Abstract:
A dynamic random access memory (DRAM) cell comprising a deep trench storage capacitor having an active transistor device partially disposed on a side wall of the trench. The side wall is aligned to a first crystallographic plane having a crystallographic orientation along a single crystal axis. A process for manufacturing such a DRAM cell comprises: (a) forming a deep trench in a substrate, (b) forming a faceted crystal region along the trench side wall having a single crystallographic orientation, and (c) forming a transistor device partially disposed on the faceted crystal region in the side wall. The faceted crystal region may be formed by growing an oxide collar, such as by local thermal oxidation under oxidation conditions selected to promote a higher oxidation rate along a first family of crystallographic axes than along a second family of crystallographic axes.

Description:
This application is a divisional of U.S. patent application Ser. No. 09/359,292, filed on Jul. 22, 1999, now U.S. Pat. No. 6,320,215. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to semiconductor transistor devices and, more specifically, to DRAM cells having non-planar transistor channel regions. 
     BACKGROUND OF THE INVENTION 
     Dynamic Random Access Memory (DRAM) cells can retain information only temporarily, on the order of milliseconds, even with power continuously applied. Therefore, the cells must be read and refreshed at periodic intervals. Although the storage time may appear to be short, it is actually long enough to allow many memory operations to occur between refresh cycles. The advantages of cost per bit, device density, and flexibility of use (i.e., both read and write operations are possible) have made DRAM cells the most widely used form of semiconductor memory to date. The earliest DRAM cells were three-transistor cells. Today, DRAM cells consisting of only one transistor and one capacitor have been implemented. 
     As DRAM circuits are scaled to small dimensions, it becomes increasingly important to form compact array cell layouts. The active device transistor is placed along the vertical side wall of a deep-trench storage capacitor in one type of array cell layout. Such a configuration forms a non-planar transistor device. 
     The non-planar transistor channel region crystal orientation can be a function of lithographic-projected image shape and the overlay between lithographically defined deep trench and active area patterns. Gate oxide thickness, surface state density, and other physical and electrical properties may be a function of the projected image shape and the overlap between the deep trench and active area patterns. These physical and electrical properties influence the transistor electrical, physical, and reliability characteristics. 
     As shown in FIG. 1, a typical deep trench having an elliptical cross section has a vertical side wall  32  that cuts across a continuum of planes including {001} and {011} crystal planes. Side wall  32  is not aligned with any particular crystal plane. Therefore, side wall  32  has associated crystal-plane-dependent properties that vary as side wall  32  makes a curved transition from one crystallographic plane to the other. 
     In accordance with standard crystallographic nomenclature, various symbols have specified meanings. Among those symbols are rounded brackets, { }, which refer to families of equivalent crystallographic planes (i.e., the {001} family of planes); parentheses, ( ), which refer to specific crystallographic planes (i.e., the (100) plane); horizontal triangles, &lt; &gt;, which refer to families of equivalent crystallographic axes (i.e., the &lt;011&gt; family of axes); and square brackets, [ ], which refer to a specific crystal axis (i.e., the [110] axis). For example, in silicon crystals, the (100) plane and the (001) plane are equivalent to one another and, thus, are both in the same {001} family of planes. 
     For optimized device performance, it is desirable to provide a side wall device aligned to a single crystallographic plane having a crystallographic orientation along a single crystal axis. It is an object of the present invention, therefore, to provide a crystal-axis-aligned, non-planar transistor structure. A related object is to provide a process for obtaining such a structure. 
     SUMMARY OF THE INVENTION 
     To achieve these and other objects, and in view of its purposes, the present invention provides a dynamic random access memory (DRAM) cell. The cell comprises a deep trench storage capacitor having an active transistor device partially disposed on a side wall of the deep trench. The side wall is aligned to a first crystallographic plane having a crystallographic orientation along a single crystal axis. The substrate surface may be aligned along a second crystallographic plane. The first and second crystallographic planes may be in the same family of equivalent crystallographic planes, such as the {001} family, or the first and second crystallographic planes may be in different families. 
