Abstract:
A one-time programmable memory cell includes a fuse and an anti-fuse in series. The memory cell has two states, an initial state and a written (programmed) state. In the initial state, a resistance of the cell is finite, typically dominated by the relatively high resistance of the anti-fuse. In the written state, the resistance is infinite because the breakdown of the fuse resulting in an open circuit. The cell may be programmed by applying a critical voltage across the cell generating a critical current to cause the fuse to become open. When critical voltage is applied, this generally causes the anti-fuse to break down, which in turn causes a pulse of high current to be applied to the fuse. The states are detected by applying a read voltage across the memory cell. If the memory has not been programmed, then a measurable amount flows. Otherwise, no current flows.

Description:
RELATED APPLICATIONS 
     The following applications of the common assignee may contain some common disclosure and may relate to the present invention: 
     U.S. patent application Ser. No. 09/964,770, entitled “VERTICALLY ORIENTED NANO-FUSE AND NANO-RESISTOR CIRCUIT ELEMENTS” (Attorney Docket No. 10012295-1); 
     U.S. patent application Ser. No. 09/924,500, filed Aug. 9, 2001, entitled “ONE-TIME PROGRAMMABLE VERTICALLY ORIENTED FUSE AND VERTICALLY ORIENTED FUSE/DIODE UNIT MEMORY CELL AND ONE-TIME PROGRAMMABLE MEMORY USING THE SAME” (Attorney Docket No. 10019168-1); 
     U.S. patent application Ser. No. 09/924,577, filed Aug. 9, 2001, entitled “ONE-TIME PROGRAMMABLE MEMORY USING FUSE/ANTI-FUSE AND VERTICALLY ORIENTED FUSE UNIT MEMORY CELLS” (Attorney Docket No. 10012495-1). 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to programmable memory cells. More particularly, the invention relates to a one-time programmable memory cells. 
     BACKGROUND OF THE INVENTION 
     The demand for semiconductor devices has increased dramatically in recent years. One can readily observe the pervasiveness of consumer electronic devices in the modern world. Most or all of the consumer electronic devices are made possible because of developments in semiconductor devices. As the consumer electronic devices become smaller, more sophisticated, and less expensive, increasingly higher densities of the semiconductor devices, including memories, are demanded at a lower cost in today&#39;s market place. 
     In the field of memories, the demand for ever increasing densities and lower cost is particularly true, especially for the non-volatile memories, i.e., those memories that do not lose data even when power is not supplied. 
     A non-volatile memory may be a one time programmable (“OTP”) or reprogrammable. As the name suggests, OTP memory is programmed once, and it is permanent for all practical purposes. Most OTP memories can be categorized into four basic types: 1) anti-fuse, 2) fuse, 3) charge storage (EPROM), and 4) mask ROM. 
     Existing OTP memory technologies described above are based on cell sizes considerably larger than 4λ 2 , the minimum cell size for a cross-point memory. In addition, in each case the memory cell consists of a single plane of memory elements constructed on a single crystal silicon substrate, with sense and programming electronics located around the periphery of the memory array. Since single crystal silicon transistors are integral components of the memory elements in the foregoing technologies, stacking memory layers on top of one another to increase density is not possible. Consequently, high density, low cost OTP memories are difficult to fabricate. 
     SUMMARY OF THE INVENTION 
     In one respect, an exemplary embodiment of a memory cell may include a top conductor extending in a first direction and a bottom conductor extending in a second direction. The top and bottom conductors define a region of overlap at an intersection between the two conductors. The top and bottom conductors are electrically connected. The memory cell may also include a fuse formed in the region of overlap between the top and bottom conductors. The fuse may also have electrical connectivity with the top and bottom conductors. Further, the memory cell may include an anti-fuse in electrical series with the fuse. The anti-fuse may also be formed between the top and bottom conductors. The fuse may be vertically oriented, i.e. the current substantially flows vertically within the fuse. 
     In another respect, an exemplary embodiment of a method of fabricating a memory cell may include forming a top conductor extending in a first direction and forming a bottom conductor extending in a second direction so as to define a region of overlap at an intersection between the top and bottom conductors. The top and bottom conductors may have electrical connectivity with each other. The method may also included forming a fuse in the cross-point between the top and bottom conductors. The method may further include forming an anti-fuse in electrical series with the fuse. 
