SEMICONDUCTOR DEVICE HAVING HIERARCHICAL STRUCTURED BIT LINES

A semiconductor device includes first and second global bit lines; first, second, third and fourth sense node; a first sense switch coupled between the first sense node and the first global bit line; a second sense switch coupled between the second sense node and the second global bit line; a third sense switch coupled between the third sense node and the first global bit line; a fourth sense switch coupled between the fourth sense node and the second global bit line; a first sense amplifier including a first terminal coupled to the first sense node and a second terminal coupled to the second sense node; a second sense amplifier including a third terminal coupled to the third sense node and a fourth terminal coupled to the fourth sense node. The first, second, third and fourth terminals respectively have first, second, third and fourth parasitic capacitances substantially equal in capacitance value.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A representative example of a technical concept for solving the problem of the present invention is described below. It is needless to mention that the contents that the present application is to claim for patent are not limited to the following technical concept, but to the description of the appended claims. That is, the present invention has a technical concept of performing a sense operation to simultaneously-accessed two memory cells by time division by using two sense amplifiers, and thereafter simultaneously performing restore operations from the two sense amplifiers to corresponding two memory cells. With this arrangement, switches are not necessary in the middle of the global bit lines, and no problem occurs when performing a restore operation by time division. Because a parasitic CR model of a first sense amplifier and that of a second sense amplifier become mutually the same, high sensitivity can be maintained.

FIG. 1is a circuit diagram showing a main part of a semiconductor device10according to a first embodiment of the present invention.

As shown inFIG. 1, the semiconductor device10according to the first embodiment is a semiconductor memory having hierarchized bit lines, and is configured to have two sense amplifiers connected to a pair of global bit lines as high-order bit lines. More specifically, the semiconductor device10according to the first embodiment includes a pair of global bit lines GBLTi and GBLBi (i=0, 1, 2, . . . ) extended in a Y direction, a sense amplifier SAi provided at one end of the pair of global bit lines GBLTi and GBLBi, and a sense amplifier SAiA provided at the other end of the pair of global bit lines GBLTi and GBLBi. Circuit formats of the sense amplifiers SAi and SAiA are not particularly limited, and a flip-flop circuit can be used, for example. The pair of global bit lines GBLTi and GBLBi have a so-called folded structure.

A sense switch SSW0i(first sense switch) is connected between one end of the global bit line GBLTi and one input/output node SI0Ti of the sense amplifier SAi, and a sense switch SSW1i(second sense switch) is connected between one end of the global bit line GBLBi and the other input/output node SI0Bi of the sense amplifier SAi. Similarly, a sense switch SSW2i(third sense switch) is connected between the other end of the global bit line GBLTi and one input/output node SI1Ti of the sense amplifier SAiA, and a sense switch SSW3i(fourth sense switch) is connected between the other end of the global bit line GBLBi and the other input/output node SI1Bi of the sense amplifier SAiA. The sense switches SSW0ito SSW3iare configured by an N-channel MOS transistor, and connection signals SH0T, SH0B, SH1T, and SH1B are respectively supplied to gate electrodes of these sense switches. Electric conduction and nonconduction of the four sense switches SSW0ito SSW3ican be mutually independently controlled. CMOS switches can be also used for the sense switches.

Local bit lines as low-order bit lines are connected to global bit lines as high-order bit lines via hierarchical switches. In the first embodiment, four local bit lines are allocated to one global bit line. These four local bit lines have mutually equal wiring lengths. Specifically, local bit lines BL0Ti, BL1Ti, BL2Ti, and BL3Ti are allocated to the global bit line GBLTi. Hierarchical switches LSW0Ti to LSW3Ti (first hierarchical switches) are respectively connected between the local bit lines BL0Ti to BL3Ti and the global bit line GBLTi. Similarly, local bit lines BL0Bi, BL1Bi, BL2Bi, and BL3Bi are allocated to the global bit line GBLBi. Hierarchical switches LSW0Bi to LSW3Bi (second hierarchical switches) are respectively connected between the local bit lines BL0Bi to BL3Bi and the global bit line GBLBi. The hierarchical switches LSW0Ti to LSW3Ti and LSW0Bi to LSW3Bi are configured by an N-channel MOS transistor, and corresponding connection signals LSW0to LSW7are supplied respectively to gate electrodes of these hierarchical switches. CMOS switches can be also used for the hierarchical switches.

As shown inFIG. 1, the local bit lines BL0Ti and BL1Bi intersect word lines WL0to WLn extended in an X direction, and memory cells MC are arranged at intersections. Similarly, the local bit lines BL1Ti and BL0Bi intersect same word line (not shown), the local bit lines BL2Ti and BL3Bi intersect same word lines (not shown), and the local bit lines BL3Ti and BL2Bi intersect same word lines (not shown).

