Abstract:
A method and structure are achieved for making an array of high-density memory cells for DRAMs. The high density is achieved by forming vertical cylindrical transistors aligned over deep-trench capacitors in a silicon substrate. The method consists of forming a field oxide around and extending inward over a portion of polysilicon trench capacitor electrodes. A gate isolation oxide and an array of word lines are formed aligned over the trench capacitor electrodes, and openings are etched in the word lines to the trench capacitor electrodes. Source contacts are implanted in the trench capacitor electrodes exposed in the openings. A gate oxide for the vertical transistors (FETs) is formed on the sidewalls in the openings, and a P doped polysilicon is formed in the openings for the FET channels. The vertical transistors are then completed by forming a drain implant in the FET channels, and a polysilicon layer is deposited and patterned to form an array of bit lines. The alignment of the vertical transistors over the trench capacitors significantly reduces the memory cell area and increases the memory cell density for future DRAM devices, while providing a cost-effective process.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to an integrated circuit semiconductor device, and more particularly to a method for making deep-trench capacitors with overlying vertical cylindrical transistors (FET) for dynamic random access memory (DRAM) devices to form a very dense array of memory cells. 
     (2) Description of the Prior Art 
     Dynamic random access memory (DRAM) circuits are used in the electronics industry for storing information as binary data. The DRAM circuit formed on chips diced from semiconductor substrates consists of an array of memory cells, and includes peripheral circuits for randomly accessing the memory cells for storing and retrieving the digital information. The individual DRAM cell is composed of a single FET, commonly referred to as a pass transistor, and a single charge storage capacitor. The storage capacitor is usually formed in the semiconductor substrate as a trench capacitor, or alternatively, formed as a stacked capacitor over the FET and within the cell area. 
     In recent years the cell density has increased dramatically on the DRAM chip because of improvements in the semiconductor technologies, such as high-resolution photo-lithography and directional plasma etching. In future DRAM technology the number of memory cells on a DRAM chip, each storing a bit of information, is expected to exceed a gigabit in the next several years. As this cell density increases on the chip it is necessary to reduce the area of each cell to maintain a reasonable chip size and to improve the circuit performance. 
     Unfortunately, as the cell size decreases, it is necessary to reduce the storage capacitor size to restrict the capacitor to within a cell area. This results in decreased charge stored on the capacitor that makes it more difficult to detect during the read cycle because of the lower signal-to-noise ratio at the read sense amplifiers. These volatile storage cells also require more frequent refresh cycles to maintain sufficient charge on the capacitor. Therefore, there is a strong need in the electronics industry to increase the capacitance of the storage capacitor while reducing the cell area. 
     Of the two approaches, the stacked capacitor has received considerable interest in recent years because of the variety of ways that the capacitor electrodes can be formed in the vertical (third) dimension over the FET and within the cell area to increase capacitance while reducing the cell area. However, the rough topography on the stacked capacitor requires an additional planarizing step to planarize the substrate surface in order to make more reliable submicrometer feature size structures, and the planarizing is a costly process that can also degrade product yield. 
     Alternatively, the DRAM cells can be made using deep-trench capacitors. In this approach the FET devices are formed adjacent to the trench capacitors and this limits reduction in cell area size. However, when the deep trench is formed in the substrate, the top surface of the substrate remains relatively planar and is available for forming the electrical interconnections having submicrometer feature sizes. 
     Several methods of making DRAMs with deep-trench capacitors have been reported. For example, Arnold, U.S. Pat. No. 5,937,296, forms a vertical transistor where the gate electrode is formed in the upper portion of the trench and source/drains are formed in the substrate. U.S. Pat. No. 5,302,541 to Akazawa shows a vertical transistor over a trench capacitor in which the source/drains are formed in a second conductive type material in the trench by out-diffusing from doped insulating oxide layers. In U.S. Pat. No. 5,744,386 to Kenney a vertical transistor is formed in the trench with an epitaxy, a gate oxide and gate electrodes formed in the trench. Lim, U.S. Pat. No. 6,018,176, forms a vertical transistor on a silicon-on-insulator substrate with a stacked capacitor over the transistor. p However, as the number of memory cells on a DRAM device continues to increase, there is still a strong need in the semiconductor industry to reduce the memory cell area while maintaining sufficient capacitance while providing a low-cost manufacturing process. 
