Abstract:
An immersion lithography system compensates for displacement of the final optical element of the optical assembly caused by the immersion fluid. The system includes an optical assembly to project an image defined by a reticle onto a wafer. The optical assembly includes a final optical element spaced from the wafer by a gap. An immersion element supplies an immersion fluid into the gap and recovers any immersion fluid that escapes the gap. A fluid compensation system applies a force to the final optical element of the optical assembly to compensate for pressure variations of the immersion fluid.

Description:
INCORPORATION BY REFERENCE 
       [0001]    This is a divisional of U.S. patent application Ser. No. 11/628,942, which is the U.S. National Stage of International Application No. PCT/US2004/042808 filed Dec. 20, 2004, which claims the benefit of U.S. Provisional Application No. 60/580,510 filed on Jun. 17, 2004. The disclosure of each of the prior applications is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    The present invention relates generally to lithography, and more particularly, to an apparatus and method for compensating for pressure exerted on the lithography lens caused by the immersion fluid. 
         [0003]    A lithography apparatus is used to transfer images defined by a reticle or other patterning element such as a programmable mirror array (see U.S. Pat. Nos. 5,296,891, 5,523,193 and PCT Application Nos. 98/38597 and 98/33096 for example, all incorporated by reference herein) onto a semiconductor wafer during fabrication. A typical lithography apparatus includes an illumination source, a reticle stage assembly for positioning the reticle, a wafer stage for supporting the wafer, and an optical assembly including lenses for projecting the image defined by the reticle onto the wafer. Control and measurement systems are also provided to control the movement of the wafer and measure the position of the wafer relative to the optical assembly respectively. 
         [0004]    Immersion lithography systems utilize a layer of immersion fluid that fills a gap between the final lens of the optical assembly and the wafer. The fluid enhances the resolution of the system by enabling exposures with numerical apertures (NA) greater than one, which is the theoretical limit for conventional “dry” lithography. The fluid in the gap permits the exposure with light that would otherwise be totally internally reflected at the optical-air interface. With immersion lithography, numerical apertures as high as the index of refraction of the immersion fluid are possible. Fluid immersion also increases the depth of focus, which is the tolerable error in the vertical position of the wafer, compared to a conventional lithography system. Immersion lithography thus has the capability of providing resolution down to 50 nanometers or lower. 
         [0005]    One potential issue with immersion lithography is that fluid pressure on the lens may cause the last lens of the optical assembly to become displaced. More specifically, the amount of force on the lens depends on the pressure exerted by the fluid and the size of the surface area of the lens, lens mount hardware, and any immersion fluid supply nozzles attached to the lens mount hardware. 
         [0006]    The fluid pressure may be caused by a number of reasons. With immersion lithography, the surface tension of the liquid at the air-fluid interface surrounding the exposure area, sometimes referred to as the meniscus, has the effect of sucking or pulling down the lens and optical assembly. Variations in the amount of immersion fluid may also cause pressure variations on the lens. The applicants have found that a positive or negative change of only 0.02 cubic centimeters will cause a change of force of approximately 50 milli-newtons on the lens with an 80 millimeter diameter. Also as water escapes the gap, the water flow also has a tendency to create a pull down force on the lens. Dynamic motion of the wafer in the horizontal plane can cause shear forces that exert pressure on the lens. Vertical motions of the wafer performed for focusing purposes may also cause unwanted vibrations or vertical coupling of the lens. 
         [0007]    Excessive forces exerted on the last lens of the optical assembly can cause a number of problems during exposure operation. If the force causes the lens to be displaced, the resulting image projected onto the wafer may be out of focus. 
         [0008]    On the other hand, if the optical assembly is too rigidly mounted to prevent the displacement, lens aberrations may result due to thermal expansion, again resulting in a blurring of the projected image. 
         [0009]    An apparatus and method for compensating for pressure exerted on the lithography lens caused by the immersion fluid is therefore needed. 
