WO2007013676A1

WO2007013676A1 - Exposure head and exposure apparatus

Info

Publication number: WO2007013676A1
Application number: PCT/JP2006/315350
Authority: WO
Inventors: Isao Tsuruma; Koichi Kimura
Original assignee: Fujifilm Corporation
Priority date: 2005-07-28
Filing date: 2006-07-27
Publication date: 2007-02-01
Also published as: JP2007033973A; TW200712796A

Abstract

An exposure head has a spatial light modulation device, such as a DMD (50), in which pixel portions for modulating irradiated light based on respective control signals is arranged. Multiple exposure is performed on the same position of a photosensitive material (150) with light from a plurality of pixel portions as the exposure head relatively moves with respect to the photoconductive material (150). A microlens array (55) including microlenses (56) arranged in an array is provided in an optical path between the spatial light modulation device and the photosensitive material (150), and each of the microlenses (56) condenses light emitted from respective pixel portions. Further, microlenses (56A through 56D) used for performing multiple exposure on the same position have different focal lengths from each other. An exposure apparatus includes the exposure head and a stage (152) for relatively moving the exposure head with respect to the photosensitive material (150).

Description

DESCRIPTION

EXPOSURE HEAD AND EXPOSURE APPARATUS

Technical Field

The present invention relates to an exposure head and an exposure apparatus . Particularly, the present invention relates to an exposure head which includes a spatial light modulation device for modulating light based on a control signal, and which exposes a photosensitive material to light by relatively moving with respect to the photosensitive material. The present invention also relates to an exposure apparatus in which suph an exposure head is provided.

Background Art Conventionally, various kinds of exposure heads for exposing a photosensitive material to light which has been modulated, based on image data, by a spatial light modulation device are well known. Further, various kinds of exposure apparatus for forming an image pattern on the photosensitive material by scanning the photosensitive material by the exposure head are well known. The spatial light modulation device is a device including a multiplicity of pixel portions formodulating irradiated light based on respective control signals, and the pixel portions are arranged side by side. An example of the spatial light modulation is a DMD (digital micromirror device) . The DMD is a mirror device in which a multiplicity of micromirrors is two-dimensionalIy arranged on a semiconductor substrate (base plate) , made of silicon or the like. In the DMD, the angle of a reflection plane of each of the micromirrors is changed based on a control signal. In Japanese Unexamined Patent Publication No .2003-337425 and Japanese Unexamined Patent Publication No. 2004-9595, an exposure apparatus using a DMD as a spatial light modulation device is disclosed. In the exposure apparatus, a microlens array is arranged in an optical path of light modulated by the spatial light modulation device . In the microlens array, microlenses are arranged in an array so as to correspond to respective pixel portions of the spatial light modulation device, and each of the microlenses condenses light emitted from the respective pixel portions. Further, in Japanese Unexamined Patent Publication No. 2004-9595, a technique in which a DMD is arranged so as to be inclinedwith respect to a scan direction, thereby causing the same scan line to be exposed to light more than once (multiple exposure) , is disclosed. This technique is intended to reduce non-uniformity in an image and to improve the resolution of the image by narrowing the intervals of scan lines in scanning. In an exposure apparatus using the microlens array, as described above, light modulated by a DMD is formed into dots by the microlenses in the microlens array. Then, the light which has been formed into the dots is projected onto a photosensitive material to expose the photosensitive material to light. In this case, resolution R depends on NA (numerical aperture) of a microlens, and the resolution R is represented by the following equation (1) :

R = kl X λ/NA (1) where kl-is a constant, λ is a wavelength, NA = n X sinθ, n is a refractive index, and θ is an angle across an exit pupil radius with respect to an image point. As equation (1) shows, it is necessary to increase the value of NA to improve the resolution by controlling the shape of a lens. Meanwhile, a focal depth Z is represented by the following equation (2) :

Z = k2 X λ/NA² (2) As equation (2) shows, the focal depth is shallower as the value of NA is larger.

