Patent Number: 
Section: description

Reference is made first to FIG. 1, which schematically depicts a first representative embodiment of an X-ray projection-exposure apparatus 100. The depicted apparatus 100 comprises a source 41 of X-ray light, an illumination-optical system 42 situated downstream of the source 41 so as to direct the X-ray beam (dashed line) from the source 41 to a mask 2, a mask stage 3 for holding the mask 2, a projection-optical system 1 situated so as to receive a beam 11a of patterned X-ray light reflected from the mask 2 and to direct the patterned beam 11b to a wafer 4, a wafer stage 5 for holding the wafer 4, a mark-position-detection system 6 associated with the wafer 4, and a mark-position-detection system (not shown, but similar to the system 6) associated with the mask 2. The X-ray source 41 can be, for example, a discharge-plasma X-ray source. The illumination-optical system 42 comprises multiple lenses, filters, and the like for forming the beam from the source 41 into a hollow beam that is directed to the mask 2. The mask 2 is a reflective mask including a multilayer-film reflective surface on which a pattern is defined. The pattern defined on the mask 2 is intended for pattern transfer (e.g., at unitary magnification or demagnification) onto the wafer 4. The projection-optical system 1 has a ring-shaped exposure-image field, and comprises multiple multilayer-film reflective mirrors and the like. Since the mask 2 is reflective, the projection-optical system 1 is non-telecentric on the mask side. The illuminated portion of the mask pattern is focused onto a corresponding location on the surface of the wafer 4 by the projection-optical system 1. Meanwhile, as both the mask 2 and wafer 4 are scanned synchronously at a constant velocity, a pattern area of the mask 2 is illuminated selectively so as to be exposed on (transferred to) the surface of the wafer 4. Since X-rays having a wavelength in the range of 1 to 30 nm are heavily attenuated by the atmosphere, at least the X-ray trajectories of the FIG. 1 apparatus are maintained at subatmospheric pressure (notably high vacuum) or in a suitable He atmosphere. (Most desirably, the atmosphere is high vacuum, as established in a vacuum chamber (not shown) containing the X-ray optical system.) The mark-position-detection system 6 is situated and configured to detect a position of a mark (not shown) on the wafer stage 5 or a mark 4a on the wafer 4. The mark-position-detection system 6 includes an optical system that directs and utilizes a light beam 12 for mark detection. Based upon data, obtained by the mark-position-detection system 6, concerning the position of the wafer 4, a wafer coordinate is obtained for use in driving the wafer stage 5, thereby ensuring that the mask pattern is exposed onto the desired location on the wafer. FIG. 2 depicts certain details of an embodiment of the mark-position-detection system 6. Specifically, this embodiment is configured as an optical microscope. The wafer stage 5 is positioned such that the wafer mark 4a is located just downstream of the optical axis 6a of the mark-position-detection system 6. The point of intersection of the optical axis 6a with the surface of the wafer 4 is termed the xe2x80x9cdetection center.xe2x80x9d The system 6 includes a detector 21, such as a charge-coupled device (CCD) for detecting an enlarged image of the mark 4a . Upon detection of the mark 4a , the actual position of the mark is determined by execution of an image-processing routine. The mark-position-detection system 6 also comprises a light source (not shown), an illumination-optical system (not shown), and a detection-optical system (comprising lenses 22, 23). The light source produces a beam of light having a wavelength to which the photoresist on the wafer surface is not lithographically sensitive. The illumination-optical system of the system 6 directs the beam of light from the source onto the mark 4a as the lenses 22, 23 of the detection-optical system project an enlarged image of the illuminated mark onto the detector 21. The detection-optical system has a numerical aperture sufficiently large to form the mark image with high contrast, thereby allowing the position of the mark 4a to be detected with high accuracy and precision. The mark-position-detection system 6 also comprises a position-adjusting device 24, discussed later below. FIG. 3, providing a plan view as viewed from above, depicts positional relationships of the exposure-image field 31 of the projection-optical system 1 with the detection centers of the mark-position-detection system 6 in the embodiment of FIG. 1. In FIG. 3 the exposure-image field 31 is shaped as a sector of an annulus (ring) formed in the image plane of the projection-optical system 1. The exposure-image field 31 encompasses an area situated between two arcs 33, 34. The arcs 33, 34 have respective radii and subtend a certain angle centered at a cross-point 32 situated at the intersection of the center axis (which may or may not be the optical axis) of the projection-optical system 1 with the image plane (an x-y plane) of the projection-optical system. The sector-shaped exposure-image