Abstract:
A trench isolation structure for a semiconductor is provided including an isolation ring and an isolation path. The isolation ring surrounds active semiconductor areas and is bordered on the outside by inactive semiconductor area. The isolation path extends from the isolation ring through the inactive semiconductor area. A first level conductor on the isolation path electrically connects or capacitively couples a device in the active semiconductor area to a location on the substrate outside the isolation ring. The isolation path has a configuration derived from the layout of the conductor.

Description:
This application is a Divisional of U.S. application Ser. No. 08/837,141, filed Apr. 14, 1997, now U.S. Pat. No. 6,063,687, which is a Divisional of U.S. application Ser. No. 08/581,680, filed Dec. 22, 1995, now U.S. Pat. No. 5,734,192. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to isolation for semiconductor structures such as integrated circuits. More particularly it relates to a layout for isolation that reduces the cost and process complexity of trench isolation. Even more particularly, it relates to a layout for isolation that provides narrow regions of isolation around active areas and under first level conductors. 
     BACKGROUND 
     Isolation is provided in a semiconductor between transistors on a semiconductor chip to prevent unwanted electrical connections therebetween. 
     Local oxidation of silicon (LOCOS) has long been the conventional process for fabricating isolation, and it has the advantage of being both relatively inexpensive and capable of providing isolation over wide areas of a chip. LOCOS has several disadvantages, however, among them the formation of a bird&#39;s beak that increases the horizontal space of the isolation, reducing the density of devices on a chip. 
     Because of its more vertical sidewalls and more planar surface trench isolation provides significant advantage over LOCOS in the quest for providing a high density of integrated circuits. Trench isolation schemes are therefore finding increasing use in semiconductor processing. However, because of the difficulty of planarizing, trench isolation is more complex and expensive to process. 
     Trench isolation process complexity is evident from a brief description of the conventional shallow trench isolation (STI) process. First, thick silicon nitride is deposited on thin silicon dioxide on the semiconductor surface. Then, in a first photolithography step, a mask is formed in the layers of insulator defining the areas where STI is desired. Next, the semiconductor is etched, using a reactive ion etch (RIE) process, to form shallow trenches. A thin layer of silicon dioxide is then thermally grown and a conformal silicon dioxide layer is deposited, filling the shallow trenches. 
     A lengthy series of steps, such as the sequence described below, is then used solely to planarize the surface. The typical planarization process is quite complex because (1) the oxide deposition is usually conformal, and therefore narrow areas of isolation may be at a higher level than wide areas of isolation; and (2) wide areas of isolation polish much more quickly than narrow areas, a problem known as dishing. 
     Dishing is a problem, first because oxide in these large areas of STI is thinner than in narrow areas, and second because in regions having a low density of devices, the nitride polish stop layer over small, isolated active area regions may be completely polished off, and then the active area may get polished down, introducing potential damage to the semiconductor crystal. 
     To overcome these difficulties the planarization steps may involve applying a planarizing material, such as spin-on-glass or photoresist. For example, one conventional process uses two photoresist applications and a masking step for planarizing. In addition, tight control over plasma etching and chemical-mechanical polishing steps are needed, all adding significantly to the process complexity of trench isolation compared to conventional LOCOS. The photoresist application and masking step planarizes by providing photoresist exclusively in the isolation areas (the low areas), and removing photoresist from the active areas (the high areas). The resist thickness is designed to equal the shallow trench depth, so the surface of the remaining resist is roughly coplanar with the surface of the exposed oxide film over active areas. The next photoresist layer is spun-on (but no expose or develop step is used) to further smooth out local variations in the developed resist pattern, resulting in a photoresist and oxide surface that is both locally and globally planar. An RIE process then etches back both the photoresist and the oxide at about the same rate to a point below irregularities in the oxide, rendering the wafer surface largely planar. A final touch-up chemical-mechanical polish (CMP) is then used to remove any local irregularities and complete the oxide removal over active areas. 
     Even with this process complexity, the results of the conventional STI process are frequently unsatisfactory. Substantial dishing is still found in areas having a large expanse of STI or a low density of devices. Furthermore, the process complexity introduced to overcome dishing increases cost and cycle time and reduces yield. Thus, an alternate scheme for providing trench isolation, without the need for a complex series of planarization steps, is needed and is provided by the following invention. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to reduce the cost and complexity of STI and to increase the yield of product having STI. 
