Imaging optical system configured with through the lens optics for producing control information

A new and useful imaging concept is provided that is designed to improve the manner in which control information for an imaging optical system such as a lithographic imaging optical system can be generated. An imaging optical system comprises imaging optics defining a primary optical path along which a primary image is imaged, and a measurement optical path is established and includes at least part of the primary optical path. The imaging optical system is configured to obtain information from the measurement optical path for use in providing control information for the imaging optical system. Such a system is particularly useful for measuring the topography of a large region of the surface under investigation, like the entire instantaneous field of a wafer, instead being limited to a small patch or set of patches.

BACKGROUND

The present invention relates to an imaging optical system, and particularly to a new and useful way of producing control information for the imaging optical system. The present invention is particularly useful in providing control information for an imaging optical system such as a lithographic imaging optical system.

In a typical lithographic imaging optical system, a spatially incoherent radiation (e.g. light) source is used to illuminate a mask or reticle, to produce an image that is projected and used to image the photoresist on a semiconductor wafer. The wafer is typically supported on a machine part known as a wafer stage that can be moved (adjusted) relative to the imaging lens. A controller drives actuators associated with the wafer stage to correctly position the wafer stage in the imaging optical system.

In an imaging optical system such as a lithographic imaging optical system, it is desirable to position the wafer properly in relation to the focal plane of the imaging lens system. One way of generating control information for positioning a wafer relative to the imaging lens system is shown in U.S. Pat. No. 5,268,744. A reflection beam is directed through the optical system to a first region of a wafer and then onto a predetermined plane to determine the focal plane of the optical system, and to establish a reference position on the predetermined plane that corresponds to the focal plane. Another reflection beam is directed to another region of the wafer and onto the predetermined plane, and generating control information for positioning the other region of the wafer on the basis of the location of the other beam with respect to the reference position.

SUMMARY OF THE INVENTION

The present invention provides imaging concepts that are designed to further improve the manner in which control information for an optical system such as a lithographic imaging optical system can be generated.

The present invention provides an imaging optical system comprising imaging optics defining a primary optical path along which a primary image is projected, and a measurement optical path that is established and includes at least part of the primary optical path. The imaging optical system is configured to obtain information from the measurement optical path for use in providing control information for the imaging optical system. The present invention is particularly useful as part of a metrology system that includes, e.g. optics, detectors, electronics, mechanics etc., which detects the information from the measurement optical path, and produces control data that is useful in the imaging optical system.

According to a preferred version of the invention, the imaging optics includes an aperture stop, and the measurement optical path is in a predetermined relationship to the aperture stop of the imaging optics. The measurement optical path preferably includes a reflection optic located in a predetermined relationship to the aperture stop of the imaging optics, and the measurement optical path provides an image of a measurement image source (real or virtual) that is inserted into the imaging optics in the optical space between the aperture stop and the image plane of the imaging optical system. The measurement optical path can also provide an image of the measurement image source that is inserted between the aperture stop and the object (reticle) plane of the imaging optical system, where it can be used to investigate the reticle.

Moreover, in a preferred imaging optical system, the image plane defines a stop for the measurement optical path, and the imaging optical system further includes a measurement pupil plane that is conjugate to the measurement image stop. The measurement optical path includes an image of a measurement image source that is projected from the measurement image stop to the measurement pupil plane.

The present invention is particularly useful in providing a means of making in-situ measurements of various aspects of the imaging of a lithographic imaging lens. For example, the present invention allows for measuring the topography of a large region of the surface under investigation, like the entire instantaneous field of a wafer, instead being limited to a small patch or set of patches.

Other features of the present invention will become further apparent from the following detailed description and the accompanying drawings.

DETAILED DESCRIPTION

As discussed above, the present invention relates to a new and useful imaging concept designed to improve the manner in which control information for an imaging optical system (e.g. a lithographic imaging optical system) can be generated. The manner in which the principles of the present invention can be implemented in imaging optical systems of the type that are useful in a lithographic imaging optical system are described herein, and from that description, the manner in which the principles of the present invention can be implemented in various types of imaging optical systems will be apparent to those in the art.

FIG. 1schematically illustrates an imaging optical system100of the type that would be useful in a lithographic imaging optical system. The imaging optical system100comprises a radiation (e.g. light) source102, a scanning slit104that is used to direct a scanning beam through an object (or reticle)106, and primary imaging optics108that image the scanned object onto an image plane110. Such aspects of a lithographic imaging optical system are well known and should not require further description to those in the art. The system100also includes illumination optics112,114and an aperture stop116that would be well known to those in the art, and should not require further explanation.

