Fly's eye lens unit for illumination system

An illumination system includes a light source, an ellipsoidal mirror for converging light emitted from the light source, a fly's eye lens unit disposed at a converging point of the ellipsoidal mirror, a collimation lens for collimating light rays exited from the fly eye lens unit, and an object disposed along the optical path of the illumination system. The fly's eye lens unit includes a first and a second frame defining an incident aperture and an exit one, a plurality of element lenses aligned in a matrix of rows closely in contact with each other, side walls disposed between the first and second frames for housing the element lenses therein, and biasing element for applying a biasing force to the element lenses sideward with respect to the rows of matrix. The respective element lenses are fixedly held without any medium therebetween.

BACKGROUND OF THE INVENTION 
The present invention relates generally to a fly's eye lens unit for an 
illumination system, and more particularly to an assemblage of element 
lenses for constituting an efficient fly's eye lens unit for use in 
yielding a uniform irradiance distribution on an object. 
It is well known that a fly's eye lens unit is used in an illumination 
system for use in a proximity or contact exposure of integrated circuit 
patterns, printed wiring circuit patterns or the like. One of such 
conventional illumination systems is shown in U.S. Pat. No. 3,296,923 
issued to Miles, in which the illumination system includes a light source, 
a ellipsoidal mirror disposed with respect to the light source, a 
collimating lens for collimating the light rays emitted from the light 
source, a cold mirror placed on the optical axis of the system for 
reflecting the thus collimated light rays, two lenticular lenses with a 
fly's eye lens structure for a converging effect on the propagated light 
and an object lens for applying a uniform illumination on an object to be 
illuminated. 
In order to constitute such a fly's eye lens unit as employed in the Miles 
patent, it is necessary to dispose element lenses of the unit at precise 
locations in two dimensions perpendicular to the optical axis of the 
illumination system. Moreover, as can be seen in the Miles patent, it is 
often required that more than one fly's eye lens unit be used in the form 
of multiple structure (hereinafter the word "fly's eye lens unit" refers 
to such a multiple structure of the matrix of element lenses as well as a 
single structure thereof). In this case, each element lens must be 
precisely disposed in two dimensions perpendicular to the optical axis, 
and furthermore each element lens of the matrices must precisely be 
disposed along the optical axis of the illumination system. 
Loss of light should also be considered, when assembling element lenses 
into a fly's eye lens unit. That is, if the light beam from light source 
is blocked by any elements constituting the fly's eye lens unit, (e.g. a 
lens holder for holding respective element lenses or adhesive agent for 
applying the adhesive effect to respective lenses) light intensity or 
quantity should be compensated, which will apparently be disadvantageous. 
Even if the loss of light can be compensated by increasing the light 
intensity or quantity of the light source, another problem, i.e. of heat, 
may be caused. The problem of heat may, of course, be caused even when the 
light intensity or quantity of the light source is not increased. Since 
the fly's eye lens unit is often placed at a position that the light beam 
from the light source is converged, the fly's eye lens unit is heated 
considerably. If any elements of the fly's eye lens unit (e.g. metallic 
lens holder elements and element lenses) have different coefficients of 
thermal expansion, a mechanical distortion may occur, which affects the 
optical characteristics of the illumination system. 
SUMMARY OF THE INVENTION 
Accordingly, it is a principal object of the present invention to provide a 
novel fly's eye lens unit suitable for an illumination system which 
illuminates an object with a uniform irradiance distribution. 
It is another object of the invention to provide a fly's eye lens unit 
which is easy for assembling element lenses into a single unit and each 
element lens of which is precisely positioned therein. 
It is a further object of the invention to provide a fly's eye lens unit 
which resolves the problem of heat, where the optical characteristics is 
kept substantially constant, even if the thermal expansion may occur in 
the fly's eye lens unit. 
It is a further object of the invention to provide an illumination system 
useful for applying a uniform illumination to an object. 
The aforementioned objects are accomplished by the present invention, with 
a fly's eye lens structure which includes a first frame for defining an 
incident aperture of the fly's eye lens thereon, a second frame for 
defining an exit aperture of the fly's eye lens unit thereon, a plurality 
of element lenses orderly aligned in two-dimensions, said element lenses 
forming a plurality of rows extending in a predetermined direction and 
being closely in contact with each other, side walls disposed between said 
first and second frames for housing said element lenses therein, and 
biasing means for applying a biasing force to the element lenses sideward 
with respect to the rows of the element lenses. 
