Optical: systems and elements – Diffraction – From zone plate
Reexamination Certificate
1999-07-23
2001-12-04
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Diffraction
From zone plate
C359S566000, C359S569000
Reexamination Certificate
active
06327086
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical diffraction device, and more particularly to a binary-type optical diffraction device. The invention also relates to an optical system including a binary-type optical diffraction device and to an optical apparatus such as an exposure apparatus, including such an optical system.
2. Description of the Related Art
A binary device is regarded in the art as an important possible technique to realize a high-accuracy optical diffraction device. The binary device is a technique to produce an optical diffraction device having a step structure in cross section such as that shown in
FIG. 1B
, which is an approximation of the blazed cross-sectional structure of an optical diffraction device such as that shown in FIG.
1
A. The optical device shown in
FIGS. 1A and 1B
is a diffraction-type Fresnel lens, which is also called a kinoform. As shown in
FIGS. 1A and 1B
, a diffraction grating having a fine structure is formed on a transparent substrate
100
or
101
. If the structure of the diffraction device is approximated by a step structure, the diffraction device can be easily produced with high precision using the semiconductor process technology which is widely used to produce an LSI or the like.
FIGS. 2A-2H
illustrate the process of producing a 4-step optical diffraction device (binary device) using the semiconductor process technology. In
FIGS. 2A-2H
, reference numeral
110
denotes a transparent substrate on which a diffraction grating is formed,
111
a resist coated on the substrate
110
, and
112
a mask used to form a grating pattern. In the first processing step shown in
FIG. 2A
, the resist
111
is exposed via the mask
112
to exposure light
113
so as to form a latent image corresponding to the mask pattern. Then in the processing step shown in
FIG. 2B
, the resist is developed so that the portions exposed to light (latent image) are removed (herein the resist is assumed to be of the positive type). In the processing step shown in
FIG. 2C
, the substrate
110
is etched to a predetermined depth by means of the reactive ion etching technique. Then, the remaining resist is removed. At this processing stage, a 2-step structure is obtained as shown in FIG.
2
D. In the following processing step shown in
FIG. 2E
, another resist
114
is coated on the surface of the substrate, and the resist
114
is exposed via a mask
114
having a grating pattern with a pitch half that of the pattern formed on the mask
112
. In the processing steps shown in
FIGS. 2F and 2G
, the resist
114
is developed so that exposed portions of the resist are removed as in the previous development process, and the substrate
110
is etched using the remaining resist as a mask. After that, the remaining resist is removed. Thus, a 4-step structure is obtained as shown in FIG.
2
H. When it is desired that the number of steps in the structure be increased, the above processing steps are repeated using a mask having a pattern with a pitch half that of the pattern formed on the mask
115
. With this technique, although the number of steps in the structure is limited to 2
n
(n: integer), it is possible to form a desired number of steps by properly selecting the number of masks and the line width of the mask pattern.
In the above technique, a desired step structure is formed by etching a substrate. Instead, it is also known in the art to form a step structure by performing a process repeatedly to deposit a film having. a thickness equal to one step on a substrate in such a manner that the film is formed at predetermined locations. In the case where the diffraction grating structure is approximated by a step structure, although it is impossible to achieve 100% efficiency, high efficiency such as 95% for an 8-step structure and 99% for a 16-step structure can be achieved, which is good enough for practical applications.
FIG. 3
illustrates, in an enlarged fashion, a part of an optical diffraction device. With reference to this figure, the step structure will be discussed in further detail below. In this structure, it is assumed that the refractive index of the substrate
120
is n
s
and the refractive index of the medium
121
on which light is incident is n
i
. The broken line
122
represents the ideal structure of the device while the solid line
123
represents the structure which approximates the ideal structure by steps. In order for the incident light to encounter an abrupt phase change of 2&pgr; at each boundary (denoted by B in
FIG. 3
) between adjacent units of the periodic structure, the height D of the ideal structure
122
should be
D=&lgr;/(n
s
−n
i
)
where &lgr; is the wavelength of the light. If each step has a height h, and the number of steps is L (six steps are shown in
FIG. 3
for convenience of explanation), the height E of the step-approximated structure
123
becomes E=(L−1)h. There is a difference, as denoted by &agr; in
FIG. 3
, between the heights D and E, wherein &agr; satisfies the following equation:
(L−1)h+&agr;=&lgr;/(n
s
−n
i
) (1)
Usually the structure is designed so that &agr;=h. In this case, it is required to meet the following condition:
h=D/L and E=D·(L−1)/L
Thus, the height of each step is given by the above equation. However, the height of each step may also be determined in another way. In the above method, when the number L of steps is determined, the height h of each step is automatically determined, and it is impossible to modify h to optimize the characteristics of the device. From this point of view, it is rather desirable that &agr; be allowed to have a value within the range 0<&agr;<h so that h may be set to an arbitrary desired value. In this case, if &agr; is represented as &agr;=k·h (0<k<1), then the following equation should be met when the height h of each step and the number L of steps are determined.
(L−1+k)h=&lgr;/(n
s
−n
i
) (2)
where k may have an arbitrary value within the range 0<k ≦1.
The surface of an optical device is generally covered with an antireflection film for suppressing the reflection of light at the surface. In the case of a dioptric lens, since the surface is smooth, it is easy to form an antireflection film on the surface. As for binary devices, a technique of forming an antireflection film on the device surface is disclosed in “Anti reflection-coated diffractive optical devices fabricated by thin-film deposition”, E. Pawlowski and B. Kuhlow, Opt. Eng. 33(11), 3537-3546 (1994). In this technique, as shown in
FIG. 4
, a material
131
for forming an antireflection film is deposited from above at right angle onto the surface of a substrate
130
by means of ion beam sputtering thereby forming a thin film
132
on the step-structured surface of the substrate
130
.
FIG. 5
illustrates a multilayer antireflection film formed on the fine step-structured surface using the sputtering technique. As can been seen from
FIG. 5
, the antireflection film
143
formed on substrate
141
using the conventional technique has uniformity in thickness due to the steps, which causes a reduction in antireflection effect.
For a similar reason, a reduction in effect occurs when a reflection enhancement film is formed on a reflection type optical diffraction device.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical diffraction device having excellent capability for antireflection or reflection enhancement.
It is another object of the present invention to provide an improved illumination optical system, projecting optical system, copying machine, camera, and exposure apparatus, using an optical diffraction device. It is a further object of the present invention to provide a method of producing a device using the optical diffraction device.
According to an aspect of the present invention, there is provided an optical diffraction device including: a plurality of step-s
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Jr. John Juba
Schuberg Darren
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