X-ray exposure apparatus

Abstract: An X-ray exposure apparatus for exposing a resist on a substrate (4) to a pattern of an original (3) includes a radiation source (1) for providing X-rays (6); and an illumination system for irradiating the original and the substrate with the X-rays such that the resist of the substrate is exposed to the pattern of the original with the X-rays; wherein the illumination system has a convex mirror (2) having a reflection surface of a shape like a cylindrical surface, for reflecting the X-rays from the radiation source to the original; and wherein the reflection surface of the mirror has such an aspherical surface shape that, with respect to a top of the reflection surface, a radiation source side and an original side are asymmetrical in shape, that, in the neighborhood of the top, the radiation source side has a radius of curvature smaller than that of the original side, and that at a peripheral portion the reflection surface has a curvature of radius larger than that at the top of the reflection surface.

Description: This invention relates to an X-ray exposure apparatus for use in the manufacture of semiconductor devices, for transferring a pattern of an original such as a reticle to a semiconductor substrate such as a wafer by using an X-ray source such as a synchrotron orbit radiation (SOR) source. In such an X-ray exposure apparatus, where a SOR source is used as an X-ray source, since the light from the SOR source consists of sheet like electromagnetic waves having a large divergence angle in a direction parallel to the electron orbit plane but having a small divergence angle in a direction perpendicular to the electron orbit plane, if the light from the SOR source is directly projected to an original, with respect to the aforementioned perpendicular direction only a limited area of the original.can be illuminated. This necessitates some measures for expanding the SOR light (X-rays) in the perpendicular direction, where the X-ray exposure apparatus uses a SOR source as an X-ray source. An Example is shown in Figure 1A wherein a grazing incidence mirror (flat mirror) 21 is disposed between a SOR source and an exposure area of a wafer 22, and the mirror 21 is oscillated by a small angle of a few milli-radians to expand the light from the SOR source, as discussed in J. Vac. Sci. Technol., B1 (4), 1983, p.1271-1275. A second example is shown in Figure 1B wherein a mirror 23 having a sectional shape which can be represented by an exponential function is provided to expand the SOR light having a Gaussian distribution, with respect to the perpendicular direction, and also to make the intensity distribution uniform, as disclosed in Japanese Laid-Open Patent Application, Laid-Open No. 60-226122. In Figure 1A, the graph at the right-hand half depicts the intensity distribution of X-rays as absorbed by a resist applied to the wafer, in the first example. In Figure 1B, the graph at the right-hand half depicts the intensity distribution of X-rays on the mask surface, in the second example. In the first method described above, at a moment only a part of the mask is irradiated. Thus, there is a high possibility of local thermal expansion of the mask during the exposure process, which leads to pattern transfer distortion. In order to avoid the effect of such thermal expansion, the oscillation period of the mirror 21 has to be made sufficiently small, and this requires use of a large driving power for oscillating the mirror 21. Also, the necessary mechanism of the driving means for the mirror 21 is complicated and, therefore, this method is not so practicable. In the second method described above, non-uniformness in the intensity of the SOR light can be reduced by the reflection with the mirror 23. However, as the SOR light passing through the mask is absorbed by the resist, the absorption rate of the resist has wavelength-dependence and, generally, the SOR light is not a monochromatic light. Therefore, non-uniformness in exposure occurs in the resist itself. Further, the mirror 23 is equipped with only such a reflection surface which corresponds to a half of a cylindrical surface at one side of its top. As a result, only a portion of the SOR light which is at the upper side or lower side of the electron orbit plane can be used. Thus, the efficiency of utilization is bad. The use of cylindrical convex mirrors in X-ray exposure apparatus for expanding the angular intensity distribution of X-ray radiation from a synchrotron source is disclosed in the "Handbook on Synchrotron Radiation" by E-E Koch (Vol. 1B North-Holland Publishing Company, NL, 1983 Chapter 13 