Patent Publication Number: US-6218720-B1

Title: Semiconductor topography employing a nitrogenated shallow trench isolation structure

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to semiconductor processing and, more particularly, to a method for forming an integrated circuit containing an improved isolation structure. 
     2. Description of the Related Art 
     The fabrication of an integrated circuit involves the formation of numerous devices within active areas of a semiconductor substrate. Isolation structures are needed to electrically isolate one device from another. Isolation structures define the field regions of the semiconductor substrate, and the device areas define the active regions. The devices may be interconnected with conducting lines running over the isolation structures. 
     A popular isolation technology used in the fabrication of integrated circuits involves locally oxidizing silicon. In local oxidation of silicon (“LOCOS”) processes, an oxide layer is first grown upon a silicon substrate. A silicon nitride (“nitride”) layer is deposited upon the oxide layer. The oxide layer serves as a pad layer for a nitride layer. The surface of a field region of the silicon substrate is then exposed by etching portions of the nitride layer and oxide layer above this region. Active regions of the silicon substrate remain covered by the nitride layer, which is used as a mask to prevent oxidation of these regions in subsequent steps. An implant is performed in the field region to create a channel-stop doping layer. The exposed portion of the silicon substrate within the field region is then oxidized. The silicon dioxide (“oxide”) grown in the field region is termed field oxide. By growing a thick field oxide in isolation (or field) regions pre-implanted with a channel-stop dopant, LOCOS processing can help to prevent the establishment of parasitic channels in the field regions. 
     Although LOCOS has remained a popular isolation technology, the basic LOCOS process described above has several problems. When growing the field oxide, oxide growth should ideally be contained within the field region. In reality, however, some oxide growth may occur in a lateral direction, causing the field oxide to grow under and lift the edges of the nitride layer. Because the shape of the field oxide at the nitride edges is that of a slowly tapering wedge that merges into the pad oxide, the wedge is often described a bird&#39;s beak. In many instances, formation of the bird&#39;s beak can cause unacceptable encroachment of the field oxide into the active regions. In addition, the high temperatures associated with field oxide growth often cause the pre-implanted channel-stop dopant to migrate towards adjacent active regions. An increase in the dopant concentration near the edges of the field oxide can create a reduction in the drain current, an outcome that is often described as the narrow-width effect. Furthermore, thermal oxide growth is significantly less in small field regions (i.e., field areas of narrow lateral dimension) than in large field regions. Because of this reduction in oxide growth, an undesirable phenomenon known as the field-oxide-thinning effect may occur in small field regions. Field-oxide-thinning can produces problems with respect to field threshold voltages, interconnect-to-substrate capacitance, and field-edge leakage in small field regions between closely spaced active areas. 
     Despite advances made to decrease the bird&#39;s beak, channel-stop encroachment and non-planarity problems, it appears that LOCOS technology is still inadequate for deep submicron technologies. Many of the problems associated with LOCOS technology are alleviated by an isolation technique known as shallow trench isolation (“STI”). 
     An isolation structure formed by a conventional shallow trench isolation process (hereinafter “the conventional STI process”) is shown in FIG.  1 . Silicon substrate  100  is typically a lightly doped wafer of single crystal silicon. The conventional STI process includes an initial step in which a relatively shallow trench (e.g., between 0.3 and 0.5 microns in depth) is etched in silicon substrate  100 . The trench is then filled with trench dielectric  102 , which is usually a deposited oxide. Some trench processes also include an intermediate step of growing oxide on the trench floor and sidewalls before filling the trench with trench dielectric  102 . After the trench is filled, the upper surface of trench dielectric  102  is then made coplanar with the upper surface of silicon substrate  100  to complete the isolation structure. 
     The conventional STI process eliminates many of the problems of LOCOS techniques, including bird&#39;s beak and channel-stop dopant redistribution. STI processes are also better suited than LOCOS processes for isolating densely spaced active devices having field regions less than one μm wide. In addition, the trench isolation structure formed in STI processes is fully recessed, offering at least the potential for a planar surface. Moreover, field-oxide thinning in narrow isolation spaces is less likely to occur when using the shallow trench process. But despite its many advantages over LOCOS techniques, the conventional trench isolation process described above nevertheless has its own set of drawbacks. 
