Abstract:
An antifuse circuit includes a terminal, an antifuse, and a select transistor. The antifuse is coupled to the terminal and has an associated program voltage. The select transistor is coupled to the antifuse and has a gate terminal coupled to receive a first select signal. The select transistor operates in a snapback mode of operation in response to an assertion of the first select signal and the program voltage at the terminal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to integrated circuits and, more particularly, to an antifuse programming circuit with a snapback select transistor. 
     Integrated circuits are interconnected networks of electrical components fabricated on a common foundation called a substrate. The electrical components are typically fabricated on a wafer of semiconductor material that serves as a substrate. Various fabrication techniques, such as layering, doping, masking, and etching, are used to build millions of resistors, transistors, and other electrical components on the wafer. The components are then wired together, or interconnected, to define a specific electrical circuit, such as a processor or a memory device. 
     Fusible elements are employed in integrated circuits to permit changes in the configuration of the integrated circuits after fabrication. For example, fusible elements may be used to replace defective circuits with redundant circuits. Memory devices are typically fabricated with redundant memory cells. The redundant memory cells may be enabled with fusible elements after fabrication to replace defective memory cells found during a test of the fabricated memory device. Fusible elements may also be used to customize the configuration of a generic integrated circuit after it is fabricated, or to identify an integrated circuit. 
     One type of fusible element is a polysilicon fuse. The polysilicon fuse comprises a polysilicon conductor fabricated to conduct electrical current on an integrated circuit. A portion of the polysilicon fuse may be evaporated or opened by a laser beam to create an open circuit between terminals of the polysilicon fuse. The laser beam may be used to open selected polysilicon fuses in an integrated circuit to change its configuration. The use of polysilicon fuses is attended by several disadvantages. Polysilicon fuses must be spaced apart from each other in an integrated circuit such that when one of them is being opened by a laser beam the other polysilicon fuses are not damaged. A bank of polysilicon fuses therefore occupies a substantial area of an integrated circuit. In addition, polysilicon fuses cannot be opened once an integrated circuit is placed in an integrated circuit package, or is encapsulated in any manner. 
     Another type of fusible element is an antifuse. An antifuse includes two conductive terminals separated by an insulator or a dielectric, and is fabricated as an open circuit. The antifuse is programmed by applying a high voltage across its terminals to rupture the insulator and form an electrical path between the terminals. Another type of antifuse may be implemented using a transistor. Under high voltage, the gate dielectric layer ruptures, causing a short to substrate. In either case, the electrical path created by programming the antifuse can later be detected and used as the basis for configuring the device. 
     Antifuses have several advantages that are not available with typical fuses. A bank of antifuses takes up much less area of an integrated circuit because they are programmed by a voltage difference that can be supplied on wires connected to the terminals of each of the antifuses. The antifuses may be placed close together in the bank, and adjacent antifuses are typically not at risk when one is being programmed. Antifuses may also be programmed after an integrated circuit is placed in an integrated circuit package, or encapsulated, by applying appropriate signals to pins of the package. This is a significant advantage over polysilicon fuses for several reasons. An integrated circuit may be tested after it is in a package, and may then be repaired by replacing defective circuits with redundant circuits by programming selected antifuses. A generic integrated circuit may be tested and placed in a package before it is configured to meet the specifications of a customer. This reduces the delay between a customer order and shipment. The use of antifuses to customize generic integrated circuits also improves the production yield for integrated circuits, because the same generic integrated circuit may be produced to meet the needs of a wide variety of customers. 
     An exemplary antifuse programming circuit  100  is shown in  FIG. 1 . The antifuse programming circuit includes a programming terminal  105  to which an external programming voltage is applied for programming an antifuse  110 . The antifuse is coupled to an isolation transistor  115  and a select transistor  120 . The isolation transistor  115  provides voltage isolation due to the relatively high programming voltage required to program the antifuse  110  thereby protecting other circuit elements from damage (e.g., the select transistor  120 ). The isolation transistor  115  only passes its gate voltage minus a threshold voltage to its source. 
