Self-aligned fabrication technique for tri-tone attenuated...

Details Self-aligned fabrication technique for tri-tone attenuated... Self-aligned fabrication technique for tri-tone attenuated...

2001-03-16

2003-04-22

Rosasco, Stephen (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Radiation modifying product or process of making

Radiation mask

C716S030000

Reexamination Certificate

active

06551750

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a tri-tone attenuated phase-shifting mask, and in particular to a self-aligned fabrication technique for a tri-tone attenuated phase-shifting mask.
2. Description of the Related Art
Lithography is a well-known process used in the semiconductor industry to form lines, contacts, and other known structures in integrated circuits (ICs). In conventional lithography, 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 resist layer provided on a wafer. During a subsequent development process, portions of the resist 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 resist 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 lithographic 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. Therefore, the projected images from these two regions destructively interfere where their edges overlap, thereby creating a clear separation between the two images. Thus, the boundary between exposed and unexposed portions of a resist 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. 1A
illustrates a simplified, phase-shifting mask
100
fabricated with an attenuated, phase-shifting region
102
formed on a clear region
101
, wherein a border
110
of attenuated, phase-shifting region
102
defines a single IC structure. Clear region
101
is transparent, i.e. a region having an optical intensity transmission coefficient T>0.9. In contrast, attenuated phase-shifting region
102
is a partially transparent region, i.e. a region having a low optical intensity transmission coefficient 0.03<T<0.1. Referring to
FIG. 1B
, which shows a cross-section of mask
100
, the phase shift of light passing through attenuated phase-shifting region
102
relative to light passing through clear region
101
is approximately 180 degrees.
As known by those skilled in the art, increasing the intensity transmission coefficient of attenuated phase-shifting region
102
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 T>0.9 (in other words, the region is transparent) yet having a phase shift of 180 degrees relative to clear region
101
. 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 providing a higher transmission is theoretically possible.
Unfortunately, the use of this higher transmission phase-shifting material increases the risk of printing certain portions of attenuated phase-shifting region
102
. Specifically, to ensure complete removal of residual resist, the actual dose used to remove the resist is typically at least twice the theoretical dose needed to remove the resist. This over-exposure can result in increasing the risk of printing certain larger portions of attenuated phase-shifting region
102
.
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. 2A
illustrates a simplified, phase-shifting mask
200
fabricated with an attenuated phase-shifting region
202
formed on a clear region
201
and an opaque region
203
formed on attenuated phase-shifting region
202
, wherein a border
210
of attenuated phase-shifting region
202
defines a single IC structure. In this embodiment, clear region
201
has an optical intensity transmission coefficient T>0.9, attenuated phase-shifting region
202
has an optical intensity transmission coefficient 0.03<T<0.4, and an opaque region
203
typically has an intensity transmission coefficient of T<0.01. Referring to
FIG. 2B
, which shows a cross-section of mask
200
, the phase shift of light passing through attenuated phase-shifting region
202
relative to light passing through clear region
201
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.
FIGS. 3A-3G
illustrate a conventional process for generating a tri-tone attenuated phase-shifting mask.
FIG. 3A
illustrates a conventional PSM blank
300
including a transparent substrate
301
on which are formed an attenuated phase-shifting layer (hereinafter attenuated layer)
302
and an opaque layer
303
. Blank
300
further includes a first resist, i.e. e-beam or photo sensitive, layer
304
formed on opaque layer
303
.
During a primary patterning operation, an e-beam scanner or a UV exposure tool (hereinafter, the patterning tool) can expose areas
305
A and
305
B of first resist layer
304
. After areas
305
A and
305
B are developed, patterned resist region
304
A is formed, as shown in FIG.
3
B. In this embodiment, an etch process is then performed to transfer the pattern in first resist region
304
A to opaque layer
303
.
FIG. 3C
shows the resulting patterned opaque region
303
A. At this point, any exposed upper surface of attenuated layer
302
and the upper surface of first resist region
304
A are subjected to a standard dry or wet etch, thereby removing all portions of attenuated layer
302
not protected by first resist region
304
A and patterned opaque region
303
A. First resist region
304
A is then stripped away, leaving the structure shown in FIG.
3
D.
Next, the structure is coated with a second resist layer
306
as shown in
FIG. 3E. A
secondary patterning operation in then performed in which the patterning tool exposes areas
307
A and
307
B of second resist layer
306
. After areas
307
A and
307
B are developed, a patterned second resist region
306
A is formed, as shown in FIG.
3
F. In this embodiment, an etch process is then performed (not shown) to transfer the pattern of second resist region
306
A to patterned opaque region
303
A. Second resist region
306
A is then stripped away, leaving the resulting twice-patterned opaque region
303
A(
1
), as shown in FIG.
3
G. At this point, the pattern necessary for the tri-tone attenuated phase-shifting mask has been completed.
However, as noted in
FIG. 3E
, patterned opaque region
303
A is not self-aligned to patterned attenuated region
302
during the manufacturing process. Thus, the distance D
1
from the edge of twice-patterned opaque region
303
A(
1
) to the edge of patterned attenuated region
302
A on one side of the structure may not equal distance D
2
on the other side of the structure. Unfortunately, any misalignment of twice-patterned opaque region
303
A(
1
) with patterned attenuated region
302
A can generate critical dimension and pattern placement errors, thereby degrading performance of the resulting structures on the IC. Moreover, in an extreme case, if either one of distances D

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