Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-12-20
2003-11-25
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06653026
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a phase-shifting mask, and in particular to a structure and method of correcting proximity effects in a tri-tone attenuated phase-shifting mask.
2. Description of the Related Art
Photolithography is a well-known process used in the semiconductor industry to form lines, contacts, and other known structures in integrated circuits (ICs). In conventional photolithography, a mask (or a reticle) having a pattern of transparent and opaque regions representing such structures in one IC layer is illuminated. The emanating light from the mask is then focused on a photoresist layer provided on a wafer. During a subsequent development process, portions of the photoresist layer are removed, wherein the portions are defined by the pattern. In this manner, the pattern of the mask is transferred to or printed on the photoresist layer.
However, diffraction effects at the transition of the transparent regions to the opaque regions can render these edges indistinct, thereby adversely affecting the resolution of the photolithography process. Various techniques have been proposed to improve the resolution. One such technique, phase-shifting, uses phase destructive interference of the waves of incident light. Specifically, phase-shifting shifts the phase of a first region of incident light waves approximately 180 degrees relative to a second, adjacent region of incident light waves to create a more sharply defined interface between the first and second regions. Thus, the boundary between exposed and unexposed portions of a photoresist illuminated through a semiconductor mask (or reticle) can be more closely defined by using phase-shifting, thereby allowing greater structure density on the IC.
FIG. 1
illustrates a simplified, phase-shifting mask
100
fabricated with an attenuated, phase-shifting region
101
formed on a clear region
102
, wherein a border
103
of attenuated, phase-shifting region
101
defines a single IC structure. Clear region
102
is transparent, i.e. a region having an optical intensity transmission coefficient T>0.9. In contrast, attenuated phase-shifting region
101
is a partially transparent region, i.e. a region having a low optical intensity transmission coefficient 0.03<T<0.1. The phase shift of light passing through attenuated phase-shifting region
101
relative to light passing through clear region
102
is approximately 180 degrees.
As known by those skilled in the art, increasing the intensity transmission coefficient of attenuated phase-shifting region
101
could increase the performance of structures formed by the photolithographic process. In fact, optimal performance would be theoretically achieved by providing an attenuated, phase-shifting region with an optical intensity transmission coefficient approaching T=1 (in other words, the region would be transparent) yet having a phase shift of 180 degrees relative to clear region
102
. In this manner, assuming partially coherent illumination, amplitude side lobes from each region would substantially cancel, thereby creating a substantially zero-intensity line at the transition between these two regions. Current material technology typically provides this phase shift with an attenuated, phase-shifting region having an optical intensity transmission coefficient of approximately T=0.4, although a higher transmission is theoretically possible and preferable.
Unfortunately, the use of this higher transmission phase-shifting material increases the risk of printing certain portions of attenuated phase-shifting region
101
. Specifically, to ensure complete removal of residual photoresist, the actual dose used to remove the photoresist is typically at least twice the theoretical dose needed to remove the photoresist. This over-exposure can result in increasing the risk of printing certain larger portions of attenuated phase-shifting region
101
.
To solve this problem, some masks, called tri-tone attenuated phase-shifting masks, include an opaque region within the larger portion(s) of the attenuated, phase-shifting region, wherein the opaque region blocks any unwanted light transmitted by the attenuated phase-shifting region.
FIG. 2
illustrates a simplified, phase-shifting mask
200
fabricated with an attenuated phase-shifting region
201
formed on a clear region
202
and an opaque region
204
formed on attenuated phase-shifting region
201
, wherein a border
203
of attenuated phase-shifting region
201
defines a single IC structure. In this embodiment, clear region
202
has an optical intensity transmission coefficient T>0.9, attenuated phase-shifting region
201
has an optical intensity transmission coefficient 0.3<T<0.9, and an opaque region
204
typically has an intensity transmission coefficient of T<0.01. Note that the phase shift of light passing through attenuated phase-shifting region
201
relative to light passing through clear region
202
remains approximately 180 degrees.
Thus, forming an opaque region on an attenuated phase-shifting region advantageously allows for the use of a significantly higher optical intensity transmission coefficient for isolated structures. Unfortunately, a tri-tone phase-shifting mask exhibits strong optical proximity effects, thereby making it difficult to utilize this mask in a single common exposure for both isolated as well as crowded patterns.
Therefore, a need arises for a structure and a method for correcting optical proximity effects on a tri-tone, attenuated phase-shifting mask.
SUMMARY OF THE PRESENT INVENTION
A structure and method are provided for correcting the optical proximity effects on a tri-tone phase-shifting mask. A tri-tone phase-shifting mask typically includes a plurality of attenuated phase-shifting regions formed on a transparent layer as well as opaque regions formed on the larger portions of the attenuated, phase-shifting regions to block any unwanted light transmitted by the attenuated phase-shifting regions. In this manner, the opaque regions prevent these larger portions of the attenuated phase-shifting regions from printing during the development process.
In accordance with the present invention, a rim, formed by an opaque region and an attenuated phase-shifting region, is kept at a predetermined width either for all structures of a certain type or for all structures across the mask. Typically, the rim is made as large as possible to maximize the effect of the attenuated phase-shifting region while still preventing the printing of larger portions of the attenuated phase-shifting region during the development process.
In accordance with one feature of the present invention, a photolithographic mask includes a plurality of structures. Some of the structures are formed with a transparent region, an opaque region, and an attenuated region. The opaque region and the attenuated region form an attenuated rim having a predetermined width. In the present invention, the width of this rim is substantially the same in the subset of structures.
In one embodiment, the transparent region provides approximately a 0 degree phase shift and has an optical intensity transmission coefficient greater than approximately 0.9, whereas the attenuated region provides approximately a 180 degree phase shift and has an optical intensity transmission coefficient between approximately 0.3 and approximately 1.0. In this embodiment, the opaque region has an optical intensity transmission coefficient of less than approximately 0.01.
In accordance with another feature of the present invention, a method of forming a plurality of structures in a tri-tone attenuated phase-shifting mask is provided. A subset of the structures are formed by a first region and a second region, wherein the first region has a phase shift relative to the second region of 180 degrees. The method includes positioning a third region within a boundary for the second region, thereby forming a rim of the second region. The third region prevents the second region from printing durin
Pierrat Christophe
Zhang Youping
Bever Hoffman & Harms LLP
Harms Jeanette S.
Numerical Technologies Inc.
Rosasco S.
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