Patent Number: 056235291
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention are described below with reference to the drawings. Embodiment 1 FIG. 1 shows the construction of an SOR exposure system in accordance with a first preferred embodiment of the present invention. In FIG. 1, reference numeral 1 denotes an SOR light source apparatus including a charge storage ring, the synchrotron radiation emitted from the charge storage ring 1 being supplied to beam lines 21 to 28 connected to exposure apparatus 31 to 38 for exposing wafers, and beam lines 4a and 4b connected to exposure apparatus 5a and 5b, respectively, for duplicating X-ray masks. In this embodiment, the mask duplicating beam lines 4a and 4b are longer than the beam lines 21 to 28 and have a smaller divergence angle (i.e., higher resolving power for exposure transfer). FIG. 2 illustrates the construction of the beam lines 21 to 28 to which the wafer exposure apparatus 31 to 38 are respectively connected, the beam lines 21 to 28 having the same construction. The synchrotron radiation 51 emitted from an emission point 50 is enlarged and vertically reflected by a convex X-ray mirror 53 in a mirror chamber 52. The radiation 51 is then passed through a vacuum duct 54, a vacuum partition 55, and a shutter unit 56 and is projected onto a mask 58 to transfer a mask pattern to a wafer 59 by exposure. The inside of an exposure unit 57 containing the mask 58 and the wafer 59 has a He atmosphere under pressure reduced to about 150 torr. The vacuum partition 55 is made of a Be foil having a thickness of about 15 .mu.m so as to decrease the attenuation of the illumination light within the region from the vacuum partition 55 to a photosensitive material on the wafer 59. The above illumination optical system is designed so that the intensity of the illumination light satisfies resolving power and productivity (throughput) which are required for manufacturing wafers. In this construction, since the distance from the emission point 50 to the X-ray mirror 53 is, for example, 3 m, and the distance from the X-ray mirror 53 to the mask 58 is, for example, 5 m, the distance from the emission point 50 to the mask 58 is 8 m. If the exposure region of the wafer 59 has, for example, a 30-mm square rectangular form, the horizontal divergence angle is 3.75 mrad, and the vertical divergence angle is 6 mrad. If the dispersion of the gap dimension between the mask 58 and the wafer 59 is 3 .mu.m, a distortion of 18 nm occurs in transfer of the pattern. Although the intensity of the illumination light depends upon the intensity of the synchrotron radiation and the sensitivity of the photosensitive material on the wafer used, the illumination optical system is designed so that the intensity of the illumination light satisfies the condition that the exposure time is about 0.3 to 1 second. Table 1, below, shows details of the design of the illumination system in the wafer exposure apparatus shown in FIG. 2. The X-ray mirror used is made of silicon carbide and is processed so that the reflecting surface has a surface roughness of 1 nm (rms), and the angle of incidence of the main SOR beam on the reflecting surface is 15 mrad. This mirror decreases the intensity of the short wavelength component of the SOR beam. The vacuum partition made of a 15-.mu.m Be foil decreases the intensity of the long wavelength component. The items of this optical system are determined so that an intensity and contrast sufficient for practical use are obtained in the intensity profile of the exposure light absorbed by a chemical sensitization type resist on the wafer when the exposure light is applied to the resist through an X-ray mask comprising a gold absorber pattern having a thickness of 0.6 .mu.m and a silicon nitride membrane having a thickness of 2 .mu.m. FIG. 3 shows the absorption intensity distribution of the resist in the illumination system having the items shown in Table 1. In FIG. 3, a solid line shows the spectrum of the exposure light transmitted by a portion of the mask without the gold absorber pattern, and a dotted line shows the spectrum of the exposure light transmitted by a portion with the gold absorber. The absorption intensity of the exposure light is obtained by integrating each of the spectra shown in FIG. 3 with wavelength. The absorption intensity in the portion without the gold absorber is approximately 2.13 mW/cm.sup.2, and the absorption intensity in the portion with the gold absorber is approximately 0.176 mW/cm.sup.2. If the optimum amount of the light absorbed by the resist required