JP2014143033A - Stage device and beam irradiation device - Google Patents

Abstract

【Task】
The present invention provides a stage apparatus that is capable of specifying the position of a sample stage over a wide range without lengthening a reflecting mirror or installing a plurality of interferometers in one direction. Objective.
[Solution]
In order to achieve the above object, according to the present invention, a stage apparatus including a sample stage and a laser interferometer, which is different from the above-described laser interferometer for specifying the position of the sample stage. A control device that switches between a measurement device, a laser interferometer, and another stage position measurement device, the sample stage is configured to be movable to a position where the laser beam is not irradiated on the reflection mirror, and the control device is connected to the reflection mirror A stage device is proposed that performs position measurement with another stage position measurement device when the sample stage is located at a position where the laser beam is not irradiated.
[Selection] Figure 2

Description

The present invention relates to a stage apparatus used in a charged particle beam apparatus, a reduced exposure apparatus or the like that processes or inspects a semiconductor device, and in particular, a stage apparatus that identifies a stage position by irradiating a beam onto a reflecting mirror, and The present invention relates to a beam irradiation apparatus including the stage device.

In recent years, as the diameter of a wafer increases, the sample chamber and stage of a charged particle beam apparatus that performs measurement and inspection of a semiconductor device tend to expand. For example, in an SEM type image acquisition apparatus that inspects a circuit pattern using an electron beam in a semiconductor inspection apparatus, the wafer size is changed from 300 mm in diameter to 450 mm in diameter.
In the SEM type image acquisition apparatus, a highly accurate stage positioning operation is necessary, and a method using a laser interferometer and a reflection mirror as a stage position measuring means at this time is known. This laser interferometer has a measurement accuracy of several tens of picometers, and since the sample stage using this laser interferometer enables high-precision positioning, it is an apparatus for inspecting fine samples such as semiconductors. Widely used in
Patent Document 1 discloses a distance measuring device that specifies a sample stage position using a laser interferometer and a reflection mirror, and in particular, a description of providing a plurality of mirrors to specify a stage position in one direction. Has been.

JP-A-6-69099 (corresponding US Pat. No. 5,523,841)

The laser interferometer is a measuring device that receives the laser interference light reflected by the reflecting mirror and measures the position. However, if the laser interference light is lost, the measured value becomes indeterminate and the stage positioning operation may continue. Can not. For this reason, with an increase in the size of a sample such as a wafer, the length of the reflection mirror has been increased so that the entire sample chamber can be covered.
If a reflecting mirror that requires high-precision flatness becomes long, it becomes very difficult in terms of workability and mounting, and the cost of purchasing the reflecting mirror becomes very expensive. Furthermore, in the case of a device that needs to keep the sample chamber in a vacuum, such as a charged particle device, a structure that covers the structure that shields the atmosphere, such as a sample chamber or a lens barrel, is required, and a long reflection is required. In order to prevent the mirror and the sample chamber from interfering with each other, there is a problem that the sample chamber is enlarged. Moreover, the expansion of the sample chamber is accompanied by an increase in manufacturing cost, an expansion of the apparatus installation area, and an increase in the apparatus transport cost.

  According to the interferometer including a plurality of mirrors as disclosed in Patent Document 1, the measurement range can be expanded without increasing the mirror surface length of the mirror, but a plurality of mirrors need to be installed. In order to cover a wide range, it is necessary to provide a plurality of interferometers (laser light sources) in one direction. The installation of such a plurality of optical elements is not only limited by installation conditions including structures in other sample chambers, but also increases the volume of the sample chamber required for the installation of a plurality of laser interferometers. Therefore, an increase in the size and cost of the sample chamber is inevitable.

  Below, a stage device and beam irradiation aiming at specifying the position of the sample stage over a wide range without lengthening the reflection mirror and installing a plurality of interferometers in one direction The apparatus will be described.

  As one aspect for achieving the above object, a sample stage, a reflection mirror provided on the sample stage, and a laser interferometer for measuring the position of the sample stage by irradiating the reflection mirror with laser light are provided. A stage apparatus equipped with another stage position measuring device different from the above-mentioned laser interferometer for specifying the position of the sample stage, and a control device for switching between the laser interferometer and the other stage position measuring apparatus, The sample stage is configured to be movable to a position where the reflection mirror is not irradiated with the laser beam, and the control device is configured so that when the sample stage is located at a position where the reflection mirror is not irradiated with the laser beam, A stage device and a beam irradiation device that perform position measurement using a position measurement device are proposed.