An equalize circuit EQ is connected to the global bit lines GBLTi and GBLBi forming a pair. When an equalize signal EQB is activated, the global bit lines GBLTi and GBLBi are equalized at a predetermined potential. An equalize operation is also called “precharge”.

Further, the sense amplifier SAi is connected to the local I/O line LIO via a column switch YSWi. The local I/O line LIO is a complementary wiring including local I/O lines LIOT and LIOB. When any one of column switches YSWi is in a conductive state, data of the global bit line GBLTi amplified by the sense amplifier SAi is transferred to the local I/O line LIOT, and data of the global bit line GBLBi amplified by the sense amplifier SAi is transferred to the local I/O line LIOB. The column switches YSWi are all configured by N-channel MOS transistors, and column selection signals YSi are supplied to gate electrodes of the column switches YSWi. The column switches can be also in a CMOS structure.

Activation signals of the connection signals SH0T, SH0B, SH1T, SH1B, and LSW0to LSW7, the column selection signals YSi, and word lines WL are generated by a control circuit12shown inFIG. 1. That is, the control circuit12is a circuit block including a row decoder, a column decoder, and a control logic.

As shown inFIG. 1, the global bit lines GBLTi and GBLBi forming a pair have twist parts in which positions in the X direction are replaced at one or two positions. Specifically, a pair of global bit lines at even-order positions have one twist part TW1. The local bit lines BL0Ti, BL1Bi, BL1Ti, and BL0Bi are arranged between the twist part TW1and the sense amplifier SAi, and the local bit lines BL2Ti, BL3Bi, BL3Ti, and BL2Bi are arranged between the twist part TW1and the sense amplifier SAiA. On the other hand, a pair of global bit lines at odd-order positions has two twist parts TW2and TW3. The local bit lines BL0Ti and BL1Bi are arranged between the twist part TW2and the sense amplifier SAi. The local bit lines BL1Ti, BL0Bi, BL2Ti, and BL3Bi are arranged between the twist parts TW2and TW3. The local bit lines BL3Ti and BL2Bi are arranged between the twist part TW3and the sense amplifier SAiA.

As a result, a twist structure of the global bit lines becomes as shown inFIG. 2. Each global bit line is divided into four areas A1to A4having a uniform length in the Y direction. Each of these areas A1to A4is an area (memory block) in which a corresponding local bit line is formed. Because complementary signals appear in global bit lines forming a pair, noise between the bit lines is cancelled by the twist structure shown inFIG. 2. For example, when the global bit line GBLT2is focused, the global bit line GBLT2is adjacent to complementary bit lines GBLBi and GBLT1in the areas A1and A2, respectively, and is adjacent to complementary bit lines GBLB3and GBLT3in the areas A3and A4, respectively. Therefore, regardless of a logic level of data appearing in the global bit liens GBLT1and GBLBi and the global bit lines GBLT3and GBLB3, noise given to the adjacent global bit line GBLT2is cancelled.

FIG. 3is a cross-sectional view showing a physical structure of the memory cell MC.

As shown inFIG. 3, in the first embodiment, the memory cell MC is configured by a series circuit of a cell transistor Tr (access transistor) and a cell capacitor C (memory element), as an example. The cell transistor Tr is a pillar MOS transistor (vertical transistor) having a pillar channel201perpendicular to a main surface of a semiconductor substrate200. Diffusion layers202(local bit line BL) and203(at a storage contact side of the memory cell MC) are provided respectively at a lower side and an upper side of the pillar channel201. A side surface is covered with a gate electrode205via a gate dielectric film204. Accordingly, when a predetermined voltage is applied to the gate electrode205, the upper and lower diffusion layers202and203are electrically connected. The gate electrode205functions as the word line WL. Based on this configuration, one memory cell MC can be formed in an area of 4F.sup.2 (F is a minimum feature size). A 4F.sup.2 memory cell MC is a one-intersection and one-cell type having memory cells arranged at all intersections of the word lines WL and local bit lines BL.

The lower diffusion layer202is connected to the local bit lines BL embedded in the semiconductor substrate200. As explained above, in the first embodiment, the local bit lines BL are embedded in the semiconductor substrate200. Therefore, there are few cross couplings of the local bit lines BL and the word lines WL as compared with those when a normal planar transistor is used. The local bit line BL can be formed by using doped polycrystalline silicon such as arsenic (As), tungsten, or a metal material.

