Patent Number: 
Section: description

Various aspects of the invention are described below in the context of representative embodiments, which are not intended to be limiting in any way. Although the various embodiments are described as utilizing an electron beam as an exemplary charged particle beam, the general principles set forth herein are applicable with equal facility to use of an alternative charged particle beam such as an ion beam. First Representative Embodiment This embodiment is depicted in FIGS. 1, 2, 3, and 4(A)-4(C). Turning first to FIG. 1, certain optical-system components of an electron-beam microlithography system in the vicinity of the wafer stage are shown. At the upstream end of the depicted apparatus an illumination beam 12 is shown incident on a reticle 11. The illumination beam 12 is emitted from an electron gun (not shown) and formed by an illumination-optical system (not shown, but well understood to be located between the electron gun and the reticle 11) so as to be collimated as it is incident on the reticle 11. The reticle 11 defines one or more xe2x80x9cmeasurement marksxe2x80x9d as used for measuring beam blur, and also can define an actual lithographic pattern. The reticle 11 also represents an xe2x80x9cobject planexe2x80x9d of the depicted system. In this embodiment, the measurement marks include a mark 13 defined as a rectangular aperture (through-hole). As the illumination beam 12 is incident on the measurement mark 13, a portion of the beam passes through the mark without experiencing any absorption or scattering of electrons by the reticle 11. The portion of the beam 12 transmitted through the measurement mark 13 is thus formed into a collimated beamlet EB having a rectangular transverse profile. First and second projection lenses 14, 15, respectively, define a two-stage projection-lens system disposed downstream of the reticle 11. A contrast diaphragm 17 is situated between the projection lenses 14, 15. The beamlet EB formed by the measurement mark 13 in the reticle 11 is converged by the first projection lens 14 to form a crossover CO in the center of an aperture 17a defined by a contrast diaphragm 17. The contrast diaphragm 17 comprises a plate 17b that blocks electrons of the beamlet EB that were forward-scattered during passage through the reticle 11 (i.e., only non-scattered electrons pass through the aperture 17a). A wafer stage 16 is situated downstream of the second projection lens 15. The wafer stage 16 is configured to hold a suitable xe2x80x9csensitivexe2x80x9d lithographic substrate such as a semiconductor wafer having an upstream-facing surface coated with a xe2x80x9cresistxe2x80x9d that is sensitive in an image-imprinting manner to exposure by the beamlet EB from the reticle 11. The wafer stage 16 includes a knife-edged reference mark 2 defined in, desirably, a silicon thin film 3 having a thickness of about 2 xcexcm, for example. The substrate and the thin film 3 on the wafer stage 16 define a plane representing an xe2x80x9cimage planexe2x80x9d of the depicted system. A knife-edge 1 of the reference mark 2 is shown. A knife-edge 1 formed on such a thin film 3 easily provides a high-quality straight edge having minimal edge roughness. Mounted to the upstream-facing surface of the wafer stage 16 is a wafer chuck (not shown but well understood in the art) on which a wafer or other suitable substrate (not shown) is mounted for lithographic exposure. As detailed in FIGS. 2 and 3, a beam-limiting diaphragm 5 is disposed downstream of the knife-edged reference mark 2. The beam-limiting diaphragm 5 comprises a plate 5b that defines a beam-limiting aperture 5a having a diameter xe2x80x9cdxe2x80x9d (FIG. 2). The axial distance (denoted xe2x80x9chxe2x80x9d in FIG. 3) from the beam-limiting diaphragm 5 to the knife-edged reference mark 2 typically is a value between a few millimeters (mm) to about 20 mm. In general, the diameter xe2x80x9cdxe2x80x9d of the aperture 5a is sufficient to block xe2x80x9cmostxe2x80x9d(at least 90%) of the electrons e2. In this regard, referring to FIG. 2, note a right triangle having an apex at the knife-edge 1, a first side extending (along the trajectory e1) from the apex to the center of the aperture 5a, a second side extending from the center to the edge 5e of the aperture 5a, and a hypotenuse extending from the edge 5e to the apex. The apex of the triangle has an angle xcex8, which represents the axial angle of the beam-limiting aperture as