Abstract:
A method for fabricating a vertical offset structure that forms a complete vertical offset on a wafer includes a first trench forming step of forming first trenches on a wafer; a first etching step of performing a first patterning for determining etching positions of second and third trenches by depositing a first thin film on the wafer, performing a second patterning for temporarily protecting the etching position of the third trench by depositing a second thin film on the first thin film and the wafer, and then forming the second trenches by etching the wafer; a second etching step of forming a protection layer on side surfaces of the second trenches and then vertically extending the second trenches by etching the wafer; a third etching step of removing the second thin film and then forming the third trench by etching a position from which the second thin film is removed; and a fourth etching step of horizontally extending the second trenches vertically extended at the second etching step and the third trench by etching the wafer.

Description:
This application claims benefit under 35 U.S.C. § 119 from Korean Patent Application No. 2004-36507, filed on May 21, 2004, the entire content of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method for fabricating a vertical offset structure, and more particularly to a method for fabricating a vertical offset structure by extending specified trenches using a single wafer. 
   2. Description of the Related Art 
   A vertical offset structure is fabricated using a MEMS (Micro Electro Mechanical System) fabrication method. The MEMS fabrication method relates to fabricating systems of the micrometer dimension on a wafer (i.e., substrate) using a silicon process, and is based on the method of fabricating semiconductor devices. Representative systems which are fabricated using the MEMS fabrication method are accelerometers for measuring acceleration of moving objects, gyroscopes for measuring angular velocity of rotating objects, optical switches capable of controlling optical paths, etc. 
   The performance indicator of the vertical offset structure is determined depending on whether upper and lower electrodes of the structure have a complete vertical offset and how narrow the vertical gap between the vertical offset structures is in order to improve the performance of vertical driving and detection. 
     FIG. 1  is a cross-sectional view illustrating an example of a conventional vertical offset structure. This vertical offset structure is disclosed in U.S. patent application Publication No. 2002/0158293A1. The vertical offset structure, in which structure members having different thickness have a vertical offset, has a good performance in applications of vertical driving and detection. However, according to the vertical offset structure as illustrated in  FIG. 1 , the vertical offset is not produced on an upper electrode of the structure, but is produced on a lower electrode thereof. Thus, the vertical offset structure as illustrated in  FIG. 1  has a degraded performance in applications using vertical driving and detection. 
     FIG. 2  is a view illustrating another example of a conventional vertical offset structure. This vertical offset structure is disclosed in U.S. Pat. No. 6,694,504 B2. According to the vertical offset structure as illustrated in  FIG. 2 , a complete vertical offset is produced on both upper and lower electrodes. However, according to the principle of the fabrication process, the horizontal gap gh between the upper and lower electrodes of the structure is limited to about 4.5 μm. Since the horizontal gap is limited, the improvement of performance of the vertical offset structure is limited. 
   SUMMARY OF THE INVENTION 
   The present invention has been developed in order to address the above drawbacks and other problems associated with the conventional arrangement. An aspect of the present invention is to provide a method for fabricating a vertical offset structure which can achieve a complete vertical offset on upper and lower electrodes of the structure and narrow a gap between the upper and lower electrodes of the structure. 
   The foregoing and other objects and advantages are substantially realized by providing a method for fabricating a vertical offset structure, according to the present invention, which comprises a first trench forming step of forming first trenches on a wafer by etching the wafer and then injecting a specified material into the first trenches; a first etching step of performing a first patterning for determining etching positions of second and third trenches by depositing a first thin film on the wafer, performing a second patterning for temporarily protecting the etching position of the third trench by depositing a second thin film on the first thin film and the wafer, and then forming second trenches by etching the wafer; a second etching step of forming a protection layer on side surfaces of the second trenches and then vertically extending the second trenches by etching the wafer; a third etching step of removing the second thin film and then forming a third trench by etching a position from which the second thin film is removed; and a fourth etching step of horizontally extending the second trenches vertically extended at the second etching step and the third trench by etching the wafer. 
   It is preferable, but not necessary, that the method further comprises the step of removing the material injected into the first trenches, the first thin film and the protection layer. 
   It is preferable, but not necessary, that upper structures, a lower structure and a base structure are formed by the first trench forming step and the first to fourth etching steps, a vertical gap between the upper and lower structures is determined by the second trenches formed at the first etching step and the third trench, and a horizontal gap between the upper and lower structures is determined by the first trenches. 
   It is preferable, but not necessary, that the wafer is a single-crystalline wafer, and especially a single-crystalline wafer having a crystal orientation of (111). 
   It is preferable, but not necessary, that the material is made of SiO2, the first thin film is a SixNy thin film, and the second thin film is a SiO2 thin film. 
   It is preferable, but not necessary, that the first to third etchings are dry etchings, and the fourth etching is a wet etching. It is preferable, but not necessary, that the wet etching is performed in an alkaline solution. 