  According to the above configuration, the position of the sample stage can be measured with high accuracy over a wide range without providing a plurality of interferometers in one direction or providing a long reflecting mirror.

It is a longitudinal cross-sectional view which shows the whole structure of an electron microscope apparatus. It is a cross-sectional view of an electron microscope apparatus, and is a figure which shows the moving range of a stage. It is a flowchart which shows the execution procedure in an electron microscope apparatus. It is a flowchart which shows the execution procedure in an electron microscope apparatus.

  When inspecting or measuring a fine structure such as a pattern formed on a semiconductor device, it is necessary to accurately place a region including the fine structure in the field of view of an electron microscope or the like. Therefore, high precision positioning accuracy is required for a sample stage such as an electron microscope. In order to perform high-precision positioning, it is necessary to provide a measurement apparatus such as a laser interferometer capable of performing high-accuracy position specification among the measurement apparatus for specifying the position of the sample stage on the sample stage.

  On the other hand, when exchanging the wafer, positioning is not required so accurately that a laser interferometer must be used. Furthermore, in an apparatus for measuring and inspecting semiconductor devices, alignment measurement can be performed to measure the amount of rotation and offset between the coordinate system of the stage called alignment measurement and the coordinate system of the wafer and update the target position. Only positioning accuracy may be good. Empirically, positioning accuracy on the order of several microns is sufficient when exchanging wafers and measuring alignment.

The inventors have achieved both high-accuracy positioning operation that requires a laser interferometer without increasing the length of the reflection mirror and low-accuracy positioning operation compared to the laser interferometer, from the above situation of the semiconductor measurement device and the like. We have come up with a stage device that can be used.
Specifically, a first measuring device for specifying a stage position, such as a linear encoder that covers the entire sample chamber, and a stage position by a laser interferometer and a reflecting mirror so that the measurement areas partially overlap. The inventors have come up with a stage device equipped with a second measuring device. In addition, a laser interferometer can be used without correcting the length of the reflection mirror by correcting the first measurement resolution and the second measurement resolution by using an alignment measurement function generally mounted in a semiconductor inspection apparatus. When inspecting a minute sample such as a semiconductor that satisfies the wafer replacement operation and the positioning operation during alignment measurement that do not require such high accuracy, it is based on the stage position information from the laser interferometer. High-precision positioning is possible.

