Abstract:
A deep trench capacitor, in accordance with the present invention, includes a deep trench formed in a substrate having a storage node formed therein. A center node is capacitively coupled to the storage node. The center node is disposed within the deep trench and formed inside the storage node. A first buried strap is provided for accessing the storage node, and a second buried strap is electrically isolated from the storage node and formed in contact with the center node and a buried plate. The center node is formed to provide additional capacitive area to the deep trench capacitor. A method for forming the deep trench capacitor in accordance with the present invention is also provided.

Description:
BACKGROUND 
     1. Technical Field 
     This disclosure relates to semiconductor devices and fabrication methods and more particularly, to a folded deep trench capacitor structure, which increases capacity for deep trench capacitors. 
     2. Description of the Related Art 
     In semiconductor memory devices, for example, dynamic random access memory (DRAM) devices, memory cells include capacitors for storing data. The data stored in the capacitors is read from and written to through bitlines. The memory cells need to maintain a relationship between the storage capacitor&#39;s capacitance and the bitline capacitance to maintain a sensible voltage difference between bitline and complement bitline voltages, that is, to distinguish between a ‘0’ and ‘1’ read from the storage capacitor. With further shrinking of minimum feature sizes, critical dimensions (CD) must be compensated for by maintaining a higher or at least equal storage capacity. 
     Storage capacitors may include deep trench capacitors. Deep trench capacitors provide the ability to extend deep into a substrate to provide adequate storage while minimizing the cost of layout area. One way of achieving the relationship of capacity between bitlines and deep trench capacitors with smaller CD&#39;s is by etching the trenches deeper or as deep as before with a smaller CD. This requires thicker resists for etching the deeper trenches, which makes the resist budget more critical and other procedures, like hard mask processes, have to be developed to accommodate deeper trenches. Further, critical dimensions must be maintained as large as possible to permit access to the smaller width of trench for processing. This makes the alignment budget more critical as well. 
     Therefore, a need exists for a deep trench capacitor structure, which increases the capacitance so that both width and depth dimensions could be relaxed, and the area occupied by deep trench is at least the same as the prior art. 
     SUMMARY OF THE INVENTION 
     A deep trench capacitor, in accordance with the present invention, includes a deep trench formed in a substrate having a storage node formed therein. A center node is capacitively coupled to the storage node. The center node is disposed within the deep trench and formed inside the storage node. A first buried strap is provided for accessing the storage node, and a second buried strap is electrically isolated from the storage node and formed in contact with the center node and a buried plate formed in the substrate surrounding the deep trench. The center node is formed to provide additional capacitive area to the deep trench capacitor. 
     In other embodiments, the center node and the storage node may have a nitride layer disposed therebetween. The center node may extend to a bottom of the deep trench. The first buried strap is preferably coupled to a transistor for enabling access to the storage node. The buried plate and the center node preferably represent a first capacitor plate of the deep trench capacitor, and the storage node preferably represents a second capacitor plate of the deep trench capacitor. 
     A method for forming a deep trench capacitor, in accordance with the invention, includes a node dielectric formed in a lower portion of the deep trench and a collar formed in an upper portion of the deep trench. The deep trench is filled with a conductive material to form a storage node. A center trench is formed into the storage node, and the center trench is centrally disposed within the storage node. A center node dielectric layer is then deposited in the center trench. The center trench is filled with the conductive material to form a center node separated from the storage node by the center node dielectric layer. A hole is formed to expose a portion of the center node and a portion of a substrate surrounding the deep trench, the hole also exposes a portion of the storage node. A dielectric cap is formed on the portion of the storage node exposed in the hole, and a conductive material is deposited in the hole to connect the center node to a buried plate surrounding the lower portion of the deep trench. 
     Another method for forming a deep trench capacitor with a center node includes forming a deep trench in a semiconductor substrate, lining the deep trench with a node dielectric layer, filling the deep trench with a conductive material, recessing the conductive material and removing the node dielectric layer from an upper portion of the deep trench to expose a portion of the substrate. A collar is formed in the upper portion of the deep trench on the exposed portion of the substrate and by partially refilling the trench with the conductive material a storage node is formed with a void which forms a center trench in the storage node. A center node dielectric layer is deposited in the center trench. The center trench is filled with the conductive material to form a center node. A hole is formed to expose a portion of the center node and a portion of the substrate, the hole also exposes a portion of the storage node. A dielectric cap is formed on the portion of the storage node exposed in the hole, and conductive material is deposited in the hole to connect the center node to a buried plate surrounding the deep trench. 
