Abstract:
A method for manufacturing a shallow trench isolation structure comprises etching a plurality of trenches into a silicon substrate. The trenches have an upright wall portion, a bottom floor portion, and a corner portion connecting the upright wall portion and the bottom floor portion. The method further comprises conformally depositing a dielectric layer into the trenches. The dielectric layer covers at least part of the upright wall portion, at least part of the bottom floor portion, and at least part of the corner portion. The method further comprises oxidizing the dielectric layer. A portion of the dielectric layer deposited over the corner portion is oxidized at a first oxidization rate, and a portion of the dielectric layer deposited over the upright wall portion is oxidized at a second oxidization rate. The first oxidization rate is less than the second oxidization rate. The method further comprises depositing an electrically insulating material into the trenches over the dielectric layer.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to integrated circuit fabrication techniques, and more specifically to formation of improved shallow trench isolation structures for use in integrated circuit devices. 
     BACKGROUND OF THE INVENTION 
     In the field of integrated circuit fabrication, the drive to reduce circuit size has led to smaller electronic devices being packed with greater density on a common substrate. Examples of typical electronic devices used in integrated circuit applications include active devices, such as transistors, and passive devices, such as resistors and capacitors. Generally, the reliability of densely-packed electronic devices depends in part on providing sufficient electrical isolation between adjacent devices in the integrated circuit. 
     One common way to electrically isolate adjacent devices from each other is through the formation of local oxidization of silicon (“LOCOS”) structures. LOCOS structures are typically formed by depositing a non-oxidizable mask, such as silicon nitride (Si 3 N 4 ), over a blank silicon wafer. The mask is patterned using photolithography, and then a film of silicon oxide (SiO 2 ) is formed on the portions of the silicon surface exposed by the photolithographic process. The oxide film functions to electrically isolate devices formed over the portions of the substrate where the non-oxidizable mask remains. 
     Another common way to electrically isolate adjacent devices from each other is to form shallow trench isolation (“STI”) structures between the devices in the substrate. STI structures are typically formed by etching shallow trenches in the substrate between areas where electronic devices are to be formed, and then filling the trenches with an electrically insulating material, such as silicon oxide or another dielectric material. After the trenches are filled, the wafer surface is planarized by a chemical mechanical polish (“CMP”) process so that the insulating material remains in the trench, with its top surface level with that of the exposed silicon substrate. The “buried” insulating material provides electrical isolation between adjacent devices formed on or within the exposed substrate surface. A typical insulating material used to fill STI trenches is oxide, thin films of which are deposited using a high density plasma chemical vapor deposition (“HDP-CVD”) process. 
     SUMMARY OF THE INVENTION 
     While LOCOS structures provide effective electrical isolation in certain applications, there are significant limitations to LOCOS technology. For example, LOCOS structures consume substantial amounts of silicon active area on the substrate surface, thereby reducing the area available to be used in the formation of electronic devices. Additionally, LOCOS structures have a non-planar surface topology that adversely affects process yield and complexity of subsequent substrate processing steps. Furthermore, lateral oxidization of the silicon underneath the silicon nitride mask tends to occur in conventional LOCOS methodology, thereby forming a “bird&#39;s beak” structure at the edges of the patterned oxide. 
     Likewise, conventional STI technology also suffers from certain disadvantages. For example, as the width of an STI trench decreases, so does the width of the insulating material filing the trench. If the defect density in the insulating material filling the trench is too high, the insulating material will have a reduced capacity to provide effective electrical isolation. In particular, STI trenches that are formed using many conventional techniques and that have a width less than about 180 Å have a significantly reduced capacity to provide electrical isolation between adjacent electronic devices. Defects in the insulating material often result from mechanical and thermal stresses placed on the substrate and/or the insulating material during formation of the STI trench structures. 
     In accordance with the foregoing, Applicant has developed an improved approach for electrically isolating densely-packed devices provided on a common semiconductor substrate. In certain embodiments, a STI structure is provided with an improved, rounded profile that reduces the density of stress-induced dislocations in the insulating material at the corners of the trench. The reduced dislocation density in the insulating material provides the STI structure with improved electrical insulation characteristics. 
