Abstract:
A manufacturing process for producing dynamic random access memories (DRAMs) having redundant components includes steps for concurrently forming normal (i.e. non-fused) contacts to components of the DRAMs and anti-fused contacts to the redundant components. The process by which the normal and anti-fused contacts are made is readily implemented using standard integrated circuit processing techniques. An anti-fuse contact ( 20 ) and a normal (i.e. non-fused) contact ( 10 ) are formed by opening respective contact areas in a dielectric ( 110 ), selectively forming an insulating layer ( 210 ) over the anti-fuse contact, applying polysilicon ( 212, 410 ) to cover the insulating layer of the anti-fuse contact and to fill the opening over the normal contact. In one embodiment of the invention, the circuit region served by the anti-fuse contact is subject to ion implantation ( 810 ) to improve its conductivity before the anti-fuse contact is formed. In another embodiment of the invention, the anti-fuse is formed in an isolated well ( 1212 ) on the integrated circuit device and a non-fused contact ( 1216 ) to the well is also provided to aid in blowing the anti-fuse.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to semiconductor devices and in particular to anti-fuse elements formed in integrated circuit devices. 
     Redundancies are commonly used in the manufacture of dynamic random access memories (DRAMs) to increase yields. DRAM devices are designed with a number of redundant component elements such that if, on testing, one component is found to be defective, one of the redundant components may be substituted for the defective component to provide a fully functional circuit. These redundant components may be individual memory rows, memory columns or even individual bit positions in the memory. 
     Once the DRAM devices are fully assembled, two types of fuse elements may be used to isolate the defective component and connect the redundant component: a conventional fuse element which is closed until it is opened at a wafer level using a laser to cut though the fuse element, and an anti-fuse element which is open until it is shorted out in response to an electric current. When a defective component is identified in the integrated circuit, it may be removed from the circuit and replaced by a redundant component by cutting conventional fuses. Next, the redundant component may be substituted into the circuitry by blowing anti-fuses in order to selectively connect the redundant component in place of the defective component. This process is known as “repairing” the device. While laser-trimmed fuse elements may be used to repair devices at the wafer level, electrically blown fuses may be used to repair devices after they are packaged. Generally, it is difficult to use electrically blown fuse elements as it is difficult to provide the high voltages and currents needed to blow the fuses once the device has been packaged. Antifuse elements, however, are typically blown using relatively low voltages and currents that can easily be provided to the packaged circuit. 
     The formation of a fuse element is relatively straightforward, a thin layer of metal is applied in a pattern that defines trace having first and second ends separated by a relatively narrow waist. A higher than normal current is then applied to the conductive trace. This current, flowing through the relatively narrow waist heats the metallization and causes it to melt, disconnecting the two ends of the trace. More commonly, however, a laser is used to cut through the metal trace on the wafer level, after the device has been tested. 
     Anti-fuses, on the other hand, are relatively more complex to manufacture. These devices are formed as a part of the process by which the integrated circuit is produced. One such anti-fuse circuit is described in U.S. Pat. No. 5,886,392 entitled ONE TIME PROGRAMMABLE ELEMENT TABBING CONTROLLED PROGRAMMED STATE RESISTANCE. The anti-fuse described in this patent is formed by depositing a first metal layer, depositing an insulating layer on top of the first metal layer and then depositing a second metal layer on top of the first metal layer. In addition, the semiconductor device is processed to define locations at which the anti-fuses are placed, to trim the metallization to prevent inadvertent short circuits from developing and to provide electrical connection to both ends of the anti-fuse device. At least some of the processing steps used to form the anti-fuse device (i.e. the two metallization steps) may be difficult to integrate into a typical DRAM manufacturing process. 
     SUMMARY OF THE INVENTION 
     The present invention is embodied in a DRAM manufacturing process that is more readily implemented using standard integrated circuit processing techniques. An anti-fuse contact and a normal (i.e. non-fused) contact are formed by opening respective contact areas in a dielectric, selectively forming an insulating layer over the anti-fuse contact applying polysilicon to cover the insulating layer of the anti-fuse contact and filling the opening over the normal contact. 
     According to one aspect of the invention, ion implantation is used to increase the conductivity of the contact area of the anti-fuse contact before the insulating layer is formed. 
     According to yet another aspect of the invention, the anti-fuse is formed in an isolated well on the integrated circuit device and a non-fused contact to the well is also provided to aid in blowing the anti-fuse. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that according to common practice, the various features of the drawings are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity and to aid in the description of the invention. Included in the drawings are the following figures. 
     FIG. 1 is a cross-section of an integrated circuit device in which channels have been opened for an array contact and an anti-fuse. 
     FIGS. 2 through 5 are cross-sectional diagrams of the integrated circuit shown in FIG. 1 which illustrate the formation of the anti-fuse and array contact. 
