Abstract:
The present invention relates to a one-time programmable (OTP) device including three fuses connected in parallel to a logic element which determines that the device is programmed when at least one of the fuses open. The present invention comprises a one-time programmable device that, before the one-time programmable device is programmed, provides, in response to a test signal, a simulation output signal that simulates an output signal that the one-time programmable device provides if the one-time programmable device is programmed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to one-time programmable (OTP) devices used in microelectronics, and more specifically to such devices using semiconductor fuses. 
     2. Discussion of the Related Art 
     OTP devices using semiconductor fuses find many applications in microelectronics. They are, for example, used to short-circuit resistors in a chain of series resistors to adjust a reference voltage in an integrated circuit. Such OTP devices may also be used to permanently store, in an integrated circuit, after its manufacturing, data enabling identification of the circuit from the outside, or required by the circuit operation. 
     FIG. 1 shows a portion  10  of an integrated circuit including a conventional OTP device  20 . Device  20  includes a logic gate  22  and a fuse  24 . Logic gate  22  receives a selection signal S from a selection device  26  and a programming signal P. Fuse  24  is arranged between the output F of logic gate  22  and an output O. A fuse here designates a device including a fusible element, means for fusing the fusible element, generally by over-current, and means for generating a binary output O indicating whether the fusible element has fused or not. The structure of this device is well known in the art and will not be detailed herein. 
     To program device  20 , signal S is activated to select device  20 , and programming signal P is activated. Then, logic gate  22  generates a signal F which causes the fusing of fuse  24 . Selection device  26  is generally controlled by an external control signal C. 
     A conventional device such as shown in FIG. 1 has several significant disadvantages. It may occur that fuse  24  does not completely fuse when receiving signal F. Thus, in some cases, the fuse may generate an incorrect value after programming. In practice, the probability of a failure is approximately 1%. When a fuse has not completely fused, it is difficult to have it completely fuse by a second programming, since its resistance is then too high to reach the fusing energy with the admissible programming voltage. 
     Further, a properly fused, that is, non-conductive fuse  24  may become conductive again in the course of time, by recombination of the fused elements. The probability of such a recombination is approximately 1%. 
     Moreover, circuit  10  may be submitted to electrostatic discharges which can, in some conditions, simultaneously activate signals P and S. Element  20  may under such circumstances be untimely programmed. 
     Eventually, when using an OTP device such as that in FIG. 1 for settings, for example, to short-circuit resistors in a chain of resistors enabling determination of a reference voltage, it may be desirable to have a way of knowing the effect of the programming of device  20  before effectively programming it. 
     Because of the combination of the above-mentioned disadvantages, those skilled in the art consider semiconductor fuse devices as unreliable. As a result, for example, such devices are not used in applications for which a high programming reliability is indispensable. In such applications, more complex programming means are then used. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a fuse one-time programmable device, the programming of which is particularly reliable. 
     Another object of the present invention is to provide such a device which is protected from electrostatic discharges. 
     Another object of the present invention is to provide such a device associated with means for knowing the effect of the programming before proceeding to the programming. 
     To achieve these objects, as well as others, the present invention provides an one-time programmable device including three fuses connected in parallel to a logic element which determines that the device is programmed when at least one of the fuses is open. 
     According to an embodiment of the present invention, the device includes means for checking the programming of the fuses and declaring the device to be not suitable if less than two fuses are open. 
     According to an embodiment of the present invention, the device is programmable by a programming signal. 
     According to an embodiment of the present invention, the logic element is provided to indicate a programmed state of the device when it receives a test control signal. 
     According to an embodiment of the present invention, the device includes a circuit for preventing the programming signal to be generated by an electrostatic discharge. 
     The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1, previously described, schematically shows a portion of an integrated circuit including an OTP device according to the state of the art; 
     FIG. 2 schematically shows a portion of an integrated circuit including a first embodiment of an OTP device according to the present invention; and 
     FIG. 3 schematically shows a portion of an integrated circuit including a second embodiment of an OTP device according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides, to avoid the reliability disadvantages of a single-fuse device, a redundant-fuse device. An immediate solution to implement this redundancy would be to connect the fuses in series. A device using series-connected redundant fuses would be particularly reliable, but the fusing of the fuses would be difficult to perform and the devices enabling this operation would use a relatively large surface area. According to the present invention, it is conversely provided to use the redundant fuses in parallel. 
     In FIG. 2, an integrated circuit  10  includes an OTP device  21  according to the present invention. This device  21  includes three fuses  24   a,    24   b,  and  24   c  of conventional type, respectively generating three output signals O a , O b , and O c  Device  21  also includes a two-input logic gate  22 . Logic gate  22  is similar to that described in relation with FIG.  1 : it receives control signals P and selection signals S and generates a programming signal F when signals P and S are both activated. Selection signal S is also generated, as in FIG. 1, by a selection device  26 . 
     Device  21  includes a logic gate  28  which receives output signals O a , O b , and O c  from fuses  24   a,    24   b,  and  24   c.  Gate  28  generates a binary signal O which is active, that is, which corresponds to a programmed state, when one at least of the three signals O a , O b , and O c  is active, that is, when one at least of the three fuses has fused. 