     It is a further object of the present invention to avoid dishing of areas with a low density of devices without adding process complexity. 
     It is another object of the present invention to eliminate extra photoresist application and exposure steps during STI processing. 
     It is another object of the present invention to provide isolation both horizontally between active semiconductor areas and vertically between a first level of conductor and the silicon surface. 
     It is a feature of the present invention that large expanses of STI are replaced with inactive semiconductor areas, there being narrow rings of STI immediately surrounding active semiconductor areas and STI paths underlying first level conductors, the paths little wider than the conductors. 
     It is a feature of the present invention that the design of an isolation mask includes the layouts of active semiconductor areas and of first level conductors, including gate conductors and local interconnects, where the first level conductors extend beyond active semiconductor areas. 
     It is an advantage of this invention that dishing of active semiconductor areas and STI is eliminated by design rather than process. 
     It is an advantage of this invention that the cost, complexity, defects, and cycle time of STI processing are significantly reduced, while yield is improved. 
     It is an advantage of this invention that process window during chemical-mechanical polishing of STI is increased. 
     These and other objects of the invention are accomplished by the isolation structure of the present invention. The structure includes a semiconductor substrate having an isolation ring having an inside edge and an outside edge. An active semiconductor area borders the inside edge, and the active semiconductor area includes a device. An inactive semiconductor area borders the outside edge. A trench isolation path extends from the outside edge to a location on the substrate outside the isolation ring. The inactive semiconductor area borders at least two sides of the isolation path. A first level conductor is on the isolation path. The first level conductor electrically connects or capacitively couples the device to the location. The isolation path electrically insulates the conductor from the inactive semiconductor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features, and advantages of the invention will be apparent from the following detailed description of the invention, as illustrated in the accompanying drawings, in which: 
     FIG. 1 is a planar diagram illustrating the layout of active semiconductor areas, isolation, first level conductors, n-well, and inactive semiconductor areas of the present invention; 
     FIGS. 1 b - 1   c  are cross sectional views along b-b′ and a-a′ of FIG. 1; 
     FIG. 2 is a planar diagram illustrating another embodiment of the layout of active semiconductor areas of the present invention; 
     FIGS. 3 a - 3   b  are planar diagrams illustrating the layout of derived isolation structures having corners that are within a minimum photolithographic dimension of each other in violation of ground rules, and a tab isolation that avoids this problem; and 
     FIG. 4 is a planar diagram illustrating the layout of a resistor located on path isolation surrounded by an inactive semiconductor area according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a ring of isolation around each active semiconductor area. The invention also provides paths of isolation through inactive semiconductor areas under first level conductors. The layout of the paths of isolation is derived from the layout of the first level conductors by shifting the edge of the conductor outward at each point a fixed dimension d. The dimension added should be sufficient to at least account for overlay tolerance. It can also be as much as 10 to 50 micrometers in most locations without leading to substantial dishing. These paths of isolation are usually quite narrow compared to the wide expanses of isolation found under first level conductors on many conventional chips. Thus, the present invention addresses the dishing problem by providing a design in which wide areas of isolation are eliminated, replaced by wide areas of inactive semiconductor through which extend paths of isolation. Because the wide areas of isolation are eliminated in the design or layout of the present invention, the need for complex and expensive process steps to planarize, while avoiding dishing, is eliminated as well. 
     The term “first level conductors,” as used in this application includes both gate conductors and local interconnects. Gate conductors are usually formed first, in a first deposition and masking step. After such steps as implants for source/drain diffusions and high temperature anneal, local interconnects are deposited and defined in a second masking step. Local interconnects may make contact with the source/drain regions or gates. 
     The term “devices” as used in this application includes structures such as transistors, diodes, resistors, capacitors, and isolated contacts. 
     The term “horizontal” as used in this application is defined as a plane parallel to the conventional planar surface of a semiconductor chip or wafer, regardless of the orientation the chip is actually held. The term vertical refers to a direction perpendicular to the horizontal as defined above. Prepositions, such as “on,” “side,” and “under” are defined with respect to the conventional planar surface being on the top surface of the chip or wafer, regardless of the orientation the chip is actually held. 
     Semiconducting substrates are formed from materials such as silicon, germanium, and gallium arsenide. Silicon will be used for illustration below. 
