Abstract:
The present invention details a method which characterizes an STI fabrication process, and more particularly provides information relating to a variation in the STI sidewall profile between trenches in a middle portion of an array and a trench on an outer portion thereof. The method comprises forming two STI arrays with an STI fabrication process, forming a conductive layer over each array, biasing each conductive layer and determining a current associated therewith. The two current are then utilized to ascertain the variation of interest.

Description:
TECHNICAL FIELD OF INVENTION 
     The present invention relates to the characterization of shallow trench isolation structures. In particular, the invention provides a method which can distinguish between an outer edge current component and a normal current component associated with an array of shallow trench isolation structures. 
     BACKGROUND OF THE INVENTION 
     Shallow trench isolation (STI) has become a common isolation method for deep submicron CMOS technologies and for some power devices. The shallow trench isolation process begins with a relatively shallow trench, which is first etched in a silicon (Si) substrate. This trench is refilled with an insulator material and the surface is planarized to complete the isolation structure. During fabrication, the shape of both the top and bottom corners of the trench are important for device performance. Sharp corners with a small bending radius or with faceting can cause high electric fields, high mechanical stress, and non uniform oxide thickness, resulting in a degradation of device performance and gate oxide integrity problems. 
     In addition, the etching process can result in shallow trench isolation regions within an array having different geometric shapes, in particular, the sidewalls of the trenches can be different. For instance, an adjacent trench formation or etching process, can influence the manner in which a given trench forms. This is especially true for STI regions located at the ends, or edges, of an array of STI regions and will be explained in detail later. 
     Prior art focused on trench corner characterization and developing process controls which are intended to minimize extreme bends in the shape of both the top and bottom corners of a trench. Models of trench shape and formation using experimental and numerical methods are available; concentrating on detection of sharp corners, extreme bends in trench sidewalls, and defects in the refill process. In addition, prior art modeled the shape of the trenches and the thickness of the oxide as accurately as possible. However, the prior art does not characterize differences between the shape of shallow trench isolation regions located on the edge of an STI array versus STI regions located in the center of an array. Prior art considered this issue as negligible, however as the industry proceeds with downward scaling of electronic devices, these differences need to be taken into account and characterized. This becomes apparent when considering the surface area of a transistor gate located adjacent to an STI region, where the STI region is located on an edge of an STI, and comparing it to the surface area of a transistor gate where the STI region is located within the center of such an array. 
     SUMMARY OF THE INVENTION 
     The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention, nor delineate the scope of the invention. Its primary purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
     The present invention details an approach in which the properties of an STI array can be easily characterized, including differences between the edge regions and the isolation regions located in the center of an STI array. This approach takes advantage of an intrinsic current enhancement inherent from the STI regions located on the outer edges of an STI array. A numerical method is then employed, which can distinguish two current components and thus provides vital characteristics of the STI sidewall structure. This approach allows for easy identification of the different characteristics for an STI array. 
     To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an exemplary prior art flash memory device in which shallow trench isolators are utilized to electronically isolate source and drain regions, respectively, of memory cells along a word line thereof. 
     FIG. 2 is a cross sectional view of the prior art flash memory device of FIG. 1 taken along line  2 - 2 ′. 
     FIG. 3 is a perspective view of an exemplary flash memory device in which the effects of down scaling STI regions is illustrated. 
     FIG. 4 is a cross sectional view of a flash memory device of FIG. 3 taken along line  3 - 3 ′ in which the variations in STI regions is illustrated. 
     FIGS. 5A &amp; 5B are cross sectional views of prior art flash memory devices which compares variations in the shape of STI edge regions and the effects of down scaling the STI array. 
     FIG. 6 is a perspective view emphasizing the effects of STI sidewall variation on edge transistor surface areas employed in an array. 
     FIG. 7 is a cross sectional view emphasizing an STI region formed on the STI array edge versus an STI formed within center portion of an STI array. 
     FIGS. 8A &amp; 8B are top and sectional views, respectively, of an STI array. 
     FIG. 9 is a flow chart illustrating a method of characterizing an STI process in accordance with the present invention. 
     FIGS. 10A &amp; 10B are top and cross sectional views, respectively, for an STI with a first isolation area. 
     FIGS. 11A &amp; 11B are top and cross sectional views respectively, for an STI with a second differing isolation area. 
     FIG. 12 is a perspective view of two differing STI arrays having a bias applied thereto. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described with respect to the accompanying drawings, in which like numbered elements represent like parts. In order to facilitate an understanding of various advantageous features of the present invention, a brief discussion of the prior art will be provided. Subsequently, the various features of the present invention will be discussed in detail in conjunction with several exemplary figures. 
     As can be seen in FIG. 1, an exemplary, simplified flash memory device comprises several transistors or stacked gate cells, separated by trench isolation regions  1 . Each cell comprises a drain  2 , a source  3  and a stacked gate  4 . The drain  2  and source  3  of each cell typically comprise an N-type material embedded in a P-type substrate  6 . The stacked gate  4  may include several layers (or stacks) of oxides and conductive materials. Typically the memory cells are arranged, for example, as shown in FIG. 1, with an insulating trench, or STI region  1 , electrically separating groups (or banks) of cells  5  along a word line. 
     FIG. 2 provides an exemplary cross sectional illustration of the flash memory device of FIG. 1 taken along line  2 - 2 ′. From FIG. 2 it is clear that trench isolation regions  1  can be formed in a relatively consistent manner, when devices and regions are not scaled down and therefore the group of memory cells  5  performance is likewise consistent. In other words, the side wall shape of the outer trenches  8 , is generally similar to that of the inner trench  7 , because the shallow trench isolators have relatively large distances between them, thus minimizing their influence on an adjacent STI etching process. 
     As flash memory devices are scaled down, or include more devices, device isolation techniques have to be modified. Shallow trench isolation (STI) has become a common isolation method for deep submicron CMOS technologies. The shallow trench isolation process begins with a relatively shallow trench, which is first etched in a silicon (Si) substrate. This trench is refilled with an insulator material and the surface is planarized to complete the isolation structure. During fabrication, the shape of both the top and bottom corners of the trench become important for device performance. Sharp corners with a small bending radius, or with faceting, can cause high electric fields, high mechanical stress and non uniform oxide thickness, resulting in a degradation of device performance and gate oxide integrity problems. In addition, the etching processes can result in shallow trench isolators with different geometric shapes, in particular, the sidewalls of the trenches can be different. This occurs during the formation or etching process of the array of respective trenches. An adjacent trench formation, or etching, process can influence the manner in which a given trench forms. This is especially true for STI regions located at the ends or edges of an array of such regions and is illustrated, for example, in FIGS. 3 and 4. 
