Abstract:
A DRAM array in an SOI wafer having a uniform BOX layer extending throughout the array eliminates the collar oxide step in processing; connects the buried plates with an implant that, in turn, is connected to a conductive plug extending through the device layer and the box that is biased at ground; while the pass transistors are planar NFETs having floating bodies that have a leakage discharge path to ground through a grounded bitline.

Description:
BACKGROUND OF INVENTION 
   The field of the invention is that of SOI integrated circuits having DRAM arrays of trench capacitor cells. 
   Dynamic Random Access Memory (DRAM) cells are well known. A DRAM cell is essentially a capacitor for storing charge and a pass transistor (also called a pass gate or access transistor) for transferring charge to and from the capacitor. Data (1 bit) stored in the cell is determined by the absence or presence of charge on the storage capacitor. Because cell size determines chip density, size and cost, reducing cell area is one of the DRAM designer&#39;s primary goals. Reducing cell area is done, normally, by reducing feature size to shrink the cell. 
   Besides shrinking the cell features, the most effective way to reduce cell area is to reduce the largest feature in the cell, typically, the area of the storage capacitor. Unfortunately, shrinking the capacitor plate area reduces capacitance and, consequently, reduces stored charge. Reduced charge means that what charge is stored in the DRAM is more susceptible to noise, soft errors, leakage and other well known DRAM problems. Consequently, another primary goal for DRAM cell designers is to maintain storage capacitance while reducing cell area. 
   One way to accomplish this density goal without sacrificing storage capacitance is to use trench capacitors in the cells. Typically, trench capacitors are formed by etching long deep trenches in a silicon wafer and, then, placing each capacitor on its side in the trench, orienting the capacitors vertically with respect to the chip&#39;s surface. Thus, the surface area required for the storage capacitor is dramatically reduced without sacrificing capacitance, and correspondingly, storable charge. 
   However, since using a trench capacitor eliminates much of the cell surface area, i.e., that portion of cell area which was formerly required for the storage capacitor, the cell&#39;s access transistor has become the dominant cell feature determining array area. As a result, to further reduce cell and array area, efforts have been made to reduce access transistor area, which include making a vertical access transistor in the capacitor trench. See, for example, U.S. Pat. No. 6,426,252 entitled “Silicon-On-Insulator Vertical Array DRAM Cell With Self-Aligned Buried Strap” and references cited in it. 
   Performance is equally as important as density to DRAM design. Silicon(SOI) has been used to decrease parasitic capacitance and hence to improve integrated circuit chip performance. SOI reduces parasitic capacitance within the integrated circuit to reduce individual circuit loads, thereby improving circuit and chip performance. However, reducing parasitic capacitance is at odds with increasing or maintaining cell storage capacitance. Accordingly, SOI is seldom used for DRAM manufacture. One attempt at using SOI for DRAMS is taught in the cited patent. 
   In the case of transistors formed on SOI those skilled in the art conventionally introduce a contact to the body to drain away holes generated in the course of operation. Such body contacts add to the area of the cell and thus defeat some of the advantages of using SOI. 
   Thus, there is a need for increasing the number of stored data bits per chip of Dynamic Random Access Memory (DRAM) products. There is also a need for improving DRAM electrical performance without impairing cell charge storage. 
   SUMMARY OF INVENTION 
   The invention relates to an SOI integrated circuit having a DRAM array that employs planar transistors. 
   A feature of the invention is a reduction in processing time by the elimination of several processing steps in the trench capacitor including the collar oxide process. 
   Another feature of the invention is a floating body pass gate transistor. 
   Another feature of the invention is the elimination of a triple well isolation structure. 
   Another feature of the invention is the connection of the buried plates in the capacitors to a reference supply by means of at least one implant below the buried oxide (BOX). 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  shows a cross section of a portion of an SOI integrated circuit at the start of the process. 
       FIG. 2  shows a portion of the DRAM array after preliminary steps of forming the buried plate, dielectric and center electrode and recessing the center electrode. 
       FIG. 3  shows a cross section of a via for establishing contact to the buried plates. 
       FIG. 4A  shows the area of  FIG. 2  after filling the recessed area in the capacitor. 
       FIG. 4B  shows the area of  FIG. 3  after filling the via. 
       FIG. 5  shows the area of  FIG. 2  after completing the cell. 
       FIG. 6  shows the area of  FIG. 3  after completing the implants for connecting to the buried plates. 
       FIG. 7A  shows a cross section of a wafer according to the prior art. 