     In other methods, the step of forming a center trench into the storage node may include the step of etching the center trench into the storage node. The step of forming a center trench into the storage node may include the step of underfilling the deep trench with conductive material to form the storage node having the center trench provided therein. The step of forming a center trench into the storage node may include the step of forming the center trench to the bottom of the deep trench. The method may include the step of forming the buried plate surrounding the lower portion of the deep trench wherein the center node is coupled to the buried plate to form a first electrode of the deep trench capacitor. 
     In still other methods, the step of forming a dielectric cap on the portion of the storage node exposed in the hole may include the steps of depositing an oxide layer in the hole to cover the exposed portion of the storage node in the hole, thinning the oxide layer and depositing a nitride layer over the oxide layer. The method may include the step of removing portions of the oxide layer and the nitride layer to form the dielectric layer over the portion of the storage node. The method may further include the step of forming a buried strap for accessing the storage node. 
     These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     This disclosure will present in detail the following description of preferred embodiments with reference to the following figures wherein: 
     FIG. 1 is a cross-sectional view showing a deep trench formed in a substrate and a buried plate formed in accordance with the prior art; 
     FIG. 2 is a cross-sectional view of the structure of FIG. 1 showing a node dielectric layer formed in accordance with the prior art; 
     FIG. 3 is a cross-sectional view of the structure of FIG. 2 showing the deep trench filled and recessed to remove a portion of the node dielectric layer and to form a collar in accordance with the prior art; 
     FIG. 4 is a cross-sectional view of the structure of FIG. 3 showing the deep trench filled with conductive material in accordance with the prior art; 
     FIG. 5A is a cross-sectional view of the structure of FIG. 4 showing a center trench formed in the deep trench filled with conductive material, the center trench may be formed by underfilling the trench with conductive material or etching the center trench, in accordance with the present invention; 
     FIG. 5B is a cross-sectional view of the structure of FIG. 4 showing a center trench formed to the bottom of the deep trench in accordance with the present invention; 
     FIG. 6 is a cross-sectional view of the structure of FIG. 5 showing the center trench being lined with a dielectric layer in accordance with the present invention; 
     FIG. 7 is a cross-sectional view of the structure of FIG. 6 showing a shallow trench formed therein in accordance with the present invention; 
     FIG. 8 is a cross-sectional view of the structure of FIG. 7 showing a dielectric layer (e.g., oxide) formed in the shallow trench in accordance with the present invention; 
     FIG. 9 is a cross-sectional view of the structure of FIG. 8 showing another dielectric layer (e.g., nitride) formed in the shallow trench in accordance with the present invention; 
     FIG. 10 is a cross-sectional view of the structure of FIG. 9 showing a dielectric cap formed at the bottom of the shallow trench in accordance with the present invention; 
     FIG. 11 is a cross-sectional view of the structure of FIG. 10 showing the shallow trench filled with conductive material to form a connection between the substrate and the center node in accordance with the present invention; 
     FIG. 12 is a cross-sectional view of the structure of FIG. 11 showing a first buried strap ( 46 ) and a trench top dielectric formed in accordance with the present invention; 
     FIG. 13 is a cross-sectional view of the structure of FIG. 12 showing a second buried strap ( 70 ) and an access transistor formed in accordance with the present invention; 
     FIG. 14 is a cross-sectional schematic view of the structure of a deep trench capacitor in accordance with the prior art; and 
     FIG. 15 is a cross-sectional schematic view of the structure of a folded deep trench capacitor in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention provides a folded deep trench capacitor structure, which provides increased capacitance without increased trench width. This provides at least a 40 to 50% higher deep trench capacity with the same area demand as conventional deep trench structures. Advantageously, the width critical dimension as well as the deep trench etch depth can be relaxed and at least the same deep trench capacity can be achieved. In one implementation of the deep trench structure of the present invention, a second buried strap is formed connecting an inner electrode to a substrate, which surrounds the deep trench. The deep trench structure of the present invention preferably employs available deep trench, collar and buried strap technology. 
     Referring now in specific detail to the drawings in which like reference numerals identify similar or identical elements throughout the several views, and initially to FIG. 1, a partially fabricated semiconductor device  10  is shown. Device  10  includes a semiconductor substrate  12 , preferably, a monocrystalline silicon substrate. FIG. 1 shows a deep trench  14  formed in substrate  12 . Deep trench  14  may be formed by conventional processes, such as a reactive ion etch (RIE) using a hardmask (not shown) patterned as is known in the art. A pad nitride layer  16  and a pad oxide layer  18  are formed before hard mask deposition and patterning as is also known in the art. Other materials and processes may be employed to form trench  14 . A buried plate  19  is formed surrounding trench  14 . Buried plate  19  may be formed by implanting dopants in substrate  12  adjacent to a lower portion of trench  14 . 
