Abstract:
A system and method are used to detect parameters regarding an exposure portion or an exposure beam. The system comprising a substrate stage and a metrology stage. The substrate stage is configured to position a substrate to receive an exposure beam from an exposure portion of a lithography system. The metrology stage has a sensor system thereon that is configured to detected parameters of the exposure system or the exposure beam. In one example, the system is within a lithography system, which further comprises an illumination system, a patterning device, and a projection system. The patterning device patterns a beam of radiation from the illumination system. The projection system, which is located within the exposure portion, projects that pattered beam onto the substrate or the sensor system.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 11/362,280 to Van De Kerkhof et al., entitled “Dedicated Metrology Stage for Lithography Applications” and filed Feb. 27, 2006 (now abandoned), which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 60/657,710, filed Mar. 3, 2005, all of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention is related to measurement of lithographic exposure system parameters, and more particularly, to a dedicated metrology stage for lithography applications. 
     2. Related Art 
     Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. One skilled in the relevant art would recognize that the description herein would also apply to other types of substrates. 
     During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by an exposure system located within a lithography system. The exposure system includes a reticle (also called a mask) for projecting the image onto the wafer. 
     The reticle is generally located between a semiconductor chip and a light source, usually mounted on a reticle stage. In photolithography, the reticle is used as a photo mask for printing a circuit on a semiconductor chip, for example. Lithography light shines through the mask and then through a series of optical lenses that shrink the image. This small image is then projected onto the silicon or semiconductor wafer. The process is similar to how a camera bends light to form an image on film. The light plays an integral role in the lithographic process. For example, in the manufacture of microprocessors, the key to creating more powerful microprocessors is the light&#39;s wavelength. The shorter the wavelength, the more transistors can be etched onto the silicon wafer. A silicon wafer with many transistors results in a more powerful microprocessor. 
     A relatively common problem in the lithographic art is a need to measure parameters of the optical systems used for lithographic exposure. As a general rule, it is desirable to be able to do such measurements without taking the lithographic exposure system offline, and without disassembly and reassembly of components. The current practice in the industry is to place sensors on the wafer stage, to the extent space permits. These sensors are generally located in the space not occupied by the wafer itself, in the corners of the wafer stage. 
     However, with the ever increasing sophistication of exposure systems, with decreasing exposure wavelengths, and with increasing complexity of the optics, the number of different sensors that end users require is increasing. At the same time, there are severe constraints on the available space. For example, it is generally impractical or undesirable to increase the dimensions of the wafer stage, since this complicates stage positioning, and stage movement, and increases the dimensions of the overall lithographic equipment, which is problematic, since clean room space inside a fabrication facility is limited. 
     Accordingly, what is needed is a way to enable positioning of measurement sensors of lithographic exposure optics without affecting overall lithographic tool dimensions. 
     SUMMARY 
     In one embodiment of the present invention there is provided a system comprising a substrate stage and a metrology stage. The substrate stage is configured to position a substrate to receive an exposure beam from an exposure portion of a lithography system. The metrology stage has a sensor system thereon that is configured to detected parameters of the exposure system or the exposure beam. 
     In another embodiment of the present invention, the system is within a lithography system, which further comprises an illumination system, a patterning device, and a projection system. The patterning device patterns a beam of radiation from the illumination system. The projection system, which is located within the exposure portion, projects that pattered beam onto the substrate or the sensor system. 
     In a further embodiment of the present invention, there is provided a method of measuring optical parameters of an exposure portion of a lithography system comprising the following steps. Moving a substrate stage away from an optical axis of the exposure portion. Moving a metrology stage to locate a sensor in the optical axis. Measuring an optical parameter of the exposure system. 
     Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure and particularly pointed out in the written description and claims hereof as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. 
         FIG. 1  illustrates a metrology stage, according to one embodiment of the present invention. 
         FIG. 2  schematically illustrates an exemplary lithographic system according to one embodiment of the present invention, which uses the metrology stage. 
         FIG. 3  shows an exemplary polarization sensor that can be used in the system of  FIG. 2 , according to one embodiment of the present invention. 
         FIG. 4  shows an exemplary arrangement of sensors on the metrology stage. 
         FIG. 5  shows an apodization sensor in the sensors, according to one embodiment of the present invention. 
         FIGS. 6 and 7  show a stray light sensor in the sensors, according to one embodiment of the present invention. 