     Referring now to FIGS. 1,  1   b  and  1   c , active silicon areas  20  and  22  are surrounded by isolation rings  24  and  26 , the isolation rings having a width w, typically in the range of about 0.4 to about 10 micrometer. Active silicon areas  20  and  22  contain n- and p-channel FETs  28  and  30  respectively. Active silicon area  20  is located on p- substrate  31  (FIGS. 1 b - 1   c ) while active silicon area  22  is located within n-well  32 . N-well  32  is surrounded by isolation ring  34  extending horizontally on both sides of n-well junction edge  36 ′, where junction  36  would otherwise intersect silicon surface  38 . 
     For highest circuit density, isolation ring width w, is equal to the photolithographic minimum dimension of the fabrication technology. However, where an isolation ring is a border between active area and an inactive area, there is no circuit density advantage in using a minimum dimension isolation ring. In this case, the dimension selected for the width of the isolation ring can then be a compromise between defect sensitivity and planarization efficiency. For example, for a technology having a minimum photolithographic dimension of 0.4 micrometer, the isolation ring width may therefore be selected to be less than about five times the minimum photolighographic dimension, or about 0.6 to about 0.8 micrometer. 
     Gate conductor  40  of N-FET  28  extends across active silicon area  20 , terminating in contact region  42 . Gate conductor  40  is located on a gate dielectric (not shown) within active area  20 . Gate conductor  40  is located on isolation ring  24  and isolation path  50  where gate conductor  40  extends outside active silicon area  20 . Similarly, gate conductor  44  is located on gate dielectric  46  (FIG. 1 c ) where gate conductor  44  extends across active silicon area  22  in n-well  32 . Gate conductor  44  is located on isolation ring  26  and isolation path  48  where it extends outside active silicon area  22 . Gate conductors  40  and  44  are preferably formed of a semiconductor material such as doped polysilicon, but they may also be formed of a metal, such as tungsten. 
     Local interconnect  60  electrically connects source/drain region  62  of n-channel FET  28  with gate conductor  44  of p-channel FET  30 . Local interconnect  60  is located on isolation path  64  (which intersects isolation rings  24 ,  26 , and  34  and isolation path  48  where local interconnect  60  extends outside active area  20 . Where local interconnect  60  intersects and makes contact with gate conductor  44 , local interconnect  60  is on top of gate conductor  44  which itself is on top of isolation path  48 . Local interconnect  60  is formed of a conductive material, such as doped polysilicon, a metal, such as tungsten, or a metal silicide, such as WSi or TiSi. 
     The layout of isolation paths  48 ,  50 , and  64  are derived from the layout of conductors  40 ,  44 , and  60  where the conductors extend outside said the respective isolation rings. Thus, isolation paths  48  and  50  follow the pattern of gate conductors  40  and  44 , and isolation path  64  follows the pattern of local interconnect  60 . Isolation paths  48  and  50  are wider than gate conductors  40  and  44 , and isolation path  64  is wider than local interconnect  60 , the isolation paths nominally extending horizontally a distance d from the edges of gate conductors  40  and  44  and local interconnect  60 . 
     Gate conductors and local interconnects are formed, however, in separate masking steps from isolation paths, so conductors and interconnects may therefore not be placed exactly centered on the isolation paths. Thus, for example, the horizontal spacing of isolation path  64  on one side of local interconnect  60  may differ from the horizontal spacing on the other side, depending on the overlay of masks. Furthermore, distance d need not be the same for gate conductors and local interconnects. 
     Inactive silicon area  66  borders isolation rings  24  and  34  and isolation paths  50  and  64 . Inactive silicon area  66 ′ within n-well  32  borders isolation rings  26  and  34  and isolation paths  48  and  64 . Both active and inactive silicon areas are defined by the placement of isolation rings and isolation paths. Active semiconductor areas are distinguished by the presence of devices within their borders that are connected to provide a circuit function for the chip. Inactive silicon areas  66  and  66 ′ do not have such connected structures within their borders. 
     The presence of inactive silicon areas  66  and  66 ′ is valuable, since the inactive silicon areas limit the area of isolation on the chip to narrow rings and paths, and these narrow remaining regions of isolation are easily planarized without dishing. The invention takes advantage of the fact that active and inactive regions of the silicon surface are inherently planar. Thus, many of the process steps for fabricating shallow trench isolation otherwise needed to address dishing concerns are eliminated. 
     Advantageously, the present invention permits the designer to specify a maximum STI width while retaining adequate isolation between active silicon areas. Ideally, the maximum width is selected to be one capable of planarization either exclusively with an etch or exclusively with CMP. 
