Abstract:
The invention relates to a memory cell configuration in which a plurality of memory cells are present in the region of a main area of a semiconductor substrate ( 10 ), and in which the memory cells each contain at least one MOS transistor having a source ( 29 ), gate (WL 1  and WL 2 ) and drain ( 60 ). The memory cells are configured in memory cell rows which run essentially parallel, in which adjacent memory cell rows are insulated by an isolation trench ( 20 ), in which adjacent memory cell rows each contain at least one bit line ( 60 ), and where the bit lines ( 60 ) of two adjacent memory cell rows face one another. The memory cell configuration is constructed in such a way that the isolation trench ( 20 ) penetrates more deeply into the semiconductor substrate ( 10 ) than the bit lines ( 60 ), and at least one of the source ( 29 ) and/or of the drain is at least partially situated underneath the isolation trench ( 20 ). The invention furthermore relates to a method for fabricating this memory cell configuration.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of copending international application PCT/DE99/00762, filed Mar. 17, 1999, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a memory cell configuration in which a plurality of memory cells are present in the region of a main area of a semiconductor substrate, in which the memory cells each contain at least one MOS transistor having a source, gate and drain, in which the memory cells are configured in memory cell rows which run essentially parallel, in which adjacent memory cell rows are insulated by an isolation trench, in which adjacent memory cell rows each contain at least one bit line, and where the bit lines of two adjacent memory cell rows face one another. The invention furthermore relates to a method for fabricating this memory cell configuration. 
     Memory cells are used in wide areas of technology. The memory cells may involve both read-only memories, which are referred to as ROMs, and programmable memories, which are referred to as PROMs (programmable ROMs). 
     Memory cell configurations on semiconductor substrates are distinguished by the fact that they allow random access to the information stored in them. They contain a multiplicity of transistors. During the reading operation, the logic states 1 or 0 are assigned to the presence or absence of a current flow through the transistor. The storage of the information is usually effected by using MOS transistors whose channel regions have a doping which corresponds to the desired blocking property. 
     A memory cell configuration of the generic type is shown in Yoshida (5,306,941). In this memory cell configuration, bit lines are configured in the edge region of memory cell webs, and the bit lines of adjacent memory cell webs face one another. In this case, the bit lines are isolated from one another in each case by an isolation trench filled with an insulating material. This document furthermore discloses a method for fabricating a memory cell configuration, in which memory cell webs are formed by etching isolation trenches into a semiconductor substrate. The etching of the isolation trenches is followed by diffusion of a dopant, bit lines being formed by the diffusion. This memory cell configuration of the generic type is suitable for feature sizes of at least 0.5 μm and for a ROM read-only memory. Electrical programming is not possible in this case. 
     A further memory cell configuration is disclosed in DE 195 10 042 A1. This memory cell configuration contains MOS transistors configured in rows. The MOS transistors are connected in series in each row. In order to increase the storage density, adjacent rows are in each case configured alternately at the bottom of strip-type longitudinal trenches and between adjacent strip-type longitudinal trenches at the surface of the substrate. Interconnected source/drain regions are designed as a contiguously doped region. Row-by-row driving enables this memory cell configuration to be read. 
     This memory cell configuration is distinguished by the fact that the area requirement that is necessary for the memory cells has been reduced from 4 F 2  to 2 F 2  where F is the minimum feature size of the photolithographic process used for the fabrication. What is disadvantageous, however, is that a further increase in the number of memory cells per unit area is not possible in this case. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a memory cell configuration and a method of producing the configuration which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type in such a way the greatest possible number of memory cells is configured in the smallest possible space. Preferably, the memory cell configuration shall also be electrically programmable. 
     In the case of a device of the generic type, this object is achieved by virtue of the fact that the isolation trench penetrates more deeply into the semiconductor substrate than the bit lines, and in that at least one partial region of the source and/or of the drain is situated underneath the isolation trench. 
