Abstract:
A method of etching and tilling deep trenches is disclosed, which includes: forming an ONO(oxide-nitride-oxide) sandwich layer on a semiconductor substrate; forming deep trenches by using top oxide of the sandwich layer as a stop layer; removing the top oxide and middle SiN of the sandwich layer; tilling the deep trenches with epitaxial film or polysilicon film; polishing the wafer to get a planarized surface by stopping at the surface of the bottom oxide layer; removing the bottom oxide layer.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of Chinese patent application number 201010200461.5, filed on Jun. 12, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a method for manufacturing a semiconductor device, particularly a superjunction device with deep trenches. 
     BACKGROUND 
     Normally, a trench with a depth of more than 10 μm is defined as a deep trench. Deep trench structures are widely used in semiconductor manufacturing in nowadays. For example, a deep trench can function as an isolation module to isolate power MOS transistors with different operation voltages; a deep trench can also be used in a superjunction MOSFET as a P type pillar in an N type drift layer structure (or an N type pillar in a P type drift layer structure) to form a P-N depletion junction to balance the electric field, so that insulation breakdown can be prevented, and high breakdown voltage can be achieved. 
     Conventional manufacturing method of superjunction MOSFET structure includes: growing an N type epitaxial layer as a drift layer on a P type substrate; forming a deep trench in the N type epitaxial layer by plasma etching; tilling the deep trench with P type epitaxial film or P type polysilicon; planarize the surface of the deep trench by CMP process. Thus, a P-N junction structure with alternating N type and P type regions is formed, wherein the deep trench is functioned as a P-pillar, the N type epitaxial layers beside the deep trench are functioned as N-drift regions. A similar function can be achieved by exchanging the N type and P type silicons. 
     The above method may have the following problems. On one hand, since N drift and P pillar regions are both made of silicon, it is difficult to distinguish the P pillar from the N drift regions during the CMP process, as a result, it is impossible to carry out selective removal, and the polishing may cause damage to the active area and hence influence the device performance. On the other hand, as the substrate and the material filled in the deep trench have the same type, it is hard to control polishing stop point during the CMP process. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     An object of the present invention is to provide a method for etching and filling deep trenches to protect the substrate from polishing damage during CMP process, and to attain good process control. 
     In order to achieve the above object, the present invention provides a method for etching and filling deep trenches, which includes: 
     step 1, deposit an ONO (oxide-nitride-oxide) layer on a substrate, the ONO layer includes a bottom silicon oxide layer, a middle silicon nitride layer and a top silicon oxide layer; 
     step 2, etch a deep trench in the substrate, which further includes:
         form a shallow trench in the ONO layer by removing at least part of the top silicon oxide layer;   etch in the shallow trench to form a deep trench by using remaining part of the top silicon oxide layer as an etch stop layer;       

     step 3, remove the top silicon oxide layer and the middle silicon nitride layer; 
     step 4, fill the deep trench with epitaxial film and/or polysilicon film; 
     step 5, planarize the surface of the deep trench by using the bottom silicon oxide layer as a stop layer; 
     step 6, remove the bottom silicon oxide layer. 
     In the present invention, the oxide layers of the ONO sandwich layer are used as stop layers. Firstly, the top oxide layer is used as an etch stop layer during the step of deep trench etch. The middle SiN layer is used to protect the bottom oxide layer from damage during deep trench etch and top oxide removal, thus remaining a good thickness uniformity of the bottom oxide layer. Finally, the bottom oxide layer is used as a stop layer during the CMP process. Compared with conventional methods, the present invention can achieve better process control during trench etching and CMP. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart of the method for etching and tilling deep trenches according to the present invention. 
         FIG. 2A˜FIG .  2 F are sectional views of the method for etching and filling deep trenches according to the present invention. 
         FIG. 3A˜FIG .  3 B are sectional views of the step of etching deep trenches according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is further detailed by embodiments in combination with the drawings. In the following descriptions, the term “N type” includes the meaning of N+ type and N− type; the term “P type” includes the meaning of P+ type and P− type. 
     Please refer to  FIG. 1  and  FIGS. 2A˜2F , the method for etching and filling deep trenches of the present invention includes the following steps: 
     Step 1, deposit an ONO layer  3  on a substrate (see  FIG. 2A ). 
     The substrate may be a silicon substrate alone or a silicon substrate having an epitaxial layer formed on it.  FIG. 2A  shows the latter structure, namely in this embodiment, the substrate is composed of a silicon substrate  1  and an epitaxial layer  2  (also called drift layer), wherein the epitaxial layer  2  is an N− type epitaxial layer. Those skilled in the art shall understand that the epitaxial layer  2  may also be a P− type epitaxial layer. The thickness of the epitaxial layer  2  is in a range of 10˜100 μm. 
