Patent Number: 06207962&
Section: description

DETAILED DESCRIPTION Reference is first made to FIG. 5 which depicts a representative embodiment of a charged-particle-beam (CPB) projection-exposure apparatus that can include the instant invention. The FIG. 5 embodiment is discussed below in the context of an electron-beam system, but it will be understood that any of various other charged particle beams can be used with such an apparatus, such as an ion beam. In FIG. 5, an electron gun 101 produces an electron beam EB that propagates in a downstream direction along an optical axis A. The electron beam EB propagates from the electron gun 101 through various components (discussed below) to a reticle 110 and then through other components (discussed below) to a substrate 114. Downstream of the electron gun 101 are situated a first condenser lens 103 and a second condenser lens 105. The electron beam EB passes through the condenser lenses 103, 105 and is converged at a crossover image C01. Downstream of the second condenser lens 105 is a beam-shaping aperture 106. The beam-shaping aperture 106 trims the electron beam EB to have a transverse profile suitable for illuminating an individual exposure unit on the downstream reticle 110. Desirably, the beam-shaping aperture 106 trims the electron beam EB to have a transverse profile slightly larger than the area and profile of the exposure unit. For example, the beam-shaping aperture 106 can shape the electron beam to have a square profile measuring slightly more than one millimeter on a side as projected onto the reticle 110, for illuminating an exposure unit measuring exactly 1 mm square. A blanking aperture 107 is situated at the same axial position, downstream of the beam-shaping aperture 106, as the crossover image C01. Immediately downstream of the blanking aperture 107 is a deflector 108. A collimating lens 109 forms an image of the beam-shaping aperture 106 on the illuminated exposure unit on the reticle 110. As used herein, an "illumination beam" denotes the charged particle beam EB between the electron gun 101and the reticle 110, and an "imaging beam" denotes the charged particle beam between the reticle 110 and the substrate 114. Similarly, the "illumination-optical system" denotes the optical system located between the source 101 and the reticle 110, and the "projection-optical system" denotes the optical system located between the reticle 110 and the substrate 114. The deflector 108 sequentially scans the electron beam EB primarily in the X direction of FIG. 5 so as to illuminate, within the optical field of the illumination-optical system, a desired exposure unit on the reticle 110. With respect to the reticle 110, although only one exposure unit (through which the optical axis A passes) is shown in FIG. 5, the reticle 110 actually extends outward in the X-Y plane (perpendicular to the optical axis) and typically comprises a large number of exposure units. As the exposure units are sequentially illuminated by the electron beam, the deflector 108 scans the electron beam, as discussed above, across the optical field of the illumination-optical system. Provided downstream of the reticle 110 are first and second projection lenses 112 and 113 and, respectively, a deflector 131. The projection lenses preferably are configured as a "Symmetric Magnetic Doublet" or "SMD." As each exposure unit on the reticle 110 is illuminated by the illumination beam, the beam passes through the illuminated exposure unit and thus acquires an ability to form an image of the illuminated exposure unit. The resulting imaging beam is demagnified by passage through the projection lenses 112, 113 and deflected as required by the deflectors 131 to form an image of the illuminated exposure unit at the desired location on the substrate 114. The reticle 110 is mounted on a reticle stage 111 that is movable within an X-Y plane. In a similar manner, the substrate (e.g., a semiconductor wafer) 114 is mounted on a wafer stage 115 that is also movable within a respective X-Y plane. Hence, continuous scanning of the exposure units of the reticle pattern can be performed (assuming the projection lenses 112, 113 are configured as an SMD) by scanning the reticle stage 111 and the wafer stage 115 in opposite directions along the Y axis. Both the reticle stage 111 and wafer stage 115 include highly accurate position-measurement systems employing laser interferometers as known in the art. The position-measurement systems, in concert with beam alignments and adjustments performed by the various deflectors of the illumination and projection optical systems, enable the images of the exposure units as formed on the substrate 114 to be accurately stitched together. The upstream-facing surface of the substrate 114 is coated with a suitable resist so as to be imprintable with the projected image of the substrate pattern. To effect such imprinting, the substrate 114 must be exposed with a proper dosage of the imaging beam. Situated upstream of the substrate 114 is a backscattered-electron detector 133 used for mark detection, as discussed below. FIG. 1 shows the vicinity of a reticle stage according