A semiconductor read only memory

Abstract: A semiconductor read only memory is disclosed. The memory has a memory cell array including a plurality of memory cell groups. Each of the plurality of memory cell groups includes a plurality of memory cells. The memory has select circuit for selecting an arbitrary memory cell group from among the plurality of memory cell groups, and an address storage circuit for storing address information of an arbitrary one of the memory cell groups; and a data storage circuit for storing data to be written into the arbitrary one of the memory cell groups.

Description: 1. Field of the Invention: The present invention relates to a semiconductor read only memory, particularly to a mask ROM (read only memory) into which information can be written at a fabrication stage. 2. Description of the Related Art: In recent years, semiconductor read only memories such as mask ROMs (Read Only Memories) have much finer device elements and larger memory capacity, and thus improvement in the production yield thereof becomes a prime issue. In order to improve the production yield, various structures of mask ROMs have been proposed so far. One example is a mask ROM provided with an error-correcting circuit, which has already been put into practical use. A second example is a mask ROM provided with redundant memory cells, such as conventionally employed in a RAM (Random Access Memory), etc., for a memory cell column (row) arranged along a bit line (a word line). According to the second example, in cases where a defective memory cell is found in a memory cell array, the redundant memory cells are substituted for the memory cell column or row including the defective memory cell. The mask ROM provided with the error-correcting circuit has an advantage that the time for dealing with and repairing the defective memory cell can be shortened, but has a disadvantage that the chip area is increased by 20% or more due to additional inclusion of the error-correcting circuit and memory cells for correcting any errors. Moreover, such a mask ROM provided with the redundant memory cells cannot readily be put to practical use unlike a case of the RAM, etc, because not only address information but much more data information should be written into the mask ROM. In cases where several bits of memory cells show some defect, not only information of the defective memory cell but also that of normal memory cells connected to the same line to which the defective memory cell is connected should be written into each fuse element of the mask ROM. Hereinafter, an exemplary structure of such a mask ROM will be described referring to Figure 8 , assuming that 1024 or more memory cells are connected to a common word line or bit line. As is shown in Figure 8 , the mask ROM comprises a sense amplifier 40 , a row decoder 60 , bit lines 90 , a word line i , a selection circuit 50 , a substitute address storage section 20 , and a substitute data storage section 21 . In cases where a defective memory cell is connected to the word line i , address information to specify the word line i is written into the substitute address storage section 20 provided with a plurality of fuse elements (not shown) by disconnecting prescribed fuse elements with radiation from laser light. Furthermore, information from all the memory cells (1024 or more memory cells) including the defective memory cell, connected to the word line i , is written into the substitute data storage section 21 provided with a plurality of fuse elements (not shown) by disconnecting prescribed fuse elements with radiation from laser light. As a result, if the memory cell connected to the word line i is accessed, a signal output from the substitute address storage section 20 is activated. Accordingly, the information stored in the substitute data storage section 21 is output via the selection circuit 50 . According to such a mask ROM, many fuse elements are inevitably disconnected with radiation from laser light, thereby substantially increasing the time needed for dealing with the laser light. Furthermore, the production yield cannot practically be improved because the percentage of success of the disconnection by laser light is decreased or the like. In cases where the source or drain of a transistor included in the memory cell array shows a defect, it often occurs- that the other memory cells connected to the bit line to which the transistor is connected cannot be read out correctly. The document EP-A-383,452 discloses a semiconductor memory array, which may be divided into cell blocks, including means for replacing defective memory cells. SUMMARY OF THE INVENTION According to the present invention, there is provided a semiconductor read-only memory according to claim 1. In one embodiment, the address storage means has a plurality of fuse elements to be disconnected by laser light or electrical means, and stores address information in accordance with the conductive or non-conductive state of each fuse element. In one embodiment, the data storage means has a plurality of fuse elements to be disconnected by laser light or electrical means, and stores data in accordance with the conductive or non-conductive state of each fuse element. In one embodiment, the address storage means has a plurality of fuse elements to be disconnected by laser light or electrical means, and stores address information in accordance with the conductive or non-conductive state of each fuse element; and the data storage means has a plurality of fuse elements to be disconnected by laser light or electrical means, and stores data in accordance with the conductive or non-conductive state of each fuse element. Thus, the invention described herein makes possible the advantage of providing a semiconductor read only memory improved in production