Planar microlens array and method of manufacturing same

A planar microlens array 1 includes an array of microlenses 3 made of a synthetic resin having a high refractive index and formed on a surface of one of a base glass plate 2 and a cover glass plate 4, and an adhesive layer 5 made of a synthetic resin having a low refractive index standing between the array of microlenses 3 and the other of the base glass plate 2 and the cover glass plate 4, wherein the equations (Eqs. 1-5) are satisfied, where, n.sub.1 represents the refractive index of the synthetic resin having a high refractive index, n.sub.2 represents the refractive index of the synthetic resin having a low refractive index, t.sub.1 represents the thickness of the thickest portion of the sphere portion of the microlenses, t.sub.2 represents said thickness of the rest portion of the microlenses and t.sub.3 represents the thickness of the thinnest portion of the adhesive layer. EQU 1.59.ltoreq.n.sub.1 .ltoreq.1.68 (Eq. 1) EQU 1.38.ltoreq.n.sub.2 .ltoreq.1.42 (Eq. 2) EQU 5.ltoreq.t.sub.1 .ltoreq.30 (.mu.m) (Eq. 3) EQU t.sub.2 .ltoreq.6 (.mu.m) (Eq. 4) EQU t.sub.3 .gtoreq.0.2 t.sub.1 (.mu.m) (Eq. 5)

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a planar microlens array used in a liquid 
crystal display element and so on. 
2. Description of the Related Art 
Prior-art liquid crystal display elements in which a planar microlens array 
and a liquid crystal layer are combined are shown in FIG. 1(A) and FIG. 
1(B) of the accompanying drawings. The liquid crystal display element 
shown in FIG. 1(A) includes a planar microlens array 1 comprising an array 
of convex microlenses 3 provided on a surface of a base glass plate 2. The 
array of convex microlenses 3 is covered with a cover glass plate 4 which 
is bonded to the array of convex microlenses 3 by an adhesive layer 5. A 
liquid crystal layer 7 is filled between the cover glass plate 4 and a TFT 
(Thin Film Transistor) glass substrate 6. The TFT glass substrate 6 
supports transparent pixel electrodes 8 on its surface facing the liquid 
crystal layer 7. The surface of the TFT glass substrate 6 includes areas 9 
that are free of the transparent pixel electrodes 8 and carry 
interconnections and TFTs which do not pass applied light. Electrodes 10 
which confront the transparent pixel electrodes 8 are mounted on a surface 
of the cover glass plate 4 that faces the liquid crystal layer 7. 
In the liquid crystal display element shown in FIG. 1(B), an array of 
convex microlenses 3 is provided on a surface of a cover glass plate 4. 
A planar microlens array according to the present invention can be applied 
to both the liquid crystal display elements shown in FIG. 1(A) and FIG. 
1(B). 
The planar microlens array 1 operates as follows: Applied light is 
converged by the convex microlenses 3 onto the transparent pixel 
electrodes 8 to brighten an image projected onto a screen. 
A process of manufacturing the planar microlens array 1, the structure of 
which is mentioned above, is as follows: As shown in FIG. 2(A), a release 
agent is coated on a shaping surface of a stamper 11 on which convex 
portions are densely arranged, and a light-curable or heat-curable 
synthetic resin material having a high refractive index is set on the 
shaping surface of the stamper 11. Next, as shown in FIG. 2(B), the base 
glass plate 2 is pushed onto the synthetic resin material, thereby 
spreading the synthetic resin material, and the synthetic resin material 
is cured by applying ultraviolet radiation or heating and is shaped to 
form the convex microlenses 3. Thereafter the stamper 11 is peeled off. 
Next, as shown in FIG. 2(C), a light-curable or heat-curable synthetic 
resin material having a low refractive index is coated onto the convex 
microlenses 3, and a glass substrate which is made into a cover glass 
plate 4 is pushed onto the synthetic resin material, thereby spreading the 
same. Thereafter, the synthetic resin material is cured and finally the 
planar microlens array 1 is formed by grinding the glass substrate to the 
thickness of the cover glass plate 4. 