     The invention thus provides for the memory cell configuration to be configured in such a way that it contains memory cell webs between which there are isolation trenches which penetrate deeply into the semiconductor substrate and thus enable effective insulation of mutually opposite bit lines. 
     An electrical connection between the sources and/or the drains of different memory cell webs is preferably effected by a partial region of the sources and/or of the drains extending from one memory cell web to a further memory cell well. 
     In this case, the sources and/or drains of different transistors are preferably located in a common well. 
     The memory cell configuration is made electrically programmable by the provision of a gate dielectric with traps for electrical charge carriers, for example a triple layer having a first SiO 2  layer, layer, an Si 3 N 4  layer and a second SiO 2  layer, or the like. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a memory cell configuration, that includes a semiconductor substrate with a plurality of memory cells each including at least one MOS transistor having a source, a gate, and a drain. The plurality of memory cells are configured in substantially parallel memory cell rows. Each of the memory cell rows include at least one bit line configured such that a bit line of one of the memory cell rows faces a bit line of an adjacent one of the memory cell rows. The semiconductor substrate includes isolation trenches insulating adjacent ones of the memory cell rows. The isolation trenches penetrate more deeply into the substrate than the at least one bit line. The at least one MOS transistor includes a region configured to be at least partially underneath the isolation trench, and the region is selected from the group consisting of the source and the drain. 
     In accordance with an added feature of the invention, the sources of adjacent ones of the MOS transistors are designed as a continuously doped region. 
     In accordance with an additional feature of the invention the drains of adjacent ones of the MOS transistors are designed as a continuously doped region. 
     In accordance with another feature of the invention, the isolation trenches penetrate from 0.1 μm to 0.5 μm more deeply into the semiconductor substrate than the at least one bit line. 
     In accordance with a further feature of the invention, the at least one bit line of each of the memory cell rows has a height of from 0.1 μm to 0.3 μm. 
     In accordance with a further added feature of the invention, there is provided a web with mutually opposite side walls configured between each two adjacent ones of the isolation trenches. Each web includes two of the memory cell rows. The at least one bit line of each of the memory cell rows adjoin one of the side walls of the web. Adjacent ones of the memory cells that are perpendicular to a course of the bit lines have a common region selected from the group consisting of a common source region and a common drain region. 
     With the foregoing and other objects in view there is also provided, in accordance with the invention, a method for fabricating a memory cell configuration, which includes steps of: etching isolation trenches into a semiconductor substrate and thereby forming webs between the isolation trenches; producing bit lines after channel regions have been produced; and subsequent to producing the bit lines, performing an etching step resulting in the isolation trenches penetrating more deeply into the semiconductor substrate. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a memory cell configuration and method for fabricating it, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross section through a semiconductor substrate after a first etching operation; 
     FIG. 2 shows the semiconductor substrate illustrated in FIG. 1 after the implantation of a first dopant; 
     FIG. 3 shows the semiconductor substrate after the implantation of a second dopant; 
     FIG. 4 shows the semiconductor substrate after a further etching operation; 
     FIG. 5 shows a circuit diagram for an electrical connection of individual regions of the semiconductor substrate illustrated in FIG. 4; 
     FIG. 6 shows a detail from a section perpendicular to the section shown in FIGS. 1 to  4 , through the upper region of the semiconductor substrate after the deposition of a dielectric layer, the deposition and patterning of a semiconductor layer and the deposition of a further insulating material; 
     FIG. 7 shows the detail from the upper region of the semiconductor substrate after the performance of anisotropic etching for the purpose of forming spacers; 
     FIG. 8 shows the detail from the upper region of the semiconductor substrate after a further etching operation; 
     FIG. 9 shows the detail from the upper region of the semiconductor substrate after the growth of a dielectric layer; 
     FIG. 10 shows the detail from the upper region of the semiconductor substrate after the application and partial etching away of an electrode layer; 
     FIG. 11 shows a plan view of the finished memory cell configuration; and 
     FIG. 12 shows the electrical circuit diagram of a detail from the cell array. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a mask  15  which is applied to a semiconductor substrate  10  that is made, for example, of n-doped monocrystalline silicon with a basic dopant concentration of, preferably, from 1×10 16  cm −3  to 1×10 17  cm −3 , for example 2×10 16  cm −3 . The mask  15  may be composed for example of silicon oxide formed according to a TEOS (Si(OC 2 H 5 ) 4 ) method. In a TEOS method, tetraethyl orthosilicate Si(OC 2 H 5 ) 4  is converted into silicon oxide SiO 2  at a temperature of approximately 700 degrees Celsius and a pressure in the region of 40 Pa. 