     In essence, the present invention provides for either polishing or etching immediately after oxide deposition without the cumbersome sequence of masking and photoresist deposition. Furthermore, the process window of the single planarization process step can also be vastly increased. With the isolation layout of the present invention, costs, defects, scrap, measurements, and cycle time associated with STI processing are all substantially reduced. Thus, the goal of providing improved design to greatly reduce process complexity is achieved. 
     The isolation layout of the present invention is derived from the layout of active silicon areas, gate conductors, and local interconnects. More specifically, the layout, of inside edge  24   a  of isolation ring  24  is identical with the layout of outside edge  20   a  active silicon area  20 . The layout of outside edge  24   b  of isolation ring  24  is obtained from the layout of active silicon area  20  by shifting edge  20   a  of active silicon area  20  outward at each point by a fixed dimension w, as illustrated in FIG. 1 a . A similar process is used to derive the layout of isolation ring  26  from the layout of active silicon area  22 . The active silicon areas are within these isolation rings. 
     Similarly, the layout for isolation paths  48  and  50  under gate conductors  40  and  44  or isolation path  64  under local interconnect  60  is obtained from the layout of gate conductor  40  and  44  or local interconnect  60  by shifting outward edge  40   a ,  44   a , or  60   a  respectively of the gate conductor or local interconnect by fixed dimensions d. Path isolation within active silicon area  20  or  22  is then eliminated from the layout, as illustrated in FIG. 1 a . Thus, gate conductors and local interconnects are designed to be located approximately in the center of isolation paths (but deviation of the conductors from the center of the isolation paths is expected because of mask overlay tolerance), and isolation paths extend only outside active silicon areas. 
     The magnitude of fixed dimension d used to derive path isolation dimensions is at least equal the overlay tolerance of the lithographic system used in the fabrication technology. The overlay tolerance is usually a fraction of the minimum resolution of the photolithographic system. Thus, for photolithographic systems having a minimum dimension of about 0.4 um, the overlay tolerance is about 0.15 micrometer. Of course, this overlay tolerance will decrease as lithographic systems improve, and so the minimum expansion fixed dimension d is likely to decrease with each generation of semiconductor technology. However, significantly larger expansion of the isolation than the photolithographic minimum may be desirable to reduce gate conductor-to-semiconductor capacitance, trading density for performance. 
     Similarly, the layout of isolation ring  34  for n-well  32  (FIG. 1 c ) is obtained from the layout of n-well  32 . For n-well isolation it is desirable that n-well junction edge  36 ′ should not reach the original silicon surface; junction edge  36 ′ should be located under isolation ring  34  (FIG. 1 c ). To provide this feature, both inside edge  34   a  and outside edge  34   b  of the isolation ring  34  are derived from the location of n-well junction edge  36 ′. The layout of inside edge  34   a  of isolation ring  34  is obtained from the layout of n-well junction edge  36 ′ by shifting n-well junction edge  36 ′ inward at each point by fixed dimension w 1 . The layout of outside edge  34   b  of isolation ring  34  is obtained from the layout of n-well junction edge  36 ′ by shifting n-well junction edge  36 ′ outward at each point by fixed dimension w 2 . Preferably, w 2  is set to be greater than w 1  since the junction diffuses outward. Thus, n-well junction edge  36 ′ is located approximately beneath the center of isolation ring  34 , and isolation ring  34  has a width of w 1 +w 2 . w 1 +w 2  is at least equal to the minimum photolithographic dimension for the fabrication technology used. 
     The layout of the present invention also provides that active silicon areas or n-wells that are close together may have only a single ring of isolation therebetween. For example, in FIG. 1, in the region between active silicon area  20  and n-well  32 , inactive silicon area  166  and side  24   a  of isolation ring  24  can both be eliminated. Active silicon area  20  and n-well  32  can be moved closer together since isolation ring  34  provides sufficient isolation therebetween. 
     Alternatively, isolation shapes that have their outer edges closer than the photolithographic minimum dimension apart are merged to form a wider ring or path of isolation. For example, for a technology having an isolation ring dimension of 0.5 micrometer and a minimum photolithographic dimension of 0.4 micrometer, two active areas that are designed to be 1.35 micrometers or closer to each other will only have isolation therebetween; two active areas that are designed to be 1.4 micrometers apart or greater will have isolation around each and a band of inactive silicon area therebetween. 