The aforementioned fly's eye lens structure is particularly adaptable for 
an illumination system comprising a light source, converging means 
disposed with respect to said light source for converging light emitted 
from the light source, a fly's eye lens unit disposed approximately at a 
converging point of the converging means, collimating means for 
collimating light rays exited from the fly's eye lens unit and an object 
disposed along the optical path of the illumination system. 
According to a preferred embodiment, a spacer means is disposed closely 
between the respective side walls and element lenses. Preferably, the 
spacer means has heat insulating characteristics and elasticity adaptable 
for absorbing the thermal expansion of elements of the fly's eye lens 
unit. The spacer means comprises, for example, a resin plate made of 
fluorine-containing polymers. 
According to a preferred embodiment, a plurality of bores are provided on 
at least one of the side walls, said bores being defined at positions 
corresponding to the respective rows of the element lenses, and the 
biasing means is accommodated in the respective bores. 
Preferably, the biasing means comprises a contact member for contacting 
with an element lenses aligned in a corresponding row of the element 
lenses, a biasing coil spring for applying a biasing force to a 
corresponding row of the element lenses and adjusting means for adjusting 
the biasing force. The contact member has, preferably, heat insulating 
characteristics and elasticity adaptable for absorbing the thermal 
expansion of elements of the fly's eye lens unit. 
Practically, it is preferable to dispose the element lenses in a matrix of 
rows, and the element lenses are formed in a plurality of matrices. 
Preferably, both the incident aperture and the exit aperture defined on the 
first and second frames have enough inner diameter not to block light rays 
received by and passing through the fly's eye lens unit. 
Having the aforementioned features, the present invention has the following 
useful advantages: 
Assembling of a fly's eye lens unit can easily be accomplished with precise 
positioning of the respective element lenses. 
Light entering and exiting from the fly's eye lens unit is not be blocked, 
since there is no intermediate between the respective element lenses, and 
accordingly substantially all light rays directed toward the fly's eye 
lens unit can be applied to an object to be illuminated without any loss 
of light. 
The problem of heat can efficiently be resolved, that is, if the fly's eye 
lens is considerably heated, the heat transmission can efficiently be 
blocked, and, even if the thermal expansion may occur in the fly's eye 
lens unit, such thermal expansion can efficiently be absorbed without 
affecting the optical characteristics of the fly's eye lens. 
Other novel features and advantages of the present invention will become 
apparent in the course of the following detailed description taken 
together with the accompanying drawings, which are directed only to the 
understanding of the present invention and not to the restriction of the 
scope of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, which shows an illumination system embodying the 
present invention, the illumination system generally comprises a light 
source LS, an ellipsoidal mirror EM disposed with respect to the light 
source, a first folding mirror M.sub.1, a fly's eye lens unit FU, a second 
folding mirror M.sub.2, a collimation system CS, a mask film MF bearing 
desired circuit patterns thereon and a photosensitive material PM, 
respectively disposed along the optical pat in order. 
The light source LS is positioned at or adjacent to the first focal point 
of the ellipsoidal mirror EM, so that light emitted from the light source 
is condensed and directed to the fly's eye lens unit FU through the 
folding mirror M.sub.1. The folding mirror M.sub.1 comprises so-called a 
cold mirror which allows most of infrared radiation to pass therethrough 
and reflects most of ultraviolet radiation. 
The fly eye lens unit FU is positioned at a point adjacent to the second 
focal point of the ellipsoidal mirror E M, which is preferred in order to 
form the secondary light source thereby, since light emitted from the 
light source is most condensed at this point. However, it is sufficient if 
the fly's eye lens unit FU is disposed at a position optically nearer to 
the light source LS, as long as the light directed toward the fly's eye 
lens unit FU is received within the effective area thereof. Light 
condensed by the ellipsoidal mirror EM enters the fly's eye lens unit FU 
to impinge upon the incident surface of the collimation system CS. 
The characteristics of the fly's eye lens unit FU as an optical system is 
designed so that the following equation is substantially established: 
EQU h=k tan .theta. 
where h is an entrance height of a light beam, k is a proportional constant 
and .theta. is an exit angle from the fly's eye lens unit. 
One such an optical system having the aforementioned characteristics, the 
detailed description is given in the co-pending U.S. Pat. Application Ser. 
Nos. 3,957 filed on Jan. 16, 1987 and 62,440 filed on June 12, 1987. 
Accordingly, the detailed discussion thereof is omitted here. 
The fly's eye lens structure is very useful for resolving the irradiance 
non-uniformity at the entrance pupil of such optical system, which enables 
the incident surface of the collimation system CS to be illuminated 
uniformly independent of the irradiance distribution at the entrance 
pupil. Moreover, respective element lenses of the fly's eye lens unit 
provide uniform irradiance through the entire area of the incident surface 
of the collimation system, thus the irradiance distribution on the 
incident surface of the collimation system will become uniform. 