     After the application of the mask  15 , an etching process is carried out, for example a multistage process with a first etching step with a gas mixture comprising CF 4  and O 2  or CHF 3  and O 2  and a second etching step with an HBr-containing gas, with the result that isolation trenches  20  are formed in the semiconductor substrate  10 . 
     There are webs  30  between the isolation trenches  20 , the distance between the centers of adjacent webs  30  being 2F. In this case, F is the minimum feature size that can be fabricated, and is preferably in the range of from 0.1 μm to 0.5 μm. This processing state of the semiconductor substrate is illustrated in FIG.  1 . 
     A first dopant  22  is subsequently implanted, with the result that side regions  25  of the webs  30  and lower bottom regions  28  of the isolation trenches  20  are p-doped. The side regions  25  of the webs  30  and the bottom regions  28  of the isolation trenches  20  form channel regions in the finished memory cell configuration. In order to produce p-type doping, boron, for example, is implanted at an implantation energy preferably of the order of magnitude of from 10 to 20 keV. The implantation dose is equal to the product of a desired concentration and the thickness of a layer to be doped. For example, the implantation dose is 4×10 12  cm −2 , given a preferred layer thickness of about 0.2 μm and an advantageous concentration of 2×10 17  cm −3 . After the implantation and drive-in, the concentration of the dopant in the side regions  25  and in the lower bottom regions  28  is approximately 2×10 17  cm −3 . As a result of the dopant drive-in, a bottom region  28 , with two side regions  25  connected to it, forms a region in which a continuous channel can form in the finished memory cell configuration. This processing state of the semiconductor substrate is illustrated in FIG.  2 . 
     A further dopant  35  is subsequently implanted, with the result that side walls  40  of the webs  30  and upper bottom regions  50  of the isolation trenches  20  are heavily doped by the opposite conductivity type to that of the side regions  25  and of the bottom region  28 . In order to produce n + -type doping, phosphorus or arsenic, for example, is implanted at an implantation energy preferably of the order of magnitude of from 40 to 80 keV, and with a dose in the region around 2×10 15  cm −2 . 
     After this implantation, the concentration of the dopant in the side walls  40  and in the upper bottom regions  50  is approximately 2×10 20  cm −3 . This processing state of the semiconductor substrate is illustrated in FIG.  3 . 
     In order to provide insulation between the individual webs  30 , a further etching process is subsequently performed. As a result of this, the isolation trenches  20  are etched more deeply and the doped upper bottom regions  50  of the isolation trenches  20  are removed. As a result of this process, bit lines  60  that are spacially separate from one another are formed from the side walls  40  of the webs  30 , mutual insulation between the bit lines being ensured by virtue of the fact that the isolation trenches  20  penetrate as far as possible into the substrate. Parts of the bit lines  60  form drains of MOS transistors in the finished memory cell configuration. The bit lines  60  have a height of approximately 200 nm. The depth of the isolation trenches  20  is greater than the height of the bit lines  60 . An effective path length  1  for a possible current path through the semiconductor substrate  10  is thereby enlarged. This processing state of the semiconductor substrate is illustrated in FIG.  4 . 