As described above, when an exposure wavelength is constant, if the value of NA is increased to improve the resolution, the focal depth becomes shallower. If the focal depth is shallow, the amount of blur on an exposure surface, which is caused by a very small shift in position in the direction of an optical axis of light incident on the photosensitive material, increases. Therefore, a problem of deterioration in the sharpness of a pattern formed on the photosensitive material arises . Particularly, when a photosensitive layer is a thick substrate (base plate) , the problem caused by the shift in position is more substantial. These problems may be solved by providing an autofocus mechanism. However, additional mechanism, such as a substrate detection mechanism, a control mechanism or the like, is required to provide the autofocus mechanism, and the structure of the exposure apparatus becomes complex. Further, an additional problem, such as a rise in cost, arises.

Disclosure of Invention

In view of the foregoing circumstances, it is an object of the present invention to provide an exposure head with a substantially deep focal depth without using a complex structure while high resolution is maintained. It is also an object of the present invention to provide an exposure apparatus in which such an exposure head is provided. An exposure head according to the present invention is an exposure head comprising: a spatial light modulation device including a multiplicity of pixel portions formodulating irradiated light basedon respective control signals, wherein the pixel portions are arranged therein side by side, and wherein multiple exposure is performed on the same position of a photosensitive material with light from a plurality of pixel portions of the multiplicity of pixel portions by relatively moving with respect to the photoconductive material, and wherein a microlens array including microlenses arranged in an array is provided in an optical path between the spatial light modulation device and the photosensitive material, and wherein each of the microlenses condenses light emitted from respective pixel portions of the spatial light modulation device, and wherein at least two of the microlenses used for performing multiple exposure on the same position of the photosensitive material have different focal lengths from each other.

At least three of the microlenses used for performing multiple exposure on the same position may have different focal lengths from each other. Further, a difference in focal length betweenmicrolenses of which the focal lengths are close to each other may be set so as to be smaller as the focal lengths of the microlenses are shorter.

Alternatively, when at least three of the microlenses used for performing multiple exposure on the same position have different focal lengths from each other, a difference in focal length between microlenses of which the focal lengths are close to each other may be set so as to be smaller as the focal lengths of the microlenses are longer.

Here, the expression "have different focal lengths from each" represents that the focal lengths are different from each other but close to each other with respect to the direction of an optical axis such that when one of the microlenses is focused, the other microlens or microlenses are not focused and when one of the microlenses is not focused, the other or another microlens is focused.

An exposure apparatus according to the present invention is an exposure apparatus comprising: the exposure head as described above; and a movement means for relatively moving the exposure head with respect to the photoconductive material.

The exposure head according to the present invention performs multiple exposure on the same position with light transmittedthrough a plurality of microlenses which have different focal lengths from each other. Therefore, the light includes both light which is focused on a point to be exposed and light which is focused on a point which is slightly different from the point to be exposed in the direction of the optical axis. Meanwhile, light power density is high at the focal position of each of the microlenses, and the light power density decreases as a distance from the focal position increases. Therefore, if an exposure amount is set so that a pattern is formed by exposing the photosensitive material to light of high light power density at the focal position, the pattern is formed only with light which is focused on the point to be exposed. A pattern is not formed with light which is not focused on the point to be exposed. Therefore, even if the focal depth of each of the microlenses becomes shallow by increasing the value of NA of the microlenses to improve the resolution, it is possible to perform exposure with high resolution and with a substantially deep focal depth.

For example, if the lenses are refractive lenses, lenses which have different focal lengths from each other may be easily produced by changing the curvature of each of the refractive lenses. Therefore, in the present invention, it is possible to achieve substantially deep focal depth by using a simple method.

When microlenses are simplest plano-convex spherical lenses and the aperture diameters of the microlenses are constant, the focal depth of a microlens becomes shallower as the focal length of the microlens is shorter. Meanwhile, it is necessary that an arbitrary position of the photosensitive material with respect to the thickness direction of the photosensitive material is within the range of the focal depth of at least one of the microlenses to focus on the arbitrary position without failure. Specifically, it is necessary that the whole range of the photosensitive material with respect to the thickness direction of the photosensitive material is covered by the range of the focal depth of at least one of the microlenses . Therefore, when at least three of the microlenses used for performing multiple exposure on the same position have different focal lengths from each other, if a difference in focal length between microlenses of which the focal lengths are close to each other is set so as to be smaller as the focal lengths of the microlenses are shorter, it is possible to efficiently cover the whole range of the photosensitive material with respect to the thickness direction of the photosensitive material by the range of the focal depth of at least one of the microlenses.