yield without enlarging the chip area nor increasing the time needed for dealing with defective memory cells. This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a structure for a memory cell array of a semiconductor read only memory according to an example of the present invention. Figure 2 shows the structure of the semiconductor read only memory according to the example. Figure 3 shows the structure of a substitute bank address storage circuit according to the example. Figure 4 shows the structure of a substitute data storage circuit according to the example. Figure 5 shows the structure of a substitute data bit identification storage circuit according to the example. Figure 6 shows the structure of a switching circuit according to the example. Figure 7 shows a structure for a memory cell array of a semiconductor read only memory according to another example of the present invention. Figure 8 shows a structure for a conventional mask ROM. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described by way of illustrating examples referring to figures. Figure 1 shows a structure for a memory cell array of a semiconductor read only memory according to an example of the present invention. The memory cell array comprises a plurality of memory cell blocks (hereinafter, referred to as "banks") shown in broken lines in Figure 1 : odd-numbered banks B m,2i-1 , B m+1,2i-1 , ..., etc. and even-numbered banks B m,2i , B m+1,2i , ..., etc. In each bank, a plurality of memory cells are arranged in a column direction. Between the odd-numbered banks ( B m,2i-1 , ..., etc.) and the even-numbered banks ( B m,2i , ..., etc.) arranged in a row direction, there are formed sub-bit lines (first bit lines) SB m,2i-2 , SB m,2i-1 , SB m,2i , ..., etc. made of a diffused layer. The sub-bit lines are each connected to the memory cells M included in each bank. Main bit lines (second bit lines) MB i-1 , MB i , ..., etc. each extend along the column direction and made of a metal layer with low-resistance. Two adjacent sub-bit lines are connected to a common main bit line via two adjacent bank select MOSFETs (transfer gate transistors), respectively. For example, sub-bit lines SB m,2i-1 and SB m,2i are connected to a main bit line MB i through bank select MOSFETs QO m,2i-1 and QO m,2i , respectively. The gates of the bank select MOSFETs QO m,2i-2 , QO m,2i-1 , QO m,2i , ..., etc. are each connected to a bank select line BO m . The gate of each memory cell (transistor) is connected to a word line ( WL 1 , ..., or WL n ). Furthermore, the sub-bit lines SB m,2i-2 , SB m,2i-1 , SB m,2i , ..., etc. are connected to bank select MOSFETs QE m,2i-2 , QE m,2i-1 , QE m,2i , ..., etc., respectively. The gates of the bank select MOSFETs QE m,2i-1 , QE m,2i , ..., etc. are each connected to a bank select line BE m . The main bit lines MB i-1 , MB i , ..., etc. are each connected to sense amplifiers and to GND via transistors Q i-1 , Q i+1 , ..., etc., respectively. Thus, the memory cell array of the present example is divided into a plurality of banks, and the plurality of banks each have a plurality of memory cells. According to such a memory cell array, in cases where some defect occurs in a diffused layer constituting a sub bit line, only memory cells connected to the diffused layer will become defective. Figure 2 shows the structure of the semiconductor read only memory according to the present example. As is shown in Figure 2 , the semiconductor read only memory comprises the memory cell array 1a having a plurality of memory cells (the structure thereof is described above); an address buffer 5 for receiving an address input signal from the outside; a row decoder 2 and a column decoder 3 for selecting an arbitrary memory cell from among the plurality of memory cells in accordance with the address input signal; a sense amplifier 4 for reading out data from the selected memory cell; and an output circuit 6 for outputting data in accordance with a signal output from the sense amplifier 4 . The semiconductor read only memory further comprises a switching circuit 7 provided between the sense amplifier 4 and the output circuit 6 ; a substitute bank address storage circuit section 8 ; a substitute data storage circuit section 9 ; and a substitute data bit identification storage circuit section 10 . The address input signal is transmitted from the outside via the address buffer 5 to the row decoder 2 and the column decoder 3 for selecting an arbitrary memory cell in an arbitrary bank from among the memory cell array 1a. The address input signal is also input into the substitute bank address storage circuit section 8 and the substitute data storage circuit section 9 via the address buffer 5 . The substitute bank address storage circuit section 8 stores the address information of the bank including the defective memory cell. In accordance with the signal output from the substitute bank address storage circuit section 8 , it is detected which bank should be substituted. The signal output from the substitute bank address storage circuit section 8 is transmitted to the substitute data storage circuit section 9 and to the substitute data bit identification storage circuit section 10 . The substitute data storage circuit section 9 thus receives the signal output from the substitute bank address storage circuit section 8 , and then stores information for each memory cell of the bank to be substituted. On the other hand, the substitute data bit identification storage circuit section 10 receives the signal output from the substitute bank address storage circuit section 8 , and then stores which one of the sense amplifiers (1 to m) will receive the address information on the defective bank to be read out from the memory cell array 1a . The switching circuit 7 masks a signal output from the sense