Data processing: generic control systems or specific application – Specific application – apparatus or process – Product assembly or manufacturing
Reexamination Certificate
2001-03-08
2003-06-03
Picard, Leo (Department: 2121)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Product assembly or manufacturing
C356S401000
Reexamination Certificate
active
06574524
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the photolithography process used in fabricating semiconductor devices. More particularly, the present invention relates to a method of aligning the dies of a wafer in succession with a photomask of photolithography equipment in the process of exposing the dies to an image borne by the photomask.
2. Description of the Related Art
The fabricating of a highly integrated semiconductor device requires forming a plurality of wiring patterns within a small area. The extent to which a large number of wiring patterns can be formed within a given area depends upon available photolithography exposure techniques more than anything else. In photolithography, respective areas of a wafer are exposed in succession to a predetermined pattern. Subsequently, the exposed areas are developed whereupon the wafer is patterned.
The precision of the exposure process must be increased if the current demand for more highly integrated and for higher quality semiconductor devices is to be met. In order to improve the precision of the exposure process, the capability of the exposure equipment to perform a self-analysis of the exposure process and the precision under which the wafer and the exposure equipment are aligned relative to one another must both be improved. In particular, currently available photolithography equipment has an analysis capability that is insufficient for determining whether the exposure process has yielded a semiconductor product of a quality meeting current requirements. Even the use of deep ultraviolet rays (DUV) to enhance the analysis capability of today's photolithography equipment poses certain limitations. Therefore, a self-aligning patterning method and the like are being explored as means to produce a pattern of a desired size at a precise position on a wafer.
More specifically, fabricating a semiconductor device involves forming a three-dimensional wiring pattern on a wafer. To form such a wiring pattern, several layers of material are deposited sequentially on the wafer, and these layers are patterned and/or processed. The patterning of a layer of material on the wafer is carried out through the execution of numerous exposure processes in which a respective pattern is transferred repeatedly onto the layer at several areas thereof, respectively. In this process, a photomask bearing the pattern is aligned with certain areas of the wafer in succession so that the pattern of the photomask is positioned relative to patterns already transferred to or to be transferred to the layer of material on the wafer.
In the alignment process, an alignment key pattern is formed on the wafer. The alignment key pattern is used as a reference mark during the process of aligning the wafer with the photomask. An overlay key pattern, formed on the photomask, is used to inspect the state of alignment between the photomask and a selected area of the wafer. The inspection process determines whether the overlay key pattern coincides with or otherwise corresponds to the alignment key pattern, i.e., whether a pattern to be transferred to an area on the wafer during the current exposure process is positioned precisely relative to the pattern that was transferred to another area on the wafer during the previous exposure process.
Image recognition and analysis equipment such as a KLA is used to quantify (measure) the state of alignment between the alignment key pattern and the overlay key pattern. Specifically, the KLA produces an image alignment deviation value and issues a signal representative of this value to the stepper of the photolithography equipment. The image alignment deviation value is used by the stepper to correct, if necessary, the state of alignment between the photomask and the wafer.
In a conventional alignment method, the state of alignment is inspected and used to position the wafer in the processes of exposing the individual areas of the wafer. That is, the inspection process is carried out in connection with the exposure of each and every die of each and every wafer. Such a method comprising numerous inspecting steps is a hindrance on the production efficiency of the exposure process and becomes particularly onerous when the wafer comprises a large number of dies.
Recently, therefore, an alignment method has been developed in which several sample dies from a wafer are selected, the state of alignment of only these sample dies is inspected, deviation data is produced from the inspecting of the state of alignment of the sample dies, and a final job file is produced from the deviation data. Basically, the final job file dictates an overall alignment corrected position for wafers loaded in the stepper. That is, once a wafer is loaded onto a stage in the stepper, the wafer is moved linearly or is rotated, or the exposure equipment is focused, on the basis of the final job file, to set the wafer at the corrected position. After the wafer is set at the corrected position, the wafer stage is moved in increments determined by the size of the dies, to execute the exposure processes without any further inspecting of the state of alignment of the individual dies.
Such an overall alignment correction method is advantageous in terms of enhancing the efficiency of the exposure process. Furthermore, under this method, the actual aligned positions of the individual dies does not deviate much from the ideal positions because the photolithography equipment, i.e., the stepper, is in general very precise. At present, however, the so-called process margin of the exposure process has become very small in order to meet the strong demand for more highly integrated semiconductor devices. Therefore, even a small deviation per die between the actual and ideal state of alignment becomes problematic.
Such small deviations are shown in FIG.
1
. In this figure, the magnitude and direction of alignment errors or deviations are represented by the vectors. As can be appreciated from
FIG. 1
, under the conventional overall alignment correction method, most of the dies will have a similar or the same deviation. Therefore, under the overall alignment correction method, the same inferior state of alignment is present throughout a significant part of the wafer.
There are several potential causes for the occurrence of such a constant alignment deviation. One of the causes might be merely the increase in size of the wafers that are being processed today. Also, seeing that many different devices make up the exposure equipment used in fabricating a semiconductor device, some characteristic particular to the device(s) can give rise to an alignment deviation which can not be overcome using the conventional overall alignment correction method. For instance, a typical piece of exposure equipment comprises a flat plate having side walls extending along X-and Y-axes, respectively, and a mirror mounted to one of the side walls. The wafer is mounted on the flat plate, whereby the plate serves as a wafer stage. The mirror constitutes an interferometer that is used to determine the distances between the side walls of the plate and reference positions as measured along the X-and Y-axes, respectively. These distances are used to precisely position the flat plate, on which the wafer is mounted, during the exposure process. However, the mirror is generally not exactly planar, and so the origin of light reflecting from the mirror can not always be exactly determined. Thus, a measurement value obtained by the interferometer to represent the distance along the X-axis between the flat plate and a reference position might not be accurate. In this case, the flat plate might deviate from its desired position when it is moved along the Y-axis as fixed in the direction of the X-axis on the basis of the measurement value produced by the interferometer.
Of course, such a deviation inherent in the exposure equipment could be compensated for to some extent by reflecting its value in an operative program of the equipment. For instance, in the example of an in
Kosowski Alexander
Picard Leo
Samsung Electronics Co,. Ltd.
Volentine & Francos, PLLC
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