Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-03-16
2002-11-12
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S052000, C355S053000, C355S055000
Reexamination Certificate
active
06480263
ABSTRACT:
BACKGROUND OF THE INVENTION
Optical lithography is a driving force behind the continual improvements in the size and performance of the integrated circuit (IC) since its inception. Feature resolution down to 0.25 &mgr;m is now routine using a 248 nm KrF excimer wavelength and optical projection tools operating at numerical apertures above 0.60 with aberration levels below 0.04 &lgr; RMS OPD. The industry is at a point where resolution is limited for current optical lithographic technologies. In order to extend capabilities toward sub-0.25 &mgr;m, modifications in source wavelength, optics, illumination, masking, and process technology are required and are getting very much attention. Off-axis illumination and phase shift photomasks contribute to extending the range of optical lithography below 0.25 microns.
However, as devices get smaller, the photomask pattern becomes finer. Fine patterns diffract light and thus detract from imaging the photomask onto the surface of a wafer.
FIG. 1
a
shows what happens when a photomask with a fine pattern
6
having a high frequency (pitch
2
d
is about several microns), is illuminated through a projection lens system
7
, The fine pattern
6
is illuminated along a direction perpendicular to the surface thereof and it diffracts the light that passes through the mask
6
, Diffraction rays
3
-
5
caused by the pattern include a zero-th order diffraction ray
5
directed in the same direction as the direction of advancement of the input ray, and higher order diffraction rays such as positive and negative first order diffraction rays
3
,
4
, for example, directed in directions different from the input ray. Among these diffraction rays, those of particular diffraction orders such as, for example, the zero-th order diffraction ray and positive and negative first order diffraction rays
3
,
5
are incident on a pupil
1
of the projection lens system
7
, Then, after passing through the pupil
1
, these rays are directed to an image plane of the projection lens system, whereby an image of the fine pattern
6
is formed on the image plane. In this type of image formation, the ray components, which are contributable to the contrast of the image, are higher order diffraction rays. If the frequency of a fine pattern increases, it raises a problem that an optical system does not receive higher order diffraction rays. Therefore, the contrast of the image degrades and, ultimately, the imaging itself becomes unattainable.
As will be shown below, some solutions to this problem rely upon shaping the rays of light impinging the photomask in order to compensate for the lost contrast due to diffraction. These techniques rely upon optical systems for shaping the rays that illuminate the photomask.
In considering potential strategies for sub-0.25 &mgr;m lithography, the identification of purely optical issues is difficult. Historically, the Rayleigh criteria for resolution (R) and depth of focus (DOF) has been utilized to evaluate the performance of a given technology:
R=k
1
&lgr;/NA
DOF=+/−k
2
&lgr;/NA
2
where k
1
, and k
2
are process dependent factors, &lgr; is wavelength, and NA is numerical aperture. As wavelength is decreased and numerical aperture is increased, resolution capability improves. Considered along with the wavelength-linear and NA-quadratic loss in focal depth, reasonable estimates can be made for system performance. Innovations in lithography systems, materials and processes that are capable of producing improvements in resolution, focal depth, field size, and process performance are those that are considered most practical.
The control of the relative size of the illumination system numerical aperture has historically been used to optimize the performance of a lithographic projection tool. Control of this NA with respect to the projection systems objective lens NA allows for modification of spatial coherence at the mask plane, commonly referred to partial coherence. This is accomplished through specification of the condenser lens pupil size with respect to the projection lens pupil in a Köhler illumination system. Essentially, this allows for manipulation of the optical processing of diffraction information. Optimization of the partial coherence of a projection imaging system is conventionally accomplished using full circular illuminator apertures. By controlling the distribution of diffraction information in the objective lens with the illuminator pupil size, maximum image modulation can be obtained.
Phase shift masking also contributes to sub 0.25-micron lithography. Prior to the work of Levenson, et. al., as reported in “Improving Resolution in Photolithography with a Phase Shifting Mask,” IEEE Transactions on Electron Devices, VOL., ED-29, Nov. 12, Dec. 1982, pp. 1828-1836, it was generally thought that optical lithography would not support the increased density patterning requirements for feature sizes under 0.5 microns. At this feature size, the best resolution has demanded a maximum obtainable numerical aperture (NA) of the lens systems. However, the depth of field of the lens system is inversely proportional to the NA, and since the surface of the integrated circuit could not be optically flat, good focus could not be obtained when good resolution was obtained and it appeared that the utility of optical lithography had reached its limit. The Levenson paper introduced a new phase shifting concept to the art of mask making which has made use of the concepts of destructive interference to overcome the diffraction effects. See also U.S. Pat. No. 5,702,848.
Ordinary photolithography, with diffraction effects, is illustrated in FIGS.
3
(
a
) to
3
(
d
). As the mask features P
1
and P
2
become closer, N becomes smaller, and as seen in FIG.
3
(
b
), the light amplitude rays, which pass through P
1
and P
2
, start to overlap due to diffraction effects. These overlapping portions result in light intensity at the wafer, FIG.
3
(
d
), which impinges on the photoresist layer. Accordingly, due to diffraction, the intensity of the wafer no longer has a sharp contrast resolution in the region between P
1
and P
2
.
As illustrated by FIGS.
4
(
a
) to
4
(
e
), it is possible to make use of the fact that light passing through the masking substrate material, FIG.
4
(
a
),
51
, (and FIG.
4
(
b
),
51
′) exhibits a wave characteristic such that the phase of the amplitude of the light exiting from the mask material is a function of the distance the light ray travels in the substrate material, i.e., thickness t.sub.1 and t.sub.2, By making the thickness t.sub.2 such that (n−1)(t.sub.2) is exactly equal to ½ .lambda., where .lambda. is the wavelength of the light in the mask material, and n=refractive index of the added or subtracted natural material, then the amplitude of the light existing from aperture P
2
is in opposite phase from the light exiting aperture P
1
. This is illustrated in FIG.
4
(
c
) showing the effects of diffraction and use of interference cancellation. The photoresist material is responsive to the intensity of the light at the wafer. Since the opposite phases of light cancel where they overlap and since intensity is proportional to the square of the resultant amplitude, as seen in FIG.
4
(
d
) contrast resolution is returned to the pattern.
FIG.
4
(
a
) and FIG.
4
(
b
) illustrate two different techniques for obtaining the interference phase shifting. In FIG.
4
(
a
), the light traverses through a longer distance via deposited layer
52
, In FIG.
4
(
b
), the light in region P
2
transverses through a shorter distance by virtue of an etched groove
52
′ in the wafer
51
′. The etched depth or shifter film thickness is designed to produce the desired 180 degree phase shift at the proper incident wavelength (for example, I-line or DUV).
Lithographic imaging for semiconductor integrated circuit (IC) device fabrication is sensitive to the lens aberrations in a projection lens. This becomes especially critical as geometries are pushed toward and below t
Adams Russell
ASML Netherlands B.V.
Nguyen Hung Henry
Pillsbury & Winthrop LLP
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