Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2002-04-22
2004-11-02
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S022000, C355S018000, C356S400000
Reexamination Certificate
active
06811939
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a focus monitoring method, a focus monitoring system, and a device fabricating method. More particularly, the invention relates to a focus monitoring method, a focus monitoring system, and a device fabricating method for use in generation of a pattern of a device.
2. Description of the Background Art
Increase in packing density and reduction in size of a semiconductor integrated circuit in recent years is remarkable. In association with this, a circuit pattern formed on a semiconductor substrate (hereinafter, simply called a wafer) is rapidly becoming finer.
In particular, the photolithography technique is widely recognized as a basic technique of generating a pattern. It has been therefore variously developed and modified until today. The pattern is continuously becoming finer and a demand on improvement of the resolution of the pattern is increasing.
The photolithography technique is a technique of transferring a pattern on a photo mask (original image) onto a photoresist applied on a wafer and patterning an underlayer to be etched by using the transferred photoresist.
At the time of transferring the photoresist, the photoresist is subjected to a developing process. In a photoresist of the positive type, the photoresist irradiated with light in the developing process is removed. In a photoresist of the negative type, the photoresist which is not irradiated with light is removed.
Generally, a resolution limit R(nm) in the photolithography technique using a stepper method is expressed as follows.
R=k
1
·&lgr;/(
NA
)
where &lgr; is a wavelength (nm) of light used, NA denotes numerical aperture of a projection optical system of a lens, and k
1
is a constant depending on image forming conditions and resist process.
As understood from the expression, to improve the resolution limit R, that is, to obtain a finer pattern, a method of setting each of k
1
and &lgr; to a small value and setting NA to a large value can be considered. That is, it is sufficient to set a lower constant which depends on the resist process, shorten the wavelength, and set a larger NA.
It is however difficult from the technical point of view to improve a light source and a lens. By shortening the wavelength and setting a larger NA, a focal depth &dgr; (&dgr;=k
2
·&lgr;/(NA)
2
) of light decreases, and a program such as deterioration in the resolution arises.
In the photolithography technique, to expose and transfer the pattern of a photo mask onto a photoresist with high resolution, the photo mask has to be exposed in a state where the photoresist is fit in the range of the depth of focus with respect to the best focus plane of the projection optical system. For this purpose, the position of the best focus plane of the projection optical system, that is, the best focus position has to be calculated by any method.
An example of a conventional focus monitor for measuring the best focus position is a phase shift focus monitor developed by Brunner of IBM corporation and sold by Benchmark Technology Co., U.S.A.
FIG. 16
is a diagram for explaining a phase shift focus monitoring method. Referring to
FIG. 16
, the phase shift focus monitoring method uses a phase shift mask
105
. Phase shift mask
105
has a transparent substrate
105
a
, a shielding film
105
b
having a predetermined pattern, and a phase shifter
105
c
formed on the predetermined pattern.
Phase shift mask
105
has a pattern in which, as concretely shown in
FIG. 17
, narrow shield pattern
105
b
is provided between sufficiently thick transmitting portions
105
d
and
105
e
. Phase shifter
105
c
is not disposed in transmitting portion
105
d
but is disposed in transmitting portion
105
e.
According to the phase shift focus monitoring method, referring to
FIG. 16
, first, phase shift mask
105
is irradiated with light. Since phase shifter
105
c
is constructed to shift the phase of transmission light by about 90 degrees, in the case where light passed through transmitting portion
105
e
travels faster than light passed through transmitting portion
105
d
by an optical path difference of 1/4&lgr;, 5/4&lgr;, . . . or in the case where the light passed through transmitting portion
105
e
travels behind the light passed through transmitting portion
105
d
by an optical path difference of 3/4&lgr;, 7/4&lgr;, . . . , the light strengthens with each other. Consequently, light passed through phase shift mask
105
has an intensity distribution asymmetrical with respect to the z axis (optical axis). Light passed through phase shift mask
105
is condensed by projection lenses
119
a
and
119
b
, and an image is formed on a photoresist
121
b
on a semiconductor substrate
121
a.
By the phase shift focus monitor, an image is formed on photoresist
121
b
in a state where the intensity distribution of diffracted light is asymmetrical with respect to the z axis. With movement of a wafer
121
in the z direction, an image of a pattern on the wafer
121
therefore moves in the direction (x-y direction, that is, the lateral direction of the drawing) perpendicular to the z axis (vertical direction of the drawing). By measuring the shift amount of the image of the pattern in the x-y direction, the position in the z direction, that is, focus can be measured.
In the conventional phase shift focus monitoring method, to obtain high detection sensitivity in the z direction (ratio of the shift amount in the x-y direction to the shift amount in the z direction), isotropic illumination having a small angle (circular shape in a pupil plane), that is, illumination having a small &sgr; value has to be used. This is described by T. A. Brunner et al., “Simulations and experiments with the phase shift focus monitor”, SPIE Vol. 2726, pp. 236-243.
FIG. 4
of this literature teaches that, when the &sgr; value is 0.3, the shift amount (focus monitor overlay error) in the x-y direction of a pattern is the largest, and the detection sensitivity in the z direction increases.
In order to obtain illumination having a small &sgr; value as described above, for example, as shown in
FIG. 18
, the diameter of an open portion
14
d
of an illumination aperture
14
has to be decreased.
However, when a device pattern is formed with illumination having the &sgr; value as small as about 0.3, coherence of light is too strong, and deformation of a two-dimensional pattern transferred onto the photoresist is remarkable. To suppress such deformation of a two-dimensional pattern, the diameter of the illumination aperture
14
used at the time of forming a device pattern is set to be larger than that of illumination aperture
14
used at the time of monitoring a focus, and the &sgr; value has to be set to, for example, 0.6 or higher. Consequently, different illumination apertures
14
have to be used for monitoring of a focus and formation of a device pattern. There is a problem such that effort and management for changing illumination aperture
14
are required.
When oxygen entered in the illumination optical system at the time of the change remains, the lens is clouded up. Consequently, the oxygen has to be purged by introducing nitrogen for long time after the change, and a problem such that the work becomes complicated occurs.
SUMMARY OF THE INVENTION
An object of the invention is to provide a focus monitoring method, a focus monitor system, and a device fabricating method with high detection sensitivity in the z direction while an illumination aperture does not have to be changed.
According to the invention, there is provided a focus monitoring method used for forming a pattern of a device, wherein a pattern of a photo mask for phase shift focus monitor, the photo mask having first and second light transmitting areas adjacent to each other while sandwiching a shielding film, is transferred onto a photosensitive member on a substrate by using modified illumination, the pattern being constructed so that a phase difference other than 180 degrees occurs between exposure light pass
Maejima Shinroku
Miyamoto Yuki
Nakao Shuji
McDermott Will & Emery LLP
Renesas Technology Corp.
Young Christopher G.
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