Improvements in or relating to fuse and antifuse link structures for integrated circuits

Abstract: A fuse and antifuse link structure, which when used with a memory integrated circuit device such as a gate array or programmable read-only memory (PROM), allows the memory circuit to be reprogrammed. The fuse and antifuse link is comprised of a fuse 12 and an antifuse 16 , connected in series, parallel, or a combination thereof. Either element of the link can be programmed initially, and the other can be programmed in a second step, to reverse the first programming. Several links can be used in one circuit to provide multiple reprogramming capability.

Description: Background of the Invention Field of the Invention: The invention relates generally to integrated circuits, and more particularly to memory devices such as programmable gate arrays or PROMs (Programmable Read-Only Memory). Poly-silicon ("poly") fuses are well known in prior art and have been used often in redundancy schemes for memory applications and for trimming resistors to set voltage reference levels in analog applications. Poly fuses are conductive links until a high current pulse is sent through them, causing them to melt and form an open circuit. More recently, antifuses which can consist of a thin dielectric between two conductors have been introduced into Field Programmable Gate Array products. Antifuses start out as circuit opens and are 'blown" by applying a high voltage pulse causing the dielectric to rupture, forming a dosed circuit U.S. Patent number 4,943,538 entitled "Programmable Low Impedance Anti-fuse Element" filed on Mar. 10, 1988 by Mahsen et al describes such an antifuse, with a capacitor-like structure having very low leakage before programming and a low resistance after programming. Another form of antifuse consists of a region of amorphous material of high resistivity placed between two conductors. To change the amorphous material from high resistivity to a conductor, current is passed through the amorphous material, heating it and converting it to a highly conductive state. This type of antifuse is described in patent number 4,752,118 entitled "Electric Circuits Having Repairable Circuit Lines and Method of Making the Same" filed on Oct. 14, 1986 by Johnson. US-A-5 200 652 discloses a circuit comprising a first fuse and antifuse connected in parallel between first and second conductors and a second fuse, or antifuse, also connected between the first and second conductors in parallel with the first fuse and antifuse. The structure may be programmed to provide open circuit and short circuit between the first and second conductors alternately by alternately blowing the fuses and antifuses. A programming conductor is provided to allow a programming voltage to be applied to the node between the first fuse and the first antifuse. Summary of the Invention The invention is defined by the independent claims 1, 2, 5 and 7. There are several types of electronic devices that can be reprogrammed (i.e. programmed more than once), such as PROMs (Erasable-Programmable Read-Only Memory), EEPROMs (Electrically Erasable-Programmable Read-Only Memory), or SRAMs (Static Random Access Memory). The circuits that are programmed are transistor-based, but are rather large, taking up space on the integrated circuit Other devices, such as the typical Gate Arrays or PROMs, can be programmed only once. These devices have constructed links made of a single fuse or antifuse, and no reprogramming or correction of errors in the programming of routings is possible (other than a hard-wire or "external jumper" to the packaged IC). A PROM or Gate Array that has been misprogrammed is generally discarded as unusable. Therefore, if such a device had the capability of being reprogrammed, programming scrap costs would be reduced. Further, it is common for engineering changes to be made which require memory devices to be reprogrammed. Again, costs could be reduced if such a device could be programmed more than once. In the new invention, a link is comprised of series, parallel, or both, combinations of fuses and antifuses. Such a link allows two or more programming steps. Multiple sublinks can be connected in various ways, providing for multiple reprogramming steps. There are many advantages to the fuse and antifuse link invention, a very important one being that the area requirement for the fuse and antifuse link is less than for transistor-based programming elements. The fuse and antifuse structures can be built vertically on the semiconductor wafer, whereas the transistors for reprogramming cells for PROMs, EEPROMs and SRAMs are generally built in a single crystal substrate, located horizontally next to the logic transistors. The fuse and antifuse link may, for example, take up an 0.8 µm 2 area, versus an SRAM cell at 70 µm 2 , and an EEPROM cell at 7.25 µm 2 . This equates to semiconductor real estate savings ranging from 89% to 99%. Other advantages are reprogrammability, decreased scrap costs, lower resistance, lower expense, ease of implementation, radiation hardness, and in situ test capability. With the fuse and antifuse