According to the configuration as described above, it is possible to prevent the reflection mirror from becoming long, and to reduce the purchase cost and make it easy to attach.
Hereinafter, a specific configuration of the stage including the laser interferometer will be described with reference to the drawings. FIG. 1 is a diagram showing an outline of a scanning electron microscope which is an embodiment of a charged particle beam apparatus used for semiconductor measurement and inspection. In the following, a scanning electron microscope will be described as an example. However, it is also possible to provide a stage device to be described later in another charged particle beam device such as a focused ion beam irradiation device. Further, the stage apparatus described later can be applied not only to the charged particle beam apparatus but also to other beam irradiation apparatuses (such as a reduced projection exposure apparatus) that require highly accurate positioning.
In FIG. 1, it is roughly divided into a sample chamber 2, a lens barrel 15, and a load chamber 21. The sample chamber 2 can be evacuated by the sample chamber vacuum pump 1, and the inside of the sample chamber 2 and the lens barrel 15 is evacuated. The load chamber 21 is provided with a load chamber vacuum pump 27 so that the gate valve 26 and the load chamber 21 can be individually evacuated.
A sample stage 7 is mounted inside the sample chamber 2, and by rotating a pulse motor 10 (abbreviated as “PM” in FIG. 1) as an actuator, the X ball screw 11 converts rotational motion into linear motion. To do. At this time, the X-sliding guide member 5 as a guide mechanism fixed and attached to the base 3 is restrained, and the X-rod 12 attached to the X-ball screw 11 is pushed and pulled, so that one direction (X direction (FIG. 1 The sample stage 7 can be moved in the middle left and right direction)). At this time, as a means for measuring the position of the sample stage 7, the position of the stage is measured using a position detector such as a laser interferometer (abbreviated as “interferometer” in FIG. 1) 28 and the linear encoder 4. The sample stage 7 is similarly configured with a Y slip guide member 6 that intersects the X slide guide member 5 at a right angle, and the sample stage 7 is arranged in one direction (Y direction (in FIG. 1) along the Y slide guide member 6. Move back and forth)).
The control device 18 performs positioning control by rotating the pulse motor 10 based on the position information measured using the laser interferometer 28 and the linear encoder 4 described above. The control device 18 includes a microprocessor and a built-in memory, sequentially compares the target position and the current position of the sample stage 7, outputs a required amount of drive command signal to the pulse motor 10, and controls the sample stage 7. It is positioned.
An electron gun 16 serving as an electron beam source, a deflector 23 that changes the trajectory of the electron beam 22, an electron lens 14 that converges the electron beam 22, and secondary electrons 24 emitted from the wafer 13 are taken into the upper portion of the sample chamber 2. A lens barrel 15 incorporating a secondary electron detector 17 is mounted. The signal from the secondary electron detector 17 is subjected to signal processing by an image processing 19 and sent to an observation monitor 20.
Next, with reference to FIG. 1, FIG. 2, and FIG. 3, a wafer inspection procedure using a semiconductor measurement or inspection apparatus (hereinafter abbreviated as a semiconductor inspection apparatus) equipped with a scanning electron microscope will be described. . FIG. 2 is a top view of the scanning electron microscope illustrated in FIG. As illustrated in FIG. 2, the reflection mirror 8 provided on the sample stage 7 may not be irradiated with the beam emitted from the laser interferometer 28 (laser light source) depending on the position of the sample stage 7. This is a result of relatively shortening the reflecting mirror 8 as compared with the case where the entire moving range of the sample stage is covered, instead of simply enlarging the reflecting mirror 8 following the enlargement of the wafer 13. . In this embodiment, the length of the reflecting mirror 8 in the direction perpendicular to the laser orbit (for example, the length of the laser light receiving portion in the Y direction in the case of a mirror for specifying the stage position in the X direction) is the wafer 13. Is set to approximately the same diameter. This is because the laser interferometer 28 is positioned at the stage position where the wafer 13 overlaps with the electron beam irradiation position of the scanning electron microscope (ideal optical axis of the electron beam (for example, the intersection of the beam trajectories of the two laser interferometers 28 in FIG. 2)). This is a condition for enabling position specification at. On the other hand, in the apparatus of this embodiment, there is a stage moving range in which the position cannot be specified by the laser interferometer 28 (the laser is not applied to the reflection mirror).
Since operations other than electron beam irradiation (wafer replacement and alignment with an optical microscope) do not require high-accuracy positioning, this embodiment does not require a reflecting mirror. A stage apparatus is proposed that uses a first measuring apparatus with inferior accuracy and a second measuring apparatus capable of highly accurate position measurement but requiring a reflecting mirror. In addition, the two measurement devices are not used together, but both measurement ranges are overlapped, and whether the operation performed after moving the stage is electron beam irradiation (or an operation that does not require highly accurate alignment). And / or a stage apparatus provided with a control device that controls the two measurement apparatuses to be used properly or in combination based on the determination of whether the irradiation position of the wafer and the beam is a superimposed stage position.