     SUMMARY OF THE INVENTION 
     A principal object of this invention is to provide a very dense array of memory cells on a DRAM chip by forming a deep-trench capacitor with a vertical cylindrical transistor aligned over the capacitor, while providing a cost-effective process. 
     It is another object of this invention to form the vertical cylindrical transistor by forming the FET channel in an opening in the word line and aligned over the deep-trench capacitor. 
     A further object of this invention is to form an array of bit lines that are orthogonal to the word lines and that are aligned over the openings that have the FET channels. 
     In accordance with the objectives of the present invention, a method and structure are described for making an array of DRAM cells having deep-trench capacitors and vertical field effect transistors aligned over the deep-trench capacitors to decrease the cell area and dramatically increase DRAM cell density. 
     The method for making this array of DRAM cells with vertical FETs over capacitors is now briefly described. The method consists of providing a semiconductor substrate, preferably a P −  doped single-crystal silicon substrate. Conventional deep-trench capacitors are formed in the substrate. The deep-trench capacitors are formed by using a patterned CVD silicon oxide/silicon nitride hard mask and anisotropic plasma etching to etch deep trenches in the silicon substrate. A thin dielectric layer is formed in the deep trenches to form a capacitor interelectrode dielectric layer. Then the trenches are filled with a first poly-silicon to form the capacitor electrodes and that also serve as the node contacts for the capacitors. Next a shallow trench isolation (STI) is formed to surround and electrically isolate the array of deep trenches. The STI also forms other active device areas, such as the peripheral device areas on the DRAM chip. The STI is formed by first removing the CVD SiO 2  portion of the hard mask. Then a shallow-trench photoresist mask and plasma etching are used to pattern the Si 3 N 4  portion of the hard mask and to etch shallow trenches in the substrate. The shallow trenches are formed to extend partly inward over the edge of the deep-trench capacitors and to leave active device areas over the first polysilicon deep-trench capacitors. The STI is completed by depositing an insulating layer and polishing back. A gate isolation oxide remains after chemical-mechanical polishing (CMP) on the surface of the first polysilicon layer in the deep-trench capacitors. Next, an N doped second polysilicon layer having a cap insulating layer is deposited and patterned to form word lines that extend over the deep-trench capacitors. An insulating layer is deposited over the word lines and polished back to expose the cap insulating layer on the word lines and to provide a planar surface. One important feature of this invention is to etch an array of openings in the cap insulating layer, in the polysilicon word lines, and in the gate isolation oxide. The openings are aligned over the first polysilicon (capacitor electrode) in the deep-trench capacitors. The source contacts for the vertical transistors are formed in the first polysilicon layer exposed in the openings by ion implantation. A gate oxide is formed on the sidewalls of the polysilicon word lines in the openings, followed by a bottom oxide etch using a directional plasma etch. Then a P doped third polysilicon layer is deposited sufficiently thick to fill the openings, and is polished back to the insulating layer to form FET channel cylinders. The drain contacts for the vertical transistors are formed in the top surface of the P doped third polysilicon layer exposed in the openings by ion implanting an N type dopant. An N doped fourth polysilicon layer is deposited and patterned to form an array of bit lines, that are orthogonal to the word lines, over the openings and electrically contacting the drain contact areas to complete the high-density array of DRAM cells. The fourth polysilicon layer may include an upper metal silicide layer to reduce electrical resistance and to improve circuit performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and advantages of this invention are best understood with reference to the attached drawings in the figures and the embodiment that follows. 
     FIGS. 1A through 6A show schematic cross-sectional views for the sequence of process steps for making DRAM cells having vertical cylindrical transistors (FETs) aligned over the DRAM storage capacitors. One of the multitude of DRAM cells in shown in the drawings. 
