Patent Document

CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority to U.S. patent application Ser. No. 10/053,371, filed Jan. 18, 2002 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0002]    This invention relates generally to methods and apparatus for removing photoresist from the surfaces of substrates, and specifically to methods and apparatus for removing photoresist from silicon semiconductor wafers using ozonated deionized water.  
         BACKGROUND ART  
         [0003]    The need for quick and efficient removal of photoresist is critical in the area of semiconductor manufacturing. In order to produce a useful semiconductor wafer, first a silicon crystal is grown, sliced into thin wafers, and exposed to a photoresist which forms a layer on the wafers. Multiple layers of photoresist can be formed on the surface of the wafers and then etched off to form patterns on the wafers.  
           [0004]    The use of DIO 3 , which is a mixture of chilled distilled water (DI) and ozone (O 3 ), to remove photoresist from surfaces of a silicon wafer has been taught by Matthews in U.S. Pat. No. 5,776,296. Matthews discloses a process and an apparatus for removing photoresist from a semiconductor wafer using DIO 3  at sub-ambient temperatures of 1 to 15° C. wherein the ozone is introduced into the process tank with “a composite element having a permeable member and a nonpermeable member, the permeable member having a top portion and a bottom portion, a means defining an open space in a center portion of the permeable member, and a means defining a trench positioned on the top portion of the permeable member between an outer periphery of the permeable member and the means defining an open space.” The Matthews system suffers from certain disadvantages in photoresist removal.  
           [0005]    Specifically, requiring that the DIO 3  be at a sub-ambient temperature necessitates the use of a chiller, which can be expensive and occupy valuable space in manufacturing clean rooms. Additionally, the Matthews system and methods are not optimal with respect to the speed, efficiency, and effectiveness.  
           [0006]    Another method of photoresist stripping is taught by Honda et al in U.S. Pat. No. 6,103,680. Honda teaches applying ozonated water to a semiconductor wafer through the use of a spray rinse to strip photoresist from the wafer. However, the strip rates achieved by merely spraying the wafers with ozonated water are less than optimal with respect to the speed, efficiency, and effectiveness. Thus, a need exists to improve this stripping process.  
           [0007]    It is an object of the present invention to provide an improved process of removal of photoresist from semiconductor wafers during the manufacture thereof. Another object is to provide a process and system at high rates and efficiency.  
         DISCLOSURE OF THE INVENTION  
         [0008]    These objects, and others which will become apparant from the following disclosure and the accompanying drawings, are achieved by the present invention which in one aspect is a method of removing photoresist from silicon wafers comprising: positioning at least one semiconductor wafer having at least one layer of photoresist in a process tank; providing ozone gas to said process tank; and spraying said semiconductor wafer with a mixture of ozone and deionized water (DIO 3 ) via at least one nozzle.  
           [0009]    Preferably, the ozone gas in the process tank will be under pressure as the semiconductor wafers are being sprayed with the DIO 3 . This pressure can be between 1 to 3 atmospheres. Also preferably, the nozzles should be high pressure nozzles. The DIO 3  can then be sprayed at a nozzle pressure between 1 to 10 atmospheres from said nozzles, preferably between 5 to 10 atmospheres.  
           [0010]    The process can also include keeping the temperature in the processing tank at or above ambient temperature. In one embodiment, the temperature is kept between 20-50° C. The process can further include the step of recirculating the DIO 3  back into said process tank via said nozzle. When recirculation is used, additional ozone should be added to the DIO 3  during recirculation thereby keeping the concentration of ozone in said mixture about constant. This mixture of deionized water and ozone can then be agitated via at least one nozzle. Also, the mixture of deionized water and ozone can be sprayed as a vapor into said process tank or can be sprayed into said process tank as droplets.  
           [0011]    In another aspect, the invention is an apparatus for the removal of photoresist from semiconductor wafers, comprising: a process tank capable of holding at least one semiconductor wafer; at least one nozzle set within said tank, said nozzle adapted to spray a mixture of deionized water and ozone onto said wafer; and a source of ozone connected to said process tank.  
           [0012]    Preferably, the apparatus will further comprise a means to pressurize said process tank and a means for recirculating said mixture of deionized water and ozone back to said nozzle. The recirculation means should comprise a filter so that photoresist that is stripped off the wafers is not reapplied by the nozzles. Additionally, the means for recirculating can be connected to the source of ozone so that the concentration of ozone in the mixture being applied to the wafers via the nozzles is approximately constant.  
           [0013]    It is further referable that the nozzles be high pressure nozzles capable of spraying said mixture of deionized water and ozone at a pressure between 1 and 10 atmospheres. The apparatus should further comprise a means for temperature control adapted to maintain temperature in said process tank between above 20-50° C.  
