Abstract:
An insulated trench isolation structure with large and small trenches of differing widths is formed in a semiconductor substrate with improved planarity using a simplified reverse source/drain planarization mask. Embodiments include forming large trenches and refilling them with an insulating material which also covers the substrate surface, masking the areas above the large trenches, etching to remove substantially all of the insulating material on the substrate surface and polishing to planarize the insulating material above the large trenches. Small trenches and peripheral trenches surrounding the large trenches are then formed, refilled with insulating material, and planarized. Since the large trenches are formed prior to and separately from the small trenches, etching can be carried out after the formation of a relatively simple planarization mask over only the large trenches, and not the small trenches. The use of a planarization mask with relatively few features having a relatively large geometry avoids the need to create and implement a complex and critical mask, thereby reducing manufacturing costs and increasing production throughput. Furthermore, because the large and small trenches are not polished at the same time, overpolishing is avoided, thereby improving planarity and, hence, the accuracy of subsequent photolithographic processing.

Description:
RELATED APPLICATIONS 
     This application claims priority from U.S. provisional patent application No. 60/148,071, filed Aug. 10, 1999, and is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method of manufacturing a semiconductor device comprising trench isolation. The invention has particular applicability in manufacturing high density semiconductor devices with submicron design features and active regions isolated by shallow insulated trenches. 
     BACKGROUND ART 
     Current demands for high density and performance associated with ultra large scale integration require submicron features of about 0.25 microns and under, increased transistor and circuit speeds and improved reliability. Such demands for increased density, performance and reliability require formation of device features with high precision and uniformity. 
     Conventional semiconductor devices comprise a substrate and various electrically isolated regions, called active regions, in which individual circuit components are formed. The electrical isolation of these active regions is typically accomplished by forming field oxide regions by thermal oxidation of the semiconductor substrate, typically monocrystalline silicon or an epitaxial layer formed thereon, bounding the active regions. 
     One type of isolation structure is known as trench isolation, wherein shallow trenches are etched in the substrate and an oxide liner is thermally grown on the trench walls. The trench is then refilled with an insulating material. The resulting structure is referred to as a shallow trench isolation (STI) structure. The active region typically comprises source/drain regions formed in the semiconductor substrate by implantation of impurities, spaced apart by a channel region on which a gate electrode is formed with a gate oxide layer therebetween. The gate electrode controls the turn-on and turn-off of each transistor. 
     A typical method of trench formation comprises initially growing a pad oxide layer on the substrate, and depositing a barrier nitride layer thereon. A photoresist mask is then applied to define the trench areas. The exposed portions of the nitride layer are then etched away, followed by the pad oxide layer. The etching continues into the substrate to form the shallow trench. When etching of the trench is completed, the photoresist is stripped off the nitride layer. 
     Next, the substrate is oxidized to form an oxide liner on the walls and base of the trench to control the silicon-silicon dioxide interface quality. The trench is then refilled with an insulating material such as silicon dioxide derived from tetraethyl orthosilicate (TEOS) to form a field oxide region. The insulating material is then planarized, as by chemical-mechanical polishing (CMP) using the barrier nitride layer as a polish stop, to remove all the oxide over the active regions so that only the trenches are filled. The nitride and pad oxide are stripped off the active areas to complete the trench isolation structure. 
     The planarization of the insulating material is a difficult process, because the field oxide regions vary widely in size. For example, one trench may have a width as little as 0.25 μ, while an adjacent trench may be several microns wide. After the insulating material is deposited to fill the trenches and cover the polish stop, fissures called “seams” exist in the deposited insulating material above the smaller trenches, and indentations called “steps” exist in the upper surface of the insulating material above the large trenches. The steps are considerably wider than the seams; however, the seams are considerably deeper than the steps. The presence of both seams and steps is problematic during polishing, in that the greater amount of polishing required to remove the seams over the small features results in the removal of too much insulating material over the large features. This overpolishing of the insulating material above the large trenches produces undesirable “dishing” of the insulating material, resulting in a nonplanar insulating surface. 