amplifier 4 when the defective bank is read out, in response to a signal (BIT) output from the substitute data bit identification storage circuit section 10 . Thus, a signal (RDATA) output from the substitute data storage circuit section 9 rather than the signal output from the sense amplifier 4 is transmitted to the output circuit 6 via the switching circuit 7 . Figure 3 shows an exemplary structure for the substitute bank address storage circuit section 8 for storing address information on the defective bank including the defective memory cell. The substitute bank address storage circuit section 8 comprises an address decoder 11 , a plurality of lines, and a plurality of transistors. The gate electrodes of the transistors are each connected to an output section of the address decoder 11 via the plurality of lines. One of the source and drain of each transistor is grounded, and the other of them is connected to an output terminal of the substitute bank address storage circuit section 8 via one of the polysilicon fuse elements ( AF 11 to AF n1 , ..., and AF 1j to AF nj ), and serially connected to two inverters. Decoded output signals Q 1 to Q n of the address decoder 11 are transmitted to the plurality of lines, respectively. Output signals RAD 1 to RAD j are output from the output terminals and then each transmitted to the substitute data storage circuit section 9 and to the substitute data bit identification storage circuit section 10 . When the address decoder 11 of the substitute bank address storage circuit section 8 receives the address signal to specify the bank, the address decoder 11 makes one of the decoded output signals Q 1 to Q n activated ("High" level). The transistor connected to the line to which the activated decoded output signal is transmitted becomes electrically conductive. As a result, each potential of the polysilicon fuse elements ( AF 11 to AF n1 , ..., and AF 1j to AF nj ) on the side of the transistor is reduced to the grounded level. In cases where a bank in the memory cell array 1a become defective, in accordance with the bank address to specify the bank, the corresponding one of each polysilicon fuse element group ( AF 11 to AF n1 , ..., and AF 1j to AF nj ) is disconnected. Thus, the address information of the defective bank including the defective memory cell is stored. Figure 4 shows an exemplary structure for the substitute data storage circuit section 9 . The substitute data storage circuit section 9 receives the address signal for selecting the defective memory cell from the address buffer 5 , and signals RAD 1 to RAD j output from the substitute bank address storage circuit section 8 . As is described above, the substitute data storage circuit section 9 stores information for each memory cell of the bank to be substituted. As is shown in Figure 4 , the substitute data storage circuit section 9 comprises an address decoder 12 , a plurality of lines, and a plurality of first transistors. The gate electrodes of the first transistors are each connected to an output section of the address decoder 12 via the plurality of lines. One of the source and drain of each first transistor is grounded, and the other thereof is connected to an output terminal of the substitute data storage circuit section 9 via a second transistor and two serially connected inverters. ,Signals P 1 to P h output from the address decoder 12 are transmitted to the plurality of lines, respectively. Output signal RDATA is output from the output terminal. The output signals RAD 1 to RAD j of the substitute bank address storage circuit section 8 are applied to the gate electrodes of the second transistors, respectively. In Figure 4 , in cases where a word line WL 1 (Figure 2 ) is selected, the signal P 1 output from the address decoder 12 is activated ("High" level). On the other hand, in cases where a word line WL h (Figure 2 ) is selected, the signal P h is activated ("High" level). Previously, the polysilicon fuse elements included in the substitute data storage circuit section 9 can selectively be disconnected in accordance with the information ("1" or "0") of memory cells M k1 to M kh of a defective bank k (Figure 2 ), so that information on each memory cell (substitute information) of the bank to be substituted can be stored. For example, in cases where some defects occur in the memory cells M k1 to M kh of the bank K, and the memory cell M k1 is accessed, the signal Q k output from the address decoder 11 (Figure 3 ) is activated ("High" level). In cases where the polysilicon fuse element AF k1 is previously disconnected, the signal RAD 1 is activated ("High" level). In cases where a bank other than the bank K is accessed, the output signal Q k of the address decoder 11 is not activated, and thus the signal RAD 1 is not activated either, thereby preventing errors in the substitution operation. In cases where a memory cell M k1 is selected from among memory cells M k1 to M kh , the signal P 1 output from the address decoder 12 (Figure 4 ) is activated ("High" level). Therefore, information stored in the polysilicon fuse DF 11 is transmitted to the RDATA. Figure 5 shows an exemplary structure for the substitute data bit identification storage circuit section 10 . The substitute data bit identification storage circuit section 10 receives the signals RAD 1 to RAD j output from the substitute bank address storage circuit section 8 . As is shown in Figure 5 , the substitute data bit identification storage circuit section 10 comprises a plurality of lines for receiving the signals RAD 1 to RAD j , and a plurality of transistors whose gate electrodes are connected to the plurality of lines, respectively. One of the source and drain of each transistor is grounded via one of the polysilicon fuse elements ( BF 11 to BF j1 , ..., and BF k1 to BF kj ), and