Abstract:
A method for operating an optical projection system including projection optics is disclosed. The projection optics project an image of the object on the substrate at an image distance from the projection optics. The image has a size dependent on the size of the object and the ratio of the image distance to the optical distance of the object from the projection optics. The image distance can change with changes in the condition of the projection optics such as changes in the temperature of elements of the projection optics. The image distance is monitored. If the image distance changes, the substrate is moved to the new image distance and the optical distance of the object is correspondingly changed such that the ratio of the image distance to the object distance stays the same.

Description:
PRIORITY  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/586,191 filed Jul. 8, 2004, the disclosure of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to using an optical projection system for projecting an image of a mask or reticle on a workpiece in laser material processing applications. The invention relates in particular to a method and apparatus for maintaining the projected image in focus while maintaining a predetermined dimension of the image independent of temperature-induced changes of the properties of the optical projection system.  
       DISCUSSION OF BACKGROUND ART  
       [0003]     Ultraviolet (UV) laser radiation is often used in laser material processing applications. In several such operations material must be processed in a particular pattern. This is usually accomplished by making a (negative) mask or reticle representing a desired pattern and projecting an image of the mask on the material being treated (workpiece or substrate). One such application is found in the manufacture of flat panel thin film transistor (TFT) displays. This application is laser crystallization of amorphous silicon (Si) films that have been deposited on a glass substrate. In one operation in such an application, the mask image is repeatedly projected over the total area of the flat panel so that all or most of the panel is crystallized in accordance with the projected features of the mask. One preferred method of laser crystallization is the sequential lateral solidification (SLS) process. This is a two-pulse method. First, a line-pattern structure is imaged onto the substrate with a first laser pulse. Then, non-illuminated area between lines of the image are illuminated with a second laser pulse, after correspondingly moving the substrate. In order to obtain good panels, high image quality, homogeneity and reproducibility are required, i.e., constant and homogeneous energy density, well-resolved line patterns, and well-defined dimensions in the image. As the resolution of flat-panel displays is increased, these requirements will become more stringent.  
         [0004]     One problem that arises during the projection operation is that a portion of the UV radiation being projected can be absorbed by optical elements of the optical projection system increasing the temperature of the elements and thereby changing imaging properties of the optical system. One significant such change is a shift of the image plane, i.e., the plane in which the image is in optimum focus, of the optical system either toward or away from the plane in which the workpiece is located. The shift referred to here is a shift in the direction of the longitudinal or z-axis of the optical system. In the absence of any compensation scheme for such an image-plane shift, definition of features can vary over the display.  
         [0005]     A commonly used such prior-art compensation scheme involves mounting the substrate on a platform that is movable along the z-axis, monitoring the focus position, (the image plane position), during the operation, and, if a change in the focus position is detected, moving the substrate-carrying platform along the z-axis to the changed focus position. A shortcoming of this method is that changing substrate position to match the image plane shift changes the ratio of the image distance to the object distance of the optical system, i.e., changes the magnification of the optical system. The image distance, here, being the z-axis distance of the image plane position from the optical system, and the object distance being the z-axis distance of the mask being projected from the optical system.  
         [0006]     It is also possible to attempt stabilize the temperature of the optical elements by including projection optics in a temperature controlled enclosure. In theory at least, this should prevent image shift. In practice, however, the change in temperature of individual elements of the projection optics due to absorbing the laser radiation will usually occur far more rapidly than the temperature can be stabilized by the enclosure. To partially compensate for this, the image-plane-shift tracking and temperature control can be used in combination, this however, will still cause image distance and magnification change. Accordingly, there is a need for a focus (image distance) tracking and compensation method that not only maintains a substrate at a position of best focus, but also maintains the magnification of the optical system constant during this focus tracking and compensation.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention is directed to apparatus for projecting an image of an object on a substrate. In one aspect, apparatus in accordance with the present invention comprises projection optics for projecting the image of the object on the substrate. The object is located at an object distance from the projection optics, and the focused image is located at an image distance from the projection optics. The image distance can vary, as discussed above, with variations in the condition of the projection optics such as the temperature of elements of the projection optics. A substrate shifter is provided for varying the distance of the substrate from the projection optics. An object distance varying arrangement is provided for varying the optical distance of the object from the projection optics. A focus monitor is provided for monitoring the distance of the image from the projection optics as that distance changes with changes of the condition of the projection optics. A controller cooperative with the focus monitor, the substrate shifter, and the object distance varying arrangement is arranged to maintain the substrate at the image distance as conditions of the projection optics vary, and to maintain a constant ratio of the image distance to the object distance.  
