Abstract:
The stacked semiconductor memory device of the present invention has the object of reducing the cost of developing a wide variety of memory devices and includes: a memory cell array chip that is equipped with memory cell arrays, an interface chip that is stacked with the memory cell array chip and that is provided with a memory configuration switching circuit for changing the input/output bit configuration of the memory cell arrays, and a plurality of interchip wires for connecting the memory cell array chip and the interface chip.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a semiconductor memory device, and more particularly to a semiconductor memory device in which a memory cell array chip and an interface chip for changing the memory configuration are stacked.  
         [0003]     2. Description of the Related Art  
         [0004]     With the miniaturization and increased integration of semiconductor integrated circuits, DRAM (Dynamic Random Access Memory) and SRAM (Static Random Access Memory) have achieved memory having greatly increased capacity. However, the miniaturization of semiconductors has limits, and there is consequently a need for the introduction of new techniques for achieving greater increases in capacity.  
         [0005]     A three-dimensional semiconductor in which semiconductor chips are stacked has been proposed as one technique for achieving greater capacity. Japanese Patent Laid-Open Publication No. H04-196263 discloses the realization of a large-scale integrated circuit in which semiconductor chips are stacked without changing the chip area. In this circuit, a memory circuit is integrated on a separate chip that is stacked on the main semiconductor integrated circuit. Japanese Patent Laid-Open Publication No. 2002-26283 and Japanese Patent Laid-Open Publication No. 2003-209222 disclose multilayer memory structures in which memory cell arrays are realized with multiple layers to achieve even greater increases in capacity.  
         [0006]     In the invention that is described in Japanese Patent Laid-Open Publication No. 2002-026283, memory peripheral circuits are not provided for each memory chip of a multilayered memory chip, but rather, each of the memory chips shares one pair of peripheral circuits to realize a reduction in the area of the peripheral circuits that occupy the chip area.  
         [0007]     In the invention that is described in Japanese Patent Laid-Open Publication No. 2003-209222, memory layers can be sorted following fabrication of a multilayered memory device to enable elimination of defective memory layers.  
         [0008]     However, although each of the above-described multilayered memory devices enables an increase in memory capacity within a limited chip area, no disclosure is made regarding the number of input/output bits and the number of banks, which are constituent elements of the memory, or regarding the data transfer rate.  
         [0009]     On the other hand, the increased performance of the CPU, that accompanies memory, calls for not only greater capacity of the memory device, but also for an increase in the input/output bits, an increase in the number of banks, and a higher memory transfer rate. The diversification of systems has resulted in a wide variety of memory configurations, and as a result, many varieties of memory devices must be developed, and the cost of developing these memory devices grows with each year.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention was achieved in order to address factors and has as its object a stacked semiconductor memory device in which memory cell arrays and circuits for changing the memory cell arrays are stacked on separate chips, and the chips then interconnected using a multiplicity of wires having little delay; this stacked semiconductor memory device thus enabling a wider range of reconfiguration of the memory configuration, particularly of the input/output bit configuration and the input/output prefetch configuration that relates to the memory transfer rate, and allowing a reduction of the cost of developing a wide variety of memory devices.  
         [0011]     The stacked semiconductor memory device of the present invention includes: a memory cell array chip that is provided with a memory cell array; an interface chip that is stacked with the memory cell array chip and that is provided with memory configuration switching circuits for changing the input/output bit configuration of the memory cell array; and a multiplicity of interchip wires for connecting the memory cell array chip and the interface chip.  
         [0012]     In this case, a plurality of memory cell array chips may be stacked, and the memory configuration switching circuit may change the input/output bit configurations of the memory cell arrays that are provided on each of the plurality of stacked memory cell array chips.  
         [0013]     In addition, the memory configuration switching circuit may include: a plurality of switches that are provided for each group of a prescribed number of the plurality of memory units that make up a memory cell array, wherein the number of input/output bits of each memory unit group changes according to the switched state of these switches; and a bit switching circuit for changing the number of input/output bits of the corresponding memory unit group by switching the switched state of the plurality of switches.  
