# Patent

## Title
Semiconductor device

## Abstract

A fuse peripheral circuit includes a fuse, a potential difference imparting circuit, a potential difference reducing circuit, a terminal, a memory circuit, a transfer gate, and a logic gate. The logic gate is connected to the input end of the transfer gate. The logic gate serves as a transmission prevention circuit preventing a signal stored in the memory circuit from being transmitted to the fuse, when the disconnection judgment takes place.

## Background
This application is based on Japanese patent application Ser. No. 2005-026709, the content of which is incorporated hereinto by reference.
BACKGROUND
1. Technical Field
The present invention relates to a semiconductor device.
2. Related Art
Fabrication of semiconductor memory cannot completely be exempt from failure in the memory cell. It is therefore general practice to configure the circuit in a redundant manner in order to rescue the semiconductor memory containing defective cells.
This configuration allows even a semiconductor memory having defective memory cells to normally operate, by replacing fault bits with redundant bits. The replacement is effected using a fuse as described in Japanese Laid-Open Patent Publication No. 6-140510. That is, interconnection is replaced by blowing the fuse.
FIG. 8 is a circuit drawing showing a fuse peripheral circuit described in the patent publication in the above. In this configuration, a fuse 100 is blown by bringing a terminal 101 to a high level to thereby turn an NFET transistor 102 on. This allows current to flow through the fuse 100, so that the fuse 100 blows. Whether the fuse 100 has been blown or not can be judged by raising the level of a terminal 103 to high, to thereby bring the node 104 down to low. Next, the terminal 103 is switched to low, to thereby turn a PFET transistor 105 on. In this case, the node 104 remains low if the fuse 100 has been blown. On the other hand, if the fuse has not been blown, both of the fuse 100 and the transistor 105 electrically conduct to thereby bring the node 104 up to high. It is therefore made possible to judge whether the fuse 100 has been blown or not based on the potential appears at the node 104. The potential is stored in a latch circuit as a signal indicating result of judgment on whether the fuse 100 has been blown or not.
SUMMARY OF THE INVENTION
The present inventor found out the subjects described below. That is, in the circuit configured as shown in FIG. 8, the signal stored in the latch circuit may flow back to the fuse 100 when the judgment of disconnection of the fuse 100 takes place, and this may produce negative feedback with respect to the potential at one end of the fuse (on the opposite side of a terminal 106). The negative feedback may result in misjudgment of disconnection of the fuse 100.
According to the present invention, there is provided a semiconductor device which includes a fuse; a memory circuit storing a signal which indicates result of judgment on whether the fuse has been blown or not; and a transmission prevention circuit preventing, when the judgment takes place, the signal stored in the memory circuit from being transmitted to the fuse.
In this semiconductor device, it is made possible to prevent a signal stored in the memory circuit from being sent back to the fuse, when the judgment on whether the fuse has been blown or not takes place. This makes it possible to suppress generation of negative feedback with respect to the potential at one end of the fuse. The risk of misjudgment on whether the fuse has been blown or not is therefore reduced.
The transmission prevention circuit may have a logic gate allowing input of a potential signal at one end of the fuse, and outputting a high or low output signal depending on the potential signal. In this case, the potential signal appeared at one end of the fuse is converted into a high or low output signal by the logic gate, and the output signal therefrom is obtained as a result of the disconnection judgment of the fuse. This makes it possible to further reduce the risk of misjudgment on whether the fuse has been blown or not.
The semiconductor device may be configured as having a transfer gate provided on a path between the logic gate and the memory circuit, and as allowing the output signal to be input through the transfer gate into the memory circuit. The transfer gate can prevent the stored signal from being transmitted from the memory circuit to the fuse after the disconnection judgment. In this case, provision of the logic gate holds the key in view of storing correct information into the memory circuit, because the transfer gate causes voltage drop.
The logic gate may be involved in a positive feedback loop. This makes it possible to rapidly determine the output signal from the logic gate.
The logic gate may be a NAND circuit. The NAND is preferably applicable to the logic gate.
The present invention can thus realize a semiconductor device capable of reducing a risk of misjudgment on whether the fuse has been blown or not.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a sectional view showing the first embodiment of the semiconductor device according to the present invention;
FIG. 2 is a circuit diagram showing the fuse peripheral circuit contained in the semiconductor device shown in FIG. 1;
FIG. 3 is a timing chart explaining a disconnection-judging operation in the circuit shown in FIG. 2;
FIG. 4 is a circuit diagram showing the fuse peripheral circuit in the second embodiment of the semiconductor device according to the present invention;
FIG. 5 is a timing chart explaining a disconnection-judging operation in the circuit shown in FIG. 4;
FIG. 6 is a circuit diagram showing a modified example of the semiconductor device of the present invention;
FIG. 7 is a circuit diagram showing another modified example of the semiconductor device of the present invention;
FIG. 8 is a circuit diagram showing a fuse peripheral circuit described in Japanese Laid-Open Patent Publication No. 6-140510;
FIG. 9 is a circuit diagram showing the fuse peripheral circuit in the third embodiment of the semiconductor device according to the present invention; and
FIG. 10 is a timing chart explaining a disconnection-judging operation in the circuit shown in FIG. 9.