     Device  21  is programmed by conventionally fusing the three fuses  24   a,    24   b,  and  24   c.  A single properly fused fuse is sufficient to guarantee a proper programming. Noting p the probability for a fuse to be properly fused, the probability for at least one fuse among the three to be fused is p 3 +3(1−p)p 2 +3(1−p) 2 p. If p=0.99, the probability of properly programming device  21  is 0.999997, which makes it adequate for many applications which are not possible with conventional single-fuse devices. 
     Similarly, noting q the probability for a fused fuse to become conductive again by recombination, the probability for the three fused fuses to all recover their conductive state is q 3 , and the probability for two fused fuses to recover their conductive state is q 2 . Thus, the probability for a properly programmed device  21  to return to an unprogrammed state is: p 3 q 3 +3(1−p)p 2 q 2  +3(1−p) 2 pq. 
     If p is equal to 0.99 and q is equal to 0.01, a value of 6.8 10 −6  is obtained, against 9.9 10 − 3 for a conventional single-fuse device. 
     Preferably, to increase the long-term reliability of the programming of an element  21 , it is provided to check whether at least two of fuses  24   a  to  24   c  are properly fused. This checking is performed by a conventional measurement of leakage currents by means of an external test equipment  25 , or by an internal automated test device (BIST), not shown. If the measurements determine that at least two of the three fuses have effectively fused, the user or, if present, the automated test device, determines that device  21  has been successfully programmed. If less than two fuses have fused in the programming, device  21  will be considered as being incorrectly programmed. It will have to be envisaged not to use it, as in the case of a misprogrammed conventional device. 
     In this case, noting p the probability for a fuse to have properly fused, the probability for at least two fuses among the three to have properly fused, that is, the probability for the device to be considered as properly programmed, is y=p 3 +3(1−p)p 2 . If p=0.99, y+0.9997 is obtained. 
     In the case where the programming of device  21  has effectively only fused two fuses, each of the two fuses will have to become conductive again, for example, by recombination, so that output O of gate  28  switches and no longer indicates that device  21  is programmed. 
     In the case where the programming of device  23  has fused all three fuses, each of the three fuses will have to become conductive again so that gate  28  no longer indicates the programming of device  23 . 
     Thus, the probability r for a device  23  considered as properly programmed to return to an unprogrammed state is such that r=p 3 q 3 +3(1−p)p 2 q 2 . 
     Taking p equal to 0.99 and q equal to 0.01, r is equal to 3.9 10 −6 . It should be noted that a still better reliability than in the previously described case, where it is not checked whether at least two fuses have fused in the programming is obtained. 
     The device shown in FIG. 2 implies a simultaneous fusing of the three fuses  24   a  to  24   c,  which requires a strong current during the programming of device  21 . This is not necessarily easy to obtain in an integrated circuit. It is thus provided to separately fuse fuses  24   a  to  24   c.    
     FIG. 3 shows an embodiment of an OTP device  23  enabling this. In this embodiment, conversely to that of FIG. 2, each fuse  24  ( 24   a  ,  24   b,    24   c ) receives the output F (F a , F b , F c ) of a respective gate  22  ( 22   a,    22   b,    22   c ), each gate  22  receiving an OTP signal P and a respective selection signal S (S a , S b , S c ). Each gate  22  ( 22   a,    22   b,    22   c ) provides the corresponding fuse  24  ( 24   a,    24   b,    24   c ) with a control signal F (F a , F b , F c ) when the corresponding signal S (S a , S b , S c ) and signal P are both activated. 
     Selection signals S a , S b , S c  are generated by a selection device  27  receiving an external control signal C. Selection device  27  is provided to sequentially activate signals S a  to S c  upon activation of signal C. Thus, by simultaneously activating signals C and P, the three fuses  24  are fused one after the other with a minimum current surge. 
     Device  23  is preferably provided with an element of protection  30  against electrostatic discharges between external programming signal P and logic gates  22   a  to  22   c.  Protection element  30  receives signal P and generates a signal P I  corresponding to the filtered received signal P, that is, rid of possible parasitic components introduced by an electrostatic discharge. Thus, when circuit  10  undergoes an electrostatic discharge, programming signal P I  remains inactivated and no untimely programming can occur. 
     In a conventional OTP device, no means of knowing the effect of the device programming in advance are available. Thus, once the device has been programmed, if the effect of the programming causes an unexpected circuit reaction, it is too late to go back and the circuit will not operate as desired. 
     To avoid this, in the embodiment shown in FIG. 3, device  23  is provided with a test signal T, generated by internal means (not shown) or means coming from outside of circuit  10 . Signal T is provided to logic gate  28 . Logic gate  28  is devised for, when receiving test signal T, activating signal O as if device  23  were programmed. Thus, when device  23  is used, for example, to adjust a parameter of circuit  10 , it will be possible, before definitively programming the device, to judge the effect of the programming upon the adjustment of said parameter. 
     It should be noted that the advantages of the OTP device according to the present invention are to be balanced with a substantial increase of the surface with respect to a conventional OTP device. However, this size increase is compensated by the fact that the device may now be used for many applications in which the reliability of conventional OTP devices was insufficient. Electronic circuits used in automobile security circuits, for example, to control the tire pressure, are one example application. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. For example, an element of protection against electrostatic discharges could be inserted directly at the level of the input of each of logic gates  22   a,    22   b,  and  22   c.    
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.