Abstract:
A trench device and method for fabricating same are provided. The trench device has a collar with a first portion that is doped and a second portion that is undoped. Fabrication of the partially doped collar can be done by deposition of a doped insulator in the trench, removal of a portion of the doped deposition, deposition of an undoped insulator in the trench and removal of a portion of the doped and undoped insulators.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. application Ser. No. 11/306,669, filed on Jan. 6, 2006, now U.S. Pat. No. 7,326,986. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to semiconductors and, more particularly, to trench memory devices and a method for manufacturing same. 
   2. Description of the Related Art 
   In trench memory, retention of an electric charge in a cell capacitor is greatly influenced by various leakage mechanisms. Trench memory devices or structures are subject to vertical parasitic leakage that degrades charge or data retention. As shown in  FIG. 1 , a vertical parasitic transistor is formed in a contemporary trench memory structure where the N+ buried strap is the drain, the N+ buried plate is the source, the N+ trench poly is the gate and the collar oxide is the gate dielectric. Vertical parasitic leakage current is generated due to the sub-threshold current of the vertical parasitic transistor, degrading the charge retention. 
   The resulting vertical parasitic leakage current can be suppressed through increased p-well doping. However, increasing p-well doping leads to other problems, such as elevated junction leakage and depressed write-back current. 
   In U.S. Pat. No. 6,818,534, it is suggested to utilize a fully doped collar in trench DRAM to improve leakage performance. As shown in  FIG. 2 , a fully boron-doped collar is utilized. However, the boron in the collar counter-dopes arsenic-doped N+ poly, causing high poly resistance. Additionally, the fully boron-doped collar is left exposed during subsequent high-temperature processes, such as, for example, the STI process. These high-temperature processes cause boron contamination and undesired auto-doping in the active area. The closeness of the heavily doped P-well also disturbs the characteristics of the array transistor. 
   Accordingly, there is a need for trench memory that reduces or suppresses vertical parasitic leakage. There is a further need for a process of manufacturing such trench memory structures or devices. 
   SUMMARY OF THE INVENTION 
   In one aspect, a trench memory cell is provided comprising a trench capacitor and a transistor. The trench capacitor is formed in a silicon substrate and has a collar comprising a doped insulator portion and an undoped insulator portion. The transistor comprises a gate, a source and a drain, wherein the drain is electrically coupled to the trench capacitor. The undoped insulator portion is above the doped insulator portion. 
   In another aspect, a deep trench capacitor is provided comprising a substrate; a trench in the substrate and having one or more walls; a buried plate of a first conductivity type positioned in the substrate near a lower portion of the trench; a node dielectric layer on the one or more walls of the lower portion of the trench; a well region of a second conductivity type in the substrate above the buried plate; a strap of the first conductivity type adjacent to the trench; a conducting material fill disposed in the trench; and a collar insulator formed upon the one or more walls of the trench above the buried plate. The collar insulator comprises a doped portion and an undoped portion. 
   In yet another aspect, a method of manufacturing a trench memory device is provided comprising: providing a substrate; forming a trench in the substrate, wherein the trench comprises sidewalls; forming a buried plate in the substrate in proximity to the bottom portion of the trench; layering a node dielectric along the sidewalls of the bottom portion of the trench; forming a first layer of conducting material in a bottom portion of the trench; forming a collar on the sidewalls of the trench above the first layer of conducting material, wherein said collar comprises a doped portion and an undoped portion; forming a second layer of conducting material in the trench above the first layer of conducting material; and forming a shallow isolation region in a top portion of the substrate, wherein the shallow isolation region caps the trench. 
   The undoped portion of the collar insulator can be positioned above the doped portion of the collar insulator. The capacitor may further comprise a shallow trench isolation adjacent to the trench and on a top portion of the silicon substrate. The first conductivity type can be N-type and the second conductivity type can be P-type. Alternatively, the first conductivity type can be P-type and the second conductivity type can be N-type. The doped portion of the collar insulator can be less than 50% of the undoped portion of the collar insulator. 
   The manufacturing method may further comprise planarizing a top surface of the substrate after forming the shallow isolation region, wherein the substrate comprises a nitride layer. The method can further comprise forming a buried-strap connected to a top portion of the trench. The method may further comprise: depositing a doped insulator layer along the substrate and into the trench; removing a portion of the doped insulator via etchback; depositing an undoped insulator layer along the substrate and into the trench; and removing a portion of the undoped insulator layer or the doped insulator layer to form the collar. The method can further comprise performing reactive ion etching. The method may further comprise performing high density plasma deposition. 