DETAILED DESCRIPTION
The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes.
Next paragraphs will detail preferable embodiments of the semiconductor device of the present invention, referring to the attached drawings. In the description of the drawings, any identical constituents will be given with the same reference numerals so as to avoid repetitive explanation.
First Embodiment
FIG. 1 is a sectional view showing the first embodiment of the semiconductor device according to the present invention. A semiconductor device 1 has a semiconductor substrate 90, a fuse provided on the semiconductor substrate 90, and a fuse peripheral circuit configured as including the fuse 10. It is to be noted that FIG. 1 shows only the semiconductor substrate 90 and the fuse 10, while omitting illustration of any other constituents.
A configuration of the fuse peripheral circuit will be explained referring to FIG. 2. The fuse peripheral circuit has the fuse 10, a potential difference imparting circuit 20, a potential difference reducing circuit 30, a terminal 40, a memory circuit 50, a transfer gate 60, and a logic gate 70 (transmission prevention circuit).
The potential difference imparting circuit 20 is configured as containing a transfer gate 22 (first transfer gate), a terminal 24 (first terminal) and a terminal 26, wherein a predetermined potential difference is given between both ends of the fuse 10 when disconnection of the fuse 10 is judged. The transfer gate 22 is a P-type FET (field effect transistor). The terminal 24 is connected to the source of the transfer gate 22. The terminal 24 is applied with a source potential Vcc (second potential). Vcc has a value larger than that of the potential (first potential) applied to an end portion 10a of the fuse 10. It is to be noted that, in the first embodiment, the first potential equals to the ground potential as described later. It is therefore obvious that the potential difference applied to the fuse 10 is given as Vcc.
The terminal 26 is connected to the gate of the transfer gate 22. The terminal 26 is applied with a potential switching the transfer gate 22 between ON and OFF. The drain of the transfer gate 22 is connected to an end portion 10b of the fuse 10. In other words, the configuration herein is such as having the terminal 24 connected through the transfer gate 22 to the end portion 10b.
The potential difference reducing circuit 30 is configured as having a transfer gate 32 (second transfer gate), a terminal 34 (second terminal) and a terminal 36, so as to reduce the potential difference applied between both ends of the fuse 10 by the potential difference imparting circuit 20. The transfer gate 32 is an N-type FET. The terminal 34 is connected to the source of the transfer gate 32. The terminal 34 is grounded. In other words, the terminal 34 is given with the ground potential (third potential). The terminal 36 is connected to the gate of the transfer gate 32. The terminal 36 is applied with a potential switching the transfer gate 32 between ON and OFF. The drain of the transfer gate 32 is connected to the end portion 10b. In other words, the configuration herein is such as having the terminal 34 connected through the transfer gate 32 to the end portion 10b. 
The terminal 40 is connected to the end portion 10a of the fuse 10. The terminal 40 is a terminal supplying therethrough a necessary current to the fuse 10 when the fuse 10 is blown. The terminal 40 in the first embodiment is grounded. In other words, the terminal 40 functions as a terminal applying disconnection potential Vcut to the fuse 10 so as to allow the current to flow therethrough when the fuse 10 is blown as the need arises, and is grounded after the state of the fuse 10 is determined. The “determined state” herein refers to a state of disconnection of the fuse 10, or a state of the fuse 10 remained in connection because there is no need of disconnecting the fuse 10. It is to be noted that the ground to which the terminal 40 is connected is, for instance, a ground frame of a package encapsulating a semiconductor chip therein. The terminal 40 may be connected with the external of the chip, or may be connected with other circuit inside the chip. The terminal 40 in the former case means a pad, for example.
The memory circuit 50 is a latch circuit storing a signal which indicates result of judgment in judgment operation described later. On the path between the memory circuit 50 and the fuse 10, the transfer gate 60 (third transfer gate) is provided. The transfer gate 60 is a CMOS switch, wherein the output end thereof is connected to the input end of the memory circuit 50. A P-type FET gate in the transfer gate 60 is connected with an inverter 62. By contribution of the inverter 62, the gate of a P-type FET and the gate of the N-type FET in the transfer gate 60 will have, respectively input therein, signals inverted from each other. A terminal 64 is applied with a potential switching the transfer gate 60 between ON and OFF, and is connected respectively to the input end of the inverter 62 and to the gate of the N-type FET.
