Abstract:
A method for sidewall etching includes providing a substrate having a trench defined therein, with the trench having fill material disposed over a bottom thereof, along a sidewall thereof, and at the trench opening. The fill material along the sidewall of the trench and at the trench opening is removed without removing the fill material disposed over the bottom of the trench. The fill material along the sidewall and at the trench opening may be removed without removing the fill material disposed over the bottom of the trench by inhibiting a reaction between an etchant and the fill material over the bottom of the trench. The reaction between the etchant and the fill material may be inhibited by causing an air bubble to form at the bottom of the trench. The air bubble may be formed by inverting the substrate, and immersing the inverted substrate in an etchant.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to semiconductor fabrication and, more particularly, to a method for sidewall etching, a method for etching during filling of a trench, and a method for semiconductor structure fabrication by shallow trench isolation in which a trench feature is formed in a substrate. 
   2. Description of the Related Art 
   In the fabrication of semiconductor devices, integrated circuit structures are typically fabricated in the form of multi-layer, also known as multi-level structures. Beginning at the substrate layer or level, transistor devices having diffusion regions are formed over and into silicon substrates. In subsequent layers, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials such as silicon dioxide. The insulating layers essentially form a substrate layer or level in a multi-layer structure. As used herein, the term “substrate” includes both a base substrate such as a silicon semiconductor wafer, and any substrate layer or level of a multi-layer structure. 
   In the fabrication of features in and on the substrate, trenches are typically formed in one material, e.g., a silicon substrate, polycrystalline silicon, metal or a dielectric substrate layer or level, and then filled with another material. An exemplary process is shallow trench isolation (STI) in which trenches are formed to define an active area of a device such as a well, a transistor, a memory cell, etc. Trenches are formed in the substrate to isolate a region that may eventually be doped to define a device. The trenches are filled with another, and often a different, dielectric material or a metal as the structures are fabricated. 
     FIGS. 1A ,  1 B, and  1 C illustrate a typical STI fabrication process. As shown in  FIG. 1A , trenches  12  are formed in a substrate  10 . For ease of illustration, substrate  10  in  FIGS. 1A ,  1 B, and  1 C is illustrated as a silicon substrate  10  such as a semiconductor wafer. As is known, fabrication in subsequent substrate levels or layers may or may not include all of the process steps illustrated and described herein, and depending on fabrication process utilized, may or may not include additional processing operations not illustrated or described herein. 
   Returning to  FIG. 1A , a pad oxide layer  14  is formed over the substrate  10 , and a nitride layer  16  is formed over the pad oxide layer  14 . By way of example, nitride layer  16  might be a layer of SiN. Trenches  12  have been formed through the nitride layer  16  and the pad oxide layer  14 , and into the substrate  10 . 
     FIG. 1B  shows a next process step of the STI fabrication process begun in  FIG. 1A . In  FIG. 1B , a liner  18  is shown in trenches  12 . In some applications, the liner  18  is an oxide or other material grown inside the trench  12 . Depending on the substrate, and the material, the liner  18  may or may not be used. 
     FIG. 1C  shows a fill layer  20  deposited over the structure filling trenches  12  and covering nitride layer  16 . The fill layer  20  is typically an oxide or metal, such as tetraethylorthosilicate (TEOS), oxide formed by using high density plasma (HDP), silicon dioxide, and the like, and may be deposited over the structure by chemical vapor deposition (CVD). Once the fabrication has reached the stage represented in  FIG. 1C , processing continues to remove the fill layer  20 , the nitride layer  16 , and the pad oxide layer  14 , leaving the trench  12  filled and prepared for continued structure fabrication. 
   A conventional technique for filling the trench  12  formed in substrate  10 , e.g., the trench  12  formed in STI fabrication operations, includes CVD of a dielectric material or of a conductive material. Due to the topography of the trench  12 , however, material deposited usually accumulates at the opening of the trench  12 , inhibiting the filling.  FIG. 2A  shows a detail view of a substrate  10  having a trench  12  formed therein, and a partially-deposited fill layer  20 . A liner  18  has been formed in trench  12 , and fill layer  20  has been deposited to fill trench  12 . As can be seen in  FIG. 2A , deposition of fill layer  20  results in significant narrowing at trench opening  22 . Continued deposition of fill layer  20  may or may not result in complete filling of trench  12 , and to prevent incomplete filling of trench  12  and formation of unacceptable voids in the fill layer  20 , additional processing to widen trench opening  22  is often required. 
     FIG. 2B  illustrates the formation of a void  24  within the fill layer  20  in trench  12  if additional processing to widen trench opening  22  is not accomplished. Typically, a process such as dry etching is utilized to remove the material that narrows trench opening  22 , so that filling can further proceed. However, when etching the material on the sidewall and trench opening  22 , the material formed at the bottom of the trench  12  is inevitably removed as well. Therefore, dry etching is not an ideal method to enhance or facilitate the access to a formed trench and enable trench filling. 
