Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-05-13
2004-11-30
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S311000, C716S030000
Reexamination Certificate
active
06824931
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to semiconductor device fabrication, and more particularly to photolithographic process windows and optical proximity correction (OPC) as used in 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.
This compensation is generally referred to as optical 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, isolated-dense proximity effect, and isolated-line depth of focus reduction.
Line-end shortening (LES) is the shortening of the end of a metal line end in the actual fabricated semiconductor device as compared to the circuit designer's originally contemplated ideal device. An example of LES is shown in FIG.
1
A. The line
102
should extend to the originally designed end
104
. However, in actuality, the line
102
may only extend to the actually fabricated end
106
. OPC can be used to correct LES by adding serifs or hammerheads to the originally designed end in the photomask, such that during photolithography, the actually fabricated end more closely approximates the location of the originally designed end. The addition of serifs is shown in
FIG. 1B
, in which the serifs
110
and
112
have been added to the line
102
at its end
104
. The addition of a hammerhead is shown in
FIG. 1C
, in which the hammerhead
120
has been added to the line
102
at its end
104
.
Corner rounding is the degree to which feature corners that should be at sharp angles are instead rounded by the lithography process. An example of corner rounding is shown in FIG.
2
A. The feature
202
should include the outside sharp corner
204
and the inside sharp corner
206
. However, in actuality, the feature
202
may only include the outside rounded corner
208
and the inside rounded corner
210
. OPC can be used to correct corner rounding by adding serifs to outside corners, which are called positive serifs, and subtracting serifs from the inside corners, which are called negative serifs, to the feature in the photomask. This is shown in
FIG. 2B
, in which the positive serif
220
has been added to the outside corner
204
of the feature
202
, and the negative serif
222
has been removed from the inside corner
210
of the feature
202
.
Isolated-dense proximity effect, or bias, 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, 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 depth of focus 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. Both serve to alter the images of isolated and semi-isolated lines to match those of densely nested lines, and improve depth of focus so that isolated lines can be focused as well as dense lines can with the lithography equipment. For example,
FIG. 3A
shows a set of SB's
300
, whereas
FIG. 3
b
shows the placement of such sets of SB's
300
near an isolated line
302
, in contradistinction to the dense sets of lines
304
and
306
.
Unfortunately, OPC is a difficult process, because determining the optimal type, size, and symmetry of the compensations to be made on the mask can be very complex, and depends on neighboring geometries and process parameters. Usually, a sophistical computer program is used to properly implement OPC. Using empirical data, OPC software creates a mathematical description of the process distortions, which can be in the form of simple shape manipulation rules, or a more detailed and intricate process model. Once this description is generated, automated software changes the shapes of the polygons in the pattern layout files, moving segments of line edges and adding features that compensate the layout for the distortions that will result. The critical levels of the photomask set can then be made using these modified, predistorted layout designs.
Like other semiconductor processes, OPC is desirably continually monitored and verified to ensure mask quality. Usually, OPC is inserted as part of a verification/tape-out activity. While OPC can more efficiently be included as part of mask data preparation, enough errors have been detected on wafers processed in this manner that many users are hesitant to make such significant changes to their pattern date without the insurance providing by repeating other verification steps after OPC has been applied. Mask inspection is also negatively impacted by OPC, since the addition of the small geometries may appear identical to features that mask inspection machines have been trained to recognize as defects. Masks and reticles with these features will appear to contain thousands of such defects, and be rejected. Manual inspection is also slow, because the technician must examine many different parts of each mask to ensure that the mask has been produced correctly. Since masks inherently differ based on the semiconductor device being fabricated, manual inspection can become a very laborious and non-standardized process. Production yield of new semiconductor devices as a result is usually reduced when using OPC.
Another is
Chen Ching-Ming
Hsieh Chin-Chuan
Liu Kun-Yi
Huff Mark F.
Sagar Kripa
Taiwan Semiconductor Manufacturing Co. Ltd
Tung & Associates
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