     The above disclosed exemplary embodiments may be capable of achieving certain aspects. For example, the size of the memory cell may be dramatically reduced. This enables providing a high density OTP memory cell at much lower cost. Also, the memory cell may be fabricated using standard semiconductor processes and materials, and thus, little to no capital investment may be required beyond that present in the current state-of-the-art manufacturing. Further, the current flow in the memory cells is substantially perpendicular (vertical) to the substrate plane. This allows the cells to be inserted between adjacent conductors. In particular, the cells can be placed at an intersection of a cross-point array of conductors to form a cross-point OTP memory array. The cross-point memory arrays can be fabricated such that the planar area of each memory cell is 4λ 2 . Planes of these arrays can be stacked on top of one another, which increases the density dramatically. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Features of the present invention will become apparent to those skilled in the art from the following description with reference to the drawings, in which: 
     FIGS. 1A illustrates a cross-sectional view of a first embodiment of a memory cell according to the principles of the present invention; 
     FIG. 1B illustrates a top view the first embodiment of FIG. 1A showing the cross-point nature of the memory cell; 
     FIGS. 1C-1D illustrate variations on the first embodiment of FIGS. 1A; 
     FIGS. 2A-2G illustrate cross-sectional views of an exemplary embodiment of a method of fabricating the first embodiment of the memory cell; 
     FIGS. 2A-2,  2 D- 2 ,  2 A- 3  and  2 D- 3  illustrate modifications to the method of fabricating the first embodiment of the memory cell to fabricate the variations shown in FIGS. 1C-D; 
     FIG. 3A illustrates a cross sectional view of a second embodiment of a memory cell according to the principles of the present invention; 
     FIG. 3B illustrates a top view the second embodiment of FIG. 3A showing the cross-point nature of the memory cell; 
     FIGS. 3C-3E illustrate variations on the first embodiment of FIG. 3A; 
     FIGS. 4A-4G illustrate cross-sectional views of an exemplary embodiment of a method of fabricating the second embodiment of the memory cell; 
     FIG. 5A illustrates a resistance characteristic of an exemplary anti-fuse according to an aspect of the present invention; 
     FIG. 5B illustrates a resistance characteristic of an exemplary fuse according to an aspect of the present invention; and 
     FIG. 5C illustrates the resistance and current characteristic of an exemplary fuse/anti-fuse series combination according to an aspect of the present invention. 
    
    
     DETAILED DESCRIPTION 
     For simplicity and illustrative purposes, the principles of the present invention are described by referring mainly to exemplary embodiments thereof. However, one of ordinary skill in the art would readily recognize that the same principles are equally applicable to many types of a memory cells and methods of fabrication thereof. 
     In general, a memory cell, according to certain aspects of the present invention, is located at a region of overlap of two conductors, for example a cross-point. The memory cell generally includes a fuse in series with an anti-fuse. The anti-fuse is an element that has an initial high resistance and changes to a relatively low resistance when a critical voltage or critical current is applied. 
     FIG. 5A illustrates a resistance characteristic of an exemplary anti-fuse according to an aspect of the present invention. As shown, the anti-fuse has an initial high resistance R 1   AF . When a critical voltage V C  is applied at time t 0 , current begins to flow through the anti-fuse. At time t 1 , the anti-fuse breaks down to a relatively low resistance R 2   AF . If voltage V C  continues to be applied, a large current flows through the anti-fuse after time t 1 . 
     An anti-fuse can be formed from insulator materials, a multilayer stack of insulator materials separated by conducting materials, a matrix of insulating material containing dispersed conductive inclusions, amorphous and crystalline semiconductor materials, phase change materials, combinations of a multi-layer stack of Si and silicide-forming metals, etc. Generally, the anti-fuse is sandwiched between two conducting materials to enable applications of voltage across the anti-fuse. Insulator materials include diamond like carbon, SiO X , SiN X , SiO X  N Y , AlO X , TaO X , TiO X , AlN X  and the like; amorphous and crystalline semiconductor materials include Si, Ge, alloys of Si and Ge, GaAs, and the like; phase change materials include alloys containing at least two elements selected from Si, Ge, As, Se, In, Sn, Sb, Te, Pb, Bi, and the like; silicide-forming metals include W, Pt, Pd, Co, Ni, Ti, and the like and alloys thereof. 