     Referring to FIG. 2, a liner  20  is formed in trench  14 . Liner  20  preferably includes a nitride material. Liner  20  is relatively thin, for example about  6  nm in thickness. Liner  20  is formed by processes known to those skilled in the art. 
     Referring to FIG. 3, a first filler material  22  is deposited to fill trench  14 . First filler material  22  preferably includes doped polysilicon. First filler material  22  is recessed in trench  14  and portions of liner  20  are removed in areas other than those covered by filler material  22 . A collar  24  is formed by local oxidation of silicon (LOCOS), by depositing an oxide or by growing an oxide within trench  14 . 
     Referring to FIG. 4, a second filler material  26  is deposited to fill trench  14 . Second filler material  26  is formed from a same material as first filler material  22 , preferably doped polysilicon. Filler materials  22  and  26  will be hereafter referred to collectively as a storage node  25 . A planarization step is preferably employed, for example, a chemical mechanical polish (CMP) to planarize second filler material  26  down to pad nitride  16 . 
     Referring to FIG. 5A, in one embodiment of the present invention, a center trench  28  is formed in trench  14  by further recessing material  22  and underfilling trench  14  shown in FIG. 4 with filler material  26 , preferably doped polysilicon, and more preferably low pressure chemical vapor deposited polysilicon. Filler material  26  is deposited such that material  26  underfills trench  14  leaving a void, which forms center trench  28 . Material  26  may then be removed from a top surface of device  10 . 
     In an alternate embodiment, a lithography process is employed to pattern center trench  28  in material  22  and  26  of FIG.  4 . With the lithography process, storage node  25  is patterned preferably by a photolithography process. For example, a resist layer (not shown) is spun onto a top surface of device  10 , exposed in accordance with a photomask and developed to open up a hole in the resist layer over storage node  25 . An etching process, such as, reactive ion etching (RIE), is preferably employed to etch out a center trench  28  of storage node  25 . It should be understood that center trench  28  may extend (e.g., be etched) to the bottom of deep trench  14 , as shown in FIG. 5B, although center trench may extend to any intermediate position within deep trench  14 , as shown in FIG.  5 A. Either process (e.g., underfill process or lithography process) may be employed to form center trench  28 ; however, the underfilling process is preferred since an additional lithography step and an etch step are advantageously avoided. 
     Referring to FIG. 6, a dielectric layer  30  is deposited to line surfaces of center trench  28 . Layer  30  preferably includes a nitride material. Layer  30  may be as thin as 2-3 nm, although thicknesses of about 6 nm may be employed. 
     Referring to FIG. 7, a third filler material  32  is deposited in center trench  28  to form a center node  34 . Third filler material  32  preferably includes doped polysilicon. Material  32 , layer  30  and nitride pad  16  are all removed from the top surface of device  10 . A shallow trench  36  is patterned into portions of collar  24 , storage node  25  and center node  34 . Shallow trench  36  is formed by preferably employing a photolithography process, including resist deposition, exposure and development, followed by an etch, such as RIE. The etching employed during the formation of shallow trench  36  exposes center node  34  and substrate  12 . A connection between center node  34  and substrate  12  will be formed in shallow trench  36  in later steps. 
     Referring to FIG. 8, a dielectric layer  38  is formed in shallow trench  36 . Dielectric layer  38  may be formed in a same way as collar  24 . In a preferred embodiment, dielectric layer  38  includes a deposited oxide. Dielectric layer provides protection of the sidewalls and bottom of shallow trench  36 . Dielectric layer  38  preferably includes a thickness of between, for example, about 20 nm and about 30 nm. 
     Referring to FIG. 9, dielectric layer  38  is subjected to an anisotropic etch, such as a RIE process, to thin dielectric layer  38  at the bottom of shallow trench  36 . Dielectric layer  38  is preferably thinned to a thickness of between about 10 nm and about 20 nm. A liner  40  is deposited over dielectric layer  38 . The thinned portion of dielectric layer  38  provides support for liner  40  at the bottom of shallow trench  36 . Liner  40  preferably includes nitride between 2-6 nm in thickness. Liner  40  is thinner along sidewalls of shallow trench  36  as is typical due to the deposition process. Liner  40  may include an oxy-nitride liner. Alternately, liner  40  may be employed without dielectric layer  38 . 
