Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
1999-04-14
2003-09-23
Smith, Zandra V. (Department: 2877)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C356S400000, C356S620000
Reexamination Certificate
active
06624433
ABSTRACT:
FIELD OF THE INVENTION
In particular, the invention relates to a method and an apparatus adapted to be used for positioning the photosensitive substrate in a direction of rotation in an exposure apparatus (a stepper, an aligner and the like) for producing a semiconductor device, a charge coupled device (CCD), a liquid crystal indication element, a thin film magnetic head and the like.
RELATED BACKGROUND ARTS
In a projection exposure apparatus such as a stepper and the like which is used in manufacturing a semiconductor device, a liquid crystal indication element and the like, it is desired to position (or align) with high accuracy a reticle which constitutes a mask with a wafer (or a glass mask and the like) which constitutes a photosensitive substrate, in order to transfer a circuit pattern formed on the reticle onto a photoresist layer on the wafer with high alignment accuracy.
There are various types of alignment sensors used in the alignment system, one of which is an LSA (Laser Step Alignment) type, such as shown in Japanese Patent Laid-Open Publication No. Hei 5-21324, in which a laser beam is irradiated to a doted-line-shaped alignment mark on a wafer and the position of the alignment mark is detected on the basis of the beam diffracted or scattered by the mark, another is an FIA (Field Image Alignment) type in which an image of an alignment mark is taken by illuminating with the light having a wide wave band width and emitted from a halogen lamp as a light source and the position of the alignment mark is measured by image-processing the obtained image data, and yet another is an LIA (Laser Interferometric Alignment) type in which a diffraction grating-shaped alignment mark on a wafer is irradiated from two different directions with two laser beams having slightly different frequencies with each other, and two diffracted beams emitted thereby interfere with each other to thereby enable the position of the alignment mark to be measured using the phase between the two diffracted beams. Alignment systems can be roughly divided into a TTL (Through-The-Lens) type in which the position of a wafer is detected through an optical projection system, a TTR (Through-The-Reticle) type in which a positional relationship between a reticle and a wafer is measured through an optical projection system and the reticle, and an Off-Axis type in which the position of a wafer is directly detected without using an optical projection system.
The position of a wafer not only in a translational direction and but also in a direction of rotation (angle of rotation) is detected by detecting the positions of at least two points on a wafer placed on a wafer stage by means of these alignment sensors. There are several alignment sensors such as a TTL and LIA (Laser Interferometric Alignment) type, and a TTL and LSA (Laser Step Alignment) type and an Off-Axis and FIA (Field Image Alignment) type as a sensor used in measuring an angle of rotation of the wafer.
For the projection exposure apparatuses, it is desired to align a reticle and a wafer with high accuracy based on detected results by these alignment sensors while reducing the time required for effecting an alignment of the reticle and the wafer and maintaining a high productivity (throughput). Therefore, it is necessary to increase productivity in all steps from a step in which a wafer is transferred to a wafer stage to the final exposure step. Referring to
FIG. 1
, the operation in a transfer process of a wafer prior to the final alignment of the wafer in the conventional exposure apparatus will be explained, hereinafter.
FIG. 1
shows a structure around a wafer stage explaining a transfer mechanism for a wafer in a conventional exposure apparatus. In
FIG. 1
, a state in which a substrate or wafer W is transferred from a wafer carrier device onto an elevating or vertically movable device g disposed through a telescoping mechanism f on an X stage a. The elevating device g includes three supporting pins (in
FIG. 1
, two supporting pins g
1
and g
2
are shown) which are loosely inserted into openings formed in each of a sample table c, a &thgr; rotation correction mechanism d and a wafer holder e with play therebetween. The elevating device operates such that three supporting pins thereof moves the wafer W up and down by upward and downward movement of the telescoping mechanism in response to a transfer operation of the wafer. Each supporting pin g
1
, g
2
or g
3
is adapted to suck the lower surface of the wafer by vacuum suction generated by an external vacuum pump to hold the wafer so that it does not move or displace when the elevating device is moved up and down.
After the wafer W is stationarily held on the wafer holder e by vacuum suction, the alignment sensor generates a detection signal of alignment marks formed on the opposite ends of the wafer W and a rotational error or angular error on a coordinate system of the wafer stage is calculated by obtaining the coordinates of the sample table c, for example, when a detected signal reaches its peak and is measured by means of a movable mirror h fixed on the end of the sample table c and an external interferometer (not shown). The rotational error of the wafer W is eliminated by driving the &thgr; rotation correction mechanism (&thgr; table) d on the sample table e based on the obtained results, thereby carrying out alignment of the reticle and the wafer W in the direction of rotation.
In the prior art as explained above, the &thgr; rotation correction mechanism d for rotating the wafer is provided between the wafer W and the sample table e which is a reference of the coordinate system of the wafer stage system and is provided with the movable mirror h thereon. This results in some inconveniences that the wafer W is displaced in a lateral direction when vacuum suction of the wafer holder for holding the wafer W is weak, that the rigidity of the entire stage becomes weak since complex mechanisms are provided on the sample table c and that the control performance of the stage cannot be increased since the weight of the entire stage increases. Therefore, it may be intended, for example, that the &thgr; rotation correction mechanism is arranged below the sample table c. In this case, however, angle of rotation of the &thgr; rotation correction mechanism is limited when the &thgr; rotation correction mechanism is driven to adjust the angle of rotation of the wafer W, since the angle of a light beam from the laser interferometer which strikes the movable mirror h on the sample table c varies. Therefore, for example, if accuracy of pre-alignment of the wafer is not good, the rotational error cannot be sufficiently corrected.
Also in the prior arts, when an alignment sensor of a diffracted light detection type such as the LAS type or the LIA type, particular detection errors are created in response to an angle of inclination between an alignment mark on a wafer W and the light beam for detecting a position.
FIGS.
2
(
a
) and
2
(
b
) illustrate a state in which laser beam is irradiated to an alignment mark. FIG.
2
(
a
) shows a state in which an alignment mark (grating-shaped) for the LIA type is irradiated with a laser beam and FIG.
2
(
b
) shows a state in which an alignment mark (doted-line-shaped mark) for the LSA type is irradiated with a laser beam. As shown in FIG.
2
(
a
), in the alignment system of the LIA type, a grating-shaped alignment mark is irradiated from two directions with two laser beams each having a rectangular irradiation region and the wafer is positioned on the basis of a phase of the interfered light of two diffracted lights form the alignment marks. In FIG.
2
(
a
), an irradiation region RA
1
of the laser beam is inclined by &Dgr;&thgr;
A
with respect to a central axis GM
y
extending along a longitudinal direction (non-measuring direction) of a grating-shaped mark GM which is formed by a plurality of gratings arranged with a predetermined pitch in a right and left direction.
Also, as shown in FIG.
2
(
b
), in the alignment system of the LSA type, a dotted-line-sh
Nishi Kenji
Okumura Masahiko
Nikon Corporation
Smith Zandra V.
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