     The problem of simultaneously planarizing an insulating material over both large and small features has been addressed by providing preliminary masking and etching steps, and then polishing, as by CMP. This technique is illustrated in FIGS. 1A-1C. Adverting to FIG. 1A, there is schematically illustrated the substrate  11 , pad oxide layer  12 , polish stop  13 , oxide liner  14 , trenches  15 , insulating layer  16 , seams  17  and steps  18 . Inverse source/drain photoresist mask  19  is formed on the insulating layer  16  to protect the seams  17  and steps  18  from overetching. Isotropic etching is then performed to remove most of the insulating material in the active areas (FIG. 1B) before the final chemical-mechanical polish, as shown in FIG.  1 C. 
     In principal this works very well. However, as the isotropic etch progresses, the contact area of photoresist mask  19  above small trenches  15  typically narrows down to a point. At this point, photoresist mask  19  detaches from insulating material  16 , and the etch process then attacks seams  17 . Seams  17  tend to be etched at a faster rate, since they are less compact, causing overetching and undesirable indentation around the seam. Ideally, it would be preferred to etch away all of insulating layer  16  on top of polish stop layer  13 , then proceed with CMP to remove the remaining portions of insulating layer  16  (called “fences”) followed by a short polish. However the indentation around seams  17  require a portion of insulating layer  16  be left on top of polish stop layer  13 , so the indentation does not extend below the top surface of polish stop layer  13 . This remaining portion of insulating layer  16  above polish stop layer  13  contributes to increased non-uniformity of the planarized top surface of insulating  16 . Moreover, as minimum device critical dimension (CD) shrinks, indentation of seams  17  happens earlier in the etch process, requiring a higher oxide polish target. 
     Furthermore, the inverse source/drain mask  19  is a “critical mask”; i.e., it is complex and difficult to design and use. Still further, due to the topography of insulating material layer  16  prior to polishing, a relatively large depth of focus is required to produce mask  19 . 
     Accordingly, there exists a need for a method of manufacturing a shallow trench isolation structure with improved field oxide planarity without the necessity of employing a complex critical mask. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is a method of manufacturing a semiconductor device having insulated trenches formed in a semiconductor substrate, wherein an insulating material which fills the trenches and acts as the field oxide is planarized using a simplified, non-critical inverse source/drain mask. 
     Additional objects, advantages and other features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the invention. The objects and advantages of the invention may be realized and obtained as particularly pointed out in the appended claims. 
     According to the present invention, the foregoing and other objects are achieved in part by a method of manufacturing a semiconductor device having a plurality of trenches formed in a semiconductor substrate or in an epitaxial layer on the semiconductor substrate, which method comprises: forming a pad oxide layer on a main surface of the substrate or epitaxial layer; forming a polish stop layer having an upper surface on the pad oxide layer; forming a relatively large trench having side surfaces; depositing a first layer of an insulating material to fill the relatively large trench and cover the polish stop layer, whereby the first layer of the insulating material has a step in its upper surface above the relatively large trench; providing a planarization mask on the first layer of the insulating material above the step; etching to remove substantially all of the first layer of the insulating material on the polish stop layer; removing the planarization mask; performing a first polish to planarize such that an upper surface of the first layer of the insulating material is substantially flush with the upper surface of the polish stop layer; forming a relatively small trench, the relatively small trench having a width at the main surface less than a width at the main surface of the relatively large trench; forming a peripheral trench surrounding the relatively large trench and abutting the side surfaces of the relatively large trench, the peripheral trench having a width at the main surface less than the width at the main surface of the relatively large trench; thermally growing an oxide layer lining the relatively small trench and the peripheral trench; depositing a second layer of the insulating material to fill the relatively small trench and the peripheral trench and cover the polish stop layer; and performing a second polish to planarize such that an upper surface of the second layer of the insulating material is substantially flush with the upper surface of the polish stop layer. 