link of this invention, memory devices such as PROMs and Gate Arrays that in the past have not been reprogrammable may now be reprogrammed many times. Design changes, updates, or corrections are capable of being implemented, reducing the number of finished integrated circuits that must be scrapped. Also, the resistance of a metal to metal antifuse is 50 ohms or less, versus an EEPROM transistor-based cell that has a resistance of thousands of ohms. The lower resistance provides less signal degradation and thus, better performance of the circuit. In addition, the fuse and antifuse structures required for a small number of programming steps are physically small and inexpensive to implement. The fuse and antifuse structures are more radiation hard than the transistor-based cells of PROMs, EEPROMs and SRAMs, which can be erroneously misprogrammed fairly easily. Another advantage made possible by in-circuit reprogramming include allowing a manufacturer to test chips in situ before shipping: gates can be tested independently of the links for Quality Assurance by using one programming step. The fuse and antifuse link is an inexpensive, space-conserving way to make a memory device reprogrammable. Several embodiments are described, all of which provide the novel capability of reprogramming these previously "once-only" programmable devices. Brief Description of the Drawings In the drawings, which form an integral part of the specification and are to be read in conjunction therewith, and in which like numerals and symbols are employed to designate similar components in various views unless otherwise indicated: Figure 1 shows schematic symbols for reprogrammable link components; Figure 2 shows a series fuse and antifuse link allowing two programming steps, where the states depicted are: 2a) the initial state (link open), 2b) after first programming (link shorted) 2c) after second programming (link open); Figures 3a through 3c show the series fuse and antifuse link without a programming node; Figure 4 shows a parallel fuse and antifuse link allowing two programming steps, where the states depicted are: 4a) initial state (link shorted), 4b) after first programming (link open), 4c) after second programming (link shorted); Figure 5 shows a link consisting of the parallel connection of n series sublinks, without programming nodes, utilizing antifuses activated by different voltages; Figure 6 shows a link consisting of the series connection of n parallel sublinks to allow 2n programming steps, without programming nodes, utilizing fuses of different current ratings; Figure 7 is a preferred embodiment of a fuse and antifuse link using dielectric breakdown antifuse where: 7a) is a cross-section of the structure, 7b) is a top view of the conductive portions of the series link, 7c) is a top view of the conductive portions of a parallel link; Figure 8 shows an alternate embodiment of a series link incorporating an antifuse using amorphous-Si to poly-Si conversion, where: 8a) is a cross-section of the structure before deposition of the amorphous-Si plug; 8b) is a cross-section of the completed antifuse structure, 8c) is a top view of the conductive portions of the series link; and Figure 9 shows an embodiment of a parallel link in which the fuse and antifuse link takes up approximately the same amount of space as a single fuse element. Detailed Description of Preferred Embodiments The making and use of the presently preferred embodiments are discussed below in detail. However, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention. The following is a description of several preferred embodiments and alternative embodiments, including schematic representations and manufacturing methods. Corresponding numerals and symbols in the different figures refer to corresponding parts unless otherwise indicated. Table 1 below provides an overview of the elements of the embodiments and the drawings. Drawing Element Generic Term Specific Examples or Alternates T 1 Input Terminal 1 T 2 Input Terminal 2 T P Programming Terminal 10 Series Fuse/Anti Fuse Link 12 Schematic symbol for fuse in initial state; shorted 14 Schematic symbol for fuse in "written" state; open 16 Schematic symbol for antifuse in initial state; open 18 Schematic symbol for antifuse in "written" state; shorted 20 Parallel Fuse and Antifuse Link 24 Series Sublink Fuse and Antifuse in series 26 Parallel Sublink Fuse and Antifuse in parallel 30 Array of series sublinks Multiple parallel connection of drawing element 24 34 Array of parallel sublinks Multiple series connection of drawing element 26 40 Poly, first layer 4500Å of poly silicon 42 Poly, second layer 44 Field Oxide 46 Fuse link 48 Insulating Oxide MLO oxide 50 Antifuse dielectric 52 Metal 1 54 Antifuse, dielectric breakdown type 56 antifuse opening 58 contact 60 amorphous silicon plug 62 passivating layer 64 Antifuse, amorphous silicon type A Antifuse A 1 = first antifuse; Higher voltage antifuse; Lower voltage antifuse. F Fuse F 1 = first fuse; Higher current fuse; Lower current fuse. First, terms will be defined which are used to describe the invention. A "link" refers to the