         [0008]     In one preferred embodiment of the inventive apparatus, the physical distance of the object from the projection optics is fixed. The optical distance varying arrangement includes two transparent counter-wedges located between the object and the projection optics. The optical distance of the object from the counter-wedges is varied by varying the lateral position of the wedges with respect to each other such that the total thickness of the wedges in the beam varies. In this arrangement, the focus monitor monitors the image distance through the projection optics and the counter wedges. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention.  
         [0010]      FIG. 1  schematically illustrates one preferred embodiment of apparatus in accordance with the present invention, including projection optics projecting an image of a mask on a substrate mounted on an axially movable platform at a focal distance from the projection optics, a focus monitor for determining the focal distance, a controller cooperative with the focus monitor for moving the substrate to maintain the substrate at the focal distance if the focal distance changes with operating conditions of the projection optics, and two wedges located between the mask and the projection optics, the wedges being movable by the controller with respect to each other for varying the optical distance of the object from the projection optics and thereby maintaining a fixed ratio of focal distance to object distance as the focal distance changes.  
         [0011]      FIG. 1A  schematically illustrates details of the object-distance varying wedges of  FIG. 1 .  
         [0012]      FIG. 2  schematically illustrates one preferred embodiment of apparatus in accordance with the present invention, similar to the apparatus of  FIG. 1 , but further including a substrate height tracking arrangement for detecting changes in distance from projection optics to substrate as the substrate is translated transverse to the optical axis of the projection optics in step-and-repeat or scanning operations. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     The  FIG. 1  and  FIG. 1A  schematically illustrate a preferred embodiment  10  of apparatus in accordance with the present invention. Apparatus  10  includes projection optics  12  having a longitudinal optical axis (z-axis)  14  folded by a dichroic mirror  16 . A mask  18 , an image of which is to be projected, is located on the optical axis and illuminated by UV light from a laser and illumination optics (not shown). The general direction of propagation of UV light from the illuminated mask is indicated by single arrowheads. Projection optics  12  forms an image of the mask on a substrate (workpiece)  20  located at a position S 0  at a focal distance I 0  from a principal plane P of the projection optics. Substrate  20  is held on a platform  22 , which is movable in forward and reverse directions along optical axis  14 , responsive to commands from a system controller  24  as indicated by arrows F. Those skilled in the art will recognize that while the term “focus position” is used herein to describe position S 0 , and the term “focal distance” is used to describe distance I 0 , this distance I 0  is not necessarily the “focal length” of the projection optics, but is the distance from the optics of one conjugate focus of the optics, corresponding to a corresponding conjugate focus of the optics in which mask  18  is located. That distance can be referred to in the alternative as an image distance, being the distance at which an optically defined (focused) image is located from the projection optics.  
         [0014]     A focus monitor  26  is provided for determining the degree of focus of the projection optics at the instant location of substrate  20  and thereby providing a measure of the image (focal) distance. This focus monitor employs a well-known method of wavefront measurement for determining focus position. The focus monitor  26 , here, includes a laser-diode  28 . The general propagation direction of light delivered by the laser diode is indicated by double arrowheads. Light from the laser diode is focused into a single-mode optical fiber  30  by coupling optics  32 . The laser diode-light is transported by the fiber and directed to a beamsplitter  34 . Beamsplitter  34  in this example is in the form of a cemented bi-prism, but that should not be considered as limiting the present invention.  
         [0015]     A portion of the laser diode light is reflected from beamsplitting surface  36  of the bi-prism and is transmitted through dichroic mirror  16  along axis  14  of the projection optics, and through two wedges  44  and  46 , the purpose of which is discussed further hereinbelow. The projection optics image the laser diode-light, i.e., the exit face  30 A of fiber  30 , onto the substrate. Diode-laser light is reflected from the substrate and returns through the projection optics, through wedges  44  and  46 , and through dichroic mirror  16  to beamsplitter bi-prism  34 . A portion of the laser diode light is transmitted by beamsplitting surface  36  of the bi-prism to monitor optics  38 . In monitor optics  38 , the laser diode light is collimated then focused onto a CCD chip (camera)  40  by a cylindrical lens array (not shown) as an image including two parallel lines. The separation distance between the parallel lines provides a measure of the focus position of the projection optics. Controller  24  processes data from CCD camera  40  to determine the line separation and adjusts the position of platform  22  to maintain substrate  22  at the optimum focal distance. The inventive system is typically used in repeated imaging operations on substrates having the same nominal thickness, and the system is calibrated prior to use for that nominal substrate thickness, such that the front surface of the substrate is what is maintained at the focal distance.  
