Image analysis – Applications – Manufacturing or product inspection
Reexamination Certificate
2000-10-10
2004-07-06
Johnson, Timothy M. (Department: 2621)
Image analysis
Applications
Manufacturing or product inspection
C250S559220, C382S199000, C382S286000, C382S288000
Reexamination Certificate
active
06760473
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to optical measurement systems. More specifically, the present invention relates to the measurement of features in semiconductor manufacturing.
BACKGROUND OF THE INVENTION
The successful manufacture of advanced sub-micron sized semiconductor devices requires the detection, measurement and evaluation of defects and other features as small as 1 micron on the photographic mask (photomask) used to pattern the wafer. Feature inspection and measurement techniques for masks therefore play an important role in mask making and quality assurance.
Thus, it is becoming increasingly important to be able to identify and to correctly size mask defects, line widths, and other features that are under 1 micron in size. Accurate sizing of these features allows masks that are below specification to be repaired and prevents the needless and costly hold up of masks that do meet specification. One of the problems of assessing reticle quality at these sub-micron levels on an automatic inspection system, however, is that the size of these features cannot always be accurately, quickly and cost-effectively measured in a production environment.
It has long been known that mask inspection tools are not measurement tools and that the size information provided by these tools has limited value. Consequently, many mask makers have incorporated measurement aids at the inspection station or move the mask to a more suitable measurement tool in order to make classification decisions. Measurement aids used at the inspection station include calipers, grids, and software based video image markers such as gates, scales, grids, boxes and circles. These aids are fairly rapid, but ultimately require the operator to “eyeball” the boundaries of the defect. This activity is very subjective and can lead to an error in the measurement of the defect.
For example, feature size is often measured by measuring the distance between opposite edges of the feature. Once a feature is identified by an inspection machine, the operator uses a video microscope and a television camera to position a cursor on one side of the feature and another cursor on the other side of the feature. The operator must judge for himself the exact boundaries of the feature and must place the cursors where he sees fit. At this point, the operator pushes a button and the software blindly computes the distance between the two cursors in order to supply a rough approximation of the dimension of the feature. This measurement technique is operator dependent in that the operator must manually position the cursors on the boundaries of what the operator believes to be the feature. The operator may misjudge the type of a feature, its boundaries, or may simply misplace a cursor even if the feature is visible. The software then calculates the distance between the cursors, without regard for the type of feature, its true boundaries, etc. The above technique may be performed with a standard video microscope but is completely subject to the operator's skill level and interpretation.
Alternatively, the mask may be removed from the automatic inspection tool and relocated on a more precise and repeatable measurement tool. However, this approach involves removing the mask from production, relocating the feature, and is thus impractical in a production environment. This technique is also costly, time-consuming and increases the handling risk. For example, an atomic force microscope (AFM) may be used to measure feature sizes; such a microscope is extremely accurate but is very slow, very expensive and is still subject to operator interpretation.
Another difficulty with light measurements of features less than 1 micron in size is that the wavelength of photons begins to interfere with the measurement of these 1 micron and less feature sizes. Many techniques do not adequately address the non-linearities associated with such measurements.
One approach that has been taken that uses calibration of an automatic inspection system in order to size defects is described in
Characterization Of Defect Sizing On An Automatic Inspection Station,
D. Stocker, B. Martin and J. Browne, Photomask Technology and Management (1993). One disadvantage with the approach taken in this paper is that it only provides a technique for measurement of defects of 1.5 microns and greater. Such sizes of defects would produce a linear relationship between reference sizes and actual measured sizes, and the paper does not account for defects less than 1 micron that would produce a non-linear relationship.
Of particular concern is the measurement of features used in Optical Proximity Correction (OPC). In general, photomasks are used to generate a desired pattern on silicon wafers using optical projection lithography. The optical projection causes blurring of the image created on the wafer surface. This blurring is especially noticeable on corners, and on edges near corners.
FIG. 1
illustrates the blurring effect on a corner during lithography. Design
10
represents a pattern for a silicon wafer that has a sharp corner. This design has been produced in a suitable computer program and is represented in a design database and is viewed by a user on a computer screen. Due to blurring during the process, mask
12
ends up having a slightly more rounded corner then the sharp corner in the design. When the mask is eventually used to produce wafer
14
, the resultant corner is much more rounded, and is substantially different from the sharp corner desired in the original design.
FIG. 2
illustrates the blurring effect for the pattern of two line ends. Lines
20
and
22
in the original design are desired to be a certain distance apart. Due to blurring, lines
24
and
25
on the mask have more rounded ends and are slightly farther apart. The resultant wafer shows lines
26
and
27
having markedly rounded ends and being further apart then desired in the original design.
The OPC technique has been developed in order to correct for the detrimental affects of blurring in lithography. In most cases OPC corrections consist of “serifs” added to corners on the photomask. These serifs add or remove chrome at the mask corners in order to compensate for the area lost due to blurring. There is a large amount of literature dealing with the design of these OPC features. OPC features have become vital to the success of microlithography over the last few years. As chip geometries continue to shrink they will become yet more critical. Despite their widespread use, it has been very difficult to measure OPC features (called “serifs”) because their sizes are small, and by nature they appear as curves on masks and wafers. Conventional metrology tools are designed to measure the distance between edges. If the edges are irregular curves, as serifs are, edge-to-edge measurement becomes difficult, user dependent and sample dependent. Accurate measurement of these serifs as printed on photomasks (and then on wafers) is essential to the development of improved OPC algorithms and production processes.
FIG. 3
illustrates correction using Optical Proximity Correction (OPC). Using OPC, an additional shape
32
(termed a serif) is added to the corner of design
30
. The resultant mask
34
from design
30
now includes a serif
36
which is more rounded in shape. Finally, the resultant wafer
38
has a fairly sharp corner which is the desired shape for the patent on the wafer.
FIG. 4
illustrates correction of blurring for two line ends using OPC. In the original design lines
40
and
42
are spaced a desirable distance apart and have serifs
41
-
44
added to each corner of each line ends. Line ends
46
and
47
on the mask produced from the design shows each line having more rounded serifs and the ends being slightly closer than desired. Finally, the resultant wafer shows a pattern of two line ends
48
and
49
having ever so slightly rounded corners and having the desired spacing between the two line ends.
One problem associated with OPC is that the added serifs may not be of
Bayat Ali
Beyer Weaver & Thomas LLP
Johnson Timothy M.
KLA-Tencor Technologies Corporation
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