       SUMMARY 
       [0010]    The present invention is related to an immersion lithography system that compensates for any displacement of the final optical element of the optical assembly caused by the immersion fluid. The system includes an optical assembly to project an image defined by the reticle onto the wafer. The optical assembly includes a final optical element spaced from the wafer by a gap. An immersion element is provided to supply an immersion fluid into the gap and to recover any immersion fluid that escapes the gap. A fluid compensation system is provided to compensate for the force on the final optical element of the optical assembly caused by pressure variations of the immersion fluid to minimize the displacement of the final optical element. The resulting force created by the varying pressure may cause the final optical element to become displaced. The fluid compensation system is configured to provide a substantially equal, but opposite force on the optical assembly, to prevent the displacement of the final optical element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is an illustration of an immersion apparatus having features of the present invention; 
           [0012]      FIG. 2  is an enlarged view of a fluid pressure compensation system for an immersion lithography lens according to one embodiment of the invention; 
           [0013]      FIG. 3  is a model diagram of a fluid pressure compensation system for an immersion lithography lens according to a second embodiment of the invention. 
           [0014]      FIG. 4  is a diagram of the optical assembly clamping the final optical element according to one embodiment of the invention. 
           [0015]      FIGS. 5A and 5B  are diagrams of the final optical element and clamp according to one embodiment of the invention. 
           [0016]      FIGS. 6A and 6B  are flow diagrams illustrating the fabrication of semiconductor devices using the immersion apparatus of the present invention. 
           [0017]      FIGS. 7A and 7B  are diagrams of a fluid pressure compensation system for an immersion lithography apparatus according to another embodiment of the invention. 
       
    
    
       [0018]    Like reference numbers refer to like elements in the Figures. 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0019]    Referring to  FIG. 1 , an immersion apparatus is shown. The immersion apparatus  10  includes a reticle stage  12 , an optical assembly  14  including a final optical element  16 , and a wafer stage  18  for supporting a wafer  20 . An immersion device  22 , sometimes referred to as a nozzle, is positioned between the final optical element  16  and the wafer  20  on the wafer stage  18 . The immersion device is responsible for supplying fluid into the gap  24  between the final optical element  16  and the wafer  20 . The immersion device is also responsible for recovering immersion fluid that escapes the gap  24 . In various embodiments, the immersion fluid may be a liquid such as water or oil. For more details on the immersion device, see PCT Application No. PCT/US04/22915, filed Jul. 16, 2004, entitled “Apparatus and Method for Providing Fluid for Immersion Lithography” (corresponding to U.S. patent application Ser. No. 11/362,833 published as U.S. 2006/0152697), assigned to the assignee of the present invention, and incorporated by reference herein for all purposes or a gas seal as described in European Application EP 1 420 298 A2, also incorporated by reference herein. 
         [0020]    Referring to  FIG. 2 , an enlarged cross-section view of a fluid pressure compensation system for an immersion lithography apparatus according to one embodiment of the invention is shown. The apparatus  10  includes a lens mount  32  used to mount the final optical element  16  to the optical assembly  14 . The final optical element  16  is positioned over the wafer  20 . The immersion device  22  (not shown for the sake of simplicity in  FIG. 2 ) is responsible for providing and recovering immersion fluid  34  from the gap  24 . A meniscus  35  of immersion fluid is created at the fluid-atmosphere interface below the outer-edge of the lens mount  32 . 
         [0021]    A fluid compensation system  36  is provided to compensate for any changes in the force on the final optical element  16  caused by changes in the pressure of fluid  34 . The fluid compensation system  36  includes a chamber  38  that surrounds the last optical element  16  and is positioned between the optical assembly  14  and the lens mount  32 . The chamber is filled with immersion fluid  34 . Passages  40  fluidly couple the immersion fluid  34  in the gap  24  with the chamber  38 . A purge device  44  is fluidly coupled to the chamber  38  through a passage  42 . For the purposes of this application, the chamber  38  is generically characterized as a device that is capable of expanding or contracting in the vertical direction but not in the horizontal direction. In various embodiments, the chamber  38  may be a bellows, piston, diaphragm, or other passive pressure responsive device. The passages  40  and  42  may be a duct or other opening fluidly connecting the immersion fluid  34  in the gap with the chamber  38  and purge device  44 . 