     One problem common to isolation structures, including those formed by the conventional STI process, is the unwanted diffusion of foreign species into the trench dielectric. For example, dopants implanted into active areas within semiconductor substrate  100  and adjacent to the trench can migrate into trench dielectric  102  during heat processing steps. Because of the fast diffusion rate of boron through silicon, boron diffusion into trench dielectrics is particularly widespread. Unfortunately, the voltage required to cause dielectric breakdown of a STI isolation structure generally decreases as the dopant density within the isolation structure increases. Consequently, when a voltage is applied across a conductor arranged horizontally above the trench isolation structure, dielectric breakdown may occur in those areas of the isolation structure having a high dopant density. 
     Another drawback of the conventional STI process results from the formation of sharp upper corners  106  near the surface of semiconductor substrate  100 . Sharp corners are those defined by a sidewall surface (or perimeter) of the trench near the top of the trench that are substantially perpendicular to the substrate upper surface. Sharp upper corners  106  are typically a result of the highly directional etch used to form the trench. 
     Sharp upper comers  106  may introduce certain undesirable effects during subsequent processing steps that can influence an integrated circuit&#39;s operation. One problem that results from sharp upper comers  106  is the production of structural stresses in the crystal structure of substrate  102  when subsequent layers are deposited over and into a previously defined trench. The structural stresses are caused by stress mismatches between the substrate bulk material (a single crystal lattice) near the edge of the active area and the overlying dielectric or conductive layers placed proximate to the active area edge or periphery. Any stress within the lattice may cause a number of dislocations in the silicon crystal near and around upper corners  106 . These dislocations usually migrate deeper into lower portions of the substrate during subsequent thermal processing steps (e.g., annealing). As these dislocations migrate away from sharp upper corners  106 , the dislocations may form convenient paths for leakage currents. Consequently, the dislocations may provide an electrical conduction bridge that allows currents flowing through one device to “leak” into a neighboring device. 
     In further processing, a dielectric layer  110  is typically deposited over the planarized surface. As shown, a conductive pattern  108  may be deposited and patterned over dielectric layer  110 . Conductive pattern  108  may be a metal line used as an electrical interconnection between devices, or alternatively, a polysilicon line used in transistor gates. Sharp upper corners  106  tend to congregate the electric fields in dielectric layer  110 , which causes bunching of electric fields in the corner area. Because of this bunching of the electric field, the corner has a lower threshold voltage (V T ) than the planar surfaces adjacent the trench corner. Consequently, transistor performance will suffer since the transistor will experience a threshold gradient from the center of the channel to the edge of the channel where the electric fields are bunched. Furthermore, the bunching of electric fields at underlying sharp corners  106  has been found to adversely impact the integrity of dielectric layer  110 . Reduction in the integrity of dielectric layer  110  may cause the layer to breakdown at lower voltages or suffer long-term reliability problems. 
     The conventional STI process also includes a step in which trench dielectric  104  is planarized (this step is done before the formation of dielectric layer  106 ). After the planarization step, the upper surface of the trench dielectric is somewhat coplanar with the upper surface of semiconductor substrate  100 . Unfortunately, subsequent processing steps may lead to the upper surface of trench dielectric  104  being displaced significantly below the surface of semiconductor substrate  100 . Recession of the upper surface of trench dielectric  104  below the surface of silicon substrate  100  can further bunch the electric fields near sharp upper comers  106 , making the reduction in isolation voltage that occurs at the comers even greater. 
     Other problems can result from recession of the upper surface of trench dielectric  104  below the surface of silicon substrate  100 . For example, chemical-mechanical polishing (“CMP”) is often used to planarize the trench dielectric. CMP is usually described as a “dirty” procedure because of the polishing-slurry particles and other residues that accumulate upon the surface of the semiconductor topography during the process. These contaminants must be cleaned from the semiconductor topography after the CMP process is complete. The RCA cleaning method commonly used to clean such contaminants also removes the upper surface of the trench dielectric to a slightly greater degree than the adjacent silicon within semiconductor substrate  100 . In addition, steps that involve the etching of oxide layers may result in the recession of the upper surface of trench dielectric  104  below the upper surface of semiconductor substrate  100 . Examples of such steps include stripping the sacrificial oxide layer commonly grown upon the silicon substrate before deposition of the gate oxide and etching an oxide layer deposited over a gate electrode to form sidewall spacers. 