     
    
    
     One or more embodiments of the present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers can indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number can identify the drawing in which the reference number first appears. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates one embodiment of the present invention. Shown in  FIG. 1  is an isometric three-dimensional view of a substrate handling mechanism  102  of an exposure apparatus (the remainder of the exposure apparatus, such as projection optics, reticle stage, illumination source, etc. are not shown for clarity in  FIG. 1 , but see discussion below relating to  FIG. 2 ). The substrate handling apparatus  102  has a frame  104 , portions of which are shown in  FIG. 1 . A robot arm  108 , or a similar mechanism, is used to move substrates  112  in and out of the substrate handling apparatus  102 . Two substrate stages, in this case, labeled  106 A and  106 B, are located within the substrate handling apparatus  102 . The substrate stages  106 A,  106 B have substrates  112 A,  112 B located thereon. In alternative examples, the substrates being exposed can be semiconductor wafers or flat panel display (FPD) substrates. Typical dimensions of the substrate stages  106 A,  106 B is slightly larger than the substrate  112  itself. For current state-of-the-art 12-inch diameter substrates, the substrate stages can be roughly square and on the order of about 13-14 inches in size. In a two-substrate-stage system, one of the substrate stages is typically used for exposure, while the other one is used for measuring of exposure results (for example, measurement of post-process substrate surface height, etc.). 
     Each substrate stage  106 A,  106 B has a corresponding actuating system  110 A,  110 B for moving the substrate stage  106 A,  106 B. The substrate stages  106 A,  106 B can have corresponding sensors mounted thereon, designated  124 A- 130 A for substrate stage  106 A, and  124 B- 130 B for substrate stage  106 B. In one example, that the location of the sensors  124 - 130  is in the corners of the substrate stages  106 , since the substrate  112  is normally in the center of the substrate stage  106 . 
     Also shown in  FIG. 1  is a metrology stage  116 , which includes sensors  140 A,  140 B,  140 C and  140 D for measurement of optical parameters. It will be understood that the number of sensors  140  on the metrology stage  116  is not particularly limited, although typically the overall dimensions of the metrology stage  116  will be smaller than the dimensions of the substrate stages  106 . 
     In one example, the vertical dimension of the sensors  124 - 130  placed on the substrate stages  106  may be limited for various reasons. For example, the minimum height of the substrate stage  106  and the location of the lowest element of the projection optics (not shown in the figure) can limit the vertical dimensions of the sensors  124 - 130  on the substrate stage  106 . In one example, with respect to the metrology stage  116 , since the metrology stage  116  can be “thinner” than the substrate stage  106 , the sensors  140  can be “taller” than the sensors  124 - 130 . Also, in one example, the substrate stages  106  can be made smaller in the X-Y (horizontal) dimension (for example, the “corners” can be “cut off,” resulting in a “footprint” of the substrate stage that is smaller than the roughly square shape shown in  FIG. 1 ), realizing space savings. 
       FIG. 2  schematically illustrates an exemplary lithographic system  200  according to one embodiment of the present invention, which uses the metrology stage  116 . As shown in  FIG. 2 , the lithographic system  200  (shown in side view) includes a light source (illumination source)  210 , such as a laser or a lamp, illumination optics  212  (such as a condenser lens), and a patterning device (e.g., reticle, mask, spatial light modulator, etc., hereinafter reticle will be used)  214 , which is usually mounted on a patterning device stage (not shown). Note that the reticle  214  can be a plate with an exposure pattern on it. In an alternative example, the reticle can be replaced with a dynamic patterning device, such as an array of programmable elements or a spatial light modulator array, such as used in maskless lithography. Light from the reticle  214  is imaged onto the substrate  112  using projection optics  216 . The substrate  112  is mounted on the substrate stage  106  (only one of the two substrate stages is shown in this figure, as the invention is not limited to any particular number of substrate stages). Also shown in  FIG. 2  is the housing  104  (which can enclose only the substrate stage  106  and the metrology stage  116 , or can enclose other components illustrated in the figure). 
     In one example, the sensors  140  (see  FIG. 1 ) can include a polarization sensor, which is particularly useful for measurement of time-varying polarization (absolute and relative) properties of the projection optics. The polarization sensor is one sensor where the ability to install sensors that have a substantial height becomes particularly important. 
       FIG. 3  shows an exemplary polarization sensor that can be used in the system of  FIG. 2 , according to one embodiment of the present invention. The polarization sensor includes a quarter wavelength plate  302 , a collimator lens  304 , a polarizer  306 , a detector  308 , and a mechanism for rotating the quarter wavelength plate  302 . 
