Abstract:
A system and method of compensating for image smear that arises when imaging onto a moving workpiece with a single pulse of radiation. The system includes a mask frame capable of supporting a mask to be imaged. The mask frame is operatively connected to a drive unit and is capable of moving in the mask plane. The mask frame is driven in an oscillatory fashion in the mask plane so that when a pulse of radiation illuminates the mask, the mask image moves in the same direction as the moving workpiece, thereby reducing image smear. The present invention is particularly applicable to single-pulse-exposure systems that utilize pulsed radiation sources having relatively long pulse duration, such as flash-lamps or certain types of lasers.

Description:
CROSS REFERENCE 
     This application is a continuation-in-part of still pending U.S. patent application Ser. No. 09/854,226, filed May 10, 2001, entitled “Lithography System and Method for Device Manufacture.” 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to lithography, and in particular relates to systems and methods for performing single-radiation-pulse exposures in a manner that reduces image smear. 
     2. Description of the Prior Art 
     The process of manufacturing certain micro-devices such as semiconductor integrated circuits (ICs), liquid crystal displays, micro-electro-mechanical devices (MEMs), digital mirror devices (DMDs), silicon-strip detectors and the like involves the use of high-resolution lithography systems. In such systems, a patterned mask (i.e., a reticle) is illuminated with radiation (e.g., laser radiation or radiation from an arc lamp) that passes through an illumination system that achieves a high-degree of illumination uniformity over the illuminated portion of the mask. The portion of the radiation passing through the mask is collected by a projection lens, which has an image field (also referred to as a “lens field”) of a given size. The projection lens images the mask pattern onto an image-workpiece to produce a pattern either in a photosensitive layer on the workpiece surface or directly on the workpiece because of a reaction between the workpiece surface and the incident radiation. The workpiece resides on a workpiece stage that moves the workpiece relative to the projection lens, so that the mask pattern is repeatedly formed on the workpiece over multiple “exposure fields.” 
     Lithography systems include an alignment system that precisely aligns the workpiece with respect to the projected image of the mask, thereby allowing the mask pattern to be precisely superimposed on previously exposed patterns. In most cases, the mask image needs to be precisely aligned to a pre-existing exposure field on the workpiece to provide the juxtaposed registration necessary to build up layers of the device being fabricated. 
     Presently, two types of lithography systems are used in manufacturing: step-and-repeat systems, or “steppers,” and step-and-scan systems, or “scanners.” With steppers, each exposure field on the workpiece is exposed in a single static exposure. With scanners, the workpiece is exposed by synchronously scanning the work piece and the mask across the lens image field. An exemplary scanning lithography system and method is described in U.S. Pat. No. 5,281,996. The projection lenses associated with steppers and scanners typically operate at 1X (i.e., unit magnification), or reduction magnifications of 4X or 5X (i.e., magnifications of ±1/4 and ±1/5, as is more commonly expressed in optics terminology). 
     The ability of a lithography system to resolve (or, more accurately, “print”) features of a given size is a function of the exposure wavelength: the shorter the wavelength, the smaller the feature that can be printed or imaged. To keep pace with the continuously shrinking minimum feature size for many micro-devices (particularly for ICs), the exposure wavelength has been made shorter. Also, historically the device size has increased as well, so that the lens field size has steadily grown. The resolution of the lithography system also increases with the numerical aperture (NA) of the projection lens. Thus, in combination with reducing the exposure wavelength, the numerical apertures of projection lenses tend to be as large as can be practically designed, with the constraint that the depth of focus, which decreases as the square of the NA, be within practical limits. 
     A novel and unconventional lithography system that performs exposures using single pulses of radiation is described in U.S patent application Ser. No. 09/854,226, filed on May 10, 2001 by the present inventors and assigned to the same assignee and entitled “Lithography system and method for device manufacture,” which application is incorporated herein by reference. This single pulse exposure system, referred to by the present assignee by the trademark CONTINUOUS LITHOGRAPHY™ lithography system, has many advantages. These include providing a high throughput equal to or greater than the most advanced lithography scanners using a smaller-than-conventional exposure field size. In the CONTINUOUS LITHOGRAPHY™ lithography system, the workpiece (wafer) moves continuously underneath the projection lens while exposure fields are formed on the workpiece with a single pulse of radiation. The temporal pulse length of the radiation pulses and the speed at which the workpiece moves is selected so that the exposure fields are imaged with a minimum of smearing of the mask image. 
     A preferred radiation source for the CONTINUOUS LITHOGRAPHY™ lithography system is a pulsed laser. However, while pulsed lasers can provide radiation pulses that are short and intense, they are also relatively expensive. This adds to the cost of the overall lithography system. To reduce the cost of the lithography system, a flash-lamp radiation source can be used. However, the temporal pulse lengths of a flash lamp are in the millisecond to microsecond range, with greater pulse energies being available from longer pulses. Thus, the image smear (blur) caused by imaging a fixed reticle onto a rapidly moving workpiece can be appreciable with a flash lamp source. 
