Static random access type semiconductor memory device.

0001The present invention generally relates to semiconductor memory devices and, more particularly, relates to cell structures in a static random access memory where integration density of memory cells can be increased.

0002Fig. 39 is an equivalent circuit diagram of one memory cell in a conventional static random access memory (hereinafter referred to as SRAM). This memory cell includes six p-type MOS transistors as loads. A pair of driver (for driving) transistors Q₁ and Q₂ (n-type MOS transistors) are connected to a pair of load transistors Q₅ and Q₆ (p-type MOS transistors) to form a flipflop circuit. The sources 110 and 111 of the pair of load transistors Q₅ and Q₆ are connected to a power supply Vcc and the sources 112 and 113 of driver transistors Q₁ and Q₂ are connected to GND. A pair of access transistors Q₃ and Q₄ (n-type MOS transistors) are connected to storage nodes 114 and 115, respectively. A bit line 107 is connected to one source/drain of access transistor Q₃ and a bit line 108 is connected to one source/drain of access transistor Q₄. The gates of access transistors Q₃ and Q₄ are connected to a word line 109.

0003Figs. 40 to 42 are plan views of the structure of an SRAM, showing three stages in order from the bottom on the surface of the substrate, respectively. Fig. 43 is a cross-sectional view of the structure taken along the line A-A in Figs. 40 to 42. Referring to Figs. 39, 40 to 43, a pair of driver transistors Q₁ and Q₂ and a pair of access transistors Q₃ and Q₄ are formed on a main surface of a p-type silicon substrate 148 of the memory cell. Driver transistor Q₁ includes a pair of source/drain regions 121 and 122 and a gate electrode 125. Driver transistor Q₂ includes a pair of source/drain regions 118 and 117 and a gate electrode 126. Access transistor Q₃ includes a pair of source/drain regions 119 and 120 and a gate electrode 109. Access transistor Q₄ includes a pair of source/drain regions 116 and 117 and a gate electrode 109. These transistors are n-type MOS transistors having source/drain regions formed on the main surface of p-type silicon substrate 148. Gate electrode 126 of driver transistor Q₂ is connected to source/drain region 120 of access transistor Q₃ through a contact 128. Gate electrode 126 of driver transistor Q₂ is connected to source/drain region 121 of driver transistor Q₁ through a contact 129. Gate electrode 125 of driver transistor Q₁ is connected to source/drain region 117 of access transistor Q₄ and source/drain region 117 of driver transistor Q₂ through a contact 127. A gate electrode 130 of a load transistor Q₅ is connected to a source/drain region 137 of a load transistor Q₆ through a contact 139. A gate electrode 131 of load transistor Q₆ is connected to a source/drain region 134 of load transistor Q₅ through a contact 138.

0004A bit line 107 is connected to source/drain region 119 of access transistor Q₃ through a contact 146 and a bit line 108 is connected to source/drain region 116 of access transistor Q₄ through a contact 147.

0005As stated above, in the memory cell of the conventional SRAM four n-type MOS transistors are arranged on the silicon substrate and p-type thin film transistors are provided as loads above them. A case where a p-type thin film transistor is used as a load of a memory cell in an SRAM has been described in IEDM 1990 Technical Digest pp. 477-480. Fig. 45 is a cross-sectional view of a typical structure of a thin film transistor used as load transistors Q₅ and Q₆. The thin film transistor has a channel region 142 and a pair of source/drain regions 141 and 143 formed in a semiconductor layer such as polycrystalline silicon and a gate electrode 140 provided opposite to channel region 142 with an insulating layer interposed therebetween. Fig. 46 is a diagram showing a current characteristic of the thin film transistor.

0006In such an SRAM, it is necessary to reduce an area occupied by each memory cell in order to increase the integration density of the memory cells. However, the conventional memory cell above had two problems to be described below.

0007The first problem is that it is difficult to reduce an element isolation region between transistors making up the memory cell. Fig. 44 is a diagram showing by a model a cross-section of the structure of a LOCOS film 124 (Fig. 43) for insulating and isolating transistors from each other in the memory cell shown in Fig. 43. In this LOCOS film 152 (Fig. 44), regions X called "bird's beak" are formed at its both ends, which expand to the region where elements are formed, so that an isolation width W becomes larger than its desired value. For this reason, the width of the isolation region cannot be reduced, so that reduction in the size of the memory cell cannot be achieved.