The illumination system shown in FIG. 1 is designed so that the entrance 
pupil of the fly's eye lens unit FU is conjugated with the incident 
surface of the collimation system CS, accordingly light passing through a 
predetermined common point in the entrance pupil will impinge upon a 
predetermined common point on the incident surface of the collimation 
system, and consequently light emitted from the light source and received 
in the entrance pupil will illuminate the predetermined area of the 
incident surface of the collimation system, without any loss of light. 
The collimation system CM comprises a single Fresnel lens, which is 
advantageous to reduce the optical path between the light source LC and an 
object to be illuminated. The photosensitive material PM is positioned 
immediately under the mask film MF, and exposure of a desired circuit 
pattern is made accurately and effectively on the photosensitive material. 
Referring to FIGS. 2 and 3, which show a structure outline of a first 
embodiment of a fly's eye lens unit FU according to the present invention, 
the fly's eye lens unit generally comprises a base frame 11 defining a 
square opening 12, side walls 13, 16, 22 and 27, spacer plates 14, 23 and 
26, a multiple structure of element lenses 31 to 34, and a lid plate 24 
defining a circular opening 25 therein. 
The base frame 11 is made of aluminum alloy, and the square opening 12 
defined thereon is adapted to pass light rays from a light source. The 
square opening 12 defines an exit aperture of the fly's eye lens unit FU, 
which has a large inner diameter so as not to block the light beam exited 
from the fly's eye lens unit. 
The respective side walls 13, 16, 22 and 27 are also made of aluminum 
alloy, and are secured upright on the respective peripheral edges of the 
base frame 11. On the side wall 16, there defined are small bores suitable 
for allowing the insertion of screws 15 therethrough. Each of the bores 17 
has a threaded section 17s and a non-threaded cylindrical section 17c 
therein. 
The threads provided in the threaded section 17s is designed to be 
engageable with those of the screw. The non-threaded section 17c has a 
larger diameter than that of the threaded section 17s. The non-threaded 
section 17c accommodates a biasing means comprising a first metal disk 
18, a coil spring 19, a second metal disk 20 of the same shape as that of 
the first disk 18, and a contact element 21 made of fluorine-contained 
polymers, in order. The respective bores 17 on the side wall 16 are 
positioned at positions corresponding to respective rows of the element 
lenses aligned in respect of the corresponding screws 15. 
The respective spacer plates 14, 23 and 26 are made of fluorine-containing 
polymers, and are closely disposed between the respective side walls 1 3, 
22 and 27 and the assembly of the element lenses 31 to 34, respectively. 
It is preferable to chose fluorine-containing polymers as the spacer 
plates because of their elasticity and heat insulating characteristics. 
However, it may be also applicable to use any other materials having 
appropriate elasticity for absorbing the thermal expansion either of the 
element lenses or metallic base frame and side walls 13, 22 and 27, and 
having appropriate heat insulating characteristics for blocking the heat 
transmission from the metallic base frame and side walls to the element 
lenses, or vice versa. 
On the inner surface of the respective side walls, there provided are a 
plurality of grooves 35 extending in the vertical direction. The positions 
of the grooves 35 are determined so as to correspond to respective 
adjacent element lenses on the matrices. Air existing between respective 
matrices of the element lenses can freely communicate with the outside 
thereof. 
Respective element lenses 31e to 34e constitute four units of matrices 31 
to 34, and each of these matrices includes 5 x 5 pieces of element lenses 
EM. Each matrix of the element lenses is maintained in position only by 
the biasing forces of the coil springs 19, without any adhesive agent or 
the like. The biasing force to be applied to each row of element lenses by 
the corresponding screw 15 is adjusted by the screw, and according to 
experimental demonstrations it was preferable to apply the biasing force 
in the range of 1.0 to 1.5 kg to respective rows of the element lenses. 
The element lenses in each of the matrices have substantially the same 
optical characteristics. Each of the element lenses EL has an incident 
surface and an exit surface having a predetermined curvature, 
respectively, and four plane side surfaces, as best shown in FIGS. 4-A and 
4-B It is especially preferable to form a precise square by the side 
surfaces in the plane view. The adjacent element lenses in each matrix are 
closely in contact with each other, without any intervening medium. 
The adjacent matrices disposed along the optical axis are spaced from each 
other by a predetermined distance, where respective optical axes of the 
element lenses EL in each matrix are precisely coincident with each other. 
