BACKGROUND

Field

[0001] Aspects of the disclosure relate to image projection systems. More specifically, aspects relate to systems and methods for viewing of images of a pupil plane of an image projection system.

Description of the Related Art

[0002] Optical systems often have problems related to proper imaging. The correct diagnosis of optical system problems sometimes depends on obtaining a view of the light distribution that occurs in the pupil of the imaging system which can strongly affect the results of the imaging. This is particularly true if the object being viewed has a periodic structure and the illumination system is somewhat coherent.

[0003] Projection systems can vary in size and content of the components used. An example lithographic projection system may have a Texas Instruments Digital Light Projector (DLP) chip wherein a number of laser diodes emitting in the 405 nm spectral region are used to illuminate the DLP. In a 1080p model of this DLP, there are approximately 2 million mirrors, arranged on a rectangular lattice with a 10.6 pm spacing between adjacent mirrors. The regular spacing between the mirrors combined with the narrow spectral bandwidth of the laser diodes leads to an energy distribution in the pupil that depends upon the pattern of“on” mirrors as well as the incident angle of the illumination and wavelength. For purposes of definition, a mirror that is“on” is a mirror that has a reflection that passes through the projection system, while a mirror that is“off’ produces a reflection that does not pass through the projection system. As will be understood by a person of skill in the art, the mirrors may rotate approximately ±12 degrees from the“off” to the“on” position in example types of projection systems.

[0004] Generally, lithography projection systems are designed to very exacting standards. The systems, while they may vary, are usually telecentric in both the object and image spaces, so that changes in the axial position of the object or image planes do not change a desired magnification ratio. In such a system, and neglecting diffraction effects, a cone of reflected illumination from each of the“on” mirrors is normal to an object plane. This ensures that the pupil of the projection system is symmetrically illuminated and the incident light is normal (90 degrees) to the substrate being exposed. If the grating nature of the DLP chip creates an unsymmetrical distribution in the pupil, then the magnification will vary with focus position and the resultant overlay and alignment accuracy will suffer a loss of accuracy.

[0005] It is desired, therefore, to ensure that the illumination across a digital lithography system pupil be as centered as possible. To ensure proper illumination, optical systems may be added to conventional imaging systems to perform a measurement of the light distribution at the pupil. Adding an optical system that will view the illumination distribution in the pupil, in a conventional manner however, has many drawbacks. Adding a separate optical system to view the illumination profile in the pupil is expensive and requires a relatively large amount of space in an area where space is scarce. Such an added optical system also has the drawback of requiring a separate camera to record and display the image of the pupil. Generally, performing such a function requires the addition of a beam-splitter or a flip mirror located between the pupil stop and the image plane to extract the pupil image as well as a fairly complex imaging system to image the pupil onto the added camera. Light reflected from the extra optical elements, including the beam-splitter and camera can contribute stray light to the projected image, thereby generating faint, but objectionable image imperfections.

[0006] Such complexities and the cost associated with the designed systems have additional drawbacks. While a pupil imaging system is needed, it is only used sporadically for trouble-shooting. For example, a pupil imaging system would be used when a shock causes the projection system to misalign or when a particular pattern on the DMD, combined with the periodic arrangement of the DMD mirrors and the narrow spectral range of the illumination, fails to create the desired pattern on the substrate. Such a system might also be used to locate a component having a damaged coating or a degraded transmission caused by solarization.

[0007] Many projection systems contain a beam-splitter between an object plane and the pupil, wherein the beam-splitter is used to reimage the projected image onto a camera along with the pre-existing patterns on the substrate onto which the image was projected. The camera image can then be used to align the projected image with the pre-existing patterns.

[0008] There is a need to provide a method for imaging the pupil onto the image plane, which may be imaged back onto the camera used for alignment, alleviating the need for an extra camera, a beam-splitter, and an extra optical system.

[0009] There is a further need to provide a cost effective system that will allow a pupil to be evaluated during operational conditions that allows operators to quickly check a digital lithography system for optimum performance.

[0010] There is also a still further need to provide a cost effective system that will be easily removable or movable to provide for evaluation of pupil illumination effects so that processing up-time is not compromised by the time taken to perform checks or calibrations of the machine components.

SUMMARY

[0011] A non-limiting summary is presented. This summary is not to be considered limiting of the embodiments that are possible as seen through the detailed description provided below.