W.R. Grobman, pages 1133-1137). The X-ray intensity distribution generated is not however uniform, as is shown in Figure 3 and discussed hereinafter. It is accordingly, a primary object of the present invention to provide an X-ray exposure apparatus by which a resist on a substrate can be exposed with a small loss of energy, uniformly without non-uniformness in intensity. The present invention provides an X-ray exposure apparatus for exposing a substrate to a pattern of an original including an X-ray mirror shaped to expand the angular intensity distribution of X-ray radiation which is used for exposing the substrate and which is incident on the mirror from an X-ray source, wherein the reflective surface of the mirror is convex, characterised in that: the reflective surface of the mirror has a vertex defined between two curved reflection surface portions, the radius of curvature of the curved reflection surface portion which during use of the apparatus lies closer to the X-ray source being smaller than that of the curved reflection surface portion, on the other side of the vertex. In an embodiment of the present invention, the reflection surface of the reflection mirror is shaped so that the radius of curvature at a peripheral portion (a portion off the optical axis and spaced from the top) is larger than that in the neighbourhood of the top (adjacent to the optical axis). As a result, a beam of peripheral part of the X-ray flux which impinges on a peripheral portion of the mirror can be directed to the original and the substrate with good efficiency. Namely, it is possible to use, for the exposure, such a peripheral part beam which could not be used for the exposure in an arrangement wherein a simple cylindrical reflection surface is used to reflect the X-rays to the original and the substrate. As for the structure of such a mirror, a substrate of SiC, a substrate of SiO 2 or a substrate of SiO 2 with Au deposition or, alternatively, a substrate of SiO 2 with Pt deposition, can be used. Thus, with the present invention, it becomes possible to expose a resist on a substrate uniformly, while reducing the loss of X-ray energy and, therefore, it is possible to accurately transfer a pattern of an original onto the resist of the substrate. Additionally, it is possible to reduce the exposure time. Embodiments of the present invention will now be described with reference to the accompanying drawings, in which:- Figures 1A and 1B are representations for explaining known examples of X-ray exposure system. Figure 2A is a perspective view, schematically showing an X-ray exposure apparatus according to an embodiment of the present invention. Figure 2B is a sectional view of the exposure apparatus of the Figure 2A embodiment. Figure 3 is a graph showing the X-ray absorption distribution of a resist on a wafer, in the exposure apparatus of the Figure 2A embodiment, and the X-ray absorption distribution of a resist on a wafer in an exposure apparatus having a cylindrical reflection surface. Figure 4 is a graph showing the shape of the reflection surface of the mirror of the Figure 2A exposure apparatus, and the shape of a reflection surface of a cylindrical reflection mirror. Figure 5 is a graph showing the ratio between the radius of curvature in the neighborhood of the top of the reflection mirror of the Figure 2A exposure apparatus and the radius of curvature at each point off the optical axis. Figure 6 is a graph showing the X-ray absorption distribution of a different resist when it is used in the exposure apparatus of Figure 2A. Figure 7 is a graph showing the X-ray absorption distribution as obtainable in the exposure apparatus of Figure 2A when the resist shown in Figure 6 is used, and the X-ray absorption distribution as obtainable in an exposure apparatus having a mirror with a cylindrical reflection surface when the same resist is used. Figure 8 is a graph showing the X-ray absorption distribution of a resist as obtainable with different shapes of mirror reflection surfaces, in the exposure apparatus of Figure 2A embodiment. Referring to Figures 2A and 2B showing an X-ray exposure apparatus according to an embodiment of the present invention, denoted at 1 is a synchrotron which is adapted to emit synchrotron radiation light (SOR light). The synchrotron 1 has a horizontal orbit plane which is parallel to the X-Y plane. The synchrotron 1 emits, toward a mirror 2, a sheet-like flux 6 of X-rays having a small width with respect to the Z direction and a large width with respect to a direction parallel to the X-Y plane. The mirror 2 is a convex mirror, the reflection surface of which has an