FIG. 2schematically illustrates how the principles of the present invention can be applied to an imaging optical system of the type shown inFIG. 1. The primary imaging optics includes a lens system120which defines a primary optical path by which radiation (light) that originates at the object or reticle106is directed through the imaging optics to form an image of the reticle on a wafer image plane122. InFIG. 2, the primary optical path is shown by image rays124. The wafer image plane122is a layer of photoresist on a semiconductor wafer that is supported by a wafer stage126. The wafer stage126can be controlled, in a manner described herein, to adjust the position of the wafer and its image plane122relative to the lens system120.

According to the principles of the present invention, a measurement optical path is established and includes at least part of the primary optical path. The imaging optical system is configured to obtain information from the measurement optical path for use in providing control information for the imaging optical system.

InFIG. 2, the measurement optical path is schematically illustrated by image rays130. Thus, inFIG. 2, the measurement optical path is that path taken by radiation (e.g. light) which passes from a measurement source132, through part of the imaging optics, reflects off of the wafer image plane122, passes back through the part of imaging optics and finally ends up on a detector156. Thus, an image of the measurement source132(i.e. a real or virtual image) that is projected by the measurement optical path is transmitted at least partially through the imaging optics120.

More specifically, the measurement optical path, as depicted inFIG. 2, begins with the measurement source132, which is collimated by a first element134. From there the measurement optical path passes through a beam splitter136, through some additional optics138, reflects off of one of the surfaces of the imaging lens (i.e. surface140), passes through an image of the source142and continues through part of the primary optical path through lens assembly120to the primary wafer image plane122. The reflection off of the wafer at the primary wafer image plane122reflects the radiation and the measurement optical path then extends back through the imaging optics108(122), through another image of the source144, reflection from the one surface of the imaging optics (i.e. surface140), through additional optics146, a beam splitter148and a lens150, which produces another image152of the measurement source in one plane and an image of the primary wafer image plane122in another plane (e.g. a pupil plane154) which may be coincident to the surface of detector156.

Moreover, while the measurement optical path inFIG. 2includes reflection from lens surface140, the measurement optical path through the imaging optics can be inserted by various surfaces in the imaging optics. For example, inFIG. 2, the imaging optics has an aperture stop158, and the measurement optical path provides an image of the measurement image source that is inserted into the imaging optics in predetermined relation to the aperture stop158(also referred to as a clear aperature). Thus, inFIG. 2, the measurement image originates outside the imaging optics. An image of the measurement image source (real or virtual) is picked off, enters the primary optical path in predetermined relation to the aperture stop158and is transmitted at least partially through the imaging optics120.

It is preferred that the image of the measurement image source is inserted into the primary optical path (i.e. represented by primary image rays124imaged through lens system129) in the optical space between the.clear aperture158and the wafer image plane122. Thus, as illustrated inFIG. 4, the measurement optical path (shown by image rays130) provides an image of the measurement image source that is reflected into the primary optical path by a reflection optic160located to reflect the image of the measurement image source into the primary optical path in the optical space between the aperture stop158and the wafer image plane122. Moreover, as illustrated inFIG. 5, another way to insert the image of the measurement image source into the primary optical path is by reflection directly from a lens element162that is located in the optical space between the aperture stop158and the wafer image plane122. A lens element such as162that is designed to reflect the image of the measurement image source into the primary optical path may have a special coating for that purpose. Moreover, it is contemplated that various surfaces of the lens system may have coatings to provide desired reflectance or transmittance for the image of the measurement image source.

Still further, as illustrated inFIG. 3, the image of the measurement image source (image rays130) can be provided at the actinic wavelength, and can be inserted into the primary optical path (e.g. by reflection) at a predetermined location relative to the aperture stop158.

The image of the measurement image source that is inserted into the primary optical path, and then projected by the measurement optical path to the detector156, provides information that is useful in controlling the relationship of the wafer to the imaging optics. Thus, in the system ofFIG. 2, the image of the measurement image source that originates outside the imaging optics is passed through the beam splitter136, so that the image is directed into and transmitted by the measurement optical path through the imaging optics (as shown by image rays130). Another image of the measurement image source is directed along a path164that is outside the imaging optics. The image of the measurement image source that is inserted into and imaged by the imaging optics is overlaid with the image that is directed along path164outside the imaging optics, via the beam splitter148. The overlaid images are then imaged on the detector156, which can be, e.g. an array of charged couple devices (CCDs) that detect the overlaid images.

