SEMICONDUCTOR MEMORY DEVICE

Abstract: A semiconductor memory (DRAM) device comprises memory cells, each of which is composed of an FET(4s, 4c, 4d, 5, 6) and a capacitor. The FET has a SOI structure. The capaci­ tor has its dielectric layer (3b) formed by the insulating layer of the SOI structure, an upper capacitor electrode formed by the semiconductor layer (4) of the SOI structure, and a lower electrode formed by the semiconductor substrate (1). This enables the substrate to be biased with a voltage at an inter­ mediate level between a first storage voltage and a secnd storage voltage.

Description: The present invention relates to a semiconductor device, more particularly, to a dynamic random access memory (DRAM) comprising memory cells, each of which is composed of a transistor and a capacitor. System expansion and developments in technology have brought about a need to increase the storage capacity of the DRAM, and have given rise to a requirement for reducing the size of a memory cell. To secure a sufficient charge in a small cell area, a trench-capacitor formed in a groove dug in a semiconductor substrate has been proposed and used as a capacitor for the charge storage of information. There are also demands for a method of increasing the capacitance of a memory storage capacitor by making its dielectric layer thin in such a device, and conversely, for a method of improving the reliability of the dielectric layer by decreasing the electric field strength applied to a capacitor having a thin dielectric layer. A DRAM device including such a capacitor and proposed by the present inventor is disclosed in EP-A-0145606. In the example shown in Figure 9 of this disclosure each memory cell comprises a field effect transistor (FET) and a trench-capacitor. The FET has a source region and a drain region which are formed in a semiconductor substrate, and the capacitor comprises a dielectric or insulation layer formed on the groove surface, a conductive layer forming an upper capacitor electrode on the dielectric layer and connected to the drain region, and a lower capacitor electrode which is a diffusion region formed in the substrate and along the groove surface. This device suffers from the parasitic capacitance. Furthermore, restrictions on integration prevent the formation of electrodes for connection with the diffusion region forming the lower capacitor electrode. Accordingly, the capacitor is supplied with a supply voltage V CC or a total voltage of V CC and a substrate bias voltage. When the capacitor using a thin dielectric layer is supplied with such a high voltage, degradation of the reliability of the thin dielectric layer becomes a problem, and as a result, the reliability of the memory device is degraded. EP-A-0108390 describes a semiconductor memory device comprising a field effect transistor and a memory cell. An insulating layer lies between the field effect transistor and the substrate and this layer forms the dielectric of the capacitor. The semiconductor layer in which the field effect transistor is formed provides one electrode of the capacitor and the substrate the other electrode. GB-A-2113910 also describes a semiconductor memory device comprising a field effect transistor and a memory cell. Again the semiconductor layer in which the transistor is formed is also separated from the substrate by a layer of insulation. A first storage capacitor is formed above the semiconductor layer and a second capacitor using the insulating layer as a dielectric is formed below the semiconductor layer. According to this invention a semiconductor memory device comprises a peripheral circuit composed of a field effect transistor formed in a semiconductor substrate of a first conductivity type and a memory cell composed of a field effect transistor and a capacitor, formed in a well region of second conductivity type, opposite the first conductivity type, in a surface portion of the semiconductor substrate; an insulating layer having a thin portion and a thick portion formed on the well region; a semiconductor layer formed on the insulating layer; and the memory cell field effect transistor being formed in the semiconductor layer; and, the memory cell capacitor having its dielectric formed by the thin portion of the insulating layer, an upper electrode of the second conductivity type formed by a portion of the semiconductor layer, and a lower electrode formed by the well portion; the well portion being arranged to be biased with a voltage at an intermediate level between a first storage voltage and a second storage voltage applied to the semiconductor layer, thereby reducing the field strength across the dielectric of the memory cell capacitor. According to the present invention, the transistor is made with the SOI structure and is not formed in the substrate, whereby the substrate as the lower capacitor electrode can be biased with the intermediate level voltage. Therefore, the voltage applied across the upper capacitor electrode of the semiconductor layer and the lower capacitor electrode of the substrate is decreased by the intermediate bias voltage to a level lower than in the first above-mentioned prior art case. Namely, the electric field strength in the dielectric layer is decreased, and as a result, the reliability of the dielectric layer of the capacitor is increased. The SOI structure provides a reduced parasitic capacitance, and therefore, the transistor has a higher operating speed. Accordingly the memory device can operate at a higher speed than that of a memory device not having the SOI structure. In accordance with this invention it is possible to use a semiconductor substrate of a first conductivity type having a well or doped region of a second conductivity type, which is formed in the surface portion of the substrate and serves as the lower capacitor electrode of the capacitor. The second conductivity type is opposite that of the first conductivity type. In this case, the memory cell is disposed on the well region and a peripheral circuit of the memory cells has a conventional structure and is formed in the substrate outside the well region. V CC (0 V) or a minus voltage can be adopted for the substrate voltage as usual, which ensures a stable performance of the peripheral circuit. Particular examples of semiconductor memory devices in accordance with this invention will now be described with reference to the accompanying drawings; in which:- Figures 1A is a partial section through a first example of memory cell of a DRAM device; Figure 1B is a partial plan of the first example; Figure 2 is a partial section through a second example of a memory cell of a DRAM device; Figure 3A is a partial section through a third example of a memory cell of a DRAM device; Figure 3B is a partial sectional view of an FET of a peripheral circuit of the third example of a DRAM device; and, Figure 3C is a partial section through a modification of the third example of memory cell. Referring to Figures 1A and 1B, a DRAM device according to the present invention comprises memory cells, each of which is composed of an FET and a trench-capacitor. In the drawings, a semiconductor substrate 1 is, for example, an n-type silicon single crystalline substrate, and has a V-groove 2 for the trench-capacitor. An insulating layer 3 of, e.g., silicon dioxide (SiO₂), is formed on the substrate 1 and consists of a thick portion 3a and a thin portion 3b. A semiconductor layer 4 of, e.g., polycrystalline silicon, is formed on the insulating layer 3. A gate insulating layer 5 of, e.g., SiO₂, and a gate electrode 6 of, e.g., polycrystalline silicon, (i.e., a word line) are formed on the semiconductor layer 4 above the thick insulating layer portion 3b. The FET is composed of the gate insulating layer 5, the gate electrode 6, an n-type source region 4s, an n-type drain region 4d, and a p-type channel region 4c. These regions 4s, 4d, and 4c are formed in the semiconductor layer 4. Thus, the FET has the SOI structure. The trench-capacitor is composed of a dielectric (insulation) layer of the thin semiconductor layer portion 3b, an upper capacitor electrode of the portion of the semiconductor layer 4, and a lower capacitor electrode of the substrate 1. The electric charge for information is stored in the thin insulating layer portion 3b. An insulating layer 7 of, e.g., SiO₂ is formed over the entire surface and has a contact hole 8 wherein a portion of the source region 4s is exposed. In Fig. 1B the layer 7 is omitted. A conductive layer 9 of, e.g., aluminum (i.e., a bit line) is formed on the insulating layer 7 and is connected with the source region 4s. In the memory cell having the above-mentioned structure, since the FET is isolated from the substrate 1, the substrate 1 can be biased with a suitable (intermediate) voltage for the capacitor between a first storage voltage (practically the supply voltage V CC of, e.g., 5 V) for high level information, and a second storage voltage (practically, the ground voltage of 0 V) for low level information. For example, where V CC is +5 V, it is preferable to bias the substrate 1 with +2.5 V, i.e., 1/2 V CC . Accordingly, the capacitor is supplied with 2.5 V. In a conventional memory device, since a substrate is usually biased with the ground voltage or a minus voltage of from -2 to -3 V, a capacitor is supplied with a voltage of 5 V or 7 to 8 V. Thus compared with the conventional device, it is possible to substantially decrease an electric field strength in the dielectric layer of the capacitor. FETs of a peripheral circuit of a memory device may have the SOI structure and