The input end of the transfer gate 60 is connected with the logic gate 70. The configuration herein is such as allowing a signal (output signal) output from the logic gate 70 to transmit through the transfer gate 60 to be input to the memory circuit 50. In other words, the memory circuit 50 can have, as an output signal from the logic gate 70 input therein, a signal indicating the result of judgment, and can store it. The logic gate 70 is a NAND circuit, and can have input of a reference signal and a potential signal at the end portion 10b of the fuse 10, and can output a NAND of them as an output signal. The logic gate 70 serves as a transmission prevention circuit preventing the signal stored in the memory circuit 50 from being transmitted to the fuse 10, when the disconnection judgment of the fuse 10 takes place.
One input end of the logic gate 70 is connected through an inverter 88 to the terminal 36. This allows a result of inversion of the potential applied to the terminal 36 to be input to the logic gate 70. The other input end of the logic gate 70 is connected to the end portion 10b. In the judgment operation, a high level (source potential Vcc) is applied as the reference signal to the logic gate 70. The output signal from the logic gate 70 will therefore have a high value or a low value, depending on a potential signal at the end portion 10b. In other words, the logic gate 70 outputs 0 if a value of the potential signal falls on the high side of the threshold value of the logic gate 70, and outputs 1 if it falls on the low (ground potential) side.
The logic gate 70 is involved in a positive feedback loop. More specifically, there is provided a P-type FET 72 having the gate connected to the output end of the logic gate 70, and the drain connected to the input end (input end connected to the end portion 10b) of the logic gate 70. The source of the P-type FET 72 is connected to the terminal 74 applied with the source potential Vcc. In other words, the configuration is such as turning the FET 72 on when the output signal of the logic gate 70 is low, and allowing the source potential Vcc applied to the terminal 74 to be input to the logic gate 70.
The circuit shown in FIG. 2 additionally has a transfer gate 82, a terminal 84 and a terminal 86. The transfer gate 82 has the source grounded and the drain connected to the end portion 10b. The terminal 84 is connected to the gate of the transfer gate 82. The terminal 84 is applied with a potential switching the transfer gate 82 between ON and OFF. The terminal 86 is connected to the output end of the memory circuit 50, so as to read result of judgment stored in the memory circuit 50.
The transfer gate 82 and the terminal 84 configure a current supply circuit supplying current to the fuse 10 when it is blown. More specifically, current is supplied to the fuse 10 to be blown, by turning the transfer gate 82 on, while keeping the terminal 40 applied with disconnection potential Vcut. The disconnection may be effected by heat or by electro-migration. It is to be noted that the fuse 10 may be blown either on a wafer or on a chip.
It is necessary to judge which of the intrinsic bit line and the redundant bit line should be used, before the semiconductor device 1 starts to operate. Whether the fuse 10 has been blown or not is therefore judged when the semiconductor device 1 is powered on.
Operation of judgment of disconnection which takes place in the circuit shown in FIG. 2 will be explained referring to the timing chart shown in FIG. 3. In FIG. 3, FCUT, F2, TRIG1, F3 and FOUT represent each potential respectively at the terminal 84, the terminal 36, the terminal 26, the terminal 64 and the terminal 86. W represents potential of an interconnection W connecting the end portion 10b of the fuse 10 and the input end of the logic gate 70. Potential at the terminal 84 is always kept low, when the disconnection judgment takes place.
First, the terminals 26, 36 are brought up to high, and the terminal 64 down to low. In this situation, the transfer gates 82, 22 and 60 are turned off, the transfer gate 32 is turned on, and the interconnection W is grounded. Bringing the terminal 36 down to low turns the transfer gate 32 off, and makes the interconnection W float while allowing it to remain under the ground potential. Bringing the terminal 36 down to low also makes output of the logic gate 70 reflect the potential of the interconnection W. Next, the terminal 26 is temporarily brought down to low so as to turn the transfer gate 22 on for a predetermined duration of time, to thereby charge the interconnection W. After the output of the logic gate 70 is stabilized, the terminal 64 is brought up to high so as to turn the transfer gate 60 on, to thereby transmit the output of the logic gate 70 to the memory circuit 50.
If the fuse 10 has been blown by this time, electric charge accumulated on the interconnection W is maintained, and the potential of the interconnection W is consequently kept at the elevated state (high state). This makes an output signal of the logic gate 70 low, and makes the potential of the terminal 86 high.