     If insulator materials are used as the anti-fuse, then the thickness of the anti-fuse is preferably between 0.5 and 50 nm. However, the thickness may be set to an arbitrary range depending on the circumstances. For instance, if appreciable current flow is desired through the anti-fuse in a pre-breakdown condition, then the insulator thickness may be chosen to be less than about 5 nm so that significant quantum mechanical tunneling current can flow at a modest voltage. If amorphous and crystalline semiconductor materials are used, the thickness is preferably between 1 and 100 nm. Again, the thickness may be varied. 
     As noted above, the anti-fuse is an element that has an initial high resistance and changes to a relatively low resistance when a critical voltage is applied. The mechanism that achieves the different resistive states is different for different materials. For example, anti-fuses formed from phase change materials have a high resistance when in an amorphous state and a low resistance when in a crystalline state. Also, anti-fuses formed from multilayer Si and silicide-forming metals have a high resistance when the multilayer has not been converted to silicide and a low resistance when said multilayer has been converted to the silicide. In both cases, many orders of magnitude separate the high and low resistance states. 
     As another example, if an insulator type of anti-fuse is used, up to the critical voltage V C , current passes through the insulating barrier of the metal-insulator-metal structure by electron tunneling, and the specific resistance of the cell can be rather large, for example, in the order of 10 7  Ω-μm 2 . However, beyond the critical voltage V C , the barrier breaks down due to metal migration through the insulator, and the specific resistance of the cell can drop to below 100 Ω-μm 2 . Similar current transport and breakdown mechanisms are operative in layered insulators and insulators containing conductive inclusions. 
     Unlike the anti-fuse, the fuse is an element that has an initial low resistance and changes to a high resistance, mostly to an open circuit when a critical current is applied. The fuse may be a thin film resistor, and may be formed from materials such as semiconductors (e.g. Si, Ge), conductors (e.g. Al, Cu, Ag, Au, Pt), low melting temperature materials (e.g. Zn, Sn, Pb, In), refractories (e.g. Ta, W), transition metals (Ni, Cr) and the like and any alloys thereof. It is even more beneficial if the fuse is vertically oriented, i.e. the direction of the current flow is substantially vertical within the fuse, since very small memory elements can be achieved with vertically oriented fuses. 
     FIG. 5B illustrates a resistance characteristic of an exemplary fuse according to an aspect of the present invention. As shown, the fuse has an initial low resistance R 1   F . The fuse maintains the low resistance until a critical current I C  is initiated at time t 1 . At this point, an I 2 R heating causes the resistance of the fuse to increase, leading to thermal runaway; i.e. the increase in resistance leads to additional I 2 R heating, which leads to further increase in resistance, and so on. Eventually the I 2 R heating causes the fuse to melt and become an open circuit R 2   F  at time t 2 . Thus, the memory cell with a fuse exhibits two states. The first, or initial, state is resistance R 1   F , which can be controlled to a specified value through the choice of fuse materials and geometry. The second, or final state, is R 2   F , an open-circuit. 
     Programming a memory cell made of a fuse and anti-fuse combination is done by either applying the voltage V C , leading to critical current I C , if the second state is desired or leaving the cell alone if the first state is desired. The first and second states may be detected by applying a read voltage V R  and detecting a presence or absence of an electrical current. Current presence indicates that the memory cell is in the first state and current absence indicates the second state. 
     As noted above, the memory cell generally includes a fuse and an anti-fuse connected in series. FIG. 5C illustrates a resistance (shown in solid line) and current (shown in dashed line) characteristics of an exemplary fuse/anti-fuse series combination according to an aspect of the present invention. Initially, the resistance of the combination is dominated by the high resistance R 1   AF  of the anti-fuse. However, when sufficiently large voltage, i.e. V C , is applied at time t 0 , the anti-fuse breaks at time t 1  down as explained previously. 
     At this point, both the fuse and the anti-fuse are low in resistance as shown by the sharp drop in the resistance line around time t 1 . Due to the low resistance, current passing through the fuse/anti-fuse combination becomes critical, i.e. critical current I C  is generated. This melts the fuse as discussed previously. 
     The thermal runaway process causes the resistance of the combination to increase until finally the fuse breaks and becomes an open circuit at time t 2 . At this point, the resistance of the combination is dominated by the open circuit R 2   F . Correspondingly, the current becomes zero at time t 2  as shown by the dashed lines in FIG.  5 C. The time between t 0  and t 2  may be extremely short, which allows rapid programming to take place. 