0008The second problem concerns a current handling capability ratio β of a driver transistor to an access transistor (= the current handling capability of the driver transistor/the current handling capability of the access transistor). If the current handling capability ratio β is small, data is destroyed when it is read out from a memory cell. This phenomenon will now be described below. Figs. 47(a) and (b) show two inverter circuits obtained by dividing the equivalent circuit of the memory cell shown in Fig. 39 in connection with the reading characteristic. In this case, load transistors Q₅ and Q₆ are not shown because the amount of the current flowing through these load transistors is little enough to be ignored compared with those of the access transistors and the driver transistors, so that is has no effect on the reading operation. The characteristic of reading from a memory cell is given from a change in voltage at one storage node obtained by fixing the bit line and the word line at Vcc and changing the gate voltage of the driver transistor (the voltage at the other storage node). Fig. 48(a) is a diagram showing the reading characteristic in a case where the current handling capability ratio β is large (about 3). The axis of abscissa represents a voltage at storage node 115 and the axis of ordinate represents a voltage at storage node 114. The curve α₁ represents the voltage change characteristic at storage node 114 in a case where the voltage at storage node 115 is changed. The curve γ₁ shows the voltage change characteristic at storage node 115 in a case where the voltage at storage node 114 is changed. The curves α₁ and γ₁ intersect each other at three points P₁, P₂ and P₃. At point P₃, storage node 114 has "High" data stored, and storage node 115 has "High" data stored at point P₁. Point P₂ is an unstable point and the condition at this point P₂ is not kept at the time of reading. In the figure, a region surrounded by a circle h is called "eye of a memory cell". As the current handling capability ratio β of the transistors is larger, the circle h becomes bigger and the reading operation is stabilized.

0009In order to reduce the size of a memory cell, the size of an access transistor or a driver transistor is reduced. The access transistor or the driver transistor is reduced in size, for example, by shortening the gate length. If the transistor width of the access transistor is reduced to 1µm or less, a so-called narrow channel effect becomes significant, so that a threshold voltage Vth of the access transistor is increased. Fig. 48(b) shows the voltage change characteristic at the storage node in a case where the threshold voltage Vth of the access transistor is increased. In Figs. 48(a) and (b), Vcc-ϑ or Vcc-ϑ' corresponds to the threshold voltage Vth of the access transistor. As shown in Fig. 48(b), if the threshold voltage of the access transistor is increased, the curves α₂ and γ₂ intersect each other at one point P₂ only and the so-called "eye of a memory cell" region disappears. As a result, the stable points of the voltage at each storage node disappear, and data stored in the memory cell is destroyed at the time of the reading operation. For these reasons, the access transistor cannot be reduced in size even though the size of the driver transistor can be reduced. If only the driver transistor is reduced in size, the current handling capability ratio β of the both transistors becomes small, making the reading operation unstable.

0010An object of the present invention is to reduce the size of a memory cell in an SRAM.

0011Another object of the present invention is to reduce an area of an element isolation region in an SRAM.

0012Still another object of the present invention is to reduce the size of a memory cell and stabilize the operation of reading out stored data in an SRAM.

0013A semiconductor memory device according to one aspect of the present invention includes a memory cell including a pair of driver transistors of a first conductivity type and a pair of load transistors of a second conductivity type making up a flipflop circuit, and a pair of access transistors. The semiconductor memory device further includes a semiconductor substrate having a main surface, an insulating layer formed on the semiconductor substrate, a first group of thin film transistors arranged on the insulating layer, an interlayer insulating layer covering the surface of the first group of thin film transistors and a second group of thin film transistors arranged on the interlayer insulating layer. The first group of thin film transistors includes at least one transistor of the driver transistors, the load transistors and the access transistors. The second group of thin film transistors includes at least one transistor of the driver transistor, the load transistor and the access transistor excluding the transistor included in the first group of thin film transistors.

0014In accordance with one aspect of the present invention, six transistors making up a memory cell are thin film transistors. Element isolation is made by providing the interlayer insulating layer between the thin film transistors. Accordingly, an area of an element isolation region can be reduced by eliminating the conventional element isolation structure using a LOCOS film.