   The vertical offset structure fabricated using the method for fabricating the vertical offset structure is applicable to, for example, accelerometers, gyroscopes and optical application devices. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above aspects and features of the present invention will be more apparent by describing certain embodiments of the present invention with reference to the accompanying drawings, in which: 
       FIG. 1  is a cross-sectional view illustrating an example of a conventional vertical offset structure; 
       FIG. 2  is a view illustrating another example of the conventional vertical offset structure; 
       FIG. 3  is a flowchart illustrating a method for fabricating a vertical offset structure according to the present invention; 
       FIGS. 4A to 4F  are cross-sectional views of a vertical offset structure according to fabrication steps of the vertical offset structure; 
       FIG. 4G  is a plan view of the vertical offset structure illustrated in  FIGS. 4A to 4F ; 
       FIG. 4H  is a view illustrating an example of a crystal orientation of a wafer used in the present invention; 
       FIG. 5  is a flowchart illustrating an example of a patterning process used in the method for fabricating the vertical offset structure according to the present invention; 
       FIGS. 6A to 6D  are cross-sectional views of the structure according to the patterning process of  FIG. 5 ; and 
       FIG. 7  is a view illustrating the conventional vertical offset structures and the vertical offset structure according to the present invention for the purpose of comparison. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Certain embodiments of the present invention will be described in greater detail with reference to the accompanying drawings. 
   Descriptions of items such as construction details and details of elements are only provided to assist in a comprehensive understanding of the invention. Thus, it is apparent that the present invention can be carried out without those details. Also, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. 
     FIG. 3  is a flowchart illustrating a method for fabricating a vertical offset structure according to the present invention.  FIGS. 4A to 4F  are cross-sectional views of a vertical offset structure according to fabrication steps of the vertical offset structure. Referring to  FIG. 4F , upper structures  611  to  614 , a lower structure  620  and a base structure  630  fabricated by a method for fabricating a vertical offset structure according to an embodiment of the present invention are illustrated. The upper structures  611  to  614  and the lower structure  620  have a comb structure.  FIG. 4G  is a plan view of the vertical offset structure illustrated in  FIGS. 4A to 4F . Referring to  FIG. 4G , the section taken along the line A–A′ is illustrated in  FIGS. 4A to 4F . Also, a part of a unit pitch in  FIG. 4F  is illustrated in  FIG. 4G . The part indicated as slanting lines is a part fixed when a voltage is applied. Referring to  FIG. 4G , only a part centered about the unit pitch of the vertical offset structure according to the present invention is illustrated. 
   Referring to  FIG. 3 , a predetermined number of first trenches are formed on a wafer (i.e., substrate) by an etching process, and a specified material is injected into the first trenches (step S 110 ). The distance between the first trenches  311  and  312  becomes a width (i.e., horizontal length) of the lower structure  620 . Referring to  FIG. 4A , it is preferable that the wafer  300  is a single-crystalline wafer, but it is not limited thereto.  FIG. 4H  is a view illustrating an example of a crystal orientation of the wafer used in the present invention. It is preferable that the crystal orientation of the wafer is (111). Referring to  FIG. 4H , the crystal orientation of (111) means a direction obtained by connecting the original point to a point P(111). 
   Referring to  FIG. 4A , the first trenches  311  and  312  are illustrated by example. The lengths L 1  of the first trenches  311  and  312  vertically formed on the wafer  300  by etching are equal to each other, and the horizontal line H 21  that connects the lowermost parts of the first trenches  311  and  312  corresponds to the lower part of the lower structure. A specified material is injected into the first trenches  311  and  312 , and preferably, this material is SiO2. The width g of the first trench  311  becomes the horizontal gap between the upper structures  612  and the lower structure  620 . By adjusting the width g of the first trench  311 , the horizontal gap can be narrowed. 
   A first patterning is performed through deposition of a first thin film on the wafer, a second patterning is performed through deposition of a second thin film on the wafer and the first thin film, and then second trenches are formed through etching of the wafer (step S 120 ). By the first and second patterning, the second trenches  321  and  322  and the position  331  of a third trench are determined. Then, only the second trenches are formed through patterning of the second thin film  316 . This is because the position  331  of the third trench on the wafer  300  is not etched due to the second thin film  316  deposited on the wafer. The length L 2  of the second trenches  321  and  322  vertically formed on the wafer  300  by etching becomes the width of the upper structures  611  to  614 . The lengths L 2  of the second trenches  321  and  322  are equal to each other, and the horizontal line H 11  that connects the lowermost parts of the second trenches  321  and  322  corresponds to the lower part of the upper structures  612 . The upper part of the upper structures  611  to  614  becomes the upper end surface of the wafer  300 . It is preferable to form the second trenches  321  and  322  into the wafer  300  by a dry etching. The trenches are formed in a vertical direction of the wafer  300  using the dry etching. 
     FIG. 5  is a flowchart illustrating an example of a patterning process used in the method for fabricating the vertical offset structure according to the present invention.  FIGS. 6A to 6D  are cross-sectional views of the structure according to the patterning process of  FIG. 5 . The thin-film patterning process will now be explained with reference to  FIGS. 5 and 6A  to  6 D. 
