Method for transferring patterns with charged particle beam

Abstract: A pattern to be transferred onto a predetermined area on a beam-sensitive substrate is divided into a plurality of small regions on a mask, projection of a charged particle beam covering each small region is repeated for all of the small regions, and pattern transfer positions are adjusted so that transfer areas corresponding to the small regions are contacted with each other on the beam-sensitive substrate. When the pattern to be transferred to the predetermined area on the beam-sensitive substrate is divided into the plurality of small regions, the small regions having the same pattern portion after division are represented by a common small region, and, upon transferring, the pattern of the common small region is transferred onto a plurality of positions on the beam-sensitive substrate.

Description: The present invention relates to a pattern transferring method used in lithography for semiconductor IC circuits, and more particularly, it relates to a method for transferring patterns formed on a mask onto a beam-sensitive substrate by illumination with a charged particle beam such as an electron beam or an ion beam. In recent years, exposure apparatuses using a charged particle beam to achieve both high resolving power of exposure and high throughput have been developed. Among such exposure apparatuses, in the past, exposure apparatuses of collective transfer type in which patterns for one die (corresponding to one of a plurality of integrated circuits formed on a single wafer) or a plurality of dies are transferred from a mask to a beam-sensitive substrate totally or collectively (in a lump) have been developed. However, in these collective transfer type exposure apparatuses, it was difficult to manufacture the mask acting as a negative and to limit aberration of an optical system below a predetermined value within a large optical field corresponding to one or more dies. In consideration of the above, exposure apparatuses of separate transfer type in which a pattern to be transferred to a wafer is divided into a plurality of small regions smaller than an area corresponding to one die and these small regions are successively transferred onto the wafer to complete the entire pattern have recently been considered. In such exposure apparatuses of separate transfer type, the small regions formed on the mask are separated from each other by opaque border zones. Since such opaque zones should not be formed on the beam-sensitive substrate (wafer), a charged particle beam passing through a given small region is deflected by a deflector so that a pattern image corresponding to current region is formed on the wafer directly adjacent to the previously formed pattern image. In the above-mentioned arrangement, since an amount of deflection is increased as the small region is spaced apart from a center of the mask, optical fields available to the mask and/or the beam-sensitive substrate are limited. An object of the present invention is to provide a charged particle beam type transferring method wherein limited optical fields can be utilized more effectively. To achieve the above object, there is provided a method according to claim 1. In this method for transferring a predetermined pattern onto a predetermined area on a beam-sensitive substrate, the pattern to be transferred to the predetermined area on the beam-sensitive substrate is divided into a plurality of small regions spaced apart from each other on a mask. Projection of a charged particle beam passing through each small region on the mask is repeated for all of the small regions, and pattern transfer positions are adjusted so that transfer areas corresponding to the small regions are contacted with each other on the beam-sensitive substrate. When the pattern to be transferred to the predetermined area on the beam-sensitive substrate is divided into a plurality of small regions, the small regions having the same pattern portion after division are represented by a common small region, and, upon transferring, the pattern of the common small region is transferred onto a plurality of positions on the beam-sensitive substrate. Fig. 1 is a schematic illustration for explaining one advantage in a first embodiment of the present invention; Fig. 2 is a schematic illustration for explaining another advantage in the first embodiment of the present invention; Fig. 3 is a schematic illustration for explaining a further advantage in the first embodiment of the present invention; Fig. 4 is a schematic illustration for explaining a still further advantage in the first embodiment of the present invention; Fig. 6 is a schematic illustration showing an electron beam type contraction transferring system which can be used for the embodiments of the present invention; Fig. 7 is a schematic illustration showing a relation between a mask and a beam-sensitive substrate in the first embodiment; Fig. 8 is a plan view showing an example of a pattern formed on a small region of the mask of Fig. 7; Fig. 9 is a sectional view of two kinds of masks used with the present invention; Fig. 15 is a schematic illustration showing an optical system of a conventional electron beam type contraction transferring system; Fig. 16 is a perspective view showing transferring sequences in the electron beam