Meanwhile, an absorption amount of exposure light by a photosensitive material is larger at a deeper position of the photosensitive material. Therefore, if a difference in focal length between microlenses of which the focal lengths are close to each other is set so as to be smaller as the focal lengths of themicrolenses are longer, it is possible to compensate a decrease in the light amount caused by absorption by the photosensitive material. Brief Description of Drawings

Figure 1 is a perspective view illustrating an exposure apparatus according to an embodiment of the present invention;

Figure 2 is a perspective view illustrating the structure of a scanner in the exposure apparatus illustrated in Figure 1;

Figure 3A is a plan view illustrating an exposed area formed on a photosensitive material;

Figure 3B is a diagram illustrating the arrangement of exposed areas formed by respective exposure head; Figure 4 is a schematic diagram illustrating a perspective view of an exposure head in the exposure apparatus illustrated in Figure 1;

Figure 5 is a cross-sectional view illustrating the exposure head illustrated in Figure 4; Figure 6 is a partially enlarged diagram illustrating the structure of a digital micromirror device (DMD) ;

Figure 7A is a diagram for explaining an operation of the DMD;

Figure 7B is a diagram for explaining an operation of the DMD;

Figure 8 is a diagram illustrating an exposed area formed by the DMD;

Figure 9 is a diagram illustrating the structure of a microlens array;

Figure 10 is a cross-sectional diagram illustrating microlenses which have different focal lengths from each other; Figure 11 is a diagram illustrating the microlenses in Figure 10 by superposing them one on another;

Figure 12 is a diagram illustrating another example of microlenses, which have different focal lengths from each other, by superposing them one on another; Figure 13 is a diagram illustrating another example of microlenses, which have different focal lengths from each other, by superposing them one on another;

Figure 14 is a diagram illustrating another example of arrangement of microlenses, which have different focal lengths from each other; and Figure 15 is a cross-sectional view illustrating the structure of another exposure head, to which the present invention can be applied.

Best Mode for Carrying Out the Invention

Hereinafter, an exposure head and an exposure apparatus according to an embodiment of the present invention will be described with reference to the attached drawings. The exposure head and the exposure apparatus according to the present embodiment perform multiple exposure with light transmitted through microlenses which have different focal lengths from each other. First, the structure of the whole exposure apparatus according to the present embodiment will be described. Figure 1 is a schematic diagram illustrating a perspective view of the exposure apparatus in the present embodiment . An exposure apparatus 100 in the present embodiment includes a stage 152, as illustrated in Figure 1. The stage 152 is a flat-plate-shaped movement means for holding a photosensitive material.150 on the surface thereof by suction. Further, two guides 158 extending along a stage movement direction are provided on the upper surface of a thick-plate-shaped base 158 supported by four legs 154. The stage 152 is arranged so that the longitudinal direction thereof directs in the stage movement direction. Further, the stage 152 is supported by the guides 158 so that back-and-forth movement of the stage 152 is allowed. Further, a stage drive (not illustrated) for driving the stage 152 as a sub-scan means along the guides 158 is provided in the exposure apparatus.

Further, a C-shaped gate 160 straddling a movement path of the stage 152 is provided at the center of the base 156. Each end of the C-shaped gate 160 is fixed onto either side of the base 156. Further, a scanner 162 is provided on one side of the gate 160, and a plurality of sensors 164 (for example, two sensors) for detecting a leading edge and a rear edge of the photosensitive material 150 is provided on the other side of the C-shaped gate 160. Each of the scanner 162 and the sensors 164 is attached to the gate 160, and placed at a fixed position above the movement path of the stage 152. Therefore, as the stage 152 moves, the scanner 162 and the sensors 164 move relative to the photosensitive material 150. Further, each of the scanner 162 and the sensors 164 are connected to a controller or controllers (not illustrated) for controlling them. The scanner 162 includes a plurality of exposure heads 166 (for example, eight exposure heads) , as illustrated in Figures 2 and 3B. The plurality of exposure heads 166 is arranged substantially in a matrix form of m rows X n columns (for example 2 rows X 4 columns) . Hereinafter, an exposure head arranged in the m-th row of the n-th column is represented by an exposure head 166mn.