On the contrary, if the fuse 10 has been kept in connection, the electric charge accumulated on the interconnection W leaks through the terminal 40 to the ground (that is, the charging will not intrinsically occur), and the interconnection W is brought down to the ground potential (low). This brings an output of the logic gate 70 up to high and the terminal 86 down to low.
After the output of the logic gate 70 is kept by the memory circuit 50, the terminal 64 is brought down to low to thereby turn the transfer gate 60 off. This makes it possible to memorize the output of the logic gate 70, that is, a result of judgment on whether the fuse 10 has been blown or not. State of the fuse 10, that is, which bit line should be used, is then judged depending on the logical state (high or low) of the terminal 86.
After the transfer gate 60 is turned off, the transfer gate 32 is turned on. This brings the potential of the interconnection W down to the ground potential, and the potential difference between both ends of the fuse 10 becomes substantially zero. This state is kept unchanged during the normal operation of the semiconductor device 1, and the potential difference applied to the fuse 10 remains zero.
Effect of the semiconductor device 1 will be explained. The semiconductor device 1 has the potential difference reducing circuit 30 provided therein. The potential difference between both ends of the fuse, applied by the potential difference imparting circuit 20 when the judgment of disconnection takes place, can therefore be reduced after the judgment. This make it possible to reduce a risk of short-circuiting of the blown fuse 10.
In particular in the first embodiment, the risk of short-circuiting can distinctively be reduced because the potential difference is reduced to as low as substantially zero by the potential difference reducing circuit 30. This is ascribable to that the potential (third potential) to be applied to the terminal 34 is set equal to the potential (first potential) applied to the end portion 10a of the fuse 10. It is, however, to be understood that the third potential is not necessarily equal to the first potential, and may be not lower than the first potential and not higher than the second potential. The second potential herein expresses the potential applied to the terminal 24 as described in the above.
The fuse 10 is blown by allowing current to flow therethrough, so that it is particularly important to install the potential difference reducing circuit 30. More specifically, such fuse generally produces only a small disconnection gap (a gap between fragments of the fuse produced by the disconnection), and tends to cause short-circuiting due to electro-migration. In contrast, the semiconductor device 1 having the potential difference reducing circuit 30 makes it possible to fully reduce the risk of short-circuiting. The fuse 10 herein is by no means limited to those disconnectable by current. It may be, for example, such as disconnectable by laser. Also for the case of laser disconnection, it is important to provide the potential difference reducing circuit 30, because the disconnection gap has been becoming smaller as the semiconductor device has been more micronized and laser output has been enhanced.
In the semiconductor device 1, the terminal 40 allowing current to flow therethrough to the fuse 10 for disconnection is grounded. This makes it possible to save power consumption of the semiconductor device 1. The terminal 40 may typically be an external terminal formed on the surface of the semiconductor device 1. The terminal 40 is, however, not necessarily be an external terminal, so far as the semiconductor device 1 per se can supply voltage or current sufficient for the disconnection of the fuse 10. If it cannot be supplied, it is all enough to configure the terminal 40 as the external terminal, and to externally supply a large voltage or current.
The potential difference imparting circuit 20 is configured as having the transfer gate 22 and the terminal 24. This successfully realizes the potential difference imparting circuit 20 based on a simple configuration. The potential difference reducing circuit 30 is configured as having the transfer gate 32 and the terminal 34. This successfully realizes the potential difference reducing circuit 30 based on a simple configuration.
The semiconductor device 1 has the memory circuit 50 and the transfer gate 60. In this case, a signal once stored in the memory circuit 50 is prevented from leaking from the memory circuit 50 to the fuse 10 side. This makes it possible to maintain, based on only a simple configuration, a state of potential difference between both ends of the fuse 10 reduced by the potential difference reducing circuit 30.
The semiconductor device 1 has the transmission prevention circuit (logic gate 70) provided therein. In this configuration, the transmission prevention circuit can prevent the signal stored in the memory circuit 50 from flowing back to the fuse 10, when the judgment of disconnection of the fuse 10 takes place. It is therefore made possible to suppress generation of negative feedback with respect to the potential at one end 10b of the fuse 10, so that the risk of misjudgment on whether the fuse has been blown or not is therefore reduced.