connection between circuit elements (called "gates") which is either connected or disconnected in the programming process which configures the gate array or memory. In current practice, a link is either a single fuse or antifuse; in this invention, a link is comprised of combinations of fuses and antifuses. A "fuse" is a circuit element which is initially a short, which in the programming process can be opened, typically with the passage of a high current through it An "antifuse" is the opposite of a fuse in that it is initially open, and during programming, typically by placing a high voltage across it to "punch through" a thin insulating layer, can be converted into a short. Schematic symbols for fuses and antifuses as used herein are defined in Figure 1 ; these symbols will be used in subsequent figures describing the fuse and antifuse link invention. Drawing element 12 is a schematic symbol for a fuse in the initial state, which is shorted. A fuse in the written state, an open, is shown in drawing element 14 . Drawing element 16 is a schematic symbol for an antifuse in the initial state, which is an open. The antifuse in the written state, a short, is shown in drawing element 18. Programming steps, in which opens are converted into shorts, or vice versa, to change the state of the links, had been, for practical purposes, irreversible for the single-element links of the prior art. It is this irreversibility which the invention is intended to circumvent by allowing the state of links to be reversed by additional programming steps. A basic link, which is twice programmable, consists of one fuse and one antifuse. In the first configuration, shown in Figure 2 , these two components are placed in series connection with the center node, T p , accessible for programming as is known. The initial state of the series fuse and antifuse link 10, shown in Figure 2a , is open. The end nodes, labeled T 1 and T 2 , are connected to gate inputs or outputs; these voltages, as well as the programming voltage, T P , which may be applied to the center node, can be controlled during the programming process. In the first programming, a large potential difference may be applied between T 1 and T P to "blow" (i.e. connect) the antifuse 16, thereby changing the antifuse to the written state 18 and shorting the link as shown in Figure 2b. It is recommended that T 1 be held low and T P taken to a high voltage, but the opposite sign of (T P -T 1 ) could also be used. In either case, T 2 should be held at the same potential as T P or be allowed to float in order to avoid high currents that could blow the fuse 12 during the first programming. The second programming step, which allows reversal of the link to its original open state, is accomplished by running a high current (not necessarily at a high voltage) through the fuse 12 in Figure 2b via the T P and T 2 nodes to blow the fuse and open it. Figure 2c shows the fuse 14 in the "written" state, with the link open. A variation of this configuration is shown in Figure 3, in which the programming nodes are omitted. An external current-limiting device (not shown) can be utilized with the schematic of Figure 3a to enable programming without using a programming node. This can be achieved by, in the first programming step, applying a high voltage across T1 and T2, with the current limited so that the fuse 12 is not blown when the antifuse 16 is blown. The effect of the first programming step is shown in Figure 3b. The second programming step is achieved by passing a high current through the fuse and antifuse link 10 (Figure 3b), which blows the fuse 14 as shown in Figure 3c, returning the circuit to the original open state. In the second configuration is a parallel fuse and antifuse link 20 , shown in Figure 4a, the fuse 12 and antifuse 16 are placed in parallel, which also allows two programming steps. The link is initially shorted, as shown in Figure 4a . The link can be programmed first to open it by applying a high current across T 1 and T 2 to blow the fuse 12 , which converts the circuit as shown in Figure 4b, with the fuse 14 open. The circuit shown in Figure 4b can be programmed a second time by applying a high voltage to short it again by blowing the antifuse 16 , leaving the circuit shown in Figure 4c, with the antifuse 18 in the shorted state. Note that no separate programming voltage node is required, since the fuse can be blown by high current at low voltage without disturbing the antifuse. Once the fuse is blown, a high voltage may be applied across the two terminals to blow the antifuse. Connections of multiple "sublinks" of the link types described above can be used to allow more than two programming steps. In an embodiment of the invention the circuit is arranged in an array of series sublinks 30, by connecting series sublinks 24 in parallel as shown in Figure 5. In this configuration, n series-type sublinks 24 may be connected in parallel to allow 2 n programming steps. The configuration of Figure 5 is without programming terminals. As in Figure 4, if the current is limited when the high voltage is applied to blow the antifuse 16 , then the