An exposure area 168, which is an area exposed to light by an exposure head 166, is a rectangular area with its shorter side directed in the sub-scan direction, as illustrated in Figure 2. The exposure area 168 is inclined at a predetermined inclination angle θ with respect to the sub-scan direction. Aband-shaped exposed area 170 is formed on the photosensitive material 150 by each of the exposure heads 166- as the stage 152 moves. Here, the sub-scan direction and the stage movement direction are opposite to each other, as illustrated in Figures 1 and 2. Hereinafter, an exposure area formed by an exposure head arranged in the m-th row of the n-th column is represented by an exposure area 168mn.

Further, as illustrated in Figures 3A and 3B, the exposure heads 166 which are linearly arranged in each row are arranged so as to be shifted from those in another row, by apredetermined distance, in the arrangement direction of the exposure heads 166. The exposure heads 166 are shifted so that adjacent band-shaped exposed areas 170 partially overlap with each other. Therefore, an unexposed portion between an exposure area 168u and an exposure area 168i₂ in the first row can be exposed to light by an exposure area 168₂i in the second row.

Each of the exposure heads 166 includes a digital micromirror device (hereinafter, referred to as a DMD) 50 as a spatial light modulation device. The spatial light modulation device modulates, based on image data, a light beam which is incident thereon for each pixel portion. The DMD 50 is connected to a controller (not illustrated) including a data processing unit and a mirror drive control unit. The data processing unit in the controller generates, based on input image data, a control signal for controlling drive of each of the micromirrors in an area of the DMD 50 for each of the exposure heads 166, and the area of the DMD 50 is an area to be controlled. Further, the mirror drive control unit controls, based on the control signal generated by the image data processing unit, the angle of a reflection plane of each of the micromirrors of the DMD 50 for each of the exposure heads 166. Control of the angle of the reflection plane will be described later.

Further, a fiber array light source 66, a lens system 67 and a mirror 69 are arranged in this order on the light-entering side of the DMD 50. The fiber array light source 66 includes a laser emission portion 68 in which emission ends (light emitting points) of optical fibers are arranged in a row along a direction corresponding to the longitudinal direction of the exposure area 168. The lens system 67 condenses laser light onto the DMD by correcting the laser light emitted from the fiber array light source 66. The mirror 69 reflects the laser light transmitted through the lens system 67 toward the DMD 50. In Figure 4, the lens system 67 is schematically illustrated.

The lens system 67 includes a condensing lens 71, a rod-shaped optical integrator (hereinafter, referred to as a rod integrator) 72 and a collimator lens 74, as illustrated in Figure 5 in detail. The condensing lens 71 condenses laser light B, which is illumination light emitted from the fiber array light source 66. The rod integrator 72 is inserted to an optical path of light transmitted through the condensing lens 71. The collimator lens 74 is placed on the light-emitting side of the rod integrator 72. The condensing lens 71, the rod integrator 72 and the collimator lens 74 cause the laser light emitted from the fiber array light source 66 to enter the DMD 50 as a light flux of substantially parallel light, of which the intensity within a cross section of the beam is uniform.

The laser light B which is emitted from the lens system 67 is reflected by the mirror 69 and transmitted through a TIR (total internal reflection) prism 70. Then, the DMD 50 is irradiated with the light transmitted through the TIR prism 70. In Figure 4, the TIR prism 70 is omitted.

