Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-03-15
2001-12-11
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S030000, C716S030000, C716S030000
Reexamination Certificate
active
06329107
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to photolithography and more particularly to an improved mask and mask design methodology.
2. Description of the Related Art
Photolithography is the technology of reproducing patterns using light. As presently used in semiconductor industry, a mask pattern for a desired circuit is transferred to a wafer through light exposure, development, etch, resist strip, etc. As the feature size on a circuit becomes smaller and smaller, the circuit shape on the wafer differs from the original mask pattern more and more. In particular, corner rounding, line end foreshortening, etc. are typically observed. These phenomena are called optical proximity effects.
One of the main reasons for optical proximity effects is light diffraction. Optical proximity effects coming from light diffraction can be overcome partly if one has the choice of using a shorter wavelength source of light, with a projection system possessing a larger numerical aperture. In practice, the wavelength of an optical light source is typically fixed (e.g., 365 nm, 248 nm, 193 nm, 157 nm etc.) and there is a practical upper limit on numerical aperture. Thus, other resolution enhancement methods, including the use of phase-shifting masks and masks with serifs, have been developed to correct optical proximity effects.
The light illumination in lithography is typically a partial coherent light illumination. The aerial image for partial coherent light illumination is given by the Hopkins equation,
I
(
r
→
)
=
K
(
r
→
,
r
→
1
)
K
*
(
r
→
,
r
→
2
)
M
(
r
→
1
)
M
*
(
r
→
2
)
J
(
r
→
1
,
r
→
2
)
ⅆ
r
→
1
ⅆ
r
→
2
,
which is a nonlinear integral involving the mask transmission function M, the coherent point-spread function (i.e., the kernel function) K, and mutual intensity function J. It is often assumed that the imaging system is translation invariant, i.e., that K({right arrow over (r)},{right arrow over (r)})=K({right arrow over (r)}−{right arrow over (r)}). In addition, a common assumption is that the mutual intensity function satisfies J({right arrow over (r)}
1
, {right arrow over (r)}
2
)=J({right arrow over (r)}
1
−{right arrow over (r)}
2
). For circular or annular aperture, the point-spread function between two points depends on their distance only, K({right arrow over (r)}−{right arrow over (r)})=K(|{right arrow over (r)}−{right arrow over (r)}|). Under these conditions, the Hopkins equation is usually simplified to
I
(
r
→
)
=
∫
∫
K
(
&LeftBracketingBar;
r
→
-
r
→
1
&RightBracketingBar;
)
K
*
(
&LeftBracketingBar;
r
→
-
r
→
2
&RightBracketingBar;
)
M
(
r
→
1
)
M
*
(
r
→
2
)
J
(
r
→
1
-
r
→
2
)
ⅆ
r
→
1
ⅆ
r
→
2
(
1
)
for aerial image calculations.
Two methods have been previously suggested for finding the best/suitable mask shapes under partial coherent light illumination. Y. Liu et al. “Binary and phase-shifting mask design for optical lithography,” IEEE Trans. Semiconductor Manufacturing 5, 138-152 (1992), incorporated herein by reference, treats a mask as a bitmap pattern, which consists of many pixels. The amplitude transmission at each pixel could be either 1 (with a possible fixed phase) or 0. The difficulty in this approach is that there are really too many combinations—it is 2 pixels. Y.C. Pati et al., “Phase-shifting masks for microlithography: automated design and mask requirements,” J. Opt. Soc. Am. A 11, 2438-2452 (1994), incorporated herein by reference, uses a method called “optimal coherent approximation,” which expresses the aerial image for a partial coherent light illumination as a sum of many coherent light illuminations. If this approximation is a good one, then perfect correction to both corner rounding and line end shortening can be achieved using a method disclosed previously in U.S. patent application Ser. No. 09/167,948,now U.S. Pat. No. 6,214,494, incorporated herein by reference, which is valid exactly for either coherent or incoherent light illuminations. The real situation for a partial coherent light illumination, however, is not this simple in general.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a structure and method for designing a photolithographic mask comprising, defining a device shape on the mask, the device shape having at least one outside corner and at least one straight edge, adding a partial circular serif to the corner to prevent rounding of the corner in an image produced using the mask, the serif having a radius, and adjusting a size of the serif such that a first aerial image intensity at the outer corner is equal to a second aerial image intensity at a point along the edge. The first aerial image intensity at the outer corner contains light diffraction contributions coming from transparent parts of mask and serif that are within a first cycle having the radius and being centered at the outer corner. The second aerial image intensity at a point along the edge contains light diffraction contributions coming from the transparent half circle of mask layout that is within a second circle having the radius and being centered at the point along the edge. The serif comprises a quarter-circle serif. The serif has a same transparency as the design shape. The adjusting comprises one of adding and removing rectangular strips to/from straight edges of the serif. The rectangular strips have an edge aligned with an edge of the device shape. The adjusting comprises one of adding and removing radial sections to/from the serif to form a fan-shaped serif. The adjusting comprises removing a partial circular central portion from the serif to form a partial ring-shaped serif.
A further embodiment of the invention is to provide a structure and method for designing a photolithographic mask comprising defining a device shape on the mask, the device shape having at least one inside corner and at least one straight edge forming a partial circular hole in the corner to prevent rounding of the corner in an image produced using the mask, the hole having a radius, and adjusting a size of the hole such that a first aerial image intensity at the inner corner is equal to a second aerial image intensity at a point along the edge. The first aerial image intensity at the inner corner contains light diffraction contributions coming from transparent parts of mask and hole that are within a first circle having the radius and being centered at the inner corner. The second aerial image intensity at a point along the edge contains light diffraction contributions coming from the transparent half circle of mask layout that is within a second circle having the radius and being centered at the point along the edge. The hole comprises a quarter-circle hole. The hole has an opposite transparency from the design shape. The adjusting comprises one of adding and removing rectangular strips to/from straight edges of the hole. The rectangular strips have an edge aligned with an edge of the device shape. The adjusting comprises one of adding and removing radial sections to/from the hole to form a fan-shaped hole. The adjusting comprises removing a partial circular central portion from the hole to form a partial ring-shaped hole.
Further the invention provides a structure and method for a photolithographic mask comprising a device shape, the device shape having at least one outside corner and at least one straight edge, and a partial circular serif centered at the corner, the serif preventing rounding of the corner in an image produced using the mask, the serif having a radius, wherein a first aerial image intensity at the outer corner is equal to a se
International Business Machines - Corporation
Kotulak, Esq. Richard M.
McGinn & Gibb PLLC
Rosasco S.
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