     The application of the program voltage ruptures the dielectric of the antifuse  110 , creating a conductive path through the transistors  115 ,  120 . After the initial rupture, the program voltage is applied for a specified time interval to allow current to flow through the antifuse  110  thereby reducing the resistance of the conductive path through the antifuse  110 . This specified time interval is commonly referred to as a soak interval. The level of soak current required to program the antifuse  110  and provide a reliable restive path is typically significant. 
     Although antifuses are typically more compact than other types of fusible elements, such as polysilicon fuses, they still consume an appreciable amount of real estate on the semiconductor device. With reference to  FIG. 1 , because the transistors  115 ,  120  must have sufficient current ratings to conduct the soak current, they are typically relatively large transistors. When the relatively large size is accumulated over the number of transistors  115 ,  120  needed to support a bank of antifuses, the amount of real estate consumed is significant. 
     This section is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The present invention is directed to overcoming, or at least reducing the effects of, one or more of the issues set forth above. 
     BRIEF SUMMARY OF THE INVENTION 
     The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later. 
     One aspect of the present invention is seen in an antifuse circuit including a terminal, an antifuse, and a select transistor. The antifuse is coupled to the terminal and has an associated program voltage. The select transistor is coupled to the antifuse and has a gate terminal coupled to receive a first select signal. The select transistor operates in a snapback mode of operation in response to an assertion of the first select signal and the program voltage at the terminal. 
     Another aspect of the present invention is seen in an antifuse circuit including a terminal, a disconnect transistor, and a select transistor. The disconnect transistor is coupled to the terminal and has a gate terminal coupled to receive a first select signal. The antifuse is coupled to the disconnect transistor and has an associated program voltage. The select transistor is coupled to the antifuse and has a gate terminal coupled to receive a second select signal. The select transistor has a first breakdown voltage less than the program voltage. 
     Yet another aspect of the present invention includes a method for programming an antifuse. The method includes providing a first transistor enabled responsive to a first select signal coupled to a terminal, a first antifuse coupled to the first transistor, and a second transistor enabled responsive to a second select signal coupled to the first antifuse. The second transistor has a breakdown voltage less than a program voltage associated with the first antifuse. A program voltage is provided at the terminal. The first and second select signals are asserted to program the first antifuse. The second transistor operates in a snapback mode of operation during at least a portion of the programming of the first antifuse. The first select signal is deasserted to isolate the second transistor from the program voltage following the programming of the first antifuse. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
         FIG. 1  is diagram of an exemplary prior art antifuse programming circuit; 
         FIG. 2  is a diagram of an antifuse programming circuit in accordance with one illustrative embodiment of the present invention; 
         FIG. 3  is a cross-section view of an exemplary select transistor in the antifuse circuit of  FIG. 2 ; 
         FIG. 4  is a timing diagram illustrating the timing of select signals for programming the antifuse circuit of  FIG. 2 ; 
         FIG. 5  is a simplified functional block diagram of a memory device incorporating the antifuse circuit of  FIG. 2 ; and 
         FIG. 6  is a simplified block diagram of an information handling system incorporating the antifuse circuit of  FIG. 2 . 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     One or more specific embodiments of the present invention will be described below. It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. Nothing in this application is considered critical or essential to the present invention unless explicitly indicated as being “critical” or “essential.” 
     The present invention will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present invention with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present invention. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. 
     Referring now to the drawings wherein like reference numbers correspond to similar components throughout the several views and, specifically, referring to  FIG. 2 , the present invention shall be described in the context of an exemplary antifuse circuit  200 . The antifuse circuit  200  includes a disconnect transistor  210 , a plurality of antifuses  220 , and a select transistor  230  associated with each of the antifuses  220 . In the illustrated embodiment, the disconnect transistor  210  is a p-channel device, and the select transistors  230  are n-channel devices. Of course other types of transistors may be used depending on the particular embodiment. 