         [0016]     It should be noted here that the wavefront-measurement method of determining focus position is well known in the art, and the forgoing brief discussion of focus monitor  26  is presented merely for completeness of description. This particular focus monitor arrangement should not be construed as limiting the present invention. Several other focus monitoring arrangements for driving what may be described as “auto-focus” mechanisms are also well known in the optical art. One skilled in the art may incorporate any one of these arrangements in the inventive apparatus without departing from the spirit and scope of the present invention. The wavefront-measuring focus monitor was found to be capable of a image distance resolution of less than about ±2.0 micrometers (μm) over a range of about 100 μm for an object-to-image distance of about 800 millimeters (mm). Other focus monitor types may provide a different resolution.  
         [0017]     As discussed above, a principal object of the present invention is to provide not only a device that maintains a mask image in focus on a substrate or workpiece, but to provide also that this focus is maintained without the image size being changed as a result of any optical system changes necessary to maintain the image in focus. The image size is determined by the mask size and the magnification of the optical system including the projection optics. A measure of this magnification is the ratio of the optical distances of the mask (object) and the image from a principal plane of the projection optics. A principle feature of the present invention is that means are provided to enable this image-distance:object-distance ratio to be maintained constant as the image distance is changed to track thermally-induced or other changes in the focal distance (image distance) during operation of the apparatus. It should be noted, that the term “magnification” is used here in a formal sense. Those skilled in the art will recognize that in a system of the type described, the image size will be usually be less than the mask size, i.e., the magnification will usually have a value less than 1.0. This, of course, should not be considered as limiting the present invention.  
         [0018]     In apparatus  10 , the position of mask  18  with respect to principle plane P is assumed to be fixed, i.e., the physical distance of the object from the projection optics is fixed. An arrangement  43  is provided for varying the optical distance of the mask from plane P. The optical distance depends on the physical distance and on the refractive index and thickness of any transparent elements located within the physical distance. In this arrangement, the optical distance can be varied by altering, by means of an actuator  41 , the position transverse to optical axis of two identical transparent wedges  44  and  46  with respect to each other, as indicated in the  FIGS. 1 and 1 A by arrows T. The wedges are located between the mask and the projection optics and arranged transversely across axis  14 . The vertices of the wedges are opposed and each wedge has the same wedge angle. Hypotenuse faces  45  and  47 , of wedges  44  and  46 , respectively, face each other and are parallel to each other, and outer faces  49  and  51  of the wedges are also parallel to each other Such opposed wedges are often referred to as counter-wedges by practitioners of the art, and provide for varying the thickness of material through which a beam must pass without varying the tilt of the beam or varying any lateral translation of the beam.  
         [0019]     Moving the wedges laterally closer to, or further from each other respectively increases or decreases the thickness of wedge material in the path of light from the mask. This is illustrated in  FIG. 1A  wherein wedge  44  is depicted in two different lateral (with respect to longitudinal axis  14 ) positions, with one position  44 A thereof shown in phantom. This increase or decrease of the wedge material thickness effectively, respectively increases or decreases the object distance. The translation is effected as depicted in  FIG. 1A , at an angle (the wedge angle) to a perpendicular to axis  14  such that the distance between the hypotenuse faces remains the same. This prevents changes in any lateral translation of the beam. Because the wedges have the same wedge-angle the beam is not tilted. In  FIGS. 1 and 1 A, for convenience of illustration, only one actuator is depicted, translating only one wedge with respect to the other. A bigger range of change in combined axial thickness of the wedges can be obtained by translating both wedges in opposite directions. This may, however, require a more complex mechanical arrangement.  
         [0020]     This variable object distance is indicated in the  FIG. 1  as an optical distance O between principal plane P and an “unfolded” mask position  18 A. Distance O is a function of the translation T of wedges  44  and  46  with respect to each other (O=f(T)) or as a function of the thickness of wedge material on optical axis  14 . Whatever method of focus monitoring is used, it is preferable that the focus is monitored not only through the projection optics but also through the counter wedges as described above.  
