Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-10-09
2004-09-21
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C716S030000
Reexamination Certificate
active
06794096
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a phase shifting mask (PSM), and particularly to a method of correcting for image intensity imbalance on the PSM using near-field images.
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 advance in lithography called phase shifting is able to generate features on the wafer that are smaller than the corresponding wavelength of the light. These ultra-small features are generated by the interference of light in adjacent, complementary pairs of phase shifters having opposite phase, e.g. 0 and 180 degrees. In one embodiment, the phase shifters can be formed on a phase shifting mask (PSM), which is used in conjunction with a trim mask including protective regions to protect the features defined by the phase shifters and to define remaining structures.
In the PSM, complementary phase shifters (hereinafter referred to as shifters) are configured such that the exposure radiation transmitted by one shifter is 180 degrees out of phase with the exposure radiation transmitted by the other shifter. Therefore, rather than constructively interfering and merging into a single image, the projected images destructively interfere where their edges overlap, thereby creating a clear and very small image between the phase shifters.
Unfortunately, the transmission of light through the complementary shifters (for ease of reference, called 0- and 180-degree shifters) can be non-uniform.
FIG. 1A
illustrates a cross-section of a simplified PSM
100
including a 0-degree shifter
102
and a 180-degree shifter
103
. Note that 0-degree shifter
102
is formed by etching through an opaque layer
104
of PSM
100
to expose substrate (e.g. quartz)
101
. In contrast, 180-degree shifter
103
is formed by etching through an opaque layer
104
and then etching into substrate
101
to a predetermined depth.
This PSM topography effect produces an aerial image imbalance. Specifically,
FIG. 1B
illustrates a graph
105
comparing a first waveform (dashed line)
106
generated by using a theoretical thin mask (i.e. Kirchoff's) model and a second waveform (solid line)
109
generated by using a simulated EMF model. The peaks of high image intensity, e.g. peaks
107
and
108
of waveform
106
and peaks
110
and
111
of waveform
109
, correspond to shifters
102
and
103
of PSM
100
(FIG.
1
A). Of interest, peaks
107
and
108
have equal image intensity. However, peaks
110
and
111
, which are generated with rigorous computations, have significantly different intensities. In particular, peak
110
(which represents 0-degree shifter
102
) has a significantly higher image intensity than peak
111
(which represents 180-degree shifter
103
).
An image intensity imbalance can cause a shift in feature location. For example, if an image intensity imbalance results from exposing PSM
100
(FIG.
1
A), then the feature produced by that exposure might not be positioned in the middle of shifters
102
and
103
, but might instead be closer to 180-degree shifter
103
. Additionally, the image intensity imbalance can also result in variations in feature size. This displacement and size variation can contribute to an undesirable feature placement and size (or critical dimension (CD)) error on the wafer.
In some cases, the image intensity imbalance caused by 0- and 180-degree shifters-can be corrected through the application of a predetermined amount of undercut in the substrate as shown in FIG.
2
A. Specifically, instead of an anisotropic etch that generates a vertical trench in substrate
204
, indicated by dashed lines
201
, an isotropic etch can be used to generate a sloped trench
202
that undercuts opaque layer
203
. More generally, any suitable etch process can be used. However, as feature sizes shrink below 100 nm, allowable undercut size becomes limited by mask making capabilities and therefore is no longer sufficient to correct the imbalance on its own.
Therefore, in such cases, the undercut methodology can be combined with space biasing as shown in FIG.
2
B. Specifically, an opening
210
forming the 0-degree shifter can be biased inward, as indicated by arrows
211
, whereas an opening
212
forming the 180-degree shifter can be biased outward, as indicated by arrows
213
. Because the size of the 0-degree shifter decreases by an amount the size of the 180-degree shifter increases, the printed feature size should remain constant, which is desirable. Note that space biasing can be performed in some embodiments without undercutting.
FIG. 3
illustrates a flow chart
300
in which shifter biasing can be incorporated into tape out (and/or mask manufacture). After a layout is received in step
301
, phase shifters can be assigned to various features in the layout in step
302
. In one embodiment shown in step
303
, these shifters can be separated to different layers based on their width. For example, shifters having a first predetermined width used for forming transistor gates can be separated from shifters having a second predetermined width used for forming interconnect lines. In one embodiment, a design-rule-checking (DRC) tool can perform this separation. A first proximity correction, e.g. rule-based optical proximity correction (OPC), can be applied in step
306
using the shifter information provided in step
303
. Note that the term “optical proximity correction” is used generically herein to refer to proximity-correction generally, e.g. resist, etch, microloading, etc., except where from the context it is clear that optical proximity effects in particular are being exclusively considered.
Rule-based OPC can include rules to implement certain changes to the layout, thereby compensating for some lithographic distortions. Specifically, these changes can form features on the wafer that are closer to the original intended layout. With respect to shifters, rule-based OPC can provide the appropriate biasing to the shifters to minimize image intensity imbalance. Note that an undercut is performed during actual manufacture of the mask and therefore does not affect the layout as does space biasing. Space biasing is generally discussed in terms of N nm, wherein the initial width of the 0-degree shifter is decreased by N nm and the initial width of the 180-degree shifter (which is the same as the width of the 0-degree shifter) is increased by N nm. Note that slightly different biases can be applied to shifters in the shifter pair, although this can increase the complexity of computation. In one embodiment, the first proximity correction can provide a single bias and the second proximity correction can provide some refinement of that bias for each shifter in the shifter pair. For simplicity, one bias being applied to both shifters in the shifter pair is described in reference to the embodiments herein.
To provide the appropriate bias information to the first proximity correction tool, methodologies using an aerial image have been developed. These methodologies generally either emulate or simulate the aerial image produced by a stepper system.
FIG. 4
illustrates a simplified stepper system in which an illumination
401
passes
Bever Hoffman & Harms LLP
Harms Jeanette S.
Numerical Technologies Inc.
Rosasco S.
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