to a first representative embodiment of the invention. As shown in FIG. 1, a reticle 1 is mounted on a reticle stage 3. A mark member 5 is situated adjacent the reticle on the reticle stage 3. The upstream-facing surfaces of the mark member 5 and the reticle 1are desirably co-planar in a "reticle plane" that is orthogonal to the optical axis. The mark member 5 desirably is made of silicon about 800 .mu.m in thickness and defines one or more "upstream" marks, such as shown in FIGS. 2(A)-2(D), useful for alignment and calibration purposes, for example. Whenever the charged particle beam 8 impinges on an upstream mark, some of the particles in the beam pass through the upstream mark and are projected onto a respective region on the substrate or wafer stage. The upstream-facing surface on the substrate or on the wafer stage where the upstream mark is projected desirably is situated in a "substrate plane" orthogonal to the optical axis. Situated upstream of the mark member 5 is a shield 7. The shield 7 desirably is made of an electrically conductive material such as tantalum or molybdenum having a thickness of approximately 0.1 to 1 mm in this embodiment. The shield 7 is supported relative to the reticle stage 3 by a leg portion 7b from which a shield plate 7c extends in a cantilever manner so as to cover the mark member 5. The gap between the mark member 5 and the shield 7 is desirably within the range of approximately 0.1 mm to several mm. Alternatively, a separate leg portion 7b can be placed along each of at least two edges of the shield plate 7c, or the shield plate can be supported relative to the reticle stage 3 in any of various other suitable ways. Flanking the shield 7b is a laser mirror 9 used by the position-measurement system of the reticle stage discussed above. The shield plate 7c defines an aperture 7a that is desirably slightly larger than the upstream mark on the mark member 5. The aperture 7a desirably is located in the center of the shield plate 7c and axially registered with the upstream mark on the mark member 5. The aperture 7a is discussed further below, with reference to FIGS. 3(A) and 3(B). The reticle 1 also can be covered with a shield 6 that defines apertures 6a in locations on the shield 6 that correspond to the locations of corresponding upstream marks on the reticle 1. Representative relationships between an upstream mark and the illumination beam are depicted in FIGS. 2(A)-2(D). FIG. 2(A) shows the area encompassed by a single exposure unit 11, with the superposed transverse profile of the illumination beam 13. (The exposure-unit area 11 encompasses that portion of the overall reticle pattern transferred from the reticle 1 to the substrate in a given instant of time.) For divided projection exposure, a typical exposure-unit area 11 would be square or rectangular in profile and have an area (on the reticle) of approximately (100 .mu.m).sup.2 to (1000 .mu.m).sup.2. With a demagnification ratio of 4:1, for example, such an exposure unit would illuminate an area of approximately (25 .mu.m).sup.2 to (250 .mu.m).sup.2, respectively, on the substrate. For a shaped-beam single-shot transfer technique such as cell projection, the typical exposure-unit area 11 would measure (100 .mu.m).sup.2 to (200 .mu.m).sup.2 on the reticle. With a demagnification ratio of 25:1, for example, such an exposure unit would illuminate an area of about (5 .mu.m).sup.2 on the substrate. In FIGS. 2(A)-2(D), the upstream marks are formed on the same membrane region of the reticle as the pattern to be projection-transferred to the substrate. The transverse area of the illumination beam 13 is slightly larger than the exposure unit 11. For example, if the exposure unit 11 were a square measuring 1000 .mu.m.times.1000 .mu.m, then the transverse area of the illumination beam 13 would be a square measuring about 1100 .mu.m.times.1100 .mu.m. FIG. 2(B) shows a relatively large (relative to the aperture 21) upstream mark 23 that has especial utility for aligning and calibrating the main field of the illumination and imaging optical systems. The mark 23 is configured as a line-and-space pattern in which each line has a width of, by way of example, 1.6 .mu.m, a length of 50 .mu.m and spacing therebetween of 3.2 .mu.m. The illumination beam illuminates the upstream mark 23. As the illumination beam illuminates the mark 23, the portion of the beam passing through the mark is projected onto the substrate (or other suitable location on the substrate plane). The projection is performed such that the projected image of the upstream mark 23 overlays a corresponding "downstream" mark on the substrate (or substrate plane). The image of the upstream mark 23 is scanned onto the downstream mark by the deflector 131 (FIG. 5). The backscattered-electron detector 133 (FIG. 5) detects backscattered electrons propagating from the overlaying marks. Based on the resulting detection signal relative to the scan signal, a measurement is performed in which a mark pattern previously imprinted on the substrate or substrate plane is aligned so as to be in registration with the newly projected mark pattern. Alternatively, a calibration