       FIG. 7B  shows a corresponding cross section of a wafer according to the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a portion of an SOI integrated circuit wafer that will have a DRAM array and also support circuitry for the DRAM array and, in the case of an embedded DRAM chip, a logic area for the logic transistors performing the function of the chip. 
   The illustrated area shows P− substrate  10 , having BOX  20  (130 nm) separating the substrate from the device layer  30  (70 nm), illustratively doped P− in the DRAM array and having P-type and N-type regions in the support areas and logic area. 
     FIG. 2  shows a portion of the DRAM array after a number of preliminary steps. A deep trench has been etched in the substrate through the pad layer  35 , illustratively a layer of thermal oxide topped by nitride, then through silicon device layer  30 , then through buried oxide (SiO2, BOX)  20 , and then down about 8 um. 
   Buried plates  105  (N+) have been diffused into the substrate in a conventional process in which highly doped polycrystalline silicon (poly) is deposited and heated to diffuse dopant into the substrate. The buried plates are shown as extending only partially up to the bottom of the BOX, so that the buried plates are separated vertically from the box by a buried plate offset. In a later step, an implant will fill in the area below the BOX in the array and extend downwardly to overlap the buried plates in a vertical overlap region. 
   Optionally, one could extend the buried plates upward closer to or reaching the bottom of the BOX, which would increase the capacitance in the capacitor. In that case, the overlap region would be co-extensive with the implant. 
   At the top of  FIG. 2 , an aperture  118  has been formed by recessing the polysilicon center electrode of the capacitor down nominally to the mid-point of BOX  20 . With the sidewalls of the trench exposed, the nitride capacitor dielectric is stripped and any desired treatment is applied to the trench sidewalls, e.g. a thin layer of thermal oxide or nitride to passivate the walls in the device layer while still permitting current to pass in and out of the capacitor. 
   In conventional trench DRAM processing, a “triple well” isolation scheme is required. Typically, the “triple well” consists of a â□□(1) “deep” p-well implant (peak concentration ˜0.7 Î¼ m beneath the Si surface), (2) “medium” p-well implant (peak concentration ˜0.3 â□□ 0.4 Î¼ m below the Si surface), (3) “shallow” p-well implant (peak concentration ˜30 â□□40 nm below the Si surface). The “deep” implant is needed to suppress the vertical parasitic device along the upper regions of the trench sidewall (plate to buried strap leakage, gated by the collar oxide &amp; N+ polyfill). The “medium” p-well implant is used to isolate cross-talk of eDRAM cells in the lateral dimension. The “shallow” implant is used to adjust the threshold voltage of the array pass transistor. Here, eDRAM cells are contained in a p-well  117  that is isolated by an n-well that surrounds the eDRAM array blocks not shown in the Figure. One of the many advantages of the floating-body eDRAM cell is that the isolated p-well is no longer required â□□ only a transistor threshold implant is necessary â□□ thereby eliminating implant steps and lowering the processing cost of SOI eDRAM. 
   Further, SOI integrated circuits that have embedded DRAMS conventionally are required to form the SOI layers only in the logic portions of the chip, with the DRAM array being formed in an area that does not have the BOX. This patterning of the BOX, which is quite expensive, is not required for the practice of the current invention, thus saving considerable expense. 
     FIG. 3  shows a portion of the chip where a contact via that will connect the buried plates to a reference (ground) has been etched. This etch passes through pad nitride  35 , SOI layer  30  and BOX  20 , with the chemistry being changed to handle the different materials in a conventional manner. 
     FIG. 4A  shows the same region as  FIG. 2  after a step of filling recess  118  in  FIG. 2  with doped poly. A dotted line denotes the separation between the portion of poly added after FIG.  2 . 
     FIG. 4B  shows via  155  of  FIG. 3  after filling it simultaneously with the recess in the DRAM array. The conductive filling (also referred to as a plug) is denoted with numeral  36 . Heavy line  112  represents an optional dielectric liner (e.g. nitride) that isolates the plug  36  from the SOI layer  30 . 
     FIG. 5  shows a completed portion of the DRAM array of FIG.  2 . An N− implant  160  (about 1E18/cm3) has been provided that extends down nominally 1 um from the lower surface of the BOX to overlap and make contact with the buried plates. This implanted area forms a conductive path to maintain the buried plates at their specified voltage of ground. 
   The device layer has NFET pass transistors  60  formed in it. The body of the pass transistors is P-doped by implantation at any convenient time. Gates  65  have been formed over a conventional gate oxide and sidewalls  62  have been formed. An N+ Source/Drain (S/D) implant has been made in areas  39 , with the sidewalls  62  reducing the implanted dose to N− in the areas  38  adjacent to the transistor bodies. 
   Passing wordlines  70  are shown as crossing the tops of the capacitors, separated vertically from the center electrodes of the capacitors  115  by trench top oxide (or other dielectric)  113 . The capacitors are separated by oxide-filled isolation trenches (STI)  32 , formed at the same time as the STI in the logic areas. Top dielectric  113  is made sufficiently thick to prevent cross talk from the passing wordlines. If the architecture of the DRAM array is not one that uses passing wordlines (referred to as a folded bitline architecture), or if the passing wordlines do not cause crosstalk, the separate dielectric may be dispensed with or the gate oxide may be the only dielectric. 