     Accordingly, it would be greatly advantageous to be able to use a high-energy, long-pulse-duration flash lamp or long-pulse-duration laser in the single-pulse CONTINUOUS LITHOGRAPHY™ lithography system without experiencing the aforementioned image smearing. 
     SUMMARY OF THE INVENTION 
     The present invention relates to lithography, and in particular relates to systems and methods for performing single-radiation-pulse exposures in a manner that reduces image smear. 
     A first aspect of the invention is a mask holder system for oscillating a mask to provide motion compensation for performing single-pulse exposures of the mask onto a moving workpiece. The mask holder system includes a mask frame for supporting the mask. A drive unit is operatively connected to the mask frame for imparting an oscillatory motion to the mask frame that corresponds to the movement of the workpiece. The oscillation is coordinated with the single-pulse exposures so that the image of the mask and the workpiece move in the same direction at substantially the same speed during exposure. 
     An exemplary embodiment of the mask holder system utilizes parallel rails slidably connected to first and second sides of the mask frame to allow for the mask frame to move in a plane defined by the parallel rails. 
     A second aspect of the invention is a method of reducing image smear when forming an exposure field with a single pulse of radiation on a moving workpiece. The method includes oscillating a mask in a mask plane, and then illuminating a mask with a pulse of radiation while the mask moves in a direction such that an image of the mask moves in the same direction as the workpiece. The method further includes projecting an image of the mask onto the moving workpiece to form an exposure field. The method is repeated to form a plurality of adjacent exposure fields, with each exposure field formed from a single pulse of radiation. The radiation pulses may be from a flash-lamp or a pulsed laser (including a modulated continuous-wave laser); in either case, the radiation pulses have a temporal pulse length of about a microsecond or greater. 
     A third aspect of the invention is a lithography system for conducting single pulse exposures that includes a pulsed radiation source (i.e., a flash-lamp or pulsed laser), an illuminator for collecting the radiation pulses, the mask holder system as described above in connection with the first aspect of the invention, a projection lens and a workpiece stage. A mask is supported in the mask holder system. The motion of the workpiece via the workpiece stage, the oscillating motion of the mask, and the emission of pulses of radiation from the radiation source are coordinated by a main control unit so that the amount of image smear from imaging onto the moving workpiece is reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a lithography system for carrying out the method of the present invention, and which includes the mask holder system of the present invention; 
     FIG. 2A is a plan view of the mask holder system of the present invention, showing the mask held in a mask frame, the rails that provide for slidable movement of the frame, and the driver unit, with the driver unit shown connected to the main controller; 
     FIG. 2B is a cross-sectional view of the mask holder system of FIG. 2A taken along the line  2 B— 2 B; and 
     FIG. 3 is a detailed plan view of an exemplary embodiment of the mask holder system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to lithography, and in particular relates to systems and methods for performing single-radiation-pulse exposures in a manner that reduces image smear. 
     With reference now to FIG. 1, there is shown a lithography system  10  suitable for use in carrying out the present invention. System  10  is also referred to herein by the trademark CONTINUOUS LITHOGRAPHY™ lithography system, as used by the present assignee. System  10  is described in great detail in aforementioned U.S. patent application Ser. No. 09/854,226, filed May 10, 2001. 
     Lithography system  10  of the present invention includes, in order along an optical axis A 1 , a radiation source  14  electrically connected to a radiation source controller  16 . In the present invention, the term “radiation source” includes a flash lamp containing a gas fill that emits flashes (i.e., pulses) of radiation in the same part of the spectrum for which the projection lens (discussed below) is corrected. For example, a xenon-filled flash lamp emits radiation in the UV part of the spectrum from 150 nm through the visible blanketing the regions where the resists used for lithography are sensitive. Using suitable band-pass filters a portion of the spectrum matched to the characteristics of the projection lens and the resist is readily selected for use. In bump lithography, the temporal pulse length from the lamp might be in the 100 microsecond to 5 millisecond range and the minimum feature formed by the optical system might be 25 to 50 microns wide. Another suitable radiation source  14  is a laser-driven xenon plasma source operating at a wavelength in the 6 nm-14 nm wavelength region of the electromagnetic spectrum. 
     Yet another radiation source  14  is a laser of the appropriate wavelength with a relatively long pulse duration (e.g., one microsecond). The relatively long radiation pulses may be formed by passing a continuous-wave laser beam through a modulator that periodically interrupts the beam. Another possible radiation source  14  is provided by the new laser diodes that operate in the near UV with good conversion efficiency. 