     Referring to FIG. 10, liner  40  is etched to remove liner  40  from sidewalls of trench  36 . A wet etch, for example an HF or buffered HF etch, is employed to remove dielectric layer  38  from sidewalls of shallow trench  36 . This creates a window between center node  34  and substrate  12 . Liner  40  and dielectric layer  38  remain as an insulator between shallow trench  36  and storage node  25 . 
     Referring to FIG. 11, a fourth filler material  42  is deposited to fill shallow trench  36 . A planarization step is employed to planarize a top surface of device  10  to remove filler material  42  from the surface. Filler material  42  preferably includes doped polysilicon. Dielectric layer  38  may function as a polish/etch stop for the planarization process (e.g., CMP). Filler material  42  provides a connection (an additional buried strap, for example) between center node  34  and buried plate  19 . 
     Referring to FIG. 12, portions of filler material  42 , storage node  25  and collar  24  are recessed back. Conductive material  44  is deposited to form a buried strap  46 . Buried strap  46  preferably includes doped polysilicon. Buried strap  46  may be enhanced by an additional doping process, by employing doping processes known in the art. Conductive material is recessed again to permit deposition of a trench top oxide  48 . This provides a folded deep trench structure in accordance with the present invention. 
     Referring to FIG. 13, processing continues to form an access transistor  60 . Access transistor  60  includes a gate structure  50  (e.g., a wordline), which includes a doped polysilicon conductor  52  and silicide layer  54 . Dielectric spacers  56  and a cap  58  are formed, preferably of nitride, to provide insulation around gate structure  50 . When gate structure  50  is activated, current is permitted to flow between diffusion region  62  and storage node  25  through a channel  64 , a diffusion region  66 , and buried strap  46 . This permits charging and discharging of storage node  25 . Storage node  25  represents a first capacitor plate of the folded deep trench structure. Center node  34  is connected to buried plate  19  to form a second plate of the deep trench capacitor in accordance with the present invention. A second buried strap  70  is formed, which outdiffuses into surrounding regions  72 . This outdiffusion region  72  combines with dopants of buried plate  19  to form the second plate of the capacitor. This outdiffusion may be assisted by performing an annealing step, such as a rapid thermal anneal or other anneal process to cause sufficient outdiffusion from buried strap  70 . To further assist outdiffusion, additional doping of buried strap  70  may be provided. By increasing dopant concentration of buried strap  70  better conduction between buried plate  19  and buried strap  70  is advantageously provided. 
     When charging or discharging storage node, the voltage of buried plate  19  is modified by a voltage pump through substrate  12 . Processing may now continue as is known in the art. 
     To calculate capacitance the following formula is used: 
     
       
           C=∈   0 ∈ R   A/S,   
       
     
     where C is the capacitance, ∈ 0  is a dielectric constant for permittivity of free space, ∈ R  is the dielectric constant for permittivity through the capacitor dielectric, A is the surface area between the capacitor plates and S is the thickness of the capacitor dielectric. Possible improvements in capacitance may come from: material with better dielectric qualities (∈ 0 , ∈ R ), thinner node dielectric (S) and/or area increase. The present invention focuses on increasing area, to achieve increased capacity. 
     Referring to FIG. 14, a schematic of a conventional deep trench capacitor is shown. The area of the capacity is determined by the size of the cylinder area of a deep trench  100 . Assuming a perfect cylinder with round ground area, the area can be calculated by the following: 
     A=2 r 2 +2 rh, where r is the radius of the cylinder and h is the height of the cylinder. Assuming a radius of 0.3 microns and a height of 6 microns the area is 11.87 microns 2 . 
     Referring to FIG. 15, a folded trench structure  200  in accordance with the present invention includes a larger area. Advantageously, the active area of the capacity could be increased by at least 40-50% over the prior art. This may also be increased by 100% or beyond. The area calculation for the present invention may illustratively be performed as follows: 
     A=2 (r 1   2 −r 2   2 )+2 r 1 h+2 r 2 h where r 1  is the radius of the storage node  25 , r 2  is the radius of the center node  34  and h is the height (assumed to be approximately the same for both nodes). Assuming a radius r 1  of 0.3 microns, r 2  of 0.15 microns and a height of 6 microns the area is 17.95 microns 2 ! 
     Having described preferred embodiments for folded deep trench capacitor and method (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.