Static RAM with thin film transistor

Abstract: A thin film transistor includes a semiconductor thin film (10) formed with a source region (12) and a drain region (13) at opposite end portions thereof and having an offset region (16) near at the drain region, a gate electrode (11) formed above the region between the source region and the offset region of the semiconductor thin film, with a gate insulating film (15) being interposed, and a conductive layer (17) formed above the gate electrode or under the semiconductor thin film, with an insulating film being interposed, the conductive layer being applied with generally the same potential as the gate electrode, wherein the resistance value of the offset region is controlled by the potential of the conductive layer. The gate electrode may be formed under the semiconductor thin film. In this case, the conductive layer is formed above the semiconductor thin film or under the gate electrode, with an insulating film being interposed. An SRAM is also provided which uses a thin film transistor constructed as above.

Description: The present invention relates to a static RAM having a first inverter including a first load transistor and a first drive transistor and a second inverter including a second load transistor and a second drive transistor. In the recent semiconductor technology, there is known a thin film transistor of the type that it is formed on an inner layer insulating film covering elements formed on the surface of a semiconductor substrate. Such a thin film transistor is, e.g., described in JP-A-4 125 970 where it is proposed to suppress potential barriers existing at the ends of a first gate electrode in a thin film transistor by a second gate electrode that is provided in a way that one end of the second gate electrode comprises the end which is located on the side of a drain electrode of the first gate electrode. The other end of the second gate electrode covers one part of an offset region. Another transistor device is described in EP-A-0 382 165 and particularly provided with a SOI-structure. Here, the source and the drain region are defined in a relation that the gate electrode is located adjacent to the source region with an intervening region formed between the gate electrode and the drain region. Further, there is provided a control electrode in correspondence to the intervening region such that the control electrode is separated from the semiconductor layer of the SOI-structure. In EP-A-0 403 279, too, a specific transistor structure is described wherein a first gate electrode is connected with a source of low potential in a time varying member so as to form an accumulation channel opposite to the gate electrode. It is proposed to improve the device performance by including a second gate electrode for forming a second accumulation channel that extends from the region of the first accumulation channel towards the drain electrode. Fig. 9 is a cross sectional view showing the structure of another thin film transistor as a load transistor of an SRAM. A semiconductor thin film 10 is formed on an interlayer insulating film (not shown) on a semiconductor substrate (not shown). A source region 12 and a drain region 13 are formed on opposite end portions of the semiconductor thin film 10, and a channel region 14 is formed between the source and drain regions. On the channel region 14, there is formed a gate oxide film 15 on which a gate electrode 11 is formed. A thin film transistor shown in Fig. 10 has its gate electrode 11 and gate oxide film 15 formed under the semiconductor thin film 10, and is symmetrical to the device shown in Fig. 9 relative to the semiconductor thin film 10. An offset region 16 having a width x is provided between each drain region 13 and gate electrode 11. This offset region 16 is provided to reduce a standby current to be consumed during an inactive state of the thin film transistor. The provision of such an offset region 16 weakens the electric field between the drain region 13 and gate electrode 11, reducing the leakage current. The offset region 16 cannot be controlled by a voltage applied between the drain region 13 and gate region 11. As a result, the offset region 16 functions as a resistor during an active state of the thin film transistor. This causes the drive current of the thin film transistor to lower, and the variation of resistance values of offset regions may change the characteristics of thin film transistors. Only recently such specifically structured thin film transistors have been found to be very useful for the design of high-performance SRAM-memory cells. Thus, in the article "A High-Performance SRAM Memory Cell With LDD-TFT-Loads" by K. Tsutsumi et al, 1991, Symposium on VLSI Technology, pp. 23, 24 a high-performance for Mb-SRAM-memory cells using top-gate PMOS-TFT loads with self-aligned LDDs have been proposed. Further features of this device are self-aligned LDD structures within the TFT-transistors that allow to achieve a very low standby current and a high stability of data holding. Further, top-gate structure for the TFT-transistors have been used as being suitable for self-alignment. It is proposed to fill buried contacts with n-type polysilicon and to silicide these contacts by means of self-alignment to avoid the formation of p-n junctions. The use of thin film transistor structures within SRAM-memory devices is also described in EP-A-0 443 549 which corresponds to the preambles of claims 1 and 2. Here a pair of load transistors of a flip-flop circuit constituting the memory device consists of thin film transistors, and channel regions of the pair of load transistors overlap drain regions of the load transistors