According to the above configuration, the length of the reflecting mirror can be shortened to the same length as the diameter of the wafer 13 without providing a complicated position measuring device that uses a plurality of laser interferometers in one direction. . However, semiconductor wafers often do not have a pattern formed at the edge, so if the device is intended only for such wafers, the length of the edge where the pattern is not formed is subtracted from the wafer diameter. It can also be a reflective mirror.
The sample stage of the present embodiment is an XY stage that can move in the X direction and the Y direction. For example, when the position of the sample stage in the X direction is measured using a laser interferometer, a laser beam is irradiated from the X direction onto a reflecting mirror having a reflecting surface parallel to the Y direction and the Z direction. The moving range in the Y direction of the reflecting mirror for measuring the position in the X direction includes both a range in which the laser beam is irradiated to the reflecting mirror by the laser interferometer and a range in which the laser beam is not irradiated.
FIG. 3 is a flowchart of a wafer inspection method using the semiconductor inspection apparatus illustrated in FIG. The sample stage 7 in FIG. 2 is moved to the wafer exchange position by step S10 in FIG. In step S20, the gate valve 26 is opened. In step S30, the transfer arm 25 is extended, and the wafer 13 in the load chamber 21 and the wafer to be unloaded in the sample chamber 2 are exchanged. Here, it is assumed that there is a wafer to be unloaded in the sample chamber 2, but if there is no wafer to be unloaded, only the transfer into the sample chamber 2 is performed. In the next step S40, the gate valve 26 is closed. In the next step S50, the sample stage 7 is moved to the visual field range of the optical microscope 9. In the next step S60, low-magnification alignment measurement is performed using the optical microscope 9, the rotation amount and the offset amount between the stage coordinate system and the wafer coordinate system are calculated, and the target position is updated. In the next step S70, since the sample stage 7 enters the measurement range by the laser interferometer 28, the laser interferometer 28 is reset. Start measuring the stage position. In the next step S80, the sample stage 7 is moved to the visual field range of the lens barrel 15 in order to perform intermediate magnification alignment by the secondary electron image. Here, intermediate alignment using a secondary electron image is performed, and updated to a target position with a more accurate rotation amount, offset amount, and scale size corrected. At this time, as the target position of the sample stage 7, the target position updated in step S60 is used to position the lens barrel 15 in the visual field range.
The alignment at the intermediate magnification is performed by, for example, template matching using an alignment pattern or the like, the electron beam 22 is irradiated onto the wafer 13, and the reflected secondary electrons 24 are acquired by the secondary electron detector 16. A desired position is specified by image processing 19 from the secondary electron image. When the desired position is specified, the rotation amount and the offset amount are further calculated, and the target position is updated with high accuracy.
In step S90, the sample stage 7 is moved to an inspection point registered in advance, a position to be inspected is specified from image recognition, and a high-magnification secondary electron image is acquired. After step S90 is repeated for the number of inspection points, the sample stage 7 is moved to the wafer exchange position, the gate valve 26 is opened in the next step S110, and the wafer 13 that has been inspected in the next step S120 is inspected next. Replace the wafer. At this time, if there is no wafer to be inspected next, the gate valve 26 is closed in the next step S130, and a series of inspection processes is completed. If there is a wafer to be inspected next, steps S40 to S130 are repeated.
Here, a method of using the semiconductor inspection apparatus according to the present embodiment having the above-described configuration will be described in detail. In general, the user can evaluate the pattern shape of the wafer 13 as follows: (1) where the desired pattern is located in the chip; (2) which chip pattern is arranged on one wafer 13. Are registered in advance for each of them using the position. At the time of inspection, the control device 18 moves the sample stage 7 to the field of view of the optical microscope 9 based on the registered content, measures several measurement points, and rotates the coordinate system of the stage and the coordinate system of the wafer. Measure the offset amount. The target position is updated based on the measured rotation amount and offset amount. Next, based on the updated target position, the sample stage 7 is moved to the field of view of the electron beam 22, the electron beam 22 is irradiated to acquire a secondary electron image, and the rotation amount and the offset amount are measured again. , Update the target position.
The electron beam 22 is irradiated onto the wafer 13 and scanned by the deflector 23 to acquire a secondary electron image of several tens of thousands to several hundred thousand times and is displayed on the monitor 20. Then, the shape of the pattern is discriminated from the change in brightness of the secondary electron image, and the dimension value of the designated shape (pattern line width, pitch, etc.) is calculated. After that, it moves to the position of the next registered chip and repeats image acquisition in the same manner. The optical microscope 7 necessary for the alignment measurement is difficult to place the optical microscope 9 on the same line as the center of the electron beam due to physical interference with the electron lens 14. I have.