0015A semiconductor memory device according to another aspect of the present invention includes a memory cell including a pair of driver transistors of a first conductivity type and a pair of load transistors of a second conductivity type making up a flipflop circuit, and a pair of access transistors of the first conductivity type. The semiconductor memory device further includes a semiconductor substrate having a main surface, a first group of transistors including the pair of access transistors and one of the driver transistor formed on the main surface of the semiconductor substrate, an interlayer insulating layer covering the surface of the first group of transistors, and a second group of transistors including the other of the driver transistors and the pair of load transistors formed on the interlayer insulating layer. Each transistor in the first group of transistors is a MIS type transistor including a pair of impurity regions formed in the semiconductor substrate and each transistor in the second group of transistors is a thin film transistor including a pair of impurity regions formed in a semiconductor layer on the interlayer insulating layer.

0016In accordance with the another aspect of the present invention, the size of a memory cell can be reduced by forming on the main surface of the semiconductor substrate the pair of access transistors and one driver transistor among six transistors making up the memory cell and providing the remaining three transistors over them with the interlayer insulating layer interposed therebetween.

0017A semiconductor memory device according to still another aspect of the present invention includes a memory cell formed on a main surface of a semiconductor substrate. The memory cell includes a pair of driver transistors of a first conductivity type and a pair of load transistors of a second conductivity type making up a flipflop circuit, a single access transistor connected to the flipflop circuit and a bit line connected to an impurity region of the access transistor.

0018In accordance with the still another aspect of the present invention, the size of a memory cell can be reduced by eliminating one access transistor among six transistors making up the memory cell.

0019In a semiconductor memory device according to yet another aspect of the present invention, a first group of transistors includes a pair of driver transistors formed on a main surface of a semiconductor substrate. A second group of transistors includes a pair of access transistors formed and a pair of load transistors on an interlayer insulating layer. Each transistor in the first group of transistors is a MIS type transistor having a pair of impurity regions formed in the semiconductor substrate. Each transistor in the second group of transistors is a thin film transistor having a pair of impurity regions formed in a semiconductor layer on the interlayer insulating layer.

0020In a semiconductor memory device according to yet another aspect of the present invention, a first group of transistors includes one of driver transistor formed on a main surface of a semiconductor substrate. A second group of transistors includes the other of driver transistor, a pair of access transistors and a pair of load transistors formed on an interlayer insulating layer. Each transistor in the first group of transistors is a MIS type transistor having a pair of impurity regions formed in the semiconductor substrate. Each transistor in the second group of transistors is a thin film transistor having a pair of impurity regions formed in a semiconductor layer on the interlayer insulating layer.

0021In accordance with the yet another aspects of the present invention, a narrow channel effect can be restrained and stabilization of the operation of reading out stored data as well as reduction in size of a memory cell can be achieved by forming a thin film transistor as an access transistor.

0022The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

0024Fig. 1 is a plan view of the structure of a memory cell in an SRAM according to a first embodiment of the present invention.

0025Fig. 2 is a plan view showing the structure of an upper layer portion of the memory cell shown in Fig. 1.

0026Fig. 3 is a cross-sectional view showing a typical structure of a thin film transistor used in this invention.

0027Fig. 4 is a diagram showing an electrical characteristic of the thin film transistor shown in Fig. 3.

0028Fig. 5 is a structural cross-sectional view taken along the line B-B shown in Figs. 1 and 2.

0029Fig. 6 is a structural cross-sectional view taken along the line C-C in Figs. 1 and 2.

0030Fig. 7 is a structural plan view of a memory cell in an SRAM according to a second embodiment of the present invention.

0031Fig. 8 is a structural plan view of an upper layer portion of the memory cell shown in Fig. 7.

0032Fig. 9 is a structural plan view of a still upper layer portion of the memory cell shown in Fig. 8.

0033Fig. 10 is a structural plan view of a still upper layer portion of the memory cell shown in Fig. 9.

0034Fig. 11 is a structural cross-sectional view taken along the line D-D in Figs. 7 to 10.

0035Fig. 12 is a structural cross-sectional view taken along the line H-H in Fig 8.

0036Fig. 13 is a structural cross-sectional view taken along the line E-E in Fig 7.

0037Fig. 14 is a structural cross-sectional view taken along the line G-G in Fig 7.

0038Fig. 15 is a structural cross-sectional view taken along the line F-F in Fig 7.

0039Fig. 16 is a structural cross-sectional view taken along the line I-I in Fig 9.

0040Fig. 17 is an equivalent circuit diagram of the memory cell in the SRAM according to the second embodiment.