     A low-resistance connection between the sources  29  is effected for example via a common well (not shown). A connection may, for example, also be effected via the semiconductor substrate or an electrically conductive layer. 
     The width of the bit lines  60  is approximately 50 nm. Given a cross-sectional area of (200×10 −9  m)×(50×10 −9  m)=1×10 −14  m 2 , the bit lines thus have a resistance of the order of magnitude of a few 100 kΩ per mm length of the bit line, a typical value being 500 kΩ/mm. Cell arrays with an edge length of about 1 mm can be realized as a result of this. 
     A typical threshold voltage of a memory cell configuration of this type is approximately 0.6 V. A circuit diagram for an electrical connection of the bit lines  60  and of word lines WL is illustrated in FIG.  5 . 
     Through a siliciding process (not shown), the resistance of the bit lines  60  can be considerably reduced, preferably by a factor of 10 or more. In the case of such a siliciding process, the bit lines  60  are converted into a suitable silicide, i.e. into a metal-silicon compound. In the present case, it is particularly expedient to produce silicides such as MoSi 2 , WSi 2 , TaSi 2 , TiSi 2 , PtSi, Pd 2 Si by siliconization. Siliconization is a process of selective silicide formation. 
     It is preferably performed by the silicide-forming metal being sputtered on alone and then being brought to a silicide reaction with the bit lines as silicon support. The application of the silicide-forming metal is followed by heat treatment at temperatures in the range of from 600 to 1000° C., thereby resulting in the formation of the metal silicide. 
     The mask  15  is subsequently removed. After the removal of the mask  15 , the isolation trenches  20  are filled with an insulating material, for example with SiO 2  formed using a TEOS method. This can be done by converting tetraethyl orthosilicate: Si(OC 2 H 5 ) 4  into silicon oxide SiO 2  at a temperature of approximately 700° C. and a pressure in the region of 40 Pa. 
     The filling of the isolation trenches  20  with the insulating material is followed by a planarization operation, preferably a process of chemical mechanical planarization. A suitable dielectric layer is then applied to the webs  30  and the isolation trenches  20 . The dielectric layer may preferably be formed by a multiple layer. It is particularly expedient if the dielectric layer is a triple layer, having a first dielectric layer  90  made of silicon oxide SiO 2  having a thickness of approximately 3 nm, a middle dielectric layer  100  made of silicon nitride having a thickness of approximately 7 to 8 nm, and an upper dielectric layer  110  made of silicon oxide having a thickness of about 4 nm. 
     The first dielectric layer  90  is formed to a desired layer thickness for example by heat treatment in an O 2 -containing atmosphere. In this case, the silicon of the webs  30  is converted into silicon oxide SiO 2 . This layer may subsequently be patterned by anisotropic etching using CHF 3 , for example. 
     The second dielectric layer  100  is preferably applied according to a CVD (Chemical vapor Deposition) method, in particular according to an LPCVD (Low Pressure CVD) method. A particularly suitable variant for forming the second dielectric layer  100  according to the LPCVD method may be performed by converting dichlorosilane (SiH 2 Cl 2 ) into silicon nitride (Si 3 N 4 ) with addition of ammonia (NH 3 ) at a temperature in the region of about 750° C. in a plasma at a pressure of between 10 Pa and 100 Pa, preferably 30 Pa. 
     The upper dielectric layer  110  is subsequently deposited by thermal oxidation, preferably in an H 2 O-containing atmosphere at a temperature of around 900° C. and for a period of about 2 hours, or according to one of the known layer-producing methods, for example an HTO method. Deposition using an HTO method may preferably be done by converting dichlorosilane SiH 2 Cl 2  into silicon oxide SiO 2  in an N 2 O-containing atmosphere at a temperature of approximately 900° C. and a pressure in the region of 40 Pa. 
