Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-12-29
2002-03-12
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06355382
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a photomask and to an exposure method using a photomask.
BACKGROUND OF THE INVENTION
Semiconductor devices are becoming more and more tightly packed in density and increasingly fine, and this has been accompanied by the need to provide finer circuit patterns on semiconductor substrates. Various improvements in lithography have been made in order to satisfy this need.
In existing lithographic techniques, raising the fineness of resist patterns that are transferred to semiconductor substrates has been dealt with mainly by developing exposure equipment and especially by raising the numerical aperture (NA) of projection lens optics. In general, the limit on a fine pattern that is capable of being resolved (i.e., critical resolution R) and NA are related as follows: R=K
1
×&lgr;/NA (where K
1
is a process-dependent constant such as the performance of a photosensitive resin and &lgr; represents wavelength). This means that the critical resolution can be reduced in inverse proportion to an increase in NA. However, depth of focus (DOF), namely the range over which displacement of focal point is allowed, and NA are related as follows: DOF=K
2
×&lgr;/NA (where K
2
is a process-dependent constant). As a consequence, raising the numerical aperture NA has the converse effect of shortening depth of focus.
Currently existing semiconductor devices are manufactured by repeating the steps of film formation, resist-pattern formation and etching, etc. As a result, a step or difference in level on the order of several microns usually develops on the semiconductor substrate.
If it is attempted to form a resist pattern on a semiconductor substrate having such a step, the focal point at the top of the step will differ from that at the bottom, making it difficult to form a highly precise fine pattern. In view of this, obtaining a large depth of focus is a major challenge in the manufacture of modern semiconductor devices.
Studies for the purpose of increasing depth of focus have been conducted taking a variety of approaches. Specifically, one example of a technique employed on the side of the illumination optics is a super-resolution technique referred to as the modified illumination method or oblique incidence (off-axis) illumination method. One example of a technique which is an improvement on the side of the photomask is to use an auxiliary pattern. It should be noted that a photomask is a master sheet for exposure formed into a pattern comprising transparent and opaque areas and is referred to in particular as a “reticle” unless the reduction ratio is 1:1. Here, however, the term “photomask” will be used regardless.
The modified illumination method, which approaches the problem of depth of focus from the side of the illumination optics, will be described in simple terms first. The modified illumination method is classified into a ring illumination method, which shields the central portion of an aperture stop and uses a ring-shaped illuminating light source, and a four-point illumination method that uses an aperture stop open only at the four corners of the sides thereof.
In an ordinary illumination optical system, light impinges upon the patterned photomask perpendicularly and transfers the pattern onto the semiconductor substrate through a projection lens system. Use is made of diffracted light of order 0 and order +1 or −1 in pattern resolution. As the pattern becomes finer, however, the angle of diffraction increases and eventually only diffracted light of order 0 impinges upon the projection lens system. As a consequence, the light that passes through the fine pattern is substantially the perpendicular component only and there is a decline in the contrast of the light-intensity distribution on the image plane.
However, with the four-point illumination method, for example, only oblique light impinges upon the photomask. As a result, either diffracted light of order +1 or −1 impinges upon the projection lens system and the contrast of the light-intensity distribution on the image plane can be improved. Since the contrast of the light-intensity distribution is thus enhanced by causing the illuminating light to impinge obliquely, satisfactory resolution can be obtained even if the position of the focal point is displaced. This makes it possible to increase depth of focus.
However, this modified illumination method is a technique that is effective in cases where the photomask pattern is a periodically repeating pattern that gives rise to diffracted light: it is not effective for an isolated pattern that lacks periodicity. Accordingly, for an isolated pattern of this kind, use is made of a method in which fine patterns (referred to as “auxiliary patterns ” hereinafter in the entire disclosure) that are not allowed to be transferred to the semiconductor substrate are provided around the isolated pattern in order to provide this pattern with periodicity.
The technique using these auxiliary patterns will be described with reference to
FIGS. 20 and 21
.
FIG. 20
illustrates a conventional photomask for the auxiliary pattern scheme, in which (a) is a view showing pattern layout on the photomask and (b) is a sectional view taken along line M-M′ of (a).
FIG. 21
is a graph illustrating the distribution of the intensity of light on the plane on which the image is formed in a case where exposure is performed using the photomask of FIG.
20
.
With this conventional technique, in a case where use is made of the four-point illumination method, auxiliary patterns
2
are disposed above and below and to the left and right of the pattern to be resolved (referred to as main pattern
1
) and are spaced a prescribed distance away from the main pattern
1
to provide the pattern with periodicity. If the position of the focal point is shifted in a case where only the isolated main pattern
1
is present, the skirt of the distribution of the intensity of light on the imaging surface broadens greatly and resolution undergoes a pronounced decline. When the auxiliary patterns
2
are provided, however, a phase-inverting effect ascribable to oblique incidence occurs between the main pattern
1
and the auxiliary patterns
2
and contrast is enhanced. Even if defocusing is performed, therefore, it is possible to suppress a decline in resolution.
SUMMARY OF THE DISCLOSURE
Various problems have been encountered in the course of intense investigation towards the present invention.
Namely, by using the oblique incidence illumination method as the method of illumination and forming the auxiliary patterns in the photomask, the contrast of light intensity on the image plane is enhanced even at an isolated pattern and resolution can be improved. However, a problem which arises is that the auxiliary patterns themselves are transferred to the semiconductor substrate when the auxiliary patterns are formed on the photomask.
More specifically, because an auxiliary pattern has the main pattern on only one side thereof and no patterns on its other sides, the contrast of light intensity on the image plane does not become as large as it does in the case of the main pattern but the intensity of light at the portion corresponding to the auxiliary pattern is not zero either. Depending upon conditions such as the size of the main pattern, therefore, the auxiliary pattern also is transferred to the semiconductor substrate. This is discriminated as a defect in a wafer inspection (KLA, etc.).
Taking into account the effects of interference of transmitted light, the closer the size of an auxiliary pattern approaches that of the main pattern, the greater the contribution to contrast enhancement of the main pattern. If the auxiliary pattern is made too large, however, a problem which arises is that the auxiliary pattern itself tends to be transferred to the semiconductor substrate. Methods have been proposed to solve this problem. For example, there is a method of suppressing transfer of the auxiliary pattern by finely splitting the auxiliary p
Ishida Shinji
Yasuzato Tadao
NEC Corporation
Rosasco S.
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