Electric fuses using CNTs (carbon nanotubes)

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.

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.

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.

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.