Abstract:
A substrate dryer includes, among other things, means for generating isopropyl alcohol bubbles, and a vibrator to atomize stored isopropyl alcohol. A heater may be provided to heat pumped isopropyl alcohol, as wells as a spray nozzle to spray the heated IPA to the vibrator. It is possible to increase the concentration of the isopropyl alcohol supplied for the purpose of drying the substrate. Improved substrate drying is achieved.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2005-3011, filed on Jan. 12, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a substrate dryer, and more particularly, to a substrate dryer in which the concentration of isopropyl alcohol supplied for drying a substrate can be increased 
     2. Description of the Related Art 
     Generally, to fabricate semiconductor devices, a lithography process, a deposition process, and an etching process are repeatedly performed on a substrate. In these processes, contaminants such as particles, metals, natural oxides, organic layers, etc. may be left on the substrate. Therefore, a cleaning process is performed on the substrate to remove these contaminants. Then a drying process is performed to remove the remaining contaminants. 
     Conventional drying methods include a spin drying method, an isopropyl alcohol (IPA) vapor drying method, an isopropyl alcohol Marangoni drying method, and the like. 
     Regarding the spin drying method, a drying apparatus is relatively simple and cheap, though it is difficult to continuously perform a cleaning process and a drying process. It is also impossible to completely dry the substrate. 
     Regarding the IPA vapor drying method, since a drying process is performed in IPA vapor, that is, in a clean space, less contaminants are generated. However, problems arise because a particular bath is required for the cleaning and drying processes and water spots are typically generated due to exposure of the substrate to oxygen when the substrate is transferred to a drying bath from a cleaning bath. 
     Regarding the IPA Marangoni drying method, the substrate is dried using a Marangoni effect by vertically drawing up the substrate from a deionized (DI) water bath, or draining the DI water bath while holding the substrate fixed, and blowing nitrogen gas and IPA at the surface of the substrate near the air/liquid interface. In the IPA Marangoni drying method, it is possible to drastically reduce the amount of IPA and to prevent water-spot occurrence due to exposure of the substrate to the air because the substrate is not transferred from a cleaning bath to a drying bath. 
     On the other hand, when the substrate is exposed to oxygen after cleaning, unnecessary oxides are generated on the substrate and then such oxides are left as residues, thereby generating water spots. When the substrate is exposed to nitrogen, water spots are generated, which have a size smaller than when it is exposed to oxygen. These water spots are generated when hydrophobic membranes such as poly-silicon, oxide, and nitride and hydrophilic membranes exist together in an active region of a semiconductor device have various step-difference structures. 
     However, in a recent semiconductor process requiring a high integration density corresponding to a process margin of 0.11 micrometers or less, in a substrate with a large aperture (for example, 300 mm), patterns include both simple structures and more complicated three-dimensional structures. As an example, with a RCAT (Recess Channel Array Transistor) process, which may be applied to improve refresh characteristics of transistors of high-integrated semiconductor devices, an active region is formed in a trench pattern, and hydrophobic and hydrophilic membranes exist together. In addition, the active region is formed to have a step-difference of about 1500 Å. In such a deep trench pattern, it is difficult to completely remove leftover water while drying the substrate. 
     As mentioned above, because such a pattern structure is considerably challenging to the success of a drying process, water spots may occur. Particularly, water spots occur intensively in the lower part of the substrate. Further, these water spots not only may act as a mask in a subsequent etching process, so that an etching residue may occur, but also are obstacles to an epitaxial process such as chemical vapor deposition (CVD) and epitaxial growth. 
     In the IPA Marangoni drying method performed in the nitrogen atmosphere, water spots are caused by the presence of valleys or holes and by water residue, which results when the substrate is directly exposed to the nitrogen atmosphere after being in DI water, and before going through IPA during the progression of the Marangoni effect. 