   A thin film (i.e., etch mask thin film)  510  is deposited on a wafer  500  (step S 210 ). After photoresist is deposited on the thin film  510 , the photoresist  520  is patterned through scanning of ultraviolet (UV) rays (step S 220 ). Referring to  FIG. 6B , the UV rays are scanned on the photoresist  520  after penetrating a photo mask  530 , but they cannot penetrate parts to which chrome  531  is attached. Accordingly, the UV rays penetrate only the parts of the photo mask  530 , to which chrome  531  is not attached, and are scanned on the photoresist  520 . Then, the patterned photoresist part  521  is removed, and the etching is performed (step S 230 ). Referring to  FIG. 6C , a thin film part  511  that corresponds to the patterned photoresist part  521  is removed by etching. Then, the thin film is patterned through removing of the photoresist part (step S 240 ). Referring to  FIG. 6D , the patterned thin film  512  is illustrated. Through the above-described processes, the thin film is patterned. 
   Referring again to  FIGS. 3 and 4A  to  4 H, the second trenches  321  and  322  are vertically extended through the etching after protection layers  325  and  326  are formed on wall surfaces of the second trenches  321  and  322  (step S 130 ). Referring to  FIG. 4C , the reason why the protection layers  325  and  326  are formed is to facilitate the vertical extension of the second trenches  321  and  322 . The vertical lengths L′ 2  of the second trenches  321 ′ and  322 ′ vertically extended are equal to each other, and the horizontal line H 3   h  that connects the lowermost parts of the second trenches  321 ′ and  322 ′ corresponds to an upper part of the base structure  630 . The distance Hb between the lower part of the lower structure  620  and the upper part of the base structure  630  is changed according to the vertical length L′ 2  of the second trenches  321 ′ and  322 ′. Preferably, a dry etching is used to vertically extend the second trenches  321  and  322 . 
   After the second thin film  316  is removed, the third trench  331  is formed through an etching process (step S 140 ). Referring to  FIG. 4D , the third trench  331  is formed in the position from which the second thin film (See  316  in  FIG. 4B ) is removed. Preferably, a dry etching is used to form the third trench  331 . The line H 2 h horizontally extending the lowermost part of the third trench  331  corresponds to the upper part of the lower structure  620 . The difference Ha between the lower part of the upper structures  612  and the upper part of the lower structure  620  is changed according to the vertical length L 3  of the third trench  331 . 
   Then, an etching for horizontally extending the second trenches  321 ′ and  322 ′ extended at the second etching step and the third trench  331  is performed (step S 150 ). It is preferable that a wet etching is performed to horizontally extend the trenches, and the wet etching is performed in an alkaline solution. The cross section of the structure obtained as a result of wet etching is illustrated in  FIG. 4E . If the thin films  315  and  316  and the protection layers  325  and  326 , which are deposited on the structure obtained as a result of wet etching, are removed from the structure, a vertical offset structure having the structure as illustrated in  FIG. 4F  is obtained (step S 160 ). In  FIG. 4F , only a part of the vertical offset structure fabricated on the wafer  300  is illustrated. In other words, the vertical offset structure as illustrated in  FIG. 4F  is repeatedly formed, and a unit pitch  370  for such repetition is indicated in  FIG. 4F . Referring to  FIGS. 4A and 4F , it can be seen that the horizontal gap g between the upper structures  612  and the lower structure  620  is equal to the width of the first trench  311 . 
   The vertical length L 1  of the first trench, the vertical length L 2  of the second trench formed at the first etching step, the vertical length L′ 2  of the second trenches vertically extended at the second etching step, and the vertical length L 3  of the third trench are in the order of L′ 2 &gt;L 1 &gt;L 3 &gt;L 2 . 
   In this case, the lower part of the upper structures  611  to  614  is formed in a position that has a difference of L 2  from the upper part of the wafer  300 , and the thickness of the upper structures  611  to  614  corresponds to L 2 . The upper part of the lower structure  620  is formed in a position that has a difference of L 3  from the upper part of the wafer  300 , and the thickness of the lower structure  620  corresponds to a length obtained by subtracting L 3  from L 1 . The upper part of the base structure  630  is formed in a position that has a difference of L′ 2  from the upper part of the wafer  300 , and the thickness of the base structure  630  corresponds to a length obtained by subtracting L′ 2  from the width of the wafer  300 . The horizontal gap between the upper structures  611  to  614  and the lower structure  620  corresponds to the width of the first trenches  311  and  312 . 
   The vertical offset structure fabricated by the fabrication method according to the present invention has a complete vertical offset in comparison to the vertical offset structure of U.S. patent application Publication No. 2002/0158293A1, and can reduce the horizontal gap between the upper structures and the lower structure to 1/4.5 in comparison to the vertical offset structure of U.S. Pat. No. 6,694,504B2. In driving the vertical offset structure, for example, the upper structures and the lower structure may be driven with different voltages applied thereto. In this case, as the horizontal gap is narrower, they can be driven even with lower voltages, and a greater force is produced when the same voltage is applied to the upper and lower structures. Accordingly, the vertical offset structure according to the present invention has an improved effect in two respects as described above in comparison to the vertical offset structure of U.S. Pat. No. 6,694,504B2. 
   Table 1 below shows items of the vertical offset structure (SAIT) of U.S. patent application Publication No. 2002/0158293A1, the vertical offset structure (SNU) of U.S. Pat. No. 6,694,504B2 and the vertical offset structure (Proposed) according to the present invention for the purpose of comparison. 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Version 
               SAIT 
               SNU 
               Proposed 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               Minimum 
               ~2 
               ~4.5 
               &lt;1 
             
