STATIC MEMORY CELL HAVING ELECTRICAL ELEMENTS ON TWO LEVELS

Abstract: A static MOS memory device having a double polysilicon structure has parasitic transistors (Q₅, Q₆). The arrangement of electrical elements, transistors (Q₁, Q₂) and resistors (R₁, R₂), making up a memory cell, which are arranged in two over­ lapping layers, is such that the associated parasitic transistors (Q₅, Q₆) are maintained non-conductive regardless of the state of the memory cell, that is the state «1» or «0» of the cell, thereby avoiding an enhancement of memory cell current due to the presence of the parasitic transistors (Q₅, Q₆). Each parasitic transistor (Q₅, Q₆) has its gate electrode and source commonly connected.

Description: STATIC MEMORY CELL HAVING ELECTRICAL ELEMENTS ON TWO LEVELS The present invention relates to a memory cell having electrical elements on two levels, for instance in a double polysilicon (polycrystalline silicon) structure for a static memory device. A static memory device formed on a silicon chip (substrate) comprises a row and column array of memory cells, having n rows and n columns, and associated peripheral circuits such as a row decoder, a column decoder and sense amplifiers. In a static MOS (metal-oxide-semiconductor) memory cell employing transmission gate selection for example two static inverters, each comprising a resistor and a transistor, are wired cross-coupled so as to provide a flip-flop operation. As the number of bits to be stored in a memory cell array increases, from 64 kilo bits to 256 kilo bits, for instance, a higher packing density of electrical elements on a memory chip is required. To meet this requirement, double polysilicon technology has been proposed. With this technology, the (electrical) elements of a memory cell are arranged in a double polysilicon layer structure formed on a chip, providing in a decrease in the area occupied by a memory cell on the chip and an increase in packing density by a factor of approximately two. As the number of memory cells on one chip increases, the standby current of the memory device increases, resulting in higher power consumption. In recent times proposals for improvements in peripheral circuits of memory chips have been made with a view to reducing power consumption by the peripheral circuits, for example by the use of CMOS transistors and other technologies. However, the inventor has turned his attention to the reduction of memory cell current itself. According to the present invention, there is provided a static memory cell comprising: a first resistor and a first field effect transistor having a first gate electrode and source-drain regions, said first resistor and said first field effect transistor being connected in series with one another, connected to one another at a first node; a second resistor and a second field effect transistor having a second gate electrode and source-drain regions, said second resistor and said second field effect transistor being connected in series with one another, connected to one another at a second node; and said first gate electrode being connected to said second node, and said second gate electrode being connected to said first node, and said first and second resistors being formed at a level above the first and second transistors, separated from the first and second transistors by an insulator layer, wherein the first and second resistors are positioned to overlie respective elements forming the transistors, such an element being a gate electrode or a source-drain region, which elements are connected to the first and second nodes respectively. The inventor has recognised that, from the point of view of memory cell current, double polysilicon layer structure technology can be improved; that is, the double polysilicon layer structure has disadvantages. The inventor has realised that when the double polysilicon technology is adopted for a memory cell, undesirable effects occur due to the overlapping arrangement of electrical memory cell elements, such as the generation of parasitic transistors and parasitic capacitors. These effects are unavoidable and give rise to problems. For example, in the case of a static MOS memory cell employing transmission gate selection, as shown in the schematic circuit diagram of Fig.2, which is formed with double polysilicon technology, two transistors (Ql,Q2) of the two inverters of the cell are formed in the memory chip directly, and two resistors (Rl,R2) of the two inverters are formed on an insulator layer covering the transistors. These electric elements are wired to one another through contact holes opened in the insulator layer. As a result, the resistors are formed so as to encounter doped regions, namely source and drain regions of the transistors formed in the chip, and the insulator layer exist therebetween. Thus, the inventor has recognised, parasitic FETs (field effect transistors) Q5,Q6 are formed as depicted by broken lines in Fig.2. The inventor has realised that these parasitic transistors are formed in parallel with the resistors Rl,R2, allowing a current to flow, causing an increase in memory cell current. Generally, a memory cell having electrical elements on two levels, for instance using double polysilicon technology, has undesirable parasitic transistors as identified above. Briefly, the arrangement of and fabrication steps for providing electrical elements of a double