Abstract:
A method for manufacturing a semiconductor device includes steps of: forming a trench on a semiconductor substrate, which is made of silicon; and filling the trench with an epitaxial layer. The epitaxial layer is made of silicon, and the step of filling the trench includes a step of performing a plasma CVD method with using a silicon source gas. By using anisotropic character of a plasma, the epitaxial layer is selectively deposited on a bottom of the trench. Thus, the trench is filled with the epitaxial layer having no void.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is based on Japanese Patent Application No. 2005-224629 filed on Aug. 2, 2005, the disclosure of which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a method for manufacturing a semiconductor device having a trench. 
   BACKGROUND OF THE INVENTION 
   As a semiconductor device manufactured in a way that a trench is formed in a semiconductor substrate including silicon (Si), and then the relevant trench is filled, a semiconductor device having a PN column layer is disclosed, for example, in JP-A-9-266311. 
     FIG. 7  is a schematic section view of a semiconductor device  100 , showing a representative example of a semiconductor device having the same PN column layer as a related art. 
   The semiconductor device  100  shown in  FIG. 7  is an N-channel vertical MOS transistor having a PN column layer  1   a , which is formed in a middle portion along a section of a semiconductor substrate  1 , and is rectangle in the section of the substrate, and has P-type conduction regions  1   p  and N-type conduction regions  1   n  in a striped repetitive pattern. The PN column layer  1   a  acts as a drift layer of the vertical MOS transistor, and configures a super junction (SJ) structure. 
   In an on-state of the vertical MOS transistor formed in the semiconductor device  100 , an electron flown out from a source region  1   s  is flown into an N-type conduction region  1   n  of the PN column layer  1   a  through a channel formed in a P-type conduction layer  1   c  in the periphery of a trench gate G, and reaches to a drain region  1   d . Therefore, impurity concentration is increased in the N-type conduction region as a drift region  1   n  of the PN column layer  1   a , thereby on-resistance of the vertical MOS transistor formed in the semiconductor device  100  can be decreased. On the other hand, in an off-state, the PN column layer  1   a  acting as the SJ structure can be perfectly depleted to have high breakdown voltage, i.e., high withstanding voltage. In the same way, conduction types of all components of the N-channel vertical MOS transistor shown in  FIG. 7  are reversed, thereby a P-channel vertical MOS transistor having the SJ structure is obtained. The semiconductor device  100  having low on-resistance and high breakdown voltage is structurally featured in having the PN column layer  1   a  in which the P-type conduction regions  1   p  and the N-type conduction regions  1   n  are in the repetitive pattern. 
   A formation method of the PN column layer  1   a  as a feature of the semiconductor device  100  is disclosed, for example, in JP-A-2004-273742, which corresponds to U.S. Pat. No. 7,029,977. 
   According to the method, a trench is formed in a surface portion of a semiconductor substrate in a first conduction type, then a semiconductor in a second conduction type is epitaxially grown in the trench to fill it by low-pressure chemical vapor deposition (LP-CVD), thereby the PN column layer  1   a  is formed. In such a formation method of the PN column layer using trench formation and subsequent filling, impurity concentration distribution can be made uniform in a depth direction in the trench (thus, thickness direction of the PN column layer  1   a ) unlike a formation method of the PN column layer using impurity diffusion. 
   To increase breakdown voltage of the vertical MOS transistor formed in the semiconductor device  100  shown in  FIG. 7 , the PN column layer  1   a  as the drift layer needs to be formed thick. For example, to obtain breakdown voltage of 600 V, the PN column layer  1   a  needs to have thickness equal to or more than 30 μm. Furthermore, to allow the PN column layer  1   a , which was formed with high impurity concentration for decreasing on-resistance, to be completely depleted, width of the PN column layer  1   a  needs to be narrowed to about 1 μm. Therefore, when the PN column layer  1   a  is formed according to the method, a trench having a large aspect ratio (a ratio between the depth of 30 μm and the width of 1 μm is equal to 30) and filling of the relevant trench are necessary for the semiconductor device  100 , which has low on-resistance and high breakdown voltage. 
   On the other hand, when the semiconductor is epitaxially grown by LP-CVD to fill the trench having the large aspect ratio, the following difficulty is given. 