     Another aspect of the present invention is a semiconductor device comprising: a substrate or epitaxial layer formed in the substrate; a relatively large trench, formed in a main surface of the substrate or epitaxial layer, having side surfaces; a relatively small trench, formed in the main surface, having a width at the main surface less than a width at the main surface of the relatively large trench; a peripheral trench, formed in the main surface, surrounding the relatively large trench and abutting the side surfaces of the relatively large trench, the peripheral trench having a width at the main surface less than the width at the main surface of the relatively large trench; a thermally grown oxide liner in the relatively small trench and the peripheral trench; and an insulating material filling the relatively large trench, the relatively small trench, and the peripheral trench. 
     Additional objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is made to the attached drawings, wherein elements having the same reference numeral designations represent like elements throughout, and wherein: 
     FIGS. 1A-1C schematically illustrate sequential phases of a method of STI formation requiring a complex inverse source/drain mask. 
     FIGS. 2A-2M schematically illustrate sequential phases of a method in accordance with an embodiment of the present invention. 
     FIGS. 3A-3J schematically illustrate sequential phases of a method in accordance with another embodiment of the present invention. 
     FIG. 4 is a cross-sectional view of a semiconductor device according to the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     The use of a critical mask to planarize the insulating material filling the trenches disadvantageously increases the manufacturing costs and reduces production throughput. The present invention addresses and solves such problems by enabling the use of a simplified, non-critical mask during planarization, by treating seams and steps separately. Moreover, the present invention results in improved planarity of the insulating surface compared to methods utilizing a critical mask for planarization. 
     According to the methodology of the present invention, a first source/drain photoresist mask is formed on a polish stop layer which, in turn, is formed on a pad oxide layer on a main surface of a semiconductor substrate or an epitaxial layer on a semiconductor substrate. As used throughout the present disclosure and claims, the term “substrate” denotes a semiconductor substrate or an epitaxial layer formed on the semiconductor substrate. 
     After masking, the substrate is etched to form large trenches, and an insulating material is deposited to fill the large trenches and cover the polish stop layer. A step is generally formed above each of the large trenches. A planarization mask is then formed on the insulating material above the large trenches, and the insulating material is etched to remove all the insulating material above the polish stop layer. Thereafter, the planarization mask is removed, and the insulating material is polished, as by CMP, until reaching the polish stop. 
     A second photoresist source/drain mask is then formed on the polish stop layer and above the large trenches, and the substrate is etched to form small trenches; i.e., trenches relatively smaller than the large trenches, whose width is smaller than that of the large trenches, and to form narrow peripheral trenches surrounding the large trenches. An oxide liner is grown in the small trenches and in the peripheral trenches. The peripheral trenches, having an oxide liner, function to provide isolation between the insulating material filling the large trenches and the silicon of the active areas of the substrate. The small and peripheral trenches are then filled with more of the insulating material, generally forming a seam above each of the small trenches and the peripheral trenches, but not forming any steps. The insulating material is then polished, as by CMP, until reaching the polish stop. 
     Since the inventive methodology forms and fills the large trenches separately from the small trenches, etching can be carried out after the formation of a relatively simple planarization mask over only the large trenches. Thus, the necessity for a critical mask is avoided. The inventive planarization mask is easier to design and implement than inverse source/drain planarization masks because it has fewer features, and its features have a relatively large geometry. Thus, the present invention enables a reduction in manufacturing costs and an increase in production throughput. Furthermore, since seams and steps are not polished at the same time, overpolishing and undesirable dishing of the insulating material above the large trenches is avoided. 
