Abstract:
A reflection prevention board of a charged particle beam irradiation apparatus of the present invention comprises a laminate sheet having a plurality of thin films and a plurality of microholes through the laminate sheet. According to the present invention the reflection prevention board can be manufactured at a lower cost, the reason being that it is easier to form microholes in the thin films and then laminate these thin films in an aligned relation than to drill holes through a thicker sheet. By doing so it is possible to achieve a better yield. Further, much deeper microholes, which might not otherwise be achieved on a thick sheet, can be formed by using more thin films and a reflection prevention effect can be improved by doing so.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a charged particle beam irradiation apparatus for use in the writing and measurement of a micropattern on a semiconductor device and, in particular, a charged particle beam irradiation apparatus having an improved reflection prevention board over a sample surface. 
     In the lithography process of the-state-of-the-art semiconductor manufacturing process, it is expected that an electronic beam writing type will become a future main-stream technique. The reason is that the electron beam writing method is outstandingly higher in image resolution than the light beam writing method. 
     The electronic beam writing apparatus is used for forming an etching mask on a sample for example. As shown in FIG. 1, an electron beam narrowly stopped down by an objective lens  61  is directed onto a resist  62  coated on a sample  63 , such as a mask, a wafer, etc., and the resist  62  is scanned with the electron beam. A pattern is formed on the resist  62 . And an etching mask is completed by doing so. 
     A portion of the electron beam thus directed for irradiation is reflected on the surface of the sample  63 . Secondary electrons are produced from the sample  63 . Some of the reflected electrons and secondary electrons  73  are reflected on the lower surface of the objective lens  61  and back onto the resist  62 , resulting in an error occurring on the resist  62 . Those reflected and secondary electrons  74  returned back onto the resist  62  are light-sensitized at other than a target location, resulting in a lowering in writing precision. In order to reduce such an error, a reflection prevention board is attached to the lower surface of the objective lens  61 , the sheet being made of a material of a lower atomic number, such as a carbon. 
     As a demand for a write precision becomes higher and higher, no adequate reflection preventing effect is obtained from the reflection prevention board made of the lower atomic number material. In order to improve the reflection prevention effect, proposals are made to provide a reflection prevention board with microholes opened perpendicular to the sheet. Further, as shown in FIG. 2, a proposal is also made to provide a honeycomb type reflection prevention board with a dense array of hexagonally prismatic microholes. 
     However, the proposed reflection prevention board has the following problems (1) and (2): 
     (1) The deeper the microholes in the reflection prevention board, that is, the thicker the reflection prevention board, the higher the reflection prevention effect. In general, with the microhole diameter set to 0.8 mm, at least 4 mm-thick sheet is preferable as the reflection prevention board. It is difficult, however, to form a regular honeycomb array of microholes in such a thick metal sheet by the mechanical working technique. And more working time is taken and, in spite of this, a poor yield and a very high manufacturing cost result. A proposal is also made to use the technique for metal-plating those holes in a thick-film resist patterned by an LIGA process, that is, the X-ray lithography. The LIGA process involves the problem of the reflection prevention board involving an upper size limitation and very high manufacturing cost. 
     (2) The conventional reflection prevention board can effectively prevent those vertically incident reflection electrons and secondary electrons. As shown, for example, in FIG. 3, it is not possible to effectively prevent those obliquely incident reflection electrons and secondary electrons. The reason is that the obliquely incident reflection electrons are reflected on the sidewall of the microhole and bounced out of the microhole. 
     As seen from the above, the existing reflection prevention board used in the electron beam writing apparatus presents the problems of being high in manufacturing cost, being unable to provide deeper microholes and being unable to effectively preventing the obliquely incident reflection electrons and secondary electrons. 
     BRIEF SUMMARY OF THE INVENTION 
     It is accordingly the object of the present invention to provide a charged particle beam irradiation apparatus which can prevent re-reflection of reflected particles/secondary particles at a lower manufacturing cost. 
     Another object of the present invention is to provide a charged particle beam irradiation apparatus which can effectively prevent re-reflection of reflected particles/secondary particles obliquely incident to a reflection prevention board. 
     A reflection prevention board of a charged particle beam irradiation apparatus of the present invention is comprised of a laminate sheet of thin films and a plurality of microholes formed therein. According to the present invention, the reflection prevention board can be manufactured at a lower cost. The reason is that it is very easier to form microholes in the thin films and then laminate these thin films in an aligned sheet than to form microholes in a thick board. By doing so, it is possible to improve a yield. Further, much deeper microholes, which might not otherwise be achieved on a thick board, can be formed by using more thin films. 