The upper diffusion layer203(at the storage contact side of the memory cell MC) is connected to a lower electrode211of the cell capacitor C via a contact plug206. The cell capacitor C is configured by the lower electrode211, an upper electrode212, and a capacitance dielectric film213provided between the lower electrode211and the upper electrode212. The upper electrode212is connected to a predetermined fixed potential. A global bit line GBL is provided above the cell capacitor C. Because an upper-layer wiring is used for the global bit line GBL, a film thickness T of the wiring becomes large. Copper (Cu) of a low electric resistance can be used for a material of the global bit line GBL. Accordingly, a wiring resistance (a specific resistance per unit length) of the global bit line GBL can be sufficiently smaller than that of the local bit line BL (for example, 1/10 or lower).

When a wiring resistance of the global bit line GBL is designed to be sufficiently smaller than the wiring resistance of the local bit line BL, the length of the global bit line GBL can be increased while suppressing the length of a local bit line BL. Accordingly, substantially a uniform access time (a time from when the word line WL is activated until when the sense amplifier SAi is activated) can be achieved regardless of a position of a memory cell in the Y direction. Consequently, the number of the local bit lines BL allocated to one global bit line GBL can be increased corresponding to a necessary memory capacitance. As a result, the number of memory cells allocated to one sense amplifier can be increased. In the first embodiment, although four local bit lines BL are allocated to one global bit line GBL, 16 or 32 local bit lines BL can be also allocated. When a conductive resistance (ON resistance) of a hierarchical switch is higher than a parasitic resistance of the global bit line GBL, a time constant (CR) of the ON resistance and the local bit line BL dominates a distribution multiplier.

The configuration of the semiconductor device according to the first embodiment is as described above. An operation of the semiconductor device is explained below.

FIG. 4is a timing chart for explaining an operation at a read time of the semiconductor device10according to the first embodiment. As shown inFIG. 4, the read operation of the semiconductor device10is performed at six steps.

A first step (S1) is a precharge operation. The connection signals SH0T, SH0B, SH1T, SH1B, and LSW0to LSW7are all activated at high level, and the equalize signal EQB is activated at low level. Accordingly, all global bit lines and all local bit lines are precharged at a predetermined potential (VDL/2) by the equalize circuit EQ.

A second step (S2) is a step of selecting the word line WL. A predetermined word line WL is activated based on a row address. In an example shown inFIG. 4, the word line WL0shown inFIG. 1is activated. Based on this, memory cells MC(a) and MC(b) are simultaneously connected to local bit lines BL0T0and BL1B0, respectively. In response to the activation of the word line WL0, the connection signals SH1T, SH1B, LSW1, and LSW3to LSW7excluding the connection signals SH0T, SH0B, LSW0, and LSW2are changed to low level. Accordingly, global bit lines GBLT0and GBLB0and a sense amplifier SA0A are interrupted, and all hierarchical switches excluding hierarchical switches LSW0T0and LSW1B0are in a nonconductive state. As a result, a potential of the local bit line BL0T0changes based on a logic level of data held in the memory cell MC(a), and a potential of the local bit line BL1B0changes based on a logic level of data held in the memory cell MC(b). The example shown inFIG. 4is a case that data of high level are held in the memory cells MC (a) and MC (b). In this case, the potential of the local bit lines BL0T0and BL1B0is slightly increased.

Because the local bit line BL0T0is connected to the global bit line GBLT0via the hierarchical switch LSW0T0, the potential of the global bit line GBLT0is also slightly increased.

On the other hand, the local bit line BL1B0is not connected to the global bit line GBLB0at this time because a corresponding hierarchical switch LSW1B0is in a nonconductive state. The global bit line GBLB0is connected to the local bit line BL0B0via the hierarchical switch LSW0B0. However, because a memory cell connected to the local bit line BL0B0is not selected, the potential of the global bit line GBLB0does not change. The hierarchical switch LSW0B0is kept conductive to match CR models of the global bit lines GBLT0and GBLB0at a sense time, thereby obtaining a high sensitivity. It is not essential to set the hierarchical switch LSW0B0conductive so long as it is possible to sense.

By the above operation, a potential difference based on read data from the memory cell MC(a) appears in one input/output node SI0T0and the other input/output node SI0B0of the sense amplifier SA0. That is, data held in the memory cell MC(a) is transferred to the sense amplifier SA0.