     FIG. 6  shows the area of  FIG. 3  after the back end processing up to metal  1 . Plug  36  of  FIG. 4B  has been topped with a Tungsten via  75  placed in an interlevel dielectric  72 . Via  75  makes contact with a portion of the M1 interconnect that links it to the correct reference voltage. 
   An N-well implant  165  has been made to establish ohmic contact between the plug  75  and the buried plate contact implant  160 . 
   The plugs will be placed as required to establish the designed impedance to the buried plates. Preferably, the plugs will not be placed in the array, as that would interfere with the most compact layout. Advantageously, the plugs are placed in the Nband diffusion guard ring or another location on the periphery of the array, but not within it. 
   Transistor formation is preferably the same as that for NFETs in the logic area. The cell transistors have floating bodies, while the logic transistors may have body contacts. Those skilled in the art would expect that there would be the usual, well known problems associated with the generation of holes in the floating bodies of the array NFETs. 
   It has been found that an operation sequence in which the bitline is returned to ground after writing to the capacitor and is maintained at ground during most of the time, provides an impedance path to ground through the transistor electrode and bitline that is sufficient to drain off holes generated during operation, and therefore avoids the need to add a body contact to the area of the DRAM cell. 
   This invention requires an additional mask for vias  155  of FIG.  3  and an additional Reactive Ion Etch (RIE) step. The Well implant  165  to make contact with the plugs can be performed simultaneously with the logic n-well process. However, as well isolation implants are typically avoided in SOI processing, a separate mask may be required such that one is allowed to implant an n-type dopant beneath the BOX, e.g. in the Nband diffusion guard ring surrounding the eDRAM arrays. 
   A number of processing steps within the trench capacitor module is eliminated in this invention. Specifically, the simplified trench process allows the elimination of â□□ (1) the collar oxide conventionally used for bulk eDRAM, (2) a trench polyfill deposition, (3) a polysilicon chemical-mechanical polish (CMP), (4) a polysilicon recess. It is conservatively estimated that 90 hours of process time can be saved by creating eDRAM in unpatterned SOI substrates â□□ using the invention described therein—vs. creating eDRAM in patterned SOI substrates. 
   As a design alternative, the buried plate could be formed with a top closer to the bottom of the BOX and therefore require a buried plate contact implant  160  having less thickness. For a given concentration, a thicker implant takes more time than a more shallow one, so a thinner implant will save additional time. In addition, the thicker implant will do more damage to the crystal structure of the device layer and to the gate oxide. 
   The Process Sequence is: 
   Provide a p-type SOI substrate with a BOX uniformly across the wafer 
   Deep Trench Module 
   Etch Deep Trench (DT) through Device Layer and through BOX 
   Form Buried Plates 
   Form DT Capacitor Dielectric up to wafer surface 
   Deposit doped center electrode 
   Recess center electrode halfway into BOX, leaving the level of the capacitor 
   Dielectric in the middle of the BOX 
   Etch contact vias through nitride, silicon device layer and BOX 
   Prepare sidewalls of DT capacitor for strap 
   Fill both the recessed deep trench and the contact vias with doped poly 
   Planarize the wafer
         Optionally
           Recess the Deep trench for the TTO (10-20 nm)   Deposit TTO (HDP oxide?)   Planarize the wafer
 
STI Module to isolate eDRAM cells in arrays and provide isolation for support logic circuits
 
Implant Module
   
               

   Implant the DRAM array with a buried plate contact implant (N−) that extends down to make contact with the buried plates and is doped heavily enough to provide a current path to a reference voltage (ground). 
   Implant N-wells as needed for sufficient contact with the array buried plates—preferably within the Nband diffusion ring surrounding the array blocks. 
   Transistor Module 
   N-Well implant in the device layer of the logic area and P-well implant in both the logic area and array in the device layer 
   STI in array and support 
   Gate Oxide 
   Poly gates 
   Sidewalls 
   S/D implant 
   Interconnect Module 
   Conventional Back End 
   The steps of forming the logic transistors and the interconnects (collectively the “back end”) will be referred to as completing the circuit. 
   In order to provide a relatively low impedance path to the buried plates, it may be desirable to etch a relatively long (compared with the dimensions of the contacts) trench in the periphery of the DRAM array. 
   It is preferred that vias for buried plate contacts not be formed within the array, but a relatively low doping concentration imposed for other engineering reasons may require it. 
   The invention has been reduced to practice with contacts only on the periphery. 
   While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.