contraction transferring system of Fig. 15; Fig. 17 is a schematic illustration showing a relation between small regions on a mask and transferred areas on a beam-sensitive substrate in the prior art; Fig. 18 is a view for explaining a problem caused at a connection portion between patterns in the prior art; Fig. 19 is a view for explaining a problem caused at a connection portion between patterns in the prior art; and Fig. 20 is a view similar to Fig. 16, but is a schematic illustration showing a conventional technique slightly different from that of Fig. 16. Before embodiments of the present invention are explained, conventional techniques will be described with reference to Figs. 15 to 20. Figs. 15 and 16 show an example of a conventional exposure apparatus of separate transfer type as mentioned above. In the apparatus shown in Figs. 15 and 16, an electron ray or beam EB emitted from a beam source (not shown) and modified to have square cross-section, is deflected to be spaced apart from an optical axis AX of an optical system by a predetermined distance 6 by means of deflectors 1 so that the beam is directed to one of a plurality of small regions or segments 2a formed on a mask 2. The small regions 2a include patterns to be transferred to a wafer 5, and the electron beam can pass through each small region. The small regions 2a are divided or separated from each other by border zones 2b for shielding or scattering the electron beam. When the electron beam EB is directed onto the small region 2a, a pattern image corresponding to a shape of the beam permeable portion of the small region 2a is projected onto a predetermined area 5b on the wafer with a predetermined reduction ratio (for example, 1/4) through a first projection leans 3 and a second projection leans 4. The pattern images projected on the areas 5b are not illustrated. A point CO is a cross-over point at which the electron beam EB which has passed through the mask 2 intersects with the optical axis AX. Upon transferring, illumination with the electron beam EB passing through each small region 2a is repeated for all of the small regions, and the pattern images corresponding to the shapes of the beam permeable portions of the small regions 2a are successively transferred onto the different areas 5b on the wafer 5. The positions of the pattern images transferred to the wafer 5 are adjusted by deflectors (not shown) disposed between the mask 2 and the wafer 5 so that the projected areas 5b (corresponding to the small regions 2a) on the wafer are contiguous to each other. More specifically, if the electron beam EB passed through the small region 2a is focused on the wafer 5 through the first and second projection lenses 3, 4, not only the image corresponding to the small region 2a but also the images corresponding to the border zones 2b are transferred to the wafer with the predetermined reduction ratio, so that non-exposed areas corresponding to the border zones 2b will be formed between the projected areas 5b. To avoid the formation of the non-exposed areas, the transfer positions of the pattern images are shifted by amounts corresponding to widths of the border zones 2b. After the pattern images corresponding to all of the small regions 2a formed on the single mask 2 are transferred to the wafer 5, the pattern transferring of one die to an area 5a on the wafer 5 is completed. Incidentally, unless otherwise provided, as shown in Figs. 15 and 16, a direction of the optical axis AX of the electron beam EB is referred to as a z-direction, a direction parallel with one side of the small region 2a on the mask is referred to as an x-direction, and a direction perpendicular to the z-direction and the x-direction is referred to as a y-direction. In the exposure apparatus of separate transfer type as mentioned above, the small regions 2a on the mask 2 correspond to the projection or transfer areas 5b on the wafer 5 at a ratio of 1:1. For example, as shown in Fig. 17, when the area 5a for one die on the wafer 5 is divided into one hundred transfer areas 5b, one hundred small areas 2a the number of which is equal to that of the areas 5b are formed on the mask 2. Since the pattern is transferred from the mask 2 to the wafer 5 with the reduction ratio, each small region 2a is greater than the corresponding transfer area 5b by the factor corresponding to the reciprocal number of the reduction ratio. In addition, not only the small regions 2a but also the border zones 2b are formed on the mask 2. Accordingly, as shown in Fig. 17, the dimension of a range 2P on the mask 2 in which the small regions 2a are provided (referred to as "pattern holding range" hereinafter) is always greater than that of the area 5a on the wafer 5 corresponding to all of the small regions 2a. In the illustrated embodiment, while an example that the pattern holding range 2P and the area 5a include regions and areas corresponding to one die is explained, these portions 2P, 5a do not always include the areas corresponding to one die. In the electron beam transferring apparatus, due to aberration of the optical system, optical error, in the resolving power, for example, is increased as illumination position of the electron beam is spaced apart from the optical axis of the optical system (in other words, as the deflection amount δ shown in Fig. 15 is increased). A