Abstract:
A method of chemical-mechanical polishing for forming a shallow trench isolation is disclosed. A substrate having a number of active regions, including a number of relatively large active regions and a number of relative small active regions, is provided. The method comprises the following steps. A silicon nitride layer on the substrate is formed. A number of shallow trenches are formed between the active regions one or more of which may constitute an alignment mark. An oxide layer is formed over the substrate, so that the shallow trenches are filled with the oxide layer. A partial reverse active mask is formed on the oxide layer. The partial reverse active mask exposes a portion of the oxide layer over the large active area and over the alignment mark. The oxide layer of each large active region and the alignment mark is removed. The partial reverse active mask is removed. The oxide layer is planarized.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 09/991,395, filed Nov. 20, 2001, which is a continuation of U.S. patent application Ser. No. 09/692,251, filed Oct. 19, 2000, now U.S. Pat. No. 6,448,159, which is a divisional of U.S. patent application Ser. No. 09/111,007 filed Jul. 7, 1998, now U.S. Pat. No. 6,169,012, which claims priority from Taiwan Application No. 87108699, filed Jun. 3, 1998, all the disclosures of which are herein specifically incorporated by this reference. 
     
    
     
       BACKGROUND  
         [0002]    1. Field of Invention  
           [0003]    This invention relates to a chemical mechanical polishing (CMP) applied in forming shallow trench isolation (STI), and more particularly, to a process of forming a STI structure combining CMP, using a partial reverse active mask.  
           [0004]    2. Background  
           [0005]    CMP is a technique ideal for applying in global planarization in very large scale integration (VLSI) and even in ultra large-scale integration (ULSI). Moreover, CMP is likely to be the only reliable technique as the feature size of the integrated circuit (IC) is highly reduced. Therefore, it is of great interest to develop and improve CMP techniques to reduce costs.  
           [0006]    As the IC devices are continuously sized down to a linewith of 0.25 μm or even 0.18 μm (deep sub-half micron), using CMP to planarize the wafer surface, especially to planarize the oxide layer on the surface of the shallow trench, becomes even more important. To prevent the dishing effect occurring at the surface of a larger trench during CMP process and to obtain a superior CMP uniformity, a reverse tone active mask was proposed, in cooperation with an etching back process.  
           [0007]    Typically, the active regions have varied sizes and the shallow trenches between the active regions also have different sizes. FIGS. 1A to  1 E are cross sectional views showing the process steps for forming shallow trench isolation, using CMP. Referring to FIG. 1A, on a substrate  10 , a pad oxide  15  and a silicon nitride layer  16  are deposited successively. By photolithography, the substrate  10 , the pad oxide layer  15  and the silicon nitride layer  16  are anisotropically etched to form shallow trenches  14   a ,  14   b ,  14   c  and define active regions  12   a ,  12   b ,  12   c ,  12   d . The sizes of the shallow trenches  14   a ,  14   b ,  14   c  are different since the sizes of the active regions  12   a ,  12   b ,  12   c ,  12   d  are varied.  
           [0008]    Next, referring to FIG. 1B, an oxide layer  18  is deposited by atmosphere pressure chemical vapor deposition (APCVD) on a substrate  10  to fill the interior of the shallow trenches  14   a ,  14   b ,  14   c . However, due to the step coverage of the oxide layer  18 , the deposited oxide layer  18  has an uneven surface and a rounded shape. Then, a photoresist layer is coated on the surface of the oxide layer  18  and patterned to form a reverse active mask  20  by photolithography. The reverse active mask  20  covers the shallow trenches  14   a ,  14   b ,  14   c  and is complementary to the active regions  12   a ,  12   b ,  12   c ,  12   d . However, during the formation of the reverse active mask, misalignment causes the oxide layer  18  to cover more than the shallow trenches  14   a ,  14   b ,  14   c.    
           [0009]    Referring to FIG. 1C, the oxide layer  18  exposed outside the reverse active mask  20  is etched until the silicon nitride layer  16  is exposed so that only a part of the silicon oxide layer  18 , the silicon oxide layer  18   a , is formed. After removing the reverse active mask  20 , as shown in FIG. 1D, it is observable that the silicon oxide layer  18   a  remained does not fully cover the shallow trenches  14   a ,  14   b ,  14   c  at one sides of the shallow trenches  14   a ,  14   b ,  14   c , therefore, forming cavities  22 , but at the other sides over-cover the shallow trenches  14   a ,  14   b ,  14   c , forming photo-overlap  24 .  
