Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-04-04
2004-07-20
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
06764794
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photomask for focus monitoring, a method of focus monitoring, a unit for focus monitoring and a manufacturing method of the electronic device.
2. Description of the Background Art
Increases in the integration and the miniaturization in semiconductor integrated circuits have been remarkable in recent years. Together with that, the miniaturization of the circuit pattern formed on a semiconductor substrate (hereinafter referred to simply as a wafer) has greatly progressed.
In particular, photolithographic technology is widely recognized as a basic technology in the pattern formation. Accordingly, a variety of developments and improvements have been carried out up to the present time. However, the miniaturization of patterns shows no signs of slowing down and demand for increase in resolution of the patterns is on the increase.
Such a photolithographic technology is a technology for transcribing patterns from a photomask (original image) to a photoresist applied on a wafer so that an etched film in the lower layer is patterned by using this transcribed photoresist.
At this time of photoresist transcription, a development treatment is carried out on the photoresist and a photoresist wherein the portion hit by light through this development treatment is removed is called a positive type while a photoresist wherein the portion hit by light is not removed is called a negative type photoresist.
In general, the resolution limit R (nm) in photolithographic technology using a downscaling exposure method is represented as:
R=k
1
·&lgr;/(
NA
)
Here, &lgr; is the wavelength (nm) of the utilized light, NA is the numerical aperture in the projection optical lens system and k
1
is a constant depending on the image formation condition and the resist process.
As is seen from the above equation, there is a method of making the values of k
1
and &lgr; smaller and of making the value of NA larger in order to achieve an increase in the resolution limit R, that is to say, to gain microscopic patterns. That is to say, in addition to making the constant, which depends on the resist process, smaller, a shortening of the wavelength and an increase of NA may be implemented.
From among these, a shortening of the wavelength of the light source is technically difficult and, therefore, it becomes necessary to increase the NA for the same wavelength. When an increase in NA is implemented, however, the focal depth &dgr;(&dgr;=k
2
·&lgr;/(NA)
2
) of light becomes shallow and, therefore, there are problems such that deterioration in form and in dimension precision of formed patterns is caused.
In order to expose a photoresist according to the patterns of a photomask with a high resolution using such photolithographic technology, it is necessary to carry out the exposure under the condition wherein the photoresist accords with the optimal image formation surface (optimal focus surface) of the projection optical system within the range of the focal depth. Therefore, it is necessary to precisely find the distance from the surface of the exposed substrate to the projection optical system. The process of finding this distance is called focus monitoring.
Concerning conventional focus monitoring, there is, for example, the method of phase shift focus monitoring developed by Brunner of IBM Corporation and sold by Benchmark Technology Corporation of the United States and the phase shift focus monitoring mask that is used in this method.
FIG. 56
is a view for describing the operational principle of the method of phase shift focus monitoring. In reference to
FIG. 56
, a phase shift focus monitoring mask
105
is used in this method of phase shift focus monitoring. This phase shift focus monitoring mask
105
has a transparent substrate
105
a
, a light blocking film
105
b
having a predetermined pattern and a phase shifter
105
c
that is formed on this predetermined pattern.
Concretely, this phase shift focus monitoring mask
105
has a pattern wherein a thin light blocking pattern
105
b
is arranged between sufficiently thick transmission portions
105
d
and
105
e
, as shown in FIG.
57
. Here, a phase shifter
105
c
is not placed in transmission portion
105
d
while a phase shifter
105
c
is placed on transmission portion
105
e.
In reference to
FIG. 56
, in this method of phase shift focus monitoring, first, phase shift focus monitoring mask
105
is irradiated with light. At this time, since phase shifter
105
c
is formed so as to shift the phase of the transmission light by approximately 90°, in the case that the light that has passed through transmission portion
105
e
precedes the light that has passed through the transmission portion
105
d
by the optical path difference of 1/4 &lgr;, 5/4 &lgr; . . . , or in the case that the light that has passed through transmission portion
105
e
succeeds the light that has passed through the transmission portion
105
d
by the optical path difference of 3/4 &lgr;, 7/4
718
&lgr; . . . , the light acts in a mutually reinforcing manner. Thereby, the light after passing through phase shift focus monitoring mask
105
has an asymmetric intensity distribution with respect to the z axis (optical axis). This light that has passed through phase shift focus monitoring mask
105
is condensed by means of projection lens
119
a
and
119
b
so as to form an image on a photoresist
121
b
, which is on a semiconductor substrate
121
a.
According to this method of phase shift focus monitoring, an image is formed on photoresist
121
b
under the condition wherein the intensity distribution of the diffracted light is asymmetric with respect to the z axis (optical axis: the longitudinal direction in the figure). Therefore, an image of the pattern shifts in the direction (x-y direction: lateral direction in the figure) perpendicular to the z axis (optical axis) on wafer
121
due to the shift of wafer
121
in the z direction. By measuring this amount of shift of the image of the pattern in the x-y direction, the measurement of the position in the z direction, that is to say the measurement of the focus, becomes possible.
In addition to the above described method of phase shift focus monitoring there is a method disclosed in, for example, Japanese Patent Laying-Open No. 6-120116(1994) that is a method of focus monitoring. In this method, a first predetermined pattern in the photomask surface is first irradiated with an exposure light of which the main light beam has the first angle of inclination and, thereby, the first image of the first predetermined pattern is exposed on a substrate of photosensitive material. After that, a second predetermined pattern that is different from the above first predetermined pattern is irradiated with an exposure light of which the main light beam has a second angle of inclination that differs from the first angle of inclination and, thereby, the second image of the second predetermined pattern is exposed on the substrate of the photosensitive material. By measuring the distance between the exposed first and second images, the distance from the position of the substrate of the photosensitive material to the optimal image formation surface can be found from the relationship between this distance and the amount of defocus.
In this method, a predetermined pattern on the photomask surface is irradiated according to the first angle of inclination or according to the second angle of inclination and, therefore, a photomask
205
having the structure as shown in
FIG. 58
is used.
In reference to
FIG. 58
, this photomask
205
has a transparent substrate
205
a
, marks for position measurement
205
b
1
, and
205
b
2
formed on the surface of this transparent substrate
205
a
and a diffraction grid pattern
205
c
formed on the rear surface of transparent substrate
205
a
. That is to say, an exposure light that has struck photomask
205
is diffracted by diffraction grid pattern
205
c
so that mark for position measurement
205
b
1
is irradiated
Maejima Shinroku
Miyamoto Yuki
Nakao Shuji
Tamada Naohisa
McDermott & Will & Emery
Renesas Technology Corp.
Young Christopher G.
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