range within which the optical error at the mask side is maintained below an acceptable value and a range within which the optical error at the beam-sensitive substrate (wafer) side is maintained below an acceptable value are both defined as circular areas each center of which is positioned on the optical axis of the optical system for the electron beam. Hereinbelow, the range within which the optical error at the mask side is maintained below the acceptable value is referred to as "mask side optical field" and the range within which the optical error at the beam-sensitive substrate side is maintained below the acceptable value is referred to as "beam-sensitive substrate side optical field". In Fig. 17, in order to transfer the patterns of all of the small regions 2a onto the wafer 5 with predetermined accuracy in a condition that the mask 2 and the wafer 5 are both stationary, a circle Cm circumscribing the pattern holding range 2P must be included within the mask side optical field and a circle Cw circumscribing the area 5a on the wafer 5 must be included within the beam-sensitive substrate side optical field. Since the circle Cm is considerably greater than the circle Cw, the dimensions of the pattern holding range 2P on the mask 2 and of the corresponding area 5a on the wafer 5 are limited in dependence upon the dimension of the mask side optical field of the transferring apparatus. In other words, at the beam-sensitive substrate side, although the optical field has a larger scope, the patterns can only be transferred to a place within a limited range smaller than the acceptable optical field. Accordingly, even when all of the areas 5a on the wafer 5 are included in the beam-sensitive substrate side optical field of the transferring apparatus, since the area 5a must be included in the smaller range corresponding to the circle Cm, it is necessary to shift the areas 5a within the limited range of the beam-sensitive substrate side optical field successively, with the result that the stage driving time for shifting the wafer 5 is increased to increase the total treating time, thereby reducing the throughput. Particularly, when the overhead times for shuttling the stage (i.e., changing the driving direction of the stage) is cumulatively totalled, waste time increased, thereby reducing the throughput greatly. In the exposure apparatus of separate transfer type as mentioned above, since the pattern for one die is formed by connecting the plurality of pattern images to each other, as shown in Fig. 18(a), the pattern images PT1, PT2 may be deviated from each other at a border position BP between the divided pattern images PT1, PT2, which results in erroneous operation of the produced device. For example, when the pattern images PT1, PT2 are separated from each other at the border position BP as shown in Fig. 18(b) and the dose amounts of the left and right pattern images PT1, PT2 are as indicated by the solid lines in Fig. 18(c), the dose amount at the border position BP corresponds to the sum of the dose amounts of the left and right pattern images PT1, PT2 as shown by the broken line in Fig. 18(c). In this case, if a development level after transferring is set to have a value greater than a minimum dose amount at the border position BP as shown by the dot and chain line, a pattern image PT after development will be discontinued to cause the poor electrical connection. On the other hand, as shown in Figs. 19(a) and 19(b), if the left and right pattern images PT1, PT2 are overlapped, the dose amount at the border position BP will be increased in comparison with the other portions, with the result that the pattern PT after development is enlarged in the widthwise direction at the border position BP, thereby causing a danger of contacting with a neighboring pattern. U.S. Patent No. 5,347,592 discloses a transferring method wherein an aperture corresponding to a repeated pattern such as a memory cell is formed in a mask and an image of the aperture is repeatedly transferred onto a plurality positions on a wafer. This method clearly differs from the present invention wherein the pattern to be formed on the beam-sensitive substrate is divided into the plurality of small regions on the mask and all of the pattern images are generated by the single mask, since, in the disclosed method, the images of the patterns other than the repeated pattern are generated by a variable shape forming stop without generating any pattern other than the repeated pattern by a single mask. Next, a principle for achieving a desirable function in the present invention will be explained. With reference to Fig. 1, a pattern to be transferred onto a predetermined area 101 on a beam-sensitive substrate is divided into a plurality of small regions SF1, SF2 on a mask which are spaced apart from each other. Illumination with a charged particle beam covering one of the small regions SF1, SF2 each time is repeated. Pattern transfer positions at which the small regions are transferred to the beam-sensitive substrate are adjusted so that transfer areas corresponding to the small regions SF1, SF2 are contacted with each other on the beam-sensitive substrate, thereby transferring the predetermined pattern to the predetermined area 101 on the beam-sensitive substrate. When the pattern to be transferred to the predetermined area 