[0012] In one non-limiting embodiment, a method for testing is disclosed comprising patterning an illumination beam from a light source passing the patterned beam through at least one imaging system, the imaging system having a pupil plane, the patterned beam passing through the pupil plane a first instance, reflecting the patterned beam off an arrangement that switches the pupil and image plane positions and reverses the direction of the beam so that the patterned beam passes back through the imaging system pupil in a second instance to a camera wherein the pupil plane is imaged at the camera.

[0013] In another non-limiting embodiment, a device is disclosed comprising at least one optical element having a first refractive surface and a second reflective surface configured to be placed near an image plane, the at least one optical element configured to reverse a direction of light traveling through an imaging system and image a pupil onto an image plane and a mounting system configured to support the at least one optical element to a projection system, wherein the mounting system allows for the at least one optical element to be moved from a first active position in the image plane to a second inactive position away from the image plane.

[0014] In another non-limiting embodiment, a device is provided comprising at least one optical element having a first convex surface and a second convex surface configured to be placed near an image plane of an imaging system, the at least one optical element configured to reverse a direction of light traveling through the imaging system and image a pupil of the imaging system onto an image plane, wherein the second convex surface is configured with a reflective coating; a mounting system configured to support the at least one optical element to a projection system, wherein the mounting system allows for the at least one optical element to be moved from a first active position in the image plane to a second inactive position away from the image plane and at least one motor configured to move the mounting system from the first active position to the second inactive position.

[0015] In another non-limiting embodiment, a device is disclosed comprising at least one optical element having a reflective surface configured to be placed near an image plane of an imaging system, the at least one optical element configured to reverse a direction of light traveling through the imaging system and image a pupil of a projection system onto an image plane and a kinematic mounting arrangement configured to position the at least one optical element with respect to the projection system.

[0016] In another non-limiting embodiment, a method to test a projection system, is disclosed comprising illuminating one of an image transducer and a mask containing a pattern with an illumination beam, passing the patterned illumination beam through at least one imaging system, the imaging system having a pupil plane, the light beam passing through the pupil plane a first instance, moving a lens arrangement into the light beam that has passed through the pupil, which switches the image and pupil plane positions and reflects the light beam so it passes through the pupil in a second instance to image the pupil plane at a camera and analyzing the light beam received at the camera. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, and may admit to other equally effective embodiments.

[0018] FIG. 1 is a cross-section of a lithographic projection system having a 3:1 reduction ratio, which also shows the position of the optical system that permits pupil viewing.

[0019] FIG. 2 is a cross-section of a pupil imaging system with incoming rays.

[0020] FIG. 3 is a cross-section of the pupil imaging system of FIG. 1 with outgoing rays.

[0021] FIG. 4 is a flow chart providing a method for pupil viewing of a digital lithography system.

[0022] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.

DETAILED DESCRIPTION

[0023] In the following description, reference is made to embodiments of the disclosure. It should be understood, however, that the disclosure is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the disclosure. Furthermore, although embodiments of the disclosure may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the disclosure. Thus, the following aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim. Likewise, reference to“the disclosure” shall not be construed as a generalization of an inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim.

[0024] Some embodiments will now be described with reference to the figures. Like elements in the various figures will be referenced with like numbers for consistency. In the following description, numerous details are set forth to provide an understanding of various embodiments and/or features. It will be understood, however, by those skilled in the art that some embodiments may be practiced without many of these details and that numerous variations or modifications from the described embodiments are possible. As used herein, the terms “above” and “below”,“up” and“down”,“upper” and“lower”,“upwardly” and“downwardly”, and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe certain embodiments.

[0025] The embodiments disclosed provide an arrangement that allows a user to diagnose optical system problems by viewing the illumination distribution at the projection system pupil plane. A lens provided with a reflective coating on one surface, or a convex mirror, may be used to enable imaging of the pupil plane along with existing components provided within a projection system. The optical system used for such diagnostic capabilities may be a movable arrangement that may be motorized and computer controlled if desired. Such an arrangement alleviates the need to provide a separate configuration of beam-splitters, optical relays, and cameras to perform such diagnosis. Different types of optical system arrangements may be provided to allow analysis of illumination of the pupil. The pupil image may be received by a camera and subsequently analyzed as needed. Such analysis may be accomplished by taking the image received by the camera and comparing the image to a reference image. In another example embodiment, the camera image may be analyzed for symmetry.

[0026] Photolithography is widely used in the manufacturing of semiconductor devices and display devices. Such devices may include, for example, liquid crystal displays. To create these displays, substrates must be prepared and processed in a number of steps to allow the display to function as intended. These displays can vary in size from small LCD screens used on portable consumer electronics products, to large flat panel displays that are used in large televisions, as well as computers, personal digital assistants, cell phones and similar electronics.