     Advantageously, the present invention permits small active silicon areas that are widely separated from other active silicon areas to have trench isolation while avoiding substantial dishing either of the trench insulation or of the small active silicon areas. As shown in FIG. 2, small active silicon areas  70   a  and  70   b  can be located adjacent large inactive area  82  which has an area at least 100 times greater than active semiconductor areas  70   a  or  70   b . In addition, small active silicon areas  70   a  and  70   b  are widely separated in all directions from other active silicon areas  74 , the separations being at least 5 times the dimension of active areas  70   a  and  70   b . Of course, active silicon areas  70   a  or  70   b  can also have conductors and isolation paths as illustrated in FIGS. 1,  1   b  and  1   c.    
     A practical problem arises either where the derived layout provides corners of two isolation structures that are closer together than a minimum dimension or where the derived layout provides corners of two isolation structures that overlap each other by less than a minimum dimension as illustrated in FIG. 3 a . The isolation structures can be either isolation rings or isolation paths. The problem is that mask inspection software may categorize the resulting subminimum dimension region as a defect. In FIG. 2 a  isolation ring  172  which has corner  174  is within a minimum photolithographic dimension of isolation ring  176  having corner  178 . Thus, inactive semiconductor area  180  therebetween has a subminimum dimension. This problem is avoided by providing isolation tab  182  in the layout between such corners, as illustrated in FIG. 3 b . Isolation tab  182  links isolation ring  172  and isolation ring  176  between corner  174  and corner  178 . Isolation tab  182  has a width that is at least a minimum dimension of the photolithographic technology used in the process of fabricating the isolation and less than about 2 micrometers. To save space while avoiding process difficulties, the width is selected to be less than about 2 times the minimum dimension of the photolithographic system used for defining the isolation. 
     Two other problems arise associated with very large inactive silicon areas. First, there may be a pair of defects shorting two separate active silicon areas to the same inactive silicon area, thereby shorting the two active silicon areas to each other. The probability of such a double shorting event can be reduced by a layout that breaks up very large inactive silicon areas into a plurality of smaller inactive silicon areas separated by narrow lines of isolation. 
     Second, because large expanses of unbroken inactive silicon area present a full surface of oxide to the polishing pad, as compared to the partially coated surface in other areas having a high density of active device areas, polishing uniformity will decrease in a layout having such diversity. This problem is also reduced by adjusting the layout to break up the large inactive silicon areas into a plurality of smaller inactive silicon areas, using enough isolation stripes to provide a roughly uniform silicon-to-isolation ratio in all regions of the chip. 
     Preferably, inactive silicon areas are doped to provide n-doping to inactive silicon areas within an n-well and p-doping to inactive silicon areas outside an n-well. Opposite doping for the inactive silicon areas is also acceptable. 
     Another embodiment of the present invention is shown in FIG. 4, in which elongate conductor  160  is located on elongate trench isolation path  164 . Trench isolation path  164  is surrounded by inactive silicon area  66 . Trench isolation path  164  extends horizontally beyond conductor  160  on all sides, preventing contact and reducing capacitive coupling between conductor  160  and inactive silicon area  66 . The layout of trench isolation path  164  is obtained from the layout of conductor  160  by shifting edge  160   a  of conductor  160  outward at each point by a fixed dimension d′. Fixed dimension d′ is at least an overlay tolerance of the photolithography system used for fabrication of the conductor. However, significantly larger expansion of the isolation than the photolithographic minimum may be desired to reduce conductor-to semiconductor capacitance or to increase process latitude. Fixed dimension d′ is typically up to about 10 micrometers. Preferably, conductor  160  is centered on isolation path  164  approximately within the overlay tolerance of the photolithographic system used for its fabrication. Isolated conductor  160  is formed of a material such as polysilicon or a metal, and is used as a resistor or as an interconnect between upper level conductors. 
     While several embodiments of the invention, together with modifications thereof, have been described in detail herein and illustrated in the accompanying drawings, it will be evident that various further modifications are possible without departing from the scope of the invention. For example, a variable dimension rather than a fixed dimension can be used to derive isolation rings and isolation paths. The dimension may vary depending on the density of active areas, the relative isolation of a structure, or other factors. Nothing in the above specification is intended to limit the Invention more narrowly than the appended claims. The examples given are intended only to be illustrative rather than exclusive.