Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2001-11-07
2004-09-28
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S022000
Reexamination Certificate
active
06797443
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a focus monitoring method, a focus monitoring apparatus and a method of manufacturing a semiconductor device, and more particularly, to a focus monitoring method used for pattern formation of a semiconductor device, a focus monitoring apparatus, and a method of manufacturing a semiconductor device.
2. Description of the Background Art
In recent years, high integration and miniaturization of semiconductor integrated circuits have been remarkable. Accordingly, miniaturization of a circuit pattern formed on a semiconductor substrate (hereinafter simply referred to as a wafer) has rapidly been advanced.
Among others, a photolithography technique is widely recognized as a basic technique in pattern formation. Thus, up to the present date, various developments and improvements have been made to the photolithography technique. However, patterns have continuously been reduced in size, and also the demand for improvement of pattern resolution is becoming stronger.
The photolithography technique is a technique in which a pattern on a photomask (an original) is transferred onto a photoresist applied on a wafer and the photoresist having the transferred pattern is used to pattern an underlayer film to be etched.
The photoresist is subjected to a developing process at the time of transferring the pattern onto the photoresist. A type of a photoresist in which a portion of the photoresist exposed to the light is removed in the developing process is referred to as a positive type photoresist, whereas a type in which a portion of the photoresist unexposed to the light is removed is referred to as a negative type photoresist.
In general, a limit of resolution R (nm) in the photolithography technique using a reduction exposure method is represented by
R=k
1
·&lgr;/(
NA
)
wherein &lgr; is an optical wavelength (nm) used, NA is a numeral aperture of a projection optical system of a lens, and k
1
is a constant depending on a resist process and image-forming condition.
As can be seen from the equation above, a possible way to improve the limit of resolution R, i.e. to obtain a microscopic pattern, is to make the values of k
1
and &lgr; smaller and to make the value of NA larger. That is, the constant depending on the resist process may be made lower while the wavelength is shortened and the NA is made higher.
However, problems arise in that it is technically difficult to improve a light source and a lens, and that the shorter wavelength and the higher NA may make the depth of focus 6 of light (&dgr;=k
2
·&lgr;/ (NA)
2
) shallower, resulting in lower resolution on the contrary.
In order to expose the pattern of the photomask onto the photoresist with high resolution in such a lithography technique, exposure must be carried out in the state where the photoresist is adjusted to the best image-forming plane, i.e. the best focus plane, of the projection optical system within a range of the depth of focus. For that purpose, it is necessary to obtain, in some way, the position of the best focus plane, i.e. the best focus position, of the projection optical system.
An example of a conventional focus monitor for measuring the best focus position is a phase shift focus monitor developed by Brunner of IBM Corp. and sold by Benchmark Technologies, Inc. in the United States.
FIG. 19
illustrates a method of phase shift focus monitoring. Referring to
FIG. 19
, a phase shift mask
105
is used in the phase shift focus monitoring method. Phase shift mask
105
includes a transparent substrate
105
a
, a light-shielding film
105
b
having a predetermined pattern, and a phase shifter
105
c
formed on the predetermined pattern.
Specifically, as shown in
FIG. 20
, phase shift mask
105
has a pattern in which a narrow light-shielding pattern is arranged between substantially wide transmitting portions
105
d
and
105
e
. It is noted that no phase shifter
105
c
is arranged at transmitting portion
105
d
, whereas phase shifter
105
c
is arranged at transmitting portion
105
e.
In the phase shift focus monitoring method, light is directed onto phase shift mask
105
. At that moment, phase shifter
105
c
is configured such that the phase of the transmission light is shifted 90°, and thus, when the light transmitted through transmitting portion
105
e
is advanced compared to the light transmitted through transmitting portion
105
b
by the optical path difference of ¼ &lgr;, {fraction (5/4)} &lgr;, . . . , or when it is delayed by ¾ &lgr;,{fraction (7/4)} &lgr;, . . . , the both light beams intensify each other. This allows the light transmitted through phase shift mask
105
to have an asymmetric intensity distribution with respect to the z axis (the optical axis). The light transmitted through phase shift mask
105
is condensed by projection lens
119
a
,
119
b
, and forms an image onto a photoresist
121
b
on a semiconductor substrate
121
a.
By the phase shift focus monitoring, an image is formed onto photoresist
121
b
in the state where the intensity distribution of diffracted light is asymmetric with respect to the z axis. Thus, as wafer
121
is moved in the z direction, a pattern image on wafer
121
is moved in the direction perpendicular to the z axis which is in the lengthwise direction in the drawing (the x-y direction, i.e. the crosswise direction in the drawing). Measurement of the amount of shift of the pattern image in the x-y direction enables measurement of a position in the z direction, i.e. measurement of a focus.
Another example of the focus monitoring method other than the phase shift focus monitoring is a method disclosed in Japanese Patent Laying-Open No. 6-120116. In this method, first, a predetermined pattern on the surface of a photomask is illuminated with exposure light having a main light beam at the first angle of inclination, to expose the first image of the predetermined pattern on a photosensitive substrate. Subsequently, the predetermined pattern is illuminated by exposure light having a main light beam at the second angle of inclination different from the first angle of inclination, to expose the photosensitive substrate to the second image of the predetermined pattern. The distance between the exposed first and second images is measured, and the relation between the measured distance and a defocused amount is used to obtain the distance from the position of the photosensitive substrate to the best focus plane.
In this method, a photomask
205
having a configuration as shown in
FIG. 21
is used for illumination of the predetermined pattern on the photomask surface at the first angle of inclination or the second angle of inclination.
Referring to
FIG. 21
, photomask
205
includes a transparent substrate
205
a
, position measurement marks
205
b
1
,
205
b
2
formed on the front surface of transparent substrate
205
a
, a diffraction grating pattern
205
c
formed on the rear surface of transparent substrate
205
a
. This means that the exposure light entered into photomask
205
is diffracted at diffraction grating pattern
205
c
to illuminate position measurement mark
205
b
1
at the first angle of inclination and to illuminate position measurement mark
205
b
2
at the second angle of inclination.
However, the phase shift focus monitoring described above requires the use of a phase shift mask having a special structure as photomask
105
. There was a problem in that such a photomask with the special structure increased the cost of the photomask.
In addition, the method disclosed in Japanese Patent Laying-Open No. 6-120116 requires formation of microscopic diffraction grating pattern
205
c
on the rear surface of the photomask, requiring a number of steps. Thus, there was a disadvantage in that the manufacturing cost of the mask was increased to a large degree.
Moreover, by the current mask fabricating techniques, it is extremely difficult to form patterns, while the relative positional relationship of the patterns on both sides of the mask subs
Miyamoto Yuki
Nakao Shuji
Tamada Naohisa
McDermott Will & Emery LLP
Renesas Technology Corp.
Young Christopher G.
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