0041Fig. 18 is an equivalent circuit diagram (a), (b) of two inverter circuits included in a flipflop circuit shown in Fig. 17.

0042Fig. 19 is a diagram showing the characteristic curve of reading out data from the memory cell according to the second embodiment.

0043Fig. 20 is a structural plan view of a memory cell in an SRAM according to a third embodiment of the present invention.

0044Fig. 21 is a structural plan view of a still upper layer of the memory cell shown in Fig. 20.

0045Fig. 22 is a structural plan view of a still upper layer of the memory cell shown in Fig. 21.

0046Fig. 23 is a structural plan view of a still upper layer of the memory cell shown in Fig. 22.

0047Fig. 24 is a structural cross-sectional view taken along the line J-J in Figs. 20 to 23.

0048Fig. 25 is an equivalent circuit diagram of the memory cell in the SRAM according to the third embodiment.

0049Fig. 26 is a structural plan view of a memory cell in an SRAM according to a fourth embodiment of the present invention.

0050Fig. 27 is a structural plan view of a still upper layer of the memory cell shown in Fig. 26.

0051Fig. 28 is a structural plan view of a still upper layer of the memory cell shown in Fig. 27.

0052Fig. 29 is a structural plan view of a still upper layer of the memory cell shown in Fig. 28.

0053Fig. 30 is a structural cross-sectional view taken along the line K-K in Figs. 26 to 29.

0054Fig. 31 is a structural cross-sectional view taken along the line M-M in Fig. 26.

0055Fig. 32 is a structural cross-sectional view taken along the line N-N in Fig. 26.

0056Fig. 33 is a structural cross-sectional view taken along the line L-L in Fig. 28.

0057Fig. 34 is an equivalent circuit diagram showing a memory cell having one access transistor.

0058Fig. 35 is an equivalent circuit diagram of a case where only a pair of load transistors are thin film transistors in the memory cell shown in Fig. 34.

0059Fig. 36 is an equivalent circuit diagram of a case where a pair of access transistors and a pair of load transistors are thin film transistors in the memory cell shown in Fig. 17.

0060Fig. 37 is an equivalent circuit diagram of a case where a pair of access transistors, a pair of load transistors and one driver transistor are thin film transistors in the memory cell shown in Fig. 17.

0061Fig. 38 is an equivalent circuit diagram of a case where one access transistor, one driver transistor and a pair of load transistors are thin film transistors in the memory cell shown in Fig. 34.

0062Fig. 39 is an equivalent circuit diagram of a memory cell in a conventional SRAM.

0063Fig. 40 is a structural plan view of a memory cell in a conventional SRAM.

0064Fig. 41 is a structural plan view of a still upper layer of the memory cell shown in Fig. 40.

0065Fig. 42 is a structural plan view of a still upper layer of the memory cell shown in Fig. 41.

0066Fig. 43 is a structural cross-sectional view taken along the line A-A in Figs. 40 to 42.

0067Fig. 44 is a cross-sectional view of the structure in the vicinity of a LOCOS film used in isolating elements in a conventional memory cell.

0068Fig. 45 is a cross-sectional view showing the structure of a typical cross section of a thin film transistor used as a conventional load transistor.

0069Fig. 46 is a diagram showing an electrical characteristic of the thin film transistor shown in Fig. 45.

0070Fig. 47 is an equivalent circuit diagram (a), (b) showing two inverter circuits obtained by dividing the flipflop circuit shown in Fig. 39.

0071Fig. 48 is a diagram (a), (b) showing the characteristic curve of reading out data of a conventional memory cell.

0072A memory cell according to a first embodiment includes pairs of access transistors Q₃ and Q₄, driver transistors Q₁ and Q₂ and load transistors Q₅ and Q₆ which are all thin film transistors. An equivalent circuit of this memory cell is equal to that shown in Fig. 39.

0073The structure of the memory cell will now be described with reference to Fig. 39 and further to Figs. 1, 2, 5 and 6. A main surface of a silicon substrate 32 has an insulating layer 33a formed thereon. A pair of access transistors Q₃ and Q₄ and a pair of driver transistors Q₁ and Q₂ are arranged on the surface of insulating layer 33a. The four transistors Q₁ to Q₄ are n-type thin film transistors. Access transistor Q₃ includes a pair of source/drain regions 1 and 3 and a channel region 2 formed in a polycrystalline silicon layer and a gate electrode 10. Access transistor Q₄ includes a pair of source/drain regions 6 and 8 and a channel region 7 formed in a polycrystalline silicon layer and a gate electrode 10. Driver transistor Q₁ includes a pair of source/drain regions 3 and 5 and channel region 4 formed in the polycrystalline silicon layer and a gate electrode 11. Driver transistor Q₂ includes a pair of source/drain regions 8 and 5 and a channel region 9 formed in the polycrystalline silicon layer and a gate electrode 12. The surfaces of these four transistors Q₁ to Q₄ are covered with a first interlayer insulating layer 33b.