     In addition to the issues discussed above, recession of the upper surface of trench dielectric  104  below the surface of semiconductor substrate  100  may also cause problems during silicide formation. After dopants have been implanted into source/drain junctions of the active regions between the gate conductor and adjacent trench dielectric structures, highly conductive ohmic contacts must be formed between the source/drain junctions and overlying interconnect. A self-aligned silicide (“salicide”) is typically formed at the juncture between the ohmic contacts and the junctions. Salicide formation involves deposition of a refractory metal across the semiconductor topography followed by heating the refractory metal such that the metal reacts with the underlying silicon. Thus, a highly conductive silicide is formed upon the junctions. 
     If a trench dielectric is recessed below adjacent sourcc/drain junctions such that sidewalls of the silicon substrate are exposed, silicide formation may also occur upon those exposed sidewalls. Unfortunately, the relatively low resistivity silicide can form a conductive pathway between the source/drain junctions and the oppositely-doped underlying silicon substrate. The presence of the conductive pathway may cause majority charge carriers to be drawn away from rather than toward the channel region of the transistor during operation. This shorting of the source-to-drain current flow can lead to inoperability of the transistor. 
     Therefore, it would be desirable to design a method for forming an isolation structure that substantially prevents the migration of dopants from adjacent active regions. It would also be beneficial to contrive a method that reduces the negative effects of sharp corners where the trench sidewall meets the silicon substrate upper surface. Furthermore, it would be advantageous to create an isolation structure that is more resistant to becoming recessed below the silicon surface. The desired isolation structure would thus have increased reliability benefits over conventional isolation structures. 
     SUMMARY OF THE INVENTION 
     The problems described above are in large part addressed by the improved isolation structure and fabrication method presented herein. Broadly speaking, a method is provided for forming an integrated circuit having a trench patterned in a field region of a semiconductor substrate. The trench is defined within the semiconductor substrate by a trench floor and trench sidewalls. A liner that primarily comprises nitride is formed upon the trench floor and sidewalls. The liner is then oxidized. 
     According to one embodiment, patterning the trench includes the steps of forming a pad layer upon the semiconductor substrate and then forming a masking layer upon the pad layer. A window may then be etched in the masking layer and the pad layer that substantially defines the lateral dimensions of the trench. A dielectric fill material is preferably deposited in the trench, and may overflow the trench such that the upper surface of the dielectric fill material is above the upper surface of the masking layer. The dielectric fill material may then be planarized to form a trench dielectric having an upper surface that is substantially coplanar with the upper surface of the masking layer. 
     After planarization of the dielectric fill material, the masking layer and the pad layer are removed. In addition to forming upon the trench floor and sidewalls, the liner also preferably forms on the exposed sidewall portions of the pad layer. Consequently, an upper surface of the liner may extend above the surface of the semiconductor substrate. Because the upper surface of the trench dielectric is planarized to be substantially coplanar with an upper surface of the masking layer, the upper surface of the trench dielectric may also extend above the surface of the semiconductor substrate. After removal of the masking and pad layers, the liner and trench dielectric may be planarized such that their upper surfaces are substantially coplanar. Preferably, however, the upper surface of the trench dielectric and the upper surface of the liner are not planarized, but are instead maintained above the surface of the semiconductor substrate while active devices are formed in adjacent active regions. As such, the likelihood of the isolation structure (i.e., the combination of the liner and the trench dielectric) becoming entirely recessed below the surface is greatly reduced. Moreover, it is believed that maintaining portions of the isolation structure above the surface of the semiconductor substrate results in a significant reduction of the edge effects discussed above (e.g., electric field bunching) compared to an isolation structure that is recessed below the semiconductor surface. 
     The liner is preferably formed upon the trench floor and sidewalls by annealing the semiconductor substrate in a nitrogen-containing ambient. The nitrogen-containing ambient may be composed of about 60-80% NH 3 . The balance of the nitrogen-containing ambient is preferably N 2 , which may be replaced by an inert gas if a slower reaction is desired. The nitrogen-containing ambient is preferably substantially free of oxygen. 