         [0014]     In this case, the memory configuration switching circuits may include: a plurality of latch circuits that are each connected to a respective one of a plurality of data buses for latching the data in each data bus, input and output of input/output data of the plurality of memory unit groups passing via these data buses; and a control circuit for controlling the number of bits of input/output data and the transfer rate by controlling the latch timing in the plurality of latch circuits.  
         [0015]     In addition, the control circuit may change the number of input/output bits of the memory unit groups by way of the bit switching circuits, and may implement control such that the number of input/output bits of the memory unit groups is equal to the number of bits of input/output data.  
         [0016]     In addition, the control circuit may be equipped with fuses that are provided in the wires, and according to the cut state of these fuses, may generate signals for controlling the number of input/output bits of the memory unit groups, or may control the number of bits of input/output data and the transfer rate.  
         [0017]     In addition, the control circuit may be a logic circuit that operates in accordance with the combination of bonding option input signals, and may generate signals that control the number of input/output bits of the memory unit groups or the number of bits of input/output data and the transfer rate. The stacked semiconductor memory device according to any of the above descriptions may be DRAM.  
         [0018]     According to the present invention, in a memory device in which memory cell array chips and interface chips are stacked, the input/output bit configuration of the memory cell array chips and the data transfer rate are changed by the memory configuration switching circuits of the interface chips. In this way, the cost of developing a wide variety of memory devices can be reduced.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  shows the configuration of a stacked memory device;  
         [0020]      FIG. 2  shows the configuration of a multilayered stacked memory device;  
         [0021]      FIG. 3  shows the switching means of the input/output bit configuration,  
         [0022]      FIG. 3   a  showing a 4-bit configuration,  
         [0023]      FIG. 3   b  showing an 8-bit configuration,  
         [0024]      FIG. 3   c  showing a 16-bit configuration, and  
         [0025]      FIG. 3   d  showing a 4n-bit configuration;  
         [0026]      FIG. 4  shows a 512-Mb stacked DRAM device for explaining a working example of the present invention;  
         [0027]      FIG. 5   a  shows the banks of a DRAM memory cell array, and  
         [0028]      FIG. 5   b  shows a 4-Mb memory unit;  
         [0029]      FIG. 6  shows the bit assignment of a memory cell array,  
         [0030]      FIG. 6   a  showing a 4-bit configuration,  
         [0031]      FIG. 6   b  showing an 8-bit configuration,  
         [0032]      FIG. 6   c  showing a 16-bit configuration,  
         [0033]      FIG. 6   d  showing a 32-bit configuration,  
         [0034]      FIG. 6   e  showing a 64-bit configuration, and  
         [0035]      FIG. 6   f  showing a 128-bit configuration;  
         [0036]      FIG. 7  shows a conceptual view of a memory configuration switching circuit;  
         [0037]      FIG. 8  shows the configuration of a bit switching circuit;  
         [0038]      FIG. 9  shows the rearrangement of wires realized by switching operations,  
         [0039]      FIG. 9   a  showing a 4-bit configuration,  
         [0040]      FIG. 9   b  showing an 8-bit configuration,  
         [0041]      FIG. 9   c  showing a 16-bit configuration, and  
         [0042]      FIG. 9   d  showing a 32-bit configuration;  
         [0043]      FIG. 10  is a conceptual view of the assignment of banks of a memory cell array chip;  
         [0044]      FIG. 11  shows the number of words that correspond to configurations of a number of bits and a number of banks of a 512-Mb DRAM;  
         [0045]      FIG. 12  shows the configuration of a circuit for switching the data transfer rate; and  
         [0046]      FIG. 13  shows the clock waveforms that are applied as input to a latch when switching the data transfer rate. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0047]     Explanation next regards working examples of the present invention with reference to the accompanying figures.  
         [0048]      FIG. 1  is a schematic view of the configuration of a working example of the stacked memory device according to the present invention.  