     FIGS. 8A to 8B  are enlarged section views of a trench  1   t  formed in the semiconductor substrate  1 , showing an aspect of the related art during trench-filling by epitaxial growth of the semiconductor using LP-CVD. 
   As shown in  FIG. 8A , in the trench  1   t  having a large aspect ratio (i.e., depth d/width w), a silicon (Si) source gas hardly reaches to a bottom of the trench  1   t  during LP-CVD. Therefore, as shown by size of arrows in the drawing, a growth rate of an epitaxial layer  1   e  is increased at an upper portion of the trench  1   t . As a result, as shown in  FIG. 8B , a top (i.e., an opening) of the trench  1   t  is closed in an early stage, consequently a void  1   v  that is imperfect filling tends to be formed within the trench  1   t . Moreover, since an epitaxial layer  1   e  is grown from sides of the trench  1   t , thereby crystallinity of the epitaxial layer  1   e  may be deteriorated in the periphery of the void  1   p . In particular, when the aspect ratio of the trench  1   t  is 30 or more, inferior crystals tend to be formed at the void  1   v . When the inferior crystals are present in the periphery of the void  1   v  as shown in  FIG. 8B  in the PN column layer  1   a  in  FIG. 7  formed by such trench-filling structure, they cause decrease in breakdown voltage of the semiconductor device  100  or inverse leakage current by defective connection. 
   Accordingly, as described before, as the aspect ratio of the trench is increased to obtain a semiconductor device  100  having lower on-resistance and higher breakdown voltage, trench-filling structure cannot be fabricated, leading to decrease in breakdown voltage of the semiconductor device  100  or inverse leakage current by defective connection due to the inferior crystals at the void  1   v.    
   SUMMARY OF THE INVENTION 
   In view of the above-described problem, it is an object of the present disclosure to provide a method for manufacturing a semiconductor device having a trench. 
   According to an aspect of the present disclosure, a method for manufacturing a semiconductor device comprising steps of: forming a trench on a semiconductor substrate, which is made of silicon; and filling the trench with an epitaxial layer. The epitaxial layer is made of silicon, and the step of filling the trench includes a step of performing a plasma CVD method with using a silicon source gas. 
   By using an anisotropic characteristic of a plasma, the epitaxial layer is selectively deposited on a bottom of the trench. Accordingly, the trench is filled with the epitaxial layer having no void. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
       FIGS. 1A to 1C  are cross sectional views explaining a step of filling a trench according to an embodiment; 
       FIG. 2A  is a schematic view explaining a plasma CVD method for growing an epitaxial layer on a bottom of the trench, and  FIG. 2B  is a graph showing a voltage waveform with high frequency used in the plasma CVD method; 
       FIGS. 3A to 3C  are cross sectional views explaining the step of filling the trench by using the plasma CVD method; 
       FIG. 4A  is a cross sectional view showing the plasma CVD method without a halide gas,  FIG. 4B  is a cross sectional view showing the plasma CVD method with supplying the halide gas at the same time, and  FIG. 4C  is a cross sectional view showing the plasma CVD method with supplying the halide gas alternately; 
       FIGS. 5A and 5B  are cross sectional views explaining a combination method of the plasma CVD method and a LP-CVD method; 
       FIGS. 6A to 6C  are cross sectional views showing a substrate having a different surface orientation; 
       FIG. 7  is a cross sectional view showing a semiconductor device having a SJ construction according to a related art; and 
       FIGS. 8A and 8B  are cross sectional views explaining a step of filling a trench according to a related art. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present disclosure provide a manufacturing method of a semiconductor device, in which the device is manufactured in a way that a trench is formed in a semiconductor substrate including silicon (Si), and then the trench is filled by an epitaxial layer. For example, a trench is formed in a semiconductor substrate in the N conduction type or P conduction type, and then the trench is filled by an epitaxial layer having a conduction type different from that of the semiconductor substrate to form the PN column layer  1   a  shown in  FIG. 7 , consequently a semiconductor device  100  having the PN column layer  1   a  is manufactured. 