     Thus the memory cell with the fuse and anti-fuse in series exhibits two states. The first state, or the initial state, exhibits a finite resistance (generally dominated by R 1   AF ). In this first state, some amount of current may flow since the resistance is finite. The second state exhibits an infinite resistance (an open circuit R 2   F ). As a result, no current may flow across the cell (see the dashed line in FIG.  5 C). 
     It should be noted that an anti-fuse is not strictly necessary for a memory cell. However, in a cross-point memory array that does not include a diode or transistor in series with the memory cell, the anti-fuse provides selectivity for programming a particular memory cell. Also the high initial resistance of the anti-fuse allows the individual resistance of the fuse to be reduced to an arbitrary value without jeopardizing the ability to sense an individual memory element in the array. 
     In addition, resistance of the anti-fuse can vary as different levels of voltage are applied to the memory cell. This characteristic can be used to enhance the memory cell selectivity function that an anti-fuse provides in memory devices. 
     FIG. 1A illustrates a cross-sectional view of a first embodiment of a memory cell  100  according to the principles of the present invention. As shown in FIG. 1A, the first embodiment of the memory cell  100  may include a bottom conductor  110  and a first insulator  120  situated above the bottom conductor  110 . The first insulator  120  is formed around a perimeter of a closed region  185 . As will be demonstrated below, the closed region  185  generally occupies a region defined by a cross-point of the memory. 
     To form the bottom conductor  110 , conductive materials such as aluminum, copper, gold, tungsten, and the like and any alloys thereof can be used. Polysilicon may also be used to form the bottom conductor  110 . To form the first insulator  120 , materials such as silicon oxides and nitrides, aluminum oxides and nitrides, silicon oxynitrides, and the like can be used. 
     The memory cell  100  may also include an anti-fuse  180 , which substantially occupies the closed region  185 . As noted previously, the anti-fuse  180  can be formed from insulator materials, multilayer stacks of insulator materials separated by conductive materials, a matrix of insulating materials with conductive inclusions, amorphous and crystalline semiconductor materials, phase change materials, combinations of a multi-layer stack of Si and silicide-forming metals, etc. FIG. 1A shows that the anti-fuse is patterned as a thin wafer. However, this is not strictly necessary. 
     The memory cell  100  may further include a fuse  130  and an insulating plug  140 . The fuse  130  and the insulating plug  140  may substantially occupy an edge and a center of the closed region  185 , respectively, above the anti-fuse  180 . Tops of the insulator  120 , the fuse  130 , and the insulating plug  140  may be coplanar. 
     To form the fuse  130 , materials such as semiconductors (e.g. Si, Ge), conductors (e.g. Al, Cu, Ag, Au, Pt), low melting temperature materials (e.g. Zn, Sn, Pb, In), refractories (e.g. Ta, W), transition metals (Ni, Cr) and the like and any alloys thereof can be used. Also, the materials used to form the first insulator  120  can generally be used to form the insulating plug  140 , although in certain embodiments it may be desirable for the insulating plug  140  to be etched away leaving a void. 
     Note that the insulating plug  140  is not strictly necessary. The insulating plug  140  helps to control the cross-sectional area of the fuse  130  in a plane parallel to the substrate plane, for example the area of the fuse  130  contacting the anti-fuse  180 . Presumably, it is possible that a memory cell can be fabricated with the appropriate amount of surface area such that the insulating plug  140  is not necessary. 
     The memory cell  100  may still further include a second insulator  150  and a top conductor  160 , both situated above the first insulator  120 , the fuse  130  and the insulating plug  140 . The top conductor  160  may be formed from similar materials used to form the bottom conductor  110  and the second insulator  150  may be formed from similar materials used to form the first insulator  120  and the insulating plug  140 . 
     FIG. 1A also shows that an inner wall of the fuse  130  is bounded by the insulating plug  140  and an outer wall is bounded by the first insulator  120 . This configuration provides for lateral thermal isolation of the fuse  130  and enables more efficient heating of the fuse  130  by application of current. However, it is not strictly necessary that the bounds of the walls of the fuse be strictly determined by the insulating plug  140  and the first insulator  120 . 
     Also while not strictly necessary, the fuse  130  may be vertically oriented, i.e. the direction of current flow within the fuse  130  is substantially vertical. This allows the memory cells to be inserted between adjacent conductors. In particular, the cells can be placed at an intersection of a cross-point array of conductors to form a cross-point OTP memory array. Planes of these arrays can be stacked on top of one another, which increases the density dramatically. The vertical height of the fuse  130  may be equal to or greater than the width of the fuse  130 , and in some cases substantially greater. 
