Photocopying – Projection printing and copying cameras – Focus or magnification control
Reexamination Certificate
2001-12-11
2004-03-09
Fuller, Rodney (Department: 2851)
Photocopying
Projection printing and copying cameras
Focus or magnification control
C355S052000, C355S053000, C355S077000
Reexamination Certificate
active
06704094
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to semiconductor device fabrication, and more particularly to such fabrication where the devices have asymmetrical patterns.
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. Improvements 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.
One particular 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 some 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 (or lack thereof), 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 shows the ranges of exposure dose and depth of focus (DOF) that permit acceptable lithographic quality.
For example,
FIG. 1
shows a graph
100
of a typical ED process window for a given semiconductor pattern feature. The y-axis
102
indicates exposure dose of the light source being used, whereas the x-axis
104
indicates DOF. The line
106
maps exposure dose versus DOF at one end of the tolerance range for the critical dimension (CD) of the pattern feature, whereas the line
108
maps exposure dose versus DOF at the other end of the tolerance range for the CD of the feature. The area
110
enclosed by the lines
106
and
108
is the ED process window for the pattern feature, indicating the ranges of both DOF and exposure dose that permit acceptable lithographic quality of the feature. Any DOF-exposure dose pair that maps within the area
110
permits acceptable lithographic quality of the pattern feature. As indicated by the area
110
, the process window is typically indicated as a rectangle, but this is not always the case, nor is it necessary.
To ensure that focus is properly maintained, some semiconductor equipment include leveling sensors. Such semiconductor equipment can include steppers and scanners. Steppers and scanners are types of semiconductor fabrication equipment used in photolithographic processing, such as aligning a mask over a wafer and exposing the pattern of the mask onto the wafer. A scanner typically uses a mirror system with a slit blocking part of the light coming from the light source. The size of the slit is smaller than the wafer, so the light beam scans across the wafer. Whereas scanning is generally performed on a per-wafer basis, a stepper is utilized on only a given part of the wafer at one time. A reticle is aligned and exposed, without scanning, and then is stepped to the next site and the process is repeated. Stepping generally allows more precise matching of larger-diameter wafers than scanners do.
Another type of semiconductor fabrication equipment combines the stepping and scanning process of steppers and scanners, and is known as step and scan aligners. At one position on the semiconductor wafer, a small-scale scanning process takes place, and then the reticle or mask is stepped to the next position, where the scanning process is repeated. As used herein, steppers, scanners, and step and scan aligners are generally encompassed under the term alignment and exposure equipment, which can include other types of specific semiconductor fabrication equipment besides steppers, scanners, and step and scan aligners. Examples of steppers, scanners, and step and scan aligners include those available from ASML Holding, N.V., of the Netherlands. Furthermore, unless otherwise and specifically noted, steppers, scanners, and step and scan aligners are used substantially interchangeably herein, such that reference to or description of one should be assumed to apply to other types of alignment and exposure equipment as well.
Leveling sensors of such steppers, scanners, and step and scan aligners ensure that the plane of the semiconductor wafer is horizontal. If the wafer is tilted, for instance, the semiconductor equipment will likely be out-of-focus for at least a part of the wafer, which can lead to improperly fabricated semiconductor devices, and thus to wafer scrap. Wafer scrap is costly. A leveling sensor detects the degree to which the semiconductor wafer is out of horizontal, and tilts the wafer so that it remains horizontal for the alignment and exposure processes, as well as potentially for other processes, to be performed by the semiconductor equipment. The leveling detection by such sensors is typically performed on a field-by-field basis. That is, when the semiconductor equipment moves such that it has a new field of view with respect to the wafer, the leveling sensor ensures thereafter that the wafer is still horizontally level, so that proper alignment and exposure can be performed.
A difficulty with at least some leveling sensors, however, is that they may incorrectly detect the semiconductor wafer as being non-horizontal, and correspondingly improperly tilt the wafer, when in fact the wafer is horizontal. This has been found to occur especially with semiconductor wafers on which asymmetrical semiconductor device patterns have been, or will be, fabricated. An asymmetrical semiconductor pattern is generally defined as one in which features of one part of the pattern are substantially dense, whereas features of another part of the pattern are substantially isolated. By comparison, a symmetrical semiconductor pattern is generally defined as one in which features of at least most parts of the pattern substantially have the same density, such that they are substantially dense, substantially isolated, substantially semi-dense, substantially semi-isolated, and so on. Density generally measures the periodicity of which the features appear in a given part of a pattern, where dense features have a low periodicity, and thus have a high frequency of appearance, and isolated features have a high periodicity, and thus have a low frequency of appearance.
Examples of asymmetrical patterns are shown in
FIGS. 2A and 2B
. In
FIG. 2A
, the pattern
202
has left-right asymmetry, where the left pattern part
204
has dense features, and the right pattern part
206
has isolated features. These isolated features can be lines, contacts, as well as other types of features. Similarly, in
FIG. 2B
, the pattern
212
has up-down asymmetry, where the upper pattern part
214
has dense features, and the lower pattern part
216
has isolated features. By comparison, examples of symmetrical patterns are shown in
FIGS. 2C and 2D
. In
FIG. 2C
, the pattern
222
has a pattern part
224
that is substantially isolated, whereas, in
FIG. 2D
, the pattern
232
has a pattern part
234
that is substantially dense.
The problem with untimely corrective tilting as a result of improper sensing by the leveling sensor is that the feature
Fuller Rodney
Taiwan Semiconductor Manufacturing Co. Ltd
Tung & Associates
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