     The present invention also provides a process for manufacturing a DRAM cell. The process comprises: (a) forming a deep trench in a substrate, (b) forming a faceted crystal region having a single crystallographic orientation along the trench side wall, and (c) forming a transistor device partially disposed on the faceted crystal region in the side wall. The faceted crystal region may be formed by growing an oxide collar, such as by local thermal oxidation under oxidation conditions selected to promote a higher oxidation rate along a first family of crystallographic axes, such as along the &lt;011&gt; family of crystal axes, than along a second family of crystallographic axes, such as along the &lt;100&gt; family of crystal axes. Other chemical and physical mechanisms may also be used to induce faceting in the trench side wall. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
     FIG. 1 is a schematic illustration plan view of an exemplary deep trench structure having crystal orientations as shown; 
     FIG. 2 is a schematic illustration plan view of the deep trench structure of FIG. 1 after a local oxidation step, highlighting the faceted side walls of the trench according to the present invention; 
     FIG. 3 is a schematic, cross-sectional illustration of the trench of FIG. 1 taken along the line  3 — 3 ; 
     FIGS. 4 through 10 are schematic, cross-sectional, in-process illustrations of the trench of FIG. 2 taken along the line  4 — 4  depicting steps in an exemplary process for manufacturing an exemplary DRAM cell of the present invention, with FIG. 4 showing an isolation collar formed in the upper region of the trench and a buried plate in the lower region of the trench; 
     FIG. 5 shows the trench of FIG. 4 after a dielectric is applied and the trench is partly filled with polysilicon; 
     FIG. 6 shows the trench of FIG. 5 after the collar is etched; 
     FIG. 7 shows the trench of FIG. 6 after a buried strap is formed; 
     FIG. 8 shows the trench of FIG. 7 after the buried strap is removed from the side walls of the trench above the polysilicon and above the pad; 
     FIG. 9 shows the trench of FIG. 8 after a trench-top dielectric is formed, the pad is stripped, a sacrificial oxide is grown on the exposed surface of the substrate and on the exposed side wall of the trench, a p-well and an n-band are created in the substrate, and diffusion regions are formed in the substrate; 
     FIG. 10 shows the trench of FIG. 9 after the sacrificial oxide is removed, a gate oxide is grown, a conductive gate layer is formed, an active area is patterned, an etching step is performed to etch shallow trench isolation (STI) regions everywhere except in the active area, the STI regions are filled and planarized to the pad nitride, and the pad nitride is stripped away; 
     FIG. 11A is a schematic illustration plan view of an exemplary DRAM cell, made according to the present invention, with the device oriented along a crystallographic plane in the same family as the substrate surface; 
     FIG. 11B is a schematic illustration plan view of an exemplary DRAM cell, made according to the present invention, with the device oriented along a different crystallographic plane in a different family from the substrate surface; and 
     FIG. 12 is a schematic cross-sectional illustration of the DRAM cell of FIG. 11A or FIG. 11B taken along the line  12 — 12 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawing, in which like reference numbers refer to like elements throughout, FIGS. 1-12 show various aspects of an exemplary DRAM cell of the present invention and intermediate steps in the process for making the cell. As shown in FIGS. 1 and 3, a typical deep trench storage capacitor  10  is formed into a pad  22  and a substrate  24  by conventional processing techniques well known in the art. For example, an optical lithography step may be used to form a pattern on pad  22 . Then a dry etching step such as reactive-ion etching (RIE), may be used to create a trench  20  to a desired depth through pad  22  and into substrate  24 . 
     The cross-sectional pattern of deep trench  20  is typically an ellipse that cuts across crystal axes A and B. For example, as shown in FIG. 1, axis A may have a [011] crystallographic orientation and axis B may have a [001] orientation. Substrate  24  is typically silicon and pad  22  is typically a silicon nitride (SiN) layer having a thickness of about 10 nm to about 100 nm. There may also be a thin thermal oxide layer (not shown), typically about 3 nm to about 10 nm thick, between substrate  24  and pad  22 . Deep trench  20  generally has a depth of about 3 μm to about 10 μm and a diameter or maximum width that is a function of the lithographic ground rule, typically about 0.5 μm to less than 0.1 μm. Trench  20  has side walls  32  and a bottom  33 . 
     As shown in FIGS. 2 and 4, in accordance with the present invention, an isolation collar  26  is formed in upper region  28  of trench  20 . Upper region  28  typically comprises 10 to 20% of the total depth of trench  20 . Collar  26  may be formed using local thermal oxidation (LOCOS), such as by the exemplary process explained below, or by other physical and chemical mechanisms, as also indicated below. 