     A semiconductor layer  120 , for example made of heavily doped polycrystalline silicon, is grown onto the upper dielectric layer  110 . A preferred doping of the polycrystalline silicon is at least 10 20  cm −3 , dopings above 10 21  cm −3  being particularly suitable. 
     By way of example, the semiconductor layer  120  is n + -doped by diffusion or implantation of phosphorus or arsenic. Implantation may be effected for example with an energy of 80 keV and a dose of 1×10 16  cm −2 . 
     A resist mask is subsequently applied to the semiconductor layer  120 . This is followed by an etching process, for example a multistage process with a first etching step with a gas mixture comprising CF 4  and O 2  or CHF 3  and O 2  and a second etching step with an HBr-containing gas. Isolation trenches  130  are thereby etched into the semiconductor layer  120 . Webs  140  are produced between the isolation trenches  130  as a result of the remaining material of the semiconductor layer  120 , the webs serving as word lines in the completed memory cell configuration. 
     An insulation layer  150  is subsequently deposited onto the webs  140  and the isolation trenches  130  according to a suitable method that is as far as possible conformal. It is particularly expedient for the insulation layer  150  to be formed according to a TEOS method. This can be done by converting tetraethyl orthosilicate Si(OC 2 H 5 ) 4  into silicon oxide SiO 2  at a temperature of approximately 700° C. and a pressure in the region of 40 Pa. 
     That detail of the semiconductor substrate which contains the dielectric layers  90 ,  100  and  110  and also the webs  140  is illustrated in FIG.  6 . In this case, FIG. 6 shows a section which runs perpendicularly to the section shown in FIGS. 1 to  4  through one of the webs  30 . 
     The insulation layer  150  is subsequently etched anisotropically, the etching removal of this etching operation corresponding to the thickness of the insulation layer  150  on planar regions. Spacers  160  therefore remain on the side walls of the webs  140 , the spacers also being referred to as TEOS spacers. This state of the semiconductor substrate is illustrated in FIG.  7 . 
     An etching process is subsequently performed, the nitride-containing dielectric layer  100  being removed by the use of a suitable agent, for example phosphoric acid with a concentration in the region of 80% and a temperature of around 150° C. The multistage etching process stops at the oxidecontaining lower dielectric layer  90 . The thin dielectric layer  90  is removed in the region of the isolation trenches  130  by means of a further etching operation, for example using a hydrofluoric acid-containing solution (HF-dip). This state of the semiconductor substrate is illustrated in FIG.  8 . 
     A new triple layer is subsequently grown. To that end, a lower dielectric layer  180 , a middle dielectric layer  190  and an upper dielectric layer  200  are formed. The lower dielectric layer  180  is preferably composed of silicon oxide SiO 2 , which is formed to a desired layer thickness using a heat-treatment method, for example. In this case, in the surface region of the webs  140  and of the semiconductor material  120 , silicon is converted into silicon oxide SiO 2  in an oxygen-containing atmosphere at a temperature of approximately 800 to 900° C. The middle dielectric layer  190  is preferably formed by a nitride layer which has been produced by means of an LPCVD method at approximately 700° C. The topmost dielectric layer  200  is preferably composed of the same material as the lower dielectric layer  180 , that is to say once again preferably of SiO 2 . In the final state, the thickness of the lower dielectric layer  180  is 3 nm, for example, the thickness of the middle dielectric layer  190  is approximately 7 to 8 nm and the thickness of the upper dielectric layer  200  is 4 nm. Such a sequence of the thicknesses of the layers is particularly expedient for storing captured charges as long as possible. This state of the semiconductor substrate is illustrated in FIG.  9 . 
     An electrode layer  210  is subsequently formed over the whole area. The electrode layer  210  is composed for example of a doped semiconductor material, preferably n-doped polycrystalline silicon, metal silicide and/or a metal. 
     However, the semiconductor material of the electrode layer  210  may also be p-doped. 
