In the photolithography process of semiconductor device manufacturing, an exposure apparatus is used to transfer the fine pattern of a reticle onto a wafer coated with a photosensitive material. The measurement of the position of a stage on which such a wafer is placed, and the stage drive, require a high degree of accuracy, and, therefore, a high-resolution laser interferometer and a reflecting mirror, which is the target of the laser beam, are used. In recent years, as semiconductor wafers have increased in diameter, the drive range of the stage has also increased, as has the length of the reflecting mirror.
However, increasing the length of the reflecting mirror leads to the following problems:
(1) In terms of manufacture and installation, it is difficult to achieve a nearly perfectly flat surface;
(2) Even when it is possible to achieve a nearly perfectly flat surface, the cost is prohibitive; and
(3) as the characteristic value of the reflecting mirror decreases, the band of control deteriorates.
In addition, the exposure apparatus is loaded with a large number of units, such as a projection optical system, a focus/leveling measurement system, and an alignment measurement system, an illumination system, and so forth. At the same time, the laser interferometer becomes inoperative if the light path is obstructed. As a result:
(4) the positioning of the laser interferometer and the reflecting mirror may be limited by the balance with the other units. In particular, with respect to the disposition of the laser interferometer and the reflecting mirror for measuring the position in the direction of the optical axis of the projection optical system (hereinafter the “Z axis”), there are many limitations, because most of the units are concentrated near the projection lens.
As one solution to the aforementioned problem, there is, for example, the construction shown in Japanese Laid-Open Patent Publication No. 2002-319541, in which positioning along the Z axis is measured by switching between a plurality of interferometers. In the apparatus described in Japanese Laid-Open Patent Publication No. 2002-319541, shown are an example of a configuration for measuring a position along the Z axis, implementing Z-axis measurement by using a plurality of laser interferometers and a plurality of reflecting mirrors, and switching interferometers while driving the stage in the direction of the Y axis. As a result of such a configuration, measurement of position in the Z axis is possible, no matter where the stage is in the X-Y plane.
When switching between interferometers, the coordinate systems of the interferometers switch. Accordingly, if just the interferometer to be used is switched, there is the possibility of a discontinuity in measurements just before and just after the switch, and, thus, it is necessary to insure the continuity of measurements when switching. For example, Japanese Laid-Open Patent Publication No. 2002-319541 describes providing overlapping intervals measurable by multiple lasers in order to prevent the occurrence of measurement discontinuity, such as that which occurs just before and just after switching, by inheriting a measurement result just before switching to a measurement result just after switching. However, because the reflecting mirrors placed in the light paths of the laser interferometers are not perfectly flat, but differ in shape from one mirror to the next, simply continuing measurements alone causes misalignments to occur. As a result, in, for example, the exposure apparatus, if such misalignments occur along the X, Y axes (that is, in the direction of movement of the X-Y stage (the wafer stage)), they can cause an accumulated error, and if such misalignments occur along the Z axis, they can cause a focusing error.
In particular, as patterns have become more detailed in recent years, ever greater position measurement accuracy is required. In addition, the position in the direction of the Z axis of a stage moving in the X-Y plane, where determined by switching between a plurality of interferometers, involves the use of reflecting mirrors extended along the X axis and reflecting mirrors extended along the Y axis in order to cover the range of motion of the stage, and, thus, the effects of the shapes of the surfaces of the mirrors are different for the X-axis position and the Y-axis position. For example, when switching interferometers at the same X-axis position, the Y-axis position might be different, and if the Y-axis position is different, the affect of the shape of the surface of the reflecting mirrors also is different.