The DMD 50 is a mirror device in which a multiplicity (for example, 1024 X 768) of very small mirrors (micromirrors) 62 is arranged in a grid on an SRZM (static random access memory) cell

(memory cell) 60. Each of the multiplicity of micromirrors 62 forms a pixel portion (pixel) . In the DMD 50, a micromirror 62 supported by a support post is provided on the top of each of the pixels. A material such as aluminum, which has high reflectance, is deposited on the surface of the micromirror 62 by evaporation. The reflectance of the micromirror 62 is greater than or equal to 90%, and the arrangement pitch of the micromirrors 62 is, for example, 13.7μm both with respect to a vertical direction and with respect to a horizontal direction. Further, an SRAM cell 60 of CMOS (complementary metal oxide semiconductor) of a silicon gate is arranged directly below the micromirror 62 through a support post including a hinge and a yoke . The SRZM cell 60 is manufactured in an ordinary production line of semiconductor memories . The whole DMD is monolithically structured.

When a digital signal is written in the SRZ\M cell 60 of the DMD 50, each micromirror 62 supported by a support post is inclined with respect to a diagonal line of the micromirror 62. The micromirror 62 is inclined at an angle within a range of ±a. degrees (for example, ±12 degrees) with respect to the substrate on which the DMD 50 is placed. Figure 7A is a diagram illustrating a micromirror 62 in an ON state, and the micromirror 62 is inclined at +α degrees. Figure 7B is a diagram illustrating a micromirror 62 in an OFF state, and the micromirror 62 is inclined at -α degrees. Therefore, the laser light B which has entered the DMD 50 is reflected to the inclination direction of each of the micromirrors 62 by controlling, based on an image signal, the inclination of each of the micromirrors 62 for each pixel of the DMD 50, as illustrated in Figure 6.

In Figure 7, a part of the DMD 50 is enlarged. Figures 7A and 7B illustrate examples of a state in which each of the micromirrors 62 is controlled so as to be inclined either at +a degrees or at -a. degrees . ON/OFF control of each of the micromirrors 62 is performed by a controller (not illustrated) connected to the DMD 50. Further, an absorption material (not illustrated) is arranged at a position in the traveling direction of laser light B reflectedby amicrorαirror 62 in an OFF state.

Here, it is assumed that the DMD 50 is formed by K block areas, each including micromirrors 62 of L rows X M columns. In Figure 8, an exposure area 168, which is a two-dimensional image obtained by a single DMD 50, and the block areas in the exposure area 168 are illustrated. In Figure 8, rows and columns which are less than actual rows and columns are illustrated to simplify illustration. In Figure 8, L = 4, M= 16, K= 4, and four block areas are block A, block B, block C and block D. As illustrated in Figure 8, the DMD 50 is inclined so that the exposure area 168 is inclined at an inclination angle θ (θ =

±tan^""1 (k/L) ) withrespect to the sub-scan direction. Here, k is a natural number which is relatively prime to L or a number equal to L. Since the exposure area 168 is inclined, the pitch of scan paths (scan lines) of an exposure beam formed by each of the micromirrors 62 becomes narrower than the pitch in a case in which the exposure area 168 is not inclined. Therefore, it is possible to improve resolution.

Since the inclination angle θ of the exposure area 168 is set to ±tan^"1 (k/L) with respect to the sub-scan direction, the same scan line is scanned with light reflected by a plurality of micromirrors 62. For example, in Figure 8, scan line Ll is scanned with four reflection light images (exposure beams) indicatedby black circles. Specifically, as the exposure head 166 relatively moves (sub-scanning) with respect to the photosensitive material 150, the same position on the photosensitive material 150 is exposed to light multiple times with light reflected by a plurality of micromirrors 62.

As illustrated in Figures 4 and 5, an imaging optical system 51 for forming an image on the photosensitive material 150 with laser light B reflected by the DMD 50 is provided on the light-reflection side of the DMD 50. The imaging optical system 51 is schematically illustrated in Figure 4. As illustrated in detail in Figure 5, the imaging optical system 51 includes an optical system, a microlens array 55 and an aperture array 59. The optical system includes lens systems 52 and 54, and light transmitted through the optical system is incident on the microlens array 55.