             
               possible gap 
                 
             
             
               between 
                 
             
             
               electrodes [μm] 
                 
             
             
               Unit electrode 
               ~11 
               ~10 
               ~12 
             
             
               pitch [μm] 
                 
             
             
               Electrode 
               Offset formed 
               Offset formed on 
               Offset formed on 
             
             
               structure 
               only on lower 
               both upper and 
               both upper and 
             
             
                 
               part of electrodes 
               lower parts of 
               lower parts of 
             
             
                 
                 
               electrodes 
               electrodes 
             
             
                 
             
           
        
       
     
   
   The compared items are the number of masks, minimum possible gap between electrodes, unit electrode pitch, electrode cross-section structure, and the change of capacitance.  FIG. 7  is a view illustrating the conventional vertical offset structures and the vertical offset structure according to the present invention for the purpose of comparison. Referring to  FIG. 7 , the vertical offset structure  610  of U.S. Patent Application Publication No. 2002/0158293A1, the vertical offset structure  620  of U.S. Pat. No. 6,694,504B2 and the vertical offset structure  630  according to the present invention are illustrated. In  FIG. 7 , the minimum possible gap g between electrodes, and the unit electrode pitch are also illustrated. The minimum possible gap g means the horizontal gap between the upper structures (i.e., upper electrodes) and the lower structure (i.e., lower electrode). 
   The vertical offset structure according to the present invention can be used as vertical combs for vertical driving and detection. Specifically, the vertical offset structure according to the present invention may be used to fabricate accelerometers, gyroscopes, micromirrors, etc. Research for the micromirrors has been widely carried out for displays, scanners, optical communications, etc. 
   As described above, according to the present invention, the vertical lengths of the second and third trenches are determined to be different from each other, and thus a vertical offset structure in which a complete vertical offset is produced between the upper and lower structures. 
   Also, according to the present invention, by adjusting the width of the first trench, the vertical offset structure, in which the horizontal gap between the upper and lower structures is greatly narrowed, can be provided. 
   Although in the embodiment of the present invention, the first to third trenches have been explained, the width and depth of the first trenches, second trenches, or third trench may be differently determined. 
   The foregoing embodiment and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. Also, the description of the embodiments of the present invention is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.