   The above-described and other features and advantages of the present disclosure will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of a prior art trench memory structure; 
       FIG. 2  is a schematic illustration of a prior art trench memory structure as described in U.S. Pat. No. 6,818,534; 
       FIG. 3  is a schematic illustration of a trench memory structure according to an exemplary embodiment of the present invention; 
       FIG. 4  is a method of manufacturing the trench memory structure of  FIG. 3 ; 
       FIG. 5A  is a first portion of a trench memory structure during manufacture by the method of  FIG. 4 ; 
       FIG. 5B  is a second portion of a trench memory structure during manufacture by the method of  FIG. 4 ; 
       FIG. 5C  is a third portion of a trench memory structure during manufacture by the method of  FIG. 4 ; and 
       FIG. 5D  is a fourth portion of a trench memory structure during manufacture by the method of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings and in particular to  FIG. 3 , an exemplary embodiment of a trench memory structure is shown and generally represented by reference numeral  5 . Trench memory structure  5  can be used in various devices and systems such as, for example, embedded DRAM, DRAM, SRAM, system-on-chip and application-specific integrated circuits. Trench memory structure  5  has one or more deep trench storage capacitors  10  and one or more transistors  15 , such as, for example, a MOSFET, in or on a substrate  20 . Trench memory structure  5  would typically comprise an array of deep trench capacitors  10  coupled with an array of transistors  15  to form an array of memory cells interconnected by rows and columns for reading data from, or writing data to, the memory cells. For simplicity, trench memory structure  5  is being described with respect to one of the deep trench capacitors  10  coupled with one of the transistors  15 , but of course any number could be used. 
   Deep trench capacitor  10  has a trench  25  in substrate  20 . The trench  25  is filled with conducting materials such as N+ polycrystalline silicon (poly)  30 ,  32 , and  34 . Other conducting materials such as metals, metallic compounds, silicides, and any combination of these materials including polysilicon can also be used to fill the trench. Near a bottom portion of trench  26 , a buried plate  40  is positioned. The buried plate is a heavily doped region. For instance, the buried plate can be doped by arsenic or phosphorous. The poly  30  and buried plate  40  are isolated from each other by a node dielectric layer  50  formed along the walls of the bottom portion of the trench  25 . A P-well  60  is positioned in the substrate  20  above the buried plate  40 . A shallow trench isolation region (STI)  70  is formed into the substrate  20  from a top surface thereof. 
   Along the walls of trench  25 , a collar  80  is formed, which is adjacent to the walls of the P-well  60 . The collar  80  comprises a first portion  90  and a second portion  100 . The first portion  90  is a doped insulator, such as, for example, boron-doped oxide. The second portion  100  is an undoped insulator such as an oxide. The doped portion  90  is positioned along a lower portion of collar  80 , while the undoped portion  100  is positioned along an upper portion of the collar. A buried strap  110  is connected at the top of the trench  25  to the drain  16  of the transistor  15 , which also has a gate  17  and a source  18  as illustrated in  FIG. 3 . 
   The trench memory structure  5  with the collar  80  comprising both doped and undoped portions  90  and  100  provides several advantages. First, a localized and heavily doped P+ region  95  is formed next to the doped portion of the collar  90  by driving the dopants in the doped collar to the substrate. This heavily doped P+ region  95  increases the threshold voltage of the vertical parasitic transistor and therefore suppresses the vertical parasitic leakage. Second, less counter-doping of the N+ poly  30  occurs due to the use of undoped portion  100  of the collar  80 , which reduces poly resistance. Third, the doped portion  90  of the collar  80  is no longer exposed (being sealed by the undoped portion  100 ) during subsequent high temperature processes, such as, for example, STI formation, so there is no contamination or undesired auto-doping in the active area. Fourth, the transistor  15  is not disturbed as the P+ region is far enough away from the transistor. Finally, the P+ region has minimal impact on substrate sensitivity as it is localized. 
   Referring to  FIGS. 4 through 5D , a method for manufacturing the trench memory structure  5  is illustrated and generally shown by reference numeral  400 . In step  400 , standard deep trench processing is used to form the deep trench capacitor  10  into the substrate  20 . This may include etching of the trench  25  to a predetermined depth, filling of the poly  30  into the trench and recessing the poly to a depth of about 700-1500 nm, positioning of the buried plate  40  into the substrate and layering of the node dielectric  50  along the wall of the trench. A pad layer  535  is positioned along the top surface of the substrate  20  adjacent the trench  25 . The pad layer, which may comprise a nitride layer with an optional underlying oxide layer, protects the substrate  20  in the subsequent process. The resulting first portion of trench memory  5  is shown in  FIG. 5A . 