         [0022]    During operation, changes in the pressure of the immersion fluid  34  may create forces on the final optical element  16 , the lens mount  32 , and the immersion device, all of which may result in displacement of the final optical element  16 . The chamber  38  is designed to create an equal but opposite force to compensate or cancel out the force created by the immersion fluid  34 . When pressure caused by the immersion fluid  34  in the gap  24  increases, an upward force is created on the final optical element  16 . The increased pressure concurrently causes a corresponding increase in the pressure in the chamber  38  via the passages  40 . The increased pressure results in the expansion of the chamber  38 , creating an equal but opposite downward force on the lens mount  32 . As a result, the final optical element  16  is not displaced. Alternatively, if the pressure of the immersion fluid  34  decreases in the gap  24 , a downward force on the final optical element  16  is created. The decreased pressure results in a corresponding decrease in pressure in the chamber  38 . Consequently, the chamber  38  compresses, causing an equal but opposite upward force on the lens mount  32 . As a result, the final optical element  16  is not displaced. 
         [0023]    In one embodiment, the horizontal surface area of the top and bottom surfaces of the lens mount  32  in contact with the chamber  38  and immersion fluid  34  are substantially the same. The substantially equal surface area ensures that the chamber  38  exerts an equal but opposite force on the top surface of the lens mount  32  as the immersion fluid  34  exerts on the bottom surface of the mount  32  and the final optical element  16  combination. Assume in  FIG. 2  that the bottom surface of the lens mount  32  has a radius R 1  and the inner and the optical assembly  14  has an outer and inner radius R 2  and R 3  respectively. When the equation πR 1   2 =πR 2   2 −πR 3   2  is satisfied, then the top and bottom surface areas in contact with the chamber  38  and immersion fluid  34  are substantially the same. For example, this relationship is satisfied when R 1 =4, R 2 =5 and R 3 =3 measurement units respectively. Although in this example the lens mount  32  and the optical assembly  14  are round, this shape should not be construed as limiting the present invention. In various embodiments, the lens mount  32  and optical assembly  14  can be any shape, including but not limited to square, rectangular, oval, etc. In other embodiments, the equal but opposite forces on the bottom and top surfaces of the lens and lens mount are substantially vertically aligned. 
         [0024]    Referring to  FIG. 3 , a diagram of a fluid pressure compensation system for an immersion lithography apparatus according to another embodiment of the invention is shown. The apparatus  50  includes a lens mount  32  used to mount the final optical element  16  to the optical assembly  14 . The final optical element  16  is positioned over the wafer  20 . The immersion device  22  (again not shown for the sake of simplicity in  FIG. 3 ) is responsible for providing and recovering immersion fluid  34  from the gap  24 . A meniscus  35  of immersion fluid is created at the fluid-atmosphere interface below the outer-edge of the lens mount  32 . 
         [0025]    A fluid compensation system  52  is provided to compensate for any changes in the force on the final optical element  16  caused by changes in the pressure of fluid  34 . The fluid compensation system  52  includes a pair of actuators  54  mechanically coupled between the lens mount  32  and the sidewalls of the optical assembly  14 . Optical position sensors  56 , mounted on struts  58  that extend from the sidewalls of the optical assembly, are provided to measure the relative position of the lens mount  32  with respect to the optical assembly  14 . A pressure sensor  60  is used to measure the pressure of the immersion fluid  34  in the gap  24 . A control system  62 , coupled to both the pressure sensor  60  and the position sensors  56 , is used to control the actuators  54 . 
         [0026]    During immersion lithography, the immersion fluid  34  may become pressurized, either positively or negatively, for the reasons described above. This pressure is applied to the bottom surface of the lens mount  32 , last optical element  16 , and the nozzle, all of which may contribute to the displacement of the final optical element  16 . Pressure variations of the immersion fluid  34  are continuously provided to the control system  62  as measured by the pressure sensor  60 . The position sensors  56  also measure the actual position of the lens mount  32 . The control system  62  in turn controls the actuators in real time to compensate for any displacement of the final optical element  16  caused by changes in pressure and detected by the position sensors  56 . For example, when an increase in pressure causes the final optical element to be displaced upward, the control system  62  directs the actuators to exert an equal but opposite downward force on the lens mount  32 . Alternatively, the control system  62  causes the actuators  54  to exert an upward force on the lens mount  32  when the sensor  60  measures a decrease in immersion fluid  34  pressure. In either case, the actuators  54  prevent the displacement of the final optical element  16 . In various other embodiments, the control system  62  may use inputs from the position sensors  56  and the pressure sensor  60  to control the actuators. Alternatively, the control system may use inputs from either the position sensors  56  or the pressure sensor  60 , but not both. In yet another embodiment, the actuators may be internal to the lens mount  32 , as opposed to being mechanically coupled between the lens mount  32  and the optical assembly  14 . 