Next, the laser interferometer 28, the reflection mirror 8, and the linear encoder 4 which are stage position measuring means used in this embodiment will be described. The laser interferometer 28 has a measurement accuracy of several tens of picometers, and the sample stage 7 using the laser interferometer 28 can be positioned with high accuracy. Widely used in inspection equipment. However, the laser interferometer 28 loses the position of the stage because the measured value becomes indeterminate when the irradiated laser light deviates from the reflecting mirror 8. For this reason, the reflecting mirror 8 has been elongated so that the position of the sample stage 7 is not lost even if the sample stage 7 moves in the entire area of the sample chamber 2. However, the lengthening of the reflecting mirror 8 that requires a high degree of flatness is not only difficult in terms of workability attachment but also very expensive in terms of cost. In the case of an apparatus that needs to keep the inside of the vacuum 2, a structure that covers the structure that shields the atmosphere, such as the sample chamber 2 and the lens barrel 15, is required. In order to prevent the side surfaces from interfering with each other, there is a problem that the sample chamber 2 is enlarged. Further, the expansion of the sample chamber is accompanied by an expansion of the apparatus installation area and an increase in the apparatus transport cost.
In order to solve this problem, the present embodiment includes a stage position measuring method using the linear encoder 4 and a stage position measuring method using the laser interferometer 28. In the series of operation sequences shown in FIG. 2, the positioning of the sample stage 7 to the wafer exchange position and the positioning at the time of alignment measurement by the optical microscope 9 are as high as the positioning of the sample stage 7 using the laser interferometer 28. Does not require accuracy. Therefore, when the entire stroke in the sample chamber is moved, the sample stage 7 is positioned using the position value of the linear encoder 4. When performing the measurements in steps S80 and S90 in FIG. 2, the control is switched to the high-precision positioning control using the laser interferometer 28.
Next, a method for switching between the linear encoder 4 and the laser interferometer 28 will be described in detail with reference to FIG. In step S401, it is determined whether the next step is acquisition of a secondary electron image. If NO, positioning of the sample stage 7 by the linear encoder 4 is performed as before. If YES, it is determined in the next step S402 whether it is within the measurable range by the laser interferometer 28, and waits until YES. If YES, the laser value is reset in the next step S403, and the laser interferometer 28 is enabled immediately. In the next step S404, the stage is positioned while the linear encoder 4 is used. Next, in step S405, the intermediate alignment measurement using an electron beam is performed, and the target position is updated. In the next step S406, high-precision positioning using the laser interferometer 28 is performed based on the updated target position.
Next, the handling of the stage measurement value when the laser is reset will be described in detail. In step S404, the laser interferometer is reset. At this time, if the stage position measurement value by the laser interferometer is XL and the stage position measurement value by the linear encoder is XE, the same position information is obtained as in (Equation 1). substitute.
Xl = XE (Equation 1)
At this time, when the resolution of the laser interferometer is ΔXL and the resolution of the linear encoder is ΔXE, the relationship is as shown in (Equation 2).
ΔXL <ΔXE (Equation 2)
From the above, as shown in the following (Equation 3), an error corresponding to the maximum ΔXE is included only by simple substitution.
XL = XE ± ΔXE (Equation 3)
However, the final target position is updated by measuring the wafer position, the rotation amount of the stage position, and the offset amount again at the time of the medium magnification alignment by the secondary electron image in step S405. Therefore, if the final target position is compared with the current stage position measured by the laser interferometer, the remaining positional deviation amount can be calculated with high accuracy. Since it is only necessary to move the sample stage 7 by using the laser interferometer for the position deviation, the error included in the laser interferometer 28 included in step S404 can be ignored.
In the next step S406, the sample stage 7 is driven by the position deviation amount calculated in step S405. Alternatively, the electron beam 22 is deflected by the deflector 23 and the next step S407 is performed to acquire a high-magnification secondary electron image.
In the next step S408, it is determined whether there is a point from which a high-magnification secondary electron image is to be obtained. If there is a point, the process is repeated from step S406. If there is no more point to acquire the secondary electron image in step S408, the stage positioning operation of only the linear encoder 4 is performed ignoring the information of the laser interferometer 28.
In this embodiment, the linear encoder 5 is used for measuring the stage position corresponding to the full stroke. However, a position sensor attached to the sample stage 7 or a rotation angle detector installed on the motor and the ball screw 11 is used. The method of measuring the stage position can also be applied.
In the present embodiment, the present invention is applied to the wafer stage. However, the present invention can also be applied to a reticle stage.
In this embodiment, the optical microscope 9 is provided for alignment measurement. However, the present invention can also be applied to a method of performing low magnification alignment using an electron microscope.

