BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a semiconductor process, more specifically, to a method for forming a deep trench capacitor of a semiconductor memory. 
   2. Description of the Prior Art 
   Deep trench capacitors have been widely employed in the conventional semiconductor memory (such as DRAM) process, in order to reduce the occupied areas and to increase capacitances.  FIGS. 1   a  and  1   b  present parts of a conventional deep trench capacitor process. The deep trench structure in  FIG. 1   a  comprises a substrate  11  usually being a silicon substrate, a pad oxide layer  12 , a pad nitride layer  13  usually being SiN, a trench  14 , a buried plate (BP)  15  (i.e., the bottom electrode), a dielectric layer  16  usually containing nitride and oxide, a conductive layer  17  used as an upper electrode and usually being polysilicon, and an insulation layer  18 , which is oxide usually and is used to form a collar oxide layer in the trench  14  in the subsequent process. Later, in  FIG. 1   b , the insulation layer  18  on the pad nitride layer  13  and the conductive layer  17  is removed by dry etching, for example, and the trench  14  is filled with the conductive layer  19 , which is used as a connection electrode, by deposition and etching, for example, to engage the conductive layer  17 . Then, a buried strap (BS) (not shown) is formed on the conductive layer  19  to engage an adjacent transistor (not shown), which is subsequently formed. 
   However, as shown in  FIG. 1   c , the aforementioned process would generates a conductive layer  17 ′ with a recess and thereby generate an insulation layer  18 ′ with a recess. Later in  FIG. 1   d , part of the insulation layer  18 ′ remains in the recess of the conductive layer  17 ′ after the removing of the insulation layer  18 ′ on the pad nitride layer  13  and the conductive layer  17 ′, and creates poor electrical connection between the conductive layer  19 ′, which is subsequently formed, and the conductive layer  17 ′, while the conductive layer  19 ′ comprises a recess as well. In an extreme situation, the residual insulation layer  18 ′ left in the recess still connects with the insulation layer  18 ′ on the sides of the trench, so that the electrical connection cannot be formed by the conductive layer  19 ′ and the conductive layer  17 ′ and the deep trench capacitor is ineffective. 
   Therefore, a need for overcoming the above problem is required. The present invention fulfils this need. 
   SUMMARY OF THE INVENTION 
   A purpose of the present invention is to provide a method for forming a trench capacitor, which can prevent the insulation layer acting as the collar oxide layer from remaining in the conductive layer acting as the upper electrode, so that a good electrical connection between the conductive layer used as the upper electrode and the conductive layer used as the connection electrode can be formed. 
   Another purpose of the present invention is to provide a method for forming a trench capacitor, which does not require the dry etch to remove the unnecessary collar oxide layer/insulation layer on the upper electrode conductive layer, so that the process is simplified. 
   In accordance with an aspect of the present invention, a method for forming a trench capacitor comprises steps of providing a substrate; forming a mask layer of a predetermined pattern on the substrate to expose a portion of the substrate; forming a trench in the exposed portion of the substrate; forming a conductive diffusion region in the substrate at the periphery of the lower portion of the trench; forming a dielectric layer on the surface of the trench; filling the trench with a photoresist layer, and making the top of the photoresist layer at least not lower than the top of the conductive diffusion region; removing the dielectric layer on the surface of the trench not covered by the photoresist layer; forming a low temperature oxide layer on the surface of the trench not coverd by the photoresist layer; removing the photoresist layer in the trench; and filling the trench with a conductive layer and making the top of the conductive layer lower than the bottom of the low temperature oxide layer. 
   In accordance with another aspect of the present invention, a method for forming a trench capacitor comprises steps of providing a substrate; forming a mask layer of a predetermined pattern on the substrate to expose a portion of the substrate; forming a trench in the exposed portion of the substrate; filling the lower portion of the trench with a photoresist layer; forming a low temperature oxide layer on the surface of the trench not covered by the photoresist layer; removing the photoresist layer in the trench; forming a conductive diffusion region in the substrate at the periphery of the portion of the trench not covered by the low temperature oxide layer; forming a dielectric layer on the surfaces of the trench and the low temperature oxide layer; and filling the trench with a conductive layer, and making the top of the conductive layer lower than the bottom of the low temperature oxide layer. 
   In accordance with another aspect of the present invention, in the aforementioned methods, the top of the conductive diffusion region or the photoresist is lower than the bottom of a transistor adjacent to the trench and subsequently formed. 
   In accordance with another aspect of the present invention, in the aforementioned methods, the low temperature oxide layer is a liquid phase deposition layer 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following drawings are not drawn according to practical dimensions and ratios and are only for illustrating the mutual relationships between the respective portions. In addition, the like reference numbers indicate the similar elements. 
       FIGS. 1   a  to  1   d  are the schematical sectional drawings illustrating the respective steps of part of the conventional process for forming a deep trench capacitor. 
       FIGS. 2   a  to  2   d  are the schematical sectional drawings illustrating the respective steps of the process for forming a deep trench capacitor in accordance with the first embodiment of the present invention. 
       FIGS. 3   a  to  3   d  are the schematical sectional drawings illustrating the respective steps of the process for forming a deep trench capacitor in accordance with the second embodiment of the present invention. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The embodiments of the present invention will be described in detail with reference to the accompanying drawings  FIGS. 2   a  to  2   d  and  3   a  to  3   d , the schematical sectional drawings illustrating the respective steps of the process for forming a deep trench capacitor in accordance with the present invention. 