     When NH 3  is used to nitridate silicon, hydrogen-containing species (e.g., NH x , —H, and —OH) are usually formed. The presence of these species can result in the formation of electron traps between the liner and the subsequently deposited trench dielectric. Oxidation of the liner reduces the concentration of hydrogen-containing species within the liner, and thus the potential for electron traps. Oxidation of the liner is preferably carried out by annealing the semiconductor substrate in an oxygen-containing ambient. 
     Furthermore, it is desirable that the nitrogen concentration at the liner-trench dielectric interface be reduced below the level present after the initial nitridation. Oxidation of the liner preferably creates a nitrogen gradient within the liner that is oriented such that the percentage of nitrogen atoms within the liner decreases in a direction away from the trench floor and sidewalls. In one embodiment, the percentage of the nitrogen atoms in the portions of the liner closest to the trench floor and sidewalls is about 5-7%, and the percentage of nitrogen atoms in those portions of the liner most distant from the trench floor and sidewalls is about 3-5%. Thus, oxidation of the liner allows the percentage of nitrogen at the liner-trench dielectric interface to be reduced to a suitable level while keeping the percentage of nitrogen at the liner-trench interface high enough to substantially prevent dopant migration. 
     In another embodiment, the pad layer is composed of oxide, and the masking layer is composed of nitride. Annealing the semiconductor substrate in a nitrogen-containing ambient preferably not only forms the liner on the trench floor and sidewalls, but also forms the liner on the exposed sidewall portions of the pad layer (as stated above). Because the masking layer is composed of nitride, the liner preferably does not form on its exposed sidewall portions. Thus, the liner covers the sharp upper comers formed at the interface between the trench sidewalls and the upper surface of the semiconductor substrate. Forming the liner in such a manner may reduce the stress mismatches between the semiconductor substrate and the overlying pad layer, thus decreasing the likelihood of dislocations being formed. The formation of the liner may also reduce the sharpness of the comers, which could lessen the bunching of electric fields that would occur if the upper comers were very sharp. 
     A semiconductor topography is also presented. The semiconductor topography contains a trench disposed within a field region of a semiconductor substrate. The trench is preferably defined in an upper portion of the semiconductor substrate by a trench floor and trench sidewalls. A liner may be arranged upon the trench floor and trench sidewalls. The liner comprises nitrogen and oxygen atoms. A nitrogen gradient exists within the liner, the nitrogen gradient being oriented such that a percentage of nitrogen within the liner increases in a direction away from the trench floor and sidewalls. A trench dielectric may be at least partially disposed within the trench. Active devices, such as MOS transistors, may be arranged in adjacent active regions. The nitrogen gradient is preferably oriented such that the liner comprises about 5-7 percent nitrogen atoms in those portions of the liner nearest to the trench floor and sidewalls and about 3-5 percent nitrogen atoms in those portions of the liner most distant from the trench floor and sidewalls. The final isolation structure (i.e., the combination of the liner and the trench dielectric) exhibits increased reliability over conventional isolation structures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view of a conventional STI structure disposed within a silicon substrate; 
     FIG. 2 is a partial cross-sectional view of a semiconductor topography in accordance with the present invention; 
     FIG. 3 is a partial cross-sectional view of the semiconductor topography, wherein a pad layer is formed upon the semiconductor substrate according to an initial processing step subsequent to FIG. 2; 
     FIG. 4 is a partial cross-sectional view of the semiconductor topography, wherein a masking layer is formed upon the pad layer according to a processing step subsequent to FIG. 3; 
     FIG. 5 is a partial cross-sectional view of the semiconductor topography, wherein a window is patterned in the masking layer and the pad layer according to a processing step subsequent to FIG. 4; 
     FIG. 6 is a partial cross-sectional view of the semiconductor topography, wherein a trench is patterned in the semiconductor substrate according to a processing step subsequent to FIG. 5; 
     FIG. 7 is a partial cross-sectional view of the semiconductor topography, wherein a liner primarily composed of nitride is formed upon the floor and sidewalls of the trench according to a processing step subsequent to FIG. 6; 
     FIG. 8 is a partial cross-sectional view of the semiconductor topography, wherein a the liner is oxidized according to a processing step subsequent to FIG. 7; 
     FIG. 8 a  is a detailed view of a section of the liner showing relative atomic concentrations of nitrogen within portions of the liner; 
     FIG. 9 is a partial cross-sectional view of the semiconductor topography, wherein a dielectric fill material is deposited within the trench according to a processing step subsequent to FIG. 8; 
     FIG. 10 is a partial cross-sectional view of the semiconductor topography, wherein the dielectric fill material is planarized to form a trench dielectric according to a processing step subsequent to FIG. 9; 
     FIG. 11 is a partial cross-sectional view of the semiconductor topography, wherein the masking layer and pad layer are removed according to a processing step subsequent to FIG. 10; and 
     FIG. 12 is a partial cross-sectional view of the semiconductor topography, wherein MOS transistors are formed in the active regions adjacent to the trench according to a processing step subsequent to FIG.  11 . 