     Further, while FIG. 1A shows that the top conductor  160  covers the entirety of the fuse  130  at the top of the closed region  185 , this is not a requirement to practice the present invention. Similarly, FIG. 1A also shows that the bottom conductor  110  covers the entirety of the anti-fuse  180  at the bottom of the closed region  185 . While complete coverage is shown, it is required only that a conductive path between the top and the bottom conductors  160  and  110  exists. 
     Thus, electrical connections should exist among the bottom conductor  110 , the fuse  130 , the anti-fuse  180 , and the top conductor  160 . It is not necessary that the bottom conductor  110 , the fuse  130 , the anti-fuse  180 , and the top conductor  160  be in physical contact with each other. 
     FIG. 1B illustrates a top view of the first embodiment of FIG. 1A showing the fuse  130  and the insulating plug  140  substantially occupying the edge and center of the closed region  185 , which is located within the cross-point  115  of the top and bottom conductors  160  and  110 . The anti-fuse  180  (not shown in FIG. 1B) can have the same shape as the insulating plug  140  and fuse  130 , or it can extend beyond the fuse  130  and assume a different shape. The top and bottom conductors  160  and  10  extend in their respective directions to form the cross-point  115  (shown as a dashed line region for illustrative purposes). Even though the closed region  185  is shown to be entirely located within the cross-point  115 , this is not strictly required. As noted above, it is only necessary that electrical connectivity is maintained between the top and bottom conductors  160  and  110  through the structure within the closed region  185 . 
     For simplicity, the first and second insulators  120  and  150  are not included in FIG.  1 B. Also, for illustrative purposes, the fuse  130  and the insulating plug  140  are shown at the cross-point. However, the top conductor  160  may cover the entirety of the fuse  130  and the insulating plug  140 . Also, even though a cross-point  115  is shown in FIG. 1B, it is only necessary that a region of overlap is created between the top and bottom conductors  160  and  110 , i.e. the first and second directions need not be different. 
     Also, in FIG. 1B, the closed region  185  is shown as being cylindrical with the fuse  130  substantially occupying an annulus of the closed region  185  and the insulating plug  140  substantially occupying a center of the closed region  185 . However, the shape of the closed region  185  is not so limited and may include other shapes as well, such as a rectangle, a square, an ellipse, or any other enclosed shapes. Again, the insulating plug  140  may only partially fill the interior of the closed region  185 . 
     FIGS. 1C-1D illustrate variations on the first embodiment of FIG.  1 A. In FIG. 1C, a thin conductor  190  may be placed as shown to enhance the performance of the memory cell. In FIG. 1D, two thin conductors  190  and  190   b  may be placed as shown for the same purpose. The thin conductors  190  and/or  190   b  enable independent control of the material adjacent to the anti-fuse  180  and provide a larger contact area between the fuse  130  and the anti-fuse  180 . The thin conductors may be formed of aluminum, copper, nickel, titanium, tungsten, gold, metal nitrides, doped silicon, tantalum, and the like and the alloys thereof. 
     In FIG. 1C, the thin conductor  190  is placed between the anti-fuse  180  and the fuse  130  in the closed region  185 . If only a single thin conductor is to be included, this is the preferred placement in order to increase the area of the top surface of the anti-fuse  180 . In FIG. 1D, the first thin conductor  190  is placed between the anti-fuse  180  and the fuse  130  as in FIG. 1C, but also includes a second thin conductor  190   b  placed between the bottom conductor  110  and the anti-fuse  180 . 
     One reason to include thin conductors  190  and/or  190   b  is to introduce a material with lower thermal conductive than present in the top or bottom conductors  160  or  110 . A layer with low thermal conductivity may help to thermally isolate the memory cell from the top or bottom conductors  160 ,  110 . Thermal isolation provides for more efficient use of heat generated by I 2 R processes. 
     Using an amorphous or crystalline semiconductors as the anti-fuse introduces additional reasons to include thin conductors  190  and/or  190   b . First, the choice of conductor material in contact with the semiconductor determines whether a rectifying or ohmic contact is formed. The nature of this contact may affect the function of the anti-fuse  180 . Second, in certain semiconductor anti-fuses, the low resistance state is created by metal migration through the semiconductor layer. This process is dependent on the metal adjacent to the semiconductor. Thin conductors  190  and/or  190   b  provide flexibility in the choice of conductors  110  and  160  and the metal layer adjacent to the semiconductor or anti-fuse. 