The detector156is in circuit communication with a measurement processor166that processes the overlaid images, to determine if an adjustment of the position of the wafer relative to the imaging optics should be made. If an adjustment of the wafer position is desirable, the processor166provides appropriate control data to a wafer stage controller168to drive the wafer stage126, thereby to provide the desired positioning of the wafer relative to the lens system.

As schematically illustrated inFIGS. 8-11, the overlaid images generate a fringe pattern that is sensed at the detector, and used to generate a surface topography map that is used in controlling the wafer stage. Thus,FIG. 8is a simulation of a surface topography map plotted as a function of position (with horizontal axes in normalized coordinates and the vertical axis interpreted as nanometers of distance or optical path, etc), andFIG. 9is a simulation of a fringe pattern related to the surface topography map ofFIG. 8. In the simulation ofFIGS. 8 and 9,FIG. 8was initially generated, and the fringe pattern ofFIG. 9generated from the surface topography map ofFIG. 8, but in an actual system, the detector156would receive the fringe pattern, and the measurement processor166would generate the surface topography map. Similarly, the simulation ofFIGS. 10,11shows how the surface topography map and the fringe pattern shift as the surface under investigation shifts up or down, and the phase of the fringes also shifts. In the simulation ofFIGS. 10,11, the surface topography map (FIG. 10) has been shifted up by 50 nm (which could be the result of an upward shift of the surface under investigation). Note that the fringes associated with the surface have shifted (FIG. 11) and the surface topography map has also shifted (FIG. 10).

A measurement optical path that is inserted into the imaging optics of a primary imaging optical system and used to provide wafer control information is particularly useful with imaging optics that has telecentricity, because with such imaging optics detection from the measurement optical path can be direct, and is simple to process and to use in a metrology system.

A measurement optical path that is configured in the manner described herein is useful with a number of imaging optical systems. For example, it can be used with “wet” imaging optical system, in which the imaging onto the image plane122is through an immersion fluid layer (170FIG. 7), and also with a “dry” imaging optical system, in which imaging onto the image plane122is through a medium172such as a gas, air or a vacuum (FIG. 6). Moreover, an imaging optical system according to the present invention is particularly useful for measuring the topography of a large region of the wafer surface under investigation, like the entire instantaneous field of the wafer, instead being limited to a small patch or set of patches of the wafer. Additionally, such a system allows for investigation of multiple interfaces in a lithographic system, i.e. glass-liquid interfaces, resist-liquid interface or glass-air interfaces. Also, it allows for investigation of optical interfaces during exposure, for investigation of artifacts on surfaces, impurities, inclusions, particles etcetera in the bulk material of the glass, air and/or immersion liquid, particulate impurities in the immersion liquid with, for example, a dark field or bright field test.

FIGS. 12-14schematically illustrate different aspects of the surface under investigation that can be investigated, using the principles of the present invention. For example, as illustrated byFIG. 12variations in surface height (also referred to as “undulations) of the surface under investigation180can be investigated by a measurement beam (represented by rays130). As illustrated inFIG. 12, undulations of the surface180under investigation can perturb the measurement beam. In depressions of the surface under investigation, rays130will travel greater distances and have greater optical path, while the opposite is true for rays130that arrive at bumps on the surface. The global tilt and z-position of the surface produce the same effect-changing the optical path of the rays130and moving the surface in relation to the conjugate position of the detector. In addition, as illustrated inFIG. 13, variations in the index of refraction of the measurement beam may also be detected. InFIG. 13, the gray-scale pattern of an immersion material182adjacent the surface under investigation180represents different indices of refraction within the immersion material182. This variation in index of refraction could arise, for example, from heating of the immersion material by the actinic radiation during exposure of the resist on the surface under investigation180. If the lighter shading is interpreted as higher index of refraction, then the rays which travel through the center of the field of view of the immersion material will have greater optical path than those which travel through the periphery of the field of view of the immersion material. Still further, as illustrated inFIG. 14, rays incident in regions A and B of a multi layer coating184on a surface under investigation180can in general have different phases due to the structure of the multilayer coating on the surface under investigation. InFIG. 14, two rays186,188, associated with a planar wavefront as shown in the upper layer, enter the surface structure from above. These two rays strike the surface under investigation in regions A and B. Because of local variations of the multilayer coatings on the substrate, these two rays can have different phases as represented by φoand φo+δφ respectively. Variations in the coatings can take the form of impurities of the coating materials, variations in thickness and index of refraction of the coating materials or other variations resulting for accidental or intention variations of the coating structure. Variations of the substrate material could, for example arise from structures printed in the previously printed lithographic layers.

