Etching a substrate: processes – Forming or treating optical article
Reexamination Certificate
2000-07-28
2003-05-13
Gulakowski, Randy (Department: 1746)
Etching a substrate: processes
Forming or treating optical article
C216S022000, C216S026000, C216S012000, C219S121140, C430S005000, C430S030000, C430S032000
Reexamination Certificate
active
06562253
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to a method of producing an optical element which may suitably be used as, for example, an optical component of a semiconductor manufacturing reduction projection exposure apparatus, such as a phase modulation plate or an optical element having a two-dimensional binary structure or a phase type CGH (Computer Generated Hologram), or an optical interconnection element.
FIG. 9A
is a fragmentary sectional view of a Fresnel lens
1
, which comprises a Fresnel type diffraction grating. It has a saw-tooth blazed shape
2
.
FIG. 9B
is a fragmentary sectional view of a binary optics element
3
, which comprises a binary type diffraction grating. It has a step-like shape
4
.
An idealistic diffractive optical element may be one having a blazed shape
2
such as shown in
FIG. 9A
, and it may assure a diffraction grating of 100% with respect to a design wavelength. However, it is very difficult to produce a complete blazed shape
2
. Therefore, while quantizing and approximating the blazed shape
2
, the binary optics element
3
having a step-like shape
4
such as shown in
FIG. 9B
is used. Although the binary optics element
3
is made on the basis of approximation of the Fresnel lens
1
, a diffraction efficiency for first-order diffraction light can be 90% or more. In consideration of this, much research has been done with respect to such binary optics
3
, by approximating a blazed shape
2
with a step-like shape.
In order that such an optical element has an enlarged power and that, through much better approximation, the optical element has an improved performance as a diffractive optical element, the processed linewidth should be made to be very fine, as much as possible. To this end, a lithographic process having been developed in the semiconductor manufacturing technology and being able to accomplish very high precision processing has been used.
FIG. 10
is a schematic view which illustrates the procedure for making a diffractive optical element having an eight-level step structure. More specifically, in (a) of
FIG. 10
, drops of a resist material are applied to a cleaned substrate
11
and, through spin coating, a resist coating of a film thickness of about 1 micron is formed on the substrate. Then, through a baking process, a resist film
12
a
is produced. In (b) of
FIG. 10
, the substrate
11
is loaded into an exposure apparatus having a performance with which a finest diffraction pattern can be printed. Then, a reticle
13
a
corresponding to a desired diffraction pattern is used as a mask, and exposure light L, with respect to which the resist film
12
a
has a sensitivity, is projected thereto. When a positive type resist is used, zones having been exposed with the exposure light L become solvable by a developing liquid. Thus, in (c) of
FIG. 10
, a resist pattern
14
a
having a desired size can be produced. Subsequently, in (d) of
FIG. 10
, the substrate
11
is introduced into an ion beam etching apparatus or a reactive ion etching apparatus by which an anisotropic etching process can be done. While using the produced resist pattern
14
a
as an etching mask, the substrate
11
is etched for a predetermined time period, by which it is etched to a predetermined depth. Then, the resist pattern
14
a
is removed. By this, a pattern
15
a
having a two-level step structure such as shown in (e) of
FIG. 10
is produced.
Again, as in (a) of
FIG. 10
, a resist film
12
b
is produced on the substrate
11
having the pattern
15
a
formed thereon. In (f) of
FIG. 10
, the substrate then is placed in the exposure apparatus, and an exposure process is performed while a reticle
13
b
formed with a pattern having a periodicity two times larger than that of the reticle
13
a
is used as a mask. The mask pattern is transferred onto the pattern
15
a
, after the two are aligned with each other at the alignment precision of the exposure apparatus. By a developing process in (g) of
FIG. 10
, the resist film
12
b
is developed such that a resist patten
14
b
is produced. Then, in (h) of
FIG. 10
, as in (d) of
FIG. 10
, a dry etching process is performed to remove the resist pattern
14
b
, by which a pattern
15
b
having a four-level step structure is produced.
Subsequently, in (i) of
FIG. 10
, as in (a) of
FIG. 10
, again a resist film
12
c
is applied to the substrate
11
. Then, an exposure process is performed while a reticle
13
c
formed with a pattern having a periodicity four times larger than that of the reticle
13
a
is used as a mask. Then, in (j) of
FIG. 10
, the resist film
12
c
is developed, by which a resist pattern
14
c
is produced. Finally, in (k) of
FIG. 10
, the resist pattern
14
c
is removed, whereby a diffractive optical element with a pattern
15
c
having an eight-level step-like shape is produced.
When a diffractive optical element having a multiple-level step-like shape is to be produced while reticles having periodicities in multiples are used as masks, such as described above, as long as no alignment error or no dimensional error occurs, a multiple-level step structure having an idealistic shape can be produced. For example, by using three reticles
21
a
-
21
c
shown in
FIG. 11
, an eight-level structure “A” having an idealistic step height “d” can be produced.
A paper in “O plus E”, No. 11, pages 95-100 (1996), discusses a procedure wherein processes of resist application, mask pattern and etching are repeated, and it mentions that a multiple-level phase type CGH (Computer Generated Hologram) having phase levels of 2
L
can be produced where L is the number of masks used.
FIGS. 12
a
-
12
c
are plan views for explaining a procedure for producing a CGH. More specifically,
FIGS. 12
a
-
12
c
show patterns of reticles
31
a
,
31
b
and
31
c
, wherein zones depicted with hatching are light blocking portions. For example, the reticle
31
a
is used to perform the etching to a depth of 61 nm, the reticle
31
b
is used to perform the etching to a depth of 122 nm, and the reticle
31
c
is used to perform the etching to a depth of 244 nm. Although the order of using these reticles
31
a
-
31
c
is not fixed, a better resist patterning precision is attainable when a reticle for a smaller etching depth is used first.
The reticle
31
a
is used first to perform resist patterning on a substrate, and it is etched to a depth of 61 nm. As a result, an etching depth distribution such as shown in
FIG. 13A
is produced, wherein numerals denote etching depths (nm). Thereafter, the resist on the substrate is removed. Then, the reticle
31
b
is used to perform resist patterning, and the substrate is etched to a depth of 122 nm, by which an etching depth distribution such as shown in
FIG. 13B
is produced. Further, the resist on the substrate is removed, and the reticle
31
c
is used to perform resist patterning. The substrate is etched to a depth of 244 nm, by which an etching depth distribution such as shown in
FIG. 13C
is produced.
FIG. 14
is a sectional view taken along a line E-e in FIG.
13
C.
In the examples described above, if an alignment error occurs in registration of reticles, it directly leads to degradation of the performance of a diffractive optical element. More specifically, at the boundary of zones where phase differences are quantized, a very small structure which is not included in the design may be produced due to the alignment error. Such a very small structure causes degradation of the function of the diffractive optical element, and thus, causes a decrease of the diffraction efficiency. Further, the light corresponding to the decrease of diffraction efficiency advances in a direction not intended in the design as unwanted diffraction light, and it causes various undesirable problems. In this manner, light rays not desired may be produced from the diffractive optical element. Such light rays may function as flare light in an optical system wherein the diffractive optical element is used, and the flare light causes degradation of the imaging performance o
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Gulakowski Randy
Winter Gentle E.
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