polysilicon technology memory cell are as follows. In a silicon substrate, source and drain regions (hereinafter SD regions) of transistors, herein enhanced (enhancement) type MOS FETs by way of example, are formed in a transistor region which is defined on the substrate (chip) by a field oxide layer, usually a thick silicon dioxide (so02) layer. The substrate is covered entirely by a thin silicon dioxide (Sio2) insulator layer which provides gate insulators for the transistors. Over the insulator layer, first polysilicon layers, providing gate electrodes of the transistors, word lines and power supply lines are formed. These polysilicon layers are covered with another insulator layer, and second polysilicon layers, providing resistors, are formed on this other insulator layer. In a previously proposed memory cell, the resistors are formed over SD regions (this is shown in Fig.3, see below) because a flat surface having no step-difference is there available and is desirable for obtaining uniform and reliable layers using various fabricating processes such as an ion implantation method of doping, chemical vapour deposition, dry etching and the like. With the previously proposed memory cell structure described above, there occur zones whereat resistor polysilicon layer, insulator layer and SD region are laminated in that order. Thus parasitic transistors are formed in such zones. In the memory cell, for such parasitic transistors, a resistor layer acts as a channel region, the insulator layer acts as a gate insulator and an SD region acts as a gate selectrode. Since such parasitic transistors are usually connected in parallel with the resistors, memory cell current is enhanced by the parasitic transistors. The inventor has realised that the elimination of parasitic transistors (from a memory cell having a double polysilicon layer structure) is not necessary to avoid memory cell current enhancement due to such parasitic transistors. The inventor has had the insight that the formation of parasitic transistors can be allowed but nevertheless the undesired effect of the paratisic transistors can be eliminated or reduced. The inventor provides that this end can be accomplished by an apparently small and simple rearrangement of electrical elements of a memory cell which is nevertheless effective to achieve the desired result. With this rearrangement the gate electrode of each such parasitic transistor is connected commonly to its source region, making the parasitic transistor non-conductive. An embodiment of the present invention can provide a memory cell of a static memory device having a structure providing a reduction in memory cell current. An embodiment of the present invention can provide a memory cell having a double polysilicon structure with an effective layout of electrical elements such as to reduce or eliminate the effects of parasitic transistors which otherwise enhance memory cell current. An embodiment of the present invention concerns the structure of a static memory cell, having a flip-flop circuit comprising two cross-coupled pairs of resistor-transistor inverters formed in double polysilicon layers. Reference is made, by way of example, to the accompanying drawings, in which: Fig.l is a schematic partial block circuit diagram of a static MOS memory device, illustrating wiring of the memory matrix array of the memory device, Fig.2 is a schematic circuit diagram of a previously proposed memory cell circuit, comprising two resistors and two transistors, formed on a memory chip, Fig.3 is a schematic plan view of a previously proposed memory cell having a double polysilicon layer structure, illustrating the layout of electrical elements of the cell formed on a memory chip, and additionally showing parasitic transistors identified as being generated in the structure, Fig.4 corresponds to the plan view of Fig.3, illustrating schematically the pattern of an SD region of the memory cell in the substrate (chip), Fig.5 is a partial cross-sectional view of the memory cell of Fig.3, Fig.6 is a schematic circuit diagram of a memory cell circuit according to an embodiment of the present invention, illustrating a modified formation of parasitic transistors as compared with Fig.2, Fig.7 is a schematic plan view of a memory cell in accordance with an embodiment of the present invention, realising the circuit (parasitic transistor) configuration of Fig.6, illustrating the layout of electrical elements of the cell, showing in particular a modified location and pattern of resistors of the cell, as compared with Fig.3, Fig.8 is a partial cross-sectiional view of the memory cell of Fig.7, illustrating modified resistor positioning, and Fig.9 is a schematic plan view of a memory cell in accordance with another embodiment of the present invention, also realizing the circuit (parasitic transistor) configuration shown in Fig.6. Before disclosing an embodiment of the present invention, general operation and construction of a memory device will be described. Fig.l is a partial schematic block circuit diagram of a static memory device formed on a chip. The device has a row and column array of memory cells of which cells C to C COO l,n-l are shown in Fig.l. The device has word lines in correspondence to rows of the array. Word lines WL and WL, are shown in Fig. 1. The device also has bit 0 line pairs BLot BL to BL BL in correspondence to 0 0 columns of the