measured at the knife-edge. In order for the beam-limiting aperture 5a to block most of the electrons e2, the diameter xe2x80x9cdxe2x80x9d of the aperture 5a desirably is such that the angle xcex8 is xe2x80x9cslightly greaterxe2x80x9d(i.e., 1.1 to 3 times greater) than the angle of convergence of the beamlet EB at the substrate (image plane). Desirably, d≈2hxcex8. If d less than 2hxcex8, then not only scattered electrons but also non-scattered electrons are blocked by the beam-limiting diaphragm 5, which reduces the signal and contrast. On the other hand, if d is much larger than 2hxcex8, then a significant portion of the scattered electrons pass through the beam-limiting aperture 5a, which reduces contrast. In this embodiment an exemplary range of the diameter d is 100 to 200 xcexcm. The plate 5b of the beam-limiting diaphragm 5 desirably is made of an electro-conductive metal and has a thickness (1 mm, for example) sufficient for absorbing electrons incident on the plate 5b.  An electron detector (sensor) 6 is situated downstream of the beam-limiting aperture 5a. The electron detector 6 desirably comprises a combination of a photomultiplier and a scintillator, a Faraday cup, or a semiconductor detector. The electron detector 6 is connected to a pre-amplifier 7, a differentiation circuit 8, and an oscilloscope (or analogous display) 9. With a device as shown in FIGS. 2 and 3, as the rectangular beamlet EB (downstream of the second projection lens 15) is scanned over the knife-edge 1, electrons not absorbed by the thin film 3 (i.e., electrons e1 that pass through the knife-edged reference mark 2 without experiencing any scattering, and electrons e2 that are forward-scattered during transmission through the thin film 3) propagate downstream. The electrons e1, e2 then reach the beam-limiting diaphragm 5. The non-scattered electrons e1 pass through the beam-limiting aperture 5a, while most (at least 90% of) the forward-scattered electrons e2 are blocked by the plate 5b. As a result, essentially only the non-scattered electrons e1 are detected by the electron detector 6. An exemplary beam current of non-scattered electrons e1 detected by the electron detector 6 is depicted as the upper graph in FIG. 4(B). This waveform is of detected beam current. As shown in FIG. 4(A), the beamlet EB is scanned over the knife-edge 1 in the direction of the xe2x80x9cSCANxe2x80x9d arrow (i.e., to the right in the figure). As the beamlet EB passes over the knife-edge 1, the proportion of the beamlet EB propagating past the knife-edge 1 progressively increases, indicated by a corresponding increase in the beam current detected by the detector 6 (FIG. 4(B)). Specifically, in FIG. 4(B), note the rise in detected beam current as indicated by the rise to the right in the upper curve. This beam current is amplified by the pre-amplifier 7 and converted to a plot of percentage change versus time by the differentiation circuit 8. An exemplary differential waveform output from the differentiation circuit 8 is shown as the lower curve in FIG. 4(B). Ideally, the differential waveform has a rectangular profile W1 if the beamlet EB has no blur. In practice, an actual differential waveform W2 has sloped sides resulting from beam blur. Referring to FIG. 4(C), the distance xe2x80x9ctxe2x80x9d over which the waveform W2 exhibits a rise is regarded as being situated within the range of 12% to 88% maximum beam intensity of the differential waveform. Beam blur is quantified by determining the distance t. The output waveform of the differentiation circuit 8 is displayed on the oscilloscope 9. Beam adjustment (e.g., calibration of focal point, astigmatism, magnification, rotation, and/or other parameters) and evaluation of imaging performance are performed on the basis of the waveform displayed on the oscilloscope 9. A specific numerical example of the angle xcex8 (see FIG. 2) is established as follows for an electron-beam microlithography apparatus. If the acceleration voltage of the illumination beam 12 is 100 kV, and the angle of convergence of the beamlet EB at the projection lens is 6 mrad, then the aperture angle xcex8 established by the edge 5e of the beam-limiting aperture 5a (as seen from the knife-edge 1) is 6 to 10 mrad. In such an instance, approximately 100% of the non-scattered electrons e1 pass through the aperture 5a, whereas no more than approximately 0.1% of the forward-scattered electrons e2 pass through. Under such conditions, beam-blur measurements