Optics: measuring and testing – By alignment in lateral direction – With registration indicia
Reexamination Certificate
2000-02-18
2002-06-25
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By alignment in lateral direction
With registration indicia
C356S399000
Reexamination Certificate
active
06411386
ABSTRACT:
INCORPORATION BY REFERENCE
The disclosures of the following priority application(s) are herein incorporated by reference:
Japanese Patent Application Laid Open No. 09-210315 filed Aug. 5, 1997.
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to an alignment apparatus and method which permit mask patterns to be precisely aligned with pattern transfer objective regions on a substrate, in accordance with coordinates predicted on the basis of, for example, a statistical technique. More specifically, the present invention is concerned with an alignment apparatus and method suitable for use in an Enhanced Global Alignment (EGA) process.
2. Description of Related Art
Semiconductor devices and liquid crystal display devices are produced by apparatus and methods that employ a lithographic process in which a reticle pattern, i.e. a mask pattern, is transferred to successive shot regions on a wafer or a glass plate coated with a photo-resist. The lithographic process is performed using a projection optical system having a projection exposure device such as a stepper. This process requires a high degree of precision of alignment between the reticle patterns, through which the exposure is to be performed, and circuit patterns which have already been formed on the respective shot regions on the wafer.
For example, U.S. Pat. No. 4,780,617 (Japanese Patent Laid-Open No. 61-44429) discloses a method of Enhanced Global Alignment (EGA), which realizes a high degree of precision of alignment between the reticle patterns and circuit patterns. According to this method, coordinate positions of alignment marks (wafer marks), provided on preselected sample shot regions on a wafer, are measured and the results of the measurement are statistically processed to determine the coordinate positions of the shot regions.
The alignment in accordance with the EGA technique is a kind of fine alignment. In order that the fine alignment be executed satisfactorily, it is necessary that a search alignment be performed such that the wafer marks on the sample shots regions fall within the sensing coverage of the alignment sensor without fail.
FIG. 9
shows a wafer
51
on which alignment is performed in accordance with the conventional EGA technique. A multiplicity of shot regions are allocated to the surface of the wafer
51
at predetermined pitches in two orthogonal directions, which will be referred to as the X and Y directions, respectively. Preselected shot regions, e.g., three shot regions, among these shot regions are provided with different search alignment marks WX, WY and W&thgr;, respectively, roughly indicating the positions in the X, Y and rotational (&thgr;) directions. Each of the other shot regions is provided with a wafer mark
55
indicative of a two-dimensional position and composed of X-axis wafer mark
54
X and Y-axis wafer mark
54
Y.
In the actual EGA process, the search alignment marks and the wafer marks are formed on the boundaries between adjacent shot regions. Such boundary regions are also referred to as “street regions.” In the EGA process, ten shot regions C
1
to C
10
, which are distributed substantially at a constant pitch in the circumferential direction, are selected as sample shots regions from among all the shot regions.
As the first step of the process, measurements are sequentially executed by a search alignment sensor. More specifically, the Y coordinate of the Y-axis search alignment mark WY, the Y coordinate of the &thgr;-axis search alignment mark W&thgr; and the Y coordinate of the X-axis search alignment mark WX are sequentially measured. A conversion parameter composed of a rotational angle and an offset, necessary for converting the sample coordinate system on the wafer
51
into a stationary coordinate system of a wafer stage, is determined based on the results of the measurement. Coordinate positions of the sample shots are then determined on the stationary coordinate system to thereby, complete the search alignment.
Thereafter, the wafer marks
55
on the sample shots C
1
to C
10
are successively moved into the sensing area of a fine alignment sensor so that the coordinates of these wafer marks
55
on the stationary coordinate system are measured. The coordinates of the shot regions on the stationary coordinate system are determined through a statistical processing of the results of measurement. Exposure is then performed for each of the shot regions while aligning the shot regions in accordance with the coordinate positions on the stationary coordinate system. It is thus possible to achieve a high degree of overlay accuracy through the EGA process.
The conventional EGA alignment essentially requires a search alignment in order to ensure that the wafer marks of the sample shots fall within the sensing area of the fine alignment sensor. For example, in the case of the wafer shown in
FIG. 9
, it is necessary that the wafer be moved to three different positions in order to measure the positions of the search alignment marks WX, WY and W&thgr;. Subsequently, the wafer is moved to ten positions for the purpose of measuring the positions of the wafer marks
55
on the ten sample shots. Consequently, a considerably long time is required for the alignment. As a result, the throughput of the exposure is undesirably reduced.
Another disadvantage encountered with the known alignment process is that the area on each shot region available for the circuit pattern is limited. This is due to the necessity of using both search alignment marks and fine alignment wafer marks which are to be formed on the shot regions or on the street line regions which are defined between adjacent shot regions.
Further, a complicated positioning control is required for the wafer stage in the conventional EGA adjustment. The degree of freedom of correction is also limited because the position of each shot region is finally corrected by controlling the position of the wafer stage in accordance with the results of the fine alignment executed by using the EGA process.
SUMMARY OF THE INVENTION
The present invention provides an alignment apparatus and method for aligning the position of each of the shot regions on a wafer based on the results of a statistical processing of the measured positions of a preselected number of the shot regions on the wafer. The alignment of the present invention employs a reduced number of measurement marks and is performed in a shorter processing time, as compared with known techniques, without impairing the measuring accuracy.
According to one aspect of the present invention, there is provided an alignment apparatus and method for successively bringing a plurality of shot regions to be processed on a substrate into alignment with a predetermined reference position. The shot regions to be processed are arranged on the substrate in accordance with design array coordinates so that a mask pattern is transferred to each of the shot regions successively. In this apparatus and method, m measurement objective regions are selected from among the plurality of shot regions. The selected measurement objective regions are successively brought into a predetermined measuring area to measure the coordinates of the measurement objective regions. A statistical computation is performed on the measured coordinates of the selected measurement objective regions thereby computing a linear error of the actual coordinates of each of the shot regions on the substrate from the design array coordinates. Then, relative correction on the position to which the substrate is to be moved is performed in accordance with the computed linear error.
The alignment method of the present invention comprises: (1) selecting k pieces of a preparatory measurement objective region from among the m pieces of measurement objective region; (2) determining the array coordinates of the plurality of shot regions based on the outline of the substrate; (3) successively bringing the k pieces of preparatory measurement objective region into a predetermined measuring area in accordance with the de
Font Frank G.
Lee Andrew H.
Nikon Corporation
Oliff & Berridg,e PLC
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