     As mentioned previously, some, or all, of the insulating plug  140  may be etched away leaving a void in the region of the insulating plug  140 . This configuration provides extremely low thermal conductivity adjacent to the fuse  130 , and provides space for molten or evaporated fuse material to enter. These features lower the power necessary to break the fuse  130 . 
     FIGS. 2A-2G illustrate cross-sectional views of an exemplary embodiment of a method of fabricating the first embodiment of the memory cell of FIG.  1 A. As shown in FIG. 2A, a conductive material may be deposited and patterned to form the bottom conductor  110 . Then an anti-fuse material  180 ′ may be deposited above the bottom conductor  110  as shown. As part of the patterning process, the bottom conductor  110  may be planarized, by using well-known methods such as chemical-mechanical polishing (“CMP”) prior to depositing the anti-fuse material  180 ′. Similarly, the anti-fuse material  180 ′ may be planarized as well. 
     Subsequently, a dielectric film  140 ′ may be deposited over the anti-fuse material  180 ′. Then, as shown in FIG. 2B, the dielectric film  140 ′ may be etched to form the insulating plug  140 . Standard lithography and etch methods may be used to form the insulating plug  140 . 
     Then, as shown in FIG. 2C, a fuse material  130 ′ may be deposited over the anti-fuse material  180 ′ and even over the insulating plug  140 . A deposition method such as atomic layer deposition (ALD) may be used to ensure a conformal coating and precise control of the thickness of the fuse material  130 ′. Afterwards, the fuse material  130 ′ may be etched to leave the fuse  130  primarily on the wall of the insulating plug  140  as shown in FIG.  2 D. This process is very well suited to fabricate the fuse  130  that is vertically oriented. The fuse  130  may be formed by anisotropic etching of the fuse material  130 ′ using ion etching, reactive ion etching, or other etching methods. 
     Note that the fuse  130 , in this case a vertically oriented fuse, is generally formed within the closed region  185 . Also note that the etching process may etch the anti-fuse material  180 ′ leaving an anti-fuse  180  so that the bottom conductor  110  is exposed in areas perimeter to the closed region  185 . In the particular instance of an insulator anti-fuse, it is not necessary to pattern the anti-fuse material  180 ′ since there is no conductivity in the plane of the film. Note further that the ratio of the vertical height ‘h’ of the vertically oriented fuse  130  to the width ‘w’ of the closed region  185  can be large such as 5 to 1 or more. When anisotropic etching is used, the process inherently leaves behind the fuse  130  primarily on the vertical sidewalls of the insulating plug  140 . Thus lateral area consumption is kept to a minimum, which allows for precise control of the lateral thickness ‘t’ of the fuse  130 . Note that the vertical height ‘h’ to lateral thickness ‘t’ ratio of the fuse  130  can be extremely large, such as 30 to 1 or more. Also, since the fuse  130  is only on the perimeter of the closed region  185 , whereas the anti-fuse  180  covers at least the entire base of the closed region  185 , the ratio of a anti-fuse area to fuse area can also be substantial. 
     Then as shown in FIG. 2E, an insulating material  120 ′ may be deposited over the bottom conductor  110  covering the area outside the perimeter of the closed region  185 . Then the insulating material  120 ′ is patterned to form the first insulator  120  as shown in FIG.  2 F. The first insulator  120  may be patterned by planarizing the insulating material  120 ′ to expose the fuse  130  and the insulating plug  140 , again using CMP and/or other planarizing method(s). Indeed, the tops of the first insulator  120 , fuse  130 , and insulating plug  140  may define a plane. At this point the vertically oriented fuse  130  is bounded on all vertical sides by insulator. This configuration reduces heat transfer from the fuse to its surroundings. 
     Then to complete the process, a top conductor  160  may be deposited and patterned in the first direction over the fuse  130 , the insulating plug  140  and the first insulator  120 . If desired, the second insulator  150  may be deposited over the top conductor  160  and first insulator  120  and planarized using CMP or other planarizing methods. The resulting structure is shown in FIG. 2G (same as FIG.  1 A). 
