Abstract:
A method of manufacturing an integrated circuit using an imaging system having a mask and an energy source that produces an exposure field. A substrate is moved across the exposure field while changing the depth of focus of the imaging system relative to the substrate. The depth of focus may be changed by moving the substrate, the mask, or both, relative to each other changes the depth of focus. The depth of focus may be oscillated according to a periodic waveform where the waveform is equal to the time for a typical point on the substrate to pass through the exposure field.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to manufacturing an integrated circuit and, more particularly, to a method of manufacturing integrated circuits using scanning systems and methods. 
     BACKGROUND OF THE INVENTION 
     Integrated circuit (IC) fabrication involves a process sequence in which patterns are generated in different material layers using, for example, a combination of deposition, lithography, and etching techniques. After the formation of a material layer on a silicon wafer, lithographic and etching techniques are used to transfer a desired pattern into the material, or to process the exposed substrate material. Typically, a radiation-sensitive material, called a resist, is spin-coated onto this material layer prior to lithographic printing. The lithographic printing step is usually performed using an imaging tool called a stepper, which has a high intensity light source, a relay lens, a reticle stage, an imaging lens and a high precision translation stage. 
     A reticle containing an IC pattern to be printed is illuminated by the high intensity light source, which may be a mercury arc lamp or a laser, at a specific wavelength that causes radiation-induced changes in the resist. The light passing through the reticle is imaged by the lens onto the resist layer on the wafer. After each exposure, the wafer is stepped by a translation stage to the next site for subsequent exposure. The wafer is positioned on the translation stage. This exposure step essentially generates a latent image of the circuit pattern in the resist, similar to the exposure of a photographic film in conventional photography. The exposed resist can then be developed to produce a patterned resist layer, which can be used as a mask in a subsequent processing step, which, for example, transfers this pattern onto the underlying material layer. 
     The pattern is formed using a thin layer of material opaque to the radiation. The opaque material may be a chrome layer formed on a quartz substrate. During the exposure step, illuminating light passes through the reticle at regions where chrome, the pattern material, is absent. The light that passes through the reticle travels through a lens to expose the resist on the substrate. The opaque material prevents light from passing through the reticle. 
     The resolution R of an optical system and the depth of focus DOF at its resolution limit are respectively expressed in equations (1) and (2) below.              R   =       k   1          λ   NA               (   1   )               DOF   =       k   2          λ       (   NA   )     2                 (   2   )                                
     λ is the wavelength, NA is the numerical aperture of the projection optical system, and k i  and k 2  are process dependent constants. 
     There is an increasing demand to shrink transistor size and increase circuit speed. One method of reducing transistor size is to enhance the resolution of the optical system by reducing the wavelength λ and increasing the numerical aperture NA. Changing these parameters reduces the depth of focus DOF. For example, a typical deep ultraviolet (DUV) stepper having a wavelength λ equal to 0.248 μm and a numerical aperture NA equal to 0.6 has a resolution R of approximately 0.3 μm and a depth of focus DOF of approximately 0.5 μm. This assumes k 1 =k 2 =0.8. Transferring the pattern from the reticle to the substrate becomes more difficult as the depth of focus decreases. 
     Efforts have been made to increase the effective depth of focus DOF. One such method is the focus latitude enhancement exposure process, also known as focus drilling. This method is described in A new method for enhancing focus latitude in optical lithography: Flex,  IEEE Electron Device Letters , EDL-8, 179, (1987) by H. Fukuda et al.; and Using multiple focal planes to enhance depth of focus, SPIE 1674, Optical/Laser Microlithography V, 285, (1992) by C. A. Spence et al. Each of these documents are herein incorporated by reference. 
     During focus drilling, the same part of the wafer is exposed at different focal positions. This is achieved by moving the wafer stage in the Z direction while the stepper shutter is open. The image on the wafer is, therefore, an integration of multi-exposures at different focal positions. This method was developed for stepper tools having wafer stages, which did not perform X-, or Y-movement while the shutter is open. In other words, the wafer stage is in a fixed position in the X or Y-direction relative to the light source when the relative position of the substrate stage is changed in the Z direction. 