     Optionally included adjacent radiation source  14  is a pulse stabilization system  18  for providing pulse-to-pulse uniformity of the radiation pulses emitted from the radiation source in the case where the radiation source pulse-to-pulse stability needs to be improved. 
     Further included in system  10  along axis A 1  is an illumination system (“illuminator”)  24  and a mask holder system  30  capable of movably supporting a mask M at a mask plane MP. Mask holder system  30  is the focus of the present invention and is described in greater detail below. 
     With continuing reference to FIG. 1, mask M includes a top surface  31  and a bottom surface  32  that includes a pattern  34 . Pattern  34  may be a binary (e.g., a chrome pattern on clear glass), or a phase mask (e.g., phase changes generated by a patterned phase-inducing dielectric material), or a combination of the two. Mask M is typically quartz or other suitable material transparent to the wavelength of radiation from radiation source  14 , except where mask M is a reflective mask and the substrate material transmittance is of no consequence. In the case where system  10  employs EUV radiation, mask M is reflective, and system  10  is folded accordingly. A binary reflective mask is created by forming a reflective layer atop a substrate and then forming a pattern atop the reflective layer using an absorber layer. Exemplary mask M suitable for use in the present invention are described in greater detail below. 
     System  10  also includes a projection lens  40  having an object plane OP arranged substantially coincident with mask plane MP, an aperture stop AS and an image plane IP. A workpiece stage  50  is arranged adjacent projection lens  40  at or near image plane IP and has an upper surface  52  capable of movably supporting a workpiece W having an image-bearing surface WS. In a preferred embodiment of the present invention, workpiece W is a semiconductor wafer, and upper surface WS is an image-bearing surface comprising a layer of photoresist. 
     With continuing reference to FIG. 1, electrically connected to workpiece stage  50  is a workpiece stage position control system  60 , which includes a metrology device  62  for accurately measuring the workpiece stage position. Metrology device  62  is electrically connected directly to radiation source controller  16  so that the motion of workpiece stage  50  and the activation of radiation source  14  can be coordinated. Stage position control system  60  is capable of positioning workpiece W with high precision relative to projection lens  40  or other reference. 
     Workpiece stage  50  preferably has movement capability in all 6 degrees of freedom. Existing air-bearing and magnetically levitated (“maglev”) workpiece stages and stage position control systems are capable of providing such movement, as well as high scan speeds (e.g., in excess of 100 mm/s) and are thus suitable for use with the present invention. Exemplary workpiece stages  50  are described in U.S. Pat. No. 5,699,621, and in the article by M. E. Williams, P. Faill, S. P. Tracy, P. Bischoff, and J. Wronosky, entitled  Magnetic levitation scanning stages for extreme ultraviolet lithography , ASPE 14 th  annual meeting, Monterey Calif., November 1999, which patent and article are both incorporated herein by reference. 
     The ability of workpiece stage  50  to move in the X- and Y-planes and rotate about the Z-axis is necessary for properly positioning mask images on image-bearing surface WS of workpiece W. Z-axis movement capability, along with angular adjustment capability about the X- and Y-axis (pitch and roll), is necessary for keeping the workpiece surface within the shallow depth of focus of projection lens  40 . The Z-position of the image-bearing surface WS (which is also the focal surface) of workpiece W can vary between exposure fields if the workpiece is not perfectly flat. Similarly, small rotations about the X- and Y-axis can also occur. Accordingly, metrology device  62 , which in an example embodiment is an interferometer, is preferably included as part of stage position control system  60  for accurately measuring the coordinates of workpiece stage  50  relative to projection lens  40 , and for providing this positioning information to radiation source control system  16 . 
     With continuing reference to FIG. 1, a focus system  70  is arranged (e.g., adjacent projection lens  40 , as shown) in operative communication with workpiece W and senses the position of image-bearing surface WS of the work piece with respect to projection lens  40 . Focus system  70  generates electrical signals, which are sent to control system  60  and result in adjusting the axial (Z) position of the workpiece by means of workpiece stage  50 . 
     System  10  further includes an alignment system  72  arranged in optical communication with workpiece W for aligning the workpiece with respect to a reference (e.g., the image of a mask alignment key imaged on the workpiece by projection lens  40 ). A workpiece handling system  80  in operable communication with workpiece stage  50  is provided for transporting workpieces between the workpiece stage and a workpiece storage unit  84 . A system controller  90  is electrically connected to radiation source controller  16 , pulse stabilization system  18 , illumination system  24 , mask holder system  30 , workpiece stage position control system  60 , focus system  72 , alignment system  70 , and workpiece handling system  80 , and controls and coordinates the operation of these systems. 
     Impact of Mask Motion on Flash-lamp Exposure 
     Assume that it is desired to perform single-pulse lithographic exposures using a 1X-projection lens having a 22×22-mm lens field (which produces a 22×22 mm exposure field) and a flash-lamp radiation source  14  that flashes (pulses) 10 times per second. In order to place one exposure field adjacent another without overlap, the workpiece stage must scan the workpiece at scanning velocity V s  given by: 
     