[0027] Flat panels can be constructed in several configurations. One such configuration includes a layer of liquid crystal material forming pixels sandwiched between two plates. When an electric field is applied across the liquid crystal material, the amount of light passing through the liquid crystal material can be controlled. By choosing which pixels pass light and which are opaque, images can be generated on the display.

[0028] Microlithography processes are used to create electrical features incorporated as part of the liquid crystal material layer forming the pixels. In microlithography, a light-sensitive material called“photoresist” is applied in an even coating over the surface of a substrate containing a layer of material that needs to be patterned. After curing, the substrate with the layer of photoresist is exposed to a patterned light source which causes chemical changes in the parts of the photoresist layer exposed to the light. Subsequent immersion of the substrate in a developer selectively removes either the exposed or unexposed parts of the resist layer, depending on whether the resist is positive or negative acting. This is generally followed by an etch step, which removes any of the underlying material, which is unprotected by the resist coating. At this point the remaining resist is removed, another layer is added followed by a resist layer and the process is repeated.

[0029] As should be apparent, very fine details to be created on a layer of photoresist are necessarily made by short-wavelength light. Longer wavelengths provide less resolution in the layer of photoresist. Wavelengths of less than 408 nm are used in most digital lithography systems. Additionally, certain types of photoresist react more strongly to specific wavelengths, therefore the spectrum of the light source employed in a lithographic imaging system is best matched to the sensitivity spectrum of the photoresist used.

[0030] In an effort to produce these ever finer details with lithographic techniques, maintaining the complex lithographic systems at an optimum performance level is increasingly important. [0031] FIG. 1 is a cross-section through a 3:1 lithographic projection system 100 used to project an image of a Texas Instrument DLP 102 onto a photoresist coated substrate. The DLP 102 is a digital micro-mirror device (DMD) which controls an array of small mirrors for projecting the necessary light. The mirror array may be connected to a heat sink, to remove excess heat generated at the mirror face. The DLP 102 may be a single chip or a multiple chip unit, as necessary.

[0032] A frustrated cube 106 located adjacent to the DLP 102 is used to efficiently illuminate the DLP 102 with an off-axis illumination beam 108 and to direct the illumination reflected from the“on” mirrors in the DLP 102 down the axis of the projection system through a lens system. The lens system can include, for example, lenses with configurations that are biconvex, plano-convex, positive meniscus, plano-concave, biconcave and negative meniscus, as non-limiting example embodiments.

[0033] A specific system consists of a first 45 degree prism 110, a second 45 degree prism 112, and a gap 114 that separates the prisms. The first 45 degree prism 110 and second 45 degree prism 112 transmits an off-axis illumination beam 108 onto the DLP surface. On mirrors on the DLP change the off-axis angle to on- axis and this component of the beam then passes down the projection system axis. The cross-section provided in FIG.1 duplicates the path between the DLP 102 and the projection system 100. A 45 degree beam-splitter 111 collects some of the energy reflected from the substrate 104 and images the energy on a camera focal plane 116 that is conjugate with the DLP focal plane.

[0034] The pupil 118 has a position as shown in FIG. 1. When viewed from either the object or image space, the pupil 118 appears to be located at infinity, which is a direct consequence of the telecentricity requirement. Thus, the center ray of each beam imaging a DLP pixel is normal to the object and image planes, neglecting grating effects generated by the regular spacing the DLP mirrors. An optical component 200 is placed at the end of the imaging system to conduct the desired testing of the projection system 100. The optical component 200 may be placed on a mounting system 201 [Not currently in Fig. 1]that is able to move the component from a first“active” position to a second“inactive” position. In the first “active position” the component 200 is properly aligned to reflect a light beam back at the camera 190 so that the light beam may go through the pupil 118 and eventually be detected by a camera 190. A motor 192 may be connected to the optical component 200 to move the component into proper alignment. The motor 192 may be activated through use of a computer as necessary.

[0035] Referring to FIG. 2, the component 200 is inserted between the last optical element in the projection system 100, which is a window 208, and the image plane 209 or slightly beyond the image plane. One of the many possible forms of component 200 is a glass element that contains two curved surfaces, the first curved surface 203 functions as a refractive lens and second curved surface 204 functions as a spherical mirror. The second curved surface 204 can be covered with high reflectivity coatings or films to produce spectral selectivity.

