Abstract:
A method for manufacturing a semiconductor device, includes: supplying a liquid resist containing a water-repellent additive to a surface of a rotating semiconductor wafer fixed to a rotary support to form a resist film to a design thickness on the surface of the semiconductor wafer; spin drying the resist film; bringing a liquid into contact with the resist film and exposing the resist film through the liquid after the spin drying; developing the resist film to form a resist pattern; and performing processing on the semiconductor wafer. 
     A condition for adjusting contact angle between the resist film surface and the liquid is controlled so that the contact angle assumes a desired value, the condition including at least one selected from the group consisting of spin drying time for the resist film, resist temperature during the supplying, pressure of an atmosphere above the semiconductor wafer surface, and humidity of the atmosphere above the semiconductor wafer surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2009-053337, filed on Mar. 6, 2009; the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a method for manufacturing a semiconductor device including a patterning process based on immersion exposure. 
     A method for manufacturing a semiconductor device typically includes numerous processes for depositing a plurality of materials, including a subject film, on a semiconductor substrate and patterning them into desired patterns. In the patterning process, a film of photosensitive material, called resist, is formed on the subject film, and the resist film is subjected to selective exposure using a mask (reticle). Subsequently, the exposed or unexposed portion of the resist film is removed by development to form a resist pattern, which is further used as a mask to process the subject film. 
     Commonly used exposure light sources include ultraviolet lasers such as KrF excimer lasers and ArF excimer lasers. However, with the miniaturization of integrated circuit patterns, the required resolution is falling below the wavelength of such ultraviolet light. Thus, exposure process margins such as exposure amount margin and focus margin are becoming insufficient. 
     Studies to increase the numerical aperture (NA) toward resolution enhancement have now led to the immersion exposure technique in which exposure is performed through a liquid filled between the resist film surface and the projection lens. In such immersion exposure, the immersion liquid is in contact with the resist film surface, and the number of defects depends on the contact angle. For instance, in scan exposure, if the contact angle of the immersion liquid in the scan direction (advancing contact angle) is too high, defects due to air inclusions are generated. On the other hand, if the contact angle of the immersion liquid on the opposite side of the scan direction (receding contact angle) is low, the immersion liquid is left behind on the resist and causes defects due to remaining liquid. 
     As a resist for immersion lithography, JP-A-2009-004478(Kokai), for instance, proposes use of a topcoat-less resist, that is, a resist requiring no protective film between the resist film and the immersion liquid. When the topcoat-less resist is used, the concentration distribution of resist components including a water-repellent additive in the film thickness direction varies with the wafer rotation speed during spin coating of the resist, causing variation in the contact angle of the immersion liquid in contact with the resist surface. Furthermore, the film thickness of the resist film is controlled by the wafer rotation speed. Hence, if the wafer rotation speed is changed with the design film thickness, there is concern about variation in the contact angle of the resist surface and generation of defects during immersion exposure. Thus, it is desirable to maintain the contact angle of the resist surface at a desired value irrespective of the design film thickness of the resist film, or the wafer rotation speed. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the invention, there is provided a method for manufacturing a semiconductor device, including: supplying a liquid resist containing a water-repellent additive to a surface of a rotating semiconductor wafer fixed to a rotary support to form a resist film to a design thickness on the surface of the semiconductor wafer; spin drying the resist film after the forming the resist film; bringing a liquid into contact with the resist film and exposing the resist film through the liquid filled between a surface of the resist film and a projection optical system after the spin drying; developing the resist film after the exposure to form a resist pattern; and performing processing on the semiconductor wafer by using the resist pattern as a mask, a condition for adjusting contact angle between the resist film surface and the liquid being controlled in accordance with rotation speed of the semiconductor wafer or the design thickness of the resist film so that the contact angle assumes a desired value, the condition including at least one selected from the group consisting of spin drying time for the resist film, resist temperature during the supplying, pressure of an atmosphere above the semiconductor wafer surface, and humidity of the atmosphere above the semiconductor wafer surface. 