     The polarizer (analyzer)  306  is located downstream of the projection optics  216  and positioned in the optical path. The polarizer  306  passes one particular polarization of the incoming light that can then be measured in the metrology stage  116 . Examples of polarizers can be such optical components as polarizing plates, polarizing beam splitters, etc. Such optical components are frequently relatively volume-intensive, for example, on the order of several cubic centimeters. Furthermore, such optical components are usually very limited in angular range (i.e., in terms of angle of incidence), usually on the order of less than 1 degree, and frequently substantially less than 1 degree. At the same time, the projection optics  216  is typically a high numerical aperture lens, or set of lenses, which is mismatched to the very small angular range of the optical components, such as polarizing beamsplitters. 
     In one example, in order to use such a polarizer  306 , it is necessary to shape the beam appropriately. In one example, such shaping is done by means of a collimator lens  304  (or set of lenses). The collimator lens  304  is also relatively difficult to miniaturize, and often has a volume of several cubic centimeters. Furthermore, it is frequently desirable to measure not just one polarization, but a range of polarizations. To accomplish this, in one example the entire polarization sensor needs to be rotated, while in another example a quarter wavelength plate  302  can be inserted into the beam path (for example, between the collimator lens  304  and projection optics  216 ) and can then be rotated to select the appropriate polarization. A detector  308 , for example a charged coupled device (CCD) array (or a photodiode), is positioned such that the detector  308  is at the proper focus and is aligned in the X-Y plane (note that this is an imaging measurement, and it is important to properly position the detector). 
     From the description above, it can be seen that the entire polarization sensor, including the quarter wavelength plate  302 , collimator lens  304 , polarizer  306 , CCD array  308 , and a mechanism for rotating the quarter wavelength plate  302 , occupies relatively large volume. For example, this volume can be on the order of several cubic centimeters, which given the “cramped” dimensions available to the designer of the lithographic tool, makes it relatively impractical to use such polarization sensors, if they need to be mounted on conventional substrate stages. However, since the metrology stage  116  can be made thinner, the polarization sensor, an example of which is described above, can be installed on the metrology stage  116 . 
       FIG. 5  shows an apodization sensor  502  in the sensors  140 , according to one embodiment of the present invention. The apodization sensor  502  measures the intensity of the exposure beam as a function of distance from the optical axis in the XY plane (image plane). This is also an imaging measurement. The apodization sensor  502  is another example of a sensor where vertical height requirements can make it impractical to mount such a sensor on a conventional substrate stage. In one example, the apodization sensor  502  includes a CCD array  504  that “looks” into the pupil of the projection optics  216 . Generally, the CCD array  504  needs to be optically conjugate with the pupil of the projection optics  216 . This requires the use of a relay lens  506  between the charge coupled device  504  and the projection optics  216 . In one example, the relay lens  504  has a dimension on the order of several millimeters or even a few centimeters. Thus, mounting such an apodization sensor  502  on a conventional substrate stage is extremely difficult. 
     In one example, a CCD array  504  for an apodization sensor  502  measures the light intensity in the image plane as a function of (X,Y) and is at least the size of the exposure field in the image plane. In one example, an exposure field is several tens of millimeters by several millimeters in size and on the order of about 26 millimeters by 10 millimeters in size (although the exposure field in many state of the art lithography tools is generally increasing in size over time). Thus, the CCD array  504  is at least as large, or somewhat larger, in size, as the exposure field. 
     In one example, the apodization sensor  502  can be used to verify the numerical aperture of the system. Such a measurement may be desired by the end user to confirm that the system performs to specification, i.e., works “as advertised.” It should be noted that the numerical aperture measurement is a one-time (or, at most, relatively rare) measurement, compared to many other measurements that need to be performed much more frequently. 
       FIG. 4  shows an exemplary arrangement of sensors on the metrology stage (with the top view and side view shown). In this case, nine sensors  140 A- 140 I are shown in a grid pattern arrangement. Any of the sensors discussed above or below can be one of these nine sensors  140 A- 140 I, having the arrangement and structures as described, which are not shown for convenience. 