through a gate insulating film. Thus, a channel length of the load transistor can be increased to reduce a leakage current of the load transistor. However, within the memory devices described in the last two documents it is difficult to prevent the lowering of the drive current and the variation of the characteristics of the thin film transistors. It is therefore an object of the present invention to provide a SRAM-memory using a thin film transistor capable of reduced standby current in an inactive state while supplying a large drive current during an active state, thus suppressing the variation of characteristics. This object is achieved for a static RAM-memory of the type outlined above in that each of the first and second load transistors comprises a semiconductor thin film formed with a source region and a drain region at opposite end portions thereof and having an offset region adjacent to the drain region; and a gate electrode formed above or under a region between the source region and the offset region of the semiconductor thin film, with a gate insulating film being interposed between the semiconductor thin film and the gate electrode, whereby the offset region of the first load transistor is positioned above a gate electrode of the first drive transistor, with an insulating film being interposed between the gate electrode of the first drive transistor and the offset region of the first load transistor and the resistance value of the offset region being controlled by the potential of the gate electrode of the first drive transistor, and the offset region of the second load transistor is positioned above a gate electrode of the second drive transistor, with an insulating film being interposed between the gate electrode of the second drive transistor and the offset region of the second load transistor, and the resistance value of the offset region being controlled by the potential of the gate electrode of the second drive transistor. This object is further achieved for a static RAM-memory of the type outlined above in that each of said first and second load transistors comprises a semiconductor thin film formed with a source region and a drain region at opposite end portions thereof and having an offset region adjacent to the drain region; and a gate electrode formed above or under a region between the source region and the offset region of the semiconductor thin film, with a gate insulating film being interposed between the semiconductor thin film and the gate electrode, and whereby the offset region of the first load transistor is positioned above a drain region of the second drive transistor, with an insulating film being interposed between the drain region of the second drive transistor and the offset region of the first load transistor, the resistance value of the offset region being controlled by the potential of the drain region of the second drive transistor, and the offset region of the second load transistor is positioned above a drain region of the first drive transistor, with an insulating film being interposed between the drain region of the first drive transistor and the offset region of the second load transistor, and the resistance value of the offset region being controlled by the potential of the drain region of the first drive transistor. Thus, within the inventive SRAM-memory there is provided a thin film transistor comprising: a semiconductor thin film formed with a source region and a drain region at opposite end portions thereof and having an offset region near at the drain region; a gate electrode formed above the region between the source region and the offset region of the semiconductor thin film, with a gate insulating film being interposed; and a conductive layer formed above the gate electrode or under the semiconductor thin film, with an insulating film being interposed, the conductive layer being applied with generally the same potential as the gate electrode, wherein the resistance value of the offset region is controlled by the potential of the conductive layer. The resistance value of the offset region, formed near at the drain region of the semiconductor thin film between the drain region and gate electrode, is controlled by the conductive layer with the potential generally the same as the gate electrode being applied. In the inactive state of the thin film transistor, the offset region weakens the electric field between the drain region and gate electrode, suppressing the leakage current. In the active state, the resistance value of the offset region is lowered, obtaining a large drive current. The gate electrode may be formed under the semiconductor thin film. In this case, the conductive layer is formed above the semiconductor thin film or under the gate electrode, with an insulating film being interposed. Further, in the SRAM-memory having the first and second load transistors made of thin film transistors, the potential of the gate electrode of the first thin film transistor is the same as the gate electrode of the first drive transistor or the drain region of the second drive transistor, and the potential of the gate electrode of the second thin film transistor is the same as the gate electrode of the second drive transistor or the drain region of the first drive transistor. The offset region of the first thin film transistor positions above the gate electrode of the first drive transistor or the drain region of the second drive transistor, and the offset region of the second thin film transistor positions above the gate electrode of the second drive transistor or the drain region of the first drive transistor. Accordingly, the resistance value of the offset region can be controlled