array. Data bit lines DB and DB are selectively connectible to the bit line pairs and are connected to a sense amplifier circuit S. V represents cc a power supply voltage. When a memory cell C11 in the 1st row and 1st column is selected, the potential of a row address signal Xl, and accordingly the potential at word line WLl, is changed from low to high by a row decoder (not shown) and the potential of a column address signal Y1 is changed from low to high by a column decoder (not shown). Accordingly, bit lines BL1 and BL1 are connected to data bit lines DB and DB respectively. The output of the memory cell C11 is fed to a sense amplifier S and sensed and amplified to TTL level. Fig.2 is a circuit diagram illustrating a previously proposed memory cell which can be used in the memory cell matrix array. The memory cell has MOS transistors Q1,Q2 and resistors Rl,R2, which are cross-coupled in a flip-flop arrangement. MOS transistors Q3,Q4 provide a transmission gate function (to selectively connect nodes Nl and N2 to bit lines-running vertically on left and right sides of Fig.2 - in dependence upon the potential of a word line running horizontally at the bottom of Fig.2). Resistor R1 and transistor Q1 are connected in series with node Nl as their common connection point. Resistor R2 and transistor Q2 are connected in series with node N2 as their common connection point. The ends of resistors Rl and R2 not connected to nodes Nl and N2 are connected to a power supply line V . The gate of Q1 cc is connected to N2. The gate of Q2 is connected to Nl. V is another power supply line (e.g. ground). ss Corresponding to a memory cell state "0" , the potentials at node N2 and node Nl are high and low respectively. Accordingly, MOS transistor Q1 is conductive and an MOS transistor Q2 is non-conductive. MOS transistors Q3 and Q4 are made conductive by high potentials applied to their gates through the word line. The memory cell can be constructed with a double polysilicon structure as mentioned above. A schematic plan view of such structure of this previously proposed memory cell is shown in Fig.3, and a cross-sectional view, taken along line W-W of Fig.3, is shown in Fig.5, illustrating the mutual positional relationship of the transistor Q2 and the resistor Rl. In the memory cell of Figs.2 to 5, the inventor has realised that parasitic transistors Q5 and Q6 are formed as depicted by broken lines in Fig.2. It should be noted that with this arrangement of electrical elements, the gates of parasitic transistors are commonly connected to the Node N2. The connection of the gate electrodes of the parasitic transistors Q5 and 6 will be described in more detail below. Since the potential of node N2 is high in a memory cell state "0", the parasitic transistor Q and the transistor Ql are both conductive, resulting in an increase in the current flowing through the Rl-Ql circuit. This leads to an enhancement of the memory cell current. In this memory cell state, in the R2-Q2 circuit, the transistor Q2 is non-conductive and no current flows. When the memory cell is in a "1" state, the potentials of the nodes N2 and N1 are low and high respectively. No current flows through the Rl-Ql circuit because the transistor Q1 is non-conductive. On the other hand, in the R2-Q2 circuit, the transistor is conductive and a current flows through the resistor R2 and transistor Q2' but the potential of the gate electrode of the parasitic transistor Q5 is low (the potential of node N2) and the parasitic transistor Q5 is non-conductive, resulting in no enhancement of the memory cell circuit. In other words, the gate electrode of the parasitic transistor Q is commonly connected to its own source contact, the connection of which always makes the enhancement type MOS transistor non-conductive. The inventor has realised that if the gate electrode of the parasitic transistor Q6 is also commonly connected to the source contact of the transistor Q6 as shown in Fig.6, the transistor Q6 would always remain non-conductive and the problem of enhanced memory cell current due to parasitic transistors would not occur. To achieve this, the inventor proposes an improved memory cell structure, a plan view of an example of which is given in Fig.7. A cross-sectional view taken along the line Z-Z of Fig.7 is given in Fig.8. The present invention provides an elegant and subtle improvement of memory cell structure which affords a reduction in memory cell current. To clarify the advantages of the present invention over the previous proposed memory cell structure, the memory cell of Figs. 3 to 5 and its associated fabrication steps will be described briefly. Herein, the memory device will be assumed to be a static n-type MOS memory. The fabrication of a static n-MOS memory cell begins with the formation of a field oxide layer, a thick silicon dioxide (SiO2) layer FO (see Fig.5), over a p-type silicon substrate SUB (see Fig.5), defining transistor regions wherein enhanced (enhancement) type MOS transistors (e.g.Ql,Q2) are to be formed. A thin silicon dioxide (SiO2) layer for providing transistor gate insulator layers is formed over the entire surface of the substrate SUB. Following this step, polysilicon layers for providing gate electrodes Gl (see Fig.3) and G2 (see Figs.3 and 5) of the transistors Ql and Q2' word lines WL (Fig.3) and power supply lines PL (Figs.3 and 5) are formed selectively using a