     According to an aspect of the invention, there is provided a method for manufacturing a semiconductor device, including: supplying a liquid resist containing a water-repellent additive to a surface of a rotating semiconductor wafer fixed to a rotary support to form a resist film to a design thickness on the surface of the semiconductor wafer; spin drying the resist film after the forming the resist film; bringing a liquid into contact with the resist film and exposing the resist film through the liquid filled between a surface of the resist film and a projection optical system after the spin drying; developing the resist film after the exposure to form a resist pattern; and performing processing on the semiconductor wafer by using the resist pattern as a mask, concentration distribution of the water-repellent additive contained in the resist film being controlled so that contact angle between the resist film surface and the liquid assumes a desired value with a thickness of the resist film maintained at the design thickness. 
     According to an aspect of the invention, there is provided a method for manufacturing a semiconductor device, including: supplying a liquid resist to a surface of a rotating semiconductor wafer fixed to a rotary support to form a resist film to a design thickness on the surface of the semiconductor wafer; supplying a solution containing a water-repellent additive to the surface of the semiconductor wafer during, before, or after the supplying a liquid resist; bringing a liquid into contact with the resist film and exposing the resist film through the liquid filled between a surface of the resist film and a projection optical system; developing the resist film after the exposure to form a resist pattern; and performing processing on the semiconductor wafer by using the resist pattern as a mask, supply amount of the solution containing the water-repellent additive being controlled in accordance with rotation speed of the semiconductor wafer or the design film thickness of the resist film so that contact angle between the resist film surface and the liquid assumes a desired value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a resist coater; 
         FIGS. 2A and 2B  are schematic views showing dropping of a topcoat-less resist for immersion exposure and formation of a water-repellent layer on a resist surface side by the wafer rotation; 
         FIG. 3  shows a graph illustrating an example of the relationship between resist film thickness and contact angle on the resist surface; 
         FIG. 4  is a flow chart showing major steps in the resist formation in this embodiment of the in the invention; 
         FIG. 5  is a schematic view showing an example of the relation ship between drying time in the spin drying step in  FIG. 4  and the contact angle on the resist film surface; 
         FIGS. 6A to 6C  are schematic views showing pattern formation steps in a method for manufacturing a semiconductor device according to this embodiment of the invention; and 
         FIGS. 7A and 7B  are schematic views showing supplying a prewet thinner and a resist in methods for manufacturing a semiconductor device according to other embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     According to an embodiment of the invention, a liquid resist is supplied to the surface of a semiconductor wafer by spin coating.  FIG. 1  is a schematic view of a resist coater used therefor. 
     This resist coater includes a cup  22  with openings  26 ,  27  formed at the top and bottom center, respectively, and a rotary support (or spin chuck)  23  is provided inside the cup  22 . A semiconductor wafer  10  is fixed to the rotary support  23  illustratively by a vacuum chuck. In this state, the space above, below, and around the semiconductor wafer  10  is surrounded by the cup  22 . 
     A nozzle  21  is provided near the upper opening  26  of the cup  22 , and its discharge port is opposed to the surface (subject surface) of the semiconductor wafer  10  fixed to the rotary support  23 . 
     The rotary support  23  is coupled to a motor  24  by a rotary shaft  23   a  penetrating through the lower opening  27  of the cup  22  and protruding out of the cup  22 . The driving force received from the motor  24  enables the rotary support  23  and the semiconductor wafer  10  fixed thereto to rotate around the rotary shaft  23   a.    
     In this embodiment, immersion exposure is performed in which a resist film is exposed through a liquid (such as pure water) filled between the resist film surface and the projection optical system (projection lens) of the exposure apparatus, and a resist requiring no protective film, or a topcoat-less resist, is used as a resist for the immersion exposure. The topcoat-less resist is a resist whose surface is made water-repellent by addition of a water-repellent additive, eliminating the need to form a water-repellent protective film separately on the resist film. 
     Spin coating is a technique in which the semiconductor water  10  is rotated together with the rotary support  23  so that a liquid resist  13   a  ( FIG. 2A ) dropped from the nozzle  21  is spread into a film with uniform thickness on the surface of the semiconductor water  10 . Here, the water-repellent additive added to the resist  13   a  is localized on the surface side and forms a water-repellent layer  14  on the surface side of the resist  13   a  as shown in  FIG. 2B . 