can be made at nearly full contrast. Second Representative Embodiment This embodiment is depicted in FIGS. 5 and 6, in which components that are similar to respective components in FIG. 2 have the same respective reference numerals. FIG. 5 is a schematic elevational view with accompanying block diagram, and FIG. 6 is a schematic plan view showing size and shape relationships of electron beams and apertures used in the method. Turning first to FIG. 6, a dummy beam EB2 having a relatively large, hollow transverse profile (with a square-shaped central void) surrounds much smaller measurement beamlets EB1-1. The dummy beam EB2 exerts a Coulomb effect on the measurement beamlets EB1-1. Hence, beam blur is measured under actual exposure conditions in which the Coulomb effect is assumed as being present. The hollow-square profile of the dummy beam EB2 is actually an aerial image of mesh holes in a pattern (defined by an upstream reticle, not shown) used for experimentally varying the pattern-element density of a projected pattern. As shown, the dummy beam EB2 is configured so as to surround the beamlets EB1-1, which are clustered at the center of the void in the dummy beam. A beam-limiting diaphragm 5 is situated downstream of the knife-edged reference mark 2. The beam-limiting diaphragm 5 comprises a plate 5b that defines an aperture 5a (usually round; FIG. 6). The edge of the plate 5b around the aperture 5a is denoted 5e. The axial distance (denoted xe2x80x9chxe2x80x9d in FIG. 5) between the beam-limiting diaphragm 5 and the knife-edged reference mark 2 can range from a few millimeters (mm) to about 20 mm. The plate 5b of the beam-limiting diaphragm 5 is made of an electro-conductive metal and has a defined thickness (e.g., 1 mm thick) sufficient for absorbing incident electrons. The electron detector (sensor) 6 is situated downstream of the beam-limiting diaphragm 5. As in FIG. 3, the configuration shown in FIG. 5 includes a pre-amplifier 7, a differentiation circuit 8, and an oscilloscope 9 connected to the sensor 6. Whenever the beamlets EB1 and the dummy beam EB2 are scanned over the knife-edged reference mark 2, electrons not absorbed by the plate 3 are transmitted downstream of the plate 3. These transmitted electrons comprise non-scattered electrons e1 of the beamlets EB1 that passed through the reference mark 2, forward-scattered electrons e2 of the beamlets EB1 that passed (with forward-scattering) through the plate 3, and forward-scattered electrons e2xe2x80x2 of the dummy beam EB2 that passed (with forward-scattering) through the plate 3. The electrons e1, e2, and e2xe2x80x2 propagate to the beam-limiting diaphragm 5. The non-scattered electrons e1 pass through the aperture 5a. Some of the forward-scattered electrons e2 are blocked by the aperture plate 5b. Also, some of the forward-scattered electrons e2xe2x80x2 of the dummy beam EB2 are blocked by the aperture plate 5b.  Third Representative Embodiment In the second representative embodiment many electrons e2 and e2xe2x80x2 pass through the aperture 5a and reach the electron detector 6. In view of these conditions and because the dummy beam EB2 is larger in transverse profile than the beamlet EB1, the number of electrons of the dummy beam EB2 that reaches the detector 6 could be sufficiently large to provide less than optimal results for certain applications. This third representative embodiment addresses these concerns. The third representative embodiment is depicted in FIGS. 7, 8, 9(A)-9(B), and 10(A)-10(C), with reference also being made to FIG. 1. In this embodiment, components that are similar to respective components in the first representative embodiment have the same respective reference numerals. Referring first to FIG. 1, certain optical-system components in the vicinity of the wafer stage are shown. At the upstream end of the depicted apparatus an illumination beam 12 is shown incident on a reticle 11. The illumination beam 12 is emitted from an electron gun (not shown) and formed by an illumination-optical system (not shown, but well understood to be located between the electron gun and the reticle 11) so as to be collimated as it is incident on the reticle 11. The reticle 11 defines one or more xe2x80x9cmeasurement marksxe2x80x9d as used for measuring beam blur, and also can define an actual lithographic pattern. In this embodiment, the measurement marks include a mark 13 defined as a rectangular aperture (through-hole). As the illumination