aspherical surface shape and is based on a cylindrical reflection surface, to be described below. The mirror 2 has a generating line which extends in the X direction. A perpendicular to the top (vertex) of the reflection surface of the mirror 2, extends in the Z direction. In the section along the Z-Y plane, the mirror 2 has a predetermined shape deformed slightly from a cylindrical shape. More particularly, with respect to the top of the reflection surface, the synchrotron 1 side and a side opposite thereto (i.e. the mask 3 side) are asymmetrical in shape. Further, in the neighborhood of the top, the radius of curvature at the synchrotron 1 side is set to be smaller than that at the mask side. The reflection surface of the mirror 2 reflects the X-rays 6 from the synchrotron 1, and while expanding the beam diameter and transforming the sectional intensity distribution thereof into a predetermined distribution, the mirror 2 directs the X-rays to the mask 3. The X-rays 6 from the mirror 2 pass through a shutter 5 and illuminate a circuit pattern of the mask 3. Those of the X-rays passing through the circuit pattern of the mask impinge on a wafer 4. By those X-rays 6, a resist applied to the wafer 4 surface is exposed in accordance with the circuit pattern of the mask 3. The surfaces of the shutter 5, the mask 3 and the wafer 4, receiving the X-rays, are placed substantially in parallel to the Z-Y plane. The mirror 2 may comprise a substrate of SiC having a machined convex reflection surface, a substrate of SiO 2 having a machined convex reflection surface, a substrate of SiO 2 having a machined convex reflection surface on which Au is deposited, a substrate of SiO 2 having a machined convex reflection surface on which Pt is deposited, or the like. Referring to Figure 3, a solid line depicts the X-ray absorption distribution (a distribution of the quantity (intensity) X-ray absorption per unit time), with respect to the Z direction, of the resist on the wafer 4 as the wafer 4 is exposed by using the exposure apparatus of Figures 2A and 2B. Also, in Figure 3, a broken line depicts the X-ray absorption distribution with respect to the Z direction, as a wafer 4 is exposed by using a mirror having a cylindrical reflection surface, in place of the mirror 2. Referring to Figure 4, a solid line depicts the sectional shape of the mirror 2 of the exposure apparatus of Figures 2A and 2B. Also, a broken line depicts the sectional shape of the afore-mentioned mirror having the cylindrical reflection surface. If such a mirror having a cylindrical reflection surface is used for exposure, as depicted by the broken line in Figure 3, the X-ray absorption distribution obtained is non-uniform. In order to correct such non-uniformness, it is necessary to use some measures to block the X-rays corresponding to the hatched area in Figure 3. If such a method for blocking the X-rays partially is used, the quantity of X-ray absorption per unit time is reduced to a lower level, such as Ic shown in Figure 3. With the mirror 2 of the present embodiment, as compared therewith, it is possible to attain the quantity of X-ray absorption per unit time, of a high level Ia which is higher than Ic. Additionally, over the whole exposure region on the resist, the X-ray absorption distribution is uniform. Thus, with the exposure apparatus of the present embodiment, it is possible to accomplish uniform exposure in a reduced time. Detailed of the mirror 2 of the present embodiment will be explained below. The quantity I(z) of X-ray absorption of the resist on the wafer 4, at a certain portion along the Z direction, can be expressed by the following equation: wherein C = [1 + (ℓ 23 /ℓ 12 )·(1 + 2ℓ 12 /ρsin)] x (ℓ 12 +ℓ 23 )/ℓ 12 where λ is the wavelength of the X-rays 6; I 1 (λ) is the intensity of the X-rays 6 impinging on the mirror 2; R(λ) is the reflectivity of the mirror 2; Tw(λ) is the transmittance as provided where a beryllium window or a filter is used; T 3 (λ) is the transmittance of the mask 3; A 4 (λ) is the absorption rate of the resist 4; and C is the expansion rate of the X-rays 6 (i.e., "beam area on the wafer 4"/"beam area on the mirror 2", of the mirror 2, as determined by the curvature radius ρ at each point on the mirror 2 surface, the distance ℓ 12 between the light emission center of the synchrotron 1 and each point on the mirror 2, the distance ℓ 23 between each point on the mirror 2 and each corresponding point (z) on the resist of the wafer 4, and the angle of incidence () of an X-ray beam impinging on each point on the mirror 2. Here, the quantity (intensity) of X-rays as absorbed per unit time by the