     The film thickness of the resist  13   a  at this time depends primarily on the rotation speed of the semiconductor water  10 . Furthermore, in the resist  13   a  containing the water-repellent additive, the manner of localization of the water-repellent additive (concentration distribution of the water-repellent additive in the film thickness direction) varies with the rotation speed of the semiconductor wafer  10 , causing variation in the contact angle between the resist surface and the immersion liquid in contact with this surface (hereinafter also simply referred to as contact angle). In general, the aforementioned contact angle tends to increase when the water-repellent additive exists more on the resist surface side. 
     Variation of the aforementioned contact angle in accordance with the rotation speed of the semiconductor wafer  10  translates into variation of the contact angle in accordance with the resist film thickness determined by the rotation speed of the semiconductor wafer  10 .  FIG. 3  shows an example of this correlation between resist film thickness and contact angle. 
     In  FIG. 3 , the relationship between film thickness (nm) and contact angle (°) is shown illustratively for two resists A and B which are different in type (or material composition). In the case of  FIG. 3 , resist A exhibits a larger rate of variation in contact angle with respect to variation in film thickness than resist B. 
     From the foregoing, when the resist film thickness is controlled by the wafer rotation speed, the contact angle of the resist surface varies with the design film thickness, and the scan tolerance during immersion exposure fluctuates with the design film thickness. To change the film thickness without varying the contact angle, it is necessary to change the amount of water-repellent additive added to the resist. This requires preparation of a plurality of types of resist with a substantially different composition for each design film thickness, leading to cost increase. 
     Thus, in this embodiment, conditions other than the wafer rotation speed are used to control the localizability (concentration distribution in the film thickness direction) of the water-repellent additive, thereby controlling the contact angle. That is, the concentration distribution of the water-repellent additive contained in the resist film is controlled so that the contact angle assumes a desired value while maintaining the film thickness of the resist film at a desired design film thickness. 
       FIG. 4  shows major steps in resist film formation. 
     First, a liquid resist  13   a  is dropped from the nozzle  21  onto the surface of the semiconductor wafer  10  fixed to the rotary support  23  (step  101 ). It may be dropped on the semiconductor wafer  10  yet to be rotated, or on the semiconductor wafer  10  already in the rotating state. 
     After the resist  13   a  is dropped, the rotation speed of the semiconductor wafer  10  is maintained at a desired constant rotation speed (step  102 ). This is the step for determining the film thickness of the resist  13   a , where the rotation is controlled at the constant rotation speed and maintained for a prescribed period of time to achieve a desired film thickness, depending on the viscosity of the resist and the type of solvent contained therein. 
     With the aforementioned constant rotation speed being maintained, when the surface of the semiconductor wafer  10  is coated with the resist  13   a  to the desired film thickness, the supply of the resist  13   a  from the nozzle  21  is stopped, making a transition to a spin drying step  103  for simply rotating the semiconductor wafer  10  to evaporate the solvent in the resist  13   a.    
     In this embodiment, the period of time for this spin drying step (drying time) is adjusted to control the concentration distribution in the film thickness direction of the water-repellent additive contained in the resist  13   a , thereby controlling the contact angle of the resist surface. This resist drying time refers to the time from when the desired film thickness of the resist is obtained on the surface of the semiconductor wafer  10  and supply of the resist to the semiconductor wafer  10  is stopped until when the semiconductor wafer  10  stops rotation (the point of time when the rotation actually stops or the point of time when a stop command signal is outputted to the motor  24 ). 
     To set this resist drying time, for instance, the correlation as illustrated in  FIG. 5  is determined in advance to prepare a data set. On the basis thereof, the drying time to achieve a desired contact angle is set in accordance with the design film thickness of the resist. 
     In  FIG. 5 , the horizontal axis represents drying time in the aforementioned step  103  for spin drying the resist, and the vertical axis represents the contact angle made between the surface of the formed resist film and the immersion liquid in contact with this surface. This relationship is determined for each design film thickness (the example of  FIG. 5  shows two film thicknesses a and b). It is noted that the film thickness can also be translated as rotation speed because the film thickness is determined by the wafer rotation speed in the aforementioned step  102  for determining the film thickness. 