The convex microlenses may be formed on the glass substrate. 
Presently available liquid crystal display panels have pixel dimensions 
ranging from about 40 .mu.m to 60 .mu.m. It is expected that the pixel 
dimensions will be reduced to about 20 .mu.m to 30 .mu.m in the future to 
meet demands for clearer displayed images. 
Smaller pixel dimensions require the convex microlenses 3 to be reduced in 
size, resulting in a shorter focal length. For efficient utilization of 
the applied light, it is necessary that the focal point of the convex 
microlenses 3 be positioned substantially on the transparent pixel 
electrodes. To meet such a requirement, the cover glass plate 4 must be 
reduced in thickness. 
Each of the convex microlenses 3 and the adhesive layer 5 is made of a 
heat-curable or ultraviolet-curable synthetic resin. The synthetic resin 
shrinks when cured. In particular, the synthetic resin having a high 
refractive index which makes the convex microlenses 3 has high Young's 
modulus and high residual stress. 
The cover glass plate 4 can withstand the shrinkage of the synthetic resin, 
provided that the cover glass plate 4 has a substantial thickness. 
However, if the cover glass plate 4 is thinner, it tends to yield and 
allow the entire planar microlens array 1 to warp upon shrinkage of the 
synthetic resin, as shown in FIG. 3 of the accompanying drawings. In 
particular, shrinkage of the synthetic resin having a high refractive 
index may result in high residual stress. As a result of this, the width 
of a cell gap which is formed between the planar microlens array 1 and the 
TFT glass substrate 6 and to which liquid crystal is applied varies 
between the center and the periphery. Presently the maximum permissible 
error range of a cell gap in size is 1.5 .mu.m. 
On the other hand, the Young's modulus of the synthetic resin having a low 
refractive index which makes the adhesive layer 5 is smaller than that of 
the synthetic resin having a high refractive index which makes the convex 
microlenses 3. That does not mainly cause the warpage but does mainly 
cause the small voids. 
Namely, as shown in FIG. 4, small voids are produced between the convex 
microlenses 3 and the adhesive layer 5 because the volume shrinkage 
percentage of the synthetic resin having a low refractive index generally 
reaches to 6-9%, and further the synthetic resin having a low refractive 
index has small values in membrane intensity and interface adhesion 
intensity. 
SUMMARY OF THE INVENTION 
To solve the above-mentioned problems, the inventors of the present 
invention tested the cause, the incident amount or the like with regard to 
warpage and small voids. 
FIGS. 5(A) and 5(B) show the relation between thickness and warpage of 
convex microlenses 3 which comprise a synthetic resin having a high 
refractive index. In the case shown in FIG. 5(A), the thickness of convex 
microlenses 3 is small. In a case shown in FIG. 5(B), a thickness thereof 
is large. A thickness of an adhesive layer 5 which comprises a synthetic 
resin having a low refractive index is equal in both cases. 
It became clear that residual stress and amount of warpage are smaller in 
the case shown in FIG. 5(A) than in FIG. 5(B). 
In the above-mentioned manufacturing method using a stamper, in addition to 
a sphere portion 3a which functions as a lens, a rest portion 3b needs to 
be formed in the convex microlenses 3 to prevent damage to the stamper. 
The amount of warpage is determined by the sum of the thickness t.sub.1 of 
the sphere portion 3a and the thickness t.sub.2 of the rest portion 3b 
(t.sub.1 +t.sub.2). 
FIGS. 6(A) and 6(B) show the relation between a thickness of an adhesive 
layer 5 comprised of a synthetic resin having a low refractive index, and 
small voids. In a case shown in FIG. 6(A), a thickness of the adhesive 
layer 5 is thin. In a case shown in FIG. 6(B), the thickness thereof is 
thick. The thickness of convex microlenses 3 comprised of a synthetic 
resin having a high refractive index is equal in both cases. 