   Please refer to  FIG. 2   a . On the substrate  21 , which is usually a silicon substrate, a pad oxide layer  22  is formed by thermal process, for example, and a pad nitride layer  23  usually being SiN is formed by deposition process, for example. These layers as a whole are used as a mask layer in the subsequent process, and a predetermined pattern is formed by etching, for example, in these layers to expose a portion of the substrate. Then, a trench  24  is formed by etching, for example, in the exposed portion of the substrate, and then a conductive diffusion region  25  is formed in the substrate at the periphery of the lower portion of the trench  24  by Gas Phase Diffusion (GDP) or by the following steps: forming an arsenic silicon glass (ASG) layer (not shown) on the surface of the lower portion of the trench  24  by deposition and photoresist etching, for example; applying a TEOS oxide layer (not shown) to protect the portion not covered by the ASG layer; and implementing drive-in to the ASG layer by heat treatment, for example, to form the conductive diffusion region  25  which is used as the lower electrode of the capacitor, also referred to as buried plate (BP). The top of the conductive diffusion region  25  is lower than the bottom of a transistor adjacent to the trench  24  and subsequently formed, to prevent the conductive diffusion  25  improperly contacting the adjacent transistor. The TEOS and ASG layers are then removed. 
   In  FIG. 2   b , a dielectric layer  26  usually being nitride is formed on the surfaces of the whole structure by deposition, for example, and a photoresist layer  27  is formed in the trench  24  by coating and etching, for example. The top of the photoresist layer  27  is at least not lower than the top of the conductive diffusion region  25  so that the top of the dielectric layer  26  finally formed is not lower than the top of the conductive diffusion region  25 , and the improper contact between the conductive diffusion region  25  of the lower electrode of the capacitor and the upper electrode subsequently formed in the trench  24  can be prevented. 
   As shown in  FIG. 2   c , the dielectric layer  26  on the surface of the trench  24  not covered by the photoresist layer  27  is removed by etching, for example, and a liquid phase deposition (LPD) oxide layer  28  used as a collar oxide layer is formed on the surface of the trench  24  not covered by the dielectric layer  26 . LPD characterizes in that it will not be formed on the photresist and can be implemented in a low temperature like the room temperature. Then the photoresist layer  27  is removed and an oxide layer (not shown) can be formed on the surface of the dielectric layer  26  by implementing heat treatment. 
   Please refer to  FIG. 2   d . The trench  24  is filled with a conductive layer  29 , which usually is polysilicon, by deposition and etching, for example. The conductive layer  29  has its top lower than the bottom of the LPD oxide layer  28 , and is used as the upper electrode of the deep trench capacitor. The main structure of the deep trench capacitor has been formed now. 
   In another embodiment shown in  FIG. 3   a , a substrate  31 , a pad oxide layer  32 , a pad nitride layer  33 , and a trench  34  are formed in the same method of forming the substrate  21 , the pad oxide layer  22 , the pad nitride layer  23 , and the trench  24  shown in  FIG. 2   a , respectively. A photoresist layer  35  is then formed in the lower portion of the trench  34  by coating and etching, for example. The top of the photoresist layer  35  is lower than the bottom of a transistor adjacent to the trench  34  and subsequently formed, so as to prevent the lower electrode of the capacitor subsequently formed from improperly contacting the adjacent transistor. 
   As shown in  FIG. 3   b , a liquid phase deposition (LPD) oxide layer  36 , used as the collar oxide layer, is formed on the surface not covered by the photoresist layer  35  in the trench  34 . The photoresist layer  35  is then removed. 
   In  FIG. 3   c , a conductive diffusion region  37  is formed in the substrate at the periphery of the portion of the trench  34  not covered by the LPD layer  36 , in the same way of forming the conductive diffusion region  25 . A dielectric layer  38  usually being nitride is formed on the surfaces of the whole structure by deposition, for example, and an oxide layer (not shown) can be formed on the surface of the dielectric layer  38  by implementing heat treatment. 
   Please refer to  FIG. 3   d . The trench  34  is filled with a conductive layer  39  usually being polysilicon by deposition and etching, for example. The conductive layer  39 , has its top lower than the bottom of the LPD oxide layer  36 , and is used as the upper electrode of the deep trench capacitor. The dielectric layer  38  not covered by the conductive layer  39  is then removed. The main structure of the deep trench capacitor has been formed now. 
   Then, as what has been well known, a connecting electrode (not shown) and a buried strap (BS) (not shown) are formed on the conductive layers  29  and  39 , so as to engage the transistor formed in the adjacent active region. 
   According to the method of the present invention for forming a deep trench capacitor, prior to forming the conductive layers  29  and  39 , which are used as the upper electrodes of the deep trench capacitors, the liquid phase deposition (LPD) oxide layers  28  and  36  used as the collar oxide layers have been formed. By that there will be no poor electrical connection created due to the residual oxide layer in the recess, as the conventional techniques presented in  FIG. 1   d , even if the recess would be generated in the conductive layers  29  and  39 . In addition, the method of the present invention does not require the use of dry etch to remove the unnecessary collar oxide layer insulation layer on the upper electrode conductive layer as the conventional techniques presented in  FIG. 1   b , so as to be capable of simplifying the process. 
   While the embodiments of the present invention are illustrated and described, various modifications and alterations can be made by persons skilled in this art. The embodiments of the present invention are therefore described in an illustrative but not restrictive sense. It is intended that the present invention may not be limited to the particular forms as illustrated, and that all modifications and alterations which maintain the spirit and realm of the present invention are within the scope as defined in the appended claims.
A method for forming a deep trench capacitor mainly utilizes a liquid phase deposition (LPD) oxide to form a collar oxide layer in the trench, followed by forming a conductive layer serving as an upper electrode of the deep trench capacitor, thereby avoiding collar oxide residue in the conductive layer and thus forming good electrical connection. And, the method of the present invention does not need a dry etch to remove the unnecessary collar oxide layer such that the process can be simplified.