         [0027]    Referring to  FIG. 4 , an enlarged diagram of the optical assembly  14  is shown. In one embodiment, kinematic clamps  70  are used to clamp the final optical element  16  to lens barrels  72  of the optical assembly  14 . Referring to  FIG. 5A , a top view of the final optical element  16  is shown. As is illustrated in the figure, the optical element includes a lens portion  16   a  and a flanged portion  16   b  extending around the periphery of the optical element  16 . Referring to  FIG. 5B , a cross section view of the final optical element  16  and a clamp  70  is shown. The clamp  70  is configured to clamp onto the flanged portion  16   b  of the final optical element  16  to hold it in place within the optical assembly  14 . Although the clamp  70  is described herein as a kinematic clamp, it should be noted that any type of mechanical clamp may be used. In yet another embodiment, glue may be used to secure the final optical element  16  to the barrels  72  of the optical assembly  14 . For the sake of simplicity, only one clamp  70  is shown. It should be noted that typically two, three or even more clamps  70  may be used around the circumference of the final optical element  16 . In embodiments where glue is used, separate or non-clamp actuators can be used to compensate for any displacement of the final optical element  16 , similar to as illustrated in the diagram of  FIG. 3 . 
         [0028]    In one embodiment, the clamps  70  are the force actuators and are responsible for both holding the final optical element  16  in place within the optical assembly  14  and for providing the equal but opposite force to compensate for any displacement caused by the immersion fluid, similar to the actuators  54  of  FIG. 3 . In various embodiments, the force actuators may be VCMs, El cores, a low stiffness piezo stack, piezo bi-morph, or other magnetic or pressure driven actuators. Regardless of the type of force actuator used, the force applied by the clamps  70  to the kinematic mount should be equal to but opposite the direction of the force created by the immersion fluid  34 . Furthermore, the forces created by each mount can be controlled by the control system  62  so that the center of the effort coincides with center of the fluid force. 
         [0029]    In the aforementioned embodiments, the control system  62  relies on both an actual instantaneous force calculation as measured by the pressure sensors  60  and position feedback as measured by the optical position sensors  56 . For example, the instantaneous force is calculated by multiplying the instantaneous pressure times the surface area of the final optical element  16 , lens mount  32 , and immersion element  22  in contact with the immersion fluid. An instantaneous counter-force can then be applied based on the outcome of the calculation. The position sensors  56  can be used for feedback to adjust the counter-force as necessary. In other embodiments, however, the control system  62  may rely on either the instantaneous force calculation as measured by the pressure sensors  60  (i.e., an open loop system) or the position feedback as measured by the optical position sensors  56  (i.e., a closed loop system), but not both. 
         [0030]    According to various embodiments, the immersion apparatus  10  can be used as a scanning type photolithography system that exposes the pattern from a reticle onto the wafer with the reticle and the wafer moving synchronously. In a scanning type lithographic apparatus, the reticle is moved perpendicularly to an optical axis of the optical assembly by a reticle stage assembly and the wafer is moved perpendicularly to the optical axis of the optical assembly  14  by a wafer stage assembly. Scanning of the reticle and the wafer occurs while the reticle and the wafer are moving synchronously. 
         [0031]    Alternatively, the immersion apparatus  10  can be a step-and-repeat type photolithography system that exposes the reticle while the reticle and the wafer are stationary. In the step and repeat process, the wafer may be in a constant position relative to the reticle and the optical assembly  14  during the exposure of an individual field. Subsequently, between consecutive exposure steps, the wafer is consecutively moved with the wafer stage assembly perpendicularly to the optical axis of the optical assembly  14  so that the next field of the wafer is brought into position relative to the optical assembly  14  and the reticle for exposure. Following this process, the images on the reticle are sequentially exposed onto the fields of the wafer, and then the next field of the wafer is brought into position relative to the optical assembly  14  and the reticle. 
         [0032]    As is well known in the art, the immersion apparatus  10  also includes an illumination system (not shown) having an illumination source and an illumination optical assembly. The illumination source emits a beam (irradiation) of light energy. The illumination optical assembly guides the beam of light energy from the illumination source to the optical assembly  14 . The illumination source can be a g-line source (436 nm), an i-line source (365 nm), a KrF excimer laser (248 nm), an ArF excimer laser (193 nm) or a F 2  laser (157 nm). 