The optical system including the lens systems 52 and 54 magnifies an image formed by the DMD 50, and the magnified image is formed on the microlens array 55. In the microlens array 55, a multiplicity of microlenses 56 corresponding to respective pixel portions of the DMD is two-dimensionally arranged. Each of the multiplicity of microlenses 56 condenses light emitted from the respective pixel portions of the DMD 50. Each of the microlenses 56 is arranged at a position at which the laser light B from a corresponding micromirror 62 is incident thereon. Further, the microlens 56 is placed in the vicinity of an image formation position of the corresponding micromirror 62, at which an image is formed by the lens systems 52 and 54. In the aperture array 59, amultiplicity of apertures (openings) corresponding to respective microlenses 56 of the microlens array 55 is formed. In Figure 5, the photosensitive material 150 is sub-scanned in the direction indicated by arrow F.

The microlens array 55 is divided into four block areas corresponding to the block areas A, B, C and D of the DMD 50, as illustrated in Figure 9. The focal length of the microlenses in each block area is different from that of the microlenses in the other block areas . Hereinafter, microlenses arranged in each of the block areas A, B, C and D are referred to as microlenses 56A, 56B, 56C and 56D, respectively. In Figure 9, rows and columns which are less than actual rows and columns are illustrated to simplify illustration.

As schematically illustrated in Figures 10 and 11, the focal lengths of microlenses become longer in the order of the microlenses 56A, 56B, 56C and 56D. Further, the range of the focal lengths substantially includes a distance from the upper surface to the lower surface of a photosensitive layer 150a of the photosensitive material 150, and a difference in the focal length is a substantially equal distance. In Figure 11, the four kinds- of microlenses, namely microlenses 56A, 56B, 56C and 56D, are superposed one on another. The arrangement direction of the microlenses in the microlens array 55 with respect to the sub-scan direction is also inclined at an inclination angle θ in a manner similar to inclination of the DMD 50, as described above. Further, in the microlens array 55, the scan line Ll is also scanned with a single microlens of each of the block areas A, B, C and D, namely a microlens 56A, a microlens 56B, a microlens 56C and a microlens 56D, as indicated by black circles in Figure 9. Therefore, the same position on the photosensitive material 150 is exposed to light multiple times. The same position on the photosensitive material 150 is exposed to light transmitted thorough the microlenses 56A, 56B, 56C and 56D, which have different focal lengths from each other.

An optical power density is high at a focal position of each microlens. The optical power density decreases as a distance from the focal position becomes longer. Therefore, if an exposure amount is set so that the photosensitive material reacts to light by exposure to light at the optical power density of the focal position, a pattern is formed onlybymicrolenses which are focused onpoints to be exposed. A pattern is not formed by unfocused microlenses.

When the number of times of exposure in multiple exposure is large, there is a possibility that the accumulation amount of exposure only by unfocused microlenses exceeds a threshold value of reaction to exposure, at which the photosensitive material reacts to light and a pattern image is formed. Then, the resolution may deteriorate. In such a case, the exposure amount should be set so that the accumulation amount of exposure only by the unfocused microlenses does not exceed the threshold value of reaction to exposure and so that the accumulation amount of exposure by the focused microlens and exposure by the unfocused microlenses exceeds the threshold value of reaction to exposure. Accordingly, a result similar to the result as described above can be achieved. When the NA. (numerical aperture) of each of the microlenses is increased to improve the resolution, the focal depth of each of the microlenses becomes shallower. However, if multiple exposure is performed using microlenses which have different focal lengths from each other, as described above, it is possible to perform exposure with high resolution and with substantially deep focal depth.

Next, an example of a method for designing and producing the microlenses which have different focal lengths from each other will be described. If a plano-convex spherical lens, which can be easily produced, is adopted as a microlens, the focal length f of the lens is simply determined only by a refractive index n and a curvature radius r, and satisfies f = r/ (n - 1) . For example, when it is assumed that quartz glass with a refractive index of approximately 1.46 is used as a material for the microlens and that the microlens is a plano-convex spherical lens with a focal length of lOOum, the curvature radius is 46μm. Further, if the exposure wavelength is 405n, NA = 0.26 and a constant k2, which is determined by resist and/or development process, is 0.2, the focal depth is approximately 1.2um.