     According to one embodiment of the present invention, a method for manufacturing a shallow trench isolation structure comprises etching a plurality of trenches into a silicon substrate. The trenches have an upright wall portion, a bottom floor portion, and a corner portion connecting the upright wall portion and the bottom floor portion. The method further comprises conformally depositing a dielectric layer into the trenches. The dielectric layer covers at least part of the upright wall portion, at least part of the bottom floor portion, and at least part of the corner portion. The method further comprises oxidizing the dielectric layer. A portion of the dielectric layer deposited over the corner portion is oxidized at a first oxidization rate, and a portion of the dielectric layer deposited over the upright wall portion is oxidized at a second oxidization rate. The first oxidization rate is less than the second oxidization rate. The method further comprises depositing an electrically insulating material into the trenches over the dielectric layer. 
     According to another embodiment of the present invention, a method comprises etching a trench into a substrate. The method further comprises forming a dielectric layer in the trench. The dielectric layer includes a trench wall portion, a trench floor portion and a trench corner portion connecting the wall portion with the floor portion at an angled joint. The method further comprises oxidizing the dielectric layer. The trench wall portion is oxidized more rapidly than the trench corner portion, thereby modifying the trench corner portion from the angled joint to a rounded joint. 
     According to another embodiment of the present invention, an semiconductor structure comprises a silicon substrate having a trench etched therein. The trench has an upper wall portion, a bottom floor portion and a corner portion connecting the upper wall portion and the bottom floor portion. The semiconductor structure further comprises an oxide layer formed on the upper wall, the bottom floor and the corner portions of the trench. The oxide layer has a first thickness adjacent to the upper wall portion of the trench, and a second thickness over the corner portion of the trench. The second thickness is less than the first thickness. The semiconductor structure further comprises an electrically insulating material positioned within the trench and over the oxide layer. The oxide layer is positioned between the electrically insulating material and the silicon substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the improved STI techniques and structures are illustrated in the accompanying drawings, which are for illustrative purposes only. The drawings comprise the following figures, in which like numerals indicate like parts. 
         FIG. 1  is a cross-sectional view of a conventional STI structure. 
         FIG. 2  is a cross-sectional view of an STI trench having a silicon nitride layer conformally deposited thereover. 
         FIG. 3  is an enlarged view of a lower corner the STI trench of  FIG. 2  after performing an in situ steam generated oxide process. 
         FIG. 4  is a cross-sectional photograph of a portion of an STI structure formed using an exemplary embodiment. 
         FIG. 5  is a cross-sectional photograph of an STI trench formed using an exemplary embodiment. 
         FIG. 6  is a flow chart illustrating an exemplary method for forming an STI trench having an insulating material reduced dislocation density deposited therein. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As described herein, the ability of an STI trench to provide adequate electrical isolation is compromised by the presence of a high defect density in the insulating material filling the trench. Defects in the insulating material often result from mechanical and thermal stresses that result from the shape of the STI trench structure. This effect is illustrated in  FIG. 1 , which is a cross-sectional view of a conventional STI structure. This structure includes a silicon substrate  100  having a silicon oxide layer  102  and a silicon nitride layer  104  formed thereover. A trench  108  is etched through the oxide and nitride layers into the substrate  100 . Using a conventional oxidization process, such as dry or wet thermal oxidization, a sidewall oxide layer  106  is formed over the surface of the trench  108 . When the trench  108  is subsequently filled with an insulating material, for example using a HDP-CVD technique, the sidewall oxide layer  106  helps to reduce stress in the insulating layer. 
     The example conventional STI trench structure illustrated in  FIG. 1  suffers from several disadvantages. For example, an oxide encroachment  107 , also commonly referred to as a “bird&#39;s beak”, is formed when H 2 O and O 2  diffuse into the interface between the silicon substrate  100  and the silicon oxide layer  102 . The encroachment  107  acts as a source for the generation of defects in the insulating material filled into the trench  108 . Additionally, the sharp corners  110  at the bottom of the STI trench also cause a high level of embedded stress in the insulating material filling the trench  108 , thereby providing yet another source for the generation of defects. In some applications, the corners  110  of the trench are etched slightly deeper than the center of the trench, thereby making the corners  110  even sharper, particularly after application of the sidewall oxide layer  106 . This further increases the level of embedded stress, and therefore the defect density, in the insulating material filling the trench  108 . As described herein, defects in the insulating material filling the trench  108  result in leakage current through the trench  108 , thereby degrading the reliability of neighboring active devices separated by the trench  108 . 