       
           V   s =(exposure field width w)(pulse rate p)=(22 mm) (10/second)=220 mm/sec.  (1) 
       
     
     Assume mask M is made to oscillate with a half-amplitude A at an angular velocity ω. The mask position x at time t is given by: 
     
       
           xω=   A  sin ω t   (2) 
       
     
     Also, the angular frequency ω is given by: 
     
       
         ω=2 nπp =20π/sec  (3) 
       
     
     The mask oscillating velocity V r  is given by: 
     
       
           V   r   =A ω cos ω t   (4) 
       
     
     Since projection lens  40  is assumed to be 1X in this example, the velocity of the mask image (“the mask image velocity”) equals the mask velocity, except perhaps for a sign change due to negative magnification (i.e., −1X magnification). If it is also assumed that exposure of workpiece W occurs where the mask image is at its maximum velocity, then: 
     
       
         
           V 
           s 
           =V 
           r max 
           =Aω 
         
       
     
     
       
           A=V   s /ω=(220 mm/sec)/(20 π/sec) 
       
     
     
       
           A =3.50 mm  (5) 
       
     
     In the present invention, the entire mask M is illuminated. Thus, with an oscillation amplitude on the order of several millimeters, it may be necessary to adjust the illumination field at mask plane MP to cover an area sufficient to fully illuminate the mask over the entire range of motion. 
     The maximum acceleration of the mask a m  is given by:                      a   m     =     A                   ω   2                   =       (     3.5                 mm     )            (     20      n        /        sec     )     2                   =     13   ,   819                 mm        /          sec   2                   =     1.4                   g   .                     (   6   )                                
     It was shown in aforementioned U.S. patent application Ser. No. 09/854,226 that the effect of image smear during a single-pulse exposure could be ignored provided that the mask image motion associated with the movement of workpiece W during the exposure has a length L that is a small fraction of the wavelength L corresponding to the highest possible spatial frequency passed by the projection system. This length is twice the minimum possible feature size and is given by: 
     