fuse 12 will not be blown. Also, in order to reprogram the fuse and antifuse link structure depicted in Figure 5, the thickness of the dielectric for the antifuses can be varied, so that all of the antifuses are not "blown" by the same voltage. More specifically, the thickness of the dielectric can be sequentially thicker for each antifuse in the array of sublinks. Similarly, the circuit of the invention can be arranged in an array of parallel sublinks 34, by connecting parallel sublinks 26 in series as shown in Figure 6, which also uses n sublinks to obtain 2 n programming steps. The circuit of Figure 6 is without programming terminals. If the fuses are constructed with varying cross-sectional areas and amperage ratings, the fuse to be programmed could be selected by controlling the current. A process flow for building either the series (Figure 2) or parallel (Figure 4) forms of the fuse and antifuse link will be described next. Figures 7a and 7b show cross-sectional views and top views (top views herein show only conductive or potentially conductive portions) of the series link using a dielectric breakdown antifuse, while Figure 7c shows the application of this process to a parallel link in a top view. Both double level poly and single level metal are used. For this embodiment, in a CMOS process, the n-wells and p-wells (not shown) are formed in the usual manner. Field oxide 44 is then formed, also in the usual manner, to isolate individual transistors from each other. Approximately 4500 Å of poly-silicon 40 is then deposited which forms (as shown in the top view) the fuse link 46, the bottom plate of a poly/dielectric/metal 1 antifuse 54, and it can, for example, be used as well to form the transistor gates (not shown). The resistivity of the poly can be in the range of 30 to 40 ohm/sq for the fuse link but is generally heavily doped with arsenic for the antifuse bottom plate. To meet these two criteria, first the poly 40 is POCl 3 doped to 30 ohm/sq resistivity followed by a patterning which exposes only the bottom plate area of the antifuse 54 . A 1E16 dose of arsenic is then implanted, and the poly 40 is patterned and etched (sources and drains, not shown, are patterned and implanted in the usual manner). The fuse link 46 is formed by etching a region of poly 1 µm wide and 4 to 8 µm long. Later, if the fuse 46 is programmed, a high current pulse will blow the fuse 46 by causing the poly to melt. A thick insulating oxide 48 is deposited to isolate the substrate and gate poly 40 from the metal interconnect 52 . A fuse opening is then patterned and etched to the fuse bottom plate poly. A thin oxide is then thermally grown and a thin (e.g. LPCVD) nitride is deposited, forming the antifuse dielectric 50 . A steam seal is then performed to improve the antifuse dielectric 50 followed by deposition of the polysilicon for the antifuse top plate 42. This polysilicon is also implant-doped with 1E16 arsenic and annealed to activate the arsenic. The poly is patterned and etched, forming the top plate 42 of the antifuse. Contacts 58 are then etched down to the poly 40 , and the sources, drains and metal are deposited, patterned and etched forming interconnect in the usual manner. In this embodiment, to blow the fuse 46, a 6 to 8 volt pulse of high (approximately 40 mA) current is applied across the fuse link 46 . To blow the antifuse 54, a high voltage pulse of 18 volts is applied across the thin dielectric. A second embodiment, incorporating a different method of forming an antifuse element, is shown in Figure 8. The CMOS processing and fuse processing proceed as described above through the first poly 40 deposition and POCl 3 doping. The thick insulating oxide 48 is then deposited and the antifuse opening is etched, exposing the two ends of the poly conductors which will make contact to the antifuse as shown in Figure 8a. An amorphous silicon plug 60 is then formed in the opening to form the antifuse material between the two poly conductors. Contacts and metal (not shown) are then formed in the usual manner to complete the process, with the final structure shown in Figure 8b . In this embodiment, an 8 to 10 volt pulse with high current can be used either to blow the fuse element 46 , or to convert the highly resistive, amorphous silicon antifuse 64 element into a highly-conductive poly-silicon link. As shown in the top view of Figure 8c, this embodiment can be used in a series link, which provides a T p terminal to individually address the two elements. It is more difficult to use the amorphous silicon antifuse in circuits without a T p terminal (e.g. with the parallel link of Figure 4 because the same type of pulse is generally required to blow both fuses and antifuses. An alternate method of implementing a parallel fuse and antifuse link in the manufacturing process is to combine the fuse and antifuse in close proximity in order to minimize space, as shown in Figure 9. In this configuration the elements of the fuse and antifuse link take up about the same space as a single fuse 46 does. While the fuse and antifuse link invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.