           [0010]    Referring to FIG. 1E, the portion of the oxide layer  18   a  higher than the shallow trenches  14   a ,  14   b ,  14   c  is polished by CMP until the surface of the silicon nitride layer  16  is exposed. Therefore, the silicon nitride layer  16  and the silicon oxide layer  18   a  are at the same level. The profile of the silicon oxide layer  18   a  formed by APCVD is rather rounded and the APCVD silicon oxide layer  18   a  is hard to planarize. Moreover, it is obvious that the silicon oxide layer  18   a  does not fully fill the shallow trenches  14   a ,  14   b ,  14   c  but forms the concaves  22 . The undesired concaves  22  may cause kink effect and consequent short circuit or leakage current which therefore influence the yield.  
           [0011]    As a result, it is important to overcome the problems coming after the formation of the concaves due to the misalignment of the reverse active mask during the process of CMP, especially, while nowadays the linewidth is decreasing.  
         SUMMARY OF THE INVENTION  
         [0012]    It is therefore an objective of the present invention to provide a method of chemical-mechanical polishing for forming a shallow trench isolation. A substrate having a number of active regions, including a number of relative large active regions and a number of relative small active regions, is provided. The method comprises the following steps. A silicon nitride layer on the substrate is first formed. A number of shallow trenches are formed between the active regions. An oxide layer is formed over the substrate, so that the shallow trenches are filled with the oxide layer. A partial reverse active mask is formed on the oxide layer. The partial reverse active mask has an opening at a central part of each relative large active region. The opening exposes a portion of the oxide layer. The opening has at least a dummy pattern. The oxide layer on the central part of each large active region is removed to expose the silicon nitride layer. The partial reverse active mask is removed. The oxide layer is planarized to expose the silicon nitride layer.  
           [0013]    In one aspect, the invention provides a method of chemical-mechanical polishing for forming a shallow trench isolation, wherein a substrate having a plurality of active regions, including a plurality of relatively large active regions and a plurality of relatively small active regions and an alignment mark. The method comprises: forming a plurality of shallow trenches between the active regions; forming an oxide layer over the substrate, so that the shallow trenches and the alignment mark are filled therewith; forming a partial reverse active mask on the oxide layer, wherein the partial reverse active mask has an opening at each relatively large active region and at the alignment mark when the reverse active mask completely covers each relatively small active region and trenches, wherein the opening exposes a portion of the oxide layer; removing portions of the oxide layer on each large active region and at the alignment mark; removing the partial reverse active mask; and planarizing the oxide layer.  
           [0014]    In another aspect, the invention provides a method of chemical-mechanical polishing in forming a multi-layered semiconductor device comprising a substrate. The method comprises forming an alignment mark in the substrate and a plurality of shallow trenches between active regions of the semiconductor substrate; forming an oxide layer over the substrate; forming a partial reverse active mask on the oxide layer, wherein the partial reverse active mask has an opening over a portion of at least one active region and over the alignment mark; removing portions of the oxide layer over at least one active region and over portions of the alignment mark to expose a portion of the oxide layer; removing the partial reverse active mask; and planarizing the oxide layer.  
           [0015]    In yet another aspect, the invention provides a method of forming a semiconductor device having an alignment mark. The method comprises forming an alignment mark in a substrate; forming at least one active area on the substrate; forming an oxide layer over the substrate, wherein the oxide layer covers at least a portion of the alignment mark; forming a partial reverse active mask on the oxide layer, wherein the partial reverse active mask has an opening over at least a portion of the alignment mark; removing portions of the oxide layer to expose a portion of the oxide layer; removing the partial reverse active mask; and planarizing the oxide layer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The invention can be more fully understood by reading the following detailed description of the preferred embodiments, with reference made to the accompanying drawings, wherein:  
         [0017]    [0017]FIGS. 1A to  1 E are cross-sectional views showing the process steps of forming a conventional shallow trench using a reverse active mask;  
         [0018]    [0018]FIGS. 2A to  2 E are cross-sectional views showing the process steps of forming shallow trenches using a partial reverse active mask according to a preferred embodiment of the invention;  
         [0019]    [0019]FIGS. 3A to  3 D illustrate the partial reverse active mask according to a preferred embodiment of the invention; and  
         [0020]    [0020]FIGS. 4A TO  4 E are cross-sectional views showing the process steps of forming shallow trenches and maintaining an alignment mark using a partial reverse active mask according to another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    The invention provides a process for forming STI, combining the partial reverse active mask and CMP, using high density plasma chemical vapor deposition (HDCVD). This process prevents the formation of concaves in the shallow trenches due to the misalignment of the reverse active mask, which consequently causes short circuit or leakage current. In another embodiment, a partial active reverse mask technique is used to permit oxide to be etched from an alignment mark in the substrate.  