0074A pair of p-type load transistors Q₅ and Q₆ are formed on the surface of first interlayer insulating layer 33b. Load transistor Q₅ includes a pair of source/drain regions 13 and 15, a channel region 14 and a gate electrode 11. Load transistor Q₅ shares gate electrode 11 with driver transistor Q₁. Load transistor Q₆ includes a pair of source/drain regions 13 and 17, a channel region 16 and a gate electrode 12. Load transistor Q₆ shares gate electrode 12 with driver transistor Q₂. The surfaces of load transistors Q₅ and Q₆ are covered with a second interlayer insulating layer 33c.

0075A pair of bit lines 107 and 108 are formed on the surface of second interlayer insulating layer 13c. Bit line 107 is connected to source/drain region 1 of access transistor Q₃ through a contact 26. Bit line 108 is connected to source/drain region 6 of access transistor Q₄ through a contact 27. Source/drain region 15 of load transistor Q₅ is connected to source/drain region 3 shared by access transistor Q₃ and driver transistor Q₁ through a contact 20. A contact 23 connects source/drain region 15 of load transistor Q₅ to an interconnection layer 18. A contact 22 connects interconnection layer 18 to gate electrode 12 of load transistor Q₆. A contact 21 connects source/drain region 17 of load transistor Q₆ to gate electrode 11 of load transistor Q₅. A contact 19 connects gate electrode 11 of load transistor Q₅ to source/drain region 8 of driver transistor Q₂.

0076Fig. 3 is a diagram showing by a model the structure of a typical cross section of a thin film transistor used in the first embodiment. Fig. 4 shows an electrical characteristic of the thin film transistor shown in Fig. 3.

0077As stated above, since all the six transistors in the memory cell are thin film transistors, it is possible to prevent an increase in the threshold voltage of the access transistor under the influence of the narrow channel effect. Accordingly, the sizes of the access transistor and the driver transistor can be determined so that the current handling capability ratio β of the driver transistor to the access transistor is large. As a result, a memory cell can be constructed in which a stable reading operation can be performed.

0078A second embodiment will now be described. As shown in Fig. 17, in the memory cell according to the second embodiment, a pair of access transistors Q₃ and Q₄ and one driver transistor Q₁ are MOS transistors formed on the surface of a substrate and the other driver transistor Q₂ and a pair of load transistors Q₅ and Q₆ are thin film transistors. Figs. 7 to 10 are structural plan views of the memory cell and Figs. 11 to 16 are structural cross-sectional views of the memory cell. The memory cell according to this embodiment has a structure in which a plurality of polycrystalline silicon layers are stacked, forming a multilayer with interlayer insulating layers interposed therebetween. The structural plan views Figs. 7 to 10 show each layer in the plurality of stacked polycrystalline silicon layers in order starting with a lower layer.

0079Referring to Figs. 7 to 16, a pair of access transistors Q₃ and Q₄ and one driver transistor Q₁ are arranged on a main surface of a silicon substrate 100. Access transistor Q₃ includes a pair of source/drain regions 31 and 32 and a gate electrode 33. Access transistor Q₄ includes a pair of source/drain regions 36 and 37 and a gate electrode 38. Driver transistor Q₁ includes a pair of source/drain regions 32 (21) and 22 and a gate electrode 23. These three transistors are n-type MOS transistors formed on the surface of the silicon substrate.

0080The other driver transistor Q₂ and two load transistors Q₅ and Q₆ formed in upper layers of silicon substrate 100 are thin film transistors. Driver transistor Q₂ includes a pair of source/drain regions 26 and 27 formed in a polycrystalline silicon layer L2 and a gate electrode 28. Load transistors Q₅ and Q₆ are provided in a still upper layer over driver transistor Q₂. In Fig. 9, load transistor Q₅ includes a pair of source/drain regions 47 and 41 formed in a polycrystalline silicon layer L4 and a gate electrode 43. Load transistor Q₆ includes a pair of source/drain regions 46 and 47 formed in polycrystalline silicon layer L4 and a gate electrode 48 (28). Gate electrode 48 which is gate electrode 28 is shared by load transistor Q₆ and driver transistor Q₂.