     FIGS. 1B through 6B show schematic top views of four adjacent DRAM cells corresponding to the sequence of cross-sectional views for making the DRAM cells with vertical transistors of FIGS.  1 A- 6 A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The method is now described in detail for making DRAMs with vertical cylindrical transistors aligned over the deep-trench capacitors for increasing cell density. Although the method is described for making only DRAM devices with deep-trench capacitors and vertical N channel transistors, it should be understood that by including additional processing steps, both conventional N channel FETs and P channel FETs can also be formed, such as required for making CMOS circuits for the peripheral circuits on the DRAM device. 
     Referring to FIG. 1A, the method for making these deep-trench capacitors having N channel vertical FETs begins by providing a semiconductor substrate  10 . The substrate is preferably a P −  doped single-crystal silicon having a &lt;100&gt; crystallographic orientation. 
     Next deep-trench capacitors are formed in the substrate  10  as is commonly practiced in the industry. Briefly, conventional photolithographic techniques and anisotropic plasma etching are used to form an array of openings in a hard mask on the substrate  10 , which are then used to etch deep trenches  2  in the substrate  10 . Typically the hard-mask layer consists of an insulating layer, such a silicon oxide (not shown), and a pad silicon nitride (Si 3 N 4 .) layer  12 . The layer  12  is deposited by chemical vapor deposition (CVD) using a reactant gas such as dichlorosilane (SiCl 2 H 2 ) and ammonia (NH 3 ), and is deposited to a preferred thickness of between about 1500 and 2000 Angstroms. The array of deep trenches  2  are then etched in the silicon substrate  10  using high-density plasma (HDP) etching or reactive ion etching (RIE), and preferably using a fluorine-based etchant gas such as NF 3  and HBr. The trenches are typically etched to a depth of about 7 to 8 micrometers (um) and typically have a width W of between about 0.2 and 0.3 um. Only a single trench and an upper portion of the trench are shown in FIG. 1A to simplify the drawing. The cross section in FIG. 1A is for the cross section through the region  1 A- 1 A′ in the top view of FIG. 1B, which shows the layout for four adjacent deep-trench openings  2  etched in the substrate  10 . After etching the trenches  2  the silicon oxide portion of the hard mask is removed and the pad Si 3 N 4  layer  12  is retained as a barrier layer to oxidation and as a polish-back etch mask. 
     Still referring to FIG. 1A, capacitor electrodes (first electrodes)  14  are formed in the silicon substrate  10  adjacent to the deep trenches  2  by diffusing a dopant, such as arsenic. Typically the capacitor electrodes  14  are doped to a concentration of between about 5.0×10 19  and 1.0×10 20  atoms/cm 3 . Next a thin dielectric layer  16  is formed on the exposed silicon surfaces in the deep trenches  2  to form the interelectrode dielectric layer  16 . Typically the dielectric layer is formed by depositing a Si 3 N 4  layer by LPCVD to a thickness of between about 40 and 50 Angstroms and the Si 3 N 4  is thermally oxidized to form a silicon oxide/silicon nitride/silicon oxide (ONO) layer and to reduce pinholes in the Si 3 N 4  dielectric layer  16 . Then the trenches  2  are filled with a first polysilicon layer  18  to form the trench capacitor electrodes (second electrodes) that are also the node contacts  18  for the capacitors. Typically the first polysilicon layer  18  is deposited by CVD and is in-situ doped with an N type dopant, such as phosphorus, to a concentration of between about 5.0×10 19  and 1.0×10 20  atoms/cm 3 . The polysilicon is polished back to the barrier layer  12  to form the polysilicon capacitor electrodes  18 . For this invention the polysilicon  18  is further recessed, using plasma etching, to about 500 Angstroms below the surface of the silicon substrate  10 . A gate isolation oxide is later formed in the recess to prevent the vertical transistor gate electrode from electrically shorting to the capacitor. 