mask, covering the channel regions of all the transistors Q1 to Q4. This is the first polysilicon layer formation. Except for portions thereof underneath the gate electrodes G1 and G2, word lines WL and power lines PL, the thin silicon dioxide layer for providing transistor gate insulator layers is removed by etching to expose the original silicon substrate SUB. The exposed zones of the substrate are heavily ion implanted with n-type dopants utilizing a mask to provide a predetermined profile. Naturally, the field oxide layer FO, gate electrodes G1 and G2 and word lines WL have a masking function. The implanted dopants are diffused to form n-type SD regions for the transistors as shown by chain lines in Fig.3. Zones of the substrate beneath the gate electrodes Gl and G2 and the word line WL are not doped by the n-type dopants, remaining as p-type channel regions for the transistors to toQ4 . For clarity, Fig.4 shows only the layout of the diffused SD regions formed in the substrate SUB. Next, the entire substrate is covered with insulator layers of silicon dioxide (six2) and silicon nitride. This is followed by the opening of contact windows for Nodes Nl and N2, and power supply lines PL. These windows are snown as hatched squares in Fig.3. Then a polysilicon layer for providing resistors R1 and R2 is formed over the insulator layer. This is the second polysilicon layer formation. Through the windows, connections of the resistors R1 and R2 to nodes N2 and Nl, and to the power supply line PL are accomplished. As illustrated in Fig.3, narrow polysilicon layer portions signified by zig-zag lines (resistor symbols) serve as the resistors Rl and R2. After this step, an insulator layer is formed over the entire substrate and windows for bit lines BL and BL, and a window for the second power supply line Vss (not shown), are opened in that insulator layer, followed by formation of wiring on the surface of that insulator layer. Thus the electrical elements and wiring of memory cells are completed. Apparently, both resistors Rl and R2 overlie the SD region connected to the node N2, underneath the insulator layer. There are thus formed laminated layer structures each of which comprises a polysilicon layer resistor, a silicon dioxide (so02) insulator layer and n-type doped SD region, in that order. Thus the laminated layer structures act as parasitic transistors Q5 and Q6 as shown in Fig.2, whereby SD region, insulator layer and resistor layer (R1 or R2) play the roles of gate electrode, gate insulator and channel region respectively. It should be noted that the gate electrodes of both parasitic transistors Q5 and Q6, namely the SD region of the transistor Q21 are connected to the node N2 together. This means that the gate electrode of the parasitic transistor Q5 is commonly connected to its source contact side (node N2), so that the parasitic transistor Q5 is always non-conductive regardless of the potential of the node N2, high or low. On the other hand, the gate electrode of the parasitic transistor Q is not connected to the source contact of but butto the node N2 (to the drain region of the parasitic transistor Q6 - if the gate electrode is commonly connected to the drain region of the parasitic transistor, the parasitic transistor is conductive and allows current to flow) and the parasitic transistor Q6 is controlled by the potential of the node N2. Corresponding to the state of "0" of the memory cell, where the potential of the node N2 is high level, the parasitic transistor Q6 becomes conductive and a current flows in parallel with a current of the resistor Rl. In other words, it can be said that the effective resistance of the resistor Rl is reduced, resulting in an increase of memory cell current. Without completely avoiding the formation of parasitic transistors, which the inventor believes would be difficult if not impossible in a memory cell having a double polysilicon structure, a solution to the identified problem of enhanced memory cell current arising due to the parasitic transistors is provided by the inventor by giving a reduction or elimination of the effects of the parasitic transistors, by making the parasitic transistors Q5 and Q6 non-conductive regardless of the state "l" or "0" of the memory cell. This can be done by connecting the gate electrode of each parasitic transistor commonly to the source contact of the transistor as shown in a circuit diagram of Fig.6, where the gate electrode of the parasitic transistor Q6 is connected to the node Nl. Now a memory cell structure in accordance with an embodiment of the present invention will be described. Fig.7 is a schematic plan view of the memory cell structure and Fig.8 is a partial cross-sectional view taken along the line Z-Z in Fig.7. In this structure the resistor Rl is formed over the gate electrode G2 and not over the SD region of the MOS transistors of the memory cell. This is a minor modification with regard to the totality of the fabrication of a memory device, because the new arrangement of the resistor R1 is obtained by changing the pattern of a mask used for the second polysilicon layer formation. With the layout of Fig.7, the parasitic transistor Q6 is provided by a laminated layer structure composed of resistor Rl, insulator layer and gate electrode G2 which act as a channel region, gate insulator and gate electrode of the parasitic transistor Q6 respectively. Since