     In one example, the sensors  140 A- 140 I can include a sensor to measure slit uniformity, if slits are used in the lithographic optics. This is a measure of illumination source quality. A typical high-end lithographic system, as noted above, exposes an area on a substrate that is several tens of millimeters by several millimeters in size, depending on the manufacturer of the lithographic system, for example, about 26 millimeters by 10 millimeters. For nomenclature purposes, the 26 millimeter dimension is usually referred to as “X,” and the 10 millimeter dimension is usually referred to as “Y.” Ideally, the optical system is able to image a perfect “rectangle” that has an intensity distribution that is uniform throughout the rectangle. A slit uniformity sensor is designed to measure whether the “uniform rectangle” that is imaged is in fact uniform, and if not, how far it deviates from uniformity. This can be accomplished, for example, through the use of a integrating precision photodiode that is scanned in the Y direction. The photodiode can have a pinhole, or a slit, “on top” of it, to limit the amount of light that reaches the photodiode. 
     It should also be noted that a charged couple device normally cannot be used for this purpose, because most CCDs drift over time. What is of primary interest in the slit uniformity measurement is absolute values of intensity (in addition to relative intensity as a function of X,Y distance), since it is important to accurately relate the voltage from the photodiode to the amount of light received by the photoresist. Note also that the use of a precision photodiode permits a better signal-to-noise ratio. The photodiode provides an integration of the received light in the Y direction, either by moving the photodiode or by integrating using a slit. 
     In one example, the sensors  140 A- 140 I can include a wavefront sensor to measure the quality of the wavefront image, as well as any aberrations. An example of such a wavefront sensor is an ILIAS sensor (Inline Lens Interferometric System) to measure the quality of the wavefront. It should be noted that although ILIAS sensors have been used in the past, one problem with such sensors in conventional systems is the need to measure the “far field” due to the lack of space for a relay lens, or a collimator lens. As discussed above, the ILIAS sensor can include such a relay and/or collimator lens, to substantially improve performance of the ILIAS sensor, and therefore, of the measurement of the quality of the wave front and the aberrations. 
     In one example, the sensors  140 A- 140 I can include a sensor to measure image contrast. The contrast sensor measures the quantity 
                 Maximum   ⁢           ⁢   Intensity     -     Minimum   ⁢           ⁢   Intensity           Maximum   ⁢           ⁢   Intensity     +     Minimum   ⁢           ⁢   Intensity             
in the image plane. One way to implement the contrast sensor is to have slits on the reticle, with the slits arranged to have a certain pitch. One photodetector, or one single photodetector per pitch can be used in the image plane. With perfect contrast, the slits in the reticle (in the object plane) will form “lines of light” in the image plane and perfectly dark areas outside of the lines of light. In practice, this may be unachievable and there is always some signal that will be measured even in the “dark areas.” The contrast sensor therefore provides a measurement of the relative intensity between the light areas and the dark areas.
 
       FIGS. 6 and 7  show a stray light sensor  602  in the sensors  140 A- 140 I, according to one embodiment of the present invention.  FIG. 7  shows an alternative embodiment of the plate portion  706  of sensor  602 , according to one embodiment of the present invention. Sensor  602  can measure stray light (which can be due to contamination of the optics). The stray light sensor  602  essentially measures intensity in the image plane as a function of radial distance from the optical axis. In one example, this is done by creating a point source in the object plane, in other words, the reticle functions as a point source, rather than as a mask for exposure. Ideally, the point source images into a point in the image plane. The stray light sensor  602  can also include a transmissive glass plate  606  with chrome  608  (or other metal) blocking the light from the point source. The glass plate  606  is positioned in the image plane. A detector  604 , for example, as a photo detector or a CCD array is positioned below the glass plate. The detector can also be an integrating photodiode. 
     In the example shown in  FIG. 7 , in addition to the central portion having chrome  708  blocking the light from the point source, a ring shaped annulus  710  is left open, with the remaining portion of the glass plate  706  also covered by chrome or metal  708 . Thus, the sensor  602  measures the amount of light received at a distance r (i.e., I(r)) from the optical axis, which, with the point source blocked, represents stray light. Different glass plates, with different radii of the annulus can be used to “step through” the various distances r. It will be appreciated that other arrangements of sources in the object plane, detectors and blocking elements (like glass plates) are possible. 
     In one example, the sensors  140 A- 140 I can include a focus sensor, to sense the location of the focus (image) plane in the vertical direction. The focus sensor is typically a photodiode that is initially placed at the expected location of the focus, and light intensity is measured. The photodiode is then moved in three degrees of freedom (X, Y and Z) to locate the maximum, which is then taken to be the location of the focus. 