     Among dryers employing a conventional IPA Marangoni drying process, for example, ones manufactured by Korea DNS. Co. Ltd, and YieldUP International of U.S.A., the concentration of IPA is in the range of 0.15 to 0.25%. Compare this to when the concentration of IPA is in the range of about 2 to 3% and water spots do not occur during the drying process. 
     On the other hand, in the IPA Marangoni dryer, as disclosed in U.S. Pat. No. 6,598,312, IPA is supplied to the inside of a cleaning bath through an IPA bubbling device, and IPA bubbles are generated by supplying nitrogen to the inside of a bubble tank. 
     As a result, it is possible to increase the concentration of IPA to some level by increasing the amount of nitrogen to be supplied, or by raising the pressure of nitrogen to be supplied. However, there is a problem that if the amount of nitrogen is increased over the reference level of a particular process, then the mixture ratio of nitrogen and IPA is not within a regulated level, so that unexpected water spots can be generated due to the excessive amount and pressure of nitrogen. 
     SUMMARY OF THE INVENTION 
     The present invention is contrived to solve the above-mentioned problems. In an embodiment of the present invention a substrate dryer in which the concentration of isopropyl alcohol (IPA) can be increased while the amount and the pressure of nitrogen supplied for the purpose of generating IPA bubbles are kept constant in the IPA Marangoni drying process. 
     Another embodiment of the present invention provides a substrate dryer which can generate and supply IPA that is atomized using ultrasonic waves along with IPA bubbles generated using nitrogen to increase the concentration of IPA in the IPA Marangoni drying process. 
     Still another embodiment of the present invention provides a substrate dryer using megasonic or ultrasonic waves and heat along with bubbles generated using nitrogen to increase the concentration of IPA. 
     Yet another embodiment of the present invention provides a method of drying a substrate by using megasonic or ultrasonic waves and heat to increase the concentration of IPA while the amount and the pressure of nitrogen supplied for generating IPA bubbles are kept constant in the IPA Marangoni drying process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments with reference to the attached drawings in which: 
         FIG. 1  is a conceptual block diagram illustrating a substrate dryer according to an embodiment of the present invention; 
         FIG. 2  is a structural diagram illustrating a substrate dryer according to an embodiment of the present invention; 
         FIG. 3  is an enlarged cross-sectional view illustrating an isopropyl alcohol supplier according to an embodiment of the present invention; and 
         FIG. 4  is a flowchart illustrating operations of a substrate dryer according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention will be described more fully with reference to the accompanying drawings. 
     As shown in  FIGS. 1 and 2 , a substrate dryer comprises a cleaning bath  100  for holding a cleaning solution for cleaning a cleaning target, that is, a substrate  10  such as a wafer. Here, deionized water is used as the cleaning solution. 
     The cleaning bath  100  is comprises an inner bath  120  for immersing and cleaning the substrate  10  in the cleaning solution, and an outer bath  110  surrounding the inner bath  120 . The substrate  10  immersed in the inner bath  120  is mounted on a substrate guide  20  and positioned in the inside of the inner bath  120 . More than one substrate  10  may be mounted on the substrate guide. 
     Upper parts of the inner bath  120  and the outer bath  110  of the cleaning bath  100  are opened, and a cover  130  is installed on the upper part of the outer bath  110 . Thus, the outer bath  110  is constructed to surround the whole of the inner bath  120 . The outer bath  110  functions to drain cleaning solution overflow when the cleaning solution is oversupplied. To operate this function, a cleaning solution supply pipe  210  is in the lower part of the inner bath  120 , while penetrating the outer bath  110 . The supply pipe  210  supplies the cleaning solution to the inside of the inner bath  120 , and a cleaning solution supplier  200  outside of the baths supplies the cleaning solution to the cleaning solution supply pipe  210 . 
