Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-03-23
2003-09-30
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06627356
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-085421, filed Mar. 24, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a photomask used in the manufacture of a semiconductor device, a photomask blank, and a method of applying a light exposure treatment to a semiconductor wafer by using said photomask.
In recent years, with progress in the degree of integration of the semiconductor device and in the miniaturization of the semiconductor element, the required pattern size is approaching the resolution limit of the light exposure apparatus. Therefore, a so-called “resolution enhancement technology” such as an oblique illumination method and a phase control mask has come to be positively employed in the pattern transfer. Also, in the patterning process of a resist, the thickness of the resist film is being decreased in an attempt to extend the resolution limit and to widen the focus latitude.
In an ideal optical system, if the pattern arrangement on a mask is the same within a range affected by the optical proximity effect, the optical contrast of the transferred pattern, the exposure latitude and the profile of the resist pattern become the same.
However, with improvement in the resolution performance, a problem is actually generated that patterns are made different in the cross sectional profile of the resist film and the focus· exposure latitude depending on the difference in the peripheral pattern (construction), even if the patterns are exactly the same in the design.
For example, a photomask having a fine pattern arranged on the entire surface is made different from a photomask having the periphery of a fine pattern covered with an opaque film in the cross sectional profile of the resist even if these photomasks are the same in the pattern design. To be more specific, in a line/space (L/S) pattern of 300 &mgr;m square, in which the influence of the ordinary optical proximity effect is negligible, the light exposure sensitivity is improved so as to lower the light exposure latitude in the case where the average covering ratio of the periphery with an opaque film is 30%, compared with the case where the entire periphery is covered with an opaque film. Therefore, if a positive resist is used in the case where the average covering ratio is 30%, the head portion of the resist profile is rendered roundish.
These phenomena are considered to be caused by the irradiation of the resist with a back ground light (flare), which is not generated in an ideal optical system, so as to lower the optical contrast.
The flare in the light exposure apparatus is evaluated in general by the method described in publication 1 [J. P. Kirk, “Scattered light in photolithographic lenses”, Proc. SPIE (1994)].
In the case of using a photomask in which a large opaque pattern, i.e., scores of micrometers square, is present on a transparent substrate, the light must not reach that region of a silicon wafer which is positioned right under the opaque pattern. However, if the exposure Dose is gradually increased, the particular region of the silicon wafer is exposed to light because of the flare. Publication 1 quoted above discloses a method of numerically expressing the amount of the flare by utilizing the phenomenon described above.
The influences given by the flare to the device pattern are described in publication 2 [C. Progler, “Potential causes of across field CD variation”, Proc. SPIE (1997)] and publication 3 [E. Luce, “Flare impact on the intrafield CD control for sub-0.25 &mgr;m patterning”, Proc. PPIE (1999)].
These publications 2 and 3 teach that the amount of the flare is distributed substantially concentrically within the light exposure region. On the other hand, it has been experimentally confirmed that the size of the resist pattern is concentrically changed.
FIG. 1
schematically shows the situation of the light exposure in the case of using a conventional photomask. In the drawing, the general flare is involved in the reflection from the surface of a lens
81
(projection optics system), from the upper surface and lower surface of a photomask
82
including a quartz glass substrate
821
and a laminate film
822
consisting of a Cr film and an oxidized Cr film and acting as a opaque film, and from the surface of a wafer
83
. The flare can be divided into a flare generated on the upstream side (illuminating optics system) of the photomask
82
and a flare generated on the downstream side (projection optics system) of the photomask
82
. A reference numeral
841
in the drawing denotes an unexposed portion of the resist, with a reference numeral
842
denoting light-exposed portion of the resist.
In each of these cases, the amount of the flare is considered to be proportional to the covering ratio (i.e., area of the opaque film/area of the quartz glass substrate) of the photomask
82
. The flare is considered to boost the light intensity distribution on the wafer
83
as a background so as to lower the contrast of the pattern.
However, as a result of an extensive research conducted by the present inventors, it has been clarified that the change in the cross sectional profile of the resist film etc., which is derived from the difference in the peripheral pattern of the photomask
82
, is irrelevant to the flare as described herein later in detail.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention, which has been achieved in view of the situation described above, is to provide a photomask having an object pattern and a peripheral pattern formed in the periphery of the object pattern, which permits performing a light exposure that is unlikely to be affected by the peripheral pattern and to provide a photomask blank used for preparing the particular photomask.
Another object of the present invention is to provide a method of applying a light exposure treatment to a semiconductor wafer by using the photomask of the present invention.
As a result of an extensive research, the present inventors have found that the problem in respect of the change in the cross sectional profile of the resist pattern described above is caused by the light re-reflected from the back surface of the photomask. The present invention is basically featured in that the re-reflected light can be effectively suppressed.
According to a first aspect of the present invention, which has been achieved for achieving the objects described above, there is provided a first photomask, comprising a transparent substrate having a first main surface and a second main surface opposite to the first main surface, the substrate transmitting the exposed light; a pattern formed on the first main surface of the transparent substrate and having at least one of a opaque film, a translucent film, and a phase control film, the opaque film not transmitting the exposed light, the translucent film transmitting partly the exposed light, and the phase control film serving to control the phase of the exposed light; and a thin film formed on the second main surface of the transparent substrate and containing calcium fluoride.
In the first photomask, a thin film consisting of calcium fluoride is formed as an antireflection coating on the back surface of the transparent substrate. The refractive coefficient of an antireflection coating of an ideal single layer structure (reflection-reduced single layer film) is n
s
1/2
, where n
s
represents the refractive coefficient of a quartz glass substrate used as the transparent substrate in the present invention.
The material having an ideal value (=n
s
1/2
) of the refractive coefficient at KrF or ArF, which is known to the art nowadays, includes only calcium fluoride (fluorite) and a mixture of calcium fluoride and a suitable additive. Therefore, if the first photomask is used in the case where a KrF laser beam or an ArF laser beam is used as the exposed light, it is possib
Kawamura Daisuke
Nomura Hiroshi
Finnegan Henderson Farabow Garret & Dunner, L.L.P.
Kabushiki Kaisha Toshiba
Rosasco S.
LandOfFree
Photomask used in manufacturing of semiconductor device,... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Photomask used in manufacturing of semiconductor device,..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Photomask used in manufacturing of semiconductor device,... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3081665