Abstract:
An improved borderless contact structure and method of making the structure including a substrate with side walls formed on the side of the shallow trench. An insulator is formed over the side walls and in the remainder of the trench such that the insulator extends above an upper surface of the substrate. The side walls are formed of a first etch selection type and the insulator is formed of a second etch selection type.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an improved structure of a borderless contact and its fabricating method, and more particularly to a structure of a borderless contact in which a nitride film side wall is formed at a shallow trench isolation region.  
           [0003]    2. Description of the Background Art  
           [0004]    Conventionally, integrated circuits are designed with a boundary region near a region where a contact is formed during a fabricating process. The boundary region near the contact is used to prevent a contact opening from expanding by exceeding a region at which a contact is to be formed.  
           [0005]    If a boundary region is not formed, a contact would be formed and depending upon variations in processes, the contact could extend into an adjacent region causing an undesirable connection. In this instance during subsequent processing, a field oxide would be removed due to over-etching and a leakage path may be formed from a metal to the substrate. Also, the over-etching of the field oxide may lead to over-etching of the silicon substrate.  
           [0006]    Though the boundary near the contact opening guarantees proper registration of the contact and protection of lower conductive devices, it also brings about the negative affect that the maximum number of devices to be integrated is restricted  
           [0007]    On the other hand, a borderless contact does not require a boundary near the contact, as is noted from the name, and thus, accomplishes improved chip integration. The borderless contact is formed by exposing an active region and an insulation region.  
           [0008]    [0008]FIG. 1A is a sectional view depicting a structure of a borderless contact in accordance with a conventional art.  
           [0009]    As shown in the drawing, a gate region  11  consisting of a gate  11   a  and a gate side wall  11   b  is formed on a substrate  10 . An active region  12  is formed at a right lower portion of the gate region  11 , and a field oxide  13  is formed next to the active region  12 .  
           [0010]    A nitride film  14  is deposited on the gate region  11 , the active region  12  and the field oxide  13 , and an interlayer dielectric film  15  is deposited on the nitride  14 .  
           [0011]    A contact opening  16  is formed in a portion of the interlayer dielectric  15 , exposing the active region  12  and the field oxide  13 . The contact opening  16  is filled with a conductive material in a subsequent process.  
           [0012]    Since the nitride film  14  and the field oxide  13  each have etching selection ratios different from each other, the nitride film  14  serves as an etch stopper when the contact opening  16  is formed. However, when the nitride film  14  is used as an etch stopper, since the nitride film  14  is also deposited on the active region  12 , it applies a stress to the active region  12 , resulting in degradation of the device characteristic.  
           [0013]    [0013]FIG. 1B is a sectional view showing a structure of a borderless contact without the use of a nitride film as an etch stopper and in accordance with the conventional art.  
           [0014]    As shown in the drawing, when the contact opening  16  is formed at the interlayer dielectric film  15 , since there is no etch stopper, the active region  12  and the field oxide  13  on the substrate  10  are over-etched.  
           [0015]    Thus, in the case that an etch stopper of nitride film is not used, a problem of field recess arises due to lack of an etch stopper. To compensate for junction leakage which increases with the field recess, an ion-implantation is conducted to form a deep junction at the over-etched portion. In this case, however, its isolation characteristic is reduced.  
         SUMMARY OF THE INVENTION  
         [0016]    Therefore, an object of the present invention is to improve a device characteristic by providing a borderless contact structure in which no stress is applied to a substrate in spite of using a nitride film as an etch stopper.  
           [0017]    To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided an improved borderless contact structure including: a substrate having a shallow trench; side walls on the sides of the shallow trench; and an insulator over the side walls and in the remainder of the trench such that the insulator extends above an upper surface of the substrate. Additionally, the structure may have an oxide film having a thickness of less than 200 Å between the side wall and the substrate.  
           [0018]    The side walls may be a nitride film and the insulator may be an oxide film. Also, the side walls are formed of a first etch selection type, while the insulator is formed of a second etch selection type.  
           [0019]    The structure also includes active regions on either side of the shallow trench; gates over the substrate; and an interlayer dielectric film over the gates, active regions, insulator and substrate. Contact openings penetrate the interlayer  112  dielectric film and extend from the upper surface of the substrate to an upper surface of the interlayer dielectric film.  