Here, use of lenses, of which the curvature apertures are for example 44um, 45um and 47um, is assumed based on the lens with the curvature aperture of 46um. The curvature apertures 44um, 45um and 47um are curvature apertures which are slightly different from the curvature aperture of 46um. For each of these lenses, an aperture which has the same diameter is arranged, and focal length and a focal depth are calculated. The calculation result is shown in Table 1.

[Table 1]

Such lenses are produced by forming lens shapes using resist.

The lens shapes are formed by using a method of producing lens shapes which have different curvature radiuses from each other, on a quartz glass substrate, by controlling a well-known gray scale gradation pattern for each block area. Alternatively, the lens shapes are formed by using a method of performing thermal reflow by changing the size of a resist pattern for each block area. Then, ICP

(inductively coupled plasma) dry etching is performed with fluorine-based mixed gas, such as CF₄, C2F15 and CHF₃, to transfer the lens shape onto the quartz glass. At this time, a selection ratio between the resist and the quartz glass maybe adjustedby controlling bias, the temperature of the substrate and/or the mixture ratio of the etching gas. Accordingly, a shape which has the curvature radius, as described above, can be obtained. In the above embodiment, the focal length is changed at substantially equal intervals. However, it is not necessary that the focal length is changed in such a manner. It is preferable that the focal length is appropriately changed, based on various kinds of condition, such as the kind of the photosensitive material and exposure process. When the focal lengths of at least three microlenses among the microlenses which are used for multiple exposure on the same position are different from each other, the focal length may be set based on a difference in the focal length.

When the aperture diameters of the plano-convex spherical lenses are constant, the focal depth is shallower as the focal length is shorter. This characteristic may be utilized so as to cover the thickness of the photosensitive material by the focal depths of a plurality of microlenses. The thickness of the photosensitive material may be covered by setting a difference in focal length between microlenses of which the focal lengths are close to each other becomes smaller as the focal lengths of the microlenses are shorter. Specifically, a number is assigned to each of a plurality of microlenses which have different focal lengths from each other in an ascending order of focal length. When the focal length of a microlens which has the i-th (i = 1, 2, 3, ...) shortest focal length is fi, the focal length is determined so as to satisfy the following equation:

Fi+l - fi < fi+2 - fi+_l

In Figure 12, these fourmicrolenses, namelymicrolenses 56A' through 56D' , are schematically illustrated by superposing them one on another.

Alternatively, a characteristic that an absorption amount of exposure light by a photosensitive material is larger at a deeper portion of the photosensitive material. In that case, the focal lengths may be set so that a difference in focal lengths between microlenses of which the focal lengths are close to each other becomes smaller as the focal length of the microlenses is longer.

Specifically, a number is assigned to each of a plurality of microlenses in an ascending order of focal length. When the focal length of a microlens which has the i-th (i = 1, 2, 3, ...) shortest focal length is f_±, the focal length is determined so as to satisfy the following equation:

In Figure 13, these fourmicrolenses, namelymicrolenses 56A' through 56D' , are schematically illustrated by superposing them one on another.

If the structure of the microlens is appropriately selected, as described above, it is possible to cope with various kinds of resist and various kind of exposure process. Further, the microlens to be used is not limited to the plano-convex spherical lens, as described above. A microlens which has a different shape may be used. Particularly, a nonspherical lens, a refractive-index-distribution-type lens (a gradient index lens) , a diffraction optical device or the like may be adopted. Further, in the above embodiment, the microlens array was divided into four block areas. However, it is not necessary that the microlens array is divided into four blocks. The microlens array may be divided into blocks areas which are more than or equal to two. It is preferable that the number of the block areas is set based on intended patterning process . Further, in the present embodiment, the focal length of microlenses in each of theblock areas is different from that of microlenses in the other block areas. However, when the number of the block areas is large, the focal length ofmicrolenses in some of the block areas may be the same . Further, it is not necessary that the microlens array is always divided into block areas in a matrix form. It is sufficient if at least two of the microlenses which are used to perform multiple exposure on the same position have different focal lengths from each other. Therefore, the microlenses in the microlens array may be arranged in various manners. For example, microlenses which have different focal lengths from each other may be arranged alternately along the path of the scan line. Alternatively, as illustrated in Figure 14, four kinds of lenses a, b, c and d, which have different focal lengths from each other, may be arranged in a so-called hound' s-tooth form so that microlenses which have the same focal length are aligned in a diagonal direction. When the microlens array is formedwith lenses with the same NAandwith different focal lengths, if the outer diameter of each of the microlenses is different from each other, it is preferable that the microlenses are arranged by considering the size of the outer diameter of each of the microlenses . Especially when a microlens is formed by performing thermal reflow by changing the resist pattern size, a hound' s-tooth arrangement in which a microlens with a large outer diameter and a microlens with a small outer diameter are arranged next to each other is effective to avoid contact between the adjacent microlenses.