     Before the oxidation step, a barrier film (not shown) may be formed along the exposed surfaces of trench  20  and pad  22  such as by a low-pressure chemical vapor deposition (LPCVD) of a SiN film having a thickness of about 2 nm to about 10 nm. The barrier film is then removed from upper region  28 , for example by filling trench  20  with photoresist (not shown) and partially etching the photoresist down into trench  20  to a depth controlled by the amount of overetch time. This step exposes the barrier film in upper region  28  while leaving the lower region  30  covered. The exposed barrier film may then be removed in upper region  28  of trench  20  and from pad layer  22 , for example, by chemical or dry etching, and then the photoresist stripped away. Other processes for isolating side wall  32  in upper region  28  while protecting side wall  32  in lower region  30  may also be used. 
     The local oxidation step is then performed at oxidation conditions that promote the oxidation rate along one family of crystal axes over another, such as, for example, promoting oxidation along &lt;011&gt; axes over &lt;001&gt; axes. For instance, the oxidation step may comprise the use of oxygen (O2) or water (H2O) at a temperature of between about 800° C. and about 1,100° C. for between about 2 minutes and about 10 minutes, not including ramping time, to achieve an oxide isolation collar  26  having a thickness of between about 10 nm to about 50 nm. Such oxidation conditions induce faceting of the underlying silicon substrate  24  during growth of collar  26 . 
     Thus, the curved trench wall  32  having an elliptical cross section, as shown in FIG. 1, facets into a polygonal cross section having distinct planar walls  32 ′ and  32 ″ aligned with crystal planes (001) and (011), respectively, as shown in FIG.  2 . Faceted walls  32 ′ and  32 ″ thus have consistent physical and electrical properties along the faceted structure, providing improved transistor electrical, physical, and reliability characteristics as compared to transistors built on unfaceted trench walls. The thermal oxide collar  26  and associated faceting are formed only on side walls  32  in upper region  28  of trench  20 ; the barrier film protects side walls  32  in lower region  30  of trench  20 . 
     Faceted side walls  32 ′,  32 ″ may be formed, as described above, by thermal oxidation. Faceted side walls  32 ′,  32 ″ may instead be formed by other physical or chemical mechanisms. Such mechanisms include, for example, preferential crystal axis etching, such as etching with potassium hydroxide (KOH), as is well-known in the art. 
     The remaining DRAM structure may be constructed according to processes well known in the art, such as the exemplary process described below. The process provided below is not intended to be a limitation of the present invention, but rather is included for illustration. In such an exemplary process, the barrier film in lower region  30  is stripped via a process that selectively leaves thermal oxide isolation collar  26  in upper region  28  of trench  20 . Buried plate  34  is then created in lower region  30 , leaving the configuration shown in FIG.  4 . Buried plate  34  may be created by doping lower region  30  of trench  20  to form an out-diffusion in substrate  24  using collar  26  as a mask for upper region  28 . The out-diffusion may be formed using arsenosilicate glass (ASG) drive-in, plasma doping (PLAD), plasma ion implantation (PIII), gas-phase diffusion of arsenic (As) or phosphorus (P), or other techniques known in the art. 
     Next, as shown in FIG. 5, a thin node dielectric  35  is created, such as by thermal nitridation, for example with ammonia (NH3), followed by LPCVD of SiN. Finally, trench  20  is filled, such as with an n+ doped LPCVD polysilicon  36 , and recessed to a desired depth D 1 . Depth D 1  is typically about 300 mn to about 700 nm, preferably between 300 to 450 nm. 
     Isolation collar  26  is then etched away, such as with a wet etch using a solution containing hydrogen fluoride (HF), to expose side walls  32  in the area where the collar  26  is not covered by polysilicon  36  and below the polysilicon level to a depth D 2 , as shown in FIG.  6 . D 2  is typically about 10 nm to about 50 nm. 
     Next, as shown in FIG. 7, a buried strap  40  is formed. Typically of LPCVD silicon, buried strap  40  is formed in a layer having a thickness of about 10 mn to about 50 mn. As shown in FIG. 8, buried strap  40  is then removed from side walls  32  of trench  20  above polysilicon  36  and above pad  22 , such as by an isotropic wet chemical or dry etching step. 