for transfer is 60 J/cm.sup.3, since the thickness of the resist is 1 .mu.m, the exposure time is approximately 2.8 seconds. In addition, the contrast between the pattern portion and the non-pattern portion is 12.1:1. FIG. 4 illustrates the construction of the beam line 4a (or 4b) to which the X-ray mask duplicating exposure apparatus 5a (or 5b) is connected. The synchrotron radiation 71 emitted from an emission point 70 is reflected twice by two plane mirrors 73 and 74 in a mirror unit 72. The radiation 71 is then passed through a vacuum duct 75 and a vacuum partition 76 and is projected onto an original mask 77 to transfer by exposure an original mask pattern to a mask substrate 78 to be exposed. In this apparatus, the distance from the light source to the mask is, for example, 30 m, and the horizontal divergence angle is 1 mrad. The whole exposure region is exposed by scanning the original mask 77 and the mask 78 to be exposed vertically to the illumination light without vertically enlarging by a mirror. In this case, the illumination light is made parallel in the vertical direction. Since the mask substrate which is flatter than the wafer subjected to various processes is used as a substance to be exposed, the dispersion of the gap dimension can be set to a small value. For example, if the dispersion of the gap dimension is 2 .mu.m, the resultant distortion is 2 nm. Table 2 shows details of a design of the illumination system in the X-ray mask duplicating exposure apparatus shown in FIG. 4. The two X-ray mirrors 73 and 74 are made of silicon carbide and are processed so that the reflecting surface of each has a surface roughness of 4 nm (rms), and the angle of incidence of the main SOR beam on each of the reflecting surfaces is 26 mrad. These mirrors have low reflectance on the short wavelength side and thus have a central wavelength longer than the central wavelength of the exposure light obtained by the wafer exposing beam lines. The vacuum partition 76 is made of a material of polyimide having a thickness of 0.5 .mu.m, and separates the He atmosphere under reduced pressure in the exposure apparatus from the vacuum in each of the beam lines. The polyimide has higher transmittance than Be on the long wavelength side, and can thus transmit the long-wavelength illumination light selected by the X-ray mirror. Although the illumination system configured as shown in Table 2 exhibits low illumination intensity and long wavelength, as compared with the illumination system shown in Table 1, it is possible to supply illumination light more suitable for the mask duplicating exposure apparatus for the reasons below. A first reason is that since the illumination intensity is low, the amount of the heat generated due to exposure energy in the original mask and the duplicate mask substrate used as the substrate to be exposed can be decreased, thereby decreasing thermal distortion and increasing the precision of the pattern transfer position. This is particularly effective for the case where the substrate to be exposed for the duplicate mask comprises a thin film. A second reason is that although a long wavelength causes deterioration in the resolving power due to the effect of the light diffracted by the mask, this can be compensated for by decreasing the proximity gap because the duplicate mask substrate as the substrate to be exposed has higher flatness than that of the wafer which was subjected to various processes. In addition, with a long wavelength, since the range of the secondary electrons generated by the X-ray used as the exposure beam is short, the resolving power is increased. For these reasons, high resolving power can be obtained by setting the proximity gap to a small value. The absorber pattern of the original mask is produced by drawing the pattern on the resist using an electron beam drawing exposure apparatus, and then etching or plating. When the thickness of the finally formed absorber is as small as possible, the stress strain generated can be decreased. The thickness distribution of the absorber can also be decreased, and the precision of the pattern line width transferred can be improved. Further, when the pattern is produced by the plating method, if the absorber has a thickness of, for example, about 0.2 .mu.m, a single-layer resist can be used in electron beam drawing. When the pattern is produced by etching, the process can be simplified. In this way, in manufacturing the original mask, the mask with higher precision can easily be manufactured by decreasing the thickness of the absorber pattern. The items of