Abstract:
A method of fabricating shallow trench isolation. In the method, a refill step of oxide layer and a step of forming a sacrificial layer on the semiconductor substrate are applied after filling insulating layer into the shallow trenches. The purpose of the steps is to protect the oxide layer on the semiconductor substrate and the corner of the shallow trenches, used to isolate the STI.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of integrated circuits manufacturing technology. More particularly, the invention relates to a method for fabrication of shallow trench isolation whereby corner erosion of shallow trench isolation is avoided and the performance of the semiconductor device is thereby improved. 
     2. Description of the Related Art 
     Filling the shallow trench in the semiconductor substrate with insulating materials, such as silicon dioxide, by high density plasma-chemical vapor deposition (HDPCVD) to form shallow trench isolation (STI) has gradually replaced the conventional Local Oxidation of Silicon (LOCOS) method to become the major method of active region isolation. The method of fabricating STI in the prior art, in general, consists of defining the shallow trench first by removing part of the pad layer and semiconductor substrate after the pad layer is formed on the semiconductor substrate. Second, “pull-back” is performed on the pad layer to “pull back” the pad layer and expose part of the surface of the semiconductor substrate and its corner formed with the opening of the shallow trench. Thermal oxide film and nitride liner film are sequentially formed to cover the surface of the semiconductor substrate and the inner walls of the shallow trench. However, because of corner thinning, the thickness of the thermal oxide film formed on the corner is thinner than that formed on the surface of the semiconductor substrate or the inner walls of the shallow trench. Then the shallow trench is filled with insulating materials, such as silicon dioxide, by high-density plasma-chemical vapor deposition (HDPCVD) and so on. Then the exposed thermal oxide film and the nitride liner film are sequentially removed by isotropic etching. The rest of the thermal oxide film and the silicon nitride pad thin film are over-etched to expose the opening of the shallow trench and the corner of the surface of the semiconductor substrate. Later, after forming the gate oxide and the gate on the surface of the semiconductor substrate, current leakage is likely to occur in the gate oxide on the corner because of corner thinning. Thus the reliability of the gate is lowered, and the performance of the metal oxide semiconductor transistors is affected. The following refers to FIG. 1A to FIG. 1F showing a cross-section of STI fabrication to describe the process thereof. 
     In FIG. 1A, a semiconductor substrate  100  is provided. A pad layer  105  with a pad silicon oxide layer  102  and a pad silicon nitride layer  104  is formed on the surface of the silicon substrate  100 . Next, part of the pad layer  105  is removed to define an opening (not shown), then pad layer  105  is used as a mask to perform isotropic etching to form shallow trenches  106  in the semiconductor substrate  100 . Next, “pull-back” is performed on the pad layer  105 : an anisotropic etching is performed to remove part of the pad layer  105  around the opening of shallow trenches  106 , then part of the surface of the semiconductor substrate and its corner  107  between the opening of shallow trenches are exposed. 
     Next, in FIG. 1B, a thermal oxide film  110  is formed on the inner walls of the shallow trenches  106  and the exposed surface of the semiconductor substrate  100  by thermal oxidation. Because of corner thinning, the thickness of the thermal oxide film  110  on the inner walls of the shallow trenches  106  and the exposed surface of the semiconductor substrate  100  is greater than that on the corner  107 . 
     In FIG. 1C, pad silicon oxide layer  102  and thermal oxide film  110  are shown as a first oxide layer  112  for convenience of explanation. A nitride liner film  120  is deposited by CVD to cover the sidewalls of the pad silicon nitride layer  140  and the surface of the first oxide layer  112  in the shallow trenches  106 . 
     Next, in FIG. 1D, an insulating layer (not shown) of, for example, high-density plasma oxide (HDP Oxide), is formed to fill up the shallow trenches  106 . Then, part of the insulating layer is removed by a Deglaze step using HF-type etching agents to form a first opening  108 . The rest of the insulating layer is represented as insulating layer  130 . The insulating layer  130  is divided into the top part  134  and the bottom part  132 , because the top part  134  is formed in the space surrounded by the “pulled-back” pad layer  105 , thus its width is greater than the bottom part  132 . 
     In FIG. 1E, anisotropic etching is performed to remove the pad silicon nitride layer  104  and part of the nitride liner film  120 , thus the top part of the insulating layer  134  and its sidewalls and part of the first oxide layer  112  are exposed. Furthermore, part of the nitride liner film  120  under the top part of the insulating layer  134  is over-etched, thus the second opening  124  is formed. The rest of the nitride liner film  120  is represented as the rest of the nitride liner film  120 ′. 
     Next, in FIG. 1F, the first oxide layer  112  not covered with the rest of the nitride liner film  120 ′ is removed by isotropic etching, thus the surface of the semiconductor substrate  100  is exposed. Furthermore, part of the first oxide layer  112  under the top part of the insulating layer  134  and the rest of the nitride liner film  120 ′ is over-etched, thus the corner  107  is exposed and the third opening  142  is formed. The rest of the first oxide layer  112  is represented as the rest of the first oxide layer  112 ′. So far the conventional steps of fabricating STI are completed. 