           [0020]    The contact opening may be entirely over an active area. The contact opening may also be in the shallow trench and partially in the insulator. When the contact opening is partially in the insulator, the contact opening does not extend lower than the active region or an upper surface of the side wall.  
           [0021]    A second embodiment of the improved borderless contact structure includes a substrate having a shallow trench; side walls formed of a first etch selection type on sides of the shallow trench; and an insulator formed of a second etch selection type over the side walls and in the remainder of the trench. Additionally, the structure may have an oxide film having a thickness of less than 200 Å between the side walls and the substrate.  
           [0022]    The side walls may be a nitride film and the insulator may be an oxide film.  
           [0023]    The structure also includes active regions on either side of the shallow trench; gates over the substrate; and an interlayer dielectric film over the gates, active regions, insulator and substrate. Contact openings penetrate the interlayer dielectric film and extend from the upper surface of the substrate to an upper surface of the interlayer dielectric film.  
           [0024]    The contact opening may be entirely over an active area. The contact opening may also be in the shallow trench and partially in the insulator. When the contact opening is partially in the insulator, the contact opening does not extend lower than the active region or an upper surface of the side wall.  
           [0025]    Also, a portion of the insulator extends above the upper surface of the substrate.  
           [0026]    In order to achieve the above object, there is also provided a method for fabricating a borderless contact structure including: forming a shallow trench in a substrate; forming sidewalls on the sides of the shallows trench; and forming an insulator over the sidewalls in the remainder of the trench such that the insulator extends above an upper surface of the substrate. The method may also include forming a thin oxide film having a thickness less than 200 Å between the sidewalls and the trench.  
           [0027]    Photoresist may be deposited on the substrate and the trench formed in an area where a portion of the photoresist removed.  
           [0028]    The method may also include removing the photoresist and forming a gate and an active region on the substrate; and depositing an interlayer dielectric film over the gate, active region and substrate.  
           [0029]    Additionally, contact openings are formed in the interlayer dielectric film and the contact openings are filled with a conductive material. The contact openings are formed such that at least one contact opening extends from an upper surface of the interlayer dielectric film to the upper surface of the substrate. Further, a contact may be entirely over the active area or a contact opening may penetrate the insulator. When the contact opening penetrates the insulator, the sidewalls are used to prevent the contact opening from extending below the upper surface of the substrate.  
           [0030]    Another embodiment of the invention provides a method for fabricating a borderless contact structure including: forming a shallow trench in a substrate; forming sidewalls having a first etch selection type on the sides of the shallow trench; and forming an insulator having a second etch selection type over the sidewalls in the remainder of the trench. The method may also include forming a thin oxide film having a thickness less than 200 Å between the sidewalls and the trench.  
           [0031]    Photoresist may be deposited on the substrate and the trench formed in an area where a portion of the photoresist removed.  
           [0032]    The method may also include removing the photoresist and forming a gate and an active region on the substrate; and depositing an interlayer dielectric film over the gate, active region and substrate.  
           [0033]    Additionally, contact openings are formed in the interlayer dielectric film and the contact openings are filled with a conductive material. The contact openings are formed such that at least one contact opening extends from an upper surface of the interlayer dielectric film to the upper surface of the substrate. Further, a contact may be entirely over the active area or a contact opening may penetrate the insulator. When the contact opening penetrates the insulator, the sidewalls are used to prevent the contact opening from extending below the upper surface of the substrate.  
           [0034]    The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.  
         [0036]    In the drawings:  
         [0037]    [0037]FIG. 1A is a sectional view depicting a structure of a borderless contact in accordance with a conventional art;  
         [0038]    [0038]FIG. 1B is a sectional view depicting a structure of a borderless contact not using a nitride film as an etch stopper in accordance with the conventional art;  
         [0039]    [0039]FIGS. 2A through 2C depict a series of processes for forming a nitride film side wall for an etch stopper in accordance with the present invention; and  
         [0040]    [0040]FIG. 2D is a sectional view depicting a borderless contact structure in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]    The present invention pertains to an improved borderIess contact structure including a substrate with a shallow trench structure that prevents a contact from being formed below the active region of the substrate.  
