Abstract:
A method for storing wafers is disclosed. A plurality of wafers are placed into the wafer cassette box. The wafer cassette box is hermetically sealed and pumped down to vacuum for the wafer storage. Alternatively, the wafers carried by a holder conveyed on a wafer conveyor are placed into a pump-down chamber enclosing a section of the wafer conveyor. The pump-down chamber is hermetic sealed and pumped down to vacuum for the wafer storage on the wafer conveyor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method of fabricating semiconductor devices, and particularly to a method of storing wafers in the interval of two semiconductor manufacturing processes. 
         [0003]    2. Description of the Prior Art 
         [0004]    In the semiconductor fabrication technology, extremely high cleanliness environment is demanded for wafer processing. The wafers for processing and loading/unloading to/from a process tool are constantly flushed with ultra-pure nitrogen that contains no oxygen and moisture, in order to eliminate micro-contamination and to reduce native oxide growth on silicon surfaces. During a Q-time between two fabricating processes, as shown in  FIG. 1 , wafers  2  are usually stored in a wafer storage container  4  without the particle contamination, moisture absorption and oxidation problems by maintaining a positive pressure of an inert gas (such as nitrogen gas) inside the container higher than the surrounding environment outside the container. The wafers are transported (also referred to as “conveyed”) from one process tool or apparatus to another. 
         [0005]    However, the ultra-pure nitrogen gas is costly. Therefore, there is still a need for a novel method for storing wafers efficiently and economically. 
       SUMMARY OF THE INVENTION 
       [0006]    One object of the present invention is to provide a method for storing wafers to prolong Q time. 
         [0007]    According to one embodiment of the present invention, a method for storing wafers includes steps as follows. A wafer cassette box is provided. A plurality of wafers are placed into the wafer cassette box. The wafer cassette box is hermetically sealed. The wafer cassette box having the wafers therein is pumped down to vacuum. Accordingly, the wafers can be stored in the wafer cassette box in vacuum. 
         [0008]    According to another embodiment of the present invention, a method for storing wafers on a wafer conveyor includes steps as follows. A wafer conveyor is provided. A pump-down chamber is formed to enclose a section of the wafer conveyor. A plurality of wafers carried by a holder are conveyed on the wafer conveyor into the pump-down chamber. The pump-down chamber is sealed. The pump-down chamber is pumped down to vacuum. Accordingly, the wafers can be stored as being carried by the holder on the wafer conveyor in the pump-down chamber in vacuum. 
         [0009]    According to further another embodiment of the present invention, a method for storing wafers on a wafer conveyor includes steps as follows. A wafer conveyor is provided. A pump-down chamber is formed to enclose a first section of the wafer conveyor. The pump-down chamber is pumped down to vacuum. A pre-pump chamber is formed to enclose a second section of the wafer conveyor adjacent to a first side of the pump-down chamber. A plurality of wafers carried by a holder is conveyed on the wafer conveyor into the pre-pump chamber. The pre-pump chamber is pumped down to a reduced air pressure. The wafers with the holder together are conveyed from the pre-pump chamber into the pump-down chamber. The pump-down chamber is pumped down with the wafers therein to vacuum for storing the wafers. 
         [0010]    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 THE DRAWINGS 
         [0011]      FIG. 1  is a schematic diagram illustrating a conventional method to store wafers in a wafer cassette box; 
           [0012]      FIG. 2  is a flowchart illustrating an embodiment of the method for storing wafers according to the present invention; 
           [0013]      FIG. 3  is a schematic diagram illustrating an embodiment of the method for storing wafers according to the present invention; 
           [0014]      FIGS. 4 and 5  are flow charts illustrating some embodiments of the method for storing wafers on a wafer conveyor according to the present invention; and 
           [0015]      FIGS. 6 and 7  are schematic diagrams illustrating some embodiments of the method for storing wafers on a wafer conveyor according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring to  FIG. 2  and  FIG. 3 , a method for storing wafers according to an embodiment includes steps described as follows. First, Step  101  is performed to provide a wafer cassette box. The wafer cassette box  10  may include a box body  12  having an opening and a cover member  14  for closing the opening. The wafer cassette box  10  may be a conventional one. The cover member  14  can be used to hermetically cover the opening to allow the wafer cassette box  10  to be in an airtight status. The wafer cassette box  10  has a valve  18  for pumping. The valve  18  may be disposed on the box body  12  or the cover member  14 . 
         [0017]    Next, Step  102  is performed to place wafers  20  into the wafer cassette box  10  through the opening of the wafer cassette box  10 . The wafers  20  may be semi-products processed from a semiconductor device manufacturing process and wait for a subsequent process. The wafers  20  may be usually carried by a holder  22 . The wafers  20  and the holder  22  may be put together into the wafer cassette box  10  for storage until being fetched out for the subsequent process. The Step  103  is performed to hermetically seal the wafer cassette box  10 . In this embodiment, sealing the wafer cassette box  10  maybe accomplished by covering the opening with the cover member  14 . An o-ring  16  may be further disposed to surround the opening or the cover member, such that when the cover member covers the opening, the o-ring  16  can be between the cover member and the opening to hermetically seal the wafer cassette box  10 . 
         [0018]    Thereafter, Step  104  is performed to pump down the wafer cassette box  10  to vacuum. The pumping may be through the valve  18 . The valve  18  may be a one-direction valve for only permit one-way airflow, or a two- or more-direction valve which can be turned off for prohibiting the air from flowing into the wafer cassette box  10  through the valve after the vacuum status is reached. A useful vacuum pump is not particularly limited as long as it can achieve the desired vacuum and does not cause pollution. After the vacuum status is reached and the valve is turned off, the wafers are in a wafer cassette box and separated from environmental air for storage. 