Moreover, in the semiconductor device 1, potential signal appeared at one end 10b of the fuse 10 is converted into a high or low output signal by the logic gate 70, and the output signal is obtained as a result of the disconnection judgment of the fuse 10. This makes it possible to further reduce the risk of misjudgment on whether the fuse 10 has been blown or not.
Meanwhile, when the potential of the interconnection W is input to the memory circuit 50 without allowing it to pass through the logic gate 70, the potential to be input to the memory circuit 50 may have a value close to the threshold voltage of the inverter in the memory circuit 50 due to voltage drop. This may fail in inverting the latch, so that a false result of judgment may undesirably be stored in the memory circuit 50. In contrast, the semiconductor device 1 is configured as allowing the output signal of the logic gate 70, having a large current supply capacity, to be stored in the memory circuit 50, so that it is made possible to make the memory circuit 50 correctly store information on whether the fuse 10 has been blown or not.
The logic gate 70 is configured so that the output signal therefrom is transmitted through the transfer gate 60 to be input to the memory circuit 50. The transfer gate 60 is effective in view of preventing the information stored in the memory circuit 50 from leaking to the fuse 10 side, as described in the above. On the other hand, the transfer gate 60 may also be a causal factor of undesirably making, due to voltage drop of its own, the potential to be input to the memory circuit 50 close to the threshold voltage of the inverter. In the semiconductor device 1, it is therefore particularly important to provide the logic gate 70 in order to make the memory circuit 50 store correct information.
The logic gate 70 is involved in a positive feedback loop. This makes it possible to rapidly determine the output signal of the logic gate 70. The first embodiment successfully realizes the positive feedback based on a simple configuration, by using the FET 72 and the terminal 74.
A NAND is used as the logic gate 70. The NAND is preferably used as the logic gate 70. The NAND, which can be configured by a relatively small number of transistors, is also contributive to saving of circuit resources. It is, however, to be noted that the logic gate 70 may be configured by using a logic gate other than NAND, such as an inverter.
Second Embodiment
FIG. 4 is a circuit diagram showing the fuse peripheral circuit in the second embodiment of the semiconductor device of the present invention. The fuse peripheral circuit has the fuse 10, the potential difference imparting circuit 20, the potential difference reducing circuit 30, the terminal 40, the memory circuit 50, the transfer gate 60 and the logic gate 70 (transmission prevention circuit). Of these components, configurations of the fuse 10, the terminal 40, the memory circuit 50, the transfer gate 60 and the logic gate 70 are same as those shown in FIG. 2. In the second embodiment, the terminal 40 is applied with the source potential Vcc.
The potential difference imparting circuit 20 is configured as having the transfer gate 22, the terminal 24 and the terminal 26, so as to produce a predetermined potential difference between both ends of the fuse 10 when the disconnection judgment takes place. The transfer gate 22 herein is an N-type FET. The terminal 24 is connected to the source of the transfer gate 22. The terminal 24 is applied with the ground potential (second potential). In the second embodiment, the second potential has therefore a value smaller than the potential Vcc (first potential) applied to the end portion 10a of the fuse 10.
The potential difference reducing circuit 30 is configured as having the transfer gate 32, the terminal 34, the terminal 36 and an inverter 38, so as to reduce the potential difference between both ends of the fuse 10 applied by the potential difference imparting circuit 20. The transfer gate 32 is a P-type FET. The terminal 34 is connected to the source of the transfer gate 32. The terminal 34 is supplied with the source potential Vcc (third potential). The terminal 36 is connected through the inverter to the gate of the transfer gate 32.
Next paragraphs will describe operations of the disconnection judgment taken place in the circuit shown in FIG. 4, referring to the timing chart shown in FIG. 5. Meanings of the individual reference numerals are same as those shown in FIG. 3. The potential of the terminal 84 is always kept low when the disconnection judgment takes place.