         [0021]     Thermally induced changes in projection optics  12  can cause a shift of the focus of the objective from a nominal focus position S 0  at which the substrate is located to a position S 1  closer to principal plane P of the projection optics, or to a position S 2  further from principal plane P. Positions S 0 , S 1 , and S 2 , are at distances (image distances) of I 0 , I 1 , and I 2 , respectively, from principal plane P of the objective, with I 0  being the nominal image distance. It is desired that the system have a fixed magnification M equal to a nominal image distance divided by a nominal object distance.  
         [0022]     If a focus shift occurs as a result of thermally-induced changes in the projection optics, focus monitor  26  detects the focus shift. A signal from the focus monitor corresponding to the shift is received by controller  24 . In a preferred correction scheme, the controller then calculates, in response to the signal, a new object distance and a new image distance that will provide a focused (optimally sharp) image and a ratio of image-distance:object-distance equal to M. The controller then simultaneously adjusts the object distance the new value and commands actuator  22  to shift the substrate to the new image distance from the projection optics. The controller changes the object distance by commanding controller  41  to translate wedges  44  and  46  with respect to each other to change O to the required value. In another correction scheme, the controller can first shift the substrate to a new image distance, then change object distance O to restore the magnification to M. In this scheme, if the change in distance O is sufficiently great, it is possible that the focal position will again change as a result of changing distance O. If this is the case, the changed focal position is again detected by the focus monitor and the substrate shifting and wedge translation are executed again. The substrate shifting and wedge translation can take place iteratively, if necessary, until a condition is reached where the substrate has a focused image thereon and the magnification is M.  
         [0023]     One potential source of inaccuracy of the above-discussed inventive optical system is that changes in the axial distance of the substrate from the projection optics can occur if the substrate is translated transverse to optical axis  14  in scanning or step-and-repeat imaging operations and the substrate surface on which the image is projected is not precisely flat or of constant thickness. Such a distance change could also occur due to differences in thickness between substrates in a batch thereof, i.e., after an exposed substrate was exchanged for a new, un-exposed substrate. In most cases, such changes would be within the potential accuracy of the focus monitor itself, however, a method of compensating for a more significant change can be advantageous. A description of one such method of compensating for such a possible inaccuracy is set forth below with reference to  FIG. 2 .  
         [0024]      FIG. 2  schematically depicts another preferred embodiment  11  of a projection system in accordance with the present invention, similar to system  12  of  FIG. 1 , but further including a height sensor  54  for detecting changes in substrate height, i.e., the distance from substrate to projection optics, that would occur if substrate  20  were not exactly flat, and the substrate were translated in either the X or Y directions transverse to optical axis  14  between exposures of the substrate to the mask image. In this example height sensor  54  includes a light source  56  and a receiver  58 . A beam from the light source is focused on a position detector (not shown) in the receiver. The signal from the detector is transmitted to the controller. Any change of beam position following a translation will be interpreted by the controller as a resulting from a change in substrate distance from the originally calibrated distance. The controller can compensate for this by changing accordingly the axial position of the substrate, i.e., by moving the substrate back to where the image instantly is. It is emphasized, here, that sensor  54  is but one form of substrate-distance determining sensor that can be used with the present invention, and should not be considered as limiting the present invention.  
         [0025]     By means of these two independent sensors, (one monitoring image distance through the projection optics and the other measuring the substrate-distance changes), controller  24  can clearly distinguish between effects resulting from changes in properties of the projection lens, and effects resulting from substrate thickness or flatness changes. Changes in properties of the projection lens are compensated for by changing both the object distance and the image distance as described above, in order to hold image focus and magnification constant. Changes in substrate thickness are compensated for by changing only the axial position of the substrate.  
         [0026]     In conclusion, the present invention provides a system for projecting an image of a mask on a substrate in which the image size is maintained constant if the distance of the substrate from projection optics of the system is changed to correspond to a monitored operational shift in focus position. This is achieved by providing that the ratio of the optical distance from the substrate to the projection optics to the optical distance to the projection optics is maintained constant as the image distance is changed to refocus the image on the substrate on the substrate. This maintains the magnification of the system, and accordingly the image size, essentially constant.  
         [0027]     The present invention is described above with reference to one preferred embodiment. In this embodiment, a wavefront measuring focus monitor is used to determine focal distance, and the image size is maintained essentially constant by varying the optical distance of the mask from the projection optics such that the ratio of the image distance to the optics distance is held constant. Persons of ordinary skill in the art may modify the above-described embodiment without undue experimentation or without departing from the spirit or scope of the present invention. By way of example, a different focus monitor may be employed or a different method of changing the optical distance of the object may be employed. These or any other such departures or deviations, however, should be construed to be within the scope of the claims appended hereto.