         [0049]     As shown in  FIG. 1 , the present working example is made up of: memory cell array chip  101 , memory unit  102 , input/output buffer  103 , address buffer  104 , interface chip  105 , interchip wires  106 , memory configuration switching circuit  107 , address buffer  108 , row decoder  109 , column decoder  110 , and memory cell array  111 .  
         [0050]     Memory cell array  111  is made up from a plurality of memory units  102  that are arranged in a matrix; this matrix being integrated with row decoder  109  and column decoder  110  to make up memory cell array chip  101 . Interface chip  105  is made up of address buffer  104 , input/output buffer  103 , and memory configuration switching circuit  107 ; is stacked with memory cell array chip  101 ; and is connected to memory cell array chip  101  by interchip wires  106 .  
         [0051]     Memory cell array  111  of memory cell array chip  101  and memory configuration switching circuit  107  of interface chip  105 , taking advantage of the stacked structure, are connected by the multiplicity j×k of interchip wires  106  to enable highly parallel data transmission. Due to the stacked configuration, the chip area is smaller than for a memory device of the same capacity having a planar configuration of the prior art. In addition, stacking a plurality of memory cell array chips  101  as shown in  FIG. 2  allows a further increase in area efficiency.  
         [0052]     Explanation next regards the means for switching the input/output bit configuration of the memory device. Memory cell array  111  is made up of n memory units  102 . Memory cell array  111  is connected to memory configuration switching circuit  107  by a data bus of four bits (DQ 0 , DQ 1 , DQ 2 , DQ 3 ) that is made up of four interchip wires  106  from each memory unit  102 , the total number of bits of the data buses being 4n.  
         [0053]     If the data buses from all n memory units are organized by DQ by means of memory configuration switching circuit  107  as shown in  FIG. 3   a , the number of input/output bits is 4. Arranging the data buses from n/2 memory units as shown in  FIG. 3   b  results in eight input/output bits, arranging the data buses from n/4 memory units as shown in  FIG. 3   c  results in 16 input/output bits, and if the data buses from n memory units are connected in parallel to the input/output buffer without alteration, 4n-bit input/output configuration is realized as shown in  FIG. 3   d.    
         [0054]     As described in the foregoing explanation, switching the data bus wires between chips having a large parallel number, by means of memory configuration switching circuit  107  on interface chip  105 , enables switching of the input/output bit configuration of the memory device over a wide range from a 4-bit to a 4n-bit configuration on one memory cell array chip  101 .  
         [0055]     In addition, providing latches on the data buses of memory configuration switching circuit  107  in interface chip  105 , prefetching data, and then controlling the timing of clocks to the latches enables a doubling or a quadrupling of the data transfer rate with an external interface without changing the clock frequency of memory cell array  111 .  
         [0056]     Explanation next regards the details of a working example of the present invention with reference to the accompanying figures.  
         [0057]      FIG. 4  shows the configuration of a working example of a 512-Mb stacked DRAM device according to the present invention. In  FIG. 4 , memory cell array chip  401 , memory units  402 , input/output buffer  403 , address buffer  404 , interface chip  405 , interchip wires  406 , memory configuration switching circuit  407 , address buffer  408 , row decoder  409 , column decoder  410 , and memory cell arrays  411  are each equivalent to memory cell array chip  101 , memory unit  102 , input/output buffer  103 , address buffer  104 , interface chip  105 , interchip wires  106 , memory configuration switching circuit  107 , address buffer  108 , row decoder  109 , column decoder  110 , and memory cell array  111 , respectively, that are shown in  FIG. 1 .  
         [0058]     Memory cell array chip  401  is stacked on interface chip  405 . Memory cell array chip  401  has a four-bank configuration (BK 0 , BK 1 , BK 2 , BK 3 ), each bank being provided with 128-Mb memory cell array  411 , row decoder  409 , and column decoder  410 . Interface chip  405  is provided with memory configuration switching circuit  407 , address buffer  404 , and input/output buffer  403 . Memory cell array  411  of memory cell array chip  401  and memory configuration switching circuit  407  of interface chip  405  are connected by data buses, and row decoder  409  and column decoder  410  are connected to address buffer  404  by an address bus. Each of these buses employs interchip wires  406 , which are through-type wires having an extremely short distance of 100 μm or less that pass through the chip; and these data buses, due to their large diameter in the order of 20 μm and low resistance, enable high-speed transfer in the GHz order.  
