Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-03-23
2003-10-21
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S311000, C716S030000
Reexamination Certificate
active
06635393
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a technique for producing a high-resolution phase-shift masking photomask, and is specifically directed towards enabling the use of an electron beam exposure tool to perform the phase level patterning operation during the mask-making process.
2. Related Art
Photomasks are a key element in the manufacture of modern integrated circuits (ICs). During lithography process steps, photomasks are used to transfer an IC layout onto a wafer surface. Each photomask typically comprises a pattern etched into a pattern layer (typically chrome) on a quartz substrate, the pattern representing one layer of the IC. Accordingly, the accuracy of the projected images formed by the photomask strongly affects the performance of the final IC.
During a lithography process step, exposure radiation (light) is used to project the pattern in the chrome layer of a photomask onto a wafer. The chrome layer pattern comprises a multitude of tiny apertures in the chrome layer. As the dimensions of modern IC devices continue to shrink, the chrome apertures must become smaller and smaller, which leads to increasing diffraction of the exposure radiation as it passes through the photomask. This diffraction can cause projected images from adjacent apertures in the chrome layer to overlap and merge, preventing the desired pattern from properly resolving at the wafer surface. Various techniques have been developed in an effort to extend the usable range of optical lithography tools. One of the most important of those techniques is phase-shift masking (PSM) technology.
In a PSM photomask, critical features are defined using pairs of complementary features (apertures) in the chrome layer. The complementary features are configured such that the exposure radiation transmitted by one aperture is 180 degrees out of phase with the exposure radiation transmitted by the other aperture. Therefore, rather than constructively interfering and merging into a single image, the projected images destructively interfere where their edges overlap, creating a clear separation between the two images. As a result, the images formed by a PSM photomask can have a much higher resolution than images formed by a conventional photomask.
To shift the phase of the exposure radiation passing through a given aperture in the chrome layer, a pocket is etched in the quartz substrate at that aperture. The quartz pocket reduces the thickness of the quartz substrate at the out-of-phase (“phased”) aperture relative to the quartz substrate thickness at the in-phase aperture. The pocket depth at the phased aperture depends on the wavelength of the exposure radiation used by the lithography tool in which the PSM photomask is to be used. By precisely configuring the relative thickness of the quartz substrate at the phased and in-phase regions, the projected images from adjacent apertures can be set to be 180 degrees out of phase.
FIGS. 1A-1F
depict a conventional process for creating a PSM photomask.
FIG. 1A
shows a conventional photomask blank
110
comprising an original resist layer
113
formed over a chrome layer
112
, which is in turn formed over a quartz substrate
111
. During a primary patterning operation, an electron beam (“e-beam”) scanner exposes regions
113
a
and
113
b
of original resist layer
113
. Exposed regions
113
a
and
113
b
are developed away, leaving patterned resist layer
113
shown in FIG.
1
B. An etch process is then performed, thereby transferring the pattern in original resist layer
113
into chrome layer
112
. Original resist layer
113
is then stripped away, leaving patterned chrome layer
112
with apertures
112
a
and
112
b
, as shown in FIG.
1
C.
At this stage, the entire layout pattern is contained in chrome layer
112
. However, to complete the PSM photomask, quartz substrate
111
must be etched to the proper depth under the out-of-phase, or “phased”, portion of the layout pattern. As shown in
FIG. 1D
, a secondary resist layer
140
is formed over patterned chrome layer
112
, and a portion
140
a
of secondary resist layer
140
is optically exposed during a “phase level patterning” operation. Exposed portion
140
a
is developed away, and chrome layer
112
is etched through, as shown in FIG.
1
E. Thus, the actual etching of quartz substrate
111
is controlled by aperture
112
b
in chrome layer
112
, i.e., the purpose of the phase level patterning operation is merely to uncover the appropriate apertures in chrome layer
112
. After the quartz etch, secondary resist layer
140
is stripped, leaving a basic PSM photomask
110
f
as shown in FIG.
1
F.
Once the quartz etch is complete, apertures
112
a
and
112
b
are complementary apertures, as the images projected by the two during a lithography process step will be 180 degrees out of phase with each other. Aperture
112
a
is designated the in-phase aperture, whereas aperture
112
b
is designated the phased aperture. The phase shift of aperture
112
b
is provided by a pocket
111
b
that thins quartz substrate
111
under aperture
112
b
. While only a single complementary pair of apertures is depicted, any number could be present in an actual PSM photomask, each of the phased apertures having a quartz pocket of depth d.
In this manner, basic PSM photomask
110
f
is configured to produce properly phase-adjusted images at the wafer surface. However, the pockets formed by the quartz etch affect not only the phase, but also the intensity, of the exposure radiation transmitted by the PSM photomask.
FIG. 2
shows how PSM photomask
110
f
would be used in a lithography process step. PSM photomask
110
f
of is placed “upside down” (i.e. with quartz substrate
111
on top) in a stepper (not shown), and exposure radiation from the stepper projects the pattern in chrome layer
112
onto a wafer (also not shown). As indicated in
FIG. 2
, diffraction of the exposure radiation at phase layer aperture A
1
starts to occur at the base of pocket
111
b
, whereas diffraction at chrome aperture A
2
originates at the surface of quartz substrate
111
(i.e. where aperture A
2
meets quartz substrate
111
). Therefore, more of the exposure radiation is “lost” inside quartz substrate
111
at phase layer aperture A
1
, resulting in unequally-sized and improperly-spaced projected features. This phenomenon is described in detail in “Phase-Shifting Mask Topography Effects on Lithographic Image Quality” by Pierrat et al., IEDM 92-53, IEEE 1992, herein incorporated by reference.
To overcome this problem, a post-processing step is typically performed on a PSM photomask after the phase layer quartz etch. The purpose of the post-processing step is to create an “undercut” beneath the chrome layer by increasing the width of the quartz pockets under the phased apertures. This undercutting process is typically accomplished by performing a wet (isotropic) etch on the quartz layer. For example, after the dry (anisotropic) etch shown in
FIG. 1E
, quartz substrate
111
could be wet etched, as shown in
FIG. 1G
(the dry etch depth would have to be reduced to maintain the final phase-shifting characteristics of the quartz after the wet etch). The wet etch gives the pocket in quartz substrate
111
a final width W
1
, which is greater than the width W
2
of the aperture in chrome layer
112
, as shown in FIG.
1
H. The diffraction effects from the base of the pocket in the quartz substrate then have much less effect on the actual radiation transmitted by the aperture in the quartz layer. This in turn allows the intensity characteristics of the radiation output from the phase layer aperture to more closely match the characteristics of the output from the chrome aperture, thereby resulting in improved PSM functionality.
This post-processing step can be performed in various ways. For example, after a dry etch of the quartz substrate to form the phase layer (as shown in FIGS.
1
E-
1
F), a wet etch could be performed on both the phase and pattern layers, as shown in FIG.
1
I. This would produ
Bever Hoffman & Harms LLP
Harms Jeanette S.
Numerical Technologies Inc.
Rosasco S.
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