     The ONO layer  3  is a sandwich structure composed of a bottom silicon oxide layer  31 , a middle silicon nitride layer  32  and a top silicon oxide layer  33  from the bottom up. The ideal thickness of the bottom silicon oxide layer  31  is in a range of 10˜5000 Å; the ideal thickness of the middle silicon nitride layer  32  is in a range of 10˜5000 Å; the ideal thickness of the top silicon oxide layer  33  is in a range of 10˜30000 Å. Although in  FIG. 2A˜2F , the three layers  31 ˜ 33  seem to have the same thickness, those skilled in the art shall understand that the thickness of the three layers  31 ˜ 33  may vary from one another. Generally, the middle and bottom layers  32 ,  31  may have similar or same thickness, while the top layer, namely the top silicon oxide layer  33  may have a much larger thickness than that of the bottom silicon oxide layer  31 . 
     The formation of the ONO layer  3  follows the below sequence: firstly, deposit the bottom oxide layer  31  with an ideal thickness of 10˜5000 Å; then, deposit the middle nitride layer  32  with an ideal thickness of 10˜5000 Å; finally, deposit the top oxide layer  33  with an ideal thickness of 10˜30000 Å. Each of the three layers  31 ˜ 33  could be formed by thermal oxidation, APCVD (Atmospheric-Pressure Chemical Vapor Deposition), LPCVD (Low-Pressure Chemical Vapor Deposition) or PECVD (Plasma Enhanced Chemical Vapor Deposition). 
     Step 2, form a deep trench  4  in the substrate by photolithography and etch (see  FIG. 2B ). During the etch process, the top silicon oxide layer  33  of the ONO layer is used as an etch stop layer. 
     Normally, the depth of the deep trench  4  is more than 10 μm. Preferably, for a deep trench having a width (CD) of 1˜4 μm, the ideal depth is 10˜100 μm. The deep trench may he formed in the epitaxial layer  2  without reaching the bottom surface of the epitaxial layer  2  (as shown in FIG. B); or the bottom of the deep trench  4  may just reach the interface of the epitaxial layer  2  and the silicon substrate  1 ; or the deep trench may be etched through the epitaxial layer  2  into the silicon substrate  1 . 
     Please refer to  FIGS. 3A˜3B , the step of forming a deep trench by photolithography and etch is as follows: 
     Step 20, as shown in  FIG. 3A , coat a photoresist layer  6  on the ONO layer  3  and form an etch window  60  in the photoresist layer  6  by exposure and development to expose the surface of the ONO layer  3 , wherein the position and dimension of the etch window is determined by the deep trench to be formed. 
     Step 21, as shown in  FIG. 3B , remove the top silicon oxide layer  33  in the etch window  60  by using the photoresist layer  6  as hardmask and then remove the photoresist layer  6 . After this step, a shallow trench  30  is formed in the ONO layer  3 . Optionally, the middle silicon nitride layer  32 , or the middle silicon nitride layer  32  and the bottom silicon oxide layer  31  in the etch window  60  may also be removed during this step. 
     Step 22, continue to etch in the shallow trench  30  to form a deep trench  4  in the substrate by dry etch. During this step, the top silicon oxide layer  33  of the ONO layer  3  is functioned as an etch stop layer to protect the middle silicon nitride layer  32 , the bottom silicon oxide layer  31  and the substrate below it from being etched. The process of deep trench etching is as follows: 
     Firstly, in step 22a, remove the remaining ONO layer in the shallow trench by etch. If in step 21, only the top silicon oxide layer  33  in the etch window  60  was removed, then in this step, remove the middle silicon nitride layer  32  and the bottom silicon oxide layer  31  in the shallow trench; if both the top oxide layer  33  and the middle nitride layer  32  were removed in step 21, then remove the bottom oxide layer  31  in the shallow trench in this step. In the above two cases, when the bottom silicon oxide layer  31  in the shallow trench  30  is removed, the thickness of the top silicon oxide layer  33  at both sides of the shallow trench  30  will also be reduced due to the effect of the etchant; however, since the top oxide layer  33  usually has a much larger thickness than the bottom oxide layer  31 , a part of the top oxide layer  33  will be remained after step 22a. If the ONO layer in the etch window  60  was totally removed in step 21, then skip to step 22b below. 
     Then, in step 22b, etch the silicon body, namely the substrate in the shallow trench  30  to form a deep trench. By using an echant having an etch selectivity of silicon over oxide, the silicon body will be etched while only a part of the top silicon oxide layer  33  outside the shallow trench is removed, thus enabling the remaining top silicon oxide layer  33  to protect the structure below it. 
