Patent Number: 
Section: description

Various components and certain imaging relationships of a charged-particle-beam (CPB) microlithographic projection-exposure apparatus according to a representative embodiment of the invention are illustrated in FIG. 3. As a representative embodiment of a CPB projection-exposure apparatus, the apparatus of FIG. 3 utilizes an electron beam. FIG. 3 also schematically depicts a control system for controlling the overall optical system of the apparatus. An electron gun 1 is situated at the most upstream end of the apparatus (top of FIG. 3). The electron gun emits an electron beam in a downstream direction along an optical axis AX (Z-direction). FIG. 3 also includes a reticle 10 and a substrate (termed herein a xe2x80x9cwaferxe2x80x9d) 15. The beam propagating between the electron gun 1 and the reticle 10 is termed an xe2x80x9cillumination beamxe2x80x9d IB and the beam propagating between the reticle 10 and the wafer 15 is termed a xe2x80x9cpatterned beamxe2x80x9d PB. From the electron gun 1, the illumination beam passes through a first condenser lens 3 and a second condenser lens 5 situated downstream of the electron gun 1. The condenser lenses 3, 5 cause the illumination beam IB to form a first crossover image C.O.1 on the optical axis AX downstream of the second condenser lens 5. Downstream of the second condenser lens is a xe2x80x9cbeam-shaping aperturexe2x80x9d 6 that defines an opening (typically rectangular or square in profile) that trims the illumination beam IB to have a transverse profile sized and profile to illuminate a single exposure unit e.g., subfield on the reticle 10. For example, the beam-shaping aperture 6 shapes the illumination beam IB to have a transverse square profile with dimensions of just over (1 mm)2 on the reticle 10 so as to just illuminate one exposure unit on the reticle 10. An image of the beam-shaping aperture 6 is formed on the reticle 10 by a third condenser lens 9 situated between the first crossover C.O.1 and the reticle 10. A blanking aperture 7 is axially disposed at the position of the first crossover C.O.1 downstream of the beam-shaping aperture 6. An illumination-beam deflector (IB deflector) 8 is also disposed downstream of the beam-shaping aperture 6. The IB deflector 8 sweeps the illumination beam IB in the X direction in FIG. 1, so as to illuminate each of multiple exposure units within the scanning range of the deflector 8 and within the field of the illumination-optical system. (The xe2x80x9cillumination-optical systemxe2x80x9d comprises the components discussed above that are situated between the electron gun 1 and the reticle 10). The third condenser lens 9 collimates the illumination beam IB and forms an image of the beam-shaping aperture 6 on a region exposure unit on the reticle illuminated by the illumination beam IB. Even though only one exposure unit (situated on the axis AX) is shown in FIG. 3, it will be understood that the reticle 10 actually extends within the X-Y plane perpendicular to the optical axis AX. The reticle 10 defines an entire pattern to be projection-exposed onto the wafer 15 at each of multiple dies (chips) on the wafer 15. As suggested above, the reticle is divided into multiple (typically thousands) of exposure units (also termed xe2x80x9csubfieldsxe2x80x9d) that define respective portions of the overall pattern. The illumination beam is deflected by the IB deflector 8 as required to illuminate individual exposure units in a sequential manner within the field of the illumination-optical system. The reticle is mounted on a reticle stage 11 that can be moved as required in the X- and Y-directions. The wafer (substrate) 15 is mounted on a wafer stage 16 that also can move as required in the X- and Y-directions. By synchronously moving the reticle stage 11 and wafer stage 16 scanningly in Y-directions that are opposite each other, exposure units linearly arrayed in the Y-direction on the reticle 10 are sequentially exposed onto corresponding regions on the wafer 15. The wafer 15 is also termed a xe2x80x9csensitive substratexe2x80x9d because the upstream-facing surface of the wafer is typically coated with a layer of a substance (termed a xe2x80x9cresistxe2x80x9d) that is imprintable with the projected images. Each of the stages 11, 16 is provided with a respective position-measurement system (not shown) employing laser interferometers. Thus, the positions of the respective stages 11, 16 can be determined and controlled extremely accurately. Such positional control of the stages 11, 16, along with other features of the FIG.3 embodiment, allows the images of the exposure units on reticle 10 to be accurately xe2x80x9cstitchedxe2x80x9d together on the wafer 15 as the images are exposed onto the wafer. Situated between the reticle 10 and the wafer 15 is a xe2x80x9cprojection-optical systemxe2x80x9d comprising a first projection lens 12, a second projection lens 14, and at least one deflector 13. Whenever the illumination beam IB strikes an exposure unit on the reticle 10, particles of the beam pass through the illuminated exposure unit and become the xe2x80x9cpatterned beamxe2x80x9d PB. The patterned beam PB is demagnified (also termed xe2x80x9creducedxe2x80x9d) by the projection lenses 12, 14 and deflected by the deflector 13 to form an image of the illuminated exposure unit at a desired location on the