[0036] In another example embodiment, the component 200 may have a dielectric mirror positioned on a rear side of the component 200. The dielectric mirror may be configured of a number of different strata that each have reflective properties for the anticipated reflective light. As a result, if a multi-emission-line light source is used, the component 200 may have a number of layers to reflect the emission-line colors necessary. In another embodiment, if a single emission-line source is used, then a minimal number of layers may be created on the rear of the component 200 to reflect the monochromatic light. In the instance of use of a dielectric mirror device, the light reflective capability can be greater than 99 percent. The first curved surface 203 may be configured with an anti-reflective (AR) coating that will prevent unintended reflections from reaching back to the camera 190. Typical types of AR coatings that may be used include single-layer interference coatings, multi-layer interference coatings and index matching coatings. Although described as a coating, it will be apparent that thin films may also be used to perform such tasks as providing reflection suppression. In either case, however, the coating or film should be configured such that the component 200 has a heat tolerance according to the expected power absorption from the illumination source that is being used. Such illumination sources can be laser, soft x-ray, conventional light, or other appropriate illumination sources.

[0037] The component 200 receives light beams 206 from the projection system 100 The first curved surface 203 is configured to shift the image plane slightly to the right and the second curved surface 204, the mirror surface, images the pupil 118 onto the shifted focal plane surface. After passing through the refractive surface (the first curved surface 203), on the way back through the lens, the pupil 118 appears to be located at the original focal plane and is therefore imaged on the DLP 102 and alignment camera 190. The alignment camera 190 may then be used for an additional purpose, that being for receiving the image of the pupil 118. The component 200 may be configured from a variety of materials including glass materials of crown glass, barium crown glass, heavy flint glass, borosilicate and barium flint. The component 200 could also be made from plastic compounds.

[0038] The return path of the reflected light beam 301 through the pupil 118 imaging component is illustrated in FIG. 3. On the return path, after passing through the first curved surface 203, the pupil 118 appears to be located at the original image plane of the projection system 100 and therefore is reimaged on the alignment camera 190 and the DLP 102.

[0039] On the reverse path through the projection system 100, the image plane becomes a pupil plane, the original pupil plane becomes an image plane and the object plane becomes and image of the pupil. Since the alignment camera 190 also images the original image plane, the image of the pupil is also displayed on the camera 190.

[0040] The pupil imaging system (with component 200) is only used sporadically, such as for testing. Such uses would be if the imaging system were to be misaligned or degraded after a large number of repeated uses. If the optical imaging system is suspected as being the source of the image defects, the component 200 could be attached to the imaging system with a kinematic mounting system 201 that eliminates the need for alignment and facilitates use.

[0041] Referring to FIG. 4, a method 400 for pupil viewing of a digital lithography system is presented. The method 400 entails illuminating an image transducer 403 with a light source at 402. The image transducer can be a DLP system operating in reflection as described above. The patterned light beam then passes through an imaging system 404. The number of lenses that the light beam passes through can vary according to the needs of the system. In the illustrated embodiment, an imaging system that reduces the object to 1/3 its size at the image plane is shown, however other configurations are possible. In one alternative embodiment, the light passing through the lens is intercepted in the vicinity of the image plane where the aberrations are well corrected and where an image of the pupil can be conveyed back to the object and camera planes with good fidelity.

[0042] In this region either a concave mirror, or a combination thick-lens/mirror combination, or a more elaborate optical system can be used to reverse the direction of the light and interchange the positions of the image and pupil planes so that a reasonably good image of the pupil is obtained at the camera component 200. The component 200, which performs the reversal and interchange, may be positioned using a motor that is computer controlled at 406. The component 200, described as a concave mirror or alternative embodiment above, is configured to reflect incident light 408 so this light travels back through the pupil 118 wherein the camera, at 110 receives the pupil image and the pupil image can be analyzed at 412. There are many possible optical designs for component 200, with the choice depending on the quality needed for the image of the pupil and the design of the projection system.

[0043] Although the projection system described above employs a 3:1 reduction type imaging system, other types of imaging systems, including lesser or greater reduction types may be used. Additionally, although the example chosen is employed as a lithography system, those of skill in the art will recognize that the methods described are equally applicable to other image projection systems, including EUV lithography systems flat panel lithography systems and bright field microscopes as non-limiting embodiments.

[0044] In one non-limiting embodiment, a method for testing is disclosed comprising projecting an illumination beam from an object having a pattern, passing the illumination beam through at least one imaging system, the imaging system having a pupil plane, the patterned beam passing through the pupil plane a first instance, reflecting the light beam off an arrangement back through the pupil in a second instance to a camera wherein the pupil plane and an image plane positions are switched and wherein the pupil plane is imaged on the camera instead of the object. [0045] In another non-limiting embodiment, the method may further comprise comparing the pupil image received at the camera to an image of an ideal and desired intensity distribution.

