Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-06-29
2002-10-01
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06458495
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit manufacturing, and more specifically, to a phase-shifting mask and a process for fabricating a phase-shifting mask.
2. Discussion of Related Art
Improvements in photolithography have allowed higher density and faster speed to be attained in semiconductor Integrated Circuits (ICs) by continually shrinking the devices. According to the Rayleigh criterion, the minimum Critical Dimension (CD) which can be resolved by a wafer stepper is directly proportional to the wavelength of the illumination source and inversely proportional to the Numerical Aperture (NA) of the projection lens. However, diffraction will degrade the aerial image when the CD becomes smaller than the actinic wavelength. The actinic wavelength is the wavelength of light at which a mask is used in a wafer stepper to selectively expose photoresist on a substrate.
Photolithography in the sub-actinic wavelength regime will benefit from wavefront engineering with Resolution Enhancement Techniques (RETs), such as Phase-Shifting Masks (PSMs), to achieve a sufficiently wide process latitude. Unlike a conventional binary mask that only modulates the amplitude of light, a PSM further controls the phase of light to take advantage of destructive interference to mitigate the detrimental effects of diffraction. An Alternating PSM (AltPSM) is particularly useful for patterning very small CDs, such as the gate length of a transistor in a device. AltPSM improves contrast by introducing a phase shift of 180 degrees between the light transmitted through adjacent clear openings to force the amplitude between the two images to zero.
A phase shift of 180 degrees can be implemented creating a difference in the optical path lengths through adjacent openings in an opaque layer, such as chrome. An additive process may be used to deposit a transparent layer, such as Spin-On-Glass (SOG), through openings in the chrome onto a transparent substrate, such as fused silica or quartz, followed by removal of the transparent layer in alternate openings. However, an additive process is susceptible to optical mismatch of materials in the light path and accompanying internal losses at the interfaces. Thus, it is more common to use a subtractive process to etch a trench into the quartz substrate in alternate openings.
However, an AltPSM that is fabricated with a subtractive process will scatter incident light off the sidewalls and bottom corners of the etched openings. The waveguide effect causes an aerial image imbalance which is manifested as a CD error and a placement error. The CD of the etched opening becomes smaller than the CD of the unetched opening. The space between the two openings appears displaced from the unetched opening towards the etched opening.
The aerial image in an AltPSM can be balanced in several ways. A CD biasing approach enlarges the etched opening relative to the unetched opening to balance the aerial image. As shown in
FIG. 1
, the phase-shifted opening
101
has a trench in the quartz
107
with a depth
117
and a width
111
. The non-phase-shifted opening
103
has no trench and a width
113
. The phase-shifted opening
101
and the non-phase-shifted opening
103
are separated by an opaque layer
105
with a width
115
. The depth
117
corresponds to a phase shift of 180 degrees between the phase-shifted opening
101
and the non-phase-shifted opening
103
. The width
111
of the trench includes a bias
109
per edge to increase transmission of the phase-shifted opening
101
relative to the non-phase-shifted opening
103
.
An etchback approach, such as with an isotropic wet etch, recesses the sidewalls and corners laterally under the chrome to balance the aerial image. The etchback approach may be one-sided or two-sided. As shown in
FIG. 2
, the one-sided version of the etchback approach undercuts the substrate below the edges of the opaque layer
205
around the phase-shifted opening
201
only. The undercut
209
per edge increases transmission of the phase-shifted opening
201
relative to the non-phase-shifted opening
203
which is not undercut. The trench in the phase-shifted opening
201
has a depth
219
before the etchback and a depth
217
after the etchback. The depth
217
corresponds to a phase shift of 180 degrees.
The etchback approach may also be two-sided. As shown in
FIG. 3
, the two-sided version of the etchback approach undercuts the substrate below the edges of the opaque layer
305
around both the phase-shifted opening
301
and the non-phase-shifted opening
303
. The phase-shifted opening
301
has a depth
119
before the etchback and a depth
317
after the etchback. The non-phase-shifted opening
303
only has a depth
339
after the etchback. A depth difference
341
is maintained between the two trenches which corresponds to a phase shift of 180 degrees. The undercut
309
per edge of the phase-shifted opening
301
and the undercut
329
per edge of the non-phase-shifted opening
303
determines the quartz width
525
underneath the chrome between the phase-shifted opening
301
and the non-phase-shifted opening
303
.
As shown in
FIG. 4
, a dual-trench approach can also balance the aerial image by etching a deep trench with a depth
417
in the phase-shifted opening
401
and an adjacent shallow trench with a depth
439
in the non-phase-shifted opening
403
. The two trenches are separated by chrome with a width
415
. A depth difference
441
is maintained between the two trenches which corresponds to a phase shift of 180 degrees.
The various approaches for balancing the aerial image have disadvantages. The CD biasing approach is constrained to the discrete values available on the design grid. The etchback approach may cause defects, such as chipping or delamination of the overhanging chrome between adjacent openings. The dual-trench approach adds complexity and cost by requiring additional processing.
SUMMARY OF INVENTION
Thus, what is needed is a phase-shifting mask with balanced transmission and phase and a process for fabricating such a phase-shifting mask.
REFERENCES:
patent: 5718829 (1998-02-01), Pierrat
Qian Qi-De
Tsai Wilman
Blakley Sokoloff Taylor & Zafman LLP
Intel Corporation
Rosasco S.
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