Further, the present invention can be also applied to an exposure head illustrated in Figure 14. In the exposure head illustrated in Figure 14, the imaging optical system 51 in the above embodiment is replaced by an imaging optical system 51' . In the imaging optical system 51' , an optical system including lens systems

57 and 58 for changing a magnification ratio or the like is inserted between the microlens array 55 and the photosensitive material 150.

In the above example, an exposure head in which a DMD is provided as a spatial lightmodulation devicewas described. However, a transmissive-type spatial light modulation device (LCD: liquid crystal display) may be used instead of the reflective-type spatial light modulation device, such as the DMD. For example, anMEMS (Micro

Electro Mechanical Systems) type spatial light modulation device

(SLM: Spatial Light Modulator) or a spatial light modulation device other than the MEMS type spatial light modulation device may be used.

The spatial light modulation device other than the MEMS type is a spatial light modulation device, such as an optical device (PLZT

(lead lanthanum zirconate titanate) device) for modulating transmission light byutilizing an electric-optic effect and a liquid crystal shutter array such as a liquid crystal light shutter (FLC: ferroelectric liquid crystal) . Here, the term "MEMS" is a general term for a micro-system using a micro-machining technique based on IC production process. The MEMS is a system in which a micro-size sensor, a micro-size actuator and a micro-size control circuit are integrated. Further, an MEMS-type spatial light modulation device refers to a spatial light modulation device which is driven by an electricmechanic operation utilizing static electric force . Further, the spatial light modulation device may be formed by two-dimensionally arranging a plurality of GLV s (Grating Light Valve) . When the reflective-type spatial light modulation device (GLV) or the transmissive-type spatial light modulation device (LCD) is used, a lamp or the like may be used as a light source instead of the laser.

Claims

1. An exposure head comprising: a spatial light modulation device including a multiplicity of pixel portions for modulating irradiated light based on respective control signals, wherein the pixel portions are arranged therein side by side, and wherein multiple exposure is performed on the same position of a photosensitive material with light from a plurality of pixel portions of the multiplicity of pixel portions by relatively moving with respect to the photoconductive material, and wherein a microlens array including microlenses arranged in an array is provided in an optical path between the spatial light modulation device and the photosensitive material, and wherein each of the microlenses condenses light emitted from respective pixel portions of the spatial light modulation device, and wherein at least two of the microlenses used for performing multiple exposure on the same position of the photosensitive material have different focal lengths from each other.

2.. An exposure head, as defined in Claim 1, wherein at least three of the microlenses used for performing multiple exposure on ^■ the same position have different focal lengths from each other, and wherein a difference in focal length between microlenses of which the focal lengths are close to each other is set so as to be smaller as the focal lengths of the microlenses are shorter.

3. An exposure head, as defined in Claim 1, wherein at least three of the microlenses used for performing multiple exposure on the same position have different focal lengths from each other, and wherein a difference in focal length between microlenses of which the focal lengths are close to each other is set so as to be smaller as the focal lengths of the microlenses are longer.

4. An exposure apparatus comprising: an exposure head, as defined in any one of Claims 1 to 3; and a movement means for relatively moving the exposure head with respect to the photoconductive material.