     Then, as shown in FIG. 9, trench-top dielectric  42  or trench-top oxide (TTO) is formed, such as by an anisotropic high-density plasma (HDP) or other bias-assisted oxide deposition step. The creation of trench-top dielectric  42  typically forms a corresponding layer (not shown) atop pad  22 , which is removed by a chemical mechanical polishing (CMP) step as is known in the art. Thus exposed, pad  22  is then stripped, typically by a wet chemical etch step selective to trench-top dielectric  42 , and a sacrificial oxide  44  is grown on the exposed surface of substrate  24  and exposed side wall  32  of trench  20 , as shown in FIG.  9 . 
     Ion implantation may then be used to create a p-well  50  and an n-band  52  below p-well  50  in substrate  24 . Similarly, ion implantation of As or P may be used to create diffusion region  62 . Another diffusion region  62 ′ is created by out-diffusion from n+ doped polysilicon region  36  through buried strap  40 . Such process steps yield the structure shown in FIG.  9 . Other device-threshold-tailoring implants may also be created at this time. 
     Next, as shown in FIG. 10, sacrificial oxide  44  is removed, such as by a chemical wet etch process with an HF-containing solution. Then, gate oxide  45  is grown and a conductive gate layer  48 , such as polysilicon having a thickness approximately equal to the diameter of trench  20 , is formed. A nitride pad (not shown) is formed having a thickness of approximately half to approximately equal to the thickness of gate layer  48 . 
     An active area  54  is patterned (see FIGS.  11 A and  11 B), typically by photolithography, and an etching step, such as RIE, is performed to etch shallow trench isolation (STI) regions  46  everywhere except in active area  54 . STI regions  46  are filled, typically with an oxide, and planarized by a CMP step down to the pad nitride. The pad nitride is then stripped away, leaving the structure shown in FIG.  10 . 
     Next, a thin seed layer of polysilicon is typically deposited, extending polysilicon gate layer  48  over the edge of trench  20  and over top diffusion region  62  in p-well  50  of substrate  24 . Middle layer  56 , which typically comprises a higher conductivity material than polysilicon gate layer  48 , such as tungsten (W) or tungsten silicide (WS), is then formed. Finally, gate cap layer  58 , comprising SiN or silicon oxide, is formed. The gate conductor layers are then patterned by lithography and dry etched, leaving the gate conductor  59  (comprising gate layer  48 , middle layer  56 , and gate cap  58 ) shown in FIG.  12 . 
     Next, side wall isolation spacers  70 , typically comprising silicon nitride, silicon oxide, or a combination of those materials, are created by processes well-known in the art to electrically isolate the wordline (gate conductor  59 ) from the bitline (the diffusion contact  64 ). Spacers  70  are typically created by depositing a conformal coating of SiN of about 10 nm to about 100 nm, and performing an anisotropic dry spacer etch process to leave the spacers  70  only on the side walls of gate conductor  59 . At this time, optional added implants may be performed to tailor source and drain regions (diffusion regions  62  and  62 ′) of the transistor. 
     Next, the isolating regions between multiple gate conductors  59  on the wafer are typically filled with an interlevel dielectric  63 , and contact holes are etched via lithography and dry etching to create the holes in which to form diffusion contact  64 . Diffusion contact  64  typically comprises doped polysilicon or a tungsten stud. The overlapping region  72  of diffusion contact  64  and of gate cap layer  58  of gate conductor  59 , as shown in FIG. 12, is typical of diffusion contacts known as borderless contacts. 
     Thus, as shown in FIGS. 11A,  11 B, and  12 , an exemplary trench-side wall array device  60  results from the exemplary process described above. As shown, n+ diffusion regions  62 ,  62 ′ under diffusion contact  64  adjacent to trench  20  serve as the source and drain of device  60 . A channel  66  results in substrate  24  (p-well  50 ) adjacent faceted side wall  32  of trench  20 . Although device  60  as shown in FIG. 11A has been fabricated across the (001) plane, device  60  may also be fabricated across the (011) plane as shown in FIG.  11 B. The surface of substrate  24  is typically along the (100) plane. Thus, device  60  may be along a crystallographic plane in the same family as the substrate surface {001}, as shown in FIG. 11A, or may be on a different crystallographic plane (011) in a different family, as shown in FIG.  11 B. 
     Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.