     Current lithography tools, however, are transitioning to scanner systems for critical level printing. In a scanner system, the reticle translation stage (the platform holding the reticle) and the substrate translation stage (the platform holding the substrate) are at constant motion when the shutter is open. As described above, the prior art technique uses a wafer stage that is maintained in a fixed position during exposure. Therefore, it would be difficult to implement the prior art focus drilling technique directly in a scanner system. Thus, it would be desirable to provide a system that improves the effective depth of focus DOF for present and future lithography technologies. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of manufacturing an integrated circuit using an imaging system having a mask (a reticle) and an energy source that produces an exposure field. A substrate is moved across the exposure field while changing the depth of focus of the imaging system relative to the substrate. The depth of focus may be changed by moving the substrate, the mask, or both, relative to each other changes the depth of focus. The depth of focus may be oscillated according to a periodic waveform where the waveform is equal to the time for a typical point on the substrate to pass through the exposure field. It is to be understood that both the foregoing description and the following detailed description are exemplary, but are not restrictive, of the invention. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice in the semiconductor industry, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
     FIG. 1 is a schematic diagram of a scanning system according to an illustrative embodiment of the present invention; 
     FIG. 2 is a top view of the scanning system of FIG. 1; 
     FIGS. 3 a - 3   f  are top views of the scanning system of FIG. 1 during successive stages of operation; and 
     FIG. 4 is a waveform diagram illustrating the movement of the substrate translation stage with respect to the reticle of the scanning system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawing, wherein like reference numerals refer to like elements throughout, FIG. 1 is a schematic diagram of a scanning system  100  according to an illustrative embodiment of the present invention. The scanning system  100  includes a high intensity light or energy source  110 , a condenser lens  180 , a relay lens  120 , an imaging lens  140 , and a reticle translation stage  150  including a reticle  130 . The resist  210  on the substrate  200  is exposed to the energy source  110  through a slit  160  which limits the area of the resist exposed by the energy source  110  through the reticle  130 . 
     During the exposure operation, both the reticle  130  and the substrate  200  are constantly moving when the resist  210  is exposed to the energy source  110 . The reticle translation stage  150  moves the reticle  130  in one or more directions and the substrate translation stage  170  moves the substrate  200  in one or more directions. For example, the reticle translation stage  150  and the substrate translation stage  170  may respectively move the reticle  130  and the substrate  200  parallel to the x-y plane. Typically, the reticle translation stage  150  is moved relative to the substrate translation stage  170  in a straight line (the scanning direction) parallel to the X-Y plane. 
     The distance between the imaging lens  130  and the substrate  200  is adjusted in the z-direction in an oscillatory fashion around a desired focal plane by moving the substrate translation stage  170 . Movement of the reticle translation stage  150 , translation stage  170 , and other components is implemented by controller and drive mechanism  190 . In an alternative embodiment, the imaging lens  140 , reticle translation stage  150 , or other component may be adjusted or moved to change the DOF. An exemplary scanning system is S 203  or S 202  produced by Nikon Inc. 
     The operation of the scanning system  100  is described below with reference to FIGS. 2-4 and continuing reference to FIG.  1 . FIG. 2 is a top view of the scanning system  100  and an image field  300  to be imaged on the substrate  200  using an exposure field defined by the slit  160 . Other components of the scanning system  100  are not shown for clarity. FIGS. 3 a  to  3   f  illustrates the exposure of the imaging field  300  at different times. 
     The period of movement of the substrate translation stage  170  (or other component) in the z-direction may be equal to the duration of time for each point in the image field  300  to pass under the slit  160  (or through the exposure field). In other words, the substrate translation stage  170  is moved through a periodic movement having a period T while the image field  300  passes under the slit  160  (or through the exposure field). The period T is equal to the time t that it takes a point in the image field  300  to pass under the slit  160  (through the exposure field). Each point in the image field  300  is exposed at the full range of the focal positions although each point within the image field  300  may start to enter the exposure field at a different phase of oscillation of the substrate translation stage  170 . 