the illumination system shown in Table 2 are determined so as to select exposure light having a wavelength which can achieve a satisfactory contrast even if the gold absorber of the original mask has a thickness of 0.2 .mu.m. If the line width dimension of the pattern is 0.2 .mu.m, the ratio of the line width to the thickness, i.e., the aspect ratio, is 1. A low aspect ratio is also advantageous for manufacturing the original mask. The X-ray mask with a higher aspect ratio can be duplicated by using the original mask. FIG. 5 shows the absorption intensity distribution of the resist in the illumination system having the items shown in Table 2. In FIG. 5, a solid line shows the spectrum of the exposure light transmitted by a portion of the mask without the gold absorber pattern, and a dotted line shows the spectrum of the exposure light transmitted by a portion with the gold absorber. The comparison with FIG. 3 reveals that the central wavelength of the spectra is longer than that shown in FIG. 3. The design having the items shown in Table 2 thus enables the achievement of illumination light having a longer wavelength. The value of absorption intensity of the exposure light is determined by integrating each of the spectra shown in FIG. 5 with wavelength. The absorption intensity in the portion without the gold absorber is approximately 0.0113 mW/cm.sup.2, and the absorption intensity in the portion with the gold absorber is approximately 0.000818 mW/cm.sup.2. The contrast between the pattern portion and the non-pattern portion is thus 13.8:1. The light having a short wavelength contained in the synchrotron radiation scatters the secondary electrons emitted from the mask substrate used as the material to be exposed and sensitizes the photosensitive material, thereby deteriorating the resolving power in duplication of the mask. The embodiment shown in FIG. 4 thus uses plane mirrors 73 and 74 for removing the adverse short-wavelength component. However, a construction without such reflecting mirrors is, in some cases, effective from the viewpoint of the characteristics of the synchrotron radiation. In this case, the construction of a mask duplicating beam line is as shown in FIG. 6. The beam line shown in FIG. 6 does not have the mirror unit containing the plane mirrors shown in FIG. 4. In FIG. 6, reference numeral 90 denotes a shielding member which has a slit-formed opening for passing as a light flux 71' a portion of the upper half of the synchrotron radiation in the vertical divergence therethrough. The shielding member 90 is made of a metal having a thickness sufficient to shield X-rays and has an edge portion in a form which is designed so that the surface area parallel with the beam is decreased for decreasing scattering of the illumination light. In this way, the central portion having the short-wavelength component of relatively high intensity is removed, and the portion having a small amount of short-wavelength component is used as the illumination light. FIG. 7 is a drawing illustrating details of the construction of the X-ray mask duplicating exposure apparatus 5a (or 5b). A mask substrate 78 to be exposed has a frame 80 for supporting the substrate, the frame 80 being held by a vacuum chuck 81. The vacuum chuck 81 is connected to a holding member 83 by a plate spring mechanism 82. A gap setting driving mechanism 84 is provided at three positions in order to move the vacuum chuck 81 in parallel with the plate spring mechanism 82 along the optical axis of the illumination light. The driving amounts of the gap setting driving mechanism 84 are set to different values so that the inclination of the vacuum chuck 81 can be adjusted together with movement of the vacuum chuck 81 along the optical axis. The gap between the original mask substrate 77 and the mask substrate 78 to be exposed can be controlled with high precision by using the detected value of a gap detector 85. The original mask substrate 77 is attached to a vacuum chuck 79 which is connected to a frame 86. A locking actuator 87 is provided between the frame 86 and the holding member 83 so that the frame 86 and the holding member 83 are connected together with high stiffness by driving the actuator 87 during exposure transfer. The original mask substrate 77 and the mask substrate 78 to be exposed are thus substantially integrated, and are scanned over the whole exposure region at right angles to the illumination light. Since the original mask substrate 77 and the mask substrate 78 to be exposed are mechanically locked in scanning exposure, the relative positional