         [0059]      FIG. 5  presents a detailed view of memory cell array  411 ,  FIG. 5   a  showing the details of the bank portion, and  FIG. 5   b  showing the internal configuration. 128-Mb memory cell array  411  is made up of 32 4-Mb memory units  501 . Four interchip wires  406  extend from each 4-Mb memory unit  501 , these interchip wires  406  constituting 4-bit data buses. 4-Mb memory unit  501  has a 4-bit 1-Mword configuration, and as shown in  FIG. 5   b , regarding the internal configuration of 4-Mb memory unit  501 , 1024 data select lines  502  from column decoder  410  and 1024 word lines  503  from row decoder  409  connect with the memory unit, and memory cells  505  exist at the four DQ portions DQ 0 , DQ 1 , DQ 2 , and DQ 3  at the intersections of data select lines  502  and word lines  503 .  
         [0060]     When reading data, the small-amplitude differential signal of the data line that depends on the data of each memory cell  505  is converted to a full-amplitude signal by sense amplifier  504  and data amplifier  507  and sent to the data buses of the four bits DQ 0 , DQ 1 , DQ 2 , and DQ 3 . When writing data, data are sent from the four-bit data bus to the four data lines that have been selected at data select lines  502  and the data are written to memory cells  505  of the four DQ portions at the intersections with selected word lines  503 . The four-bit data bus exchanges data with interface chip  405  through four interchip wires.  
         [0061]      FIG. 6  shows the DQ bit assignment to memory cell array  411  when changing the input/output bit configuration inside banks. Changing the combinations of 128 data bus wires that connect from one bank of memory cell array chip  401  by means of memory configuration switching circuit  407  of interface chip  405  changes the number of input/output bits. Using the 4-Mb memory units of the four DQ as the smallest units, bit configurations of ×4, ×8, ×16, ×32, ×64, and ×128 can be assigned as shown in  FIGS. 6   a - 6   f.    
         [0062]      FIG. 7  is a block diagram showing the configuration of memory configuration switching circuit  702  of memory interface chip  405 . Memory configuration switching circuit  702  is made up from bit-switching circuit  708 , prefetch switching circuit  701 , and switching control circuit  706 . Bit-switching circuit  708  rearranges the data bus lines from memory cell array chip  401 .  
         [0063]      FIG. 8  is a block diagram showing the details of the configuration of bit-switching circuit  708 . The four interchip wires  406  that extend from 4-Mb memory unit  501  in memory cell array chip  401  that is shown in  FIG. 4  and  FIG. 5   a  are connected to four-bit wiring unit  803  of bit-switching circuit  708  that is shown in  FIG. 8 , and as with the memory units, these bit-switching circuits  708  are provided in eight horizontal rows and four vertical columns for a total of 32. Connections between each of wiring units  803  are realized by 4-switch arrays  801  and  804  that are made up of the transfer gates of four transistors. In addition, wires from bit-switching circuit  708  are connected to input/output buffer  412  by way of prefetch switching circuit  701 .  
         [0064]      FIG. 9  shows a portion in which eight 4-bit wiring units  803  are arranged horizontally inside bit-switching circuit  708 , wherein the number of bits is switched by switching 4-switch arrays  804  between 4-bit wiring units  803 .  
         [0065]     Turning ON 4-switch arrays  804  that are enclosed by boxes in  FIG. 9  enables setting the bit configurations of ×4, ×8, ×16, and ×32 that are shown in  FIGS. 9   a - 9   d . Further connecting the 32-bit configuration that is shown in  FIG. 9   d  to four rows vertically and then switching the interposed switch arrays enables setting the input/output bit configurations of ×32, ×64, and ×128 that correspond to  FIGS. 6   d - 6   f.    
