Abstract:
An integrated circuit semiconductor memory device having the BOX layer removed from under the gate of a storage transistor to increase the gate-to-substrate capacitance and reduce the soft error rate. The increased node capacitance thus obtained is achieved without requiring a corresponding increase in area.

Description:
BACKGROUND ART 
     This invention relates to an integrated circuit semiconductor memory device such as an SRAM (Static Random Access Memory) or latch that provides increased node capacitance for protection against soft errors. 
     Semiconductor memory devices such as RAM (random access memory) generally include a number of memory cells each formed of a number of transistors. Generally, two storage transistors are coupled between two pass gate transistors, and a bit line is coupled to each of the pass gate transistors. Each pass gate transistor has a gate electrode coupled to a word line, and an address signal is provided on the word line associated with a particular memory cell in order to select that memory cell and read out the stored data therefrom. With the memory cell so selected, its data is read out from the memory node of the memory cell (or data is written therein) through the pass gate transistors via the bit lines. It is of course important that the data stored in the memory cell remain unchanged until it is read out. 
     There has been a growing difficulty in preserving such stored data as the scale of integration grows higher and higher and the physical size of the memory cell elements decreases. This difficulty arises from what are known as soft errors, which are caused primarily by an alpha particle striking one of the memory nodes, or may be caused by circuit noise. This can cause the voltage on the memory nodes to change, sometimes sufficiently so that a logic 1 is transformed into a logic 0 or vice versa. The amount of voltage change for a given alpha particle hit is inversely proportional to the capacitance on the memory node, and so a relatively large capacitance on the memory node reduced the amount of voltage change for a given alpha particle hit and correspondingly reduces the chance of a soft error. 
     With the relatively large devices of smaller scale integration, there was sufficient node capacitance to prevent soft errors most of the time. However, as the dimensions of the memory cells are scaled down to fit more devices on the chip, the node capacitances are getting correspondingly very low. In addition, the applied voltage Vdd is also being scaled down with device size, again leading to reduced node capacitance. The result is a high susceptibility to circuit noise and radiation, which in turn leads to an unacceptably high soft error rate. 
     It would therefore be highly desirable to increase the node capacitances of SRAMs, latches and the like without resorting to increasing the device size again. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a semiconductor memory device that avoids the above-discussed difficulties of the prior art. 
     It is another object of the present invention to provide a semiconductor memory device having increased node capacitance to enable a low soft error rate. 
     It is yet another object of the present invention to provide a semiconductor memory device having increased node capacitance without requiring an increase in area. 
     The above objects, as well as additional advantages, will be realized in the practice of the invention as herein described. In its broadest embodiment, the integrated circuit semiconductor memory device comprises a substrate with a first dielectric layer covering a first portion of the substrate, the first dielectric layer being absent from a second portion of the substrate. The device further comprises a second dielectric layer having a property different from the first dielectric layer, the second dielectric layer at least partly covering the second portion of the substrate. A source region is formed in a first doped region on the first dielectric layer, a drain region is formed in a second doped region on the first dielectric layer, and a gate is formed over the second dielectric layer and between the first and second doped regions. In accordance with an important aspect of the present invention, the property of the second dielectric layer provides a gate capacitance of the gate with respect to the substrate that is greater than a theoretical capacitance of a gate formed over the first dielectric layer on the substrate. 
     In an advantageous aspect of the invention, the memory device is an SRAM memory cell, advantageously an FET or especially a FinFET. 
     In one preferred embodiment, the first dielectric layer is a buried oxide layer and the second dielectric layer is a thin oxide layer providing less insulating effect than the buried oxide layer, the gate being capacitively coupled to the substrate. 
     In another preferred embodiment, the device is a FinFET having a fin and further comprises a gate dielectric layer between the gate and the fin, wherein the second dielectric layer has less leakage than the gate dielectric layer. 