     Still referring to FIG. 1A, shallow trench isolation regions are formed to surround and electrically isolate the array of deep trenches  2  and to form other active device areas. The shallow trench isolation is formed by using a photoresist mask  20  and plasma etching to pattern the Si 3 N 4  layer  12  of the hard mask and to etch shallow trenches that extend partly inward over the edge of the deep-trench capacitor electrodes  18  leaving a polysilicon portion  18 ′ over the deep trench capacitors. The portion of the photoresist mask  20  over the deep trench capacitors is shown in FIG.  1 A. The shallow trenches are also etched to form active device areas elsewhere on the substrate. 
     Referring to FIG. 2A, after etching the shallow trenches to a depth of between about 3000 and 4000 Angstroms, a brief thermal oxidation is carried out to reduce surface damage of the silicon substrate  10  in the shallow trenches. A CVD SiO 2    22  is deposited and polished back to form the shallow trench isolation (STI)  22 . A portion of the CVD SiO 2    22  remains over the polysilicon portion  18 ′ to serve as a gate isolation oxide  24  to prevent the capacitor nodes from shorting to the gate electrodes of the vertical transistors. Alternatively, the CVD oxide  22  can be selectively etched back to the surface of the polysilicon portion  18 ′ and an additional gate isolation oxide  24  can be deposited to a preferred thickness of between about 400 and 600 Angstroms, and more specifically to a thickness of about 500 Angstroms. FIG. 2B shows a top view of four adjacent memory cells, and the cross section through the region  2 A- 2 A′ of FIG. 2B is shown in FIG. 2A for one of the four adjacent memory cells. The dashed line  3  in FIG. 2B is the perimeter of the top portion of polysilicon portion  18 ′ aligned to the capacitor opening  2 . 
     Referring to FIG.  3 A and more specifically to the method of the invention, an N doped second polysilicon layer  26  having a cap insulating layer  28  is deposited and patterned to form word lines  26  that extend over the deep-trench capacitor portions  18 ′. FIG. 3B shows a top view of four adjacent memory cells, and the cross section through the region  3 A- 3 A′ of FIG. 3B is shown in FIG. 3A for one of the four adjacent memory cells. The second polysilicon layer  26  is deposited by LPCVD using, for example, silane (SiH 4 ) as the reactant gas, and is in-situ doped or by ion implanting an N type dopant, such as phosphorus, to a final concentration of between about 1.0×10 19  and 5.0×10 19  atoms/cm 3 . Second polysilicon layer  26  is deposited to a preferred thickness of between about 1800 and 2500 Angstroms, and more specifically to a thickness of about 2000 Angstroms. Second polysilicon layer  26  is patterned using conventional photolithographic techniques and anisotropic plasma etching to form the word lines  26 . The cap insulating layer  28  is Si 3 N 4  or SiO 2 , deposited by CVD to a thickness of between about 300 and 800 Angstroms, and more specifically to a thickness of about 500 Angstroms. Next, an insulating layer  30  is deposited over the word lines  26 . Layer  30  is SiO 2 , or a doped SiO 2  such as borophosphosilicate glass (BPSG), and is deposited by CVD to a thickness at least greater than the combined thicknesses of layers  26  and  28 . Layer  30  is chemically-mechanically polished back to the cap insulating layer  28  on the word lines  26  to provide a planar surface. 