         [0066]     The switch control signals of bit-switching circuit  708  are sent from switching control circuit  706 . As a configuration of a switching control circuit, a configuration can be proposed in which fuses are provided in the wires of switching control circuit  706 , the circuit being configured to supply the control signals of switches that realize a desired memory configuration by cutting these fuses in accordance with the specifications of the memory configuration following fabrication of the chip. As an alternative to this configuration, a configuration can also be proposed in which switching control circuit  706  is a logic circuit that operates according to a combination of bonding option input signals, the logic circuit being configured to output the control signals of switches that realize a desired memory configuration by causing bonding between bonding option pads and package pins in accordance with the desired input/output bit configuration specifications when packaging the chip.  
         [0067]     Although only one memory cell array chip was shown in the present working example, adopting a form in which a plurality of chips are stacked enables an increase in the memory capacity of the memory device according to the number of chips and allows similar switching of the memory configuration. Further, although a DRAM was used as the memory device in the present working example, a similar configuration is possible using SRAM.  
         [0068]     The memory cell array chip allows alteration of the number of banks as one bank, two banks, or four banks as shown in  FIG. 10 , and when the memory capacity of the memory cell array chip is 512 Mb, 21 types of memory configurations are possible depending on the number of banks and the number of input/output bits as shown in  FIG. 11 . When a configuration having a plurality of banks is adopted, addresses can be designated in bank units and memory cells then accessed, whereby, when accessing a memory cell in a particular bank, interleaving operations can be carried out in other banks such as precharging, or further, operations for selecting word lines and activating bit lines, and refresh operations. As a result, banks can be accessed successively and without interruption and data transferred.  
         [0069]     Explanation next regards a means for, without changing the operating frequency of the memory core, switching the clock timing and prefetch number in prefetch switching circuit  701  and changing the data transfer rate between a memory device and the outside.  
         [0070]     In  FIG. 12 , data from bit-switching circuit  708  are sent to prefetch switching circuit  701  by way of four sets of 32-bit data buses  1201 . Prefetch switching circuit  701  is provided with four latch circuits  1203  that correspond to each of four sets of 32-bit data buses, clock control circuit  1202  is connected to clock line  705  and control signal line  707 ; prefetch switching circuit  701  regulates clock signals that are sent by way of clock line  705  in accordance with control signals that are sent by way of control signal line  707 ; and supplies these clock signals to each latch circuit  1203  as the timing signals by which each latch circuit  1203  latches data.  
         [0071]     The timing at which data are latched in prefetch switching circuit  701  is controlled, the latched data are sent to input/output buffer  704 , and the transfer rate to the outside is switched to double or quadruple the original rate.  
         [0072]     When clocks with a frequency of 200 MHz, which is the same as the clock frequency of memory cell array chip  401 , are supplied as input to latches and all of the data of the four sets of 32-bit data buses  1201  are sent to input/output buffer  704  at the rise time of the clocks as shown by the clock waveform in  FIG. 13   a , the input/output data transfer rate reaches 200 Mbps at 128 input/output bits. When two sets of the four sets of 32-bit data buses  1201  are latched at the rise time of the clocks and the other two sets are latched at the fall of the clocks, the data transfer rate is doubled to 400 Mbps at 64 input/output bits ( FIG. 13   b ). When the clocks that are applied to latches are further set to two phases that are phase-shifted by π/2 as shown in  FIG. 13   c  and both the rise time and fall time of the clocks are used to realize latching of the data of four sets of 32-bit data buses at a timing of 800 MHz cycles, the data transfer rate reaches 800 Mbps at 32 input/output bits. The control signals for switching the input clocks to each latch circuit  1203  are sent from the switching control circuit, as with the control signals for switching the input/output bit configuration. In this way, changing the prefetch number to 1, 2 and 4 enables switching of not only the input/output bit configuration, but the data transfer rate as well.