     FIG. 3 illustrates an exemplary flash memory device that has been scaled down or includes more devices. This scaled down flash memory device comprises several memory cells arranged in a manner similar to that of FIG. 1, separated by shallow trench isolation regions. Each memory cell comprises a drain  2 , a source  3  and a stacked gate  4 . The drain  2  and source  3  of each memory typically comprises an N-type material embedded in a substrate  6 . The gate  4  may include several layers (or stacks) of oxides and/or conductors such as polysilicon. The memory cells are arranged, as shown in FIG. 3, with an insulating trench, or shallow trench isolation region, electrically separating memory cells  11 ,  12  along a word line. As the STI regions  10  decrease in size, variations in the side portions thereof on those regions on the outside of the array have a larger influence on the operation of memory cells associated therewith. Note that such variations in STI trench profiles in FIG.  3  and other figures are not necessarily drawn to scale, but rather are illustrated as such for purposes of clarity. 
     FIG. 4 provides an exemplary cross sectional illustration of the flash memory device of FIG. 3 taken along  3 - 3 ′. As clearly seen in FIG. 4, the shallow trench isolation regions  10  located on the edges of the array, have a different sidewall shape  9  than the sidewalls associated with the shallow trench isolation regions located within the center of the STI array due to micro-loading in the formation thereof. This difference can cause the memory cells  11  associated with the ends of the STI array to behave differently than the memory cells  12  located in the center of the STI array. This is especially true if the surface areas  13  of source/drain regions of the edge memory cells  11  are not equal or similar to the source/drain regions of memory cells  11  in the center of the array (e.g., different surface areas  13  can lead to different memory cell injection currents causing non-uniformities in the programming and erasing of memory cells across the memory cell array). 
     FIG. 5A provides a cross sectional illustration of the prior art device of FIG. 1 while FIG. 5B provides a cross sectional illustration of the device of FIG.  3 . These figures are presented in order to emphasize the trench shape differences  8 ,  9  which can occur as memory cell density is increased. As can be seen, the source/drain areas  14  of the outside memory cell requires a certain degree of process control. However, as more memory cells are added to a device and STI regions become smaller, this surface area  13  becomes more difficult to control; therefore the process controls must become more stringent, including identification of key process parameters. 
     FIG. 6 presents an exemplary three dimensional illustration emphasizing the effects of device downscaling on edge transistor surface areas. A shallow trench isolation region, formed in the center portion of a STI array has a shape similar to  20 , in which the STI&#39;s top portion  23  corresponds to  26  representing an area of the STI region. The surface area of the STI&#39;s top portion  23 , effectively blocks injection current from entering a portion of a transistor gate of cell adjacent the region  20 . However, a STI region formed on an array&#39;s edge would include area  21 , for example, resulting in STI top portions  23  and  24  corresponding to a surface area  26  and  27 . Therefore an STI region on an array edge reduces a surface area of the source/drain regions  25  associated therewith. Since a current drive of a transistor or memory cell is a function of the width/length ratio (W/L) of the device, the variation in shape of STI regions between the center of an STI array and an edge thereof causes corresponding variations in transistor or memory cell behavior across the array, which in many cases is undesirable. 
     FIG. 7 provides a cross sectional view of an STI region formed on the STI array edge  31  versus an STI formed within center portion of an array  30  (with the differences being amplified or exaggerated for purposes of illustration). Trenches formed within the center of the array are shaped similar to  30 . Trenches formed on an array&#39;s edge undergo a different formation  31 , for example. This formation results in a trench shape similar to  31  and occurs generally on one side of the trench (the side which does not have a trench adjacent to it). After the trench is formed, the array is filled with an insulator material  32  and the STI is complete. 
     FIGS. 8A and 8B illustrate top and cross sectional views of an STI array, respectively. Because the outer edge STI regions  40  and  41  have a different intrinsic shape  43 , memory cells associated therewith will have different electrical characteristics from the center portions of the STI array  42 . The intrinsic shape  43  of the STI outer edges  41  causes some injection current variation for transistors or memory cells associated therewith versus devices associated with center STI shapes  42 . 
     FIG. 9 is a flow chart illustrating a method  100  of characterizing an STI process in accordance with the present invention. As discussed above, an STI process employed in generating an STI array will produce variations between STI regions in the center and on the outer edges of the array. By characterizing an STI process, various processes may be evaluated to determine which type of STI process provides optimal uniformity with respect to the position of an STI region within an STI array. While the exemplary method  100  is illustrated and described herein as a series of acts or events, it will be appreciated that the present invention is not limited by the illustrated ordering of such acts or events, as some acts or events may occur in different orders or concurrently with other acts or events apart from that shown or described herein, in accordance with the invention. In addition, not all illustrated acts or events may be required to implement a methodology in accordance with the present invention. Moreover, it will be appreciated that the method  100  may be implemented in association whit apparatus and systems illustrated and described herein and as well as in association with other systems not illustrated. 
     In FIG. 9, the method  100  begins by forming a first array of STI regions at  102 , wherein each of the STI regions within the array generally have a first length and the first array has a first area associated therewith. For example, as illustrated in FIGS. 10A and 10B, which represent a top plan view and cross section, respectively, the STI fingers  50  each have a length L 1  and array width W 1  such that an array area A 1  is calculated by L 1 ×W 1 . Continuing in FIG. 9, a second array of STI regions is formed at  104 , with the same STI fabrication that was used to form the first array, wherein the second array has a second length and a second area associated, wherein the first and second lengths are different and the first and second areas are different, respectively. For example, as illustrated in FIGS. 11A and 11B, a second array of STI regions  51  have a second length L 2  and a second area A 2  calculated by L 2 ×W 2 , wherein L 1 ≠L 2  and A 1 ≠A 2 . 
     The method  100  continues at  106 , wherein first and second conductive layers are formed over the first and second STI arrays, respectively. In one example, as illustrated FIG. 12, if the first and second STI arrays are integrated onto a single substrate, the first and second conductive layers may comprise a single conductive layer such as a metal or polysilicon film separated into two sections  61  and  64  with a space or insulating region  63  disposed therebetween. As illustrated in FIG. 12, the first conductive layer  64  overlies the first STI array of regions  50  while the second conductive layer  61  overlies the second STI array of regions  51 . 
     At  108 , the first conductive layer  64  is biased with a voltage  62  and a first current (I g1 ) is determined, for example, using a current meter  65 . At  110 , the second conductive layer  61  is biased with a voltage  60 , and a second current (I g2 ) is determined using, for example, a current meter  66 . The method  100  then ascertains an amount of STI region variation between STI regions on an outer edge of an array from STI regions not on the outer edges of arrays (for the STI fabrication process employed to form the first and second arrays) using the first and second currents at  112 . 
     In accordance with one aspect of the present invention, act  112  comprises using the measured currents I g1  and I g2 , to set up two equations with two unknowns. For the first array: 
     