     According to the conventional process, the corner is exposed after the pad oxide layer on the semiconductor substrate is removed. Later in the semiconductor manufacturing process, after the gate oxide and the gate are formed on the semiconductor substrate, current leakage is likely to occur at the gate oxide formed on the corner because of corner thinning. Thus the reliability of the gate is lowered, and the performance of the metal oxide semiconductor transistor is affected. 
     SUMMARY OF THE INVENTION 
     Therefore, the purpose of the invention is to provide a method of fabricating STI, in which a refill step of oxide layer and a step of forming a sacrificial layer on the semiconductor substrate are applied after filling an insulating layer into the shallow trenches. The purpose of the steps is to protect the oxide layer on the semiconductor substrate and the corner of the shallow trenches, used to isolate the STI. 
     Thus, the invention provides a method of fabricating shallow trench isolation on a semiconductor substrate, comprising: forming a pad layer on the semiconductor substrate; removing part of the pad layer to form an opening, then using the pad layer as mask to define a shallow trench in the semiconductor substrate; removing part of the pad layer around the opening of the shallow trenches to expose the surface of the semiconductor substrate thereunder and form a corner between the surface of the semiconductor substrate and the opening of the shallow trenches; forming a thermal oxide film on the surface of the semiconductor substrate exposed in the sidewalls of the shallow trenches to constitute a first oxide layer with the pad layer; forming a nitride liner film to cover the surface of the first oxide layer on the sidewalls of the shallow trenches and the pad layer on the opening of the shallow trenches; forming an insulating layer to fill the shallow trenches; forming a first opening by removing part of the insulating layer in the shallow trenches, such that the rest of the insulating layer is divided into top and bottom parts; removing the pad silicon nitride layer and the nitride liner film not covered with the top part of the insulating layer to expose the top part of the insulating layer, its sidewalls and part of the first oxide layer, such that the nitride liner film under the top part of the insulating layer is over-etched to form a second opening; forming a second oxide layer to cover the upper surface of the top part of the insulating layer and the surface of the first oxide layer exposed at the sidewalls of the top part of the insulating layer, and to fill the second opening; removing the second oxide layer and the first oxide layer not covered with the top part of the insulating layer and the surface of the semiconductor substrate is exposed, wherein part of the first oxide layer under the top part of the insulating layer is removed by over-etching, thus the third opening is formed; forming a sacrificial oxide layer on the exposed semiconductor substrate such that part of the third opening is filled to constitute a third oxide layer with the rest of the first oxide layer; forming a fourth oxide layer to cover the upper surface of the top part of the insulating layer, its sidewalls and the surface of the third oxide layer, and to fill the third opening; and removing the fourth oxide layer and part of the third oxide layer such that the upper surface of the top part of the insulating layer, its sidewalls, and the surface of the semiconductor substrate are exposed to form a shallow trench isolation comprising the top part and the bottom part of the insulating layer. 
     According to the preferred embodiment of the present invention, the semiconductor substrate is silicon or germanium. The substrate is formed by Epitaxy or silicon-on insulating materials. The insulating layer, the second oxide layer and the third oxide layer are all boron-phosphorous-silicon glass (BPSG), non-doping silicon glass (NSG), high-density plasma (HDP) oxide, or tetraethylethoxide (TEOS) by chemical vapor deposition (CVD), atmospheric-pressure CVD (APCVD), sub-atmospheric-pressure CVD (SAPCVD), low-pressure CVD (LPCVD), plasma-enhanced CVD (PECVD), or high-density plasma CVD (HDPCVD). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description, given by way of example and not intended to limit the invention solely to the embodiments described herein, will best be understood in conjunction with the accompanying drawings, in which: 
     FIGS. 1A to  1 F show cross-sections of the manufacturing process of the conventional shallow trench isolation; and 
     FIGS. 2A to  2 H show cross-sections of the manufacturing process of shallow trench isolation in accordance with the preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 2A, a semiconductor substrate, for example silicon substrate  200 , is provided. A pad layer  205  with a pad silicon oxide layer  202  and a pad silicon nitride layer  204  is formed on the surface of the silicon substrate  200 . The pad silicon oxide layer  202  about 100-600 Å thick is formed on the surface of the silicon substrate  200  by oxidization method, and the pad silicon nitride layer  204  about 1600-3000 Å thick is formed by CVD. Next, part of the pad layer  205  is removed to define an opening (not shown), then the pad layer  205  is used as a mask to perform anisotropic etching to form shallow trenches  206  in the silicon substrate  200 . Next, “pull-back” is performed on the pad layer  205 : an anisotropic etching is performed to remove part of the pad layer  205  around the opening of shallow trenches  206  and thus enlarge the opening of shallow trenches  206 , then part of the surface of the silicon substrate  200  and its corner  207  between the opening of shallow trenches  206  are exposed. Then a thermal oxide film  210  is formed on the exposed surface of silicon substrate  200  inside the shallow trenches  206  by thermal oxidation. The thickness of the thermal oxide film  210  is about 80-140 Å. For convenience, the pad silicon oxide layer  202  and the thermal oxide film  210  are represented together as a first oxide layer  212 . Then a nitride liner film  220  is deposited evenly by CVD on the surface of the first oxide layer  212  at inner walls of the shallow trenches  206  and the sidewalls of the pad layer  205  around the opening of the shallow trenches  206 . The thickness of the nitride liner film  220  is about 80-140 Å. 