         [0042]    The shallow trench structure, as shown in FIG. 2C includes a substrate  20  with a shallow trench  22  formed therein and sidewalls  23  formed on the trench sides  19 . Also, an insulator  24  is formed in the trench  22 . The shallow trench structure serves as an insulator or field region between active regions  25  (see FIG. 20) of the substrate  20 .  
         [0043]    As shown in FIG. 2D, the upper surface of the substrate  20  includes gates  28 . Gate insulators  32  are formed under the gates  28  and gate insulator side walls  27  are formed on the sides of the gates  28 . An interlayer dielectric film  26  is formed over the substrate  20 , gates  28  and insulator  24 . Also, contact openings  29  and  29 ′ are formed in the interlayer dielectric film  26 , and a conductive material  31  is formed in the contact openings  29  and  29 ′.  
         [0044]    As shown in FIG. 2D, the contact openings  29  and  29 ′ may be formed on various positions of the substrate. For example, contact opening  29 ′ is formed entirely over the active region  25  while contact opening  29 ′ is misaligned with the active region  25  and formed partially in insulator  24  and partially over the active region  25 . Sidewalls  23  prevent the field region corresponding to contact opening  29 ′ or other similarly situated field regions from forming below the active region  25 .  
         [0045]    An oxide film may be used as the insulator  24 , and the upper portion of the insulator is formed higher than the active region, as shown in FIGS. 2C and 2D. If the insulator is formed lower than the active region, the active region is reduced in the width and a threshold voltage drops rapidly, causing degradation of the device characteristic. Such a narrowed-width effect can be prevented by maintaining the field region at a higher position in the active region.  
         [0046]    Though side wall  23  is used as an etch stopper for a field oxide film in the process of the borderless contact, it exists only inside the field region. As a result, the degradation of device characteristic due to the stress applied to the silicon substrate by the nitride film as in the case of the conventional art can be improved. Further, side wall  23  may be composed of a nitride film or any other suitable etch stopper well known in the art.  
         [0047]    [0047]FIGS. 2A through 2C show a series of processes for forming the shallow trench structure in accordance with the present invention.  
         [0048]    With reference to FIG. 2A, after a photoresist  21  is deposited on a substrate  20 , the photoresist  21  is partially removed. The substrate  20  at the region where the photoresist has been exposed is etched to form a shallow trench  22  with sides  19 .  
         [0049]    Next, with reference to FIG. 2B, a side wall  23  is formed at both sides  19  of the shallow trench  22 . Before formation of the side wall  23 , to alleviate the stress between the substrate  20  and the side wall  23 , a thin oxide film  33  having a thickness of less than 200 Å may be formed in the trench structure.  
         [0050]    With reference to FIG. 2C, after an insulator  24  or field oxide film is filled in the trench  22 , etching by chemical mechanical polishing (CMP) is carried out. The insulator  24  is etched in a manner that the upper portion of the insulator  24  is maintained higher than the active region. During the CMP process, a slurry is used which selectively etches the substrate rather than the insulator  24  material. Thus, the photoresist  21  is removed and the insulator  24  remains.  
         [0051]    As shown in FIG. 2D, the active region  25  and gates including gate  28 , gate side wall  27 , and gate insulator  32  are formed on the substrate  20 . Also, an interlayer dielectric film  26  is deposited on the gates, the active region  25  and the insulator  24 . Further, contact openings  29 ,  29 ′ are formed by etching into the interlayer dielectric film  26 . If the etching of the contact opening  29  is misaligned as shown by contact opening  29 ′, the side wall  23  serves as an etch stopper. A conductive material  31  is filled in the contact openings  29 ,  29 ′, thereby completing a borderless contact structure.  
         [0052]    As described, according to the borderless contact structure of the present invention, since the side wall  23  used as an etch stopper for the field oxide film is formed only at the inside of the field region, the degradation of device characteristic due to a stress applied by the side wall  23  to the silicon substrate can be improved. In other words, the nitride film formed, as an etch stopper, only at the side wall of the field oxide film contributes to reduce the stress applied to the active region, thereby remarkably improving a device characteristic and its reliability.  
         [0053]    As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the meets and bounds of the claims, or equivalence of such meets and bounds are therefore intended to be embraced by the appended claims.