     FIGS. 1A to 1C  are cross section views of a trench  1   t  formed in a semiconductor substrate  1 , schematically showing a filling process of the trench  1   t , which is provided to avoid the difficulty of the inferior crystal at the void  1   v  as described in  FIG. 8B . 
   To avoid the difficulty of the inferior crystal at the void  1   v  in  FIG. 8B , as shown in  FIG. 1A , a silicon (Si) source  2   s  is preferentially allowed to reach to a bottom of the trench  1   t . Thus, as shown in  FIG. 1B , a growth rate of an epitaxial layer  1   e  is made large at the bottom of the trench  1   t  compared with sides of the trench  1   t , so that as shown by an outline arrow, the epitaxial layer  1   e  including silicon (Si) is preferentially grown from the bottom of the trench  1   t . In this case, since a growth face of the epitaxial layer  1   e  is sequentially raised from the bottom of the trench  1   t  to a top (i.e., an opening) of the trench, as shown in  FIG. 1C , the void is not formed in the epitaxial layer  1   e  for filling the trench lt. 
   To realize growth of the epitaxial layer  1   e  from the bottom of the trench  1   t  as shown in  FIGS. 1A to 1C , the epitaxial layer  1   e  is grown by plasma CVD using the silicon (Si) source gas to fill the trench  1   t.    
   Plasma has been used for anisotropic etching of a silicon substrate, or deposition of an amorphous film having excellent coverage performance at low temperature. The epitaxial layer  1   e  is grown using plasma CVD to fill the trench  1   t  as seen in  FIGS. 1A to 1C . Unlike the trench-filling using low-pressure CVD (LP-CVD) as in  FIGS. 8A to 8B  in the related art, in trench-filling using the plasma CVD, the following various kinds of deposition control can be performed, and therefore the epitaxial layer  1   e  can be selectively grown from the bottom of the trench by using anisotropy of plasma. Accordingly, trench-filling can be performed without formation of the void. 
     FIGS. 2A to 2B  are views for illustrating the plasma CVD used for growth of the epitaxial layer from the trench bottom, wherein  FIG. 2A  shows main components of the plasma CVD, and  FIG. 2B  shows a high-frequency voltage waveform used for plasma formation. 
   As shown in  FIG. 2A , the plasma CVD is carried out in a way that a semiconductor substrate (wafer)  1  including silicon (Si) is set in a chamber, then a silicon (Si) source gas  3  such as a SiH 4  gas is supplied from a gas supply port into the chamber, and then high-frequency voltage of about 500 MHz as shown in  FIG. 2B  is applied and thus plasma  2  is formed. As the silicon source gas  3 , for example, inexpensive silane (SiH 4 ) can be used. When the trench  1   t  is filled by the epitaxial layer  1   e  to form the PN column layer  1   a  in  FIG. 7 , an impurity gas having a conduction type different from that of the Si substrate  1  (P conduction type: B 2 H 6  and the like, N conduction type: PH 3 , AsH 3  and the like) is supplied into the chamber together with the Si source gas  3 . 
   As shown in  FIG. 2A , the Si source gas  3  is preferably supplied from an upside of the wafer  1 . Thus, the Si source gas  3  can be supplied evenly in a plane of the wafer  1 , therefore in-plane evenness of growth thickness (growth rate) or impurity concentration can be improved. On the other hand, when the Si source gas  3  is supplied in a lateral direction as shown by a dot line in  FIG. 2A , in-plane distribution tends to occur in the growth thickness and the like. 
   The Si source gas  3  supplied from the gas supply port into the chamber is excited by the plasma  2 , thereby each radical of SiH 3 , SiH 2 , SiH or Si, or each ion such as SiH 3   +  is formed, which is silicon (Si) source  2   s  of epitaxial growth. 
   According to a preliminary examination, among the respective radicals of SiH 3 , SiH 2 , SiH and Si excited from the Si source gas  3  by the plasma  2 , the radical of SiH 3  most contributes to growth of the epitaxial layer including Si. Thus, among the respective radicals of SiH 3 , SiH 2 , SiH and Si, the radical amount of SiH 3  is maximized, thereby an epitaxial layer having excellent film quality can be grown at high growth rate. 
