Optics: measuring and testing – By light interference – Having polarization
Reexamination Certificate
2001-06-05
2003-04-22
Hannaher, Constantine (Department: 2878)
Optics: measuring and testing
By light interference
Having polarization
C356S492000, C356S508000, C356S509000
Reexamination Certificate
active
06552798
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to a position detecting method and a position detecting system. Also, the invention concerns an exposure apparatus, a device manufacturing method, a semiconductor manufacturing factory, and/or a maintenance method for an exposure apparatus, using such a position detecting system. In another aspect, the invention is directed to an interference microscope for measuring the position of a pattern on an object, to be observed through an imaging optical system, and for detecting the position of that object on the basis of it.
For example, the invention is applicable to an alignment detecting system in an exposure apparatus for semiconductor manufacture, or an overlay inspecting system to be used for inspection of distortion or any performance such as alignment precision, in an exposure apparatus.
Projection exposure apparatuses for semiconductor manufacture are required to have a performance of projecting and printing a circuit pattern formed on the surface of a reticle onto the surface of a wafer with a high resolving power, to meet the increasing density of integrated circuits. The projection resolving power for a circuit pattern can be improved, for example, by enlarging the numerical aperture (NA) of a projection optical system while keeping the wavelength of exposure light unchanged. Alternatively, it can be done by shortening the wavelength of exposure light, such as changing light from g-line to i-line, from i-line to excimer laser emission wavelength, or to F
2
laser emission wavelength or SOR light.
On the other hand, there are requirements for higher precision alignment of a reticle (having an electronic circuit pattern) and a wafer, to meet further miniaturization of circuit patterns. The required precision is generally one-third or less of the circuit pattern. For a 1 gigabit DRAM with a 0.18 micron rule circuit pattern, for example, an overlay precision of 60 nm or less (alignment over the whole exposure region) is required. Further, as for an overlay inspection system for measuring this overlay precision, a precision of one-tenth of the overlay precision is required. Thus, a precision of 6 nm or less is necessary, for a 1 gigabit DRAM.
There is a TIS correction method for reducing the influence of a TIS (Tool Induced Shift), which is a detection system factor among measurement error producing factors, to attain high precision measurement.
Referring to
FIG. 1
, the TIS correction method will be explained. In
FIG. 1
, as an example, a surface step (level difference) is produced on a silicon wafer by an etching process, and, after alignment with that pattern, the relative relationship with respect to a resist image pattern defined by exposure and development is measured. In the TIS correction method, the measurement is carried out twice. The second measurement is made with the wafer being rotated 180 degrees, as compared with the first measurement. Thus, the result in the first measurement is called a “0-deg. measured value”, and the result in the second measurement is called a “180-deg. measured value”. In the TIS correction method, a value (&Dgr;TIS correction) which is obtainable by subtracting the 180-deg. measured value from the 0-deg. measured value and then by bisecting the remainder, is taken as a measured value. With this procedure, the error due to the detection system factor is reduced to attain high precision measurement. Here, the value obtained by adding the 0-deg. measured value and the 180-deg. measured value and then by bisecting the sum, is called a “TIS”.
Most of the overlay inspection systems or alignment detection systems, currently available, use a bright-field image processing method.
FIG. 2
shows a known example of an overlay inspection system.
In the inspection system of
FIG. 2
, exclusive marks
2
and
3
are formed on a wafer
1
. Images of these marks are formed through an optical system upon an image pickup device
14
such as a CCD, and the position is detected by processing an electric signal from the CCD.
The imaging performance most required in this optical system is the symmetry of image. If there is something in the optical system that deteriorates the image symmetry, a TIS is produced.
These types of detecting systems use a high optical magnification such as 100x, for example, and in most cases, a region close to the optical axis is used. Therefore, the major factor causing degradation of the image symmetry is not abaxial aberrations but non-uniformness of the illumination system and eccentric coma aberration close to the optical axis of the optical system.
In recent semiconductor processes, flattening has been advanced, and a CMP (Chemical Mechanical Polishing) process is carried out at plural steps.
However, the flattening technology raises a critical problem to alignment or overlay inspection machines. As a result of the flattening process, the level difference (surface step) of a mark to be used for the detection is eliminated. In the bright-field image processing method which is used most prevalently and with a good precision, the contrast of a mark image to be used for the measurement becomes very poor, which directly leads to deterioration of the detection precision.
As a measure for such a problem, a phase difference detecting method has been proposed. However, this method needs a phase plate in a portion of the optical system, and this is a factor for producing a TIS as described above. Therefore, while the contrast may be better, a good precision may not be attainable. Particularly, since the phase plate should be made mountable/demountable into and out of the optical system so as to allow coexistence with an ordinary bright-field system, this creates a factor for a large TIS.
An interferometer detecting method may be a detecting method not sensitive to a TIS. An example will now be described, in conjunction with FIG.
3
.
First, a conventional overlay inspecting system of
FIG. 2
will be described, and second, the interferometer detecting system shown in
FIG. 3
will be described.
In
FIG. 2
, an etching pattern mark
2
is produced on a silicon etching wafer
1
, through a lithographic process, a development process and an etching process. Then, upon the etching pattern mark
2
, a resist pattern mark
3
is formed through a lithographic process and a development process. In an overlay inspecting system, the relative positional relation between these two marks
2
and
3
is just going to be measured. To this end, the marks
2
and
3
are illuminated with light
6
emitted from a halogen lamp
5
. More specifically, the light
6
from the lamp goes through a fiber
7
and an illumination system
8
, and S-polarized light is reflected by a polarization beam splitter
9
. Thereafter, the light passes through a relay
12
and via a mirror
4
, and then through a quarter waveplate
10
and an objective lens
11
, to illuminate the marks
2
and
3
.
Reflected light from the marks
2
and
3
passes backwardly through the objective lens
11
, the quarter waveplate
10
and the relay
12
. Since the polarization direction is P-polarization, the light is transmitted through the polarization beam splitter
9
. Then, by means of an erector
13
, an image is formed on the image pickup surface of a CCD camera
14
. The observation image thus formed on the image pickup surface is photoelectrically converted by the CCD camera
14
, and a corresponding signal is applied to a computer (calculating means)
15
through a line. The computer
15
performs image processing to the received signal, and it detects the relative positional relationship between the two marks.
Here, the polarization beam splitter
9
and the quarter waveplate
10
are used for the sake of efficient use of the light quantity. If the light source has a large power or the object to be observed has a large reflection factor such that the loss of light quantity can be disregarded, use of a polarization beam splitter or a quarter waveplate may be omitted. A half mirror may be used, for example, in plac
Ina Hideki
Kitaoka Atsushi
Suzuki Takehiko
LandOfFree
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