without providing an additional conductive layer. The offset region of the first load transistor may be positioned above the drain region of the second drive transistor, with an insulating film being interposed, and the resistance value of the offset region is controlled by the potential of the drain region of the second drive transistor. The offset region of the second load transistor may be positioned above the drain region of the first drive transistor, with an insulating film being interposed, and the resistance value of the offset region is controlled by the potential of the drain region of the first drive transistor. Overall, the use of the gate electrode of the other transistor as the conductive layer without forming an additional conductive layer, simplifies the device structure and the manufacturing process. Further, the resistance in the offset region declines, thus preventing the lowering of the drive current and the variation of characteristics of the thin film transistors within the SRAM-memory. Further features and advantages result from the following detailed description of different embodiments thereof with particular reference to the accompanying drawings in which: Fig. 1 is an elevational cross section showing the structure of a thin film transistor used in the present invention; Fig. 2 is an elevational cross section showing the structure of a thin film transistor used in the present invention; Fig. 3 is an elevational cross section showing the structure of a thin film transistor used in the present invention; Fig. 4 is an elevational cross section showing the structure of a thin film transistor used in the present invention; Fig. 5 is an elevational cross section showing the structure of an SRAM according to a first embodiment of the present invention; Fig. 6 is an elevational cross section showing the structure of an SRAM according to a second embodiment of the present invention; Fig. 7 is a circuit diagram of an SRAM having load transistors made of thin film transistors according to the present invention; Fig. 8 is a graph showing the drive current of the thin film transistor used in the present invention, as compared with the drive current of a conventional thin film transistor; Fig. 9 is an elevational cross section showing the structure of a conventional thin film transistor; and Fig. 10 is an elevational cross section showing the structure of another conventional thin film transistor. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described with reference to the accompanying drawings. Fig. 1 is a cross sectional view showing the structure of a thin film transistor used in the present invention. An interlayer insulating film (not shown) is formed on a semiconductor substrate (not shown). On this interlayer insulating film, a semiconductor thin film 10 is formed. This semiconductor thin film 10 is formed by depositing amorphous silicon and growing it into polysilicon in a solid phase, and has a film thickness of about 300 angstroms. A source region 12 and a drain region 13 are formed in the semiconductor thin film 10 at its opposite end portions. On a channel region 14 between the source and drain regions 12 and 13, there is formed a gate oxide film 15 having a film thickness of about 300 angstroms on which film a gate electrode 11 having a thickness of about 1000 angstroms is formed. Between the drain region 13 and gate electrode 11, an offset region 16 having a width x is provided as with a conventional thin film transistor. A conductive layer 17 is formed under the semiconductor thin film 10, with an insulating film (not shown) being interposed therebetween. It is necessary for the distance between the conductive layer 17 and semiconductor thin film 10 to be set greater than the film thickness of the gate oxide film 15. The thin film transistor constructed as above operates as follows. In the inactive state, because of the provision of the offset region 16, the electric field between the drain region 13 and gate electrode 11 weakens similarly to the conventional case, thereby reducing the leakage current. Since the conductive layer 17 is positioned more remotely from the semiconductor thin film 10 than the gate electrode 11, the effect of the conductive layer 17 is small even if the conductive layer 17 were applied with the same potential as the gate electrode 11. In the active state, the potential of the conductive layer 17 is set to the same potential as the gate electrode 11. With this setting, the offset region 16 is controlled by the conductive layer 17 so that this region operates in the same manner as the channel region 14 under the gate electrode 11. The resistance of the offset region 16 lowers accordingly, preventing the drive current from being reduced. In addition, the variation of resistance values of the offset regions 16 is suppressed, maintaining the characteristics of thin film transistors with less change. In this first form, the gate electrode 11 is formed above the semiconductor thin film 10. In contrast, the gate electrode 11 of the second form shown in Fig. 2 is formed under the semiconductor thin film 10. Also in the second form, the potential of the offset region 16 between the drain region 13 and gate electrode 11 is controlled by the conductive layer 17 in the active state. Therefore, the resistance of the offset region 16 lowers, preventing the lowering of the drive current and the variation of characteristics of thin film transistors. In the first and second forms, the conductive layer 17 is formed under the Semiconductor thin film 10. In contrast, in a third form shown in Fig. 3 and fourth form shown in Fig. 4, the conductive layer 17 is formed above the semiconductor