Optics: measuring and testing – By alignment in lateral direction
Reexamination Certificate
1998-10-27
2001-11-06
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By alignment in lateral direction
C356S401000, C356S508000
Reexamination Certificate
active
06313916
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates generally to a projection exposure apparatus having high alignment function and, more particularly, to a position detecting system and a projection exposure apparatus with the same, wherein position detection is performed by use of an interference image of diffractive light produced by a grating mark, having a periodicity and being provided on an object such as a wafer to be detected.
Semiconductor device manufacturing technology has been advanced remarkably, and microfabrication techniques have also been improved considerably. Particularly, optical processing techniques have entered a microfabrication region of submicron resolution order, after production of 1 MDRAM semiconductor chips.
With increases in density of a semiconductor chip along with miniaturization of IC or LSI, the range of tolerance for registration between a mask pattern and a wafer (photosensitive substrate) becomes very narrow. Usually, positional information of a wafer is obtained by use of alignment marks provided on the wafer. There are various methods for observation of such alignment marks.
Most prevalent method is one based on use of video image. Japanese Laid-Open Patent Application, Laid-Open No. 343291/1993 shows a method which is based on an interference image obtainable by extracting (±n)-th order light (n=1, 2, 3, . . . ) among reflectively diffractive light as produced by a grating mark having periodicity and being provided on a wafer surface. Such interference image is then FFT processed and the phase is detected. The detection resolution is therefore high, and the system is incorporated into a projection exposure apparatus as an alignment microscope.
FIG. 1
shows a schematic arrangement of the
15
system of the aforementioned Japanese Laid-Open Patent Application, Laid-Open No. 343291/1993. Lithographic transfer of a pattern is performed by projecting, through a projection lens
1
and in a reduced scale, an electronic circuit pattern formed on a reticle R surface as illuminated by exposure light from an illumination system IL, onto a wafer W surface placed on a wafer stage ST.
In an alignment operation, an interference image is formed by (+−n)-th order light (n=1, 2, 3, . . . ) produced by a grating mark with periodicity, as provided on the wafer surface. The interference image detected is FFT processed, whereby a phase thereof is detected, such that it is transformed into positional information. Here, components of the alignment system have the following functions.
Denoted at
51
is a detection optical system having a reference mark GS. Denoted at
101
is an image pickup means having a solid image pickup device. Denoted at GW is a wafer mark (grating mark or alignment mark) which is provided on the wafer surface.
For the alignment operation, within the projection exposure apparatus, the relative position of the reticle R relative to the projection lens
1
, the detection optical system
51
and the image pickup means
101
is predetected by using an appropriate detection system. In this state, the position of a projected image of the wafer mark GW of the wafer W with respect to a projected image of the reference mark GS within the detection optical system
51
, is detected upon an image pickup surface of the image pickup device
101
. Thus, relative alignment of the reticle R and the wafer W is performed indirectly.
In conventional projection exposure apparatuses, the position of the wafer mark GW will be detected in the manner to be described below, for positioning the wafer W with respect to a predetermined position.
Denoted at
2
is a HeNe laser which produces rectilinearly polarized light of alignment wavelength &lgr; different from the exposure light (to be used for the exposure process). The light from the laser
2
enters an acoustooptic optical element (AO element)
3
, by which the quantity of light directed to a lens
4
is controlled. The acoustooptic element
3
has a function of blocking light completely when it is in a certain state. The light passing through the acoustooptic element
3
is collected by the lens
4
and, after this, it impinges on a polarization beam splitter
5
while the illumination range is spatially restricted by means of a field stop SIR which is disposed on a plane I optically conjugate with the wafer W.
The polarization beam splitter
5
reflects the received light, and the reflected light goes via a quarter waveplate
6
, a lens
7
, a mirror
8
, a lens
9
and the projection lens
1
. The light then illuminates the wafer mark GW on the wafer W surface, perpendicularly.
Here, the illumination light passes the portion I
0
(
FIG. 2
, (A) and (C)) upon the pupil plane
31
of the optical system which is constituted by the projection lens
1
, the lens
9
and the lens
7
. In
FIG. 2
, V and W denote coordinates on the pupil plane
31
, and the portion I
0
represents incidence angle distribution of the illumination light with respect to the wafer W surface. The pupil plane
31
corresponds to the Fourier's transformation surface for the wafer W surface which is an image plane, and therefore the illumination light impinges on the wafer W surface substantially perpendicularly.
As shown in
FIG. 2
, (B) and D), the wafer mark GW formed on the wafer W surface comprises a diffraction grating pattern (grating mark) of a pitch P. The portions with hatching in (B) and (D) of
FIG. 2
denoted that theses regions have a difference in level (height) as compared with other regions on the wafer surface, or they have a difference in phase, thus providing the function as a diffraction grating.
The light reflected by the wafer mark GW passes through the projection lens
1
and, thereafter, it goes via the lens
9
, mirror
8
, lens
7
, quarter waveplate
6
and polarization beam splitter
5
, in this order. Then, it goes via the lens
10
and the beam splitter
11
, and it forms an aerial image of the wafer mark GW, at position F. The aerial image of the wafer mark GW formed at the position F passes through a Fourier transformation lens
12
whereby it is Fourier transformed. Stopper
14
serves to selectively transmit only light of predetermined orders, out of the reflectively diffractive light from the wafer mark GW.
If, for example, reflectively diffractive light of n-th order is to be transmitted, what to be selected is reflectively diffractive light corresponding to ±sin
−1
(n&lgr;/P). The light selected by the stopper
14
goes through a Fourier transformation lens
16
, and it forms an interference image of the wafer mark GW upon a solid image pickup device
20
. The produced interference image is an image of the wafer mark GW that provides a diffraction grating of pitch P as illuminated by monochromatic light. Therefore, it has sufficiently high and stable signal light quantity and contrast as compared with a dark field image in a case where scattered light is used.
In the optical arrangement described above, the lenses
7
and
10
provide a correction optical system
52
in respect to the imaging of the wafer mark GW, which serves to correct aberrations such as on-axis chromatic aberration or spherical aberration, for example, to be produced by the projection lens
1
with respect to the alignment wavelength. It is not necessary for the correction optical system
52
to perform aberration correction with respect to all the light beams to be received. Aberration correction should be made only with respect to the reflectively diffractive light passed through the stopper
14
. Thus, the system
52
can be accomplished with a simple structure. In excimer steppers that use light of an excimer laser as exposure light, the amount of aberration produced with respect to the alignment wavelength which illuminates the wafer mark GW is large as compared with that in projection exposure apparatus using conventional g-line or i-line light. The restricted necessity of aberration correction described above is therefore very effective.
On the other hand, a
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Font Frank G.
Punnoose Roy M.
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