Abstract:
A resettable fuse device is fabricated on one surface of a semiconductor substrate ( 10 ) and includes: a gate region ( 20 ) having first and second ends; a source node ( 81 ) formed in proximity to the first end of the gate region; an extension region ( 52 ) formed to connect the source node to the first end of the gate region; and a drain node ( 80 ) formed in proximity to the second end of the gate region and separated from the gate region by a distance (D) such that upon application of a predetermined bias voltage to the drain node a connection between the drain node and the second end of the gate region is completed by junction depletion. A gate dielectric ( 30 ) and a gate electrode ( 40 ) are formed over the gate region. Current flows between the source node and the drain node when the predetermined bias is applied to both the drain node and the gate electrode.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a simple resettable fuse device that can be used in digital and analog integrated circuits for the adding or removing circuit elements on a dynamic basis.  
         [0003]     2. Background Information  
         [0004]     Fuse devices are useful in integrated circuits for disconnecting or disabling failed circuit elements. In a RAM cell array, for example, it is well known in the art to fabricate on the silicon chip miniature metallic fuses that are in series with the memory data lines. If in the process of testing during the manufacturing cycle, a defective memory location is detected, the appropriate hard fuse can be severed, rendering the bad location inoperable. While simple to fabricate, a disadvantage of the hard fuse is that the change is permanent. Moreover, once the RAM cell array is sealed after testing, any further failures of memory locations cannot be cannot corrected by severing fuse links, and the entire device may have to be scrapped.  
         [0005]     Another disadvantage of hard fuses is they are severed by laser cutting and therefore must be placed away from other sensitive parts of the chip. This tends to increase the distance that signals on the data lines must travel, an undesirable feature in high speed, high density, microelectronic circuits.  
         [0006]     Soft fuse devices disclosed in the prior art overcome some of the disadvantages of using hard fuses in integrated circuits. For example, U.S. Pat. No. 6,384,665 to Smoszna discloses the use of a programmable soft fuse for disabling a signal line in a RAM cell array, which includes an electronic switching circuit, a latch, a control circuit and a reset circuit. In Smoszna, these circuits are used in conjunction with an address decoder to test the RAM cell array to locate failing bits, word lines or bit lines on the array, and enter the location of the bad bits into the decoder. When the address does not correspond to a failed bit, the latches of the corresponding programmable soft fuse are set so as to allow the address signal to go to the array. However, if the address corresponds to one of the failed bits, then the latches are set so as to prevent the address signal form going to the array, and in that case, additional shifting circuitry connected to the address decoder can reroute the data to another redundant word line or bit line.  
         [0007]     While soft fuses disclosed in the prior art address some of the shortcomings of hard fuses, what is needed is simple soft fuse device that can be adapted to use in a variety of digital and analog integrated circuits.  
       SUMMARY OF THE INVENTION  
       [0008]     It is, therefore, a principle object of this invention to provide a resettable fuse device and method of fabricating the same.  
         [0009]     It is another object of the invention to provide a resettable fuse device that solves the above-mentioned problems.  
         [0010]     These and other objects of the present invention are accomplished by the resettable fuse device and method of fabricating the same disclosed herein.  
         [0011]     In an exemplary aspect of the invention, a resettable fuse device is fabricated on one surface of a semiconductor substrate and includes: a gate region having first and second ends; a source node formed in proximity to the first end of the gate region; an extension region formed to connect the source node to the first end of the gate region; and a drain node formed in proximity to the second end of the gate region and separated from the gate region by a distance such that upon application of a predetermined bias voltage to the drain node a connection between the drain node and the second end of the gate region is completed by junction depletion. A gate dielectric and a gate electrode are formed over the gate region. Current flows between the source node and the drain node when the predetermined bias is applied to both the drain node and the gate electrode. Preferably, the predetermined bias voltage applied to the drain node is no greater than a maximum voltage based on a gate dielectric breakdown model.  
         [0012]     The exemplary resettable fuse device also includes a halo region formed between the extension region and the first end of the gate region. The halo region is formed of a different impurity type than the extension region.  
         [0013]     The resettable fuse device may be either an NFET device having p-type impurities implanted in the channel region, or a PFET device having n-type impurities implanted in the channel region, depending upon the application.  
