining CNTs  610   a - 610   h  to touch the conductive layer  110  (after the CNT  610   i  is blown, as can be seen in  FIG. 9 ). After fuse blow, in one embodiment, the voltage V 2  is removed from the catalyst region  210  and the conductive layer  110 . As a result, none of the CNTs  610   a - 610   i  electrically couples the catalyst region  210  to the conductive layer  110 . In other words, the structure  100  switches from the first state to the second state. 
     FIG. 10  illustrates the method for bringing the structure  100  of  FIG. 9  from the second state to the first state after the CNT  610   i  is blown, in accordance with embodiments of the present invention. More specifically, in one embodiment, the voltage V 1  is applied between the catalyst region  210  and the conductive layer  110  via the resistor  810 . As a result, the CNTs  610   a - 610   h  are attracted towards the conductive layer  110 . In one embodiment, the CNT segment  610   h ′ touches the conductive layer  110  in a manner similar to the manner the CNT segment  610   i ′ touches the conductive layer  110  as described above. In one embodiment, the resistor  810  is selected such that the current flowing through the CNT  610   h  is not high enough to blow the CNT  610   h  after the CNT segment  610   h ′ comes into direct physical contact with the conductive layer  110 . After that, in one embodiment, the voltage V 1  is removed from the catalyst region  210  and the conductive layer  110 . In one embodiment, after the removal of the voltage V 1 , the CNT  610   h  remains in direct physical contact with the conductive layer  110 . As a result, one of the CNTs  610   a - 610   i  electrically couples the catalyst region  210  to the conductive layer  110 . In other words, the structure  100  switches from the second state to the first state. 
   In summary, the state of the structure  100  of  FIG. 7C  can be changed from the second state to the first state by causing a CNT to touch the conductive layer  110 . In one embodiment, the state of the structure  100  can be changed from the first state to the second state by blowing the CNT that electrically couples the catalyst region  210  to the conductive layer  110 . It should be noted that the state of the structure  100  can be changed from the second state to the first state N times, wherein N is the number of CNTs. In the case of the structure  100  of  FIG. 7A , the state of the structure  100  can be changed from the second state to the first state nine times because it has the nine CNTs  610   a - 610   i . As a result, in one embodiment, the structure  100  of  FIG. 7C  can be used as a memory cell whose content can be changed eighteen times. 
   While particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.