     In the case of  FIG. 5 , assume that θ, for instance, is to be obtained as the contact angle of the resist film surface. In this case, the desired contact angle θ is achieved by setting the drying time to t 1  for design film thickness a, and setting the drying time to t 2  for design film thickness b. 
     In the step for spin drying the resist, contact angle control is given a higher priority than drying to set the drying time. Even if the drying time is relatively short and results in insufficient drying, the solvent can be completely evaporated from the resist in the next baking step  104  to obtain a solid-phase resist film. 
     The parameter used to control the contact angle (the condition at the time of forming the resist film) is not limited to the resist drying time, but it is also possible to use resist temperature, the pressure of the atmosphere above the semiconductor wafer surface, and the humidity of this atmosphere. 
     The resist temperature is the temperature of the liquid resist discharged from the nozzle  21 . Increase of this resist temperature accelerates evaporation of the solvent in the dropped resist, which results in a larger amount of water-repellent additive migrating in the resist toward the surface in association with the evaporation of the solvent, and tends to increase the contact angle of the resist surface. 
     In the coater of  FIG. 1 , the pressure of the atmosphere above the semiconductor wafer surface is the atmosphere pressure in the cup  22 . Increase of this pressure suppresses evaporation of the solvent in the dropped resist and also suppresses migration of the water-repellent additive toward the resist surface, and the contact angle of the resist surface tends to decrease. 
     The humidity of the atmosphere above the semiconductor wafer surface is the humidity in the cup  22  of the coater. Increase of this humidity suppresses evaporation of the solvent in the dropped resist and also suppresses migration of the water-repellent additive toward the resist surface, and the contact angle of the resist surface tends to decrease. 
     Also for these resist temperature, atmosphere pressure, and atmosphere humidity, like the resist drying time, the aforementioned correlation as illustrated in  FIG. 5  is determined in advance to prepare a data set. On the basis thereof, the resist temperature, atmosphere pressure, and atmosphere humidity to achieve a desired contact angle are set in accordance with the design film thickness. 
     Here, it is possible to control the contact angle by adjusting one of the resist drying time, resist temperature, atmosphere pressure, and atmosphere humidity, or to control the contact angle by adjusting two or more of these conditions. 
     Next, the process subsequent to resist film formation is described with reference to  FIG. 6 . 
     In  FIG. 6 , the semiconductor wafer  10  is illustratively made of a silicon or other semiconductor substrate  11  with a subject film  12  such as a silicon oxide film formed thereon. However, a plurality of films may be formed on the semiconductor substrate  11 , or the semiconductor wafer  10  may consist only of the semiconductor substrate  11 . Furthermore, between the semiconductor wafer  10  and a resist film  13 , other films such as an anti-reflection coating may be formed as needed. 
     After the resist film  13  is formed on the surface of the semiconductor wafer  10 , a mask (or reticle), not shown, is used to perform exposure on the resist film  13  as shown in  FIG. 6A . This is immersion exposure in which exposure is performed on the resist film  13  through a liquid (not shown) filled between the surface of the resist film  13  and the projection optical system (projection lens). 
     Next, after post-exposure baking, the resist film  13  is developed. For instance, for a chemically amplified resist containing a photoacid generator, an alkaline developer can be used. By this development, the exposed or unexposed portion of the resist film  13  is removed, and a resist pattern  15  is formed as shown in  FIG. 6B . 
     Next, the resist pattern  15  is used as a mask to perform processing such as ion implantation and etching on the semiconductor wafer  10 . In this embodiment, for instance, the resist pattern  15  is used as a mask to dry etch the subject film  12 . Thus, as shown in  FIG. 6C , the subject film  12  is patterned. 