         [0014]     In another aspect of the invention, a method for fabricating the inventive resettable fuse device includes the steps of: providing a semiconductor substrate having a hard mask formed on one surface; etching the hard mask to form a hole exposing a portion of the semiconductor substrate; implanting impurities in the exposed portion of the semiconductor substrate to form a gate region having a first end and a second end; forming a dielectric layer in the hole, over the gate region; forming spacers on the sidewalls of the hole at the first and second ends of the gate region; forming a gate electrode over the dielectric layer and the sidewall spacers, so as to fill the hole in the hard mask; removing the hard mask and the sidewall spacer at the first end of the gate region by etching; implanting impurities in the surface of the substrate exposed by the removal of the hard mask and the sidewall spacer to form an extension region extending from the first end of the gate region, and a halo region between the extension region and the first end of the gate region; removing the hard mask at the second end of the gate region by etching; forming another sidewall spacer adjacent to the gate electrode at the first end of the gate region after forming the extension region and the halo region; implanting impurities in the substrate surface in proximity to the first end of the gate region to form a source node which is in contact with the extension region; and implanting impurities in the substrate surface in proximity to the second end of the gate region to form a drain node which is separated from the gate region by a distance such that upon application of a predetermined bias voltage to the drain node a connection between the drain node and the second end of the gate region is completed by junction depletion.  
         [0015]     To preclude ion implantation in the vicinity of the second end of the gate region during formation of the extension region and the halo region, a photo-resist mask may be temporarily disposed over portions of the hard mask and the gate electrode. In the alternative, the same objective may be achieved without the use of a mask by aligning the semiconductor substrate on the apparatus used for ion implantation so as to preclude implantation in the vicinity of the second end of the gate region.  
         [0016]     In the exemplary embodiment of the resettable fuse device, the semiconductor substrate is preferably a silicon substrate or wafer, the hard mask and the sidewall spacers are preferably composed of silicon nitride, and the gate electrode may be composed of an intrinsic polysilicon, an in-situ doped polysilicon, or a metallic material. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a cross-sectional view illustrating a first stage in the fabrication of a resettable fuse device according to the present invention;  
         [0018]      FIG. 2  is a cross-sectional view illustrating a second stage in the fabrication of the inventive resettable fuse device;  
         [0019]      FIG. 3  is a cross-sectional view illustrating a third stage in the fabrication of the inventive resettable fuse device;  
         [0020]      FIG. 4  is a cross-sectional view illustrating a fourth stage in the fabrication of the inventive resettable fuse device;  
         [0021]      FIG. 5  is a cross-sectional view illustrating a fifth stage in the fabrication of the inventive resettable fuse device;  
         [0022]      FIG. 6  is a cross-sectional view illustrating a sixth stage of fabrication of the inventive resettable fuse device;  
         [0023]      FIG. 7  is a cross-sectional view illustrating a seventh stage of fabrication of the inventive resettable fuse device;  
         [0024]      FIG. 8  is a cross-sectional view illustrating the operation of the inventive resettable fuse device with the application of high drain bias;  
         [0025]      FIG. 9  is a schematic representation of a single soft fuse transistor according to the present invention;  
         [0026]      FIG. 10  is a schematic representation of an array of soft fuses according to the present invention;  
         [0027]      FIG. 11  is a graph of maximum operating gate voltage versus lifetime for an exemplary embodiment of the inventive resettable fuse device; and  
         [0028]      FIG. 12  is a graph of depletion width versus junction-to-bulk potential for an exemplary embodiment of the inventive resettable fuse device. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     The invention will now be described in more detail by way of example with reference to the embodiments shown in the accompanying figures. It should be kept in mind that the following described embodiments are only presented by way of example and should not be construed as limiting the inventive concept to any particular physical configuration.  
         [0030]     Further, if used and unless otherwise stated, the terms “upper,” “lower,” “front,” “back,” “over,” “under,” and similar such terms are not to be construed as limiting the invention to a particular orientation. Instead, these terms are used only on a relative basis.  
         [0031]     The present invention is directed toward a simple resettable fuse device that can be used in digital and analog integrated circuits for the adding or removing circuit elements on a dynamic basis.  
         [0032]     An exemplary method of fabricating the inventive resettable fuse device is illustrated in  FIGS. 1-7 , and will help in understanding the operation of the device, discussed below.  FIG. 1  shows a starting wafer or substrate  10 , typically of silicon, having an isolation oxide  11  already formed using a standard technique, such as shallow trench isolation.  FIG. 1  further shows a pre-existing dielectric layer  12 , typically silicon oxide, formed on the surface of substrate  10  and a hard mask  13 , typically composed of silicon nitride, formed on top of the dielectric layer.  
         [0033]     Using a photo-resist layer (not shown) the hard mask  13  is patterned and etched to form an open region  20 , as illustrated in  FIG. 2 , in which a gate region will be formed.  