     A drain  400  is provided to the lower part of the cleaning bath  100 . The drain  400  comprises an outer bath drain  411  in the bottom of the outer bath  110  to drain any cleaning solution overflow in the outer bath  110  to the outside, and an inner bath drain  421  in the bottom of the inner bath  120  to drain the cleaning solution from the inner bath  120  to the outside. A first drain valve  412  on the outer bath drain  411  controls the flow in the outer bath drain  411 . A second drain valve  422  on the inner bath drain  421  controls the flow in the inner bath drain  421 . The outer bath drain  411  with the drain valve  412  and the inner bath drain  421  with the drain valve  422  are connected to each other and form a common drain. A special pump (not shown) may be installed in the drains. 
     A nitrogen supply nozzle  131  is in the cover  130  of the outer bath  110  to supply nitrogen gas to the inside of the inner bath  120 . The nitrogen supply nozzle  131  is connected to a first nitrogen supply source  300  that is outside of the cleaning bath  100 , and supplies heated nitrogen of a desired temperature to form nitrogen atmosphere in the inner bath  120 . Nitrogen is supplied to the inside of the cleaning bath  100  by the first nitrogen supply source  300  to stably perform a drying process stably. The nitrogen is heated in the first nitrogen supply source  300  to be in the range of 100 to 150° C., and the temperature of the inside of the cleaning bath  100  is controlled to be about 80° C. 
     Separately from the cleaning bath  100 , an IPA supplier  500  is provided to supply IPA, a liquid having a lower surface tension than the cleaning solution, to the inside of the cleaning bath  100 , for drying the substrate  10 . The IPA supplier  500  is connected to the cleaning bath  100  through an IPA supply pipe  520 . The IPA supply pipe  520  penetrates the cover  130  and is divided into a plurality of paths, and positioned so that their nozzles open towards the lower part of the inner bath  120 . The IPA supplier  500  supplies bubble-state IPA and atomized IPA to the cleaning solution stored in the inner bath  120  to form an IPA layer having a desired thickness on the cleaning solution. 
     The IPA supplier  500  comprises an IPA supply tank  510 . The IPA supply tank  510  is constructed to continuously receive IPA from a separate IPA supply line (not shown). 
     As shown in  FIG. 3 , in the IPA supply tank  510 , a bubble nozzle  530  extends from the exterior to within the IPA in the IPA supply tank  510 . The bubble nozzle  530  emits nitrogen supplied from a second nitrogen supply source  540  from within the IPA to form an IPA atmosphere in a bubble-state in the IPA supply tank  510 . Besides nitrogen, another inert gas such as argon and helium may be used as a supply gas. 
     A vibrator  550  is installed in the upper part of the inside of the IPA supply tank  510 . The vibrator  550  is connected to an oscillator  560  outside the IPA supply tank  510  to generate ultrasonic oscillations. Below the vibrator  550 , a spray nozzle  570  pumps IPA stored in the IPA supply tank  510  and sprays onto the vibrator  550 . A pump unit  580  is outside of the IPA supply tank  510  to pump the IPA to the spray nozzle  570 . 
     The pump unit  580  comprises a pump pipe  583  with a lower end that is extended to within the IPA in the lower part of the IPA supply tank  510 . A pump  581  is incorporated with the pump pipe  583 , as well as a heater  582  for heating the pumped IPA to a desired temperature. 
     The heater  582  heats the IPA to up to the boiling point of the IPA. As an example, with IPA, because the boiling point is about 83° C. in 1 atm, it is desirable to be in the range of about 25 to 80° C. The IPA is heated to keep an atmosphere of the IPA supplied to the inside of the inner bath  120  and to improve the drying effect of the substrate  10 . 
     The vibrator  550  atomizes the IPA which is sprayed on the vibrator  550  through the IPA spray nozzle  570 . The concentration of IPA in the IPA supplier  500  may be raised so that the concentration of the IPA in the inner bath  120  can, in turn, be raised. 
     The vibration frequency of the vibrator  550  can be controlled as needed. That is, depending on the concentration of IPA being supplied, the vibrator  550  may be turned off or the vibration frequency may be increased or decreased. In the present embodiment, ultrasonic waves of 20 to 400 KHz, megasonic waves of 700 KHz to 1.2 MHz, or any other range may be used as vibration frequency. When the megasonic frequency is used, IPA may be atomized and supplied to have a concentration much higher than if the IPA bubbles were conventionally generated. 