101 on the beam-sensitive substrate is divided into the plurality of small regions SF1, SF2 on the mask, the small regions PF1 (PF2) having the same pattern shape after division are treated as a common small region SF1 (SF2), and, upon transferring, the pattern of the common small region SF1 (SF2) is transferred onto a plurality of positions on the beam-sensitive substrate. With the arrangement as mentioned above, it is possible to set a mask side optical field of an optical system used in the transferring to become smaller than a mask side optical field which would be required when the common small regions SF1, SF2 are not used. In an arrangement as shown in Fig. 2, a predetermined area 111 on the beam-sensitive substrate is set to become greater than a beam-sensitive substrate side optical field FSw of a charged particle transferring apparatus in one direction (shown by the arrow Aw) of the beam-sensitive substrate, so that, upon transferring, pattern images of all of small regions SF on the mask can be transferred within the predetermined areas 111 by shifting the beam-sensitive substrate in said one direction. In an arrangement as shown in Fig. 3, the dimension of a predetermined area 121 on the beam-sensitive substrate is selected to be included in the beam-sensitive substrate side optical field FSw of the charged particle transferring apparatus, and the pattern to be transferred onto the predetermined area 121 on the beam-sensitive substrate is divided into a plurality of small regions SF disposed in an area 120 greater than a mask side optical field FSm of the charged particle transferring apparatus in one direction (shown by the arrow Am) of the mask, so that, upon transferring, pattern images of all of small regions SF on the mask can be transferred within the predetermined areas 121 by shifting the mask in said one direction. In an arrangement as shown in Fig. 4, the dimension of a predetermined area 131 on the beam-sensitive substrate is selected to become greater than the beam-sensitive substrate side optical field FSw of the charged particle transferring apparatus in one direction (shown by the arrow Aw) of the beam-sensitive substrate and the pattern to be transferred onto the predetermined area 131 on the beam-sensitive substrate is divided into a plurality of small regions SF disposed in an area 130 greater than the mask side optical field FSm of the charged particle transferring apparatus in one direction (shown by the arrow Am) of the mask, so that, upon transferring, pattern images of all of small regions SF on the mask can be transferred within the predetermined area 131 by shifting the mask in said one direction of the mask and shifting the beam-sensitive substrate in said one direction of the beam-sensitive substrate. As shown in Fig. 1, when the predetermined area 101 on the beam-sensitive substrate is divided into one hundred transferred areas PF, it is assumed that the pattern images of the transfer areas PF1 positioned below the line I-I in Fig. 1 are the same as each other and the pattern images of the transfer areas PF2 positioned above the line I-I are the same as each other. In this case, the pattern to be transferred onto the transfer areas PF1 positioned below the line I-I is formed in the small region SF1 and the pattern to be transferred onto the transfer areas PF2 positioned above the line I-I is formed in the small region SF2. As a result, the dimension of a range 100 including the small regions SF1, SF2 can be reduced in comparison with the conventional cases. Accordingly, the range 100 including the small regions SF1, SF2 on the mask can be smaller than the predetermined area 101 on the beam-sensitive substrate, and, therefore, the predetermined area 101 can be widened to the maximum extent corresponding to the beam-sensitive substrate side optical field. In the arrangement shown in Fig. 2, as the beam-sensitive substrate is shifted in one direction (shown by the arrow Aw), an overlapped area (hatched area in Fig. 2) between the predetermined area 111 of the beam-sensitive substrate and the beam-sensitive substrate side optical field FSw is gradually changed. Thus, the pattern can be transferred onto the area 111 greater than the beam-sensitive substrate side optical field FSw by directing the charged particle beam on the small regions SF successively while maintaining the mask in the stationary condition so that the area 110 including all of the small regions SF is included in the mask side optical field FSm. In the arrangement shown in Fig. 3, as the mask is shifted in one direction (shown by the arrow Am), an overlapped area (hatched area in Fig. 3) between the area 120 of the mask including the small regions SF and the mask side optical field FSm is gradually changed. Thus, even when some of the small regions SF to be transferred to the predetermined area 121 of the beam-sensitive substrate are disposed in the area 120 out of the mask side optical field FSm, the pattern images of all of the small regions SF can be transferred onto the predetermined area 121 of the beam-sensitive substrate. In the arrangement shown in Fig. 4, as the beam-sensitive substrate is shifted in one direction (shown by the arrow Aw), an overlapped area (hatched area in Fig. 4) between the predetermined area 131 of the beam-sensitive substrate and the beam-sensitive