Projection exposure system

Details Projection exposure system Projection exposure system

2003-05-08

2004-10-19

Fuller, Rodney (Department: 2851)

Photocopying

Projection printing and copying cameras

Illumination systems or details

C359S349000, C359S754000

Reexamination Certificate

active

06806942

ABSTRACT:

The invention relates to a projection exposure system for imaging a first object in a region of a second object. In particular, the projection exposure system is provided to be used in a process, in particular, in a lithographic process for manufacturing miniaturized devices. Accordingly, the projection exposure system is configured to enable an imaging with comparatively high resolution and correspondingly high numerical aperture.
In a conventional step of a lithographic process, an image of the geometry of the structure to be produced is provided as the first object or a mask or reticle. The mask is illuminated with light of a first wavelength band having a width &dgr;&lgr; about a central working wavelength &lgr;. With this light, the projection exposure system images the mask on the second object which is usually a wafer coated with a radiation-sensitive layer or resist. After having been exposed, the radiation-sensitive layer is subjected to further lithographic steps in order to form the miniaturized structures in the wafer in further process steps. The imaging of the mask on the radiation-sensitive layer is usually effected reduced in size, for example, with a scale of 4:1.
The present projection exposure system is particularly configured for effecting the illumination and imaging with light of a relatively wide wavelength band. Accordingly, the objective must have a relatively high chromatic correction in order to ensure also with such wide wavelength bands a highly resolved and sufficiently error-free imaging. A relative width &dgr;&lgr;/&lgr; of the wavelength band particularly aimed at is larger than 0.005. As a result, the projection exposure system is particularly suited for light of the Hg-I line which comprises light of a spectrum of a wavelength band of 365.5 nm ±2 nm. However, the present projection exposure system is not limited to the use of the Hg-I line. Rather, the use of other radiation sources and thus other wavelength bands is contemplated as well.
FIG. 1
schematically shows a beam path through an earlier design of a projection exposure system or objective
101
for imaging a mask positionable in an object plane
103
at an image plane
105
. The conventional objective
101
is provided for imaging with the Hg-I line and a wavelength &lgr; of 365.5 nm, the imaging being effected with a demagnification of 4 to 1 and a numerical aperture NA=0.65. A plurality of lenses is disposed along an optical axis
107
. In
FIG. 1
, a continuous numbering of the lenses is indicated above the same proceeding from an object plane
103
to an image plane
105
. Below the lenses, there is indicated a continuous numbering of the refractive surfaces of the lenses in the same order. Here, the lens surface of the first lens disposed towards the object plane
103
is designated by 2. The surface of said lens disposed towards the image plane
105
is designated with 3 and the lens surface of lens
2
disposed towards the object plane
103
is designated by 4 etc.
In
FIG. 1
there is further indicated: an upper marginal beam
109
as well as a lower marginal beam
111
of a field point
113
disposed on the optical axis
107
and an upper marginal beam
115
, a main beam
117
and a lower marginal beam
119
of an outermost field point
121
, respectively. Furthermore, a system diaphragm
123
is shown wherein the upper marginal beams
109
and
115
of the central field point
113
and the outermost field point
121
, respectively, approximately intersect. The lower marginal beams
111
and
119
of the field points
113
and
121
, respectively, approximately intersect therein as well.
As is evident from
FIG. 1
, the depicted objective is a so-called “three-bulge system”. This means that beam cross-sections of the imaging light comprise three expanded portions and, accordingly, two constricted portions disposed between the expanded portions. These constricted portions are referred to as beam waists, whereas the expanded portions are referred to as beam bulges. This beam path with three bulges and two waists is formed by accordingly arranging lens groups successively, wherein the lens groups in which the beam path is bulged predominantly provide a positive refractive power and, correspondingly, lens groups in which the beam path is constricted predominantly provide a negative refractive power. In
FIG. 1
, these lens groups are referred to hereafter as G
1
, G
2
, G
3
, G
4
and G
5
, the lens groups G
1
, G
3
and G
5
providing a positive refractive power and the lens groups G
2
and G
4
providing a negative refractive power.
The lenses indicated in hatched outline with lens numbers
1
,
3
,
9
,
13
,
14
,
15
,
16
,
22
,
27
,
30
and
31
are made of a high-dispersion material similar to a flint glass. The other lenses
2
,
4
,
5
,
6
,
7
,
8
,
10
,
11
,
12
,
17
,
18
,
19
,
21
,
23
,
24
,
25
,
26
,
28
and
29
are made of a low-dispersion material similar to a crown glass.
Although, according to calculations, the above-described earlier design of a projection exposure system exhibits satisfactory imaging properties as far as the numerical aperture and aberrations are concerned, comparative experiments performed on components of the system showed that the quality of the imaging through the components and the system, respectively, could diminish during operation in the course of time, in particular, when a high production rate is applied in the exposure of wafers, for example.
Accordingly, it is an object of the present invention to provide a projection exposure system of the above-described type which provides specific imaging properties with increased stability over time.
In this respect, the invention proceeds from a projection exposure system positionable between a first object and a second object for imaging the first object in a region of the second object with light of a wavelength band which has, in particular, a relative width &dgr;&lgr;/&lgr; of more than 0.002, preferably, of more than 0.005. Such a width of the wavelength band is relatively large in so far as specific wavelength bands of radiation which is likewise conventionally applied in lithography and which is, for example, provided by lasers, are considerably narrower.
Furthermore, the projection exposure system is a so-called three-bulge system, that is, it comprises five lens groups, wherein, in the order of the arrangement of the lens groups between the first object and the second object, each one of a first, a third and a fifth lens group has, as a whole, a positive refractive power, and each one of the lens groups respectively disposed therebetween, that is, the second and the fourth lens group, each has, as a whole, a negative refractive power.
Due to the wide wavelength band to be used, the projection exposure system exhibits a chromatic correction. To this end, the materials used to manufacture the individual lenses are selected from two material groups. A first material group thereof comprises materials having Abbe numbers which are higher than a limit value and, accordingly, materials of a second material group have Abbe numbers which are lower than the limit value. This means, that materials of the first material group have a lower dispersion than materials of the second material group. At least one of the lenses of the projection exposure system is made of a material of the first material group and has a positive refractive power, whereas at least another one of the lenses of the projection exposure system is made of a material of the second material group and has a negative refractive power.
The invention is based on the finding that certain glasses suitable for use at the predetermined working wavelength have a reduced long-term stability than other ones of such glasses. Due to the light passing through the lens material during operation, changes occur in the lens materials which can also change the optical properties of the lenses. Known mechanisms of such a kind are mechanisms known as “compaction”, “lens heating” or also “solarization”. It has now

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