     If a void is desired in the region of the insulating plug  140 , then the insulating material can be removed by either wet or dry etching after definition of the top conductor  160 . Access to the insulating plug  140  may be possible when the top conductor  160  does not completely cover the insulating plug  140 . In other words, to generate a void region, the top conductor  160  and insulating plug  140  may be misaligned with respect to one another such that a portion of the insulating plug  140  is exposed for etching. After creating the void, the second insulator  150  can be deposited and patterned to complete the memory cell. 
     The steps indicated by FIGS. 2A-2G may be modified to fabricate the variations as shown in FIGS. 1C-1D. For example, to fabricate the thin conductor  190  between the anti-fuse  180  and the fuse  130  as shown in FIG. 1C, the fabrication steps illustrated in FIG. 2A may be replaced by FIG. 2A-2. As shown in FIG. 2A-2, a thin conductor material may be deposited and patterned above the anti-fuse material  180 ′. The dielectric material  140 ′ may be deposited on top of the thin conductor material  190 ′ afterwards. The fabrication then may proceed as described above and in FIGS. 2B-2G. Note that when the etching takes place to form the fuse  130 , the thin conductor  190  and the anti-fuse  180  are etched to expose the bottom conductor  110  as shown in FIG. 2D-2. 
     To fabricate the first and second thin conductors  190  and  190   b  as shown in FIG. 1D, the fabrication steps illustrated in FIG. 2A may be replaced by FIG. 2A-3. As shown, a thin conductor material may be deposited above the bottom conductor  110  to form a second thin conductor material  190   b ′, and then the anti-fuse  180  may be formed as described above. And then another thin conductor material may be deposited to form a first thin conductor material  190 ′. The dielectric material  140 ′ may be deposited on top of the first thin conductor material  190 ′ subsequently. The fabrication may proceed as described above and in FIGS. 2B-2G. Note that when the etching takes place to form the fuse  130 , the first and second thin conductors  190  and  190   b , respectively, and the anti-fuse  180  are etched to expose the bottom conductor  110  as shown in FIG. 2D-3. 
     While not shown, other variations are possible in addition to the variations shown in FIGS. 1C and 1D. For example, more thin conductors or other placements of the thin conductors may take place to augment the performances of the fuse  130  and/or the anti-fuse  180 . 
     FIG. 3A illustrates a cross-sectional view of a second embodiment of a memory cell  300  according to an aspect of the present invention. As shown, the memory cell  300  may include a fuse  330  and an insulator  320  formed on either side of the fuse  330 . As will be seen later, the interior of the fuse  330  may or may not be completely filled. 
     The cell  300  may also include a bottom conductor  310 . Note that vertical portions of the use  330  and the bottom conductor  310  make up a ‘U’ region  385 . This ‘U’ region concept is better illustrated in FIG. 3D where the two vertical portions of the fuse  330  and the bottom conductor  310  make up the ‘U’ region  385 , i.e. there is no horizontal portion to the fuse  330 . The horizontal portion of the fuse  330  of FIG. 3A is not necessary to practice the invention. 
     The cell  300  may further include an insulating plug  340  occupying some or substantially all of the interior of the ‘U’ region  385 . The cell  300  may still further include an anti-fuse  380  and a top conductor  360  above the above the ‘U’ region  385  and the insulator  320 . Note that the fuse  330  and the insulating plug  340  may define a plane. 
     Materials used to form the various parts of the memory cell have been discussed above, and thus will not be repeated. Again, for reasons discussed before, the insulating plug  340  is not strictly necessary. Further, when the insulating plug  340  is present, top surfaces of the insulator  320 , fuse  330 , and the insulating plug  340  may be coplanar. 
     FIG. 3B illustrates a top view of the second embodiment of FIG.  3 A. As shown, the top conductor  360  may extend in a first direction. Note that the anti-fuse  380  (not visible in FIG. 3B) may also extend in the first direction. The anti-fuse  380  may also extend in a second direction on top of the fuse  330  and insulating plug  340 . Indeed, if the anti-fuse material  380 ′ is an insulator, the anti-fuse  380  does not require patterning since it is by definition insulating in the plane of the film. The fuse  330 , and thus the ‘U’ region  385 , including the insulating plug  340  and the bottom conductor  310  (not shown in FIG. 3B) may extend in the second direction and thereby defining a cross-point at the intersection. 
     FIGS. 3C-3E illustrate variations on the second embodiment of FIG.  3 A. In FIG. 3C, a thin conductor  390  may be placed between the fuse  330  and the anti-fuse  380  to enhance performance of the memory cell  300  as discussed previously with respect to the variations on the first embodiment. Note that the placement of the thin conductor  390  may be varied and is not limited to the placement as shown in FIG.  3 C. 