0081Bit lines 107 and 108 are provided above load transistors Q₅ and Q₆. Bit line 107 is connected to source/drain region 31 of access transistor Q₃ through a contact 56. Bit line 108 is connected to source/drain region 37 of access transistor Q₄ through a contact 57. A contact 51 connects source/drain region 32 (21) shared by access transistor Q₃ and driver transistor Q₁ to gate electrode 28 (48) of driver transistor Q₂ and load transistor Q₆. A contact 52 connects gate electrode 23 of driver transistor Q₁ to source/drain region 26 of driver transistor Q₂. A contact 54 connects source/drain region 36 of access transistor Q₄ to gate electrode 43 of load transistor Q₅. A contact 53 connects gate electrode 48 of load transistor Q₆ to source/drain region 41 of load transistor Q₅. A contact 58 connects gate electrode 43 of load transistor Q₅ to source/drain region 46 of load transistor Q₆.

0082A description will now be made of the operation in reading from the memory cell according to the second embodiment. Fig. 18(a), (b) shows two inverter circuits divided in connection with the reading characteristic in the equivalent circuit of the memory cell shown in Fig. 17. Here, the second embodiment is different from the conventional technique in that the inverter circuit shown in Fig. 18(b) is a CMOS inverter circuit including two thin film transistors only. The reading characteristic of the memory cell is given by fixing bit line 107 and a word line 109a at Vcc and a word line 109b at GND. Fig. 19 shows a curve of a reading characteristic of the memory cell given in this way. Fig. 19 shows a reading characteristic in a case where the current handling capability ratio β of driver transistor Q₁ to access transistor Q₃ is small, i.e., in the order of 1.5. The axis of abscissas represents a voltage at a storage node 115 and the axis of ordinates represents a voltage at a storage node 114. In Fig. 19, a curve α₃ represents a voltage characteristic at storage node 114 when a voltage at storage node 115 of Fig. 18(a) is changed. A curve γ₃ represents a voltage characteristic at storage node 115 when a voltage at storage node 114 of Fig. 18(b) is changed. The curves α₃ and γ₃ intersect each other at three points P₁, P₂ and P₃. This memory cell is stabilized at the intersection P₁ or P₃. At point P₁, storage node 115 has "High" data stored, and storage node 114 has "High" data stored at point P₃. The point P₂ is an unstable point and the condition at this point is not maintained in a reading operation.

0083In the memory cell according to this embodiment, as the inverter circuit shown in Fig. 18(b) is a CMOS circuit, a voltage at each storage node rapidly changes from High level to Low level or from Low level to High level with a certain voltage (voltage of about 1/2Vcc) Accordingly, the voltage characteristic as indicated by the curve γ₃ is obtained, so that a large circle h called "eye of a memory cell" can be secured. Therefore, a stable operation can be performed in reading data from the memory cell. That is, even if the current handling capability ratio β is small, the characteristic of a CMOS can be used by means of the n-type thin film transistor as the driver transistor and the p-type thin film transistor as the load transistor, thereby stabilizing the reading operation.

0084In the operation of writing data into the memory cell, for example, in order to bring storage node 114 to High level, storage node 115 is brought to Low level and storage node 114 to High level by operating access transistor Q₄ only and bringing bit line 108 to Low level. In order to bring storage node 115 to High level, only access transistor Q₃ is operated and bit line 107 is brought to Low level, so that storage node 114 attains Low level and storage node 115 is set at High level.

0085In this embodiment, the two access transistors Q₃ and Q₄ do not operate simultaneously. Accordingly, one bit line can be used as bit lines 107 and 108.

0086A third embodiment is directed to a memory cell including a single bit line as stated above. As shown in Fig. 25, source/drain regions of a pair of access transistors Q₃ and Q₄ are connected to a common bit line 107. Six transistors making up the memory cell have the same structures as those in the second embodiment. Figs. 20 to 23 are structural plan views of the memory cell having a multilayered structure, showing each layer in order starting with a lower layer.