     An embodiment of the present invention is illustrated in FIGS. 2A-2M, wherein sequential phases in forming a semiconductor device in accordance with the present invention are depicted. Referring to FIG. 2A, substrate  201  is prepared having a substantially planar surface, typically a semiconductor substrate comprising doped monocrystalline silicon or an epitaxial layer formed on a semiconductor substrate in accordance with conventional practices. A pad oxide layer  202  is then grown on the substrate  201 . Pad oxide layer  202  is typically silicon oxide and can be thermally grown on the substrate or deposited by chemical vapor deposition (CVD). In another embodiment, a pad oxide containing a thinned thermally-grown silicon oxide layer and a buffer polycrystalline silicon layer is employed as the pad layer. FIG. 2A illustrates silicon substrate  201  and the pad oxide layer  202 . 
     After formation of the pad oxide layer  202 , a polish stop layer  203  is deposited on the pad oxide layer  202 , as shown in FIG. 2B, such as a silicon nitride layer by CVD. Silicon oxide pad layer  202  functions as a buffer layer cushioning stresses between substrate  201  and polish stop layer  203 . Polish stop layer  203  functions as an oxidation mask as it is resistant to the diffusion of oxygen and water vapor therethrough, thereby preventing an oxidizing species from reaching the underlying silicon substrate, as well as acting as a polish stop. 
     A first photoresist source/drain mask  204  is then formed on polish stop layer  203 , as shown in FIG.  2 C. First photoresist source/drain mask  204  has a pattern defined by openings  240 , which generally have a width  241  substantially corresponding to the width of subsequently formed large trenches at the main surface  201   a  of the substrate  201 . The polish stop layer  203  is then etched away, and the etching continues through the pad oxide layer  202  and into the substrate  201  to form the shallow large trenches  205  as shown in FIG.  2 D. The large trenches  205  are typically etched to a depth of up to about 4000 Å. In practicing the present invention, a trench depth of about 2500 Å to about 3000 Å has been found particularly suitable. When the etching of the large trenches  205  is completed, the first photoresist mask  204  is stripped off the polish stop layer  203 . 
     Thereafter, large trenches  205  are filled with a first layer  206  of a suitable insulating material, as shown in FIG. 2E, to a height above polish stop layer  203 . Such insulating material  206  can comprise silicon dioxide derived from TEOS by LPCVD or derived from silane by LPCVD. The large trenches  205  can also be filled with a high density plasma (HDP) oxide. The thickness of insulating material  206  is such that the upper surface  206   a  of insulating material  206  above trenches  205  is slightly higher than the upper surface of polish stop layer  203 . 
     Subsequent to trench filling, a photoresist planarization mask  207  is formed on the first layer  206  of insulating material above the large trenches  205 , as shown in FIG. 2F, and the first layer  206  of insulating material is isotropically etched, as shown in FIG. 2G, to remove substantially all of the insulating material  206  over the polish stop layer  203 . Planarization mask  207  is then removed, leaving “fences”  206   b,  which are then polished away, as by CMP. A short polish is then performed to ensure that the upper surface  206   c  of insulating material  206  is flush with the upper surface  203   a  of polish stop layer  203 , as shown in FIG.  2 H. 
     A second photoresist source/drain mask  208  is then formed on polish stop layer  203  and first layer  206  of insulating material, as shown in FIG.  2 I. Second photoresist source/drain mask  208  has a pattern defined by openings  280 ,  290  which generally have a width  281 ,  291  substantially corresponding to the width of the subsequently formed trenches at the main surface  201   a  of the substrate  201 . The polish stop layer  203  is then etched away, and the etching continues through the pad oxide layer  202  and into the substrate  201  to form the shallow small trenches  209  and shallow peripheral trenches  210 , as shown in FIG.  2 J. The peripheral trench openings  210  surround the large trenches  205  and abut the side surfaces of the large trenches  205 . The peripheral trenches  210  have a width about equal to the minimum width required by the design rules of the semiconductor device; e.g., about 0.3 μ. The trenches  209 ,  210  are typically etched to about the same depth as the large trenches  205 ; i.e., up to about 4000 Å, with a depth of about 2500 Å to about 3000 Å being particularly suitable. When the etching of the trenches  209 ,  210  is completed, the photoresist  208  is stripped off the polish stop layer  203 . 