The distance between the adjacent matrices is determined on the basis of 
the optical characteristics on an object to be illuminated. 
The lid plate 24 is also made of aluminum alloy, and defines an circular 
opening 25 thereon. The circular opening 25 defines an incident aperture 
of the fly's eye lens unit FU, where the inner diameter of the opening 25 
is designed so that the light beam converging on the fly's eye lens unit 
should not be blocked by the lid plate. 
In the aforementioned embodiment, assemblage of the respective element 
lenses is accomplished only by the biasing forces of the biasing coil 
springs 19, which are applied to the rows aligned with the screws from a 
longitudinal direction and are appropriately adjusted only by manipulating 
respective screws. 
Referring to FIG. 5, which shows a second embodiment of a fly's eye lens 
unit FU according to the present invention, the same reference numerals as 
in FIG. 3 denote the same elements. In this embodiment, a side wall 27' is 
formed substantially in the same manner as the side wall 16, and the 
spacer plate 26 in FIG. 3 is replaced by assemblies of biasing means as 
those in FIG. 3, each of which includes screw 15', a first disk plate 18', 
a coil spring 19', a second disk plate 20' and a contact element 21'. The 
biasing force applied to the respective rows in respect of the screws 15' 
is adjusted to be smaller by approximately 500g than that applied to the 
respective rows of the screws 15. 
Referring to FIGS. 6 through 8, when assembling the fly's eye lens unit, 
first both the side walls 13 and 22 are fixed to the base frame 11, and 
the spacer plates 14 and 23 are mounted along these side walls Then an 
appropriate square plate 51 corresponding to the square opening 12 is 
placed within the opening. The fourth (i.e. lowermost) matrix 31 of 
element lenses are aligned, on the square plate 51, along the side walls 
13 and 22. After that, the side wall 16 is mounted to the base frame 11, 
where the respective bores 17 accommodates a biasing means comprising the 
contact element 21, the coil spring 19 and a pair of the metal disks 18 
and 19. Then, Screws 15 are set to the bores 17, applying the biasing 
forces to the rows aligned in respect of the screws. Thus, the respective 
rows of the element lenses is fixedly held by the biasing forces. 
Next, as shown in FIG. 7, an adjusting plate 71 is placed on the fourth 
(i.e. lowermost) matrix 31 of the element lenses, where the adjusting 
plate 71 has a thickness precisely corresponding to the distance between 
the fourth matrix 31 of the element lenses and the third matrix 32 
thereof. The element lenses of the third matrix 32 are aligned in the same 
manner as those of the fourth matrix. 
In such a manner as above, four matrices of element lenses are superposed 
in order. Respective rows of the element lenses in the each matrix are 
fixedly held by the biasing forces. Then, as shown in FIG. 8, the 
respective adjusting plates 71 interposed between the matrices are drawn 
in the direction of an arrow A, and are removed from the fly's eye lens 
unit. 
Then, the spacer plate 26 and the side wall 27 are fixedly mounted on the 
base frame 11, in the case of the first embodiment shown in FIG. 3, or the 
side wall 27' having substantially the same structure as the side wall 16 
is fixedly mounted on the base frame 11, in the case of the second 
embodiment shown in FIG. 5. After that, the lid plate 24 is mounted on the 
side walls, whereby assembling of the fly's eye lens unit is accomplished. 
As mentioned above, since the fly's eye lens unit FU is positioned at a 
point adjacent to the second focal point of the ellipsoidal mirror EM, the 
fly eye's lens unit FU will be heated considerably, in spite of 
interposition of the cold mirror M.sub.1. However, since the spacer plates 
having heat insulating characteristics are interposed between the element 
lenses and a housing comprising the base frame, side walls and lid plate 
of aluminum alloy, the thermal transmission from the housing to the 
element lenses, or vice versa, is efficiently blocked. Even if the thermal 
expansion may occur in the fly's eye lens unit, it is efficiently absorbed 
by the spacer plates 14, 23 and 26 and the biasing means, and accordingly 
the optical characteristics of the fly's eye lens unit is kept 
substantially constant. 
Since there is no intermediate between respective element lenses of the 
fly's eye lens unit, light entering the fly's eye lens unit is not blocked 
and passes therethrough without any loss of light. 
Assembling of the fly's eye lens unit can be accomplished with ease and 
with precise positioning, as can be apparently understood from the 
aforementioned description. 
While the invention has been illustrated and described as embodied an 
illumination system, it is not intended to be limited to the details 
shown, since various modifications and structural changes may be made 
without departing in any way from the spirit of the present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from standpoint of prior art, fairly constitute essential characteristics 
of the generic or specific aspects of this invention.