   As process technology evolves, resulting in smaller features and more complex and dense semiconductor structures fabricated in and on substrates, trenches become smaller and narrower, and trench openings become increasingly susceptible to blockage during trench-filling processes. As is well known, voids in trench filling material are unacceptable. As such, a method of etching to be used in trench-filling processes that enables sufficient access for complete trench-filling without voids and without removal of desired material at the bottom of the trench is needed. 
   SUMMARY OF THE INVENTION 
   Broadly speaking, the present invention fills this need by providing a method for sidewall etching that improves access to trench structures or features without removing deposited material at a bottom of a trench or gap. Several exemplary embodiments of the present invention are described below. 
   In accordance with one aspect of the present invention, a method for etching during filling of a trench is provided. In this method, a semiconductor substrate having a trench defined therein and a fill layer deposited over the substrate and at least partially filling the trench is provided. The semiconductor substrate is inverted, and the inverted substrate is immersed in an etchant to remove portions of the fill layer on the sidewall and at an opening of the trench. When the inverted substrate is immersed in the etchant, air is trapped at a bottom of the trench. This trapped air inhibits a reaction between the etchant and a portion of the fill layer at the bottom of the trench such that the portion of the fill layer at the bottom of the trench remains after the portions of the fill layer on the sidewall and at the opening of the trench have been removed. 
   In accordance with another aspect of the present invention, a method for sidewall etching is provided. In this method, a substrate having a trench defined therein is provided. The trench has fill material disposed over a bottom of the trench, along a sidewall of the trench, and at the trench opening. The fill material along the sidewall of the trench and at the trench opening is removed without removing the fill material disposed over the bottom of the trench. In one embodiment, the removing of the fill material along the sidewall of the trench and at the trench opening without removing the fill material disposed over the bottom of the trench includes inhibiting a reaction between an etchant and the fill material over the bottom of the trench. In one embodiment, the inhibiting of the reaction between the etchant and the fill material includes causing an air bubble to form at the bottom of the trench. 
   In accordance with yet another aspect of the present invention, a method for semiconductor feature fabrication by shallow trench isolation in which a trench feature is formed in a substrate is provided. In this method, a fill layer is deposited over the substrate and into the trench feature. The fill layer at least partially fills the trench feature and is deposited such that a trench opening and a trench sidewall includes fill material restricting access into an interior of the trench feature and preventing complete filling of the trench feature without unacceptable voids in the fill material. The fill layer is etched to remove the fill layer from over the substrate at the trench opening and along the trench sidewall without removing the fill layer at the bottom of the trench feature. Thereafter, additional fill material is deposited over the substrate to completely fill the trench feature with fill material free of unacceptable voids. In one embodiment, the etching of the fill layer includes causing an air bubble to form at the bottom of the trench feature, with the air bubble preventing the etching of the fill layer at the bottom of the trench feature. In one embodiment, the etching of the fill layer includes inverting the substrate, and immersing the inverted substrate in an etchant. 
   The present invention provides a number of significant advantages. One notable benefit and advantage of the invention is the ability to etch away material blocking a trench opening without removing the material at the bottom of the trench. As features continue to be fabricated smaller and more densely populating a semiconductor substrate, susceptibility to blocked trenches or gaps when depositing a fill layer increases. In accordance with embodiments of the present invention, intermediate etching of the fill layer at a trench opening, and at least part of the trench sidewall, is accomplished while the fill material at the bottom of the trench is protected from etching. Subsequent deposition of fill material can completely fill the trench without unacceptable voids. 
   Other advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the principles of the invention. 
       FIG. 1A  illustrates a typical shallow trench isolation fabrication process. 
       FIG. 1B  shows a next process step of the shallow trench isolation fabrication process begun in  FIG. 1A . 
       FIG. 1C  shows a fill layer deposited over the structure filling the trenches and covering the nitride layer. 
       FIG. 2A  shows a detail view of a substrate having a trench formed therein, and a partially-deposited fill layer. 
       FIG. 2B  illustrates the formation of a void within the fill layer in the trench if additional processing to widen the trench opening is not accomplished. 
       FIG. 3A  shows a substrate for fabrication in accordance with one embodiment of the present invention. 
       FIG. 3B  shows etching of the partially deposited fill layer in accordance with one embodiment of the present invention. 
       FIG. 3C  shows the substrate following etching of partially deposited fill layer in accordance with an embodiment of the invention. 
       FIG. 4  is a flow chart diagram illustrating the method operations performed when etching a trench opening and a trench sidewall in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Several exemplary embodiments of the methods of the present invention will now be described with reference to the accompanying drawings.  FIGS. 1A-1C  and  2 A- 2 B have been discussed above in the “Background of the Invention” section. 