     Thereafter, the surface of the trenches  209 ,  210  is thermally oxidized to form an oxide liner  211  on the inner surface of trenches  209 ,  210 , typically at a temperature of about 1000° C. or higher. FIG. 2K shows the trenches  209 ,  210  with the completed liners  211 . Subsequent to formation of the oxide liners  211 , trenches  209 ,  210  are filled with a second layer  212  of the insulating material to a height above polish stop layer  203  using any of the techniques and materials discussed in reference to the deposition of the first layer  206  of insulating material, as shown in FIG.  2 L. Alternatively, second layer  212  of insulating material may be a spin on layer, since the geometry to fill is relatively small, such as below 1 micron in width. Such materials may need an additional thermal cycle to cure. Due to the nature of the insulating material, after deposition the second layer  212  of insulating material has a seam  212   a  above each of the small trenches  209  and the peripheral trenches  210 . 
     Subsequent to trench filling, the second layer  212  of insulating material is polished, as by CMP, such that the upper surface  212   b  of the second layer of insulating material  212  is substantially flush with the upper surface  203   a  of the polish stop layer  203  and the upper surface  206   c  of the first layer  206  of insulating material, as depicted in FIG.  2 M. This polishing step is monitored in a conventional manner, as by measuring oxide over the polish stop layer. 
     In another embodiment of the present invention, as depicted in FIGS. 3A-3J, the small trenches and the peripheral trenches are formed first, followed by the relatively large trenches. Referring now to FIG. 3A, after pad oxide layer  202  and polish stop layer  203  are formed on main surface  201 , a first photoresist source/drain mask  301  is formed on polish stop layer  203 . First photoresist source/drain mask  301  has a pattern defined by openings  380 ,  390  which generally have a width  381 ,  391  substantially corresponding to the width of the subsequently formed trenches at the main surface  201   a  of the substrate  201 . 
     The polish stop layer  203  is then etched away, and the etching continues through the pad oxide layer  202  and into the substrate  201  to form the shallow small trenches  302  and shallow peripheral trenches  303 , as shown in FIG.  3 B. The peripheral trench openings  303  are sized to surround the subsequently formed large trenches and abut the side surfaces of the large trenches. As in the embodiment of FIGS. 2A-2M, the peripheral trenches  303  have a width about equal to the minimum width required by the design rules of the semiconductor device; e.g., about 0.3 μ, and the trenches  209 ,  210  are typically etched to about the same depth as the large trenches  205  described above. When the etching of the trenches  302 ,  303  is completed, the photoresist  301  is stripped off the polish stop layer  203 . 
     Thereafter, as shown in FIG. 3C, the surface of the trenches  302 ,  303  is thermally oxidized to form an oxide liner  304  on the inner surface of trenches  302 ,  303 , typically at a temperature of about 1000° C. or higher. Subsequent to formation of the oxide liners  304 , as shown in FIG. 3D, trenches  302 ,  303  are filled with a first layer  305  of the insulating material discussed in the embodiment of FIGS. 2A-2M to a height above polish stop layer  203  using any of the techniques and materials discussed above in relation to the formation of second insulating layer  212  (e.g., by deposition or a spin-on technique). Due to the nature of the insulating material, after formation the first layer  305  of insulating material has a seam  305   a  above each of the small trenches  302  and the peripheral trenches  303 . 
     Subsequent to trench filling, the first layer  305  of insulating material is polished, as by CMP, such that its upper surface  305   b  is substantially flush with the upper surface  203   a  of the polish stop layer  203 , as depicted in FIG.  3 E. This polishing step is monitored in a conventional manner, as by measuring oxide over the polish stop layer. 
     Next, a second photoresist source/drain mask  306  is then formed on polish stop layer  203 , as shown in FIG.  3 F. Second photoresist source/drain mask  306  has a pattern defined by openings  360 , which generally have a width  361  substantially corresponding to the width of subsequently formed large trenches at the main surface  201   a  of the substrate  201 , and cover small trenches  302  and peripheral trenches  303 . The polish stop layer  203  is then etched away, and the etching continues through the pad oxide layer  202  and into the substrate  201  to form the shallow large trenches  307  as shown in FIG.  3 G. 