A third step (S3) is a step of activating the sense amplifier SA0. Specifically, sense switches SSW20and SSW30and all hierarchical switches are set in a nonconductive state by inactivating the connection signals SH1T, SH1B, and LSW0to LSW7at low level. The sense amplifier SA0is activated in this state. The sense amplifier SA0is activated by setting a sense activation signal SAPT0at high level and by setting a sense activation signal SANT0at low level. Accordingly, a potential difference appearing in the input/output nodes SI0T0and SI0B0of the sense amplifier SA0is amplified by the sense amplifier SA0. Amplified read data is held in the sense amplifier SA0.

A fourth step (S4) is a step of precharging (equalizing) the global bit lines GBLT0and GBLB0at a predetermined potential (VDL/2) again by the equalize circuit EQ. During this period, a column selection signal YS0is activated, thereby transferring read data held in the sense amplifier SA0, that is, data read from the memory cell MC(a), to the local I/O line LIO. The connection signals SH0T and SH0B are set at low level, and the connection signals SH1T and SH1B are set at high level, thereby changing over sense switches SSW00and SSW10to a nonconductive state and changing over the sense switches SSW20and SSW30to a conductive state.

A fifth step (S5) is a step of transferring data of the memory cell MC(b) to the sense amplifier SAGA and activation of the sense amplifier SAGA. Specifically, by setting the connection signals LSW1and LSW3at high level, the local bit line BL1B0is connected to the global bit line GBLB0, and the potential of the global bit line GBLB0is changed corresponding to data read from the memory cell MC(b). On the other hand, although a local bit line BL1T0is connected to the local bit line BL1T0via a hierarchical switch LSW1T0, a memory cell connected to the local bit line BL1T0is not selected. Therefore, the potential of the global bit line GBLT0does not change. In this case, it is not essential to set the hierarchical switch LSW1T0conductive so long as it is possible to sense.

By the above operation, a potential difference based on read data from the memory cell MC(b) appears in one input/output node SI1T0and the other input/output node SI1B0of the sense amplifier SA0A. That is, data held in the memory cell MC(b) is transferred to the sense amplifier SA0A.

Next, by inactivating the connection signals LSW1and LSW3at low level, all hierarchical switches are set in a nonconductive state, and the sense amplifier SA0A is activated in this state. The sense amplifier SA0A is activated by setting a sense activation signal SAPT1at high level and by setting a sense activation signal SANT1at low level. As a result, a potential difference appearing in the input/output nodes SI1T0and SI1B0is amplified by the sense amplifier SA0A. The amplified read data is held in the sense amplifier SA0A. While the sense amplifier SA0A is performing a sense operation, read data transferred to the local I/O line LIO is output to outside of the semiconductor device10.

A sixth step (S6) is a step of restoring data read from the memory cells MC(a) and MC(b) into the memory cells MC(a) and MC(b). Specifically, by setting the selection signals SH0T, SH1B, LSW0, and LSW1at high level and by setting the selection signals SH0B, SH1T, and LSW0to LSW7at low level, sense switches SWW0and SWW3and the hierarchical switches LSW0TO and LSW1B0are electrically conducted. Accordingly, data amplified by the sense amplifiers SA0and SA0A are simultaneously restored into the memory cells MC (a) and MC (b), respectively. Thereafter, the word line WL0is inactivated, thereby completing a series of read operations.

After completing a series of read operations, the process returns to the first step (S1), and the global bit lines and the local bit lines are precharged.

In the above example, a read operation and a restore operation are performed for the memory cell MC(b) although data read from the memory cell MC (b) is not readout to outside. This is because data held in the memory cell MC(b) is unavoidably destroyed following an access to the memory cell MC(a). Therefore, contrary to the above example, when the memory cell MC(b) is accessed, it suffices that data of the memory cell MC(a) is sensed by the sense amplifier SA0A, and data of the memory cell MC (b) is sensed by the sense amplifier SA0.

As explained above, in the read operation of the semiconductor device10according to the first embodiment, data of selected two memory cell are amplified by time division by using two sense amplifiers connected to a pair of global bit lines. Consequently, it is not necessary to provide switches in the middle of the global bit lines as required in the case of Japanese Patent Application Laid-open No. 2000-114491 and Japanese Patent Application Laid-open No. H11-163292. Accordingly, the parasitic capacitance of the global bit lines can be reduced. Further, because the operation condition of the sense amplifier SAi and that of the sense amplifier SAiA become equal, a sufficient operation margin of the sense amplifiers can be secured. Because restore operations are performed simultaneously, unlike Japanese Patent Application Laid-open No. H8-87880, problems accompanying with restore operations by time division do not happen.

Because data amplified by the sense amplifier SA0can be output to outside while the sense amplifier SA0A is performing a sense operation, an access time is not delayed.

A second embodiment of the present invention is explained next.

FIG. 5is a circuit configuration showing a main part of a semiconductor device20according to the second embodiment.