It became clear that the larger the ratio of the thickness t.sub.3 of the 
thinnest portion of the adhesive layer 5 to the thickness t.sub.4 of the 
thickest portion thereof (t.sub.4 /t.sub.3) is, the easier it is to 
produce small voids. 
In addition, in the cases shown in FIG. 5(A) and FIG. 5(B), even if 
thickness of the adhesive layer 5 is equal, small voids are easier to 
produce in a case where the thickness of the convex microlenses 3 is thick 
than in a case where the thickness thereof is thin. 
Pixel dimensions of liquid crystal display panels are determined within 
14-60 .mu.m. A epoxy resin having a refractive index (n.sub.1) of 
1.59-1.68 is used as a synthetic resin having a high refractive index. A 
fluoro epoxy resin or a fluoro acrylic resin having a refractive index 
(n.sub.2) of 1.38-1.42 is used as a synthetic resin having a low 
refractive index. 
When pixel dimensions of liquid crystal display panels are determined, a 
focal length of a lens is automatically determined. When a focal length of 
a lens and a kind of a synthetic resin having a high refractive index are 
determined, a thickness (t.sub.1) of a sphere portion of a convex lens is 
automatically determined. 
A thickness (t.sub.1) of a sphere portion varies according to differences 
in pixel dimensions, a kind of a synthetic resin and so on. However, 
generally it is within the scope of 5.ltoreq.t.sub.1 .ltoreq.30 (.mu.m). 
As a result, among elements which take part in causing warpage and small 
voids, including a thickness (t.sub.1) of a sphere portion of a convex 
lens, a thickness (t.sub.2) of a rest portion and a thickness t.sub.3 of 
the thinnest portion of an adhesive layer, the thickness (t.sub.1) of a 
sphere portion is determined by another element. Therefore, the thickness 
(t.sub.2) of a rest portion and the thickness t.sub.3 of the thinnest 
portion of an adhesive layer have room for adjustment. 
According to the present invention, optical variations in a case where each 
thickness t.sub.1, t.sub.2 and t.sub.3 mentioned above is made to vary, 
and room for adjustment in the thickness t.sub.2 and t.sub.3 are watched, 
and the most suitable conditions are estimated thereby. 
A planar microlens array includes an array of microlenses made of a 
synthetic resin having a high refractive index and formed on a surface of 
one of a base glass plate and a cover glass plate, and a adhesive layer 
made of a synthetic resin having a low refractive index standing between 
the array of microlenses and the other of the base glass plate and the 
cover glass plate, wherein the equations (Eqs. 1-5) are satisfied, where, 
n.sub.1 represents the refractive index of said synthetic resin having a 
high refractive index, n.sub.2 represents the refractive index of said 
synthetic resin having a low refractive index, t.sub.1 represents the 
thickness of the thickest portion of the sphere portion of said 
microlenses, t.sub.2 represents the thickness of the rest portion of said 
microlenses and t.sub.3 represents said thickness of the thinnest portion 
of said adhesive layer. 
EQU 1.59.ltoreq.n.sub.1 .ltoreq.1.68 (Eq. 1) 
EQU 1.38.ltoreq.n.sub.2 .ltoreq.1.42 (Eq. 2) 
EQU 5.ltoreq.t.sub.1 .ltoreq.30 (.mu.m) (Eq. 3) 
EQU t.sub.2 .ltoreq.6 (.mu.m) (Eq. 4) 
EQU t.sub.3 .gtoreq.0.2 t.sub.1 (.mu.m) (Eq. 5) 
In addition, it is preferable to satisfy the relation expressed by 
10.ltoreq.t.sub.1 +t.sub.2 +t.sub.3 .ltoreq.60 (.mu.m) because a necessary 
focal length cannot be obtained in a case where the value of t.sub.1 
+t.sub.2 +t.sub.3 is smaller than 10 .mu.m, and warpage is produced in a 
case where it is larger than 60 .mu.m.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiment 1 
The base glass plate has a thickness of 0.95 mm, the cover glass plate has 
a thickness of 0.15 mm, the base glass plate and the cover glass plate 
have dimensions of 27.times.20 mm, the lenses are arranged squarely and 
densely, a pitch of the lenses is 32 .mu.m, a refractive index (n.sub.1) 
of the synthetic resin having a high refractive index which makes up the 
lenses is 1.59, a refractive index (n.sub.2) of the synthetic resin having 
a low refractive index which makes up the adhesive layer is 1.38, a 
thickness (t.sub.1) of a sphere portion of the lenses is 10 .mu.m, a 
thickness (t.sub.3) of the thinnest portion of the adhesive layer is 10 
.mu.m and a thickness (t.sub.2) of a rest portion of the microlenses is 
made to vary. 