beam 12 is incident on the measurement mark 13, a portion of the beam passes through the mark without experiencing any absorption or scattering of electrons by the reticle 11. The portion of the beam 12 transmitted through the measurement mark 13 is thus formed into a collimated beamlet EB having a rectangular transverse profile. First and second projection lenses 14, 15, respectively, define a two-stage projection-lens system disposed downstream of the reticle 11. A contrast diaphragm 17 is situated between the projection lenses 14, 15. The beamlet EB formed by the measurement mark 13 in the reticle 11 is converged by the first projection lens 14 to form a crossover CO in the center of an aperture 17a defined by a contrast diaphragm 17. The contrast diaphragm 17 blocks electrons of the beamlet EB that were scattered during passage through the reticle 11 (i.e., only non-scattered electrons pass through the aperture 17a). A wafer stage 16 is situated downstream of the second projection lens 15. The wafer stage 16 is configured to hold a suitable xe2x80x9csensitivexe2x80x9d lithographic substrate such as a semiconductor wafer having an upstream-facing surface coated with a xe2x80x9cresistxe2x80x9d that is sensitive in an image-imprinting manner to exposure by the beam from the reticle 11. The wafer stage 16 includes a knife-edged reference mark 2 defined in, desirably, a silicon thin film 3 having a thickness of about 2 xcexcm, for example. A knife-edge 1 of the reference mark 2 is shown. A knife-edge 1 formed on such a thin film 3 easily provides a high-quality straight edge having minimal edge roughness. Mounted to the upstream-facing surface of the wafer stage 16 is a wafer chuck (not shown but well understood in the art) on which a wafer or other suitable substrate (not shown) is mounted for lithographic exposure. Turning now to FIG. 9(A), a subfield pattern (e.g., having dimensions of 250 xcexcm square on the reticle 11) is shown. The depicted subfield has a peripheral zone 11a having a square-frame configuration. The peripheral zone 11a surrounds a square center portion 11b connected to the peripheral zone 11a by four connecting portions 11c. The subfield also includes four electron-transmissive regions 11d situated between the peripheral zone 11a and the center portion 11b. A respective group of rectangular measurement marks 13 is defined at each of five locations: four locations in the peripheral zone 11a and one location in the center portion 11b. Two measurement marks 13 are in each group, one extending in the X direction and the other extending in the Y direction. The electron-transmissive regions 11d can define respective portions of the overall pattern, defined by the reticle 11, to be lithographically transferred to a substrate. Referring now to FIG. 9(B), incidence of an illumination beam 12 on a group of measurement marks 13 in the reticle 11 produces two corresponding collimated rectangular measurement beamlets EB1. Portions of the illumination beam passing through the transmissive regions 11d become respective dummy beams EB2 that serve to adjust beam current for purposes of correcting space-charge effects. The dummy beams EB2 exert a Coulomb effect on the measurement beamlets EB1. Producing the dummy beams EB2 allows measurements of beam blur to be obtained during an actual pattern exposure. FIG. 1 schematically illustrates one measurement beamlet EB1 produced by passage of a corresponding portion of the illumination beam 12 through a single measurement mark 13. As shown in FIGS. 7 and 8, a beam-limiting diaphragm 5 is situated downstream of the knife-edged reference mark 2. The beam-limiting diaphragm 5 comprises a plate 5b desirably made of an electro-conductive metal and having a thickness (e.g., 1 mm) sufficient for absorbing incident charged particles. The aperture 5a defined by the plate 5b desirably has a diameter (denoted xe2x80x9cdxe2x80x9d in FIG. 8) of 50 xcexcm or less (most desirably approximately 10 xcexcm) to ensure adequate blocking of the dummy beams. In FIG. 7 the axial distance xe2x80x9chxe2x80x9d is shorter than xe2x80x9chxe2x80x9d in FIG. 3. Desirably, in FIG. 7, h less than d/2xcex1, wherein xcex1 is the beam-convergence angle at the image plane. The axial distance h between the aperture 5a and the knife-edged reference mark 2 is such that the angle (denoted xe2x80x9cxcex8xe2x80x9d in FIG. 8) of the edge 5e from the knife-edge 1 is slightly greater than the angle of convergence of