Abstract:
The invention provides a method for forming a bottle-shaped trench. A semiconductor substrate having a trench and a pad stack layer formed thereon is provided. A masking layer is then formed in the lower portion of the trench. Plasma nitridation is then performed to form a nitride layer covering the sidewalls of the trench, followed by removing the masking layer to expose the sidewalls of the trench. The lower portion of the trench is then expanded by etching to form a bottle-shaped trench.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a bottle-shaped trench fabrication method, and more particularly to a fabrication method for forming a bottle-shaped trench by plasma nitridation. 
   2. Description of the Related Art 
   Generally speaking, capacitors widely used in dynamic random access memory (DRAM) are formed by two conductive layers (electrode plates) having an insulation layer therebetween. The ability to store the electric charge of a capacitor depends on the thickness and electrical characteristics of the insulation layer, and the surface area of the electrode plate. Recent developments to reduce the size of semiconductor elements to enhance integration density require the memory cell area to be continuously reduced to hold a large number of memory cells, and thereby increase density. The electrode plates of a capacitor in a memory cell however must provide sufficient surface area to store sufficient electric charge. 
   Nevertheless, when element size is continuously reduced, the trench storage node capacitance of DRAM is also reduced. As a result, storage capacitance must be increased to maintain excellent memory operating performance. 
   Currently, the method for increasing DRAM storage capacitance increases the width of the bottom of a trench, thereby increasing surface area by forming bottle-shaped trench capacitors. 
     FIGS. 1A to 1K  are cross-sections showing the conventional method for forming a bottle-shaped trench. In  FIG. 1A , a substrate  100  with a trench therein is provided, a pad layer is formed on the substrate  100 , and the pad layer comprises an oxide layer  102  and a nitride layer  104 . A sidewall oxide layer  106  is formed on the trench sidewall by thermal oxidation. A protective nitride layer  108  and a polysilicon layer  110  are formed thereon by chemical vapor deposition (CVD). In  FIG. 1B , the polysilicon layer  110  is oxidized forming a protective oxide layer  120 . In  FIG. 1C , a mask layer  122  is formed to cover the lower portion of the trench, the mask layer  122  comprises, for example, a photoresist material. In  FIG. 1C , the protective oxide layer  120  not protected by the mask layer  122 , is removed to form a protective oxide layer  120 ′. Then the mask layer  122  is removed as in  FIG. 1E . In  FIG. 1F , a sidewall nitride layer  124  is formed on the upper portion of the trench by nitridation. The protective oxide layer  120 ′ is then removed as shown in  FIG. 1G . In  FIG. 1H , the protective nitride layer  108  is then removed to form a protective oxide layer  108 ′. The sidewall nitride layer  124  is removed as shown in  FIG. 1I . In  FIG. 1J , the lower portion of sidewall oxide layer  106  is removed to form a sidewall oxide layer  106 ′. In  FIG. 1K , the lower portion of the trench is etched to form a bottle-shaped trench by wet etching. 
   The above-mentioned bottle-shaped trench fabrication method is complicated, and due to the continuous reduction in semiconductor scaling, this fabrication method is progressively more complicated. Therefore, a simple fabrication method for forming the bottle-shaped trench is necessary. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the invention is to provide a simple method for forming a bottle-shaped trench. 
   To achieve the above object, the invention provides a method for forming a bottle-shaped trench comprising the following steps, providing a substrate with a trench and a pad layer thereon, filling a mask layer in the lower portion of the trench, using plasma nitridation to form a sidewall nitride layer on the trench sidewall, removing the mask layer, etching the sidewall and bottom of the trench not isn&#39;t protected by the sidewall nitride layer, to obtain the bottle-shaped trench. 
   To achieve the above object and improve the nitride layer resistance, the invention further provides a method for forming a bottle-shaped trench comprising an added step before plasma nitridation of etching a small portion of pad oxide near the trench to reveal a corner of the substrate. Due to this treatment, the nitride layer has better adhesion during the sequent etching process. 

   
     DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, reference is made to a detailed description to be read in conjunction with the accompanying drawings, in which: 
       FIGS. 1A to 1K  are cross-sections showing the conventional method for a forming bottle-shaped trench; 
       FIGS. 2A to 2E  are cross-sections showing a method for forming a bottle-shaped trench according to the first embodiment of the present invention; 
       FIGS. 3A to 3F  are cross-sections showing a method for forming a bottle-shaped trench according to the second embodiment of the present invention; and 
       FIG. 4  is a schematic showing the plasma nitridation reactor according to the embodiment of the present invention. 
   

   REFERENCE NUMERALS IN THE DRAWINGS 
     4  RF electrode 
     6  exhaust 
     8  magnet ring 
     10  reactor 
     12  gas inlet 
     14  gas injection plate 
     16  plasma 
     100 ,  200 ,  300  substrate 
     102 ,  202 ,  302  oxide layer 
     104 ,  204 ,  304  nitride layer 
     106  sidewall oxide layer 
     108  protective nitride layer 
     110  polysilicon layer 
     120  oxidative polysilicon layer 
     122 ,  222 ,  322  mask layer 
     124 ,  230 ,  330  sidewall nitride layer 
     140 ,  240 ,  340  bottle-shaped trench 
     225 ,  325  trench 
   DETAILED DESCRIPTION OF THE INVENTION 
   FIRST EMBODIMENT 
     FIGS. 2A to 2E  are cross-sections showing a method for forming a bottle-shaped trench according to the first embodiment of the present invention. 