   In step  420 , a doped insulating material  590  is deposited along the substrate  20 , into trench  25  and above the poly  30 . The doped insulator  590  can be oxide, oxynitride, nitride, other dielectric materials such as “high-k” materials, or any suitable of combination of these materials. Preferably, the insulator  590  is an oxide that is doped with a P-type dopant such as boron or indium with a concentration of 0.1-6% in weight and more preferably 1-2% in weight. The process for depositing the insulator  590 , includes but is not limited to, chemical vapor deposition (CVD), atomic layer deposition (ALD), physical vapor deposition (PVD), sputtering, plating, or any suitable combination of these processes. Preferably, the insulator  590  is deposited by a high density plasma (HDP) CVD process. Due to the anisotropic nature of HDP process, i.e., the deposition rate of HDP process is higher in the vertical direction than in the lateral direction), the oxide thickness on top of the poly  30  and pad layer  535  is greater than on trench sidewall. 
   For example, the oxide thickness on trench sidewall is only one third of the oxide thickness on top of the poly and pad layer by a typical HDP deposition process. Preferably, the oxide thickness ranges from 50-200 nm on top of the poly and pad layer and 15-70 nm on trench sidewall after HDP deposition. Optionally, an oxide liner (not shown) of approximately 2-6 nm may be formed by thermal oxidation before HDP deposition to protect the trench sidewall from the attack of plasma during HDP process. In one embodiment, the insulator  590  is in-situ doped during deposition. In another embodiment, the insulator  590  is doped after deposition. For example, ion implantation of boron after deposition can be used to form a P-type doped insulator  590 . 
   Portions of the doped insulator  590  are then removed from trench sidewall by an etchback in step  430 . When the doped insulator is oxide deposited by HDP process, a timed wet etch comprising buffered HF (BHF) or diluted HF (DHF) can be used. Approximately the same amount of HDP oxide is removed from the top of the poly and pad layer, resulting approximate 30-150 nm doped oxide on top of the poly and the pad nitride after etch. The optional oxide liner, if present, may be removed along with the HDP oxide by BHF or DHF. The resulting first portion of trench memory  5  is shown in  FIG. 5B . 
   In step  440 , undoped insulator  600  is deposited into the trench  25  and above the doped insulator  590 . The undoped insulator  600  can be oxide, oxynitride, nitride, or any other suitable dielectric materials deposited by any suitable deposition techniques, such as chemical vapor deposition (CVD), thermal oxidation, atomic layer deposition (ALD), physical vapor deposition (PVD), sputtering, plating, and any combination of these techniques. The undoped layer may be oxide, oxynitride, nitride, other dielectric materials such as “high-k” materials, or any suitable of combination of these materials. Preferably, the undoped insulator  600  is an oxide deposited by a conformal process such low-pressure CVD process. In another embodiment, the undoped insulator  600  is an oxide formed by thermally oxidizing the exposed trench sidewall. When thermal oxidation is used, the undoped collar is formed only on trench sidewall. deposited by a conformal process such low-pressure CVD process. The thickness of the undoped insulator ranges from 10 nm to 50 nm, and more preferably 20-30 nm. The resulting second portion of trench memory structure  5  is shown in  FIG. 5C . 
   In step  450 , the insulator  600  and  590  are removed from the top of the poly  30  and the pad layer  535  to form a collar  80  on trench sidewall. The collar  80  comprises an undoped portion  100  and doped portion  90 . A reactive ion etching (RIE) can be used to form the collar. The RIE etchback removes portions of the undoped insulator  600  so that the collar  80  is formed with a doped insulator lower portion  90  and an undoped insulator upper portion  100 . The doped insulator has a height of 30-150 nm and the undoped insulator has a height of 500-1200 nm. Dopant in the doped portion of the collar is driven into the substrate by the subsequent thermal process to form a localized doped region  95 . In one embodiment, the doped collar portion is doped with boron and thus the localized doped region  95  in the substrate is P-type. In another embodiment, the localized doped region in the substrate is self-aligned to the buried plate. The resulting third portion of trench memory structure  5  is shown in  FIG. 5D . 
   In step  460 , standard trench memory processing is used to form the remaining components of the trench memory  5  shown in  FIG. 3 . This may include filling the formed collar  80  with a conducting material  32 , recessing the conducting material  32 , removing the exposed collar, buried strap  110  formation by deposition and recess of strap material  34 , and isolating active areas by STI formation, such as, for example, by anisotropic etching, filling with an oxide, planarizing to the surface of the substrate  20  and capping the deep trench capacitor  10 . As needed by the trench capacitor  10 , well implants, gate oxidation, gate conductors, and source-drain diffusions are formed. Other structural features, such as gate conductors and wiring, are common in the field of trench memory, and, as such, are omitted for brevity. Hence, the remainder of the deep trench capacitor is formed so as to produce a trench memory structure  5  such as that shown in  FIG. 3 , for example. 
   While the instant disclosure has been described with reference to one or more exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope thereof. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.