the other thereof is connected to an output terminal of the substitute data bit identification storage circuit section 10 via two serially connected inverters. In order to identify a sense amplifier signal bit (1 to m) corresponding to the defective bank, one of each polysilicon fuse element group ( BF 11 to BF j1 , ..., and BF k1 to BF kj ) is previously disconnected. Accordingly, only one of the output signals BIT 1 to BIT m can be activated. Figure 6 shows an exemplary structure for the switching circuit 7. The switching circuit 7 receives a sense amplifier signal S k output from the sense amplifier 4, an RDATA signal output from the substitute data storage circuit section 9 , and a BIT k signal output from the substitute data bit identification storage circuit section 10 . In accordance with the BIT k signal, the switching circuit 7 masks the sense amplifier signal S k of the defective memory cell information, and then outputs the RDATA signal output from the substitute data storage circuit section 9 as an output signal D k to the output circuit 6 . In cases where one of the other memory cells M k2 to M kh included in the bank K (Figure 1 ) is accessed, the same operation will be performed. As is described above, in cases where several bits of memory cells become defective, all the information of normal memory cells connected to the line to which the defective memory cells are connected is not required to be written into each fuse element. Therefore, the number of fuse elements to be disconnected can greatly be reduced, resulting in remarkable reduction in the repair process time. Furthermore, the production yield can be improved because the percentage of success of disconnection by the laser light is improved. Thus, according to the present example, the production yield can be improved while almost maintaining the chip area and the process time for repairing defective memory cells. The present invention is not limited to a ROM having the memory cell array 1a shown in Figure 1 but can be applied to a ROM having a memory cell array such as shown in Figure 7 . The memory cell array 1b is divided into a plurality of memory cell banks. Each of the memory cell banks includes a memory column in which a plurality of memory cells are connected in series, and select MOSFETs which are serially connected to the memory cell column. The memory cell banks are connected to an associated main bit line in parallel. Furthermore, the method for disconnecting the fuse elements is not limited to the above mentioned method using laser light, but another method such as an electrical means can also be employed. Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.

Claims: A semiconductor read only memory comprising: a memory cell array (1a) including a plurality of memory cell banks (B); selecting means (QO, QE) for selecting an arbitrary memory cell bank from among said plurality of memory cell banks; address storage means (8) for storing address information of an arbitrary one of said memory cell banks; and data storage means (9) for storing data to be written into said arbitrary one of said memory cell banks; characterised in that said memory cell array comprises a plurality of first bit lines (SB) made of impurity diffusion layers and a plurality of second bit lines (MB) made of metal layers; and said selecting means interconnects said first bit lines to said second bit lines. A semiconductor read-only memory according to claim 1, wherein said plurality of memory cell banks (B) each includes a plurality of memory cell MOS transistors provided between two adjacent first bit lines forming source/drain regions of said plurality of MOS transistors. A semiconductor read-only memory according to claim 1, wherein each of the plurality of memory cell banks (B) includes a memory cell column in which a plurality of memory cells are connected in series and a plurality of select MOSFETs, which are serially connected to said memory cell column, and the plurality of memory cell banks (B) are connected to an associated main bit line in parallel. A semiconductor read-only memory according to any preceding claim, wherein said address storage means (8) has a plurality of fuse elements (AF) disconnectable by laser light or electrical means, and stores information in accordance with the conductive or non-conductive state of each fuse element (AF). A semiconductor read-only memory according to any preceding claim, wherein said data storage means (9) has a plurality of fuse elements (DF) disconnectable by laser light or electrical means, and stores data in accordance with the conductive or non-conductive state of each fuse element (DF). A semiconductor read-only memory according to any preceding claim, wherein the address storage means (8) stores the address information of one of said memory cell banks (B) including a defective memory cell, and it is detected which memory cell bank (B) should be substituted in accordance with a signal output from the address storage means (8). A semiconductor read-only memory according to any preceding claim, wherein the signal output from the address storage means (8) is transmitted to a substitute data storage circuit section (9) and to a substitute data bit identification storage circuit section (10). A semiconductor read-only memory according to claim 7, wherein the substitute data bit identification storage circuit section (10) receives the signal output from the substitute bank address storage circuit section (8) and then stores which one of the sense amplifiers (1 to m) receives the address information on the defective memory cell bank (B) to be read out from the memory cell array (1a).

Inventors: HOTTA YASUHIRO, HOTTA, YASUHIRO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1993
Main classification: G06F11
IPC codes: G11C29, G11C17