field 31 solves the difficulty of having to provide a wide image field in the vicinity of the center axis of a reflective optical system. The detection center (e.g., point 35a, see FIG. 3) of the mark-position-detection system 6 for the wafer is laterally displaced, as shown in FIG. 2, a predetermined distance from the center axis 1a of the projection-optical system 1. The predetermined distance reflects space limitations caused by mechanical interference between the mark-position-detection system 6 and the projection-optical system 1. Normally, the mark-position-detection system 6 is mounted on the side wall of the optical column of the projection-optical system 1. The distance between the center axis 6a (corresponding to the detection center) of the mark-position-detection system 6 and the center axis 1a of the projection-optical system 1 is equal or nearly equal to the sum of the radius r (e.g., 50 mm) of the optical column of the mark-position-detection system 6 plus the radius R (e.g., 300 mm) of the optical column of the projection-optical system 1. As indicated in FIG. 3, regarding the cross-point 32 as an origin, an x-y coordinate system is established such that the center 36 of the exposure-image field 31 is located at a position x on the x-axis, wherein x less than 0, and the detection center 35a of the mark-position-detection system 6 is located within a region denoted by xxe2x89xa60 (namely, the area 39 denoted by hatching). In other words, establishing a line connecting the cross-point 32 (corresponding to the center axis of the projection-optical system 1 and serving as an origin) with the center 36 of the exposure-image field 31 as an x-axis, the x-coordinate of the center 36 is at a respective xe2x88x92x value, relative to the cross-point 32. Similarly, the detection center 35a also has a respective xe2x88x92x coordinate. Thus, the detection center 35a is located in the xe2x88x92X-direction relative to the exposure-image field 31 and relative to the center axis 1a of the projection-optical system 1. This x-axis is parallel to or coincident with the axis along which the wafer stage 5 is moved during scanning exposure. By situating the detection center 35 in the area 39 as discussed above, the base line BL (namely, the distance between the detection center 35 and the center 36 of the exposure-image field) can be made shorter than conventionally. As a result, the distance over which the wafer stage 5 is moved from the alignment position to the exposure position can be reduced, thereby correspondingly improving the stability of the base line. More specifically, and by way of example, whenever the detection center is situated at the point 35a on the x-axis, the stability of the base line is improved. Similarly, by situating the detection center at a point 35b or a point 35c on a straight line (namely, a line extending parallel to the y-axis) passing through the center 36 of the exposure-image field, coordinates are established in a similarly simple manner that improves the stability of the base line. The mark-position-detection system 6 cannot be located, even within the hatched area 39, where it may interfere with the optical column of the projection-optical system 1. However, the mark-position-detection system 6 desirably is located at a position separated from the cross-point 32 by a distance greater than (R+r) (FIG. 2) (namely, the radius of the optical column of the projection-optical system plus the radius of the mark-position-detection system). The shortest length of the base line BL is such a case is established when the detection center 35a is located on the x-axis (FIG. 3). A dimension of the base line BL in this case is obtained by subtracting a distance F (between the origin 32 and the center 36 of the exposure-image field) from the sum (R+r). Whereas visible light, infrared light, or ultraviolet light can be irradiated as detection light onto the mark 4a on the wafer 4, the mark-position-detection system 6 can be configured to detect detection light reflected, scattered, and/or diffracted from the mark and having a wavelength in any of these ranges. By employing an optical detection principle, higher detection accuracy is obtained. In particular, by increasing the wavelength band of the detection light, a decreased interference effect of light within the resist on the wafer 4 is achieved, which improves detection accuracy. Especially in embodiments in which the mark-position-detection system 6 is situated adjacent the projection-optical system 1, at least a portion of its optical system can be situated within the vacuum environment used for making microlithographic exposures. As a result, the optical elements of the optical system (which have refractive indices typical of glass) have refractive indices that are relative to the refractive index of vacuum, which is a desirable refractive-index difference for achieving high-resolution detection of mark position. On the other hand, if any portion of the optical system of the mark-position-detection system 6 is situated at atmospheric pressure, then that portion can be adjusted easily without having to manipulate or release the vacuum established in the