     In yet another preferred embodiment, the substrate has an upwardly-facing first surface at an upper level and an upwardly-facing second surface at a lower level, the first dielectric layer being a buried oxide layer formed on the first surface and the second dielectric layer being a thin oxide layer formed on the second surface. 
     In a further preferred embodiment, a bulk process produces a layout in which the first dielectric layer is a buried oxide layer and the second dielectric layer is a thin oxide layer providing less insulating effect 
     These and other objects, features and aspects of the present invention will be apparent from the following description of the preferred embodiments taken in conjunction with the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will be further described with reference to the drawings in which similar elements in different drawings are represented by numbers having the final two digits in common. 
         FIG. 1  is a schematic top plan view of a conventional FinFET. 
         FIG. 2  is a schematic cross-sectional view of the conventional FinFET taken along arrows A-A in  FIG. 1 . 
         FIG. 3  is a schematic top plan view of a FinFET in accordance with a first preferred embodiment of the present invention. 
         FIG. 4  is a schematic cross-sectional view of the FinFET in accordance with the first preferred embodiment of the present invention taken along arrows B-B in  FIG. 3 . 
         FIG. 5  is a schematic cross-sectional view of a portion of a FinFET in accordance with a second preferred embodiment of the present invention. 
         FIG. 6  is a schematic cross-sectional view of a portion of a FinFET in accordance with a third preferred embodiment of the present invention. 
         FIG. 7  is a schematic cross-sectional view of a portion of a FinFET in accordance with a fourth preferred embodiment of the present invention. 
         FIG. 8  is a schematic illustration of an SRAM layout including FinFETs in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following discussion, the prior art and the embodiments of the present invention will be described in the context of FinFETs. A FinFET is a double-gate MOSFET that is formed by defining and etching a thin, vertical fin in the silicon body of an SOI wafer to connect the source and drain regions. Polysilicon gate electrodes are defined surrounding the fin. In the embodiments discussed below, the double gates are on the right and left sides of the fin and are connected by a portion of the gate passing over the fin. When the FinFET is turned on, the current flow is from source to drain along both the left and right vertical edges of the fin. 
     It will be apparent to those of ordinary skill in the art that the following discussion and the accompanying drawings do not reference the complete structure of FinFETs generally or any FinFET in particular, but rather schematically define and compare only those elements of a FinFET useful for explaining the present invention. The elements omitted or simplified do not affect the following discussion. Accordingly, it should be understood that the invention is to be applied in the context of actual memory cell structures incorporating all necessary elements. 
     Thus, with reference to  FIG. 1 , a conventional FinFET  10  is schematically illustrated as forming an element of a semiconductor memory device on an integrated circuit chip, with  FIG. 2  being a side cross-sectional view as indicated. The conventional FinFET  10  is formed with a substrate  12  which has on its upper surface  14  a buried oxide (BOX) layer  16 . The FinFET  10  has a source region  18  formed in a first doped region on the BOX layer  16 , a drain region  20  formed in a second doped region on the BOX layer  16  and a vertically projecting fin  22  connecting the source and drain regions  18 ,  20 . As shown in  FIG. 2 , the fin  22  is also formed on the BOX layer  16 , and includes sidewalls  24 ,  26  of thin oxide. In addition, the FinFET  10  includes a gate  28  that serves as the control electrode for activating the FinFET  10 , with two gate portions  28   a ,  28   b , one on each side of the fin  22 . With this construction, the BOX layer  16  under the gate  28  provides insufficient capacitance in larger scale integrations where the area of the gate  28  has been substantially reduced. 