     One approach to reduce defect density in the insulating material filling STI trenches is to sequentially deposit alternating layers of insulating materials having tensile stress and compressive stress. Theoretically, these alternating layers of insulating material will function to compensate for the overall structural stress in the bulk insulator. However, from a practical production standpoint, it is difficult or impossible to reduce the structural stress to a level that provides adequate reduction of structural defects in the bulk insulator material using this technique. Additionally, as a result of the deposition of multiple layers of insulator material, it is difficult to accurately control the subsequent CMP process that is used to provide the integrated circuit with a planar surface. 
     Another approach to reduce defect density in the insulator material used to fill STI trenches is to perform a high-temperature anneal after filling the trench. The anneal is intended to reduce the mechanical stress and therefore the dislocation density in the insulating material deposited in the trench. However, in embodiments wherein a sidewall oxide layer  106  is provided on the surface of the isolation trench, the anneal step disadvantageously causes thermal stress to be induced in this layer, and this thermal stress acts as yet another source for dislocations in the insulating material. 
     In accordance with the foregoing, Applicant has developed an improved approach for reducing defect density in the insulating material deposited in an STI trench structure. As shown in the exemplary embodiment illustrated  FIG. 2 , an STI trench  208  is etched through a silicon nitride layer  204 , an oxide layer  202 , and into a substrate  200 . In an exemplary embodiment, the substrate  200  is a silicon substrate with a &lt;100&gt; crystallographic orientation. However, in other embodiments, the substrate comprises other semiconductor substrates, such as a silicon-on-insulator (“SOI”) substrate. In an exemplary embodiment, the oxide layer  202  comprises a silicon dioxide layer formed by a thermal growth process and having a thickness between about 20 Å and about 300 Å. In an exemplary embodiment, the silicon nitride layer  204  has a thickness between about 100 Å and about 2000 Å. In a modified embodiment, the silicon nitride layer is substituted by an appropriate alternate material other than silicon nitride. 
     The depth of the STI trench  208  depends, at least in part, on the type of device to be isolated by the trench. For example, in flash memory applications, the trench is usually between about 2500 Å and about 4500 Å. To separate logic devices, such as metal oxide semiconductor (“MOS”) transistors, the trench is usually between about 2000 Å and about 4000 Å. In an exemplary embodiment, the trench is formed by anisotropic etching, such as reactive ion etching, although other etching techniques are used in other embodiments. 
     A thin, conformal silicon nitride layer  212  is then conformally deposited over the etched trench  208 . In one embodiment the conformal silicon nitride layer  212  has a thickness between about 25 Å and about 300 Å, in another embodiment the conformal silicon nitride layer  212  has a thickness between about 30 Å and about 200 Å, in another embodiment the conformal silicon nitride layer  212  has a thickness between about 50 Å and about 150 Å, and in another embodiment the conformal silicon nitride layer  212  has a thickness between about 75 Å and about 125 Å. 
     In such embodiments, the silicon nitride layer  212  is then selectively oxidized by an in situ steam generated (“ISSG”) oxidization process. An example of a processing tool that is capable of performing an ISSG oxidization process is a Centura® 5000 system marketed by Applied Materials Corporation (Santa Clara, Calif.). During the ISSG process, the silicon nitride on the bottom corners of the trench  208  has a relatively low oxidization rate as compared to the silicon nitride on the other (upright and horizontal) portions of the trench  208 . Consequently, in the time used to oxidize the silicon nitride at the bottom corners of the trench  208 , not only will the silicon nitride on the other portions of the trench  208  also be oxidized, but a portion of the underlying silicon substrate  200  will be oxidized as well. Because the ISSG oxidization process oxidizes different surfaces of the trench  208  at different rates, this results in a modified sidewall oxide layer in the trench  208  having a variable thickness, as illustrated in  FIG. 3 . Specifically,  FIG. 3  illustrates that the ISSG oxidization process causes the conformal silicon nitride layer  212  to have a relatively larger thickness t 1  in the upright and horizontal portions of the trench  208 , and a relatively smaller thickness t 2  in the corner portion of the trench  208 . 