     According to this embodiment described above, the film thickness of the resist film is determined by the wafer rotation speed during spin coating of the resist, and parameters other than the wafer rotation speed (conditions for adjusting the contact angle, such as resist drying time, resist temperature, atmosphere pressure, and atmosphere humidity) are used to control the localizability (concentration distribution in the film thickness direction) of the water-repellent additive in the resist, thereby controlling the contact angle of the resist film surface. This can prevent generation of defects during immersion exposure due to variation of the contact angle with the design film thickness, and accurate patterning can be performed. 
     For instance, if the film thickness of the film to be etched is increased and the aspect ratio of an opening (or hole) to be formed by etching is increased, or if the etching selection ratio of the film to be etched with respect to the resist film is relatively low, the film thickness of the resist film may be increased to enhance the etching resistance of the resist film. According to this embodiment, such change in the film thickness of the resist film, that is, change in the wafer rotation speed during spin coating, can also be addressed without varying the contact angle of the resist film surface. 
     Next, other embodiments of the invention are described with reference to  FIG. 7 . 
     The embodiment shown in  FIG. 7A  includes, before supplying the liquid resist  13   a  to the surface of the semiconductor wafer  10 , the process for supplying a prewet thinner  32  to the surface of the semiconductor wafer  10  to increase wettability of the surface of the semiconductor wafer  10 , and a water-repellent additive is added to the prewet thinner  32 . Discharge of the prewet thinner  32  to the surface of the semiconductor wafer  10  is performed before discharge of the resist  13   a  using the same nozzle  21  as the resist  13   a  is discharged. 
     In the embodiment shown in  FIG. 7B , simultaneously with resist supply, the prewet thinner  32  containing a water-repellent additive is discharged from a nozzle  31  different from the nozzle  21  for discharging the resist  13   a.    
     In these embodiments, the amount of the prewet thinner  32  supplied to the surface of the semiconductor wafer  10  is adjusted to control the amount of the contained water-repellent additive supplied to the semiconductor wafer  10 , thereby controlling the contact angle of the resist surface. To set the supply amount of the prewet thinner  32 , like the previous embodiment, the correlation among the design film thickness (wafer rotation speed during coating), the supply amount of the prewet thinner, and the contact angle is determined in advance to prepare a data set. On the basis thereof, the supply amount to achieve a desired contact angle is set in accordance with the design film thickness of the resist. 
     Also in the embodiments shown in  FIGS. 7A and 7B , the film thickness of the resist film is determined by the wafer rotation speed, and a parameter other than the wafer rotation speed (supply amount of the prewet thinner  32  containing the water-repellent additive) is used to control the contact angle of the resist film surface. This can prevent generation of defects during immersion exposure due to variation of the contact angle with the design film thickness, and accurate patterning can be performed. 
     In the embodiments of  FIGS. 7A and 7B , after the prewet thinner  32  and the resist  13   a  are dropped, they are mixed on the surface of the semiconductor wafer  10 . Hence, the water-repellent additive added to the prewet thinner  32  becomes contained in the resist film covering the surface of the semiconductor wafer  10 . The water-repellent additive may or may not be added to the resist  13   a  before being dropped. 
     Preparing and controlling a plurality of types of resist containing a water-repellent additive with different concentrations requires much effort and cost. In contrast, in this embodiment, the contact angle of the resist film surface can be controlled simply by controlling the amount of the prewet thinner  32  containing the water-repellent additive supplied to the surface of the semiconductor wafer  10  irrespective of the film thickness (wafer rotation speed during coating). It is only necessary to prepare one type of resist. For this one type of resist, the film thickness is controlled by the wafer rotation speed during coating, and the contact angle of its surface is controlled by the supply amount of the prewet thinner. 
     Here, the solution containing the water-repellent additive is not limited to the prewet thinner, but other solutions may also be used. Furthermore, the timing for supplying the solution containing the water-repellent additive is also not limited to before or during supplying the resist, but may be after supplying the resist. Any modification is possible as long as the resist is mixed with the solution containing the water-repellent additive so that the water-repellent additive is contained in the resist covering the surface of the semiconductor wafer. 
     It is noted that adding a water-repellent additive to a prewet thinner and supplying it to a semiconductor wafer does not mean addition of a new step to the existing process including the step for supplying a prewet thinner.