resist of the wafer 4 is determined as Ia, on the basis of the nature of the used resist and the intensity of the X-rays 6. For example, based on the X-ray absorption distribution such as depicted by the broken line in Figure 3 which is obtainable when the resist is exposed by using a mirror having a cylindrical reflection surface, the quantity of X-rays absorbed by the resist is integrated, with respect to the z direction, from an end of the illumination region to the other end, and an integrated level Ir is obtained. Then, the integrated level Ir is divided by the length Ar of the illumination region in the Z direction, by which Ia can be determined as Ia = Ir/Ar. Here, if the length of the exposure area in the Z direction is denoted by Dr, it is necessary that the length Ar of the illumination region is made larger than the length Dr of the exposure area. Also, if the former is too large, the intensity of X-rays (the X-ray quantity to be used for the exposure) decreases. In consideration of these points, preferably the structure is arranged to satisfy the relation that Dr < Ar < 3Dr. After the quantity Ia of X-ray absorption per unit time is determined, the curvature radius ρ 0 (unknown quantity) of the mirror 2 at a portion adjacent the top (vertex) thereof is determined. To this end, I = Ia and ρ = ρ 0 are substituted into equations (1) and (2), and calculations are made in accordance with these equations. If Ia and ρ 0 are determined in this manner and the quantity I 0 of X-rays reflected by the top of the mirror 2 and absorbed by (the center of) of the exposure area of the resist is determined as I 0 = Ia, then the curvature radius ρ = ρ(y) at each point on the reflection surface of the mirror 2 can be determined in accordance with the following equation: 1/ρ = {ℓ 12 /[ℓ 23 (ℓ 12 +ℓ 23 )]·C-(ℓ 12 +ℓ 23 )/(ℓ 12 ·ℓ 23 )} x sin/2 wherein C = I/I 0 · [1+(ℓ 23 /ℓ 12 ) (1+2ℓ 12 /ρ 0 sin)] x (ℓ 12 +ℓ 23 )/ℓ 12 where ℓ 12 , ℓ 23 and  are those corresponding to the variables in equations (1) and (2), and I is the quantity of X-ray absorption per unit time at each point (z) on the resist of the wafer 4, as the resist is exposed with an X-ray beam reflected at each point (y) on the mirror with a cylindrical reflection surface having a curvature radius ρ 0 . In the present embodiment, I 0 and ρ 0 are set beforehand and, in accordance with equations (3) and (4), the curvature at each point (y) of the reflection surface of the mirror 2 is determined successively, in an order from the vertex of the mirror 2 to the end, so as to assure that the X-ray absorption distribution of the resist in the exposure area becomes uniform such as depicted by the solid line in Figure 3. Figure 5 shows the change in the curvature ρ(y), wherein ρ 0 /ρ(y) is taken on the axis of ordinate. In Figure 5, the sign of the curvature ρ(y) is illustrated as being positive (ρ(y) > 0) when the curvature center is below the mirror 2 surface, while the sign of the curvature ρ(y) is illustrated as being negative (ρ(y) < 0) when the curvature center is above the mirror 2 surface. However, it will be readily understood from the drawing that in the present embodiment the shape of the reflection surface of the mirror 2 is determined in accordance with a range -0.5 < ρ 0 /ρ(y) < 1.5 and, additionally, ρ 0 /ρ(y) > 0 is satisfied. In the present embodiment, the reflection surface of the mirror 2 has such a shape that, with spacing from the top of the mirror 2, the surface gradually shifts from a cylindrical reflection surface having the same curvature radius as the curvature radius (ρ 0 at the portion adjacent the top (vertex) of the mirror 2). The curvature radius at a point off the optical axis and sufficiently spaced from the vertex, is larger than the curvature radius ρ 0 in the neighborhood of the vertex. Also, in order to ensure uniform X-ray absorption distribution of the resist in the exposure area, the left-hand and right-hand sides of the vertex of the reflection surface have asymmetrical shapes. The mirror 2 so shaped has a function for collecting peripheral portions of the X-ray flux from the synchrotron 1, which portions are off the axis and which can not be used for the exposure conventionally, toward the wafer 4 as well as a function for providing uniform X-ray absorption distribution of the resist on the wafer 4. As a result, it is possible to reduce the exposure time and also to assure uniform exposure, that is, accurate pattern transfer. As regards the uniformness of the X-ray absorption distribution, desirably it is such that the non-uniformness is not greater than 2 %, preferably, not greater than 0.2 %. Advantageous effects of the present embodiment will be explained by using specific numerical values. For determination of the intensity Ia of resist absorption, the integrated intensity Ir' as absorbed by the resist is determined as 23 mW/cm 3 . Here, the mirror material is SiO 2 , the mask material is Si 3 n 4 , the resist material is PMMA and the angle of incidence of the center beam is 10 mrad. Here, if the exposure area has a size of 3 cm square while the illumination area has a size of 6 cm square, then the quantity Ia of X-ray absorption can be determined by: Ia = I 0 = 24/6 = 4.0 (mW/cm 2 ) Here, the curvature radius ρ 0 of the mirror 2 in the neighborhood of the top thereof is 50 m, for example, and the shape of the reflection surface can be determined in accordance with equations (3) and (4). As compared therewith, if a mirror with a cylindrical reflection surface having a curvature radius of 50 m is used, at the opposite ends of the exposure area, the quantity Ic of X-ray absorption was 3.5 mW/cm 2 . As described, with the mirror 2 of the present embodiment, as compared with a mirror having a cylindrical reflection surface of the same curvature radius, the quantity of X-ray absorption is increased by 15 %. Thus, with simple comparison in respect to the intensity, the exposure time can be reduced to 87 % of that as required by a mirror having a cylindrical reflection surface. Figure 6 illustrates the results of comparison, in respect to the X-ray absorption distribution, between the resist used in this embodiment and a different type resist. In Figure 6, a broken line depicts the distribution as obtained with the resist (resist 1) used in the present embodiment, while a solid line depicts the distribution as obtained with a different type resist (resist 2). It is seen from the drawing that, with a different resist, a different distribution is provided, and there is a possibility of non-uniformness (hatched area). However, as shown in Figure 7, the non-uniformness resulting from the different resist (resist 2) is 3 % at the maximum, and this is very small as compared with the non-uniformness of 15 % in the case using a mirror having a cylindrical reflection surface. Thus, with a simple comparison in respect to the intensity, the exposure time can be reduced to 89 % of that as required when the mirror with cylindrical reflection surface is used. The concept of increasing the intensity and resultant reduction of exposure time, based on use of an aspherical surface shape of the reflection mirror, is not limited to a form satisfying equations (3) and (4). There is a certain effective range. But is, when the surface shape of the mirror that satisfies equations (3) and (4) is denoted by Zρ(y) and the surface shape of a mirror with a cylindrical reflection surface having a curvature radius ρ 0 , which is a basic curved surface on which the mirror of the present embodiment is based, is denoted by Z 0 (y), then the effective surface shape Z(y) can be represented by: Z(y) = Z 0 (y) + K(Zρ(y)-Z 0 (y)) wherein 0 < K ≦ 1.5 Figure 8 shows the quantity (intensity) distribution of X-rays are absorbed by the resist, with different values for "K". "K = 0" corresponds to the cylindrical reflection surface, while "K = 1" corresponds to the surface shape of the mirror. It is to be noted here that Z 0 (y) can be expressed, while taking the origin on the vertex, as follows: Z 0 (y) = ρ 0 2 -y 2 - ρ 0 While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the scope of the following claims.

Claims: An X-ray exposure apparatus, for exposing a substrate to a pattern of an original, including an X-ray mirror (2) shaped to expand the angular intensity distribution of X-ray radiation which is used for exposing the substrate and which is incident on the mirror (2) from an X-ray source (1), wherein the reflective surface of the mirror (2) is convex, characterised in that: the reflective surface of the mirror (2) has a vertex defined between two curved reflection surface portions, the radius of curvature of the curved reflection surface portion which during use of the apparatus lies closer to the X-ray source (1) being smaller than that of the curved reflection surface portion, on the other side of the vertex. An X-ray exposure apparatus according to Claim 1, further comprising: a radiation source (1) for providing X-rays (6); and said mirror (2) being included in an illumination system for irradiating a patterned original (3) and a substrate (4) arranged with respect to the X-rays such that a resist carried on the substrate is exposed through the pattern of the original to the X-rays; wherein said mirror (2) has a partially cylindrical reflection surface, said reflection surface of said mirror having an aspherical surface shape in that, with respect to the vertex the sides of the mirror nearer and further from the radiation source (1) are asymmetrical in shape, in that at a peripheral portion said reflection surface has a curvature of a radius larger than the portion of said reflection surface at the vertex of said reflection surface. An apparatus according to Claim 2, wherein said radiation source comprises a synchrotron radiation source (1). An apparatus according to Claim 2, wherein, in the neighbourhood of said vertex, said reflection surface has a radius of curvature P 0 ; wherein the intensity of the X-ray beam reflected by said vertex and then absorbed by the resist is I 0 ; wherein, when a y coordinate is defined along an axis corresponding to a tangent to said vertex taking said vertex as an origin the surface shape of said reflection surface is represented by a function Z(y); and wherein equations (1) - (5) below are substantially satisfied: Z(y) = Z 0 (y) + K(Z ρ (y)-Z 0 (y)) 0 < K ≤ 1.5 1/ρ = {L 12 /[L 23 (L 12 +L 23 )].C-(L 12 +L 23 )/(L 12 .L 23 )} x sin/2 C = I/I 0 .[1+(L 23 /L 12 )(1+2L 12 /ρ 0 sin)] x (L 12 +L 23 )/L 12 Z 0 (y) = √ ρ 0 2 -y 2 - ρ 0 where Z 0 (y) is the function which represents the surface shape of a cylindrical reflection surface of a curvature radius ρ 0 , corresponding to a basic curved surface on which the reflection surface of said mirror is based; Zρ(y) is a function which represents the surface shape of the reflection surface of said mirror that substantially satisfies the conditions (3) and (4), ρ is the radius of curvature at each point on the reflection surface of said mirror along the coordinate y;  is the angle of incidence of each X-ray beam impinging on each point on the reflection surface of said mirror along the coordinate y; L 12 is the distance between the emission center of said radiation source and each point on the reflection surface of said mirror along the coordinate y; L 23 is the distance between each point on the reflection surface of said mirror along the coordinate y and the point of incidence on the resist of each beam reflected by each point on the reflection surface; and I is the intensity of each X-ray beam as reflected by each point on the reflection surface along the coordinate y and absorbed by the resist as the resist is exposed with the X-rays reflected by the reflection surface having a surface shape of Z 0 (y). An apparatus according to Claim 4, wherein K = 1. An apparatus according to Claim 2, wherein, in the neighbourhood of said top, said reflection surface has a radius of curvature ρ 0 ; wherein the intensity of the X-ray beams reflected by said vertex and then absorbed by the resist is I 0 ; wherein, when a y coordinate is defined along an axis corresponding to a tangent to said vertex taking said vertex as an origin, the surface shape of said reflection surface is represented by a function Z(y); and wherein equations (1) and (2) below are substantially satisfied: 1/ρ = {L 12 /[L 23 (L 12 +L 23 )].C-(L 12 +L 23 )/(L 12 .L 23 )} x sin/2 C = I/I 0 .[1+(L 23 /L 12 )(1+2L 12 /ρ 0 sin)] x (L 12 +L 23 )/L 12 where: ρ is the curvature radius at each point on the reflection surface of said mirror along the coordinate;  is the angle of incidence of each X-ray beam impinging on each point on the reflection surface of said mirror along the coordinate; L 12 is the distance between the emission center of said radiation source and each point on the reflection surface of said mirror along the coordinate; L 23 is the distance between each point on the reflection surface of said mirror (2) along the coordinate and the point of incidence on the resist of each beam reflected by each point on the reflection surface; and I is the quantity of each X-ray beam as reflected by each point on the reflection surface and absorbed by the resist, as the resist is exposed with the X-rays reflected by the cylindrical reflection surface with a curvature radius ρ 0 , which corresponds to a basic curved surface on which the reflection surface of said mirror (2) is based.

Inventors: EBINUMA RYUICHI, FUKUDA YASUAKI, MOCHIZUKI NORITAKA, UNO SHINICHIRO, WATANABE YUTAKA, EBINUMA, RYUICHI, FUKUDA, YASUAKI, MOCHIZUKI, NORITAKA, UNO, SHINICHIRO, WATANABE, YUTAKA, EBINUMA, RYUICHI, C/O CANON KABUSHIKI KAISHA, FUKUDA, YASUAKI, C/O CANON KABUSHIKI KAISHA, MOCHIZUKI, NORITAKA, C/O CANON KABUSHIKI KAISHA, UNO, SHINICHIRO, C/O CANON KABUSHIKI KAISHA, WATANABE, YUTAKA, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: G03F7
IPC codes: G21K1, G03F7