First, the terminal 36 is brought up to high, and the terminals 26, 64 are brought down to low. In this situation, the transfer gates 82, 22 and 60 are turned off, the transfer gate 32 is turned on, and the interconnection W is supplied with the source potential Vcc. Bringing the terminal 36 down to low turns the transfer gate 32 off, and makes the interconnection W float while allowing it to retain the source potential Vcc as is in the state immediately before. Bringing the terminal 36 down to low also makes output of the logic gate 70 reflect the potential of the interconnection W. Next, the terminal 26 is temporarily brought up to high so as to turn the transfer gate 22 on for a predetermined duration of time, to thereby leak the charge accumulated in the interconnection W to the ground. After the output of the logic gate 70 is stabilized, the terminal 64 is brought up to high so as to turn the transfer gate 60 on, to thereby transmit the output of the logic gate 70 to the memory circuit 50.
If the fuse 10 has been blown by this time, the interconnection W is brought down to the ground potential. This makes the output of the logic gate 70 high and the terminal 86 low.
On the contrary, if the fuse 10 has been kept in connection, the interconnection W is charged by the source potential Vcc applied to the terminal 40. This makes the output of the logic gate 70 low, and the potential of the terminal 86 high.
After the output of the logic gate 70 is held by the memory circuit 50, the terminal 64 is brought down to low to thereby turn the transfer gate 60 off. This makes it possible to memorize the output of the logic gate 70, that is, a result of judgment on whether the fuse 10 has been blown or not. State of the fuse 10, that is, which bit line should be used, is then judged depending on the logical state (high or low) of the terminal 86.
After the transfer gate 60 is turned off, the transfer gate 32 is turned on. This brings the potential of the interconnection W into the source potential Vcc, and the potential difference between both ends of the fuse 10 becomes substantially zero. This state is kept unchanged during the normal operation of the semiconductor device 1, and the potential difference applied to the fuse 10 remains zero.
Also the semiconductor device of the second embodiment, having the potential difference reducing circuit 30 provided therein, can reduce the potential difference, which has been applied between both ends of the fuse by the potential difference imparting circuit 20 when the judgment takes place, after the judgment. This successfully reduces a risk of short-circuiting of the blown fuse 10. Moreover, it is made possible to distinctively reduce the risk of short-circuiting, because the potential difference is reduced to as low as substantially zero by the potential difference reducing circuit 30. It is, however, to be understood that the third potential is not necessarily equal to the first potential, and may be higher than the first potential and not higher than the second potential.
There is also provided a transmission prevention circuit (logic gate 70). The transmission prevention circuit can prevent the signal stored in the memory circuit 50 from flowing back to the fuse 10, in the disconnection judgment of the fuse 10. It is therefore made possible to suppress generation of negative feedback with respect to the potential at one end 10b of the fuse 10, and consequently to prevent misjudgment on whether the fuse 10 has been blown or not.
Third Embodiment
FIG. 9 is a circuit diagram showing the fuse peripheral circuit in the third embodiment of the semiconductor device according to the present invention. The fuse peripheral circuit has the fuse 10, the potential difference imparting circuit 20, the potential difference reducing circuit 30, the terminal 40, the memory circuit 50, the transfer gate 60, and a transmission prevention circuit 71. Of these constituents, configurations of the fuse 10, the potential difference imparting circuit 20, the potential difference reducing circuit 30, the terminal 40 and the transfer gate 60 are same as those shown in FIG. 2. The terminal 40 in the third embodiment is grounded.
The memory circuit 50 has a P-type FET 52 and an N-type FET 54 composing a first inverter, and a P-type FET 56 and an N-type FET 58 composing a second inverter. The input end of the first inverter is connected to the transfer gate 60. The output end of the first inverter is connected to the input end of the second inverter.
Between the FET 56 and the FET 58, there is provided a P-type FET 73 and an N-type FET 75 More specifically, the source of the FET 73 is connected to the drain of the FET 56, and the source of the FET 75 is connected to the drain of the FET 58. The FETs 73, 75 are connected with each other through the drains thereof. The gate of the FET 73 is connected to the terminal 64. On the other hand, the gate of the FET 75 is connected through the inverter 62 to the terminal 64. In this configuration, FETs 73, 75 are turned off when the terminal 64 is high, and turned on when low. The FETs 73, 75 in the third embodiment configures a transmission prevention circuit. More specifically, the FETs 73, 75 nullify the second inverter in inverter in the disconnection judgment, that is, bring the FET 56 and the FET 58 into a non-conductive state. This successfully prevents the signal stored in the memory circuit 50 from being transmitted to the fuse 10.
