Abstract:
An apparatus and method to polish a wafer using abrasive flow machining (AFM) is provided. Under a high-pressure condition, the wafer is polished by flowing abrasive media with high viscosity, on the wafer in order to planarize the wafer. Therefore, the polishing efficiency is higher, and the attained roughness is lower than the conventional method. In addition, the selectivity of this method is lower.

Description:
FIELD OF THE INVENTION 
     The invention relates to an apparatus and method to polish a wafer using abrasive flow machining (AFM), and in particular to a polishing apparatus and method with high efficiency and stability. 
     DESCRIPTION OF THE RELATED ART 
     Planarization is a very important technology in wafer processing. Currently, chemical mechanical polishing (CMP) is a common manner to planarize the wafer. However, CMP technology has a lot of disadvantages, as enumerated below: 
     (1) a lot of water is needed, 
     (2) a lot of chemical treatment using slurry is required, 
     (3) short life time of every consumable part of the polishing apparatus, 
     (4) short PM time, 
     (5) dishing and other errors, 
     (6) a lot of vibration and noise is produced, 
     (7) the processed wafer is easily scratched; 
     (8) the quality of the polishing pad is unstable; 
     (9) precise control is required, 
     (10) the pre-polish topography will affect the result of the polish, 
     (11) the polishing apparatus is easily corroded, 
     (12) the supply tube and the discharge pipe of the slurry is easily clogged, and 
     (13) chemical evaporation results in pollution. 
     SUMMARY OF THE INVENTION 
     In view of the above problem, an object of the invention is to provide an apparatus and method to polish a wafer using abrasive flow machining. Under a high-pressure condition, the wafer is polished by abrasive media with high viscosity in order to planarize the wafer. Therefore, the polishing efficiency is higher, and the attained roughness is lower than by the conventional method. In addition, the selectivity of this method is lower. 
     To attain the above object, the polishing apparatus comprises a table, a carrier, an abrasive media supply device, a pressing device and an abrasive media-removing device. The table is used for supporting the wafer disposed thereon. The carrier is disposed on the table in an detachable manner. The abrasive media supply device, communicating with the carrier, is used for supplying abrasive media to a space defined by the carrier and the table. The pressing device, communicating with the carrier, is used for applying pressure to the abrasive media inside the space. The abrasive media-removing device, communicating with the carrier, is used for supplying high-pressure air to the space to remove the abrasive media. 
     Furthermore, the abrasive media supply device consists of a first tube, a first valve and a pump. The first tube is communicated with the carrier at one end. The first valve is attached to the first tube. The pump is communicated with the other end of the first tube. 
     Furthermore, the pressing device consists of a rod and a first cylinder. The rod is located inside the carrier partly and in contact with the abrasive media at one end. The first cylinder is connected with the other end of the rod. 
     Furthermore, the abrasive media-removing device consists of a channel, a check valve, a second tube, a second valve and an air source. The channel is located inside the rod. The check valve is attached to one end, in contact with the abrasive media inside the carrier, of the channel. The second tube is communicated with the other end of the channel. The second valve is attached to the second tube. The air source is communicated with the other end of the second tube. 
     Furthermore, the apparatus further comprises a robot, a lift pin, a second cylinder, a release hole and a cooling path. The robot is used for transferring the wafer. The lift pin is moveably disposed inside the table. The second cylinder, connected with the carrier, is used for moving the carrier. The release hole is disposed on the carrier. The cooling path is disposed inside the table. 
     The method of the invention comprises the steps of: (a) providing the polishing apparatus; (b) transferring the wafer on the lift pin by means of the robot; (c) positioning the wafer on the table by lowering the lift pin; (d) positioning the carrier on the table by means of the second cylinder to form the space between the carrier and the table; (e) supplying the abrasive media to the space through the first tube and the first valve by means of the pump; (f) shutting off the first valve when the abrasive media inside the space reaches a predetermined amount; (g) applying pressure to the abrasive media inside the space by moving the first cylinder to polish the wafer and discharge the abrasive media through the release hole; (h) supplying the high-pressure air from the air source through the second tube, the second valve, the channel and the check valve to the space to remove the abrasive media through the release hole; (i) after separating the carrier from the table by means of the second cylinder, separating the wafer from the table by raising the lift pin; and (j) removing the wafer from the table by means of the robot. 
     Furthermore, the size of the abrasive media is from 800 to 0.5 μm. The temperature during the polishing is from 25 to 80° C. The abrasive media is aluminum oxide, silica, silicon carbide or diamond. The pressure applied to the abrasive media is from 100 to 3000 psi. The discharge rate through the release hole is from 7 to 250 L/min. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is hereinafter described in detail by reference to the accompanying drawings in which: 
     FIG. 1 is a schematic diagram showing the apparatus to polish a wafer using abrasive flow machining of an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, it shows a method and apparatus to polish a wafer using abrasive flow machining of an embodiment of the present invention. 