     The electrode layer  210  is formed to a thickness which suffices to fill the isolation trenches  130  between the webs  140  forming the word line. The electrode layer  210  is therefore deposited to a thickness of approximately 0.2 μm to 0.6 μm, preferably 0.4 μm. 
     The electrode layer  210  is subsequently patterned. The electrode layer  210  is patterned in a method which has a number of steps. Firstly, the electrode layer  210  is removed by a planarization process, for example a CMP (Chemical Mechanical Planarization) step. In this case, the middle dielectric layer  190  acts as a stop layer. 
     The dielectric layer  170  is subsequently removed above the webs by the removal of its partial layers  180 ,  190  and  200 . This is followed by further etching back or a process of chemical mechanical planarization (CMP) (FIG.  10 ). 
     In the memory cell configuration, memory cells are realized by MOS transistors each formed from part of one of the bit lines  60 , which acts as drain, the adjoining side region  25 , which acts as a channel region, one of the sources  29  and the dielectric layer  90 ,  100 ,  110 , which acts as a gate dielectric, and one of the webs  140 , which acts as a gate electrode, or the triple layer  180 ,  190 ,  200 , which acts as a gate dielectric, and part of the patterned electrode layer  210 , which acts as a gate electrode. 
     Since the webs  140  and the patterned electrode layer  210  are fabricated in a self-aligned manner with respect to one another, the memory cell configuration can be fabricated with a distance between the centers of adjacent gate electrodes along one of the webs  30  of a minimum feature size F that can be fabricated. The distance between the centers of adjacent webs  30  is a minimum of 2F given the use of a mask  15  which is fabricated with the aid of photolithographic process steps. Since the webs  30  each have two adjacent memory cells perpendicular to the course of the bit lines  60 , the space requirement per memory cell is F 2 . 
     If the mask  15  is formed with the aid of a spacer technique, then a distance between the centers of adjacent webs  30  of F is achieved. This results in a space requirement per memory cell of 0.5×F 2 . 
     A plan view of the finished memory cell configuration is illustrated in FIG.  11 . This illustration shows the configuration of the bit lines  60  and of first word lines WL 1  and second word lines WL 2 . The first word lines WL 1  and the second word lines WL 2  are formed by the webs  140  and by the patterned electrode layer  210  (see FIG.  10 ), respectively. 
     It can be seen here that, of the two bit lines  60  that are present on a web  30 , one bit line  60  in each case is connected to a contact  220  in the upper region of the cell array. The respective other bit line  60  of the web  30  is connected to the lower edge of the cell array in a manner that is not illustrated. 
     FIG. 12 illustrates the electrical circuit diagram of a detail from the cell array. 
     Interconnection between the bit lines  60 ,  60 ′,  60 ″ and word lines WL 1  and WL 2  can be seen here. 
     In order to clarify the method of operation of the electrical circuit, those voltages which are necessary to write to a memory cell  230  are illustrated by way of example. 
     The memory cell  230  is written to by the tunneling of electrical charge. A gate voltage of, preferably, from 9 V to 10 V is applied to the memory cell  230  via the associated word line WL 2 . The common sources of all the memory cells are at a common, elevated potential of 5 V, for example. A drain voltage of 0 V is applied to the memory cell  230  via the bit line  60 ″. Either a gate voltage 0 or a positive drain voltage of 5 V, for example, is applied to the other cells. This prevents cells that have already been written to from being erased. 
     A memory cell is read preferably in such a way that the common sources of the memory cells are at 0 V, that the bit line associated with the cell is at a positive potential, and that the word line associated with the cell is at a potential of 3 V, for example. 
     All of the memory cells are erased simultaneously preferably by the common sources of the cells being at 0 V, by all the bit lines  60 ,  60 ′,  60 ″ being at the potential 0 V, and by a negative gate voltage of −10 V, for example, being applied via the word lines WL 1  and WL 2 . 
     The invention is not restricted to the exemplary embodiments described. In particular, the n-type and p-type dopings can be interchanged.