     The disconnect transistor  210  is coupled between a terminal  240  and the antifuses  220  for selectively applying a voltage to an antifuse  220  selected for programming by its associated select transistor  230 . For example, the terminal  240  may be an external pin of the device with which the antifuse circuit  200  is associated. Control logic  250  is provided for generating various select signals for programming the antifuses  220 . For example, a program voltage may be applied to the terminal  240 , and the disconnect transistor  210  may be selected by asserting the BANK SEL signal. The appropriate select transistor  230  may be enabled by asserting the SEL signal to select the particular one of the antifuses  220  to be programmed. Although the antifuses  220  are illustrated as being plate-type antifuses, the application of the present invention is not so limited, and the antifuses  220  may be of the transistor type as well. 
     The antifuse circuit  200  is illustrated as having one disconnect transistor  210  to service a bank  225  of antifuses  220 . Generally, the disconnect transistor  210  is provided for isolating the bank  225  from the program voltage at the terminal  240  between program events. It is contemplated that in some embodiments, each antifuse  220  may have its own disconnect transistor  210 . The n designations on the BANK SEL and SEL signals indicate that multiple banks  225  and multiple select signals may be provided. 
     The disconnect transistor  210  and select transistors  230  are designed and fabricated such that the select transistor  230  enters a snapback mode of operation after the initial rupture of the antifuse  220 , while the disconnect transistor  210  remains in a normal mode of operation. In a snapback mode of operation the select transistor  230  exhibits increased current conduction with a given applied voltage, as compared to a transistor operating in the normal mode below the breakdown voltage. Generally, this increased current conduction mode allows the select transistor  230  to conduct sufficient soak current to perfect the programming of the antifuse  220  without requiring an increased device size. Hence, the select transistors  230  consume less real estate on the semiconductor device, as compared to the space that would be required for transistors sized to carry the same amount of soak current in a normal mode of operation. 
     The relative breakdown voltages of the disconnect transistor  210  and the select transistor  230  are tailored to achieve the desired snapback mode of operation for the select transistor  230 . For example, if the program voltage associated with the antifuse is approximately 5V, the breakdown voltage of the select transistor  230  may be approximately 4-4.5V. In the illustrated embodiment, the disconnect transistor  210  is a p-channel device with essentially conventional p-channel implants. However, the n-channel select transistor  230  is provided with a sharper n+/p− sub junction at its drain to decrease the breakdown voltage and encourage avalanche breakdown and thus snapback. This sharper junction may be formed using a locally higher n-type implant or a locally higher p-type implant (or both) at the drain. For example, a halo implant may be performed at the drain to achieve this characteristic. In either case (i.e., more n-type or more p-type), the implant is conducted with sufficient energy to be located at the n+/p− sub junction (i.e., it is not a surface implant). 
     Turning briefly to  FIG. 3 , a simplified diagram of an exemplary select transistor  230  is provided to illustrate the tailoring of the breakdown voltage. The select transistor  230  is formed on a substrate  300  (e.g., P type). A gate  310  is formed on the substrate  300 . For brevity, and to avoid obscuring the present invention, not all features of the gate  310  are shown. Typically, the gate  310  includes a gate oxide layer formed over the substrate  300 , a conductive layer over the gate oxide, and an insulative layer over the conductive layer. The conductive and insulative layers may each include more than one layer. For example, the conductive layer may include a polysilicon layer covered by a silicide layer, and the insulative layer may include an oxide layer covered by a nitride cap layer. Various gate  310  embodiments may be used, and their specific constructs are well known to those of ordinary skill in the art. 
     As seen in  FIG. 3 , source/drain regions  320  are formed in the substrate  300  using one or more implantation steps, as is well known in the art. Spacers  330  may be used to tailor the profile of the source/drain regions  320 . Of course, multiple spacers of differing sizes may be used, or spacers may be omitted entirely, to tailor the source/drain regions  320  as desired. In the example construction of  FIG. 3 , the source/drain regions  320  include a highly-doped region  340 , a lightly-doped region  350 , and a halo region  360 . To sharpen the n+/p− sub junction (e.g., between the source/drain region  320  and the substrate  300 ), the halo region  360  may be formed using a p-type dopant. 
     Returning to  FIG. 2 , the disconnect transistor  210  is controlled to disconnect the select transistor  230  from the terminal  240  after a programming event, thereby isolating the select transistor  230  from the program voltage to allow the select transistor  230  to exit from snapback mode without requiring the cycling of the program voltage. In this manner a programming event that programs multiple antifuses  220  in the bank  225  may be conducted by cycling the disconnect transistor  210 , not by cycling the program voltage, thereby reducing the time required for the programming event. 
