Abstract:
A wafer, having at least a spindle region and at least two through regions alongside the spindle region, is provided. The wafer in the spindle region is partially removed from the bottom surface. Thereafter, the bottom surface is bonded to a carrier with a bonding layer, and the wafer in the through regions is completely removed from the top surface.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention pertains to a method of double-sided etching, and more specifically, to a method of forming a micro spindle by double-sided etching. 
   2. Description of the Prior Art 
   Micro electromechanical system (MEMS) technology is an emerging technology which highly integrates electronics and mechanics, and has been broadly adopted to fabricate various devices having electro and mechanical characteristics, such as micro sensors, micro actuators, micro motors, photosensitive switches, etc. MEMS devices generally have mechanical structures more complicated than semiconductor devices, and thus cannot be directly fabricated by standard semiconductor processes. Take the micro spindle, one of the most common structures in MEMS devices, for example, accurate shape and smooth surface condition are strictly required for ensuring reliability and stress-bearing capability. 
   Please refer to  FIG. 1  to  FIG. 3 .  FIG. 1  is a schematic diagram of a micro spindle  10 ;  FIG. 2  and  FIG. 3  are schematic diagrams illustrating a conventional method of forming a micro spindle. As shown in  FIG. 1 , the micro spindle is a suspension structure capable of being driven by a voltage, light beams, or a magnetic field, and thereby rotates in the direction as the arrow indicates shown in  FIG. 1 . Therefore, the micro spindle  10  must have an accurate shape, a smooth surface, and a uniform axis so as to guarantee reliability and stress-bearing capability. The conventional method of fabricating the spindle  10  is described below. 
   As shown in  FIG. 2 , a wafer  20  is provided. Then, an etching stop layer  22  and a photo resist pattern  24  are respectively formed on the bottom surface and the top surface of the wafer  20 . As shown in  FIG. 3 , an etching process is followed to etch through the wafer  20  not covered by the photo resist pattern  24  until the etching stop layer  22 . 
   According to the conventional method, however, equilibrium of the etching process and the thickness uniformity of the wafer  20  are not taken into consideration. Thus, the yield of the etching process cannot be well controlled because etching rates in different regions of the wafer  20  are not equal. For example, on the occasion of etching through the wafer  20 , the overall area being etched varies dramatically, and leads to unexpected changes during the etching process. In addition, when the etching process is performed down to the etching stop layer  22 , side etching effect tends to occur and therefore results in an undercut  26  as shown in  FIG. 3 . As mentioned earlier, if the shape accuracy of the micro spindle degrades, reliability of the micro spindle will be seriously affected. 
   SUMMARY OF INVENTION 
   It is therefore a primary object of the present invention to a method of double-sided etching to overcome the aforementioned problems. 
   According to a preferred embodiment of the present invention, a method of forming a micro spindle is disclosed. The method includes the following steps: 
   providing a wafer comprising at least a spindle region and two through regions, the two through regions being respectively positioned on both sides of the spindle region; 
   partially removing the wafer in the spindle region from a first surface of the wafer; and 
   removing the wafer in the two through regions from a second surface of the wafer until the wafer is removed through to the first surface. 
   The method forms the micro spindle by means of double-sided etching, and thus is able to prevent excessive variation in the overall area being etched and prevent the side etching problem while the etching process is performed down to the etching stop layer. Consequently, the reliability and stress-bearing capability of the micro spindle is ensured. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic diagram of a micro spindle. 
       FIG. 2  and  FIG. 3  are schematic diagrams illustrating a conventional method of forming a micro spindle. 
       FIG. 4  through  FIG. 8  are schematic diagrams illustrating a method of forming a micro spindle according to a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 4  through  FIG. 8 .  FIG. 4  through  FIG. 8  are schematic diagrams illustrating a method of forming a micro spindle according to a preferred embodiment of the present invention. As shown in  FIG. 4 , a wafer  50 , such as a silicon wafer, is provided, and a photo resist pattern  52  is formed on the bottom surface of the wafer  50  to define the dimension of a spindle region  54 . As shown in  FIG. 5 , an etching process, such as a reactive ion etching (RIE) process, is performed to etch the wafer  50  unprotected by the photo resist pattern  52  down to a predetermined depth. It is appreciated that the predetermined depth must be greater than the sum of the deviation of the etching process to be performed and the deviation of the thickness of the wafer  50 , and therefore the structure of the micro spindle will not be damaged during the etching process to be performed. 
   As shown in  FIG. 6 , the photo resist pattern (not shown) is stripped, and a bonding layer  56  is utilized to bond the bottom surface of the wafer  50  and a carrier  58  together. Afterward, another photo resist pattern  60  is formed on the top surface of the wafer  50  to define the dimensions of two through regions  62 . In this embodiment, the material of the bonding layer  56  is selected from photo resist, metal, silicon oxide, benzocyclobutene (BCB), polyimide, tape, UV tape, wax, and so on, which can be easily removed by wet etching, heating, or irradiating. The carrier  58  is selected from materials compatible to semiconductor processes, such as silicon, glass, quartz, and ceramics. 
   As shown in  FIG. 7 , another etching process, such as a RIE process, is performed to etch the wafer  50  unprotected by the photo resist pattern  60  in the through regions  62 . It is noteworthy that when the etching process is performed down to the depth shown in  FIG. 7 , the wafer  50  in the spindle region  54  is suspended. Meanwhile, the variation of overall area being etched is equal to the area of the wafer  50  in the through regions  62  minus the area of the wafer in the spindle region  54 , and thus the overall area is not dramatically varied. In addition, since the etching process does not reach the bonding layer  56 , side etching effect will not occur. It is also appreciated that the dimension of the spindle region  54  is slightly larger than the actual dimension of the micro spindle to be formed for increasing alignment tolerance of the second etching process, so that the shape and size of the micro spindle is more accurate. 
   As shown in  FIG. 8 , the etching process is continued until etching through wafer  50  in the through regions. The photo resist pattern  60  on the top surface of the wafer  50  and the bonding layer  56  on the bottom surface of the wafer  50  are then removed for accomplishing fabrication of the micro spindle. The bonding layer  56  works to bond the wafer  50  and the carrier  58 , and functions as an etching stop layer as well. When the etching process is performed down to the bonding layer  56 , the overall area being etched varies extremely, and side etching may occur. However, the wafer  50  in the spindle region  54  is suspended, and the predetermined depth being etched in the first etching process takes the variations of the second etching process and the wafer  50  thickness into consideration in advance, and thus the wafer  50  in the spindle region  54  is not affected. Consequently, the structure of the micro spindle is ensured. 
   In comparison with prior art, the method of the present invention forms the micro spindle by means of double-sided etching, and thus is able to prevent excessive variation in overall area being etched and the side etching problem while the etching process is performed down to the etching stop layer. Consequently, the reliability and stress-bearing capability of the micro spindle is ensured. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.