substrate side optical field FSw is gradually changed, and, as the mask is shifted in one direction (shown by the arrow Am), an overlapped area (hatched area in Fig. 4) between the area 130 of the mask including the small regions SF and the mask side optical field FSm is gradually changed. Thus, even when some of the small regions SF to be transferred to the predetermined area 131 greater than the beam-sensitive substrate side optical field FSw are disposed in the area 130 out of the mask side optical field FSm, the pattern images of all of the small regions SF can be transferred onto the predetermined area 131 of the beam-sensitive substrate. Next, a first embodiment of the present invention will be explained with reference to Figs. 6 to 9. Fig. 6 is a schematic illustration of an electron beam contraction transferring apparatus used in the first embodiment. An electron beam or ray EB emitted from an electronic gun 10 is changed to parallel beam by means of a condenser lens 11, which parallel beam is deflected in an x-y plane (parallel with the X-axis and Y-axis) by means of field selection deflectors 12 to be directed to one of small regions (not shown) on a mask 50 which will be described later. The electron beam EB passed through the mask 50 is deflected by a predetermined amount by means of deflectors 13, and then is focused on a predetermined position on a beam-sensitive substrate 60 with predetermined reduction ratio (for example, 1/4) through lenses 14, 15. The mask 50 is attached to a mask stage 16 so that the mask is maintained in parallel with the x-y plane. The mask stage 16 is continuously shifted in the X-direction and is driven in the y-direction step-by-step by means of a drive means 17. A position of the mask stage 16 in the x-y plane is detected by a laser interferometer 18, and the detected result is sent to a controller 19. Incidentally, the x-direction corresponds to a direction perpendicular to the plane of Fig. 6. The beam-sensitive substrate 60 is attached to a movable stage 21 supported on a beam-sensitive substrate support 20 so that the beam-sensitive substrate is maintained in parallel with the x-y plane. The movable stage 21 can be continuously shifted by means of a drive means 22 in a direction opposite to the direction that the mask stage 16 is continuously shifted. The reason why the movable stage is shifted in the opposite direction is that the pattern images are turned over by the lenses 14, 15. A position of the movable stage 21 in the x-y plane is detected by a laser interferometer 23, and the detected result is sent to the controller 19. The controller 19 serves to calculate the deflection amount of the electron beam EB caused by the deflectors 12, 13 on the basis of exposure data inputted from an input device 24 and position information regarding the mask stage 16 and the movable stage 21 detected by the laser interferometers 18, 23, and also calculate information (for example, positions and shifting speeds) required for controlling the operations of the mask stage 16 and the movable stage 21. The calculated deflection amount is outputted to deflection amount setting devices 25, 26 so that the deflection amounts of the deflectors 12, 13 are adjusted by the deflection amount setting devices 25, 26, respectively. The calculated information regarding the stages 16, 21 is outputted to the drivers 27, 28. The drivers 27, 28 serve to control the operations of the drive means 17, 22 so that the stages 16, 21 are driven on the basis of the calculated results. The input device 24 may be of the type which can read magnetic information formed by an exposure date generating means or can read exposure date registered in the mask 50 and the beam-sensitive substrate 60 when they are used. Fig. 7(a) shows an area 500 including small regions 501 on the mask 50. In the illustrated embodiment, the small regions 501 are disposed on the area 500 greater than a mask side optical field FSm of an electron beam directing optical system in the x-direction. The small regions 501 are separated from each other by border zones 502 for blocking or dispersing the electron beam. Each small region 501 includes electron beam permeable portions (white areas) BT corresponding to the pattern shape to be transferred onto the beam-sensitive substrate, for example, as shown in Fig. 8. The mask used for transferring using the electron beam may be, for example, a so-called scattering mask wherein the electron beam permeable portions BT are formed by a film made of silicone nitride (SiN) and tungsten scattering portions BS are provided on the film as shown in Fig. 9(a), or, a so-called stencil (aperture) mask wherein through holes formed in tungsten scattering portions BS are used as the electron beam permeable portions BT as shown in Fig. 9(b). A hatched portion 600 on the beam-sensitive substrate 60 shown in Fig. 7(b) shows an area on which the combination of the small regions 501 on the mask 50 can be transferred. The area 600 corresponds to a half of an area 601 for one die to be formed on the beam-sensitive substrate 60 and is greater than a beam-sensitive substrate side optical field FSw in the x-direction. The beam-sensitive substrate side optical field FSw has a dimension which includes about 1/4 of the area 601 of one die. When the pattern to be transferred onto the area 