   In  FIG. 2A , a substrate  200  is provided. A pad stack layer is then formed on the substrate  200  by CVD. The pad stack layer is comprised of an oxide layer  202  and a nitride layer  204 . Thickness of the oxide layer  202  is 20˜100 Å and the nitride layer  204  is 1000˜3000 Å. The pad stack layer is used as an etching stop layer for dry etching or chemical mechanical polishing (CMP). An opening is then formed in the pad stack layer by lithography and dry etching. The dry etching method comprises reactive ion etching (RIE) and plasma etching. Next, a trench  225  is formed by plasma etching. 
   In  FIG. 2B , a mask layer  222  is filled to the trench  225 . The mask layer  222  is a photoresist layer. Next, the mask layer  222  is etched to a level below the substrate surface 1000˜3000 nm by dry etching back. 
   A key point of the present invention is described in the following. In  FIG. 2C , a sidewall nitride layer  230  is formed on the exposed trench sidewall by plasma nitridation. The thickness of the sidewall nitride layer  230  is 40˜50 Å. Therefore fabrication of the sidewall nitride layer protecting the upper portion of the trench in the present invention is achieved in one step and is simpler than the conventional method. 
   In  FIG. 2D , the mask layer  222  is removed by the conventional method. 
   In  FIG. 2E , the lower portion of the trench is etched to form a bottle-shaped trench  240  by wet etching. The wet etching solution is ammonia (NH 4 OH+H 2 O). The bottle-shaped trench of the present invention is thus obtained. 
   The key point of the present invention is plasma nitridation step. Nitridation is executed by modified magnetron type nitridation (MMT nitridation) and the reactor is shown in  FIG. 4 . In the reactor  10 , a plasma  16  is induced by an RF electrode  4 . Preferably, the N source gas is selected from the group consisting of N 2 , NO, N 2 O, NH 3  and combinations thereof, or the above-mentioned gas mixes with He, Ne, Ar, Kr, or Xe. The N source gas enters the reactor  10  from the gas inlet  12  and gas injection plate  14 , passes through the RF electrode  4  the plasma  16  is then induced and deposited on the wafer, then exhausted from the exhaust  6 . Preferably, the plasma nitridation temperature is 25˜100° C. When the temperature is above 100° C., the photoresist materials of the mask layer may evaporate. Preferably the pressure range is 30˜50 Pa and the RF power is 500˜1000 W. 
   SECOND EMBODIMENT 
     FIGS. 3A to 3F  are cross-sections showing a method for forming a bottle-shaped trench according to the second embodiment of the present invention. 
   In  FIG. 3A , a substrate  300  is provided. A pad stack layer is then formed on the substrate  300  by CVD. The pad stack layer is comprised of an oxide layer  302  and a nitride layer  304 . Thickness of the oxide layer  302  is 20˜100 Å and the nitride layer  304  is 1000˜3000 Å. The pad stack layer is used as an etching stop layer for dry etch or chemical mechanism polishing (CMP). An opening is then formed in the pad stack layer by lithography and dry etching. The dry etching method comprises reactive ion etching (RIE) and plasma etching. Next, a trench  325  is formed by plasma etching. 
   In  FIG. 3B , a mask layer  322  is filled into the trench  325 . The mask layer  322  is a photoresist layer. Next, the mask layer  322  is etched to a level below the substrate surface 1000˜3000 nm by dry etching back. 
   The following step is the key difference between this and the first embodiment. In  FIG. 3C , a portion of the pad oxide  302  is etched by diluted HF (DHF) to reveal the substrate corner. The undercut is then filled with nitride layer by subsequent plasma nitridation. The nitride layer with this structure achieves better adhesion during the subsequent etching process. 
   Another key point of the present invention is described in the following. In  FIG. 3D , a sidewall nitride layer  330  is formed on the exposed trench sidewall and the pad oxide undercut by plasma nitridation. The plasma nitridation is executed by modified magnetron type nitridation using the same equipment as the first embodiment. Preferably, the nitridation N source gas is selected from the group consisting of N 2 , NO, N 2 O, NH 3 , and combinations thereof, or the above-mentioned gas mixed with He, Ne, Ar, Kr, or Xe. Preferably the plasma nitridation temperature is 25˜100° C. When the temperature is above 100° C., the PR materials of the mask layer may evaporate. The pressure range is 30˜50 Pa and the RF power is 500˜1000 W preferably. The thickness of the sidewall nitride layer  330  is 40˜50 Å. Therefore fabrication of the sidewall nitride layer protecting the upper portion of the trench in the present invention is achieved in one step and is simpler than the conventional method. 
   In  FIG. 3E , the mask layer  322  is removed by the conventional method. 
   In  FIG. 3F , the lower portion of the trench is etched to form a bottle-shaped trench  340  by wet etching. The wet etching solution is ammonia (NH 4 OH+H 2 O). The bottle-shaped trench of the present invention is thus obtained. 
   While the invention has been described by way of examples and in terms of the two preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To 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.