         [0022]    Referring to FIG. 2A, active regions  42   a ,  42   b  are defined on a substrate  40  first by depositing a pad oxide layer  45  and a silicon nitride layer  46 , and then by photolithography and trench etching to form shallow trenches  44  between the active regions  42   a ,  42   b . The sizes of the shallow trenches are varied since the sizes of the active regions  42   a ,  42   b  are different. Then, a silicon oxide layer  48  is deposited over the substrate  40  and filling the trenches  44 , preferably by high density plasma chemical vapor deposition (HDPCVD). The profile of the silicon oxide layer  48  on the active region  42   a ,  42   b  is at a higher level than that of the silicon oxide layer  48  on the shallow trenches  44  since the shallow trenches are fallen in the substrate  40 . The HDPCVD oxide layer  48  on the active region  42   a ,  42   b  has a sharp profile, as shown in FIG. 2B, which is different from the conventional.  
         [0023]    Referring to FIG. 2C, a photoresist layer is coated on the oxide layer  48  and defined to form a partial reverse active mask  50  by photolithography. The partial reverse active mask  50  has an opening  52  at the central part of the larger active region  42   a . Since the opening  52  exposes only the central part of the silicon oxide layer  48  at the larger active region  42   a , the silicon oxide layer  48  over the shallow trenches  44  will not be exposed even though misalignment occurs.  
         [0024]    Referring to FIG. 2D, using the reverse active mask  50  as a mask, the exposed silicon oxide layer  48  at the larger active region  42   a  is etched back until the silicon nitride layer  46  is exposed. The reverse active mask is then peeled. Then, only the oxide layer  48   b  on the smaller active region  42   b  and a small portion of the silicon oxide layer  48   a  through etching back on the larger active region  42   a  remain. The remaining silicon oxide layer  48   a  and  48   b  formed preferably by HDPCVD have sharp profiles and are therefore easy to be planarized by CMP. Also, the sizes of the remained silicon oxide layer  48   a  and  48   b  are more or less similar so that the consistency of CMP is increased.  
         [0025]    Next, referring to FIG. 2E, the remained silicon oxide layer  48   a  and  48   b  (as shown in FIG. 2D) are polished by CMP, using the silicon nitride layer  46  as an etching stop layer so that the silicon oxide layer  48   c  in the shallow trenches and the silicon nitride layer  46  are almost at the same level.  
         [0026]    In the above embodiment, a partial reverse active mask is employed for forming a shallow trench isolation. In FIGS. 3A to  3 D, a method of forming a partial reverse active mask is shown. As shown in FIG. 3A, to define a photomask pattern, active regions are formed first. The active regions include a larger active region pattern  60  and a smaller active region pattern  62 .  
         [0027]    Referring to FIG. 3B, the larger active region pattern  60  and the smaller active pattern region  62  are shrunk as shown in the figure. The shrinking larger active region pattern and the shrinking smaller active region pattern are denoted as  60   a  and  62   a  respectively.  
         [0028]    Referring to FIG. 3C, the shrinking process is continued until the shrinking smaller active region pattern  62   a  disappears. The shrinking distance is about 0.5 μm to 2 μm each side so that active region patterns with maximum radius of less than 0.1˜0.4 μm will disappear. Next, the shrinking larger active region  60   a  is enlarged until the profile of it is a little bit smaller than the profile of the original larger active region pattern. The profile of the larger active region pattern at the stage is denoted as  60   b . The shrinking large active region pattern  60   a  is enlarged with a dimension of about 0.2 μm to 2 μm each side. This enlarged dimension is smaller than the shrinking distance mentioned above.  
         [0029]    Referring to FIG. 3D, the partial reverse active mask  60   b  is located at the central part of the larger active region  60  but slightly smaller than the larger active region. One characteristic of the present invention is that the partial reverse active mask pattern  60   b  at the larger active region  60  has dummy pattern  64  so that dishing effect at the larger active region  60  can be avoided. By applying this photo-mask pattern in forming a shallow trench isolation, the central part of an active region is exposed, whereas the edge part of the active region is covered by a photo-resist. A partial reverse active mask pattern is thus obtained.  