         [0034]     In  FIG. 3 , an ion implant (not shown) is used to add dopant impurities to the open region  20  in the hard mask  13  to form the gate region of the device. The energy is sufficient to allow ions to penetrate the silicon substrate  10 , but not to penetrate the hard mask  13 . Typical Implants are 5-10 Kev Boron with doses in the 12E12 to 10E13/cm 2  for surface implants. Higher energies are used to set punch through and well resistances. The figure shows one device type, in this case an NFET with p-type impurities. The dual is also possible, i.e., PFET devices with n-type impurities in the channel of the device. If both device types are required then a mask would have to be used to cover up one of the “holes” during fabrication of the other type device. If only one device type is used then such a “covering” mask is not required.  
         [0035]     Following the implant process, a gate oxide ( 30 ) is grown. Optionally it can be grown in place of dielectric layer  12 , in addition to dielectric layer  12 , or dielectric layer  12  can be utilized instead. A silicon nitride spacer is  31  is then formed on the sidewall of the hard mask  13 .  
         [0036]      FIG. 4  shows that a damascene gate electrode  40  (which may, for example be intrinsic polysilicon, in-situ doped polysilicon, or a metal gate) is next deposited on the entire wafer, and then chemically/mechanically polished (which is a standard practice known in the art) so as to fill only the hole  20  in the hard mask  13 .  
         [0037]     In  FIG. 5 , a photo resist mask  50  is used to expose one end of the device. The hard mask ( 13 ) and the sidewall spacer ( 31 ) are removed from the other end of the device by etching. A single side halo ( 51 ) and junction extension implant ( 52 ) are then formed by implantation. The gate electrode masks these from the internal body of the device, and the PR mask and the hard mask  13  keep the implants from entering the other end of the device. Note that the gate undercut naturally formed by the spacer ( 31 ) removal affords a region where gate oxide  30  is not covered by the gate electrode  40 . This will become an important fact later on.  
         [0038]     It should be noted that if the devices were all arranged on the wafer unidirectionally, then the masked used in  FIG. 5  would not be required. In this alternative, the hard mask and spacer material could have all been etched, and the wafer directionally aligned to the implanter so as to place the junction extension  52  (p type for an NFET) and halo  51  (n type for an NFET) into only one side of the device. This type of device alignment is becoming standard practice in 65 nm technology.  
         [0039]     In  FIG. 6 a  new spacer  60  is defined on the side of the gate that has been implanted with halo  51  and junction extension  52 , and the photo resist mask  50  is removed. Again, if the gates were designed unidirectionally, this mask would not have been required.  
         [0040]     In  FIG. 7 , the hard mask  13  is entirely removed, and a common N++ (for an NFET) is ion implanted, forming a high voltage node  80  and a low voltage node  81 . As shown, the implant is not linked to the channel on one end of the gate, as it does not have junction extension  52  to assist in the link. The unlinked portion  80  is separated from the channel by the gate oxide  30  having the spacer  31  on top of it. In principle, this device can now support a high voltage on the un-linked end because the equivalent dielectric thickness in the unlinked portion  80  is much greater than in the linked portion  81 . This feature is necessary in order to have the device function as a resettable fuse.  
         [0041]      FIG. 8  shows how the device operates. Under a high bias on node  80 , the channel is completed by junction depletion and the device can switch as a standard FET. This is the program mode.  
         [0042]      FIG. 9  is a device schematic showing the terminals for the device, namely, high voltage fuse control node  80 , common source node  81 , and low voltage fuse scan node  82 . In this case the device depicted is an NFET and current flow between nodes  80  and  81  will occur only when both nodes  80  and  82  are high.  
         [0043]     The following is a consideration of the reliability aspects of the resettable fuse device described above, which bear on the way it can be used in various applications.  
         [0044]     The channel of the device can be made long to eliminate any degradation due to hot carriers, but the device can also be made short and the hot carrier problem largely avoided if, when the fuse is to be energized, the gate voltage is applied to the gate node  82  before the high voltage node  80  is energized. Once the gate node  82  is high, the voltage at the high voltage node  80  can be raised. By doing so, switching the gate while the high junction is going positive is avoided, thus the device does not pass through the maximum substrate current condition (Vgs=0.5 Vds), which is the worst case for hot carriers. If the gate node  82  is high first, the hot carrier effect will be minimal. Of course, the operating conditions for the fuse should not be such that Vgs=0.5 Vds. Using this approach, any damage or degradation to the gate dielectric at the high voltage junction end is also minimized. By having the gate high first, the FET is prevented from being in gate breakdown condition at the high voltage node, which is when the diffusion is high and the gate is low. This is not a desirable condition. Because the device is energized only during soft fusing, the effect on the gate dielectric is minimized.  