     As shown in  FIG. 2B , after step 22b, the deep trench  4  is formed in the substrate. 
     Step 3, as shown in  FIG. 2C , remove the top silicon oxide layer  33  and the middle silicon nitride layer  32 . It is preferable to remove the top silicon oxide layer  33  first by using the middle silicon nitride layer  32  as a protection layer to the bottom silicon oxide layer  31 , so that the thickness of the bottom silicon oxide layer  31  will remain unchanged, in other words, the bottom silicon oxide layer  31  will remain to have a good thickness uniformity. Then, the middle silicon nitride layer  32  is removed. In practice, the two layers  33 ,  32  may both be removed by dry etch or wet etch. HF may he used for silicon oxide wet removal, hot H3PO4 may be used for silicon nitride wet removal. A combination of silicon oxide dry etch and silicon nitride wet removal or silicon oxide wet removal and silicon nitride dry etch is also feasible. 
     If the top silicon oxide layer  3  is removed by wet etch, the bottom silicon oxide layer  31  will be etched in the horizontal direction due to the effect of lateral erosion. In such a case, part of the substrate surface on both sides of the deep trench  4  will be exposed after step 3 (see  FIG. 2C ). 
     Step 4, as shown in  FIG. 2D , fill the deep trench  4  with epitaxial film or polysilicon film or the combination  5 . For example, the deep trench  4  may be filled by epitaxial growth of a monocrystal silicon film or by deposition of a polysilicon film or by firstly growing a monocrystal silicon epitaxial film and then depositing a polysilicon film. A CVD equipment is preferred for epitaxial growth. During the process of epitaxial growth, a mixture of silicon source gas, halide gas and doping gas are used as reaction gas, wherein the silicon source gas may be selected from silane, dichlorosilane, trichlorosilane, etc.; the halide gas may be HCl, which is used to prevent epitaxial growth at top of the deep trench  4  so that the opening of the deep trench  4  will not be closed before the deep trench  4  is completely filled; the doping gas may be boron hydride for P type epitaxial growth, phosphine or arsenic hydride for N type epitaxial growth. LPCVD is preferred during the process of polysilicon deposition if the deep trench  4  is tilled by a combination of monocrystal silicon epitaxial film and polysilicon film. 
     Step 5, as shown in  FIG. 2E , planarize the surface of the deep trench  4  by CMP process, wherein the bottom silicon oxide layer  31  of the ONO layer is used as a stop layer during this process; in other words, the CMP process is stopped at the surface of the bottom silicon oxide layer  31 , so that no silicon is remained on the bottom silicon oxide layer  31  after the CMP process. In practice, after this step, the surface of the deep trench  4 , namely the surface of the silicon  5  filled in the deep trench  4  will be lower than the surface of the bottom silicon oxide layer  31  due to different erosion rates of slurry to different materials during CMP. 
     Step 6, as shown in  FIG. 2F , remove the bottom silicon oxide layer  31 . Similar to step 3, the bottom silicon oxide layer  31  may be removed either by dry etch or by wet etch. Finally, a P type pillar  5  in an N type drift region  2  is formed. 
     Although the above embodiment is described by taking the formation of one P type pillar in N type drift region as example, those skilled in the art shall understand that the method for etching and filling deep trenches according to the present invention may also be used to simultaneously form more than one deep trenches in the substrate; besides, N type pillars in P type drift regions can be easily implemented by changing the type of dopants. 
     In a summary, the present invention forms an ONO layer  3  on the substrate before etching the deep trenches. The top oxide layer  33  acts as a stop layer during the step of etching deep trenches. The middle SiN layer  32  is used to protect the bottom oxide layer  31  from damage during the removal of top oxide layer  33 , so as to remain a good thickness uniformity of the bottom oxide layer  31 , which is advantageous for CMP process control. 
     If only one oxide layer is deposited on the substrate, and is used as a stop layer both in deep trench etching process and CMP process, the oxide layer may have poor thickness uniformity after deep trench etching due to the fluctuation of process. This poor thickness uniformity will be disadvantageous for CMP process control. 
     By adopting the ONO structure, the bottom oxide layer will not be damaged during the steps of deep trench etching and filling, so that the thickness of the bottom oxide layer can be adjusted according to the process and device requirement. Generally, the thinner the bottom oxide layer is, the less effect of lateral erosion will arise during the step of wet etch, and the better device performance and reliability can be achieved. 
     Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, the disclosure is not for limiting the scope of the invention. Persons having ordinary skill in the art may make various modifications and changes without departing from the scope and spirit of the invention. Therefore, the scope of the appended claims should not be limited to the description of the preferred embodiments described above.