   Furthermore, according to another preliminary examination, among respective ions formed from the Si source gas  3  by the plasma  2 , the ion of SiH 3   +  most contributes to separation of hydrogen (H) terminating a surface of the trench. Thus, among the respective ions formed by the plasma  2 , the ion amount of SiH 3   +  is maximized, thereby H terminating the trench surface can be efficiently separated. 
   As described later, the separation of H by the SiH 3   +  ion is preferably combined with supply of Si by the SiH 3  radical to a site where H was separated, thereby the epitaxial layer having excellent film quality can be grown at high growth rate. 
   Since the plasma  2  has charge, when deposition is performed while the wafer  1  is grounded, or while bias voltage Vb is applied, the plasma  2  or each ion formed by the plasma  2  is attracted to a wafer  1  side, consequently anisotropic epitaxial growth can be performed. 
   The plasma formation voltage shown in  FIG. 2B  is high-frequency pulse voltage. Thus, the following control can be performed, which cannot be performed in the case of forming the plasma  2  by a continuous high-frequency wave. First, by using difference between an on-state and off-state of a high-frequency pulse, the ion and the radical can be alternately supplied into the trench. Second, by using a fact that lives of the radicals of SiH 3 , SiH 2 , SiH and Si excited by the plasma  2  are different from each together, a duty ratio (on-time Td/cycle time Tc) between on and off of the high-frequency pulse shown in  FIG. 2B  is appropriately set in accordance with a life of a required radical. Thus, the amount of the radical of SiH 3  can be maximized. At that time, the ion amount of SiH 3   +  is maximized at the same time. 
   A condition of maximizing the radical amount of SiH 3  and the ion amount of SiH 3   +  can be previously examined, and deposition can be carried out at an obtained, fixed condition during deposition by the plasma CVD. However, the radical amount of each radical of SiH 3 , SiH 2 , SiH or Si excited by the plasma  2 , or the ion amount of each ion formed from the Si source gas  3  by the plasma  2  is preferably monitored during the deposition by the plasma CVD. Thus, even if the radical amount of each radical or the ion amount of each ion is varied during deposition, it can be monitored and used for feedback control to the optimum deposition condition. 
     FIGS. 3A to 3C  are views schematically showing a most preferable filling process of the trench  1   t  by the plasma CVD. 
   First, as shown in  FIG. 3A , a surface of the trench  1   t  in the Si substrate (wafer)  1  is subjected to fluorinated acid (HF) treatment before starting the plasma CVD, so that Si in the surface of the trench  1   t  is previously terminated by hydrogen (H)  1   h . Thus, the surface of the trench  1   t  is in a uniform and stable surface condition, thereby the following separation effect of H by the SiH 3   +  ion can be stably exhibited. 
   Next, the Si wafer  1  after HF treatment is set in a plasma CVD chamber as shown in  FIG. 2A , then the duty ratio of the high-frequency pulse shown in  FIG. 2B  is appropriately set, and then deposition is performed at a condition where the radical amount of SiH 3  and the ion amount of SiH 3   +  are maximized. Plasma formation using the high-frequency pulse shown in  FIG. 2B  allows the SiH 3   +  ion and the SiH 3  radical to alternately reach to the Si wafer  1  during the on-state and the off-state of the high-frequency pulse. 
   As shown in  FIG. 3B , when the high-frequency pulse is in the on-state and the SiH 3   +  ion is predominant, a SiH 3   +  ion  2   si  impinging in a vertical direction to the Si substrate  1  reaches to the bottom of the trench  1   t , and separates hydrogen (H) terminating a bottom surface of the trench  1   t . Thus, an adsorption site  1   q  is formed on the bottom surface of the trench  1   t.    
   As shown in  FIG. 2A , bias voltage Vb is preferably applied to the Si wafer  1  during operation of the plasma CVD. Thus, the SiH 3   +  ion  2   si  is accelerated to advance further straightly, consequently allowed to preferentially reach to the bottom of the trench  1   t . The SiH 3   +  ion  2   si  that has reached to the bottom of the trench  1   t  separates H terminating the surface as described before, and the adsorption site  1   q  is preferentially formed on the bottom surface of the trench  1   t.    