Claims: A reprogrammable electrical circuit (30) comprising: a first terminal (T 1 ), a second terminal (T 2 ), and a first sublink (24) connected between said first and second terminals comprising a first fuse (12) and a first antifuse (16) connected in series between said first and second terminals, characterised in that the electrical circuit further comprises: a second sublink (24) comprising a second fuse and a second antifuse connected in series between said first and second terminals, the second antifuse being arranged to blow at a higher voltage than the first antifuse, whereby the circuit is reprogrammable so as to alternately open and close an electrically conductive path between said first terminal and said second terminal. A reprogrammable electrical circuit (34) comprising: a first terminal (T 1 ), a second terminal (T 2 ), and a first sublink (26) comprising a first fuse (12) and a first antifuse (16) connected in parallel, characterised in that the electrical circuit further comprises: a second sublink (26) comprising a second fuse and a second antifuse connected in parallel, the first sublink and the second sublink being connected in series between the first and second terminals, said second fuse being arranged to blow at a higher current than said first fuse, whereby the circuit is reprogrammable so as to alternately open and close an electrically conductive path between said first terminal and said second terminal. A reprogrammable circuit as claimed in claim 1 or claim 2 and further comprising programming means connected to the first and second terminals (T 1 ,T 2 ) for supplying signals for blowing the said fuses and antifuses. A gate array or a programmable read only memory (PROM) comprising a reprogrammable circuit as claimed in any preceding claim. A method of programming and reprogramming a reprogrammable electrical circuit (30) by alternately opening and closing an electrically conductive path between first and second terminals (T 1 , T 2 ) of the electrical circuit, the electrical circuit comprising a first sublink (24) comprising a first fuse (12) and a first antifuse (16) connected in series between said first and second terminals and a second sublink (24) comprising a second fuse and a second antifuse connected in series between said first and second terminals, the method comprising the steps of: blowing the first antifuse to program it closed by applying a first voltage between the first and second terminals of the electrical circuit, the first voltage being sufficient to blow the first antifuse but not sufficient to blow the second antifuse; blowing said first fuse to program it open by applying a current between the first and second terminals of the electrical circuit, the current being sufficient to blow the first fuse; blowing the second antifuse to program it closed by applying a second voltage between the first and second terminals of the electrical circuit, the second voltage being sufficient to blow the second antifuse; and blowing said second fuse to program it open by applying a current between the first and second terminals of the electrical circuit, the current being sufficient to blow the second fuse. A method as claimed in claim 5, wherein the steps of applying a voltage to blow an antifuse include limiting the current to prevent a fuse in the electrical circuit (30) from being blown. A method of programming and reprogramming a reprogrammable electrical circuit (34) by alternately closing and opening an electrically conductive path between first and second terminals (T 1 , T 2 ) of the electrical circuit, the electrical circuit comprising a first sublink (26) comprising a first fuse (12) and a first antifuse (16) connected in parallel and a second sublink (26) comprising a second fuse and a second antifuse connected in parallel, the first and second sublinks being connected in series between said first and second terminals, the method comprising the steps of: blowing the first fuse to program it open by applying a first current between the first and second terminals of the electrical circuit, the first current being sufficient to blow the first fuse but not sufficient to blow the second fuse; blowing said first antifuse to program it closed by applying a voltage between the first and second terminals of the electrical circuit, the voltage being sufficient to blow the first antifuse; blowing the second fuse to program it open by applying a second current between the first and second terminals of the electrical circuit, the second current being sufficient to blow the second fuse; and blowing said second antifuse to program it closed by applying a voltage between the first and second terminals of the electrical circuit, the voltage being sufficient to blow the second antifuse. A method as claimed in claim 7, wherein the steps of applying a current to blow a fuse include limiting the voltage to prevent an antifuse in the electrical circuit (34) from being blown.

Inventors: MAGEL GREGORY A, STOLTZ RICHARD A, MAGEL, GREGORY A., STOLTZ, RICHARD A.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1995
Main classification: G11C17
IPC codes: H01L27, H01L21, G11C17