Optics: measuring and testing – By alignment in lateral direction
Reexamination Certificate
1998-10-14
2001-06-26
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By alignment in lateral direction
C355S052000
Reexamination Certificate
active
06252662
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to a projection exposure apparatus and a device manufacturing method using the same. More particularly, the invention is suitably usable for the manufacture of devices such as semiconductor devices or liquid crystal devices, for example, wherein an illumination region on a mask having a pattern to be transferred is illuminated and, through full-field exposure or scan exposure with respect to the illumination region, the mask pattern is transferred to a photosensitive substrate, whereby a pattern image is printed thereon.
Recent semiconductor device manufacturing exposure apparatuses of the type that use an excimer laser are provided with a high driving frequency and high power excimer laser for improvement of throughput. As compared with conventional Hg lamps, excimer lasers produce discontinuous pulse light. Also, the peak value of its light intensity is high. For these reasons, there is a possibility that a glass material used in a projection optical system absorbs the energy which causes a change in optical performance.
Typical examples of optical performance to be changed by it are the transmission factor or refractive index. From the standpoint of deterioration of optical performance of a projection optical system, a change in refractive index of a glass material cannot be disregarded.
More specifically, the path of light passing through a projection optical system changes, before and after the refractive index change. This causes a large change in optical performance. Such a change in refractive index of a glass material is attributable to a contraction phenomenon (called “compaction”) due to two-photon absorption of the glass material. As regards the amount of change in refractive index caused thereby, as is known in the art, it is determined in accordance with equation (1) below.
Here, &Dgr;n′ is the change in relative refractive index (ppb=10
−9
), N is the pulse number (10
6
), and I is the energy density (mW/cm
2
) per one pulse. The refractive index change coefficient &agr;of quartz glass material in this case is about 0.01 in terms of a wavelength of 248 nm, and it is about 1.0 with a wavelength of 193 nm. Further, with the wavelength of 193 nm, there is an example wherein saturation (saturation efficient &ggr;) at a higher level is taken into account, as equation (2), below. It is known that the refractive index change coefficient &bgr; of the quartz glass material in this example is about 4.4 (saturation coefficient &bgr; is 0.4-0.8) in terms of a wavelength of 193 nm. Equation (1) corresponds to a case where the saturation coefficient is not considered (&ggr;=1).
&Dgr;
n′=&agr;×NI
2
(1)
&Dgr;
n
′=&bgr;×(
NI
2
)&ggr; (2)
It is seen that, in accordance with the above equation, a change in refractive index is large in a portion within the projection optical system where the pulse energy density is high, that is, the portion where the exposure light is collected. Projection optical systems used in semiconductor device manufacturing exposure apparatuses generally comprise a reduction system of 1:5 or 1:4. Thus, light is concentrated at a portion adjacent to the image plane.
In consideration of the above, in projection optical systems of recent exposure apparatuses having an excimer laser, in order to avoid damage of glass materials due to compaction, a lens closest to the image plane (wafer surface) is placed away from the image plane, as much as possible, so that the lens is disposed on a light path where the exposure light is expanded more. In other words, optical design is made under the restrictive condition that the distance between the projection optical system and the object (substrate) to be exposed, i.e., back focus, should be made longer.
The restrictive condition that the back focus of a projection optical system has to be long, in the optical design of an exposure apparatus in which strictest optical performance is required also from the point of wave optics, significantly narrows design latitude, and it may cause deterioration of optical performance.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a projection exposure apparatus and/or a device manufacturing method using the same, wherein a glass material or materials constituting a projection optical system are less deteriorated and high optical performance is assured, through appropriate setting of components of the projection optical system or a pulse light source for projection exposure of a mask pattern onto a substrate to be exposed.
In accordance with an aspect of the present invention, there is provided a projection exposure apparatus, comprising: an illumination optical system for illuminating a pattern of a mask with pulse light from a light source; and a projection optical system for projecting the pattern of the mask onto a rectangular region on a substrate; wherein said exposure apparatus satisfies a relation
&tgr;>(&bgr;/&Dgr;
n′
max
)
1/&ggr;
NI
0
2
f
2
(
D
)
where &tgr; is the width of a single pulse of the pulse light, D is the distance from the last surface of the projection optical system to the surface of the substrate, &bgr;, &ggr; and &Dgr;n′ are the coefficient of refractive index change of, the saturation coefficient of and the tolerable change in relative refractive index of the glass material of the final lens of the projection optical system, respectively, N is the number of pulses irradiated, and I
0
is the energy density per pulse upon the image plane; and
when L
1
and L
2
are half lengths of two sides of the rectangular region (L
1
≦L
2
), A is the numerical aperture of the projection optical system, and &sgr; is the coherence factor determined by an illumination condition,
f
(
D
)
=
{
1
(
R
<
L
1
)
1
-
(
2
/
π
)
[
φ
1
-
sin
(
2
φ
1
)
/
2
]
(
L
1
≦
R
<
L
2
)
1
-
(
2
/
π
)
{
φ
1
+
φ
2
-
[
sin
(
2
φ
1
)
+
sin
(
2
φ
2
]
/
2
}
(
L
2
≦
R
<
L
1
2
+
L
2
2
)
4
L
1
L
2
/
π
R
2
(
L
1
2
+
L
2
2
≦
R
)
wherein
R
=
[
A
σ
/
I
-
(
A
σ
)
2
]
D
,
cos
φ
1
=
L
1
/
R
and
cos
φ
2
=
L
2
/
R
.
In accordance with another aspect of the present invention, there is provided a projection exposure apparatus, comprising: an illumination optical system for illuminating a pattern of a mask with pulse light from a light source; and a projection optical system for projecting the pattern of the mask onto a rectangular region on a substrate; wherein a pulse light emission time ratio of the pulse light is approximately not less than 2×10
−5
, and wherein the distance from the last surface of the projection optical system to the surface of the substrate is approximately not greater than 12 mm.
In this aspect of the present invention, when the light source has an emission frequency of 1 KHz, the pulse width of a single pulse of the pulse light may be approximately not less than 20 ns.
In accordance with a further aspect of the present invention, there is provided a projection exposure apparatus, comprising: an illumination optical system for illuminating a pattern of a mask with pulse light from a light source; and a projection optical system for projecting the pattern of the mask onto a rectangular region on a substrate; wherein a pulse light emission time ratio of the pulse light is approximately not less than 4×10
−5
, and wherein the distance from the last surface of the projection optical system to the surface of the substrate is approximately not greater than 8 mm.
In this aspect of the present invention, when the light source has an emission frequency of 1 KHz, the pulse width of a single pulse of the pulse light may be approx
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Font Frank G.
Lee Andrew H.
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