     At this point it is useful to look into the interrelation between the concentration of the IPA and the ultrasonic waves. As known conventionally, for a frequency of 1 MHz, the amount of atomization indicates a particle acceleration of about 1,000,000 G (9.8 m/sec 2 ). Therefore, due to atomization, according to a Newton&#39;s law of motion (F=ma, where m is the particle mass of the IPA, and a is the particle acceleration of the IPA), it is possible to provide an IPA having a concentration much higher than that of a conventional case of simple bubbles. 
     However, the concentration of the IPA has to be controlled to be in a proper range. That is, when the concentration is relatively low, for the case of a highly-integrated semiconductor device, effective drying is not obtained. But when the concentration is much higher, the number of IPA particles left on the substrate  10  after a drying process is increased, so that poor cleaning may occur. Thus, it is desirable to have, for example, a concentration of IPA that is in the range of 0.2 to 5.0%, so the thickness of the IPA formed on the cleaning solution is in the range of 1 to 3 mm. 
     Besides the IPA, ethyl alcohol, methyl alcohol, tetrahydrofuran, acetone, perfluoro hexane, hexane, and ether may be employed and used in the IPA supplier  500 . 
     Hereinafter, the operation of the substrate dryer according to the present embodiment described above will be explained. 
     As shown in  FIG. 4 , in a substrate dryer according to the present embodiment, when a substrate guide  20 , on which a substrate  10  is mounted, is positioned in the inner bath  120 , a cleaning solution is supplied through a cleaning solution supply pipe  210 , and then a cleaning process starts. The cleaning solution is supplied from the lower part of the inner bath  120  until it overflows the top of the inner bath  120 . Any cleaning solution overflow is collected in the lower part of the outer bath  110 . At the same time, when the first drain valve  412  is opened, the cleaning solution is drained from the lower part of the outer bath  110  through the outer bath drain  411  (S 10 ). 
     If the cleaning process is completed, a substrate drying process starts. After the cleaning process, the atmosphere of the inside of the outer bath  110  is converted into a drying atmosphere by supplying heated nitrogen gas to the inside of the outer bath  110  through a nitrogen supply nozzle to form nitrogen atmosphere in the outer bath  110 . At this point, the nitrogen atmosphere is kept at about 80° C. and in the range of 2 to 4 kg/cm 3  (S 20 ). 
     Then, a portion of the cleaning solution in the inner bath  120  is drained through the inner bath drain  421  to a level above where the substrate is not exposed over the cleaning solution. And then the IPA is supplied from the IPA supplier  500  through the IPA supply pipe  520 . 
     In supplying the IPA, bubble-state IPA and atomized IPA are supplied selectively or together (S 30 ) (S 31 ). For example, at first only bubble-state IPA may be supplied. Then when the surface of the cleaning solution drops to under the half height of the substrate  10 , atomized IPA may be supplied together with the bubble-state IPA. That is, the substrate  10  is dried efficiently by raising the concentration of IPA gradually. 
     But, using another method, bubble-state IPA and atomized IPA may be supplied together from the beginning. Depending on the type of substrate  10 , semiconductor device, or integration degree and type of display substrate, the concentration of IPA can be controlled to be various levels. 
     For the bubble-state IPA, when nitrogen is supplied from a second nitrogen supplier  540  to the inside of the IPA supplier  500  through a bubble nozzle  530 , bubbles are generated in the IPA, and then the IPA supply tank  510  fills with these bubbles. These bubbles are discharged to the inside of the cleaning bath  100  through the IPA supply pipe  520  (S 30 ). 
     For the atomized IPA, IPA is pumped by the pump  581  in the IPA pump  580 , heated by heater  582 , and sprayed on the vibrator  550  through the spray nozzle  570 . Then, the vibrator  550  atomizes the sprayed IPA and the IPA supply tank  510  is filled with the atomized IPA. As a result, the bubble-state IPA and the atomized IPA together fill the IPA supply tank  510  and this combination is supplied to the inside of the cleaning bath  100  through the IPA supply pipe (S 31 ). 