     Again, instead of extending in the first direction like the top conductor  360 , the thin conductor  390  may occupy a part of the ‘U’ region  385  above fuse  330  and below the top conductor  360 . In other words, the thin conductor  390  may be substantially limited to an area defined by a cross-point  315 . 
     FIG. 3D, in addition to clarifying the ‘U’ region  385 , also illustrates a variation of the on the second embodiment of FIG.  3 A. As noted above, the horizontal portion of the fuse  330  is not necessary to practice the invention. FIG. 3D demonstrates this concept. 
     Further in FIG. 3E, the thin conductor  390  need not cover the entirety of the ‘U’ region  385 . In this variation, the thin conductor  390  is formed substantially within the interior of the ‘U’ region  385  and the fuse  330  is in contact with the anti-fuse  380 . Note that many other variations are possible and are within the scope of the invention. 
     While the foregoing descriptions of the memory cell associated FIGS. 3A-3E indicate that the fuse  330 , insulating plug  340 , and ‘U’ region  385  extend in a second direction along with the bottom conductor  310 , this orientation is not required to practice the present invention. Indeed, the fuse  330  can be associated with the top conductor  360  and extend in a first direction. In this case the vertical portions of the fuse  330  and the top conductor  360  now make up an inverted ‘U’region  385 . An insulating plug  340  can once again occupy some or substantially all of the inverted ‘U’ region  385 . The memory cell  300  may still further include an anti-fuse  380  substantially occupying the bottom of the inverted ‘U’ region  385  above bottom conductor  310 . 
     FIGS. 4A-4G illustrate cross-sectional views of an exemplary embodiment of a method of fabricating the second embodiment of the memory cell  300  of FIG.  3 A. As shown in FIG. 4A, an insulator material may be deposited and patterned to form the insulator  320 . The insulator  320  may be patterned to define a trench where the ‘U’ region  385  will be formed. Again, the height to width ratio of the closed region  385  can be large (5 to 1 or more). 
     Then, as shown in FIG. 4B, a fuse material  330 ′ may be deposited into the trench and even over the insulator  320 . The deposition naturally creates the ‘U’ shape of the fuse  330 . Conformal coating of the first insulator  320 , including vertical walls, may be achieved using deposition methods as ALD and the like. Then a conductor material  310 ′ is deposited over the fuse material  330 ′ including into the ‘U’ region  385 . 
     Then as shown in FIG. 4C, the fuse material  330 ′ and the conductor material  310 ′ may be planarized using standard methods such as the CMP. At this point, the insulator  320 , the bottom conductor  310 , and the fuse  330  may be coplanar. 
     Then, as shown in FIG. 4D, the bottom conductor  310  may be preferentially etched using etching techniques such as wet etching, reactive ion etching, ion milling, and the like to a prescribed depth so that the bottom conductor  310  forms a lateral portion of the ‘U’ region  385 . 
     Then, as shown in FIG. 4E, an insulating plug material  340 ′ may be deposited to fill the interior of the ‘U’ region  385 , and the resulting surface may be planarized. At this point, the insulating plug  340 , the insulator  320 , and the fuse  330  may be coplanar as shown in FIG.  4 F. 
     Then, to complete the process, an anti-fuse material and another conductor material may be deposited and patterned to form the anti-fuse  380  and the top conductor  360  as shown in FIG. 4G (same as FIG.  3 A). Note that prior to depositing the conductor  360 , the anti-fuse  380  may be planarized. Also, planarizing the top conductor  360  may be part of the fabrication process. 
     The steps indicated by FIGS. 4A-4G may be modified to fabricate the variations as shown in FIGS. 3C-3E by one of ordinary skill. And again, a void may be created similar to as discussed with reference to the first embodiment. 
     While the invention has been described with reference to the exemplary embodiments thereof, those skilled in the art will be able to make various modifications to the described embodiments of the invention without departing from the true spirit and scope of the invention. The terms and descriptions used herein are set forth by way of illustration only and are not meant as limitations. In particular, although the method of the present invention has been described by examples, the steps of the method may be performed in a different order than illustrated or simultaneously. Those skilled in the art will recognize that these and other variations are possible within the spirit and scope of the invention as defined in the following claims and their equivalents.