         [0030]    It will be appreciated by one of skill in the art that the reverse partial active mask techniques described hereinabove may be used to facilitate etching oxide from an alignment mark in the substrate. This removes the oxide from the alignment so that a CMP process over the alignment mark is unnecessary. Thus, the alignment mark is clearly exposed without the need for additional steps.  
         [0031]    This is illustrated in FIGS. 4A to  4 E, which demonstrate the use of a reverse partial active mask technique to etch oxide from an alignment mark in the substrate. In an exemplary embodiment, an alignment mark may be a shallow trench formed in the substrate. Referring to FIG. 4A, active regions  442   a ,  442   b  are defined on a substrate  440  first by depositing a pad oxide layer  445  and a silicon nitride layer  446 , and then by photolithography and trench etching to form shallow trenched  444  between the active regions  442   a ,  442   b . In addition, an alignment mark  460  is formed in the substrate  440 . The sizes of the shallow trenches may be varied since the sizes of the active regions  442   a ,  442   b  may be different. Then, a silicon oxide layer  448  is deposited over the substrate  440  and filling the trenches  444  and the alignment mark  460 , preferably by high density plasma chemical vapor deposition (HDPCVD). The profile of the silicon oxide layer  448  on the active region  442   a ,  442   b  may be at a higher level than that of the silicon oxide layer  448  on the shallow trenches  444  and alignment mark  460 , since the shallow trenches  444  and alignment mark  460  follows the contour of the substrate  440 . The HDPCVD oxide layer  448  on the active region  442   a ,  442   b  may have a sharp profile, as shown in FIG. 4B, which is different from the conventional.  
         [0032]    Referring to FIG. 4C, a photoresist layer is coated on the oxide layer  448  and defined to form a partial reverse active mask  450 , e.g., by photolithography. The partial reverse active mask  450  may have an opening  452 , e.g., near the central part of the larger active region  442   a . Since, in one exemplary embodiment, the opening  452  exposes only the central part of the silicon oxide layer  448  of the larger active region  442   a , the silicon oxide layer  448  over the shallow trenches  444  will not be exposed even though misalignment occurs. In addition, the partial reverse mask  450  may have an opening  462  over at least a portion of the alignment mark  460 .  
         [0033]    Referring to FIG. 4D, using the reverse active mask  450  as a mask, the exposed silicon oxide layer  448  at the larger active region  442   a  is etched back until the silicon nitride layer  446  is exposed. The reverse active mask  450  may then be peeled. Then, only the oxide layer  448   b  on the smaller active region  442   b  and a small portion of the silicon oxide layer  448   a  through etching back on the larger active region  442   a  remain. The remaining silicon oxide layer  448   a  and  448   b , which may be formed by high-density plasma chemical vapor deposition (HDPCVD) have sharp profiles and therefore are easy to planarize by CMP. Also, the sizes of the remained silicon oxide layer  448   a  and  448   b  are more or less similar so that the consistency of CMP is increased.  
         [0034]    Next, referring to FIG. 4E, the remained silicon oxide layer  448   a  and  448   b  (as shown in FIG. 4D) are polished by, e.g., CMP, using the silicon nitride layer  446  as an etching stop layer so that the silicon oxide layer  448   c  in the shallow trenches and the silicon nitride layer  446  are almost at the same level.  
         [0035]    Advantages of the invention include:  
         [0036]    (1) The oxide layer formed by HDCVD has a pyramid-like profile, so that using chemical-mechanical polishing, the oxide layer is planarized easily.  
         [0037]    (2) Using a partial reverse active mask to etch away the oxide layer on the central part of an active region, only the oxide layer on the edge part of the active region and on a small active region is remained. The profile of the remaining oxide layer is pyramid-like and has a better uniformity. Therefore, a recess formed while polishing a large trench is avoided.  
         [0038]    (3) The dishing effect on the large active region is avoided since the partial reverse active mask has a dummy pattern.  
         [0039]    (4) Since only the oxide layer on the central part of an active region is etched away by using a partial reverse active mask, even when a misalignment occurs, the oxide layer within the trench is not etched. The kink effect is prevented. As a consequence, the current leakage and the short circuit caused by kink effect are avoided, so that the yield of wafer is enhanced.  
         [0040]    Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention, as hereinafter claimed.  
         [0041]    The words “comprise,” “comprising,” “include,” “including,” and “includes” when used in this specification and in the following claims are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, or groups.