be formed by using the polycrystalline silicon layer 4 on the thick SiO₂ layer 3a. The memory device of Figs. 1A and 1B is produced in the following manner. An n-type single crystalline silicon substrate 1 is thermally oxidized to form a thick SiO₂ layer 3a. The SiO₂ layer 3a is selectively etched to expose a portion of the substrate 1. The exposed portion is anisotropically etched to form a V-groove 2 or a U-groove. The substrate 1 is thermally oxidized to form a thin SiO₂ layer 3b on the groove surface. Then, a p-type polycrystalline silicon layer 4 is deposited over the entire surface by a chemical vapor deposition (CVD) method and is patterned by an etching method. In order to form a thin SiO₂ layer including a gate insulating layer 5 on the polycrystalline silicon layer 4, a thermal oxidation treatment is carried out. A polycrystalline silicon layer is formed over the exposed surface by a CVD method and is then selectively etched to form a word line 6 including a gate electrode. Donor impurities (e.g., phosphorous) are implanted in the polycrystalline silicon layer 4 by an ion-implantation method to form an n-type source region 4s and an n-type drain region 4d. An SiO₂ layer 7 is then formed over the entire surface and is selectively etched to form a contact hole 8. An aluminum layer is formed on the SiO₂ layer 7 and is patterned by etching to form a bit line 9 connecting with the n-type source region 4s. Thus the memory cell having the FET and the trench-capacitor is obtained, as shown in Fig. 1A. A memory device according to another embodiment of the present invention is illustrated in Fig. 2. In this case, a semiconductor layer 4 is of a single crystalline silicon and the V-groove is filled with a polycrystalline silicon filler 21. In Fig. 2, the same reference numerals as those shown in Figs. 1A and 1B indicate the same portions. The device is produced in a similar manner to the above-mentioned production manner of the device shown in Figs. 1A and 1B, except for the formation of the polycrystalline silicon filler 21 and single-crystallization of the semiconductor layer 4. After the formation of the thin SiO₂ layer 36, a polycrystalline silicon layer is deposited over the entire surface by a CVD method so as to fill the V-groove, and then is selectively removed by a suitable etching method, to form a substantially flat surface, as shown in Fig. 2. A polycrystalline silicon layer is deposited over the entire flat surface by a CVD method, and is recrystallized by applying a laser beam to form a single crystalline silicon layer. The single crystalline silicon layer is patterned by etching to obtain the semiconductor layer 4 of single crystalline silicon. If the V-groove is not filled with polycrystalline silicon filler, and thus a polycrystalline silicon layer which is not flat is heated by the laser beam, the polycrystalline silicon layer will melt and a portion thereof will flow to the bottom of the V-groove to accumulate therein. As a result, the silicon layer separate into portions on the thick SiO₂ layer 3a and in the bottom of the V-groove, respectively, so that an upper capacitor electrode cannot be formed. In this case, since the FET is made by using the single crystalline silicon layer, the performance of the FET is better than that of the FET which is made by using the polycrystalline silicon layer. A memory device according to still another embodiment of the present invention is illustrated in Figs. 3A, 3B and 3C. In this case, an n-type well portion 32a, 32b is formed in a p⁻-type silicon single crystalline substrate 31 and serves as the lower capacitor electrode. The well portion 32a or 32b is biased with the intermediate voltage (e.g., +2.5 V) through a conductor line (not shown) connecting therewith. The p⁻-type substrate can be biased with the ground voltage (0 V) or a minus voltage (e.g., -2 to -3 V), so that an FET of the peripheral circuit of the DRAM device is conventionally formed by using the p-type substrate, as shown in Fig. 3B. The performance of such an FET is better than that of the FET having the SOI structure, since the crystal properties of the p⁻-type substrate are better than those of the semiconductor layer having the SOI structure. In Fig. 3B, a source (n⁺-type) region 34s and a drain (n⁺-type) region 34d of the FET are formed directly in the p⁻-type substrate 31. A field (thick) insulating (SiO₂) layer 35 and a gate insulating (thin SiO₂) layer 36 are formed on the substrate 31. A gate electrode 37 made of, e.g., polycrystalline silicon, is formed on the gate insulating layer 36. Another insulating (SiO₂) layer 38 covers the entire surface and has contact holes therein. A source electrode 39s and a drain electrode 39d of, e.g., aluminum, are formed. The memory cell shown in Fig. 3A is produced in the following