     In one example, the sensors  140 A- 140 I can include a sensor to measure alignment of the reticle, functioning in a manner similar to the focus sensor (to find the maximum intensity at a point of alignment). 
     Throughput is a critical performance driver in current semiconductor manufacturing processes. Use of a dedicated metrology stage as described above can diminish throughput when measurements are made in a serial fashion. That is, in one approach to using the dedicated metrology stage, either substrate stage  106  is located underneath the projection lens so as to expose wafers, or metrology stage  116  is located underneath the projection lens, but not both. Further to the penalty imposed by the serial nature of these measurements, there is a time penalty resulting from the need to swap substrate stage  106  with metrology stage  116 , and vice versa. In addition, there are different volume requirements imposed on sensors that can be located on substrate stage  106  versus the more relaxed volume requirements imposed on sensors that can be located on metrology stage  116 . Such volume requirements can result, for example, in the height of metrology stage  116  being less than the height of substrate stage  106 . Alternatively, where the height of metrology stage  116  and the height of substrate stage  106  are similar, certain sensors with substantial size requirements are precluded from being located on substrate stage  106 . 
     In accordance with an embodiment of the present invention, a hybrid mode of metrology is described that addresses the above throughput requirements. In an embodiment of the hybrid mode, sensors, such as sensors  140 , are divided into two sets. The first set of sensors includes those sensors that provide relatively rapid measurements with modest accuracy, while a second set of sensors includes those sensors providing more accurate measurements, albeit more slowly. Thus, more frequently needed measurements of relevant imaging parameters are made using the first set of sensors located on substrate stage  106 , while less frequently needed measurements are made using the second set of sensors  140  located on metrology stage  116 . In this embodiment, the metrology stage sensors and associated measurements are typically more accurate and offer better repeatability than substrate stage sensors, since less of a compromise is required between the available building space and environment provided to the sensors on the one hand and sensor performance on the other. Moreover, a hybrid mode speeds up the measurements on the metrology stage by shifting a portion of the measurement time needed to the first set of sensors (the faster sensors) on substrate stage  106 . Thus, without any real performance sacrifice, throughput is gained in two ways, namely through less stage-swap overhead and through faster average measurements. 
     In addition, sensors on substrate stage  106  and their measurements may be calibrated against measurements made by corresponding sensors on metrology stage  116 . Thereafter, the sensors on substrate stage  106  may be used in a relative mode. Such a calibration that permits sensor usage in a relative mode reduces an absolute accuracy requirement for the sensors. As such, the relative mode significantly improves the attractiveness of the compromise situation between sensor performance, sensor volume, environment and measurement time. Therefore, for example, a sensor can be used in relative mode on substrate stage  106 , while the same type of sensor can be used in the much slower absolute mode on metrology stage  116 . 
     In a further embodiment of the present invention, an alternative hybrid approach is to measure a first set of relevant imaging parameters using sensors located on substrate stage  106 , and to measure a second set of relevant imaging parameters using sensors located on metrology stage  116 . Again, the sensors on substrate stage  106  and metrology stage  116  include sensors  140 . In an embodiment, the relevant imaging parameters chosen to be included in the second set of relevant imaging parameters are those that to remain constant or change very slowly over time, which means the measurements may be made on a relatively infrequent basis. Similarly, the relevant imaging parameters chosen to be included in the first set of relevant imaging parameters are those that vary more rapidly than their counterparts in the second set of relevant imaging parameters. In a further variation on this approach, when correlations are established between parameters measured on substrate stage  106  and parameters measured on metrology stage  116 , these parameters may be measured using sensors on substrate stage  106  over the working range established by the correlation. When measurements are required outside the established working range, a new working point may be established by a measurement using a sensor on metrology stage  116 . 
     In a still further embodiment of the present invention, metrology stage  116  (including the sensors located on metrology stage  116 ) may be detached from close proximity to substrate stage  106 . Metrology stage  116  may be stored separately from substrate stage  106 . In addition, metrology stage  116  may be moved from one location to another, from one substrate stage  106  to the next, as measurement requirements dictate. In particular, moving from one location to another includes moving from one machine (or even building) to another. Following a move from one location to another, metrology stage  116  is typically actively aligned in position and orientation to the particular measurement situation prior to measurements being taken. 
     CONCLUSION 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 
     It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections can set forth one or more, but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way.