wafer 15. A second crossover C.O.2 is formed at an axial location at which the distance between the reticle and the wafer 15 is subdivided by the xe2x80x9cdemagnification ratioxe2x80x9d of the projection-optical system. (As used herein, the demagnification ratio is the integer ratio factor by which an image as formed on the wafer 15 is smaller than the corresponding exposure unit on the reticle 10.) A contrast aperture 17 blocks charged particles in the patterned beam PB that were scattered as the illumination beam IB passed through the reticle 10. Thus, such scattered particles do not reach the wafer 15. The combination of the illumination-optical system and the projection-optical system is termed herein the xe2x80x9celectron-optical systemxe2x80x9d (or more generally xe2x80x9cCPB-optical systemxe2x80x9d if the beam is other than an electron beam). The FIG. 3 embodiment also includes a controller 31 that is connected to each lens 3, 5, 9, 12, 14 via respective coil power supplies 3a, 5a, 9a, 12a, 14a, and to each deflector 8, 13 via respective coil power supplies 8a and 13a. The controller 31 is also connected to the reticle stage 11 and wafer stage 16 via respective stage drivers 11a, 16a. Thus, energization of each lens and deflector, and actuation of each stage, are directly controlled by the controller 31. Under such control, the various exposure units on the reticle 10 are sequentially illuminated by the illumination beam IB, and corresponding images of the exposure units are projected onto corresponding locations on the wafer 15. The exposure locations on the wafer are carefully determined so as to xe2x80x9cstitchxe2x80x9d together the demagnified images of the exposure units and hence achieve transfer of the entire reticle pattern to a die on the wafer. An example of a reticle used to perform divided-pattern projection-transfer according to the invention using an electron beam is shown in FIG. 4. Strong contrast is imparted to the image as formed on the wafer by providing on the reticle 10 corresponding regions defined by an electron-scattering material exhibiting a large scattering angle, and other regions defined by an electron-scattering material exhibiting a relatively small scattering angle. Thus, only electrons having a small scattering angle are allowed to pass through the contrast aperture 17 to form an image on the wafer 15. (The contrast aperture 17 in FIG. 3 is disposed at the pupil plane of the projection-optical system.) The reticle of FIG. 4 can be of either of two main types. The first type, termed a xe2x80x9cscattering-membranexe2x80x9d reticle, comprises a thin silicon membrane (e.g., approximately 0.1 xcexcm thick), upon which regions of a scattering material (e.g., heavy metal) are deposited to define pattern features. A second type, termed a xe2x80x9cscattering-stencilxe2x80x9d reticle, comprises a relatively thick (e.g., approximately 2 xcexcm thick) silicon membrane that defines voids through holes corresponding to pattern features. In the scattering-membrane reticle, the silicon membrane is a low-scattering-angle electron-scattering material, whereas, in the scattering-stencil reticle, the silicon membrane is a high-scattering-angle electron-scattering material. In FIG. 4, regions denoted by the squares 41 correspond to single exposure units (subfields) each measuring approximately (0.5 mm)2 to (5 mm)2 at the reticle. The peripheral area 43 surrounding each exposure unit 41 is termed a xe2x80x9cskirt.xe2x80x9d The skirt 43 is made from a scattering material exhibiting a relatively high scattering angle so as to trim the patterned beam PB propagating downstream of the respective exposure unit 41. The width of each skirt 43 is approximately 10 to 100 xcexcm. A strut member 45 extends perpendicularly (in the Z-direction) from each skirt 43. The struts 45 are collectively termed xe2x80x9cgrillage.xe2x80x9d Each strut 45 extends approximately 0.5 to 1 mm in the Z-direction and approximately 100 xcexcm in the X- or Y-direction; thus, the grillage provides the reticle with substantial rigidity and mechanical strength. In FIG. 4, four exposure units 41 form a single linear group arrayed in a respective row in the X-direction on the reticle 10. (Actual reticles typically have more than four exposure units 41 per row. The number of exposure units in each such row is defined by the maximal sweep angle of the illumination beam IB in the X-direction that can be achieved within the field of the illumination-optical and projection-optical systems.) Multiple such rows are arrayed in the Y-direction to form a xe2x80x9cstripexe2x80x9d 49. The reticle 10 typically comprises multiple stripes 49 arrayed in the X-direction. Wide struts 47 extend in the Y-direction between adjacent stripes 49. The wide struts 47 provide additional rigidity to the reticle to further reduce flexing of the reticle. The wide struts 47 are typically several mm wide in the X-direction. The width of each stripe 49 corresponds to the maximal deflection of the illumination beam IB that can be achieved within the field of the illumination-optical and projection-optical systems. During projection-transfer exposure, the features defined in the various exposure units are stitched together on the wafer 15 without projection of the non-patterned areas such as the skirts 43 and grillage. With a typical demagnification ratio of 1/4 or 