vacuum chamber. However, optical systems designed to exhibit an optimal degree of aberration correction under vacuum conditions usually do not exhibit optimal aberration correction under atmospheric-pressure conditions. For example, changing the pressure from vacuum to atmospheric or vice versa typically produces a change in focal length. Hence, it is desirable that the optical system of the mark-position-detection system 6 include a mechanism for correcting focal position. Providing a focal-position-correction mechanism allows accurate detection of mark position even under atmospheric conditions and allows easy adjustment of the mark-position-detection system 6. An embodiment of a mark-position-detection system 6 including a focal-point-correction mechanism as described above includes an optical system comprising multiple lenses. A subset (one or more) of the lenses is movable by the focal-point-correction mechanism along the optical axis of the optical system. All the lenses of the optical system can be located in the vacuum chamber so as to be in the vacuum environment during use. Alternatively, a subset of the optical elements can be situated within the vacuum chamber while another subset of the optical elements is situated outside the vacuum chamber. The more desirable configuration has all the optical elements located in the vacuum environment, as shown in FIG. 2, in which a position-adjusting device 24 (e.g., an actuator such as a motor that moves at least one lens and thus, in combination with the at least one lens, serves as the focal-point-correction mechanism) is associated with at least one of the optical elements in the vacuum environment and is configured so as to adjust the lens 23 as required for optimal resolution of mark detection. The position-adjusting device 24 can be remote-controlled. Alternatively, the position-adjusting device 24 can be situated outside the vacuum chamber, which may be desirable because of the typically lower cost of such an arrangement. The mark-position-detection system 6 is not limited to having an optically based detection system (i.e., an optical system based on visible, IR, or UV light). Alternative embodiments can utilize an electron beam or other charged particle beam, or an X-ray beam irradiated onto a mark. The mark-position-detection system 6, in such alternative embodiments, detects electrons, charged particles, or X-rays, respectively, that are reflected, scattered, or discharged energetically from the mark 4a , or that pass through the mark. If the projection-exposure apparatus utilizes an X-ray beam for exposure, the exposure environment is typically a high vacuum, which is an ideal environment for the electron beam, charged particle beam, or X-ray beam of the mark-position-detection system 6 (because all these types of beams are readily absorbed and attenuated by air). FIG. 4 depicts an X-ray projection-exposure apparatus 110 corresponding to a second representative embodiment. The apparatus 110 includes an X-ray source (not shown), an X-ray illumination-optical system (not shown), an X-ray projection-optical system 1, a mask stage 3 for holding a mask 2, a wafer stage 5 for holding a wafer 4, a mark-position-detection system 6 associated with the wafer 4, a mark-position-detection system (not shown) associated with the mask 2, a vacuum chamber 7, a base member 8 for supporting an optical column, and a vibration-damping base 9 for supporting the base member 8. The X-ray light source in this embodiment is a laser-plasma X-ray source. X-rays generated by this are irradiated via the illumination-optical system onto the mask 2. An exemplary X-ray wavelength produced by the source is 13.5 nm, which requires that the mask 2 be an X-ray-reflective mask. The X-ray beam 11a reflected from the mask 2 pass through the projection-optical system 1 and, now as the beam 11b, is projected onto the wafer 4. By way of example, the mask pattern is transferred, with demagnification, onto the surface of the wafer 4. The projection-optical system 1 of this embodiment comprises six reflective mirrors, has a demagnification ratio of xc2xc, and has a ring-shaped exposure-image field having a width of 2 mm and a length of 30 mm. The six reflective mirrors are supported within an optical column made of Invar or analogous material to suppress thermal deformations of the projection-optical system 1. Each of the reflective mirrors has an aspherical reflective surface on which a respective Mo/Ru -Si multi-layer film has been applied so as to render the reflective surfaces highly reflective to incident X-rays. The Mo layers and Ru layers are formed alternatingly with respect to Si layers in a superposed manner and under conditions in which the internal stress exhibited by the multi-layer film desirably is no greater than 30 MPa. During exposures, the mask 2 is moved in a scanning manner by the mask stage 3 as the wafer 4 is moved in a scanning manner by the wafer stage 5. If the projection-optical system 1 has a demagnification ratio of xc2xc, the scanning velocity of the wafer 4 is continuously synchronized at xc2xc the scanning velocity of the mask 2. The mark-position-detection system 6 associated