     In the charged particle beam irradiation apparatus of the present invention, a reflection prevention board is so formed as to have a plurality of inclined microholes therein. In this case, reflected particles and/or secondary particles obliquely incident to the reflection prevention board goes more deeply into the microholes so that a reflection prevention effect is improved. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a view showing a write error conventionally occurring due to electrons which are reflected from a sample irradiated with an electron beam and/or secondary electrons emitted at that time being re-reflected back onto a resist; 
     FIG. 2 is a plan view showing a reflection prevention board of a conventional honeycomb type; 
     FIG. 3 is an explanatory view showing a problem with a conventional reflection prevention board; 
     FIG. 4 is a structural view showing a charged beam irradiation apparatus according to a first embodiment of the present invention; 
     FIG. 5 is a perspective view showing a thin film for a reflection prevention board of FIG. 4; 
     FIG. 6 is a perspective view showing a laminate sheet of which a reflection prevention board of FIG. 4 is comprised; 
     FIG. 7 is a cross-sectional view showing a reflection prevention board according to a second embodiment of the present invention; 
     FIG. 8 is a cross-sectional view showing a reflection prevention board according to a third embodiment of the present invention; 
     FIG. 9 is an enlarged view showing a dotted line area in FIG. 8; 
     FIG. 10 is a partially cross-sectional view showing a reflection prevention board formed by a thermocompression bonding; 
     FIG. 11 is a cross-sectional view showing a reflection prevention board according to a fourth embodiment of the present invention; and 
     FIG. 12 is a cross-sectional view showing a reflection prevention board according to a variant of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiments of the present invention will be explained below with reference to the accompanying drawing. 
     First Embodiment 
     As shown in FIG. 4, an electronic beam writing apparatus of the present invention includes an electron gun  1  for generating an electron beam, a condenser lens  3  for focusing the electron beam, a deflector  4  for deflecting the electron beam, an objective lens  5  for focusing the beam onto a sample  7 , and a reflection prevention board  6  provided on the lower surface side of the objective lens  5 , that is, on a side opposite to the sample  7 . 
     The reflection prevention board  6  is provided to prevent some of the electrons which are directed onto the sample  7  and reflected on the surface of the sample, and/or those secondary electrons which are knocked out from the sample, from being re-reflected back onto the sample  7  for irradiation. 
     FIG. 5 shows one thin film and FIG. 6 shows a reflection prevention board  6  made up of a laminated thin film structure. The thin film  11  is formed with a honeycomb array of hexagonally prismatic microholes  12  and has positioning holes  13  at four places. The thin film is made of, for example, a copper sheet 0.2 mm thick and the diameter  2 R of the microhole  12  is, for example, 0.8 mm and a width d of a beam  14  separating adjacent microholes  12  from each other is, for example, 0.1 mm. In this case, a hole ratio of all the microholes  12  to the surface area of the thin film  11  reaches about 75%. By further decreasing the width d of the beam  14  it is possible to increase the hole ratio. Since the sheet  11  is thin, microholes can be easily formed by mechanical punching or chemical etching. 
     For example,  20  films  11  are, as shown in FIG. 6, laminated, are jointed by a pin  15 . Needless to say, the microholes  12  are aligned in the thickness direction to pass through the board  6  and give a depth of 4 mm each. In this case the aspect ratio is 5 (=4 mm/0.8 mm). According to the experiments conducted by the present inventors, the reflectance was below 1% for an electron of 30 KeV in the case where it is incident to the center of the microhole whose bottom is aluminum and was smaller by above one order of magnitude in comparison with a Be flat sheet. In the present system, it is possible to readily obtain a higher aspect ratio by preparing many more thin films of the same structure. 
     According to the present invention, a plurality of thin films  11  with microholes  12  formed are joined in a positionally aligned relation to provide a reflection prevention board  6  with a high aspect ratio, that is, very deep microholes  12 . The deep microholes  12  can effectively suppress the re-reflected electrons and knocked-out secondary electrons. In this case, no deep etching is necessary for the formation of deep microholes and it is only necessary to perform a simpler working process of opening microholes  12  in the thin film. It is, therefore, possible to manufacture the reflection prevention board at a much lower manufacturing cost. 
     It is to be noted that the reflection prevention board  6  may be formed with more kinds of thin film materials, instead of one kind of thin film material. For example, a reflection prevention board  6  may be made up of a laminated structure of Al, Au and Cu thin films. Further, another combination may be possible, such as a combination type of a low rigid material allowing the better absorption of reflected electrons and/or secondary electrons and a high rigid material but allowing the lower absorption of reflected electrons and/or secondary electrons. In this case, it is possible to get both the readier handling of it and an improved reflection prevention effect. 
     Second Embodiment 
     FIG. 7 is a cross-sectional view for explaining a reflection prevention board of an electron beam writing apparatus according to a second embodiment of the present invention. 
     In this structure, a reflection preventing plate is of a single layer structure, not of a multi-layered structure as in the first embodiment, where a specific design consideration is given to the configuration of its microholes. That is, the reflection prevention board  10  is relatively thick, for example, 4 mm and has a plurality of microholes  16 . The microholes  16  are inclined with respect to the reflection prevention board  10 . Stated in more detail, the center axes of the microholes  16  is converged at an electron beam incident spot  22  and the oblique angles of the microholes vary from place to place. 
     In such an arrangement, reflected electrons and/or secondary electrons  24  penetrate deeply through the microholes without collision on the sidewall of these microholes, thus there being less chance of these electrons being re-reflected back from the sidewall of these microholes. Therefore, it is possible to improve a reflection prevention effect on the reflection prevention board  10  and ensure an improved write precision on the electron beam writing apparatus. 