can be performed in which one or more of demagnification ratio, rotation, distortion, lateral position, and focus position, for example, is adjusted as required. FIG. 2(C) shows a relatively small (relative to the aperture 31) upstream mark 33 that has especial utility for calibrations and corrections of distortion of exposure units as projected onto the substrate. The upstream mark 33 is detailed further in the enlargement shown in FIG. 2(D), in which the mark comprises multiple lines 35 each having, by way of example, a width of several .mu.m, a length of about 10 .mu.m, and spaces therebetween each having a width of 2 .mu.m. The mark patterns shown in FIGS. 2(B) and 2(C) are significantly smaller than the transverse profile of the illumination beam 13. As a result, many (if not most) of the charged particles in the illumination beam are not used to illuminate the marks per se but rather used to illuminate the vicinity of the marks. I.e., most of the charged particles impinge on the mark member 5 (or the reticle if the upstream marks are defined on the reticle) and cause localized heating and consequent thermal deformation of the mark member (or reticle). Such thermal deformation causes the shapes and positions of the upstream marks (and of the lines or elements thereof) to change. Such changes degrade alignment and calibration accuracy, which degrade the accuracy with which the reticle pattern can be transferred to the substrate. The shields 6, 7 shown in FIG. 1 alleviate this problem. Details of a shield 6, 7 according to two example embodiments are shown in FIGS. 3(A) and 3(B), respectively. Turning first to FIG. 3(A) the shield 6, 7 is shown in plan view. The perimeter of the shield 6, 7 encloses an area that is larger than the transverse area and profile of the illumination beam 13. For example, if the illumination beam 13 has a 1100 .mu.m.times.1100 .mu.m transverse profile, then the shield 6, 7 has at least a slightly larger area. The center of the shield 6, 7 defines an aperture 6a, 7a measuring, by way of example, 55 .mu.m.times.55 .mu.m. The aperture 6a, 7a is situated such that the upstream mark 23 (which, by way of example occupies an area of approximately 50 .mu.m.times.50 .mu.m) when viewed axially is approximately centered in the aperture 6a, 7a. To illuminate the upstream mark 23, the illumination beam first passes through the aperture 6a, 7a; the shield 6, 7 blocks most of the illumination beam from reaching anything downstream other than the upstream mark 23. As a result, only that portion of the illumination beam that is actually required to illuminate the upstream mark 23 strikes the mark member 5. The amount of heating imparted to the mark member 5 is thus much less than if the shield 6, 7 were absent. The example embodiment of the shield shown in FIG. 3(B) is especially useful whenever the space between the lines of the upstream mark 23 is relatively wide. Rather than having a single large aperture 6a, 7a, as used in the FIG. 3(A) embodiment, the shield 6', 7' in the FIG. 3(B) embodiment defines individual slit-shaped apertures 6a', 7a' for each respective line of the mark 23. By way of example, each slit-shaped aperture 6a', 7a' has a width of 5.5 .mu.m and a length of 51 .mu.m. Thus, each slit-shaped aperture 6a', 7a' is slightly larger than the corresponding line of the mark 23. The FIG. 3(B) configuration further reduces the electron dose received by regions of the mark member 5 (or reticle) outside the upstream mark 23. This, in turn, further reduces thermal deformation of the mark member (or reticle). Turning now to FIG. 4 showing another representative embodiment, a shield 51 defining an aperture 51a is axially separated from a mark member 57. I.e., the shield 51 is situated upstream of the mark member 57, and a lens 53 is situated between the shield and the mark member. An illumination beam 55, having passed through the aperture 51a in the shield 51, is projected by the lens 53 onto (and imaged on) an upstream mark 57a on the mark member 57. In this configuration, the upstream mark 57a on the mark member (or reticle) is illuminated selectively by the illumination beam. This avoids thermal deformation of the mark member (or reticle) due to excessive localized irradiation by the illumination beam. Therefore, the present invention provides a shield situated over a location on a reticle plane (e.g., a mark member or reticle) defining an upstream mark. The shield effects more localized irradiation of the upstream mark during instances in which the upstream mark is being irradiated by the illumination beam. Consequently, excess irradiation of the vicinity of the upstream mark is prevented, which correspondingly reduces thermal deformation of the mark and increases the accuracy of mark detection. Whereas the invention has been described in connection with multiple representative embodiments, it will be apparent that the invention is not limited to those embodiments. On the contrary, the invention is intended to encompass all alternatives, modifications, and equivalents as may be encompassed within the spirit and scope of the invention as defined by the appended claims.