           [0014]    The apparatus can further comprise an ozonator in fluid connection with said nozzle. The source of ozone can be an ozone generator and the nozzle is preferably at or near the top of the process tank.  
           [0015]    Preferably a pressure plenum set in excess of one atmosphere, a temperature control system, an ozonator, a filter connected to the tank, and a recirculating pump are included. It is further preferred that the temperature controller is set to maintain the liquid temperature at 20-21° C. or higher.  
           [0016]    The DIO 3  water mixture can be exposed to the photoresist in the form of a fog and/or tiny droplets of water. The concentration level of the gaseous and dissolved ozone can be monitored using inline ozone analyzers.  
           [0017]    Agitation of the DIO 3  water on the photoresist layers raises the rate of photoresist removal, i.e., the “strip rate” for photoresist treated with DIO 3  water is linked to the velocity rate of the DIO 3  water. Notably, an increase in the fluid velocity reduces the boundary layer thickness, thereby resulting in a higher rate of O 3  oxidizing the photoresist (also known as “the etching rate”). Fluid velocity of the mixture coming from the nozzles increases with nozzle pressure. In addition, the use of sonic energy also reduces the boundary layer thickness, again resulting in a higher rate of O 3  oxidizing the photoresist or etch rate. Thus, the higher the kinetic energy and O 3  concentration, the shorter the strip time. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a schematic of an embodiment of a photoresist stripping apparatus according to the present invention.  
         [0019]    [0019]FIG. 2 is a chart of the rate of etching of the photoresist versus the ambient temperature.  
         [0020]    [0020]FIG. 3 is a chart of the rate of etching of the photoresist versus the nozzle supply pressure of DIO 3  water. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Inasmuch as the etch rate of photoresist utilizing a solution of O 3  in DI water increases linearly with the increase in O 3  concentration, the object of the present invention is to provide a method which significantly increases the O 3  concentration in a DI water solution from the methods currently available. In addition, the series of nozzles seek to increase the velocity rate of the DIO 3  water so as to reduce the boundary layer thickness and therefore increase the rate of etching. The increase in velocity rate of the DIO 3  water coming out of the nozzles is a function of nozzle pressure. The higher the nozzle pressure, the higher the velocity. Additionally, the O 3  concentration in the DIO 3  water can be increased by providing a pressurized ozone gas atmosphere in the process tank as the nozzles spray the wafers with the DIO 3  water. By providing an ozone gas atmosphere within the tank and under pressure, the O 3  more readily diffuses into the DIO 3  water as it being sprayed from the nozzles and contacting the wafers. The increase in diffusion increases the O 3  concentration, thus increasing strip rates.  
         [0022]    In processing semiconductor wafers, the wafers are often transported and processed in cassettes that can hold a plurality of wafers. In the present invention, semiconductor wafers having a layer or multi-layers of photoresist are exposed to pressurized DIO 3  water at ambient temperature and with a velocity produced by a series of nozzles. The process tank is pressurized and/or contains an ozone gas atmosphere when the wafers are positioned therein for processing. This results in an etching or removal of the photoresist at a higher rate than previously known.  
         [0023]    Referring to FIG. 1, the illustrated photoresist removal apparatus includes a process tank  10  which holds semiconductor wafers  20  in a cassette  30 . The semiconductor wafers  20  have a layer or multiple layers of photoresist baked onto them. The semiconductor wafers  20  are first loaded into cassette  30 . Loaded cassette  30  is then positioned within process tank  10 . As illustrated, wafers  20  are in a substantially vertical position and are spaced so that the photoresist can be removed as quickly and completely as possible. Alternatively, cassette  30  does not have to be used and wafers  20  can be positioned and supported in process tank  10  in any acceptable manner. Moreover, the present invention is applicable to single wafer processing methods and apparatus.  
         [0024]    Once wafers  20  are positioned in process tank  10 , lid  15  is closed. The semiconductor wafers  20  are exposed to DIO 3  water  70  through spray nozzles  80 . Process tank  20  and DIO 3  water  70  is maintained or above ambient temperature. The temperature can range from 20-50° C.  
         [0025]    DIO 3  water  70  is produced in the following manner. First, oxygen (O 2 )  45  is fed into ozone generator  60 . Ozone generator  60  converts oxygen  45  into pure ozone gas  50  using conventional methods. Pure ozone gas then feeds into ozonator  100  where it can take one of two routes: (1) pure ozone gas  50  can pass directly into process tank  10  by passing through  03  pressure plenum  90 ; or (2) pure ozone gas  50  will be combined with deionized water  40  by ozonator  100 , thus forming DIO 3  water  70  which is then fed into process tank  10  via nozzles  80 . In performing a photoresist stripping process according to one embodiment of the present invention, pure ozone gas  50  is first produced in ozone generator  60  as described above. This pure ozone gas  50  flows into ozonator  100 . A portion of this pure ozone gas  50  is allowed to flow directly into process tank  10  through  03  pressure plenum  90  until a pressurized ozone gas atmosphere is created in process tank  10  at a desired pressure between 1 to 3 atmospheres.  