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, FIGS. 2-12 illustrate a process sequence for forming an isolation structure within a field region of a semiconductor substrate. FIG. 2 shows a partial cross-sectional view of semiconductor substrate  100 . Semiconductor substrate  100  is preferably a wafer of single crystal silicon. 
     FIG. 3 depicts the formation of pad layer  202  upon semiconductor substrate  200 . Pad layer  202  preferably is preferably composed of oxide, and is either deposited or grown in a heated, oxygen-containing ambient. The thickness of pad layer  202  is preferably about 50-200 angstroms. 
     FIG. 4 shows the formation of masking layer  204  upon pad layer  202 . Masking layer  204  is preferably composed of a material that is substantially resistant to oxidation and substantially impervious to the diffusion of oxidants into any underlying materials. Suitable materials for masking layer  204  include nitride. Nitride may be deposited to form masking layer  204  by a variety of CVD methods, including low-pressure, plasma-enhanced, and remote plasma-enhanced techniques. Masking layer  204  is preferably about 500-1800 angstroms thick. If masking layer  204  is made of nitride, a pad layer  202  made of oxide can reduce the stresses that inherently exist between deposited nitride (in this case, masking layer  204 ) and single crystal silicon (semiconductor substrate  200 ). 
     FIG. 5 depicts a subsequent processing step in which masking layer  204  and pad layer  202  are patterned to form window  205 . Formation of window  205  preferably occurs by use of a dry etch process with a high degree of anisotropy. As such, the sidewalls of masking layer  204  and pad layer  202  exposed by the formation of window  205  are preferably vertical. Photoresist (not shown) may be spun on, exposed, and developed as is well known in the art to define the pattern used to form window  205 . 
     FIG. 6 depicts a subsequent processing step in which trench  208  is patterned within a field region of semiconductor substrate  200 . The trench is preferably defined within an upper portion of semiconductor substrate  200  by trench floor  206  and trench sidewalls  207 . Trench  208  is preferably formed by removal of portions of semiconductor substrate  200  in a highly anisotropic dry etch process. The lateral dimensions (i.e., length and width) of trench  208  may be substantially defined by the corresponding lateral dimensions of window  205 . The width of trench  208  may be as narrow as the minimum resolvable feature size of the process used to form the trench. In a preferred embodiment, trench  208  is about 0.20 microns wide. Trench  208  is preferably about 0.15-0.30 microns deep. 
     Although trench sidewalls  207  are shown as being substantially perpendicular to trench floor  206  (thus forming sharp corners at the intersections), it should be understood that the particular relationship shown is not fixed. On the contrary, the bottom corners of the trench may be substantially rounded, the angle between trench sidewalls  207  and trench floor  206  may deviate a large degree from vertical, or any number of other modifications may be made that would be apparent to one skilled in the art having the benefit of this disclosure. 
     FIG. 7 illustrates an annealing process in which semiconductor substrate  200  is exposed to thermal radiation  209  in a nitrogen-containing ambient. The nitrogen within the ambient may be in the form of NH 3  and/or N 2 . In a preferred embodiment, the nitrogen-containing ambient comprises from about 60-80% NH 3 , with the balance being N 2 . The N 2  may be replaced with an inert gas such as helium or argon at later stages of the nitridation step to slow the nitridation reaction. Preferably, annealing takes place in a rapid thermal processing chamber (“RTP”) for about 30-120 seconds at a temperature of about 900-1100° C. Annealing of semiconductor substrate  200  preferably nitridates trench floor  206  and trench sidewalls  207  such that liner  210  is formed. Preferably, liner  210  is primarily composed of nitride. Liner  210  may also form on the exposed sidewalls of pad layer  202  (as shown in FIG.  7 ). Because pad layer  202  is about 50-200 angstroms thick, the upper surface of liner  210  preferably extends about 50-200 angstroms above the surface of semiconductor substrate  200 . Liner  210  is preferably about 50-100 angstroms thick at this point in the process. 