deviation between both substrates, which is caused by vibration or the like during scanning, can be decreased, thereby providing an X-ray mask with higher precision. Embodiment 2 FIG. 8 is a drawing illustrating the construction of a second preferred embodiment of the present invention. The same members as those shown in FIG. 2 are denoted by the same reference numerals. The apparatus of this embodiment is configured so as to be used for both producing semiconductor devices and duplicating masks. The construction of the system comprises a plurality of exposure apparatus radially connected to a common SOR light source apparatus, as in the construction shown in FIG. 1. An X-ray intensity attenuation means 100 shown in FIG. 8 is provided on any one or all of the beam lines of the exposure apparatus. FIGS. 9 and 10 are drawings illustrating details of the construction of the intensity attenuation means 100. The inside of a chamber 102 is in a state under the same reduced pressure as in the beam port. When a semiconductor is exposed, i.e., when high X-ray intensity is required for obtaining high productivity, an attenuation filter 103 is retracted from the use region (exposure region) 101 of the illumination light, as shown in FIG. 9, so that the X-rays are introduced into the exposure apparatus without being attenuated. On the other hand, when an X-ray mask is duplicated, i.e., when a high resolving power is required, the attenuation filter 103 is placed on the use region 101 of the illumination light, as shown in FIG. 10, so that the X-rays attenuated in intensity are introduced into the exposure apparatus. The two states are switched by driving a cylinder 104. Since heat is generated in the attenuation filter due to absorption of a portion of X-ray energy in the state shown in FIG. 10, wafer cooling means 105 is provided for preventing the temperature from rising due to the heat generated. The attenuation filter 103 has a filter comprising a thin plate of silicon, silicon nitride, silicon carbide, beryllium or the like, and a frame member for fixing the filter. The thickness of the filter may be set so that thermal strain is within a desired permissible range in view of the intensity of the illumination light, the heat transfer passage from the original X-ray mask to the chuck, etc. The mechanism for attenuating X-ray intensity is not limited to the above form, and some modified examples can be considered. FIG. 11 is a drawing illustrating another example of the attenuation filter. This example comprises a plurality of filters which have different attenuation factors and which are provided in a frame member 110. The filter selected from the plurality of filters is placed on the use region of the illumination region. The X-ray illumination light having an appropriate intensity can be obtained by switching the filters. FIG. 12 is a drawing illustrating a further example of the attenuation filter. A filter 115 having a transmission region with a width greater than the width of the use region of the illumination light is provided on the illumination optical path, and the angle of the filter 115 with respect to the illumination light is adjusted. Since the apparent thickness of the filter 15 can be changed during transmission of the illumination light, the intensity of the illumination light can be arbitrarily attenuated. FIGS. 13A and 13B are drawings illustrating a still further example for attenuating the apparent intensity of the illumination light. Two shielding plates 120 and 121 having a substantially semicircular form are provided in the illumination optical path in order to shield the illumination light so as to pass the illumination light through the gap between the two shielding plates. The two shielding plates are simultaneously rotated while maintaining the gap therebetween. If the gap is moved at a speed at which the time required for moving the gap through the exposure region is sufficiently smaller than the time constant of the heat transfer passage from the X-ray mask to the chuck, the substantial intensity of the illumination light applied to the exposure region is attenuated. The attenuation factor can also be adjusted by adjusting the dimension of the gap. In FIG. 13A, the gap is moved by synchronously rotating the two shielding means using motors 122 and 123, respectively, and the gap dimension is adjusted by adjusting the rotational phases of the two motors. In the above embodiments, since the intensity of the illumination light in an X-ray exposure apparatus which uses the common light source for synchrotron