Moreover, it should be noted that in an imaging optical system according to the present invention, when insertion and pick-off of the image projected by the measurement optical path are nearly conjugate (e.g.FIG. 2) obstruction of the primary optical path is or can be minimized or entirely eliminated. Further, fringes generated resulting from reflections from various surfaces could be isolated interferometrically using a broad band source. Also, it can be advantageous to compensate for aberrations in such a way that the wavefront is well corrected at the surface under investigation (either the wafer or the reticle), and this can be achieved with wavefront compensation in the measurement optical path prior to, or succeeding its overlap with the primary optical path. Still further, the wavelength, and bandwidth, of the measurement image are limited only by the transmission of the optics of the system since chromatic aberrations (as well as any others) can be corrected outside the primary optical path in the same way.

Also, it should be noted that the optics which allow the measurement optical path to overlap the primary optical path can be inside or outside the optical space between the aperture stop and the image plane, it is believed that placing them inside that optical space (e.g. with the use of small mirror inserted into the primary imaging path) may be at the expense of obscuration. Moreover, it will be appreciated by those in the art that the measurement image can be projected at either actinic or visible or infrared wavelengths. Also, it will be appreciated that the images of the measurement source may or may not be symmetric conjugates with respect to the surface of reflection (e.g. wafer plane or reticle plane). Additionally, the measurement optical path may or may not enter and leave the primary imaging optical path in conjugate optical spaces; that is the measurement optical path need not enter and exit the primary imaging optical path in the same air or lens space. The measurement optical path can begin or end the overlap with the primary imaging optical path in air, through the side of a lens or by reflection from a lens surface. Still further, the measurement optical path could be conditioned in several ways, including polarization, wavelength, bandwidth, pulse characteristics, phase, position and direction etcetera. In addition, the measurement optical path may or may not contain optics which compensate for aberrations generated by the imaging optics. This compensation could be achieved with reflective, refractive or diffractive nulling optics, and these optics could be placed before or after overlap with the imaging optical path.

Additionally, while disclosed in connection with one form of metrology system (e.g. for a lithographic imaging optical system ), the principles of the present invention can be used with various types of lithographic imaging optical systems. For example, inFIG. 1, the lithographic imaging optical system shown in full lines is a scanning lithographic imaging optical system, in which the scanning slit104and the reticle106have openings (shown in full lines) that move in synchronism to produce the image at the image plane110. The lithographic imaging optical system could also be of the “step and repeat type”, which is well known to those in the art, and in which the scanning slit104, the reticle106have larger openings that are shown in dashed lines, and are moved in a stepped fashion to produce the image shown in dashed lines in the image plane110. In addition, an imaging optical system according to the principles of the present invention provides a measurement image that can produce input to any number of metrology systems including but not limited to a Shack-Hartmann wavefront sensor, a confocal microscope, interferometric confocal microscope, a distance measuring interferometer, a phase measuring interferometer, bi-homodyne interferometer, heterodyne interferometer, star test, knife-edge test, wire test, Hartmann test, shearing interferometer, curvature sensor, etc. Still further, an imaging optical system according to the present invention can be configured with a measurement beam that examines a surface under investigation other than a wafer located at an image plane. For example, in a lithographic imaging optical system of the type shown inFIG. 1, the principles of the present invention can be used to examine the reticle106as a surface under investigation.

Also, this invention can be utilized in an immersion type exposure apparatus that takes suitable measures (e.g. pressure and/or height) for a liquid (e.g. a liquid reservoir of an immersion lithography apparatus). For example, PCT patent application WO 99/49504 discloses an exposure apparatus in which a liquid is supplied to the space between a substrate (wafer) and an imaging lens system in an exposure process. The pressure and/or height of liquid in a liquid reservoir of an immersion lithography apparatus is obtained by a measurement device. The pressure and/or height can be used to determine the height and/or tilt of the substrate. U.S. Pat. No. 7,038,760 corresponds to WO 99/49504. As far as permitted, the disclosures of WO 99/49504 and U.S. Pat. No. 7,038,760 are incorporated herein by reference.

Further, the principles of the present invention can be applied to a general optical system such as an imaging optical system for a microscope or inspection system. Lastly, it should be noted that although the invention as described and illustrated inFIG. 2includes symmetrical optical systems at the insertion side and the exit side of the primary imaging optics, this does not necessarily have to be the case. In various embodiments of the invention, the optics on the insertion side and the exit side need not be identical, but rather, can be non-symmetrical and independently implemented as desired.

With the foregoing disclosure in mind, it is believed that various ways that a measurement image can be inserted into an optical pathway, and imaged to a measurement image detector, to produce control information for use with a primary imaging optical system, according to the principles of the present invention, will be apparent to those in the art.