     FIG. 8 depicts a subsequent annealing step in which semiconductor substrate  200  is exposed to thermal radiation  211  in an oxygen-containing ambient. Annealing preferably takes place in an RTP chamber at temperatures of about 1000-1100° C. for 1 to 3 minutes. Alternately, annealing may be carried out in a tube furnace at temperatures of about 1000-1100° C. for 15-30 minutes. Annealing preferably oxidizes liner  210  such that a nitrogen gradient is created. FIG. 8 a  shows detailed section  213  of liner  210 . As can be seen in this figure, the nitrogen gradient is preferably oriented such that the percentage of nitrogen atoms  212  within liner  210  is higher in the portions of liner  210  closest to trench floor  206  and trench sidewalls  207  than in the portions of liner  210  most distant from trench floor  206  and trench sidewalls  207 . For example, the portions of liner  210  closest to trench floor  206  and trench sidewalls  207  may have about 5-7 percent nitrogen atoms, while the portions of liner  210  most distant from trench floor  206  and trench sidewalls  207  may have about 3-5 percent nitrogen atoms. The oxidation of liner  210  also preferably increases the thickness of liner  210  to about 100-300 angstroms. The oxygen-containing ambient may also comprise an inert gas to moderate the rate of oxidation during the annealing process. 
     FIG. 9 depicts a subsequent processing step in which dielectric fill material  214  is deposited in trench  210  such that an upper surface of the dielectric fill material  214  is above an upper surface of the masking layer  204 . Dielectric fill material  214  may be composed of oxide or silicon oxynitride (“oxynitride”) deposited in a CVD process. 
     Dielectric fill material  214  is then planarized to form trench dielectric  216 . After planarization, the upper surface of trench dielectric  216  is preferably substantially coplanar with the upper surface of masking layer  204 . Planarization of dielectric fill material  214  may be carried out by use of CMP or a selective etchback technique. If planarization of dielectric fill material  214  is carried out by CMP methods, masking layer  204  preferably serves as a polish stop. Because masking layer  205  is preferably about 500-1800 angstroms thick and the underlying pad layer is about 50-200 angstroms thick, the upper surface of trench dielectric  216  preferably extends about 550-2000 angstroms above the surface of semiconductor substrate  200 . 
     FIG. 11 depicts a subsequent processing step in which masking layer  204  and pad layer  202  are removed. The removal of masking layer  204  may be accomplished by use of dry or wet etch processes. If masking layer  204  is composed of nitride, then a heated solution of phosphoric acid may be used to remove the layer. Pad layer  202  may also be removed by use of dry or wet etch processes. Regardless of the method used, removal of masking layer  204  and pad layer  202  is preferably undertaken such that minimal amounts of trench dielectric  216 , liner  210 , and semiconductor substrate  200  are removed. 
     FIG. 12 depicts the formation of active devices, in this case MOS transistors  218 , in the active areas adjacent to the isolation stricture defined by the liner and the trench dielectric while maintaining the upper surfaces of liner  210  and trench dielectric  216  above the upper surface of semiconductor substrate  200 . Liner  210  and trench dielectric  216  may be planarized so that their upper surfaces are coplanar with the upper surface of semiconductor substrate  200 . However, it is preferred that liner  210  and trench dielectric  216  not be planarized, but instead extend significantly above the surface of semiconductor substrate  200 . As such, it is highly unlikely that these structures will be removed by subsequent processes to a point where their upper surfaces are recessed below semiconductor substrate  200  (unless, of course, removal of these structures is the desired goal of such processes). 
     It will be appreciated to those skilled in the art having the benefit of this disclosure that both the improved isolation structure and the method for making an improved isolation structure described herein are capable of being used with numerous applications involving active area isolation within integrated circuits. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. For example, the present process could be incorporated into a modified LOCOS process where a field oxide is first grown that partially fills a trench formed in a semiconductor substrate. This step could be followed by nitridation of the trench to form a liner and a subsequent oxidation of the liner, as described herein. It is intended that the following claims be interpreted to embrace all such modifications and changes and, accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.