radiation or the like can be adjusted without influences on the illumination light intensity in another exposure apparatus, a high resolving power can be obtained by attenuating the X-ray intensity in duplication of the X-ray mask, and a high productivity can be obtained by increasing the X-ray intensity in wafer exposure. Not only when the X-ray mask is duplicated but also when a device with higher precision is produced, i.e., when high precision exposure is desired in spite of the need for much exposure time, the X-ray intensity may be attenuated, and a semiconductor device with a higher degree of integration can be manufactured. Embodiment 3 An embodiment of the device manufacturing method using the above-described exposure apparatus is described below. FIG. 14 shows a manufacture flowchart of a microdevice (an IC or LSI semiconductor chip, a liquid crystal panel, CCD, a thin-film magnetic head, a micromachine, etc.). The circuit of the device is designed in Step 1 (circuit design). A mask having the designed circuit pattern formed thereon is manufactured in Step 2 (mask manufacture). The manufacture of the mask employs the above-described exposure apparatus. On the other hand, a wafer is manufactured by using material such as silicon or the like in Step 3 (wafer manufacture). Step 4 (wafer process) is referred to as a pre-process for forming an actual circuit on the wafer by the lithographic technique using the prepared mask and wafer. Next Step 5 (assembly) is referred to as a post-process for forming a semiconductor chip using the wafer manufactured in Step 4, the post-process comprising the assembly step (dicing, bonding), the packaging step (chip sealing) and so on. In Step 6 (inspection), tests such as a device operation confirmation test, durability test, etc., are performed on the device manufactured in Step 5. The device is completed through these processes and then delivered (Step 7). FIG. 15 shows details of the flowchart of the above wafer process. The surface of the wafer is oxidized in Step 11 (oxidation). An insulating film is formed on the surface of the wafer in Step 12 (CVD). Electrodes are formed on the wafer by evaporation in Step 13 (electrode formation). An ion is implanted into the wafer in Step 14 (ion implantation). A sensitizing agent is coated on the wafer in Step 15 (resist treatment). The circuit pattern of the mask is baked and exposed by the above exposure apparatus in Step 16 (exposure). The exposed wafer is subjected to development in Step 17 (development). Portions other than the developed resist image are cut off in Step 18 (etching). The unnecessary resist after etching is removed in Step 19 (resist separation). These steps are repeated to form a circuit pattern in multiple layers on the wafer. The use of the manufacturing method of this embodiment enables manufacture of a device which cannot be easily manufactured by a conventional method, with a high degree of integration and high productivity. TABLE 1 ______________________________________ Example of Illumination System for Wafer Exposure Apparatus ______________________________________ Light source Electron energy 700 MeV Orbital radius 0.582 m Stored current 300 mA Beam size (.sigma..sub.y) 0.5 mm Beam divergence angle (.sigma..sub.y') 0.2 mrad Critical wavelength 9.49.ANG. Mirror Reflecting surface material SiC Surface roughness 10 .ANG. Surface shape Cylindrical Radius of curvature 40 m Incident angle 15 mrad X-ray Material Be window Thickness 15 .mu.m Mask Supporting film material Si.sub.3 N.sub.4 Thickness 2 .mu.m Resist Type Chemical amplified type Thickness 1 .mu.m Arrangement SOR-mirror 3 m SOR-mask 8 m X-ray window-mask 0.4 m ______________________________________ TABLE 2 ______________________________________ Example of Illumination System for Mask Duplicating Exposure Apparatus ______________________________________ Light source Electron energy 700 MeV Orbital radius 0.582 m Stored current 300 mA Beam size (.sigma..sub.y) 0.5 mm Beam divergence angle (.sigma..sub.y') 0.2 mrad Critical wavelength 9.49.ANG. First mirror Reflecting surface material SiC Surface roughness 40 .ANG. Surface shape Plane Incident angle 26 mrad Second Mirror Reflecting surface material SiC Surface roughness 40 .ANG. Surface shape Plane Incident angle 26 mrad X-ray Material polyimide window Thickness 0.5 .mu.m Mask Supporting film material Si.sub.3 N.sub.4 Thickness 2 .mu.m Resist Type Chemical amplified type Thickness 3 .mu.m Arrangement SOR-first mirror 3 m SOR-second mirror 4 m SOR-mask 30 m X-ray window-mask 0.4 m ______________________________________