         [0026]    Ozonator  100  also combines a portion of incoming pure ozone gas  50  with incoming deionized water  40 , thus forming DIO 3  water  70  to be pumped into process tank  10  via multiple nozzles  80 . The multiple nozzles  80  thus produce a DIO 3  fog wherein the DIO 3  fog interacts with the photoresist on the semiconductor wafers  20 . Alternatively, nozzles  80  can be adjusted to spray DIO 3  water  70  over wafers  70  as droplets. The multiple nozzles  80  can produce DIO 3  droplets varying in diameter which then interact with the photoresist on the semiconductor wafers  20 . The droplet size of the sprayed deionized_water will range from a few microns in the fogging stage to a few millimeters in size once collected on the semiconductor wafers  20 .  
         [0027]    The level of ozone in the DIO 3  water  70  is kept in constant through regulation of a first O 3  gas sensor located downstream of ozonator  100  but before nozzles  80 . If the ozone level is high enough, the pure ozone gas  50  is allowed to pass directly into process tank  10  through O 3  pressure plenum  90 . On the other hand, if the ozone level is too low, the ozone gas  50  is combined with deionized water  40  in ozonator  100  where more ozone is added until it reaches the proper level, at which time the DIO 3  water  70  passes into the process tank  10 .  
         [0028]    Upon condensation of the DIO 3  water  70  as droplets upon the semiconductor wafers  20  the DIO 3  water  40  is collected in the bottom of the process tank  10  as liquid  150 . The DIO 3  liquid  150  is recirculated through process tank  10  by flowing the DIO 3  liquid  150  from the process tank  10 , into pump  120 , through filter  140 , and back through ozonator  100  for introduction back into tank  10  via nozzles  80 .  
         [0029]    Before recirculated DIO 3  liquid  150  passes into ozonator  100 , it passes through a second dissolved O 3  gas sensor (not illustrated) which measures the concentration of O 3  present in the recirculated DIO 3  liquid  150 . If the O 3  concentartion level is too low, a signal is sent to ozonator  100  to add more pure ozone gas  50  to the recirculated liquid  150  as it passes therethrough.  
         [0030]    As mentioned above the pressure in the process tank  10  is maintained at or above the atmospheric pressure to help maintain a high ozone concentration in the DIO 3  water  70  contacting wafers  20 , thus enhancing the stripping rate. Further, since the process tank  10  is kept pressurized the temperature within the process tank  10  is increased above ambient temperature, preferably process tank  10  is maintained between 20 and 50 degrees Celsius through the use of temperature sensor  160  which will be operably connected to a source of heat and a properly programmed processor. Alternatively, the temperature sensor can be operably connected to measure (and adjust if necessary) the temperature of DIO 3  water  70  prior to being sprayed by nozzles  80 .  
         [0031]    Referring now to FIG. 2, the rate relationship between the ambient temperature and concentration of ozone in the DIO 3  water  70  indicates that a process time of 15-25 minutes can be used to strip about 15000 Angstrom of positive hard baked photoresist at ambient temperature. The photoresist strip rate depends on the dissolved O 3  concentration and average fluid velocity.  
         [0032]    Referring now to FIG. 3, which presents experimental data showing the relationship between the etching rate and the velocity of DIO 3  water  40 , the higher the kinetic energy (from the fluid velocities) and ozone concentration, the shorter the strip time. By increasing the fluid velocity and turbulence intensity, ozone is introduced to the wafer surface and penetrates the boundary layer. The series of nozzles play a significant role to reduce the process time significantly when optimized. The removal rate has shown to depend on the fluid velocity, turbulence intensity, and ozone concentration. As mentioned earlier, as the nozzle pressure of nozzles  80  is increased, the fluid velocity also increases. It has been fond that a nozzle pressure of 1 to 10 atmospheres is acceptable, with a nozzle pressure of 5-10 atmospheres being preferable.  
         [0033]    The nozzle pressure of the DIO 3  water directly affects the O 3  concentration on the boundary layer and correspondingly affects the etch rate. In summary, the etch rate is affected by the O 3  concentration in the DI water which is in turn affected by the temperature and pressure of the DIO 3  water. Further, the etch rate is directly affected by the velocity rate of the DIO 3  water.  
         [0034]    Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this invention is not limited to the illustrative embodiments set forth herein.

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