   As described above, in the fabrication of semiconductor structures, trenches or gaps are often defined in a substrate such as those features defined in shallow trench isolation (STI) processes. STI features and processes are used herein as exemplary features and processes. It should be understood that any other fabrication process in which trenches, gaps, interconnect lines, holes, and the like are formed or defined in a substrate can benefit from the present invention. The terms “trench” and “trenches” as used herein should be understood to include gaps, interconnect lines, holes, and any manner of similar feature. 
   Once defined in a substrate of one type of material, e.g., silicon or a desired dielectric, the trench is filled with another type of, or a different, material, e.g., a desired dielectric, polysilicon, or metal. During the trench-filling process, however, trench-filling material commonly builds up on the sidewall of the trench and at the trench opening resulting in incomplete trench filling, or voids within the trench-filling material. Embodiments of the present invention provide for methods of intermediate etching of the structure in which trenches are to be filled to enable complete trench-filling without removal of deposited material at the bottom of the trench. 
   In one embodiment of the present invention, a substrate in which semiconductor structures are defined that include trenches to be filled is provided. The trenches may or may not include a liner layer within the trench, and the substrate surface may or may not include multiple layers such as nitride layers, barrier layers, and the like through which the trenches are defined into the substrate. The substrate is provided with a trench-filling layer at least partially deposited thereon. As is well known, a trench-filling layer is deposited over the substrate to fill the trenches. 
   An exemplary process utilized to deposit the trench-filling layer is chemical vapor deposition (CVD). Exemplary materials deposited for trench-filling by CVD include dielectrics such as tetraethylorthosilicate (TEOS), silicon dioxide (SiO 2 ), borophosphosilicate glass (BPSG), oxide formed by high density plasma (HDP), and the like, and conductive materials including polysilicon, and metals such as tungsten, copper, aluminum, titanium, and the like. 
   In one embodiment, after trench-filling material is at least partially applied so that the trenches are partially filled, the substrate is immersed in a tank filled with an etchant so that the trench-filling layer is in contact with the etchant to cause a reaction. The substrate is inverted so that the trench openings face the etchant in which the substrate is immersed. The trench-filling material is thus removed from the trench or gap openings and the sidewall at or near the gap openings. The material in the bottom of the trench is protected from etching by an air bubble that is trapped within the trench when the inverted substrate is immersed in the etchant. Preferred etchants are selected depending on the trench-filling material to be etched and the desired selectivity to any barrier that may exist, e.g., the substrate material, and include diluted HF, H 2 O 2 , H 3 PO 4 , or a mixture of HCl and HNO 3 . 
     FIG. 3A  shows a substrate  100  for fabrication in accordance with one embodiment of the present invention. Substrate  100  includes a trench  110  formed therein. Trench  110  is formed in accordance with known semiconductor fabrication processes for semiconductor manufacture of which STI is an exemplary process. A barrier  112  is shown formed in trench  110 , and a partially deposited fill layer  114  is shown deposited over substrate  100  and into trench  110 . The partially deposited fill layer  114  is shown to be partially filling trench  110  with a void remaining within trench  110 , and the accumulated fill material narrowing the opening of trench  110  at the top indicated at  116 . 
   Following the partial deposition of fill layer  114 , e.g., by a process such as CVD, a method of etching the partially deposited fill layer  114  to enhance access to the trench opening  116  and enable complete filling of trench  110  without unacceptable voids is provided. It should be understood that material deposited by CVD or other semiconductor fabrication process may form miniscule or inconsequential voids within the material. As used herein, the phrase “unacceptable voids” refers to voids that result in a completed structure being rendered defective, faulty or otherwise unacceptable due to a void in the material. In one embodiment, the opening of a trench feature, and the upper sidewall of the trench feature are etched while the fill material at the bottom of the trench is protected from etching to enable complete filling of the trench feature in subsequent deposition of fill material. 
     FIG. 3B  shows etching of the partially deposited fill layer  110  in accordance with one embodiment of the present invention. Substrate  100  is inverted and immersed in an etchant  118 . Suitable etchants are selected depending on the material to be etched, any desired selectivity to barrier, liner, and substrate material, and other known considerations, and include diluted HF, H 2 O 2 , H 3 PO 4 , or mixture of HCl and HNO 3 . Other suitable etchants are readily available and known to those skilled in the art. In one embodiment, the inverted substrate  100  is positioned in etchant  118  so that partially deposited fill layer  114  is in contact with etchant  118 . The contact between the partially deposited fill layer  114  and the etchant  118  results in a reaction between the material of the partially deposited fill layer  114  and the etchant  118 , etching the partially deposited fill layer  114 . 