DESCRIPTION OF SYMBOLS 1 Vacuum pump for sample chambers 2 Sample chamber 3 Base 4 Linear encoder 5 X slide guide member 6 Y slide guide member 7 Sample stage 8 Reflective mirror 9 Optical microscope 10 Pulse motor 11 Ball screw 12 X rod 13 Wafer 14 Electron lens 15 Lens tube DESCRIPTION OF SYMBOLS 16 Electron gun 17 Secondary electron detector 18 Control apparatus 19 Image processing 20 Monitor 21 Load chamber 22 Electron beam 23 Deflector 24 Secondary electron 25 Transfer arm 26 Gate valve 27 Vacuum pump for load chamber 28 Laser interferometer

Claims (14)

In a stage apparatus comprising a sample stage, a reflection mirror provided on the sample stage, and a laser interferometer for measuring the position of the sample stage by irradiating the reflection mirror with laser light,
Another stage position measuring device different from the laser interferometer for specifying the position of the sample stage, and a control device for switching between the laser interferometer and the other stage position measuring device, the sample stage, The control device is configured to be movable to a position where the laser beam is not irradiated on the reflection mirror, and when the sample stage is located at a position where the laser beam is not irradiated on the reflection mirror, the other stage A stage apparatus characterized by performing position measurement with a position measuring apparatus. In claim 1,
A stage apparatus characterized in that a position measurement range of the sample stage by the laser interferometer overlaps a position measurement range of the other stage position measurement apparatus. In claim 1,
The stage device characterized in that the control device switches between the laser interferometer and the other stage position measuring device according to the operation or position of the movement destination of the sample stage. In claim 1,
A beam light source for irradiating the sample supported on the sample stage with a beam light source; and the control device is configured to move a position of the sample by the sample stage to irradiate the sample with the beam from the beam light source. In this case, the stage apparatus performs position measurement by the laser interferometer. In claim 4,
A stage device comprising a sample exchange mechanism for exchanging the sample, wherein the control device performs position measurement by the other stage measurement device when exchanging the sample by the sample exchange mechanism. In claim 1,
The other stage position measuring device includes a linear encoder attached to a ball screw for driving the sample stage, or a position sensor for specifying the position of the sample stage. In claim 1,
The other stage position measuring device includes a rotation angle detector for detecting a rotation angle of a ball screw for driving the sample stage or a rotation angle of a motor for driving the sample stage. . In claim 1,
A stage device comprising a pulse motor for driving the sample stage, wherein the other stage position measuring device specifies the position of the sample stage according to the number of pulses to the pulse motor. A position of the sample stage is specified by irradiating a laser beam to a beam light source, a sample stage that movably supports the sample irradiated with the beam, a reflection mirror provided on the sample stage, and the reflection mirror In a beam irradiation apparatus equipped with a laser interferometer for
Another stage position measurement device different from the laser interferometer for specifying the position of the sample stage, and a control device for switching the laser interferometer and the other stage position measurement device, the reflection mirror, The length of the position measurement direction by the laser interferometer and the length perpendicular to the optical axis direction of the beam emitted from the beam light source is equal to or greater than the diameter of the sample supported on the sample stage, and the reflection mirror A beam irradiation apparatus, wherein a movement range when moving along a reflection surface of a reflection mirror includes a range in which the reflection mirror is not irradiated with the laser light. In claim 9,
The stage device characterized in that the control device switches between the laser interferometer and the other stage position measuring device according to the operation or position of the movement destination of the sample stage. A stage configured to be able to move two-dimensionally, a reflection mirror provided on the stage, and a first measurement unit that measures the position of the stage by irradiating the reflection mirror with laser light; In a stage apparatus provided with a control device for controlling the stage drive means based on this measurement position, and provided with an alignment measurement means for measuring the position of the sample mounted on the stage and the stage,
Compared with the first measurement means for measuring the position of the stage, the measurement means for the second stage position, which is less accurate, is arranged so as to overlap the first measurement region,
The first measuring means and the second measuring means for measuring the position of the stage are switched to measure the position of the stage, the stage position is controlled based on the stage position, and the first measuring means is adjusted by the alignment measuring means. A stage apparatus comprising a control device that compensates for the measurement accuracy of the measuring means and the second measuring means.   The stage apparatus according to claim 11, wherein a detector using a linear encoder or a position sensor is used as the second measuring means on the sample stage or the ball screw.   The stage apparatus according to claim 11, wherein a rotation angle detector that detects a rotation angle of a ball screw or a motor is used as the second measuring means.   12. The stage apparatus according to claim 11, wherein a second motor is used as a substitute by using a pulse motor as the stage driver and estimating the stage position from the number of pulses commanded to the pulse motor.