Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-03-01
2004-09-07
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S022000, C430S030000, C716S030000, C716S030000
Reexamination Certificate
active
06787272
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to semiconductor device fabrication, and more particularly to the use of assist features in conjunction with off-axis illumination (OAI) for such fabrication.
BACKGROUND OF THE INVENTION
Since the invention of the integrated circuit (IC), semiconductor chip features have become exponentially smaller and the number of transistors per device exponentially larger. Advanced IC's with hundreds of millions of transistors at feature sizes of 0.25 micron, 0.18 micron, and less are becoming routine. Improvement in overlay tolerances in photolithography, and the introduction of new light sources with progressively shorter wavelengths, have allowed optical steppers to significantly reduce the resolution limit for semiconductor fabrication far beyond one micron. To continue to make chip features smaller, and increase the transistor density of semiconductor devices, IC's have begun to be manufactured that have features smaller than the lithographic wavelength.
Sub-wavelength lithography, however, places large burdens on lithographic processes. Resolution of anything smaller than a wavelength is generally quite difficult. Pattern fidelity can deteriorate dramatically in sub-wavelength lithography. The resulting semiconductor features may deviate significantly in size and shape from the ideal pattern drawn by the circuit designer. These distortions include line-width variations dependent on pattern density, which affect a device's speed of operation, and line-end shortening, which can break connections to contacts. To avoid these and other optical proximity effects, the semiconductor industry has attempted to compensate for them in the photomasks themselves, as well as by using other approaches.
This compensation in the masks themselves is generally referred to as optic al proximity correction (OPC). The goal of OPC is to produce smaller features in an IC using a given equipment set by enhancing the printability of a wafer pattern. OPC applies systematic changes to mask geometries to compensate for the nonlinear distortions caused by optical diffraction and resist process effects. A mask incorporating OPC is thus a system that negates undesirable distortion effects during pattern transfer. OPC works by making small changes to the IC layout that anticipate the distortions. OPC offers basic corrections and a useful amount of device yield improvement, and enables significant savings by extending the lifetime of existing lithography equipment. Distortions that can be corrected by OPC include line-end shortening, corner rounding, and isolated-dense proximity effect.
Isolated-dense proximity effect, or bias, in particular refers to the degree to which the mean of measured dense features differs from the mean of like-sized measured isolated features. Isolated-dense bias is especially important in the context of critical dimensions (CD's), which are the geometries and spacings used to monitor the pattern size and ensure that it is within the customer's specification. CD bias, therefore, refers to when the designed and actual values do not match. Ideally, bias approaches zero, but in actuality can measurably affect the resulting semiconductor device's performance and operation. Isolated features, such as lines, can also negatively affect depth of focus (DOF), such that they cannot be focused as well with the lithography equipment as can dense features.
OPC can be used to correct the isolated-dense proximity effect and the isolated-feature DOF reduction by adding scattering bars (SB)'s) and anti-scattering bars (ASB)'s) near the edges of opaque and clear features, respectively, on a photomask. SB's are sub-resolution opaque-like features, whereas ASB's are sub-resolution clear-like features. SB's and ASB's are specific examples of assist features, which are features added to the mask that are not specifically part of the intended semiconductor design, but which assist the proper imprinting of the design on the photoresist. Both SB's and ASB's serve to alter the images of isolated and semi-isolated lines to match those of densely nested lines, and improve DOF so that isolated lines can be focused as well as dense lines can with the lithography equipment. For example,
FIG. 1A
shows a set of SB's
100
, whereas
FIG. 1B
shows the placement of such sets of SB's
100
near an isolated line
102
, in contradistinction to the dense sets of lines
104
and
106
.
Another issue that impacts the quality of lithography is focus variation, which is nearly ubiquitous in IC manufacturing because of the combined effects of many problems, such as wafer non-flatness, auto-focus errors, leveling errors, lens heating, and so on. A useful lithographic process should be able to print acceptable patterns in the presence of focus variation. Similarly, a useful lithographic process should be able to print acceptable patterns in the presence of variation in the exposure dose, or energy, of the light source being used. To account for these simultaneous variations of exposure dose and focus, it is useful to map out the process window, such as an exposure-defocus (ED) window, within which acceptable lithographic quality occurs. The process window for a given feature, with or without OPC to compensate for distortions, shows the ranges of exposure dose and defocus that permit acceptable lithographic quality.
For example,
FIG. 2
shows a graph
200
of a typical ED process window for a given semiconductor pattern feature. The y-axis
202
indicates exposure dose of the light source being used, whereas the x-axis
204
indicates defocus. The line
206
maps exposure dose versus defocus at one end of the tolerance range for the CD of the pattern feature, whereas the line
208
maps exposure dose versus defocus at the other end of the tolerance range for the CD of the feature. The area
210
enclosed by the lines
206
and
208
is the ED process window for the pattern feature, indicating the ranges of both defocus and exposure dose that permit acceptable lithographic quality of the feature. Any defocus-exposure dose pair that maps within the area
210
permits acceptable lithographic quality of the pattern feature. As indicated by the area
210
, the process window is typically indicated as a rectangle, but this is not always the case, nor is it necessary.
Besides OPC, another approach that can be used to improve patterning is off-axis illumination (OAI). OAI is the shifting of the direction of the exposure beam during lithography from perpendicular, which interrupts the interference pattern that causes standing waves in the underlying photoresist being patterned. OAI particularly has the ability to significantly improve both the resolution and DOF for a given optical lithographic technology. For dense features, especially those having line-to-space duty ratios on the order of 1:1 to 1:2, such improvements are straightforward. Performance improvements are realized when illumination is obliquely incident on a mask at an angle so that the zeroth and first diffraction orders are distributed on alternative sides of the optical axis.
Examples of OAI are shown in
FIGS. 3A and 3B
. In
FIG. 3A
, the original center of illumination
302
has an illumination mask
304
positioned thereover. When an off-axis light source is instead used for illumination through the mask
304
, first diffraction orders
306
a
and
306
b
result. The OAI of
FIG. 3A
is referred to as conventional OAI because the illumination mask
304
has a standard disc shape. In
FIG. 3B
, the original center
302
has an illumination mask
310
positioned thereover, resulting in diffraction orders
312
a
and
312
b
. The OAI of
FIG. 3B
is referred to as annular OAI because the illumination mask
310
has a ring shape. Other types of OAI include dipole, quadrupole, and quasar, which vary from one another and from conventional and annular OAI based on the illumination mask shapes used in such types of OAI.
Unfortunately, OAI does no
Sagar K.
Taiwan Semiconductor Manufacturing Co. Ltd
Tung & Associates
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