         [0033]    Semiconductor devices can be fabricated using the above described systems, by the process shown generally in  FIG. 6A . In step  601  the device&#39;s function and performance characteristics are designed. Next, in step  602 , a mask (reticle) having a pattern is designed according to the previous designing step, and in a parallel step  603  a wafer is made from a silicon material. The mask pattern designed in step  602  is exposed onto the wafer from step  603  in step  604  by a photolithography system described hereinabove in accordance with the present invention. In step  605  the semiconductor device is assembled (including the dicing process, bonding process and packaging process), finally, the device is then inspected in step  606 . 
         [0034]      FIG. 6B  illustrates a detailed flowchart example of the above-mentioned step  604  in the case of fabricating semiconductor devices. In  FIG. 6B , in step  611  (oxidation step), the wafer surface is oxidized. In step  612  (CVD step), an insulation film is formed on the wafer surface. In step  613  (electrode formation step), electrodes are formed on the wafer by vapor deposition. In step  614  (ion implantation step), ions are implanted in the wafer. The above mentioned steps  611 - 614  form the preprocessing steps for wafers during wafer processing, and selection is made at each step according to processing requirements. 
         [0035]    At each stage of wafer processing, when the above-mentioned preprocessing steps have been completed, the following post-processing steps are implemented. During post-processing, first, in step  615  (photoresist formation step), photoresist is applied to a wafer. Next, in step  616  (exposure step), the above-mentioned exposure device is used to transfer the circuit pattern of a mask (reticle) to a wafer. Then in step  617  (developing step), the exposed wafer is developed, and in step  618  (etching step), parts other than residual photoresist (exposed material surface) are removed by etching. In step  619  (photoresist removal step), unnecessary photoresist remaining after etching is removed. 
         [0036]    Multiple circuit patterns are formed by repetition of these preprocessing and post-processing steps. 
         [0037]    Referring to  FIG. 7A , a fluid pressure compensation system for an immersion lithography apparatus according to another embodiment of the invention is shown. The apparatus  70  includes a lens mount (not shown) used to mount the final optical element  16  to the optical assembly  14  (not shown). The final optical element  16  is positioned over the wafer  20 . The immersion device  22  is responsible for providing and recovering immersion fluid  34  from the gap  24 . An immersion fluid duct  72  is provided to supply immersion fluid into the gap  24 . A gas seal  74  including air nozzle  76  for providing pressurized air or gas and a vacuum port  78  are provided around the periphery of the gap  24 . The gas seal is used to confine or seal the immersion fluid  34  in the gap. In various embodiments, the air seal  74  may contain two or more air nozzles  76  and/or vacuum ports  78 . The gas seal and/or bearing  74  may also be used as an air or gas bearing to support the immersion element  22  over the wafer  20 . For more details on the gas seal  74 , see European Patent Application EP 1 420 298 A2 incorporated by reference herein for all purposes. The apparatus  70  further includes a pressure sensor  80  fluidly coupled to the immersion fluid  34  in the gap by a duct  82 . The pressure sensor  80  is used to measure the pressure of the immersion fluid  34  in the gap  24 . The pressure information can then be used to control the velocity of the gas or air exiting nozzle  76  to selectively adjust the height of the gap  24  between the wafer  20  and the immersion device  22 . In other words, the velocity of the air or gas exiting the one or more nozzles  76  can be selectively adjusted to control the force used to support the immersion device  22 . 
         [0038]    Referring to  FIG. 7B , a block diagram of a control system  84  is shown. The pressure sensor  80  provides instantaneous pressure measurements to the controller  86 . In turn, the controller generates control signals to a gas flow controller  88 . The gas flow controller  88  controls the velocity and pressure of the gas exiting the nozzle or nozzles  76  of the gas seal  74 . By controlling the pressure and velocity, the gap  24  between the wafer  20  and the immersion device  22  can be selectively controlled. For example, the pressure and velocity can be altered on the fly to maintain a constant gap  24  as the pressure of the immersion fluid varies. Alternatively, the pressure and velocity can be altered to selectively control the height of the gap. In various embodiments, the controller  86  and gas flow control can be one device or separate devices. 
         [0039]    While the particular exposure apparatus as shown and disclosed herein is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.