0087Referring to Fig. 25, and further to Figs. 20 to 24, a pair of access transistors Q₃ and Q₄ and one driver transistor Q₁ are arranged on a main surface of a silicon substrate 100. One driver transistor Q₂ which is a thin film transistor is provided over it. Load transistors Q₅ and Q₆ which are thin film transistors are further provided over it. Furthermore, above it are provided one bit line 107 and a GND line 112 extending parallel to the bit line. Bit line 107 is connected to a source/drain region 31 of access transistor Q₃ through a contact 65 and is connected to a source/drain region 37 of access transistor Q₄ through a contact 66. In the figure, the same reference numerals designate the same elements as in the second embodiment. As stated above, if bit line 107 is used as one common interconnecting line, the other of the two heretofore used as bit lines can be employed as a GND line. Also, each memory cell can contact GND line 112 independently. Since a metal interconnecting line for a GND line in a conventional memory device was connected to a unit of a plurality of memory cells (for example, eight memory cells), there was a problem that the GND potential was likely to be raised in a memory cell far from this connection. However, the difficulty of the conventional art can be overcome as each memory cell can contact the GND line independently in accordance with this embodiment.

0088Also as a modified embodiment, one GND interconnecting line may be provided for two memory cells adjacent to each other.

0089Only one access transistor (103) may be provided as shown in an equivalent circuit diagram Fig. 34. In this case, referring to the plan views in Figs. 20 to 23 showing the third embodiment, the portions (36, 37, 38 and 66) making up access transistor Q₄ are eliminated. If these portions are replaced by an isolation region 124, a memory cell having only one access transistor can be obtained in an SRAM.

0090In a fourth embodiment, a driver transistor Q₂ may not be formed in thin film polysilicon but may be formed in a silicon substrate 100 in an equivalent circuit shown in Fig. 34. The equivalent circuit in this case is shown in Fig. 35. Figs. 26 to 29 show structural plan views and Figs. 30 to 33 show structural cross-sectional views in this case. In this embodiment, polypads 149 and 150 formed of a polycrystalline silicon layer L2 shown in Fig. 27 respectively connect source/drain region of the load transistor to source/drain region of the driver transistor.

0091In the figure, the same reference numerals designate the same elements as in the second embodiment. As stated above, it is also possible to form a memory cell in an SRAM with driver transistor Q₂ being formed in the silicon substrate.

0092In addition to those shown in the embodiments described above, a memory cell of an SRAM with various combinations of transistors can be easily made, for example, at least one of the access transistors and the pair of load transistors are thin film transistors, or the pair of access transistors, one of the driver transistors and the pair of load transistors are thin film transistors. In particular, in a case where the access transistor is thin film transistor, a narrow channel effect or a back gate effect caused when it is formed in the semiconductor substrate may be ignored and the margin of the reading operation can be expanded, so that it is possible to stabilize the reading operation.

0093Fig. 36 shows an equivalent circuit diagram with a pair of access transistors Q₃ and Q₄ and a pair of load transistors Q₅ and Q₆ being thin film transistors in the memory cell of the SRAM shown in Fig. 17. Fig. 37 shows an equivalent circuit with one driver transistor Q₂, a pair of access transistors Q₃ and Q₄ and a pair of load transistors Q₅ and Q₆ being thin film transistors. Fig. 38 is an equivalent circuit diagram with the one access transistor Q₃ being also a thin film transistor in the memory cell of the SRAM shown in Fig. 34.

0094In a case where access transistors are thin film transistors as shown in Figs. 36, 37 and 38, for example, access transistors Q₃ and Q₄ may be p channel MOS transistors as shown in Fig. 37.

0095As stated above, in accordance with one aspect of the present invention, all the transistors making up the memory cell are thin film transistors and the transistors are insulated and isolated from each other without using a LOCOS film, so that miniaturization of the cell structure by reduction in the size of the isolation region can be realized.

0096In accordance with another aspect of the present invention, a stable operation of reading stored data as well as reduction in the size of a memory cell can be achieved with a structure in which three of the transistors making up the memory cell are thin film transistors stacked on the substrate.

0097In accordance with still another aspect of the present invention, reduction in the size of a memory cell can be achieved by eliminating one access transistor.

0098In accordance with yet another aspect of the present invention, at least a pair of access transistors and a pair of load transistors among transistors making up a memory cell are thin film transistors, so that reduction in the size of the memory cell can be achieved and an expanded operation margin of reading stored data can be obtained, making it possible to perform a stable reading operation.

0099Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.