     Here, ideally, as shown in FIG. 7, the microhole  16  is so formed as to have a hexagonally pyramidal configuration with its diameter shorter on the sample  20  side and greater on its base side, the microhole being gradually widened in a direction away from the incident spot  16  on the sample. Needless to say, the microhole  16  may be of a truncated conical configuration or truncated polygonally pyramidal configuration type. Forming the microhole  16  in such a specific configuration allows the reflected electrons and/or secondary electrons  24  to penetrate deeply through the microholes  16  in the reflection prevention board  10  almost without collision on the sidewall surface of these microholes. As a result, these electrons have less possibility of being re-reflected from the sidewall of these microholes  16 . It is evident that, even if the diameter of the microhole  16  does not depend from its depth, the reflection electron prevention effect is improved. 
     Third Embodiment 
     FIG. 8 is a cross-sectional view for explaining a reflection prevention board of an electronic beam writing apparatus according to a third embodiment of the present invention. 
     This embodiment corresponds to first and second embodiments of the present invention. As in the first embodiment, microholes  12  are formed in a thin film  11  by etching and a plurality of such thin films are laminated one over another into a single layer structure. By gradually varying the diameter and position of each corresponding microhole  12  in the thin films  11 , microholes  16  are so joined as to allow the central axes of the microholes  16  to converge at an electron beam incident position  22  as in the second embodiment, that is, to provide a truncated hexagonally conical configuration. 
     FIG. 9 is an enlarged view of a portion as indicated by a dotted line in FIG.  8 . When a reflection prevention plate is formed as set out above, a sidewall  17  of the microhole  16 , is strictly speaking, stepped as shown in FIG.  9 . If the thin films  11  are very thin each, a resultant fine stepped sidewall  17  has an advantage as a non-stepped sidewall, because the reflected/secondary electrons go deeper into the microholes without striking against the fine stepped sidewall  17 . As a result, it is possible to further reduce the re-reflection from the microholes  16 . 
     It is to be noted that the thin films, being jointed (integrated) by pins  15 , may be adhesively bonded or thermocompression-bonded into an integral structure. If the thermocompression is done under the condition of a temperature lower than its melting point, a “solid-phase diffusion” phenomenon occurs and hence the atoms are mutually diffused between the adjacent thin films  11 . As a result, it is not possible for a finished product to provide a multilayered structure or to identify any multilayered structure as shown in FIG.  10 . Even in this case, the sidewall  17  of the microhole  16  is maintained as a stepped structure. 
     According to the present invention, it is possible to reduce the manufacturing cost of the reflection prevention board as in the first embodiment and to enhance the reflection prevention effect as in the second embodiment. Therefore, both the reduction of the cost and enhancement of the write precision can be accomplished simultaneously. 
     Fourth Embodiment 
     FIG. 11 is a cross-sectional view for explaining a reflection prevention board of an electronic beam writing apparatus according to a fourth embodiment of the present invention. 
     This embodiment, though corresponding to a combination of the first and second embodiments, differs from the third embodiment in that microholes  12  in all thin films  11  are the same in size but differ in their position, that is, a joined microhole  16  becomes a truncated hexagonally prismatic configuration. Needless to say, the joined microhole  16  may take a truncated conical or truncated polygonally pyramidal configuration in accordance with the cross-section. In this case, this embodiment, though being somewhat inferior to the second and third embodiments, can ensure an adequate reflection prevention effect. 
     According to this embodiment, since the microholes  12  in the thin film  11  are all the same in size and different only in their position, they can be formed very easily. Further, the microholes  12  may be formed all vertical to a surface in which case the microholes  12  is more easily provided. That is, a reflection prevention board higher in reflection prevention efficiency than in the conventional counterpart can be easily manufactured at a lower cost. 
     The present invention is not restricted to the above-mentioned embodiments. The reflection prevention board thus explained in conjunction with the embodiments is effective to the apparatus using not only the electron but also an ion, a neutron and a photon beam. It is, therefore, possible to apply the present invention to not only the electron beam writing apparatus but also an ion beam writing apparatus. It is also possible to apply the present invention not only a charged particle beam writing apparatus but also a measuring apparatus using an electron beam microanalyzer, etc. The reflection prevention board  6 ′ may be located under the sample  7 . In this case, any secondary electrons radiated under the sample  7  can be prevented from being returned back to the sample  7 . 
     Further, since the thin film can be readily deformed by a pressing operation, etc., it is possible to form, as a reflection prevention structure, not only a flat one but also a curved one as shown in FIG.  12  and any proper combination of curved ones. If a thin film of a lower atomic number material, such as beryllium and carbon, is attached to a thin film surface or a surface of reflection prevention board comprised of thin films, then it has an effect of suppressing the reflectance at a remaining portion of that opening or at a sidewall of the microhole and it is effective to further improve the reflection particle prevention efficiency. 
     Further, the configuration and array of the microholes are not restricted to a honeycomb type and it may be considered that they are an array of, for example, rectangles and of concentric circles divided in an angular direction, and so on. Various changes or modifications can be made without departing from the spirit and scope of the present invention. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.