Next paragraphs will describe operations of the disconnection judgment taken place in the circuit shown in FIG. 9, referring to the timing chart shown in FIG. 10. Meanings of the individual reference numerals in FIG. 10 are same as those shown in FIG. 3. The potential of the terminal 84 is always kept low when the disconnection judgment takes place.
First, the terminals 26, 36 are brought up to high, and the terminal 64 is brought down to low. In this situation, the transfer gates 82, 22 and 60 are turned off, the transfer gate 32 is turned on, and the interconnection W is grounded. Bringing the terminal 36 down to low in the next turns the transfer gate 32 off, and makes the interconnection W float while allowing it to retain the ground potential as is in the state immediately before. Next the terminal 64 is brought up to high to thereby turn the transfer gate 60 on, to thereby make the potential of the interconnection W transmittable to the memory circuit 50. At the same time, the terminal 26 is temporarily brought down to low so as to turn the transfer gate 22 on for a predetermined duration of time, to thereby charge the interconnection W.
If the fuse 10 has been blown by this time, electric charge accumulated on the interconnection W is maintained, and the potential of the interconnection W is consequently kept at the elevated state (high level). The memory circuit 50 therefore has a high input, and makes the potential of the terminal 86 low.
On the contrary, if the fuse 10 has been kept in connection, the electric charge accumulated on the interconnection W leaks through the terminal 40 to the ground (that is, the charging will not intrinsically occur), and the interconnection W is brought down to the ground potential (low level). The memory circuit 50 therefore has a low input, and makes the potential of the terminal 86 high.
The terminal 64 is then brought down to low to thereby turn the transfer gate 60 off, and turn the FETs 73, 75 on. This makes it possible to memorize a result of judgment on whether the fuse 10 has been blown or not into the memory circuit 50. State of the fuse 10, that is, which bit line should be used, is then judged depending on the logical state (high or low) of the terminal 86.
After the transfer gate 60 is turned off, the transfer gate 32 is turned on. This brings the potential of the interconnection W down to the ground potential, and makes the potential difference between both ends of the fuse 10 substantially zero. This state is kept unchanged during the normal operation of the semiconductor device 1, and the potential difference applied to the fuse 10 remains zero.
Also the semiconductor device of the third embodiment, having the potential difference reducing circuit 30 provided therein, can reduce the potential difference, which has been applied between both ends of the fuse by the potential difference imparting circuit 20 when the judgment takes place, after the judgment. This successfully reduces a risk of short-circuiting of the blown fuse 10. Moreover, it is made possible to distinctively reduce the risk of short-circuiting, because the potential difference is reduced to as low as substantially zero by the potential difference reducing circuit 30.
There is also provided a transmission prevention circuit 71. The transmission prevention circuit 71 makes it possible to prevent the signal stored in the memory circuit 50 from flowing back to the fuse 10, when the disconnection judgment of the fuse 10 takes place. It is therefore made possible to suppress generation of negative feedback with respect to the potential at one end 10b of the fuse 10, and consequently to prevent misjudgment on whether the fuse 10 has been blown or not.
The semiconductor device of the present invention is by no means limited to the above embodiments, and may be modified in various ways. For example, as shown in FIG. 6, the logic gate 70 may be configured as being not involved in the positive feedback loop. The circuit shown in this drawing corresponds to the circuit shown in FIG. 2, having the FET 72 and the terminal 74 excluded therefrom.
It is also allowable, as shown in FIG. 7, to use a single transfer gate both as the transfer gate of the potential difference reducing circuit and as the transfer gate supplying current to the fuse for the disconnection. The circuit shown in this drawing corresponds to the circuit shown in FIG. 6, having the transfer gate 32, the terminal 34 and the inverter 88 excluded therefrom. In this circuit, the transfer gate 82, the terminal connected to the transfer gate 82 and the terminal 84 configure the potential difference reducing circuit.
It is still also allowable to adopt a configuration having no potential difference reducing circuit 30 provided therein. Even this configuration makes it possible to reduce the risk of misjudgment on whether the fuse 10 has been blown or not, by virtue of provision of the transmission prevention circuit.
It is apparent that the present invention is not limited to the above embodiments, and may be modified and changed without departing from the scope and spirit of the invention.