1/5, if a single chip (e.g., for a 4-gigabit DRAM) on the wafer measures 27 mmxc3x9744 mm, then the corresponding pattern defined on the reticle including the non-patterned regions is approximately (120 mm to 150 mm)xc3x97(230 mm to 350 mm). As noted above, exposure units in a row in a stripe are sequentially exposed by sweeping the illumination beam. Movement as required from one row to the next and from one stripe to the next is achieved by appropriate movements of the reticle stage 11 and wafer stage 16 in a synchronous and coordinated manner. Pattern division, displacement, and exposure according to a representative embodiment of a method according to the invention are now described with reference to FIGS. 1(a)-1(c). FIG. 1(a) depicts, in plan view, a pattern portion consisting of four spaced-apart square features 30 with interconnecting lines 31. Inside each square feature 30 is an island region 32 that is defined on the reticle with a corresponding xe2x80x9cblackxe2x80x9d region. The squares 30 and lines 31 are defined on the reticle with corresponding xe2x80x9cwhitexe2x80x9d regions. On a stencil reticle, the xe2x80x9cwhitexe2x80x9d regions are voids (through-holes) in the reticle membrane, and the xe2x80x9cblackxe2x80x9d regions are regions occupied by the reticle membrane. In FIG. 1(a), xe2x80x9cwhitexe2x80x9d regions 56 are denoted by hatching and shading and comprise first and second feature portions 57, 59, respectively. If the FIG. 1(a) reticle 51 is a stencil reticle, complementary reticles are required to fully expose the portion shown in FIG. 1(a). This is because, as discussed above, the island portions 32 cannot be defined in a single exposure using a stencil reticle. Therefore, the FIG. 1(a) reticle is divided into a first reticle 53 shown in FIG. 1(b) and a second reticle 55 shown in FIG. 1(c). (It will be understood that the first and second reticles 53, 55 need not be on physically separate reticles, but rather can be different regions of the same reticle.) The first reticle 53 defines the first feature portions 57, and the second reticle 55 defines the second feature portions 59. The portion of the first reticle 53 shown in FIG. 1(b) is divided into four exposure units 61. To simplify this discussion, boundaries indicated by the dashed lines 63 extend between the exposure units 61. Similarly, the portion of the second reticle 55 shown in FIG. 1(c) is divided into four exposure units 65, and boundaries indicated by the dashed lines 67 extend between the exposure units 65. According to the invention, as can be seen in FIGS. 1(b)-1(c), the boundaries between adjacent exposure units are situated so as not to cross features defined in the exposure units. As a result of such configurations, the boundaries between subfields in the first and second reticles would not be in register if the features defined by the reticles were in register with each other. In other words, the seams 63 between exposure units 61 in the first reticle 53 are displaced from the seams 67 between exposure units 65 in the second reticle 55. More specifically, as shown in FIG. 1(a) (depicting a superposition of the first and second reticles with proper registration of the respective feature portions 57, 59) the dashed lines 63 denote exposure-unit seams of the first reticle 53, and the dashed lines 67 denote exposure-unit seams of the second reticle 55. In FIG. 1(a), although the feature portions defined by the first and second reticles are shown in proper registration with each other, the seams 63, 67 are displaced from each other by the distance S (in a first dimension) and T (in a second dimension). There are similar displacements between the exposure units 61 of the first reticle 53 relative to the exposure units 65 of the second reticle 55. The features 56 are divided (to form feature portions 57, 59) in a manner such that no feature portion 57, 59 crosses a seam 63, 67, respectively. For example, the feature portions 57 (denoted by shading in FIG. 1b)) are situated in each exposure unit 61 such that they are surrounded by the dashed lines 63. Meanwhile, the feature portions 59 denoted by hatching in FIG. 1(c)) are situated in each exposure unit 65 such that they are surrounded by the dashed lines 67. Hence, as a result of selectively dividing the exposure units from one another in each of the first and second reticles (or reticle portions) 3053, 55, respectively, no feature portions cross over seams between adjacent exposure units 61, 65, respectively, on the respective reticle (or reticle portion) 53, 55, respectively. The pattern shown in FIG. 1(a) is produced on the wafer by two exposures on a corresponding region on the wafer. The first exposure is of the exposure units 61 of the first reticle (or reticle portion) 53 and the second exposure is of the exposure units 65 of the second reticle (or reticle portion) 55. After making an exposure using the first reticle (or reticle portion) 53, an exposure using the second reticle (or reticle portion) 55 is made after displacing the second reticle (or reticle portion) 55 according to the displacements S, T relative to the first reticle or reticle portion 53. In such a manner, the images of all the projected exposure units can be stitched together properly on the region of the wafer. The displacements S, T can be achieved by appropriate movements of one or both the