     The relative rates of silicon nitride deposition and oxidization (by ISSG) are provided in Table A for various portions of the STI trench. 
                             TABLE A                       ISSG Oxidization       Local Topology   SiN Deposition Rate   Rate       of STI Trench   (flat = 1.00)   (flat = 1.00)                   flat   1.00   1.00       (trench sides and floor)       out-swinger   1.00   0.98       (trench top corners)       in-swinger   1.00   0.25       (trench bottom corners)                    
As illustrated in Table A, while the conformal silicon nitride layer  212  is deposited across the substrate at a uniform rate, the ISSG oxidization technique allows that layer to be oxidized non-uniformly based on the local topology. In particular, in the one exemplary embodiment for which data are provided in Table A, the silicon nitride located in the bottom corners of the STI trench is oxidized approximately four times slower than silicon nitride location on other portions of the STI trench structure. The data provided in Table A were obtained using transmission electron microscopy.
 
     In one embodiment, the ISSG oxidization process uses a hydrogen gas percentage (% H 2 ) of between approximately 1% and approximately 50%, in another embodiment the ISSG oxidization process uses a % H 2  of between approximately 5% and approximately 33%, an in another embodiment the ISSG oxidization process uses a % H 2  of between approximately 10% and approximately 25%. The hydrogen gas percentage % H 2  is defined by the following expression:
 
% H 2 =(H 2  flow rate)×(H 2  flow rate+O 2  flow rate) −1  
 
     By forming a variable-thickness silicon nitride layer in the STI trench, a modified sidewall oxide layer  206  is provided in the STI trench. As illustrated in  FIG. 3 , the modified sidewall oxide layer  206  has a rounded corner, as compared to the sharp corner  110  of the conventional STI trench illustrated in  FIG. 1 . When an insulating material is deposited into the STI trench having the modified sidewall oxide layer  206 , the result is reduced mechanical stress in the insulating material by virtue of the rounded corner. This results in a lower dislocation density in the insulating material. 
       FIG. 4  is a photograph of a portion of an STI trench  308  formed using certain of the exemplary embodiments disclosed herein. The photograph illustrates a modified sidewall oxide layer  306  having a first thickness t 1  in flat portions of the trench wall, and a second thickness t 2  in the corners of the trench, wherein t 1 &gt;t 2 . A similar structure is illustrated in  FIG. 5 . The photographs of  FIGS. 4 and 5  were obtained using a transmission electron microscope and a scanning electron microscope, respectively. 
     An exemplary embodiment of the technique disclosed herein is schematically illustrated in the flowchart of  FIG. 6 . In such embodiments, a trench is etched through nitride and oxide layers and into an underling silicon substrate in an operational block  400 . A silicon nitride layer is then conformally deposited over the trench in an operational block  402 . An ISSG process is then performed to oxidize the conformally-deposited silicon nitride layer in an operational block  404 . In an exemplary embodiment, the ISSG process is tuned to oxidize the silicon nitride at an asymmetrical oxidization rate depending on the local topology, such that silicon nitride located at the bottom corners of the STI trench is oxidized slower than other portions of the silicon nitride. This results in thinner oxidation around the corners of the STI trench. In such embodiments, this asymmetrical oxidization causes a modified sidewall oxidization layer to be formed in the STI trench with rounded bottom corners. An insulating material is then deposited into the trench in an operational block  406 , and this is optionally followed by a CMP process in operational block  408  to provide an overlying planar surface. 
     Certain of the exemplary embodiments disclosed herein provide advantages over many conventional techniques for forming STI trenches, and allow even narrower trenches to be formed without sacrificing the ability to provide effective electrical isolation. In particular, certain of the exemplary embodiments disclosed herein allow a STI structure to be formed that has reduced defect density. Such embodiments include a rounded sidewall oxide layer that provides reduced structural stress in the insulation material filing the trench. 
     SCOPE OF THE INVENTION 
     While the foregoing detailed description discloses several embodiments of the present invention, it should be understood that this disclosure is illustrative only and is not limiting of the present invention. It should be appreciated that the specific configurations and operations disclosed can differ from those described above, and that the methods described herein can be used in contexts other than shallow trench isolation structures.