As shown inFIG. 5, the semiconductor device20according to the second embodiment is different from the semiconductor device10according to the first embodiment in that a local I/O line LIO0is allocated to the sense amplifier SAi, and a local I/O line LIO1is allocated to the sense amplifier SAiA. Because other features of the semiconductor device20are identical to those of the semiconductor device10, like elements are denoted by like reference numerals and redundant explanations thereof will be omitted.

The local I/O line LIO0is a complementary wiring including local I/O lines LIOT0and LIOB0. When a column switch YSW0iis in a conductive state, data of the global bit line GBLTi amplified by the sense amplifier SAi is supplied to the local I/O line LIOT0, and data of the global bit line GBLBi amplified by the sense amplifier SAi is supplied to the local I/O line LIOB0. Similarly, the local I/O line LIO1is a complementary wiring including local I/O lines LIOT1and LIOB1. When a column switch YSW1iis in a conductive state, data of the global bit line GBLTi amplified by the sense amplifier SAiA is supplied to the local I/O line LIOT1, and data of the global bit line GBLBi amplified by the sense amplifier SAiA is supplied to the local I/O line LIOB1. The column switches YSW0iand YSW1iare configured by an N-channel MOS transistor, and corresponding column selection signals YS0iand YS1iare supplied respectively to gate electrodes of the column switches YSW0iand YSW1i.

FIG. 6is a timing chart for explaining an operation at a read time of the semiconductor device20according to the second embodiment. As shown inFIG. 6, the read operation of the semiconductor device20is performed at seven steps.

Operations at a first step (S11) to a fifth step (S15) are the same as those of the first step (S1) to the fifth step (S5) shown inFIG. 4, and thus redundant explanations thereof will be omitted.

A sixth step (S16) is a step of outputting data amplified by the sense amplifier SA0A to the local I/O line LIO1. That is, the column selection signal YS10is activated, thereby transferring read data held in the sense amplifier SA0A, that is, data read from the memory cell MC(b) is transferred to the local I/O line LIO1.

An operation at a seventh step (S17) is the same as that of the sixth step (S6) shown inFIG. 4, and thus redundant explanations thereof will be omitted.

After completing a series of read operations, the process returns to the first step (S11) again, and the global bit lines and the local bit lines are precharged.

As explained above, in the semiconductor device20according to the second embodiment, the local I/O lines LIO0and LIO1are respectively allocated to two sense amplifiers SAi and SAiA connected to the pair of global bit lines GBLTi and GBLBi. Therefore, data of the two memory cells MC(a) and MC(b) to be accessed can be output to outside. Because the data of the memory cells MC(a) and MC(b) are transferred to the local I/O lines LIO0and LIO0at different timings, the data transferred to the I/O line LIO0can be output to outside while the sense amplifier SAiA is performing a sense operation, and thereafter the data amplified by the sense amplifier SAiA can be output to outside continuously.

Therefore, the basic technical concept of the present specification is not limited to that explained above, and circuit formats of the sense amplifiers and switches are not limited to those explained in the above embodiments. Further, an equalize voltage of bit lines is not limited to a so-called ½ voltage (VDL/2), and can be a low voltage at a VSS side and a high voltage at a VARY side, for example. Further, the present invention may be applied to various kinds of FETs (Field Effect Transistors) such as MISs (Metal-Insulator Semiconductors) and TFTs (Thin Film Transistors), other than MOSs (Metal Oxide Semiconductors). The present invention may be applied to various kinds of FETs such as transistors. The transistors may partially include bipolar transistors.

The basic technical concept of the present invention can be applied not only to an exclusive storage device, but to various types of semiconductor devices. For example, the present invention can be applied to semiconductor devices in general, such as a CPU (Central Processing Unit), an MCU (Micro Control Unit), a DSP (Digital Signal Processor), an ASIC (Application Specific Integrated Circuit), or an ASSP (Application Specific Standard Circuit), which has an information storage function. For example, an SOC (System on Chip), an MCP (Multi Chip Package), and a POP (Package on Package) can be mentioned as product formats of such a semiconductor device to which the present invention is applied. The present invention is applicable to semiconductor devices with the above arbitrary product format or package format.

Also, NMOS transistors (n-channel MOS transistors) are typical examples of the transistors of the first conductivity type, and PMOS transistors (p-channel MOS transistors) are typical examples of the transistors of the second conductivity type.

Various combinations and selections of the components disclosed herein may be made within the scope of the invention. In other words, the present invention of course includes various changes and modifications that are obvious to those skilled in the art according to all the disclosure including the claims and the technical concept.