Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-03-03
2001-10-09
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C430S030000, C430S321000, C430S394000, C356S399000, C356S401000
Reexamination Certificate
active
06301001
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to an optical element manufacturing method and, more particularly, to a method of manufacturing a binary type diffractive optical element.
Many types of optical systems with a diffractive optical element using light diffraction phenomenon have been proposed. As examples of such diffractive optical elements, there are a Fresnel zone plate, a diffraction grating and a hologram.
Generally, for a diffraction type optical element having a blazed shape, the manufacture becomes more difficult with the decrease in pitch. As regards the shape of the diffractive optical element, if a shape of binary type is selected, semiconductor device manufacturing techniques can be applied to the manufacture of it, and a fine pitch can be accomplished relatively easily. For these reasons, research and developments have recently been made on binary type diffractive optical elements wherein the blazed shape is approximated by use of a step-like (with levels) shape.
For details of a binary type diffractive optical element, reference may be made to the following publications:
a) G. J.Swanson, “Binary Optics Technology: The Theory and Design of Multi-level Diffractive Optical Elements”, Massachusetts Institute of Technology Lincoln Laboratory, Technical Report 854, Aug. 14, 1989.
b) G. J. Swanson, “Binary Optics Technology: Theoretical Limits on the Diffraction Efficiency of Multilevel Diffractive Optical Elements”, Massachusetts Institute of Technology Lincoln Laboratory, Technical Report 914, Mar. 1, 1991.
Referring to
FIG. 5
showing the manner of manufacturing an optical element according to the present invention, the manner of manufacturing a binary type diffractive optical element of four-level structure will be briefly explained.
Denoted in the drawing at
100
is a transparent glass plate of a refractivity n, and denoted at
101
is a resist. Denoted at
102
is a mask to be used for a first exposure. Denoted at
103
is exposure light. In this example, the resist
101
comprises a positive type resist.
First, in process A, a pattern of a mask
102
is transferred to the resist
101
by use of the exposure light
103
. In process B, development of the resist
101
is performed. In process C, etching of the glass substrate
100
is performed while the resist
101
after being developed is used as a mask pattern. Then, in process D, unnecessary resist on the substrate
100
is removed, whereby a binary type optical element of two-level step structure is accomplished.
The etching depth d
1
in process C is determined, when the wavelength to be used with the binary type optical element is &lgr;, by the following equation:
d
1
=&lgr;/[2(n−1)]
Subsequently, to the glass substrate
100
on which a binary type optical element of two-level structure has been formed, a resist material (
104
) is applied again, and in process E a mask
105
is used to perform a second exposure. The pattern of the mask
105
has a pitch a half of the pattern of the mask
102
. The exposure is performed while correctly aligning the edge of a light blocking portion of the mask
105
pattern with the edge of the two-level binary structure. By these procedures and after the development treatment at process F, a resist pattern as illustrated is formed.
Subsequently, in process G, second etching is performed by using the resist pattern formed in process F as a mask pattern. In process H, unnecessary resist is removed, whereby a binary type optical element of four-level structure is accomplished. Here, the etching depth d
2
in process G is determined by the following equation:
d
2
=&lgr;/[4(n−1)]
While the foregoing description has been made in relation to a four-level structure, as is well known in the art, a binary type optical element of eight-level structure or sixteen-level structure can be manufactured by repeating the above-described procedure while changing the mask pitch.
In process E of the processes for manufacturing a binary type diffractive optical element described above, it is not easy to align the mask
105
for the second exposure with respect to the mask
102
of the first exposure. Usually, there occurs a registration error (alignment error) of some degree.
The effect of such alignment error will be described with reference to
FIGS. 1A
,
1
B,
2
A and
2
B. Denoted in the drawings at
110
is a glass substrate on which a diffraction grating of two-level structure has been formed. Denoted at
111
is a mask for use in second exposure.
Here, one period of the two-level structure is T, and the mask
111
has a light blocking portion of a width T/4. Denoted at
112
is a coordinate axis for explanation, and the pattern formed on the glass substrate
110
has a periodicity in X-axis direction.
FIG. 1A
shows a state in which the mask
111
is deviated from an idealistic position, in positive X-axis direction by aT/4 (a>0).
If processes E-H in
FIG. 5
are performed in this state, then an accomplished diffractive optical element will have a shape such as shown in FIG.
1
B.
FIG. 2A
shows a state in which the mask
111
is deviated from an idealistic position, in negative X-axis direction by |aT/4| (a<0). If processes E-H in
FIG. 5
are performed in this state, then an accomplished element will have a shape such as shown in FIG.
2
B.
With these shapes, as a matter of course, there occurs a decrease of diffraction efficiency. When the diffraction efficiency of first-order diffraction light is calculated by using the value “a” as a parameter, it follows that:
|C
1
|
2
=(8/&pgr;
2
){1−sin[&pgr;(|a|/2)]} (1)
Equation (1) is derived in accordance with scalar theory, and details of it will be described later with reference to embodiments of the present invention. The result can be shown in a graph, such as that of FIG.
3
. In the case where there is no alignment error, that is, when a=0, an idealistic diffraction efficiency of 81% is attainable with respect to the four-level structure. However, with enlargement of alignment error, the diffraction efficiency decreases considerably. Since the decrease of diffraction efficiency leads to various problems such as a decrease of usable light quantity or an increase of unnecessary diffraction light, causing flare or the like, it should be suppressed as much as possible.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical element manufacturing method in which an alignment error, if any, may cause small reduction of diffraction efficiency.
In accordance with an aspect of the present invention, there is provided an optical element manufacturing method, comprising: a first process for forming a mask pattern on a substrate; and a second process for forming a step-like structure on the substrate by use of the mask pattern; wherein the first and second processes are repeated N times, and wherein, before execution of the (k)th time second process where 2≦k≦N, there is a process for determining a relative alignment error between a mask pattern as formed through the (k)th time first process and a mask pattern as formed through the (k−1)th time first process; and wherein the height of the step-like structure to be defined by the (k)th time second process is determined in accordance with the alignment error.
In accordance with another aspect of the present invention, there is provided an optical element manufacturing method, comprising: a first process for forming a first mask pattern on a substrate; a second process for forming a step-like structure on the substrate by use of the first mask pattern; a third process for forming a second mask pattern on the substrate; a fourth process for determining a relative alignment error between the first and second mask patterns; and a fifth process for forming a step-like structure on the substrate by use of the second mask pattern, wherein the height of the step-like structure to be formed throu
Adams Russell
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Fuller Rodney
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