thin film 10. In the third form, the gate electrode 11 is positioned above the semiconductor thin film 10, and in the fourth form, the gate electrode 11 is positioned under the semiconductor thin film 10. In both the forms, in the active state, the conductive layer 17.is applied with the same potential as the gate electrode 11 to control the offset region 16 and reduce the resistance thereof. In the first to fourth forms described above, the positions of the gate electrodes 11 relative to the semiconductor thin films 10 differ from each other, and the positions of the conductive layers 17 relative to the semiconductor thin film 10 differ from each other. In any one of the first to fourth forms, the resistance of the offset region 16 can be reduced in the active state. Next, an SRAM according to a first embodiment according to the present invention will be described. In this embodiment, a thin film transistor is used as a load transistor for each cell of the SRAM. As shown in Fig. 7, an SRAM cell has two inverters, one being formed by a P channel transistor P1 and an N channel transistor N1 and the other being formed by a P channel transistor P2 and an N channel transistor N2. The P channel transistors P1 and P2 as the load transistors are made of thin film transistors. As shown from Fig. 7, the gate electrode of the P channel transistor P1 is applied with the same potential as the gate electrode of the N channel transistor N1 operating as the drive transistor. The gate electrode of the N channel transistor N1 is used as the conductive layer of the P channel transistor P1 of the thin film transistor. Use of the gate electrode of the other transistor as the conductive layer without forming an additional conductive layer, simplifies the device structure and the manufacturing processes. Fig. 5 is an elevational cross section showing the structure of an SRAM according to the first embodiment of the present invention. On the surface of a semiconductor substrate 21, field oxide films 24a and 24b are formed. Between the field oxide films 24a and 24b, a gate oxide film 22 is formed on which a gate electrode 23 is formed. Also formed are gate oxide films 25 and gate electrodes 26 for N channel transistors N3 and N4 operating as the transfer transistors. A bit line contact 27 for a bit line BL is connected to the source region of the transfer transistor. An interlayer insulating film 28 having a thickness from 4000 to 7000 angstroms is formed on the semiconductor substrate 21. A gate electrode 29 and a gate oxide film 36 of the P channel transistor P1 are formed on the surface of the interlayer insulating film, in this order from the bottom. A semiconductor thin film 30 is formed on the surfaces of the gate electrode 29, nearby interlayer insulating film 28, and gate electrode 35 of the other P channel load transistor P2. A source region 31 and a drain region 33 are formed in the semiconductor thin film 30 at opposite end portions of the film. A channel region is provided above the gate electrode 29 between the source region 31 and drain region 33. An offset region 34 having a width x is provided between the gate electrode 29 and drain region 33. The offset region 34 positions above the gate electrode 23 of the N channel transistor N1. As described previously, the gate electrode 23 is applied with the same potential as the gate electrode 29 of the thin film transistor, the gate electrode 23 having the same function as the conductive layer 17 of the first to fourth forms. In the active state of the thin film transistor, therefore, the resistance of the offset region 34 is controlled by the gate electrode 23 of the N channel transistor N1, and so the offset region 34 provides the same function as the channel region 32. The resistance of the offset region 34 therefore declines, preventing the lowering of the drive current and the variation of characteristics of the thin film transistors. As shown in Fig. 7, the potential of the gate electrode of the P channel transistor P1 formed as the thin film transistor is the same as not only the gate electrode of the N channel transistor N1 but also the drain region of the N channel transistor N2 of the other inverter. In consideration of this, in an SRAM according to a second embodiment according to the present invention, the offset region of a thin film transistor is positioned above the drain region of an N channel transistor N2. Fig. 6 is an elevational cross section showing the structure of an SRAM of the second embodiment. On the surface of a semiconductor substrate 41, field oxide films 42a and 42b are formed. Between the field oxide films 42a and 42b, a drain region (not shown) of the N channel transistor N2 is formed. Also formed as in the fifth embodiments are gate oxide films 44 and gate electrodes 45 for N channel transistors N3 and N4 operating as the transfer transistors, and a bit line contact 46. An interlayer insulating film 47 is formed on the surface of the semiconductor substrate 41. A gate electrode 51 and a gate oxide film 58 of the P channel transistor P1 as the thin film transistors are formed on the interlayer insulating film 47, in this order from the bottom. A semiconductor thin film 52 is formed on the surfaces of the device. A source region 52 and a drain region 56 are formed in the semiconductor thin film 52 at opposite end portions of the film. The drain region 56 is electrically connected to the gate electrode 57 of the other P channel load transistor P2. The gate electrode 57 is connected to the gate electrode 43 of the N channel transistor N2. A channel region 54 in the semiconductor thin film 52 is positioned above the gate electrode 51. Between the gate electrode 