[0046] In another non-limiting embodiment, the method may further comprise examining a symmetry of the illumination beam received at the camera using at least one image transducer pattern to determine how various patterns affect a light distribution in the pupil.

[0047] In another non-limiting embodiment, the method may further comprise passing the illumination beam through a frustrated prism assembly before passing a patterned light beam through the projection system.

[0048] In another non-limiting embodiment, the method may be performed wherein the reflecting the light beam off an arrangement back through the pupil, comprises: passing the light beam through a refractive lens and bouncing the light beam off a curved mirror.

[0049] In another non-limiting embodiment, the method may be performed wherein a refractive lens shifts an image plane to create a shifted focal plane for the light beam.

[0050] In another non-limiting embodiment, the method may be performed wherein a curved mirror of the arrangement images the pupil on to the shifted focal plane.

[0051] In a further embodiment, a device may be disclosed comprising: at least one optical element having a first refractive surface and a second reflective surface configured to be placed near an image plane, the at least one optical element configured to reverse a direction of light traveling through an imaging system and image a pupil onto an image plane and a mounting system configured to support the at least one optical element to a digital lithography system, wherein the mounting system allows for the at least one optical element to be moved from a first active position in the image plane to a second inactive position away from the image plane.

[0052] In another non-limiting embodiment, the device may further comprise a motor configured to position the at least one optical element. [0053] In another non-limiting embodiment a device is disclosed comprising at least one optical element having a first convex surface and a second convex surface configured to be placed near an image plane of an imaging system, the at least one optical element configured to reverse a direction of light traveling through the imaging system and image a pupil of the imaging system onto an image plane, wherein the second convex surface is configured with a reflective coating, a mounting system configured to support the at least one optical element to a digital lithography system, wherein the mounting system allows for the at least one optical element to be moved from a first active position in the image plane to a second inactive position away from the image plane and at least one motor configured to move the mounting system from the first active position to the second inactive position.

[0054] In another non-limiting embodiment, the device may be provided wherein the at least one motor is computer controlled.

[0055] In another non-limiting embodiment, the device may be configured to comprise at least one optical element having a reflective surface configured to be placed near an image plane of an imaging system, the at least one optical element configured to reverse a direction of light traveling through the imaging system and image a pupil of a projection system onto an image plane, and a kinematic mounting arrangement configured to position the at least one optical element with respect to the projection system.

[0056] In another non-limiting embodiment, the device may be configured wherein the reflective surface is concave.

[0057] In one non-limiting embodiment, a method to test a projection system, is disclosed comprising illuminating one of an image transducer or a mask containing a pattern illuminated with a beam producing a patterned illumination beam, passing the patterned illumination beam through at least one imaging system, the imaging system having a pupil plane, the light beam passing through the pupil plane a first instance, moving a lens arrangement into the light beam that has passed through the pupil, reflecting the patterned illumination beam off of the lens arrangement to pass through the pupil in a second instance to image the pupil plane wherein an image plane and the pupil plane positions are switched by the lens arrangement, receiving the patterned illumination beam at a camera and analyzing the pupil of the patterned illumination beam received at the camera.

[0058] In another non-limiting embodiment, the method may be performed wherein the moving of the lens arrangement into the light beam is through a motor.

[0059] In another non-limiting embodiment, the method may further comprise moving a kinematic mounting arrangement to position the at least one lens arrangement with respect to the projection system.

[0060] In another non-limiting embodiment, the method may be performed wherein the analyzing entails comparing the picture received at the camera to a desired intensity distribution.

[0061] In another non-limiting embodiment, the method may be performed wherein the analyzing entails determining an asymmetry of the picture received at the camera using various images to determine how asymmetry and therefore alignment accuracy varies with focus position and image composition.

[0062] In another non-limiting embodiment, the method may be performed wherein the camera is incorporated into the projection system.

[0063] In another non-limiting embodiment, the method may be performed wherein the reflecting the light beam off of the lens arrangement entails reflecting the light beam off a concave reflective surface.

[0064] While embodiments have been described herein, those skilled in the art, having benefit of this disclosure will appreciate that other embodiments are envisioned that do not depart from the inventive scope of the present application. Accordingly, the scope of the present claims or any subsequent related claims shall not be unduly limited by the description of the embodiments described herein.