Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-04-14
2002-03-05
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06352800
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reticle for use in exposing a semiconductor, a method of producing the reticle, and a semiconductor device, and more particularly, to a reticle to be used in exposure for producing a resist pattern, a method of producing the reticle, and a semiconductor device.
2. Description of Related Art
A photolithography process for manufacturing a semiconductor device comprises a photoresist application process, an exposure process, and a development process. Of these processes, the exposure process is a process for faithfully reproducing an integrated circuit pattern formed on a reticle onto a photoresist pattern formed on a wafer, through use of a stepper. An optical reduction-projection exposure system is widely used as an exposure system.
FIG. 4
shows an optical reduction-projection exposure system
100
(hereinafter referred to as a “stepper”) using a conventional reticle
110
. In
FIG. 4
, reference numeral
101
designates alight source;
102
designates a shutter for adjusting light emitted from the light source
101
;
103
designates a beam-shaping optical system for shaping the light emitted from the light source
101
by way of the shutter
102
so as to assume a desired geometry;
104
and
105
designate Fly's eye lenses for uniformly illuminating the area to be exposed;
106
designates a vibration mirror for guiding the light emitted from the Fly's eye lens
104
to the Fly's eye lens
105
;
107
designates a reticle blind for covering an area on a reticle
110
other than the area onto which a circuit pattern is to be projected through exposure;
109
designates a condenser lens for illuminating the entire surface of the reticle
110
;
108
designates a mirror for guiding to the condenser lens
109
the light which has passed through the reticle blind
107
;
110
designates a reticle on which is formed a circuit pattern to be projected onto the wafer
112
;
111
designates a reduction-projection lens for projecting, in a reduced manner and onto the wafer
112
the light which has passed through the reticle
110
; and
112
designates a wafer on which a circuit pattern formed on the reticle
110
is patterned through exposure.
FIG. 5
shows diffraction of light caused when the reticle
110
is exposed to coherent light formed from a plane wave of coherent wavelength and phase. In
FIG. 5
, the same reference numerals as those provided in
FIG. 4
designate the same elements, and hence their explanations are omitted here. In general, as shown in
FIG. 5
, in a case where the reticle
110
is exposed to coherent light, light
50
which has entered the reticle
110
at right angles is divided into light
52
(0-order light) which travels straight ahead and light beams
51
and
53
(±m-order light beams, where m=1, 2, 3, . . . ) which are diffracted, by the surface (lower surface) of the rectile
110
. Here, provided that an angle of diffraction; for example, an angle of diffraction exemplified by an angle formed between the straightly-traveling light
52
and the diffracted light
51
is &thgr;; a pattern pitch—which is in the proportion of one line representing the width of linear patterns formed on the rectile
110
to one space representing the space between the lines—is taken as P; an index of refraction of the reticle
110
is taken as “n”; the numeric aperture expressed as NA=nsin&thgr; is taken as NA; and the wavelength of the incident light
50
is taken as &lgr;, there stands a relationship between the pattern pitch P and the numeric number PA, as expressed below.
P=m&lgr;/NA (1)
As represented by Eq. (1), the numeric aperture NA and the angle of diffraction &thgr; increases with a reduction in the pattern pitch P. In contrast, if the pattern pitch P is constant, diffracted light of greater order “m” has a greater numeric aperture NA, and the diffraction angle &thgr; increases. As mentioned above, Eq. (1) represents the minimum pattern pitch P at which m-order light can be collected at the predetermined wavelength &lgr; and the numeric aperture NA. For instance, when the wavelength &lgr; is 248 nm and the numeric aperture NA is 0.55, the minimum pattern pitch P at which light of m=±1 order can be collected can be expressed as P=1×248 (nm)/0.55=451 nm=0.45 &mgr;m. In the case of a circuit pattern whose width is smaller than 0.45 &mgr;m, diffracted light of m=±1 order cannot be collected. If only 0-order light is used for exposure, an image patterned on the wafer
112
loses contrast and is not resolved. Accordingly, as a circuit pattern formed on the reticle
110
becomes more minute, the contrast of an image of the circuit pattern projected on the wafer
112
through exposure is reduced. If the circuit pattern becomes smaller than a certain size, the circuit pattern will not be resolved, thereby hindering formation of the circuit pattern (e.g., a resist pattern) onto the wafer
112
.
Conventionally, to solve the foregoing problem, the contrast of an image to be resolved on the wafer
112
is increased through use of modified illumination, thereby resolving a more minute resist pattern.
FIGS. 6A and 6B
are descriptive views of modified illumination, wherein
FIG. 6A
shows exposure without use of modified illumination and
FIG. 6B
shows exposure using modified illumination. In
FIG. 6
, those reference numerals which are the same as those provided in
FIGS. 4 and 5
designate the same elements, and hence repetition of their explanations is omitted here. In
FIG. 6A
, reference numeral
66
designates a circuit pattern formed on the surface (lower surface) of the reticle
110
;
64
designates a depth of focus of the light resolved on the wafer
112
; and
65
designates the contrast of light resolved.
Modified illumination refers to an illumination technique for causing a luminous flux to which a reticle is to be exposed to enter an optical system obliquely, through use of a diaphragm provided outside the optical axis of the optical system.
FIG. 6B
shows a case where a luminous flux
60
of exposing radiation is caused to enter the reticle
110
obliquely through use of modified illumination. In
FIG. 6B
, reference numeral
62
designates 0-order light diffracted by the reticle
110
;
61
designates +1-order light diffracted by the reticle
110
;
63
designates −1-order light diffracted by the reticle
110
;
67
designates the depth of focus (DOF) of the light resolved on the wafer
112
; and
68
designates the contrast of the light resolved on the wafer
112
. A comparison between exposure without use of modified illumination and exposure using modified illumination reveals that the focal depth DOF
67
is greater than the focal depth DOF
64
, and that the contrast
68
is greater than the contrast
65
. The contrast of the image formed on the wafer
112
can be improved by means of increasing the luminous flux of exposing radiation
60
that enters obliquely, through use of modified illumination. Consequently, a more minute resist pattern formed on the reticle
110
can be resolved.
FIGS. 7A and 7B
show a case where the previously-described modified illumination is applied to a stepper
100
shown in
FIG. 4
, wherein
FIG. 7A
shows exposure without use of modified illumination and
FIG. 7B
shows exposure using modified illumination. In
FIGS. 7A and 7B
, those reference numerals which are the same as those provided in
FIGS. 4 through 6
designate the same elements, and hence repetition of their explanations is omitted here. In
FIG. 7A
, reference numeral
109
designates a condenser lens corresponding to the previously-described condenser lens; and
40
designates the vertical distance (positive and negative) over which the stepper
100
is moved with reference to a horizontal position
41
of the reduction-projection lens
111
, which is taken as 0. In
FIG. 7B
, reference numeral
74
designates a diaphragm plate in which a diaphragm
McDermott & Will & Emery
Rosasco S.
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