51 and drain region 56, there is provided an offset region 55. The offset region 55 is positioned above the drain region of the N channel transistor N2. In the active state of the thin film transistors, therefore, the drain region operates as the conductive layer so that the resistance of the offset region 55 declines, preventing the lowering of the drive current and the variation of characteristics of the thin film transistors. In the first and second embodiments, the P channel transistor P1 of the two load transistors has been used for the description of the embodiments. The other P channel transistor P2 can also be configured in the same manner. Namely, the offset region is positioned above the gate electrode of the N channel transistor N2 or the drain region of the N channel transistor N1. Fig. 8 shows the results of comparison between the drive current of the thin film transistor of the second form shown in Fig. 2 and the drive current of the conventional thin film transistor shown in Fig. 10. The dimensions of the second form thin film and the conventional thin film were both set such that the gate electrode 11 had a width 0.7 µm and a length 1.0 µm and the offset region 16 had a width x of 0.6 µm, The conductive layer 17 for the second form thin film transistor was made of tungsten polycide. The drive current indicated by a curve a of the second form is larger than that indicated by a curve b of the conventional thin film transistor. It was verified that a large drive current was obtained by controlling the offset region between the drain region and gate electrode by the conductive layer. The above-described embodiments are illustrative only, and are not intended to limit the scope of the present invention. The conductive layer of the invention may be made of any other conductive material such as polysilicon silicide, polycide, and metal, as with the material for the gate electrode.

Claims: A static RAM having a) a first inverter including a first load transistor (P1) and a first drive transistor (N1), and b) a second inverter including a second load transistor (P2) and a second drive transistor (N2), c) each of said first and second load transistors (P1, P2) comprising: c1) a semiconductor thin film (30) formed with a source region (31) and a drain region (33) at opposite end portions thereof and a channel region (32); and c2) a gate electrode (32) formed above or under said channel region (32) of said semiconductor thin film (30), with a gate insulating film (36) being interposed between said semiconductor thin film (30) and said gate electrode (32), characterised in that said semiconductor thin film (30) further comprises an offset region (34) between said channel region (32) and said drain region (33) whereby the gate electrode is also formed above or under said offset region, whereby c3) said offset region (34) of said first load transistor (P1) is positioned above a gate electrode (23) of said first drive transistor (N1), with an insulating film (28) being interposed between said gate electrode (23) of said first drive transistor (N1) and said offset region (34) of said first load transistor (P1) and the resistance value of said offset region (34) being controlled by the potential of said gate electrode (23) of said first drive transistor (N1), and c4) said offset region of said second load transistor (P2) is positioned above a gate electrode of said second drive transistor (N2), with an insulating film being interposed between said gate electrode of said second drive transistor (N2) and said offset region of said second load transistor (P2), and the resistance value of said offset region being controlled by the potential of said gate electrode of said second drive transistor (N2). A static RAM having a) a first inverter including a first load transistor (P1) and a first drive transistor (N1), and b) a second inverter including a second load transistor (P2) and a second drive transistor (N2), c) each of said first and second load transistors (P1, P2) comprising: c1) a semiconductor thin film (52) formed with a source region (53) and a drain region (56) at opposite end portions thereof and a channel region (54); and c2) a gate electrode (51) formed above or under said channel region (54) of said semiconductor thin film (52), with a gate insulating film (58) being interposed between said semiconductor thin film (52) and said gate electrode (51), characterised in that said semiconductor thin film (52) further comprises an offset region (55) between said channel region (54) and said drain region (56) whereby the gate electrode is also formed above or under said offset region, whereby c3) said offset region (55) of said first load transistor (P1) is positioned above a drain region of said second drive transistor (N2), with an insulating film (47) being interposed between said drain region of said second drive transistor (N2) and said offset region (55) of said first load transistor (P1), the resistance value of said offset region (55) being controlled by the potential of said drain region of said second drive transistor (N2), and c4) said offset region of said second load transistor (P2) is positioned above a drain region of said first drive transistor (N1), with an insulating film being interposed between said drain region of said first drive transistor (N1) and said offset region of said second load transistor (P2), and the resistance value of said offset region being controlled by the potential of said drain region of said first drive transistor (N1).

Inventors: KASAI KUNIHIRO, KASAI, KUNIHIRO
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1993
Main classification: H01L27
IPC codes: H01L21, H01L27, H01L29