         [0045]     The number of safe fusing operations is determined from the gate dielectric breakdown model of the technology in question. As an example, for a 1.3 nm gate dielectric thickness, and with a soft fuse device oxide area of 0.5 μm 2 ,  FIG. 11  shows that the relationship between V max , the safe maximum gate voltage, and lifetime in hours at 27° C. For this example, with a nominal technology voltage of 1.2 V, the soft fuse gate voltage should be set to the technology voltage, or a voltage that is less than or equal to the V max  of the technology, which  FIG. 11  indicates is 2.26 volts for 10 years of operation at 27° C.  
         [0046]     Shown in  FIG. 12  is a graph of the depletion width of a junction, in μm, versus junction to bulk potential, Vj, for a structure with Nd=3E19, Nb=5E16 and a junction depth of 0.5 μm.  FIG. 12  can be used to set the allowed tolerance of the spacer  31 , shown in  FIG. 3 , which essentially determines the depletion width D shown in  FIG. 7 . From this example, a 225-angstrom spacer having a 22-angstrom tolerance, which is an unusually large tolerance, can be employed.  
         [0047]     To minimize hot electron effects in the exemplary embodiment being considered, it is necessary, as discussed earlier, to avoid the condition Vgs=0.5 Vds, i.e., the peak hot electron injection condition, so as to minimize this source of device degradation. Incorporating a typical design tolerance of +/−50 mV, the actual maximum voltage should be limited to 2.3 V. Referring to  FIG. 12 , it appears that the proposed tolerance for spacer  31  is consistent with this added limitation, thus allowing for a workable and reliable invention. In other words,  FIG. 12  shows that at 2.3 V, the depletion width is in excess of the worst-case spacer thickness of approximately 247 angstroms. Thus, the channel would be completed and the soft fuse would activate with a voltage on node  80  of 2.3 V, and the gate voltage rising first, also to 2.3V.  
         [0048]     As illustrated in  FIG. 10 , the present invention provides the means for constructing soft fuse devices whereby an array  110  of such devices can be implemented on a integrated circuit chip having individual fuse inputs  100  at the drains of the soft fuse devices, a fuse common  101 , and a common control gate  102 . When an error, or “fail” is detected in an integrated circuit due to a failing element on the circuit chip, a signal is sent to the drain of one of the soft fuses in the array of soft fuses, causing that particular soft fuse device in the array to conduct providing a path for replacing the failing circuit element by another. The failing element, as an example, could be a bit line in a memory array, or an inductor or capacitor in an analog or RF circuit. The soft fuses of the present application are intended for dynamic use, meaning that these soft fuses can be used throughout the lifetime of the integrated circuit chip, and not just during the initial manufacturing or testing phase. Thus, the dynamic fuse can be applied to repair fails which occur during the actual lifetime usage of the integrated circuit chip in which it is integrated. The soft fuse can be implemented to temporarily repair a fail that may or may not to be followed by a permanent repair. The possible applications include: 
        1) In an ECC (error correction code) environment, to take care of SER (soft error rate) events. As CMOS scaling continues to increase aggressively, the concern about SER is becoming one of the major challenges.     2) Repair of “soft fails”, e.g. fails that occur in a certain temperature range or at a high or low-temperature. This type of fail could be time-dependent and occur at some point during lifetime usage due to some parametric shift of (e.g., threshold voltage, device current) of the device during operation.     3) Repair of “soft fails” that occur only in a certain mode of operation of the integrated circuit chip. For example, high or low voltage ranges of operation may occur for the same input voltage mode or may occur because of switching from one mode of voltage to another in a certain circuit of the integrated chip. This type of fail could also be time-dependent, occurring either at initial use or during the lifetime operation of the integrated circuit chip.     4) Repair of fails that are not classified as “soft fails”, but are time-dependent fails such as capacitor fails leading to leakage degradation of the capacitor isolation, or inductor fails leading to either short-circuit of at least part of the inductor coil or high resistance developing in the inductor. Examples also include bit line failure during operation in a memory chip.     5) Repair of fails due to ESD (electrostatic discharge) events that may occur at some point during the lifetime usage of the integrated chip.        
 
         [0054]     It should be understood that the invention is not necessarily limited to the specific process, arrangement, materials and components shown and described above, but may be susceptible to numerous variations within the scope of the invention.  
         [0055]     It will be apparent to one skilled in the art that the manner of making and using the claimed invention has been adequately disclosed in the above-written description of the preferred embodiments taken together with the drawings.  
         [0056]     It will be understood that the above description of the preferred embodiments of the present invention are susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.