     IPA supplied to the cleaning bath  100  overflows the upper part of the inner bath  120  and forms an IPA layer of a desired thickness on the cleaning solution (S 40 ). Meanwhile a portion of this IPA layer vaporizes. The vaporized IPA is included in the nitrogen atmosphere, so a mixed atmosphere of IPA and nitrogen gas is formed in the upper part of the inner bath  120 . 
     Next, a drying process is started by draining the cleaning solution through the inner bath drain  421  of the inner bath  120  (S 50 ). At this point, the cleaning solution is drained through the inner bath drain  421  at a constant rate. In another embodiment, the pump  581  may be used for draining the cleaning solution described above. 
     However, if the cleaning solution is drained too rapidly, the IPA layer may be lost, and a sufficient Marangoni effect will not be obtained. On the other hand, much slower draining will decrease process efficiency. Thus, in a preferred embodiment, the level of the cleaning solution falls at the rate of from about 1.5 to about 2.5 mm/sec. 
     The thickness of the IPA layer should be kept constant or increased, so water residue on the substrate  10  is removed by the Marangoni effect. Simply explained, as the substrate is extracted from the cleaning solution (e.g. the cleaning solution is slowly drained, or in another embodiment, the substrate is slowly lifted out of the cleaning solution), the residue on the surface falls off. This extraction of the substrate causes the surface of the cleaning solution to pass across the surface of the substrate to be dried, e.g. a substantial portion or the entire substrate. Then, due to surface tension, at the surface level of the cleaning solution and the IPA layer, these two solutions closely adhere to the substrate  10 . But, at this point, because the IPA has a smaller surface tension than the cleaning solution, when the water level is falling off, liquid flows rapidly from the IPA layer with a small surface tension, to the cleaning solution with a large surface tension. Thus, water left on the substrate  10  is removed, so that the substrate  10  is dried. 
     As described above, if the concentration of IPA is kept constant or increased, it is possible to completely remove water on a semiconductor device, even having a complicated three-dimensional structure, a circuit pattern having a large step-difference, or a substrate where a hydrophobic membrane and a hydrophilic membrane exist together. 
     On the other hand, it may be desirable that the IPA is supplied on the cleaning solution in the inner bath  120  continuously while the cleaning solution is drained. By supplying the IPA vapor continuously to keep the mixed atmosphere of the IPA and nitrogen gas, the IPA layer on the cleaning solution is maintained at a constant amount while the cleaning solution is drained, so that the drying effect may even be improved. 
     When the water level of the cleaning solution is below the wafer, the supply of IPA is stopped. The supply of heated nitrogen gas is also stopped. Then, after the cleaning solution left under the substrate  10  is completely drained through the drain  421 , the cover  130  is opened and the substrate guide  20  on which the substrate is mounted is extracted from the cleaning bath  100 . Cleaning and drying of the substrate  10  is then complete. 
     Besides embodiments described above, other embodiments may be executed by changing various elements. That is, process conditions may be changed, a portion of the construction may be modified, a vibrator may be installed to the lower part or side of a supply tank, or vibrator may be installed on the supply tank without installing a spray nozzle to the supply tank. 
     In a substrate dryer, according to the embodiment of the present invention described above, IPA bubbles using nitrogen and atomized IPA, using ultrasonic frequencies, may be supplied together while keeping the amount of the nitrogen and nitrogen supply pressure constant. This may generate IPA bubbles in the IPA Marangoni drying method, so that the concentration of IPA is raised above conventional amounts to improve drying efficiency. Further, in the case of semiconductor devices, manufacturing efficiency of a substrate is improved by effectively drying the substrate which may have a complicated three-dimensional structure, a hydrophobic membrane and a hydrophilic membrane together, or a pattern with a large step-difference. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.