The relation between t.sub.2 and an amount of warpage (.mu.m) is shown in 
TABLE 1. 
TABLE 1 
______________________________________ 
Amount of warpage 
t.sub.2 (.mu.m) 
(.mu.m) 
______________________________________ 
2 0.62 
3 0.89 
4 1.20 
5 1.43 
6 1.50 
7 1.78 
8 2.15 
9 2.37 
10 2.62 
11 2.81 
12 3.08 
______________________________________ 
As mentioned above, presently the permissible warpage is below 1.5 .mu.m 
with regard to a planar microlens array. 
As shown in TABLE 1, it is required that t.sub.2 be below 6 .mu.m in order 
that the amount of warpage is below 1.5 .mu.m. 
Embodiment 2 
A thickness (t.sub.2) of a rest portion of the microlenses is fixed at 3 
.mu.m and the other conditions are the same as EMBODIMENT 1. A thickness 
(t.sub.1) of a sphere portion of the microlenses and a thickness (t.sub.3) 
of the thinnest portion of the adhesive layer are made to vary. 
The relation between t.sub.3 and an amount of warpage (.mu.m) is shown in 
TABLE 2. 
TABLE 2 
______________________________________ 
Small Small Small 
t.sub.3 (t.sub.1 = 10) 
voids t.sub.3 (t.sub.1 = 15) 
voids t.sub.3 (t.sub.1 = 20) 
voids 
______________________________________ 
1 X 1 X 1 X 
2 .largecircle. 
2 .largecircle. 
2 .largecircle. 
3 .largecircle. 
3 .largecircle. 
3 .largecircle. 
4 .largecircle. 
4 .largecircle. 
4 .largecircle. 
5 .largecircle. 
5 .largecircle. 
5 .largecircle. 
6 .circleincircle. 
6 .largecircle. 
6 .largecircle. 
7 .circleincircle. 
7 .circleincircle. 
7 .largecircle. 
8 .circleincircle. 
8 .circleincircle. 
8 .circleincircle. 
9 .circleincircle. 
9 .circleincircle. 
9 .circleincircle. 
10 .circleincircle. 
10 .circleincircle. 
10 .circleincircle. 
11 .circleincircle. 
11 .circleincircle. 
11 .circleincircle. 
______________________________________ 
X . . . Small voids were produced all over. 
.largecircle. . . . Small voids were produced only in the periphery of th 
lens. 
.circleincircle. . . . No small void was produced. 
As shown in TABLE 2, it becomes clear that no small void is produced in a 
case of t.sub.3 .gtoreq.0.2 t.sub.1 (.mu.m). 
As mentioned above, according to the present invention, in a planar 
microlens array manufactured using a stamper, it is possible to 
effectively prevent a warpage and small voids from arising by making the 
relation among a thickness (t.sub.1) of the thickest portion of a sphere 
portion of the microlenses, a thickness (t.sub.2) of a rest portion of the 
microlenses and a thickness (t.sub.3) of the thinnest portion of the 
adhesive layer satisfy the equations, 5.ltoreq.t.sub.1 .ltoreq.30 (.mu.m), 
t.sub.2 .ltoreq.6 (.mu.m) and t.sub.3 .gtoreq.0.2 t.sub.1 (.mu.m).