     When the etching of the large trenches  307  is completed, the second photoresist mask  306  is stripped off the polish stop layer  203 , and large trenches  307  are filled with a second layer  308  of the insulating material to a height above polish stop layer  203 , using any of the deposition techniques discussed above. The thickness of insulating material  307  is such that its upper surface above trenches  307  is slightly higher than the upper surface of polish stop layer  203 . 
     Subsequent to trench filling, a photoresist planarization mask  309  is formed on the second layer  308  of insulating material above large trenches  307 , as shown in FIG. 3H, and second layer  308  of insulating material is isotropically etched, as shown in FIG. 3I, to remove substantially all of the insulating material  308  over the polish stop layer  203 . Planarization mask  309  is then removed, leaving “fences”  308   b,  which are then polished away, as by CMP. A short polish is then performed to ensure that the upper surface  308   c  of insulating material  308  is flush with the upper surface  203   a  of polish stop layer  203 , as shown in FIG.  3 J. 
     An embodiment of a semiconductor device in accordance with the present invention is described with reference to FIG.  4 . The inventive semiconductor device comprises large trenches  31 , small trenches  32  and peripheral trenches  33  formed in a main surface  30   a  of substrate  30 . Small trenches  32  have a width W 2  at main surface  30   a  less than a width W 1  at main surface  30   a  of large trenches  31 . The peripheral trenches  33  surround the large trenches  31  abutting side surfaces  31   a  of the large trenches  31 , and have a width W 3  at main surface  30   a  less than the width W 1  at main surface  30   a  of the large trenches  31 . Small trenches  32  and peripheral trenches  33  have a thermally grown silicon dioxide liner  34 , and a silicon dioxide insulating material  35  fills the large trenches  31 , the small trenches  32 , and the peripheral trenches  33 . The peripheral trenches typically have a width W 3  about equal to the minimum width required by the design rule of the semiconductor device; e.g., about 0.3 μ at the main surface  30   a.  The trenches  31 ,  32 ,  33  typically have a depth D of about 2500 Å to about 4000 Å; e.g., about 3000 Å. 
     According to the methodology of the present invention, the layers of insulating material filling the large trenches and the small trenches are planarized separately from each other. Thus, the steps above the large trenches and the seams above the small trenches are polished in separate operations. The inventive methodology thereby enables the use of a planarization mask which is simpler to make and use than reverse source/drain planarization masks, since it is only necessary to protect the thinner insulating material (i.e., the steps) over the large trenches The inventive planarization mask is generated by a relatively simpler algorithm than the reverse source/drain masks, since it is not necessary to locate and protect small features like seams. Moreover, because only larger and less numerous areas such as large trenches are masked, the planarization mask is relatively easy to use; e.g., it is easier to align, etc. Still further, since the steps and the seams are not polished at the same time, overpolishing of the insulating material above the large trenches is prevented, thereby improving planarity. Thus, the inventive method provides an increase in production throughput and an attendant economic benefit. The present invention is applicable to the manufacture of various types of semiconductor devices having STI, particularly high density semiconductor devices having a design rule of about 0.25 μ and under. 
     The present invention can be practiced by employing conventional materials, methodology and equipment. Accordingly, the details of such materials, equipment and methodology are not set forth herein in detail. In the previous descriptions, numerous specific details are set forth, such as specific materials, structures, chemicals, processes, etc., in order to provide a thorough understanding of the present invention. However, as one having ordinary skill in the art would recognize, the present invention can be practiced without resorting to the details specifically set forth. In other instances, well known processing structures have not been described in detail, in order not to unnecessarily obscure the present invention. 
     Only the preferred embodiment of the invention and but a few examples of its versatility are shown and described in the present disclosure. It is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein.