Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2002-09-17
2004-11-16
Mathews, Alan (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S067000
Reexamination Certificate
active
06819403
ABSTRACT:
DESCRIPTION OF THE INVENTION
1. Field of the Invention
The present invention pertains to an illumination optical system, exposure apparatus, and microdevice manufacturing method. In particular, the present invention relates to an illumination optical system capable of illuminating a mask, reticle, or other such object; the optical system's illumination having a uniform illuminance distribution. The present invention further relates to an exposure apparatus equipped with such an illumination optical system and capable of being used, among other things, during the manufacture of semiconductor elements, liquid crystal display elements, image pickup elements, thin-film magnetic heads, and/or other such microdevices. The present invention also relates to a microdevice manufacturing method employing such an exposure apparatus.
2. Background of the Invention
Exposure apparatuses may be employed during photolithographic operations—such operations representing a portion of the operations for the manufacture of semiconductor elements, liquid crystal display elements, image pickup elements, thin-film magnetic heads, and/or other such microdevices—to transfer patterns formed on masks or reticles (hereinafter referred to collectively as “mask”) onto wafers, glass plates, substrates or the like (hereinafter referred to collectively as “substrate”) which have been coated with photoresist or other such photosensitive material. To be able to illuminate a mask with illumination of uniform illuminance distribution, such an exposure apparatus may be equipped with an illumination optical system capable of causing light emitted from an excimer laser or other such light source to possess a uniform illuminance distribution within a beam formed so as to have a prescribed cross-sectional shape.
If the illuminance distribution of the light emitted from such an illumination optical system varies over the surface of the mask or substrate, there will be nonuniformity in linewidth throughout the pattern formed on the substrate. This variation occurs because the exposure dose of the light irradiating the substrate will vary in correspondence to that illuminance distribution. However, high uniformity in linewidth is demanded during the manufacture of semiconductor elements employed in logic circuits, such semiconductor elements representing one category among the semiconductor elements mentioned above as examples of microdevices. Linewidth uniformity is required because nonuniformity in pattern linewidth will result in decreased operational speed. As an example of the significance of this fact, central processor units (CPUs) operating at frequencies of several GHz have in recent years become standard, and because further increases in operating speed can be expected to be achieved in the future, increased uniformity of pattern linewidth is likely to be extremely important.
To cause light irradiating a substrate to have a uniform exposure over the surface of the substrate, conventional exposure apparatuses have employed illumination optical systems possessing condenser lenses having distortion. The value of the distortion being varied so as to achieve a uniform exposure dose across the surface of the substrate. Referring to
FIG. 11
, the principle by which the illuminance distribution might be varied by varying distortion of a condenser lens is briefly described.
FIG. 11
is a drawing to assist in description of the principle by which an illuminance distribution might be adjusted by means of a condenser lens.
In
FIG. 11
, P
1
represents a light source,
100
represents a condenser lens, and P
2
represents the plane of an object to be illuminated (“object plane”). This object plane P
2
might for example be the plane in which the pattern on a mask is formed. In the discussion below, &thgr; represents the exit angle of a light beam emitted from light source P
1
(the exit angle of a light beam emitted so as to be parallel to optical axis AX being taken to have &thgr;=0), f represents the focal length of condenser lens
100
, and h represents the distance from optical axis AX to a location on object plane P
2
at which the light beam emitted from light source P
1
at exit angle &thgr; is incident thereon.
Assuming standard Koehler illumination, the relationship describing projection by the condenser lens will, in general, be given by FORMULA (1), below:
h=f·g
(&thgr;) (1)
Note that at FORMULA (1), above, g(&thgr;) is a function of &thgr;.
If we assume that light source P
1
is a perfectly diffusing surface (a photometrically ideal surface illuminant), then illumination at object plane P
2
will be uniform when g(&thgr;)=sin(&thgr;). We therefore take the distortion of condenser lens
100
to be zero when g(&thgr;)=sin(&thgr;).
Let us first consider the case in which the distortion of condenser lens
100
is zero. In such a case, the infinitesimal area dS of the locus on object plane P
2
of a light beam of infinitesimal solid angle d&OHgr; emitted from light source P
1
is given by FORMULA (2), below:
dS=dh d&psgr; h=f
2
sin &thgr; cos &thgr; d&thgr; d&psgr; (2)
. . . where &psgr; is an angle of rotation about optical axis AX.
We next consider the case in which condenser lens
100
has nonzero distortion. The relationship describing projection when there is n% distortion at some image height is given by FORMULA (3), below:
h=f
sin &thgr;(1
−n
/100) (3)
Now, because the dimensions of condenser lens
100
are fairly uncomplicated, there is little generation of aberration of order three or higher. It is therefore sufficient to likewise only consider distortion attributable to aberration up to the third order. Upon making such an assumption, since distortion is now assumed to be proportional to the square of image height, we can express this in the form n=&agr;sin
2
&thgr;, where &agr; is a constant.
In such a case, the infinitesimal area dS of the locus on object plane P
2
of a light beam of infinitesimal solid angle d&OHgr; emitted from light source P
1
is given by FORMULA (4), below:
d
S
=
d
h
d
ψ
h
f
2
sin
θ
cos
θ
d
θ
d
ψ
(
1
-
4
α
sin
2
θ
/
100
+
3
α
2
sin
4
θ
/
10000
)
(
4
)
FORMULA (2), above, gives the infinitesimal area dS of the locus on object plane P
2
of a light beam of infinitesimal solid angle d&OHgr; emitted from light source P
1
for zero distortion at condenser lens
100
. FORMULA (4), above, gives the infinitesimal area dS of the locus on object plane P
2
of a light beam of infinitesimal solid angle d&OHgr; emitted from light source P
1
for nonzero distortion at condenser lens
100
.
FORMULAS (2) and (4) determine the infinitesimal areas dS of the loci on object plane P
2
produced by light beams of identical infinitesimal solid angle d&OHgr; emitted from light source P
1
. Using FORMULAS (2) and (4), one obtains a smaller infinitesimal area dS when there is distortion as compared with the infinitesimal area dS obtained when there is no distortion. This is so despite use of the same infinitesimal solid angle d&OHgr;. From these results, one can conclude that illuminance will be greater by a corresponding amount.
If we now take the ratio of the expressions at the right sides of FORMULAS (2) and (4), above, we find that infinitesimal area dS is foreshortened due to distortion by a factor given by:
1−4&agr; sin
2
&thgr;/100+3&agr;
2
sin
4
&thgr;/10000.
Since the term 3&agr;
2
sin
4
&thgr;/10000appearing in this formula can be ignored when distortion is exceedingly small, i.e., for &agr;<<1, the factor by which infinitesimal area dS is foreshortened due to distortion can in such case be said to be substantially given by
Finnegan Henderson Farabow Garrett & Dunner LLP
Mathews Alan
Nikon Corporation
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