     The apparatus of this invention consists of a table  4 , a carrier  5 , an abrasive media supply device, a pressing device and an abrasive media-removing device. The table  4 , serving as a base of the apparatus, is used for supporting a wafer  1  and the carrier  5  disposed thereon. The carrier  5 , transferred by means of a second cylinder  6 , is hood-typed. When the carrier abuts against the table  4 , abrasive media  7  to be flowed into the carrier  5  flows into a predetermined space defined by the carrier  5  and the table  4 . 
     The abrasive media supply device, consisting of a first tube  9 , a first valve  10  and a pump  8 , is used for supplying the abrasive media  7  into the space between the carrier  5  and the table  4 . One end of the first tube  9  is communicated with the carrier  5 , and the other end thereof is communicated with the pump  8  to supply the abrasive media  7  to the carrier  5 . The first valve  10  is attached to the first tube  9  to control the amount of the abrasive media  7  through the first tube  9 . The pump  8  forces the abrasive media  7  to flow into the carrier  5 . 
     The pressing device, consisting of a rod  11  and a first cylinder  12 , is used for applying pressure to the abrasive media  7  inside the carrier  5 . Part of the rod  11 , partly located inside the carrier  5 , is hollow, and one end thereof abuts against the abrasive media  7  inside the carrier  5 . The first cylinder  12 , connected with the other end of the rod  11 , is used for moving the rod  11 . 
     The abrasive media-removing device, consisting of a channel  16 , a check valve  17 , a second tube  20 , a second valve  14  and an air source  13 , is used for supplying high-pressure air into the carrier  5  to remove the remained abrasive media  7 . The channel  16 , used for the path of the high-pressure air flowing into the carrier  5 , is the hollow part of the rod  11 . The check valve  17 , used for keeping the high-pressure air from the channel  16  flowing toward the carrier  5 , is attached to one end, contact with the abrasive media  7  inside the carrier  5 , of the channel  16 . One end of the second tube  20 , used as a path of the high-pressure air flowing into the channel  16 , is communicated with the other end of the channel  16 , and the other end thereof is communicated with the air source  13 . The second valve  14 , used for controlling the high-pressure air, is attached to the second tube  20 . The air source  13 , used for supplying the high-pressure air, is communicated with the other end of the second tube  20 . 
     In addition, the apparatus further comprises a robot  2 , a lift pin  3 , a second cylinder  6 , a release hole  15  and a cooling path  19 . The robot  2  is used for transferring the wafer  1 . The lift pin  3 , making the deposition and the separation between the wafer  1  and the table  4  easier, is moveably disposed inside the table  4 . The second cylinder  6 , connected with the carrier  5 , is used for moving the carrier  5 . The release hole  15 , used for discharging the abrasive media  7 , is disposed on the carrier  5 . The cooling path  19  is disposed inside the table  4 . Since the cooling water  18  flows inside the cooling path  19 , the cooling path  19  can keep the temperature of the wafer  1  constant during the polishing process. 
     The method of this invention comprises the steps of: (a) transferring the wafer  1  on the lift pin  3  by means of the robot  2 ; (b) positioning the wafer  1  on the table  4  by lowering the lift pin  3  while the robot  2  returning to home; (c) positioning the carrier  5  on the table  4  by means of the second cylinder  6  to form a predetermined space between the carrier  5  and the table  4 ; (d) supplying the abrasive media  7  to the space through the first tube  9  and the first valve  10  by means of the pump  8 ; (e) shutting off the first valve  10  when the abrasive media  7  inside the space reaches a predetermined amount; (f) applying pressure to the abrasive media  7  inside the space by moving the first cylinder  11  to polish the wafer  1  and discharging the mixture of the abrasive media  7  and the substance produced from the surface of the wafer  1  through the release hole  15 ; (g) supplying the high-pressure air from the air source  13  through the second tube  20 , the second valve  14 , the channel  16  and the check valve  17  to the space to remove the remaining abrasive media  7  through the release hole  15 ; (i) after separating the carrier  5  from the table  4  by means of the second cylinder  6 , removing the wafer  1  from the table  4  by raising the lift pin  3 ; and (j) removing the wafer  1  from the lift pin  3  by means of the robot  2 . 
     Furthermore, the powder size of the abrasive media in this invention is from 800 to 0.5 μm in order to be stiff to fluid. During the polishing process, the temperature of the wafer is from 25 to 80° C. The abrasive media is aluminum oxide, silica, silicon carbide or diamond preferably. The pressure, applied to the wafer by the pressing means, is from 100 to 3000 psi. The discharge rate of the abrasive media through the release hole is from 7 to 250 L/min. 
     In summary, the advantages of this invention is described as follows: 
     (1) high removal rate: typically 0.0013 to 0.5 mm/min (determined by the polishing condition), 
     (2) better roughness (Ra): 2 to 20 μin (typically {fraction (1/10)} of prior roughness) 
     (3) low selectivity: since the polishing method is physical, it will not have the same drawback of the conventional chemical method. In other word, the polishing result of this invention will not depend on the material of the wafer. 
     It is to be understood that although the present invention has been described with reference to particular preferred embodiments, it should be appreciated that numerous modifications, variations and adaptations may be made without departing from the scope of the invention as defined in the claims.