       
           L =0.5 λ/NA   (7) 
       
     
     where NA is the numerical aperture of projection lens  40  and λ is the exposure wavelength. 
     If it is assumed that the image smear is linear and extends over a distance s, then the loss in image modulation amplitude at the highest possible spatial frequency L is given by: 
     
       
           L =1−(sin  x )/ x   (8) 
       
     
     and 
     
       
           x=πs/L   (9) 
       
     
     Using equation (7) for L: 
     
       
           x =2 π ( s )( NA )/λ  (10) 
       
     
     Using equation (8) and setting the allowable image modulation amplitude loss equal to 5%, the result is: 
     
       
         (sin  x )/ x =0.95 (11)  (11) 
       
     
     and solving for x: 
     
       
           x =0.5519 
       
     
     Using equation (10), it is now possible to solve for the corresponding amount of image smear s for, say, NA=0.16 and λ=365 nm (i-line):              s   =     x                   λ   /   2        nNA                 =       (   .5519   )            (     .365                 µ     )     /   2          n        (   0.16   )                     =     0.2                 µ                                  
     This is the maximum allowable amount of linear, image smear for the example lithography system  10 . In this case image smear is introduced by differences between the harmonic motion of the mask and the linear motion of the workpiece and is a cubic type of smear i.e., the velocity of the mask is slightly too slow at the beginning and at the end of the exposure flash. However, to a first order approximation we can assume the limit for cubic smear is the same as that for linear smear. 
     Thus, the mask image smear S generated by the oscillatory motion of the mask is given by: 
     
       
           S=A  sin ω t−Aωt   (12) 
       
     
     where t is the time measured from the center of the exposure pulse. Thus, 
     
       
           S=A (ω t −1/6 (ω t ) 3 +. . . )− Aωt   
       
     
     
       
           S≈A (ω t ) 3 /6  (13) 
       
     
     Since an equal amount of image smear is generated before and after perfect synchronization, the maximum pulse duration D is given by: 
     
       
           D =2 t =2(6 S/A ω 3 ) 1/3   
       
     
     
       
           D =2(6(0.2 μ)/3.5×10 3  μ) (20 π/sec) 3 ) 1/3   
       
     
     
       
           D =0.00223 seconds  (14) 
       
     
     If the reticle is held stationary the corresponding duration D s  of the exposure would be: 
     