     In the illustrated embodiment, the antifuses  220  are programmed using a voltage of approximately 4-5 volts. In other embodiments, the antifuses  220  may require a higher program voltage (e.g., 7-8) volts. In such a case, the antifuse circuit  200  may include isolation transistors, such as the isolation transistor  115  of  FIG. 1  disposed between the antifuse  220  and the select transistor  230 . Even with the addition of isolation transistors  115 , the real estate consumed by the antifuse circuit  200  is less than a conventional circuit due to the size savings associated with the select transistors  230 . 
     Turning now to  FIG. 4 , a timing diagram showing the control signal provided for programming selected antifuses  220  in the bank  225  is provided. The program voltage V PGM  is asserted at the terminal  240  to initiate the programming operation. The BANK SEL signal is asserted to select the bank  225 . The SEL1 signal is asserted to select a first select transistor  230  and its associated antifuse  220 . The select transistor  230  enters a snapback mode of operation following the rupture of the antifuse  220  to provide increased soak current. The SEL1 and BANK SEL signal are deasserted following a predetermined time interval to allow the select transistor  230  to exit the snapback state. 
     Subsequently, the bank  225  is selected again by asserting the BANK SEL signal, and a second select transistor  230  is enabled by asserting an SEL2 signal. Once again, the SEL2 and BANK SEL signal are deasserted following the programming of the antifuse  220  to allow the select transistor  230  to exit the snapback state. 
     The length of the predetermined program time interval depends on the particular characteristics of the antifuse circuit  200 , including the program voltage, the time required to rupture the antifuse  220 , and the soak time required to perfect the antifuse  220 . Likewise, the particular time intervals between assertions of the BANK SEL signal and the time the BANK SEL signal remains deasserted to allow the select transistor  230  to exit snapback mode depend on the particular implementation and device characteristics. The time intervals illustrated are merely intended to be illustrative of the programming sequence, not the actual relative timing or time periods. 
     Referring now to  FIG. 5 , a block diagram of a memory device  500  is shown according to another embodiment of the present invention. The memory device  500  includes an array  510  of memory cells that are accessed according to address signals provided to the memory device  500  at a number of address inputs  520 . An address decoder  530  decodes the address signals and accesses memory cells in the array  510  according to the address signals. Input/output (I/O) circuitry  540  is provided for controlling read and write events to the memory array  510  in the locations specified by the address inputs  520 . Control inputs  550  are provided for defining the type of transaction being conducted (e.g., typical control inputs  550  include a chip enable signal, a write enable signal, and an output enable signal) DQ lines  560  are provided for the exchange of read or write data with the memory array  510 . For example, data is written to the memory cells in the array  510  when a write enable signal and a chip enable signal are both low. The data is received by the memory device  500  over the DQ lines  560 . The data is provided to the memory cells in the array  510  from the DQ lines  560  through the I/O circuitry  540 . Data is read from the memory cells in the array  510  when the write enable signal is high and the output enable signal and the chip enable signal are both low. 
     The antifuse circuit  200  may be used in the memory device  500  for configuring the memory array  510 . For example, defective memory cells may be replaced with redundant cells by programming certain antifuses  220 , as is well known in the art. The antifuse circuit  200  may be integrated with the memory array  510  or may be a separate circuit on the memory device  500 . 
     A block diagram of an information-handling system  600  is shown in  FIG. 6  according to yet another embodiment of the present invention. The information-handling system  600  includes a memory system  610 , a processor  620 , a display unit  630 , and an I/O subsystem  640 . The processor  620 , the display unit  630 , the I/O subsystem  640 , and the memory system  610  are coupled together by a suitable communication line or bus  650  over which signals are exchanged between them. The processor  620  may be, for example, a microprocessor. One or more of the memory system  610 , the processor  620 , the display unit  630 , and the I/O subsystem  640  may include one or more of the circuits and devices described above with respect to  FIGS. 1-5  according to embodiments of the present invention. 
     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.