reticle stage and wafer stage, by appropriately shifting the image field, and/or by appropriately deflecting the patterned beam as projected onto the wafer. Because the feature portions 57, 59 do not cross seams between adjacent exposure units on the same reticle (or reticle portion), better linewidth control is achieved, especially in regions of the projected pattern where adjacent exposure units are stitched together. The beneficial result of improved pattern-transfer accuracy achieved with the present invention can be illustrated in the simplified example shown in FIGS. 2(a)-2(e). FIGS. 2(a), 2(b), and 2(c) depict, in a manner similar to FIGS. 1(a)-1(c), respectively, an exemplary manner in which a linear feature 81 is divided into first feature portions 83 and second feature portions 85. The first feature portions 83 are defined in exposure units 91 of a first reticle (or reticle portion), as shown in FIG. 2(b), and the second feature portions 85 are defined in exposure units 93 of a second reticle (or reticle portion), as shown in FIG. 2(c). Referring to FIG. 2(b), the feature portion 83 is situated approximately in the center of the respective exposure unit 91. Similarly, in FIG. 2(c), the feature portion 85 is situated approximately in the center of the respective exposure unit 93. For convenience in this example, the lengths of each feature portion 83, 85 are equal. Thus divided, the two reticles (or reticle portion)s are projection-transferred to the wafer. Two exposures are required for exposure of the pattern, one for each exposure unit on the first reticle (or reticle portion) and another for each respective exposure unit on the second reticle (or reticle portion). Assume that a similar exposure-unit rotational error was present in the projected image of each exposure unit. Under such conditions, respective feature-portion images 83xe2x80x2, 85xe2x80x2 would be formed on the wafer at a slant, as shown in FIG. 2(d). However, because the feature-portion images 83xe2x80x2, 85xe2x80x2 exist in nearly the same respective locations in the respective exposure-unit images 91xe2x80x2, 93xe2x80x2, the rotational displacements of the feature-portion images 83xe2x80x2, 85xe2x80x2 due to the error is relatively small. For comparison, FIG. 2(e) shows a similar situation in which a division of the feature was not performed according to the invention. I.e., the feature portions 83xe2x80x3 and 85xe2x80x3 extend to the seams of the respective exposure units 91xe2x80x3, 93xe2x80x3. As a result, the error caused a relatively large rotational displacement of the feature portions 83xe2x80x3 and 85xe2x80x3 relative to each other. Therefore, in exposure situations requiring the use of complementary reticles (or reticle portion)s to achieve projection-transfer of a pattern defined by a segmented reticle, by not forming feature portions on or near the seams of respective exposure units, much better linewidth accuracy can be achieved,especially at seams between adjacent exposure units as projected on the wafer. FIG. 6 is a flowchart of an exemplary semiconductor fabrication method to which apparatus and methods according to the invention can be readily applied. The fabrication method generally comprises the main steps of wafer production (wafer preparation), reticle production (reticle preparation), wafer processing, device assembly, and inspection. Each step usually comprises several sub-steps. Among the main steps, wafer processing is key to achieving the smallest feature sizes (critical dimensions) and best inter-layer registration. In the wafer-processing step, multiple circuit patterns are successively layered atop one another on the wafer, wherein the formation of each layer typically involves multiple sub-steps. Usually, many operative semiconductor devices are produced on each wafer. Typical wafer-processing steps include: (1) thin-film formation involving formation of a dielectric layer for electrical insulation or a metal layer for connecting wires; (2) microlithography to form a resist pattern for selective processing of the thin film or the substrate itself; (3) etching or analogous step to etch the thin film or substrate according to the resist pattern, or doping as required to implant ions or impurities into the thin film or substrate according to the resist pattern; (4) resist stripping to remove the resist from the wafer; and (5) chip inspection. Wafer processing is repeated as required (typically many times) to fabricate the desired semiconductor chips on the wafer. FIG. 7 provides a flow chart of typical steps performed in microlithography, which is a principal step in wafer processing. The microlithography step typically includes: (1) resist-coating step, wherein a suitable resist is coated on the wafer substrate which can include a circuit element formed in a previous wafer-processing step; (2) exposure step, to expose the resist with the desired pattern; (3) development step, to develop the exposed resist; and (4) annealing step, to enhance the durability of the resist pattern. Methods and apparatus according to the invention can be applied to a semiconductor fabrication process, as summarized above, to provide substantially accuracy and resolution. Whereas the invention has been described in connection with 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 invention as defined by the appended claims.