   Next, as shown in  FIG. 3C , when the high-frequency pulse is in the off-state and the SiH 3  radical is predominant, the SiH 3  radical  2   sr  entering into the trench  1   t  is absorbed by the adsorption site  1   q  preferentially formed on the bottom surface of the trench  1   t . Thus, the epitaxial layer  1   e  is selectively grown from the bottom of the trench  1   t . Since Si on an upper surface of the trench  1   t  is terminated by H, growth of the epitaxial layer  1   e  is inhibited in an upper part of the trench  1   t.    
   Processes shown in  FIGS. 3B and 3C  are alternately repeated in synchronization with the on and off cycle of the high-frequency pulse shown in  FIG. 2B , thereby the epitaxial layer  1   e  can be grown from the bottom of the trench  1   t  stably and continuously, which allows filling of the trench  1   t . Thus, the trench  1   t  by the epitaxial layer  1   e  can be filled without formation of the void shown in  FIG. 1C . 
   Substrate temperature of 900 to 1200° C. is necessary for growing the epitaxial layer  1   e  by the LP-CVD. On the other hand, by using the plasma CVD, epitaxial growth can be made at low temperature (for example, room temperature to 300° C.). However, even in the plasma CVD, higher substrate temperature is preferable to improve crystallinity of the epitaxial layer  1   e . Moreover, the Si substrate (wafer)  1  after trench-filling by the plasma CVD may be subjected to heat treatment in hydrogen atmosphere, nitrogen atmosphere, or the like. This can also improve crystallinity of the epitaxial layer  1   e  grown in the trench  1   t.    
   In the plasma CVD shown in  FIG. 2A , not only the Si source gas  3 , but also a halide gas may be supplied into the chamber. 
     FIGS. 4A to 4C  are views for illustrating an effect of supplying a halide gas  4 , wherein  FIG. 4A  shows a case that the halide gas  4  is not supplied,  FIG. 4B  shows a case that the halide gas  4  and the Si source gas  3  are supplied at the same time, and  FIG. 4C  shows a case that the halide gas  4  and the Si source gas  3  are alternately supplied. As the halide gas  4 , for example, as shown in the figures, either of inexpensive hydrochloric (HCl) gas or chlorine (Cl 2 ) gas can be used. 
   As shown in  FIG. 4A , when the halide gas  4  is not supplied, the epitaxial layer  1   e  is formed not only in the trench  1   t  but also on a surface of the Si substrate  1  by the Si source  2   s  formed by the plasma  2 . Therefore, in the case that the halide gas  4  is not supplied as in  FIG. 4A , surface polishing of the Si substrate  1  is necessary after filling of the trench  1   t.    
   As shown in  FIG. 4B , when the halide gas  4  and the Si source gas  3  are supplied at the same time, the halide gas, particularly Cl atoms are terminating the surface of silicon on the periphery of the top of the trench  1   t , and cover the periphery of the top. In this way, the halide gas  4  and the Si source gas  3  are supplied at the same time, thereby growth of epitaxial layer  1   e  can be inhibited in the periphery of the top of the trench  1   t.    
   As shown in  FIG. 4C , when the halide gas  4  and the Si source gas  3  are alternately supplied, a deposition process by the Si source and etching of the periphery of the top of the trench  1   t  by a halide gas  2   h  can be alternately performed. In this way, an etching process for the periphery of the top of the trench  1   t  is introduced, thereby the epitaxial layer  1   e  growing in the periphery of the top of the trench  1   t  is appropriately removed, and consequently filling performance of the trench  1   t  can be improved. 
   As shown in  FIGS. 8A to 8B , in filling of the trench  1   t  using the LP-CVD, when the aspect ratio (depth d/width w) of the trench  1   t  is 30 or more, the inferior crystal tends to be formed at the void  1   v . On the contrary, in a manufacturing method using the plasma CVD, even in the trench  1   t  having the large aspect ratio, the epitaxial layer can be grown from the bottom of the trench  1   t , and consequently formation of the void can be prevented. Therefore, the manufacturing method using the plasma CVD is preferable for filling of the trench  1   t  having an aspect ratio of 30 or more in which the epitaxial layer  1   e  is hard to be grown excellently in the LP-CVD. 