     Next, in FIG. 2B, an insulating layer (not shown) of HDP Oxide is formed to fill up the shallow trenches  206  by, for example, CVD. Then, part of the insulating layer is removed by a Deglaze step using HF-type etching agents to form a first opening  208 . The rest of the insulating layer is represented as the insulating layer  230 . The insulating layer  230  is divided into the top part  234  and the bottom part  232  by position, because the top part  234  is formed in the space surrounded by the “pulled-back” pad layer  205 , thus its width is greater than the bottom part  232 . 
     Next, in FIG. 2C, etching, for example, isotropic etching, is performed to remove the pad silicon nitride layer  204  and part of the nitride liner film  220  not covered with the top part of the insulating layer  234 , thus the top part of the insulating layer  234 , its sidewalls and part of the first oxide layer  212  are exposed. Furthermore, part of the nitride liner film  220  under the top part of the insulating layer  234  is over-etched, thus the second opening  224  is formed. The rest of the nitride liner film  220  is represented as the rest of the nitride liner film  220 ′. 
     Next, in FIG. 2D, a second oxide layer  240  of about 100-200 Å is deposited on the exposed upper surface of the top part of the insulating layer  234 , its sidewalls, and the exposed surface of the first oxide layer  212 , to fill up the second opening  224 . The second oxide layer  240  of, for example, HDP Oxide, is formed by, for example, CVD. 
     FIG. 2E shows etching, for example isotropic etching, performed to remove the second oxide layer  240  and the first oxide layer  212  not covered with the top part of the insulating layer  234 , thus the surface of the silicon substrate  200  is exposed. Furthermore, part of the first oxide layer  212  under the top part of the insulating layer  234  is removed by over-etching and drawn back under the rest of the nitride liner film  220 ′, thus the third opening  324  is formed. During this etching, the top part of the insulating layer  234  is etched as well due to similar composition with the first oxide layer  212  and the second oxide layer  240 . The rest of the top part of the insulating layer is represented as the rest of the top part of the insulating layer  234 ′, and the rest of the first oxide layer  212  is represented as the rest of the first oxide layer  212 ′. The purpose of this step is to confirm that corner  207  is well covered with the first oxide layer  212  to avoid the drawback of short protection of the rest of the first oxide layer  212 ′, thereby exposing the corner  207 . 
     Next, in FIG. 2F, oxidization is performed to form a sacrificial oxide layer  250  on the exposed surface of the silicon substrate  200 . Part of the third opening  242  is filled with the sacrificial oxide layer  250 . 
     Next, in FIG. 2G, a third oxide layer  260  of about 100-200 Å is deposited on the upper surface of the rest of the top part of the insulating layer  234 ′, its sidewalls, and the surface of the sacrificial oxide layer  250 . The third opening is filled up with the third oxide layer  260 . The third oxide layer  260  of, for example, HDP Oxide, is formed by, for example, CVD. For convenience, the rest of the first oxide layer  212 ′ and the sacrificial oxide layer  250  are represented together as oxide layer  255 . 
     Next, in FIG. 2H, etching, for example an isotropic etching, is performed to remove the third oxide layer  260  and part of the oxide layer  255 , thus the upper surface of the rest of the top part of the insulating layer  234 ′, its sidewalls, and the surface of the silicon substrate  200  is exposed. The rest of the oxide layer  255  is represented as the rest of the oxide layer  255 ′. The purpose of this step is to confirm that the corner  207  is covered with the rest of the oxide layer  255 ′ to avoid the drawbacks of the prior art. During this etching, the top and the sidewalls of the rest of the top part of the insulating layer  234 ′ is etched as well due to similar composition with the third oxide layer  260  and the oxide layer  255 , thus its width and height are slightly reduced. Eventually, the rest of the top part of the insulating layer  234 ′ is cut at the same level with the rest of the oxide layer  255 ′ and the sidewalls of the rest of the nitride liner film  220 ′. The rest of the top part of the insulating layer  324 ′, together with the bottom part of the insulating layer  232 , composes the shallow trench isolation (STI). So far the fabrication of the STI is completed. 
     While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.