     The present invention provides a solution to this problem without requiring an increase in physical size of the memory cell elements.  FIGS. 3 and 4  are respectively top plan and side cross-sectional views of a first preferred embodiment of the present invention, corresponding to  FIGS. 1 and 2 . In  FIGS. 3 and 4 , a FinFET  100  is formed with a substrate  112  which has on a first portion  114   a  of its upper surface  114  a buried oxide (BOX) layer  116 , which is a dielectric material having defined properties. The FinFET  100  has a source region  118  formed in a first doped region on the BOX layer  116 , a drain region  120  formed in a second doped region on the BOX layer  116  and a vertically projecting fin  122  connecting the source and drain regions  118 ,  120 . As shown in  FIG. 4 , the fin  122  is also formed on the BOX layer  116 , and includes sidewalls  124 ,  126  of thin oxide. 
     In accordance with the present invention and in distinction from the prior art, the BOX layer  116  does not cover the entire portion of the substrate  112  underlying the FinFET  100 , but rather is absent from at least a second portion  130  of the substrate  112 . Instead, a second, different dielectric layer  132  is provided on this second portion  130 . The second dielectric layer  132  is formed of a dielectric material that has different properties from the dielectric material forming the BOX layer  116 , in particular a different dielectric coefficient and/or a different thickness. The gate  128  of the FinFET  100  is formed on the second dielectric layer  132 , which in this embodiment is a thin oxide layer. 
     In the present invention, the thin oxide layer  132  increases the node capacitance by replacing the BOX layer  116  and thereby permitting the gate  128  to be capacitively coupled to the substrate  112 . That is, this thin oxide (second dielectric) layer  132  provides a gate capacitance of the gate  128  with respect to the substrate  112  that is greater than a theoretical capacitance of a gate formed over the BOX (first dielectric) layer  116 . 
     Of course, there may be other portions of the substrate  112  not covered by either the BOX layer  116  or the thin oxide layer  1132 . 
     One method for producing the FinFET  100  would be to add the steps of forming a block mask for BOX removal and then etching the BOX. 
       FIG. 5  is a schematic side cross-sectional view of another preferred embodiment. In the FinFET  200  of  FIG. 5 , the thin oxide second dielectric layer  132  of the first embodiment is replaced by a thicker oxide layer  232  which has a different dielectric coefficient than that of the material forming the BOX layer  216  to reduce gate leakage. The oxide layer  232  may be formed by additional process steps to deposit or grow the dielectric material and then mask and etch the dielectric material into the proper areas. 
     In this and in other embodiments, the side walls on either side of the fin are advantageously in the form of thin dielectric layers between the gate and the fin, and are made of a dielectric material having less leakage than the Box layer  116 . 
       FIG. 6  illustrates a third preferred embodiment. Here, when the BOX layer  316  is removed by, for example, etching in making the FinFET  300 , the etching continues into the substrate  312  so that the lower portions  328   a ,  328   b  of the gate  328  fit into depressions  312   a ,  312   b  in the substrate  312 . Accordingly, the substrate  312  has an upwardly-facing first surface  314   a  at an upper level and an upwardly-facing second surface  314   b  at a lower level. The additional etching of the silicon substrate  312  can be an additional process or a continuation of the BOX etching process. This structure then uses thin oxide layers  332   a ,  332   b  along the bottom and sides of the lower portions  328   a ,  328   b , as well as thin oxide sidewalls  324 ,  326  on the sides of the fin  322 . 
     In a bulk process for creating plural memory cells, it is advantageous to grow a thick oxide layer or deposit dielectric under the gate to create a dielectric layer that is different from the dielectric material on the transistor to reduce leakage from the gate to the substrate.  FIG. 7  illustrates schematically a single FinFET  400  from such a bulk process, having this second dielectric layer  432  on the substrate  412  under the gate  428 . The fin  422  includes sidewalls  424 ,  426  of thin oxide.  FIG. 8  illustrates a FinFET SRAM layout  550  in which the FinFETs have the structure of FinFET  552  produced in accordance with the present invention. 
     In an alternative process, the substrate may be doped so that oxide grows much faster on the substrate than on the FinFET, so that the difference in dielectric effect, and hence in capacitance, arises from the differential thickness of the oxide layer.