the gate electrode G2 and resistor Rl are connected to the node Nl, the gate electrode and the source contact of the parasitic transistor Q6 are commonly connected thereby realizing the circuit shown in Fig.6. Thus the parasitic transistor Q6 is kept non-conductive during flip-flop operation of the memory cell. From the point of view of fabrication, the positioning of the resistor R1 over the gate electrode G2 induces a small drawback for the formation of the second polysilicon layer, namely the resistor Rl, as compared with the previously proposed memory cell structure. The flatness of the surface whereon the second polysilicon layer is to be formed (to provide resistor Rl), is rather worse than that of the surface whereon the resistor R1 is formed in the previously proposed memory cell structure, as can be seen from Fig.8. However, this drawback can be overcome by modern MOS device fabrication technology. Another memory cell structure in accordance with an embodiment of the present invention is shown in a plan view of Fig.9. The resistors R1 and R2 are formed separated (remote) from each other as shown in Fig.9. the transistor-resistor pairs making up the circuits Rl-Ql and R2-Q2 are connected to nodes N2 and Nl respectively. With the laminated layer structure of Fig.9 parasitic transistor Q6 is composed of resistor Rl, insulator layer and a zone of the SD region, which act as channel region, gate insulator and gate electrode of the parasitic transistor. This zone of the SD region is connected to the node N1 (differently from the zone of the SD region forming parasitic transistor Q5 which is connected to node N2 - see e.g. Fig.4). The layout and wiring of Fig.9 provide parasitic transistors Q5 and Q6 respectively with commonly connected gate electrodes and source contacts, realizing the circuit arrangement of Fig.6. The slight fabrication drawback of the structure of Fig.7 is also avoided. The structures in accordance with embodiments of the present invention as described above relate to n-type static MOS memory devices, but it will be clear to those skilled in the art that the present invention is also applicable to p-type MOS memory devices.

Claims: CLAIMS 1. A static memory cell comprising: a first resistor and a first field effect transistor having a first gate electrode and source-drain regions, said first resistor and said first field effect transistor being connected in series with one another, connected to one another at a first node; a second resistor and a second field effect transistor having a second gate electrode and source-drain regions, said second resistor and said second field effect transistor being connected in series with one another, connected to one another at a second node; and said first gate electrode being connected to said second node, and said second gate electrode being connected to said first node, and said first and second resistors being formed at a level above the first and second transistors, separated from the first and second transistors by an insulator layer, wherein the first and second resistors are positioned to overlie respective elements forming the transistors, such an element being a gate electrode or a source-drain region, which elements are connected to the first and second nodes respectively. 2. A static memory cell comprising: a first resistor and a first field effect transistor having a first gate electrode and source-drain regions, said first resistor and said first field effect transis'tor being connected in series at a first node; a second resistor and a second field effect transistor having a second gate electrode and source-drain regions, said second resistor and said second field effect transistor being connected in series at a second node; and a power supply line to which said first and second resistors are connected, said first gate electrode being connected to said second node, said second gate electrode being connected to said first node, said first resistor being positioned over said second gate electrode and said second resistor being positioned over the one of said source-drain regions of said second field effect transistor which is connected to said second node. 3. A static memory cell comprising: a first resistor and a first field effect transistor having a first gate electrode and source-drain regions, said first resistor and said first transistor being connected in series at a first node; a second resistor and a second field effect transistor having a second gate electrode and source-drain regions, said second resistor and said second field effect transistor being connected in series at a second node; and a power supply line to which said first and second resistors are connected, said first gate electrode being connected to said second node, said second gate electrode being connected to said first node, said first resistor and said second resistor are connected to said power supply line, said first resistor being positioned over the one of said source-drain regions of said second field effect transistor which is connected to said first node, and said second resistor being positioned over the one of said source-drain regions of said second field effect transistor which is connected to said second node.

Inventors: YAMAUCHI, TAKAHIKO C/O FUJITSU LIMITED
Applicants: FUJITSU LIMITED
Application year: 1984
Main classification: G11C11
IPC codes: H01L27, H01L21, G11C11