         [0019]      FIGS. 4 and 5  further illustrate flow charts according to some other embodiments.  FIGS. 6 and 7  are schematic diagrams for illustrating the embodiments. According to the embodiments, the wafers are stored in a holder on a wafer conveyor. The holder may be a cassette for carrying the wafers to process tools or apparatus. The wafer conveyor is for conveying the wafers carried by the cassette to process tools or apparatus. In the embodiments of the present invention, the wafers carried by the holder are stored on the wafer conveyor, such that loading and unloading procedures may be not needed. Accordingly, it is time saving and convenient. 
         [0020]    Referring to  FIG. 4  and  FIG. 6 , a method for storing wafers on a wafer conveyor according to the embodiment includes steps described as follows. First, Step  201  is performed to provide a wafer conveyor  24 . The wafer conveyor is preferably a roller conveyor. Next, Step  202  is performed to form a pump-down chamber  26  to enclose a section of the wafer conveyor  24 . The pump-down chamber  26  may include an inlet with a door  28  and an outlet with a door  30  to allow the wafer conveyor  24  to go through the pump-down chamber  26 . The pump-down chamber  26  further includes at least one valve  32  for pumping down the pump-down chamber  26  to vacuum. The valve  32  may be as aforesaid. Thereafter, Step  203  is performed to convey the wafers  34  carried by a holder  36  on the wafer conveyor  24  into the pump-down chamber  26  from, for example, the inlet. The wafers  34  and the holder  36  may be placed within a box  38 , such as a SMIF (standard mechanical interface) pod. Step  204  is performed to hermetically seal the pump-down chamber  26 , which may be achieved by closing the doors  28  and  30 . Preferably, an o-ring is disposed between each of the doors and each of the openings of the inlet and the outlet for securing the airtight status. Step  205  is performed to pump down the pump-down chamber  26  to vacuum, and accordingly the wafers  34  are stored in the pump-down chamber  26  on the wafer conveyor  24 . 
         [0021]    Referring to  FIG. 5  and  FIG. 7 , a method for storing wafers on a wafer conveyor according to the embodiment includes steps described as follows, in which a pre-pump down step is performed. First, Step  301  is performed to provide a wafer conveyor  24 . The wafer conveyor is preferably a roller conveyor. Next, Step  302  is performed to forming a pump-down chamber  26  to enclose a first section of the wafer conveyor  24 . The pump-down chamber  26  may be as aforesaid. Thereafter, Step  303  is performed to pump down the pump-down chamber  26  to vacuum. 
         [0022]    Not limited to the time order with respect to Steps  301  to  303 , Step  304  is performed to form a pre-pump chamber  40  to enclose a second section of the wafer conveyor  24 . The pre-pump chamber  40  is disposed adjacent to a side of the pump-down chamber  26  and may also include an inlet with a door and an outlet with a door to allow the wafer conveyor  24  to go through. The outlet with the door may be merged with the inlet with the door  28  of the pump-down chamber  26  to be one, as shown in  FIG. 7 . The pre-pump chamber  40  may further include at least one valve  44  for pumping. Preferably, an o-ring is disposed between each of the doors and each of the openings of the inlet and the outlet for securing the airtight status. 
         [0023]    Step  305  is performed to convey the wafers  34  carried on the holder  36  on the wafer conveyor  24  into the pre-pump chamber  40 . Step  306  is performed to pump down the pre-pump chamber  40  to a reduced air pressure through the valve  44 , with the doors closed. This step allows the wafers not to suffer from a drastic change of air pressure in the moment when the door  28  is opened. Accordingly, the reduced air pressure may be less than that in the environment and greater than that in the pump-down chamber  26 . Step  307  is performed to convey the wafers  34  carried by the holder  36  on the wafer conveyor  24  from the pre-pump chamber  40  into the pump-down chamber  26 . Step  308  is performed to pump down the pump-down chamber  26  with the wafers  34  therein through the valve  32 , preferably with the doors  28  and  30  closed, to vacuum for storing the wafers  34 . Thereafter, the valve  32  is turned off if it is desired. Accordingly, the wafers  34  can be stored on the wafer conveyor  24  for waiting for the subsequent process. 
         [0024]    Furthermore, referring to  FIG. 7 , an open-pump chamber  46  may be further formed to enclose a third section of the wafer conveyor  24  adjacent to another side of the pump-down chamber  26 . The open-pump chamber  46  may include an inlet with a door and an outlet with a door  48  to allow the wafer conveyor  24  to go through. The inlet with the door may be merged with the outlet with the door  30  of the pump-down chamber  26  to be one, as shown in  FIG. 7 . The open-pump chamber  46  may further include at least one valve  50  for pumping. Preferably, an o-ring is disposed between each of the doors and each of openings of the inlet and the outlet for securing the airtight status. When the wafers are about to be conveyed to a subsequent processing tool or apparatus, the open-pump chamber  46  may be pumped down through the valve  50  to a reduced air pressure, such that the wafers  34  will not suffer from a drastic change of air pressure in the moment when the door  30  is opened. Accordingly, the reduced air pressure may be less than that in the environment and greater than that in the pump-down chamber  26 . Thereafter, the wafers  34  are conveyed from the pump-down chamber  26  into the open-pump chamber  46 . Thereafter, the wafers  34  are conveyed out of the open-pump chamber  46 . 
         [0025]    In the present invention, when the wafers are stored in vacuum, the wafers are separated from contaminates, such as oxygen gas, moisture, and particles. Accordingly, inert gas, such as nitrogen, is not required to purge or fill the wafer cassette box, to reduce production cost. 
         [0026]    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.