     FIGS. 6 through 11 are cross-sectional diagrams of the integrated circuit device shown in FIG. 1 which illustrate an alternative method for forming an array contact and an anti-fuse device. 
     FIG. 12 is a cross-section of a semi-conductor device which illustrates the configuration of the anti-fuse device shown in FIGS. 5 and 11, along with additional circuitry which is used to blow the anti-fuse. 
    
    
     DETAILED DESCRIPTION 
     The present invention is embodied in a semiconductor process by which anti-fuse contacts to redundant integrated circuit elements may be made at the same time as normal contacts. FIGS. 1 through 11 are cutaway views of a semi-conductor element showing two sections. Section  10  corresponds to a normal contact in the integrated circuit while Section  20  corresponds to an anti-fuse contact. 
     While the invention is described in terms of bit-line contacts for a DRAM device, it is contemplated that it may be used for other types of integrated circuits where it is desirable to concurrently form both normal contacts anti-fuse contacts. 
     In the exemplary embodiment of the invention, both a normal contact and an anti-fuse contact are formed to provide a bit-line contact to a pass transistor on the integrated circuit. The bit-line contact (not shown) is in the semi-conductor substrate  100 . In the first step of the process, an insulating layer of, for example, silicon oxide  110  is deposited on the substrate  100 . Next, a photo resist material  112  is patterned onto the insulating layer  110  and standard dry etch techniques are used to form openings in the insulating layer of  110  down to the bit-line contacts on the substrate  100 . 
     Next, as shown in FIG. 2, a thin insulating layer  210  is grown or deposited in at least the contact regions. The insulating layer may be deposited over the entire surface of the silicon wafer or it may be “grown” by oxidizing the exposed silicon in the regions  10  and  20 . The insulating layer  210  may be, for example, a 2 to 5 nm silicon oxide layer grown, for example, using a rapid thermal processing (RTP) tool. Alternatively, the insulating layer may be silicon nitride or other insulating material that is either grown or deposited into the contact areas. 
     After forming the insulating layer  210 , a thin layer of doped polysilicon is deposited over at least the array contact areas of the silicon wafer. In the exemplary embodiment of the invention the polysilicon is N+ doped and has a thickness of 50 to 100 nm. Both the insulating layer and the polysilicon layer may be formed in a single furnace cycle. 
     The next step in the process is shown in FIG.  3 . In this step, a photo resist pattern  310  is formed exclusively over the anti-fuse regions. The polysilicon  212  is then etched out of the normal contact region  10 , using common dry etching techniques. This step leaves only the oxide  210  in the normal array contact area  10 , but leaves the polysilicon  212  and the oxide layer  210  in the anti-fuse contact area  20 . 
     FIG. 4 shows the next steps in the process. As a first step, the resist pattern is stripped from the circuit and the wafers are cleaned using, for example, sulfuric peroxide, ammonium hydroxide peroxide and hydrogen chloride peroxide. Buffered hydrofluoric acid or some similar acid is then used to etch away the thin insulating layer  210  from the normal contact regions. The polysilicon layer  212  protects the oxide in the anti-fuse regions  20  from being removed by the etchant. Next, a thick doped polysilicon layer  410  approximately 300 to 400 nm thick is deposited on the wafer. This fills all of the contact areas. In the exemplary embodiment of the invention the polysilicon layer  410  is N+ doped. 
     The next steps in the process are shown in FIG.  5 . As shown in this figure, a dry etch is used to remove the polysilicon from the surface of the insulator  110  while simultaneously recessing the polysilicon in the contract regions  10  and  20 . In the exemplary embodiment of the invention, the polysilicon is recessed to a depth of 200 to 300 nm below the surface of the insulator  110 . Also, as shown in FIG. 5, the insulator  110  is patterned to form contact regions and the contact regions are filled with a metallic material  510  (e. g. aluminum). 
     FIG. 5 shows the completed structure for both a normal bit-line contact  10  and a anti-fuse coupled bit-line contact  20 . The normal bit-line contact is formed by a metal layer  510  and polysilicon layer  410  positioned above and in contact with the bit-line pass transistor (not shown) on the substrate  100 . The anti-fuse coupled bit-line contact is formed from a metallic layer  510  to polysilicon regions  410  and  210  and a thin insulating layer  210  positioned above the bit-line pass transistor. 
     Although the process described above deposited doped polysilicon  410  to form the body of the normal contact  10  and the anti-fuse contact  20 , it is contemplated that a metallic material (e.g. aluminum) may be deposited in place of the polysilicon  410 . 
     As described below with reference to FIG. 12, the anti-fuse contact my be blown by applying a voltage across the insulating layer  210  by applying a potential to the metallic layer  510  and to the bit-line contact area using one or more normal contacts which directly connect to the bit-line contact area. 
     Using this technique, the inventors have been able to produce anti-fuses having blown resistances of between 10 kilohms and 100 kilohms with unblown resistances of one megohm. 