## Claims

1. A semiconductor device comprising:
a fuse;
a memory circuit storing a signal which indicates result of judgment on whether said fuse has been blown or not;
a transmission prevention circuit preventing, when said judgment takes place, said signal stored in said memory circuit from being transmitted to said fuse; and
a transfer gate provided between said fuse and said memory circuit, wherein said transfer gate is turned on when said judgment takes place, and is turned off when said judgment does not take place.

2. The semiconductor device as claimed in claim 1, wherein said transmission prevention circuit has a logic gate allowing input of a potential signal at one end of said fuse, and outputting a high or low output signal depending on said potential signal to said memory circuit.

3. The semiconductor device as claimed in claim 2, wherein said logic gate is involved in a positive feedback loop.

4. The semiconductor device as claimed in claim 2, wherein said logic gate is a NAND circuit.

5. A semiconductor device comprising:
a fuse;
a memory circuit storing a signal which indicates result of judgment on whether said fuse has been blown or not; and
a transmission prevention circuit preventing, when said judgment takes place, said signal stored in said memory circuit from being transmitted to said fuse,
wherein said transmission prevention circuit has a logic gate allowing input of a potential signal at one end of said fuse, and outputting a high or low output signal depending on said potential signal to said memory circuit.

6. The semiconductor device as claimed in claim 5, further comprising a transfer gate provided on a path between said logic gate and said memory circuit;
said output signal being input through said transfer gate into said memory circuit.

7. The semiconductor device as claimed in claim 5, wherein said logic gate is involved in a positive feedback loop.

8. The semiconductor device as claimed in claim 5, wherein said logic gate is NAND circuit.

9. A semiconductor device comprising:
a fuse;
a memory circuit storing a signal which indicates result of judgment on whether said fuse has been blown or not; and
a transmission prevention circuit provided between said fuse and said memory circuit, said transmission prevention circuit preventing, when said judgment takes place, said signal stored in said memory circuit from being transmitted to said fuse.

10. The semiconductor device as claimed in claim 9, wherein said transmission prevention circuit has a logic gate allowing input of a potential signal at one end of said fuse, and outputting a high or low output signal depending on said potential signal to said memory circuit.

11. The semiconductor device as claimed in claim 10, further comprising a transfer gate provided on a path between said logic gate and said memory circuit;
said output signal being input through said transfer gate into said memory circuit.

12. The semiconductor device as claimed in claim 10, wherein said logic gate is involved in a positive feedback loop.

13. The semiconductor device as claimed in claim 10, wherein said logic gate is a NAND circuit.