manner. Donor impurities (e.g., phosphorous) are selectively implanted in the p⁺-type substrate by an ion-implantation method. The substrate is thermally oxidized to form a thick insulating SiO₂ layer 41a, and to simultaneously form the n-type well portion 32a, by diffusion. Then the thick SiO₂ layer 41a is selectively etched, and the p⁻-type substrate 31 is anisotropically etched to form a V-groove. The flow of the production steps is similar to those mentioned for the cell shown in Figs. 1A and 1B. The same reference numerals in Fig. 3A as these shown in Figs. 1A and 1B indicate the same portions. Where the V-groove is deep, it is necessary to make the depth of the well region deeper than the groove. However, it is difficult to form a very deep well region. To solve this problem, the structure of Fig. 3C is adopted instead of that of Fig. 3A. In this case, the n-type well region 32b is formed in two steps. Donor impurities (e.g., phosphorous) are selectively but not so deeply implanted in the p⁻-type substrate by an ion-implantation method. The substrate is thermally oxidized to form a thick SiO₂ layer 41b, and simultaneously, to form a portion of the well region 32b, which extends toward but does not reach the bottom peak of the V-groove. Then the thick SiO₂ layer 41b is selectively etched, and the substrate 31 is anisotropically etched to form the V-groove extending through the formed well region portion. Donor impurities are introduced into the substrate 31 from the V-groove surface by a thermal diffusion method, to form another portion of the well region 32b which is along the V-groove surface. Thus, the n-type well region 32b shown in Fig. 3C is obtained. The following production steps are similar to those mentioned for the cell shown in Figs. 1A and 1B, and the same reference numerals in Fig. 3C as those in Figs. 1A and 1B indicate the same portions. As explained above, according to the present invention, the adoption of the SOI structure can increase the speed of operation of device, decrease an electric field strength applied to a capacitor with a thin dielectric layer, and improve the reliability of the device.

Claims: A semiconductor memory device comprising: a peripheral circuit composed of a field effect transistor (34d, 34s, 36) formed in a semiconductor substrate (31) of a first conductivity type and a memory cell composed of a field effect transistor (4s, 4c, 4d) and a capacitor (3b, 4, 32), formed in a well region (32) of second conductivity type, opposite the first conductivity type, in a surface portion of the semiconductor substrate (31); an insulating layer having a thin portion (3b) and a thick portion (41) formed on the well region (32); a semiconductor layer (4) formed on the insulating layer (41, 42); and the memory cell field effect transistor (4s, 4c, 4d) being formed in the semiconductor layer (4); and, the memory cell capacitor having its dielectric formed by the thin portion (3b) of the insulating layer, an upper electrode of the second conductivity type formed by a portion of the semiconductor layer (4), and a lower electrode formed by the well portion (32); the well portion being arranged to be biased with a voltage at an intermediate level between a first storage voltage and a second storage voltage applied to the semiconductor layer, thereby reducing the field strength across the dielectric of the memory cell capacitor. A semiconductor memory device according to claim 1, wherein the field effect transistor comprises a source region (4s) and a drain region (4d) formed in the semiconductor layer (4) and a gate electrode (6) formed on a gate insulating layer (5) formed on the semiconductor layer (4). A semiconductor memory device according to claim 1 to 2, wherein the capacitor is a trench type capacitor. A semiconductor memory device according to claim 3, wherein the semiconductor layer is composed of a polycrystalline silicon portion (21) filling the trench and a single crystalline silicon portion (4) formed on the polycrystalline silicon portion (21) and on the insulating layer (3a)(Figure 2). A semiconductor memory device according to claim 1 or 2, wherein the semiconductor layer is formed entirely of poly-crystalline silicon. A semiconductor memory device according to any one of claims 1 or 2, wherein the semiconductor layer is formed entirely of single crystal silicon. A semiconductor memory device according to any one of the preceding claims, wherein the first storage voltage is a supply voltage and the second storage voltage is ground voltage. A semiconductor memory device according to claim 7, wherein the intermediate level voltage is half the supply voltage.

Inventors: TAKEMAE, YOSHIHIRO
Applicants: FUJITSU LIMITED
Application year: 1986
Main classification: H01L27
IPC codes: H01L21, H01L27, G11C11