   On the other hand, as shown in  FIGS. 5A to 5B , the plasma CVD and the LP-CVD can be used in a combined manner.  FIG. 5A  shows first filling of the trench  1   t  using the plasma CVD, and  FIG. 5B  shows second filling of the trench  1   t  using the LP-CVD. 
   First, as shown in  FIG. 5A , using the plasma CVD, the Si source  2   s  such as SiH 3  radical formed by the plasma  2  is supplied into the trench  1   t , so that an epitaxial layer  1   e  (P) is grown to fill the trench  1   t  halfway. Next, as shown in  FIG. 5B , using the LP-CVD, the Si source  3   s  by the Si source gas is decomposed on the Si substrate  1  heated to high temperature of 900° C. or more. Thus, a second epitaxial layer  1   e  (L) including silicon (Si) is grown in the trench  1   t  to completely fill the trench  1   t.    
   In a method in a combination of the plasma CVD and the LP-CVD as shown in  FIGS. 5A and 5B , in the first plasma CVD of  FIG. 5A , the epitaxial layer  1   e  (P) is grown on the bottom of the trench  1   t  and thus the trench  1   t  is filled to a level where the void is not formed even if the LP-CVD is used, so that the aspect ratio of the trench  1   t  is decreased. Next, using the LP-CVD having the large deposition rate as shown in  FIG. 5B , the epitaxial layer  1   e  (L) is grown to completely fill the trench  1   t . Thus, epitaxial growth by the plasma CVD that enables low-temperature growth, but has small growth rate is compensated, and consequently throughput of trench-filling can be improved as a whole. The plasma CVD and the LP-CVD may be treated in separate chambers respectively, or may be treated in the same chamber. 
   The filling of the trench  1   t  by growth of the epitaxial layer  1   e  using the plasma CVD has small dependence on a plane direction of the semiconductor substrate  1  or a plane direction of a side of the trench  1   t . Therefore, semiconductor substrates and trenches in various plane directions shown in  FIGS. 6A to 6C  can be used. 
     FIGS. 6A to 6B  show a case that, as the semiconductor substrate  1 , a Si (100) substrate is used, which provides most excellent characteristics when it is formed into a device. For example, in the case that a MOS transistor is formed in a surface portion of the Si substrate  1 , when a channel of the MOS transistor is formed in a (100) plane using the Si (100) substrate, excellent characteristics are obtained in mobility and interface state compared with a case of using a Si substrate in another plane direction. Compared with the Si (100) substrate, in  FIG. 6A , sides of the trench  1   t  are configured in (110) planes. In  FIG. 6B , the sides of the trench  1   t  are configured in (100) planes. In this case, all the sides and the bottom of the trench  1   t  are in the (100) plane. Both of the trenches  1   t  in  FIGS. 6A to 6B  are formed using dry etching. 
   The filling of the trench  1   t  by growth of the epitaxial layer  1   e  using the plasma CVD can be performed to any of the Si substrates  1  and the trenches it in  FIGS. 6A to 6B . 
   In  FIG. 6C , a Si (110) substrate is used as the semiconductor substrates  1 , and sides of the trench  1   t  are configured in (111) planes. The configuration of these planes is that most excellent in filling performance in filling using the LP-CVD, and is preferable for a case of using the plasma CVD and the LP-CVD in a combined manner as shown in  FIGS. 5A to 5B . Moreover, in the plane-direction configuration, wet etching using TMAH, KOH or the like can be used for formation of the trench  1   t.    
   As shown hereinbefore, the manufacturing method of the semiconductor device is provided, the device being manufactured in a way that the trench  1   t  is formed in the semiconductor substrate  1 , and then the relevant trench  1   t  is filled; wherein a trench can be filled without formation of the void even if the trench  1   t  has a large aspect ratio. Accordingly, the method is preferable for manufacturing the semiconductor device  100  having the PN column layer  1   a  shown in  FIG. 7 , which has low on-resistance and high breakdown voltage. 
   While the invention has been described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the preferred embodiments and constructions. The invention is intended to cover various modification and equivalent arrangements. In addition, while the various combinations and configurations, which are preferred, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.