Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
1998-12-18
2001-06-05
Lee, Eddie C. (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000
Reexamination Certificate
active
06243159
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection aligner and an exposure method using the same. More particularly, the present invention relates to a projection aligner for projecting a mask pattern and an exposure method using the same.
2. Description of the Background Art
A conventional projection aligner will be first described in the following.
FIG. 4
is a schematic view showing the structure of a conventional projection aligner. Referring to
FIG. 4
, the conventional projection aligner demagnifies a pattern on a reticle (mask)
11
and projects it onto a photo resist (photosensitive material) on a wafer
20
, and has a light source
13
, an illumination optical system
12
from light source
13
to reticle
11
, and a projection optical system
110
from reticle
11
to wafer
20
.
Specifically, projection optical system
110
has a lens barrel
101
and a plurality of lenses
102
a
to
102
e
arranged in lens barrel
101
.
In the exposing operation, a light beam emitted from light source
13
is first projected through illumination optical system
12
onto reticle
11
on which a pattern is formed. The pattern image thus produced is projected onto the photo resist applied to wafer
20
through lenses
102
a
to
102
e
in projection optical system
110
.
Here, the reference character AX in the figure denotes the optical axis of the light beam emitted from light source
13
.
In manufacturing semiconductor integrated circuits, projection aligners are extensively employed to form fine patterns. For the recent demand for finer patterns, the exposure wavelength has become smaller to the g ray (436 nm), the i ray (365 nm) and further to the KrF excimer laser light beam (248 nm). Further, in order to form patterns of at most the wavelength of an exposure light beam, such super-resolution technique as the use of a phase shift mask is required.
Since the phase shift mask is a technique to improve resolution by interference of a light beam in an opposite phase, the light beam interference has to be large to some extent. Thus, the interference between mask apertures is generally made larger to attain its effect in the super-resolution technique. It is therefore necessary to carry out exposure by increasing the interference of illumination (reducing the “&sgr; (coherency) value”). In this case, an image may be formed by the interference using only part of a pupil, and this increases the influence of aberration as distortion of a lens and deteriorates the imaging property.
Although the lens aberration is attributable to the accuracy of polishing the lens, lens form distortion due to the lens weight after assembling a projection aligner is also one of the major causes in the conventional projection aligner.
For example, for a parallel plate lens
2
arranged in lens barrel
101
as shown in
FIG. 5
, its own weight causes the central portion of the lens to bend more than the peripheral portion in a direction in which gravity works. When lens
2
does not bend, the path A of a beam passing through lens
2
is linear. However, the path B of a beam passing through bent lens
2
is refracted by lens
2
and is not linear. Thus, the beam is displaced, causing aberration.
Such aberration causes the following effects in forming device patterns.
Pattern displacement due to comma is one example. Hole patterns and interconnection patterns are used in LSIs (Large Scale Integrated circuits). As shown in
FIG. 6
, the structure in which a hole
26
for electrically connecting a lower layer interconnection
22
and an upper layer interconnection
28
passes through an intermediate layer interconnection
24
is generally used. In this structure, hole
26
is formed in interlayer insulating films
23
,
25
. Hole
26
is filled with a conductive layer
27
, and lower layer interconnection
22
is formed on a silicon substrate
21
, for example.
In the structure shown in
FIG. 6
, when relative displacement between hole
26
and intermediate layer interconnection
24
is caused by aberration while a pattern is transferred, conductive layer
27
filling hole
26
and intermediate later interconnection
24
may be electrically short-circuited.
When the displacement due to aberration during pattern transfer is uniform in the exposure field, the influence of displacement due to aberration is substantially eliminated if exposure is carried out by deliberately shifting the amount of displacement during overlay exposure. In an actual stepper, however, the amount of displacement (including the directions) due to aberration is non-uniformly distributed in the field. Further, since displacement varies from one pattern to another (due to the pitch of a line/space pattern, for example), eliminating this effect is considered to be difficult. Even in a portion having the same patterns and the same image fields, the hole patterns and the interconnection patterns are relatively displaced by changing the exposure method (for example, modified illumination and normal illumination).
In order to form patterns not to cause such an electric short circuit, therefore, the space between hole
26
and intermediate layer interconnection
24
has to be widely designed. Then, the patterns can not be arranged closely, the chip size becomes larger, the number of production per wafer decreases, and production is lowered.
The typical amount of relative displacement for an actual stepper is considered to be 20-30 nm with a device having a 0.20 &mgr;m rule. Since the amount of displacement (conventional “overlay error” not caused by aberration) when the same patterns are overlayed and exposed under the same exposure conditions is 40-50 nm, such displacement between patterns that can not be ignored is caused by aberration.
FIG. 3
of C. M. Lim “Analysis of nonlinear overlay errors by aperture mixing related with pattern asymmetry”, SPIE Vol. 3051, pp. 106-115 shows the distribution, in an exposure field, of displacement between large size (~10 &mgr;m □) patterns due to a difference in the type of illumination (modified illumination).
FIG. 3
(a) is the measured amount of displacement, and
FIG. 3
(b) shows the amount of displacement without a parallel movement component and a magnification error component (the amount of displacement which can not be corrected by a stepper when the second layer is exposed). It can also be seen from this figure that the amount of displacement which can not be corrected is at most 30 nm.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a projection aligner capable of forming a highly-accurate and fine pattern by suppressing occurrence of aberration due to the weight of a lens itself and an exposure method using the same.
A projection aligner of the present invention for irradiating a mask, on which a pattern is formed, with an energy ray and projecting the image of the pattern onto a photosensitive material through a projection optical system includes a cylindrical member, an optical member and pressure adjusting means. The cylindrical member passes the image of the pattern. The optical member is held in the cylindrical member to separate first and second spaces in the cylindrical member while the first and second spaces are kept in an air-tight state. The pressure adjusting means adjusts the pressures in the first and second spaces so that force having substantially the same magnitude as gravity working on the optical member works on the optical member in an opposite direction to the gravity.
Since the force can be caused to work on the optical member to cancel its gravity in the projection aligner of the present invention, distortion of the optical member form due to its own weight and occurrence of aberration according to the distortion are suppressed. Therefore, a fine pattern can be formed.
In the above described projection aligner, the pressure adjusting means is preferably formed to be able to adjust the pressures in the first and second spaces so that the product of a pressure difference between the first and s
Lee Eddie C.
McDermott & Will & Emery
Mitsubishi Denki & Kabushiki Kaisha
Nguyen Hung Henry
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