Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-04-23
2004-06-01
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S030000
Reexamination Certificate
active
06743554
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photomask for aberration measurement, an aberration measurement method, a unit for aberration measurement and a manufacturing method for the unit.
2. Description of the Background Art
In recent years increases in integration and miniaturization of semiconductor integrated circuits have been remarkable. Together with that, miniaturization of circuit patterns formed on a semiconductor substrate (hereinafter referred to simply as wafer) has made rapid progress.
In particular, photolithographic technology is widely recognized as a basic technology used for pattern formation. Accordingly, a variety of developments and improvements have recently been implemented. However, the miniaturization of patterns shows no sign of slowing down and demand for increase in pattern resolution has continued to increase.
This photolithographic technology is a technology wherein patterns on a photomask (original image) are transferred to a photoresist applied to a wafer and wherein the etched film, which is the layer beneath the photoresist, is patterned by using the transcription on the photoresist.
At the time of the transcription onto the photoresist, a development process is carried out on the photoresist in which the type of photoresist wherein the portion struck by light is removed through this development process is called a positive type photoresist while the type of photoresist wherein the portion that has not been struck by light is removed is called a negative type photoresist.
In general, the resolution limit R (nm) in the photolithographic technology using a scale down exposure method is represented as:
R=k
1
·&lgr;/(
NA
)
wherein &lgr; is the wavelength (nm) of the utilized light, NA is a numerical aperture in the projection optical system of the lenses and k
1
is a constant that depends on the image formation condition and on the resist process.
As can be seen from the above equation, a method of reducing the values of k
1
and &lgr; while increasing the value of NA may be adopted in order to achieve an increase in the resolution limit R, that is to say, in order to gain microscopic patterns. Namely, the constant that depends on the resist process is reduced while a shorter wavelength and a greater NA value are utilized.
Among these, it is technically difficult to shorten the wavelength of the light source and, therefore, it is required to increase the NA value for the same wavelength. The introduction of a greater NA value, however, makes the focal depth &dgr;(&dgr;=k
2
·&lgr;/(NA)
2
) of light shallow so that there is a problem wherein deterioration of form and of dimensional precision occurs in the formed patterns.
It is necessary to carry out a pattern design wherein the aberration of the projection lens, and the like, are taken into consideration in order to transcribe the patterns of the photomask to the photoresist with a high precision in the above described photolithographic technology. In order to achieve this, it is necessary to precisely measure the aberration of the projection lens, and the like.
As for a conventional method of measuring the aberration of the projection lens, there is a method shown in the following Reference 1.
Reference 1: H. Nomura et al., “Higher order aberration measurement with printed patterns under extremely reduced &sgr; illumination”, Proc. SPIE Vol. 3679, (1999), pp. 358-367.
In this method aberration is measured by forming a pattern using a photomask as described in the following Reference 2. In the following, this measurement method is described.
Reference 2: H. Nomura et al., “Overlay Error due to Lens Coma and Asymmetric Illumination Dependence on Pattern Feature”, Proc. SPIE Vol. 3332, pp. 199-210.
FIGS. 17A
,
17
B and
17
C are views showing the configurations of the pattern of the photomask that is used for the aberration measurement method described in the above reference and the pattern for aberration measurement. In addition,
FIGS. 18A
to
18
F are schematic cross sectional views showing the formation method for the pattern for aberration measurement described in the above Reference 2 in process order.
Referring to
FIG. 18A
, first, a wafer
121
is prepared wherein a photoresist
121
b
is applied to a semiconductor substrate
121
a.
Referring to
FIG. 18B
, the pattern of a photomask
105
A, shown in
FIG. 17A
, is exposed to photoresist
121
b
of wafer
121
. Through this first exposure, photoresist
121
b
is selectively exposed to light. Here, the portions of photoresist
121
b
that are exposed to light are shown as white areas while the portions that are not exposed to light are shown as diagonally hatched areas going from the upper left to the lower right.
Referring to
FIG. 18C
, the pattern of a photomask
105
B, shown in
FIG. 17B
, is further exposed to photoresist
121
b
. Through this second exposure photoresist
121
b
is selectively exposed to light. Here, the portions of photoresist
121
b
that are not exposed to this exposure light are shown as diagonally hatched areas going from the upper right to the lower left.
FIG. 18D
is an enlarged view showing the region R
1
while
FIG. 18E
is an enlarged view showing the region R
2
.
After this, photoresist
121
b
is developed and, then, only the regions (regions shown by cross hatching) that are not exposed to the exposure light through the first and second exposures remain so as to form resist pattern
121
b
as shown in FIG.
18
F. This resist pattern
121
b
has a form as shown in
FIG. 17C
as represented in a plane manner.
Here,
FIG. 18F
corresponds to the cross sectional view along line XVIII—XVIII of FIG.
17
C.
Thus, only a portion of the line and space pattern (L/S pattern) is extracted by eliminating the portions other than the L/S pattern portion through a double exposure according to a conventional method. The relative movement amount between the extracted L/S pattern and the formed pattern of large dimensions is measured with respect to many L/S patterns of which the pitch and the direction differ so that an odd aberration is found from the change in the pitch of this movement amount.
In addition, an even aberration is found from change in the pitch and the direction of the optimal focus after finding the optimal focus from the range of the resolution focus with respect to a large number of L/S patterns of which the pitch and direction differ.
According to this method, however, it is necessary to carry out a very large number of measurements, as shown in
FIGS. 4 and 5
of Reference 1, in order to find the lens aberration in projection optical system and there is a problem wherein a great deal of labor and effort are required.
In addition, it is necessary to carry out the exposure by reducing the aperture size of the iris for the detection of the movement amount the optimal focus of the pattern, of which the pitch is large, and there is also a problem wherein a great deal of labor and effort are required for the exposure.
In addition, it is necessary to carry out the exposure by greatly reducing the aperture size of the iris in order to find the aberration in the vicinity of the center part of the iris and there is also a problem wherein this cannot be implemented by means of a commercially available exposure unit.
In addition, a large mask region becomes necessary since a large number of pattern types are used in order to carry out the measurement at a specific field point so that the inside of the field cannot be examined in detail. Concretely, there is a problem wherein the measurement can only be carried out to the degree wherein 8 mm is divided into three portions as shown in
FIG. 3
of Reference 1.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a photomask for aberration measurement, an aberration measurement method, a unit for aberration measurement and a manufacturing method for the unit that can measure the aberration of a lens that requires little labor and that ramifies the inside of the field.
A photomas
McDermott & Will & Emery
Renesas Technology Corp.
Young Christopher G.
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