with the wafer 4 is an optical microscope in this embodiment. The optical microscope forms an enlarged image of the mark 4a, formed on the wafer 4, and the enlarged image is detected by a CCD or the like of the mark-position-detection system 6. The mark image is digitally processed to obtain data concerning the position of the wafer 4. As indicated in FIG. 3, a detection center of the mark-position-detection system 6 associated with the wafer can be situated at the point 35a on the x-axis. In this position, the mark image can be observed readily. The mark-position-detection system 6 also is arranged such that the base line BL (FIG. 3), between the center 36 of the exposure-image field and the detection center 35a, is minimized in length. The mark-position-detection system 6 is mounted to the base member 8, which is mounted on the vibration-attenuating base 9. The vibration-attenuating base 9 is configured to prevent propagation of vibrations (e.g., from the mask stage 3 and/or wafer stage 5) to the base member 8 and hence to the projection-optical system 1 and the mark-position-detection 6. In addition, a shock-absorber 10 such as a bellows or the like is situated between the vacuum chamber 7 and the base member 8. Thus, the positional relationships of the projection-optical system 1 and the mark-position-detection system 6 relative to each other are maintained substantially constant, thereby allowing the position of a mark on the wafer to be detected to within 10 nm or less with high accuracy. Such accurate detection also provides a base-line stability of 10 nm or better. With such accuracy in mark detection and stability of the base line achieved using the apparatus of FIG. 4, microlithographic exposures of fine patterns are performed with extremely good overlay accuracy. For example, the apparatus 110 can produce transferred patterns, in the resist on the wafer 4, having a minimum feature size of 0.07 xcexcm over the entire die area (area of a single semiconductor chip as formed on the wafer) with good shape fidelity of the transferred pattern elements, at high yield and high throughput. An exemplary exposure of a resist-coated wafer, using the apparatus of FIG. 4, is performed as follows. First, multiple marks formed on the surface of the wafer 4 are detected, in advance of exposure, by the mark-position-detection system 6. From these positional data, the respective intervals between the marks are determined and compared to respective as-designed data. Any differences from as-designed data serve as the bases for calculating respective magnification-correction data. The magnification-correction data are used to correct the positions of the mask 2 and/or wafer 4 (in the direction of the optical axis of the projection-optical system 1) during exposure so as to achieve optimal exposure at each location on the wafer. Next, a baseline, corresponding to a distance between the center of an exposure-image field and a detection center, is measured. In a similar manner, respective marks defined on the wafer surface and on the mask surface are detected by the respective mark-position-detection systems. As required, and based on data obtained by these systems, the wafer stage 5 and mask stage 3 are driven so as to adjust the wafer position relative to the mask position. Thus, on the wafer 4, a projected pattern is formed on the previously formed circuit pattern at a desired overlay accuracy, based on upon the previously obtained data concerning the base line. Thus, for example, a resist pattern having a minimum feature size of 0.07 xcexcm or less can be obtained over the entire die area on the wafer, at an overlay accuracy of 10 nm or better. The X-ray projection-exposure apparatus and methods described above can be used for manufacturing a microelectronic device. For example, the microelectronic device can be a 16-GB (gigabytes) DRAM. Typically, these devices comprise approximately 22 layers, of which at least 15 layers have sufficiently narrow linewidths to require exposure by X-ray projection-exposure. The remaining 7 layers have pattern linewidths of at least 0.15 xcexcm, which allow them to be exposed by excimer-laser microlithography. The microlithography steps are accompanied by respective steps of resist-coating, doping, annealing, etching, and metal depositions, as required. Completing formation of all the requisite layers completes formation of the respective microelectronic devices on the wafer. The wafer is cut up (xe2x80x9cdicedxe2x80x9d) into individual xe2x80x9cchips,xe2x80x9d and each chip is encased in a package of, e.g., ceramic. As described above, X-ray projection-exposure apparatus as described above provide enhanced base-line stability. As a result, during microlithography performed using the apparatus, increased accuracy of layer overlay is obtained, even with wafers exhibiting large deformation. Such increased accuracy is realized without sacrificing throughput. Whereas the invention has been described in connection with several representative embodiments, it will be understood that the invention is not limited to those embodiments. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the appended claims.