     Referring to FIG. 4A, an array of openings  4  is etched in the cap insulating layer  28 , in the polysilicon word lines  26 , and in the gate isolation oxide  24  to the first polysilicon (capacitor electrode)  18 ′ in the deep-trench capacitors. The openings  4  are etched for the channel cylinders for vertical transistors (FETs). The word lines  26  typically have a width of about 0.3 to 0.5 micrometers (um), and the openings  4  have a preferred diameter of between about 0.25 and 0.3 um. The openings  4  are aligned over the first polysilicon portion  18 ′, as shown in the top view of FIG.  4 B. The cross section in FIG. 4A is through the region  4 A- 4 A′ in FIG.  4 B. The openings  4  are etched using photolithographic techniques and anisotropic plasma etching. The plasma etching is carried out in a high-density plasma etcher. The insulating cap layer  28  is etched selectively to the second polysilicon layer  26  (word lines) using an etchant gas mixture such as CHF 3 , O 3 , and CF 4 . The second polysilicon  26  is etched selectively to the gate isolation layer  24  using an etchant gas mixture such as CHF 3  and NF 3 . And the gate isolation oxide  24  is etched selectively to the polysilicon  18 ′ using an etchant gas mixture such as CHF 3 , O 3 , and CF 4 . 
     Still referring to FIG. 4A, an ion implantation is carried out to form the source contacts  32  in the first polysilicon portion  18 ′ in the openings  4  for the vertical transistors. The source contacts  32  are formed by implanting a dopant such as phosphorus. The contacts  32  are doped to form an N −  doped source to achieve a final concentration of between about 1.0×10 14  and 5.0×10 14  atoms/cm 3 . Next, a gate oxide  34  is formed on the sidewalls of the polysilicon word lines  26  in the openings  4 , as shown in FIG.  4 A. The gate oxide  34  is formed by a thermal oxidation in oxygen, and is grown to a thickness of between about 60 and 80 Angstroms. The gate oxide that is unintentionally formed on the surface of polysilicon  18 ′ is then removed selectively using a high-density plasma etch at low pressure. The gate oxide  34  is also depicted in the top view of FIG.  4 B. 
     Referring to FIG. 5A, a boron-doped third polysilicon layer  36  is deposited sufficiently thick to fill the openings  4 . Layer  36  is deposited using LPCVD and is in-situ doped to achieve a final boron concentration of between about 1.0×10 12  and 5.0×10 12  atoms/cm 3 . Third polysilicon layer  36  is then polished back to the insulating layer  30  to form FET channel cylinders  36  in the openings  4 . Next the drain contacts  38  for the vertical transistors are formed in the top surface of the P doped channel cylinders  36 . The drain contacts are formed by ion implanting an N type dopant, such as arsenic ions (As 75 ), to achieve an N −  final dopant concentration of between about 1.0×10 14  and 5.0×10 14  atoms/cm 3 . The cross-sectional view in FIG. 5A is shown for the region  5 A- 5 A′ in the top view of FIG. 5B, and includes the channel cylinder  36  with the drain contact  38 . 
     Referring to FIG. 6A, a fourth polysilicon layer  40  is deposited, for example, by LPCVD using a reactant gas such as SiH 4 , and is doped with phosphorus by ion implantation, or in-situ doped during deposition using, for example, phosphine. The fourth polysilicon layer  40  is deposited to a thickness of between about 2000 and 3000 Angstroms, and is doped to a final concentration of between about 5.0×10 19  and 1.0×10 20  atoms/cm 3 . Alternatively, the fourth polysilicon layer  40  may include an upper metal silicide layer (not shown), forming a polycide layer, to reduce electrical resistance and to improve circuit performance. For example, a tungsten silicide (WSi x ) layer can be deposited by CVD using tungsten hexafluoride (WF 6 ) and SiH 4  as the reactant gases, and typically would be deposited to a thickness of about 500 to 800 Angstroms. Layer  40  is then patterned to form an array of bit lines  40 . Conventional photolithographic techniques and anisotropic plasma etching are used to etch layer  40  to form the bit lines. Preferably the etching is carried out in a HDP etcher or reactive ion etcher using a chlorine-based etchant gas such as HCl, Cl, and NF 3  for etching the WSi x , and HCl and Cl 2  for etching the polysilicon. The bit lines  40  are formed orthogonal to the word lines  26 , as depicted in the top view of FIG.  6 B. The bit lines  40  are also formed aligned over the openings  4  and electrically contacting the drain contact areas  38  to complete the vertical transistors and the high-density array of DRAM cells. 
     While the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.