       
           D   s   =s/V   s =0.2 μ/(220×10 3  μ/sec)=0.91×10 −6  seconds 
       
     
     Thus, the oscillatory motion imparted to mask M extends the allowable exposure time by a factor of about 2,500. If a flash lamp radiation source is used, this extension in the allowable exposure time increases the amount of energy that is attainable by a factor of about 50. 
     For single-pulse lithographic applications, the range of half-amplitudes A of the oscillatory motion of the mask will typically range from about 1 mm to about 1 cm, and the range of angular frequencies will typically range from about 20 radians/second to about 200 radians/second. 
     The Mask Holder System 
     With reference now to FIGS. 2A and 2B, there is shown a schematic diagram of mask holder system  30  comprising a mask support structure  206  having a mask frame  210  with opposing sides  212 , third and fourth sides  214  and  215 , and a lip  220  that supports mask M at mask bottom surface  32 . Mask support structure  206  includes two parallel rails  230  with ends  232  and  234 . Frame  210  is slidably connected to rails  230  at sides  212  so that the frame (and thus mask M held therein) can move in the plane defined by the rails. Operably connected to side  214  is a drive unit  240  for driving mask frame  210  in an oscillating motion, as indicated by double-headed arrow  246 . The direction of oscillatory motion is along the line workpiece W moves beneath projection lens  40  when conducting the single-pulse exposures using system  10 . Drive unit  240  is electrically connected to system controller  90 , which coordinates the oscillatory movement of mask M with the movement of workpiece W. 
     Exemplary Mask Holder System 
     With reference now to FIG. 3, there is shown an exemplary embodiment of mask holder system  30  of the present invention. Mask holder system  30  is preferably designed to cause minimal vibration and shaking of lithography system  10  when mask M is oscillated. One way to achieve isolated oscillatory motion of mask M is by moving a counterweight in the opposite direction so that no net force is applied between mask holder system  30  and the rest of system  10 . 
     Exemplary mask holder system  30  includes, as discussed above in connection with FIGS. 2A and 2B, a support structure comprising mask frame  210  and rails  230 . Frame  210  is slidably connected to parallel rails  230  by air bearings  302 ,  304  and  306 . Rails  230 , in turn, are slidably connected to lithography system  10  by air bearings  310 ,  312  and  314 . Rails  230  are also connected to and held apart by opposing endplates  330  and  332  at respective rail ends  232  and  234 . A damping pad  336  is attached to endplate  332  for damping the motion of mask holder  30  as a whole. 
     In the present embodiment of mask holder system  30 , drive unit  240  comprises a voice coil assembly  340  operatively coupled to a magnetic assembly  346  and to mask frame  210  at edge  214 . Magnetic assembly is slidably connected to rails  230  via air bearings  350 ,  352  and  354  and thus acts as a movable counterweight. Magnetic assembly  346  is also movably connected to endplate  332  by a central rod  360  having two sections  366  and  368  separated by a spring unit  380 . The latter may be, in an exemplary embodiment, a flexure. Likewise, mask frame  210  is movably connected to endplate  330  by a central rod  390  having two sections  396  and  398  separated by a second spring unit  380 . 
     A precision measurement gauge  424 , such as a laser gauge or interferometer, is provided on isolated structure  426  (FIG. 1) (including at least projection system  40 , focus system  70 , alignment system  72 , wafer stage metrology system  62  and the grounded parts of the structure shown in FIG. 3) that supports the projection lens and mask holder system  30 . Gauge  424  may also be fixed to another reference position. Gauge  424  is in operable (e.g., optical) communication with mask frame  210  so that the position, velocity and acceleration of the mask at any particular point in time can be determined. Gauge  424  is connected to main controller  90  so that the position, velocity and acceleration information pertaining to mask M can be utilized in exposing workpiece W, as described in greater detail below. 
     Mask holder system  30  further includes a small drive unit  440  fixed to endplate  330  and operatively connected to frame  210  at edge  215 , and a movement sensor  450  mounted to a fixed reference on system  10  (i.e., a fixed reference member). Sensor  450  may be, for example, a capacitance gauge in communication with end plate  332 , as shown. 
     With continuing reference to FIGS. 2A,  2 B and FIG. 3, in operation, mask holder system  30  moves mask M in an oscillatory fashion in the plane defined by rails  230 . The oscillatory motion is initiated by passing an electric current to voice coil  340  in FIG. 3 (drive unit  240  in FIG.  2 A). This generates equal and opposite forces on mask frame  210  and magnet assembly  346 . Once set in motion, mask frame  210  and magnet assembly  346  oscillate in opposition due to the presence of spring units  380  by sliding on rails  230  on their respective air bearings. The oscillation frequencies of mask frame  210  and magnet assembly  346  can be made identical (or very nearly so) by ensuring that the mass of each times the spring constant of the respective spring units  380  (i.e., the deflection per unit force) are equal or nearly so. 
     As discussed above, the motion of mask frame  210  and thus mask M supported therein is coordinated with the emission of radiation pulses from radiation source  14  and the movement of workpiece stage  50 . The motion of mask M is such that it causes the mask image to move in the direction of workpiece W when the workpiece is being exposed with a pulse of radiation. In between radiation pulses, the mask moves in the opposite direction in preparation for the next radiation pulse, as the workpiece continues to move beneath the projection lens. The smooth oscillation of mask stage  210  and magnetic assembly  346  is maintained by monitoring the amplitude of motion via sensor  424 . The information from sensor  450 , which indicates the amount of vibration of the mask stage assembly, is feed to main controller  90 , which can initiate driver unit  440  to correct the oscillatory motion imbalance by applying a correcting force. 
     The many features and advantages of the present invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the described apparatus that follow the true spirit and scope of the invention. Furthermore, since numerous modifications and changes will readily occur to those of skill in the art, it is not desired to limit the invention to the exact construction and operation described herein. Accordingly, other embodiments are within the scope of the appended claims.