600 of the beam-sensitive substrate 60 is divided into the small regions 501, the small regions having the same shape after division are treated as a common small region 501. As a result, the number of the small regions 501 becomes smaller than the number of the divisions of the area 600. With the arrangement as mentioned above, a correspondence relation between the small regions 501 and the transfer positions on the beam-sensitive substrate 60 is previously inputted from the input device 24 to the controller 19 as the exposure data. In the transferring, the deflection amounts of the deflectors 12, 13 and the positions of the stages 16, 21 are controlled on the basis of the exposure data so that the pattern images of the small regions 501 are transferred onto the designated positions on the beam-sensitive substrate 60. During the transferring, the mask stage 16 and the movable stage 21 are continuously shifted in opposite directions along the x-axis. As a result, the overlapped portion between the area 500 and the optical field FSm and the overlapped portion between the area 600 and the optical field FSw shown in Fig. 7 are gradually changed, with the result that the pattern images of all of the small regions 501 included in the area 500 can be transferred onto the area 600 on the beam-sensitive substrate 60. During the continuous shifting movement of the stages, the adjustment of the illumination position for the electron beams EB and the pattern transferring position in the x-direction are effected by the deflectors 12, 13. After the transferring of the pattern images to the area 600 is finished, the movable stage 21 is shifted in the y-direction by a distance corresponding to the width of the area 600, and the pattern images are transferred onto the remaining area. In this way, the transferring of the pattern images to the area 601 for one die is completed.

Claims: A method of transfer with a charged particle beam for transferring a predetermined pattern onto a predetermined area on a substrate (101), wherein said pattern to be transferred to said predetermined area on said substrate (101) is divided into a plurality of small regions (SF1, SF2) spaced apart from each other on a mask, said method comprising repeatedly directing a charged particle beam to said small regions (SF1, SF2) on said mask, with each small region being treated as a unit of projection with the beam, and adjusting pattern transfer positions so that transfer areas corresponding to said small regions (SF1, SF2) are contacted with each other on said substrate, characterised in that when said pattern to be transferred to said predetermined area on said substrate is divided into a plurality of small regions (PF1, PF2), the small regions (PF1, PF2) having the same pattern portion after division are represented by a common small region (SF1, SF2), and, upon transferring, the pattern of said common small region (SF1, SF2) is transferred onto a plurality of positions on said substrate. A method of transfer according to claim 1, wherein a mask side optical field of an optical system used in the transferring is set to be smaller than the mask side optical field which would be required when said common small region (SF1, SF2) is not used. A method of transfer according to claim 1, wherein said predetermined area (111) on said substrate is set to be greater than a substrate side optical field (FSw) of a charged particle beam type transferring apparatus in one direction of said substrate, and, upon transferring, said small regions (SF1, SF2) on said mask are transferred onto said predetermined area (111) on said substrate by shifting said substrate in said one direction. A method of transfer according to claim 1, wherein said predetermined area (121) on said substrate is set to be included within a substrate side optical field (FSw) of a charged particle beam type transferring apparatus and the pattern to be transferred onto said predetermined area (121) is divided into small regions (SF) disposed in a range (120) greater than a mask side optical field (FSm) of said charged particle beam type transferring apparatus in one direction of said mask, and wherein, upon transferring pattern portions of said small regions (SF) of said mask are transferred onto said predetermined area (121) on said substrate by shifting said mask in said one direction. A method of transfer according to claim 1, wherein said predetermined area (131) on said substrate is set to be greater than a substrate side optical field (FSw) of a charged particle beam type transferring apparatus in one direction of said substrate and the pattern to be transferred onto said predetermined area (131) is divided into small regions (SF) disposed in a range (130) greater than a mask side optical field (FSm) of said charged particle beam type transferring apparatus in one direction of said mask, and wherein, upon transferring, pattern portions of said small regions (SF) of said mask are transferred onto said predetermined area (131) on said substrate by shifting said mask in said one direction of said mask and shifting said substrate in said one direction of said substrate.

Inventors: NAKASUJI MAMORU, OKINO TERUAKI, OKINO, TERUAKI, NAKASUJI, MAMORU
Applicants: NIPPON KOGAKU KK, NIKON CORPORATION
Application year: 1995
Main classification: H01J37
IPC codes: H01J37