     For a scanner system with a scanning speed of 80 mm/sec and a slit width of 8 mm, for example, the time needed for any point to pass through the image field is 0.1 second. Thus, the period T for the substrate translation stage  170  motion is 0.1 second. In other words, the period of time for the substrate translation stage  170  to move through its range of motion and return to its starting position is equal to the period of time for a point in the image field  300  to pass through the exposure field. FIG. 4 shows the movement of the image field  300  relative to the movement of the substrate translation stage  170  according to a sinusoidal waveform. 
     Returning to FIGS. 3 a - 3   f , lines A through E represent positions in the imaging field along a line in the scan direction in that field. At time t0, line A enters the slit when the substrate translation stage  170  is at a position to produce a desired DOF or optimal DOF. This position is identified as Z equals zero (Z=0). Z is a normalized number where Z equals zero (Z=0) is the center of the substrate translation stage  170  movement and where Z equals ±1 (Z=±1) corresponds to the full range of movement of the substrate translation stage  170  relative to the reticle translation stage 150 in the Z-direction. At time t1, after a quarter period (1/4 T), the line A is passing under the slit and the line B starts to pass under the slit while the stage is at a top location of Z=1. The lines C, D, and E follow one after another in order, passing under the slit while line A moves further towards the other edge of the slit and the substrate translation stage  170  moves down and up. At time t=T, the substrate translation stage  170  has undergone the full cycle of movement in the Z-direction back to the Z=0 position, at substantially the same time or at the same time that the line A passes through the entire slit. 
     Each line in the image field will experience the same route as line A except that they may enter the slit while the stage is at different positions in the Z direction. Because the image field passes under the slit continuously, it is not necessary to setup a starting position for the Z motion (motion in the Z-direction) relative to the X or Y motion (motion in the scanning direction). The two motions should be implemented so that the period of the Z motion is equal to the time for a point to pass from one end of the slit to the other end of the slit. The period of motion may be continuously implemented. 
     A typical range of Z motion may be about ±0.5 μm. This motion may be implemented using a piezoelectric-driven mechanism in the controller and drive mechanism  190 . The process described above may be implemented in hardware or software or a combination thereof. For example, the controller and drive mechanism  190  may include a processor that controls mechanical and/or moving the different components. 
     FIG. 4 illustrates the sinusoidal movement of the substrate translation stage  170  in the Z-direction with reference to lines A through E as they pass under the slit. Although the movement is shown to be sinusoidal, different functions or waveforms having periodic form may characterize the movement of the substrate translation stage  170 . For example, a periodic triangular waveform having a periodic movement may be used to define the motion of the translation stage. In addition, the frequency of the substrate translation stage  170  oscillation may be increased. The frequency of the translation stage  170  oscillation is defined in equation (3) below.              f   =     n   T             (   3   )                                
     n is an integer and T is the period of time for a point in the image field to pass from one edge to another edge of the slit  160  (or through the exposure field). 
     Using the process described above, the DOF may be doubled. The doubling of the DOF by this method may also alleviate the stringent requirement of exact focus tracking during the scan, which may increase the throughput of the scanner system. Although movement in the Z-direction of the substrate translation stage was described above for increasing the DOF, other components may be moved or adjusted to change the DOF. For example, the reticle  130  may be moved in the z-direction in an oscillatory fashion to change the DOF. Alternatively, more than one component may be moved or adjusted to change the DOF. For example, the substrate translation stage and the reticle may both be moved. 
     Although the invention has been described with reference to exemplary embodiments, it is not limited to those embodiments. Rather, the appended claims should be construed to include other variants and embodiments of the invention which may be made by those skilled in the art without departing from the true spirit and scope of the present invention.