   Within trench  110  of inverted substrate  100 , a bubble  120  forms protecting the fill material at the bottom of the trench  110 . As illustrated in  FIG. 3B , the substrate  100  is inverted. The bottom of the trench  110  is therefore in an upper region of inverted substrate  100  opposite the trench opening  116 . Turning back to  FIG. 3A , the trench opening  116  opens from a surface of substrate  100  into the ambient environment. The trench  110  extends into the substrate  100  creating a space or void. When the substrate  100  is inverted and immersed in a fluid as shown in  FIG. 3B , the air within the space or void is trapped as a bubble  120  against the bottom of the trench  110 . 
   In one embodiment, bubble  120  protects fill material at the bottom of trench  110  from etchant  118 . As the etchant  118  effects etching of partially deposited fill layer  114 , the fill material at trench opening  116  and the sidewall of trench  110  near trench opening  116  are etched which facilitates access to trench  110 . Fill material at the bottom of trench  110  remains intact or as originally deposited. Subsequent deposition of fill material can completely fill trench  110  without unacceptable voids. 
     FIG. 3C  shows substrate  100  following etching of partially deposited fill layer  114  in accordance with one embodiment of the invention. Substrate  100  has been removed from etchant  118  (see  FIG. 3B ) and remains inverted. Fill layer  114  remains only at the bottom of trench  110 , having been protected from etching by bubble  120  (see  FIG. 3B ). Fill material has been etched away from trench opening  116 , as well as the upper sidewall of trench  110  to facilitate access to interior of trench  110 . Subsequent deposition of fill material can completely fill trench  110  without voids. 
     FIG. 4  is a flow chart diagram  150  illustrating the method operations performed when etching a trench opening and trench sidewall in accordance with one embodiment of the present invention. The method begins with operation  152  in which a substrate for semiconductor manufacture is provided. The substrate, as described above, may also be a substrate layer or level of a multi-layer semiconductor structure. In the substrate, a trench feature is fabricated and a fill layer is deposited to partially fill the trench feature. An exemplary trench feature is a trench as fabricated in STI semiconductor manufacturing processes. The trench feature is typically fabricated into the substrate, which can be silicon, a dielectric material, metal, etc. 
   The fill layer is typically deposited over the substrate and into the trench feature by a manufacture or fabrication process such as CVD. The fill layer can be of any material as desired, and is usually a dielectric material such as TEOS, SiO 2 , BPSG, oxide formed by HDP, a conductive material such as polysilicon, or a metal such as tungsten, aluminum, copper, and titanium. When the fill material is deposited over the substrate and into the trench, the opening of the trench significantly narrows as the fill material builds up on the sidewall and edges of the trench at the trench opening. 
   The method continues with operation  154  in which an etchant is selected to react with the fill layer. If a barrier has been fabricated within the trench feature, the etchant that is typically selected is selective to the barrier. In one embodiment, the fill layer is partially deposited over the substrate and into the trench feature. The opening of the trench feature is significantly narrowed from build up of the fill material on the sidewall and on the edges at the trench opening. Continued deposition of fill material can result in unacceptable voids and incomplete filling of blocked trench openings of the trench features, and an etchant is selected to etch the fill material and facilitate access to an interior region of the trench feature and enable complete filling without unacceptable voids. Preferred etchants are selected depending on the trench-filling material to be etched and the desired selectivity to the barrier, the substrate or substrate level or layer material, and include diluted HF, H 2 O 2 , H 3 PO 4 , or a mixture of HCl and HNO 3 . 
   Next, in operation  156 , the substrate is immersed in the etchant with the trench feature facing downward. The substrate, with the trench feature fabricated therein, is inverted and immersed in the etchant so that the fill layer is in contact with, and therefore reacts with, the etchant. Because the trench feature is fabricated into the substrate, when the substrate is inverted and immersed in the etchant, air is trapped as a bubble at the bottom of the trench. The bubble blocks the etchant from contact with the fill material at the bottom of the trench feature, and the fill material remains intact as deposited at the bottom of the trench. 
   The method concludes with operation  158  in which the substrate is removed from the etchant for continued semiconductor manufacture. In one embodiment, the continued manufacture includes subsequent deposition of the fill layer over the fill material remaining at the bottom of the trench. In this manner, the trench feature is completely filled without unacceptable voids. With the removal of the substrate from the etchant, the method is done. 
   In summary, the present invention provides a method for sidewall etching, a method for etching during filling of a trench, and a method for semiconductor structure fabrication by shallow trench isolation in which a trench feature is formed in a substrate. The invention has been described herein in terms of several exemplary embodiments. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention. The embodiments and preferred features described above should be considered exemplary, with the scope of the invention being defined by the appended claims and their equivalents.