       
           I   g1   =J   n   A   1   +J   e   L   1 , 
       
     
     wherein, J n  comprises the current density associated with one finger or region of the first STI array, and J e  represents a current per unit length associated with an outer edge of an STI region of the outer edge of the first array. Similarly, for the second array: 
     
       
           I   g2   =J   n   A   2   +J   e   L   2 . 
       
     
     Since A 1 , L 1 , A 2  and L 2  are known and I g1  and I g2  have been measured, we have two equations with two unknowns (J n  and J e ), solving the two equations, J n  and J e  are determined wherein J e  provides information relating to the character of the outer edge of an STI array made by the STI fabrication process being evaluated. By repeating such analysis for additional STI arrays fabricated with different STI fabrication processes, STI fabrication processes can be characterized with respect to the variation in STI regions from outer edges to inside portions of the array. 
     Although the invention has been shown and described with respect to a certain implementation or implementations, it will be appreciated, by those skilled in the art, that equivalent alterations and modifications will occur, to others skilled in the art, upon the reading and understanding of this specification and the annexed drawings. In particular, regard to the various functions performed by the above described components (assemblies, devices, circuits, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiments of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations or applications of the invention, such features may be combined with one or more other features of the other implementations, as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the term, “includes”, “has”, “having”, and/or variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the terms “comprises” and “comprising”.