the beamlet EB1 at the second projection lens 15. A desired range that falls within the scope of xe2x80x9cslightly greaterxe2x80x9d is from 1.1 to 3 times the angle of convergence. By way of example, the angle xcex8 is 10 mrad, the angle of convergence of the beamlet EB1 is 5 mrad, and h is approximately 2.5 mm. The electron detector 6, situated downstream of the beam-limiting diaphragm 5, desirably comprises a combination of a photomultiplier and a scintillator, a Faraday cup, or a semiconductor detector. The detector 6 is connected to a pre-amplifier 7, a differentiation circuit 8, and an oscilloscope (or analogous display) 9. Referring to FIG. 7, as the beamlet EB1 and the dummy beams EB2 pass through the second projection lens 15 and are scanned over the knife-edge 1, electrons not absorbed by the thin film 3 propagate downstream. These downstream-propagating electrons consist of non-scattered electrons e1 of the beamlet EB1 that pass through the reference mark 2, forward-scattered electrons e2 of the beamlet EB1, and forward-scattered electrons e2xe2x80x2 of the dummy beams EB2. The electrons e1, e2, and e2xe2x80x2 then reach the aperture 5a through which the non-scattered electrons e1 of the beamlet EB1 pass. xe2x80x9cNearly allxe2x80x9d (i.e., at least 90% of) the forward-scattered electrons e2 of the beamlet EB1 and the forward-scattered electrons e2xe2x80x2 of the dummy beams EB2 are blocked by the plate 5b. As a result, essentially only the non-scattered electrons e1 are detected by the electron detector 6. The beam current of the non-scattered electrons e1 detected by the electron detector 6 produces a waveform as shown in the upper graph in FIG. 10(B). As shown in FIG. 10(A), as the beamlet EB1 passes over the knife-edge 1, the proportion of the beamlet EB1 propagating past the knife-edge 1 progressively increases, indicated by a corresponding increase in the beam current detected by the detector 6 (FIG. 10(B)). Specifically, in FIG. 10(B), note the rise in detected beam current as indicated by the rise to the right in the upper curve. This beam current is amplified by the pre-amplifier 7 and converted to a plot of percentage change versus time by the differentiation circuit 8. An exemplary differential waveform output from the differentiation circuit 8 is shown as the lower curve in FIG. 10(B). Ideally, the differential waveform has a rectangular profile W1 if the beamlet EB1 has no blur. In actual practice, an actual differential waveform W2 has sloped sides resulting from beam blur. Referring to FIG. 10(C), the distance xe2x80x9ctxe2x80x9d over which the waveform W2 exhibits a rise is regarded as being situated within the range of 12% to 88% maximum beam intensity of the differential waveform. Beam blur is quantified by determining the distance t. The output waveform of the differentiation circuit 8 is displayed on the oscilloscope 9. Beam adjustment (e.g., calibration of focal point, astigmatism, magnification, rotation, and/or other parameters) and evaluation of imaging performance are performed on the basis of the waveform displayed on the oscilloscope 9. Hence, as a result of most of the forward-scattered electrons e2xe2x80x2 of the dummy beams EB2 being blocked by the plate 5b of the beam-limiting diaphragm 5, beam blur can be measured accurately and with low noise. Fourth Representative Embodiment This embodiment is depicted in FIG. 11, in which components that are similar to respective components discussed in the second representative embodiment have the same respective reference numerals. This embodiment is essentially the same as the second representative embodiment except that this third representative embodiment includes not only a first beam-limiting diaphragm 5 but also a second beam-limiting diaphragm 4. This embodiment including two beam-limiting diaphragms is especially useful whenever one or more dummy beams is used. The second beam-limiting diaphragm 4 comprises a plate 4b that defines a respective beam-limiting aperture 4a and is situated between the first beam-limiting diaphragm 5 and the electron detector 6. The second beam-limiting diaphragm 4 is configured as an electro-conductive metal plate having a defined thickness of approximately 1 mm to ensure absorption by the plate 4b of incident charged particles. The axial distance (denoted xe2x80x9cHxe2x80x9d in FIG. 11) between the second beam-limiting diaphragm 4 and the knife-edged reference mark 2 desirably is in the range of