     For some applications, it may be desirable to enhance the bit contact region through ion implantation techniques prior to forming the fuse regions. The implanted region acts to reduce the contact resistance of the fuse. It may also be desirable to block any implant from other array regions to prevent the alteration of transistors in those regions. These steps my be performed without adding additional mask levels as described below with reference to figures six through eleven. 
     As shown in FIG. 6, the contact areas  10  and  20  are opened in the insulating layer  110  using common photolithography and dry etch techniques. A thick doped polysilicon layer, 300 to 500 nm thick, is then deposited over the entire surface of the insulating layer  110 . This polysilicon layer fills both the normal bit-line contacts and the anti-fuse contacts. 
     As shown in FIG. 7, a photo resist pattern  710  is then formed over the surface of the polysilicon  610  to provide openings in the photo resist over the anti-fuse areas  20 . The polysilicon in the anti-fuse areas  20  is then etched down to leave approximately 200 to 400 nm of polysilicon in the anti-fuse areas  20 . All of the steps as shown in FIGS. 6 and 7 may be performed in a single multi-step dry etch process. 
     Next, as shown in FIG. 8, the remaining photo resist is stripped and the wafer is etched to remove all of the polysilicon from the anti-fuse areas  20  and to recess the polysilicon in the normal contact regions  10  to approximately 100 to 300 nm below the surface of the insulating layer  110 . The etchant chosen for this process desirably etches the polysilcon at a faster rate than single crystalline silicon so as not to etch too deeply into the substrate  100  near the fuse contact region  20 . The fuse regions are then implanted with a selected dopant. In the exemplary embodiment of the invention, the dopant is phosphorus or arsenic and is implanted in a concentration of 10 14  atoms per square centimeter using a 30 Kev implantation process. 
     As shown in FIG. 9, the next step in the process is to grow a thin (e.g. 2 to 5 nm) fuse oxide layer  910  over the polysilicon  610  remaining in the normal contact region  10  as well as the implanted region  810  in the anti-fuse area  20 . A second layer of doped polysilicon  912  is then deposited. The polysilicon film  912  is desirably 300 to 500 nm thick and fills both the fuse contact areas and the array contact areas. 
     Next, as shown in FIG. 10, the polysilicon layer  912  is recessed using a dry etchant. This etching operation is stopped when the oxide area  910  in the normal contact areas  10  are exposed. This results in a polysilicon layer that is recessed approximately 100 to 300 nm below the surface of the insulating layer  100  in the anti-fuse regions  20 . 
     Finally, as shown in FIG. 11, the silicon oxide  910  is removed from the normal contact regions  10  using, for example, buffered hydrogen fluoride. Both the normal contact areas  10  and anti-fuse contact areas  20  are then patterned to form contacts and metallized to deposit a metal layer  1110  (e.g. aluminum) over both the normal contact and the anti-fuse contact areas. 
     Using the steps described above with reference to FIGS. 6 through 11, both normal contact and anti-fuse contact areas have been formed and the anti-fuse contact regions have been enhanced by implanted region  810  in the bit-line contact area (not shown). 
     FIG. 12 shows a typical application for the anti-fuse elements described above with reference to FIGS. 1 through 11. The exemplary anti-fuse element includes an insulating layer  210 , polysilicon layers  410  and  212 , and a metallization layer  1216 . As shown in FIG. 12, the anti-fuse is formed above a bit-line region  1212  in a silicon substrate covered by an insulating layer  110 . In addition to the anti-fuse area  20 , two metallic contact areas  1216  are formed above the bit-line region  1212 . The anti-fuse is blown by applying a potential of approximately 5 to 7 volts between the contact area of the anti-fuse  20  and the contact areas of the two metallic contacts  15 . A current flowing in response to this potential punches through the insulating layer  210  at the bottom of the anti-fuse region  20 , locally heating the area and causing the polysilicon  212  to flow into contact with the N+ contact region  1212 . 
     While the programming contact regions  15  are shown as being metallic contacts, it is contemplated that they may be ordinary non-fused contacts formed in the same process as used to form the normal contact regions  10  as described above with reference to FIGS. 1 through 11. 
     The anti-fuse contact  20  and the programming contacts  15  are shown in a typical configuration which includes a buried N+ region formed, for example, by a high voltage ion implantation and diffusion process in a P-well 100. The P-well 100 is separated from the remaining circuitry by isolation diffusions  1214  formed, for example, by standard surface deposition and diffusion techniques. The bit-line contact  1212  is formed in the P-well using standard surface deposition and diffusion techniques. In the exemplary embodiment of invention, the P-well 100 is N+ doped using, for example, phosphorous or arsenic as the dopant to form the bit-line contact  1212 . 
     While the invention has been described in terms of an exemplary embodiment, it is contemplated that it may be practiced as described above within the scope of the appended claims.