Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2003-05-02
2004-10-19
Mohamedulla, Saleha R. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06806007
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to a reflective mask for use in lithography, such as extreme ultra-violet lithography, and to a methodology for making the same.
BACKGROUND OF THE INVENTION
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been, and continues to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such a high device packing density, smaller feature sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as the corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. With regard to semiconductor fabrication, a silicon slice (e.g., a wafer) is coated uniformly with a radiation-sensitive film (e.g., a resist). The coated substrate can be baked to evaporate solvents in the resist composition and to fix the resist coating onto the substrate. An exposing source (e.g., light, x-rays, an electron beam) can then be utilized to illuminate selected areas of the surface of the film through an intervening master template (e.g., a mask or reticle) to affect the transfer of a pattern formed within the template onto the wafer. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image from the intervening master template is projected onto the resist coating, it is indelibly formed therein.
Light projected onto the resist layer during photolithography changes properties (e.g., solubility) of the layer of material such that different portions thereof (e.g., the illuminated or un-illuminated portions, depending upon the type of resist utilized) can be manipulated in subsequent processing steps. For example, regions of a negative resist become insoluble when illuminated by an exposure source such that the application of a solvent to the resist during a subsequent development stage removes only non-illuminated regions of the resist. The pattern formed in the negative resist layer is, thus, the negative of the pattern defined by opaque regions of the template. By contrast, in a positive resist, illuminated regions of the resist become soluble and are removed via application of a solvent during development. Thus, the pattern formed in the positive resist is a positive image of opaque regions on the template.
By way of example, prior art
FIGS. 1-6
generally depict the fundamental operation of positive and negative type resists in a photolithography process. A cross-sectional side view of a portion of one or more layers of a wafer
100
whereon semiconductor structures are produced is illustrated in the figures to facilitate the explanation. In
FIG. 1
, a resist layer
102
is deposited on a thin film
104
, such as via spin-coating, for example. The thin film
104
may include, for example, silicon dioxide (SiO
2
) and overlies a substrate
106
that can comprise silicon, for example. In
FIG. 2
, the resist layer
102
is selectively exposed to radiation
108
(e.g., ultraviolet (UV) light) via apertures
110
formed within a mask or reticle
112
to generate one or more exposed regions
114
in the resist layer
102
.
When the exposed regions
114
are made soluble, a positive image of the mask
112
is produced in the resist layer
102
. These features
114
are revealed when a specific solvent or developer is subsequently applied across the wafer
100
as illustrated in FIG.
3
.
In this situation, the resist material is therefore referred to as a “positive resist”. Areas
116
of the film
104
underlying the removed regions
114
of the resist layer
102
may then be subjected to further processing (e.g., etching) to thereby transfer the desired pattern from the mask
112
to the film
104
, as illustrated in
FIG. 4
(wherein the remaining portions of the resist layer
102
has been stripped away or otherwise removed).
Conversely, when the exposed regions
114
are made insoluble by radiation, a negative image of the mask
112
is produced in the resist layer
102
. These features
114
remain when the rest of the resist layer
102
is removed via application of a specific solvent or developer across the wafer
100
, as is illustrated in FIG.
5
. In this situation the resist material is referred to as a “negative resist.” Revealed areas
118
in the film
104
may then be subjected to further processing (e.g., etching) to thereby transfer into the film
104
the desired features
120
from the mask
112
, as illustrated in
FIG. 6
(wherein the remaining portions of the resist layer
102
have once again been stripped away or otherwise removed).
Projection lithography is a powerful and important tool for integrated circuit processing. However, as feature sizes continue to decrease, optical systems are approaching their limits due to the wavelengths of the optical radiation utilized. A recognized way of further reducing feature sizes is to lithographically image them with radiation of a shorter wavelength. Extreme ultraviolet (EUV) or “soft” x-rays, which have wavelengths within a range of about 30 to 700 Angstroms (i.e., about 3 to 70 nm), can, for example, be considered as an alternative radiation source in photolithography processing in an effort to achieve smaller desired feature sizes.
Prior art
FIG. 7
is a schematic diagram illustrating the fundamentals of an exemplary EUV lithography system
700
. The prior art system
700
depicted in
FIG. 1
is designed to delineate a latent image (not shown) of a desired circuit pattern (e.g., having feature dimensions of 0.13 &mgr;m and less) onto a wafer
702
, and more particularly onto one or more die on the wafer
702
, by illuminating a reflective mask
704
with EUV radiation and having at least a portion of that radiation reflected onto the wafer (e.g. via a system of mirrors). The portion of the radiation reflected onto the wafer
702
corresponds to the desired circuit pattern that is to be transferred onto the wafer
702
. It will be appreciated that
FIG. 7
is a simplified schematic representation of such a system wherein certain components are not specifically shown.
By way of example, EUV radiation
706
having a wavelength of between about 3 nm to 70 nm, for example, can be generated from a light source
708
, such as a synchrotron or a laser plasma source that can include optical filtering elements
710
and a reflective condenser
712
. The condenser and filtering elements can collect the EUV radiation and project one or more beams
714
onto the reflective mask
704
through a slit (not shown), for example, having a particular width and length. The reflective mask
704
absorbs some of the EUV radiation
716
and reflects other portions of the EUV radiation
718
corresponding to one or more features or circuit patterns formed on the mask. The reflective system can include, for example, a series of high precision mirrors
720
(e.g., concave and/or convex mirrors) which can cause the radiation to converge and/or diverge in projecting a de-magnified or reduced image of the pattern(s) to be transferred onto the wafer
702
, which is coated with a resist material. Typically, the reflective mask
704
and wafer
702
are mounted to stages (not shown) such that a scanner can move the mask
704
and the wafer
702
at respective orientations and speeds relative to one another (e.g., in a step and scan fashion) to effect a desired mask-to-image reduction and to facilitate pattern transfers onto one or more different die on the wafer.
The mask
704
of prior art
FIG. 7
is an important component in the EUV lithography system
700
. Unlike conventional UV lithography systems which predominately use refractive optics, many EUV lithography systems, such as the system
700
depicted in prior art
FIG. 7
, utilize reflec
Abdo Amr Yehia
LaFontaine Bruno
Advanced Micro Devices , Inc.
Eschweiler & Associates LLC
Mohamedulla Saleha R.
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