approximately 10 to 20 mm. Desirably, h less than d/2xcex1 (i.e., d greater than 2hxcex1), wherein xcex1 is the beam-convergence angle at the image plane. The aperture 4a desirably has a diameter of approximately 200 to 400 xcexcm""s. Note that the aperture 4a has a larger diameter than the aperture 5a in FIG. 7. The aperture 5a in FIG. 7 desirably has a diameter no greater than about 50 xcexcm to ensure adequate blocking of the dummy beams. In contrast, the aperture 4a in FIG. 11 does not have to block dummy beams (because the aperture 5a already does so). As shown in FIG. 11, as the beamlet EB1 and the dummy beams EB2 are scanned over the knife-edge 1, the electrons e1 of the beamlet EB1 that passed through the reference mark 2 without scattering propagate toward the first beam-limiting diaphragm 5. The electrons e2 of the beamlet EB1 that were forward-scattered during transmission through the thin film 3, and the electrons e2xe2x80x2 of the dummy beams EB2 that were forward-scattered during transmission through the thin film 3 also propagate downstream to the first beam-limiting diaphragm 5. The non-scattered electrons e1 pass through the beam-limiting aperture 5a, and many of the forward-scattered electrons e2 are blocked by the first beam-limiting diaphragm plate 5b. Essentially all the electrons e2xe2x80x2 of the dummy beams EB2 are blocked by the first beam-limiting diaphragm plate 5b. Many of the other forward-scattered electrons e2xe2x80x3 (of the electrons e2 from the beamlet EB1) that passed through the first beam-limiting aperture 5a are blocked by the second beam-limiting diaphragm plate 4b. As a result, essentially only the non-scattered electrons e1 are detected by the electron detector 6, which further enhances contrast and allows beam blur to be measured with even greater accuracy. Fifth Representative Embodiment FIG. 12(A) is a plan view of a reticle subfield 31 showing an exemplary pattern of reference marks 33 useful for measuring beam blur according to this embodiment, and FIG. 12(B) is a plan view of the aerial image produced by the subfield of FIG. 12(A). The reticle 31 of FIG. 12(A) (having dimensions of, e.g., 250 xcexcm square) defines multiple pairs of rectangular apertures (reference marks 33). In each pair of reference marks 33, one aperture extends in the X direction and the other extends in the Y direction. The pairs of reference marks 33 are arranged in five rows and five columns within the subfield 31. The aerial image shown in FIG. 12(B) comprises groups of paired rectangular beamlets EB3. The groups of beamlets EB3 of FIG. 12(B) can be used for measuring not only beam blur but also residual strain in a reticle subfield. Residual strain is based on differences in beam blur as measured at different locations within the subfield using respective pairs of beamlets. For example, the beam-limiting aperture 5 is disposed at the location shown in FIG. 12(B), and the respective pair of beamlets EB3 is scanned over the knife-edge 1. The locations of the beamlets EB3 with respect to the knife-edge 1 are determined from the scanning waveform detected by the detector (not shown, but see FIG. 2). After performing measurements using one pair of beamlets EB3, the wafer stage 16 is moved as required in the X and Y directions to reposition the beam-limiting aperture 5a, and the measurement is repeated for another pair of beamlets EB3. This protocol can be repeated to obtain measurements involving each of the pairs of beamlets EB3 produced by the subfield 31. The overall strain of the subfield is determined based on the beam-blur-distribution data obtained with the various pairs of beamlets EB3. Based on the results of these measurements, the CPB microlithography apparatus can be adjusted as required (e.g., adjustments made to the CPB-optical system) to compensate for the strain. The compensation enables the CPB microlithography apparatus to produce optimal exposure results despite the strain in the subfield. The compensations (e.g., adjustment of focal point, astigmatism, magnification, rotation, and/or other parameters as required) can be performed in real time. Therefore, high-accuracy measurements of beam blur can be made at multiple measurement locations within a reticle subfield, thereby facilitating exposure adjustments made in real time so as to realize high-accuracy exposures. Whereas the invention has been described in connection with multiple 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.