Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2003-10-14
2004-12-14
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06830854
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of alternating phase-shifting masks, and in particular to a method of correcting three-dimensional (3D) effects in alternating phase-shifting masks using two-dimensional (2D) analysis.
2. Description of Related Art
To fabricate an integrated circuit (IC), a physical representation of the features of the IC, e.g. a layout, is transferred onto a plurality of masks. The features make up the individual components of the circuit, such as gate electrodes, field oxidation regions, diffusion regions, metal interconnections, and so on. A mask is generally created for each layer of the IC. To create a mask, the data representing the layout for a corresponding IC layer can be input into a device, such as an electron beam machine, which writes IC features onto the mask. Once a mask has been created, the pattern on the mask can be transferred onto the wafer surface using a lithographic process.
Lithography is a process whose input is a mask and whose output includes the printed patterns on a wafer. As printed patterns on the IC become more complex, a need arises to decrease the feature size. However, as feature sizes shrink, the resolution limits of current optical-based lithographic systems are approached. Specifically, a lithographic mask includes clear regions and opaque regions, wherein the pattern of these two regions defines the features of a particular semiconductor layer. Under exposure conditions, 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 this 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. In this manner, the projected images from these two regions destructively interfere where their edges overlap, thereby improving feature delineation and allowing greater feature density on the IC. A mask that uses such techniques is called a phase-shifting mask (PSM).
In one type of PSM, called an alternating (aperture) PSM, apertures between closely spaced features are processed so that light passing through any aperture is 180 degrees out of phase from the light passing through an adjacent aperture.
FIGS. 1A and 1B
illustrate one embodiment of an alternating PSM
100
including closely spaced opaque (e.g. chrome or some other absorbing material) features
101
,
102
,
103
, and
104
formed on a transparent, e.g. quartz, substrate
105
. Thus, apertures
106
,
107
, and
108
are formed between features
101
-
104
.
To provide the phase-shifting in this embodiment, the areas of substrate
105
under alternating apertures can be etched, thereby causing the desired 180 degree phase shift. For example, substrate
105
can be etched in the area defined by aperture
107
to a predetermined depth. In this manner, the phase shift of light passing through aperture
107
relative to light passing through apertures
106
and
108
is approximately 180 degrees.
Unfortunately, the use of a PSM can introduce an intensity imbalance problem.
FIG. 1C
illustrates a graph
130
that plots intensity (0 to 1.0) versus position on alternating PSM
100
. In graph
130
, waveforms
131
that are shown nearing 1.0 intensity correspond to apertures
106
and
108
, whereas waveform
132
that is shown at approximately 0.84 intensity corresponds to aperture
107
. The intensity imbalance between the 180 degree phase-shifting region (i.e. aperture
107
) and the 0 degree phase-shifting regions (i.e. apertures
106
and
108
) is caused by the trench cut into substrate
105
, thereby causing diffraction in the corners of aperture
107
and degrading the intensity of the corresponding waveform. This industry-recognized diffraction effect is called a three-dimensional (3D) effect.
Intensity imbalance can adversely affect printing features and overlay on the wafer. Typically, a feature on a binary mask has a pair of corresponding phase-shifting regions on a PSM. For example, referring to
FIG. 1D
, a feature
140
can have a corresponding 0 degree phase-shifting region
141
placed relative to one side of feature
140
and a corresponding 180 degree phase-shifting region
142
placed relative to the other side of feature
140
. Of interest, if phase-shifting regions
141
and
142
are the same size, the electric field associated with region
141
is stronger than the electric field associated with region
142
, thereby resulting in the maximum interference of these fields to occur to the right of centerline
143
on feature
140
. Thus, under these conditions, feature
140
will actually print on the wafer to the right of the desired location as shown by feature
150
and its associated centerline
153
.
Moreover, any defocus in the system can exacerbate the 3D effect and cause significant deviation from desired feature placement on the wafer. Because any wafer production line requires at least some acceptable range of defocus, e.g. typically within 0.4 microns, feature placement is frequently adversely affected when using alternating PSM. Therefore, those in the industry have proposed various methods to address the intensity imbalance problem.
In one proposed method shown in
FIG. 1E
, an additional etching step can be performed on substrate
105
, thereby providing an under-cut etch
160
of features
101
-
104
. Under-cut etch
160
increases the intensity by attempting to localize the diffraction effects under features
101
-
104
. Unfortunately, under-cut etch
160
can also create mechanical instability of features
101
-
104
on the mask. In fact, the more the diffraction effects are localized, the greater the probability of mechanical instability during subsequent processing steps, such as mask cleaning. Thus, under-cut etch
160
provides an incomplete solution with the potential of causing complete mask failure.
SUMMARY OF THE INVENTION
In accordance with one feature of the present invention, an accurate, cost-effective system and method for correcting three-dimensional effects on an alternating phase-shifting mask (PSM) is provided. To facilitate this correction, a method of building a library used for creating the alternating PSM can be provided. The method can include determining a first group of 180 degree phase-shifting regions, wherein the first group of 180 degree phase-shifting regions have a common first size. Three-dimensional (3D) simulation can be performed based on this first size. Of importance, a transmission and a phase can be altered in a 2D simulation based on this first size until a shape dependent transmission and a shape dependent phase allow the 2D simulation to substantially match the 3D simulation. Finally, a modified first size can be chosen using the shape dependent transmission and the shape dependent phase such that a 2D simulation based on the modified first size substantially matches the 3D simulation based on the first size. The library can associate the first size with the modified first size, the shape dependent transmission, and the shape dependent phase.
This method can be repeated for a plurality of groups of 180 degree phase-shifting regions for the alternating PSM, each group of 180 degree phase-shifting regions having a common size that is a different size than any other group. The size can refer to a width, a length, a width/length combination, or an area. In one embodiment, altering a transmission and a phase in the 2D simulation includes substantially matching a Fourier spectrum for the 3D simulation with a Fourier spectrum for the 2D simulation.
A method of designing a lithographic mask using this library is also provided. The method includes placing 0 degree phase-shifting regions and 180 degree phase-shifting regions on the lithographi
Liu Hua-Yu
Liu Yong
Bever Hoffman & Harms LLP
Harms Jeanette S.
Numerical Technologies Inc.
Rosasco S.
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